PATENT DOCUMENT

Publication Number: US-12115771-B2
Application Number: US-202217716911-A
Country: US
Kind Code: B2

Title: Electronic devices with multilayer adhesive

Abstract:
In devices with flexible displays, multilayer adhesive stacks may be included. A multilayer adhesive may attach a flexible display panel to the display cover layer in an electronic device. Including multiple layers of adhesive in the adhesive stack (as opposed to a single layer) provides more degrees of freedom for the tuning and optimization of the properties of the adhesive stack. The multilayer adhesive stack therefore has better performance than if only a single layer of adhesive is used. The multilayer adhesive stack may include one or more layers of soft adhesive, hard adhesive, hard elastomer, hard polymer, and/or glass to optimize the mechanical and optical performance of the multilayer adhesive stack. Soft adhesive layers may be included to optimize lateral decoupling (e.g., during folding and unfolding) of the adhesive stack. Harder layers may be included to provide rigidity and prevent denting during impact events.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 a flexible display panel; and 
 a multilayer adhesive stack that is attached to the flexible display panel, wherein the multilayer adhesive stack includes a first layer of adhesive formed from a first material, a second layer of adhesive formed from a second material and in direct contact with the first layer of adhesive, and a third layer of adhesive formed from a third material and in direct contact with the second layer of adhesive, wherein the first layer of adhesive has a first magnitude for a property, and wherein the second layer of adhesive has a second magnitude that is different than the first magnitude for the property. 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the property is modulus and wherein the first magnitude is less than 100 kPa. 
     
     
       3. The electronic device defined in  claim 2 , wherein the second magnitude is between 500 MPa and 10 GPa. 
     
     
       4. The electronic device defined in  claim 2 , wherein the second magnitude is between 0.01 MPa and 1 GPa. 
     
     
       5. The electronic device defined in  claim 2 , wherein the second magnitude is greater than 10 GPa. 
     
     
       6. The electronic device defined in  claim 1 , wherein the first material is a soft adhesive. 
     
     
       7. The electronic device defined in  claim 6 , wherein the second material is a hard adhesive. 
     
     
       8. The electronic device defined in  claim 6 , wherein the second layer of adhesive is a high-adhesion layer having a higher adhesion than the first layer of adhesive. 
     
     
       9. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack further comprises a hard polymer. 
     
     
       10. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack further comprises a hard elastomer. 
     
     
       11. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack further comprises glass. 
     
     
       12. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack also includes an adhesion promotion layer that is interposed between the first layer of adhesive and the second layer of adhesive. 
     
     
       13. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack includes a repeating unit of layers that includes the first layer of adhesive and the second layer of adhesive. 
     
     
       14. The electronic device defined in  claim 1 , wherein the property is glass transition temperature. 
     
     
       15. The electronic device defined in  claim 1 , wherein the property is transmission of ultraviolet light. 
     
     
       16. The electronic device defined in  claim 1 , wherein the second layer of adhesive is formed by modifying a portion of the first layer of adhesive by physical or chemical treatment. 
     
     
       17. The electronic device defined in  claim 1 , wherein the third material is different than the first and second materials. 
     
     
       18. The electronic device defined in  claim 17 , wherein the adhesive stack includes a fourth layer of adhesive formed from a fourth material that is different than the first, second, and third materials. 
     
     
       19. The electronic device defined in  claim 1 , wherein the first and second layers of adhesive are coplanar. 
     
     
       20. The electronic device defined in  claim 1 , wherein the first and second layers of adhesive are vertically stacked. 
     
     
       21. The electronic device defined in  claim 1 , wherein the flexible display panel is a scrollable display. 
     
     
       22. The electronic device defined in  claim 1 , wherein the multilayer adhesive stack includes a gradient interface layer between the first layer of adhesive and the second layer of adhesive. 
     
     
       23. The electronic device defined in  claim 1 , wherein the third material is the same as the first material, wherein the second layer of adhesive is interposed between the first and third layers of adhesive, and wherein the second layer of adhesive is part of an array of pillar structures formed from the second material. 
     
     
       24. An electronic device, comprising:
 a foldable housing that is configured to bend about a bend axis; 
 a flexible display panel that overlaps the bend axis; 
 a display cover layer that overlaps the flexible display panel; and 
 a multilayer adhesive stack that attaches the flexible display panel to the display cover layer, wherein the multilayer adhesive stack includes at least first, second, and third layers that are formed from different materials, and wherein the third layer has a lower glass transition temperature than the first and second layers. 
 
     
     
       25. The electronic device defined in  claim 24 , wherein the first layer is thicker than the second layer and wherein the first layer is softer than the second layer. 
     
     
       26. The electronic device defined in  claim 24 , wherein the first layer is formed from a first material, wherein the second layer is formed from a second material, wherein the first and second layers are coplanar, and wherein the second layer overlaps the bend axis. 
     
     
       27. The electronic device defined in  claim 24 , wherein the first layer comprises a high-adhesion layer and the second layer comprises a soft adhesive layer. 
     
     
       28. An electronic device, comprising:
 a flexible display panel; 
 a multilayer adhesive stack that is attached to the flexible display panel, wherein the multilayer adhesive stack includes a first layer formed from a first material and a second layer formed from a second material and wherein the first and second layers are coplanar; and 
 a display cover layer that overlaps the flexible display panel, wherein the multilayer adhesive stack is interposed between the flexible display panel and the display cover layer, wherein the flexible display panel has a bend axis, and wherein the first layer extends in a strip that overlaps the bend axis. 
 
     
     
       29. The electronic device defined in  claim 28 , wherein the second material is different than the first material.

Description:
This application claims the benefit of U.S. provisional patent application No. 63/197,882, filed Jun. 7, 2021, which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD 
     This relates generally to electronic devices, and, more particularly, to electronic devices with displays. 
     BACKGROUND 
     Electronic devices often have displays. Portability may be a concern for some devices, which tends to limit available real estate for displays. 
     SUMMARY 
     Electronic devices may be provided with displays. Displays may be used for displaying images for users. Displays may be formed from arrays of light-emitting diode pixels or other pixels. A device may have an organic light-emitting diode display or a display formed from an array of micro-light-emitting diodes (e.g., light-emitting diodes formed from crystalline semiconductor dies). 
     The displays may include flexible displays. Flexible displays may be bent about a bend axis to allow an electronic device to be folded and/or may be rolled around rollers. This allows the flexible display to be stored in an electronic device housing when a compact device arrangement is desired and to be pulled from within the electronic device housing when an enlarged display area is desired. 
     In devices with flexible displays, multilayer adhesive stacks may also be included. For example, a multilayer adhesive may attach a flexible display panel to the display cover layer in an electronic device. Including multiple layers of adhesive in the adhesive stack (as opposed to a single layer) provides more degrees of freedom for the tuning and optimization of the properties of the adhesive stack. The multilayer adhesive stack therefore has better performance than if only a single layer of adhesive is used. 
     The multilayer adhesive stack may include one or more layers of soft adhesive, hard adhesive, hard elastomer, hard polymer, and/or glass to optimize the mechanical and optical performance of the multilayer adhesive stack. Soft adhesive layers may be included to optimize lateral decoupling (e.g., during folding and unfolding) of the adhesive stack. 
     Harder layers may be included to provide rigidity and prevent denting during impact events. One or more adhesive layers may be included that are optimized for low temperature performance. One or more adhesion promotion layers may be included in the adhesive stack. 
     The multilayer adhesive stack may have different, coplanar materials in addition to different materials in the thickness direction. Softer materials may be included in a strip that overlaps a bend axis of the device, as one example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG.  2    is a perspective view of an illustrative electronic device with a display in accordance with an embodiment. 
         FIG.  3    is a cross-sectional side view of an illustrative electronic device in accordance with an embodiment. 
         FIG.  4    is a cross-sectional side view of an illustrative display with a multilayer adhesive stack that is interposed between a display panel and a display cover layer in accordance with an embodiment. 
         FIG.  5    is a cross-sectional side view of an illustrative display with a multilayer adhesive stack that is interposed between first and second portions of a display panel in accordance with an embodiment. 
         FIG.  6    is a cross-sectional side view of an illustrative display with a multilayer adhesive stack that is interposed between a display panel and an additional component in accordance with an embodiment. 
         FIG.  7    is a cross-sectional side view of an illustrative display with a multilayer adhesive stack that is interposed between first and second components in accordance with an embodiment. 
         FIG.  8    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a repeating unit of first and second layers in accordance with an embodiment. 
         FIG.  9    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a repeating unit of first, second, and third layers in accordance with an embodiment. 
         FIG.  10    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a repeating unit of first, second, third, and fourth layers in accordance with an embodiment. 
         FIG.  11    is a cross-sectional side view of an illustrative multilayer adhesive stack where the final repeating unit on one side of the multilayer stack includes less than all of the layers of the repeating unit in accordance with an embodiment. 
         FIGS.  12  and  13    are cross-sectional side views of illustrative multilayer adhesive stacks that include coplanar adhesive layers formed from different materials in accordance with an embodiment. 
         FIG.  14    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a soft adhesive layer that is interposed between first and second high-adhesion layers in accordance with an embodiment. 
         FIG.  15    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a soft adhesive layer and an ultraviolet light blocking layer in accordance with an embodiment. 
         FIG.  16    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes an adhesive layer with a low glass transition temperature in accordance with an embodiment. 
         FIG.  17    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes a high-damping adhesive layer in accordance with an embodiment. 
         FIG.  18    is a cross-sectional side view of an illustrative multilayer adhesive stack that includes adhesion promoting layers in accordance with an embodiment. 
         FIG.  19    is a cross-sectional side view of an illustrative multilayer adhesive stack with a gradient interface layer between vertically adjacent adhesive layers in accordance with an embodiment. 
         FIG.  20    is a cross-sectional side view of an illustrative multilayer adhesive stack with a gradient interface layer between horizontally adjacent adhesive layers in accordance with an embodiment. 
         FIG.  21    is a cross-sectional side view of an illustrative multilayer adhesive stack with an array of pillar structures in accordance with an embodiment. 
         FIG.  22    is a cross-sectional top view of the multilayer adhesive stack of  FIG.  21    in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with displays. Displays may be used for displaying images for users. Displays may be formed from arrays of light-emitting diode pixels or other pixels. For example, a device may have an organic light-emitting diode display or a display formed from an array of micro-light-emitting diodes (e.g., light-emitting diodes formed from crystalline semiconductor dies). 
     The displays may include flexible displays. Flexible displays may be bent about a bend axis to allow an electronic device to be folded and/or may be rolled around rollers. This allows the flexible display to be stored in an electronic device housing when a compact device arrangement is desired and to be pulled from within the electronic device housing when an enlarged display area is desired. An electronic device may incorporate both foldable and scrollable displays or may have foldable displays and/or scrollable displays in a housing that also includes one or more rigid displays. 
     A schematic diagram of an illustrative electronic device having a flexible display is shown in  FIG.  1   . Device  10  may be a cellular telephone, tablet computer, laptop computer, wristwatch device or other wearable device, a television, a stand-alone computer display or other monitor, a computer display with an embedded computer (e.g., a desktop computer), a system embedded in a vehicle, kiosk, or other embedded electronic device, a media player, or other electronic equipment. Configurations in which device  10  is a cellular telephone, tablet computer, or other portable electronic device may sometimes be described herein as an example. This is illustrative. Device  10  may, in general, be any suitable electronic device with a display. 
     Device  10  may include control circuitry  20 . Control circuitry  20  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  20  may use a display and other output devices in providing a user with visual output and other output. 
     To support communications between device  10  and external equipment, control circuitry  20  may communicate using communications circuitry  22 . Circuitry  22  may include antennas, radio-frequency transceiver circuitry (wireless transceiver circuitry), and other wireless communications circuitry and/or wired communications circuitry. Circuitry  22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  10  and external equipment over a wireless link (e.g., circuitry  22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link). Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 6 GHz and 300 GHz, a 60 GHz link, or other millimeter wave link, cellular telephone link, wireless local area network link, personal area network communications link, or other wireless communications link. Device  10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  10 . 
     Device  10  may include input-output devices such as devices  24 . Input-output devices  24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  24  may include one or more displays such as display  14 . Display  14  may be an organic light-emitting diode display, a liquid crystal display, an electrophoretic display, an electrowetting display, a plasma display, a microelectromechanical systems display, a display having a pixel array formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display. Configurations in which display  14  is an organic light-emitting diode display or microLED display are sometimes described herein as an example. 
     Display  14  may have an array of pixels configured to display images for a user. The pixels may be formed as part of a display panel that is flexible. A flexible display (e.g., an organic light-emitting diode display formed on a sheet of polymer or other flexible substrate and/or other flexible display pixel array structures) may be used to permit device  10  to be bent and/or stretched to allow display  14  to be folded and/or scrolled (e.g., to allow the visible area of display  14  to be expanded by moving display  14  in or out of a housing using a roller). A flexible display may be folded and unfolded about a bend axis. For example, a flexible (bendable) display in device  10  may be folded so that device  10  may be placed in a compact shape for storage and may be unfolded when it is desired to view images on the display. 
     Sensors  16  in input-output devices  24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors (e.g., a two-dimensional capacitive touch sensor integrated into display  14 , a two-dimensional capacitive touch sensor overlapping display  14 , and/or a touch sensor that forms a button, trackpad, or other input device not associated with a display), and other sensors. If desired, sensors  16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (lidar) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, and/or other sensors. In some arrangements, device  10  may use sensors  16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  10  may include additional components (see, e.g., other devices  18  in input-output devices  24 ). The additional components may include haptic output devices, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
       FIG.  2    is a perspective view of electronic device  10  in an illustrative configuration in which device  10  is a portable electronic device such as a cellular telephone or tablet computer. As shown in  FIG.  2   , device  10  may have a display such as display  14 . Display  14  may cover some or all of the front face of device  10 . Touch sensor circuitry such as two-dimensional capacitive touch sensor circuitry may be incorporated into display  14 . 
     Display  14  may be mounted in housing  12 . Housing  12  may form front and rear housing walls, sidewall structures, and/or internal supporting structures (e.g., a frame, an optional midplate member, etc.) for device  10 . Glass structures, transparent polymer structures, and/or other transparent structures that cover display  14  and other portions of device  10  may provide structural support for device  10  and may sometimes be referred to as housing structures. For example, a transparent housing portion such as a glass or polymer housing structure that covers and protects a pixel array in display  14  may serve as a display cover layer for the pixel array while also serving as a housing wall on the front face of device  10 . In configurations in which a display cover layer is formed from glass, the display cover layer may sometime be referred to as a display cover glass or display cover glass layer. The portions of housing  12  on the sidewalls and rear wall of device  10  may be formed from glass or other transparent structures and/or opaque structures. Sidewalls and rear wall structures may be formed as extensions to the front portion of housing  12  (e.g., as integral portions of the display cover layer) and/or may include separate housing wall structures. 
     Housing  12  may have flexible structures (e.g., bendable housing wall structures) and/or hinge structures such as hinge  30 . Hinge  30  may have a hinge axis aligned with device bend axis  28 . Hinge  30  and/or flexible housing structures that overlap bend axis  28  may allow housing  12  to bend about bend axis  28 . For example, housing  12  may have a first portion on one side of bend axis  28  and a second portion on an opposing side of bend axis  28  and these two housing portions may be coupled by hinge  30  for rotational motion about axis  28 . 
     As housing  12  is bent about bend axis  28 , the flexibility of display  14  allows display  14  to bend about axis  28 . In an illustrative configuration, housing  12  and display  14  may bend by 180°. This allows display  14  to be folded back on itself (with first and second outwardly-facing portions of display  14  facing each other). The ability to place device  10  in a folded configuration in this way may help make device  10  compact so that device  10  can be stored efficiently. When it is desired to view images on display  14 , device  10  may be unfolded about axis  28  to place device  10  in the unfolded configuration of  FIG.  2   . This allows display  14  to lie flat and allows a user to view flat images on display  14 . The ability to fold display  14  onto itself allows device  10  to exhibit an inwardly folding behavior. Display  14  may be sufficiently flexible to allow device  10  to be folded outwardly and/or inwardly. 
     Device  10  of  FIG.  2    has a rectangular outline (rectangular periphery) with four corners. As shown in  FIG.  2   , a first pair of parallel edges (e.g., the left and right edges of device  10  in the example of  FIG.  2   ) may be longer than a second pair of parallel edges (e.g., the upper and lower edges of device  10  of  FIG.  2   ) that are oriented at right angles to the first pair of parallel edges. In this type of configuration, housing  12  is elongated along a longitudinal axis that is perpendicular to bend axis  28 . Housing  12  may have other shapes, if desired (e.g., shapes in which housing  12  has a longitudinal axis that extends parallel to bend axis  28 ). With an arrangement of the type shown in  FIG.  2   , the length of device  10  along its longitudinal axis may be reduced by folding device  10  about axis  28 . 
       FIG.  3    is a cross-sectional side view of an illustrative foldable electronic device. Device  10  of  FIG.  3    may bend about bend axis  28 . Bend axis  28  may be aligned with display cover layer  14 CG or other structures in device  10 . For example, bend axis  28  may pass through a portion of display cover layer  14 CG or may be located above or below layer  14 CG. 
     As shown in  FIG.  3   , display  14  includes an array of pixels P forming display panel  14 P under an inwardly facing surface of display cover layer  14 CG. Display panel  14 P may be, for example, a flexible organic light-emitting diode display or a microLED display in which light-emitting pixels are formed on a flexible substrate layer (e.g., a flexible layer of polyimide or a sheet of other flexible polymer). Flexible support layer(s) for display  14  may also be formed from flexible glass, flexible metal, and/or other flexible structures. 
     Display cover layer  14 CG may be formed from polymer, glass, crystalline materials such as sapphire, other materials, and/or combinations of these materials. To locally increase flexibility, a portion of layer  14 CG that overlaps and extends along bend axis  28  may be locally thinned (e.g., this portion may be thinned relative to portions of layer  14 CG that do not overlap bend axis  28 ). The thickness of layer  14 CG (e.g., the non-thinned portions of layer  14 CG) may be 50-200 microns, 70-150 microns, 100-200 microns, 100-600 microns, at least 100 microns, at least 200 microns, less than 600 microns, less than 400 microns, less than 250 microns, less than 150 microns, less than 100 microns, at least 50 microns, or other suitable thickness. 
     In the example of  FIG.  3   , housing  12  has a portion on rear face R that forms a rear housing wall and has side portions forming sidewalls 12 W. The rear housing wall of housing  12  may form a support layer for components in device  10 . Housing  12  may also have one or more interior supporting layers (e.g., frame structures such as an optional midplate, etc.). These interior supporting layers and the rear housing wall may have first and second portions that are coupled to opposing sides of a hinge that is aligned with bend axis  28  (see, e.g., hinge  30  of  FIG.  2   ) or may be sufficiently flexible to bend around bend axis  28 . 
     Electrical components  32  may be mounted in the interior of device  10  (e.g., between display  14  and the rear of housing  12 . Components  32  may include circuitry of the type shown in  FIG.  1    (e.g., control circuitry  20 , communications circuitry  22 , input-output devices  24 , batteries, etc.). Display  14  may be mounted on front face F of device  10 . When device  10  is folded about axis  28 , display cover layer  14 CG, display panel  14 P, and the other structures of device  10  that overlap bend axis  28  may flex and bend to accommodate folding. 
       FIG.  4    is a cross-sectional side view of display  14  showing how a multilayer adhesive stack may be used to attach the display panel  14 P to the display cover layer  14 CG. Multilayer adhesive stack  42  (sometimes referred to as multilayer adhesive  42  or adhesive  42 ) may include two or more layers. Although some of the layers in the adhesive stack may have higher adhesion than other layers, all of the layers in multilayer adhesive stack  42  may be referred to as adhesive layers. Including multiple layers in adhesive stack  42  (as opposed to a single layer) may allow for optimization of the properties of the adhesive for the particular display application (e.g., a foldable display). 
     In the example where display  14  is a foldable display (e.g., that bends around a bend axis as in  FIG.  3   ), it may be desirable for adhesive  42  to be soft in order to have high decoupling in the lateral direction (to allow easy folding and unfolding). Specifically, the modulus of the adhesive may desirably be low in the lateral direction. However, it may also be desirable for adhesive  42  to be sufficiently hard to minimize denting in the thickness direction. The adhesive also needs to have sufficient adhesive strength to attach components within the display. The adhesive also may need to have sufficient performance at a wide range of operating temperatures. To balance these considerations, multiple layers of adhesive may be used in the adhesive stack. Including multiple layers of adhesive in the adhesive stack provides more degrees of freedom for the tuning and optimization of the properties of the adhesive stack. The multilayer adhesive stack therefore has better performance than if only a single layer of adhesive is used. 
       FIG.  4    shows an example where the multilayer adhesive is interposed between and in direct contact with display cover layer  14 CG and display panel  14 P. In other words, the multilayer adhesive has an upper surface that is in direct contact with the display cover layer  14 CG and a lower surface that is in direct contact with the display panel  14 P. This example is merely illustrative. Multilayer adhesive stacks may be incorporated in other locations within an electronic device with a flexible display. 
       FIG.  5    is a cross-sectional side view of a display showing an example where multilayer adhesive  42  is interposed between a first component  14 P- 1  in the display panel and a second component  14 P- 2  in the display panel. In other words, multilayer adhesive  42  may be used to attach adjacent layers within the display panel (different portions of the display panel). The multilayer adhesive has an upper surface that is in direct contact with display panel component  14 P- 1  and a lower surface that is in direct contact with display panel component  14 P- 2 . 
       FIG.  6    is a cross-sectional side view of a display showing an example where multilayer adhesive  42  is interposed between display panel  14 P and an additional component  44 . Additional component  44  may be a structural component (e.g., a housing wall, a midplate, etc.) and/or an electronic component (e.g., a printed circuit board, an input-output component, etc.). The multilayer adhesive has an upper surface that is in direct contact with display panel  14 P and a lower surface that is in direct contact with component  44 . 
       FIG.  7    is a cross-sectional side view of a display showing an example where multilayer adhesive  42  is interposed between component  44 - 1  and component  44 - 2 . Each one of components  44 - 1  and  44 - 2  may be a structural component (e.g., a housing wall, a midplate, etc.) and/or an electronic component (e.g., a display component, a printed circuit board, an input-output component, etc.). The multilayer adhesive has an upper surface that is in direct contact with component  44 - 1  and a lower surface that is in direct contact with component  44 - 2 . 
     When the multilayer adhesive is formed above the light-emitting portion of display panel  14 P (e.g., as in  FIGS.  4  and  5    but not  FIGS.  6  and  7   ), the multilayer adhesive may have a high transparency (e.g., greater than 80%, greater than 90%, greater than 95%, greater than 99%, etc.). When the multilayer adhesive is not formed above the light-emitting portion of display panel  14 P (e.g., as in  FIGS.  6  and  7   ), the multilayer adhesive need not necessarily have a high transparency and may have any desired transparency (e.g., may be opaque). 
     The multilayer adhesive stack may be formed from a repeating unit of two or more adhesive layers.  FIG.  8    is a cross-sectional side view of a multilayer adhesive stack  42  formed from a repeating unit of two adhesive layers. As shown, repeating unit  46  includes adhesive layer  42 A and adhesive layer  42 B. Adhesive layers  42 A and  42 B may be formed from different materials having different properties in order to optimize the performance of the adhesive. Each adhesive layer  42 A may be formed from the same material (e.g., a first material) and each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material). The repeating unit  46  is repeated throughout the multilayer adhesive stack. 
     The example in  FIG.  8    of the repeating unit including two different adhesive layers is merely illustrative. In another example, shown in  FIG.  9   , the repeating unit includes three different adhesive layers ( 42 A,  42 B, and  42 C). Adhesive layers  42 A,  42 B, and  42 C may be formed from different materials having different properties in order to optimize the performance of the adhesive. Each adhesive layer  42 A may be formed from the same material (e.g., a first material), each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material), and each adhesive layer  42 C may be formed from the same material (e.g., a third material that is different than the first and second materials). The repeating unit  46  is repeated throughout the multilayer adhesive stack. 
     In another example, shown in  FIG.  10   , the repeating unit includes four different adhesive layers ( 42 A,  42 B,  42 C, and  42 D). Adhesive layers  42 A,  42 B,  42 C, and  42 D may be formed from different materials having different properties in order to optimize the performance of the adhesive. Each adhesive layer  42 A may be formed from the same material (e.g., a first material), each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material), each adhesive layer  42 C may be formed from the same material (e.g., a third material that is different than the first and second materials), and each adhesive layer  42 D may be formed from the same material (e.g., a fourth material that is different than the first, second, and third materials). The repeating unit  46  is repeated throughout the multilayer adhesive stack. 
     If desired, the final repeating unit in the multilayer stack may not be included in its entirety.  FIG.  11    shows an example where, at the bottom of the multilayer adhesive stack, only adhesive layer  42 A is included (instead of layers  42 A and  42 B as in the preceding instances of the repeating unit). Each adhesive layer  42 A may be formed from the same material (e.g., a first material) and each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material). This concept may be applied to a multilayer adhesive stack regardless of the number of adhesive layers in the repeating unit. For example, when the repeating unit has three layers as in  FIG.  9   , only one of the three layers or only two of the three layers may be included on one side of the multilayer stack if desired. When the repeating unit has four layers as in  FIG.  10   , only one of the four layers, only two of the four layers, or only three of the four layers may be included on one side of the multilayer stack if desired. 
     In general, the multilayer adhesive  42  may include any desired number of adhesive layers (e.g., two layers, three layers, four layers, five layers, six layers, seven layers, eight layers, nine layers, ten layers, more than ten layers, more than twelve layers, more than twenty layers, more than forty layers, less than fifty layers, less than twenty layers, less than ten layers, less than six layers, between (inclusive) two and ten layers, etc.). The adhesive layers in multilayer adhesive may include a repeating unit of any desired number of layers (e.g., two layers, three layers, four layers, more than four layers, etc.). The final repeating unit on one side of the multilayer stack may optionally include less than all of the layers of the repeating unit. 
     In addition to including multiple layers in the thickness direction, the multilayer adhesive stack may be heterogenous within the XY-plane.  FIG.  12    shows how the multilayer adhesive stack  42  may include a repeating unit  46 . Similar to as in  FIG.  8   , the repeating unit  46  includes adhesive layers  42 A and  42 B. However, each adhesive layer  42 A is interrupted by a coplanar adhesive layer  42 C within the XY-plane and each adhesive layer  42 B is interrupted by a coplanar adhesive layer  42 D within the XY-plane. Adhesive layers  42 C and  42 D may overlap a bend axis of a foldable display, as one example. Adhesive layer  42 C may be surrounded (within the XY-plane) by adhesive layer  42 A and adhesive layer  42 D may be surrounded (within the XY-plane) by adhesive layer  42 B. Adhesive layers  42 A,  42 B,  42 C, and  42 D may be formed from different materials having different properties in order to optimize the performance of the adhesive. Each adhesive layer  42 A may be formed from the same material (e.g., a first material), each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material), each adhesive layer  42 C may be formed from the same material (e.g., a third material that is different than the first and second materials), and each adhesive layer  42 D may be formed from the same material (e.g., a fourth material that is different than the first, second, and third materials). The repeating unit  46  is repeated throughout the multilayer adhesive stack. 
       FIG.  13    is a cross-sectional side view of another multilayer adhesive stack with one or more layers that are heterogeneous within the XY-plane. Similar to as in  FIG.  8   , the repeating unit  46  includes adhesive layers  42 A and  42 B. However, each adhesive layer  42 B in  FIG.  13    is interrupted by an adhesive layer  42 A within the XY-plane. Adhesive layers  42 A (that are coplanar with adhesive layer  42 B) may overlap a bend axis of a foldable display, as one example. Adhesive layers  42 A overlapping the bend axis may be surrounded (within the XY-plane) by adhesive layer  42 B. Adhesive layers  42 A and  42 B may be formed from different materials having different properties in order to optimize the performance of the adhesive. Each adhesive layer  42 A may be formed from the same material (e.g., a first material) and each adhesive layer  42 B may be formed from the same material (e.g., a second material that is different than the first material). The repeating unit  46  is repeated throughout the multilayer adhesive stack. 
     In yet another possible arrangement, the adhesive stack may include only a single layer in the thickness direction but may be heterogeneous within the XY-plane. Within the single adhesive layer, adhesive of a first material may overlap a bend axis of a foldable display (as one example). The adhesive of the first material may be surrounded (within the XY-plane) by an adhesive layer of a second material that is different than the first material. The first material may be softer than the second material. 
     Any of the multilayer adhesive stacks in  FIG.  8 - 13    may be used in any of the locations shown in  FIGS.  4 - 7   . Any desired materials may be used in the aforementioned multilayer adhesive stacks of  FIGS.  8 - 13   . Illustrative materials that may be used for any of the adhesive layers include soft adhesive, hard adhesive, hard polymer, hard elastomer, glass, etc. Generally, materials may be characterized by Young&#39;s modulus and tensile modulus (E) that measure tensile stress and tensile strain behavior and shear modulus (G) that measure shear stress and sheer strain. Modulus is a complex number with a real part characterizing the elastic behavior and an imaginary part characterizing viscous behavior. For example, tensile modulus (E) is a complex modulus (E*) with a real part (storage modulus E′) and an imaginary part (loss modulus E″). Shear modulus (G) is a complex modulus (G*) with a real part (storage modulus G′) and an imaginary part (loss modulus G″). Each layer in the multilayer adhesive stack may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.001 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, greater than 1 MPa, between 1 MPa and 1 gigapascal (GPa), between 1 MPa and 500 MPa, between 500 MPa and 1 GPa, between 500 MPa and 1.5 GPa, greater than 1 GPa, greater than 10 GPa, greater than 20 GPa, etc. Each layer in the multilayer adhesive stack may have a modulus at various other temperatures (e.g., −20 degrees Celsius, −40 degrees Celsius, 65 degrees Celsius, 80 degrees Celsius, greater than −20 degrees Celsius, greater than 0 degrees Celsius, greater than 20 degrees Celsius, between (inclusive) −20 degrees Celsius and 70 degrees Celsius, less than 70 degrees Celsius, less than 20 degrees Celsius, less than 0 degrees Celsius, less than −20 degrees Celsius, less than −40 degrees Celsius, etc.) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.001 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, greater than 1 MPa, between 1 MPa and 1 gigapascal (GPa), between 1 MPa and 500 MPa, between 500 MPa and 1 GPa, between 500 MPa and 1.5 GPa, greater than 1 GPa, greater than 10 GPa, greater than 20 GPa, etc. 
     Modulus characterizes small deformation behavior in the linear regime. Materials (e.g. materials used in the adhesive stack) may also be characterized by large deformation behavior. For example, each layer in the multilayer adhesive stack may have a shear stress (at room temperature and 500% shear strain using 200%/min strain rate) of between 5 kPa and 50 kPa, less than 1 kPa, less than 0.1 kPa, less than 10 kPa, less than 100 kPa, less than 1000 kPa, less than 1 GPa, more than 1 kPa, more than 0.1 kPa, more than 10 kPa, more than 100 kPa, more than 1000 kPa, more than 1 GPa, between 1 kPa and 100 kPa, etc. This example of shear stress test conditions is merely illustrative. In general, at any desired temperature (e.g., −20 degrees Celsius, −40 degrees Celsius, 65 degrees Celsius, 80 degrees Celsius, greater than −20 degrees Celsius, greater than 0 degrees Celsius, greater than 20 degrees Celsius, between (inclusive) −20 degrees Celsius and 70 degrees Celsius, less than 70 degrees Celsius, less than 20 degrees Celsius, less than 0 degrees Celsius, less than −20 degrees Celsius, less than −40 degrees Celsius, etc.), any desired strain level (e.g., 500%, 200%, 2000%, less than 200%, less than 500%, less than 2000%, more than 200%, more than 500%, more than 2000%, between 200% and 2000%, etc.), and any desired strain rate (e.g., 200%/min, 2000%/min, 50,000%/min, more than 200%/min, more than 2000%/min, more than 50,000%/min, less than 200%/min, less than 2000%/min, less than 50,000%/min, between 200%/min and 2000%/min, between 200%/min and 50,000%/min, etc.), each layer in the multilayer adhesive stack may have a shear stress of between 5 kPa and 50 kPa, less than 1 kPa, less than 0.1 kPa, less than 10 kPa, less than 100 kPa, less than 1000 kPa, less than 1 GPa, more than 1 kPa, more than 0.1 kPa, more than 10 kPa, more than 100 kPa, more than 1000 kPa, more than 1 GPa between 1 kPa and 100 kPa, etc. 
     Each layer in the multilayer adhesive stack may have a glass transition temperature that is greater than −20 degrees Celsius, greater than 0 degrees Celsius, greater than 20 degrees Celsius, between (inclusive) −20 degrees Celsius and 70 degrees Celsius, less than 70 degrees Celsius, less than 20 degrees Celsius, less than 0 degrees Celsius, less than −20 degrees Celsius, less than −40 degrees Celsius, etc. Each layer in the multilayer adhesive stack may have a transparency to visible light that is greater than 99.9%, greater than 99%, greater than 95%, greater than 90%, greater than 80%, less than 80%, less than 70%, less than 50%, between (inclusive) 90% and 100%, etc. Each layer in the multilayer adhesive stack may have a thickness that is less than 200 microns, less than 100 microns, less than 50 microns, less than 40 microns, less than 30 microns, less than 20 microns, less than 10 microns, less than 5 microns, less than 1 micron, greater than 1 micron, greater than 5 microns, between 1 and 40 microns, between 3 and 25 microns, etc. The total thickness of multilayer adhesive stack  42  may be less than 200 microns, less than 100 microns, less than 75 microns, less than 50 microns, greater than 35 microns, between 25 microns and 75 microns, between 25 microns and 200 microns, etc. 
     One illustrative multilayer adhesive stack may include alternating layers of soft adhesive and hard polymer. For example, adhesive layer  42 A in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a soft adhesive and adhesive layer  42 B in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a hard polymer. The soft adhesive layer  42 A may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, less than 0.01 MPa, less than 0.001 MPa, between 0.01 MPa and 0.5 MPa, between 0.01 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. The hard polymer layer  42 B may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of greater than 1 GPa, greater than 10 GPa, between 0.5 GPa and 5 GPa, etc. 
     The hard polymer layers  42 B may have a greater modulus (E′, E″, E*, G′, G″, or G*) than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, a factor of 100,000 or more, etc.). The hard polymer layers  42 B may have a greater shear modulus at 10 kHz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, a factor of 100,000 or more, etc.). 
     The soft adhesive layers  42 A provide a low shear modulus in the lateral direction in order to optimize decoupling during folding of the display. The hard polymer layers  42 B make the multilayer adhesive stack more rigid in the thickness direction to prevent denting of the adhesive stack during impact events. The hard polymer material may be polyimide or polyethylene terephthalate, as examples. 
     Properties of the overall multilayer adhesive stack may be tuned by tuning the thicknesses of each individual layer within the stack. For example, the shear modulus of the multilayer adhesive stack at a given frequency may be determined according to the equation 1/P 0 =a/(P 1 )+b/(P 2 )+c/(P 3 )+d/(P 4 ) . . . , where P 0  is the shear modulus for the overall multilayer stack at the given frequency, P 1  is the shear modulus for the first layer in the stack at the given frequency, a is the coefficient of linear combination for the first layer in the stack (e.g., a is equal to the thickness of the first layer divided by the total thickness of the stack), P 2  is the shear modulus for the second layer in the stack at the given frequency, b is the coefficient of linear combination for the second layer in the stack (e.g., b is equal to the thickness of the second layer divided by the total thickness of the stack), etc. 
     The thicknesses of the individual layers may therefore be tuned to tune the overall properties of multilayer adhesive stack  42 . Layers of the same material may have different thicknesses or the same thicknesses (e.g., a first layer  42 A may have the same thickness or a different thickness than a different layer  42 A). Layers of different materials may have different thicknesses or the same thicknesses (e.g., a layer  42 A may have the same thickness or a different thickness than a layer  42 B). 
     Another illustrative multilayer adhesive stack may include alternating layers of soft adhesive and hard elastomer. For example, adhesive layer  42 A in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a soft adhesive and adhesive layer  42 B in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a hard elastomer. The soft adhesive layer  42 A may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.001 MPa and 0.01 MPa, between 0.01 MPa and 0.5 MPa, between 0.01 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. The hard elastomer layer  42 B may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of greater than 0.1 MPa, greater than 1 MPa, between 1 MPa and 1 gigapascal (GPa), between 1 MPa and 500 MPa, between 500 MPa and 1 GPa, etc. 
     The hard elastomer layers  42 B may have a greater modulus at 1 Hz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, etc.). The hard elastomer layers  42 B may have a greater modulus at 10 kHz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, etc.). 
     The soft adhesive layers  42 A provide a low shear modulus in the lateral direction in order to optimize decoupling during folding of the display. The hard elastomer layers  42 B make the multilayer adhesive stack more rigid in the thickness direction to prevent denting of the adhesive stack during impact events. 
     Another illustrative multilayer adhesive stack may include alternating layers of soft adhesive and glass. For example, adhesive layer  42 A in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a soft adhesive and adhesive layer  42 B in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from glass. The soft adhesive layer  42 A may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.001 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. The glass layer  42 B may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and an oscillation frequency of 1 Hz) of greater than 1 GPa, greater than 10 GPa, greater than 20 GPa, greater than 30 GPa, between 10 GPa and 30 GPa, between 15 GPa and 25 GPa, etc. 
     The glass layers  42 B may have a greater modulus (E′, E″, E*, G′, G″, or G*) at 1 Hz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, a factor of 100,000 or more, etc.). The glass layers  42 B may have a greater modulus (E′, E″, E*, G′, G″, or G*) at 10 kHz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, a factor of 1,000, a factor of 10,000 or more, a factor of 100,000 or more, etc.). 
     The soft adhesive layers  42 A provide a low shear modulus in the lateral direction in order to optimize decoupling during folding of the display. The glass layers  42 B make the multilayer adhesive stack more rigid in the thickness direction to prevent denting of the adhesive stack during impact events. 
     Another illustrative multilayer adhesive stack may include alternating layers of soft adhesive and hard adhesive. For example, adhesive layer  42 A in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a soft adhesive and adhesive layer  42 B in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from a hard adhesive. The soft adhesive layer  42 A may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and at 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.001 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. The hard adhesive layer  42 B may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and 1 Hz) of greater than 0.5 MPa, greater than 0.3 MPa, between 0.5 MPa and 1 MPa, less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.01 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. 
     The hard adhesive layers  42 B may have a greater modulus at 1 Hz (E′, E″, E*, G′, G″, or G*) than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, etc.). The hard adhesive layers  42 B may have a greater modulus (E′, E″, E*, G′, G″, or G*) at 10 kHz than soft adhesive layers  42 A (by a factor of 1.2 or more, a factor of 1.5 or more, a factor of 2 or more, a factor of 10 or more, a factor of 100 or more, etc.). 
     As one specific example, soft adhesive layers  42 A may have a modulus of less than 1 MPa (e.g., between 0.5 MPa and 1 MPa) at an oscillation frequency of 10 kHz whereas hard adhesive layers  42 B may have a modulus of greater than 1 MPa (e.g., between 1 MPa and 6 MPa) at an oscillation frequency of 10 kHz. 
     The soft adhesive layers  42 A provide a low shear modulus in the lateral direction in order to optimize decoupling during folding of the display. The hard adhesive layers  42 B make the multilayer adhesive stack more rigid in the thickness direction to prevent denting of the adhesive stack during impact events. 
     Another illustrative multilayer adhesive stack may include alternating layers of adhesive optimized for normal temperatures (e.g., room temperature) and adhesive optimized for low temperatures (e.g., temperatures below 0 degrees Celsius). For example, adhesive layer  42 A in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from an adhesive optimized for normal temperatures and adhesive layer  42 B in  FIG.  8    (or  FIGS.  9 - 13   ) may be formed from an adhesive optimized for low temperatures. Adhesive layers  42 A (optimized for normal temperatures) in the multilayer adhesive stack may have a glass transition temperature that is greater than −20 degrees Celsius, greater than 0 degrees Celsius, greater than 20 degrees Celsius, between (inclusive) −20 degrees Celsius and 70 degrees Celsius, less than 70 degrees Celsius, etc. Adhesive layers  42 B (optimized for low temperatures) in the multilayer adhesive stack may have a glass transition temperature that is less than 0 degrees Celsius, less than −20 degrees Celsius, less than −40 degrees Celsius, etc. 
     The glass transition temperature for adhesive layers  42 B may be lower than the glass transition temperature for adhesive layers  42 A (e.g., by 5 degrees Celsius or more, by 20 degrees Celsius or more, by 40 degrees Celsius or more, by 60 degrees Celsius or more, by 100 degrees Celsius or more, etc.). 
     The layers optimized for low temperatures may have shear stress (at room temperature and 500% shear strain using 200%/min strain rate) of between 1 kPa and 30 kPa, between 1 kPa and 50 kPa, less than 1 kPa, less than 0.1 kPa, less than 10 kPa, less than 100 kPa, less than 1000 kPa, less than 1 GPa, more than 1 kPa, more than 0.1 kPa, more than 10 kPa, more than 100 kPa, more than 1000 kPa, more than 1 GPa between 1 kPa and 100 kPa, etc. The layers optimized for low temperatures may have shear stress (at −20 degrees Celsius and 500% shear strain using 200%/min strain rate) of between 1 kPa and 30 kPa, between 1 kPa and 50 kPa, less than 1 kPa, less than 0.1 kPa, less than 10 kPa, less than 100 kPa, less than 1000 kPa, less than 1 GPa, more than 1 kPa, more than 0.1 kPa, more than 10 kPa, more than 100 kPa, more than 1000 kPa, more than 1 GPa between 1 kPa and 100 kPa, etc. The layers optimized for low temperatures may have shear stress (at 0 degrees Celsius and 500% shear strain using 200%/min strain rate) of between 1 kPa and 30 kPa, between 1 kPa and 50 kPa, less than 1 kPa, less than 0.1 kPa, less than 10 kPa, less than 100 kPa, less than 1000 kPa, less than 1 GPa, more than 1 kPa, more than 0.1 kPa, more than 10 kPa, more than 100 kPa, more than 1000 kPa, more than 1 GPa between 1 kPa and 100 kPa, etc. 
     Including adhesive layers that are optimized for different temperatures may optimize the performance of the multilayer adhesive stack across a wide range of temperatures. 
     In a multilayer adhesive stack that varies in the lateral direction (within the XY-plane) in addition to the thickness direction (e.g., as in  FIGS.  12  and  13   ), soft adhesives may be included in a hinge area to optimize for decoupling during folding. For example, in  FIG.  12   , adhesive layers  42 C and  42 D may be formed from softer materials (e.g., having smaller modulus at the same conditions) than adhesive layers  42 A and  42 B. Adhesive layers  42 C and  42 D may overlap a bend axis (e.g., hinge area) of the flexible display (see bend axis  28  in  FIG.  3   ). The soft materials in the hinge area may be optimized for folding whereas the harder materials ( 42 A and  42 B in  FIG.  12   ) are optimized for surface dent prevention and recovery. 
     In  FIG.  13   , adhesive layers  42 A may be formed from a softer material (e.g., having smaller modulus at the same conditions) than adhesive layers  42 B. Adhesive layers  42 A that are coplanar with adhesive layers  42 B may overlap a bend axis (e.g., hinge area) of the flexible display (see bend axis  28  in  FIG.  3   ). The hinge area therefore only includes the soft adhesive material  42 A (to optimize for decoupling during folding) whereas the surrounding non-hinge areas include a harder material ( 42 B in  FIG.  13   ) that is optimized for surface dent prevention and recovery. In  FIG.  13   , layers  42 A may be formed from a soft adhesive whereas layers  42 B may be formed from a hard polymer (as one example). 
     The example in  FIG.  13    of having different adhesive materials in the hinge area (that extends in a strip parallel to bend axis  28  across the width of the display/device and adjacent on both sides to non-hinge areas) is merely illustrative. In general, the multilayer adhesive stack may vary within the XY-plane in any desired pattern (e.g., with any desired number of areas having different adhesive materials). 
     In yet another possible arrangement, the adhesive stack may include only a single layer in the thickness direction (e.g., in the Z-direction in  FIG.  12   ) but may be heterogeneous within the XY-plane (e.g., only the top layer of  FIG.  12    or only the second layer of  FIG.  12    may be included in the adhesive stack). Within the single adhesive layer, adhesive of a first material may overlap a bend axis of a foldable display (as one example). The adhesive of the first material may be surrounded (within the XY-plane) by an adhesive layer of a second material that is different than the first material. The first material may be softer than the second material. 
       FIG.  14    is a cross-sectional side view of another multilayer adhesive stack  42 . The adhesive stack in  FIG.  14    includes a soft adhesive layer  42 B. The soft adhesive layer  42 B may provide high decoupling to allow the multilayer adhesive stack to be repeatedly bent during operation of the flexible display. However, soft adhesives (such as adhesive layer  42 B) may tend to have lower adhesion than harder adhesives. Accordingly, first and second high-adhesion layers  42 A may be formed on one or both sides of the soft adhesive layer  42 B. The adhesive layers  42 A may have a higher adhesion than adhesive layer  42 B. This allows the multilayer adhesive stack to strongly couple to adjacent components within the electronic device. Additionally, because adhesive layers  42 A are thin, the overall decoupling capability of the multilayer adhesive stack is not significantly affected. 
     The example of two high-adhesion layers of the same type being used in the multilayer adhesive stack is merely illustrative. If desired, high-adhesion layers formed from different materials may be included in the multilayer adhesive stack. As one specific example, a first high-adhesion layer at the top of the stack may be optimized to adhere to a glass layer that is formed above the multilayer adhesive stack. A second high-adhesion layer at the bottom of the stack may be optimized to adhere to a polymer layer that is formed below the multilayer adhesive stack. The first and second high-adhesion layers may be formed from different materials. 
     Each adhesive layer  42 A may be thinner than adhesive layer  42 B. Each adhesive layer  42 A may have a thickness that is less than 50% of the thickness of adhesive layer  42 B, may have a thickness that is less than 30% of the thickness of adhesive layer  42 B, may have a thickness that is less than 15% of the thickness of adhesive layer  42 B, may have a thickness that is less than 10% of the thickness of adhesive layer  42 B, may have a thickness that is less than 5% of the thickness of adhesive layer  42 B, etc. Adhesive layer  42 B may be formed from a softer material (e.g., having smaller modulus at the same conditions) than adhesive layers  42 A. As one specific example, soft adhesive layer  42 B may have a modulus of less than 0.1 MPa (e.g., between 0.01 MPa and 0.1 MPa) at an oscillation frequency of 1 Hz whereas adhesive layers  42 A may have a modulus of greater than 0.1 MPa (e.g., between 0.1 MPa and 0.3 MPa) at an oscillation frequency of 1 Hz. 
     In some applications, it may be desirable for the multilayer adhesive stack to block ultraviolet light. In these cases, a thin layer of ultraviolet light blocking adhesive may be included in addition to a soft adhesive layer.  FIG.  15    is a cross-sectional side view of a multilayer adhesive stack with an ultraviolet blocking layer. As shown, the adhesive stack in  FIG.  15    includes a soft adhesive layer  42 A. The soft adhesive layer  42 A may provide high decoupling to allow the multilayer adhesive stack to be repeatedly bent during operation of the flexible display. The soft adhesive layer  42 A may have a modulus (E′, E″, E*, G′, G″, or G*) (e.g., at room temperature and 1 Hz) of less than 1 megapascal (MPa), less than 0.5 Mpa, less than 0.1 MPa, less than 0.05 MPa, between 0.01 MPa and 0.5 MPa, between 0.001 MPa and 0.1 MPa, between 0.1 MPa and 0.2 MPa, etc. 
     Soft adhesive layer  42 A may be optimized for the desired mechanical properties of the multilayer adhesive stack. Ultraviolet light blocking adhesive layer  42 B, meanwhile, is optimized for the desired optical properties of the multilayer adhesive stack. Ultraviolet light blocking adhesive layer  42 B may block more than 30% of ultraviolet light, more than 50% of ultraviolet light, more than 70% of ultraviolet light, more than 80% of ultraviolet light, more than 90% of ultraviolet light, more than 95% of ultraviolet light, more than 99% of ultraviolet light, etc. Said another way, ultraviolet light blocking adhesive layer  42 B may transmit less than 70% of ultraviolet light, less than 50% of ultraviolet light, less than 30% of ultraviolet light, less than 20% of ultraviolet light, less than 10% of ultraviolet light, less than 5% of ultraviolet light, less than 1% of ultraviolet light, etc. This may prevent ultraviolet light from damaging an underlying display panel (e.g., as in  FIG.  4   ). To ensure that light from the display panel is still visible to a viewer, ultraviolet light blocking adhesive layer  42 B may have a transparency to visible light that is greater than 99.9%, greater than 99%, greater than 95%, greater than 90%, greater than 80%, etc. 
     Adhesive layer  42 B may be thinner than adhesive layer  42 A. Adhesive layer  42 B in  FIG.  15    may have a thickness that is less than 50% of the thickness of adhesive layer  42 A, may have a thickness that is less than 30% of the thickness of adhesive layer  42 A, may have a thickness that is less than 15% of the thickness of adhesive layer  42 A, may have a thickness that is less than 10% of the thickness of adhesive layer  42 A, may have a thickness that is less than 5% of the thickness of adhesive layer  42 A, etc. Adhesive layer  42 B may transmit less ultraviolet light than adhesive layer  42 A. 
     In the example of  FIG.  14   , a soft adhesive layer is interposed between first and second high-adhesion layers that are harder than the soft adhesive layer. To further optimize for different temperature conditions, the multilayer adhesive stack may also include an adhesive layer that is optimized for low temperatures.  FIG.  16    is a cross-sectional side view of a multilayer adhesive stack that includes an adhesive layer  42 C that is optimized for low temperatures. Adhesive layer  42 C is interposed between a soft adhesive layer  42 B and a harder, high-adhesion layer  42 A (both already described in connection with  FIG.  14   ). 
     Adhesive layers  42 A and  42 B (optimized for normal temperatures) in the multilayer adhesive stack may have a glass transition temperature that is greater than −20 degrees Celsius, greater than 0 degrees Celsius, greater than 20 degrees Celsius, between (inclusive) −20 degrees Celsius and 70 degrees Celsius, less than 70 degrees Celsius, etc. Adhesive layers  42 C (optimized for low temperatures) in the multilayer adhesive stack may have a glass transition temperature that is less than 0 degrees Celsius, less than −20 degrees Celsius, less than −40 degrees Celsius, etc. 
     To further optimize the multilayer adhesive stack, the multilayer adhesive stack may also include an adhesive layer that is optimized for high damping and cushioning performance under high frequency of stress to reduce the impact force to the delicate layers in the display during impact events.  FIG.  17    is a cross-sectional side view of a multilayer adhesive stack that includes a soft adhesive layer  42 C (similar to layer  42 B as described in  FIGS.  14  and  16   ) that is optimized for high decoupling, an adhesive layer  42 D (similar to layer  42 C as described in  FIG.  16   ) that is optimized for low temperatures, high-adhesion layers  42 A as described in  FIGS.  14  and  16   , and a high-damping adhesive layer  42 B. In  FIG.  17   , the high-damping adhesive layer  42 B is interposed between high-adhesion layer  42 A and soft adhesive layer  42 C. The high-damping adhesive layer  42 B may have a high loss modulus to storage modulus ratio (tan delta) at impact frequency to maximize impact energy absorption and dissipation. Alternatively or in addition, the high-damping adhesive layer  42 B may have a low storage modulus at impact frequency to prolong the impact duration and thus reduce the instantaneous peak stress. The high-damping adhesive layer  42 B may add impact damping or cushioning capability to the multilayer adhesive stack to prevent panel failure during impacts. 
     Adhesive layer  42 B in  FIG.  17    may be formed from a more viscous material or softer material under high frequency of stress (e.g., having a higher tan delta or lower storage modulus at the same conditions) than adhesive layer  42 C. 
     To ensure adequate adhesion between adjacent layers within the multilayer adhesive stack, one or more adhesion promoting layers may be included in the multilayer adhesive stack.  FIG.  18    is a cross-sectional side view of a multilayer adhesive stack that includes adhesion promoting layers. First and second adhesion promoting layers  42 B are included in the multilayer stack of  FIG.  18   . Each adhesion promoting layer  42 B is interposed between a respective high-adhesion layer  42 A (as described in connection with  FIG.  14   ) and soft adhesive layer  42 C (similar to layer  42 B as described in connection with  FIG.  14   ). 
     Adhesion promoting layer  42 B may be formed from a material that has a high adhesion to both adhesive layers  42 C and  42 A. With the presence of adhesion promoting layer  42 B, the adhesion between adhesive layers  42 A and  42 C may be stronger than if adhesive layers  42 A and  42 C were attached directly together (e.g., as in  FIG.  14   ). 
     The order of the layers in the multilayer adhesive stacks shown herein are merely illustrative. The orders of the layers within the multilayer adhesive stacks may be changed if desired. 
     It should be understood that the interface between layers within adhesive  42  may not necessarily be a sharp interface with a well defined boundary. Instead, the interface between layers  42  within adhesive  42  may sometimes follow a composition gradient.  FIG.  19    is a cross-sectional side view of a multilayer adhesive stack  42  of this type. As shown, a gradient interface layer  42 I is interposed between adhesive layer  42 A and adhesive layer  42 B. Adhesive layers  42 A and  42 B may be formed from different materials having different properties in order to optimize the performance of the adhesive (as discussed in detail above). Gradient interface layer  42 I may have a composition that is 100% equal to the composition of adhesive layer  42 A at the border between layers  42 I and  42 A. Gradient interface layer  42 I may have a composition that is 100% equal to the composition of adhesive layer  42 B at the border between layers  42 I and  42 B. The composition of layer  42 I may gradually change while moving from the border adjacent to layer  42 A towards the border adjacent to layer  42 B (e.g., with an increasing proportion of composition equal to the composition of adhesive layer  42 B and a decreasing proportion of composition equal to the composition of adhesive layer  42 A). 
     The gradient can be in the vertical direction and/or in the horizontal direction.  FIG.  19    shows an example where the gradient is in the vertical direction (parallel to the Z-axis).  FIG.  20    shows an example where the gradient is in the horizontal direction (parallel to the X-axis). 
     Gradient interface layer  42 I may be formed naturally due to inter-diffusion between adjacent layers (e.g., immediately after fabrication and/or slowly over time). Alternatively, gradient interface layer  42 I may be introduced by design to achieve certain performance benefits. 
     The gradient may be a material/chemistry gradient (e.g., a gradient of additive concentration, material type, molecular weight, polymer cross-linking level, polymer chain branching, etc.) and/or a property gradient (e.g., a gradient of modulus, glass transition temperature, refractive index, etc.). 
     Gradient interface layer  42 I has a thickness  52  (in the direction of the gradient). 
     Thickness  52  may be less than 1 centimeter, less than 1 millimeter, less than 1 micron, less than 1 nanometer, greater than 1 centimeter, greater than 1 millimeter, greater than 1 micron, greater than 1 nanometer, etc.). The gradient across thickness  52  may be linear or non-linear. 
       FIGS.  21  and  22    show yet another possible arrangement for a multilayer adhesive stack.  FIG.  21    is a cross-sectional side view of a multilayer adhesive stack  42  with an array of pillar structures.  FIG.  22    is a cross-sectional top view of the multilayer adhesive stack of  FIG.  21   . 
     As shown in  FIG.  21   , first and second adhesive layers  42 A may be separated by a gap. An array of patches of adhesive layer  42 B (sometimes referred to as pillars  42 B) are interposed between the first and second adhesive layers  42 A. The pillars of adhesive layer  42 B are discrete pillars arranged in a grid (as one example). The space  54  between the patches of adhesive layer  42 B may be filled with air or another desired filler material (e.g., an additional adhesive layer  42 C). Adhesive layers  42 A and  42 B may be formed from different materials having different properties in order to optimize the performance of the adhesive (as discussed in detail above). 
     The magnitude of the gap between first and second adhesive layers  42 A in  FIG.  21    (which is also equal to the thickness of adhesive layers  42 B) may be less than 1 centimeter, less than 1 millimeter, less than 1 micron, less than 1 nanometer, greater than 1 centimeter, greater than 1 millimeter, greater than 1 micron, greater than 1 nanometer, etc.). 
     Any of the multilayer adhesive stacks in  FIG.  8 - 21    may be used in any of the locations shown in  FIGS.  4 - 7   . Any desired materials may be used in the aforementioned multilayer adhesive stacks of  FIGS.  8 - 21   . 
     In any of the adhesive stacks shown in  FIGS.  8 - 21   , a first layer that is adjacent to a second layer (e.g., vertically stacked or horizontally adjacent) may be formed by a treatment on the second layer. For example, a physical or chemical treatment may be applied to the second layer that modifies a portion of the second layer to form the first layer. As one illustrative example, a second layer may receive an edge treatment (e.g., around its periphery) such as UV overcuring. The UV overcuring may form a first layer with modified properties (e.g., the first layer may be harder than the second layer). The first layer may prevent the softer second layer from oozing out of an edge of the device. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20220408
Publication Date: 20241015
Grant Date: 20241015
Priority Date: 20210607
Inventors: WU, XIAOWEI
KIM, HOON SIK
ZHAO, Yuxi
LAM, TERRY C
AFSAR, YASMIN F
HUANG, CHANG-CHIA
LALGUDI VISWESWARAN, BHADRINARAYANA
GOYAL, SUPRIYA
DRZAIC, PAUL S
Assignee: APPLE INC
CPC Classifications: [{"code": "C09J7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "C09J2301/124", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2203/318", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2301/312", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J9/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/03", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/0017", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B17/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B7/027", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B7/02", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B2250/42", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B7/022", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B7/023", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B27/36", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B27/281", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C09J2301/312", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2301/1242", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2203/318", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": true, "tree": "[]"}, {"code": "C09J7/10", "inventive": true, "first": true, "tree": "[]"}, {"code": "C09J2301/312", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2301/124", "inventive": false, "first": false, "tree": "[]"}, {"code": "C09J2203/318", "inventive": false, "first": false, "tree": "[]"}, {"code": "B32B2457/20", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K5/03", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/0017", "inventive": true, "first": false, "tree": "[]"}, {"code": "C09J9/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "C09J7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B32B7/12", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 84284842