Abstract:
An assembly including and method of forming arbitrary localized wrinkles upon a surface utilizing a shape memory polymer substrate and rigid overlay, wherein the geometrical distribution of the wrinkles is produced by recovering a lateral strain history within the substrate and buckling the overlay, and the localized wrinkles are used to create, among other things, optically three-dimensional engaging surfaces, structural colors, modified surface texturing, and haptic alerts.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure generally relates to assemblies for and methods of producing localized surface wrinkles, and more particularly, to an assembly for and method of producing localized surface wrinkles using a shape memory polymer substrate and rigid overlay. 
         [0003]    2. Discussion of Prior Art 
         [0004]    Surface wrinkles have been used to effect, modify, or control various benefits/conditions, including surface adhesion, texturing, coefficients of friction, structural colors, metrology, and haptic alerts. Methods of producing surface wrinkles preexisting in the art include using a stretched substrate overlaid by a rigid (e.g., metal) film. Wrinkles are instantaneously or selectively produced in the film, upon the release of energy by the substrate, if the compressive strain in the film exceeds the critical bucking strain. As a result, these conventional methods produce generalized wrinkles that co-extend with the entire surface defined by the overlay. This method is in fact behind wrinkles commonly encountered, for example, on human skin and dehydrated apples. Of particular interest is that the wrinkle geometry is closely related to the material properties. Precisely controlled wrinkle structures have found many interesting applications including nano-metrology, stretchable electronics, biosensors, and manipulation of material topographic properties. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention recites a novel assembly for and method of producing localized wrinkles within a surface, and more specifically, to an assembly for and method of producing localized surface wrinkles using a shape memory polymer substrate and rigid overlay. The present invention is useful for modifying the surface texture, and/or coefficient of friction of a select portion of a continuous surface. Where achieving a visible wavelength, the inventive wrinkling is also useful for producing structural colors in a predetermined pattern within the continuous surface; and as such, is further useful to produce a three-dimensional engaging surface (e.g., indicia, logo, shape, or picture) on a two-dimensional surface. 
         [0006]    In a first aspect of the invention, a method of forming localized wrinkles upon a surface is presented. The method includes indenting a substrate at least partly formed of shape memory polymer presenting a glass transition temperature, so that the substrate defines a first area experiencing purely compressive strain and a second area adjacent the first area and experiencing both a vertical compressive strain and a lateral tensile strain. Next, a overlay is attached to the substrate, so as to overlay the first and second areas. The overlay defines the surface. The polymer is then activated, so as to recover the strains, and cause wrinkles to form in the overlay. Finally, the polymer is deactivated, so as to lock in the wrinkles. 
         [0007]    A second aspect of the invention, includes a method of determining the cracking strain/stress of a nanoscopicallythin overlay, which includes observing the wrinkles in the above process, so as to determine a crack formation occurred for a given tensile strain. 
         [0008]    Thus, in a third aspect, the invention presents an assembly for forming localized wrinkles within a continuous surface. The assembly includes a substrate at least partially formed of a shape memory polymer presenting a glass transition temperature and first elastic modulus when activated. The substrate is indented so as to define a first area experiencing a purely compressive strain, and a second area adjacent the first area and experiencing both a vertical compressive strain and a lateral tensile strain, when the polymer is deactivated. The substrate is operable to recover the strains when the polymer is activated. The assembly further includes a relaxed overlay fixedly attached to and configured to cover the first and second areas. The overlay defines the surface, a second elastic modulus, and a height, wherein the second elastic modulus is greater than the first modulus. The moduli, height, and strains are cooperatively configured to cause buckling in the overlay, when the polymer is activated and recovers the strains. 
         [0009]    The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]    A preferred embodiment(s) of the invention is described in detail below with reference to the attached drawing figures of exemplary scale, wherein: 
           [0011]      FIGS. 1   a - d  is a multi-elevation progression showing a method of forming localized wrinkles upon a surface, wherein a protruding press is used to indent the substrate, and the wrinkles are further shown in enlarged caption at  FIG. 1   d , in accordance with a preferred embodiment of the invention; 
           [0012]      FIG. 2   a - c  is a multi-elevation progression showing a method of forming localized wrinkles upon a surface, wherein a recessed press is used to form a projection upon the substrate, in accordance with a preferred embodiment of the invention; 
           [0013]      FIG. 3  is a perspective view of the wrinkles formed in  FIGS. 1   d , and  3   d , in accordance with a preferred embodiment of the invention; and 
           [0014]      FIG. 4  is a plan view of a surface presenting a plurality of wrinkles and cracks formed using the inventive method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. As described and illustrated herein, a novel assembly  10  for and method of forming arbitrary localized wrinkles (i.e., wrinkle structures)  10   a  within a surface  12  includes and utilizes, respectively, a locally and plastically deformed shape memory polymer (SMP) based substrate  14  and a thin, high modulus overlay  16  ( FIGS. 1-4 ); however, it is certainly within the ambit of the invention to utilize the benefits of the assembly  10  with other equivalent selectively activated active materials exhibiting shape memory effect, and/or in other applications and configurations discernable by those of ordinary skill in the art. 
         [0016]    As used herein, the term “shape memory polymer (SMP)” shall generally refer to a group of polymeric materials that demonstrate the ability to return to some previously defined shape when subjected to an appropriate thermal stimulus, as is known in the art. Shape memory polymers are capable of undergoing phase transitions in which their shape is altered as a function of temperature. The previously defined or permanent shape can be set by curing for thermoset polymers or melting or processing the polymer at a temperature higher than the highest thermal transition for thermoplastic polymers. For a thermoplastic shape memory polymer, a temporary shape can be set by heating the material to a temperature higher than T g  or T m  of the soft segment, but lower than the T g  or melting point of the hard segment. For a thermoset shape memory polymer, a temporary shape can be set by heating the material to a temperature higher than T g  or T m . The temporary shape is set by cooling. The material can be reverted back to the permanent shape by heating the material above the shape memory transition temperature. 
         [0017]    The temperature needed for permanent shape recovery can be set at any temperature between about −63° C. and about 120° C. or above. Engineering the composition and structure of the polymer itself can allow for the choice of a particular temperature for a desired application. A preferred temperature for shape recovery is greater than or equal to about −30° C., more preferably greater than or equal to about 0° C., and most preferably a temperature greater than or equal to about 50° C. Also, a preferred temperature for shape recovery is less than or equal to about 150° C., and most preferably less than or equal to about 150° C. and greater than or equal to about 80° C. 
         [0018]    Suitable shape memory polymers include thermoplastics, thermosets, interpenetrating networks, semi-interpenetrating networks, or mixed networks. The polymers can be a single polymer or a blend of polymers. The polymers can be linear or branched thermoplastic elastomers with side chains or dendritic structural elements. Suitable polymer components to form a shape memory polymer include, but are not limited to, polyolefins, epoxy polymers, polyphosphazenes, poly(vinyl alcohols), polyamides, polyester amides, poly(amino acid)s, polyanhydrides, polycarbonates, polyacrylates, polyalkylenes, polyacrylamides, polyalkylene glycols, polyalkylene oxides, polyalkylene terephthalates, polyortho esters, polyvinyl ethers, polyvinyl esters, polyvinyl halides, polyesters, polylactides, polyglycolides, polysiloxanes, polyurethanes, polyethers, polyether amides, polyether esters, and copolymers thereof. Examples of suitable polyacrylates include poly(methyl methacrylate), poly(ethyl methacrylate), ply(butyl methacrylate), poly(isobutyl methacrylate), poly(hexyl methacrylate), poly(isodecyl methacrylate), poly(lauryl methacrylate), poly(phenyl methacrylate), poly(methyl acrylate), poly(isopropyl acrylate), poly(isobutyl acrylate) and poly(octadecyl acrylate). Examples of other suitable polymers include polystyrene, polypropylene, polyvinyl phenol, polyvinylpyrrolidone, chlorinated polybutylene, poly(octadecyl vinyl ether) ethylene vinyl acetate, polyethylene, poly(ethylene oxide)-poly(ethylene terephthalate), polyethylene/nylon (graft copolymer), polycaprolactones-polyamide (block copolymer), poly(caprolactone) dimethacrylate-n-butyl acrylate, poly(norbornyl-polyhedral oligomeric silsequioxane), polyvinylchloride, urethane/butadiene copolymers, polyurethane block copolymers, styrene-butadiene-styrene block copolymers, and the like. 
         [0019]    In the present invention, pre-patterning on the substrate overlay  16  is produced to create wrinkle structures  10   a  ( FIGS. 1   d ,  2   c ,  3 , and  4 ). The method is based on the principle that the compressive strain on the surface  12  can be altered by indentation or pressing. Depending on the shape and dimension of the indenter or press, the resulting wrinkle structures  10   a  can be manipulated in a local and arbitrary fashion. 
         [0020]    More particularly, in an exemplary embodiment, the substrate  14  was at least partially formed of a solid epoxy shape memory polymer consisting, for example, of an aromatic diepoxide (EPON 826, 3.6 g or 0.01 mol), an aliphatic diepoxide (NGDE, 2.16 g or 0.01 mol), and an aliphatic diamine curing agent (Jeffamine D-230, 2.3 g or 0.01 mol). This mixture was cured at 100° C. for 1 hour and at 130° C. for 1 hour to obtain a shape memory polymer presenting a glass transition temperature of approximately 40° C., and a permanent default shape. The default shape preferably defines smooth exterior surfacing (i.e., curved or flat but having no indentations or protrusions). The substrate  14  may be rectangular ( FIGS. 1-2   c ), oblong, define a molding, such as an auto trim, or be of any shape, so long as it is large enough to support a surface  12  suitable for displaying the intended wrinkle structure  10   a . The substrate  14  may include other components, in addition to a body of SMP, such as an external interface layer (not shown) that facilitates bonding with the overlay  16 , or non-active sectors where wrinkles are not desired, for example, to better withstand purely compressive forces. 
         [0021]    To effect the inventive method, a press (e.g., indenter)  18  with a protruded ( FIGS. 1   a - d ) or recessed ( FIGS. 3   a - d ) defining surface  18   a  that defines a relief (e.g., three-dimensional logo, indicia, shape, image, or picture) is pressed either manually or automatically onto the shape memory polymer substrate  14  until deformation results ( FIG. 1   b ,  2   b ). More preferably, the press  18  is applied after the substrate  14  has been preheated, e.g., at 65° C. for 10 min in the exemplary embodiment, to a temperature above its glass transition temperature, so as to reduce the modulus of elasticity of the substrate  14 , thereby making it easier to indent and deform. The relief can be of any arbitrary shape and dimensions. 
         [0022]    In a first directly indented area  14   a , only a compressive strain in the vertical direction is created ( FIGS. 1   b ,  2   b ). On the other hand, no strain is induced in areas sufficiently away from the indented area  14   a . Owing to the material continuity, a second transition area  14   b  around the edge of the indented area  14   a  is produced. In the transition area  14   b , the strain consists of a compressive component in the vertical direction and a tensile component in the lateral direction. It is appreciated that the spatial distribution of the lateral strain in the transition area  14   b  is highly dependent on the shape of the press  18 . As such, the resulting wrinkle structures  10   a  can be manipulated by alternating the shape of the press  18 . The substrate  14  is then cooled back to a temperature below its transition temperature under the load, so as to lock in the deformation. 
         [0023]    Next, the relatively rigid overlay  16  is securely attached to the substrate  14 , so as to cover the first and second areas  14   a,b  ( FIG. 1   c ,  2   c ). In the exemplary embodiment, the deformed substrate  14  is coated at room temperature with a “white gold” film (e.g., palladium/gold alloy composition)  16  using a sputtering system (not shown). Here, the film  16  thickness (e.g., approximately 10 nm) is controlled by deposition time and may be measured directly by a scanning electron microscopic analysis of the cross-sections. 
         [0024]    After film deposition is complete, and when the formation of wrinkles are desired, the assembly  10  is heated to activate the polymer substrate  14  and create wrinkles  10   a  due to shape recovery induced local compression ( FIGS. 1   d ,  2   c ,  3 , and  4 ). As such, it is certainly appreciated that the overlay (e.g., film)  16  must be non-reactive at temperatures at least equal to the transition temperature of the SMP. In the exemplary embodiment, the polymer may be heated to 90° C. for 10 minutes to ensure complete activation and shape recovery. In the direct indented area  14   a , the shape recovery simply moves the thin film  16  upwards in the vertical direction and induces no strain therein. Again, in the transition area  14   a  compressive strain is recovered as the thin film  16  moves upwards. Here, however, tensile strain is also recovered, which creates lateral compression in the thin film  16 . 
         [0025]    If the lateral compression strain exceeds a critical buckling value defined by the assembly  10 , wrinkles  10   a  will form. In a preferred embodiment, the critical buckling strain, ε c , may be pre-determined according to the following formula: 
         [0000]      ε c =[9 E   s   2 /64 E   f   2 ] 1/3   (1)
 
         [0000]    wherein E s  is the modulus of the substrate, and E f  is the modulus of the film; and accordingly the resultant wrinkle amplitude, A, may be determined by the following formula: 
         [0000]        A=h [(ε/ε c )−1] 1/2   (2)
 
         [0000]    wherein ε is the strain currently experienced by, and h is the thickness of the overlay  16 . Thus, it is appreciated that for rigid substrates, i.e., large E s  critical strain is large, amplitude is small, and wrinkles are difficult to form. Once the wrinkles  10   a  are formed, the substrate  14  is again cooled to a temperature below the transition temperature of the SMP, so as to lock in the wrinkles  10   a , which makes them more robust than those conventionally produced by soft substrate assemblies. 
         [0026]    It is appreciated that circularly distributed wrinkles ( FIG. 4 ) are created with a spherical press defining surface  18   a , square-shaped wrinkles would be created by a square shaped press defining surface ( FIGS. 1-2   c ), and a logo would be created by a logo-shaped defining surface  18   a . By contrast, the wrinkles  10   a  may be generated using a Vickers indenter and the first indentation step may be conducted on a non-preheated shape memory polymer substrate  14  using a Nano Scratch Tester (CSM Instruments) under a predetermined load. 
         [0027]    In the exemplary embodiment, the wrinkles  10   a  were analyzed using an atomic force microsopy (AFM) due to the microscopic scales resulting therefrom. AFM characterization of wrinkles was conducted at room temperature in a contact mode using Dimension 3100 manufactured by Veeco™. The wavelength, a, and amplitude or height, A, of the wrinkles  10   a  were obtained by measuring 80-100 individual wrinkles using the section analysis function in the Nanoscope software (Nanoscope 5.31r1). One sample, presented a wavelength and amplitude of 800 nm of 80 nm, respectively. 
         [0028]    It is appreciated that the wrinkle wavelength decreases linearly with strain, whereas wrinkle amplitude is independent of strain. Increasing the overlay thickness on the other hand, increases both wrinkle wavelength and amplitude. With respect to the impact of strain, the classical wrinkle theory based on elastic energy minimization suggests that wrinkle wavelength should be strain independent according to the following formula: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    where E, v, h, and ε represent respectively modulus, Poisson ratio, film thickness, and compressive strain, and the subscripts s and f denotes substrate and film (i.e., overlay). Finally, it is appreciated that the linear dependence between wavelength and strain under the present invention provides a benefit in creating localized wrinkles  10   a  of different wavelength on the same surface, but deviates from the above relationship, when finite deformation is considered. 
         [0029]    Where the wavelength falls within the visible spectrum, it is appreciated that a structural color will result. That is to say the wrinkles  10   a  will cause a color to be perceived by altering the way light travels at different dimensions, as opposed to chemical colors that rely on the absorption of certain wavelength lights by pigment molecules. It is appreciated that the colors are highly angle dependent; that is to say, the viewing angle contributes to the actual color perceived. It is further appreciated that this process presents advantages over conventional structural color forming techniques that require the use of lithographic templates, which can be relatively expensive and require dedicated equipment not widely accessible. 
         [0030]    Through the creation of structural colors, arbitrary images can be captured and displayed using wrinkle based diffraction colors. For example, where the relief  18   a  presents a protruded logo or indicia, the letters can be made to appear to protrude out of the surface  12  (i.e., three-dimensionally), while in fact the surface  12  is macroscopically smooth. This illusion results because the edge of the letters is colored to resemble shading even though no pigment is introduced in the process. By using a recessed relief ( FIGS. 2   a - c ), an engaging surface or logo may be produced with the letters colored, instead of the edges of the letters. With this change, the transition strain area  14   b  resides in the letter or image face. 
         [0031]    As shown in the circularly distributed wrinkles of  FIG. 4 , it is appreciated that the wavelength increases with the radial distance from the center of the indent. In certain instances, cracks  20  may also be produced in the wrinkle structures  10   a  ( FIG. 4 ). The existence of cracks corresponds to surpassing a critical strain above which cracks are formed. Thus, it is also appreciated that the present invention can be used as a convenient method of measuring the cracking strain/stress of a nanoscopicallythin film  16 , which could otherwise be challenging using conventional methods. 
         [0032]    This invention has been described with reference to exemplary embodiments; it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.