Abstract:
A reversible adhesive system for coupling together two objects may consist of two shape memory polymers with molecular “hooks and loops” on the surfaces (i.e. the surface away from each of the objects). Utilizing the shape memory properties of the polymers, the molecular hooks and molecular loops may be brought together to form non-covalent bonds, leading to macroscopic adhesion. Upon heating, the adhesive bond can be separated with a small peeling force. The adhesive bonding and debonding can be repeated for multiple cycles with significant adhesion retention.

Description:
TECHNICAL FIELD 
     The field to which the disclosure generally relates includes polymer chain adhesion methods and more specifically to a high strength, reversible adhesion method for a solid polymer-polymer interface. 
     BACKGROUND 
     Hook and loop fasteners such as Velcro® are well known ways for mechanically fastening together two separate objects or things without the use of an adhesive like glue. These fasteners including a hook portion coupled to one of the two objects and a loop portion coupled to the other of the two objects. To fasten the objects together, simply press the hook portion into the loop portion. An interesting characteristic of hook and loop fasteners is that the mechanical bonds created are relatively strong if one attempts to pull apart the hook portion in a direction perpendicular to the loop portion, while the mechanical bonds are less strong if one simply peels the hook portion away from the loop portion in a direction not perpendicular to the loop portion. 
     By contrast, conventional chemical bonding allows objects or things to be coupled together by applying an adhesive between the two surfaces that cures to couple together the two objects. The objects are bonded together by curing the adhesive. This creates essentially an irreversible bond between the objects (i.e. the bond cannot subsequently be debonded and bonded for multiple cycles). 
     SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     A reversible adhesive system for coupling together two objects may consist of two shape memory polymers with molecular “hooks and loops” on the surfaces (i.e. the surface away from each of the objects). Utilizing the shape memory properties of the polymers, the molecular hooks and molecular loops may be brought together to form non-covalent bonds, leading to macroscopic adhesion. Upon heating, the adhesive bond can be separated with a small peeling force. The adhesive bonding and debonding can be repeated for multiple cycles with significant adhesion retention. 
     In one exemplary embodiment, the non-covalent bonding may be achieved wherein both shape memory polymers contain hydrogen bonding moieties on their surfaces. Thus, multiple hydrogen bonds may be formed at the interface that leads to adhesion between the respective surfaces. As the temperature increases above the transition temperatures of the shape memory polymers, they become soft (significant modulus drop), which allows the adhesive bond to be separated in a peeling mode. In the peeling separation mode, the interfacial adhesion force is overcome gradually. Therefore, only a small peeling force is needed for bond separation. In the absence of an increased temperature, a significant amount of peel strength is necessary to break the same hydrogen bonds due to the rigidity of the shape memory polymers. 
     In another exemplary embodiment, the non-covalent bonding may be achieved wherein the first shape memory polymer contains positive charges on its surface, while the second shape memory polymer contains negative charges on its surface. Thus, ionic bonds may be formed that leads to adhesion between the respective surfaces. 
     In still another exemplary embodiment, the non-covalent bonding may be achieved wherein the first shape memory polymer and second shape memory polymer surfaces contain aromatic moieties that are capable of aromatic interaction, otherwise known as π-π interaction. 
     In yet another exemplary embodiment, the non-covalent bonding can result from weak intermolecular interactions, collectively referred to as van der Waal forces, between molecules on the first shape memory polymer and second shape memory polymer side chains. 
     In a further exemplary embodiment, the non-covalent bonding between the respective side chains can result from any combination of the above listed non-covalent bonding interactions. 
     Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  is an illustration of a generic exemplary embodiment of a reversible chemical adhesive system in an uncoupled form; 
         FIG. 1B  is an illustration of a reversible chemical adhesive system of  FIG. 1A  in a coupled form; 
         FIG. 2  is a two-dimensional illustration of the bonding of a portion of a reversible chemical adhesive system according to one exemplary embodiment; 
         FIG. 3  is a dynamic mechanical analysis curve for the cured epoxy shape memory polymer backbone of  FIG. 2 ; 
         FIG. 4  is a two-dimensional illustration of two epoxy shape memory polymers grafted with low molecular weight and high molecular weight BPEI, respectively; 
         FIG. 5  is a graphical illustration of the pull-off strengths of various epoxy BPEI based epoxy shape memory polymers in accordance with one or more exemplary embodiments; 
         FIG. 6  is a graphical illustration of the impact of pull-off rate on pull-off strength for a BPEI-3 based epoxy shape memory polymer according to one exemplary embodiment; and 
         FIG. 7  is a graphical illustration of the pull-off strength for the BPEI-3 based epoxy shape memory polymer according to one exemplary embodiment as a function of pH. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring now to  FIGS. 1A and 1B , a generic embodiment of the principles of a reversible chemical adhesive system  10  may be illustrated in an uncoupled and a coupled position. 
     The reversible chemical adhesive system  10  may be formed by reversibly coupling together a first polymeric chain  12  (i.e. polymer  12 ) and a second polymeric chain  14  (i.e. polymer  14 ) using a series of molecular hooks  16  and molecular loops  18 . 
     The molecular hooks  16  and molecular loops  18  represent side chains  15  coupled onto one or both of the polymeric backbone portions  22 ,  24  of the first polymeric chain  12  and second polymeric chain  14 , respectively, that will reversibly interact with one another (i.e. a molecular hook  16  on the first polymeric chain  12  will interact with a molecular loop  18  on the second polymeric chain  14 , and/or alternatively a molecular loop  18  on the first polymeric chain  12  will interact with a molecular hook  16  on the second polymeric chain  14 ) to create a non-covalent bond of varying adhesive strength. 
     The polymeric backbones  22 ,  24  of the first polymeric chain  12  and second polymeric chain  14 , respectively, in one exemplary embodiment, may be formed from shape memory polymers (SMPs). SMPs represent responsive polymers that can fix to deformed temporary shapes and recover to their permanent (original) shapes only upon external stimuli. SMPs may be available exhibiting a dual shape memory effect (DSME), wherein the SMP can only memorize one temporary shape in addition to its permanent shape in each shape memory cycle. It is also contemplated that SMPs may be available exhibiting a triple shape memory effect (TSME) or greater, wherein the SMP can memorize two distinct temporary shapes (for a TSME) or more in addition to its permanent shape in each memory cycle. 
     In general, to transform an SMP from its permanent shape to its temporary shape, the permanent shape may be heated to a first elevated temperature and then deformed under stress to yield the first temporary shape, a shape which may be different in visual appearance from the permanent shape. By definition, the first elevated temperature is a temperature sufficiently high to ensure a phase transition of the SMP (i.e. is a temperature above the glass transition temperature (T g ) of SMP). The SMP may then be cooled under stress to a temperature below the glass transition temperature of one SMP, wherein the stress may be relieved while maintaining the first temporary shape. To recover the permanent shape from the first temporary shape, the SMP may be reheated to the first elevated temperature in the absence of stress. 
     The ability of an SMP to change between its original permanent shape and at least one temporary shape allows the intimate contact of the molecular hooks  16  and molecular loops  18  on polymers  12  and  14 . Thus, more or less non-covalent interactions between molecular hooks  16  of one of the polymeric backbones  22  or  24  and molecular loops  18  on the other of the polymeric backbones  22  or  24  may occur. A higher degree of non-covalent interactions may result in stronger bonding between the first polymeric chain  12  and the second polymeric chain  14 . Conversely, a lower degree of non-covalent interactions may result in weaker bonding. Thus, by changing one, or the other, or both of the shape memory polymeric backbones  22 ,  24  from their original permanent shape to a deformed temporary shape, the amount of non-covalent interactions (and hence the bond strength) between the respective molecular hooks  16  and molecular loops  18  of the polymeric backbones  22 ,  24  available for coupling the chains  12 ,  14  together may be altered in a predictable manner. 
     The polymeric system  10  may be formulated such that the maximum amount of non-covalent bonding between the respective side chains  15  (i.e. a maximum amount of interaction occurs between the molecular hooks  16  of one polymeric chain  12  with the molecular loops  18  of the other polymeric chain  14 ) occurs when each of the shape memory polymeric backbones  22 ,  24  is in their respective temporary states and such molecular interaction is maintained after cooling due to the ability of shape memory properties of polymers  12  and  14 . Further, an intermediate amount of non-covalent bonding occurs when one of the shape memory polymeric backbones  22  or  24  is in its respective temporary shape, while the other of the shape memory polymeric backbones in its original permanent state. Thus, it may be preferable wherein the glass transition temperature of the shape memory polymeric backbone  22  is sufficiently different than the glass transition temperature of the shape memory polymeric backbone  24  to allow precise control over the amount of non-covalent bonding between the respective polymers  12  and  14 . 
     The side chains  15  coupled onto the shape memory polymeric backbones  22 ,  24  on the polymers  12 ,  14  that provide the molecular hooks  16  and molecular loops  18  may take on many different forms as described in the exemplary embodiments below. These different forms therein provide one or more kinds of non-covalent interactions that therefore for bonds between the molecules of varying strength. 
     For example, the non-covalent bonding may be achieved wherein the side chain  15  of the polymeric chain  12  and  14  both contain hydrogen bonding moieties (both hydrogen bonding acceptors and donors). In another exemplary embodiment, the side chain  15  of the first polymeric chain  12  includes hydrogen bonding acceptors as the molecular hooks  16  and hydrogen bonding acceptors donors as the molecular loops  18  on its surface, while the side chain  15  of the second polymeric chain  14  includes hydrogen bonding donors as the molecular loops  18  and hydrogen bonding acceptors as molecular hooks  16  on its surface. 
     Thus, multiple hydrogen bonds may be formed between the respective hooks  16  on one of the side chains  15  of the polymer chain  12  or  14  and loops  18  on a side chain  15  of the other of the polymer chains  12  or  14  that may lead to adhesion between the respective surfaces. As the moduli of polymers  12  and  14  drop significantly when temperature is increased above the shape memory polymer transition temperature, the surfaces can be debonded by simply increasing the temperature to a temperature above the glass transition temperature of each SMP chain  12 ,  14  and peeling one polymeric backbone  22  away from the other polymeric backbone  24 . In the absence of increased temperature (i.e. when the polymer chains  12 ,  14  are in their stressed temporary shape), a significant amount of peel strength is necessary to break the same hydrogen bonds. Moreover, the amount of force necessary to break the hydrogen bonds by moving the polymeric backbone  22  in a direction substantially normal with respect to the polymeric backbone  24  may be sufficiently greater than the amount of force to peel the polymeric backbone  22  with respect to the polymeric backbone  24 . 
     Alternatively, the non-covalent bonding may be achieved wherein the side chain  15  of one of the polymer chains  12  or  14  includes positive charges (i.e. the side chain  15  may be a positively charged molecule) as the molecular hooks  16  on its surface, while one of the side chains  15  of the other polymer chain  12  or  14  includes negative charges (i.e. the side chain  15  may be a negatively charged molecule) as the molecular loops  18  on its surface. In yet another exemplary embodiment, the side chain  15  of both of the polymer chains  12 ,  14  includes positive charges as the molecular hooks  16  and negative charges as the molecular loops  18  on its surface. 
     Thus, ionic bonds may be formed between the positively charged molecule of one side chain  15  and the negatively charged molecule of another side chain  15  that leads to adhesion between the respective surfaces of the first polymer chain  12  and the second polymer chain  14 . Similar to hydrogen bonding, the ionic bonds on the surfaces may be debonded by simply peeling one polymeric backbone  22  away from the other polymeric backbone  24 , especially when thermally activated above the glass transition temperatures of each SMP chain  12 ,  14 . A significantly greater force may be necessary to break the ionic bonds by moving the polymeric backbone  22  in a direction substantially normal with respect to the polymeric backbone  24 . 
     Non-covalent bonding may also be achieved wherein adhesion occurs as a result of both ionic bonding and hydrogen bonding. Thus, the side chains  15  of both the first polymeric chain  12  and the second polymeric chain  14  may have any of an infinitely available combination of side chains constituting positive charges, negative charges, hydrogen bond acceptors, and hydrogen bond donors. In any of these exemplary embodiments, the non-covalent bonds may be more easily broken by peeling the polymeric backbone  22  relative to the polymeric backbone  24 , as opposed to moving the polymeric backbone  22  in a direction substantially normal with respect to the polymeric backbone  24 , after heating the SMP chains  12 ,  14  above their respective glass transition temperatures. 
     Non-covalent interactations may also be achieved when the side chains  15  include organic compounds containing aromatic moieties. Aromatic interaction (or π-π interaction) between the side chains  15  of the first polymeric chain  12  and the second polymeric chain  14  may thus occur. π-π interactions are caused by intermolecular overlapping of porbitals in π-conjugated systems, so they become stronger as the number of π-electrons increases. 
     In addition, the composition and relative orientations of the molecules of the side chains  15  may provide other forms of intermolecular interactions, collectively referred to as van der Waals forces, which may contribute to non-covalent interactions. The name van der Waals force refers to the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules forces. Van der Waals forces include momentary attractions between molecules, diatomic free elements, and individual molecules. Van der Waals forces are relatively weak compared to normal chemical bonds. Examples of types of van der Waal forces that may occur include dipole-dipole interactions, dipole-induced dipole forces, and London forces. 
     In either of these additional exemplary embodiments, similar to the other embodiments described above, the amount of non-covalent interactions may be tailored to be maximized when each of the polymer chains  12 ,  14 , is in their first temporary shape. Moreover, the non-covalent bonds may be more easily broken by peeling the polymeric backbone  22  relative to the polymeric backbone  24 , as opposed to moving the polymeric backbone  22  in a direction substantially normal with respect to the polymeric backbone  24 , after heating the SMP chains  12 ,  14  above their respective glass transition temperatures. 
     In further exemplary embodiments, the non-covalent bonding between the respective side chains can result from any combination of the above listed non-covalent bonding interactions. Thus, the first polymer chain  12  and second polymer chain  14  may include side chains  15  that provide hydrogen bond donors and acceptors for hydrogen bonding, positively and negatively charged molecules for ionic bonding, aromatic moieties for π-π interactions, or other components that induce van der Waal forces, or any combination thereof. Thus, by transforming either the first polymer chain  12 , or the second polymer chain  14 , or both chains, from their respective permanent shapes to temporary shapes, the amount of non-covalent interactions may be precisely controlled and understood. Further, by increasing the temperature after coupling above the glass transition temperatures of each SMP chain  12 ,  14 , the chains can be uncoupled by a simple peeling mechanism with little force. 
     EXPERIMENTAL 
     In one exemplary embodiment, a chemical system exhibiting shape memory effect was synthesized that included a branched polyethyleneimine (BPEI) polymer of varying molecular weights grafted onto a crosslinked epoxy SMP backbone. 
     The crosslinked epoxy SMP backbone formulation contained an aromatic diepoxide, an aliphatic diepoxide, and an aliphatic diamine as a curing agent, with the total amount of epoxide being six mole percent in excess. The actual materials utilized, synthesis, and testing of the materials is described further below. 
     The cured epoxy SMP backbone exhibited a tightly crosslinked nature, as shown in  FIG. 2 , with little possibility of chain entanglement due to the fact that the only free chain ends of the network are short monoepoxide segments A′ (MW of 341 or 247). In addition, the cured epoxy SMP backbone was expected to exhibit shape memory properties, as illustrated in a dynamic mechanical analysis curve as shown in  FIG. 3 , with a glass transition temperature of about 68 degrees Celsius. Further, examination of the polymer structure revealed it contained both hydrogen bond donors (hydrogen atoms on the hydroxyl groups of segments A and A′, respectively) and hydrogen bond acceptors (all the nitrogen and oxygen atoms in the epoxy structure). 
     Next, three BPEI polymers (hereinafter the copolymers created are designated BPEI-1, BPEI-2, and BPEI-3) with molecular weights of 1,800; 10,000; and 60,000 were grafted onto the crosslinked epoxy SMP backbone resin by reacting the active hydrogens coupled to the nitrogen on the BPEI polymers with the unreacted epoxy rings on the crosslinked epoxy backbone structure. Since unreacted epoxy rings are randomly distributed on the backbone and the reactive hydrogen atoms are densely populated on the BPEI molecule, as shown in  FIG. 4 , each BPEI molecule may react only once where the molecular weight of BPEI is low, or multiple times in the BPEI molecular weight is sufficiently high. These facts were confirmed by atomic force microscopy height imaging on the non-grafted and grafted BPEI epoxy polymers and by nitrogen atomic percentages as obtained by x-ray photoelectron spectroscopy analysis. 
     Since BPEI is a partially ionized polymer (with the degree of ionization being pH dependent), BPEI polymers grafted onto the SMP epoxy backbone introduced hydrogen bonds and ionic bonds to the copolymer. The adhesive strength (i.e. pull-off strength) of crosslinked epoxy SMP polymers having BPEI side chains, wherein the polymer is stressed to its first temporary shape, as summarized in  FIG. 5  increased to about 636 N/cm 2  for BPEI-2 and BPEI-3, as compared with 436 N/cm 2  for the ungrafted epoxy crosslinked SMP polymer and for BPEI-1. The increase in adhesive strength in BPEI-2 and BPEI-3 is attributed to nearly full surface coverage, and due to the fact that the hydrogen bond donors and acceptors on the epoxy polymers are buried beneath the BPEI grafts. 
     In addition, as shown in  FIG. 6 , the measured adhesion did not vary greatly with the pull-off rate used during adhesion tests, with only an 18-percent increase in adhesive strength experienced for crosslinked epoxy SMP polymers having BPEI-3 side chains (stressed to their first temporary shape) as the pull off rate increased from about 5 to 40 N/s. Further, as shown in  FIG. 7  for BPEI-3, a slightly higher adhesion strength was experienced at lower pH due to more contribution from ionic interactions. At the higher pH of 11.5, the degree of ionization of BPEI-3 was very low, and the adhesion was primarily hydrogen bonding based. 
     While the BPEI copolymers described above achieved significant pull-off strength, experimental analysis revealed that that coupled BPEI copolymer polymeric chains could easily be detached using a peeling mechanism. To achieve this, the BPEI copolymers were heated to about 90 degrees Celsius in the absence of stress, which is a temperature above the glass transition temperature for the crosslinked epoxy SMP backbone, to allow the polymer chains to return to their permanent shape from the temporary shape. The epoxy polymers became flexible due to the modulus drop and due to the transition from the first temporary shape to their relaxed permanent shape, which allowed the two surfaces to be separated via a peeling mechanism with a peeling force less than about 1 N/cm 2 . 
     In addition, after detachment, experimental analysis confirmed that the BPEI copolymers could be bonded again (reattached) by returning the polymers to their first temporary shape (by increasing the temperature under stress to achieve the first temporary shape and then cooling the polymers to a temperature below their glass transition temperature) and bringing the polymer chains in close proximity without a significant drop in adhesion strength (a 67% adhesion retention was achieved for the BPEI polymer systems after two bonding-debonding cycles), suggesting the reversible nature and adhesive strength of the polymeric system. 
     Thus, by simply determining the rigid structure of the SMP polymeric backbones, and the degree of non-covalent interactions achieved by the added side chains, a wide variety of polymeric systems can be designed having significant adhesion strength in the SMP polymers temporary shape and reversible adhesion characteristics when the SMP polymer is heated above its glass transition temperature to its revert to its permanent shape. As shown above, these cycles are repeatable for many coupling and uncoupling cycles, given the reversible nature of the system. 
     Materials 
     Diglycidyl ether bisphenol A epoxy monomer (EPON 826) and poly(propylene glycol)bis(2-aminopropyl) ether curing agent (Jeffamine D-230) were obtained from Hexion and Huntsman, respectively. Neopentyl glycol diglycidyl ether (NGDE) was purchased from TCI America. All the other materials including branched poly(ethyleneimine) polymers (BPEI) were purchased from Aldrich. All chemicals were used without further purification. 
     Material Synthesis 
     Synthesis of Crosslinked Epoxy Shape Memory Polymer Backbone. 4.68 g of EPON 826 was melted at 80 degrees Celsius and mixed with 1.51 g of NGDE and 2.16 g of Jeffamine D-230. The liquid mixture was degassed under vacuum for 30 minutes, cured under ambient pressure in an aluminum pan at 100 degrees Celsius for 1 hour, and post cured at 130 degrees Celsius for another hour. After cooling to room temperature, the cured epoxy polymers were demolded. 
     BPEI Surface grafting onto Crosslinked Epoxy Shape Memory Polymer Backbone. Epoxy polymers without being subjected to post cure at 130 degrees Celsius were used. Excess BPEI (50 wt % aqueous solution) was spread onto the epoxy surface and the grafting reaction proceeded at 80 degrees Celsius for 2 hours. Afterwards, the sample was sonicated in methanol for 10 minutes at room temperature, and the process was repeated two more times with fresh methanol. The sample was post cured at 130 degrees Celsius for 1 hour, cut into small pieces, rinsed with isopropanol, and blow dried prior to use. 
     Adhesive Bonding and Testing 
     Each adhesive system consists of a pair of polymers. The back (ungrafted) sides of the two polymers were first bonded onto two separate holders using Superglue® and left at room temperature for at least 1 hour. They were then placed in an oven at 90 degrees Celsius for 10 minutes. Immediately after the samples were taken out of the oven, 50 μL of methanol was dropped onto the front (grafted) side of either polymers, onto which the front side of the other polymer was pressed with a preload of 8.3×0.3 N/cm 2 . The samples were cooled under load for 10 minutes at room temperature. The load was removed and the bonding was formed. 
     The pull-off force to separate the bonding between the polymers was measured using a ROMULUS Universal Mechanical Strength Tester (Quad Group Inc.) fitted with a homemade ball-joint fixture (see appendix). A pull-off rate of 20 N/s was used unless otherwise noted. The pull-off strength was defined as the force per unit bonding area (N/cm 2 ). 
     Characterization 
     The dynamic mechanical analysis (DMA) was conducted in a dual cantilever mode using a DMA Q800 (TA instruments). The testing parameters were: constant frequency=1 Hz; oscillation amplitude=30 μm; heating rate=1 degree Celsius/minute. 
     The tapping mode atomic force microscopy (AFM) measurement was carried out in ambient air using a Dimension 3100 (VEECO Instruments) with silicon probes (Budget Sensors, Tap 300). The z-range was set to 15 nm and flattened height images are shown in this report. 
     The x-ray photoelectron spectroscopy (XPS) analysis was conducted using a PHI Quantera Scanning X-ray Microprobe (ULVAC-PHI, Inc). A 200-micrometer-diameter Al Kα x-ray beam was used, with electron and ion neutralizers to compensate for charging. The nitrogen atomic percentages were calculated using the Multipak software. 
     The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.