Abstract:
One embodiment of the invention includes a method of joining two substrates with multilayer thermo-reversible dry adhesives and separating the two bonded substrates by completely thermally reversing the adhesion via heating.

Description:
TECHNICAL FIELD 
     The field to which the disclosure generally relates includes thermo-reversible dry adhesives. 
     BACKGROUND 
     Gecko feet pads, with nanohair structures on them, are examples of smart dry adhesives. The working principle of the Gecko adhesion is that the nanohair structure allows the foot pad to make maximum contact with a counter surface regardless of its roughness and chemical composition. This is accomplished by nanohairs that are relatively long and protruding from the foot pad at an angle so that adjacent nanohairs can contact the counter surface regardless of its topography. The maximum contact further allows for accumulation of millions of small van der Waals (in the range of microNewtons) interactions between the Gecko foot pad and the counter surface, leading to an overall adhesion force (pull-off force) of about 10 N/cm 2 . When the detaching force is employed in a peel-off mode, however, the complete detachment is achieved gradually by overcoming small adhesion forces corresponding to very small areas. Thus, the adhesion is easily reversed. Overall, the attractiveness of the Gecko adhesion lies in the combination of adhesive strength (10 N/cm 2 ), reversibility, and the ability to adapt to a variety of surfaces in terms of both the surface roughness and composition. The above unique features of the Gecko adhesion has stimulated scientific research efforts to produce synthetic smart dry adhesives that work using the same principle as the Gecko feet. Up to now, the two best synthetic Gecko adhesives show maximum pull-off strength of 3 and 10 N/cm 2  towards glass. Both adhesives suffer from severe adhesion loss after only one or two attaching/detaching cycles, as a result of breakdown of the nano structures or lateral collapse of the nano structures, with the latter referring to the bonding of adjacent nano-hairs. In addition, typical synthetic Gecko adhesives are expensive to produce and large-scale manufacturing is practically too difficult. 
     SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     One embodiment of the invention includes a method of joining two substrates with multilayer thermo-reversible dry adhesives and separating the two bonded substrates by completely thermally reversing the adhesion via heating. 
     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. 1  illustrates a product according to one embodiment of the invention. 
         FIG. 2  illustrates a product according to one embodiment of the invention. 
         FIG. 3  illustrates a product according to one embodiment of the invention. 
         FIG. 4  illustrates a product according to one embodiment of the invention. 
         FIG. 5  illustrates a product according to one embodiment of the invention. 
         FIG. 6  illustrates a product according to one embodiment of the invention. 
         FIG. 7  illustrates a product according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , one embodiment includes a multilayer thermo-reversible dry adhesive  11 . In one embodiment, the multilayer thermo-reversible dry adhesive  11  may be a double layer adhesive (DLA)  10 . The DLA  10  may include an elastomeric dry adhesive layer  12  and a shape memory polymer (SMP) layer  14 . 
     In various embodiments, the dry adhesive layers may be an epoxy elastomeric dry adhesive. In various embodiments, the shape memory polymer may be an epoxy. In various embodiments, the components of the dry adhesive or the components of the shape memory polymer may include a rigid epoxy and a flexible epoxy. The range of possible crosslinking chemistries which may be used to achieve a dry adhesive or shape memory polymer may include alpha, omega-diaminoalkanes, organic multi-caroxylic acid, anhydride, or catalytic (as in imidazole type) crosslinking reactions. There are many different ways to achieve the appropriate relationships between the molecular properties. For example, the dry adhesives or shape memory polymers may include a rigid epoxy, an epoxy extender, and a crosslinking agent; or a rigid epoxy, a flexible crosslinking agent, and a flexible epoxy; or a rigid epoxy, a rigid crosslinking agent, and a flexible epoxy; or a rigid epoxy, a flexible epoxy, and a catalytic curing agent; or a rigid epoxy, a crosslinking agent, and a diluent; or a flexible epoxy, a crosslinking agent, and a diluent; or a rigid epoxy and a flexible crosslinking agent; or a flexible epoxy and a catalytic curing agent; or a flexible epoxy and a crosslinking agent; and wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, the epoxy extender has one epoxide group, and the crosslinking agent is one of a multi-amine, an organic multi-carboxylic acid, or an anhydride, and the diluent is a monoamine or a mono-carboxylic acid. In various embodiments, the catalytic curing agent (or catalytic cure) promotes epoxy-to-epoxy or epoxy-to-hydroxyl reactions. The catalytic curing agent may include, but is not limited to, tertiary amines, amine salts, boron trifluoride complexes, or amine borates. In one embodiment, the components of the dry adhesive may be present in an amount sufficient to provide, upon curing of the composition, a dry adhesive having a glass transition temperature (T g ) of −90° C. to 200° C. and having a pull-off strength of 1-200 N/cm 2  from a substrate. In another embodiment, the dry adhesive may have a glass transition temperature of −90° C. to 25° C. In one embodiment, the components of the shape memory polymer composition may be present in an amount sufficient to provide, upon curing of the composition, an epoxy shape memory polymer having a change in storage modulus of 2 to 3 orders of magnitude before and after its glass transition. In one embodiment, the shape memory polymer has a T g  of 25° C. to 200° C. 
       FIG. 1  shows the original curvature of the DLA  10  including the dry adhesive layer  12  and the SMP layer  14 , according to one embodiment of the invention. In  FIG. 1 , the DLA  10  is positioned on a flat substrate  18 . In various embodiments, the substrate  18  may be, for example but not limited to, stainless steel alloy 304, glass, aluminum alloy 5657, polypropylene, or Teflon (polytetrafluoroethylene).  FIG. 2  shows the DLA of  FIG. 1  from another angle. As shown in  FIG. 2 , a contact area  16  between the dry adhesive layer  12  and the substrate  18  is small due to the curvature of the DLA. In one embodiment shown in  FIG. 3 , the DLA  10  includes a non-adhesive portion  20  formed in the dry adhesive layer  12 . In one embodiment, the non-adhesive portion  20  may be formed at approximately the center of the dry adhesive layer  12 . In various embodiments, the non-adhesive portion  20  may be introduced by molding or coating methods. The non-adhesive portion  20  may comprise a non-adhesive material, for example but not limited to, glass, metal, or Teflon (polytetrafluoroethylene). In one embodiment, the non-adhesive portion  20  may be an opening in the dry adhesive layer  12 . In another embodiment the non-adhesive portion  20  may be a non-adhesive material over the dry adhesive layer  12  or the non-adhesive material may be received in an opening or recess in the dry adhesive layer  12 . 
     In one embodiment, a method is provided for reversible adhesion of the DLA  10 . The DLA  10  is positioned on the substrate  18 . Then the DLA  10  with the non-adhesive portion  20  is heated to a temperature higher than the glass transition temperature of the SMP layer  14 . Then a load is imposed on the DLA  10  while the DLA  10  is cooled. In one embodiment, the load pressure may be about 0.1 N/cm 2  to about 20 N/cm 2 . In one embodiment, the DLA  10  may be cooled to about 25° C. The DLA  10  deforms and complies with the substrate  18 , as shown in  FIG. 4  (side view of the DLA) and in  FIG. 5  (top view of the DLA). A strong adhesive bond is formed between the DLA  10  and the substrate  18 . In one embodiment, adhesive strength of 60 N/cm 2  may be achieved. As shown in  FIG. 5 , the DLA  10  may not adhere to the substrate  18  at the location of the non-adhesive portion  20 . 
     According to the method of one embodiment, the DLA  10  is then released from the substrate  18  by heating the DLA  10  to a temperature above the glass transition temperature of the SMP of the SMP layer  14 , and not applying a load. The DLA  10  recovers to its original curvature and the residue adhesion is approximately zero, or complete adhesion reversal.  FIG. 1  shows a side view of the recovered DLA  10  that has been released from the substrate  18 . 
     Referring to  FIG. 6 , in another embodiment, the multi-layer thermo-reversible dry adhesive  11  may be a quadruple dry adhesive  26 , including two layers or portions of an elastomeric dry adhesive and two layers or portions of a shape memory polymer. The quadruple dry adhesive  26  may include a first dry adhesive layer  28  and a second dry adhesive layer  30 , wherein the curvatures of the layers  28 ,  30  point in the opposite directions. At least one of the first adhesive layer  28  or the second adhesive layer  30  may include the non-adhesive portion  20 , as shown in  FIG. 6 . The outer surface of each of the layers  28 ,  30  may have a generally convex shape. The first and second dry adhesive layers  28 ,  30  meet each other at the outer edges  32  and  34 . The quadruple dry adhesive  26  may also include a first shape memory polymer portion  36  and a second shape memory polymer portion  38 . The first shape memory polymer portion  36  may be positioned adjacent to the first adhesive layer  28 . The second shape memory polymer portion  38  may be positioned adjacent to the second adhesive layer  30 . A cavity  40  may be formed between the first and second shape memory polymer portions  36 ,  38 . 
     In one embodiment, the quadruple dry adhesive  26  is positioned between a first substrate  42  and a second substrate  44  and heated to a temperature above the glass transition temperature of the SMP of the first and second shape memory polymer portions  36 ,  38 . 
     Then a load is imposed on the quadruple dry adhesive  26  while is cooled to attach the first substrate  42  to the second substrate  44 , as shown in  FIG. 7 . The quadruple dry adhesive  26  may be cooled to about 25° C. The quadruple dry adhesive  26  deforms and complies with the substrates  42  and  44 . Upon cooling under the load, the cavity  40  may become very small or may disappear altogether. A good contact and thus strong adhesive bond is formed between the quadruple dry adhesive  26  and the substrates  42  and  44 . The quadruple dry adhesive  26  may not adhere to the first substrate  42  or the second substrate  44  at the non-adhesive portion(s)  20 . 
     Then the quadruple dry adhesive  26  may be released from the first and second substrates  42  and  44  by heating the quadruple dry adhesive  26  to a temperature above the glass transition temperature of the SMP of the first and second shape memory polymer portions  36 ,  38 , and not applying a load. Upon heating, each of the first and second dry adhesive layers  28  and  30  peel from the substrates  42 ,  44  respectively, first from near the outer edges  32  and  34 , and progressing towards a center  46 . Upon heating, the cavity  40  forms in between the first and second polymer layers  36  and  38 . The quadruple dry adhesive  26  recovers to its original curvature. In an embodiment where the first and second adhesive layers  28 ,  30  include the non-adhesive portion  20 , the residue adhesion is approximately zero, or complete adhesion reversal. 
     One embodiment of the invention includes a method of making a multilayer thermo-reversible dry adhesive  11  comprising heating 3.6 g of EPON 826 (a Bisphenol A based epoxy resin) to about 75° C. and mixing the same with 2.16 g of neopentyl glycol diglycidyl ether (NGDE) and 2.3 g of a diamine such as Jeffamine D-230. Jeffamine D-230 is a polyetheramine that is difunctional, primary amine with an average molecular weight of about 230. The primary amine groups are located on secondary carbons at the end of the aliphatic polyether chain. Jeffamine is available from Huntsman. 
     The mixture may then be poured into an aluminum pan and cured in an oven at about 100° C. for 1.5 hours. Then a mixture of 2.16 g of NGDE and 1.15 g of an amine such as Jeffamine D-230 may be poured into the aluminum pan on top of the first cured epoxy layer and cured for 1.5 hours at 100° C. In a third step, the oven temperature may be raised to 130° C. for post-curing for about one hour. At the end of the post-curing, the cured double layer epoxy may be demolded and cut into small pieces, if desired. A double layer epoxy may be obtained with the first layer which had a thickness of about 2 mm and functioned as a shaped memory polymer with a glass transition of about 50° C. and second layer as a dry adhesive having a thickness of about 1 mm. The non-adhesive portion  20  can be formed by selectively depositing a non-adhesive metal coating such as aluminum. 
     Another embodiment of the invention includes a method of making a dry adhesive layer  12  comprising mixing 4.32 g of neopentyl glycol diglycidyl ether (NGDE) with 2.3 g of an amine such as Jeffamin D-230. The liquid mixture was then poured into an aluminum mold. Curing was conducted in an oven for about 1.5 hours at 100° C. and then for about one hour at 130° C. The cured epoxy may then be demolded and cut into small pieces, if desired. 
     Another embodiment of the invention includes a method of making an SMP layer  14  comprising mixing 3.6 g of EPON 826 with 2.16 g of NGDE and 2.3 g of Jeffamine D-230. The mixture was poured into a circular aluminum pan and cured at 100° C. for 1.5 hours and postcured at 130° C. for 1 hour. The cured epoxy may then be demolded and cut into small pieces, if desired. 
     The dry adhesive layer  12  may provide a continuous contact surface or the dry adhesive layer may include a plurality of spaced apart fingers each providing a relative small contact surface so the overall contact surface of the adhesive layer is not continuous. 
     Numerous shaped memory polymers may be utilized in various embodiments of the invention. For example, staring with a typical aromatic diepoxy/diamine system with a T g  of about 90° C., the aromatic epoxy component is replaced systematically with an aliphatic diepoxy to yield a series of epoxy shape memory polymers with T g &#39;s ranging from 3° C. to 90° C. As such, a shape memory polymer may be tailored for use with a dry adhesive as desired for a particular application operated within certain temperature ranges. 
     The following is another embodiment of the invention providing a method of making a shape memory polymer layer  14  with T g &#39;s ranging from 3° C. to 90° C. EPON 826 was weighed into a glass bottle and placed into an oven preset at 70° C. to melt. The melting took about 1 hour. Immediately after the bottle containing the EPON 826 was taken out of the oven, weighed Jeffamine D-230 and NGDE were added to the bottle. The bottle was then shaken vigorously by hand for about ten seconds to mix the components. The detailed formulations of the five epoxy SMP samples prepared according to the method are summarized in Table 1. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Formulations of epoxy samples 1-5 
               
             
          
           
               
                   
                 Sample 
                 EPON 826 
                 NGDE 
                 Jeffamine D-230 
               
               
                   
                 # 
                 (mole) 
                 (mole) 
                 (mole) 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 0 
                 0.02 
                 0.01 
               
               
                   
                 2 
                 0.005 
                 0.015 
                 0.01 
               
               
                   
                 3 
                 0.01 
                 0.01 
                 0.01 
               
               
                   
                 4 
                 0.015 
                 0.005 
                 0.01 
               
               
                   
                 5 
                 0.02 
                 0 
                 0.01 
               
               
                   
                   
               
             
          
         
       
     
     Next, the mixture was poured into an aluminum pan. The epoxy samples were thermally cured at 100° C. for 1.5 hours and postcured at 130° C. for 1 hour. Upon the completion of the cure, the epoxy samples were demolded and cut into desirable shapes. 
     In another embodiment, a series of epoxy amine shape memory polymers with various crosslink densities were synthesized in the following manner. The epoxy formulations for samples 6-11 are given in Table 2 below. For each sample, 0.02 mole of EPON 826 was weighed into a glass bottle, which was placed into an oven preset at 75° C. and kept there for half an hour. Immediately after the bottle containing EPON 826 was taken out of the oven, Jeffamine D-230 and decylamine were introduced into the bottle according to the amounts specified in Table 2. The bottle was then shaken vigorously by hand for about ten seconds to mix the components and the mixture was poured into an aluminum pan. All epoxy samples were thermally cured at 100° C. for 1.5 hours and postcured at 130° C. for 1 hour. Upon the completion of the cure, the epoxy samples were demolded and cut into desirable shapes. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Formulations of epoxy samples 6-11 
               
             
          
           
               
                   
                 Sample 
                 EPON 826 
                 Jeffamine D-230 
                 decylamine 
               
               
                   
                 # 
                 (mole) 
                 (mole) 
                 (mole) 
               
               
                   
                   
               
             
          
           
               
                   
                 6 
                 0.02 
                 0.01 
                 0 
               
               
                   
                 7 
                 0.02 
                 0.0075 
                 0.005 
               
               
                   
                 8 
                 0.02 
                 0.005 
                 0.01 
               
               
                   
                 9 
                 0.02 
                 0.0025 
                 0.015 
               
               
                   
                 10 
                 0.02 
                 0.0005 
                 0.019 
               
               
                   
                 11 
                 0.02 
                 0 
                 0.02 
               
               
                   
                   
               
             
          
         
       
     
     In one embodiment, the system consists of EPON 826, Jeffamine D-230 as the crosslinker, and decylamine as the monoamine. As shown in Table 2, from sample 6 to 11, the fraction of the crosslinker is systematically reduced, while the total amounts of epoxy functionality and active hydrogen functionality on the amines are maintained equal. Among these samples, sample 11 was used as a reference sample because it contains no crosslinker and is not expected to possess shape memory properties. 
     In one embodiment, the substrate(s) may be flat and the multilayer thermo-reversible dry adhesive  11  may be curved. In another embodiment, the substrate(s) may be curved and the multilayer thermo-reversible dry adhesive  11  may be flat. 
     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.