Abstract:
One exemplary method includes providing a first polymer and a second polymer each comprising a first shape memory polymer backbone having at least one surface free side chain, the first polymer and the second polymer each transformable between a permanent shape and a temporary shape; creating an adhesive bond between the first polymer and the second polymer, wherein the creating of the adhesive bond transforms the first polymer to its temporary shape and transforms the second polymer to its temporary shape; and wherein the at least one surface free chain of the first polymer in its temporary shape is interdiffused with the at least one surface free chain of the second polymer in its temporary shape by the creation of the adhesive bond.

Description:
TECHNICAL FIELD 
       [0001]    The technical field generally relates to polymer coupling methods and more specifically to a reversible welding process for polymers. 
       BACKGROUND 
       [0002]    Welding, or fusion welding, of thermoplastic polymer composites is a well-known process for joining composites. Fusion welding is accomplished wherein portions of the polymers to be joined are partially melted (or softened) to allow the polymer chains at the interface to diffuse into one another. The interdiffusion occurs in a large length scale, allowing chain entanglement to form at the interface. Essentially, two separated polymers become one. This polymer joining method is non-reversible, as it relies on the polymer chains at the interface to fuse into each other and form one phase. 
       SUMMARY OF EXEMPLARY EMBODIMENTS 
       [0003]    One exemplary method includes providing a first shape memory polymer (SMP) and a second SMP each comprising chains with one free end and the other chain end attached to the polymer surfaces. The SMPs are each transformable between a permanent shape and a temporary shape; creating an adhesive bond between the first SMP and the second SMP, wherein the creating of the adhesive bond transforms SMP to its temporary shape and transforms the second SMP to its temporary shape; and wherein the at least one surface free side chain of the first SMP in its temporary shape is interdiffused with the at least one surface free side chain of the second SMP in its temporary shape to create the adhesive bond. 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 
         [0004]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0005]      FIG. 1A  illustrates a fully crosslinked SMP; 
           [0006]      FIG. 1B  illustrates a partially crosslinked SMP; 
           [0007]      FIG. 1C  illustrates a lightly crosslinked SMP; 
           [0008]      FIG. 2A  illustrates two fully crosslinked SMP brought in close contact in their permanent shape; 
           [0009]      FIG. 2B  illustrates two fully crosslinked SMP of  FIG. 2A  transformed from their permanent shape to a temporary shape by heating above their shape memory transformation temperature and brought in close contact under a load; 
           [0010]      FIG. 2C  illustrates two fully crosslinked SMP of  FIG. 2B  maintained in their temporary shapes wherein the load has been removed; 
           [0011]      FIG. 2D  illustrates two fully crosslinked SMP chains of  FIG. 2B  transformed from their temporary shape to their permanent shape upon heating in the absence of a load; 
           [0012]      FIG. 3A  illustrates two partially crosslinked SMP brought in close contact in their permanent shape; 
           [0013]      FIG. 3B  illustrates two partially crosslinked SMP of  FIG. 3A  transformed from their permanent shape to a temporary shape by heating above their shape memory transformation temperature and brought in close contact under a load; 
           [0014]      FIG. 3C  illustrates two partially crosslinked SMP of  FIG. 3B  maintained in their temporary shapes wherein the load has been removed; 
           [0015]      FIG. 3D  illustrates two partially crosslinked SMP of  FIG. 3B  transformed from their temporary shape to their permanent shape upon heating in the absence of a load; 
           [0016]      FIG. 4A  illustrates two lightly crosslinked SMP brought in close contact in their permanent shape; 
           [0017]      FIG. 4B  illustrates two lightly crosslinked SMP of  FIG. 4A  transformed from their permanent shape to a temporary shape by heating above their shape memory transformation temperature and brought in close contact under a load; 
           [0018]      FIG. 4C  illustrates two lightly crosslinked SMP of  FIG. 4B  maintained in their temporary shapes wherein the load has been removed; and 
           [0019]      FIG. 4D  illustrates two fully crosslinked SMP of  FIG. 4B  transformed from their temporary shape to their permanent shape upon heating in the absence of a load. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0020]    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. 
         [0021]    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. 
         [0022]    In general, to transform an SMP from its permanent shape to its temporary shape, the permanent shape may be subject to external stimuli. For example, the SMP 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. 
         [0023]    Many crosslinked polymers also possess SMP properties. However, to adhere two polymer crosslinked SMP&#39;s together, it appears that good surface contact as well as interdiffusion may be prerequisites for good adhesive bonding. 
         [0024]    Referring first to  FIGS. 1A-1C , a crosslinked SMP polymer  20  may be illustrated as having three separate crosslinking densities, namely a fully crosslinked SMP polymer  22  as shown in  FIG. 1A , a partially crosslinked SMP polymer  24  as shown in  FIG. 1B , and a lightly crosslinked SMP polymer  26  as shown in  FIG. 1C . 
         [0025]    Each of the crosslinked SMP polymers  20  illustrated in  FIGS. 1A-1C  may include one or more polymeric backbone portions  30  and one or more free surface chain portions. For illustrative purposes and as shown in  FIGS. 1A-1C , the composition of the polymeric backbone portions  30  of each respective SMP  22 ,  24 ,  26  may be virtually identical. 
         [0026]    The fully crosslinked SMP polymer  22  as used herein and as shown in  FIG. 1A , may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions  32 . In other words, there may be virtually no free side chain portions extending from any of the polymeric backbone portions  30 . 
         [0027]    The partially crosslinked SMP polymer  24  as used herein and as shown in  FIG. 1B  in its permanent shape, may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions  34 . In addition, the partially crosslinked SMP polymer  24  may include one or more additional free side chain portions  36  having a first end  38  coupled to the surface of  24  a second, and free end  40 , that may not be reacted with a corresponding free side chain portion  36  of an adjacent polymeric backbone portion  30 . 
         [0028]    The lightly crosslinked SMP polymer  26  as used herein and as shown in  FIG. 1C  in its permanent shape, may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions  43 . In addition, the lightly crosslinked SMP polymer  26  may include one or more additional free side chain portions  44  having a first end  46  coupled to the surface of  30  and a second end, or free end  48 , that may not be reacted with a corresponding free side chain portion  44  of an adjacent polymeric backbone portion  30 . 
         [0029]    The lightly crosslinked SMP polymer  26 , by definition, has less crosslinked portions per unit area than the partially crosslinked SMP polymer  24  (i.e. there are more crosslinked portions  32  in the partially crosslinked SMP polymer  24  per unit area than corresponding crosslinked portions  43  in the lightly crosslinked SMP polymer  26 ). In addition, the lightly crosslinked SMP polymer  26  may be characterized wherein the length of the free side chain portions  44  are longer than the corresponding length of the free side chain portions  36  of the partially crosslinked SMP polymer  24 . 
         [0030]      FIGS. 2 ,  3  and  4  illustrate the process for bringing together two separate crosslinked SMP chains of  FIGS. 1A-1C  above to form an adhesive bond there between. The resultant bonded materials may have varying degrees of adhesive strength that depends on a function of the “shape” of the SMP polymer (i.e. whether in its permanent shape or in its temporary shape), the degree and type of crosslinking, and the length of available surface free chains for interdiffusion. 
         [0031]    Referring first to  FIG. 2A , two fully crosslinked SMP polymeric chains  22 ,  23  may be illustrated as being brought in close contact in their respective permanent shapes  22 A,  23 A in the absence of load. Here, the fully crosslinked SMP chains  22 ,  22  in their permanent shapes  22 A,  23 A may be macroscopically flat but microscopically rough rigid polymers. 
         [0032]    Next, as shown in  FIG. 2B , the fully crosslinked SMP chains  22 ,  23  of  FIG. 2A  have been heated to a temperature above their glass transition temperatures and placed under a load sufficient to transform the fully crosslinked SMP polymeric chains  22 ,  23  from their permanent shapes  22 A,  23 A to their temporary shape (i.e. hot pressed together), as shown by reference numerals  22 B,  23 B. The transformation to their temporary shapes  22 B,  23 B provides an interface  54 , and hence better contact between the two chains  22 B,  23 B to form a bonded material  53 . 
         [0033]    However, while good contact at the interface  54  was achieved, little adhesive strength may be realized between the polymeric chains  22 B,  23 B in bonded material  53 , thus allowing the two fully crosslinked SMP polymer chains in their temporary shapes  22 B,  23 B to be easily separated by cooling the SMP below the glass transition temperatures and subsequently removing the load, as shown in  FIG. 2C , wherein the polymers were maintained in their temporary shapes  22 B,  23 B. A similar separation occurred if the bonded material  53  was maintained at a temperature above the glass transition temperatures of the polymers  22 ,  23  when the load was removed, as shown by the transformation from  FIG. 2B  to  FIG. 2D , or when the SMP was cooled below the glass transition temperature, followed by a load removal, and then heated back above the glass transition temperature, as shown in the transformation from  FIG. 2C to 2D , wherein the polymeric chains were transformed to their permanent shapes  22 A,  23 A. 
         [0034]    Referring now to  FIG. 3A , two partially crosslinked SMP  24 ,  25  may be shown in close proximity to one another in the absence of load in their permanent shape  24 A,  25 A. Here, the partially crosslinked SMP  24 ,  25  in their permanent shapes  24 A,  25 A may be macroscopically flat but microscopically rough rigid polymers. 
         [0035]    Next, in  FIG. 3B , the partially crosslinked SMP  24 ,  25  may have been heated to a temperature above their glass transition temperatures and placed under a load sufficient to transform the polymer chains from their permanent shapes  24 A,  25 A to their temporary shape, as shown by reference numerals  24 B and  25 B. The transition may provide an interface  64 , and hence better contact between the two chains  24 B,  25 B to form a bonded material  63 . In addition, the transformation from their permanent shapes  24 A,  25 A to their temporary shapes  24 B,  25 B may allow diffusion between the respective surface side chain portions  44  to create an interdiffusion thin layer  76 . The interdiffusion thin layer  76  includes a plurality of surface free chain portions  36 ,  36 ′ from the first SMP  24 B and second SMP  25 B that are in an overlapping position in a common place. 
         [0036]    In  FIG. 3C , the polymeric chains may be allowed to cool below their glass transition temperatures under load, wherein the load was removed, thus maintaining the polymers in their temporary shapes  24 B,  25 B. As shown in  FIG. 3C , the surface free chain portions  36 ,  36 ′ may remain substantially frozen and interdiffused, thus possibly providing some degree of resistance from allowing the polymeric chains  24 B,  25 B to easily separate. 
         [0037]    When the polymeric chains  24 ,  25  were heated back to a temperature above the glass transition temperature in the absence of load, as shown in the transformation from  FIG. 3C  to  FIG. 3D , or when the load was removed while the polymers  24 ,  25  were maintained at a temperature above the glass transition temperature, as shown in the transformation from  FIG. 3B  to  FIG. 3D , the polymers  24 ,  25  may be transformed back to their original permanent shapes  24 A,  25 A and allows frozen free side chain portions  36 ,  36 ′ to become mobile, which may allow the polymers to separate. 
         [0038]    Referring now to  FIG. 4A , two lightly crosslinked SMP polymeric chains formed from the polymeric material  26 ,  27  shown in  FIG. 2C  may be brought in close contact in their permanent shapes  26 A,  27 A. Here, the lightly crosslinked SMP chains  26 ,  27  in their permanent shapes  26 A,  27 A may be macroscopically flat but microscopically rough rigid polymers. 
         [0039]    Next, in  FIG. 4B , the lightly crosslinked SMP chains  26 ,  27  may have been heated to a temperature above their glass transition temperatures and placed under a load sufficient to transform the polymer chains from their permanent shapes  26 A,  27 A to their temporary shapes, as shown by reference numerals  26 B and  27 B. The transition may provide an interface  74 , and hence better contact between the two chains  26 B,  27 B to form a bonded material  73 , or composite material  73 . In addition, the transformation from their permanent shapes  26 A,  27 A to their temporary shapes  26 B,  27 B may allow diffusion between the respective free side chain portions  44 ,  44 ′ to create a small interdiffusion layer  86 . The degree of interdiffusion of the interdiffusion layer  86  in  FIG. 4B  may be greater than the degree of interdiffusion in interdiffusion layer  76  of  FIG. 3B . 
         [0040]    In  FIG. 4C , the polymeric chains may be allowed to cool below their glass transition temperatures under load, wherein the load was removed, thus maintaining the polymers in their temporary shapes  26 B,  27 B. As shown in  FIG. 4C , the surface free chain portions  44 ,  44 ′ may remain substantially frozen, thus not allowing the polymeric chains  26 B,  27 B to easily separate. The degree of force necessary to separate the polymeric chains  26 B,  27 B in  FIG. 4C  may be greater than the degree of force necessary to separate the polymeric chains  24 B,  25 B of  FIG. 3C  (whose degree of force was greater than the force necessary to separate the polymeric chains  22 B,  23 B of  FIG. 2C ), which suggests the adhesive strength of the formed composite material  73  in  FIG. 4C  may be more than the corresponding adhesive strength of the bonded material  63  of  FIG. 3C  and the bonded material  53  of  FIG. 2C . 
         [0041]    This suggests that the degree of interdiffusion in the interdiffusion layers may contribute to the adhesive strength of the formed composite material. A greater degree of interdiffusion may lead to greater adhesive strength between the SMP in their temporary shapes. Along those lines, the degree of interdiffusion may be related the length of the surface free chain portions  44 ,  44 ′. 
         [0042]    In addition, the degree of interdiffusion, and hence the adhesive strength of the polymers when reversibly coupled, may also be affected by the number of available free side chain portions per unit area of the shape memory polymer. The degree of interdiffusion corresponds to the amount of overlap, or intermingling, of the surface free chain portions when a pair of SMP are coupled. An increased number of available surface free side chain portions may increase the degree of interdiffusion. Conversely, a large amount of crosslinking of side chains in a shape memory polymer chain, and hence a smaller amount of available surface free chains, may reduce the degree of interdiffusion, and hence the adhesive strength. 
         [0043]    When the polymer may be heated back to a temperature above the glass transition temperature in the absence of load, thus transforming the polymers back to their original permanent shapes  26 A,  27 A from their temporary shapes  26 B,  27 B as shown in the transformation from  FIG. 4C to 4D , or wherein the load is simply removed while the chains  26 B,  27 B are maintained above their glass transition temperature, as shown in the transformation from  FIG. 4B to 4D , the frozen free side chain portions  44 ,  44 ′ may become mobile, thus allowing the polymer chains  26 A,  27 A to easily separate. 
         [0044]    Thus, the exemplary embodiments illustrate that lightly crosslinked SMP chains having long and mobile side chain portions may be welded together to form composite material having a degree of adhesive strength. Moreover, by simply heating the SMP polymers in the absence of load to transform the SMP polymers back to their permanent shape, such coupled SMP polymers may be easily separated and subsequently rewelded. 
       Experimental Confirmation 
       [0045]    Two lightly crosslinked polystyrene samples with identical crosslink density were produced by polymerizing a mixture of 0.5 weight percent BPO initiator and 2.0 mole percent of divinylbenzene with styrene at seventy-five degrees Celsius for about sixteen hours. The samples were sulfonated using concentrated sulfuric acid at ninety degrees Celsius for about 5 minutes. The sulfonated crosslinked polystyrene samples were pressed together at one hundred forty five degrees Celsius for about 30 minutes. After cooling under load, adhesive strength of 40 N/cm2 was obtained. The bonded samples, when subjected to heating back to one hundred forty five degrees Celsius, in the absence of load, separated from each other without any external separating force. Overall, such a phenomenon may be referred to as reversible welding. 
         [0046]    When two fully crosslinked epoxy polymer samples were subjected to a similar bonding procedure under load, no measurable adhesion was obtained. This appears to confirm that presence of free chains on the polymer surface may be necessary to achieve interdiffusion of the polymer chains, and hence the reversible welding of the polymer chains together under load. 
         [0047]    In another case, a sulfonated crosslinked polystyrene was hot pressed to a non-sulfonated crosslinked polystyrene. In this example, no adhesion was observed. This appears to confirm that the miscibility of the surface free chains may be an additional requirement for reversible welding. 
         [0048]    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.