Reversible welding process for polymers

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.

TECHNICAL FIELD

The technical field generally relates to polymer coupling methods and more specifically to a reversible welding process for polymers.

BACKGROUND

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

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.

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.

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 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 (Tg) 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.

Many crosslinked polymers also possess SMP properties. However, to adhere two polymer crosslinked SMP's together, it appears that good surface contact as well as interdiffusion may be prerequisites for good adhesive bonding.

Referring first toFIGS. 1A-1C, a crosslinked SMP polymer20may be illustrated as having three separate crosslinking densities, namely a fully crosslinked SMP polymer22as shown inFIG. 1A, a partially crosslinked SMP polymer24as shown inFIG. 1B, and a lightly crosslinked SMP polymer26as shown inFIG. 1C.

Each of the crosslinked SMP polymers20illustrated inFIGS. 1A-1Cmay include one or more polymeric backbone portions30and one or more free surface chain portions. For illustrative purposes and as shown inFIGS. 1A-1C, the composition of the polymeric backbone portions30of each respective SMP22,24,26may be virtually identical.

The fully crosslinked SMP polymer22as used herein and as shown inFIG. 1A, may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions32. In other words, there may be virtually no free side chain portions extending from any of the polymeric backbone portions30.

The partially crosslinked SMP polymer24as used herein and as shown inFIG. 1Bin its permanent shape, may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions34. In addition, the partially crosslinked SMP polymer24may include one or more additional free side chain portions36having a first end38coupled to the surface of24and a second end, or free end40, that may not be reacted with a corresponding free side chain portion36of an adjacent polymeric backbone portion30.

The lightly crosslinked SMP polymer26as used herein and as shown inFIG. 1Cin its permanent shape, may be characterized wherein the molecular chains in the bulk are connected via crosslinking chain portions43. In addition, the lightly crosslinked SMP polymer26may include one or more additional free side chain portions44having a first end46coupled to the surface of30and a second end, or free end48, that may not be reacted with a corresponding free side chain portion44of an adjacent polymeric backbone portion30.

The lightly crosslinked SMP polymer26, by definition, has less crosslinked portions per unit area than the partially crosslinked SMP polymer24(i.e. there are more crosslinked portions32in the partially crosslinked SMP polymer24per unit area than corresponding crosslinked portions43in the lightly crosslinked SMP polymer26). In addition, the lightly crosslinked SMP polymer26may be characterized wherein the length of the free side chain portions44are longer than the corresponding length of the free side chain portions36of the partially crosslinked SMP polymer24.

FIGS. 2,3and4illustrate the process for bringing together two separate crosslinked SMP chains ofFIGS. 1A-1Cabove 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.

Referring first toFIG. 2A, two fully crosslinked SMP polymeric chains22,23may be illustrated as being brought in close contact in their respective permanent shapes22A,23A in the absence of load. Here, the fully crosslinked SMP chains22,22in their permanent shapes22A,23A may be macroscopically flat but microscopically rough rigid polymers.

Next, as shown inFIG. 2B, the fully crosslinked SMP chains22,23ofFIG. 2Ahave been heated to a temperature above their glass transition temperatures and placed under a load sufficient to transform the fully crosslinked SMP polymeric chains22,23from their permanent shapes22A,23A to their temporary shape (i.e. hot pressed together), as shown by reference numerals22B,23B. The transformation to their temporary shapes22B,23B provides an interface54, and hence better contact between the two chains22B,23B to form a bonded material53.

However, while good contact at the interface54was achieved, little adhesive strength may be realized between the polymeric chains22B,23B in bonded material53, thus allowing the two fully crosslinked SMP polymer chains in their temporary shapes22B,23B to be easily separated by cooling the SMP below the glass transition temperatures and subsequently removing the load, as shown inFIG. 2C, wherein the polymers were maintained in their temporary shapes22B,23B. A similar separation occurred if the bonded material53was maintained at a temperature above the glass transition temperatures of the polymers22,23when the load was removed, as shown by the transformation fromFIG. 2BtoFIG. 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 fromFIG. 2C to 2D, wherein the polymeric chains were transformed to their permanent shapes22A,23A.

Referring now toFIG. 3A, two partially crosslinked SMP24,25may be shown in close proximity to one another in the absence of load in their permanent shape24A,25A. Here, the partially crosslinked SMP24,25in their permanent shapes24A,25A may be macroscopically flat but microscopically rough rigid polymers.

Next, inFIG. 3B, the partially crosslinked SMP24,25may 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 shapes24A,25A to their temporary shape, as shown by reference numerals24B and25B. The transition may provide an interface64, and hence better contact between the two chains24B,25B to form a bonded material63. In addition, the transformation from their permanent shapes24A,25A to their temporary shapes24B,25B may allow diffusion between the respective surface free side chain portion36to create an interdiffusion thin layer76. The interdiffusion thin layer76includes a plurality of surface free chain portions36,36from the first SMP24B and second SMP25B that are in an overlapping position in a common place.

InFIG. 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 shapes24B,25B. As shown inFIG. 3C, the surface free chain portions36,36′ may remain substantially frozen and interdiffused, thus possibly providing some degree of resistance from allowing the polymeric chains24B,25B to easily separate.

When the polymeric chains24,25were heated back to a temperature above the glass transition temperature in the absence of load, as shown in the transformation fromFIG. 3CtoFIG. 3D, or when the load was removed while the polymers24,25were maintained at a temperature above the glass transition temperature, as shown in the transformation fromFIG. 3BtoFIG. 3D, the polymers24,25may be transformed back to their original permanent shapes24A,25A and allows frozen free side chain portions36,36′ to become mobile, which may allow the polymers to separate.

Referring now toFIG. 4A, two lightly crosslinked SMP polymeric chains formed from the polymeric material26,27shown inFIG. 2Cmay be brought in close contact in their permanent shapes26A,27A. Here, the lightly crosslinked SMP chains26,27in their permanent shapes26A,27A may be macroscopically flat but microscopically rough rigid polymers.

Next, inFIG. 4B, the lightly crosslinked SMP chains26,27may 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 shapes26A,27A to their temporary shapes, as shown by reference numerals26B and27B. The transition may provide an interface74, and hence better contact between the two chains26B,27B to form a bonded material73, or composite material73. In addition, the transformation from their permanent shapes26A,27A to their temporary shapes26B,27B may allow diffusion between the respective free side chain portions44,44′ to create a small interdiffusion layer86. The degree of interdiffusion of the interdiffusion layer86inFIG. 4Bmay be greater than the degree of interdiffusion in interdiffusion layer76ofFIG. 3B.

InFIG. 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 shapes26B,27B. As shown inFIG. 4C, the surface free chain portions44,44′ may remain substantially frozen, thus not allowing the polymeric chains26B,27B to easily separate. The degree of force necessary to separate the polymeric chains26B,27B inFIG. 4Cmay be greater than the degree of force necessary to separate the polymeric chains24B,25B ofFIG. 3C(whose degree of force was greater than the force necessary to separate the polymeric chains22B,23B ofFIG. 2C), which suggests the adhesive strength of the formed composite material73inFIG. 4Cmay be more than the corresponding adhesive strength of the bonded material63ofFIG. 3Cand the bonded material53ofFIG. 2C.

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 portions44,44′.

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.

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 shapes26A,27A from their temporary shapes26B,27B as shown in the transformation fromFIG. 4C to 4D, or wherein the load is simply removed while the chains26B,27B are maintained above their glass transition temperature, as shown in the transformation fromFIG. 4B to 4D, the frozen free side chain portions44,44′ may become mobile, thus allowing the polymer chains26A,27A to easily separate.

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

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.

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.

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.

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.