Functionally Graded Shape Memory Polymer

Abstract

A functionally graded shape memory polymer (SMP) that has a range of transition temperatures that are spatially distributed in a gradient fashion within one single article. The SMP is formed by post-curing a pre-cured glassy SMP in a linear temperature gradient that imposes different vitrification temperature limits at different positions along the gradient. Utilizing indentation-based surface shape memory coupled with optical measurements of photoelastic response, the capability of this material to respond over a wide range of thermal triggers is correlated with the graded glass transition behavior. This new class of SMP offers great potential for such applications as passive temperature sensing and precise control of shape evolution during a thermally triggered shape recovery.

1 . A functionally graded shape memory polymer comprising a shape memory polymer comprising:
 a first end;   a second end; and   a plurality of shape memory glass transition temperatures (T g 's) that are spatially located and distributed in a gradient fashion from said first end to said second end.   
     
     
         2 . The functionally graded shape memory polymer of  claim 1 , wherein said gradient is linear and one dimensional from T min  at said first end to T max  at said second end. 
     
     
         3 . The functionally graded shape memory polymer of  claim 2 , wherein said shape memory polymer is structured to respond to a range of temperatures T, where T min <T<T max , upon application of substantially spatially uniform external heating yielding a spatially dependent elastic modulus for a given temperature and a spatially dependent shape recovery response. 
     
     
         4 . The functionally graded shape memory polymer of  claim 3 , wherein said response comprises recovery of the shape memory polymer from a temporary deformed or strained configuration with lower conformational entropy to a permanent configuration with higher conformational entropy in a wavelike-fashion upon said spatially uniform external heating, wherein said recovery begins at said first end where the transition temperature is lowest and propagates in the direction of increasing transition temperature towards said second end. 
     
     
         5 . The functionally graded shape memory polymer of  claim 3 , further comprising a plurality of portions at least partially separated from one another and spaced along the transition temperature direction from said first end to said second end, wherein each of said plurality of portions comprises a different localized T g  and is structured to relatively independently recover from a temporary deformed or strained configuration with lower conformational entropy to a permanent configuration with higher conformational entropy upon said spatially uniform external heating. 
     
     
         6 . The functionally graded shape memory polymer of  claim 5 , wherein each of said plurality of portions is further structured to recover from a temporary deformed or strained configuration with lower conformational entropy to a permanent configuration with higher conformational entropy when said uniform external heating is greater than a respective portion's localized T g . 
     
     
         7 . The functionally graded shape memory polymer of  claim 1 , wherein said shape memory polymer is a thermoset selected from the group consisting of P(MMA-co-VP)-PEG semi-IPNs, copolyester, P(AA-co-MMA)-PEG, corn oil copolymer, PMMA-PBMA copolymers, epoxy, fish oil copolymers, PET-PEG copolymer, P(MA-co-MMA)-PEG, soybean oil copolymers with styrene and DVB, styrene copolymer, thermosetting PU, dehydrochlorinated cross-linked PVC, thermosets formed by thiol-ene reaction, polyacrylates, and polymethacrylates. 
     
     
         8 . The functionally graded shape memory polymer of  claim 3 , wherein said shape memory polymer further comprises a film coating on at least one surface of said shape memory polymer that is more rigid than said shape memory polymer. 
     
     
         9 . The functionally graded shape memory polymer of  claim 8 , wherein said response comprises recovery of the shape memory polymer from a temporary deformed or strained configuration with lower conformational entropy towards a permanent configuration with higher conformational entropy, wherein said surface with said film coating forms wrinkles upon said spatially uniform external heating, and wherein said recovery and buckling begins at said first end where the transition temperature is lowest and propagates in the direction of increasing transition temperature towards said second end resulting in a change in color of said surface. 
     
     
         10 . The functionally graded shape memory polymer of  claim 8 , wherein said film coating comprises a coating selected from the group consisting of a metallic coating and a polymeric coating. 
     
     
         11 . The functionally graded shape memory polymer of  claim 10 , wherein said metallic coating comprises gold. 
     
     
         12 . The functionally graded shape memory polymer of  claim 10 , wherein said polymeric coating comprises a polymer with a modulus of elasticity at least 10 times greater than that of a rubbery state of said shape memory polymer. 
     
     
         13 . The functionally graded shape memory polymer of  claim 12 , wherein said polymeric coating comprises a polymer selected from the group consisting of polystyrene, polycarbonate, poly(alkyl methacrylate)s, poly(alkyl acrylate)s, polyimides, and poly(arylene ether ketone)s. 
     
     
         14 . A method of preparing a functionally graded shape memory polymer, said method comprising the steps of:
 providing a shape memory polymer comprising a first end and a second end;   applying an increasing temperature gradient to said shape memory polymer from said first end to said second end, wherein said application produces a corresponding increasing gradient in crosslink density and glass transition temperatures (T g 's) to said shape memory polymer from said first end to said second end.   
     
     
         15 . The method of  claim 14 , further comprising the step of photocuring said shape memory polymer by use of a radiation source. 
     
     
         16 . The method of  claim 15 , wherein said shape memory polymer is a curable thermoset. 
     
     
         17 . The method of  claim 16 , wherein said curable thermoset is selected from the group consisting of polyacrylates, polymethacrylates, thermosets formed by thiol-ene reactions, polyurethanes, and epoxy resins. 
     
     
         18 . A method of preparing a functionally graded shape memory polymer, said method comprising the steps of:
 providing a shape memory polymer comprising a first end and a second end;   photocuring said shape memory polymer by use of a radiation source through a gradient photomask, wherein said gradient photomask allows an increasing amount of radiation to reach said shape memory polymer from said first end to said second end, wherein said photocuring produces a corresponding increasing gradient in crosslink density and glass transition temperatures (T g 's) to said shape memory polymer from said first end to said second end.   
     
     
         19 . The method of  claim 18 , wherein said photocuring is performed when said shape memory polymer is at a temperature greater than a maximum glass transition temperature (T g ) allowable by said shape memory polymer. 
     
     
         20 . The method of  claim 18 , wherein said shape memory polymer is a radiation-curable thermoset. 
     
     
         21 . The method of  claim 18 , wherein said radiation is ultra-violet radiation. 
     
     
         22 . A method of preparing a functionally graded shape memory polymer, said method comprising the steps of:
 providing a shape memory polymer comprising a first end and a second end;   photocuring said shape memory polymer by use of a radiation source and an opaque mask placed in between said radiation source and said shape memory polymer, wherein said opaque mask moves at a predetermined velocity from said second end to said first end to allow an increasing amount of radiation to reach said shape memory polymer from said first end to said second end, wherein said photocuring produces a corresponding increasing gradient in crosslink density and glass transition temperatures (T g 's) to said shape memory polymer from said first end to said second end.   
     
     
         23 . The method of  claim 22 , wherein said photocuring is performed when said shape memory polymer is at a temperature greater than a maximum glass transition temperature (T g ) allowable by said shape memory polymer. 
     
     
         24 . The method of  claim 22 , wherein said shape memory polymer is a radiation-curable theremoset. 
     
     
         25 . The method of  claim 22 , wherein said radiation is ultra-violet radiation. 
     
     
         26 . A temperature sensor device comprising the functionally graded shape memory polymer of  claim 1 , wherein said device is a label structured to be attached to a surface of an object and is pre-deformed by stretching, bending, indenting, or embossing. 
     
     
         27 . The temperature sensing device of  claim 26 , wherein said device is adapted to respond to an environmental temperature, T, in the range of T min <T<T max . 
     
     
         28 . The temperature sensing device of  claim 27 , wherein said device is capable of generating a spatially dependent recovery when exposed to the environmental temperature, T, in the range of T min <T<T max .