Shape memory polymers (SMP) are materials that have the capability of changing shape when exposed to some external stimulus. The most common type of shape memory material is one for which the shape change is achieved by changes in temperature, i.e., a thermally-induced shape memory effect. Such materials possess at least two independent crosslinked networks, at least one of which is thermally reversible. The materials have a permanent shape due usually to a covalently crosslinked network, but they can be reshaped above a thermal transition (Tc) of a second, reversible network and fixed into a temporary shape when cooled under stress to below Tc. When reheated above the critical temperature, the material remembers and reverts to the permanent shape due to entropy elasticity. The objective of the proposed work is to develop and understand the structure and properties of a new class of SMP based on blends of an elastomeric ionomer and low molar mass fatty acids or their salts. A major goal is to understand the molecular origin of shape memory in such materials. Nanophase separation of the ionomer or covalent crosslinking of the ionomer will be used to develop the permanent network. Very strong ionic or dipolar interactions between the polymer and the fatty acid (salt) will allow crystals of the fatty acid (salt) to function as thermally reversible, physical crosslinks that can provide a temporary shape. Since, the melting point of the fatty acid (salt) serves as Tc, the temperature of the shape memory effect can be easily controlled by choosing an appropriate fatty acid (salt) for the compound. This will provide significant versatility in the tailoring of Tc, as well as the mechanical properties of the SMP.
NON-TECHNICAL SUMMARY
Shape memory polymers (SMP) have applications as medical devices (orthodontic wires, polymer stents with drug delivery capabilities, biodegradable implants, smart surgical sutures), actuators, sensors, artificial muscles, switches, smart textiles and self-deployable structures. This project will develop a new, versatile type of SMP and train scientists to work in the field of smart materials, which is an enabling technology in many high-tech applications. The grant specifically supports the work of two graduate students and undergraduate chemical engineering students will participate in the project through independent study or as REU students. The PI and students will also participate in education and outreach programs designed to enhance exposure of K-12 students and teachers to science and technology, specifically in the field of smart materials. These include the University?s residential Engineering 2000 program that targets minority students, the DaVinci Project that helps math and science teachers (grades 7-12) to integrate engineering into the classroom, and the NSF-funded Galileo Project that introduces high school students and K-12 educators to core engineering concepts and problem-solving practices. In addition, the NSF-funded Louis Stokes Alliance for Minority Participation' (LSAMP) will be used to introduce minority students to research. The results of the research will be published in prestigious, peer-reviewed journals and presented at international scientific congresses.
Intellectual Merit. Intellectual Merit. The objectives of the research were to develop and understand the structure and properties of a new class of shape memory polymer based on blends of an elastomeric ionomer and low molar mass fatty acid or their salts (FAS). A major goal was to understand the molecular origin of shape memory in such materials and determine the dynamics of such systems. Three specific designs of SMPs were investigated: 1) soft compositions derived from compounds of FASs and an elastomeric ionomer, the zinc salt of sulfonated poly{ethylene-co-ran-propylene-co-ran-(5-ethylidene-2-norbornene)}, Zn-SEPDM; 2) semi-crystalline thermoplastic compositions of zinc stearate, ZnSt, and the zinc salt of poly(ethylene-co-ran-methacrylic acid), ZnPEMA; and 3) high temperature compounds containing sodium oleate, NaOl, and sulfonated poly(ether ether ketone), SPEEK. Strong intermolecular interactions between the FAS and the ZnSEPDM provide strong physical bonds between the components which are capable of forming a crosslink network. Physical crosslinks due to nanophase separation of the ionic species in the ionomer serve as the "permanent" crosslinks that provide the permanent shape. However, the ionic network exhibited creep, which produced a permanent set; the temporary network did not creep. The permanent set was eliminated by crosslinking the residual unsaturation. Tunable shape memory was achieved with neat ZnPEMA due to its broad melting transition. Any value for Tc, within the melting range of the ionomer, can be achieved by partially melting the crystals, deforming the compound and recrystallizing the melted material to form the temporary network. Multiple shape memory was achieved by partially melting the polymer, deforming it and fixing the temporary network and then repeating that procedure at lower temperature(s) to establish multiple temporary steps. Like the ZnSEPDM/FAS SMPs the ionic network exhibited produced non-ideal shape fixing and recovery, which was resolved by crosslinking the ionomer. Amorphous SPEEK ionomers exhibited shape memory (Tc ~ 250°C) For the neat ionomers, Tc = Tg, which can be varied by the choice of the ionomer cation. Zn-SPEEK ionomer (Tg ~ 250°C) exhibited fixing (F) and recovery (R) efficiencies of ~90% and 100%. The addition of NaOl produced near ideal two-way shape memory (F = 96%; R = 100%) with Tc ~ 260°C, and three-way shape memory Tc’s = 220°C and 260°C. The research produced 14 publications. Broader Impacts. Training and Development. Three graduate students (two female, one male) were involved with the research and trained in the field of structure and properties of polymers and, specifically, the physics of shape memory behavior and smart materials. One graduate student graduated and is currently employed by United Technologies Corp. Two postdoctoral research associates (both female), PDAs,were involved with this project and were mentored by the PI with regard to developing research strategies and skills and preparing for employment in academe or industry. One PDA has left UA and is working for Saint Gobain Corp. The PDAs were also been given the opportunity to teach some lectures in the PI's undergraduate courses on Polymer Science for Engineers and Polymer Morphology. Two undergraduate REU students (one male and one female) and a female elementary school science teacher (RET program) were mentored by the PI, the PDAs and the graduate students. In addition to gaining experience in carrying our hypothesis-driven research, the REU and RET participants were trained in proper laboratory safety procedures and provided instruction for using a variety of polymer characterization instruments. Two female high school students also worked on the project and were supervised closely by the PI, the PDAs and the graduate students. One of the high school students has now graduated and is enrolled in a science curriculum at Ohio State University. She also won several local science fair awards for her work on shape memory polymers and was a member of the 2011 team that won the State of Ohio STEM competition.
