An integrated and comprehensive study of new triblock copolymers comprising cholesteric liquid crystalline units in the central block and ionic imidazolium or pyridinium groups in the end blocks for shape-memory applications is the main goal of this proposal. Synthetic methods to prepare monomers bearing cholesteric units, B, and monomers bearing imidazole or pyridine units, A, are described. Steroidal molecules that will be used as side-chain mesogens are chosen for their functionality and their cholesteric LC property. Sequential monomer addition in the order of A, B, and A, by controlled polymerization methods, will be used to prepare ABA triblock copolymers. Due to incompatibility between the different blocks, through a bottom-up approach, a phase-segregated morphology will result from the melts of these ABA triblock copolymers. Cholesteric units are confined to a nanodomain and the cholesteric mesogens are known to arrange in a helical fashion; imidazole or pyridine groups will phase-segregate into separate nanodomains. Triblock copolymer, ABA, will be converted to the ionic form, A'BA'. With the support of this CAREER award, the Kasi group will (1) perform thermal, optical, and morphological analysis of these polymers, (2) investigate the shape-memory behavior of ABA and A'BA' triblock copolymers, (3) establish the influence of confined cholesteric helices on shape-memory behavior of these triblock copolymers and (4) establish the effect of thermal actuation of cholesteric helices which will result in pitch change of helices allowing the design of thermo-responsive selective light reflection media.
NON-TECHNICAL SUMMARY
Shape-memory polymers exhibit reversible shape changes in the presence of stimulus such as thermal energy. These polymers are used in biomedical applications such as degradable sutures and implants. A step forward is the design and structure-property study of new nanostructured liquid crystalline block copolymers that show a dual-response of shape and light reflection with temperature, which is the primary goal of this CAREER research plan. These new responsive liquid crystalline polymers will find applications in medical devices, smart textiles that change shape and color, and display technologies. The educational and outreach activities that have been initiated by the principal investigator (PI) will link different educational levels by using polymer based research training as an educational tool. Graduate, undergraduate, and high school students working in the PI's group at the University of Connecticut will gain experience in the cross-disciplinary field of responsive polymers. Additionally, the PI has initiated two different science outreach programs targeted towards middle- and high school students from a federally designated high-need district in Connecticut. The goal of the middle-school outreach program is to link science syllabus with innovations in science and technology of ubiquitous materials like polymers through hands-on experimentation. The high school outreach program is a collaborative summer workshop established by the PI and five Chemistry faculty members. The goal of this program is to provide talented under-represented high school students research experience in five sub-fields of chemistry. Introducing these students to exciting research and innovations in chemistry could be a viable method to keep them interested in sciences.
Intellectual Merits We investigate how polymers respond and show a shape change with or without color change when subjected to thermal or magnetic fields. We also want to understand the self-assembly of polymers at the nanoscopic level and how this translates to macroscopic material properties. These soft materials designed and developed have applications as responsive materials, actuators, artificial muscles, thermochromics, sensors, tissue engineering scaffold, and biomedical delivery devices. We chose to use side-chain liquid crystalline polymers (SCLCPs) in which mesogenic side-groups are linked to a polymeric backbone through a flexible spacer, typically methylene units. Depending on the length of flexible spacer, motional decoupling of polymer backbone and mesogenic groups are varied, and this influences the resulting liquid crystalline mesophases morphology and thermal transitions. We established methodologies to synthesize several new libraries of hierarchically structured side-chain liquid crystalline polymers (SCLCP) using metal-catalyzed and control radical polymerization techniques. Here, cholesteryl moiety is used as a chiral mesogenic side-chain and several types of monomers and polymers bearing side-chain cholesteryl group linked with different flexible methylene spacers have been synthesized. In our SCLCPs, the side-chain cholesteryl mesogen self-assembles into layered smectic A (SmA) mesophase. Generally, these polymers are thermally responsive and the self-assembled SmA mesophase will clear to an isotropic state at the clearing temperature (Tcl). Interestingly, in our SCLCPs varied methylene spacer length (n = 5, 10 and 15) between polymer backbone and cholesteryl side-chain induces diversified SmA polymorphism in nanometer scale from bilayer (SmA2) for n = 5 to monolayer (SmA1) for n = 15 and mixed layers of SmA2 and SmA1 for n = 10 depending on the extent of interdigitation of cholesteryl mesogens. We are interested in investigating the influence of SmA polymorphism and hierarchical ordering on macroscopic properties such as shape memory, morphing and actuator effects. My group has demonstrated that the microstructure evolution of the SmA phase at the nanoscale actually determines macroscopic strain response (elongation and contraction) during shape memory cycle. This interdigitation-based thermostrictive property has not been observed before and our new library of side-chain random copolymers can provide a new mechanism to develop thermal actuators and unique shape changing or morphing materials. Our polymer framework is very modular and with minor chemical modifications, we prepared new polymers that are used in (1) Preparing composites with magnetic nanoparticles to improve the mechanical and thermal properties of these materials and create multi-stimuli responsive materials, (2) Designing polymers comprising cholestryl moieties that can function as thermochromics and thermomechanochromics, (3) new library of biomaterials that can form stable nanoparticles in water and can entrap hydrophobic cancer drugs and contrast agents for delivery within animal models. Broader impacts During this grant period we published several papers on new polymers that respond to thermal, magnetic and mechanical fields and showed shape change as well as color change. These materials can be used in temperature and mechanical sensors. Additionally, these polymers are biocompatible and can be used to entrap and deliver biological molecules, drugs and contrast agents. Human resource development was the major outreach goal of this grant. This research was a medium used to develop outreach activities targeted towards high school students. These include student workshops as well as students working in my research lab through UConn Mentor Connection. These programs were held every year to motivate students to participate in polymer based research activities and to interest them in the field of applied sciences. Additionally several undergraduate participated in this research and a few of them have since started graduate school in polymeric materials. All graduate students who participated in this research have since started their independent careers as scientists and engineers working in polymer industries within the United States.
