Synthesis and Characterization of Novel Double-Functionalized Surface Modified Thermoplastic Elastomers

University Of Akron, Akron OH

Investigators

Abstract

Summary. This project in polymer science is co-funded by the Polymers Program of the Division of Materials Research and the Central Europe and Eurasia Program of the Office of International Science and Engineering. It is aimed at the synthesis and characterization of novel non-polar self-assembling nanostructured elastomeric block copolymers, whose surface is decorated with polar hydroxyl functional groups. Specifically, amorphous block copolymers comprising a core of dendritic (arborescent) polyisobutylene (arbPIB) carrying a primary hydroxyl group at each branching point, flanked by poly(styrene-co-p-thymylmethylstyrene) glassy blocks constituting about 30wt% of the polymer, will be synthesized (arbPIB(OH)-b-P(St-co-TMeSt). Living carbocationic polymerization of isobutylene (IB) by using 4-(1,2-oxirane-isopropyl)-styrene as inimer (initiator-monomer) in conjunction with TiCl4 will introduce one polar primary OH group at each branching points of the core (arbPIB(OH)). Preliminary evidence suggests that polar groups migrate to the surface of non-polar polymers, available for further modification. The arbPIB(OH) core will be characterized by NMR, SEC, FTIR, XPS and other appropriate techniques. After establishing optimum synthesis conditions, block copolymers will be synthesized by living carbocationic polymerizations using sequential monomer addition, followed by appropriate functionalization of the end blocks. Self-assembly of the blocks is governed by the phase separation of the rubber and plastic phases, leading to reinforcement of the material. Both the static and dynamic properties, these latter critical in biomedical applications, will be optimized by introducing reversible hydrogen bonding into the styrenic phases. This will be accomplished by the living carbocationic copolymerization of St with pClMeSt, followed by converting the pendant benzylic Cl groups of the arbPIB-b-P(St-co-ClMeSt) into TMe groups, yielding a novel double-functionalized block copolymer (arbPIB(OH)-b-P(St-co-TMeSt). This block copolymer will be a thermoplastic elastomer (TPEs), displaying rubber-like behavior at room temperature while processing as plastics at above the glass transition temperature (Tg) of the glassy blocks. The bulk and surface properties of the novel block copolymer will be investigated. Specifically, the dynamic fatigue and creep properties will be investigated and compared to currently used biomaterials (silicone rubber, polyesters and polyurethanes). It is expected that the combination of the branched PIB core and the hydrogen bonding between the thymine functional groups in the styrenic hard phases will dramatically improve dynamic fatigue and creep properties of this novel biomaterial. The surface hydroxyl groups are expected to improve biocompatibility. Fatigue testing will be carried out at the University of Bayreuth, Germany, in collaboration with Professor Volker Altsdt, and biocompatibility testing will be carried out at the Pomeranian Medical Academy in Szczecin, Poland, in collaboration with Professor Miroslawa El Fray. Intellectual Merit. Linear and star-branched PSt-b-PIB-b-PSt TPEs (coinvented by the PI) have recently received FDA approval for use as drug-eluting coronary stent coating. arbPIB-b-PSt block copolymers are the newest members of the family of PIB-based TPEs. The current proposal charts new synthetic routes to introduce OH groups at the surface of arbPIB-based block copolymers, and to functionalize the glassy end blocks. This research would lead to two new families of materials: arbPIB(OH)-b-P(St-co-ClMeSt), and arbPIB(OH)-b-P(St-TMeSt). The P(St-co-TMeSt) end blocks are expected to have higher Tg than PSt (Tg ~100 C), due to hydrogen bonding. In addition, the validity of the hypothesis of improving the dynamic fatigue and creep properties of amorphous block TPEs by hydrogen bonding will be tested (hydrogen bonding is believed to be the reason for the excellent fatigue properties of polyurethanes). This would be a new, fundamental development with general applicability. The new materials may emerge as an alternative to medical grade silicone rubber. Broader Impacts. The new materials, just like other TPEs, are environmentally friendly rubbers since they can be reprocessed. The project to produce and test these new biomaterials has strong international aspects, with directly relevant collaborations in Germany (mechanical testing) and Poland (biocompatibility testing). It is complementary to proposals submitted to the German and Polish Research Foundations (DFG) to investigate the dynamic creep and fatigue properties of novel nanostructured elastomeric materials that have the potential to replace silicone rubber in specific biomedical applications. Silicone-based breast implants were the single "choice" for a quarter of a million American women who underwent the procedure last year. This aspect is very popular with young people, especially females, so this program will be appealing to potential female American graduate students, still under-represented in the field of chemistry and material science. The interdisciplinary nature of the projects will expose students to a great variety of scientific disciplines (polymer chemistry, organic chemistry, material science, biochemistry, surface science, polymer engineering, biomedical engineering). They will also have a chance to carry out part of their research in Germany and Poland. Exposure to different cultures, organizations and work ethics will prepare the students to work better in the global economy, making them more attractive to potential employers.