Novel Thermoplastic Elastomers based on Benzofulvene: Synthesis and Mechanical Properties

Thermoplastic elastomers (TPEs) are of great importance both academically and technologically. Currently TPEs are the predominated form of styrene-diene copolymers. However, these styrenic TPEs have serious limitations in applications, especially at higher temperature, because of their low upper service temperature (UST). The work described in this dissertation aimed to developing thermoplastic elastomers with a higher UST and lower cost.

In order to develop TPEs with a higher UST, we employed benzofulvene, an anionically polymerizable monomer in hydrocarbon solvent at room temperature, as the glassy block and copolymerized it with isoprene to prepare polybenzofulvene-polyisoprene-polybenzofulvene (FIF) triblock copolymers. Among all triblock copolymers studied, FIF with 14 vol% [volume percentage] of PBF [polybenzofulvene] exhibited a maximum stress of 14.3 MPa [megapascal] and strain at break of 1394 % from tensile tests. Dynamic mechanical analysis showed that the upper service temperature of FIF is 145°C. Microphase separation of FIF triblock copolymers was observed by small angle X-ray scattering, however, without long range order.

Additionally, we report the effects of partial and complete hydrogenation on the thermal stability, mechanical and morphological properties of high temperature thermoplastic elastomers comprised of polybenzofulvene-polyisoprene-polybenzofulvene (FIF) triblock copolymers. After hydrogenation of polyisoprene and unsaturated carbon bonds in the five member ring of PBF, ultimate tensile stress was reduced to 11.2 MPa with strain at break of 750%. The upper service temperature also decreased to 125 °C. The fully hydrogenated triblock copolymer demonstrated an ultimate stress of 17. 4 MPa at 744 % strain. The glass transition temperature (T_g) of fully hydrogenated PBF was 130 °C. Thermal stability was greatly improved by both partial and complete hydrogenation.

Lastly, we developed a cost efficient method to prepare high molecular weight “comb-shaped” graft copolymers, poly(isoprene-g-styrene), with polyisoprene as the backbone and polystyrene as side chains. The grafted polymers were synthesized via free radical emulsion polymerization by copolymerization of isoprene with a polystyrene macromonomer synthesized using anionic polymerization. A material incorporating 29 wt% [weigh percentage] polystyrene exhibits a disordered microphase separated morphology and elastomeric properties. These materials show promise as new, highly tunable, and potentially low cost thermoplastic elastomers.