Synthesis, Characterization, and Surface Functionalization of Polyisobutylene Based Biomaterials

Synthesis of the inimer (initiator-monomer) 4-(1,2-oxirane-isopropyl)styrene (EPOIM) and copolymerization of this inimer with isobutylene (IB) to form arborescent polyisobutylene (arb_PIB) was carried out using TiCl₄ coinitiator. The effect of reaction conditions was investigated. Size exclusion chromatography (SEC) was used to show incorporation of EPOIM across the molecular weight distribution. The average number of branches was measured by selective link destruction. Polymer architecture analysis was carried out using branching parameters based on the radii of gyration (R_g) and hydrodynamic radii (R_h) determined by multidetector SEC. In situ FTIR was utilized to monitor the polymerization of EPOIM. Block copolymers were synthesized by the blocking of arb_PIB with p-methylstyrene to form novel 4th generation SIBS materials, arb_poly[isobutylene(OH)-b-(isobutylene-co-pmethylstyrene] [arb_IB(OH)-MS]. Proof of the presence of hydroxyl groups in the polymer was obtained using ¹H-NMR spectroscopy and by silylation with chlorotrimethylsilane.

The reinforcement and surface functionalization of thermoplastic elastomers (TPEs) for soft tissue replacement was investigated. The materials investigated were soft TPE based PIBs, with surfaces more hydrophilic than poly(styrene-b-isobutylene-b-styrene) (SIBS) and silicone rubber. The material reinforced with carbon black (CB) [arb_IB(OH)-MS_CB] had an interpenetrating network structure, in which the T_g of the dispersed phase increased to 126 °C, making the material steam-sterilizable. It also had the lowest water contact angle at 82°, and superior mechanical properties in comparison with silicone rubber. After 180 days implantation into rabbits arb_IB(OH)-MS_CB displayed the thinnest capsule around the implant in comparison to arb_IB(OH)-MS and silicone.

A new method of surface functionalization to increase the hydrophilicity of arb_IB(OH)-MS was developed. Using a low molecular weight thymine-functionalized polyisobutylene (PIB-T), a layer of PIB-T was spin coated on top of a layer of arb_IB(OH)-MS. This resulted in the “gluing” of the low molecular weight PIB-T onto the surface of the arb_IB(OH)-MS, and showed a 15° decrease in contact angle indicating an increase in hydrophilicity. Evidence of nitrogen and oxygen near the surface of the polymers containing thymine was obtained by XPS. These low molecular weight PIB-Ts will be electrosprayed onto the surface of the arb_IB(OH)-MS; the products are expected to show properties similar to those obtained by spin coating. Surfaces with PIB-T will allow for hydrogen bonding of various proteins to allow the surface to be modified.