Thermoplastic Superelastomers Based on Poly(isobutylene)-graft-Poly(l-lactide) Copolymers: Enhanced Thermal Stability, Tunable Tensile Strength, and Gas Barrier Property

Abstract

A mechanically adjustable reinforced thermoplastic superelastomer system, with tunable gas-permeability, was developed. The superelastomer is based on a graft copolymer structure, using commercial butyl rubbers (or poly(isobutylene-co-isoprene), P(IB-co-I)) and l-or d,l-lactide (LLA or LA) derived from renewable feedstocks, by a "grafting from" controlled polymerization. First, hydroxyl-functionalized PIB (PIB-g-(OH)) macroinitiators were prepared through epoxidation using an economical alternative to m-chloroperoxybenzoic acid and ring-opening reaction. Second, PIB-based graft copolymers with end-hydroxylated poly(lactide) as hard side-chains, PIB-g-(P(L)LA-OH)s, were synthesized to target fP(L)LA of 0.18-0.45, to achieve mechanical reinforcement and an additional gas barrier. They were subsequently acetylated with an acetic anhydride to produce PIB-g-(P(L)LA-Ac)s with improved thermal stability. The well-defined molecular structures indicated controlled P(L)LA lengths, and the resulting superelastomers demonstrated improved thermal stability with increased Td,5%; microphase-separated structures having spherical and/or elongated features; thermoplastic behaviors proved by TODT, which were much lower than the resulting Td,5%; and superior and adjustable mechanical characterizations, proving to control elastomeric-to-ductile properties. An oxygen permeability value as low as 27 mL mm m-2 day-1 atm-1 was achieved by increasing fP(L)LA to 0.45, comparable to polyethylene terephthalate. The partially biodegradable and processable gas barrier films based on these PIB-g-PLLA thermoplastic superelastomers have great potential for the flexible packaging of food and medical products.