Modified Biomedical Poly(urethane) Block Copolymers: Nanocomposites and Polyisobutylene Comb Polymers

Introduction Poly(urethane urea) multiblock copolymers [PUU] are used in variety of biomedical applications, including as blood-contacting components in ventricular assist systems. One of the drawbacks of these materials (particularly when used in completely implantable devices) is their relatively high permeability to air and moisture. Conventional biomedical PUU elastomers are generally composed of ~80 wt% soft segment, most often poly(tetramethylene oxide) [PTMO]. It is penetrant diffusion through the continuous, low Tg polyether phase that gives rise to the high permeability. One approach to resolving this problem is to synthesize a polymer architecture that has backbone chemistry nearly identical to a typical biomedical PUU, and possesses polymeric combs of a material with superior barrier properties.1 The PUU backbone would render the comb polymer ‘compatible’ with existing PUU multiblock copolymers, while the combs, which are anticipated to microphase separate, would be expected to reduce the transport of air and water vapor. Polyisobutylene (PIB) was chosen for the combs due to the combination of its good barrier properties and its controllable (living cationic) polymerization. A second approach involves dispersing organically-modified layered silicates (OMS) in typical biomedical PUUs.2 As a result of their high aspect ratio, the introduction of modest amounts of intercalated or exfoliated OMS inorganic layers in PUUs can dramatically enhance the barrier properties. We describe our initial experiments on polymer/inorganic hybrids based on a PUU having a composition comparable to typical biomedical PUUs and an alkyl-ammonium modified montmorillonite, a biocompatible layered silicate frequently used in cosmetics and food supplements.