Morphology and physical properties of poly(styrene- b-isobutylene- b-styrene) block copolymers

A series of linear and three-arm star poly(styrene- b-isobutylene- b-styrene) (PS-PIB-PS) block copolymers with varying block compositions was synthesized via living carbocationic polymerization using the initiation system 1,3-di(2-chloro-2-propyl)-5- tert-butylbenzene (or 1,3,5- tris-(2-chloro-2-propyl)benzene)/TiCl 4/pyridine/2,6-di- tert-butylpyridine in a 60 40 (v/v) hexane/MeCl solvent mixture at −80°C. High resolution gel permeation chromatography showed that the compositions of the copolymers were complex, consisting of higher molecular weight coupled products, and products of lower molecular weight including, probably, homo-PS. Morphology and physical properties were characteristic of microphase-separated block copolymers, and were affected strongly by the PIB span molecular weight and volume fraction of PS. When the latter was in the range 0.20–0.42 vol%, the morphology was characterized by cylinders of PS in a continuous phase of PIB; however, at least two samples exhibited mixed morphologies in which regions of PS cylinders coexisted with regions of PS spheres and lamellae, respectively. Linear samples showed much better long-range morphological order than star-branched samples. The dynamic mechanical response of block copolymers consisted of separate PIB and PS relaxations whose relative intensities scaled well with copolymer composition. The low-temperature relaxation was broad, which is an inherent characteristic of PIB; the high-temperature PS relaxation was narrow, suggesting well defined phase separation and a sharp PIB-PS interface. Melt rheological studies of representative samples showed that microphase separation persisted up to 265°C. Tensile properties varied with PIB span molecular weight and PS content. An abrupt change occurred from 15–22 vol% PS, indicating a minimum PS content needed for strong network formation. Properties changed from elastomeric within the range 20–37 vol% PS, to ductile in the range 37–45 vol%, to brittle in the range 45–55 vol%. Samples with PS vol% ⩾ 35 yielded improved elastomeric properties as a result of annealing above the glass transition temperature of PS. Within the elastomeric range, the PIB span molecular weight was the dominant factor effecting elongation at break. A high tensile strength of 24 MPa was obtained, and most samples displayed strengths ⩾ 15 MPa.