Physical Characterization of Pivalolactone-Grafted Elastomers. I. General Morphology

Abstract

Abstract The pivalolactone graft copolymers represent a new type of thermoplastic elastomer, and their morphology appears to be unique. A comparison of some of the properties of the three principal types of thermoplastic elastomer is given in Table III. Over a fairly wide composition range (10–40% polyPVL), the general morphology of the polyPVL graft copolymers remains the same: discrete crystallites in a continuous elastomeric phase. The poly(ethyl acrylate)-based grafts, having much higher values of HSL and SSL, tend to form spherulitic structures. These spherulites are composed, however, of discrete polyPVL crystallites. Clean phase separation, as indicated by invariance of Tg with composition, is partly due to the relative compatibility of the backbone and grafted side-chain polymers. Nevertheless, the extent of phase separation between hard and soft phases should always be greater for ABA block and polyPVL graft copolymers than for segmented copolymers because of the free end possessed by each hard segment in the first two cases. The melting temperature of the hard segments in polyPVL grafts depends primarily on the HSL, which can be varied easily during synthesis. The melting temperature of segmented and triblock copolymers cannot easily be changed without large changes in physical properties. The low initial (Young's) modulus of the polyPVL graft copolymers and some ABA triblock polymers is a consequence of a continuous elastomeric phase and a discrete hard segment phase. When the hard phase becomes cocontinuous with the soft phase, a high initial modulus more like that of a plastic than of an elastomer may result. Annealing of crystalline segmented and polyPVL graft copolymers causes improvement in such physical properties as compression set, creep resistance, and tensile strength. This is attributed tc relaxation of molded-in stress, accompanied by recrystallization of the hard segments. Since the glassy hard segments in ABA triblock polymers lack crystalline order, similar improvement in properties would not be expected. PolyPVL crystallites act as both crosslinks and reinforcing filler in these copolymers. The size of the crystallites depends upon both the length of the grafted chain (HSL) and the separation of chains along the elastomer backbone (SSL). Generally, large values of HSL and small values of SSL cause the largest crystallites to form. Based on experimental evidence from transmission electron microscopy, x-ray diffraction, thermal analysis, and solvent swelling, a morphological model has been developed.