VISCOELASTIC CONTRAST AND KINETIC FRUSTRATION DURING POLY(ETHYLENE OXIDE) CRYSTALLIZATION IN A HOMOPOLYMER AND A TRIBLOCK COPOLYMER - COMPARISON OF ULTRASONIC AND LOW-FREQUENCY RHEOLOGY

We report a combined study using an ultrasonic shear wave reflection t echnique and conventional low-frequency rheology for the investigation of the crystallization kinetics and melting of a poly(ethylene oxide) (PEO) homopolymer and a poly(ethylene oxide)-polystyrene-poly(ethylen e oxide) (PEO-PS-PEO) triblock copolymer. Both isochronal heating and isothermal/isochronal kinetic measurements of the complex dynamic shea r modulus G have been performed with high-frequency (ultrasonic) and conventional low-frequency rheology at frequencies of 3.5 MHz and 0.16 Hz, respectively. The different frequencies create a different viscoe lastic contrast between the two phases in the two experiments. In both experiments the system can be regarded as a composite material made o f spherulites within the amorphous matrix, and the increase of the she ar modulus with time is attributed to the growing of spherulites at th e expense of the amorphous matrix. The kinetics of crystallization are analyzed in the framework of the Avrami equation using different mech anical models that describe a two-phase composite. Although a simple p arallel model composed of an amorphous and a spherulitic phase suffice s to describe the ultrasonic shear experiment, a more complex model in volving phase inversion is required to account for the abrupt dependen ce of the shear modulus on time in rheology. The effect of external sh ear is to speed up the kinetics. The ultrasonic shear experiments for the homopolymer crystallization are compared with those of the tribloc k copolymer, and it is found that the final modulus in the latter is c onsiderably lower than that in the former experiment. This is explaine d by the less perfect structure of the lamellae and/or the spherulites in the copolymer and the kinetic frustration of the crystallization ( for low undercooling) due to the glassy PS microphase. It is shown tha t the comparison of high and low-frequency rheology can provide new in sights on the crystallization process and the final morphology in semi crystalline polymers.