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 technique 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(ethylene oxide) (PEO-PS-PEO) triblock copolymer. Both isochronal heating and isothermal/isochronal kinetic measurements of the complex dynamic shear 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 viscoelastic contrast between the two phases in the two experiments. In both experiments the system can be regarded as a composite material made of spherulites within the amorphous matrix, and the increase of the shear modulus with time is attributed to the growing of spherulites at the expense of the amorphous matrix. The kinetics of crystallization are analyzed in the framework of the Avrami equation using different mechanical models that describe a two-phase composite. Although a simple parallel model composed of an amorphous and a spherulitic phase suffices to describe the ultrasonic shear experiment, a more complex model involving phase inversion is required to account for the abrupt dependence of the shear modulus on time in rheology. The effect of external shear is to speed up the kinetics. The ultrasonic shear experiments for the homopolymer crystallization are compared with those of the triblock copolymer, and it is found that the final modulus in the latter is considerably lower than that in the former experiment. This is explained 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 that the comparison of high and low-frequency rheology can provide new insights on the crystallization process and the final morphology in semicrystalline polymers.