Viscoelastic Contrast and Kinetic Frustration during Poly(ethylene oxide) Crystallization in a Homopolymer and a Triblock Copolymer. Comparison of Ultrasonic and Low-Frequency Rheology

Abstract

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...