Masters Theses

Date of Award

12-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Kevin M. Kit

Committee Members

J. E. Spruiell, R. S. Benson

Abstract

Miscible rigid-rigid polymer blend systems, although not always tough, tend to show better modulus and yield strength than the rubber-toughened systems. The understanding of the relationships between structure and energy absorption mechanisms in these blends is still lacking. The objective of this work is to characterize melt miscible syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends which form a rigid-rigid system. Specifically, the effect of the spherulitic texture and the scale of segregation of PPO on mechanical properties and fracture are discussed.

The sPS/PPO blends, prepared by melt blending, were found to be miscible in melt phase at all compositions studied. These miscible blends were crystallized from melt as well as quenched state at different temperatures. Dynamic mechanical analysis was then used to study the change in composition of the amorphous phases formed after crystallization. In melt crystallized blends, the scale of segregation of PPO increased with slow crystallization at high temperatures. In cold crystallized blends, the extent of segregation of the PPO was much lower. Tensile testing of the bulk samples showed that for melt-crystallized blends, tensile properties are a strong function of the crystallization temperature. The samples crystallized the slowest showed the highest tensile strength, strain at break and tensile toughness. The tensile properties of the cold crystallized blends varied weakly with the crystallization temperature. The observation of the spherulitic morphology in the scanning electron microscope revealed that bundle thickness, bundle spacing and size of amorphous domains increase with increasing crystallization temperatures in melt crystallized blends. The cold crystallized blends did not show substantial change in morphology at different crystallization temperatures. The SEM micrographs of the fractured surfaces of melt crystallized blends suggest that while intraspherulitic fracture occurs at low crystallization temperatures, interspherulitic fracture takes place at high crystallization temperatures. Wide Angle X-ray diffraction studies and density measurements suggest complex polymorphic behavior of the sPS after blending with PPO.

The correlation of the morphology and mechanical properties suggests that redistribution of the amorphous PPO strongly influences the ultimate tensile properties of the sPS/PPO blends. Blends melt crystallized slowly formed larger interfibrillar amorphous phase domains and showed better tensile properties. It is interesting to note that slow crystallization of the neat sPS resulted in poorer mechanical properties than fast crystallization. The fracture surface of these tougher blends suggest that some stress concentration effect led to drawing of the large amorphous domains between the bundles.

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