Morphological effects on Glass Transitions in Immiscible Polymer Blends
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Morphological effects on Glass Transitions in Immiscible Polymer Blends Vivek M. Thirtha, Richard L. Lehman and Thomas J. Nosker Dept of Ceramic and Materials Engineering, Rutgers University, Piscataway NJ 08854, USA ABSTRACT This paper describes the effects of structures on the glass transition of glassy polymers blended with a semi-crystalline polymer. Immiscible blends of PS/PP and PS/HDPE were prepared from commercially available polymers using melt processing and extrusion without additives. The weight fractions of the components were varied from 0 to 1. SEM analysis of the blends showed a range of morphologies over the composition range from small inclusions at low volume concentrations through intertwined co-continuous structures at specific intermediate compositions, and a reversal of this configuration at high volume fractions. The glass transition of the glassy polymer was measured with differential scanning calorimetry using the sensitive and high resolution modulated DSC method. A systematic change in glass transition of glassy polymers is observed as a function of composition in various immiscible polymer blends. Results show that the glass transition of polystyrene increases with a reduction in volume fraction, by approximately 5.4°C in polypropylene and 6.5°C in polyethylene. Probable models which might explain this effect are mentioned. INTRODUCTION Immiscible Polymer Blends (IMPB’s) are of significant interest due to their potential to provide properties near the constituent rule of mixtures without requiring alloying or compatibilization of the blend. The combination of a glassy polymer and a semi-crystalline one combines the complementary properties of strength and toughness of the respective polymers. These types of blends have been investigated by various researchers due to their attractive enduse applications. [1-3] The morphologies of these blends, ranging from dispersed to cocontinuous structures are capable of influencing certain physical properties of the components such as crystallization temperature and crystal growth rates and the crystallinity.[4] Whereas glass transition variations with compositions are widely observed in miscible polymer blend systems due to their chemical interactions, this effect is not expected with immiscible polymer systems.[5] Tg of polystyrene films were observed to change with their thickness and size.[6, 7] This effect was seen in polymer having dimensions on the order of nanometers. Tg shifts in the soft phase, such as polybutadiene have also been observed in the presence of a hard matrix, such as polystyrene in ABS systems, due to differences in thermal expansion coefficients of the constituent phases.[8, 9] Mucha concurred that there are physical interactions between phases which cause the Tg of aPS and iPP to change in an aPS/iPP blend.[10] Greco et al. did not see a linear variation in the Tg with composition in PS/PP systems, but saw the Tg increase to a single higher value from the homopolymer value at a certain composition.[11] The authors
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