Glass Transition Phenomena in Melt-Processed Polystyrene/Polypropylene Blends Vivek Thirtha, Richard Lehman, Thomas Nosker AMIPP Advanced Polymer Center, School of Engineering, 607 Taylor Road, Rutgers University, Piscataway, New Jersey 08854-8065 Blends of an amorphous and a semi-crystalline poly- mer—polystyrene and polypropylene, respectively— were prepared by melt processing in an extruder at 220°C. These polymers are known to be immiscible and the composite morphologies were characterized by electron microscopy and thermal analysis. Fine micron- scale morphologies, ranging from 0.5 to 20 microns were observed. Thermal analysis and dynamic mechanical analysis showed changes in both the polystyrene and polypropylene glass transition temperatures (T g ) over the composition range. The major effect was a sharp increase in polystyrene T g with increasing polypropylene content in the blend. A T g elevation of 5.5°C was ob- served at 85% polypropylene. The polypropylene T g also increases with increasing polypropylene content, start- ing at a depressed value in discrete polypropylene do- main environments and approaching the bulk polypro- pylene value after the phase inversion is crossed. Qualitative structural models are proposed based on spatial and mechanical interactions between the com- ponents. POLYM. ENG. SCI., 45:1187–1193, 2005. © 2005 So- ciety of Plastics Engineers INTRODUCTION Immiscible polymer blends have been of considerable interest because of their inherent capability to combine complementary functionalities of the component systems [1]. Low cost, high value materials can be easily and inex- pensively prepared from immiscible polymers if blends follow the rule of mixtures without compatibilizers. The morphology of the blends and the subsequent inter- action between the component phases is a critical feature of immiscible blends that has the ability to transform proper- ties such as the melting temperature (T m ), glass transition temperature (T g ), and the degree of crystallization [2]. Lin- ear glass transition variations have been observed in thin films of polystyrene by various researchers [3, 4]. The T g of certain polymers in the physical form of thin films or nanoparticulates has been shown to decrease as a function of the film or particle dimension [5]. The authors attributed this to surface area/volume effects, although the small di- mensions present in the nanoparticulates would certainly constrain the size of any polymeric entity, thus altering its structural behavior. Other studies reported a change in T g of polyethylene terephthalate (PET) in a blend with polycar- bonate. The T g of PET in the blend was higher than that of the pure polymer. The presence of a rigid polycarbonate matrix as PET cools through its glass transition gives rise to a “wall” effect, causing the T g of PET to increase [6]. The T g of polybutadiene in polycarbonate/ABS blends was shown to decrease with decreasing ABS content. This was attributed to different thermal expansion coefficients of the polybutadiene particles and the surrounding matrix [7, 8]. Polystyrene/polypropylene (PS/PP) blends are attractive for their potential use as packaging materials due to their water vapor resistance [9], although there have been few papers dealing with such blends and their properties in the uncompatibilized form [2, 10, 11]. The T g of polypropylene in a PS/PP blend has been shown to decrease with decreas- ing polypropylene composition, apparently due to the ther- mal expansion coefficient mismatch, while the T g of poly- styrene did not change [12]. Mucha [13] concurred that there are physical interactions between phases that cause the T g of aPS and iPP to change in an aPS/iPP blend. Greco et al. [14] did not see a linear variation in the T g with com- position in PS/PP systems, but saw the T g increase to a single higher value from the homopolymer value at a certain composition. They attributed this to the migration of low molecular weight species into the polypropylene phase. Our laboratory has focused on combining immiscible polymers by melt processing. The blending of a glassy polymer with a semi-crystalline polymer, e.g., polysty- rene/HD polyethylene, has produced materials that gener- ally show rule of mixtures behavior but that also demon- strate an important synergy of blend properties at certain compositions [15]. This behavior has been attributed to the unique morphology at the phase inversion composition of the components. An enhanced mechanical interaction at the Correspondence to: R. Lehman; e-mail: rllehman@rutgers.edu Contract grant sponsor: New Jersey Commission for Science and Tech- nology. DOI 10.1002/pen.20387 Published online in Wiley InterScience (www.interscience.wiley.com). © 2005 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2005