Selected Physical Characteristics of Polystyrene/High Density Polyethylene Composites Prepared from Virgin and Recycled Materials Jayant Joshi, Richard Lehman, Thomas Nosker Rutgers University School of Engineering, Piscataway, New Jersey 08854 Received 15 September 2004; accepted 7 April 2005 DOI 10.1002/app.22492 Published online 6 December 2005 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Mixtures of polystyrene and high density polyethylene were injection molded from recycled and virgin polymers to generate cocontinuous structures. The mechanical properties of these blends were evaluated to assess their conformance to rule of mixtures behavior in general and to identify areas of synergy or incompatibility in specific. Flexural and tensile data for recycled blends showed that generally the properties are not additive, except in a cocontinuous region of composition near 35/65 PS/ HDPE that has been identified previously for recycled ma- terials. Analysis of crystallinity in the HDPE phase of these blends by differential scanning calorimetry indicates a marked reduction in the level of HDPE crystallinity at the 35/65 PS/HDPE composition. Similar blends of virgin PS/ HDPE polymer do not show the differing regions of incom- patibility and synergy illustrated by the recycled materials, but rather show approximate conformance to the rule of mixtures. Furthermore, the virgin blends show virtually no crystallinity suppression and a more pronounced T g shift in the polystyrene compared to recycled materials. Detailed characterization of the recycled materials in terms of poly- mer and particulate impurities should improve understand- ing of these differences and perhaps provide direction for obtaining enhanced synergistic behavior in virgin polymer blends. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2044 –2051, 2006 Key words: blends; crystallization; immiscible; modulus; structure/property relationships INTRODUCTION Immiscible polymer blends have been studied ex- tensively as an approach to the synthesis of new engineering materials. 1 Most of these blends rely on the use of compatibilizers to improve the strength of the mechanical linkages between the component polymers. Blends without compatibilizers have not been highly successful, although some studies in our laboratories have shown that promising me- chanical properties can be obtained from immiscible blends if the composition and processing parame- ters are properly selected. 2,3 The microstructural morphology plays a key role in imparting unique properties to such blends, and such microstructures have been studied extensively for various blends. 4–6 Studies of cocontinuous materials have attributed the enhanced mechanical properties to mechanical clamping between the phases with concomitant stress transfer, 7 a feature that also leads to reduced crystallinity. 8,9 In this article we extend the concept of mechanically interlocking structures and intro- duce the terminology “mechanical grafting.” This term describes immiscible polymer blends that have nonbonded interfaces and yet have a sufficiently fine interlocking cocontinuous structure that the composites exhibit rule of mixtures mechanical be- havior and are stable when annealed near the T g of the glassy phase. Mechanical grafting is different from traditional chemical grafting in that there are no chemical bonds between the immiscible phases but, rather, all load transfer is affected by morphology and me- chanical clamping. The morphology is generated by composition and processing conditions, and the clamping is generated by volumetric changes in each phase during cooling from the melt processing temperatures. Mechanical grafting is similar to chemical grafting in that the mechanical properties of the blends follow rule of mixtures relationships, as if strong chemical bonds spanned the interface. The goal of the present work was to produce im- miscible polymer composites from blends of polysty- rene and HD polyethylene in the laboratory and to assess under controlled conditions the degree of me- chanical grafting that occurs as a function of compo- sition and raw material purity as represented by vir- gin versus recycled raw materials. Correspondence to: R. Lehman (rllehman@rci.rutgers.edu). Journal of Applied Polymer Science, Vol. 99, 2044 –2051 (2006) © 2005 Wiley Periodicals, Inc.