Introduction

Polymers offer a wide range of material properties, from the flexibility of polyethylene in garbage bags to the high strength of poly(phenylene terephthalamide) in bulletproof jackets. This wide range of polymer properties has led to the widespread use of polymers in many materials applications. However, when a new application arises in industry, often a single polymer can not fulfill all of the material requirements that are associated with that application. In this case, development of a new polymeric material may be needed to fulfill the material property requirements. In these situations, industry has often utilized mixing of two (or more) polymers to develop new materials with targeted properties. By combining two polymers with diverse properties, it is possible to create 8 new material that retains physical characteristics of both polymers. However, it is also well known that two long chain molecules will rarely mix on a thermodynamic level due to their low entropy of mixing. The resultant two-phase structure will have inferior properties to the initial components, primarily due to the presence of a sharp biphasic interface that does not provide entanglement between the polymers in the separate phases. This lack of entanglement across the interface results in poor transfer of stress, which in turn degrades the macroscopic properties of the mixture.

Due to the importance of the presence of a biphasic interface on the macroscopic properties of a polymer blend, substantial work has been completed towards understanding and improving the interface and thus the macroscopic properties of the mixture. In particular, the effect of adding a copolymer to act as an interfacial modifier has received abundant attention. Much of this work has centered on the ability of a copolymer to strengthen the biphasic interface, lower interfacial tension (to create a finer dispersion), and inhibit coalescence during processing. Each of these mechanisms apparently contributes to the improvement of macroscopic properties of biphasic polymer blends upon addition of a copolymer and the importance of each has been the subject of some debate in the literature.

Specific results that have been published include the work of Balasz et. al. who have completed extensive self-consistent calculations and Monte Carlo simulations to evaluate the effect of adding copolymers to a blend on the interfacial tension of the biphase.1-9 Additionally, Kramer and others have utilized experimental techniques to determine the mechanism of fracture that occurs at a biphasic interface in the presence or absence of copolymer10-27 Macosko, however, has viewed the problem from an engineering perspective and has examined the role of added copolymer on the blend morphology that results from typical processing 28-31condition.

Much of the work on copolymers as interfacial modifiers has utilized block copolymers as additives. The role of copolymer molecular weight, composition, and other molecular parameters on the ability of a block copolymer to improve the properties of a biphasic blend is well understood. However, block copolymers are expensive and difficult to synthesize. Therefore, their use as interfacial modifiers in commercial applications has been limited. Other copolymer structures, including random copolymers, may also act as compatibilizers. However, there exist conflicting results regarding the utility of random copolymers as interfacial modifiers.11, 1121325

These results include studies on mixtures of polystyrene (PS) and poly(methyl methacrylate) (PMMA) with random copolymers of PS and PMMA as compatibilizer which show that the random copolymer does not improve the strength of the biphasic interface as well as a block copolymer.1213 Alternatively, if the same experiment is completed on mixtures of PS and poly(2-vinyl pyridine) (P2VP) with a PS/P2VP random copolymer, the random copolymer substantially improves the interfacial strength between the two homopolymer1125 Additionally, seminal work by Fayt et. al. 32-36howed that addition of tapered (which can also be thought of as a type of random) copolymers of PS/PE (polyethylene) to a blend of PS and PE improved the elongation and ultimate strength of that blend MORE than the addition of a similar diblock copolymer. In this paper, Monte Carlo simulation studies will be discussed which provide insight into the underlying factors that affect the ability of a copolymer to strengthen and interface and compatibilize a polymer blend. The interpretation of these results will then be correlated to the experimental evidence that currently exists in the literature. It is expected that the results of this work will provide important fundamental information on the underlying physics that govern the interfacial behavior of copolymers. In turn, this information can be utilized to develop processing schemes by which materials can be efficiently created from polymer mixtures with optimized and tunable properties.

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