Modifying a Polystyrene/Poly(methyl methacrylate) Interface with Poly(styrene-co-methyl methacrylate) Random Copolymers Mohan Sikka, † Nicole N. Pellegrini, † Edward A. Schmitt, ‡ and Karen I. Winey* ,† Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272, and The Rohm and Haas Company, Bristol, Pennsylvania 19007 Received September 3, 1996; Revised Manuscript Received November 26, 1996 X ABSTRACT: Joints of polystyrene (PS) and poly(methyl methacrylate) (PMMA) modified with ∼50 nm of poly(styrene-co-methyl methacrylate) random copolymer [P(S-ran-MMA)] were investigated. Copoly- mers having styrene compositions of fS ) 0.48 and fS ) 0.73 were used. Transmission electron microscopy reveals that the copolymers phase separate to form a distinct layer at the joint such that there is an interface with each homopolymer. Interfacial fracture toughness measurements, using the asymmetric double cantilever beam geometry, show a strong effect of the PS to PMMA sheet thickness ratio; that is, the phase angle influences the measured interfacial toughness. Reflection infrared spectroscopy on fracture surfaces indicates that the crack propagates at or near the PS/copolymer interface for all thickness ratios and for both copolymers. In-plane crazing was not observed in front of the crack tip for these systems. Rather, strengthening appears to be exclusively a consequence of oblique crazes in the more compliant PS sheet which form at 45° or 135° relative to the crack direction. Joints modified with P(S0.73-ran-MMA) exhibit denser oblique crazes than those modified with P(S0.48-ran-MMA), resulting in a higher measured fracture toughness at all sheet thickness ratios or phase angles. Introduction Reinforcing homopolymer mixtures with copolymers continues to be an area of keen commercial and funda- mental interest. Efficient interfacial agents should be easy to synthesize, thermodynamically favored to in- habit the joint region between the homopolymers, and effective in strengthening the joint. Block copolymers are one class of materials that have been shown to affect phase size and mechanical properties in polymer blends. 1,2 Block copolymer films (∼10-100 nm) coated between homopolymer sheets demonstrably increase the fracture toughness of the joint. 3,4 However, diblock copolymers remain expensive to synthesize commer- cially. Also, the possibility of other low-energy states, such as micelles or segregation to free surfaces, com- plicates the ideal picture of individual block copolymer chains moving in adequate numbers to the desired interfaces in a commercial process. 5,6 Another promising route to interfacial strengthening is to use random copolymers. Random copolymers are relatively inexpensive to synthesize using standard free radical polymerization methods and do not form mi- celles. In a recent study, Kramer and co-workers 7 used the asymmetric double cantilever beam (“crack-open- ing”) method to show that the compositionally sym- metric poly(styrene-ran-2-vinylpyridine) random copoly- mer (i.e., having styrene composition f S ≈ 0.5) is remarkably effective at reinforcing the joint between homopolystyrene and homopoly(2-vinylpyridine). The effectiveness of the random copolymers decreased as f S increased or decreased relative to 0.5, that is, as the copolymer composition became asymmetric. Brown’s study 3 of poly(styrene-co-methyl methacry- late) block copolymers at polystyrene (PS)/poly(methyl methacrylate) (PMMA) joints included a small section on a commercial, polydisperse poly(styrene-co-methyl methacrylate) random copolymer [P(S-ran-MMA)] with an average styrene composition of f S ) 0.7. Copolymer films of thickness 10-40 nm coated between PS and PMMA sheets were found to significantly strengthen the joint. These results were from the so-called “static blade test”, which consists of wedging a blade between ho- mopolymer sheets and measuring the crack in front of the blade after 24 h. It is also important to note that these increased interfacial fracture toughness results were measured for a joint having equal PS and PMMA sheet thicknesses and with the PS sheet adhered to an Al plate. Until recently, theoretical work 8,9 on AB random and alternating copolymers at the A/B homopolymer joint surmised a “stitch-like” organization for a single copoly- mer chain at the joint. Analytical or molecular dynamic approaches were utilized in these studies, and the free energy was found to be lowered when sections along the copolymer chain which were rich in A units formed loops on the A side of the joint and vice versa. Kramer and co-workers 7 rationalized their results for the PS:PVP system in terms of these theoretical predictions. They conclude that, for low coverage by a long, composition- ally symmetric copolymer, the observed fracture tough- ness would imply about 10 connections across the PS/ PVP joint for a random copolymer of length N ≈ 8000 monomers. An interesting issue that emerges from work in the Kramer group is that the observed strengthening per- sists when the random copolymer layer becomes com- parable to or larger than the radius of gyration of the copolymer chains. 10 Brown’s results using P(S 0.70 -ran- MMA) random copolymers at PS/PMMA joints are similar; interfacial toughness continues to increase as the copolymer layer thickness becomes 2 and 4 times the average radius of gyration of the copolymer chains (∼10 nm). These results cannot adequately be ex- plained by the “stitch” model, because random copoly- mers in “thick” layers cannot cross between the A and * To whom correspondence should be addressed at the Depart- ment of Materials Science and Engineering. † University of Pennsylvania. ‡ The Rohm and Haas Co. X Abstract published in Advance ACS Abstracts, January 15, 1997. 445 Macromolecules 1997, 30, 445-455 S0024-9297(96)01302-2 CCC: $14.00 © 1997 American Chemical Society