In-Situ Deformation Studies of Rubber Toughened Poly(methyl methacrylate): Influence of Rubber Particle Concentration and Rubber Cross-Linking Density Chaobin He, Athene M. Donald,* and Michael F. Butler Polymers and Colloids Group, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, U.K. Received March 21, 1997; Revised Manuscript Received October 3, 1997 ABSTRACT: The deformation behaviour of rubber toughened poly(methyl methacrylate) (PMMA) was studied using real time small angle X-ray scattering (SAXS) with a synchrotron radiation source. The invariant analysis method was used to analyze the influence of rubber particle concentration and rubber cross-linking density on craze concentration quantitatively. It is shown that both rubber particle concentration and the cross-linking density of the rubber layer have a significant effect on craze density. The craze density increases as the concentration of rubber particles increases from 10 wt % up to 30 wt %; thereafter, it decreases as the concentration of rubber particles increases further. For all polymer matrices with different cross-linking densities, a maximum craze concentration appears at ∼30 wt % of rubber particle, which corresponds to a true rubber concentration of about 15 wt %. The influence of cross-linking density on craze concentration is similar to that of rubber particle concentration. As the cross-linking density increases, the craze density first increases and then decreases. The appearance of crazing coincides with the occurrence of the macroscopic yield point of the polymer sample, corresponding to the beginning of plastic deformation. It appears that a sufficient concentration of crazes is needed to achieve an optimum toughness of the PMMA matrix. For the same cross-linking density of the rubber, high toughness requires a sufficient rubber particle concentration which can generate many crazes, while, for a given rubber particle concentration, high toughness is associated with a lower craze density. Introduction Rubber toughening of poly(methyl methacrylate) (PMMA) is a frequently used strategy to overcome the normal brittle response to deformation of PMMA. Modern polymerization methods have led to the pos- sibility of preparing rubber particles with a range of different internal morphologies: they may contain several alternating layers of rubber and PMMA, and the properties of the rubber itself can be altered, for instance, by cross-linking. However, an understanding of how changing this structure may affect the subse- quent deformation mechanism is somewhat rudimen- tary, and indeed there is still controversy over what mode of deformation may be most effective. It has long been asserted that the role of the rubber particles is to nucleate a high density of crazes so that a high energy is absorbed before any single craze fails. 1 More recently, it has been demonstrated on the basis of real time X-ray studies using synchrotron radiation that cavitation and shear may both occur before crazing for some toughened styrenic systems, and the two processes combined may contribute more than 50% of the total deformation. 2 Very recently it has been shown that, in a series of model rubber toughened polystyrenes, the impact tough- ness increases with rubber content, although the amount of crazing itself strongly decreases. 3 Thus, although empirical strategies have been devised to improve performance, it is clear the rationale for these strategies is still muddled. Small angle X-ray scattering (SAXS) is a useful method for the study of deformation microstructure, and in particular the use of high-intensity synchrotron radiation allows the possibility of an in-situ deformation study. In our previous report on the deformation behavior of core shell rubber particles in a PMMA matrix under tensile strain, 4 we have shown that core shell rubber particles (consisting of PMMA core-rubber middle layer-PMMA outer layer) first undergo inho- mogeneous deformation (the PMMA core and surround- ing rubber layer deform to different extents), followed by progressive debonding or cavitation in the rubber layer beyond the yield point. Recently methods have been developed 5,6 to characterize the amount of crazing which occurs in rubber toughened glassy polymers, using the so-called invariant obtained from SAXS analysis. This invariant analysis is based on the idea that a craze can be effectively modeled within a two phase model and that there are no other scattering sources. However, when core shell particles are used for toughening, the analysis may be affected since the particles themselves can scatter strongly, even in the absence of crazes or cavitation. In our previous paper 7 we have explored the effect of such core shell rubber particles and their deformation on invariant analysis and set up a framework for separating the contribution of the particle form factor from the total X-ray scatter- ing. In this way the contribution of the crazes to the total invariant can correctly be assessed. It is shown that, 7 for a submicron rubber particle system, the scattering from the form factor for core shell rubber particles dominates the SAXS pattern of undeformed samples, and the intensity of this scattering increases as the concentration of rubber particle increases. This scattering needs to be subtracted from the experimental data before the invariant can be quantitatively analyzed for the amount of crazing as deformation proceeds. Subsequent deformation of the core shell rubber par- ticles increases the contribution of the form factor to * To whom correspondence should be addressed. 158 Macromolecules 1998, 31, 158-164 S0024-9297(97)00398-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/13/1998