Rubber-Modified Glassy Amorphous Polymers Prepared via Chemically Induced Phase Separation. 2. Mode of Microscopic Deformation Studied by in-Situ Small-Angle X-ray Scattering during Tensile Deformation B. J. P. Jansen, S. Rastogi,* H. E. H. Meijer, and P. J. Lemstra Eindhoven Polymer Laboratories, The Dutch Polymer Institute, Eindhoven University of Technology, PO Box 513, 5600 MB, Eindhoven, The Netherlands Received October 19, 2000 ABSTRACT: The mode of microscopic deformation in rubber-modified amorphous polymers has been investigated by small-angle X-ray scattering during tensile deformation. Synchrotron experiments were performed for blends consisting of poly(methyl methacrylate) (PMMA) with a finely dispersed rubbery epoxy phase. These blends were prepared via chemically induced phase separation, as shown in the first paper of this series. On macroscopic deformation these blends show that the toughness of brittle amorphous polymers can be significantly enhanced by the introduction of submicron size rubber particles. The objective of the present study is to establish the relationship between the morphology and the macroscopic mechanical properties of the blends. As observed for neat PMMA, crazing is found to occur for the macroscopically brittle PMMA/epoxy 90/10 blend. In contrast, the ductile blend with 20 wt % epoxy deforms via shear yielding which is preceded by cavitation. Shear yielding also occurs for blends having even higher epoxy contents, although it is not accompanied by the occurrence of dilatation processes. The changes in the scattering patterns during deformation are attributed to morphological changes like orientation. Cross-linking of the epoxy phase appears to have an important influence on the mode of microscopic deformation. A blend with 20 wt % un-cross-linked epoxy appears to deform via crazing instead of cavitation. The change in deformation mechanism is associated with the plasticization of crazes on a local level. The local strain is defined as the local deformation of the sample exposed to the incident beam as measured by recording the beam intensity in front of, and after, the sample during the drawing process. Thus, the local strain in the beam can accurately be measured and related to the corresponding scattering patterns. The local strain values obtained are in agreement with those from macroscopic tensile tests. 1. Introduction Generally, the mechanical properties of brittle amor- phous polymers are enhanced by the introduction of a dispersed rubbery phase. Toughness improvement in high-impact polystyrene (HIPS) is the result of multiple crazing on the microscopic level which is initiated by the presence of micron-sized rubber domains. 1 Recently, it has been shown that in polystyrene (PS) toughness enhancement can also be obtained by decreasing the absolute ligament thickness between rubber particles to a submicron level. In this case the improvement is the result of a transition in the mode of microscopic deformation from crazing to shear yielding. 2-4 These two examples indicate the importance of studying the mode of microscopic deformation and a necessity to establish the relation between the morphology and the mechan- ical properties of a blend, at both microscopic and macroscopic levels. In this study, the microscopic de- formation of poly(methyl methacrylate) (PMMA) con- taining an extremely finely dispersed rubbery epoxy phase 5 is monitored in situ by performing small-angle X-ray (SAXS) experiments during tensile deformation. Time-resolved experiments were carried out using syn- chrotron radiation. SAXS as a tool to monitor deforma- tion development at the microscopic level has been used earlier for both amorphous polymers 6-12 and semicrys- talline polymers. 13-16 Detailed analysis of these SAXS data clearly results in more quantitative data, as compared to other techniques like dilatometry and microscopy. For rubber-modified amorphous polymers, the occur- rence of dilatation processes such as debonding, cavita- tion, and crazing can easily be distinguished from shear yielding. Crazing can be identified by SAXS as it results in typical crosslike scattering patterns consisting of two perpendicular streaks. 17,18 One is positioned along the tensile direction and the other perpendicular. The intense streak in the tensile direction, often indicated as “anomalous peak”, is the result of scattering caused by reflection from craze surface. The less intense streak, which develops parallel to the craze plane, is attributed to the scattering of the craze fibrils and can be used to calculate the amount of crazing and the craze fibril diameter. 6,7,19 In several synchrotron studies this fibril scattering is used to determine the contribution of crazing to the total amount of strain in rubber-modified thermoplastics. 6,7 Bubeck et al. 6 showed that, during impact deformation of HIPS and ABS (acrylonitrile- butadiene-styrene), only half of the total strain could be ascribed to crazing. Other dilatation processes and shear yielding accompany the remaining strain. He et al. 8,9 showed that the craze density decreases as the toughness increases in commercial rubber-toughened PMMA. The craze density was found to first increase and then decreases with increasing rubber particle concentration or cross-link density. Besides crazing, Lovell et al. 10 also observed other dilatation processes for rubber-modified PMMA which contained multilay- ered core-shell rubber particles. The scattering pat- terns found are ascribed to the occurrence of rubber particle cavitation and debonding. Earlier, Ijichi et al. * To whom correspondence should be addressed. 4007 Macromolecules 2001, 34, 4007-4018 10.1021/ma001810y CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001