Design of Mechanically Robust High-T g Polymers: Mechanical Properties of Glassy Poly(ester carbonate)s with Cyclohexylene Rings in the Backbone Xiangyang Li †,§ and Albert F. Yee* ,‡ Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, and Materials Science and Engineering, Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received May 26, 2004; Revised Manuscript Received July 20, 2004 ABSTRACT: The incorporation of trans cyclohexylene (C-ring) groups into the main chain glassy polymers has been found to increase chain motion. With the increase in trans C-ring content, Young’s modulus and yield stress both decrease. A linear correlation among Young’s modulus, yield stress, and the inverse of the coefficient of thermal expansion has been found in this work. With the increase in trans C-ring content, post-yield stress drop (PYSD) decreases, the tendency to necking in tensile test decreases, and the activation volume increases. The results are consistent with the idea that thermal expansion, Young’s modulus, and yield stress are predominantly controlled by intermolecular forces. Necking and PYSD are related not only to chain stiffness but also closely to polymer chain motion. On the basis of the results, the hypothesis that the incorporation of trans C-ring enhances local segmental motions, which effectively modulates interchain interactions, is proposed. 1. Introduction Main chain segmental motions in polymer glasses have been proposed to play a crucial role in their mechanical properties, such as yield stress, craze stress, and impact strength; 1-4 it has been further proposed that these molecular motions can be enhanced by conformational transitions in the main chain, such as the ring inversion of cyclohexylene (C-ring). 2-5 In previ- ous papers, 6,7 the synthesis, characterization, secondary relaxation behavior, and physical properties of three series of high-T g glassy polymers with the incorporation of main chain C-rings have been reported. The chemical structures of these polymers are shown in Figure (1), and their T g s are listed in Table (1). In this paper, the effects of main chain C-ring incorporation on some mechanical properties, such as Young’s modulus, yield stress, and post-yield stress drop (PYSD), are reported. Conventional efforts to design and synthesize high- T g polymers often result in compromised mechanical properties. For example, by introducing two methyl groups ortho to the carbonate group of BPA-PC, tet- ramethyl bisphenol A polycarbonate (TMBPA-PC) is obtained with a T g of 200 °C; 8 by replacing BPA with SBI, a polycarbonate (SBI-PC) with T g of 230 °C is produced 9 (for structures see Figure 2). But these two polymers are extremely brittle at room temperature. A common feature of these two polymers is that they are both restricted in the types of possible local molecular motions. In SBI-PC, SBI is locked into a twisted bulky banana shape, and SBI-PC has a very limited, confined local motion. 6 In TMBPA-PC, the local motion is re- stricted by the steric interaction between the carbonate groups and their ortho methyl groups. 10 By constrast, TmcPC has a high T g of 240 °C, but its mechanical properties are comparable to those of BPA-PC. 11 The motions are also restricted in TmcPC by the severe steric hindrance between the axial methyl group and the axial phenyl ring, but motions of the carbonate group are not restricted. 6 These examples point out the very important influence of molecular motions on me- chanical properties. † Macromolecular Science and Engineering. ‡ Materials Science and Engineering, Macromolecular Science and Engineering. § Current address: Lexan Technology, GE Advanced Materials, Mount Vernon, IN 47620. * Corresponding author: e-mail afyee@uci.edu. Figure 1. Chemical structures and short names for all the polycarbonates, polyesters, and poly(ester carbonate)s. 7231 Macromolecules 2004, 37, 7231-7239 10.1021/ma0489568 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004