Identifying the Phase Behavior of Biodegradable Poly(hexamethylene succinate-co-hexamethylene adipate) Copolymers with FTIR Xiangyang Li, † Zhenfei Hong, † Jie Sun, ‡ Yong Geng, § Youju Huang, † Haining An, † Zhe Ma, † Baijin Zhao, † Chunguang Shao, † Yapeng Fang, † Chuanlu Yang, § and Liangbin Li* ,† National Synchrotron Radiation Laboratory and Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, China; Institute of Chemistry Materials, China Academy of Engineering Physics, Mianyang, China; and Department of Physics and Electrons, Ludong UniVersity, Yantai, China ReceiVed: NoVember 4, 2007; ReVised Manuscript ReceiVed: January 5, 2009 Whether a phase separation or a cocrystallization occurs in poly(hexamethylene succinate-co-hexamethylene adipate) (P(HS-co-HA)) copolymers was studied with a combination of wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy. With HA as the majority, the presence of HS comonomers leads to weakening and broadening of (10l) peaks in the X-ray fiber diffraction patterns, while a crystal structure similar to PHS is formed in the copolymer with HS as the majority. The X-ray diffraction patterns imply possible cocrystallization between HS and HA comonomers, but cannot lead to an unambiguous conclusion, which was clarified with the compensative tool of FTIR. Following the characteristic absorption bands of crystals, cocrystallization of HS and HA comonomers was observed in copolymers with HA comonomer as the majority during which HA initiated the nucleation at high temperatures. With HA as minority, cocrystallization of HS and HA can still be achieved with a fast quenching to below 0 °C, while a phase separation occurs and only HS comonomer crystallizes at high temperatures. This demonstrates that P(HS-co-HA) has an asymmetric phase diagram. Because of the sensitivity to local conformations, FTIR spectroscopic method is demonstrated to be a powerful tool on study phase behaviors of polymers with similar crystal structure. Introduction Copolymerization is a generic tool to create new materials with desirable properties. Leibler’s recent review advocates that block copolymer will be the future of the plastics industry, which leads to the era of polymer nanoalloy. 1 In this respect, random copolymer, resembling many biopolymers like protein, is expected to render polymer alloy on a molecular scale, which indeed has attracted polymer industry for many years. However, putting two different comonomers in one chain leads to some complexities in their phase behaviors. Crystallization and microphase separation may couple and compete with each other, which determines the final structure and the consequent me- chanical and other properties. 2-7 Three choices may occur: (i) comonomers are miscible and remain in the amorphous state; (ii) comonomers are immiscible and microphase-separated, where each species of comonomers may crystallize separately or stay as amorphous; and (iii) comonomers cocrystallize into isomorphism or isodimorphism states. 8 Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction are two common techniques to identify the phase behaviors of crystalline random copolymers. 9-21 Cocrys- tallization of two comonomers gives one melting peak, while two melting peaks are expected to be observed if comonomers crystallized separately. However, without structure information, DSC alone cannot give a conclusive result. One melting peak may correspond to copolymers with two comonomers in crystalline and amorphous states, respectively. Two melting peaks are also not necessary to indicate separated crystallization of two comonomers, as double melting peaks are also rather common in homopolymers. 22 As WAXD gives directly the structural information, it is possible to give a conclusive result with WAXD alone. Most reported studies are on comonomers with different lateral size. 8-10 Thus, lattice expansion is generally observed with X-ray diffraction, where diffraction peaks of (hk0) shift to low-angle direction. Nevertheless, the shift of diffraction peaks is a relative value, which may be affected by other factors such as temper- ature, deformation, and perfection of crystals. Thus, the confirmation of cocrystallization of comonomers generally requires a systematic study on samples with different contents of comonomers, which may give a direct correlation between the shifted value of peak position and the content of comono- mers. Even when cocrystallization is confirmed with X-ray diffraction on the final crystallized samples, it may still be difficult to reveal the role of different comonomers in the crystallization process. For example, which comonomer initiates the nucleation? Comonomers with different lengths rather than lateral size may be another challenge for WAXD. In principle, it is possible for diffraction technique to follow the (00l) peaks, similar to (hk0) peaks for comonomers with different lateral sizes men- tioned before. However, polymers generally form “lamellar crystal” with a large lateral size but small thickness from several nanometers to tens of nanometers. Thus, (00l) peaks are in general weak compared to (hk0), which is much more difficult to follow during the crystallization process. Without orientation as that for fiber diffraction, the weak (00l) peaks are averaged * To whom correspondence should be addressed. E-mail: lbli@ustc.edu.cn. † University of Science and Technology of China. ‡ China Academy of Engineering Physics. § Ludong University. J. Phys. Chem. B 2009, 113, 2695–2704 2695 10.1021/jp8061866 CCC: $40.75  2009 American Chemical Society Published on Web 02/11/2009