Elsevier

Polymer

Volume 41, Issue 12, June 2000, Pages 4719-4727
Polymer

Crystal structures of α and β forms of poly(tetramethylene succinate)

https://doi.org/10.1016/S0032-3861(99)00659-XGet rights and content

Abstract

Crystal structures of the α and β modifications of poly(tetramethylene succinate) (PTMS) were analyzed by X-ray diffraction: the β form appeared with an application of stress. These two modifications belonged to the monoclinic system with the space group of P21/n. In both cases, a unit cell included two chemical repeating units. For the α form, the cell dimensions were a=0.523nm,b=0.912nm,c (fiber axis)=1.090nm, and β=123.9°; for the β form, a=0.584nm,b=0.832nm,c (fiber axis)=1.186nm, and β=131.6°. The difference in the fiber periods of the two crystalline forms was attributed mainly to the conformational difference in the tetramethylene unit, i.e. TGTḠT of the α form and TTTTT of the β form. It was also found that in PTMS, the packing coefficient, K, which was defined by the ratio of the intrinsic volume with respect to the true volume of the α form was almost equal to that of the β form. This observation could be contrasted to those obtained in poly(butylene terephthalate) (PBT), where the K of the α form was considerably greater than that of the β form. The difference between PTMS and PBT was attributed to the difference between the unit cell volumes of the α and β forms of these polymers.

Introduction

Biodegradable aliphatic polyesters have received much attention from industry, particularly from the ecological viewpoint [1]. Mechanical properties of such crystalline polymers depend strongly on their crystal structures, which could be changed by pressure, temperature and strain, as well as on the crystallinity of the polymers [2]. Recently, we have discovered crystal modifications (α and β forms) in poly(tetramethylene succinate) (PTMS). The transition occurred reversibly under the application and removal of strain: the β form appeared under strain [3]. The conformations of the two forms were reported to be (T7GTḠ)[4] and (T10) [3] for the α and β forms, respectively, where T, G and Ḡ denoted trans, gauche and minus gauche, respectively. In this case, the conformation change occurred in the tetramethylene units [3]. In addition, the crystal transition mechanisms were investigated in detail by the authors [5].

The crystal structure analyses of several aliphatic polyesters, particularly those with an ethylene glycol unit, have been conducted [6], [7], [8], [9]. In addition, the crystal structure of poly(trimethylene sebacate) was recently investigated by Jourdan et al. [10]. Despite many investigations on the crystal structure of ethylene series of aliphatic polyester, few works were reported on the tetramethylene series; the crystal structures of the α form in a uniaxially oriented fiber [4] and in a single crystal [11] were reported only in PTMS. However, no detailed crystal structures were presented in both cases.

Crystal transitions induced by strain (or stress) have been discovered in many polymers, and their crystal structure analyses were conducted [12], [13], [14], [15], [16], [17]. However, most of them showed irreversible transitions [15], [16], [17]. For a reversible system, crystal structure analyses were conducted only in poly(butylene terephthalate) [PBT] [12], [13] and PEO [14]. In PBT, for instance, two kinds of crystal modifications (α and β forms) were reported [12], [13]: the β form appeared under strain. Yokouchi et al. [12] reported that the space groups of both α and β forms were P1̄, and that their conformations in the tetramethylene units were ḠḠTGG (α form) and TS̄TST (β form), where S and S̄ denoted skew and minus skew, respectively. At the same time, Hall et al. [13] reported that the space groups of these two forms were P1̄, while the conformation of the β form to be TTTTT.

In this paper, the crystal structures of both the α and β forms of PTMS were analyzed by the X-ray diffraction method.

Section snippets

Materials

The polymer material used in this study was poly(tetramethylene succinate) (PTMS), so called Bionolle, which was supplied from Showa Highpolymer Co. Ltd. No further purification was applied. The weight average molecular weight was determined to be 1.6×105 by size exclusion chromatography with poly(methylmethacrylate) standards. The melting (Tm) and glass transition temperatures (Tg) were measured by DSC and found to be 114 and −32°C, respectively. Further information can be seen elsewhere [3].

Sample preparation

Unit cell and space group

Fig. 1(a)shows the fiber diffraction pattern of the α form. The observed reflections can be indexed based on the monoclinic cell, the cell dimensions of which are summarized in Table 1. The space group is determined to be P21/n from the systematic absence rule [20]. The reflections appear only for h0l (h+l: even), h00 (h: even), 0k0 (k: even) and 00l (l: even): we have confirmed these absence rule up to the fifth layer line. This space group has been reported also in a single crystal [11] and

Crystal structure of α form

Recently, Pazur et al. [23] proposed a molecular model of the α form of PTMS based on the energy calculation of a single chain. Their final molecular structure corresponded to the model B in the present study. This was, however, ruled out in the stage of the 2D analysis. Nevertheless, 3D refinement of the model B was performed for comparison. After several refinement cycles, the model B showed R=0.36 and Rw=0.35, which were considerably higher than those for the model C (R=0.19 and Rw=0.18).

Conclusion

Structure analyses of the two crystal modifications (α and β) of PTMS ([–O(CH2)4OCO(CH2)2CO–]n), were performed by X-ray diffraction. Both of the modifications belonged to the monoclinic system with the space group of P21/n. In both the cases, the unit cell contained two molecular chains; the cell dimensions were a=0.523nm,b=0.912nm,c (fiber axis) =1.090nm, and β=123.9° for the α form; a=0.584nm,b=0.832nm,c (fiber axis) =1.186nm, and β=131.6° for the β form. The molecular conformations of the α

Acknowledgements

The authors would like to acknowledge the fruitful and stimulus discussion with emeritus Professor Y. Chatani of Tokyo University of Agriculture and Technology. This research was partially supported by a Grant-in-aid for Scientific Research (09750984) from the Ministry of Education, Science, Sports and Culture of Japan.

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  • Cited by (0)

    1

    Present address: Kawasaki Plastics Laboratory, Showa Denko K.K., 3-2, Chidori-cho, Kawasaki-ku, Kawasaki 210-0865, Japan.

    2

    On leave from Showa Denko K.K. Kawasaki Plastics Laboratory.

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