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The phase structure of isotactic polypropylenes isothermally crystallized from the melt has been examined at different temperatures by solid-state high-resolution 13C NMR spectroscopy using VT/MAS (variable temperature/magic angle spinning). On the basis of 13C spin-lattice relaxation and computer line-shape analyses, it has been found that polypropylene samples are composed of not only crystalline and amorphous components but also another noncrystalline component with 31-helical conformation. Similar NMR analyses at different temperatures have revealed that a reversible structural change between the 31-helical and random conformations occurs in the noncrystalline region at 60-110 "C.

ABSTRACT:Poly(tetramethylene succinate) (PTMS) showed a crystal transition between the r:J. (T7 GTG) and /J (T10) form under the strain and relaxation conditions, where T, G, and G denoted trans, gauche, and minus gauche, respectively. We have investigated the mechanisms of this crystal transition by FT-IR and X-ray diffraction. In the FT-IR, the absorbance peaks at 920 cm -1 and 955 cm -1 , corresponding to the fl. form, started decreasing at strain of s 8%, while the absorbance at 977cm-1 , corresponding to the /J form, appeared at e~8%, then increased with strain. In addition, the isobestic point was observed at 965 cm -1 , indicating that the crystal transition occurred only between the fl. and /J form, where no amorphous part contributed. In the X-ray diffraction, the meridional reflection of fl. (at 20=25.1°) started decreasing at e~8%. In addition, the reflection of /i (at 20=22.5°) appeared at s~8%, then increased withe. These FT-IR and X-ray results were thus consistent with each other. The molar fraction of the /J form, Xp, was determined as a function of stress, rr, by X-ray. The Xp showed a drastic increase at a critical value of rr= 140MPa. It was hence concluded that the thermodynamic first-order phase transition was the operative mechanism of the transition. Such a crystal transition mechanism had been also reported in poly(butylene terephthalate) (PBT). The free energy difference between the fl.and /J form, .JG, was determined to be .JG~ 1.6 (kJ mol-1 of monomer unit), being close to the reported value of LlG-1.4 (kJmol-1 of monomer unit) for the crystal transition in PBT. The stress-strain curve was measured. The rr increased withe when r,<8%, then remained approximately constant up to s~ 16%, followed by the second increase fore> 16%. Such a stress-strain characteristics could be explained in terms of the crystal transition. KEY WORDS Poly(tetramethylene succinate) / Crystal Transition Mechanism/ Crystal Structure/ X-Ray Diffraction/ FT-IR/ Stress-Strain Curve/ As represented by poly(tetramethylene succinate) (PTMS), which is crystalline polymer, biodegradable aliphatic polyesters have received a great attention from industry, particularly from the ecological viewpoint. 1 • 2 Mechanical properties of such crystalline polymers, in general, depend strongly on their crystal structure as well as on the crystallinity of the polymers. 3 Crystal structures can be controlled by pressure, temperature and strain.Recently, we have discovered such crystal modifications in PTMS (a and /3 form): the trans1t10n occurred under application or removal of strain, and the /3 form appeared under strain. 4 The conformations of the two forms were (T 7 GTG) 5 and (T 10 )4 for the a and f3 form, respectively: the conformational modification occurred in the tetramethylene units.Strain induced crystal modifications have t Part of this work was presented at

KEY WORDSPoly(ethylene succinate) / Crystal Structure / Crystal Modification / X-Ray Diffraction / Mechanical properties of crystalline polymers, in general, depend strongly on their crystal structures as well as on the crystallinity and orientation of the polymers 1 . Crystal structures can be controlled by pressure, temperature, and strain. Strain induced crystal modifications have been reported in some polyesters and polyethers, such as in poly-(butylene terephthalate) (PBT) 2 -10 and poly-(ethylene oxide) (PEO) 11 as well as in poly-(oxacyclobutane) (POCB). 12 In addition, the authors have recently discovered a new crystal modification in poly(tetramethylene succinate) (PTMS), 13 · 14 which is a biodegradable aliphatic polyester. In these cases, the conformation changes from gauche to trans in the (CH 2 )xO unit (x = 2, 3, and 4) cause such crystal modifications, which result in a change of mechanical characteristics. Chatani et al. have determined the skeletal conformation of poly(ethylene succinate) (PES) to be T3GT/'J, where T, G, and G denote trans, gauche, and minus gauche, respectively, 15 suggesting a possibility of a new crystal modification by an application of strain. Despite many investigations on the crystal modification in PBT and polyethers, 2 -12 few works have been made on aliphatic polyesters. 13 • 14 In this paper, we show t To whom correspondence should be addressed. the existence of a new crystal modification in PES.EXPERIMENT AL PES utilized in this work was commercially available and supplied in a pellet form from Showa Highpolymer Co., Ltd., so-called "Bionolle" 16 ; no further purification was performed on the polymer. The melting point and the glass transition temperature of PES are 99°C and -11 °C (by DSC), respectively. Uniaxially oriented fiber specimens were obtained by melt spinning at 190°C, followed by drawing up to 5.5 times at room temperature. The resulting fibers were then annealed at 80°C in vacuum under a constant length; we referred them original fibers hereafter. One of the original fiber was stretched up to 30%, under which a small plastic strain is incorporated, by means of a conventional stretching device; we referred it strained fiber. The applied strain was determined based on the ratio of the strained fiber length to that of the original one. X-Ray fiber photographs were taken for both the original and strained fibers, using a cylindrical camera (diameter of 100mm) at room temperature as reported before. 13 • 14 The fiber

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