Polymer Journal (2013) 45, 1107–1114 & 2013 The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/13 www.nature.com/pj ORIGINAL ARTICLE Phase-transition behavior of a crystalline polymer near the melting point: case studies of the ferroelectric phase transition of poly(vinylidene fluoride) and the b-to-a transition of trans-1,4-polyisoprene Paramita Jaya Ratri and Kohji Tashiro Many crystalline polymers exhibit phase transitions between the low- and high-temperature phases during the heating process. Sometimes, however, the temperature-dependent measurement of the X-ray diffraction peaks and vibrational spectra leads us to the incorrect conclusion that these two phases transform in a direct solid-to-solid transition mode instead of the melt and recrystallization process. As a reliable method to confirm the transition behavior just below the melting region, we have developed a temperature-jump cell system, with which a time-dependent measurement of the X-ray diffraction peaks (and Fourier-transform infrared spectra) is performed in the rapidly changing temperature process from the low- to the high- temperature phase. This method was applied to two types of polymers and revealed that (i) the high-temperature phase transition of poly(vinylidene fluoride) is not a direct ferroelectric transition between the ferroelectric and paraelectric phases, as proposed in our previous studies (Polymer (1983), Polym. Bull. (1983)), but a melt of the ferroelectric phase (form I) followed by the recrystallization into the nonpolar crystalline phase (form III or V) and (ii) the b-to-a phase transition of trans-1,4-polyisoprene had been also controversial for many years but it was revealed to occur via a melt-recrystallization mechanism. Polymer Journal (2013) 45, 1107–1114; doi:10.1038/pj.2013.42; published online 1 May 2013 Keywords: melt-recrystallization; phase transition; poly(vinylidene fluoride); time-resolved FTIR spectral measurement; time-resolved X-ray diffraction measurement; trans-1,4-polyisoprene INTRODUCTION based on the phase diagram produced from a series of VDF-TrFE Crystalline polymers exhibit various types of crystalline forms copolymers was that the phase transition of PVDF form I should depending on the sample-preparation conditions. These crystalline occur in the temperature region near the melting point. Actually, the forms can exhibit phase transitions by changing the external condi- ferroelectric-paraelectric phase transition was reported to occur tions, for example, the thermally induced phase transitions immediately below the melting point.15,16 This reported observation of polytetrafluoroethylene,1,2 aliphatic nylons,3–6 isotactic poly- has not been validated because a competing possibility of melt- butene-17,8 and so on, and the stress-induced transitions of recrystallization cannot be ignored.17,18 trans-1,4-polyisoprene (TPI; poly(tetramethylene terephthalate)9,10 and poly(ethylene oxide).11 gutta percha) was also reported to exhibit the solid-state transition The temperature-dependent measurement of X-ray diffraction can from the b form to the a form by heating,19–21 but this observation provide insight into the phase-transition behavior. Unfortunately, has yet to be confirmed.22–25 A similar situation might exist for the some unsolved problems remain even after carefully performing other types of crystalline polymers. temperature-dependent X-ray diffraction experiments. For example, Ambiguity remains regarding the phase transition of crystalline vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers are polymers. Which transition mechanism is correct between the quite unique polymers because they exhibit a reversible ferroelectric following two manners: (i) the direct solid-to-solid phase transition phase transition between the ferroelectric low-temperature phase and or (ii) the crystal melts to an amorphous phase or it disorders to a the paraelectric high-temperature phase during the heating and phase similar to the liquid-crystalline phase, followed by recrystalliza- cooling processes.12–14 Poly(vinylidene fluoride) (PVDF) form I, tion to another crystalline form (melt-recrystallization mechanism). being an extrapolation to a VDF content of 100%, is assumed to In some cases, the apparently direct solid-to-solid phase transition, exhibit this reversible ferroelectric phase transition. The prediction that is, mechanism (i) stated above, might not be the actual Department of Future Industry-oriented Basic Science and Materials, Toyota Technological Institute, Nagoya, Japan Correspondence: Dr K Tashiro, Department of Future Industry-oriented Basic Science and Materials, Toyota Technological Institute, Tempaku, Nagoya 468-8511, Japan. E-mail: [email protected] Received 14 February 2013; revised 3 March 2013; accepted 4 March 2013; published online 1 May 2013 Phase-transition behavior of a crystalline polymer near the melting point PJ Ratri and K Tashiro 1108 mechanism in play; rather, the phase-transition mechanism between EXPERIMENTAL PROCEDURE these two solid phases can occur via an intermediate amorphous or Samples liquid-crystalline phase, that is, mechanism (ii) stated above. PVDF form I. Kynar, the PVDF sample, was supplied by Arkema (Elf Mistakenly, ignoring such an intermediate phase may lead us to the Atochem Inc., Colombes, France). The melting point was approximately wrong conclusion regarding the phase-transition mechanism. 167 1C, slightly lower than that of the KF series (174 1C) supplied by Kureha How can such a mistake occur in the clarification of the phase- Co. Ltd., Tokyo, Japan. The sample was dissolved into the highly polar solvent 28 transition behavior? Normally, we measure the X-ray diffraction and/ hexamethylene phosphoric triamide at room temperature. This solution was cast onto a glass slide at an ambient temperature for 1 month. This quite slow or infrared and Raman spectra during the course of the heating (and evaporation of hexamethylene phosphoric triamide resulted in a highly cooling) process. Unfortunately, the phase-transition region is not crystalline and unoriented pure form I film. very narrow for general crystalline polymers, but it occurs over a relatively wide temperature region; in addition, the crystalline TPI b-form. The synthetic TPI sample with 99% trans content was purchased polymers do not exhibit very sharp and well-separated X-ray from Polysciences Inc. (Warrington, PA, USA). As described in a previous diffraction peaks. As a result, the existing phases overlap over a wide study,29 and the unoriented pure b-form sample was prepared by quenching temperature region, and the signature of the phase-transition process the melt at liquid N2 temperature followed by slowly heating to room becomes ambiguous, leading us to produce an incorrect conclusion temperature. regarding the phase-transition behavior. We propose one way to avoid such a serious problem, which Measurements involves performing time-resolved measurement of the X-ray diffrac- Differential scanning calorimetry. A differential scanning calorimeter (DSC) tion peaks and the Fourier-transform infrared (FTIR) spectra with TA DSC Q1000 was used to measure the DSC thermograms in a nitrogen gas atmosphere at a heating rate of 1 1CminÀ1. high temporal resolution during the rapid temperature-jump process from the low-temperature phase region to the high-temperature phase X-ray diffraction measurements. Wide-angle X-ray diffraction (WAXD) pro- region. During a steep temperature change, the individual phase may files of the unoriented samples were measured in the stepwise heating process appear and disappear in sequence because of a slight difference in the using a Rigaku RINT-TTR III diffractometer (Rigaku, Tokyo, Japan) in a existing temperature region. Correspondingly, the X-ray diffraction symmetric reflection mode. The incident X-ray beam was a graphite-mono- ˚ and vibrational bands of the phases are observed at slightly shifted chromatized Cu-Ka (l ¼ 1.5418 A) line, with an incident angle that was times. The proposed method was applied to two concrete cases that scanned over the diffraction angle (2y) range of 5–351, and the diffracted had remained unresolved for a long time in spite of their significance X-rays were detected using a two-dimensional semiconductor detector (D’Tex; Rigaku). During the X-ray diffraction measurement, the DSC thermogram was in the structural science of polymers: the high-temperature phase also measured simultaneously at a scanning rate of 1 1CminÀ1. transition of PVDF form I and the b-to-a phase transition of TPI. The The WAXD measurements were performed in both the stepwise heating (i) reasons for choosing these two polymers are stated below. and temperature-jump (ii) modes. (i) The stepwise heating experiments were PVDF form I is one of the most common ferroelectric polymers and performed using a Rigaku R-axis VII X-ray diffractometer equipped with a flat exhibits a polar crystal structure consisting of the parallel array of CF2 imaging plate as a detector set at a distance of 110 mm from the sample, where 26,27 dipoles of the planar-zigzag chains along the b axis. Ferroelectric a graphite-monochromatized Cu-Ka line was used as an incident X-ray beam. crystals generally exhibit a phase transition between the polar (ii) In the temperature-jump process, the WAXD data were measured using a (ferroelectric) and nonpolar (paraelectric) phases during the heating Rigaku Nanoviewer X-ray diffractometer. The sample was set in a Linkam (or cooling) process.14 A possible