Relaxation of New-Tpi Thermoplastic Polyimide Studied by Thermally Stimulated Depolarization Current
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INTRODUCTION Semicrystalline polyimide, RegulusTM NEW-TPI, is a thermoplastic polyimide produced by Mitsui Toatsu. This aromatic polyimide possesses outstanding heat resistance, high tensile strength, excellent electrical properties, and strong solvent resistance. It has become the subject of recent study in our group [1-6] and others [7-13]. The flexible ether and meta linkages in the monomer unit lower its glass transition temperature (Tg) to 250'C in quenched amorphous material. The chemical repeat unit has been reported previously [1,13] and is shown below:
[o 0
a 0
NEW-TPI is a melt-processible semicrystalline polymer with a melting temperature (Tm) of 385'C from differential scanning calorimetry (DSC). Our previous studies [ 1,5] have shown that Tg shifts only slightly to higher temperature for semicrystalline polymer and the crystallinity is low (less than 30%). A very small amount of tightly bound, or rigid, amorphous phase exists and relaxes completely about 30'C above T9. Further investigation of molecular relaxation behavior of semicrystalline NEW-TPI is carried out in the present work using a more powerful technique, thermally stimulated depolarization current (TSDC) along with DSC and wide angle x-ray scattering (WAXS).
EXPERIMENTAL RegulusTM NEW-TPI film was kindly supplied by Mitsui Toatsu Chemical Co. The asreceived film was transparent and 100l m thick. It is considered amorphous as no peaks were observed from Bragg scattering in the wide angle x-ray diffractogram and equal heats for crystallization and melting were observed in DSC. No orientation was seen in the plane of the film using polarizing optical microscopy. Crystallinity of NEW-TPI is obtained from the ratio of the heat of fusion of the semicrystalline Sample to that of the perfect crystal, which is reported as 139 J/g [9]. The film was dried in a vacuum oven at 100 0 C for 20 hrs prior to any further treatment. Some of the samples were annealed at 260*C for lhr. As no crystallization was found, these samples will be referred to as 'amorphous' in our later discussion. Some of the samples were cold crystallized at 300TC for lhr, and will be referred to as 'semicrystalline'. Crystallization and melting behavior of RegulusTM were studied using a Perkin Elmer DSC-4 with Indium calibration for temperature and heat flow. Sample mass around 8 mg and a scan rate of 10°C/min were used for all the samples. WAXS data were obtained at room temperature using a Rigaku RU-300 x-ray diffractometer which was operated at 50kV and 200mA with Cu-Ka radiation and a diffracted beam graphite monochromator. The measurement range was from 20 = 5°-55' with a step scan interval of 0.02 degrees and a 20 scan rate of 10/min. 197 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
TSDC measurement was carried out on our self-designed and self-assembled apparatus. The major part is the sample cell which contains two layers. The first layer is for liquid nitrogen cooling. The second layer is for heating and Faraday shielding. The triaxially shiel
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