How did we find the rigid amorphous phase? |
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Authors: | J D Menczel M Jaffe |
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Institution: | (1) Alcon Laboratories (retired), 412 Stampede Court, Fort Worth, TX 76123, USA |
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Abstract: | The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich 1]:
when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers poly(ethylene
terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] 2], the absence of the hysteresis peak at the lower temperature
glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled.
Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source
of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity
was confirmed 1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC
p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity
concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses
of polymers.
When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC
p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline
polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that
ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min−1 rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF,
Nylon 66 and polycaprolactone 1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate 3] were added
to this list. |
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