The rigid amorphous fraction in semicrystalline macromolecules |
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Authors: | Joseph D Menczel |
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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 fraction (RAF) was reported by Menczel and Wunderlich
for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these
polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate)
(PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC
p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated
from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most
semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated
from the ΔC
p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to
the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional)
amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous
content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease
of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also
measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could
be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that
time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling.
Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid
crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions.
After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition
in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was
shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which
can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality
of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration. |
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