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  • Author or Editor: J. Dobbertin x
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Confinement of the glass-forming regions in the nanometer range influences the α-relaxation which is associated with the glass transition. These effects were investigated for semicrystalline poly(ethylene terephthalate) by dielectric spectroscopy and differential scanning calorimetry. The results are discussed within the concept of cooperative length, i.e. the characteristic length of the cooperative process of glass transition. Both experiments showed a dependence of the glass transition on the mean thickness of the amorphous layers. For the dielectric relaxation, the loss maximum was found to shift to higher temperatures with decreasing thickness of the amorphous layers, but no differences were observed in the curve shape for the differently crystallized samples. For the calorimetric measurements, in contrast, there was no correlation for the glass transition temperature, whereas the curve shape did correlate with the layer thickness of the mobile amorphous fraction. From the structure parameters, a characteristic length of approximately (2.5±1) nm was estimated for the unconfined glass relaxation (transition).

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The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.

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The relaxation strength at the glass transition for semi-crystalline polymers observed by different experimental methods shows significant deviations from a simple two-phase model. Introduction of a rigid amorphous fraction, which is non-crystalline but does not participate in the glass transition, allows a description of the relaxation behavior of such systems. The question arises when does this amorphous material vitrify. Our measurements on PET identify no separate glass transition and no devitrification over a broad temperature range. Measurements on a low molecular weight compound which partly crystallizes supports the idea that vitrification of the rigid amorphous material occurs during formation of crystallites. The reason for vitrification is the immobilization of co-operative motions due to the fixation of parts of the molecules in the crystallites. Local movements (Β-relaxation) are only slightly influenced by the crystallites and occur in the whole non-crystalline fraction.

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