✍ Scribed by M. Beiner; F. Garwe; E. Hempel; J. Schawe; K. Schröter; A. Schönhals; E. Donth
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Specific heat C,, dynamic shear modulus G*, and dielectric compliance E* are measured in the splitting region a c* (Y + p of poly(n butyl methacrylate). The characteristic length of the glass transition is estimated from the calorimetric data to be about one nanometer before the breakdown of cooperativity between 15 and 21°C. The merging of the dielectric a and p traces is between 70 and 8O"C, more than fifty degrees higher.
In recent years the glass transition became of growing interest in condensed matter physics [l]. This transition is of well-known practical importance, and now there are the first theoretical approaches.
The idea of molecular cooperativity is generally accepted and is assumed to be the cause for the typical non-Debye relaxation functions and the non-Arrhenius temperature dependence of the relaxation times. It seems natural to ask which typical lengths, sensitive to the different responses, reflect the cooperativity (see, e.g., the introductory lecture by C.A. Angel1 in [l]).
The characteristic length 5, of the glass transition, a half correlation length sensitive to thermal response, can be obtained from calorimetric data [2]. For many amorphous polymers, values of about 2 nm are obtained at Tg [3-51. Above Tg, 5, decreases with increasing temperature [2,6]. A minimal cooperativity of N, = lo-30 particles (units), corresponding to (Fin = 1 nm [7], was predicted for the onset of the dynamic glass transition (Y in the splitting region S: a ++ (Y + /3, where a is the activated high temperature relaxation and p the local mode. To check this prediction it is necessary to get precise thermal response data in S. Such data can be taken from entropy compliance, C;(o), measurements by periodic calorimetry, or by dynamic scanning calorimetry DSC after connection of structural relaxation.
Entropy response cannot be reduced to density-density (eo) correlations. A comparison of C; to shear (also not retractable to ee correlations) and to Elsevier Science Publishers B.V.
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