ポリエチレンの粘性流動における分枝の役割 : 高分子レオロジー
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概要
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It has been shown by Peticolas and Watkins that the melt viscosity of high pressure (branched) polyethylene is far smaller than that of low pressure (linear) polyethylene of the same molecular weight, and they ascribed the fact to the free volume increase associated with short chain branchings. Later on, Moore interpreted the same fact in terms of long chain branchings. In fact, it does not seem likely that the change in the free volume has a large effect upon the viscosity of polymer in such a temperature range at least 150℃ higher than the glass transition temperature. Instead, it is expected, from several evidences found in low molecular weight paraffins, that the effect of short chain branchings is manifested in the difference of activation energy related to the flow of the two kinds of polyethylene. In order to verify the expectation, melt viscosities of various polyethylenes were measured by a rotational viscometer of the cone-and-plate type. Test specimens were (i) high pressure polyethylenes, (ii) low pressure polyethylenes (marlex 6000-50, 35, 15, 9, 2), (iii) long-chain-branched polyethylenes obtained from Marlex 6000-50 by irradiation with small does of γ-rays of Co 60. Experimental results so far obtained are summarized as follows: (1) In case of temperatures ranging from bout 150℃ down to melting point, the viscosity of linear polyethylenes (ii) as well as (iii) of lower molecular weight becomes less temperature dependent and therefore the activation energy E becomes smaller than in a temperature range above 150℃. The anomalous behavior, which was shown to be time dependent, may be an indication of the nematic state with hexagonal close packed structure suggested by the X-ray diffraction method. (2) E of linear polyethylene was found to increase with molecular weight ; E=11 Kcal/mol for M_w=10^5 where M_w=weight-average molecular weight. In this respect, the long-chain-branched polyethylene behaved in the same manner as the linear polyethylene. (3) An experimental formula which shows molecular weight dependence of E in the case of branched polyethylene of large molecular weight was given by Porter and Johnson. According to it, E=15 Kcal/mol for M_η=10^5, where M_η=viscosity-average molecular weight. Though the ratio of M_w to M_η varies among specimens, it may well be assumed to be far smaller than that of M_w to M_n, where M_n=number-average molecular weight. Therefore, it is concluded with some confidence that the well-known difference of E between polyethylenes of high and low pressure does exist, but does not exceed 4 Kcal/mol in its magnitude, if both are of the same molecular weight×10^5.
- 社団法人日本材料学会の論文
- 1961-05-15