抄録
Viscoelastic measurements were made of a molten“amorphous”polypropylene obtained by extracting it from isotactic polypropylene with hot ether. The results indicate the existence of a kind of physical (crystalline) crosslinks that connect molecules in the specimen in its molten state. The crosslinks are removed by raising the temperature or by giving it a large deformation.
The dynamic rigidity G' and viscosity η' as functions of frequency were measured at various temperatures with a rotational elastoviscometer of cone-and-plate type. The experimental results are that i) G' and η' observed at each frequency become lower with increasing temperature, and that ii) the frequency variation of G' at 129 and 135°C is expressed as G'∝ω2 in the frequency range used, as is demanded by Rouse theory. The method of reduced variables can be applied to the two curves, giving 12kcal/mol as the apparent activation energy consistent with the value obtained from the steady-state viscosity data in the same temperature range. The exponent is less than 2 at lower temperatures, and the G' curve at each temperature levels off at lower frequencies, thus suggesting the existence of plateau. As the temperature is raised, η' becomes less dependent on frequency until it is wholly independent of frequency at 125°C. The method of reduced variables can not be applied to the present case in general.
Creep measurements as well as dynamic ones were made of the same specimen before and after it was given a large deformation by rotating the plate of the cone-and-plate viscometer ten times. As a result of the large deformation, it was found that the creep compliance J(t) was increased at 91°C and the retarded elastic compliance J(t)-t/η was decreased. If the retarded elasticity function ψ(t)=[J(t)-t/η]/Jd is plotted against the time logarithmically, it is seen that the large deformation given beforehand makes the subsequent retarded elastic deformation occur faster, i.e. makes the retardation times become shorter (Jd=the steady state compliance). It was also observed that G' and η' were decreased at 75°C by the previous deformation.
It has been reported that non-Newtonian flow of molten polymers is observed when the molecular weight exceeds the critical molecular weight Mc. Mc of polypropylene is inferred to be larger than or comparable with the molecular weight of our specimen 2×104. Nevertheless, at temperatures lower than 125°C, our experimental results, for example the decrease of η' with increasing frequency, show the existence of large molecules with very long relaxation times. It is very probable that our specimen is composed of randomly coiled molecular chains, some of which are connected to each other by a kind of crosslinks. Some of the segments with stereoregular sequence remaining eventually in this extracted specimen tend to array themselves side by side, thus forming embryonic crystallites which serve as crosslinks. The crystallites melt at higher temperatures and are destroyed by a large deformation, and the apparently large molecules are separated into intrinsic small molecules.
