Inhomogeneity of hydrolytic polyamide 6 with respect to the average molecular mass

Abstract

The point of view was advanced that in spinning fibres from polymer melts, the molecular inhomogeneity of the melts is not due to the polydispersity of the polymer but to important differences in the average molecular mass of the individual granules. Using the example of three industrial samples of hydrolytic PA 6, it was shown that the standard deviation of the average molecular mass of the polymer with respect to the individual granules from its mean statistical values is at the level of 10-11% for a practically significant (for up to several percent of granules) deviation of 30-50%. The use of samples of PA 6 with the mass provided by GOST 18245-72, OST 6-06-S9-76, and GOST 11034-82 in determining the viscosity a solution of the polymer in H 2 SO 4 allows correctly estimating its average molecular mass but makes it impossible to obtain reliable information relative to the real degree of inhomogeneity of the granulate based on this index. In analyzing the dependence of the physicomechanical properties of fibres spun both from solution and from polymer melt on the molecular-weight characteristics of the polymer, it is believed that the molecular inhomogeneity of these liquid systems is exclusively due to the polydispersity of the initial polymer. However, if this assumption can be considered valid when applied to solutions of polymers, the validity of extending it to melts is doubtful. Actually, in preparing solutions of polymers (due to their relatively low viscosity in comparison to melts and the duration of the process), it is always possible in principle to attain a degree of homogenization of the system at which the differences in the average molecular mass and degree of polydispersity with respect to microvolumes of the initial (solid) polymer are totally leveled. Conversion of polymer granules into a melt is very rapid and a degree of homogenization similar to that attained in solution is probably practically impossible to attain. For this reason, inhomogeneity of the polymer granules with respect to the average molecular mass could be the basic cause of the molecular inhomogeneity of the melt on the microvolume level. The seriousness of the problem becomes evident if we consider that several meters of fibre can at the minimum be formed from a microvolume of melt formed in melting an ordinary granule weighing approximately 3⋅10 -2 g. We report the results of an analysis of polyamide 6 (PA 6) from Shchekinsk Khimvolokno Co. taken directly after the stage of synthesis on a cascade polyamidation line (polymer I) and PA 6 from Kuibyshevazot Co. (Tol’yatti) having gone through the stage of extraction of low-molecular-weight compounds and drying (polymer II), as well as treatment with an aqueous solution of a thermostabilizer and additional drying (polymer III). The essence of the experiment consisted of establishing the character of distribution of the polymer granules with respect to the average molecular mass based on determination of this quantity for 100 granules.