The role of post-crystallization of polyamide-6 in its solid-phase polycondensation

Abstract

The character of the effect of additional crystallization of the polymer on the kinetic parameters of its solid-phase polycondensation is discussed based on data on the kinetics of the change in the molecular weight an density of polyamide-6 on heating in a vacuum at 453-483 K. This reaction is polychronic and consists of pseudomonomolecular dehydration of contact pairs formed by the amino and carboxyl groups in the macromolecules in crystallization of the polymer melt.Solid-phase polycondensation of polyamide-6 (PA-6) purified of unreacted caprolactam and cyclic oligomers (demonomerized) by aqueous extraction at 433-483 K in a vacuum or inert gas stream in conditions of an irreversible reaction has become [1] an important process stage in production of the polymer used in manufacture of industrial fibres and different injection-molded items.However, in this temperature region, additional crystallization (post-crystallization) of the polymer should and actually does take place [2, 3], which reduces the volume and perhaps also alters the structural organization of the amorphous regions of PA-6 in which the polycondensation reaction strictly takes place.As we noted previously in [3][4][5][6], the main difference between the solid-phase polycondensation that takes place in amorphous regions of the partially crystalline polymer and the reaction in a melt consists of the absence of terminal groups in the macromolecules of translational mobility in the first case. For this reason, the reaction can essentially take place only because of the groups that formed contact pairs in crystallization of the melt.From a kinetic point of view, this is equivalent to knowing that solid-phase polycondensation takes place as a pseudomonomolecular reaction of dehydration of such contact pairs, which can be complicated by the different "readiness" of the latter for carrying out the chemical act.These reactions are called [7] polychronic and in the case of a first-order irreversible reaction, the kinetic curves are linearized in coordinates of the equation [8] , ln 1 ) 76 . 1 ln( 1 1 max 0where N 0 and N are the initial and current amounts of active sites in the reaction volume; k max is the maximum rate constant; S is the parameter of the kinetic nonequivalence of these sites, equal to , min max RTwhere ΔE max and ΔE min are the maximum and minimum values of the activation energy of the reaction.