On the kinetics of the reversible repolyamidation of polycaproamide below the melting point of the polymer

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

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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 .

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“…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.…”

Solid-phase synthesis of polyamide-6

“…(17) does not hold. The results of the analysis of the kinetics of additional polycondensation of PA-6 performed previously in [17,30] and re-examined in consideration of this situation, are reported in Table 3.…”

“…Equation (17) actually applies to the analysis of the kinetics of reversible additional polycondensation of PA-6 in the solid phase, and this not only concerns the direct reaction, but also the reverse reaction which, as noted above, can formally take place with any amide groups and molecules of water in the amorphous phase of the polymer [30,32]. ____________ *In the given case, the concept of readiness also includes the possibility of finding the members of a contact pair at different distances from each other as a result of their vibrational movements.…”

“…From a kinetic viewpoint, this is equivalent to knowing that condensation takes place as a pseudofirst-order reaction complicated by different readiness of each concrete contact pair for executing the chemical event* [30]. This formally means that the reaction is characterized by not just one rate constant, but by a set that obeys some distribution law.…”