Memory Effects in the Quiescent Crystallization of Polyamide 12: Self-Seeding, Post-Condensation, Disentangling, and Self-Nucleation beyond the Equilibrium Melting Temperature

Abstract

This paper addresses the role of memory effects from earlier thermal protocols on the non-isothermal crystallization behavior of polyamide 12 (PA12). It turns out that memory effects can induce differences in crystallization temperatures as large as 30 °C when cooled from the melt at 10 °C/min. DSC experiments reveal that memory effects in PA12 result from (1) self-nucleation by the action of crystalline residues (self-seeding), (2) chemical post-condensation, and (3) polymer chain disentangling and re-entangling. Self-nucleation and disentangling facilitate crystallization, while melt post-condensation and re-entangling hamper it. It was demonstrated that repeated crystallization stimulates disentangling and that re-entangling in the liquid state proceeds more rapidly the higher the temperature. Effects due to solid-state post-condensation are very intriguing and particularly relevant to laser sintering based additive manufacturing. It was observed that solid-state post-condensation at 175 °C for 1 h negatively affects crystallization in a subsequent heating/cooling cycle but that solid-state annealing at 175 °C for 20 h leads to an enhanced crystal perfection and chain rearrangements whose non-crystalline self-nucleating remnants in the melt at 240 °C need more than 1 h to relax and no longer stimulate crystallization during cooling. Importantly, 240 °C is higher than the equilibrium melting temperature of PA12.

Influence of Fusion Temperature on Nonisothermal Crystallization Kinetics of Polyamide 6

“…It seems that 2 min fusion time is not sufficient to erase previous processing history. In the literature [29,30], 5 min fusion time is usually used prior to crystallization. However, in terms of crystallization kinetics, the inverse value of crystallization is preferable.…”

“…It seems that 2 min fusion time is not sufficient to erase previous processing history. In the literature [29,30] Equation ( 2) can be modified into crystallization time t and t0 instead of the crystallization temperature T and 𝑇 , respectively. The relationship between the crystallization temperature and the crystallization time can be summarized in the following equation (Equation ( 8)) [31].…”

Unusual Memory Effect of Crystallization Raised by 4-Phenyl-1-butene and 4-Anthryl-1-butene Comonomers in 1-Butene-Based Copolymers

“…Consequently, the thermal stability of the condensed structures in polymers extends beyond the crystal melting point and resists the formation of other possible structures associated with original crystallization. , The crystallization of polymers depends on the thermal properties. The residual structures retained in the molten state owing to the memory effect influence subsequent recrystallization, − accelerating the kinetics of crystallization. ,− Thus, the underlying mechanism governing the memory effect of the condensed structures of polymers should be studied for controlling the morphology and properties of the final crystallites.…”

Self-Nucleation Ability and Intermolecular Interactions Mechanism in Fluoropolyolefins

“…Since its discovery, the SN behavior has been widely observed in a variety of polymers, such as polyesters, − polyamides, − polyethers, , and polyolefins. ,− But, the intrinsic nature is still under debate. According to the literature, numerous factors impact the SN behavior of polymers, including temperature, , treatment time, , chemical structure, , molecular weight, ,− chain topology, ,− and confinement conditions. , As a result, several hypothetical models have been proposed, including “unmolten crystal fragments”, ,, “phase topology or nonisotropic melts”, , “topological constraints (i.e., entanglements)”, , “precursor phase”, and “short-range ordered conformations”. ,, Yet, the lack of direct detection techniques has made it difficult to precisely describe the exact structure of self-nuclei, as the detailed difference between the isotropic melt and the self-nucleated melt is not easily captured.…”