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On the Structure and Properties of Polyamides. XXVIII. Crystallization of Polycaprolactam Without a Thermal History

1967; Wiley; Volume: 16; Issue: 1 Linguagem: Inglês

10.1002/polc.5070160107

1935-3065

J. Tomka, J. Šebenda, O. Wichterle,

Polymer Nanocomposites and Properties

Abstract The time lags for crystallization of polycaprolactam without a thermal history have been measured by nephelometry. The effects of crystallization temperature, degree of polymerization, diluent, and crosslinking have been investigated. Polymer without a thermal history was prepared by activated alkaline polymerization at the desired crystallization temperature, so that the polymer had never previously been crystalline. The dependence of the time lag τ on temperature T and the degree of polymerization P obeys the theoretical relation τ = const. ( T m — T )‐ − b P c If we substituted for T m the melting temperatures corrected for the content of noncrystallizable material we obtained b = 5.85. This value is close to the theoretical value 6, derived under the assumption that the diffusion of segments into the nucleus is proportional to the number of chains forming the surface of the cylindrical nucleus. Measurement of time lags for polycaprolactam crosslinked by means of methylenebiscaprolactam has corroborated the assumption that the viscosity near the surface of the crystal is not identical with the macroscopic viscosity of the melt, in agreement with the fact that the determined value of c = 1.58 is lower than 3.4. The effect of the diluent, N ‐methyl‐caprolactam on the depression of T m outweighs that of the viscosity decrease. The dilatometric measurements of the kinetics of crystallization of polycaprolactam without a thermal history have confirmed that both the time lag and the secondary crystallization can appreciably affect the value of the exponent n in Avrami's equation. If we evaluate the kinetics of the primary crystallization in terms of Avrami's equation using experimental values of time lags, the value of the exponent n is close to 4, which corresponds to a sporadic nucleation and three‐dimensional growth. If, however, the secondary crystallization is not separated and the start of the crystallization is considered to be the moment when the volume change is perceivable experimentally, then we obtain n ≈ 2. This indicates that conclusions on the mechanism of crystallization drawn from the value of the exponent n may be questionable.