Crystallization behavior of poly(N‐methyldodecano‐12‐lactam). III. Kinetics of isothermal crystallization

Abstract

Differential scanning calorimetry, combined with Avrami theory, was used to investigate the kinetics of three steps of the complex crystallization process of poly(Nmethyldodecano-12-lactam) (MPA): (1) primary melt crystallization at respective crystallization temperature (T c ), (2) additional crystallization at 30°C, and (3) recrystallization at 54°C. Kinetics of the three steps was discussed with respect to T c . The Avrami exponent n of primary melt crystallization decreased between 2.5 and 1.9 in the range of T c values of Ϫ10 to 20°C, which suggests heterogeneous nucleation, followed by two-dimensional growth, with a larger involvement of homogeneous thermal nucleation at greater supercoolings. The crystallization rate constant k decreased with increasing T c . The value of n ϭ 1.5 for additional crystallization implies a two-dimensional diffusion-controlled crystal growth with a suppressed nucleation phase. For T c values ranging from Ϫ10 to 0°C and 0 to 20°C, k showed weak and quite strong decreasing dependencies on T c , respectively. The recrystallization mechanism involved partial melting of primary crystallites and two-dimensional rearrangement of chains into a more perfect structure. The rate of this process was almost independent of T c . The values of activation energies were derived for the three steps of MPA crystallization using the Arrhenius equation.

Crystallization behavior of poly(N‐methyldodecano‐12‐lactam). IV. Nonisothermal crystallization

“…From the average values of n = 2.12–2.22 we might speculate that at later stages of nonisothermal crystallization covered by the Tobin treatment, that is, under higher supercoolings, heterogeneous nucleation suggested above on the basis of the Avrami equation might be accompanied by a homogeneous mechanism leading to increased values of n . Similar trends were revealed in our studies3 on isothermal crystallization of MPA, where the value of the Avrami exponent n increases from approximately 2.0 to 2.5 with increasing supercooling. These findings, however, should be supported by independent (such as microscopic) studies.…”

“…The values of exponent n of all three samples of MPA closely oscillate around 2.0, without any significant dependency on either cooling rate or molecular weight. The average values of 2.01 to 2.05 are in substantial agreement with the results of our studies3 on kinetics of isothermal crystallization of MPA and support the idea of a two‐dimensional crystal growth combined with heterogeneous nucleation at lower supercoolings, that is, at higher temperature of isothermal crystallization and at initial stages of nonisothermal crystallization.…”

“…After passing the glass‐transition temperature T g at about −30°C, the curves show the first exotherm, the minimum of which shifts from 0 to −15°C as the rate of the previous cooling run increases. The second deep exotherm, with the minimum at about 30°C, is associated with additional crystallization of MPA 1, 3. The primary crystalline structure formed during the preceding crystallization stages melts at about 51°C.…”

“…In the third part of our series3 we studied kinetics of the three steps of the isothermal crystallization process of MPA: (1) primary crystallization from melt at a given T c , (2) additional crystallization at 30°C, and (3) recrystallization at 54°C.…”

Long‐term X‐ray stimulated crystallization of poly(N‐methyldodecano‐12‐lactam) in blend with poly(styrene‐stat‐acrylic acid)

“…However, this temperature increase, if any, cannot explain the observed stimulating effect of X-rays because at LT the crystallization of PMDL is a nucleation-controlled process and its rate decreases with increasing temperature. 27 Moreover, additional crystallization, which is a unique feature of PMDL associated with cooperative trans/cis transition with the optimum at 30 8C, 1,27 that is, close to LT, cannot play any role in the observed phenomena either. Additional crystallization manifested by a pronounced exotherm on DSC curves at 30 8C is the crystallization stage taking place on heating PMDL after completed primary isothermal 1 or nonisothermal 28 crystallization.…”