Secondary crystallization kinetics

Abstract

The overlap between primary and secondary crystallization is the cause of the repeated observations of fractional constant n values, of no theoretical significance, in the analysis of polymer crystallization by the Avrami equation. The time dependence of the growth nuclei in primary and secondary crystallization are explained in terms of reptation and nucleation theory. A composite equation involving both primary and secondary crystallization has been derived to account for the development of the total fractional crystallinity, Xt, with time, Xt = Xp,∞ (1–exp (–Zptn).(1 + ks t1/2) and procedures derived for determining the rate constants, Zp and ks, and the limiting fractional crystallinity of the primary process, Xp,∞, such that they are not treated as adjustable parameters. The fit of the equation to experimental crystallinity/time data was within experimental error. This interpretation accounted for n values in excess of those expected for the crystallization mechanisms adopted in deriving the Avrami equation but values >3.5 and <4.0 are attributed to the presence of induction times due to homogeneous primary nucleation of the lamellae. Evidence is presented for the presence of two simultaneous processes, primary and secondary, in the crystallization of polymers, which occur with different mechanisms and are readily distinguished by their dependence on time: an exponential increase and an increase with the square root of time. Differences arise from the critical growth nuclei, whose growth is limited by chain entanglements in the case of primary and local reptation in the case of secondary.

The 3-Phase Structure of Polyesters (PBT, PET) after Isothermal and Non-Isothermal Crystallization

“…It becomes clear that the initial mobile amorphous phase is converted into both the rigid amorphous phase and the crystalline phase over a long period of time. The temporal evolution of the additional crystallinity is in a √ t-relation, which proves the interpretation as secondary crystallization [36].…”

The Effect of Physical Aging and Degradation on the Re-Use of Polyamide 12 in Powder Bed Fusion

“…When repeatedly heated to 205 °C, the melting and crystallisation behaviour of PA−12 shows what can conveniently be described as a parabolic response. This could be explained by a "self-seeding" process [49,50]. Although 205 °C is greater than the observed melting point, some thicker lamellae may not fully melt, which can result in residual lamellae.…”

Singling Out the Role of Molecular Weight in the Crystallization Kinetics of Polyester/Clay Bionanocomposites Obtained by In Situ Step Growth Polycondensation

“…The following quantitative analysis is based on the fitting of the α­(t) data sets by eq . It is well known that the Avrami model describes in a generally accurate way the first part of the crystallization process (typically up to α values of 30 to 60%), while in the final stage, the so-called secondary crystallization mechanism prevails . In all cases analyzed in this work, the overlap of the Avrami fit with the experimental points throughout the first half of the isothermal crystallization process is excellent (nonlinear fitting performed with Origin Pro 2015 Data Analysis and Graphing Software, based on the Levenberg–Marquardt algorithm).…”

“…It is well known that the Avrami model describes in a generally accurate way the first part of the crystallization process (typically up to α values of 30 to 60% 40 ), while in the final stage, the so-called secondary crystallization mechanism prevails. 43 In all cases analyzed in this work, the overlap of the Avrami fit with the experimental points throughout the first half of the isothermal crystallization process is excellent (nonlinear fitting performed with Origin Pro 2015 Data Analysis and Graphing Software, based on the Levenberg−Marquardt algorithm 44 ). This is clearly shown in three plots reported in Figure 3, representative of progressively slower crystallization processes at increasing crystallization temperature.…”