Transient and athermal effects in the crystallization of polymers. II. Nonisothermal crystallization

Abstract

Nonisothermal crystallization of several polymers was investigated with differential scanning calorimetry and optical microscopy. The results indicated that as in the case of isothermal processes, crystallization starts with nucleation on noncompletely melted crystalline residues. It is assumed that if the crystalline residues are subcritical at melting temperatures, they can become stable by an athermal mechanism during cooling. There is also some contribution of nucleation on heterogeneities. The next mechanism of nucleation is a classical homogeneous process occurring by thermal fluctuations. The results showed the non-steady-state character of the nonisothermal crystallization of polymers. In the investigated range of cooling rates, the non-steadystate character of nonisothermal crystallization of polymers is dominated by the transient thermal effects. In the range of high temperatures, the transient homogeneous nucleation can be interpreted with the Ziabicki model, and the steady-state rate determined from nonisothermal experiments coincides with the rate determined in isothermal crystallization. The athermal nucleation occurring at the beginning of crystallization from noncompletely melted aggregates seems to be independent of the applied cooling rate.

Nucleation parameter and size distribution of critical nuclei for nonisothermal polymer crystallization: The influence of the cooling rate and filler

“…23 This is the so-called transient or relaxational effect. (8), the nonisothermal DSC scan is approximated with a finite number of isothermal segments.…”

“…23 To become stable, the nuclei have to achieve the critical thickness, which according to eq. 23 To become stable, the nuclei have to achieve the critical thickness, which according to eq.…”

“…23 To become stable, the nuclei have to achieve the critical thickness, which according to eq. 23 Accordingly, the athermal effect is believed to accelerate the crystallization rate in the whole temperature range by stabilizing the subcritical nuclei, which would otherwise be dissolved. If the cooling rate is relatively high, the thin subcritical nuclei that were unstable under the initial condition will not dissolve, but will stay in the system and achieve the critical value at some lower temperature.…”

“…As a consequence, the current crystallization rate deviates from that defined by the current temperature. 23 This is the so-called transient or relaxational effect. 24 The transient effect causes the lag of crystallization behind the changes of external conditions.…”

“…The second nonisothermal, so-called athermal effect, is proportional to the cooling rate. 23 To become stable, the nuclei have to achieve the critical thickness, which according to eq. (2) decreases when lowering the temperature.…”