https://doi.org/10.3390/polym12122796
“It is widely accepted that melt memory effect on polymer crystallization depends on thermal history of the material, however a systematic study of the different parameters involved in the process has been neglected, so far. In this work, poly(butylene succinate) has been selected to analyze the effect of short times and high cooling/heating rates that are relevant from an industrial point of view by taking advantage of fast scanning calorimetry (FSC). The FSC experiments reveal that the width of melt memory temperature range is reduced with the time spent at the self-nucleation temperature (Ts), since annealing of crystals occurs at higher temperatures. The effectiveness of self-nuclei to crystallize the sample is addressed by increasing the cooling rate from Ts temperature. The effect of previous standard state on melt memory is analyzed by (a) changing the cooling/heating rate and (b) applying successive self-nucleation and annealing (SSA) technique, observing a strong correlation between melting enthalpy or crystallinity degree and the extent of melt memory. The acquired knowledge can be extended to other semicrystalline polymers to control accurately the melt memory effect and therefore, the time needed to process the material and its final performance.”
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To study melt memory effects, the following self-nucleation procedure (see ), proposed by Fillon et al. [
2,
3], was employed: first the material is heated to 25–30 °C above its melting peak to erase the thermal history of the sample, in this case the sample was heated to 160 °C. Then the sample was cooled down to −50 °C to obtain a standard crystalline state and maintained at this temperature for 0.1 s. The sample was heated to the selected self-nucleation temperature,
Ts, and was maintained at this temperature for a certain period of time, usually 5 min. After that, the sample was cooled down and heated again. Depending on the conditions employed to perform the self-nucleation procedure and on the
Ts temperature, the sample can show three different
self-nucleation Domains. The temperature region corresponding to each
Domain can be determined by analyzing the cooling scan from the
Ts temperature and the subsequent heating scan.
Figure 1. Standard self-nucleation procedure employed to investigate melt memory effects.
As a reference the results obtained applying a heating/cooling rate of 10 K/s were used, this rate was selected considering two factors: to measure the sample at the lowest rate possible without losing accuracy in the flash DSC, to allow the material to fully crystallize. The second factor considered was that no lower rates were employed since otherwise the sample spends longer times at high temperatures (as the measurement takes longer time), which might result in degradation.
In this work melting enthalpy values are discussed, which are directly correlated to the crystallinity level, since the mass of the sample employed in the flash DSC was not determined in this case.
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3.1. Self-Nucleation of PBS
shows the results obtained after applying a standard self-nucleation procedure (). In this case, a cooling and heating rate of 10 K/s has been employed except in the final heating, in which 1000 K/s was used to avoid reorganization of crystals. The sample was kept at 0.1 s at the self-nucleation temperature. Under these conditions, the crystallization temperature was constant above
Ts temperatures equal to 118 °C. So, for temperatures equal or above 118 °C, the sample was in
Domain I or the
melting Domain [
2,
3,
4]. However, when the
Ts temperature was reduced to 117 °C or below, an increase in the crystallization temperature with respect to the standard crystallization temperature was observed, which marks the transition to
Domain II or the
self-nucleation Domain. The enhancement of the crystallization temperature comes from the presence of self-nuclei and self-seeds, which increase the nucleation density.
Figure 2. (a) Differential scanning calorimeter (DSC) cooling scans of PBS from the indicated self-nucleation temperatures and (b) subsequent heating scans. (c) Crystallization temperature (right-hand side y-axis) as a function of self-nucleation temperature (x-axis) on top of the PBS DSC heating scan (the DSC heating scan is represented in colors that match the different SN Domains).
PBS crystals were molten at 109 °C according to the FSC results (), thus for temperatures above 109 °C there were no crystal fragments and consequently the increase of crystallization temperature in this region corresponded to the presence of self-nuclei; therefore, this temperature region is known as the
melt memory Domain or
Domain IIa [
4,
24], see c. For temperatures below 109 °C, there are some crystal fragments (evidenced by incomplete melting in the DSC trace) that act as self-seeds responsible for the increase in crystallization temperature. This
Ts temperature range is called
Domain IIb or the
self-seeding Domain [
4,
24], see c.
For self-nucleation temperatures equal or lower than 106 °C, if the subsequent heating scan is analyzed (b), an additional melting peak is observed (signaled by an arrow), which corresponds to the melting of annealed crystals. The lower melting peak corresponds to less stable crystals with thin lamellae whereas the higher melting temperature corresponds to recrystallized or annealed crystals. During annealing the crystals reorganize and form more stable crystals, with thicker lamellae, which results in higher melting temperatures [
33]. In b a shift of the melting peak to higher temperatures is observed, in the case of the measurement in
Domain III the lowest melting peak was considered (the highest melting peak corresponds to annealed crystals, as mentioned before). This shift resulted from the crystallization of the material at higher temperatures when cooling from
Ts, which led to crystals with thicker lamellae and thus, higher melting peaks. Summarizing, at temperatures equal or below 106 °C, the sample is in
Domain III or the
self-nucleation and annealing Domain [
2,
3,
4].
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