Half crystallization time

Fig. 9 Inverse of the crystallization half-time as a function of isothermal crystallization temperature for PCL11 homopolymer and for the PCL block of the indicated copolymers. All experiments were performed after the PPDX block had been previously crystallized until saturation. Solid lines are fits to the Lauritzen and Hoffman theory. (From [103]. Reproduced with permission of the Royal Society of Chemistry)...

Figure 4.11 The influence of comonomer content and type on the crystallization half-time of PET [51]. Reproduced from Sakellarides, S. L., ANTEC, 96, 938-942 (1996), with permission from the Society of Petroleum Engineers...

Crystallization of PCT is relatively rapid, but because of its higher Tg (90 °C) the maximum rate of crystallization occurs at a higher temperature than is typical of other crystalline polymers such as PET (Tg at about 70 °C) or PBT (Tg at about 35 °C). Figure 7.2 compares the crystallization half-times of PET and PCT from both the glass and the melt (data were obtained via DSC measurements). The effect of the higher Tg on the temperature of maximum crystallization rate (i.e. minimum half-time) is most clearly seen in the data from the melt. The basic rapid crystallization rate of PCT allows it to be used as a high-performance injection molding material. 

Figure 7.2 Crystallization half-times for PET and PCT from (a) the glassy state, and (b) the melt...

In polymer melt processing, the crystallization half-time, 0/2, is often a more useful parameter than the Avrami rate constant. An experienced individual can often estimate the processing conditions of a new polymer by comparing its 0/2 value with those of known polymers. Figure 11.6 compares the ty2 parameters of PET, PTT and PBT, with PTT falling in between the other two polyesters. 

Figure 11.6 Comparison of PET, PTT and PBT crystallization half-times at the same degree of undercooling from the melts...

Crystallization at 200°C. While normally PBT is known to have a faster crystallization rate than PET, at 200°C its undercooling is only 24°C while that of PET is 65°C. Therefore, the crystallization rate at 200°C is higher for PET than for PBT, which was already observed by DLI and now can be verified by IR and density. While the crystallization half-time of PBT in the blends remains quite constant, that of PET increases with PBT content, showing a decrease in the crystallization rate... 

Figure 16. PET-crystallization half times (plotted logarithmically) vs. PET percent at 110°C...

Because of the closeness of the annealing temperature (150°C) to the temperature of maximum rate of crystallization, both PBT and PET show at this temperature their lowest crystallization half-times. Since the distance from the changing Tg is quite high, the changes in crystallization rate are not as strong as those observed at lower annealing temperatures. 

Crystallization at 90°C. The crystallization at 90°C is very slow and since the samples were not crystallized long enough to allow for a definite determination of the crystallization half-times, these have not been plotted. The plotted degrees of crystallinity for PBT and PET are those achieved after 30 minutes at the crystallization temperature. 

Overall Crystallization Behavior. If instead of plotting the crystallization half times vs. blend composition we plot the crystallization rates for each component in the blends vs. temperature of crystallization, we obtain a series of curves which show a maximum. The right side of these curves is nucleation controlled while the left side is diffusion controlled (Figures 18 and 19). 

In our case since we have a very fast crystallizing polymer (PBT), we wanted to see if there was any difference in the crystallization of the blends depending on how the study was performed. Two sets of studies were done, in which the samples were crystallized at 110°C in one of them from the glass and in the other from the melt. The crystallization half-times and ultimate crystallinities obtained for each component were plotted in Figures 24, 25, 26, and 27. 

Figure 25. PBT-crystallization half times vs. PBT percent. Samples crystallized from glass (X) or melt (0) at 110°C.

Figure 29. PBT-crystallization half times vs. time in the melt for the 50/50 blend crystallized at 90°C...

It is possible to observe how the intrinsic viscosity remains quite constant for up to five minutes in the melt, after which there is a sharp decrease. Similar results are observed with the crystallization half-times which remain constant for up to three minutes in the melt after that there is a sharp increase. 

DSK data demonstrate raise of PET crystallinity rate in the presence of nano-partieles. It is confirmed by ealeulations of crystallization half-time at melt freezing. Crystallization halftime (ti/2) for nanoeomposites is always lower than pristine PET. Nanoeomposite containing 5% of nalchikit-M has the lowest ti/2 (1,35 min) and its cristallinity rate (2,25 min) is 1,67 times higher than pristine PET. 

This equation deals with the temperature-dependence and crystallite-size- dependence of crystallinity. Frequently, the crystallization half-time, ti/2, is reported in the research data. The time to reach one half of the total crystallization is ti/ . The time to achieve maximum crystallization is T J°°). Figure 10.2 shows the relationship between ti/2 and temperature, T, for silica-filled PDMS. The value for is... 

The above observations are similar to those obtained for peroxide-crosslinked polyethylene. An addition of filler, such as silica, results in an increased crystallization rate and a decrease in the crystallization half-time, ti/2. ... 

Crystallization half-time as a function of crystallization temperature... 

Shingankuli [1990] studied the crystallization behavior of PP in the presence of solidified PVDF domains. A higher crystallization temperature of the PP matrix phase was observed, indicating an enhanced nucleation in the blends. The degree of crystallinity of PP was found to increase by about 30 to 40% with increasing PVDF content. Isothermal crystallization studies also confirmed the acceleration of the overall crystallization rate in terms of shorter crystallization half-times for PP. 

Nadkami and Jog [1986] have reported on PPS/HDPE blends. The degree of crystallinity of HDPE was reduced when HDPE was the minor phase. Lurthermore, the T shifted to somewhat lower temperatures (by about 5°C) but only in those blends with a low HDPE content. Isothermal crystallization half-times for HDPE in its blends with PPS decreased as the HDPE content decreased, indicating an enhanced nucleation from the solidified PPS interfaces. 

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