Recrystallization rates

Homopolymerization of macroazoinimers and co-polymerization of macroinimers with a vinyl monomer yield crosslinked polyethyleneglycol or polyethyleneglycol-vinyl polymer-crosslinked block copolymer, respectively. The homopolymers and block copolymers having PEG units with molecular weights of 1000 and 1500 still showed crystallinity of the PEG units in the network structure [48] and the second heating thermograms of polymers having PEG-1000 and PEG-1500 units showed that the recrystallization rates were very fast (Fig. 3). 

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

A method based on quantitative phase analysis and the instantaneous nucleation model has been developed to quantify form II nuclei in both GSX and MSX samples (50). The method involves measurement of the recrystallization rate of form II from the form I melt in DSC at different scanning speeds, and analysis of the data (expressed as a-time curves) by appropriate kinetic models, as explained next. 

Donhowe and Hartel (1996) also compared recrystallization rates of ice crystals in ice cream at different amplitudes of temperature fluctuations. Above temperature fluctuations of 0.50 °C, significantly faster recrystallization occurred, as compared to constant temperature conditions. Recrystallization rates were about twice as fast with temperature fluctuations of 1.00°C compared to 0.010 °C. The shape of the crystals after recrystallization at fluctuating temperatures was more jagged and uneven, whereas recrystallization at constant temperature produced a rounding effect. This was evidence that melt-refreeze and accretion were the main mechanisms of recrystallization under fluctuating temperatures. 

FIGURE 3.55 (a) Time to peak and (b) maximum recrystallization rate versus maximum temperature in melt for polypropylene fibers and films. (From Jaffe, M. In Thermal Methods in Polymer Analysis, Shalaby, S.W. ed., Franklin Institute Press, Philadelphia, 1977, p. 93. With permission.)... 

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