Crystallization, kinetics for

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

FIGURE 3.54 Crystallization kinetics for polypropylene fibers spun at high (sample A) and low (sample B) spin-line stress levels. (From Jaffe, M. In Thermal Methods in Polymer Analysis, Shalaby, S.W. ed., Franklin Institute Press, Philadelphia, 1977, p. 93. With permission.)... 

A related problem of interest is when a polymer is not crystalline as prepared, but is potentially crystallizable. This situation is commonly encountered in crys-tallizable copolymers, and is also found in homopolymers. Some typical examples of this phenomenon are found in poly(styrene) synthesized by means of alfin type catalysts,(50) poly(methyl methacrylate), prepared by either free-radical or ionic methods,(39,51,52) and poly(carhonate).(53) Treatment with particular solvents or diluent at elevated temperatures can induce crystallinity in these polymers. The reason for the problem is kinetic restraints to the crystallization process. Treatment with appropriate diluents alleviates the problem. The principles involved, and the diluent requirements will he enunciated in the discussion of crystallization kinetics. For present purposes it should be recognized that the crystallizability of a polymer, particularly a copolymer, cannot be categorically denied unless the optimum conditions for crystallization have been investigated. Thus, in light of the previous discussion regarding the minimum concentration of chain units required for crystallization, and the need to have favorable kinetic conditions, the lack of crystallization in any given situation needs to be carefully assessed. 

Hwang SY, Lee WD, Lim JS, Park KH, Im SS. Dispersibility of clay and crystallization kinetics for in situ polymerized PET/pristine and modified montmoriUonite nanocomposites. J Polym Sci B 2008 46 1022-1035. 

Non-isothermal crystallization kinetics for nylon-6 and nanocomposite samples were studied. The onset of the crystallization, crystallization temperature and the heat of the crystallization were noted during the cooling cycle. Using Universal Analysis software, running integration function was performed for crystallization peak in heat flow (J/g) vs. time (minutes) plot to get the values of relative crystallinity X, for different values of time t. The time required for the sample to crystallize 50% ti/2) was noted for the nylon-6 and nanocomposite samples. Then the chart of In [-In (1 - Xj)] vs. In t was plotted. The Avrami equation was used to determine values of n and k from the slopes and the interception of the best fit (Equation 14.4). 

Overall crystallization kinetics can be described experimentally by the ratio Q i)IQ of the heat Q t) liberated between the onset of crystallization and time t to the total heat of crystallization <2- (see Fig. 15.4). To a first approximation, this ratio can be assimilated to the volume fraction a transformed into spherulites, and described by the laws for overall crystallization kinetics, for example, Ozawa s equation (see Section 15.3.2). Logically, the a T) curves are shifted to lower temperatures when the cooling rate increases. Until recently, classical calorimetry allowed us to determine the kinetic law only at low or moderate cooling rates, typically from 0.125 to 40°C/min. This raised questions when these data were used to model crystallization in processes where the cooling rates were much higher. As a result... 

Sakurai K., MacKnight W. J., Lohse D. J., Schulz D. N., and Sissano J. A. (1994) Blends of amorphous-crystalline block copolymers with amorphous homopolymers. 2. Synthesis and characterization of poly(ethylene-propylene) diblock copolymer and crystallization kinetics for the blend with atactic polypropylene. Macromolecules 27 4941-4951. 

Application of the kinetic analysis to the obtained fractional crystallinity is presented in Figure 2. The BaTi03 reaction kinetics from the amorphous gel exhibit a single-stage rate law where a linear regression yields m = 1.09. An m value equal to 1.09 implies a phase-boundary controlled mechanism where the initial crystallization is controlled by an interfacial chemical reaction (79). Conversely, the crystallization kinetics for BaTi03... 

Figure 32 Crystallization kinetics for the poly(ethylene oxide) block in triblock terpolymers with a rubbery end biock (poiybutadiene-Wocfr-polystyrene-b/oc/f-poly(ethylene oxide)) or a crystalline end block (polyethylene-b/oc/f-polystyrene-Wock -poiy(ethylene oxide)) (a) deveiopment of the relative crystallinity with crystallization time during isothermal crystallization at 49.5 °C, and (b) inverse of experimentai crystaiiization haif-time as a function of crystallization temperature. Reprinted with permission from Boschetti-de-Fierro, A. etal. Macromol. Chem. Phys. 2008,209,476- 87. ...

Analysis of the crystallization kinetics for polymers subject to either biaxial or shear deformation is hampered by the lack of knowledge of the equilibrium melting temperatures for these situations. This is true for both theoretical and experimental attempts to obtain this quantity. In addition, there is also a paucity of appropriate experimental data to guide efforts in obtaining these melting temperatures. 

In Figure 4 the half-time (xi/2 ) for Pebax 7033 composites crystalhzation at different crystallization temperatures is shown as derived from the crystallization kinetics for the composite materials presented in Table 2. Plotting half-time crystallization as a function of crystalhzation temperatiu e is a method for assessing the time required for crystal growth conversion. It is seen that the PPTA AS fiber composites had a decrease in the half-time for the crystallization from that of the virgin Pebax. This trend is attributed to a nucleating effect of the aramid fibers on the Pebax 7033 crystallization. It was found from the optical microscopy transcrystahization studies, that the largest amoimt of transcrystalhzation occmred for the succinyl chloride treated PPTA Pebax 7033 composites. The half time of... 

Figure 21, DSC and chip-based calorimetry data coincide, indicating similar crystallization kinetics for the milligram-sized DSC and the nanogram-sized chip-based calorimetry samples. For PBT, this is not the case (see Figure 22).

---