Cold crystallization isothermal

Liao, R., Yang, B., Yu, W. and Zhou, C. (2007) Isothermal cold crystallization kinetics of polylac-tide/nucleating agents. Journal of Applied Polymer Science, 104,310-317. 

The three-phase composition of PLA after isothermal cold-crystallization at different temperatures is reported in Figure 5.4 [45]. The plot also displays the estimated fraction of the a-modification [44]. The data show that the crystallinity increases with increasing crystallization temperature, with a discontinuity around 110-120 "C, discussed below. The MAF decreases with increasing crystallization temperature, whereas the RAF displays a nonmonotonous trend [45]. The early decrease in the RAF, at temperatures lower than 100 °C, is interpreted by presence of defective and constrained a -crystals, while with increasing crystallization temperature, more perfect, less constrained a -crystals develop. As a consequence, the RAF content reduces. For crystallization temperatures higher than 100 "C,... 

Figure 5.4 Crystalline, mobile, and rigid amorphous fractions (bottom) and fraction of a-crystals on total crystalline fraction of PLA (top) after isothermal cold-crystallization at various temperatures. Adapted from...

Regarding the pathway of nudeation on the crystallization rate, it has been shown for PLA with 4.25% D-unit content that isothermal cold-crystallization is faster than melt-crystallization at identical temperature [53]. The half-time... 

Wu, L. and Hou, H. (2010) Isothermal cold crystallization and melting behaviors of poly(L-lactic acid)s prepared by melt polycondensation. J. Appl. Polym. Sci., 115, 702-708. 

Additionally, Quero et al studied the isothermal cold-crystallization of PLA/PBAT blends with and without acetyl tributyl citrate (ATBC). SEM results showed that the blends exhibited two phases, but partial miscibility changes in both, Tg and Tn, of the PLA phase, a relationship of the spherulitic growth rate with the blend eomposition and the occlusion of PBAT droplets within PLA spherulites. ATBC acted like a plasticizer for both phases (neat PLA and PBAT). In the ease of blends, ATBC prefers to be included inside the PBAT-rieh phase. There was a synergistic effect on the overall crystallization rate of PLA when both ATBC and PBAT were present in the blend. 

Figure 21 Tg2 vs. T max for the PEEK samples isothermally cold-crystallized at low temperature (153 C, 3.0 h) and progressively reheated stepwise to increasingly higher temperature, rcmax- Filled sq uares correspond to the initially cold-ctystallized sample and to Tcmax of 170,176,186,194,205,227,257,271, and 305 °C. The lines are guides to the eye. With permission from Ivanov, D. A. Jonas, A. M. Legras, R. PolymerZm, 41,3719. ...

Isothermal Cold Crystallization. When a polymer sample is quenched, and its temperature is raised, isothermal cold crystallization can also be carried out. This is not a very popular technique, but it is useful when the rate constant (K) of crystallization from Eqs. (2.65)-(2.67) needs to be determined in a wide temperature range. 

Vasanthan, N., Ly, H., Ghosh, S., 2011. Impact of nanoclay on isothermal cold crystallization kinetics and polymorphism of poly(L-lactide acid) nanocomposites. J. Phys. Chem. B 115, 9556-9563. 

Wellen, R.M.R., Rabello, M.S., 2005. The kinetics of isothermal cold crystallization and tensile properties of poly(ethylene terephthal-ate). J. Mater. Sci. 40, 6099—6104. 

Liu, T., and Petremann, J., 2001, Multiple melting behavior in isothermally cold-crystallized isotactic polystyrene. Polymer Al 6453-6461. 

The cold crystallization starts at about 400 K at a supercooling not affected by modulation and registers as nonreversing. For separation of such nonreversing transitions, several modulation periods must occur across the transition, otherwise the pseudo-isothermal analysis would not develop the proper sinusoidal oscillations about , as can be seen from the modeling in Figs. 4.100-102 (loss of stationarity). 

The reversing heat capacity and the total heat-flow rate of an initially amorphous poly(3-hydroxybutyrate), PHB, are illustrated in Fig. 6.18 [21]. The quasi-isothermal study of the development of the crystallinity was made at 296 K, within the cold-crystallization range. The reversing specific heat capacity gives a measure of the crystallization kinetics by showing the drop of the heat capacity from the supercooled melt to the value of the solid as a function of time, while the total heat-Uow rate is a direct measure of the evolution of the latent heat of crystallization. From the heat of fusion, one expects a crystallinity of 64%, the total amount of solid material, however, when estimated from the specific heat capacity of PHB using the ATHAS Data Bank of Appendix 1, is 88%, an indication of a rigid-amorphous fraction of 24%. 

Quasi-isothermal TMDMA data for PEN are shown in Fig. 6.51 for slow cold crystallization at 418 K [42]. The method consists of dynamic mechanical analysis, DMA (see Sect. 4.5.4), to which temperature modulation was added. The insert is a... 

The slow, irreversible cold crystallization is followed in Fig. 6.53 for more than 10 days with quasi-isothermal TMDSC to a fixed value of RAF. At the end of the crystallization there is no frequency dependence of the heat-capacity. The crystallization and the glass transition to the RAF occur simultaneously (see also Fig. 6.18). 

The cold crystallization of PET was analyzed by running the series of quasi-isothermal experiments at 388 K and evaluated as shown in Fig. A.13.3. The reversing Cp ( ) decreases, as expected from the lower Cp of the crystallized sample. The same decrease can be calculated from the integral of the total heat-flow rate over... 

Another form of isothermal crystallization is observed with polymers which exhibit cold crystallization. In this case the sample is heated from the glassy state at the fastest rate possible to a temperature in the vicinity of the cold crystallization exotherm. The zero time is determined using the conventions cited above. 

Relaxation profiles obtained as loss peak in imaginary dielectric curve and as step jump of real permittivity line Partial miscibility might exhibit multiple relaxation peaks overlaying or towards individual components, or display diverse relaxation rate even >T, Non-isothermal crystallization by isochronal temperature sweep DSC (cold crystallization and melting discernible as sudden drop and steep rise of static dielectric permittivity, respectively) Crystallization onset predicted using DRS relaxation time in coupUng model faster than the experimental time... 

It is also important to note that samples undergoing a first-order phase transition (melting, cold crystallization, or crystal-crystal transition) during a ramp heating will not be at the same temperature as the oven, which continues to increase in temperature throughout the transition, while the sample remains isothermal until the transition is completed. Another case where the sample and oven temperatures may vary is for thermosets undergoing cure. Because the cure reaction is exothermic, the sample temperature may exceed the oven temperature. 

Wang et al. [33] studied the spherulitic growth rates and the microstructure of SPS cold-crystallized isothermally at various temperatures in the range 115-240 °C, by small-angle light scattering (SALS), optical microscopy, and transmission electron microscopy (TEM Fig. 9.2). 

Figure 9.2 Light scattering patterns and phase contrast micrographs of SPS cold-crystallized at various isothermal crystallization. Reprinted from Wang et al. [33], with permission from Elsevier.

Andrade et al. investigated the transport properties and the solvent induced-crystallization phenomena in poly(ethylene terephthalate) (PET) and PET clay nanocomposites, prepared by melt intercalation. Results of non-isothermal crystallization showed that cold crystallization temperature, and percent of crystallinity of nanocomposites are higher than those of pure PET. The sorption of all die solvents is accompanied by a large-scale stmctural rearrangement, leading to the induced crystallization of the original amorphous state. The solvent induced crystallization caused an increasing of more than four times the percent of crystallinity. 

The second route toward increased crystalline level is the addition of a plasticizer. This enhances the polymer chain mobility and thus increases the crystallite growth rate. In the context of a non-isothermal crystallization, plasticization also plays a role by decreasing the Tg thus widening the crystallization temperature window [8]. Plasticizers such as Citrate esters, triacetine and poly(ethylene glycol) have been used in PLA as a mean to increase its ductility. It has been reported that addition of plasticizers also decreased the cold crystallization temperature [9]. 

In calorimetry, the cold crystallization enthalpy on heating or at isothermal conditions was found to be dependent on the number of previously formed nuclei.This was used to study isothermal nucieation and crystallization in PCL, covering times from 10" to 10 s and temperatures from about 25 K below the glass transition up to 330 K, which is close to the equilibrium melting. With the fast scanning calorimeter, it was possible to follow the development of crystals at one temperature over 9 orders of magnitude in time. ... 

To study the kinetics of nudeation under isothermal conditions, experiments were performed for which the sample was cooled so fast (10 000 Ks" ) that no homogeneous nudeation occurs. If a sample were amorphous after annealing, it would show zero overall latent heat, but the cold crystallization enthalpy as well as its melting enthalpy would depend on the number of active nuclei in the sample. 

From these curves, the overall crystallization kinetics and the nudeation kinetics were deduced and are summarized in the activation diagram in Figure 37. ° In most measured curves shown in Figure 36, two contributions - one for the mdting of isothermally formed crystals and the other for the cold crystallization or reorganization - are separated suffi-dently for analysis. The presence of a nudeation -induced increase in cold crystallization is visible up to 290 K (black line with stars). For temperatures up to 230K, the nudeation effect of cold crystallization saturates at times before major crystallization at the annealing temperature is seen in the total latent heat. 

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