Nonisothermal melt crystallization

Nonisothermal crystallization Guerra et al. [ 16] reported on sPS/PPE blends of various compositions, prepared by compression molding (T ax = 290°C and Wx = 10 min) and then cooled to room temperature at a rate of 10°C/min. Their DSC results show that, below 50wt%, sPS is completely amorphous. Moreover, WAXD spectra of the same samples indicate that the amount of the a form, which is 100wt% in neat sPS, decreases on increasing PPE and rmax (Figure 20.1a and b) in favor of the (3 form. The authors reported that the loss of the memory of the a form, which they suggested for melt-crystallized neat sPS, is more rapid for the same temperature and time when PPE is present [16]. 

Xu C, Qiu Z (2009) Nonisothermal melt crystallization and subsequent melting behavior of biodegradable poiy(hydroxybutyrate)/ multiwalled carbon nanotubes nanocomposites. J Pol) Sci, Part B Polym Phys 47 2238-2246... 

Thus, in this section we describe melt crystallization, in which the crystal formation takes place from the polymer melt. Melt crystallization of polymers can be subdivided into (1) isothermal crystallization, a crystallization process that takes place at a definite, constant temperature and (2) nonisothermal melt... 

Nonisothermal Melt Crystallization. In routine DSC crystallization measurements of polymers, nonisothermal crystallization is typically used.This method is simple and gives practical results. At the same time, the information obtained is limited to the characteristic temperatures of crystallization. The sample is heated beyond the equilibrium melting point of the polymer, held there for several minutes for temperature equilibration, and also to destroy all nuclei, and then it is cooled at a specific constant rate to room temperature or below to record the crystallization exotherm. 

Figure 2.42. Melting and nonisothermal crystallization curve of poly(butylene tere-phthalate) at a heating and cooling rates of 10°C/min (sample from Scientific Polymeric Products Polymer Kit). Considerable supercooling r = 226.5°C, = 2(X).0°C.

Cai, J., Han, Y. Morphology, structure, and kinetic analysis of nonisothermal cold- and melt-crystallization of syndiotactic polystyrene. J. Appl. Polym. Sci, 103,1311-1324 (2007). 

Figure 10.11 SEM micrographs of etched samples of ethylene-propylene copolymer (EPS 3.0 mol-% ethylene), propylene-(l-butene) copolymer (BPS 7.6 mol-% 1-butene), and their blends crystallized nonisothermally from the melt, (a) BPS (b) EPS (c) BPS/EPS 75/25 (d) BPS/EPS 50/50. Reprinted from Bartczak et al. [66], Copyright 2004, with permission from Elsevier.

Even though the nonisothermal crystallization leads to just small changes in the subsequent melting behavior of different types of triblock copolymers, isothermal experiments employed to calculate the equilibrium melting temperature, T, have shown that this parameter can exhibit significant changes depending on composition. It has been reported that in PS-fc-PB-fc-PCL tri-... 

The present study has shown that low crystallinity ePP crystallizes from the melt into well-defined morphologies. This smdy presented definitive evidence that this class of materials, when crystallized isothermally from the melt, exhibits morphologies that are reminiscent of classical semicrystalline polymers. The presence of lamellae, crosshatching, and spherulites was revealed by high resolution tapping mode AFM and optical microscopy. The nonisothermally crystallized ePP specimens also display the hierarchical ordering as seen in the case of iPP. 

Jinan, C. et al. (1989). Nonisothermal orientation-induced crystallization in melt spinning of polypropylene. J. Appl.Polym. Sci, Vol. 37, Issue 9, pp. 2683-2697. 

Run M, Song A, Wang Y, Yao C (2007) Melting, crysttillization behaviors, and nonisothermal crystallization kinetics of PET/PTT/PBT ternary blends. J Appl Polym Sci 104 3459-3468 Salaneck WR (1997) Conjugated polymer surfaces and interfaces. Philos Trans R Soc Lond A... 

Presence of TCL changes the properties of crystalline matrix. Transcrystallization of isotactic polypropylene in the presence of different fibers has been thoroughly analyzed. Gray as the first one provided detailed description of isotactic polypropylene behavior in the presence of wood fibers using polarized light microscopy. He observed that when melted polymer is cooled down, it crystallizes in spherulite forms in nonisothermal and isothermal conditions, creating additionally a TCL. 

The overall crystallization rate is used to follow the course of solidification of iPP. Differential scanning calorimetry (DSC), dilatometry, dynamic X-ray diffraction and light depolarization microscopy are then the most useful methods. The overall crystallization rate depends on the nucleation rate, 1(0 and the growth rate of spherulites, G(0. The probabilistic approach to the description of spherulite patterns provides a convenient tool for the description of the conversion of melt to spherulites. The conversion of melt to spherulites in the most general case of nonisothermal crystallization is described by the Avrami equation ... 

Recently the statistical approach was developed [5] for the description of the kinetics of conversion of melt to spherulites and the kinetics of formation of spherulitic pattern during both isothermal and nonisothermal crystallizations. The final spherulitic pattern can also be described. The rates of formation of spherulitic interiors and boundaries (boundary lines, surfaces and points) as well as the their final amounts could be predicted if spherulite growth and nucleation rates are known. Applied to iPP crystallized during cooling with various rates, the approach allowed for the predictions of tendencies in the kinetics of formation of spherulitic structure and its final form. 

It is well known that the extent to which isothermal crystallization in polymers occurs can be profoundly influenced by molecular orientation. Recently Nakamura et al. (275) have performed a detailed theoretical analysis of the relationships between crystallization temperature, crystallinity, orientation and cooling conditions for nonisothermal processes. These investigators have also evaluated the crystallization rates under molecular orientation during melt spinning experiments and... 

If the ordinate of Fig. 12-21 is taken to be a temperature-dependent crystallization rate constant then, for a nonisothermal case (as in melt spinning), the degree of crystallinity v is approximately as shown below ... 

By measuring the fraction of crystallized material, with time t, a value of the constant n equal to 4 was evaluated by Miyata and Masuko [37] who observed, in nonisothermal crystallization experiments from the melt, a remarkable increase in the polymer crystallization at decreasing cooling rates and a maximum value of the isothermal crystallization rate at 105°C. A similar trend was observed by lannace and Nicolais [36]. However, these results could be affected by the polymer thermal degradation induced at a low cooling rates, as previously described by Migliaresi et al. [27]. 

---