Crystallization from the Melt State

By melt crystallization, the a form can be obtained pure or mixed with the P form, depending on the crystallization conditions. Many studies [18-22] have focused on the following factors influencing the polymorphic behavior in samples crystallized on cooling from melt  

The results showed that the a crystal packing can be favored, or becomes an alternative route in SPS crystallization under three conditions  

By comparison, melt crystallization at most accessible temperatures produces solidified SPS containing both a-type and P-type crystals of various fractions, although higher temperatures tend to favor larger fractions of P-type crystals [23]. 

The p-type crystals become the only discernible species if SPS is melt-crystallized at temperatures equal to or higher than 260 °C, suggesting that in conditions approaching equilibrium the p-crystals lamellae are the favored packing. 

By pressure annealing of the a form, the more densely packed p form is obtained [24], while the application of pressure on samples in the a form at temperatures below Tg results in a loss of the three-dimensional crystalline order [25]. 

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FIGURE 19.2 Polyethylene lamella crystallized from the melt state showing some regular chain folds, loose loops, and tie molecules (switchboard model). Image courtesy of Eric Wysocki, Exponent, Inc. 

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

As a first example of applying the techniques described in section 2 let us look at the chain motion of linear polyethylene (LPE). A detailed study of a perdeuterated sample, isothermally crystallized from the melt, has been carried out in our laboratory24,25,44). Since all of this work is published and, in fact, has been reviewed extensively17 we can restrict ourselves to stating the main conclusions here ... 

Finally, we were led to the last stage of research where we treated the crystallization from the melt in multiple chain systems [22-24]. In most cases, we considered relatively short chains made of 100 beads they were designed to be mobile and slightly stiff to accelerate crystallization. We could then observe the steady-state growth of chain-folded lamellae, and we discussed the growth rate vs. crystallization temperature. We also examined the molecular trajectories at the growth front. In addition, we also studied the spontaneous formation of fiber structures from an oriented amorphous state [25]. In this chapter of the book, we review our researches, which have been performed over the last seven years. We want to emphasize the potential power of the molecular simulation in the studies of polymer crystallization. 

Fig. 5 Difference intensity SAXS curves of PET after subtraction of the intensity of the melt-quench sample crystallized from the glassy state at 80 C for 3-122 min (a) and 157-313 min (b) [7]...

Fig. 25 Annealing time evolution of the difference SAXS intensity in the induction period (a) and the crystallization period (b) for the melt crystallization of PET at 244 °C [18]. This system corresponds to crystallization from the metastable state where a nucle-ation and growth type of primary phase separation first occurs followed by the spinodal decomposition type secondary phase separation...

Random copolymers of VF2/F3E when crystallized from the molten state above the Curie temperature show a microstructure in the form of very thin needle-like morphological units which are probably semicrystalline. Figure 5a illustrates the needle-like microstructure of the copolymer 80/20 melt crystallized in the paraelectric phase observed at 140 °C. After codling at room temperature the microstructure of the ferroelectric crystals is such that what appear in the optical microscope as radial fibers are, in fact, stacks of thin platelet-like morphological units (see Fig. 5b). 

The process of crystallization from solution is generally understood in terms of three interdependent conditions supersaturation, nucleation and growth. For crystallization from the melt or the gas phase, supersaturation is simply interpreted as undercooling. Of the essence is a metastable phase, at chemical potential higher than that of the crystalline phase under the same conditions. The transition from the metastable state to the state of equilibrium between phases corresponds to nucleation. 

Many polymers have the capability to crystallize. This capability basically depends on the structure and regularity of the chains and on the interactions between them. The term sernicrystalline state should be used rather than crystalline state, because regions in which the chains or part of them have an ordered and regular spatial arrangement coexist with disordered regions typical of the amorphous state. X-ray diffraction studies of samples of polymers crystallized from the melt reveal diffuse zones, char-... 

In conclusion, TAG crystallization from the melt involves nucleation mainly by two mechanisms (i.e., local-order of TAG in the liquid state and sporadic nude-... 

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