Crystal growth crystallization temperature

The growth of ECC under equilibrium conditions is too slow. Moreover, no macroscopic orientation appears and a structure of the type shown in Fig. 3 c is formed. Therefore this procedure cannot be used in practice. Usually, under real conditions, macroscopically oriented ECC are obtained from the melt stretched to the values of > /3cr at relatively low crystallization temperatures. Under these conditions, the formation of ECC proceeds by another mechanism. 

Primary crystallization occurs when chain segments from a molten polymer that is below its equilibrium melting temperature deposit themselves on the growing face of a crystallite or a nucleus. Primary crystal growth takes place in the "a and b directions, relative to the unit cell, as shown schematically in Fig. 7.8. Inevitably, either the a or b direction of growth is thermodynamically favored and lamellae tend to grow faster in one direction than the other. The crystallite thickness, i.e., the c dimension of the crystallite, remains constant for a given crystallization temperature. Crystallite thickness is proportional to the crystallization temperature. 

Figure 13 Top, plot of linear growth rates of polyfethylene adipate) spherulites as a function of crystallization temperature for indicated molecular weight fractions. Spherulites shown correspond to the indicated range of temperatures. (A) Crystallization at the lower temperature range (B) at intermediate temperatures (C) crystallization at high temperatures. Reproduced with permission from Ref. [216]. Copyright 1956,...

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. 

The initial melt of Ciooo was cooled down to various crystallization temperatures. Typical developments of crystalline domains at 370 K are shown in Fig. 37 here again the lamellae with the marked tapered shape are observed. Quite surprising is that the crystallization of Ciooo is rather fast in spite of the much longer chains. Any growth of lamellae of Ciooo at the temperature of 370 K, where Cioo did not show any appreciable growth, will be an indication of the molecular weight effect. 

Substantial evidence in a number of existing experimental studies can be easily reconciled with the models discussed in the present contribution. For example segregation of short chains reported during crystal growth [1] may be thought to arise with chains which are too short to form bundles and are thus unable to provide a sufficient amount of simultaneous attractive interactions with the crystal to yield stable adsorption. We recall in this respect that one of us obtained the correct trend of the minimum chain length of PE for crystal inclusion vs. the crystallization temperature, using the bundle approach [8]. 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

Conclusively, the calculated Avrami exponents reveal a three-dimensional growth of the crystalline regions for each blend. The rate of crystallization of each blend increased with the decrease in crystallization temperature, and the rate of crystallization of the (PHB80-PET20)/PEN blend was faster than that of the (PHB 80-PET20)/PET blend. 

The major drawbacks to standard sol-gel synthesis include slow growth rate and the typically amorphous product, rather than defined crystals, which requires crystallization and post annealing steps. Growth rate and crystallization of the fabricated hybrid can be improved via solvothermal, reflux [224], sonication, and microwave [225] treatment. However, the air oxidation of CNTs (600 °C) and graphene (450 °C) may still be lower than MO crystallization temperature. Moreover, it has been shown that the MO coatings on CNTs can drastically affect their thermal oxidation, particularly with easily reducible metal oxides (e.g. TiOz = 520 °C, Bi203 = 330 °C) [180]. It appears that metal oxides can catalyze the oxidation of CNTs via a Mars van Krevelen mechanism, limiting the maximum temperature of their synthesis as well as applications (i.e. catalysis, fuel cells). 

Montalenti, F., Sorensen, M.R., Voter, A.F. Closing the gap between experiment and theory crystal growth by temperature accelerated dynamics. Phys. Rev. Lett. 2001, 87, 126101-1 4. 

The expression for Gi nicely shows its nonmonotonic dependence on the crystallization temperature. Close to the equilibrium melting temperature, the growth is nucleation-dominated and is given essentially by Gi, . For temperatures far below T , but closer to Tq, the Gi d term dominates and the growth rate precipitously decreases with supercooling. 

The growth rate in the three regimes, at a crystallization temperature Tc can be written as (Gedde 1995 Lauritzen and Hoffman 1960)... 

Fig. 5.34 Schematic of the growth rate in the three regimes of the Lauritzen Hoffman theory. Here AT = 7 J, - T where J 2, is the equilibrium melting point and Tc is the crystallization temperature,...

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