Crystallization kinetics free-growth

The lateral surface free energy a is a key parameter in polymer crystallization, and is normally derived from crystallization kinetics. In polydisperse polymers, where the supercooling dependence of growth rate is affected both by changing... 

An alternative possibility arises from considerations related to the development of crystalline structure in polyethylene [22], The main feature of this structure is the periodic folding of the polymer chains in the crystal. Theoretically this is explained within the context of the kinetics of crystal nucleation and growth from solution. According to Cormia, Price, and Turnbull [22], the fold-surface energy in polyethylene crystals is comparable to the end-interfacial energy of rodshaped nuclei. These surface free energies are of the order of 10 to... 

Figure 11-2 shows the kinetics of crystallization in the presence of a P40 in an Al-free reaction mixture. To follow the kinetics, crystallization was carried out in a 5 liter-autoclave that could be probed directly. It is seen that crystallization is accelerated with increasing temperature, when compared to syntheses listed in Tables 11-2 to 11-4. Furthermore, this experiment emphasized the metastable character of the new layer silicates. Quartz instead of cristobalite is observed as the final product of the crystallization sequences in the Al-free cases. Crystallization starts at 4 hours and is completed after 9-10 hours. Quartz appears as a product component after 11-12 hours. The growth of quartz ceases after 40-44 hours. From the gradient of the crystallization curve, a growth rate of 15% per hour is determined for this novel silicate under these conditions. 

The kinetics of metal deposition on crystal faces free from screw dislocations is determined by the rate of nucleation /, nuclei cm sec of new lattice nets and their rate of propagation v, cm sec over the face. Almost simultaneously Nielsen/ Chernov/ and Hillig " showed that, depending on the values of these two parameters, two different mechanisms can be distinguished, whereby the extension of the surface plays a significant role (i) a layer by layer growth and (ii) a multinuclear multilayer growth. 

The process of crystallization proceeds via two distinct processes crystal nucleation and growth (Garside, 1985). The nucleation kinetics in fine droplets is often different from nucleation in the same liqnid in bulk. In a fine emulsion, the number of droplets exceeds the number of potential nncleation catalysts (impurities) present in the liquid oil. Thus, a proportion of the lipid is effectively catalyst free and must nucleate by other mechanisms. This may either be completely spontaneous homogeneous nucleation or, more probably, some catalytic effect of the droplet surface (Coupland, 2002). In either case, the crystallization temperature is greatly reduced below the melting point and depends both on particle size and on the nature of the emulsifier selected. For example, Higami and co-workers (2003) showed that the crystallization temperature of trilaurin molecules decreased from 18.9 °C, the crystallization temperature of the bulk lipid, to -9.5 °C when emulsified into droplets smaller than 100 nm. 

Figure 10.8 Variation of the surface free energy of folding, Oe with the volume fraction of crystalline component for PEO-lOOK/PMMA blends. Lower curve data from spherulite growth rate (Eq. 10.13) upper curve data from overall crystallization kinetics (Eq. 10.17). Reprinted from Martuscelli et al. [37], Copyright 1984, with permission from Elsevier.

Large variations of the thermodynamic parameters, such as surface free energy of crystals and equilibrium melting temperature, and the overall crystallization kinetics (depending on the combined effect of primary nucleation and growth) are generally observed for mis-... 

The profound influence of molecular weight on the crystallization kinetics gives a clue as to a possible reason for the adequacy of the free-growth concept at the early stages and the deviation from the expectations from the Avrami theory. Initially, prior to the onset of crystallization, the melt of a polymer is composed of entangled chains, loops, and knots, as well as other structures that can be considered to be... 

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