Melt crystallization kinetics

Buchner, S., Wiswe, D. and Zachman, H. G., Kinetics of crystallization and melting behaviour of poly(ethylene naphthalene-2,6-dicarboxylate),... 

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

Even when the principles of interface reaction and diffusion are thought to be understood, the integrated results may still require major new work. For example, the growth rate of an individual crystal in an infinite melt can be predicted if parameters are known, but the growth rates of many crystals (and different minerals), i.e., the kinetics of crystallization of a magma, is not quantitatively understood. 

Kirkpatrick R.J., Robinson G.R., and Hays J.F. (1976) Kinetics of crystal growth from silicate melts. /. Geophys. Res. 81, 5715-5720. 

Zhang Y. (1988) Kinetics of Crystal Dissolution and Rock Melting a Theoretical and Experimental Study. Thesis, Columbia University, New York. 

Fig. 9. Melting kinetics and crystallization kinetics of polymeric selenium (right) and polyethylene (left). The equilibrium melting temperatures are 494.2 and 414. 6K. The dotted curve indicates that on crystallization of the macromolecule from small molecules Sc2 there is no molecular nucle-ation necessary as in the melt crystallization (see also ref. 43 for a more detailed discussion of Se crystallization and melting). Drawn after Ref. 4,)...

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

There is no comprehensive theory for crystallization in block copolymers that can account for the configuration of the polymer chain, i.e. extent of chain folding, whether tilted or oriented parallel or perpendicular to the lamellar interface. The self-consistent field theory that has been applied in a restricted model seems to be the most promising approach, if it is as successful for crystallizable block copolymers as it has been for block copolymer melts. The structure of crystallizable block copolymers and the kinetics of crystallization are the subject of Chapter 5. 

Attempts have been made with some success to produce other polymers that exhibit this property of natural rubber. Although the melting temperature can be matched by appropriately disrupting the crystallizable structure through controlled introduction of another monomer, an exact match is not possible because the extent of crystallinity and the kinetics of crystallization will differ. 

In Chapter 8, in the section on morphology, we have seen that polymers are only partially crystalline and when crystallized from solution or the melt they form chain-folded lamellar structures, as illustrated schematically in Figure 10-17 earlier. This is a consequence of the kinetics of crystallization, which we will explore here. 

Wu and Woo [26] compared the isothermal kinetics of sPS/aPS or sPS/PPE melt crystallized blends (T x = 320°C, tmax = 5 min, Tcj = 238-252°C) with those of neat sPS. Crystallization enthalpies, measured by DSC and fitted to the Avrami equation, provided the kinetic rate constant k and the exponent n. The n value found in pure sPS (2.8) points to a homogeneous nucleation and a three-dimensional pattern of the spherulite growth. In sPS/aPS (75 25 wt%) n is similar (2.7), but it decreases with increase in sPS content, whereas in sPS/PPE n is much lower (2.2) and independent of composition. As the shape of spherul-ites does not change with composition, the decrease in n suggests that the addition of aPS or PPE to sPS makes the nucleation mechanism of the latter more heterogeneous. 

The kinetics of crystallization of polyethylene-naphthalene-2,6-dicarboxylate (PEN) and of copolyesters of this material with p-hydroxybenzoic acid (PHB) was studied by Wiswe, Gehrke, and Zachmann. PEN crystallizes in two different crystal modifications. One modification is obtained by crystallization at comparatively low temperatures, the other one is obtained sometimes when the material is crystallized near the melting point. Fig. 55 shows the change in the wide angle scattering during isothermal crystallization at 167 °C and 245 °C. 

The kinetics of crystallization of PE-containing diblocks have been studied using simultaneous SAXS and WAXS [9,11]. The development of PE crystallites (with an enhanced scattering contrast compared with the initial melt state) was followed via the SAXS invariant using Eq. (2). For PE-PEE diblocks (and PE... 

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