Nucleation molecular

Naturally, one can also propose a tertiary nucleation step for starting growth on a smooth ledge as seen at position 2 in Fig. 3.68. Its nucleation barrier would be even smaller than for secondary nucleation and even less likely to slow the crystal growth significantly. 

The concept of molecular nucleation was developed to explain some observations not in agreement with the above model of crystallization limited by secondary nucleation [29]. In Fig. 3.73 the molecular length is plotted which is rejected by a crystal growing from the melt or from solution (curves 1 and 2, respectively). These data were obtained by measuring the molar mass of the noncrystaUized molecules of a 

This concludes the discussion of the nucleation dynamics of macromolecules. It shows that the usually assumed constant number of heterogeneous nuclei and linearly increasing number of homogeneous nuclei is a simplification, and secondary nucleation as the basis for crystal growth is a doubtful concept. Finally, molecular nucleation is not a well enough understood concept to quantitatively explain facts such as the molar mass dependence of crystallization. The further work needed to understand the basis of nucleation in polymers is a big challenge for new research in solid-state polymer science. 

Based on data from polyethylene, selenium. poly(oxymethylene). and poly(oxyethylene) of different molar mass 

This short introduction to possible paths and mechanisms of melting and crystallization indicates how difficult it is to develop a detailed kinetics that links the molecular mechanism to the observed, macroscopic kinetics. Considerable molecular scale information is needed for the interpretation of the macroscopic thermal analysis data. Few data on melting rates are available for polymers, due to the tendency of large crystals to superheat [31], and small crystals to melt very fast. 

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Note, however, that a) the size of a stable nucleus depends on the model used (see Sect. 3.5.1), and b) molecular nucleation theory (Sect. 3.8.1) rules out such a mechanism. 

First we consider the theory initiated by Wunderlich and Mehta [145] which they termed molecular nucleation . The we turn to recent work by Hikosaka [146, 147] who introduced the idea of sliding diffusion . 

Note Molecular nucleation may give rise to a new crystal or increase the size of a pre-existing one. 

The lower limit of molecular length which needs molecular nucleation on melt crystallization was explored for paraffins and polyethylenes by DSC and TMDSC described in Sects. 4.3 and 4.4. Figure 3.75 illustrates the supercooling necessary for measurable crystallization. The broader molecular mass fractions of polyethylene are marked by labels like PEI 150, where the number gives the average molar mass in Da. A critical length of about 75 chain atoms can be derived for polyethylene. 

Data on the supercooling of aliphatic polyoxides in Fig. 3.91 can be compared to the paraffin and polyethylene data used for the illusdation of the limits of molecular nucleation in Fig. 3.75. In the same range of chain length all three polymers, polyethylene, PE, poly(oxyethylene), POE, and POTM, decrease in amount of supercooling due to molecular nucleation. The polyoxides, however, show a second... 

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