Kinetic mechanism of chain folding in polymer crystallization

I develop a kinetic mechanism to explain chain folding in polymer crystallization which is based on the competition between the formation of stems, which is due to frequent occupations of trans states along the chains in the supercooled polymer melt, and the random coil structure of the polymer chains. Setting equal the average formation time of stems of length $d_l$ with the Rouse time of a piece of polymer of the same arc length $d_l$ yields a lower bound for the thickness of stems and bundles. The estimated lamellar thickness is inversely proportional to the supercooling. The present approach emphasizes the importance of repulsive interactions in polymer crystallization, which are expected to be responsible for the logarithmic lamellar thickening and the increase of lamellar thickness with pressure. An expression for the growth rate for formation and deposition of stems is derived by considering the growth as a dynamic multistage process.

Figure 1

Visualization of fluctuating stems along the polymer coil. The two first and two last stems are likely to constitute longer stems, while the third stem (from the left) is likely to build a fold with the second stem.

Figure 2

Open circles: experimental data for poly($\epsilon$ -caprolactone) from [54]. Dashes: Our fit of the crystallization line. Filled circles and the continuous line: the melting line.