Roughening Transition and Quasi-continuous Melting of Monolayers of Ultra-long Alkanes: Why Bulk Polymer Melting Is Strongly First-Order

RB Zhang and WS Fall and KW Hall and GA Gehring and XB Zeng and G Ungar, MACROMOLECULES, 54, 10135-10149 (2021).

DOI: 10.1021/acs.macromol.1c01377

Because non-polymers melt in a sharp, strongly first-order transition, it is taken for granted that semicrystalline polymers do the same. However, nearly continuous melting has been reported recently in monolayers of ultra-long n-alkane C390H782 adsorbed on graphite. Here we present similar observations in a series of other n-alkanes from C60H122 to C246H494 by atomic force microscopy, mean-field theory, and molecular dynamics simulations. Consistent with the previous report, with increasing temperature, in all n-alkanes the ordered lamellar fraction decreases continuously and reversibly from similar to 100% down to <50% before final melting up to , similar to 80 K above bulk melting point. In comparison, surface overcrowding prevents significant premelting in the bulk. An additional novel feature, not observed in the bulk, is a reversible phase transition from tilted to perpendicular chains on heating, attributed to roughening of the lamellar surface. This "untilting" transition happens above the bulk melting point and unlocks the continuous melting process.

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