Phase selection pathways in ultrathin film crystallization of a low molecular weight poly(ethylene oxide) fraction on mica surfaces

Abstract

Utilizing in situ atomic force microscopy, we monitored the phase selection pathways of ultrathin film crystallization of a low molecular weight poly(ethylene oxide) fraction with two hydroxyl end groups and a number-average molecular weight of 3000 g/mol (HPEO3k) on mica surfaces. The sample forms integral folded chain (IF) monolayer crystals. From a thermodynamic point of view, in the vicinity of the melting temperature [Tm(1)] of the once-folded chain crystal [IF(1)], the system studied provides a three-phase model composed of the melt phase, the stable phase of extended chain crystals [IF(0)], and the metastable phase of IF(1). Four phase selection pathways, namely, melt → IF(0) (S0), melt → IF(1) (S1), IF(1) → IF(0) (S10), and a composite pathway built of the latter two (S2) have been revealed experimentally. The first selected pathway at crystallization temperatures near Tm(1) depends on the supercooling, in agreement with the predication of Gránásy-Oxtoby theory. Below a bifurcation point located slightly lower than Tm(1), the pathway selection of isothermal crystallization is also time dependent, wherein a coexistence of S1, S10, and S0 at the late stage of crystallization is observed. This phenomenon is beyond expectation and may be related to the crystal growth mechanism switching from nucleation-limited to diffusion-limited. © 2011 American Chemical Society.