Growth mechanism changes in pseudo-dewetted monolayer poly(ethylene oxide) crystallization

Crystal growth mechanism changes have been observed in pseudo-dewetted monolayers of low molecular weight (LMW) (PEO) on freshly cleaved hydrophilic mica surfaces [HPEO(4250) which have -OH groups at both ends and MHPEO(4700) which has one -OH and one -OCH$_{3}$ as end groups]. X-ray scattering reflectivity measurements show a wetted monolayer of molten PEO with a thickness of $\sim $ 4.5 nm on the mica surface. Non-adsorbed PEO droplets sit on top of the wetted monolayer. A two-step process for PEO single crystal growth under isothermal conditions was identified utilizing in-situ atomic force microscopy at different crystallization temperatures ($T_{x})$. In the first step, the crystal grows within the droplet which supplies the molten PEO that participates in the crystal formation. In this second-step, the wetted monolayer at the growth front is depleted by about 1.5 - 2.5 nm. The growing crystal lateral sizes obey a power law of $t^{\alpha }$ ($t$: time). At a high $T_{x}$ of 63 \r{ }C for MHPEO(4700), the growth behavior obeys \textit{r $\propto $ t} ($\alpha $ = 1). While in the case of HPEO(4250), its growth behavior follows \textit{r $\propto $ t}$^{0.5}$ ($\alpha $ = 0.5) in the whole $T_{x}$ range. With decreasing $T_{x}$, the growth of MHPEO(4700) falls into a scaling law of \textit{r $\propto $ t}$^{\alpha }$ (0.5 $< \quad \alpha \quad <$ 1).