Crystal Growth Mechanism Changes in Pseudo-Dewetted Poly(ethylene oxide) Thin Layers

Changes in the crystal growth mechanism were observed in pseudo-dewetted, thin layers of low molecular weight (MW) poly(ethylene oxide) (PEO) on a hydrophilic mica surface. The studies were conducted using two PEO fractions (HPEO and MHPEO) with similar MW's (number-average MW of around 4500 g/mol) but different end-group chemistries. For the HPEO, both ends of the chain are capped by −OH groups. For the MHPEO, one end is capped by an −OH group, while the other end is capped by an −OCH3 group. Utilizing in-situ atomic force microscopy, the growth of single crystals as a function of time (t) was monitored at different crystallization temperatures (T x ). Depending on the end-group chemistry and T x applied, two growth laws were observed, which state that the crystal lateral size r or the crystal volume V can be linearly proportional to t (r ∝ t or V ∝ t). Combined with morphological observations of the single crystals, it could be deduced that when r ∝ t, the crystal growth was nucleation-limited (NL), while in the case of V ∝ t, and thus r ∝ t 0.5 at constant crystal thickness, the crystal growth was diffusion-limited (DL). A change of the crystal growth mechanism from the NL to the DL process was also observed with decreasing T x in the MHPEO sample.