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Wang, M. ; Braun, H.-G. ; Meyer, E.
Transition of Crystal Growth as a Result of Changing Polymer States in Ultrathin Poly(ethylene oxide)/Poly(methyl methacrylate) Blend Films with Thickness of <3 nm

The transition of crystal growth is revealed in ultrathin poly(ethylene oxide)/poly(methylmethacrylate) (PEO/PMMA) blend films (film thickness D < 3 nm) as the crystallization condition changesfrom vacuum to humidity. The PEO crystallizes into a fractal-like branched structure in a vacuum froma state mixed with PMMA, but in humidity into one reminiscent of dense branching morphology (DBM).The crystal lamellae in both conditions orient flat-on, but the DBM-like structure is much thicker andwider, indicating a crystallization more close to equilibrium. Results show that as a PEO/PMMA blendfilm is exposed to humidity, PEO chains demix with PMMA and segregate on a PMMA layer before orduring crystallization. The crystal growth transition is attributed to the change in the factors that dominatethe crystal growth at crystal growth fronts, that is, the kinetic and surface tension effects, as a result ofchanging the polymer states in the blend films. The crystal growth of the demixed PEO on the PMMAlayer is dominated by a strong surface tension effect, while crystal growth of PEO in a blend/mixed stateis dominated by either a kinetic effect or a weak surface tension effect depending on the film composition.The compositional dependence of the crystal growth in humidity is also elucidated on the basis of thechanging of polymer states in the films.

Macromolecules 37, 437-445


February 2004
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