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fulltext - J Mater Sci(2007 42:45214529 DOI...

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Crystal morphologies in thin films of PEO/PMMA blends B. C. Okerberg Æ H. Marand Received: 24 March 2006 / Accepted: 25 May 2006 / Published online: 15 February 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Crystallization in thin films of poly(ethylene oxide) in blends with poly(methyl methacrylate) has been studied. The film thickness is fixed at 120 nm, while the blend composition, PMMA molar mass, and crys- tallization temperature are varied. Blends with a com- position of 50/50 (wt% PEO/wt% PMMA) exhibit a variety of morphologies that are highly dependent on PMMA molar mass and crystallization temperature. A needle morphology not previously reported in this system is also observed. For 40/60 blends, dendrites and DBM are observed at high PMMA molar mass. At low PMMA molar mass, a number of morphologies are observed over small changes in the experimental controls. In 35/65 blends, dendritic growth is observed with sidebranches at 45 ° and 90 ° to the dendrite trunk at low undercooling and only at 90 ° for larger undercool- ing. For 30/70 blends, dendritic growth is observed over a large range of PMMA molar mass and crystallization temperature. Maps demonstrating the role of the con- trol parameters on morphological development are reported. The observed morphologies are believed to result from the combined effects of the lack of crystal- lizable chains at the growth front, low dimensionality, rejection of non-crystallizable chains, and variation of the effective levels of noise and/or anisotropy. Introduction Pattern formation during crystallization has been stud- ied for a number of years. Many studies have focused on dendritic crystallization of metals and small molecules because these dendritic microstructures are known to play a large role in the resulting mechanical properties [ 1 ]. Pattern formation in facet-forming materials, such as polymers, has been studied in less detail, although early studies have shown that the presence of facets does not necessarily preclude pattern formation [ 2 4 ]. One of the outstanding problems in pattern formation is an under- standing of the processes by which substances that normally crystallize in faceted manner can be driven to crystallize in non-equilibrium, diffusion-limited mor- phologies. In general, faceted crystals have atomically smooth interfaces that grow through a layer-by-layer mecha- nism, such as secondary surface nucleation (nucleation- limited growth). On the other hand, non-faceted crystal morphologies involve rough interfaces charac- terized by many favorable sites for atom attachment. In the latter case, crystal growth is controlled by diffusion of heat or mass away from the melt-crystal interface (diffusion-limited growth). Jackson suggested that the nature of the interface (faceted vs. non-faceted) correlates with the magnitude of the entropy of fusion [ 5 ]. Smooth interfaces are observed in materials exhibiting large entropies of fusion (polymeric mate- rials), while rough interfaces are typically found in metals and other materials with small entropy of fusion.
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