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

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Crystal morphologies in thin flms 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 flms o± poly(ethylene oxide) in blends with poly(methyl methacrylate) has been studied. The flm thickness is fxed at 120 nm, while the blend composition, PMMA molar mass, and crys- tallization temperature are varied. Blends with a com- position o± 50/50 (wt% PEO/wt% PMMA) exhibit a variety o± 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 o± 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 ° ±or larger undercool- ing. For 30/70 blends, dendritic growth isobserved over a large range o± PMMA molar mass and crystallization temperature. Maps demonstrating the role o± the con- trol parameters on morphological development are reported. The observed morphologies are believed to result ±rom the combined e±±ects o± the lack o± crystal- lizable chains at the growth ±ront, low dimensionality, rejection o± non-crystallizable chains, and variation o± the e±±ective levels o± noise and/or anisotropy. Introduction Pattern ±ormation during crystallization has been stud- ied ±or a number o± years. Many studies have ±ocused on dendritic crystallization o± metals and small molecules because these dendritic microstructures are known to play a large role in the resulting mechanical properties [ 1 ]. Pattern ±ormation in ±acet-±orming materials, such as polymers, has been studied in less detail, although early studies have shown that the presence o± ±acets does not necessarily preclude pattern ±ormation [ 2 4 ]. One o± the outstanding problems in pattern ±ormation is an under- standing o± the processes by which substances that normally crystallize in ±aceted manner can be driven to crystallize in non-equilibrium, di±±usion-limited mor- phologies. In general, ±aceted crystals have atomically smooth inter±aces that grow through a layer-by-layer mecha- nism, such as secondary sur±ace nucleation (nucleation- limited growth). On the other hand, non-±aceted crystal morphologies involve rough inter±aces charac- terized by many ±avorable sites ±or atom attachment. In the latter case, crystal growth is controlled by di±±usion o± heat or mass away ±rom the melt-crystal inter±ace (di±±usion-limited growth). Jackson suggested that the nature o± the inter±ace (±aceted vs. non-±aceted) correlates with the magnitude o± the entropy o± ±usion [ 5 ]. Smooth inter±aces are observed in materials exhibiting large entropies o± ±usion (polymeric mate- rials), while rough inter±aces are typically ±ound in
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This note was uploaded on 03/16/2010 for the course AMS 203 taught by Professor Tucker during the Spring '10 term at SUNY Stony Brook.

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

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