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473 results on '"Woo, Eamor M."'

115. Cracks in Polymer Spherulites: Phenomenological Mechanisms in Correlation with Ring Bands.

Author

Woo, Eamor M. and Lugito, Graecia

Subjects
*SPHERULITES (Polymers), *POLYLACTIC acid, *BUTYRATES, *THERMAL expansion, *DEVIATION (Statistics)
Abstract

This article reviews possible mechanisms of various crack forms and their likely correlations with interior crystal lamellae and discontinuous interfaces in spherulites. Complex yet periodically repetitive patterns of cracks in spherulites are beyond attributions via differences in thermal expansion coefficients, which would cause random and irregular cracks in the contract direction only. Cracks in brittle polymers such as poly(L-lactic acid) (PLLA), or poly(4-hydroxyl butyrate) (PHB), or more ductile polymers such as poly(trimethylene terephthalate) (PTT) are examined and illustrated, although for focus and demonstration, more discussions are spent on PLLA. The cracks can take many shapes that bear extremely striking similarity to the ring-band or lamellar patterns in the same spherulites. Crack patterns may differ significantly between the ring-banded and ringless spherulites, suggesting that the cracks may be partially shaped and governed by interfaces of lamellae and how the lamellar crystals assemble themselves in spherulites. Similarly, with some exceptions, most of the cracks patterns in PHB or PTT are also highly guided by the lamellar assembly in either ring-banded spherulites or ringless spherulites. Some exceptions of cracks in spherulites deviating from the apparent crystal birefringence patterns do exist; nevertheless, discontinuous interfaces in the initial lamellae neat the nuclei center might be hidden by top crystal over-layers of the spherulites, which might govern crack propagation. [ABSTRACT FROM AUTHOR]

Published
2016
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117. Unusual Large-Pitch Banding in Poly(L-lactic acid): Effects of Composition and Geometry Confinement.

Author

Woo, Eamor M., Lugito, Graecia, Ya-Ting Hsieh, and Nurkhamidah, Siti

Subjects
*POLYLACTIC acid, *CRYSTALLINE polymers, *POLYETHYLENE oxide, *SCANNING electron microscopy, *MOLECULAR weights
Abstract

Lamellar patterns and orientations in blends of two crystalline polymers: poly(ethylene oxide) (PEO) and low-molecular-weight poly(L-lactic acid) (PLLA) were investigated using polarizing light optical microscopy (POM), and atomic and scanning electron microscopy (AFM, SEM). Specific etching off of PEO was used to reveal the complex earlier-grown PLLA lamellae patterns with various PEO content in blends. Banding of extremely long pitch (50 ?m) in crystallized PLLA spherulites was induced by two kinetic factors: geometry confinement by top cover and introduction of diluent such as PEO. The mechanisms and correlation among the lamellar assembly, ring bands, and cracks are exemplified. Lamellar patterns and ring-band types in blends were found to vary with respect to not only blend compositions, but also confinement of top-cover. [ABSTRACT FROM AUTHOR]

Published
2014
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150. Effects of top confinement and diluents on morphology in crystallization of poly( l-lactic acid) interacting with poly(ethylene oxide).

Author

Woo, Eamor M., Lugito, Graecia, and Tsai, Jia‐Hsuan

Subjects
*POLYLACTIC acid, *POLYETHYLENE oxide, *CRYSTAL morphology, *CRYSTALLIZATION, *SPHERULITES (Polymers)
Abstract

ABSTRACT Effects of top confinement and diluent poly(ethylene oxide) (PEO) on poly( l-lactic acid) (PLLA) crystal morphology have been investigated. When crystallized at 120 °C, uncovered neat PLLA sample exhibits higher growth rate ringless spherulites; while the covered sample exhibits lower growth rate ring-banded spherulites. As PEO is introduced into PLLA, the morphology also undergoes significant changes. For the same Tc,PLLA = 120 °C, the PEO/PLLA blend with PEO composition greater than 25% exhibits ring-banded patterns even in uncovered sample. However, in much greater PEO composition (>80 wt %), uncovered samples exhibit ring bands diverging into dendritic patterns, while top covered samples tend to maintain the spiral ring-band patterns. Both PEO inclusion in PLLA and top cover on films impose growth kinetic alterations. Additionally, the top glass cover tends to prevent the lower surface tension PLLA to be accumulated on the surface, resulting in the formation of ring-band pattern. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1160-1170 [ABSTRACT FROM AUTHOR]

Published
2015
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