Reversal of the isotopic effect in the surface behavior of binary polymer blends

We report neutron reflectivity measurements on the surface behavior of isotopic polystyrene blends of symmetric and disparate molecular weights near an air surface. For the symmetric blends we find, in agreement with past findings, that the segments of the deuterated polymer always partition to the...

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Bibliographic Details
Published in:The Journal of chemical physics 1993-03, Vol.98 (5), p.4163-4173
Main Authors: ARVIND HARIHARAN, RUSSELL, T. P, KUMAR, S. K
Format: Article
Language:English
Subjects:
360602 > Other Materials-- Structure & Phase Studies
AIR
Applied sciences
ASYMMETRY
BINARY MIXTURES
Blends
COHERENT SCATTERING
COMPRESSIBILITY
DEUTERIUM COMPOUNDS
DIFFRACTION
DISPERSIONS
Exact sciences and technology
FLUIDS
Forms of application and semi-finished materials
GASES
HYDROGEN COMPOUNDS
INTERFACES
ISOTOPE EFFECTS
ISOTOPES
MATERIALS
MATERIALS SCIENCE
MEAN-FIELD THEORY
MECHANICAL PROPERTIES
MIXTURES
MOLECULAR WEIGHT
NEUTRON DIFFRACTION
ORGANIC COMPOUNDS
ORGANIC POLYMERS
PETROCHEMICALS
PETROLEUM PRODUCTS
PLASTICS
Polymer industry, paints, wood
POLYMERS
POLYOLEFINS
POLYSTYRENE
POLYVINYLS
SCATTERING
SEGREGATION
SURFACE PROPERTIES
SYNTHETIC MATERIALS 360606 > Other Materials-- Physical Properties-- (1992-)
Technology of polymers
WETTABILITY
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Summary:We report neutron reflectivity measurements on the surface behavior of isotopic polystyrene blends of symmetric and disparate molecular weights near an air surface. For the symmetric blends we find, in agreement with past findings, that the segments of the deuterated polymer always partition to the air surface. These results, which are driven purely by energetic effects, can then be modeled in the framework of a mean-field lattice theory with a constant surface energy difference parameter. In contrast, for the asymmetric blends we find that the segments of either polymer can partition to the surface, and the controlling variable is the disparity in the molecular weights of the two components. These new results, which cannot be predicted with the constant density lattice models utilized for symmetric blends, can be modeled if one balances the energetic preference of placing the deuterated segments near the surface with entropic effects, which are caused by the presence of a density gradient at the air surface, preferring the surface segregation of the short chains. These findings emphasize the need for the inclusion of free-volume effects when modeling the segregation to a free surface, and we show that a recent mean-field compressible lattice model does capture these effects adequately.
ISSN:0021-9606
1089-7690
DOI:10.1063/1.465024