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Studies on whitening phenomena induced by some non‐solvents on highly oriented glassy polymers
Author(s) -
Sung N. H.,
Gahan R. E.,
Haven R. E.
Publication year - 1983
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760230607
Subject(s) - materials science , polycarbonate , polymer , crazing , thermodynamics , solvent , glass transition , sorption , polymer chemistry , composite material , chemistry , organic chemistry , physics , adsorption
Whitening phenomena, characterized by optically opaque regions when a highly oriented glassy polymer is immersed in some liquids, were investigated as a function of temperature, extent of molecular orientation, different solvent and molecular weight, using oriented polymethyl methacrylate (PMMA) obtained by hot‐stretching and cold drawn polycarbonate (PC). Whitened PMMA is highly porous resembling foamed plastics and shows the same T g as in unwhitened PMMA, while whitened PC is denser with much smaller pores and exhibits melting behavior characteristic of solvent‐induced crystallinity. Whitening progresses with a sharp boundary between whitened and unwhitened layers, which advanced at a constant velocity. This feature is similar to Case II transport. However, whitening differs from Case II sorption in that it only occurs in highly oriented polymers in contact with liquids of negligible sorption. Activation enthalpy of 44.1 and 39.7 kcal/mol has been found in a certain temperature range for PMMA/N‐methyl formamide (N‐MF) and PC/di‐isopropyl amine (DIPA) respectively. At a given temperature, a higher extent of orientation leads to faster whitening process. Entropy correlation theory, in which the decrease in the configurational entropy (Δ S c ) due to orientation is assumed to approximate an inerease in the activation entropy, was employed to correlate the dependency of the whitening rate on orientation. For hotstretched PMMA, where Δ S c is proportionl to \documentclass{article}\pagestyle{empty}\begin{document}$ (\alpha ^2 + 2/\alpha - 3) $\end{document} since affine deformation applies, a good correlation is observed. For cold drawn PC, correlation was poor, probably because Δ S c cannot be expressed to be proportational to \documentclass{article}\pagestyle{empty}\begin{document}$ (\alpha ^2 + 2/\alpha - 3) $\end{document} . In PMMA, when the molecular weight is in the same order as the average molecular weight between entanglements, the orientation and the whitening rate are both lower than higher molecular weight speciments. This may be due to a smaller number of chain entanglements and consequent chain slippage. In PMMA, whitening induced liquids seem to have a solubility parameter somewhat greater (2.5 ∼ 5.0) than that of PMMA. For PC, no consistent tendency is found.