Dynamic IR Studies of Microdomain Interphases of Isotope-Labeled Block Copolymers
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DYNAMIC IR STUDIES OF MICRODOMAIN INTERPHASES OF ISOTOPE-LABELED BLOCK COPOLYMERS I. NODA, S. D. SMITH, A. E. DOWNEY, J. T. GROTHAUS, and C. MARCOTT The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 398707, Cincinnati, OH 45239-8707
ABSTRACT By probing the localized segmental motion of isotope-labeled block copolymers, the physical nature of the interphase region between microphaseseparated domains of block polymers was examined. Dynamic infrared linear dichroism (DIRLD) spectroscopy, which measures the reorientations of submolecular structures induced by a small-amplitude oscillatory strain, was combined with specific isotope-labeling using deuterium-substituted monomers. The latter technique enabled us to differentiate the dynamic responses of well-defined parts of block segments, e.g., near the segment junction, chain end, or middle of the block. The degree of segmental interactions near the interphase region of styrene-isoprene diblock copolymers were studied as a function of the segment location and temperature. The reorientational motion of the polystyrene segment, especially near the block junction, was monitored around the glass transition temperature of the polyisoprene matrix. From this result, the degree of segmental mixing in the interphase region which leads to local plasticization of the polystyrene segment was determined.
INTRODUCTION Block copolymers owe their unique properties to the molecular architecture consisting of different polymer segments joined together by a covalent bond. The repulsive interactions between dissimilar block segments often results in microphase separation where the size of the phase domain is restricted to a scale comparable to the block segment length. It has been postulated for some time that the boundary between the adjacent microphase domains is not a sharp two-dimensional layer but rather a region of finite thickness characterized by a substantial intermixing of different block segments. The existence of the microdomain interphase 11] due to the diffuse concentration gradient across the boundary has been predicted by statistical thermodynamic theories based on the mean-field approach [2,31. The results of several experimental works (e.g., the systematic deviation of SAXS intensity profiles from the behavior of sharp-boundary systems described by Porod's law [4,5] and the modeling of rheological behavior measured by dynamic mechanical analysis [6]) support the view of a segmentally mixed interphase. Different models, such as a coarse interface with a sharp boundary [7], may also account for some of the observed results. Spectroscopic techniques which can provide direct information at a molecular scale are especially suited for probing the presence of segmental mixing at the interphase. Recent NMR studies [8] of isotope-labeled block copolymers with relatively low molecular weights, for example, reveal the existence of substantial segmental interactions at the domain boundary. We used a recently developed analytical technique called dynamic infrared con
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