The Ordered Bicontinuous Double Diamond Structure in Binary Blends of Diblock Copolymer and Homopolymer.
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THE ORDERED BICONTINUOUS DOUBLE DIAMOND STRUCTURE IN BINARY BLENDS OF DIBLOCK COPOLYMER AND HOMOPOLYMER. Karen I. Winey* and Edwin L. Thomas** * University of Massachusetts, Polymer Science and Engineering Department, Amherst, MA 01003 ** Massachusetts Institute of Technology, Materials Science and Engineering Department, Cambridge, MA 02139
ABSTRACT We report the observation of the ordered bicontinuous double diamond (OBDD) structure in binary blends of poly(styrene-isoprene) diblock copolymer and homopolystyrene. The overall polystyrene volume fraction range is 64 67 PSvol% for the OBDD structure in binary blends of a lamellar diblock (SI 27/22) and a homopolymer (14.0 hPS). This composition range is approximately within the polystyrene volume fraction range established for pure diblock copolymers in the strong segregation regime having the OBDD structure. Ordered lamellae are observed at approximately 65 PSvol% when the homopolystyrene molecular weight is greater than the molecular weight of the polystyrene block of the copolymer. This observation is discussed in terms of the decreased degree of mixing between the homopolymer and the corresponding block and the resultant effect on the interfacial curvature.
INTRODUCTION Diblock copolymers contain two polymer chains which are covalently bonded to one another at one end. Immiscible polymer blends macrophase separate, whereas the connectivity within diblock copolymers limits the size of the phase separated domains and gives rise to periodic microphase separated morphologies. In the strong segregation limit the volume percent of the components in the diblock copolymer determines which morphology is observed: spheres on a body-centered cubic lattice, cylinders on a hexagonal lattice, the ordered bicontinuous double diamond (OBDD) morphology, or lamellae. The polymer chains are amorphous within these ordered domains. The ordered bicontinuous double diamond microstructure is periodic in three dimensions and bicontinuous in that both the majority and minority components are continuous throughout the microstructure. The minority component is divided into two interpenetrating networks having diamond cubic symmetry. A color computer-generated image will assist the reader in visualizing this microstructure 1 . Both minority channels are separated from the majority component by a surface of approximately constant mean curvature. Similar bicontinuous structures have been proposed for surfactant-oil-water systems 2 . The OBDD microstructure has been observed previously in linear diblock copolymers and star diblock copolymers 3 . This paper will discuss a new method of producing the OBDD microstructure: binary blends of diblock copolymer and homopolymer. Mat. Res. Soc. Symp. Proc. Vol. 171. 'c)1990 Materials Research Society
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EXPERIMENTAL We limit our discussion to diblock copolymers which by themselves exhibit the lamellar morphology. The diblock copolymers used in this study were anionically synthesized by Dr. L. J. Fetters of Exxon. Poly(styrene-isoprene) and poly(st
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