Morphologies in Blends of Diblock Copolymer and Homopolymer: Morphology Diagrams and the Intermaterial Dividing Surface
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MORPHOLOGIES IN BLENDS OF DIBLOCK COPOLYMER AND HOMOPOLYMER: MORPHOLOGY DIAGRAMS AND THE INTERMATERIAL DIVIDING SURFACE KAREN I. WINEY AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 ABSTRACT Binary blends of diblock copolymer (AB) and homopolymer (hA) self assemble upon solvent evaporation into a great variety of microphase separated morphologies. The ordered lamellar, bicontinuous double diamond, cylindrical and spherical morphologies were observed by TEM and SAXS in our studies, as well as a range of micellar morphologies. The mean curvature (H) and the area per copolymer junction (0j), which characterize the intermaterial dividing surface, increased with increasing homopolymer concentration in the blend and/or with decreasing homopolymer molecular weight. These trends were generally obeyed both between and within ordered morphology types. The increase in H and aj was related to an increased degree of mixing between the homopolymer and the block of the copolymer. Two types of isothermal morphology diagrams were constructed to consolidate the extensive morphological data and to illustrate the general morphological transitions in AB/hA blends. The constant molecular weight morphology diagrams illustrated the interdependence of the copolymer composition and the homopolymer concentration. The constant copolymer composition diagrams emphasized the importance of the relative homopolymer molecular weight and the overall blend composition. INTRODUCTION The morphologies of a large number of blends containing a diblock copolymer (AB) and a homopolymer (hA) have previously been determined in our laboratory under the condition that the components A and B are immiscible. Ordered morphologies include lamellae, the ordered bicontinuous double diamond (OBDD) morphology, cylinders on a hexagonal lattice and spheres on a cubic lattice as presented elsewhere [1-4]. Disordered micelles of various shapes have also been observed [1,5,6]. Current theoretical predictions for AB/hA blends do not account for the extensive variety of observed morphologies. The early theories of AB/hA blends predict the critical micelle concentration (cmc) and the nature of micelles above the cmc, but are applicable only at high homopolymer concentrations where micelles do not interact [7-8]. Theories have also been developed for the onset of long range order between micelles, but only for the case of spherical micelles [910]. Whitmore and Noolandi's calculations predict the phase behavior of the AB/hA system over the entire homopolymer concentration range as a function of temperature [11]. For our purposes, the Whitmore-Noolandi results have two limitations: their theoretical approach is most reliable near the order-disorder transition and only a single morphology, lamellae,is considered. The majority of the phase diagrams by Roe and Zin were constructed from cloud point measurements which provide no firm evidence of microdomain shape or symmetry and may or may not accurately indicate macrophase separation. We present here two types of
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