Defect-Mediated Rheology of Block Copolymers
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where n is a unit vector normal to the layers, and e and V • n are the layer dilational and bending strain, respectively. B and K are the corresponding moduli. These have been measured only recently for block copolymers.3,4 The ratio of these moduli yields a characteristic microscopic length: (2)
which is calculated to be a fraction of the layer spacing do-s Similarities are also found in the dynamic properties of polymeric and smallmolecule smectics.6 These similarities are found at low strain rate and amplitude where the rheological response is controlled by their similar layered structures and associated defects. During either steady or dynamic shear, certain defect and domain motions are induced such that a steady-state alignment of the
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director has been observed.7-11 In this report, we discuss defect structure and how the much more viscous nature of these polymeric layered materials influences the distribution and relative population of each defect type. We also discuss how these defects can control material stress-relaxation behavior. The
Figure 1. TEM micrograph showing an isolated defect in a well-aligned specimen of polystyrene/poly(methyl methacrylate) (PS/PMMA). The shear direction is horizontal and the velocity-gradient direction is vertical. An elementary edge dislocation is evident in this field of view. While the PMMA microdomain is discontinuous for this defect, for other dislocations the PS microdomain is discontinuous. Copolymer asymmetry is one factor that can bias the dislocation core structure. The defect density, estimated from a sampling of TEM images, is approximately 1(f cm/cm3. No pairing or clustering of dislocations was found.
discussion will focus on a recent study4 of the response of lamellar block copolymers to compression and dilation of their lamellae. Experimental Procedure In a study of the response of a block copolymer to layer compression and dilation, the materials used were polystyrene/poly (methyl methacrylate) (PS/PMMA) symmetric diblock copolymers. They were synthesized anionically under nitrogen atmosphere, as described in a related article.4 For copolymers having a total molecular weight of 31 and 41 kg/mol, a lamellar microstructure was confirmed by TEM, and the corresponding order-disorder-transition (ODT) temperatures, 182 and 296°C, respectively, were measured by rheological methods. Vacuum compression molded disks (8-mm diameter, 1 mm thick) were analyzed using a Rheometrics RMS 800 torsional rheometer with parallel-plate geometry, as described elsewhere. 4 Ultrathin sections (—70 nm) of various orientations were taken from different locations within the specimen, mounted on copper grids, stained with RuC>4 vapor, and examined by TEM at 100 kV.4 Studies of the response to layer compression and dilation were performed by applying normal strains to block copolymers with a well-aligned lamellar microstructure in the direction of the lamellar normal n. These experiments required a well-aligned microstructure. Alignment was achieved by large-amplitude shear fro
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