Rheology and Shear-Aligning of Block Copolymers

The earliest rheological studies on block copolymers were carried out on triblocks with hexagonally ordered cylindrical microdomains (Chung and Gale 1976 Gouinlock and Porter 1977). In these triblocks, the end blocks were much smaller than the center 

Behavior that is intermediate between that of a solid and that of a liquid is perhaps not surprising for a block copolymer with hexagonally ordered cylinders, since such a material has solid-like order in the two directions perpendicular to the cylinders and liquid-like order parallel to the cylinders. Similar behavior is observed in lamellar block copolymers, which has solid-like order in the direction normal to the lamellae and has liquid-like order in the other two directions. For lamellar block copolymers, solid-like behavior at low frequencies typically arises from the disrupting effect of defects, such as those present in smectic liquid crystals (see Section 10.4.8). 

High densities of defects are frequently present in ordered block-copolymer phases. 

As discussed below, the quality of the alignment (and even its direction in the case of lamellar morphology), is influenced by temperature, as well as the frequency and strain amplitude of the aligning shear field. No general theory for the alignment of block-copolymer phases has yet been developed. However, studies of a number of different block-copolymer systems show that in ordered states with cylindrical domains, shear orients the cylinders parallel to the flow, while for lamallar microdomains, two different shear-induced orientations are commonly found, depending on alignment conditions in both of these orientations, the flow direction lies in the plane of the lamellae. 

The steady-state flow properties of block copolymers are often hard to measure. In steady shear, the shear stress often does not reach a clear steady-state value (Lyngaae-Jorgensen 1985). In cone-and-plate rheometers, steady shearing of an ordered block copolymer can result in edge fracture and flow irregularities, as might be expected when one forces a quasi-solid structure to flow (Winey et al. 1993a). 

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The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

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