Jeffrey H. Rosedale

Fluctuation effects in a symmetric diblock copolymer near the order–disorder transition

The thermodynamic and dynamic properties of a partially deuterated poly(ethylene‐propylene)–poly(ethylethylene)(PEP–PEE) diblock copolymer containing 55% by volume PEP were characterized above and below the order–disorder transition (ODT) by small‐angle neutron scattering (SANS) and rheological measurements, respectively. Both experimental techniques produced unambiguous evidence of composition fluctuations well above TODT(T−TODT≲50 °C) in the disordered state, which increase in magnitude as the weak first‐order transition is approached. Based on the SANS results, which are nearly predicted by a recent fluctuation theory, we conclude that the (equilibrium) instantaneous morphology in the disordered state closely resembles a spinodally decomposed binary mixture. Below TODT, long‐range order can be obtained by the application of a shear field as evidenced by the resulting highly anisotropic (one‐dimensional) SANS pattern. As the ODT is approached in the shear‐oriented ordered state, an isotropic scattering ...

Fluctuations, conformational asymmetry and block copolymer phase behaviour

Phase behaviour near the order–disorder transition (ODT) of 58 model hydrocarbon diblock copolymers, representing four different systems, is summarized. Six distinct ordered-state microstructures are reported, including hexagonally modulated lamellae (HML), hexagonally perforated layers (HPL) and a bicontinuous cubic morphology with Iatext-decoration:overline3d space group symmetry. Two non-classical parameters, Iµ and text-decoration:overlineN, control the occurrence and distribution of these phases, in addition to the classical variables ƒ and χN, where ƒ, χ and N are the composition, segment–segment interaction parameter and degree of polymerization, respectively. Iµ accounts for differences in the conformational and volume-filling characteristics of each block. Conformational asymmetry, Iµ≠ 1, produces an asymmetric phase diagram around ƒ= 1/2. The importance of fluctuation effects are inversely related to the magnitude of text-decoration:overlineN, a type of Ginzburg parameter that is proportional to N. As text-decoration:overlineN decreases, the bicontinuous Iatext-decoration:overline3d phase appears adjacent to the ODT. Development of this cubic phase can be rationalized based on chain-packing frustration near the lamellar hexagonal state. Apparently the Iatext-decoration:overline3d cubic state is stabilized by fluctuations since it disappears when text-decoration:overlineN becomes large. These findings provide new insights into the origins of phase complexity in condensed soft matter.