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

The thermodynamic and dynamic properties of a partially deuterated poly(ethyletie-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 T_ODT (T - T_ODT ≲ 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 T_ODT, 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 component develops (evident at wavevectors transverse to the principle line of reflections) in qualitative agreement with the fluctuation theory. We speculate that the (equilibrium) ordered morphology can be represented by the superposition of a static lamellar structure with a fluctuating spinodal pattern. Temperature quenching into the ordered region from the disordered state in the absence of an external field produces a macroscopically isotropic (nonequilibrium) material with a complex, poorly defined morphology. Associated with each of these different morphologies and phase states are gross differences in the low-frequency rheological properties that we have interpreted based on the SANS results and fluctuation theory.