Order, disorder, and fluctuation effects in an asymmetric poly(ethylenepropylene)-poly(ethylethylene) diblock copolymer

The thermodynamic and dynamic properties of a partially deuterated asymmetric poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE) diblock copolymer containing 77% by volume PEP were characterized below and above the order-disorder transition (ODT). Both small-angle neutron scattering (SANS) and rheology experiments provided unambiguous evidence of composition fluctuations between the order-disorder transition temperature (T_ODT) and at least 40 °C above T_ODT.The strength of the fluctuations increases as the weak first-order ODT is approached. An ordered state morphology of hexagonally packed PEE rich cylinders was established from the anisotropic SANS patterns obtained from specimens in which long-range order had been introduced by the application of a shear field. When heated such a sample exhibits clear evidence of a transition from the hexagonal morphology to a disordered state at 155 °C. This is in accordance with the predictions of a recent fluctuation theory. Also in qualitative accordance with the fluctuation theory is the observation that an isotropic scattering component develops in the ordered state as the ODT is approached. Quenching a specimen from the disordered state into the ordered state in the absence of an external shear field produces a macroscopically isotropic material. Gross differences are observed in the low-frequency rheological properties for the different morphologies and phase states which we have interpreted based on SANS results.