Multiple Ordered Phases in a Block Copolymer Melt

A poly(ethylenepropylene)‒poly(ethylethylene) (PEP‒PEE) diblock copolymer containing 65% by volume PEP was investigated using small-angle neutron scattering (SANS) and rheological measurements. Four distinct phases have been identified as a function of temperature: three ordered phases at low temperatures and a disordered phase at elevated temperatures. Evaluation of the ordered phases was facilitated by the introduction of long-range order using a shear-orientation technique. SANS data were acquired as a function of temperature for three specimen orientations corresponding to the principle Cartesian axis associated with the shear-orientation direction. Dynamic rheological properties were determined using a simple shear geometry with the strain direction coincident with the direction of macroscopic orientation. The order‒disorder and order‒order transitions were determined to be first-order based on discontinuities in the SANS pattern symmetries and intensities and dynamic elastic moduli. At the lowest experimental temperatures the material exhibits a (rippled) lamellar phase. At intermediate temperatures two new ordered phases appear. Above the order‒disorder transition temperature a fluctuating disordered state exists. The microstructure of the two intermediate phases has not been conclusively determined although the rheological and SANS results rule out the classical morphologies. These findings are inconsistent with the generally accepted concept of universal block copolymer phase behavior based on composition and χN alone, where χ and N are the Flory‒Huggins interaction parameter and degree of polymerization, respectively.