The disorder -to -order phase transition in poly(styrene-<i>block</i>-n-butyl methacrylate): The effect of pressure
The effect of hydrostatic pressure on the lower disorder-to-order transition (LDOT) in poly(d-styrene-block-n-butyl methacrylate) having symmetric and asymmetric block lengths was investigated by in situ small-angle neutron scattering (SANS). Currently, linear diblock copolymers having styrenic and methacrylic monomers are the only systems that display a thermally accessible phase transition from the disordered homogeneous melt to the ordered microphase-separated state upon heating. The location of this phase transition was mapped as a function of temperature and pressure by analyzing one dimensional SANS intensities, where discontinuities in the width and height of the scattering peak indicated the traversal of the transition isothermally or isobarically.
The T-P phase diagram of p(d-S-b-nBMA) built using this method shows an expansion of the disordered, homogenous region with increasing pressure. For the diblock copolymer with a lamellar morphology and Mw = 8.5 × 104, the slope of the phase transition line was ∼150 K/kbar, and was approximately linear over a range of 1 kbar. Increasing the molecular weight of p(d-S- b-nBMA) resulted in a vertical shift of the phase transition line in the phase diagram and no detectable change in slope. Similar effects were observed for diblock copolymers with a cylindrical morphology. The bulk enthalpy and volume changes at the phase transition, which dictate the pressure coefficient for a one component system, were either at the limit of experimental resolution or unobservable due to kinetic factors. In-situ X-ray reflectivity experiments, however, showed a significantly reduced thermal expansion coefficient in the disordered phase and a discontinuous increase in film thickness as a function of temperature at the bulk LDOT.
The pressure coefficient, dTLDOT/dP, for these materials is greater by a factor of five than currently observed in diblock copolymers with conventional UODT phase behavior, i.e. ordering upon cooling . This dramatic phase behavior allows rapid and convenient access to the order-disorder phase transition, isothermally, and suggests that LDOT block copolymers could be employed in blend and multi-component systems as minor components to impart pressure-induced compatibilization, surface activity, or flow properties.