Phase behavior of deuterated polystyrene-block-poly(n-pentyl methaerylate) copolymers

Abstract

The phase behavior of deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers (dPS-PnPMA) was investigated by using small-angle X-ray (SAXS) and neutron (SANS) scatterings and rheology. This block copolymer exhibited a closed-loop type of phase behavior as did hydrogenated PS-PnPMA copolymers. The closed loop consists of two transitions: lower disorder-to-order transition (LDOT) and upper order-to-disorder transition (UODT) occurring at a lower and higher temperature, respectively. The segmental interaction parameter (chi) between dPS and PnPMA blocks at various temperatures was obtained by fitting the incompressible random phase approximation theory to SANS results using a low molecular weight of dPS-PnPMA exhibiting a disordered homogeneous state over the entire temperature range. We found that at higher temperatures chi increased initially with increasing temperature (T), achieved a maximum, and then decreased. Such behavior is in keeping with polymer blends or block copolymers that would exhibit a closed-loop phase behavior. However, at temperatures below LDOT, chi did not decrease monotonically with decreasing T. Rather, with decreasing T, chi decreased initially, achieved a minimum, and then increased again. This strongly suggests that another transition of order-to-disorder transition (ODT) might be expected at lower temperatures.