Microphase separation in poly(isoprene-b-ethylene oxide) diblock copolymermelts. I. Phase state and kinetics of the order-to-order transitions

The phase state and the kinetics of the order-to-order transitions have bee n studied in a series of poly(isoprene-b-ethylene oxide) (PI-PEO) diblock c opolymers with a PI volume fraction in the range 0.25<f(PI)<0.92, using sma ll angle x-ray scattering (SAXS), and rheology. The mean-field theory (MFT) structure factor is used to describe the SAXS profiles in the disordered p hase and to extract the temperature dependence of the interaction parameter chi(T). In general, an agreement is found with the phase diagram proposed by an extended MFT, except at f(PI) = 0.61 where the following sequence of phases was found: L-c-->Hex-->Gyroid-->Dis (L-c is the crystalline lamellar phase, Hex signifies hexagonally packed cylinders, Gyroid is the bicontinu ous cubic network with the Ia (3) over bar d symmetry, and Dis is the disor dered phase). We found that crystallization disrupts the amorphous ordered morphologies and imposes a layered structure (L-c). The study of the kineti cs of the Hex to L-c and the Hex to Gyroid transitions is facilitated by th e different viscoelastic contrast and the distinctly different scattering p atterns of the three phases involved (L-c, Hex, Gyroid). Our studies show t hat it is possible to undercool and overheat ordered phases just as we can undercool the disordered phase. The transformation from the Hex to the L-c phase proceeds via a heterogeneous nucleation and growth process and result s in the formation of a spherulitic superstructure composed from stacks of lamellar crystals. The transformation of the Hex to the Gyroid phase involv es two steps. The first step-which is too fast to be picked up by rheology- involves fluctuations of the hexagonal phase. The second "slow" step involv es a nucleation and growth process of elongated objects. The transformation proceeds nearly epitaxially and has an activation energy of 47 kcal/mol wh ich is typical for a collective process. (C) 1999 American Institute of Phy sics. [S0021-9606(99)52001-1].