G. Floudas et al., Microphase separation in poly(isoprene-b-ethylene oxide) diblock copolymermelts. I. Phase state and kinetics of the order-to-order transitions, J CHEM PHYS, 110(1), 1999, pp. 652-663
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].