Microphase separation in correlated random copolymers

In this paper we present the results of a calculation of the phase diagram of a highly polydisperse multiblock copolymer in the weak segregation limit. The theory for polydisperse systems developed by Erukhimovich and Dobrynin [Erukhimovich, I.; Dobrynin, A.V. Macromol.Symp, 81, 253 (1994)] has been used. The model of the copolymer has the following characteristics: the blocklengths, as well as the molecule lengths are highly polydisperse (M(w)/M(n) = 2). The average number of blocks per molecule is very large and the effects of the finiteness of the blocklengths (the fluctuation corrections) are ignored. The resulting phase diagram shows some remarkable differences with the phase diagram of a regular monodisperse multiblock. Known differences are e.g. the order of the transition from the homogeneous state, and the variation of the period of the microstructure with the chi-parameter. Ln addition, we found a peculiar feature at the critical point: the phase boundaries have discontinuous derivatives

Fluctuation Corrections for Correlated Random Copolymers

Fluctuation effects on the order-disorder transition (ODT) in correlated random copolymers (polydisperse A/B multiblock copolymers with block lengths having an exponential Flory distribution, and a large average number of blocks per chain) are studied with due regard for the strong temperature dependence of the period of the arising ordered phases, characteristic for the system under consideration. To this end, following a field theoretical variational method, the free energy is minimized with respect to both the concentration profile ψ and the correlation function G, assumed to belong to certain classes of trial functions. The trial function for G contains an extra adjustable parameter as compared to the situation typical for monodisperse A/B block copolymer melts. The shape of the correlation function and its temperature dependence are determined both for the disordered phase and for the ordered phases. In the vicinity of the critical point the phase diagram is calculated and presented in a universal form by using reduced variables. It is shown that near the ODT and for A-monomer fractions f close to 1/2, the profiles are strongly fluctuating: in the ordered phase the amplitude of the fluctuations is equal to the amplitude of the average profile, and in the disordered phase the concentration inhomogeneities are comparable to those in the ordered phase. In the same region the disordered phase has an anomalously large correlation length, indicating some kind of local ordering. In connection with this, we discuss the close relationship between the disordered phase and the random wave structure.

Self-assembling block copolymer systems involving competing length scales: A route toward responsive materials

The phase behavior of block copolymers melts involving competing length scales, i.e., able to microphase separate on two different length scales, is theoretically investigated using a self-consistent field approach. The specific block copolymers studied consist of a linear A-block linked to an alternating (A-alt-B)-block. The large length scale microphase separation is closely related to the overall length scale of the block copolymer, whereas the short length scale microphase separation is associated with the length scale of the repeat unit of the alternating block. Because of the presence of competing intrinsic length scales, the periodicity of the lamellar structure is extremely temperature sensitive. For a range of polymer compositions a first-order phase transition occurs from a lamellar morphology with a large periodicity to a lamellar or hexagonal morphology with a much smaller periodicity. Such phase transitions could potentially form the basis for responsive materials

Self-Assembling Block Copolymer Systems Involving Competing Length Scales: A Route toward Responsive Materials

The phase behavior of block copolymers melts involving competing length scales, i.e., able to microphase separate on two different length scales, is theoretically investigated using a self-consistent field approach. The specific block copolymers studied consist of a linear A-block linked to an alternating (A-alt-B)-block. The large length scale microphase separation is closely related to the overall length scale of the block copolymer, whereas the short length scale microphase separation is associated with the length scale of the repeat unit of the alternating block. Because of the presence of competing intrinsic length scales, the periodicity of the lamellar structure is extremely temperature sensitive. For a range of polymer compositions a first-order phase transition occurs from a lamellar morphology with a large periodicity to a lamellar or hexagonal morphology with a much smaller periodicity. Such phase transitions could potentially form the basis for responsive materials.