Thermal composition fluctuations near the isotropic Lifshitz critical point in a ternary mixture of a homopolymer blend and diblock copolymer

Abstract

We have studied thermal composition fluctuations of a ternary symmetric homopolymer/diblock copolymer system of PEE/PDMS/PEE-PDMS ͓PEE and PDMS being poly͑ethyl ethylene͒ and poly͑dimethyl siloxane͒, respectively͔ in its disordered state with small angle neutron scattering for concentration ⌽ of diblocks up to 15%. The phase diagram shows three characteristic regimes; ͑1͒ below the Lifshitz concentration ⌽ LL Х9%; ͑2͒ in the very near vicinity of the Lifshitz concentration; and ͑3͒ above ⌽ LL . In the regime ͑1͒ of low diblock content the maximum neutron intensity is obtained at Qϭ0 and phase separation into macroscopic large domains is observed at low temperatures. With increasing diblock content the thermal fluctuations indicate a crossover from 3d-Ising to isotropic Lifshitz critical behavior with critical exponents of the susceptibility ␥ ϭ(1.62Ϯ0.01) and correlation length ϭ(0.99Ϯ0.04) appreciably larger than in the 3d-Ising case. In the structure factor this crossover is accompanied by a strong reduction of the Q 2 term leading to the dominance of the Q 4 term; the restoring force of the thermal fluctuations is strongly reduced as the Q 2 term is proportional to the surface energy. Near the Lifshitz critical temperature a further crossover was observed leading to the appreciably larger critical exponents ␥ϭ(2.44Ϯ0.08) and ϭ(1.22Ϯ0.08) and a stabilization of the disordered regime visible through a decrease of the phase boundary by nearly 10 K. This crossover is interpreted by the formation of fluctuation induced inhomogeneous diblock distribution at the interface of the thermal fluctuations. ͑2͒ In the intermediate regime between 9% and 12% diblock content the Lifshitz line was crossed twice upon increasing the temperature from low to high temperatures; at low and high temperatures the structure factor S(Q) shows diblock character ͑maximum of S(Q) at Q 0͒ while at intermediate temperature blendlike character ͑maximum of S(Q) at Qϭ0͒. At low temperatures a transition to a bicontinuous microemulsion phase is proposed. ͑3͒ At diblock content of 15% a weak order-disorder transition was observed. The data in the Lifshitz critical range and larger than the Lifshitz line could be interpreted by a recently developed theory of Kielhorn and Muthukumar who considered the effect of thermal fluctuations in ternary homopolymer/diblock copolymer samples and from which the Flory-Huggins parameter could be evaluated.

Formation and structure of the microemulsion phase in two-dimensional ternary AB+A+B polymeric emulsions

“…It is possible that the GµE phase coincides with the "microemulsion" (µE) phase indicated in the experimental phase diagram of Schwahn et al [41]. Strictly speaking, however, the two systems cannot be compared directly to each other, because our simulations were carried out in two dimensions only.…”

“…Nevertheless, they found that dynamic light scattering provided a clear signal for this transition, which they proposed to be the homopolymer/homopolymer Lifshitz line originally introduced by Holyst and Schick [40]. In a similar experimental system, Schwahn et al [41] found evidence for the existence of three different regimes in the disordered phase, a "disordered blend", a "microemulsion", and a "disordered copolymer" region. The three regimes differ from each other by the value of the peak wavevector q * in the structure factor -zero in the disordered blend, large in the disordered copolymer, intermediate in the microemulsion.…”

On the Behavior of the Lifshitz Line in Ternary Homopolymer/Diblock Copolymer Blends

“…Note, that for the polymer blends, [13,14] whose description is the goal of our work, due to the incompressibility condition, the order parameter is a scalar, i. e. n ¼ 1, as is indeed clear from the Hamiltonian (1). The D 1 diagram is q-independent and is therefore of no relevance to the renormalization of c 1 .…”

“…We would like to stress that the present study predicts that the LL approaches its mean-field counterpart for high and deep temperatures, the prediction which is also in agreement with experimental behavior of the LL. [13,14] However, on the basis of the current data of ref. [13,14] it is not clear which of three cases concerning the value of the lower critical dimension of the system which we have discussed above actually takes place.…”

External potential dynamics simulation of the compatibility of T-shaped graft copolymer compatibilizing two immiscible homopolymers

“…Incompatible blends with these macroscopic phase-separated regions often exhibit poor mechanical properties. Considerable attention has been focused on the block copolymer as compatibilizers to increase the miscibility and to control the dispersion within the past decades [4][5][6][7][8][9][10][11]. However, block copolymers are relatively expensive to be synthesized, which limits the practical application of block copolymers as compatibilizer in a large amount.…”