Microphase Separation and Shear Alignment of Gradient Copolymers: Melt Rheology and Small-Angle X-Ray Scattering Analysis

Abstract

The degree of microphase or nanophase segregation in gradient copolymers with compositions varying across the whole copolymer backbone is studied via low-amplitude oscillatory shear (LAOS) measurements and small-angle X-ray scattering (SAXS). Studies are done as a function of comonomer segregation strength, molecular weight (MW), gradient architecture and temperature. Controlled radical polymerization is used to synthesize strongly segregating styrene/4-acetoxystyrene (S/AS) and the more weakly segregating S/n-butyl acrylate (S/nBA) gradient copolymers. Results are compared to those from S/AS and S/nBA random and block copolymers. The higher MW S/AS gradient copolymer exhibits LAOS behavior similar to the highly microphase segregated S/AS block copolymer, while the lower MW S/AS gradient copolymer exhibits complex, nonterminal behavior indicative of a lower degree of microphase segregation. The S/nBA gradient copolymers demonstrate more liquidlike behavior, with the lower MW sample exhibiting near-Newtonian behavior, indicative of a weakly segregating structure, while the higher MW, steeper gradient sample shows behavior ranging from solidlike to more liquidlike with increasing temperature. With the exception of the lower MW S/nBA case, the gradient copolymers exhibit temperature-dependent LAOS behavior over a wide temperature range, reflecting their temperature-dependent nanodomain composition amplitudes. The S/AS samples have SAXS results consistent with the degree of microphase segregation observed via rheology. Shear alignment studies are done on the higher MW S/AS gradient copolymer, which is the most highly microphase segregated gradient copolymer. Rheology and SAXS provide evidence of shear alignment, despite the gradual variation in composition profile across the nanodomains of such gradient copolymers. A short review of the nomenclature and behavior of linear copolymer architectures is also provided.

Block Copolymers in External Fields: Rheology, Flow‐Induced Phenomena, and Applications

“…The role of macromolecular architecture-i.e., block sequences like triblocks, multiblocks, and gradient copolymers or branching like star and graft block copolymers-has been explored in length with respect to the different aspects of the rheological response discussed above (mainly linear viscoelasticity, ODT, and OOT). The key findings are qualitatively similar to those reported above [92][93][94]. The observed quantitative differences are attributed to the architecture and often rationalized theoretically [90,95,96].…”

“…Moreover they do not have to be pure homopolymers. Many block copolymers used in applications are gradient or tapered block copolymers whose composition varies gradually along the chain or the blocks from A-rich at one end to B-rich at the other end [12][13][14][15][16][17]. It is nearly impossible to list the huge variety of block copolymers that have been synthesized and investigated.…”

Highlighting the Role of the Random Associating Block in the Self-Assembly of Amphiphilic Block–Random Copolymers

“…60 Reports in the literature have shown that the control of the gradient strength was an elegant way of driving the self-assembly of such amphiphilic copolymers. 61−63 ■ ASSOCIATED CONTENT…”

Critical micelle concentrations of block and gradient copolymers in homopolymer: Effects of sequence distribution, composition, and molecular weight

“…(For a review and comparison of gradient copolymer structure relative to that of other copolymers, see ref. 49.) Using self-consistent field theory, Shull 41 predicted that gradient copolymers exhibit a substantially lower CMC than do block copolymers of similar MW and composition.…”