Tuning the Phase Behavior of Polystyrene-block-poly(n-alkyl methacrylate) Copolymers by Introducing Random Copolymer for Methacrylate Block

Abstract

Block copolymers have gained increasing interest because of their self-assembly into periodic morphologies such as lamellae, hexagonally packed cylinders, spheres (body-centered cubic), and gyroids with nanometer length scale. The phase behavior of a block copolymerdepends on the volume fraction ( f ) of oneof the blocks, the degree of polymerization (N), and the Flory-Huggins segmental interaction parameter (χ). Among many block copolymers, weakly interacting polystyrene-block-poly(n-alkyl methacrylate) copolymers (PS-b-PnAMA) show various phase behaviors depending on the length of alkyl chain in PnAMA: order-to-disorder transition (ODT), lower disorder-to-order transition (LDOT), and a closed-loop (immiscibility loop) with both LDOT and upper order-to-disorder transition (UODT) upon heating. The phase behavior of PS-bPnAMA was successfully explained by a compressible random phase approximation developed by us. Very recently, we found closed-loop phase behavior for polystyrene-block-poly(n-butyl-ran-n-hexyl)methacrylate copolymers (PS-b-Pn(B-r-H)MA) similar to polystyrene-block-poly(n-pentyl methacrylate) copolymers (PS-b-PnPMA), whereas PS-bPnHMA showed only ODT-type phase behavior, but PS-bPnBMA exhibited LDOT-type phase behavior. Also, the pressure coefficient of the transition temperatures for PS-b-Pn(B-r-H)MA (dTLDOT/dP=þ595 C/kbar and dTUODT/dP = -700 C/kbar) was much greater than that of PS-b-PnBMA (dTLDOT/dP=þ147 C /kbar) and PS-b-PnHMA (dTUODT/ dP=-60 C/kbar). Since the carbonnumbers in the alkyl group of PnBMA and PnHMA are 4 and 6, respectively, the random copolymer, Pn(B-r-H)MA, consisting of the same molar ratio of PnBMA and PnHMA, would exhibit a similar phase behavior to PnPMA with n=5. Namely, an average force field of Pn(B-r-H) MA having the same weight fraction of PnBMA and PnHMA would be similar to that of PnPMA. Ruzette et al. reported that polystyrene-block-poly(methacrylate-ran-lauryl methacrylate) copolymers (PS-b-P(MMA-rLMA)) exhibited the LDOT, even though both PS-b-PMMA and PS-b-PLMA exhibited the ODT. They explained this interesting phase behavior by the fact that the specific volume and the solubility parameter of P(MMA-r-LMA) are very similar to those of neat PnBMA, which exhibited the LDOT. They also found that the dT/dP of the LDOT of PS-b-P(MMA-r-LMA) was 150 C/kbar, which is similar to that (147 C/kbar) of PS-bPnBMA. Thus, it is interesting to investigate whether the closed-loop phase behavior is observed for polystyrene-block-poly(n-alkylran-n0-alkyl methacrylate) copolymers with very different values of n and n0. To prove this postulation, we chose polystyreneblock-poly(n-octyl-ran-methyl) methacrylate copolymers (PS-bPn(O-r-M)MA) (namely, n=8 and n0=1). We found that when the total molecular weight and the composition of the random copolymer block were judiciously controlled, PS-b-Pn(O-r-M)MA exhibited closed-loop phase behavior having both LDOT and UODT within an experimentally accessible temperature range. This is an interesting result because both PS-b-PMMA and PS-b-PnOMA showed only ODT during heating. This is distinctly in contrast to PS-b-Pn(B-r-H)MAwhere PS-b-PnBMA showed LDOT, whereas PS-b-PnOMA exhibited UODT. We consider that the closed-loop phase behavior observed for PS-bPn(O-r-M)MA is attributed to the fact that the average force field of Pn(O-r-M)MAwith∼70/30 (w/w) PnOMA/PMMAcomposition would be similar to that of PnPMA. Furthermore, PS-bPMMA exhibited small barotropicity with dTODT/dP=þ23 C/ kbar (that is, themiscibility between PS andPMMAdecreases with increasing P), while PS-b-PnOMA showed very weak baroplasticity with dTODT/dP = -5 C/kbar (that is, the miscibility between two block components is slightly enhanced upon pressurization). However, PS-b-Pn(O-r-M)MA showed excellent baroplasticity similar to PS-b-PnPMA and PS-b-Pn(B-r-H)MA. We explained this phase behavior by using a compressible random phase approximation.