While styrene acrylic-acid block copolymers show quite rich morphologies, styrene acrylic-acid random copolymers (PS-r-PAA) are believed to form only colloidal particles in water, meaning that many PS-r-PAA molecules randomly aggregate to form micrometer-sized particles that disperse in water to form a milky white solution. However, this colloidal particle is just one of their rich polymorphisms. Recently we found that PS-r-PAA creates an ultrathin (thickness is about 4 nm) and several hundred micron-sized sheets. The sheet area appeared to reach a few mm2, amazingly large compared with its thickness. Such a sheet can only be obtained when PS-r-PAA was dissolved in a strong alkaline solution (NaOH pH &gt;12) and slowly neutralized by adding HCl. The same structural transition was observed for styrene maleic acid alternate copolymers (PSMA). From the SAXS, TEM, and FFF-MALS measurements, we can conclude that (1) PS-r-PAA and PSMA are in a molecularly dispersed unimer state in alkaline conditions with the morphology of a two-phase ellipsoidal micelle, (2) Their structure show a clear transition to an ultrathin and wide sheet with decreasing pH to neutral, (3) The size of the sheet and the elliptical core is comparable (or twice) the size of the phenyl side chain of PS, (4)There was no significant difference between PS-r-PAA and SMA, except for the transition pH, and (5) Both the ellipsoid and the sheet are very uniform in shape. Both structures and the structural transition may be rationalized by the combination of the microphase separation induce by sidechain segregation and the packing parameter principle. Based on (4) and (5), we can speculate that most of the phenyl side chains are segregated into the hydrophobic domain and thus the backbone chain of PS-r-PAA and SMA must be strictly anchored on the boundary of the core-sheal of the sheet. Such a boundary of the sheet can be regarded as pseudo two dimensions.

An alternating copolymer exhibiting the microphase-structure transition from ultrathin plate to unimeric sphere

ACS Spring 2022

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