Morphological Phase Diagram for a Ternary System of Block Copolymer PS₃₁₀-b-PAA₅₂/Dioxane/H₂O

Abstract

The partial ternary phase diagram was investigated for the polystyrene 310 -block-poly(acrylic acid) 52 copolymer in dioxane/water mixtures in regions in which self-assembled nanoaggregates of various morphologies are seen. Both fractionated and unfractionated copolymers were used; the unfractionated copolymer contains homopolystyrene. The study was carried out over the range of water contents from 0 to 45 wt % and copolymer concentrations from 0.1 to 10 wt %. Freeze-drying transmission electron microscopy (TEM), turbidity measurements, as well as static and dynamic light scattering were employed. Because of the proximity of the melting points and boiling points of water and dioxane, quenching and subsequent freeze-drying of solution samples can be employed to preserve aggregate morphologies. The morphologies can then be observed using TEM. The reversibility of various morphological transitions was examined by means of TEM and turbidity measurements. With increasing water content, the sequence of copolymer structures in solution follows the order of single chains, spheres, sphere and rod mixtures, rods, rod and vesicle mixtures, and finally pure vesicles. The morphologies observed here are under thermodynamic control. Not only the water content but also the polymer concentration affects the morphologies and the sizes of the aggregates. For the unfractionated polymer, the single-chain/sphere boundary shifts to lower water contents relative to that of the fractionated copolymer, while the other morphological boundaries move to higher water contents. On the basis of the progressive changes of the aggregate morphologies with the addition of water, possible pathways of the morphological transitions are suggested and discussed briefly. Also, approximate thermodynamic functions are estimated for the morphological transitions on the basis of the morphological boundaries. The combination of freeze-drying TEM techniques with turbidity measurements is very useful in exploring the morphological behavior of block copolymers in solution.

Self‐assembled, wormlike‐cylinder network of polystyrene‐block‐poly(ethylene oxide) micelles in solution

“…14 It has been observed that the morphological changes from wormlike cylinders to vesicles in PBD 103 -b-PAA 75 occur via branched cylinders and a wormlike-cylinder network, 14 whereas in the case of the crew-cut morphology of PS 310 -b-PAA 52 in DMF/water, the change from cylinders to vesicles occurs without the formation of a network. 39 In our case (block copolymer PS 962 -b-PEO 227 ), when the solvent mixture is DMF/water, the change from cylinders to vesicles occurs via branched cylinders and the network, whereas in the DMF/acetonitrile solvent mixture, the change occurs without the formation of the network. Again, the solvent plays a crucial role in determining the morphological pathway of the change from cylinders to vesicles.…”

Density functional simulation of spontaneous formation of vesicle in block copolymer solutions

“…This agree with the experimental results qualitatively. Shen and Eisenberg [49,50] reported that by controlling solvent condition, one can control the morphology of the block copolymer micelles. The control of the morphology is especially important for application use such as the drug delivery system.…”

“…The polymerization index of PS-PEO diblock copolymer used in the experiments [51] is typically ≃ 1000, thus we expect that χ PS,PEO N is sufficiently larger than the critical value χ c N = 10.495 [14]. Note that the polyelectrolytes such as poly(acrylic acid) (PAA) is often used as the hydrophilic subchian [49,50]. Such polyelectrolytes can be dissolved into water easily, and thus we expect the χ parameter between the polyelectrolytes and the water is negative.…”

Preparation of block copolymer vesicles in solution

“…The dependence of the morphology on the concentration can be seen very clearly in the phase diagram (discussed later). 43 Shen and Eisenberg 41 showed that in the phase diagrams for different PS-b-PAA copolymer systems, when the water content is at a fixed value, the initial copolymer concentration is important in determining if vesicles can be formed. For example, at a fixed water content of 25 wt %, vesicles only form at concentrations greater than approximately 0.6 wt % PS 310 -b-PAA 52 copolymer.…”