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.

Static light scattering was used to study the phase separation behavior of homopolystyrene and polystyrene-b-poly(acrylic acid) (PS-b-PAA) in DMF as a function of added water content. It was found that the critical water content (cwc), at which phase separation starts, depends on both the polymer concentration and the molecular weight. The higher the polymer concentration and the molecular weight, the lower the cwc. For PS homopolymer, phase separation involves the precipitation of the polymer chains. In the copolymer solution, phase separation results in the formation of regular crew-cut micelles consisting of a PS core and a PAA corona; thus, it is preferably referred to as microphase separation. The change of the micelle fraction as a function of water addition can be estimated from the relationship between the cwc and the initial copolymer concentration. The influence of added electrolytes, i.e., NaOH, HCl, NaCl, CaCl2, or Ca(Ac)2, on the self-assembly process of the copolymers in DMF was also explored. Since the micelle cores are highly swollen by DMF in the early stages of their formation, the structures are labile. As the added water content increases, the cores become gradually less swollen and the mobility of the polymer chains in the cores decreases. A study of polymer chain exchange among the micelles was performed by mixing two solutions of micelles of different sizes at different water contents and studying the micelle core size distribution by transmission electron microscopy. It is shown that the copolymer chain exchange within a 1 day period is significant at a water content of 6 wt %, but becomes negligible when the water content is increased to ∼11 wt %.

Multiple changes in the aggregate morphologies of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblocks have been observed as a function of the apparent pH (pH*) in DMF/H2O mixtures. The pH* changes were induced by adding HCl (in the concentration range 400 nM−20 mM) or NaOH (100 nM−20 mM). On the acid side, as the pH* increases from 7 (20 mM HCl) to 12.3 (the pH* of the original polymer solution without any additional microions), the aggregate morphology changes from large compound micelles (LCMs) to a mixture of spheres, rods, and vesicles (pH* = 8), to spheres (pH* = 8.4), to rods (pH* = 11.8), and then back to spheres (pH* = 12.3). In the presence of NaOH, as the pH* increases from 12.3 to 18 (20 mM NaOH), the morphology changes to rods (pH* = 12.6), then back to spheres again (pH* = 17.5), and finally to a mixture of spheres, rods, lamellae, and vesicles (pH* = 18). This level of morphological complexity as a function of pH* is unprecedented. The reasons for the behavior can be ascribed to the amphiprotic nature of P4VP in DMF. The addition of either an acid or a base introduces ionic groups into the corona chains. Thus electrostatic repulsion is introduced and the aggregate morphology changes generally in the direction of bilayers to spheres. However, due to the existence of multiple equilibria, some of the added microions are free, which decreases the steric−solvation interaction and decreases the electrostatic repulsion by shielding. This decrease in the corona repulsion tends to decrease the coil dimensions in the corona. As a result, the morphology is driven in the direction of spheres to bilayers. Therefore, a competition between unshielded electrostatic repulsion and shielding coupled with a decrease of the steric−solvation interaction is induced. At relatively low concentrations, the decrease of the steric−solvation interaction dominates, while at relatively high concentrations, the shielding dominates. In intermediate regions, the unshielded electrostatic repulsion is dominant. The morphological transitions induced by extremely low concentrations of HCl or NaOH (100 nM−1 μM) are very surprising. The effect of a neutral salt (NaCl) on the neutral copolymer and the effect of pH* on a quaternized copolymer were also explored.

The kinetics and mechanism of the rod-to-vesicle transition in aggregates of polystyrene310-b-poly(acrylic acid)52 diblock copolymers in dioxane/water mixtures are explored. The transition is induced by a jump in the water content from a point at which the morphologies are under equilibrium control. The transition mechanism is monitored by transmission electron microscopy (TEM). The transition intermediates are trapped by quenching solution samples to liquid nitrogen temperature and then preserving the aggregate morphologies using the freeze-drying technique. It is shown that the morphological transition goes through a lamellar intermediate state. The corresponding kinetic data are obtained from turbidity measurements. The analysis of the plot of turbidity versus time suggests two consecutive first order steps. The first step appears to be the flattening of short rods in favor of irregularly shaped or circular lamellae, which are the intermediate morphologies observed using TEM. The second step involves the closing of the lamellae to vesicles. Since each of the three species, i.e., rods, lamellae, and vesicles, makes a distinct contribution to the turbidity, the changes in the contribution of each species to the turbidity as a function of time in the transition process can be calculated quantitatively. The kinetic analysis shows that the relaxation times (τ 1 and τ 2) of the morphological transition are influenced by the initial water content and the polymer concentration. The relaxation times increase with increasing initial water content, while the reciprocals of the relaxation times approach zero when the initial water content reaches the upper boundary of the rod stability region. An increase in the polymer concentration leads to an increase in the relaxation times. The size of the jump in the water content has a little effect on the kinetics.

The block length dependence of the morphological phase diagram was investigated for PSb-PAA copolymers in dioxane/water using transmission electron microscopy and light scattering techniques. The study was performed on copolymers with PS block lengths ranging from 49 to 310 repeat units and PAA block lengths from 7.2 to 26 repeat units. The water content ranged from 0 to 50 wt % and the copolymer concentration from 0 to 10 wt %. For all the copolymers with increasing water content, the sequence of morphologies, in general, follows the order of single chains, spheres, sphere and rod mixtures, rods, rod and bilayer mixtures, pure bilayers, and finally mixtures of bilayers and inverted structures. The bilayers here include vesicles, lamellae, and more complicated structures. It was found that the boundaries of various morphological transitions generally shift to lower water contents with increasing total block length and with decreasing PAA block length. Most importantly, it was found that long core-forming blocks and high water contents favor the formation of vesicles and that short coreforming blocks and low water contents favor open bilayers (e.g., lamellae). This finding suggests that in block copolymer systems the increase of bending modulus favors vesicle formation, which reinforces the conclusion of a theoretical study for mixed small molecule surfactant systems. The Gibbs free energies for the single chain to sphere transition were also estimated from the morphological phase diagram. It was found that at the same water content the free energy becomes less negative both with decreasing total block length and with increasing PAA block length.

Mesosize crystal-like aggregates with an internal structure of hexagonally packed hollow hoops (HHH) in a polystyrene matrix have been prepared in solution by self-assembly of asymmetric polystyrene-b-poly(acrylic acid) diblock copolymers. Most of the aggregates are cylindrical or in the shape of truncated cones. The external surface of the aggregates and the internal surface of the hollow hoops are lines with short poly(acrylic acid) chains. The hoop morphology is imposed because the end-capping energy of a rod on this size scale is more important than the curvature energy. A strong interdependence between the external shape and the internal structure in these mesosize particles is demonstrated. [S0031-9007 (97)04815-1] PACS numbers: 61.41. + e

No abstract

The thermodynamics of formation of "crew-cut" micelles of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymers were investigated. The PAA/PS ratios were in the range 0.08-0.33, and N,Ndimethylformamide/water (DMF/H 2 O) mixtures were used as the selective solvent. For the block ratios investigated here, the copolymers form spherical crew-cut micelles with a large PS core and a thin PAA corona. The temperature dependence of the critical micelle concentration (cmc) was determined under different conditions from scattered light intensity measurements as a function of temperature. The closed association model was found to describe the micellization process and was used to calculate the standard Gibbs free energies (∆G°), the standard enthalpies (∆H°), and the standard entropies (∆S°). The effects of solvent composition, copolymer composition, and added NaCl content on the thermodynamic functions were studied. From the experimental data, a single set of combined equations for the standard thermodynamic functions was obtained. These are the following: ∆G°2 98K (kJ mol -1 ) ) 20.4 -0.00859N PS + 0.00938N PAA -10.2W H2O -3.65mM NaCl ; ∆H°(kJ mol -1 ) ) -151 + 0.0906N PS -0.0692N PAA + 7.38W H2O -22.9mM NaCl ; ∆S°(J mol -1 K -1 ) ) -574 + 0.333N PS -0.264N PAA + 59.0W H2O -64.5mM NaCl . N PS and N PAA are the block lengths of the two copolymer components; W H2O is the weight percent of water in DMF; mM NaCl is the millimolar concentration of NaCl. ∆G°, ∆H°, and ∆S°values were all found to be negative under experimental conditions, which indicates that the standard entropy is unfavorable and that the standard enthalpy is solely responsible for micelle formation. However, it was also found that the enthalpy-driven process could be transformed to an entropy-driven process by increasing the water content or increasing the PS block length. The effect of homopolystyrene on the light scattering behavior during micellization was also studied. It was found that the addition of homopolystyrene increased the critical micelle temperatures (cmts) significantly.