Thermoreversible Morphology Transitions of Poly(styrene-b-dimethylsiloxane) Diblock Copolymer Micelles in Dilute Solution

Abstract

Dilute solutions of a poly(styrene-b-dimethylsiloxane) diblock copolymer with block molecular weights of 4 and 12 kDa, respectively, were prepared in a series of styrene-selective dialkyl phthalates: dioctyl phthalate (DOP), dibutyl phthalate (DBP), and diethyl phthalate (DEP). The phthalates were chosen because the interfacial tension between the core block and the solvent can be continuously varied by mixing the solvents in varying proportions. The morphologies were characterized by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS). By increasing the selectivity of the mixed solvent at room temperature the equilibrium micelle morphology changed from spheres (DOP) to cylinders (DBP) to vesicles (DEP). The selectivity of the solvent was then reduced by increasing the temperature, and we observed the reverse transitions: cylinders → spheres (in DBP) and vesicles → cylinders → spheres (in DEP). Since the core block is always above its glass transition temperature (T g ≈ −123 °C), micellar rearrangement was possible within the time scale of the experiment (i.e., minutes). An interesting consequence of these thermotropic transitions is that the viscosity of the solution can be increased upon heating. For example, in the mixed solvent DEP/DBP (1:1), the diblock forms vesicles at room temperature, but when heated, the micelle morphology changes to cylinders and the viscosity of the solution increases by an order of magnitude.

Structural and thermodynamic aspects of the cylinder-to-sphere transition in amphiphilic diblock copolymer micelles

“…calc cyl (14) where f spheres is the fraction of spherical micelles; I(Q) As demonstrated by the solid line in Fig. 2 The data above 51% d-DMF could be well described using the spherical core-shell model described in Section 2.…”

Effect of the Soluble Block Size on Spherical Diblock Copolymer Micelles

“…This is in good agreement with the CMC variations predicted by theory as shown by the lines in Figure 3 (see Appendix 1 for the corresponding equations). The observed discrepancy between theory and experiment in Figure 3 (especially for sample [19][20][21][22][23][24][25][26]) is attributed to exponential sensitivity (eq A1, Appendix 1) of CMC measurements to small variations in temperature and diblock composition. Nevertheless, the agreement is regarded as good because the discrepancy remains within the same order of magnitude.…”

“…[19][20][21] To this end, much work has been done to study the micellization kinetics. [22][23][24][25][26] While these studies have provided a wealth of information, the kinetics of micellization appears to be very dependent on specifics of the studied systems.…”

Ethanolic RAFT Dispersion Polymerization of 2-(Naphthalen-2-yloxy)ethyl Methacrylate and 2-Phenoxyethyl Methacrylate with Poly[2-(dimethylamino)ethyl Methacrylate] Macro-Chain Transfer Agents

“…For example, changes in temperature, [7][8][9][10][11] pH, [12][13][14][15][16] or electrolyte concentration [17][18][19][20][21][22][23] can be employed to induce conformational changes, self-assembly, and/or disassembly in block copolymers, and although a more rare phenomena, can actually cause morphology changes. [7,[24][25][26] Accessing such nanoaggregates has, historically, been accomplished in relatively dilute solutions (for example, ,1 wt-% is common) employing pre-prepared, well-defined block copolymers (often having utilized synthetically demanding approaches such as living anionic polymerization or quasiliving cationic polymerization as synthetic tools), followed by assembly in a selective solvent employing various, often timeconsuming, protocols. While a perfectly valid and welldocumented approach for the preparation and subsequent study and application of soft matter nano objects, a more facile method facilitating access to such materials, especially less common higher ordered species such as worms or cylinders, is clearly desirable.…”