Micellization and Gelation of Triblock Copolymers of Ethylene Oxide and Styrene Oxide in Aqueous Solution

Abstract

Nine triblock copolymers of ethylene oxide and styrene oxide (type EmSnEm, E ) oxyethylene, S ) oxyphenylethylene, n and m ) number-average block lengths) were prepared by sequential oxyanionic polymerization. Surface tensiometry was used to determine critical micelle concentrations (cmc's) and standard enthalpies of micellization, and isothermal titration calorimetry was used to confirm the enthalpy of micellization. Light scattering was used to determine micellar association numbers and hydrodynamic radii. Phase diagrams defining regions of hard and soft gel were determined by tube inversion and Couette rheometry. Comparison is made with reported results for diblock copolymers of ethylene oxide and styrene oxide and, so far as possible, with results for triblock copolymers of ethylene oxide and styrene. Compilation of values of the cmc for three series of triblock copoly(oxyalkylene)s, EmSnEm, EmBnEm (B ) oxybutylene), and EmPnEm (P ) oxypropylene), reveals a discontinuity in the block length dependence of log(cmc) at S6 and B12.

Characterization of triblock copolymer E67S15E67–surfactant interactions

“…A similar profile to that obtained for SLS data is derived. On the other hand, the hydrodynamic radius of E 67 S 15 E 67 micelles obtained for the surfactant-free solution was 8.1 nm, which is very similar to that obtained by Yang et al [18] (8.3 nm).…”

“…Therefore, it was necessary to choose a concentration and a temperature where the copolymer systems demonstrate a well-defined relaxation time distribution with micelles as the single scattering species. At this respect, copolymer E 67 S 15 E 67 in water at 25°C has an average aggregation number of 25 monomers and an apparent hydrodynamic radius (radius of the hydrodynamically equivalent hard sphere corresponding to the apparent diffusion coefficient) of 8.3 nm [18]. It has been also shown that the cmc of this copolymer is almost temperature independent, with a value of ca.…”

“…The synthesis and experimental characterization to determine purity and stoichiometry of the triblock copolymer E 67 S 15 E 67 was described in detail by Yang et al [18]. Table 1 shows the molecular characteristics of the copolymer.…”

“…This fact might be related to the lower dry micellar density of the triblock copolymer (0.14 g cm À3 ) if compared to that of the diblock (0.16 g cm À3 ), which would involve an even more open micellar structure full of solvent in the former case. Furthermore, the presence of two junctions in the core forming block of the micelles does not allow a so tightly packing of the micelle core as in the case of the diblock copolymers, for which the micellization enthalpy change is null indicating that the hydrophobic block of E m S n copolymer is significantly coiled even in the unimer state [18,27]. However, for triblock E m S n E m copolymers this enthalpy change is important, confirming a less dense packing of the S block which results in lower aggregation numbers and hence, a slight reduction in hydrodynamic radii.…”

“…However, to our best knowledge, no attention has been paid to the mixtures of surfactants and block copolymer comprising a hydrophilic poly(oxyethylene) block and a second hydrophobic poly(oxyphenylethylene) block. In recent papers, we have studied the micellization and gelation properties of both diblock and triblock architectures of this class of copolymers [17][18][19] (denoted as E m S n or S n E m for diblocks depending on polymerisation sequence, and E m S n E n for triblocks, where E denotes the oxyethylene unit, OCH 2 CH 2 , and S denotes the oxyphenylethylene, OCH 2 CH(C 6 H 5 ), and the subscripts m and n denote the number-average block lengths in repeat units). In a recent paper, we have described the interactions between a diblock (styrene oxide-ethylene oxide) copolymer with cationic and anionic surfactants by different experimental techniques [20][21][22].…”