Self-association of block copoly(oxyalkylene)s in aqueous solution. Effects of composition, block length and block architecture

Abstract

The article deals with the association behaviour in dilute aqueous solution of block copoly(oxyalkylene)s in which hydrophilic poly(ethylene oxide) is combined with hydrophobic poly(propylene oxide), poly(1,2-butylene oxide) or poly(styrene oxide). Polymers with three simple architectures are considered, i.e. copolymers of type EmAn, EmAnEm and AnEmAn, where E denotes an oxyethylene unit, A denotes a hydrophobic oxyalkylene unit, and the subscripts m and n denote number-average block lengths in repeat units. The aim is to examine how composition, block length and block architecture govern two fundamental properties, critical micelle concentration (cmc) and micelle association number (N), for systems which are in dynamic equilibrium. Copolymers with properties known to be greatly affected by heterogeneity in composition are excluded from consideration. A uniform pattern of behaviour emerges when log(cmc) is plotted against reduced hydrophobic block length (x), consistent with the micellisation equilibrium changing from one between unimers and multimolecular micelles at low values of x, to one between unimolecular micelles and multimolecular micelles at high values of x. Support for this model is provided by the enthalpy of micellisation, values of which fall effectively to zero as x is increased. Values of the micelle association number are used to define a critical hydrophobic block length for micellisation (n(cr)) for each class of diblock copolymers, values of which apply equally well to the half-length of the central block of corresponding EmAnEm triblock copolymers. Given these values, and irrespective of block architecture, the overall scaling law for the weight-average association number of the micelles is shown to be Nw = n'(1.07)m(-0.63) where m is the length (or half-length) of the hydrophilic block, and n' is the effective length of the hydrophobic block, equal to its length (or half-length) minus the critical length, i.e. n' = n-n(cr).

Some Thoughts on The Definition of a Gel

“…Gelation of low molar mass gelators, amino acid derivatives [67,68], porphyrin [69], and copoly(oxyalkylene)s [57,70] have been studied extensively, and they all seem to have a common feature different from polymeric gels, i.e., the gelating fibres are short and the gel is not rheologically robust; the linear viscoelastic regime is very narrow. Hvidt et al [62] found two different gels of 25.5 wt% triblock copolymers of ethylene oxide and propylene oxide (EO) 21 (PO) 47 (EO) 21 at lower temperature range from 30 to 40 C and at higher temperature range from 55 to 75 C. He called a lower temperature gel a ''hard gel'' and a higher temperature gel a ''soft gel'' because the former showed the storage modulus 10 4 Pa which is two orders of magnitude greater than the latter.…”

Synthesis of Symmetrical Triblock Copolymers of Styrene and N‐isopropylacrylamide Using Bifunctional Bis(trithiocarbonate)s as RAFT Agents

“…[21] The choice of an appropriate hydrophobic ''B'' block seems a priori nearly unlimited from the pool of water-insoluble polymers. Whereas still for micellar systems, hydrophobic blocks with low glass transition temperatures are favored in order to avoid ''frozen'' hydrophobic domains, [22][23][24][25][26] such ''frozen'' systems [27,28] seem attractive for hydrogel applications, as a high glass transition temperature of the physical cross-links should enhance the mechanical stability of the gels formed.…”

Synthesis of Double‐Hydrophilic BAB Triblock Copolymers via RAFT Polymerisation and their Thermoresponsive Self‐Assembly in Water

“…The self-assembly of such symmetrical amphiphilic triblock copolymers depends sensitively on the sequence of the blocks. [30][31][32] With A representing the hydrophilic block and B the hydrophobic block, ABA block copolymers tend to form core-shell micelles in aqueous solution, [33] while copolymers with a BAB architecture tend to give more complex structures, such as flower-like or interconnected micelles and networks, [34][35][36] which have also been discussed from a theoretical point of view. [37][38][39] Network formation is an important feature which distinguishes BAB block copolymers from self-associated AB diblock chains.…”