Origins of Anomalous Micellization in Diblock Copolymer Solutions

Abstract

The phenomenon of "anomalous" micellization is investigated in dilute solutions of two nearly symmetric poly(styrene-b-isoprene) diblock copolymers, via dynamic light scattering. In two polystyrene-selective solvents, diethyl phthalate and dimethyl phthalate, the critical micelle temperatures (cmt) are clearly determined by rather abrupt increases in the hydrodynamic radius and scattered intensity upon cooling; this corresponds to "normal" micellization. In contrast, for the same polymers dissolved in two polyisopreneselective solvents, tetradecane and squalane, anomalous micellization was consistently observed: the hydrodynamic radius exceeded 1000 Å and the intensity was unusually large, over a modest temperature interval just above the cmt. We propose that anomalous micellization is due to the incipient phase separation of small amounts of polystyrene homopolymer, resulting from incomplete crossover during the sequential anionic polymerization of styrene and isoprene. In one sample, the presence of homopolymer (ca. 1 wt % of copolymer) was confirmed by chromatography, and removal of this impurity eliminated the anomalous micellization. Addition of similar amounts of polyisoprene homopolymer to the same sample induced anomalous micellization in the polystyrene-selective solvents, with the onset of the effect corresponding to the cloud point for the polyisoprene homopolymer. These observations support the proposed hypothesis; the extent to which this hypothesis may extend to previous reports of anomalous micellization is discussed. LA0268808

Solubility of fluorinated homopolymer and block copolymer in compressed CO2

“…From a qualitative point of view, when the solvent quality is improved, the PFDA becomes more soluble, the micelles start to break up and the PS-blocks, which were initially protected inside the micelles, are more exposed to CO 2 . The observation of larger aggregates instead of the formation of unimers can be explained in accordance with results from a recent micellization study of diblock copolymer solutions [50], where a solvent-phobic homopolymer residue as low as 1 wt.% within the sample influenced a drastic size increase of polymeric aggregates when the solvent quality was tuned and the micelle-to-unimers transition was approached [50]. Below the micelle-to-unimer transition, the solvent-phobic homopolymer is solubilized inside the micelles and it has no discernable effect on the solution properties of the sample; close to the micelle-to-unimer transition, part of the diblocks having the longest soluble chains are released as unimers and the formation of large aggregates is attributed to the flocculation of particles poorly stabilized by the diblock and rich in solventphobic homopolymer.…”

Characterization of oligosaccharide-functionalized hyperbranched poly(ethylene imine) and their complexes with retinol in aqueous solution

“…The rate of growth and aggregate size will depend on the probability of binding sites on the PEI and PEI-Mal structures where postulated hydrophobic interactions play an important role for the growth of aggregates. The observed growth of aggregates in the presence of retinol may be related to anomalous micellization that has been reported by different research groups [36][37][38]. This phenomenon is accompanied with the growth of large structures (hydrodynamic radius usually more than 100 nm) and this effect is frequently ascribed to minute hydrophobic impurities, and perhaps retinol in our systems can act as this type of impurity.…”

“…This leads to isolated oligosaccharide units on the periphery and the presence of unreacted primary amino groups that can be efficient in the formation of intermolecular hydrogen bonds with retinol and thereby create sticky molecules. The growth of aggregates discussed above may be related to the phenomenon called anomalous micellization discussed by several research groups [36][37][38]. This effect usually resulted in very large hydrodynamic radius (more than 100 nm) of the species and it was attributed to minute hydrophobic impurities, and it is possible that the small amount dissolved retinol may play a similar role for the present systems.…”

Temperature-dependent micellar structures in poly(styrene-b-isoprene) diblock copolymer solutions near the critical micelle temperature

“…36 The intensity correlation functions in this regime can be fitted to a sum of two exponentials; the faster mode corresponds to the single chains and the slower model is due to the large assemblies, which we proposed to be emulsionlike droplets. We have demonstrated previously that the anomalous micellization is due to the incipient phase separation of small quantities of PS homopolymer, resulting from incomplete crossover to the second block during the sequential living anionic polymerization.…”