Associative phase behaviour and disintegration of copolymer aggregates on adding poly(acrylic acid) to aqueous solutions of a PEO-PPO-PEO triblock copolymer

The influence of adding poly(acrylic acids) of two different chain lengths (PAA(25) and PAA(6000)) on the phase behaviour of the amphiphilic triblock copolymer Pluronic (R) P104 (P104 = (EO)(27)(PO)(61)(EO)(27), where EO = ethylene oxide, PO = propylene oxide) in H2O was investigated. The resulting phase equilibria and structures were investigated by visual inspection, using crossed polarizers, and small angle X-ray scattering (SAXS). At low and intermediate P104 concentrations an associative phase separation was found, where a concentrated phase separated out from a dilute one on adding PAA. The corresponding phase separation region appeared in the phase diagram as a closed, droplet-shaped miscibility gap that was significantly larger for the longer PAA. Neither of the coexisting phases contained any long-range ordered structures. At higher concentrations of P104 (above ca. 25 wt%) no miscibility gap appeared but, remarkably, the various ordered liquid crystalline phases observed in binary P104/H2O mixtures were eventually destroyed upon the replacement of H2O by PAA. A similar effect was found when propionic acid (PrA), corresponding to the repeating unit of the PAA chain, was added to aqueous P104. A decrease in the PAA length, in the series PAA(6000) - PAA(25) - PrA, increased the efficiency to destroy the structured phases. NMR self-diffusion measurements showed that the self-assembled P104 aggregates dissolved on replacing water with PrA. The same mechanism was found to be responsible for the effect of added PAA, that is, PAA (or PrA) acts as less selective "solvent'' for the PEO and PPO blocks compared to water

Transport Phenomena in Particle Suspensions: Sedimentation and Thermophoresis

This chapter deals with transport phenomena induced in colloidal suspensions and complex fluids either by gravity, which is the well established but nevertheless still stimulating subject of sedimentation, or by thermophoresis, a subtler and very intriguing effect that is still partly understood. Specifically, I shall highlight the wealth of information one can get by investigating the particle concentration profiles generated at equilibrium by sedimentation, or at steady-state by thermophoresis, and discuss some novel optical techniques that have been fruitfully exploited to study them

Diverging geometric and magnetic size distributions of iron oxide nanocrystals

An important reason to prepare magnetic nanoparticles of uniform size and shape is to ensure uniform magnetic properties. However, here, we demonstrate that magnetic iron oxide crystals of 20 nm or less with a low polydispersity of the geometric size can nevertheless have a strikingly broad distribution of the magnetic dipole moment. A comparative study was performed on nanoparticles with near-perfect crystallinity, twinning defects, or a high density of dislocations. Size, shape, and crystal defects were characterized with electron microscopy and X-ray diffraction, and magnetic dipole moments were determined from magnetization curves of dilute colloidal dispersions. The largest divergence was found for spherical particles with 3.5% geometric size polydispersity and 35% magnetic size polydispersity due to crystal lattice defects that disrupt single-domain magnetic spin coupling. This is in stark contrast with the usual implicit assumption that uniform size and shape guarantee welldefined magnetic properties of the individual particles