125 research outputs found
A Viscoelastic model of phase separation
We show here a general model of phase separation in isotropic condensed
matter, namely, a viscoelastic model. We propose that the bulk mechanical
relaxation modulus that has so far been ignored in previous theories plays an
important role in viscoelastic phase separation in addition to the shear
relaxation modulus. In polymer solutions, for example, attractive interactions
between polymers under a poor-solvent condition likely cause the transient
gellike behavior, which makes both bulk and shear modes active. Although such
attractive interactions between molecules of the same component exist
universally in the two-phase region of a mixture, the stress arising from
attractive interactions is asymmetrically divided between the components only
in dynamically asymmetric mixtures such as polymer solutions and colloidal
suspensions. Thus, the interaction network between the slower components, which
can store the elastic energy against its deformation through bulk and shear
moduli, is formed. It is the bulk relaxation modulus associated with this
interaction network that is primarily responsible for the appearance of the
sponge structure peculiar to viscoelastic phase separation and the phase
inversion. We demonstrate that a viscoelastic model of phase separation
including this new effect is a general model that can describe all types of
isotropic phase separation including solid and fluid models as its special
cases without any exception, if there is no coupling with additional order
parameter. The physical origin of volume shrinking behavior during viscoelastic
phase separation and the universality of the resulting spongelike structure are
also discussed.Comment: 14 pages, RevTex, To appear in Phys. Rev
ABA triblock copolymers: from controlled synthesis to controlled function
The ABA amphiphilic block copolymers, poly(hydroxyethyl methacrylate-hlock-methylphenylsilane-block-hydroxyethyl methacrylate) (PHEMA-PMPS-PHEMA) and poly[oligo(ethylene glycol) methyl ether methacrylate-block-methylphenylsilane-block-oligo(ethylene glycol). methyl ether methacrylate] (POEGMA-PMPS-POEGMA) were successfully synthesised via atom transfer radical polymerisation (ATRP). Macroinitiators suitable for the ATRP of oligo(ethylene glycol) methyl ether methacrylate and 2-hydroxyethyl methacrylate were synthesised from the condensation reaction of alpha,omega-dihalopolymethylphenylsilane and 2'-hydroxyethyl 2-bromo-2-methylpropanoate. The copolymers were characterised using H-1 NMR and C-13 NMR spectroscopy and molecular weight characteristics were determined using size exclusion chromatography and H-1 NMR. The aggregation behaviour of some of the copolymers in water was studied using transmission and scanning electron microscopy and dynamic light scattering. These revealed the prevalent aggregate species to be micelles. Larger aggregates of 300-1000 nm diameter were also observed. The UV induced degradation of the aggregates was studied by UV-Vis spectroscopy. The thermal behaviour of selected copolymers was studied by differential scanning calorimetry and microphase separation of the two components was demonstrated
Self-assembled foam-like graphene networks formed through nucleate boiling
Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400 degrees C) SFG film were 11.8 S.cm(-1) and 91.2 Omega square(-1), respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (similar to 10 S.cm(-1))(.) The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs)open232
Étude de l’agrégation des copolymères séquences polystyrène-polyparaphénylène par diffusion de la lumière et des neutrons
Les copolymères séquencés polystyrène-polyparaphénylène (PS-PPP) sont solubles dans les solvants du PS. Leur caractérisation par Chromatographie de Perméation sur Gel (CPG) démontre qu’ils sont agrégés en solution. Nous présentons ici une étude de ces agrégats par diffusion de la lumière et par diffusion des neutrons. La diffusion de la lumière montre que ces copolymères ont un caractère amphiphile lorsque la composition en PPP est suffisamment faible. L’étude par diffusion des neutrons montre que le nombre d’agrégation et la compacité de la structure du coeur dépendent de la composition des copolymères et de la longueur de conjugaison. Le coeur des agrégats est en moyenne faiblement anisotrope
- …