Addition Of Polymer Chains Research Articles

Laponite/poly(2-methyl-2-oxazoline) hydrogels: Interplay between local structure and rheological behaviour

HypothesisDispersions of Laponite in water may form gels, the rheological properties of which being possibly tuned by the addition of polymer chains. Laponite-based hydrogels with poly(ethylene oxide) (PEO) were the most widely investigated systems and the PEO chains were then found to reduce the elastic modulus. ExperimentsHere, hydrogels based on Laponite and poly(2-methyl-2-oxazoline) (POXA) were considered. The adsorption behavior and the local structures within these nanocomposite gels were investigated by small-angle neutron scattering and NMR. The same materials were macroscopically characterized using rheology. FindingsAn original evolution of the storage modulus G' with the POXA concentration is evidenced compared to Laponite/PEO hydrogels. At low POXA concentrations, a continuous reduction of G' is observed upon increasing the polymer content, as with PEO, due to the screening of electrostatic interactions between the clay platelets. However, above a critical value of the POXA concentration, G' increases with the polymer content. This difference with PEO-based hydrogels is correlated to the stronger affinity of POXA chains for the clay surfaces, which results in the reduction of the inhomogeneities for the Laponite disks within the gels. Steric repulsions would then counterbalance the effect of electrostatic repulsions and lead to the strengthening of the POXA-based hydrogels.

Molecular dynamics simulation for shape change of water-in-oil droplets

We performed molecular dynamics (MD) simulations of water-in-oil droplet shape transformations induced by the addition of polymer chains. In a prior experiment, transformations of spherical droplets to rod-like, worm-like and network-like droplets were observed. In our previous study, we reproduced rod-like droplets via coarse-grained MD simulations, and the mechanism for the droplet shape change was elucidated by considering the contact area between the chains and the surfactant head groups. However, in that simulation model, we could not reproduce the worm-like and network-like droplets. In this study, we improved the simulation model. For a small number of chains, several spherical droplets were obtained. As the number of chains increased, the spherical droplets were transformed to rod-like, worm-like and network-like shapes by coalescence of the droplets. The calculated and experimental results agreed well, and we verified that the mechanism for the droplet shape transformations observed in the present simulations could be explained by the mechanism suggested in the previous study.

Molecular dynamics simulation for morphological change of water-in-oil microemulsion droplets induced by addition of polymer chains

We carried out molecular dynamics simulations to investigate the morphological change of water-in-oil microemulsion (ME) droplets by addition of polymer chains. Several experiments have shown an increase in the polydispersity of ME droplet size and the morphological transition of spherical ME droplets to a rod-like shape by encapsulating the polymer chain in ME droplets. In order to understand these changes in the ME droplets, we studied the effects of the chain concentration and the interaction strength between chains and surfactant. When the interaction strength was weak, we obtained spherical droplets rather than rod-like droplets. Two types of spherical ME droplets were observed: large spherical droplets containing polymer chains and small spherical droplets without chains, which resulted in a large size polydispersity. However, when the interaction strength was great, a coalescence of ME droplets containing polymer chains was observed and the morphology of the droplets finally changed to a rod-like shape with an increase in the chain concentration. The morphological change can be explained by the competition between the chain–surfactant interaction energy and the surfactant monolayer bending energy, thus the interaction energy prefers a rod-like shape to gain a large contact area between the chains and the surfactant monolayer.

Effect of attraction on the dynamical arrest of soft colloids

We consider various scenarios leading to molecular control of the rheological state of concentrated solutions of soft colloids, based on the star polymer prototype system. At high values of star functionality, where a dynamically arrested state exists, we discuss the mechanisms that bring about a restoration of ergodicity upon addition of polymer chains, and the theoretical evidence for this effect which is supported by previously published experimental results. For intermediate functionalities, a systematic shift of the glass transition to higher star densities upon addition of additive-induced attractions is established. Finally, for low functionalities, we put forward evidence for a novel phenomenon that is specular to that appearing at high functionalities: a glassy state, which is absent for the pure star system, and that can be induced by the introduction of suitable additives. The latter phenomenon takes place when the effective star–star interaction is modified to display short-range attractions and longer-range repulsions.

The Role of Fullerene-Like Structures in Carbon Black and Their Interaction with Dienic Rubber

The identification of fullerene-like sites on carbon black surface and in particular the pentagonal or corannulene-type sites, has permitted us to explain the chemical interaction between filler and polymer (diene rubber) occurring during mixing in terms of free radical and Diels-Alder addition of polymer chains on the fullerene-like sites of carbon black. The free radicals and diene sites are formed on rubber chains during mixing by the mechanochemical degradation, which is caused by shear stresses. We have also noticed the curious analogy between the turbostratic morphology of carbon black particles and the onion-like graphitic particles.

Reaction of “living” poly(ethylene oxide) chains with C 60

To determine the reactivity of fullerene toward oxanions, we have studied the grafting of “living” poly(ethylene oxide) (PEO-K +) and ethylene oxide “capped” polystyrene chains (PSCH 2CH 2O-K +) onto C 60 in THF. We demonstrate that, in this polar solvent, electron transfer from the oxanion to the highly electron accepting fullerene takes place. This transfer is limited to two electron per fullerene, and is accompanied by addition of polymer chains onto the C 60 probably through grafting of the generated radicals. SEC characterization of (PEO) XC 60 indicates the presence of products with molar masses corresponding to X≥6. That may result from aggregation occurring in this case as, using the same active species, practically only mono- and di-adducts are obtained if the chain is PS.