Ion aggregation in polymer gels

This brief review deals with our early experimental studies of ion aggregation in polymer gels proceeding via the condensation of counterions on the oppositely charged monomer units of the network with the formation of ion pairs and their clustering into multiplets. The two particular cases of the emergence of ion aggregates are considered: (a) for monovalent counterions in media of low polarity and (b) for multivalent counterions in water.     

Colloidal aggregation in polymer blends

We consider here a low-density assembly of colloidal particles immersed in a critical polymer mixture of two chemically incompatible polymers. We assume that, close to the critical point of the free mixture, the colloids prefer to be surrounded by one polymer (critical adsorption). As result, one is assisted to a reversible colloidal aggregation in the nonpreferred phase, due the existence of a long-range attractive Casimir force between particles. This aggregation is a phase transition driving the colloidal system from dilute to dense phases, as the usual gas-liquid transition. We are interested in a quantitative investigation of the phase diagram of the immersed colloids. We suppose that the positions of particles are disordered, and the disorder is quenched and follows a Gaussian distribution. To apprehend the problem, use is made of the standard phi(4) theory, where the field phi represents the composition fluctuation (order parameter), combined with the standard cumulant method. First, we derive the expression of the effective free energy of colloids and show that this is of Flory-Huggins type. Second, we find that the interaction parameter u between colloids is simply a linear combination of the isotherm compressibility and specific heat of the free mixture. Third, with the help of the derived effective free energy, we determine the complete shape of the phase diagram (binodal and spinodal) in the (Psi,u) plane, with Psi as the volume fraction of immersed colloids. The continuous "gas-liquid" transition occurs at some critical point K of coordinates (Psi(c) = 0.5,u(c) = 2). Finally, we emphasize that the present work is a natural extension of that, relative to simple liquid mixtures incorporating colloids.     

Colloid aggregation arrested by caging within a polymer network

The aggregation of colloidal particles within the confines of a polymer network has been studied. An isorefractive covalently cross-linked polymer gel in dimethyl sulfoxide was formulated so that the multicomponent system that is the gel is essentially invisible to light scattering. The high dielectric solvent was chosen so that electrostatics could be used to control the state of aggregation of a colloid dispersed within the gel matrix. Smoluchowski's population balance equations were solved for the case where aggregates larger than the gel's mesh spacing are immobile. Light scattering intensities predicted from the evolution of the aggregate population were calculated. The observed asymptotic increase in scattering intensity is consistent with this model and indicates that the aggregation process becomes arrested by the spatial constraints imposed by the mesh of the polymer network. Essentially once the aggregates reach a certain size, they become caged within the mesh of the gel network and thus no longer aggregate. Evidence is also given that indicates that formulating for specific gel properties can lead to controlled final aggregate size.     

Shear-induced aggregation of carboxylated polymer latices

Orthokinetic flocculation of latex particles containing high levels of surface carboxylation was studied by shearing for short times in a rotational viscometer and measuring changes in particle size using photon correlation spectroscopy. Results can be interpreted in terms of a comparison between the DLVO repulsive force between a pair of particles and the hydrodynamic shear force opposing it. This enables a prediction of the critical shear rate, , required to initiate aggregation. Experimental and calculated values of showed good agreement.The stability of the carboxylated latices under shear was much reduced at pH values >7 when the surface groups are ionised. The increase in suspension viscosity with pH was shown to be critical in determining the onset of aggregation via hydrodynamic forces.     

Coupling between polymer adsorption and colloidal particle aggregation

Studies of the adsorption of high molecular weight polymers on colloidal latex and silica particles and their subsequent flocculation were carried out. Neutral polyethylene oxide samples with both a narrow and a broad molecular weight distribution were used together with low charged cationic copolymers. The influence of the particle concentration and polymer dose on the flocculation were systematically investigated under quiescent conditions.Equilibrium bridging only occurred with polyelectrolyte, even in very dilute suspensions, at high particle coverage. In contrast to this, non-equilibrium bridging occurred with both neutral polymer and polyelectrolytes but only for more concentrated suspensions and small amounts of adsorbed polymer. Polymer adsorption in dilute suspensions, which did not show particle aggregation was measured an electrophoretic technique. In more concentrated suspensions, where flocculation takes place, we found that aggregation prevents further polymer adsorption and induces both an excluded volume and a surface effect. The consequences on the shape of the isotherms differ according to the aggregation mechanism.A significant decrease of the amount, , of adsorbed polymer is observed with non-equilibrium bridging. When both mechanisms simultaneously contribute to the aggregation, the value of depends on their relative importance. In the intermediate range of copolymer dose their respective contributions are critically sensitive to the details of the mixing step and stirring, leading to non reproducible experimental results.     

Structure and phase transitions of a liquid crystalline polymer

Summary The structures, textures as well as thermodynamic properties of a side chain polymer exhibiting a liquid crystalline phase in addition to a partially crystalline state and the isotropic fluid state were investigated. Furthermore the kinetics of phase transitions between these states were analyzed. It was found that the properties of this polymer are intermediate between that of low molecular weight liquid crystals and common polymers. In particular it was observed that the relation between the liquid crystalline texture and the structure is different from that of low molecular weight liquid crystals and that the properties of the crystalline and liquid crystalline state depend strongly on the tacticity of the polymer.

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Zusammenfassung Für ein Seitenkettenpolymeres, das zusätzlich zum teilkristallinen Zustand und zur isotropen Schmelze einen flüssig-kristallinen Zustand aufweist, wurden die Strukturen und Texturen untersucht sowie thermodynamische Eigenschaften. Außerdem wurde die Kinetik der Phasenumwandlungen analysiert. Es zeigt sich, daß die Eigenschaften zwischen denen üblicher Polymerer und denen von niedrigmolekularen flüssigen Kristallen liegen. Insbesondere zeigte sich, daß die Beziehung zwischen molekularer Struktur und Textur von der bei niedrigmolekularen flüssigen Kristallen abweicht, und daß die Taktizität der Kette einen starken Einfluß auf die Eigenschaften nicht nur der kristallinen Phase sondern auch der flüssig-kristallinen Phase ausübt.

     

The effect of inhomogeneous stickiness on polymer aggregation

The aggregation of polymers is important in the formation of marine aggregates and the vertical transport of material in the ocean. A polymer may be inhomogeneous along its length, with associating groups at some points along its length where bonds are more likely to form. In this paper we investigate the effects of inhomogeneous 'stickiness' along the polymer length. We describe the results of three-dimensional off-lattice simulations of polymer-polymer aggregation for four different types of polymer: polymers which are sticky along their entire length, polymers which are sticky at the ends only and two types of polymer which are slightly sticky along their entire length. We examine the mean radius of gyration and the fractal dimension of the resulting aggregates and the dynamics of aggregation. The slightly sticky polymers and the polymers which are sticky only at the ends form aggregates with a higher fractal dimension than the polymers which are sticky along their entire length. However, the mean radius of gyration of the aggregates formed by polymers which are sticky only at the ends is significantly larger than that of the aggregates formed from slightly sticky polymers. The aggregation dynamics are also different for the polymers which are sticky only at the ends compared to the slightly sticky polymers. A single 'stickiness value' is therefore likely to be inadequate to describe a polymer. We also examine the effect of polymer rigidity; it seems that the effect of inhomogeneous stickiness is greater for almost-straight polymers than for coiled chains.     

Optimizing polymer aggregation and blend morphology for boosting the photovoltaic performance of polymer solar cells via a random terpolymerization strategy

Compared to regular conjugated polymers,the random conjugated terpolymers are usually not beneficial to achieve highly efficient non-fullerene (NF)-based polyme...     

Structure and dynamics of a bisurea-based supramolecular polymer in n-dodecane

The structure and dynamics of a supramolecular polymer formed by a bisurea-type compound, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in an apolar solvent, n-dodecane (C12), were examined in detail. The EHUT/C12 organo-gel system forms long, dynamic chain-like supramolecular polymers, which lead to an entangled network showing remarkable viscoelastic behavior with two major relaxation modes. A slow relaxation mode with an approximately constant relaxation time, tauS, was observed in a flow region and the other, fast, relaxation mode with a time tauF1 (相似文献   

Scaling of the kinetics of slow aggregation and gel formation for a fluorinated polymer colloid

The aggregation and gelation kinetics in moderately concentrated (0.004 相似文献   

Protein PEGylation attenuates adsorption and aggregation on a negatively charged and moderately hydrophobic polymer surface

Covalent grafting of poly(ethylene glycol) chains to proteins ("PEGylation") is emerging as an effective technique to increase the in vivo circulation time and efficacy of protein drugs. PEGylated protein adsorption at a variety of solid/aqueous interfaces is a critical aspect of their manufacture, storage, and delivery. A special category of block copolymer, PEGylated proteins have one or more water-soluble linear polymer (PEG) blocks and a single globular protein block that each exert distinct intermolecular and surface interaction forces. We report the impact of PEGylation on protein adsorption at the interface between aqueous solutions and solid films of poly(lactide-co-glycolide) (PLG), a moderately hydrophobic and negatively charged polymer. Using the model protein lysozyme with controlled degrees of PEGylation, we employ total internal reflection fluorescence techniques to measure adsorption isotherms, adsorption reversibility, and the extent of surface-induced aggregation. Lysozyme PEGylation reduces the extent of protein adsorption and surface-induced aggregation and increases the reversibility of adsorption compared to the unconjugated protein. Results are interpreted in terms of steric forces among grafted PEG chains and their effects on protein-protein interactions and protein orientation on the surface.     

Structure of a semidilute polymer solution under steady shear

We use Brownian dynamics computer simulations to investigate the structure of a semidilute polymer solution undergoing a steady, uniform shear flow. We find that the contributions to structure factor from intra- and interchain correlations, which cancel each other almost completely for an equilibrium semidilute solution, are modified in different ways by the shear flow. Incomplete cancellation of these contributions leads to anisotropic patterns that resemble those observed in light scattering experiments on sheared semidilute solutions [Wu et al., Phys. Rev. Lett. 66, 2408 (1991)]. For small wave vectors the structure factor change is dominated by the interchain contribution. We also monitor the distortion of the pair correlation function and show that for small distances it is dominated by the intrachain contribution. Finally, we investigate nonlinear shear viscosity and find that, like the short-distance part of the distortion of the pair correlation function, it is predominantly of intrachain origin.     

一种梳型两亲聚合物的超分子结构

利用DSC、WAXD、IR及POM等手段对梳型两亲聚合物聚(2-丙烯酰胺基十六烷磺酸)(PAMC_(16)S)的超分子结构作了研究,对其组织形式作了考察,结果表明聚合物表现出梳型聚合物典型的层状堆砌,给出了其堆砌模型,对其液晶性也作了讨论。     

Binding and photoreactivity of psoralen linked to triple helix-forming oligonucleotides

Triple helix-forming oligonucleotides conjugated to a psoralen (psoTFO) have been designed to bind to three distinct purine-rich sequences within the human interstitial collagenase (MMP1) gene. Gel mobility shift assays indicate that these psoTFO bind to and photoreact with model target DNA sequences following ultraviolet A (UVA) irradiation. The dissociation constants for binding of the psoTFO to their targets range from 0.3 to 4 microM. Psoralen monoadducts with the purine-rich target strand and interstrand crosslinks are efficiently formed on targets containing either 5'-ApT-3' or 5'-TpA-3' sequences adjacent to the TFO binding sequence. The dependence of adduct formation on UVA dose has provided quantitative estimates of the overall rate constants for psoralen monoadduct and crosslink formation in the presence of a TFO. When psoralen is tethered to a TFO, the rate of monoadduct formation exceeds that of crosslinking for all sequences studied. This contrasts with the relatively low rate of monoadduct formation that has been reported for free psoralens, suggesting that the bound TFO facilitates the initial photochemistry that generates monoadducts, but does not significantly affect interstrand crosslink formation. psoTFO and UVA treatment inhibit DNA cleavage by a restriction endonuclease when the psoralen covalently reacts directly at the endonuclease site. The particular TFO studied do not completely inhibit endonuclease activity when they are noncovalently bound or when the covalent psoralen adduct does not coincide with the endonuclease site. Our findings confirm that TFO are capable of directing psoralen photoadducts to specific DNA targets and suggest that TFO can significantly modulate psoralen photoreactivity and DNA-protein interactions.     

Structure of interpenetrating polymer networks

A possible model for the formation of interpenetrating polymer networks is suggested. Phase separation is assumed to be faster than gelation. This implies that domains rich in either component grow first until late stages of spinodal decomposition. In these domains, short linear chains are crosslinked, leading to large branched macromolecules. Growth of the domains is slowed down by the presence of crosslinked polymers. It is assumed that it is stopped when the sizes of the domains and of the branched macromolecules are comparable. The resulting domains are significantly larger than the average distance between crosslinks. These results are supported by recent neutron scattering results on a poly(carbonate-urethane)/polyvinyl pyridine interpenetrating network. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1507–1512, 1998     

Structure and properties of nickel-organosilicon polymer composites

Preparation of an electrically conducting heat-resistant composite from nickel powder (conducting component) and plasticized or unplasticized polymethylphenylsiloxane (binding matrix) was studied. The optimal preparation conditions and component ratio were determined. The phase composition, thermal resistance, and electrical conductivity of a coating based on this composite were studied in relation to the preparation temperature.