Dynamic scaling of polymer gels comprising nanoparticles

We present dynamic light scattering (DLS) measurements of soft poly(methyl-methacrylate) (PMMA) and polyacrylamide (PA) polymer gels prepared with trapped bodies (latex spheres or magnetic nanoparticles). We show that the anomalous diffusivity of the trapped particles can be analyzed in terms of a fractal Gaussian network gel model for the entire time range probed by DLS technique. This model is a generalization of the Rouse model for linear chains extended for structures with power law network connectivity scaling, which includes both percolating and uniform bulk gel limits. For a dilute dispersion of strongly scattering particles trapped in a gel, the scattered electric field correlation function at small wavevector ideally probes self-diffusion of gel portions imprisoning the particles. Our results show that the time-dependent diffusion coefficients calculated from the correlation functions change from a free diffusion regime at short times to an anomalous subdiffusive regime at long times (increasingly arrested displacement). The characteristic time of transition between these regimes depends on scattering vector as approximately q(-2), while the time decay power exponent tends to the value expected for a bulk network at small q. The diffusion curves for all scattering vectors and all samples were scaled to a single master curve.     

Viscoelastic properties of ϰ-carrageenan gels

Summary A study has been made of the creep behaviour of -carrageenan in the region of linear viscoelasticity in the temperature interval 9–50 °°C and concentration range 1.0–2.5%. It is shown that the rate of the elementary relaxation processes is independent of the concentration of -carrageenan. The following three temperature intervals where -carrageenan gels exist have been established: (1) the temperature interval corresponding to the stable helix conformation of the macromolecules; (2) the temperature interval of macromoleculer unwinding; (3) the high-temperature interval where the gel three-dimensional network is formed by the unwound macromolecules. The break-down enthalpy of the gel junction zones is 36, 234 and 36 KJ/mole, respectively.     

Nanostructured polymer gels

A brief review of our recent work on polymer gel opals is given in memory of late Professor Tanaka. The central idea is to first synthesize monodisperse polymer gel nanoparticles, then self-assemble them into a 3D network, and eventually covalently bond them. The covalent bonding contributes to the structural stability, while self-assembly provides them with crystal structures that diffract light, resulting in colors. N-isopropylacrylamide (NIPA) gel has been used as a model system for this study. The nanostructured NIPA gel, which contain up to 97 wt% water, displays a striking iridescence like precious opal but are soft and flexible like gelatin.     

Inhomogeneity control in polymer gels

Inhomogeneities, that is, nonrelaxing frozen concentration fluctuations, are inevitably present in polymer gels because they are introduced during the crosslinking of the constituent polymer chains in a solvent. Therefore, inhomogeneities increase as the number of crosslinks increases in a gel. The ionization of polymer gels is one of the methods used to suppress inhomogeneities. However, because crosslinking also means a freezing‐in of the conformation and topology of polymer chains in a solvent according to the chemistry of crosslinking, inhomogeneity control is quite sophisticated. In this article, we discuss the relationship between the inhomogeneities and the molecular/environmental parameters of polymer gels, such as the polymer concentration, the degree of crosslinking, the degree of ionization, and the interaction parameter, by considering the memory effect of gels. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 617–628, 2005     

Concentration polydispersity in polymer gels

For swollen polymer networks there is no generally accepted relation between the macroscopic osmotic properties and the scattering behaviour. Detailed information on the relationship between these properties can, however, be deduced from complementary measurements of osmotic and elastic behaviour, small angle neutron and X-ray scattering (SANS and SAXS) and quasi-elastic light scattering. We describe such an investigation in two types of networks, differing in the mechanism of cross-linking. The SANS spectra yield information on the structure, which is generated both by the dynamics of the system and by long range static constraints. The former, arising from thermodynamic concentration fluctuations, governs the macroscopic osmotic and elastic moduli of the swollen network. The static superstructure in the gel reflects local variations in the cross-link density. The resulting concentration polydispersity, <δφ²>/φ², is determined by the details of the cross-linking procedure. Its concentration dependence as a function of gel swelling can be expressed in terms of the same macroscopic osmotic and elastic variables as those that govern the thermodynamic properties of the gel.     

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.     

Smart nanocomposite polymer gels

The combination of polymers with nanomaterials displays novel and often enhanced properties compared to the traditional materials. They can open up possibilities for new technological applications. The magnetic polymer gel represents a new type of composites consisting of small magnetic particles, usually from the nanometer range to the micron range, dispersed in a highly elastic polymeric gel matrix. Combination of magnetic and elastic properties leads to a number of striking phenomena that are exhibited in response to impressed magnetic fields. Giant deformational effects, high elasticity, anisotropic properties, temporary reinforcement and quick response to magnetic field open new opportunities for using such materials for various applications.     

Liquid flow through polymer gels

Slow neutron transmission technique was applied to investigate features of the solvent flow through quasi-porous matrices of gelatin, poly (acrylamide) and silica gels of different concentration/mesh sizes. We find the macromolecular friction coefficient for all gels to be proportional to the concentration of polymer network which correlates well with the predictions of the effective medium approach under the condition of negligible impact of hydrodynamic self-interactions. Determined values of the friction coefficient are used for evaluating the effective mesh sizes, which are in good agreement with the appropriate results of quasi-elastic light scattering for homogeneous poly (acrylamide) gels.     

Magnetic applications of polymer gels

In this paper we report results of both, material preparation and magnetic characterisation, on CoFe₂O₄ particles of nanometric size formed by in‐situ precipitation within polymer gels. The size of the particles was controlled within a very narrow volume distribution and its average value was shifted from 2 to 10 nm. The existence of nanoparticles showing, at room temperature, coercive field values between 500 and 900 Oe and saturation magnetisations of about 500 emu/cm³, suggest to use these systems to get magnetic recording media with ultra high density. Poly(vinyl alcohol) (PVA) and Polystyrene (PS) films were prepared from this nanocomposite material. After a magnetic field treatment nanoparticles within the PVA films are free to rotate in response to an applied magnetic field. This PVA based nanocomposite film portends a new class of magnetic material with very little or no electrical and magnetic loss.     

Osmotic deswelling of polymer gels

The osmotic deswelling of polymer networks swollen in a good solvent by transferring it into a solution of a linear polymer in the same solvent is investigated using the modified Flory model proposed previously. The predicted results obtained using this simple model are compared to the experimental data available in the literature. We further examine the variation of the degree of deswelling, the degree of swelling and the partition coefficient with the molecular weight, and the volume fraction of the linear polymer chains in the surrounding polymer solution. Also, the role of the packing factor is briefly discussed.© 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2525–2535, 1998     

Viscoelastic theory for nematic polymer interfaces

A macroscopic theory for the dynamics of compressible nematic polymer‐viscous fluid interfaces is developed from first principles. The theory is used to define and characterize the basic interfacial viscoelastic material properties of the ordered interfaces. The theory is based on a decomposition of the kinematic fields and nematic tensor order parameter that takes into account the symmetry breaking of the interface. The interfacial rate of entropy production used to identify the interfacial viscoelastic modes is given in terms of surface rate of deformation tensor and the surface Jaumann derivative of the tangential component nematic tensor order parameter. The derived surface viscous stress tensor is asymmetric and thus describes surface flow‐induced changes in the tensor order parameter. Consistency with the Boussinesq surface fluid appropriate for Newtonian interfaces is established. The interfacial material functions are identified as the dynamic surface tension, the interfacial dilational viscosities, and the interfacial shear viscosities. The interfacial material functions depend on the surface tensor order parameter and as a consequence anisotropy is their characteristic feature. Two characteristic interfacial tensions and two dilational viscosities are predicted depending on the director orientation. In addition six interfacial shear viscosities arise as the directors sample the velocity, velocity gradient, and vorticity directions. Finally the theory provides for the necessary theoretical tools needed to describe the interfacial dynamics of nematic polymer interfaces, such as capillary instabilities, Marangoni flows, and wetting phenomena.     

Dynamic scaling properties of TeO2-based gels

Homogeneous, transparent, and mechanically rigid gels have been successfully synthesized in the tellurium isopropoxide-isopropanol-citric acid and water system. The sol to gel transition and the gels microstructure have been studied by using small angle X-ray scattering (SAXS) experiments. For any value of the two key synthesis parameters, which are the citric acid ratio and the alkoxide concentration, very small Te-rich elementary particles, about 1-1.5 nm in radius, form immediately when the water is added, leading to colloidal sols. During gelation, these elementary particles stick progressively together to build up fractal aggregates by a pure hierarchical aggregation process which has been identified as a reaction-limited cluster aggregation (RLCA) mechanism. The SAXS curve analysis, based on scaling concepts, shows that the gelling network exhibits a time and length scale invariant structure factor characterized by self-similarity. This self-similarity is also displayed for a wide range of chemical compositions and the gel microstructures only differ in their fractal aggregate size according to the tellurium isopropoxide concentration as well as the citric acid ratio.     

Metal complex catalysts immobilized in polymer gels

A number of polymer gels have been prepared using tertiary ethylene‐propylene‐ethylidenenorbomene copolymer as a rubber base with grafted poly‐4‐vinylpyridine, polymethacrylic acid and polymethacrylamide ligand chains. The grafted copolymers were crosslinked and complexes of nickel, zirconium and titanium were immobilized in the formed crosslinked copolymers. After treatment with organoaluminium compounds the obtained catalysts demonstrate high catalytic activity in the reactions of dimerization of lower olefins. Structures of the complexes and the catalytic activity of the gel immobilized catalysts have been investigated.     

Self-oscillatory systems based on polymer gels

Chemical and mechanical oscillations arising when the Belousov-Zhabotinsky reaction is performed in composite gels based on polyacrylamide and silica gel and gels of poly(acrylamide-co-sodium acrylate) have been investigated. The catalyst of the reaction can be incorporated into a gel through electrostatic interactions. Mechanical and absorption properties of polymer matrices are characterized, and the periods and amplitudes of mechanical oscillations arising in them are estimated. The mechanism of the phenomenon under consideration is advanced.     

Viscoelastic dewetting of constrained polymer thin films

Thin films of fluids are playing a leading role in countless natural and industrial processes. Here we study the stability and dewetting dynamics of viscoelastic polymer thin films. The dewetting of polystyrene close to the glass transition reveals unexpected features: asymmetric rims collecting the dewetted liquid and logarithmic growth laws that we explain by considering the nonlinear velocity dependence of friction at the fluid/solid interface and by evoking residual stresses within the film. Systematically varying the time so that films were stored below the glass-transition temperature, we studied simultaneously the probability for film rupture and the dewetting dynamics at early stages. Both approaches proved independently the significance of residual stresses arising from the fast solvent evaporation associated with the spin-coating process. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3022–3030, 2006     

New class of dynamics in concentrated polymer gels

We show that concentrated poly(methyl methacrylate) solution exhibits a new class of coupled dynamics, which can be regarded as an intermediate between the collective diffusion of solutions and the structural relaxations of glasses. This class of dynamics have a relaxation rate that is directly proportional to the wave vector. The transition from diffusive to coupled collective dynamics occurs at smaller length scales with increasing polymer concentration and decreasing temperature. The experimental observations can be understood by considering the contributions from physical cross-links interconnected by stiff polymer segments.     

Thermodynamic interaction parameters in polymer solutions and gels

Polymer-solvent interaction parameters are reported for poly(vinyl acetate)-acetone, poly(vinyl acetate)-toluene, and poly(dimethyl siloxane)-toluene systems using different techniques. Results obtained by osmotic deswelling are compared with those from quasi-elastic light scattering and small-angle neutron scattering (SANS). In gels, the latter techniques involve separation of the time-dependent from the static component of the scattered radiation+ Separation is achieved in quasi-elastic light scattering through the heterodyning properties of the light, and in SANS by subtracting an appropriate static structure factor. The interaction parameters obtained by different separation procedures are consistent with measurements using the osmotic method. © 1995 John Wiley & Sons, Inc.     

Thermodynamic scaling of polymer dynamics versus T-T(g) scaling

A thermodynamic scaling law for the relaxation times of complex liquids as a function of temperature and volume has been proposed in the literature: τ(T,V) = f(TV(γ)), where γ is a material-dependent constant. We test this scaling for six materials, linear polystyrene, star polystyrene, two polycyanurate networks, poly(vinyl acetate), and poly(vinyl chloride), and compare the thermodynamic scaling to T-T(g) scaling, where τ = f(T-T(g)). The thermodynamic scaling law successfully reduces the data for all of the samples; however, polymers with similar structures but different glass transition (T(g)) and pressure-volume-temperature (PVT) behavior, i.e., the two polycyanurates, cannot be superposed unless the scaling law is normalized by T(g)V(g) (γ). On the other hand, the T-T(g) scaling successfully reduced data for all polymers, including those having similar microstructures. In addition, the T-T(g) scaling is easier to implement since it does not require knowledge of the PVT behavior of the material. The relationship between T(g)V(g) (γ)∕TV(γ) and T-T(g) scaling is clarified and is found to be weakly dependent on pressure.     

Intelligent polymer gels controlled by magnetic fields

 In this paper we summarise the effects induced by electric and magnetic fields on the mobility and shape of polymer gels containing a complex fluid as a swelling agent. Magnetic-field-sensitive gel beads and monolith gels have been prepared by introducing magnetic particles of colloidal size into chemically cross-linked poly(N-isopropylacrylamide) and poly(vinyl alcohol) hydrogels. The influence of uniform and nonuniform fields has been studied. In uniform magnetic fields the gel beads form straight chainlike structures, whereas in nonhomogeneous fields the beads aggregates due to the magnetophoretic force directed to the highest field intensity. The ability of magnetic-field-sensitive gels to undergo quick, controllable changes in shape can be used to mimic muscular contraction. Received: 26 July 1999/Accepted: 27 August 1999