Nanostructured ultraviolet resistant polymer coatings

Nanoparticle-embedded acrylic coatings that can absorb copious amounts of UV radiation yet scatter little were developed to protect base fabrics from sun-induced degradation. Zinc oxide and titanium dioxide nanoparticles with diameters ranging from 15 to 70 nm were used. Nanoparticles (5 wt%) were dispersed in acrylic emulsions. Nanoparticle-embedded acrylic films of 10 μm and 20 μm thick were prepared and bonded to Kevlar fabric. Mechanical tests as well as infrared, visible and UV spectroscopy were used to characterize nanoparticle-embedded acrylic emulsions and coated Kevlar fabric.The changes in mechanical and chemical properties of Kevlar fabric after a day and week of intense UV exposure were assessed using tear and strip tensile testing, UV, visible and infrared spectroscopy, and wide and small angle X-ray analysis. Tear and tensile data, with support from UV results, showed that 20 μm nanoparticle-embedded coatings largely prevented degradation of Kevlar fabric, allowing only 5% of the degradation that occurred in the unprotected fabric after a week of UV exposure.     

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.     

Viscoelastic scaling in polymer gels

New scaling laws for chain networks are derived to describe the fundamental relationships between the viscosity exponent (k), viscoelastic exponent (m), stretched exponent (β), spatial dimension (d). fractal dimension (d_f), and a universal constant (γ). The scaling of the total number of monomers and the radius of gyration is defined by d_f. We have discovered γ = m/β to be a universal constant which relates the shear modulus of a polymer gel melt to the shear modulus near the glass transition. Analyzing the size-dependent shear viscosity, we have determined γ = 3d_fcd/(7d−5d_fc) = 2.647 for d = 3 where d_fc is the fractal dimension of critical clusters at the gel point. By using γ, the present theory extends previous work pertaining to systems near the sol-gel transition, and shows how properties far from the critical point can be explained. The theoretical prediction is in good agreement with viscoelastic measurements.     

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     

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     

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.     

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.     

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     

Self-oscillating swelling and deswelling of polymer gels

Novel gel systems demonstrating rhythmically pulsatile mechanical motion similar to that of a heartbeat were developed. Self-oscillations of swelling and deswelling for the polymer hydrogels were realized by coupling pH and temperature sensitive hydrogels with a non-linear chemical reaction in the external solution media. The novel gel dynamics exhibiting cyclic and rhythmical oscillations may establish a new concept for functional materials that work under dynamic oscillating states.     

Mechanochemical energy conversion of neutral polymer gels

Mechanochemical energy conversion exhibited by water-swollen poly(vinyl alcohol) gels has been studied. The effective mechanical work produced as well as the chemical energy converted were measured simultaneously. It is shown theoretically and experimentally that the mechanical work developed during the swelling of the mechanochemical system increases with increase of the external load (m), the degree of cross-linking (DC) and the deswelling ratio (ϕ/ϕ_e, i.e. the ratio of the volume fraction of network polymer (ϕ) to that of the same gel in equilibrium with pure diluent (ϕ_e)). The chemical energy converted was found to be practically independent of m. The efficiency of energy conversion increased with increase of m and DC, and significantly decreased with increase of ϕ/ϕ_e. Comparison is made between theoretical predictions and experimental findings.     

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.     

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.     

Theoretical and computational modeling of self-oscillating polymer gels

The authors model wave propagation in swollen, chemoresponsive polymer gels that are undergoing the oscillatory Belousov-Zhabotinsky (BZ) reaction. To carry out this study, they first modify the Oregonator model for BZ reactions in simple solutions to include the effect of the polymer on the reaction kinetics. They then describe the gel dynamics through the framework of the two-fluid model. The polymer-solvent interactions that are introduced through the BZ reaction are captured through a coupling term, which is added to the Flory-Huggins model for polymer-solvent mixtures. The resulting theoretical model is then used to develop the gel lattice spring model (gLSM), which is a computationally efficient approach for simulating large-scale, two-dimensional (2D) deformations and chemical reactions within a swollen polymer network. The 2D calculations allow the authors to probe not only volume changes but also changes in the sample's shape. Using the gLSM, they determine the pattern formation and shape changes in 2D rectangular BZ gels that are anchored to a solid wall. They demonstrate that the dynamic patterns depend on whether the gel is expanded or contracted near the wall, and on the sample's dimensions. Finally, they isolate a scenario where the detachment of the gel from the wall leads to macroscopic motion of the entire sample.     

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.