Kinetic effects of excluded volume and selective adsorption in macromolecule solutions

Kinetic effects of excluded volume and selective adsorption in macromolecule solutions have been studied for an arbitrary position of reacting groups along the chain. It has been shown that the reactivities of similar functional groups can depend upon their location along the chain and also upon the nature of the solvent.     

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.     

Thermodynamics of swelling in unfilled and filler-loaded networks

Within the framework of the van der Waals-network model a consistent interpretation of swelling and simple extension in differently crosslinked networks is presented. It is observed that the excess parameters in the Staverman-Koningsveld-Kleintjens version do not depend on the degree of crosslinking. Swelling of filler-loaded rubbers shows universal features because of not depending on the type and the properties of the filler. By introducing the Einstein-Smallwood modification in an adequate manner one understands this phenomenon without any further parameter adjustments. It is the consequence of having quasi-permanent filler-to-matrix contacts that are not modified in presence of solvent molecules. The excess-parameters in the swollen matrix are not affected. The entropy elastic stress due to the swelling induced deformation of the matrix is apparently too small as to enforce chain-slippage. The strength of the adhesion of the polymer inhibits filler-to-solvent contacts. These results defend the mean-field treatment of the boundary problem as presented by Einstein-Smallwood, and allows a valuable proof of the thermodynamics of swelling in networks.Dedicated to Prof. H. H. Kausch on the occasion of his 60th birthday     

Universal consequences of the presence of excluded volume interactions in dilute polymer solutions undergoing shear flow

The role of solvent quality in determining the universal material properties of dilute polymer solutions undergoing steady simple shear flow is examined. A bead-spring chain representation of the polymer molecule is used, and the influence of solvent molecules on polymer conformations is modelled by a narrow Gaussian excluded volume potential that acts pairwise between the beads of the chain. Brownian dynamics simulations data, acquired for chains of finite length, and extrapolated to the limit of infinite chain length, are shown to be model independent. This feature of the narrow Gaussian potential, which leads to results identical to a delta-function repulsive potential, enables the prediction of both universal crossover scaling functions and asymptotic behavior in the excluded volume limit. Universal viscometric functions, obtained by this procedure, are found to exhibit increased shear thinning with increasing solvent quality. In the excluded volume limit, they are found to obey power law scaling with the characteristic shear rate beta, in close agreement with previously obtained renormalization group results. The presence of excluded volume interactions is also shown to lead to a weakening of the alignment of the polymer chain with the flow direction.     

Quasichemical approach to crosslinked polymer solutions and the swelling equation for polycondensation networks

The change of free enthalpy and the chemical potential of solvent for mixing of solvent with crosslinked structures formed by stepwise reactions of the polycondensation type has been derived by an adaptation of the quasichemical equilibrium method. Each monomer unit is supposed to bear a different number of crosslinkable contact points, the coupling of which gives rise to chemical bonds, and of uncrosslinkable contact points, involved in interactions with the solvent, which may be of different type. The results are used to analyze the effect of network topology on the parameters of the swelling equation, particularly on a parameter characterizing the network composition. This parameter may be obtained from analytical data or by using the crosslinking statistics; and model calculations show how it varies with monomer conversion.     

Concentration dependence of excluded volume effects

The concentration dependence of the excluded volume effects in polymer solutions is investigated. Through thermodynamic arguments for the interpenetration of polymer segments and the free energy change, we show that the disappearance of the excluded volume effects should occur at medium concentration. The result is in accord with the recent experimental observations.     

Application of the padé approximant method to excluded volume effects in polymer chains

Closed formulas for the expansion factor α and the h function appearing in the second virial coefficient are derived by the Padé approximant method, using the first two or three series coefficients. The equations for α agree well with experimental results for small z, the excluded volume parameter. The equation for h agrees well with data over the entire accessible range of z.     

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.     

Molecular silverware. I. General solutions to excluded volume constrained problems

General mathematical solutions to excluded volume constrained problems in computational chemistry are reported. The solutions have been used to create a new family of molecular modeling algorithms to facilitate the study of molecular interactions in condensed phases. The new algorithms, collectively known as Molecular Silverware, are for the most part interactive and designed for packing, solvating, and sampling molecules embedded in simple or complex topological environments. Multifolded, disconnected, or porous molecular structures are permitted. Molecular Silverware assists the preparation of Monte Carlo and molecular dynamics simulations at a small fraction of the total simulation time. Primary targets for applications include the study of molecular recognition mechanisms and the selective binding of DNA, RNA, peptides, saccharides and other biopolymers in solution as well as the prediction of phase separation behavior and physical properties of non-crystalline condensed phases such as bulk polymers, polymer blends, organic liquids, membranes, micelles, gels, crosslinked networks, glasses, and amorphous heterogeneous catalysts. As a result of this new approach to excluded volume constraints, the computer simulation of noncrystalline condensed phases is no longer hampered by the lack of a general and efficient method for the creation and configurational sampling of small and large molecular assemblies at high densities.     

Studies on mechanical and swelling behavior of polymer networks on the basis of the scaling concept. 3. The osmotic compressibility of gels

The osmotic compressibility of chemically crosslinked poly(vinyl acetate) (PVAc) gels in equilibrium with pure diluent has been determined by the method of decreasing equilibrium swelling in different swelling agents. The concentration dependence of the osmotic compressibility has been found to follow a scaling law with an exponent which was greater than that predicted by the mean-field theory. A comparison has been made between the elastic moduli determined by unidirectional deformation measurements and the compressional moduli obtained from the osmotic compressibility data.     

Field-theoretic model of inhomogeneous supramolecular polymer networks and gels

We present a field-theoretic model of the gelation transition in inhomogeneous reversibly bonding systems and demonstrate that our model reproduces the classical Flory-Stockmayer theory of gelation in the homogeneous limit. As an illustration of our model in the context of inhomogeneous gelation, we analyze the mean-field behavior of an equilibrium system of reacting trifunctional units in a good solvent confined within a slit bounded by parallel, repulsive walls. Our results indicate higher conversions and, consequently, higher concentrations of gel following the gelation transition near the center of the slit relative to the edges.     

Thermodynamics and viscosities of dilute polymer solutions in binary solvents

Experiments aimed at comparison of calculated and experimental intrinsic viscosities of PS and PMMA in some binary solvents used as eluents in chromatography are performed. The intrinsic viscosities are determined in mixed solvents of different thermodynamic qualities, including a θ solvent, in a wide composition range. The analysis of the thermodynamic and rheological theoretical equations for ternary systems allowed the interpretation of anomalies observed on the experimental curves of intrinsic viscosity versus solvent composition.     

Microscopic inhomogeneities in weakly-charged temperature sensitive polymer gels and solutions

A comparison of small-angle neutron scattering (SANS) intensity functions, I(q), was made between gels and solutions of poly(N-isopropyl acrylamide-co-acrylic acid) (NIPA/AAc), where q is the magnitude of the scattering vector. I(q)'s were strongly dependent on polymer concentration as well as temperature. At low temperatures, I(q)'s for both gels and solutions were similar to each other and were monotonous decreasing functions. However, at high temperatures above θ, the so-called theta temperature of poly-NIPA in aqueous solution, both had a distinct peak and a significant difference appeared in I(q) due to the presence or absence of crosslinks. Origins of the peak and the differences in I(q) are discussed.     

Stretching semiflexible polymer chains: evidence for the importance of excluded volume effects from Monte Carlo simulation

Semiflexible macromolecules in dilute solution under very good solvent conditions are modeled by self-avoiding walks on the simple cubic lattice (d = 3 dimensions) and square lattice (d = 2 dimensions), varying chain stiffness by an energy penalty ε(b) for chain bending. In the absence of excluded volume interactions, the persistence length l(p) of the polymers would then simply be l(p) = l(b)(2d - 2)(-1)q(b) (-1) with q(b) = exp(-ε(b)/k(B)T), the bond length l(b) being the lattice spacing, and k(B)T is the thermal energy. Using Monte Carlo simulations applying the pruned-enriched Rosenbluth method (PERM), both q(b) and the chain length N are varied over a wide range (0.005 ≤ q(b) ≤ 1, N ≤ 50,000), and also a stretching force f is applied to one chain end (fixing the other end at the origin). In the absence of this force, in d = 2 a single crossover from rod-like behavior (for contour lengths less than l(p)) to swollen coils occurs, invalidating the Kratky-Porod model, while in d = 3 a double crossover occurs, from rods to Gaussian coils (as implied by the Kratky-Porod model) and then to coils that are swollen due to the excluded volume interaction. If the stretching force is applied, excluded volume interactions matter for the force versus extension relation irrespective of chain stiffness in d = 2, while theories based on the Kratky-Porod model are found to work in d = 3 for stiff chains in an intermediate regime of chain extensions. While for q(b) ? 1 in this model a persistence length can be estimated from the initial decay of bond-orientational correlations, it is argued that this is not possible for more complex wormlike chains (e.g., bottle-brush polymers). Consequences for the proper interpretation of experiments are briefly discussed.     

NMR investigations of phase transition in aqueous polymer solutions and gels

The use of NMR spectroscopy in investigations of phase transitions in aqueous polymer solutions and gels is reviewed. Results on this subject as obtained mostly for thermoresponsive polymers (e.g., poly(N-isopropylacrylamide) and its copolymers, poly(N-isopropylmethacrylamide) and its copolymers, poly(vinyl methyl ether)) from temperature dependences of ¹H and ¹³C NMR spectra, spin–lattice and spin–spin relaxation times, diffusion coefficients and NMR images are discussed.     

Electrophoretic dynamics of large DNA stars in polymer solutions and gels

We have developed a procedure for synthesizing large stable branched DNA structures that enables visualization via fluorescence microscopy. Using this procedure we have synthesized large DNA stars and observed their electrophoretic behavior in polymer solutions and gels. In dilute polyacrylamide solutions, the DNA stars move as random coils and appear to experience only brief collisions with the polymer chains in solution. The effect of polymer solution concentration on the electrophoretic mobility of stars in the dilute regime is found to be in good accord with predictions of the transient entanglement coupling (TEC) model. In semidilute polymer solutions, the star arms extend in the field direction and drag the core through the matrix. The star arms form several U-shaped conformations as they collide and engage with polyacrylamide chains. The U-shaped conformations occasionally evolve into J-shaped conformations as the star arms slide off the matrix chains they engage during electrophoretic migration. In concentrated polymer solutions, the arms of the star extend and form V-shaped structures with the core as the apex. The arms then pull the core through the matrix. These V-shaped conformations are much longer-lived than U-shaped ones and, unlike the latter, do not transform to J-shaped conformations. In polyacrylamide and agarose gels, where matrix entanglements are fixed, DNA stars become trapped when entanglements with matrix molecules prevent the core from being pulled through the matrix by the extended arms. This trapping was observed at all gel concentrations and electric fields studied.