Desorption Of Polymer Chains Research Articles

Investigation of the time-dependent friction behavior of polyacrylamide hydrogels

Hydrogels have attracted enormous frictional investigation owing to their great potential in hydration lubrication. Researchers have revealed that the lubrication behavior of hydrogel depends on a number of parameters such as contact area, sliding velocity, and solution condition. However, less effort has been devoted to understanding the time-dependent friction behavior of polymeric hydrogels. In this work, the time-dependent behavior of hydrogel friction was systematically investigated through three typical polyacrylamide hydrogels (PAM, PAM-PEI, and PAM-PAA) on a rheometer. Interestingly, the friction coefficients (μ) of the hydrogels varied with time, and two distinct regimes (viz., the friction-decrease regime and friction-constant/increase regime) were observed. In the friction-decrease regime, all the friction coefficients for the three PAM-based hydrogels dramatically decreased (e.g. μ decreased from ∼ 0.107 to ∼ 0.034 for PAM, μ decreased from ∼ 0.059 to ∼ 0.021 for PAM-PAA). The decrease of friction coefficients can be explained by frictional relaxation mechanism, where the deformation of polymers in the shear direction and the desorption of polymer chains from solid surface play dominant roles. Tribological results show that the increase in friction is determined by the charge behavior and polymer network of the PAM-based hydrogels. Besides, shear velocity and pH of the solution also significantly affected the friction behavior of hydrogels on the glass substrate. The understanding of the time-dependent friction behavior of PAM hydrogels provides insights to develop water-based lubricants with engineering applications.

A Phenomenological Model for Shakedown of Tough Hydrogels Under Cyclic Loads

Most tough hydrogels suffer accumulated damages under cyclic loads. The damages may stem from breakage of covalent bonds, unzipping of ionic crosslinks, or desorption of polymer chains from nanoparticle surfaces. Recent experiments report that when a tough hydrogel is subject to cyclic loads, the stress–stretch curves of tough hydrogels change cycle by cycle and approach a steady-state after thousands of cycles, denoted as the shakedown phenomenon. In this paper, we develop a phenomenological model to describe the shakedown of tough hydrogels under prolonged cyclic loads for the first time. We specify a new evolution law of damage variable in multiple cycles, motivated by the experimental observations. We synthesize nanocomposite hydrogels and conduct the cyclic tests. Our model fits the experimental data remarkably well, including the features of Mullins effect, residual stretch and shakedown. Our model is capable of predicting the stress–stretch behavior of subsequent thousands of cycles by using the fitting parameters from the first and second cycle. We further apply the model to polyacrylamide (PAAM)/poly(2-acrylanmido-2-methyl-1-propanesulfonic acid) (PAMPS) and PAAM/alginate double-network hydrogels. Good agreement between theoretical prediction and experimental data is also achieved. The model is hoped to serve as a tool to probe the complex nature of tough hydrogels, through cyclic loads.

Computer simulation of adsorption properties of polymer on surface under external driving force

Monte Carlo simulation is performed to study the adsorption properties of polymers on an attractive surface. Annealing method is adopted to simulate the adsorption characteristics and conformational changes of polymer chains driven by an external driving force F. In simulations using cooperative motion algorithm, the ensembles of monomers located at lattice sites are connected by non-breakable bonds. When the external force is F=0, the finite-size scale method can be used to determine the critical adsorption temperature (Tc) of the polymer chain on the attractive surface, but when the external force is F>0, the dependence of the average number of surface contacts M> on the chain length N is unrelated to temperature T. Therefore, Tc cannot be obtained by the finite-size scale method. However, the pseudo-critical adsorption temperature Tc can be estimated by a function of the average number of surface contacts M> and the temperature T for the chain length N=200. And then Tc decreases with external force F increasing. The phase diagram is obtained for the polymer chain between the desorbed state and the adsorbed state under temperature T and external driving force F. Furthermore, the influence of the external driving force on the conformation of the polymer chain is analyzed by the mean square radius of gyration of polymer chains. The critical adsorption point Tc can be checked roughly by the minimum location of the mean square radius of gyration or by the variation of its components in the Y and Z direction perpendicular to the external force. With the increase of the external force F for adsorbed polymer, the temperature T can determine whether polymer is changed from the adsorption state to the desorption state and where the force is located at the transformation. There are two different cases, that is, the polymer can be desorbed at the temperature Tc* TTc and the polymer cannot be desorbed at T Tc*. In this paper, we discuss these two cases for the adsorption of polymer on the attractive surface:weak and strong adsorption. In the first case, the adsorption is strongly influenced by the external driving force. By contrast, in the strong adsorption, the adsorption is weakly influenced by the external force. Our results unravel the dependence of adsorption of polymer on external driving force, which is also consistent with the phase diagram of adsorption and desorption of polymer chains.

Pseudoelasticity and Nonideal Mullins Effect of Nanocomposite Hydrogels

The polymer network of a nanocomposite (NC) hydrogel is physically crosslinked by nanoclay. Recently reported high toughness of nanocomposite (NC) hydrogels highlights the importance of their dissipative properties. The desorption of polymer chains from clay surface may contribute mostly to the hysteresis of NC hydrogels. Here, we proposed a mechanistically motivated pseudoelastic model capable of characterizing the hysteresis of NC hydrogels. The two parameters in the proposed damage variable can be determined by the experiments. We applied the model to the uniaxial tension and reproduced the ideal Mullins effect of NC hydrogels. Furthermore, we considered two nonideal effects: residual deformation and nonideal reloading in multicycle test, using newly proposed damage parameters. A power law with the order of 1/3 is established between the residual fraction of the stretch and the re-adsorption ratio of polymer chains. Finally, we demonstrated the dissipative properties of various NC hydrogels with the model.

Foam Films from Thermosensitive PNIPAM and SDS Solutions

Thin liquid foam films stabilized with the addition of a temperature-sensitive neutral polymer, poly(N-isopropylacrylamide) (PNIPAM), in the presence of varying amounts of an anionic surfactant, SDS, were investigated using the thin-film balance. Data were analyzed in light of new and previously reported neutron reflectivity data describing the adsorption behavior of PNIPAM/SDS mixtures at the single air-water interface. In the case of thin films prepared from pure polymer solutions, unexpectedly thick and stable films were observed, and increases in solution temperature or polymer molecular weight induce corresponding increases in the thin-film thickness. This behavior is attributed to a stabilization of the films by repulsive entropic forces between long dangling polymer tails adsorbed at the interfaces. The addition of SDS causes a striking decrease in the thin-film thickness that is very well correlated with the desorption of polymer chains from the interface resulting from polymer-surfactant interactions in the bulk. Upon SDS introduction, repulsive forces stabilizing the films vary from purely entropic forces for small amounts of surfactant to supramolecular oscillatory forces at high SDS concentrations, whereas a combination of electrostatic and entropic forces is expected in intermediate concentration regions. Properties of thin films made from PNIPAM and SDS are thus highly sensitive to parameters such as the polymer-surfactant ratio, temperature, and molecular weight, offering precise control of the thin-film thickness that could be used to tailor foam characteristics.

Salt-Induced Multilayer Growth: Correlation with Phase Separation in Solution

We report salt-induced phase separation of polyelectrolyte complexes in solution correlates with deposition of polymer multilayers at a surface. Multilayer growth was followed in situ using FTIR-ATR (Fourier transform infrared spectroscopy by attenuated total reflection). The polymer system was quaternized poly(4-vinylpyridine) (QPVP) and poly(methacrylic acid) (PMAA), with various ratios of PMAA to QPVP chain lengths. In solution, phase diagrams were produced for QPVP/PMAA mixtures containing excess of PMAA or QPVP, in both cases showing salt-induced phase separation. These phase diagrams controlled deposition of PMAA or QPVP chains at the surface and explained a bell-shaped dependence of the multilayer thickness on the salt concentration. The order of chain removal (occurring at the step of QPVP or PMAA deposition) could also be predicted from the phase state of QPVP/PMAA mixtures in solution. Finally, we show that, by using short equilibration times with solution, robust multilayers can be produced at surfaces even in conditions when multilayer deposition is aggravated by desorption of polymer chains.

Effects of chemical impurities on the adsorption of polymer chains from a semidilute solution

Results are presented from a series of lattice Monte Carlo simulations of the adsorption and desorption of polymer chains from semidilute solution onto a solid/liquid interface, where either the chains or the adsorbing surface contain chemical impurities. We compare these results with our previous results for chemically pure systems with regard to the growth, equilibrium structure, and dynamics of the adsorbed layers. Generally, the presence of impurities causes a reduction in the total adsorbance, but does not gravely affect the de Gennes-type self-similar grid found for the pure system. In one case, however, the impurities are found to increase the total adsorbance. We find some correspondence between the static chain characteristics for cases of impurities within the chains and impurities within the surface. There are, however, visible differences between the dynamics of the two cases, particularly in their growth and exchange kinetics. Dramatic changes in behavior are shown to arise when the chain impurities impede the “reeling in” motion of the chains onto the surface, altering the chains’ relaxations. We compare our results with previous theoretical predictions for cases of random copolymer adsorption, and adsorption onto heterogeneous surfaces.

In situ fluorescence experiments for real-time monitoring of annealed high-T latex film dissolution

A new technique, based on steady-state fluorescence measurements, is introduced for studying dissolution of polymer films. These films are formed from naphthalene and pyrene labeled poly(methyl methacrylate) (PMMA) latex particles, sterically stabilized by polyisobutylene. Diffusion of solvent (chloroform) into the annealed latex film was followed by desorption of polymer chains. Annealing was performed above Tg at various temperatures for 30-min time intervals. Desorption of pyrene labeled PMMA chains was monitored in real time by the pyrene fluorescence intensity change. Desorption coefficients were found to be between 1 and 4 × 10−10 cm2/s and two different dissolution mechanisms were detected. © 1996 John Wiley & Sons, Inc.

Kinetics of adsorption and desorption of water‐soluble associative polymers on planar titanium dioxide surfaces by ellipsometry

AbstractThe adsorption and desorption kinetics of water‐soluble associative polymers with different molecular weights on crystals of titanium dioxide [TiO2(001)] have been studied by ellipsometry. The model water‐soluble associative polymers used in the kinetic study are nonionic polyurethanes based on polyethylene glycol, and have average molecular weights of 17,000, 51,000, and 100,000 with a C16H33 linear alkyl group on each end of the molecule. It is shown that the adsorption and desorption kinetics over a wide range of polymer concentrations is governed by: (1) the kinetics of adsorption and desorption of polymer chains at the interface, (2) the kinetics of adsorption and desorption at the interfaces as well as simultaneous diffusion in the adsorbed layer, and/or (3) diffusion in the adsorbed layer. The existence of regimes (1), (2), and (3) of the adsorption and desorption kinetics are justified by using the experimental data for the adsorption and desorption of water‐soluble associative polymers with different molecular weights. It is shown that the adsorption is an irreversible process for the strongly convex adsorption isotherms. Equations were derived to calculate: (a) the rate constant for polymer adsorption and desorption processes, (b) the coefficient of diffusion, (c) the activation energy of diffusion in the adsorbed layer, and (d) the time needed to attain the equilibrium states for the adsorption and desorption processes by using adsorption and desorption kinetics data with associative polymers of different molecular weights at different polymer concentrations. © 1995 John Wiley & Sons, Inc.

Topological influences on polymer adsorption and desorption dynamics.

The desorption of polymer chains through an overlayer of strongly adsorbed chains was studied to determine the influence of topological constraints on the polymer desorption process. The desorption time of linear chains fits a power law, ${\mathit{M}}^{\mathit{a}}$, where M is a molecular weight and a=2.3\ifmmode\pm\else\textpm\fi{}0.2. A comparison of linear and star-branched chains shows that desorption of star-branched chains was greatly suppressed. These findings are reminiscent of entanglement effects in bulk systems. They suggest a unifying perspective from which to analyze polymer mobility at surfaces.

Effect of surface geometry on adsorption and desorption of polymer chains

Adsorption and desorption of polystyrenes (PS) at three different porous silica surfaces in cyclohexane is investigated as a function of the size ratio of the average pore diameter to twice the radius gyration of a PS chain. The adsorption isotherms for the porous silica surfaces are almost the same as for the nonporous silica in their shape. However, the plateau adsorbed amount increases with an increase in the size ratio. A similar surface geometry effect is also observed for the desorption of polystyrenes by dioxane. These phenomena can be interpreted in the terms of the surface fractal when the whole polymer chain is regarded as the yardsticks