Adsorption Of Polymer Chains Research Articles

Flow behavior of sterically stabilized suspensions: II. Brownian motion effects in laminar shear

Sterically stabilized latex supensions can exhibit, under some circumstances, an Ostwald-type flow curve at low shear rates, even though the suspension is thermodynamically stable. It is shown that the behavior can be explained by assuming that at low shear rates the formation of doublets by both brownian motion and the shearing process must be considered. Some doublets can separate slowly by brwnian motion, but the energy dissipation for the flow process (and, hence, the flow curve) must be calculated by determining the number of doublets which are separated by the shear field. The separation energy in this latter case is much higher than for brownian separation, since the shear field tears the particles apart in a time which is short compared to the relaxation time of the adsorbed polymer chains which confer stability on the system. Separation energies of about 5 kT are observed for PMMA systems stabilized by adsorbed polyethylene oxide.

Theory of the stabilization of colloids by adsorbed polymer

The configurational free energy of random flight polymer chains adsorbed by one end onto a plane surface as a function of the distance from a parallel plane surface is expressed to a good approximation in simple analytic form. The result is used to discuss the stabilization of a colloid suspension by adsorbed polymer. According to this theory two types of aggregation of colloid particles may occur. If LI < AS/2π 3 NkT , where l is the link length and L the contour length of a polymer chain, A is the Hamaker constant, N /S is the number of adsorbed polymer chains per unit area and kT is the Boltzman constant multiplied by temperature, the particles adhere closely, but if AS/2π 3 kT < IL < AS/nkT lg 2N a looser association is formed. It is expected that the presence of excluded volume effects would greatly increase the stability against the looser association.

Kinetic aspects of flocculation by cationic polymers

AbstractFlocculation of latex particles by a series of cationic polymers has been investigated, with special attention being paid to the effect of particle concentration. Both initial rate and ultimate extent of flocculation have been examined over a range of polymer concentrations, so that the onset of flocculation and of restabilisation could be established. At low particle concentrations, the effect of the polymers seems to be predominantly charge neutralisation; even high molecular weight polymers do not appear to form ‘bridges’, presumably because of the relatively flat configuration of the adsorbed polymer chains. However, at high particle concentrations, bridging flocculation may occur because adsorbed polymer chains require a finite time to attain their equilibrium configuration.Some practical implications of these findings are briefly discussed.

Use of numerical methods in the prediction of polymer adsorption

AbstractAverage polymer segment densities and thermodynamic properties of polymer adsorbed at liquid–solid interfaces were computed by extension of the polymer adsorption theory of Forsman and Hughes. Expressions were derived for the total free energy of adsorbed polymer chains by using the Flory‐Huggins theory to represent free energy of mixing. A square‐well potential was used to represent segment–surface interaction, and configurational entropy was calculated from the probability density function for the radius of gyration of random‐flight chains. For each specified amount of surface coverage the free energy of the adsorbed polymer was minimized by varying the density of segments normal to the adsorbing surface and using a modified gradient search algorithm on a digital computer. Two different segment densities were considered, and they both gave qualitatively the same results. The two densities were (1) the sum of two Gaussian distributions and (2) a two‐step density distribution. Isotherms were then calculated by equating the partial molal free energy of polymer at the surface to that of polymer in bulk solution for each specified amount of surface coverage. The results showed that for the initial region of the isotherms the distribution of polymer segments normal to the surface consisted of a high‐density layer adjacent to the surface and a low‐density “tail” extending far out into the solution. At higher amounts of adsorbed polymer, i.e., in the general concentration range of the pseudo‐plateau, the tail of the polymer density distribution was predicted to thicken, and a single Gaussian distribution best described the segment density. Predicted adsorptions were in good agreement with reported experimental values.

Use of numerical methods in the prediction of polymer adsorption

AbstractAverage polymer segment densities and thermodynamic properties of polymer adsorbed at liquid–solid interfaces were computed by extension of the polymer adsorption theory of Forsman and Hughes. Expressions were derived for the total free energy of adsorbed polymer chains by using the Flory‐Huggins theory to represent free energy of mixing. A square‐well potential was used to represent segment–surface interaction, and configurational entropy was calculated from the probability density function for the radius of gyration of random‐flight chains. For each specified amount of surface coverage the free energy of the adsorbed polymer was minimized by varying the density of segments normal to the adsorbing surface and using a modified gradient search algorithm on a digital computer. Two different segment densities were considered, and they both gave qualitatively the same results. The two densities were (1) the sum of two Gaussian distributions and (2) a two‐step density distribution. Isotherms were then calculated by equating the partial molal free energy of polymer at the surface to that of polymer in bulk solution for each specified amount of surface coverage. The results showed that for the initial region of the isotherms the distribution of polymer segments normal to the surface consisted of a high‐density layer adjacent to the surface and a low‐density “tail” extending far out into the solution. At higher amounts of adsorbed polymer, i.e., in the general concentration range of the pseudo‐plateau, the tail of the polymer density distribution was predicted to thicken, and a single Gaussian distribution best described the segment density. Predicted adsorptions were in good agreement with reported experimental values.

On the general theory of polymer absorption at a surface

AbstractPrevious theoretical work dealt with polymer adsorption in which the adsorbed segments (trains) interact in the surface layer only. In this work the treatment is extended to Include also interaction between the loops dangling in solution. Fortunately, the excluded volume effects for adsorbed polymer chains are found to be small. Therefore an approximate treatment is possible that leads to equations that govern polymer chains under a wide variety of conditions.

Carbon Black Reinforcement in Pre-Swollen Rubbers

Abstract The mechanism of carbon black reinforcement was studied by examining stress-strain properties in SBR 1500 vulcanizates cured while swollen, then de-swollen. Carbon black-polymer attachments started contributing to the modulus of rubber at much higher elongations (ca. 300 per cent for a swelling ratio of 4.0) than in normally cured, unswollen specimens. The modulus in pre-swollen rubbers is considerably lower than in normal vulcanizates but it can be increased by heating the same specimens 30 minutes at 155° C in their de-swollen state. The observed increase in modulus of vulcanizates containing graphitized and regular carbon blacks after such a treatment was found to be related to carbon black “activity” and loading. Bound rubber does not seem to function as a distinct phase contributing to reinforcement, but it can be used as a measure of the number of attachments of carbon black and polymer. Additional information on the molecular mechanism of the phenomenon was obtained through radioactive tracer analysis. It was also found that regular vulcanizates can be softened by heating swollen samples at elevated temperatures (150-170° C). This softening was related to carbon black “activity” and was fully reversible: the original modulus could be recovered by additional heating of de-swollen vulcanizates for a further thirty minutes at 155° C. The above observations are discussed in terms of mobile adsorption of polymer chains on carbon black surface and the steric effect of fixing carbon black particles into an already extended polymer network.