Adsorption, polymer from poor solvents

In a poor solvent (cyclohexane at 5°C), a polymer chain takes on a condensed globular state because constituent molecules are repulsed by the solvent molecules. Nanofishing of this chain revealed a perfectly different force-extension curve, as shown in Figure 21.5. It was observed that constant force continued from about 30 to 130 nm after nonspecific adsorption between a... 

Then we extended the 2D-model to a 3D one [21]. We considered crystallization of a single polymer chain C500 from a vapor phase onto a solid substrate, taking into account detailed interactions between the chain and the substrate. Though the polymer molecule in a vacuum was collapsed, like in a very poor solvent, under the influence of bare van der Waals interactions between atoms, the molecule was found to show quick adsorption and crystallization into a rather neat chain folded lamella. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

The final factor results from the mutual interaction between segments and solvent molecules. In a poor solvent, the segment-solvent interaction is unfavourable. This forces the polymer out of the solution, thus promoting adsorption. In a good solvent, the segment-solvent interaction is favourable, while the aggregation of segments is unfavourable and as a result the adsorbed amount is less. 

The interesting phenomenon where a mixture of two poor solvents or nonsolvents for a polymer provides a medium that acts as a good solvent for die polymers has been the objective of many studies, by light scattering, " viscometry, " sorption equilibrium, and fluorescence. From these techniques, it has been possible to appreciate how the second virial coefficient Aj and the intrinsic viscosity [11] preferential adsorption coefficient A, and excimer and monomer emission ratio Ie/Im are involved by changing solvent composition. They present ([T)], A2) a maximum or a variation at a certain solvent composition where the polymer behaves as through it were dissolved in a good solvent. 

Steric stabilization by adsorbed homopolymers suffers from the conflicting requirements that the liquid be a poor solvent to ensure strong adsorption but a good solvent to impart a strong repulsion when the adsorbed polymer chains overlap. At low polymer concentrations, an individual polymer chain can become simultaneously adsorbed on two (or more) surfaces, resulting in an attractive interaction known as bridging flocculation (see Sect. 4.6.4). 

The above discussion on the two components of should lead to a better understanding of physical adsorption. Theoretically, polymer adsorption(so) can be treated by the Scheutjens-Fleer (SF)(si) mean-field theory, the Monte Carlo (MC) method,( 2) or the scaling approach. (83) In Figure 10, two profiles are given for the cases of adsorption (x = 1) and depletion (x = 0) using the SF theory, where x is the Flory-Huggins interaction parameter(84) between a polymer and a solvent with respect to pure components. The polymer coil expands if X < 0.5 and contracts if x These two cases are referred to as good and poor solvents, respectively. From the volume fraction profile c )(z), we can calculate other adsorption parameters, such as F, the adsorbed amount ... 

For polymer adsorption from the poor solvents, when the macromolecule volume is small, the number of macromolecules adsorbed on the unit surface area is greater than the adsorption from good solvents. A change of the solvent from poor to good leads to an increase in the volume of adsorbed macromolecules and partial desorption of macromolecules occurs. 

The adsorption of polymers typically increases with polymer molecular weight and is higher and best (e.g. thicker, denser adsorbed layers) from a solution containing a poor solvent. For these reasons, block-copolymers are excellent steric stabihzers. Block copolymers like poly(ethylene oxide) surfactants are suitable as one part of the stabilizer has a high tendency to adsorb onto the particle surface and the other has a high affinity for the solvent. 

Good solvent Poor solvent Steric stabilization Figure 12.6 Steric stabilization using polymers. For proper stabilization, the steric barrier needs to extent at least 10 nm from the particle surface and therefore polymers or oligomers are used (low molecular surfactants are too small). It works best with thick and dense layers, good solvents, strong adsorption and complete particle coverage... 

Usually," " more polymer is adsorbed from poorer solvents. Variation in solvent-substrate interactions can complicate this simple finding, which was predicted theoretically by Scheutjens and Fleer. In good solvents, repulsion between segments e.g. in tails) of adsorbed polymers will be greater than in poor solvents, leading to lower adsorption in the former. The lower adsorption in good solvents has been wrongly attributed to less-extended conformations. 

---