Chains interaction

In linear and branched polymers only secondary interaction forces act between the different chains, whereas in network polymers the various chains are bound to each other covalently. Secondary intermolecular forces are considerably weaker than primary covalent bonds. Nevertheless they have a strong effect on the physical properties of the polymer (and consequently on its permeability) because of the large number of interactions possible. Three different types of secondary force can be considered  

All the parameters discussed above such as molecular conformation, molecular configuration, chain interaction and chain length are important in deiermini% the overall. state of the polymer and will be funher di.scusscd in the next section. 

---

By using an effective, distance-dependent dielectric constant, the ability of bulk water to reduce electrostatic interactions can be mimicked without the presence of explicit solvent molecules. One disadvantage of aU vacuum simulations, corrected for shielding effects or not, is the fact that they cannot account for the ability of water molecules to form hydrogen bonds with charged and polar surface residues of a protein. As a result, adjacent polar side chains interact with each other and not with the solvent, thus introducing additional errors. 

Figure 10.18 Side-chain interactions in the leucine zipper structure, (a) The hydrophobic side chains in spikes a and d (see Figure 10.17) form a hydrophobic core between the two coiled a helices, (b) Charged side chains in spikes and g can promote dimer formation by forming complementary charge interactions between the two a helices.

Long side chain Interaction in long side chain Flexible backbone... 

These types of polar monomer provide sites for hydrogen bonding which increase the cohesive strength of the PSA because of strong inter-chain interaction, and they can also allow for hydrogen bonding or other polar interactions with some substrates. 

Since these microscopic simulations typically can only treat short times and small samples, it is important to avoid surface effects. It is common to employ periodic boundary conditions. A special trick often used for these kinds of simulation is, instead of simulating a melt of many chains, to simulate one very long chain which falls back again and again into the box. In this way, the effect of the chain ends, which introduces artificially high free volume can be reduced. However, one should keep in mind that this chain interacts with its own periodic images. It is known that this may... 

Nearest neighbors along a chain interact by means of a FENE (finitely extendible nonlinear elastic) potential... 

Concluding this section, one should mention also the method of molecular dynamics (MD) in which one employs again a bead-spring model [33,70,71] of a polymer chain where each monomer is coupled to a heat bath. Monomers which are connected along the backbone of a chain interact via Eq. (8) whereas non-bonded monomers are assumed usually to exert Lennard-Jones forces on each other. Then the time evolution of the system is obtained by integrating numerically the equation of motion for each monomer i... 

In the following section, step by step a qualitative picture is formed describing the impact of intcrmolecular interactions oil the absorption and luminescence of organic conjugated chains. The present calculations do not distinguish between dimers and aggregates (for which the wavefunctions of adjacent chains interact in the ground state, due to, for instance, solid-state effects) and excimers (where overlap occurs only upon photoexcitation) [29]. 

Many properties of silicates can be understood in terms of the type of network lattice formed. In the one-dimensional networks, shown in Figure 17-8, the atoms within a given chain are strongly linked by covalent bonds but the chains interact with each other through much weaker forces. This is consistent with the thread-like properties of many of these silicates. The asbestos minerals are of this type. 

A more accurate analysis of this problem incorporating renormalization results, is possible [86], but the essential result is the same, namely that stretched, tethered chains interact less strongly with one another than the same chains in bulk. The appropriate comparison is with a bulk-like system of chains in a brush confined by an impenetrable wall a distance RF (the Flory radius of gyration) from the tethering surface. These confined chains, which are incapable of stretching, assume configurations similar to those of free chains. However, the volume fraction here is q> = N(a/d)2 RF N2/5(a/d)5/3, as opposed to cp = N(a/d)2 L (a/d)4/3 in the unconfined, tethered layer. Consequently, the chain-chain interaction parameter becomes x ab N3/2(a/d)5/2 %ab- Thus, tethered chains tend to mix, or at least resist phase separation, more readily than their bulk counterparts because chain stretching lowers the effective concentration within the layer. The effective interaction parameters can be used in further analysis of phase separation processes... 

Different methods have been studied to reduce the chain interactions or to promote a better interaction between a solvent and the polymeric chain ... 

In the linear konjac mannan, the degree of acetylation profoundly affects the solubility and flow properties of this hydrocolloid. The acetyl substitution prevents self-association of the mannan chains, but following deacetylation chain interactions become more energetically favorable [230]. 

So far, we have not introduced a specific model of the polymer network chains. This problem can be rigorously solved for cross-linked polymer networks consisting of phantom chains [13], or even in the more general case of filled networks where the chains interact, additionally, with spherical hard filler particles [15]. 

---