Hydration of polymer chains

Water-soluble polymers often collapse upon heating. Such inverted CG transitions cannot be explained by a simple excluded-volume interaction of the type (1.71), because v T) increases with temperature and hence monomers on the chain repel each other, resulting in the chain swelling at high temperature. For a chain to collapse at high temperature, we should consider additional molecular interaction such as hydrogen bonding and hydrophobic association. 

Hydration of a neutral polymer can roughly be classified into two categories direct hydrogen bonds (referred to as H-bonds) between a polymer chain and water molecules (p-w), and the hydrophobic hydration of water molecules surrounding a hydrophobic group on a chain in a cage structure by water-water (w-w) H-bonds. In this section, we extend the combinatorial method for the partition function presented in the previous section to suit for the problem of solvent adsorption, and study polymer conformation change in aqueous solutions due to the direct p-w H-bonds. 

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The transport of many compounds takes place through interstices of polymer chains filled with aqueous medium [52], In such cases, the rate of mass transport is directly proportional to the degree of hydration of the membranes [53]. The most widely accepted method for determining the hydration of membranes is to equilibrate the membranes in water or buffer and weigh these membranes after blotting [54], In a newer method, the matrices to be studied are placed on a sintered glass funnel which is attached to a capillary filled with water. The absorption of water results in the movement of the capillary front [55],... 

When using more active emulsifiers of the C-10 type (Yeliseyeva, 1966) the effective rate and the equilibrium value of adsorption increase in the polymerization of polar monomers, which reduces the intensity of the processes of initial flocculation. Moreover, in this case the charge density is not the only stabilizing factor it is su(f>lemented by the enthalpy factor which arises from hydration of oxyethylenic chains of emulsifier molecules. The presence of these emulsifiers creates conditions that enable tbe obtain-ment of polar polymer latexes of high stability. 

Hydroxypropylmethyl cellulose Hydroxypropyhnethyl cellulose (HPMC) (Figure 28.2) is water-soluble cellulose ether and it can be used as hydrophilic polymer for the preparation of controlled-release tablets. Water penetrates the matrix and hydrates the polymer chains, which eventually disentangle from the matrix. Drug release from HPMC matrices follows two mechanisms, drug diffusion through the swelling gel layer and release by matrix erosion of the swollen layer. " ... 

For the retention of solutes, even when rejection in aqueous solution is high, the rejection of the same compound in organic solvents can be very low due to the absence of the formation of hydration shell around the solute (resulting in a smaller size) in the latter case. Finally, the structure of the surface layer of a membrane can be altered by exposure to orgauic solveuts. As a result, flows (and rejection) are modified as a function of the time of exposure to solvents. Because these changes in the structure of polymer chains are dependent on the interaction between solvent and exposure time, the concept of stability to organic solvents becomes ambiguous [23]. 

In the postgel regime where hydrated gel networks exist, one needs consider the number N i) of polymer chains of type I involved in the network. Their number density is given by (i) = (i) / S2, and their volume fraction by (i) = n (i) (i). 

In particular, the number of polymer chains of hydration type i that remain unassociated in solution is given by A (jo mo(i)). Similarly, the number of bound water molecules is... 

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