Properties molecular mass effect

Broutman and McGarry [98] examined the effects of crosslinking on toughness as early as 1965. Bell [99] observed a threefold increase in notched impact strength as the molecular mass between crosslinks was increased. Schmid et al. [100] and Lohse et al. [101] pointed out the dominating effect of molecular strand length on the ultimate properties and the toughness of crosslinked polymers. Later, Batzer et al. [46], Schmid [44], and Fischer et al. [45] compared the behavior of various networks composed of epoxy resins. 

Fig. 8.1. Effects of crosslinking on various properties (X) of the polymer. The deviation [X — XcoJ/Xa, from the thermoplastic property (X ) is plotted against Me1, the inverse molecular mass between crosslinks.

Polyethylene glycol (PEG) consists of repeating units of ethylene glycol forming linear or branched polymers with different molecular masses. Pegylation is the process by which PEG chains are covalently attached to lEN molecules. Pegylation confers a number of properties on lEN-a molecules, such as sustained blood levels that enhance antiviral effectiveness and reduce adverse reactions, as well as a longer half-life and improved patient compliance (Kozlowski et al. 2001). 

Properly functionalised additives can react with polymer substrates to produce polymer-bound functions which are capable of effecting the desired modification in polymer properties, hence the use of the term reactive modifiers. As an integral part of the polymer backbone, reactive modifiers are useful vehicles for incorporating the desired chemical functions to suit the specialised application. Being molecularly dispersed, the problem of solubility expressed under 2 above is avoided. Implicitly, the bound-nature of the function is not subjected to the normal problems of the loss of additives from the surface which are common with both high and low molecular mass additives. The bound nature of the function must be fully defined for the conditions of service. 

Another perspective provided by this model is the effect of three physiochemical parameters—solubility, distribution coefficient, and molecular mass—on transcoreal flux. All of these properties can be influenced by molecular design. The effects of these properties are illustrated in Fig. 13, in which the logarithm of the flux is plotted as a function of solubility and distribution coefficient for two different Mr. Several features of the model are depicted, and these qualitative, or semi-quantitative, aspects presumably encompass the principles of corneal permeation. 

Note 1 In many cases, especially for synthetic polymers, a molecule can be regarded as having a high relative molecular mass if the addition or removal of one or a few of the units has a negligible effect on the molecular properties. This statement fails in the case of certain properties of macromolecules which may be critically dependent on fine details of the molecular structure, e.g., the enzymatic properties of polypeptides. 

Note that nuclear mass does not appear in the electronic Schrodinger equation. To the extent that the Bom-Oppenheimer approximation is valid, this means that mass effects (isotope effects) on molecular properties and chemical reactivities are of different origin. 

Since the quality of the separation is determined by the properties of the filter used, it is essential that the investigator should understand the causes of their non-ideal behaviour and how these can be minimised. The non-ideal behaviour is due to nonspecific adsorption of the constituents to be separated on the filter, Donnan equilibria, leakage of high molecular mass constituents and hindered passage of low molecular mass species through the filter. Some of these effects and how they influence the speciation results have been described by Gardiner and Delves... 

The ratio of elastic constants Ku, calculated for the S-effect according to the equation (4) appeared to be (Kn (polymer XIV)/Kn (polymer XIII)) x 1 100 and (Ku (polymer XVI)/Kn (polymer XV)) x 1 36. Yet, as we have just indicated, taking into account molecular masses of the LC polymers and reducing k, values for various polymers to equal values of DP one may come to substantially different values for ratios of constants presented. It is necessary to note that up to date no quantitative data on the determination of elastic constants of LC polymers has been published (excluding the preliminary results on Leslie viscosity coefficients for LC comb-like polymer127)). Thus, one of the important tasks today is the investigation of elastic and visco-elastic properties of LC polymers and their quantitative description. 

Mucoadhesive polymers exhibiting strong complexing properties are capable of inhibiting intestinal brush border membrane-bound proteases through a far distance inhibitory effect [65]. In vivo, the mucoadhesive polymer is separated from the brush border membrane by a mucus layer [30]. Although there is no direct contact between polymer- and membrane-bound enzymes, it could be shown that inhibition takes place. The exploitation of this far distance effect seems to be a very promising alternative to small molecular mass inhibitors, which are currently used as inhibitors of brush border membrane-bound proteases. 

---