Kuhn segment, cellulosics

Table L Length of the Kuhn segment A and chain diameter d for cellulose esters and ethers according to the data of sedimentation - diffusion studies and viscometry... 

The flexibility of the chain is caused by a rotation about the O-C and 0-C bonds between neighboring glucose rings. If a real chain is replaced by an equivalent chain each unit of which consists of two parallel A/2 bonds about which rotation is possible and one 6 bond (normal to the two first bonds) about which no rotation takes place, it can be shown that the number of monomer units Sf in the Kuhn segment of a cellulose chain with unhindered rotation is given by... 

As an example of the use of Eq. (28) (p. 114), the data for fractions of cellulose carbanilate, the molecule of which can be represented by a partially drained worm-like coil, are plotted in Fig. 64. The dependence of the expression of the left-hand side of Eq. (28) on is approximated by a straight line the slope of whidi yields the length of the Kuhn segment A and the intercept the hydrodynamic diameter of the chain d. The curves in Fig. 64 provide the values of A = 160 A and d = 6 A for the cellulose carbanUate chain. 

Sophisticated experimental methods allow the development of models for polymers in dilute and semi-dilute solutions. Chain stiffness may be represented by the Kuhn segment lengths and determined in dilute solution. Models for cellulose and cellulose derivatives have recently been published whose main features are the irreversible aggregation of chains, if hydrogen bonding is possible even in dilute solutions. Trisubstituted cellulose derivatives or cellulose in hydrogen bond breaking solvents exist as molecular dispersed chains. How-... 

EMM Ac with /k=24.8 nm [11]. In contrast, if commercially available methyl cellulose (DS = 1.6-1.7) is substituted to DS=3 with the appropriate phenylcarbamate groups (3C1, 4C1), these derivatives exhibit no mesophase in dioxane, in fact only gels are formed, although the Kuhn segment length is comparable with those of 3C1-CTC (in dioxane this is 49.5 nm) with 32.8 nm (3C1) and 26.8 nm (4C1) [11]. 

Polymers with intermediate I values are usually called semi-rigid ones. It should be pointed out that the rigid macro-molecules in which the length of the Kuhn segment of the chain, 1, is much greater than the thickness (diameter) of the chain, d, should easily form an LC phase. Some examples of such compounds are a-helical polypeptides, macromolecules of DNA, aromatic polyamides, a number of cellulose ethers, and some polyisocyanates. Macromolecules of such polymers can be approximated in the form of long rods (Figure 4(b)). 

For values of the Kuhn segment of cellulose of the order of 100 A in direct solvents, the calculation shows that the formation of the liquid-crystalline state should be expected for an approximately 40 wt, % concentration of polymer. For a solution of cellulose in N-methylmorpholine oxide, the appearance of the liquid-crystalline state was observed in [31] with 25-55 wt. % concentrations of cellulose, which indicates the possibility of phase transitions in such systems with the appearance of the liquid-crystalline phase. 

The Kuhn segment A for the a-hdical confonnaticm of PBG is 2000 A, while the semirigid-chain macromolecules of cellulose ethers are characterized by a value of A from 130 to 330 A as a function of the chemical structure the substituents and the type of sdvent used [33]. 

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