Stiffness conformational

Region I here the molecular chains partly assume helical (stiff) conformation from random coils where the persistence length gradually increases. 

Water, occasionally modified by a small volume of ethanol and electrolytes, is the universal food solvent. In this medium, a polysaccharide polyanion s molar volume (vm) is maximum, because charge repulsion forces the molecules into the stiff (uncurled) conformation of a rigid rod that cannot be extended further, but can be oriented (Odell, 1989). vm is larger for extended, stiff conformations than for compact random-coil conformations (Berth, 1992). As solvent quality deteriorates, e.g., by addition of salt or... 

Fluorenylidene linkages have been widely used as conjugation interrupters due to their stiff conformation and to the ability to increase solubility of the resulting polymer and were therefore incorporated in several PPP based polymers by the Suzuki polycondensation reaction [127]. Following this concept a blue emitting polymer based on... 

But the concentrated phase is in equilibrium with a diluted phase which is not consistently described by a Flory-Huggins model it is difficult to imagine that a diluted solution of the two types of polyelectrolytes with an opposite global charge and a rather stiff conformation can be treated by the lattice formalism. 

Conversely, azacalixarenes with conformational inflexibility in solution were reported. Fukushima et al. [30] reported that the NH protons of azacalixarenes 35b, 35d, 35f, 36b, and 36d were observed at S 5.6-5.S ppm as broad signals at room temperature and were sharpened at -50 °C. It was suggested from the NMR studies that intramolecular NH OR hydrogen bonding interactions brought about their stiff conformations (cf. Fig. 4 in Sect. 3.1.2), which thus existed in a single conformation at the lower temperature. 

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

In a study of the transition in conformation from random coil to stiff rod by poly(acrylic acid), it was found that the point of transition depended on a number of factors, including the nature of the solvent, the temperature, the particular counterion used and the degree of dissociation (Klooster, van der Trouw Mandel, 1984). 

The polymer we consider here is a semi-flexible chain which has some bending stiffness (Eq. 3). We first estimated the chain conformation in the melt. The calculated mean-square end-to-end distance R2n between atoms n-bond apart has shown that the chains have an ideal Gaussian conformation R2 is a linear function of n (see Fig. 35 given later). The value of R2 for n = 100... 

Conformational Change and Parallel Orientation of Stiff Segments (PSs). 210... 

The time evolution of the absorption intensity of each component is plotted in Fig. 19. Since the conformation in the iPS crystal is 3/1 helix (TG)3, two GTGT bands at 567 and 586 cm-1 may be assigned to a 3/1 helical conformation, which corresponds to the stiff segments. Here, it should be noted... 

Polyisocyanates have attracted much attention owing to their liquid crystalline properties, stiff-chain solution characteristics, and induced optical activities associated with helical chain conformation (Scheme 8). Pattern and Novak [39]... 

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