Interaction interchain

It was outhned in the introductory section (cf. 4.4.1) that the periodically arranged monosaccharide sequence in a polysaccharide can be interrupted by nonperiodic segments. Such 

Initially, a periodic sequence of altering units of P-D-galactopyranose- 4-sulfate (I, conformation Ci) and a-D-galactopyranose-2,6-disulfate (II, conformation Ci) is built up in carrageenan biosynthesis  

In this way, a gel is formed with a three-dimensional network in which the solvent is immobilized. The gel properties, e. g., its strength, are influenced by the number and distribution of a-D-galactopyranosyl-2,6-disulfate residues, i.e. by a structural property regulated during polysaccharide biosynthesis. 

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The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

In the previous section we assumed that disorder results in random fluctuations of the order parameter around some average value A<). Such an approach is, essentially, a mean field treatment of the lattice. It requires sufficiently strong interchain interactions, whose role is to establish a coherence between the phases of the order parameter in different chains. 

While for strong interchain interactions large deviations of the order parameter from its average value are unlikely, for weak interactions the minimal-energy lattice configuration of disordered chains contains a finite density of kinks and anti-... 

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57], Table 4-1 collects the INDO/SCl-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

In the Flory-Huggins picture, the interchain interaction parameter should be related to the intersegment interaction parameter by... 

Basically, the first approach to correlate the polyimide chain organization to the monomer structure was to take into consideration the electron affinity of the anhydride and the ionization potential of the diamine,10 as shown in Fig. 5.3. The strongest interactions between the polymeric chain are expected when the polyimide is prepared with the dianhydride having the highest electron affinity and die diamine with the lowest ionization potential. The strongest interchain interaction leads to high Tg and low solubility. 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

In polyelectrolyte solutions, the counterion condensation on linear polyelectrolyte chains is known to occur when the charge density along the chain exceeds the critical value [40]. Our work indicates the existence of a critical value for the separation distance between chains, where the interchain interaction changes drastically, most likely due to the transition in the binding mode of the counterions (see Fig. 13). Many peculiar forms of behavior, which are often interpreted by the cluster formation or the interchain organization of polyelectrolytes, have been reported for high concentrations of aqueous polyelectrolytes... 

Lee et al. [60] investigated the adhesion of a single pair of DNA strands. They identified two types of forces interchain forces associated with Watson-Crick base pairing between complementary strands, and intrachain forces associated with the elasticity of single strands. For studying interchain interactions, complementary oligomers (ACTG)s and... 

The introduction of bulky side chains that contain adamantyl groups to poly(p-phenylenevinylene) (PPV), a semiconducting conjugated polymer, decreases the number of interchain interactions. This action will reduce the aggregation quenching and polymer photoluminescence properties would be improved [93]. 

In absence of external salt, strong electrostatic interchain interactions exist in solution causing very large increase of the reduced viscosity and the formation of a pseudo electrostatic 3D network this was recently discussed [24], It implies the salt sensitivity of the viscosity which decreases when neutral salt is added due to a screening effect on the long range electrostatic repulsions. 

Whatever is the mechanism of interchain interactions to form a three dimensional network, the basis is the formation of a cooperative junction whose stability depends on the specific energy of the linkages and the number of units cooperatively bound, as well as the number of chains involved. 

According to Hess, the relative strength of the entanglement friction can be related to the more microscopic parameter q , describing the range of the true interchain interaction potential. A value of q 1 = 7 A, close to the average interchain distance of about 4.7 A, is obtained. 

Turning to polymers giving thermodynamically stable mesophases we must assume that, since we have described bundles as an inherent structural feature of undercooled polymer melts, such structures should occur, at least in principle, also in such systems, to the extent that attractive interchain interactions which account for bundle formation play a significant role. On the other hand, rigorously speaking Class II mesophases are entropy-stabilized and inter-chain... 

The phantom network behaviour corresponding to volumeless chains which can freely interpenetrate one through the other and thus to unrestricted fluctuations of crosslinks should be approached in swollen systems or at high strains (proportionality to the Mooney-Rivlin constant C-j). For suppressed fluctuations of crosslinks, which then are displaced affinely with the strain, A for the small-strain modulus (equal to C1+C2) approaches unity. This situation should be characteristic of bulk systems. The constraints arising from interchain interactions important at low strains should be reflected in an increase of Aabove the phantom value and no extra Gee contribution to the modulus is necessary. The upper limit of the predicted equilibrium modulus corresponds therefore, A = 1. 

The Gge contribution of the form given Eq.(2) represents the simplest form of permanent interchain interactions. The value of Gee at Teg=1 and w =1, i.e. the Gge contribution of a perfect network, has been assumed equal to the plateau modulus of the corresponding linear polymer (10,15,23). This assumption has not always been confirmed and, therefore, for the purpose of this work we prefer to consider g of g" as proportionality constants. 

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