Stiffness of chain

Although the two possible modes of chain transfer to polymer (i.e., intra- and intermolecular reactions) does not have to be directly interrelated (intramolecular reaction for example may be prohibited due to the stiffness of chain), having the same origin, they should in many systems proceed simultaneously. Thus, if one of these processes is detected in any system, this is a strong indication that the other reaction also occurs. 

The model approximating a polymer chain by N linked segments of length A each i.s widely used. The value of A is determined by the interactions between the neighbouring monomer units, such as. short-range ones (the framework effect, the stiffness of chain). The distribution of N segments in space i.s well described by the Gaussian function. 

If the functionality of the hydroxyl or isocyanate component is increased to three or more, branched or crosslinked polymers are formed. The properties of urethane polymers are dependent primarily upon molecular weight, degree of crosslinking, effective inter-molecular forces, stiffness of chain segments, and... 

For some problems, however, the shortness of the chains is still a limitation, as well as the lattice structure (which prevents the study of critical dynamics, later stages of spinodal decomposition, etc., where the fluid nature of real polymer melts and blends is essential). While future work could clearly address questions such as mixtures of block copolymers and homopolymers, asymmetry effects (different stiffness of chains, etc.) and interfacial structure, one needs to develop complementary molecular dynamics methods for the study of dynamical phenomena in mixtures and block copolymer melts. 

Intrinsic stiffness of chains must also be considered for three-dimensional phonon propagation (26). It holds for bending modes 8 and... 

The parameter /r tunes the stiffness of the potential. It is chosen such that the repulsive part of the Leimard-Jones potential makes a crossing of bonds highly improbable (e.g., k= 30). This off-lattice model has a rather realistic equation of state and reproduces many experimental features of polymer solutions. Due to the attractive interactions the model exhibits a liquid-vapour coexistence, and an isolated chain undergoes a transition from a self-avoiding walk at high temperatures to a collapsed globule at low temperatures. Since all interactions are continuous, the model is tractable by Monte Carlo simulations as well as by molecular dynamics. Generalizations of the Leimard-Jones potential to anisotropic pair interactions are available e.g., the Gay-Beme potential [29]. This latter potential has been employed to study non-spherical particles that possibly fomi liquid crystalline phases. 

The wide variety of ketomethylene and amino ketone monomers that could be synthesized, and the abiUty of the quinoline-forming reaction to generate high molar mass polymers under relatively mild conditions, allow the synthesis of a series of polyquinolines with a wide stmctural variety. Thus polyquinolines with a range of chain stiffness from a semirigid chain to rod-like macromolecules have been synthesized. Polyquinolines are most often prepared by solution polymerization of bis(i9-amino aryl ketone) and bis (ketomethylene) monomers, where R = H or C H, in y -cresol with di-y -cresyl phosphate at 135—140°C for a period of 24—48 h (92). 

Well, that is the case at the low temperature, when the rubber has a proper modulus of a few GPa. As the rubber warms up to room temperature, the Van der Waals bonds melt. (In fact, the stiffness of the bond is proportional to its melting point that is why diamond, which has the highest melting point of any material, also has the highest modulus.) The rubber remains solid because of the cross-links which form a sort of skeleton but when you load it, the chains now slide over each other in places where there are no cross-linking bonds. This, of course, gives extra strain, and the modulus goes down (remember, E = [Pg.61]

On the other hand, it has also become clear that a materials-oriented synthesis of conjugated poly(phenylene)s cannot narrow its attention to properties of molecules only in solution, but has to include aspects of processing and supramolecu-lar ordering as well. The rigid-rod character of PPPs therefore suggests the use of chain stiffness as a structure-forming principle in the design of supramolecular motifs. 

Systematic studies67) to determine the properties of solutions of the obtained graft copolymers showed that graft copolymers containing stiff main chains (PAA,... 

Insertion of flexible blocks into the stiff polymer chain of PAN ensures the possibility of raising the resistance of modified PAN fibres to multiple deformations and, especially, increases significantly the abrasive resistance of fibres made of block copolymers. 

High degree of chain-to-chain attraction as a result of polarity (high cohesive energy density) results in chain stiffness and immobility... 

Few sets of K,a parameters were given in references 17 and 22 K and a are parameters which may depends on the fine structure i.e. rhamnose content, DE... when they play on the stiffness of the chain. Nevertheless, it seems that the viscometry must be used in carefully defined conditions to avoid aggregation which often surestimates the viscosity in that conditions, [ti] is related to the physical properties of the solution (tickening properties) but not directly to My. 

As discussed in the last 3 years, polysaccharides behave in solution under a worm like chain [26] the local stiffness of the chain is characterized by a persistance length (Ip) the larger Ip is, the larger the chain deviates from the gaussian behaviour in the usual molecular weight range of these natural polymers [27], This makes difficult to use the relations given in litterature for synthetic... 

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). 

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