Random coil crosslinked

Figure 1. Representation of a noncovalent network of random coils crosslinked by interchain association (a) or by microcrystalline domains (b) non-covalent network of rod-like polymers or polymer aggregates whose "crosslinking" is a manifestation of a brush-pile of rods (c) or fibers (aggregates of aligned rods) (d), or a result of non-nucleated phase separation kinetics (e).

Small deformations of the polymers will not cause undue stretching of the randomly coiled chains between crosslinks. Therefore, the established theory of rubber elasticity [8, 23, 24, 25] is applicable if the strands are freely fluctuating. At temperatures well above their glass transition, the molecular strands are usually quite mobile. Under these premises the Young s modulus of the rubberlike polymer in thermal equilibrium is given by ... 

Perhaps the most viable short-term use for dendritic macromolecules lies in their use as novel catalytic systems since it offers the possibility to combine the activity of small molecule catalysts with the isolation benefits of crosslinked polymeric systems. These potential advantages are intimately connected with the ability to control the number and nature of the surface functional groups. Unlike linear or crosslinked polymers where catalytic sites may be buried within the random coil structure, all the catalytic sites can be precisely located at the chain ends, or periphery, of the dendrimer. This maximizes the activity of each individual catalytic site and leads to activities approaching small molecule systems. However the well defined and monodisperse size of dendrimers permits their easy separation by ultrafiltration and leads to the recovery of catalyst-free products. The first examples of such dendrimer catalysts have recently been reported... 

Elastomers exhibit this behavior due to their unique, crosslinked structure (cf. Section 1.3.2.2). It has been found that as the temperatme of an elastomer increases, so does the elastic modulus. The elastic modulus is simply a measme of the resistance to the uncoiling of randomly oriented chains in an elastomer sample under stress. Application of a stress eventually tends to untangle the chains and align them in the direction of the stress, but an increase in temperatme will increase the thermal motion of the chains and make it harder to induce orientation. This leads to a higher elastic modulus. Under a constant force, some chain orientation will take place, but an increase in temperatme will stimulate a reversion to a randomly coiled conformation and the elastomer will contract. 

Table II. Gel Permeation Characteristics for Crosslinked Random Coils...

Table II provides some examples of how the effects of disulfide crosslinks on Re are reflected in the observed Mapp s for a series of proteins of known molecular weight. The divergence between the true M and the Mapp observed for crosslinked random coils clearly demonstrates the dependence of the method on linear dimensions for the polymer under investigation. It also shows the need for coupling this method with exact methodology for molecular weight determination if incontrovertible data are required.

The crosslinking must be sufficiently infrequent (about one crosslink per hundred repeat units) as to allow the polymer to adopt a random coil configuration between crosslink sites and so exhibit entropic recovery when deformed. The chemistry of rubber crosslinking is discussed later. 

It is known that PMLG takes an a-helix form in CHCI3, and the random coil form in a mixture of TFA and CFICI3 [47]. When the solvent composition is changed in the crosslinked PMLG gel, its volume and conformation are changed. 

J. Stone I think there are no basic differences at all. Actually our dextrans are not crosslinked but similar to the P.E.G. s in that they are long chain molecules which randomly coil in solution to form spherical molecules. One reason we prefer the sugars and dextrans is that they are optically active and thus we can use a precision polarimeter for measurement of concentration. We also have a very wide range of molecular sizes. The dextrans have been well characterized by the physical chemistry group at Uppsala and we use the data which we get from them. As far as our basic conclusions are concerned, I don t think there is any dispute at all. We both agree that wood isn t digestible because the pores in wood are too small for the enzyme to get in. ... 

In a crosslinked sample, the hexagonal crystalline phase withstands even 230 °C. This apparently abnormally high temperature of melting may be explained by the fact that polymer chains were forced to retain an extended chain arrangement within the crystallites of the highly crosslinked and diemically modified amorphous domains and the entropy of their fusion would thus be smaller than the value for a random coil in the melt. In amorphous domains, a denser structure of crosslinks is probably formed and this has a high temperature. 

The most utilized PAI congeners, namely B-PEI and L-PEI, have quite different solubility behaviour. B-PEI is soluble in water, independent of solution pH, and various organic solvents, while L-PEI in its free base form is insoluble in water and most organic solvents at room temperature, except lower alcohols, due to the formation of insoluble L-PEI crystals. Aqueous solutions of the L-PEI freebase also display temperature-dependent solubility behaviour as it becomes soluble in water above 64 °C. FTIR spectroscopy has confirmed that this phase transition is due to a melting transition from a crystalline zig-zag state to the hydrated random coil state.When cooled from the heated soluble state, the polymer forms a crystalline fibre-based hydrogel, which can be chemically crosslinked with glutaric anhydride. ... 

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