Randomly crosslinked melts

Fig. 4.22 Time-dependent modulus G i) for randomly crosslinked melts. Open squares and crosses show results for U°°, while full squares and circles give results for LJ interaction. The lower two curves correspond to the original chains, while the upper give the results of constructed walks of length N = 100. (From Ref. 159).

To our knowledge there are only two sets of simulations on completely mobile randomly crosslinked polymer melts, the MD simulation of Duering et and the bond fluctuation MC simulation of Schulz and... 

The bond fluctuation lattice simulations investigate the relaxational properties of randomly crosslinked polymer melts, starting out from an equih-brated melt and then add additional bonds between monomers. Besides studying the gelation threshold they are concerned with two cases. One is the shp link problem, the other the collective relaxation of the whole network. 

A novel reactor for pyrolysis of a PE melt stirred by bubbles of flowing nitrogen gas at atmospheric pressure permits uniform temperature depolymerisation. Sweep-gas experiments at temperatures 370-410 C allowed pyrolysis products to be collected separately as reactor residue (solidified PE melt), condensed vapour, and uncondensed gas products. MWDs determined by GPC indicated that random scission and repolymerisation (crosslinking) broadened the polymer-melt MWD. 19 refs. USA... 

An important difference between randomly branched and linear polymers is that the fractal dimension of branched polymers is larger than the dimension of space (d—3). This severely limits the applicability of the mean-field theory to the crosslinking of long linear chains, called vulcanization. Long chains in the melt have a fractal dimension of P = 2, which leaves lots of room inside the pervaded volume of the chain (i.e., filled by other chains in a polymer melt). The extra room created by the linear sections between crosslinks allows the fractal dimension of P = 4 to exist in three-dimensional space on a certain range of length scales (see Section 6.5.4). 

An interesting sub-dass of lonomers are telechelic lonomers, of which there are several noteworthy examples. The term "telechelic" indicates that the ions are attached exclusively at the chain termini and that every chain end contains an ionic moiety. Such placement provides a network free of dangling chain ends, and minimizes melt viscosity, since the telechelic polymer molecular weight is of the same order as the elastically effective molecular wei t between crosslinks. This is in direct contrast to "random"... 

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. ... 

So far, various methods of crosslinking have been discussed. However, decrosslinking, the opposite effect, can also be used to achieve certain properties. As the simplest possible example, the heating of a semicrystalline polymer above its melting point leads to decrosslinking, for now the polymer can flow. In a somewhat more sophisticated manner, decrosslinking of a specific crosslink site can be done, rather than a random degradation of the polymer to a soluble condition. 

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