Polymer chain entanglement, formation

In the perspective discussed in the present contribution, bundle formation occurs within the amorphous phase and in undercooled polymer solutions. It does not imply necessarily a phase separation process, which, however, may occur by bundle aggregation, typically at large undercoolings [mode (ii)]. In this case kinetic parameters relating to chain entanglements and to the viscous drag assume a paramount importance. Here again, molecular dynamics simulations can be expected to provide important parameters for theoretical developments in turn these could orient new simulations in a fruitful mutual interaction. 

The deformation of polymer chains in stretched and swollen networks can be investigated by SANS, A few such studies have been carried out, and some theoretical results based on Gaussian models of networks have been presented. The possible defects in network formation may invalidate an otherwise well planned experiment, and because of this uncertainty, conclusions based on current experiments must be viewed as tentative. It is also true that theoretical calculations have been restricted thus far to only a few simple models of an elastomeric network. An appropriate method of calculation for trapped entanglements has not been constructed, nor has any calculation of the SANS pattern of a network which is constrained according to the reptation models of de Gennes (24) or Doi-Edwards (25,26) appeared. 

There are three types of cross-linking one is chemical and two physical. Physical cross-linking results from formation of crystalline regions within polymer structures and from chain entanglement. Cross-linked materials have good dimensional memory. Chemically cross-linked materials do not dissolve and do not melt. 

The chemical structure of a polymer can be analysed by many of the techniques used to characterise molecular species (see Chapter 3). Multinuclear NMR, IR and UV-visible spectroscopy, for example, are widely used key characterisation tools. Most polymers will dissolve in at least some readily available solvents (although the rate of dissolution may be slow due to chain entanglement effects). In cases where polymers are insoluble, solid-state NMR techniques can be used to provide excellent structural characterisation. Due to structural imperfections, unknown end groups and incomplete combustion problems as a result of ceramic formation (Section 8.2.5), elemental analysis data obtained by... 

Gibson and coworkers also found that the melt viscosity of a polymer is also altered by the formation of a polyrotaxane [19], Poly(ester rotaxane) 60 containing 42C14 as the cyclic component had a melt viscosity equivalent to that of the parent polyester with 2.5-fold higher molecular weight. This result clearly indicates that there is less chain entanglement in the polyrotaxane than in the backbone polymer by itself. 

Very high molar mass polyethylene (M>2,000,000 g mol-1) crystallises without the, formation of a clear superstructure, sometimes referred to as the random lamellar structure [116]. The great many chain entanglements present in high molar mass polymers obstruct crystallisation and the crystals become small and their orientation less correlated with surrounding crystal lamellae. 

S. L. Shenoy, W. D. Bates, H. L. Frisch, G. E. Wnek. 2005.Role of chain entanglements on fiber formation during elecfrospinning of polymer solutions good solvent, nonspecific polymer-polymer interaction limit. Polymer, 46. pp. 3372-3384. 

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