Polymers molecular motions

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

Results obtained with different initial concentrations have not been reported, but, evidently, such data could be normalized if first-order kinetics are obeyed for each decay curve. This first-order decay implies that a definite population can be isolated in a given temperature interval, while in the case of radicals and ions, there is a continuous distribution of pairs. The difference between these two cases is thus not very great. Heating induces in the polymer molecular motions which correspond to destruction of the traps with activation energies of the order of a few kcal mole-1. 

If the polymer is glassy, the solvent lowers the Tg by a plasticizing action. Polymer molecular motion increases. Diffusion rates above Tg are far higher than below Tg. Thus diffusion may depend on the concentration of the diffusing species (63,64). 

Many reactions of free radicals, of electronic excited states and of oppositely charged ions are found to be diffusion controlled in condensed phases. When these reactions involve polymeric reactants, or take place in a polymer medium, the diffusion step will be dependent on the nature of the polymer molecular motions in the reaction zone. 

The formation of a seven-membered ring is usually less favoured than that of a six-membered ring in the liquid and vapour phases, however, it is favourable in the sohd state, where polymers molecular motions are very limited below the glass transition temperature (Tg). It is possible that the conformation of the macromolecules might be such that tertiary hydrogen atoms are close to the acetate group of the next monomeric unit. 

The mechanisms responsible for the property enhancements are generally accepted as associated with the inhibition of polymer molecular motions near the filler surface. Macroscopic measurements of composite properties show that for the nylon/clay system, basic mechanical properties such as modulus, strength and impact strength cease to improve b ond a concentration of 5 wt% (equivalent to 2 vol%) [1]. If we accept the primary role of filler surface, and therefore filler surface area, the existence of a ceiling on property enhancement... 

Doi, M. and Edwards, S.F., 1978. Dynamics of concentrated polymer systems 1. Brownian motion in equilibrium state, 2. Molecular motion under flow, 3. Constitutive equation and 4. Rheological properties. J. Cheni. Soc., Faraday Trans. 2 74, 1789, 1802, 1818-18.32. 

Many simulations attempt to determine what motion of the polymer is possible. This can be done by modeling displacements of sections of the chain, Monte Carlo simulations, or reptation (a snakelike motion of the polymer chain as it threads past other chains). These motion studies ultimately attempt to determine a correlation between the molecular motion possible and the macroscopic flexibility, hardness, and so on. 

A basic theme throughout this book is that the long-chain character of polymers is what makes them different from their low molecular weight counterparts. Although this notion was implied in several aspects of the discussion of the shear dependence of viscosity, it never emerged explicitly as a variable to be investi-tated. It makes sense to us intuitively that longer chains should experience higher resistance to flow. Our next task is to examine this expectation quantitatively, first from an empirical viewpoint and then in terms of a model for molecular motion. 

The rigid structure of the polymer molecule leads to a material with a high Tg of 208°C. There is also a secondary transition at -116°C and the small molecular motions that this facilitates at room temperature give the polymer in the mass a reasonable degree of toughness. 

Kashiwabra, H., Shimada, S., Hori, Y. and Sakaguchi, M. ESR Application to Polymer Physics — Molecular Motion in Solid Matrix in which Free Radicals are Trapped. Vol. 82, pp. 141 -207. 

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

The results also indicate that there is a significant descrease in the chemical shift anisotropy in going from the segmented polymer B (which contains very few soft segments, 0(CH2)4 to the polymer C (which contains 6 times more soft segments). The difference also seems to reflect increased molecular motion of the phenyl rings in the softer of the two segmented polymers. A similar conclusion may be drawn from the Tl-values, which for polymer B is 3 s. as oposed to 0.25 s. for the C polymer. 

Up to now it has been tacitly assumed that each molecular motion can be described by a single correlation time. On the other hand, it is well-known, e.g., from dielectric and mechanical relaxation studies as well as from photon correlation spectroscopy and NMR relaxation times that in polymers one often deals with a distribution of correlation times60 65), in particular in glassy systems. Although the phenomenon as such is well established, little is known about the nature of this distribution. In particular, most techniques employed in this area do not allow a distinction of a heterogeneous distribution, where spatially separed groups move with different time constants and a homogeneous distribution, where each monomer unit shows essentially the same non-exponential relaxation. Even worse, relaxation... 

As we have seen previously the presence of crosslinks between macromolecules influences the way in which these materials respond to heat. Uncrosslinked polymers will generally melt and flow at sufficiently high temperatures they are usually thermoplastic. By contrast, crosslinked polymers cannot melt because of the constraints on molecular motion introduced by the crosslinks. Instead, at temperatures well above those at which thermoplastics typically melt, they begin to undergo irreversible degradation. 

Again because of the crosslinks, such brittle behaviour occurs whatever the temperature unlike brittle materials based on linear polymers, there is no temperature at which molecular motion is suddenly freed. In other words, the Tg, if there is one, does not produce dramahc changes in mechanical properties so that the material is changed from one that undergoes brittle behaviour to one that exhibits so-called tough behaviour. 

The topic of molecular motion is an active one in experimental and theoretical polymer physics, and we may expect that in time the simple reptation model will be superseded by more sophisticated models. However, in the form presented here, reptation is likely to remain important as a semi-quantitative model of polymer motion, showing as it does the essential similarity of phenomena which have their origin in the flow of polymer molecules. 

Experimental values of the molar mass exponent close to 2 have been obtained. For example, for poly(methyl methacrylate), a value of 2.45 has found (see P. Prentice, Polymer, 1983, 24, 344—350). As with values of selfdiffusion coefficient, this has been regarded as close enough to 2 for reptation to be considered a good model of the molecular motion occurring at the crack tip. 

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

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