Self-diffusion rubbers

Table 7 Relaxation time (t), self-diffusion coefficient (D), and monomer friction coefficient (Co) of unfilled (B) and nanoclay-filled (BCLNA8) BIMS rubber...

The ability to infiltrate the surface of a host material decreases with molecular size. Molecules ofM > 5 x 103 can hardly diffuse through a porous-free membrane. Self-diffusion is when a molecule moves, say in the melt, during crystallization. Also, when bonding rubber, the so-called tack is explained by the self-diffusion of the molecules. The diffusion coefficient for self-diffusion is of the order of... 

The diffusion of small molecules in rubbers is of both theoretical and practical importance. The theories of diffusion based on consideration of free volume can be tested by measurement of self-diffusion using methods such as pulsed field gradient NMR. Selfdiffusion of small molecules must be understood for applications of rubbers as seals in contact with solvents, and for example for diffusion of plasticisers and other small molecules. 

Functional properties and stability of rubbery materials Chapters 1, 3, 4, 7, 12 and 13, give examples of applications of spectroscopic techniques for the characterisation of thermal stability and degradation, kinetics of thermal decomposition, ageing, oxidation and weathering, self-diffusion of small molecules in rubbery materials, adhesion of rubbers to metals, fluid adsorption and swelling. 

Bueche et al. (1952) derived that the coefficient for self-diffusion of poly(n-butyl acrylate) is inversely proportional to the bulk viscosity of this polymer. Also in the natural rubber (polyisoprene) diffusion system a clear connection appears to exist between diffusion coefficient and bulk viscosity. In general the following expression may be used as a good approximation ... 

It is possible that the lower than required values of D2 reflect a problem with incorrect values of Q, which if too large would result in smaller values of D2. In an interferometric study of the diffusion of toluene in an uncrosslinked natural rubber sample, Mozisek (15) reported results for the mutual diffusion coefficient which were similar to the results of Hayes and Park. In the absence of thermodynamic data from Mozisek s work, correction factors calculated for the present work were applied to his data. The results are shown in Figure 7, which reproduces Mozisek s data along with the values for D2. The extrapolated value at 1, would exceed the self diffusion coefficient for toluene by about two orders of magnitude, similar to the discrepancy seen with Hayes and Park s data. This indicates that the fault with the results in the present case is not due to overly high values of the correction factors. Moreover, the method of calculating D from D12 has been confirmed experimentally by Duda and Vrentas (16) in a comparison of vapor sorption results for toluene diffusion in molten polystyrene with the values of D1 obtained directly using radio-labeled toluene. 

Figure 7. Comparison of Dj versus volume fraction of solvent in natural rubber, calculated from results of Von Mozisek using thermodynamic correction factors from present study, with self-diffusion coefficient for pure toluene. D, obtained using Q from Rory—Huggins (Chi=0.36).

Figure 8. Dj for heat corrected data in unfilled, crosslinked, natural rubber sample, versus volume fraction of solvent, compared with empirical extrapolation (dashed line) to the self-diffusion constant for toluene.

In the EjE type, the segments, and to some extent also the macromolecules of the adherent and the adhesive themselves, are able to move. Therefore, they can diffuse into one another. If adherent and adhesive are chemically equal, then self-diffusion is observed for this type. Self-diffusion leads to self-adhesion (autoadhesion) which is responsible for the tack of freshly cut natural rubber. 

The self-diffusion coefficient D) of swollen liquids in elastomers can be measured using NMR-MOUSE with a bar magnet (192). The method was shown to be particularly useful for measuring D of small penetrant molecules in elastomers without the need for measin-ements of the transverse relaxation rates. The selfdiffusion coefficient of toluene in a series of cross-linked natimal rubber samples was measured and correlated with the cross-link density. 

M. Tirrell, Polymer self-diffusion in entangled systems. Rubber Chem. Technol. 57, 523 (1984). 

M. Tirrell, "Polymer Self-diffusion in Entangled Systems," Rubber Chem, Technol, 51, 523-556 (1984). 

Fig. 37. Self-diffusion coefficients D of various elastomers as a function of molecular weight M. (Taken from Ref. 177, published by Rubber Division, American Chemical Society.)...

Matthew Tirrell, Polymer Self-diffusion in entangled systems . Rubber Chem. and Technology, 57,1984, pp 523-556. 

The predicted self-diffusion coefficients depend principally on the quality of the force fields used to model the interactions not only between the penetrant and polymer matrix, but also intramolecular interactions between polymer chains. These last ones, strongly affect the quality of the amorphous polymer cell and in particular the total free volume its distribution and dynamics which in their turn affect the predicted values of diffusion coefficients. The role of chain relaxation and matrix fluctuations in explaining the diffusion mechanism of small gas penetrants as N2 in rubber polymer membranes has been clearly... 

Tirrel, M. 1984. Polymer Self-Diffusion in Entangled Systems. Rubber Chem. TechnoL, 57, 523-556. 

The diffnsion theory of adhesion is basically a very simple concept, the original use of which is due to Voyutskii. He was concerned with the self-adhesion (antohesion) of nnvnlcanized (not cross-linked) rnbber (see also Rubber-based adhesives). He snggested that if two snch polymer surfaces are in sufficiently close contact, parts of the long-chain molecnles will diffuse across the interface. They will interpenetrate and eventnally the interface will disappear and the two parts will have become one. It is clear that, if this... 

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