Rubber-solvent interaction

The sorption and transport of four aliphatic hydrocarbons (n-hexane, n-heptane, n-octane and n-nonane) into NR crosslinked using conventional, efficient, dicumyl peroxide(DCP) and mixed sulphur/peroxide vulcanisation systems were investigated at temp, of 28 to 60C. The NR vulcanised by DCP exhibited the lowest penetrant uptake of the systems studied. It was observed that the kinetics of liquid sorption in every case deviated from the regular Fickian trend, characteristic of sorption of liquids by rubbers. The diffusion coefficient, activation energy of sorption, enthalpy, entropy and rubber-solvent interaction parameter were evaluated for the four systems from the swelling data. 30 refs. 

Table 2. Polymer—Solvent Interaction Parameters for Polyisobutylene and Butyl Rubber...

Table XXXVI.—Solvent Interaction with Natural Rubber ...

RA Orwell. The polymer-solvent interaction parameter %. Rubber Chem Technol 50 451-456, 1977. 

In order to understand the relationship between the difference in the interaction parameter of rubber-solvent (Xab) and clay-solvent (xcd) systems and the properties of HNBR/SP nanocomposites, the plots of modulus at 100% elongation and tensile strength versus Xab-Xcd are represented in Fig. 45a, b. An exponential decay in both modulus and tensile strength is observed with the increase in difference of interaction parameter. 7) and 7max follow the same trend as above. 

It is well known that the lower the AGM value, the better is the rubber-filler interaction. As for the Ch/MEK solvent combination x is zero, hence the A CN, cp2X term of (27) is zero for such a solvent combination. In all other solvent combinations, where x 0. the A l N r tp2X term of (27) is positive. Thus, AGM of the system for the Ch/MEK solvent combination is the least, and dispersion (if clay is in the rubber matrix) is also best in this solvent combination, giving rise to the highest polymer-filler interaction. 

The kinetic theory of rubber elasticity is so well known and exhaustively discussed (17, 27, 256-257, 267) that the remarks here will be confined to questions which relate only to its application in determining the concentration of elastically effective strands. In principle, both network swelling properties and elasticity measurements can provide information on network characteristics. However, swelling measurements require the evaluation of an additional parameter, the polymer-solvent interaction coefficient. They also involve examining the network in two states, one of which differs from its as-formed state. This raises some theoretical difficulties which will be discussed later. Questions on local non-uniformity in swelling (17) also complicate the interpretation. The results described here will therefore concern elasticity measurements alone. 

Flory [3] formalized the equation of state for equilibrium swelling of gels. It consists of four terms the term of rubber-like elasticity, the term of mixing entropy, the term of polymer solvent interaction and the term of osmotic pressure due to free counter ions. Therefore, the gel volume is strongly influenced by temperature, the kind of solvent, free ion concentrations and the degree of dissociation of groups on polymer chains. 

There is only an alternative way of determining crosslink density by a non empirical method, using the theory of equilibrium swelling of a network in a solvent (Flory and Rehner, 1943). This method, usual in the domain of rubbers, needs the knowledge of a polymer solvent interaction coefficient proportional to (8p — 8S)2, which is not very easy to determine accurately. Furthermore, damaging by swelling stresses and the need to work at elevated temperatures complicate the analysis seriously for the usual thermosets... 

One of the most characteristic properties of crosslinked rubbers is the ability to swell in appropriate solvents to a constant volume. Not only is this property exploited for estimation of parameters such as crosslink densities and polymer-solvent interaction parameters, but the resultant change in nuclear magnetic resonance (NMR) parameters allows a large number of new and interesting NMR experiments. It is the aim of this chapter to introduce some simple concepts of polymer swelling and to examine the information obtainable for the range of NMR experiments possible on swollen gels. 

NMR spectroscopy. On the other hand it was found that T2 of the solute in the rubber cured in the presence of triallyl cyanurate (TAC) increased on curing, despite the equilibrium swelling ratio decreasing. This surprising result was explained by a change in the polymer-solvent interactions on copolymerisation with TAC. 

Figure 15.33 shows benzene uptake by natural rubber samples. Filled samples absorb less solvent (lower swelling). The carbon black containing sample had a lower benzene uptake than the silica filled sample. The lower swelling of the carbon black containing sample is due to high bound rubber content, the crosslink density of the black filled vulcanizate, and a strong rubber-filler interaction. 

R. A. Orwoll, The Polymer-Solvent Interaction Parameter x. Rubber Chem. Technol 50, 451... 

The viscosity dependence on polymer molecular weight is demonstrated in Figure 2.21 whereas Figure 2.22 shows the effect of polymer/solvent interaction. In the latter, the viscosity of solutions of a thermoplastic rubber at constant concentration and temperature is given in various hydrocarbon(blend)s. In this example hydrogen bonding or polar effects play no or only a very limited role. It is clear that in this case the solution viscosity is very much influenced by the Hildebrand solubility parameter of the solvent in relation to that of the polymer (8.2-9.1). 

PAT Patterson, D., Bhattachaiyya, S.N., and Picker, P., Thermodynamics of chain-molecule mixtures heats of mixing Unear methylsiloxanes, Trans. Faraday Soc., 64, 648, 1968. 1968SEE Seeley, R.D., Thermodynamic properties and polymer solvent interaction parameters for silieone rubber netwoiks. Rubber Chem. Technol, 41,608, 1968. 

The swelling data and Ye detemined from the compressive stress-strain measurements in the swollen state were used to determine the polymer solvent interaction pea ameters Xi, and their dependence on V2- The value of the solubility parameter fpr the rubber has been deduced from these data to be 11.6 (cal/cm ). ... 

The possibility of obtaining spin-labelled rubbers by interaction of their solutions in the inert solvents with a mixture of nitrogen dioxide and oxygen has been demonstrated in the work [34]. Such rubbers can be prepared simply and rapidly by reactions of block polymeric samples with gaseous NOj [35], The experiments were carried on 1,4-cw-polyisoprene (PI) and copolymer of ethylene, propylene and dicyclopentadiene. The samples had the form of cylinders of 1.5 cm height and 0.4 cm in diameter. On exposure these polymers to NO2 (10 -2.3xl0" molxk ) at 293 K, identical EPR spectra were registered. The spectra represent an anisotropic triplet... 

Figure 9.5 Influence of composition on the polymer-solvent interaction parameter. Experimental values of the interaction parameter Xi are plotted against the volume ftaction (j)2 of polymer. Data for polydimethylsiloxane (M = 3850) in benzene (A), polystyrene in methyl ethyl ketone ( ), and polystyrene in toluene (O) are based on vapor-pressure measurements. Those for rubber in benzene (T) were obtained using vapor-pressure measurements at higher concentrations and isothermal distillation equilifaratian with solutions of known activities in the dilute range. (Reprinted fiom Paul J. Flory, Principles of Polymer Chemistry. Copyright 1953 Cornell University and copyright 1981 Paul J. Flory. Used by permission of the Puhhsher, Cornell University Press.)...

Sheehan, C. J., and A. L. Bisio, Polymer/Solvent Interaction Parameters, Rubber Chem. Tech., 39, 149-192, 1966. 

Figure 1. The polymer-solvent interaction parameter % in reduced form as a function of cross-link density for poly(isoprene) rubber cross-linked with dicumyl peroxide and showing the effect of cross-link density on x- (After reference 4).

Different characteristics of solvents seriously affect the sol-gel reaction in solution. This in turn influences the physico-mechanical properties of the resultant rubber-silica hybrid composites. Bandyopadhyay et al. [34,35] have carried out extensive research on stmcture-property correlation in sol-gel-derived rubber-sihca hybrid nanocomposites in different solvents with both chemically interactive (ENR) and noninteractive (ACM) mbber matrices. Figure 3.12 demonstrates the morphology of representative ACM-sihca and ENR-sihca hybrid composites prepared from various solvents. In all the instances, the concentration of TEOS (45 wt%), TEOS/H2O mole ratio (1 2), pH (1.5), and the gelling temperature (ambient condition) were kept unchanged. 

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