NOTES. Melting Behavior of a Natural Rubber Network Under Stress

Transcription

1 JOURNAL OF POLYMER SCIENCE: Polymer Physics Edition VOL. 11 (1973) NOTES Melting Behavior of a Natural Rubber Network Under Stress Introduction It is well known that the melting temperature of a crystallizable elastic network increases with deformation. There are two theoretical treatments of this phenomenon; the fist was given by Floryl and the second, more recently, by Krigbaum and Roe.2 In both treatments the expression for the thermodynamic melting temperature is where T,,, and T,' are the thermodynamic melting temperatures for a strained and unstrained network; R is the gas constant, N, is the number of monomeric units between two crosslink points; AH, is the molar enthalpy of fusion per monomeric unit; and F(a) is a function of the strain ratio a, given by a different analytical expression in the two treatments: Flory's treatment' yields F(a) = (24N/r)'/' Q! - [a* + (l/a)l with N = N,/S, where S is number of monomer units per statistical chain segment. According to Krigbaum and Roe, the expression is F(a) = [a* + (l/a) - 31 Both theories are based on the hypothesis that the network chains follow Gaussian statistics.a~' On the other hand, it is well known that the Gaussian statistics has not been experimentally verified and that deviations are observed for the mechanical and thermodynamic behavior. The dependence of the melting temperature on the extension ratio has been studied for several networksj6' and results are not in good agreement with the theory. Further, the experimental stressatrain behavior of elastomer networks show a deviation from the Gaussian treatment and close agreement with the phenomenological Mooney-Rivlin equation.8 In previous paperg-'* the elastic and thermoelastic behavior of rubbers vulcanized in the swollen state has been studied. Results clearly indicate that this curing technique gives materials which follow Gaussian theory. In the present paper the melting behavior under stress of natural rubber vulcanized in the swollen state has been analyzed. The aim is to investigate the dependence of the melting temperature on the strain for a network with exhibiting ideal mechanical behavior. Experimental The sample was vulcanized in the way previously described.o The solvent used in the vulcanization mixture was chlorobenzene; the volume fraction of natural rubber in the mixture was Dicumyl peroxide (3% on the weight of rubber) was used as crosslinking initiator. Vulcanization was carried out at 110 C for 2 hr. After vulcanization the solvent was extracted with benzene and then the sample was dried under vacuum. Stressdrain isotherms were obtained at 30 C, the sample being stretched in steps at intervals of 30 min by John Wiley & Sons, Inc. (2) (3)

2 2292 NOTES The apparent melting temperatures T,,,' were determined by a method first described by Gents and used in our previous w0rk.1~0'~ According to this method the melting point is obtained from plots of forcef vewus temperature and is taken as the point where the trend becomes linear.601a-14 The heating rate was 1 C/min. The thermodynamic melting temperature Tm was obtained by determining at each strain the apparent melting temperature T,' as a function of the crystallization temperature T, and using the extrapolation method of Hoffman and Weeks16 according to the equation: Tm' = Tm [1 - (l/r)l + (Tc/2r) (4) where 7 is a parameter which depends on the initial and final dimensions of the crystals. Crystallizations were carried out for 20 hr in a thermostatted bath controlled to f0.05"c. Results and Discussion Stress+train data analyzed in terms of the Mooney-Rivlin equation give C, = 0.56 kg/cmz, CZ = 0.23 kg/cmz, and thus Ce/Cl = This ratio (compared with that for conventional vulcanizates of natural rubber, which is in the range 1.6 to 1N6) indicates elastic behavior in good agreement with the Gaussian theory.3 From the value of CI, the molecular weight of a network chain has been calculated as M, = 19,790 and therefore N, = 291. The melting temperatures are listed in Table I as a function of crystallization temperature for various extension ratios and plotted in Figure 1, where the extrapolation according to eq. (4) is also shown. The thermodynamic melting temperatures Tm are listed as a function CI in Table I1 and graphed in Figure 2 following Flory's treatment and for S = From Figure 2 we obtain AH, = 1176 cal/mole and T," = 15 f 1 C. In Figure 3 the melting data are graphed according to eqs. (1) and (3) and give AHu = oal/mole and T," = 18 f 1 C. For natural rubber the values AH, = lo00 cal/ mole" and T," = 28 f l CITv" have been reported. TABLE I Melting Temperature and Crystallization Temperature at Various Extension Rat& Extension ratio CI To, "C T,,,', "C

3 NOTES 2293 TABLE I1 Calculated Thermodynamic Melting Temperature Extension ratio a TmJ "C s 25- a= ,' 25 a=2.07 A Fig. 1. Data of Table I plotted according to eq. (4). The extrapolated points give the thermodynamic melting temperature. The results clearly indicate very good agreement between expeanenta1 data and the theoretical treatment, giving F(a) in the form of eq. (2). Of course this result depends on the value used for S. However, on taking into account that S occurs in F(a) 1/dJ and that S for natural rubber is near unity: the errors in S do not have a sensitive influence on the experimental AH.. On the other hand, similar agreement has not been observed when F(a) has the form of eq. (3). In fact, the experimental value of AH", to which the strain dependence of the thermodynamic melting temperature is related, deviates from the literature value by 17.6% when calculated according Flory's equation

4 2294 NOTES I I I I I I I b F(8) Fig. 3. Data of Table I1 plotted according to eqs. (1) and (3). and -85.4% by the Hrigbaum treatment. Taking into account that our sample shows a mechanical behavior that follows the Gauasian theory, we can suggest that in this particular case the hypothesis tbat supports eqs. (1)-(3) is verified. Results seem to indicate that, when this hypothesis is verified, only eqs. (1) and (2) give a satisfactory description of the experimental behavior. A similar conclusion was obtained in a previous paper," where the melting behavior of a polyoxyethylene network was analyzed. In that case the stress&& isotherm gave C1 = 2.76 kg/cml and C1 = 0.0, and the deviations obaerved for AHu were ca. +4% for eqs. (1) and (2) and ca. -85% for eqs. (1) and (3). In conclusion we consider the experimental values observed for T,". In both treatr ments we have obtained valuw lower than Mandelkern's value. This is attributable to a depression due to crosslinks which generally is of the order of 5-10 C but in our case could be greater for the reasons previously suggested.10

5 NOTES 2295 References 1. P. J. Flory, J. Chem. Phys., 15,397 (1947). 2. W. R. Krigbaum and R. J. Roe, J. Polym. Sci. A, 2,4394 (1964). 3. P. J. Flory, J. Amer. Chem. SOC., 78,5222 (1956). 4. P. J. Flory, Statistical Mechanics of Chain Molecules, Wiley, New York, A. Gent, J. Polym. Sci. A, 3,3787 (1965). 6. K. J. Smith, A. Green, and A. Ciferri, Kolloid-Z. Z. Polym., 194,49 (1964). 7. W. R. Krigbaum, J. V. Dawkins, and G.H. Via, J. Polym. Sci. A-i?,4,475 (1966). 8. L. R. G. Treolar, The Physics of Rubber Elasticity, Oxford Univ. Press, C. Price, G. Allen, F. de Candia, M. C. Kirkham, and A. Subramaniam, Polymer, 11,486 (1970). 10. F. de Candia, Macromolecules, 5,l (1972). 11. F. de Candia, L. Amelino, and C. Price, J. Polym. Sci. A-i?,10,975 (1972). 12. C. Price, K. Evans, and F. de Candia, Polymer, 14,338 (1973). 13. F. de Candia, Makromol. Chem., 141,177 (1971). 14. F. de Candia and V. Vittoria, Makrml. Chem., 155,17 (1972). 15. J. Hoffmann and J. J. Weeks, J. Res. Natl. Bur. Sfad. A66,13 (1962). 16. F. de Candia and L. Amelino, J. Polym. Sci. A-2,10,715 (1972). 17. D. E. Roberts and L. Mandelkern, J. Am. Chem. SOC., 77,781 (1955). 18. L. Mandelkern, J. Polym. Sci., 47,494 (1950). Laboratorio di Ricerche su Tecnologia del Polimeri e Reologia C.N.R. Arc0 Felice, Napoli, Italia Received May 22, 1973 Revised July 17, 1973 F. DE CANDIA G. ROMANO V. VITTORIA