JOURNAL OF POLYMER SCIENCE: Polymer Physics Edition VOL. 11 (1973)

NOTE

ESR Spectra of SBR Rubber and SBR-Carbon Black Masterbatches

Much attention has been given to the interaction between carbon blacks and elasto- mers1e2 because of the technological importance of these composite materials. Here we wish to report certain new features of the ESR spectra of SBR rubber alone and SBR con- taining a reinforcing carbon black (50 phr of Vulcan 6, an ISAF carbon black).

The ESR spectra of carbon blacks have been reported extensively but often without agreement regarding the line widths. The interaction of three blacks with solvents was the subject of a recent in which it was confirmed that the free electrons in the blacks were assessable to the surface of the carbon black particles but that spin-pairing with 0 2 molecules did not take place. The line width was narrowed when the blacks were treated with solvent, which prevented 02 molecules from interacting (without spin-pair- ing) with the free electrons of the carbon black.

There is considerable literature on the ESR spectra of polymers which have been sub- jected either to mechanical working, radiation or other methods which produce free radi- cal chain ends by homolytic cleavage of covalent bonds between the atoms, usually car- b~n-carbon.~ Waldrop and Kraus6 have reported measurements at low temperatures ( - 196°C) finding that SBR “pure” polymer and gum vulcanizates did not have a detectr able ESR signal but that mixtures of carbon black with SBR had a narrow “rubber” res- onance signal as well as that attributable to the carbon black. This was also observed by Sullivan and Wise.6

In measure- ments at ambient temperatures (23°C) (Fig. l), the ESR signal is broad, complex and asymmetric with respect to g for the free electron. The ESR spectrum of a SBR-carbon black masterbatch contains both the original carbon black and rubber signals (Fig. 2). We were unable to detect the narrow rubber signal that Waldrop and Kraus report, al- though we used small sweeps with a low field modulation. This would be consisteni with their assignment of this narrow signal to a polymer free radical. Such free-radical centers would be short-lived at room temperature. With repeat determinations cf the ESR spectrum we observed that this new broad resonance is present in samples: up to 5 months old and thus is essentially permanent. A further feature is that there is an orientational dependence of this new signal (Figs. 1 and 2) which was found by rotation of the sample about an axis perpendicular to the plane defined by the main magnetic field Ho and the rf

We have found a pronounced ESR signal for “pure” masticated SBR.

I 1000 Gauss

I H-

Fig. 1. X-band (9.1 kMHz) ESR spectrum of pure masticated SBR rubber (Intol. The dotted line shows the spectrum obtained on rotating the sample through 20” 1500).

in the magnetic field. 1461

@ 1973 by John Wiley & Sons, Inc.

1462 NOTE

1000 Gauss R H

Fig. 2. X-band (9.1 kMHz) ESR spectrum of the SBR-carbon black masterbatch The dotted line shows the spectrum Intol 1500:Vulcan 6, 100:50 parts by weight).

obtained on rotating the sample through 20” in the magnetic field.

field HI. The position of the carbon black resonance is essentially unaltered by rotation, only the new broad signal has an orientational dependence (Fig. 2).

Since the alignment of the chains with respect to the direction of the main magnetic field HO is not perfect and varies with position in the sample, it is not possible to give a full analysis of the ESR spectrum observed for the masticated SBR sample. However, the origin of this signal is unlikely to be due to the presence of a polymer radical because of the the large line width and also because of the permanence of the signal. Polymer free rad- icals would decay well within 5 months in a sample at room temperature due to reaction with molecular oxygen. Thus, it would seem that the species responsible for this signal must be some additive present in the rubber. A possible residue is ferric dimethyl dithio- carbamate which is present in Intol 1500 (Fe concentration <0.5%) formed by reaction of the iron containing redox initiator with sodium dimethyl dithiocarbamate which is added &s a sh~rt-stop.~

An iron(II1) complex could be responsible for the signal observed, since the ESR spectra of Fe(II1) may be very broads with g = 2 for crystal fields of octahedral sym- metry which is the case of iron(II1) dithiocarbamate comple~es.~ Also the line-width is sensitive to the ligands attached to the irori aturn.Io Also, it is possible that the narrow signal observed by Waldrop and Kraus5 for the sample prepared by adsorption of SBR from solution onto carbon black for which they could not offer an explanation may be due ro some residue present in the SBR.

An interesting observation from the present results is the permanence of the “memory” of the rubber, even the gum sample. Also we have shown that it is possible to use para- magnetic probes to assess orientational effects in processed elastomers.

We are grateful to Mi-. Briggs of Cabot Carbon Limited for samples and to Dr. E. W. Duck of the International Synthetic Rubber Company for information regarding Intol 1500.

References

1. P. B. Stickney and R. D. Falb, Rubber Chem. Technol., 37,1299 (1964). 2. G. Kraus, Adv. Polym. Sci., 8,156 (1971). 3. J. F. Baugher and B. Ellis, J . Colloid Interface Sci., 38.658 (1972). 4. D. Campbell, Macromol. Rev. (J. Pclynt. Sci. D), 4,91 (1970). 5. M. A. Waldrop and G. Kraus, Rubber Chem. Twhnol., 42,1155 (1969). 6. A. B. Sullivan and R. W. Wise, Proceedings of the Fifth International Rubber Con-

7. E. W. Duck, private correspondence. 8. D. Loveridge and S. Parke, Phys. Chem. Glasses, 12,19 (1971).

ference, Brighton, England, 1967, p. 235, Maclaren & Son, London, 1968.

NOTE 1463

9. R. M. Golding, W. C. Tennant, C. R. Kaneker, R. L. Martin, and A. H. White,

10. W. Lohmann, C. F. Fowler, W. H. Parkms, and J. L. Sanders, Nature, 209, 908

Bryan Ellis

J . Chem. Phys., 45,2688 (1966).

(1966).

Department of Glass Technology University of Sheffield Elmfield, Northumberland Road Sheffield, England

J. F. Baugher

Department of Physics Illinois Institute of Technology Chicago, Illinois 60616, USA

Received February 21, 1973