Joumal of Analytical and Applied Pyrolysis, 3 (1981) 49-57
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

DETERMINATION OF THE CROSS-LINK DNNSITY OF VULCANIZEDPOLYISOPRENE BY PYROLYSIS-GAS CHROMATOGRAPHY-MASS
SPECTROMETRY *DIN XIGAO * and LI HUIMING

Institute of Chemistry, Academia Sinica, Beijing (Peoples Republic of China)(Received December 3rd, 1980; accepted January 20th, 1981)
SUMMARY
The determination of the cross-link density of vulcanized polyisoprene by means ofpyrolysis-gas Chromatography-mass spectrometry was investigated. The main degradation products were isoprene, 1,5-dimethy1-5-vinylcyclohexene, 1-methy1-4-isopropy1cyclohexene (dipentene) and 3-methyl-1 ,3-pentadiene. The quantitative results indicatedthat the amounts of monomer and dimer decreased with increasing cross-link density, but
the yield of 3-methy1-1,3-pentadiene was independent of the cross-link density. A linear
relationship exists between dipentene to 3-methy1-1,3-pentadiene (ratio of the peak areasor peak heights) and the cross-link density. The reproducibility of the results was generally
good (the relative standard deviation was less than 2.0%).
INTRODUCTION
The determination of cross-link density is an important parameter characterizing a vulcanized rubber. As vulcanized rubber is an insoluble and nonflowing polymer network, usually containing many additives, conventional
analytical methods, such as IR and NMR spectroscopy, are difficult to apply.
Although the swelling equnibrium method, based on a firm theoreticalfoundation, is an important classical method for determining cross-link density, it is time consuming. Determination of the content of combined sulphur
is widely used in industry. In this method, the free sulphur must be extracted
completely, otherwise considerable errors may result.
In pyrolysis-gas chromatography (Py-GC), a polymer sample is rapidly
degraded under specified conditions into smaller molecular fragments, which
are then separated and identified by gas chromatography-mass spectrometry(GC-MS) in order to obtain structural information about the sample [1,2].
Cross-linked polymers and composite materials containing addivites can also
be directly analysed by Py-GC without resorting to elaborate separations.

*Presented at the 29th Annual Meeting of the Japan Society for Analytical Chemistry,
Fukuoka, October 11-15, 1980.

Much work on the analysis of the composition and the micro-structure of
rubber by Py-GC has been published [1-3], but few studies have been
made of the relationship between pyrograms and the cross-link structure ofrubber. In this work, we compared the pyrograms of vulcanized polyisoprene
with various cross-link densities and analysed the structure and compositionof the degradation fragments.
EXPERIMENTAL

Perparation of the samplesSix samples with various cross-link densities were prepared by a conventional method using 1-6% sulphur as the curing agent, and vulcanization wascarried out at 143℃ for 40 min. The vulcanized rubber was sectioned into
small pieces, extracted with acetone for 8 h in a Soxhlet extractor and dried
under vacuum at 40℃ for 24 h. The formulation of the samples is listed inTable1.
Calibration of the cross-link density was carried out by the swelling
equilibriu-m method.
The densities of the samples were determined by dilatometry. The samples
were subsequently kept in benzene for 1 week at 25± 0.1℃in order toreach equilibrium swelling.
Pyrolysis-gas chromatography
A schematic diagram of the equipment used is shown in Fig. 1. It consists of a JHP-2 Curie-point pyrolyser by Japan Analytical Industry (Tokyo,Japan), attached to a Chromatron GCHF 18.3 and/or a Pye Model 104 gas
chromatograph with a flame ionization detector (FID). About 1 mg of eachsample was pyrolysed for 4 s by using various ferromagnetic foils having
different Curie points, ranging from 358 to 920℃. Other operating conditions are given in Table 2.

Mass spectrometry

The pyrolytic products appearing on the pyrograms were identified withan AEI Model MS 50 mass spectrometer connected with the GC system. The
operating conditions are given in Table 2.

Determination of glass transition temperature (Tg) by viscoelastic spectrometw
For determination of the Tg, an Iwamoto Model VES viscoelastic spectrometer was used at a frequency of 3.3 Hz with linear temperature programming at a rate of 2.0℃/min.

Content of combined sulphur
The content of combined sulphur was determined by elemental analysis(barium chloride test).
RESULTS AND DISCUSSION

Calibration of the cross-link density
The volume fraction of the polymer after swelling equilibrium was measured, then the average molecular weight between cross-linking points (Mc)was calculated from the values of V2 according to the Flory-Rehner equation:

where u is the interaction coefficient of the polymer10lvent syste〜m (fornatural rubber-benzene, u = 0.437), p is the density of the sample, Vis themolar volume of the solvent (for benzene, 〜V = 89.3). The M-c values obtainedfor various samples are given in Table 3.
Qualitative analysis of Py-GC-MS results
Fig. 2 shows a typical pyrogram obtained from vulcanized rubber that
had been degraded at 434oC. There are four major fragment peaks, A, B, Cand D, the mass spectra of which are illustrated in Fig. 3. Peak A is monomeric isoprene, B is 3-methy1-1,3-pentadiene, C is 1,5-dimethyl-5-vinylcyclohe2(ene and D is 1-methyl-4-isopropylcyclohexene (dipentene). A
simple degradation mechanism which shows that dipentene came from 1,41,4 units along the polymer chain is indicated below. We believe that chain
scission occurs at the P-carbon atom adjacent to a C= C bond and thencyclization takes place by "back-biting" involving two isoprene units to produce the dimer [4,5]:

Of course, the probability of chain scission at an ct-carbon atom is much lessthan that of β-scission, so the yield of B is much less than those of themonomer and dipentene.

Relationship between yield of degradation products and cross-link density
The yields of various degradation products obtained at a degradation temperature of 434℃ were determined (Table 3). The contents of monomer anddimer decrease with increase in cross-link density, and this tendency is greaterfor the dimer than for the monomer (Fig. 4), because the retro-propagation
was retarded by the presence of cross-link sites. For the formation of fragments of higher molecular weight, this effect becomes more pronounced. As
the probability of α-scission is much less than that of β-scission, the effect ofMc on the yield of 3-methyl-1,3-pentadiene can be neglected (Fig. 5). For
this reason, when the peak-height (or peak-area) ratio of dipentene to
3-methy1-1,3-pentadiene is plotted against the degree of cross-linking, alinear relationship is obtained, as shown in Fig. 6. Py-GC has been used todetermine the composition of blended and copolymerized rubber, without
considering the effect of cross-link density on the yield of the monomer anddimer. However, as is clearly shown in Fig. 6, the influence of the cross1ink density is significant, and should not be neglected, otherwise, considerable errors may be introduced.

Experimental conditions and reproducibility
Experimental conditions such as pyrolysis temperature, temperature risetime, sample size, volume of pyrolysis chamber and other GC conditions,should be carefully controlled in order to obtain high reproducibility in
Py-GC. By comparing the yield of dipentene at various temperatures (from385 to 920oC), we found that the maximal yield of the dipentene peakoccurs at 434℃, where the experimental error in peak area or peak height isminimal (Fig. 7). For Py-GC, the smaller the sample size, the better is thereproducibility, but owing to the non-uniformity of the industrial rubbersample, 1 mg of sample would be appropriate. The analytical data obtained
from sample 4 by using the peak-area ratio of dipentene to 3-methyl-1,3pentadiene are shown in Table 3. The reproducibility was generally good,with a standard deviation of less than 2% (Table 4).

Comparison with other measurements
Other methods (e.g., swelling equilibrium, dynamic mechanical measurement of Tg and elemental analysis) can be used for the det3irmination ofcross-link density. Fig. 8 shows the relationship between Mc and Tg as
obtained by viscoelastic measurements. However, the relationship is notlinear, in agreement with earlier reports [9]. The relationship between Mc
and content of combined sulphur is shown in Fig. 9. It was found that when
the content of sulphur is high, the extraction of free sulphur will not becomplete, and when the content of sulphur is low the experimental error
becomes very large.
The determination of the cross-link density of vulcanized polyisopreneby Py-GC-MS has been demonstrated to be advantageous with regard tosimplicity (no pre-treatment is needed) and rapidity (about 15 min for eachrun), only a small amount of sample is needed and the reproducibility isgood.