Makromol. Chem., Rapid Commun. I, 319 - 323 (1986) 319

Influence of the structure on the photodegradation of PVC, 1

The effect of tacticity on the quantum yield of photodehydrochlorination

Franckca CastilIo, Gerard0 Martinez, Roberto Sastre, J o d Luk MiIl&n *

C.S.I.C., Instituto de Plhsticos y Caucho, Juan de la Cierva, 3, 28006 Madrid, Spain

(Date of receipt: December 13, 1985)')

Introduction

Since the ordinary monomeric units of poly(viny1 chloride) (PVC) contain no chromophore absorbing in the UV-Vis solar emission range, the known photoinsta- bility of PVC is generally thought to be due either to chromophoric defect structures or to absorbing impurities incorporated into the polymer. The nature of these poten- tial photosensitizers has been recently surveyed') and a few interesting attempts to correlate some of them with the photodegradation process have also been reported*). Nevertheless, these authors gave no quantitative correlations between the quantum yields of dehydrochlorination, which is the major reaction of the overall degradation process, and the likely chromophoric structures, probably because of the difficulties for an easy detection of the latter. As a result, the reasons for the PVC photochemical degradation remain very controversial.

None of the above mentioned attempts has investigated the potential influence of the tacticity on the initiation of the PVC photodegradation. On the contrary, this influence has been extensively studied in earlier works on thermal degradation3). They revealed that a definite type of isotactic triads, which are postulated to be the GTTG* conformation, results in both an enhanced instability of the polymer and an abnor- mally high concentration of polyenes of 7 - 9 double bonds3). Thus, we thought it interesting to compare the behaviour of four PVC samples of different tacticities in photodegradation, using a polychromatic light (UV-Vis region), under inert atmo- sphere at 35 "C.

The preliminary results are reported in this paper.

Experimental part

Preparation of polymers: Samples 3 and 4 were prepared by bulk polymerization using AIBN and UV irradiation to initiate polymerization at temperatures of 0 and - 50 "C, respectively. Samples 1 and 2 were prepared at 90 and 60 "C, respectively, by the same method except for the UV irradiaton, which was not used. The method used was the usual procedure in our labora- tory, the full details of which have already been published elsewhere3).

a) Revised manuscript of February 25, 1986.

0173-2803/86/$01 .OO

320 F. Castillo, G. Martinez, R. Sastre, J. L. Milldn

Charucterization of samples: The osmometric measurements were carried out at 34 "C with solutions in cyclohexanone using a Knauer membrane osmometer. The tacticities wre compared from IR absorbance ratio A,,, cm -1 /A,,,, ,,-i. The spectra were recorded using a Perkin- Elmer model 783 spectrometer. The tacticities were also measured by means of the 13C NMR spectra obtained at 7 5 3 MHz and 80°C using an XL-300 Varian spectrometer. The samples were examined as 10 wt.-% solutions in mixtures of deuterated dimethyl sulfoxide/l,2-di- chlorobenzene (vol. ratio 1 : 4), using 10 mm sample tubes. The calculations were carried out by measuring the areas with a compensating polar planimeter. The obtained values are given in Tab. 1, where Pmm, P,, or P,, refer to the probability of isotactic, heterotactic and syndiotactic triads, respectively.

Preparution of f i lm: The films were made by casting from solution in a mixture of tetra- hydrofuran/cyclohexanone (vol. ratio 2 : 3). After a slow evaporation the last traces of solvent were removed by extraction with diethyl ether in a Soxhlet and were dried under vacuum for 5 h at 50 "C. The thickness of the film was 90 - 150 f 3 pm.

Irradiations: They were performed on a thermostated (37 f 0,Ol "C) cylindrical reactor with an OSRAM high-pressure mercury lamp (HBO 500W). PVC films were placed just in the middle of the cell and the hydrochloric acid formed under irradiation was removed from the reactor by means of a stream of pure nitrogen. The hydrochloric acid evolved was measured by a conductimetric method. The details of the irradiation and the measurement of hydrogen chloride have been published,). The quantum yield was obtained taking into account that, because the absorbance in the UV-Visible region of the PVC samples changes during irradia- tion, it was necessary to devise a method for determining, in a continuous way, the incident light intensity on the sample and the fraction of the incident light absorbed by the film4).

Results and discussion

As illustrated by Tab. 1, the main pecularity of the polymers is that the content of isotactic triads decreases markedly from sample 1 to sample 4, which makes it likely to attribute the differences in behaviour to the tacticity.

Tab. 1. Characteristics of polymers and quantum yield of dehydrochlorination (&cI)

Sample M, . I O - ~ p r r a) P, a) pmm &Cl. I@

1 21,5 0,281 0,505 0,214 1,96 2 38,9 0,299 0,508 0,193 1,61 3 85,O 0,378 0,468 0,154 1,33 4 54,8 0,426 0,442 0,132 0,91

a) Probability of syndiotactic (P,), heterotactic (Pmr) and isotactic triads (Pmm), from 13C NMR measurements.

The dehydrochlorination kinetic cuves (Fig. 1) show that, during the irradiation, the amount of HCl evolved increases from sample 4 to sample 1, that is, with decreas- ing content of isotactic triads. The difference between samples 1 and 2 is apparent only for the fiist stage of irradiation. Nevertheless, it is well-known that photodegra- dation gives rise to big changes in absorption due to the polyene structure. On the

Influence of the structure on the photodegradation of PVC, 1 321

P f d

- Fig. 1. Hydrogen chloride evolution

photodehydrochlorination of PVC films. (0): Sample 1; (A): sample 2; (0): sample 3; (0): sample 4

0

with irradiation time during z 5

-q 0

E

0 50 100 150 200 s

irradiation time in min

other hand, these changes were demonstrated to depend on the tacticity in thermal de- hydrochl~rination~). Thus, it is difficult to derive definite conclusions from direct rate analysis.

Bearing this in mind, we have measured the HCl evolved as a function of the ab- sorbed dose. Because the so obtained plots are straight lines throughout the irradia- tion time (Fig. 2), the quantum yields of dehydrochlorination, &,-,, may be deter- mined from the slopes. As can be seen from the obtained values (Tab. I), the quantum yield appears to be higher as the isotactic triad content increases.

Fig. 2. Hydrogen chloride evolved during irradiation of PVC films as a function of the number of photons absorved. (0): Sample 1; (A): sample 2; (0): sample 3; ( 0 ) : sample 4 "3 -

0 0 25 50 75 100 lO'.Absorbed dose in Einstein

As shown by Fig. 3, the above relationship is quite linear. Thus, the isotactic triads prove to be related to the photodegradation of PVC.

Within experimental errors, extrapolation of the straight line to zero quantum yield indicates that there appears to exist a fraction of unreactive isotactic triads. In this connection, it must be emphasized that the isotactic triads in the form of GTTG* conformation, whose fraction depends directly on the overall isotactic triads content, have been proposed to be thermally labiles). Whether or not this conformation is also responsible for PVC photoinstability cannot be assessed at the current stage of our

322 F. Castillo, G . Martinez, R. Sastre, J . L. Millan

work. However, the above results make it clear that some isotactic triads are involved in the process of photodehydrochlorination, as they are in the initiation of thermal degradation”. Furthermore, the photodehydrochlorination appears to be highly res- ponsive to any change of the content in isotactic triads, as indicated by the slope of the straight line in Fig. 3. Because no similar correlation has ever been reported for other PVC structures, whether they are defects or normal structures, the above con- clusions allow to assess that the occurrence of a definite type of isotactic triads is one prevailing factor in the photodegradation of PVC.

Fig. 3. Relatonship between quantum yields of dehydrochlorination (&, ) and isotactic content of the PVC samples

% 0.5 loOL 0 0.10 0.20

Pmm

Because these triads do not absorb in the UV-Visible region, a reliable mechanism for their observed effect cannot be given at the current stage of the work. However, some tentative conclusions are to be drawn. As shown by Fig. 2, the changes in behaviour only occur during the stationary state and after a pseudo induction period is over. On the basis of a similar result in thermal degradation, the pseudo induction and stationary periods have been suggested to be related to the photo-sensitive struc- tures and the labile isotactic triads, respectively4).

The chromophores, most likely to initiate dehydrochlorination, are alkenes the original amount of which in PVC do not seem to affect the quantum yidd of de- hydrochlorination@. On the other hand, the difference in content of defect structures over a broad range of polymerization conditions has been found to be very small relative to that of isotactic triads7#*). In the light of these results, the polymers used in this work should not be so different in content of structural irregularities. Conversely, the difference in content of isotactic triads is that expected from the changes in poly- merization temperature5).

With reference to the influence of labile chlorine atoms in some isotactic triads on the dehydrochlorination quantum yield, any of the following mechanisms might be invoked: i) a secondary thermolysis through the bulk thermal energy deriving from internal conversion process, as proposed by Reinisch et aL9) for other labile atoms; ii) the formation of an intermediate exciplex from excited chromophoresl); iii) a process of energy transfer from polyenes to labile sites of PVC’O).

To our knowledge, such mechanisms have been suggested, but never rigorously demonstrated, to explain the initiation by labile structures whether they are chromo-

Influence of the structure on the photodegradation of PVC, 1 323

phores (e. g. allylic chlorine) or not. Therefore, any of these mechanisms could very well apply if some chlorines in isotactic GTTG* conformation were of low dissocia- tion energy as suggested in prior works5).

’) W. H. Starnes, Jr., ACS Symp. Ser. 151, 197 (1981) *) C. Decker, Eur. Polym. J. 20, 149 (1984) 3, G. Martinez, C. Mijangos, J. Millan, J. Appl. Polym. Sci. 28,33 (1983) and references cited

4, F. Castillo, G. Martinez, R. Sastre, J. Millhn, V. Bellenger, J . Verdu, Polym. Degrad. Stab. therein

13, 211 (1985) G. Martinez, C. Mijangos, J. Millan, J. Appl. Polym. Sci. 29, 1735 (1984) C. Decker, “Degradation and Stabilization of PVC”, edited by E. D. Owen, Appl. Science Publ., Barking, UK, 1984, p. 81

7, G. Martinez, C. Mijangos, J. Millh, Eur. Polym. J. 21, 387 (1985) *) T. Hjertberg, E. SBrvik, J. Vinyl Technol. 7 , 53 (1985) 9, R. F. Reinisch, H. R. Gloria, G. M. Androes, “Photochemistty of Macromolecules”,

lo) E. D. Owen, “Developments in Polymer Photochemistty-3”, edited by N. S. Allen, Appl. edited by R. F. Reinisch, Plenum Press, New York 1970, p. 185

Science Publ., Barking, UK, 1982, p. 165