Thermal Degradation of PS in Air. - Mahendra Singh - U. S. Nandi

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THERMAL DEGRADATION OF POLYSTYRENE IN AIRTh e degradation of polystyrene has been a subject of many investigationsand a large volume of literatu re exists on th e subje ct. More recently Cascavalet al. (1) have reported t hat the weight loss of polystyrene containing endgroups derived from azobisisobutyronitrile (AIBN) seems to be diff eren t from

that of anionically polymerized samples. We have been wo rking for some timewith polystyrene having different end groups, and in this communication wewould like to report some interesting observations related to the decompositionof some of these polymers.

Purification of MonomerStyrene was freed from stabilizer by washing with 5% aqueous caustic soda

followed by repeated washing with distilled water to remove excess of alkali.The monomer was dried over fused calcium chloride by keeping it overnightand wi th occasional shaking. It was filtered a nd finally distilled in vacuumand the fraction between 52-53C at 28 mm pressure was collected.

Purification of InitiatorsThe benzoyl peroxide and AIBN were purified by recrystallizing three timesAll solvents were of analytical reagent grade and were used without furtherfrom G.R. grade methanol and drying in vacuum at room temperature.

purification..Polystyrene samples were prepared by the usual sealing-tube techn ique . Re-quired amo unts of the m ono me r, transferring a gent, and initiator, if any, were

intro duc ed in a cleaned ampule a nd sealed off in vacuum after degassing byrepeated freezing and melting. Th e sealed tubes were suspended in a therm o-stat maintained at 95 ? 0.1"C. The tubes were taken out at definite intervalsof time, chilled into an ice-salt mix ture, and broken ope n. The polymer wasprecipitated by pouring the reaction mixture into an excess of methanol. Thepolymers thus o btained were dissolved in benzene, reprecipitated with m eth-anol, and dried in vacuum at 50C for 12 hr.loss studies a t 308-360C in air. The thermograv imetric analysis was carriedout in a TGA apparatus constructed in our laboratory and similar to that de-scribed by McBain and Baker (2) and by Vasudevamurthy et al. (3). Theweight loss with respect to time was determined by the measurement of con-traction of a helical quartz spring (supplied by Messrs Thermal Syndicate,U.K). with uniform sensitivity of 0.034 cm/mg. A cathetometer with a low-est count of 0.001 cm was used for the measurement of contraction of thespring.

Th e polystyrene samples prepared as above were used fo r isoth erm al weight

To obtain the rate of decomposition of the polymers a furnace maintainedJournal of Polymer Science: Polymer L etters Edition, Vol. 17, 121-124 (1979)0 1979 John Wiley & Sons, Inc. 0360-6384/79/0017-0121$01.00


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122 POLYMER LETTERS EDITIONTABLE I

Total Percent Weight Loss for Polystyrene Samplesa%male So . l o l j r s t yre ne % w t . l o s s . D.P.w i t h t r a n s -f e r r i n g a g e n t s and o r i n i t i a t o r .

12

345

10

AI 3x 94.4Ber noy l 94 . 1p e r o x i d eC a r b o n t e t r a - 4 0 . 0c h l o r i d e .P r onobenzene 59.3Benzene 57.2- 77.63 t h y l a c e t a t e 73.4a c e t i c a c i d 7 9.4 re th y1 e t h y l 7 4 . ~k e t o n ei e t h y l a a i n e 6 5 .?

366461.

6 0 .2506 .4 l O .5 41.1Q57 .1016.1 5 1 7 .

~ ~~~aExperimental temperature: 308C; total time: 2 hr; atmo sphere : air. Allsamples except 1 and 2 were thermally polymerized.at c onstant temp erature within 5 1 C was used in the experimental setup. Thetemperature of the furnace was measured with the help of a chrome-Alumeltherm ocou ple with one of its junctions in the c enter of the furnace and justbelow the sample holde r. Before determining the rate of decom position, thefurnace was maintained at the required temperature and was kept for 8 - 12hr to attain equilibrium.

For each run a sample of 50.0 mg was placed into a thin-walled cylindricalplatinum hollow buc ket (made of foil of 0.02 5 mm thickness). The bucke twas placed in the furnace maintained at the required temperature and equilib-rium was found to be attained within less than 1 min. Th e zero-time readingwas taken after 1 min of insertion of the sample in the furnace and due cor-rection m ade fo r any weight loss during this per iod . All the samples werestudied under similar expe rimen tal conditions. The percent weight loss ineach case was measured with definite intervals of time for 2 hr. Th e residualmass was checked gravimetrically. The percentage weight loss with respect t otime is represented in Figure 1 and the t ota l % weight loss is given in Table I .The weight loss experiment has been carried out at different temperatures


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POLYMER LETTERS EDITION 123

TEMPERATURE 308 O C25 5 0 ? 5 100I I I I00 p

tf

WI?3

5

5

Fig. 1. Percent weight loss of polystyrene samples a t diffe ren t times. Num-bers refer to the sample as depicted in Table I .

(308-360C) and Table I is a representative one for 308C. The experimentswere repeated at least four times and found to be reproducible within experi-mental error.The foregoing results clearly show the significant difference in the extent ofdecomposition as depe nden t on the type of end group . Thus polystyrene w ithhalogen end groups prepared in the presence of carbon tetrachloride undergoesonly 40% weight loss as compared to 94.4% or samples prepared in the pres-ence of AIBN or benzoyl peroxide and 77% for thermally polymerized sample.The extent of decomposition can be easily seen to be a function of the typeof end group indicating that the cause of decomposition is end initiated.styrene sample having DP =60.0 for carbon tetrachloride as compared to thethermally polymerized sample of DP =5546.0,n actual experiment the re-verse trend is notic ed. Such wide variation is larger than would be acc oun tedfor by molecular weight effe cts. Also there is significant difference between

Although we would have expected a higher rate of decomposition for poly-


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124 POLYMER LETTERS EDITIONsamples 5 and 6 which is qu ite surprising. We have carried ou t IR , UV, andmass spectral analysis, an d have some tentative clue to their m ode of behavior.At present we refrain from suggesting any mechanistic model for such differ-ences until further work is carried out.

References(1) C. N. Cascaval, S. Straus, D. W. Brown, and R. E. Florin, J . Polym.(2) J. W. McBain and A. M. Baker, J. Am. Chem. SOC.,48, 690 (1926).(3) A. R. Vasudevamurthy, D. S. Bharadwaz, and R. M. Mallya, Chem.

Sci. Symp., 57, 81-88 (1976).

Ind., 16, 300 (1956). Mahendra SinghU. S . Nandi

Department of Inorganic andIndian Institute of ScienceBangalore-560 01 2India

Physical Chemistry

Received May 9, 1978Revised October 6, 1978

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Thermal Degradation of PS in Air. - Mahendra Singh - U. S. Nandi