1 Journal of Research of the National Bureau of Standards Vol. 60, No. 2, February 1958 Research Paper 2832 Infrared Spectra of Thermally Degraded Poly(Vinyl Chloride} * Robert R. Stromberg, Sidney Straus, and Bernard G . Achhammer The cha nges in chemical structure occuring in poly (vinyl chloride) as a res ul t of heating ina vacuum in t he range 100° to 400° C were studied using infrared spectrophoto metry. The principa l changes occurri ng in t he residue during p yrolysis in a vacuum were the forma­ t ion of unsaturated structures a nd a change from an aliphatic spectrum to one showing aromatic absorption. The data are used to support a previously proposed mechanism of decomposition for poly(vinyl chloride). 1. Introduction 2. Experimental Procedure The kine Li cs of the thermal decomposition of Three pol~- (v in~- l chloride) polymers were studied. l)oly(vinyl chloride) in a vacuum have been pre­ Th e ~- were prepared from vinyl chloride monomer a viously reported [1).1 Under pyrolytic conditions, follows: pol.v(viuyl chloride) decomposes predominantely into hydrogen c hloride and a colorcd solid residue. ,--P Olymer The color of the residue has been attributed to a Preparation polyene structure exi ting after the d eh~- dro chloriua­ tion [2] . The role of oxygen in the decomposition PVC- /, ___ _ Init iator- /, radiation from 0.3 Curie m echanism has not ~r et been resolved although Co- 50 source PVC- bp ___ _ Init iator- O.l mole percent of bcnzo.d hydroperoxide m echanisms have been proposed [3]. pero xid e; 40° C r R ecently group theory has been applied to the PVC- [l,ZO __ _ Initiator- 0.02 m ole percent of 2,2' interpretation of the infrared spectra of poly(vinyl azobis (isobutyronitrije); 40° C chloride) and the spectrum has been analyzed in a comprehensive manner [4]. The head-to-tail ar­ rangement of the monomer units [5], and the alter­ All the polym er samples were powdered by grinding nate arrangements of the chlorine atoms in the plane under liquid nitrogen . The white powders were of the carbon chain [6], with the conclusion that the sieved through a 325 mesh screen to a particle siz e crystallographic repeating unit consists of two less than 44 microns in diameter . monomer units, were further verified b~1 the work of Specimens of these polymer were pyrolyzed in a Krimm and Liang [4]. Polarized spectra were used vacuum for 30 min at temperatures up to 400° C by Krimm and Liang to make band assignments. after outgassing at 110° C for approximatcl~T 1 hour. Other, more limited analyses of the infrared spectra At temperatures above 180° C volatile products were of poly (vinyl chloride) have also been reported [7 , 8]. evolved . The color of the residue changed to li ght The branching of poly (vinyl chloride) has been brown at the lower pyrolysis temperatures and to measured [9J by determining the methyl group con­ dark brown at the more elevated temperatures. centration of the reduced polymer. The application After the exposure to pyrolytic conditions wa of infrared spectrophotometry to the explanation of completed , the degraded powder samples were re­ the degradation process occurring in poly (vinyl moved and prepared for infrared analysis. 1Jntreated chloride) has, however, not been extensively reported poly(vinyl chloride) is soluble in only a few solvents, in the literature. Campbell and Rauscher [3] which include cyclohexanone, tetrabydrofuran, di­ u tilized changes in the infrared spectra of poly(vinyl methyl formamide and to a limited extent, ketones. chloride) in a study of the base-accelerated degrada­ Some of the degraded material had an even more t ion of this polymer. Others [10, 11] limited their limited solubility. Cyclobexanone and tetrahydro­ studies to changes in only one portion of the spec­ furan tend to form hydroperoxicLes, which decompose trum. easily and may initiate decomposition of the polymer. This paper describes the results of an investigation If these solvents are retained in the polymer film, in which infrared spectrophotometry was used to they also can lead to erroneous conclusions about study changes in chemical structure occurring in the the structure of the polymer. This is especially true solid residue during p~'r o l ysis of poly (vinyl chloride) when hydroperoxide breaks down to give carbonyl in vacuum. and hydroxyl groups. Therefore, the solid phase pellet m ethod was used to prepare the material for . Presen ted at the 8th Ammal Pittsburgb Conference of Analytical Chemistry infrared analysis. Potassium bromide (Harshaw and Applied Spectroscop y. Mardh 1957. I Figures ill brackets indicate the literature refe rences at the end of this pape.-. Chemical Co.) was used for the suspending medium 147 because it has a refractive index, n D of l.56 whi ch approximates that of untreated poly(vinyl chloride) (l.55) . The polym er and th e potassium bromide in the ratio of 1 to 50 were mixed in a dry box and the pellets wer e prepared in the manner described pre­ viously [12]. Although the ratio by weigh t of polym er to potassium bromide was th e same for all samples, there was an increase in the molal' ratio of degraded polym er to potassium bromide as a result of the loss of h~T dl'o gen chloride. A P erkin-Elmer :YIodel 2] double-beam infrared spectrophotometer with a sodium chloride prism was used for m easurements in the 5000 to 670 cm- I region. For increased resolution a P erkin-Elmer YIodell2- B instrumen t wi th a lithium fluoride prism was used for measuremrnts in the 3000 to 26C Ocm - 1 regIOn . 2919 F I GURE 3 , POT/ion oj infraTed spec/rum cf Geon-l Ol PIT 3 . Infrared Spectra and Discussion using lithium filw1'il:e prism. 3.1. Untreated Polymers The infrared sprctra of th e three poly(vinyl chlo­ ride) polymers studied are shown in fi gure l. These T A B LE L .1ssign1llents oj absorption bands /07' untuated patterns wcrc obtain ed by placing a potassium bro­ poly(vinyl chl'JI'ide) mide pellet containing the un treated poly(vinyl chloride) in the sample beam and a blank potassium bromide pellet in the reference beam. A commercial Freque ncy As~ i g n mem I References I 2 , polym er, Geon 101 was studied with a lithium cm- 1 fluoride prism in the region of 3000 to 2820 em - I, 690 0 - 0 1 stretch"""""""""",."",."""" 34 833 chain stretch"""""""""""".""""", 4 This spectrum is given in fig ure 2. 963 cba in stretch".".""""""""".""."",,, 1 1095 perpend icuhr chain stretch ." , .. " ... ,,"""" 4 The assignments of the principle absorption bands 11 20 parallel chain stretch .. , ''".''''''''''"'''''''' 4 of th e basic polymer are given in table 1. Krimm 1200 OH wag, out of phasc " .. "". ,,"""""""'" 4 1250 OH bend, in phase"""""""""".,,._""" 4 and Liang [4] reported a bsorptions in the region of 1330 OH be nel, out of phase, ""."".,,"",,"""" 4 1 1352 OH , wag, in phase ____ ,, __________ ___ ___________ . 4 3000 cm - that varied slightly from those given in 1427 OH , be nd, in phasc ____ ,, __ . __ ,, __ ,, __ "' ~,, ____ , ~.4 table 1 and fi gure 2. It is possible that this sm all 2823 OH stretch, in phase __ "" __ ,,,,""""_________ 4 2854 OH , stretch, symmetrical, in phase " __ ",, ____ ,, 4,13 difference is a result of th e nature and perhaps 29 19 Clb stretch, asym metrical, in phase _________ 4. 13 2977 on stretch , ou t of phase, on n ei~ h bori n g OHOI scatter of the pellet specimen. Krimm and Liang groups""" " __ .,, __ " "'" ,,,.,,"'''''''''' 2 B. F . Goodrich Ohemical 00. 650 100 80 0~ w ~ 60 <l I-- I-- ~ en 4 0 z <l cr I-- 20 0 2 8 9 WAVELENGTH, fL FIGUR E 1. Inf rared spectra of the poly(vinyl chloride) polymers stu died . , , . , PVC-/, - ---- PVC-bp - . - .. - PVC- azo 148 obtained their spectra from a film and the spectrum the order of 300 0 C and higher , there is a marked shown in figure 2 was obtained from a pellet. over-all departure from the spectrum of poly(vinyl A weak absorption band at 1590 cm- 1 is observed chloride) . It was shown in a previous study ill in the spectrum of the PVCr sample. This is that at approximately 300 0 C practically all of the assigned to a carbon- carbon double bond stretching chloride atoms have been removed from the polymer vibration [13]. The double bond structure is as HCl without any apparen t appreciable break­ formed as a result of some dehydrochlorination down of the polymer chain . This removal of the [1] of the polymer while exposed to the 'Y-radiation. highly elec tronegative halogen from the chain could The curve obtained from the PVC-bp sample cause frequenc:v shifts in some absorption bands as exhibits several absorp tion bands not observed in well as other spectral changes r es ulting from t he the other polymers. The bands at 1795 cm-1 and new structural configurations. Substantial break­ 1773 cm- 1 are assigned to the carbonyl vibration of down of the polymer chain occurs at 400 0 C [1] . t he catalyst, benzoyl peroxide [1 31. The band at In the region of the carbon-·hydrogen stretching 1733 cm - 1 is probably from t he carbonyl stretching absorption, 3000 cm- l, a new band is formed at vibration of an aldehyde or ketone [1 3], formed from 3030 cm- l that mav be associated with th e carbon­ interaction of the catalyst with vinyl chloride.