Journal of Research of the National Bureau of Standards Vol. 51, No.3, September 1953 Research Paper 21:45 Study of Degradation of Polystyrene, Using Ultraviolet Spectrophotometry Mary Jane ReineYI Max Tryon/ and B. G . Achhammer The role of ultraviolet radiant energy and effect of presence of monomer on degradation of polystyrene were studied by means of ultaviolet absorption techniques. It was found that ~xposure of pure pol,Ystyrene to ultravio~e t .radiant energy resulted in increased absorption III t h ~ ultravIOlet regIOn of 280 to 400 millimicrons, whICh advanced progressively as exposure t une lllcreased. The presence of monomer increased the rate of degradation without alter­ ing the general type of abso~·p.tion. A post-radiation effe~t was noted in both the pUlified polymer and polymer contallllllg monomer styrene. Agam, t he presence of monomer in­ creased the ra~e of progress of the post-radiation effect. Possible mechanisms are postulated for t he ultravIOlet degradatIOn, and the concept of e ntrapped free radicals is considered as a possible explanation for the post-radiation effect observed. 1. Introduction The spectrophotometric measurements on thes~ samples as received arc designated by the sample It has been shown by infrared spectrophotometric letters alone. The measurements made after partial measuremel!-ts that carbonyl and hydroxyl groups purification of the samples by a "frozen benzene are formed III polystyrene on exposure to ultraviolet technique" [2J are designated by Lhe sample letLers radiant energy in the presence of oxygen [1, 2J. 1 followed by P. Sample X was also subjected to The volatile products given off and measured by purification by solution in metbyl ethyl ketone and mass spectrometry and the changes in olubility and precipitation with isopropanol three times; the oxygen content on exposure of polystyrene to heat samples from the fir t, second, and Lhird precipita­ and/or ultraviolet radiant energy in vacuum and in tions are designated Xl, X2, and X3, respectively. oxygen have also been reported [3J. This report The technique for preparation of the samples ill presents the changes in th e ultraviolet spectrum of film form of various Lhicknesses has been described polystyrene as a result of exposure to ultraviolet previously [2J. Casting of the thicker films on a radiant energy in air. A mechanism is sugaestcd surface of regenerated cellulose, instead of directly to describe possible chemical changes in the polymer on glass, facilitated their removal because wetting that could be responsible for the variation of the the opposite side of the cellulose with watcr swelled ultraviolet spectrum of polystyrene, both on exposure the cellulose and broke the bond between it and the of the polymer to ultraviolet radiant energy and on polystyrene film. subsequent storage in the absence of light. Exposure of the fllms to ultraviolet radiant energy was accomplished by placing the film in hold ers on a 2. Materials a nd Methods revolving table 6 in. from an S- l sunlamp. The exposure was done in air, and Lhe temperature at The polystyrene samples used in this study are the table was 60° C. This sunlamp equipment is described in table 1. All samples were prepared bv described in method o. 6021 of Federal Specification the Dow Chemical Co. Samples A and El at'-e L-P-406a [4]. commercial resins; they do not contain any com­ A Beckman model DU spectrophotometer with pounding ingredients and were used as received. ultraviolet accessories was used for all Lhe transmis­ Samples W, X, and Y were prepared by polymerizing sion measurements.2 For soluLion work, siliea cells purified monomer without catalvst or solvent in a of 1.000-cm path length were u ed. nitrogen atmosphere. - 3. Effect of Film Thickness on Transmittance TABLE 1. Description of samples of polystyrene In the same manner that the concentration of a Appro,imate l'olymeri7.a· solution for a given light path length will dictate the Sample molecular tion tem­ weight perature spectrum of polystyrene obtained, the thickness of films of the polymer will be the controlling factor in o C determining its absorption spectrum. The effect of A __________ _ • 308, 000 EL ________ _ • 299,000 film thickness on the spectrum of polystyrene films W __________ _ b 500 000 X __________ _ 60 b 237: 000 120 of sample XP is shown in figure 1. Y __________ _ b 110, 000 180 Films of thickness equivalent to that of curve E of figure 1 were selected for the present study because • Weight average obtained by light scattering. they are thick enough to be mechanically stable; b Number average ol)tainco from osmotic pressure. , The various spectral transmission and absol'ption terms used in this paper arc based on tb~ terminology recom mended by Kasson S. Gibson in NBS Circular 1 Figures in brackets indicate the literature references at the end of this paper. 484, Spectrophotometry (200 to 1,000 mUlirnicrons) (Sept. 15, 1949). 267518-53--3 155 .80 .eo __ XF' ."..,----- .60 .60 ,.­ w /' ~wp ~ / .. -.,- !! I I- / '" ~z 1 / ~.40 I .. I- 1/ Ii I-' MOLECULAR WEIGHT FILM THICKNESS FILMS OF PARTIALLY PURIFIED SAMPLE XP / .20 yp 110,000 0.0059 IN. ely. 0,00004 INCH // EI 299,000 0.aOB8 IN. 0,0001 INCH 1/ A 308,000 0.0061 IN. C 0.0004 INCH 1/ XP 2.37,000 0.0077 IN. /..' WP 500,000 0.0068 IN . (/ 300 :320 340 360 380 WAVELENGTH, MILLIMICRONS FIGURE 1. E.tJect of film thickness on the ultraviolet FIGURE 2. Ultraviolet transmittance of partially transmittance of polystyrene. purified polystyrenes. they arc not;too thick to show the shape of the absorption curve in the region 275 to 350 mJ.L where the greatest differences between the polymers studied were obtained; and their strong absorptancr 3 1.00 below 270 mJ.L is relatively unimportant to this study. --------------yp Smakula [5] states that the absorptance of poly­ styrene at about 260 mJ.L is due to the phenyl residues. BO These would not be expected to change appreciably _ ........... XP on degradation of the polymer. Furthermore, mono­ ----_ ... --- mer styrene, which has been suggested as playing an ,-- important part in the discoloration phase of the ,/// __ wp degradation of the polymer [1], is known to absorb .BO I __ ----.-- in the ultraviolet at 291.5 mJ.l. The fact that the " ,/ absorption band at 291.5 mJ.L is present in curve E ;" / / indicates that the purification of the sample was not .40 ! / complete and that some monomer remained in the :; sample. :/ il SOlUTIONS ARE leg!/ IN eENZENE 4. Transmittance of Undegraded Polymers . 20 j SA"Pl.E MOLECULAR WEIGHT yp 110,000 y" XP 237,000 Figure 2 presents the spectra of films of the various ____ ,1 WP 500,000 polymer samples studied. It.is thought that the O'L-~2e~0-----'~~0'----'~----~~~-----­ differing spectra cannot be ascrIbed to th~ moderate thickness variations of the films. This IS substan­ WAYELENGTH, MILLIMICRONS FIGURE 3. Ultraviolet transmittance of partially tiated by the measurements made on solutions of 18 purified pOlystyrenes. gjliter concentration of WP, XP, and YP i.n benzene, which are presented in figure 3. The SO!utIOl~ spectra of these three samples bear out the relatlOnshlp shown affected the spectrum of polystyrene at the shorter by the spectra of these samples in film form. wavelengths. Moreover, the variation in the spectra cannot be The spectrum of polystyrene, however, is a func­ ascribed to the molecular weight differences of -the tion of the purity of the sample. Sample X was care­ samples. McGovern and coworkers [6] found that fully purified by repeated solution in methyl ethyl the same spectrum was obtained at wavelengths of ketone and precipitation with isopropanol. The X3 282 and 291 mJ.L, regardless of the molecular weight curve in figure 4 represents the pure polymer, and of the polystyrene. Meehan [7] showed ~hat the the other spectra denote varying degrees of contami­ specific extinction coefficient, referred.to. weIght con­ nation retained by the polymer. TheXpolymershows centration of polystyrene, at 260 mJ.L IS mdependent marked regions of absorptance not characteristic of of molecular weight. Newall [8] measured five poly­ pure polystyrene. Figure 5 combines the spectra of styrene samples and observed that n~ith~r molecu.lar 18 glliter solutions of X3 and X with those of the weight nor variables of the polymenzatlOn reactlOn unpurified polymers Wand Y. The difference in 3 Absorptance is defined as "I-Transmittance." purity of the polymers W, X , and Y is clearly shown, 156 . 90 ,90 / / /' / / / XI / / / / / / / .70 ,70 / / / W / g --------- ~ / X >- / I i I •• 0 '"~ .50 / I I SOLUTIONS ARE 18 gIl IN CHLOR OFORM >- I SOLUTIONS ARE IB91 1 IN CHLOROfORM I I X3 THIRD PRECIPITATE OF X y UNPURI FIEO 180·C SA MPLE X3 THIRD PPT. OF X I X UNPURIFIEO 120· C SAM PLE X2. SECOND PPT. OF )( I I w UNPURIFIEO 60·C SAMPLE . 30 X I FIRST PPT. OF )( ,30 I XP PARTIALLY PURIFIED X (BENZENE SOLUTION) I I UNPURIFIED 120·C SAMPLE I I I I . 10 '-!:':2a~0-'----'--;:!c:----:C32~0-----:34:-'-0:-----=3-'c60:------:3~aO::-----' "0'-----2~a::-0--...lL:3~00:-----=-32:-::0------:34,.,.0,,-------=3~60-...L--3,,-Ja'='0--....J WAVELENGT H. MILLIMIC RONR WAVELENGTH, MILLIMICRONS FIGURE 4. Ultraviolet transmittance of poly,~tyrene of varying FIGU RE 5. Ultraviolet tmnsmittance oj polystyrenes of va7'ying degree of purity. degree of purity. ,ao .aOr ..---- I " I .60 ,6 0 I I / SOLUTIONS ARE 18 gIl IN CHLOROF ORM / X3 · THI RD PPT. OF )( A • X3 PLUS 0.1% MONOMER ~ / SOLUTIONS ARE 9911 IN CHLOROFORM B • X3 PUtS 0.4% MONOMER A· X3 PLU S 2 .0 % MONOMER C • X3 PLUS 0.8"1.