Transformations of styrene at high pressureD. Gourdain, J. C. Chervin, and Ph. Pruzan Citation: The Journal of Chemical Physics 105, 9040 (1996); doi: 10.1063/1.472738 View online: http://dx.doi.org/10.1063/1.472738 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/105/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Amorphization from quenched highpressure phase in IIIV compounds AIP Conf. Proc. 309, 311 (1994); 10.1063/1.46335 The effect of high pressure on model membranes AIP Conf. Proc. 309, 1435 (1994); 10.1063/1.46250 High pressure FTIR spectroscopy for biomedical and cancer research AIP Conf. Proc. 309, 1431 (1994); 10.1063/1.46249 On pressure figures in single crystals, mechanical strength, and high stresses AIP Conf. Proc. 309, 445 (1994); 10.1063/1.46139 Desorption kinetics of hydrogen from silicon surfaces using transmission FTIR AIP Conf. Proc. 167, 50 (1988); 10.1063/1.37144

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Transformations of styrene at high pressureD. Gourdain, J. C. Chervin, and Ph. PruzanPhysique des Milieux Condense´s, CNRS-URA 782, B 77, Universite´ Pierre et Marie Curie,F-75252 Paris Cedex 05, France

~Received 28 May 1996; accepted 23 August 1996!

Phase transformations of styrene~C8H8! at room temperature and up to 32 GPa have been studiedby visual observations, Fourier transform infrared spectroscopy, and Raman scattering. At roomtemperature, styrene freezes at 0.3 GPa and undergoes a solid-to-solid transformation around 3.5GPa. Around 15 GPa the extent of the reaction monostyrene→polystyrene becomes significant. At32 GPa the reaction is nearly completed and, contrary to the general trend, the phenyl group is stillstable at this pressure. This uncommon behavior is very likely correlated with the amorphouscharacter of the polystyrene formed under pressure. ©1996 American Institute of Physics.@S0021-9606~96!51444-3#

I. INTRODUCTION

At room pressure monostyrene~MS! ~C6H5CH5CH2!freezes at 242 K; at 93 K the structure was found to beorthorhombic either with space groupPmm2 ~[C2v

1 ! orPmmm~[D2h

1 ! depending on whether the molecule is pla-nar or with no symmetry.1,2 This solid is hereafter calledsolid I. High pressure investigations, under hydrostatic con-ditions, were performed by Bridgman.3,4 Bridgman and Co-nant reported that at 298 K freezing takes place at 0.31 GPaand an incomplete irreversible transformation, characterizedby a partially polymerized product, is observed on compres-sion to 0.9 GPa.4 Kobekoet al.studied the contraction due topolymerization up to 0.6 GPa at various temperatures.5 Un-der shear stress, at 1 GPa, Zharovet al. showed that styreneproduces polystyrene~PS!.6

Depending on the molecular shape~linear or cyclic sys-tems! of the initial product, and on the multiple bond mutualorientations and distances in the solid, polymerization of sys-tems with unsaturated bonds occurs at various pressures.7–10

For instance at 330 K ethylene polymerizes at 2.5 GPa,10 andat 293 K benzene transforms to a polymer at 21 GPa.7 This isa general trend, all solids with unsaturated bonds were ob-served to polymerize at pressure below;25 GPa, this pro-cess leading to the destruction of multiple bonds. With itsphenyl and vinyl groups styrene might be expected to giverise under compression and according to the final pressure, tovarious types of polymerized products.

In the present work, styrene was investigated in the dia-mond anvil cell~DAC! at 293 K up to 32 GPa~the phasediagram is given in Fig. 1! using optical observation, Fouriertransform infrared~FTIR! spectroscopy, and Raman scatter-ing. Our discussion lies mainly on our infrared results, ourRaman results being mentioned only when needed or rel-evant. The variations under pressure of the fundamental fre-quencies of styrene allow us to identify a solid–solid trans-formation at;3.5 GPa. As shown by FTIR spectroscopy,Raman scattering, and the amount of recovered product, thereaction MS→PS was observed to proceed significantlyslightly below 15 GPa. At 32 GPa the reaction is nearlycompleted, at this pressure the basic molecular structure of

PS, and specifically the phenyl group, is stable. In otherwords, for compression above 25 GPa the compound formedis still polystyrene.

II. EXPERIMENT

A membrane diamond anvil cell~MDAC!11 was used forIR and Raman spectroscopy. This DAC, with two 2338°optical apertures, was equipped with type IIa diamonds witha 0.5 mm culet. For FTIR spectroscopy, the DAC was placedon the sample stage of a Perkin–Elmer microscope with Cas-segrain optics and a MCT detector. This system was coupledto a Perkin–Elmer 1600 Fourier transform interferometer;the spectral range of the setup extends from 500 to 4000cm21. The Raman scattering setup has been described in Ref.12. The optical arrangement was a micro-Raman setup usedin a backscattering geometry. The spectrometer was aX–YDilor with a diode array detector, the spectral domain ofwhich lies in the range of 50–4000 cm21.

Prior to filling with styrene, several ruby spheres wereplaced in the hole of a 301-SS gasket forin situ pressuremeasurement by theR1 line shift method.13 The gasket holewas then filled with 99% purity liquid styrene fromPROLABO.

III. RESULTS

A. Visual observations

At room temperature, seven high pressure runs were per-formed at different maximum pressures. Visual observationswere done with a polarizing microscope. Depending on thedelay in freezing, crystallization was observed between 0.3and 0.5 GPa. For slow pressure increases, crystallites have anelongated shape with rounded edges; rapid upstrokes yieldcrystallites with rodlike shapes and striations which appearin the whole solid sample. On further compression no sig-nificant changes occurred. From pressure runs up to 4 and 12GPa, we retrieve a very limited quantity, around a fewper-cent, of a translucent product. On the other hand after com-pressions to 30 GPa and more, this quantity was estimated tobe more than 90%. This fractional conversion, plotted in Fig.5 ~see Sec. III B 3!, was found to depend only on pressure

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and not on time, at least on the time scale of the experiment.At the end of downward runs, the presence of untransformedstyrene was observed: liquid styrene plus the translucentwhite product were recovered on opening the cell. As shownthereafter, the recovered product, after low or high final pres-sure, is polystyrene. It is to be noted that a marked photo-chemical effect may be caused by the laser beam used eitherfor the measurement of the ruby shift or for the Raman in-vestigation. The percentages of recovered product givenabove were obtained with a weak intensity laser beam fo-cused only on the ruby grains or avoidingin situ pressuremeasurement. In this latter case the pressure was estimatedfrom the ram pressure of the cell.

B. FTIR spectroscopy in the DAC

1. General features and assignment

FTIR spectra of styrene were recorded during upwardpressure runs in the 500–4000 cm21 frequency range. Pre-cisely 58 peaks were observed in the liquid state, and 87 inthe solid phase. Due to diamond two-phonon absorption inthe 1700–2625 cm21 range, data could not be obtained afterspectral substraction in the 1900–2400 cm21 range. Assign-ments in the liquid and solid phases were given by compari-son with the published data.2,14–16 The fundamentals arenumbered according to the Herzberg convention.2,16 The

molecule is assumed to be planar withm ~[Cs! symmetry.The observed modes of styrene which are all Raman andinfrared active in the liquid state~A8 and A9 species!, areidentified with their benzene and ethylene counterparts. It isuseful to characterize the fundamentals according to theirtype of vibration which are:a in-plane ring deformation,bin-plane bending,g out-of-plane bending,f out-of-planering deformation, andn stretch, the various functional groupsof interest being C–X~X5vinyl group!, C–C–C, C5C,C–C, and C–H. As for benzene and benzene derivatives, thespectrum of styrene is characterized by three bands with in-tense resolved peaks@Fig. 2~a!#. These three bands concernthe various vibrations:~i! f~C–C–C!, a~C–C–C!, g~C–H!,b~C5C!R , g~C5C!W ~500–1100 cm21!; ~ii ! y~C–C!, y~C5C!, b~C–H! ~1350–1650 cm21!; ~iii ! y~C–H! ~2900–3200cm21! whereR,W, stand for rocking and wagging. Weakerpeaks are observed in between these large bands, they areassigned toy~C–X! andb~C–H! vibrations in the 1100-1350cm21 range and to combinations above 1650 cm21. By com-parison to the literature data, the assignments in the liquidphase were straightforward. Apart from three weak peakslocated in the 1950–2000 cm21 range, due to combinationsand overtones, all modes reported in the literature were ob-served. Three weak extra peaks, not reported in the literature,observed at room pressure at 750, 2495, and 2627 cm21,were followed under pressure. The last two are very likelycombinations. Also two very weak intensity peaks, probablydue to combinations of a vibron~in the n24 region! with alattice mode, were observed under pressure at 1050 and 1080cm21 at 0.5 and 12 GPa, respectively@Fig. 3~b!#. Other peaksobserved under pressure, and not due to pure styrene, will bediscussed in Sec III B 2.

2. Main features of the modification of the spectrumunder pressure

The IR spectra of solid styrene at 0.7, 15, and 32 GPaare given in Figs. 2~a!–2~c! and wave numbers of variousmodes versus pressure, are plotted in Figs. 3~a!–3~d!. Underpressure, in the solid phase, a number of splittings and shoul-ders were observed; because of the occurrence of PS theycannot be clearly assigned to MS. To identify the possiblemodifications of the styrene spectrum by the polystyreneformed under pressure, it is necessary to compare the highpressure spectra of these two compounds. To this end a PSsample, formed from a powder, was investigated in DACwith FTIR spectroscopy up to 32 GPa. In the following, thecomparison of the data from these two compounds is takeninto account for the description of the evolution of the MSspectrum and the corresponding assignments. On the freez-ing of styrene, between 0.3 and 0.5 GPa, frequency and in-tensity jumps of a number of modes were observed, as wellas splitting of the various modesn35, n32, n20, andn7. Fromgroup theoretical analysis, our observations indicated a crys-tal structure which is different from the structure of solid Irecalled in Sec. I. For instance, in the Pmm2 structure,modes ofA8 species are expected to produce two infrared-active components. Actually the splitting of theA8 modeswas observed only for two modesn20 andn7 ~the number of

FIG. 1. Phase diagram of styrene, pressure~GPa! as a function of tempera-ture ~K!. At 298 K and above 1 GPa—this work. I—solid investigated at 93K ~Ref. 1!. D—freezing points at atmospheric pressure and at 298 K. Solidline—melting curve according to the Simon equation fitted to the literaturedata. Thin vertical line—isotherm 298 K investigated during the presentwork. Short dashed line at;3.5 GPa—location of the II–III solid–solidtransition.@PS#↑ and thick dashed line at 15 GPa—pressure where the con-version rate to PS increases markedly. PS—at;32 GPa the conversion isnearly completed.

9041Gourdain, Chervin, and Pruzan: Styrene at high pressure

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A8 modes is 29!, furthermore, unexpected splittings were ob-served for twoA9 species modes:n35 and n32. For similarreasons thePmmmstructure does not comply with our ob-servations. Accordingly the solid obtained under pressure at300 K is not solid I, in the following this phase will bereferred to as solid II. On further compression, in the 2.5–5GPa pressure range, frequency jumps, slope changes, andsplittings of a number of modes were observed. These fea-tures, listed in Table I, indicate the occurrence of a newphase referred to as solid III. One may note also that somesmall intensity modes were not observed above;3 GPa,e.g., then32 andn20 components andn15. At this point it isworth recalling that a lot of molecular compounds whichcrystallize under pressure, at room temperature, exhibit inthis pressure range a solid-to-solid transformation related toan orientational ordering of the lattice. The features found inthe 2800–3000 cm21 frequency range call for a few com-ments@Fig. 3~d!#.

~i! Very small intensity peaks, especially those at 2850and 2920 cm21, assigned to the symmetric and anti-symmetric modes@ns~CH2! andna~CH2!# of the CH2group of the PS chain,17 were followed under pres-sure. The evolution of these peaks with pressure willbe discussed hereafter.

~ii ! A peak which is very weak at low pressure and cor-responds to the 2n12 overtone~2990 cm21! was alsofollowed under pressure. From 4 GPa it increases inheight to be of medium intensity at 15 GPa@Fig.

2~b!#. This variation may correspond to a Fermi reso-nance in solid III of 2n12 with n7.

In the 0.5–6 GPa pressure range the Gru¨neisen param-etersg5d ln n/d ln v, wherev is the specific volume, werecomputed using the Murnaghan equation of state fitted to theBridgman data.3 Values of the slopesdn/dp, which wereobserved to depend on the type of vibration, and the Gru¨n-eisen parameters are given in Table II. Largedn/dp wereobserved for the out-of-plane ring deformations, the out-of-plane bending deformations of the vinyl group, and the phe-nyl C–H stretchings.

On further compression and from 5 GPa, peaks of then~C–H! stretching band began to overlap and to increase inintensity. Overlapping in the 500–1700 cm21 frequencyrange occurred from;12 GPa up. In the 15–20 GPa pres-sure range the following correlated features were observed.

~1! The band which is assigned tona~CH2! increased in in-tensity and broadened@Fig. 2~b!#. This indicates an in-crease of the extent of the MS→PS reaction. This prob-lem is discussed in Sec. III B 3.

~2! A strong intensity drop of the Raman signal occurredfrom 15 GPa for all the modes~lattice and internal vi-brations!, and above 18 GPa it was not possible to iden-tify individual peaks. This fact may be correlated to thehighly disordered nature of the product formed at highpressure.

~3! Plots of then vs pressure from our infrared data, exhibitfor a number of modes change in slope in the range17–20 GPa~n38,n37,n21,n141n18,2n12!. For other modesthis change is associated with a small dicontinuity in therange 15–17 GPa ~n27,n25,n34,n24,n31,n23,n20,n19,n18,n14,n13,n12,n11,n10, n9,n131n15,n7,n6!.

The comparison of our infrared data of MS and PS underpressure demonstrates that features~3! are due to the occur-rence of PS. The spectrum of PS has strong similarities tothat of MS; the resulting effects of the increase of the extentof reaction are positive or negative frequency shifts accord-ing to the respective peak positions. At 30~or 32! GPa, dueto the strong displacement of the chemical reaction to theright, the whole spectrum was completely modified com-pared to the low pressure one: it is composed of stronglyoverlapping bands of weak intensity as shown in Fig. 2~c!.Actually this spectrum is similar to the one obtained from thecompression of PS powder at;30 GPa~see Sec. III B 3!.During decreasing pressure runs from 32 GPa, only a fewintense PS peaks were followed down to atmospheric pres-sure.

3. FTIR spectroscopy of the recovered product. Thechemical transformation under pressure

a. Recovered products.Our infrared and Raman resultsindicate that the transformation MS→PS occurs markedlyaround 15 GPa. However as mentioned in Secs. I and III A,the transformation takes place at much lower pressure with alow extent of reaction. The recovered PS samples after acompression to 4, 11, 30, and 32 GPa were analyzed by

FIG. 2. Infrared spectra of solid styrene~and formed polystyrene! at variouspressures:~a! 0.7 GPa,~b! 15 GPa,~c! 32 GPa.na ~CH2!—aliphatic carbonCH stretching band assigned to the formation of polystyrene. 2n12—Fermiresonance~see Sec. III B 2!.

9042 Gourdain, Chervin, and Pruzan: Styrene at high pressure

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infrared spectroscopy and Raman scattering. The infraredspectrum of the recovered product from a run to 30 GPa isgiven in Fig. 4. The spectra of the products formed at lowerpressures exhibited a weaker band overlapping especially inthe range of the C–H stretching vibrations. A comparison of

these spectra with that of a commercial polystyrene sample~powder or film! shows that the main polystyrene peaks areobserved. In order of increasing frequency, the bandsn37@f~C–C–C!# at 700 cm21 and n36 @g~C–H!phenyl# at 760cm21, remain comparatively strong; around 1000 cm21 there

FIG. 3. Wave number as a function of pressure of the styrene peaks for various frequency ranges.L—infrared atmospheric pressure data~Ref. 14!.d—intense modes, large and small open symbols, small filled symbols: weak modes and shoulders. Assignments according to the literature~Refs. 2, 14–17!.Vertical dotted lines—melting pressure and II–III solid–solid transition pressure~see the text and Table I!. Thin vertical line at;15 GPa—zone where theMS spectrum begins to be perturbed by the occurrence of PS. Thick and thin solid lines—manual fit. Main peaks in~a!–~d!: ~a! n37@F~C–C–C!#,n25@a~C–C–C!#, n34@g~C–H!vinyl#. ~b! n24@n~C–C!phenyl#, n23@b~C–H!R#, n21@b~C–H!phenyl#, n18@n~C–X!#. ~c! n14@b~C–H!vinyl#; n13, n12, andn11 @n~C–C!phenyl#,n9@n~CvC!#. ~d! Range 2850–2950 cm21, *—aliphatic carbon C–H stretching,ns ~CH2!, na ~CH2!, na ~CH3!; from 15 GPa up the intensity ofna ~CH2!increases~* !, dashed line—approximate location ofns ~CH2!. The intensity of 2n12 increases from 4 GPa~hj !. Above 3000 cm21—region of CH stretchingpartially presented,n7@n~C–H!vinyl#, n6, n4, andn3 @n~C–H!phenyl#.

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is a marked overlapping and above 1400 cm21 the n~C–C!stretching vibrations of the phenyl group~n13,n12,n10! arewell resolved. The aliphatic C–H stretching region is char-acterized by strong overlapping peaks at 2850 and 2920cm21 assigned to the symmetric@ns~CH2!# and the antisym-metric @na~CH2!# vibrations. Weak peaks at 2870 and 2960cm21, which may be assigned to the methyl group vibrations~chain ends!, were clearly observed in the sample formed at12 GPa. The C–H stretching vibrations of the phenyl group~above 3000 cm21! were well resolved. This band is lessintense for the sample formed at 30 GPa. It is hardly visible

for the sample obtained after a run up to 32 GPa. The Ramanspectrum of the recovered products after runs up to 5and 12 GPa, respectively, exhibited only the main featuresof commercial PS i.e.,n26 @a~C–C–C!#, n24@n~C–C!ring#,n10@n~C–C!ring#, a broad band for na~CH2! andn4@n~C–H!phenyl#. From the sample obtained after a run up to30 GPa it was not possible to observe significant Ramansignals. As mentioned above, this observation suggests thatthe product formed is strongly disordered.

b. The chemical transformation under pressure.Fromthe increase of the infrared absorption of thena~CH2! peakand the attenuation of all the Raman peaks at 15 GPa, weconclude that the chemical transformation is significantabove this critical pressure. The results of the comparisonbetween our MS and PS data are as follows:~i! from lowpressure measurements the only signs of the occurrence ofPS were weak peaks, in the aliphatic carbon C–H stretchingrange, assigned tona~CH2! andns~CH2! vibrations@see Figs.2 and 3~d!#. At 15 GPa thens~CH2! peak was overlapped bythena~CH2! band whose intensity increased;~ii ! in the otherfrequency ranges the modification of the MS spectrum by PSwas also found around 15 GPa and at 30 GPa the spectrum isthat of PS.

At pressurep, the integrated intensity of thena~CH2!band is proportional to the quantity of PS formed. The inten-sity ratio of this band to that obtained from the run on PSpowder can provide an estimate of the fractional conversionx of PS formed. A plot ofx vs pressure is given in Fig. 5.The fractional conversion which is about 1024 at atmo-spheric pressure~MS sample was not purified!, is about 1021

at 13 GPa, on further compression it increases sharply then

TABLE I. Features observed at the II-III transition from FTIR spectroscopy.

Observation ModesaPressure range

GPa

Frequency jump n38 ~;650! 3.5n281n29

b ~681! 4n19 ~1181! 2n13 ~1441! 3.5n141n18 ~2595! 5

Slope change n21 ~1083! 3.5n351n37 ~;1540! 2n331n36 ~1688! 3.5n331n35 ~1749! 4n131n15 ~2815! 3.5

Splitting n3 ~3080! 3.5n27 ~554! 5n26 ~618! 3n25 ~776! 4n35 ~839! 3.5n34 ~908! 2 and 7n24

c ~991! 7n30 ~;1000! 2n23 ~1020! 4.5n14 ~1412! 4n13 ~1449! 6n10 ~1600! 4n9 ~1629! 42n10 ~;3200! 4

Fermi resonance 2n12 ~2989! 4

aBetween bracket: atm. pressure wave number.bMay be either assigned ton281n29 or to a new peak.cDue to a bad resolution of then24 component splitting may occur below 7GPa.

TABLE II. Pressure shift coefficientsdn/dp and Gruneisen parameters ofsolid styrene in the 0.5–6 GPa range.W[wagging,T[torsion.

Type of vibrationdn/dp

cm21/GPaa g5d ln n/d ln vb

a ~C–C–C! 1.95–2.47 0.072f ~C–C–C! 3.89 0.13

g ~C–H! 1.6–2.38 0.04–0.06g ~C–H!T 1.13 0.026g ~CvC!W 4.63 0.16n ~C–C! 1.98–2.45 0.033n ~CvC! 3.15 0.044n ~C–H! 3.92 0.03–0.055

aWhen required maximum and minimum slopes are given.bThe volumev was computed using the equation of state of Murnaghanv5v0[(B01B8p)/B0]

21/B8 fitted to the Bridgman data in solid styrene~v051.008,B052.46,B858.77!.

FIG. 4. ~a! Infrared spectrum of the recovered product after a pressure run to30 GPa. The assignments of the main bands are given in the figure.~b!Infrared spectrum of PS powder in the DAC at atmospheric pressure.

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above 15 GPa a slower regime is observed. Checks using ourdata from the various pressure runs on MS, show that theerror onx is around630%. This inaccuracy mainly origi-nates from the irregularities of the gasket hole deformationduring the compression, causing a sample thickness pressuredependence which is different from run to run.

IV. DISCUSSION AND CONCLUSION

It is usually accepted that multiple bond systems areabruptly destabilized at a critical pressure to give rise to apolymerized state through a chemical transformation with alarge extent of reaction. For styrene the reaction is not soabrupt: the extent of reaction is very low at low pressure. Itevolves slowly up to;10 GPa to become sharp at;15 GPa.On further compression the conversion increases to be nearlycompleted at 32 GPa. The weak extent of reaction at lowpressure is consistent with the chemical instability of styreneto external factor, e.g., natural light. Other features foundduring the present work are summarized as follows.

~1! During the present work two solid phases~termed II andIII ! were found at 0.4 and 3.5 GPa, respectively.

~2! The MS→PS transformation leads to a disordered poly-merized state. This disorder increases with the maximumpressure of the run.

~3! For compression above 25 GPa, the product formed isstill polystyrene. In particular the phenyl group is notdestabilized after a compression to 32 GPa.

Point ~3!, which was checked starting from MS or PS, isan important result concerning the stability of the phenylgroup at high pressure. It is known, in organic solid statechemistry, that the reactivity between adjacent molecules,e.g., polymerization reaction, is favored if multiple bonds areparallel and no farther apart than 4 Å. In pure benzene, the

transformation to a polymerized state occurs at 21 GPa. Atthis pressure, the distance between parallel double bonds ofadjacent molecules is close to 3.1 Å, whereas it is 3.8 Å atlow pressure. In PS, the nearest phenyl groups pertain toneighboring macromolecules, the shorter distance betweenadjacent phenyl groups may be estimated at room pressurearound 3.6 Å.18 The compressibility of PS is lower than thatof benzene, but a positive pressure shift of the critical dis-tance is not expected because the specific volume of PS atatmospheric pressure is 6% lower than that of benzene. Thecause of the stability of the phenyl group, in compressed MSor PS, very likely arises from the amorphous nature of thesolid formed at high pressure. Both high pressure systems,formed from the compression of either MS or PS, are com-posed above;20 GPa of disordered PS~the infrared bandsof the compressed PS weakened and strongly overlapped inthis pressure range!. In an amorphous solid, as compared to acrystalline state, the conditions of reaction, in particular con-cerning multiple bond orientation, cannot be reached, and sothe disorder prevents the reactivity.

ACKNOWLEDGMENTS

We thank F. Lafuma for suggesting that we investigatethe transformation of styrene under pressure. We are in-debted to M. Brigodiot for helpful information and for kindlyproviding us polystyrene powder. We have benefitted fromdiscussions with J. M. Besson.

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FIG. 5. Fractional quantityx of PS formed as a function of pressure~Sec.III B 2 !. s—computed from the ratio of the integrated intensities of thena~CH2! band of MS and PS runs. Solid curve—manual fit.

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