J . Phys. Chem. 1986,90, 5715-5719 hydrogen abstraction is constant for all sites (in reality we expect that there exists also a certain distribution for the photochemical rate constant kchwhich may or may not be correlated with the distribution for the deactivation constant kdes),then after the first shot those molecules which deactivate slowly to the ground state have a larger chance to abstract a hydrogen than those which are quenched very easily. As those benzophenone molecules which abstract a hydrogen are irreversibly transformed into LAT, they can no more be excited by the next laser shot. So with each shot the site distribution shifts to molecules which have a larger deactivation constant, and the mean rate constant ko increases as seen in our experiments. Horie and co-workersz7have recently investigated the nonexponential phosphorescence decay of benzophenone in acrylic acid methacrylic polymers. They have explained their data by the introduction of a the-dependent transition term for the dynamic quenching of the T1 state by ester groups of the matrix polymers. At this stage we cannot discriminate between the two models. For a direct observation of the diffusion processes postulated by Horie et al., the fringe spacing in a grating experiment would have to be of the order A = (DT) V2 = [(10-13)(4 X 10-5)]1/2 cm = 0.02 nm. The C W holographic determination of the quantum yield 4 = 0.2 and the mean rate constant ko = 2.6 X lo4 s-l measured in this transient grating experiment allow us to evaluate the pure photochemical rate constant kch. Assuming that the dominant branching takes place in the triplet state, this rate constant for hydrogen abstraction kch = 4ko = 5 X lo3 s-',
Conclusions and Prospects We have demonstrated in this paper that it is indeed possible to perform single-shot transient grating experiments. So in principle every photoinduced irreversible process can be investigated through transient grating techniques. We have additionally shown that photochemical intermediates can be detected and their kinetics be evaluated. Together with the total rate constant and quantum yield obtained through a C W holographic study, photochemical rate constants can be extracted. The transient grating method offers several advantages for the investigation of photoprocesses compared to traditional absorption
5715
techniques. It is inherently a method of high sensitivity as the signal due to the intermediates is recorded on a zero background, whereas absorption techniques often have to rely on the evaluation of a small difference between two large quantities. The grating method is much more versatile as the wavelength of the probe beam used can be chosen outside of the absorption bands of the photochemical species and one can still follow the process in the sample through the pure phase grating. In this case one can additionally increase the sensitivity of the experiment by increasing the intensity of the probe beam. We do not run the risk of damaging the sample as-in contrast to the conventional methods-the probe beam is not absorbed by the sample. What we think is the most promising prospect of the experiment described here is the application to diffusion-controlled chemical reactions. It is possible to probe the diffusion of reaction species over small distances (micrometer scale), and one can separate the contributions of the diffusion constant D and the reaction rate constant k by varying the fringe spacing A. The limitations of this method are naturally given by the minimum accessible fringe spacing Aminwhich determines the minimum diffusion constant D and the maximum rate constant K which can be evaluated. As a consequence it seems unrealistic to investigate such a reaction in the solid state where the grating decay constant will always be dominated by the reactive part of the process. The situation is different in liquids: here an appropriate choice of concentrations and fringe spacings should allow us to follow diffusive and reactive processes simultaneously. A general problem of the grating experiment remains: it is impossible to identify reactive intermediates by a pure grating technique. One can expand the technique described here in a straightforward manner to the study of more complex reaction schemes as, e.g., a two-photon process by performing two-pulse experiments, etce41
Acknowledgment. For support of this work we thank the VW-Stiftung and the Fonds der Chemischen Industrie. Registry No. PMMA, 901 1-14-7; benzophenone, 119-61-9; azulene, 275-51-4. (41) Deeg, F. W.
Ph. D. Thesis, Universitat Miinchen, 1985.
Hydrogen Abstraction of Benzophenone from Polymer Matrices: Evaluation of Quantum Yields and Photomechanical Effects F. W. k e g , J. Pinsl, and Chr. Brauchle* Institut fur Physikalische Chemie der Universitat Miinchen, 0-8000Miinchen 2, West Germany (Received: March 20, 1986; In Final Form: June 24, 1986)
We have investigated the hydrogen abstraction of benzophenone in polymer matrices by holographic grating and absorption spectroscopic procedures. The four polymers used, poly(methy1 methacrylate), poly(isobuty1 methacrylate), poly(buty1 methacrylate), and poly(viny1 acetate), are characterized by different glass transition temperatures ( T J . We find a strong dependence of the photochemical quantum yield I#J on Tg,Le., the rigidity of the polymer matrix. The holographic grating experiments are very sensitive to changes of the overall density of the sample, and a comparison to the absorption experiments allows an accurate determination of photomechanical effects in the sample. The different magnitude of these photomechanical effects in the matrices investigated supports the assumption that the rigidity and microviscosity of the matrix are the rate-determining factors for the photochemial reaction.
1. Introduction The influence of the physical properties of polymeric media on the reactions of incorporated photoactive molecules has been an area of great interest in the past years.' The investigators have focused on reversible processes as the cis-trans isomerization of (1) Smets, G. Adu. Polym. Sci. 1983, 50, 17 and references therein.
aromatic azo compoundsZand the ring opening/closure reaction Of spirobenzopyran derivative^.^ It has been shown for these (2) (a) Lovrien, R. Proc. Natl. Acad. Sci. U.S.A. 1967,57, 236. (b) Blair, H. S.; Pogue, H.I.; Riordan, E. Polymer 1980,21, 1195. (c) Irie, M. et al. Macromolecules 1981.14, 262. (d) Irie, M.; Schnabel, W. Macromolecules 1981, 14, 1246. (e) Matejka, L.; Dusek, K. Makromol. Chem. 1981, 182, 3223.
0022-3654/86/2090-5715$01SO/O 0 1986 American Chemical Society
5716 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 reactions that generally it is not the chemistry of the reaction but the physical state of the glassy polymer, Le., the distribution of the free volume in the polymer matrix, that is the ratedetermining That means the rigidity of the polymeric matrix and the chain segment mobility of the polymer respective to the glass transition temperature Tg govern the isomerization, and photoactive molecules can be used as probes for the detection of local chain movements. It has also been demonstrated that these photochemical reactions can induce conformational changes and under certain conditions changes of dimensions of the bulk polymer and generate a so-called photomechanical effect.] All these processes are unimolecular reactions in which the photoactive molecule undergoes a structural rearrangement. The molecule needs a flexible environment to perform the necessary rotations. Such rearrangements are therefore impossible at low temperat ~ r e . ~ ~ A different type of reaction is photoaddition in which two molecules must meet and so must be able to move in the solid matrix. A common primary process of many photoadditions is a hydrogen abstraction. It has been proven that this reaction can even take place at liquid helium temperatures if the H acceptor ,]~ and H donor are closely and suitable spaced in the m a t r i ~ . ~ A very thoroughly studied photochemical process is the hydrogen abstraction of benzophenone (BP).” This reaction proceeds as follows: the excited benzophenone relaxes to the lowest triplet state TI and then abstracts a hydrogen from the hosts to form a radical pair. The ketyl radical (BPH*) can then form a photoadditive product, the so-called “light absorbing transient” (LAT). This intermediate reacts upon further irradiation to form the final photoproduct~.]~-~~ BP
+ RH 2 3BP* + R H I step
-
BPH’
+ R’
-
LAT
hv
products Here we are interested in the first step of the reaction and the question how the microviscosity of the polymer matrix influences the hydrogen abstraction and the formation of the LAT. Several groups have studied the hydrogen abstraction of benzophenone in poly(methy1 methacrylate) (PMMA)15-17 at room temperature. Karpukhin and Kutsenova (KK)15 found a photochemical quantum yield = 0.25, whereas Salmassi et a1.I6 and Horie et a1.I’ could not detect any considerable photoconversion at this temperature well below the glass transition point Tg. Salmassi et al. found free ketyl radicals and irreversible photochemistry only for T > Tg. KK concluded from their scavenger experiments that only small amounts of ketyl radicals are set free and the formation of LAT is predominantly an in-cage process with the radical pair as a direct precursor. Murai et al.1s319investigated the same reaction in alcoholic
+
(3) (a) Irie, M. et al. J . Polym. Sci., Polym. Lett. 1979, 17, 29. (b) hie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176. (c) Menju, A.; Kayashi, K.; Irie, M. Macromolecules 1981, 14, 755. (d) Gehrtz, M.; BrBuchle, Chr.; Voitlander, J. J . Am. Chem. SOC.1982, 104, 2094. (4) Eisenbach, C. Makromol. Chem. 1978, 179, 2489. (5) Kryszewski, M.; Grachowska-Kapienis, D.; Nadolski, B. J . Polymn. Sci., Polym. Chem. Ed. 1973, 1 1 , 2423. (6) Eisenbach, C. Ber. Bunsenges. Phys. Chem. 1980, 84, 680. (7) Eisenbach, C. Polym. Bull. 1980, 2, 169. (8) Smets, G.; Thoen, J.; Aerts, A. J. Polym. Sci., Polym. Symp. 1975, 51, 119. (9) Furrer, R.; Heinrich, M.; Stehlik, D.; Zimmermann, H. Chem. Phys. 1979, 36, 27. (10) Prass, B.; Fujara, F.; Stehlik, D. Chem. Phys. 1983, 81, 175. (11) Wagner, P. J. Top Curr. Chem. 1976, 66. (12) (a) Schenck, G. 0.;Cziesla, M.; Eppinger, K.; Matthias, G.; Pape, M. Tetrahedron Lett. 1967, 193. (b) Schenck, G. 0.;Matthias, G. Ibid. 1967, 699. (13) Filipescu, N.; Minn, F. L. J . Am. Chem. SOC.1968, 90, 1544. (14) Chilton, J.; Giering, L.; Steel, C. J. Am. Chem. SOC.1976, 98, 1865. (15) Karpukhin, 0. N.; Kutsenova, A. V. Vysokomol. Soedin. Ser. B. 1977, 19, 344. (16) Salmassi, A.; Schnabel, W. Polym. Photochem. 1984, 5 , 215. (17) Horie, K.; Morishita, K.; Mita, I. Macromolecules 1984, 17, 1746. (18) Murai, H.; Obi, K. J . Phys. Chem. 1975, 79, 2446. (19) Murai, H.; Jinguji, M.; Obi, K. J . Phys. Chem. 1978, 82, 38.
Deeg et ai. solvents below and above the glass transition temperature Tgwith the help of ESR. They concluded that at all temperatures hydrogen abstraction can take place and a radical can be formed but only for T > Tgcan the ketyl radical escape from the solvent cage. That is, for T < Tgthe radical pair can only disappear by chemical reaction within the solvent cage whereas for T > Tg radicals can escape from the cage and diffuse in the matrix. Obviously matrix rigidity and viscosity should play an important part in both reaction steps: abstraction of the hydrogen and in-cage reaction or escape from the cage. To clarify the influence of the polymer matrix we have investigated the hydrogen abstraction of benzophenone in a series of polymers characterized by their different glass transition temperatures Tg. We have (a) measured the quantum yield for the formation of LAT and (b) investigated the microviscosity of the polymers by evaluating the photomechanical deformations, Le., the density changes concomitant with the photoreaction. For this purpose we have applied a recently developed grating technique which is very sensitive to changes of the index of refraction and so to changes of the overall density. This technique has been demonstrated to be a very versatile tool in the investigation of photochemical reactions in the solid state, offering several advantages compared to conventional techniques.2*22 In a very recently published paper we have shown how this method can be used for reliable evaluations of photochemical quantum yields.23 The principal procedure is as follows: Two coherent beams of the same intensity interfere on the sample and produce a modulated intensity pattern. This interference pattern induces a spatially nonuniform photochemistry in the photoactive sample. As the change in species concentration results in a change of the optical constants of the medium as are absorption coefficient a and refractive index n, a holographic grating is formed in the sample. A beam striking the grating under its appropriate Bragg angle is diffracted, and the diffraction efficiency T , Le., the relative amount of light diffracted, is proportional to the progress of the Photochemical reaction in the matrix. If we have only to consider a change of the refractive index n-which is always possible through the appropriate choice of the wavelength of the reading beam-then the time dependence of the diffraction efficiency is given by (see eq 6 and 8 in ref 20) 7 = at2 where the square root of the proportional constant reads
(1)
in the case of a one-photon reaction. Here h denotes the wavelength of the reading beam, 6 is its angle of incidence, d is the thickness of the sample, and n is its average index of refraction. e denotes the extinction coefficient of the photoactive molecule at the wavelength of the hologram-producing beams, I is the sum of the intensity of the two beams, and 6 is the photochemial quantum yield. g is a geometry factor which reflects the influence of the size of hologram-producing and readout beams. R is a dimensionless unit characteristic for the refraction of the sample
R
= (1 /NA)CciRi i
(3)
Here i runs over all components of the sample, ci being their number densities [ ~ m - ~Ri ] , their molar refractions [cm3/mol], and NA Avogadro’s number. A change of R during the photoreaction can in general have two contributions: one from the transformation of the photoactive molecule A into a product molecule B with a different molar refraction R so that ARl = NA-IcA(O)(RB - RA) = NA-IcA(O)LWBA, a second from an overall change of dimension of the sample through photomechanical (20) Brauchle, Chr.; Burland, D. M. Angew. Chem. 1983,95,612; Angew. Chem., Int. Ed. Engl. 1983, 22, 582. (21) Brauchle, Chr. Mol. Cryst. Liq. Crysr. 1983, 96, 83. (22) Burland, D. M.; Brauchle, Chr. J . Chem. Phys. 1982, 76, 4502. (23) Deeg, F. W.; Pinsl, J.; Brauchle, Chr.; Voitllnder, J. J . Chem. Phys. 1983, 79, 1229.
:;
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5717
Hydrogen Abstraction of Benzophenone
-25
,
.
cwr.co*II.I Slop. 0975 1.11
200
m,
250
arnmi
350
Figure 1. Absorption spectra of benzophenone in PVAc (thickness of polymer film d = 39 rm) after various irradiation times. Spectrum 1 is for a fresh sample, and spectrum 2 corresponds to the absorption of the LAT after the end of the first reaction step ( I = 147 mW/cm2, 7 = 100 s). Spectrum 3 depicts the spectral changes after long-lasting further irradiation ( I = 147 mW/cm, 7 = 1 h).
-45
/
// @ PMMA Holography
I'
effects, AR2 = NA-ICiAciRi. In the samples investigated here the concentration of the doped photoactive molecule is much smaller than that of the polymeric host so that in a very good approximation hR2= NA-lA~,,Rp(the index p refers to polymer). Altogether then AR is given by = ( ~ / N A ) ( C A (MEA ~) +
(4)
In the case of a rigid polymeric host, the second term in eq 4 is zero. Then a knowledge of the parameters in eq 2, which are easily determined by sample preparation and experimental setup, and AI?, which is accessible through the absorption spectra of educt and product,23allows us to calculate the quantum yield 4 out of the hologram growth curves. This is not possible if the second term in eq 4 contributes to AR as we do not know Acp. However, if we know 4 from other procedures (in this paper direct absorption techniques) we can solve eq 2 and 4 for Acpand calculate Acp and the overall density change of the sample (a reliable value of R, can be obtained through the bond refraction^).^^ By this way we have obtained information about the photomechanical effects associated with the photoreduction of benzophenone in polymeric hosts as shown in section 3. 2. Experimental Section Most of the details of the setup for the holographic experiments have been described previously.20 The hologram was produced by the 350.7-nm line of a krypton ion laser (Coherent 3000 K) and read out by a 1-mW HeNe laser striking the sample at the appropriate Bragg angle. The diffracted HeNe beam was chopped and its intensity measured by a photodiode and a lock-in amplifier. The absorption spectroscopic procedures have also been explained earlier.20 Photochemistry was again induced by the 350.7-nm line of the krypton ion laser. The absorption spectra were recorded with a spectrophotometer Model 330 from Perkin-Elmer. Benzophenone (purum) was obtained from Fluka and recrystallized twice from methanol. Poly(methy1 methacrylate) (PMMA), poly(isobuty1 methacrylate) (PIBMA), poly(viny1 acetate) (PVAc), and poly(buty1 methacrylate) (PBMA)-all secondary standard-were purchased from Aldrich and used without further purification. The polymers were dissolved in acetone to form a viscous solution and 1 wt % of benzophenone was added to this solution. Thin films (50-200 pm) were produced by casting the mixture in a mold and allowing the solvent to evaporate slowly. The glass transition point of the samples was determined by differential thermal analysis and found to be 86 O C (PMMA), 57 "C (PIBMA), 37 O C (PVAc), and 27 OC (PBMA). 3. Results and Discussion Figure 1 shows the absorption spectra of benzophenone in PVAc for various irradiation times. Spectrum 1 is the spectrum of a fresh not yet irradiated sample with two bands corresponding to ~~
~
~~
(24) Tomlinson, W. J.; Chandross, E. A. Adu. Phorochem. 1980,12, 201.
Figure 2. Holographic and absorption data for the hydrogen abstraction of benzophenone in PMMA. (a) d 2as defined in eq 1 as a function of the hologram-producing laser intensity I in a log-log plot. uo = 1 s-', Io =1 W/cm2. (b) The total reaction constant k as a function of the laser intensity I in a log-log plot. ko and I, are given by 1 s-' and 1 W/cm2, respectively. The slope of these plots gives the number of photons involved in the reaction.
the lowest m* (252 nm) and n r * (337 nm) state of benzophenone. Graph 2 shows the spectrum of the LAT after all benzophenone has been transformed with the typical band around 335 nm which has been observed by several other groups in various host materials.13J4 From this graph 2 the extinction coefficient cLAT of the LAT has been determined. Only for much longer irradiation times do we observe again changes in the spectral properties of the sample as is shown in spectrum 3 which reflects the consecutive reaction of the LAT to final photoproducts. Here we were only concerned with the fast step I of the reaction which can be easily separated from the slow step I1 because of the great disparity in the time scale of the two reactions. The spectral changes concomitant with the photochemistry of benzophenone in PMMA, PIBMA, and PBMA are the same and have been published earlier for the case of PMMA.ZS In Figures 2-5 the results of the experiments in the four polymeric hosts are depicted. The upper part in each figure (denoted a) documents the holographic data and shows a plot of the slope all2 of the hologram growth curves as defined in eq 1 vs. the light intensity I . All data are corrected for the finite OD of the sample as are all numbers mentioned in this paper. The lower parts of the figure show a plot of the total rate constant k vs. the light intensity I as found in absorption spectroscopic measurements. The slope of the respective log-log plots demonstrates that one photon is enough to trigger the phototransformation from benzophenone to LAT. This is contradictory to earlier results2S but has been confirmed in all absorption and holographic measurements. (25)
Briiuchle, Chr.; Burland, D. M.; Bjorklund, G. C. J . Phys. Chem.
1981, 85, 123, 618.
5718 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
-=I
,
slope
Cwr. C M I t .
lg[3
Deeg et al.
..
n95
I
0.w
swr.
/
OM
4 //
PlBMA Holography
;"Ac
@
Holography
I
-io
-io
-23
-+i
-& ig[+-]
-io
L
$kl
-1
/
/
/ '
I
Absorption PWA@
-1
-4 /
Figure 3. Holographic and absorption data for the hydrogen abstraction of benzophenone in PIBMA. The various symbols have the same meaning as in Figure 2.
Figure 4. Holographic and absorption data for the hydrogen abstraction of benzophenone in PVAc. The various symbols have the same meaning as in Figure 2.
TABLE I: Values Used in the Evaluation of the Photochemical Quantum Yields and Density Changes
an apparent quantum yield which is identical with the true quantum yield only in the case of a rigid host. In the PBMA sample, step I1 of the reaction, the transformation of LAT to final products, is so fast (certainly because of the soft matrix) that it is not possible to separate it clearly from step I and we cannot evaluate tLAT(351 nm) and AI?. Nevertheless we have calculated approximate values of 4A, $H, etc.-all given in parentheses (see Tables I and 11)-by using the average FLAT and as found in the three other matrices. The third column in Table 11, the quantum yield +A as measured by absorption techniques, demonstrates the dependence of the photochemical quantum yield on the glass transition temperature Tg.Whereas in PMMA, the host with the highest Tg, is only 0.19, in the other matrices with distinctly lower Tgthe quantum yield is close to 1. The chemical H-donor properties of PVAc should be slightly better than those of the methacrylates PMMA, PIBMA, and PBMA. Here the data in Table I1 indicate that this fact has no influence on the quantum yield, the reaction rate being determined by the physical properties of the matrix. It should be noted that the value 4A = 0.19 found in PMMA is very close to the quantum yield 4 = 0.25 found by Karpukhin et al. for the same system.15 In column 4 of Table I1 the apparent quantum yields c $ ~as obtained from the holographic experiments are listed. W e recognize that in the rigid PMMA matrix (high T,) 4Hand 4Aagree very well within the accuracy of measurement. This proves that
BPI BPI BPI PB/ PMMA PIBMA PVAc PBMA 632.8 632.8 632.8 632.8
parameter Arcad, Bread,
deg
2.13
13 1.48 1.15
0.065
0.060
126 1734
115
13
1.49
n g
~ ~ ( 0mol/L 1, cBp(351nm) cLAT(351 nm) AR(632.8 nm), cm3/mol Rpolymcr, cm'lmol
-0.51 24.9
1160 -0.47 38.8
13
13
1.47 2.90 0.065
1.48 1.15
0.059
-0.60
105 (1278) (-0.53)
20.3
38.8
118 941
From the absorption spectroscopic data and the well-known relation between quantum yield and the total rate constant for a one-photon process23 4A
= k/2303tI
(5)
one can easily obtain +5h The 4Ashown in Table I1 are calculated in each case from the average of all absorption data (see Figures 2b-Sb). In the case of the holographic experiments we have taken the average of the a1I2/Zvalues in Figures 2a-Sa, respectively, together with the parameters listed in Table I and evaluated a quantum yield C # J ~ by assuming zero overall density change. As has been explained in the Introduction the so-calculated 4His only
TABLE II: Results of the HolomaDhic and AhrDtion Exwriments for the Hvdronen Abstraction of BenzoDhenone in Polvmer Glasses
poly(methy1 methacrylate) poly(isobuty1 methacrylate) poly(viny1 acetate) poly(buty1 methacrylate)
Tg,O C
4A
86
0.19 f 0.04 0.89 f 0.09 0.80 f 0.14 (0.82 f 0.06)
57 37 27
apparent 4" 0.25 f 0.08
Aclc
0.41 f 0.14 1.66 f 0.63 (1 S O f 0.66)
(1.4 f 0.2) x 10-4 (3.5 1.0) x 10-4 ((3.0 f 1.0) X lo4)
Hydrogen Abstraction of Benzophenone
4 -35
PBMA Holography
-L
/// 1Slop.
r m 1.02
-1
PBMA Absorption
Figure 5. Holographic and absorption data for the hydrogen abstraction of benzophenone in PBMA. The various symbols have the same meaning as in Figure 2.
the assumption of zero density change is justified and that indeed the matrix is not deformed through the photochemical process. The situation is different in the three other hosts with lower Tg, cbH being far off the value of cbk This is obviously due to the fact that the sample density changes during the phototransformation
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5719 and the second term in eq 4 can no more be neglected. Following the procedure outlined in the Introduction we can calculate the density changes from aknowledge of 4Aand dH. If we assume that the sample contracts due to the photoreaction-which seems very plausible for this photoaddition to the polymer-the reaction and density effects have opposite signs (AR is negative in all cases, see Table I) and one can calculate the relative density changes Ac/c listed in the last column of Table 11. We find that Ac/c increases with decreasing glass transition temperature. This demonstrates the increasing deformability and plasticity of the polymeric matrix with the lowering of TB. We think that the large error margins which are found for the holographic experiments in the matrices with low T8are also due to the physical properties of the amorphous polymer matrices. The microviscosity of these softer and more flexible polymers is not the same in the whole sample but depends to a certain degree on the specific spot where the experiment is performed and so do the measured density changes. It should be noted that the photomechanical effects encountered in the photoreduction of benzophenone are 1-2 orders of magnitude smaller than effects found for photoisomerizations in polymers' but comparable to those found in the polymerization of residual monomer in poly(alkyl-cr-cyanoacrylates).26 4. Conclusions We have demonstrated that the rate-determining factor for the hydrogen abstraction of benzophenone in polymer matrices below the glass transition temperature Tgare the physical properties of the matrix, i.e., the rigidity and microviscosity of the polymeric network. The quantum yield measured by absorption techniques shows a strong dependence on Tg, being close to 1 for the samples with Tg6 57 OC. The holographic experiments performed give us simultaneous information about the changes of dimension of the samples concomitant with the photoreaction. The relative density changes found that way strongly support the quantum yield data showing the same dependence on Tg.Whereas the PMMA matrix can be considered as rigid during the photoreaction, the other softer matrices suffer increasing deformations.
Acknowledgment. We thank the Volkswagen-Stiftung and the Fonds der Chemischen Industrie for support of this work. Registry No. PMMA, 901 1-14-7; PIBMA, 901 1-IS-8;PVAc, 900320-7;PBMA, 9003-63-8; benzophenone, 119-61-9; hydrogen, 1333-74-0. (26) Pinsl, J.; Deeg, F. W.; BrBuchle, Chr. Appl. Phys. E . 1986, 40, 77.
