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Self-sensitized photopolymerization of a diacetylene single crystal pTS
Volume 175, number 1,2
CHEMICAL PHYSICS LETTERS
30 November 1990
Self-sensitized photopolymerization
of a diacetylene single crystal pTS M. Bara a,b, M. Schott a and M. Schwoerer b a Groupe de Physique des Salides, UniversitPParis VII, Tour 23-2, place Jussieu, 7525 1 Paris Cedex OS, France b Physikaiisches Institut and Bayreuther Institut ftir Makromofekiilforschung (BMF), Universityof Bayreuth,
W-8500 Bayreuth, Germany Received 4 September 1990
The influence of already formed polymer chains on the photopolymerization of the diacetylene crystal pTS has been studied using the holographic method. It is shown that, for wavelengths between 340 and 400 nm, photopolymerization occurs and is faster in more polymerized regions. This sensitized process does not involve polymer excited states, but most probably the monomer triplet. It is a two-quantum process. It is proposed that this “sensitization” is in fact due to the elastic effects of polymer chains into the surrounding monomer matrix.
1. Introduction The photopolymerization of diacetylene crystals irradiated in their first electronic absorption band below circa 1% 3 IO nm has been thoroughly studied [ l-3 1, The best known such material is pTS or TS6, in which the diacetylene R-C = C-C = C-R has side groups R with formula -CH*-S03-C6H4-CH3. At low temperature, all intermediate states, between the monomer and the stable oligomer terminated with unreactive chain ends, could be identified in several different monomer crystals and their electronic structures studied by absorption spectroscopy, and ESR [2]. At room temperature, the intermediate states have a short lifetime, and cannot be studied independently. It has been shown, however, that in pTS, photopolymerization proceeds through the same steps at 4 and 300 K [ 41. Usually, the very first (“photophysical”) step in diacetylene photopolymerization is absorption of a photon to form the lowest singlet excited state of the diacetylene moiety, with a threshold near 325 nm at room temperature [ 51, where the monomer absorption coefficient is a few cm-‘. However, there are a few reports in the literature of diacetylene photopolymerization effected by longer wavelength photons. Bubeck et al. [ 61 found that, in Elsevier Science Publishers B.V. (North-Holland)
partiallypolymerizeddiacetyleneLangmuir-Blodgett films [ 71, photons with 300
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zation is studied, using a simple setup, in the following way: a spatially modulated polymer concentration grating is first produced at 308 nm, i.e. in wellstudied conditions [ 51. The whole crystal is then uniformly excited at the wavelength at which sensitization is to be studied. On sensitization by the polymer being modulated, the efficiency of the grating is expected to vary. This variation is simultaneously studied by diffraction of a wavelength (633 nm ) which is neither efficient in producing polymerization nor absorbed.
2. Experimental method pTS crystals were grown from acetone solutions following Huber [ 141 and Wegner [ 151. Approximately 100 pm thick platelets were cleaved along ( 100) planes. Experiments must be performed below room temperature, to avoid thermal polymerization during the experiments. The samples were therefore mounted in a cryostat (Oxford Instruments CF-204 with temperature contrdller DTC2). “Monomer” pTS crystals always contain some polymer, which at low concentration gives to the crystal a pale-pink color. Plates with the faintest colorations were selected, to ensure at the beginning of the experiment the lowest polymer content (estimated in the order of 10m4to 10d5 by weight). The experiment is made in two steps with the same experimental setup (fig. 1). First, a holographic grating is written onto the crystal using the 308 nm
Diode 1
30 November 1990
line of an excimer laser polarized parallel to the b crystal axis (the polymer chain propagation direction) , The experimental conditions of the first step correspond to thick phase gratings as shown by the values of the factors Q and p introduced in refs. [ 16] and [ 17 ] respectively:
/7=)L~(A2Zngl,)-‘s
10
with, in these experiments, the reading wavelength A,=633 nm, crystal thickness d= 100 urn, grating period n = 3 urn, mean refractive index no zz 1.5 and the grating refractive-index modulation nl x 3 x 10m3.It was shown already [ 51 that such holograms are thickphase holograms. A low-efficiency grating, q= 1%, is written, two orders of magnitude less than the maximum attainable efficiency [ 5 1. This corresponds to the production of far less than 0.11 polymer. Then, one records the modification of the hologram by homogeneous irradiation using an Osram HBO-200 Hg-lamp through a KG5 filter to stop IR (A> 850 nm) and short-wavelength UV light (A<300 nm), and one of the following sharp cut-off filters (Schott): WG345, WG360, WG400 and WG420. The numbers correspond to the wavelength (nm) of 50% transmission. The actual cut-off wavelengths ( < 1% transmission) are respectively 328, 345, 387 and 408 nm. Light intensity was varied by adjusting the lamp electrical supply, and calibrated. Polarization of this beam was effected by a Glan-Thomp-
Filted
L2 Chopper
Fig. I, Sketch of the optical part of the experimental setup. The solid line is the light path for the first step. The dashed line is the light pathfor the second step. The He-Ne laser is not shown. L are lenses and P is the polarizing prism.
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son prism. Throughout the process, the efficiency q of the holographic grating is determined by directly measuring the ratio of the intensities of the first-order diffracted and incident beams at 633 nm using a 1 mW Spectra Physics He-Ne laser. Typically, an irradiation of 1O3to 10”s yielded a significant (larger than loo/o) change in q.
30November 1990
the variation in refractive index is proportional to the variation in polymer content, An(633)ccAP, whereas the absorption a(633) remains negligible. It is then possible to linearize eq. ( 1) and to neglect the exponential damping term, so that
3. Model where A is a known constant factor and Since low-efficiency gratings are written in the first step, and efficiency changes in the second step are small, it will be possible to linearize most equations. The experimental conditions are close to those assumed in Kogelnik’s theoretical treatment [ 161, that is, sine gratings read under Bragg conditions with sizeable diffraction intensity in first order only; so, this treatment has been used rather than the more general Magnusson and Gaylord theory [ 181. Defining the efficiency q as the ratio of the intensity of the first-order diffracted beam to the incident beam, one obtains, for a uniform transmission grating of thickness d,
x exp
(1)
where 0, is the Bragg angle, a(&) the average absorption. The modulation of refractive index n and absorption a! along the direction of modulation x is
P, =P(O) -P( f/f)
is the modulation in polymer content of the grating. Therefore, observation of an increase of q under homogeneous irradiation proves the occurrence of sensitized photopolymerization at the wavelength used: API > 0. Note that if ci!were not negligible in eq. (l), linearization would indroduce a multiplicative term in (2) of the form
p&P) with d=cr,,Pand Z&P(O)+P(jA), in which case q would decrease in absence of sensitization, and only a large-enough sensitization effect would offset the decrease; again, observation of an increase of r] corresponds to sensitized photopolymerization. To use eq. (2 ), P, has to be calculated as a function of the experimental parameters. The simplest possible process uses only one light quantum from monomer M to product: M 7 M* t product .
n(x)=no+
f n,cos(kjx)) I
and similarly for a(x). n, and CX,in eq. (1) are the coefficients of the first term in the Fourier series. In our experiments, these are the only significant ones. it, and (Y~are not strictly constant in depth, since at 308 nm the light-penetration thickness is almost equal to d. Thus, the average values of n, and aI are about half their maximum values at the surface; this has no influence on the qualitative conclusions which will be drawn from these experiments. As discussed in appendix A, in our experimental conditions at the reading wavelength A,=633 nm,
Neglecting monomer depletion, and taking into account that each reaction incorporates n monomer units into the polymer fraction, yield at steady state the polymer concentration, P=CY-
kn k+j3'-
Eqs. (3) and (2) lead to q’/2cczt,
whereas experiment (see section 4) yields y1112 0: I’?. Since thelight fluxes (<5x 10” cm-2 s-‘) are 25
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too low for sizeable two-photon excitation of M* to occur, a more complicated reaction scheme occurs. One obvious two-quanta process is 012 (11) MC=+ MT&
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h
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($g)
MS 2 product.
82
Since polychromatic light is used, the efficient fluxes I, and I2 for the first and the second step may be different. CTis an average absorption cross section over the whole wavelength range of 12, which should be as IO-l6 cm2 so alz
If Z,ccl,, eqs. (4) and (2) lead to ~‘%t1:.
4. Results The grating efficiency increases quadratically with time under constant homogeneous irradiation: Aq ’/‘a t. This demonstrates the occurrence of polymer-sensitized photopolymerization in this wavelength range. Under the same conditions, the efficiency increases almost as the fourth power of light intensity; the results with filters KG5 and WG360 are reported in fig. 2 as log ( AqI”/ At) as a function of log I: the linear dependence found corresponds to an exponent 1.8, suggesting that eq. (4) is applicable. In all cases studied, the relation uoct213-6f.0.4was obeyed. Since the effect is a rapidly decreasing function of light intensity, the action spectrum cannot be obtained using monochromatic light. Using filter WG420, with cutoff at 407 nm (less than 1% transmission), we find no variation of 9: the threshold is near 400 nm. In this, pTS crystal apparently differs from LB films [ 61. This also shows that the effects observed are not due to unwanted thermal polymerization. The difference between the responses with, for instance WG345 and WG360, is due to wavelengths between their respective cutoffs, in this case 3309.~350 nm. Since this may slightly change the 26
t
I
-2
-1
Lnl
c
Fig. 2. Intensity of the rate of change (s-’ ) of the grating et% ciency, on a logarithmic scale, using KG5 + WG360 filters at 270 K. The experimental points are shown with their experimental uncertainties and correspond to the linear variation shown with a reliability factor R=0.97.
ratio between I, and i2 in eq. (4), an accurate action spectrum is not obtained. At constant total intensity, the response (change of $j2) is approximately independent of wavelength from 340 to 400 nm. Below 330 nm, the usual absorption by the singlet states takes over [ 5 1, the experimental conditions change, and the model of section 3 does not apply anymore. In the same wavelength range 340 5A5400 nm, the effect is only weakly dependent upon the incident light polarization; the variation with polarization is less than 30°& This result is only qualitative, since pTS is a biaxial crystal, and the experimental setup, particularly the use of a cryostat, made it difficult to control birefringence effects. Finally, the temperature dependence of the process was investigated in the range 250 5 Tg 300, using KG5 and WG360 filters (fig. 3), yielding Atp2 -a At
exp -
5. Discussion 5.1. Relevance of the diacetylene photopolymerization
triplet in
The growth of holographic efficiency upon humogeneous illumination of a preformed grating dem-
CHEMICAL PHYSICS LETTERS
Volume 175, number 1,2
30 November 1990
That the diacetylene triplet is involved in pTS photopolymerization has been suggested already several years ago [ 2 I,22 1. In this work we give new, more direct, evidence, in favor of this assumption. The fact that a second quantum is needed indicates that, despite its 3 eV electronic energy, the monomer triplet is still separated from the dimer in the crystal by quite a high barrier. 5.2. Origin of the sensitization effect
I
3.3
c
3.8
4.3
10%
Fig. 3. Temperature dependence of Aq’/‘/A! in the same experimental conditions as in fg 2. The straight line shown corresponds to an activation energy of 0.2 eV.
onstrates “self-sensitization”; polymerization is faster in regions already more polymerized. The holographic method is indeed well suited for such studies. Self-sensitization can be produced, either if preexisting polymer chains are directly involved in further reaction, or if their presence modifies the reactivity of the monomer matrix. The absence of sensitization at the wavelength of maximum absorption in the visible (570 nm) region shows that it is not a thermal effect. The absence of polarization dependence rules out involvement of electronically excited states of the polymer chains present [ 191, even if a higher-lying excited (ionized?) state of these chains were present above 3 eV, neither of the two quanta involved in the sensitized reaction can be absorbed by the polymer. The threshold at 400 nm corresponds to the energy of the lowest-triplet excited state of diacetylene [ 201, and what is known of the action spectrum is consistent with the triplet absorption spectrum. We conclude that the limiting step in the sensitized photopolymerization process we have studied is the generation of the diacetylene triplet. Since two quanta are needed, either two triplets are used in a bimolecular process to generate the reactive species, or, more likely, this species results from photon absorption by the triplet during its lifetime (see appendix B). The activation energy is more likely to be associated to the chain propagation step [ 41.
The thermal polymerization rate of pTS monomer-polymer mixed crystals increases with polymer content. This is well documented in calorimetric studies [ 23 1, and it has been related theoretically to elastic properties and unit-cell-dimension changes in the direction of chain growth [ 1,241. Thus, any polymer concentration difference will be amplified in further polymerization. Indeed, this has been observed, as an increase in holographic grating efliciency upon thermal annealing of a preformed grating [ 12,13,25,26]. This effect produces a “homogeneous” sensitized polymerization, in that the polymer present affects the crystal properties over large distances, and not only in its immediate vicinity; a partially polymerized crystal has a single set of Bragg reflections corresponding to a single, average, structure [ 27,281. Photopolymerization will be affected in the same way as thermal polymerization, since the increased reactivity corresponds to an increased average chain length, rather than to an increased thermal chain-initiation rate [ 23 ]_ Indeed, the activation energy found here is similar to the one found in other pTS photopolymerization studies [ 1,4,20,2 11. This is plausible, but not proven here, since in the present experiment, polymer content, and its variations, are small or very small (well below 1%)) and there are few experimental data in that range. For instance, heat is evolved at an almost constant rate at the beginning of thermal polymerization, corresponding to an almost constant polymerization rate (initiation rate and average chain length) if it is assumed that the heat evolved per reacting monomer is constant [ 231. And indeed, on extrapolation to very small polymer content, the rate of change of b yields a decrease by 10m38, for 0. I % polymer, hardly enough to produce a large reactivity increase [27,29]. 27
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The increase in grating efficiency upon thermal postannealing reported in ref. [ 121 was observed at a polymer content of 2-3%. Clearly, further work on very slightly polymerized pTS would be needed. If the “homogeneous sensitization” process proposed here is not operative, then the existing polymer chains must be involved through their effect on the monomer-crystal excited states, so that among all decay channels, polymer initiation (dimer formation) is favored. Unfortunately, as already pointed out, for instance by Blssler [ 1,s 1, processes in which polymer acts as a quencher are easily found, but not processes in which energy or charge is transferred the other way. For instance, the triplet energy could conceivably be transferred to neighboring chains, thus quenching photopolymerization. That sensitization occurs shows that this process is inefficient in pTS, perhaps due to the fairly large distance, circa 7 A, between a monomer and the nearest possible chain.
30 November
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The variation of ;10with P in partially polymerized pTS crystals is still a matter of debate. Bloor et al. [ 301 report a continuous variation of A0 with P, at a rate = 100 cm-’ per % polymer at the beginning of polymerization, while others find that 1, is constant up to at least 32 polymer [ 3 l-3 3 1. In the present experiments, P is always smaller than 0.11, so that the effect of a possible variation of L,, is negligible, and the change of refractive index at 633 nm can be assumed to be proportional to the polymer content An aDa P, to first order in P. Bloor and Preston [ 191 have measured the refractive index for light propagating lb and polarized lib, nb at 635 nm. They find that, from the almost pure monomer (PS 10p3) to a crystal containing an estimated P= 1.1 X 10m2,nb increases by 5%, while Bauer [ 251 found 6% for Px (7 -C 1) x 10 -2. This would be an upper limit of n, in eqs. ( 1) and (2). At maximum efficiency, nl z 4X10e3 [5].Inthepresentexperiments,n,
Acknowledgement
Appendix B We thank H.D. Bauer for his assistance in the experimental setup and for valuable discussions. One of us (MB ) acknowledges support of his stay in Bayreuth through SFB 2 13. Collaboration between Paris and Bayreuth was supported by the German-French PROCOPE program.
Appendix A
In many cases, the photoproduct absorption spectrum is independent of its concentration. This is not so in pTS photopolymerization; while the wavelength of maximum absorption of the first polymer chains formed is about 570 nm, that of the pure polymer is 6 15 nm [ 301. The use of a Sellmeier formula for the refractive index n2(n)=n;+
*
-6
indicates that the refractive index of the pTS samples at the reading wavelength of 633 nm may be dependent, not only on D, which is usually proportional to the photoproduct (here the polymer) concentration P, but on & as well. 28
The singlet-triplet absorption coefficient in molecular crystals is usually low, except in the presence of heavy atoms [ 19 1. In pTS, the heaviest atom is sulfur, and the corresponding absorption enhancement is certainly modest. It is then not obvious that enough triplets are formed in the experiments described above to account for the observed photopolymerization. Since almost none of the relevant parameters is known, only order of magnitude evidence can be given. Assuming an absorption coefficient I&T of 1O-3 to 10m2cm-‘, the total number of triplet formed (per cm3) during a typical experiment in which a light flux II s 5x 10” cmB2 s-l is incident onto the grating for a time t x lo2 s is NT = aYSTZI 6 or 5 x 10 I6 to 10” cmm3. To produce a PI x 1 to 5 x 10p4, a total concentration of 2X IO” to lOI monomers/cm3 must be incorporated into polymer chains. Since the length of these chains is, at the beginning of polymerization, 20 to 30 monomers in pTS [l-3], 1 to 5X lOI initiation events per cm3 during these lo2 s are needed. Thus, something like 10% of all triplets formed should initiate a chain, so at least the same number must be photoexcited. With typical values of absorption cross
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section 0~ IO-r7 cm*, I25 10” crnm2s-l and triplet lifetime N 10m3s, this is indeed the case. Another two-quantum process would be the generation of the higher excited MT, the reacting state, by triplet-triplet annihilation. But via the same parameter values as above, plus an annihilation rate constant yTT< lo-” cm3 s-’ quite typical of a molecular crystal, the total number of triplet-triplet annihilations per cm3 during time f is 5 2.5 X 10” cmm3, too small by about two orders of magnitude. But with more strongly absorbed light, a process becomes conceivable by which the singlet would be formed, then would generate the triplet by ISC, and where finally triplet bimolecular annihilation would generate the reacting state.
References [ 1] H. Biissler, Advan. Polym Sci. 63 ( 1984) 1. [2] H. Sixl, Advan. Polym. Sei. 63 (1984) 49. [ 31D. Bloor and R.R. Chance, eds., Polydiacetylenes, NATO AS1Series, Vol. E 102 (Nij hoff, The Hague, 1985 ) [4] H. Niederwald and M. Scbwoerer, Z. Naturforsch. 38a (1983) 749. (51H.D. Bauer, Th. Vogtmann, I. Miiller and M. Schwoerer, Chem. Phys. 133 (1989) 303. [6] C. Bubeck, B. Tieke andG. Wegner, Ber.Bunsen&s. Physik. Chem. 86 (1982) 495. [7] B. Tieke, G. Wegner, D. Naegle, H. Ringsdorf, S. Baneyre, D. Day and J.B. Lando, Colloid. Polym. Sci. 255 (1977) 521. [8 ] F. Braunschweig and H. Biissler, Chem. Phys. Letters 90 (l982)41. [9] G.C. Bjiirklund, D.M. Burlandand D.C. Alvarez, J. Chem. Phys. 73 ( 1980) 432 1. [lo] Chr. Briuchle, D.M. Burland and G.C. Bj8rklund, J. Phys. Chem. 85 ( 1981) 123.
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[ I 1 ] D.M. Burland and Chr. Bdchle, J. Chem. Phys. 76 ( 1982) 4502. [ 121 K.H. Richter, W. Gtlttler and M. Schwoerer, Appl. Phys. A 32 (1983) 1. [ 131 B.E. Kohler, H.-D. Bauer, BE. Kohler, W. Giittler and M. Schwoerer, Chem. Phys. Letters 125 ( 1986 ) 25 1. 1141R. Huber, Diplomarbeit, University of Stuttgart (1976). [ 151 G. Wegener, Z. Natutforsch. 24b ( 1969) 824. [ 161H. Kogelnik, Bell Syst. Tech. J. 48 (1969) 2909. [ 171M.G. Moharam and L. Young, Appl. Opt. 17 (1978) 1757. [ 181R Magnusson and T.K. Gaylord, J. Opt. Sot. Am. 67 ( 1977) 1165. [ 191D. Bloor and F.H. Preston, Phys. Stat. Sol. (a) 37 ( 1976) 427; 39 (1977) 607. [ 201 M. Bertault, J.L. Fave and M. Schott, Chem. Phys. Letters 62 (1979) 161; M. Bertault, P. Peretti, P. Ranson and M. Schott, J. Luminescence 33 (1985) 123. [21] R.R. Chance and G.N. Pate], J. Polym. Sci. Polym. Phys. Ed. 16 (1978) 859. [ 221 G. Wegner, Makromol. Chem. 134 ( 1970) 219. [23] M. Bertault, M. Schott, M.J. Brienne and A. Collet, Chem. Phys. 85 ( 1984) 48 1, and references therein. [ 241 R.H. Baughman, J. Chem. Phys. 68 (1978) 3 110. (251 H.D. Bauer, Ph.D. Thesis, University of Bayreuth (1990). [ 261 M. Bara, unpublished results. [27] V. Enkelmann, Advan. Polym. Sci. 63 (1984) 91. [28] J.P. Aim&M. Schott, M. Bertault and L. Toupet, Acta Cryst. B44 (1988) 617. (291 J.P. Aim& M. Bertault, J. Lefebvre and M. Schott, to be published. [ 301 D. Bloor, R.L. Williams and D.J. Ando, Chem. Phys. Letters 78 (1981)66. [ 3 1] R.R. Chance and J.M. Sowa, J. Am. Chem. Sot. 99 ( 1977) 6703. [ 321P.A. Albouy, These de 3e cycle, Universitd Paris-Sud ( 1982), unpublished. [ 331M. Bertault, unpublished results.
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CHEMICAL PHYSICS LETTERS
30 November 1990
Self-sensitized photopolymerization
of a diacetylene single crystal pTS M. Bara a,b, M. Schott a and M. Schwoerer b a Groupe de Physique des Salides, UniversitPParis VII, Tour 23-2, place Jussieu, 7525 1 Paris Cedex OS, France b Physikaiisches Institut and Bayreuther Institut ftir Makromofekiilforschung (BMF), Universityof Bayreuth,
W-8500 Bayreuth, Germany Received 4 September 1990
The influence of already formed polymer chains on the photopolymerization of the diacetylene crystal pTS has been studied using the holographic method. It is shown that, for wavelengths between 340 and 400 nm, photopolymerization occurs and is faster in more polymerized regions. This sensitized process does not involve polymer excited states, but most probably the monomer triplet. It is a two-quantum process. It is proposed that this “sensitization” is in fact due to the elastic effects of polymer chains into the surrounding monomer matrix.
1. Introduction The photopolymerization of diacetylene crystals irradiated in their first electronic absorption band below circa 1% 3 IO nm has been thoroughly studied [ l-3 1, The best known such material is pTS or TS6, in which the diacetylene R-C = C-C = C-R has side groups R with formula -CH*-S03-C6H4-CH3. At low temperature, all intermediate states, between the monomer and the stable oligomer terminated with unreactive chain ends, could be identified in several different monomer crystals and their electronic structures studied by absorption spectroscopy, and ESR [2]. At room temperature, the intermediate states have a short lifetime, and cannot be studied independently. It has been shown, however, that in pTS, photopolymerization proceeds through the same steps at 4 and 300 K [ 41. Usually, the very first (“photophysical”) step in diacetylene photopolymerization is absorption of a photon to form the lowest singlet excited state of the diacetylene moiety, with a threshold near 325 nm at room temperature [ 51, where the monomer absorption coefficient is a few cm-‘. However, there are a few reports in the literature of diacetylene photopolymerization effected by longer wavelength photons. Bubeck et al. [ 61 found that, in Elsevier Science Publishers B.V. (North-Holland)
partiallypolymerizeddiacetyleneLangmuir-Blodgett films [ 71, photons with 300
Volume175,number 1,2
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zation is studied, using a simple setup, in the following way: a spatially modulated polymer concentration grating is first produced at 308 nm, i.e. in wellstudied conditions [ 51. The whole crystal is then uniformly excited at the wavelength at which sensitization is to be studied. On sensitization by the polymer being modulated, the efficiency of the grating is expected to vary. This variation is simultaneously studied by diffraction of a wavelength (633 nm ) which is neither efficient in producing polymerization nor absorbed.
2. Experimental method pTS crystals were grown from acetone solutions following Huber [ 141 and Wegner [ 151. Approximately 100 pm thick platelets were cleaved along ( 100) planes. Experiments must be performed below room temperature, to avoid thermal polymerization during the experiments. The samples were therefore mounted in a cryostat (Oxford Instruments CF-204 with temperature contrdller DTC2). “Monomer” pTS crystals always contain some polymer, which at low concentration gives to the crystal a pale-pink color. Plates with the faintest colorations were selected, to ensure at the beginning of the experiment the lowest polymer content (estimated in the order of 10m4to 10d5 by weight). The experiment is made in two steps with the same experimental setup (fig. 1). First, a holographic grating is written onto the crystal using the 308 nm
Diode 1
30 November 1990
line of an excimer laser polarized parallel to the b crystal axis (the polymer chain propagation direction) , The experimental conditions of the first step correspond to thick phase gratings as shown by the values of the factors Q and p introduced in refs. [ 16] and [ 17 ] respectively:
/7=)L~(A2Zngl,)-‘s
10
with, in these experiments, the reading wavelength A,=633 nm, crystal thickness d= 100 urn, grating period n = 3 urn, mean refractive index no zz 1.5 and the grating refractive-index modulation nl x 3 x 10m3.It was shown already [ 51 that such holograms are thickphase holograms. A low-efficiency grating, q= 1%, is written, two orders of magnitude less than the maximum attainable efficiency [ 5 1. This corresponds to the production of far less than 0.11 polymer. Then, one records the modification of the hologram by homogeneous irradiation using an Osram HBO-200 Hg-lamp through a KG5 filter to stop IR (A> 850 nm) and short-wavelength UV light (A<300 nm), and one of the following sharp cut-off filters (Schott): WG345, WG360, WG400 and WG420. The numbers correspond to the wavelength (nm) of 50% transmission. The actual cut-off wavelengths ( < 1% transmission) are respectively 328, 345, 387 and 408 nm. Light intensity was varied by adjusting the lamp electrical supply, and calibrated. Polarization of this beam was effected by a Glan-Thomp-
Filted
L2 Chopper
Fig. I, Sketch of the optical part of the experimental setup. The solid line is the light path for the first step. The dashed line is the light pathfor the second step. The He-Ne laser is not shown. L are lenses and P is the polarizing prism.
24
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son prism. Throughout the process, the efficiency q of the holographic grating is determined by directly measuring the ratio of the intensities of the first-order diffracted and incident beams at 633 nm using a 1 mW Spectra Physics He-Ne laser. Typically, an irradiation of 1O3to 10”s yielded a significant (larger than loo/o) change in q.
30November 1990
the variation in refractive index is proportional to the variation in polymer content, An(633)ccAP, whereas the absorption a(633) remains negligible. It is then possible to linearize eq. ( 1) and to neglect the exponential damping term, so that
3. Model where A is a known constant factor and Since low-efficiency gratings are written in the first step, and efficiency changes in the second step are small, it will be possible to linearize most equations. The experimental conditions are close to those assumed in Kogelnik’s theoretical treatment [ 161, that is, sine gratings read under Bragg conditions with sizeable diffraction intensity in first order only; so, this treatment has been used rather than the more general Magnusson and Gaylord theory [ 181. Defining the efficiency q as the ratio of the intensity of the first-order diffracted beam to the incident beam, one obtains, for a uniform transmission grating of thickness d,
x exp
(1)
where 0, is the Bragg angle, a(&) the average absorption. The modulation of refractive index n and absorption a! along the direction of modulation x is
P, =P(O) -P( f/f)
is the modulation in polymer content of the grating. Therefore, observation of an increase of q under homogeneous irradiation proves the occurrence of sensitized photopolymerization at the wavelength used: API > 0. Note that if ci!were not negligible in eq. (l), linearization would indroduce a multiplicative term in (2) of the form
p&P) with d=cr,,Pand Z&P(O)+P(jA), in which case q would decrease in absence of sensitization, and only a large-enough sensitization effect would offset the decrease; again, observation of an increase of r] corresponds to sensitized photopolymerization. To use eq. (2 ), P, has to be calculated as a function of the experimental parameters. The simplest possible process uses only one light quantum from monomer M to product: M 7 M* t product .
n(x)=no+
f n,cos(kjx)) I
and similarly for a(x). n, and CX,in eq. (1) are the coefficients of the first term in the Fourier series. In our experiments, these are the only significant ones. it, and (Y~are not strictly constant in depth, since at 308 nm the light-penetration thickness is almost equal to d. Thus, the average values of n, and aI are about half their maximum values at the surface; this has no influence on the qualitative conclusions which will be drawn from these experiments. As discussed in appendix A, in our experimental conditions at the reading wavelength A,=633 nm,
Neglecting monomer depletion, and taking into account that each reaction incorporates n monomer units into the polymer fraction, yield at steady state the polymer concentration, P=CY-
kn k+j3'-
Eqs. (3) and (2) lead to q’/2cczt,
whereas experiment (see section 4) yields y1112 0: I’?. Since thelight fluxes (<5x 10” cm-2 s-‘) are 25
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too low for sizeable two-photon excitation of M* to occur, a more complicated reaction scheme occurs. One obvious two-quanta process is 012 (11) MC=+ MT&
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h
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($g)
MS 2 product.
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Since polychromatic light is used, the efficient fluxes I, and I2 for the first and the second step may be different. CTis an average absorption cross section over the whole wavelength range of 12, which should be as IO-l6 cm2 so alz
If Z,ccl,, eqs. (4) and (2) lead to ~‘%t1:.
4. Results The grating efficiency increases quadratically with time under constant homogeneous irradiation: Aq ’/‘a t. This demonstrates the occurrence of polymer-sensitized photopolymerization in this wavelength range. Under the same conditions, the efficiency increases almost as the fourth power of light intensity; the results with filters KG5 and WG360 are reported in fig. 2 as log ( AqI”/ At) as a function of log I: the linear dependence found corresponds to an exponent 1.8, suggesting that eq. (4) is applicable. In all cases studied, the relation uoct213-6f.0.4was obeyed. Since the effect is a rapidly decreasing function of light intensity, the action spectrum cannot be obtained using monochromatic light. Using filter WG420, with cutoff at 407 nm (less than 1% transmission), we find no variation of 9: the threshold is near 400 nm. In this, pTS crystal apparently differs from LB films [ 61. This also shows that the effects observed are not due to unwanted thermal polymerization. The difference between the responses with, for instance WG345 and WG360, is due to wavelengths between their respective cutoffs, in this case 3309.~350 nm. Since this may slightly change the 26
t
I
-2
-1
Lnl
c
Fig. 2. Intensity of the rate of change (s-’ ) of the grating et% ciency, on a logarithmic scale, using KG5 + WG360 filters at 270 K. The experimental points are shown with their experimental uncertainties and correspond to the linear variation shown with a reliability factor R=0.97.
ratio between I, and i2 in eq. (4), an accurate action spectrum is not obtained. At constant total intensity, the response (change of $j2) is approximately independent of wavelength from 340 to 400 nm. Below 330 nm, the usual absorption by the singlet states takes over [ 5 1, the experimental conditions change, and the model of section 3 does not apply anymore. In the same wavelength range 340 5A5400 nm, the effect is only weakly dependent upon the incident light polarization; the variation with polarization is less than 30°& This result is only qualitative, since pTS is a biaxial crystal, and the experimental setup, particularly the use of a cryostat, made it difficult to control birefringence effects. Finally, the temperature dependence of the process was investigated in the range 250 5 Tg 300, using KG5 and WG360 filters (fig. 3), yielding Atp2 -a At
exp -
5. Discussion 5.1. Relevance of the diacetylene photopolymerization
triplet in
The growth of holographic efficiency upon humogeneous illumination of a preformed grating dem-
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That the diacetylene triplet is involved in pTS photopolymerization has been suggested already several years ago [ 2 I,22 1. In this work we give new, more direct, evidence, in favor of this assumption. The fact that a second quantum is needed indicates that, despite its 3 eV electronic energy, the monomer triplet is still separated from the dimer in the crystal by quite a high barrier. 5.2. Origin of the sensitization effect
I
3.3
c
3.8
4.3
10%
Fig. 3. Temperature dependence of Aq’/‘/A! in the same experimental conditions as in fg 2. The straight line shown corresponds to an activation energy of 0.2 eV.
onstrates “self-sensitization”; polymerization is faster in regions already more polymerized. The holographic method is indeed well suited for such studies. Self-sensitization can be produced, either if preexisting polymer chains are directly involved in further reaction, or if their presence modifies the reactivity of the monomer matrix. The absence of sensitization at the wavelength of maximum absorption in the visible (570 nm) region shows that it is not a thermal effect. The absence of polarization dependence rules out involvement of electronically excited states of the polymer chains present [ 191, even if a higher-lying excited (ionized?) state of these chains were present above 3 eV, neither of the two quanta involved in the sensitized reaction can be absorbed by the polymer. The threshold at 400 nm corresponds to the energy of the lowest-triplet excited state of diacetylene [ 201, and what is known of the action spectrum is consistent with the triplet absorption spectrum. We conclude that the limiting step in the sensitized photopolymerization process we have studied is the generation of the diacetylene triplet. Since two quanta are needed, either two triplets are used in a bimolecular process to generate the reactive species, or, more likely, this species results from photon absorption by the triplet during its lifetime (see appendix B). The activation energy is more likely to be associated to the chain propagation step [ 41.
The thermal polymerization rate of pTS monomer-polymer mixed crystals increases with polymer content. This is well documented in calorimetric studies [ 23 1, and it has been related theoretically to elastic properties and unit-cell-dimension changes in the direction of chain growth [ 1,241. Thus, any polymer concentration difference will be amplified in further polymerization. Indeed, this has been observed, as an increase in holographic grating efliciency upon thermal annealing of a preformed grating [ 12,13,25,26]. This effect produces a “homogeneous” sensitized polymerization, in that the polymer present affects the crystal properties over large distances, and not only in its immediate vicinity; a partially polymerized crystal has a single set of Bragg reflections corresponding to a single, average, structure [ 27,281. Photopolymerization will be affected in the same way as thermal polymerization, since the increased reactivity corresponds to an increased average chain length, rather than to an increased thermal chain-initiation rate [ 23 ]_ Indeed, the activation energy found here is similar to the one found in other pTS photopolymerization studies [ 1,4,20,2 11. This is plausible, but not proven here, since in the present experiment, polymer content, and its variations, are small or very small (well below 1%)) and there are few experimental data in that range. For instance, heat is evolved at an almost constant rate at the beginning of thermal polymerization, corresponding to an almost constant polymerization rate (initiation rate and average chain length) if it is assumed that the heat evolved per reacting monomer is constant [ 231. And indeed, on extrapolation to very small polymer content, the rate of change of b yields a decrease by 10m38, for 0. I % polymer, hardly enough to produce a large reactivity increase [27,29]. 27
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The increase in grating efficiency upon thermal postannealing reported in ref. [ 121 was observed at a polymer content of 2-3%. Clearly, further work on very slightly polymerized pTS would be needed. If the “homogeneous sensitization” process proposed here is not operative, then the existing polymer chains must be involved through their effect on the monomer-crystal excited states, so that among all decay channels, polymer initiation (dimer formation) is favored. Unfortunately, as already pointed out, for instance by Blssler [ 1,s 1, processes in which polymer acts as a quencher are easily found, but not processes in which energy or charge is transferred the other way. For instance, the triplet energy could conceivably be transferred to neighboring chains, thus quenching photopolymerization. That sensitization occurs shows that this process is inefficient in pTS, perhaps due to the fairly large distance, circa 7 A, between a monomer and the nearest possible chain.
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The variation of ;10with P in partially polymerized pTS crystals is still a matter of debate. Bloor et al. [ 301 report a continuous variation of A0 with P, at a rate = 100 cm-’ per % polymer at the beginning of polymerization, while others find that 1, is constant up to at least 32 polymer [ 3 l-3 3 1. In the present experiments, P is always smaller than 0.11, so that the effect of a possible variation of L,, is negligible, and the change of refractive index at 633 nm can be assumed to be proportional to the polymer content An aDa P, to first order in P. Bloor and Preston [ 191 have measured the refractive index for light propagating lb and polarized lib, nb at 635 nm. They find that, from the almost pure monomer (PS 10p3) to a crystal containing an estimated P= 1.1 X 10m2,nb increases by 5%, while Bauer [ 251 found 6% for Px (7 -C 1) x 10 -2. This would be an upper limit of n, in eqs. ( 1) and (2). At maximum efficiency, nl z 4X10e3 [5].Inthepresentexperiments,n,
Acknowledgement
Appendix B We thank H.D. Bauer for his assistance in the experimental setup and for valuable discussions. One of us (MB ) acknowledges support of his stay in Bayreuth through SFB 2 13. Collaboration between Paris and Bayreuth was supported by the German-French PROCOPE program.
Appendix A
In many cases, the photoproduct absorption spectrum is independent of its concentration. This is not so in pTS photopolymerization; while the wavelength of maximum absorption of the first polymer chains formed is about 570 nm, that of the pure polymer is 6 15 nm [ 301. The use of a Sellmeier formula for the refractive index n2(n)=n;+
*
-6
indicates that the refractive index of the pTS samples at the reading wavelength of 633 nm may be dependent, not only on D, which is usually proportional to the photoproduct (here the polymer) concentration P, but on & as well. 28
The singlet-triplet absorption coefficient in molecular crystals is usually low, except in the presence of heavy atoms [ 19 1. In pTS, the heaviest atom is sulfur, and the corresponding absorption enhancement is certainly modest. It is then not obvious that enough triplets are formed in the experiments described above to account for the observed photopolymerization. Since almost none of the relevant parameters is known, only order of magnitude evidence can be given. Assuming an absorption coefficient I&T of 1O-3 to 10m2cm-‘, the total number of triplet formed (per cm3) during a typical experiment in which a light flux II s 5x 10” cmB2 s-l is incident onto the grating for a time t x lo2 s is NT = aYSTZI 6 or 5 x 10 I6 to 10” cmm3. To produce a PI x 1 to 5 x 10p4, a total concentration of 2X IO” to lOI monomers/cm3 must be incorporated into polymer chains. Since the length of these chains is, at the beginning of polymerization, 20 to 30 monomers in pTS [l-3], 1 to 5X lOI initiation events per cm3 during these lo2 s are needed. Thus, something like 10% of all triplets formed should initiate a chain, so at least the same number must be photoexcited. With typical values of absorption cross
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section 0~ IO-r7 cm*, I25 10” crnm2s-l and triplet lifetime N 10m3s, this is indeed the case. Another two-quantum process would be the generation of the higher excited MT, the reacting state, by triplet-triplet annihilation. But via the same parameter values as above, plus an annihilation rate constant yTT< lo-” cm3 s-’ quite typical of a molecular crystal, the total number of triplet-triplet annihilations per cm3 during time f is 5 2.5 X 10” cmm3, too small by about two orders of magnitude. But with more strongly absorbed light, a process becomes conceivable by which the singlet would be formed, then would generate the triplet by ISC, and where finally triplet bimolecular annihilation would generate the reacting state.
References [ 1] H. Biissler, Advan. Polym Sci. 63 ( 1984) 1. [2] H. Sixl, Advan. Polym. Sei. 63 (1984) 49. [ 31D. Bloor and R.R. Chance, eds., Polydiacetylenes, NATO AS1Series, Vol. E 102 (Nij hoff, The Hague, 1985 ) [4] H. Niederwald and M. Scbwoerer, Z. Naturforsch. 38a (1983) 749. (51H.D. Bauer, Th. Vogtmann, I. Miiller and M. Schwoerer, Chem. Phys. 133 (1989) 303. [6] C. Bubeck, B. Tieke andG. Wegner, Ber.Bunsen&s. Physik. Chem. 86 (1982) 495. [7] B. Tieke, G. Wegner, D. Naegle, H. Ringsdorf, S. Baneyre, D. Day and J.B. Lando, Colloid. Polym. Sci. 255 (1977) 521. [8 ] F. Braunschweig and H. Biissler, Chem. Phys. Letters 90 (l982)41. [9] G.C. Bjiirklund, D.M. Burlandand D.C. Alvarez, J. Chem. Phys. 73 ( 1980) 432 1. [lo] Chr. Briuchle, D.M. Burland and G.C. Bj8rklund, J. Phys. Chem. 85 ( 1981) 123.
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[ I 1 ] D.M. Burland and Chr. Bdchle, J. Chem. Phys. 76 ( 1982) 4502. [ 121 K.H. Richter, W. Gtlttler and M. Schwoerer, Appl. Phys. A 32 (1983) 1. [ 131 B.E. Kohler, H.-D. Bauer, BE. Kohler, W. Giittler and M. Schwoerer, Chem. Phys. Letters 125 ( 1986 ) 25 1. 1141R. Huber, Diplomarbeit, University of Stuttgart (1976). [ 151 G. Wegener, Z. Natutforsch. 24b ( 1969) 824. [ 161H. Kogelnik, Bell Syst. Tech. J. 48 (1969) 2909. [ 171M.G. Moharam and L. Young, Appl. Opt. 17 (1978) 1757. [ 181R Magnusson and T.K. Gaylord, J. Opt. Sot. Am. 67 ( 1977) 1165. [ 191D. Bloor and F.H. Preston, Phys. Stat. Sol. (a) 37 ( 1976) 427; 39 (1977) 607. [ 201 M. Bertault, J.L. Fave and M. Schott, Chem. Phys. Letters 62 (1979) 161; M. Bertault, P. Peretti, P. Ranson and M. Schott, J. Luminescence 33 (1985) 123. [21] R.R. Chance and G.N. Pate], J. Polym. Sci. Polym. Phys. Ed. 16 (1978) 859. [ 221 G. Wegner, Makromol. Chem. 134 ( 1970) 219. [23] M. Bertault, M. Schott, M.J. Brienne and A. Collet, Chem. Phys. 85 ( 1984) 48 1, and references therein. [ 241 R.H. Baughman, J. Chem. Phys. 68 (1978) 3 110. (251 H.D. Bauer, Ph.D. Thesis, University of Bayreuth (1990). [ 261 M. Bara, unpublished results. [27] V. Enkelmann, Advan. Polym. Sci. 63 (1984) 91. [28] J.P. Aim&M. Schott, M. Bertault and L. Toupet, Acta Cryst. B44 (1988) 617. (291 J.P. Aim& M. Bertault, J. Lefebvre and M. Schott, to be published. [ 301 D. Bloor, R.L. Williams and D.J. Ando, Chem. Phys. Letters 78 (1981)66. [ 3 1] R.R. Chance and J.M. Sowa, J. Am. Chem. Sot. 99 ( 1977) 6703. [ 321P.A. Albouy, These de 3e cycle, Universitd Paris-Sud ( 1982), unpublished. [ 331M. Bertault, unpublished results.
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