TkBIZ 1 Characteristics of the Samples. Sample % Styrene Microstructure of Tg (in mass) PI or PB phase ( C) % % % cis trans vinyl

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2 192 L. Bokobza et al. required for the excimer formation process depends on the free volume as described by the WLF equation. The 10,10'-diphenyl-bis-9- anthrylmethyloxide called 'diphant' for convenience: was chosen as the intramolecular excimer forming probe on account of its high fluorescence quantum yield and of its good excimer stability. Both time-resolved and photostationary-state measurements were carried out in order to analyze the emission behavior of diphant dispersed in polymer host matrices whose characteristics are summarized in Table 1. These matrices are polybutadiene (PB), polyisoprene (PI), and random copolymers styrene-butadiene (SBR) and styrene-isoprene (SIR). All these rubbers are crosslinked but similar results were obtained with an uncured sample of polybutadiene. The interesting feature of the family of styrene-butadiene copolymers is the variation of the glass transition temperature T~ induced by changing either the microstructure of the butadiene phase or the styrene content. The polymers were swollen with solutions of the probe in cyclohexane and then dried under vacuum. The final probe concentration in bulk samples was chosen so that only intramolecular interaction between the chromophores could occur. Analysis of the experimental data was performed according to the TkBIZ 1 Characteristics of the Samples Sample % Styrene Microstructure of Tg (in mass) PI or PB phase ( C) % % % cis trans vinyl PB Diene 45 NF SBR Solprene SBR Stereon SBR Stereon PI IR SIR

3 - /31) Excimer fluorescence as a probe of mobility in bulk polymers 193 conventional kinetic scheme: 4 ~_..._jm. k.. ' (M ~l)* kvm and km are the rate constants for the monomer and excimer emission, kim and kid are the non-radiative rate constants of the monomer and excimer, and kom and k~ are the excimer formation and dissociation rate constants, respectively. This scheme yields the following expression for the ratio of excimer to monomer fluorescence intensity ID/IM: /M kvo [kvd+ kom.] kfm kid + kmdj This ratio is an approximate measure of the efficiency of the excimer sampling mechanism and thus reflects the mobility of the probe in the host matrix. The time dependence of monomer and excimer fluorescence decays can be described by the following equations: 5 (X - [e_,2, _~ (/32-- x) e_b,,] Io(t)=kmkD~M[e-~,'--e-B~ '] where /31.z = ½{(x - y) q: [(x - y)2 + 4kDMkMD]I/Z} and x = kvm + k~m + kdm = 1,r M y =km + kid + kmd = 1 TD where ~'M and ~-D represent the monomer and excimer lifetimes, respectively. The fluorescence lifetime measurements allow direct access to the dynamic properties of diphant especially by the determination of the rate constant of excimer formation kdm.

4 194 L. Bokobza et al. RESULTS AND DISCUSSION Figure 1 represents, as an example, the temperature-dependent fluorescence spectra of diphant embedded in polybutadiene. The structured emission corresponds to the locally excited chromophore, so-called 'monomer', and the broad structureless band with a maximum near 550 nm is ascribed to the intramolecular excimer. The excimer and monomer emission bands are well separated, which reflects a large stabilization energy of the excimer complex. The spectra show an isoemissive point near 500 nm, even at relatively high temperatures. In Fig. 2, the temperature dependence of the 40 *C 60 C so c.. "... ', "X / --":2"'"'".-;2... mg ~o 5~o 5~o ~o Wavelength (nm) 1. Temperature dependence of the emission spectrum of diphant in polybutadiene.

5 Excimer fluorescence as a probe of mobility in bulk polymers I D 0 ~ - : ~ Tempersture I c)l(~o Fig. 2. Temperature dependence of excimer (ID) and monomer (IM) fluorescence intensities for diphant in polybutadiene. fluorescence intensities of the monomer (IM) and excimer (Io) for diphant dispersed in polybutadiene are displayed. IM is approximately independent of temperature between -100 and -20 C while Io increases very slightly. The non-zero value of ID in this range of temperature means that the excimer is not formed by the usual

6 196 L. Bokobza et al. 8t I" M (n-) 9 - Methoxymethyl Phenylanthracene , -1oo -~ o ~o I~o 'Temperl ure ( C) Fig. 3. Fluorescence lifetimes versus temperature of the excited monomer of diphant (*) and of the monomeric model compound (ll) in polybutadiene.

7 Excimer fluorescence as a probe of mobility in bulk polymers 197 process of conformational change but more likely arises from excitation of a chromophore pair preformed in the ground state. Above -20 C IM is observed to decrease significantly while ID increases. At temperatures exceeding 80 C the isoemissive point becomes less defined and both emission bands decrease. Excimer dissociation is responsible for the diminution of the excimer band. The transient measurements are in agreement with the photostationary results. The temperature dependence of the monomer lifetime of diphant in polybutadiene is given in Fig. 3. Below -20 C the monomer lifetime is nearly independent of temperature which indicates that the conformational change of the probe is hindered below that temperature. This figure also shows the lifetimes of the 9-methoxymethyl-10- phenylanthracene used as the monomeric model compound of diphant. The two sets of measurements were made under identical experimental conditions. At temperatures where the rotational motion of the probe is frozen, the model lifetime is always slightly larger than the monomer lifetime of diphant. A similar result was observed in solution by De Schryver et al. 6 and was explained by an intramolecular interaction between the chromophores of diphant in the ground state. Nevertheless, assuming that the monomer decay time of diphant in the absence of excimer formation exhibits the same temperature dependence as that of the model compound, we can easily deduce the rate constant of intramolecular excimer formation kdm. In Fig. 4, the ID/IM ratios for diphant in polybutadiene and in the styrene-butadiene copolymers are plotted against temperature. The data clearly show that the nature and the morphology of the polymer influence the rate of conformational change of the probe. Figure 5 gives the temperature dependence of the monomer lifetime of diphant in polybutadiene and in its copolymers. It is interesting to note that the monomer fluorescence decay was found to be monoexponential in each matrix and at all temperatures. This indicates that excimer dissociation into monomer-excited groups is negligible in the range of our investigation. Once more the results reveal that at the same temperature the monomer lifetime, and thus the rate constant of excimer formation which reflects the probe mobility, is affected by the host matrix. On the other hand, as is seen in Fig. 6, the excimer lifetime is nearly independent of the host matrix and exhibits the same temperature dependence in each polymer.

8 198 L. Bokobza et al. ID/IM * PB Olene45 NF Tg =-91"C 0 SBR Solprene "C 0.20 SBR Stereon "C u SBR Stereon "C ) 5(1 Temperature {*C }1()0 Fig. 4. Temperature dependence of the ID/IM ratio for diphant dispersed in polybutadiene and its copolymers.

9 Excimer fluorescence as a probe of mobility in bulk polymers 199 T M {n$) ~k o o ~k PB Dlene 45 NF T9 =-91 C 0 SBR Solprene C SBR Stereon ~C o SBR Stereon C o ~ ~o 16o Temperature ( C) $. Fluorescence monomer lifetime versus temperature for diphant polybutadiene and its copolymers. in

10 200 L. Bokobza et al. "r D (.s) PB Diene 45 NF o SBR Solprene SBR Stereon 704 [] SBR Stereon PI IR 307 SIR 30. ~+ o e o.k. "~, D 20. o 's ~'o 7'5 Temper,,ture ('C) Fig. 6. Temperature dependence of the excimer lifetime of diphant.

11 Excimer fluorescence as a probe of mobility in bulk polymers 201 ) I- I ~ ~ 04 ~ o,.=_ o e ~ Q 4 o "0 x r: 0

12 a Log ~otc -6,0- t Log10 a T " I T-Tg I 80.5 (T-Tg PB Diene 45NF Tg=-91 C o SBR Solprene ~C SBR Stereon C I~ Log at+ 0.1 ~,, lo o SBR Stereon ~C o. o * o, o % -7,5- o o o ,x % ()0 T-Tg Fig. 8. Logarithmic plot of the correlation times versus (T-T~) for diphant in: (a) polybutadiene and the styrene-butadiene copolymers, and (b) polyisoprene and the styrene-isoprene copolymers.

13 Excimer fluorescence as a probe of mobility in bulk polymers 203 b Log 10 I"C Log 10 at " (T-Tg) (T-Tg I + PI IR307 Tg =-580C -6.5 SIR -270C ~/L 9 10 at ~o 1~o 2~o T - Tg F~. g--contd.

14 204 L. Bokobza et al. In order to discover whether the rotational process is associated directly with the glass-rubber relaxation phenomena of the polymer matrix itself, the ID/IM ratios were plotted (Fig. 7) against (T- Tg) for each polymer. A master curve is obtained for each family of polymers, one for PI and its copolymer and the other for PB and its copolymers. Within a given family of polymers the rotational mobility of the probe appears to reflect the glass-rubber relaxation of the host matrix. Besides, it is clear that at the same (T-T g) the mobility of diphant is higher in PI than in PB. It is interesting to see if the excimer formation process is controlled by the free volume theory as described by the WLF equation: 7,~(T) C~(T- T~ log,o at=logxorc(rg )- C2+(T-T~) This equation describes the temperature dependence of the ratio of the correlation time at temperature T to its value at a reference temperature Tg in rubbery polymers. For each family of polymers, the set of C1 and C2 parameters are those given by Ferry 7 for elastomers of microstructure similar to that of the samples investigated in the present study: ~C1= 16-8 pb ~C1= 11"2 PI LC2 = 53.6 I.C2 = 60.5 For all the polymers, the correlation times determined by the reciprocal of the rate constant for intramolecular excimer formation were plotted versus (T-T~). Figure 8(a) represents the correlation times obtained in polybutadiene and in styrene-butadiene copolymers. The related WLF equation drawn as a dotted line gives a good fitting of the experimental data. The results obtained for polyisoprene and its copolymer are given in Fig. 8(b). Once more the data show that the excimer sampling process of diphant can be described by the appropriate WLF equation represented by the dotted line. At a constant (T-T~), the correlation time in PI is smaller than that found in PB, which confirms the photostationary measurements. This difference in intrinsic mobility observed between the two systems may be related to their different chemical structure. It is well known that the glass transition is not an activated process. Nevertheless in a limited temperature range and far above T~ it seems reasonable to fit the temperature variation of the rate constant

15 Excimer fluorescence as a probe of mobility in bulk polymers 205 l 9 -L 18. EWLF'- LnlO RCIC2 T2 (C2 *T-Tg ) PB (EDM"9 Kcal.mo1-1 / % (EWLF= ) 16- ~_ p, (,=o,,,, ~,,, Kc,,.,,,o,-1) (EwLF ) 15. I! 2.8 3'.o 3:2 ~'.4 T "I, K'l(x10 3 ) Fig. 9. Arrhenius plot of the rate constant of excimer formation kdm of diphant in polyisoprene (PI) and polybutadiene (PB).

16 206 L. Bokobza et al. for intramolecular excimer formation with an Arrhenius equation. The activation energies of the rotational process are calculated as between 40 and 80 C from Arrhenius plots of the rate constant for intramolecular excimer formation; as can be seen from Fig. 9, they are about 9 kcal mo1-1 for PB and 14 kcal mo1-1 for PI. These values seem to reflect the energies of segmental motions of the polymer matrix needed to provide the free volume for the excimer formation process. This conclusion is supported by comparison with the values deduced by differentiation of the WLF equation at 60 C, which is an average value of the temperature range investigated. The activation energies obtained in bulk polymers are higher than the value of 3.5 kcal mo1-1 found in methylcyclohexane by De Schryver et al. 6 This last value is in good agreement with conformational energy calculations performed in our laboratory. Thus the results observed in bulk polymers cannot be attributed to the rotational process itself but reflect chain segmental mobility. CONCLUSIONS We have shown that the excimer fluorescence technique of a bichromorphoric molecule non-inserted in a chain backbone may afford a way to probe molecular motions in bulk polymers. The excimer sampling process of diphant dispersed in the above elastomers seems to be controlled by the molecular motions of the chains surrounding the probe. The rotational mobility of the probe appears to reflect the glass-rubber relaxation in agreement with the WLF equation. Further investigations are in progress with other probes and other polymers. REFERENCES 1. Georgescauld, D., Desmasez, J. P., Lapouyade, R., Babeau, A., Richard, H. and Winnik, M., Photochem. Photobiol., 31 (1980) Kano, K., Goto, H. and Ogawa, T., Chem. Lett., (1981) Turro, N. J., Okubo, T., Chung, C. J., Emert, J. and Catena, R., J. Am. Chem. Soc., 104 (1982) Chandross, E. A. and Dempster, C. J., J. Am. Chem. Soc., 92 (1970) 3586.

17 Excimer fluorescence as a probe of mobility in bulk polymers Johnson, G. E., J. Chem. Phys., 63 (1975) De Schryver, F., Demeyer, K. and Huybrechts, J., J. Photochem., 20 (1982) Ferry, J. D., Viscoelastic properties of polymers, 2nd edn, Wiley, New York, 1970.