Monomer excitation

Not much attention has been paid to the two different kinds of excitation in photo-induced copolymerizations. A number of authors do not mention which species was excited, one of the partners or the CT complex. However, there are investigators who have made sure to excite the CT complex only by selecting the wavelength of the incident light [23]. Neither St nor FN absorbs 365-nm light, 

The following conclusions can be drawn about the effect of D/A strength on polymerization  

Some D/A combinations do not interact strongly enough to bring about thermal copolymerization, but readily yield copolymers under irradiation, for example, p-t-butylstyrene/MAn [23], Some undergo very slow thermal polymerization, while the photopolymerization is much faster, such as St/FN system. Table 2 [37] shows the results of thermal (80 °C) and photocopolymerizations of some monomer pairs. The acceptor strength decreases in the following order  

The numbers in the brackets show the e values of Price-Alfrey equation. The donor strength decreases in the following order  

Spontaneous thermal copolymerization is usually observed in strong donor (D)/strong acceptor (A) pairs, for example a-MSt/MAn, because only in these pairs is the charge-transfer interaction strong enough to produce initiating radicals. On the other hand, photo-induced copolymerizations encompass a wider range of donor/acceptor combinations. 

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LB PMMA film transferred at 7 dyn/cm. The excimer excitation (dashed curve) was monitored at 500 nm and monomer excitation (solid curve) was monitored at 376.5 nm. 

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

The method using intermolecular excimer formation is based on the same principle because this process is also diffusion-controlled. Excimers should, of course, be formed during the monomer excited-state lifetime. In Section 4.4.1, it was shown that the ratio Ie/Im of the intensities of the excimer and monomer bands is proportional to ki provided that the transient term can be neglected. When the dissociation rate of the excimer is slow with respect to de-excitation, the relationship is... 

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

A1C1)2 and their corresponding monomers. Excitation spectra of the... 

In a first successful approach, Kochi, Renzepis, and co-workers [41] chose EDA complexes of 9-cyanoanthracene (14) and tetracyanoethene (TCNE, 15) since their charge transfer (CT) absorption bands are well separated from the absorption bands of the monomers. Excitation with a 25 ps laser pulse produced two transient absorption bands near 460 and 750 nm, which decayed simultaneously within ca. 60 ps. As was shown in the chloranil-enolether system 9—10, cf. Fig. 6), the transients can be identified with the arene radical cation (14a+ ) and the olefin radical anion (/5- ), respectively (Scheme 5). 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

Fig. 16a-e. Changes of X-ray diffraction patterns upon heat treatment of as-polymerized poly-DSP crystals and photopoly-merization of DSP crystals, (a) As-polymerized poly-DSP, (b) heat-treated sample (1), (c) poly-DSP obtained after 50-min irradiation of the monomer with a xenon lamp, (d) heat-treated sample (2), and (e) DSP oligomer obtained by selective monomer excitation, (source Ref. 65)... 

It is of great interest that when the heat-treated sample (1) of poly-DSP in Fig. 16 is heated to 330°C (with the same scanning speed), the X-ray diffraction pattern of the resulting sample (2) changes into a pattern which is exactly the same as that of the as-polymerized oligomer crystals prepared by selective monomer excitation (see Sect. m.b.). 

As soon as plasma is created, the gas phase is no longer the vapor of the original monomer but becomes a complex mixture of the original monomer, ionized species, excited species of the original monomer, excited species of fragments from the monomer, and gas products that do not participate in polymer formation such as H2 and F2. Perfluorocarbons represent perhaps the most extreme case of ablation competing with polymer formation. The principle found with perfluorocarbons, however, should be applicable to nearly all cases. 

There is an early report in the literature claiming absence of the autocatalytic reaction enhancement in TS if the reaction is induced by UV-excitation of the monomer crystal. The implication would be that thermal and UV-polymerization involve different mechanisms. Later on, however. Chance and Patel found this to be an artifact caused by the neglect of spatially inhomogeneous absorption by polymer molecules which effectively competes with monomer excitation at increasing conversion and prematurely terminates the reaction. Although it is difficult to correct X(t)-curves obtained under UV-excitation for polymer absorption quantitatively, particularly if irradiation is done with unpolarized non-monochromatic light, it turns out that there is a qualitative agreement between X(t)-curves obtained under y-and UV-irradiation. Application of this correction, however, does not solve the puzzle why in case of y- or UV-polymerization of TS, the reaction rate increases less dramatically with conversion, than observed upon thermal conversion. 

The latter effect can be explained by taking into account that a partially polymerized diacetylene crystal is a molecular crystal containing dopant molecules with lower lying optical transition acting as traps for monomer excitations . Take k as the rate constant for non-radiative energy transfer from a donor to a trap, Tq as the intrinsic donor lifetime, and c, as the relative trap concentration, then the lifetime of a donor in presence of traps would be T = -I- k cj Since the probability that an excited... 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E" need not be and in general is not identical with the energy... 

Owing to the bimolecularity of the initiation reaction the quantum yield of the dimer molecules (M2/Nji ) is proportional to the absorbed light quanta N bs and to the ratio kj/ko, characterizing the competition of the chemical dimer initiation process (kj) with the deactivation processes (ko) of the monomer excitation. A comparison of the dimer A absorption intensities of different diacetylene crystals shows that the ratio kj/ko is about a factor of 10 to 10 larger in the TS-6 crystals than in... 

If very rapid deactivation processes exicst for the monomer excited state (very short lifetime) little or no complex formation will be observed. 

Complex formation can be fast compared with other deactivation processes of the monomer excited state, but dissociation can be negligible. The dissociation can be inefficient for two reasons ... 

Though the quantum yield for photopolymerization with UV light is high at low polymer conversions, it rapidly decreases as the polymer concentration increases [2). The limiting yield for UV polymerization in 4BCMU is about 35%. It is believed that this effect has its origin in quenching of the monomer excited states by the polymer chains, i.e., the monomer excited states created on irradi-... 

The UV-visible absorption spectra were measured with a Cary 210 spectrophotometer manufactured by Varian. The fluorescence spectra were taken with a spectrofluorometer that has been described previously. (11) The excitation spectra were measured by a Spex Fluorolog 212 spectrofluorometer. The monomer excitation spectrum was monitored at 376nm and the excimer excitation spectrum was at SOOnm in the scanning excitation range between 300nm and 370nm. 

According to exciton theory [82,83], the excited state energy level of the monomeric dye splits into two upon aggregation, one level being lower and the other higher in energy than the monomer excited state. The transition to the higher state is forbidden for head-to-tail (J-type) dimers, whereas the lower... 

The second way we used excitation spectra was to follow the changes in excitation maxima as a function of the molar ratio of the poly(carboxylic acid) to the PEG (41). We observed that the initial red shift of the excimer excitation spectrum relative to the monomer for Py-PEG-Py(4800) remained approximately constant as the PAA(890,000) content was increased. There was, however, an enhancement in the relative shift of the excimer for PMAA(9500) up to the stoichiometric equivalence of PMAA to PEG. Thereafter, the shift between excimer and monomer remained constant. One form of comparison of these results is shown in Figure 3, where we present the monomer excitation spectra for Py-PEG Py(4800) free in aqueous solution and complexed with PMAA(9200) at [PMAA] to [PEG] ratio of 5 1. 

Figure 3. Monomer excitation spectrum monitored at 376 nmfor 100% tagged...

Upon adsorption onto colloidal silica, the monomer excitation spectrum remained essentially unchanged from that of the solution. In this system, the excimer persisted, allowing study of its excitation spectrum. In this experiment, a substantial shift in the excimer excitation spectrum occurs, indicating that, although excimer-forming sites remain, they exist in a strained state. This observation is further supported by the time dependence of the excimer fluorescence decay. Whereas chains remain adsorbed on the colloidal silica surface, the excimers formed are substantially shorter lived than those in free solution. 

A measure of the dipole-dipole interaction energy between two adjacent monomers and of the anharmonicity of the chain can be obtained by an inspection of the bandwidth of the absorption band of the monomer excitation, that forms the exciton. For a-helix proteins in which inhomogeneous broadening has been eliminated by ordering processes of the samples, the bandwidth of the C = 0 absorption at 1660 cm gives evidence of the excitonlike collectivization of the vibrational C = O excitation along the chain. This is a prerequisite for the existence of Davydov solitons on the chain. 

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