The quenching by oxygen of the lowest electronically excited singlet (S1) and triplet (T1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S1 state, Δ , at 0.1 MPa fell in the range from 14 to 16 cm3/mol, which is significantly smaller than Δ (25 cm3/mol) determined from the pressure dependence of the solvent viscosity, η, whereas that for the T1 state, Δ , at 0.1 MPa changed from +6.1 (anthracene) to −15.2 cm3/mol (triphenylene) and correlated approximately linearly with the triplet energy, E T. However, Δ at 350 MPa was positive and almost independent of E T (3−6 cm3/mol). The pressure dependence of was interpreted in the framework reported previously by us, and that of was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T1 state molecules is involved.

Photocrosslinking reaction kinetics of poly(2-chlorostyrene) performed inside the spinodal region of poly(2-chlorostyrene)/poly(vinyl methyl ether) (P2CS/ PVME) blends was investigated by means of ultraviolet (UV)-visible absorption spectroscopy. The reaction was performed via photodimerization of anthracene moieties chemically labeled on the P2CS chains. The crosslinking kinetics of (P2CS/PVME) blends submitted to a temperature jump from the one-phase into the spinodal regions was observed by monitoring the irradiation time dependence of the absorbances of anthracene as well as of the blend in two regions of wavelengths. One is inside and the other is outside the absorption range of anthracene. The contribution of the sample cloudiness to the absorbance of anthracene was subtracted from the absorption data by using an empirical power law experimentally established between the incident wavelengths and the absorption of the blends. It was found that the reaction kinetics approximately follows the mean-field kinetics inside the spinodal region, resembling the behavior of the crosslinking reaction performed in the miscible region at relatively low crosslinking densities. On the other hand, the method described here fails to estimate the crosslinking densities when the phase separation proceeds rapidly, overcoming the reaction.