Spread monolayers of poly(styrene)-poly(ethylene oxide) diblock copolymers (PSm-PEOn, m ) 38, n ) 90, 148, 250, and 445) have been studied at the air-water interface by measuring the surface pressurearea (π-A) isotherms at several temperatures. The π-A isotherms exhibit several regions which can be ascribed to different conformations of the polymer chains: a pancake structure at low surface pressures and high areas when the isolated chains are adsorbed by both the PS globule and the PEO segments at the interface; an intermediate structure, quasi-brush, when the PEO segments are solubilized in the subphase; and finally a brush developed at low surface areas when the PEO chains are obliged to stretch away from the interface to avoid overlapping. At surface pressures near 10 mN/m there is a transition between a high-density pancake and the quasi-brush regime. The compression and the subsequent expansion curves superpose at the transition and quasi-brush regions but not at the brush and pancake stages. This points to a high cohesion in the brush structure after compression and to some irreversible entanglement and hydration of the PEO chains when immersed in the subphase. These two local hystereses depend differently on the PEO chain length and temperature. The hysteresis observed at high surface pressures (brush conformation) decreases with the PEO length and temperature, whereas the low surface pressure hysteresis (pancake) increases with PEO chain length, decreases with temperature in the range 283-298 K, and increases in the range 298-315 K. A negative mean transition entropy change was obtained from the temperature dependence of the quasi-SSAL-quasi-brush transition. The results indicate that the extensive properties of the present diblock copolymers at the interface, such as the pancake limiting area and the mean transition entropy, when expressed by PEOmer, are independent of the PEO length.

A numerical method is presented to recover the excimer formation rate coefficient from the experimental monomer and excimer decay curves. This method was applied to study pyrene monomer-excimer kinetics in cyclohexanol from 25 'C up to 85 OC. We observed that the time evolution of the rate coefficient deviates from the Smoluchowski equation, showing a minimum at high temperatures and concentrations, when the reversibility effects are more pronounced.K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS). The ratios were derived from the observed decreases of MBMS CH2 signals upon substituting 25% of the helium bath gas by 0.5 Torr of methane, which selectively scavenges CH2('Al); in the absence of CH, the singlet CH2 is mainly collisionally converted to CH2(!Bl). In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained. As the rate of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with 0, both the ratios kt/kzb and k3s/k3b can be. extracted from the data. Thus, including also probably systematic errors, the HCCO yield of C2H2 + 0 is found to be k2,/kz = 85?% and the CH2('AI) yield of HCCO + H, k 4 k 3 = 92 * 15% (95% confidence intervals).In addition, the rate constant of CH#B1) + H -CH(211) + H2 (rl) was denvcd relative to the known k(3CH2+O) from CH2 signals at different [H]/[O] ratios:kl = (1.6 f 0.6) X lo1' cm3 mol-' s-l.

Cyclization rate measurements were carried out on polystyrene chains of different molecular weights and labelled at both ends with I-pyrenyl groups, using as solvents cyclopentane (a thetasolvent), acetone (a poor solvent), and their mixtures. These mixtures exhibit a cosolvency effect for polystyrene. The results show that the rate constants for cyclization and ring-opening are solvent-and chain-length-dependent. This was interpreted in terms of the influence of excluded volume effects on these processes. The results indicate that the solvent giving the largest excluded volume effect is an equimolar mixture of cyclopentane and acetone, which agrees with the light scattering and viscometry results for polystyrene in cyclohexane/acetone mixtures.