Détermination expérimentale du rapport isomérique du 136I dans la fission thermique et par neutrons de 3 MeV de 235U

Specific volume measurements carried out on linear and star-shaped liquid polystyrene versus the number average molecular weight, M,, scaling between 5 000 and 2 . lo6, are in good agreement with a review of the experimental results of others. These results confirm the occurrence of a loss of volume per monomeric unit above a molecular weight of 10000 already observed for polystyrene in solution. We attribute the influence of this specific volume variation against M,, on the thermal dilatometric coefficient a = (l/u)do/dT and the temperature of vitrous transition. We also attempted to give an explanation for this phenomenon. 0250-973318

Volumes spécifiques de polystyrènes en étoile à l'état liquide

Sarazin

François

1981

Excluded volume effect on polymer chains: Internal parts vs. free chain expansion

Ganazzoli

1986

Date of receipt: May 21, 1985)a) SUMMARY:A self-consistent Fourier configurational approach is used to describe the swelling of a polymer chain and of its inner parts in a good solvent. Minimization of the free energy yields the full spectrum of the expansion coefficients for the interatomic mean-square distances assumed to have a Gaussian distribution with the actually perturbed second moment. Due to the repulsive effect of the outer segments, the internal parts are more expanded than a free chain with the same molecular weight under identical conditions, in good agreement with recent experimental findings, but contrary to what is predicted by mean-field theories and by the blob model.

Relative reaction probabilities in polymer‐polymer reactions, 1. Investigations with Monte‐Carlo model chains

Olaj

Zifferer

1988

Pairs of chains taken from ensembles of five-way cubic lattice chains (athermal or theta) were analysed for the (relative) probability of contact formation between specified segments belonging to different chains within this pair. This probability may be taken as a measure of the rate constant which characterizes bimolecular polymer-polymer reactions which occur between active sites attached to or localized at these segments, relative to the reaction between the same active sites, if these were not bound to the polymer chains. It turns out that the (expected) effect which results from the interaction between the chains as a whole (external shielding) is of comparatively small extent and is fairly independent of chain length. The much more important contribution, however, is made by a novel phenomenon whichexhibiting a strong dependence on chain length nis responsible for an "internal" shielding: the compatible pair configurations (i. e. those which are free of intermolecular intersections) are characterized by largely increased segment-segment distances. As a consequence, the probability of contact formation is much lower than it would be expected from a convolution of the (mutually independent) radial distributions of the segments participating in contact formation. The overall shielding factor K can be represented as a product of the fairly independent contributions of external (KG) and internal (K,) shielding. The overall chain length dependence is described by a power law K = Dnd, where the exponent d depends on whether two (Type I contacts), one (Type 11) or no endsegment at all (Type 111) is involved in contact formation. For athermal chains the exponents are d = -0,16 (Type I), d = -0,30 (Type 11), and d = -0,50 (Type 111), the pre-factor being of the order of unity throughout. @-systems roughly show the same general characteristics as the athermal ones, however, to a lesser extent. Thus, neither shielding nor its chain length dependence vanish, the exponents being about -0,06 (Type I), -0, I (Type 11), and -0,2 (Type III), respectively. 0025-I 16X/88/$03 .OO Tab. 1. Parameters D (pre-factor) and d (exponent) of the power laws Dnd characterizing the dependence of the shielding factor K , the external shielding factor K, , the internal shielding factor K, , and the product K,