From cylindrical to non‐cylindrical foreland basin: Pliocene–Pleistocene evolution of the Po Plain–Northern Adriatic basin (Italy)

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Particle state dependent radical desorption and its effect on the kinetics of emulsion polymerization

Radical desorption from polymer particles is a kinetic event peculiar to the emulsion polymerization process. A careful modeling of this phenomenon is highly valuable in order to achieve accurate predictions of polymerization rate and average properties of molecular weight. In this work, radical desorption is described accounting for an aspect fully neglected in previous modeling literature. Specifically, particle state dependent desorption coefficients are used instead of a single average coefficient, and the corresponding rate expressions are developed and applied to the solution of the well-known Smith-Ewart equations. Parametric model simulations show that the higher level of detail introduced in the description of radical desorption improves the accuracy of the predicted values of the average number of radicals per particle, especially in the cases of high desorption rate and slow reactions in the aqueous phase. © 2014 American Chemical Society

Role of active chain segregation in emulsion polymerization

The effects on the molecular weight development of chain segregation in emulsion polymerization are discussed both theoretically and experimentally. A large delay on gel formation induced by carrying out the polymerization reaction in emulsion instead of in bulk is predicted by the model. Two systems of practical interest (polyvinyl acetate and polybutyl acrylate) are then examined. In the first case, literature experimental data are compared with model predictions. The synergic effect of the different branching mechanisms on the molecular weight polydispersity is highlighted. In the second case, original experimental data obtained in a calorimetric reactor are presented. The predicted onset of gel formation at conversion values larger than those expected in bulk polymerization is experimentally verified. (C) 2001 Elsevier Science Ltd. All rights reserved

Calculation of molecular weight distributions in free-radical polymerization with chain branching

The molecular weight distribution (MWD) of a branched polymer has been calculated by using approximate techniques based on the partition of the overall polymer chain population into classes according to molecular dimension or the number of branches. These techniques are especially useful in segregated systems, such as emulsion polymerization, where rigorous methods are too onerous. A comparison with the correct MWD obtained through a detailed solution method has shown that, of these techniques, 'numerical fractionation' predicts in some cases a marked shoulder at the high molecular weights that does not exist in the true MWD. On the other hand, besides offering an intelligent description of gel formation, 'numerical fractionation' permits to calculate quickly the correct average molecular weights and constitutes a significant improvement compared to the classical method of moments when a complete MWD is required. In the latter case, however, the most accurate approximate technique has been found to be based on the subdivision of the polymer chains according to the number of branches. This technique, though more time-consuming than 'numerical fractionation', provides also the branching distribution along with the MWD. A quick method is suggested to estimate the minimum number of branches required by this technique to calculate the correct complete MWD

Molecular weight distribution of crosslinked polymers produced in emulsion

A kinetic model for evaluating the chain length distribution of a branched polymer produced in emulsion was developed. Chain branching occurring through any intermolecular mechanism is considered, namely, crosslinking, chain transfer to polymer and propagation to terminal double bond. The model accounts for active chain compartmentalization and, when coupled to a model able to describe the evolution of the polymerization system, allows evaluation of the cumulative properties of the produced polymer both in the pregel and postgel phases. The numerical difficulties related to the description of a rather wide polymer chain population and of gel formation are overcome by using the 'numerical fractionation' technique. A parametric analysis of both instantaneous and cumulative properties is reported and discussed, with special attention to the role of radical compartmentalization in determining the molecular weight properties of a polymer produced in emulsion. Significant differences with the molecular weights computed using models developed for homogeneous :non compartmentalized) systems have been found. A comparison with the predictions of Flory's statistical theory is also reported in terms of gel point and gel weight fraction. (C) 1998 John Wiley & Sons, Inc

Nonlinear chain-length distributions in free-radical polymerization

By using results previously reported in the literature, the role and interactions of various termination mechanisms in determining the molecular weight distribution of nonlinear polymer chains produced by different processes are discussed. The analysis is focused on the instantaneous polydispersity of nonlinear chain distributions generated by chain transfer to polymer, cross-linking, and terminal double-bond propagation. Cumulative properties are considered, and criteria for the occurrence of polymer gelation for each of the above reactions are developed. Finally, the role of active radical compartmentalization, which is peculiar of emulsion polymerization, is discussed

Molecular weight distribution in emulsion polymerization: Role of active chain compartmentalization

A comparison of the results of two models with different levels of complexity has shown that errors can be made in the calculation of the polydispersity ratio of a polymer, produced in emulsion in the presence of termination by combination, if compartmentalization is not taken into account by properly considering the distribution of the lengths of the pairs of chains belonging to the same particle. These errors have been shown to be significant for values of the average number of free radicals per particle between about 0.4 and 2, which are typical of many emulsion systems. Accounting correctly for compartmentalization becomes crucial when the termination by combination mechanism is responsible for gelation, as in the case where branching through chain transfer to polymer is present in the system. The nature of the effects of compartmentalization on molecular weight has been analyzed both from a physical and a mathematical point of view. The importance of using a detailed model has been discussed for three polymerization systems: styrene, methyl methacrylate, and vinyl acetate