A comprehensive model for kinetics and development of film structure in interfacial polycondensation

A detailed model for interfacial polyconclensation (IP) reaction, which accounts for the salient equilibrium and rate processes in reaction and phase separation, is reported here. The modeling of nucleation phenomena is more rational and fundamentally based than so far attempted in the literature on this process. Simpler models are proposed for situations in which one of transport and reaction resistances is the dominant one, and criteria developed that guide the selection of the model. The model explains the empirical findings on different interfacial systems, which have been reported in the literature. An extensive parametric study has been carried out in order to explain the effect of the important dimensionless parameters that arise, and analysis shows that most of the effects observed can be rationalized with respect to certain types of asymptotic behavior. The model makes it possible to predict the time course of development of the important film properties such as thickness, MWD and crystallinity, and to relate these to the preparation conditions and system parameters, as embodied in the different dimensionless parameters. It should therefore be possible to use this model to choose synthesis conditions and system-dependent parameters to achieve desired properties. (C) 200

Understanding interfacial polycondensation: Experiments on polyurea system and comparison with theory

Interfacial polycondensation, with its multiphase character and the involvement of several rate and equilibrium processes, presents unique challenges to our ability to understand and design processes for achieving desired properties. In a recent study [1], we have presented a detailed model for the process and shown that it explains the salient features as reported in the literature for several interfacial systems. In the present paper, we report extensive experimental studies on the polyurea system and their comparison with the model. Two geometries - the spherical geometry of the microcapsule and the flat-film geometry - have been used to study qualitative and quantitative features of the polycondensation and the nature of the film that forms. While some aspects, such as the manner in which the solvent influences the kinetics, confirm earlier findings, inadequacies have been identified in the sample preparation protocols followed in earlier work, because of which property estimations may carry a large error. Improved protocols have accordingly been developed and used to study the development of film properties in time and as a function of the important preparation variables. Detailed molecular weight distributions have been determined using a GPC technique and used to derive important properties of the polyurea (such as Mark-Houwink parameters) as well as to gain insights into mechanisms. The data have been used to determine the rate parameters in the Dhumal and Suresh [1] model. The predictions of the model, as far as trends are concerned, are shown to be satisfactory given the level of uncertainty about parameter values and the complexity of the system being studied. Where discrepancies exist, the reasons have been established and the areas for improvement of the model identified. The findings reported are of interest to applications such as controlled release and membrane separations, in which permeation rate through the membrane is of importance and depends upon various membrane properties like crystallinity, morphology, etc

An experimental study of polyurea membrane formation by interfacial polycondensation

Interfacial polycondensation has been studied for many applications, and most importantly for such niche applications such as microencapsulation and membrane synthesis. The fast kinetics and the complexity of the process involving the interplay of several rate and equilibrium processes make it difficult to study the reaction and explore the effect of kinetics on the properties of the polymer film that forms. We show in this study that a dispersed phase configuration such as used in microencapsulation, with a fast on-line pH measurement, is a convenient way to study these reactions. Apart from the ease of obtaining kinetic information, the technique results in self-supporting films which can be recovered and characterized for structure by a variety of techniques. In this study, the intrinsic variables which influence observed reaction velocities, such as reactant concentrations and film thickness, have been varied through experimental parameters that can be more conveniently set. Although the intrinsic kinetics is fast, it is still possible that intrinsic chemical kinetics play a significant role in the overall mechanism, since the films formed are very thin. Data obtained under such conditions, with an excess of the organic-side monomer (hexamethylene-1,6-diisocyanate, HMDI), show a first order dependence of monomer consumption rate on the aqueous-side monomer (hexamethylene-1,6-diamine, HMDA). The effect of solvent on the observed rates shows some interesting characteristics, counterintuitive if a correlation is sought with the partition coefficient of HMDA into the solvent. It is shown that rates correlate better with solvent polarity. The polymer formed is of low molecular weight in general. The molecular weight shows a dependence on the mole ratio of monomers, with a ratio (HMDI/HMDA) slightly in excess of 1 being the most conducive to molecular weight development. The polymer is of semicrystalline structure, with the crystallinity determined by the conditions of reaction. These findings are of interest to applications such as controlled release and membrane separations, in which permeation rate through the membrane is of importance.© Elsevie

Interfacial polycondensation—modeling of kinetics and film properties

Interfacial polycondensation (IP) is an important technique used in the encapsulation of a variety of active ingredients and synthesis of thin film composite membranes. The present work seeks to advance our understanding of the mechanisms underlying the reaction, phase separation and film formation in this process, and hence, of how the film properties are influenced by preparation conditions. The model presented here incorporates all the essential physicochemical processes at a fundamental level through simple phenomenologies: ionic equilibria in the aqueous phase, resistances due to external mass transfer, diffusion through polymer film, interfacial reaction, thermodynamics of phase separation, and formation of a coherent film. The model has been tested against the data previously communicated [S.J. Wagh, Studies in interfacial polycondensation. Ph.D. Thesis. IIT Bombay, 2004; S.J. Wagh, S.S. Dhumal, A.K. Suresh, An experimental study of polyurea membrane formation by interfacial polycondensation, Journal of Membrane Science, submitted for publication] on polyurea microcapsules. The influence of the model parameters and preparation conditions, on the properties of the polymer and film and their development during reaction, have been studied. The study provides important insights into the process and should help in designing synthesis methodologies to suit the application.© Elsevie