Polycondensation Processes in Detail

Polymer precipitation is taken into account through the model of Kamide et al. [92] with a phenomenological rate of nucleation knuc,n (nil for n = 1, taken as independent of molecular weight for n 1) [93]. The saturation concentrations of polymer species are taken as the values of their concentrations in the lower branch of the spinodal curve for the liquid-liquid equilibrium with organic solvent. The earlier paper by Karode et al. [89] considered only spinodal decomposition. 

A coherent film is predicted to form when the sum of projected areas for all phase-separated polymer nuclei (assumed spherical) is equal to the interfacial area. Film thickness is predicted through the overall mass balance of precipitated polymer. 

Besides thermodynamic data on the liquid-liquid equilibria of polymer/solvent and hydrophilic monomer polymer/solvent, this model needs the rate of nucleation knuc, considered as an adjustable parameter, it also tries to fit the diffusion coefficient of monomer in the polymer film Daxa, and) as it postulates a constant width of the reaction zone, it has also to fit the kinetic parameters. 

---

There are two basic methods by which this reaction may be applied, referred to as interfacial polycondensation and solution polycondensation. Morgan [6] describes these processes in detail. 

Rheokinetic processes for polymerizing media have clearly defined specific properties. These are as follows a complex kinetic scheme, variation of molecular-mass distribution (MMD) of products with time, and viscosity growth during reaction. The first two peculiarities were investigated in detail in numerous works both in general form and in connection with different types of polymerization and polycondensation... 

The nonequilibrium polycondensation processes kinetics is the problem studied in detail every [48]. However one hasn t got any general description at... 

The aim of this chapter is to describe the possibilities offered by the miniemulsion process for performing chain polymerization, polyaddition, polycondensation, and modifications of polymers, and to outline the current trends in this field of research. Whilst the different polymerization types performed in miniemulsion are discussed in detail, descriptions of so-called secondary or artificial miniemulsions (i.e., miniemulsions with a preformed polymer) will not be included at this point... 

Sol-gel processes in general are known to involve both hydrolysis and polycondensation reactions leading up to the formation of network structures in liquid solution with varying degrees of cross-linking. This network connectivity depends on a number of reaction parameters, which are easy to describe phenomenologically but rather cumbersome to imderstand in detail. Perhaps the central parameter is the type of precursor used for the... 

Microencapsulation by a polycondensation process, which may be either normal dispersion polycondensation or interfacial polycondensation, is especially attractive for liquid active agents. The important advantage of this method is that in most cases very high active agent loadings can be obtained. These methods are well reviewed by Arshady [6], and are discussed in detail elsewhere in this book. 

Recently, DHBCs have been used as a good stabilizer for the in-situ formation of various metal nanocolloids and semiconductor nanocrystals such as Pd, Pt [328-330], Au [280,328-330], Ag [331], CdS [332], and lanthanum hydroxide [333]. PAA-fe-PAM and PAA-fc-PHEA were used as stabihzer for the formation of hairy needle-Uke colloidal lanthanum hydroxide through the complexation of lanthanum ions in water and subsequent micelhzation and reaction [333]. The polyacrylate blocks induced the formation of starshaped micelles stabilized by the PAM or PHEA blocks. The size of the sterically stabilized colloids was controlled by simply adjusting the polymer-to-metal ratio, a very easy and versatile synthesis strategy for stable colloids in aqueous environment [333]. The concept of induced micelhzation of anionic DHBCs by cations was also apphed in a systematic study of the direct synthesis of highly stable metal hydrous oxide colloids of AP+, La +, Ni +, Zn ", Ca ", or Cu " via hydrolysis and inorganic polycondensation in the micelle core [334,335]. The AP+ colloids were characterized in detail by TEM [336], and the intermediate species in the hydrolysis process by SANS, DLS, and cryo-TEM [337]. 

A thin polymer him can be formed in situ on the surface of a porous substrate membrane by an interfacial polycondensation process. A classical example of this method was described by Rozelle et al. in detail for the formation of the North Star NS-100 membrane 30]. According to their description, the polysulfone support films are placed, shiny surface upwards, into a 0.67% aqueous polyethyicnimine (PEI) solution in an aluminum tray. After 1 min, the PEI solution is poured off, and the tray held in a vertical position for 1 min to allow the excess solution to drain from the surface of the him. Then the wet surface is contacted with a 0.5% solution of toluene 2,4-diisocyanate (TDI) for 1 min at room temperature. After draining the excess TDI solution, the u y is placed horizontally at 11S C for 10 min. After the heat curing, the composite membrane is easily peeled off from the aluminum surface. 

Examples of catalysts are shown on the first line, whereas all the other compounds are coreactants including dicyandiamide, ureas, imidazoles, aliphatic polyamines, cycloaliphatic polyamides, and cycloaliphatic dicarboxylic acid anhydrides. As all the corresponding reaction mechanisms have been previously disclosed in detail," the following presentation is limited to the initial reaction steps leading to the active species involved in the polymerization or polycondensation processes. These primary attacks are enlightened in Fig. 12.6, which displays only one epoxy group reacting with catalysts or coreactants. 

---