Drying polycondensation

The polymer is then dried thoroughly and stored for subsequent processing. Whenever a polyester is made by melt polycondensation, a small amount of cycHc oligomer is formed which is in equiHbrium with the polymer. This can be extracted with solvents from soHd polymer but when the... 

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

Polycondensation At room temperature, 0.4% mass of Sn(II) chloride dihydrate (SnCl2-2H20) and 0.4% mass of p-toluenesulfonic acid monohydrate (p-TSA) are introduced into the mixture. The mixture is heated to 180°C under mechanical stirring. The pressure is reduced stepwise to reach 13 mbar, and file reaction is continued for 20 h. The reaction system becomes gradually viscous, and a small amount of L-lactide is formed and refluxed through the reflux condenser. At file end of the reaction, the flask is cooled down, file product is dissolved in chloroform and subsequently precipitated into diethyl ether. The resulting white fibrous solids are filtered and dried under vacuum (average yield 67%). 

Interfacial or solution polycondensation, with or without stirring, was the general procedure utilized for the preparation of the polyamides and polyureas.l a Details are given in Table I. An important point to be noted is that, in the unstirred interfacial condensation polymerization of 1 with sebacoyl chloride or tere-phthaloyl chloride in the organic phase and triethylamine as the proton acceptor, immediate film formation took place at the interface. The polyamide films were removed after 1 h, dried, and utilized for taking electron micrographs. 

Conditions to be met in oven drying enamels depend also on the composition of the binder. Paint systems containing melamine-formaldehyde or urea-formaldehyde resins, for instance, harden by polycondensation with other resins, such as epoxy resins, short-oil alkyd or acrylic resins at elevated temperatures. Baking is carried out at temperatures between 100 and almost 200°C and may last from a few minutes to more than an horn. A general trend towards energy conservation has shifted public attention towards binders which require low baking temperatures. 

Tang et al. used columns packed with a slurry of beads suspended in supercritical C02. This packed column was filled with a dilute sol solution prepared by hydrolysis and polycondensation of tetramethoxysilane and ethyltri-methoxysilane precursors. The column was dried using supercritical C02 and heated first to 120 °C for 5 h followed by another 5 h at a temperature of 250 °C [108-110]. Column efficiencies of 127,000 and 410,000 plates/m were reported... 

The occurrence of fluorescence is often related to inappropriate processing conditions in molten-state and solid-state polycondensation (SSP) (presence of oxygen, high temperature, long retention time, etc.), as well as the later drying of chips where prolonged residence times can occur. 

The condensation polymerization process, employed recently by Skourlis et al. (1993) and Duvis et al. (1993), involves immersion of carbon fibers in a solution containing hexamethylenediamine and sodium carbonate. Dried carbon fibers are then immersed in a dipolychloride solution in carbon tetrachloride where the interfacial polycondensation reaction takes place. The result is that a thin layer of polyamide (nylon 6,6) coating is deposited on the continuous carbon fiber, whose thickness is controlled though by varying the diamine concentration. 

The viscosity of the solution gradually increases and so does the temperature in the flask. In order to maintain a quick rate of distillation (and therefore polycondensation), each time the internal temperature reaches 130 °C about 25 ml of pure toluene are added. After about 25 h the flask is cooled to room temperature and the solution is added drop-wise to a tenfold amount of methanol the polymer is filtered off and dried to constant weight in vacuum at 40 C. Yield about 90%. 

The increase of molecular weight during polycondensation can be followed by taking samples from time to time (initially at short intervals) and determining the limiting viscosity numbers of the precipitated and dried samples the carboxyl and hydroxy end groups can also be quantitatively determined (see Example 4-1). 

The solution polycondensation usually works at room temperature in approximately 10% solution in toluene, methylene chloride, N-methylpyrrolidone (NMP), or dry tetrahydrofuran (THE). Tertiary amines (triethylamine) or dispersed calcium hydroxide are added as acid acceptor. This procedure has the following advantages The polycondensation is carried out at low temperature (0-40 °C) it is nevertheless very fast, the reaction usually being over in a few min-... 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

Attention was then turned to aromatic heterocyclic ladder polymers other than BBL. Due to the complexities in their synthesis, ladder polymers were not reported extensively in the literature at that time. A sample of polyfluoflavine (I) having an inherent viscosity in methanesulfonic acid of 2.5 was obtained from professor C. S. Marvel at the University of Arizona. The ladder polymer was prepared [5] from the A-B polycondensation of 2,3-dihydroxy-6,7-diamino-quinoxaline hydrochloride in PPA. Transparent blue sheets were observed on precipitation of this polymer from methanesulfonic acid, and when collected and dried, it formed gold films much like the color of the BBL films. It was felt at that time that all ladder polymers of sufficient molecular weight would form precipitated films. 

Kaneda et al. synthesized [61] a series of high molecular weight extended chain copolyimides (XV) by the reaction of PMDA and 3,3, 4,4 -biphenyltetra-carboxylic dianhydride (PPDA) with 3,3 -dimethyl-4,4 -diaminobiphenyl. Solvents used for the one-step synthesis to the fully cyclized imide structure were phenol, p-chlorophenol, m-cresol, p-cresol and 2,4-dicholorophenol. The polycondensations were performed at 180°C for 2h with a monomer concentration of 6% by weight and p-hydroxybenzoic acid used as a catalytic accelerator. A maximum of 50 mol % of PMDA could be used before the copolymer precipitated from solution. Reconstituted copolymers as isotropic dopes (8-10% by weight) in p-chlorophenol were dry-jet wet spun between 80 and 100 °C [62]. 

The long-chain a-amino acid esters (40), (41), and (42) form bilayers on sonication in water under acidic conditions. Liposomes prepared from (40) and (42) precipitate if the aqueous medium is neutralized by titration with NaOH. Only liposomes made from (41) are stable even in basic solutions, as shown by electron microscopy52). Polypeptide formation in oriented spherical vesicles was confirmed by FT-IR spectroscopy. The liposomal solution of (41) was freeze-dried and the spectrum obtained from the residue was comparable with one of the polycondensed monolayers. The formation of polypeptide vesicles is illustrated in Scheme 4. 

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