Polymeric mixtures, transport propertie

Given the vast number of possible matrix-reinforcement combinations in composites and the relative inability of current theories to describe the viscosity of even the most compositionally simple suspensions and solutions, it is fruitless to attempt to describe the momentum transport properties of composite precursors in a general manner. There are, however, two topics that can be addressed here in an introductory fashion flow properties of matrix/reinforcement mixtures and flow of matrix precursor materials through the reinforcement. In both cases, we will concentrate on the flow of molten polymeric materials or precursors, since the vast majority of high-performance composites are polymer-based. Fnrthermore, the principles here are general, and they apply to the flnid-based processing of most metal-, ceramic-, and polymer-matrix composites. 

A supercritical fluid (SCF) can be defined as a substance or mixture which has both a temperature and pressure that exceed its critical temperature (Tc) and critical pressure (Pc), and a density at or above its critical density (1) (Fig. 1). Near this critical point, the density, transport properties (such as diffusivity and viscosity), and other physical properties (such as dielectric constant and solvent strength) can be varied in a continuum from gas-like to liquid-like, with relatively small changes in temperature or pressure. The tunability of SCFs makes them very attractive as solvents for polymerization reactions. 

Obviously, temperature trajectories are not anployed in continuous reactors. Instead, the various vessels in a series of polymerization reactors may operate at different tanperatures. Then, the polymerizing mixture will see diffeoit tanpera-tures as it passes from one vessel to the next Likewise, monomer trajectories are replaced in continuous systans with intermediate injection of a more reactive monomer between polymerization vessels. This strategy can be exploited to adjust the CCD. Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If online MWD is available, initiator flow rate or reactor temperature can be manipulated to adjust MWD. Polymer quality and end-use property control are hampered as in batch polymerization case by infrequenL ofif-Une measurements, hi addition, online measurements may be severely delayed due to the constraints of the process flow sheet. For example, even if online viscometry (via melt index) is available every 1-5 min, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the develops and the viscometer where the is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data are old. 

Gapillary column IGC was used to determine the diffiisivity, solubility and transport properties of various test solutes in polymers and polymeric mixtures. These... 

Chen YD, Yang RT (1994) Preparation of carbon molecular sieve membrane and diffusion of binary mixtures in the membrane. Ind Eng Chem Res 33 (12) 3146-3153 Moaddeb M, Koros WJ (1997) Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles. J Membr Sci 125 (1) 143-163 Ash R, Barrer RM, Lowson RT (1973) Transport of single gases and of binary gas mixtures in a microporous carbon membrane. J Chem Soc Faraday Trans I 69 (12) 2166-2178 Bird AJ, Trimm DL (1983) Carbon molecular sieves used in gas separation membranes. Carbon 21 (3) 177-163... 

This work offers a contribution to the understanding of some fundamental aspects of sorption and diffusion in glassy polymers. The research focuses on an extensive experimental study of sorption and mass transport in a specific polymeric matrix. A high free volume polymer, (poly l-trimethylsilyl-l-propyne) [PTMSP], has been used here in order to emphasise aspects of sorption and transport which are peculiar to polymer/penetrant mixtures below the glass transition temperature. The discussion of the experimental data presented in this work permits a clarification of concepts which are of general validity for the interpretation of thermodynamic and mass transport properties in glassy systems. 

The experimental results are briefly discussed in terms of thermodynamic and mass transport properties in the glassy polymer mixture. The aim of the discussion is to highlight peculiarities of solubility and difiusivity in polymeric systems below the glass transition temperature and to consider possible interpretations. The focus is on the effect of swelling on the thermodynamic and transport properties in glasses. Indeed, it is well known that, contrary to the case of rubbery systems, the solute partial specific... 

The separation of gas mixtures by polymeric membranes has become a commercially important methodology for a number of different systems (1). Several recent review articles have discussed the interaction between polymer structure and gas permeability properties (2,3). The quantification of the effect of polymer structure on gas transport properties recently has been reported (4,5) and it is now possible to optimize gas transport properties for well defined polymeric materials. For those materials which do not have a well defined data base it is necessary to prepare and measure the gas transport properties. The polyamide-imides (PAI) are a class of polymeric materials which do not have an extensive data base for gas transport properties (6,7). Work by Yamazaki and coworkers (8) demonstrated that PAI materials could be prepared easily and in a manner whereby the amide bond could be prepared from a phosphite activated carboxylic acid and an aromatic amine. Yang and CO workers (9-11) have shown that novel dicarboxyl ic acids could be prepared from trimellitic acid anhydride (TMA) and aromatic diamines and that these dicarboxylic acids could be coupled with a second diamine to form regiospecific PAI materials. Our focus was to examine the effects of a phenylene diamine and its alkylated analogs on the gas transport properties of regiospecific PAI materials and to identify those structures which maximized both permeability and selectivity. 

An adequate understanding of the kinetics, thermodynamics and transport properties of the polymerizing mixture should allow a calculation of the yield, molecular weight distribution and molecular structure of pol3 ethylene produced in batch and simple t3q)es of continuous reactors. Except in very special cases, this aim has not been realized, in spite of the voluminous technical literature dealing with this subject. One is led to inquire into the reasons for this. 

Lokaj and Bila (2003) studied the PV of EtOH-water mixtures through styrene-substituted V-phenylmaleimide copolymer membranes. The object was to estimate the effect of incorporated maleimide units on the PV properties on the polymeric membranes. For this, a number of copolymers of styrene with substituted V-phenylmaleimide were synthesized and their solutions in chloroform were used in the casting of homogeneous membranes. These membranes were characterized by the separation factor related to transported water and by the flux of the permeate. In contrast to the membranes made from copolymers of styrene with V-phenyhnaleimide, the separation factor of the membranes containing substituted V-phenyhnaleimide increased with increasing amount of EtOH in the feed solution. 

In fact, the copolymers (143), from the ROM co-polymerization of mixtures of the endo-exo norbornenemethoxy-cyclotriphosphazenes (142a) and (142b) (both prepared from 139), were successively reacted with KOBu, aqueous HCl, and LiOH to transform the 4-(propylcarboxalato)phenoxy side groups (R ) first into the -COOH and finally into the COO Li to synthesize novel lithium-ion conductive polymers as prospective membranes for Lithium-Seawater Batteries. The dependence of ion transport and hydrophobic properties on the polymer composition were discussed. ... 

From a reactor engineering point of view, polymerizations form a special case. Several reactions take place at the same time, often at very low molecular reactant concentrations. Intrinsic reaction rates are generally high and there are usually transport limitations. The presence of the produced polymer generally influences the physical properties of the mixture considerably, and heat of reaction has to be removed under unfavourable conditions. In addition, the requirements for product quality are usually very high. Though it is common practice to develop reactors for these processes in a purely empirical way, it is nevertheless possible to apply reactor engineering principles here with some success, even if only qualitative predictions can be made. 

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