Solvent imbibition

A very valuable indication of the porous nature of a polymer support can be deduced from observations of solvent imbibition or uptake. Furthermore such measurements can be reasonably quantitative. It is, however, important to understand the possible processes that can occur when a polymer is contacted with a solvent otherwise the data obtained can be misleading or misinterpreted. 

Increasingly there is a need to evaluate simple physical properties such as solvent imbibition in a more rapid manner for high-throughput materials studies, or for real-time evaluation. Recently this issue has been addressed and a very elegant solution developed [75] based on earher work [76, 77]. In this a polypropylene disposable syringe containing a known mass of polymer is suspended from a microbalance and the balance reading set to zero. A reservoir of the solvent to be inves- 

Typical adsorption and desorption isotherms for a porous soHd are indicated in Fig. 1.13. Following the adsorption branch, the steep initial rise (A) is caused by adsorption in the most energetic regions of the soHd (single layer adsorption). Thereafter, in the flat region (B), gas molecules adsorb on sites already occupied by other molecules (multilayer adsorption). 

The abrupt rise in the middle (C) is caused by starting bulk condensation of the adsorbing gas in small pores. On increasing the pressure, bulk condensation occurs in even larger pores, and if all pores are filled, finally the isotherm becomes horizontal again (D). Adsorption and desorption branches retrace each other only for nonporous materials or special cases with pores of certain conical geometry. 

Usually, a hysteresis loop appears because of different adsorption and desorption mechanisms and network or connectivity effects. 

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Solvent imbibition would result in swelling of the coal particles with its consequent effect upon the porous structure. This consideration does not seem important in the present experiments since little swelling of the coal has been reported (4). It is known that solvents which swell the matrix can release... 

It is much more time efficient to design the experiment to allow the use of 1-D STRAFI profiles to give the required information. A number of such studies of both pure and composite rigid polymers have been reported, virtually all as precursors to solvent imbibition and/or diffusion. However, a very few have been intended to obtain information about the relationships between the molecular... 

Commercial lecithin is insoluble but infinitely dispersible in water. Treatment with water dissolves small amounts of its decomposition products and adsorbed or coacervated substances, eg, carbohydrates and salts, especially in the presence of ethanol. However, a small percentage of water dissolves or disperses in melted lecithin to form an imbibition. Lecithin forms imbibitions or absorbates with other solvents, eg, alcohols, glycols, esters, ketones, ethers, solutions of almost any organic and inorganic substance, and acetone. It is remarkable that the classic precipitant for phosphoHpids, eg, acetone, dissolves in melted lecithin readily to form a thin, uniform imbibition. Imbibition often is used to bring a reactant in intimate contact with lecithin in the preparation of lecithin derivatives. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Fig. 10. Schematic of casting machine used to make microporous membranes by watervapor imbibition. A casting solution is deposited as a thin film on a moving stainless steel belt. The film passes through a series of humid and dry chambers, where the solvent evaporates from the solution, and water vapor is absorbed from the air. This precipitates the polymer, forming a microporous membrane that is taken up on a collection roU (25).

Set-up for Ethylene Glycol Lignin Production. A process development unit (PDU), previously described by Chornet and co-workers (11), was used for the experiments. A typical preparation consists of initially mixing 1-1.2 kg of wood meal with 10 1 of ethylene glycol. The mixture is allowed to stand overnight for imbibition to take place. To enhance solvent to substrate penetration, the slurry is homogenized at 200°C in the pretreatment section of the PDU. It is then pumped through the treatment section which consists of a tubular reactor at 220°C. The product slurry is collected in a receiver. The detailed procedure and choice of conditions above have been published elsewhere (11,12). 

There is a relationship between yield of the extract and the saturation sorption or imbibition of solvent that is independent of the rank coal or the particular amine solvent (Dryden, 1951). An adsorption isotherm for ethylenediamine vapor on an 82% carbon coal exhibited three main features (1) chemisorption up to 3 to 6% adsorbed, (2) a fairly normal sorption isotherm from the completion of chemisorption up to a relative pressure of at least 0.8, and (3) a steeply rising indefinite region near saturation that corresponded to observable dissolution of the coal. 

Soybean lecithin is soluble in aliphatic and aromatic hydrocarbon solvents, partially soluble in ethyl alcohol (principally the inositol fraction), and practically insoluble in acetone (less than 0.003% weight/volume at 5°C) and in water (73). When mixed with water, soybean lecithin hydrates to a thick emulsion that can be thinned with water to almost any desired dilution. Acetone does dissolve readily in lecithin and will form a thin, uniform imbibition as long as the quantity of acetone is insufficient to precipitate the phospholipids. Lecithin is soluble in... 

Imbibition - consisting of spreading off the liquid solvent into the colloidal cluster... 

The left-hand member in Eqs. (3.132) and (3.133) represents the lowering of the chemical potential owing to mixing of polymer and solvent, while the right-hand side represents the increase of the chemical potential due to the elastic reaction of the network. Ifx, Fi, and Me are known, 02m is uniquely determined from Eq. (3.132). However, a more direct method of expressing the degree of swelling is as the maximum imbibition Qm, defined as the volume of solvent imbibed per unit volume of polymer. Qm is related to 02m by... 

FIGURE 11.4 Variation of (a) imbibition of the solvent and (b) degree of extraction with carbon content (From Dryden, I.G.C., Fuel, 30, 39, 1951.)... 

Imbibition, consisting in spreading off the liquid solvent into the colloidal cluster, and reducing the cohesive forces between the coUoidal beads. This process corresponds to the wetting of a dry, porous solid by the liquid. 

Gels may expel or take up solvent. These phenomena, known as syneresis and imbibition, are briefly described in Section 12.7. 

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