Polymer concentrations

The release kinetics of proteins may also be increased or decreased by varying the concentration of polymer in the formulation. In general, as the concentration of polymer increases in a formulation, the release of any protein is retarded as the density of the polymeric matrix increases. This effect is more prominent in the initial release of dmg and is exemplified in Fig. 2 which shows the cumulative release profiles for formulations of increasing PLA concentration that were incorporated with follicle-stimulating hormone (FSH). In vitro analysis was done as discussed in Section 2.1, 

For porous membrane.s obtained by instantaneous demixing, the separation properties are mainly determined by the choice of solvent/nonsolvent. Indeed this type of structure can almost be considered to be independent of the choice of polymer. Table IE -9 gives a list of polymers from which ultrafiltration membranes have been made using DMAc or DMF as the solvent and water as the nonsolvent. The polymer concentration varied from 10-20% and immersion precipitation occurred at room temperature. 

Instantaneous demixing occurs in both cases (confirmed experimentally by light transmission measurements, see figure III - 44). but with a higher initial pol  

System water/DMAc/polysulfone AP = 3 bar, T = 20 C. t very low in terms of uitrafiltration fluxes 

Fornonporous membranes (obtained by using poorly miscible so)venl/nonsol vent pairs), the influence of the polymer concentration is also very dear. As the delay time for liquid-liquid demixing is increased the distance from the fllm/bath interface in the film also increases, so that the first formed nuclei of the dilute phase are formed at a greater distance in the film from the fllm/bath interface. Thus the thickness of the dense top layer increases with increasing polymer concentration, as is clearly shown in figure III - 47 for the polysulfone/DMAc/i-propanol system. 

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The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

The adliesion and fiision mechanisms between bilayers have also been studied with the SEA [M, 100]. Kuhl et al [17] found that solutions of short-chained polymers (PEG) could produce a short-range depletion attraction between lipid bilayers, which clearly depends on the polymer concentration (fignre Bl.20.1 It. This depletion attraction was found to mduce a membrane fusion widiin 10 minutes that was observed, in real-time, using PECO fringes. There has been considerable progress in the preparation of fluid membranes to mimic natural conditions in the SEA [ ], which promises even more exciting discoveries in biologically relevant areas. 

In dilute solutions, tire polymer chains are isolated from one anotlier and only interact during brief encounters. Witli increasing polymer concentration, a point is reached where tire chains start to overlap, tliis point referred to as... 

In summary, we see now how tire change from tire expanded chains in dilute solutions to tire ideal chains in a melt is accomplished. Witli increasing polymer concentration, tire chain overlap increases and tire lengtli scale over... 

Additives, whether hydrophobic solutes, other surfactants or polymers, tend to nucleate micelles at concentrations lower than in the absence of additive. Due to this nucleating effect of polymers on micellization there is often a measurable erne, usually called a critical aggregation concentration or cac, below the regular erne observed in the absence of added polymer. This cac is usually independent of polymer concentration. The size of these aggregates is usually smaller than that of free micelles, and this size tends to be small even in the presence of added salt (conditions where free micelles tend to grow in size). 

At equilibrium, in order to achieve equality of chemical potentials, not only tire colloid but also tire polymer concentrations in tire different phases are different. We focus here on a theory tliat allows for tliis polymer partitioning [99]. Predictions for two polymer/colloid size ratios are shown in figure C2.6.10. A liquid phase is predicted to occur only when tire range of attractions is not too small compared to tire particle size, 5/a > 0.3. Under tliese conditions a phase behaviour is obtained tliat is similar to tliat of simple liquids, such as argon. Because of tire polymer partitioning, however, tliere is a tliree-phase triangle (ratlier tlian a triple point). For smaller polymer (narrower attractions), tire gas-liquid transition becomes metastable witli respect to tire fluid-crystal transition. These predictions were confinned experimentally [100]. The phase boundaries were predicted semi-quantitatively. 

In practice, colloidal systems do not always reach tlie predicted equilibrium state, which is observed here for tlie case of narrow attractions. On increasing tlie polymer concentration, a fluid-crystal phase separation may be induced, but at higher concentration crystallization is arrested and amorjihous gels have been found to fonn instead [101, 102]. Close to the phase boundary, transient gels were observed, in which phase separation proceeded after a lag time. 

RT In a3 + nV3 for the side containing the polymer. If the polymer concentration is low, the term nVa is insignificant compared to the terms arising from the electrolyte itself therefore... 

This relationship with a = 1 was first proposed by Staudinger, but in this more general form it is known as the Mark-Houwink equation. The constants k and a are called the Mark-Houwink coefficients for a system. The numerical values of these constants depend on both the nature of the polymer and the nature of the solvent, as well as the temperature. Extensive tabulations of k and a are available Table 9.2 shows a few examples. Note that the units of k are the same as those of [r ], and hence literature values of k can show the same diversity of units as C2, the polymer concentration. 

Hoffman Degradation. Polyacrylamide reacts with alkaline sodium hypochlorite [7681-52-9], NaOCl, or calcium hypochlorite [7778-54-3], Ca(OCl)2, to form a polymer with primary amine groups (58). Optimum conditions for the reaction include a slight molar excess of sodium hypochlorite, a large excess of sodium hydroxide, and low temperature (59). Cross-linking sometimes occurs if the polymer concentration is high. High temperatures can result in chain scission. 

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

Solutions of welan are very viscous and pseudoplastic, ie, shear results in a dramatic reduction in viscosity that immediately returns when shearing is stopped, even at low polymer concentrations (230). They maintain viscosity at elevated temperatures better than xanthan gum at 135°C the viscosity half-life of a 0.4% xanthan gum solution is essentially zero, whereas a welan gum solution has a viscosity half-life of 900 minutes (230). The addition of salt to welan solutions slightly reduces viscosity, but not significantly. It has excellent stabiUty and theological properties in seawater, brine, or 3% KCl solutions... 

High molecular weight polyacrylamides are used as viscosity builders in freshwater muds (53) or as bentonite extenders. The ionic nature of the polyacrylamide may range from nonionic to anionic (30% hydrolyzed) depending on the situation. Molecular weights ranging from >3 x 10 are used for this purpose. Polymer concentrations of 0.7—2.8 kg/m (0.25—1.0 Ib/bbl) are used depending on the appHcation. 

Aqueous poly(ethylene oxide) solutions of higher molecular weight (ca 10 ) become stringy at polymer concentrations less than 1 wt %. At concentrations of 20 wt %, solutions become nontacky elastic gels above this concentration, the solutions appear to be hard, tough, water-plasticized polymers. 

Tempera.ture Effect. Near the boiling point of water, the solubiUty—temperature relationship undergoes an abmpt inversion. Over a narrow temperature range, solutions become cloudy and the polymer precipitates the polymer caimot dissolve in water above this precipitation temperature. In Figure 4, this limit or cloud point is shown as a function of polymer concentration for poly(ethylene oxide) of 2 x 10 molecular weight. 

Macroscopically, the solvent and precipitant are no longer discontinuous at the polymer surface, but diffuse through it. The polymer film is a continuum with a surface rich in precipitant and poor in solvent. Microscopically, as the precipitant concentration increases, the polymer solution separates into two interspersed Hquid phases one rich in polymer and the other poor. The polymer concentration must be high enough to allow a continuous polymer-rich phase but not so high as to preclude a continuous polymer-poor phase. 

Fig. 5. Effect of surfactant type on surface resistivity, (a) Concentration of surface-active compound in low density polyethylene (LDPE) requked to achieve 10 Q/sq surface resistivity and (b) effect on surface resistivity of an acrylic polymer. Concentration of surface-active compound is 0.3%.

As the polymer molecules form and dissociate from the catalyst, they remain ia solution. The viscosity of the solution increases with increasing polymer concentration. The practical upper limit of solution viscosity is dictated by considerations of heat transfer, mass transfer, and fluid flow. At a mbber soflds concentration of 8—10%, a further increase in the solution viscosity becomes impractical, and the polymerisation is stopped hy killing the catalyst. This is usually done by vigorously stirring the solution with water. If this is not done quickly, the unkilled catalyst continues to react, leading to uncontrolled side reactions, resulting in an increase in Mooney viscosity called Mooney Jumping. 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

The. T-value is, however, rather dependent on the method of measurement and in the past different suppliers have used difficult solvents and different polymer concentrations. For this reason it is now becoming more common to quote the... 

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