Poly critical concentration

Concentration of the polymeric adhesive. In general, the more concentrated the poly-merie adhesive, the lower its bioadhesive strength. The coiled molecules become solvent poor in a concentrated solution which, in turn, reduces the available chain length for interpenetration into the mueus layer. Therefore, a critical concentration of the polymeric adhesive is required for optimum bioadhesion [37]. 

Figure 5 shows the dependence of the swelling ratio, X, on the acetone content, a, in a water-acetone mixtures of ionized poly(acrylamide) networks the charges onto PAAm chains were introduced by the copolymerization of acrylamide with a low amount of sodium methacrylate [11] (the molar fraction xMNa = 0, 0.004, 0.008, 0.012, 0.016 and 0.024 for series A, B, C, D, E and F, respectively). While in series A and B the dependence of X on composition of the mixtures is continuous, in the other series (C-F) with xMNa > 0.008 a collapse takes place. The extent of the transition and the critical concentration of the acetone at which the collapse appears, ac, increase with increasing xMNa. 

The effectiveness of a polymeric flow enhancer is influenced decisively by the state of solution and the solvation characteristics. In the case of polyelectrolytes, in particular, the chemical nature plays a significant role, e.g., it was found for poly(acrylamide)-coacrylate that a significant increase in effectiveness arises with the increasing number of ionic groups. It is therefore necessary to consider, for example, such factors as the question of critical concentration, polymer-polymer and polymer-solvent interactions, the thermodynamic quality of the solvent, the proportion of ionic molecular groups and their behavior in the presence of lower-molecular-weight charge carriers. 

These critical values differ from solvent to solvent. With the solvents most fully investigated, sulphuric acid for poly-(p-phenylene terephthalamide) and dimethyl acetamide/lithium chloride for poly (p-benzamide), there are also critical values of solvent composition the sulphuric acid must exceed a critical strength, the lithium chloride in the dimethyl acetamide must exceed a critical concentration. The critical values are, of course, interdependent rather than absolute. Diagrams that display some of the critical values for the two systems cited have been published in patents10,1 l Figures 3 and 4 illustrate the type of information available. 

Recent studies in our laboratory (3) using polystyrene latex dispersions stabilised by the above "comb" dispersing agent have shown that polymers such as poly(ethylene oxide) induce wealc flocculation above a certain critical concentration of free polymer which was dependent on the molecular weight of the chain. 

As is known, the polypeptide a-helix molecules are rod-shaped if the helical internal structures are smeared out. Therefore, we may expect a phase separation in their solutions also. Indeed, Robinson (27) found in 1956 a phase separation in several solutions of the a-helix, poly-y-benzyl-L-glutamate, in which the second phase, separated out as small droplets, showed an optical birefringence. ITie critical concentration is of course a function of the molecular length. 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

The melt behavior of sulfonated polystyrene ionomers was studied by Lundberg and coworkers41. As was shown in the case of a sulfonated elastomer30, sulfonation of polystyrene leads to an increase in the melt viscosity (measured at 250 °C) of the SPS upon neutralization, indicating increased association of the sodium poly-(salt). A sudden jump of the melt viscosity occurs at the point of complete neutralization, where a critical concentration of Na polystyrenesulfonate is reached, apparently resulting in a sharp phase separation between the ionic and hydrophobic domains (Figure 5). 

The concentration dependence of the Brookfield viscosity (Figures 9 and 10) indicates a rapid drop in viscosity upon dilution below a critical concentration. Bagley (7) attributed such rheological behavior to a structure of swollen, deformable gel particles closely packed in intimate contact. Davidson (8) later attributed the thickening efficiency of a cross-linked poly(acrylic acid) to the dispersed rather than the continuous phase. In general, pseudoplastic and viscoplastic rheology is characteristic of dispersions with low... 

Heller and Pugh (1954) used a slight modification of Zsigmondy s gold number experiment to measure the effect that four samples of poly-(oxyethylene) had on the stabUity of a Faraday gold sol when potassium chloride was added. Their results are shown in Table 2.2. Listed therein are the values of the critical concentration of potassium chloride (cJ q), required to induce the red- blue transformation in the gold sol in the presence of the polymer at a given concentration (1 -0 g dm ). This concentration of polymer... 

As mentioned earlier, poly(NIPAAm-co-AAc) where AAc content is small (2-5 mol%) forms loose hydrogels above the LOST. This makes the system an attractive candidate for tissue engineering application as an extracellular matrix (ECM). Above a critical concentration, aqueous polymer solutions of W-isopropylacrylamide copolymers with small amounts of acrylic acid, synthesized in benzene by free radical polymerization, exhibited four distinct phases as the temperature increased a clear solution, an opaque solution, a gel, and a shrunken gel. The transition between the opaque solution phase and the gel phase was in the range of 30-34°C and was... 

Some hydrogels are sensitive to specific ions and molecules. For instance, PNIPAAm and poly (diallyldimethylammonium chloride) hydrogels are sensitive to a critical concentration of sodium chloride in aqueous solution. The LCST of the hydrogel is lowered by inereasing the chloride concentration, although the mechanism of this ion-sensitivity is unknown (Park and Hoffman, 1993). 

A typical example is offered by Pluronics, i.e. triblock copolymers of PEG and poly(propylene glycol) (PPG) Pluronic suspensions, above a critical concentration, are typically liquid at room temperature, but undergo a very fast... 

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