Polymer solutions turbidity

One factor which affects the extent of polymer-solvent interactions is relative molar mass of the solute. Therefore the point at which a molecule just ceases to be soluble varies with relative molar mass, which means that careful variation of the quality of the solvent can be used to fractionate a polymer into fairly narrow bands of polymer molar masses. Typically, to carry out fractionation, the quality of the solvent is reduced by adding non-solvent to a dilute solution of polymer until very slight turbidity develops. The precipitated phase is allowed to settle before removing the supernatant, after which a further small amount of non-solvent is added to the polymer solution. Turbidity develops once again, and again the precipitated phase is allowed to settle before removal of the supernatant. Using the technique polymers can be separated, albeit slowly, into fractions of fairly narrow relative molar mass. 

Other experiments. Precipitation of PPE from toluene with methanol yielded the same kind of agglomerates as with CO2. A major difference between the experiments was that only a few drops of methanol had to be added to turn the clear polymer solution turbid white, whereas when CO2 is used the liquid level first rises considerably. The way in which the solubility declines is therefore different for methanol and CO2. hi both cases the amount of crystalline material was approximately 25 %, and does not change significantly when methanol is used instead of CO2. 

If turbidity itself is the quantity which is used to characterize a polymer solution, then Eqs. (10.53) and (10.54) are used as derived above ... 

The determination of the turbidity of polymer solutions will be discussed briefly in the following pages, and the essentials involved in the deduction of molecular weights and molecular dimensions will... 

That the turbidities of dilute polymer solutions agree with the corresponding theoretical relationship, given by Eq. (VII-37), was shown in Figs. 47 and 48. 

Process in which a precipitant is added incrementally to a highly dilute polymer solution and the intensity of light scattered by, or the turbidity due to, the finely dispersed particles of the polymer-rich phase is measured as a function of the amount of precipitant added. 

The intensity of scattered light or turbidity (t) is proportional to the square of the difference between the index of refraction (n) of the polymer solution and of the solvent ( o), to the molecular weight of the polymer (M ), and to the inverse fourth power of the wavelength of light used (A). Thus ... 

Rayleigh X Rs, particle size Applicable for (R/X) < 1/20 extension of the Rayleigh equation to solutions allows the measurement of osmotic pressure, molecular weight, and turbidity of colloidal or polymer solutions see Section 5.3... 

Window autoclaves have been built to allow one to determine visually the appearance of turbidity in a homogeneous polymer solution, indicating the onset of phase separation, and to observe the resulting number of liquid and gaseous phases and optionally to determine their composition. Lentz [40] developed a mechanically driven stirrer fitted in a 2000-bar window autoclave. This apparatus allowed fast homogenation of concentrated polymer solutions and blends having low viscosity. 

CPs of copolymers can be determined using the optical transmittance method (Chung et al., 1999), and even using a visual observation method (Pandya et al., 1993). Optical transmittance of aqueous polymer solutions at various temperatures can be measured using a UV/visible spectrometer. Sample and reference cells are thermostated with a circular water jacket, and the sample is monitored for the onset of turbidity. For the visual observation method, copolymer solutions at different concentrations are measured by heating them in glass tubes immersed in a well-stirred heating bath. The temperature of Lrst appearance of turbidity is taken as the CP. For both methods, the samples must be well-stirred. 

Recent theoretical developments have related the turbidity of polymer solutions to the size and shape of the dissolved molecules. The light scattered by a solution in excess of that scattered by pure solvent (r) can be shown12 to be related to the molecular weight by the continued series equation... 

The weight average molecular weight of the sample is determined directly by measurement of the turbidity of the polymer solution. 

A dilute polymer solution has a turbidity of 0.0100 cm . Assuming that the solute molecules are small compared to the wavelength of the incident light, calculate the ratio of the scattered to incident light intensities at 90° angle to the incident beam and 20 cm from 2 ml of solution. Assume that all the solution is irradiated. 

The following experiment is conducted to And out the pH at which the polymer will precipitate, and induce drug release from the liposomes, once anchored on the membrane. The phase transition pH is determined at 37°C by measuring the turbidity of the polymer solutions at different pHs. As the polymer undergoes coil-to-globule transition and aggregates, the turbidity of the polymer solution increases. 

The thermodynamic parameters 1/) and K introduced above, pertaining to polymer-solvent interactions in dilute solutions, may be determined from thermodynamic studies of dilute solutions of the polymer, e.g., from osmotic pressure or turbidity measurements at different temperatures. These parameters may also be determined, at least in principle, from viscosity measurements on polymer solutions (see Frictional Propcitics of Polymers). The parameter ij, which is a measure of the entropy of mixing, appears to be related to the spatial or geometrical character of the solvent. For those solvents having cyclic structures, which are relatively compact and symmetrical (e.g., benzene, toluene, and cyclohexane), xp has relatively higher values than for the less symmetrical acyclic solvents capable of assuming a number of different configurations. Cyclic solvents are thus more favorable... 

This expression provides us with a way to determine molecular weights of polymers from Light scattering of polymer solutions as shown below. Moreover, a comparison of Eqs. (4.74) and (4.63) shows that the relationship between the turbidity and the Rayleigh ratio is... 

The phase separation experiments were conducted at 30.00°C by titrating the starting polymer solutions with base until turbid. The value of a at incipient turbidity, aT, was sharp and reproducible. Back-titration of a turbid solution with hydrochloric acid caused the solution to become clear exactly at aTi indicating the reversibility of the phase separation effect. 

Solution Rheology. Polymers were hydrated in distilled, filtered water and were agitated gently until dissolution was complete. To prepare polymer solutions containing salt, concentrated sodium chloride solutions were added to polymer previously dissolved in distilled water. An alternative procedure was used to evaluate the effect of sahnity on solution rheology. Solid sodium chloride was slowly added to various concentrations of polymer in solution. To ensure complete dissolution, the solutions were allowed to equilibrate for approximately 24 h before viscometric measurements were obtained. Turbidity measurements were made with a turbidimeter (Hach) on 1500-ppm solutions in 3% NaCl and 0.3% CaCU brine, which we called 3.3% brine. 

Table III shows the results obtained with two crystalline preparations of the polysaccharide. Single crystals and a "quench precipitate" (prepared by quickly chilling a polymer solution in ice to precipitate the polysaccharide with minimal crystallinity) were compared. The extent of degradation of lamellar crystals is highly temperature dependent, with the total fraction of polymer digested at any temperature being finite and quite reproducible from one batch of crystals to another. Incubation up to 20 hours causes no additional decrease in turbidity nor do the crystals inactivate the enzyme. The plateau in absorbance is evidence of a "two region" model of crystal morphology, i.e., one with both accessible and inaccessible zones. The extent of digestion at 20°C of the lamellar crystalline material is to be compared with that of the "quench precipitate" form at the same temperature.

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