Determination of relative molecular mass of water soluble polymer by viscosity method

Determination of relative molecular mass of water soluble polymer by viscosity method

Abstract: in the study of polymers, the relative molecular mass is an indispensable important data Because it not only reflects the size of polymer molecules, but also directly related to the physical properties of polymers.

In the study of polymers, the relative molecular mass is an important data. Because it not only reflects the size of polymer molecules, but also directly related to the physical properties of polymers. However, unlike ordinary inorganic or low molecular organic matter, the polymer is a mixture of different molecular weight, so the relative molecular mass is usually an average value. There are many methods to determine the relative molecular weight of polymer. Compared with other methods, the viscosity method is simple and convenient to operate.

The viscosity of polymer solution is particularly large, because its molecular chain length is much larger than that of the solvent molecule, and the solvent is added to make it more resistant to friction.

In the flow of viscous liquid, it is necessary to overcome the internal friction resistance and work. The size of the coefficient of resistance available viscosity of viscous liquid in the flow process (the viscosity) to express (kg M-1 S-1).

The viscosity of dilute polymer solution is a reflection of the size of the internal friction force. The pure solvent viscosity reflects the internal friction between the solvent molecules, denoted ETA 0, polymer solution viscosity is high internal friction between the molecules, polymer molecules and solvent molecules and internal friction between three and 0. At the same temperature, usually ETA > ETA 0, relative to the solvent, the viscosity of the solution increases the fraction called viscosity, denoted as SP, i.e.

1）The ratio of the viscosity of the solution and the pure solvent viscosity called relative viscosity, denoted R, i.e.

2）The viscosity behavior of ETA R is also reflected in the solution, and SP means has been deducted from the solvent internal friction between the molecules effect only reflects the polymer molecules and solvent molecules and polymer molecules between the internal friction effect.

The polymer solution viscosity eta SP tended to increase with increasing concentration of C. For comparison, the unit concentration shows the specific viscosity of SP / C is called viscosity, R / C and LN ETA is known as logarithmic viscosity. When the solution is diluted, the polymer molecules are far apart from each other, and their interaction can be ignored

3）[η] called intrinsic viscosity, it reflects the polymer molecules and solvent molecules in dilute solution of infinite internal friction between the size and shape, its value depends on the properties and polymer molecules of solvent. Because R and SP are both ETA ETA dimensionless quantity, so their unit is the reciprocal of concentration of C units.

In the dilute polymer solution is sufficient, SP / C and C / C and LN R and C respectively conform to the following formula:

4）

5）The two and the Huggins and Kramer are the constants in the. This is the two linear equations, the SP / C ETA on C or ln / C ETA r in C mapping, extrapolated to C=0 is the intercept []. Obviously, for the same polymer, the two linear equations are extrapolated to the same point, as shown in figure 1.

Figure 1: extrapolation method [η]        Figure 2: viscometer

The relationship between intrinsic viscosity and polymer molar mass of polymer solution, usually expressed with two parameters of Mark - Houwink equation:

6) In the formula  means the average molecular weight of viscosity, K is the proportionality constant, and alpha is an empirical parameter related to the molecular shape. The value of K and alpha is related to the temperature, the properties of the polymer and the solvent. The value of K is affected by temperature obviously, alpha value mainly depends on the polymer coils in a certain temperature, the degree of a stretch of solvent. K and alpha can be determined by some absolute experimental methods (such as membrane osmotic pressure, light scattering, etc.).

The capillary tube method was used to determine the viscosity, and it was obtained by measuring the time required for a certain volume of liquid to flow through a capillary with a certain length and radius. Ubbelohde viscometer used in the experiment as shown in figure 2. When the liquid passes through the capillary under gravity, it follows Poiseuille's law:

7) Type of eta (kg M-1 s-1) is the viscosity of the liquid; liquid density; capillary length; capillary radius; outflow time; the average height of liquid column of liquid flows through the capillary; for the acceleration of gravity; V is the volume of liquid flows through the capillary; and the geometric shape of the instrument is constant. When take m=1.

For a specified viscometer, many parameters are specified, so that

,.

So 7) can be changed as,

8) In the Formula , when

,the equation on the right side of the second can be ignored. The density of the solution
 is approximately equal to the solvent density
. In this way, the T and t0 of the solution and solvent can be calculated

:  

9) So the only solution was determined and solvent in capillary outflow time can be obtained by ηr.

Operation steps

1 cleaning equipment

The viscometer with lotion, tap water and distilled water clean, pay special attention to the capillary, rinse with ethanol, and then dried standby.

It takes longer to dry the viscometer, especially for a long period of time. After drying, the viscometer is placed in a constant temperature bath and is clamped vertically with an iron clamp.

2 polyvinyl alcohol solution

Accurate weighing about 1g solid polyvinyl alcohol, into the beaker, add 60mL distilled water to dissolve, can be properly heated to accelerate the dissolution, to be solid polyvinyl alcohol completely dissolved, the solution (solution after cooling) into 100mL volumetric flask volume.

1.0111g polyvinyl alcohol is said to be solid and dissolved in an electric furnace. After the dissolution of polyvinyl alcohol 10min. Transferred to the capacity of the bottle at 35 degrees Celsius under constant temperature and volume, the volume of the solution was found to have some foam.

3 constant temperature

Open the thermostat, set the temperature of 35 + 0.01. The viscometer (Figure 2) in the thermostat temperature 10min (note that vertically). Pure water as well as samples to be measured at 35 degrees C.

In addition, the utility model can be conveniently placed in a constant temperature water tank with a conical bottle filled with distilled water.

4 Determination of solvent outflow time t0

The water is injected into the viscometer 20mL, and the temperature is measured after 2 minutes. The method is as follows: the C tube is not ventilated. Wash the ear with a ball at the upper end of the suction pipe B, the water from the F ball by D ball, E ball, capillary pumped to G at 2/3; the clip on the release of C tube, the ventilation, the D solution in the ball back into the F ball, the capillary suspended above the liquid; the liquid falls above the capillary. When the concave surface the lowest scale a flows through the line, immediately press the stopwatch start time, stop time to B. Note the time required for the liquid to flow through a, B. Repeated measurement three times, the deviation is less than 0.3s, the average value is t0.

Distilled water is taken from the 3 in the conical flask, to ensure that the liquid drops when the liquid level does not bubble, the data recorded in 5.

5 Determination of time of solution outflow

Drying viscometer, with the pipette has been a good solution in advance of 10mL, the A tube into the viscometer, in 25 degrees of water temperature for about 2 minutes, the determination of the outflow time of three times, the average.

Pure solution viscosity is very large, with the ear when the ball to slowly inhale to rise to the surface of the G ball, otherwise the bubble will be a lot of.

6 Determination of the outflow time of different concentration solution

2, 3, 10 mL distilled water was added to the viscometer, and the T values of different concentrations were measured by the above methods, respectively. After each dilution, the liquid is mixed (2min) with an ear washing ball, and the E ball, G ball and capillary part of the viscosity are repeatedly washed.

In accordance with the steps in the operation 3-4 times after the solution is roughly mixed. Data records see 5.

7 cleaning equipment

Turn off the power supply, clean the viscometer 3 times, fill it with water. To ensure that the viscometer is not used when it is clean, otherwise there is a foreign body for a long time to clean up the impact of experimental results.

Matters needing attention

1 the use of black viscosity timing, in the same viscometer to determine a series of concentrations into a simple proportional relationship between the outflow time of the solution. In order to avoid the volume change caused by the temperature change, the solution and the solvent should be removed at the same temperature.

2 every time when adding solvent dilution solution, the viscometer must be mixed in the liquid, but also to the solution to the E line above the ball two times, wash capillary, otherwise the time of the solution is poor repeatability.

3 in the use of organic matter as a solvent for the polymer, the polymer solution contained a glass instrument, should be the first to use this solvent soaking and washing to be washed and moist, dry polymer organic material such as solvent, can be used to soak chromic acid, or organic materials are made of chromium in pickling solution of potassium dichromate reduction, lotion will fail.

4 viscometer must be clean, such as hanging on the capillary wall with water, blowing dryer.

5 Determination of the viscometer to be placed vertically, otherwise affect the accuracy of the results.

6 in this experiment, the dilution of the solution is carried out directly in the viscometer, so the solvent must be in constant temperature with the solution in the same temperature tank, and then accurately measured with the pipette and fully mixed can be measured.

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