Physical Chem lab report：Viscosity

In this experiment, will measure the viscosities of polyvinyl alcohol solutions and solutions of degradation products of this material. Polyvinyl alcohol is often written in the following manner.

The formula in parentheses is the monomer unit. The monomer unit has a head (the CHOH group) and a tail (the CH2 group). This formula does not indicate how adjacent monomer groups are oriented with respect to each other. One might suppose that they form regular chains, with the head of one monomer bonded to the tail of its neighbor, and so on, down the chain.

—(CH2CHOH)—(CH2CHOH)—(CH2CHOH)—(CH2CHOH)—(CH2CHOH)—CH2CHOH)—

Nothing is that perfect, so the chains often contain runs of regularly arranged monomers, with occasional head-to-head defects, i.e., the arrangement where the central linkage is a head-to-head, 1, 2-glycol structure.

—(CH2CHOH)—(CH2CHOH)—(CH2CHOH)—(COHCH2)—(OHCHCH2)—(OHCHCH2)—

A measure of the polymer’s structure is the fraction, , of monomer linkages that are head-to-head. In this experiment, treatment with periodate specifically cleaves 1,2-glycol linkages, making smaller chains. The size of the smaller chains will depend on the number of head-to-head linkages in the particular sample of poly(vinyl alcohol), which can be estimated from the number-average molar masses of the polymer and its cleaved products.

Thus, one examines the structure of the polymer indirectly through a chemical reaction. A reaction that acts specifically at the 1,2-glycol linkages to cleave a polymer breaks it into smaller units, the size of which is determined by the number of monomer units between these sites. Treatment with periodate can cause this cleavage. On the assumption that all 1,2-glycol linkages are cleaved and that none of the head-to-tail linkages are cleaved during treatment with periodate, is equal to the increase in the number of molecules after cleavage divided by the total number of monomer units represented by all molecules presented in the system. Since these numbers are in inverse proportion to the respective weights,

(1)

where Mn is the number-average molar mass of the polymer before cleavage, Mn’is the number-average molar mass of the polymer after cleavage and M0 is the molar mass of the monomer, equal to 44. Thus,

(2)

The following equation can also be used:

(3)

which allows for to be calculated directly from viscosity averages.

PROCEDURE

Preparation of the polymer solution takes considerable time. If a stock solution of the polymer is not available, it should be prepared as follows: Weigh out accurately 8.0 − 9.0 grams of the dry polymer. Transfer to a 250 mL beaker. Fill three-fourths full with distilled water, and stir, on a hot plate until the polymer is dissolved. Optimal temperature to ensure dissolution of the polymer as well as to avoid coagulation is 90 − 95 °C. Since the final concentration of the solution of the polymer is important, it is necessary to know the actual amount of the polymer that has dissolved in the water. The amount of dry polymer that coagulated on the bottom of the beaker can be determined (by weighing) and used to adjust for the actual amount of the polymer that has dissolved in the water.

Cool[footnoteRef:1] the solution of the polymer to room temperature and transfer quantitatively to a 250 mL volumetric flask avoiding foam as much as possible by letting the solution run down the side of the flask. Make up to the ‘mark’ with distilled water, and mix gently (to avoid foam) but thoroughly. If the solution appears contaminated with insoluble material that would possible interfere with the viscosity measurements, filter it, e.g., through Pyrex wool. Wash all glassware very thoroughly with water as soon as possible after use. [1: In the interest of time, the mixture is stirred at room temperature on a second hotplate that already is room temperature.]

Pipette 50 mL of the stock solution into a 100 mL volumetric flask, and make up to the mark with distilled water observing the above precautions to prevent foaming. Mix thoroughly and place in the water bath to equilibrate. In this and other dilutions, rinse the pipette very thoroughly with water and dry by rinsing with acetone and air-drying.

To obtain a third concentration, pipette 50 mL of the previously diluted solution into a 100 mL volumetric flask, and make up to the mark with distilled water. Place in the bath to equilibrate.

A fourth concentration can be similarly obtained by pipetting 50 mL of the previously diluted solution into a 100 mL volumetric flask, and making it up to the mark with distilled water.

Viscosity will be measured following the instructions detailed in the manual. Measure the viscosity of the stock solution and of each of the dilutions.

Viscosity Measurement : Make measurements of solutions in random order to avoid any systematic error. The higher the concentration, the more viscous the solution and the easier the instrument locks onto the viscosity. Measure each solution at least three times to get information on the uncertainty in the measurement. See Manual for additional details.

Polymer cleavage/degradation

To cleave the polymer, pipette 50 mL of the stock solution into a 250 mL Erlenmeyer flash and add up to 25 mL of distilled water and 0.25 g of solid KIO4. Warm the flask to about 70 C, and stir until all the salt is dissolved. Then clamp the flask in a thermostat bath and stir until the solution is at 25 °C. Transfer quantitatively to a 100 mL volumetric flask, and make up to the mark with distilled water. Mix and place in the bath to equilibrate. In the absence of a water bath, allow the flask to equilibrate on the laboratory bench-top (assume room temperature is ~ 25 °C.

To obtain subsequent concentrations, dilute the stock solution of the cleaved polymer with distilled water to obtain approximately the following dilutions ¾, ½ and ¼. Place in the bath to equilibrate. Make sure that you dilute properly. It is important that the concentrations be accurately known.

Calculations

1. For each of the polymer solutions studied, calculate the viscosity  and the concentration c in grams of polymer per 100 mL of solution. Then calculate sp/c and (1/c)(ln / 0).

2. Plot both sp/c and (1/c)(ln / 0) vs c and extrapolate linearly to c = 0 to obtain [] for the original and for the degraded polymer.

3. Calculate v and nfor both the original polymer and the degraded polymer, then obtain a value for . Report these figures together with the polymerization temperature for the sample studied. Discuss the relationship between  and the rate constants kand k.

OH

n

H

H

H