EXPERIMENT 33 MOLECULAR WEIGHT DETERMINATION OF POLYMERS BY CAPILLARY FLOW METHOD OBJECTIVE The objetive of this experiment is to determine the intrinsi visosity of a polymersolvent system and to determine the visosity average moleular weight of a polymer from measurements on Ubbelohde visometer. PRELIMINARY WORK In this experiment, instead of writing a report, emphasis will be plaed on lab notebook keeping. You are required to do all alulations in your lab-notebooks individually DURING THE EXPERIMENT. Your notebooks will be heked and graded at the end of the experiment. Therefore, it is strongly reommended that you are prepared before the laboratory session about the ated onepts, proedure and experimental setup. You have to write the introdution and experimental proedure parts briefly in your lab-notebook before laboratory session. 1. Bring your laptops to laboratory for alulations and ensure that EXCEL is installed. 2. Read the bakground information; try to obtain Eqn. 2 and the study the determination of visosity average moleular weight of polymers graphially (Huggins plot) from the referenes. 3. Understand and write the definitions of number average, weight average and visosity average moleular weight. See Referene 4. 4. Wath an Ubbelohde apillary visometer demonstration from web. Familiarize yourself with the instrument.. Think about the data you will have at the end of the experiment and prepare a data sheet aordingly. BACKGROUND INFORMATION Visosity It is a general property of fluids that an applied shear fore whih produes flow in the fluid is resisted by a fore proportional to the gradient of the flow veloity. Newton s law of visosity shows the ation between the veloity gradient (du/dx) and the shear fore (F) where A is the area of ontat between the moving layers of fluid, du F A (1) dx and the proportionality onstant "η" is known as visosity of the fluid. Measurement of Visosity There are several experimental methods to determine the visosity of fluids suh as (a) flow of fluids through an aperture in a plate, (b) torque or visous drag on a rotating disk or ylinder immersed in the fluid, () fall of solid heres through fluids, (d) flow through 1/
the apillary tubes. Dilute solution visosity of polymers is usually measured by apillary methods using Ostwald-Fenske and Ubbelohde visometers (see Figure 1). Figure 1. Ubbelohde visometer The Ubbelohde method makes use of the Hagen-Poiseuille equation for the determination of liquid visosities by measuring the time of flow of a liquid through a vertial apillary tube under the influene of gravity. For a virtually inompressible fluid this flow is governed by the Poiseuille's Law in the form of r 4 Pt (2) 8Vl where t is the time required for the upper menisus of the liquid in the middle arm of the visometer to fall from the upper to the lower fiduial mark. The hydrostati pressure head P is gh, being the density and P is variable sine h varies during the measurement. However, its initial and final values are known. Thus, Equation (2) an be written (with some simplifying assumptions) as 4 r ght 8 Vl At where A is a onstant for a given visometer be determined through alibration with a liquid of known visosity and density. It follows that, for two liquids 1 and 2, the ative visosity, is given as, t 1 1 1 (4) 2 2t2 Sine the densities of dilute polymer solutions differ little from that of solvent, t 1 1 () 2 t2 (3) 3/2/
Determination of Moleular Weight: Relation between visosity and MW All polymers inrease the visosity of the solvent in whih they are dissolved. This inrease allows for a onvenient method of determining the moleular weight of polymers. The solution visosity will depend on the solvent visosity, temperature, solute onentration and the partiular polymer and solvent, beause interations between the polymer and solvent influene the onformation of the polymer moleules and entanglements between the polymer moleules. Several important visosity funtions are used in visosity studies. Speifi visosity ( ) is found by alulating the frational inrease in visosity to get rid of the effet of solvent visosity. 0 1 (6) 0 where η = visosity of the solution η 0 = visosity of the solvent The redued visosity ( red ) is the fluid visosity inrease per unit of polymer solute onentration, (g/dl). red (7) The intrinsi visosity [] is the limit of the redued visosity as the polymer solute onentration approahes zero. The intrinsi visosity is also the limit of the inherent visosity, as the onentration of polymer solution approahes zero. Both redued visosity =η / and inherent visosity =ln(η )/ show a reasonably linear onentration dependene at low onentrations. lim 0 ln( lim 0 ) By extrapolation to zero solute onentration, the influene of entanglements on visosity is eliminated. The intrinsi visosity then should be a funtion of the moleular weight of the polymer in solution, the polymer-solvent system and the temperature. If the experiment is done with eified solvent for a partiular polymer under onstant temperature, intrinsi visosity should be quantitatively ated to the moleular weight of the polymer. For non-herial partiles inluding polymeri moleules, Mark-Houwink- Sakurada obtained an empirial equation whih ates the intrinsi visosity to the moleular weight as a KM V where K and a are experimentally determined empirial onstants. Although the experimental determination of intrinsi visosity is quik and easy, Equation 9 gives only a visosity average moleular weight for the polymer, sine generally a polymer sample onsists of mixtures of moleules with different degrees of polymerization. (8) (9) 3/3/
EQUIPMENT AND CHEMICALS Experimental setup is plaed in E-Blok 1 st Floor - Basi Measurements Laboratory. Ubbelohde visometer, pipettes, pump, baker, stopwath, distilled water, polyethylene glyol (PEG), polyvinyl alohol (PVA), aetone or sulphuri aid will be used. EXPERIMENTAL PROCEDURE NOTES : 1. In this part of the experiment, avoid foaming of solutions as muh as possible. This requires areful pouring when a solution is to be transferred from one vessel into another. 2. Temperature has an effet on visosity so do not forget to reord the room temperature. If you have problems in keeping the temperature onstant, use a onstant temperature water bath at 2 o C and plae the visometer inside the water bath. 3. It is neessary to rinse the immediately and very thoroughly with the solvent used, all the glassware (visometer, pipette, et.) that was in ontat with the polymer solution, beause one the polymer has dried on the glass surfae it is quite diffiult to remove. PROCEDURE: Polymeri material will be given to you by your assistant. Choose 4- different onentrations in the range of 4 g/dl-0.2 g/dl (η < 2) for the experiment and prepare the polymer solution with highest onentration. 1. Introdue the polymer solution from the beaker into the reservoir of the visometer by pipetting a known amount. Measurements at other onentrations an be made by suessive dilutions in the visometer beause Ubbelohde visometer has the advantage that the measurement is independent of the amount of solution in the visometer. 2. Pump the solution in the reservoir into the apillary arm until it fills the feed bulb and the menisus should be well above the upper fiduial mark. 3. Remove the pressure and let the fluid flow by its own weight. Reord the time required for the upper menisus of the liquid in the feed bulb to pass two alibration marks (A and B in Figure 1). 4. Repeat the measurements of fluid efflux time at least for three times (The reading should agree to within 1% of the average flow time).. Repeat steps 2- for other onentrations. Rinse the visometer with a few millimeters of the solution to be investigated after eah hange of onentration to remove the previous solution remaining in the apillary. 6. At the end of the experiment, rinse the visometer with aetone or sulphuri aid and lastly rinse with distilled water. CALCULATIONS 1. Using the data for pure water, alulate the visometer onstant A. The density and visosity of water for use in this equation an be found in literature. 3/4/
2. Prepare a table whih gives the values of, t, 0 seonds and 0 denotes the visosity of solvent.,, η /, ln(η )/ where t is in 3. Plot η / versus and ln(η )/ versus, determine the intrinsi visosity of the polymer from eah graph, whih should be a straight line. Take the average of determined from the two plots and find the moleular weight of the polymer using this average value. (Mark-Houwink-Sakurada onstants, K and a, an be found from Referenes 1 and 6) SUGGESTED READING 1. Brandrup, J., Immergut, E.H., Polymer Handbook, Intersiene Publishers, New York, 1966. 2. MCabe, W.L., and Smith, J.C., Unit Operations of Chemial Engineering, MGraw- Hill, New York, 1976. 3. Moore W.J., Physial Chemistry, Prentie-Hall, Englewood Cliffs, New Jersey, 1972. 4. Rosen, S.L., Fundamental Priniples of Polymeri Materials, Wiley, New York, 1982.. Shoemaker D.P., Garland, C.W., Steinfeld, I.J., Experiments in Physial Chemistry, MGraw-Hill, New York, 1974. 6. Riedl, B., Vohl, M.J., Calve, L., Moleular size and solvation of low moleular weight poly(ethylene oxide) and phenol-formaldehyde resols in different solvents, Journal of Applied Polymer Siene, 1990. 3//