Intrinsic Viscosity PDF

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Intrinsic Viscosity: Polyvinyl alcohol Danna Gomes Physical Chemistry CH408 03/27/2018

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Abstract: Using an Oswald Viscometer, the intrinsic viscosities of uncleaved and cleaved (KIO4 treated) Polyvinyl Alcohol were determined. The intrinsic viscosity for the uncleaved polymer was calculated to be 0.5103088 cm3/g, while the intrinsic viscosity for the cleaved polymer was 0.5093270 cm3/g. Additionally, the average molar mass of the uncleaved sample was found to be 4243±853 g/mol and the average molar mass of the cleaved polymer was found to be 1172±72 g/mol.

Introduction: Polyvinyl alcohol is a non-toxic, water soluble polymer used in the food industry as a thickening agent. Polyvinyl alcohol is a macro molecule formed of multiple repeating vinyl alcohol groups joined by a carbon-carbon bond formed by spitting the double bond on the vinyl group. In order to know anything useful about a polymer, it is critical to first estimate the size of the polymer. For linear polymer, the longer the polymer, the greater the mass and if in solution, the more viscous the solution. Viscosity itself can be defined in multiple ways however the most applicable way to think of it is the resistance to shear force in a fluid, or rather a frictional force in the fluid itself. More viscous fluids will be thicker and act more jellies while a less viscous fluid is easy to pour and will be more like water. In this experiment the longer the PVOH chains are will directly result in a greater viscosity coefcient ( ɲ). ɲ =Bt ρ The above equation relates the viscosity coefcient ɲ and density

ρ

apparatus constant B (of the Oswald viscometer) and flow time t in seconds.

to the

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Background: When vinyl alcohol polymerizes, there are two possible orientation which it can do so, head to tail, or the less common head to head linkage. Because the head to head linkage is a vicinal diol, this group is susceptible to cleavage by KIO 4 and will result in smaller chains K β A β e− Eaβ/ RT −Ea/ RT =S e = −Eaα / RT K α Aα e The above equation relates the two rate constants Kα and Kβ that correspond to head to tail and a head to head linkage respectively. (S) is the steric constant that determines if the polymer is sterically hindered to the point of affecting the linkages and ΔEa= - Eaα= Eaβ such that this term should be positive given Eaβ is much larger. Using known values for the density of water and the viscosity coefcient of water, the time water takes to flow through the viscometer can be measured and B can be calculated by rearranging equation to be in the form of:

B=

ɲ ρt

This can be used to experimentally determine the viscosities of our solutions. We can also make use of the specific viscosity of the solutions η ηsp = −1 η0 Above, η0 is the viscosity of the pure solvent. By definition, intrinsic viscosity is defined as the ratio of specific viscosity to the weight concentration of the solute as concentration goes to zero. Physically, this value describes the contribution of the solute to the total viscosity of the solution and can be given mathematically by

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η η0 ¿ ¿ ¿ C ln ¿ ¿ ηsp =lim ¿ C C →0 [ η ]=lim ¿ C→0

Plotting these against concentration and extrapolating to c=0 will give the intrinsic viscosities for the polymer samples. Flory and Leutner also showed that the intrinsic viscosity of a polymer in solution can be related to the viscosity-average molar mass by the following empirical relationship [ η ] =2.0x10-4M0.76 M =7.6x104[ η ]1.32 To calculate the average molar mass of the polymer, the gamma function is employed ∞

t−1 −x Γ ( t )=∫ x e dx 0

Flory showed that the ratio of the average-viscosity molar mass and that average molar mass (� n) can be described as 1

(1+a)Γ (1+a)¿ a M´ v =¿ M´ n

For PVOH, the value of a is 0.76. Therefore, the above equation reduced to

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M´ v =1.89 M´ n

The promise of this calculation is that cleavage only occurs between 1,2-glycol structures, marked by the red bond above. Therefore, because occurrences of molecules are inversely proportional to molar mass, it can be subtracted the inverse molar mass of the cleaved polymer by the inverse molar mass. Then, it can be divided by the total occurrences, which can be represented by the inverse molar mass of the monomer (Vinyl Alcohol, MW=44.0 g/mol), and rearrange to get the following formula

∆=83(

1 1 − ) ' M´ v M´ v

Procedure: 

Clean the viscometer with cleaning solution, rinse with DI water and dry with acetone and air;



Immerse in a 25 ℃ thermostat bath to equilibrate;

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Weigh out accurately in a weighing bottle or in a watch glass 4.0 or 4.5g of the dry polymer. Add with stirring to about 200ml of hot DI water in a beaker;

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When all polymer has dissolved, let the solution cool and transfer it carefully and quantitatively into a 250ml volumetric flask;

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Make the solution up to the mark with DI water and mix by slowly inverting a few times. Avoid foaming as much as possible;

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Pipette 50ml of the stock solution into a 100ml volumetric flask and make up to the mark with DI water. Mix the liquids and place the flask in the bath to equilibrate;

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Pipette 50ml of the stock solution into a 250ml flask and add up to 25ml of DI water and 0.25g of solid KIO4. Warm the flask to about 70 ℃ , and stir until all the salt is dissolved. Then clamp the flask in a thermostat bath and stir until the solution is at 25 ℃ ;

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Transfer to a 100ml volumetric flask and make up to the mark with DI water. Mix carefully and place it in the bath to equilibrate;

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At this point, two “initial” solutions have been prepared, one cleaved with periodate and one uncleaved of the same concentration. To obtain a second concentration of each material, pipette 50ml of the “initial” solution into a 100ml volumetric flask and make up to the mark with DI water. Place all solutions in the thermostat bath to equilibrate;

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The viscometer should be mounted vertically in a constant-temperature bath so that both fiducial marks are visible and below the water level. The temperature should be maintained within

± 0.1℃ of 25 ℃ during a

run; 

Pipette the required quantity of solution (or water) into the viscometer;

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By suction with a pipette bulb through a rubber tube, draw the solution up to a point well above the upper fiducial mark. Release the suction and measure the flow time between the upper and lower marks with a stopwatch or timer. Obtain two or more additional runs with the same filling of the viscometer. Three runs agreeing within about 1% should sufce;

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Each time the viscometer is emptied, rinse it very thoroughly with DI water then dry with acetone and air.

Results:

nsp/c vs c (uncleaved) 1.8 1.6 1.4

nsp/c

1.2

f(x) = 0.86 x + 0.62 R² = 0.71

1 0.8 0.6 0.4 0.2 0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000 1.1000

c

nsp/c vs c (cleaved) 0.25 0.2

f(x) = − 0.11 x + 0.24 R² = 0.89

nsp/c

0.15 0.1 0.05 0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000 1.1000

c

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1/c ln n/no vs c (uncleaved) 1.2

1/c ln n/no

1 0.8

f(x) = 0.16 x + 0.75 R² = 0.28

0.6 0.4 0.2 0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000 1.1000

c

1/c ln n/no vs c (cleaved) 0.25

1/c ln n/no

0.2

f(x) = − 0.12 x + 0.24 R² = 0.9

0.15 0.1 0.05 0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000 1.1000

c

Discussion: Viscosity is a very difcult quantity to measure accurately, as the slightest change in a number of variables can affect it. The operation of an Oswald viscometer involves measuring the time required for a fluid to flow from the upper to lower fiducial mark. Because the viscometer used in this experiment was not automated, this measurement needed to be completed by stopwatch. This introduces operator error into the experimental measurements. The solutions were prepared by having

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0.50, 0.40, 0.25, 0.10 concentrations, for cleaved and uncleaved. The solutions prepared in this experiment were created to an appreciable degree of accuracy. We started with solid PVOH and measuring out precise masses and volumes for highly accurate concentrations, the solutions made were estimated for the percentages PVOH required and filled out with DI water to the specific mark. The reasoning for filling out inti lth right mark is to know exactly the volume for the solution. These measurements were taken with specific burettes for each concentration, which is the best piece of equipment to use because highly accurate results were desired. In collecting data for this experiment, the initial experiment showed less than stellar results. The cleaved data did not show a proper correlation, as the time required did not increase with increasing percentage PVOH. After retrial, this data appeared to follow the correct trend. Initially, the uncleaved data followed a correct trend, but calculations showed significant error. Results for the medium power were completely off, in comparison with low power and high power. Reasons for this can be temperature was not ideal for the experimental, temperature is very important on measuring viscosity, especially handling and preparing the solution the temperature had to be specific so the polymer could be prepared....