viscosity average molecular weight

7/27/2019 Viscosity Average Molecular Weight

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INTRODUCTION

Viscosity of a fluid is a measure of resistance to flow. Normally viscosity of a fluid depends on the

fluid properties such as molecule sizes and interaction with those molecules as well as the external

environmental conditions such as temperature.

When considering polymers their average molecular weight is related to the viscosity of the polymer

under specific conditions. Molecular weight measure using viscosity technique is known as viscosity

average molecular weight.

In the practical intrinsic viscosity of a polymer is used to determine the viscosity average molecular

weight of the poly styrene. Here solutions of poly styrene in different concentrations is prepared using

toluene as a solvent and time spent each solution to travel between the marked point of viscometer is

measured. Then by using given equations, relationship between intrinsic viscosity and viscosity

average molecular weight is developed using graphical method.


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CALCULATION

Concentration(dl/g) Time (s) r sp sp/C ln ( r )/C

0.25 76 1.1875 0.1875 0.75 0.68740.5 89.5 1.3984 0.3984 0.7969 0.6707

1 121 1.8906 0.8906 0.8906 0.63692 203 3.1719 0.1719 1.0859 0.5772

Graph:

According to the above two plots, those two plots will cut the y axis around (0.7008+0.702)/2 = 0.7014

y = 0.1921x + 0.7008

y = -0.0629x + 0.702

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5

sp/C

ln ( r)/C

Linear ( sp/C)

Linear (ln ( r)/C)


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DISCUSSION

Introduction to viscosity and viscosity average molecular weight:

Viscosity is a measure of resistance to flow of a fluid. This resistance is due to the friction produced bythe fluids molecules . Also viscosity of a fluid governs the flow characteristics, heat transfer and masstransfer. Viscosity of a fluid decreases with increasing of temperature as intermolecular forcesdecreases and increases when fluid become cooler. More viscous fluids are shows sticky behavior.Viscosity of a material depends on the size and shape of the molecules. So fluids with larger andcomplex molecules show higher viscosity. Viscosity is an important factor for mixing characteristics.

Intrinsic viscosities of polymers are generally measured to get an idea about the molecular weight of polymers. Normally intrinsic viscosity of polymers is measure by preparing a solution of polymer andcomparer the time spent for solution and pure solvent to travel through a capillary. This is relative easymethod and average molecular weight can be obtained by graphical method (when concentration of tends to zero). Obtained average molecular weight is depends on the type of solvent used. Theobtained molecular weight is known as viscosity average molecular weight. Also intrinsic viscosity isrelated to the degree of polymerization. As the intrinsic viscosity is measure of molecular weight &degree of polymerization, it gives an idea about the properties related to molecular weight such asmaterials melting point, c rystallinity and tensile strength.

Molecules in a polymer have different sizes of chain length and hence cant fine specific singlemolecular weight. So when dealing with the polymers, molecular weight is measured as average value.Molecular weight measured using viscometer according to above method (intrinsic viscosity) is calledas viscosity average molecular weight.

Where = Intrinsic viscosity, M = viscosity average molecular weight and K & are constants for a particular polymer solvent system. Normally polymer having lower viscosity average molecular weight has shorter chains and showing less viscous. Also polymer having higher average molecular weight has longer chains and showing high viscous. Normally is varying from 0.5 to 0.85 and if is0.5 it is flexible polymer in ideal solvent.

Also by considering the individual molecule viscosity average molecule can be expressed as:

where

Analysis of the results and errors:

According to the results viscosity average molecular weight of poly styrene for toluene solvent isobtain as According to the literature viscosity average molecular weight for polystyrene is about 3 X 10 5 at 130 C . So our practical value is higher than literature value. This maymainly due to practical errors such as measuring errors in stop watch, contamination of solution etc.solvent and solution amount prepared for practical washing the equipment is not sufficient. Socontamination can occur. Also environmental condition is not same as standard conditions. So it canaffect to the viscosity of the solution and hence leads to error. But results were able to plot in linear manner and their intercept are also nearly close. This is an easy method to predict the molecular weight.


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Importance of the practical and viscosity average molecular weight:

Determination of average molecular weight using viscosity average molecular weight technique is veryeasy and simple. Also calculation methods are also simple and rapid. So results can be obtained veryquickly. One drawback of this method is results depend on the solvent used.

Viscosity average molecular weight is reflection of the average molecular weight of the polymer. Soviscosity average molecular weight gives an idea about properties with related to average molecular weight such as stiffness, strength, viscoelasticity, toughness, and viscosity. Also it gives an idea of thetransition temperature of the polymers to liquid. So if a polymer has low molecular weight then itshows low mechanical properties and low transition temperature and vice versa. Also averagemolecular weight gives an idea about the complexity and length of the polymer molecule. If moleculehas higher molecular weight then the complexity and length of the molecule is high. Also thisviscosity average molecular weight is useful in comparing different polymer samples. By doing so,idea about the properties of different polymers can be gain. Also when comparing the compounds of same polymer sample from different origin or different polymerization techniques. Polymer with broadmolecular weight distribution shows less viscosity.

Important to industrial applications:

When using a polymer for specific application or manufacturing processes in industry, transitiontemperature of that polymer should be considered according to the application. As average molecular weight related to the transition temperature in proportional manner, by looking at viscosity averagemolecular weight transitions temperature of the polymer can be predicted. Also mechanical propertiesof the polymer are related with average molecular weight. So when selecting particular polymer for specific application, correct polymer can be chosen by looking at the viscosity average molecular weight.

Also flow properties of a polymer is depends on viscosity and therefore it is depends on averagemolecular viscosity. Most of the time viscosity of polymer determines the process and processconditions. So viscosity average molecular weight gives an important conception about the processwhich should be used and controlling of that process according to the given polymeric material.

Normally viscous polymeric materials are used for injection molding process and low viscous polymers are used for blow molding or extrusion processes.


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REFEERENCES

http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosity

http://www.azom.com/article.aspx?ArticleID=4577 http://www.springerreference.com/docs/html/chapterdbid/218710.html http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/MolWeight.pdf

http://link.springer.com/article/10.1007%2FBF01974771
http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://www.azom.com/article.aspx?ArticleID=4577http://www.azom.com/article.aspx?ArticleID=4577http://www.springerreference.com/docs/html/chapterdbid/218710.htmlhttp://www.springerreference.com/docs/html/chapterdbid/218710.htmlhttp://www.ias.ac.in/initiat/sci_ed/resources/chemistry/MolWeight.pdfhttp://www.ias.ac.in/initiat/sci_ed/resources/chemistry/MolWeight.pdfhttp://www.ias.ac.in/initiat/sci_ed/resources/chemistry/MolWeight.pdfhttp://www.springerreference.com/docs/html/chapterdbid/218710.htmlhttp://www.azom.com/article.aspx?ArticleID=4577http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosityhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Viscosity

viscosity average molecular weight

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