Viscosity Average Molecular Weight

March 21, 2019 | Author: Sihanu Subasingha | Category: Polymers, Molecular Mass, Viscosity, Solution, Molecules
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Viscosity Average Molecular Weight...



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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  

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  

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Concentration (dl/g)

Time (s)




























ln (Ƞr )/C


1.2 y = 0.1921x + 0.7008 1 0.8 Ƞsp/C 0.6

ln (Ƞr)/C

y = -0.0629x + 0.702

Linear (Ƞsp/C) 0.4

Linear (ln (Ƞr)/C)

0.2 0 0






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

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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 by the fluid’s molecules. Also viscosity of a fluid governs the flow characteristics, heat transfer and mass transfer. Viscosity of a fluid decreases with increasing of temperature as intermolecular forces decreases 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 and complex 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 and comparer the time spent for solution and pure solvent to travel through a capillary. This is relative easy method 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. The obtained molecular weight is known as viscosity average molecular weight. Also intrinsic viscosity is related 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 as material’s melting point, crystallinity and tensile strength. Molecules in a polymer  have different sizes of chain length and hence can’t fine specific single molecular 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 called as viscosity average molecular weight.

   Where Ƞ = Intrinsic viscosity, M = viscosity average molecular weight and K & α 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 α is varying from 0.5 to 0.85 and if α is 0.5 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 is obtain as  According to the literature viscosity average molecular weight for poly styrene is about 3 X 105 at 130 °C. So our practical value is higher than literature value. This may mainly 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. So contamination can occur. Also environmental condition is not same as standard conditions. So it can affect 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.

Importance of the practical and viscosit y average molecular weight:

Determination of average molecular weight using viscosity average molecula r weight technique is very easy and simple. Also calculation methods are also simple and rapid. So results can be obtained very quickly. 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. So viscosity 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 the transition temperature of the polymers to liquid. So if a polymer has low molecular weight then it shows low mechanical properties and low transition temperature and vice versa. Also average molecular weight gives an idea about the complexity and length of the polymer molecule. If molecule has higher molecular weight then the complexity and length of the molecule is high. Also this viscosity 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 broad molecular weight distribution shows less viscosity. 

Important to industrial applications:

When using a polymer for specific application or manufacturing processes in industry, transition temperature 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 average molecular weight transitions temperature of the polymer can be predicted. Also mechanical properties of 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 average molecular viscosity. Most of the time viscosity of polymer determines the process and process conditions. So viscosity average molecular weight gives an important conception about the process which 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.

REFEERENCES  r_Forces/Viscosity

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