Determination of molecular weight by osmotic pressure method

Determination of molecular weight by osmotic pressure method.

Osmotic pressure method is also called membrane osmometry. This method is widely used to determine the number average molecular weight of polymers. This method is based on the phenomenon of osmosis. If a pure solvent is separated from a solution through a semipermeable membrane, due to concentration (chemical potential) difference between the solvent and solution, the solvent will flow into the solution through semipermeable membrane. The pressure applied on the solution to completely stop the flow of solvent into it through semipermeable membrane is called osmotic pressure.

The theory of osmotic pressure also applies to a solution of polymers.

Ordinary solution obeys Van't Hoff equation i.e

π=cRT/M

Where π is osmotic pressure, c is concentration in mass per unit volume, R is gas constant, T is temperature and M is molar mass.

The polymer solution are non ideal. Taking into account their deviation and using thermodynamics, Memillian and Mayer modified van't Hoff equation for polymers as

π=cRT/Mₙ+ Bc²+Dc³

=> π/c=RT/Mₙ+Bc+Dc²

For dilute polymer solution concentration is small,thus

π/c=RT/Mₙ+Bc

If graph is plotted between π/c vs c ,we get a straight line having slope B and intercept RT/Mₙ.

π/c vs c.

Thus Mₙ can be calculated.

Difference between organic and inorganic polymers.

Difference between organic and inorganic polymers. Definition Organic polymers are the polymers that essentially contain carbon atom in the backbone. Inorganic polymers are the polymers that have no carbon atom in the backbone. Structure Most organic polymers have simple structures. Almost all inorganic polymers are highly branched and have complex structures. Electrical Conductivity In most of the aqueous solutions, organic polymers are typically poor conductors of electricity and heat. Inorganic polymers in aqueous solutions are good conductors of electricity, this is because they have high ability to ionise and this makes them better conductors. Flammability Organic polymers are flammable whereas inorganic polymers are nonflammable. Effect on nature Organic polymers are environmental friendly as these are biodegradable. Inorganic polymers are not environmental friendly as these are non biodegradable. Examples Organic polymers include polysaccharide...

Monomers.

Monomers. A monomer is a molecule that can be reacted together with other monomer molecules to form a larger polymer chain. Some monomers are given below with their polymers . Tetrafluoroethene ; Tetrafluoroethene is the monomer of Teflon . Teflon and its monomer. Acrylonitrile (vinyl cyanide); Acrylonitrile is the monomer of Polyacrylonitrile . Polyacrylonitrile and its monomer. Isoprene; Isoprene is the monomer of Polyisoprene (natural rubber). Polyisoprene and its monomer. Chloroprene; Chloroprene is the monomer of Neoprene . Neoprene and its monomer. Vinyl chloride; Vinyl chloride is the monomer of Poly(vinyl chloride) . Polyvinyl chloride and its monomer. Vinyl acetate; Vinyl acetate is the monomer of Poly(vinyl acetate). Poly(vinyl acetate) and its monomer. Propylene; Propylene is the monomer of Polypropylene . Polypropylene and its monomer. Styrene; Styrene is the monomer of Polystyrene . Polystyrene and i...

Determination of molecular weight of polymers by viscosity method.

Determination of molecular weight of polymers by viscosity method. It is a simple method for determining the molecular weight of polymers. Addition of polymers in the solvent increases the viscosity of the solvent, due to introduction of inhomogeneities by the polymers. If η₀ is the viscosity of the solvent and η is the viscosity of the solution at the same temperature, then the relative viscosity is given by Relative viscosity. Specific viscosity: It is defined as the relative increase in viscosity and is given as Specific viscosity. Reduced viscosity: It is defined as the relative increase in viscosity per unit concentration (C) of polymer and is given as Reduced viscosity. Intrinsic viscosity: The reduced viscosity is dependent on concentration (C). If a graph is plotted between reduced viscosity vs concentration then the extraplotation value when C=0 is called intrinsic viscosity. Intrinsic viscosity. Graph of reduced vi...

Polymers

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