Polymers

Name three ways of classifying polymers, and the categories within these

Source (natural vs synthetic)
Structure (condensation vs addition)
Mechanism (step growth vs chain growth)

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Define DP

Degree of polymerisation; number of monomer units repeated within the polymer chain

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Define p (row)

Conversion on a scale of 0 to 1, where 1 is full monomer conversion.

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Define Mn

Number average molecule weight
=sum (Ni*Mi)/sum(Ni)

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Define Mw

Weight average
=sum(Ni*Mi^2)/sum(Ni*Mi)

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Define Đm

Polydispersity Index; Mw/Mn. Always >1

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Define Dh

Polymer hydrodynamic volume; The size of the sphere the polymer forms as it collapses

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Define Rg

Radius of gyration; the RMS (root mean squared) distance of a chain segment from the centre of mass of the molecule.

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Why do some polymers look cloudy?

At high molecular weight, polymers scatter light (Rayleigh scatterying) and therefore look cloudy. This is solvent dependent.

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Give two methods of analysing polymers

GPC (Gel Permeation Chromatography) or SEC (Size Exclusion Chromatography)

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What are the principles of GPC/SEC?

Polymer solution washed down a chromatographic column packed with particles. Seperation based on size in solution, which is dependent on solvent and temperature. There is no interaction between the media surface and the sample.

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Define homopolymer

A polymer derived from one species of monomer (but often used to describe a polymer which has a single type of repeating unit).

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Define copolymer

A polymer derived from more than one species of monomer. More often used to describe polymers which contain 2 or more types of repeating unit.

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What is the name given to the growth of a polymer chain?

Propagation

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True or false; Synthetic polymers such as polystyrene possess a well-defined molecular weight that can be defined as a population of molecules of identical mass.

False; Even synthetic polymers have a range of chain length

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True or false; SEC, calibrated with narrow molecular weight standards, directly measures the molecular weight of a polymer sample

False. It measures retention time, which can be related to molecular weight by calibrating.

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Define Tm

Crystalline melting temperature

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Define Tg and what kind of transition is it?

Glass transition temperature; the temperature at which the Gibbs free energy is such that Ea for the cooperative movement of 50 elements of the polymer is exceeded. A second order transition.

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What happens when a liquid polymer is cooled?

Translational, rotational and vibrational energies of the polymer decrease.When translational and rotational energies are zero, the polymer can crystalise (Tm - not all polymers crystalise). As temperatures continue to decrease, long range motion ceases (

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If a polymer only has Tg, what is it's morphology?

Amorphous

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If a polymer only has Tm, what is it's morphology?

Crystalline

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Define free volume

Space in a solid or liquid which is not occupied by the polymer molecules

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Name 5 factors that affect Tg

Chain flexibility/stiffness, tacticity, polarity and symmetry, sterics and molecular weight.

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How does symmetry affect Tg and why?

Increase in symmetry reduces Tg. The free volume effect means it's harder to pack, which means less thermal energy is needed to create the free volume effect for chain rotaion.

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Polymers

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