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L. D. COLLEGE OF ENGINEERING

RUBBER TECHNOLOGY

Sub:- V.E.E(Viscoelasticity of elastomer)

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Topic:-Molecular weight affect the Tg

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Molecular weight affect the tg

• The glass transition temperature of a polymer is influenced by its molecular weight, at least up to around a value of 20,000.

• Beyond this, the effect of the mw. Is not pronounced.

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Molecular weight affect the tg

• The Flory–Fox equation relates the number-average molecular weight, Mn, to the glass transition temperature, Tg, as shown below:

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Molecular weight affect the tg

• where Tg,∞  is the maximum glass transition temperature that can achieved at a theoretical infinite molecular weight and K is an empirical parameter that is related to the free volume present in the polymer sample. It is this concept of “free volume” that is observed by the Flory–Fox equation.

• Free volume can be most easily understood as a polymer chain's “elbow room” in relation to the other polymer chains surrounding it.

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Molecular weight affect the tg

• The more elbow room a chain has, the easier it is for the chain to move and achieve different physical conformations.

• Free volume decreases upon cooling from the rubbery state until the glass transition temperature at which point it reaches some critical minimum value and molecular rearrangement is effectively “frozen” out, so the polymer chains lack sufficient free volume to achieve different physical conformations.

• This ability to achieve different physical conformations is called segmental mobility.

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Molecular weight affect the tg

A polymer chain (represented by the white circles) exhibits more free volume (represented by the gray shading) at the ends of the chain than from units within the chain.

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Molecular weight affect the tg

• Free volume not only depends on temperature, but also on the number of polymer chain ends present in the system.

• End chain units exhibit greater free volume than units within the chain because the covalent bonds that make up the polymer are shorter than the intermolecular nearest neighbor distances found at the end of the chain.

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Molecular weight affect the tg

• In other words, end chain units are less dense than the covalently bonded inter chain units.

• This means that a polymer sample with long chain lengths (high molecular weights) will have fewer chain ends per total units and less free volume than a polymer sample consisting of short chains.

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Molecular weight affect the tg

• In short, chain ends can be viewed as an “impurity” when considering the packing of chains, and more impurity results in a lower Tg.

• Thus, glass transition temperature is dependent on free volume, which in turn is dependent on the average molecular weight of the polymer sample. This relationship is described by the Flory–Fox equation.

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Molecular weight affect the tg

• Thus the mobility of chain segments influences the glass transition temperature.

• At the end of chain (chain segment) , chain ends have more freedom for motion .

• Same way on other hand the segment that are away from the chain ends, although part of the main chain, is connected at the two ends to the main chain and, hence their mobility is rather restricted.

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Molecular weight affect the tg

• For a given weight of the polymer, a low molecular weight sample will have more chain end segment than a high molecular weight sample.

• The larger the number of chain end segment , the larger will be the effective segmental motion.

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Molecular weight affect the tg

• Low molecular weight values result in lower glass transition temperatures whereas increasing values of molecular weight result in an asymptotic approach of the glass transition temperature to Tg,∞  . 

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