This is note about on the factors affecting glass transition temperature. by Molecular weight in detail

1. L. D. COLLEGE OF
ENGINEERING
RUBBER TECHNOLOGY
Sub:- V.E.E(Viscoelasticity of elastomer)
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2. Topic:-Molecular weight affect
the Tg
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3. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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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13. 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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14. 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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