Chapter 1 Introduction
1.1 Glass transition temperature
Many plastics lose their strength at relatively low
temperature. Continuous-service-temperature comparison of plastics reveals that
most common plastics can endure temperature more than 150oC when
under low or no stress. Glass transition temperature or Glass point
(Tg) is the point at which polymers act as glass or become viscous
liquids. That's a very important factor for polymers to evaluate their
processing and application performances.
The Glass transition is a reversible change that occurs when a
resin polymer is heated to a certain temperature (Tg), resulting in
a sudden change or transition from rigid polymer to a flexible, rubbery
material or a viscous liquid. When the polymer is cooled below this
temperature, it becomes hard and brittle, like glass. Certainly there are a few
polymers used above their Glass transition temperatures, however majority of
polymers are used below. Popular hard plastics like polystyrene
(Tg=100oC) is used below their glass transition
temperatures; that is its glassy state. Their Glass transition temperatures are
well above room temperature, both at around 100oC.
Rubbers elastomers like polyisoprene and polyisobutylene are
used above their Glass transition temperatures; that is in the rubbery sate
where they are soft and flexible.
The glass transition differs from the melting transition by
the fact that the former is a transition which happens to amorphous polymers
and the latter is a transition which occurs in crystalline polymers. But even
crystalline polymers will have some amorphous portions; this portion usually
makes up 40-70% of the polymer sample. This is why the same sample of a polymer
can have both a Glass transition temperature and a melting temperature. But
only the amorphous portion undergoes the glass transition and only the
crystalline portion undergoes melting. This change in mobility with temperature
happens because heat is really a form of kinetic energy.
The exact temperature at which the polymer chain undergoes
this big change in mobility depends on the structure of the polymer. A polymer
chain that can move around fairly easily will have a very low Tg, while one
that doesn't move so well will have a high one. The more easily a polymer can
move the less heat it takes for the chains to commence wiggling and break out
of the rigid glassy state and into the soft rubbery state. A given polymer
sample does not have a unique value of Tg because the glass phase
is not at equilibrium.
1.1.1 Factors governing Tg
The height of the glass-rubber transition temperature is, in
the first instance, governed by the competition between thermal motion and the
attraction forces between the chains.
The thermal motion is depend on the freedom of the chain to
undergo changes in conformation .When this freedom is higher, the chain is
subjected to a stronger thermal motion than a chain which, e.g. as result of
hindrance in rotation, is more rigid, the chain stiffness plays an important
role.
The primary criteria are:
-chain flexibility
-chain interactions
| 
