Physical Properties of Polymers

Muhammad Zafar Iqbal

Date: 15-05-2008

Today

Glassy State and Glass Transition Temperature

• Introduction• States of aggregates• States of phases• Transitions and physical Properties

• Factors affecting Tg• Measurement techniques for Tg from different instruments

What is Glass Transition Temperature?

• A rubber ball if cooled below -70 0C becomes so hard and brittle that it will break into pieces like a ball falling on a hard surface.-------------------- ?

• The temperature below which a polymer is hard, brittle and above which it is soft is called Glass Transition Temperature.

• The hard or brittle state is called glassy state• The soft flexible state is called rubbery state

• On further heating, if the polymer is un- crosslinked, then it will become a viscous liquid and start flowing. The temperature at which this occurs is called flow temperature.

Glassy Solids and Glass TransitionStates of Aggregation• Low mol. wt. compounds have three states of aggregation:

Solid, liquid, gas. Polymers have only two: solid, liquid.• In a crystalline solid (low mol. wt.), there is no brownian

movement (molecular translation). Only vibrational motion is there.

• The molecules occupy definite positions within the crystal lattice and this well defined arrangement is repeated several times in all three dimensions of the crystal and give rise to LONG RANGE ORDER (LRO).

• A low molecular weight crystalline solid is defined by:1- position & condition of individual molecules2- geometry of the crystal lattice3- response to an external force

• As the temperature is raised, more energy is added and vibrational motion increases sufficiently to cross the energy barrier which holds the molecules together. Brownian movement is activated and it becomes vigorous at higher temperatures.

• The molecules are diffused all over randomly. This state is called liquid state.

• The temperature at which this change of state occurs is called melting point Tm.

• In polymers, as we increase temperature, localized units (segments) within the long chain molecule are first mobilized before the whole molecule start moving.

• At this stage, we have two kinds of movements:1- localized mobility of chain segments: IBM2- The total mobility of the molecule as a whole: EBM

• As temp. raised, IBM starts at first. Further heating, EBM is also activated. At this stage, the material behaves like a very viscous liquid.

• In polymers there is also an Intermediate stage where IBM is activated but EBM does not. The localized segments where mobility is already activated correspond to liquid state while the molecules as a whole, where mobility is forbidden is in solid state.

• This state which is a combination of liquid and solid, is called the rubbery state.

• Under the influence of an applied stress, it exhibits the properties of a viscous fluid as well as an elastic solid and undergoes what is called Visco-elastic deformation.

States of Phases• A phase is a part of a system separated from the other parts

of the same system by an interface.• Different phases within the same system differ with regard to

the order of arrangement of their molecules.• The crystalline state has LRO.• The liquid state does not have any LRO. Although the

molecules in the near vicinity of any given molecule may exhibit some orderly arrangement (short range order:SRO), due to aggregation or association, but even this limited orderliness ceases to exist at a small distance from the given molecule.

• Two situations arise in liquid phase:1- Where molecular mobility is frozen2- Where molecular mobility is activated

• In the first case, as there is no molecular mobility, so it is solid state. At the same time, it does not posses any LRO, so unable to crystallize. Such solids which are unable to crystallize are called Amorphous solids. Amorphous solids are often compared with soda glass and therefore amorphous state is often called the glassy state.

• As temperature is raised, BM is activated and there is molecular mobility. Due to onset of molecular motion, solid becomes liquid (because liquids have molecular mobility).

• There is no change in the internal order of the molecular arrangement, but a change in energy of motion.

• Therefore amorphous to liquid is not a true melting.

Conclusion Remarks

1- both amorphous and crystalline solids are in solid state of aggregation while the crystalline solid exists in crystalline state

2- Amorphous solids, although physically are aggregated solids, but are in liquid state

3- Both amorphous and crystalline substances can exist in a molten liquid state.

Cont’dWhen a crystalline substance passes from a molten liquid state

to a solid crystalline state, there is not only a physical change but also a phase change, involving transition from an equilibrium state of short range order to an other equilibrium state of long range order. In other words it is a real phase transition.

When an amorphous substance changes from a molten liquid to a glassy solid, there is no change occurring in molecular order.

However there is decrease energy of thermal motion, molecules loose their mobility and physically a solid is formed.

The glassy solid is considered to be a super-cooled liquid. The transition from the molten liquid to a solid involves only a physical change and no phase change.

Cont’d • A polymer that does not possess LRO cannot exist in the

crystalline state• It can exist in glassy rubbery or liquid state• At low temperatures, where molecular as well as segmental

mobility is ceased, it exists as a glassy solid• On heating, segmental mobility begins and it becomes rubbery.• The temperature at which an amorphous polymer passes from

glassy to rubbery state is called Tg.• The temperature at which the molecular mobility sets in and

polymer passes from rubbery to liquid state is called Tf.

• Due to the absence of a crystal lattice in amorphous polymers, no true melting takes place and nothing called melting point is associated with it.

Crystalline Polymers:• A polymer capable of LRO is called Crystalline polymer• The segments are held very firmly and do not posses any

mobility.• The energy required to overcome such forces forming the

crystallites is quite high and almost equal to the energy required to initiate the molecular mobility.

• That’s why crystalline polymers do not pass from solid state to rubbery state and do not possess any Tg.

• The temperature at which transition from crystalline to liquid state occurs is called Tm.

• Most of the commercially available polymers are not 100% crystalline or 100% amorphous. Therefore they possess both Tg and Tm.

Transitions and Physical Properties• The most important physical property that can reveal the

changes during transitions is specific volume.• Normally this is measured in dilatometers

• When a low mol. Wt. crystalline solid melts,the change of state is associated by discontinuity in T.d. variables.

Transitions in Polymers