POLYMER SCIENCE; TEX: 103

SECTION: BTOPIC: GLASS TRANSITION

TEMPERATURECOURESE ADVISER

DR. ENGR. MD. SAIFUR RAHMAN

HEAD OF THE DEPARTMENT OF B.S.T.E

GROUP MEMBERS

NAME ID

Md. Shahjahan Ali

Sazzad Hossain

Md. Tajul Islam

06313200

06313257

06313222

Md. Riyaz-Ul-Islam

Md. Habibur Rahman

Mithun Ghos

Md. Syedur Rahman

06313241

06313077

06313253

06212150

The glass transition temperature is different for each polymer, but many polymers are above Tg at room temperature. In many cases the polymers are at least partially crystalline at room temperature and the temperature at which the crystals melt (Tm) is above room temperature. The graph below shows how some polymers are above Tg but below Tm at room temperature. Such polymers are rubbers (so long as they are largely amorphous) at room temperature. However, the polymer may flow like a liquid over long time periods as its amorphous component relaxes under the polymer's weight

Introduction

Glass transition temperature

The temperature, below which it becomes hard and above which it becomes soft, is called “glass transition temperature”.

Glass transition temperature is represented by the symbol Tg. At glass transition temperature the hard, brittle state is known as “the glassy state” and the soft, flexible state is known as “the rubbery” or “viscous elastic state”. On heating polymer becomes a highly viscous liquid and starts flowing is known as “viscous fluid state” (Tf).

Glass state Rubbery or Viscous fluid state

Viscous elastic state

(Brittle state) (Tough plastics & (Polymer melts)

Rubbers)

Tg Tf temperature(Glass transition temperature) (Flow temperature)

Fig: change of state with temp. in polymeric materials

There are several methods available to measure the glass transition temperature, some of which are given below. Since the value of the glass transition temperature depends on the strain rate and cooling or heating rate, there cannot be an exact value for Tg.

Measurement of Tg

Mechanical Methods

It is possible to calculate a value for the glass transition temperature by measuring the elastic modulus of the polymer as a function of the temperature, for example by using a torsion pendulum. Around Tg there is a large fall in the value of the modulus. The frequency of the oscillation is important, since Tg depends on the time allowed for chain segment rotation.

A more common method is dynamic mechanical thermal analysis (DMTA), which measures the energy absorbed when a specimen is deformed cyclically as a function of the temperature, and a plot of energy loss per cycle as a function of temperature shows a maximum at Tg.

As was shown in the thermodynamic approach to glasses, the enthalpy of a polymer decreases as the temperature decreases, but with a change in slope in the graph at Tg. Taking the derivative of this graph with respect to temperature, the specific heat capacity can be plotted, as below. The specific heat capacity, Cp, can be measured using calorimetry, e.g. differential scanning calorimetry (DSC). The value of Tg depends on the heating or cooling rate.

Thermal Methods

The changes in conformation that occur above Tg require more volume, so plotting a graph of specific volume or thermal expansion coefficient against temperature will give a value for Tg. The actual volume of the molecules stays the same through Tg, but the free volume (the volume through which they can move) increases.

Volume Methods

If a varying electric field is applied to a polymeric material, any polar groups will align with the field. Below Tg rotation of the bonds is not possible, so the permittivity will be low, with a big increase around Tg. At higher temperatures the increased thermal vibrations cause the permittivity to drop again. If the frequency of the field is increased, the polar groups have less time to align, so the glass transition occurs at a higher temperature.

Dielectric Constant

Glass transition temperature of some materials Polymer Tg (°C)

Nylon 6 50

Nylon 6,6 50

Polyethylene (high density) -125

Polyethylene terephthalate 69

Polystyrene 100

Poly(vinyl chloride) (PVC) 81

Polystyrene 95

Polypropylene (isotactic) 0

Poly-3-hydroxybutyrate (PHB) 0

Poly(methylmethacrylate) (atactic) 105

Chalcogenide AsGeSeTe 245

ZBLAN 235

Tellurite 279

Avatrel; Polynorbornene 215

Fluoroaluminate 400

Tyre Rubber 160

Soda-lime glass 520-600

Quartz glass 1175

Glass transition temperature and molecular weight

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 molecular weight is not pronounced and this can be seen from Fig. many attempts have been made to compute mathematical relationships, among whish the following two are frequently used:

Ts=Ts-K/Mn

1/Ts=1/Ts+A/Mn

Where Ts is the glass transition temperature at infinite molecular weight and K and A are arbitrary constants.

Molecular weight around 20,000

Molecular weight

Tg

Fig. Plot showing effect of molecular weight on glass transition temperature of polymers.

Glass transition temperature and plasticizers

Plasticizers are low molecular weight non-volatile substances (mostly liquids), which when added to a polymer, improve its flexibility, possibility and, hence, utility. The plasticizer substantially reduces the brittleness of many amorphous polymers because its addition even in small quantities markedly reduces the Ts of the polymer. This effect is due to a reduction in cohesive forces of attraction between polymer chains. Plasticizer molecules (being relatively small in size compared to polymer molecules) penetrate into the polymer matrix and establish polar attractive forces between it and the chain segments. These attractive forces reduce the cohesive forces between the polymer chains and increase the segmental mobility, thereby reducing the Ts value.

Importance of glass transition temperature

The glass transition temperature is an important parameter of a polymeric material. It is used as a measure for evaluating the flexibility of a polymer molecule and the type of response the polymeric material would exhibit to mechanical stress. The Tg value of a polymer, therefore, decides whether a polymer at the use temperature will behave like rubber or plastic. As present, it would suffice to know that polymers above their Tg will be soft and flexible and exhibit a delayed elastic response (viscoelasticity). While those below their Tg will be hard and brittle and will possess dimensional stability.The Tg value along with the Tm value give an indications of the temperature region at which a polymeric material transforms from a rigid solid to a soft viscous state. This helps in choosing the right processing temperature, i.e., the temperature region in which the material can be converted into finished products through different processing techniques such as molding, calendaring and extrusion.

Conclusion

Reference Books:*V R Gowariker, N V Viswanathan, Jayadev Sreedhar, Polymer Science, 17th Edition.New Age International (P) Limited, Publishers. New Delhi,India.Reference Links:*www.macrogalleria.com*www.google.com*www.yahoo.com

THE END &

THANK YOU ALL FOR

WATCHING OUR PRESENTATION.