Erdem Ozdemir 030070307 - 1st Week Answers

The Glass Transition Temperature, T_g

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The glass transition is a phenomenon observed in linear amorphous poly­mers, such as poly(siyrene) or poly(methyl methacrylate). It occurs al a fairly well-defined temperature when the bulk material ceases to be brittle and glassy in character and becomes less rigid and more rubbery.

Many physical properties change profoundly at the glass transition tem­perature, including coefficient of thermal expansion, heat capacity, refractive index, mechanical damping, and electrical properties. All of these are dependent on the relative degree of freedom for molecular motion within a given polymeric material and each can be used to monitor the point at which the glass transition occurs. Unfortunately, in certain cases, the values obtained from these various techniques can vary widely. An example is the variation found in the measured values of T_f for poly(mcthyl methacrylate). which range from 110 °C using dilatometry (i.e. where volume changes are moni­tored) to 160 °C using a rebound elasticity technique. This, though, is an extreme example; despite the fact that the measured value of T_s does vary according to the technique used to evaluate it, the variation tends to be over a fairly small temperature range.

The glass transition is a second-order transition. In this it differs from gen­uine phase changes that substances may undergo, such as melting or boiling, which arc known as first-order transitions. These latter transitions arc char­acterised by a distinct volume change, by changes in optical properties (i.e. in the X-ray diffraction pattern and the infrared spectrum) and by die exis­tence of a latent heat for the phase change in question. By contrast, no such changes occur at the glass transition, though the rate of change of volume with temperature alters at the T_$, as illustrated in Figure 3.5.

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The glass transition can be understood by considering the nature of the changes that occur at the temperature in question. As a material is heated to this point and beyond, molecular rotation around single bonds suddenly becomes significantly easier. A number of factors can affect the ease with which such molecular rotation takes place, and hence influence the actual value that the glass transition temperature takes. The inherent mobility of a single polymer molecule is important and molecular features which cither increase or reduce this mobility will cause differences in the value of 7_?. In addition, interactions between polymer molecules can lead to restrictions in molecular mobility, thus altering the 7* of the resulting material.

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Figure 3.5 Plot of volume against temperature for a typical polymer passing through its glass transition.

Briefly, the following features are known to influence the glass transition temperature:

(a)  The presence of groups pendant to the polymer backbone, since they increase the energy required to rotate the molecule about primary bonds in the main polymer chain. This is especially true of side chains or branches.

(b)  The presence of inherently rigid structures in the backbone of the mole­cule, e.g. phenylene groups.

(c)        Crosslinking.

(d)  Hydrogen bonds between polymer chains.

(e)  Relative molar mass, which influences 7_g because higher molar mass polymers have less case of movement and more restrictions in their overall molecular freedom than polymers of lower molar mass.

if) The presence of plasticisers. These are discussed in detail in the next section of this chapter.

(The Chemistry of Polymers, John W. Nicholson, Pg:48,49)

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The glass transition temperature (Tg) is a key parameter in thermosetting polymers, not only from the product performance point of view, but also from the processing point of view, since it may strongly affect the reaction kinetics. The glass transition temperature marks the boundry bbetween the glassy, rigid state of a polymer and the soft, flexible (or fluid) state of the polymer. Below the glass tranition temperature, the available energy is insufficient to allow the molecules coordinated mobility (although there may be some localized motion), so the material is rigid; above the glass transition, the molecules can flow past each other above the glass transition temperature - the polymer is a "melt". In the case of thermoset polymers above the glass transition temperature, the chemical crosslinks prevent the molecules from flowing, but there is enough mobility for molecules to cooperatively relax, and the polymer becomes flexible and "rubbery".

(Cheng S.Z.D., Handbook of thermal analysis and calorimetry: applications to polymers and plastics, 2002, pg.315,316)