Thermal transitions in polymers

In this section we will briefly review the character of the principal thermal transitions that occur in polymers, crystallization, melting and the glass transition, then treat these individually in more detail. Some background material has been covered in our discussion of States of Matter, which you should also review if you ve got a memory lilce a sieve. 

FIGURE 10-15 Schematic plots of the volume changes on cooling a polymer. 

FIGURE 10-16 Schematic plots showing the dependence of the T on the rate of cooling. 

If you are unfamiliar with materials and their properties and have a basic knowledge of thermodynamics, you may by now be thinking To hell with kinetics, why should a polymer, or any other material, crystallize at all You may have recalled that in the disordered state characteristic of the melt, the system has a higher entropy than in the ordered 

Hopefully, you will recall that we can express the overall change in entropy in terms of parameters describing changes within the system alone, using (for a fixed pressure and temperature) the Gibbs free energy, Equation 10-21 in the preceding section (AG = -TA AZf-TAS). 

The term transition refers to a change of state induced by changing the temperatures or pressure. TWo major thermal transitions are the glass transition and the melting, the respective temperatures being called Tg and 

The different types of thermal response in the transition of a thermoplastic polymer from the rigid solid to an eventually liquid state can be illustrated in several ways. One of the simplest and most satisfactory is to trace the change in speci c volume, as shown schematically in Fig. 2.19. 

The volume change in amorphous polymers follows the curve ABC. In the region C-B, the polymer is a glassy sohd and has the characteristics of a glass, including hardness, stiffness, and brittleness. But as the sample is heated, it passes through a temperature Tg, called the glass transition temperature, above which it softens and becomes rubberhke. This is an important temperature 

In a perfectly crystalline polymer, all the chains would be contained in regions of three dimensional order, called crystallites, and no glass transition would be observed. Such a polymer would follow the curve G-F-A, melting at to become a viscous liquid. 

Perfectly crystalline polymers are, however, rarely seen in practice and real polymers may instead contain varying proportions of ordered and disordered regions in the sample. These semicrystalline polymers usually exhibit both Tg and Tm (not 7 ) corresponding to the disordered and ordered regions, respectively, and follow cirrves similar to E—H—D-A. Tm is lower than 7 and more often represents a melting range, because the semicrystalline polymer contairrs crystallites of various sizes with many defects which act to depress the melting temperature. 

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The initial four chapters of the book concern several important aspects of polymer science which are relevant to a course in polymer chemistry. Following Chapter 1, which is a general introduction aimed at giving the reader an appreciation for the language, applications, and versatility of synthetic polymers. Chapter 2 is devoted to polymer characterization dealing with the size and shape of a polymer chain, polymer isomerism, polymer conformation, and thermal transitions in polymers. 

In the-preceding chapters we showed that several thermo-analytical techniques are available to study thermal transitions in polymers. The thermal transitions of new, polymeric systems are usually first investigated by DSC measurements (small amount of sample needed/high scanning speed). The DSC technique is, however, often not sensitive enough to detect weak and/or secondary relaxation effects. DMA (rigid polymers) or dielectric (rigid/rubbery/viscous systems) experiments are then necessary. The sensitivity of the dielectric measurements depends on the polymer s polarisability (see 5.1.2). Besides, DC conduction effects can seriously hamper the detection of the relaxation effects studied. The TSD analysis technique offers in such a case an attractive and sensitive alternative. 

Figure 1.2 Schematic representation of the main thermal transitions in polymers in a plot of specific volume-temperature.

In addition, there is a still more fundamental difference between the thermal behavior of polymers and simple molecules. To understand, first recall that molecular motion in a polymer sample is promoted by its thermal energy. It is opposed by the cohesive forces between structural s ments (groups of atoms) along the chain and between neighboring chains. These cohesive forces and, consequently, thermal transitions in polymers depend on the structure of the polymer. In this regard, two important temperatures at which certain physical properties of polymers undergo drastic changes have been identified ... 

Thermal analysis is an essential tool to study the necessary thermal transitions in polymers, without which the processing and fabrication of polymeric materials is not possible. The thermal behaviour of a polymer has significant technological importance. For example, the T of rubber determines the lower limit of the use of rubber and the upper limit of the use of an amorphous thermoplastic. The most important techniques will be briefly discussed below. 

Study of thermal transitions in polymers and other materials (Gorbunov et al. 1999, 2000c). The information on thermal properties garnered from such studies need not be an end in itself and is indeed used more often as a tool for understanding material structure and processes. The microanalogs of traditional thermal analysis techniques yield results that are similar to those obtained using macroscale techniques, but with the additional benefit that these results are spatially resolved. 

In contrast to small organic molecules, longmass diffusion, but also by small-scale diffusion of a few monomer units. It is generally observed that even monomolecular reactions could happen only if these motions are unfrozen. In the wider sense, the dependence of reaction efficiency on polymer morphological structures can be described in terms of the free volume concept, and of diffusion constants [6]. These molecular characteristics are themselves dependent on the thermal transitions in polymers, the most important of which being ffie glass transition temperature. 

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