Effect of Temperature on Polymers

We have observed earlier that solid polymers tend to form ordered regions, such as spherulites (see Chapter 11 for complete details) these are termed crystalline polymers. Polymers that have no crystals at all are called amorphous. A real 

In this section, only the effect of chain structure on Tg is examined—other factors will be discussed in Chapters 10-12. In order to understand the various transitions for polymeric systems, we observe that a molecule can have all or some of the following four categories of motion  

Long cooperative wriggling motion of 40-50 C—C bonds of the molecule, permitting flexing and uncoiling 

The glass transition temperature, Tg, is the temperature below which the translational as well as long and short cooperative wriggling motions are frozen. In the robbery state, only the first kind of motion is frozen. The polymers that have their Tg values less than room temperature would be rubbery in nature, such as neoprene, polyisobutylene, or butyl rubbers. The factors that affect the glass transition temperatures are described in the following subsections. 

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Ej has a value of about —60 kJ mol-1 for thermal initiator decomposition, and Xn decreases rapidly with increasing temperature. Ej is about the same for a purely thermal, self-initiated polymerization (Fig. 3-16). For a pure photochemical polymerization Ej is positive by approximately 20 kJ mol-1, since Ed is zero and X increases moderately with temperature. For a redox polymerization, Ej is close to zero, since Ed is 40-60 kJ mol-1, and there is almost no effect of temperature on polymer molecular weight. For all other cases, Xn decreases with temperature. 

Fig. 7-3 Effect of temperature on polymer molecular weight in the BF3 polymerization of tetra-hydrofuran. After Burrows and Crowe [1962] (by permission of Wiley-Interscience, New York).

The effect of temperature on polymer stresses can also be measured. Again, this could be done in situ. Figure 14 presents a plot of stress as a function of temperature 13). The data for a polyimide in Figure 14 indicates that interfacial stress decreases with an increase in temperature. This could be effectively utilized to determine the effect of frictional heating on polymer stress as well as wear. 

The effect of temperature on polymers is of practical importance because most polymers are processed at relatively high temperature and have applications in a wide temperature range. The temperature dependence of thermal cOTiductivity of polymers has been studied from extremely low temperatures at 0.1 K to above melting point [11,13-18,20]. Generally, with increase in temperature, thermal cOTiductivity for amorphous polymers increases gradually... 

The homopolymerization of several important a-olefins by C2- and Q-symmetric ansa-metallocenes was thoroughly investigated in the past decade. Kaminsky and Grumel developed highly active syndioselective flni fl-zirconocene catalysts (5, 6 respectively) for the polymerization of 1-pentene, 1-hexene, and 1-octene, and studied the effects of temperature on polymer molecular weight and microstructure.It was found that polyma- syndiotacticity depended strongly upon... 

Ricci, G Italia, S. Porri, L. Polymerization of (Z)-l,3-pentadiene with CpTiCl3/MAO. Effect of temperature on polymer structure and mechanistic implications. Macromolecules 1994, 27, 868-869. 

The fluorescence properties of these probes permits us to study the rotational relaxation in various polymers and even during polymerization reactions and thereby obtain information on the microscopic rigidity of the media. In the following discussion a description of the photophysical properties of the dyes 1-3 will be given, with particular emphasis on the excited-state conformational relaxation in various media. This will be followed by a discussion related to the application of these probes to study polymerization reactions, the effect of polymer molecular structure on free-volume, the dependence of polymer chain relaxation on molecular weight, and the effect of temperature on polymer conformation and free-volume. 

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