Glass-rubber transition temperature determination

The volume resistivity/temperature curve clearly illustrates the strong difference in the temperature dependency of the volume resistivity of a polymer in its glassy phase and in its rubbery phase. The Tg-value, obtained by drawing two tangents near this glass-rubber transition, is determined at 1000/T 3.58 or 6°C. This Tg-value is a real static Tg-value and its hypothetical frequency [f(h)l in the frequency/temperature plane will be lower than the f(h) = lxB-2 to lxE-4 claimed by Phillips [12] for dilatometric experiments. A good fit on the Arrhenius plot was obtained assuming an f(h) of lxE-6 for this Tg-value, see below. 

The temperature at which the spin-spin relaxation of protons begins to involve the Lorentzian relaxation process due to liquid-like polymer protons in addition to the Gaussian-type relaxation process due to solid-like polymer protons is considered to be a glass/rubber transition temperature. Basically, this is a critical temperature of molecular mobility as determined by NMR relaxation measurements, and is analogous to the glass transition temperature (Tg) determined by DSC. This critieal mobility temperature is referred as T- c-... 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

It is clear from the described characteristics that the secondary transitions have specific features, different from those of the glass-rubber transition. Furthermore, to determine these characteristics, as well as the existence of single or multiple motional processes, measurements have to be performed at various temperatures and frequencies. 

As is well known, the glass-rubber transition is of considerable importance technologically. The glass transition temperature (Tg) determines the lower use limit of a rubber and the upper use limit of an amorphous thermoplastic material. With increasing molar mass the ease of "forming" (shaping) diminishes. 

Usually the primary (P) glass-rubber relaxation cannot be resolved from the secondary relaxation at hypersonic frequencies. However, this is not always the case. The Brillouin frequencies Awd) and tan 8 for polypropylene glycol (PPG) (13) are plotted versus temperature in Figure 9. Two tempratures of maximum loss are observed. The higher temperature loss at 100 °G and a frequency of 4.40 GHz correlates very well with the primary glass-rubber relaxation line determined by dielectric relaxation at gigahertz frequencies (13), The lower temperature loss at 50°G and a frequency of 5.43 GHz correlates with an extension of the secondary transition line. The transition map is shown in Figure 10. 

Thermomechanical Analysis (TMA) can be defined as the measurement of a specimen s dimensions (length or volume) as a function of temperature whilst it is subjected to a constant mechanical stress. In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored. Many materials will deform under the applied stress at a particular temperature which is often connected with the material melting or undergoing a glass-rubber transition. Alternatively, the specimen may possess residual stresses which have... 

Many properties are involved in determining a material s ultimate usefulness. For homopolymers, the temperature of the glass-rubber transition, presence or absence of crystallinity, crosslinking, molecular weight characteristics, permeability, and toughness all make a contribution. When two different polymers are mixed or joined in some fashion the mode of mixing becomes important as well. Tables 2.1-2.3 describe some of the many actual or proposed applications for two-polymer combinations. In many cases, the patent literature has been referred to, as this is an excellent source of such information. With polymer blends, blocks, and grafts, many new properties and hence uses appeared that were previously unavailable. 

Glass transition temperature determined by middle-point method neoprene -37 C, butyl rubber -61 C, cis-polybutadiene -104 °C. 

A most important conclusion can be drawn immediately and it concerns the nature of the main part, the glass-rubber transition. As we find a systematic shift of the time range of the transition with temperature, it is obvious that we are dealing here w ith a purely kinetical phenomenon rather than with a structural transition like the melting process or a solid-solid phase change. Curves demonstrate that whether a sample reacts like a glass or a rubber is just a question of time. Temperature enters only indirectly, in that it determines the characteristic time which separates glassy from rubbery behavior. 

These changes being reversible, the glass-transition temperature determines whether a particular polymeric material will behave as a glass (hard and slilT plastic) or elastomer at a given temperature. In Table A are listed (he approximate Tg values for some of the polymers from which it is clear that rubbers have their Tg considerably below room temperature, whereas Tg for plastii-s is much above room temperature. Tg for fibres is also substantially higher than room temperature. 

It is unfortunate that test methods for soft plastics and for rubbers, although very similar, are not identical, for example differences in tensile stress strain, tear and hardness methods. If they were aligned, much of debate about which method to use would be eliminated. For some properties, there is a distinct difference in approach. For example, glass transition temperature is frequently determined for plastics whilst various low temperature tests have been specifically developed for rubbers. 

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