Polymer, thermal property softening temperature

Mechanical and Thermal Properties. The first member of the acrylate series, poly(methyl acrylate), has fltde or no tack at room temperature it is a tough, mbbery, and moderately hard polymer. Poly(ethyl acrylate) is more mbberflke, considerably softer, and more extensible. Poly(butyl acrylate) is softer stiU, and much tackier. This information is quantitatively summarized in Table 2 (41). In the alkyl acrylate series, the softness increases through n-octy acrylate. As the chain length is increased beyond n-octy side-chain crystallization occurs and the materials become brittle (42) poly( -hexadecyl acrylate) is hard and waxlike at room temperature but is soft and tacky above its softening point. 

Thermal Properties. Some wholly N-methylated aromatic polyamides have been reported previously and found to be considerably lower in thermal stability than the corresponding unsubstituted polymers. Thus, Koton reported that the softening point of VII was 247°C, a temperature much below that of PPD-T, which shows no softening point below 400°C. 

We will discuss in this section the various ways that can be used to improve the thermal stability of polymers. The synthesis and thermal behaviour of some typical heat-resistant polymers (sometimes commercially available) will then be given. The volatilization of these materials has very seldom been thoroughly studied orders of reaction, activation energies and pre-exponential factors have generally not been determined. Therefore the thermal stability of the polymers will be characterized in an arbitrary way for the purpose of comparison. It must be stressed, however, that the physical properties of a polymer are at least as important for use at high temperature as the volatilization characteristics an infusible polymer is very difficult to process, and a heat resistant polymer with a low softening temperature is often useless. The softening temperature corresponds to the loss of mechanical properties. It can be measured by the standard heat deflection test. 

The mutual repulsion between substituents may cause some displacement. As a result, the plane of symmetry is bent in the form of a helix. This occurs also in biopolymers (double-helix of deoxyribonucleic acid (DNA)). Different stereoisomers have different mechanical and thermal properties. For example, atactic polystyrene is an amorphous polymer whereas syndiotactic polystyrene is a crystalline substance. The chemical design of macromolecules determines their properties as extent of crystallization, melting point, softening (glass transition temperature), and chain flexibility which in turn strongly influence mechanical properties of the materials. 

In this second edition of handbook, discussions regarding low-temperature brittle point, Vicat softening point, oxidative induction time, melt and crystallization parameters using DSC, and thermal degradation using TGA are added in order to bring around completion of comprehension on thermal properties of polymers and blends. 

It is possible to use differential thermal analysis (DTA) and thermogravi-metric analysis (TGA) to evaluate the thermal properties of several types of polyurethane elastomers. For example, a typical elastomer prepared from MDI, a polyether and an aliphatic diamine extender will show from DTA two very small endothermic changes, one at 150°C and one at 205°C that is identical with the softening temperature. The polymer is found to melt at 2WC and exhibit weight loss in two steps, respectively, beginning at 280°C and 325X. 

Looking at the thermal properties, the polymer has a heat distortion temperature of 217°C at 264 psi. The small difference between softening point and heat distortion temperature suggests the polymer has very good property retention at elevated temperatures. This is confirmed by measurements of the flexural modulus of the material versus temperature, which shows that even at 180°C, the flexural modulus is over 300,000 psi. Add to that the well-known thermal-oxidative stability of polyimides (in general) and it should not be at all surprising that the continuous use temperature is 170°C or higher. 

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