Melting temperatures relationship with glass transition

The temperature dependence of melt viscosity at temperatures considerably above T approximates an exponential function of the Arrhenius type. However, near the glass transition the viscosity temperature relationship for many polymers is in better agreement with the WLF treatment (24). 

In order to select materials that will maintain acceptable mechanical characteristics and dimensional stability one must be aware of both the normal and extreme thermal operating environments to which a product will be subjected. TS plastics have specific thermal conditions when compared to TPs that have various factors to consider which influence the product s performance and processing capabilities. TPs properties and processes are influenced by their thermal characteristics such as melt temperature (Tm), glass-transition temperature (Tg), dimensional stability, thermal conductivity, specific heat, thermal diffusivity, heat capacity, coefficient of thermal expansion, and decomposition (Td) Table 1.2 also provides some of these data on different plastics. There is a maximum temperature or, to be more precise, a maximum time-to-temperature relationship for all materials preceding loss of performance or decomposition. Data presented for different plastics in Figure 1.5 show 50% retention of mechanical and physical properties obtainable at room temperature, with plastics exposure and testing at elevated temperatures. 

Relationship Between Molecular Structure and Composition of Poly(ethylene-co-p-MS) Copolymers. From above discussion, we have shown that poly(ethylene-co-p-MS) copolymers with a wide range of compositions can be achieved by varying the p-MS monomer concentration in the feed. To fully understand the properties of this new class of materials, it is very important to know the correlation between the copolymer compositions and their molecular structures. In this section, we focus on the effect of p-MS concentration on molecular weight, molecular weight distribution, melting point (Tm), crystallinity and glass transition temperature (Tg) of the copolymer. A series of copolymers with various p-MS concentrations were analyzed by GPC and DSC. 

In laboratories where preparative work is done, softening points are often determined with the Kofler bar, which consists of a metal plate with a temperature gradient along it. The sample is moved with a brush from the colder to the warmer points on the metal plate. At a given point, the sample will remain stuck to the plate the temperature associated with this point is taken as the softening point. Since this temperature depends on both the viscosity of the sample and its adhesion to the metal surface, the softening point thus determined is very approximate. It often bears no simple relationship to the glass transition or melt temperature. 

The paracrystal model is rather eonvenient for explaining some observations, sueh as the broadening of the x-ray diffraction pattern and the essentially linear relationship between the melting temperature and the spedfie volume of nylon samples that are eharacterized by rather different thermal and processing histories [278]. On the other hand,, for example, both the existence of a glass transition, which is affected by plasticizing agents, and the observed relationships between density and the diffusion and absorption of water and dyes are less compatible with this model. These phenomena and processes seem to be more readily explained in terms of a two-phase model. 

For the members of the series which were thermotropic, the glass transition temperatures, T, melting points, T, and clearing temperatures, T, decrease in a Uniform manner, for he most part, with increasing alkyl group size. These polymers were all nematic, but the temperature range over which they were liquid crystalline, AT, did not follow a simple relationship with substituent size. The... 

Fig. 14.2 Temperature dependence of the number of boron atoms in boroxol rings, /, as obtained from the Raman peak at 808 cm . The solid lines represent graphical interpolations of experimental data from various authors [67-69] using different normalisation procedures. The experimental data of Walrafen et al. [67, 68] were modelled by these authors with the function ln = B/T + C where A, B, C are constants (A is the value of / at Tg). The values of [A, B, C] are (0.644, 3237.66, -2.58893 in [67] and (0.7882, 2490.5, -2.3734 in [68]. The data from Hassan et al. which were digitised from Fig. 9 of [69] originally provide fat, the number of atoms in boroxol rings and are represented here as /, using the relationship / = fat- The vertical arrows indicate the glass transition Tg) tind melting (J ,) temperatures...

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