Transition crystalline melting temperature

Relatively few processible polyimides, particularly at a reasonable cost and iu rehable supply, are available commercially. Users of polyimides may have to produce iutractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions iu order to meet property requirements such as thermal and thermoxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the stmcture—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

The transition temperatures that are combined in Figure 2 show the disappearance of crystallinity in the copolymers as the Ter and Tm flow together moving away from either homopolymer. This reflects the random distribution of monomer units in these copolymers. If the copolymer reactions had given homopolymer mixtures, there would be two separate crystalline melting temperatures. In addition, the 13C NMR indicates that the copolymer products contain a random distribution of C5 and C8 units and that the resulting double bonds are cis from the C8 monomer and largely trans from the C5 monomer (52). 

Among the spectrum of melt-spinnable fibers such as polyolefins and nylons, PET stands at the upper end in terms of crystalline melt temperature and glass transition temperature. This provides superior dimensional stability for applications where moderately elevated temperatures are encountered, e.g. in automobile tires or in home laundering and drying of garments. The high thermal stability results from the aromatic rings that hinder the mobility of the polymer chain. 

Fig. 1 9 Determination of glass transition and crystalline melting temperatures by changes in specific volume.

The PET studied was Mylar C film (DuPont) of 12 to 25 micron thickness. Such films are biaxially oriented and are about 50% crystalline. The glass transition and melting temperatures are about 70° and 250°C., respectively. The polymer contains several parts per thousand of inorganic materials, moisture, and 1 to 2% cyclic oligomers (mostly trimer) (8). The number-average molecular weight of the polymer molecules is... 

In this chapter it will be demonstrated that the two main transition temperatures, viz. the glass-rubber transition temperature and the crystalline melting temperature can be correlated with the chemical structure by means of a method based on group contributions. 

Table 15.2 Typical polymer glass transition and semi-crystalline melting temperatures...

The glass transition temperature of amorphous polymers is a function of the chemical structure of the polymer chain. It varies widely with the types of skeletal atoms present, with the t T)es of side groups, and with the tacticity of side groups along the polymer backbone. Table 14.11 demonstrates the effects of structural variations on the crystalline melting temperature and glass transition temperature for several polymers. 

The glass transition temperature, T, the crystalline melting temperature, and the enthalpy for melting. A//, for PMMA, PETG... 

Amorphous materials usually exhibit an apparent second-order thermal transition, the glass transition, at about two-thirds of the crystalline melting temperature (measured in Kelvin)... 

The transition temperatures were essentially the same for a similar type of polymers regardless of molecular weight range In this study. The lower transition (-86 C) In the B-CL of 70-30 wtX dlblock Is associated with Tg of the polybutadiene block and the upper transition (56 C) with the crystalline melting temperature of polycaprolactone segment No polycaprolactone Tg (about -60 C for Its amorphous part) was observed In the B-CL dlblock, evidently because of Its lowr content and proximity to the polybutadiene Tg. It can be detected at -59 C, however. In the S/B-Cl of 33/44-23 wt% dlblock terpolymer. In the latter, the Tg of the S/B block (-20 ) was essentially the same as the random S/B copolymer control which Is coded as S/B-CL 33/44-0 iftZ In Table IV. 

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