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 polymer obtained is white, opaque, and tough and has a crystalline melting temperature of265° C. The inherent viscosity (r]ir ) is 1.0—1.2 in 96% sulfuric acid (1.0 % solution, 25° C). In the past, prepolymers were prepared from dry PA salt on a small scale in sealed heavy-walled glass tubes. As these heavy-walled tubes are not safe to handle, they should no longer be used. 

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

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). 

Figure 7.6 Effect of ethylene glycol concentration on the crystalline melting temperature of PCTG copolyesters...

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.

Copolymers offer the twin advantages of reduced crystalline melting temperature and introduction of crosslinking sites. One of the best for. both of these purposes that has been reported so far is the thiocarbonyl fluoride-allyl chloro-formate copolymer (39). Products containing as little as 2-3 mole % of allyl chloroformate melt below 0°C and are readily crosslinked by incorporation of zinc oxide followed by heating. 

The first report of the successful synthesis of high molecular weight PAEH by aromatic nucleophilic displacement reaction appears to be from the reaction of 2,5-di(4-fluorophenyl)-l,3,4-oxadiazole and 2,2-di(4-hydroxyphenyl)propane (bisphenol A) [1]. A polymer with a relative viscosity of 0.50 dL/g and a Tg of 180°C was obtained. The 1,3,4-oxadiazole unit activates the fluoro group sufficiently to allow displacement by the bisphenolate. The corresponding dichloro compound gave low molecular weight polymer. Another PAEH with a Tg of 180 °C and a crystalline melt temperature (Tm) of 250 °C was prepared from the reaction of 3,6-dichloropyridazine with bisphenol A [1]. 

Fig. 2. Crystalline melting temperatures of poly(vinylidene chloride) (40).

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. 

Other thermo physical properties (Chap. 6-9) crystalline melting temperature... 

Microscopic crystalline regions found within a solid polymer below the crystalline melting temperature, (p. 1237)... 

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