Crystalline polymers melting temperature

Fig. 4.8.7. Typical DSC curves of a crystalline polymer melting temperature (T, ), crystallization temperature (7V), glass transition temperature (Tg), and cold crystallization temperature (T )...

Linear amorphous polymers are glasslike at low temperatures and become leathery at temperatures slightly higher than the glass transition temperature (Tg). These leathery polymers become rubbery at slightly higher temperatures, and crystalline polymers melt at the melting point (Tm). 

On the other hand, the temperature of melting and crystallinity measurement can describe only those stereosequences that crystallize. In propylene oxide polymers which we have studied, for example, isotactic sequences are the only ones that crystallize. Therefore, in these polymers we can determine only the stereosequence length of isotactic units and the average length of the uncrystallizable units. The syndiotactic and atactic sequences in the uncrystallizable blocks are not distinguishable from the crystallinity and melting temperature measurements. 

As already mentioned, positional isomerism is important for the solubility and fusibility of aromatic LC polyesters. Consequently, polyesters made from symmetrical 2,5-disubstituted or 2,3,4,5-tetrasubstituted monomers should result in polymers that are less soluble and less fusible. This is in general the case with short lateral substituents. Ballauff and others reported that the series of poly( 1,4-phenylene-2,5-dialkoxy tereph-thalate)s with long flexible alkoxy side chains at the terephthalic moiety result in tractable LC polyesters [20] (Fig. 12). These polyesters exhibit a high degree of crystallinity with melting temperatures below 300 °C. Polyesters with short side chains (2350°C for m = 2... 

We find an increase in the transition temperature linear in Ve, and a continuous second order transition above a certain critical external field. The external field can cause ordering in the isotropic phase (fig. 5), As an application, the magnetic birefringence in a isotropic liquid crystalline polymer melt has been calculated and good agreement with measurements of Haret is found. 

For certain uses, it is desirable to prepare biodegradable PVA plasticized with biodegradable plasticizers that specifically assist in lowering the polymer melt temperature and the extrusion temperature and at the same time maximize water solubility by minimizing crystallinity. A PVA composition has been described that is transesteri-fied with a polyol as plasticizer (16). 

For semicrystalline polymers, melting temperature (Tu) is correlated to the thickness of crystalline lamellae. If all other variables are held constant, thinner lamellae will melt at lower temperatures than thicker ones. Thus, the melting temperature Tu, defined as the maximum temperature within a melting regime M, can be used to approximate the lamellar thickness. Within the first DSC run curves of polypropylenes prepared by 9a, typically several melting transitions. Mi, M2, and M3, with maxima 7mi, Tm2, and Tu, can be seen (Figure 9.13). 

Plasticizers are small molecules added to soften a polymer by lowering its glass transition temperature or reducing its crystallinity or melting temperature. The most widely plasticized polymer is poly (vinyl chloride). The distinctive odor of new vinyl shower curtains is caused by the plasticizer, for example. 

Copolymerization is one of the most efficient synthetic techniques to decrease the crystallinity and melting temperature of a given polymer. The melting-point depression occurring in copolyesters (Vectra) based on p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (HNA) is in fact relatively moderate in comparison with that of other copolymers (Fig. 6.35). The minimum in melting point occurs at about 40 mol% HNA. The decrease in isotropization temperature in the copolyesters is moderate, leading to the desired expansion of the temperature region of a nematic phase. 

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