Polypropylene crystalline melting point

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

Polypropylene is the second most important commercial polyolefin. Isotactic PP has the lowest density (0.90-0.91 g/mL) of the major plastics. It has a high crystalline melting point of 165°C. The first commercial production of polypropylene was in the 1950s following the discovery of Z-N catalysts. 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

Many high molecular weight synthetic polymers, such as polyethylene and polypropylene, have a large percentage of their molecules in the crystalline state. Prior to dissolution, these polymers must usually be heated almost to their melting points to break up the crystalline forces. Orthodichlorobenzene (ODCB) is a typical mobile phase for these polymers at 150°C. The accuracy and stability of the Zorbax PSM columns under such harsh conditions make them ideal for these analyses (Fig. 3.8). 

The beta crystalline form of isotactic polypropylene differs from the alpha form by having a lower crystalline density and lower melting point. The beta form is metastable to the alpha form and will rearrange to the alpha structure when heated to approximately 100 °C or placed under strain. Beta lamellae form parallel stacks as shown in Fig. 19.4. 

In contrast, the curve E2 (isotactic polypropylene) is characteristic for partially crystalline polymers. The modulus is three decades higher than in an elastomer. At the glass transition temperature [T (2) 0 °C] the decay of the E modulus is small it does not drop to the lower level of the molten state before the melting point. 

The polypropylene so obtained has a high molecular weight and is crystalline.The proportion of isotactic polymer, determined by extracting with heptane for 10 h in a Soxh-let apparatus, is 98.5%. Isotactic polypropylene shows similar solubility behavior to polyethylene, but has a higher melting point (crystalline melting range 165-171 °C). 

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