Ethylene propylene random, crystalline

Calculate the glass transition temperature, Tg for a noncrystalline ethylene-propylene random copolymer that contains 20 —CH3 groups per 100 main-chain carbons, knowing that it reduces the degree of crystallinity of the copolymer to 0%. 

Wright, K.J., Lesser, A.J., Crystallinity and mechanical behavior evolution in ethylene-propylene random copolymers. Macromolecules 2001,34 3626-3633. 

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. 

For example, a PE-fe-poly(ethylene-co-propylene) diblock composed of crystalline PE and amorphous ethylene/propylene copolymer segments was synthesized from ethylene and ethylene/propylene. The addition of MAO and Ti-FI catalyst 40 (Fig. 25) to an ethylene-saturated toluene at 25 °C resulted in the rapid formation of a living PE (Mn 115,000, MJMn 1.10). The addition of ethylene/propylene (1 3 volume ratio) to this living PE formed a PE-/>poly(ethylcnc-co-propylcnc) block copolymer (Mn 211,000, MJMn 1.16, propylene content 6.4 mol%) [30], As expected, the polymer exhibits a high Tm of 123 °C, indicating that this block copolymer shows good elastic properties at much higher temperatures than the conventional random copolymers of similar densities. 

A common example of a copolymer is an ethylene-propylene copolymer. Although both monomers would result in semi-crystalline polymers when polymerized individually, the melting temperature disappears in the randomly distributed copolymer with ratios between 35/65 and 65/35, resulting in an elastomeric material, as shown in Fig. 1.19. In fact, EPDM rubbers are continuously gaining acceptance in industry because of their resistance to weathering. On the other hand, the ethylene-propylene block copolymer maintains a melting temperature for all ethylene/propylene ratios, as shown in Fig. 1.20. 

It is interesting that some heterogeneous superhigh-activity Ziegler-Natta catalysts such as MgC /TiCU/LB—AlEt3 also yield random ethylene/propylene copolymers. These copolymers, however, exhibit a blocky nature and highly isotactic propylene sequences (with no 2,1-inserted propylene units) that contribute to undesired crystallinity [68,456]. 

Ethylene-propylene rubbers (EPR) are basically random copolymers of ethylene and propylene, with 60-70% (w/w) ethylene. Polyethylene and polypropylene are homopolymers that display too high a degree of crystallinity to be used as elastomers. Nevertheless, random copolymerization produces linear chains with sufficient structural irregularity to prevent crystallization. The copdlymerization process leads to amorphous, fully saturated chains. 

Polypropylene, PP, was blended with a random crystalline terpolymer of 96-85 wt% propylene, 1.5-5.0 wt% ethylene, and 2.5-10 wt% C4 g alpha-olefin. The blends were used to manufacture strands of multiple monofilamcmts m staple fibers with high resiliency and shrinkage, for pile fabric. 

Infrared microscopy, pFTlR, is a highly attractive technique to map the crystallinity and additive content of polymer samples. " The spatial distribution of the p-nucleating agent in random ethylene propylene copolymer was determined by pFTIR from the sum... 

It is worth noting that behavior of ethylene copolymers containing bulkier a-olefins is completely different from that of EP copolymers. In fact, bulkier comonomer units are excluded from the crystalline phase, which remains orthorhombic for high comonomer contents, giving rise only to small increases of the a and b unit cell parameters (as shown for ethylene/styrene random copolymers in Figure 12.10). Moreover, as shown in Figure 12.12, as the ethylene molar content in EP copolymers decreases, the concomitant decrease in crystallinity is less rapid for EP copolymers than for ethylene/styrene and similar ethylene copolymers containing bulkier a-olefins. This is due to the inclusion of propylene units and the exclusion of bulkier units from the crystalline phase. ... 

In terms of IR and Raman spectroscopy, an ethylene-propylene copolymer may be viewed as an extreme case because both ethylene and propylene may exist in crystalline forms. The IR and Raman spectra of a random copolymer with one major component will reflect the crystalline nature of that... 

Random copolymers do not crystallize—this is a common technique for making elastomers (section 1.13). Thus, in ethylene/propylene rubbers (EP rubbers) and ethylene/vinyl acetate (EVA) the crystallinity of the PE is destroyed. A 50/50 copolymerization of the following polyacrylates yields an amorphous copolymer, 180°C. 

Sequence The difference in reactivity between comonomers affects the composition and also alters the placement of the monomer units along the chain. In the case of living polymerization, sequential monomer addition leads to the formation of block copolymers. However, when a random copolymer is targeted, reactivity differences can lead to nonrandom distribution of monomer units. If the incorporation of a comonomer B is intended to disrupt crystallinity of poly( A), uninteimpted sequences of A can lead to domains of crystallinity. For example, block copolymers of ethylene-propylene are highly aystaUine, while random copolymers are completely amorphous. 

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