Crystallinity random copolymers

Because of the geometric regularity present, gutta-percha (/ra/w-polyi-soprene) and stretched NR from Hevea brasitiensis (c/s-polyisoprene) are crystalline. Random copolymers are usually amorphous, but some may be crystalline if the comonomers, such as ethylene and tetrafluoroethylene, are similar in size. Block copolymers may have crystalline domains if either of the com-... 

On the other hand, cocrystallization or grinding together of 12a and S-ethyl 4-[2-(2-pyrazinyl)ethenyl]thiocinnamate (12b) forms a 45 55 molar mixed crystal (a solid solution), which upon UV irradiation gave a crystalline random copolymer (Scheme 10) [14]. 

Hanna S, Windle AH (1988) Geometrical limits to order in liquid crystalline random copolymers. Polymer, 29 207... 

Ciassification Propylene copolymer Definition Amorphous or crystalline random copolymer... 

The theory of cold crystallization is derived on the basis of copolymers by Wunderlich B (1958) Theory of Cold Crystallization of High Polymers. J Chem Phys 29 1395-1404. Further applications of such computer-generated matching of chemical stracture and crystals were shown by Hanna S, Windle AH (1988) Geometrical Limits to Order in Liquid-crystalline Random Copolymers. Polymer 29 207-223. 

Polyolefin blends comprised 25-95 wt% of a crystalline random copolymer, EPRl (of propylene with ethylene and/or an alpha-olefin), and 5-75 wt% of a mixture consisting of PE and EPR2. The density of EPRl was about equal that of the mixture. The blends had good transparency and impact resistance even at low temperatures and were used to manufacture food containers, medical, packaging films, etc. 

Hanna, S. Windle, A. H., Geometrical Limits to Order in Liquid Crystalline Random Copolymers. Polymer 1988, 29,207-223. 

In order to meet the military performance specification, a foil laminate structure is currently used in the MRE food pouch. However, current research is investigating alternative multilayer polymeric packaging structures. [4, 5] Ethylene co-vinyl alcohol (EVOH) is one polymer that is being researched. EVOH is a crystalline, random copolymer of ethylene and vinyl alcohol. EVOH is known to be a superior barrier to oxygen however the barrier properties of EVOH diminish as the humidity increases. Also, the hydrophilic nature of the material becomes a problem with retort packaging applications because the process requires water and steam for sterilization. EVOH resins absorb moisture as a function of humidity and temperature, decreasing the barrier properties of the resin during the retort process. 

Random copolymers are similar to PEO but when the regular helical structure of the chains is demolished, the crystallinity is also destroyed. One of the simplest and most successful amorphous host polymers is an oxyethylene- oxymethylene structure in which medium length but statistically variable EO units are interspersed with methylene oxide groups. First described in 1990 [37], aPEO has the general structure... 

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

Figure 3 Non-linear least-squares curve fitting of the orthorhombic WAXS profile of an ethylene 1-decene random copolymer with 2.7 mol% branches. The two crystalline reflections and the amorphous halo are shown. 

Fig. 12 Cooling (solid lines) and heating (dotted and dashed lines) crystallinity curves of random copolymers with variable comonomer mole fractions as denoted near the curves. The dashed lines start from the reduced temperature of 2 and meet the dotted curves at high temperatures [52]... 

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. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

Interestingly, the lactones copolymerization is responsible for a decrease in both and degree of crystallinity of the copolyesters when compared to the parent homopolymers. This behavior is illustrated in Fig. 1 in the case of po-ly( CL-co-6VL) random copolymers [35]. 

The value of the modulus and the shape of the modulus curve allow deductions concerning not only the state of aggregation but also the structure of polymers. Thus, by means of torsion-oscillation measurements, one can determine the proportions of amorphous and crystalline regions, crosslinking and chemical non-uniformity, and can distinguish random copolymers from block copolymers. This procedure is also very suitable for the investigation of plasticized or filled polymers, as well as for the characterization of mixtures of different polymers (polymer blends). 

Although hdpe and it-PP are crystalline, the commercial random copolymer of ethylene and propylene (EP) is an amorphous elastomer. The most widely used EP copolymer (EPDM) is produced by the copolymerization of ethylene and propylene with a small amount of an alkyldiene this permits cross-linking or vulcanization. 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

Copolymers of vinyl chloride are less crystalline and may he more flexible than unplastidzed PVC The random copolymer of vinyl chloride (17%) and vinyl acetate (11%) (Vinylite) contains the following repeating units ... 

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