Polyallomers

From a practical point of view, incorporation of a few percent of polyethylene in block form as a tail on the polypropylene chain lowers the material s brittle temperature and increases impact strength. Table 6.4 

Comparison of Properties of Propylene-Ethylene Polyallomers and Polypropylene  

Unnotched Izod impact strength (23°C), ft-lb/in 24 No break No break No break 

While these materials do contain two crystalline phases, it appears that small quantities of amorphous polyethylene are important in the mechanical property improvements noted above. It should be noted that the amorphous portions of polyethylene have a glass temperature near — 80°C, compared to — 10°C (Nielsen, 1962, p. 16) for polypropylene. 

Special polymerization can produce ethylene-propylene block copolymers that exhibit crystallinity similar to that seen in the homopolymers. This thermoplastic, which is insoluble in hexane and heptane, has many of the properties of HOPE and PP, along with an annealed density of 0.91, a brittleness temperature as low as — 40°C (—40°F), and an Izod impact strength (notched) of640 J/m (12 ft.-lb./in.) with less notch sensitivity. 

Polyallomers mold as easily as PPs, yet they have better dynamic flexural fatigue resistance, as in living hinges, particularly at low temperatures. Their flow and moldability is better than that of linear PE and has a superior softening point as well as greater hardness, stress-crack resistance, and mold shrinkage. Polyallomers are superior to rubber-modified PPs in color, clarity, moldability, electrical properties, and blush resistance when bent or stretched. 

This thermoplastic is used for blow-molding bottles, wire coating, extruding film, sheet, and pipe, and thermoforming various packaging applications. 

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Properties Polybutylene extrusion Homopolymer Copolymer Impact copolymer Polyallomer Injection molding ... 

Properties extrusion Homopolymer Copolymer copolymer Polyallomer molding reinforced... 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

Use a long Pasteur pipette to layer 2.2 ml of the 10% sucrose solution in a Beckman polyallomer tube (14 x 89 mm, 331372). Then, underlay 2.2 ml of the 20% sucrose solution by inserting the tip of the pipette to the bottom of the tube and slowly pipetting the 20% sucrose solution under the 10% solution. Underlay the 30, 40, and 50% sucrose solutions in the same manner. Cover the tube with aluminum foil and store overnight at 4° to establish a linear gradient. 

Method Density gradient, Rate-zonal Gradient 10-40% continuous sucrose Rotor/run conditions SW 55 Tl rotor at 55000 rpm for approximately 6 hours Potential tube materials Polyallomer, Ultra-Clear... 

Potential tube materials Polyallomer, Ultra-Clear. SpinPro checks that all gradient materials, samples, and solvents are compatible with the tube materials. The affects of acids, bases, oils, organic solvents, and salts on the tube materials are considered. 

Glass, polyallomer, or polypropylene centrifuge tubes (see Note 2). 

Polyallomer Ethylene block copolymers Eastman Chemical... 

Copolymers of propylene and a small amount of ethylene (3%) are called polyallomers. These copolymers are not as flexible as EPDM, but they are more ductile than //-PP. 

POLYALLOMER RESINS. These are block copolymers prepared by polymerizing monomers in the presence of anionic coordination catalysts. The polymer chains in polyallomers are composed of homopolymerized segments of each of the monomers employed. The structure of a typical polyallomer can be represented as ... 

The word polyallomer is derived from the Greek words alios, meros. and poly. Alios means oilier and denotes a differentiation from the normal. The word meros means parts and the prefix poly is added to show that these materials are polymeric. Since allomerism is defined as a constancy of crystalline form with a variation in chemical composition, the polyallomers are examples of allomerism in polymer chemistry. 

Evidence for the crystalline nature of the polyallomers includes, x-ray diffraction patterns, infrared spectra, and for olefin polyallomers, low solubility in hydrocarbon solvents. 

The physical properties of polyallomers are generally intermediate between those of the homopolymers prepared from the same monomers, but frequently represent a better balance of properties than blends of the homopolymers. This is illustrated by comparing the properties of a propylene-ethylene polyallomer containing 2.5% ethylene with polypropylene, high-density polyethylene, and a blend of 5% high-density polyethylene and 95% polypropylene. 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

Isotactic poly(4-methyl-l -pentene) Ethylene/isotactic propylene block (compact) copolymers (polyallomers)... 

The polyolefins are high-molecular-weight hydrocarbons. They include low-density, linear low-density, and high-density polyethylene, polyallomer, polypropylene, and polymethylpentene. All are break-resistant, nontoxic, and... 

PP/PA (polypropylene/polyallomer). These plastics are similar to polyethylene, but each unit of their chains has a methyl group attached. They are translucent, autoclavable, and have no known solvents at room temperature. They are slightly more susceptible than polyethylene to strong oxidizing agents. They offer the best stress-cracking resistance of the polyolefins. Products made of polypropylene are brittle at ambient temperature and may crack or break if dropped from benchtop height. 

Beckman polyallomer tubes (1.43 X 8.9 cm used in a SW41 rotor). Before loading the gradient, heat the pooled samples to 37° for 10 min to ensure that the enzyme sites are free and not annealed. Centrifuge at 35,000 rpm for 6 to 7 hr at 20°. 

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