Amorphous or atactic polypropylene

Amorphous polypropylene (aPP) is characterized by a random steric orientation of the methyl pendant groups on the tertiary carbon atoms along the molecular chain. The random sequence of these methyl substituents is linked to an atactic configuration. Due to its fully amorphous nature, aPP is easy soluble (even at ambient temperatures) in a great number of aliphatic and aromatic hydrocarbons, esters and other solvents in contrast to the isotactic PP (iPP) of semicrystalline feature. 

Due to its tacky consistency, aPP is available mostly in block form. In order to reduce the tackiness and blocking properties, spraying by chalk or talc powders or wrapping in silicon-coated papers are widely used. Pelletized, granulated aPP grades are either of highly crystalline nature or compounded, modified grades. 

Property Test Method or ASTM standard Unit Commercial Byproduct Metallocene- synthesized HMW-aPP 

Designations and M weight- and number-average molecular weight, respectively  

melt flow index (230°C, 2.16 kg) Tg and Tm, glass trairsition and melting temperature, respectively GPC, gel permation chromatography DSC, differential scanning calorimetry. 

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Karger-Kocsis J (1999) Amorphous or atactic polypropylene, in Polypropylene An A-Z Reference (Ed. Karger-Kocsis J) Kluwer, Dordrecht, Netherlands, pp. 7-12. 

MAJOR APPLICATIONS Low molecular weight atactic polypropylene is used as a component of hot melt adhesives and sealants. Atactic" polypropylene which is produced as a by-product of isotactic PP production is not ideally atactic or completely amorphous. Ideally atactic polypropylene has been prepared by hydrogenation of poly(2-methyl-l,3-pentadiene), that is, poly(l,3-dimethyl-l-butenylene) or PDMB/ Recently, directly synthesized atactic pwlypropylene and other amorphous poly(a-olephins) (APAO or APO) have been developed. 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Figure 13.4 shows the variations between the tensile test parameters for the PP/interfacial modifier/talc composites as a function of either the grafted group, succinic anhydride (SA) or succinyl-fluorescein (SF) attached to an atactic polypropylene, as well as the differences on tensile strength and strain levels at yield or at break points, depending on the amorphous or semicrystalline nature of the interfacial modifier as it was fully discussed elsewhere (29,31). 

Amorphous or non-crystalline polymers - These polymers do not have any degree of crystallinity. Examples of these polymers are butadiene rubber and atactic polypropylene. 

Polypropylenes can be a semicrystalline polymer (atactic/ isotactic), (atactic/syndiotactic) or a purely amorphous polymer (atactic). The effect of crystallinity on the yield of trapped electrons (Table 52.2) has already been discussed. 

In addition to polypropylenes in which the entire polymer chain consists of isotactic, syndiotactic, hemiisotactic, or atactic chains, polypropylenes in which the chain consists of alternating blocks of two microstructures have been prepared. Perhaps the most interesting and useful of these stereoblock polymers consists of the combination of a crystalline block, such as a unit of isotactic or syndiotactic polypropylene, and an amorphous block, such as atactic polypropylene. Polymers containing this combination of microstructures often behave as a thermoplastic elastomer and have properties... 

Catalysts that form ethylene-propylene copolymers can be used to produce a material known as ethylene-propylene rubber (EPR). If an isospecific catalyst for the formation of polypropylene incorporates a small amount of ethylene, a crystalline copolymer is formed that has a lower melting point than isotactic PP. If an aspecific catalyst is used, or if more ethylene is incorporated into the polymer, an amorphous ethylene-propylene rubber (EPR) is formed. EPR generally has a lower glass transition temperature than atactic PP, and is a useful material for low-temperature applications. Catalysts that form isotactic or syndiotactic polypropylene can also generate polymers possessing defined stereochemistry within the propylene units in an EP copolymer. 

Block copolymers made from ethylene and propylene are valuable industrial materials. They can be used as thermoplastic elastomers and as compatibOizing agents for homopolymer blends. The properties of this type of copolymer depend on the microstructure of the blocks, the relative lengths of the blocks, and the overall molecular v eight. An ABA triblock copolymer structure containing crystalline A blocks and an amorphous B block can exhibit elastomeric behavior. The crystalline "hard" blocks can consist of isotactic or syndiotactic polypropylene (iPP or sPP) units or linear polyethylene (PE). The amorphous "soft" blocks can consist of atactic polypropylene (aPP) or ethylene-propylene copolymer (ethylene-propylene rubber, EPR). 

Molecular dynamics (MD) is an invaluable tool to study structural and dynamical details of polymer processes at the atomic or molecular level and to link these observations to experimentally accessible macroscopic properties of polymeric materials. For example, in their pioneering studies of MD simulations of polymers, Rigby and Roe in 1987 introduced detailed atomistic modeling of polymers and developed a fundamental understanding of the relationship between macroscopic mechanical properties and molecular dynamic events [183-186]. Over the past 15 years, molecular dynamics have been applied to a number of different polymers to study behavior and mechanical properties [187-193], polymer crystallization [194-196], diffusion of a small-molecule penetrant in an amorphous polymer [197-199], viscoelastic properties [200], blend [201,202] and polymer surface analysis[203-210]. In this article, we discuss MD studies on polyethylene (PE) with up to 120,000 atoms, polyethylproplyene (PEP), atactic polypropylene (aPP) and polyisobutylene (PIB) with up to 12,000 backbone atoms. The purpose of our work has been to interpret the structure and properties of a fine polymer particle stage distinguished from the bulk solid phase by the size and surface to volume ratio. 

The applications of these techniques is illustrated by data on a variety of polymers, although the bulk of the discussion concerns a typical amorphous polymer, atactic polystyrene and a crystalline polymer, polyethylene oxide. A number of results are quoted which seem to indicate that phenyl group rotations or oscillations are not an important mechanism in the relaxation of polystyrene. Other materials mentioned are polypropylene oxide, starch and ionized copoly-... 

The properties of some polymers are dependent on their microstructure for example isotactic polypropylene is crystalline whereas atactic polypropylene is amorphous. Microstructure effects are also exemplified by polybutadienes, where the mode of addition of the diene to the growing chain leads to 1,2-addition, 1,3-addition and 1,4-addition, which may be as or trans. The fraction of different addition species changes the mechanical properties of the polymer. Another example is provided by the chemical composition of a copolymer and its sequence distribution, which together determine its ultimate properties. It is thus of great importance to be able to characterize polymer micro structure. This is generally done using spectroscopic methods, specifically infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

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