Atactic plastic

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Poly(vinyl acetate) is too soft and shows excessive cold flow for use in moulded plastics. This is no doubt associated with the fact that the glass transition temperature of 28°C is little above the usual ambient temperatures and in fact in many places at various times the glass temperature may be the lower. It has a density of 1.19 g/cm and a refractive index of 1.47. Commercial polymers are atactic and, since they do not crystallise, transparent (if free from emulsifier). They are successfully used in emulsion paints, as adhesives for textiles, paper and wood, as a sizing material and as a permanent starch . A number of grades are supplied by manufacturers which differ in molecular weight and in the nature of comonomers (e.g. vinyl maleate) which are commonly used (see Section 14.4.4)... 

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts. The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures Isotactic, with all methyl groups in the same plane makes the best plastic syndiotactic, in which the methyl groups alternate in the same plane and atactic, with the methyl groups randomly in and out of the plane is soft and rubbery. Polypropylene is used as film and in many structural forms. It is also used as fibers for carpet manufacture and for thermal clothing. 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

In the case of homopolymers, the tacticity of the chains directly influences the material properties. Atactic PHB has an oily consistency and is of less use for plastic industry, whereas completely isotactic PHB shows comparably interesting... 

Polychloroethene (polyvinyl chloride), as usually prepared, is atactic and not very crystalline. It is relatively brittle and glassy. The properties of polyvinyl chloride can be improved by copolymerization, as with ethenyl ethanoate (vinyl acetate), which produces a softer polymer ( Vinylite ) with better molding properties. Polyvinyl chloride also can be plasticized by blending it with substances of low volatility such as tris-(2-methylphenyl) phosphate (tricresyl phosphate) and dibutyl benzene-1,2-dicarboxylate (dibutyl phthalate) which, when dissolved in the polymer, tend to break down its glasslike structure. Plasticized polyvinyl chloride is reasonably flexible and is widely used as electrical insulation, plastic sheeting, and so on. 

Poly(vinyl chloride), also known as PVC, is prepared by radical polymerization to produce material composed of an average of 10,000 to 24,000 monomer units. It is atactic and therefore amorphous, but it has a relatively high Tg because of the large size of its molecules and its polar carbon-chlorine bonds. It is a rigid material and is used to make pipe, panels, and molded objects. About 68% of PVC is used in the building and construction industry. A more flexible form of PVC is produced by adding a plasticizer such as dioctyl phthalate. This is used to prepare electric wire coatings, film, and simulated leather or vinyl. ... 

Side groups in atactic structures and chain branching hinder crystallization of the thermoplastic polymers in contrast to unbranched and isotactic configurations which lead to increased crystallinity. With increasing crystallinity the density, strength and stiffness are increased but the transparency and processability of the plastic decrease. 

The polyvinylethers form a further group of thermoplastics which are not used as containers or packaging films. They are atactic polymers forming oils, sticky soft resins or nonsticky rubber elastic materials according to their molecular weight and composition. All polyvinylethers are very resistant to saponification by dilute acids and alkalis. They can subsequently be used as unsaponifiable polymer plasticizers and for the manufacture of glues. 

A common example of a plasticized polymer is poly(vinyl chloride). The common atactic form has a Tg of about 80 °C, well above room temperature. Without a plasticizer, vinyl is stiff and brittle. Dibutyl phthalate (see the structure at left) is added to the polymer to lower its glass transition temperature to about 0 °C. This plasticized material is the flexible, somewhat stretchy film we think of as vinyl raincoats, shoes, and even inflatable boats. Dibutyl phthalate is slightly volatile, however, and it gradually evaporates. The soft, plasticized vinyl gradually loses its plasticizer and becomes hard and brittle. 

Explain how chain branching, cross-linking, stereochemistry (isotactic, syndiotactic, or atactic), and plasticizers affect the properties of polymers. 

In the case of sPS, the problem of its brittleness can be even more acute since it has to compete with engineering plastics which possess an inherent toughness superior to that of sPS. For this reason, a good impact modification of this product is of paramount importance and may even be essential for its survival as a commercial thermoplastic. For this reason a chapter of this book has been dedicated to the impact modification of sPS using elastomers. Since rubber modification plays such an important role for styrene polymers, whether atactic or sydiotactic, we will first look at the methods of energy dissipation in these homopolymers on impact. 

An effort to investigate the kinematics of plastic deformation in glassy atactic polypropylene was presented by Mott, Argon, and Suter.Using an atomistic simulation for strains up to 20%, the authors observed that the plastic rearrangement of the structure was revealed in the microstructural stress—strain behavior (i.e., smooth reversible portions bounded by irreversible sharp drops in the stress values). 

The feedstock materials used in this work included medium density polyethylene (PE), atactic-polypropylene (aPP), isotactic-polypropylenc (iPP), beech wood, pine wood, cellulose and hydrolytic lignin. The size of the wood biomass and the plastic particles was less 0,1 mm. 

The origin of plastic has more influence on the distribution of products of biomass / plastic co-pyrolysis. According to Table 3 the yield of light liquids from beech-wood / plastic mixtures (1 2 weight ratio) was decreased in the following sequence isotactic-polypropylene > atactic-polypropylene > polyethylene, whilst the yield of heavy liquids was increased in the same order,... 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

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