Polymer physical characteristics

With the discovery of crystalline polypropylene in the early 1950 s, polymer stereochemical configuration was established as a property fundamental to formulating both polymer physical characteristics and mechanical behavior. Although molecular asymmetry was well understood, polymer asymmetry presented a new type of problem. Both a description and measurement of polymer asymmetry were essential for an understanding of the polymer structure. 

Rabek, J.F. Photodegradation of Polymers (Physical Characteristics and Applications) Springer Berlin, 1996. 

Polymers having broad MW distributions not only exhibit improved polymer molding, but the MW breadth can also increase some of the polymer physical characteristics, including impact and tear resistance, toughness, creep and hoop stress resistance in pipe, and especially resistance to chemical attack, or stress crack (ESCR). These improved properties result because a broad MW distribution usually contains more of the long chains that can act as tie molecules to connect the crystalline lamellae (Figure 42). 

J.F. Rabek, Photodegradation of Polymers Physical Characteristics Applications, p. 60, Springer, New York (1996). 

Rabek, J. F. (1996). Photodegradation of polymers Physical characteristics cmd applications. Springer. 

Even though no loss in physical properties was observed for aromatic poly(anhydride)s subjected to gamma irradiation, it is important to establish the polymers physical characteristics as a function of exposure time in-vitro. This is a necessary requirement, since past work has shown that absorbable polymers, and devices formed from them (PDS, Vicryl ), subjected to cobalt may indicate little change in physical properties, but when tested in-vitro rapidly lose strength. However, no difference is observed in in-vitro properties between coupons subjected to cobalt versus unirradiated coupons. In fact, yield strength as a function of weeks in-vitro appears to follow a linear decrease profile for annealed test coupons. However, unnanealed test coupons display an induction period (6 weeks) prior to the linear decrease in physical strength. 

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Biological reactors play a valuable role in tlie conversion of substrates by microorganisms and mammalian cells into a wide range of products such as antibiotics, insulin, and polymers. Figures 11-12, 11-13, and 11-14 illustrate various types of biological reactor, and Figure 11-15 shows the physical characteristics of a typical coimuercial fermentation vessel. 

Table 13 Physical Characteristics of Uniform Porous Polymer Particles (Mw.i.p = 1.49 x 10, 15% DVB)...

The introduction of large gas phase volumes into the polymer alters the physical characteristics of the material volume weight, permeability to fluids and gases, and physico-mechanical properties. Moreover, the properties of the polymer matrix itself are changed (owing to orientation effects, supermolecular structure of the polymer in the walls, ribs and tension bars of cells), which drives up the value of specific strength on impact, and results in anisotropy of elasticity. 

In fact, considering the basic structure of these materials (vide supra), it can be immediately realized that the basic features of poly(organophosphazenes) are the result of two main contributions. The first one is fixed and is basically related to the intrinsic properties of the -P=N- inorganic backbone, while the second is variable and mostly connected to the chemical and physical characteristics of the phosphorus substituent groups. Skeletal properties in phos-phazene macromolecules intrinsically due to the polymer chain are briefly summarized below. 

Among the physical characteristics of these nonlinear condensation polymerizations, the occurrence of a sharp gel point is of foremost significance. At the gel point, which occurs at a well-defined stage in the course of the polymerization, the condensate transforms suddenly from a viscous liquid to an elastic gel. Prior to the gel point, all of the polymer is soluble in suitable solvents, and it is fusible also. Beyond the gel point, it is no longer fusible to a liquid nor is it entirely soluble in solvents. Linear polymers, on the other hand, remain soluble in suitable solvents and fusible to liquids as well (unless the melting point is above the temperature of thermal decomposition), regardless of the extent of condensation. 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

Stereodefects in syndiotactic polymers have similar effects to those in their isotactic counterparts, reducing crystallinity levels and changing the associated physical characteristics. 

We are interested in the effect of weathering on polymers for two distinctly different reasons. We may wish to retard it, so that our products survive longer in outdoor applications, or we may wish to accelerate it, so that products degrade rapidly when exposed to the elements. In either case, we need a way of predicting the response of polymers to the factors that produce measurable changes in their chemical and physical characteristics. Ideally, we would like to be able to obtain these results in as short a period of time as possible. 

The successful use of crop plants as a production method for biopolymer not only depends on the amount of PHA accumulated in plants but also on the type and quality of the PHA synthesized. Since poly(3HB) is a polymer with poor physical characteristics [16], it was important to engineer plants for the synthesis of PHA co-polymers with better physical characteristics. Poly(3-hydroxybu-tyrate-co-3-hydroxyvalerate) [poly(3HB-co-3HV)] is the best studied co-poly-mer. Poly(3HB-co-3HV) has lower crystallinity, and is more flexible and less brittle than poly(3HB) homopolymer [16]. Synthesis of poly(3HB-co-3HV) in bacteria was first achieved by fermentation of R. eutropha on glucose and propionic acid [2]. For a number of years, production of poly(3HB-co-3HV),... 

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