Synthetic polymer Repeat units

Several materials have been proposed and commercialized as electrolytes for HT-PEFCs. As introduced before, the main polymers used materials from the PBI family and the Advent tetramethyl pyridine sulfone (TPS) family, both being basic polymers allowing chemical interaction with mineral acids (e.g., phosphoric acid) see Fig. 1. Differences can be fotmd both in the chemistry and in the synthetic process especially among the different PBIs, yielding different physicochemical properties such as glass transition temperatures, mechanical stabilities, proton conductivities, and achievable phosphoric acid doping levels (defined as either the ratio of phosphoric acid molecules per polymer repeat unit or the weight ratio of polymer and included phosphoric acid) for a summary, see Table 1. 

Just as it is not necessary for polymer chains to be linear, it is also not necessary for all repeat units to be the same. We have already mentioned molecules like proteins where a wide variety of different repeat units are present. Among synthetic polymers, those in which a single kind of repeat unit are involved are called homopolymers, and those containing more than one kind of repeat unit are copolymers. Note that these definitions are based on the repeat unit, not the monomer. An ordinary polyester is not a copolymer, even though two different monomers, acids and alcohols, are its monomers. By contrast, copolymers result when different monomers bond together in the same way to produce a chain in which each kind of monomer retains its respective substituents in the polymer molecule. The unmodified term copolymer is generally used to designate the case where two different repeat units are involved. Where three kinds of repeat units are present, the system is called a terpolymer where there are more than three, the system is called a multicomponent copolymer. The copolymers we discuss in this book will be primarily two-component molecules. We shall discuss copolymers in Chap. 7, so the present remarks are simply for purposes of orientation. 

An important application of Eq. (3.39) is the evaluation of M, . Flory et al.t measured the tensile force required for 100% elongation of synthetic rubber with variable crosslinking at 25°C. The molecular weight of the un-cross-linked polymer was 225,000, its density was 0.92 g cm , and the average molecular weight of a repeat unit was 68. Use Eq. (3.39) to estimate M. for each of the following samples and compare the calculated value with that obtained from the known fraction of repeat units cross-linked ... 

Polymer Solutions. Perhaps the most extensively studied macromolecular Hquid crystals are the synthetic polypeptides, such as poly( y-benzyl L-glutamate) [25513-40-0] (PBLG). PBLG is a homopolymer of the L-enantiomorph of a single amino acid with the foUowiag repeat unit. 

These polymers, typical of polyamides with fewer than four main chain carbon atoms in the repeating unit, decompose before melting and have to be processed from solution. Several of the polymers may, however, be spun into fibres. Over thirty years ago Courtaulds produced silk-like fibres on an experimental commercial scale from poly-(L-alanine) and from poly-(a-methyl-L-glutamate). The latter material is also said to be in use as a synthetic leather in Japan. The... 

We ve seen on several occasions in previous chapters that a polymer, whether synthetic or biological, is a large molecule built up by repetitive bonding together of many smaller units, or monomers. Polyethylene, for instance, is a synthetic polymer made from ethylene (Section 7.10), nylon is a synthetic polyamide made from a diacid and a diamine (Section 21.9), and proteins are biological polyamides made from amino acids. Note that polymers are often drawn by indicating their repeating unit in parentheses. The repeat unit in polystyrene, for example, comes from the monomer styrene. 

Polymer (Sections 7.10, 21.9, Chapter 31 introduction) A large molecule made up of repeating smaller units. For example, polyethylene is a synthetic polymer made from repeating ethylene units, and DNA is a biopolymer made of repeating deoxyribonucleotide units. 

Polymers are examples of organic compounds. However, the main difference between polymers and other organic compounds is the size of the polymer molecules. The molecular mass of most organic compounds is only a few hundred atomic mass units (for reference, atomic hydrogen has a mass of one atomic mass unit). The molecular masses of polymeric molecules range from thousands to millions of atomic mass units. Synthetic polymers include plastics and synthetic fibers, such as nylon and polyesters. Naturally occurring polymers include proteins, nucleic acids, polysaccharides, and rubber. The large size of a polymer molecule is attained by the repeated attachment of smaller molecules called monomers. 

The overwhelming majority of synthetic polymers is organic in nature, and it is on these that we will concentrate. The simplest and most common synthetic polymer is polyethylene, which will be our first example. Figure 1.1 shows the basic chemical structure of polyethylene. Pairs of hydrogen atoms are attached to the carbon atoms that make up the backbone. The repeat unit in this structure contains two carbon atoms and is derived from the ethylene monomer. In the case of polyethylene, the number of monomer residues, which is known as the polymerization... 

The word "polymer" (first proposed by Berzelius in 1833) is made of "poly" from the ancient Greek word "mlvq" meaning "many" and "pepot " meaning "part". Polymers are molecules built up from numerous identical chemical "units" spatially repeated to form a chain. From the early times and still nowadays, a distinction is often made between "natural" and "synthetic" polymers, but it is somewhat artificial as natural polymers can now sometimes be synthesized (e.g., synthetic "natural rubber") and some synthetic polymers, which are never found in nature, can be synthesized by natural ways (enzymatic syntheses). 

The main parameters used to describe a polymer chain are the polymerization index N, which counts the number of repeat units or monomers along the chain, and the size of one monomer or the distance between two neighboring monomers. The monomer size ranges from a few Angstroms for synthetic polymers to a few nanometers for biopolymers. The simplest theoretical description of flexible chain conformations is achieved with the so-called freely-jointed chain (FJC) model, where a polymer consisting of N + I monomers is represented by N bonds defined by bond vectors r/ with j= Each bond vector has a fixed length r,j = a corresponding to the... 

A second approach to biodegradable packaging is to blend polyethylene with a second synthetic polymer with polar repeating units that are capable of degradation, such as ester linkages (chapter 12). Poly(caprolactone) represents such a class of polymer, which has a long history of compatibility ( with a variety of polymers and degradability (5) recently, improved miscibility and Glm properties have been reported when poly(caprolactone) is blended with commodity plastics... 

Coals are considered macromolecular solids.(l) Although they are not polymers in the sense that they possess a repeating unit, they do possess several fundamental properties typical of synthetic crosslinked polymers.(2) One of these properties is the ability of coals to swell in organic solvents without dissolving. 

Each specific protein molecule has a specific chain length, like classical small molecules, and is said to be monodisperse with respect to chain length or molecular weight. However, most synthetic commercial polymers such as HDPE are composed of molecules of different lengths. Thus, the numerical value for the number of repeat units, n, or the degree of... 

While the specific chemistry and physics dealing with synthetic polymers is complicated, the chemistry and physics of natural polymers is even more complex because of a number of related factors, including (1) the fact that many natural polymers are composed of different, often similar but not identical, repeat units (2) a greater dependency on the exact natural polymer environment (3) the question of real structure of many natural polymers in their natural environment is still not well known for many natural polymers and (4) the fact that polymer shape and size are even more important and complex in natural polymers than in synthetic polymers. 

Organic polymers are responsible for the very life—both plant and animal—that exists. Their complexity allows for the variety that is necessary for life to occur, reproduce, and adapt. Structures of largely linear natural and synthetic polymers can be divided into primary structures, which are used to describe the particular sequence of (approximate) repeat units secondary structures, which are used to describe the molecular shape or conformation of the polymer tertiary structures, which describe the shaping or folding of macromolecules and quaternary structures, which give the overall shape to groups of tertiary-structured macromolecules. The two basic secondary structures are the helix and the sheet. 

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