Polymer Structure and Chemistry (Структура и химия полимеров). Учебное пособие

UNIT 9

BIOPOLYMERS

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Biopolymers are a class of polymers produced by living organisms. Biopolymers have been around for billions of years longer than synthetic polymers like plastics. Well-known biopolymers include starch, proteins, DNA, and RNA. Together these make up much of our bodies and the majority of the biosphere.

The starch polymer is made up of sugar monomers. When you consume starch, it gets broken down into sugar within the body. Starch provides an extended-release form of nutrition in contrast to more quicklymetabolized sugars.

Protein and peptide biopolymers have amino acids as their constituents. This is why amino acids are often referred to as “the building blocks of life”. DNA and RNA are made up of nucleic acids, which alternate in precise patterns to encode large quantities of data.

Biopolymers are used as an environmentally friendly alternative to petroleum-based polymers, which may take thousands of years to biodegrade. Biopolymers can be produced without toxic byproducts and biodegrade quickly, leaving a minimal human footprint on the environment.

In contrast to synthetic polymers, biopolymers tend to have a welldefined structure. Perhaps, this is because evolution tends to select for chemical reactions and structures that are largely predictable. Biopolymers have an evenly distributed set of molecular weights and are built using a template-directed process.

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Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. DNA is a long polymer made of repeating units called nucleotides. DNA polymers can be enormous molecules containing millions of nucleotides.

The backbone of the DNA strand is made of alternating phosphate and sugar residues. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings.

The DNA double helix is stabilized by hydrogen bonds between the bases attached to the two strands. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide.

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Polysaccharides are carbohydrate polymers consisting of tens to hundreds to several thousand monosaccharide units. All of the common polysaccharides contain glucose as the monosaccharide unit. Polysaccharides are synthesized by plants, animals, and humans to be stored for food, structural support, or for energy.

The term “starch” is used to describe a biopolymer system comprising predominantly of two polysaccharides — amylose and amylopectin. Amylose molecules consist of single mostly-unbranched chains with a molecular weight of up to 20,000. Each amylopectin molecule is branched and contains up to two million glucose residues in a compact structure with hydrodynamic radius of 21-75 nm.

Glycogen is a polysaccharide that is found in animals and is composed of a branched chain of glucose residues. It is stored in liver and muscles.

The structural components of plants are formed primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose.

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1.The structure of DNA is illustrated by a right handed double helix, with about 10 nucleotide pairs per helical turn.

2.Certain animals can digest cellulose, because bacteria possessing the enzyme are present in their gut. The classic example is the termite.

3.The sequence, or protein primary structure is specified by the sequence of the DNA containing a gene for that protein, and the sequence is unique to the individual protein.

4.Knowledge of the tertiary structure of proteins is often necessary to understand, since their most significant actions may involve only a single small active site on a very large molecule.

5.Biopolymers could also prove an asset to waste processing. Replacing the polyethylene used in coated papers by a biopolymer could help eliminate plastic scraps occurring in compost.

6.Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues.

7.Enzymes are very specific protein catalysts. Complementary shape, charge and hydrophilic/hydrophobic characteristics of enzymes and substrates are responsible for this specificity.

8.Biochemists define four levels of protein structure: primary, the amino acid sequence; secondary, helix stabilized by hydrogen bonds; tertiary, the overall shape of a protein molecule; quaternary, two or more polypeptides.

9.Ribonucleic acid or RNA is a nucleic acid, consisting of many nucleotides that carry information about a protein sequence to the ribosomes, the protein synthesis factories in the cell.

10.Unlike petrochemical polymers, biopolymers are renewable, because they are made from biodegradable plant materials and are ecologically friendly.

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1.What is a genetic code?

2.What are biological catalysts?

3.Describe the main biopolymers of human living cells.

4.Why do we think of biopolymer materials as environmentally friendly?

5.What happens to starch when we consume it?

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UNIT 10

RESEARCH METHODS IN POLYMER CHEMISTRY

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A variety of lab techniques are used to determine the properties of polymers. Techniques such as wide angle X-ray scattering, small angle X- ray scattering and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the number average molecular weight, weight average molecular weight, and polydispersity. FTIR, Raman and NMR can be used to determine composition.

Thermal properties such as the glass transition temperature and melting point can be determined by differential scanning calorimetry and dynamic mechanical analysis. Pyrolysis followed by analysis of the fragments is one more technique for determining the possible structure of the polymer.

An important tool of the polymer chemist is infrared spectroscopy, or “IR”. IR spectra are acquired on a special instrument, called an IR spectrometer. IR is used to gather information about compound’s structure, assess its purity, and sometimes to identify it.

The scanning electron microscopy SEM is especially useful for routine examination of plastics, elastomers and biopolymers.

There are many instruments that test the mechanical and viscoelastic properties of polymer materials. These instruments apply varied shear stress and shear rate to polymer samples to estimate their response to deformation and mechanical strength.

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Thermogravimetric analysis, or TGA, is a type of testing that is performed on samples to determine changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change. As many weight loss curves look similar, the weight loss curve may require transformation before results may be interpreted. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent. Again, interpretation is limited without further modifications and deconvolution of the overlapping peaks may be required.

TGA is commonly employed in research and testing to determine characteristics of materials such as polymers, to determine degradation temperatures, absorbed moisture content of materials, the level of inorganic and organic components in materials, decomposition points of explosives, and solvent residues. It is also often used to estimate the corrosion kinetics in high temperature oxidation.

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Wide angle X-ray scattering (WAXS) is an X-ray diffraction technique that is often used to determine the crystalline structure of polymers. According to this method the sample is scanned in a wide angle X-ray unit.

When X-rays are directed in solids they will scatter in predictable patterns based upon the internal structure of the solid. A crystalline solid consists of regularly spaced atoms (electrons) that can be described by imaginary planes. Every crystalline solid will have a unique pattern of X- ray scattering.

Small-angle X-ray scattering is carried out at very low angles (typically, 0.1 — 10°). This method is used for the determination of the microscale or nanoscale structure of particle systems in terms of such parameters as averaged particle sizes, shapes and distribution.

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1.Small-angle X-ray scattering is used to study the structure of ordered systems like lamellar structures, fractal-like materials and etc.

2.Dynamic mechanical analysis is a technique used to study and

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characterize polymer materials and it is also useful for observing their viscoelastic nature.

3.Infrared spectroscopy is widely used in both research and industry as a simple and reliable technique for measurement, quality control and dynamic measurement.

4.Differential scanning calorimetry is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature.

5.The atmosphere inside the thermogravimetric analysis unit may be purged with an inert gas to prevent oxidation or other undesired reactions.

6.Oxidation stability of polymers is studied by heating them to a certain high temperature when they are oxidized much faster and then estimating their oxidation rate in normal conditions.

7.Liquid samples for IR-spectroscopy are placed between two plates of a high purity salt (commonly sodium chloride), that are transparent to the infrared light.

8.Differential thermal analysis curve provides data on the transformations that have occurred, such as glass transitions, crystallization, melting and sublimation.

9.Raman spectroscopy is a spectroscopic technique used in chemistry to study vibrational, rotational, and other low-frequency modes in a system.

10.Small angle neutron scattering is a laboratory technique similar to the light scattering that provides valuable information on chemical aggregation, colloids, proteins, viruses and macromolecules.

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