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

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Liquid crystals can be considered to be crystals which have lost some or all of their positional order, while maintaining full orientation order.

Imagine a large number of toothpicks put into a rectangular box and shaken. When you open the box, the toothpicks will be facing in about the same direction, but will have no definite spatial organization. They are free to move, but like to line up in about the same direction. This is a primitive model for nematic liquid crystals.

Smectic liquid crystals are different from nematics in that they have one more degree of orientational order than do the nematics. Smectics generally form layers within which there is a loss of positional order, while orientational order is still preserved.

If a nematic liquid crystal is made of chiral molecules, i.e. the molecules differ from their mirror image, a cholesteric liquid crystal (from cholesterol acetate, the first kind of this type) is obtained. Locally, cholesterics can be practically not distinguished from nematics but the preferred orientation forms a helical structure, with the helical axis perpendicular to the director.

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Liquid crystals usually consist of steric “rod-like” or “disk-like” organic molecules which tend to align themselves with a long range order due to anisotropic intermolecular forces. As a result of orientational order, most physical properties of liquid crystals are anisotropic. Examples are heat diffusion, magnetic susceptibility, dielectric permittivity or optical birefringence.

Liquid crystal state transition is typical of many polymers. Liquid crystal polymers (LCPs) consist of densely packed fibrous polymer “chains”.

Liquid crystal polymers are capable of forming regions of highly ordered structure while in the liquid phase. However, the degree of order is somewhat less than that of a regular solid crystal. Typically LCPs have outstanding mechanical properties at high temperatures, excellent chemical

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resistance, inherent flame retardancy and good weatherability. Liquid crystal polymers come in a variety of forms from sinterable high temperature to injection moldable compounds.

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1.Liquid crystals are also not quite liquid and not quite solid. Physically, they are observed to flow like liquids, but they have some properties of crystalline solids.

2.A lyotropic liquid crystal consists of two or more components that exhibit liquid-crystalline properties in certain concentration ranges.

3.If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the thermotropic liquid crystal phase, pushing the material into a conventional isotropic liquid phase.

4.Smectic liquid crystals are different from nematics in that they have one more degree of orientational order than do the nematics.

5.The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap.

6.Nematics are polarizable rod-like organic molecules on the order of 20 Angstroms in length.

7.In the lyotropic phases solvent molecules fill the space around the compounds to provide fluidity to the system.

8.Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field.

9.Knowledge in the field of liquid crystals is crucial for an understanding of biological membranes, thus biologists, medical researchers and pharmacists are also interested in liquid crystal research.

10.There is a range of temperatures at which we observe thermotropic liquid crystal; and most of them have several “subphases” (nematic, smectic etc.), which we may observe by modifying temperature.

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in… — 3 3 — exhibit a phase transition — 3 — distinct texture —

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1.Can we consider a liquid crystal state to be a fourth state of matter?

2.What physical properties should a polymer have to form a liquid crystal?

3.What are the two main criteria of liquid crystals classification?

4.What is anisotropy of properties in liquid crystals?

5.Describe some of the main advantages of LCD monitors.

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UNIT 8

POLYELECTROLYTES

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Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions, making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers, and are sometimes called polysalts. Like salts, their solutions are electrically conductive.

Like polymers, polyelectrolyte solutions are often viscous. Charged molecular chains play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while their unique properties are being exploited in a wide range of technological and industrial fields.

One of the major roles of polyelectrolytes seems to be the one played in biology and biochemistry. Many biological molecules are polyelectrolytes. For instance, polypeptides (thus, all proteins) and DNAs are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.

The physical properties of polyelectrolyte solutions are usually strongly affected by this degree of charging. Since the polyelectrolyte dissociation releases counter-ions, this necessarily affects the solution’s ionic strength, and, therefore, the Debye length. This, in turn, affects other properties, such as electrical conductivity.

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Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they can be used to either stabilize colloidal suspensions, or to initiate flocculation (precipitation). They can also be used to impart a surface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used as thickeners, emulsifiers, conditioners, flocculents, and even drag reducers. They are used in water treatment and for oil recovery.

Many soaps, shampoos, and cosmetics incorporate polyelectrolytes. Additionally, they are added to many foods. Some of the polyelectrolytes that appear on food labels are pectin, carrageenan, alginates, polyvinylpyrrolidone and carboxymethyl cellulose. All but the last two are of natural origin. Finally, they are used in a variety of materials, including cement.

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In order to make polymers dissolve in water, something that is of considerable technological interest, it is necessary to add charged groups along the hydrophobic backbone. These charged macromolecules display a rich variety of properties not found in neutral polymers and are therefore of considerable interest from a basic scientific standpoint. For example, altering the solvent conditions may cause coil-globule and coil-rod transitions in polyelectrolytes, which are not always observed in neutral polymers under similar conditions.

While their importance as biological molecules such as proteins and nucleic acids has been known for a long time, their significance in industry has only been realized in the last two decades. They now have a wide range of technological uses: as processing aids such as flocculants, dewatering agents, demulsifiers, and drag reduction agents; as additives in detergents and cosmetics; and in the manufacture of membranes, ion-exchange resins, gels, and modified plastics, wastewater treatment, separations, colloid science, and petroleum recovery.

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1.Polyelectrolytes are used in water treatment systems for a variety of

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applications including coagulation, flocculation, ballasted sedimentation, filtration and dissolved air flotation.

2.If the solution is free of added electrolytes the polymer coil expands as the polymer concentration decreases. This is known as the “polyelectrolyte effect”.

3.The conformation of any polymer is affected by a number of factors: notably, the polymer architecture and the solvent affinity. In the case of polyelectrolytes, charge also has an effect.

4.Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and gels.

5.It is known that the presence of polyelectrolytes induces aggregation of the oppositely charged surfactant.

6.There is currently much research in using biocompatible polyelectrolytes for implant coatings, for controlled drug release, and other applications.

7.Polyelectrolytes which bear both cationic and anionic repeating groups are called polyampholytes. Many proteins are polyampholytes, as some amino acids tend to be acidic while others are basic.

8.DNA molecule functioning as long-term storage of genetic information is in fact a polyelectrolyte, which is highly stable in weakly alkaline aqueous solutions.

9.If the solution contains a great deal of added salt, the charges will be screened and consequently the polyelectrolyte chain will collapse to a more conventional conformation.

10.Although the statistical conformation of polyelectrolytes can be captured using variants of conventional polymer theory, the long-range Coulomb interactions are necessary to properly model polyelectrolyte chains.

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1.What is the main difference between polyelectrolyte and ordinary polymer?

2.What structure does polyelectrolyte have in water solution?

3.How will you classify polyelectrolytes?

4.Give 2-3 examples of application fields for polyelectrolytes.

5.Provide examples of natural polyelectrolytes.

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