Polymer - Wikipedia
Here you'll learn the definition and properties of polymers, another name for plastics. The simplest definition of a polymer is a useful chemical made of many. Structure/property relationships in polymer membranes for water purification and Polymer membrane-based processes dominate the desalination market, and. Structure-property relationships. Given the large number of possible configurations in polymers, what guides to likely properties are available? We have.
Glass transition temperature[ edit ] A parameter of particular interest in synthetic polymer manufacturing is the glass transition temperature Tgat which amorphous polymers undergo a transition from a rubbery, viscous liquid, to a brittle, glassy amorphous solid on cooling. The glass transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizer.
In general, polymeric mixtures are far less miscible than mixtures of small molecule materials.
This effect results from the fact that the driving force for mixing is usually entropynot interaction energy. In other words, miscible materials usually form a solution not because their interaction with each other is more favorable than their self-interaction, but because of an increase in entropy and hence free energy associated with increasing the amount of volume available to each component.
This increase in entropy scales with the number of particles or moles being mixed. Since polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture is far smaller than the number in a small molecule mixture of equal volume.
The Basics: Polymer Definition and Properties
The energetics of mixing, on the other hand, is comparable on a per volume basis for polymeric and small molecule mixtures. This tends to increase the free energy of mixing for polymer solutions and thus make solvation less favorable. Thus, concentrated solutions of polymers are far rarer than those of small molecules.
Furthermore, the phase behavior of polymer solutions and mixtures is more complex than that of small molecule mixtures. Whereas most small molecule solutions exhibit only an upper critical solution temperature phase transition, at which phase separation occurs with cooling, polymer mixtures commonly exhibit a lower critical solution temperature phase transition, at which phase separation occurs with heating.
In dilute solution, the properties of the polymer are characterized by the interaction between the solvent and the polymer.
In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions. In a bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. In the theta solventor the state of the polymer solution where the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent repulsion balances exactly the intramolecular monomer-monomer attraction.
Under the theta condition also called the Flory conditionthe polymer behaves like an ideal random coil. The transition between the states is known as a coil-globule transition. Inclusion of plasticizers[ edit ] Inclusion of plasticizers tends to lower Tg and increase polymer flexibility. Plasticizers are generally small molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and reduced interchain interactions.
A good example of the action of plasticizers is related to polyvinylchlorides or PVCs. An uPVC, or unplasticized polyvinylchloride, is used for things such as pipes.
A pipe has no plasticizers in it, because it needs to remain strong and heat-resistant. Plasticized PVC is used in clothing for a flexible quality. Plasticizers are also put in some types of cling film to make the polymer more flexible.
Chemical properties[ edit ] The attractive forces between polymer chains play a large part in determining polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Different side groups on the polymer can lend the polymer to ionic bonding or hydrogen bonding between its own chains. These stronger forces typically result in higher tensile strength and higher crystalline melting points.
The intermolecular forces in polymers can be affected by dipoles in the monomer units. These strong hydrogen bonds, for example, result in the high tensile strength and melting point of polymers containing urethane or urea linkages.
Dipole bonding is not as strong as hydrogen bonding, so a polyester's melting point and strength are lower than Kevlar 's Twaronbut polyesters have greater flexibility. Ethene, however, has no permanent dipole.
Just as quenching can produce amorphous arrangements, processing can control the degree of crystallinity for those polymers that are able to crystallize. Some polymers are designed to never be able to crystallize. Others are designed to be able to be crystallized. The higher the degree of crystallinity, generally, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer can be directly affected by its crystallinity.
Crystallinity creates benefits in strength, stiffness, chemical resistance, and stability. Scientists and engineers are always producing more useful materials by manipulating the molecular structure that affects the final polymer produced.
Manufacturers and processors introduce various fillers, reinforcements and additives into the base polymers, expanding product possibilities.
Characteristics of Polymers The majority of manufactured polymers are thermoplastic, meaning that once the polymer is formed it can be heated and reformed over and over again.
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This property allows for easy processing and facilitates recycling. The other group, the thermosets, cannot be remelted.
Once these polymers are formed, reheating will cause the material to ultimately degrade, but not melt. Every polymer has very distinct characteristics, but most polymers have the following general attributes. Polymers can be very resistant to chemicals. Consider all the cleaning fluids in your home that are packaged in plastic. Reading the warning labels that describe what happens when the chemical comes in contact with skin or eyes or is ingested will emphasize the need for chemical resistance in the plastic packaging.
While solvents easily dissolve some plastics, other plastics provide safe, non-breakable packages for aggressive solvents. Polymers can be both thermal and electrical insulators. A walk through your house will reinforce this concept, as you consider all the appliances, cords, electrical outlets and wiring that are made or covered with polymeric materials.
Thermal resistance is evident in the kitchen with pot and pan handles made of polymers, the coffee pot handles, the foam core of refrigerators and freezers, insulated cups, coolers, and microwave cookware. The thermal underwear that many skiers wear is made of polypropylene and the fiberfill in winter jackets is acrylic and polyester.
Generally, polymers are very light in weight with significant degrees of strength. Consider the range of applications, from toys to the frame structure of space stations, or from delicate nylon fiber in pantyhose to Kevlar, which is used in bulletproof vests. Some polymers float in water while others sink. But, compared to the density of stone, concrete, steel, copper, or aluminum, all plastics are lightweight materials.
Polymers can be processed in various ways. Extrusion produces thin fibers or heavy pipes or films or food bottles. Injection molding can produce very intricate parts or large car body panels. Plastics can be molded into drums or be mixed with solvents to become adhesives or paints. Elastomers and some plastics stretch and are very flexible. Some plastics are stretched in processing to hold their shape, such as soft drink bottles. Polymers are materials with a seemingly limitless range of characteristics and colors.
Polymers have many inherent properties that can be further enhanced by a wide range of additives to broaden their uses and applications. Polymers can be made to mimic cotton, silk, and wool fibers; porcelain and marble; and aluminum and zinc. Polymers can also make possible products that do not readily come from the natural world, such as clear sheets and flexible films. Polymers are usually made of petroleum, but not always.
Many polymers are made of repeat units derived from natural gas or coal or crude oil. But building block repeat units can sometimes be made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Some plastics have always been made from renewable materials such as cellulose acetate used for screwdriver handles and gift ribbon. When the building blocks can be made more economically from renewable materials than from fossil fuels, either old plastics find new raw materials or new plastics are introduced.
Polymers can be used to make items that have no alternatives from other materials. Polymers can be made into clear, waterproof films.