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Prior to the early ‘s, chemists doubted the existence of molecules having molecular weights greater than a few thousand.
This limiting view was challenged by Hermann Staudingera German chemist grrek experience in studying natural compounds such as rubber and cellulose. In contrast to the prevailing rationalization of these substances as aggregates of small molecules, Staudinger proposed they were made up of macromolecules composed of 10, or more atoms.
greei He formulated a polymeric structure for Chelsea Shirt fabregas 17 4 Away Adidas Kids Originals Black q4H77Fbased on a repeating isoprene unit referred to as a monomer.
For his contributions to chemistry, Staudinger received the Nobel Prize. The terms polymer and monomer were derived from the Greek roots poly manymono one and meros part. Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties.
Indeed, applications of these materials as fibers, flexible films, adhesives, resistant paints and tough but mehhoody solids have transformed modern society.
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Some important examples of these substances are discussed in the following sections. The repeating structural unit of most simple polymers not only reflects the monomer s from which the polymers are constructed, but also provides a concise means for drawing structures to represent these macromolecules.
For polyethylene, arguably the simplest polymer, this is demonstrated by the following equation. Here ethylene ethene is the monomer, and the corresponding linear polymer is called high-density polyethylene HDPE. If Y and Z represent moles of monomer and polymer respectively, Z is approximately 10 -5 Y.
This polymer is called polyethylene rather than polymethylene, -CH 2 – nbecause ethylene is a stable compound methylene is notand it also serves as the synthetic precursor of the polymer.
The two open bonds remaining at the ends of the long chain of carbons colored magenta are normally not specified, because the atoms or groups found there depend on the chemical process used for polymerization. The synthetic methods used to prepare this and other polymers will be described later in this chapter.
Unlike simpler pure compounds, most polymers are not composed of identical molecules. The HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer units. Because of this, polymer molecular weights are usually given as averages. Two experimentally determined values are common: M nthe number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and M wthe weight average molecular weight, is calculated from the weight fraction distribution of different sized molecules.
These are defined below. Since larger molecules in a sample weigh more than smaller molecules, the weight average M w is necessarily skewed to higher values, and is always greater than M n.
The influence of different mass distributions on M n and M w may be examined metohody the aid of a simple mass calculator. To use this device. Many polymeric materials having chain-like structures similar to polyethylene are known.
Polymers formed by a straightforward linking together of monomer units, with no loss or gain of material, are called addition polymers or chain-growth polymers.
A listing of some important addition polymers and their monomer precursors is presented in the following table. Rubber and cellulose molecules have similar mass ranges, but fewer monomer units because of the monomer’s larger size. The physical properties of these three polymeric substances differ from each other, and of course from their monomers.
It greei not as easily stretched and deformed as is LDPE. HDPE is insoluble in water and most organic solvents, although some metgoody may occur on immersion in the latter. HDPE is an excellent electrical insulator. Films made from LDPE stretch easily and are commonly used for wrapping.
LDPE is insoluble in water, but softens and swells on exposure to hydrocarbon solvents. It swells to more than double its size in nonpolar organic solvents like toluene, eventually dissolving, but is impermeable to water.
The C 5 H 8 monomer isoprene is a volatile liquid b. Cotton absorbs water readily, but is unaffected by immersion in toluene or most other organic solvents. Cellulose fibers may be bent and twisted, but do not stretch much before breaking. The monomer of cellulose is the Grdek 6 H 12 O 6 aldohexose D-glucose.
To account for the differences noted here we need to consider mrthoody nature of the aggregate methoodg structure, or morphologyof each substance. Because polymer molecules are so large, they generally pack together in a non-uniform fashion, with ordered or crystalline-like regions mixed grwek with disordered or amorphous domains. In some cases the entire solid may be amorphous, composed entirely of coiled and tangled macromolecular chains. Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three-dimensional matrix.
In the diagram on the right, crystalline domains are colored blue.
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Increased crystallinity is associated with an increase in rigidity, tensile strength and opacity due to light scattering. Amorphous polymers are usually less rigid, weaker and more easily deformed. They are often transparent. Three factors that influence the degree of crystallinity are: As noted earlier, HDPE is composed of very grerk unbranched hydrocarbon chains.
These pack together easily in crystalline domains that alternate with amorphous segments, and the resulting material, while relatively strong and stiff, retains a degree of mwthoody. In contrast, LDPE is composed of smaller and more highly branched chains which do not easily adopt crystalline structures.
This material is methooxy softer, weaker, less dense and more easily deformed than HDPE. As a rule, mechanical properties such as ductility, tensile strength, and hardness rise and eventually level off with increasing chain length. The nature of cellulose supports the above analysis and demonstrates the importance of the third factor iii.
To begin with, cellulose chains easily adopt a stable rod-like conformation.
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Methoosy molecules align themselves side by side into fibers that are stabilized by inter-chain hydrogen bonding between the three hydroxyl groups on each monomer unit.
Consequently, crystallinity is high and the cellulose molecules do not move or slip relative to each other. The high concentration of hydroxyl groups also accounts gteek the facile absorption of water that is characteristic of cotton.
Natural rubber is a completely amorphous polymer. Unfortunately, the potentially useful methoofy of raw latex rubber are limited by temperature dependence; however, these properties can be modified by chemical change. Metboody cis-double bonds in the hydrocarbon chain provide planar segments that stiffen, but do not straighten the chain. If instead, the chains of rubber molecules are slightly cross-linked by sulfur atoms, a process called vulcanization which was discovered by Charles Goodyear inthe desirable elastomeric properties of rubber are substantially improved.
The following illustration shows a cross-linked methoofy of amorphous rubber. By clicking on the diagram it will change to a display of the corresponding stretched section. The more highly-ordered chains in the stretched conformation are entropically unstable and return to their original coiled state when allowed to relax click a second time. On heating or cooling most polymers undergo thermal transitions that provide insight into their morphology.
These are defined as the melt transition, T mand the glass transition, T g. T m is the temperature at which crystalline domains lose their structure, or melt.
As crystallinity increases, so does T m. T g is the temperature below which amorphous domains lose the structural mobility of the polymer chains and become rigid glasses. T g often depends on the history of the sample, particularly previous heat treatment, mechanical manipulation and annealing. It is sometimes interpreted as the temperature above which significant portions of polymer chains are able to slide past each other in response to an applied force.
The introduction of relatively large and stiff substituents such as benzene rings will interfere with this chain movement, thus increasing T g note polystyrene below. The introduction of small molecular compounds called plasticizers into the polymer matrix increases the interchain spacing, allowing chain movement at lower temperatures.
The outgassing of plasticizers used to modify interior plastic components of automobiles produces the “new-car smell” to which we are accustomed. T m and T g values for some common addition polymers are listed below. Note that cellulose has neither a T m nor a T g. Rubber is a member of an important group of polymers called elastomers. Elastomers are amorphous polymers that have the ability to stretch and then return to their original shape at temperatures above T g.
This property is important in applications such as gaskets and O-rings, so the development of synthetic elastomers that can function under harsh or demanding conditions remains a practical goal.
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At temperatures below T g elastomers become rigid glassy solids and lose all elasticity. A tragic example of this caused the space shuttle Challenger disaster.
The unexpectedly low temperatures on the morning of the launch were below this T gallowing hot rocket gases to escape the seals. Symmetrical monomers such as ethylene and tetrafluoroethylene can join together in only one way. Monosubstituted monomers, on the other hand, may join together in two organized ways, described in the following diagram, or in a third random manner. Most monomers of this kind, including propylene, vinyl methoodt, styrene, acrylonitrile and acrylic esters, prefer to join in a head-to-tail fashion, with some randomness occurring from time to time.
The reasons for this regioselectivity will be discussed in the synthetic methods section. If the polymer chain is drawn in a zig-zag fashion, as shown above, each of the substituent groups Z will metjoody be located above or below the plane defined by the carbon chain.
Consequently we can identify three configurational isomers of such polymers. If all the substituents lie on one side of the chain the configuration is called isotactic.
If the substituents alternate from one side to another in a regular manner the configuration is termed syndiotactic. Finally, a random arrangement of substituent groups is referred to as atactic. Examples of these configurations are shown here. Many common and useful polymers, such as polystyrene, polyacrylonitrile and poly vinyl chloride are atactic as normally prepared. Customized catalysts that effect stereoregular polymerization of polypropylene and some other monomers have been developed, and the improved properties associated with the increased crystallinity of these products has made this an important field of investigation.
The following values of T g have been reported. The properties of a given polymer will vary considerably with its tacticity. Thus, atactic polypropylene is useless as a solid construction material, and is employed mainly as a component of adhesives or as a soft matrix for composite materials.