Intermolecular forces
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogens in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole। The attractive forces between polyethylene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogens in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole। The attractive forces between polyethylene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
Branching
During the propagation of polymer chains, branching can occur। In radical polymerization, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
During the propagation of polymer chains, branching can occur। In radical polymerization, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
Stereoregularity
Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.
In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.
Syndiotactic substituent groups alternate regularly in opposite directions। because of this regularity, syndiotactic chains can position themselves close to each, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.
Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.
In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.
Syndiotactic substituent groups alternate regularly in opposite directions। because of this regularity, syndiotactic chains can position themselves close to each, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.
Copolymerization
Copolymerization is polymerization with two or more different monomers. Already mentioned are the twenty amino acid monomers that make up protein chains. Copolymerization of different monomers can result in varied properties of polymers, just as different amino acids result in different shapes of proteins. For example, copolymerising ethene with small amounts of hex-1-ene is one way to form linear low density polyethene (LLDPE) (See Polyethylene). The C4 branches resulting from the hexene lower the density and prevent such large crystalline regions within the polymer as in HDPE. This means that LLDPE can withstand strong tearing forces whilst remaining flexible.
The following image shows a specific type of copolymerization called a step-growth polymerization, or condensation polymerization। In this particular polymerization a small molecule is released upon polymerization. In the following reaction scheme, water is given off and nylon is formed. The type of nylon (name and properties) are governed by the R and R' groups in the monomers used.
Copolymerization is polymerization with two or more different monomers. Already mentioned are the twenty amino acid monomers that make up protein chains. Copolymerization of different monomers can result in varied properties of polymers, just as different amino acids result in different shapes of proteins. For example, copolymerising ethene with small amounts of hex-1-ene is one way to form linear low density polyethene (LLDPE) (See Polyethylene). The C4 branches resulting from the hexene lower the density and prevent such large crystalline regions within the polymer as in HDPE. This means that LLDPE can withstand strong tearing forces whilst remaining flexible.
The following image shows a specific type of copolymerization called a step-growth polymerization, or condensation polymerization। In this particular polymerization a small molecule is released upon polymerization. In the following reaction scheme, water is given off and nylon is formed. The type of nylon (name and properties) are governed by the R and R' groups in the monomers used.
Polymer characterization
A variety of laboratory techniques are used to determine the properties of polymers. Techniques such as wide angle xray scattering, small angle xray 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 is 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. Thermal degradation followed by analysis of the fragments is one more technique for determining the possible structure of the polymer.
1 comment:
Need More On Polymer. What about Plasma process? any flowchart?
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