Infrared absorption spectroscopy is one of the most important methods for the study of high polymers, and its use of wide range of application in this field:
1. Chemical structure, detection of particular types of chemical bonds,
2. conformation and configuration molecules
3. crystal structure
4. cystallinity, crystallization,
7. chemical analysis, identification
Infra red spectroscopy studies of high polymers have the following characteristic features.
For most polymers, film samples of suitable thickness for infrared measurement can be easily prepared.
Crystallization effects appear in the spectra. One can between amorphous polymers such as polymethyl methacylate, polyvinyl acetate, and polystyrene and crystalline polymers such as polyamides, polyethylene, and polyethylene terepthalate.
Chain molecules and crystallites can be oriented by stretching or rolling. By measuring the polarized infrared spectra of such oriented polymer sample, we can obtain useful information about the molecular or structure orbital. Infrared radiation plane-polarized after transmission trough plates of silver chlorine inclined at Brewster’s angler or through an oriented film or crystal, the intensity of a selected absorption band will be at maximum when the polarization vector of the vibration coincident in direction with the electric vector in incident beam.
Although almost synthetic high polymer consist sequence of more or less simple structural units, their overall structures at first glance appear to be highly complicated, and the application of a detailed theoretical analysis has been considered to be difficult. The polymer molecules arranged regularly in the crystalline region can, however, be treated theoretically by utilizing the symmetry properties of the chain. Factor analysis gives number of normal modes and the selection rules infrared and Raman spectra. To use the speed electronic computer has made it possible to calculate the normal modes vibrations of many crystalline polymers.
Comparatively few solvents are applicable for high polymers, but the data obtained from solution give us information on the isolated polymer molecules. The infrared spectra of CCl4 solution of polyethylene provide an estimate of the degree of branching without interference from crystallization effects. The solution spectra of stereoregular polymethyl methacrylate, isotactic polystyrene, etc, indicate the presence of special (helical) conformations in solution, especially in low temperature.
Low molecular weight compound can usually be purified to a high degree and brought into define state. In a strict sense, this is almost impossible with high polymers. The problem separates the mixture polymer homologues into fraction of identical molecules-not only with respect to a molecular weight but also with respect to degree and type branching-is almost insuperable. In additional, it is very difficult to bring sample into exactly the same state of crystallinity and orientation.
To provide more chemical structure in polymer, IR spectroscopy can also give more information about the physical structure which allows influence strongly the physical properties of polymer.
INFRARED SPECTROSCOPY ON POLYMER ANALYSIS
Identification techniques of polymer using infrared spectroscopy are based on vibrators of the atoms of some molecule. An infrared atom spectrum is obtained by passing infrared radiation is absorbed at a particular energy. The energy at every peak in absorption spectrum appears corresponds to the frequently of vibration of part of the sample molecule.
The are a number of methods available for examining polymers sample. If the polymer is thermoplastic it can be softened by warming and pressed into a thin film by using hydraulic press. Some polymers, such as cross-linked synthetic rubbers, can be mirotomed, like cut into pieces with a blade, if the polymer is a surface coating, reflectance techniques may be use.
The infrared spectrum can be divide into three regions, namely far infrared (<400 cm-1), the mid infrared (400-4000 cm-1) and the near-infrared (4000-13000 cm-1). Spectrum interpretation is simplified by the fact that the bands that appear can be assigned to particular parts of molecule, thus producing what are know as group frequency.
The most widely used method to characterize polymer structure is in infrared spectroscopy, particularly FTIR spectroscopy. Polymer sample for IR analysis can have variety of forms including thin film, solution, or a pellet containing a mixture of the granulated polymer and an IR transparent powder such as potassium bromide. Bulk sample can be analyze by reflection or attenuated total reflectance (ATR).
The properties of polymer dependent upon the average size of the molecular size that is present. Synthetic polymer consists of a mixture of molecules with various chain lengths and molecular weight. Because virtually all polymers are mixtures of many large molecules, one must resort to averages to describe molecular weight. Among many possible ways of reporting averages, three are commonly used: the number average, weight average, and z-average molecular weights. The weight average is probably the most useful of the three, because it fairly accounts for the contributions of different sized chains to the overall behavior of the polymer, and correlates best with most of the physical properties of interest. The different sized chains can facilitate detection by vibrational spectroscopy, even at low levels at which they occur in the many polymers.
Low-angle laser light-scattering (LALLS) photometer and a differential refractometer are used for the accurate measurement of weight-average molecular weights of coal macromolecules and synthetic polymers used in modeling coal structures. Combined with gel permeation chromatography, the LALLS gives an accurate measurement of the molecular weight distribution of the samples. The optical anisotropy of the macromolecules can also be obtained with the LALLS by using polarizing filters. These data give information on the shape or form of the molecules.
The ends of polymer chains often consist of functional group different with the monomer which constitutes the polymer. End group analysis involves the determination different type in given mass of polymer. Obvious application of ability to measure the concentration of end groups in polymer is the determination of number average molecular weight, which has an important effect on many properties in polymer. For example is polyethylene which provides a particularly simple and striking ability in infrared spectroscopy.
Polymerization is actual mechanism that involve in polymerization process. As infrared spectra of monomers are different to those of the corresponding polymers, it is possible to monitor polymerization reaction. Attenuated total reflectant (ATR) spectroscopy is particularly useful for monitoring infra red spectra of reaction mixtures short time intervals and for the characterizing reaction progress.
Composition of Copolymer
Copolymer is a mixture that made by two or more types of monomer which polymerize by similar mechanism. The composition of copolymers may be determined by using copolymer equation. For the copolymerization of a monomer M1 and other monomer M2, it is assumed that the rate of additional of monomer to the growing free radical depends only on the nature of the end-groups on the radical chain. At fundamental level, the molecular level of copolymer can often provide information of polymerization mechanism. The principal copolymer composition by infrared is on a term a superposition of the spectra of the relevant homopolymers, but also noted that this approach is potentially subject to error, particularly for random copolymers containing short sequence of individual monomer units.
The application of infrared spectroscopy in considered relate with copolymer is determination of the composition of copolymer and mixture of photopolymers. For example in analysis of mixture of low molecular weight materials, is the examination of a mixture polymer or of a copolymer when the pure homopolymers are available and when all the polymers are soluble in solvent that is reasonably transparent. If possible to work with a conventional liquid cell, for which x can be measured with adequate precision, and to measured with adequate precision, and to measure the various kind directly. For example the analysis of vinyl chloride/vinyl acetate copolymers provides a particularly good example. A solution of tetrahydrofuran is placed in a cell thickness 0.1-0.2 mm and the absorbance of the carbonyl peak at 1740 cm-1 is measured and compared with the corresponding value for poly(vinyl acetate) because poly(vinyl acetate) does not absorb at the frequency. This method assume that spectrum of poli(vinyl acetate) in the copolymer same with the homopolymer. This is substantially so in the case of block polymers but there is so many effects with random copolymers, particularly in the view of the fact that the concentration of vinyl chloride unit several times that of the vinyl acetate unit in commercial copolymers in this type.
The effect first that frequency of vibration of a group mode of a unit of type A may depend on whether the unit is attached directly to other unit A or unit B, and corresponding effect may occur for units in type B. Corresponding peak in spectrum may broadened, or maybe become asymmetry, and maybe shifted somewhat in position. Secondly if one type of monomer unit is present as a minor component it will occur as relatively isolated units in a matrix in major component. When the infrared spectra of series random ethylene/vinyl acetate copolymers are examined, the peak to due to the carbonyl stretching mode is found move from 1736, 8 cm-1 for material containing 95% of vinyl acetate units. The fact of the absorption peak changes with composition suggests the presence of more than component peak.
Cross-linking involves the formation of covalent bonds between polymer chains. The presence of cross-linking can have significant effect on the resulting properties of materials. Some polymers are cross-link by the application of the heat and pressure, while other polymers can be cross-linked via chemical reaction occurring temperature room.
For example, phenolic resin can be formed from two types of prepolymers, novolaks and resole. Condensation using acid on phenolic resin gives structural units in which aromatic rings are linked by methylene bridges, which known as novolaks. Subtitutions occurs mainly in the ortho and para position, and the proportion of )-0 and 0-p linkages depend on the reaction conditions. This type of resin usually subsequently cross-linked with reaction of hexamine. Condensation under alkaline conditions gives resol resins, in which predominant basic structure. Self condensation occurs which contain of a methylene eter functional group. This type of reaction predominates during the self curing (cross-linking) of resoles resins which take place after held at 150°C for about two hours. In view of the considerable structural complexity of the phenolic resins as whole discussion spectra will be restricted to an outline of the use of the group frequency approach.
The terms configuration and conformation are used to describe the geometric structure of a polymer and are often confused. Configuration refers to the order that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed. Conformation refers to order that arises from the rotation of molecules about the single bonds.
Stereoregularity is the term used to describe the configuration of polymer chains. Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain. A syndiotactic polymer chain is composed of alternating groups and ataxic is a random combination of the groups.
For some polymers, polymerization can result in different configuration. In stereoisomers, the atoms are linked in the same order in a head-to-tail configuration, but differ in their spatial arrangement. Especially for asymmetric monomers, the orientation of each monomer adding to the growing chain is described as tacticity. Infrared spectroscopy has been successfully applied to the study of tacicity of number of vinyl polymers. There is relationship between the stereoregularity and the present of regular chains and leads to the appearance of infrared bands due such chains. For example, the infrared of the isotactic, syndiotactic, and atactic at figure 9.
The absorbance values at 970 and 1460 cm-1 do not upon tacicity, where is absorbance at 840, 1000, and 1170 cm-1 are characteristic of isotatic PP, and at absorbance 870 cm-1 is syndiotactic PP. Such different helical structures present in the isomers and can be used to estimate the fractions of isotactic and syndiotactic sequences in particular samples.
The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angle. If the two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations Since different conformations represent varying distances between the atoms or groups rotating about the bond, and these distances determine the amount and type of interaction between adjacent atoms or groups, different conformation may represent different potential energies of the molecule.
Conformations are part of physical state of polymer chain. One conformation can be transformed into the other one by rotating certain groups around single bond. Conformation band originating in define short or long range conformational order, should be sensitive against any change of the state that influences the conformation in polymer change. Thus, the influence of solvent or temperature on conformational order in revealed by changes in infrared spectrum. In some example configurational and conformational isomerism are both present in the same polymer, which complicates the task of assigning the various peaks to the two type of structure. There are polymers that exist in one configurational state only but which exhibit conformational isomerism that is clearly manifest in vibrational spectrum.
When two or more conformational isomers are present in a liquid the concentrations in which they are present are determined by the Gibbs free energy differences, ΔG, between them and in thermodynamic equilibrium C1 and C2 any two forms are given by equation.
This relationship applies for low molecular weight halo-alkanes, which have been extensively investigated and some of which will be referred to below. Comparison of the spectra in the molten and solid states, particularly with and without quenching to vary the degree of chrystallinity present, usually permits assignment peaks to various conformers present.
For example, the comparison of the infrared spectra of poly (ethylene terephtalate) in molten form and in solid samples with different degrees of cristallinity show that there are five pairs peaks where the intensity of one increases as the other decreases, a clear indication for conformation isomers. They are 1470 and 1445 cm-1, 1340 and 1370 cm-1, 1120 and 1110 cm-1, 975 and 1045 cm-1, and 850 and 896 cm-1. The first pair is are due to
bending mode and the second wagging mode, which suggest that trans/gauche isomerism about the C-C bond of the –CH2- CH2- group in the basic structural unit is the origin of the pairs modes.
Several regions in vibrational spectrum of poly(vinyl chloride) or PVC are sensitive to configurational and conformational isomerism. In particular, the deformation mode and C-Cl stretching and bending modes are each split into several components due to various isomeric forms. The C-Cl stretching region (600-700 cm-1) is particularly complex.
Infrared spectroscopy can be used to investigate the effect environmental exposure of polymers. The exposure to ultraviolet radiation, radical attract by free radicals produced from natural process, and microbiological attack can all be studied by using infrared spectroscopy. The role of this technique in polymer degradation is illustrated by its application to the thermally and photoxidized polyethylenes. During the heat oxidation process with PE, ranges of carbonyl containing compounds are formed. This composition give arise to abroad C=O stretching at 1725 cm-1 because of the saturated keton. When the oxidized samples are treated with an alkali, peak at 1715 cm-1 will disappear and replaced by peak near 1610 cm-1. This band due to C=O stretching of COO- ion of salt., indicating at the shoulder 1715 cm-1 is saturated carboxylic acids. Another shoulder at 1735 cm-1 is characteristic of saturated aldehyde. The broad C=O also present in the infrared spectrum of photooxidized PE sample, which also show additional bond at 990 and 910 cm-1. The latter bands are characteristic of vinyl groups and their presence show that chains terminating unsaturated groups are being formed.
Source: My Assignment on Molecular Spectroscopy, USM 2007
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