Fisika Polimer Ariadne L. Juwono. Sem /2007

Transcription

1 Chapter 9. Analysis and testings of polymer 9.1. Chemical analysis of polymers 9.2. Spectroscopic methods 9.3. X-Ray diffraction analysis 9.4. Microscopy 9.5. Thermal analysis 9.6. Physical testing

2 9.1. Chemical analysis of polymers There are 2 common techniques for analyzing low-molecular-weight polymers of reactions of polymers. Mass spectroscopy The polymer is reacted to form low-molecular-weight fragments that are condensed at liquid-air temperature. Then, the fragments are Volatilized, ionized, and separated based on the mass and charge by the action of electric and magnetic fields in a typical mass spectrometer analysis. Gas Chromatography A method of separation in which gaseous or vaporized components are distributed between a moving gas phase and a fixed liquid phase or solid absorbance. The components pass through chromatography column and are detected their number, nature, and amounts.

3 9.2. Spectroscopic methods Infrared spectroscopy (IR) Infrared spectroscopy works because chemical bonds have specific frequencies at which they vibrate corresponding to energy levels. The resonant frequencies or vibrational frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, eventually by the associated vibronic coupling. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. Thus, the frequency of the vibrations can be associated with a particular bond type. Simple diatomic molecules have only one bond, which may stretch.

4 More complex molecules may have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. The atoms in a CH 2 group, commonly found in organic compounds can vibrate in six different ways, symmetrical and asymmetrical stretching, scissoring, rocking, wagging and twisting. In order to measure a sample, a beam of infrared light is passed through the sample, and the amount of energy absorbed at each wavelength is recorded. This may be done by scanning through the spectrum with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once.

5 From this, a transmittance or absorbance spectrum may be plotted, which shows at which wavelengths the sample absorbs the IR, and allows an interpretation of which bonds are present. This technique works almost exclusively on covalent bonds, and as such is of most use in organic chemistry. Clear spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures however.

6 Sample preparation Gaseous samples require little preparation beyond purification, but a sample cell with a long pathlength (typically 5-10 cm) is used as gases show relatively weak absorbances. Liquid samples can be sandwiched between two plates of a high purity salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, and so the sample, washing reagents and the like must be anhydrous (without water).

7 Solid samples can be prepared in two major ways. The first is to crush the sample with a mulling agent (usually nujol) in a marble or agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass.

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11 Nuclear magnetic resonance spectroscopy (NMR) Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the magnetic properties of an atom's nucleus. All nuclei that contain odd numbers of nucleons and some that contain even numbers of nucleons have an intrinsic magnetic moment. The most commonly used nuclei are hydrogen-1 and carbon-13, although certain isotopes of many other elements nuclei can also be observed. NMR studies a magnetic nucleus, like that of a hydrogen atom (protium being the most receptive isotope at natural abundance) by aligning it with a very powerful external magnetic field and perturbing this alignment using an electromagnetic field. The response to the field by perturbing is what is exploited in nuclear magnetic resonance spectroscopy and magnetic resonance imaging.

12 NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about a molecule. It is the most powerful technique that can provide detailed information on the three-dimensional structure of biological molecules in solution. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary quantum computers. Spin angular momentum is a vector quantity. The z component of which, denoted I z, is quantized: This magnetic moment is intrinsically related to I with a proportionality constant γ, called the gyromagnetic ratio:

13 The energy of a magnetic moment µ when in a magnetic field B 0 : The frequency of electromagnetic radiation required to produce resonance of a specific nucleus in a field B is:

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15 Chemical shifts of NMR

16 Electron paramagnetic resonance spectroscopy (EPR/ESR) Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a spectroscopic technique which detects species that have unpaired electrons, generally meaning that the molecule in question is a free radical, if it is an organic molecule, or that it has transition metal ions if it is an inorganic complex. Because most stable molecules have a closed-shell configuration without a suitable unpaired spin, the technique is less widely used than nuclear magnetic resonance (NMR). The basic physical concepts of the technique are analogous to those of NMR, but instead of the spins of the atom's nuclei, electron spins are excited. Because of the difference in mass between nuclei and electrons, weaker magnetic fields and higher frequencies are used, compared to NMR. For electrons in a magnetic field of 0.3 tesla, spin resonance occurs at around 10 GHz.

17 EPR is used in solid-state physics, for the identification and quantification of radicals (i.e., molecules with unpaired electrons), in chemistry, to identify reaction pathways, as well as in biology and medicine for tagging biological spin probes. Since radicals are very reactive, they do not normally occur in high concentrations in biological environments. With the help of specially designed nonreactive radical molecules that attach to specific sites in a biological cell, it is possible to obtain information on the environment of these so-called spin-label or spin-probe molecules.

18 An electron has a magnetic moment. When placed in an external magnetic field of strength B 0, this magnetic moment can align itself parallel or antiparallel to the external field. The former is a lower energy state than the latter (this is the Zeeman effect), and the energy separation between the two is E = g e µ B B 0, where g e is the gyromagnetic ratio of the electron (see also the Landé g-factor) the ratio of its magnetic dipole moment to its angular momentum, and µ B is the Bohr magneton. To move between the two energy levels, the electron can absorb electromagnetic radiation of the correct energy: E = hν = g e µ B B 0 The paramagnetic centre is placed in a magnetic field and the electron caused to resonate between the two states; the energy absorbed as it does so is monitored, and converted into the EPR spectrum.

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21 9.3. X-ray diffraction (XRD) X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. This generally leads to an understanding of the material and molecular structure of a substance. The spacing in the crystal lattice can be determined using Bragg's law. The electrons that surround the atoms, rather than the atomic nuclei themselves, are the entities that physically interact with the incoming X-ray photons. This technique is widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds, DNA, and proteins. X-ray diffraction is commonly carried out using single crystals of a material, but if these are not available, microcrystalline powdered samples may also be used, although this requires different equipment, gives less information, and is much less straightforward.

22 Bragg's Law nλ = 2d sinθ

23 X-ray diffraction pattern from a layered structure vermiculite clay.

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25 9.4. Microscopy Light microscopy Electron microscopy Scanning electron microscopy (SEM)

26 9.5. Thermal analysis Differential scanning calorimetry (DSC)

27 Thermogravimetric analysis (TGA) A method of measuring the weight change of a sample as a function of temperature, using a sensitive balance. Some application of TGA in the assessment of thermal stability and decomposition temperature, curing in condensation polymers, composition in distribution in copolymers, and composition of filled polymers Thermomechanical analysis This technique measures the mechanical response of a polymer system as the temperature is changed. The measurements include dilatometry, heat deflection, torsion modulus, and stress-strain behaviour.

28 Thermal properties The most interesting thermal properties is the softening temperature of plastics. Some measurements to obtain this property are: Using an indentor under fixed load to penetrate the plastics, HDT, Polymer melt of stick temperature test, Zero-strength temperature test. Another test is a flammability test to obtain the burning rate.

29 9.6. Physical testing Mechanical properties Stress-strain properties The most informative mechanical measurement is obtained from stress-strain curve in tension. The tensile test is conducted by measuring force developed as the sample is elongated at constant rate of extension. Another mechanical tests are flexure, compression, and torsion tests Fatigue tests Impact tests

30 Impact tests

31 Instron machine

32 Tear resistance Tear resistance test specimen Hardness tests Barcoll hardness tester

33 Rockwell hardness tester Brinell hardness tester Vickers hardness tester

34 Optical properties Transmittance and reflectance Transmittance is the major appearance of a transparent materials. Transmittance is the ratio of the intensities of light passing through and light incident on the material. Reflectance is the major appearance of an opaque materials. Transmittance is the ratio of the intensities of the reflected and the incident lights. A translucent substance is one that transmits part and reflects part of the light incident on it. (units : luminous transmittance and luminous reflectance [flux]).

35 Gloss Gloss is the geometrically selective reflectance of a surface responsible for its shinny of lustrous appearance. Haze Haze is the percentage of transmitted light that is passing through the specimen deviates from the incident beam by forward scattering. Transparency Transparency is the state permitting perception of objects through or beyond the specimen. A sample of low transparency may not exhibit haze, but objects seen through it will appear blurred of distorted. These properties are objective physical properties that belong to the materials.

36 Color Color is the subjective sensation in the brain resulting from the perception of those aspects of the appearance of objects that result from the spectral composition of the light reaching the eye. Color depends largely on the spectral power distribution of a light source, the spectral reflectance of the illuminated object, and the spectral response curves of the eye. Color technology includes visual perception, the measurement of color and color difference, color matching, and the coloring of plastics.

37 Electrical properties Dielectric constant, dielectric strength and loss factor Dielectric constant of an insulating material is the ration of the capacities of a parallel plate measured w or w/o the dielectric material placed between the plates. Loss angle, δ, is a phase difference between changes in the field and the polarization. Power factor sin δ, dissipation factor tan δ. Loss factor dielectric constant X power factor. For insulators, high voltage decreases the resistance and deteriorates the dielectric.

38 Resistivity In most polymers, the resistance is very high. Both surface and volume resistivity are important for applications of polymers as insulating materials Electronic properties Polymers are possible to produce unusual electronic properties. These include electrical conductivity, charge storage, energy transfer, and, contact electrification (triboelectricity). Examples: PE, PET, PVF Conductivity of some polymer can have 20 X conductivity usual polymer. Examples: polyacetylene, poly-p-phenylene.

39 Chemical properties Resistance to solvent The effect of solvents on polymers may take several forms: a. Solubility, b. Swelling (inc absorption of water), c. Environmental stress cracking (materials fail by mechanical stress in the presence of an organic liquid or an aqueous solution), d. Craszing (material fails by the development of small cracks in the presence of an organic liquid or its vapour, w or w/o mechanical stress Vapour permeability The permeability of a polymer to a gas or vapour is the product of the solubility of the gas or vapour in the polymer and its diffusion coefficient. Permeability rate of transfer vapour through unit thickness of the polymer in film film/m 2 and p.

40 Weathering Artificial weathering for convenience and reproducibility. The real weathering test takes a long time and expensive.