Measurement techniques

Transcription

1 Measurement techniques 1 GPC GPC = gel permeation chromatography GPC a type of size exclusion chromatography (SEC), that separates analytes on the basis of size. The column used for GPC is filled with a microporous packing material. The column is filled with the gel. The smaller analytes can enter the pores more easily and therefore spend more time in these pores, increasing their retention time. These smaller molecules spend more time in the column and therefore will elute last. Conversely, larger analytes spend little if any time in the pores and are eluted quickly. All columns have a range of molecular weights that can be separated. When characterizing polymers, it is important to consider the polydispersity index (PDI) as well the molecular weight. Polymers can be characterized by a variety of definitions for molecular weight including the number average molecular weight (M n ), the weight average molecular weight (M w ) (see molar mass distribution). GPC allows for the determination of PDI. Figure 1: GPC setup 2 DSC DSC = differential scanning calorimetry DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program 1

2 for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned. When the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid, it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. DSC is used widely for examining polymeric materials to determine their thermal transitions. The observed thermal transitions can be utilized to compare materials, although the transitions do not uniquely identify composition. The composition of unknown materials may be completed using complementary techniques such as IR spectroscopy. Melting points and glass transition temperatures for most polymers are available from standard compilations, and the method can show polymer degradation by the lowering of the expected melting point, T m. Figure 2: DSC curve 3 TGA TGA = thermogravimetric analysis TGA is a method of thermal analysis in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate), or as a function of time (with constant temperature and/or constant mass loss). TGA can provide information about physical phenomena, such as second-order phase transitions, including vaporization, sublimation, absorption, adsorption, and desorption. The TGA instrument continuously weighs a sample as it is heated. As the temperature increases, various components of the sample are decomposed and the weight percentage of each resulting mass change can be measured. Results are plotted with temperature on the X-axis and mass loss on the Y-axis. 2

3 Figure 3: Here polyester (71% of the polymer), polystyrene (29% of the polymer), fiberglass (22.9% of the whole) and CaCO 3 (49.3% of the whole) were easily identified by their different temperatures of combustion or evaporation. 4 NMR NMR = nuclear magnetic resonance NMR is the most powerful tool available for organic structure determination. It exploits the magnetic properties of a certain atomic nuclei ( 1 H, 13 C, 15 N, 19 F, 31 P,...) to determine physical and chemical properties of atoms or the molecules in which they are contained. It provides detailed information about the structure, dynamics, reaction state and chemical environment of molecules. The principle of NMR usually involves three sequential steps: ˆ Magnetic nuclear spins align (anti)parallel to the applied constant magnetic field B 0. Protons precess around the field lines of the applied magnetic field with a frequency that depends on the magnitude of th magnetic field. ˆ By applying an RF perturbation on top (adding energy) with frequencies that are the same as the precessing frequency more magnetic nuclear spins align antiparallel to to the magnetic field and the precession movement will start to be in phase. The net magnetic vector will now be perpendicular to the applied magnetic field. ˆ When the RF pulse is switched off, spin-lattice recovery and spin-spin recovery occur. Figure 4: NMR mechanism 3

4 Figure 5: NMR mechanism Depending on their local chemical environment, different nuclei (of the same sort) in a molecule absorb at slightly different frequencies. Figure 6: Example of NMR spectrum 5 FTIR FTIR = Fourier transform infrared The IR portion of the EM spectrum is divided into three regions; the near,-mid,-and far-ir. The mid-ir (λ = µm) may be used to study the fundamental vibrations ans associated rotational-vibrational structure. The goal of any absorption spectroscopy is to measure how well a sample absorbs light at each wavelength. The most straightforward way to do this, the dispersive spectroscopy technique, is to shine a monochromatic light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. FTIR is a less intuitive way to obtain the same information. Rather than shining a monochromatic beam of light at the sample, this technique shines a beam containing many frequencies of light at once, and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is repeated many times. Afterwards, a computer takes all these data and works backwards to infer what the absorption is at each wavelength. The frequencies that are absorbed are characteristic of their structure and are resonant frequencies for a bond or a group that vibrates. 4

5 Figure 7: Example of FTIR spectrum Figure 8: Vibrational modes 6 XPS XPS = x-ray photoelectron spectroscopy XPS is a surface-sensitive quantitative spectroscopic technique that measures the elemental composition at the parts per thousand range, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 0 to 10 nm of the material being analyzed. E binding = E photon (E kinetic + Φ) (1) 5

6 Figure 9: XPS mechanism 7 TPA TPA = texture profile analysis TPA is a test that is performed to examine the recoveryproperties of a sample (often hydrogels) after compression. The sample is compressed twice by the plunjer, moving with the same speed. The obtained curve always represents force as a function of time. From the surface area below the curves and from the measured forces, different parameters can be calculated. REMARK: this technique is often used on foods and is called the two-bites-test. Figure 10: TP analysis curve 8 Fatigue test Fatigue tests are similar to TPA. In contrast to TPA, where only two compressive cycles are applied, the sample now undergoes a large amount of cycles in order to examine a possible change in mechanical properties after repeated loading. 6

7 9 Fracture test Fracture tests are similar to TPA. In contrast to TPA, where only two compressive cycles are applied, the sample now undergoes only one compression cycle until it breaks.parameters such as fracture force and fracture deformation are obtained. 10 Rheology Rheology is the study of the deformation and flow of matter under the influence of an applied stress. If a certain stress is applied to a sample, it will deform. When the stress is removed, an oscillating stress or strain is applied. Different responses can occur: ˆ Ideally elastic response (reversible deformation) ˆ Ideally viscous response (irreversible deformation) Polymers behave as visco-elastic materials. Two important parameters are the storage modulus G and the loss modulus G. ˆ G is a measure for the deformation energy stored by the sample during deformation ˆ G is a measure for the deformation energy consumed (e.g. heat generation) by the sample during deformation Figure 11: Rheometer setup 11 Texturometry In contrast to rheology, texturometry applies large deformations. By mean of a plunjer or probe, which compresses the sample at a constant rate, a compression force is applied onto the testing material. 7