Polymer Characterization: Spectroscopic techniques Flashcards
What is spectroscopy?
A broad range of techniques that measure the emission, absorption and transmission of electromagnetic radiation from materials
- uses a wide array of wavelengths (not visible)
What is Fourier-transform infrared spectroscopy (FTIR)?
- popular non-destructive technique
- uses mid-infrared range (4000-400 cm^-1)
The radiation passes through a beamsplitter, causes interference to create different wavelengths, these wavelengths pass through the sample and are detected on the other side. moveable mirror moves to change the wavelength
- fourier transform is used to convert time domain (mirror position) results to frequency domain
How does FTIR work?
As radiation passes through the sample, energy is absorbed through the vibration of molecules in the sample (hits dipole and excites it)
- certain frequencies make the molecule resonate - these correspond to the peaks in FTIR
- the location and intensity of absorption peaks depend on the change in dipole moment induced by the vibration
what are the different vibrations when radiation hits dipole?
- stretch vibrations (symmetric and asymmetric)
- bending vibrations (in-plane rocking, in-plane scissoring, out-of-plane wagging, out-of-plane twisting)
What does FTIR tell you?
- matches stretching to types of bonds
- can see intramolecular bonds (interactions with neighbouring chains influences the vibrational modes) )and intermolecular bonds
- can determine grafting
What is Raman spectroscopy?
similar to FTIR, but uses Raman scattering (in-elastic scattered light) - they are emitted at different frequency than they are absorbed at (so only use a single frequency light source)
*need more powerful lasers since the intensity of Raman scattering is much less
What are the pros/cons of Raman spectroscopy compared to FTIR?
pros:
- wider spectral range
- Raman can ‘look through’ containers/solutions
- less sensitive to thermal noise
- higher spatial resolution
- easier to combine with microscopy and mapping techniques
cons:
- requires a much more powerful radiation source (laser)
- requires more sensitive detector (generally CCD)
- more complicated to calibrate
- highly sensitive to fluorescence
- issues of photodegradation
Raman Spectroscopy wavelength?
- about 0.000001% as intense as incident radiation
- requires lots of photons
- often use 785 nm laser as most popular tradeoff of signal strength and sample fluorescence
How does Raman spectroscopy work?
incident beam fired at beam splitter and focused on sample. The reflected wavelengths are filter with a notch filter (blocks Rayleigh scattering) and is focused into a CCD detector
What is sample mapping with Raman Spectroscopy?
Raman techniques allow beam focusing - thus allowing sample across the XY plane to be mapped with Raman spectroscopy points and used to map heterogeneous systems
What is confocal Raman microscopy?
Raman combined with confocal microscopy to allow mapping in the Z-direction. This allows us to look through the surface of samples and record Raman spectra beneath the surface (not possible with FTIR)
- this is ideal for multilayer systems (ex. polymer packaging) as it can quantify gradients and highlight interdiffusion between polymer layers
What is nuclear magnetic resonance (NMR) spectroscopy?
Uses strong magnetic fields to cause excitation of atomic nuclei (requires non-zero quantum spin nuclei - isotopes with odd number of protons+neutrons)
When the nuclei align in a magnetic field, it gives info about how they are bonded to other atoms in the molecule
How does NMR work?
the energy required to orient an active nucleus results in the emission of radiation. We measure this energy as chemical shift of the signal frequency compared to standard reference material
What does NMR find?
characterization of polymer structure (NMR provides precise platform for determining the structure and configuration of the chain and pendant groups)
- allows for identification of polymer
- structure analysis of polymer
What is energy dispersive x-ray (EDS) spectroscopy?
Used for analyzing elemental composition of a given sample, but doesn’t tell us much about structure
*usually used with SEM since these x-rays scatter off when SEM occurs (these are the x-rays scattered when an electron ejects an electron from an inner shell and another electron drops down to fill that hole)
