General exam cb_05_24_10


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General exam cb_05_24_10

  1. 1. Temperature Dependence of Local Domain Photophysics Chris Bingham Reid Lab General Exam Practice
  2. 2. Why do we care? • Mission of CMDITR – Create new materials for information technologies • A lot of interest lies in using chromophores and polymers – Chromophores and polymers show a lot of promise for these new devices • We are generally interested in the electro-optic (EO) activity of these
  3. 3. Why aren’t they used? • EO activity is not inherent in composite materials, as it is with some inorganics (like LiNbO3). – EO activity arises from χ(2) processes, so order within the composite is important. – Rotational studies • Chromophore robustness (irreversible photo-decomposition) – Fluorescent intermittency studies
  4. 4. What are the objectives of my research? • Understand the processes behind creating order and photo-decomposition • Understand the relation between the local environment (domain) and the chromophore (probe) • Understand the probe-domain interactions that give rise to the population and depopulation of the dark state • Increase understanding of SM photophysics, which are surprisingly complex
  5. 5. What variables can we control? • Probe – Size – Shape – Functional groups • Domain – Glass transition temperature (Tg) – Side groups • Temperature – How does a change in the thermal energy of the system affect photophysics?
  6. 6. Experimental setup • Confocal fluorescence microscopy
  7. 7. Experimental conditions • Violamine R (VR) • Poly(vinyl alcohol) (PVOH) • Nano-molar concentrations • Temperature range from 23C to 85C
  8. 8. Fluorescence traces
  9. 9. Power Law Plots • Power law (PL) plots are indicative of distributed kinetics • Roll off on the plots means that it is moving to a more single exponential distribution
  10. 10. Memory Plots • A density of points along the diagonal indicates memory • Memory is related to domain exchanges • However, due to poor statistics, SM memory plots
  11. 11. Problems with VR • One of the downsides to using VR is that it comes as a relatively low dye content (60-65%) • The question is what is the other ~40%? – Is it also fluorescent, particularly at 532 nm? • Also, the fluorescence of VR is solvent dependent
  12. 12. Solid Walled Pockets • In studies performed by Orrit and coworkers (Zondervan, 2007), they showed that some polymers exhibit domains that are slow to exchange with the local environment, even above Tg. • The evidence for a slow exchange is that there is not a drastic jumps in the rotational timescales of the probe molecules
  13. 13. Support for SWPs • PVOH is able to hydrogen bond – Is known to h-bond with itself – Orrit and coworkers showed SWPs for glycerol and o-terphenyl, both which are capable of intra-polymer bonding • H-bonds in glycerol • Stacking in o-terphenyl • PMA does not hydrogen bond, or at least not as easily – Memory plots are very different than those of PVOH
  14. 14. Domain stress • The free volume of the polymers is much smaller than that of the Van der Waals radii of the probes • Free volume expansion by introduction of probe may increase side chain interactions, such as hydrogen bonds • This may be a cause for SWPs
  15. 15. Differences between domains • PVOH – Simple structure – Can form hydrogen bonds between itself as well as the probe • KAP – Crystal – Domain is constant – Oxygen impermeable • PMA – Longer side chain – Has a relatively high chain mobility
  16. 16. Reduced temperature scale • Reduced temperature scale is relative to a polymers Tg • By looking at T/ Tg, we are able to directly compare chain mobilities, even though the absolute temperatures may be different
  17. 17. Comparing domains in the reduced scale • The results of VR/PMA (23˚C) compare to the VR/PVOH (85˚C) – T/ Tg is ~1.04 • The overall shape of the PL plots are the same – Both exhibit roll offs at the ends • However, one big difference is the density of points is much greater in
  18. 18. What do we get out of this? • So, if PL distributions are similar, but memory is different, what does that tell us? • The processes that are responsible for the population and depopulation of the dark state are consistent between the two domains, but in PMA, the domain is able to exchange much more quickly than PVOH, resulting in a
  19. 19. Where do we go? • PVOH and PMA share a complication. – It is difficult to properly span the Tg in both polymers. – PMA has a relatively low Tg (~9˚C) – PVOH has a relatively high Tg (~72˚C) • Use a polymer that has a Tg that can be easily covered – Poly(isobutyl methacrylate) (PiBMA)
  20. 20. Why PiBMA? • PiBMA has a mid-range Tg (~55˚C) • The structure is similar to PMA • PiBMA has been used in previous EO/ SHG studies (Dhinojwala, 1993) • PiBMA is a fairly common polymer
  21. 21. Change the probe • Size • Shape • Functional groups • DCM is a common laser dye, which has also been used in previous studies by our group
  22. 22. DCM • Dye content is 98%, as opposed to VR which has a dye content of 60% • Hydrogen bonds formed with DCM may be weaker than those formed with VR
  23. 23. Europium • Europium (Eu) is fluorescent • Adding ligands increases size • Does the size of the Eu complex affect domain exchanges? • Will an increase in stress on the polymer result in slower exchanges?
  24. 24. What have we done? • First Act – Violamine R (VR) in poly(vinyl alcohol) (PVOH) • Second Act – VR in potassium acid phthalate (KAP) • Third Act – VR in poly(methyl acrylate) (PMA)
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