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Macromolecular Chemistry

Macromolecular Chemistry. Lecture 10 . Midterm Exam. Where: Right here….in the lecture room WEL 3.502 When: Next Tuesday 2/24/09 at 3:30 – 5PM What: Covers lectures through Thursday 2/18 Bring: Pencil, eraser, Calculator only…closed book!

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Macromolecular Chemistry

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  1. Macromolecular Chemistry Lecture 10

  2. Midterm Exam • Where: Right here….in the lecture room WEL 3.502 • When: Next Tuesday 2/24/09 at 3:30 – 5PM • What: Covers lectures through Thursday 2/18 • Bring: Pencil, eraser, Calculator only…closed book! • Do: Study lecture notes, homework, reading assignments and graduate presentations • Do not: Memorize the free radical kinetics equations…but do know the principles! Go over homework problems. • Please: Do a good job!

  3. Measuring Molecular Weight • Membrane Osmometry  Alfredo • Vapor Phase Osmometry  Linda • Viscometry  GW • Gel Permeation Chromatography • Size exclusion Chromatography • Light Scattering • MALDI • Others • End group analysis You  GW  GW  GW

  4. Grubisic, Rempp & Benoit, JPS Pt. B, 5, 753 (1967)

  5. Scattering (Mie, Rayleigh) Absorption Incident Beams Emission (Fluorescence) Light scattering Particles The interaction of light with a particle in terms of scattering, absorption, and emission. 4

  6. Rayleigh Scatter • Molecules and very small particles do not absorb, but scatter light in the visible region (same freq as excitation) • Rayleigh scattering is directly proportional to the electric dipole and inversely proportional to the 4th power of the wavelength of the incident light The sky looks blue because the gas molecules scatter more light at shorter (blue) rather than longer wavelengths (red)

  7. Building a detector In the lab…

  8. Principle 1 The amount of light scattered is directly proportional to the product of the polymer molar mass and concentration. Principle 2 The angular variation of the scattered light is directly related to the size of the molecule. Basic light scattering principles

  9. p I 8 a 4 2 = + ( 1 cos ) 2 q l I r 4 2 o Rayleigh Scattering particles much smaller than the wavelength of the light Iθ r Io ● r = distance from particle a = polarizability λ = wavelength

  10. KC æ ö 1 1 = + 2 A c ç ÷ è ø R M P ( ) 2 q q Static Light Scattering Some nasty math later……Raleigh gets the average scattering intensity as a function of the (particle) molecular weight and the 2nd virial coefficient. K = Optical constant C = Concentration M = Molecular weight Rθ = Rayleigh ratio A2 = 2nd Virial coefficient P(θ) = Shape (or form) factor Rayleigh Equation

  11. p q 16 n R æ ö 2 2 2 dn p 2 æ ö 2 q = + 2 P 1 sin ç ÷ 2 0 g = K n ç ÷ l KC 2 è ø 3 2 dc æ ö 1 1 l 4 è ø N 0 = + 0 2 A c ç ÷ 0 A è ø R M P ( ) 2 q q I n 2 = R R A 0 q I n T 2 T T λo = laser wavelength NA = Avogadros number no = Solvent RI dn/dc = differential RI increment Rg = Radius of gyration θ = Measurement angle IA = lntensity of analyte (sample I – solvent I) no = Solvent RI IT = Intensity of standard (toluene) nT = Standard (toluene) RI RT = Rayleigh ratio of standard (toluene)

  12. KC 1 KC æ ö 1 1 = + 2 A c = + 2 A c ç ÷ R M 2 è ø R M P ( ) 2 q q q Light Scattering For Rayleigh scatterers, P(θ) = 1 and the equation is simplified to (y = b + mx) Therefore a “Debye plot” of KC/Rθ versus c should give a straight line whose intercept at zero concentration will be 1/M and whose slope will be A2!

  13. Molecular Weight Example

  14. Molecular Wt Example

  15. Light Scattering Experiment Measure I/I0 = f(θ)

  16. For SEC • This concept has been embodied in detectors • These measurements give an absolute value for Mw • Remember that there are some assumptions made that limit the utility of the “simple” detectors to Molecular weights below abut 150,000 Daltons • dn/dc can be measured with an RI detector • A “significant” difference in the refractive index of the sample and the solvent is required for good signal.

  17. Study Hard

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