Chemistry 367L/392N Macromolecular Chemistry Lecture 10

Chemistry 367L/392N Midterm Exam Where: Where: Right here….in the lecture room WEL When: When: Next Tuesday 2/24/09 at 3:30 – 5PM What: What: Covers lectures through Thursday 2/18 Bring: Bring: Pencil, eraser, Calculator only…closed book! Do: Do: Study lecture notes, homework, reading assignments and graduate presentations Do not: Do not: Memorize the free radical kinetics equations…but do know the principles! Go over homework problems. Please: Please: Do a good job!

Chemistry 367L/392N Measuring Molecular Weight Alfredo Membrane Osmometry Alfredo Linda Vapor Phase Osmometry Linda GW Viscometry GW Gel Permeation Chromatography –Size exclusion Chromatography Light Scattering MALDI Others You –End group analysis You GW GW

Setting the Standard for GPC. Grubisic, Rempp & Benoit, JPS Pt. B, 5, 753 (1967 )

Setting the Standard for GPC.

Chemistry 367L/392N Light scattering 4 Scattering (Mie, Rayleigh) Absorption Emission (Fluorescence) Incident Beams Particles The interaction of light with a particle in terms of scattering, absorption, and emission.

Chemistry 367L/392N Rayleigh Scatter Molecules and very small particles do not absorb, but scatter light in the visible region (same freq as excitation) Rayleigh scattering Rayleigh scattering is directly proportional to the electric dipole and inversely proportional to the 4 th 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)

Chemistry 367L/392N Building a detector In the lab…

Chemistry 367L/392N Basic light scattering principles 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.

Chemistry 367L/392N Rayleigh Scattering ● )cos1(    r aI I o IoIo IθIθ r r = distance from particle a = polarizability λ = wavelength particles much smaller than the wavelength of the light

Chemistry 367L/392N 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 R θ = Rayleigh ratio A 2 = 2nd Virial coefficient P(θ) = Shape (or form) factor Static Light Scattering Rayleigh Equation KC)(   P cA MR       

Chemistry 367L/392N I A = lntensity of analyte (sample I – solvent I) n o = Solvent RI I T = Intensity of standard (toluene) n T = Standard (toluene) RI R T = Rayleigh ratio of standard (toluene)KC)(   P cA MR               dc dn n N K A  λ o = laser wavelength N A = Avogadros number n o = Solvent RI dn/dc = differential RI increment        2 sin   g Rn P Rg = Radius of gyration θ = Measurement angle T TT A R nInIR  

Chemistry 367L/392N Light Scattering Therefore a “Debye plot” of KC/R θ versus c should give a straight line whose intercept at zero concentration will be 1/M and whose slope will be A 2 !KC)(   P cA MR        For Rayleigh scatterers, P(θ) = 1 and the equation is simplified toKC2 1 2  cA MR  (y = b + mx)

Chemistry 367L/392N Molecular Weight Example

Chemistry 367L/392N Molecular Wt Example

Chemistry 367L/392N Light Scattering Experiment Measure I/I 0 = f(θ)

Chemistry 367L/392N For SEC This concept has been embodied in detectors This concept has been embodied in detectors These measurements give an absolute value for Mw 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 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 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. A “significant” difference in the refractive index of the sample and the solvent is required for good signal.

Chemistry 367L/392N Study Hard