Chemistry 367L/392N

Macromolecular ChemistryMacromolecular Chemistry

Lecture 10 Lecture 10

Chemistry 367L/392N

Midterm ExamMidterm Exam Where:Where: Right here….in the lecture room WEL 3.502 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 WeightMeasuring Molecular Weight Membrane Osmometry AlfredoAlfredo Vapor Phase Osmometry LindaLinda Viscometry GWGW Gel Permeation Chromatography

– Size exclusion Chromatography Light Scattering MALDI Others

– End group analysis YouYou

GWGW

GWGW

GWGW

Setting the Standard for GPC.

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

Setting the Standard for GPC.

Chemistry 367L/392N

Light scatteringLight scattering

4

Scattering(Mie, Rayleigh)

Absorption

Emission(Fluorescence)

Incident BeamsIncident Beams

Particles

The interaction of light with a particle in terms of scattering, absorption, and emission.

Chemistry 367L/392N

Rayleigh ScatterRayleigh Scatter Molecules and very small particles do not

absorb, but scatter light in the visible region (same freq as excitation)

Rayleigh scatteringRayleigh 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 The sky looks blue because the

gas molecules scatter more gas molecules scatter more light at shorter (blue) rather light at shorter (blue) rather than longer wavelengths (red)than longer wavelengths (red)

Chemistry 367L/392N

Building a detectorBuilding a detector

In the lab…

Chemistry 367L/392N

Basic light scattering principlesBasic light scattering principlesPrinciple 1Principle 1The amount of light scattered is directly proportional to the product of the polymer molar mass and concentration.

Principle 2Principle 2The angular variation of the scattered light is

directly related to the size of the molecule.

Chemistry 367L/392N

Rayleigh ScatteringRayleigh Scattering

●

)cos1(8 2

24

24

raI

Io

Io

Iθr

r = distance from particle a = polarizabilityλ = 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 = ConcentrationM = Molecular weight Rθ = Rayleigh ratio A2 = 2nd Virial coefficient P(θ) = Shape (or form) factor

Static Light ScatteringStatic Light Scattering

Rayleigh EquationRayleigh Equation

KCKC))((

11221122

PPccAA

MMRR

Chemistry 367L/392N

IA = lntensity of analyte (sample I – solvent I)no = Solvent RIIT = Intensity of standard (toluene)nT = Standard (toluene) RIRT = Rayleigh ratio of standard (toluene)

KCKC))((

11221122

PPccAA

MMRR

22

0044

00

2222

dcdc

dndnnn

NNKK

AA

λo = laser wavelengthNA = Avogadros numberno = Solvent RIdn/dc = differential RI increment

22sinsin

331616

11 22

22

00

2222

00

22

ggRRnn

PP

Rg = Radius of gyrationθ = Measurement angle

TT

TTTT

AA RRnnIInnIIRR 22

22

00

Chemistry 367L/392N

Light ScatteringLight Scattering

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!

KCKC))((

11221122

PPccAA

MMRR

For Rayleigh scatterers, P(θ) = 1 and the equation is simplified to

KCKC2211

22

ccAAMMRR

(y = b + mx)(y = b + mx)

Chemistry 367L/392N

Molecular Weight ExampleMolecular Weight Example

Chemistry 367L/392N

Molecular Wt ExampleMolecular Wt Example

Chemistry 367L/392N

Light Scattering ExperimentLight Scattering Experiment

MeasureI/I0 = f(θ)

Chemistry 367L/392N

For SECFor SEC This concept has been embodied in detectorsThis concept has been embodied in detectors These measurements give an absolute value for MwThese measurements give an absolute value for Mw Remember that there are some assumptions made that Remember that there are some assumptions made that

limit the utility of the “simple” detectors to Molecular limit the utility of the “simple” detectors to Molecular weights below abut 150,000 Daltonsweights below abut 150,000 Daltons

dn/dc can be measured with an RI detectordn/dc can be measured with an RI detector A “significant” difference in the refractive index of the A “significant” difference in the refractive index of the

sample and the solvent is required for good signal. sample and the solvent is required for good signal.

Chemistry 367L/392N

Study HardStudy Hard