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Lecture8 Notes

# Lecture8 Notes - Lecture 8 Solution Characterization of...

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Lecture 8 Solution Characterization of Polymers 2 ANNOUNCEMENT: Review Session will be Tuesday during normal lecture time. Practice exam is posted; equation sheet coming soon Exam will cover lectures 1-9 1 Monday, October 18, 2010

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Characterizing Macromolecules with EM 2 Monday, October 18, 2010
Today’s Topic: Light Scattering Static Light Scattering Dynamic Light Scattering SANS SAXS 3 Monday, October 18, 2010

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Interactions between light and matter Reflection specular reflection: when a light beam is reflection in a very definite direction by a flat surface such as a mirror diffuse reflection: when light is reflected in various directions from an irregular surface such as a piece of white paper Refraction: the light ray is deviated or bent from its original direction Diffraction: the light ray is deviated slight from its original path upon interaction with matter; may also be considered a spatial redistribution of waves Absorption: a process that removes energy from the incident light beam by converting it to another form 4 Monday, October 18, 2010
Light Scattering Interactions between light and matter may produce different effects, depending on the wavelength of light and the characteristics of the material. Light scattering is the alteration of the direction and intensity of a light beam that strikes an object. This is due to the combined effects of reflection, refraction and diffraction Scattering occurs when the electron cloud of the scatterer removes energy from the light beam (photon by photon) and then re-emits this energy (photon by photon) without otherwise altering it 5 Scattering = Reflection + Refraction + Diffraction Monday, October 18, 2010

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What’s so great about light scattering? non-invasive and non-destructive small sample volumes size range: 2 nm to 10 um! M w 1K to 10 million! Wide range of acceptable concentrations can detect aggregation, can be used for complex samples Static Light Scattering: can get M w , R g , and A 2 looks at the intensity of scattered light at different concentrations and different angles Dynamic Light Scattering (also referred to as Quasi-Elastic Light Scattering, or QELS): can get R h and D looks at fluctuations in scattered light intensity due to Brownian motion at a single angle 6 Monday, October 18, 2010
Light Scattering Light is an electromagnetic field The electric field of light drives oscillatory motions of the electrons in a molecule (induced dipole) The magnitude of the dipole moment is: p = α E = α E 0 cos 2 π c s t λ E = E 0 cos ω t 7 Monday, October 18, 2010

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Light scattering 8 Monday, October 18, 2010
Light Scattering The oscillating dipole scatters the light The intensity of the scattered light is given by: I ' = I 0 α 2 8 π 4 r 2 λ 4 1 + cos 2 θ ( ) 9 Monday, October 18, 2010

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Light Scattering We can normalize this scattering to the number of molecules per unit volume: I ' = I 0 α 2 8 π 4 r 2 λ 4 1 + cos 2 θ ( ) α = 1 2 π dn dc M N Av Number of molecules per unit volume
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Lecture8 Notes - Lecture 8 Solution Characterization of...

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