LIGHT SCATTERING
(Hiementz- 659)
This is an excellent reference, especially with regard to the
Zimm Plot
calculation
The Hiementz reference is math-heavy, with 10 different equations for
turbidity, the last of which is shown below (Hiementz-685):
is refractive index (Hiementz-669)
is concentration. Hiementz-713 lists a problem
where the units of concentration are grams per centimeter cubed.
Until I read otherwise I'm
going to assume the 2 is used to denote that it is the concentration of
the polymer (similar to what we saw with
volume fractions where subscript 1 stood for solvent, and subscript 2
stood for polymer.)
-
M is molecular weight.
-
has the units molecules per cubic
centimeter.
-
B is the second virial coefficient. (See Flory-299,
eq. 37 and Flory 649.)
Do this simple light scattering experiment:
Try to align your thumb and your index finger as parallel as possible, and
spaced about 1 mm apart. Balance the hand on your nose and your forehead
so that your finger and thumb are close to your eye. Lift your head so
that an overhead fluorescent light shines through the slit you are forming.
Bring your finger and thumb together slowly. As the slit width approaches
the dimensions of the wavelength of light, there is an interference pattern
blocking the transmission of light as your finger and thumb are about to touch.
The light previously transmitted through to your eye is scattered.
Disclaimer: I can't say at this time that it is complete accurate to call
the above a scattering experiment. Look up "Fraunhofer diffraction pattern
of a single slit" if you are interested in the physics of this simple
experiment. University physics illustrates Fresnel diffraction off of
a razer when a point source of light was used. The point source is used
to magnify the effect (other light sources might shine light in the areas
of darkness. More on these "waves": your finger has a sharp "edge" when
it is far from your eye with respect to the overhead light, but when
your finger is real close to your eye, then the "waves" appear.
around the edges.
Full picture available-
University of Southern Mississippi Polymer Science Department
photo
Notice the tubes that circulate water to maintain the temperature control of
the intstrument.
Light scattering is a technique for determining molecular weight that does
not require calibration (as is the case for viscometric determinations which
make use of the
Mark Houwink Equation.
Light scattering increases with increasing particle size (for a series
of solutions where particle concentration is held constant.)
The data from a light scattering experiment is used to construct a
Zimm Plot.
Last Update- December 29, 1994- wld