Procedure by William Schreiter, Richard Amankwah and Karen
Nordell, Chemistry Department, Lawrence University.
Evaporation of a dilute suspension of polymer nanospheres
creates a close packed layer on the surface of a glass slide suspended in the
solution. The particle size can be calculated from the visible absorption maximum
of this colloidal crystal.
Procedure
Wear eye protection
Dilute 50 microliters of previously prepared PMMA nanospheres in
20 mL distilled water. (Synthesis)
Using a diamond pencil, score and break lengthwise a large glass
cover slip (20 mm x 40 mm).
Gently attach one of the glass pieces to a toothpick using tape.
Clean the glass by soaking in isopropanol for 10 minutes followed
by soaking in pure water for 5 minutes.
Suspend the glass slide into the diluted PMMA nanosphere suspension
you prepared earlier. If desired, more than one glass slide can be suspended
in each solution as long as they do not touch each other.
To promote the growth of high quality close packed layers on the glass
slide, evaporate the solution in a controlled temperature environment
such as an oven. At 50 degree C, the solution will evaporate in about
36 hours.
If an oven is not available, the solutions can be placed in a room
temperature location where they will be undisturbed such as a window sill.
Under these conditions, evaporation takes between 5 and 7 days.
After the solution has evaporated, a dry film of the close-packed
nanospheres remains on the glass slide.
Observe the color of the film under reflected light conditions by holding
the slide under a bright light source. Observe the color of light transmitted
through the film by holding the glass slide up and looking at a bright
light source through the slide. How are the colors of reflected and transmitted
light related?
Obtain the visible absorption or transmission spectrum when the glass
slide is perpendicular to the beam.
Use a modified Bragg equation to calculate
the size of the nanoparticles:
λp = 1.633 Ds [ns2
f + nv2 (1-f)]1/2
λp = the wavelength at maximum absorbance (in nm)
Ds = the diameter of the close packed spheres (in nm)
ns = the index of refraction for the spheres
nv = the index of refraction for the void spaces (presumably air
in a dry film)
f = the filling fraction for a close packed structure (0.76)
Materials
PMMA nanospheres
Microscope cover slips
Diamond tip pencil
Visible spectrometer
