1 Assignment 4: November 2 2009 Due date: November 13 2009 ...

Assignment 4:
November 2 nd 2009
Due date: November 13 th 2009
Problems:
1. Shown in Figure 1 is a portion of the H 2 O-NaCl phase diagram:
Figure 1
a) Using this diagram, briefly explain how spreading salt on ice that is on a temperature
blow 0º C can cause the ice to melt.
b) What concentration of salt is necessary to have 50% ice – 50% liquid brine at - 10 ºC?
2. Consider a specimen of ice that is at – 10 º C and 1 atm pressure. Using Figure 2 , the
pressure – temperature phase diagram for H2O, determine the pressure to which the
specimen must be raised or lowered to cause it
a) to melt and b) to sublime.
Figure 2
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3.We were discussing the distribution function of the end-to-end distances of a polymer
and have seen an example calculated for a chain made of 2000 C-C bonds in an ideal =
good solvent (Tanford page 175).
How will this distribution curve change by choosing a bad solvent?
Describe it in words or draw a scheme.
4. Imagine you have a macroscopic semi crystalline polymer sample. It has the form of a
right angled box with dimensions 5 mm x 10 mm x 1 mm.
The molecules are oriented or aligned parallel to the longest axis in the sample, because it
had been drawn apart along that direction.
You place this sample into a polarization microscope with the (10 mm x 5 mm)-face
perpendicular to the optical path. What do you see? Which refractive index components
are you probing here?
Then you turn the sample by 90 degree so that the (10 mm x 5 mm)-face is perpendicular
to the optical path? Any changes in the image? What refractive indices are probed now?
Turn it again, so that the (1 mm x 5 mm)-face is perpendicular to the optical path. (Let's
assume the microscope has enough space to do so and the sample does not fall over.)
What refractive indices are probed now?
Help: Draw a picture of the sample and the alignment direction of the polymer chains and
define the refractive indices as well as directions in there!
5. Does Tg increase or decrease if the characteristic ratio increases?
6. Consider you have to design a band aid from a polymeric material that is supposed to
be a drug delivery system for a fever drug. (Something like the "NIKOTINEL" - anti
smoke band aids, which supplies nicotine to people how want to stop smoking or are not
allowed to smoke, e.g. in a plane, but need nicotine.)
What kind of polymer would you chose? Do not name a chemical formula; describe the
physical characteristics of that polymeric material with the attributes you know so far.
7. The Brewster angle (tan α Brewster = n(dielectric material)) is the angle where light with
p-polarization is not back reflected into the air from a surface made out of a dielectric
material. (Polarizers are used by photographers to diminish reflections on taking
photographs from fish in an aquarium.)
Imagine you have an unperturbed water surface (no waves). Water is a dielectric
material, so you can point an expanded laser beam to that surface exactly under the
Brewster angle in p-polarization. In addition you also have a screen which images the
reflected light.
a) What do you see on the screen if you have a clean water surface at the Brewster angle
in p-polarization? What happens if you change to s-plarization?
b) Imagine you put little tiny droplets of oil on the water surface. What do you see now
under p-polarization? What happens now if you change the angle of the incoming laser
beam?
c) Imagine you have two molecules, which are immiscible with each other and which are
both amphiphilic. Amphiphilic means that a molecule has a part which is hydrophilic
(that part likes water and dissolves in it) and has a second part which is hydrophobic
(does not like water and does not dissolve in it).
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You mix both molecules and dissolve the mixture with a very low concentration in a
solvent which evaporates very quickly - say chloroform. This solution you drop very
carefully onto your clean water surface in such an amount that the entire water surface
will be covered by theses molecules in a monolayer. This process is called spreading.
You give the system time to let the solvent evaporate. The film you formed is called a
Langmuir-Blodgett film.
What do you see now on your screen at the Brewster angle of clean water? And what
happens if you change the incident angel of the laser beam?
Please answer qualitatively. You do not need to calculate anything!
8. When you want to pass a ray of light from an optically thick medium (e.g. water with n
=1.33) to an optically thin medium (e.g. air with n = 1.0) your success depends on the
angle of incidence onto that optical interface water/air. If you are above the critical angle
of total internal reflection, all the light will be reflected from that surface, no beam can be
transmitted or be refracted here. On the other hand if you are above the critical angle of
total internal reflection and you look very closely to your interface now, you will find a
film of light present on the low refractive index side of your interface in a thickness
regime close to the wavelength of the used light. This is called an evanescent field. Could
you see any use of such a film of light due to a total internal reflection? Or maybe the
total internal reflection by itself can be of any use? Any idea? Of cause not necessarily at
an air/water interface! You can choose any air/dielectric interface.
I do not expect a specific answer. Just think about it and write it down.
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