Ammonium vs potassium dichromate -- Bolte

Hi Loris,
Here's the information you asked for (sorry, I've lost the initial
post, so I'm doing this by memory):

1. How is it determined that holograms made with ammonium dichromate
are not as stable as those made with potassium dichromate.

I don't know the basis for the statement that it has been "long
established that holograms formed from ammonium dichromate were of
better quality than holograms formed from potassium dichromate"
since I'm not working in the holographic field, but if it's a well-
known fact, it seems like it must be pretty self-evident. Perhaps
Martin could elucidate that for us.

In this study, the measure was the percentage of the irradiated film
that was insoluble after being stirred in water for 24 hours, which
one would assume would serve as a reasonable measure of sturdiness of
the matrix; up to the point of maximum insolubility, as the
percentage of the film that is insoluble increases, the more stable
the image will be, as Chris pointed out the other day.

They also looked at the amount of insoluble material formed
throughout irradiation of films including ammonium dichromate and
potassium dichromate (calculated to contain the same ratio of Cr(VI)
to PVA); I commend to you the graph (figure 10) in this article
comparing the insoluble fraction of the film to the irradiation time,
for each of the dichromates. The bottom line: "With ADC a maximum
value of 87% was reached after 20 minutes, while with KDC a value of
68% was obtained after 30 minutes. "

2. I don't think you asked this question, but I was curious myself
how they came to the conclusion that the reason for the greater
insolubility of the ammonium dichromate matrix involves amide groups
as additional chelating agents:

"The structural modification of the polymer matrix in ADC (ammonium
dichromate) was monitored... The ammonium cations observed before
irradiation were very rapidly consumed during the first step of the
(chromium) reduction. The most probable reaction could be the one
involving them and ester groups leading to amide functions -CO-NH2.
Accordingly, we looked at the eventual formation of amide groups
characterized by two vibration bands which nicely matched the
experimental observation of bands at 1658 and 1620 cm-1. The
formation of such bands were only observed with ADC doped PVA. There
was an additional confirmation of the assumption through the linear
correlation between the formation of the amide and the consummation
of NH4-.

3. You asked about my own experience with potassium vs ammonium
dichromate. My own experience wouldn't be relevant to the question,
because I've not compared the two with weights calculated to result
in films containing exactly the same amount of Cr(VI) to PVA. I've
only compared saturated ammonium dichromate to saturated potassium
dichromate, and of course in that comparison, the ammonium will
produce a stronger matrix. But like I say, not a fair comparison to
test the question of whether the cation itself is related to
sturdiness of layer.

I don't remember what else you asked, but if there's something else
you want to know, ask again. But I would recommend reading the
article for yourself.
Katharine
Received on Sat Apr 1 11:15:59 2006