Re: A PVA for printing "gum" from Mike Ware

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@sask.usask.ca>; <kthayer@pacifier.com>
Sent: Friday, March 31, 2006 6:50 PM
Subject: Re: A PVA for printing "gum" from Mike Ware

> I didn't read Bolte's 2005 paper, but in my understanding of her work
> from a couple of years ago, she is recently interested in the behavior
> of completely *dried,* dichromated *pure* PVA *film* with no
> postexposure rinsing. If you read her papers carefully, you'll know
> that moisture content of dichromated PVA or introduction of a few
> percents of carboxyl groups (for example, by oxidation) can change the
> results dramatically.
>
> There is no good way to make a usable hologram from dichromated
> gelatin without rinsing. Gelatin also contains significant fraction of
> carboxyl groups. So in dichromated gelatin, Cr(V) is not found after
> the exposure is turned off and the material is rinsed.

It depends. You're right that real-time efficiency of DCG is usually low
compared to PVA, DCPAA etc. In the case of gelatin heat treatment doesn't
enhance diffraction efficiency either. However, there are some methods to
produce reasonably high efficiciency without wet processing. E.g. there's
so-called "dark self-enhancement" and a Russian research group showed DCG
could be formed in thick semi-wet layers (mixing gelatin with glycerol). In
that case fairly good holograms could be recorded in real-time.

> In the case of dichromated PVA *solution,* Cr(V) is rapidly dropped
> after the exposure is turned off. This indicates that the presence of
> moisture changes the reaction and Cr(V) becomes unstable; it's
> converted to Cr(III). However, the measurement methods used by Bolte's
> lab is not very efficient in detecting Cr(III) because it gives very
> weak UV-vis spectroscopic signal at the amount sufficient to crosslink
> the polymer, so absence of Cr(III) discussion/data in their paper does
> not mean absence of Cr(III). Cr(III) can be measured more effectively
> by low frequency dielectric loss, for example.

In another paper (Lafond/Pizzocaro/Lessard/Bolte, Primary photochemical
process in films of dichromated gelatin: a quantitative spproach, Opt. Eng.
39, March 2000) the authors showed Cr(V) to be quite stable both in
dichromated gelatin and dichromated PVA. One of the most interesting aspects
(at least for holographers) of Cr(V) consists in its strong absorption band
around 520nm. That suggests it might be possible to "activate" a dichromated
colloid (gelatin, PVA, possibly gum) with a weak overall UV exposure, which
willl turn the Cr(VI) into Cr(V). Then the image exposure is carried out at
a visible (non-UV) wavelength, preferably around 520nm.

Martin
Received on Sat Apr 1 07:55:39 2006