There was a Lab on a Chip "Chips & Tips" segment on something similar
to what you need published last year:

"Chips & Tips: An inexpensive and durable epoxy mold for PDMS"
http://www.rsc.org/Publishing/Journals/lc/Chips_and_Tips/epoxy_mould.asp

- Kevin

On Sun, Oct 10, 2010 at 9:42 PM, Shao Guocheng  wrote:
> Dear all,
>
>    I have a structure that can only be fabricated with SU-8 in a positive
> manner (that is, my SU-8 structure is the same as my final structure, instead
of
> being a negative as in most SU-8 mold fabrication). But I do need to have this
> structure in PDMS. So my plan is to do regular PDMS molding using the SU-8
> structure and get a negative PDMS mold. With the negative mold and a second
PDMS
> molding process, I should have a PDMS structure same as the original SU-8 one.
> Has anyone done something like this before? How did you overcome the PDMS to
> PDMS sticking problem? I am thinking about surface silanization on the PDMS
> negative mold, however is concerning about how well the silanization can be
done
> on PDMS surface and how long it can last. So if anyone has previous experience
> with similar procedure, please give some advice about if this is doable or not
> (I do have high aspect ratio structure, 10um wide, 100um tall). Or, is there
any
> other material that I use to make a negative mold from SU-8 master so the
> following PDMS mold maybe easy on that material?
>
>    Any input is greatly appreciated and thx in advance.
>
> Guocheng Shao
>