even this paper can help:

Carrillo F, Gupta S, Balooch M, Marshall S J, Marshall G W, Pruitt L and
Puttlitz C M "Nanoindentation of polydimethylsiloxane elastomers: effect of
crosslinking, work of adhesion, and fluid environment on elastic modulus"
J. Mater. Res. 20.10: 2820-2830 (2005)

A 3:1 or 5:1 should do for you.
Otherwise try h-pdms.

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>From      : mems-talk-bounces@memsnet.org
To          : "General MEMS discussion" mems-talk@memsnet.org
Cc          :
Date      : Sat, 28 Jun 2008 12:04:59 -0400
Subject : [mems-talk] pdms base/curing agent ration

> Hi,
>
>   I use the 10 to 1 base to curing agent ration for PDMS. I would like
> to make the PDMS more rigid.  Would adding more curing agent make it
> more rigid?