[Corribus]

I need to make some general comparisons of hardness (particularly scratch hardness) or brittleness (tendency to fracture) of various organic polymers. In particular at the moment I'm interested in Nylon 6 and polypropylene, but a table that compares the mechanical properties of numerous common organic polymers would be useful. I know hardness and brittleness is dependent on a number of factors but I'm interested in sort of general ranges so that I can say that, in general, Nylon 6 is a harder or more brittle material (e.g.) than polypropylene. Does anyone know where I could find such a table?

[Enthalpy]

The best source is in German, so in case you don't speak it, just learn it 
Kunststofftabellen, from Bodo Carlowitz (some 90€, whoop)
it contains many polymers, including variants charged with powder, fibres... and the copolymers, the molecular weight and so on. Among the data (like 6 pages for every polymer variant), you have the fracture toughness.

Then, the eternal problem of toughness is that no data is useable in a computation, no decent theory exists, and as a consequence, there are a dozen of different figures which can't be converted in an other. You can only make comparisons, and in the best case have two different measures for one polymer which helps you calibrate the comparisons among different units. Bad luck.

For a limited set of polymers, I'd suggest to go to the websites of manufacturers or sellers. They use to give toughness figures. Beware it depends on everything: details of production and shaping (chemists often deliver granules, injection is made elsewhere), temperature, charges... Where you see units like MPa*m^0.5 it's there.
http://www.matbase.com/
http://www.matweb.com/
but manufacturers are more reliable.

Be especially wary of polyamides, polyolefines, polyvinyls, polystyrenes, whose toughness varies a lot.

[Corribus]

Thanks Enthalpy. I found another site that offers comparisons, but it seems like too general a comparison to make. Too many variables involved. I will have to just make my conclusion more generally.