  We present the analysis of the Polycyclic Aromatic Hydrocarbon (PAH) spectra
of a sample of 92 typical star forming galaxies at 0.03 < z < 0.2 observed with
the Spitzer IRS. We compare the relative strengths of PAH emission features
with SDSS optical diagnostics to probe the relationship between PAH grain
properties and star formation and AGN activity. Short-to-long wavelength PAH
ratios, and in particular the 7.7-to-11.3 micron feature ratio, are strongly
correlated with the star formation diagnostics D_n(4000) and H-alpha equivalent
width, increasing with younger stellar populations. This ratio also shows a
significant difference between active and non-active galaxies, with the active
galaxies exhibiting weaker 7.7 micron emission. A hard radiation field as
measured by [OIII]/H-beta and [NeIII]_15.6/[NeII]_12.8 affects PAH ratios
differently depending on whether this field results from starburst activity or
an AGN. Our results are consistent with a picture in which larger PAH molecules
grow more efficiently in richer media and in which smaller PAH molecules are
preferentially destroyed by AGN.
