  Ductility quantifies a material's capacity for plastic deformation, and it is
a key property for preventing fracture driven failure in engineering parts.
While some brittle materials exhibit improved ductility at small scales, the
processes underlying this phenomenon are not well understood. This work
establishes a mechanism for the origin of ductility via an investigation of
size-affected fracture processes and polymer degree of conversion (DC) in
two-photon lithography (TPL) fabricated materials. Microscale single edge notch
bend ($\mu$SENB) specimens were written with widths from 8 to 26 $\mu$m and
with different laser powers and post-write thermal annealing to control the DC
between 17\% and 80\%. We find that shifting from low to high DC predictably
causes a $\sim$3x and $\sim$4x increase in strength and bending stiffness,
respectively, but that there is a corresponding $\sim$6x decrease in fracture
energy from 180 $J/m^2$ to 30 $J/m^2$. Notably, this reduced fracture energy is
accompanied by a ductile-to-brittle transition (DBT) in the failure behavior.
Using finite element analysis, we demonstrate that the DBT occurs when the
fracture yielding zone size ($r_p$) approaches the sample width, corresponding
with a known fracture size-affected transition from flaw-based to
strength-based failure. This finding provides a crucial insight that ductility
is a size-induced property that occurs when features are reduced below a
characteristic fracture length scale and that strength, stiffness, and
toughness alone are insufficient predictors of ductility.
