  Surface stress and surface energy are fundamental quantities which
characterize the interface between two materials. Although these quantities are
identical for interfaces involving only fluids, the Shuttleworth effect
demonstrates that this is not the case for most interfaces involving solids,
since their surface energies change with strain. Crystalline materials are
known to have strain dependent surface energies, but in amorphous materials,
such as polymeric glasses and elastomers, the strain dependence is debated due
to a dearth of direct measurements. Here, we utilize contact angle measurements
on strained glassy and elastomeric solids to address this matter. We show
conclusively that interfaces involving polymeric glasses exhibit strain
dependent surface energies, and give strong evidence for the absence of such a
dependence for incompressible elastomers. The results provide fundamental
insight into our understanding of the interfaces of amorphous solids and their
interaction with contacting liquids.
