  Liquid argon is commonly used as a detector medium for neutrino physics and
dark matter experiments in part due to its copious scintillation light
production in response to its excitation and ionization by charged particle
interactions. As argon scintillation appears in the vacuum ultraviolet (VUV)
regime and is difficult to detect, wavelength-shifting materials are typically
used to convert VUV light to visible wavelengths more easily detectable by
conventional means. In this work, we examine the wavelength-shifting and
optical properties of poly(ethylene naphthalate) (PEN), a recently proposed
alternative to tetraphenyl butadiene (TPB), the most widely-used
wavelength-shifter in argon-based experiments. In a custom cryostat system with
well-demonstrated geometric and response stability, we use 128~nm argon
scintillation light to examine various PEN-including reflective samples'
light-producing capabilities, and study the stability of PEN when immersed in
liquid argon. The best-performing PEN-including test reflector was found to
produce 34% as much visible light as a TPB-including reference sample, with
widely varying levels of light production between different PEN-including test
reflectors. Plausible origins for these variations, including differences in
optical properties and molecular orientation, are then identified using
additional measurements. Unlike TPB-coated samples, PEN-coated samples did not
produce long-timescale light collection increases associated with solvation or
suspension of wavelength-shifting material in bulk liquid argon.
