  This paper proposes a physio-chemically-based constitutive framework to
simulate and predict the response of semi-crystalline low-density polyethylene
(LDPE) to severe photo-oxidation. Photo-oxidation induced by exposure to
Ultra-Violet (UV) light and oxygen is the dominant degradation mechanism
affecting the lifespan of LDPE. In this work, we propose evolution functions
for the material properties in the constitutive equations of
\cite{boyce2000constitutive} to incorporate the effects of photo-oxidation on
the mechanical response of LDPE. The evolution functions are based on
chemically verified processes that are responsible for material degradation,
namely the change in crystallinity and mass loss relative to the initial
pristine films over exposure time. Changes in crystallinity and mass loss are
characterized by Differential Scanning Calorimetry (DSC) and Quartz Crystal
Microbalance with Dissipation Monitoring (QCM-D) experiments, respectively.
Connecting the physio-chemical processes affecting polymer network evolution to
the mechanical response of LDPE bypasses the need for defining fitting
parameters that carry no physical meaning. The developed constitutive framework
is validated with respect to a series of in-house uniaxial tensile tests
performed on LDPE aged for different UV exposure times. Comparison of the
constitutive framework versus experimental mechanical tests also confirms the
accuracy of DSC and QCM-D as rigorous techniques to monitor and characterize
degradation in LDPE films. The outcome shed light on the evolution of the
macromolecular network in LDPE under extreme photo-oxidation and the evolution
of the associated mechanical material properties.
