The mechanical, structural, and morphological properties of polystyrene
under fatigue combined with accelerated aging were investigated by
mechanical fatigue, gel permeation chromatography (GPC), gel percentage,
thermogravimetric analysis (TGA), differential scanning calorimetry
(DSC), and scanning electron microscopy (SEM).
The mechanical fatigue behavior was evaluated under load control
(Instron universal testing machine 8874) following the ASTM-D3479
[13] standard. The experimental conditions used included a load rate
of LR = 0.5, frequency of 5 Hz at 25°C, and 50% relative humidity. For
each group, five samples of type 1 ASTM-D638 [14] were tested.
The microhardness test was carried out using a Shimadzu Microdurometer
with a pyramidal penetrator containing a square diamond base with an
angle of 136° in each of the specimens to assess the surface hardness,
according to the ASTM E384-11 standard testing method [15]. A load
of 0.05 kgf was used for 30 s.
GPC was used to determine the weight-average molecular weight (Mw) and
polydispersity (Mw/Mn) of the samples, before and after UV irradiation
on the investigated times. These GPC tests were performed on a Shimadzu
chromatography system (RID 20A) at 25°C using PA-grade chloroform as
both the solvent and mobile phase, at a flow rate of 1 mL/min. Mass
standards of monodisperse polystyrene were used to calibrate the GPC
system.
The percentage of gel produced by the radiation action, before and after
exposure to UV-B, was determined by extracting the non-crosslinked
fraction in a Soxhlet extractor following the ASTM D-2765 standard
testing method [16]. Decahydronaphthalene P.A. was used as a solvent
for 6 h under reflux (189–192 °C) at a rate of 40 drops per min. The
insoluble fraction was dried at 110 °C for 12 h until the mass remained
constant. The degree of crosslinking was estimated using Equation 3,