Despite
the great potential in fabrication of biodegradable and
eco-friendly air filters by electrospinning poly(lactic acid) (PLA)
membranes, the filtering performance is frequently dwarfed by inadequate
physical sieving or electrostatic adsorption mechanisms to capture
airborne particulate matters (PMs). Here, using the parallel spinning
approach, the unique micro/nanoscale architecture was established
by conjugation of neighboring PLA nanofibers, creating bimodal fibers
in electrospun PLA membranes for the enhanced slip effect to significantly
reduce the air resistance. Moreover, the bone-like nanocrystalline
hydroxyapatite bioelectret (HABE) was exploited to enhance the dielectric
and polarization properties of electrospun PLA, accompanied by the
controlled generation of junctions induced by the microaggregation
of HABE (10–30 wt %). The incorporated HABE was supposed to
orderly align in the applied E-field and largely promote the charging
capability and surface potential, gradually increasing to 7.2 kV from
the lowest level of 2.5 kV for pure PLA. This was mainly attributed
to HABE-induced orientation of PLA backbone chains and CO
dipoles, as well as the interfacial charges trapped at the interphases
of HABE–PLA and crystalline region–amorphous PLA. Given
the multiple capturing mechanisms, the micro/nanostructured PLA/HABE
membranes were characterized by excellent and sustainable filtering
performance, e.g., the filtration efficiency of PM0.3 was
promoted from 59.38% for pure PLA to 94.38% after addition of 30 wt
% HABE at a moderate airflow capacity of 32 L/min and from 30.78 to
83.75% at the highest level of 85 L/min. It is of interest that the
pressure drop was significantly decreased, mainly arising from the
slip effect between the ultrafine nanofibers and conjugated microfibers.
The proposed combination of the nanostructured electret and the multistructuring
strategy offers the function integration of efficient filtration and
low resistance that are highly useful to pursue fully biodegradable
filters.
Vulcanized, foamed latex rubber features high density
of intercommunicated
cells and controlled elastomeric properties, rendering wide applications
from elastic sponges to rubbery coatings. This provides an emergent
incentive for properly recycling, devulcanizing and reusing the waste
latex rubber (WLR). Here, a microwave-assisted devulcanization (MAD)
approach was disclosed to cleave the cross-links without severe rupture
of backbone rubber chains, strategically involving the use of a microwave-assisted
hydrothermal reaction at 180 °C for several minutes and incorporation
of graphene nanosheets serving as the nanoabsorbers of microwave irradiation.
The MAD processing duration was examined to be a vital parameter controlling
the macromolecular characteristics: a high devulcanization degree
of 83.6% was obtained with 8 min MAD, whereas the soluble content
was undesirably increased to 76.2% after 16 min. The devulcanization
efficacy was examined by melt compounding with poly(lactic acid) (PLA),
displaying extensive interfacial interactions and thereby, creating
strong and resilient interfacial adhesion within the PLA composites.
With the addition of 10 wt % devulcanized WLR by 8 min MAD, the tensile
strength and elongation at break of PLA composites were largely promoted
to 58.2 MPa and 16.6%, increasing nearly 62% and 240% compared to
those of pure PLA, respectively. The proposed MAD approach may open
an environment-respecting and reliable pathway to sustainable recycling
of waste rubber, yielding useful components to toughen PLA without
sacrificing the strength.
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