Additive manufacturing
or, as also called, three-dimensional (3D)
printing is considered as a game-changer in replacing traditional
processing methods in numerous applications; yet, it has one intrinsic
potential weakness related to bonding of layers formed during the
printing process. Prior to finding solutions for improvement, a thorough
quantitative understanding of the mechanical properties of the interface
is needed. Here, a quantitative analysis of the nanomechanical properties
in 3D printed photopolymers formed by digital light processing (DLP)
stereolithography (SLA) is shown. Mapping of the contact Young’s
modulus across the layered structure is performed by atomic force
microscopy (AFM) with a submicrometer resolution. The peakforce quantitative
nanomechanical mapping (PF-QNM) mode was employed in the AFM experiments.
The layered specimens were obtained from an acrylate-based resin (PR48,
Autodesk), containing also a light-absorbing dye. We observed local
depressions with values up to 30% of the maximum stiffness at the
interface between the consecutively deposited layers, indicating local
depletion of molecular cross-link density. The thickness values of
the interfacial layers were approximately 11 μm, which corresponds
to ∼22% of the total layer thickness (50 μm). We attribute
this to heterogeneities of the photopolymerization reaction, related
to (1) atmospheric oxygen inhibition and (2) molecular diffusion across
the interface. Additionally, a pronounced stiffness decay was observed
across each individual layer with a skewed profile. This behavior
was rationalized by a spatial variation of the polymer cross-link
density related to the variations of light absorption within the layers.
This is caused by the presence of light absorbers in the printed material,
resulting in a spatial decay of light intensity during photopolymerization.
A critical complication in handling nanoparticles is the formation of large aggregates when particles are dried e.g. when they need to be transferred from one liquid to another. The particles in these aggregates need to disperse into the destined liquid medium, which has been proven difficult due to the relatively large interfacial interaction forces between nanoparticles. We present a simple method to capture, move and release nanoparticles without the formation of large aggregates. To do so, we employ the co-non-solvency effect of poly(N-isopropylacrylamide) (PNIPAM) brushes in water-ethanol mixtures. In pure water or ethanol, the densely end-anchored macromolecules in the PNIPAM brush stretch and absorb the solvent. We show that under these conditions, the adherence between the PNIPAM brush and a silicon oxide, gold, polystyrene or poly(methyl methacrylate) colloid attached to an atomic force microscopy cantilever is low. In contrast, when the PNIPAM brushes are in a collapsed state in a 30-70 vol% ethanol-water mixture, the adhesion between the brush and the different counter surfaces is high. For potential application, we demonstrate that this difference in adhesion can be utilized to pick up, move and release 900 silicon oxide nanoparticles of diameter 80 nm using only 10 × 10 μm PNIPAM brush.
Marine plastic pollution
is a worldwide challenge making advances
in the field of biodegradable polymer materials necessary. Polylactide
(PLA) is a promising biodegradable polymer used in various applications;
however, it has a very slow seawater degradability. Herein, we present
the first library of PLA derivatives with incorporated “breaking
points” to vary the speed of degradation in artificial seawater
from years to weeks. Inspired by the fast hydrolysis of ribonucleic
acid (RNA) by intramolecular transesterification, we installed phosphoester
breaking points with similar hydroxyethoxy side groups into the PLA
backbone to accelerate chain scission. Sequence-controlled anionic
ring-opening copolymerization of lactide and a cyclic phosphate allowed
PLA to be prepared with controlled distances of the breaking points
along the backbone. This general concept could be translated to other
slowly degrading polymers and thereby be able to prevent additional
marine pollution in the future.
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