A resin strategy lets 3D printed polymers organize internally during curing, creating ordered nanostructures that can later be locked into stronger materials.
(Nanowerk Spotlight) Light-based 3D printing builds polymer objects from a liquid resin. A pattern of light hardens selected regions of the resin into one thin layer, then the printer repeats the step until the full part forms. The printer can define the outside of the part with precision, but not the architecture hidden inside the polymer.
That internal architecture can matter as much as the outer geometry. Many polymer properties depend on tiny repeated domains that guide stress, heat, ions, molecules, or light through the material. Block copolymers can form those domains by self-assembly, but they need enough molecular motion to organize. In fast light-based printing, the printed part may have the right outer shape while its interior lacks the ordered pathways needed for advanced function.
A paper in Advanced Materials (“Additive Manufacturing of Ordered Polymer Nanostructures”) reports a resin strategy that addresses this timing problem by changing the sequence of events during curing. The method, called Polymerization-Induced Arrangement of Nanostructures with Order-tunability, or PIANO, lets polymer chains separate and organize during printing before a later heat treatment locks the structure into a stronger permanent network.
The challenge sits at the intersection of two manufacturing ideas. Additive manufacturing can create complex three-dimensional shapes. Self-assembly can create ordered features far below the printer’s resolution. Connecting printed macro-scale architecture with nanoscale order remains a central goal for hierarchical materials, as earlier work on combining 3D printing with self-assembly has also shown.
Schematic illustration of the competing kinetics and morphological outcomes in PIMS and PIANO during photopolymerization-based 3D printing. (A) Conventional polymerization-induced microphase separation (PIMS). In the presence of divinyl crosslinkers (ethylene glycol diacrylate, EGDA), the rapid formation of covalent networking kinetically arrests the developing BCP chains, which leads to “frozen” disordered morphologies that lack long-range nanoscale order. (B) Polymerization-induced arrangement of nanostructures with order-tunability (PIANO). By replacing permanent crosslinkers with ethylene glycol (EG) (small-molecule mobility mediator), PIANO decouples mobility regulation from crosslinked network formation, thereby opening a kinetic window that allows rapid evolution toward ordered lamellar (LAM), or hexagonally packed cylindrical (HEX) phases during the printing timeframe. Subsequent thermal annealing triggers esterification of the latent crosslinker (EG), locking the ordered nanostructure, and enhancing mechanical robustness. (Image: Reproduced from DOI:10.1002/adma.73395, CC BY) (click on image to enlarge)
Block copolymers offer a chemical route to that internal order. They contain different polymer segments joined in one chain. When those segments resist mixing, they can separate into repeating nanoscale patterns, including lamellae, which are layered domains, and cylinders packed in ordered arrays. These materials can create patterned internal structures without requiring each feature to be carved directly.
The obstacle in 3D printing is not simply making the polymer segments separate. Conventional polymerization-induced microphase separation can already generate nanoscale domains as polymer chains grow. The problem is that printable resins often use crosslinkers that rapidly form a permanent covalent network. That network helps each printed layer hold its shape, but it freezes the developing morphology before the domains can develop long-range order.
PIANO replaces that early covalent lock with temporary physical support. The resin contains poly(n-butyl acrylate) precursors, acrylic acid monomer, a photoinitiator, and ethylene glycol. When light drives polymerization, acrylic acid grows into poly(acrylic acid) blocks that become incompatible with the poly(n-butyl acrylate) blocks. This incompatibility drives nanoscale separation. Ethylene glycol keeps the chains mobile enough for that separation to become ordered.
Ethylene glycol plays the unusual role of a temporary helper rather than an immediate crosslinker. During printing, it acts as a mobility mediator while forming hydrogen bonds with poly(acrylic acid) regions. Those interactions give the material enough cohesion to withstand layer-by-layer printing stresses without creating a permanent network too soon. The resin remains printable while preserving a short window for molecular rearrangement.
The critical test was whether ordering could occur on a printing-relevant timescale. Small-angle X-ray scattering, which detects repeating nanoscale spacing, captured the emergence of ordered signatures under light exposure. Depending on formulation, the materials formed lamellar layers or hexagonally packed cylinders within roughly 30 to 60 s. That speed matters because traditional block copolymer ordering often requires much longer thermal or solvent treatment.
A control experiment showed why the timing change matters. Substituting ethylene glycol diacrylate, a conventional divinyl crosslinker, for ethylene glycol still allowed nanoscale phase separation. It did not produce the secondary scattering features that mark long-range order, meaning the repeated layers or cylinders never fully developed. The early permanent network trapped the polymer blocks in disordered nanodomains.
To matter for manufacturing, the ordered structures had to survive real printing. Using a commercial LCD 3D printer, the researchers printed films and three-dimensional objects from several PIANO formulations. The printed materials contained ordered nanostructures with domain spacings of about 20 to 60 nm. The larger printed parts showed features around 200 µm, so one object carried structure from hundreds of micrometers down to tens of nanometers.
That multiscale control came from the resin formulation rather than the printer hardware. Changing the amount of polymer precursor shifted the internal pattern from hexagonally packed cylinders to lamellar layers. Increasing the precursor length increased the distance between repeated domains. The chemistry therefore acted as a design dial for the hidden architecture inside the part, while the printer controlled the part’s external form.
Direct imaging showed that the ordered domains were not only scattering signatures. Thin slices from printed cubes showed the same cylindrical and layered motifs inferred from scattering measurements, and their spacing followed the same formulation trends. The images also clarified the limits of the approach. Longer chains and higher precursor loadings increased viscosity, which reduced chain mobility and weakened long-range order. PIANO does not remove kinetic constraints. It makes them adjustable.
The balance matters because mobility alone would not make a printable material. More ethylene glycol gives polymer chains more freedom to organize, but too much can weaken the material before final strengthening. Slowing polymerization gives the chains more time, but it also slows printing. The useful window lies between those failures: enough mobility for nanoscale order, enough cohesion for each printed layer to keep its shape.
The same ethylene glycol that opens this window during printing then helps close it after printing. Heating triggers reactions between ethylene glycol and carboxylic acid groups in the poly(acrylic acid) domains, forming ester crosslinks. This converts the temporary hydrogen-bonded material into a covalently crosslinked network, compensating for the weakness that mobility mediation can introduce. In one formulation, extended annealing at 100 °C raised tensile strength by about 5-fold while preserving the ordered nanoscale morphology.
That strengthening step did not undo the printed geometry. Complex geometries printed from different formulations kept their intended features after heating, and thermal treatment caused no appreciable shrinkage. A strengthening step that warps the part would defeat the purpose of printing it precisely. In PIANO, the nanoscale structure and the macroscopic shape survived the same post-printing treatment.
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ORCID information
Cyrille Boyer (University of New South Wales, Sydney)
, 0000-0002-4564-4702 corresponding author
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