The research, led by Chloé Melin of the École de Technologie Supérieure in Montreal and INSA-Lyon, together with Jean-Fabien Capsal, Ricardo Zednik, Angelo Pommella, Nicole Demarquette, and Jean-Marc Chenal, set out to answer a question that has lingered in the literature for years: does PLA actually help PVDF become a better piezoelectric material, or does it merely look like it does on a spectroscopy plot? The team attacked the problem from two processing directions at once. They melt-blended PVDF with PLA at weight ratios of 95/05 and 60/40 using a twin-screw extruder at 210 degrees Celsius, then compression-molded films and stretched them uniaxially at draw ratio 3 across a range of temperatures. In parallel, they prepared the same blend compositions as electrospun fibrous membranes using a semi-industrial twenty-needle unit, with both polymers dissolved together in a dimethyl sulfoxide and acetone mixture.

The central character in this story is the crystal structure of PVDF. The polymer can solidify into several distinct lattice forms, and they are not equal partners. The alpha phase, which forms readily when PVDF cools from the melt, is non-polar and piezoelectrically useless. The beta phase, in which polymer chains adopt an all-trans conformation, is highly polar and delivers the strongest piezoelectric response when the molecular dipoles are aligned by a high-voltage poling treatment. Converting alpha to beta is therefore the name of the game, and the two classic routes are uniaxial stretching of solid films and electrospinning, in which a jet of polymer solution is whipped by intense electric fields into nanometer- and micrometer-scale fibers, combining extreme elongational deformation with rapid solvent evaporation.

Morphology turned out to be the first fork in the road. In the melt-processed blends, scanning electron microscopy revealed that the 95/05 formulation produced a beaded, droplet-in-matrix structure, with discrete PLA domains dispersed in the PVDF, while the 60/40 blend formed a co-continuous architecture in which both polymers formed interpenetrating networks. Differential scanning calorimetry showed that adding PLA left the overall crystallinity of PVDF essentially unchanged at roughly 42 percent, before and after stretching, a consequence of the micrometric PLA domain size and the fact that PVDF crystallizes at a higher temperature than PLA. But the phase composition told a very different story. In the stretched 95/05 blend, the beta-phase fraction matched that of pure PVDF, whereas in the co-continuous 60/40 blend the beta fraction collapsed to 36 percent from the 86 percent achievable in well-processed samples, leaving the material dominated by the inert alpha phase.

The reason for that collapse is a lesson in mechanics. In a co-continuous blend, the macroscopic deformation applied during stretching is shared between the two continuous polymer networks, so the PVDF phase simply never sees enough local stress to drive the alpha-to-beta conformational transformation. Meanwhile, in the beaded 95/05 blend, the team observed cavitation, tiny voids opening at the PVDF-PLA interfaces during stretching, which relaxed local stresses and prevented the hoped-for enhancement of stress transfer to PVDF crystals. Earlier hypotheses in the literature had suggested that PLA could act as a stress concentrator, boosting beta formation through cavitation and heterogeneous deformation. This study’s data call that idea sharply into question: the beta fraction in the stretched blend was no better than in neat PVDF, and in some conditions worse.

Then came the electrical measurements, and the results were even more striking. After poling at 100 volts per micrometer for one hour in dielectric oil, the stretched 95/05 blend showed a piezoelectric coefficient d33 that was 34 to 45 percent lower than pure PVDF stretched under equivalent conditions, despite having a nearly identical beta-phase fraction. The culprit, the researchers showed, is dielectric mismatch. PVDF has a relative permittivity of about 10.8, while PLA’s is only 2.7. During poling, electric field lines preferentially route through the low-permittivity PLA inclusions and through the air-filled cavities left by stretching-induced voids, starving the surrounding PVDF matrix of the field it needs to rotate its dipoles. Finite element simulations in COMSOL Multiphysics, built on a representative volume element containing a PLA inclusion surrounded by an air ellipse, reproduced exactly this field concentration. When the team measured the piezoelectric response of pure PVDF poled at the reduced effective field of roughly 65 volts per micrometer, they obtained a d33 of about 6 piconewtons per coulomb, in close agreement with the blend’s measured value. The physics, in other words, checks out.

The co-continuous 60/40 blend, being overwhelmingly alpha phase, registered no measurable piezoelectric response at all, a d33 of zero. But the electrospinning results added their own twist. Pure electrospun PVDF membranes outperformed their stretched-film counterparts, delivering a d33 roughly 30 percent higher, thanks to about 29 percent greater overall beta-phase content and the superior chain alignment imparted by the enormous elongational forces inside the spinning jet. Interestingly, electrospinning also introduced a significant gamma-phase fraction, an intermediate polar conformation rarely seen in the stretched films, which the authors attribute to the rapid solvent evaporation and extreme deformation rates preserving the initially beta-nucleated chains only partially, letting them relax toward the gamma conformation rather than all the way to alpha.

Yet even electrospinning could not rescue the blend. The electrospun PVDF/PLA 95/05 membranes, despite achieving an electroactive phase fraction of about 86 percent, essentially identical to neat electrospun PVDF, showed a d33 of exactly zero after poling. The team points to a convergence of factors: the finely dispersed, highly elongated PLA domains formed during fiber formation disrupt the local electric field and hinder chain and lamellar orientation; the vastly increased interfacial area places PLA, which is stiffer than amorphous PVDF at the poling temperature, immediately adjacent to the crystal-amorphous interfaces where dipole switching initiates; and interfacial interactions locally stiffen the material, impeding the conformational kinks that must propagate through PVDF chains during poling. Raising the poling temperature triggered electrical breakdown, and lowering the field produced no effective dipole alignment, leaving the membranes piezoelectrically mute.

The study also challenges another widespread assumption. None of the electrospun membranes in this work showed measurable piezoelectric activity before an external poling step, even though the poling field used was nearly twice that of most previous studies reporting self-poled electrospun PVDF. The through-thickness dipole orientation required for a d33 response, the authors note, evidently does not emerge from the electrospinning process alone under these conditions, contradicting a body of literature that has treated self-poling as an intrinsic benefit of the technique.

The broader takeaway is a warning against a seductive shortcut. In the drive toward sustainable, flexible sensors and energy harvesters, blending PVDF with biobased PLA seems like an obvious win: PLA is renewable, biodegradable, stiff, and easy to process. But this work demonstrates that a high beta-phase fraction alone does not guarantee functional piezoelectricity. Electric field distribution, interfacial cavitation, domain size, chain mobility, and processing route all conspire to determine whether a material that looks piezoelectric under an infrared spectrometer actually produces a measurable voltage under stress. For engineers designing the next generation of self-powered wearables and structural health monitors, the message is clear: composition, morphology, and processing must be engineered together, and the role of PLA in enhancing the beta phase of PVDF, once considered promising, is now very much in doubt.

Subject of Research: Piezoelectric performance of PVDF/PLA polymer blends prepared by melt processing and electrospinning

Article Title: Piezoelectric properties of PVDF/PLA blends prepared by melt processing and electrospinning

Article References: Melin, C., Capsal, J.-F., Zednik, R., Pommella, A., Demarquette, N., & Chenal, J.-M. (2026). Piezoelectric properties of PVDF/PLA blends prepared by melt processing and electrospinning. Journal of Materials Science: Polymers, 1(1), Article 13. https://doi.org/10.1007/s44493-026-00010-9

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00010-9

Keywords: PVDF, PLA, piezoelectricity, polymer blends, electrospinning, melt processing, beta phase, energy harvesting, flexible sensors, poling, crystallinity, dielectric permittivity

The materials at the centre of the study are covalent organic frameworks, or COFs, a class of substances that has captivated chemists for the better part of two decades. COFs are constructed from organic molecules stitched together by strong covalent bonds into extended, ordered lattices riddled with pores measured in nanometres. This marriage of crystalline precision and internal void space makes them tantalising candidates for everything from electronic devices and chemical sensors to energy storage systems. Yet there has always been a catch. In their pristine form, most COFs are electrical insulators, and the standard remedy has been post-synthetic doping, in which external substances are introduced to supply the electrons or holes needed for conduction. That remedy comes at a price, because dopants can disrupt the framework’s structure, diminishing its crystallinity, its porosity and its long-term stability.

The CiQUS researchers, working across three groups led by Manuel Souto, Diego Peña and Francisco Rivadulla, proposed a fundamentally different route. Rather than injecting charge carriers after the framework has been assembled, they designed the building blocks to carry them from the outset. The key ingredients are neutral trioxotriangulene radicals, often abbreviated TOT, a family of organic molecules distinguished by a highly delocalised spin and a remarkable resistance to degradation. Because these radicals are stable in their neutral state, they can be woven into the framework as genuine structural components rather than added later as guests, and their unpaired electrons stand ready to serve as charge carriers without the need for counterions or any other external chemical species.

The results reported in the journal Angewandte Chemie International Edition are striking on several fronts. The radical-embedded framework crystallised into an ordered lattice and exhibited genuine semiconducting behaviour, with room-temperature electrical conductivity ranking among the highest values ever recorded for a neutral, non-doped COF. Crucially, the material did not sacrifice the property that makes COFs so attractive in the first place. The framework retained a specific surface area exceeding 1,200 square metres per gram, a figure that speaks to an internal landscape of pores preserved intact despite the electronic functionality now built into its walls.

What distinguishes this approach from earlier attempts to electrify COFs is the intimate integration of function and structure. The TOT radicals are not additives suspended within the pores or molecules grafted onto the surface as an afterthought. They are part of the molecular backbone itself, positioned by design so that their unpaired electrons can participate in charge transport. The ordered arrangement of these radical units throughout the framework creates pathways along which charges can move, turning the entire crystalline edifice into a conduit for electricity. In conventional doped systems, by contrast, the charge carriers and the framework often coexist uneasily, with the dopant acting as an intruder whose presence is tolerated rather than celebrated.

The significance of the achievement lies in the simultaneous preservation of three properties that chemists have historically struggled to reconcile: electrical conductivity, crystallinity and porosity. Materials that conduct well tend to be dense and disordered; materials that are porous and crystalline tend to be insulating. By embedding stable radicals into the framework’s architecture, the Spanish team has demonstrated that these attributes need not be mutually exclusive. The study further shows that the framework can be modified by selecting different molecular components and linkages, offering chemists a tunable dial for adjusting the electronic properties of the resulting materials. That modularity is one of the great promises of COF chemistry, and the new work suggests it can now be extended into the semiconducting regime without compromise.

The practical implications stretch across a remarkably broad technological canvas. Conductive, porous and crystalline organic frameworks could serve in electronics, where their processability and structural diversity offer advantages over rigid inorganic semiconductors. They could underpin spintronics, a field that exploits electron spin rather than charge alone, since the embedded radicals carry intrinsic spin character. Their porosity makes them natural candidates for sensors, where target molecules can diffuse into the material and modulate its electrical response, and for electrochemical devices where ion and electron transport must be coordinated. Energy storage looms particularly large on the horizon, because the TOT units can reversibly accept electrons, a property that points toward their use as active materials in batteries. The researchers are careful to note that these applications remain prospects for future investigation rather than demonstrated realities, but the foundational chemistry is now in place.

Behind the paper lies a story of interdisciplinary synergy within a single research centre. Synthesising a COF is a synthetic chemist’s challenge, demanding the precise design of molecular building blocks and the control of polymerisation conditions; characterising its electronic behaviour is a physicist’s task, requiring careful measurement of conductivity and charge transport; and understanding how structure governs function demands expertise spanning both. The groups of Souto, Peña and Rivadulla contributed complementary perspectives on the design, synthesis, characterisation and property studies of the new materials. The collaboration also extended beyond Spain’s borders, drawing in scientists at the CICECO-Aveiro Institute of Materials at the University of Aveiro in Portugal, whose contributions helped complete the picture of the framework’s structure and behaviour.

The work has already attracted attention beyond the specialist literature. Chemical & Engineering News, the news magazine of the American Chemical Society, recently highlighted the study as a new route to producing semiconducting COFs without dopants, a signal that the broader chemistry community regards the strategy as more than an incremental advance. Published under the title Semiconducting Covalent Organic Frameworks Based on Spin-Delocalized Trioxotriangulene Neutral Radicals, the paper arrives at a moment when demand is surging for organic electronic materials that can be manufactured cheaply, tuned molecularly and deployed in applications where silicon cannot follow. CiQUS, which holds María de Maeztu Unit of Excellence accreditation and CIGUS recognition from the Xunta de Galicia, and receives support from the European Union through the Galicia FEDER 2021–2027 Programme, has positioned itself at the forefront of that effort. If the radical-embedded framework approach proves general, the era of doping organic semiconductors may be drawing to a close, replaced by materials that carry their own charge, pore by pore, bond by bond.

Subject of Research: Dopant-free semiconducting covalent organic frameworks based on spin-delocalized trioxotriangulene neutral radicals

Article Title: New strategy to produce porous organic semiconductors without doping

Article References: New strategy to produce porous organic semiconductors without doping. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: covalent organic frameworks, COFs, organic semiconductors, trioxotriangulene radicals, dopant-free conductivity, charge transport, porosity, crystallinity, energy storage, spintronics, CiQUS, Angewandte Chemie