Milijana Jovic

Thermosets underpin many structural, coating, adhesive and electronic applications, but their permanent crosslinked architectures make damage repair, reshaping and end-of-life recycling intrinsically difficult. At the same time, many epoxy thermosets require flame retardancy, creating a design challenge in which dynamicity and fire safety are typically introduced through separate molecular components. Here we report β-amino phosphine oxide junctions as a multifunctional dynamic motif that simultaneously enables thermally activated network rearrangement and intrinsic phosphorus-based flame retardancy in epoxy covalent adaptable networks. Di- (DVPPO)and trifunctional (TVPO)vinyl phosphine oxides were covalently incorporated into epoxy–piperazine networks through a controlled aza-Michael addition, establishing reversible aza-Michael/retro-aza-Michael chemistry from vinyl phosphine oxide Michael acceptors in polymer networks while providing a practical formulation window distinct from rapidly reacting β-amino ester systems. The resulting materials undergo stress relaxation, pressure-assisted healing of surface damage, and repeated thermomechanical reconstruction over five reprocessing cycles while retaining useful thermal and mechanical performance. At 6 wt% phosphorus, the DVPPO-based network retains a UL-94 V-0 classification after five reprocessing cycles, whereas the TVPO-based network gives the largest initial peak heat-release-rate reduction. Coupled cone calorimetry, DEP-MS and SEM–EDX analyses show that molecular architecture controls the dominant fire-retardant pathway: DVPPO promotes phosphorus volatilization and gas-phase flame inhibition, while TVPO favors phosphorus retention and condensed-phase char protection. These results establish β-amino phosphine oxides as dynamic phosphorus junctions for designing repairable, reprocessable, and fire-safe thermosets from a single molecular platform.

Thermosetting polymers are widely employed in high-performance applications owing to their excellent mechanical properties, thermal stability, and chemical resistance. However, their permanently crosslinked architecture severely limits recyclability, while their intrinsic flammability remains a major concern for safety-critical applications. Herein, hybrid epoxy–amino ester covalent adaptable networks (CANs), incorporating phosphorus-containing polyelectrolyte complex (PEC) micro-particles, are developed to simultaneously address these two challenges. The developed networks undergo thermally activated rearrangement and can be thermomechanically reprocessed while retaining an overall crosslinked architecture. The aqueous, organic-solvent-free preparation of PEC from comparatively low-hazard ingredients provides an environmentally preferable route to a phosphorus–nitrogen flame-retardant additive. PEC preserves the overall curing behavior and dynamic network properties. The moderate increase in apparent curing activation energy from 44 to 51 kJ mol⁻¹ indicates a limited kinetic influence that does not require modification of the curing procedure. The resulting materials show stable glass transition temperatures, efficient stress relaxation, and retained thermal properties after one thermomechanical reprocessing cycle. An optimized phosphorus loading of 3 wt% enables the material to achieve a UL-94 V-0 classification, while maintaining its flame-retardant performance after one thermomechanical reprocessing cycle. In contrast, formulations containing commercial ammonium polyphosphate (APP), used as a practical commercial reference under the present processing conditions, did not achieve a UL-94 V-0 classification even at 4 wt% phosphorus. Carbon-fiber-reinforced composites fabricated using the developed matrix exhibited high flexural strength, although PEC incorporation reduced the flexural modulus relative to the phosphorus-free composite, demonstrating the feasibility of using the developed CAN as a composite matrix.

Rigid polyurethane foams combine excellent thermal insulation with low density but remain inherently flammable, motivating the development of reactive flame-retardant components that can be incorporated without relying on conventional particulate additives. Here, the Pudovik reaction is used as a modular route to prepare two classes of phosphorus-containing polyol components: positional-isomeric small-molecule α-hydroxyphosphonate diols and oligomeric phosphorus polyols obtained by partial functionalization of P–H-containing polyphosphonates. Their incorporation into conventional pMDI-based rPUFs reveals a strong dependence of foam formation, phosphorus retention, and fire behavior on molecular architecture. The small-molecule P-diols progressively increase the limiting oxygen index from approximately 22 to 24–25 vol% and achieve HF-1 classification at 4 wt% phosphorus. Under cone calorimetry, the small-molecule phosphonate diols reduce the peak heat release rate by approximately 20–26%, while causing little change in total heat release. In contrast, the polymeric P-polyol produces only a limited reduction in peak heat release rate but decreases total heat release by approximately 38%. Incorporation of the phosphorus-containing components promotes earlier thermal degradation and increased char formation, while thermal conductivity is only moderately affected. Leaching, solid-state 31P NMR, evolved-volatile analysis, and SEM–EDX further reveal architecture-dependent phosphorus retention: the meta-substituted diol is more effectively retained than its para-substituted analogue, whereas the P-polyol generates a comparatively homogeneous phosphorus-containing char. Smoke production increases relative to the neat foam, particularly for the small-molecule P-diols, highlighting a remaining performance trade-off. Overall, the results demonstrate that phosphorus content alone does not determine flame-retardant performance in rigid foams; molecular structure, network retention, and macromolecular architecture govern the balance between heat-release suppression, char formation, smoke generation, and foam processability. Pudovik-derived α-hydroxyphosphonates therefore provide a versatile platform for the molecular design of reactive phosphorus components for polyurethane foams.

Thermosetting polymers are widely employed in high-performance applications owing to their excellent mechanical properties, thermal stability, and chemical resistance. However, their permanently crosslinked architecture severely limits recyclability, while their intrinsic flammability remains a major concern for safety-critical applications. Herein, hybrid epoxy–amino ester covalent adaptable networks (CANs), incorporating phosphorus-containing polyelectrolyte complex (PEC) micro-particles, are developed to simultaneously address these two challenges. The hybrid network combines associative transesterification and dissociative retro Aza-Michael addition exchange reactions, enabling efficient thermomechanical reprocessing while preserving the crosslinked architecture. The incorporation of PEC, prepared through a simple aqueous process from poly(allylamine hydrochloride) and sodium polyphosphate, provides an environmentally friendly phosphorus-nitrogen flame-retardant additive without altering the curing behavior or dynamic network properties. The resulting materials exhibit stable glass transition temperatures, efficient stress relaxation, and excellent retention of thermal properties after recycling. An optimized phosphorus loading of 3 wt% enables the material to achieve a UL-94 V-0 classification, while maintaining its flame-retardant performance after thermomechanical recycling. In contrast, equivalent formulations containing commercial ammonium polyphosphate (APP) flame retardant failed to achieve a UL-94 V-0 classification, even at 4 wt% phosphorus, demonstrating the superior flame-retardant efficiency of PEC. Finally, carbon-fiber-reinforced composites fabricated using the developed matrix demonstrate excellent flexural properties, highlighting the potential of these recyclable flame-retardant CANs for sustainable structural applications.

Covalent adaptable networks (CANs) are predominantly employed as bulk polymer matrices, while their potential as dynamically crosslinked functional additive phases within conventional thermoplastics remains largely unexplored. Here, phosphorus-containing CANs (P-CANs) with systematically varied molecular architectures are employed as durable functional additives for polyamide 6 (PA6), enabling network architecture to be related to additive dispersion, retention, melt processability, and functional performance. Transitioning from a rigid, small-molecule-derived network to flexible polyphosphonate-based architectures tunes the glass transition temperature from 160 to -5 °C and markedly alters network dynamics and dispersion during melt processing. The rigid P-CAN forms larger phosphorus-rich domains in PA6, whereas the most flexible architecture exhibits substantially more homogeneous dispersion, reduced processing resistance, and pronounced shear-thinning behavior. This balance enables translation from laboratory-scale screening to pilot-scale twin-screw compounding and continuous extrusion of uniform PA6 films containing 5–10 wt% CANFR15. Despite low final phosphorus contents of only ~0.25–0.60 wt%, P-CAN incorporation increases the limiting oxygen index of PA6 from 25.8% to as high as 33.0%, promotes char formation, and reduces peak heat release measured in microscale combustion calorimetry. Embedding phosphorus within the CAN architecture additionally suppresses its extraction relative to conventional phosphorus-containing additive flame retardants under accelerated solvent-leaching conditions. These results establish molecular architecture as a key design parameter for balancing network rigidity, additive dispersion and retention, functional performance, and melt processability, while demonstrating P-CANs as a promising class of durable macromolecular additives for scalable thermoplastic processing.

Polyamide 6,6 (PA6,6) fabrics are widely used in textiles due to their high mechanical strength and chemical stability. However, their inherent flammability and melting behavior under fire pose significant safety challenges. In this study, a dual-layer flame-retardant system was developed by integrating atomic layer deposition (ALD) of ZnO with a phosphorus–silane-based flame retardant (DOPO-ETES). ALD allowed precise control of ZnO layer thickness (50, 84, and 199 nm), ensuring uniform coating. Thermal analysis (TGA) and microscale combustion calorimetry (MCC) revealed that ZnO altered the degradation pathway of PA6,6 through catalytic effects, promoting char formation and reducing heat release. The combination of ZnO and DOPO-ETES resulted in further reductions in heat release rates. However, direct flame tests showed that self-extinguishing behavior was not achieved, emphasizing the limitations related to the melting of PA6,6. TG-IR and cone calorimetry confirmed that ZnO coatings suppressed the release of smoke-related volatiles and incomplete combustion products. These findings highlight the potential of combining metal-based catalytic flame retardants like ZnO with phosphorus-based coatings to improve flame retardancy while addressing the specific challenges of polyamide textiles. This approach may also be adapted to other fabric types and integrated with additional flame retardants, broadening its relevance for textile applications.

The effects of combining 0.1–5 wt % graphene nanoplatelet (GNP) and 3–30 wt % phosphorous flame retardant, 9,10- dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as fillers in epoxy polymer on the mechanical, flame retardancy, and electrical properties of the epoxy nanocomposites was investigated. GNP was homogeneously dispersed into the epoxy matrix using a solvent-free three-roll milling process, while DOPO was incorporated into the epoxy resin by mechanical stirring at elevated temperature. The incorporation of DOPO reduced the crosslinking density of the epoxy resin. When using polyetheramine as a hardener, the structural rigidity effect of DOPO overshadowed the crosslinking effect and governed the flexural moduli of epoxy/DOPO resins. The flexural moduli of the nanocomposites were improved by adding GNP up to 5 wt % and DOPO up to 30 wt %, whereas the flexural strengths deteriorated when the GNP and DOPO loading were higher than 1 wt % and 10 wt %, respectively. Limited by the adverse effects on mechanical property, the loading combinations of GNP and DOPO within the range of 0–1 wt % and 0–10 wt %, respectively, in epoxy resin were further studied. Flame retardancy index (FRI), which depended on three parameters obtained from cone calorimetry, was considered to evaluate the flame retardancy of the epoxy composites. DOPO showed better performance than GNP as the flame retardant additive, while combining DOPO and GNP could further improve FRI to some extent. With the combination of 0.5 wt % GNP and 10 wt % DOPO, improvement in both mechanical properties and flame retardant efficiency of the nanocomposite was observed. Such a combination did not affect the electrical conductivity of the nanocomposites since the percolation threshold was at 1.6 wt % GNP. Our results enhance the understanding of the structure–property relationship of additive-filled epoxy resin composites and serve as a property constraining guidance for the composite manufacturing.

The role of various additives (emulsifier, anti-dripping agent) and formulation procedures (pre-dispersion of solid additives in polyol via milling) which influence the flame retardancy of 6,6′-[ethan-1,2-diylbis(azandiyl)]bis(6H-dibenzo[c,e][1,2]oxaphosphin-6-oxid) (EDA-DOPO) containing flexible polyurethane foams has been investigated in this work. For comparison, the flame retardancy of two additional structurally-analogous bridged 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based compounds, i.e., ethanolamine-DOPO (ETA-DOPO) and ethylene glycol-DOPO (EG-DOPO) were also evaluated together with EDA-DOPO in flexible PU foams of various formulations. The flame retardancy of these three bridged-DOPO compounds depends on the type of PU formulation. For certain PU formulations containing EDA-DOPO, lower fire performance was observed. Addition of emulsifier and polytetrafluoroethylene (PTFE) to these PU formulations influenced positively the flame retardancy of EDA-DOPO/PU foams. In addition, dispersion of EDA-DOPO and PTFE via milling in polyol improved the flame retardancy of the PU foams. Mechanistic studies performed using a microscale combustion calorimeter (MCC) and its coupling to FTIR showed no difference in the combustion efficiency of the bridged-DOPO compounds in PU foams. From MCC experiments it can be concluded that these bridged-DOPO compounds and their decomposition products may work primarily in the gas phase as flame inhibitors. The physiochemical behavior of additives in PU formulation responsible for the improvement in the flame retardancy of PU foams was further investigated by studying the dripping behavior of the PU foams in the UL 94 HB test. A high-speed camera was used to study the dripping behavior in the UL 94 HB test and results indicate a considerable reduction of the total number of melt drips and flaming drips for the flame retardant formulations. This reduction in melt drips and flaming drips during the UL 94 HB tests help PU foams achieve higher fire classification.