Frontal Polymerization Research Articles

One-Pot Synthesis of Robust Fluorescent Nanocomposite Gel via Frontal Polymerization.

Fluorescent nanocomposite gels have attracted increasing attention due to their excellent optical properties, as well as enhanced mechanical strength originating from the nanoparticles. At present, two-step methods are usually employed, where fluorescent nanoparticles are firstly prepared, followed by mixing with gel precursor to achieve the final products after gelation, which suffer from the disadvantages of a tedious and time-consuming process. Thus, the development of a facile strategy is highly desirable, which still remains an obstacle. Herein, a new one-pot synthesis method towards robust fluorescent nanocomposite gels via frontal polymerization (FP) is proposed, where small molecular precursors (citric acid (CA) and urea, or L-cysteine) and gel precursor (vinyl monomers) are mixed together as co-reactants. During the FP process, a lot of heat release gives rise to the generation of carbonized polymer dots (CPDs). Thus, companying with the propagating of the polymerization, the production of fluorescent CPDs/gel composite is completed. In addition, as a nanofiller, CPDs dramatically enhance the mechanical property of the CPDs/gel composite. This work proposes a new fast and efficient one-pot strategy for the production of CPDs/gel composite, which will guide the development of high-performance polymer nanocomposites through an in situ synchronous reaction fashion.

Redox cationic frontal polymerization: a new strategy towards fast and efficient curing of defect-free fiber reinforced polymer composites.

Frontal polymerization of epoxy-based thermosets is a promising curing technique for the production of carbon fiber reinforced composites (CFRCs). It exploits the exothermicity of polymerization reactions to convert liquid monomers to a solid 3D network. A self-sustaining curing reaction is triggered by heat or UV-radiation, resulting in a localized thermal reaction zone that propagates through the resin formulation. To date, frontal polymerization is limited to CFRCs with a low fiber volume percent as heat losses compromise on the propagation of the heat front, which is crucial for this autocatalytic curing mechanism. In addition, the choice of suitable epoxy monomers and thermal radical initiators is limited, as highly reactive cycloaliphatic epoxies as well as peroxides decarboxylate during radical induced cationic frontal polymerization. The resulting networks suffer from high defect rates and inferior mechanical properties. Herein, we overcome these shortcomings by introducing redox cationic frontal polymerization (RCFP) as a new frontal curing concept. In the first part of this study, the influence of stannous octoate (reducing agent) was studied on a frontally cured bisphenol A diglycidyl ether resin and mechanical and thermal properties were compared to a conventional anhydride cured counterpart. In a subsequent step, a quasi-isotropic CFRC with a fiber volume of >50 vol%, was successfully cured via RCFP. The composite exhibited a glass transition temperature > 100 °C and a low number of defects. Finally, it was demonstrated that the redox agent effectively prevents decarboxylation during frontal polymerization of a cycloaliphatic epoxy, demonstrating the versatility of RCFP in future applications.

Preparation of SA/P(U-AM-ChCl) composite hydrogels by frontal polymerization and its performance study.

Deep eutectic solvent (DES) was synthesized from urea (U), acrylamide (AM) and choline chloride (ChCl), sodium alginate (SA) was selected as filler, and SA/P(U-AM-ChCl) composite hydrogel was prepared by thermal initiation frontal polymerization (FP). The hydrogels were characterised by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The effects of SA on the swelling properties, mechanical properties and self-healing properties of the composite hydrogels were investigated. The results show that the swelling properties of the composite hydrogel with the addition of SA are greatly enhanced due to the large number of hydroxyl groups contained in the SA chain. The tensile strength of the hydrogel gradually increased with increasing SA content, with the maximum tensile strength increasing by a factor of 2.89. The self-healing efficiency of the composite hydrogel gradually increased with the increase of SA, and the healing rate of FP5 reached 94.4% after 48 h of healing. This study provides a simple and rapid method for the preparation of composite hydrogels with good mechanical properties and self-healing properties.

Preparation of P(AM-co-MA) composite hydrogels by frontal polymerization and its performance study

Deep eutectic solvent (DES) was synthesized from choline chloride (ChCl) and acrylamide (AM), and P(AM-co-MA) hydrogels were prepared by adding different contents of maleic acid (MA) to DES by frontal polymerization.

Frontally polymerized foams: thermodynamic and kinetical aspects of front hindrance by particles.

Frontal polymerization (FP) is a solvent-free, energy-efficient process where a self-propagating polymerization reaction with a characteristic sharp temperature gradient at the front head propagates through the resin to provide the curing conditions. It relies on the enthalpic balance, which spreads the reaction to unreacted resin in the neighborhood. Therefore, the FP is sensitive to the presence of non-reactive volumes, such as boundaries, fillers, or other additives, that retain heat from the front but produce no enthalpy in return. On the other hand, the front's high temperature could be used to initiate other processes, such as foaming, incorporating them into a simple single-step fabrication procedure. This study used silica particles of two different sizes (14 nm and 200-300 nm) in an epoxy-based FP foam as a representative filler to probe the constraints imposed by non-reactive additives. The presence of particles visibly hindered the front propagation, increased the foam density and even corrupted the frontal regime in some cases. We show that preheating or chemical composition changes are viable approaches to address the fillers' adverse effects. Furthermore, we present evidence that the reduced reaction enthalpy caused by silica nanoparticles, was balanced by the lower heat capacity of our model system. At the same time, the front hindrance was attributed to changes in reaction kinetics and the heat distribution around the front. These results set up essential narratives for the design and practical applications of frontally polymerized foams with non-reactive fillers.

Preparation of novel β-CD/P(AA-co-AM) hydrogels by frontal polymerization.

In this paper, betaine (Bet) was used as a hydrogen bond acceptor (HBA), and acrylic acid (AA) and acrylamide (AM) were used as hydrogen bond donors (HBD) and mixed to form a deep eutectic solvent (DES). Different concentrations of β-cyclodextrin (β-CD) were dispersed in the DES, and a novel β-CD/P(AA-co-AM) hydrogel was prepared by frontal polymerization (FP). The characteristic structure and morphology of the hydrogels were analyzed using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM), and the properties of the hydrogels were investigated. The results show that the mechanical properties of the hydrogel were improved by β-CD acting as a second cross-linking agent in the polymerization process, thus increasing the cross-link density of the hydrogel. Because the carboxyl groups contained in the acrylic acid dissociate under alkaline conditions, the composite hydrogel shows excellent pH responsiveness under alkaline conditions. Tetracycline hydrochloride was used as a drug model to test the drug loading and drug release performance of the hydrogels. With the increase of β-CD content, the loading capacity of the hydrogels for tetracycline hydrochloride gradually increased. The data of drug release indicated that the hydrogel has good drug delivery performance and has promising applications in drug delivery systems and other areas.

Fast Healing of Covalently Cross-Linked Polymeric Hydrogels by Interfacially Ignited Fast Gelation

We report a new design concept that utilizes the Ag+/Fe3+-catalyzed fast gelation of acrylic acid (AA) or AA/comonomer aqueous solutions (e.g., 56 s for the gelation of the 20 wt % AA solution) to achieve fast healing of covalently cross-linked polymer hydrogels. The fast-generated poly(acrylic acid) (PAA) or AA copolymer hydrogel with a covalent network is highly active for the interfacially igniting gelation (IIG) of AA or the AA/comonomer in water through a frontal polymerization process. Using the AA or AA/comonomer aqueous solution as the “repairing liquid,” the fast-generated hydrogels (polymer concentration of 20 wt %) are repeatedly healed at room temperature through the IIG reaction without extra energy input. This IIG reaction-mediated healing (IIG-healing) has healing efficiencies higher than 90%. The healing time can be shortened from ∼60 min to only ∼1 min by increasing the solid concentration of the fast-generated hydrogel to 92 wt %. The IIG reaction also enabled the coating of a hydrophobic polymer layer on the hydrogel, resulting in higher tensile strength, toughness, and much better water retention capacity. We expect that this Ag+/Fe3+-catalyzed fast gelation strategy will pave a facile and energy-efficient pathway to fabricate versatile and multifunctional healing polymers.

Energy-efficient manufacturing of multifunctional vascularized composites

The retention and transport of different fluids inside synthetic microvascular fiber-reinforced polymer (FRP) composites enable environmentally adaptive functions, including thermal regulation, self-healing, and electromagnetic modulation. However, manufacturing of vascularized components involves an energy- and time-intensive multistep process to cure the host matrix (several hours at elevated temperature) and then evacuate the embedded sacrificial template (12–24 h at 200°C under vacuum). Here, we demonstrate rapid (minutes), energy-efficient, and scalable fabrication of vascularized FRP composites at room temperature using the exothermic frontal polymerization of a dicyclopentadiene host matrix. The chemical energy released during frontal curing of the host resin facilitates the endothermic depolymerization of an embedded sacrificial thermoplastic to create structures with high-fidelity microchannels, reducing the thermal energy for fabrication by nearly four orders of magnitude compared to previous methods. The presence of fiber reinforcement in this tandem curing and vascularization strategy presents several challenges related to successful frontal curing and microchannel formation. Increasing the volume fraction of fiber reinforcement ( Vf) decreases the volume of the host resin matrix, generating less energy for sustaining the curing and vascularization processes. Heat retention for several minutes after completion of frontal curing using thermally insulating tooling is crucial for obtaining clear microchannels in composite specimens with Vf = 60%. Simulation of the vascularization process confirms the slower depolymerization of the sacrificial templates in high- Vf composites. A nominal decrease in channel circularity also occurs with an increase in the compaction pressure required for high Vf of composite panels. We leverage this rapid manufacturing strategy to fabricate hybrid composites with vascular networks that span the bulk of the composite and a surface coating for potential self-healing applications.

Preparation of FeCo/C-N and FeNi/C-N Nanocomposites from Acrylamide Co-Crystallizates and Their Use as Lubricant Additives.

FeCo and FeNi nanoalloy particles encapsulated in a nitrogen-doped carbonized shell (FeCo/C-N and FeNi/C-N) were synthesized by thermolysis at 400 °C of polyacrylamide complexes after frontal polymerization of co-crystallizate of Fe and Co or Ni nitrates and acrylamide. During the thermolysis of polyacrylamide complexes in a self-generated atmosphere, Co(II) or Ni(II) and Fe(III) cations are reduced to form FeCo and FeNi nanoalloy particles, while polyacrylamide simultaneously forms a nitrogen-doped carbon shell layer. This unique architecture provides high chemical and thermal stability of the resulting nanocomposites. The average crystallite size of FeCo and FeNi nanoparticles is 10 and 12 nm, respectively. The nanocomposites were studied by X-ray diffraction, atomic force microscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The nanocomposites have been tested as antifriction and antiwear additives in lubricating oils. The optimal concentrations of nanoparticles were determined, at which the antifriction and antiwear properties of the lubricant manifest themselves in the best possible way.

Multiphysics modeling of frontal polymerization-assisted layer-by-layer additive manufacturing of thermoset polymer components

Frontal polymerization (FP) is a novel curing strategy that relies on a self-propagating exothermic reaction front to polymerize thermoset resins rapidly. Due to its energy-efficiency and rapid curing of thermosets, a series of applications related to polymer and polymer composite manufacturing have been developed based on FP, in particular additive manufacturing (AM). While the current research demonstrates successful printing of 1D, 2D and 3D structures based on FP, a key challenge is the determination of the printing parameters through trial-and-error, which hinders its large-scale application. To better understand how different process parameters affect the front behavior and the printing process, and eventually enable fast printing of complex parts without trial-and-error, computational modeling of the printing process is highly desired. For the first time, we develop a multiphysics finite element model for simulating FP-assisted AM which accounts for both the thermal-chemical process and the ink deposition in a real printing process. To simulate the dynamic ink deposition process, element activation is used that constantly adds elements to the simulation domain following a predefined printing path and velocity. A coupled thermo-chemical partial differential equation system is solved over the changing ink domain to simulate the heat transfer and chemical reaction during the printing process. The model is first validated by comparing the front temperature history during the printing process with the experimental infrared thermal measurements. The validated model enables the determination of proper printing velocity range, within which the polymerization front can follow the printing such that the ink will not deform before curing, which is critical to ensure printing accuracy. Furthermore, the simulation reveals the change of front temperature and degree of cure between different layers, and their dependency on layer length and printing velocity, providing insights into the printing experiments for processing parameters selection.

Rapid and Energy-Efficient Frontal Curing of Multifunctional Composites Using Integrated Nanostructured Heaters.

Current technologies for the manufacture of fiber-reinforced polymer composites are energy-intensive, environmentally unfriendly, and time-consuming and require expensive equipment and resources. In addition, composites typically lack key nonstructural functionalities (e.g., electrical conductivity for deicing, lightning strike protection, and structural health monitoring), which are crucial to many applications such as aerospace and wind energy. Here, we present a new approach for rapid and energy-efficient manufacturing of multifunctional composites without using traditional expensive autoclaves, ovens, or heated molds used for curing of composites. Our approach is predicated on embedding a thin conductive nanostructured paper in the composite layup to act as a resistive heater for triggering frontal polymerization of the matrix thermosetting resin of the composite laminate. Upon passing electric current, the nanostructured paper quickly heats up and initiates frontal polymerization, which then rapidly propagates through the thickness of the laminate, resulting in rapid curing of composites (within seconds to few minutes) irrespective of the size of the composite laminate. The integrated nanostructured paper remains advantageous during the service of the composite part by imparting new functionalities (e.g., deicing) to the cured composite, owing to its excellent electrical conductivity and electrothermal properties. In this work, we first study the influence of several composite processing parameters on the electrothermal properties of the nanostructured paper and determine the power required for rapid initiation of frontal polymerization. We then successfully fabricate a 10 cm × 10 cm composite panel within 1 min using only 4.49 kJ of energy, which is 4 orders of magnitude less than the energy consumed by the traditional bulk, oven-curing technique. Detailed experiments are conducted to provide an in-depth understanding of the effect of heater position, tooling material, and input power on frontal curing of composite laminates. The multifunctional response of produced composites is demonstrated by performing a deicing experiment, where a 50 × 50 × 3 mm3 cube of ice is completely melted within 3 min using an input power of 77 W.

Frontal polymerization-triggered simultaneous ring-opening metathesis polymerization and cross metathesis affords anisotropic macroporous dicyclopentadiene cellulose nanocrystal foam

Multifunctionality and effectiveness of macroporous solid foams in extreme environments have captivated the attention of both academia and industries. The most recent rapid, energy-efficient strategy to manufacture solid foams with directionality is the frontal polymerization (FP) of dicyclopentadiene (DCPD). However, there still remains the need for a time efficient one-pot approach to induce anisotropic macroporosity in DCPD foams. Here we show a rapid production of cellular solids by frontally polymerizing a mixture of DCPD monomer and allyl-functionalized cellulose nanocrystals (ACs). Our results demonstrate a clear correlation between increasing % allylation and AC wt%, and the formed pore architectures. Especially, we show enhanced front velocity (vf) and reduced reaction initiation time (tinit) by introducing an optimal amount of 2 wt% AC. Conclusively, the small- and wide-angle X-ray scattering (SAXS, WAXS) analyses reveal that the incorporation of 2 wt% AC affects the crystal structure of FP-mediated DCPD/AC foams and enhances their oxidation resistance.

Iodonium Borate Initiators for Cationic Photopolymerization and Their Application in Radical-Induced Cationic Frontal Polymerization

Iodonium antimonates and borates are well established initiators in the field of cationic photopolymerization and preferred substances for radical-induced cationic frontal polymerization (RICFP). Extensive research in the fields of ionic liquids and lithium ion battery electrolytes made many superacids based on the borate anions available. Cyanide-ligated borane B2 has lower molecular weight compared to state-of-the-art borates and is less expensive to synthesize from commercial starting materials. By comparing reactivity in the epoxy-based monomer bisphenol-A-diglycidylether, B2 outperforms all commercial systems in a broad range of temperatures with excellent epoxy group conversions of up to 95%. Besides its good performance as a cationic initiator, B2 is also capable of thermally and photochemically induced RICFP. Exceptional are transparent B2 specimens in contrast to the tested commercial systems, which are black. High reactivity, good frontal parameters, easy synthetic access, and sufficient pot life of formulations make cyanide-ligated iodonium borates a compelling alternative to current state-of-the-art initiators.

Development of periodic colored bands via frontal polymerization

The development of periodic brown-white colored structural patterns during the frontal polymerization process has been studied in Methacrylic acid (MAA), Resorcinol, Benzoyl peroxide (BP), and N, N Dimethyl aniline (DMA) reaction system. The reaction has been carried out in a thin glass test tube under ambient conditions. Pattern formation and their dependency on the concentration of MAA monomer, Resorcinol, and DMA initiator have largely been observed. Methanol has also been used as a solvent to optimize the effect of dilution levels on the periodicity of colored structural patterns. The materials characterization is carried out using NMR, SEM, IR, CHNS, and UV–visible analyses to show the possible reaction mechanisms and elemental composition of the polymerization reaction. It has been observed that white and brown colored polymeric phases are formed during FP reactions and polymerized simultaneously. In this context, a possible chemical mechanism has been proposed which shows the formation of white colored polymethacrylic acid and brown phase poly (2–hydroxy, 1, 4 napthaquinone methacrylic acid). The analytical data observed during the study has a good agreement with the proposed chemical mechanism. The characteristics of colored patterns found depending on the concentration of resorcinol, BP, and EtOH have been studied and properly discussed.

Frontal Polymerization in Short-Fiber-Reinforced Thermoset Composites

Frontal polymerization (FP) has recently been introduced as a faster, more energy-efficient manufacturing process for high-performance thermoset composites. This numerical study focuses on the FP-based manufacturing of short-fiber-reinforced polymeric matrix composites. The investigation involves a combination of multiphysics finite element analyses and analytical predictions. The developed model is also applied to a short-fiber composite with spatially varying fiber orientation obtained from a sample produced by vacuum injection method. The results show how the steady-state front velocity and inclination angle depend on the fiber orientation angle and volume fraction. Analytical expressions are derived to capture that dependence and show excellent agreement with the numerical steady-state results. Simulations of FP in short-fiber composites based on experimental distributions of the fiber orientation indicate the feasibility of FP in samples prepared by injection molding.

Surface pattern formation induced by oscillatory loading of frontally polymerized gels

Frontal polymerization (FP) is a rapid, energy-efficient technique for the manufacturing of polymeric materials and composites. It has also emerged as a way to rapidly alter the shape of partially cured polymeric materials through mechanical deformation. The first objective of this paper is to introduce a coupled thermo-chemo-mechanical theory capable of describing within the framework of nonlinear kinematics the evolution of deformation and temperature fields during frontal polymerization of gels. At the heart of the proposed theory is the introduction of eigenstrains corresponding to the phase transformation prompted by the frontal curing process. Another objective is to introduce a novel bio-inspired oscillatory loading-induced patterning technique in which we form thermoset polymeric materials with periodic surface topography patterns by applying oscillatory uniaxial loads to partially cured gels during FP. After a detailed presentation of the experimental methodology, the theoretical predictions are compared with experimental results.

3D Printing of Frontal-polymerized Multiscale Epoxy Thermoset and Composites

Epoxy thermosets and their composites demonstrate an intensive application due to their high strength, chemical resistance, and lightweight while the conventional manufacturing methods are very difficult to fabricate complex geometry and also time-consuming and energy-intensive. Frontal polymerization based in-situ curing provides an alternative efficient way in the additive manufacturing of epoxy thermosets with both design and manufacturing freedom. However, low reactivity of epoxy resins makes it difficult to integrate frontal polymerization of epoxy resin into energy-efficient additive manufacturing. Carbon nanotubes (CNTs) catalyzed frontal polymerization of epoxy resins afforded a new way to free-form manufacturing of epoxy thermosets with low energy consumption and design flexibility. In this paper, frontal curing of epoxy resins was tuned by CNT catalysation and then integrated to fast printing of multiscale epoxy composites. Specifically, discontinuous CNTs and continuous carbon fibers (c-CF) were integrated to print frontal-cured multiscale epoxy composites. The mechanical tests indicated that the CNTs/c-CF/epoxy thermosets composites demonstrated a tensile strength of 1.2GPa while the CNTs/epoxy thermosets only showed a tensile strength of around 50 MPa. The young’s modulus of the CNTs/c-CF/Epoxy thermosets reached 95 GPa, around 9-fold higher than that of the CNTs/Epoxy thermosets. This emerging technology provides a new direction for thermosets and composites manufacturing.

Switching Frontal Polymerization Mechanisms: FROMP and FRaP

Two frontal polymerization (FP) mechanisms, frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene and frontal radical polymerization (FRaP) of benzyl acrylate and hexanediol diacrylate, were combined for rapid manufacturing of welded thermoset materials. Leveraging the immiscibility of the two different FP resins, welded thermosets and gradient foams of varying composition were achieved by switching of FP mechanisms. The adhesion strength of the welded thermoset materials differed depending on the originating mechanism. Finally, welded thermoset foams of varying porosity and homogeneity were generated through initiation from the bottom of the two resins.

Rapid preparation of conductive and self-healing ionic gels with tunable mechanical properties via frontal polymerization of deep eutectic monomers

Rapid preparation of conductive and self-healing ionic gels with tunable mechanical properties via frontal polymerization of deep eutectic monomers

Facile synthesis of self-healing gels via frontal polymerization toward acid–base regulatable wound dressing

Facile synthesis of self-healing gels via frontal polymerization toward acid–base regulatable wound dressing