Frontal Polymerization Process Research Articles

Redox cationic frontal polymerization: a rapid curing approach for carbon fiber-reinforced composites with high fiber content

Conventional frontal polymerization processes for epoxy-based composites rely on cations and radicals generated by a short (and local) light or heat stimulus in the presence of an iodonium salt and a radical thermal initiator. However, due to heat losses, the propagation of the exothermic curing front is often limited by sample geometry and filler concentration. Redox cationic frontal polymerization (RCFP) is a promising approach to radically expand the composition and design options of frontally cured epoxy-based composites. By adding stannous octoate as reducing agent, a higher number of radicals and cations are generated at lower temperature, which yields highly cured composite even at elevated filler content. In the current study, RCFP was used to cure standard unidirectional carbon fiber-reinforced composites based on a commercially available epoxy resin and the properties were compared with its anhydride hardener-cured counterpart. Cure degree and thermal properties of the resins were determined by ATR FT-IR spectroscopy and DMA analysis. Subsequently, unidirectional composites with a fiber volume content of ~ 60% were produced via vacuum infusion and subjected to DMA, tensile, compression, and inter-laminar shear tests. The results showed a remarkable similarity between mechanical properties of RCFP and anhydride hardener-cured composites. The RCFP-cured composites exhibited even a higher damping resistance and compression strength than anhydride hardener-cured composites. The results show that RCFP allows for a significant reduction in the curing time (from several hours to 60 min), while it yields composites with properties comparable to classic anhydride-cured systems.Graphical abstract

Assessment of Photoinduced Frontal Polymerization Processes for a Stable 1K System

AbstractFrontal polymerization (FP) has attracted increasing interest in recent years in various applications. This polymerization method can be very promising for the polymerization of thick materials with high fillers content in the range of 50–80% (weight) by local application of a reasonable amount of energy. In this work, recent advances in controllable and predictive behavior for photoinduced frontal photopolymerization are reported. Here, tert‐butyl peroxybenzoate (Luperox‐P) is selected to initiate thermal polymerization at depth because its high polymerization ability and its decomposition temperature is in a promising range, i.e., neither extremely high (monomer decomposition) nor very low (storage stability issues). Thermal imaging experiments are used to follow the temperatures in the samples in real time. The number of cured layers and the depth of cure are also determined. This paper investigates various factors such as the contents of both photo and thermal initiators, the light intensity, the fiber contents, the irradiation time, etc., resulting in a statistical design of experiments with the factors: 1) content of Luperox P and 2) the irradiation time used to investigate the influence on photoinduced frontal polymerization. Markedly, FP appears to be fully controllable for a storage‐stable, tunable 1K system.

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.

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.

Modeling the through‐thickness frontal polymerization of unidirectional carbon fiber thermoset composites: Effect of microstructures

AbstractThe frontal polymerization is a technique that creates a self‐sustaining “cure front” that propagates through the thermoset resin material. Such a technique is potentially capable of substantially reducing the curing time of thermoset resin fiber composites from several hours to only a few minutes or even seconds, which is promising for the additive manufacturing and repair of thermoset fiber composites. In this study, the effect of the microstructures (i.e., fiber volume fraction, fiber tow size, and fiber tow shape) of unidirectional fiber composites on the frontal polymerization initiated in the through‐thickness direction of the composites is investigated through computational modeling. The computational model is verified through comparisons with experimental data. The simulation results show that the frontal polymerization process is largely affected by the fiber volume fraction and the fiber tow shape and is insensitive to the fiber tow size. The average front velocity decreases significantly as the fiber volume fraction increases from 0% to 30% and then decreases mildly from 30% to 46%. Above 46%, the average front velocity plateaus. Moreover, the average front velocity decreases in an approximately linear fashion as the ratio between the major radius and minor radius of the elliptical cross section of fiber tow increases.

On novel hydrogels based on poly(2-hydroxyethyl acrylate) and polycaprolactone with improved mechanical properties prepared by frontal polymerization

This work aimed to improve the mechanical properties of poly(2-hydroxyethyl acrylate) (PHEA)-based hydrogels by performing their polymerization in the presence of an ad hoc synthesized star-shaped polymer, whose arms are composed of a biopolymer, i.e., polycaprolactone (PCL) end-capped with acrylic units to act as a crosslinker for PHEA. Indeed, this system was designed to have (i) biocompatible, biodegradable, and mechanical strong arms as well as (ii) a star structure with acrylic functionalities to promote the crosslinking of the material. 1H NMR, IR and TGA measurements confirmed the star PCL functionalization, which was carried out starting from a hydroxyl-terminated polymer. Hydrogels were prepared by varying the concentration of the synthesized star polymer, and the properties of the materials obtained by frontal (FP) and bulk polymerization (BP) were compared. Moreover, to evaluate the specific effect of PCL-star-4-2 k-tetraacrylate (PCL-TA) on the crosslinking of the systems, samples were also synthesized using a commercial acrylate crosslinker. For what concerns the frontal polymerization process, fronts were found to be stable in the presence of PCL-TA. The thermal characterization results showed a significant decrease in the PHEA glass transition temperature with increasing PCL-TA content, which was particularly evident in the samples prepared by FP. This result can be attributed to the partial miscibility of the two polymers, which become compatible during the polymerization process by forming a copolymer system. This was confirmed by analyzing the thermal behavior of hydrogels polymerized in the presence of a hydroxyl terminated PCL, i.e., a polymer without active functional end groups. The samples synthesized by FP with the star polymer and the commercial crosslinker, as well as neat PHEA, were subjected to mechanical tests. The mechanical behavior of PCL-based hydrogels was outstanding, exhibiting three time higher modulus than that of crosslinked PHEA, and were structurally stable even under high compression loadings. This remarkable property, combined with the fast and efficient polymerization method and the environmentally friendly properties of PCL, make the developed hydrogels promising systems for practical applications in various fields.

Radical photoinduced cationic frontal polymerization in porous media

AbstractTwo different interpenetrating phase composites were produced using a radical photoinduced cationic frontal polymerization process. The composites were based on polyurethane (PU) and aluminium open‐cell foams impregnated with a formulation of a cycloaliphatic epoxy with different concentrations of a cationic photoinitiator and a thermal initiator. The influence of both types of initiators on the frontal polymerization features was systematically evaluated for the PU foam. It was found to occur only when the concentration of both initiators was greater than 0.5 wt%, leading to full conversion of the epoxy in the whole volume of the 15 mm thick composite samples within less than 100 s. The maximum temperature reached by the propagation front was in the range 275–305 °C depending on the type of formulation, leading to pores in the epoxy phase and extensive degradation of the PU phase. In the case of the opaque aluminium foam, an additional layer of pure resin was required on the UV‐exposed surface, which corresponded to a critical mass of a few grams to ensure sufficient heat generation and trigger the front propagation. © 2020 Society of Chemical Industry

Frontal polymerization for smart intrinsic self‐healing hydrogels and its integration with microfluidics

ABSTRACTFrontal polymerization (FP) is applied for the synthesis of β‐cyclodextrin/poly(vinylimidazole‐co‐N‐vinylcaprolactam‐co‐acrylic acid) (β‐CD/P(VI‐co‐NVCL‐co‐AA)) copolymers. The dependence of frontal velocity and temperature on the initiator and cross‐linker are discussed. The synthesized copolymers have been characterized by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The thermo‐pH dual‐stimuli responsive behavior of the hydrogel is determined by swelling measurement at different temperatures and pH values. Besides, the hydrogels show intrinsic self‐healing behavior and their healing efficiency is determined by the mechanical tests. Interestingly, we integrate FP with microfluidic technology, which may realize the execution of FP under continuous condition. Such simple microfluidics‐FP integrated approach has both methodological and practical value for the synthesis of functional materials. This paper mainly presents the synthesis and characterization of β‐cyclodextrin/poly(vinylimidazole‐co‐N‐vinylcaprolactam‐co‐acrylic acid) (β‐CD/P(VI‐co‐NVCL‐co‐AA)) copolymers by using thermal frontal polymerization (TFP). Hydrogels were found to be self‐healing with good mechanical performance and show dual thermo‐pH responsive behavior. Low‐cost, energy‐saving and efficient method of thermal frontal polymerization process was integrated with microfluidics technology to prepare supraball hydrogel. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1412–1423

Study on the Frontal Polymerization of Polyurethane/Hollow Glass Microsphere Hybrid Material

In this article, we use propylene glycol (PPG), Chain expansion agent 1, 4-Butylene Glycol (BG), toluene diisocyanate (TDI) and hollow glass microsphere Sol as raw materials to prepare polyurethane/hollow glass microsphere hybrid materials by frontal polymerization. Here we studies how the front position varies with time and how the polymer viscosity influences the reaction rate in the frontal polymerization process, and through the comparisons with autoclave polymerization, draw to a conclusion that we can also get polyurethane/hollow glass microsphere hybrid materials by this new preparation and processing method.

Effects of Synthesis-Solvent on Characteristics of Poly(<i>N-Isopropylacrylamide</i>) Hydrogels Synthesized by Frontal Polymerization

The synthesis of thermo-responsive hydrogels poly(N-isopropylacrylamide )(PNIPAm) by frontal polymerization (FP) was carried out in four mixed solvents, i.e. dimethylsulfoxide (DMSO) mixing with water, ethanol, tetrahydrofuran (THF) and acetone, respectively. The influences of mixed solvent on frontal parameters, pore morphologies, swelling behavior of PNIPAm hydrogels were investigated. The pore of PNIPAm hydrogel synthesized in THF/DMSO was observed a big honeycomb cells, others were channel-like cells. Furthermore, the sample obtained in THF/DMSO had higher swelling ratio as compared to others, and the one synthesized in Water/DMSO showed the lowest. Above results indicated that fine-tuned poly(N-isopropylacrylamide) hydrogels can be prepared in short time and with easy protocol by adjusting mixed solvents in frontal polymerization process.

Advances and problems of frontal polymerization processes

The main stages of the development of frontal polymerization are presented. The processes taking place at cryogenic temperatures, under high (up to 5 kbar) pressures and usual conditions, when the polymerization is performed in glass ampoules without excessive pressure, are discussed. Depending on the Semenov parameter, the conditions of polymerization in low-temperature quasi-isothermal and high-temperature adiabatic or frontal thermal modes are considered. Theoretical and experimental data for the dependence of the front velocity on the various parameters. The influence of the nature of initiator, monomer, and dispersed inorganic on the reaction order with respect to the initiator is analyzed. Monomers polymerizing in the reaction front propagation are classified regarding their polymerization rates and boiling temperatures. The articles of various authors devoted to the synthesis of polymer materials and polymer-based composites difficult to obtain under conventional conditions are considered. The reactors of frontal polymerization in turbulent and laminar flows, widely applied in practice, are studied. Certain conclusions are made on the further development of frontal polymerization processes on the basis of data available in the literature.

Frontal Polymerization Synthesis of Starch‐Grafted Hydrogels: Effect of Temperature and Tube Size on Propagating Front and Properties of Hydrogels

The frontal polymerization process was used to produce superabsorbent hydrogels based on acrylic acid monomers grafted onto starch. Using a simple test tube which was nonadiabatic and permitted contact with air, the effects of initial temperature and tube size on the propagating front of grafting copolymerization and the properties of hydrogels were explored. The unrestricted access of the reaction mixture to oxygen delayed the formation of self-propagating polymerization front. The ignition time was markedly lengthened with the increasing of tube size attributed to the formation of large amounts of peroxy radicals. The front velocity dependence on initial temperature could be fit to an Arrhenius function with the average apparent activation energy of 24 kJ mol(-1), and on tube size to a function of higher order. The increase of the initial temperature increased the front temperature, which lead to more soluble oligomers and higher degree of crosslinking. The interplay of two opposite effects of oligomer and crosslinking determined the sol and gel content. An increase in tube size had two effects on the propagating front. One was to reduce heat loss. The other effect was to increase the number of escaping gas bubbles. The combined action of the two effects resulted in a maximum value of front temperature, an increase in sol content and a reduction in gel content with tube size. The highest swelling capacity of hydrogels was obtained when the initial temperature or tube size favored a formation of porous microstructure of hydrogels.

A numerical study of one-step models of polymerization: Frontal versus bulk mode

In free-radical polymerization, a monomer-initiator mixture is converted into a polymer. Depending on initial and boundary conditions, free-radical polymerization can occur either in a bulk mode (BP) or in a frontal mode (FP) via a propagating self-sustaining reaction front. The main goal of this paper is to study the role that bulk polymerization plays in frontal polymerization processes for various one-step kinetics models. We use numerical simulations to study the influence of reaction kinetics on one-dimensional frontal polymerization. We show that the long-time behavior of systems modeled with discontinuous distributed kinetics (e.g. step-function kinetics) significantly departs from the long-time behavior of systems modeled with Arrhenius kinetics. The difference is due to slow BP in the initial mixture of reagents, which influences both the speed and the long-time stability of the reaction front. Further, we show that for distributed kinetics a “true” FP is only possible for a steadily propagating, traveling-wave reaction front. When a front propagates in a pulsating mode, we demonstrate the existence of pockets of unreacted monomer behind the front. These pockets evolve via a bulk polymerization mechanism. A mathematical model of one-step free-radical frontal polymerization is identical to the model of gasless combustion, so bulk reactions play a role in the latter context, as well. However, fronts propagate much faster in combustion than in polymerization, and slow bulk reactions in regions ahead of the burning front can generally be neglected.

Propagating waves of network formation induced by light

The exposure of a photopolymerizable liquid (e.g., multifunctional thiol-ene) to ultraviolet radiation often leads to a propagating wavefront of network formation that invades the unpolymerized material from the illuminated surface of the photosensitive material. We theoretically describe this light-driven frontal photo-polymerization (FPP) process, which is the basis of many commercially important fabrication methods, in terms of an order parameter ϕ(x,t) characterizing the extent of monomer-to-polymer conversion, the temporally and spatially evolving optical attenuation μ(x,t) of the medium, and the height h(t) of the resulting solidified material. The non-trivial aspects of this frontal polymerization process derive from the coupling of μ(x,t) and the growing non-uniform network ϕ(x,t) and we consider limiting situations in which the optical attenuation increases (‘photodarkening’) or decreases (‘photobleaching’) in time to illustrate the general nature of FPP front propagation and the essential variables on which it depends. Since FPP fabrication of complex three-dimensional structures containing components having different material characteristics would greatly extend the practical utility of this method, we explore the influence of nanoparticle (silica, titania, and multi-wall carbon nanotube) additives on FPP front propagation. We also characterize the influence of temperature on the kinetics of FPP since this factor can often be controlled in practice. The experiments elucidate basic physical aspects of FPP and are well described by our model with a sensible variation of relevant physical parameters (optical attenuation and chemical rate constant) governing frontal growth. Our results are of interest both from the standpoints of complex structure fabrication and for understanding the fundamental nature of the FPP process.

Bistability of propagating front with spin‐mode in a frontal polymerization of trimethylopropane triacrylate

AbstractWe have studied a nonlinear dynamic behaviour in a frontal polymerization process. Frontal polymerization is a mode of converting a monomer into a polymer via a localized reaction zone that propagates as a front. Interest in studying the dynamic behaviour of such propagating fronts is growing. Many studies focus on the spin‐mode front propagation characterized by a nonplanar front having one or more high temperature regions which move in a helical path along the axis of the reaction vessel. In this study, the bistable character of the spin‐mode induced by reaction vessel diameter has been analyzed. First, a reactor with a discontinuously varying diameter for preliminary tests, and second, a conical reactor with a continuously varying diameter have been used. We have obtained evidence of bistability of the spin‐mode front in the frontal polymerization of trimethylopropane triacrylate under certain reaction conditions. Copyright © 2003 Society of Chemical Industry

Polymerization in microgravity as a new process in space technology

The paper briefly reviews the results of orbital experiments and terrestrial simulations performed by research teams from Russia, USA, Uzbekistan, Italy, and Germany to investigate the basic mechanisms of gravitational polymerization sensitivity. The occurrence of convective instability, the evolution of convective flows and the front instability are analyzed for different frontal polymerization processes. It is shown that these effects are responsible for inhomogeneity of the polymer synthesized in terrestrial conditions. The different ways of instability suppression and polymerization control are proposed. The possibilities of synthesising more homogeneous polymer materials in space compared to terrestrial specimens are demonstrated.

Interoperability parallel programs approach to simulate 3D frontal polymerization processes

The main object of this paper is to demonstrate the feasibility of coupling parallel codes in the framework of large scale scientific computing. The second aim is to provide a numerical tool to solve 3D frontal polymerization in liquid problems where the multiple physical scales require intensive computation on MIMD architecture. We extend our 2D modelization [14] of such phenomena by coupling a 3D reaction diffusion system well known in solid combustion [22] with the 3D Navier–Stokes equations written in Boussinesq approximation. We develop two separate parallel codes for each physical model, each code running on its own topological network of processors with its own data structures. The non-blocking communications, manage the interaction terms of the two physical models on each code, are performed through a Portable Inter Program Communication Library ( PIPCL). This communication library computes array-based communication schedules creating a one-to-one implicit mapping between each data representation. The data distribution of each code is hidden to the others, allowing an easy parallel program inter-operability on heterogeneous computers.

Mathematical modeling of interfacial gel polymerization

Frontal Polymerization (FP) is a process in which a spatially localized reaction zone propagates into a monomer converting it into a polymer. Two types of frontal polymerization processes have been observed experimentally. One is exothermic FP, which occurs due to diffusion of heat released in the polymerization reactions and which we have previously studied. The other is an isothermal FP process, also referred to as interfacial gel polymerization, which is due to mass diffusion of the species coupled with the gel effect. We propose and study a model of interfacial gel polymerization. We determine, both numerically and analytically, important characteristics of the process including the time-dependent propagation velocity of the reaction zone, the structure of the wave, and the distance traveled by the front before it breaks down due to reactions ahead of the front.

A model of frontal polymerization including the gel effect

Frontal polymerization is a process in which a spatially localized reaction zone propagates into a monomer, converting it into a polymer. This new approach to polymer production requires both theoretical and experimental study. Agreement between the existing theoretical and experimental work done on this subject has generally been fairly good. However, experimental results tend to show a higher degree of conversion than theoretical results. The reason for this discrepancy may be attributed to an autoacceleration of the polymerization rate which occurs when conversion has reached a certain point. This autoacceleration is due to a decrease in the termination rate caused by a phenomenon known as the gel effect. In this paper, we develop a mathematical model of the frontal polymerization process, taking the gel effect into consideration. Specifically, we determine how it will affect the degree of conversion, maximum temperature, and propagation velocity of the system.

Mathematical Modeling of Free-Radical Polymerization Fronts

Frontal polymerization is a process in which a spatially localized reaction zone propagates into a monomer, converting it into a polymer. In the simplest case of free-radical polymerization, a mixture of a monomer and initiator is placed into a test tube. Upon reaction initiation at one end of the tube, a self-sustained thermal wave, in which chemical conversion occurs, develops and propagates through the tube. We develop a mathematical model of the frontal polymerization process and analytically determine the structure of the polymerization wave, the propagation velocity, maximum temperature, and degree of conversion of the monomer. Specifically, we examine their dependence on the kinetic parameters of the reaction, the initial temperature of the mixture, and the initial concentrations of the initiator and monomer. Our analytic results are in good quantitative agreement with both direct numerical simulations of the model and experimental data (on butyl acrylate polymerization), which are also presented in the paper.