Free-radical Polymerization Systems Research Articles

Computer-aided design of initiator utilization strategies for vinyl chloride suspension polymerization with constant reaction rate

From the viewpoint of polymer reaction engineering (PRE), polymerization with constant reaction rate is beneficial for reducing manual operations and promoting production efficiency. Using vinyl chloride (VC) suspension polymerization as a model system, manipulating the radical concentration by radical concentration-dependent optimization (RCO)-based initiators mixing (IM) strategy or continuous initiator dosing (CiD) strategy enables the polymerization with constant reaction rate. Case studies have been done to illustrate how the RCO-based IM strategy works for binary initiator combination screening and how to conduct VC suspension polymerizations with different reaction rates using RCO-based IM strategy. Importantly, decomposition kinetic parameters estimated for virtual initiators can be used for guiding the design of the capable initiators. Moreover, in the single initiator systems, the applicability of the RCO-based CiD strategy has been demonstrated by conducting VC suspension polymerization with different reaction rates and different temperatures. Optimal dosing rate profiles of the initiator can be provided for the cases demonstrated. It is expected that both RCO-based IM and CiD strategies can be extended to other free radical polymerization systems.

Overcoming the penetration–saturation trade-off in binder jet additive manufacturing via rapid in situ curing

Binder jetting (BJ) enables the production of green parts for structurally complex ceramic, metal, or composite parts. However, the existence of a penetration–saturation trade-off (PSTO) in conventional BJ 3D printing hinders its development. Since an increase in saturation is often accompanied by an increase in penetration distance of binder (P), it is difficult to produce high strength and high precision green parts; this limitation is not conducive to the development of new material systems. To overcome this limitation, a methacrylate-based, thermally activated free radical polymerization system was used in this study as a binder to produce green parts under in situ curing conditions, created by increasing the temperature of the powder bed. The penetration of the powder bed by the binder is controlled by regulating its curing status during printing. The proposed rapid in situ curing printing process overcomes the PSTO associated with conventional BJ and can inform the optimization of binder formulation, equipment design, and binder jet printing. • Acrylate-based binder cures by thermally activated free radical polymerization. • The binder cures rapidly in a heated powder bed, limiting penetration distance. • In situ cured green parts demonstrate high dimensional accuracy and strength.

Terthiophene Derivative-Based Photoinitiating Systems for Free Radical and Cationic Polymerization under Blue LEDs

Terthiophene derivatives were prepared and utilized as photoinitiators of photopolymerization reactions upon exposure to LEDs (410 and 445 nm). The photochemical mechanisms of the terthiophene-based photoinitiating systems were investigated and confirmed by steady-state photolysis, fluorescence quenching, and electron paramagnetic resonance spin-trapping techniques. The photoinitiation ability of terthiophene derivatives in combination with the iodonium salt and optionally with a tertiary amine in the free radical polymerization of multifunctional acrylates and cationic polymerization of epoxides under mild light irradiation was investigated. Exceptionally, a sodium salt form of a terthiophene derivative presented exceeding water solubility and admirable efficiency in rapid hydrogel formation from the 2-hydroxy ethyl acrylate and water blend. The storage stability of the effective photoinitiating systems in the formulations was also studied under moderate conditions.

Naphthoquinone derivatives: Naturally derived molecules as blue-light-sensitive photoinitiators of photopolymerization

Two naphthoquinone derivatives, i.e. 5-hydroxy-1,4-naphthoquinone (5HNQ) and 2-hydroxy-1,4-naphthoquinone (2HNQ), combined with different additives such as diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), phenacyl bromide (R-Br), N-phenyl glycine (NPG) and ethyl 4-dimethylamino benzoate (EDB), are investigated as photoinitiating systems (PISs) for free radical polymerization of trimethylolpropane triacrylate (TMPTA) and cationic polymerization of triethyleneglycol divinyl ether (DVE-3). The photopolymerization of the thiol-ene system trimethylolpropane tris(3-mercaptopropionate) (Trithiol)/DVE-3 with 5HNQ or 2HNQ based PISs are also investigated. 5HNQ shows maximum absorption in visible light region (blue light) which thus has the potential to perform under the irradiation of blue LED light. For the free radical polymerization of TMPTA, 5HNQ/Iod/NPG shows the highest photoinitiating ability among all the naphthoquinone derivative-based PISs, while 2HNQ-based systems are inefficient. The addition of Trithiol to TMPTA can significantly increase the double bond conversion of TMPTA during the photopolymerization. For the cationic photopolymerization of DVE-3, 2HNQ/Iod shows the highest photoinitiating ability among all the naphthoquinone derivative-based photoinitiating systems. 5HNQ-based systems also demonstrate good photoinitiation ability for DVE-3, but with slower polymerization rate compared to 2HNQ-based systems. In thiol-ene photopolymerization, 5HNQ PISs shows better photoinitiation ability than 2HNQ PIs systems. The results illustrate that the position of hydroxyl group in 1,4-naphthoquinone molecules plays an important role on photoinitiation ability of naphthoquinone derivatives. Steady state photolysis and electron spin resonance trapping experiments are also carried out to evaluate the relevant photochemical mechanism.

Novel ketone derivative-based photoinitiating systems for free radical polymerization under mild conditions and 3D printing

Novel ketone derivatives as efficient photoinitiators for the free radical polymerization of acrylates and application in 3D printing.

A geometric observer design for a semi-batch free-radical polymerization system

In this work, a geometric observer (GO) is formulated and tested using experimental data from the Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) system for a semi-batch free-radical polymerization reactor that synthetizes polyacrylamide. The available measurements include the weight-average molecular weight, the concentration of monomer, and the volume of internal contents. Different combinations between innovated states and measurements offer a number of possible structures of the GO. The computation of the minimum singular values and condition numbers permit to select the most promising structures. To overcome inadequacies observed in full-order architectures, low order GOs with passive structures are explored. The best observer is selected, compared with a standard estimation strategy and tested under various experimental operating conditions. The observer performance is evaluated qualitatively and quantitatively as a tradeoff between dynamic property estimation and signal processing.

Mathematical modeling of frontal polymerization

Frontal polymerization is way to convert liquid resin into a solid material with a self-propagating reaction. The reaction spreads like a flame from the heat of the reaction that diffuses into neighboring regions, starting more reaction. The frontal velocity has been accurately modeled for free-radical polymerization systems. The dynamics of fronts have been studied theoretically and experimentally. If the viscosity of the initial medium is low, then fronts can become unstable due to buoyancy-driven convection. A fascinating aspect of frontal polymerization is that fronts often do not propagate as a plane waves but exhibit complex modes such as “spin modes” and chaos. The kinetics of the polymerization significantly affects the onset of these modes. Multifunctional acrylates exhibit more complex dynamics than monoacrylates. Using multifunctional acrylates and inorganic fillers, 3P LLC created “cure-on demand” systems that do not require mixing before use, have a long shelf life and can be hardened in seconds to minutes. We consider two commercial products using frontal polymerization. The first is a wood filler that can be applied to a damaged section of wood and hardened in a few seconds by the application of heat to the surface. The second product is QuickCure Clay (QCC). QCC has an unlimited working time during which it can be sculpted. QCC is then cured by heating part of the object to 100 °C, setting off the propagating curing front. The modeling of frontal polymerization helped guide the development of these products.

A Passivation Approach to the Control Design of Non-Passive Nonlinear System

This work proposes a tracking error passivity-based multivariable control via feedback passivation for a class of free-radical polymerization systems in a continuously stirred tank reactor (CSTR). Firstly, this system dynamics with high nonlinearity is passivized by input coordinate transformations, the resulting passive system is then re-written into a canonical form strongly related to the so-called Port Control Hamiltonian structure. Actually, this representation allows to show the physical meanings of system dynamics such as dissipative, non-dissipative terms and supply rate. From this, a feedback controller based on tracking error is designed for the globally exponential stabilization at an arbitrarily chosen reference trajectory passing the desired equilibrium point. The theoretical developments are then illustrated for polystyrene polymerization system in the CSTR. The numerical simulations show that the trajectories of styrene polymerization system considered as an illustrative example of FRP system converges globally exponentially to the imposed trajectories.

A Passivity Approach to the Stabilization of Free-Radical Polymerization Reactor

This work proposes a tracking error passivity-based multivariable control viafeedback passivation for a class of free-radical polymerization systems in a continuouslystirred tank reactor (CSTR). Firstly, this system dynamics with high nonlinearity ispassivized by input coordinate transformations, the resulting passive system is then rewritten into a canonical form strongly related to the so-called Port Control Hamiltonianstructure. Actually, this representation allows to show the physical meanings of systemdynamics such as dissipative, non-dissipative terms and supply rate. From this, a feedbackcontroller based on tracking error is designed for the globally exponential stabilization atan arbitrarily chosen reference trajectory passing the desired equilibrium point. Thetheoretical developments are then illustrated for polystyrene polymerization system inthe CSTR. The numerical simulations show that the trajectories of styrene polymerizationsystem considered as an illustrative example of FRP system converges globallyexponentially to the imposed trajectories.

Copper-Based (Photo)redox Initiating Systems as Highly Efficient Systems for Interpenetrating Polymer Network Preparation

Simultaneous redox free radical polymerization (FRP) and redox cationic polymerization (CP) are combined for the synthesis of methacrylate/diepoxide interpenetrating polymer networks (IPN). At first, the Cu(acac)2/phosphine/iodonium salt operates according to the principles of free radical promoted cationic polymerization (FRPCP). A photoactivation of the reaction for that system was necessary to enhance the mild methacrylate and diepoxide conversions. Second and at least, two complementary copper catalytic cycles are used simultaneously: the recently developed Cu(II)/reducing agent/peroxide FRP system combined with the older Cu(II)/reducing agent/iodonium salt redox CP system. For this latter hybrid system, outstanding efficiency was observed with more than 90% of epoxy functions conversion for the cationic difunctional monomers and 78% conversion for the vinylic functions conversion for difunctional monomers. The radical and cation generations are discussed in order to fill the interrogations raised by ...

Improving mechanical and electrical properties of poly(vinyl alcohol-g-acrylic acid) nanocomposite films by reinforcement of thermally reduced graphene oxide

Graphene has drawn an astounding research interest in recent years, owing to its exceptional properties. The scaled-up and trustworthy production of graphene derivatives, such as graphene oxide (GO) and thermally reduced graphene oxide (TRGO), offer wide variety of possibilities to synthesize graphene-based polymer materials for various applications. In the present work, poly(vinyl alcohol-g-acrylic acid) films were prepared by grafting polyacrylic acid chains onto polyvinyl alcohol backbone employing a free radical polymerization system in the presence of crosslinking agent, N,N′-methylenebisacrylamide (MBA). The graphene was homogeneously dispersed into the prepared polymer using thermal mechanical agitation technique. The so prepared nanocomposites were undertaken for structural and morphological characterization using FTIR, SEM, XRD, Raman Spectroscopy, DLS, Zetasizer and AFM analysis, respectively. The electrical conductive and mechanical properties of prepared nanocomposite films were also investigated.

New copper(i) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference

Novel copper complex based initiating systems for redox free radical polymerization (FRP) of methacrylate resins under mild conditions are proposed.

Double role of squarylium dye: Fluorescence probe for biomolecule determination and photosensitizer in dyeing photoinitiating system

In the present paper, the double role of one of the squarylium dyes is presented. First it was shown the role of 1,3-bis(N-ethyl-2-methylenebenzoxazole)squarine as a spectroscopic probe for bovine serum albumin determination. Next the photoinitiating ability of squarylium dye incorporated in two- and three-component systems for free radical polymerization of trimethylolpropane triacrylate has been investigated. In such system squarylium dyes act as sensitizer (primary light absorber). The radical photopolymerization of an acrylate was observed upon irradiation with ion-argon laser.

Studies of Free Radical Polymerization Initiated by Visible Light Photoredox Catalysis

A visible-light-induced free radical polymerization has been developed. Distinct from living radical polymerizations, this polymerization process is not controlled in the chain growth step but the initiation step, by in situ generation of a constantly low concentration of initiator radicals. “On”–“off” experiments demonstrate the ease of controllability of the process, which indicate that it may be an improved method for radical polymerization, since it can be initiated immediately by powering on a light, without any process of preheating the reaction mixture, and can also be sharply stopped by turning off the light, without any cooling process. By adjusting the photo irradiation strength, reaction temperature, and the amount of the initiator system, the molar mass as well as conversion rate can be accurately tuned, which is not easy to realize in other free radical polymerization systems. Notably, a molar mass as high as 5.64 × 105 g mol−1 (Đ = 1.71) is achieved with this new methodology.

Prediction interval-based modelling of polymerization reactor: A new modelling strategy for chemical reactors

Precise and reliable modelling of polymerization reactor is challenging due to its complex reaction mechanism and non-linear nature. Researchers often make several assumptions when deriving theories and developing models for polymerization reactor. Therefore, traditional available models suffer from high prediction error. In contrast, data-driven modelling techniques provide a powerful framework to describe the dynamic behaviour of polymerization reactor. However, the traditional NN prediction performance is significantly dropped in the presence of polymerization process disturbances. Besides, uncertainty effects caused by disturbances present in reactor operation can be properly quantified through construction of prediction intervals (PIs) for model outputs. In this study, we propose and apply a PI-based neural network (PI-NN) model for the free radical polymerization system. This strategy avoids assumptions made in traditional modelling techniques for polymerization reactor system. Lower upper bound estimation (LUBE) method is used to develop PI-NN model for uncertainty quantification. To further improve the quality of model, a new method is proposed for aggregation of upper and lower bounds of PIs obtained from individual PI-NN models. Simulation results reveal that combined PI-NN performance is superior to those individual PI-NN models in terms of PI quality. Besides, constructed PIs are able to properly quantify effects of uncertainties in reactor operation, where these can be later used as part of the control process.

Molecular weight/branching distribution modeling of low-density-polyethylene accounting for topological scission and combination termination in continuous stirred tank reactor

We present a comprehensive model to predict the molecular weight distribution (MWD),11Molecular weight distribution. and branching distribution of low-density polyethylene (ldPE),22Low density polyethylene. for free radical polymerization system in a continuous stirred tank reactor (CSTR).33Continuous stirred tank reactor. The model accounts for branching, by branching moment or pseudo-distributions. The common free radical polymerization reactions including chain scission have been considered in the model. Non-linear or the so-called topological scission has been modeled using approximate fragment length distributions derived from scission, applied to branching topologies. To model the distributions, the Galerkin-FEM method based on the same principles as PREDICI® has been applied and implemented in MATLAB©. The fundamental numerical problem arising from topological scission has been solved. Thus, the model provides more accurate results, allowing a precise comparison to earlier results and to Monte Carlo simulations.

Negative Contrast Curves for Two-photon Free Radical Polymerization Systems and Their Potential Applications in Sub-diffraction Limited Two-photon Photolithography

ABSTRACT:Two-photon fabrication is a powerful method of fabricating complex microstructures. Superresolution by methods analogous to stimulated emission depletion (STED) has been described previously, enabling sub-100 nm imaging with 800 nm light. STED-related methods of enhancing imaging resolution require photoresists with exposure conditions for which the photoresist exhibits negative contrast, i.e., image density decreases with increasing exposure from the depletion beam. We have observed decreasing voxel size with increasing exposure during two-photon initiated polymerization of acrylate- and methacrylate-based photoresists, that is, negative imaging contrast, γ < 0, independent of the type of photoinitiator. Negative contrast is not observed in epoxy-type photoresists containing photoacid generators. An investigation of the exposure conditions has led us to conclude that radical-radical recombination at high exposure is responsible for negative contrast. Results of the investigation, discussion of the proposed mechanism for negative contrast and implications for two-photon superresolution will be presented.

Silver Nanoparticles Coated with Thioxanthone Derivative as Hybrid Photoinitiating Systems for Free Radical Polymerization

A new type of photoinitiator for free radical polymerization was synthesized and characterized. 2-(11-Mercaptoundecyloxy)thioxanthone (1) was anchored at the surface of silver nanoparticles (NPs), and the interaction of plasmon field generated in the immediate vicinity of Ag NPs carrying the chromophores was evaluated. The optical features and structure of the silver-initiator nanoassemblies (Ag@1) were characterized by UV-vis and fluorescence spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). TEM and XRD studies revealed the presence of ca. 5-6 nm diameter Ag NPs, and XPS also confirmed the successful anchorage of 1 at their periphery. The nanoassemblies Ag@1 were successfully used as macroinitiator for radical polymerization of acrylate monomers, triggered photochemically, to obtain Ag(0)-polyacrylate nanocomposite materials. The nanocomposite materials synthesized with the use of Ag@1 exhibit attractive possibilities for patterning the surface of thin films.

Free radical formation in three-component photoinitiating systems

N,N′-diethylthiacarbocyanine iodide (Cy) with two different compounds acting as co-initiators were used in the three-component photoinitiating systems for free radical polymerization of acrylate monomer. The steady state and time-resolved techniques were used to study the fluorescence quenching by co-initiators, as well as laser flash photolysis to investigate an electron transfer process, that occurs in these photoinitiating system (PIS). To investigate the key factors involved with visible-light activated radical polymerizations involving three-component photoinitiators, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with thiacarbocyanine dye as a photosensitizer. The Rehm–Weller equation was used to verify the thermodynamic feasibility for electron transfer reaction. Experiments show, that the main interaction between the dye and both co-initiators occurs through its excited singlet state. It was concluded, that the key kinetic factors for efficient visible-light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step to consuming dye-based radical and regenerating the original photosensitizer.Basing on the study, it is known that, there are two possible mechanisms of generation of free radicals, which can start polymerization involving photoreducible series mechanism and parallel series mechanism (photoreducible–photooxidizable series mechanism).

Photoinitiating behavior of benzophenone derivatives covalently bonded tertiary amine group for UV‐curing acrylate systems

AbstractA series of benzophenone derivatives (N‐BPs) containing tertiary amine group used as hydrogen abstraction‐type (type II) photoinitiators were synthesized through the addition reaction of secondary amines with 4‐(2,3‐epoxypropyloxy) benzophenone. The chemical structures were characterized with 1H NMR, FTIR spectroscopy, and UV spectrum measurements. The N‐BPs showed the higher absorption in 300–400 nm than benzophenone (BP). The photoinitiating activity was examined based on the photopolymerization of 1,6‐hexanediol diacrylate using photo‐DSC method. The results showed that the photoinitiating efficiency was negatively affected by the molecular structure of alkyl group connected to the tertiary amine with the order of isopropyl (N‐BPI) < methyl (N‐BPM) < ethyl (N‐BPE) < propyl (N‐BPP). Moreover, the diethanolamine‐modified benzophenone derivative (N‐BPOH) had the highest‐photoinitiating efficiency for free radical polymerization systems among the N‐BPs. Copyright © 2011 John Wiley & Sons, Ltd.