Enthalpy Of Polymerization Research Articles

In‐plane and in‐depth frontal polymerization behaviors of continuous fiber‐reinforced epoxy composites

AbstractFrontal polymerization (FP) is a self‐sustaining reaction that relies on polymerization exothermicity and heat transfer. This study explores the in‐plane and in‐depth FP mechanisms of continuous fiber‐reinforced epoxy composites. First, the effects of initiator and diluent concentrations on the FP behaviors of neat epoxy resins were examined. It was found that the frontal velocity and frontal temperature of the neat resins increase with an increase of either the initiator concentration or the diluent content, depending on the polymerization enthalpy and curing kinetics. Then, the FP behaviors of continuous carbon fiber and glass fiber‐reinforced epoxy woven composites were investigated. The results showed that the in‐plane FP behavior of the composites was primarily controlled by the thermal conductivity and specific heat capacity of the continuous fibers, whereas the in‐depth FP behavior mainly depended on the pores of the woven fabrics.Highlights Frontal velocity and temperature of neat resins are controlled by the polymerization enthalpy and curing kinetics. The in‐plane FP behavior of continuous fiber reinforced composites is dependent on the thermal conductivity and specific heat capacity of the fibers. The in‐depth FP behavior of composite is determined by both the thermal conductivity of fibers and the pores of fabrics.

The Effects of Ionic Liquid Addition on the Heat of Polymerization of Additive Manufacturing Resins

Ionic liquids (ILs) are utilized for a wide range of applications today due to their low vapor pressure, near infinite combination of cations and anions with varying chemical functionalities, organic and inorganic species solubility, as well as their superb thermal and chemical stabilities. In this work, four ILs are used as additives for 3D printing resins to add flexibility to the final printed parts. The ILs of interest are 1-n-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide ([C4mim][NTf2]) and 2-(1-methylimidazolium)-ethyl methacrylate chloride ([mimEMA][Cl]), which were synthesized in-house, as well as tetraethylammonium bis(trifluoromethanesulfonyl)imide ([TEA][NTf2]) and Butyltrimethylammonium bis(trifluoromethanesulfonyl)imide ([BTMA][NTf2], which were purchased and used as received. These ILs were added to commercial Formlabs CV4 resin from 10 to 50 wt%. The change in temperature of polymerization is characterized via container-less acoustic levitation and FLIR thermography and the heat capacity is characterized with differential scanning calorimetry (DSC). From these two measurements, the enthalpy of polymerization was calculated via a simple equation relating the change in droplet temperature to the reaction enthalpy equation and compared to literature values. The degradation temperature of each was also characterized via thermal gravimetric analysis (TGA). Each IL lowered the enthalpy of polymerization while the substances with [NTf2] counterions raised the decomposition temperature and the only substance containing Cl anions, [mimEMA][Cl], lowered the decomposition temperature.

Synthesis and photoinitiated cationic polymerization of epoxidized phenylpropanoid and α-pinene derivatives

Phenylpropanoides and myrtenol were converted into laurates in a first reaction step followed by epoxidation of the olefinic structure using m-chloroperbenzoic acid to synthesize new epoxy monomers. These are epoxy eugenyl laurate (EEL), epoxy isoeugenyl laurate (EiEL), epoxy cinnamyl laurate (ECL), and epoxy myrtenyl laurate (EML). Cationic photoinitiated polymerization of the new epoxy monomers melting below or slightly above room temperature results in oligomers with a degree of polymerization between 2 and 6. The low degree of polymerization causes a glass transition temperature of the new oligoethers below the polymerization temperature indicating the neglecting influence of vitrification on the photoinitiated polymerization of these epoxy monomers. Significant differences were found regarding the kinetics of the cationic photoinitiated polymerization between the single new epoxy monomers and epoxy anethol (EA) as well as epoxy estragol (EE). The molar polymerization enthalpy determined for epoxy monomers undergoing quantitative conversion differs from the value for ethylene oxide that is known from the literature. This indicates the influence of the substituents bound at the epoxy group on the polymerization kinetics.

Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation.

A central challenge in the development of next-generation sustainable materials is to design polymers that can easily revert back to their monomeric starting material through chemical recycling to monomer (CRM). An emerging monomer class that displays efficient CRM are thiolactones, which exhibit rapid rates of polymerization and depolymerization. This report details the polymerization thermodynamics for a series of thiolactone monomers through systematic changes to substitution patterns and sulfur heteroatom incorporation. Additionally, computational studies highlight the importance of conformation in modulating the enthalpy of polymerization, leading to monomers that display high conversions to polymer at near-ambient temperatures, while maintaining low ceiling temperatures (Tc). Specifically, the combination of a highly negative enthalpy (-19.3 kJ/mol) and entropy (-58.4 J/(mol·K)) of polymerization allows for a monomer whose equilibrium polymerization conversion is very sensitive to temperature.

Toward Recyclable Polymers: Ring-Opening Polymerization Enthalpy from First-Principles.

Ring-opening polymerization (ROP) enthalpy ΔHROP is an important thermodynamic property controlling the polymerization of cyclic monomers. While ΔHROP can be measured, computing ΔHROP for realistic polymer systems with an error of ≃5-10 kJ/mol is critical for designing new monomer systems for depolymerizable polymers. We have developed a first-principles computational scheme in which multiple challenges in computing ΔHROP are resolved definitively including extensive exploration of conformational states and adequately addressing finite size effects. This scheme is validated on a diverse benchmark set of 42 ROP polymers for which reliable experimental values of ΔHROP are available. For this set, the ΔHROP root-mean-square error is ≃7 kJ/mol, about 3-times smaller than conventional approaches. This development opens up new pathways to build up a high-quality database of ΔHROP for downstream predictive machine-learning models and ultimately to accelerate the design of depolymerizable polymers with desired properties.

Antimicrobial agents in dental restorative materials: Effect on long-term drug release and material properties.

The present study reports on the long-term drug release and mechanical properties of bioactive dental filling materials based on chlorhexidine diacetate (CHX) or octinidine (di)hydrochloride (ODH) incorporated in a composite based on dimethacrylates or an ormocer. CHX or ODH were added to a nano-hybrid ormocer (O) and a nano-hybrid composite (C) with the amount of 2wt% to achieve four matrix-drug combinations: O-CHX, O-ODH, C-CHX, and C-ODH. Drug extraction and release were measured using high-performance liquid chromatography with diode-array detection (HPLC-DAD), while drug distribution was assessed by using energy dispersive X-ray spectroscopy (EDX). Drug release in water at 37°C was observed over 87 d. To determine the material properties, the water absorption, water solubility, flexural strength and hardness were measured and compared to the reference materials. Persistent drug release over 87 d was observed for both ODH-based systems and both ormocer-systems, with the longest duration of activity seen for the O-ODH combination. Persistent drug release was achieved via the loosening of the polymer network indicated via decreasing polymerization enthalpies, enhanced water absorption, and water solubility. As a consequence, the flexural strengths of the materials were reduced. However, surface hardness was hardly reduced. ODH seems to be more adequate than CHX for the design of bioactive dental filling materials based on nano-hybrid ormocer and composites.

Antimicrobial agents in dental restorative materials: Effect on polymerization, short-term drug release and biological impact.

With the aim to design bioactive dental restorative material, the present study investigated the influence of the antimicrobial agents chlorhexidine diacetate (CHX) and octinidine (di)hydrochloride (ODH) when incorporated in two different materials. Selected parameters were polymerization enthalpy, short-term drug release, and the effect on Streptococcus mutans as well as human gingival fibroblasts. Samples were made by mixing a nano-hybrid ormocer (O) and a methacrylate-based nano-hybrid composite (C), each with a mass fraction of 2% CHX or ODH. Release profiles and concentrations of active agents from the resins were assessed, and the cell proliferation of human gingival fibroblasts as well as Streptococcus mutans cultured with the eluates were evaluated. The influence on polymerization was assessed by means of differential scanning calorimetry. Both drugs, especially ODH, showed a decreasing effect on polymerization enthalpies associated with a lowered crosslinking degree. At the same time ODH appeared to be released more persistently than CHX. Moreover, ODH was more efficient with regard to bacteria growth inhibition but also more cytotoxic in terms of reduction of cell viability. ODH is deemed more appropriate for application in a dental resin-based drug delivery system, because of the more persistent drug release than seen for CHX.

Renewable furan-derived diamine as primary amine source to prepare fully bio-based bis-benzoxazine monomer under solvent-free condition

Fully bio-based benzoxazine is classified into sustainable high-performance thermosetting resin and has become current focus in polybenzoxazine field. But lack of bio-based primary diamine restricted the preparation of bis-benzoxazine with high biomass content. In this report, renewable furan-containing diamine (PDFA) derived from furfurylamine was condensed with cardanol/eugenol and formaldehyde under solventless condition to obtain two biomass-based bis-benzoxazine monomers of BBOZ-Ca and BBOZ-Eu. The curing behaviors of two monomers were investigated by non-isothermal DSC method and the results showed that BBOZ-Ca possessed broad processing window comparing with BBOZ-Eu, the characteristic polymerization temperature and activation energy of two monomers were very close to each other, but the total heat releasing (enthalpy of polymerization) of BBOZ-Eu was higher than that of BBOZ-Ca due to its high oxazine content. The TGA results indicated that cured BBOZ-Eu had better thermal resistance than cured BBOZ-Ca, the char yield of cured BBOZ-Eu was 65.4% at 850 °C. Copolymerization with BBOZ-Eu effectively increased the char yield of cured BBOZ-Ca. Isothermal DMA was utilized to further investigate the curing kinetics and the results showed that the storage modulus increased rapidly when polymerization was initiated. The surface properties of cured films were studied using scanning electron microscope (SEM) images and contact angle measurement.

A Thermodynamic Roadmap for the Grafting-through Polymerization of PDMS11MA.

Grafting-through atom transfer radical polymerization (ATRP) was used to polymerize a sterically hindered poly(dimethylsiloxane) methacrylate (PDMS11MA, Mn = 1000) macromonomer to high conversion as a function of temperature, solvent, initial monomer concentration, and pressure. Higher polymerization yields were obtained when polymerizations were conducted at (i) lower temperature (T), (ii) in a poor solvent for the side chain, (iii) higher initial monomer concentration ([M]0), and (iv) higher pressure by mitigating the contribution of the equilibrium monomer concentration ([M]eq). The enthalpy of polymerization (ΔHp) and entropy of polymerization (ΔSp) were more negative in poor solvents. Polymerizations at ambient pressure required higher [M]0, use of a poor solvent, and lower temperatures to reach higher conversion with good control, whereas high pressure ATRP (HP-ATRP) displayed better control under dilute conditions. Grafting-through polymerization at high P and higher [M]0 was less controlled, plausibly due to limited solubility and mobility of the copper catalyst in the highly viscous medium.

Synthesis and Characterization of Low-Cost Cresol-Based Benzoxazine Resins as Potential Binders in Abrasive Composites.

A series of cresol-based benzoxazines were synthesized for potential application as a polymer matrix in abrasive composites. The chemical structures of the obtained benzoxazine resins were investigated in detail using Fourier transform infrared spectroscopy (FTIR) and hydrogen-1 as well as carbon-13 nuclear magnetic resonance spectroscopy (1H NMR, 13C NMR) with an additional analysis using two-dimensional NMR techniques (2D NMR 1H-1H COSY, 1H-13C gHSQC and gHMBC). Structural analysis confirmed the presence of vibrations of -O-C-N- at ~950 cm−1 wavenumber, characteristic for an oxazine ring. The thermal properties of benzoxazine monomers were examined using differential scanning calorimetry (DSC) analysis. The polymerization enthalpy varied from 143.2 J/g to 287.8 J/g. Thermal stability of cresol-based benzoxazines was determined using thermogravimetry (TGA) analysis with additional analysis of the amount of volatile organic compounds (VOC) emitted from the synthesized benzoxazines during their crosslinking by static headspace coupled with gas chromatography technique (HS-GC). The amount of residual mass significantly differed between all synthesized polybenzoxazines in the range from 8.4% to 21.2%. The total VOC emission for benzoxazines decreased by 46–77% in reference to a conventional phenolic binder. The efficiency of abrasive composites with the benzoxazine matrix was evaluated based on abrasion tests. Performed analyses confirmed successful synthesis and proper chemical structure of cresol-based benzoxazines. All the experiments indicated that benzoxazines based on different cresol isomers significantly differ from each other. Good thermal performance and stability of the abrasive composites with the polybenzoxazine matrix and significantly lower VOC emission allow us to state that benzoxazines can be a promising and valuable alternative to the phenolics and a new path for the development of modern, eco-friendly abrasives.

Poor Solvents Improve Yield of Grafting-Through Radical Polymerization of OEO19MA.

Radical polymerization of poly(ethylene glycol) methyl ether methacrylate (OEO19MA, Mn ∼ 950) at an initial monomer concentration of 150 mM was investigated as a function of solvent composition. Conventional and controlled radical polymerizations in anisole at 60 °C converged at approximately the same equilibrium monomer concentration ([M]eq) of ∼38 mM, suggesting that livingness or diminished termination did not affect the thermodynamic parameters of polymerization. Conventional radical polymerizations (RPs) in anisole, dimethylformamide (DMF), toluene, and 1×PBS buffered water were taken to approximately 98% thermal initiator decomposition to determine [M]eq at reaction completion within a broad temperature range. The enthalpy (ΔHp) and entropy (ΔSp°) of polymerization were solvent-dependent. Polymerizations in 1×PBS were the most thermodynamically favorable, followed by those in DMF, toluene, and anisole. -ΔHp and -ΔSp increased with the square of the difference in the Hansen solubility parameters of poly(ethylene glycol) and the solvent. It is proposed that poor solvents favor polymer-polymer interactions over polymer-solvent interactions, which improves the thermodynamic polymerizability.

Effect of Additives on Cure Kinetics of Pultrusion Resins

Cure kinetics of pultrusion resins differ from the kinetics of resins used in other methods of composites production due to the presence of technological additives. Such additives add useful properties to the composite, for example, reduce the friction of the composite on a die or reduce the combustibility of the composite. This work is devoted to studying the effect of additives on vinyl ester resin cure kinetics. The curing process was characterized using differential scanning calorimetry (DSC). The effects of the addition of Zinc-stearate and Aluminum-hydroxide on vinyl ether ATLAC 430 resin were analyzed with dynamic scans at different heating rates. The obtained kinetics clearly showed that additives influence the resin polymerization enthalpy. The effect of additives on the enthalpy also transferred to the obtained kinetic constants of their models. Moreover, additives can effect on the model error - this effect clearly demonstrates the melting of zinc stearate, which is superimposed on the final polymerization stage of the studied resin. Results show that the calculation of kinetics in industrial production should be carried out considering all the technological additives to optimize the process and to obtain a large number of high-quality pultrusion profiles. Also, the results clearly show that there is a need to further study the effect of additives on the kinetics of polymerization of pultruded resins.

Effects of multiwall carbon nanotubes on the polymerization model of aniline

In this paper, chemical polymerization of aniline and aniline/multi-walled carbon nanotubes (MWCNTs) is investigated, and a modified polymerization model is proposed based on the obtained experimental results. In the experimental section, the variation in temperature of the reaction mixture was measured during the polymerization reaction. According to the obtained thermographs, it was concluded that the polymerization of aniline or aniline/MWCNTs takes place in three different steps. In the first step, some nuclei of phenazine-type trimers are formed for pure aniline while for aniline/MWCNTs, aniline molecules are adsorbed on the nanotubes surfaces. In the second step, cation-radical polymerization occurs at a high speed where there are a significant number of monomers around the growing chains. It is proposed that in the third step a new polymerization reaction begins on the pre-synthesized polymers. For pure PANI, it seems that only oligomers are created in this step while for PANI/MWCNTs, long chain polymers can also grow. Experimental results show that the enthalpy of polymerization reduces while polymerization yield increases with the weight percent of MWCNTs, which can be explained by the new observed polymerization model.

Examining the Influence of Organophosphorus Flame Retardants on the Thermal Behavior of Aromatic Polybenzoxazines

Abstract2,2‐bis(3,4‐Dihydro‐3‐phenyl‐2H‐1,3‐benzoxazine)propane (BA‐a) is blended with three commercial organophosphorus compounds, bis(4‐ hydroxyphenyl)phenylphosphine oxide (BHPPO), diphenyl phosphorami‐date (DPPA), and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO), at different loadings (1–10 wt%). Incorporation of all dopants results in a reduction in the initiation temperature for the curing mechanism (by some 30–60 K), but only the DOPO truly moves the curing into a lower temperature regime by decreasing the end temperature by 25 K (DPPA lowered this by 5 K). DPPA and DOPO are also shown to have a significant effect on the enthalpy of polymerization each decreasing the value by 50 J g−1. Differential scanning calorimetry rescan indicates all three additives have a positive impact on the glass transition temperature of poly(BA‐a) with increases of 9 K (BHPPO) and 14 K (DPPA, DOPO). Thermogravimetric analysis indicates that all additives have little effect on the magnitude of char formed (BHPPO and DPPA both increased the char yield by 3%, whereas DOPO reduces the nal char yield by 3%). However, importantly, the mechanism of thermal degradation for poly(BA‐a) differs for all three additives and is shown to change with increasing concentra‐ tion. DOPO has the greatest effect on promoting the formation of char at elevated temperatures.

Ring-opening copolymerization thermodynamics and kinetics of γ-valerolactone/ϵ-caprolactone.

The general misconception that γ-lactones are not thermodynamically polymerizable has limited the development of all γ-lactone-based copolymers. A few studies have reported copolymerization of these five-membered cyclic esters with more reactive monomers, yet a systematic investigation of kinetics and thermodynamics is still lacking. To explore the feasibility of the reaction, we combined equilibrium and non-isothermal syntheses for the copolymerization of γ-valerolactone with ϵ-caprolactone, initiated with methoxy polyethyleneglycol and catalyzed by Tin(II) 2-ethylhexanoate. Here, we present the polymerization kinetic and thermodynamic parameters for different monomer ratios in the reaction feed. We observed the dependency of enthalpy and entropy of polymerization upon monomer ratio changes, and estimated a linear increase in the activation energy by increasing the γ-valerolactone fraction in the starting monomer mixture. Our data demonstrate that γ-valerolactone can copolymerize with ϵ-caprolactone, but only under specific conditions. The reaction parameters determined in this study will enable preparation of additional γ-valerolactone-based copolymers and development of a family of degradable materials with improved properties in respect to commonly used polyesters.

An assessment of dynamics of electrochemical polymerization of pyrrole

In this work, the dynamics of electrochemical polymerization of pyrrole was assessed by measuring the system current and polymer yield under different cell dimensions and polymerization conditions. The effects of distance between electrodes on the yield and system current were described. Also, the effects of monomer concentration and electrode size on monomer-to-polymer conversion were critically analyzed with respect to the energy supplied to the system. The enthalpy of the process was also determined. It was observed that the yield was directly proportional to the product of system current and time when sufficient monomer and electrode space for deposition of polymer were available otherwise the electrical energy was transformed more into heat energy. Electrochemical polymerization of pyrrole was found to be an exothermic process, and the enthalpy of polymerization increased with an increase in polymerization temperature.

Effect of enthalpy of polar modifiers interaction with n-butyllithium on the reaction enthalpy, kinetics and chain microstructure during anionic polymerization of 1,3-butadiene

The influence of interaction enthalpy (ΔHMOD/BuLi) of μ, σ, σ+μ and σ-μ complexing polar modifiers with n-butyllithium on the 1,3-butadiene anionic polymerization enthalpy (ΔHBD), polymerization reaction rate (kp) and polybutadiene microstructure was studied. It has been found that enthalpy of interaction depends on complex type, molar ratio of polar modifier to n-butyllithium (MOD/BuLi) and temperature. For the first time it has been proven that with increasing ΔHMOD/BuLi the content of 1,2 butadiene (vinyl) units in the chain increases individually for each complex type but the value of ΔHBD decreases similarly for all complexes containing σ-donor groups, exhibiting linear nature. Since ΔHMOD/BuLi controls the content of butadiene isomeric structures in the chain its value was compared with polymerization reaction rate, ranging from ∼46 to ∼5500 moL/L·min, and discussed on mechanistic level.

Thermochemistry of Acrylamide Polymerization: An Illustration of Auto-acceleration and Gel Effect

Addition polymerization of alkenes is an exothermic process due to the relief of the bond strain from C-C double bonds (π bonds) in the monomer molecules. Control of exothermic reactions is crucial for preventing runaway reactions/explosions in the industrial setting. This experiment was designed for undergraduate students to demonstrate the exothermic nature of addition (chain-growth) polymerization and to estimate the enthalpy of polymerization using adiabatic calorimetry technique. Acrylamide (AAM) was polymerized using ammonium persulfate (APS) and Tetramethylethylenediamine (TMEDA) as redox initiators at room temperature. The concentration of AAM monomer was varied in the range of 5-25 wt. %, and the heat of polymerization was calculated from the recorded temperature rise (ΔT). The enthalpy of AAM polymerization (∆Hp) was estimated to be ≈ 16.5–19 kcal/ mole, depending on the monomer concentration. Enthalpy of polymerization was increased with increasing the monomer concentration due the auto-acceleration effect, which was clearly observed for solutions with AAM concentrations of ≥ 20 wt. %. The auto-acceleration phenomenon was identified from the fast and non-linear increase in the rate of temperature rise which reached a maximum of (0.5°C/ s) for solutions with AAM of 25 wt. %. For AAM concentrations of ≥ 20 wt. %, the auto-acceleration was accompanied by the polymer gelation (gel effect) due to the formation of the high molecular weight polyacrylamides.

Differential Scanning Calorimetry study on curing kinetics of diglycidyl ether of bisphenol A with amine curing agents for self-healing systems

ABSTRACT Epoxy resin is one of the most important polymeric matrixes for diverse uses, such as aeronautical, mobile, electronics and others. Providing a material capable of self-healing microcracks will allow it to have a more reliable and longer use. In order to study this new property of self-healing to an epoxy resin, a Differential Scanning Calorimetry (DSC) study was carried out to measure physical chemical properties such as activation energy, enthalpy of polymerization, order of reaction and pre-exponential factor based on an Arrhenius equation modelling. The materials used are diglycidyl ether of bisphenol A (DGBA) as the core material, triethylenetetramine (TETA) and an aminated polydimethylsiloxane (ADM), as curing agent and poly(urea-formaldehyde) (PUF) as microcapsule wall. Tests were done by dynamic and isothermal DSC of systems containing the resin and microencapsulated curing agent and also the resin with the free curing agent. The results showed the necessity of heating for the cure and the self-healing to be initiated.

A Precision Ethylene-Styrene Copolymer with High Styrene Content from Ring-Opening Metathesis Polymerization of 4-Phenylcyclopentene.

Ring-opening metathesis polymerization of 4-phenylcyclopentene is investigated for the first time under various conditions. Thermodynamic analysis reveals a polymerization enthalpy and entropy sufficient for high molar mass and conversions at lower temperatures. In one example, neat polymerization using Hoveyda-Grubbs second generation catalyst at -15 °C yields 81% conversion to poly(4-phenylcyclopentene) (P4PCP) with a number average molar mass of 151 kg mol(-1) and dispersity of 1.77. Quantitative homogeneous hydrogenation of P4PCP results in a precision ethylene-styrene copolymer (H2 -P4PCP) with a phenyl branch at every fifth carbon along the backbone. This equates to a perfectly alternating trimethylene-styrene sequence with 71.2% w/w styrene content that is inaccessible through molecular catalyst copolymerization strategies. Differential scanning calorimetry confirms P4PCP and H2 -P4PCP are amorphous materials with similar glass transition temperatures (Tg ) of 17 ± 2 °C. Both materials present well-defined styrenic analogs for application in specialty materials or composites where lower softening temperatures may be desired.