Advanced Calorimetric Techniques in Polymer Engineering

Summary: In polymer synthesis, reaction calorimetry (RC) is an appropriate technique for on-line process monitoring, since polymerization reactions are highly exothermic. Measurements are noninvasive, rapid, and straightforward. Nowadays RC is the technique recognized as the most powerful way to study such process in near-to-the- industrial conditions. Our approach was focused on temperature oscillation calorimetry (TOC). Two different reaction calorimeters were used, i.e. a isoperibolic calorimeter and a Calvet type high sensitivity differential calorimeter, respectively. A special attention was paid to the interpretation of the measured signals in order to obtain reliable calorimetric data. The evolution of heat transfer coefficient UA was followed by performing appropriate Joule effect calibrations, before and after the reaction. A convolution differential method of the measured heat flow by the generated one was used for determining the time constants and deconvoluting the measured heat flow.     

Analysis of mass transport limitation during emulsion polymerization using reaction calorimetry and conductivity measurement

The seeded emulsion polymerization of styrene was investigated by reaction calorimetry in combination with conductivity measurements using the RC1e calorimeter (Mettler Toledo). Varying the stirrer speed showed the existence of mass transport limitation in the lower speed range. For the case of using the autoclave HP60 with an anchor stirrer a minimum stirrer speed of 400 rpm is required to be sure of adequate mixing in the reactor.     

Thermal polymerization of acrylamide by differential scanning calorimetry

Thermal polymerization of acrylamide was studied by differential scanning calorimetry. Latent heat of fusion ΔH_f and enthalpy of polymerization ΔH_p values were found to be 36 and ?18.0 kcal mol^?1, respectively. The overall activation energy E for the polymerization was calculated to be 19 k cal mol^?1 up to 60% conversion. The added free-radical inhibitor (benzoquinone) was found to desensitize the thermal polymerization of acrylamide suggesting the polymerization to be a free-radical type. The existing rate equation for the heterogeneous bulk polymerization in the presence of initiators has been modified for the thermally initiated bulk polymerization of acrylamide. The experimental overall E value was found to agree well with the calculated E value when considering only the propagation and termination steps, thereby suggesting the process to be similar to postpolymerization of acrylamide.     

Solubility diagrams in solvent-antisolvent systems by titration calorimetry

Isothermal titration calorimetry (ITC) has been used to develop a method to construct the solid-liquid equilibrium line in ternary systems containing the solute to precipitate and an aqueous mixed solvent. The method consists in measuring the heat of dissolution of a solid component (the solute) during successive additions of the liquid solvent. The cumulated heat, resulting from the successive heat peaks obtained for the different injections of known volumes of solvent, plotted vs. the ratio of the numbers of moles n _solvent/n _solute is represented by two nearly straight lines. The intercept of the two lines gives the solubility limit and the corresponding enthalpy of dissolution of the solute in the solvent. Solubility diagrams have been established at 303.15 K in binary mixed solvents ethanol-water over the whole concentration range for seven compounds of pharmaceutical interest, namely: urea, phenylurea, l-valine, dl-valine, l-valine ethyl ester hydrochloride, tris(hydroxymethyl)amino methane.     

热动力学的滴定量热法研究2: 单底物酶促反应的热动力学

用滴定量热法分别建立了滴定期和停滴反应期单底物酶促反应热动力学的数学模型。根据这两种模型,可由一次实验的滴定量热曲线同时解析出单底物酶促反应的热力学参数(Δ~rH~m)和动力学参数(K~m和k~2)。用滴定量热法研究了一个经典的单底物酶促反应---过氧化氢酶催化分解过氧化氢反应的热动力学,由滴定期和停滴反应期热动力学模型解析出在298.15K和pH=7.0时该反应的米氏常数K~m分别为(5.41±0.24)×10^-^3和(5.43±0.21)×10^-^3mol.L^-^1,酶转换数k~2分别为(3.58±0.33)×10^3和(3.60±0.41)×10^3s^-^1,摩尔反应焓为(-86.75±0.88)kJ.mol^-^1,实验结果验证了上述热动力学模型的正确性。     

Influence of different initiators on methyl methacrylate polymerization,studied by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to measure the decomposition rates of four commercially used initiators, 2,2′-azobis(isobutyronitrile) 2,2′-azobis(2,4-dimethylvaleronitrile), dilauroyl peroxide and bis(4-t-butylcyclohexyl)peroxydicarbonate, in dynamic mode, while the courses of methyl methacrylate polymerization with the listed initiators at 65, 75 and 85°C were measured isothermally. From the DSC curves, the polymerization enthalpies, the overall reaction rate constants and the activation energies for the initial steady-state polymerization were calculated. It was found that the polymerization enthalpy and the kinetic parameters depended on the type of the initiator. An initiator with a shorter decomposition half-lifetime shifted the onset of the gel effect to a higher conversion, intensified it and decreased the average molar mass of the polymer.     

Enzyme-catalysed hydrolysis of L-amino acid esters in a low water organic solvent studied by isothermal calorimetry

Isothermal calorimetry and UV-visible spectrophotometry have been used to study the thermochemistry of the enzyme-catalyzed hydrolysis of hydrophobic L-amino acid esters in organic solvents with low water content at 298 K. The p-nitrophenyl esters of Z-L-tyrosine and Z-L-phenylalanine were used as model hydrophobic substrates. Acetonitrile was used as a model organic solvent. A special preparation protocol of the reactants in the calorimetric vessel was applied in order to determine the heat effects accompanying the enzyme-catalyzed hydrolysis reaction in organic mixtures with low water content and the Tris buffer ionization enthalpies over the whole range of water content in acetonitrile. It was found that the molar enthalpy of the hydrolysis of p-nitrophenyl esters and buffer ionization enthalpy depend significantly and similarly on the water content in acetonitrile. However, the reaction enthalpy corrected for the buffer ionization enthalpy does not depend on the water content in organic solvent mixtures. An explanation of the effect of the selected organic solvent on the thermochemical parameters was provided on the basis of the IR spectroscopic data for the hydrogen bond network of water in acetonitrile. The results obtained show that the state of water in organic solvents is an important factor that determines the reaction enthalpy as well as buffer ionization enthalpy.     

Dynamical properties of ethylene glycol and glucose studied by temperature-modulated differential scanning calorimetry and Brillouin scattering

The fragility of ethylene glycol and glucose aqueous solution systems has been investigated by temperature-modulated differential scanning calorimetry (TMDSC). The frequency and temperature dependences of complex specific heat have been observed in the vicinity of a glass-transition temperature T _g . It is shown that the value of the fragility index m can be determined from the temperature dependence of the α-relaxation times observed by TMDSC. We have also studied the elastic properties of these aqueous solutions by micro-Brillouin scattering, and determined these relaxation times of elastic properties in the gigahertz range.     

Modulated differential scanning calorimetry

Modulated-temperature differential scanning calorimetry was used to measure the glass transition temperature,T _g, the heat capacity relaxation in the glassy state and the increment of heat capacity, C_p, in the glass transition region for several polymers. The differential of heat capacity with respect to temperature was used to analyseT _g and C_p simply and accurately. These measurements are not affected by complex thermal histories.     

Applications of reaction calorimetry in chemical engineering

Reaction calorimetry strongly penetrated process development laboratories in the fine chemicals industry. Applications of calorimetry to different fields of process optimization, chemical reactions and physical unit operations were developed. Applications were first developed in the field of process safety. The thermal data of reaction obtained in the calorimeters allow us to check if a reaction will be controllable at full scale under normal operating conditions and in case of equipment failure. Further, the accurate temperature control and heat flow measurement opened the door to more engineering related data, in the fields of phase equilibria like vapour liquid, solubilities, crystallization and also in the mixing techniques. Some examples of developments in these different fields will be reviewed.     

Investigation of copper(II) complexation by glycylglycine using isothermal titration calorimetry

Isothermal titration calorimetry (ITC) and potentiometric titration methods have been used to study the process of proton transfer in the copper(II) ion-glycylglycine reaction. The stoichiometry, conditional stability constants, and thermodynamic parameters (ΔG, ΔH, and ΔS) for the complexation reaction were determined using the ITC method. The measurements were carried out at 298.15 K in solutions with a pH of 6 and the ionic strength maintained with 100 mM NaClO₄. Carrying out the measurements in buffer solutions of equal pH but different enthalpies of ionization of its components (Mes, Pipes, Cacodylate) enabled determination of the enthalpy of complex formation, independent of the enthalpy of buffer ionization. The number of protons released by glycylglycine on account of complexation of the copper(II) ions was determined from calorimetric and potentiometric measurements.     

Polypropylene Modification by the Radiation Graft Polymerization of N-Vinylcaprolactam

The radiation graft polymerization of N-vinylcaprolactam onto polypropylene films was studied. The radiation graft polymerization was performed with preirradiation in air (peroxide method) or using a direct method in aqueous solutions and organic solvents. The effects of radiation dose, reaction time, monomer concentration, and homopolymerization inhibitor on the radiation graft polymerization were studied. It was found that the radiation graft polymerization from a monomer emulsion in water occurred at the highest rate. The IR spectra of grafted polymers were obtained. With the use of differential scanning calorimetry and equilibrium swelling in water, it was found that the modified polymers of polypropylene with grafted poly(N-vinylcaprolactam) chains exhibited thermoresponsive properties.     

Thermal hazard analysis of cumene hydroperoxide using calorimetry and spectroscopy

Organic peroxides have been widely used in industries and are known to be self-reactive chemicals. In this paper, thermal and infrared spectroscopic analyses were carried out to obtain a better understanding of the thermally hazardous behavior of cumene hydroperoxide (CHP) with cumene solvent. The temperature and heat flow profiles of different concentrations of CHP at scanning and isothermal conditions were measured with a small scale reaction calorimeter. Furthermore, probe type in situ infrared spectroscopic measurements were performed and the reaction mechanism will be discussed in regards to both energy release and product identification.     

Reactivity of fly ash in cement paste studied by means of thermogravimetry and isothermal calorimetry

Four paste mixtures with varying replacement level of the cement content by fly ash have been studied. Due to fly ash, the acceleration period decreased and a third hydration peak was noticed with isothermal calorimetry. The total heat after 7 days increased with increasing fly ash content. From 1 to 7 days, thermogravimetry showed a higher chemically bound water and Ca(OH)₂-content for the pastes with fly ash. Between 7 and 14 days the calcium hydroxide started to be depleted due to the pozzolanic reaction. A unique relation was found between calcium hydroxide and total heat development.     

Polymerization processes at conditions of moderately short kinetic chains by calorimetry

A method has been developed for the determination of the heat of the acts of chain growth q_g and of the summary heats of the acts of chain initiation and termination q^** for the radical polymerization of vinyl monomers at the conditions of moderately short kinetic chains, when the length of the kinetic chain v < 100, but the degree of polymerization is ¯P > 20. The analysis of the experimental data, obtained by investigating the kinetics of polymerization of vinyl monomers in solution by calorimetry has led to the conclusion, that at v > 100 an approximation of the long material chains can be used (when the contribution of q^** is negligibly small) with a permissible experimental error; however, in the region of moderately short chains a correction for q^** must be introduced, in order to obtain the correct kinetics of the process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 695–701, November–December, 1988.     

Determination of kinetic parameters for the reaction between phenol and formaldehyde by differential scanning calorimetry

The kinetic parameters of the complex reaction between phenol and formaldehyde in the presence of sodium hydroxide (NaOH) have been obtained by differential scanning calorimetry (DSC). The two dominant reactions appear to be addition of formaldehyde to phenol with formation of o-hydroxymethyl-phenol and subsequent condensation of the latter. For both reactions, the activation energy (E_a), reaction order and rate constants at different temperatures have been determined. E_a for addition changes from 23·7 to 19·3 kcal mole^?1 and for condensation from 22·9 to 19·1 kcal mole^?1 when the amount of NaOH is increased from 0·25 to 1·00 per cent. The reaction order for addition is 2 and for condensation 1. Thus DSC appears useful for studying the kinetics of more complex polymerization reactions.     

Polymerization of trimethylene carbonate in aqueous solutions: Reaction mechanism and characterization

Polymerization of a trimethylene carbonate (TMC) in an aqueous solution was investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. The polymerization reaction proceeded rapidly in the aqueous solution and high conversion was achieved in a relatively short time. 1,3‐Propanediol (PPD) formed by hydrolysis of TMC was used as the initiator. The TMC oligomer obtained by ring‐opening polymerization had a TMC unit backbone with terminal 3‐hydroxypropyl groups at both chain ends. The oligomer underwent transesterification reaction with elimination of PPD, resulting in a gradual increase in the molecular weight of the product. The molecular weight was affected by the concentration of TMC. The thermal properties of the polymers were investigated by differential scanning calorimetry. Polymers within the molecular weight (M_n) range from 6.0 × 10³ to 2.3 × 10⁴ g/mol crystallized, and endothermic peaks corresponding to the melting temperature were observed. The glass transition temperature increased with the molecular weight of the polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1485–1492, 2010     

Determination of kinetic parameters of polymerizations by differential scanning calorimetry

Differential scanning calorimetry is well suited to record heat productions of chemical and physical processes as data for the following kinetic analysis. To obtain kinetic parameters of complex reactions, nonlinear optimization methods have to be used. Such complex reaction systems are polymerizations. We tried to evaluate measurements of the epoxy cure and the polymerization of β-propiolactame with simple and complex models. In both cases the simple models did not produce satisfactory results. But by using complex models a successful fitting of the measured data was possible. Our investigation shows that the combination of DSC and modern nonlinear evaluation methods presents a suitable tool for the kinetic investigation of polymerizations.     

Polystyrene‐block‐poly(butyl acrylate) and polystyrene‐block‐poly[(butyl acrylate)‐co‐styrene] block copolymers prepared via controlled free‐radical miniemulsion polymerization using degenerative iodine transfer

Polystyrene‐block‐poly(butyl acrylate) and polystyrene‐block‐poly[(butyl acrylate)‐co‐styrene] block copolymers were prepared in an aqueous dispersed system via controlled free‐radical miniemulsion polymerization using degenerative iodine transfer. The first step is batch miniemulsion polymerization of styrene in the presence of C₆F₁₃I as transfer agent. The second step consists of the addition of butyl acrylate to this seed latex, either in one shot or continuously. The addition was started before the consumption of styrene was complete in order to perform a copolymerization reaction able to moderate the rate of propagation in the butyl acrylate polymerization step and, therefore, to favor the transfer reaction. Kinetics of polymerization and control of the molar masses were examined according to the experimental conditions and particularly to the rate of butyl acrylate addition. The formed block copolymers were analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR).     

Calorimetric and diffractometric study of the room-temperature transitions of polytetrafluoroethylene from aqueous suspension polymerization

The room-temperature transitions of native (never-melted) polytetrafluoroethylene (PTFE) obtained by aqueous suspension polymerization have been studied through differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS). Two crystal-crystal transitions are observed, instead of the four shown for native PTFE from aqueous emulsion polymerization. One crystalline component seems to be present, which may correspond to the low-transient, already evidenced for PTFE from aqueous emulsion polymerization.