Use of differential scanning calorimetry to study polymer crystallization kinetics with longer half times

A method has been developed to determine polymer crystallization parameters with longer half times using differential scanning calorimetry. This method can be used irrespective of the crystallization temperature, sample weight, nature of the polymer and the sensitivity of the instrument. The results obtained from this method for polyethylene samples are compared with those obtained from dilatometry. Similar values for the kinetic parameters are obtained using both techniques. The calorimetric method has been used for other polymers such as polypropylene and poly(4-methyl-1-pentene).     

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.     

Thermal behaviour of milk powders studied by differential thermal analysis and heat flow calorimetry

The technique of heat flow calorimetry was used to study the thermal behaviour of different milk powders above 20°C. Exothermic reactions of different types occurred when measurements were made with the samples in sealed cells. According to our interpretation of the calorimetric curves, these reactions correspond to: crystallization of amorphous lactose, Maillard reaction between milk proteins and lactose, oxidation of milk fat and decomposition of lactose. Their relative importance is discussed. In addition, the technique of high-pressure differential thermal analysis was used for studying spontaneous ignition and combustion of milk powders, under a large oxygen excess. The role of fat, carbohydrate and protein decomposition is underlined. Some self-ignition temperature values are given.     

Isothermal differential scanning calorimetry on an exothermic phenomenon during thermal decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O

An exothermic phenomenon and a simultaneous rapid evolution of a small amount of carbon dioxide at ?500°C during thermal decomposition of hydromagnesite 4 MgCO₃ · Mg(OH)₂ · 4 H₂O was studied by isothermal DSCTG in a carbon dioxide atmosphere. It was quantitatively confirmed that the exothermic phenomenon was due to crystallization of MgCO₃ from the amorphous phase and that the evolution of carbon dioxide was due to decomposition of the MgCO₃ by the heat of crystallization (?3.4 kcal mole^?1.     

Vapor pressures by differential thermal analysis: application to some nickel chelates

A simple and rapid DTA procedure is described for determining the vapor pressures of metal-organic compounds. We have applied the method to six volatile nickel β-ketoamine chelates which have proved useful, in previous work, for the chemical vapor deposition of nickel metal. The volatilities of these compounds are strongly influenced by the configuration (cis or trans planar) about the bonded metal atom and by the presence or absence of fluorine as part of the organic portion of the molecules. Some of the chelates have vapor pressures of several hundred torr at temperatures where thermal decomposition is not significant.     

Transport and kinetics at microheterogeneous electrodes: Part III. Application to the photoelectrochemical decomposition of water

The transport and kinetics in a system involving two sets of microheterogeneous electrodes, which are designed to use solar radiation to split water into H₂ and O₂, are described. A reaction scheme involving fourteen different processes is considered. The conditions for an efficient system are derived. It is shown that the main problems are electrode selectivity and the loss of H₂ and O₂ by back reactions.     

A high pressure differential thermal analysis apparatus

A high pressure differential thermal analysis apparatus is described which is capable of operation in the pressure range from 1-600 atm of nitrogen gas and at temperatures from 25 to 500°C. Use of the apparatus is illustrated by the deaquation reactions of CuCl₂·2H₂0 and CoSO₄·7H₂O at pressures from 1 to 69 atm.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Normalized potential sweep voltammetry: Part III. Elimination of the effect of uncompensated Resistance and the application to heterogeneous kinetics of rapid charge-transfer processes

Uncompensated resistance (R_u) has a distoring effect on normalized potential sweep voltammetry (NPSV) slopes. This provides a simple and effective method to determine R_f^o, the value of the potentiostat feedback resistance necessary for full compensation. If the NPSV range is divided into overlapping segment, 1 and 2, corresponding to I_N of 0.20–0.50 and 0.50–0.80 respectively, the slopes m₁ and m₂ differ significantly when R_f differs form R_f^o. The difference, m₁-m₂, is negative for R_f<R_f^o and positive for R_f>R_f^o. Fine tuning of the potentiostat R_f setting so that the average value of the difference is the theoretical value can be accomplished in a minimum of time. Under these conditions, m₁ and m₂, as well as m_T, the slope of the entire correlation have very nearly the same values. Linear equations were derived from theoretical data which allow heterogeneous rate constants to be obtained directly from NPSV slopes. The precision in the NPSV slopes was observed to be of the order of ±0.002 which implies that the method should give reliable rate constants as great as 10cm s^? at a voltage sweep rate of 100V s^?1. The method is demonstrated using the reduction of benzonitrile and perylene in N,N-dimethylformamide and acetonitrile as examples.     

Reaction kinetics in thermal analysis. Part 1. The sensitivity of kinetic equations to experimental errors. A mathematical analysis

The frequent publication of contradicting or meaningless kinetic parameters and the resulting criticism of the “ill-conditioned nature” of non-isothermal reaction kinetics led the authors to an examination of the sensitivity of kinetic parameters to experimental errors. Using simple mathematical deductions, conditions were given at which about 10% precision of the kinetic parameters can easily be achieved. To obtain a graphic picture about the information content of a thermoanalytical curve and the effect of the systematic measurement errors, mathematical relationships were deduced to show the dependence of the kinetic parameters on the formal (geometric) characteristics of the thermoanalytical curves.     

Proton diffusion reaction in a protein polyelectrolyte membrane and the kinetics of electromechanical forces

A theoretical model is developed for proton transport in a charged, protein polyelectrolyte membrane. Protons bind to fixed charge groups of the membrane. This binding process also modulates electromechanical tensile forces in the membrane, due to the change in the electrostatic interactions between the charged membrane molecules. We used this force as an experimental measure of the space—time evolution of proton transport inside the membrane. A step change in the bath pH produced a measurable change in the isometric tensile force of the membrane. H⁺ ion transport was modelled by a diffusion reaction theory. The kinetics of the measured force were compared to the H⁺ diffusion reaction dynamics predicted by the theory. Experimental and theoretical time constants were in good agreement for reasonable values of the H⁺ diffusivity, the equilibrium dissociation reaction constant and the number of available binding sites. This suggests that the diffusion reaction of H⁺ into the membrane is rate-limiting in force generation. Other nonequilibrium processes, including molecular reconformation and osmotic swelling, must be proceeding at least as rapidly as H⁺ transport. The combination of theory and experiment provides a useful, non-destructive technique for characterizing the kinetics of binding, and other transport processes, in charged polymers and in certain biological tissues.     

New differential permeation rate method for determination of membrane transport parameters of gases

A new method for determining the membrane transport parameters (diffusivity, permeability and solubility) of gases through nonporous polymeric membranes is described. The method employs a continuous permeation chamber containing a flat membrane. The most important feature of this method is that, instead of a step concentration change, a rectangular pulse or impulse is sent to the upstream side of the membrane. Consequently, no steady state is approached but a signal peak of typical form can be recorded. The permeability and the diffusivity can be estimated from the height and half-width of the peak, respectively. The method was applied to measure the permeability of hydrocarbons through a polyethylene membrane, the permeation rate being measured by a flame ionization detector. The method and the derived relations are valid for other detectors and gas—membrane combinations as well. The advantages of this novel method are that all the membrane transport parameters can be directly evaluated from data of the response peak, whilst approaching the steady state is not necessary and thus the measuring time can be shortened. Finally, the known and new differential permeation rate methods are compared by generalization of the relationship between the input and output (response) functions.     

A simultaneous electrical conductivity—differential thermal analysis apparatus

A highly sensitive simultaneous electrical conductivity—differential thermal analysis apparatus is described. The apparatus can simultaneously record both the electrical conductivity and differential thermal analysis curves of a sample under the same conditions of furnace heating rate and atmosphere. Application of the apparatus is illustrated by curves of CuSO₄·5H₂O, [Co(NH₃)₅H₂0]Br₃ and Cu₂HgI₄.     

Theoretical considerations on the temperature and pressure dependence of the kinetics of reversible thermal decomposition processes of solids

Whereas the temperature dependence of reaction rates of reversible solid-state decomposition processes of the type A_solid ? B_solid + C_gaseous has been outlined in numerous theoretical as well as in experimental studies, its simultaneous dependence on the actual pressure of the gaseous product has not been taken into account sufficiently. Theoretical considerations elucidate, however, that specific and reproducible kinetic data, in particular umambiguous values for activation energies, can only be determined under vacuum conditions or in some cases by fulfilling the following precondition: $\text{p}{}_{\mathrm{<mtext>exp</mtext>}}\text{p}{}_{\mathrm{<mtext>equ</mtext>}}\text{= f(T) =}\text{constant}$, where p_exp is the actual pressure and p_equ the equilibrium pressure of the gaseous product.     

Subtractive differential pulse anodic stripping voltammetry at a stationary mercury-coated glassy carbon electrode

The application of subtractive mode differential pulse anodic stripping voltammetry (SDPASV) at a stationary mercury-coated glassy carbon electrode for the analysis of labile Zn(II), Cd(II), Pb(II) and Cu(II) is described. It is shown that the method has an improved sensitivity to Cu(II) owing to elimination of high background currents normally encountered in normal mode DPASV at the TFME. The sensitivity limits of the present method to Cd(II) and Cu(II) is estimated to be 0.025 and 0.067 ppb respectively, when a 2 min deposition time is used. It is suggested that the high sensitivity of the method coupled to the relative simplicity of the stationary electrodes could make the method useful in environmental and natural water studies.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.