Isothermal kinetics of photopolymerization and thermal polymerization of bis-GMA/TEGDMA resins

The purpose of this work is to analyse certain kinetic features related to thermoinduced and photoinduced isothermal curing in the 25/75 mass% bis-GMA/TEGDMA system. The kinetic parameters associated with photo and thermal curing were determined and compared using an isoconversional procedure and the kinetic model was obtained by means of a reduced master plot. In photocuring, the kinetic results obtained by means of this phenomenological methodology were compared with those obtained on the basis of mechanistic considerations. In this case, we estimated the propagation and termination constants associated with photocuring at different conversions. When the phenomenological procedure is performed, the rate constant decreases slightly during the curing process and the autoacceleration effect of the process is demonstrated in the kinetic model, which is autocatalytic. However, in the mechanistic model, this same effect is noted through an increase in the rate constants, while it is assumed that the kinetic model is in the order of n with n=1.     

Chemical surface modifications of microfibrillated cellulose

Microfibrillated cellulose (MFC) was prepared by disintegration of bleached softwood sulphite pulp through mechanical homogenization. The surface of the MFC was modified using different chemical treatments, using reactions both in aqueous- and organic solvents. The modified MFC was characterized with fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Epoxy functionality was introduced onto the MFC surface by oxidation with cerium (IV) followed by grafting of glycidyl methacrylate. The length of the polymer chains could be varied by regulating the amount of glycidyl methacrylate added. Positive charge was introduced to the MFC surface through grafting of hexamethylene diisocyanate, followed by reaction with the amines. Succinic and maleic acid groups could be introduced directly onto the MFC surface as a monolayer by a reaction between the corresponding anhydrides and the surface hydroxyl groups of the MFC.     

二氧化硅脱除羟基的动力学研究

醇盐水解法制备的纳米二氧化硅粉体中含有大量羟基,结合差热分析,采用Doyle-Ozawa法和Kissinger法研究二氧化硅粉体脱除羟基的动力学,得到反应表观活化能为134.78kJ.mol-1;并采用Coats-Redfern法进行验证,结果为140.22 kJ.mol-1.采用SEM和XRD对样品进行检测,表明非晶态二氧化硅样品颗粒大小没有变化,晶态没有改变.     

原位担载Fe_2S_3催化剂煤的热解动力学研究

用加压热天平研究了原位担载纳米级Fe2 S3 催化剂的大柳塔次烟煤的加氢热解动力学 ,考查了 2 0MPa氢气氛或氮气氛下原煤及担载催化剂的煤热解失重过程 ,计算了活化能E和指前因子A等热解动力学参数。结果表明 ,煤原位担载Fe2 S3 催化剂后热失重速率比原煤有较大程度的增加 ,最大热解速率的特征温度也比原煤降低。特别是煤在表面活性剂十六烷基三甲基溴化铵溶液中担载Fe2 S3 催化剂后的热解反应速率高于水溶液中担载同样催化剂的热解反应速度。原位担载Fe2 S3 的煤及原煤的热解反应过程符合一级反应动力学。     

Influence of Process Parameters on the Reaction Kinetics of the Chromium‐Catalyzed Trimerization of Ethylene

In this paper we report the results of an extensive experimental kinetic study carried out on the novel ethylene trimerization catalyst system, comprising the chromium source [CrCl₃(thf)₃] (thf=tetrahydrofuran), a Ph₂P‐N(iPr)‐P(Ph)‐N(iPr)H (PNPNH) ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum (AlEt₃) as activator. It could be shown that the initial activity shows a first‐order dependency on the ethylene concentration. Also, a first‐order dependency was found for the catalyst concentration. The initial activity follows a typical Arrhenius behavior with an experimentally determined activation energy of 52.6 kJ mol^?1. At elevated temperatures (ca. 80 °C), a significant deactivation was observed, which can be tentatively traced back to a ligand rearrangement in the presence of AlEt₃. After a fast initial phase, a pronounced ‘kink’ in the ethylene‐uptake curve is observed, followed by a slow, almost linear, further increase of the total ethylene consumption. The catalyst composition, in particular the ligand/chromium and the cocatalyst/chromium molar ratio, has a strong impact on the catalytic performance of the trimerization of ethylene.     

Influences of evolved gases on the thermal decomposition of zinc carbonate hydroxide evaluated by controlled rate evolved gas analysis coupled With TG

The influences of atmospheric CO₂ and H₂O on the kinetics of the thermal decomposition of zinc carbonate hydroxide, Zn₅(CO₃)₂(OH)₆, were investigated by means of controlled rate evolved gas analysis (CREGA) coupled with TG. Although CO₂ and H₂O were evolved simultaneously in a single mass-loss step of the thermal decomposition, different effects of those evolved gases on the kinetic rate behavior were observed. No distinguished effect of atmospheric CO₂ was detected within the possible range of self-generated CO₂ concentration. On the other hand, apparent acceleration effect by the increase in the concentration of atmospheric H₂O was observed as the reduction of reaction temperature during the course of constant rate thermal decomposition. The catalytic effect was characterized by the decrease in the apparent activation energy for the established reaction with increasing the concentration of atmospheric H₂O, accompanied by the partially compensating decrease in the pre-exponential factor.     

Reduction kinetics of glycinate complexes of palladium(II) on a dropping-mercury electrode in acid perchlorate solutions

Polarograms for the reduction of glycinate complexes of palladium(II) (5 × 10^?5 M) are obtained in equilibrium solutions of pH 0.8–3.0 with different protonated-glycine concentrations c _Hgly (supporting electrolyte, 0.5 M NaClO₄). It is established that the irreversible wave of reduction of complexes Pd(gly)₂ corresponds to the diffusion limiting current I _d ⁽²⁾ . A similar wave at pH 1.5 and c _Hgly = 0.005 M, as well as at pH 1.0 and c _Hgly = 0.05–0.5 M is preceded by the diffusion limiting current I _d ⁽¹⁾ . Values of the I _d ⁽²⁾ /I _d ⁽¹⁾ ratio are close to the ratio between equilibrium concentrations of Pd(gly)₂] and [Pdgly⁺], calculated using the step stability constant for Pd(gly)₂. This fact testifies to the reduction of complexes Pdgly⁺ in the vicinity of I _d ⁽¹⁾ and complexes Pd(gly)₂, in the vicinity of I _d ⁽²⁾ . At pH 0.8–1.2 and [H₂gly⁺] = 1 × 10^?4 to 5 × 10^?3 there is observed the diffusion-kinetic limiting current of the first wave I ₁ ⁽¹⁾ , which increases with increasing [H⁺] and decreasing [H₂gly⁺]. The nature of the slow preceding chemical stage that occurs during the reduction of complexes Pdgly⁺ is discussed.     

Tryptophan-based peptides to synthesize gold and silver nanoparticles: a mechanistic and kinetic study

Synthetic oligopeptides with a tryptophan residue at the C-terminus have been used for the synthesis of gold and silver nanoparticles at pH 11. The tryptophan residue in the peptides is responsible for the reduction of metal ions to the respective metals, possibly through electron transfer. A mechanistic pathway has been proposed to explain the reductive properties of the tryptophan moiety of the peptide based on some spectroscopic techniques, such as UV-visible and fluorescence spectroscopy. This study reveals that some of the peptide molecules are converted to its corresponding ditryptophan, kynurenine form and some cross-linked products, all of which are highly fluorescent species. The resultant peptide-functionalized metal nanoparticles have also been characterized by UV-visible spectroscopy, transmission electron microscopy, and Fourier transform IR spectroscopy and thermogravimatric analysis.     

The DMAP-catalyzed acetylation of alcohols--a mechanistic study (DMAP = 4-(dimethylamino)pyridine)

The acetylation of tert-butanol with acetic anhydride catalyzed by 4-(dimethylamino)pyridine (DMAP) has been studied at the Becke3 LYP/6-311 + G(d,p)//Becke3 LYP/6-31G(d) level of theory. Solvent effects have been estimated through single-point calculations with the PCM/UAHF solvation model. The energetically most favorable pathway proceeds through nucleophilic attack of DMAP at the anhydride carbonyl group and subsequent formation of the corresponding acetylpyridinium/acetate ion pair. Reaction of this ion pair with the alcohol substrate yields the final product, tert-butylacetate. The competing base-catalyzed reaction pathway can either proceed in a concerted or in a stepwise manner. In both cases the reaction barrier far exceeds that of the nucleophilic catalysis mechanism. The reaction mechanism has also been studied experimentally in dichloromethane through analysis of the reaction kinetics for the acetylation of cyclohexanol with acetic anhydride, in the presence of DMAP as catalyst and triethylamine as the auxiliary base. The reaction is found to be first-order with respect to acetic anhydride, cyclohexanol, and DMAP, and zero-order with respect to triethyl amine. Both the theoretical as well as the experimental studies strongly support the nucleophilic catalysis pathway.     

Mechanism for nucleation and growth of electrochemical deposition of palladium(II) on a platinum electrode in hydrochloric acid solution

Palladium(II)and chloride ions tend to form complexes in aqueous solution.Both theoretical and experimental(by UV spectrum)results indicate that there are four complexes formed in aqueous solution containing 3 mol/L hydrochloric acid and 20mmol/L PdCl2.This work evaluates the kinetics of electrochemical deposition of palladium on a Platinum electrode.For this purpose,palladium electrodeposition was investigated by means of cyclic voltammetry(CV),potentiostatic current-time transients(CTTs)and Tafel curve.By CTTs curves,the regions corresponding to the charge transfer control,mixed control and diffusion control were identified.In the diffusion control region,palladium electrodeposition mechanism was characterized as progressive nucleation with three-dimensional(3D)growth under diffusion control;as for the mixed control region,an adsorption(IAds),ion transfer(IIT),and nucleation and growth(ING)model were proposed to analyze the current-time transients quantitatively,which could separate the IAds,IIT and ING perfectly.