Different Degrees Of Conversion Research Articles

Flame retardance and thermal properties of a novel phosphorus-containing unsaturated polyester resin/SiO2 hybrid nanocomposite

AbstractA novel phosphorus-containing unsaturated polyester resin/SiO2 hybrid nanocomposite was prepared from the in-situ curing of unsaturated polyester resin (UP) and triethoxysilane (DI) which was synthesized from the nucleophilic addition reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and 3- (trieoxysilyl) isocyanate(icteos). The formation of unsaturated polyester resin/SiO2 hybrid nanocomposite (UP-DI) was confirmed by infrared spectra (FTIR) and scanning electron microscopy (SEM). With the increase of DI contents, the limiting oxygen index (LOI) value of the cured UP-DIs was raised from 19 to 29, suggesting a significant improvement in flame retardance. The improved flame retardance could also be revealed by the variations of weight and morphology of char residues. Thermogravimetric analysis (TGA) both in air and in nitrogen was used to estimate mechanism of thermal degradation and activation energy (Ea) of different degree of conversion (α) was calculated by Ozawa’s method. The UP-DI containing 10%DI (UP-DI10) had higher Eathan the pure UP as α>5% because the degradation mechanism was altered by the incorporation of DI.

Knoop Microhardness and FT-Raman Spectroscopic Evaluation of a Resin-Based Dental Material Light-Cured by an Argon Ion Laser and Halogen Lamp: Anin VitroStudy

The purpose of this study was to evaluate in vitro the Knoop microhardness (Knoop hardness number [KHN]) and the degree of conversion using FT-Raman spectroscopy of a light-cured microhybrid resin composite (Z350-3M-ESPE) Vita shade A3 photopolymerized with a halogen lamp or an argon ion laser. Optimal polymerization of resin-based dental materials is important for longevity of restorations in dentistry. Thirty specimens were prepared and inserted into a disc-shaped polytetrafluoroethylene mold that was 2.0 mm thick and 3 mm in diameter. The specimens were divided into three groups (n = 10 each). Group 1 (G1) was light-cured for 20 sec with an Optilux 501 halogen light with an intensity of 1000 mW/cm(2). Group 2 (G2) was photopolymerized with an argon laser with a power of 150 mW for 10 sec, and group 3 (G3) was photopolymerized with an argon laser at 200 mW of power for 10 sec. All specimens were stored in distilled water for 24 h at 37 degrees C and kept in lightproof containers. For the KHN test five indentations were made and a depth of 100 microm was maintained in each specimen. One hundred and fifty readings were obtained using a 25-g load for 45 sec. The degree of conversion values were measured by Raman spectroscopy. KHN and degree of conversion values were obtained on opposite sides of the irradiated surface. KHN and degree of conversion data were analyzed by one-way ANOVA and Tukey tests with statistical significance set at p < 0.05. The results of KHN testing were G1 = 37.428 +/- 4.765; G2 = 23.588 +/- 6.269; and G3 = 21.652 +/- 4.393. The calculated degrees of conversion (DC%) were G1 = 48.57 +/- 2.11; G2 = 43.71 +/- 3 .93; and G3 = 44.19 +/- 2.71. Polymerization with the halogen lamp (G1) attained higher microhardness values than polymerization with the argon laser at power levels of 150 and 200 mW; there was no difference in hardness between the two argon laser groups. The results showed no statistically significant different degrees of conversion for the polymerization of composite samples with the two light sources tested.

Specific features of solid-phase reactions induced by shock waves

Kinetic and thermodynamic aspects of solid-phase reactions in recovery ampoules under shock compression are considered. Attention is paid to incompleteness of chemical conversion owing to the “cold” character of exothermic reactions in shock waves and to the dependence of the heat-release profile on the grain-size distribution of reagents. Different degrees of conversion at an identical level of pressure under loading and unloading are noted.

Silicon‐29 and carbon‐13 nuclear magnetic resonance identification of intermediates developed during the formation of a hybrid based on tetraethoxysilane (TEOS) and 4‐[(5‐dichloromethyl)(silyl)pentyloxy]cyanobenzene (DCN)

AbstractDuring the formation of organic–inorganic hybrid materials by the sol–gel process, the observation of multiphase systems at different stages of the reaction is not uncommon. An assessment of the chemical species formed before and after gelation is not an easy task because complex mixtures are generally found in these systems. In this study, the identification of intermediates developed during the formation of a hybrid obtained from tetraethoxysilane (TEOS) and 4‐[(5‐dichloromethylsilyl)pentyloxy]cyanobenzene monomer has been undertaken. Solution and solid‐state 29Si‐ and 13C‐NMR spectra, recorded at different degrees of conversion, allow the elucidation of the mechanism of hybrid formation. 4‐[(5‐dichloromethylsilyl)pentyloxy]cyanobenzene, (DCN), DCN–TEOS cyclosiloxanes and low‐molecular‐weight linear copolysiloxanes are formed in the early stages of the process and before gelation occur. Before the gel is formed, cyclic species undergo an acid‐catalyzed ring‐opening copolymerization with silica to produce a random copolymer. Other intermediates of the Qij type (where Q indicates a four‐functional silicon, the first subscript indicates the number of siloxane bridges connected to the silicon site, and the second subscript indicates the number of silanol groups) that develop before the gel is formed have also been identified. Spectral peak patterns of the final hybrid, observed in solid‐state 29Si and 13C‐NMR spectra, are also discussed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 520–531, 2006

Photosensitive 2,5-distyrylpyrazine particles produced from rapid expansion of supercritical solutions

Solvent-free, photoreactive particles of 2,5-distyrylpyrazine (DSP) monomer were developed by rapid precipitation from an expanding supercritical chlorodifluoromethane solution. DSP polymer particles were produced by solid-state photopolymerization. DSP particles below a critical diameter of about 0.5 μm were found to be mechanically stable and did not fragment upon photopolymerization. The rate of DSP photopolymerization was shown to be size-sensitive. Nano-scale particles demonstrated superior photoreactivity in the solid state in comparison to micro-scale crystals. UV spectra of DSP at different degrees of conversion were investigated and the extinction coefficients were calculated for the DSP monomer and polymer in sulfuric acid.

Comparing true countercurrent and simulated moving‐bed chromatographic reactors

AbstractSimulated Moving Bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single continuous and countercurrent unit. This integration promises substantial improvements in process performance, especially when applied to equilibrium‐limited reactions involving such heat‐sensitive products as fine chemicals and pharmaceuticals. In this work, the interplay among the relevant process design parameters (dimensionless ratios of the fluid and solid flow rates, and the Damköhler numbers for each section of the unit) is investigated. For this, an analytical solution of differential mass‐balance equations for the corresponding true countercurrent process (TCC), using as a model system the reaction A⇌B+C with each species exhibiting linear adsorption behavior, was developed. Based on this solution, criteria were derived for the optimum process design with respect to productivity and solvent consumption. Comparing these results with numerical simulations of an SMBR unit shows that the TCC model does not apply to SMBR units with a finite number of columns per section, that is, units of practical relevance, because the two units exhibit different residence time distributions and, hence, lead to different degrees of conversion.

In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites

Polymer composites have been used in dental applications for more than 25 years. Although their properties and behavior have been systematically improved, they still are not able to produce dental restorations chemically, dimensionally and mechanically stable for long periods of time. Low degrees of monomer conversion and poor processing control are some of the main features responsible for the materials instability. In this work, the monomer conversion of dental composites during visible light irradiation was in situ monitored by infrared spectroscopy. The relationship between degree of conversion and mechanical properties was obtained by evaluating the microhardness of composites with different degrees of conversion. The relationship obtained was then used to identify the evolution of the mechanical properties during photopolymerization.

Kinetics of Radical Polymerization at High Conversion I. GPC Measurements of Molecular Mass Distribution

A series of poly(methyl methacrylate) (PMMA) samples was prepared with different monomer concentrations and to different degree of conversion in benzene, at 50°C. Average molecular weights and intrinsic viscosity values in the eluent (THF) measured by GPC using an online viscometer are reported. At monomer concentrations up to 50 mol %, no gel effect is observed even in the case of very high conversions (∼90%); both molecular weights and kinetic parameters which determine the length of the polymer chain remain practically unchanged independent of conversion. From monomer concentrations of 75 mol %, significant increase of both of these values is observed depending on conversion. The average branching numbers of polymer chains for all samples remain close to zero. The polydispersity values are generally near 1.7. These results denote that there is no significant chain transfer to polymer and the disproportionation probability in chain termination reactions agree with earlier results described in the literature.

Addition of a reactive diluent to a catalyzed epoxy-anhydride system. II. effect on enthalpy relaxation

Enthalpy relaxation in an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a reactive diluent cured with methyl-tetrahydrophthalic anhydride (MTHPA) with an accelerator was investigated by differential scanning calorimetry. The reactive diluent (RD) added was an aliphatic diglycidyl ether which was mixed in a proportion of 50 parts by weight (pbw) per 100 parts of DGEBA, with the stoichiometric quantity of MTHPA. The key parameters of the enthalpy relaxation investigated were the nonlinearity parameter, x, the apparent activation energy,Δh*, and the nonexponentiality parameter, β. The results were compared with other data obtained previously in similar epoxy-anhydride systems without an RD but with different degrees of conversion in order to analyze the effects of (a) the introduction of aliphatic chains of the RD in the epoxy structure and (b) a reduction in the crosslink density of the resin. © 1997 John Wiley & Sons, Inc.

Addition of a reactive diluent to a catalyzed epoxy‐anhydride system. II. effect on enthalpy relaxation

Enthalpy relaxation in an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a reactive diluent cured with methyl-tetrahydrophthalic anhydride (MTHPA) with an accelerator was investigated by differential scanning calorimetry. The reactive diluent (RD) added was an aliphatic diglycidyl ether which was mixed in a proportion of 50 parts by weight (pbw) per 100 parts of DGEBA, with the stoichiometric quantity of MTHPA. The key parameters of the enthalpy relaxation investigated were the nonlinearity parameter, x, the apparent activation energy,Δh*, and the nonexponentiality parameter, β. The results were compared with other data obtained previously in similar epoxy-anhydride systems without an RD but with different degrees of conversion in order to analyze the effects of (a) the introduction of aliphatic chains of the RD in the epoxy structure and (b) a reduction in the crosslink density of the resin. © 1997 John Wiley & Sons, Inc.

Crystallization kinetics of poly(ethylene adipate) investigated by differential scanning calorimetry

Crystallization of poly(ethylene adipate) from the amorphous state under non-isothermal conditions with several heating rates has been investigated. The activation energy values, evaluated for different degrees of conversion, using the Ozawa plot, show a slightly decreasing tendency for the higher degrees of conversion.

Study of polyester dimethacrylates with a system of conjugated bonds

IR spectroscopic and densitometric methods have been used to study polymers of the OEA class 1,6-dimethacryloyloxy-2,4-hexadiyne ( I) and 1,6-dimethacryloyloxyhexamethylene ( II) with different degrees of conversion. It is shown that polymerization of I occurs both via the methacryl- and triple bonds with the formation of a three-dimensional polymer with a system of conjugated bonds.

Calorimetric determination of H+/M+ (M = Li, Cs) ion exchange in?-titanium hydrogen phosphate

H+/M+ (M = Li, Cs) ion exchange inγ-titanium phosphate (γ-TIP) at 25°C and under static conditions has been studied. Titration and hydrolysis curves and the exchange isotherms were determined. The substitution was followed by X-ray diffraction. Direct calorimetric measurements were carried out at different degrees of conversion and the variation of the exchange enthalpy was obtained. The shape of the calorimetric curves is discussed. The results are compared to the values of ΔH 0 obtained from titration data.

Circular dichroism and exciton effect of syndiotactic methacrylic polymers with chiral diacyl hydrazide side chains: 3. [formula omitted] and [formula omitted] and their N-t-butyloxycarbonyl protected derivatives

The phenomenon of exciton coupling, which is very well known from circular dichroism (c.d.) spectroscopy studies of biopolymers, has now also been found for syndiotactic vinyl polymers in solution. The syndiotactic polymers were prepared by reacting poly(methacrylic acid hydrazide) with the N-protected amino acids l-(+)-serine and l-(+)-alanine, yielding poly[methacrylic acid-N β(N( t-butyloxycarbonyl) l-seryl) hydrazide ] and poly[methacrylic acid-N β(N( t-butyloxycarbonyl) l-alanyl) hydrazide ] . On removal of the N-t-butyloxycarbonyl group, the corresponding deprotected polymers were also obtained. All four polymers possess functional and optically active lateral chains which are capable of mutual interactions by hydrogen bonding, dipolar and electrostatic forces. At strongly alkaline pH, the diacyl hydrazide moiety is deprotonated to confer polyelectrolyte character to the macromolecule. The interactions between lateral chains lead in the deprotonated state to spatial and conformational order between the side chains, causing sizeable excition effects in the c.d. spectra. The presence of exciton effects was supported by the c.d. spectroscopic behaviour of the polymers at different pH and by that of copolymers, the latter being obtained by different degrees of conversion of the poly(methacrylic acid hydrazide). The degree of order is strongly influenced by changing temperature and the addition of methanol and salts. By comparison with previously studied polymers derived from syndiotactic poly(methacrylic acid hydrazide) and l-(+)-lactic acid or d-(−)-lactic acid, the structural features necessary for exciton coupling are discussed. A model for the main-chain conformation which was proposed earlier was found to be compatible also with the results obtained for the present polymers.

Composites--characterization of composite filling materials: reactor response.

Dental composite restorative materials consist of an organic matrix, ceramic fillers, and the interface between the inorganic fillers and the matrix. Marked variations in the composition of the composite materials, as well as different degrees of conversion after polymerization, have been observed. These circumstances lead to substantial differences in the properties of polymerized composite materials. The variations in clinical behavior of the different composite materials, e.g., discoloration or lack of wear resistance, may be explained on the basis of differences in composition. This review paper describes the composition, conversion, and properties of presently available composite materials.

Calorimetric determination of H+/M+ (M=Na, K) ion exchange in ?-titanium phosphate

H+/M+ (M=Na, K) ion exchange on γ-titanium phosphate (γ-TiP) at 25°C and under static conditions has been studied. Titration and hydrolysis curves and the exchange isotherms were determined. The substitution was followed by X-ray diffraction. Direct calorimetric measurements were carried out at different degrees of conversion and the variation of the exchange enthalpy and the hydrolysis enthalpy of γ-TiP were obtained. The results are compared to the values of ΔH° obtained in previous works from titration data.

Analysis of Linear Viscoelasticity of a Crosslinking Polymer at the Gel Point

We suggest a very simple memory integral constitutive equation for the stress in crosslinking polymers at their transition from liquid to solid state (gel point). The equation allows for only a single (!) material parameter, the strength S[Pas1/2], and it is able to describe every known viscoelastic phenomenon at the gel point. Measurements were performed on polydimethylsiloxane model networks with balanced stoichiometry for which the crosslinking reaction has been stopped at different degrees of conversion. At the gel point, the loss and storage moduli were found to be congruent and proportional to ω1/2 over a wide range of temperature (−50°C to +180°C) and five decades of frequency ω. The hypothesis is made that this behavior is valid in the entire range 0<ω<∞. This congruence hypothesis is consistent with the Kramers‐Kronig relation and leads to a constitutive equation which shows that, for our polymer, congruent functions G′(ω)=G″(ω) are as much a rheological property at the gel point as are infinite ...

Role of Amines in the Isomerization Reaction of Polycondensed Maleic Anhydride and Diethylene Glycol

Abstract Aniline and N-ethylaniline were used as model compounds to study the effect of aromatic primary and secondary amines on maleate-fumarate isomerization. Amines were added to the copolymerization reaction of maleic anhydride and diethylene glycol at different degrees of conversion (P). The effect of amines on the relative amount of isomerization was followed with proton NMR. The efficiency of fumarate production decreases in the order P = 0.62 > 0.76 > 0.35 > 0. Mechanisms to account for the effect of aniline and N-ethylaniline on the maleate-fumarate isomerization are outlined.

Remaining methacrylate groups in composite restorative materials

The quantity of the remaining unreacted methacrylate groups in polymerized composite materials has been determined. Six proprietary composites were investigated by infrared multiple internal reflection spectroscopy. Infrared reflectance measurements were made before polymerization and repeated after the composites were subjected to polymerization at 37 degrees C for 24 hours. The quantities of remaining unreacted methacrylate groups were determined and the data expressed as percentages of the total amount of methacrylate groups in the unpolymerized materials. The specimens were specially prepared to ensure that the surface properties simulated the bulk properties of the polymerized composites. The quantities of remaining methacrylate groups in the six composites determined by this surface measuring technique ranged from 25 to 48%. The results demonstrate that commercially available composite materials exhibit different degrees of conversion 24 hours after the start of polymerization. These differences can be correlated to the different monomer compositions of the composite restorative resins.

Copolymerization of 1-vinylimidazole with 4-vinylpyridine

Abstract A study was made of copolymerization of 1-vinylmidazole (VIM) with 4-vinylpyridine (VP) under conditions of free radical initiation of AID at 60° in ethanol. Constants of relative activities of VIM and VP were calculated by non-linear approximation using a computer. It was shown that VP is a more active monomer. The relative activity of VP in relation to the VIM radical is 4·76. Diagrams were plotted to show copolymer compositions calculated with different degrees of conversion and different component ratios.