Infra-red spectrum of the tight (110) fold in n-C198H398

The alkane n-C₁₉₈H₃₉₈ has been crystallised in both extended chain and once-folded forms and annealed to produce materials with low concentrations of gauche bonds. The concentrations of the specific conformers detected by FTIR spectroscopy at −173 °C are calculated, using measurements on liquid n-hexadecane for calibration: values are all generally less than 2.0 per 100 carbon atoms, with extended chain samples showing values less than 1.0 per 100 carbon atoms. A subtraction spectrum (Once-folded chain sample minus Extended chain sample) shows positive bands at 1298, 1340, 1347 and 1369 cm⁻¹, which are predicted in earlier calculations for a (110) fold, while additional positive bands at 1353 and 1363 cm⁻¹ are assigned, respectively, to gg conformers and (tentatively) to strained gtg or gtg′ conformations.     

Pure and mixed gas CH4 and n-C4H10 sorption and dilation in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C₄H₁₀ and CH₄ sorption and dilation in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures ranging from −20 to 35 °C. The presence of n-C₄H₁₀ in the mixture considerably reduces CH₄ solubility. For example, CH₄ solubility (in the limit of zero CH₄ fugacity) at 25°C decreases from 4.0 (pure gas) to 0.78 cm³(STP)/(cm³ polymer atm) in the presence of n-C₄H₁₀ at an activity of 0.60. At −20 °C, CH₄ solubility decreases by almost an order of magnitude, from 10.2 (pure gas) to 1.22 cm³(STP)/(cm³ polymer atm) in the presence of n-C₄H₁₀ at an activity of 0.61. In contrast, n-C₄H₁₀ mixture sorption properties are not measurably affected by the presence of CH₄. The dual mode sorption model parameters for CH₄ and n-C₄H₁₀ in PTMSP were determined from pure and mixed gas sorption measurements, and this model can adequately describe the sorption data. The n-C₄H₁₀/CH₄ mixed gas solubility selectivity in PTMSP decreases as temperature increases and as n-C₄H₁₀ activity increases. For example, at 25 °C, the n-C₄H₁₀/CH₄ solubility selectivity decreases from 250 to 120 as n-C₄H₁₀ activity increases from 0.02 to 0.25. At −20 °C and an n-C₄H₁₀ activity of 0.24, the n-C₄H₁₀/CH₄ solubility selectivity is 590. Penetrant-induced volume dilation of PTMSP can be adequately modeled by assuming that all swelling is caused by penetrant molecules sorbed in the polymer's dense equilibrium region (i.e., the Henry's law region) during sorption. However, the best fit partial molar volumes in the Henry's law region for the dilation data are considerably lower than the penetrant partial molar volumes in liquids, suggesting that further theoretical efforts are needed to develop predictive models of volume dilation in high free volume glassy polymers.     

Structural behaviour of molecular alloys: n-pentacontane (n-C50H102)-n-pentacosane (n-C25H52); n-pentacontane (n-C50H102)-n-tricosane (n-C23H48) at room temperature

This paper displays a study of binary mixtures of n-alkanes whose ratio of chain length is around two. The systems composed of n-tricosane (n-C₂₃H₄₈)-n-pentacontane (n-C₅₀H₁₀₂) and n-pentacosane (n-C₂₅H₅₂)-n-pentacontane (n-C₅₀H₁₀₂) have been studied by means of X-ray analyses. These latter, performed at room temperature, showed in both cases, the existence of a large domain where the phases characteristic of each pure component coexist. These mixtures obey Kravchenko's rule relative to the solubility of the n-alkanes according to the chain length of each component. The mixtures studied do not form an intermediate solid solution. In other words, there is no particular arrangement of the shorter molecules inside the crystallographic unit of the longer.     

Crystallization process of poly(?-caprolactone)-poly(ethylene oxide)-poly(?-caprolactone) investigated by infrared and two-dimensional infrared correlation spectroscopy

The crystallization of poly(?-caprolactone)-poly(ethylene oxide)-poly(?-caprolactone) (PCL-PEO-PCL) triblock copolymer was studied using FTIR and 2D FTIR spectroscopies. The weight ratio of PCL/PEO in the investigated sample was about 20:1. Although it is such a low amount of PEO that it cannot form any crystals, the PEO block undergoes some structural change in the cooling process. It was established that the PCL constituent crystallized quickly, and then forced the noncrystallizable PEO constituent to form a tighter structure (helical conformation) from the trans zigzag conformation. Besides, through the 2D IR analysis, more exact and detailed assignments of the overlapped CH₂ bands have been made - the 1193 cm⁻¹ band is attributed to methylene next to the carbonyl group, whereas the 1162 cm⁻¹ and 1295 cm⁻¹ bands are assigned to other common methylenes.     

Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water☆

Partial oxidative gasification of n-hexadecane (n-C₁₆) and organosolv-lignin (lignin) was studied by use of a batch type reactor in supercritical water: 673 K, 0.52 cm⁻³ of water density (40 MPa of water pressure at 673 K), and 0.3 of O/C ratio for the n-C₁₆ experiments; 673 K, 0.35 cm⁻³ of water density (30 MPa of water pressure at 673 K), and 1.0 of O/C ratio for the lignin experiments. The experiments without O₂ were also conducted for lignin (lignin decomposition). For all the cases (n-C₁₆ partial oxidation, lignin decomposition, lignin partial oxidation), NaOH or zirconia (ZrO₂) was added in the system as catalysts. Through n-C₁₆ studies, the catalytic effect of NaOH and ZrO₂ on partial oxidation in supercritical water were examined. In the case of lignin partial oxidation, we studied the possibility of partial oxidation in supercritical water for gasification technique of wastes. The yield of H₂ from n-C₁₆ and lignin with zirconia was twice as same as that without catalyst at the same condition. The H₂ yield with NaOH was 4 times higher than that without catalyst. Thus, a base catalyst has a positive effect on partial oxidation of n-C₁₆ and lignin to produce H₂. The catalytic effect of NaOH and ZrO₂ was found to be enhancement of decomposition of intermediate (aldehyde and ketone) into CO, through n-C₁₆ studies. In the case of lignin studies, the enhancement of decomposition of the carbonyl compounds by catalytic effect of NaOH and ZrO₂ inhibit char formation and promotes CO and thus H₂ formation.     

In situ FTIR spectroscopic studies of (bi)sulfate adsorption on electrodes of Pt nanoparticles supported on different substrates

Nanostructured Pt electrodes were prepared by electrodeposition of Pt nanoparticles on different substrates (GC, Pt and Au) under cyclic voltammetric conditions and with various number (n) of potential cycling, and were denoted as nm-Pt/S(n) (S = GC, Pt and Au). Adsorption of (bi)sulfate on the nm-Pt/S(n) was studied by in situ FTIR reflection spectroscopy. It has been revealed that the nanostructured Pt electrodes exhibit anomalous IR properties for (bi)sulfate adsorption regardless of the different reflectivity of substrate, i.e. the IR absorption of (bi)sulfate species adsorbed on all the nm-Pt/S(n) electrodes is significantly enhanced and the IR band direction is completely inverted in comparison with the same species adsorbed on a bulk Pt electrode. The two IR bands around 1200 and 1110 cm⁻¹ attributed to adsorbed (bi)sulfate species are shifted linearly with increasing electrode potential, yielding Stark tuning rates () of 152.1 and 21.1 cm⁻¹ V⁻¹ on nm-Pt/GC(20), respectively. Along with increasing n, the Stark tuning rate of the IR band around 1200 cm⁻¹ decreases quickly and declined to 7.6 cm⁻¹ V⁻¹ on nm-Pt/GC(80), while the Stark tuning rate of the IR band near 1100 cm⁻¹ is fluctuated between 23.0 and 16.2 cm⁻¹ V⁻¹. It has determined that the enhancement of IR absorption of (bi)sulfate adsorbed on nanostructured Pt electrode is varied with substrate material and n, and a maximal 16-fold enhancement of the IR band near 1200 cm⁻¹ has been measured on the nm-Pt/GC(30) electrode. The in situ FTIR studies illustrated that the adsorption of (bi)sulfate occurs mainly in the double layer potential region, and reaches a maximum around 0.80 V. The results demonstrated also that the competitive adsorption of CO and oxygen species can inhibit completely (bi)sulfate adsorption, which has evidenced a weak interaction of (bi)sulfate with nm-Pt/S(n) electrode surface.     

Effect of the alkyl group on the synthesis and the electrochemical properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids

The effect of the alkyl side group on the synthesis and the electrochemical properties of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR_1ATFSI) ionic liquids (ILs) is reported. The investigation was focused on the PYR_1ATFSI ionic liquid family because of the interesting electrochemical properties of the members with propyl and butyl side chains. Side alkyl groups (A = C_nH_2n+1 with n ranging from 1 to 10) of different length and structure were used for the synthesis of PYR_1ATFSI materials. NMR and DSC have shown that the ionic liquids were correctly synthesized with the exception of the compounds with tertiary side chains. Most of the materials exhibited a conductivity higher than 10⁻³ S cm⁻¹ already at 12 °C. In the molten state a moderate conductivity decrease was observed with increasing the length and the branching of the side chain (C₂H_2n+1) group according with the change of viscosity of the ionic liquids. Most of the PYR_1ATFSI samples exhibited an electrochemical stability window exceeding 5 V.     

Synthesis and mesophase behaviors of 2,5-disubstituted styrene-based random copolymers: Effect of difference in side-group length on liquid crystallinity

Five series of binary copolymers, poly[2,5-di(ROOC)styrene-co-2,5-di(R′OOC)styrene]s (R = n-C₃H₇-, R′ = C₂H₅-, n-C₄H₉-, and n-C₅H₁₁-; R = n-C₆H₁₃-, R′ = n-C₄H₉-, and n-C₅H₁₁-), were synthesized via free radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities of the analogous monomers and proved by ¹³C NMR analysis. It was also implied by the smoothly varying glass transition temperatures and further supported by the monotonous variation in d spacings for the liquid crystalline copolymers, where both corresponding homopolymers are liquid crystals. Subtle difference of R and R′ was found to have significant impact on the mesomorphic properties of the copolymers. When the pair of the corresponding homopolymers has the same mesogenic structure, a difference of one carbon atom in the alkyl chain can be tolerated over the whole copolymer composition range; however, the liquid crystalline structure soon disappears with the incorporation of a co-unit of an homopolymer that is not liquid crystal. For the copolymers with alkyl chain length differing by two carbon atoms, the liquid crystalline structure is lost with the incorporation of relatively low co-unit content despite the pair of corresponding homopolymers having the same mesogenic structure.     

FTIR studies of conformational disorder: crystal perfecting in long chain n-alkanes

The low temperature FTIR spectra of long chain n-alkanes C₁₉₈H₃₉₈ and C₂₄₆H₄₉₄, crystallised from solution in both extended and once-folded forms, have been investigated in the CH₂ wagging region. Successive annealing and cooling stages result in a significant reduction in the number of non-planar C-C bond conformers in all cases. This is evidence of a ‘perfecting’ process within the crystals. The time dependence of this behaviour shows that, on raising the annealing temperature, the level of disorder reaches a maximum before declining. In addition, in a continuous heating run on once-folded C₁₉₈H₃₉₈, all types of conformational disorder except end-gauche bonds are shown to pass through a maximum near 118 °C, in the region of the transformation to extended chain crystals.     

Non-premixed fluidized bed combustion of C1-C4n-alkanes

The non-premixed combustion of C₁-C₄n-alkanes with air was investigated inside a bubbling fluidized bed of inert sand particles at intermediate temperatures: 923 K ? T_B ? 1123 K. For ethane, propane and n-butane, combustion occurred mainly in the freeboard region at bed temperatures below T₁ = 923 K. On the other hand, complete conversion occurred within 0.2 m of the injector at: T₂ = 1073 K. For methane, the measured values of T₁ and T₂ were significantly higher at 1023 K and above 1123 K, respectively. The fluidized bed combustion was accurately modeled with first-order global kinetics and one PFR model to represent the main fluidized bed body. The measured global reaction rates for C₂-C₄n-alkanes were characterized by a uniform Arrhenius expression, while the global reaction rate for methane was significantly slower. Reactions in the injector region either led to significant conversion in that zone or an autoignition delay inside the main fluidized bed body. The conversion in the injector region increased with rising fluidized bed temperature and decreased with increasing jet velocity. To account for the promoting and inhibiting effects, an analogy was made with the concept of induction time: the PFR length (b_i) of the injector region was correlated to the fluidized bed temperature and jet velocity using an Arrhenius expression. These results show that the conversion of C₂-C₄n-alkanes can be estimated with one set of critical bed temperatures and modeled with one Arrhenius kinetics expression.     

Thermally induced conformational and structural disordering in polyethylene crystal studied by near-infrared spectroscopy

Thermally induced structural and conformational changes in polyethylene (PE) samples were explored by using near-infrared (NIR) spectroscopy. The differences in the temperature-dependent structural disordering process among six PE samples were depicted by monitoring the intensities of NIR bands characteristic of orthorhombic crystalline phase. The temperature dependency of bands in the NIR region that have been considered to be due to orthorhombic crystalline lattice was compared to that of a band at 1378 cm⁻¹ due to the methyl symmetric bending mode. The intensity decrease of the band in the mid-infrared (MIR) region seems to sensitively reflect the overall disordering of orthorhombic crystalline structure. As a result of this study, the intensity decrease of the bands in the NIR spectral region was found to proceed at lower temperature than that of the band at 1378 cm⁻¹. This finding suggests the status of orthorhombic crystalline structure probed by the intensity of the band at 1378 cm⁻¹ and that by the “crystalline” bands in the NIR spectral region may not be identical. The NIR spectra were further analyzed by two-dimensional (2D) correlation spectroscopy to provide the in-depth analysis of NIR bands. The 2D correlation spectroscopy has detected the presence of two NIR bands at 4342 and 4290 cm⁻¹ due to orthorhombic crystalline phase and those at 5840 and 5640 cm⁻¹ due to amorphous phase. The hetero-spectral 2D correlation analysis was carried out between the NIR spectral region of 4365-4240 cm⁻¹ and the well-established MIR spectral region for CH₂ wagging deformation region of 1390-1240 cm⁻¹, where bands due to nonplanar conformer are detected. This approach allowed us to determine NIR bands, which behave in a way similar to MIR bands originating from conformational defect sequences that exist in the orthorhombic crystalline lattice, the amorphous domain and the chain fold regions. As a result of the hetero-spectral 2D NIR-MIR correlation spectroscopic studies on the development of conformational defect sequence in three types of PE samples, it was concluded that the intensity of a band at 4265 cm⁻¹ changes in the same manner as the MIR bands at 1368, 1353 and 1308 cm⁻¹ assignable to gtg, gg and gtg′ (kink) conformations. This finding means that the state of conformational disorder in PE crystal can be studied by monitoring the intensity of the NIR band at 4265 cm⁻¹. The use of NIR spectroscopy makes it possible to directly probe the degree in the formation of conformational defect sequences in thick PE products typically produced in industry, which cannot be studied by MIR spectroscopy. This paper thus provides in-depth fundamental understandings on NIR spectra of PE as well as the results of our study regarding structural and conformational changes in PE crystals probed by NIR spectroscopy.     

CO tolerance effects of tungsten-based PEMFC anodes

The performance of proton exchange membrane fuel cells (PEMFC) fed with CO-contaminated hydrogen was investigated for anodes with PtWO_x/C and phosphotungstic acid (PTA) impregnated Pt/C electrocatalysts. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with anodes containing 0.4 mg PtWO_x cm⁻² and also for those with 0.4 mg Pt cm⁻² impregnated with ca. 1 mg PTA cm⁻². A decay of the single cell performance with time is observed, and this was attributed to an increase of the membrane resistance due to the polymer degradation promoted by the crossover of the tungsten species throughout the membrane.     

Characterization by Fourier transform infrared spectroscopy of hydroxyapatite co-doped with zinc and fluoride

Pure and Zn²⁺ and/or F⁻ doped hydroxyapatite (HA) were synthesized by the precipitation method and detection of ion incorporations into the HA structure was investigated by a non invasive Fourier transform infrared (FTIR) spectroscopic technique. The synthesized materials were sintered at 1100 °C for 1 h. The Zn²⁺ addition amount was kept constant at 2 mol% whereas F⁻ amount was changed. The weight fractions of the HA and CaO were calculated by Rietveld analysis by using GSAS. Co-doping of Zn²⁺ and F⁻ ions increased the stability of HA. A detailed analysis of FTIR spectroscopy was performed to observe whether HA structure was formed or not. The bands corresponding to the (PO₄³⁻) functional group and (OH⁻) functional group were observed. Moreover, the ion incorporation into the HA structure and the amount of the ions were analyzed by FTIR spectroscopy. The OH…F bands were observed at 711 cm⁻¹ and 3543 cm⁻¹. The Zn–O stretching band was observed at 3403 cm⁻¹ and 433 cm⁻¹. The area calculation under the OH…F bands and (OH⁻) stretching and librational modes of the bands revealed that as the F⁻ amount increased, the area under the bands at 711 cm⁻¹ and 3543 cm⁻¹ increased whereas the area under the (OH⁻) stretching and librational modes of the bands decreased due to the fact that F⁻ ion replaced with (OH⁻) ion in HA structure. All these results showed that Zn²⁺ and F⁻ ions were successfully incorporated into the HA structure. Moreover, the amount of F⁻ ions in the HA structure was successfully confirmed by determination of the area under the F⁻ and (OH⁻) related bands.     

Regeneration of Bombyx mori silk by electrospinning. Part 3: characterization of electrospun nonwoven mat

Regenerated Bombyx mori silk fibroin in formic acid was electrospun and the morphological, chemical and mechanical properties of these nanofibers were examined by field emission environmental scanning electron microscopy (FESEM), Raman spectroscopy (RS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and tensile testing. FESEM indicated that the average fiber diameter was less than 100 nm and circular in cross section. This paper maps the silk fibroin molecular conformations of each step of the sample preparation and the electrospinning process. The secondary structural compositions (random and β-sheet) of the fibroin were determined by FTIR and RS. The crystallinity index of the electrospun fiber, calculated as the intensity ratio of 1624 (β-sheet) and 1663 (random) cm⁻¹ FTIR bands was higher than that of the pristine fiber. Raman spectra of the amide I (1665 cm⁻¹, random) to amide III (1228 cm⁻¹, β-sheet) ratio of the electrospun fiber was less than that of the pristine fiber indicative of higher β-sheet content. The fiber crystallinity, determined by XRD, showed a lower value for the electrospun fiber. The electrospun fiber shows small but significant increases in the β-sheet content in comparison with the pristine fiber. Dissolution of fibroin in formic acid enhances β-sheet crystallization and may facilitate β-sheet formation in electrospun fiber. The electrospun random silk mat had a Young's modulus, ultimate tensile strength and strain of 515 MPa, 7.25 MPa and 3.2%, respectively.     

Swelling behavior of amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate

The amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate with different 1-bromoalkanes (1-C_nH_2n+1Br, n = 2, 4, 6, 8, 12) were synthesized by radiation-induced polymerization and crosslinking. The length of alkyl side chains had significant influence on the swelling behavior of the resulting gels. The swelling degree of the gels decreased with the increase of side chain length, and the gel hardly swelled when n = 12. The effect of temperature and ionic strength on the swelling behavior of the resulting gels revealed that (1) the gels with longer side chains (n ≥ 8) had upper critical solution temperature, while other gels were not thermo-sensitive. (2) Antipolyelectrolyte effect was observed when immersing the gels (n ≥ 8) in NaCl solutions in certain concentration range. The dramatic difference in swelling behavior was attributed to the different gel structures. The gels with short side chains (n ≤ 6) had cellular structure of normal polyelectrolyte gels. The gels (n ≥ 8) had an aggregation gel structure caused by the hydrophobic interaction among alkyl groups and the formation of ion-cluster between tetra-alkyl ammonium cation and Br⁻, which had been analyzed with the aid of SEM, Br⁻-selective electrode and fluorescence molecular probe.     

Pure and mixed gas CH4 and n-C4H10 permeability and diffusivity in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C₄H₁₀ and CH₄ permeability coefficients in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures from −20 to 35 °C. CH₄ partial pressures range from 1.1 to 14.6 atm, and n-C₄H₁₀ partial pressures range from 0.02 to 1.8 atm. CH₄ permeability decreases with increasing n-C₄H₁₀ upstream activity (f/f_sat) in the feed. For example, at −20 °C, CH₄ permeability decreases by more than an order of magnitude, from 52,000 to 1700 Barrer, as n-C₄H₁₀ activity increases from 0 to 0.73. In contrast, n-C₄H₁₀ mixed gas permeability is essentially unaffected by the presence of CH₄. The depression of CH₄ permeability in mixtures is a result of competitive sorption and blocking effects, which reduce both CH₄ mixture solubility and diffusivity, respectively. Diffusion coefficients of n-C₄H₁₀ and CH₄ in mixtures were calculated from mixture permeability and mixture solubility data. The CH₄ concentration-averaged diffusion coefficient generally decreases as n-C₄H₁₀ activity increases. On the other hand, the n-C₄H₁₀ diffusion coefficient is essentially unaffected by the presence of CH₄. Pure and mixed gas activation energies of permeation and diffusion of CH₄ and n-C₄H₁₀ are reported. The mixed gas n-C₄H₁₀/CH₄ permeability selectivity increases with increasing n-C₄H₁₀ activity and decreasing temperature, and it is higher than pure gas estimates would suggest. Mixture diffusivity selectivity also increases with increasing n-C₄H₁₀ activity. The difference between pure and mixed gas permeability selectivity arises from both solubility and diffusivity effects. The dual mode mixed gas permeability model describes the mixture permeability data reasonably well for n-C₄H₁₀. However, the model must be modified to accurately describe the methane data by accounting for the decrease in methane diffusivity due to the presence of n-C₄H₁₀ (i.e., blocking). Even though the penetrant concentrations are rather significant at some of the conditions considered, no evidence is observed for phenomena such as multicomponent coupling that would require a model more complex than the binary form of Fick's law. That is, Fick's law in its simplest form adequately describes the experimental data.     

Thermolysis of the polyyne C8H2 in hexane and methanol: Experimental and theoretical study

The mean lifetimes of polyyne C₈H₂ in hexane were determined at 50, 60, 80, and 100 °C and in methanol at 60 °C. The reactions are second order at all temperatures: ln k₂ = 20.5 ± 1.5-10303 ± 520T⁻¹ and the corresponding activation energy is 85.7 ± 6.3 kJ mol⁻¹ (7164 cm⁻¹). Extrapolation suggests that solutions at 1 mM concentration are significantly unstable at room temperature. Quantum chemical calculations show that polyynes C_mH₂ + C_nH₂ (m + n = 16) could be products, but these were not detected. Alternatively, C₁₆H₂ isomers could form. IR spectra of the solid residues from hexane and methanol solutions were obtained.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Thermodynamics of sorption in and free volume of poly(5,6-bis(trimethylsilyl)norbornene)

Thermodynamics of sorption of n-alkanes C₄-C₁₁ in poly(5,6-bis(trimethylsilyl)norbornene) was studied using the Inverse Gas Chromatography (IGC). Temperature dependences of the specific retention volume V_g and solubility coefficient S of various alkanes were obtained in the range 60-140 °C. For solubility coefficients, the following equation holds: where T_c is the critical temperature of solutes, T is the experimental temperature and S is expressed in cm³(STP)/cm³ atm. The partial molar enthalpies ΔH_m and entropies ΔS_m of mixing of different n-alkanes in this glassy polymer vary in much wider range than in rubbers and display positive correlations: for more exothermic mixing process the larger negative ΔS_m values are observed. It was shown that the ΔH_m in poly(5,6-bis(trimethylsilyl)norbornene) pass through a minimum, when the size of solutes increases. The coordinates of ΔH_m at minimum versus solute size provide an estimate of the size of free volume elements in this polymer. This conclusion was supported by variation of permeability in different glassy polymers studied using IGC and by the results of other methods for probing free volume in glassy polymers.     

Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph

Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3₁ helices of isotactic sequences of different n length (n, number of monomer units). The second type corresponds to vibrational coupling of short sequences of the chain that include the 1-hexene comonomer. Among the latter are absorbances at 910 and 1025 cm⁻¹ which are markers for the formation of a trigonal phase in these copolymers. They remain unchanged prior to and during crystallization in copolymers with the 1-hexene units rejected from the crystallites (<13 mol% 1-hexene) and increase in intensity when the comonomer is an integral part of the crystallites (>13 mol% 1-hexene). Analysis of the real time evolution of IR regularity bands during isothermal crystallization of these copolymers confirms the beginning of crystallization at a critical helical sequence length (n∗) of ∼12 isotactic units (841 cm⁻¹), and enables details of the early and final stages of crystallization. In the homopolymer and copolymers, the intensity of regularity bands with n ≤ 10 is constant in the initial undercooled melt, and increases simultaneously with the appearance of helices with n ≥12, in support of a classical crystallization mechanism of nucleation and growth. Due to density fluctuations in the initial melt, the short helices eventually collapse in aggregates or precursors that spontaneously (within the experimental macroscopic time frame) extend to stable nuclei (n ≥ 10). Stable nuclei further extend and grow cooperatively dragging additional short sequences as inferred by the simultaneous temporal evolution of helices with n = 10 and greater. The intensity of the 998 cm⁻¹ (n = 10) band prior to nucleation, correlates directly with the isotactic sequence length of the copolymer and is independent of the final structure that evolves, either monoclinic or trigonal. This feature infers a nucleation event driven preferentially by the initial steady-state content of short helices in iPP and iPP-based copolymers. The temporal evolution of the 841 cm⁻¹ band is an excellent avenue to study the crystallization kinetics of copolymers, including those with very low crystallinities. Via FTIR, the mechanism of the formation of mesomorphic crystallites in copolymers with ∼10 mol% 1-hexene at low temperatures is contrasted with the formation of alpha crystallites at higher temperatures in the nucleation driven range. The intensity of the 841 cm⁻¹ band (n = 12) at the end of the transformation correlates linearly with the degree of crystallinity obtained by WAXD.