Melting behavior of poly(3-hydroxybutyrate) investigated by two-dimensional infrared correlation spectroscopy

The melting behavior of a bacterially synthesized biodegradable polymer, poly(3-hydroxybutyrate) (PHB), was investigated by using generalized two-dimensional infrared (2D IR) correlation spectroscopy. Temperature-dependent spectral variations in the regions of the C-H stretching (3100-2850 cm(-1)), C=O stretching (1800-1680 cm(-1)), and C-O-C stretching (1320-1120 cm(-1)) bands were monitored during the melting process. The asynchronous 2D correlation spectrum for the C=O stretching band region resolved two crystalline bands at 1731 and 1723 cm(-1). The intense band at 1723 cm(-1) may be due to the highly ordered crystalline part of PHB, and the weak band at 1731 cm(-1) possibly arises from the crystalline part with a less ordered structure. These crystalline bands at 1731 and 1723 cm(-1) share asynchronous cross peaks with a band at around 1740 cm(-1) assignable to the C=O band due to the amorphous component. This observation indicates that the decreases in the crystalline components do not proceed simultaneously with the increase in the amorphous component. In the 3020-2915 cm(-1) region where bands due to the asymmetric CH3 stretching and antisymmetric CH2 stretching modes are expected to appear, eight bands are identified at 3007, 2995, 2985, 2975, 2967, 2938, 2934, and 2929 cm(-1). The bands at 2985 and 2938 cm(-1) are ascribed to the amorphous part while the rest come from crystal field splitting, which is a characteristic of polymers with a helical structure.     

Specific crystal structure of poly(3-hydroxybutyrate) thin films studied by infrared reflection–absorption spectroscopy

Infrared reflection–absorption (IR-RAS) and transmission spectra were measured for poly(3-hydroxybutyrate) (PHB) thin films to explore its specific crystal structure in the surface region. As IR-RAS is sensitive to the vibration mode of perpendicular orientation of the surface, differences between IR-RAS and transmission spectra indicate an orientation of the lamella structure in the surface of PHB thin films. The relative intensity of the crystalline CO stretching band in the IR-RAS spectrum is significantly weaker than that in the transmission spectrum. It may be concluded that the transient dipole moment of the CO stretching mode of the crystalline state is not oriented perpendicular but nearly parallel to the substrate surface. On the other hand, the relative intensity of the band at 3009 cm⁻¹ due to the C–H stretching mode of the C–HOC hydrogen bonding is similar between the IR-RAS and transmission spectra, suggesting that the C–H bond is oriented neither perpendicular nor parallel to the substrate surface but in an intermediate direction. Since the CO group of the C–HOC hydrogen bonding is oriented nearly parallel to the surface and its C–H group is in the intermediate direction, it is very likely that the C–HOC hydrogen bonding has a somewhat bent structure. These results are in good agreement with our previous conclusion that the C–HOC hydrogen bonding of PHB exists along the a-axis (not the b-axis) between the CH₃ group of one helix and the CO group of another helix.     

Composition analysis of poly(ethylene glycol)-poly(L-lactide) diblock copolymer studied by two-dimensional column chromatography

We used two-dimensional column chromatography to analyze the composition of a sample of presumably a diblock copolymer of poly(ethylene glycol) (PEG) and poly(L-lactide) synthesized from monomethoxy-terminated PEG. The first dimension of the separation is phase fluctuation chromatography to prepare fractions that contain various components of the copolymer in different ratios. The second dimension is size-exclusion chromatography, NMR, and HPLC at the critical condition of PEG. The PEG initiator has small amounts of diol-terminated dimeric components. We found that the copolymer sample contains a triblock copolymer and low-molecular-mass components in addition to the main part of the diblock copolymer. The SEC chromatograms show that the main part consists of two components with distinct peak lengths for the PLLA block. The low-molecular-mass components have a broad distribution in chemical composition. Phase fluctuation chromatography enriched the triblock copolymer and the diblock copolymer with the longer PLLA block in early fractions when the column was packed with carboxymethyl-modified porous silica. When the porous medium was PLLA-grafted silica, size exclusion dominated, but the low-molecular-mass components were separated according to their chemical composition.     

Determination of tacticity in poly(vinyl chloride) by infrared spectroscopy

From the temperature dependence of infrared spectra of poly(vinyl chloride) samples prepared by different methods, the intensity of the band at 690 cm.¹ (proportional to the number of isotactic diads in the sample), as well as that of the tacticity-independent C? H stretching band, was found to be independent of the crystallinity of the sample. These lines were therefore applied for the tacticity determination in poly(vinyl chloride), measured in the form of KBr pellets. The numerical tacticity value was obtained from the known values of absorbance coefficients of SCH and SHH type C? Cl stretching bands in solution, and from the shape of the spectrum.     

Isothermal crystallization behavior of water in poly(vinyl methyl ether) aqueous solution investigated by infrared and two-dimensional infrared correlation spectroscopy

Isothermal crystallization behavior of water at −30 °C in PVME aqueous solution with the PVME concentration in the range of 40-60 wt% was investigated in detail by time-dependent infrared spectroscopy and two dimensional correlation analysis. The result suggests that when the PVME concentration is between 40 and 60%, the crystallization rate decreases with increasing PVME concentration, and the crystallization of water in low temperature is kinetically controlled. Of particular interest is that the so-called “unfrozen bound water” can be frozen slowly when PVME aqueous solution is annealed at a suitable low temperature. The crystallization mechanism of water in PVME/water system is elucidated by 2D correlation analysis.     

Conformational composition of meta- and ortho-fluoro(trifluoromethoxy)benzene as studied by vibrational spectroscopy

The IR spectra of o- and m-F-C₆H₄OCF₃ in the gas, liquid, and solid (glass and crystal) phases were analyzed along with the Raman spectra of these compounds in the liquid and solid (glass and crystal) phases. This investigation includes both experimental and theoretical studies of the spectra. Vibrational frequencies and normal modes for all possible stable conformers of m- and o-fluoro(trifluoromethoxy)benzene were calculated using B3LYP/cc-pVTZ harmonic quantum-chemical force fields. It was found that m-F-C₆H₄OCF₃ exists as a mixture of the orthogonal and two planar conformers in the gas, liquid, and amorphous solid (glass) phases and as one orthogonal conformer in the crystal state, while o-F-C₆H₄OCF₃ exhibits only one orthogonal conformer in every phase.     

Resorbable materials of poly(L-lactide)

Poly(L-lactide) PLLA materials with various porosity in the range of 5 to 300 Μm were produced by crystallization of the polymer in presence of low molecular weight additives from solution in good solvents, followed by the diluent extraction. For the concentration of diluent in PLLA in the range of 10 to 50 per cent by weight, the pore size increased with increasing concentration of additive, while the additive was easily extracted from the polymer. For the concentration of additive in PLLA in the range of 60 to 80 per cent by weight, the pore size increased or/and decreased with increasing concentration of the additive, while the latter was hardly extractable from the polymer. Porous, resorbable tubes or rods of various diameter well accepted by living organism were produced by this method. Tracheal prostheses, pins or porous reservoirs made from PLLA or modified porous PLLA could be some of many potential application of these materials.     

Conformational isomerism of 1-butene secondary ozonide as studied by means of matrix isolation infrared absorption spectroscopy

Theoretical calculations of structures, stability and vibrational spectra of 1-butene secondary ozonide (SOZ) conformers were performed using DFT method B3LYP with a 6-311++G(3df, 3pd) basis set. The calculations predict six staggered structures of 1-butene SOZ. The FTIR spectra of 1-butene SOZ isolated in Ar, N₂ and Xe matrices were recorded. It was found that nitrogen is the best suited for the matrix isolation of 1-butene SOZ. The bandwidth of the spectral bands of the ozonide isolated in nitrogen was as narrow as 2 cm⁻¹. For the first time the existence of five conformers of 1-butene SOZ were confirmed experimentally by means of matrix isolation infrared absorption spectroscopy. The equatorial gauche (∠OCCC=−66.1°) conformer was proved theoretically and experimentally to be the most stable. It was found that due to high potential barriers of the conformational transitions annealing of the matrix is useless for the assignment of spectral bands to various conformers of 1-butene SOZ. Using the hot nozzle technique the van’t Hoff experimental plots were made for three additional conformers of 1-butene SOZ and experimental ΔH values for these additional conformers were established. The crystallization problems of 1-butene SOZ are discussed which accounts for the rich conformational diversity of the ozonide as well as high conformational barriers for axial-equatorial transitions.      

Electrospun chitosan-coated fibers of poly(L-lactide) and poly(L-lactide)/poly(ethylene glycol): preparation and characterization

Fibrous poly(L-lactide) (PLLA) and bicomponent PLLA/poly(ethylene glycol) mats were prepared by electrospinning and then were coated with chitosan. The presence of chitosan coating was proved by scanning electron microscopy and by fluorescence microscopy. On contact with blood, the chitosan coating led to changes in erythrocyte shape and in their aggregation. The haemostatic activity of the mats increased with increasing chitosan content. Microbiological studies against Staphylococcus aureus revealed that the chitosan coating imparts antibacterial activity to the hybrid mats. The combined haemostatic and antibacterial activities render these novel materials suitable for wound-healing applications.     

Radiolysis of ferrocyanide solutions studied by infrared spectroscopy

The behavior of the neutral and basic aqueous ferrocyanide system under irradiation is investigated using the coupling of a LINAC with infrared spectroscopy. The comparison between the neutral and basic system evidences the formation of the hydroxopentacyanoferrate (III) ions and gives information on the reaction mechanisms. The pseudo-protective effect of the dissolved dioxygen on the ferrocyanide is explained via a mechanism implying the superoxide radical anion.     

Ordered structures of syndiotactic poly(methyl methacrylates) studied by a combination of infrared,Raman, and NMR spectroscopy.

The C ? O stretching vibrations in Raman and infrared spectra of poly(methyl methacrylate) (PMMA) samples of various stereoregularity in solutions and in the solid state were measured. It was found that for ordered structures of syndiotactic (s) PMMA, transition dipole coupling of ester groups in ordered s-PMMA sequences leads to a splitting of the C ? O stretching vibration into three components with wavenumbers equal in the solid state and in solution, and of different activity in Raman and infrared spectra. A comparison of the time dependences of self-aggregation of s-PMMA as measured by NMR and infrared spectroscopy indicates that the first phase of the aggregation process is detected by both methods. NMR analysis of the effect of the degree of stereoregularity on the self-aggregation of s-PMMA shows that the prerequisite for the generation of a stable ordered structure is the mutual interaction of two s-sequences of a minimum length of 9 monomer units. The experimental data obtained lead to the conclusion that, in self-aggregated s-PMMA, some of the ester groups are in close contact, with the interacting s-sequences probably forming a double helix.     

Thermal oxidation of poly(1-trimethylsilylprop-1-yne) studied by IR spectroscopy

Thermal oxidation of poly(1-trimethylsilylprop-1-yne) was studied by IR spectroscopy in the 20—245 °C temperature interval. In the 20—160 °C temperature range, the reaction proceeds predominantly at the C—Me group as revealed by the decrease in the intensity of the bands of the methyl group bound to the C atom and the appearance of the bands of the hydroperoxide and methylene groups. The decomposition of hydroperoxides produces aldehydes and ethers. At 160—200 °C, oxidation occurs via two routes: at the C—Me and C=C groups, while the Me₃Si group remains unchanged. At 230—240 °C, the rate of the reaction occurring at the C=C bond is higher than the rates of the processes involving the MeC and Me₃Si groups. The relative content of the structural units was calculated for the samples oxidized at different temperatures. Plausible mechanisms of thermal oxidation of poly(1-trimethylsilylprop-1-yne) were considered on the basis of the data obtained.     

Conformational behavior of simple furanosides studied by optical rotation

Experimental and theoretical specific optical rotations (OR) of anhydro, epithio, and epiminoderivatives of methyl tetrofuranosides in chloroform solutions have been compared and used as a tool for exploring their conformational behavior. The potential energy surfaces of these saccharides with reduced flexibility were examined with the density functional theory and the MP2 and CCSD(T) wavefunctions methods. Theoretical ORs were obtained by Boltzmann averaging of values calculated for local minima. Resultant rotations could be used to assess the quality of the DFT and MP2 relative conformer energies. OR values calculated for equilibrium geometries in vacuum were significantly improved when the solvent was accounted for by a polarizable continuum model and first and diagonal second OR derivatives were used for an anharmonic vibrational averaging. The DFT used as a default method reproduced the experimental data fairly well. A modified B3LYP functional containing 70% of HF exchange further improved the results. Because of the strong dependence of OR on the conformation, not only the absolute configuration could be determined, but also the conformational populations were estimated. Likewise, the predicted dependence of OR on the light wavelength well agreed with experiment. The increasing precision of the contemporary computational methods thus makes it possible to relate the specific rotation to more detailed features in molecular structure. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

Conformational dynamics of bistable RNAs studied by time-resolved NMR spectroscopy

The structural transition between two alternate conformations of bistable RNAs has been characterized by time-resolved NMR spectroscopy. The mechanism, kinetics, and thermodynamics underlying the global structural transition of bistable RNAs were delineated. Both bistable RNA conformations and a partial unstructured RNA of identical sequence could be trapped using photolabile protecting groups. This trapping allowed for an investigation of the initial folding from an unfolded RNA to one of the preferred conformations of the bistable RNA and of the structural transitions involved. Folding of the secondary structure elements occurs rapidly, while the global structural transition of the bistable RNA occurs on a time scale of minutes and shows marked temperature dependence. Comparison of these results with bistable systems previously investigated leads to the prediction of activation enthalpies (DeltaH++) associated with global structural transitions in RNA.     

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.Dedicated to Prof. Dr. Erhard W. Fischer on the occasion of his 75th birthdayAn erratum to this article can be found at     

Copper-on-copper homoepitaxy studied by infrared spectroscopy of adsorbed CO

Infrared spectroscopy of adsorbed CO was used to characterize the dependence of surface structure on deposition temperature during homoepitaxial growth on Cu(100). Intensity borrowing due to dipole coupling greatly enhances the absorption signal due to defect-bonded CO, making it possible to detect and quantify defect concentrations at the level of a few percent. For deposition temperatures between 300 and 400 K, the defect density increases slightly with decreasing deposition temperature but remains below 2%. There is a sharp increase in defect density, up to 5%-6%, as the deposition temperature is decreased from 300 to 250 K. At lower deposition temperatures, there is some sign of a leveling off in defect density, but the IR absorption spectrum becomes so broad that meaningful analysis becomes impractical, while visible degradation of the low-energy electron diffraction pattern indicates worsening surface order. No indication of "re-entrant" ordering at low temperatures was observed for deposition temperatures down to 150 K.     

LCST and UCST behavior of poly(N-isopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy

Temperature-responsive phase separations of poly(N-isopropylacrylamide) (PNiPAm)/dimethylsulfoxide (DMSO)/water mixtures have been investigated by infrared and confocal micro-Raman spectroscopy. The ternary mixtures exhibited lower critical solution temperature (LCST) and upper critical solution temperature (UCST) phenomena at low and high DMSO concentrations, respectively. The amide I band of PNiPAm consists of two components; the intensity of the 1650 cm-1 component increased, and that of the 1625 cm-1 component decreased with increasing temperature during both LCST and UCST phase transitions. Gradual red shifts of the C-H stretching and the amide II bands with increasing temperature or increasing DMSO concentration indicate a removal of water molecules from the alkyl and N-H groups. Raman microscopic measurements showed that DMSO is excluded from the polymer-rich phases upon both LCST and UCST phase separation. On the basis of the experimental results and the quantum chemical calculations, a model that explains the solvation change of the polymer during phase transitions was proposed.     

Synthesis and characterization of poly(ethylene glycol) grafted poly(L-lactide)

Poly(ethylene glycol) grafted poly(L -lactide) was prepared by ring opening polymerization of L -lactide and epoxy-terminated poly(ethylene glycol) methyl ether (PEGME). Stannous octoate and Al(Et)₃·0.5 H₂O were tested as polymerization catalysts, and Al(Et)₃·0.5 H₂O was found to be more effective for the ring-opening of the epoxy group of the modified PEGME monomer. The synthesized polymers were characterized by NMR and the efficiency of the incorporation of epoxy-terminated PEGME in the copolymer was determined.