Stretching Raman Mode Research Articles

Interrelation of macroscopic mechanical properties and molecular scale thermal relaxation of biodegradable and non-biodegradable polymers

Comparative analysis of macroscopic mechanical properties of a biodegradable polymer polypropylene carbonate (PPC) is carried out concerning two most commonly used, non-biodegradable synthetic polymers-high-density polyethylene (HDPE) and linear-low density polyethylene (LLDPE). Responses of the films of these polymers when subjected to mechanical and thermal stresses are analyzed. Response to tensile stress reveals the highest elongation at break (EB) in PPC films (396 ± 104 mm), compared to HDPE (26 ± 0.5 mm) and LLDPE (301 ± 143 mm), although the elastic modulus (YM) of PPC is around 50 ± 6 MPa, 3-fold lesser than LLDPE (YM = 153 ± 7 MPa) and 6-fold lesser than HDPE (YM = 305 ± 32 MPa). The plastic deformation response of PPC is intermediate to that of HDPE and LLDPE; initial strain softening is followed by strain hardening in LLDPE, a plateau region in PPC, and prolonged strain softening in HDPE. Crystalline domains in HDPE produce restriction on molecular motion. Crystallinity abruptly decreases by 70% in HDPE following a thermal quench, showing the possibility of free chain molecular mobility during plastic deformation. High correlation among Raman modes for all polymers reveals cooperative relaxation processes after thermal quench; C-C stretching modes and C-H bending, CH2wagging modes have Pearson's correlation coefficient 0.9. The integrated peak intensity and width of the C-C stretching Raman mode is 3-fold higher in PPC than HDPE after a thermal quench, showing enhanced molecular mobility and contributing modes in PPC. The peak width of this mode shows a strong negative correlation of -0.7 with the YM and a strong positive correlation of 0.6 with EB, showing that higher amorphicity leads to enhanced molecular mobility and EB at the cost of YM. This study reveals importance of molecular-scale response in governing the macroscopic properties of polymers.

Competing Interactions: Evolution of Inter and Intramolecular Hydrogen Bonds in Salicylic Acid at High Pressures.

Benzoic acid derivatives are important molecular systems in the pharmaceutical industry. Salicylic acid is distinct among the derivatives of benzoic acid due to the presence of an intramolecular hydrogen bond. With a view to study the evolution of inter and intramolecular hydrogen bonding at shorter length scales, in situ high pressure Raman spectroscopic measurements, angle dispersive X-ray diffraction experiments, and density functional theory (DFT) based first principle calculations have been carried out on crystalline salicylic acid. Subtle structural modifications are noted across ∼1 GPa leading to structural phase transition to a new crystalline phase above 7 GPa which is reversible. Substantial softening of the OH stretching Raman mode associated with intramolecular hydrogen bond is observed prior to the transition pressure. Possible molecular configurations associated with tautomerization are discussed with the aid of DFT calculations.

Infrared and multi‐wavelength Raman spectroscopy of regio‐regular P3HT and its deutero derivatives

AbstractThe recently synthesized regio‐regular poly(3‐hexylthiophene‐2,5‐diyl) selectively deuterated on the main chain backbone and/or on the hexyl side chain have given the opportunity to record their infrared (IR) and Raman spectra, and to carry out a spectroscopic study supported by density functional theory calculations. The Effective Conjugation Coordinate associated with the collective C=C stretching mode, with Raman scattering and IR absorption near 1,450 cm−1, is used as a probe of the electronic structure and the molecular conformation of the chain backbone. With the help of the data collected from the deuterated species, the vibrational assignment for the structurally relevant 1,600–1,300 cm−1 region has been clarified. The excitation‐dependent wavenumbers and intensities of the C=C stretching Raman modes are discussed. Raman spectra excited in‐resonance or off‐resonance show the existence of a multimodal distribution of effective conjugation lengths that are ascribed to a phase hairy‐A with a practically flat backbone chain, a phase hairy‐B where the conformation of the backbone is slightly distorted, and an amorphous phase. IR spectra provide additional information on the Effective Conjugation Coordinate.

Pressure induced transformations in sorbic acid

This research reports a pressure dependent Raman study of the sorbic acid between 0.0 and 10.0GPa. The unpolarized Raman spectra were measured in the spectral range of 20–3000cm−1. The high-pressure Raman scattering study of the sorbic acid showed that it underwent a gradual, disordering process. At the room temperature and at the ambient pressure conditions, the crystal structure of the sorbic acid belongs to the monoclinic system with a C2/c (C2h6) space group. The pressure increase induced a higher disorder in the monoclinic unit cell, since a single bending mode, and only very broad stretching Raman modes are present at pressure of ~10GPa. Upon pressure release the high-pressure phase transforms directly into the ambient-pressure phase. The presence of the internal vibrational modes is a guarantee that the molecular structure is maintained. Beyond this, the presence of external modes shows that the crystal has a memory to reverse the process and suggest that the crystal, which was in high disorder (broad Raman bands), does not suffer decomposition in the crystalline structure. The DFT calculations for the sorbic acid were performed in order to understand the vibrational properties. The theoretical study showed that the volume of the unit cell and beta angle decrease significatively when passing from the 0.0GPa to 8.0GPa. The decreases in the volume and beta angle of this particular unit cell were supposed to induce the larger increase in the bandwidths of the observed bands, pointing to some disorder in the monoclinic phase.

Ultra‐Low Fire Glass‐Free Li3FeMo3O12 Microwave Dielectric Ceramics

A new ultra‐low fire glass‐free microwave dielectric material Li3FeMo3O12 was investigated for the first time. Single phase ceramics were obtained by the conventional solid‐state route after sintering at 540°C–600°C. The atomic packing fraction, FWHM of the Ag oxygen‐octahedron stretching Raman mode and Qf values of samples sintered at different temperatures correlated well with each other. The sample with a Lower Raman shift showed a higher dielectric constant. Interestingly, the system also showed a distinct adjustable temperature coefficient of resonant frequency (from −84× 10−6/°C to 25 × 10−6/°C).

De‐intercalation process from Stage‐1 to Stage‐2 graphite intercalation compounds revisited

AbstractThe identification and contribution of the different phonons present in the G‐line Raman response in Stage‐1 and Stage‐2 graphite intercalation compounds (GIC) is crucial for a correct stage determination. Different factors like laser induced de‐intercalation play an important role in the precise stage assignment of these phases, and their intrinsic Raman response. In this contribution, an in situ micro‐Raman analysis was conducted under high‐vacuum conditions. Local heating of the samples was induced by using a high laser power (8.5 mW) in order to study the de‐intercalation process from Stage‐1 to Stage‐2 GICs. A detailed Raman line‐shape analysis was performed from the recorded spectra to determine the changes from the G‐line response of KC8, CaC6, and LiC6. We confirmed the assignment of the broad E2g Fano mode at ∼1510 cm−1 to the intrinsic Stage‐1 Raman response in GICs. Additionally, the most evident change from Stage‐1 to Stage‐2 was observed in an asymmetric Fano mode in the range of 1565–1610 cm−1. This mode is linked to the first‐order stretching Raman mode of Graphite, which tends to increase in frequency and decrease in width as function of de‐intercalation. Finally, the response of the Stage‐2 phase after de‐intercalation was confirmed to be a useful benchmark for the identification of the intercalation stage in highly doped GICs.

Structural and Magnetic Property of Ion Irradiated TiO2 Single Crystals

Ferromagnetism is induced in pure TiO2 single crystals by oxygen ion irradiation. The ferromagnetism is observed up to room temperature and is with weak temperature dependence. By combining X-ray diffraction, Rutherford backscattering/channelling, Raman scattering, and electron-spin resonance spectroscopy, supperconducting quantum interference device, displacement per atom, we measured the lattice damage accumulation with increasing fluences. A defect complex, i.e., Ti3+ on the substitutional accompanied by oxygen vacancies, has been identified in the irradiated TiO2. This kind of defect complex results in a local (TiO6−x) stretching Raman mode. We elucidate that Ti3+ with one unpaired 3d electron provide the local magnetic moments.

Symmetric C–C Stretching Mode Splitting versus CH2-Chain Conformation Order in Sodium Montmorillonite Modified by Cetyltrimethylammonium Bromide

Exploiting Raman spectroscopy and computational modeling, for the first time, we report and explain an interesting phenomenon in clay modified by cetyltrimethylammonium bromide. A splitting of the CH(2)-chain's symmetric C-C stretching Raman mode found at ~1128 cm(-1) in cetyltrimethylammonium bromide into two bands at 1128 and 1139 cm(-1) in clay modified by cetyltrimethylammonium bromide is observed. We demonstrate that this splitting appears if two types of trans-segments with nonequivalent lengths and terminal groups coexist in the CH(2)-chain of the alkylammonium ion embedded into the clay interlayer space. We report Raman experimental evidence for a CH(2)-chain bending within the clay galleries, resulting in the symmetric C-C stretching band splitting, as was also suggested by computational modeling. Noteworthy, we postulate that this unique behavior based on CH(2)-chain bending provides a general understanding of conformation reorganization and switching within long CH(2)-chain molecules confined within modified clay interlayer galleries. For all modifier concentrations, we show that the intercalated cetyltrimethylammonium ions exist in a liquid-like state, consisting mainly of trans conformations (~86%) of two types in approximately equal proportions. Moreover, we demonstrate that the integral Raman intensity ratio I(1295)(CH(2))/I(705)(clay) provides a rapid nondestructive quantification of the relative content of alkylammonium ions in modified clays. These results demonstrate that a simple direct monitoring of specific modifier-dependent interlayer conformational states is possible, which is of great importance for a tunable fabrication of modified clays-based nanocomposites with desired properties.

Synthesis, Thermal Expansion Properties and Raman Spectroscopic Study of Ca<sub>1-x</sub>Sr<sub>x</sub>Zr<sub>4</sub>P<sub>6</sub>O<sub>24</sub> Ceramics

A simplified method with much lower time and energy costs is presented for the rapid synthesis of low thermal expansion materials of CaZr4P6O24, SrZr4P6O24 and their solid solution Ca0.5Sr0.5Zr4P6O24. The coefficients of thermal expansion of CaZr4P6O24, SrZr4P6O24 and Ca0.5Sr0.5Zr4P6O24 are measured to be -1.45×10-6, 2.1×10-6 and 0.26×10-6, respectively. Raman spectroscopic study confirms the formation of the solid solution of Ca0.5Sr0.5Zr4P6O24 though its symmetric and asymmetric stretching modes are obviously broadened with respect to those of CaZr4P6O24 and SrZr4P6O24 due to the lattice deformation by incorporation of Ca2+ and Sr2+ with different cation size. The shifts of the asymmetric stretching Raman modes and the librational/translational modes with temperature in Ca0.5Sr0.5Zr4P6O24 are between those in CaZr4P6O24 and SrZr4P6O24, suggesting a cancelling effect of Ca2+ and Sr2+ cations in the thermal expansion of Ca0.5Sr0.5Zr4P6O24.

Resonance Chemical Imaging of Polythiophene/Fullerene Photovoltaic Thin Films: Mapping Morphology-Dependent Aggregated and Unaggregated C═C Species

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of approximately 1450 (I(C=C)(agg)) and approximately 1470 cm(-1) (I(C=C)(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I(C=C)(agg) and I(C=C)(un) species (R = I(C=C)(agg)/I(C=C)(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both R and frequency dispersion resonance Raman images of I(C=C)(agg) and I(C=C)(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.

Origin of magnetic moments in defectiveTiO2single crystals

In this Brief Report we show that ferromagnetism can be induced in pure ${\text{TiO}}_{2}$ single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman scattering, and electron-spin resonance spectroscopy, a defect complex, i.e., ${\text{Ti}}^{3+}$ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated ${\text{TiO}}_{2}$. This kind of defect complex results in a local $({\text{TiO}}_{6\ensuremath{-}x})$ stretching Raman mode. We elucidate that ${\text{Ti}}^{3+}$ ions with one unpaired $3d$ electron provide the local magnetic moments.

Exploring Three-Dimensional Nanosystems with Raman Spectroscopy: Methylene Blue Adsorbed on Thiol and Sulfur Monolayers on Gold

Resonant Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl- and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.

Vicker’s hardness and Raman spectroscopy evaluation of a dental composite cured by an argon laser and a halogen lamp

We present the results of the Vicker's hardness test and the use of near-infrared Raman spectroscopy (RS) to measure in vitro the degree of conversion (DC) of a bis(phenol)-A-glycidyl-dimethacrylate-based composite resin, photoactivated by both a halogen lamp (power density=478 mW/cm(2); 8-mm diameter spot) and an argon laser (power density=625 mW/cm(2); 7-mm diameter spot). The degree of conversion was estimated by analyzing the relative intensities between the aromatic C=C stretching Raman mode at 1610 cm(-1) and the methacrylate C=C stretching Raman mode (1640 cm(-1)) on top and bottom surfaces. For the hardness evaluation, the samples were embedded in polyester resin and three indentations with a 50-g load for 10 s were made on the top surface. The higher relative DC values achieved by the photoactivation of a composite resin by the argon laser suggest a better biocompatibility in the bottom surface. The correlation test showed that the higher Vicker's hardness number (VHN) values were associated with higher DC values. The derivative analysis showed a greater curing rate from 5 to 20 s of exposure. The comparison of VHN and DC values with both light sources at each curing time showed that a small change in conversion is related to a large change in hardness. Raman spectroscopy is more sensitive to changes in the first stages of curing reaction than later ones, and the Vicker's hardness assay is more sensitive to changes in the last stages.

Raman and FTIR studies of phase transitions in the organic superconductor κ-(BEDT-TTF) 2Cu[N(CN) 2]Cl

The Raman active CC stretching modes and CH 2 deformation modes and the infrared active CN mode are studied as a function of temperature in κ-(BEDT-TTF) 2Cu(N(CN) 2)Cl under pressure. Effects of the structural and superconducting transitions on these excitations are analyzed. The CH 2 and CN modes are sensitive to the thermal expansion around 80 K and to a possible superlattice transition around 160 K while the CC stretching Raman modes ν 2(A g) are influenced by the formation of the superconducting state.

A vibrational study of various KAuCl 4− xBr x solid solutions

Infrared and Raman vibrational studies have been undertaken on various powdered KAuCl 4− x Br x (0≤ x≤4) solid solutions. Characteristic variations of the spectra clearly demonstrate the existence of distinct individual planar anions. From a comparison of the vibrational data with results of valence force field calculations, we are able to discuss the domains of existence of several anions and to characterise with confidence the following halide mixed species: • The (AuCl 3Br) − anion is characterised in Infrared by four stretching (at 354, 343, 330 and 236 cm −1) and three deformation bands (at 196, 178 and 148 cm −1) and in Raman by essentially the intense line at 236 cm −1(ν Au–Br). • The (AuCl 2Br 2) − ‘ trans’ displays intense infrared (at 357 and 254 cm −1) and Raman (at 340 and 203 cm −1) stretching bands. • The (AuClBr 3) − anion exhibits three stretching Raman modes (at 236, 212 and 203 cm −1) and one stretching (at 336 cm −1) plus three bending Infrared modes (at 178, 134 and 118 cm −1). Furthermore, from intensity variations of the Raman lines at 203 and 236 cm −1, it is evidenced that the 236 cm −1 signal is mainly due at low ‘ x’ concentration to the only (AuCl 3Br) − entity, but at higher ‘ x’ values ( x>30%) to at least two species, including perhaps the (AuCl 2Br 2) − ‘ cis’ anion.

Raman and microwave surface impedance measurements in the organic superconductor κ-Et 2Cu[N(CN) 2]Br xCl 1−x

We report a Raman study of CH 2 and C=C vibrations for single crystals of the organic superconductor κ-Et 2Cu[N(CN) 2]Br xCl 1−x (nominal value x=.5) in both the normal and superconducting states (T c=l 1.3 K). The average charge transfer ρ=0.5 is estimated from the frequency of the C=C ring stretching Raman mode ν 3 (Ag). Raman results are discussed in connection with our previous study of the normal and superconducting states of Et 2Cu[N(CN) 2]Br. High sensitivity microwave surface impedance is also studied. The microwave resistance R s and microwave reactance X s are reported as a function of temperature in order to complement our Raman experiments and to determine precisely the critical temperature T c and the type of superconductivity.

Rheo-optical Raman study of chain deformation in uniaxially stretched bulk polyethylene

A rheo-optical study has been developed to examine the nature of the structural changes, other than the well known chain orientation, that can appear during uniaxial drawing of polyethylene samples. Stretching was performed at room temperature in samples of melt-crystallized high-and low-density polyethylene. Raman spectra were simultaneously collected along with stress and strain data in order to gain insight into the molecular basis of the properties resulting from the mechanical process. The need for real-time studies was to detect possible transient occurrences that cannot be observed with non-rheo-optical studies. These facts originate from the rapid relaxation phenomena of this polymer. By these means, a band shift and asymmetric band broadening were observed on the CC asymmetric stretching Raman mode (1064 cm −1), which has been attributed to some degree of backbone chain deformation. The band shift and, accordingly, the rate of bond deformation did not show a linear stress dependence. There is no substantial band shift during the first moments of the stretching. That initial zone was followed by a stage, during formation of the transition fronts, where the band shift begins to be appreciable. Finally, a third stage was observed when the observation zone falls into the neck. In that stage, a linear dependence was found between the band shift and the applied load. Qualitatively, the observed trend can be thought of as the consequence of the initial isotropic nature of the stretched sample. For that kind of sample, the transference of the load on the polymer backbone is not effective until the sample shows a high degree of orientation in the stretching direction. The band shifts were reversible as the load was removed.

Anomalous pressure effects on the Raman spectra in hydrogen-bonded molecular chain systems

Effects of hydrostatic pressure on the Raman spectra have been investigated for four kinds of 1,3-diketone crystals with hydrogen-bonded molecular chains. In all the crystals we studied, intense CO stretching Raman mode shows pressure-induced softening reflecting compression of the hydrogen bonds. Furthermore, application of pressure broadens the both C=O and C–O stretching modes and intensifies several specific vibrational bands. We ascribed these spectral changes to formation or growth of the kink-type defects of the hydrogen-bonded sequence.

The role of water in biological systems

Abstract A general model of the liquid water structure consistent with its physical, chemical, structural, and also biological properties has not been described in the literature so far. Therefore, different structure models of liquid water have been taken into account in relation to its OH stretching Raman modes. Some factors (structure factor, H-bond cooperativity) typical of liquid and biological water have been considered. Also, the different roles played by water in biological systems are elucidated and discussed by means of a few general structure models, suitable for explaining the perturbations induced on liquid water by solutes (ionic, acids and bases, apolar) and surfaces. In particular, the possibility of strong acid—base H-bond interactions between different groups has been explored together with the biological role of water. Furthermore, the modulating effect of water on the H-bond strength and the consequent properties have been hypothesized. Finally, the behaviour of water structure in tissues and its possible correlation with normal and pathological conditions are reported.