Molecular orientation of poly(ethylene terephthalate) and poly(butylene terephthalate) probed by polarized Raman spectra: a parallel study

Peculiar behavior of the ester carbonyl vibrational modes in anisotropic aliphatic and semi-aromatic polyesters

The current work reports on a systematic study related to the vibrational modes of the ester carbonyl group in drawn polyesters. We have observed and try to explain how the presence of aromatic units in the molecular structure substantially affects the respective elements of the Raman tensor in contrast to the dipole moment derivative vector which is only marginally influenced. The work is based on the collection of polarized Raman spectra and FTIR dichroism measurements on the one hand and on DFT calculations on the other. The experimental data were obtained from uniaxially stretched aliphatic and semi-aromatic polyesters. The calculations were applied on relevant oriented oligomers and allowed the extraction: (i) of reliable Raman/FTIR vibrational spectra and (ii) the components of the dipole moment derivative and Raman tensor of the vibrational modes and in particular the ones involving the ester carbonyl group. Experimental data indicate that the intensity of the ester carbonyl band is considerably enhanced in the Raman spectra of semi-aromatic polyesters, which results from a considerable enhancement of the related coupling coefficient. Furthermore, the angles of the principle Raman tensor axis are rotated so that the element of the tensor with the greatest value is oriented towards the direction designated by the segment. The latter explains the peculiar experimentally indicated anisotropy, through the ester carbonyl stretching, for the case of semi-aromatic polyesters, which is totally different with that observed in the aliphatic ones.

Orientation of Poly(ε-caprolactone) in Its Poly(vinyl chloride) Blends Crystallized under Strain: The Role of Strain Rate

The blends of high and low molecular weights poly(ε-caprolactone) (PCL) with poly(vinyl chloride (PVC) were prepared. The samples before and after the crystallization of PCL were uniaxially stretched to different draw ratios. The orientation features of PCL in a stretched crystalline PCL/PVC blend and crystallized from the amorphous PCL/PVC blends under varied strains were studied by wide-angle X-ray diffraction (WAXD) diffraction. It was found that a uniaxial stretching of crystalline PCL/PVC blend with high molecular weight PCL results in the c-axis orientation along the stretching direction, as is usually done for the PCL bulk sample. For the stretched amorphous PCL/PVC blend samples, the crystallization of high molecular weight PCL in the blends under a draw ratio of λ = 3 with a strain rate of 6 mm/min leads to a ring-fiber orientation. In the samples with draw ratios of λ = 4 and 5, the uniaxial orientation of a-, b-, and c-axes along the strain direction coexist after crystallization of high molecular weight PCL. With a draw ratio of λ = 6, mainly the b-axis orientation of high molecular weight PCL is identified. For the low molecular weight PCL, on the contrary, the ring-fiber and a-axis orientations coexist under a draw ratio of λ = 3. The a-axis orientation decreases with the increase of draw ratio. When the λ reaches 5, only a poorly oriented ring-fiber pattern has been recognized. These results are different from the similar samples stretched at a higher strain rate as reported in the literatures and demonstrate the important role of strain rate on the crystallization behavior of PCL in its blend with PVC under strain.

Poly(ethylene Terephthalate) Carbon-Based Nanocomposites: A Crystallization and Molecular Orientation Study

Hybrid polymeric materials incorporating carbon nanostructures or inorganic constituents stand as a promising class of materials exhibiting distinct but also complementary features. Carbon nanotubes have been proposed as unique candidates for polymer reinforcement; however, sustained efforts are further needed in order to make full use of their potential. The final properties of the reinforced polymer are controlled in part by the morphology and the eventual molecular orientation of the polymer matrix. In the present study, multiwall carbon nanotubes (MWCNTs) were utilized in order to reinforce polyethylene terephthalate (PET) composites. The effect of CNTs on the crystallization and the orientation of the structurally hybridized polymeric material has been investigated from the perspective of assessing their impact on the final properties of a relevant nanocomposite product. Functionalized MWCNTs were used to achieve their optimal dispersion in the polymer matrix. The physical properties of the composites (i.e., crystallinity and orientation) were characterized via differential scanning calorimetry, X-ray diffraction, and polarized Raman microscopy. The addition of well-dispersed CNTs acted as a nucleation agent, increasing the crystallization of the polyethylene terephthalate matrix and differentiating the orientation of both CNTs and macromolecular chains.

Effect of carbon nanotube modification on poly (butylene terephthalate)-based composites

The influence of the chemical modification of carbon nanotubes on the mechanical, thermal and electrical properties of poly(butylene terephthalate)-based composites was investigated. Polymer composites based on poly(butylene terephthalate) were obtained via in situ polymerisation or extrusion. Commercially available multi-walled carbon nanotubes (Nanocyl NC7000) at different loadings (mass %: 0.05, 0.25, 1, 2) were used as fillers. The functionalisation process took place under a chlorine atmosphere followed by a reaction with sodium hydroxide. The effect of carbon nanotube modification was analysed according to the changes in the polymer thermal and mechanical properties. An addition of modified carbon nanotubes in the amount of 0.05 mass % improved the mechanical properties of the composites in terms of both Young’s modulus and tensile strength by 5–10 % and 17–30 % compared with composites with unmodified carbon nanotubes and neat poly(butylene terephthalate), respectively. The in situ method of composite preparation was a more effective technique for enhancing the matrix–filler interactions, although a significantly lower amount of fillers were used than in the extrusion method.

Novel method for quantifying molecular orientation by polarized Raman spectroscopy: a comparative simulations study

Polarized Raman spectroscopy is widely used to quantify the level of molecular orientation of various types of materials. By using a simplified procedure we call the depol (depolarization) constant (DC) method, since it assumes that the depolarization ratio is a constant. However, our ability to quantify orientation by using the DC method is often limited by the need for a completely isotropic sample showing the same chemical and phase composition as the oriented sample of interest to obtain information on the depolarization ratio. In this paper, we propose a new method for orientation quantification, the most probable distribution (MPD) method, based on the hypothesis that the population distribution is the most probable one. In contrast to the conventional DC procedure, this new method does not require knowledge of the depolarization ratio and eliminates the assumption that it does not evolve on orientation. Simulations show the wide applicability of the MPD method for large sections of the 〈P2〉 〈P4〉 diagram, especially for coordinates that are most likely to be observed in experimental conditions. They also highlight the significant inaccuracies produced by the conventional DC method due to depolarization ratio errors.

Non-invasive detection of antibiotics and physiological substances in the aqueous humor by raman spectroscopy

BACKGROUND AND OBJECTIVES

Laser Raman spectroscopy is an inelastic light scattering technique able to characterize molecules in aqueous environments. The purpose of this work is to develop a non-contact and non-invasive spectroscopic method to identify and eventually quantify the presence of medicines (e.g., antibiotics) and physiological substances (e.g., glucose) in the aqueous humor of the eye.

STUDY DESIGN/MATERIALS AND METHODS

A new laser light delivery probe has been developed and adapted to a Raman spectroscopic system with the ability of favorable collection of the Raman light at 90 degrees scattering geometry while scanning the anterior chamber of the eye. Different amounts of ceftazidime, amphotericin B, and glucose had been injected in the aqueous humor of porcine eyes, maximum 24 hours after death and extraction, in-vitro. Raman measurements were excited with a visible (514.5 nm) laser beam at a power of 25 mW and an exposure/acquisition time of 1 second.

RESULTS

The specific collection optics and Raman analysis components used in the present work have resolved the Raman signatures of probed molecules and low concentrations of ceftazidime (0.9 mg/mL), amphotericin B (9 microg/mL), and glucose (2 mg/ml) separately injected in the anterior chamber of porcine eyes were detected in vitro.

CONCLUSION

This special illumination design gives the opportunity of avoiding the direct exposure to the laser light of basic cordial tissues of the eye, like lens and retina, although an optimum collection of scattered light is accomplished. Concentrations close to the minimum inhibitory concentration (MIC) have been detected for ceftazidime and amphotericin b; the detection of glucose has been realized at concentrations close to the early pathological levels of patients with diabetes.