Christian Pellerin

Orientation and relaxation of orientation of amorphous poly(ethylene terephthalate)

Abstract Poly(ethylene terephthalate) (PET) has been uniaxially stretched at different draw ratios and draw rates above its glass transition temperature, in the 80–105°C temperature range. Molecular orientation and relaxation have been followed by birefringence. A decrease in temperature reduces the mobility of the oriented chains resulting in a slow relaxation while an increase in stretching rate results in higher orientation values and rapid relaxation after the extension. The same relaxation behavior has been observed from birefringence and polarization modulation infrared spectroscopy. Rouse relaxation times have been estimated from rheological master curves and birefringence relaxation data, while retraction and the reptation times have been deduced from the scaling laws proposed by Doi and Edwards.

Submolecular Plasticization Induced by Photons in Azobenzene Materials

We demonstrate experimentally for the first time that the illumination of azobenzene derivatives leads to changes in molecular environment similar to those observed on heating but that are highly heterogeneous at the submolecular scale. This localized photoplasticization, which can be associated with a free volume gradient, helps to understand the puzzling phenomenon of photoinduced macroscopic material flow and photoexpansion upon illumination far below the glass transition temperature (T(g)). The findings stem from the correlation of infrared (IR) spectral band shifts measured upon illumination with those measured at controlled temperatures for two amorphous DR1-functionalized azo derivatives, a polymer, pDR1A, and a molecular glass, gDR1. This new approach reveals that IR spectroscopy can be used as an efficient label-free molecular-scale thermometer that allows the assignment of an effective temperature (T(eff)) to each moiety in these compounds when irradiated. While no band shift is observed upon illumination for the vibrational modes assigned to backbone moieties of pDR1A and gDR1 and a small band shift is found for the spacer moiety, dramatic band shifts are recorded for the azo moiety, corresponding to an increase in T(eff) of up to nearly 200 °C and a molecular environment that is equivalent to thermal heating well above the bulk T(g) of the material. An irradiated azo-containing material thus combines characteristic properties of amorphous materials both below and above its bulk T(g). The direct measurement of T(eff) is a powerful probe of the local environment at the submolecular scale, paving the way toward better rationalization of photoexpansion and the athermal malleability of azo-containing materials upon illumination below their T(g).

Performance and Application of a New Planar Array Infrared Spectrograph Operating in the Mid-Infrared (2000–975 cm -1 ) Fingerprint Region

A no-moving-part planar array infrared spectrograph (PA-IR) equipped with a 256 × 256 mercury cadmium telluride (MCT) focal plane array has been designed and constructed. The performance of the instrument, whose frequency range extends from 2000–975 cm−1, has been assessed in terms of resolution, bandwidth, and signal-to-noise ratio. The PA-IR spectrograph is able to record spectra with an 8.7 ms time resolution and has peak-to-peak noise levels as low as 2.4 × 10−4 A.U. As a demonstration of the potential of PA-IR, the dynamics of reorientation of a liquid crystalline sample exposed to a single electric field pulse has been studied. It was shown that PA-IR can be used for the simultaneous acquisition of two orthogonally polarized spectra. The advantages and limitations of PA-IR, step-scan Fourier transform infrared (FT-IR), and ultra-rapid-scanning FT-IR for real-time studies of reversible and irreversible phenomena are thoroughly discussed.

Solid-State NMR Structure Determination of Whole Anchoring Threads from the Blue Mussel Mytilus edulis

The molecular structure of the blue mussel Mytilus edulis whole anchoring threads was studied by two-dimensional (13)C solid-state NMR on fully labeled fibers. This unique material proves to be well ordered at a molecular level despite its heterogeneous composition as evidenced by the narrow measured linewidths below 1.5 ppm. The spectra are dominated by residues in collagen environments, as determined from chemical shift analysis, and a complete two-dimensional assignment (including minor amino acids) was possible. The best agreement between predicted and experimental backbone chemical shifts was obtained for collagen helices with torsion angles (-75°, +150°). The abundant glycine and alanine residues can be resolved in up to five different structural environments. Alanine peaks could be assigned to collagen triple-helices, β-sheets (parallel and antiparallel), β-turns, and unordered structures. The use of ATR-FTIR microscopy confirmed the presence of these structural environments and enabled their location in the core of the thread (collagen helices and antiparallel β-sheets) or its cuticle (unordered structures). The approach should enable characterization at the molecular level of a wide range of byssus macroscopic properties.

Structure and phase behavior of the poly(ethylene oxide)-thiourea complex prepared by electrospinning.

Electrospinning was used for the first time to prepare nanofibers of the host/guest complex between poly(ethylene oxide) (PEO) and thiourea. It is shown by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) that the stoichiometry of the complex is (EO)(12)-(thiourea)(8), settling a series of conflicting values in literature reports. The complex crystallizes in a monoclinic unit cell with a = 9.15 A, b = 18.88 A, c = 8.25 A, and beta = 92.35 degrees. On the basis of WAXD, infrared spectroscopy, and polarized Raman scattering measurements, it is proposed that the complex adopts a layered structure in which alternating PEO and thiourea layers are stabilized by intermolecular hydrogen bonds. This structure is highly reminiscent of that of the beta complex between PEO and urea. A phase diagram was determined and shows that the complex melts incongruently at 110 degrees C to form a peritectic liquid and crystals of pure thiourea. The nanofibers of the PEO-thiourea present a very large molecular orientation with a (c) value of 0.76, among the largest reported for electrospun materials.

Study of polymer orientation and relaxation by polarization modulation and 2D-FTIR spectroscopy

´ a CERSIM, Departement de chimie, Uni˝ersite La˝al, Cite uni˝ersitaire, La˝al, Quebec, Canada G1K 7P4 ´´ ´ ´ b Laboratoire de physicochimie moleculaire, Uni˝ersite de Bordeaux I, Bordeaux, 33405 Talence, France ´´ Abstract . Polarized Fourier transform infrared FTIR spectroscopy has been used extensively to study polymer orientation. The dichroic ratio and the dichroic difference are normally obtained from spectra recorded sequentially with the infrared radiation polarized parallel and perpendicular to a reference direction. To improve the sensitivity of this technique and to be able to follow accurately the dynamics of orientation, FTIR spectroscopy has been coupled with the polarization modulation .PM technique. With this technique, the dichroic difference spectrum is recorded directly, thus minimizing instrumental and sample fluctuations. The results obtained demonstrate the high efficiency of polarization modulation infrared linear . dichroism PM-IRLD to determine quantitatively the time dependence of the orientation function of several chemical groups during the orientation and relaxation processes. Examples of the application of this technique to study in situ the . . . dynamics of orientation will be presented for both stretched films of polystyrene PS and poly vinyl methyl ether PVME blends and optically oriented copolymers containing azobenzene side chains. The use of the PM-IRLD technique has allowed the direct determination of the relaxation kinetics of the components in polymer blends. In the case of the azopolymers, the time-dependent spectra obtained by PM-IRLD have also been analyzed by two-dimensional Fourier . transform infrared 2D-FTIR spectroscopy. The results obtained with this technique show that the spectral resolution is significantly enhanced in the asynchronous maps that also provide valuable information about the relative movement of the different chemical groups of the polymers. q 1998 Elsevier Science B.V. All rights reserved.

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.

Tg and rheological properties of triazine-based molecular glasses: incriminating evidence against hydrogen bonds.

Bis(mexylamino)triazines have been identified as a family of compounds showing an exceptional propensity to form glassy phases as opposed to crystals. The particularities of this family of compounds are their ability to self-assemble through hydrogen bonding in well-defined patterns to form supramolecular aggregates which pack poorly and the wide range of glass transition temperatures (T(g)) that can be attained through minor structural modifications. Representative bis(mexylamino)triazines were studied by rheology to establish correlations between their rheological properties and their molecular structure, and all compounds were found to behave in a similar fashion except for the temperature at which glass transition takes place. FTIR and NMR spectroscopy experiments were performed on the molecular glasses studied herein; comparisons between the viscosity, T(g), hydrogen bonding, and association constant (K(a)) in CDCl3 solution have revealed a relationship between the rheological properties, the T(g) of the molecular glasses, and the extent and strength of hydrogen bonding present in the material.

Role of hydrogen bonding in the formation of glasses by small molecules: a triazine case study

Diaminotriazine derivatives with 3,5-dimethylphenyl groups have been shown to be capable of readily forming extremely stable glassy phases under ambient conditions, and self-assembly through hydrogen bonding has been thought to be a key element in this process. Herein, we probed the role played by hydrogen bonding in glass formation in this family of compounds by studying the crystal structures of both molecular glasses and closely similar compounds that crystallize under ambient conditions. We also compared a molecular glass in the crystalline and amorphous states by FTIR spectroscopy to monitor changes at the molecular level as the material underwent glass transition. We have shown that hydrogen bonding favors glass formation through the formation of aggregates that pack poorly, and breaking of hydrogen bonds plays a role in the physical changes associated with glass transition.

New developments in planar array infrared spectroscopy

A planar array infrared (PA-IR) spectrograph offers several advantages over other infrared approaches, including high acquisition rate and sensitivity. However, it suffers from some important drawbacks, such as a limited spectral range and a significant curvature of the recorded spectral images, which still need to be addressed. In this article, we present new developments in PA-IR spectroscopy that overcome these drawbacks. First, a data processing method for the correction of the curvature observed in the spectral images has been developed and refined. In addition, a dual-beam instrument that allows the simultaneous recording of two independent spectral images has been developed. These two improvements have been combined to demonstrate the real-time background correction capability of PA-IR instruments. Finally, the accessible spectral range of the PA-IR spectrograph has been extended to cover simultaneously the methylene stretching (3200–2800 cm−1) and the fingerprint (2000–1000 cm−1) spectral regions.