Experimental determination of the fiber orientation parameters and the Raman tensor of the 1614cm−1 band of poly(ethylene terephthalate)

Raman Analysis of Orientation and Crystallinity in High Tg, Low Crystallinity Electrospun Fibers

Electrospun fibers of amorphous or low-crystallinity polymers typically exhibit a low molecular orientation that can hamper their properties and application. A key stage of the electrospinning process that could be harnessed to mitigate the loss of orientation is jet rigidification, which relates closely to the solvent evaporation rate. Here, we establish quantitative Raman methods to assess the molecular orientation and crystallinity of weakly crystalline poly(2,6-dimethyl-1,4-phenylene oxide) fibers with varying diameters. Our findings demonstrate that solvent volatility can be leveraged to modulate the orientation and crystallinity through its impact on the effective glass transition temperature (Tg,eff) of the polymer jet during the electrospinning process. Specifically, a highly volatile solvent yields a higher and more sustained orientation (median ⟨P2⟩ of 0.53 for diameters < 1.0 µm) because its fast evaporation rapidly increases Tg,eff above room temperature. This vitrification early along the jet path promotes the formation of an oriented amorphous phase and a moderate fraction of strain-induced crystals. Our data reveals that a high Tg is a crucial parameter for reaching high orientation in amorphous or low-crystallinity polymer systems.

Raman Investigation of the Processing Structure Relations in Individual Poly(ethylene terephthalate) Electrospun Fibers

*These authors contributed equally.Electrospun fibers often exhibit enhanced properties at reduced diameters, a characteristic now widely attributed to a high molecular orientation of the polymer chains along the fiber axis. A parameter that can affect the molecular organization is the type of collector onto which fibers are electrospun. In this work, we use polarized confocal Raman spectromicroscopy to determine the incidence of the three most common types of collectors on the molecular orientation and structure in individual fibers of a broad range of diameters. Poly(ethylene terephthalate) is used as a model system for fibers of weakly crystalline polymers. A clear correlation emerges between the choice of collector, the induced molecular orientation, the fraction of trans conformers, and the degree of crystallinity within fibers. Quantitative structural information gathered by Raman contributes to a general description of the mechanism of action of the collectors based on the additional strain they exert on the forming fibers.

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.

Orientation-independent spectra for biaxial systems in polarized Raman microspectrometry

From an extension of the scattering intensity expressions in polarized Raman macrospectrometry to the case of micro-Raman backscattering experiments, we have established theoretical expressions to calculate orientation-independent intensity sums. This approach makes use of the K2 Raman invariant and of correction coefficients due to integration of the Raman scattering over the cone of collection of the objective lens, and it may be applied to uniaxial as well as to biaxial symmetry systems. The intensity sums thus obtained are expected to be orientation insensitive and allow one to compare conformational changes in various biaxially oriented polymer samples, either amorphous and/or semicrystalline thin films. As application examples we have compared the polarized Raman results obtained for various biaxially oriented amorphous and semicrystalline polystyrene (PS), poly(ethylene terenaphthalate) (PEN), and poly(ethylene terephthalate) (PET) thin film samples with their respective "isotropic spectrum" and have considered the different main conformational changes in related polymer systems. The method is thus tested on these macromolecular systems and appears quite successful in providing evidence for the molecular conformational changes without interferences from spectral contributions due to orientation.

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

Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) samples uniaxially drawn above Tg and beyond the yield point exhibit significant differences in their molecular orientation behavior as probed by polarized Raman spectra. The quasi-amorphous PET samples, drawn close to the Tg, manifest considerable molecular orientation development; however, when drawn above Tg + 30 degrees C, they exhibit significant molecular orientation relaxation. The semi-crystalline PBT samples maintain prominent molecular orientation even when drawn 110 degrees C above Tg. The drawing process, in PET samples, when resulting in molecular orientation, is accompanied by a gauche-trans transformation of the glycol linkage and a concurrent initiation of crystallinity development. In PBT specimens, it gives rise to a coexistence of alpha- and beta-type crystalline phases. Phase alpha is predominant at high draw temperatures, i.e., Tg + 110 degrees C, while phase beta dominates at low draw temperatures, i.e., Tg + 10 degrees C. PBT samples, with beta-phase predominance, left at relevant draw temperatures without stress, exhibit a beta-alpha phase change though no molecular orientation relaxation occurs. A note is made of the fact that complete molecular orientation analysis of PBT segments utilizing the depol method gives more reliable results than the simplified analysis assuming a cylindrical tensor for the 1614 cm(-1) symmetric stretch of the para-disubstituted benzene ring of PBT. In this context, segments of PBT specimens rich in alpha-phase exhibit higher molecular orientation than those with beta-phase predominance.

Effects of Electrospinning and Solution Casting Protocols on the Secondary Structure of a Genetically Engineered Dragline Spider Silk Analogue Investigated via Fourier Transform Raman Spectroscopy

Micrometer and submicrometer diameter fibers of recombinant dragline spider silk analogues, synthesized via protein engineering strategies, have been electrospun from 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and compared with cast films via Raman spectroscopy in order to assess changes in protein conformation that may result from the electrospinning process. Although the solvent casting process was shown to result in predominantly beta-sheet conformation similar to that observed in the bulk, the electrospinning process causes a major change in conformation from beta-sheet to alpha-helix. A possible mechanism involving electric field-induced stabilization of alpha-helical segments in HFIP solution during the electrospinning process is discussed.

The effect of the molecular orientation on the release of antimicrobial substances from uniaxially drawn polymer matrixes

A new method of controlled release of low molecular weight biocides incorporated in polymer matrixes is described. The molecular orientation of uniaxially drawn biocide doped polymer films is suggested as a significant parameter for controlled release monitoring. Triclosan, a well-established widespread antibacterial agent, has been incorporated into high density polyethylene (HDPE) films that have been subsequently uniaxially drawn at different draw ratios. The molecular orientation developed was estimated utilizing polarized mu-Raman spectra. Biocide incorporated polymer films, drawn at different draw ratios, have been immersed in ethanol-water solutions (EtOH) and in physiological saline. The release of Triclosan out of the polymer matrix was probed with UV-Vis absorption spectroscopy for a period of time up to 15 months. In all cases, although the film surface of the drawn samples exposed to the liquid solution was higher than the undrawn one, the relevant release rate from the drawn specimens was lower than the non-stretched samples depending on the molecular orientation developed during the drawing process. A note is made of the fact that no significant molecular orientation relaxation of the polyethylene films has been observed even after such a long time of immersion of the drawn films in the liquid solutions.

Determination of the molecular orientation of poly(propylene terephthalate) fibers using polarized Raman spectroscopy: a comparison of methods

For the first time, four different methods to determine the degree of molecular orientation from polarized Raman spectroscopy measurements are compared. The great influence of molecular orientation on the properties of polymers has driven the development of multiple experimental techniques and procedures. This study is based on the C(1)-C(4) ring stretching vibration of poly(propylene terephthalate) (PPT) at 1614 cm(-1). It is shown that simply ratioing the band intensity obtained with the polarization parallel and perpendicular to the unique axis of the sample provides a good qualitative method to observe the evolution of orientation in a series of similar samples. To quantitatively compare the degree of orientation one needs to utilize a more complex method yielding the second- and fourth-order parameters of the orientation distribution function (P(2) and P(4), respectively). To date, most studies have been based on the assumption of a cylindrically symmetric polarizibility tensor. It is shown that this assumption is highly questionable although this method has been used fairly successfully in the past. This method results in orientation parameters that are clearly different from those obtained with the two more complex procedures. The most complex method, both theoretically and experimentally, requires the most measurements per sample. Major problems have occurred when trying to calculate the desired parameters, in particular for samples with high birefringence. These problems are related to experimental complexities occurring for measurements when the samples are tilted with respect to the polarization direction of the incident light. These measurements are replaced by a simple determination of depolarization ratio in the third method. This method assumes that the depolarization ratio is independent of changes in molecular orientation and structure. It was found that this assumption is not correct. Thus, the most complex method is the method of choice to quantitatively determine the second- and fourth-order parameters of the orientation distribution function, unless one has knowledge of the depolarization ratio of each sample being studied. That knowledge permits the use of an experimentally simpler method to obtain the desired parameters.

Rotational invariants for polarized Raman spectroscopy

A new method has been developed to determine an orientation-independent Raman scattered intensity based on various polarized Raman measurements. The equivalent term in infrared spectroscopy is the structural absorbance, which has existed for many years. As with the structural absorbance, the calculated Raman intensity allows one to observe spectral changes that are due uniquely to morphological changes in a set of different samples in the presence of orientation differences. The full theoretical development is presented, followed by an example based on a set of polymer fibers processed under different conditions leading to different morphologies and degrees of molecular orientation.