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

Abstract

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.

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

“…By comparison, fibers of amorphous and low-crystallinity polymers spun under similar conditions typically exhibit a d 0 inferior to 0.7 µm, a lower orientation at d < d 0 , and a negligible orientation at d > d 0 . 16,33 This unusual behavior is likely due to the very high T g of PPO (∼210 °C) and to the high volatility of CHCl 3 (vapor pressure of 26.2 kPa at 25 °C61 ).…”

“…Crystallization during fiber formation can strongly impact molecular orientation, as shown in previous works on the molecular structure of fibers of various crystallinity. 17,32,33 The presence of a crystalline phase in PPO has already been documented for bulk samples, 51 but the ability of PPO to crystallize during electrospinning and its possible impact on the orientation process remain unknown. To our knowledge, quantitative markers of crystallinity have not been established in the Raman spectrum of PPO.…”

“…15,30 Studies at the mat level thus provide imprecise information on the change of orientation as a function of fiber diameter and specific experimental parameters. Some characterization techniques can alleviate these limitations by interrogating individual fibers rather than mats of fibers, most notably polarized confocal Raman microscopy, 16,17,[31][32][33][34][35] selected area electron diffraction, [36][37][38][39][40][41] and IR photothermal-induced resonance techniques with atomic force microscopy or optical detection. 42 Among these, Raman microscopy stands out because it allows faster and nondestructive quantification of molecular orientation in both crystalline and amorphous phases, thereby enabling a more efficient collection of diameter-dependent structural information.…”

“…34 In contrast to PEO, fibers of lowcrystallinity poly(ethylene terephthalate) (PET) show a much lower orientation that depends on the collector used. 33 The formation of oriented crystalline or mesomorphous phases was monitored simultaneously by Raman and also depended on the type of collector, a result that was mainly attributed to the viscosity of the PET jet at the point where the collector induces additional stretching. 33 This survey highlights that reaching a high orientation in fibers of amorphous or low-crystallinity polymers requires balancing the strain exerted on the jet during the electrospinning process and the mobility of the polymer chains.…”

“…33 The formation of oriented crystalline or mesomorphous phases was monitored simultaneously by Raman and also depended on the type of collector, a result that was mainly attributed to the viscosity of the PET jet at the point where the collector induces additional stretching. 33 This survey highlights that reaching a high orientation in fibers of amorphous or low-crystallinity polymers requires balancing the strain exerted on the jet during the electrospinning process and the mobility of the polymer chains. One way to control the mobility aspect is by choosing a solvent, or a mixture of solvents, whose volatility will dictate the viscosity of the polymer solution during fiber formation.…”