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

Abstract

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 (T_g,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 T_g,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 T_g is a crucial parameter for reaching high orientation in amorphous or low-crystallinity polymer systems.

Keywords: Anisotropic; Raman spectroscopy; applications; materials; polymers; quantitative analysis.

Graphical Abstract

This is a visual representation of the abstract.

Figure 1.

Sets of polarized Raman spectra collected on two representative PPO fibers of smaller and larger diameters.

Figure 2.

Diameter dependence of molecular orientation for PPO fibers electrospun using high-volatility CHCl₃ and low-volatility 50 : 50 CHCl₃/ClBz solvent systems.

Figure 3.

(a) Calculated pure component spectra for the crystalline and amorphous phases of PPO. (b) PLS calibration relating the crystallinity (X_c) values of bulk PPO samples predicted from the Raman spectra to those determined by DSC.

Figure 4.

Diameter dependence of crystallinity for PPO fibers electrospun using high-volatility CHCl₃ and low-volatility 50 : 50 CHCl₃/ClBz solvent systems. The crystallinities were computed from the average of structural spectra within 500 nm diameter windows.

Figure 5.

Schematic representation of the T_g,eff as a function of jet trajectory and the corresponding fiber orientation for (i) high-T_g polymer with high volatility solvent, (ii) high-T_g polymer with low volatility solvent, (iii) low-T_g polymer (T_g > RT) with high volatility solvent, and (iv) low-T_g polymer (T_g > RT) with low-volatility solvent. Vitrification occurs when T_g,eff exceeds RT. The progressive change in the color of the curves reflects the evolution of the jet composition toward the pure polymer due to solvent evaporation.