Lignin, a highly aromatic biopolymer extracted as a coproduct of wood pulping, was investigated as a suitable precursor for carbon fibers. Lignin was chemically modified and blended with poly(lactic acid) (PLA) biopolymer before melt spinning into lignin fibers. The chemical modification of raw lignin involved butyration to form ester functional groups in place of polar hydroxyl (-OH) groups, which enhanced the miscibility of lignin with PLA. Fine fibers were extracted and spooled continuously from lignin/PLA blends with an overall lignin concentration of 75 wt.%. The influence of chemical modification and physical blending of lignin with PLA on the resulting fiber was studied by analyzing the microstructure of the fibers using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The influence of blend composition on the phase behavior was studied by differential scanning calorimetry (DSC). The effect of composition on the mechanical properties was studied by tensile tests of the lignin/PLA blend fibers. The thermal stability and carbon yield of the blended fibers with different concentrations of lignin were characterized by thermogravimetric analysis (TGA). The microstructure analysis of carbon fibers produced from lignin/PLA blends revealed composition dependent microporous structures inside the fine fibers.

The uniaxial mechano-optical behavior of a series of amorphous l-phenylalanine-based poly­(ester urea) (PEU) films was studied in the rubbery state. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during deformation. When the materials were subjected to deformation at temperatures near the glass transition temperature (T g), the photoelastic behavior was manifested by a small increase in birefringence with a significant increase in true stress. At temperatures above T g, PEUs with a shorter diol chain length exhibited a liquid–liquid (T ll) transition (rubbery–viscous transition) at about 1.06T g (K) under the tested strain rate of 0.017 s–1 (stretching speed of 20 mm/min), above which the material transforms from a heterogeneous “liquid of fixed structure” to a “true liquid” state. The initial photoelastic behavior disappears with increasing temperature, as the initial slope of the stress optical curves becomes temperature independent. Fourier transform infrared spectroscopy (FTIR) was used to study the effect of hydrogen bonding on the physical properties of PEUs as a function of temperature. The average strength of hydrogen bonding diminishes with increasing temperature. For PEUs with the longest diol chain length, the area associated with N–H stretching region exhibits a linear temperature dependence. However, a three-stage temperature dependence was observed for PEUs with shorter diol chain length. The presence of hydrogen bonding enhances the “stiff” segmental correlations between adjacent chains in the PEU structure. As a result, the photoelastic constant decreases with increasing hydrogen bonding strength.

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The uniaxial mechano-optical behavior of a series of amorphous L-valine-based poly(ester urea) (VAL-PEU) with varying diol lengths was studied to elucidate the molecular mechanism associated with their thermal shape memory properties. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during the temporary shape programming at stretching temperatures above the glass transition temperature (T g ). The mechanooptical response of VAL-PEUs exhibits an initial photoelastic behavior related to enhanced segmental correlation at low temperatures above the T g . A characteristic temperature, defined as the liquid−liquid (T ll ) transition (rubbery−viscous transition), was found at about 1.05 T g (K) (at T g + 15 °C) at strain rate of 0.017 s −1 , above which the segmental contacts largely "melt" and the initial slope of the stress-optical curves becomes temperature independent. This temperature corresponds to the temperature where mean relaxation time for the polymer is maximized. Real-time infrared spectroscopy (IR) and in situ wideangle X-ray scattering (WAXS) revealed a strain-induced intersegmental structural change during stretching. The intermolecular hydrogen bonding between the urea−urea and/or urea−carbonyl groups was found to adopt different bonding modes at the onset of strain hardening with a concurrent increased separation distance between adjacent segments. The hydrogen bonding strengthened supramolecular packing. The compromised shape recovery is a result of the disruption of the rigid segmental correlation, induced either by large extension at T < T ll or by progressive thermal effects at T > T ll , both of which may be minimized at T ll .