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We herein report the influences of chain orientation and packing on the photoluminescent (PL) and photothermal properties of polybenzimidazole fibers with meta-linkage under different excitation wavelengths (λex) and powers of laser sources. For the purpose, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PmBI) was synthesized and then processed into fibers via solution spinning and heat treatment. The chain orientation and packing of the as-spun PmBI fiber decreased and increased owing to the heat treatment, respectively, which resulted from the nonlinear chain conformation of PmBI with meta-linkage. It was found that the visible PL intensity under λex = 325 nm, which is associated with an efficient interchain photorelaxation between benzimidazole rings, was much higher for the as-spun fiber with higher chain orientation, while the infrared PL intensity under λex = 514 nm, which is dominated by an intrachain photorelaxation, was far higher for the heat-treated fiber with higher chain packing. Accordingly, the heat-treated PmBI fiber exhibited excellent photothermal heating behavior by attaining maximum temperature increments (ΔTmax) of ~31.7 °C under λex = 405 nm and 18.4 mW, which was even higher than the as-spun fibers (ΔTmax ~ 27.5 °C). For the PmBI fibers, the ΔTmax values were also found to increase with the decrease of λex. [ABSTRACT FROM AUTHOR]

Electrospinning has been widely used to prepare polymerfiberswith diameters ranging from a few nanometers to micrometers. Whilemost studies focus on controlling the sizes and morphologies of electrospunpolymer fibers by changing electrospinning conditions, the effectof post-treatments such as thermal annealing on the properties ofelectrospun polymer fibers has been less studied. Here, we investigatethe effect of thermal annealing on the morphology changes of electrospunpolystyrene (PS) fibers on substrates. Different from annealing thefibers in a uniform environment, annealing the fibers on substratesresults in a substrate-dependent morphology transformation. When theelectrospun PS fibers are annealed on a glass substrate, wetting ofthe fibers on the glass substrate occurs. When the electrospun PSfibers are annealed on a poly(methyl methacrylate) (PMMA)-coated substrate,a Rayleigh-instability-driven morphology transformation is observed.The polymer fibers transform into hemispherical polymer particlescaused by the lower surface tension of PS than that of PMMA and theinterfacial tension between PS and PMMA. This transformation processis influenced by the annealing time and temperature. The characteristictime of the transformation process is shorter when the sample is annealedat a higher temperature because of the lower polymer viscosity. Thesize of the polymer particles fits well with the theoretical prediction,which is dependent on the initial fiber diameter and is independentof the annealing temperature. [ABSTRACT FROM AUTHOR]