Browsing by Author "Lee, Ting-Hsien"

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Annealing Effect on Electrospun Polymer Fibers and Their Transformation into Polymer Microspheres
(2012-02-27)
Fan, Ping-Wen
;
Chen, Wan-Ling
;
Lee, Ting-Hsien
;
Chen, Jiun-Tai
;
應用化學系
Electrospinning is a simple and convenient technique to produce polymer fibers with diameters ranging from several nanometers to a few micrometers. Different types of polymer fibers have been prepared by electrospinning for various applications. Among different post-treatment methods of electrospun polymer fibers, the annealing process plays a critical role in controlling the fiber properties. The morphology changes of electrospun polymer fibers under annealing, however, have been little studied. Here we investigate the annealing effect of electrospun poly(methyl methacrylate) (PMMA) fibers and their transformation into PMMA microspheres. PMMA fibers with an average size of 2.39 mu m are first prepared by electrospinning a 35 wt% PMMA solution in dimethylformamide. After the electrospun fibers are thermally annealed in ethylene glycol, a non-solvent for PMMA, the surfaces of the fibers undulate and transform into microspheres driven by the Rayleigh instability. The driving force of the transformation process is the minimization of the interfacial energy between the polymer fibers and ethylene glycol. The sizes of the microspheres fit well with the theoretical predictions. Longer annealing times are found to be required at lower temperatures to obtain the microspheres.
Rayleigh-Instability-Driven Morphology Transformation by Thermally Annealing Electrospun Polymer Fibers on Substrates
(1970-01-01)
Fan, Ping-Wen
;
Chen, Wan-Ling
;
Lee, Ting-Hsien
;
Chiu, Yu-Jing
;
Chen, Jiun-Tai
"Electrospinning has been widely used to prepare polymer fibers with diameters ranging from a few nanometers to micrometers. While most studies focus on controlling the sizes and morphologies of electrospun polymer fibers by changing electrospinning conditions, the effect of post-treatments such as thermal annealing on the properties of electrospun polymer fibers has been less studied. Here, we investigate the effect of thermal annealing on the morphology changes of electrospun polystyrene (PS) fibers on substrates. Different from annealing the fibers in a uniform environment, annealing the fibers on substrates results in a substrate-dependent morphology transformation. When the electrospun PS fibers are annealed on a glass substrate, wetting of the fibers on the glass substrate occurs. When the electrospun PS fibers 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 particles caused by the lower surface tension of PS than that of PMMA and the interfacial tension between PS and PMMA. This transformation process is influenced by the annealing time and temperature. The characteristic time of the transformation process is shorter when the sample is annealed at a higher temperature because of the lower polymer viscosity. The size of the polymer particles fits well with the theoretical prediction, which is dependent on the initial fiber diameter and is independent of the annealing temperature."
Rayleigh-instability-driven morphology transformation of electrospun polymer fibers imaged by in situ optical microscopy and stimulated Raman scattering microscopy
(2014-01-01)
Lee, Ting-Hsien
;
Chiu, Yu-Jing
;
Lai, Yu-Cheng
;
Fan, Ping-Wen
;
Kuo, Tyng-Yow
Electrospinning is one of the most common methods to prepare polymer fibers with sizes ranging from several nanometers to hundreds of micrometers. In most studies of electrospun polymer fibers, the properties and morphologies of polymer fibers are controlled by changing the electrospinning conditions. Few studies focus on the post-treatments of polymer fibers, which are critical for many fiber-based applications. In this work, we investigate the morphology transformation of electrospun polystyrene (PS) fibers annealed on top of poly(methyl methacrylate) (PMMA) film-coated glass substrates. In situ optical microscopy and stimulated Raman scattering (SRS) microscopy are used to observe the transformation process, which is driven by the Rayleigh instability, the surface tensions, and the interfacial tensions of polymers. Depending on the thickness of the underlying PMMA films, the electrospun PS fibers may transform into hemispheres or disks. The growth rates of the undulating amplitude are also affected by the film thickness.