Session F21: Focus Session: Polymeric Fibers and Superstructures

Edge electrospinning: a facile needle-less approach to realize scaled up production of quality nanofibers

Utilizing unconfined polymer fluids (e.g., from solution or melt), edge electrospinning [1] provides a straightforward approach for scaled up production of high quality nanofibers through the formation of many parallel jets. From simple geometries (using solution contained within a sharp-edged bowl [2,3] or on a flat plate [4]), jets form and spontaneously re-arrange on the fluid surface near the edge. Using appropriate control of the electric field induced feed rate, comparable per jet fabrication as traditional single-needle electrospinning can be realized, resulting in nanofibers with similar diameters, diameter distribution, and collected mat porosity. The presence of multiple jets proportionally enhances the production rate of the system, with minimal experimental complexity and without the possibility of clogging. Extending this needle-less approach to commercial polyethylene polymers, micron scale fibers can be melt electrospun using a similar apparatus. [1] N. M. Thoppey et al., \textit{Polymer} \textbf{51}, 4928 (2010). [2] N. M. Thoppey et al., \textit{Nanotechnology} \textbf{22}, 345301 (2011). [3] N. M. Thoppey et al., \textit{Macromolecules} \textbf{45}, 6527 (2012). [4] M. P. Roman et al., \textit{Macromolecules} \textbf{46}, 7352 (2013).   

A new method for the alignment of electrospun nanofibers by oxygen plasma treatment

An effective way of controlling the alignment of electrospun nanofibers using oxygen plasma treatment was introduced. Poly (dimethylsiloxane) (PDMS) was selected as a base material for electrospinning and polyvinyl alcohol (PVA) was chosen as an electrospun-nanofiber material. It was found that most of PVA nanofibers were selectively deposited on the O2 plasma-treated area of PDMS, while only a few PVA nanofibers were randomly deposited on the untreated area of the PDMS film. Interestingly, a number of PVA nanofibers were neatly aligned along the border of the untreated area and the O2 plasma-treated area of PDMS. The surface structures and the morphology of the PDMS films with PVA nanofibers were analyzed by scanning electron microscopy, water contact angle measurements, and X-ray photon spectroscopy. By selecting the optimized ratio of treated and untreated area of PDMS film, it was found that more than 80{\%} of PVA nanofibers could be deposited parallel to the border of the treated and untreated area of PDMS. We used PVA as a reference material for the nanofiber alignment in this study, but similar deposition behavior was also observed for polyurethane (PU) fibers.   

Molecular dynamics simulations of electron irradiated PVDF nanofibers

High-resolution, aberration corrected transmission electron microscopy was used to observe morphological changes and segmental motion of electrospun poly(vinylidene fluoride) nanofibers in an 80 kilovolt electron beam. Atomic and molecular scale high-resolution images of fibers were made with an aberration corrected electron microscope. Chemical and morphological changes, which include the breaking of the fiber, loss of fluorine atoms and cross-linking of chains, caused by the high-energy electron beam were observed. We present the results of molecular dynamics (MD) simulations of such atomic and molecular level observations. The calculational models include the influence of chain scission, chain recoiling, and torsional defects on the morphology of a nanofiber. The effects of the loss of fluorine atoms and the applied tension on the morphology of the fibers were also investigated.   

Synchrotron X-ray Scattering Studies of Poly(lactide) Electrospun Fibers Containing Carbon Nanotubes

Carbon nanotubes(CNTs) often serve as an effective nucleating agent that facilitates the crystallization of semicrystalline polymers. Here we study the influence of CNTs on thermal and structural properties of Poly-lactide (PLA), which is well-known as a biodegradable and biocompatible thermoplastic polymer. The effect of CNTs on the crystallization and melting behavior of electrospun fibers of poly (L-lactide) (PLLA, with 100{\%} L-isomer) and poly (D-lactide) (PDLA, containing 4{\%} D-isomer) was systemically studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform spectroscopy(FT-IR) and real time synchrotron wide-angle X-ray scattering (WAXS) . Multi-walled CNTs were co-electrospun with the poly(lactides) in weight ratios ranging from 0.1 to 4.0 wt{\%} MW-CNT. PLA/carbon nanotubes composite electrospun fibers were successfully produced by appropriate choice of processing conditions and solution concentration. The morphologies of neat and CNT-filled electrospun nanofibers were observed by scanning electron microscopy. WAXS and DSC results show that lower content of CNTs contributes to higher speed of crystallization. However the results also showed that at the highest concentration of CNTs the ultimate crystallinity was reduced. FTIR and X-ray results show that PLA fibers have different crystal forms at high and low crystallization temperature. DSC results also show that D-lactide has reduced crystallinity compared to L-lactide.   

Nanofibers from Melt Blown Fiber-in-Fiber Polymer Blends

Nanofibers were generated by melt blowing three sets of polymer blends each comprised of pairs of immiscible components. Blends containing minority phases of poly(ethylene-\textit{co}-chlorotrifluoroethylene) (PECTFE) in poly(butylene terephthalate) (PBT), PECTFE in poly(styrene) (PS), and PBT in PS, were melt blown into long (\textgreater 100 microns) fibers with average diameters of several microns. Electron microscope revealed that melt blowing transformed the initial spherical dispersions into a nanofibers-in-fiber morphology. Macroscopic mats of nonwoven PBT and PECTFE nanofibers, with average diameters as small as 70 nm, were isolated by selectively removing the majority phase with a solvent. This method provides a potentially inexpensive, high throughput, one step route to scalable quantities of polymeric nanofibers.   

Multi-scale modeling for the self-assembly of DNA-functionalized nanoparticle into supperlattice and Wulff polydedra

Since 1996, DNA hybridization has proven robust for programmable self-assembly of nanoparticles (NPs). Recently, we showed that through a ``slow cooling'' method, DNA functionalized nanospheres or so-called ``programmable atom equivalents'' can assemble into crystals with a specific and uniform habit. Here we perform molecular dynamics simulations on multi-scale models to study and predict the corresponding shapes. Firstly, we use a scale-accurate coarse-grained model with explicit DNA chains to estimate surface energy ratios for different surface orientations, and predict the corresponding Wulff polyhedra based on these values. Secondly, we use a colloidal model in which each DNA coated nanosphere is represented by a single bead to simulate the growth dynamics of the crystals. By this method, we confirm the shape for the body-centered-cubic system to be a (110)-enclosed rhombic dodecahedron. But the face-centered-cubic system does not show any uniform shape yet except triangular features with (111) and (100) facets due to crystallographic defects including twinning and stacking faults. These simulated crystal shapes agrees very well with experiments.   

Giant Polyhedra based on Nano-atoms

In order to create new functional materials for advanced technologies, both Precisely control over functionality and their hierarchical structures and orders are vital for obtaining the desired properties. Among all the giant molecules, giant polyhedra are a class of materials which are utilized via deliberately placing precisely functionalized polyhedral oligomeric silsesquioxane (POSS) and fullerene (C60) molecular nano-particles (MNPs) (so-called ``nano-atoms'') at the vertices of a polyhedron. These giant polyhedra capture the essential structural features of their small-molecule counterparts in many ways but possess much larger sizes, and therefore, they are recognized as size-amplified versions of those counterparts. One of the most illustrating examples is a series of novel giant tetrahedral which possessing precisely-defined amphiphilic MNPs with different geometric combinations. With both geometrical and chemical symmetry breakings, these tetraphedra perform as building blocks to construct When specific interactions are introduced, these polyhedral are functioned as building blocks to construct into hierarchical ordered structures. A range of ordered super-lattice structures of this class of materials have been investigated in the condensed bulk state. The study has also expanded to other types of giant polyhedra to identify the general role in their assembly processes.   

Programmable Nanoparticle clusters via DNA linking

Due to selective recognition, short complementary DNA strands have been widely used as linkers to direct the crystallization or the formation of larger assemblies of nanoparticles. We study the self-assembly of small clusters through DNA hybridization in a binary mixture of spherical nucleic acid gold nanoparticles (SNA-GNPs) with the larger SNA-GNPs in excess by performingthe molecular dynamics simulationson the graphical processing unit (GPU).The resultant structures are self-assembled clusters with a varying number of large SNA-GNPs clustersaround the small ones, and the structure of the clusters varies as the ratio of large to small hydrodynamic radii changes.   

Fluorinated Polyhedral Oligomeric Silsesquioxane Based Giant Molecular Shape Amphiphiles: Hierarchical Self-Assembly with Unusual Chain Conformation

The fluorous phase has thus been considered as the third phase that repels both oil and water due to its ultra-low surface energy. Incorporation of fluorinated component into hydrophilic/hydrophobic polymers is anticipated to bring novel self-assembly behaviors in the bulk, solution and thin film states, which are not only academically intriguing but also technological relevant. Among them, fluorous molecular clusters are of particular interest. A topologic isomer pair of giant molecular shape amphiphiles can be constructed by tethering molecular nanoparticle at different location of block polymers. In this study, a fluorinated polyhedral oligomeric silsesquioxane (FPOSS) was precisely fixed onto polystyrene\textit{block}poly(ethylene oxide) (PS-$b$-PEO) at chain end (FPOSS-PS-$b$-PEO), or junction point [PS-(FPOSS)-PEO]. The interplay between nanoparticle and block polymers results in hierarchical structures with three types of order. The incommensuration of cross-sectional area between FPOSS and block polymer stretches polymer chains, which found to enhance the immiscibility between PEO and PS block.   

Large-scale electrohydrodynamic organic nanowire printing, lithography, and electronics

Although the many merits of organic nanowires (NWs), a reliable process for controllable and large-scale assembly of highly-aligned NW parallel arrays based on ``individual control (IC)'' of NWs must be developed since inorganic NWs are mainly grown vertically on substrates and thus have been transferred to the target substrates by any of several non-individually controlled (non-IC) methods such as contact-printing technologies with unidirectional massive alignment, and the random dispersion method with disordered alignment. Controlled alignment and patterning of individual semiconducting NWs at a desired position in a large area is a major requirement for practical electronic device applications. Large-area, high-speed printing of highly-aligned individual NWs that allows control of the exact numbers of wires, and dimensions and their orientations, and its use in high-speed large-area nanolithography is a significant challenge for practical applications. Here we use a high-speed electrohydrodynamic organic nanowire printer to print large-area organic semiconducting nanowire arrays directly on device substrates in an accurately individually-controlled manner; this method also enables sophisticated large-area nanowire lithography for nano-electronics. We achieve an unprecedented high maximum field-effect mobility up to 9.7 cm$^{2}$$\cdot$V$^{-1}$$\cdot$s$^{-1}$ with extremely low contact resistance (\textless 5.53 $\Omega \cdot$ cm) even in nano-channel transistors based on single-stranded semiconducting NWs. We also demonstrate complementary inverter circuit arrays consist of well-aligned p-type and n-type organic semiconducting NWs. Extremely fast nanolithography using printed semiconducting nanowire arrays provide a very simple, reliable method of fabricating large-area and flexible nano-electronics.