Electrospinning/Electrospray
POLYMERIC BIOMIMETIC MATERIALS
Electrospinning and Polymer Nanofibers
The recently fast developing technology “electrospinning” is a unique way to produce novel polymer nanofibers with diameters typically in the range from 50 nm to 500 nm. Polymer nanofibers can be made from a variety of polymer solutions or melts, and are of substantial scientific and commercial interests including composite, filtration, protective clothing, biomedical and electronic applications. Carbon nanofibers made from polymeric precursors further expand the list of possible uses for nanofibers. Polymer nanofibers could have many extraordinarily properties including, small diameter (and the resulting large surface area to mass ratio), highly oriented crystalline structures (and the resulting high strength), etc. Meanwhile, the non-woven fabrics made of polymer nanofibers offer unique capabilities to control pore size and have been researched to be the novel scaffold for cell growth.
The followings are two projects in this area.
Study of the Technology of Electro spinning to Produce Polymer Nanofibers with the Controllable Morphology and Properties.
The process of electrospinning is a complicated combination of polymer science, electronics and fluid mechanics. Both solution properties and processing variables can significantly affect the electrospinning process. To date, a fundamental mechanism of the process of electrospinning is still characterized only qualitatively. The absence of the comprehensive knowledge of electrospinning has resulted in the polymer nanofibers with less controllable morphology and properties, and has significantly affected the polymer nanofibers to be used as a functional material. It is the purpose of this research to systematically study the process of electrospinning to produce polymer nanofibers with controllable morphology and properties. Several key objectives are outlines as follows:
Design and construct of a comprehensive electrospinning station, for controllable and reproducible electrospinning. <?XML:NAMESPACE PREFIX = O />
Systematically investigate the process of electrospinning. The process will be studied based on three stages: jet initiation, jet elongation (bending instability), and nanofiber formation. For each stage, systematical observations and measurements will be carried out.
Clarify of the polymer solution characteristics (viscosity, conductivity, surface tension, etc.) and the process variables (electrostatic field, flow rate, polarity of the power supply, etc.) as well as environment conditions (temperature, pressure, solvent vapor pressure, etc.) on the electrospinning process, and their effects on the morphology and properties of polymer nanofibers.
Explore of the effects of the external electro-magnetic field on the morphology and properties of the electrospun polymer fibers.
The research will further be expanded to prepare and evaluate (a) novel carbon (and/or graphite) nanofiber nano composites, and (b) high efficiency polymeric photovoltaic devices.
Highly oriented crystalline, strong carbon nanofibers and very porous, extremely large surface area carbon nanofibers produced from electrospun precursors
This research is to study the formation and physical properties of the carbon nanofibers made from the electrospun precursors.
Two kinds of carbon nanofibers will be produced and studied:
Highly graphite-crystalline ordered, strong carbon nanofibers made from mesophase pitch or PAN nanofiber precursors.
Very porous carbon nanofibers made from PAN or PVA nanofiber precursors, with extremely large specific surface area. (Note: Different approaches will be employed to generate micropores. For PAN based carbon nanofibers, steam and/or CO2 treatment will be used; while for PVA based carbon nanofibers, micropores will be generated in situ with carbonization, through thermal decomposition of (NH4)2HPO4.)
Further objectives include:
Study the electrospinning process to make precursor nanofibers with desirable morphological and physical properties.
Investigate the stabilization, carbonization, graphitization and activation conditions of the nanofiber precursors.
Prepare (polymer/strong carbon nanofibers) nanocomposite, and evaluate the mechanical properties.
Explore the advantages of substituting activated carbon black by highly porous carbon nanofiber nonwoven fabrics.
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Polymeric Nanocomposites
Nanocomposites are a new class of composites, that are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. Over the last decade, the utility of layered silicate nanoparticles as additives to enhance polymer performance has been established. Nanoscale fillers result in physical behavior that is dramatically different from that observed for conventional microscale counterparts. For instance, increased moduli, gas barrier, increased strength and reduced thermal expansion coefficients are observed with only a few percent additions of nanofiller; thus maintaining polymeric processability, cost and clarity.
The following are two potential projects in this area:
Preparation of polymer/layered silicate nanocomposite, or polymer/rigid rod polymer molecular level blend (molecular composite), via emulsion polymerization.
Emulsion polymerization offers a viable, flexible route for nanocomposite fabrication from nanoscale spheres, rods and plates. Combining emulsion generated polymer particles that are ionically stabilized in aqueous solution with a dispersion of nanoparticles of opposite sense results in an interfacial exchange reaction and co-precipitation. As shown in Fig. 2, the idea is to make a few percentage of the surfactants, which have positive charges, with double bonds so that the surfactant molecules can be copolymerized. The resulting macromolecules, which will carry certain amounts of charges, will then co-precipitate with charged exfoliated clay particles or charged rigid rod macromolecules, to make polymer/layered silicate nanocomposites or rigid rod molecular composites.
Protect polymeric materials against aggressive space environment through nanocomposite
Polymers are very attractive and desirable materials for use in space applications, in particular for addressing multi-functional requirements. With modifications these materials could potentially solve many of the weight-based and process-based problems plaguing the space industry and offer new capabilities for future systems. Polymers are remarkable materials but there are, as with any material, problems associated with their use, especially in a harsh space environment. Degradation is a most prominent concern with using polymers in space, and the existence of atomic oxygen in Low Earth Orbit (LEO) is one of the major reasons of degradation.
This research is to investigate the “Multi-functional (self-passivating/self-rigidizing/self-healing) polymeric materials for space survivable structures based on polymer/layered silicate nanocomposites”. The previous results showed that Nylon 6/layered silicate nanocomposites are able to self-generate a silicate passivation layer upon exposure to oxygen plasma. The resulting layer is strongly interferometric. The thickness of the layer varies from a few hundred nanometers to one micron, and chemical composition of the layer is nearly completely inorganic. The formation of the layer is due to the preferential oxidation of the polymer (Nylon 6) from the nanocomposite, and the corresponding deposition of the nanoscale layered silicate on the surface. The structure of the inorganic layer is turbostratic, with the average distance between silicate layers of about 1 to 4 nanometers. After the passivation layer forms, the degradation of the polymer underlying can be significantly retarded during exposure to oxygen plasma. Thus, nanocomposite may potentially be used to protect the degradation of polymers, especially against atomic oxygen in low earth orbit.
Polymeric biomimetic materials
POLYMERIC DENTAL RESTORATIVE COMPOSITES
Developed over 40 years ago, dental restorative composite, consisting of a tough, wear-resistant polymeric resin matrix and glass or ceramic fillers, presented opportunities never before equaled in modern dentistry, and was rapidly accepted by the profession. The resin matrix is usually cured (hardened) by photo-initiated free radical polymerization. Camphorquinone (CQ) is a common visible light initiator and ethyl-4-(N,N’-dimethylamino) benzoate (4EDMAB) is a common accelerator. The monomer 2,2’-bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA) is the most commonly used base monomer.
Bis-GMA is a very viscous, honey-like liquid. To improve the handling qualities, a low viscous diluent monomer, such as tri- (ethylene glycol) dimethacrylate (TEGDMA), is added to thin the resin. In the Bis-GMA/TEGDMA system, Bis-GMA functions to limit shrinkage and enhance resin reactivity, while TEGDMA provides for increased resin conversion.
Improving mechanical properties and reducing internal stresses (induced by polymerization shrinkage) have been among the major research efforts for polymeric dental restorative composites for decades. The recently fast developing technology “electrospinning” and its novel product “polymer nanofibers”, provide a potential way to better solve the problems. In the process of electrospinning, the key phenomenon “bending instability” results in extremely large elongational flow rate of up to 1,000,000 s-1. Such huge elongational flow rate can well align macromolecule chains as well as nanofillers (e.g. layered silicates, carbon nanotubes, etc.) along the nanofiber axis; therefore, electrospun nanofibers can be extraordinary strong. The electrospun polymer nanofibers, typically collected as non-woven fabrics, can be soaked with and embedded into dental monomers (e.g. Bis-GMA/TEGDMA mixture). After polymerization, the composite resins are in the form similar to the interpenetration network (IPN). In this research, for the first time, the electrospun polymer nanofibers will be introduced to prepare dental materials. In particular, the use of strong nanofibers of Nylon 6/layered silicate nanocomposites (NLS) to improve the strength, and the use of elastomeric nanofibers of ethylene-propylene-diene elastomer (EPDM) to reduce the internal stresses, will be studied. Other objectives include (1) investigating the process of electrospinning to make nanofibers with desirable morphological and physical properties, (2) improving interfacial properties between the filler of nanofibers and the matrix resin, through both chemical modifications of the macromolecules and plasma surface treatment of the nanofibers, (3) evaluating hybrid embedding of Nylon 6 and EPDM nanofibers to maximize strength, and to minimize internal stress, and (4) characterizing long-term mechanical and physical properties, such as wear, water aging and thermal cycling, for the selected composite resins with desirable properties (e.g. high strength and/or minimum internal stresses).
Polymeric Artificial Bone Materials
Bone can be described as a connective tissue, helping to support and bind together various parts of body. Bone is a composite material consisting of both fluid and solid phases. Bone obtains its hard structure because the organic extracelluar collagenous matrix is impregnated with inorganic materials, principally hydroxyapatite Ca10(PO4)6(OH)2 consisting of the minerals calcium and phosphate. Calcium and phosphate account for roughly 65 to 70% of the bone’s dry weight. Collagen fibers compose approximately 95% of the extracellular matrix and account for 25 to 30% of the dry weight of bone. The organic material gives bone its flexibility while the inorganic material gives bone its resilience.
In this research topic, there are two projects as follows
Formation of artificial bones with designed shape
Utilizing the transplantation and growth of cells on a scaffold, shows promise as a viable therapeutic option to organ transplantation. Preliminary results showed nanofiber nonwoven fabrics provided perfect scaffolds for living cells to grow. The objective is the creation of a bio-artificial bone in the form of a tube, on the scale of the leg bone of a small chicken. The scaffold will be made in the form of a three dimensional network of polymer nanofibers. The nanofibers provide excellent mechanical support to living cells without interfering with their essential biological functions. The nanofibers will be electrospun from the biopolymers that will be synthesized for the purpose. The living cells will be incorporated into the tube shaped scaffold as the nanofibers are spun and wound onto a removable mandrel. Nutrients will be supplied and waste products will be removed by the controlled flow of appropriate solutions through and around the tube. Preliminary work also suggested the nanofiber nonwoven fabric scaffold with grown skin cells might be used to replace the burnt skin to avoid the formation of skin scar tissue for the cosmetic application.
Investigation of calcium phosphate cement (CPC) reinforced by collagen and water-soluble elastin.
CPC self-setting to form hydroxyapatite has been used in a number of bone procedures. Applications have been limited, however, because of problems with brittleness and low strength. The objective of this study is to reinforce CPC with collagen and water-soluble elastin fibers to improve the physical properties. Meanwhile, the embedded water-soluble elastin will eventually be dissolved to create macropore channels for the bone cells to grow. Preliminary results showed the reinforced CPC composite achieved a flexural strength 2.5 times, and work-of-fracture (related to toughness) more than 100 times, greater than the unreinforced CPC. The strength and toughness of the reinforced CPC could maintain for 2 to 4 weeks upon saline immersion. After that, macropore channels could be observed, and the formed macropore channels are suitable for the vascular ingrowth.