NWRI Graduate Research Fellowship Progress Report Natalia Hoogesteijn von Reitzenstein, Arizona State University October 2015
Background Electrospun polymer fibers with diameters in the submicron to nanometer range have unique characteristics which have led to increasing interest in their applications as reinforcements for composite materials, filtration, soft tissue prostheses, wound dressing, cosmetics, protective clothing, sensors, and purification of air or water [1,2]. Electrospinning uses an electrically charged jet of polymer solution to produce polymer filaments by applying a high power potential between 10-40 kv and a grounded collector. The surface tension on the fluid droplet at the tip of the syringe is overcome by the strength of the electric field and a charged jet of fluid stretches from the syringe tip and is deposited onto the grounded collector, forming a mat of fibers with diameters in the micro- and nanometer scale. NP addition into polymers produces nanocomposites known to improve mechanical strength, resistance to wear, and thermal stability [3]. Additionally, NP-polymer electrospun fiber composites also enhance the performance of these fibers due to the multifunctionality of NPs as biocides, sorbents, and photocatalysts. For example, the addition of antimicrobial silver NP to a mat of electrospun fibers grafted onto a membrane could help prevent bacterial membrane fouling [4]. Carbon-based nanomaterials such as multi-walled carbon nanotubes [5], graphene [6], and graphene oxide [7] have been embedded as additives in electrospun polymeric fibers to enhance the mechanical, electrical and thermal stability of polymer fibers [6]. However, the adsorptive properties of carbon-based nanomaterials when spun with a polymer has not been comprehensively tested yet. Adsorption of organic contaminants with carbon-based nanomaterials have been extensively studied in the last decade. One of the primary limitations for all carbon-based nanomaterials is their tight aggregation in water causing depletion of available sorption sites. By incorporating nanomaterials in electrospun polymers, good dispersion can be obtained and exposed surfaces of nanomaterials can show superior sorption behaviors when compared to aggregate nanoparticles in suspension. The current phase of this project is focused on achieving multiple functionalities within one fiber unit. To do this, various methods of fiber functionalization are being developed and tested. Currently, I am focusing on increasing the porosity of the fibers. Optimizing the porosity of the fibers is critical for the use of these fibers as a three-dimensional sorbent and an ion exchanger. Research Plan: Phase I - System Build (complete) Phase II Spinning with PVP (complete) Phase III Spinning with PVP and PS with Indium oxide and titanium dioxide nanoparticles as fillers (complete) Phase IV Development of a multifunctional fiber a. Optimization of polymer(s) (complete) b. Optimization of ENP(s)/fiber functionalities and fiber feature tuning Phase V Fiber Testing: Performance comparison of 5 nanocomposites Phase VI Reactor Design and Testing Experimental MATERIALS. Polystyrene (MW 350,000 g/mol, Aldrich Chemistry) functions as the base polymer for its mechanical integrity. N,n-dimethylformamide (DMF, Sigma-Aldrich) was used as the organic solvent. Multi-walled carbon nanotubes (MWNT, Sigma-Aldrich, St. Louis, MO), powdered activated carbon (PAC, Clemson University, Clemson, SC), and graphene platelets (Angstron Materials, Dayton, OH) were sonicated in DMF at 1 wt% concentration, and then 20 wt% PS was added to the solution and stirred for 24 hours at 40 C. 1
ELECTROSPINNING. Electrospinning was performed using a high voltage power supply that provided up to 40 kv (Gamma High Voltage, Ormond Beach, FL), a syringe pump (New Era NE-300, Farmingdale, NY), a 10 ml plastic syringe, and a grounded aluminum foil coated collector which was placed 15 cm away from the syringe tip. Experimental procedure consisted of loading the solution into a plastic 10 ml syringe fitted with a stainless steel needle which was connected to the high voltage power supply. The composite solution was injected at varying feed rates through a stainless steel, 22-gauge needle (Sigma-Aldrich stainless steel 304 syringe needle) onto which an alligator clip was attached to charge the needle and the polymer solution as it exited the capillary tip. The entire system was enclosed in order to mitigate the effects of air currents on the system, as well as for safety. ANALYTICAL METHODS. Scanning electron microscope (SEM) images of fibers were obtained using a JEOL 2010F FEI. Results Electrospinning of neat PS fibers produced long fibers with rough surfaces. The addition of graphene platelets, MWNT, and PAC to the PS solution produced fibers with surface morphologies that were rough and also porous (Figures 1-4). Pores were seen especially well in the polymer beads that form along the fibers. This not only makes the polymer surface itself more desirable, but may provide access to carbon-based fillers inside the fiber that can function as adsorbents. The porous polymer itself may also trap contaminants. 2
Figure 1: PS fiber without fillers shows a rough surface without pores. Figure 2: Pores visible on the surface of a bead in PS-graphene platelet fiber. Figure 3. Pores visible on the surface of a bead (left) and on the fiber surface in PS-MWNT fiber (right). Figure 4. Pores visible on the surface of a bead (left) and on the fiber surface in PS-activated carbon fiber (right). 3
Future Work Immediate future work consists of batch absorption experiments in order to further characterize absorption capacity of PS fibers containing carbon-based fillers, as well as TEM imaging of internal fiber structure to determine the distribution of filler material in the polymeric fibers. The next step after the completion of batch absorption experiments is the development of a fiber with the capacity for ion exchange. Currently, ion exchange fibers are functionalized using a variety of toxic chemicals and lengthy treatments after they are spun. The goal is to synthesize an ion exchange fiber with minimal post-spinning treatment. References [1] Huang Z-M, Zhang Y-Z, Kotaki M, Ramakrishna S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 2003;63:2223 53. doi:10.1016/s0266-3538(03)00178-7. [2] Li D, Xia Y. Electrospinning of Nanofibers: Reinventing the Wheel? Adv Mater 2004;16:1151 70. doi:10.1002/adma.200400719. [3] Mangal R, Srivastava S, Archer L a. Phase stability and dynamics of entangled polymer nanoparticle composites. Nat Commun 2015;6:1 9. doi:10.1038/ncomms8198. [4] Xu X, Yang Q, Wang Y, Yu H, Chen X, Jing X. Biodegradable electrospun poly(l-lactide) fibers containing antibacterial silver nanoparticles. Eur Polym J 2006;42:2081 7. doi:10.1016/j.eurpolymj.2006.03.032. [5] Bayley GM, Mallon PE. Porous microfibers by the electrospinning of amphiphilic graft copolymer solutions with multi-walled carbon nanotubes. Polymer (Guildf) 2012;53:5523 39. doi:10.1016/j.polymer.2012.08.058. [6] Barzegar F, Bello A, Fabiane M, Khamlich S, Momodu D, Taghizadeh F, et al. Preparation and characterization of poly(vinyl alcohol)/graphene nanofibers synthesized by electrospinning. J Phys Chem Solids 2015;77:139 45. doi:10.1016/j.jpcs.2014.09.015. [7] Das S, Wajid AS, Bhattacharia SK, Wilting MD, Rivero I V, Green MJ. Electrospinning of Polymer Nanofibers Loaded with Noncovalently Functionalized Graphene 2013:4040 6. doi:10.1002/app.38694. 4