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NTC Project: C03-CD01s

National Textile Center Final Report: November 2004

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Nano-Porous Ultra-High Surface Area FibersC03-CD01s

You-Lo HsiehUniversity of California, Davis, CA 95616-8722

Steve B. WarnerUniversity of Massachusetts, Dartmouth

Heidi Schreuder-GibsonUS Army Natick Soldiers Center

Goal

The goal of this seed project is to create ultra high specific surface fibers with 102 nm rangediameters and nanometer-size pores. Several chemical and synthetic strategies were exploited tocreate novel fibers with these new characteristics from both natural and synthesized polymers.Due to the nanoscale sizes of these fibers and pores, a much smaller quantity of these newmaterials is needed to achieve the anticipated functions. Furthermore, it is highly probable that,unanticipated but potentially useful properties may be discovered. It is envisioned that materialswith these characteristics are potentially useful in many areas of technical application, such asselective chemical reactivity, solid support catalysts, membrane supported smart materials, andmembranes for immobilizing biological and pharmacologically active agents and molecules.

Abstract

The main focus of this one-year seed project was to investigate the phase separation phenomenain polymer mixtures to create new nano-scale structures within ultra-fine fibers. Solutions ofpolymer mixtures of various compositions were investigated for the formation of fibers withdiameters ranging from around 100 nm to 1000nm. These fiber sizes are about two orders ofmagnitude smaller than fibers conventionally produced, translating to two order of magnitudehigher specific surface areas. By evaluating a selection of compatible polymer pairs in commonsolvents, phase separation in the solid fibers was evaluated. In fibers with proven phase-separated morphology, the removal of the phase-separated polymer from the matrix polymergenerated pores in these ultra-fine fibers. Preliminary findings give strong support for thisapproach to create nano-porous fibers. The fibers generated exhibit not only ultra-high specificsurface areas and nanometer size pore structures but also potentially unique chemicalfunctionalities. Further development of this research will contribute to the knowledge base offiber materials science and the development of new fibrous materials to meet emerging needs inadvanced technical applications, e.g., chemically and biologically protective coatings,recyclable catalysts, reactive and smart materials, and targeted separation membranes.



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Background

Fibers have intrinsically high specific surface areas, namely surface-to-volume or surface-to-mass values. Ultra-fine fibers such as nanofibers are typically two to three orders of magnitudesmaller than fibers conventionally produced and therefore have \ two to three orders ofmagnitude higher specific surface areas. These ultra-high specific surface characteristics are ofsignificant scientific interest and highly desirable for applications that rely on materials’ surfacecharacteristics, including separation/filtration membranes, composite reinforcement, and surface-activated and surface-supported chemical reactions.

Current melt and solution spinning technologies are capable of producing fibers with diametersin the 20-50 mm range at relatively high speeds and with good uniformity. Althoughtechnological advancement has resulted in efficient production of micrometer diameter fibers, todate, electrospinning remains the only direct method by which fibers of less-than-micrometersizes can be generated. In electrospinning, polymer solutions or melts are charged to highvoltage under proper conditions, their surface tension forces can be overcome to form fine jetstoward a grounded target. Most work has focused on the formation of fibers from wide varietiesof polymers [1] and DNA molecules [2]. Current challenges include forming fibers with lateraldimensions in the nanometer range. The extremely high surface area characteristics of nanofibermembranes make them excellent carriers and supports for reactive compounds [3]. Twoprevious NTC projects (M01-D22, M98-D01) have focused on the engineering aspects ofelectrospinning, such as the design of the apparatus (e.g., multiple spinnerets, rotating collector),the effects of charging/ionized field, improved productivity and the characterization of fiberphysical properties. Much of the experimental work has centered on polyethylene oxide (PEO),polyacrylonitrile (PAN), and polycaprolactone (PCL).



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Concept

Removal of dispersed & phaseseparated component (polymer B)

Polymer BPolymer A

Electrospinning

Homogeneous mixture of A+Bin common solvent

Ultra-thin bicomponent A+B fiber

Nano-porous ultra-thin fiber

Findings

Ultra-fine Fiber Formation. From previous collaboration with the Natick Material Sciencegroup, we have devised an electrospinning apparatus, consisting of a high voltage power supply(ES30P/100, Gamma High Voltage Research Inc.), a polymer solution reservoir, and a target orcollector. The solution is fed through a glass tube with capillary opening of approximately 1mm in diameter. A metal (stainless steel or copper) pin immersed in the solution serves as theelectrode and is connected to a high voltage source. With the adjustment of an electrical field, theelectrostatic force overcomes the surface tension of the drop, ejecting the jet toward the target.Changes of the polymer jets cause “splaying” or longitudinal splitting of the jet into finerstreams. Upon evaporation of the solvent, dried fibers are collected on the counter electrode in afibrous web. By controlling the solvent systems and solution properties, fibers with diameters of100-500 nm have been produced from several polymers (bar=1 mm).

Bicomponent Fiber Formation. The first successful approach for nanopore formation is phaseseparation and selective dissolution employing two polymers, A and B. Both polymers are



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organic soluble and polymer B is also water-soluble. Electrospinning of organic solutions ofthese two polymers at varying polymer B contents yields ultra-fine cylindrical fibers of smoothsurfaces and homogeneous dimensions. The fiber diameters ranged from ~500 nm down to ~100nm with increasing amounts of polymer B (Figure 1). The finer fibers have 200 times the specificsurfaces of conventional fibers. Scanning electron microscopy of these fibers has shown thatfiber diameters are generally <500 nm and surface pores <50 nm.

a b c

Figure 1. SEM of electrospun fibers from mixtures of A and B polymers: (a) 15% B, (b) 30% B;(c) 50% B (bar=2 mm); viscosities of 8% polymer solutions are 31.2 and 2.7 cP, for polymers Aand B, respectively.

Phase-Separation. The distinct thermal behaviors of the individual polymers detected by DSC,confirm clear phase separation of the two, in the form of cast films as well as electrospun fibers(Figure 2).



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Figure 2. DSC of electrospun fibers from polymers A and B: before and after water treatmenta. A; b. B; c. electrospun A/B fibers; d. after removal of polymer B

Removal of Phase-Separated Domains. The aqueous-soluble polymer B was removed bydissolution in water and the mass losses were 17%, 31% and 49%, very close to the masses ofpolymer B in the original compositions. The fiber surfaces become rough (Figure 3). Themembranes remain fibrous upon prolonged exposure to aqueous media.

100 200 300 400 500

T/°C

a

b

c

d

endo



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Figure 3. Electrospun 50/50 A/B fibers: before (left) and after (right) removal of polymer B.

FTIR confirms that differential solubility is an efficient means to remove the phase-separateddomains of polymer B (Figure 4).

Nanoporosity!!!

Preliminary experiments using a liquid inclusion method show that inter-fiber porosity can becontrolled between 0.3 to 0.95. Porosity within the fibers can be measured by gas adsorptionisotherm based on Lagmuir Brunauer, Emmett and Teller (BET) method. Intra-fiber porosity hasshown to also significantly increase with the removal of polymer B. For fibers in Figure 1c,BET measurements shows that removal of polymer B significantly increases the intra-fiber porevolume (50% from 0.26 cc/g to 0.37 cc/g) and specific surface (three fold from 18.9 to 49.7 m2/g)of the fibers. The diameters of these nanopores range from 8 to 60 nm, depending upon themethods and substrates.

Our preliminary results indicate that (a) electrospinning of solvent-compatible yet phase-separated polymers generates fibers with high efficiency; (b) fiber sizes range from around 100nm to 1 mm; (c) fiber size and inter-fiber porosity can be easily controlled by polymercompositions and solution properties; and (d) differential solubility removes phase-separateddomains and generates nanoporosity.



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Figure 4. FTIR of polymers A, B, electrospun A/B, washed A/B, and cast films from B

Future Plan and Collaboration

These investigators seek funding to further develop this line of research. Future prospectstoward this research include exploring chemical strategies, such as in-situ polymerization andcopolymerization, synthesis of interpenetrating networks, and chemical crosslinking, to createnanoporous fiber structures. One application advantage of these ultra-high specific surface fibersis the miniaturization and/or invisibility of devices and sensors. These fibrous materials can alsobe incorporated with conventional textiles as well as other structures like coatings, laminates,blends and additives. Examples of applications include chemical conversion, solid supportcatalysts, selective separation, membrane-supported smart materials, scaffolds for tissue/cellgrowth, and membranes for immobilizing biological and pharmacologically significant agentsand molecules.

A/B washed

10% B

5% B

1% B

A

843962

B

740 760 780 800 820 840 860 880 900 920 940 960 980 1000 1020 1040 1060Wavenumbers (cm-1)



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Collaboration and industrial partners from the fiber/textile industry as well aschemical/polymer/consumer product sector will be sought for further development of theconcept and transferring of the developed technology to all affiliated industries, including thosein the West Coast and California. Furthermore, collaboration with the government isanticipated. Researchers at US Army Soldier Biological & Chemical Command centers andMedical Command have been investigating methods towards the development of absorbent andfibrous materials for decontaminating equipment and personnel upon exposure to toxicchemicals and biological agents. These groups have an expressed interest in collaborating onmilitary applications.

Project Websitehttp://trc.ucdavis.edu/textiles/ntc%20projects/C03.CD01s.12.html

AcknowledgementsThis project is made possible by the experimental work of Lifeng Zhang and Corine Cecile.Their significant contribution is greatly appreciated.

References:1. a. Fong, H.; Reneker, D.H. Chapter 6 in Structure Formation in Polymeric Fibers, Ed. D.R.

Salem and M.V. Sussman, Hanser 225-246.6. b. Fong, H., I. Chun and D.H. Reneker,Polymer, 1999, 40(16): 4585.

2. Perkins, T.T., D.E. Smith and S. Chu, Science, 1994, 264: 819.3. Schreuder-Gibson, H., P. Gibson, K. Senecal, M. Sennett, J. Walker, W. Yeomans, D.

Ziegler and P.P. Tsai, Protective materials based on electrospun nanofibers (Personalcommunication).

4. Schreuder-Gibson, H.L., P.W. Gibson, and Y.L Hsieh, Transport properties of electrospunnonwoven membranes", International Nonwovens Journal, Vol. 11, No. 2, pg. 21 (2002)

5. Liu, H. and Y.-L. Hsieh, Ultra-fine Fibrous Cellulose Membranes from Electrospinning ofCellulose Acetate, J. Polymer Science, Polymer Physics, 40:2119-2129 (2002).

6. Liu, H. and Y.-L. Hsieh, Surface methacrylation and graft-co-polymerization of Ultra-FineCellulose fibers, J. Polymer Science, Polymer Physics (41):953-964 (2003).

7. Xie, J. and Y.-L Hsieh, Ultra-high surface fibrous membranes from electrospinning of naturalproteins: casein and lipase enzymes, Symposium book, J. Material Science 38, 2125-2133 (2003)

8. Chen, H. and Y.-L. Hsieh, Dual sensitive hydrogel composites, the Fiber Society, Fallmeeting, Lake Tahoe, NV October, 2001. Pages 120,121.

9. Wang, Y. and Y.-L. Hsieh, Cellulose substrates funcationalized by diacrylchloride, the FiberSociety, Fall meeting, Lake Tahoe, NV October, 2001. Pages 117-119.

10. Hsieh, Y.-L., Ultra-high Surface Membranes from Natural Proteins and Enzymes, PolymerFibres, Manchester, UK, July, 2002.

11. Hsieh, Y.-L., Nano-Porous Materials from Natural Polymers, invited paper, USDA Nano-Scale Science and Engineering Workshop, Washington DC, November 17-18, 2002.



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12. Chen, H. and Y.-L. Hsieh, Functionalization of deacetylated cellulose acetate (DCA)membranes by poly(acrylic acid)brushes, Division of Polymer Chemistry, 225th NationalMeetings American Chemical Society, New Orleans, March 23-27, 2003, Polymer.

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