RESEARCH NEWS

June 2004 17

Polymer nanofibers could be used in

composites for biomedical applications

such as tissue engineering, bandages,

and drug release systems, as well as

for optoelectronic devices, photonic

crystals, and flexible photocells.

However, using conventional

electrospinning to produce these fibers

is difficult because electrical forces

imposed by the capacitor-like electric

field on the droplet of polymer solution

result in the formation of an unstable

jet. This causes looping and stretching

of the jet elements. After the solvent

evaporates, nanofibers with diameters

in the submicron range are deposited

on the counter-electrode. A single jet

issues from a single needle and, to

achieve a high production rate, many

needles are needed. However,

processes using many needles are

complex and the needles get plugged.

Using multiple upward jets from the

free surface of polymer solutions could

enable electrospinning of multiple jets

without using needles. Researchers at

Technion in Israel use this principle to

develop a novel polymer fiber

electrospinning process [Yarin and

Zussman, Polymer (2004) 45, 2997].

The system comprises two layers, a

lower ferromagnetic suspension layer

and an upper layer of polymer solution.

When the system is subjected to a

magnetic field, vertical spikes of the

magnetic suspension perturb the

interlayer interface and the free

surface of the polymer layer. The

perturbations of the free surface

become sites of upward jetting by

applying a normal electric field.

Multiple jets undergo strong stretching

by the electric field and bending

instability, the solvent evaporates, and

solidified nanofibers are deposited on

an upper electrode, as in a

conventional electrospinning process.John K. Borchardt

Spinning outfibersPOLYMERS

Sol-gel processes are used to produce nanoscaleorganic-inorganic hybrid materials with varyingstructural characteristics and particle morphology.Compared with polyblend composites, thecomponents of these hybrid materials –interpenetrated (IPN), semi-interpenetrated (semi-IPN), and encapsulated (Encap) organic-inorganicsystems – can have improved compatibility.Encapsulation involves the enclosure of micro- ornano-sized particles of the first component insidethe matrix of the second. Nano-sized particles oftenprovide superior interfacial interactions comparedwith micro-sized ones. Isam M. Arafa and coworkers at Jordan Universityof Science and Technology prepared silica-basedurea-formaldehyde (UF/SiO2) composite materials ofeach type using different sol-gel syntheticprocedures to produce an interpenetrating IPN-UF/SiO2, a micro-sized UF resin encapsulated insidea silica shell (Encap-UF/SiO2), a micro-sized silicaencapsulated inside UF shell (Encap-SiO2/UF), and a‘conventional’ UF/SiO2 blend [Arafa et al., Euro.Polymer J. (2004), doi: 10.1016/ j.eurpolymj.2004.02.014]. The IPN-UF/SiO2 and Encap-UF/SiO2 hybrids have silica surfaces, while Encap-

SiO2/UF has a UF surface. The formation of thefirst composite requires a two-step manufacturingprocess involving acid-catalyzed hydrolysis ofSi(OCH2CH3)4 and polycondensation using urea.Most of the urea molecular aggregates are trappedin the silica pores. Formaldehyde diffuses acrossthe porous silica surfaces into the interior poreswhere condensation polymerization reactions withthe trapped urea molecules take place. UF polymerchains grow inside the silica network until all thetrapped urea is consumed. The chains and silicaframework are interlocked. Since the urea isuniformly distributed in the pores, the UF domain isexpected to be homogeneous throughout theframework. To prepare an Encap-SiO2/UF hybrid,silica powder is dispersed in urea containing aminimum amount of water/ethanol. Urea isadsorbed at the silanol groups on the silica surface.Addition of formaldehyde initiates condensationpolymerization to form hybrid composite particleswith a silica core and a UF shell. This shell isexpected to dominate the surface characteristics ofthe hybrid. A composite can also be prepared byenclosing micro-sized UF particles in a silica matrix. John K. Borchardt

Nano-sized hybrids are superior COMPOSITES

Electrolytes for electric car batteriesPOLYMERS

Li-ion secondary batteries are widely used inconsumer electronics. Novel highperformance Li-ion secondary batteries usingpolymer electrolytes are being developed foruse in hybrid electric vehicles (HEVs). HEVsrequire high-rate discharging and charging;processes likely to reduce the service life andperformance of conventional Li-ion batteries.Masataka Wakihara and coworkers at theTokyo Institute of Technology report that theaddition of Lewis acids to polymerelectrolytes greatly increases the chargetransfer rate and thus the rate of batterycharging [Kato et al., Angew. Chem. Int. Ed.(2004) 43, 1966]. This provides high powerdensity and discharging, resulting in goodautomotive performance. The polymerelectrolytes studied by Wakihara are basedon Li and Mg salts of polyethers.Poly(ethylene glycol) (PEG)-borate esterincreases the ionic conductivity andtransport number of Li or Mg ions ofpolymer electrolytes. The polymer electrolyteused in this case is poly(ethylene glycol)

dimethyl ether (PEGDME). The borate estergroups act as a Lewis acid and enhance thedissociation of Li or Mg salts in the polymerelectrolyte. Since the charge-transferreaction rate is proportional to the activityof metal ions, this reaction rate and, thus,the rate of battery charging and dischargingis enhanced by the addition of the Lewis acid. What the researchers call “drasticincreases” in the exchange current densitiesof the electrolytes are found when the PEG-borate ester is added to PEGDME. At alltest temperatures, the exchange currentdensity is at a maximum when the PEG-borate ester is 25% by weight of thePEGDME polymer electrolyte. Thiscorresponds to an almost 1:1 molar ratio ofthe PEG-borate ester to the anion.This novel approach promises to aidachievement of high charge-transfer reactionrates and, thus, the development of highpower density Li-ion batteries for electric,hybrid, or fuel cell vehicles.John K. Borchardt