RESEARCH NEWS

April 200518

Flexible biodegradable polymers would

be very useful in biomedical

applications. Most biodegradable

polymers in current use are relatively

stiff with limited extendibility,

making them unsuitable for many

applications.

Now, researchers at The Hebrew

University of Jerusalem, Israel have

developed highly flexible, biodegradable

thermoplastic elastomers that can be

tailored for different soft-tissue

applications [Cohn and Hotovely-

Salomon, Polymer (2005),

doi:10.1016/j.polymer.2005.01.012].

They synthesized and studied

poly(ethylene oxide)/poly(L-lactic acid)

or (PEO/PLA) copolymers. These

polymers are based on multiblock

backbones that integrate flexible, ‘soft’

PEO segments with ‘hard’,

biodegradable PLA blocks.

Differential scanning calorimetry (DSC)

and X-ray analyses indicate that,

depending on copolymer composition,

amorphous matrices, as well as

materials comprising one or two

crystalline phases, can be synthesized.

The PEO segment prevents

crystallization of shorter PLA blocks

of 12 and 17 lactoyl units. Only when

the PLA blocks are sufficiently long –

about 36 lactoyl units – do they

crystallize.

The amorphous PEO chains can be

considered as molecular springs while

the crystalline PLA blocks form strong,

noncovalent crosslinked domains. This

phase-segregated morphology provides

excellent mechanical properties.

Ultimate tensile strength values are as

high as 30 MPa and elongation at

break levels are well above 1000%.

Even when fully hydrated, the block

copolymers exhibited significant

strength of 8-9 MPa.John K. Borchardt

Biodegradablepolymers shownew flexibilityPOLYMERS

The mechanical, electrical, and optical properties ofmultiwalled carbon nanotubes (MWNTs) make themattractive as reinforcing fillers for advanced polymercomposites. While carbon nanotubes have a hightensile strength because of their stiffness,composites fabricated with nanotubes often exhibitinferior toughness. Researchers from BostonCollege, the Center of Advanced European Studiesand Research, Germany, and the Adam MickiewiczUniversity in Poland have found that a silica shell onthe surface of a nanotube enhances MWNTstiffness and rigidity, and can improve thenanomechanical properties of polymer composites[Olek et al., Langmuir (2005), doi: 10.1021/la0470784].The group studied the nanomechanical properties ofpoly(methyl methacrylate), or PMMA, compositesreinforced with 1-5 wt.% silica-coated MWNTs. Theproperties were determined using an atomic forcemicroscope fitted with a nanoindenter. This depth-sensing and load-control device measures elasticand plastic properties at the nanoscale level.The absence of steps and discontinuities on load-displacement curves of the silica-coated MWNT-PMMA nanocomposites indicates that no cracksand fractures occur when making indentations on

test samples. The hardness and Young’s modulus ofthe composites increases strongly with increasingcontent of silica-coated MWNTs. In sharp contrast,without the silica coating, the MWNTs did not alterthe nanomechanical properties of the composites. Synthesis of the silica-coated MWNTs was a keypart of the research. Michael Giersig andcoworkers have developed a method to coatMWNTs with silica particles using a technique thatis amenable for scaleup to commercial production.First, the MWNTs are functionalized withpoly(allylamine hydrochloride) and then dispersed inwater. The MWNT suspension is added to silica soland sonicated to form a homogeneous mixture.After 12 hours, the mixture is centrifuged fourtimes to wash the nanotubes with ethanol. Finally,the solid is dispersed in a solution of ammonia inethanol, and tetraethoxysilane (TES) is added withstirring. The TES polymerizes to form a uniform,thick layer on each individual MWNT. The originalMWNTs have diameters in the range of 5-20 nm,while the diameters of the silica-coated MWNTsrange from 70-80 nm. This indicates that the silicashell thickness is roughly equal to that of the corenanotubes.John K. Borchardt

Silica coats give strong nanotube compositesNANOTECHNOLOGY

Light-emitting polymers turn bluePOLYMERS

More efficient blue chromophores for light-emitting diode (LED) applications, such asthe next generation of flat-panel displays,remain a target of intense research. Researchers at Université Pierre et MarieCurie, ONERA, Université de Rennes, andCEA Saclay in France have synthesized aseries of blue-light-emitting 6-(arylvinylene)-3-bromopyridine derivatives (Leclerc et al.,Chem. Mater. (2005), 17 (3), 502). The group’s versatile synthetic routeproduces high yields of asymmetrical andsymmetrical chromophores end-capped withreactive functional groups. These buildingblocks can be coupled to produce polymericchromophores. Nicolas Leclerc andcoworkers found that the resulting liquidcrystal polymers show intense fluorescenceand are promising for nonlinear opticsapplications. One symmetrical compoundexhibited intense blue fluorescence emissionand was an active laser dye. It also provided

good efficiency when evaluated in an LED. Meanwhile, University of Washingtonresearchers have synthesized copolymers of9,9’-dioctylfluorene and 2,3-bis(p-phenylene)quinoxaline and evaluated them asblue-light-emitting materials in LEDs[Kulkarni et al., Macromolecules (2005), 38(5), 1553]. Their objective was to synthesizefluorene copolymers containing an electron-deficient comonomer that provides stableblue-light emission with high brightness andimproved quantum efficiency.All the copolymers emit blue light in dilutesolution and the solid state. A copolymercontaining 5 mol.% 2,3-bis(p-phenylene)-quinoxaline exhibits a significantenhancement in electroluminescence. SamsonJenekhe and coworkers believe the light-emitting properties and improved thermalstability of the copolymers make thempromising materials for blue polymer LEDs.John K. Borchardt