Some natural biomaterials, for example silk and bone, show incredible properties of strength, extensibility, stiffness and toughness. What surprise people most are how these biomaterials become a combination of all these properties? Recent research unravels the key to these mechanical gifts, which can be attributed to a regular incorporation between the nanocrystallites and the protein matrices [ 1 , 2 ]. This incorporation throws a paradigm to polymer engineering, but it's not easy to tune hydrophobic polymers with inorganic, often hydrophilic nanoparticles.

There comes a try by Liff and the colleagues [ 3 ]. Through an efficient solvent-exchange procedure, they successfully incorporate unmodified the clay nanoparticles of Laponite into Elasthane 80A matrix. This composite polymer exhibits expected thermomechanical reinforcement that is competent for high temperature applications. Certainly, the technique is similar to what the spider will do with its silk spinning—modulate both the stiffness and extensibility of the silk by selectively putting nanocrystallites into polypeptide chain.

The Polymer Elasthane 80A

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Elasthane 80A: Elasthane 80A is an elastomeric block copolymer composed of MDI-BDO HS and PTMO SS. HS (length 3–11 nm) phase segregate to form hard crystalline domains via hydrogen bonding.

Credit:Scidea Art 2007 Source: ScideaNews.com

Elasthane 80A is a block polymer and composed of hard segments (HS) and soft segments (SS). It becomes stiffer and stronger when the HS contents increases, while its extensibility increases with increment of SS content. By controlling the content of HS and SS, thermomechanical properties of the polymer could be altered. Practically, it is very difficult to enhance the stiffness without reducing the extensibility of the polymer via a synthetic approach. However, Liff et al. found a solution for sufficient insertion by matching the size of the clay particle with that of HS domain (3-11 nm in length).

Besides this, full exfoliation of nanoparticles is also a challenge for well incorporating. Because these small nanoparticles are tend to agglomerate tightly. One possible way to exfoliate the nanoparticles is to chemically modify the nanoparticles' surface. However, for a wide flexibility, it's meaning for the authors' selection of unmodified nanoparticles, and more importantly, their two-solvent dispersal technique can efficiently exfoliate the Laponite nanoparticles, which is found to be especially helpful to the enhancement of hard microdomain of the polymer.

The incorporation of Laponite into polyurethane can greatly impact the elastic modulus of the polymer. At 20 wt% loading, the modulus is found to be 23-fold higher than pure polyurethane. On the other hand, the extensibility, strength and overall toughness are not dramatically increased with the change of nanoparticle concentration. The little change of extensibility in turn suggests the nanoparticles are mainly sitting in the hard domains of polymer. Moreover, the heat distortion temperature of the nanocomposite exhibits more than 100 ^oC increase, which allows it to be used at higher temperatures.

So, it's indeed a surprise that a single procedure can change things much. Beyond the underlying refinement and its contribution to material science, this research really pulls us back to think for a while something simple.□

* Xiaofeng Qiu is in the Chemistry Department of Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA.

Received 20070315, Online 20070316.