Self-assembling block copolymers (BCPs) could be used to make well-defined periodic nanopatterns with feature sizes ranging from nanometres to hundreds of nanometres. Most of the research in this field has focused on ever-decreasing the size of the features (with 3 nm being the record smallest to date) but there are a number of applications that require features on the scale of hundreds of nanometres. However, the problem here is that very high molecular weight BCPs must be used to produce such large-sized features – something that is far from ideal, since these polymers self-assemble much more slowly than their lighter counterparts.

Although researchers have tried speeding up the assembly rate of these heavy BCPs – by swelling the polymers with CO2 at high temperatures, for example – such tactics have been far from successful. So-called "brush" BCPs might come into their own here, according to new experiments by Thomas Russell and Robert Grubbs at the University of Massachusetts in Amherst and the California Institute of Technology together with colleagues at Yonsei University, Korea and Samsung. These materials consist of a linear polymer chain with branching polymer side chains densely attached to the main backbone.

Faster self-assembly

Thanks to a phenomenon called steric hindrance between the side chains, which forces the brushes to separate from each other, high molecular weight brush BCPs appear to self-assemble much faster than conventional linear polymers with the same molecular weight. Indeed, the team has found that even ultrahigh molecular weight brush BCPs will rapidly self-assemble into well-ordered microphase-separated structures with extremely large feature sizes of around a tenth of a micron. What is more, the feature sizes depend on the molecular weight of the backbone chain, which can thus be changed at will by choosing different brush BCPs.

And that is not all: the researchers have also observed that the lamellar microdomains formed by these brush BCPs are oriented perpendicular to the substrate without the surface having to be modified in any way. According to the team, such structures might be used as templates and scaffolds for nanopattern transfer or backfilling processes to fabricate electronics devices on either hard or flexible surfaces (that may or may not be transparent). Such templates could be ideal for making polarizers and photonic band gap materials for visible, ultraviolet, and even infrared light. Russell says that Samsung is already interested by this new technology.

"We now plan to quantitatively understand such mixed systems in which the periodicity can easily be manipulated,” he told nanotechweb.org. "There are also fundamental questions regarding the packing of the polymer chains that we need to urgently answer."

More information on the research can be found in ACS Nano DOI: 10.1021/nn402639g.

Further reading

Self-assembly goes square-shaped (Sep 2008)
Block copolymer blends form tunable nanotemplates (Jan 2011)
Universal assembly directs ultrahigh-density storage devices (Nov 2010)