Here is a recent work on the Poisson's ratio of carbon nanotube sheet [1]. From the experimental results, the researchers found that the Poisson's ratio could change from 0.06 to -0.20 as the content of MWNTs increasing from 0~100%. The interesting zero Poisson's ratio and auxetic materials were thus found.  

Ref. [1] Sign Change of Poisson's Ratio for Carbon Nanotube Sheets. Science 25 April 2008: 504-507

Ying Li

Department of Engineering Mechanics, Tsinghua University