Preferential Association of Segment Blocks in Polyurethane Nanocomposites

LaShanda T. James Korley, Shawna M. Liff, Nitin Kumar, Gareth H. McKinley, and Paula T. Hammond

Department of Chemical Engineering, Department of Mechanical Engineering, and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Received June 20, 2006

Revised Manuscript Received August 1, 2006

Abstract.  Controlling the level of dispersion of silicate layers in polymer matrices
through intermolecular interactions and exploiting these interactions to enhance
thermomechanical behavior are key challenges in the field of polymer anocomposites.  In this investigation, unmodified Laponite platelets are dispersed in a segmented polyurethane containing polar, hydrophilic soft segment and a hydrophobic hard segment using a novel solvent exchange method and compared to polyurethane nanocomposites containing more hydrophobic hard and soft domains.  It was determined that the silicate layers were preferentially, but not exclusively, attracted to the hydrophilic, polar soft domains. An apparent micro-phase segregated morphology was observed in transmission electron microscopy for this system, revealing regions of exfoliation and intercalation. According to polarizing optical microscopy, strain-induced alignment is inhibited for this polyurethane nanocomposite, which is reflected in dramatic reductions in tensile strength and ultimate extensibility.   In comparison, the Laponite discs appear to be preferentially, but not exclusively, embedded to the hard domains in the segmented polyurethanes containing more hydrophobic hard and soft domains.  Exfoliation of the clay platelets leads to enhanced modulus and toughness without a reduction in extensibility. This study provides clues for exploiting silicate-polymer interactions to tune material properties without chemical modification.