Preferential Association of Segment Blocks in Polyurethane Nanocomposites
LaShanda T. J. Korley1, Shawna M. Liff2, Paula T. Hammond3, and Gareth H. McKinley2. (1) School of Chemical and Biomolecular Engineering, Cornell University, Olin Hall, Room 120, Ithaca, NY 14853, (2) Massachusetts Institute of Technology, NE47-511, 500 Technology Square, Cambridge, MA 02467, (3) Chemical Engineering, Massachusetts Institute of Technology, 77 Massachuhsetts Avenue, 66-550, Cambridge, MA 02139
Vast improvements in thermomechanical properties have been reported with the exfoliation and/or intercalation of layered silicates within polymer matrices. Recently, a novel exchange method to exfoliate unmodified Laponite within a segmented polyurethane matrix was developed, which resulted in dramatic enhancements in toughness, initial modulus, and tensile strength. As an extension of this research, we have explored the role of specific polyurethane-unmodified silicate interactions on the thermomechanical properties of thermoplastic polyurethane elastomers containing polar and/or hydrophilic soft and hard blocks. An unexpected reduction in tensile properties was obtained in these nanocomposite materials containing poly(ethylene oxide) (PEO) copolymer soft segments, which exhibit both polarity and hydrophilicity. We propose that a preferential attraction between the PEO copolymer soft segments and the Laponite platelets restricts the mobility of the soft segments, affecting the ultimate tensile strength and elongation. Although the soft segment thermal properties remain essentially unchanged in these nanocomposites, a broadening of the tan δ peak in dynamic mechanical analysis is observed at higher temperatures, reflecting multiples modes of segmental motion and a reduction in mobility. Based on transmission electron microscopy images and confirmed with wide-angle X-ray difraction patterns, regions of exfoliation and intercalation are observed within the polyurethane. Thermal characterization confirms that the nanocomposites are segregated into hard and soft domains and that the hard segment melting transition remained relatively unchanged upon clay loading. Our results suggest that the material properties of segmented polyurethane/clay nanocomposites may be tailored by selecting blocks with varying degrees of polarity and hydrophilicity.