Intelligent Viscoelastic Polyurethane Intrinsic Nanocomposites
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LIGHT weight polymer nanocomposites are attractive candidates for many demanding load-bearing applications. Composites based on polymers constituting nanosubstructure do have the ability to dissipate energy in the viscoelastic matrix. Currently, there is increased interest in the energy absorption by viscoelastic elastomers based on polyurethanes formed by the polyaddition reaction of hydroxyl (–OH) containing prepolymer chains and diisocynates (–NCO). It is a multiphase material composed of hard (–NCO) and soft (–OH) segments. The (–OH) differs in its backbone character. It could be polyether, polyester, or a polybutadiene structure; the type of isocyanate (–NCO) chosen depends on the degree of its reactivity.[1] These systems provide energy absorption even at low ambient temperature and, if their dynamic properties are appropriately tuned, under high strain rates prevalent during shock or impact loading. As depicted in Figure 1, the hard blocks in the polyurethane (PU) mass are elastic, while the soft M. BILAL KHAN, Dean and Director Projects, is with the School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12 Islamabad, Pakistan. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Mechanical Behavior of Nanostructured Materials,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Electronic, Magnetic, and Photonic Materials Division, the TMS Materials Processing and Manufacturing Division, the TMS Structural Materials Division, the TMS Nanomechanical Materials Behavior Committee, the TMS Chemistry and Physics of Materials Committee, and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online February 6, 2010 876—VOLUME 41A, APRIL 2010
blocks are viscous in nature. In a segmented PU, these are microphase segregated at the nanolevel.[2] The microstructure of a PU is characterized by hard block globules and hard segment spherulites, impeded by amorphous or featureless regions of the soft segment, which can crystallize under extensive nanophase segregation, as detected by X-ray diffraction at an angle of incidence of approximately 12 deg.[3] Attenuated total reflection–infrared (ATR-IR/FT-IR) has been previously employed to assess the onset of nanosegregation phenomenon.[1,4] Fu and Schneider[4] carried out IR spectroscopy for the entire three major precursors polymer chains, i.e., butadiene, polyester, and polyether, and found phase segregation, as revealed by an enhanced –NH stretching peak at 3320 cm–1 together with a shift in the carbonyl peak at approximately 1750 cm–1. Hydrogen bonds form between the carbonyl oxygen and the amine hydrogen when urethane hard segments associate with each other.[5,6] This results in the frequency shift in the C=O peak. During polymerization, the C=O peak increases as isocyanate groups [–NCO] convert to urethane [–NHCOO]. It is this association of hard segments that leads
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