Nanostructure to Microstructure Self-Assembly of Aliphatic Polyurethanes: The Effect on Mechanical Properties Abhinay Mishra, † Vinod K. Aswal, ‡ and Pralay Maiti* ,† School of Materials Science and Technology, Institute of Technology, Banaras Hindu UniVersity, Varanasi 221 005, India and Solid State Physics DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ReceiVed: January 21, 2010; ReVised Manuscript ReceiVed: March 18, 2010 We report the step by step self-assembly from nanostructure to microstructure (bottom-up approach through X-ray diffraction (1.6 nm), small angle neutron scattering (SANS) (11.6 nm), atomic force microscopy (70 nm smaller crystallite from enlarged image and 450 nm greater crystallites), and polarizing optical microscope (2 µm)) of aliphatic polyurethanes (PU) in contrast to aromatic polyurethanes depending on hard segment content (HSC). Polyurethanes of 10 to 80% HSC have been synthesized by using appropriate amount of polyol and chain extender. The effect of self-assembled patterns on mechanical properties both in solid and liquid state has been established exhibiting structure-property relationship of supramolecular polyurethanes. The crystallinity enhances but the degradation temperature decreases with increasing HSC. The characteristic length (measure of gap between lamellar crystallites), as revealed from SANS, gradually decreases with increasing HSC suggesting compactness of the crystallites through extensive hydrogen bonding. The Young’s modulus increases with increasing HSC with a percolation threshold of hard segment (50%) while the toughness improves up to 30% HSC followed by gradual decrease in presence of bigger crystallites which promote brittle fracture. The origin of self-assembly in aliphatic PUs has been demonstrated through electronic structure calculations to form a loop structure with minimum intermolecular distance (2.2 Å) while that distance is quite large in aromatic polyurethanes (4.6 Å) that cannot form hydrogen bonds. The unique splintering of domain structure and its subsequent reformation under dynamic shear experiment has been established. Introduction Synthetic polymers like polyurethanes having the properties of biocompatibility and biodegradability are a special class of materials and are being used in biomedical applications, high- performance elastomeric product, biomaterials, durable coating, and shape memory materials. 1–9 Polyurethanes are most versatile polymer constituted by hard and soft segments. Thermodynamic incompatibility between hard and soft segments is responsible for unparalleled combinations of strength, stiffness, and tough- ness by altering the composition of the constituents. The explicit feature of these hard and soft segments drives the polymer system into two phase morphology in which hydrogen-bonded, crystalline hard microdomains form amid the rubbery soft domains. 10–14 Depending on the specific segmental composition and the interactions between soft-soft and hard-soft, the hard microdomains can form fibrillar, globular, cylindrical, or lamel- lar structures within a continuous soft matrix or form an interconnected hard-domain network. The flexibility of urethane chemistry enables the structure/shape of the domains to control various properties of the polyurethane. 15,16 Cooper and co-workers 17–19 have defined the presence and behavior of the hydrogen bonds on mechanical properties of polyurethanes. Velankar et al. 20 have reported the phase mixing through small-angle X-ray scattering (SAXS) and rheological measurements. Further, temperature-resolved SAXS experiments have demonstrated gradual microphase mixing of the mi- crophase-separated materials with increasing temperature. 21 Koberstein and co-workers 22,23 have demonstrated temperature- induced changes in the hard microdomain structure. The temperatures corresponding to the microcrystalline melting and microphase disordered endotherms increase with both the hard segment content and annealing temperature. The morphological changes that accompany these alterations in thermal properties have been studied by simultaneous SAXS and differential scanning calorimetry (DSC) measurements. 24–26 A more detailed discussion of the peak temperature trends and the consequences of this behavior in terms of structure-property relations have been presented therein. Klinedinst et al. 27 have worked on the polyurethaneurea based on hydrogenated diphenyl methane diisocyanate and hexamethylene diisocyanate in addition to either ethylene diamine or 2-methyl-1,5-diaminopentane as the chain extender. The morphologies of these materials have developed microphase separation with wide service windows as measured using SAXS and dynamic mechanical analyzer (DMA). In addition to the broad temperature insensitive plateau, they displayed a unique, near linear, Hookean-like stress-strain response until fracture at very high levels of strain in excess of 900% in some cases. Direct visual evidence of the microphase- separated morphology has also been reported by using atomic force microscopy (AFM) for polyurethaneurea-based materials. Recently Vaia and co-workers 28 have reported transient micro- structure of low hard segment thermoplastic polyurethane under uniaxial deformation. Further, they have concluded from X-ray experiments that the neat low hard segment polyurethane system shows a transient morphology at low strains, dominated by soft segments crystallites, involving the coexistence of two inde- pendent crystalline phases. The subsequent mechanical proper- ties are associated with these soft segment crystallites, which * To whom correspondence should be addressed. E-mail: pmaiti.mst@ itbhu.ac.in. † Banaras Hindu University. ‡ Bhabha Atomic Research Centre. J. Phys. Chem. B 2010, 114, 5292–5300 5292 10.1021/jp100599u  2010 American Chemical Society Published on Web 04/02/2010