A large array of easily available small-molecule (as opposed to industrial oligomeric) triisocyanates and aromatic polyols render polyurethanes a suitable model system for a trend-based systematic study of structure−property relationships in nanoporous matter as a function of the monomer structure. Molecular parameters of interest include rigidity, number of functional groups per monomer (n), and functional group density (number of functional groups per phenyl ring, r). All systems were characterized from gelation to the bulk properties of the final aerogels. Molecular and nanoscopic features of interest, including skeletal composition, porous structure, nanoparticle size, and assembly, were probed with a combination of liquid-and solid-state 13 C and 15 N NMR, rheometry, N 2 -and Hg-porosimetry, SEM, and small-angle Xray scattering (SAXS). Macroscopic properties such as styrofoam-like thermal conductivities (∼0.030 W m −1 K −1 ), foam-like flexibility, or armor-grade energy absorption under compression (up to 100 J g −1 ) were correlated with one another and serve as a top-down probe of the interparticle connectivity, which was again related to the monomer structure. Overall, both molecular rigidity and multifunctionality control phase-separation, hence, particle size and by association porosity (e.g., meso versus macro) and internal surface area. With sufficiently rigid monomers, skeletal frameworks include intrinsic microporosity, rendering the resulting materials hierarchically nanoporous over the entire porosity regime (micro to meso to macro). Most importantly, however, clear roles have been identified not only for the absolute number of functional groups per monomer, but also for parameter r. The latter is expressed onto the surface of the skeletal nanoparticles (controls the surface functional group density per unit mass) and becomes the dominant structure-directing as well as property-determining parameter. By relating the molecular functional group density with the functional group density on the nanoparticle surfaces, these results establish that for three-dimensional (3D) assemblies of nanoparticles to form rigid nanoporous frameworks, they have first and foremost to be able to develop strong covalent bonding with one another. These findings are relevant to the rational design of 3D nanostructured matter, not limited to organic aerogels.

A high birefringence of over 0.21 for the yttrium vanadate (YVO4) crystal in the middle wavelength infrared (i.e., 3-5 microm) was measured. A Fourier transform infrared spectrometer was employed in the channel spectra technique to obtain the measurements.

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Polydicyclopentadiene (pDCPD) is a polymer of emerging technological significance from separations to armor. It is a paradigm of ring opening metathesis polymerization (ROMP) and some of its remarkable properties (e.g., strength) have been attributed to crosslinking of the pendant cyclopentenes. pDCPD should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of the wet-gels precursors in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. Based on spectroscopic evidence (IR, solid state 13 C NMR and several liquid 1 H NMR controls), only 4-5% of the pendant cyclopentene double bonds of pDCPD are engaged in crosslinking, via Wagener-type olefin coupling. Deformation was rectified via free radical polymerization of methylmethacrylate (MMA) in the pores of pDCPD wet-gels. The uptake of PMMA was varied in the 13-28% w/w range by varying the concentration of MMA. Evidence (e.g., differential scanning calorimetry) though suggests that PMMA remains a linear polymer, hence the pDCPD/PMMA network resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nano-topology of the two components (see next paper of this issue). With cylindrical monoliths available, the nature of the interparticle chemical bonding in pDCPD/PMMA aerogels was probed top-down with thermal conductivity and compression testing, using linearpolynorbornene (pNB) aerogels as a control system. The latter, with no pendant cyclopentenes, has no chance for interpolymer chain crosslinking. The solid thermal conduction and stiffness of pDCPD/PMMA and pNB aerogels scale similarly, pointing to a common mechanism for interparticle bonding. That was assigned to cross-metathesis, effectively extending the polymer chains of one nanoparticle into another, and was reflected on very high polydispersities (8-13).

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The mechanical behavior of human tympanic membrane (TM) has been investigated extensively under quasistatic loading conditions in the past. The results, however, are sparse for the mechanical properties (e.g., Young's modulus) of the TM at high strain rates, which are critical input for modeling the mechanical response under blast wave. The property data at high strain rates can also potentially be converted into complex modulus in frequency domain to model acoustic transmission in the human ear. In this study, we developed a new miniature split Hopkinson tension bar to investigate the mechanical behavior of human TM at high strain rates so that a force of up to half of a newton can be measured accurately under dynamic loading conditions. Young's modulus of a normal human TM is reported as 45.2-58.9 MPa in the radial direction, and 34.1-56.8 MPa in the circumferential direction at strain rates 300-2000 s(-1). The results indicate that Young's modulus has a strong dependence on strain rate at these high strain rates.