RESUMO
One of the challenges in polymer science has been to prepare large-polymer single crystals. We demonstrate a visible light-triggered quantitative topochemical polymerization reaction based on a conjugated dye molecule. Macroscopic-size, high-quality polymer single crystals are obtained. Polymerization is not limited to single crystals, but can also be achieved in highly concentrated solution or semicrystalline thin films. In addition, we show that the polymer decomposes to monomer upon thermolysis, which indicates that the polymerization-depolymerization process is reversible. The physical properties of the polymer crystals enable us to isolate single-polymer strands via mechanical exfoliation, which makes it possible to study individual, long polymer chains.
RESUMO
An understanding of the mechanical behavior of polymers is critical towards the design, implementation, and quality control of such materials. Yet experiments and method for the characterization of material properties of polymers remain challenging due the need to reconcile constitutive assumptions with experimental conditions. Well-established modes of mechanical testing, such as unconfined compression or uniaxial tension, require samples with specific geometries and carefully controlled orientations. Moreover, producing specimens that conform to such specifications often requires a considerable amount of sample material. In this study we validate a micromechanical indentation device, the Tissue Diagnostic Instrument (TDI), which implements a cyclic indentation method to determine the material properties of polymers and elastomeric materials. Measurements using the TDI require little or no sample preparation, and they allow the testing of sample materials in situ. In order to validate the use of the TDI, we compared measurements of modulus determined by the TDI to those obtained by unconfined compression tests and by uniaxial tension tests within the limit of small stresses and strains. The results show that the TDI measurements were significantly correlated with both unconfined compression (p<0.001; r(2) = 0.92) and uniaxial tension tests (p<0.001; r(2)=0.87). Moreover, the measurements across all three modes of testing were statistically indistinguishable from each other (p=0.92; ANOVA) and demonstrate that TDI measurements can provide a surrogate for the conventional methods of mechanical characterization.
RESUMO
Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites. One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters. The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers. Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts. The overall cylindrical lattice is capped at its upper end by a terminal sieve plate and rooted into the sea floor at its base by a flexible cluster of barbed fibrillar anchor spicules. External diagonally oriented spiral ridges that extend perpendicular to the surface further strengthen the lattice. A secondarily deposited laminated silica matrix that cements the structure together additionally reinforces the resulting skeletal mass. The mechanical consequences of each of these various levels of structural complexity are discussed.
