Abiotic tooth enamel.

Tooth enamel comprises parallel microscale and nanoscale ceramic columns or prisms interlaced with a soft protein matrix. This structural motif is unusually consistent across all species from all geological eras. Such invariability-especially when juxtaposed with the diversity of other tissues-suggests the existence of a functional basis. Here we performed ex vivo replication of enamel-inspired columnar nanocomposites by sequential growth of zinc oxide nanowire carpets followed by layer-by-layer deposition of a polymeric matrix around these. We show that the mechanical properties of these nanocomposites, including hardness, are comparable to those of enamel despite the nanocomposites having a smaller hard-phase content. Our abiotic enamels have viscoelastic figures of merit (VFOM) and weight-adjusted VFOM that are similar to, or higher than, those of natural tooth enamels-we achieve values that exceed the traditional materials limits of 0.6 and 0.8, respectively. VFOM values describe resistance to vibrational damage, and our columnar composites demonstrate that light-weight materials of unusually high resistance to structural damage from shocks, environmental vibrations and oscillatory stress can be made using biomimetic design. The previously inaccessible combinations of high stiffness, damping and light weight that we achieve in these layer-by-layer composites are attributed to efficient energy dissipation in the interfacial portion of the organic phase. The in vivo contribution of this interfacial portion to macroscale deformations along the tooth's normal is maximized when the architecture is columnar, suggesting an evolutionary advantage of the columnar motif in the enamel of living species. We expect our findings to apply to all columnar composites and to lead to the development of high-performance load-bearing materials.

Viscoelasticity of glycerol at ultra-high frequencies investigated via molecular dynamics simulations.

We present a calculation of the shear and longitudinal moduli of glycerol in the gigahertz frequency regime and temperature range between 273 K and 323 K using classical molecular dynamics simulations. The full frequency spectra of shear and longitudinal moduli of glycerol between 0.5 GHz and 100 GHz at room temperature are computed, which was not previously available from experiments or simulations. We also demonstrate that the temperature dependence of the real parts of the shear and longitudinal moduli agrees well with available experimental counterparts obtained via time-domain Brillouin scattering. This work provides new insights into the response of molecular liquids to ultra-high frequency excitation and opens a new pathway for studying simple liquids at high frequencies and strain rates.

Computational exploration of polymer nanocomposite mechanical property modification via cross-linking topology.

Molecular dynamics simulations have been performed in order to study the effects of nanoscale filler cross-linking topologies and loading levels on the mechanical properties of a model elastomeric nanocomposite. The model system considered here is constructed from octafunctional polyhedral oligomeric silsesquioxane (POSS) dispersed in a poly(dimethylsiloxane) (PDMS) matrix. Shear moduli, G, have been computed for pure and for filled and unfilled PDMS as a function of cross-linking density, POSS fill loading level, and polymer network topology. The results reported here show that G increases as the cross-linking (covalent bonds formed between the POSS and the PDMS network) density increases. Further, G is found to have a strong dependence on cross-linking topology. The increase in shear modulus, G, for POSS filled PDMS is significantly higher than that for unfilled PDMS cross-linked with standard molecular species, suggesting an enhanced reinforcement mechanism for POSS. In contrast, in blended systems (POSS/PDMS mixture with no cross-linking) G was not observed to significantly increase with POSS loading. Finally, we find intriguing differences in the structural arrangement of bond strains between the cross-linked and the blended systems. In the unfilled PDMS the distribution of highly strained bonds appears to be random, while in the POSS filled system, the strained bonds form a netlike distribution that spans the network. Such a distribution may form a structural network "holding" the composite together and resulting in increases in G compared to an unfilled, cross-linked system. These results are of importance for engineering of new POSS-based multifunctional materials with tailor-made mechanical properties.

Heterogeneous directional mobility in the early stages of polymer crystallization.

Recently, we demonstrated via large-scale molecular dynamics simulations a "coexistence period" in polymer melt ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism [Gee et al., Nat. Mater. 5, 39 (2006)]. Here, we present an extension of this work, where we analyze the directional displacements as a measure of the mobility of monomers as they order during crystallization over more than 100 ns of simulation time. It is found that the polymer melt, after quenching, rapidly separates into many ordered hexagonal domains separated by amorphous regions, where surprisingly, the magnitude of the monomer's displacement in the ordered state, parallel to the domain axial direction, is similar to its magnitude in the melt. The monomer displacements in the domain's lateral direction are found to decrease during the time of the simulation. The ordered hexagonal domains do not align into uniform lamellar structures during the timescales of our simulations.

Molecular dynamics simulations of ordering of poly(dimethylsiloxane) under uniaxial stress.

Molecular dynamics simulations of bulk melts of poly(dimethylsiloxane) (PDMS) are utilized to study chain conformation and ordering under constant stress uniaxial extension at room temperature. We find that large extensions induce chain ordering in the direction of applied stress. During the extension, we also find that voids are created via a cavitation mechanism. At the end of our simulations, by visual inspection, we distinguish cavity, fibril, and amorphous regions that coexist together. The surrounding material about the formed cavities is fibril-like, while the remaining material remains amorphous. We also estimate the surface energy of the cavity. The cavity size continually increases in the dimension of applied stress but saturates in the lateral dimensions, most likely due to the finite size of the system. Despite chain orientation and ordering in the direction of applied stress, crystallization is absent in the time and stress range of our simulation. This study represents a baseline for the future study of mechanical properties of PDMS melts enriched with fillers under stress.

Atomistic simulations of spinodal phase separation preceding polymer crystallization.

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental and theoretical efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a 'coexistence period' in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.