With increasing density imposed by external osmotic pressure, DNA in univalent salt solutions (e.g., NaCl) is known to go through a set of ordered mesophases, eventually crystallizing into an orthorhombic crystal. While the transition from the cholesteric to the line hexatic (LH) phase has been observed before, it has remained unclear whether the transition is of second order or first order. We use the small but accurately measurable temperature dependence of the osmotic pressure of a PEG solution to fine-regulate the osmotic stress with which it acts on the DNA subphase. This allows us to set the osmotic pressure to an accuracy never achieved before. This advance in experimental methodology allows us then to detect small but nevertheless finite changes in the density of DNA as it goes through the cholesteric → LH transition. In this way, we first determine experimentally the small density change that occurs at the cholesteric → LH phase transition. Further, we establish that this small density discontinuity of Na-DNA is merely increased when polyvalent salt Co(NH3)6Cl3, i.e. CoHex, is added to the solution. Increasing CoHex concentration finally leads to a phase separation at zero imposed osmotic pressure. Establishing a continuity of thermodynamic states for the cholesteric → LH transition and DNA condensation, thought to be completely unrelated before, represents an important advance in our understanding of DNA polymorphism in electrolyte solutions.

Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its behavior at a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water, and above this temperature, polymer chains collapse and transform into a globule state. The conformational dynamics of single chains of polymer in a solution is known to be different from those of grafted structures that comprise of an ensemble of such single chains. In this study, we have carried out MD simulations of a mesoscopic nanostructure of PNIPAM polymer chains consisting of 60 monomer units grafted onto gold nanoparticles of different diameters, to study the effect of temperature and core particle size on the polymer conformations. Additionally, we have also studied the effect of grafting density on the coil-to-globule transition exhibited by PNIPAM through the LCST. The systems investigated consisted of ∼3 and ∼6 million atoms. Simulations were carried out below and above the LCST of PNIPAM, at 275K and 325K. Simulation trajectories were analyzed for radius of gyration of PNIPAM chains.

A computational algorithm has been developed to simulate the transport properties of oriented and un-oriented thin film nanocomposites of isotactic Polypropylene (iPP) and carbon nanotubes (CNT) with increasing CNT concentration. Our goal is to be able to design materials with optimal properties using these simulations. We use a cellular automata approach in a Matlab 3-D array environment. The percolation threshold is reproduced in the simulations, matching experimental data. Upon percolation, the thermal transport in the films increases sharply, due to the large difference in the thermal conductivities of the CNTs and the polymer. To verify the simulation, the thin-film samples were sheared in the melt at 200C at 1 Hz in a Linkan microscope shearing hot stage. The thermal conductivity measurements were performed on the same cell arrangement with the transport perpendicular to the thin-film plane using a DC method. The thermal conductivity is higher for the un-sheared as compared to the sheared samples. Our cellular automata simulations provide information about the microstructuremacroscopic property relation in the thin film nanocomposites and can be extended to simulations of other important materials.

We investigate the effect of nanoparticles on polymer structure, polymer dimensions and topological constraints (entanglements) in polymer melts for nanoparticle loading above percolation threshold as high as 40.9% using stochastic molecular dynamics (MD) simulations. We show unambiguously that short polymer chains are not disturbed by the presence of repulsive nanoparticles. In contrast entangled polymer chains can be perturbed by the presence of attractive nanoparticles when the polymer radius of gyration is larger than the nanoparticle radius. They can expand under the presence of attractive nanoparticles even at low nanoparticle loadings of very small nanoparticle size. We observe an increase in the number of entanglements (decrease of Ne with 40.9% volume fraction of nanoparticles dispersed in the polymer matrix) in the nanocomposites as evidenced by larger contour lengths of the primitive paths. Attraction between polymers and nanoparticles affects the entanglements in the nanocomposites and alters the primitive path.

Semi-crystalline structures were prepared from different processing condition. Biaxial oriented melt were crystallized at 375 K and atmospheric pressure for 10 nanoseconds (ns), to generate a lamellar semi-crystalline structure. Similar structures were also prepared from deformation of a cubic amorphous initial structure isothermally at 375 K. For comparison, two different thermostats, the constant stress (NPT) and constant volume (NVT) conditions were applied to the system during 10 ns of crystallization. The semi-crystalline samples shared common morphological features such as in the crystallinity, crystal orientation, lamellae thickness and density distribution, etc. However, during the subsequent uniaxial tensile deformation test of the samples to strain of 0.5, different stress-strain behaviors were demonstrated. By combining the observations of morphologies during deformation tests and analysis of the stress-strain curves, conclusions were made that the effectiveness of the network had a strong influence on the mechanical property and strain hardening behavior. The oriented network from the constant stress crystallization, owing to the taut chains, gave rise to optimal mechanical response with substantial strain-hardening.

The optical properties and electronic structure of AlPO4, SiO2, Type I collagen, and DNA were examined to gain insight into the van der Waals-London dispersion behavior of these materials. Interband optical properties of AlPO4 and SiO2 were derived from vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and showed a strong dependence on the crystals’ constituent tetrahedral units, with strong implications for the role of phosphate groups in biological materials. The UV-Vis decadic molar absorption of four DNA oligonucleotides was measured, and showed a strong dependence on composition and stacking sequence. A film of Type I collagen was studied using spectroscopic ellipsometry, and showed a characteristic shoulder in the fundamental absorption edge at 6.05 eV. Ab initio calculations based on density functional theory corroborated the experimental results and provided further insights into the electronic structures, interband transitions and vdW-Ld interaction potentials for these materials.

Remora fish have evolved a unique dorsal pad capable of fast, reversible adhesion to a large range of natural and artificial surfaces. The effectiveness of adhesion is due in part to the pad’s ability to dynamically conform and adapt to the geometry of its host. Simulations based on measured material properties and geometry can provide useful design metrics for biologically inspired design, and furthermore, serve as platform for virtual experiments. The pad itself consists of a lamellar, composite structure composed of mineralized and soft tissue. In this work, finite element models based on μCT scans and measured viscoelastic material properties elucidate the pad’s complex moduli frequency spectrum and response to different loading configurations.