Molecular dynamics simulations were carried out to investigate carbon nanotube (CNT) interactions and dispersion in a polyethylene oxide (PEO)/water solution. The potential of mean forces (PMF) which embodies the entropic and enthalpic contributions by the solvent and the polymer molecules were computed. The relative enthalpic and entropic contributions to the PMF were studied in order to understand the CNT interaction mechanisms in solution. An adaptive biasing force (ABF) method was used to speed up the PMF calculations. The simulation results provide detailed atomic arrangements and atomic interactions between the CNTs and surrounding molecules (PEO and water). This molecular level computational study provides insights into the CNT’s interactions with PEO polymer/water systems.

Molecular dynamics simulations are used to compute the potential of mean force (PMF) governing the interactions between carbon nanotubes (CNTs) in water/surfactant systems. The effects of CNT length, diameter, chirality (armchair and zigzag) and surfactant structures on CNT interaction and dispersion in water/surfactant systems are investigated for (5, 5), (5, 0), and (10, 10) single walled CNTs with two commonly used surfactants [viz., sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS)] at room conditions. An adaptive biasing force method was used to speed up the calculations. Simulations revealed that CNT length and diameter as well as optimum amount of surfactant addition and its structures can significantly affect CNT interactions (i.e., PMFs vary significantly). Surfactant molecules were found to adsorb at the CNT surface and reduced interaction strength between CNTs. SDBS surfactant contributed weaker interactions between CNTs as compared with that of SDS surfactant by a factor of about 10 indicating that SDBS is better than SDS for dispersing CNTs in an aqueous suspension. This phenomenon agrees qualitatively with the experimental results reported in the literature. The understanding of detailed atomic arrangements and atomic interactions between CNTs and surrounding molecules reported in this study is significantly helpful to computationally screening different surfactants and improving the CNT dispersion in aqueous solution. The method will also facilitate the reduction of time and cost required to produce CNT reinforced nanocomposite materials as well as homogeneous CNT dispersed solutions for many biological applications.

The properties of nanocomposite materials depend on the dispersion of the nanoparticles/nanofibers within the matrix. The addition of surfactants and varied processing techniques are used to increase the dispersion of the nanoparticles in the final composite. A method for the quantitative prediction of the interactions between nanoparticles in solution would aid in the design of processing schedules. In this study, molecular dynamics simulations are used to compute for the potential of mean force as a function of the distance and orientation between a pair of single-walled carbon nanotubes (CNTs) in water. An adaptive biasing force method is used to speed up the calculations. Simulation results show that CNT orientation and the addition of surfactant can significantly affect CNT interactions and inturn dispersion.

Enzymatic-detergent decellularization treatments may use a combination of chemical reagents to reduce vascular tissue to sterilized scaffolds, which may be seeded with endothelial cells and implanted with a low risk of rejection. However, these chemicals may alter the mechanical properties of the native tissue and contribute to graft compliance mismatch. Uniaxial tensile data obtained from native and decellularized longitudinal aortic tissue samples was analyzed in terms of engineering stress and fit to a modified form of the Yeoh rubber model. One decellularization protocol used SDS, while the other two used TritonX-100, RNase-A, and DNase-I in combination with EDTA or sodium-deoxycholate. Statistical significance of Yeoh model parameters was determined by paired t-test analysis. The TritonX-100/EDTA and 0.075% SDS treatments resulted in relatively variable mechanical changes and did not effectively lyse VSMCs in aortic tissue. The TritonX-100/sodium-deoxycholate treatment effectively lysed VSMCs and was characterized by less variability in mechanical behavior. The data suggests a TritonX-100/sodium-deoxycholate treatment is a more effective option than TritonX-100/EDTA and SDS treatments for the preparation of aortic xenografts and allografts because it effectively lyses VSMCs and is the least likely treatment, among those considered, to promote a decrease in mechanical compliance.

Growth and remodeling are fundamental processes in the development of tissues in normal and pathological conditions. Mechanical quantities such as stress, strain or strain energy in the tissue can modulate its growth and remodeling; however, it is not clear yet which mechanical quantity takes this role. Experimental data can be found to support both. Furthermore, the driving-mechanism may be tissue-dependent and therefore, a universal growth law may not exist [7,8]. This field has been an important research topic in biomechanics over the recent decades. The review articles by Humphrey [4] and Taber [8] contain numerous related references. An important contribution was made by Rodriguez et al. [7] to the area of volumetric growth of soft elastic tissue which allowed for the coupling between stress and finite growth through multiplicative decomposition of the deformation gradient into elastic and growth parts. This theory has been followed in our study. Our goal is to model growth of the hind foot cartilage anlagen in newborn infants, and explore effect of congenital anomalies on the otherwise normal development.Copyright © 2009 by ASME

Mechanical characterization of human cartilage anlagen is required to effectively model congenital musculoskeletal deformities. Such modeling can effectively explore the effect of treatment procedures and potentially suggest enhanced treatment methods. Using serial MRI, we have noted shape changes of the cartilaginous hindfoot anlagen in patients with clubfoot, suggesting they are soft and deformable. We therefore determined the stress relaxation behavior of cartilage plugs obtained from third-trimester stillborn fetuses in unconfined and confined compression geometries. The material parameters determined were the aggregate modulus HA = 0.15 ± 0.07 MPa, Poisson’s ratio ν = 0.4 ± 0.06, Young’s modulus Es = 0.06 ± 0.03 MPa, and permeability coefficients k0 = 2.01 ± 0.8 × 10−14 m4 N−1 s−1 and M = 4.6 ± 1.0. As compared with adult articular cartilage, stiffness was an order of magnitude lower than the values reported in the literature, suggesting the relative softness of the tissue, and the permeability was an order of magnitude higher, indicating relative ease of flow in the tissue. Poisson’s ratio also was close to the higher end of the range reported in previous studies. Such material is expected to deform and relax to larger extents. These findings are consistent with the deformability of the cartilage anlagen during manipulation and casting for treatment of clubfoot.

Understanding the interaction between a carbon nanotube and biological macromolecules such as lipid bilayers is important for the design and development of nanovectors for gene and drug delivery. The forces of penetration and the free energies of rupture of lipid bilayers during nanotube penetration were studied using nonequilibrium, all-atom molecular dynamics simulations for pure POPC and POPC/cholesterol bilayers. The presence of cholesterol did not alter the magnitude of the rupture force and minimally increased the estimated free energy of rupture. However, the ability of the nanotube to disrupt the membrane leading to membrane poration increased with increasing cholesterol content.

The proteoglycan and collagen composition of cartilage is known to change during fetal and postnatal development. The current report represents the first attempt to semi-quantitatively determine the changes in the composition of developing human fetal cartilage. Human fetal talus bones were obtained from late 2nd and 3rd trimester specimen. Fetal bones are comprised of an intramembranous tissue commonly referred to as cartilage anlagen. During maturation the anlagen develops an ossific nucleus. Fourier Transform Infrared Spectroscopy (FT-IRS) and Fourier Transform Infrared Imaging Spectroscopy (FT-IRIS) were used to assess the changes in composition relating to tissues main constituents, collagen (COL) and proteoglycan (PG). FT-IRS was used to obtain average values of composition across the entire anterior-posterior length of each bone. Relative percent composition values of COL and PG were calculated by multivariate least-squares analysis of model compound spectral features associated with COL (Amide I spectral absorbance) and PG (C-O-C sugar absorbance). It was shown that PG/Amide I values decreases from 4.9 +/−3.4 to 2.9 +/−3.2 over development. These values were translated to a relative percent compositional drop of PG from 49.9% +/−16.2% to 36.4% +/−8.1%. FT-IRIS was used to observe the spatial changes in composition from the subchondral region to the articulating surfaces. Collagen was observed to be distributed away from the articulating surfaces with increase in development. Proteoglycans were observed to have uniform concentrations with a marked decrease in PG across developmental stages. A noticeable benchmark in development is the ossific nucleus which was absent in the 2nd trimester. The findings of the current study demonstrate that cartilage anlage contains approximately triple the amount of proteoglycan in the 2nd trimester as compared to that previously reported in hyaline articular cartilage. The proteoglycan decreases over development, resulting in double the proteoglycan in the 3rd trimester as compared to previously reported adult values. No site-specific, macroscopic (FT-IRS), differences in PG content were found while microscopic assessment (FT-IRIS) observed heterogeneity with marked changes in PG content.

Carbon nanotubes (CNTs) are dispersed into polyacrylonitrile polymer solution and then assembled into continuous nanocomposite yarns through the drum-tape co-electrospinning process to facilitate the translation of CNT properties to higher order structures. We explore the dispersion of CNTs in a polymer matrix, the process of obtaining continuous yarn through electrospinning, and the surface morphology and mechanical properties of the nanocomposite yarn.

Mechanical characterization of human cartilage anlagen is required in order to effectively model congenital musculoskeletal deformities. Such modeling can effectively explore the effect of treatment procedures and potentially suggest enhanced treatment methods. We therefore determined the stress relaxation behavior of cartilage plugs obtained from third-trimester still-born fetuses in unconfined and confined compression geometries. The material parameters determined were the aggregate modulus HA = 0.15 ± 0.07 MPa, permeability coefficients k0 = 2.01 ± 0.8 × 10−14m4N−1s−1 and M = 4.6 ± 1.0, Young’s modulus Es = 0.06 ± 0.03 MPa, and Poisson’s ratio ν = 0.4 ± 0.06. As compared to adult articular cartilage, stiffness was an order of magnitude lower than the values reported in the literature, inferring the relative softness of the tissue; and the permeability was an order of magnitude higher indicating relative ease of flow in the tissue. Poisson’s ratio also was close to the higher end of the range found in previous studies. Such material is expected to deform and relax to larger extents.

The anisotropic elastic constants of crystalline octacyclopentyl polyhedral oligomeric silsesquioxane (CpPOSS) were determined using molecular dynamics. The force field used for these calculations was shown to model accurately the rhombohedral and triclinic crystal structures of octasilsesquioxane and CpPOSS, respectively, as well as the vibrational frequencies of octasilsesquioxane. The moduli for CpPOSS are anisotropic, with a Reuss-averaged bulk modulus of 7.5 GPa, an isotropic averaged Young's modulus of 11.78 GPa, and an isotropic averaged shear modulus of 4.75 GPa. These isotropic averages or, alternatively, the full anisotropic stiffness tensor of the crystal can be used with micromechanical composite models to calculate the effective elastic properties of polymer nanocomposites that contain crystalline aggregates of CpPOSS.

The influence of blending polyhedral oligomeric silsesquioxane substituted with cyclopentyl rings (CpPOSS) into a polyethylene (PE) matrix was probed using atomistic simulations. Composites of 5, 15, and 25 wt % CpPOSS were simulated. Interactions between POSS particles were found to promote organization of POSS within the polymer, where clear signs of aggregation were observed. Both particle and polymer dynamics were monitored. Particular attention was given to the structure and dynamics of the interface between particle and polymer. The interface was found to consist of a 3-5 angstrom thick shell in which the structure of the matrix was altered from that of the bulk. In this region, the local polymer backbone orientation is biased toward a configuration parallel to the particle surface. Dynamically, the interface consists of a thick shell about 11 angstrom thick in which polymer mobility is damped in the direction normal to and enhanced circumferentially to the surface of the CpPOSS particle. These results suggest that the CpPOSS particles exert an influence on the matrix material which mimics the effects of a rigid surface. A potential of mean force between CpPOSS particles in a polyethylene matrix at 500 K is derived from these atomistic simulation results.

The behavior of a glassy polyethylene-like polymer undergoing active compressive deformation was investigated via molecular dynamics simulation. Several important features can be identified within the stress–strain response of the system. Namely, the system deforms elastically, yields, softens, and then at large strains exhibits strain hardening. Simulations reveal that the actively deforming polymer exhibits several distinct characteristics at the molecular scale. Active deformation is found to significantly increase the transition rate between different dihedral angle states as well as promote the propagation of dihedral angle flips along the chain. When deformation is stopped, the transition rates decrease and propagation of these transitions along the chain is once again hindered. Below the glass transition temperature, transitions are heterogeneously distributed within the system. However, a local density-transition rate correlation study shows that this transitional heterogeneity is not attributable to heterogeneity in the local density. Instead, the high local transition rates must be caused by stresses propagated along the chain backbone as indicated by changes in neighbor correlations with stress. The yield stress is determined as a function of strain rate between strain rates of 108s−1 and 5×1010s−1. The activation volume within the context of the Eyring model is calculated to be 0.21 nm3 for this system.

Molecular dynamics simulation is used to reveal the origin of increased molecular mobility that accompanies plastic deformation of a glassy amorphous polymer under an applied stress. Significant increases in torsional transition rates are observed during active deformation prior to and just beyond the yield point. The transition rate drops when active deformation ceases. Increased transition rates are not contingent upon dilation. These simulations verify recent experimental observations of increased mobility during active deformation.

As the demand for polymers with superior properties increases, an understanding of the fundamental connections between the mechanical behavior and underlying chemical structure becomes imperative. In this work, the thermo-mechanical behavior and the molecular-level origins of plastic deformation of an amorphous glassy polymer were studied using atomistic simulations. Understanding of the molecular response will aid the development of physics-based continuum level models for these materials. A polyethylene-like molecular network was numerically constructed using a Monte Carlo algorithm and then subjected to uniaxial deformation over a wide range of strain rates and temperatures using Molecular Dynamics. The model exhibits many experimentally observed characteristics such as an initial elastic response followed by yield then volume preserving plastic deformation. The stress response was decomposed into intra and inter molecular components and analyzed throughout deformation. In the glassy regime, activation parameters were calculated in the context of the Eyring Model of flow in a solid. In addition, observations were made of the evolution of chain configuration and the correlation of transitions between dihedral angle states. Below the glass transition, dynamic heterogeneity is observed. It was also observed that mobility, as measured by the transitioning between dihedral angle states, increases during plastic deformation to levels observed at much higher temperatures under zero stress. At temperatures near the glass transition temperature, the mobility approaches levels of undeformed samples at the glass transition temperature. Thesis Supervisor: Mary C. Boyce Title: Professor Thesis Supervisor: Gregory C. Rutledge Title: Associate Professor

An oxidation‐resistant interphase for layered alumina composites was prepared by aerosol spray deposition of submicrometer alumina powder. A model composite specimen was made by placing the interphase between thin layers of monolithic alumina. The composite sandwich was hot‐pressed to control the interphase fracture resistance for successful crack deflection. Specimens were tested under four‐point bending in air at two crosshead speeds at ambient temperature, 1000°C, and 1200°C. The fracture behavior was temperature dependent, with a higher work of fracture at higher temperatures. Interphase delamination and composite toughening behavior were very pronounced at all temperatures. At the highest temperature, the transition to multiple widely distributed cracks and increased crack deflection may be related to inelastic deformation in the alumina.