Atomistic simulations of elastic-plastic deformation of amorphous polymers

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.Includes bibliographical references (p. 121-125).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.by Franco Mario Capaldi.S.M

Effect of Decellularization Protocol on the Mechanical Behavior of Porcine Descending Aorta

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

CHARACTERIZATION OF COMPRESSIVE BEHAVIOR OF DEVELOPING HUMAN TALUS

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 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

Molecular Mechanisms of Light Harvesting in the Minor Antenna {CP}29 in Near-Native Membrane Lipidic Environment

CP29, a chlorophyll a/b-xanthophyll binding protein, bridges energy transfer between the major LHCII antenna complexes and photosystem II reaction centers. It hosts one of the two identified quenching sites, making it crucial for regulated photoprotection mechanisms. Until now, the photophysics of CP29 has been studied on the purified protein in detergent solutions since spectrally overlapping signals affect in vivo measurements. However, the protein in detergent assumes non-native conformations compared to its physiological state in the thylakoid membrane. Here, we report a detailed photophysical study on CP29 inserted in discoidal lipid bilayers, known as nanodiscs, which mimic the native membrane environment. Using picosecond time-resolved fluorescence and femtosecond transient absorption (TA), we observed shortening of the Chl fluorescence lifetime with a decrease of the carotenoid triplet formation yield for CP29 in nanodiscs as compared to the protein in detergent. Global analysis of TA data suggests a (1)Chl* quenching mechanism dependent on excitation energy transfer to a carotenoid dark state, likely the proposed S*, which is believed to be formed due to a carotenoid conformational change affecting the S-1 state. We suggest that the accessibility of the S* state in different local environments plays a key role in determining the quenching of Chl excited states. In vivo, non-photochemical quenching is activated by de-epoxidation of violaxanthin into zeaxanthin. CP29-zeaxanthin in nanodiscs further shortens the Chl lifetime, which underlines the critical role of zeaxanthin in modulating photoprotection activity.Published under an exclusive license by AIP Publishing

Conceptualizing the Human Health Outcomes of Acting in Natural Environments: An Ecological Perspective

Human-nature interactions have been presented as important for promoting and sustaining wellbeing and health benefits. Research has shown that pictures of nature, interacting with nature, physical activity in nature, immersion in nature and even feeling connected to nature can improve health. While considerable research supports this notion that nature can have positive health impact, theoretical and conceptual frameworks that help explain how the natural environment provides benefits to human health and wellbeing have proved limited. In extreme cases, theoretical approaches reinforce a problematic notion where nature is viewed as a separate entity, as a treatment to be taken as prescribed to remediate health problems that arise. Such approaches are limited as they fail to address how beneficial person-nature relations may be captured in interventions. There is a need for a deeper understanding of the processes underlying the observed benefits of the person-nature link in order to design effective research and interventions. It is especially important to consider the implications of research on person-nature relations for people living in urban contexts. In this paper, we present an ecological perspective building on James Gibson’s conceptualization of human behavior. Specifically, we outline a framework that emphasizes the person-environment system as the most appropriate scale of analysis. We present three relevant concepts from the ecological approach: form of life, affordances and niche construction, as helpful for appreciating how acting in natural environments might benefit human health and wellbeing. This approach urges policy makers and urban designers to rethink environmental designs to provide and support a landscape of affordances that makes use of the richness of natural environments

Biochemical and histological characterization of tomato mutants

Biochemical responses inherent to antioxidant systems as well morphological and anatomical properties of photomorphogenic, hormonal and developmental tomato mutants were investigated. Compared to the non-mutant Micro-Tom (MT), we observed that the malondialdehyde (MDA) content was enhanced in the diageotropica (dgt) and lutescent (l) mutants, whilst the highest levels of hydrogen peroxide (H2O2) were observed in high pigment 1 (hp1) and aurea (au) mutants. The analyses of antioxidant enzymes revealed that all mutants exhibited reduced catalase (CAT) activity when compared to MT. Guaiacol peroxidase (GPOX) was enhanced in both sitiens (sit) and notabilis (not) mutants, whereas in not mutant there was an increase in ascorbate peroxidase (APX). Based on PAGE analysis, the activities of glutathione reductase (GR) isoforms III, IV, V and VI were increased in l leaves, while the activity of superoxide dismutase (SOD) isoform III was reduced in leaves of sit, epi, Never ripe (Nr) and green flesh (gf) mutants. Microscopic analyses revealed that hp1 and au showed an increase in leaf intercellular spaces, whereas sit exhibited a decrease. The au and hp1 mutants also exhibited a decreased in the number of leaf trichomes. The characterization of these mutants is essential for their future use in plant development and ecophysiology studies, such as abiotic and biotic stresses on the oxidative metabolism

Can Simulated Green Exercise Improve Recovery From Acute Mental Stress?

This exploratory study enhances previous research into green exercise and addresses a gap in the research by exploring the contribution of individual and combined senses in the recovery of mood and stress after a psychological stressor, whilst rigorously controlling exercise intensity. The hypotheses were: (i) recovery of mood and stress from a state of psychological stress would be greater following simulated green exercise compared to rest, (ii) green exercise would facilitate better recovery than exercise alone and (iii) these effects would remain 10 minutes following intervention. iv) visual stimuli alone would enhance recovery from a state of psychological stress compared to sound. Fifty participants were randomly assigned to one of five groups: REST, exercise, exercise with nature sounds, exercise with nature visual and exercise with nature sound and visual. An initial visit to obtain predicted peak power output values and to familiarize participants with the equipment being used was followed by a second visit, where participants experienced one test condition. Baseline measures of heart rate, blood pressure, total mood disturbance and perceived stress were taken, before participants completed a stressor based on the Trier Social Stress test. Measures of heart rate and blood pressure were recorded in the last 30 seconds of the stressor to assess efficacy of the stressor. Immediately post stressor, measures of mood and perceived stress were taken followed by the intervention assigned (one of five described above). Measures of mood and perceived stress were taken again immediately post intervention and ten minutes post-intervention. Results showed that green exercise improved mood and stress scores more than exercise alone or REST. For both total mood disturbance and perceived stress, improvements in all simulated nature conditions were significantly improved compared to REST or exercise alone immediately post-intervention. There were no significant changes 10 minutes post intervention in either mood or perceived stress compared to immediately post-intervention values in any of the groups. This study suggests that environmental exercise settings including nature sounds, visual or both combined should be considered as important in the use of exercise as a therapeutic activity or recovery from acute psychological stress

Studying Adolescent Male Sexuality: Where Are We?

This article critically reviews the literature about adolescent males’ sexuality in order to describe the state of the science and to identify promising concepts and research designs that have the potential to guide the next generation of research. A critique was conducted on 94 peer-reviewed studies of sexual behaviors that included a sample of adolescent males; 11 scholarly texts and 2 dissertations. Most studies lacked a theoretical foundation and had cross-sectional designs. For those studies with a theoretical base, 3 perspectives were most often used to guide research: cognitive, biological, or social-environmental. Studies frequently relied on older adolescents or young adult males to report behaviors during early adolescence. Male-only samples were infrequent. Findings include (a) the measurement of sexual activity is frequently limited to coitus and does not explore other forms of “sex”; (b) cognitive factors have been limited to knowledge, attitudes, and intent; (c) little is known about younger males based on their own self-reports; (d) little is known about the normative sexuality development of gay adolescent males; and (e) longitudinal studies did not take into account the complexities of biological, social, and emotional development in interaction with other influences. Research on adolescent sexuality generally is about sexual activity, with little research that includes cognitive competency or young males’ sense of self as a sexual being. The purpose of the paper is to critically review the literature about male sexuality in order to describe the state of the science as well as to identify potential directions to guide the next generation of adolescent male sexual being research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45299/1/10964_2005_Article_5762.pd

Atomistic simulations of CpPOSS/PE nanocomposites

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005."June 2005."Includes bibliographical references.As the scientific community develops the ability to create composites which incorporate nanoscopic filler particles, the detailed atomic arrangement and atomic interactions become significant in determining the composite properties. Nanoscopic fillers such as carbon nanotubes, polyhedral oligomeric silsesquioxane (POSS), and layered silicates have already been successfully used to improve the thermal and mechanical properties of polymers. On this length scale, details describing particle organization, interaction between particles, and interactions between particle and matrix are needed to understand the behavior of the composite. In this thesis, we use atomistic simulations to investigate the detailed behavior of a blended octacyclopentyl polyhedral oligomeric silsesquioxane (CpPOSS) / polyethylene (PE) nanocomposite. The model potential employed to describe the atomic interactions in these systems is capable of reproducing the experimental vibrational and crystal structures for the POSS well. The mechanical properties of an infinite crystal were calculated. The), are anisotropic with a Reuss average isotropic elastic modulus of 11.78 GPa. Simulations of CpPOSS/PE composites revealed that the POSS had a stiffening effect on the polymer. Simulations revealed that both PE and POSS dynamics as measured by translational and rotational diffusivities decreased, the glass transition temperature increased, and both the density and modulus increased with increasing POSS content. Micromechanical models were fit to the composite modulus which allowed effective mechanical particle sizes to be determined.(cont.) The POSS was found to aggregate into small necklace structures which have internal ordering similar to the crystal phase. The formation of crystallites was found to be energetically favorable in this system. A coarse grained potential which accounts for both the attraction and orientation between particles was developed to aid the further study of aggregation and crystallization in these composites. The interface between the POSS and the polymer is found to consist of polymer chains aligned tangentially to the POSS cage. This layer has increased mobility tangential to the surface of the particle and decreased mobility in the radial direction. Though it is very thin, consisting of only 1 or 2 monolayers of polymer, due to the small size of the POSS particle, the weight fraction of polymer in the interfacial region is as high as 43 % in the 25 weight percent CpPOSS/PE composite. These simulations reveal the formation of structure on both the angstrom length scale in the polymer near the interface and the mesoscopic length scale between the POSS particles.by Franko M. Capaldi.Ph.D

Outcomes of Using an Infinitely Explorable Online Learning System

<p>A next generation online learning system has been developed at Merrimack College with the goal of revolutionizing online engineering education by “understanding a subject” rather than simply delivering content. The software delivers content in an interactive, three-dimensional environment. When presented with a problem, student input is not constrained in format or to only the final solution. Students have the ability to enter equations, diagrams, graphs, or text related to any portion of the problem including the solution, intermediate steps, or even extraneous aspects of the problem. These entries are evaluated by the software and feedback is provided regarding the correctness of the entry in the context of the presented problem. This grants the student freedom to explore the problem and receive instantaneous feedback as though they were interacting with the instructor in real-time. A number of studies have found that students provided with immediate feedback when working problems are more actively engaged in the learning process and demonstrate greater retention of the information3,4,5. This feedback can take the form of either standard correct/incorrect responses or an answer-until-correct approach. However, some evidence suggests that the answer-until-correct approach has a more significant impact on information retention6. Further, the use of immediate feedback in the setting of the online educational software allows the student to experience a learning environment more consistent with that of individual instruction while not requiring the direct input of the instructor.</p> <p>In this presentation, results of a pilot study on the effectiveness of this new online learning system will be presented. The specific aims of this study include:</p> <p>1) Test the effectiveness of using this software in enhancing learning in engineering courses.</p> <p>2) Evaluate the underlying mechanisms used in the software (active learning, immediate feedback, and an interactive three-dimensional environment).</p> <p>3) Study the usage scenarios when students are allowed to use the software with little instruction.</p