Transient moisture conditions can accelerate the viscoelastic behavior of certain materials over that of constant moisture conditions. This is termed the mechano-sorptive phenomenon. The thrust of this research effort is to study the mechano-sorptive effects on the creep behavior of synthetic fibers and composite materials. This study consisted of two main parts: 1). a phenomenological investigation of the transient moisture effects in synthetic fibers and composite materials and 2). mechanistic studies of the observed phenomenon. The materials studied included Kevlar® fibers, Kevlar® fiber reinforced composites, Technora fibers, poly(methyl methacrylate) (PMMA) fibers, and Nylon 6,6 fibers. Unidirectional ( 0° ) Kevlar® 49/7714 epoxy coupons undergoing desorption exhibited an increase in tensile and bending creep deformations, a decrease in storage modulus, and an increase in the loss tangent (Tan δ) when compared to coupons maintained at a constant (saturated) moisture content. However, the transient moisture effects were not seen in composite coupons along the matrix direction. Experimental results showed that aramid fibers exhibited logarithmic creep behavior under tensile load. Even though different constant moisture conditions did not have appreciable effects on the creep behavior of aramid fibers, the creep process increased substantially under transient moisture conditions. The logarithmic creep rates and the mechano-sorptive effects increased with temperature. The creep activation energies of Kevlar® fibers are: 4.84 Kcal/mole for the cyclic moisture conditions and 1.04 Kcal/mole for the constant (saturated) moisture condition. Increases in stress may increase the logarithmic creep rates but may reduce the mechano-sorptive effect. In addition, the creep behavior under transient moisture conditions was nonlinearly dependent on stress. The fiber elastic compliance was observed to increase after creep deformation. Moreover, it was found that the fiber elastic compliance has correlation with the logarithmic creep rates. Aramid fibers contain hydrogen bonds between rod-like crystallites oriented at small angles relative to the fiber axis. These hydrogen bonds may be disrupted during a transient moisture process. The breakage of these hydrogen bonds may cause slippage of hydrogen bonded crystallites and result in accelerated crystallite rotations, thus causing increases in logarithmic creep rate. Analysis indicated that the obtained activation energy (4.84 Kcal/mole) and the reduction in fiber elastic compliance due to creep deformation support the proposed mechanisms. / Ph. D.

The main focus of this dissertation was on characterizing the viscoelastic behavior of a set of four flexible slabstock water-blown polyurethane foams with varying hard segment content as well as solid plaques made from these foams. Three viscoelastic tests; tensile stress relaxation, compression load relaxation, and compression creep, were utilized to evaluate the behavior of these materials at constant temperature and/or relative humidity, RH. The tensile stress relaxation tests were performed at a 25 percent strain level. The majority of the compression load relaxation tests were conducted at a 65 percent level since this is the strain level used for the common indentation load deflection test for flexible foams and the relaxation behavior was rather independent of strain at this level. Over a three hour testing period, a near linear relationship for the log of tensile stress or compressive load versus log time is observed for most conditions. The slope from this linear relationship in tension or the stress decay rate is similar for all the foams and their respective plaques; thus indicating that the tensile stress relaxation of these materials is governed by the solid portion of the foams and is therefore independent of the cellular textures. In addition, the rates of relaxation for rather linear behavior in tension and compression are also comparable for these foams and this implies that the relaxation in compression is mostly independent of the cellular texture of the foams at a 65 percent strain level. After a short induction period, the compressive creep behavior exhibits rather linear behavior for linear strain vs log time over a three hour period. The slope of this relationship is dependent on the initial strain level and goes through a maximum with initial strain at 40 percent. This maximum is believed to be due to the buckling of the foam’s struts. The results for the creep behavior were evaluated at a 65 percent initial strain since the creep behavior is believed to be mostly independent of the cellular texture of the foam at this level and greater. A greater amount of viscoelastic decay, i.e. tensile stress relaxation, compression load relaxation and com- pression creep is observed for the higher hard segment foams. Temperature has a similar effect on the results obtained from the three viscoelastic tests. Likewise, relative humidity at a constant temperature also has a similar effect on the viscoelastic behavior of the three tests. Up to 100°C, temperature accelerates the viscoelastic decay of these foams over a three hour time period. For all three viscoelastic tests, a significant increase in the viscoelastic decay at temperatures greater than 100°C is observed. The FTIR thermal analysis of the plaques indicated that this significant increase is due to additional hydrogen bond disruption as well as possible degradation in the urea and urethane links. Increasing relative humidity at a given temperature does bring about a steady decrease in the initial load or initial stress as well as a small increase in the rate of viscoelastic decay. Overall, the effects of temperature are greater on the viscoelastic decay than humidity. The morphology and the viscoelastic behavior of another set of flexible slabstock foams were characterized. These additional foams are rather unique in that some of their morphological features, the urea aggregate structure in particularly, are altered by adding a small amount of LiCl to the formulation. As discussed within the body of this dissertation, these observed changes in morphology are believed to have a significant effect on the viscoelastic nature. / Ph. D.

In this work, a finite element formulation and associated computer program is developed for the transient large deformation analysis of laminated composite plate/shell structures. In order to satisfy the plate/shell surface traction boundary conditions and to have accurate stress description while maintaining the low cost of the analysis, a newly assumed displacement field theory is formulated by adding higher-order terms to the transverse displacement component of the first-order shear deformation theory. The laminated shell theory is formulated using the Updated Lagrangian description of a general continuum-based theory with assumptions on thickness deformation. The transverse deflection is approximated through the thickness by a quartic polynomial of the thickness coordinate. As a result both the plate/shell surface tractions (including nonzero tangential tractions and nonzero normal pressure) and the interlaminar shear stress continuity conditions at interfaces are satisfied simultaneously. Furthermore, the rotational degree of freedoms become layer dependent quantities and the laminate possesses a transverse deformation capability (i.e. the normal strain is no longer zero). Analytical integration through the thickness direction is performed for both the linear analysis and the nonlinear analysis. Resultants of the stress integrations are expressed in terms of the laminate stacking sequence. Consequently, the laminate characteristics in the normal direction can be evaluated precisely and the cost of the overall analysis is reduced. The standard Newmark method and the modified Newton Raphson method are used for the solution of the nonlinear dynamic equilibrium equations. Finally, a variety of numerical examples are presented to demonstrate the validity and efficiency of the finite element program developed herein. / Ph. D.

Content: Preface Selected publications Statistics (Publications and patents, Concluded scientific degrees; Appointments and honors; Invited conference contributions, colloquia, lectures and talks; Conferences, workshops, colloquia and seminars; Exchange of researchers; Projects) Doctoral training programme Experimental equipment User facilities and services Organization chart and personnel

The first year of membership of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in the Helmholtz Association of German Research Centers (HGF) was a year of many changes also for the Institute of Ion Beam Physics and Materials Research (IIM). The transition period, however, is not yet over, since the full integration of the Center into the HGF will only be completed in the next period of the so-called program-oriented funding (POF). This funding scheme addresses the six core research fields identified by the Helmholtz Association (Energy; Earth and Environment; Health; Key Technologies; Structure of Matter; Aeronautics, Space and Transport) to deal with the grand challenges faced by society, science and industry. Since the Institute has strong contributions to both core fields “Key Technologies” and “Structure of Matter”, intense discussions were held amongst the leading scientists of the Institute, across the Institutes of the HZDR, and finally with leading scientists of other Helmholtz centers, to determine the most appropriate classification of the Institute’s research. At the end we decided to establish ourselves in Structure of Matter, the core field in which most of the large-scale photon, neutron and ion facilities in Germany are located. As a consequence, the Ion Beam Center (IBC) of the Institute submitted an application to become a HGF recognized large-scale facility, providing more than 50% of its available beam time to external users. This application perfectly reflects the development of the IBC over more than a decade as a European Union funded infrastructure in the framework of the projects “Center for Application of Ion Beams in Materials Research (AIM)” (1998-2000, 2000-2003, 2006-2010) and subsequently as the coordinator of the integrated infrastructure initiative (I3) “Support of Public and Industrial Research using Ion Beam Technology (SPIRIT)” (2009-2013). Another part of the Institute’s activities is dedicated to exploit the infrared/THz free-electron laser at the 40 MeV superconducting electron accelerator ELBE for condensed matter research. This facility is also open to external users and funded by the European Union.

A parallel-plate capacitively-coupled plasma deposition system was designed and built for the growth of polyethylene glycol-like films. Deposition rate, bonding structure and dissolution and swelling behavior was characterized as a function of input RF power, reactor pressure and substrate temperature to provide information on the relationship between input plasma parameters and film properties. For the conditions studied in this thesis, deposition rates increased at increasing input powers and operating pressures and decreasing substrate temperatures. The PEG-like coatings resembled higher molecular weight solution-polymerized PEG films with a higher crosslinked structure. Manipulation of plasma deposition conditions allowed control of film crosslink density and resulted in tunable dissolution and swelling properties of the PEG-like polymer. At higher applied powers, lower operating pressures, and higher substrate temperatures, films had a higher crosslink density, thus leading to slower dissolution rates and smaller extents of swelling. Void space openings of swelled-state, PEG-like films were determined using electrophoretic drift and diffusion-controlled transport of fluorophore-tagged PAMAM dendrimers into the bulk of the coating. PAMAM dendrimers were used because of their well-defined sizes and negatively-charged succinamic acid surface groups as a means to probe pore sizes of the plasma films. It was estimated that the upper bound of pore size diameters in the plasma polymer was approximately equal to ~5.5-6.0 nm. Positron annihilation lifetime spectroscopy was used to determine average pore sizes and was estimated to equal ~0.60-0.65 nm.

This research investigates the application of the AISC Direct Analysis Method for stability bracing design of columns, beams, beam-columns and frames. Emphasis is placed on out-of-plane flange bracing design in metal building frame systems. Potential improvements and extensions to the 2005 AISC Appendix 6 stability bracing provisions are studied and evaluated. The structural attributes considered include various general conditions encountered in practical metal building design: unequal brace spacing, unequal brace stiffness, nonprismatic member geometry, variable axial load or bending moment along the member length, cross-section double or single symmetry, combined bending and axial load, combined torsional and lateral bracing from girts/purlins with or without diagonal braces from these components to the inside flanges, load height, cross-section distortion, and non-rigid end boundary conditions. The research addresses both the simplification to basic bracing design rules as well as direct computation for more complex cases. The primary goal is improved assessment of the demands on flange bracing systems in metal building frames.

Atherosclerosis is the number one cause of death in the United States and one of the most common treatments is the implantation of a stent. In order to eliminate the two most common complications - restenosis and thrombosis - a novel covered stent is investigated. A covered stent membrane should be able to undergo large stretch, prevent restenosis, and be relatively non-thrombogenic. Polyvinyl alcohol (PVA) cryogels are examined as a candidate material for covered stent membranes. Mechanical testing included uniaxial tensile testing, puncture testing, and the fabrication and expansion of PVA cryogel covered stents. Uniaxial testing showed PVA cryogels to have sufficient ultimate stretch which was similar to bare metal stents during deployment. Puncture testing revealed that PVA cryogels are not likely to puncture in vivo. No tears were seen in the PVA cryogel membrane after expansion of the covered stents. Finite element analysis was used to determine a PVA cryogel membrane's effect on artery wall stress. PVA cryogel covered stents reduced both artery wall stress and tissue prolapse when compared to equivalent uncovered stents. Migration assays were used to determine if PVA cryogels are able to block the smooth muscle cell migration seen during restenosis. PVA cryogels significantly reduced cellular migration in modified Boyden chambers - suggesting that they would be able to prevent restenosis in vivo. Thrombogenicity was tested in vitro with a gravity-fed flow loop using porcine blood and in vivo with a sheep model. PVA cryogels were found to be less thrombogenic than polyester controls with the flow loop system. The sheep study demonstrated the feasibility of implanting PVA cryogel covered stents and good early patency. After explantation, the PVA cryogel membranes were intact - providing in vivo evidence for the durability of PVA cryogel covered stents. Overall, this work provides evidence that covered stents made with PVA cryogels are a feasible device in terms of their mechanics, ability to prevent restenosis, and low thrombogenicity. This work represents a major advancement in the development of PVA cryogel covered stents and provides necessary safety and feasibility data for future clinical trials.

Thanks to their extraordinary properties, carbon nanotubes reveal a promising potential for applications on the nanometre scale. When filled with metals or ferromagnets, nano-wires and magnets with a protecting carbon shell are realised. Different synthesis routes are described, such as laser ablation and chemical vapour deposition. Probes for magnetic force microscopy based on ironfilled carbon nanotubes are presented, and demonstrate a high spatial resolution, with the carbon shells at the same time providing effective wear resistance. We show also the potential of carbon nanotubes for biomedical applications, in particular their suitability as magnetic nano-heaters, drug-carrier systems or sensors for diagnostic and therapeutic usage on the cellular level. / Außergewöhnliche Materialeigenschaften machen Kohlenstoffnanoröhren zu einem vielseitigen nanoskaligen Werkstoff. Füllt man sie zum Beispiel mit metallischen oder ferromagnetischen Materialien, so ergeben sich durch eine Kohlenstoffhülle geschützte „Nano- Kabel“ oder Nano-Magnete. Neben verschiedenen Syntheseverfahren wie der Laserablation und der Chemischen Gasphasenabscheidung werden grundlegende physikalische Eigenschaften sowie Anwendungen in der Messtechnik und in der Medizin vorgestellt. In der Magnetkraftmikroskopie versprechen magnetisch gefüllte Kohlenstoffnanoröhren eine hohe laterale Auflösung bei gleichzeitigem Schutz des magnetischen Messsensors durch die Außenhülle. Im Bereich der biomedizinischen Anwendungen stellen Kohlenstoffnanoröhren ein nanoskaliges Transportmedium dar, das zum Transfer von Funktionsmaterialien in einzelne Zellen, zum Beispiel für magnetische Sensorik oder für Medikamententransporte, angewendet werden kann.

The year 2013 was the third year of HZDR as a member of the Helmholtz Association (HGF), and we have made progress of integrating ourselves into this research environment of national Research centers. In particular, we were preparing for the evaluation in the framework of the so-called program oriented funding (POF), which will hopefully provide us with a stable funding for the next five years (2015 – 2019). In particular, last fall we have submitted a large proposal in collaboration with several other research centers. The actual evaluation will take place this spring. Most of our activities are assigned to the program “From Matter to Materials and Life” (within the research area “Matter”). A large fraction of this program is related to the operation of large-scale research infrastructures (or user facilities), one of which is our Ion Beam Center (IBC). The second large part of our research is labelled “in-house research”, reflecting the work driven through our researchers without external users, but still mostly utilizing our large-scale facilities such as the IBC, and, to a lesser extent, the free-electron laser. Our in-house research is performed in three so-called research themes, as depicted in the schematic below. What is missing there for simplicity is a small part of our activities in the program “Nuclear Waste Management and Safety” (within the research area “Energy”).