Rheological Investigations of Self-Assembled Block Copolymer Nanocomposites with Complex Architectures

The self-assembly of block copolymers (BCP) into microphase separated structures is an attractive route to template and assemble functional nanoparticles (NP) into highly ordered nanocomposites and is central to the “bottom up” fabrication of future materials with tunable electronic, optical, magnetic, and mechanical properties. The optimization of the co-assembly requires an understanding of the fundamentals of phase behavior, intermolecular interactions and dynamics of the polymeric structure. Rheology is a novel characterization tool to investigate these processes in such systems that are not accessible by other means. With the combination of X-ray scattering techniques, structure-property relationships are determined as a function of NP loading in self-assembled hybrid composites. This thesis examines two classes of BCP templates used for nanocomposite assembly. First, low molecular weight, disordered (low χN) BCP surfactants are considered. The addition of NPs with enthalpically favored interactions between the NP and one of the BCP domains boosts the phase segregation strength and drives self-assembly, resulting in highly filled nanocomposites (φNP ~ 30 vol.%) with small domain spacing (d0 ~ 10 nm) due to the low N. The effect of NPs on the self-assembly dynamics, material properties, and temperature dependent phase transitions are considered in the high loading regime. Oscillatory shear rheology reveals a transition from liquid-like to solid-like behavior with increasing NP content. The addition of stiff NPs to a soft polymer matrix, along with favorable intermolecular interactions, produces a x103 increase in the magnitude of G*. Phase transitions are investigated by correlating time-resolved rheology and time-resolved SAXS. Structure development and viscoelasticity scale with the NP content, and a general master curve describing behavior across all NP loadings is constructed. The access to new material properties and transitional phenomenon provides further insight into the complex structure-property relationships of this class of nanocomposites. The second BCP template is microphase separated bottlebrush block copolymers (BBCP), macromolecules with discrete blocks of densely grafted side chains tethered to a molecular backbone. Highly extended backbone conformations and significant repulsion between grafted side chains are believed to suppress chain entanglements, resulting in rapid self-assembly (order of minutes) into large nanostructures (d0 \u3e 100 nm) advantageous for optically active materials. A systematic study of model poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) BBCPs with short side chains below entanglement molecular weight is conducted. We measure dynamic moduli G’(ω) and G”(ω) over a wide range of timescales. The scaling relationships in dynamic data show distinct power law behavior analogous to critical gels. The relaxation mechanisms are a consequence of the reduced entanglements and mobile microstructure. This interplay of high molecular mobility and rapid self-assembly contrasts the viscoelasticity of linear BCP materials with comparable microstructure. The role that applied shear plays on the directed alignment of microphase separated lamellae in bulk BBCP samples is considered. The periodic structures are found to align at exceptionally low strain amplitudes and mild processing temperatures as confirmed by SAXS. Alignment over several mm3 is realized by high throughput synchrotron experiments and we hypothesize that this method can be applied as a means of fabricating and processing BCP-based hybrid materials with exceptional long-range order. Building on the understanding from the highly loaded NP/BCP composites, similar considerations are taken towards the investigation of phase behavior, morphology, and rheological response in NP/BBCP hybrids. The goal is to understand how NPs and intermolecular interactions impacts the unique relaxation processes inherent to BBCP melts. From oscillatory shear rheology measurements, systematic transitions in the long-time relaxations towards solid-like behavior is observed with increasing NP loading, suggesting the NP inhibits the highly mobile microstructure and rapid side chain relaxations. The structure-property relationships realized by both rheology and SAXS lay the groundwork as we explore future manipulation and processing of these diverse structures for both well-established and emergent applications

Microbial Consumption of Atmospheric Isoprene in a Temperate Forest Soil

Isoprene (2-methyl-1,3 butadiene) is a low-molecular-weight hydrocarbon emitted in large quantities to the atmosphere by vegetation and plays a large role in regulating atmospheric chemistry. Until now, the atmosphere has been considered the only significant sink for isoprene. However, in this study we performed both in situ and in vitro experiments with soil from a temperate forest near Ithaca, N.Y., that indicate that the soil provides a sink for atmospheric isoprene and that the consumption of isoprene is carried out by microorganisms. Consumption occurred rapidly in field chambers (672.60 +/- 30.12 to 2,718.36 +/- 86.40 pmol gdw day) (gdw is grams [dry weight] of soil; values are means +/- standard deviations). Subsequent laboratory experiments confirmed that isoprene loss was due to biological processes: consumption was stopped by autoclaving the soil; consumption rates increased with repeated exposure to isoprene; and consumption showed a temperature response consistent with biological activity (with an optimum temperature of 30 degrees C). Isoprene consumption was diminished under low oxygen conditions (120 +/- 7.44 versus 528.36 +/- 7.68 pmol gdw day under ambient O(2) concentrations) and showed a strong relationship with soil moisture. Isoprene-degrading microorganisms were isolated from the site, and abundance was calculated as 5.8 x 10 +/- 3.2 x 10 cells gdw. Our results indicate that soil may provide a significant biological sink for atmospheric isoprene

Consumption of Atmospheric Isoprene in Soil

Natural vegetation annually emits 503 Tg yr−1 of isoprene (2-methyl-1,3 butadiene) to the global atmosphere where it reacts very rapidly with hydroxyl radicals and strongly regulates atmospheric chemistry. Current models of the compound\u27s chemical behavior assume the atmosphere is the only significant sink; however, there is evidence that soil may consume isoprene. Here we show through field and laboratory studies that soil exposed to isoprene at low mixing ratios removed isoprene to concentrations below those commonly observed in forest canopies, and that the removal of isoprene was biologically mediated. On the basis of laboratory studies with soil from several different ecosystems worldwide, we provide a first approximation of a global annual soil sink for isoprene of 20.4 Tg yr−1, suggesting a soil sink should be included in models that attempt to describe the effect of isoprene emission on atmospheric chemical processes

Mountain Pine Beetle Outbreaks in the Rocky Mountains: Regulators of Primary Productivity?

We consider the hypothesis that mountain pine beetles function as cybernetic regulators of primary productivity in ecosystems of lodgepole pine forest through their selective killing of dominant trees and the subsequent redistribution of resources. Following a recent major beetle outbreak in Yellowstone and Grand Teton national parks, surviving trees did grow significantly faster (P \u3c .1); wood production was redistributed among canopy, subcanopy, and understory trees; and annual wood production per hectare usually returned to pre-attack levels or exceeded them within 10-15 yr. However, reconstructions of annual wood production over the last 70-80 yr indicate that the beetle outbreak did not reduce the variation in productivity; rather, the beetles introduced more variation than would have existed in their absence. Hence, our results do not support the hypothesis that the beetles function as cybernetic regulators (in the strict sense). Nevertheless, the beetle-pine system that we studied shows great resilience, and the effects of beetles on primary productivity do not appear to be as severe as conventional wisdom maintains. Annual wood production per hectare returned quickly to previous levels in the stands we studied, and associated ecological changes can be considered generally benign or even beneficial

Photoadaptable Hydrogels to Probe Intestinal Organoid Crypt Morphogenesis

The adult small intestine contains an active stem cell compartment confined to the intestinal crypt that functions to rapidly renew intestinal tissue over the course of days. Intestinal stem cell (ISC) behavior, and thus intestinal function, is regulated by the coordinated delivery of biochemical and biophysical signaling cues from the extracellular environment. Dysregulation of these environmental cues can lead to the onset of intestinal disease and cancer, and so it is important to understand how ISCs sense and respond to their environment. Intestinal organoids are in vitro tissue constructs that recapitulate some structural and functional features of the intestine, and therefore serve as models study how environmental factors, particularly biophysical signaling cues, regulate crypt function. However, the utility of organoid models is limited by a lack of appropriate adaptable materials to properly interrogate and control dynamic matrix properties in vitro. Specifically, photoadaptable hydrogels permit user defined, spatiotemporal control over matrix properties that define the cell culture environment. This thesis focuses on the development of photoadaptable poly(ethylene glycol) based synthetic macromers reacted through bio-click reactions to create responsive hydrogel environments to study the effects of dynamic cell-cell and cell-matrix interactions on intestinal organoid crypt morphogenesis, including crypt architecture, activation of mechanotransduction pathways, and distribution of crypt cell populations.&nbsp; First, we use a rapidly degradable hydrogel based on allyl sulfide chemistry to initially support organoid growth and then to collect intestinal stem cells for long term culture while maintaining differentiation potential. Allyl sulfide photodegradation is then used to partially soften hydrogels, generating defined changes in matrix mechanical properties. Here the extent of softening enables control over the resulting organoid architecture, including the length of organoid crypts and the number of crypts per organoid. Following investigations into the effect of matrix stiffness on crypt formation, we next utilize allyl sulfide mediated viscoelasticity to modulate epithelial curvature and cell-cell contacts. By generating dynamic changes in curvature, we are able to investigate the activation of ion channel dependent mechanotransduction programs that initiate early crypt budding and symmetry breaking. Finally, hydrogels based on nitrobenzyl ether photodegradation are used to generate softened hydrogel regions that direct the architecture of crypts, including crypt width, length, curvature, and spacing. Generation of engineered crypts in vitro that resemble their in vivo counterparts enables investigations into the effect of epithelial curvature on the localization of crypt cell populations. We find that the localization of Paneth cells, another crypt cell type, is dependent on the local epithelial curvature in the crypt. Collectively, this thesis highlights the use of photoadaptable and photoresponsive materials to precisely control the matrix environment and to study the matricellular influences on crypt morphogenesis and cellularity.</p

Legacy mercury and stoichiometry with C, N, and S in soil, pore water, and stream water across the upland‐wetland interface: The influence of hydrogeologic setting

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99104/1/2012JG002250R_Appendix_C_120728.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99104/2/jgrg20066.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99104/3/2012JG002250R_Appendix_B_100903.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99104/4/2012JG002250R_Appendix_A_100907.pd

Seasonal Changes in Methanogenesis and Methanogenic Community in Three Peatlands, New York State

Fluctuating environmental conditions can promote diversity and control dominance in community composition. In addition to seasonal temperature and moisture changes, seasonal supply of metabolic substrates selects populations temporally. Here we demonstrate cascading effects in the supply of metabolic substrates on methanogenesis and community composition of anaerobic methanogenic archaea in three contrasting peatlands in upstate New York. Fresh samples of peat soils, collected about every 3 months for 20 months and incubated at 22 ± 2°C regardless of the in situ temperature, exhibited potential rates of methane (CH4) production of 0.02–0.2 mmol L−1 day−1 [380–3800 nmol g−1 (dry) day−1). The addition of acetate stimulated rates of CH4 production in a fen peatland soil, whereas addition of hydrogen (H2), and simultaneous inhibition of H2-consuming acetogenic bacteria with rifampicin, stimulated CH4 production in two acidic bog soils, especially, in autumn and winter. The methanogenic community structure was characterized using T-RFLP analyses of SSU rRNA genes. The E2 group of methanogens (Methanoregulaceae) dominated in the two acidic bog peatlands with relatively greater abundance in winter. In the fen peatland, the E1 group (Methanoregulaceae) and members of the Methanosaetaceae were co-dominant, with E1 having a high relative abundance in spring. Change in relative abundance profiles among methanogenic groups in response to added metabolic substrates was as predicted. The acetate-amendment increased abundance of Methanosarcinaceae, and H2-amendment enhanced abundance of E2 group in all peat soils studied, respectively. Additionally, addition of acetate increased abundance of Methanosaetaceae only in the bog soils. Variation in the supply of metabolic substrates helps explain the moderate diversity of methanogens in peatlands

Seasonal changes in soil organic matter after a decade of nutrient addition in a lowland tropical forest

© 2015, US Government. Soil organic matter is an important pool of carbon and nutrients in tropical forests. The majority of this pool is assumed to be relatively stable and to turn over slowly over decades to centuries, although changes in nutrient status can influence soil organic matter on shorter timescales. We measured carbon, nitrogen, and phosphorus concentrations in soil organic matter and leaf litter over an annual cycle in a long-term nutrient addition experiment in lowland tropical rain forest in the Republic of Panama. Total soil carbon was not affected by a decade of factorial combinations of nitrogen, phosphorus, or potassium. Nitrogen addition increased leaf litter nitrogen concentration by 7 % but did not affect total soil nitrogen. Phosphorus addition doubled the leaf litter phosphorus concentration and increased soil organic phosphorus by 50 %. Surprisingly, concentrations of carbon, nitrogen, and phosphorus in soil organic matter declined markedly during the four-month dry season, and then recovered rapidly during the following wet season. Between the end of the wet season and the late dry season, total soil carbon declined by 16 %, total nitrogen by 9 %, and organic phosphorus by between 19 % in control plots and 25 % in phosphorus addition plots. The decline in carbon and nitrogen was too great to be explained by changes in litter fall, bulk density, or the soil microbial biomass. However, a major proportion of the dry-season decline in soil organic phosphorus was explained by a corresponding decline in the soil microbial biomass. These results have important implications for our understanding of the stability and turnover of organic matter in tropical forest soils, because they demonstrate that a considerable fraction of the soil organic matter is seasonally transient, despite the overall pool being relatively insensitive to long-term changes in nutrient status

Distinct responses of soil respiration to experimental litter manipulation in temperate woodland and tropical forest

Global change is affecting primary productivity in forests worldwide, and this, in turn, will alter long‐term carbon (C) sequestration in wooded ecosystems. On one hand, increased primary productivity, for example, in response to elevated atmospheric carbon dioxide (CO2), can result in greater inputs of organic matter to the soil, which could increase C sequestration belowground. On other hand, many of the interactions between plants and microorganisms that determine soil C dynamics are poorly characterized, and additional inputs of plant material, such as leaf litter, can result in the mineralization of soil organic matter, and the release of soil C as CO2 during so‐called “priming effects”. Until now, very few studies made direct comparison of changes in soil C dynamics in response to altered plant inputs in different wooded ecosystems. We addressed this with a cross‐continental study with litter removal and addition treatments in a temperate woodland (Wytham Woods) and lowland tropical forest (Gigante forest) to compare the consequences of increased litterfall on soil respiration in two distinct wooded ecosystems. Mean soil respiration was almost twice as high at Gigante (5.0 μmol CO2 m−2 s−1) than at Wytham (2.7 μmol CO2 m−2 s−1) but surprisingly, litter manipulation treatments had a greater and more immediate effect on soil respiration at Wytham. We measured a 30% increase in soil respiration in response to litter addition treatments at Wytham, compared to a 10% increase at Gigante. Importantly, despite higher soil respiration rates at Gigante, priming effects were stronger and more consistent at Wytham. Our results suggest that in situ priming effects in wooded ecosystems track seasonality in litterfall and soil respiration but the amount of soil C released by priming is not proportional to rates of soil respiration. Instead, priming effects may be promoted by larger inputs of organic matter combined with slower turnover rates