16 research outputs found
Crystallinity and chain stem length dependence of yield kinetics in polyethylene
Researchers have long sought to predict the mechanical behavior of polyethylene from its microstructure. In particular, the yield strength and yield kinetics have been reported to be dependent on crystallinity and crystal thickness, but the relative importance of these two microstructural attributes has not been shown. In the present work, a series of microstructures was obtained through a combination of controlled quench rates from the melt and inclusion of various amounts of hexene comonomer. The yield strength for a wide range of strain-rates was linearly dependent on the crystallinity, and independent of crystal thickness (chain stem length), both measured by Raman spectroscopy. Similarly, yield kinetics described by a Ree-Eyring two-process stress activated model showed linear dependence on crystallinity and no dependence on crystal thickness. The results of the present work call into question models of yield kinetics dependent on screw dislocation nucleation, which depend on crystal thickness
Iron Catalyzed Synthesis and Chemical Recycling of Telechelic, 1,3-Enchained Oligocyclobutanes
Closed-loop recycling offers the opportunity to help mitigate plastic waste through reversible polymer construction and deconstruction. While examples of the chemical recycling polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers that are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n’-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock
Long-Chain Hyperbranched Comb Block Copolymers: Synthesis, Microstructure, Rheology, and Thermal Behavior
Microstructure of Crystallizable α‑Olefin Molecular Bottlebrushes: Isotactic and Atactic Poly(1-octadecene)
Isotactic
and atactic poly(1-octadecene) (iPOD and aPOD) have been
synthesized by organometallic coordinative insertion polymerization
of 1-octadecene. Analyzing X-ray and neutron scattering data of POD
melts identifies their bottlebrush structures as flexible rods where
the rod length is the extended backbone length and rod radius is the
side chain coil dimension. Upon cooling, both iPOD and aPOD melts
crystallize by fully extending their coiled side chains to form orthorhombic
alkane crystals in iPOD and nematically ordered rotator alkane crystals
in aPOD, as determined by X-ray scattering and Raman spectroscopy.
Molecular dynamics simulations of isotactic and atactic 48-mers of
1-octadecene were applied to define and verify melt and crystalline
structures and scattering peak assignments, respectively. Modeling
suggests that side chains of both crystallized isotactic and atactic
PODs align at 70° and 160° to the 4/1 spiral backbone of
equal probability, at an average of 115°, and POD chains pack
in an antiparallel pattern. Large wheat-sheaf structural assembly
of fibril bundles can be observed in aPOD, which render high opacity
to these samples. Each of those fibrils is made of several bottlebrush
molecules packed into a hexagonal lattice. Faster crystallization
observed in iPODs hinders the formation of large crystallites, which
results in translucent samples
Next Generation Development of Hybrid Continuous Flow Pediatric Total Artificial Heart Technology: Design-Build-Test.
To address the unmet clinical need for pediatric circulatory support, we are developing an operationally versatile, hybrid, continuous-flow, total artificial heart ( Dragon Heart ). This device integrates a magnetically levitated axial and centrifugal blood pump. Here, we utilized a validated axial flow pump, and we focused on the development of the centrifugal pump. A motor was integrated to drive the centrifugal pump, achieving 50% size reduction. The motor design was simulated by finite element analysis, and pump design improvement was attained by computational fluid dynamics. A prototype centrifugal pump was constructed from biocompatible 3D printed parts for the housing and machined metal parts for the drive system. Centrifugal prototype testing was conducted using water and then bovine blood. The fully combined device (i.e., axial pump nested inside of the centrifugal pump) was tested to ensure proper operation. We demonstrated the hydraulic performance of the two pumps operating in tandem, and we found that the centrifugal blood pump performance was not adversely impacted by the simultaneous operation of the axial blood pump. The current iteration of this design achieved a range of operation overlapping our target range. Future design iterations will further reduce size and incorporate complete and active magnetic levitation
Long-Chain Hyperbranched Comb Block Copolymers: Synthesis, Microstructure, Rheology, and Thermal Behavior
A series of poly(ethylene-<i>co</i>-acrylic acid)-<i>cb</i>-atactic polypropylene
(EAA-<i>cb</i>-aPP) comb block copolymers were synthesized
by grafting aPP-OH macromonomers onto a commercial EAA copolymer made
by the high-pressure free radical process. The starting EAA copolymer
contains 11 wt % of EAA units and has a significant amount of long
chain branches. Therefore, the EAA-<i>cb</i>-aPP copolymers
can be classified as hyperbranched. Room temperature atomic force
microscopy and X-ray scattering measurements reveal strong, finely
textured, phase segregation of the amorphous aPP and semicrystalline
EAA domains, which persists in the melt state. The amorphous aPP side
chains have an unexpected nucleating effect that facilitates crystallization
of the EAA backbone, as evidenced by an increase in crystallization temperature.
Moreover, phase segregation has a strong effect on both the linear
and nonlinear viscoelastic response of the copolymers. Increases in
both the branching density and branch chain length result in an improvement
of melt strength as well as an increase in the extensional strain
hardening (SH). We postulate that the SH enhancement may arise from
the interfacial anchoring of the aPP side chains in the aPP homopolymer
domains. This would produce additional resistance for the EAA backbone
to stretch under uniaxial load due to an energetically unfavorable
process of pulling the aPP arms into the EAA phase where they would
face strong repulsions
Revitalization of a Forward Genetic Screen Identifies Three New Regulators of Fungal Secondary Metabolism in the Genus Aspergillus
The study of aflatoxin in Aspergillus spp. has garnered the attention of many researchers due to aflatoxin's carcinogenic properties and frequency as a food and feed contaminant. Significant progress has been made by utilizing the model organism Aspergillus nidulans to characterize the regulation of sterigmatocystin (ST), the penultimate precursor of aflatoxin. A previous forward genetic screen identified 23 A. nidulans mutants involved in regulating ST production. Six mutants were characterized from this screen using classical mapping (five mutations in mcsA) and complementation with a cosmid library (one mutation in laeA). The remaining mutants were backcrossed and sequenced using Illumina and Ion Torrent sequencing platforms. All but one mutant contained one or more sequence variants in predicted open reading frames. Deletion of these genes resulted in identification of mutant alleles responsible for the loss of ST production in 12 of the 17 remaining mutants. Eight of these mutations were in genes already known to affect ST synthesis (laeA, mcsA, fluG, and stcA), while the remaining four mutations (in laeB, sntB, and hamI) were in previously uncharacterized genes not known to be involved in ST production. Deletion of laeB, sntB, and hamI in A. flavus results in loss of aflatoxin production, confirming that these regulators are conserved in the aflatoxigenic aspergilli. This report highlights the multifaceted regulatory mechanisms governing secondary metabolism in Aspergillus. Additionally, these data contribute to the increasing number of studies showing that forward genetic screens of fungi coupled with whole-genome resequencing is a robust and cost-effective technique. IMPORTANCE In a postgenomic world, reverse genetic approaches have displaced their forward genetic counterparts. The techniques used in forward genetics to identify loci of interest were typically very cumbersome and time-consuming, relying on Mendelian traits in model organisms. The current work was pursued not only to identify alleles involved in regulation of secondary metabolism but also to demonstrate a return to forward genetics to track phenotypes and to discover genetic pathways that could not be predicted through a reverse genetics approach. While identification of mutant alleles from whole-genome sequencing has been done before, here we illustrate the possibility of coupling this strategy with a genetic screen to identify multiple alleles of interest. Sequencing of classically derived mutants revealed several uncharacterized genes, which represent novel pathways to regulate and control the biosynthesis of sterigmatocystin and of aflatoxin, a societally and medically important mycotoxin.National Institute of General Medical Sciences [PO1-GM084077]; UW-Madison Food Research Institute; USDA Hatch Formula Fund [WIS01710]; Predoctoral Training Program in Genetics - National Institutes of Health [5 T32 GM007133-40]; National Institutes of Health [1R01GM112739-01]; NSF [DGE-1256259]; U.S. Forest Service, Northern Research StationThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]