655 research outputs found
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
How men view genetic testing for prostate cancer risk: findings from focus groups
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65567/1/j.1399-0004.2000.580303.x.pd
Self-assembling behavior in decane solution of potential wax crystal nucleators based on poly(co-olefins)
The control of the precipitation and gelation of long chain paraffins from oil remains an enduring technological challenge regarding the processing and recovery of refined fuels and waxy crudes. Wax crystal modifiers based on polyethylene -poly(ethylene-propylene) (PE-PEP) diblock copolymers function as efficient nucleators for wax crystals in middle distillate fuels. These diblock polymers self-assemble in oil to form expansive platelike aggregates consisting of a PE core cloaked behind the amorphous PEP brush layer. The PE core thus promotes nucleation of solubilized long chain alkanes. Additional candidate structures for wax crystal nucleators include linear and star copolyolefins where the composition variation signals the alteration between crystalline and amorphous segments. This study focuses upon the self-assembling behavior in solution of these materials. The characteristics of the single chains and the aggregates formed at lower temperatures were determined via small-angle neutron scattering. Both plates and needlelike structures were found. The placement of the amorphous and crystalline blocks in the arms of the star shaped polymers was found to influence the architecture of the self-assembled micelles. As a point of comparison a commercial copolymer of ethylene-vinyl acetate (EVA) was also investigated. The EVA material was found to be relatively "undisciplined" in comparison to the structurally more uniform anionically prepared counterparts
Interaction of paraffin wax gels with ethylene/vinyl acetate copolymers
The commercial grades of ethylene/vinyl acetate (EVA) co-polymers have found application as pour point" depressants in refined fuels. This study focuses on their behavior as additives to crude oils, where the intent is to reduce the yield stress of the gels that can form when the oil exits the reservoir. The model crude oils consisted of 4 wt % wax in decane. At EVA dosage levels of similar to200 ppm, the reduction in yield stress is 3 orders of magnitude for the C-36 wax, whereas the reduction is 1 order of magnitude for C-32 and only 3-fold for the C-28 wax. This decrease in efficiency with decreasing wax carbon number indicates that the EVA materials would not provide an adequate reduction in yield stress to ensure against gelation in pipeline transport. Neutron scattering studies, as a function of temperature, of the self-assembly of the EVA co-polymers show dramatically different aggregated structures in decane. The EVA with the lowest ethylene content shows scattering that increases with a power-law exponent of similar to1.6. This scattering behavior is typical for weakly aggregating polymer gels. In contrast, the EVA with the higher ethylene content shows a transition from surface scattering (found for strongly segregated objects) to a plateau whose height is dependent on temperature. Micrographs of the wax crystal morphology indicate that the ethylene-poor EVA alters the wax crystal habit at higher concentrations more effectively than does its higher-ethylene-content counterpart, whereas the latter EVA grade seems to form more wax crystals at low concentrations
Interaction of paraffin wax gels with random crystalline/amorphous hydrocarbon copolymers
The control mechanisms involved in the modification of wax crystal dimensions in crude oils and refined fuels are of joint scientific and practical interest. An understanding of these mechanisms allows strategies to be developed that lead to decreases in crude oil pour points or (for refined fuels) cold filter plugging points. The attainment of these goals involves the control and modification of wax crystals that spontaneously form in mixed hydrocarbon systems upon decreasing temperature. This work reports on the influence of random crystalline-amorphous block copolymers (ethylene-butene) upon the rheology of model oils. In a parallel fashion small-angle neutron scattering was exploited to gain microscopic insight as to how added poly(ethylene-butene) copolymers modify the wax crystal structures. The copolymers with different contents of polyethylene are highly selective with respect to wax crystal modification. Thus, the copolymer with the highest crystalline tendency is more efficient for the larger wax molecules while the less crystalline one is more efficient for the lower waxes
Adolescent contraception and abortion-related care: a three country comparison and Malawi case study
Partial and Incremental PCMH Practice Transformation: Implications for Quality and Costs
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102699/1/hesr12085.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102699/2/hesr12085-sup-0002-AuthorMatrix.pd
Measuring co-authorship and networking-adjusted scientific impact
Appraisal of the scientific impact of researchers, teams and institutions
with productivity and citation metrics has major repercussions. Funding and
promotion of individuals and survival of teams and institutions depend on
publications and citations. In this competitive environment, the number of
authors per paper is increasing and apparently some co-authors don't satisfy
authorship criteria. Listing of individual contributions is still sporadic and
also open to manipulation. Metrics are needed to measure the networking
intensity for a single scientist or group of scientists accounting for patterns
of co-authorship. Here, I define I1 for a single scientist as the number of
authors who appear in at least I1 papers of the specific scientist. For a group
of scientists or institution, In is defined as the number of authors who appear
in at least In papers that bear the affiliation of the group or institution. I1
depends on the number of papers authored Np. The power exponent R of the
relationship between I1 and Np categorizes scientists as solitary (R>2.5),
nuclear (R=2.25-2.5), networked (R=2-2.25), extensively networked (R=1.75-2) or
collaborators (R<1.75). R may be used to adjust for co-authorship networking
the citation impact of a scientist. In similarly provides a simple measure of
the effective networking size to adjust the citation impact of groups or
institutions. Empirical data are provided for single scientists and
institutions for the proposed metrics. Cautious adoption of adjustments for
co-authorship and networking in scientific appraisals may offer incentives for
more accountable co-authorship behaviour in published articles.Comment: 25 pages, 5 figure
ABC Triblock Copolymer Worms: Synthesis, Characterization, and Evaluation as Pickering Emulsifiers for Millimeter-Sized Droplets
Polymerization-induced self-assembly (PISA) is used to prepare linear poly(glycerol monomethacrylate)–poly(2-hydroxypropyl methacrylate)–poly(benzyl methacrylate) [PGMA–PHPMA–PBzMA] triblock copolymer nano-objects in the form of a concentrated aqueous dispersion via a three-step synthesis based on reversible addition–fragmentation chain transfer (RAFT) polymerization. First, GMA is polymerized via RAFT solution polymerization in ethanol, then HPMA is polymerized via RAFT aqueous solution polymerization, and finally BzMA is polymerized via “seeded” RAFT aqueous emulsion polymerization. For certain block compositions, highly anisotropic worm-like particles are obtained, which are characterized by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The design rules for accessing higher order morphologies (i.e., worms or vesicles) are briefly explored. Surprisingly, vesicular morphologies cannot be accessed by targeting longer PBzMA blocks—instead, only spherical nanoparticles are formed. SAXS is used to rationalize these counterintuitive observations, which are best explained by considering subtle changes in the relative enthalpic incompatibilities between the three blocks during the growth of the PBzMA block. Finally, the PGMA–PHPMA–PBzMA worms are evaluated as Pickering emulsifiers for the stabilization of oil-in-water emulsions. Millimeter-sized oil droplets can be obtained using low-shear homogenization (hand-shaking) in the presence of 20 vol % n-dodecane. In contrast, control experiments performed using PGMA–PHPMA diblock copolymer worms indicate that these more delicate nanostructures do not survive even these mild conditions
Study protocol: becoming and being a mother living with HIV - a multicentre longitudinal mixed methods study among pregnant women living with HIV, non-pregnant women living with HIV and pregnant women not living with HIV in a high-income setting (the 2B MOM study)
Introduction The success of combination antiretroviral therapy has decreased the risk of perinatal HIV transmission and normalised pregnancy in women living with HIV (WLWH). Despite these advances, WLWH still face complex medical and psychosocial issues during pregnancy and postpartum, and there is a gap of knowledge on the experiences of becoming and being a mother living with HIV in today’s context. The overall aim of this study is to investigate psychosocial outcomes and experiences of WLWH in Scandinavia during pregnancy and early motherhood.Methods and analysis This is a multicentre longitudinal convergent mixed methods study consisting of a quantitative survey study, a qualitative interview study and a mixed methods analysis. The survey study aims to examine psychosocial outcomes of WLWH across the pregnancy – postpartum trajectory. Participants are pregnant WLWH living in Scandinavia. Two control groups of HIV-negative pregnant women and non-pregnant WLWH are also included. Data is collected in the third trimester, 3 and 6 months postpartum using standardised questionnaires. Statistical analysis will assess changes over time and identify predictors of adverse outcomes. The interview study seeks to understand experiences of pregnancy and becoming a mother while living with HIV. Pregnant WLWH who are enrolled in the survey study will be asked to participate in individual interviews in the third trimester and 6 months postpartum. Data will be analysed using narrative analysis. The survey and interview results will be merged in a mixed methods analysis to assess confirmation, expansion or discordance between the data sets.Ethics and dissemination Approval from the Danish Data Protection Agency (VD-2018–253), and the Finnish and Swedish Ethics Committees have been obtained (HUS/1330/2019 and Dnr: 2019–04451, respectively). Study results will be disseminated to patient organisations, through publications in peer-reviewed journals and at scientific conferences
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