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

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

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

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

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

Clinical trialist perspectives on the ethics of adaptive clinical trials: a mixed-methods analysis

Abstract Background In an adaptive clinical trial (ACT), key trial characteristics may be altered during the course of the trial according to predefined rules in response to information that accumulates within the trial itself. In addition to having distinguishing scientific features, adaptive trials also may involve ethical considerations that differ from more traditional randomized trials. Better understanding of clinical trial experts’ views about the ethical aspects of adaptive designs could assist those planning ACTs. Our aim was to elucidate the opinions of clinical trial experts regarding their beliefs about ethical aspects of ACTs. Methods We used a convergent, mixed-methods design employing a 22-item ACTs beliefs survey with visual analog scales and open-ended questions and mini-focus groups. We developed a coding scheme to conduct thematic searches of textual data, depicted responses to visual analog scales on box-plot diagrams, and integrated findings thematically. Fifty-three clinical trial experts from four constituent groups participated: academic biostatisticians (n = 5); consultant biostatisticians (n = 6); academic clinicians (n = 22); and other stakeholders including patient advocacy, National Institutes of Health, and U.S. Food and Drug Administration representatives (n = 20). Results The respondents recognized potential ethical benefits of ACTs, including a higher probability of receiving an effective intervention for participants, optimizing resource utilization, and accelerating treatment discovery. Ethical challenges voiced include developing procedures so trial participants can make informed decisions about taking part in ACTs and plausible, though unlikely risks of research personnel altering enrollment patterns. Conclusions Clinical trial experts recognize ethical advantages but also pose potential ethical challenges of ACTs. The four constituencies differ in their weighing of ACT ethical considerations based on their professional vantage points. These data suggest further discussion about the ethics of ACTs is needed to facilitate ACT planning, design and conduct, and ultimately better allow planners to weigh ethical implications of competing trial designs.http://deepblue.lib.umich.edu/bitstream/2027.42/111302/1/12910_2015_Article_22.pd

Strong-Segregation Theory of Bicontinuous Phases in Block Copolymers

We compute phase diagrams for $A_nB_m$ starblock copolymers in the strong-segregation regime as a function of volume fraction $\phi$, including bicontinuous phases related to minimal surfaces (G, D, and P surfaces) as candidate structures. We present the details of a general method to compute free energies in the strong segregation limit, and demonstrate that the gyroid G phase is the most nearly stable among the bicontinuous phases considered. We explore some effects of conformational asymmetry on the topology of the phase diagram.Comment: 14 pages, latex, 21 figures, to appear in Macromolecule