Topological versus rheological entanglement length in primitive path analysis protocols

Primitive path analysis algorithms are now routinely employed to analyze entanglements in computer simulations of polymeric systems, but different analysis protocols result in different estimates of the entanglement length, N_e. Here we argue that standard PPA measures the rheological entanglement length, typically employed by tube models and relevant to quantitative comparisons with experiment, while codes like Z or CReTA also determine the topological entanglement length. For loosely entangled systems, a simple analogy between between phantom networks and the mesh of entangled primitive paths suggests a factor of two between the two numbers. This result is in excellent agreement with reported values for poly-ethylene, poly-butadiene and bead-spring polymer melts.Comment: 3 pages, no figure

The Middle-earth Epic and the Seven Capital Vices

Examines Tolkien’s use of habitual evil choices of free beings leading to vices. Defines the seven capital sins more properly as vices, or habitual patterns, based on the work of Thomas Aquinas, and shows how Tolkien used them to give depth and motivation to characters in Middle-earth

Tolkien and Christian Concepts of Evil: Apocalypse and Privation

Argues that Tolkien’s conception of evil in Arda comes from two Christian sources: its personification (as in Revelation), and medieval concepts (primarily from Augustine through Aquinas) of evil as privation or corruption of initial good

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

A simple theory for glassy polymeric mechanical response which accounts for large scale chain relaxation is presented. It captures the crossover from perfect-plastic response to strong strain hardening as the degree of polymerization $N$ increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain activated relaxation arising from the need to maintain constant chain contour length reduces the $N$ dependence of the characteristic relaxation time by a factor $\sim \dot\epsilon N$ during active deformation at strain rate $\dot\epsilon$. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.Comment: The theoretical treatment of the mechanical response has been significantly revised, and the arguments for coherent relaxation during active deformation made more transparen

Thermal Fluctuations and Rubber Elasticity

The effects of thermal elastic fluctuations in rubber materials are examined. It is shown that, due to an interplay with the incompressibility constraint, these fluctuations qualitatively modify the large-deformation stress-strain relation, compared to that of classical rubber elasticity. To leading order, this mechanism provides a simple and generic explanation for the peak structure of Mooney-Rivlin stress-strain relation, and shows a good agreement with experiments. It also leads to the prediction of a phonon correlation function that depends on the external deformation.Comment: 4 RevTeX pages, 1 figure, submitted to PR

Strain Hardening in Polymer Glasses: Limitations of Network Models

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While traditional entropic network models can be fit to the total stress, their underlying assumptions are inconsistent with simulation results. There is a substantial energetic contribution to the stress that rises rapidly as segments between entanglements are pulled taut. The thermal component of stress is less sensitive to entanglements, mostly irreversible, and directly related to the rate of local plastic arrangements. Entangled and unentangled chains show the same strain hardening when plotted against the microscopic chain orientation rather than the macroscopic strain.Comment: 4 pages, 3 figure

Direct optical observations of surface thermal motions at sub-shot noise levels

We measure spectral properties of surface thermal fluctuations of liquids, solids, complex fluids and biological matter using light scattering methods. The random thermal fluctuations are delineated from random noise at sub-shot noise levels. The principle behind this extraction, which is quite general and is not limited to surface measurements, is explained. An optical lever is used to measure the spectrum of fluctuations in the inclinations of surfaces down to $\sim 10^{-17}\rm rad^2/Hz$ at $1\sim10 \mu$W optical intensity, corresponding to $\sim 10^{-29} \rm m^2/\rm Hz$ in the vertical displacement, in the frequency range $1{\rm}\rm kHz\sim10 MHz$. The dynamical evolution of the surface properties is also investigated. The measurement requires only a short amount of time and is essentially passive, so that it can be applied to a wide variety of surfaces.Comment: 5pp, 5 figure

Smectic-C tilt under shear in Smectic-A elastomers

Stenull and Lubensky [Phys. Rev. E {\bf 76}, 011706 (2007)] have argued that shear strain and tilt of the director relative to the layer normal are coupled in smectic elastomers and that the imposition of one necessarily leads to the development of the other. This means, in particular, that a Smectic-A elastomer subjected to a simple shear will develop Smectic-C-like tilt of the director. Recently, Kramer and Finkelmann [arXiv:0708.2024, Phys. Rev. E {\bf 78}, 021704 (2008)] performed shear experiments on Smectic-A elastomers using two different shear geometries. One of the experiments, which implements simple shear, produces clear evidence for the development of Smectic-C-like tilt. Here, we generalize a model for smectic elastomers introduced by Adams and Warner [Phys. Rev. E {\bf 71}, 021708 (2005)] and use it to study the magnitude of Smectic-C-like tilt under shear for the two geometries investigated by Kramer and Finkelmann. Using reasonable estimates of model parameters, we estimate the tilt angle for both geometries, and we compare our estimates to the experimental results. The other shear geometry is problematic since it introduces additional in-plane compressions in a sheet-like sample, thus inducing instabilities that we discuss.Comment: 8 pages, 5 figure

At the intersection of marginalised identities: Lesbian, gay, bisexual and transgender people’s experiences of injecting drug use and hepatitis C seroconversion

While the levels of injecting drug use among lesbian, gay, bisexual and transgender (LGBT) populations are high we know little about their experiences of injecting drugs or living with hepatitis C virus (HCV) infection. The loss of traditional family and cultural ties means connection to community is important to the wellbeing of LGBT populations. While some kinds of drug-use are normalised within many LGBT communities, injecting drug use continues to be stigmatised. This exploratory qualitative study of people with newly acquired HCV used semi-structured interviews to explore participants’ understandings and awareness of HCV, seroconversion, testing, diagnosis and treatment. We present a secondary thematic analysis of eight LGBT participants of the experience of injecting drugs, living with HCV and having a marginalised sexual or gender identity. Community was central to the participants’ accounts. Drug-use facilitated connection to a chosen community by suppressing sexual or gender desires allows them to fit in to the mainstream; enacting LGBT community norms of behaviour; and connection through shared drug-use. Participants also described feeling afraid to come out about their drug-use to LGBT peers because of the associated stigma of HCV. They described a similar stigma associated with HIV within the PWID community. Thus the combination of being LBGT/living with HIV (a “gay” disease) and injecting drugs/living with HCV (a “junkie’s” disease) left them in a kind of no man’s land. Health professionals working in drug and HCV care services need to develop capacity in providing culturally appropriate health care for LGBT people who inject drugs. Key words: Hepatitis C; HIV; Drug Use; Sexuality; Culturally-appropriate; health car