Entropic phase separation in polymer-microemulsion networks

We study theoretically a model system of a transient network of microemulsion droplets connected by telechelic polymers and explain recent experimental findings. Despite the absence of any specific interactions between either the droplets or polymer chains, we predict that as the number of polymers per drop is increased, the system undergoes a first-order phase separation into a dense, highly connected phase, in equilibrium with dilute droplets, decorated by polymer loops. The phase transition is purely entropic and is driven by the interplay between the translational entropy of the drops and the configurational entropy of the polymer connections between them. Because it is dominated by entropic effects, the phase behavior of the system is extremely robust and is independent of the detailed properties of either polymers or drops.

Nonhomogeneous textures and banded flow in a soft cubic phase under shear

We report evidence for two distinct strain-induced orientation transitions in a lyotropic bcc cubic crystal submitted to increasing shear rates. The crystal is built up from copolymer spherical micelles in a selective solvent. The distribution of orientations is characterized by x-ray diffraction: in the two oriented states, the dense <111> rows align along the flow, but they differ from each other in the orientations of the dense layers with respect to the shear plane. These orientation transitions have clear rheological signatures in the form of two stress plateaus each associated with the coexistence of two states of orientation. We compare this behavior with the well documented shear-induced orientation transition in wormlike micelles.

Quantitative analysis of lyotropic lamellar phases SANS patterns in powder oriented samples

We have developed a detailed numerical method based on the Caillé model to fit Small Angle Neutron Scattering profiles of powder-oriented lyotropic lamellar phases. We thus obtain quantitative values for the Caillé parameter and the smectic penetration length from which we can derive the smectic compression modulus and the membrane mean bending modulus. Our method, applied to a surfactant lamellar phase system decorated by amphiphilic copolymers, provides excellent fits for any intermembrane spacing or membrane concentration over the entire q-range of the SANS experiments. We compare our fits with those obtained from the model of Nallet et al. (J. Phys. II 3, 487 (1993)), which is reviewed. Good fits are obtained with both methods for samples exhibiting "hard" smectic order (sharp Bragg peak, moderate small angle scattering). Only our procedure, however, gives good fits in the case of "soft" smectic order (smooth Bragg peak, strong small angle scattering). A quantitative criterion to discriminate between these "soft" and "hard" samples is also proposed, based on a simple analogy with smectic-A liquid crystal in contact with an undulating solid surface. This allows us to anticipate the type of thermodynamic information that can be derived from the fits.

Metastable versus unstable transients at the onset of a shear-induced phase transition

We measure the transient stress response at the onset of shear of a wormlike micelles solution which shows an inhomogenous banded flow pattern above a characteristic rate (.)gamma(t.). Depending on the magnitude of the excess rate above (.)gamma(t.), we identify two distinct types of stress response that correspond, respectively, to the metastable and to the unstable regimes, for the formation of the banded flow. This feature further underlines the analogy between the spontaneous formation of a banded flow and a first-order phase transition.

Microscopic mechanisms of the brittleness of viscoelastic fluids

We show that a large class of viscoelastic fluids, i.e., transient networks, are brittle according to the Griffith's theory of solid fracture. However, contrary to solids, cracks are intrinsic to the material arising from the equilibrium nature of the fluid microstructure. The brittleness of these fluids comes from thermal fluctuations of bonds distribution. In this approach, the rupture stress is predicted to be on the order of the Young modulus, in very good agreement with experimental values.

Loose Complexation of Weakly Charged Microemulsion Droplets and a Polyelectrolyte

The effect of polyelectrolyte addition on the properties of an oil-in-water (O/W) microemulsion of weakly charged spherical micelles is studied. The 81 A radius O/W droplets in this system can be charged by the partial substitution of the nonionic surfactant by a cationic surfactant. The effect of the addition of poly(acrylic acid) (PAA), which is a charged pH-dependent polyelectrolyte, on the interactions between charged or noncharged droplets has been investigated using SANS. We have characterized the phase behavior of this pH-smart system as a function of the microemulsion and the polyelectrolyte concentration and the number of charges per droplet at three pH values: pH = 2, 4.5, and 12. In particular, an associative phase separation due to the bridging of the droplets by the neutral PAA chains through H-bonds is observed with extremely low PAA addition at low pH. At the opposite, an addition of PAA at pH = 4.5 generates a strong repulsive contribution between neutral droplets. Electrostatic bonds between charged droplets and PAA, controlled by the number of charges per droplet, are responsible for a pH drift and then for an associative phase separation similar to that observed at low pH. Finally, at high pH, the creation of electrostatic bonds between fully charged PAA and charged droplets liberates sufficiently counterions in solution at high droplet charge density to screen the electrostatic interactions and to allow an associative phase separation.

Structure and rheological properties of model microemulsion networks filled with nanoparticles

Model microemulsion networks of oil droplets stabilized by non-ionic surfactant and telechelic polymer C18 -PEO(10k)- C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering (SANS). A Reverse Monte Carlo (RMC) approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R = 10 nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%. q(A-1).

Real-world impact of laparoscopic surgery for rectal cancer: a population-based analysis

Background

Randomized trials have demonstrated equivalent oncologic outcomes and decreased morbidity in patients with rectal cancer who undergo laparoscopic surgery (lapsx) compared with open surgery (opensx). The objective of the present study was to compare short-term outcomes after lapsx and opensx in a real-world setting.

Methods

A national discharge abstract database was used to identify all patients who underwent rectal cancer resection in Canada (excluding Quebec) from April 2004 through March 2015. Short-term outcomes examined included same-admission mortality and length of stay (los).

Results

Of 28,455 patients, 82.4% underwent opensx, and 17.6%, lapsx. The use of lapsx increased to 34% in 2014 from 5.9% in 2004 (p < 0.0001). Same-admission mortality was lower among patients undergoing lapsx than among those undergoing opensx (1.08% and 1.95% respectively, p < 0.0001). On multivariable analysis, the odds of same-admission mortality with lapsx was 36% lower than that with opensx (odds ratio: 0.64; p = 0.003). Median los was shorter after lapsx than after opensx (5 days and 8 days respectively, p = 0.0001). The strong association of lapsx with shorter los was maintained on multivariable analysis controlling for patient, surgeon, and hospital factors.

Conclusions

For patients with rectal cancer, shorter los and decreased same-admission mortality are associated with the use of lapsx compared with opensx.

Microrelief changes in chronically sun-exposed human skin

The effects of solar exposure on the microtopography of the human skin can be followed, in vivo, by comparing 2 nearby areas, protected and non-protected, of the arm. This has been possible in the case of professional cyclists, whose short-sleeved outerwear provides a clear demarcation of the 2 zones. The microrelief was measured from negative replicas by an automatic image analysis method. The skin pattern showed clear changes between the 2 zones. Results are discussed in an attempt to outline similarities and differences between the effects of the actual aging process and solar radiation.