Identification of nonmonotonic behaviors and stick-slip transition in liquid crystal polymers

International audienceThe recent identification of shear-induced phases in the isotropic melts of liquid crystal polymers shows that these materials are expected to display original nonlinear behaviors. We have investigated the flow behavior of a nematic sidechain polymer above its isotropic-nematic transition temperature. Nonlinear rheology and bire-fringence measurements indicate the appearance, above a critical shear rate, of the shear-induced isotropic-nematic phase transition. The rheological behavior of this induced phase is characterized by undamped time-periodic shear stress oscillations. These sustained oscillations are interpreted in terms of a stick-slip mechanism alternating high-friction static state and low-friction kinetic state. PACS number͑s͒: 83.80.Xz, 47.20.Hw, 83.50.Ax, 64.70.Md Polymers are non-Newtonian fluids ͓1͔ whereas liquid crystals do not behave as simple fluids close to phase transitions ͓2͔. When these two complex fluids are coupled to form a melt of sidechain liquid crystal polymers ͑SCLCPs͒, the resulting rheological behavior is expected to be peculiar. The very first flow studies ͓3͔ have indeed indicated that SCLCP melts display strong nonlinear behaviors above the isotropic-nematic transition temperature (T NI). This behavior looks similar to the well-studied shear-induced behavior of giant micelle solutions which display a shear-induced IN transition above T NI ͓4,5͔. The SCLCP shear-induced transition was revealed by flow birefringence and via the existence of a stress plateau in the stress versus shear rate curve. The stress plateau can be explained by entering an unstable flow region; above a critical shear rate, the region is characterized by a decreasing stress with increasing shear rate. The system is then supposed to phase separate into homogeneous bands ͑shear banding͒ to maintain the imposed shear rate ͓5͔. The existence of such nonequilibrium states opens the question of identification of the coupling parameters associated with the critical shear rate. Clearly, the shear induced SCLCP critical times are not associated with the lifetime of the pretransi-tional fluctuations, suggesting a coupling with slower time scales which could be rather consistent with the existence of macroscopic heterogeneities as proposed by Collin et al. ͓6͔. The shear-induced phase conformation of the polymer main chain was also determined using small angle neutron scattering. For a LC polymer characterized in the equilibrium nem-atic phase by a perpendicular main chain/mesogen coupling ͑oblate conformation͒, we observed that the initially perpendicular coupling is inverted in the shear-induced nematic phase to a parallel coupling with the main-chain conforma-tion becoming prolate ͓3͔. This structural rearrangement can be proposed as a working hypothesis to explain the appearance of shear-induced transitions in SCLCP isotropic melts. The purpose of the present paper is to analyze the flow behavior produced above T NI by a SCLCP whose main-chain conformation is already prolate in the equilibrium nematic phase ͓7͔. The experimental techniques used are nonlinear rheology and flow birefringence. A nonequilibrium phase compatible with shear-banding is identified together with the observation of a second nonlinear behavior corresponding to an oscillating regime. The SCLCP chosen, PA 4-CN, is characterized as a prolate nematic polymer ͓7͔. The monomers have been synthesized at the Laboratoire Lé on Brillouin and polymerized by Poly-merExpert via controlled radical polymerization. The polymer described here corresponds to a molecular weight of M W ϭ85 800 and a polydispersity index of Iϭ1.1. This molecular weight corresponds to a nonentangled polymer and no rubbery plateau was found in viscoelastic measurements. This PA 4-CN presents the following succession of me-sophases: Tg-30 °C-N-116 °C-I and corresponds to the formul

Dynamic study of blood-brain barrier closure after its disruption using ultrasound: a quantitative analysis.

International audienceDelivery of therapeutic or diagnostic agents to the brain is majorly hindered by the blood-brain barrier (BBB). Recently, many studies have demonstrated local and transient disruption of the BBB using low power ultrasound sonication combined with intravascular microbubbles. However, BBB opening and closure mechanisms are poorly understood, especially the maximum gap that may be safely generated between endothelial cells and the duration of opening of the BBB. Here, we studied BBB opening and closure under magnetic resonance (MR) guidance in a rat model. First, MR contrast agents (CA) of different hydrodynamic diameters (1 to 65 nm) were employed to estimate the largest molecular size permissible across the cerebral tissues. Second, to estimate the duration of the BBB opening, the CA were injected at various times post-BBB disruption (12 minutes to 24 hours). A T(1) mapping strategy was developed to assess CA concentration at the ultrasound (US) focal point. Based on our experimental data and BBB closure modeling, a calibration curve was obtained to compute the half closure time as a function of CA hydrodynamic diameter. These findings and the model provide an invaluable basis for optimal design and delivery of nanoparticles to the brain

Early liver transplantation for severe alcohol-related hepatitis not responding to medical treatment: a prospective controlled study

peer reviewedBackground: Early liver transplantation for severe alcohol-related hepatitis is an emerging treatment option. We aimed to assess the risk of alcohol relapse 2 years after early liver transplantation for alcohol-related hepatitis compared with liver transplantation for alcohol-related cirrhosis after at least 6 months of abstinence. Methods: We conducted a multicentre, non-randomised, non-inferiority, controlled study in 19 French and Belgian hospitals. All participants were aged 18 years or older. There were three groups of patients recruited prospectively: patients with severe alcohol-related hepatitis who did not respond to medical treatment and were eligible for early liver transplantation according to a new selection scoring system based on social and addiction items that can be quantified in points (early transplantation group); patients with alcohol-related cirrhosis listed for liver transplantation after at least 6 months of abstinence (standard transplantation group); patients with severe alcohol-related hepatitis not responding to medical treatment not eligible for early liver transplantation according to the selection score (not eligible for early transplantation group), this group did not enter any further liver transplantation processes. We also defined a historical control group of patients with severe alcohol-related hepatitis unresponsive to medical therapy and non-transplanted. The primary outcome was the non-inferiority of 2-year rate of alcohol relapse after transplantation in the early transplantation group compared with the standard transplantation group using the alcohol timeline follow back (TLFB) method and a prespecified non-inferiority margin of 10%. Secondary outcomes were the pattern of alcohol relapse, 2-year survival rate post-transplant in the early transplantation group compared with the standard transplantation group, and 2-year overall survival in the early transplantation group compared with patients in the not eligible for early transplantation group and historical controls. This trial is registered with ClinicalTrials.gov, NCT01756794. Findings: Between Dec 5, 2012, and June 30, 2016, we included 149 patients with severe alcohol-related hepatitis: 102 in the early transplantation group and 47 in the not eligible for early transplantation group. 129 patients were included in the standard transplantation group. 68 patients in the early transplantation group and 93 patients in the standard transplantation group received a liver transplant. 23 (34%) patients relapsed in the early transplantation group, and 23 (25%) patients relapsed in the standard transplantation group; therefore, the non-inferiority of early transplantation versus standard transplantation was not demonstrated (absolute difference 9·1% [95% CI –∞ to 21·1]; p=0·45). The 2-year rate of high alcohol intake was greater in the early transplantation group than the standard transplantation group (absolute difference 16·7% [95% CI 5·8–27·6]) The time spent drinking alcohol was not different between the two groups (standardised difference 0·24 [95% CI −0·07 to 0·55]), but the time spent drinking a large quantity of alcohol was higher in the early transplantation group than the standard transplantation group (standardised difference 0·50 [95% CI 0·17–0·82]). 2-year post-transplant survival was similar between the early transplantation group and the standard transplantation group (hazard ratio [HR] 0·87 [95% CI 0·33–2·26]); 2-year overall survival was higher in the early transplantation group than the not eligible for early transplantation group and historical controls (HR 0·27 [95% CI 0·16–0·47] and 0·21 [0·13–0·32]). Interpretation: We cannot conclude non-inferiority in terms of rate of alcohol relapse post-transplant between early liver transplantation and standard transplantation. High alcohol intake is more frequent after early liver transplantation. This prospective controlled study confirms the important survival benefit related to early liver transplantation for severe alcohol-related hepatitis; and this study provides objective data on survival and alcohol relapse to tailor the management of patients with severe alcohol-related hepatitis. Funding: The present study has been granted by the French Ministry of Health—Programme Hospitalier de Recherche Clinique 2010

Identification et étude de la transition isotrope-nématique induite sous cisaillement dans les polymères cristaux liquides en peigne

Dans ce travail de thèse, une transition isotrope-nématique induite par cisaillement dans les fondus de polymères cristaux liquides en peigne a été pour la première fois mise en évidence. Cette transition de phase hors équilibre est caractérisée principalement par l'apparition, au delà d'un taux de cisaillement critique, d'une phase fortement biréfringente en phase isotrope (T > T_NI) et par un plateau de contrainte dans les courbes d'écoulement de contrainte en fonction du taux de cisaillement. Ces résultats sont interprétés comme révélateurs d'une transition de phase hors équilibre analogue à une transition du premier ordre, donnant lieu à une région du diagramme de phase hors équilibre (contrainte en fonction de la température) où la phase isotrope et la phase nématique induite coexistent. Deux bandes de biréfringences différentes sont directement visualisées dans le plan de cisaillement (vitesse, gradient de vitesse). Enfin, le mécanisme envisagé donnant lieu à cette transition isotrope-nématique hors équilibre est la déformation de chaînes connectées dans des amas élastiques. Des expériences de diffusion des neutrons aux petits angles, réalisées in situ, dans une cellule de Couette spécialement conçue, permettent l'étude de la conformation des chaînes en phase nématique induite. On a ainsi pu démontrer que la conformation des chaînes subit une réorientation de perpendiculaire au directeur de la phase nématique à l'équilibre à parallèle dans la phase nématique induite. Enfin, dans le cas d'un polymère caractérise par une conformation de chaîne de type parallèle au directeur, en phase nématique induite ainsi qu'à l'équilibre, nous avons relevé l'apparition d'oscillations temporelles auto-entretenues de la contrainte en réponse à un taux de cisaillement imposé en phase nématique induite. Ce phénomène oscillant est interprété en termes de déformation élastique suivie d'une relaxation du milieu.In this work, a shear induced isotropic to nematic phase transition in melts of side chain liquid crystal polymers is pointed out for the first time. This non-equilibrium transition has been revealed through rheo-optical measurements: a high birefringence emerges from the isotropic phase when sheared above a critical shear rate. Furthermore, a stress plateau in the flow curves of stress versus shear rate is observed and interpreted as relevant of a non-equilibrium transition achieved through shear banding. Two bands of different birefringence have been observed in the shear plane (velocity, velocity gradient). The mechanism considered for the formation of the shear induced nematic phase is the deformation of connected chains forming elastic clusters. Small angle neutron scattering experiments, performed in situ, in an especially designed Couette shear cell, allow the determination of the main-chain conformation in the shear induced nematic phase. The oblate conformation observed in the equilibrium nematic phase (at T < T_NI) is changed into a prolate one characterizing the shear induced nematic phase. Lastly, in the case of a side chain liquid crystal polymer characterized by a prolate main chain conformation, in the induced nematic phase as well as in the equilibrium one, we pointed out the appearance of undamped stress oscillations in response to an applied shear rate in the shear induced nematic phase. This oscillating phenomenon is interpreted in terms of an elastic deformation followed by a relaxation of the sample.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Chelate nano emulsion for MRI

The present invention relates to an oil-in-water nanoemulsion composition for MRI, including: an aqueous phase which is 70 to 90 wt % of the composition, advantageously 75 to 85 wt %, more advantageously 78 to 82 wt %; a lipid phase including an oil, which is 9.5 to 29.5 wt % of the composition, advantageously 14 to 25 wt %, more advantageously 17 to 21 wt %; a surfactant at the interface between the aqueous and lipid phases, the surfactant including at least one amphiphilic paramagnetic metal chelate and optionally an amphiphilic lipid; the total content by weight of the surfactant relative to the oil being 4 to 10 wt %, advantageously 5 to 8 wt %; the total content by weight of the surfactant relative to the composition being 0.35 to 2.95 wt %, advantageously 0.5 to 2 wt %; the oil including at least 70 wt %, advantageously at least 80 wt %, more advantageously at least 95 wt %, in particular at least 97 wt %, of C6-C18 saturated fatty acids, advantageously C6-C14, and more advantageously C6-C10. (FR)La présente concerne une composition de nanoémulsion huile dans eau pour IRM, comprenant : une phase aqueuse, représentant 70 à 90% en poids de la composition, avantageusement 75 à 85%, plus avantageusement de 78 à 82% une phase lipidique comprenant une huile, représentant 9.5 à 29.5% en poids de la composition, avantageusement 14 à 25%, plus avantageusement 17 à 21%, un tensioactif à l 'interface entre les phases aqueuse et lipidique, le tensioactif comprenant au moins un chélate de métal paramagnétique amphiphile et éventuellement un lipide amphiphile; - la teneur totale en tensioactif en poids par rapport à l'huile étant comprise entre 4 et 10%, avantageusement entre 5 et 8 %; - la teneur totale en tensioactif en poids par rapport à la composition étant comprise entre 0.35 et 2.95%, avantageusement entre 0.5 et 2 %; - l'huile comprenant au moins 70%, avantageusement au moins 80%, de façon avantageuse au moins 95% en poids, notamment au moins 97 %, d'acides gras saturés en C6-C18, avantageusement en C6-C14, plus avantageusement en C6-C10

Ultrasmall iron oxide nanoparticles for biomedical applications: Improving the colloidal and magnetic properties

A considerable increase in the saturation magnetization, M s (40%), and initial susceptibility of ultrasmall (<5 nm) iron oxide nanoparticles prepared by laser pyrolysis was obtained through an optimized acid treatment. Moreover, a significant enhancement in the colloidal properties, such as smaller aggregate sizes in aqueous media and increased surface charge densities, was found after this chemical protocol. The results are consistent with a reduction in nanoparticle surface disorder induced by a dissolution-recrystallization mechanism. © 2011 American Chemical Society.Peer Reviewe

Transmission Electron Microscopy Of Lipid Vesicles For Drug Delivery

Iron oxides nanocrystals are largely used for biomedical applications due to their high magnetization. Furthermore for in vivo applications these nanoparticles must be covered with a non-toxic material. Inside the numerous nano-systems for drug delivery, lipid structures, such as Solid Lipid Nanoparticles (SLNs), have been largely developed for various administration routes. In this work SLNs and iron-oxide nanocrystals covered with a lipid shell are characterized by Transmission Electron Microscopy. This technique has revealed to be essential to investigate the ultrafine compositional and morphological properties of these systems