Alain Lapp

Improvement of data treatment in small-angle neutron scattering

Small-angle neutron scattering has been successfully used since the 1970s. As a general rule, methods to extract the useful signal from that received by the detector are well known and give good results. At the Laboratoire Leon Brillouin, for example, these methods have been employed for a long time. However, the data reduction software has been changed for the following reasons. Problems are encountered if the container of the sample gives a spurious signal or if the scattering angle is so large that its cosine cannot be approximated by 1. In the present paper, generalizations of formulas are made in order to account for these difficulties. The decrease of scattered intensity delivered by an incoherent sample that is often observed at large angles is shown to be only due to a geometrical effect. The consequent modifications of the relations used for the normalization of cells of position-sensitive detectors and for the absolute calibration are given. As for the inherent background of the sample, the contribution of density fluctuations is usually neglected. This contribution is formally given as a function of the contrast lengths and of the isothermal compressibility of the sample. This new result allows numerical evaluations of the different terms of the inherent sample background. Practical examples are given. Finally, several methods, developed at the Laboratoire Leon Brillouin, are given to determine the background properly. They are based on systematic measurements of transmissions and background levels of suitably prepared blank samples.

Synthesis and characterization of high molecular weight polyrotaxanes: towards the control over a wide range of threaded α-cyclodextrins

This work focuses on the synthesis of polyrotaxanes with high molecular weight template poly(ethylene glycol) PEG (20 kg mol) having various and well-defined amounts of α-cyclodextrins (α-CD) per chain from 3 up to 125. is the complexation degree of the polyrotaxane defined to be the average number of cyclodextrin molecules per template chain. The usual route has been used for high values of , while sparsely complexed polyrotaxanes have been synthesized with an original one pot synthesis in water. Furthermore, a systematic study was carried out to understand and control the complexation degree of the polyrotaxane as a function of the complexation time, the temperature and the initial ratio of α-CD to template polymer. It has been shown that a high temperature thermal plateau leads to the formation of very sparsely complexed (low ) pseudo-polyrotaxanes for which, the threaded α-CD act like nuclei and generate a favourable driving force for the final complexation at lower temperature.

Hierarchical structures based on self-assembled diblock copolymers within honeycomb micro-structured porous films

This article details the preparation of hierarchically ordered microporous films using the so-called breath figure approach combined with the self-assembly of well-defined poly(n-butyl acrylate)-block-polystyrene or poly(tert-butyl acrylate)-block-polystyrene copolymers synthesized by nitroxide-mediated polymerization. The first level of organization was the hexagonal pattern of pores at the micrometre length scale leading to iridescence properties and surface hydrophobicity of the honeycomb structured films. Optical microscopy with the corresponding 2-Dimensional Fast Fourier Transform highlighted the ordering of the pores over a large scale (∼1 cm2). The second level of structuring was provided by the diblock copolymers chosen for their ability to self-assemble into ordered nanophases. The nanoscale morphology of both the honeycomb films and the corresponding thermally annealed continuous films was systematically investigated by atomic force microscopy (AFM) and small angle neutron scattering (SANS). The film characterization revealed a nanostructuration of the acrylate-based coil–coil diblock copolymer within the walls of the highly ordered microporous films obtained via a simple solvent evaporation method under humid atmosphere. The four synthesized diblock copolymers exhibited different macromolecular features with respect to the Flory–Huggins interaction parameter, the glass transition temperature of each block and the weight fraction of each monomer that influenced the quality of either the micropores structuration or the nanophase segregation.

Formation and Self-Organization Kinetics of α-CD/PEO-Based Pseudo-Polyrotaxanes in Water. A Specific Behavior at 30 °C†

alpha-Cyclodextrins (alpha-CDs) have the ability to form inclusion complexes with poly(ethylene oxide) (PEO) polymer chains. These pseudo-polyrotaxanes (PPRs) can be obtained by quenching an alpha-CD/PEO mixture in water from 70 degrees C down to a lower temperature (typically in the range from 5 to 30 degrees C) thanks to favorable interactions between alpha-CD cavities and PEO chains. Moreover, starting from a liquid alpha-CD/PEO mixture at a total mass fraction of 15% w/w at 70 degrees C, the formation of PPRs with time at a lower temperature induces a white physical gel with time, and phase separation is observed. We established that PPR molecules are exclusively found in the precipitated phase although unthreaded alpha-CD molecules and unthreaded PEO chains are in the liquid phase. At 30 degrees C, the physical gel formation is much slower than at 5 degrees C. At 30 degrees C, we established that, in a first step, alpha-CDs thread onto PEO chains, forming PPR molecules which are not in good solvent conditions in water. At a higher length scale, rapid aggregation of the PPR molecules occurs, and threaded alpha-CD-based nanocylinders form (cylinder length L = 5.7 nm and cylinder radius R = 4.7 nm). At a higher length scale, alpha-CD-based nanocylinders associate in a Gaussian way, engendering the formation of precipitated domains which are responsible for the high turbidity of the studied system. At the end of this first step (i.e., after 20 min), the system still remains liquid and the PPRs are totally formed. Then, in a second step (i.e., after 150 min), the system undergoes its reorganization characterized by a compacity increase of the precipitated domains and forms a physical gel. We found that PPRs are totally formed after 20 min at 30 degrees C and that the system stays in a nongel state up to 150 min. This opens new perspectives regarding the PPR chemical modification: between these two characteristic times, we can easily envisage an efficient chemical modification of the PPR molecules in water, as for instance an end-capping reaction leading to the synthesis of polyrotaxanes.

pH-triggered reversible sol–gel transition in aqueous solutions of amphiphilic gradient copolymers

We demonstrate the possibility of reversible pH-controlled sol–gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly(styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized vianitroxide-mediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nano-domains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.

Synthesis and pH- and salinity-controlled self-assembly of novel amphiphilic block-gradient copolymers of styrene and acrylic acid

A novel type of amphiphilic ionic copolymer comprising a hydrophilic poly(acrylic acid) (PAA) block and an amphiphilic poly(acrylic acid)-grad-poly(styrene) (PAA-grad-PS) copolymer block was synthesized using a one step direct nitroxide-mediated polymerization (NMP). A strong influence of the macroinitiator on the values of the reactivity ratios of the co-monomers is confirmed by 1H NMR. The aggregation behaviour of the copolymers in the aqueous medium was studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) and by transmission electron microscopy (TEM) in a wide range of pHs and ionic strengths. It has been demonstrated that PAA-b-(PAA-grad-PS) copolymers are soluble in alkaline water at room temperature without the special experimental procedures (addition of co-solvent, heating, etc.) that are usually required for solubilisation of classical PAA-b-PS diblock copolymers. The self-assembly of the PAA-b-(PAA-grad-PS) copolymers into nano-scale aggregates at low/moderate pH and/or high ionic strength was demonstrated by SANS and DLS experiments. The SANS spectra for the copolymer solution exhibit a correlation peak pointing to the formation of micelles with repulsive coronae. TEM images indicate that the micelles have an approximately spherical shape and exhibit a wide size distribution. Our results prove, that in contrast to “frozen” aggregates formed by PAA-b-PS copolymers in aqueous media, the micelles of PAA-b-(PAA-grad-PS) amphiphilic copolymers exhibit “dynamic” pH-responsive properties, i.e. they can reversibly change their aggregation number upon a variation in the pH or salinity of the solution.

Understanding the complex rheological behavior of PEO–PPO–PEO copolymers in aqueous solution

The viscoelastic properties of a 4-branched poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) copolymer in aqueous solutions have been studied in all parts of its phase diagram. In the unimer zone, the solution behaves like a Maxwellian fluid. The formation of micelles induces the presence of a secondary relaxation process on the dynamic mechanical response of the system. When the micelles condense into a crystalline body centered structure, the solution exhibits rheological properties similar to the behavior of an entangled polymer. The crystal’s terminal relaxation time τ is strongly dependent on the value of the stress used in the dynamic mechanical test. Stress-time relationships can also be observed with yield stress measurements. First of all, the structural origin of these phenomena has been explored. The relevance of several already proposed mechanisms has been analyzed and discrepancies between rheological results and SANS experiments discussed. We have been able to demonstrate that the evolution of the crystalline phase’s terminal relaxation time with stress can be described with an approach derived from Eyring’s theory. Moreover, this rheological relaxation time has been interpreted as being characteristic of a micelle’s diffusion in a crystalline structure. Finally, the values of the diffusion length and the intermicellar energy have been calculated and compared to already existing theories.The viscoelastic properties of a 4-branched poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) copolymer in aqueous solutions have been studied in all parts of its phase diagram. In the unimer zone, the solution behaves like a Maxwellian fluid. The formation of micelles induces the presence of a secondary relaxation process on the dynamic mechanical response of the system. When the micelles condense into a crystalline body centered structure, the solution exhibits rheological properties similar to the behavior of an entangled polymer. The crystal’s terminal relaxation time τ is strongly dependent on the value of the stress used in the dynamic mechanical test. Stress-time relationships can also be observed with yield stress measurements. First of all, the structural origin of these phenomena has been explored. The relevance of several already proposed mechanisms has been analyzed and discrepancies between rheological results and SANS experiments discussed. We have been able to demonstrate tha...

A direct measurement of the polyion conformation in aqueous solutions at different temperatures. Small angle neutron scattering of PSSNa using zero average and full contrast

The intrachain correlations in polystyrene sulfonate of sodium (PSSNa) have been measured as a function of the temperature, in semidilute solutions, using the zero average contrast technique. No temperature effect is observed. We also display and discuss data on interchain correlation, which are obtained from a scattering experiment at full contrast.

Multiblock copolymer behaviour of α-CD/PEO-based polyrotaxanes: towards nano-cylinder self-organization of α-CDs

The present work demonstrates that α-cyclodextrin/poly(ethylene oxide) (α-CD/PEO)-based polyrotaxanes (PRs) behave as multiblock copolymers. One block type consists of a rod-like tube made of 6 to 7 weakly stacked α-CDs threaded along the PEO chain. The other one is made of a naked PEO segment, i.e. the PR part that is not covered by α-CDs. This multiblock behaviour induces the self-organization of PRs in concentrated solution in dimethyl sulfoxide (DMSO) at room temperature leading to the formation of nano-cylinders. These nano-cylinders consist of assemblies of roughly 60 α-CD rod-like tubes through hydrogen bonding. Moreover, crystallites of naked PEO segments are formed since PEO is in poor solvent conditions (DMSO) at room temperature. Furthermore, the formation of α-CD nano-cylinders as well as of naked PEO segment crystallites leads to physical gelation of PRs in DMSO.

Structure of biodiesel based bicontinuous microemulsions for environmentally compatible decontamination: A small angle neutron scattering and freeze fracture electron microscopy study

Most toxic industrial chemicals and chemical warfare agents are hydrophobic and can only be solubilized in organic solvents. However, most reagents employed for the degradation of these toxic compounds can only be dissolved in water. Hence, microemulsions are auspicious media for the decontamination of a variety of chemical warfare agents and pesticides. They allow for the solubilization of both the lipophilic toxics and the hydrophilic reagent. Alkyl oligoglucosides and plant derived solvents like rapeseed methyl ester enable the formulation of environmentally compatible bicontinuous microemulsions. In the present article the phase behavior of such a microemulsion is studied and the bicontinuous phase is identified. Small angle neutron scattering (SANS) and freeze fracture electron microscopy (FFEM) measurements are used to characterize the structure of the bicontinuous phase and allow for an estimation of the total internal interface. Moreover, also the influence of the co-surfactant (1-pentanol) on the structural parameters of the bicontinuous phase is studied with SANS.