Christophe Derail

Effect of the synthetic methodology on molecular architecture: from statistical to gradient copolymers

Styrene-butyl acrylate (S-BuA) copolymers were synthesized by nitroxide-mediated controlled radical polymerization using an alkoxyamine as an initiator. Using different synthetic methodologies, statistical copolymers were be obtained by batch nitroxide mediated polymerization while the gradient composition was a forced gradient by continuous addition of S during BuA polymerization (semi-batch process). These gradient copolymers have been studied by H NMR and size exclusion chromatography to characterize the gradient composition molecular structure. The evolution of the composition was correlated with the glass transition temperature () of the copolymers. The gradient copolymers exhibit one with a value in between the of polystyrene and poly(butyl acrylate), indicating that the materials did not present well defined microphase separation. Specific organization at the air-polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra. This bulk soft matter assembly was confirmed by AFM analysis, which showed a different morphology at the surface and in the bulk following removal of the top layer. Moreover, for the most well defined gradient composition, a specific nano-structuring was demonstrated by small angle neutron scattering. The preliminary rheological properties of these gradient copolymers were studied and are discussed in relation with their molecular structure.

pH Sensitive Hierarchically Self-Organized Bioinspired Films

In the present manuscript, we have demonstrated that hierarchically structured smart porous polymer films based on honeycomb-patterned surface can be elaborated from PS-b-P4VP pH-responsive block copolymer using the breath figure process. Despite the fast film formation by a bottom-up process, the copolymer nanostructuration was observed inside the walls of the honeycomb porous film. Atomic force microscopy (AFM), small angle X-ray and neutron scattering (SAXS and SANS) measurements were used to reveal both the hexagonal arrays formed by the pores at the micrometer length scale and the hexagonal copolymer self-assembly at the nanometer length scale. Contact angle (CA) measurements were used to point out the reversible pH-responsive wettability character of the surface. The PS-b-P4VP honeycomb film shows a contact angle variation of 20° between pH 9 and pH 3. An increase of the roughness was obtained with the pincushions hexagonal array enhancing the pH responsiveness of the polymer film with a switching CA gap of 75° when pH tuned from pH 9 to pH 3. This work presents the first report on honeycomb porous and pincushion films exhibiting a reversible pH-responsive character.

Hydrogel nanocomposites as pressure-sensitive adhesives for skin-contact applications

This study investigates the effects of monodisperse polystyrene nanoparticle fillers on the network formation, rheological properties and adhesion performance of hydrogel nanocomposites based on polyacrylamide and poly(acrylamide-hydroxyethyl methacrylate). We demonstrated a simultaneous increase in elasticity and tack of these humid composite materials. A 1H-NMR kinetic study showed quasi-total conversion of these monomers during the polymerization–reticulation process and the formation of inhomogeneities within the hydrogel network structure due to the difference in reactivity ratios of the comonomers: acrylamide (AM) and hydroxyethyl methacrylate (HEMA)(rAM = 0.41 ± 0.01 and rHEMA = 7.4 ± 0.3). The rheological properties of these materials were found to be affected by their chemical composition (HEMA content, presence of nanoparticles and heterogeneities). We investigated the adhesion properties of our materials using a probe test tack. Measurements were carried out on a human skin substitute to compare with metal and investigate the potential use of these hydrogel nanocomposites as dermatological patches. The adhesion energy was found to be related to the chemical composition and rheological properties of the hydrogels, as well as to the surface properties of both the adhesive and the substrate.

Rheological properties of hot melt pressure-sensitive adhesives based on styrene--isoprene copolymers. Part 1: A rheological model for [sis-si] formulations

The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, y . In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.

Rheology and Adherence of Pressure-Sensitive Adhesives

We have studied the relationship between rheological and peeling properties for hot-melt pressure-sensitive adhesives based on homopolymers or copolymers blended with tackifying resins. In this article, we particularly try to demonstrate that it is possible to define a quantitative link between rheology and adherence when the model formulations are deposited on substrates with strong (thermodynamic) adhesion. We describe the experimental results obtained on these model formulations and discuss the quantitative relationships obtained. In the case of “adhesion modulation” (derived from different treatments of the substrates), we show that the relationships become much more complicated, even with the same model adhesives. At the end, we discuss on the competition between adhesion and dissipation in the case of poor adhesion.

Synthetic methodology effect on the microstructure and thermal properties of poly(n-butyl acrylate-co-methyl methacrylate) synthesized by nitroxide mediated polymerization

In this work, we report the synthesis of poly(n-butyl acrylate-co-methyl methacrylate) copolymers by the nitroxide mediated polymerization (NMP) technique, using N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide (SG1) as a control agent and 2-methylaminoxypropionic-SG1 alkoxyamine (BlocBuilder®) as the initiator. The copolymers are synthesized either by batch or semi-batch processes and the gradient profile is examined via the determination of the instantaneous fraction of monomer incorporated in the copolymer. A control of the molar mass together with low molar mass distribution (Mw/Mn < 1.4) is observed. The dependence of the copolymer glass transition temperature with conversion was followed by differential scanning calorimetry. The copolymers are investigated by carbon nuclear magnetic resonance and heteronuclear multiple bond correlation (HMBC) NMR sequences to study the effect of the monomer addition mode on the microstructure of copolymers. The thermomechanical properties of gradient copolymers are finally reported to establish the effect of the composition on the mechanical behaviour of the copolymers.

Hierarchically structured hybrid honeycomb films via micro to nanosized building blocks

Metal oxide nanoparticles were chemically modified by surface initiated polymerization to core@shell building blocks (CSBBs). These CSBBs were then self-assembled using the bottom up breath figure (BF) method to design honeycomb (HC) porous hybrid films exhibiting hexagonal pore arrays. Depending on the nature, the size and the shape of the building blocks, honeycomb hybrid films with tunable pore sizes can be achieved.

Photonic properties of hybrid colloidal crystals fabricated by a rapid dip-coating process

The enhancement of the capillarity fabrication of well-ordered two-dimensional (2D) and three-dimensional (3D) opal photonic crystal is described herein. The quality enhancement and the reduction of the fabrication time are improved by using core@soft adhesive shell (Silica@PolyButylAcrylate) particles dispersed in an organic solvent with a high boiling point. The hybridization by an elastomeric corona polymer, grafted from the SiO(2) surface, has offered adhesive properties naturally tunable by changing the polymer state from a solvated to a dry one. Such properties involve drastic changes of the self-assembly behavior and qualities. Their use, as elementary building blocks, for colloidal crystal fabrication have required a high withdrawal rate (up to 4000 μm s(-1)), i.e. involving a three order of magnitude reduction in time compared to a classic vertical deposition method (1 to 10 μm s(-1)) and a good control/prediction of the coating thickness can be tuned by varying the withdrawal rate and the particle concentration. In addition, an analysis of the 2D synthetic iridescence of the hybrid photonic crystal was performed under white light, revealing the adhesive shell bridge influence on the dissipation energy of cracks linked to the crystal quality and the photonic properties.

RHEOLOGICAL PROPERTIES OF HOT-MELT PRESSURE-SENSITIVE ADHESIVE (HMPSAs) BASED ON STYRENE–ISOPRENE COPOLYMERS. PART 2: INNOVATIVE MOLECULAR DESIGN FROM PREDICTIVE FORMUlATION

This article is the second in a series that deals with the viscoelastic properties of Hot-melt pressure-sensitive adhesives (HMPSAs) based on formulations of block copolymers and tackifying resins. The viscoelastic properties of HMPSAs govern, to a large extent, their adhesion, processing, and end-use properties. In the first part of this article, we present a brief description of the rheological behavior of styrene isoprene styrene–styrene isoprene [SIS–SI] copolymer blends at room temperature. We then present an original approach that may lead to the design of new block copolymers (tetrablock and radial copolymers) that mimic the rheological behavior at room temperature of optimized SIS–SI blends used in adhesive formulations. We describe the concept and calculations that lead to the design of the characteristics of these new molecules. In the third part of this article, we discuss in detail the rheological behavior of these new block copolymers compared with the observed behavior of equivalent SIS–SI. In the last part we also demonstrate how the molecular model of the rheological behavior developed in the first article of this series can be applied to these new molecules. We propose, in particular, to apply the blending law (presented in the first article) on the complex shear modulus instead of the relaxation modulus, which simplifies calculations and even leads to a better agreement with experimental data. As a conclusion, we show how this original approach can bring really innovative solutions for the formulation of adhesives with specific properties by using molecular concepts of viscoelasticity.

Unexpected behaviour of multi-walled carbon nanotubes during “in situ” polymerization process: When carbon nanotubes act as initiators and control agents for radical polymerization

Raw multi-walled carbon nanotubes (MWCNTs) were used, without an initiation agent, to initiate and self-control polymerization of poly(acrylic acid) and poly(methyl methacrylate) copolymers. To enhance the livingness of the radical polymerization, a stable nitroxide can be added to strongly reduce the rate energy of the macromolecular chains growth and their dispersion. During the auto-initiation and self-control processes of the radical polymerization, the grafting of the copolymers onto the MWCNTs surface has been observed and characterized by transmission electronic microscopy (TEM). In particular, the grafting can be achieved thanks to an “in situ” radical polymerization that can simultaneously lead to the grafting of the MWCNTs and improve the dispersion. This work represents a real insight and breakthrough because in one step, MWCNTs acting as chemical motors are grafted and the growth of copolymer chains is well controlled.