Abdel Khoukh

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.

Gradient or statistical copolymers by batch nitroxide mediated polymerization: effect of styrene/methyl acrylate feed

Monomer reactivity ratios in controlled radical copolymerization of styrene (S) and methyl acrylate (MA) monomers at 120 °C were determined. The Fineman–Ross method was used to calculate rs and rMA. Using this method, monomer reactivity ratio values of 0.89 and 0.22 were calculated for styrene and methyl acrylate, respectively. Because of the different reactivity ratios between the two monomers, and according to the molar fractions of two monomers, S/MA statistical or gradient copolymers were synthesized by nitroxide-mediated polymerization. Indeed, for different monomer ratios, the same statistical or gradient copolymers can be obtained by batch nitroxide mediated polymerization. These copolymers have been characterized by 1H nuclear magnetic resonance and size exclusion chromatography, and evolution of the composition has been correlated with the glass transition temperature measured by differential scanning calorimetry. Integrated intensities of the three observed peaks of (–OCH3) group in the 1H NMR spectra were used to determine the MA/MA/MA, MA/MA/S and S/MA/S triad sequences in the copolymers. 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.

Interactions between quaternized chitosan and surfactant studied by diffusion NMR and conductivity

Surfactant-polysaccharide complexes (SPEC) based on oppositely charged sodium 1-decanesulfonate and quaternized chitosan were studied using two techniques. The first one, the conductivity, is a very often used even when diffusion NMR (DOSY) technique was considered for the first time for such systems involving surfactant and chitosan derivatives and more generally polysaccharides. The physico-chemical characteristics of pure surfactant solutions as well as SPECs were determined and compared according to the considered experimental technique. Close results were obtained and the great advantage of DOSY technique is the capacity to study simultaneously the two components of the systems, allowing more information on the nature of interactions between the surfactant and the polysaccharide as well as the mechanism involved during these interactions. This may be of great interest to understand how these complexes can alter the properties of formulations in which these components are involved, which is one of the big challenges of the industrial research.

Physico‐chemical study of sucrose and calcium ions interactions in alkaline aqueous solutions

Interactions between sucrose and calcium hydoxide have been the subject of our work. They were studied in dilute solutions (sucrose concentration < 40% w/w, with a molar stoichiometric ratio Ca(OH) 2 / sucrose < 2) by several physico-chemical methods, titration of free and bound calcium ions, 13 C NMR spectroscopy, static light scattering and low shear viscosimetry. From these analyses we propose a polymer-like complex formation with calcium ions bound to at least two sucrose molecules. Moreover, this polymer-like aggregate can be described as a linear short chain at low Ca(OH) 2 and sucrose concentrations and as dense fractal particles at higher concentrations.

RAFT/MADIX emulsion copolymerization of vinyl acetate and N-vinylcaprolactam: towards waterborne physically crosslinked thermoresponsive particles

Well-defined poly(N-vinylcaprolactam-co-vinyl acetate) thermoresponsive particles physically crosslinked by means of hydrophobic interactions were synthesized by polymerization-induced self-assembly. It was highlighted that a xanthate-terminated poly(ethylene glycol) (PEG-X) efficiently acted as both a stabilizer and a macromolecular chain transfer agent for the RAFT/MADIX batch emulsion copolymerization of N-vinylcaprolactam (VCL) and vinyl acetate (VAc), enabling the direct synthesis in aqueous dispersed media of PEG-b-P(VAc-co-VCL) block copolymers. It was emphasized that a fraction of 47 mol% of hydrophobic VAc in the second block of the copolymer was suitable for maintaining the integrity of the self-assembled PEG-b-P(VAc-co-VCL) block copolymer particles at low temperature while exhibiting a temperature-induced phase transition. The well-defined physically crosslinked particles interestingly behaved as thermoresponsive colloids analogous to chemically crosslinked microgels. The PEG-b-P(VAc0.47-co-VCL0.53) particles were able to undergo a reversible swollen-to-collapse transition with increasing temperature in the absence of hysteresis. The PEG-b-P(VAc0.17-co-VCL0.83) block copolymer with a lower fraction of VAc in the copolymer (17 mol%) behaved oppositely as very small objects were present in the aqueous phase at low temperature (T < 20 °C) and self-assembled into large aggregates by increasing the temperature. Finally, the statistical copolymers based on VAc and VCL were successfully hydrolyzed into promising thermoresponsive biocompatible statistical copolymers based on vinyl alcohol and N-vinylcaprolactam co-monomer units.

Diffusion-ordered spectroscopy NMR DOSY: an all-in-one tool to simultaneously follow side reactions, livingness and molar masses of polymethylmethacrylate by nitroxide mediated polymerization

In this work, we report the use of the diffusion-ordered spectroscopy (NMR DOSY) nuclear magnetic resonance (NMR) technique to follow the nitroxide-mediated polymerization (NMP) of methyl methacrylate (MMA), well known as a difficult monomer to polymerize by NMP. Two syntheses were carried out either in the presence or absence of a small amount of styrene at 80 °C, 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. Diffusion coefficients determined from NMR DOSY experiments could be accurately correlated to the weight average molar masses Mw of PMMA, and could thereby be used to confirm that the polymerization of pure MMA by NMP is uncontrolled due to disproportionation side reactions leading to non-reversible termination (i.e. dead chains). In a similar manner, the livingness of the PMMA chains grown in the presence of a small amount of styrene was confirmed by the linear evolution of the diffusion coefficient versus time (conversion) and the retention of the alkoxyamine chain end. Livingness was further assessed by NMR DOSY analysis of chain extensions with n-butyl acrylate. This all-in-one technique permits disproportionation side reactions, livingness, and evolution of the molar masses with time to be simultaneously followed.

Smart self-assembled microgel films as encapsulating carriers for UV-absorbing molecules

This article reports the ability of oligo(ethylene glycol)-based stimuli-responsive microgels to spontaneously form self-assembled microgel films under different conditions such as the presence or absence of water soluble polymers (WSP, formed during the microgel synthesis) and the type and/or amount of salt in the microgel dispersion. The unusually high encapsulation of four different cosmetically active molecules has been achieved via hydrophobic interactions and hydrogen-bonding interactions between the –OH groups of active molecules and ether functions of self-assembled microgel films. The loaded-films present absorption higher than 50% of the UVA–UVB wavelengths and can be used for skin protection against sun. The nature of the interactions was determined by NOESY-NMR spectroscopy in the gel state. Moreover, confocal microscopy was used to show that the water solubility of the active molecule plays a crucial role in obtaining a more homogeneous distribution in films after loading.