Chemical analysis and aqueous solution properties of charged amphiphilic block copolymers PBA-b-PAA synthesized by MADIX®

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Abstract

We have linked the structural and dynamic properties in aqueous solution of amphiphilic charged diblock copolymers poly(butyl acrylate)-b-poly(acrylic acid), PBA-b-PAA, synthesized by controlled radical polymerization, with the physico-chemical characteristics of the samples. Despite product imperfections, the samples self-assemble in melt and aqueous solutions as predicted by monodisperse microphase separation theory. However, the PBA core are abnormally large; the swelling of PBA cores is not due to AA (the Flory parameter χPBA/PAA, determined at 0.25, means strong segregation), but to h-PBA homopolymers (content determined by liquid chromatography at the point of exclusion and adsorption transition, LC-PEAT). Beside the dominant population of micelles detected by scattering experiments, capillary electrophoresis CE analysis permitted detection of two other populations, one of h-PAA, and the other of free PBA-b-PAA chains, that have very short PBA blocks and never self-assemble. Despite the presence of these free unimers, the self-assembly in solution was found out of equilibrium: the aggregation state is history dependant and no unimer exchange between micelles occurs over months (time-evolution SANS). The high PBA/water interfacial tension, measured at 20 mN/m, prohibits unimer exchange between micelles. PBA-b-PAA solution systems are neither at thermal equilibrium nor completely frozen systems: internal fractionation of individual aggregates can occur.

Graphical abstract

The self-assembly of amphiphilic diblocks in water (large core size of micelles, low CMC but presence of free unimers, hysteretic fractionation) are explained via a precise physico/chemical analysis of samples.

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Introduction

Water-soluble surface active block copolymers present a great potential as alternatives or additives to common small-molecule surfactants. Typical applications of amphiphilic block copolymers enclose all small-molecule surfactant-based formulations for detergency, suspension stabilization, emulsion polymerization, wetting modification, etc. Advantages of using block copolymers rather than small-molecule surfactants are, for instance, a longer range of repulsion (steric and/or electrostatic) between surfaces covered by amphiphilic molecules or a better anchoring of the stabilizers on the surfaces. However, the potential of amphiphilic diblocks for applications has long been hindered by the difficulty to synthesize block copolymers in a simple and cheap enough way for industrial-scale production. Anionic polymerization is the best controlled way to produce block copolymers, with index of polydispersities as low as 1.01. Such samples have proven very precious for academic research but their synthesis conditions are too expensive for most commercial applications. Pluronics® polymers, produced industrially from an anionic process, are an exception to this rule. Still, side reactions with PPO lead to poorly controlled products and the advantage of using an anionic polymerization process is lost. The other major limitation of anionic synthesis process comes from the narrow range of chemical compounds that can be properly polymerized (PB, PS, PI, PtBA, PMMA, and PEO). Fortunately, in the last decade, controlled radical polymerizations [1] like RAFT [2] and ATRP [3], [4] have been developed as an alternative route to block copolymer synthesis, opening a new era for block copolymer science and industry. These processes allow relatively cheap and large-scale production of block copolymers out of a wide variety of chemistries. The cons are that (i) chains are more polydisperse and (ii) side-products are generated in larger quantities. The index of polydispersity is typically around 1.2–2, which is much better than the polydispersity of radical polymerization products (typically 3), but significantly higher than the one of anionic polymerization products (as low as 1.01 in best cases). Side products are mainly homopolymers of each block and, to a lesser extent, multiblocks. As a consequence, physico-chemical properties of these products are expected to be strongly dependent on the composition of the products and are expected to differ from the targeted properties of a pure block copolymer sample. This subject has been largely overlooked in the literature and is systematically discussed in this paper. We investigate here the physical properties in melt and in aqueous solution of diblock copolymers produced by controlled radical polymerization. Our aim is to understand the non ideal physical properties on the basis of a precise chemical characterization of the products. This approach appeared sane and necessary for controlled radical polymerization samples, which are known to be imperfect. However, the same approach could also be enlightening with anionic polymerization samples, whose imperfections are not always negligible [5], [6], [7], [8], [9], [10], [11]. We have investigated samples of poly(butyl acrylate)-b-poly(acrylic acid), or PBA-b-PAA, synthesized by the Rhodia patented MADIX® process [12]. These polymers were designed for their potential surface active properties in aqueous solution, which are most valued for applications. These products have already been successfully tested in formulations to increase stability of direct and inverse emulsions. Our final goal with these PBA-b-PAA samples is to achieve an academic understanding of their surface properties, e.g. to link their properties in solution and their properties at surfaces or in the presence of small-molecule surfactant. The present paper deals with the first step of this study, i.e. the physical properties of PBA-b-PAA samples in bulk aqueous solutions. The originality of the present study is that it links the synthesis mechanism, the chemical composition of the samples [determined by GPC, NMR, capillary electrophoresis (CE), and liquid chromatography at the point of exclusion adsorption transition (LC-PEAT)], and the physical properties of the solutions (characterized by DLS, SANS, and SAXS). We will focus on the structural parameters of self-assembled aggregates (in melt and in solution), the values of CMC's in solution, and the reversibility of the self-assembly in solution. In the end, this paper gives a new insight on the self-assembling properties of diblock copolymer in aqueous solution in relation to physical parameters, e.g. the interfacial tension between the PBA and water, and to chemical parameters, e.g. the presence of homopolymers.

Section snippets

Materials

Monomers butyl acrylate (Aldrich, 234923, 99%), acrylic acid (Aldrich, 147230, 99%) and deuterated butyl d9-acrylate (Polymer Source, D9nBUA), initiator 2,2′-azobis(2-methylbutanenitrile) (AMBN, DuPont Vazo 67), controlling agent (2-mercaptopropionic acid, methyl ester, o-ethyl dithiocarbonate) (Rhodixan A1, Rhodia, 99% NMR purity) and solvent ethanol (Aldrich, 459844) were used without further purification. Water was purified using a Milli-Q plus water purification system.

Process

All polymers were

Chemical analysis of components

We present here the measurements of the main analytical characteristics of our PBA-b-PAA systems (cf. Table 1). These data will later serve for the interpretation of bulk and aqueous solution properties.

Summary

We have presented a link between an in depth chemical and physical characterization of amphiphilic diblock copolymers samples PBA-b-PAA and their self-assembling properties in aqueous solutions. The samples were synthesized by controlled radical polymerisation MADIX®. Despite imperfections of the products, these PBA-b-PAA samples present the general characteristics expected for ideal diblock systems: they self-assemble in the melt state and in aqueous solution with different topologies

Acknowledgements

Thanks to: M. Destarac and G. Lizarraga for advises on the synthesis part, C. Bauer, H. Mauermann and D. Radtke for active participation in the analysis measurements and preparative GPC experiments, L. Porcar from NIST and F. Cousin from LLB for help on the scattering runs, A. Checco and P. Guenoun for technical help with AFM experiments, and also M. Airiau and A. Vacher for help with cryo-TEM experiments.

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