Two-dimensional liquid chromatography of polystyrene–polyethylene oxide block copolymers
Highlights
► LCCC of PS-b-PEO copolymers. ► Hyphenation of LCCC with SEC, LCCC and FTIR. ► Quantification of homopolymers content. ► Calculation of chemical composition of real block copolymers. ► Comparison of direct analysis of raw samples and analysis after LCCC-fractionation.
Introduction
Amphiphilic copolymers generally consist of covalently bonded hydrophilic and hydrophobic blocks. Due to the incompatibility of these blocks with each other and their different affinities for different solvents, they exhibit surface activity. Recently, these materials have attracted considerable attention due to their applications in several different fields: for the modification of surfaces to control protein adsorption, nano- and biomaterials, controlled release, nanofiltration membranes, surface modification of biomaterials, synthetic lubricants, emulsifiers, and for biomedical applications [1], [2], [3], [4].
In most cases polyoxyethylene is the hydrophilic block, while alkyl, aryl alkyl, poly(ether), poly(ester), poly(styrene), etc. are the hydrophobic blocks. The block copolymers consisting of ethylene oxide and styrene are amphiphilic in nature. The amphiphilic nature of PS-b-PEO has been exploited for many different industrial applications such as polymeric surfactants in the oil, pharmaceutical, agriculture, paper and detergent industries, as well as compatibilisers in polymer blending, dispersions, stabilisers, and templates for the preparation of inorganic nanoparticles [5]. Exploiting the self-assembly of PS-b-PEO and PS-b-PEO-b-PS to ordered nanostructures, novel applications such as the preparation of mesoporous silica films with different pore sizes [6] and electrolytes for rechargeable batteries have been developed [7]. Darling demonstrated in a review that the size and shape of self-assembled amphiphilic block copolymers are tuneable through the synthetic chemistry of the constituent molecules [8].
During the synthesis of PS-b-PEO copolymers, side reactions can take place leading to the formation of homopolymers with different functionalities of both types of monomers involved, along with the targeted block copolymers. The presence of homopolymers and the size of the different blocks have a significant effect on the final properties of these materials [8]. Therefore, characterisation of these block copolymers is vital to develop the structure–property relationships and to improve synthesis procedures. The application of spectroscopic methods such as NMR, MS, and FTIR to the bulk products only yields the average composition of the sample, which is not always conclusive with regard to information about all the components. These methods are, however, very often informative when used after fractionation by HPLC or other fractionation techniques.
Liquid chromatography is one of the most powerful methods for the characterisation of the molecular heterogeneity of complex polymers and liquid chromatography at critical conditions (LCCC) has been employed successfully for the fractionation of many different types of polymer blends, block copolymers, and polymers with different functionalities, etc. LCCC is one of the major modes of liquid chromatography employed as the first dimension in multidimensional liquid chromatography [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31].
In this study, liquid chromatography at the critical conditions of PS and PEO is used to separate the block copolymers from respective homopolymers. In one of our previous papers, the critical conditions of PS were established using a tetrahydrofuran (THF)–water mobile phase. However, due to the limited solubility of the polymers being investigated, this system only works for chains with a molar mass of up to 10 kg/mol of each block [32]. In this study we used a different mobile phase system in order to extend the molar mass range. This approach should provide information on the molar mass distribution of the non-critical block in the block copolymer and allows separation of critical homopolymers from the rest of the sample if there are any. An ELS detector is calibrated under the above conditions and the homopolymer content is quantified. Comprehensive two-dimensional liquid chromatography is used to obtain the molar mass distribution of the fractions separated at the critical conditions of both PS and PEO. As a next step, fractions are collected and subjected to further analysis at the critical conditions of the other block, as well as by FTIR for qualitative as well as quantitative analysis. The study is further extended by combining the results obtained at both critical conditions to quantify the homopolymer content as well as the chemical composition distribution of the real block copolymers. Recently, this approach has been demonstrated for PDMS-b-PS copolymers [33], [34] to obtain valuable information regarding the complex polymer system. In this study, we applied a similar approach to other commercially important PS-b-PEO copolymers.
Section snippets
HPLC of polymers
The separation mechanism in liquid chromatography of polymers depends on the size/pore size ratio and on the so-called interaction parameter c, which describes the interaction of the structural unit with the stationary phase [35]. This parameter is negative in size exclusion chromatography (SEC), in which retention decreases with an increase in molar mass, and is positive in liquid adsorption chromatography (LAC) where retention increases exponentially with the number of repeat units.
In both
Synthesis of PS-b-PEO diblock copolymers
PS-b-PEO diblock copolymers were produced by Polymer Standards Service GmbH (Mainz, Germany) by sequential living anionic polymerisation of styrene and ethylene oxide [46]. Briefly, styrene was polymerised at −78 °C in THF using cumylpotassium as initiator. After 1 h, an aliquot was taken to recover the polystyrene precursor for monitoring purposes. Ethylene oxide (EO) was then cryodistilled into the reactor. At this stage, the orange colour of the solution disappeared and a pale yellowish colour
Separations at the CAP of PS
Liquid chromatography at the critical conditions of PS allows the molar mass independent elution of PS homopolymers and the separation of the PS-b-PEO block copolymers which is governed by the PEO block irrespective of the size of the PS block. These conditions are very useful for the analysis of complex polymer mixtures. In our previous study under critical conditions of PS for PS-b-PEO copolymers, only low molar mass samples could be analysed [32]. In the current study, the critical
Conclusions
Detailed analysis of PS-b-PEO copolymers at the critical conditions of PS and PEO showed that most of the products contain quite a large amount of both types of homopolymers formed during synthesis. The combination of comprehensive 2D-LC and the hyphenation of LCCC with LCCC of the other block, along with FTIR, gave much more insight into the product composition. The agreement of the quantification results for the homopolymers content, both by calibration of the ELS detector used for LCCC, and
Acknowledgements
MIM thanks the Claude Leon Foundation for a postdoctoral fellowship. We extend our gratitude to Prof. Wolf Hiller for providing samples.
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