Characterisation of Block Copolymers by On-line HPLC-NMR

Block copolymers are macromolecules consisting of two or more chemically different polymer segments of a single type of monomer unit, covalently bound together. They represent a versatile class of functional materials for a multitude of applications because they combine the properties of incompatible but well known polymers. Among other properties, the ability of these polymers to modify interfacial properties and to enhance the compatibility of polymer blends makes this polymer type attractive for applications ranging from thermoplastic elastomers, information storage, drug delivery and photonic materials. With the development of living anionic polymerisation1 the synthesis of block copolymers, especially those with complex architectures, has recently received increased attention due to interests in both academia and industry. Diblock copolymers of polystyrene (PS) and poly (methyl methacrylate) (PMMA) [PS-b-PMMA] have been extensively used to make templates for fabrication of nanostructured materials.2 Block copolymers of polyisoprene (PI) and PMMA [PI-b-PMMA] have been used as emulsifiers for the fabrication of polyester nanoparticles.3 Copolymers of PI-b-PMMA are interesting because they can be used for rubber production, as effective compatibilisers for natural rubber/acrylic polymer blends4, and as potential materials for medical applications5. Diblock copolymers of PS and PI [PS-b-PI] are thermoplastic elastomers. Chemical modification of these polymers, for example sulphonation, can give access to functional materials. These block copolymers can be used as templates for nanolithographic processes6. Block copolymers are complex materials. The physical properties of block copolymers are determined by their molecular characteristics, such as molar mass, chemical composition and chain architecture. In order to establish a detailed relationship between the molecular characteristics and macroscopic properties of a block copolymer, it is essential to perform a comprehensive analysis to determine their chemical composition distribution (CCD) and molar mass distribution (MMD). Generally, block copolymers are synthesised by sequential monomer addition, in which several factors should be controlled effectively, including the initiation efficiency of the macroinitiator (MI), the desired total molar mass and the molar mass distribution of each block. Standard characterisation methods such as nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR), cannot differentiate the block copolymer from homopolymer blends. In other words, they cannot determine the existence of unreacted macroinitiator and/or the newly generated homopolymers in the final block copolymer product7. Since the 1950’s, high performance liquid chromatography (HPLC) has emerged as a powerful technique to analyse various molecular distributions in synthetic (co)polymers. Size exclusion chromatography (SEC) is the most prevalent example of the use of HPLC for polymer characterisation separating macromolecules with regard to their hydrodynamic volume in solution. Because of the simple relationship between hydrodynamic volume and the molar mass for linear homopolymers, SEC has become the established method to determine the molar mass and MMD of synthetic polymers8. However; two intrinsic reasons hinder SEC from being an effective tool in fully characterising block copolymers. The first reason is the low resolution of SEC, which in most cases cannot fully separate the block copolymer from its precursor macroinitiator. The second reason is that the hydrodynamic volume of a copolymer is influenced by both molar mass and chemical composition. Specifically, SEC cannot provide information on the MMD of each individual block in the block copolymer. Therefore, new HPLC methods, such as liquid adsorption chromatography (LAC)9-11,14 and liquid chromatography at critical conditions (LC-CC)26,44 were developed, which consider the contribution of the enthalpic interactions between the analyte and the stationary phase in the column as a factor for polymer separation. Since chromatographic methods do not provide information about the microstructure of the monomer units in the block copolymers it is necessary to couple these selective separation techniques on-line with spectroscopic techniques such as NMR. The on-line coupling of HPLC and 1H-NMR is a powerful and time saving tool for the analysis of complex mixtures. To our knowledge, there are no applications of LC-CC-NMR for the characterisation of block copolymers yet. The main focus of this research work is to develop chromatographic methods for the characterisation of block copolymers. The developed separation methods are then directly coupled on-line with 1H-NMR for fast and complete characterisation of these copolymers. In the first experimental chapter PS-b-PMMA copolymers will be investigated. These block copolymers are synthesised by living anionic polymerisation. When block copolymers are synthesised by this method in addition to the copolymer there is also a possibility for the formation of homopolymer fractions. To get an exhaustive description of the MMD and CCD of the block copolymers as well as the homopolymers formed during synthesis, chromatographic techniques shall be developed and coupled with NMR to comprehensively characterise the samples. By using chromatography at critical conditions the copolymers shall be separated from the corresponding homopolymers. The sizes of the individual blocks shall be calculated. By using NMR as detector the tacticity of the PMMA block in the block copolymers shall be analysed selectively. In the second experimental chapter blends of homopolymers of 1,4-PI and 3,4-PI will be investigated. Chromatographic techniques shall be developed for separation of these blends. The homopolymers of 1,4-PI and 3,4-PI are not homogeneous and each of them contains different isomeric structures of monomeric units such as 1,4-PI, 3,4-PI and 1,2-PI. The chemical composition of the blends and the microstructure of the homopolymers shall be determined by NMR. PS-b-PI copolymers will be investigated in the third experimental chapter. These copolymers are synthesised by two different approaches: sequential living anionic polymerisation and coupling of living precursor blocks. When the copolymers are synthesised by these methods homopolymers are also formed. Samples shall then be analysed by developing chromatographic methods. The block lengths of the individual blocks, the chemical composition of the block copolymers and the microstructure of the PI blocks shall be analysed. The fourth experimental chapter is dedicated to the analysis of PI-b-PMMA copolymers. These block copolymers are synthesised by living anionic polymerisation. New chromatographic methods shall be developed for the analysis of these samples. By coupling chromatographic techniques with NMR the block lengths of the individual blocks as well as the chemical composition of the copolymers shall be calculated. By using NMR as detector the microstructure of the individual blocks shall be identified and calculated.