Synthesis, Characterization and Competitive Adsorption of Comb Copolymer Dispersants

Abstract

Poly(carboxylate ether) (PCE) superplasticizers have become an indispensable part in modern concrete formulations. On the market, a broad variety of chemically different PCEs is available. However, the performance of a PCE does not exclusively depend on the chemical nature of the monomers. Indeed, their molecular structure and dispersity have a significant impact on performance and plasticizing abilities. To this date, a lot of research empirically describes the impact of average molecular parameters on the performance of PCEs, but falls short of predictive models considering parameter distributions. At the same time, there is a lack of analytical protocols that enable a comprehensive analysis of molecular dispersity. Indeed, the notion dispersity is often applied to acknowledge solely the width of the molar mass distribution (MMD). However, PCEs are characterized by a high degree of molecular heterogeneity. This means that dispersity is not restricted to variations in molar mass, but also applies to distributions of the molecular parameters, such as backbone length, grafting ratio and potentially side chain length. A better understanding of dispersity is crucial for the prediction of structure-performance relations of PCEs. This thesis draws attention to their complex dispersity and reveals how molecular parameters affect the competitiveness of PCEs for adsorption at the cement surface. More specifically, the thesis contributes to the field of PCE research in four main aspects: First of all, versatile protocols for the synthesis of PCE model structures with low dispersity and systemic variations of the molecular parameters are presented. Well-defined model structures are crucial for identifying structure-performance relations, but also to develop analytical methods that allow to characterize molecular heterogeneity. Subsequently, this thesis opts for a better use of liquid chromatography (LC) for PCE analytics. LC-based protocols are developed that enable to investigate molecular heterogeneity within PCEs. Such versatile and refined analytical tools are essential to get an insight into the polymeric fractions of PCEs that ultimately determine their qualities as superplasticizers. For instance, size exclusion chromatography (SEC) with dual concentration detection is suggested to quantify the comonomer composition of PCEs. While common SEC analysis solely targets the MMD, the dual concentration method gives information on the PCE composition in dependence on their molecular size. Moreover, it is revealed how and to what extent the synthesis pathway affects the chemical dispersity of PCEs. Furthermore, reverse-phase high performance liquid chromatography (RP-HPLC) with solvent gradients is presented as a suitable tool to separate PCEs according to the grafting ratio. Thus, this method gives an insight into the chemical composition distribution (CCD). Limitations of RP-HPLC are discussed, and complementary SEC experiments allow a profound 2D-analysis of PCEs targeting the CCD in the first and MMD in the second chromatography dimension. Besides providing analytical tools to study dispersity, the effect of molecular parameters on PCE adsorption and competition is studied. For this purpose, a novel experimental approach is presented that enables to quantify the competitive adsorption between PCE molecules of different molecular parameters, i.e. different charge density, backbone length and side chain length. The presented results confirm the existence of competitive adsorption phenomena among PCE molecules as a consequence of their molecular characteristics. In this context, the impact of molecular heterogeneity of PCEs is discussed. A thermodynamic framework is established to interpret the results. It offers the first theoretical model capable to discriminate the adsorption competitiveness of a PCE in relation to its molecular structure. All in all, this thesis presents a comprehensive approach to synthesize and analyze PCEs before investigating their adsorption behavior. Herein, the thesis opts for a better use of polymer analytics and polymer physics in combination with basic thermodynamics and fundamental cement chemistry to provide a better understanding of competitive adsorption phenomena. Most importantly, it is emphasized that reliable structure-performance relations can only be established when the applied PCEs are well-characterized regarding their molecular parameters.