In this Thesis, addressable macromolecules based on two classes of stimulus respon-sive polymers, temperature responsive poly(N-isopropylacrylamide) (PNIPAM) and redox responsive poly(ferrocenylsilane) (PFS), are described. Our goal was to lay the foundation for a molecular crawler based on PFS-g-PNIPAM graft copolymers, which could exploit the dual stimulus responsiveness of these branched polymer architectures for directed motion. To this end, we established a reliable functionalization method for attaching PNIPAM as side chains to a PFS backbone, with the aid of the Huisgen cycloaddition click reaction. This allowed us to tune the structure of the formed molecular bottlebrushes, and create a grafting density gradient along the chain. The addressability of the bottlebrushes was assessed by gauging the redox response of the PFS backbone by cyclic voltammetry as a function of temperature and by studying the reversible change in molecular size by atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements as a function of temperature. Furthermore, for the study of stimulus responsiveness of single chains and their ensembles, atomic force microscopy (AFM) methods were used.
Chapter 1 describes a brief introduction of the topics relevant for this thesis and a motivation for the presented work. Responsive polymers as well as multi-responsive materials that react reversible to external stimuli have attracted much attention over the last decades.
Chapter 2 provides an overview of stimulus responsive polymer architectures and summarizes the relevance of such materials in the area of materials science. The first part of the Chapter describes chemical methods to construct addressable macromolecules. Physical properties of such polymer structures are discussed. Furthermore, relevant tools to study the responsive behavior of these macromolecular architectures, such as dynamic light scattering and atomic force microscopy are introduced. The second part of the Chapter focuses on the application of atomic force microscopy for the investigation of polymer properties at the nanoscale. To investigate nanomechanical properties of end grafted polymer layers (brushes) we employed AFM.
In Chapter 3, as a representative example, a zwitterionic poly(sul-fobetaine methacrylate) (PSBMA) brush, grafted from a planar Si surface and a poly(methacrylic acid) (PMAA) brush, grown on a colloidal AFM probe, were studied. Force-distance curves were obtained and the grafting density based on the theory of de Gennes was determined. The apparent value of the Young’s modulus, analyzed by the Hertz model, was also determined.
AFM based single molecule force spectroscopy (AFM-SMFS) is often used for the detection and mechanical characterization of single molecules under environmentally controlled conditions.
As a next step to observe single chain responsive properties, in Chapter 4 the molecular stretching behavior of temperature responsive poly(N-isopropylacrylamide) (PNIPAM) chains was studied by AFM-SMFS. Force-extension curves obtained in water below and above the lower critical solution temperature (LCST), in the co-nonsolvent mixture water/methanol, and in dimethyl sulfoxide (H-bond blocking) follow the same trajectory, regardless whether the chain was pulled from a collapsed or from a solvated state. This result indicates that for a single PNIPAM chain the formation of intra chain H-bonds in the precipitated state does not cause measurable chain stiffening at the single chain level.For stimulus-propelled molecular crawlers we chose a system that exhibits dual responsive behavior, consisting of a PFS backbone and PNIPAM side chains (PFS-g-PNIPAM).
In Chapter 5 corresponding molecular bottlebrushes were discussed. These macromolecules were obtained by forming the backbone first, followed by side chain grafting. To prepare the dual stimuli responsive macromolecules a ’grafting to’ as well as a ’grafting from’ process were used. The ’grafting to’ method involved a Huisgen cycloaddition click reaction between an azide functionalized PFS backbone and alkyne end-functionalized PNIPAM chains. According to 1H and 13C NMR spectroscopy, quantitative azido functionalization of the backbone was achieved. The click reaction between the backbone and the side chains afforded bottlebrushes with average molar masses of Mn = 2 - 4 million g/mol, and when clicking PNIPAM chains, approximately 75% of the azide groups were consumed, yielding bottlebrushes with relatively high grafting densities. Functionalization of the PFS backbone with smaller moieties such as pendant initiator groups via click chemistry proceeded with quantitative conversion. From one of these macroinitiators, bearing ATRP groups, PNIPAM chains were grown from the organometallic PFS macroinitiator via a ’grafting from’ ARGET-ATRP process. Gel permeation chromatography measurements proved that the PFS backbone was converted into macromolecules of much higher molar mass after the ’grafting to’ or ’grafting from’ reactions.
Three dimensional objects such as the water-soluble graft bottlebrushes can undergo a change in their size and/or shape in response to an external environmental change. PFS-g-PNIPAM bottlebrushes were imaged by AFM. Height images of bottlebrush molecules deposited on a HOPG surface were obtained in air and in aqueous NaClO4. These images confirmed the cylindrical shape of the bottlebrush molecules. In the dry state, the average length of the molecules was 105 nm, the width was 15 nm and the height 5 nm. In liquid environment the bottle brush molecules retained their shape, but they swelled about 3 - 5 times their original size in width and height. The length of the molecules did not change significantly. The dimensions of the bottlebrushes obtained by AFM matched well with those obtained by dynamic light scattering.
In Chapter 6 the response to temperature of the PNIPAM side chains, and variation in the oxidation state of ferrocene in the backbone were studied in detail as a function of the structure of the bottlebrushes. For all PFS-g-PNIPAM bottlebrushes, in aqueous solution, LCST behavior was observed, and TLCST was determined to be 32 °C, which is identical to that of PNIPAM. As monitored by dynamic light scattering measurements bottlebrushes undergo a reversible linear decrease in size over a wide temperature range, until the LCST is reached where either intra-or inter-molecular collapse was observed. PFS-g-PNIPAM macromolecules deposited on a HOPG surface showed two different types of responses to electrochemical signals. In most of the cases a double-wave cyclic voltammogram typical of PFS was observed. The shape of the voltammogram changed when the temperature was increased above the LCST, indicating that the redox response is influenced by the temperature response of PNIPAM. In contrast to these measurements, bottlebrush with long PNIPAM side chains displayed ’break in’ behavior upon oxidation at 22 °C, and only one oxidation peak was observed at 0.5 V.A bottlebrush with a gradient in side chain grafting density was obtained by creating a composition gradient within the PFS backbone consisting of functionalizable monomer units and nonfunctionalizable ones.
Chapter 7 describes the synthesis of the asymmetrical PFS-g-PNIPAM bottlebrushes. By controlling the feed ratio of the two silaferrocenophane monomers, featuring ethyl-methyl or methyl-chloropropyl groups at the silicon atom, a continuous composition decay of the latter units was achieved. After backbone formation, the chloropropyl moieties were converted into azidopropyl groups, and according to 1H NMR measurements the final product contained 35% azide groups and 65% of inert ethyl-methyl substituted units. The obtained monomodal peak in gel permeation chromatography traces and the steadily increasing molar mass with time confirmed that copolymerization of the monomers occurred. Alkyne end-functionalized PNIPAM chains were attached in a ’grafting to’ process via click reaction. Based on 1H NMR- and ATR-FTIR spectroscopy we estimate that the click reaction between the backbone and the side chains afforded a gradient bottlebrush with a molar mass of M„ = 2.7 million g/mol. Dual stimuli responsive properties of the gradient bottlebrush were assessed by cyclic voltammetry (CV) measurements. In organic electrolyte, reversible double wave oxidation curves were obtained, while in aqueous environment the shape of the voltammograms was highly influenced by the inhomogeneous distribution of the water soluble PNIPAM chains along the backbone and further changed when the temperature was raised above the LCST. In principle, during oxidation-reduction cycles, the PFS chain periodically extends and retracts along the chain direction, while the phase transition of PNIPAM manifests itself in an adhesion change towards the underlying surface. Therefore, a sequential variation in the externally applied stimuli should result in directed motion of the molecule on a suitable surface.
In Chapter 8 we summarize principles of the molecular movement of a nanocrawler and discuss how characteristics of the underlying surface can be in principle tuned to assist in directional molecular motion.