Polarity in materials and biological systems: a pyroelectric and nonlinear optical study

This thesis deals with polarity as a fundamental property of condensed matter, its fundamental origin on the microscopic scale, its manifestation on the macroscopic scale as a spontaneous electrical polarization, and its detection by means of related properties such as pyro-, piezo-, ferroelectricity and nonlinear optical phenomena. Polarity appears to be a recurring trait in nature that is encountered in most connective biological tissue, where the role of polarity is hypothesized to be functional in some cases while possibly occurring as a side effect in others. In addition, the study of polarity in crystals, polymers and ionic liquids aids in understanding the occurrence and detection of polar material properties. There are no direct means to detect or measure the polarity of a material; it is a material property that can only be deduced by observing the electromagnetic response of the material after a thermal, electrical or mechanical perturbation. This thesis utilizes two principal techniques; one relies on the pyroelectric response after a small thermal perturbation, while the other measures the second harmonic, nonlinear optical activity, which can be considered a large electrical perturbation. Combining these two techniques leads to complementary results on the physics of polarity. The first two chapters describe the material properties and imaging techniques. Here, focus is given to the photo- pyroelectric approach and related imaging techniques in view of the large body of literature based on SHG techniques. A wide variety of materials was examined for polar properties, which include crystals, polymers, ionic liquids, biological tissues, biological systems and animal cells. The original emphasis lay on biological samples, while the crystal, polymer and ionic liquid functioned as reference systems and stepping stones to new and improved techniques. It will be shown that these 'test' systems revealed a couple of unexpected and intriguing findings that deserve an adequate presentation within this thesis. The molecular crystals CNS and DBANS are used as reference materials to compare the local pyroelectric response with the local second harmonic generation response. Both techniques yielded consistent results by confirming a longitudinal macroscopic polarity axis. Applying the same methodology, several Metal Organic Frameworks (MOF) are probed for polarity, yielding two possibly ferroelectric MOFs : Mil-53(Fe) and CAU-10-NO2. Measurements on an ionic liquid revealed a polarization distribution indicative for electrode polarization as directly confirmed by dielectric relaxation spectroscopy (DRS), yet occurring on a timescale not fully matching corresponding dielectric relaxation times. Polarization measurements in ionic liquids were used as an intermediate step toward biological systems in saline liquids, such as the coagulation of blood plasma, where a pyroelectric response is found at the coagulation front and in the polymerized fibrin. The presence of a spontaneous electrical polarization, and the consequential piezoelectric and pyroelectric properties, appears to be a recurring trait in biological materials. The polarity can be functional when it becomes part of sensory receptors for e.g. mechanical (piezoelectric) and thermal (pyroelectric) stimuli in the environment. A set of scleroproteins: fibroin (caterpillar silk), spidroin (spider silk), keratin (nails and skin) and chitin (insect wings) are investigated for polar properties. The scleroproteins used by insects display both pyroelectric and nonlinear optical activity and are suspected to be functional in nature, while the polarization in keratin is only detected via photothermal means, and is assumed to be non-functional. A major achievement is the photothermal pyroelectric detection of the lipid bilayer membrane potential and nucleus membrane potential of epithelial cells. These results are regarded as a step toward non-contact potential monitoring of cell and organelle membrane during cell activity. The copolymer PVDF-TrFE is revisited and subjected to a wide array of measurements, to determine the known asymmetric behavior during and after electrical poling. It is concluded that the negative poling electrode gives rise to an additional charge generating mechanism via electron injection that results in a large pyroelectric and SHG response. In thin films of PET, LIMM measurements yield first evidence about the length and shape of adsorption induced polarity that supports recent insights about polymer adsorption in both a qualitative and a quantitative way.Abstract Abbreviations Contents 1 Introduction 2 Electrical material properties 2.1 Microscopic and macroscopic electrical polarization 2.2 Piezo- and pyroelectricity 2.2.1 Piezoelectricity 2.2.2 Pyroelectricity 2.3 Electrical polarizability, hyperpolarizability and ferroelectricity 2.3.1 Hyperpolarizability 2.3.2 Ferroelectricity 3 Imaging techniques 3.1 Scanning PyroElectric Microscope (SPEM) 3.1.1 Detection of local pyroelectric activity 3.1.2 Experimental SPEM setup 3.2 Laser Intensity Modulation Method (LIMM) 3.2.1 Scale Transform method 3.2.2 Monte Carlo Deconvolution 3.2.3 Experimental Details 3.3 Second Harmonic Generation Microscope (SHGM) 3.4 Time resolved LIMM and SHGM measurements 3.5 Artifacts and false positives 3.5.1 Secondary and tertiary pyroelectric effect 3.5.2 Mechanical piezoelectric resonance 3.5.3 Photoelectric effect 3.5.4 Relating pyroelectric response and spontaneous polarization 3.5.5 Higher order nonlinear effects 3.6 Comparing photothermal and nonlinear optical results 3.7 Experimental results 4 Spontaneous polarization of (molecular) crystals 4.1 CNS 4.2 DBANS 4.3 MOF 4.3.1 MIL-68 4.3.2 CAU-10 4.3.3 MIL-53 5 Spontaneous polarization of biological tissue 5.1 Fibroin: Caterpillar silk 5.2 Spidroin: Synthetic spider silk 5.3 Keratin: Feline and human nails 5.4 Chitin: Insect wings 5.5 Conclusions 6 Spontaneous and induced polarization of polymers 6.1 PVDF-TrFE : A ferroelectric copolymer 6.1.1 Asymmetric polarization and hysteresis behavior in ferroelectric P(VDF-TrFE) (76:24) copolymer thin films spatially resolved via LIMM 6.1.2 Spatially resolved kinetics of polarization switching 6.1.3 Conclusion 6.2 PET : Interface polarization 7 Polarization evolution in liquids 7.1 Electrode polarization in an ionic liquid 7.2 Coagulation of blood plasma 8 Cells: Membrane polarization 8.1 Cardiac pacemaker cells 8.2 Stratified epithelial cells 9 Conclusions A Complex numbers, polar formalism and waves B Thermal Waves C Electric Dipole Radiation Bibliography Curriculum Publicationsnrpages: 198status: publishe