Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
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Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. These polymers are characterized by a high permeability and reasonable permselectivity. The latter point is somehow surprising because for microporous systems a more Knudson-like diffusion is expected then a size dependent temperature activated sieving process. It was argued in the framework of a random gate model that molecular fluctuations on a time scale from ps to ns are responsible for the permselectivity.
Here series of polymers of intrinsic microporosity (PIMs) as well as microporous polynorbornenes with bulky Si side groups and a rigid backbone are considered. The polymers have different microporosity characterized by high BET surface area values.
First inelastic time-of-flight neutron scattering measurements were carried out to investigate the low frequency density of state (VDOS). The measured data show the characteristic low frequency excess contribution to the VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. It was found that the frequency of the maximum position of the Boson peak correlates with the BET surface area value.
Secondly elastic scans as well as quasielastic neutron scattering measurements by a combination of neutron time-of-flight and backscattering have been out. A low temperature relaxation process was found for both polymers. This process was assigned to the methyl group rotation. It was analysed in terms of a jump diffusion in a three-fold potential. The analysis of the dependence of the elastic incoherent structure factor on the scattering vector yields the number of methyl groups which might be immobilized.
The neutron scattering experiments were accompanied by fast scanning calorimetry and broadband dielectric investigations as well as atomistic molecular dynamic simulations.
We show that mechanochemically driven polymorphic transformations can require extremely long induction periods, which can be tuned from hours to days by changing ball milling energy. The robust design and interpretation of ball milling experiments must account for this unexpected kinetics that arises from energetic phenomena unique to the solid state. Detailed thermal analysis, combined with DFT simulations, indicates that these marked induction periods are associated with processes of mechanical activation. Correspondingly, we show that the pre‐activation of reagents can also lead to marked changes in the length of induction periods. Our findings demonstrate a new dimension for exerting control over polymorphic transformations in organic crystals. We expect mechanical activation to have a much broader implication across organic solid‐state mechanochemistry.
Ionic Liquid Crystals are ionic liquids that exhibit liquid crystalline mesomorphism together with ionic conductivity. As known confined liquid crystal mesophases can show an anomalous dynamics and phase behavior. Investigations considering the factors controlling the macroscopic properties of ILCs in confinement are scare in the literature. This study reports the molecular mobility, and the phase transition behavior of a guanidinium based columnar ILC confined in the nanopores of self-ordered anodic aluminum oxide membranes of various pore diameters (25 – 180 nm) using Broadband Dielectric Spectroscopy (BDS), calorimetry and X-ray scattering. It is aimed to reveal in which way the pore size as well as the pore surface wettability (hydrophobic or hydrophilic) alters the molecular dynamics, and phase transition behavior for this system. These properties are crucial for applications. The DSC investigations reveal: (i) the phase transition temperature for the transition from the plastic crystalline to the crystalline-liquid state has non-monotonic dependence versus the inverse pore diameter and (ii) the transition from the liquid crystalline to the isotropic phase is suppressed for all nanoconfined samples. This transition suppressed in the thermal signal was evidenced by BDS and X-ray scattering. It is discussed as a continuous phase transition taking place in the pores instead of a discontinuous first order transition as observed for the bulk. BDS investigations show different relaxation processes for the bulk and the nanoconfined ILC. Molecular origins for various relaxation processes are discussed and suggested. It is further shown that the self-assembly of this ILC is dynamic in nature which might apply for other ILCs too. The obtained results will have implications for the nanoscale applications of ionic liquid crystals.
Thin polymeric films are of great importance of high number of high-tech applications for instance in sensors and nanoelectronics. Form the scientific point of view thin films with thickness below 100 nm are ideal model systems to study confinement effects on its properties for instance on the molecular relaxation processes. In this contribution an overview is presented about the behavior of different systems as investigated by nanosized relaxation spectroscopy like broadband dielectric spectroscopy employing nano structured capacitors and AC chip calorimetry complimented by ellipsometry. The systems considered are PVME1, PVME/PS blends2,3 P2VP4, PBAC5 and polysulfone6. Besides the film also the adsorbed layer on the substrate prepared by a leaching approach and investigated by AFM is considered.1,4-7. For these investigationsss it is found that the adsorbed layer itself exhibits a relaxation dynamics which might be assigned either to molecular motions or to adsorptions desorption kinetics.
Inelastic incoherent neutron time-of-flight scattering was employed to investigate the low-frequency vibrational density of states (VDOSs) for a series of glassy Janus-poly(tricyclononenes), which consist of a rigid main chain and flexible alkyl side chains. Here, the length of the flexible side chains was systematically varied from propyl to octyl. Such materials have potential applications as active separation layers in gas separation membranes as a green future technology, especially for the separation of higher hydrocarbons. From the morphological point of view, the Janus polynorbornenes undergo a nanophase separation into alkyl side chain-rich nanodomains surrounded by a rigid polynorbornene matrix. Here, the influence of the nanophase-separated structure on the low-frequency VDOS is investigated from a fundamental point of view. The low-frequency VDOSs of these Janus polynorbornene show excess contributions to the Debye type VDOS known as the Boson peak (BP) for all side chain lengths. Due to the high incoherent scattering cross-section of hydrogen, most of the scattering comes from the alkyl side chain-rich domains.
Compared to conventional glass-forming materials, in the considered Janus polynorbornenes, the BP has a much lower intensity and its frequency position is shifted to higher values. These experimental results are discussed in terms of the nanophase-separated structure where the alkyl chain-rich domains were constrained by the surrounding matrix dominated by the rigid backbone. With increasing alkyl chain length, the size of the alkyl chain-rich domains increases. The frequency position of the BP shifts linearly to lower frequencies with the size of these nanodomains estimated from X-ray measurements. The obtained results support the sound wave interpretation to the BP
Copolymer thin films with two types of functional groups have excellent performance as sensors, for example. The formation and deposition of allyl alcohol‐acrylic acid copolymer films by pulsed high frequency plasma is a complex process. As usual, the chemical composition of the top surface of the films was investigated by XPS and FTIR measurements. Furthermore, contact angle measurements with water were used to characterise the hydrophilicity and wettability of the polymer films.
After plasma deposition, a significant decrease in functional groups (OH and COOH) was observed compared to the classically copolymerised equivalent.
The remaining functional groups, i.e. the majority of these groups, were sufficient for application as sensor layers. Segmental mobility and conductivity, important for sensor applications, were analysed by broadband dielectric spectroscopy.
Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0–3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmAd) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Colh ), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli DTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.
The growth kinetics of the adsorbed layer of poly(2-vinylpiridine) on silicon oxide is studied using a leaching technique which is based on the Guiselin brushes approach. The adsorbed layer is grown from a 200 nm thick P2VP film for several annealing time periods at different annealing temperatures. Then the film is solvent-leached, and the height of the remaining adsorbed layer is measured by atomic force microscopy. At the lowest annealing temperature only a linear growth regime is observed, followed by a plateau. Here, the molecular mobility of segments is too low to allow for a logarithmic growth. At higher annealing temperatures, both linear and logarithmic growth regimes are observed, followed by a plateau. At even higher annealing temperatures, the growth kinetics of the adsorbed layer changes.
A linear growth followed by logarithmic growth kinetics is observed for short annealing time periods. For longer annealing time periods, an upturn of the growth kinetics is observed. At the highest annealing temperature, only a logarithmic growth regime is found. The change in the growth kinetics is discussed by an alteration in the structure of the adsorbed layer. Moreover, the interaction between the polymer segments and the substrate becomes weaker due to both enthalpic and entropic effects. Therefore, at high annealing temperatures the polymer segments might more easily desorb from the substrate.
The glass transition behavior of thin films of poly(bisphenol A carbonate) (PBAC) was studied employing ellipsometry. The glass transition temperature increases with the reduction of the film thickness. This result is attributed to the formation of an adsorbed layer with a reduced mobility compared to bulk PBAC. Therefore, for the first time, the growth kinetics of the adsorbed layer of PBAC was investigated, prepared by leaching samples from a 200 nm thin film which were annealed for several times at three different temperatures. The thickness of each prepared adsorbed layer was measured by multiple scans using atomic force microscopy (AFM). Additionally, an unannealed sample was measured. Comparison of the measurements of the unannealed and the annealed samples provides proof of a pre-growth regime for all annealing temperatures which was not observed for other polymers. For the lowest annealing temperature after the pre-growth stage only a growth regime with a linear time dependence is observed. For higher annealing temperatures the growth kinetics changes from a linear to a logarithmic growth regime at a critical time. At the longest annealing times the films showed signs of dewetting where segments of the adsorbed film were removed from the substrate (dewetting by desorption). The dependence of the surface roughness of the PBAC surface on annealing time also confirmed that the films annealed at highest temperatures for the longest times desorbed from the substrate.