Intrinsic Artefacts in Optical Oxygen Sensors—How Reliable are our Measurements?

Optical oxygen sensing is of broad interest in many areas of research, such as medicine, food processing, and micro‐ and marine biology. The operation principle of optical oxygen sensors is well established and these sensors are routinely employed in lab and field experiments. Ultratrace oxygen sensors, which enable measurements in the sub‐nanomolar region (dissolved oxygen), are becoming increasingly important. Such sensors prominently exhibit phenomena that complicate calibration and measurements. However, these phenomena are not constrained to ultratrace sensors; rather, these effects are inherent to the way optical oxygen sensors work and may influence any optical oxygen measurement when certain conditions are met. This scenario is especially true for applications that deal with high‐excitation light intensities, such as microscopy and microfluidic applications. Herein, we present various effects that we could observe in our studies with ultratrace oxygen sensors and discuss the reasons for their appearance, the mechanism by which they influence measurements, and how to best reduce their impact. The phenomena discussed are oxygen photoconsumption in the sensor material; depletion of the dye ground state by high‐excitation photon‐flux values, which can compromise both intensity and ratiometric‐based measurements; triplet–triplet annihilation; and singlet‐oxygen accumulation, which affects measurements at very low oxygen concentrations.     

Contact angle hysteresis: study by dynamic cycling contact angle measurements and variable angle spectroscopic ellipsometry on polyimide

The phenomenon of contact angle hysteresis was studied on smooth films of polyimide, a polymer type used in the microelectronic industry, by dynamic cycling contact angle measurements based on axisymmetric drop shape analysis-profile in combination with variable angle spectroscopic ellipsometry (VASE). It was found that both advancing and receding contact angles became smaller with increasing the number of cycles and are, therefore, not a property of the dry solid alone. The changes of the wetting behavior during these dynamic cycling contact angle measurements are attributed mainly to swelling and/or liquid retention. To reveal the water-induced changes of the polymer film, the polyimide surface was studied before and after the contact with a water droplet by VASE. Both the experimental ellipsometric spectrum for Delta and that for Psi as well as the corresponding simulations show characteristic shifts due to the contact with water. The so-called effective medium approximation was applied to recover information about the thickness and effective optical constants of the polymer layer from the ellipsometrically measured values of Delta and Psi. On the basis of these results, the swelling and retention behavior of the polyimide films in contact with water droplets were discussed.     

Influence of block composition on crack toughness behaviour of styrene-b-(styrene-random-butadiene)-b-styrene triblock copolymers

The deformation and fracture behaviour of symmetric and asymmetric styrene-b-(styrene-random-butadiene)-b-styrene (S-SB-S) triblock copolymers with variations in their molecular architectures in terms of their outer PS block and the random SB middle block composition ratios have been investigated using essential work of fracture approach based on post yield fracture mechanics concept. The present investigations on crack resistance behaviour of these S-(S/B)-S triblock copolymers where effective interaction parameter (χ_eff) is systematically varied through the variation of block compositions and architecture is in continuation to our earlier communicated short article highlighting the phase behaviour-morphology and mechanical property interrelation. The crack initiation and propagation behaviours are correlated to morphology and dynamic mechanical properties as obtained from TEM, SAXS and DMA measurements. The influence of interaction parameter (χ-parameter) space which has been manipulated through the variation of block compositions has clearly manifested in their morphologies and in their mechanical properties. Further the kinetic aspects of fracture mechanical response have also been investigated where all the materials have clearly revealed block composition dependence. SEM analysis was carried out to understand the fracture modes prior to failure.     

Strain Field Analysis and Kinetics of Crack Propagation of Binary Triblock Copolymer Blends

Summary: The crack toughness behaviour of binary styrene‐butadiene (SB) triblock copolymer blends of a thermoplastic block copolymer (LN3) and a thermoplastic elastomer (LN4) with different molecular architecture was studied using essential work of fracture (EWF) concept and was correlated to the morphological features from transmission electron microscopy (TEM). An increase in the crack toughness behaviour between 60 and 80 wt.‐% LN3 has been observed and is attributed to a change from cylindrical to lamellar morphology. The time‐resolved crack propagation studies have offered new dimensions to understand the kinetic aspects of fracture behaviour while the strain field analysis has explained the time‐dependent deformation behaviour to characterise the time dependence of the strain energy dissipation modes.

Load‐displacement diagrams of non essential work of fracture values of LN3/LN4 blends.     

Influence of Phase Miscibility on the Crack Propagation Kinetics of Nanostructured Binary S‐(S/B)‐S Triblock Copolymer Blends

Summary: The crack propagation kinetics of binary styrene‐(styrene/butadiene)‐styrene triblock copolymer blends based on one with symmetrical (LN4) and another with asymmetrical (LN3) molecular architecture is discussed with respect to post‐yield crack‐tip blunting and stable crack propagation behavior while highlighting the dynamic mechanical properties of the blends. The crack‐tip opening displacement (CTOD) rate is revealed to be sensitive to phase behavior, which is in agreement with a transition in phase miscibility in a critical composition range of 40–60 wt.‐% of LN3. Analyses of R‐curves from CTOD‐values reveal that kinetics of crack propagation is controlled by phase behavior, whereas the resistance to stable crack initiation is largely dependent on the composition. Our investigation offers new possibilities to tailor and optimize the crack resistance (crack propagation stability) of block copolymer blends through the control of phase miscibility and hence, fundamentally, adds a new dimension to the development of novel materials based on toughened nanostructured polymers.

Crack resistance curves for LN3 blends having different compositions.     

Influence of molecular architecture of S–S/B–S triblock copolymers on rheological properties

The influence of middle and outer block composition of symmetric triblock copolymers consisting of a polystyrene–polybutadiene (S/B) random middle block and two polystyrene (PS) outer blocks on morphology and rheological behavior has been investigated. Master curves are obtained by shifting the experimental data measured at different temperatures using time‐temperature superposition principle, the validity of which was confirmed in the linear viscoelastic regime. The rheological properties are observed to be strongly influenced by the relative composition of the S‐SB‐S triblock copolymers. Increasing the S/B ratio from 1:1 to 1:2 in the middle block has lead to a change in morphology from wormlike to lamellar, which is also accompanied with broad and sharp tan δ peaks in the dynamic mechanical measurements, respectively. The storage and loss modulus have been observed to increase with the increase in PS contents in the outer blocks and PB content in the middle block. The triblock copolymer with wormlike structure showed terminal linear viscoelastic behavior, whereas the ones with lamellar morphology showed nonterminal flow behavior in the similar low‐frequency regime. The relaxation modulus (G_t) has been observed to increase four times when the S/B ratio is increased from 1:1 to 1:2, whereas it increases threefold when the PS‐content in the outer block was increased by just 8 wt %. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2776–2788, 2006     

Bemerkung zu dem Aufsatz von K. H. Meyer: „Über die Viskosität und das Molekulargewicht von Hochpolymeren“

Zusammenfassung Zur Berichtigung des Aufsatzes von K. H. Meyer in der Kolloid-Z.95, 70 (1941) werden einige experimentelle Belege für die Gültigkeit des Viskosit?tsgesetzes für linearmakromolekulare Verbindungen gebracht. 288. Mitteilung: H. Staudinger, Cellulosechem. Januar-Heft1942.     

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