88 Vivid Scientific Research at the University of Silesia


Birefringence, a property of a crystalline material describes how within the material there are two distinct indices of refraction. Birefringent materials are used to create polarising filters or interference colours.


Krystian Roleder, the Regional Editor for the Central and Eastern Europe edition of the multinational journal ‘Phase Transitions’ and Researcher at the Institute of Physics, University of Silesia, Poland, has been studying birefringence measurements for a number of years. Mr Roleder has been using the THMSG600 stage to study birefringence in several materials, including BaTiO3. He commented: “This Linkam stage is indispensable for my scientific work."


In this research, Roleder and his colleagues have focused on the materials properties above crystallisation temperature. BaTiO3 has been the focus of many research projects since its first recognition in the 1940s. Extensive studies have looked at the phase transitions and domain structure along with the optical, electrical and mechanical properties. The cubic to tetragonal phase change is described as the order-disorder type and is displacive in nature. It is widely accepted that the Ti ion becomes displaced from its position within the oxygen octahedron, and this is the cause of the polar region formation within the crystal. It was found that the BaTiO3 crystals were birefringent over a broad temperature range but disappeared over 160-170°C.


The group has demonstrated the temperature dependence of the birefringence above crystallisation temperature and the behaviour of anomalous birefringence can be attributed to the existence of polar clusters. These polar clusters originate due to the interdependence of the structural and polar soft modes. These polar clusters interrupt the cubic symmetry of the crystal and about crystallisation temperature the crystal become birefringent. These polar regions are connected to the movement of the Ti molecule within the oxygen cage.


Samples were tested using the THMSG600 stage on an Oxford Cryosystems Metripol Birefringence Imaging System and heated at a constant 0.2°C/min.


The THMS600 is one of the most widely used heating and freezing microscope stages on the market. With its excellent 0.1°C accuracy and stability, the THMS600 is used in many applications including birefringence research. The stage gives researchers the option to characterise their sample quickly by heating to within a few degrees of the required temperature at a rate of up to 150°C/min with no overshoot.


MORE INFO. 250


Launch of Next Generation Turnkey Optical Tweezers System


JPK unveiled the new NanoTracker 2 optical tweezers system at the 2013 Biophysics Annual Meeting held in Philadelphia. Visitors to the conference and exhibition had a first impression about the new system first hand.


JPK, with a reputation as the ‘force tool shop’ for researchers in techniques such as Bio AFM and force spectroscopy, decided to develop a system to meet the needs of the practising applications scientist. These are people who work in the areas that include biophysics, biochemistry, polymer science, biology, single molecule mechanics groups, cell sorting and manipulation.


In total, this means all people who are working with small particles interacting with their environment - and measuring their forces. This optical tweezers platform provides force and interaction measurement in parallel with optical microscopy using complementary spectroscopic techniques.


By talking with users of optical tweezers systems, both the home-builders and of their first- generation system, JPK has come up with a system which minimises user interactions on a platform which has been built with increased stability and lower noise to improve detection. With force measurements being so important to users, JPK has enabled new open routines for the user to design their own enhanced measurements. It also provides new force clamp and signal multiplexing and de-multiplexing capabilities by utilising acousto-optic deflectors which spatially control the optical beams.


As CTO, Torsten Jähnke, described the system, "We have developed a more integrated-design approach than before. For example, it makes single molecule experiments such as DNA stretching or cell/particle experiments much more straightforward. This saves time for the user who now has the full control of all parameters in one control and analysis software package."


The new system comes with an overall improved performance especially for the beam steering through highly accurate pivot-point piezo mirrors and the detection system which benefits from a complete redesign of the optical pathway.


The resulting better linearity and diminished crosstalk improve all sensitive force measurements. New functions are also implemented for a more precise trap calibration which allows the extraction of additional material properties.


The full capabilities of the system are described in detail in a new eight page product brochure which is available free at JPK's web site.


MORE INFO. 251


Tensile Temperature Stage used to study the Influence of DMDBS on the Mechanical Properties of Polypropylene Films


Polypropylene (PP) is a common, commercial engineering plastic that is tough, flexible and can be manufactured via extrusion film casting (EFC). Most commercial PP is isotactic with all methyl groups orientated on the same side of the backbone of the polymer chain. This orientation of the methyl groups within the polymer relative to each other directly influences the ability of the polymer to form crystals.


DMDBS (dimethyl dibenzylidene sorbitol)


is a butterfly shaped molecule that is used as a nucleating agent in the manufacture of PP via EFC. It forms crystalline nanofibres (approx 5nm diameter) when it precipitates out of a hot melt


of isotactic PP (iPP). These nanofibres form a mesh. At cooler temperatures, iPP (α-form) crystals nucleate on the surface of these fibres.


Guruswamy Kumaraswamy and a group of scientists from the National Chemical Laboratory (NCL) of India have used a Linkam TST350 tensile testing temperature stage to look at the influence of this semi-crystalline morphology on the mechanical characteristics of the film.


Homopolymer iPP pellets were coated with DMDBS using a DMDBS solution in acetone. These pellets, with 0.2%, 0.4%, or 0.8% (by weight) DMDBS were extruded through a ThermoHaake PolyLab single screw extruder. This created a film of a constant thickness of 0.45mm as the film was taken up on chill rolls (10°C). To achieve six different draw ratios, the speed of the chill rolls was varied.


These films were tested using the Linkam TST350 stage. The stage is built to maintain perfect uniform vertical and horizontal alignment during testing. Temperature control and accuracy is second to none, with a range from -196 to 350°C with 0.1°C control and up to 60°C/min heating rate.


It was observed that at TDIE=200°C voids form within the 0.8% DMDBS film during extrusion. This, the scientists hypothesised, was the reason why the 0.8% film exhibiting a decrease in yield stress and modulus values compared to neat iPP film and the 0.2% & 0.4% films. The 0.2% and 0.4% DMDBS PP films exhibit a ≈50% increase in modulus and yield strength compared to neat PP. This is observed to be higher in the transverse direction. More information can be found in the paper ‘the influence of DMDBS on the morphology and mechanical properties of polypropylene cast films’ by K. Sreenivas, Harshawardham V.Pol, and Guruswamy Kumaraswamy.


MORE INFO. 252


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