Research

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Research

Numbers refer to ‘Selected papers’.

As research scientist at the Philips’ Research Laboratories in Eindhoven, I worked mainly on the origin and properties of liquid crystals. work on the orientational transition of a nematic twisted nematic film in a magnetic field [12], and on the dielectric properties [28, 29, R2] and the elastic constants [32, 41] of nematic liquid crystals. Subsequently I started a new group on liquid crystal physics at the Solid State Physics Laboratory of the University of Groningen. Further work on the properties of nematic liquid crystals included the magnetic susceptibilities [44] and the viscosity coefficients [48, 55]. Much work from those years is summarized in my book Physical Properties of Liquid Crystalline Materials. In addition theoretical and experimental work was devoted to the origin of smectic liquid crystalline behavior [47, 54, R8].

X-ray research on phase transitions in liquid crystals and ordering at surfaces was done during visits to the Risø National Laboratory (Roskilde, Denmark) and to the synchrotron at Hasylab (Hamburg). Upon moving to the FOM-Institute AMOLF a high-resolution x-ray apparatus was set up, which could also be used for x-ray reflectivity studies. In addition extensive use was made of the synchrotron source at Brookhaven (NSLS) , and later also the ESRF at Grenoble (including a sabbatical in spring 1995). This work included high-resolution x-ray scattering of the critical behavior of the phase transition nematic-smectic A [76, R9]. Also an analysis was made of the x-ray lineshapes of smectic polymers crosslinked into a percolating elastomer network. We established that for such smectic elastomers the logarithmic divergence of the fluctuations (Landau-Peierls instability) is no longer observed [92, R15].

Using diffuse x-ray reflectivity, we performed pioneering studies of freely suspended smectic films (smectic membranes). The in-plane wave-vector dependence of the fluctuations of the smectic layers was investigated down to molecular dimensions, At long lateral length scales top and bottom of the film fluctuate in unison, while at smaller in-plane length scales this conformality is lost [84, 86, 93, R12]. Coherent x rays were used to study the dynamics of the fluctuations in smectic membranes by XPCS [103]. This allowed mapping out the dispersions relations for the fluctuation spectrum, including surface- en bulk-elasticity dominated regimes. To do so the fast side of XPCS had to be further developed, bringing the timescales down to the nanosecond range [111]. In this region we combined for the first time XPCS with neutron spin-echo techniques [117, 132, R12].

After becoming full time associated at AMOLF, I made a transition from liquid crystal to polymer physics. the in-house x-ray facilities were extended with a SAXS-WAXS setup. Ordering and crystallization in (block co)polymers – using external fields as control parameters – was chosen as a leading scientific theme. In particular shear was used to influence bulk polymer crystallization and confinement in thin films to control ordering and crystallization in block copolymers.

Crystallization in diblock copolymer films with a crystallizable and an amorphous (rubbery) block was found to lead to an expansion of the lamellar period. This could be attributed to an expansion of the crystalline layers in order to accommodate a (half) integer number of folded stems [112, 131, R11].

Crystallization under shear was investigated for several types of polymers (iPP, nylon-12, PBT) [113, 122, 140], including the influence of nucleators [145] Furthermore the coupling and competition between microsphase separation and crystallization has been studied for several types of block copolymers with a crystallizable block [115, 120, 131].

Nanostructures in thin diblock copolymer films with a smectic and an amorphous block were controlled exploiting the competition between smectic layering and block microphase separation. This allowed stabilizing the orientation of either lamellae [88] or cylinders [133, 143] perpendicular to the substrate [R14].

During a sabbatical at UMass (Amherst, USA) I was involved in some quite different work on wrinkling of thin polymer films floating on water [142].

Upon moving to Amherst and Aachen, my work has focused on transferring my x-ray scattering and soft matter knowledge to a younger generation. Still block copolymer structures and morphologies – both in bulk and in thin films – remained of continuing interest. In particular the structure and applications of ordered nanostructures in thin films of substituted ologothiophenes has been investigated [157, 159].

 

Selected invited lectures of more than local importance

 

1976 Invited plenary lecture at the ‘VIth International Liquid Crystal Conference’ (Kent, Ohio, USA).

1977 Invited lecture at the NATO Advanced Study Institute ‘The Molecular Physics of Liquid Crystals’ (Cambridge, UK).

1980 Invited lecture at the ‘VIIIth International Liquid Crystal Conference’ (Kyoto, Japan) .

1981 Invited plenary lecture at the Bunsen-Kolloquium ‘Strukturen und Phasenumwandlungen Thermotroper Flüssigkristalle’ (Paderborn, FRG).

1982 Invited lecture at the Discussion Meeting of the Royal Society ‘The Physics, Chemistry, and Applications of Thermotropic Liquid Crystals’ (London, UK).

1983 Invited plenary lecture at the Bunsen-Kolloquium ‘Materialeigenschaften und Ordnungszustände von Flüssigkristallen’ (Berlin, FRG).

1985 Invited lecture at the ‘Gordon Research Conference on Liquid Crystals’ (Wolfeboro, NH, USA).

1986 Invited lecture at the NATO Advanced Study Institute ‘Structural Incommensurability in Crystals, Liquid Crystals and Quasi-Crystals’ (Boulder, Colorado, USA).

1987 X-ray studies of liquid-crystalline surfaces, Invited plenary lecture at the ‘Seventh Liquid Crystal Conference of Socialist Countries’ (Pardubice, Czechoslovakia).

1988 Invited lecture at the DFG-colloquium ‘Flüssige Kristalle und Anisotrope Fluide’ (Berlin, FRG).

1991 Microscopic aspects of liquid crystal phases, invited lecturer at the NATO Summer school ‘Phase Transitions in Liquid Crystals’ (Erice, Sicily, Italy).

1994 Fluctuations in freely suspended smectic films as measured by specular and diffuse x-ray scattering, invited lecture at the ‘XVth International Liquid Crystal Conference’ (Budapest, Hungary).

1995 Conformality and fluctuations in free-standing smectic films as studied by off-specular x-ray reflectivity, invited lecture at the ‘Gordon research conference on liquid crystals and liquid crystalline polymers’ (Wolfeboro, NH, USA).

1996 Thin films of hybrid smectic/isotropic block copolymers, invited lecture at the NATO Advanced Research Workshop: ‘Manipulation of organisation in polymers using tandem molecular interactions’ (Il Ciocco, Castelvecchio, Italy).

1996 Fluctuations in freely suspended smectic-A films, invited lecture at the ‘XVIth International Liquid Crystal Conference’ (Kent, Ohio, USA).

1996 Understanding the packing of mesogens in orthogonal smectic phases, Invited lecture at the symposium ‘Liquid crystals, past, present and future’ (Capri, Italy).

1997 Orientational wetting in hybrid liquid crystalline/isotropic block copolymers, invited lecture at the ‘Fourth international conference on polymers for advanced technologies’ (Leipzig, Germany).

1998 Fluctuations and surface crystallization in freely suspended smectic films, invited lecture at the ‘XVIIth International Liquid Crystal Conference’ (Strassbourg, France).

1998 Orientational wetting in thin films of hybrid smectic/isotropic block copolymers, invited lecture at the ‘Second international workshop on wetting and self-organisation in thin liquid films’ (Münich, Germany).

Also invited lecture at the workshop ‘Unifying principles for engineering soft materials’ (Roskilde, Denmark).

1999 On the interpretation of x-ray reflectivity experiments: resolution and other effects, invited lecture at the ‘Sixth International conference on surface x-ray and neutron scattering’ (Noordwijkerhout, Netherlands).

2000 Crystalline smectic B films as fluctuating systems, invited lecture at ‘Surface Science 2000: Self-organization at interfaces and in thin films’ (Grenoble, France).

2000 Possibilities of coherent dynamic x-ray scattering of freely suspended films of soft matter, invited lecture at ‘Surface and interface physics with the X-FEL’ (Hamburg, Germany).

2001 Coherent diffraction from free standing smectic films, invited lecture at the ‘Gordon conference on x-ray science’ (New London, CT, USA).

2002 On the use of liquid crystallinity to obtain polymeric nanopatterns, invited lecture at the International workshop ‘Pattern formation by molecular self-organization at interfaces’ (Berlin, Germany).

2002 Smectic membranes in motion: surface diffraction and dynamic x-ray scattering, invited lecture at the workshop ‘Frontiers for synchrotron research on soft matter and biomaterials’ (Tarrytown, NY, USA).

2003 Probing the dynamics of smectic fluctuations by x-ray photon correlation spectroscopy and neutron spin echo, invited lecture at the ‘Gordon Research Conference on Liquid Crystals’ (New London, CT, USA).

2004 Smectic membranes in motion: The limits of x-ray photon correlation spectroscopy, invited lecture at the ‘Eighth International Conference on Surface X-Ray and Neutron Scattering’ (Bad Honnef, Germany).

Also invited lecture at the OSA conference ‘Photon Correlation and Scattering’ (Amsterdam, Netherlands).

2004 Controlling block copolymer nanopatterns by liquid crystallinity, invited lecture at the ‘World Polymer Congress Macro 2004’ (Paris, France).

2004 X-ray scattering of nanostructured soft matter films, invited lecture at ‘XTOP 2004, 7th Biennial Conference on High Resolution X-Ray Diffraction and Imaging’ (Prague, Czechoslovakia).

2005 Order and disorder by random crosslinking in smectic elastomers, invited lecture at the ‘Third International Conference on Liquid Crystal Elastomers’ (Cambridge, UK).

2006 Dynamics of ordering and crystallisation in polymers: What can we learn from x-rays? ESRF Symposium ‘Dynamics of Soft Matter’ (Grenoble, France).

2006 Nanostructures in block copolymers with liquid-crystalline amphiphilic molecules, invited lecture at the ‘COST P12 Conference’ (Mittelwehr, France).

2007 Order and disorder in low-dimensional soft matter systems, invited plenary lecture at the HASYLAB Users Meeting (Hamburg, Germany).

2007 On the subtleties of smectic liquid crystal ordering, invited lecture at the symposium ‘Modern X-Ray Physics’ (Copenhagen, Denmark).

2009 Order, disorder and strain in smectic elastomers, invited lecture at the ‘Fifth International Conference on Liquid Crystal Elastomers’ (Kent, Ohio, USA).

2011 Block copolymers with liquid-crystalline amphiphilic molecules, invited lecture at the 2nd FAPS Polymer Congress (Beijing, China).

2011 The smectic-nematic phase transition in liquid-crystalline elastomers, invited lecture at the ‘Sixth International Conference on Liquid Crystal Elastomers’ (Lisbon, Portugal).

2011 X-ray lineshape and order-disorder in liquid-crystalline elastomers, invited lecture at the ‘Integrated workshop for SAS and XPCS in soft matter’, National Synchrotron Radiation Research Center (Hsinchu, Taiwan).

 

 

 

 

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