Experimental Soft Matter Physics
September 2016

New paper on polymer-stablized liquid crystal shells in Advanced Materials

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Congratulations to JungHyun and Benjamin for their new Adv. Mater. paper "Taming Liquid Crystal Self-Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization" on polymer-stabilization of nematic and smectic liquid crystal shells, and the sometimes unexpected consequences for the liquid crystal self-assembly. By polymerizing a small fraction of the reactive mesogen RM257 in shells of 8CB or its homologues, certain defect configurations can be locked in place, the exact result depending sensitively on the mixture composition and the temperature at which polymerization is carried out. Surprisingly, when polymerizing close to a phase boundary, a transition into the adjacent phase can be induced. The new phase can be more or less ordered, depending on the starting situation. The lifetime of the shells is dramatically enhanced, as is the temperature stability. By tuning the conditions, the self-assembled structure can be made fully permanent, being visible even upon heating to the isotropic phase of the non-polymerized component, or it can be retained in a latent state, allowing macroscopic loss of order on heating to the isotropic phase yet with a memory of the chosen defect configuration when cooling down. Apart from proposing explanations for the various observations we discuss possibilities to apply the polymer-stabilized shells, for instance in advanced materials generation or in sensing.

Here you can find the full paper (don't forget to check out the rich supporting information, including many spectactular movies), and here is a layman's abstract for the paper.

New paper on quantitative optical characterization of CNC films

IrenasCNC-Paper
We have a new article out in the journal Cellulose, describing the results from a collaboration with Sweden and Slovenia. The article, entitled "Correlation between structural properties and iridescent colors of cellulose nanocrystalline films", describes an optical and electron microscopy study of dried films of cellulose nanocrystals (CNC), prepared with or without circular shear flow. This is a follow-up study of our previous study on the influence of shear flow when drying CNC samples to make iridescent films, and the new component is a quantitative spectrophotometric study as a function of location in the films, together with a high-resolution electron microscopy characterization of the fractured films. It turns out that the reflection spectra are surprisingly similar between the two types of films, but the film cross section is very different. Whereas films dried with shear flow have a clearly periodic helical structure throughout the film, the ones dried without shear flow are disordered near the air interface. Because there is still a sufficiently thick internal regime, close to the bottom substrate, that is helical, the reflection spectra are similar, but the structural difference at the film top can have a strong effect on birefringence. The reflection spectra are much broader than what is expected for a cholesteric liquid crystal with well-defined pitch, indicating a variation in pitch within the film, which may be related to the polydispersity of CNC samples.

Read the article on the Cellulose website.

New paper in Langmuir on CNC dispersion in non-aqueous polar solvents

JohannaLangmuirToC
Congratulations to Johanna and co-authors for the publication in Langmuir of the article "Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents". The team studied dispersion of Cellulose Nanocrystals (CNC) and the associated liquid crystal formation in water and in non-aqueous but polar solvents. Johanna developed a new method for exchanging the solvent of CNC suspensions from the water used during synthesis to formamide, N-methylformamide (NMF) and N,N-dimethylformamide (DMF), without inducing aggregation of the nanorods. She found striking differences between the solvents concerning the liquid crystal formation. The four solvents span a large range of dielectric permittivity, a parameter that turns out to be of key importance. The cholesteric helical superstructure develops much faster in high-permittivity NMF than in water and the pitch depends much less on CNC concentration than when water is the solvent. In low-permittivity DMF the first trace of liquid crystal formation coincides with kinetic arrest of the whole sample, preventing any helix formation since an equilibrium liquid crystal phase never develops. We propose that this is due to aligned aggregation of the CNC nanorods. The experimental results are corroborated by computer simulations done in Tanja Schilling's group, which furthermore indicate that the nematic order parameter goes up for high-permittivity solvents.

If you have an institutional subscription to Langmuir, you can download the article here. If you do not, you can try the following link: http://pubsdc3.acs.org/articlesonrequest/AOR-uVuBGBqpFzkXHmtihG36 . The first 50 to try this link can download the article for free, regardless of subscription.

We regret that the published Acknowledgments section unfortunately did not mention that Rick Dannert is a member of the Laboratory for the Physics of Advanced Materials of the University of Luxembourg and that this group kindly made their equipment (AFM, rheometer and refractometer) available for some of the experiments described in the paper. We deeply apologize for having forgotten this information in the published paper. Here we would like to express our deep gratitude for the support from the Laboratory for the Physics of Advanced Materials.