Responsive cholesteric polymer networks : humidity sensitive interpenetrating polymer network based on liquid crystals
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AuthorRenedo Gil, Elda
ABSTRACT: A humidity responsive cholesteric network has been created using an interpenetrating polymer network (IPN).
For building the IPN a liquid crystal mixture has been used due to its special properties. This experiment is based on cholesteric liquid crystals. Two liquid crystalline monomers have been used as the first polymer network; the chiral dopant LC 756 (5wt%) has been incorporated to the nematic liquid crystal LC 242 (95 wt%) with the aim of getting a helical structure of the chiral nematic phase. Also 5CB (4-Cyano-4'-pentylbiphenyl), a non polymerizable mesogen, has been added to this mixture with the aim of replacing it later with another monomer and create the two different phase separated networks. To carry out the photo-polymerization a photoinitiator has been added, plus a surfactant, which gives a better planar alignment.
As the second polymer network, different monomers have been used such as Styrene, Acrylic Acid, Methyl methacrylate, Hexanediol diacrylate, etc. It has been tested that the acrylic acid and styrene are the ones which work best for the purpose of the experiment.
As a result an IPN has been obtained which respond to changes in humidity after being activated with 0,1M KOH.
On the other hand making a switchable surface topography has been tried. In this case some parts of the sample swell due to the increasing humidity and others do not. For this purpose the monomer Styrene has been used, placing it in the sample, and polymerizing it with a photo-mask, therefore only a half of the styrene is transform in polystyrene, the rest is evaporated.
After this step the acrylic acid is added filling the parts without polystyrene in the sample. Afterwards it is polymerized.
The results show a higher swelling of the cholesteric when the styrene it is not used, due to the presence of this one, the cholesteric cannot swell as much as it does when it is not present.