Chitosan as matrix for bio-polymer dispersed liquid crystal systems
Highlights
► Polymer dispersed liquid crystal composites based on chitosan polymer matrix were obtained. ► The 4-cyano-4′-pentylbiphenyl liquid crystal droplets have a radial configuration. ► Submicronic droplets were obtained for low content of liquid crystal. ► The micronic dispersion of liquid crystal into chitosan matrix results in a better luminescence.
Introduction
Among the composite materials, polymer dispersed liquid crystal (PDLC) systems are a class of materials largely developed in the last decades due to their use in the building of a large spectrum of devices. Starting with projection displays (Kikuchi, Fujii, Kawakita, Fujikake, & Takizawa, 2000), smart windows (Liu et al., 2011), or holographic systems (Su, Chu, Chang, & Hsiao, 2011), studies on PDLC systems are continuing at present, the newest ones being directed to bring the use of the PDLC systems to bio-applications, e.g. smart food packaging (Perju, Marin, Grigoras, & Bruma, 2011) or tunable artificial iris modulating light intensity through human eyes for assisting patients of aniridia (Hsu, Lu, Huang, & Shih, 2011). To apply PDLC systems as new biomaterials for the next generation of materials for biotechnology and medicine, friendly soft composites based on biocompatible polymers need to be developed. To meet this requirement, this paper is focused on the obtaining of new PDLC systems based on chitosan as a polymer matrix.
A short overview of the literature shows that various polymeric matrices were used for the obtaining of PDLCs: poly(styrene)s (Wang et al., 2007), poly(methylmethacrylate)s (Lv, Liu, Li, Tang, & Zhou, 2012), poly(dimethylsiloxane)s (Formentin, Papacios, Ferre-Berrull, Palares, & Marsal, 2008), poly(acrylate)s and branched poly(acrylate)s (Zhou, Collard, Park, & Srinivasalao, 2002), thiol–ene polymers (White, Natarajan, Tondiglia, Bunning, & Guymon, 2007), UV curable resin NOA65 (Norland Products) (Russed, Peterson, Imrie, & Heeks, 1995), polysulfone (Marin & Perju, 2009), polyvinylalcohol and so on (Mucha, 2003), but none reports chitosan as the polymeric matrix.
Chitosan, the linear and partly acetylated (1–4)-2-amino-2-deoxy-beta-d-glucan-isolated from marine chitin (Muzzarelli et al., 2012, Muzzarelli et al., 1986) is a promising candidate as polymer matrix for bio-PDLC systems due to its outstanding intrinsic biological properties: it is biocompatibile, fungistatic, immunoadjuvant, it has the ability to improve wound healing or blood clotting (Ravi Kumar, Muzzarelli, Muzzarelli, Sashiwa, & Domb, 2004). Furthermore, chitosan meets the main requirements of polymers as components of blends or composites with liquid crystals: film forming ability, transparency, inertness in relation to a liquid crystal and immiscibility with a liquid crystal in solid state (Mucha, 2003). Moreover, chitosan can be cross-linked with a large variety of binder agents (Berger et al., 2004, Muzzarelli, 2009), increasing its potential to be used as matrix for the confinement of liquid crystal droplets.
For all these reasons, we set out to obtain new PDLC systems using chitosan as polymer matrix, by encapsulation method. As liquid crystal, 4-cyano-4′-pentylbiphenyl (5CB) has been chosen, an archetypal example of a small molecule nematic liquid crystal usually involved to obtain PDLC systems due to its technological importance (Zou & Fang, 2011).
Section snippets
Materials
Low molecular weight chitosan, 4-cyano-4′-pentylbiphenyl 98% and acetic acid 99% were purchased from Sigma–Aldrich. The molecular weight of the chitosan was calculated to be 125 kDa and its degree of acetylation (DA) value was evaluated to be 13.5% (Brugnerotto et al., 2001, Marin et al., 2012). The refractive index of chitosan (n = 1.54) is within the variation range of the liquid crystal 5CB (no = 1.53 and ne = 1.71).
PDLC obtaining
The targeted PDLC composites were obtained by means of the encapsulation technique,
Results and discussions
Five new PDLC composites, named C1–C5, based on chitosan as polymer matrix and 4-cyano-4′-pentylbiphenyl (5CB) as liquid crystal were obtained for the first time, by means of encapsulation, using different weight ratios between the two components (Table 1). The composites gave free standing flexible films. Polarized light optical microscopy (POM), electron scanning microscopy (SEM), transmission scanning microscopy (TEM), Raman spectroscopy and differential scanning calorimetry (DSC) were used
Conclusions
PDLC composites based on chitosan matrix and 5CB liquid crystal have been successfully obtained for the first time by encapsulation method. The micron-size droplets were obtained by air-evaporation and submicron-size droplets were obtained by thermal evaporation under vacuum. The chitosan matrix generates the homeotropic alignment of the liquid crystal due to the positive charge of its protonated state. The nematic liquid crystal droplets have a radial configuration with the point defect in the
Acknowledgements
The financial support of European Social Fund – “Cristofor I. Simionescu” Postdoctoral Fellowship Programme (ID POSDRU/89/1.5/S/55216), Sectoral Operational Programme Human Resources Development 2007–2013 is acknowledged.
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