Pim
van der Asdonk
,
Hans C.
Hendrikse
,
Atang C.
Sauli
,
Stijn P. M.
Kraaijkamp
and
Paul H. J.
Kouwer
*
Department of Molecular Materials, Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands. E-mail: p.kouwer@science.ru.nl
First published on 12th August 2016
Controlling the spatial and temporal organization of functional polymers is essential for the development of switchable soft-matter based electro-optical devices. By using a combination of a liquid crystal template, a photopatternable substrate (for spatial control) and electric fields (for dynamically switching) we show that we are able to dynamically control the spatial organization of polymers across multiple length scales (conveyed through the patterned liquid crystal template). The polymer that we organize is an azobenzene-functionalized polyisocyanide, whose stiff polymeric helix and laterally attached pendant azobenzene units induce tangential anchoring to the liquid crystal host. Due to the donor–acceptor functionalized azobenzene units, the polymeric material is strongly absorbing in the visible range for characterization purposes. We find that polymers align locally to the liquid crystal director field and reversibly change its orientation by the application of an electric field. Since this hybrid technique can be easily applied to other functional polymeric materials and relies on simple techniques, such as spincoating, photoalignment and electric fields, we believe it has great potential for the development of a wide range of switchable electro-optical devices.
In this manuscript, we use liquid crystal templating (LCT) to generate complex and dynamic structures of polymer materials. LCT has been successfully applied to organize various soft materials in bulk liquid crystals. Key examples include supramolecular aggregates,2,3 (semi)conducting organic polymers,4–9 and carbon nanotubes.10–12 To a lesser extent, this approach has been applied to aqueous lyotropic or chromonic templates, for example to organize carbon nanotubes,13,14 motile bacteria,15–17 and a spatially patterned chromonic–silica hybrid18 and self-assembled peptide amphiphiles,19 both on photopatternable substrates. The major advantages of LCT are its versatility, its high dimensional control and its dynamic addressability. The versatility stems from the absence of distinct molecular interactions between the template and the polymer. Dimensional control is given by the substrate; micrometer patterning techniques are already commercially applied in the LCD industry.20 Lastly, the liquid crystal template is readily manipulated using electric fields, again analogous to LCD technology, which introduces spatially controlled dynamics to the dispersed polymers. The promises of LCT has only partly been realized: by far the majority of studies are focused on plain long-range unidirectional alignment and only rare examples use complex structured substrates18,19 or electric fields4,8,10–12 to take full advantage of this approach.
In this paper, we use an approach to spatially and temporally control the organization of functional polymers, by applying a combination of a liquid crystal template, a photopatternable substrate and electric fields. This is to our knowledge the first example where both complex patterned in-plane alignment of functional polymers is obtained, combined with controlled reversible homeotropic switching of these materials.
In order to benefit from the anisotropic environment that the liquid crystalline matrix offers, one would like to use rigid or semi-flexible polymers with a persistence length larger or of the same order as the contour length. Polyisocyanides are such class of polymers through the formation of a rigid helix which is the result of the polymerization mechanism. The helical backbone conformation, supported by a β-sheet-like hydrogen-bond pattern at the periphery yields stiff polymer chains,21 which can be decorated by a variety of functional groups.22 We used an azobenzene-functionalized polyisocyanide (AzoPIC). The azobenzene units increase solubility of the monomer (and polymer) in the liquid crystal and are laterally attached to induce tangential anchoring in the liquid crystal host. Although we designed the polymer for optimal compatibility with the liquid crystal host, the strength of liquid crystal templating is that no specific molecular interactions with the host are necessary to achieve the desired alignment.4,8,12 Furthermore, the azobenzene units carry donor and acceptor groups to push the absorbance deep into the visible for characterization purposes. We find that polymers align locally to the liquid crystal director field and reversibly change its orientation by the application of an electric field. Moreover, we find that for its assemblies also intermediate alignment can be realized.
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Fig. 1 Molecular structures (a) of AzoPIC, LC template 5CB and command layer material PMAz. Schematic 3D depiction (b) of AzoPIC. |
The synthesis of AzoPIC is outlined in Scheme 1. Details of the synthesis and characterization are given in the ESI.† Boc-protected alanine was equipped with a short spacer through a standard EDC coupling reaction. The hydroxyl-functionalized push–pull azobenzene252 was first deprotonated with t-BuOK and then added to bromide 1 to give the functionalized boc-protected alanine 3. Deprotection with EtOAc·HCl, formylation and dehydration using Burgess' reagent yielded the isocyanide monomer 5 in an 18% overall yield. The polymerization of monomer 5 was initiated with a Ni2+ salt in the presence of a small amount of alcohol. The reaction was carried out in chloroform (which is common solvent for isocyanide polymerization reactions) or in 4-cyano-4′-pentyl-1,1′-biphenyl (5CB). In 5CB, the monomer concentration studied was relatively low (c = 0.035 wt%), due to the limited solubility of 5 in this liquid crystal (0.064 wt%).
As for the liquid crystal template, we used the liquid crystal workhorse 4-cyano-4′-pentylbiphenyl (5CB, Fig. 1), which forms a nematic LC phase at room temperature. This well-studied thermotropic liquid crystal has also been applied in several organic LCT examples3,9–11,26 and is highly susceptible to electric fields due to its dipolar molecular structure and typical long range cooperative interactions. Furthermore, its biphenyl aromatic core is anticipated to have favorable interactions with the azobenzene side groups of AzoPIC.
The photopatternable command layer is an azobenzene-functionalized polymethacrylate (PMAz, Fig. 1); its synthesis and characterization were described earlier.19 By using simple procedures based on multiple illumination steps with polarized light and photomasks, micron-scale patterns of locally aligned azobenzenes (covalently attached to a PMA matrix) can be imprinted on a wide range of substrates.27,28 This locally patterned PMAz command layer transfers the organization to many bulk liquid crystals. Since PMAz is applied to a substrate by spincoating, we can easily coat conducting ITO/glass substrates (CEC100S, PGO), which allows for the application of electric fields and can ultimately be integrated with standard pixel-based LCD technology. In our study we constructed ∼1 cm2 20 μm spaced liquid crystal cells from the PMAz-coated ITO plates.
Polymer formation is best followed with circular dichroism (CD). Whilst the isocyanide monomer 5 is CD silent (like many chromophore-substituted isocyanides), the polymer shows a Cotton effect right at the absorption wavelength of the azobenzene side groups (Fig. 2). This is a clear indication that in the AzoPIC polymer, the chromophores are stacked in a helical configuration along the polymer backbone.29 The reaction proceeds slowly due to the low concentration of monomer and catalyst. The yield of the isocyanide polymerization reaction, often measured by disappearance of the characteristic isocyanide stretch in IR spectroscopy was not determined as the large number of cyano groups of the liquid crystal template obstructed quantification. The very small amounts of polymer (approximately 0.5 ng per cell) impeded other standard polymer characterization techniques, such as molecular weight analysis. Following the formation of AzoPIC in chloroform did show the disappearance of the isocyanide stretch (Fig. S1, ESI†). Both 5 and AzoPIC have a slight influence on the thermal properties of the liquid crystalline matrix; the presence of these compounds in both situations lowers the nematic–isotropic transition temperature TNI of 5CB by 0.3 °C.
For the microscopy studies, we introduced the polymerizing mixtures in two types of cells: standard parallel-rubbed polyimide (PI)-coated glass cells and custom-made photopatterned cells (detailed information about the sample and cell preparation can be found in the ESI†). The latter were prepared by spin coating a thin layer of PMAz on an electrode (indium tin oxide, ITO) covered glass substrate and patterning the PMAz layers in following three consecutive irradiations steps (with three different polarization directions) using a photomask designed after the logo of our IMM institute (Fig. S2, ESI†). After filling the cells, we observed that the LC locally aligned with respect to the patterned command layer (Fig. 2a), where the direction of the 5CB mesogens is parallel to the photoaligned azobenzene moieties of PMAz (Fig. S3, ESI†). After a few hours, we observed the formation of micron-sized red colored bundles aligned parallel to the local LC director (Fig. 2b and c). Control samples with 5 in 5CB without catalyst present in rubbed polyimide (Fig. S4, ESI†) or photopatterned PMAz (Fig. S5, ESI†) cells did not show any sign of the formation of (aligned) red bundles. After several days, a parallel-rubbed polyimide cell with 5/5CB/catalyst solution was carefully opened, the LC was washed away and scanning electron microscopy (SEM) showed small bundles organized on the substrate in the rubbing direction (Fig. 3d).
The approx. 10 μm long structures are obviously longer and wider than single polyisocyanide chains and we expect them to be bundles of chains. These bundles are formed as a result of depletion interactions (which depend on time, temperature and the AzoPIC length and concentration) between the anisotropic LC solvent and 1D AzoPIC. Fig. 3c shows that the 5CB background however, is also still colored. This red shade originates from AzoPIC that is molecularly dissolved or in very small (microscopically invisible) bundles, or may come from unreacted isocyanide monomer as a result of the incomplete polymerization reaction. Further microscopy studies with polarized light showed that the red color is, in fact, polarization dependent and thus also aligns with the 5CB matrix (see backgrounds in Fig. 4).
The AzoPIC bundles, which are locally aligned to the template show strong linear polarized absorption characteristics. In Fig. 4, the polarization of the light source is rotated 360° in four 90° steps. When the polarizer is directed parallel to the anisotropic AzoPIC bundles, the bundles appear dark (Fig. 4a and b), caused by the strong absorbance of the D–π–A azobenzene dyes grafted on the polymer backbone. While rotating the polarizer perpendicular to the bundles, they become almost indistinguishable from their background as the absorbance is minimal (Fig. 4c and d). The azobenzene's main absorption band (at λmax = 497 nm) is attributed to the π–π* transition. The corresponding dipole moment (along the long axis of the molecule) thus is oriented parallel to the director of the 5CB LCT. This also means that the attached azobenzene moieties are oriented largely parallel to the PIC backbone, as is schematically represented in Fig. 1. The orientation of monomer, dissolved polymer and or small bundles follows the same trend and here the dyes are also oriented parallel to the director (see for instance in the backgrounds of the top areas of Fig. 4, panels a and c or the bottoms of panels b and d).
Through POM analysis, we quantitatively determined the switching times of the AzoPIC bundles in relation to their bundle length (Fig. 5f) by measuring the response rates of a number of individual bundles of different sizes. As could be expected based on their inertia, for smaller bundles the switching times are shorter while larger bundles respond slower. For the longest bundles, we anticipate that the bundles interact with the substrate, which can contribute to the observed slower in-plane to homeotropic transition. The response rate of the bundles is of the order of seconds, much slower than the polymer PFO nanowires in prepared the nematic host E7, for which 100 ms response times were recorded.8 The latter one-dimensional structures were smaller (2 μm) and were switched at a slightly higher electric field (0.6 V μm−1).
From the POM analysis, we also were able to measure the change in orientation of an AzoPIC bundle as a function of the field strength (Fig. 6). After a threshold field of approximately 0.05 V μm−1, the 14 μm long AzoPIC bundle rotates from an in-plane orientation into a complete homeotropic orientation at 0.43 V μm−1. At intermediate electric field strengths (between 0.1 and 0.43 V μm−1), the bundle adopts a stable partially rotated alignment where the degree of reorientation is related to the applied field. We find that this electric field range also depends on the length of the specific bundle.
Since in general, delicate molecular interactions between the LCT and the dispersed materials are absent, this hybrid approach can be applied to a wide range of other functionalized materials. Both the unique ability to dynamically switch the orientation of complex patterns of in-plane aligned materials and the adoption of photosensitive surfaces by the industry, should pave the way for the development and mass-manufacturing of soft matter based switchable optical and electric applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6tc02790c |
This journal is © The Royal Society of Chemistry 2016 |