Redox- and mechano-chromic response of metallopolymer-based elastomeric colloidal crystal ﬁ lms † J. Mater. Chem. C , 2014,

A novel and convenient route for the preparation of monodisperse ferrocene-containing core-interlayer-shell particles using emulsion polymerisation protocols is described. These metallopolymer-based particles can be used to produce highly ordered elastomeric opal ﬁ lms with fascinating distinct re ﬂ ection colours. The present work additionally describes the combined addressability of both stimuli, redox-chemistry and mechanical stress. The obtained materials are interesting since application of the melt-shear process followed by crosslinking of the matrix provides access to large-area, mechano-responsive elastomeric opal ﬁ lms featuring an additional redox response. Both, basic synthesis aspects and ﬁ rst steps towards application in the ﬁ eld of stimuli-responsive sensing of respective materials are discussed.


Introduction
Recent efforts in the eld of tunable colloidal crystals based on monodisperse organic or inorganic particles featuring a stimuliresponsive core-shell structure or a polymer matrix have led to fascinating stimuli-responsive materials. These materials are promising candidates for, e.g., opto-electronic sensing devices or as actuation systems. [1][2][3][4][5][6][7] By applying external triggers, i.e., by changing the solvent, temperature, ionic strength, light, or by applying an electrical eld or mechanical stress, the lattice distances and/or the refractive index of these materials can be tremendously inuenced. The response of such materials is accompanied by a remarkably fast change of the optical properties. A great advantage of colloidal crystal structures is their inexpensive and convenient bottom-up preparation resulting in a good optical performance with iridescent reection colours caused by Bragg diffraction of visible light. [8][9][10][11][12][13][14][15] For example, a crystalline colloidal array of hydrogel-based photonic crystals has been applied as a sensor showing distinct diffraction colour changes when treated with different transition metal cations or by changing the pH value. 16 Very recently, Zhao and co-workers reported the variation of the lattice distances in elastomeric polymeric opals in dependency of an applied voltage. 12 However, manifold strategies were reported to obtain wellordered and large-scaled 3D arrays based on colloidal spheres.
Drying or spin coating of the particle dispersions are the most common and convenient methods thereof. [17][18][19][20] On the other hand, the so-called melt-ow technique allows the convenient preparation of perfectly ordered polymer and silica particles in elastomeric lms. The process for the convenient preparation of large-area self-supporting opal lms involves the compression of the monodisperse particles between the plates of a press. [21][22][23] The melt-shear technology allows the organization of such coreshell particles consisting of a rigid core and a so, elastomeric shell into a highly ordered fcc lattice. The so shell is a basic prerequisite for this process. Elastomeric polymer-based colloidal crystals, also referred to as opal lms, show a reversible mechanochromic response while stretching with excellent colour changes due to a modication of the 111 (200) plane spacing. 24,25 Electrically addressable photonic band gap materials have garnered a great deal of attention in the last few years. Intrinsically conductive polymers such as polyaniline, polypyrrole or polythiophene could be used for this purpose. [26][27][28] Compared to those, reports about stimuli-responsive colloidal structures based on metallopolymers are rather scarce. A major breakthrough in synthesis pathways leading to manifold applications for ferrocene-containing polymers was achieved by Manners' discovery of the ring-opening polymerization of ansaferrocenophanes. [29][30][31][32] The most prominent example regarding metallopolymers and colloidal crystals was reported by Arsenault et al. The material properties investigated by this group was based on the redox-activity of poly(silaferrocenophane) (PFS) derivatives as matrix materials. 13,33 The optical Bragg diffraction of the embedded silica spheres can be varied by oxidizing the ferrocene moieties inducing an increase in the lattice distances.
Due to recent efforts in the synthesis of metal-containing polymers featuring the ferrocene motif in the side-chain, the rst steps towards potential applications have been investigated in the last few years. [34][35][36][37] Within this contribution, we now expand the eld of side-chain ferrocene-containing polymers to novel photonic band gap elastomeric lms with combined redox-and mechanochromic properties. The controlled bottomup synthesis of monodisperse redox-responsive particles featuring the ferrocene moieties in the particle shell is accessible using convenient one-pot emulsion polymerization protocols using 2-(methacryloyloxy)ethyl ferrocenecarboxylate (FcMA) as a comonomer with ethyl acrylate (EA). Dynamic light scattering (DLS) is used to prove the tremendous swelling behaviour of the polymer particles aer chemical oxidation. Furthermore, the melt-shear technique can be applied to prepare large-scaled redox-responsive opal lms which are investigated regarding their combined optical and mechanooptical properties. Our studies provide a convenient method to prepare reversibly and individually addressable well-ordered opal lms with fascinating mechanical and redox properties. We expect these functional lms to be promising candidates as next generation materials with potential applications in elds of optoelectronic and sensing devices.

Results and discussion
Free radical (co)polymerisation of FcMA With regard to the goal of manufacturing a redox-responsive elastomeric opal lm with all its advantages, the use of 2-(methacryloyloxy)ethyl ferrocenecarboxylate (FcMA) for the bottom-up synthesis of core-shell particles obtainable via emulsion polymerisation was investigated. First experiments for the free radical homo-and copolymerisation of the FcMA monomer were carried out to prove the suitability in emulsion polymerisation.
Ferrocene-containing monomers, particularly featuring an electron-rich ferrocene motif, are known for having issues in free radical polymerisation. The radical propagating chain is able to undergo a single electron transfer as evidenced by Georges and Hayes for the vinylferrocene (VFc) monomer. 38,39 The ability of FcMA to be (co)polymerised with monomers such as ethyl acrylate (EA) in a radical manner is essential for its use in emulsion polymerisation. Studies on polymerisation of the FcMA monomer via ATRP protocols have recently been reported. 34,40,41 However, more important for the successful manufacture of elastomeric opal lms by a convenient melt-shear procedure concerns the resulting glass transition temperature (T g ) of the shell polymer which builds the matrix in the nal opal lm. Thus, the free radical copolymerisation of FcMA with ethyl acrylate (EA) is investigated beforehand in this work. Both, homopolymerisation of FcMA and copolymerisation with EA were conducted in solution with AIBN as a thermal initiator. The resulting polymers were characterised by using size-exclusion chromatography (SEC, Fig. S1 †), DSC ( Fig. 1) and NMR (Fig. S2 †). The experiments succeeded with respect to reasonable molar masses and monomer ratios of FcMA and EA used in the corresponding copolymers (see ESI †). The T g of the copolymer obtained (PFcMA 15% -co-PEA 85% ) and the corresponding homopolymers (PFcMA and PEA) 22,34 are compared by using DSC measurements (Fig. 1) proving the successful copolymerization yielding a melt-ow processable ferrocene-containing polymer. Due to the moderate T g of 11 C for the shell polymer, the obtained lm will also show elastomeric behaviour aer appropriate matrix crosslinking.

Emulsion (co)polymerisation of the FcMA monomer
Since the monodispersity of core-shell particles is a basic prerequisite for colloidal crystallisation, the introduction of the FcMA monomer in emulsion polymerisation is a crucial step in order to produce a ferrocene-containing elastomeric opal lm in the next step. Fig. 2A (top) shows schematically the stepwise bottom-up growth of the core-interlayer-shell (CIS) particles starting from hard PS core particles followed by the formation of a slightly crosslinked interlayer. In the nal step, FcMA was copolymerized with EA to generate the so, redox-responsive shell. Evidence for the desired monodispersity and the attainment of an appropriate particle diameter is given by TEM images (Fig. 2). Additionally, the growth was followed by dynamic light scattering (DLS) aer each synthetic step (Fig. 2).
The DSC measurement in Fig. S3 † shows a T g for the synthesized particles above the T g of PEA, conrming the statistic incorporation of FcMA into the shell polymer. Furthermore, the T g is still lower than the ambient temperature for the desired lm properties. The second T g above 110 C can be assigned to the crosslinked PS cores.
Altogether, we were able to synthesize monodisperse CIS particles with redox responsive FcMA in the shell polymer and the obtained latex matches in all requirements for the manufacture of elastomeric opal lms via the melt-shear procedure. Before we turn to the investigation of redox-responsive behaviour of the particles, the attempts for opal lm preparation will be described in the next section.

FcMA containing elastomeric opal lms
The next step aer the successful synthesis of FcMA-containing CIS particles was fabrication of elastomeric opals via the meltshear technique. Therefore, the FcMA-containing particles were precipitated and manufactured into elastomeric opal lms in the melt-shear process based on previously described routes. 21,22 For elastomeric behaviour and, thus, for the mechanochromic response, benzophenone as an UV-active crosslink agent was homogeneously incorporated during the manufacturing process and the nal lm was irradiated by UV light.
The average particle diameters were chosen to be 176 AE 5 nm for the core and 269 AE 6 nm for the CIS particles respectively as determined by using DLS measurements (Fig. 2). These particle diameters correspond to a green reection colour of the prepared lms at normal light incidence (angle of light incidence q ¼ 90 ) for the nal multi-stimuli responsive material. The size of the redox-responsive CIS particles can be adjusted in the range of 150 to 500 nm. The obtained colours conrmed expectations ensuing from the determined particle size as can be calculated by using Bragg's law. The resulting lm disk revealing diameters of about 15 cm show remarkably brilliant reection colours which can be switched by changing the angle of view or by stretching the lms to a dened strain 3. Photographs of the obtained opal lm featuring the expected reection colours are shown in Fig. 3A and B. In contrast, the non-ordered actual material prior to the applied melt-shearing process revealed a brownish yellow colour.
Additionally, the optical properties of the obtained lms were characterised by UV/VIS spectroscopy to conrm the optical similarity of the new material to conventional elastomeric opals. Fig. 3C shows the typical blue shi when decreasing the angle of incidence from 90 to 30 . The blue shi induced by the strain up to 50% is depicted in Fig. 3D and can be assigned to the decreasing 111 lattice plane spacing of the colloidal crystal structure.
Chemical redox-response of FcMA-containing latex particles In the next step, the redox-response of FcMA-containing CIS particles was studied to evaluate the possibility of redoxinduced swelling of the particles inside the elastomeric colloidal crystalline structure. Therefore, the oxidation of CIS particles in aqueous media is supposed to cause signicant swelling behaviour as illustrated in Fig. 4 comparable to previous results obtained for PFcMA-brushes immobilized using organic polystyrene particles. 34 The polymer shell of CIS particles designed for the preparation of elastomeric opal lms tended to peel off aer oxidation (as evidenced by TEM measurements, Fig. S4 †). This phenomenon is caused by the absence of any crosslinking agent in the so polymer shell. Thus, it is worthy to note that a crosslinking of the polymer shell with allyl methacrylate (ALMA) was sine qua non for oxidation experiments, since only a few Fig. 2 Scheme of built-up particle architecture (top), TEM images of each stage of synthesis and the stepwise particle growth obtained by using DLS measurements.  polymer chains were graed to the interlayer. Concerning these aspects, emulsion polymerisation with a modied mechanism was conducted, resulting in latex particles whereas the shell polymer consists of 82% EA, 15% FcMA and 3% ALMA (experimental details are given in the ESI †).
Chemical oxidation of the obtained ferrocene-containing latex was carried out by treatment with FeCl 3 in water. Collaborative TEM studies revealed the stability of the PFcMA-containing latex aer oxidation and subsequent reduction with ascorbic acid (Fig. 5). These results prove the feasibility of the PFcMA-containing particles to be reversibly addressable by the redox stimulus used. DLS measurements of the particle dispersion before and aer oxidation prove the successful redox-induced switching of the ferrocene/ferrocenium moieties (Fig. 6). The particle polydispersity was found to be 9.2% before oxidation and 15.3% aer, calculated using a cumulant analysis. The correlation time s is related to the translational diffusion coefficient D by D ¼ 1/(sq 2 ). The expected linear variation of 1/s with q 2 was well represented by our data. By using the Stokes-Einstein relation a hydrodynamic radius R h ¼ k B T/6phD could be calculated, with k B , T, and h as the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. Oxidised PFcMAcontaining particles show a tremendous increase of additional 24 nm of the hydrodynamic radius R h suitable for intended swelling-induced optical changes for the novel opal lms.
These results unambiguously prove the redox-induced swelling capability of CIS particles in water.
In summary, the redox properties of the CIS particles in water enabled a signicant increase of the particle diameter aer oxidation due to the oxidation of the ferrocene moieties. These oxidation protocols were applied for switching the opal lm properties regarding their polarity and underlying lattice plane spacing accompanied by a signicant change of reection colours as described in the following sections.
Redox response of PFcMA-containing elastomeric opal lms Treatment of the opal lm with FeCl 3 in THF caused a more homogeneous invasion of the oxidation agent into the lm compared to the rst attempts with FeCl 3 in water. The original lm was capable of swelling in THF but not in water. Hence, oxidation had to be carried out in a good solvent for the particle shell. In the dry state, the lm retains its original green reection colour independent of the presence of an oxidation reagent used (Fig. 7). That means that in the bulk state, the chemically induced swelling was negligible.
Interestingly, the dried opal lm revealed a tremendous change of its reection colour from initial green to red aer treatment with water caused by the swelling of the oxidised matrix polymer (Fig. 8).   6 Intensity autocorrelation functions for the original (A) and oxidised (B) PFcMA-containing particles in water at 2q: 50-130 , measured at 20 C. The solid lines represent the fit of the data using a cumulant analysis. (C) q 2 dependence of inverse relaxation times to determine the diffusion coefficient and the resultant hydrodynamic radii for different particles before and after oxidation. Prior to oxidation, the hydrodynamic radius could be determined to be 142 nm (284 nm diameter) and 166 nm (332 nm diameter) after oxidation. Errors for the hydrodynamic radii are less than 2%. Fig. 7 Photographs of the PFcMA-containing elastomeric opal films before and after chemical oxidation followed by drying. The optical response in the presence of an oxidation reagent was negligible in the dry state. A blank test (treated with THF only) is given for comparison. The water induced colour change of the oxidised opal lm is fully reversible, once the residual moisture is evaporated. Time dependence and reversibility are determined by UV/VIS reection measurements. The shi of the Bragg peak dependent on time, and therefore, dependent on the swelling degree is demonstrated in Fig. 9.
The UV/VIS spectra shown in Fig. 9 (top) conrm a swelling induced reection colour shi of slightly more than 100 nm, which means a change from green to red reection colour under normal light incidence. It is clearly observable that the initial Bragg peakcorresponding to a green wavelength rangeis decreasing immediately in the presence of water. A second signal appears at $600 nm and moves up to 690 nm upon further swelling.
Also, the spectra of the oxidised lm prior to swelling and aer water evaporation (Fig. 9 bottom) conrm the complete reversibility of the swelling process featuring the original stop band (shied less than 5 nm) aer relaxation. The 3D structure persisted due to the crosslinks of the matrix polymer.
Besides the convenient large-scalable opal lm manufacture, the mechanical response of the lm is an important advantage compared to other colloidal crystal materials. Concerning this aspect, the combination of redox and mechanical responses is of particular importance. Since the elasticity is due to matrix polymer crosslinking, the mechanochromic behaviour is still available in the oxidized and the swollen states. The individual addressability of combined stimuli is schematically depicted in Fig. 10.
As shown in Fig. 10, the swelling process as well as the mechanical deformation of the elastomeric opal lm leads to a change of lattice distances. This change is responsible for the resulting angle dependent reection colour.
For the determination of the strain induced colour shi, the ferrocenium-containing lm was swollen in water and xed in a stretch gadget (see Fig. 11, top). The corresponding UV/VIS reection measurements are shown in Fig. 11.   The UV/VIS reection spectra of swollen, oxidised lms conrm the mechanocromic behaviour of this novel metallopolymer-based photonic crystal material.

Electric response of PFcMA-containing elastomeric opal lms
For the rst steps towards future applications of herein investigated redox-responsive elastomeric lms in elds of sensing applications and display technologies, the electro-chemical response of the obtained opal lms was investigated by cyclic voltammetry experiments. The corresponding cyclic voltammograms are given in Fig. 12. For these experiments the lm was xed on a conductive indium tin oxide (ITO) plate in the presence of tetrabutylammonium hexauorophosphate as the electrolyte in acetonitrile.
The cyclic voltammograms show the typical redox signals for ferrocene-containing polymers with minima and maxima lying in between 400 and 900 mV as previously reported for the diluted polymers. 34 The endurance of the manufactured material during 80 iterations conrms the reversible redox responsiveness of ferrocene repetitive units in the opal lm matrix. These rst experiments unambiguously reveal the suitability for the as-prepared lms to be repeatedly addressable by applying a current which gives rise to manifold future applications in the eld of so sensing.

Conclusions
The ferrocene-containing monomer 2-(methacryloyloxy)ethyl ferrocenecarboxylate (FcMA) was successfully radically polymerised leading to monodisperse metallopolymer-based stimuli-responsive particles featuring a core-interlayer-shell (CIS) architecture by applying convenient emulsion polymerisation protocols. These novel materials were excellent candidates for a feasible preparation of large-scaled elastomeric opal lms by the so-called melt-shear technique featuring distinct iridescent reection colours corresponding to Bragg's law. Essential for the successful lm preparation-which was exclusively based on the particle architecturewas the narrow particle size distribution as well as an appropriate glass transition temperature of the CIS-building segments. For this purpose, parameters for copolymerising FcMA and ethyl acrylate (EA) monomers in solution and in emulsion polymerisation were thoroughly investigated and successfully adjusted to the process. The ferrocene-containing repetitive units revealed the intrinsic capability to change the particle polarity aer oxidation accompanied by tremendous particle swelling which was investigated by using DLS measurements before and aer chemical oxidation. The nally prepared ferrocene-containing elastomeric opal lms were studied regarding their mechanochromic response showing brilliant and iridescent reection colours corresponding to Bragg's law. The excellent redoxresponsive behaviour for the particles in dispersion and for the elastomeric opal lm was evidenced by DLS measurements and UV/VIS reection spectroscopy, respectively. Moreover, we were able to conrm the typical mechanochromic behaviour of the obtained opal lms in the dry state prior to and aer oxidation. For this purpose, different strains for a water-swollen oxidised ferrocene-containing lm were applied showing the individual addressability of both stimuli used, redox chemistry and mechanical stress. The fully reversible ferrocene reduction/ oxidation cycles inside the prepared opal lms were studied by cyclic voltammetry for at least 80 cycles. We expect herein prepared novel functional materials as interesting candidates for manifold sensing applications and future display technologies based on stimuli-responsive polymeric materials.

Reagents
Styrene, EA, and butanediol diacrylate (BDDA) were obtained from BASF SE; ALMA and MMA from Evonik Rohm GmbH; and Dowfax 2A1 from Dow Chemicals. FcMA was synthesised according to the literature. 30 All other chemicals were purchased from VWR and Sigma Aldrich and used as received, if not otherwise stated. Prior to use in emulsion polymerisation, the stabilisers were removed from the monomers. For this purpose, EA and MMA were extracted with 1 M sodium hydroxide solution, washed with water until the solution was neutral, and then dried over sodium sulfate. S was distilled under reduced pressure. ALMA and BDDA were destabilised using an ion exchanger (De-Hibit 200, PolySciences Europe GmbH).
Instrumentation NMR spectra were recorded on a Bruker DRX 500 spectrometer working at 500 MHz ( 1 H NMR). NMR chemical shis are referenced relative to tetramethylsilane. Standard SEC was performed with THF as the mobile phase (ow rate 1 mL min À1 ) on a SDV column set from PSS (SDV 1000, SDV 100000, SDV 1000000) at 30 C. Calibration was carried out using PS standards (from Polymer Standard Service, Mainz). TEM experiments were carried out on a Zeiss EM 10 electron microscope operating at 60 kV. All shown images were recorded with a slowscan CCD camera obtained from TRS (Tröndle) in bright eld mode. Camera control was computer-aided using the ImageSP soware from TRS. CV measurements were carried out on a multi-potentiostat VMP2 (Princeton Applied Research) with a custom-made cell in dry acetonitrile with tetrabutylammonium hexauorophosphate (TBAHFP) as the electrolyte (0.1 M) under a nitrogen atmosphere. The Ag/AgCl reference electrode and Pt counter electrodes were chosen and a scan rate of 100 mV s À1 in a range of À0.2 V to 1.6 V was applied. Dynamic light scattering (DLS) experiments were carried out with a set-up based on a He-Ne laser (l ¼ 632.8 nm) as the light source. Polarization of the primary beam is dened using a Glan-Thomson prism. The scattered beam polarization was analyzed in the verticalvertical geometry. The scattered intensity was detected with an optical ber coupled to two avalanche photodiodes. The intensity autocorrelation functions, calculated with ALV 5000 soware for both photodiodes, were measured in steps of 20 , usually in the angle range between 2q ¼ 50 to 130 . The measurement time for each angle was 5 min. All measurements were performed at 20 C using cylindrical cuvettes (Hellma) for the samples in a temperature controlled index matching bath. DSC measurements were recorded with a Mettler Toledo DSC1 from À50 C to 100 C with a heating rate of 10 K min À1 . Additional DLS measurements of the particles were performed on a Nanophox photon cross-correlation spectrometer (Sympatec). The experiments on diluted dispersions of the particles aer each step of the synthesis were carried out at an angle of 90 at 20 C. Reection spectra were recorded using a vis-NIR ber spectrophotometer (USB 4000, Ocean Optics). For reection measurements a deuterium/tungsten halogen lamp (HL-2000, Ocean Optics) was used. Reection measurements as a function of strain 3 respectively in air and water were carried out at normal light incidence. Therefore, a 1 Â 2 cm strip of the lms was uniformly stretched with a custom-made micrometrecontrolled sample holder, while the optical properties were measured simultaneously. Angle-dependent reection measurements were carried out using a custom-built goniometer setup. Swelling of the oxidized opal lm was tracked aer setting a drop of water on the top of the opal lms. For the timedependent measurement of the swelling process at room temperature (22 C) spectra were recorded at intervals of 1 min. Reversible switching of the stop band was followed by recording reection spectra aer each cycle.

Synthesis of CIS particles
2-(Methacryloyloxy)-ethyl ferrocenecarboxylate (FcMA) containing CIS particles were synthesised by starved feed emulsion polymerization. First, the PS cores were produced in a 1 L vessel equipped with a stirrer, a reux condenser and nitrogen feeding at 75 C as follows. An ice cooled mixture of 320 mg sodium dodecyl sulfate (SDS), 280 g deionised water, 3.6 g styrene and 0.4 g BDDA was lled in the vessel, immediately followed by the addition of 36 mg sodium disulte (NaDS), 520 mg sodium persulfate (NaPS) and 36 mg NaDS in this order (each component is dissolved in 5 mL water). Then, 10 min aer clouding, an emulsion of 230 mg SDS, 400 mg potassium hydroxide (KOH), 220 mg Dowfax 2A1, 90 g water, 70 g styrene and 7 g BDDA continuously added with a rate of 1 mL min À1 . Aer complete addition of monomer emulsion, the reaction was nished by holding the temperature for an additional hour. The CIS particle synthesis was conducted in a 250 mL vessel with the same equipment at 75 C. 113.75 g of occurred PS latex was lled in the vessel. Aer adding 25 mg NaDS, 100 mg NaPS and 25 mg NaDS (dissolved each in 3 mL water), the monomer emulsion containing 13 mg SDS, 53 mg Dowfax 2A1, 8 g deionised water, 5.55 g EA and 0.75 g ALMA was added with a ow rate of 0.25 mL min À1 , followed by the addition of 100 mg SDS, 100 mg KOH, 300 mg Dowfax 2A1, 40 mg water, 30.17 g EA and 5.33 g FcMA under similar conditions. Aer complete addition of monomer emulsion, the reaction was nished by maintaining the temperature for an additional hour.

Opal lm preparation
For the preparation of elastomeric opal lms, the obtained CIS particles were precipitated in methanol, containing a small amount of saturated sodium chloride solution. The precipitate was ltered, water washed and dried under reduced pressure. The resulting elastomeric mass was compacted and mixed with benzophenone (3 wt%) in a microextruder (microl, DSM Research) at 130 C. For enhanced colour contrast, 0.15-0.5% of carbon nanotubes (BAYTUBES C 150 P, Bayer Material Science) were additionally incorporated. A 3 g portion of the rubbery polymer was covered with a PET lm and heated to 130 C between the plates of a Collin 300E laboratory press. Melt ow was induced by applying 150 bar for 3 min, resulting in an elastomeric opal lm of about 15 cm in diameter. Finally, the lm was irradiated with an industrial-type mercury lamp (UV Cube 2000, Dr Hoenle) with an output power of 1000 W. The PET-covered lms were treated at a distance of 4 cm for 2 min each side.