Light-Adaptive Supramolecular Nacre-Mimetic Nanocomposites

Jul 25, 2016 - Indeed, light manipulation of mechanical properties has so far mostly focused on global changes in soft and elastic materials, such as ...
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Light-Adaptive Supramolecular Nacre-Mimetic Nanocomposites Baolei Zhu, Manuel Noack, Remi Merindol, Christopher Barner-Kowollik, and Andreas Walther Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02127 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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Light-Adaptive Supramolecular Nacre-Mimetic Nanocomposites

Baolei Zhua, Manuel Noacka, Remi Merindola, Christopher Barner-Kowollikb, Andreas Walthera,*

a

DWI-Leibniz Institute for Interactive Materials, Forckenbeck Str.50, 52074, Aachen, Germany b

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie,

Karlsruhe Institute of Technology (KIT) Engesserstr. 18, 76128 Karlsruhe, Germany and Institut für Biologische Grenzflächen (IBG), Karlsruhe Institute of Technology (KIT), Herrmann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. [email protected]

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Abstract: Nature provides design paradigms for adaptive, self-healing and synergistic highperformance structural materials. Nacre’s brick-and-mortar architecture is renowned for combining stiffness, toughness, strength and lightweightness. While elaborate approaches exist to mimic its static structure and performance, and to incorporate functionalities for the engineering world, there is a profound gap in addressing adaptable mechanical properties, particularly using remote, quick, and spatiotemporal triggers. Here, we demonstrate a generic approach to control the mechanical properties of nacre-inspired nanocomposites by designing a photothermal energy cascade using colloidal graphene as light-harvesting unit and coupling it to molecularly designed, thermoreversible, supramolecular bonds in the nanoconfined soft phase of polymer/nanoclay nacremimetics. The light intensity leads to adaptive steady-states balancing energy uptake and dissipation. It programs the mechanical properties and switches the materials from high stiffness/strength to higher toughness within seconds under spatiotemporal control. We envisage possibilities beyond mechanical materials, e.g. light-controlled (re)shaping or actuation in highly reinforced nanocomposites.

Keywords: Bioinspired Materials, Adaptive Materials, Supramolecular Polymers, Mechanical Properties, Toughness

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Biological composites with high levels of aligned reinforcements have spurred the quest for nextgeneration nanocomposites realizing unique combinations of stiffness and toughness by control of hierarchical structures and energy dissipation mechanisms. Particular success was achieved for nacre-mimetic materials,1-11 for which functional properties such as transparency, conductivity and gas barriers were added to realize multifunctional bioinspired nanocomposites.8, 9, 12 However, all of these approaches only consider static material properties, whereas natural materials can be highly adaptive to their environment and change their properties on demand and quickly. This can be found in the color adaptation of chameleon, or in the ability of the sea cucumber to change its body stiffness.13 Clearly, realizing an adaptation to external signals would provide new levels of control in bioinspired structural materials. A real advance requires pathways towards a rapid material response, and facile, external control mechanisms with high levels of spatial and temporal control. This is a particular challenge for bioinspired nanocomposites, because their high reinforcement content (typically above 50 wt%) only leaves little flexibility on manipulating the soft minority phase, whose molecular interactions are decisive in adaptive material settings. Light is a uniquely powerful external trigger as it provides remote, spatiotemporally and intensitycontrolled means to manipulate materials rapidly within the bulk, because light does not require diffusive processes.14, 15 Light-induced switches either rely on photochemistry, e.g. phototriggered dimerization/ligation/isomerization,16-18 or on photothermal effects generating heat via nonradiative relaxation of absorbing materials.14,

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While the former may suffer from

photodegradation and insufficient dynamics in a glassy bulk, the latter can be realized using mild NIR irradiation and is independent of bulk dynamics. Photothermal effects are most effective when targeting changes in mechanical properties upon coupling to thermo-reversible bonds or possibly order/disorder phase transitions.14, 15, 21-23, 25 Additionally, in terms of adaptation of properties of a material to the light intensity, it is important to conceptually distinguish between (i) the reachable properties of molecular photochemistry and (ii) 3 ACS Paragon Plus Environment

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the adpativness provided by the photothermal energy cascade. For classical photochemical reactions (e.g. photoinduced radical crosslinking, photoinduced dimerization or photochemical ligation)14, 26, an intermediate steady-state with a fractional level of conversion is only obtained at one point in time at a given light intensity and under continuous irradiation. Over time such reactions always run to the full conversion of their system depending on the intensity provided (see Supplementary Note 1 and Figure S1). Hence, classical irreversible photochemistry cannot result in light intensityadaptive properties that are stable under continued irradiation. The solution to reach materials adaptive to the light intensity, i.e. achieving a steady-state with fractional conversion, is to balance the constant energy uptake by an energy dissipation mechanism. While this may be in reach for solution-based materials using reversible photoisomerization of e.g. azobenzenes28, which already relax back at room temperature, such molecular relaxations are strongly hindered in bulk27, 28, and particularly difficult to engineer in highly reinforced, stiff nanocomposites. In contrast, our targeted photothermal energy cascade is not hindered by low bulk dynamics, and a dissipative effect is achieved by radiation of excess heat from the material as a function of laser intensity. A steady-state forms that balances energy uptake and energy dissipation. In summary, classical molecular photochemistry typically provide an “on/off switch” independent of the light intensity at sufficient time scales, while the photothermal cascade including the heat dissipation can provide adaptive steady-state properties as a function of the light intensity under continued irradiation – a process reminiscent of non-equilibrium self-assembly concepts, where transient states are only stable as long as the energy input is present.29 Indeed, light manipulation of mechanical properties has so far mostly focused on global changes in soft and elastic materials, such as photoinduced changes in liquid crystalline elastomers or hydrogels, and simple binary light switches (on/off) were in focus rather than realizing adaptive behavior.15, 16, 20-23, 30 However, even such simple light switches (on/off) have not been realized for stiff and strong bioinspired nanocomposites with low dynamics in the bulk and high reinforcement content. 4 ACS Paragon Plus Environment

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Here, we demonstrate a generic approach to manipulate the mechanical properties in highly reinforced bioinspired nanocomposites over orders of magnitudes by incorporation of a rapid photothermal energy transfer cascade. It is realized by co-assembling small amounts of thermally reduced graphene oxide (RGO) nanoplatelets within self-assembled nacre-inspired nanocomposites formed by high aspect ratio nanoclay and a supramolecularly linked soft polymer phase. The RGO serves as efficient NIR photothermal converter, allowing rapid, spatiotemporally controlled and localized heat generation within the nacre-mimetics used to break the supramolecular bonds. The breakage of the supramolecular bonds leads to a significant localized softening and controls stress relaxation properties as well as a stiff to tough transition in the mechanical properties. We describe in detail the photothermal effect, spatiotemporal control and its coupling to mechanical properties. This study demonstrates the first light-adaptive bioinspired nanocomposites with high levels of reinforcements and large changes in mechanical properties. The nacre-mimetics are hierarchically self-assembled from synthetic sodium fluorohectorite nanoclay (NHT, thickness ≈ 1 nm; aspect ratio ≈ 750)8 and poly(ethylene glycol methacrylate) copolymers, containing different molar amounts of thermo-reversible supramolecular 4-fold hydrogen bonding motifs (ureidopyrimidinone, UPy31, 32; 13 and 30 mol%; abbreviated U13, U30), in a 50/50 w/w ratio (Figure 1, Table S1). The polymers have relatively low degrees of polymerization (DPn,U13 = 162 and DPn,U30 = 137) and a low glass transition temperature (Tg,U13 ≈ -52 °C and Tg,U30 ≈ -34 °C) so that the main internal cohesion arises from supramolecular bonds, rather than entanglements.33 This leads to highly defined layered nanocomposites, in which the macroscopic mechanical properties are controlled by the molecular interactions, meaning the fraction of supramolecular bonds.9 To impart light-adaptive properties, we co-assemble a small fraction (1 wt%) of a thermally reduced graphene oxide (RGO, thickness 1 nm, aspect ratio ≈ 2000). The preparation of the RGO is described elsewhere,34 but its important characteristics are (i) the restoration of a large part of the aromatic framework for an efficient photothermal effect, and (ii) a tuned degree of reduction to maintain large fractions of unilamellar sheets in water (Figure S2). A key to providing 5 ACS Paragon Plus Environment

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a homogeneous photothermal effect throughout the thickness of the materials is to only use small amounts of RGO, which prevents excessive light adsorption on the top. We abbreviate the films as UXX-C-RGO, where RGO indicates the presence of 1 wt% RGO.

Figure 1. Light-adaptive nacre-mimetic nanocomposites exploiting a photothermal energy transfer cascade by connecting light-harvesting RGO colloids with supramolecular thermo-reversible bonds. (a) Formation of artificial nacre by aqueous hierarchical co-assembly of synthetic nanoclay (NHT, aspect ratio = diameter/thickness = d/t ≈ 750), reduced graphene oxide (RGO, d/t ≈ 2000) and Uxx copolymers containing 4-fold UPy hydrogen bonds (xx indicates the molar fraction of UPy). The weight ratios are [NHT]:[RGO]:[Uxx]=50:1:50 (b) Localized light irradiation leads to spatiotemporally controlled photothermal heating and light-adaptive breakage of supramolecular bonds on the molecular scale, and a macroscale material transition from stiff/strong to soft/ductile.

The incorporation of 1 wt% RGO proceeds smoothly and has no negative effect on the structure formation as seen by scanning electron microscopy (SEM) and x-ray diffraction (XRD). SEM displays highly aligned cross-sections resembling the brick-and-mortar structure of nacre (Figure 2a). The RGO nanosheets cannot be identified due to similar sizes as the nanoclay. Further quantification via 1D XRD reveals similar periodicities in all materials with multiple higher reflections of the primary diffraction peak (Figure 2b), confirming highly ordered lamellar structures with long-range order. The d-spacings of all nacre-mimetics center around 3.1 ± 0.1 nm. Therefore, differences in the mechanical properties can be directly related to the interactions of the 6 ACS Paragon Plus Environment

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components. The RGO converts the fully transparent nacre-mimetic (light transmission ~ 90%) into a strongly light-absorbing film, serving as the base for the photothermal effect (Figure 2c).

Figure 2. Structural characterization of the nacre-mimetic films. (a) Representative SEM of the cross section of U30-C-RGO displays a nacre-inspired, highly aligned, layered nanocomposite structure, scale bar: 5 µm. (b) 1D-XRD of all nacre-mimetics showing similar d-spacing as calculated from the primary diffraction peak. Pure NHT nanoclay is shown for comparison to demonstrate full delamination and absence of tactoids. (c) UV-Vis spectra of the nacremimetics films with and without RGO doping, thickness normalized to 25 µm. Inset shows a photograph of U13-C-RGO, scale bar: 2 mm.

Next we turn to the mechanical performance. To clearly depict the role of RGO we compare tensile testing curves of nacre-mimetics with and without RGO (Figure 3a). Overall it is evident that the amount of UPy governs the mechanical properties by providing supramolecular linkages in the matrix phase and tight adhesion to the nanoclay platelets. The materials containing U30 are very stiff and strong, and lack inelastic deformation, while those based on U13 display dynamics (stick/slip interactions, sacrificial bonds), inelastic deformation, and realize interesting combinations of high stiffness and toughness (Figure 3b). The toughness is also indicated by stable crack propagation despite 50 wt% nanoclay. The addition of RGO generally leads to slightly lower stiffness and higher ductility. One underlying mechanism for lower stiffness and increased ductility is enhanced interfacial dynamics at the RGO/polymer interface, as the interactions at the RGO(nonpolar)/polymer interface are lower compared to the nanoclay(polar)/polymer interface and can lead to slippage/frictional de-bonding and sacrificial bonding type of behavior. In summary, the structural and mechanical property characterization evidence well-defined RGO-doped nacremimetics, which span a remarkable property range with two distinct sets of attractive properties: (i) very stiff and strong (U30), and (ii) stiff and ductile/tough (U13). These are the foundation for 7 ACS Paragon Plus Environment

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understanding light-adaptive mechanical properties in diverse settings of nacre-mimetic materials, or bioinspired materials in general. Importantly, the stiffness and tensile strength values are orders of magnitude higher than in liquid crystalline elastomers or hydrogels, which have been previously targeted by light switching mechanisms.15, 16, 20-23, 30

Figure 3. Tensile mechanical properties of the nacre-mimetics. (a) Tensile testing curves and (b) comparison of Young’s modulus (E), tensile strength (σUTS), strain-to-failure (εb), work-of-fracture (wof, area under the curve). All tests are performed at 60 % relative humidity.8

We study the photothermal effect using an intensity-modulated fiber-guided NIR laser (808 nm). We focus on NIR light because it is comparably mild, as opposed to direct UV irradiation often needed for molecular photoswitches14-18, and because it is relevant for in vivo applications of such concepts in the long run. We use a FLIR camera (forward looking infrared) to record the spatiotemporal photothermal heating (at 90°), while the laser irradiates with an incident angle of ca. 70° at a beam diameter adjusted to 1 mm (Figure 4a). Figure 4b depicts a typical FLIR image series during irradiation at 35 mW (45 mW/mm2), showing concentric heat profiles that reach a steadystate balancing photothermal heating and dissipation.

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Figure 4. Photothermal effect in RGO-doped nacre-mimetic films (U30-C-RGO). (a) Schematic FLIR camera/laser setup. (b) FLIR images of U30-C-RGO irradiated at 35 mW (scale bar: 4 mm). (c) Time-dependent photothermal heating for varying laser output power. (d) Intensity-dependent cross-sectional temperature profiles after 10 seconds. (e) Steady-state temperature vs. laser output power. (Absence of RGO does not allow heating as shown in Figure S3).

A time-dependent monitoring of the peak temperature in the center for different laser intensities quantifies the photothermal heating rates (Figure 4c). All heating curves show similar profiles with a swift temperature increase in the first 2-3 seconds, followed by the steady-state. The final temperature can be controlled between 40-160 °C depending on the laser intensity, which is in the range where the UPy dimerization breaks.32 Higher temperatures can be reached using laser powers above 40 mW or higher RGO contents (not considered yet, because 1 wt% RGO suffices), but we restrict the temperature to 160 °C to prevent polymer degradation. There is no significant heating in the absence of RGO (Figure S2). Strikingly, the development of the dissipative steady-state is a key advantage for adaptive materials over molecular photocrosslinking, because photocrosslinking reactions would always run to maximum conversion with increasing illumination time (independent of the intensity, but on different time scales) and cannot maintain a steady-state with only fractional conversion (Supplementary Note 1).

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A plot of the quasi-equilibrium temperature of the plateau region versus laser output power shows a strictly linear increase, demonstrating a high level of predictability (Figure 4e). This heating is homogeneous throughout the thickness of the film (as checked by FLIR monitoring on the backside) because we only use small amounts of RGO to prevent excessive light adsorption on the top. Further studies on the dependence on the material thickness and high degrees of RGO loading are underway. More importantly, however, the heat is spatially confined to the dimensions of the laser spot and there is no excessive lateral heat transport into non-irradiated areas, as can be seen from the intensity-dependent cross-sectional profiles (Figure 4d). The profiles are close to a Gaussian curve and the full width at half maximum (FWHM) center around 1.45 mm, corresponding well to the beam diameter of 1 mm. Excess lateral heat transport is intentionally prevented using only small amounts of the RGO to avoid percolation. From an adaptive materials perspective, this is relevant because it shows that the photothermal effect cannot only be temporally modulated with a rapid response time (< 3 s), but also occurs with great spatial definition. Focusing the attention now on the objective to prepare light-adaptive bioinspired nanocomposites, we discuss first stress relaxation measurements under spatially localized irradiation, and subsequently turn to tensile tests under global light irradiation. Relaxation tests are powerful tools to understand molecular relaxation processes in bulk materials. Therefore, we stretched both RGO-doped nacre-mimetics, U13-C-RGO and U30-C-RGO, to ε ≈ 0.9 % (before the yield point), in a tensile tester and allowed them first to relax for 20 s at ambient conditions, and thereafter for another 20 s with spatially confined laser irradiation using a 1 mm spot shining on the specimen center. As discussed above, changing the laser intensity modulates the local temperature only at this spot with a steady-state heating plateau reached in less than 3 s. Figure 5a,b display the temporal evolution of the stress, which show a two-step decay profile corresponding to stress relaxation at ambient conditions and under light irradiation. During the first relaxation period, elastic stress is released due to molecular reorientation of chains, which are above 10 ACS Paragon Plus Environment

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their Tgs. The process reaches a quasi-plateau after ca. 15 s. The ambient relaxation is comparably slow, and a strongly accelerated relaxation takes place upon light irradiation. Importantly, the extent of relaxation in both parts is inverse in both materials, and in fact reminiscent of their different tensile behavior. While U13-C-RGO (stiff and tough/ductile) undergoes comparably strong relaxation in the first period and smaller relaxation in the second period, U30-C-RGO (very stiff and strong) shows only a minor relaxation at ambient conditions, but a very strong stress release once the photothermal effect is triggered. Note that both polymers have nearly the same molecular weight and similar Tgs, hence differences in their relaxation behavior can be directly linked to the extent of supramolecular binding. Furthermore, U30-C-RGO displays light-adaptive stress relaxation and the finally achieved plateaus depend on the light intensity, which confirms that the supramolecular bonds are dynamized and opened up (Figure 5b, c). The light-accelerated dynamics favor a larger scale reorientation of the otherwise tightly physically linked chains. A critical temperature can be observed (ca. 80 °C), whereafter the stress drops only insignificantly further. Hence, below this temperature, the fraction of effective UPy/UPy linkages depends on the temperature, whereas when exceeding this point, the UPy/UPy dissociation is practically complete and does not enhance the relaxation processes anymore. This temperature range corresponds well with the dissociation temperature reported for this UPy motif in polyacrylate bulk material.32

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Figure 5. Light-adaptive mechanical properties under (a-e) spatially confined and (f-g) global irradiation. (a-b) Relaxation tests of (a) U13-C-RGO and (b) U30-C-RGO as follows: Stretching to ε ≈ 0.9 %, relaxation for 20 s, and intensity-controlled laser irradiation in the specimen center (beam diameter 1 mm) for 20 s (laser on); further relaxation for 20 s (laser off); re-stretching of the full specimen till failure. (c) Stress decay vs. laser-intensity/temperature during laser irradiation (12) (d) Strain fields monitored by digital image correlation of a related relaxation test of U30-CRGO irradiated at 30 mW with shortened first halt period (5 s). Time-laps strain maps depict the start, the halt stage (first relaxation), and after the laser irradiation. The FWHM of the laser beam is indicated in d as a black circle at the area of A. The strain color code is from 0 – 3.3 % (e) Time-dependent average strain values in the two areas extracted from DIC in (d). A: laser-irradiated area, B: non-irradiated area. (f) Light-adaptive tensile properties of U30-C-RGO achieved by global laser irradiation to the temperature indicated, and (g) corresponding Young’s modulus, E, and workof-fracture, wof. Lines in c,e,g are guides to the eye.

The localized strain field adaptation can be further highlighted from in-situ digital image correlation (DIC) of a stochastic speckle pattern sprayed on a U30-C-RGO specimen surface during a relaxation test (Figure 5d,e). Note that the first relaxation period is shortened to 5 s to maximize relaxation during the laser irradiation. The strain field is homogeneous at the start and mostly homogeneous during the first relaxation. The slightly higher strain in the center part may be due to the dogboneshaped specimen. However, once the laser is switched on (here 30 mW), the strain fully 12 ACS Paragon Plus Environment

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concentrates as inelastic deformation in the irradiated spot, A. This even leads to some relaxation at the outer parts of the specimens, B, due to recovery of stored elastic stress. Hence DIC unambiguously confirms a high level of spatial control. For the characterization of the global light-adaptive tensile properties, we use a beam spreader to cover the full tensile specimen, and higher laser powers to compensate the spatial intensity loss. We focus the investigation on U30-C-RGO, because this material opens the possibility for large scale tunability starting from very stiff and strong mechanical behavior. Figure 5g,h summarize the lightadaptive tensile curves at different irradiation intensities, but listed for the reached surface temperatures. At room temperature and in absence of the light, the nacre-mimetic behaves as a very stiff and strong material with high stiffness, high tensile strength and a small elongation at break (1.3 ± 0.2 %). Strikingly, upon light irradiation, the material continuously adapts its mechanical performance to the light flux and obtained temperature. A considerable softening occurs, which can be traced by lower Young’s moduli and lower maximum strengths. The stiffness shows a distinct drop at ca. 60 °C. This indicates that UPy/UPy dimers are dynamic at this deformation rate and transition into the opened state in this regime, again corresponding to the dissociation regime for this UPy derivative.32 More importantly, a yield point occurs and the material is capable of undergoing significant inelastic deformation. The strain-to-failure reaches up to 4.0 ± 0.3 %, which is roughly 300 % larger than the untreated film, and the work-of-fracture, as an estimate of toughness, has a maximum before the drop in stiffness occurs, hence in a region where significant molecular exchange of UPy/UPy connections occurs. In addition, stable cracks occur at temperatures above 45 °C, again corresponding to more dynamic and more loosely bound situations as found above in U13-C-RGO at room temperature (Figure 3a). Even though one could argue that the very long inelastic deformation of the U13-C-RGO nacre-mimetic cannot be reached yet (Figure 3), it needs to be kept in mind that U13-C-RGO is one of the most ductile and toughest clay-based nacre-mimetics prepared so far,7-9 13 ACS Paragon Plus Environment

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and that photothermal heating changes the dynamics in the polymer structures differently than reducing the fraction of UPy units at constant temperature. We suggest that adjusting the dynamics and transition temperature of the supramolecular bonds will provide key pathways to tailoring the light-adaptive tensile behavior further. In conclusion, we demonstrated a generic and simple energy transfer concept to impart fast, remote, spatially controlled, light-adaptive modulation of mechanical tensile properties in highly reinforced, supramolecular nacre-mimetics by co-assembly of small amounts of RGO. The formation of the light-adaptive, dissipative steady-state represents a key advantage over molecular photochemistry as the same molecular state can be maintained independent of the irradiation time. The use of low Tg, self-healing polymers with different amounts of thermo-reversible supramolecular motifs is crucial for understanding and realizing large property changes. This molecular design opens ample possibilities to engineer future bioinspired light-adaptive systems, also of other architectures, and targeting advanced temperature response using combinations of hydrogen bonds of different strength or strong glass formers (vitrimers) with fundamentally different temperature response. While we now realized spatially confined light-adaptive strain field engineering, and a transition from a stiff/strong nacre-mimetic to a softer and tougher one on a material level, we foresee these principles to be also relevant for on demand shaping/reshaping of bioinspired nanocomposites, for highly reinforced actuators or adaptive gas separation. ASSOCIATED CONTENT

Supporting Information. Materials and Experimental Details. Supporting Note on “Adaptation to light intensity”. TEM of RGO. Heating curves of materials without RGO. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENT

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We acknowledge financial support from the Volkswagen foundation. This work was performed in part at the Center for Chemical Polymer Technology, supported by the EU and North RhineWestphalia (EFRE 30 00 883 02). We acknowledge R. Mülhaupt for providing RGO. C.B.K. acknowledges additional support from the Helmholtz association for the Science and Technology of Nanosystems (STN) and the BioInterfaces in Technology and Medicine programs (BIFTM). AUTHOR INFORMATION

Corresponding Author *[email protected]. REFERENCES

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