Letter | Published:

Photoswitching topology in polymer networks with metal–organic cages as crosslinks

Abstract

Polymer networks can have a range of desirable properties such as mechanical strength, wide compositional diversity between different materials, permanent porosity, convenient processability and broad solvent compatibility1,2. Designing polymer networks from the bottom up with new structural motifs and chemical compositions can be used to impart dynamic features such as malleability or self-healing, or to allow the material to respond to environmental stimuli3,4,5,6,7,8. However, many existing systems exhibit only one operational state that is defined by the material’s composition and topology3,4,5,6; or their responsiveness may be irreversible7,9,10 and limited to a single network property11,12 (such as stiffness). Here we use cooperative self-assembly as a design principle to prepare a material that can be switched between two topological states. By using networks of polymer-linked metal–organic cages in which the cages change shape and size on irradiation, we can reversibly switch the network topology with ultraviolet or green light. This photoswitching produces coherent changes in several network properties at once, including branch functionality, junction fluctuations, defect tolerance, shear modulus, stress-relaxation behaviour and self-healing. Topology-switching materials could prove useful in fields such as soft robotics and photo-actuators and also provide model systems for fundamental polymer physics studies.

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Change history

• 20 August 2018

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Acknowledgements

We thank the National Science Foundation (CHE-1629358 for J.A.J. and CHE-1506722 for X.L.) for support of this work. This work was supported in part by the MRSEC Program of the National Science Foundation under award number DMR-1419807. Y.G. thanks ExxonMobil for an ExxonMobil–MIT Energy Fellowship. A.P.W. and E.A.A. acknowledge funding from the Research Corporation for Scientific Advancement through a Cottrell Scholars Award. X-ray scattering experiments were performed at the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (DE-AC0206CH11357). We thank G. Clever for discussions.

Reviewer information

Nature thanks S. Sheiko and T. Sirk for their contribution to the peer review of this work.

Author information

Y.G. and J.A.J. conceived the idea. Y.G. designed and conducted the synthesis and characterization studies. E.A.A. and A.P.W. conducted the simulations. H.W. and X.L. conducted the mass spectrometry studies. Y.G. and J.A.J. wrote the manuscript. All authors read and edited the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Jeremiah A. Johnson.

Extended data figures and tables

Extended Data Fig. 1 Solution self-assembly of DTE-based bis-pyridine ligand and Pd2+.

a, DTE-containing bis-pyridine photoswitch reversibly interconverts between open (o-L) and closed (c-L) forms. Note that the ring-closure reaction produces trans-isomers (racemic). In the presence of Pd2+, o-L and c-L form Pd3(o-L)6 and Pd24(c-L)48 MOCs, which can be interconverted using light. b, Aromatic regions of the solution 1H NMR spectra (CD3CN, 25 °C, ω/2π = 500 MHz) for o-L and c-L, and corresponding MOCs prepared from these ligands and Pd(CH3CN)4(BF4)2. Photoswitching steps are indicated by black arrows. c, 1H DOSY measurements indicate that Pd2+ forms a smaller assembly with o-L (green spectrum) and a larger assembly with c-L (blue spectrum). From a series of 1H NMR spectra generated from 1H DOSY measurements, the decay of peak intensities was fitted to provide the diffusion value (D) for Pd3(o-L)6 (green shaded region) and Pd24(c-L)48 (blue shaded region), respectively. From the measured diffusion values, the hydrodynamic radii of Pd3(o-L)6 and Pd24(c-L)48 were calculated. d, ESI mass spectrum of Pd3(o-L)6. The charge states of intact assemblies due to the loss of counterions are marked. Inset shows the simulated and observed isotopic patterns of [Pd3(o-L)6 + 2BF4]4+. e, Cold spray ionization mass spectrum of Pd24(c-L)48. The charge states of intact assemblies due to the loss of counterions are marked.

Extended Data Fig. 2 Frequency sweeps in oscillatory rheometry at 0.5% strain.

a, Data for three o-gel samples. b, Data for three o-gel samples after UV irradiation. c, Data for three o-gel samples after UV irradiation followed by green-light irradiation. d, Data for three c-gel samples prepared directly from c-PL.

Extended Data Fig. 3 Fitting of high-q SAXS profile of the c-gel.

a, High-q regime of the SAXS profile for the c-gel. Five peaks were identified (dashed lines) and indexed. b, Experimental results were fitted with the form factor of a spherical particle with radius 2.9 nm.

Extended Data Fig. 4 Topology switching in the presence of free ligand as defects.

a, Frequency sweep in oscillatory rheometry at 0.5% strain for three o-gel′ samples. b, Frequency sweep in oscillatory rheometry at 0.5% strain for three o-gel′ samples after UV irradiation. c, Frequency sweep in oscillatory rheometry at 0.5% strain for three o-gel′ samples after UV irradiation followed by green-light irradiation. d, SAXS curves for the o-gel′ before and after UV irradiation.

Extended Data Fig. 5 Simulations of network topology.

a, Snapshots of in silico polyMOCs before UV irradiation. Left: MOC junctions are shown as grey spheres and polymer chains connecting MOC junctions are shown in blue. Right: A zoom-in view of the region in the green cube in the left panel. Looped and non-looped polymer chains are shown in red and blue, respectively. b, Snapshots of in silico polyMOCs after UV irradiation. Left, MOC junctions are shown as grey spheres, and polymer chains connecting MOC junctions are shown in blue. Right, A zoom-in view of the region in the green cube in the left panel. Looped and non-looped polymer chains are shown in red and blue, respectively. The blue dashed circle shows a representative case in which two Pd24L48 clusters are connected by multiple polymer chains (that is, multiple secondary loops). c, A representative polymer network which is abundant in secondary loops. The connectivity of each junction is calculated on the basis of the total connections, which describes the number of polymer chains connecting MOCs. d, The same polymer network is represented in another way by calculating the connectivity of each junction on the basis of the active connections.

Extended Data Fig. 6 Simulated results for average connections per MOC for a series of polyMOCs with various PdxLy stoichiometries.

Two types of connections between MOCs are defined: total connections and active connections. These connections correspond to two classical models of elasticity: the affine model (total connections, red curve) and the phantom model (active connections, black curve). Both affine (yellow stars) and phantom (blue stars) models were used to calculate the average branch functionality for the o-gel and c-gel based on measured G′, and the results were compared with simulated active connections and total connections. For the o-gel, the phantom-model based experimental calculation agrees well with simulated active connections; for the c-gel, the affine-model based experimental calculation agrees well with simulated total connections. The experimental and simulated results suggest that the o-gel is best described as a phantom network while the c-gel is best described as an affine network.

Extended Data Fig. 7 Simulation studies of fatigue behaviours.

a, Simulation results for $${\bar{f}}_{{\rm{ac}}}$$ of Pd3L6 gel obtained by assuming that a certain fraction of ligand is inactive. b, Simulation results for $${\bar{f}}_{{\rm{tc}}}$$ of Pd24L48 gel obtained by assuming that a certain fraction of ligand is inactive.

Extended Data Fig. 8 1H NMR of the aromatic regions of model compound o-L during UV irradiation.

The o-L was completely converted to c-L after 5 h, with approximately 2% side product as indicated by the presence of a second set of peaks. After 18 h of UV irradiation, about 20% of c-L was converted to the side product. By 45 h, more than 80% of the c-L had undergone side reaction(s).

Supplementary information

Supplementary Information

This file contains synthetic methods and additional characterization data.

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