Recent publications

On the origin of elasticity and heat conduction anisotropy of liquid crystal elastomers at gigahertz frequencies
Nat. Communication 2022

Elastic anisotropy of liquid crystal elastomers (LCEs) is typically measured at low frequencies for the applications such as soft robotics, actuators, and origami. The exploration of the elasticity and thermal conduction anisotropy at gigahertz frequencies will benefit its emerging high-frequency applications such as 5G cellular networks. 
MPIP researchers in collaboration with J. Liu and S. Yang (University of Pennsylvania), M. Ryu and J. Morikawa (Tokyo Institute of Technology) study the elasticity anisotropy of LCEs and its strain dependence at gigahertz frequencies utilizing Brillouin light spectroscopy. They found the unexpectedly lower Young’s modulus anisotropy than that measured by tensile testing, suggesting disparity between the local mesogenic orientation and the larger scale orientation of the network strands. Along with the observed directional heat conductivity, this work reveals the different length scales involved in the thermoelastic anisotropy and provides insights for programming liquid crystal elastomers on-demand for high-frequency applications.

The project is funded by ERC AdG SmartPhon (Grant No. 694977) more
Origin of the Acoustic Bandgaps in Hypersonic Colloidal Phononics: The Role of the Elastic Impedance
J. Phys. Chem. B 2022

Concurrence of structure periodicity and strong acoustic resonant components in colloidal crystals enable control of phonon propagation at hypersonic frequencies. The elastic mismatch plays a pivotal role in opening interference- and resonance-induced bandgaps. The deliberate engineering of colloid elasticity and interfacial connectivity control phonon‒matter interactions in hybrid materials.
MPIP researchers in collaboration with R. Sainidou and P.  Rembert (Laboratoire Ondes et Milieux Complexes UMR CNRS), G. Magnabosco and N. Vogel (Friedrich-Alexander University Erlangen-Nürnberg) report on the experimental hypersonic phonon dispersion in hard and soft fcc colloidal crystals infiltrated in liquid polydimethylsiloxane with different elastic impedance contrast using Brillouin light spectroscopy. The two systems reveal distinct differences in the phonon dispersion discussed by first-principle full elastodynamic multiple-scattering calculations. Topology strongly impacts both the speed of sound and the nature of the particle vibration resonance-induced hybridization bandgap, respectively, at long and short wavelengths. In view of many conversational parameters, predictive tuning of phonon propagation in soft-matter based hypersonic phononics remains still challenging. 
The project is funded by ERC AdG SmartPhon (Grant No. 694977) more
Unusual High-Frequency Mechanical Properties of Polymer-Grafted Nanoparticle Melts
Phys. Rev. Lett. 2022

Transport mechanisms (of gas, ions, sound, thermal phonons) in polymer-grafted nanoparticle (GNP) melts are radically different if interpenetrated chain segments can be “pushed out of the way” or not. This provides a facile new means for manipulating the properties of these materials.
MPIP researchers, in collaboration with M. Jhalaria and S. K. Kumar (Columbia University, New York, USA), and Y. Huang and B. Benicewicz (University of South Carolina, Columbia, USA), investigated high-frequency mechanical properties of polymer-grafted nanoparticle melts. Brillouin light spectroscopy is used to measure the elastic moduli of GNP melts as a function of chain MWn at fixed grafting density (0.47 chains/nm-2) and nanoparticle radius (8 nm). The results reveal a novel dependence of the elasticity on graft chain MWn with a crossover behavior caused by the short, interpenetrated chain fragments being pushed out of the way. The unique elasticity-structure relationship advances the current understanding of transport mechanisms (of gas, ions, sound, thermal phonons) of spatially inhomogeneous materials. PHYSICAL REVIEW LETTERS 128, 187801 (2022)

The project is funded by ERC AdG SmartPhon (Grant No. 694977) more
Nano-structured, nacre-mimetic hybrid Bragg stacks show unique orientation- and composition-dependent mechanical and thermal properties that hold a promise for thermal management applications
ACS Appl. Nano Mater. 2022

Layered nanomaterials fascinate researchers for their mechanical, barrier, optical, and transport properties. Nacre is a biological example thereof, combining excellent mechanical properties by aligned submicron inorganic platelets and nanoscale proteinic interlayers. Mimicking nacre with advanced nanosheets requires ultra-confined organic layers when aiming at nacre-like high reinforcement fractions. MPIP researchers, in collaboration with Theresa Dörres, Kai Herrmann, Marius Schöttle, Daniel Wagner, Markus Retsch, and Josef Breu (University of Bayreuth, Bayreuth, Germany) and Olli Ikkala (Aalto University, Espoo, Finland), fabricated hybrid Bragg stack thin films consisting of alternating fluorohectorite-clay and polyethylene-glycol layers. Brillouin light spectroscopy (BLS) allowed determination of the orientation-dependent mechanical properties of the thin films, revealing an unprecedentedly high in-plane Young’s modulus of 162 GPa, which is more than five times the value obtained from tensile tests that are more susceptible to material defects than BLS. Complementary thermal measurements by lock-in thermography and the photoacoustic technique exhibited low cross-plane thermal conductivities κ = 0.11 ~ 0.15 W m-1 K-1 and high thermal anisotropies κ/κ = 28 ~ 33. Conceptually, this work reveals the ultimate elastic and thermal properties of aligned layered clay nanocomposites in flaw-tolerant conditions. It paves the way to unraveling the complexity of designing hybrid stacks, and could eventually lead to combined molecular and nanostructured design approaches that allow for an independent adjustment of the mechanical and thermal properties.

This project is funded by ERC SmartPhon (No. 694977). more
Intermixed time dependent self-focusing and defocusing nonlinearities in polymer solutions 
ACS Photonics. 2022

Refractive δn index profiles (at the entrance and normal to the beam), propagation beam (left) and phase images (simulation and experiment) for self-focusing case (positive case), self-defocusing case (negative case) and co-existence of two cases (solvent mixtures). Laser propagation axis: left to right.

MPIP researcher, in collaboration with A.Bogris, N.Burger and B Loppinet (Institute of Electronic Structure and Laser ,Heraklion Greece) and K.Makris (University of Crete, Heraklion, Greece), investigated experimentally and theoretically the pattern formation in polymer binary solutions by low power visible light. DOI:

We demonstrate the presence of two nonlinearities of opposite effects in the same photo-reactive polybutadiene (PB) solutions. Depending of the solvent, these present either self-focusing or defocusing nonlinearity. Both responses are local in space and nonlocal in time with time integrating response, but with different kinetics. In ternary PB solutions in two good solvents, specific light propagation and formation of unique refractive index patterns are realized and well reproduced by the model of coexistence of self-focusing and defocusing optical responses. This supports the hypothesis that the two nonlinearities have different physico-chemical origin and can coexist when mixture of solvents are used. We expect such complex response could be engineered in other polymeric materials with the requirement of two independent mechanisms leading to opposite local change of refractive index. Such novel type of highly nonlinear polymer medium may lead to complex patterning and novel lithographic techniques utilizing the large induced refractive index contrast, δn change compared to other nonlinear materials, e.g., semiconductor crystals. Recently, the self-focusing response was utilized by some of the present authors to create low loss deformable optical fiber interconnects.
About the Project
This project is partially funded by ERC SmartPhon (No. 694977).
Fundamentals, Progress and Perspectives on High-frequency Phononic Crystals
Journal of Physics D: Applied Physics, 2022

Phononic crystals (PnCs) are capable to manipulate the flow of elastic energy through their periodic structures and emerge as a promising field. Thanks to the advances in microfabrication technologies and developments of multifunctional materials, the engineering of periodic structures moves forward to the nanometer scale. Hence, the relevant frequencies of elastic waves are pushed toward the gigahertz regime where strong photon-phonon interactions trigger the applications of PnCs towards information and communication technologies.

MPIP researcher in collaboration with Yu Cang (Tongji University), Yabin Jin, (Tongji University), and Bahram Djafari-Rouhani (University of Lille, Lille, France) reviewed the experimental achievements on hypersonic PnCs involving microfabrication technologies to realize the desired structures and characterization of their band structures for unraveling phonon propagation modulation. Some application-orientated research directions were proposed in terms of advances in fabrication and characterization technologies and the development of electro-optomechanical systems.

This work was supported by the ERC AdG SmartPhon (Grant 694977), Shanghai Pujiang Program (Grant No. 20PJ1413800) and the National Natural Science Foundation of China (Grant No. 12102304). more
Quantization of acoustic modes in dumbbell nanoparticles
Phys. Rev. Lett. 2022

MPIP researchers, in collaboration with Hojin Kim and Eric M. Furst (University of Delaware, Delaware, USA), Maria Secchi and Maurizio Montagna (University of Trento, Trento, Italy), and Bahram Djafari-Rouhani (University of Lille, Lille, France), investigated the acoustic eigenmodes of dumbbell-shaped nanoparticles by Brillouin light scattering experiments and theoretical calculations. The results reveal the frequencies, mode shapes, and Brillouin activities of the characteristic modes of this unique type of building blocks for achieving tunable, anisotropic phononic crystals. (Physical Review Letters. DOI:

The vibrational eigenmodes of dumbbell-shaped polystyrene nanoparticles are recorded by Brillouin light spectroscopy (BLS), and the full experimental spectra are calculated theoretically. Different from spheres with a degeneracy of (2l + 1), with l being the angular momentum quantum number, the eigenmodes of dumbbells are either singly or doubly degenerate owing to their axial symmetry. The BLS spectrum reveals a new, low-frequency peak which is attributed to the out-of-phase vibration of the two lobes of the dumbbell. The quantization of acoustic modes in these molecule-shaped dumbbell particles evolves from the primary colloidal spheres as the separation between the two lobes increases. The elucidation of the vibration eigenmodes of the dumbbell-shaped nanoparticles and the well-controlled tunability of their geometric parameters (i.e., D1, D2, d12) are envisaged to promote the development of a wide range of DB-based phononic crystals with direction-dependent, tunable bandgaps, which could have potential applications in areas such as nanomaterials and on-chip devices by providing more degrees of freedom in wave-based filtering and computing.

About the Project
This project was funded by ERC SmartPhon (No. 694977). more
Probing the thermal actuation of polydopamine nanomembranes
Nano Letters, 2021

Polydopamine (PDA) is a multifunctional mussel-inspired polymer with a highly complicated molecular structure, and a broad range of applications from smart coatings and adhesives to optoelectronics. Now, a combination of contactless optical techniques shows that pure PDA nanomembranes can also exhibit fast light-driven motion. This work paves the way for developing bio-inspired photoactuators, soft jointless nanorobots, and artificial muscles.
Using micro-Brillouin light scattering (Fig. 1), optical reflectivity and microscopy, as well as surface-examination with sum-frequency-generation spectroscopy, we showed that ultrathin PDA membranes exhibit pseudo-negative thermal expansion due to water desorption. The light-driven motion of PDA membranes can be as fast as 140 μs.
This work is a collaboration between the group of Bartlomiej Graczykowski in the Adam Mickiewicz University in Poznan, Poland, and researchers at the Max Planck Institute for Polymer Research in Mainz (George Fytas, Ellen H. G. Backus, Tanja Weil et al.).
This work was supported by the Polish Science Foundation (POIR.04.04.00-00-5D1B/18), three Marie Skłodowska-Curie Actions (PLASMMONS, BORGES and REWIRE) and the ERC AdG SmartPhon (Grant 694977).

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Direct Visualization and Characterization of the Interfacially Adsorbed Polymer atop Nanoparticles and within Nanocomposites
Macromolecules 2021

BLS vibration spectra of silica nanoparticles bearing different polystyrene (PS) adsorbed layers controlled by the annealing, tann , time. The splitting of the fundamental (1,2) mode increases with  PS thickness. SEM of nanoparticles bearing a PS adsorbed layer before (top) and after (bottom) heating
MPIP researcher in collaboration with D. Cangialosi (Centro de Física de Materiales, San Sebastián, Spain) and K.Randazzo,, B. Zuo, D. Priestley (Chemical and Biological Engineering, Princeton University,USA) developed techniques to determine the glass transition and elasticity of thin Interfacially and irreversibly adsorbed polystyrene layers atop of silica nanoparticles.

Irreversible adsorption at polymer/substrate interfaces influences glassy properties in thin films. However, consideration has yet to be extended to the nanocomposite geometry wherein a large interfacial area and high processing temperatures afford for irreversible adsorption at the polymer/nanoparticle interface. Here, we present an approach for directly measuring the site-specific glassy properties and the elasticity at the polystyrene (PS)-adsorbed layer interface in- and the elasticity of-PS-silica model nanocomposites. We measured a depressed compared to the bulk PS Tg, glass transition temperature (Tg) via fluorescence and the nanoparticle fundamental elastic vibration sensitively depending on the PS adsorbed layers atop silica nanoparticles via Brillouin light spectroscopy (BLS). By measuring the thickness via transmission electron microscopy, the drop of Tg was observed below about 10nm PS adsorbed layers, whereas the interactions between nanoparticles significantly increased with PS thickness. Our results provide compelling evidence that adsorbed layer formation within polymer nanocomposites can have a profound impact on local interfacial properties. more
Influence of Surfactant-Mediated Interparticle Contacts on the Mechanical Stability of Supraparticles
J.Phys.Chem.C 2021

Supraparticle of PS nanoparticles with distinct macroscopic mechanical behavior and PS vibration spectrum dependent on the surfactants used in the supraparticle assembling.

MPIP researcher in collaboration with J. Wang, U.Sultan,B.Merle ,A.Inayat and N.Vogel (Institute of Particle Technology, University Erlangen-Nürnberg) developed techniques to fabricate colloidal supraparticles and  determine the interactions between the constituent primary particles vital for the  mechanical integrity upon external compressional forces.

Colloidal supraparticles are micron-scale spherical ordered or disordered assemblies of uniform primary submicron particles, which exists as a dispersible powder. The mechanical integrity during processing is a prerequisite of their realization and requires understanding of how the internal structure relates to the resultant mechanics of a supraparticle. For the fabrication, stable emulsions can be generated by either nonionic block copolymers (PPP) or  anionic fluorosurfactants (Krytox) both leading to structurally similar supraparticles. However, whilePPP induces superior mechanical stability and ductile behavior, Krytox leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin Light spectroscopy (BLS) that is very sensitive to the interparticle interactions. These are critically determined by the subnanometer thick adsorbed surfactant layers atop of the PS nanoparticles. While Krypton does not significant impact the interparticle bonding, the aphiphilic PPP drastically strengthen this nanoparticle bonding to the point that the PS fundamental vibration is not resolved in the experimental BLS spectrum. The results demonstrate that seemingly subtle changes in the physicochemical properties of suprapartcles can drastically impact their resultant macroscopic mechanics.

About the Project: This project is funded by ERC SmartPhon (No. 694977).
Optically formed rubbery waveguide interconnects 
Optics Letters, 2021

Laser light (at 670nm) induced fiber formation between two single mode optical fibers immersed in a semidilute polyisoprene solution (left) and light guide performance (at 780nm) monitoring. Bright (c) and dark field (d) optical microscopy images demonstrate the formation of a light-induced-self-written waveguide. 

Researchers in the Institute of Electronic Structure and Laser(IESL), Heraklion ,Crete, Greece (G.Violakis, A.Bogris, B.Loppinet and S.Pissadakis) in  collaboration with MPIP (G.Fytas) utilized the light-induced patterning of entangled polyisoprene solutions to form flrxible waveguide optical interconnects with significant optical transmission in near IR.

About two decades ago G.Fytas and coworkers of IESL have reported the facile formation of mater patterns in non-dilute polydiene solutions in good solvents using low power and weakly focused CW laser light in the visible.(Science 297,67, 2002. The light modifies the materials refractive index leading in the most of the cases to its self-focusing. In the case of polydiene solutions, laser light with wavelength lower than about 700nm induces limited but very effective crosslinking only in the laser beam. In this publication, we report an application of this effective self-focusing to fabricate deformable interconnects between optical fibers while keeping significant optical transmission. Extensibility up to 800% with typical loss of about 1.5dB/mm was demonstrated. The mechanical and optical performance along the facile fabrication pave the way for long-length elastic optical interconnects, adjustable power couplers, and wearable sensors.   
About the Project: This project is funded by ERC SmartPhon (No. 694977).
Fluid dynamics simulations illustrate the flow and concentration distributions in the coating bead of the meniscus-guided coating of organic semiconductors
Advanced Functional Materials 2021.

MPIP researchers published a paper on meniscus-guided coating of organic semiconductors, which demonstrates how optimized charge transport properties can be achieved by controlling the fluid dynamics and crystallization.

The effects of the casting speed and solute concentration on the crystallization of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) during meniscus-guided coating (MGC) are investigated, and three morphological subregimes with increasing casting speed are identified: I) an isotropic domain-like structure; II) unidirectionally aligned crystalline bands; and III) a corrugated dendritic morphology. Interestingly, increasing the solute concentration does not affect these morphologies but merely the associated transition velocities. Numerical simulation of both the fluid dynamics in the coating bead and the crystallization itself not only explains these morphological trends but also the decrease in the width of the crystalline bands of morphology II with the casting speed. We demonstrate that the latter provides an optimal processing window for organic field-effect transistors, with minimized charge trapping, maximized on/off ratio, and reliability factor. more
Polymer cyclization for the emergence of hierarchical nanostructures

Synthetic polymer nanoobjects with well-defined hierarchical structures are important for a wide range of applications, e.g., nanomaterial synthesis, therapeutics. Inspired by the programmability and precise three-dimensional architectures of biomolecules, T. Weil’s group demonstrated the strategy of fabricating controlled hierarchical structures through self-assembly of folded synthetic polymers. Linear poly(2-hydroxyethyl methacrylate) of different lengths are folded into cyclic polymers and their self-assembly into hierarchical structures is elucidated by various experimental techniques including dynamic plight scattering and molecular simulations. The work establishes a versatile approach for constructing biomimetic hierarchical assemblies.

About the Project
In this project, G.Fytas  and Dr. K.Wunderlich are partially funded by ERC SmartPhon (No. 694977). more
Lifting restrictions on coherence loss when characterizing hypersonic non-transparent phononic crystals
Scientific Reports 2021

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry-Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. To address this problem, we demonstrated a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately: We injected coherent phonons into a non-transparent PnC using a pulsed laser and acquired the acoustic transmission spectrum on a TFPI, where pumped appeared alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determined the bandgap and compared with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump-probe technique was found to retain key performance features of the established one and going forward will likely be preferred for disordered samples. more
Internal microstructure dictates interactions of polymer-grafted nanoparticles in solution
Macromolecules 2021

MPIP researcher in collaboration with L Gury, S. Kamble, D. Parisiand D. Vlassopoulos (IESL-FORTH, Greece) and J. Zhang, J. Lee, A. Abdullah, K. Matyjaszewski a nd  M.R. Bockstaller (CMU, Pittsburgh, PA  USA) found thatthe second virial coefficient, A2 displays an unexpected crossover behavior from the anticipated positive to unexpected negative values in a good solvent for the polymer grafts.

Understanding the effect of polymer brush architecture on particle interactions in solution is a requisite to enable the development of functional materials based on self-assembled polymer-grafted nanoparticles (GNP). Static and dynamic light scattering of polystyrene-grafted silica particle solutions in toluene reveals that the pair-interaction potential, inferred from the second virial coefficient, A2, is strongly affected by the grafting density,σ, and degree of polymerization, N, of tethered chains. In the limit of intermediate σ (~0.3-0.6 nm-2) and high N, A2 is positive and increases with N in agreement with the good solvent conditions. In contrast, for high σ > 0.6 nm-2 and low N, A2 displays an unexpected reversal to negative values, thus indicating poor solvent conditions. These findings are rationalized by means of a simple analysis based on a coarse-grained brush potential which balances the attractive core-core interactions and the excluded volume interactions imparted by the polymer grafts.. The effect of grafting density also illustrates the opportunity to tailor physical properties of hybrid materials by altering geometry (or architecture) rather than variation of the chemical composition.

About the Project: This project is funded by ERC SmartPhon (No. 694977). more
Analysis of the 3D trajectories of capsid-protein-encapsulated fluorescent nanodiamonds in living HeLa cells: Transition from subdiffusion at short lag time to normal diffusion at  intermediate time
J Mater Chem B 2021

MPIP researchers revealed the intracellular diffusion behaviors of nanoparticles by recording and analyzing long-term 3D trajectories of capsid-protein-encapsulated fluorescent nanodiamonds in living HeLa cells. (Journal of Materials Chemistry B. DOI: 10.1039/D1TB00890K)
Long-term tracking of nanoparticles to resolve intracellular structures and motions is essential to elucidate fundamental parameters as well as transport processes within living cells. High stability, biocompatibility, and cellular uptake of fluorescent nanodiamond NDs under physiological conditions are required for intracellular applications. Herein, highly stable NDs encapsulated with Cowpea chlorotic mottle virus capsid proteins (ND-CP) exhibiting excellent biocompatibility both in vitro and in vivo are prepared. Long-term 3D trajectories of the ND-CP with fine spatiotemporal resolutions are recorded; their intracellular motions are analyzed by different models, and the diffusion coefficients are calculated. The ND-CP with its brilliant optical properties and stability under physiological conditions provides us with a new tool to advance the understanding of cell biology, e.g., endocytosis, exocytosis, and active transport processes in living cells as well as intracellular dynamic parameters.

About the Project
In this project Dr. Zuyuan is funded by ERC SmartPhon (No. 694977).
Optomechanic Coupling in Ag Polymer Nanocomposite Films
J Phys Chem C 2021

MPIP researchers in collaboration with Adnane Noual (Université Mohammed Premier, Oujda, Morocco), Tanmoy Maji and Manos Gkikas (University of Massachusetts Lowell, Massachusetts USA), and Bahram Djafari-Rouhani (University of Lille, Villeneuve d’Ascq, France investigated the strong amplification of the inelastic light scattering near surface plasmon resonance of metallic nanoparticles. (
J. Phys. Chem. C 2021, 125, 14854−14864)

Access to the particle elastic vibrations of nanoparticles embedded in polymers is essential for understanding their elasticity and interactions relevant to applications (e.g., sensors, coatings, acoustics). However, the weakly localized particle resonances in a fluid or solid medium renders their detection difficult. The strong optomechanic phonon−plasmon-based coupling in metallic nanoparticles (NPs) allowed not only the detection of single NP eigenvibrations but also the interparticle interaction effects on the acoustic vibrations of NPs. The “rattling” and quadrupolar modes of Ag/polymer and polymer-grafted Ag NPs with different diameters in their assemblies are probed by Brillouin light spectroscopy (BLS). We present thorough theoretical 3D calculations for anisotropic Ag elasticity to quantify the frequency and intensity of the “rattling” mode and hence its BLS activity for different interparticle separations and matrix rigidity. Theoretically, a liquid like environment, e.g., poly(isobutylene) (PIB) does not support rattling vibration of Ag dimers but unexpectedly hardening of the extremely confined graft melt renders both activation of the former and a frequency blue shift of the fundamental quadrupolar mode in the grafted nanoparticle Ag@PIB film.
About the Project This project is funded by ERC SmartPhon (No. 694977).
Brillouin light spectroscopy provides a noninvasive access to the anisotropic elasticity of a wide range of hybrid, micro- or nano-structured materials
MPIP researchers published a review on Brillouin light spectroscopy and its recent applications in studying the anisotropic elasticity of a wide range of hybrid, micro- or nano-structured materials, including brush-particle thin film, spider silk, molecular glass, and clay/polymer Bragg stack. (Encyclopedia of Polymer Science and Technology. DOI: 10.1002/0471440264.pst673)
Understanding the elastic properties of polymer-based materials is essential for many applications (e.g., biofibers, coatings, interfacial fillers, organics semiconductors). Often those materials are anisotropic, posing a challenge for characterizing their direction-dependent elasticity. As an optical technique, Brillouin light spectroscopy (BLS) allows determination of the complete elastic tensor of materials in a noncontact, nondestructive manner. The elastic properties of many polymer-based materials have been investigated by BLS in the past five decades. In this review, we present the working principles of BLS and a few recent applications of BLS to the determination of the complete elasticity of polymer-based, nanostructured, anisotropic materials. Considering its unique power in elasticity characterization, we expect BLS to find more applications in the future, particularly in the research fields of materials science, biomedical science, biomechanics, and nanotechnology.

About the Project
This project is funded by ERC SmartPhon (No. 694977).
Determination of the elastic moduli of CVD graphene by probing graphene/polymer Bragg stacks
2D Mater. 8 (2021) 035040

MPIP researchers in collaboration with C. Pavlou, Prof. C. Galiotis (University of Patras), introduced a new technique (Brillouin Light Spectroscopy) for a unique determination of the Young’s modulus of wrinkled CVD graphene. (2D Mater. DOI. 10.1088/2053-1583/abfedb)

CVD graphene has been widely used in nanocomposites with enhanced mechanical properties. However, it’s mechanical properties usually suffers from the inevitable wrinkles from the synthesis and transfer processes, and it remains unknown how the wrinkles affect the mechanical properties of graphene. Here, we report a new approach to accessing the elastic modulus of the CVD graphene. By probing the effective phonon propagation of Gr/PMMA hybrid Bragg stacks using BLS and combining the phononic band structure calculations, we determined the Young’s and shear moduli of the CVD graphene with a maximum height of 6 nm to be 680 ±16 and 290 ±10 GPa, respectively. Compared with other contact methods, in probing the thermal phonons, no strain was introduced, eliminating the possibility of strain hardening upon loading. This work sheds light on the elastic properties of CVD graphene and provides a method that can be extended to studying the wrinkle-induced softening effect in other two-dimensional materials.
About the Project
This project is funded by ERC SmartPhon (No. 694977). Bohai Liu is supported by the Chinese Scholarship Council.
Optomechanical crystals for spatial sensing of submicron sized particles
Sci. Rep. 2021

MPIP researchers in collaboration with Dr .D. Navarro-Urrios snd Prof. C. M. Sotomayor-Torres (ICN2, Barcelona, Spain), Dr.  B. Graczykowski (MPIP and Adam Mickiewicz University, Poznan, Poland) demonstrated the working principle of optomechanical crystal cavities (OMC) operating under ambient conditions as a sensor of submicron particles ( Sci. Rep. 2021, 11 (1), 7829). Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria. About the Project This project is partially funded by ERC SmartPhon (No. 694977). more
Large Tg Shift in Hybrid Bragg Stacks through Interfacial Slowdown
Macromolecules, 2021

Studies of glass transition under confinement frequently employ supported polymer thin films, which are known to exhibit different transition temperature Tg close to and far from the interface. Various techniques can selectively probe interfaces, however, often at the expense of samples designs very specific to a single experiment. Instead, we showed how to translate results on confined thin film Tg to a 'nacre-mimetic' clay/polymer Bragg stack, where polymer molecular layer number was precisely tunable. Exceptional lattice coherence multiplied signal manifold, allowing for interface studies with both standard Tg and broadband dynamic measurement. For the monolayer, we not only observed a dramatic increase of Tg (~ 100 K), but also used X-ray photon correlation spectroscopy (XPCS) to probe platelet dynamics, originating from interfacial slowdown. This was confirmed from the bilayer, which comprised both “bulk-like” and clay/polymer interface contributions, as manifested in two distinct Tg processes. Since platelet dynamics of mono- and bilayers were similar, while segmental dynamics of the latter were found to be much faster, we concluded that XPCS is sensitive to the clay/polymer interface. Thus, large Tg shifts can be engineered and studied once lattice spacing approaches interfacial layer dimensions.

For more information, see: “Large Tg Shift in Hybrid Bragg Stacks through Interfacial Slowdown” by Konrad Rolle, Theresa Schilling, Fabian Westermeier, Sudatta Das, Josef Breu and George Fytas (Macromolecules)
Surface contacts strongly influence the elasticity and thermal conductivity of silica nanoparticle fibers
Physical Chemistry Chemical Physics, 2020

Granular materials are often encountered in science and engineering disciplines, in which controlling the particle contacts is one of the critical issues for the design, engineering, and utilization of their desired properties. The achievable rapid fabrication of nanoparticles with tunable physical and chemical properties facilitates tailoring the macroscopic properties of particle assemblies through contacts at the nanoscale. Models have been developed to predict the mechanical properties of macroscopic granular materials; however, their predicted power in the case of nanoparticle assemblies is still uncertain. Here, we investigate the influence of nanocontacts on the elasticity and thermal conductivity of a granular fiber comprised of close-packed silica nanoparticles. A complete elastic moduli characterization was realized by non-contact and non-destructive Brillouin light spectroscopy, which also allowed resolving the stiffness of the constituent particles in situ. In the framework of effective medium models, the strong enhancement of the elastic moduli is attributed to the formation of adhesive nanocontacts with physical and/or chemical bondings. The nanoparticle contacts are also responsible for the increase in the fiber thermal conductivity that emphasizes the role of interface thermal resistance, which tends to be ignored in most porosity models. This insight into the fundamental understanding of structure–property relationships advances knowledge on the manipulation of granular systems at the nanoscale.
For more information, see: “Surface contacts strongly influence the elasticity and thermal conductivity of silica nanoparticle fibers” by Yu Cang, Bohai Liu, Sudatta Das, Xiangfan Xu, Jingli Xie, Xu Deng and George Fytas, Physical Chemistry Chemical Physics (2020).
Transparent Hybrid Opals with Unexpected Strong Resonance‐Enhanced Photothermal Energy Conversion
Advanced Materials, 2020

Photothermal energy conversion is of fundamental importance to applications ranging from drug delivery to microfluidics and from ablation to fabrication. It typically originates from absorptive processes in materials that-when coupled with non-radiative dissipative processes-allow the conversion of radiative energy into heat. Microstructure design provides versatile strategies for controlling light-matter interactions. In particular, the deliberate engineering of the band structure in photonic materials is known to be an effective approach to amplify absorption in materials. However, photonic amplification is generally tied to high optical contrast materials which limit the applicability of the concept to metamaterials such as microfabricated metal–air hybrids. This contribution describes the first observation of pronounced amplification of absorption in low contrast opals formed by the self-assembly of polymer-tethered particles. The dependence of the amplification factor on the length scale and degree of order of materials as well as the angle of incidence reveal that it is related to the slow photon effect. A remarkable amplification factor of 16 is shown to facilitate the rapid “melting” of opal films even in the absence of “visible” absorption. The results point to novel opportunities for tailoring light-matter interactions in hybrid materials that can benefit the manipulation and fabrication of functional materials.

For more information, see: “Transparent Hybrid Opals with Unexpected Strong Resonance‐Enhanced Photothermal Energy Conversion” by Yu Cang, Jaejun Lee, Zuyuan Wang, Jiajun Yan, Krzysztof Matyjaszewski, Michael R. Bockstaller, George Fytas, Advanced Materials (2020): 2004732.
Glass Transition of Disentangled and Entangled Polymer Melts: Single-Chain-Nanoparticles Approach
Macromolecules, 2020

We study the effect of entanglements on the glass transition of high molecular weight polymers, by the comparison of single-chain nanoparticles (SCNPs) and equilibrated melts of high-molecular weight polystyrene of identical molecular weight. SCNPs were prepared by electrospraying technique and characterized using scanning electron microscopy and atomic force microscopy techniques. Differential scanning calorimetry, Brillouin light spectroscopy, and rheological experiments around the glass transition were compared. In parallel, entangled and disentangled polymer melts were also compared under cooling from molecular dynamics simulations based on a bead-spring polymer model. While experiments suggest a small decrease in the glass transition temperature of films of nanoparticles in comparison to entangled melts, simulations do not observe any significant difference, despite rather different chain conformations.
For more information, see: “Glass Transition of Disentangled and Entangled Polymer Melts: Single-Chain-Nanoparticles Approach” by Manjesh K Singh, Minghan Hu, Yu Cang, Hsiao-Ping Hsu, Heloise Therien-Aubin, Kaloian Koynov, George Fytas, Katharina Landfester, Kurt Kremer, Macromolecules 53 (17), 7312-7321.
Self-Assembly of Colloidal Nanoparticles into Well-Ordered Centimeter-Long Rods via Crack Engineering
Advanced Materials Interfaces, 2020

Self-assembly of colloidal nanoparticles (NPs) is widely employed in nanofabrication to have regulated shape with fascinating functions. However, to have a specific desired shape at centimeter-scale without template is challenging. Herein, by harnessing the colloidal nanoparticle thin film crack engineering, robust and highly transparent centimeter-scale rods with uniform width and thickness are obtained. The dimension of these rods can be tailored via controlling the solvent composition, NPs volume fraction, and suspension descending rate. Their mechanical stiffness and elastic properties can be further improved by thermal annealing. It is demonstrated that these rods can be used as probes for surface enhanced Raman scattering detection making use of their rich nanostructured surface. This crack engineering strategy can be used as a universal method to assemble the nanoscale colloids into centimeter-scale rods for analytical and photoelectrical applications.
For more information, see: “Self-Assembly of Colloidal Nanoparticles into Well-Ordered Centimeter-Long Rods via Crack Engineering” by Jingli Xie, Junchang Guo, Dehui Wang, Yu Cang, Wenluan Zhang, Jiajia Zhou, Bo Peng, Yanbo Li, Jiaxi Cui, Longquan Chen, George Fytas, Xu Deng, Advanced Materials Interfaces (2020): 2000222.
Hypersound and its interaction with charges studied with Pumped-BLS
Science Advances, 2020

Mobile electronic devices, like smartphones, use hypersonic gigahertz (GHz) phonons to mediate signal processing with microwave radiation, and charge carriers for the operation of the various microelectronic components. The potential interactions of GHz phonons and charge carriers can now be explored with a newly developed technique termed pumped-Brillouin Light Spectroscopy (pumped-BLS). Pumped-BLS uses femtosecond laser pulses to excite charge carriers and phonons, and Brillouin Light Spectroscopy to probe GHz phonons with frequency- and momentum-resolution. The interaction of GHz phonons and charge carriers manifests as a Fano resonance, which can be used to extract the phase and lifetime of phonons. Moreover, pumped-BLS can reveal the propagation direction of GHz phonons and thus it is useful for studying hypersonic signals in sophisticated phononic metamaterials.
This technique has been applied for the first time on a model, spatially confined semiconductor: a 260 nanometer thick Silicon membrane. Our work paves the way for studies of hypersonic signals in novel nano- and meta-materials with promising applications in next-generation wireless communications, like high-band-5G and 6G networks.
This project is a collaboration between the Faculty of Physics of the Adam Mickiewicz University in Poznan and the Max Planck Institute for Polymer research in Mainz.

 For more information, see: “Frequency-domain study of nonthermal gigahertz phonons reveals Fano coupling to charge carriers” by T. Vasileiadis, H. Zhang, H. Wang, M. Bonn, G. Fytas, and B. Graczykowski. Science Advances 18 Dec 2020 : eabd4540.
Flash Brillouin Scattering: A Confocal Technique for Measuring Glass Transitions at High Scan Rates
ACS Photonics 2020

Glass transition temperatures Tg are most commonly measured by differential scanning calorimetry, a method that has been extended to the flash scanning calorimetry (FSC) regime by reducing sample volumes. However, significant manual preparation effort can render FSC impractical for e.g. local probing of spatially heterogeneous specimens. Another strategy can be to select a small volume by focusing down a laser beam, where Brillouin Light Scattering (BLS) is a proven method for confocal Tg measurement.  Thus, we have introduced Flash Brillouin Scattering (FBS), which extends BLS to fast scan rates, achieved by periodically heating the probed region with an infrared laser. For comparison with conventional BLS, we first characterized Tg of pure glycerol, and showed how rapid quenching produced a less packed glass with downshifted sound velocity. We then turned towards its aqueous solutions, which crystallize too fast for a non-flash approach, and demonstrated scan rates in excess of 105 K/s. These results are of interest not only because glycerol is a model system for hydrogen-bonded glass formers, but also because of its applications as a cryoprotectant for frozen biological samples. Light scattering studies of the latter, currently limited to cryo-Raman spectroscopy, are likely to be complemented by the technique we introduced.

About the Cover: ACS Photonics 8,Issue 2,2021 Flash heating of glass-forming liquids by a laser can be used to bypass crystallization during the measurement of elastic properties by Brillouin light scattering. The cover image shows a water‒glycerol mixture at three time instants, with ice crystal formation occurring as temperatures increase above the glass transition. Illustration courtesy of Stefan Schuhmacher.

For more information, see: “Flash Brillouin Scattering: A Confocal Technique for Measuring Glass Transitions at High Scan Rates” by Konrad Rolle, Hans-Jürgen Butt and George Fytas, ACS Photonics, 28/12/2020 more
Rigidification of Poly(p-phenylene)s through ortho-Phenyl Substitution
Macromolecules (2020)

A sterically
π-congested ortho-phenylated poly(p-phenylene) (PPP) hasbeen synthesized with unprecedentedly high molecular weights up to 29 kDa afterfractionation, as confirmed by gel permeation chromatography coupled with a multianglelaser light scattering detector. The chain translation diffusion coefficient obtained fromdynamic light scattering experiments displayed strong scaling (Lw0.8) to the chaincontour length, indicating a rodlike shape with remarkably high rigidity of this novelPPP. These results provide an interesting insight into the relationship between thestructure and the chain stiffness of PPP-based polymers and challenge the validity of theexisting diffusion models in polymer physics
Mechanical reinforcement of polymer colloidal crystals by supercritical fluids
J. Coll. Interf. Science (2020)

Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a plat-
form for various applications from assembling photonic and phononic crystals, acoustic metamaterials tocoating applications. However, the fragility of these systems limits their application horizon. In this workthe uniform mechanical reinforcement and tunability of 3D polystyrene colloidal crystals by means ofcold soldering are reported. This structural strengthening is achieved by high pressure gas (N2or Ar) plas-ticization at temperatures well below the glass transition. Brillouin light scattering is employed to mon-itor in-situ the mechanical vibrations of the crystal and thereby determine preferential pressure,temperature and time ranges for soldering,i.e.formation of physical bonding among the nanoparticleswhile maintaining the shape and translational order. This low-cost method is potentially useful for fab-rication and tuning of durable devices including applications in photonics, phononics, acoustic metama-terials, optomechanics, surface coatings and nanolithography. more
Extreme Elasticity Anisotropy in Molecular Glasses
Advanced Functional Materials (2020)

In article number 2001481, Mark D. Ediger, George Fytas, and co‐workers reveal the strong correlation between elastic anisotropy and molecular orientation in vapor‐deposited organic glasses of rod‐shaped itraconazole. Extreme elastic anisotropy, the ratio of the orthogonal Young's moduli, is observed in the vertically orientated molecules exceeding that in liquid crystals and even crystalline solids. more
Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement

Angewandte Chemie
International Edition (2020)
Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity
Nanoletters (2020)
High frequency phonons in the GHz-THz regime corresponding to sub-micrometer wavelengths are responsible for hypersound and heat transport, thus having importance for wireless communication, optomechanics and thermal energy harvesting. Periodically structured materials that take advantage of the wave-like nature of phonons, such as Phononic Crystals (PnCs) have been shown to enable new mechanical and acoustic features, including negative effective moduli and densities, frequency filtering and acoustic cloaking. In this work we study the phonon propagation in large-area 2D PnCs composed of spherical polystyrene (PS) nanoparticles self-assembled on an ultra-thin Si3N4 membrane. We use micro-Brillouin Light Scattering to record the dispersion of thermally populated GHz phonons. We find significant modification of the phonon dispersion revealing three types of band gaps that are related to: nanoparticle-membrane contact resonance (hybridization), lattice period (Bragg) and  local resonances of nanoparticles (hybridization). All these mechanisms contribute to multi-band filtering of hypersound that can be easily tailored by means of the nanoparticle size governing lattice spacing, frequencies of local resonances and adhesion forces. Noteworthy, for the latter we find a size effect and significant discrepancy from macroscopic predictions. more
Probing Nanoparticle/Membrane Interactions by Combining Amphiphilic Diblock Copolymer Assembly and Plasmonics

Probing Nanoparticle/Membrane Interactions by Combining Amphiphilic Diblock Copolymer Assembly and Plasmonics

The Journal of Physical Chemistry (2020)

Probing Nanoparticle/Membrane Interactions by CombiningAmphiphilic Diblock Copolymer Assembly and PlasmonicsAmelie H.R. Koch, Svenja Morsbach, Tristan Bereau, Gaëtan Lévêque, Hans-Jürgen Butt,Markus Deserno, Katharina Landfester, and George Fytas*Cite This:J.Phys.Chem.B2020, 124, 742750Read OnlineACCESSMetrics & MoreArticle Recommendations*sıSupporting InformationABSTRACT:Understanding the interactions between nanoparticles (NPs) andboundaries of cells is crucial both for their toxicity and therapeutic applications.Besides specific receptor-mediated endocytosis of surface-functionalized NPs,passive internalization is prompted by relatively unspecific parameters, such asparticle size and charge. Based on theoretical treatments, adhesion to and bendingof the cell membrane can induce NP wrapping. Experimentally, powerful tools areneeded to selectively probe possible membrane-NP motifs at very dilute conditionsand avoid dye labeling. In this work, we employ surface resonance-enhanceddynamic light scattering, surface plasmon resonance, electron microscopy, and simulations for sensing interactions betweenplasmonic AuNPs and polymersomes. We distinguish three different interaction scenarios at nanomolar concentrations by tuning thesurface charge of AuNPs and rationalize these events by balancing vesicle bending and electrostatic/van der Waals AuNP and vesicleadhesion. The clarification of the physical conditions under which nanoparticles passively translocate across membranes can aid inthe rational design of drugs that cannot exploit specific modes of cellular uptake and also elucidates physical properties that rendernanoparticles in the environment particularly toxic.
Precision Anisotropic Brush Polymers by Sequence Controlled Chemistry
J. Am. Chem. Soc. (2020)

The programming of nanomaterials at molecularlength-scales to control architecture and function represents apinnacle in soft materials synthesis. Although elusive in syntheticmaterials, Nature has evolutionarily refined macromolecularsynthesis with perfect atomic resolution across three-dimensionalspace that serves specific functions. We show that biomolecules,specifically proteins, provide an intrinsic macromolecular backbonefor the construction of anisotropic brush polymers withmonodisperse lengths via grafting-from strategy. Using humanserum albumin as a model, its sequence was exploited to chemicallytransform a single cysteine, such that the expression of saidfunctionality is asymmetrically placed along the backbone of theeventual brush polymer. This positional monofunctionalization strategy was connected with biotinstreptavidin interactions todemonstrate the capabilities for site-specific self-assembly to create higher ordered architectures. Supported by systematicexperimental and computational studies, we envisioned that this macromolecular platform provides unique avenues andperspectives in macromolecular design for both nanoscience and biomedicine.
Determination of the Complete Elasticity of Nephila pilipes Spider Silk
Biomacromolecules  (2020)

Spider silks are remarkable materials designed bynature to have extraordinary elasticity. Their elasticity, however,remains poorly understood, as typical stressstrain experimentsonly allow access to the axial Youngs modulus. In this work, micro-Brillouin light spectroscopy (micro-BLS), a noncontact, non-destructive technique, is utilized to probe the direction-dependentphonon propagation in theNephila pilipesspider silk and hencesolve its full elasticity. To the best of our knowledge, this is thefirstdemonstration on the determination of the anisotropic Youngsmoduli, shear moduli, and Poissons ratios of a single spiderfiber.The axial and lateral Youngs moduli are found to be 20.9±0.8and 9.2±0.3 GPa, respectively, and the anisotropy of the Youngs moduli further increases upon stretching. In contrast, the shearmoduli and Poissons ratios exhibit very weak anisotropy and are robust to stretching. more
Brillouin light scattering under one-dimensional confinement: Symmetry and interference self-canceling
Physical Review B (2019)

We present the spontaneous Brillouin light scattering (BLS) under simultaneous one-dimensional confinementof sound and light and show that the photon-phonon coupling results from nontrivial interplay of the photoelasticand moving-interface effects. We reveal two types of BLS self-canceling: governed by mode symmetry anddriven by destructive interference of the two effects. We show that the latter can be adjusted by the lightpolarization and phonon wave number. Furthermore, we present a measurement of the shear-horizontal waves inthin membranes. more
Disentangling the Role of Chain Conformation on the Mechanics of Polymer Tethered Particle
Nano Lett. (2019)

The linear elastic properties of isotropicmaterials of polymer tethered nanoparticles (NPs) areevaluated using noncontact Brillouin light spectroscopy.While the mechanical properties of dense brush materialsfollow predicted trends with NP composition, a surprisingincrease in elastic moduli is observed in the case of sparselygrafted particle systems at approximately equal NPfillingratio. Complementary molecular dynamics simulations revealthat the stiffening is caused by the coil-like conformations ofthe grafted chains, which lead to stronger polymerpolymerinteractions compared to densely grafted NPs with shortchains. Our results point to novel opportunities to enhancethe physical properties of composite materials by the strategicdesign of themolecular architectureof constituents to benefit from synergistic effects relating to the organization of thepolymer component. more
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