Directly Probing the Fracture Behavior of Ultrathin Polymeric Films

Plane Stress Fracture Toughness Testing of Freestanding Ultra-Thin Nanocrystalline Gold Films on Water Surface

Fracture toughness, which is the resistance of a material to crack propagation, is a critical material property for ensuring the mechanical reliability of damage-tolerant design. Recently, damage-tolerant design is introduced to flexible electronics by adopting micro-cracked ultra-thin nanocrystalline (NC) gold films as stretchable electrodes in a plane stress state. However, experimental investigation of the plane stress fracture toughness of those films remains challenging due to the intrinsic fragility from their sub-100 nm thicknesses. Here, a quantitative method for systematically evaluating the plane stress fracture toughness of freestanding ultra-thin NC gold film on water surface platform is presented. After effectively fabricating single-edge-notched-tension samples with femtosecond laser, mode I stress intensity factors are measured in the plane stress state on water surface. Moreover, investigation regarding the effect of notch length, notch sharpness, and notch tip plasticity validates this method based on linear elastic fracture mechanics theory. As a demonstration, the thickness-dependent plane stress fracture toughness of ultra-thin NC gold films is qualitatively unveiled. It is revealed that the thickness confinement effect on grain boundary sliding induces a transition in fracture behavior. This method is expected to further clarify the fracture-related properties of various ultra-thin films for next-generation electronics.

Evaluating Free Thermal Expansion and Glass Transition of Ultrathin Polymer Films on Heated Liquid

Thermomechanical properties of ultrathin films are crucial for fabrication and use of reliable thin electronic devices. Due to the lack of precise measurement techniques, the thermal deformation behavior of ultrathin films has not yet been clarified. Here, we propose a film on heated liquid (FOHL) method to simultaneously measure the coefficient of thermal expansion (CTE) and glass transition temperature (Tg) of multiple ultrathin polymer films. Free thermal expansion of thin films without substrate interaction can be guaranteed when the thin films are afloat on a liquid surface. To investigate the thermal behavior in a wide temperature range, glycerol is adopted as a thermally stable heating platform owing to its high boiling point of 290 °C. The thin films are transferred onto the glycerol surface from the water surface using the hygroscopic properties of glycerol. Highly accurate and high-throughput thermal strain measurement is achieved using three-dimensional digital image correlation (3D-DIC). The thermomechanical properties of ultrathin polystyrene thin films of various thicknesses (25-400 nm) are precisely characterized utilizing the FOHL and 3D-DIC method.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface

The phase behavior of block copolymers (BCPs) at the water-oil interface is influenced by the segmental interaction parameter ( χ ${\chi }$ ) and chain architecture. We synthesized a series of star block copolymers (s-BCPs) having polystyrene (PS) as core and poly(2-vinylpyridine) (P2VP) as corona. The interaction parameters of block-block ( χ ${\chi }$ PS-P2VP ) and block-solvent ( χ ${\chi }$ P2VP-solvent ) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized-P2VP (Q-P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s-BCP matrix, while nanotubes comprised Q-P2VP as corona and PS as core. Control over pore size, d-spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s-BCPs. Large-scale nanoporous films were obtained by freeze-drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal-packed rigid spherical micelles with Q-P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well-controlled pore structure.

Microstructural Evolution of P(NDI2OD-T2) Films with Different Molecular Weight during Stretching Deformation

Conjugated polymers exhibit excellent electrical and mechanical properties when their molecular weight (Mw) is above the critical molecular weight (Mc). The microstructural changes of polymers under strain are crucial to establish a structure-performance relationship. Herein, the tensile deformation of P(NDI2OD-T2) is visualized, and cracks are revealed either along the (100) crystal plane of side chain packing or along the main chain direction which depends on the Mw is below or above the Mc. When Mw < Mc, the film cracks along the (100) plane under small strains. When Mw > Mc, the polymer chains first undergo stretch-induced orientation and then fracture along the main chain direction at large strains. This is attributed to the fact that the low Mw film exhibits large crystalline domains and the absence of interdomain connectivity, which are vulnerable to mechanical stress. In contrast, the high Mw film displays a nearly amorphous morphology with adequate entanglements, the molecular chains can endure stresses in the stretching direction to release substantial strain energy under greater mechanical deformation. Therefore, the film with Mw > Mc exhibits the optimal electrical and mechanical performances simultaneously, i.e., the electron mobility is retained under 100% strain and after 100 stretching-releasing cycles.

Tough-interface-enabled stretchable electronics using non-stretchable polymer semiconductors and conductors

Semiconducting polymer thin films are essential elements of soft electronics for both wearable and biomedical applications^1-11. However, high-mobility semiconducting polymers are usually brittle and can be easily fractured under small strains (<10%)^12-14. Recently, the improved intrinsic mechanical properties of semiconducting polymer films have been reported through molecular design^15-18 and nanoconfinement¹⁹. Here we show that engineering the interfacial properties between a semiconducting thin film and a substrate can notably delay microcrack formation in the film. We present a universal design strategy that involves covalently bonding a dissipative interfacial polymer layer, consisting of dynamic non-covalent crosslinks, between a semiconducting thin film and a substrate. This enables high interfacial toughness between the layers, suppression of delamination and delocalization of strain. As a result, crack initiation and propagation are notably delayed to much higher strains. Specifically, the crack-onset strain of a high-mobility semiconducting polymer thin film improved from 30% to 110% strain without any noticeable microcracks. Despite the presence of a large mismatch in strain between the plastic semiconducting thin film and elastic substrate after unloading, the tough interface layer helped maintain bonding and exceptional cyclic durability and robustness. Furthermore, we found that our interfacial layer reduces the mismatch of thermal expansion coefficients between the different layers. This approach can improve the crack-onset strain of various semiconducting polymers, conducting polymers and even metal thin films.

The Interplay of Polymer Bridging and Entanglement in Toughening Polymer-Infiltrated Nanoparticle Films

Polymer-nanoparticle composite films (PNCFs) with high loadings of nanoparticles (NPs) (>50 vol %) have applications in multiple areas, and an understanding of their mechanical properties is essential for their broader use. The high-volume fraction and small size of the NPs lead to physical confinement of the polymers that can drastically change the properties of polymers relative to the bulk. We investigate the fracture behavior of a class of highly loaded PNCFs prepared by polymer infiltration into NP packings. These polymer-infiltrated nanoparticle films (PINFs) have applications as multifunctional coatings and membranes and provide a platform to understand the behavior of polymers that are highly confined. Here, the extent of confinement in PINFs is tuned from 0.1 to 44 and the fracture toughness of PINFs is increased by up to a factor of 12 by varying the molecular weight of the polymers over 3 orders of magnitude and using NPs with diameters ranging from 9 to 100 nm. The results show that brittle, low molecular weight (MW) polymers can significantly toughen NP packings, and this toughening effect becomes less pronounced with increasing NP size. In contrast, high MW polymers capable of forming interchain entanglements are more effective in toughening large NP packings. We propose that confinement has competing effects of polymer bridging increasing toughness and chain disentanglement decreasing toughness. These findings provide insight into the fracture behavior of confined polymers and will guide the development of mechanically robust PINFs as well as other highly loaded PNCFs.