November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
Determination of the mechanical response of polymeric materials with dimensions less than 100 nm is a continuing challenge. Here we describe a novel membrane (“nano-bubble”) inflation method we have developed for the purpose of making measurements of the creep response of ultrathin polymer films and show two major findings. The first is that the material dynamics as measured by the creep response of the membranes depends dramatically on film thickness. For example, in polystyrene films, the dynamics is accelerated so much that the glass transition temperatureTg of a 11 nm thick film is reduced by approximately 50 K relative to the macroscopic Tg . Furthermore, we have discovered that the nominal rubbery plateau in ultrathin films is stiffened by upwards of two orders of magnitude relative to the macroscopic state and the rate of stiffening (stiffening index S) correlates with the shape of the segmental relaxation in accordance with a recent model proposed by Ngai, Prevosto and Grassia. We have elaborated this finding further and observe a strong correlation with the fragility index m that is related to glass formation according to the Angell categorization  of super cooled liquids. These results will be discussed in terms of current understanding of the impact of nanoconfinement on the glass transition behavior of polymers. In addition to being able to characterize the creep response of the ultrathin polymer films, we have also succeeded in adapting the bubble inflation method to make measurements on a graphene/polymer nano-sandwich structure and show that the method can be used to not only extract the stiffness of the graphene inner layer of the composite but that the method can be used to extract the interfacial shear strength of the polymer-graphene couple . P.A. O&’Connell, S.A. Hutcheson and G.B. McKenna, Journal of Polymer Science: Part B: Polymer Physics, 46, 1952-1965 (2008). K.L. Ngai, D. Prevosto and L. Grassia, Journal of Polymer Science: Part B: Polymer Physics, 51, 214-224 (2013). C.A. Angell, Journal of Non-Crystalline Solids, 73, 1-17 (1985). X. Li, J. Warzywoda and G.B. McKenna, Polymer, 55, 4976-4982 (2014).
The measurement of thin layer properties is important for both fundamental and technological reasons. Measurement of stress versus strain mechanics in one of the fundamental tenets upon which materials engineering is based. Direct measurements of elastic moduli and plasticity in nanometer thick polymeric glass layers is non-trivial, and one must often turn to indirect probes or small scale mechanical measurements that require complex interpretation. We introduce a novel strain-gradient free nanoscale compression test for thin soft layers on high stiffness substrates, which permits direct extraction of stress versus strain mechanics of the layer in elastic and plastic states from a single loading curve. Confined layer compression testing is built upon the compression of a precisely aligned flat probe into the layer forming a confined, stationary volume of highly compressed material in a well-defined, invariant geometry. Due to natural confinement from the surrounding material, we show that a state of uniaxial strain is created beneath the probe under small axial strains. Under a uniaxial strain stress state lateral strains within the film are zero throughout the entire volume under the probe. By this methodology we are able to directly probe via load-displacement measurement the stress-strain relationship in uniaxial flows under both reversible elastic and irreversible plastic conditions. Such nanoscale uniaxial strain testing is in some sense the complementary antithesis of nanopillar compression testing that has become so valuable for mechanical testing of size effects in hard materials. However, nanoscale uniaxial strain testing is suitable for in situ testing of both soft materials and continuous coatings on solid supporting substrates, in both elastic and plastic states. Confined compression measurements via instrumented nanoindentation apparatus or force microscopy apparatus provide new means to investigate mechanics and physics of thin films of structured or free volume materials such as glasses and biomaterials, including the plasticity of molecular materials, non-equilibrium states and behavior under the presence of high shear stress and hydrostatic pressure.
Secreted proteins experience multiple collisions with other biomolecules. To address how a single protein copes with collisions requires to measure its mechanical response in physiological-like conditions. Force microscopy methods have been applied to study the mechanical properties1,2 of proteins. However, most of the studies involving folded proteins are performed on two dimensional arrays3, and characterizing the nanomechanical response of a single, isolated protein remains a challenge. Here, we develop a force microscopy method to measure the stress-strain curve of a single antibody pentamer by applying forces in the 20 to 300 pN range. Elastic recovery is shown for up to 0.4 compressive strains. We report the Young modulus and the yield strength of the protein&’s central region, respectively, 3.1 MPa and 2.1 pN/nm2. The yield strength explains the capability of the protein to sustain multiple collisions without any loss of biological functionality. It also indicates the upper forces that could be used in force microscopy for the non-invasive imaging of β-sheet domains.Martinez-Martin , D., Herruzo, E. T., Dietz, C., Gomez-Herrero, J. & Garcia, R. Noninvasive protein structural flexibility mapping by bimodal dynamic AFM. Phys. Rev. Lett. 106, 198101 (2011).Dong, M.D. & Sahin, O. Determination of protein structural flexibility by microsecond force microscopy. Nature Nanotech.4, 514-517 (2009).Medalsy, I.D.; Muller, D.J. Nanomechanical properties of proteins and membranes depend on loading rate and electrostatic interactions. ACS Nano.2013,7, 2642-2650.
The Atomic Force Microscope (AFM) is an ideal tool for probing the nanometer-scale mechanical response of a free interface, with industrial applications in the design and quality control of nano-composite materials. Recent developments with dynamic AFM have enabled quantitative determination of tip-surface force with unprecedented resolution, speed and accuracy. The dominant paradigm for understanding these tip-surface forces builds upon contact mechanics, where force is considered to be a function of tip position, arising from elastic deformation in the contact volume. However, fundamental scaling arguments suggest that surface forces should dominate over volume forces at the nano-scale, especially with soft materials. Furthermore, many applications desire an understanding of the viscous, or energy-dissipating mechanical response of soft nano-fillers and their interphase to the surrounding matrix.To understand both the viscous and elastic nature of surface forces we must go beyond contact mechanics with its assumption of quasi-static force equilibrium, and consider the tip-surface interaction as a dynamic two-body problem. We describe a multifrequency measurement technique and analysis method for AFM which, similar to Dynamic Mechanical Analysis (DMA) extracts the force that is in phase with, and quadrature to the harmonic tip motion . We present measurements on several soft polymer blends that clearly show the significant effect of viscous forces arising from motion of the soft material surface. We introduce a new type of dynamic interaction model that takes in to account surface motion, allowing for a correct extraction of the elastic stiffness and viscous damping constants of the soft material interface. D. Platz et al. Nature Comm. 4, 1360 (2013).
Nanoindentation techniques have recently been adapted for the study of hydrated materials, including biological materials and hydrogels. There are unique challenges associated with testing hydrated materials in commercial instruments not designed for this application. Some key results from recent works using nanoindentation to evaluate hydrated materials including soft biological tissues, bulk hydrogels and thin hydrogel layers will be reviewed. Both natural and synthetic hydrogels have been characterized using indentation and nanoindentation across a wide range of experimental length-scales. The material response is shown to be greatly dependent on the chemical bonding within the hydrogel, i.e. whether the network is physically or chemically cross-linked. Hydrogels in particular are an attractive system for studying structure-properties relationships, as the water fraction can be systematically varied for a single polymer, and different polymers with the same water fraction can be compared. Based upon knowledge of the properties of each individual component, composite hydrogels can be created to mimic the overall response of complex biological materials to create multi-component tissue engineering scaffolds.
The application of polymeric materials in engineering is rapidly increasing, in both bulk and coating form, in technologies from automotive to plastic electronics to medical applications. Determining the mechanical properties is important for structural applications, high throughput screening of materials and understanding of polymer physics. Nanoindentation is a convenient means to determine mechanical properties but the response is often difficult to interpret. Unlike metal or ceramic materials, the mechanical response of polymers can depend on strain, strain rate, strain state and load history. The test method, contact geometry and load cycle can also strongly influence the measured response. In indentation, assumptions about the contact area derived from displacement measurements may not hold, and the induced strain gradients can lead to variable time-dependent responses within the strained volume, making determination of material parameters more problematic. It is often difficult to obtain values equivalent to tensile, flexure or dynamic test methods, with measured indentation modulus and hardness values often too high, leading to the supposition of size effects.Here we present the results of an inter-comparison study of indentation methods on a range of engineering polymers. Different indentation geometries and test methods are compared to tensile and dynamic data from the same materials. The results show that the choice of indenter geometry, load cycle and analysis method have a large influence on the measured mechanical property values obtained. The results imply that confinement of the polymer within the high pressure beneath the indenter can significantly change the elastic and flow properties, creating apparent size effects. However, comparative values for time-dependent elastic properties can be obtained using appropriate geometries and analysis methods. The work reported here formed part of the European ‘MeProVisc&’ project, providing background data for the planned ISO 14577 part 5, instrumented indentation measurement of time dependent materials.
There is a rapidly growing interest in using plate-like nano-fillers such as graphene to reinforce polymers. It is also known that the spatial orientation of the reinforcing elements in polymer-based composites plays a vital role in controlling mechanical properties. There is, however, no generally-accepted way of quantifying the spatial orientation at the nanoscale of plate-like fillers in nanocomposites.It was found in our previous study that the intensity of scattering of the Raman band is dependent on the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane and it was demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) could be used to quantify the spatial orientation of the graphene. Based on this approach, polarized Raman spectroscopy has been used to quantify, as an example, the level of spatial orientation of graphene oxide (GO) flakes in different nanocomposites using a variety of polymeric matrices. It is demonstrated further how it is possible to relate the spatial orientation of nanoplatelets to the mechanical properties of the in the nanocomposites and, from the stress-induced Raman band shifts, to stress transfer to the reinforcement. In particular, it has been possible to determine the well-known Krenchel orientation factor for these plate-like fillers directly from the experimental polarized Raman data.Apart from the quantification method for the spatial orientation of nanoplatelets in the nanocomposites, another significant finding of this study is that the Krenchel factor for 3D randomly-oriented nanoplatelets is 8/15. This means that random orientation of fillers such as graphene should reduce the Young&’s modulus of the nanocomposites by less than a factor of 2 compared with the fully-aligned material. Compared to the reduction in the modulus of a factor of 5 going from aligned to 3D randomly-oriented fibres and nanotubes, it means that better levels of reinforcement should be achievable with misaligned nanoplatelets compared with, for example, nanotubes and there is less need to ensure accurate alignment of nanoplatelets in composites. Beyond just graphene and GO, this approach should be more widely applicable to the determination of the orientation of other nanoplatelet fillers for which well-defined Raman spectra can be obtained. Moreover, the effect of spatial orientation upon the mechanical properties of composites predicted in te