No abstract

An atomic force microscope (AFM) based fast dynamic scanning indentation (DSI) nano-DMA method, which relies only on the commonly available capabilities of commercial AFMs to provide quantitatively accurate high-resolution (∼15 nm) spatial maps of local viscoelastic mechanical properties (E′, E″, and tan ϕ) in heterogeneous soft adhesive material systems, is described. The versatility of the DSI approach is demonstrated by successfully employing it on three industry-leading commercial AFMs/modules (Asylum’s Cypher ES and MFP-3D Infinity AFMs with the FastForceMapping module, and Bruker’s Dimension Icon AFM with the PeakForce QNM module). Frequency sweep thermorheological DSI experiments were performed to generate quantitatively accurate nano-DMA master curves spanning an unprecedented frequency range of 5 decades. Quantitative agreement between DSI nano-DMA and bulk DMA measurements is demonstrated for two different homogeneous elastomers (styrene butadiene rubber, SBR, and synthetic natural rubber, SNR). The capability of the DSI methodology in acquiring quantitatively accurate viscoelastic property maps of heterogeneous soft solids was validated through experiments on an SBR-SNR blend sample. Experimental factors affecting DSI data quality (e.g., shift factor and AFM tip size) are also discussed.

Stiffness gradients in geometrically confined polymers as measured by nanoindentation are influenced by opposing roles of the polymers viscoelastic state and the degree of confinement.

No abstract

Atomic force microscopy (AFM) is a powerful technique for imaging polymer nanocomposites as well as other systems with heterogeneous material properties on the nanoscale. However, the quantitative measurement of modulus is highly susceptible to convoluting structural effects due to the finite tip radius and stress field interactions with particles and substrates which are often termed the "substrate effect" or "thin film effect". We present an empirical master curve that can model the change in measured modulus (E MC ) due to structural effects in an AFM indentation on a soft material near a stiff filler, using N121 and N660 carbon black−styrene−butadiene rubber nanocomposites as examples. Finite element analysis is combined with experimental AFM data across an interface at increasing indentation depths to create a robust method for confirming or rejecting the presence of an interphase layer in AFM. From the raw data, which is initially inconclusive, we reasonably estimate the width of the loosely bound layer (ξ int ) surrounding each (strongly interacting) N121 particle to be 50−60 nm after deconvolving the substrate effect. In comparison, we found no significant loosely bound layer around the (weakly interacting) N660 particles. While we have demonstrated this technique for polymer nanocomposites, we believe the strategy is broadly applicable to multiphase soft materials.

Purpose The purpose of this study is to understand how printing parameters and subsequent annealing impacts porosity and crystallinity of 3D printed polylactic acid (PLA) and how these structural characteristics impact the printed material’s tensile strength in various build directions. Design/methodology/approach Two experimental studies were used, and samples with a flat vs upright print orientation were compared. The first experiment investigates a scan of printing parameters and annealing times and temperatures above the cold crystallization temperature (Tcc) for PLA. The second experiment investigates annealing above and below Tcc at multiple points over 12 h. Findings Annealing above Tcc does not significantly impact the porosity but it does increase crystallinity. The increase in crystallinity does not contribute to an increase in strength, suggesting that co-crystallization across the weld does not occur. Atomic force microscopy (AFM) images show that weld interfaces between printed fibers are still visible after annealing above Tcc, confirming the lack of co-crystallization. Annealing below Tcc does not significantly impact porosity or crystallinity. However, there is an increase in tensile strength. AFM images show that annealing below Tcc reduces thermal stresses that form at the interfaces during printing and slightly “heals” the as-printed interface resulting in an increase in tensile strength. Originality/value While annealing has been explored in the literature, it is unclear how it affects porosity, crystallinity and thermal stresses in fused filament fabrication PLA and how those factors contribute to mechanical properties. This study explains how co-crystallization across weld interfaces is necessary for crystallinity to increase strength and uses AFM as a technique to observe morphology at the weld.

No abstract

No abstract