The mechanical properties of electrospun fiber networks are critical in a range of applications from filtration to tissue engineering and are dependent on the adhesion between contacting fibers within the network. This adhesion is complex as electrospun networks exhibit a variety of contacts, including both cross-cylinder and parallel fiber configurations. In situ atomic force microscopy (AFM) was used to quantify the work of adhesion between a pair of individual electrospun polyamide fibers using controlled orientations and measurable contact areas. The work of adhesion was found to depend strongly on the fiber–fiber contact, with the separation of fibers in a parallel fiber configuration exhibiting considerably higher work of adhesion across a range of contact lengths than a cross-cylinder configuration. Our work therefore highlights direction-dependent adhesion behavior between electrospun fibers due to a suggested polymer chain orientation mechanism which increases net van der Waals interactions and indicates the variability of adhesion within a random electrospun fiber network.

The mechanical properties of graphene oxide (GO) paper are critically defined both by the mechanical properties of the constituent GO sheets and the interaction between these sheets. Functional carbonyl and carboxyl groups decorating defects, expected to be predominantly sheet edges of the GO, are shown to transfer forces to the in-plane carbon–carbon bonding using a novel technique combining atomic force microscopy (AFM) to mechanically deform discrete volumes of GO materials while synchrotron Fourier-transform infra-red (FTIR) microspectroscopy evaluated molecular level bond deformation mechanisms of the GO. Spectroscopic absorption peaks corresponding to in-plane aromatic CC bonds from GO sheets were observed to shift during tensile tests. Importantly, FTIR provided information on clear absorption peak shifts from CO bonds linking along the GO sheet edges, indicating transfer of forces between both CC and CO bonds during tensile deformation. Grüneisen parameters were used to quantitatively link the macroscopic FTIR peak shifts to molecular level chemical bond strains, with relatively low bond strains prevalent when applying external forces to the GO paper suggesting probing of hydrogen bonding interactions. We propose a mechanistic description of molecular interactions between GO sheets in the paper from these experiments, which is important in future strategies for further modification and improvement of GO-based materials.

The mechanical properties of electrospun fiber networks are critical in a range of applications from filtration to tissue engineering and are dependent on the adhesion between contacting fibers within the network. This adhesion is complex as electrospun networks exhibit a variety of contacts, including both cross-cylinder and parallel fiber configurations. In situ atomic force microscopy (AFM) was used to quantify the work of adhesion between a pair of individual electrospun polyamide fibers using controlled orientations and measurable contact areas. The work of adhesion was found to depend strongly on the fiber–fiber contact, with the separation of fibers in a parallel fiber configuration exhibiting considerably higher work of adhesion across a range of contact lengths than a cross-cylinder configuration. Our work therefore highlights direction-dependent adhesion behavior between electrospun fibers due to a suggested polymer chain orientation mechanism which increases net van der Waals interactions and indicates the variability of adhesion within a random electrospun fiber network.

Graphene oxide (GO) paper is a promising candidate for novel applications in energy storage systems such as electrical batteries, supercapacitors and multi-layered composites where the material undergoes deformation mechanisms. In particular, the strength of graphene oxide paper is critical in such applications and is defined by the interaction between the GO sheet constituents of the paper. The deformation behavior and tensile strength of focused ion beam (FIB) fabricated GO micro-beams was measured using in situ atomic force microscopy (AFM). GO sample deformation and failure was dependent on both the size of the micro-beams and the environmental testing conditions. Specifically, the failure stress of GO paper micro-beams tensile tested in air was found to increase when compared to testing in vacuum. This environmental dependent tensile strength of GO paper is attributed to water promoting stress transfer between GO sheets within the paper for higher strength during air testing while vacuum conditions remove water, leading to poor stress transfer between GO sheets for lower tensile strength results. A two-parameter Weibull distribution is introduced to quantify the micro-beam size dependent strength, which is attributed to interfacial defects determining GO paper failure strength.

The depth distribution of a TiO2 pigment within the polyurethane (PU) coil coatings is investigated using step scan phase modulation photoacoustic (SS-PM-PA) FTIR. Coil coatings with different pigment contents were prepared and the modulation frequency (MF) of the SS-PM-PA FTIR varied to record the depth distribution of the pigment within the coating. The TiO2 pigment was shown to contribute significantly to the SS-PM-PA FTIR signal. A TiO2 aggregated region within the topcoat is found close to the topcoat-primer interface and further away from the topcoat surface. A deeper TiO2 aggregated region can be identified when pigment content is relatively low. The SS-PM-PA FTIR signal shows a considerable contribution from the primer originated signal, provided the TiO2 pigment content is sufficiently high and the modulation frequency applied is relatively low. SEM cross-section imaging results show a strong correlation of the TiO2 depth distribution with SS-PM-PA FTIR results, which confirms the applicability of the SS-PM-PA FTIR technique to the depth profiling study of TiO2 pigmented coil coatings.Highlights► Step scan phase modulation photoacoustic (SS-PM-PA) FTIR reveals TiO2 depth profile. ► TiO2 aggregates close to topcoat-primer interface in coil coating topcoat. ► Lower pigment content gives deeper TiO2 aggregated region. ► High TiO2 content, low modulation frequency result in high primer signal contribution. ► TiO2 depth profiling is verified by scanning electron microscopy (SEM).

A step scan phase modulation photo-acoustic (SS-PM-PA) Fourier transform infrared (FTIR) peak fitting method has been developed and applied to study (i) the degradation of the polyurethane (PU) coatings crosslinked with different isocyanates, (ii) the harshness of the natural exposure sites, and (iii) the correlation between the accelerated and natural exposure sites in terms of the degradation. Methyl ethyl ketoxime (MEKO) blocked hexamethylene diisocyanate biuret (HDI-BI), MEKO blocked hexamethylene diisocyanate cyclic trimer (HDI-CT) and 3,5-dimethyl pyrazole (DMP) blocked isophorone diisocyanate cyclic trimer (IPDI-CT) were used as-received to crosslink a cycloaliphatic saturated polyester resin binder. It was found that HDI-CT crosslinked PU coating is more durable compared to the HDI-BI. IPDI-CT crosslinked PU coating gives higher durability than the HDI-CT and the HDI-BI. The areas of deconvoluted peaks with Centre X (cm−1) = 1573, 1553 and 1535 (amide II, NH–CO) and Centre X (cm−1) = 1832, 1813, 1792 and 1770 (acid, anhydride, peracid) are used for degradation index calculation, due to their consistent decreasing and increasing trends, respectively, along with the degradation. It has been found that the natural weathering site in Kuala Lumpur, Malaysia (KL) is harsher than that at Vereeniging, South Africa (SA). The harshness of one year SA and KL natural weathering is comparable to 300 h–900 h of the QUV A exposure test. FTIR peak fitting method outperformed the integration method by giving a better correlation between the accelerated and natural weathering tests in terms of the degradation.

Electrospinning using positive and negative polarity applied voltages is used to produce polyamide nanofibers with tailored surface functionality. The surface free energy of the resultant nanofibers is characterized from individual nanofiber wetting experiments. The polar contribution to the total nanofiber surface energy is seen to vary with the polarity of the applied voltage used. A mechanism to describe the change in the nanofiber surface free energy with electrospinning polarity is proposed, based on the formation of either positive or negative charges at the liquid jet–air interface during the electrospinning process. These charges at the liquid jet–air interface cause molecular orientation of chemical functional groups of the polymer chains during electrospinning and the mechanism is supported by subsequent grazing angle X-ray photoelectron spectroscopy (XPS) of the nanofiber surfaces. A one-step electrospinning process is therefore demonstrated to tailor specific chemical functionalities at polymer nanofiber surfaces.

Engineered fiber reinforced polymer composites require effective impregnation of polymer matrix within the fibers to form coherent interfaces. In this work, we investigated solution interactions with electrospun fiber mats for the manufacture of nanocomposites with optimized mechanical properties. Void free composites of electrospun nonwoven PA6 nanofibers were manufactured using a PVA matrix that is introduced into the nonwoven mat using a solution-based processing method. The highest failure stress of the composites was reported for an optimum 16 wt % of PVA in solution, indicating the removal of voids in the composite as the PVA solution both impregnates the nanofiber network and fills all the pores of the network with PVA matrix upon evaporation of the solvent. These processing methods are effective for achieving coherent nanofiber–matrix interfaces, with further functionality demonstrated for optically transparent electrospun nanofiber composites.Keywords: composite; electrospinning; mechanical properties; nylon 6; PVA;

The mechanical properties of rat bone at micron length scales have been evaluated as a function of environmental conditions using an in situ atomic force microscope (AFM) setup while observing using scanning electron microscopy (SEM). Focused ion beam fabricated rat bone cantilever samples were tested in both low and high vacuum conditions in the SEM as well as wet in air using the AFM to measure their elastic modulus. The elastic modulus of rat bone at micron length scales is shown to be independent of the environmental testing conditions and indicates water is bound to bone material even under relatively high vacuum conditions. Our work therefore shows how in situ mechanical testing of bone while observing using high resolution SEM can provide results similar to testing wet in air.Highlights► AFM-SEM in situ technique to mechanically test micro cantilever beams of rat bone. ► Modulus of bone at micron level does not change with vacuum testing conditions. ► Results show water is still bound to bone after two hours in SEM environment. ► In situ SEM observation of hydrated bone micro mechanical testing is possible.

The elastic modulus of individual electrospun polyvinyl-alcohol (PVA) fibers was measured using atomic force microscopy (AFM) based bending tests. Results indicated an increase in the elastic modulus as the fiber diameter decreased. Consideration of electrospun fibers as a composite structure consisting of a shell region of aligned polymer chains surrounding a bulk-like isotropic core was analytically modeled and showed good agreement with the experimental data. Phase contrast AFM imaging of focused ion beam (FIB) prepared PVA fiber cross-sections provided a method of observing the shell–core structure directly and supported the model proposed. These findings indicate the potential for considerable polymer chain alignment for the production of high performance fibers using electrospinning methods.Graphical abstract

The fracture toughness of a noncontinuum fibrous network produced by electrospinning polyamide 6 nanofibers is investigated. The mechanical properties of the nanofiber network is observed to be independent of various incorporated macroscopic crack lengths, resulting in an apparent increase in fracture toughness with increasing crack length as evaluated using conventional fracture mechanics. Strain mapping of the nanofiber network indicates stress delocalization mechanisms operating around these macroscopic cracks in the network. The deformation behavior of the nanofiber network will therefore depend on the volume of fibers being loaded in the network and not the number of fibers in the cross-sectional width defining continuum sample mechanics. These results indicate a propensity for both the synthetic electrospun nanofibrous network in this work and potentially other nanofibrous networks to resist failure from macroscopic cracks incorporated within the material.Keywords: electrospinning; fiber networks; mechanics; stress delocalization

Nanofibers of polyamide have been synthesized using electrospinning processes and their wetting properties determined directly from a nanoscale Wilhelmy balance approach. Individual electrospun polyamide nanofibers were attached to atomic force microscope (AFM) tips and immersed in a range of organic liquids with varying polar and dispersive surface tension components. AFM was used to measure nanofiber-liquid wetting forces and derive contact angles using Wilhelmy balance theory. Owens−Wendt plots were used to show a considerable increase in the polar component of the surface free energy of the polyamide nanofibers compared with bulk film of the same polymer. Chemical surface analysis of the polyamide nanofibers and films using X-ray photoelectron spectroscopy provided evidence for enhanced availability of polar oxygen groups at the electrospun nanofiber surface relative to the film. Our results therefore confirm chemical group orientation at the electrospun polyamide nanofiber surface that promotes availability of polar groups for enhanced wetting behavior.

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•Phase contrast AFM used to investigate stiffness variation in FIB milled polymers.•Low ion beam energy induces a stiffening of the polymer surface.•High ion beam energy produces surfaces representative of bulk material.•Simulation and EDS data used to propose mechanism.•Optimal range of ion beam energies suggested for milling polymeric samples.Focused ion beam (FIB) microscopy uses Ga+ ions to remove material from a sample for a variety of imaging and preparation techniques. While considerable work has examined the effects of FIB exposure on a number of materials, optimized FIB conditions for use with softer polymeric materials are yet to be determined. In this report we use phase contrast AFM to measure local changes in the elastic modulus of polycarbonate surfaces parallel to a sectioning FIB at varying beam energies. We show that polycarbonate surfaces exposed to lower FIB energies appear stiffer than the bulk material whereas surfaces exposed to the higher beam energies of up to 25 keV are more representative of the bulk material. Energy dispersive spectroscopy (EDS) indicates that the polymer surfaces become stiffer because of Ga+ implantation from the FIB. Our experimental observations are supported by computer simulations showing an increase in the residual Ga+ concentration near-surface at lower FIB energies. A high energy FIB is therefore shown to be less invasive, producing a surface more representative of the bulk material, than using low energy FIB when sectioning polymers.

Electrospinning using positive and negative polarity applied voltages is used to produce polyamide nanofibers with tailored surface functionality. The surface free energy of the resultant nanofibers is characterized from individual nanofiber wetting experiments. The polar contribution to the total nanofiber surface energy is seen to vary with the polarity of the applied voltage used. A mechanism to describe the change in the nanofiber surface free energy with electrospinning polarity is proposed, based on the formation of either positive or negative charges at the liquid jet–air interface during the electrospinning process. These charges at the liquid jet–air interface cause molecular orientation of chemical functional groups of the polymer chains during electrospinning and the mechanism is supported by subsequent grazing angle X-ray photoelectron spectroscopy (XPS) of the nanofiber surfaces. A one-step electrospinning process is therefore demonstrated to tailor specific chemical functionalities at polymer nanofiber surfaces.