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Resilience may be referred to as the capacity for positive adaptation and to quickly recover from difficulties and significant adversity. After examining operational definitions of related concepts, the article discusses resilience building exercises for functional fitness at the individual soldier level, to include among others, self-monitoring, self-evaluation, self-reinforcement, emotional regulation exercises, mindfulness training, relaxation and grounding exercises and importance of maintaining discipline and routine in the military. Using an acronym CARRIES, the article examines efforts to enhance resilience building through empirically validated cognitive, behavioural and intrapersonal skills originating from best practices learned in the military.

Casein finds application as an eco-friendly adhesive for paper, wood, glass, etc. Casein being a protein can undergo conformational and microstructural changes during various processing steps involved in interfacial bonding. This study aims at understanding the multiscale contributions of these changes in casein to its adhesion to cellulose pressboards. Investigations spanning from molecular structure to macroscopic adhesion characteristics have been used in this work. The lap shear strength of casein bonded cellulose pressboards is found to increase with the increase in casein concentration. It was observed from Fourier transform infrared spectroscopy (FTIR) investigations along with microscopy and rheological studies that casein dispersions result in more α-helical conformations during the preconcentration process of casein dispersions. This results in increased hydrophobicity of the casein particles/aggregates, which in turn affects the wetting characteristics and the adhesion behavior. Casein compositions lacking α-helices were found to enhance the bonding strength of casein with cellulose. The present study shows that the adhesion between casein and microporous cellulose substrate has contributions at the multiscale originating from the polar-polar interactions of casein and cellulose molecules, conformational changes in the protein structure of casein during drying, microstructure of casein particles in the dispersion, and the microporous nature of the cellulose boards. These interactions at multiple scales can be tuned to suit different adhesive applications using casein.

Background: Individual differences in personality and resilience are related to a variety of social behaviors. The current study sought to answer the question of whether BMW drivers exhibit different personality profiles and resilience levels compared with drivers of other car brands. Participants and procedure: An international study was carried out in India, Latvia, Lithuania, Poland, Romania, Slovakia, and Spain on a sample of 448 athletes using the 20-item Mini-IPIP and the Resilience Scale. The results of BMW drivers (n = 91) were compared with the results of drivers of other German car brands (n = 357). Results: BMW drivers were characterized by higher neuroticism compared with drivers of other German car brands. They also showed higher resiliency, both in terms of total score and scores on the subscales of: personal coping competences and tolerance of negative emotions, tolerance of failures and perceiving life as a challenge, and optimistic attitude towards life and capacity for self-mobilization in difficult situations. The greatest difference was observed for the factor of tolerance of failures and perceiving life as a challenge. Using the Dwass-Steel-Critchlow-Fligner (DSCF) pairwise comparison test, gender differences between athletes (as BMW drivers and drivers of other German car brands, respectively) were discussed. Additionally, the results of the main logistic regression analyses emphasized that neuroticism represents a better predictor of BMW preference in the case of athletes (as drivers) than the scores obtained for resilience. Conclusions: BMW drivers differed from drivers of other German car brands only with regard to neuroticism. A higher level of neuroticism can affect mental health and the overall quality of life in athletes; aggression and distress management are essential. Athletes (as BMW drivers) also showed differences in resiliency levels. Understanding the mechanisms of behavior among BMW drivers is possible through considering their personality and individual differences.

A supramolecular hydrogel with 3D self-shrinking, without any assistance, and a shape memory performance at room temperature is discovered from an unnatural amino acid derivative

Plant cell walls undergo multiple cycles of dehydration and rehydration during their life. Calcium crosslinked low methoxy pectin is a major constituent of plant cell walls. Understanding the dehydration-rehydration behavior of pectin gels may shed light on the water transport and mechanics of plant cells. In this work, we report the contributions of microstructure to the mechanics of pectin-Ca gels subjected to different extents of dehydration and subsequent rehydration. This is investigated using a pectin gel composition that forms ‘egg-box bundles’, a characteristic feature of the microstructure of low methoxy pectin-Ca gels. Large Amplitude Oscillatory Shear (LAOS) rheology along with Small Angle Neutron Scattering and Near Infrared (NIR) spectroscopy on pectin gels are used to elucidate the mechanical and microstructural changes during dehydration-rehydration cycles. As the extent of dehydration increase, the reswelling ability, strain-stiffening behavior and the yield strain decreases. These effects are more prominent at faster rates of dehydration and are not completely reversible upon rehydration to the initial undried state. Microstructural changes due to the aggregation of egg-box bundles and single chains and the associated changes in the water configurations lead to these irreversible changes.

In this article, researchers will introduce readers to the concept of leadership through perspectives of different schools of leadership thoughts. We intend to bring to notice an important discussion on leadership in military organisations and its implications on non-military organisations and institutions. There is a plethora of literature especially borrowing from military literature, which can set the stage for our understanding of what could make up for a robust leadership model comprising of competencies like- power of personal example and influence, adaptive resilience, making critical decisions amid uncertainty, regulated leadership behaviors and thoughts, preparedness, communicating with teams and building trust.

While a considerable number of ultra-short/short amyloid peptides have been reported to form 3D supramolecular hydrogels, they all possess high minimum gelation concentration(MGC)(≥1wt%), which preclude their applications. In this context, we demonstrate that functionalisation of a well-known amyloidogenic ultra-short peptide fragment NFGAIL(IAPf) of human Islet amyloid polypeptide with a π-system (Fluorenyl, Fm) at the N-terminus of the peptide(Fm-IAPf) yield not only highly thermostable hydrogel at physiological pH but also exhibited super gelator nature as the MGC(0.08wt%) falls below 0.1wt%. Various experimental results confirmed that aromatic π-π interactions from fluorenyl moieties and hydrogen bonding interactions between the IAPf drive the self-assembly/fibril formation. Fm-IAPf is the first super hydrogelator derived from amyloid-based ultra-short peptides, to the best of our knowledge. We strongly believe that this report, i.e., functionalization of an amyloid peptide with π-system, provides a lead to develop super hydrogelators from other amyloid-forming peptide fragments for their potential applications.

Hydration and crosslinking in hydrophilic ionic polymers give rise to microstructural features that affect the diffusion of water and proton conductivity. In this work, we show that the local heterogeneities arising from domains that are more cross-linked and regions that are less cross-linked result in differential swelling behavior during diffusion of water and differential stresses during oscillatory deformation. Distinct signatures in the water uptake kinetics and the dynamic mechanical behavior are shown to be due to these heterogeneities, which are prominent at high and intermediate water contents and are unnoticeable or absent at low water contents. Using polyvinyl alcohol (PVA) cross-linked with sulfosuccinic acid (SSA) as a sample system, we show that differential swelling can lead to anomalous diffusion. In addition, the deformation of these polymers, carried out through large amplitude oscillatory shear experiments, results in strain hardening behavior due to the increased viscous dissipation arising from the differential movements of the polymer network due to the local heterogeneities. The proton conductivity of these materials is affected not only by these heterogeneities but also by the water distribution among the two types of hydrophilic groups (–SO3H and –OH) present in the system. Overall, we demonstrate the important role of crosslink heterogeneities and competitive hydration on the transport and deformation behavior of cross-linked hydrophilic ionic polymer systems. Hydration and crosslinking in hydrophilic ionic polymers give rise to microstructural features which affect diffusion of water and proton conductivity in them. Crosslinking in these systems gives rise to the presence of crosslink heterogeneities resulting in more-crosslinked domains and less-crosslinked regions. Because of the difference in elasticity between the two regions the polymer matrix swells differentially resulting in anomalous water sorption kinetics. The diffused water is distributed among hydroxyl and sulfonic groups and the crosslinking alters this distribution resulting in an increase in conductivity.

Flow of fluids through porous media such as soil, rocks, bones, wood and concrete has been an area of interest for engineering communities that deal with porous materials. The tools developed to describe flow through porous regions are generally applied to bulk materials with single composition. In this paper, however, we demonstrate their applicability to an interface of any two contacting surfaces particularly those of the elastomer and metal in the context of sealing applications. We propose a relationship to account for change in the permeability and the thickness of the seal interface with contact pressure at the interface and fluid pressure of fluid flowing through the interface. We hence develop a porous-media-based model to describe leak of gas through metal–elastomer interface. We validate the model using nitrogen gas leak experimental data for different elastomer–metal interfaces.

Elastomeric seals are devices that are widely used to prevent fluid leakage through the interface of a variety of mating industrial components. Gas leak through these seals involves gas flow through the interface of the seal and its corresponding mating component. This flow is understood to initiate due to separation of the surfaces at the sealing interface due to gas pressure. Modelling this separation can aid in optimizing seal designs and in estimating seal failure. In this paper, we develop a simple wedge penetration-based model to describe the separation. The frame work of the model is generic so that it can represent gradual as well as complete seal failures. We illustrate the applicability of this model for elastomer–metal interfaces which involve gradual separation. Nitrogen gas leak rate experimental data for stainless steel with three different elastomers are described using the model. We further discuss the processes and challenges involved in employing this model to estimate leak rate of gas through elastomeric seals of different geometries.

Evaluation of transport properties of a drug in a hydrogel matrix is of engineering importance in the assessment of drug delivery systems. Water sorption and solute release are the two important aspects that determine the transport of water/solute in a hydrogel. A novel custom-made diffusion cell is used to examine the transport properties in the hydrogels by chemically cross-linking hyaluronic acid in various alkaline solutions at different cross-linker-to-polymer ratios. The tracer diffusion coefficient in the swollen hydrogel matrix in its rubbery state is estimated using a custom-made diffusional cell. The release kinetics is followed at perfect sink conditions, which transform from a glassy polymer dry membrane state to a swollen rubbery gel state. It is shown that the diffusion of solvent (water) through the hydrogel is case II transport, while the diffusion through the swollen hydrogel is case I (Fickian). The diffusion coefficient is proved to be independent of mesh size and pH of the initial solution. However, the release kinetics is influenced by the diffusion of solute and the relaxation of the polymer in hydrogel. The relaxation coefficient is a function of alkalinity in the solution with which the gel is being made. The contribution of relaxation to the transport during solvent sorption and solute release is found to be similar.

Under circumstances such as the COVID-19 pandemic, decompression is essential to slowly overcome the lockdown stressors as a transition period between lockdown and resuming work in a manner that is no longer going to be ‘business as usual’. Firstly, we examine what is decompression in the context of reducing overwhelming and unwanted pressure emanating from the pandemic. Secondly, we reiterate the objectives and goals of decompression. Thirdly, we list ways in which one can decompress as a suitable way of endowing us with better psychological and much needed emotional support in pandemic times. Finally, the chapter offers guidelines for future research as this aspect has not been researched much and opens up new avenues in the field of psychosocial research in civilian as well as military contexts which brings the need for psychological debriefing to the forefront. The results of psychological disaster research to foresee, reduce and soothe the psychological effects of mass disasters – in this case, the global COVID-19 pandemic, maybe reconstructive.

In several applications, a protein such as casein in dispersion form undergoes multiple processing steps including drying. In this work, the rheological and microstructural features of casein dispersions concentrated by evaporation of the solvent (drying dispersions) were studied in comparison with those of equal concentrations of the as-prepared dispersions without drying. The molecular assembly of casein is affected by drying along with the conformational composition changes in the secondary structures such as α-helix, β-sheets, turns and random structures of the protein. Modeling of the rheological data indicates that these changes also affect the packing of casein molecular assemblies and these molecular assemblies in alkaline dispersions can behave as soft deformable particles. During drying, casein dispersions show prominent shear thinning for concentrations higher than 20 wt% along with the prevalence of α-helices and β-sheets. In comparison, the as-prepared dispersions show different microstructural features, and therefore different rheological responses. A detailed analysis shows that alkalinity changes during drying is the crucial factor controlling the microstructural changes of the soft casein particles and hence the rheology.

Microcapsules are widely used by researchers in self-healing composites. In this study, multi-walled carbon nanotubes (CNT) were incorporated into the core of the microcapsules, along with the self-healing agent. Dicyclopentadiene (DCPD) and urea-formaldehyde (UF) were chosen as the core and shell materials respectively, and DCPD–CNT–UF based dual core microcapsules were synthesized. Two types of microcapsules, namely, DCPD–UF and DCPD–CNT–UF were successfully synthesized by the in situ polymerization technique. The novelty of this work is the development of dual core microcapsules with DCPD–CNT–UF combination. Surface morphology characterization and elemental analysis of the microcapsules were carried out using a scanning electron microscope (SEM-EDX). TGA and DSC analysis show that DCPD–CNT–UF microcapsules have better thermal stability than DCPD–UF microcapsules. These novel DCPD–CNT–UF microcapsules were found to be compatible with epoxy base resin for making resin castings. The presence of CNT is found to improve the mechanical, thermal and electrical properties of the resin cast specimens without compromising on self-healing efficiency.

Microencapsulation is a widely used method for making healing agents used in self-healing composites. In this study, a novel two-stage process was used to make double-walled microcapsules. Dicyclopentadiene–urea formaldehyde (DCPD–UF) microcapsules were synthesized by in situ polymerization of oil-in-water emulsion followed by siloxane coating through ‘sol–gel process’ (DCPD–UF–siloxane microcapsules). Average diameter of microcapsules, UF shell thickness and siloxane coating thickness were found to be 300, 1.4 and 16 µm, respectively. The effect of addition of microcapsules on rheological properties of epoxy was studied. Breaking pattern of single-walled and double-walled microcapsules immersed in epoxy was analyzed by continuous monitoring of the deformation behavior through a rheometer–microscope arrangement, confirming improved mechanical properties of the double-walled microcapsules. In this study, epoxy resin cast specimens with and without microcapsules were prepared and the effect of microcapsules on mechanical properties was examined. Epoxy specimens with double-walled microcapsules were found to be having improved mechanical properties compared to those with single-walled microcapsules. Finally, healing efficiency of DCPD–UF–siloxane microcapsules in epoxy was observed to be marginally higher, and therefore, this double-walled microcapsule system is shown to be a promising candidate for further self-healing composite investigations.

Pluronic F-127 (Pl), a triblock copolymer that exhibits thermoreversible sol-gel transition, is being used with additives such as polysaccharides for enhanced gel stability and mechanical strength for controlled drug delivery applications. This study aims at elucidating the gelation behavior and structure-property relationship (at physiological temperature) of pluronic in the presence of hyaluronic acid (HA) using linear and nonlinear rheology. Small amplitude oscillatory shear of the gels indicated weak frequency dependence of G ′ and G ′ ′. Large amplitude oscillatory shear (LAOS) response indicated flow induced yielding with both the moduli following power law decay beyond the critical strain amplitude. Stress waveform analysis, Fourier analysis, and Chebyshev decomposition indicated similar qualitative features for gels with different HA content. Interestingly, there is a non-monotonic variation in the dynamic light scattering timescales, G ′ (linear regime), and the critical stress values (at corresponding critical strains) with HA content. The physical insights obtained were understood in terms of a cage picture of soft glassy materials. Relaxation times corresponding to yielding transition obtained from strain rate frequency superposition measurements showed similar qualitative variation as those obtained from elastoviscoplastic modeling of the yielding processes occurring during an LAOS cycle. Based on these analyses, we obtain quantitative description of nonlinear rheological response of the Pl-HA gels.Pluronic F-127 (Pl), a triblock copolymer that exhibits thermoreversible sol-gel transition, is being used with additives such as polysaccharides for enhanced gel stability and mechanical strength for controlled drug delivery applications. This study aims at elucidating the gelation behavior and structure-property relationship (at physiological temperature) of pluronic in the presence of hyaluronic acid (HA) using linear and nonlinear rheology. Small amplitude oscillatory shear of the gels indicated weak frequency dependence of G ′ and G ′ ′. Large amplitude oscillatory shear (LAOS) response indicated flow induced yielding with both the moduli following power law decay beyond the critical strain amplitude. Stress waveform analysis, Fourier analysis, and Chebyshev decomposition indicated similar qualitative features for gels with different HA content. Interestingly, there is a non-monotonic variation in the dynamic light scattering timescales, G ′ (linear regime), and the critical stress values...

Supramolecular gels are three-dimensional network structures formed by the hierarchical self-assembly of small molecules through weak noncovalent interactions. On the basis of the various interactions contributed by specific functional groups/moieties, gels with different architectures can be constructed that are smart to the external stimuli such as pH, type of solvent, stress, temperature, etc. In the present work, we explore the oscillatory shear response of supramolecular self-assembled systems formed by the low-molecular-weight (LMW) gelator based on difunctionalized amino acid, florenylmethoxycarbonyl (Fmoc)-lysine(Fmoc), Fm-K(Fm) in aqueous buffer solution, at two different pH (6 and 7.4). Small amplitude oscillatory shear (SAOS) reported weak frequency dependence of moduli indicating a gel-like network structure. Large amplitude oscillatory shear (LAOS) indicated flow regimes dictated by yielding and subsequent network dynamics analogous to cagelike soft glassy events reported for colloidal systems. The three interval thixotropy test (3iTT) indicated recovery of moduli due to regelation contributed by the reversible interactions. A generalized network model framework is utilized to investigate the transient network characteristics of the Fm-K(Fm) gels. The "network creation" and "network loss" rates were chosen as exponential functions of scaled shear stress (=

Understanding the rheological behaviour of thermoplastic nanocomposites is important to obtain a concrete knowledge of their processability. The viscoelastic properties of nanocomposites are a reflection of their morphology. The study of flow and deformation of nanocomposites provides essential information related to prevalent interactions in the system as well as contribution from the dispersion of incorporated nanofillers. In the present study, plasticized polyvinyl chloride/reduced graphene oxide nanocomposites (PPVC/RGO) were fabricated using melt mixing technique with different filler concentration. Flow behaviour of the nanocomposites was analyzed using small amplitude oscillatory shear (SAOS) measurements and it indicated an enhancement in the storage modulus (G′), loss modulus (G′′) and complex viscosity (η*) with RGO content. This can be attributed to very good dispersion and reinforcing effect of RGO in PPVC matrix as supported by TEM and FTIR results. Weak gel model is used to fit the rheological parameters and is found to be in excellent agreement with the SAOS experiments. Thermal history of the prepared nanocomposites was learned using differential scanning calorimetry. A shift in glass transition temperature (Tg) to higher temperature region could be seen, that manifest the effect of RGO in the amorphous portion of PPVC. An interesting property called secondary crystallinity was also found in these materials.

Non-linear mechanical responses observed in networks of many biopolymers such as pectin are important for their functioning as biological systems. Additionally, pectins derived from plant sources are also used for several food and biomedical applications. In the present work, the possible contributions of egg-box bundles in the large deformation response of calcium crosslinked gels of low methoxy pectin are explored using large amplitude oscillatory shear (LAOS). The gels exhibit a significant overshoot in the loss modulus (G'') and intra-cycle strain-stiffening, more prominent at greater extents of egg-box bundling. This observation signifies the dissipation characteristics of the egg-box bundles in pectin gels, hitherto not reported. The observed non-linear signatures diminish when the extent of bundling as well as the bundle radius decreases below a critical value. We identify different pectin/Ca concentration regimes based on the semi-flexible/flexible nature of the gel network and the non-linear signatures. Monovalent salt addition prior to crosslinking is shown to modify the extent of bundling, thereby influencing the magnitude of G'' overshoot and strain-stiffening. The intensity of the G'' overshoot and the extent of strain-stiffening are correlated with the radius of the egg-box bundles obtained from small angle neutron scattering (SANS) data. However, analysis using strain-stiffening models indicates the possible contributions from the semi-flexible nature of egg-box bundles and single chains.

Enhanced upward mobility of a non aqueous phase liquid (NAPL) present in wet sand during natural drying, and in the absence of any external pressure gradients, is reported for the first time. This mobility was significantly higher than that expected from capillary rise. Experiments were performed in a glass column with a small layer of NAPL-saturated sand trapped between two layers of water-saturated sand. Drying of the wet sand was induced by flow of air across the top surface of the wet sand. The upward movement of the NAPL, in the direction of water transport, commenced when the drying effect reached the location of the NAPL and continued as long as there was significant water evaporation in the vicinity of NAPL, indicating a clear correlation between the NAPL rise and water evaporation. The magnitude and the rate of NAPL rise was measured at different water evaporation rates, different initial locations of the NAPL, different grain size of the sand and the type of NAPL (on the basis of different NAPL-glass contact angle, viscosity and density). A positive correlation was observed between average rate of NAPL rise and the water evaporation while a negative correlation was obtained between the average NAPL rise rate and the NAPL properties of contact angle, viscosity and density. There was no significant correlation of average NAPL rise rate with variation of sand grain size between 0.1 to 0.5mm. Based on these observations and on previous studies reported in the literature, two possible mechanisms are hypothesized -a) the effect of the spreading coefficient resulting in the wetting of NAPL on the water films created and b) a moving water film due to evaporation that "drags" the NAPL upwards. The NAPL rise reported in this paper has implications in fate and transport of chemicals in NAPL contaminated porous media such as soils and exposed dredged sediment material, which are subjected to varying water saturation levels due to drying and rewetting.

Crosslinked polymers are three-dimensional network structures formed by interactions that range from weak physical associations to strong covalent bonding. The inherent complexity in their structures results in rich and diverse rheological response at large deformations. In the present work, we investigate two crosslinked materials based on poly(vinyl alcohol) (PVA) using large amplitude oscillatory shear (LAOS). Physically crosslinked PVA-borax exhibited Giesekus-like linear response, and a gradual intra-cycle strain stiffening and shear thinning at large strains. These signatures are attributed to temporary junction points and stretching of chains between crosslinks in the system. In contrast, chemically crosslinked PVA-hyaluronic acid (PVA-HA) gels exhibited a weak frequency dependence of linear moduli and intra-cycle strain stiffening and shear thickening at large strains. PVA-HA contains permanent crosslinks that move apart, along with stretching of crosslinked and uncrosslinked chains, and these features are responsible for such LAOS signatures. Based on these insights, we propose modifications to existing models and obtain quantitative description of LAOS response of both PVA-borax and PVA-HA crosslinked systems.

The properties of the supramolecular hydrogel built from the self-assembly of low molecular weight (LMW) compound play a crucial role in selecting the hydrogel for specific or targeted applications. Instead of developing new hydrogels, tweaking the properties of existing, well-known hydrogels using guest compounds may provide a way to design new hydrogel materials with desired properties. In this context, the influence of a bifunctional molecule (guest), namely 4,4′-Azopyridine (APy), on the thermal stability and viscoelastic properties of the supramolecular hydrogels built from a single (Fluorenylmethoxycarbonyl-L-Phenylalanine, FmF / Fluorenylmethoxycarbonyl-L-Tyrosine, FmY) and two-component (FmF+FmY) systems were analyzed. APy can connect the gelators through hydrogen bonding, and thereby it can tweak the property of the hydrogel. Various biophysical studies indicated that APy increased the gelation time of single and two-component systems while significantly improving the viscoelastic nature of both systems. Unlike FmF hydrogel, the regelation (thixotropy) efficiency of a two-component FmF+FmY gel system with APy showed complete recovery, suggesting that tweaking of thixotropic efficiency depends on the type of component gel systems. The Tgel of FmF gel was unaltered, while the two-component gel system had decreased stability in the presence of APy, suggesting that guest molecule can modulate the thermal stability of the two-component gel system. Gel characterization using various spectroscopic and microscopic techniques suggested that supramolecular self-assembly of gelators in a single and two-component systems with APy leads to fibrillar morphology, which is stabilized through intermolecular aromatic π-π stacking and hydrogen bonding interactions. NMR and FTIR analysis confirmed the interaction of APy with gelators in the self-assembled structure. Thus, the contemporary study demonstrated that the mechanical and thermal stability of the existing and/or new supramolecular gel systems could be modulated by co-assembly with guest molecules.

The rheological behavior such as yielding of fat crystal networks are dictated by many variables. Among these variables, the shape of the constituent fat cluster is important yet relatively unexplored. In this work, we describe the rheological investigations of a fat-oil system which can be formulated to either contain bundles of needles or spherical clusters by controlling the cooling rate and fat concentration. Fat-oil mixtures containing high-fat concentrations exhibited weak frequency dependence of storage modulus (G ′) and loss modulus (G ″). The yielding behavior of the mixtures were investigated by large amplitude oscillatory shear (LAOS) rheology using strain and stress controlled modes. Lissajous-Bowditch plots and orthogonal set of Chebyshev polynomials were used to analyze the non-linearities associated with the yielded fat-oil mixtures. For a given fat concentration, the yield stress of fat networks obtained at low cooling rates (bundles of needles) were similar to that of networks obtained at high cooling rates (spherical clusters). However, after yielding, Lissajous-Bowditch plots suggested that the mixtures comprising of bundles of needles exhibited viscous-like behavior while the spherical clusters exhibited a plastic-like behavior. This was further supported by microscopy images of yielded fat-oil mixtures. Overall, for a given fat concentration, the two different shapes of fat clusters can give rise to networks of similar yield stress values but different behaviors after yielding.

In this study, the effect of hygrothermal aging on the strength of pure and carbon nano-filler reinforced epoxy adhesive joints is presented. Epoxy adhesives containing three different carbon nano-fillers, namely, multi-walled carbon nanotubes (CNT), graphene nanoplatelets (GNP) and single walled carbon nanohorns (CNH) were prepared by using a high shear rate mixer. Thermogravimetric analysis (TGA) of the nano-filler reinforced epoxy indicated an improvement in the thermal stability compared to pure epoxy. Aging studies were carried out on single lap-shear specimens as per EN ISO 9142:2003 standard (cycle D3). The strength of the joints was found to decrease with hygrothermal aging time. The nano-filler reinforced joints showed less reduction in joint strength with aging compared to pure epoxy joints. The GNP/epoxy joints showed the best resistance to aging followed by CNT and CNH reinforced joints. Fourier Attenuated Total Reflectance Transformation Infrared Spectroscopy (ATR-FTIR) analysis of the hygrothermal aged samples indicated lower degradation in terms of hydrolysis of epoxy groups for nano-filler reinforced epoxy. Gravimetric analysis of samples immersed in water showed lower weight gain for nano-filler reinforced epoxy composites. Fracture surface analysis was carried out to study the change in failure mechanism with aging time.

In this work, microcrystalline cellulose (MCC) and sawdust (SD) green fillers have been demonstrated as reactive fillers and moisture reservoirs for enhanced curing and adhesion strength of one-component moisture-curing polyurethane (PU) adhesives. The chemical reaction between the MCC or SD fillers and isocyanates present in the PU adhesives was tracked using Fourier-transform infrared spectroscopy (FTIR), while the joint strength was determined using lap-shear tests. The results showed that the addition of MCC or SD leads to an accelerated curing rate through the early consumption of isocyanates. The lap-shear strength of unreinforced PU adhesive joints was achieved in 12 hours. In turn, the curing time in MCC and SD reinforced joints was achieved in 5 hours. The addition of MCC and SD fillers also enhanced the lap-shear strength of the joints under full cure conditions. An autocatalytic growth model was used to describe the lap-shear strength variation with respect to curing time.

The supramolecular gels as soft material grasp more attention due to their increasing applications in diverse fields. Although numerous low molecular weight (LMW) gelators, which gelate either aqueous or organic solvents, have been developed in the past, the ambidextrous gelators (gelate both aqueous and organic solvents) are very limited. While many gelators have been discovered serendipitously, gelator’s rational design is still a challenge, especially ambidextrous gelators. In these two contexts, herein, we demonstrate the organogelation of a hydrogelator Nα, Nβ-di-fluorenylmethoxycarbonyl(Fm) functionalized L-2,3-diaminopropionic acid [G1] and thereby introduced a new ambidextrous gelator with an unnatural amino acid. Further, comparing with previous ambidextrous gelators, a correlation between molecular structure and ambidextrous gelation has been proposed to design new aromatic ambidextrous gelators. G1 found to gelates various commonly used organic solvents with a diverse polarity ranging from polar (methanol and ethanol) to non-polar (benzene and toluene) solvents. Interestingly, among the non-polar solvents, gelator G1 showed selectivity in gelating aromatic solvents as it cannot gelate acyclic and cyclic non-polar solvents. Organogels have been thoroughly characterized using various biophysical techniques. All the organogels show an entangled nanofibrillar morphology with ordered self-assembly of gelators, which are stabilized by aromatic π–π interactions of fluorenyl moieties and hydrogen bonding interactions between ‒CO‒NH‒ groups. The mechanical property (viscoelastic and thixotropic) of G1 organogels are comparable with reported supramolecular gels in polar and non-polar solvents, except toluene. The current findings, together with previous reports of fluorene and pyrene-based ambidextrous gelators, suggest that the amidation of LMW diamine compounds containing a polar group by two larger aromatic moieties could provide a chemical structure (dumb-bell arrangement)/self-assembling property to yield “ambidextrous” gelator. While the organogelation of G1 has been identified and characterized extensively, the present contemporary study, together with the previous observation, attempted to establish a correlation between molecular structure and ambidextrous gelation, leading to the design new LMW ambidextrous gelator by following a “dumb-bell” design.

Microcrystalline cellulose (MCC) and sawdust (SD) green fillers have great potential to act as moisture reservoirs and reactive fillers. In the present study, a one-component moisture curable polyurethane (PU) adhesive was reinforced with MCC and SD fillers. The durability of aluminum/steel joints produced using unreinforced and reinforced PU adhesives was studied using dynamic mechanical analysis (DMA), fatigue, and hygrothermal aging experiments. Lap-shear tests were conducted to evaluate the effects of aging on joints strength. Fourier-transform infrared spectroscopy (FTIR) experiments were performed on adhesive joints after hygrothermal aging to understand the changes in chemical structure due to aging, and the influence of cellulosic fillers on the aging phenomena. DMA results showed that the glass transition temperature of the reinforced PU was similar to that of the unreinforced PU. Fatigue performance of MCC and SD reinforced PU adhesive joints was found to be better than unreinforced PU joints. Further, MCC reinforced PU adhesive joints were found to be more resistant to hygrothermal aging than SD reinforced and unreinforced PU joints. Overall, MCC reinforced PU adhesive joints exhibited good fatigue and superior long-term properties.

The mechanical properties of fat–oil mixtures are dictated by the morphology of the fat crystal network, which in turn is strongly affected by the processing conditions. In this work the effect of uniform (linear) cooling rates on the size and shape of tribehenin (fat) clusters in isopropyl myristate (oil) was studied. The state of the fat–oil mixtures was comprehensively analyzed by establishing pseudo-phase diagrams via visual observations, brightfield and polarization microscopy at various cooling rates and fat concentrations. The microstructure of the constituent fat clusters undergo a shape transition from bundles of needles at low cooling rates to spherical clusters at high cooling rates. The network structures formed with these different shaped fat clusters were examined using rheology. At a given cooling rate, the storage modulus (Gʹ) of fat–oil mixtures versus fat concentration followed a power law relationship. For a given fat concentration, the fat crystal networks comprising of bundles of needles showed higher Gʹ values as compared to fat networks made of spherical clusters. While the fractal dimension of networks cooled at different cooling rates varied over a small range of 2.78–2.86, considerable change in the pre-exponential factor (γ) was observed. The results show the critical importance of processing conditions on the shape and size of fat clusters and its impact on the rheological properties of the fat crystal network.

Although a few Fmoc-functionalised amino acids (Fmoc-AA) are capable of forming hydrogels, the exact levels of hydrophobicity, hydrogen bonding, and ionic nature of the Fmoc-AA gelator required for hydrogel formation remains uncertain. Here, the role of hydrophobicity of amino acid side chain, particularly in the formation of hydrogel, was studied by using Fmoc-norleucine (Fmoc-Nle) and its simple sulfur analogues such as Fmoc-methionine (Fmoc-M) in which the γCH2 of Fmoc-Nle is replaced by sulfur. Results indicate that Fmoc-M forms thermally reversible hydrogels in water (pH ca. 6.8), whereas Fmoc-Nle fails to display any gelation under similar conditions. The result suggests that substitution of the sulfur atom likely reduces the hydrophobicity of the alkyl side chain in Fmoc-Nle to the optimum level, which is sufficient to induce supramolecular hydrogelation in Fmoc-M. The difference in the self-association behaviour of Fmoc-M and Fmoc-Nle emphasise the importance of weak noncovalent interaction between side chains (in addition to the hydrogen-bond and aromatic interactions) to stabilise supramolecular self-assembly of Fmoc-functionalised compounds. The current observations provide a lead to the design of new sulfur-based low molecular weight gelators for various potential applications.

Water added to a solution of microcrystalline cellulose (MCC) in 1-allyl-3-methylimidazolium chloride (AmimCl) reduces the solvent quality and causes significant changes in the flow properties and microstructure due to restructuring and aggregation of cellulose molecules. We report an experimental investigation by means of polarization optical microscopy (POM) and rheology of the distinct phases formed in 5-20 wt% MCC/AmimCl solutions due to the addition of water. With increase in the cellulose concentration, the MCC/AmimCl/water mixtures showed different morphologies such as the non-aligned cholesteric liquid crystalline (LC) domain, the coexistence of spherulite-like structures within the LC domain and a space-spanning network of spherulite-like structures at high concentrations of water. In situ microscopy during shear and POM observations pre and post shear revealed a significant increase in the size of the birefringent domains as the shear rate is increased, which continued to exist even after the cessation of shear. With an increase in the concentration of water, the zero shear viscosity of the MCC/AmimCl/water mixtures was found to go through a minimum, beyond which the aggregation of cellulose commenced. The corresponding oscillatory shear response showed a sol-gel transition with an increase in water concentration. Moreover, at high cellulose concentrations (12-20 wt%), the MCC/AmimCl/water gels exhibited self-similarity and followed the Chambon-Winter (CW) criterion. The similar phase behavior and rheological response observed for MCC dissolved in 1-butyl-3 methylimidazolium chloride (BmimCl) indicated the generality of the presented results.

For mesoscale structural studies of polymers, obtaining maximum level of coarse-graining that maintains the chemical specificity is highly desirable. Here we present a systematic coarse-graining study of sulfonated poly(ether ether ketone), sPEEK, and show that a 71:3 coarse-grained (CG) mapping is the maximum possible map within a CG bead-spring model. We perform single chain atomistic simulation on the system to collect various structural distributions, against which the CG potentials are optimized using iterative Boltzmann inversion technique. The potentials thus extracted are shown to reproduce the target distributions for larger single chains as well as for multiple chains. The structure at the atomistic level is shown to be preserved when we back-map the CG system to re-introduce the atomistic details. By using the same CG mapping for another repeat unit sequence of sPEEK, we show that the nature of the effective interaction at the CG level depends strongly on the polymer sequence and cannot be assumed based on the nature of the corresponding atomistic unit. These CG potentials will be the key to future mesoscopic simulations to study the structure of sPEEK based polymer electrolyte membranes.

High proton conductivity in hydrophobic backbone-based polymers such as Nafion is known to be due to the formation of organized ionic clusters and channels upon hydration. However, a lower proton conductivity in hydrophilic, ionic polymers and the role played by the microstructure are not well understood. In this work, we demonstrate the importance of heterogeneity in crosslinked ionic polymer networks in explaining proton conductivity. Poly(vinyl alcohol) (PVA) crosslinked with sulfosuccinic acid (SSA) is used as the model polymer system for the study. Evolution of the microstructure with hydration and the effect on proton conductivity are analyzed using ATR-FTIR spectroscopy, dielectric spectroscopy, and small-angle neutron scattering. We show that the presence of the two hydrophilic groups in PVA-SSA (hydroxyl and sulfonic acid), as opposed to Nafion, results in competition for water and a lower proton conductivity. The crosslinked polymer–water system contains heterogeneous domains of crosslink nodes which are conductive. These domains (of size 20–35 A) interconnect with each other and form tortuous percolating domains through which proton conduction takes place. The presence of hydroxyl groups results in some of the domains being ineffective for proton transport, resulting in a lower conductivity. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016

Proton exchange membrane fuel cell (PEMFC) performance with a cross-linked poly (vinyl alcohol)/sulfosuccinic acid (PVA/SSA) polymer is compared with Nafion ® N-115 polymer. In this study, PVA/SSA (≈5 wt. % SSA) polymer membranes are synthesized by a solution casting technique. These cross-linked PVA/SSA polymers and Nafion are used as electrolytes and ionomers in catalyst layers, to fabricate different membrane electrode assemblies (MEAs) for PEMFCs. Properties of each MEA are evaluated using scanning electron microscopy, contact angle measurements, impedance spectroscopy and hydrogen pumping technique. I–V characteristics of each cell are evaluated in a H 2 –O 2 fuel cell testing fixture under different operating conditions. PVA/SSA ionomer causes only an additional ≈4% loss in the anode performance compared to Nafion ionomer. The maximum power density obtained from PVA/SSA based cells range from 99 to 117.4 mW cm −2 with current density range of 247 to 293.4 mA cm −2 . Ionic conductivity of PVA/SSA based cells is more sensitive to state of hydration of MEA, while maximum power density obtained is less sensitive to state of hydration of MEA. Maximum power density of cross-linked PVA/SSA membrane based cell is about 35% that of Nafion ® N-115 based cell. From these results, cross-linked PVA/SSA polymer is identified as potential candidate for PEMFCs.

Squeeze flow of fluids between surfaces is important in rheology, material processing, lubrication and biomedical applications. The surfaces for squeezing can be solid and/or porous. In this work, the squeeze flow between a porous surface and a solid surface is examined with Newtonian model fluids and model fabrics used in composite processing. Given that the fluid is expected to impregnate the fabric, the permeability and the wettability of the fluid-fabric combination are of utmost importance. Constant velocity squeeze flow experiments were carried out with polyester and polyol resins, while glass fabrics with different sizings were used as the porous surfaces. To understand the variation of the normal force during squeeze flow, its scaling with squeeze gap is evaluated. The scaling is observed to vary from -3 to -1 for different squeeze flow experiments. Squeeze flow theories are used to examine the normal force variation for different fluid-fabric combinations. It is shown that a slip based squeeze flow model and a permeability based squeeze flow model can be used to understand the wettability and the permeability of a fabric with a specific fluid. � 2014 Elsevier Ltd.

Charge-transfer (CT) gel materials obtained from low-molecular-weight (LMW) compounds through a supramolecular self-assembly approach have received fascinating attention by many researchers because of their interesting material property and potential applications. However, most of the CT gel materials constructed were of organogels while the construction of CT gels in the form of a hydrogel is a challenge because of the solubility issue in water, which considerably limits the use of CT hydrogels. Herein, for the first time, we report a new LMW gelator [Nα-(fluorenylmethoxycarbonyl)-Ne-(δ-butyric-1-pyrenyl)-l-lysine, (FmKPy)], composed of two functional moieties such as fluorenylmethoxycarbonyl and pyrene, which not only parade both hydro and organo (ambidextrous) supramolecular gel formation but also exhibit CT ambidextrous gels when mixed with an electron acceptor such as 2,4,7-trinitro-9-fluorenone (TNF). This finding is significant as the established CT organogelator in the literature did not form an o...

Thixotropic materials belong to the important class of time dependent, evolving, and aging systems. Their characteristics are currently understood through rheological measurements of steady and step shear wherein the steady states reached at large deformations and the transition from one steady state to another are monitored. Small deformation protocols are not well explored for such systems. This is contrary to the common approach of probing equilibrium state and small perturbations from the equilibrium state initially by focusing on linear viscoelasticity. This work addresses the question of how to understand thixotropic materials by focusing on both the small and large deformations. A model suspension of fumed silica in polyisobutylene/paraffin oil is characterized using steady shear, transient shear, and oscillatory shear rheology. The physical processes occurring due to structural changes during each of the tests are interpreted from the corresponding response. Several structural kinetics and elastoplastic/viscoelastic phenomenological models are evaluated, and a novel combination of models is proposed to obtain the description of the responses to various tests. The advantage of this strategy is highlighted in terms of possible difference in the mechanisms at small and large deformations. Kramers–Kronig relations are used to examine the experimental and simulated dataset obtained from oscillatory shear to identify possible violation of linearity or time independence. We demonstrate that the analysis of the oscillatory response using Kramers–Kronig relations leads to important insights about these systems.

Supramolecular gel material built from low-molecular-weight (LMW) gelators finds potential applications in various fields, especially in drug delivery, cell encapsulation and delivery, and tissue engineering. The majority of the LMW gelators in these applications are based on functionalized peptides/amino acids consisting of proteinogenic amino acids which are proteolytically unstable. Herein, we have developed a new LMW gelator containing non-proteinogenic amino acid namely 2,3-diaminopropionic acid (Dap), a key precursor in the synthesis of many antibiotics namely viomycin and capreomycin, by functionalizing with fluorenylmethoxycarbonyl at both amino terminals of Dap [Fm-Dap(Fm)]. Hydrogelation test at different pH indicates that Fm-Dap(Fm) can form a hydrogel in a wide range of pH (4.9 to 9.1) with minimum hydrogelation concentration depends on the pH. The mechanical strength and thermal stability of the Fm-Dap(Fm) hydrogel material are found to decrease with increasing pH (acidic > neural/physiological > basic). The thermal stability of Fm-Dap(Fm) hydrogels is pH-dependent and elicits high stability at acidic pH. Also, Fm-Dap(Fm) hydrogels exhibit strong thixotropic property where regelation (self-healing) occurs upon release of stress. Morphological analysis indicates the formation of fibrils, which are entangled to form three dimensional network structures. Several spectroscopic measurements provided evidence for the self-assembly of Fm-Dap(Fm) molecules through intermolecular aromatic π-π stacking and hydrogen bonding interactions during hydrogelation. Interestingly, Fm-Dap(Fm) not only exhibits hydrogel formation but also shows cell viability and enhanced cell proliferation at physiological pH (7.4). Further, Fm-Dap(Fm) forms a hydrogel upon co-incubation with vitamin B12 and also exhibits release of vitamin B12 over a period. The current study thus demonstrates the development of a new hydrogel material, based on LMW gelator containing the non-proteinogenic amino acid, which can elicit cell viability, enhanced cell proliferation, drug encapsulation, and drug release properties. Hence, Fm-Dap(Fm) hydrogel could be an ideal material for biomedical applications, especially in tissue engineering and drug delivery.

Large amplitude oscillatory shear [LAOS] helps to investigate non-linear rheology and the dynamic behaviour of materials when subjected to large deformations. Qualitative characterization of non-linear rheology can be easily done but quantification and physical interpretation of non-linear rheological characteristics is one of the most challenging aspects. In this work, LAOS tests are done on different materials generally known as 'living polymers', with similar linear response, in order to study their non-linear response using both quantitative and qualitative analysis. The effect of living polymer networks on LAOS was examined using different concentrations of 2,4 bis(2-ethyl hexyl ureido)toluene [EHUT] gels. Cetyl trimethyl ammonium bromide [CTAB] wormlike micellar solution is prepared to compare and contrast with EHUT gels since both these materials show similar linear viscoelastic response. Elastic loading/unloading and plastic deformations are identified as important processes that lead to the distinguishing behaviour for different materials. An equivalence is observed for Lissajous plots in the concentration frequency plane for EHUT. Giesekus model and Upper Convected Maxwell model coupled with transient network model [TNM], two well established models, are solved numerically for comparison with the experimental results. A modification of the Giesekus model to incorporate the network breakage/reformation as a linear function of strain amplitude, and nonlinear function of strain rate within an oscillation period is suggested. The proposed model is able to capture the competition between reptation, network breakage and reforming during LAOS. Moreover, the modified model suggests that the secondary loops during LAOS for EHUT gel occur soon after yielding due to competition between the network breaking and reforming at high shear rates and reptation at low shear rates within a cycle. � 2014 Elsevier B.V.