Reduction In Storage Modulus Research Articles

Effect of Graphene on Mechanical and Flowability Properties of Low-Density Polyethylene Composites

Abstract The logical outline and assembly of structural–functional materials are progressive tendencies of materials knowledge. In this study, it was found that the homogenous graphene (Gr) spread in low-density polyethylene (LDPE) considerably increases the mechanical properties of LDPE/Gr composites. The high specific surface areas and superior properties of Gr improved thermal strength, conductivity, storage modulus, and mechanical properties of composites. Results display that the inclusion of Gr (0–3 wt.%) and frequency growth enhanced the viscosity, loss modulus, and storage modulus of LDPE/Gr composites significantly. The addition of Gr also augmented the Young’s modulus, Young’s strength, tensile modulus, and elongation modulus, but the tensile strength values of LDPE/Gr composites have not changed significantly. The elongation test displayed reduction values with further addition of Gr in LDPE/Gr composites. The effect of temperature rise caused the decrease in the storage modulus with Gr addition; however, further inclusion of Gr mitigated the trend of storage modulus reduction of the composites considerably. This study also presents that Gr inclusion leads to limit polymer chain mobility, which reduces maximum strain and increases the melt flow indexes during material processing.

Mechanical characterization of a cross-sectional TiO2 acrylic-based paint by nano-indentation

ABSTRACT The mechanical response of an embedded acrylic-based paint was analyzed using quasi-static and dynamic nano-indentation. The aim of this study was to propose and evaluate a methodology for selecting indentation test parameters for characterizing the mechanical properties of cross-sectional paint samples, particularly with respect to the inherent issues of embedding materials. The experimental methodology measured the reduced modulus (E r), dynamic reduced storage modulus (), and creep (C IT) of cross-sectional paint samples and compared the results with those obtained for a reference free-film paint sample. Although nano-indentation testing on cross-sectional paint samples has been conducted previously, a systematic approach focusing on the effects of sample size and compliance of the embedding material has yet to be introduced to the conservation field. Nano-indentation was performed on two cross-sectional samples of TiO2 acrylic paint of varying size to demonstrate the practical application and constraints of the technique.

Extensional-shear coupled flow-induced morphology and phase evolution of polypropylene/ultrahigh molecular weight polyethylene blends: Dissipative particle dynamics simulations and experimental studies

The property of ultrahigh molecular weight polyethylene (UHMWPE)-based materials are significantly dependent on the morphology and phase behavior evolution subjected to extensional-shear coupled flow fields. In this study, we utilize polypropylene (PP)/UHMWPE blends as model and present a combination of numerical simulation and experimental study on their association behavior under extensional-shear coupled flow field. In terms of simulation, combined molecular dynamics (MD) and dissipative particle dynamics (DPD) method are applied, where two kinds of extensional-shear coupled flow fields are produced, named as coupledⅠand coupled Ⅱ. In response to coupled Ⅰ, shear-stretched alignment of PP chains is observed at shear rate of γ˙ = 0.01, followed by a quick disordering transition with increasing the shear rates and a final phase-separation at shear rate of γ˙ = 0.1. When subjected to coupled Ⅱ, the weak segregation start to connect with the adjacent ones, giving a homogeneous phase when further increasing the applied pressure. The presence of pressure-driven flow helps in bridging different chains, causing the percolating polymer network reinforcement. Besides, experimental studies by means of Raman spectroscopy, Raman mapping and rheological measurement are compared with the above simulations. It confirms that it is more efficiently broken for melted drops under an extensional dominated coupled flow than those under a shear dominated coupled flow, in which an effective lubricating phase forms, leading to the induced miscibility in polymer blends and reduction in storage modulus and viscosity. These experimental findings are consistent with simulation results.

Deterioration in the Physico-Mechanical and Thermal Properties of Biopolymers Due to Reprocessing.

Biopolymers are an emerging class of materials being widely pursued due to their ability to degrade in short periods of time. Understanding and evaluating the recyclability of biopolymers is paramount for their sustainable and efficient use in a cost-effective manner. Recycling has proven to be an important solution, to control environmental and waste management issues. This paper presents the first recycling assessment of Solanyl, Bioflex, polylactic acid (PLA) and PHBV using a melt extrusion process. All biopolymers were subjected to five reprocessing cycles. The thermal and mechanical properties of the biopolymers were investigated by GPC, TGA, DSC, mechanical test, and DMA. The molecular weights of Bioflex and Solanyl showed no susceptible effect of the recycling process, however, a significant reduction was observed in the molecular weight of PLA and PHBV. The inherent thermo-mechanical degradation in PHBV and PLA resulted in 20% and 7% reduction in storage modulus, respectively while minimal reduction was observed in the storage modulus of Bioflex and Solanyl. As expected from the Florry-Fox equation, recycled PLA with a high reduction in molecular weight (78%) experienced 9% reduction in glass transition temperature. Bioflex and Solanyl showed 5% and 2% reduction in molecular weight and experienced only 2% reduction in glass transition temperature. These findings highlight the recyclability potential of Bioflex and Solanyl over PLA and PHBV.

Ageing of aqueous TEMPO-oxidized nanofibrillated cellulose dispersions: a rheological study

The study concerns the development of rheological properties of aqueous TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanofibrillated cellulose (NFC) suspensions prepared from a commercially available TEMPO-NFC powder. Ageing of hydrogels with concentrations from 1 to 3 wt% of TEMPO-NFC was investigated by monitoring shear flow and linear viscoelastic properties after different days from sample preparation. For quantitative representation of the results time dependencies of zero shear viscosity and storage modulus are given. The increase of rheological parameters is described by a sigmoidal mathematical model. By superposition of individual data sets master curves displaying reduced values of the most significant parameters are constructed, showing the effect of ageing kinetics. The reduced storage modulus and zero-shear viscosity values increase for 2 and 3 decades, respectively, before reaching final values. Obtained results are useful for several applications of nanocellulose hydrogels, especially for those, where pumping, mixing, coating or injecting is necessary. For example, in biomedical field, a less viscous suspension can be injected to the spot of interest, where the final gel properties are developed.

Confinement driven crystallization of ABA crystalline-soft-crystalline block copolymers synthesized via RAFT mediated miniemulsion polymerization

Waterborne ABA block copolymers of stearyl acrylate (SA) and 2-ethylhexyl acrylate (2EHA) containing crystalline A block and soft middle B block, were synthesized via two-step reversible addition fragmentation chain transfer (RAFT) polymerization, using S,S-dibenzyl trithiocarbonate (DBTTC) bifunctional RAFT agent. First, miniemulsion polymerization was used for the synthesis of the crystalline domains. Then in the second step 2EHA was fed to the system as a pre-emulsion and soft domains were formed. For the sake of simplicity sequential monomer addition was performed without any removal of the unreacted monomer in the first step. Thermal properties of the obtained block copolymers were investigated by means of DSC, which revealed reduced crystallinity by the formation of the block copolymers compared to the crystallinity of the polySA initial block. Morphological properties were studied by means of microscopic techniques after film formation at different conditions. Lamellar phase separation induced by confined crystallization in the particles was shown in the films dried at room temperature. Particle coalescence and higher ordering of the lamellar structure was obtained upon film annealing above the melting temperature of the crystalline domains and the structure obtained was comparable with the one of films casted from THF solution, indicating that equilibrium morphology was already obtained by annealing. Mechanical properties studied using DMTA revealed two transition temperatures and reduced storage modulus as the length of the middle block and casting temperature of the film was increased.

Enzymatically degradable alginate hydrogel systems to deliver endothelial progenitor cells for potential revasculature applications

The objective of this study was to design an injectable biomaterial system that becomes porous in situ to deliver and control vascular progenitor cell release. Alginate hydrogels were loaded with outgrowth endothelial cells (OECs) and alginate lyase, an enzyme which cleaves alginate polymer chains. We postulated and confirmed that higher alginate lyase concentrations mediated loss of hydrogel mechanical properties. Hydrogels incorporating 5 and 50 mU/mL of alginate lyase experienced approximately 28% and 57% loss of mass as well as 81% and 91% reduction in storage modulus respectively after a week. Additionally, computational methods and mechanical analysis revealed that hydrogels with alginate lyase significantly increased in mesh size over time. Furthermore, alginate lyase was not found to inhibit OEC proliferation, viability or sprouting potential. Finally, alginate hydrogels incorporating OECs and alginate lyase promoted up to nearly a 10 fold increase in OEC migration in vitro than nondegradable hydrogels over the course of a week and increased functional vasculature in vivo via a chick chorioallantoic membrane (CAM) assay. Overall, these findings demonstrate that alginate lyase incorporated hydrogels can provide a simple and robust system to promote controlled outward cell migration into native tissue for potential therapeutic revascularization applications.

Effect of the penetration of isocyanates (pMDI) on the nanomechanics of wood cell wall evaluated by AFM-IR and nanoindentation (NI)

AbstractThe effects of the penetration of polymeric diphenyl methane diisocyanate (pMDI) on the chemical structure as well as the static and dynamic mechanical properties of wood cell walls (CWs) were investigated by atomic force microscopy with infrared radiation (AFM-IR) and nanoindentation (NI). Results indicated that the possible penetration of some pMDI molecules into the CW affected the mechanical properties of wood CW significantly. The physical and chemical interactions between pMDI and CW may strengthen the connections between the cell-wall materials and thus improved the static elastic modulus and short-term creep resistance of the CW. The elastic modulus (Er) of CWs was increased from 16.5 to 17.7 GPa; the creep ratio of the CWs decreased by 15% after the penetration of pMDI. Dynamic NI properties indicated that the effective penetration of pMDI had a positive effect on the reduced storage modulus (Er′), whereas it negatively affected the loss modulus (Er″) and the damping coefficient (tanδ) of wood CW in a large frequency scale.

Molecular mechanism and characterization of self-assembly of feather keratin gelation

Protein gels with controlled viscoelastic properties could find numerous material and biomedical applications. Feather keratin is naturally abundant protein while its gelation property has not been explored. In this study hydrogel from fully reduced feather keratin was prepared by dialysis. The objectives of this work were to study the molecular mechanism of self-assembly of feather keratin gel and to characterize the structural and viscoelastic properties of hydrogels prepared under various pHs (3–9) and temperatures (50–90°C). Re-oxidation of free cysteine thiols and formation of hydrophobic interactions and hydrogen bond were determined as the main stabilizing forces in self-assembly of feather keratin gel. Adding thiol blocking agent of N-ethylmaleimide leads to reduced storage modulus of keratin gel; gelation was completely inhibited at 82% blockage of free thiols. Increasing temperature decreased storage modulus, while gelation at pH 3 resulted in stiffer gels compared to pHs of 5, 7 and 9. Feather keratin gels with tunable viscoelastic properties could find applications as engineered scaffolds for different tissues.

Gelation behaviours and gel properties of a dihydrazide derivative

The gelation behaviour and gel properties of the organogels based on a twin-tapered dihydrazide derivative gelator, oxalyl acid N′,N′-di (3,4,5-tripentyloxybenzoyl) hydrazide (FH-T5) in mixtures of ethanol and ethylene glycol were investigated. It was demonstrated that the xerogels in the mixtures of ethanol and ethylene glycol showed the co-existence of sphere-like particles or short rod-like aggregates and fibers, which showed Colh phase. The more ethylene glycol in the mixtures, the shorter the gelation time of FH-T5 when the volume contents of ethylene glycol increased from 0% to 30%, and the kinetics of gelation of FH-T5 showed that Df value of the fibers increased from 1.302 in ethanol to 1.426 in the mixture with 30% ethylene glycol. Gels from the mixture of ethanol and ethylene glycol showed reduced storage modulus, higher Tg and the xerogels obtained from the mixtures exhibited higher wetting angle compared to that from ethanol.

One-pot two-step perfluoroalkylsilane functionalization of multi-walled carbon nanotubes for polyurethane-based composites

A facile approach has been developed to prepare perfluoroalkylsilane functionalized multi-walled carbon nanotubes (MWCNTs) with high grafting content through one-pot two-step reactions. In this process, hydroxyl derivatized MWCNTs (hydroxylized MWCNTs) were covalently functionalized with 3-aminopropyl triethoxysilane by silanisation reaction, and subsequently reacted with hexafluorobutyl acrylate or dodecafluoroheptyl acrylate by Micheal-addition reaction. SEM, FT-IR and TGA characterizations demonstrated that the perfluoroalkylsilane were covalently attached onto the surface of MWCNTs and the higher molecular weight of perfluoroalky acrylate yielded higher grafting content. The perfluoroalkylsilane functionalized MWCNTs showed improved dispersibility and strong interfacial adhesion in/with polyurethane matrix. An increase in the grafting content led to a decrease the crystallinity and hydrogen bonding in the PU composites. As a result, the composites showed increased elongation and unimproved tensile strength, but reduced storage modulus and loss factor. Meanwhile, the higher grafting content offered their composite with higher hydrophobicity, lower percolation threshold and more enhanced electronic conductivity.

A multi-level comparative analysis of human femoral cortical bone quality in healthy cadavers and surgical safe margin of osteosarcoma patients

Osteosarcoma is the most common primary malignancy of bone. However, the potential variation it brings to the adjacent undamaged bone tissue is seldom investigated. In this study, we conducted a multi-level comparison of human femoral cortical bone quality in healthy cadavers (aged 42±11 years) and in resected safe margin of osteosarcoma patients (aged 49±15 years). The objective of this study was to document the changes with exposure to osteosarcoma condition in bone mechanical strength, structural morphology and elementary composition, evaluated by static and dynamic mechanical analysis (DMA), scanning electron microscopy (SEM) imaging, X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The resected clinical samples from healthy and osteosarcoma conditioned bones were grouped into the Tumor and Healthy groups, and shaped into regular beam specimens or pulverized according to testing protocols. The results of mechanical tests confirmed osteolytic deteriorations in mineral phase of the Tumor group, in terms of significant reductions in storage modulus (−53.6%, at 1Hz), bending strength (−53.0%) and stiffness (−64.8%) as compared to the Healthy group. However, the energy dissipation/absorption ability of the collagen phase in the Tumor group did not differ from the Healthy group statistically. Although specimens showed no visible morphological difference, a decline in osteocyte lacunar density in bone specimens from the Tumor group was discovered (−54.4%, p<0.001). Higher magnification observation showed that the cell-free lacunae in the Tumor group were occluded by randomized overgrown collagen fibers. XRD and FTIR analysis further demonstrated a lower hydroxyapatite crystallinity and decreased mineral:matrix ratio in pulverized samples from the Tumor group, mainly due to the prominent Amide peaks. Taken together, these data revealed a previously unrecognized effect of osteosarcoma on the adjacent “normal” bone quality. The current study may provide insights on bone tumor pathology and an advanced understanding of surgical safe margin.

Rheology Guided Rational Selection of Processing Temperature To Prepare Copovidone-Nifedipine Amorphous Solid Dispersions via Hot Melt Extrusion (HME).

The production of amorphous solid dispersions via hot melt extrusion (HME) relies on elevated temperature and prolonged residence time, which can result in potential degradation and decomposition of thermally sensitive components. Herein, the rheological properties of a physical mixture of polymer and an active pharmaceutical ingredient (API) were utilized to guide the selection of appropriate HME processing temperature. In the currently studied copovidone-nifedipine system, a critical temperature, which is substantially lower (∼13 °C) than the melting point of crystalline API, was captured during a temperature ramp examination and regarded as the critical point at which the API could molecularly dissolve into the polymer. Based on the identification of this critical point, various solid dispersions were prepared by HME processing below, at, and above the critical temperature (both below and above the melting temperature (Tm) of crystalline API). In addition, the resultant extrudates along with two control solid dispersions prepared by physical mixing and cryogenic milling were assessed by X-ray diffraction, differential scanning calorimetry, hot stage microscopy, rheology, and solid-state NMR. Physicochemical properties of resultant solid dispersions indicated that the identified critical temperature is sufficient for the polymer-API system to reach a molecular-level mixing, manifested by the transparent and smooth appearance of extrudates, the absence of API crystalline diffraction and melting peaks, dramatically decreased rheological properties, and significantly improved polymer-API miscibility. Once the critical temperature has been achieved, further raising the processing temperature only results in limited improvement of API dispersion, reflected by slightly reduced storage modulus and complex viscosity and limited improvement in miscibility.

REACTIVE COMPATIBILIZATION OF THE BLENDS OF THERMOPLASTIC POLYURETHANE AND POLYDIMETHYLSILOXANE RUBBER

ABSTRACT The effect of ethylene-co-methacrylate (EMA) as polymeric chemical compatibilizer on the mechanical, dynamic mechanical, phase morphology, adhesion, and rheological properties of the blends of thermoplastic polyurethane (TPU)–polydimethylsiloxane rubber (PDMS) was investigated at different blend ratios. Melt blending technique was used to prepare the compatibilized blends. Enhancement of the mechanical properties and the reduction of dispersed PDMS domain size in the alloy confirmed the compatibilization effect of EMA on TPU-PDMS blends. Dynamic mechanical properties such as storage modulus, loss modulus, and the damping factor were evaluated to assess the compatibilization effect of EMA on TPU-PDMS blends. Creep tests revealed that compatibilization led to better dimensional stability. Compatibilized blends with finer PDMS rubber domains showed relatively less reduction in storage modulus as compared with uncompatibilized blends during stress relaxation studies. Rheological analysis suggested that the incorporation of EMA decreased the interfacial slip between the blend constituents, and this also confirmed the compatibilization effect of EMA on TPU-PDMS rubber blends as a polymeric reactive compatibilizer.

Bio-based thermosetting bismaleimide resins using cardany linolenate and allyl cardanyl ether

Cardanyl linolenate (CDLN) and allyl cardanyl ether (ACDE) were synthesized from cardanol (CD) which is one of the inexpensive natural resources. Prepolymerized compounds of CDLN/4,4′-bismaleimidodiphenylmethane (BMI) and ACDE/BMI with the molar ratios of 1/2, 1/2.5 and 1/3 at 200°C were compression-molded at 250°C to produce cured CDLN/BMI and ACDE/BMI resins (cCDLN/BMI and cACDE/BMI), respectively. The thermal and mechanical properties of cCDLN/BMI and cACDE/BMI were investigated in relation to the difference of reactivity between the linolenate group of CDLN and the allyl group of ACDE. The 1H NMR and mass spectral analyses of the reaction product of methyl linolenate and N-phenylmaleimide (PMI) revealed that PMI-rich oligomer was produced by the addition copolymerization. FT-IR analyses of cCDLN/BMI and cACDE/BMI revealed that the terminal olefins have a higher reactivity toward the addition copolymerization or ene reaction with maleimide groups than the internal olefins. Although some components which were not incorporated into the polymer networks for cCDLN/BMIs caused a little reduction of storage modulus (E′) before the glass transition starts, E′s of cACDE/BMIs did not decrease until the glass transition starts at ~300°C. The 5% weight loss temperatures (444–453°C) of cACDE/BMIs were higher than those (327–426°C) of cCDLN/BMIs. Also, cACDE/BMIs displayed much better flexural properties than cCDLN/BMIs. The fact that cACDE/BMIs showed better thermal and mechanical properties was attributed to a higher functionality (1.41) of terminal olefins for ACDE than that (0.41) for CDLN.

Microstructure and rheology of globular protein gels in the presence of gelatin

The microstructure and rheological response of globular protein gels (whey protein isolate (WPI) and soy protein isolate (SPI)) in the presence of gelatin (type A, type B and hydrolyzed type A) was investigated. Microstructural information was obtained using a combination of confocal laser scanning microscopy (CLSM) and spin echo small-angle neutron scattering (SESANS). Addition of gelatin led to a coarsening of the globular protein gel structure and to a reduction in storage modulus of globular protein gels. The presence of hydrolyzed gelatin on the other hand did not induce these changes in the globular protein gels. Results were obtained at conditions where proteins only interact via hard body interactions. For these conditions it was concluded that the ratio of molecular sizes is the most important determinant for the occurrence or absence of rheological and microstructural changes in globular protein gels prepared in the presence of gelatin.

Quasi-static and dynamic nanoindentation to determine the influence of thermal treatment on the mechanical properties of bamboo cell walls

Abstract Bamboo was thermally treated at 180°C and 200°C, and the micromechanical properties of its cell walls were investigated by means of quasi-static and dynamic nanoindentation experiments. With increasing treatment temperatures, the average dry density and mass of the bamboo decreased, whereas the already reduced elastic modulus at 180°C of the fiber cell walls did not change, but the hardness showed increasing tendencies. Dynamic nanoindentation revealed reduced storage modulus ( E ′ r ) $({E'_{\rm{r}}})$ and loss modulus ( E ″ r ) $({E''_{\rm{r}}}\,)$ for the thermotreated bamboo cell walls compared with the untreated bamboo fibers in all frequency regions. Moreover, E ′ r , E ″ r , ${E'_{\rm{r}}},{\rm{ }}{E''_{\rm{r}}},$ and loss tangent (tan δ) of treated bamboo decreased with increasing treatment temperature.

Influence of temperature on tensile behavior of multiwalled carbon nanotube modified epoxy nanocomposites

Abstract

Dynamic mechanical properties and correlation with dynamic fragility of sisal reinforced composites

In this study, the reinforcement effect in sisal/polyester composites containing distinct reinforcement content was studied ranging from fragile to strong classification. The results indicate that the reduced storage modulus changes steadily, and the loss modulus and the tan δ peak are broader for composites containing more fiber (dynamically strong composites). As more fiber was incorporated in the resin, lower peak height for the tan delta curve was obtained, which may be indicative of lower energy dissipation due to a greater relative amount of interface. Also, the use of reduced dynamic mechanical curves (similar to dynamic fragility) is an alternative to facilitate the study of the material behavior. POLYM. COMPOS., 36:161–166, 2015. © 2014 Society of Plastics Engineers

High strain rate mechanical properties of a cross-linked epoxy across the glass transition

Molecular dynamics simulations were used to study the high strain rate mechanical properties of a cross-linked epoxy system comprised of diglycidyl ether of bisphenol A (DGEBA) that is cross-linked by a poly(oxypropylene) diamine with three propylene oxide moieties per diamine. Atomistic network structures were characterized using volume-temperature behavior and their response to mechanical deformation. The Young's modulus was determined as a function of temperature across strain rates spanning three decades in magnitude, and collapsed onto a single “master curve” using the time–temperature superposition principle (TTSP). The master curve obtained from molecular dynamics simulation data shows good agreement with a similar master curve of the reduced storage modulus as a function of frequency, which was obtained using experiments. At higher strain rates, the simulation master curve deviated from the experimental master curve. This deviation could be attributed to the lack of occurrence of sub-Tg motions on the time scale of simulations due to the use of higher strain rates in simulations compared to experiments. Our work demonstrates the utility of TTSP in connecting the thermo-mechanical behavior of polymers at high strain rates and high temperatures to experiments performed at much different conditions. To the best of our knowledge, the use of the time–temperature superposition to compare mechanical properties determined from molecular simulation and experiments is the first reported effort of its kind.