Shear Rheology Experiments Research Articles

Mechanistic Study on the Optimization of Asphalt-Based Material Properties by Physicochemical Interaction and Synergistic Modification of Molecular Structure

In order to investigate the relationship between the molecular structure of fibers and the differences in physicochemical interactions between fibers and asphalt on the performance of fiber-modified asphalt, this paper chose two types of fibers with different chemical structures: straw fiber and polyester fiber. First, the differences in molecular interactions between the two fibers and asphalt were explored using molecular dynamics, then the differences in the adsorption capacity of the two fibers on asphalt components were tested by attenuated total reflection infrared spectroscopy experiments, and finally, the differences in the rheological properties of the two fiber-modified asphalts were tested by dynamic shear rheology and low-temperature creep experiments. The molecular dynamics simulation findings reveal that polyester fibers may intersperse into asphalt molecules and interact with them via structures such as aromatic rings, whereas straw fibers are merely adsorbed on the asphalt’s surface. Straw fibers and asphalt exhibit hydrogen bonding, whereas polyester fibers and asphalt display van der Waals interactions. The results of attenuated total reflectance infrared spectroscopy indicated that polyester fiber absorbed asphalt components better than straw fiber. The rheological tests revealed that the polyester fiber had the highest complex shear modulus in the temperature range of 46–82 °C, and at 64 °C, the phase angle was 4.289° lower than that of the straw fiber-treated bitumen. Polyester fiber-modified asphalt had a 32.48%, 15.72%, and 6.09% lower creep modulus than straw fiber-modified asphalt at three low-temperature conditions: −6 °C, −12 °C, and −18 °C. It is clear that fibers with aromatic rings as a chemical structure outperform lignin-based fibers in terms of improving asphalt characteristics. The research findings can serve as a theoretical foundation for the selection of fibers to produce fiber-modified asphalt.

Rheological performance regulation of solidified soil slurry for mine reclamation: Superplasticizer selection and structural characterization based on in-situ techniques

This paper focuses on the low cement content characteristic of soil slurry materials for mine reclamation, investigating the effects of three types of superplasticizers (polyester-based polycarboxylate (PCE), sodium lignosulfonate (SLS), and naphthalene-based (PNS)) on the flow properties of freshly mixed solidified soil slurry. It proposes a method combining shear rheology and micro-rheology experiments to complete the selection of superplasticizers. The rheological properties of the freshly mixed slurry were compared at different dosages of superplasticizers and various resting times. The results indicate that PCE at 0.4 % dosage exhibits the best dispersion effect in the solidified soil slurry. In-situ ATR FTIR, in-situ low-field NMR, and ζ-potential tests confirmed that PCE can effectively delay the solidification process of the soil slurry to maintain its fluidity. At a PCE dosage of 0.4 %, the spatial hindrance between slurry particles is reduced, increasing the proportion of “free water” in the solidified soil, with the ζ-potential not being the main factor affecting the rheological properties of the soil slurry. This study provides a theoretical foundation and solutions for regulating the early fluidity of solidified soil slurry materials used in the land reclamation of coal gangue in the Northwest mining areas of China.

Tuning the thermostability of GHG gels by salts at different positions on the Hofmeister scale

The influence of Hofmeister cations (NH4+, Na+, Mg2+) and anions (H2PO4−, CH3COO−, Cl−, NO3−) on the thermostability of a GHG hydrogel was investigated. The combined results of UV circular dichroism (UVCD) and Small Amplitude Oscillatory Shear Rheology experiments reveal that the addition of salt reduces the stability of the gel phase and the underlying fibrils. In line with the cationic Hofmeister hierarchy, the chaotropic Mg2+ ions caused the greatest thermal destabilization of the gel phase with the gel → sol transition temperature Tgs value lowered by 10 °C. In the absence of salt, the gel → sol transition probed by the storage modulus and microscopy is biphasic. In the presence of salt, it becomes monophasic. Contrary to expectations the presence of Hofmeister anions leads to a nearly identical reduction of the gel → sol transition temperatures. However, UVCD spectra suggest that they affect the ππ-stacking between imidazole groups to a different extent. We relate the absence of ion specificity regarding the solubility of fibrils (probed by UVCD) to the observed enthalpy-entropy compensation of the dissolution process. Our results combined show how CD spectroscopy and rheology combined yields a more nuanced picture of the processes underlying the gel → sol transition.

Effect of PCL chain length on rheological and mechanical properties of PCL‐PDMS‐PCL triblock copolymer films

AbstractThe polycaprolactone‐polydimethylsiloxane‐polycaprolactone (PCL‐PDMS‐PCL) triblock copolymer films of variable PCL chain are analyzed for rheological characteristics in this paper. The dynamics of viscoelastic behavior of photocrosslinked films above their crystal melting temperature (Tcm) is measured using oscillatory shear rheology experiments. The frequency sweep tests are analyzed at 0.1% strain as obtained from linear viscoelastic region. Shear thinning is observed in the study of complex viscosity. It has been noted that viscosity rises with increase in crosslink density and decrease in with PCL chain length. Time‐dependent effect on the copolymer films has been analyzed by creep recovery behavior. The Burgers model is fitted in creep compliance data to analyze its viscoelastic and viscous region. An examination of structure recovery revealed that recovery increased as PCL chain length decreased. Tensile strength and percentage elongation are studied at ambient conditions with a universal testing machine, while storage modulus and tan δ are analyzed by a dynamic mechanical analyzer at varying temperatures.

The role of rolling resistance in the rheology of wizarding quidditch ball suspensions

To elucidate the effect of particle shape on the rheology of a dense, viscous suspension of frictional, non-Brownian particles, experimental measurements are presented for suspensions of polystyrene particles with different shapes in the same solvent. The first suspension is made of spheres whereas the particles which compose the second suspension are globular but with flattened faces. We present results from steady shear and shear-reversal rheological experiments for the two suspensions over a wide range of stresses in the viscous regime. Notably, we show that the rheology of the two suspensions is characterised by a shear-thinning behaviour, which is stronger in the case of the suspension of globular particles. Since the shear-reversal experiments indicate an absence of adhesive particle interactions, we attribute the shear thinning to a sliding friction coefficient which varies with stress as has been observed previously for systems similar to the first suspension. We observe that the viscosity of the two suspensions is similar at high shear stress where small sliding friction facilitates particle relative motion due to sliding. At lower shear stress, however, the sliding friction is expected to increase and the particle relative motion would be associated with rolling. The globular particles attain a higher viscosity at low shear stress than the spherical particles. We attribute this difference to a shape-induced resistance to particle rolling that is enhanced by the flattened faces. Image analysis is employed to identify features of the particle geometry that contribute to the resistance to rolling. It is shown that the apparent rolling friction coefficients inferred from the rheology are intermediate between the apparent dynamic and static rolling friction coefficients predicted on the basis of the image analysis. All three rolling resistance estimates are larger for the globular particles with flat faces than for the spherical particles and we argue that this difference yields the stronger shear thinning of the globular particle suspension.

Improving food sustainability by converting orange peel waste products into hydrogels using stirred media milling

There is a need for natural gelling ingredients within the food industry. We have shown that orange peel suspensions (OPS) form hydrogels when treated with a stirred media mill. Gels were formed when an aqueous 3% OPS was milled for longer than 60 min. Their gel strength increased with milling time, which was attributed to progressive disruption of the plant cell tissues and release of pectin and other polysaccharides. The water holding capacity and amount of bound water in the orange peel gels increased with milling time. Shear rheology experiments showed that the gels exhibited shear thinning and were predominantly elastic-like materials. FTIR analysis and addition of bond disruptive agents suggested that hydrophobic interactions between methoxy groups on the pectin were mainly responsible for crosslinking and gelation. Overall, our results suggest that stirred media milling may improve the utilization of orange peel as a functional ingredient in the food industry.

Application of Novel Magnetic Surfactant-Based Drilling Fluids for Clay Swelling Inhibition

Water-based drilling fluids are widely employed in the oilfield industry for the efficient and smooth drilling of wellbore reservoirs. In this study, a novel drilling formulation was prepared by mixing a magnetic surfactant in the base mud formulation. Several drilling fluid tests were conducted, which include steady shear and dynamic rheology, filtration, clay sedimentation, clay swelling, and particle size distribution analysis. Different analytical techniques─XRD, FTIR, and TGA analyses─were performed to study the interactions of magnetic surfactant with the clay platelets. The experimental results of the surfactant-modified formulations were compared to the base mud formulation. Thermal gravimetric analysis of the surfactant-modified mud showed improved thermal stability compared to the base mud. FTIR analysis results of the base mud and surfactant-modified mud were compared. In the steady shear rheology experiments, the flow curves of the base mud and surfactant-modified mud showed that the addition of surfactant in the base mud affects the shear stress of formulation. The shear stress data of rheology was fitted to the Herschel–Bulkley model, and model parameters trends were assessed with and without the addition of surfactant in the base mud. Shale inhibition tests showed that the incorporation of magnetic surfactant minimized the clay hydration of swelling, thus proving a superior shale inhibitor. The particle size analysis showed that the average particle size of clay particles increased with the increasing concentration of the magnetic surfactant. The linear swelling analysis of clay showed that the rate of swelling was 90% in the surfactant-modified mud as compared to the swelling rate in deionized water. According to the zeta potential, the combination of surfactants significantly affects the electrokinetic behavior of dispersion. Further, the XRD proved that the interlayer spacing of clay increasing while interacting with magnetic surfactant molecules indicates the ability of magnetic surfactant as a shale swelling and hydration inhibitor for water-based drilling formulations.

Effect of the xanthan gum on the rheological properties of alginate hydrogels

Hydrogels are three-dimensional polymeric structures with remarkable physicochemical and mechanical properties. The selection of biopolymers as constituents of hydrogels is of great importance to enable their use in a variety of fields. The present study proposes a mixed hydrogel of alginate and xanthan gum prepared by the in-situ gelation method. Since alginate gelation is induced by calcium crosslinking, two calcium concentrations have been used in this study. In detail, the influence of xanthan gum and calcium cations on the rheological behavior of alginate hydrogels were investigated using steady shear rheology, dynamic and transient shear rheological experiments. The results showed that the hydrogels behaved like a shear-thinning material, as Carreau fluids, and that the overall viscosity of the hydrogel was dependent on both Ca2+ and xanthan gum concentrations. The length of linear viscoelastic range was influenced by xanthan gum content only at low calcium concentration, while the storage modulus (G′) was found to increase with xanthan gum and calcium content. The interpolation of mechanical spectra with a power law demonstrated that G′ and G″ were slightly dependent on the frequency and that G′ became less frequency dependent with the addition of xanthan gum. Even though the elastic contribution of the hydrogel increased with calcium and xanthan concentration, the hydrogels obtained were not able to fully recover their original structure after the application of a high load. The behavior at large amplitude oscillation strain, mimicking real processing conditions, showed that hydrogels crosslinked with a low calcium concentration were affected by xanthan gum even at lower concentrations than hydrogels crosslinked with high calcium content. Such hydrogel mixtures could find application in research in the food industry.

Degradation characteristics of SBS polymer and its contribution to weathering aging of modified asphalt

Under the weathering environment, the aging behavior of SBS-modified asphalt (SBSMA) is complex, including oxidation of matrix asphalt, degradation of SBS, and disruption of SBS-asphalt interaction. However, most studies on the weathering mechanism of SBS-modified asphalt have mainly been considered from its overall perspective and based on environmental variables. In order to investigate the aging mechanism of SBSMA more clearly, the paper proposes a novel method of SBS-independent weathering to determine the aging behavior of SBS under accelerated weathering conditions and its effect on the performance of modified asphalt. Finally, by correlating its samples (M1) with the behavior of different fresh SBSMA(M2) and weathering aged SBSMA(M3), the SBS effective content equation under weathering conditions and the conversion coefficient between different aging methods were determined. Furthermore, by analyzing the physicochemical properties of SBS polymers before and after weathering and comparing them with the calculated results of SBS effective content within modified asphalt, it is discussed that the swelling effect of SBS under this method differs due to different aging levels, which affects the complete separation of matrix asphalt oxidation and SBS degradation. The macroscopic rheological behavior of samples was evaluated by dynamic shear rheology experiments (DSR), the variation of their chemical component and molecular weight were analyzed by Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC), and the microscopic network was observed by fluorescence microscopy (FM). The results illustrate that the degradation pattern of weathering SBS follows a single exponential decay function. The SBS effective content decreases to 66.7 %, 50 %, and 33.3 % after accelerated weathering for 5.8 h, 10.5 h, and 14.5 h, respectively. In addition, the conversion relationship between the effective content of SBS under two modes of SBS independent weathering aging and SBSMA weathering aging was established. The effective content of SBS within SBSMA at different service stages and its remaining modification capacity can be reasonably predicted by simple SBS weathering tests.

Modification effects of multi-walled carbon nanotubes on the mechanical and rheological properties of epoxy asphalt

Epoxy asphalt (EA) was widely used in road engineering fields, but previous research found that there were compatibility problems between epoxy (EP) and asphalt. In practical engineering applications, the compatibility of EA was improved by adding complex auxiliary agents. Although the composite of multi-walled carbon nanotubes (MWCNTs) and epoxy could improve the adhesion, corrosion resistance, and toughness of asphalt, it was difficult to achieve an excellent modification effect because of the agglomeration of MWCNTs. To improve the composite effect of epoxy and MWCNTs, carboxyl functional groups were added to MWCNTs under mixed acid conditions to reduce the agglomeration and achieve a better combination with epoxy. Then, MWCNTs/EP was prepared under physical stirring conditions. The MWCNTs/EP was observed by scanning electron microscopy (SEM), and it was found that MWCNTs could be uniformly dispersed in epoxy. To research the modification effect of MWCNTs on mechanical and rheological properties of EA, a variety of MWCNTs/EA with different proportions were prepared. The impact of different mixing ratios of MWCNTs/EP on the mechanical and rheological properties of asphalt was evaluated by dynamic shear rheological (DSR) experiments. The results showed that the addition of the appropriate amount of MWCNTs could further improve the high-temperature stability of EA. In this case, MWCNTs played a positive role in the modification of epoxy. However, the improper introduction of MWCNTs would have a negative weakening effect, which would weaken the high-temperature performance of EA. Through comparison, the optimal mixing ratios of MWCNTs and epoxy were finally determined as 6 % EP + 1.4 % MWCNTs and 20 % EP + 0.125 % MWCNTs.

Tribological Properties and Seasonal Freezing Damage Evolution of Rotating Spherical Hinge Self-Lubricating Coating

The spherical hinge is an important part of rotating bridge construction, but over a long period of time, spherical hinge self-lubricating coating is easily eroded by water vapor. In this paper, the tribological properties and seasonal freezing damage evolution characteristics of a variety of rotating spherical hinge self-lubricating coating materials were studied by means of friction coefficient measurement experiments, friction and wear experiments and shear rheological experiments based on a self-developed indoor spherical hinge rotational friction coefficient tester. The results show that the self-developed indoor spherical hinge rotational friction coefficient tester can effectively and truly represent the working state and tribological properties of self-lubricating coating in practical engineering. A seasonal freezing environment has obvious influence on the tribological properties of spherical hinge self-lubricating coating, which is an irreversible process of deterioration. With the increase in the freezing–thawing cycle, the friction coefficient and viscosity of self-lubricating coating materials increase gradually, and the thixotropy and elastic recovery become worse and worse. When the content of graphene is 0.1%, the performance is the best. At room temperature and in a freeze–thaw environment, the friction coefficient of graphene grease is lower than that of PTFE 0.007 and 0.008, respectively. The diameter of the grinding plate is less than 0.075 mm and 0.001 mm, respectively. The maximum bite load without card is higher than 8.1% and 11.5%. The area of the thixotropic ring is lower than 41% and 42%. Phase transition points were higher than 42% and 64%. The apparent viscosity was higher than 6.6% and 74%. Graphene greases show the greatest bearing capacity, thixotropy and structural strength in conventional and seasonal freezing conditions and exhibit excellent tribological properties.

Polyethylene terephthalate/organoclay nanocomposites: Improvement of morphology and viscoelastic properties by using a chain-extender

A major challenge in the industrialization of polymer-clay nanocomposites at high processing temperatures is the thermal degradation of the matrix induced by the commercial organoclays. This work investigated the reactive extrusion of polyethylene terephthalate (PET) nanocomposites using a styrene–acrylic multi-functional oligomeric chain extender, Joncryl® ADR 4368 (Joncryl), to address the thermal degradation of the matrix and improve the microstructure of the nanocomposites. The matrix degradation in the nanocomposites induced by the introduction of 2 wt% Cloisite® 30B (C30B) and Nanomer® I28E (Nanomer) was estimated using the high-frequency data and the Maron-Pierce equation. While PET/C30B nanocomposite showed a highly exfoliated morphology, large tactoids were observed for the sample containing Nanomer particles. Therefore, the chain extender at different concentrations (0.5 to 2 wt%) were introduced in the sample containing Nanomer to achieve a highly exfoliated morphology. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed a significantly improved morphology for the nanocomposites containing the chain extender. The effect of the chain extender on the control of the degradation of PET nanocomposites containing Nanomer particles was assessed through small amplitude oscillatory shear (SAOS) rheological experiments. The mechanism of stabilization, through the formation of long chain branched (LCB) molecular structure, was considered by comparing the loss angle plots of the unmodified and chain extended samples. Re-linking the degraded PET chains by the multifunctional epoxy-based chain extender led to increase by more than ten times of the complex viscosity of PET nanocomposites containing Nanomer at low frequencies.

Enabling thermally enhanced vibration attenuation via biomimetic Zr–fumarate MOF-based shear thickening fluid

This work reported a biomimetic shear thickening fluid (STF) with thermally enhanced shear thickening effect for smart vibration attenuation system. It was developed by introducing Zr–fumarate metal-organic framework (MOF) crystals into a solvent mixture comprised of polyethylene glycol, polyacrylic acid and Ca2+, which endowed it with the cooperative effect of electrostatic and hydrophobic interactions. As the temperature raised from 25 to 35 °C, the STF containing 55 vol% MOF (STF-55) yielded a favorable shear thickening reinforcement with a notable viscosity growth of 1760-7953 Pa s. Besides, its storage modulus increased from 14 to 3036 Pa at 25–55 °C under a 0.1 Hz shear frequency, revealing a significant improvement in mechanical performance, as also demonstrated by transient shear rheological experiments. STF was incorporated into sandwich structures to improve the damping performance, with the natural frequency and damping ratio investigated at various ambient temperatures. Since the thermal enhanced viscosity could promote energy dissipation, the damping ratio of STF-55 filled sandwich structure obtained a substantial improvement from 0.85% to 1.93% at 25–55 °C. Thus, STF-55 based turbine blade and building damping pile were fabricated, which not only reduced the external vibration stimuli with average responsive accelerations of 7.3 and 0.8 m2s-1, but also presented attenuated levels of 6.9 and 0.6 m2s-1 at increasing temperatures, respectively. In conclusion, this work provided guiding approach for highly adaptive vibration attenuation at high-temperature environment.

Multi–scale observation of oxidative aging on the enhancement of high–temperature property of SBS–modified asphalt

This study employed a molecular dynamics (MD) simulation to investigate the rheological and aging characteristics based on dynamic shear rheology experiments, fluorescence microscopy and atomic force microscopy (AFM) microstructure analysis. The simulation process considered both the functional groups and microstructure evaluation. Rutting factor, complex modulus, damping factor, recovery (R) and unrecoverable creep compliance (Jnr) were adopted to describe the rheological properties of virgin and aged SBS-modified asphalt (5 wt%). The microscopic morphology structure of the binder samples was characterized by AFM and fluorescence microscopy. The results showed that oxidative aging improved the rutting resistance and shear resistance of the modified asphalt, enhanced the elastic component in the asphalt matrix and promoted the generation of bee-like structures; however, it destroyed the polymer-rich phase network. Molecular simulations explained the above phenomena. Oxidative aging intensified the competition between asphaltenes and polymers, and the new equilibrium was not favorable for the development of polymer-rich phases.

Zein-Bonded Graphene and Biosurfactants Enable the Electrokinetic Clean-Up of Hydrocarbons.

Nonaqueous phase liquids (NAPL, e.g., hydrocarbons and chlorinated compounds) are common groundwater pollutants. Electrokinetic remediation of NAPLs uses electric fields to draw them toward electrodes and remove them from groundwater. The treatment requires NAPL mobility. Emulsification increases mobility, but at a risk for downstream receptors. We propose using alkaline aqueous solutions of zein and graphene nanoparticles (GNP) to form conductive materials, which could also act as barriers to control NAPL migration. Alkaline zein-GNP solutions can be injected in the polluted soil and solidified by neutralizing the pH (e.g., with glacial acetic acid, GAA). Shear rheology experiments showed that zein-GNP composites were cohesive, and voltammetry showed that GNP increased electrical conductivity of zein-based materials by 3.5 times. Gas chromatography-mass spectroscopy (GC-MS) demonstrated that the electrokinetic treatment of model sandy aquifers yielded >60% and ∼47% removal of emulsified toluene in freshwater and in salt solutions, respectively (with 30 min treatment using a 10 V differential voltage between a zein-GNP and an aluminum electrode. NaCl was used as model salt contaminant. The conductivity of surfactant solutions was lower in saline water than in freshwater, explaining differences in toluene removal. Toluene-water emulsions were stabilized using the natural surfactants lecithin and saponin. These surfactants acted synergistically in stabilizing emulsions in either freshwater or salt solutions. Lecithin and saponin likely interacted at toluene-water interfaces, as indicated by the morphology, interfacial tension and compressional rigidity of toluene-water interfaces with both components (relative to interfaces of either lecithin or saponin alone). The compressional behavior of interfacial films was well-described by the Marczak model. Electrokinetic treatment of saturated model sandy aquifers also decreased the turbidity of emulsions of water and either tricholoroethylene (TCE, by ∼41%) or diesel (by ∼75%), in the presence of a bacterial biosurfactant. This decrease was used as semiquantitative indicator of NAPL removal from water.

Thermal Stabilization of Recycled PET Through Chain Extension and Blending with PBT

This study investigates the effect of using a multifunctional epoxide chain extender (Joncryl® ADR 4468) on the thermal stabilization and rheological properties of recycled polyethylene terephthalate (R-PET) and its blends with polybutylene terephthalate (PBT). The R-PET samples were prepared without and with chain extender (CE) contents of 0.4 wt% and 0.8 wt%. R-PET/PBT blends with weight ratios of 75w/25w, 50w/50w and 25w/75w were also prepared without and with a given CE content of 0.2 wt%. The thermal stability of the melt blended samples was analyzed through small amplitude oscillatory shear (SAOS) rheological experiments. The structure of the samples was evaluated using a Fourier transform infrared (FTIR) spectrometer. While the dynamic rheological properties of R-PET were improved with the addition of Joncryl and by blending with PBT, during the SAOS rheological experiments, the complex viscosity of R-PET further increased due to the concurrent polycondensation of R-PET and the resumption of Joncryl reaction with R-PET molecules. These reactions during the rheological experiments were further expedited with increasing the testing temperature. On the other hand, in R-PET/PBT blends, the reactivity of Joncryl was more noticeable in blends with higher R-PET contents due to the higher available internal reactive sites of much shorter R-PET molecules. It was observed that the addition of only 0.2 wt% Joncryl to the blends of R-PET/PBT (75w/25w) dramatically improves the thermal stability and dynamic rheological properties of R-PET and most likely its processability.

Compatibility and high temperature performance of recycled polyethylene modified asphalt using molecular simulations

ABSTRACT The objective of this study was to explore the interaction mechanism between recycled polyethylene (RPE) and base asphalt to reveal the microscopic mechanism of the two–phase structure formation, separation, and high–temperature stability of RPE–modified asphalt. Density functional theory (DFT) and molecular dynamics (MD) simulations were used to explore the interaction of RPE–modified asphalt at the atomic and molecular scales, and the simulation results were verified by fluorescence microscopy and dynamic shear rheology experiments. The results show that the binding energy of RPE with light components (aromatics and saturates) (730–1030 Ha) was smaller than that of asphaltenes with light components (2750–4330 Ha) in the following order of magnitude: phenol > pyrrole > thiophene > RPE. The charge transfer numbers of RPE with asphalt molecules indicate that RPE interacted weakly with asphaltenes and resins. In addition, MD simulation results show that RPE elevates the diffusion coefficient of asphalt molecules. The two–phase structure of RPE–modified asphalt was observed using fluorescence microscopy, and the RPE phase became larger with increasing development time. The dynamic shear rheology experiment results show that RPE reduced the damping factor while enhancing the complex modulus and rutting factor of asphalt.

Thermal and Environmentally Induced Degradation Behaviors of Amorphous and Semicrystalline PLAs Through Rheological Analysis

The degradation behaviors of an amorphous and a semicrystalline PLA (i.e., aPLA and cPLA) with similar molecular weights are compared at elevated temperatures and after being treated under various environmental conditions through using small amplitude oscillatory shear rheological experiments. The degradation behaviors are also studied by differential scanning calorimetry and Fourier-transform infrared spectroscopy analysis. Melt thermal degradation analysis shows that the d-lactic acid content does not affect the degradation behavior of PLAs. This is while the increase in temperature beyond 190 °C dramatically, but still similarly, increases the degradation rate of aPLA and cPLA samples. It is also shown that processing of aPLA through a twin-screw extruder at temperatures below 190 °C and at different screw speeds has a negligible effect on its thermal degradation. The degradation of the PLA samples treated under various humidity levels as well as when exposed to different environments such as tap and sea water, regular garden soil and commercial grade microbial fertilizer, is expedited more dramatically around and beyond the glass transition temperature. This is while the cPLA, with around 50% crystallinity, reveals a much less degradation under the aforementioned conditions. This is because the bulk diffusion of water molecules or micro-organisms is harder in cPLA with a compacted crystalline structure. It should, however, be emphasized that the fertilizer degrades the PLA samples much faster and more pronounced than other environments. The tap and sea water exhibits a similar effect on the degradation of PLA samples. Notably, the temperature is the most effective parameter in expediting the degradation rate of PLA, suggesting that the PLA degradation could gradually occur in the environments with extreme warm weather. The increase in humidity also accelerates such degradation.

Rheological Properties of Graphene Modified Asphalt Binders

Recently, low-cost, high-quality graphene can be obtained readily, so it is potential to prepare conductive graphene modified asphalts (GMAs). In this paper, GMAs were prepared with 0%, 2%, 4%, 6%, 8%, and 10% of graphene by weight of composites. Dynamic shear rheological experiments conducted from −30 to 120 °C illustrate that elasticity at above ambient temperatures and rutting resistance at higher temperatures are enhanced and, especially, the conceived percolation of GMAs occurs at graphene contents (GC) above 8% which were verified from three changes as GC increases, i.e., the curve characteristics of complex moduli, storage moduli at temperatures over 100 °C, temperatures when the phase angle reaches 90° and the trend of TG′=G″. The modification mechanisms are different before and after percolation. Before the percolation threshold, graphene which has a molecular structure similar to asphaltene enhances asphalt, like increasing asphaltene components, and after threshold, graphene improves asphalt because of the formed graphene networks. Rotational viscosities test results show that the higher the GC is, the higher the operating temperatures are, but the operating temperatures are higher than 200 °C when GC is above 4%. The percolation helps to further develop conductive asphalt concrete for intelligence pavement, but the operating properties of GMAs need to be improved.

Shear-Induced Isotropic-Nematic Transition in Poly(ether ether ketone) Melts.

In a previous work on a poly(ether ether ketone) (PEEK) melt, above its nominal melting temperature (Tm ≅ 335 °C), a severe Cox-Merz rule failure was observed. The abrupt decrease in the apparent shear viscosity was ascribed to the formation of flow-induced crystallization precursors. Here shear rheology and reflection polariscope experiments are utilized to unravel the structural changes occurring under shear on a similar PEEK melt above Tm. Three regimes of the flow curve were identified from low (0.01 s-1) to high shear rates (1000 s-1): (I) an isotropic structure with weak birefringence due to polymer chain orientation and mild shear thinning for γ̇ < 1 s-1, (II) an isotropic-nematic transition accompanied by strong birefringence, two steady-state viscosities, and large nematic polydomain director fluctuations, and (III) shear-thinning behavior with an η ∼ γ̇-0.5 dependence for γ̇ > 20 s-1, typically found in nematic fluids. The findings reported in this experimental work suggest that the nematic phase may represent the early stage of the formation of shear-induced crystallization precursors.