Search

Ethyl cellulose (EC) was blended with poly(β-hydroxybutyrate) (PHB), aiming at controlling crystallization and mechanical properties of PHB. The obtained PHB/EC blend is an immiscible system, and the discrete EC phase behaves dual characteristics in the PHB matrix, as the viscoelastic droplets during processing, and as the rigid filler particles during shear flow. This is confirmed by the rheological tests. The presence of EC domains acts as the tackifier, sharply increasing system viscosity from 1000 Pa s to 5000 Pa s, and as a result, has large influence on the spherulite morphology of PHB and its crystallization kinetics. The PHB spherulite growth rate reduces in the presence of inert EC, accompanied by decreased degree of crystallinity and reduced lamella defects. These affect the mechanical properties of PHB strongly, together with reinforcing effect of EC itself. At the lower content level, EC can act as both reinforcement and toughener. The presence of 1 wt% EC enhances the tensile strength of PHB by about 22%, from 27.5 MPa to 33.3 MPa, accompanied by 15% increase of impact strength. This work provide an easy way to control the structure and properties of PHB using EC. [ABSTRACT FROM AUTHOR]

Poly(butylene adipate-co-terephthalate) (PBAT) biocomposites containing carbon nanotubes (CNTs) were prepared by melt compounding. The linear viscoelasticity of composites, including creep, creep recovery and stress relaxation in the solid state, and dynamic shear flow in the molten state, were studied and then further described by the viscoelastic models, aiming at exploring relaxation of hierarchical structures of CNTs and obtaining structural parameters of CNTs in PBAT matrix. The results reveal that CNTs are dispersed mainly as flocs or small aggregates in PBAT, which is further confirmed by the structural parameters of CNTs obtained through model simulations on the dynamic rheological responses of composites. The presence of CNTs retards overall kinetics of creep and stress relaxation of PBAT. This is attributed to highly restrained viscoelastic and viscoplastic deformation of PBAT chain coils, rather than slightly confined elastic deformation of chain segments because the chain coils is in the same size-scale with the CNT flocs. [ABSTRACT FROM AUTHOR]

The poly(trimethylene terephthalate) (PTT) containing mesoporous silica (SBA-15) prepared by in situ polymerization were used as the master batch in this work and mixed with the commercial PTT to fabricate PTT/mesoporous silica composites. The mechanical properties and viscoelastic behaviors such as creep and creep recovery of as-obtained composite system were then studied in detail. The results reveal an evident reinforcing effect of the mesoporous SBA-15 particles at the lower loading levels. Compared with those of the pure PTT, the tensile strength and impact strength of the composite with 0.5 wt% SBA-15 particles increase by about 26% and 10%, respectively. This is attributed to the physical crosslinking between SBA-15 and PTT chain caused by the mesoporous structure of SBA-15 particle, which is confirmed by the increased glass transition temperature of the composites relative to the pure PTT. At the higher loading levels, however, the presence of mesoporous particle reduces the overall strength of PTT because the concentration of lower molecular weight PTT in the master batch also increases with increasing loading of SBA-15. Therefore, superfluous addition of master batch has negative contribution to the mechanical strength of PTT composites. Besides, the composite system presents the higher creep levels and the lower recovery rates than the pure PTT, indicating that the presence of mesoporous SBA-15 particles restrains the movements of PTT chain segments during creep and creep recovery. This is further confirmed by the viscoelastic model predictions. POLYM. COMPOS., 36:1386-1393, 2015. © 2014 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Poly(trimethylene terephthalate)/poly(butylenes succinate) (PTT/PBS) blends and their ester-exchanged system were prepared by melt mixing for the phase behavior and the viscoelasticity studies. A typical two-phase structure can be seen on the blends because two polymers are immiscible thermodynamically. The phase inversion behavior of the blends can be well determined by the blending ratio dependence of their dynamic rheological responses, which can also be predicted by the viscous Utracki model based on the viscosity ratio. However, the dynamic viscoelastic responses of the blends cannot be well described by the emulsion model because two polymers are highly asymmetric in their viscoelasticity. Besides, transesterification is an effective approach of reducing interfacial tension and improving final phase morphology of the blends, which can be evaluated qualitatively from viscoelastic response alterations after ester exchange reaction. The mechanical properties of PTT/PBS blends were also studied. The results reveal that the ester-exchanged blends show mechanical strengths even lower than the pristine ones because of bulk degradation accompanied with transesterification, despite their improved phase structure. However, they can be used as the good compatibilizer to improve phase adhesion of the pristine blends, enhancing strengths of the PTT based blends or toughness of the PBS based blends evidently. [ABSTRACT FROM AUTHOR]

The carbon nanotubes (CNTs)–filled poly(vinylidenefluoride) (PVDF) composites (PCTs) were preparedby melt compounding for rheological study. The steady and oscillatoryflow behaviors were then explored. The results show that the presenceof CNTs enhances the pseudoplastic flow accompanied by the increasedflow activation energy. However, the linear flow region is not sensitiveto the temperature whether driven by shear rate or by strain. Duringoscillatory shear flow, the solid-like response is attributed to thepercolation of CNTs, but the formation of a percolated CNT networkis temperature-dependent, and the percolation threshold values reducewith an increase of temperature. The two-phase viscoelastic modelwas then used to further describe the linear responses of composites,aiming at relating hierarchical structures of the CNTs to flow behaviorsof the composites. [ABSTRACT FROM AUTHOR]

Abstract: In this work, thermal behaviors and viscoelasticity of the PPS/epoxy resin mixtures prepared by melt mixing were studied, and the rheology was used as a tool to further explore the relation between chain structures and properties of the mixtures. The results show that the epoxy resin can promotes degradation and branching of the PPS during melt mixing due to its poor thermal stability. The formation of long chain branched and partly crosslinked structures in the mixtures together with the residual decomposed components hence result in broadening of molecular weight distribution in comparison with those of the neat PPS. As a consequence, the mixtures show the viscoelastic behaviors far different from the neat PPS: the presence of crosslinked structure leads to the strong solid-like responses in the small amplitude oscillatory shear (SAOS) flow, and the presence of long chain branched structure to the weak strain overshoots in the large amplitude oscillatory shear (LAOS) flow. Moreover, after quiescent annealing, a strain-scaling transient stress behavior is observed in the mixtures in the reverse flow. [Copyright &y& Elsevier]

Abstract: Polylactide (PLA) nanocomposites containing various functionalized multi-walled carbon nanotubes (MWCNTs) were prepared directly by melt compounding. The linear rheology and thermal stability of the PLA nanocomposites were, respectively, investigated by the parallel plate rheometer and TGA, aiming at examining the effect of surface functionalization on the dispersion of MWCNTs by using viscoelastic and thermal properties. Among three MWCNTs used in this work, the carboxylic MWCNTs present better dispersion in PLA matrix than the hydroxy and purified MWCNTs because the corresponding composite shows the lowest rheological percolation threshold, which is further confirmed by the TEM and solution experiments. The presence of all these three MWCNTs, however, nearly cannot improve the thermal stability effectively at the initial stage of degradation and the temperature corresponding to a weight loss of 5wt% (T 5wt%) only shows slight increase in contrast to that of the neat PLA while with increase of decomposition level, the presence of carboxylic and purified MWCNTs retards the depolymerization of PLA evidently, showing remarkable increase in the temperature corresponding to maximum rate of decomposition (T max). Both the dispersion state and the surface functionalization of MWCNTs are very important to the thermal stability of PLA matrix. [Copyright &y& Elsevier]

Abstract: Polylactide (PLA)/polycaprolactone (PCL) blends with various blend ratios were prepared via melt mixing. The morphology, linear and non-linear viscoelastic properties of the blend were studied using scanning electron microscope (SEM) and cone-plate rheometer. Three typical immiscible morphologies, i.e., spherical droplet, fibrous and co-continuous structure can be observed at various compositions. The elasticity ratio was proposed to play an important role together with the viscosity on the phase inversion because PLA/PCL blend presents a high viscosity ratio between two components. Two emulsion models were used to predict the linear viscoelastic properties of the blend with various morphologies. The Palierne model gives better fit compared with the G–M model, but both fail to predict the viscoelastic properties of the co-continuous blend. The viscoelastic behavior of those blends shows different temperature dependence due to their different morphologies. The principle of time–temperature superposition (TTS) is only valid for the co-continuous blend while fails with the rheological data of those blends with discrete spherical and fibrous domain structure. Moreover, although the discrete phase is difficult to be broken up due to the high viscosity ratio of the systems, the change of viscoelastic responses of those blends before and after preshear shows large difference, indicating that different morphologies have different sensitivity to the steady shear flow. [Copyright &y& Elsevier]

This article discusses a study which examined the viscoelastic properties of polyarylene ether nitriles (PEN)/thermotropic liquid crystalline polymer (TLCP) blend. The study used scanning electron microscopy (SEM), rheometry and dynamnic mechanical analysis (DMA) in the examination. It revealed temperature dependence of the blend by investigating its linear dynamic viscoelastic behavior.

Acetylated chitin nanocrystals (ChNCs) were used as stabilizer in this work to prepare sunflower seed oil- in -water emulsions for the morphological and rheological studies. The results revealed that the acetylation with moderate degree of substitution (0.38) reduced hydrophilicity and increased surface charge level of rod-like ChNCs, and as a result, significantly improved the emulsifying ability of ChNCs. At the same oil/water ratio and particle loading, the emulsions stabilized with the acetylated ChNCs had far smaller droplet size (∼3 μm) as compared to the emulsions stabilized with the pristine ChNCs (5–7 μm). The increased droplets numbers and improved surface coating level resulted in the enhanced viscous resistance and yield stress level, which improved the physical stability of the acetylated ChNC-stabilized emulsions as a result. In addition, the droplet clusters easily formed in this system, contributing to weak strain overshoot and decreased large-deformation sensitivity during dynamic shear flow. Therefore, the acetylated ChNC-stabilized system showed enhanced transient stress overshoot during startup flow and weakened thixotropy during cyclic ramp shear flow as compared to the pristine ChNC-stabilized system. The relationships between surface acetylation of ChNCs and flow behavior of emulsions were then established, which provide valuable information on the modulation of the ChNC-stabilized Pickering emulsions. • Acetylation reduced hydrophilicity and increased surface charge level of ChNCs. • Acetylated ChNCs showed far higher emulsifying capacity than pristine ChNCs. • The system with acetylated ChNCs had reduced strain-sensitivity and thixotropy. • Viscoelasticity can scale the surface treatment-dependence of ChNC as stabilizer. [ABSTRACT FROM AUTHOR]

Xylan nanocrystals (XyNCs) have attracted increasing attention recently because this kind of polysaccharide nanocrystals is fabricated via the 'bottom-up' way, instead of the 'top-down' way. As compared to the other polysaccharide nanocrystals, the studies on the applications of XyNCs are limited yet, especially on developing the emulsions stabilized with XyNCs. The reported studies on this new type of Pickering emulsions mainly focused on using plate-like XyNCs as stabilizer. In this work, rod-like XyNCs were prepared for stabilizing emulsions. The results disclose that the rod-like XyNCs have superior emulsifying ability to the olive oil- in -water emulsions. The rheological responses are closely related to the XyNC loading-dependent morphological evolution. Therefore, the catastrophic viscoelasticity during the ramp shear flow and thixotropic cycles can be used as the probe to evaluate the interfacial stabilization mechanism, in this way to prepare the emulsions with optimized XyNC loadings. This work proposes an interesting approach of formulating XyNC-stabilized emulsions, and also provides useful information on diversifying applications of xylan-based nanomaterials. [Display omitted] [ABSTRACT FROM AUTHOR]

Graphene nanosheets (GNS) with and without surface functionalization were used as the reinforcements to prepared the composites with thermoplastic polyester elastomer (TPEE) by melting mixing. The results show that the presence of GNS improves modulus, yield and tensile strengths of TPEE, and the surface functionalization of GNS further improves its reinforcing effect because of enhanced interfacial interactions. However, the elongation levels also increase in the presence of GNS, which indicates the increased elastoplasticity and viscoplasticity of system. This is because the discrete hard poly(butylene terephthalate) domains of TPEE are enriched on the surface of GNS and form ‘ball bearing’ structure in composites, leading to increased deformation ability of soft continuous poly(tetramethylene glycol) of TPEE during large-scale deformation. Thus, the composites show more amounts of tensile cycles with shorter true strain plateau associated with onset of chain disentangling during cyclic tensile tests. This ‘bearing ball’ effect, however, is not evident in small-scale viscoelastic deformation because the presence of GNS restrains creep of TPEE. In this case, the impeding effect of GNS on the deformation of TPEE chain coils is the dominant role. This work provides useful information on structure design and control of the GNS filled thermoplastic elastomer composites. [ABSTRACT FROM AUTHOR]

Discriminating the roles of different networks in the multiply cross-linked hydrogels is vital to optimize their overall performance. Poly(vinyl alcohol)/cellulose nanofiber composite hydrogels were used as template for the study. Three types of characteristic networks, including chemical network cross-linked with boronic ester bonds, physical network cross-linked with microcrystallites, and coexistence of these two networks, were constructed in the system, and the viscoelastic responses were used to detect the characteristic relaxation behavior of those networks. The physical network is more sensitive to stress-induced deformation, whereas the chemical network more sensitive to strain-induced one. The former has lower level of viscous dissipation and higher level of elastic storage as compared to the latter, and dominates linear viscoelasticity of hydrogels as the two networks coexist. Their synergistic effect can be well defined by the scaling behavior of hysteretic work. This work proposes an interesting method of probing networks in the multiply cross-linked hydrogels. [Display omitted] • Physical network cross-linked by PVA microcrystallites is stress-sensitive. • Chemical network cross-linked via boronic ester bonding is strain-sensitive. • Molecular crosslinking in 3D bulk skeleton dominates mechanical properties. • Synergistic effect of two networks leads to scaling behavior of elastoplasticity. [ABSTRACT FROM AUTHOR]

[Display omitted] Using the platelet-like starch nanocrystals (SNCs) to stabilize emulsions is attractive because as-prepared emulsions have promising applications in cosmetics and food fields. Limited studies mainly focus on the oil- in -water system, and another important system, the water- in -water emulsions stabilized by SNCs, has not yet been unveiled. Two surface modification strategies, crosslinking and acetylation, were applied to tune surface property and aggregation of SNCs, and a common all-aqueous system (dextran/poly(ethylene glycol)) was used here as template. The viscoelasticity and morphology of emulsions were studied in terms of the SNC loadings and polymer ratios. Crosslinking results in aggregation of SNCs, and the particle size increases (from 110 nm to 370 nm) with increased levels of substitution. This favors improving emulsifying ability of particles. Acetylation decreases the particle size (∼90 nm) and weakens the affinity of SNCs to the two aqueous phases, improving the emulsifying efficiency of SNCs. More intriguingly, the two emulsion systems show different phase inversion behaviors. The depletion-stabilization mechanism for the cross-linked SNCs and the diffusion-controlled mechanism for the acetylated SNCs are proposed using the emulsion viscoelasticity as probe. This study makes a comprehensive insight into the regulation of water- in -water emulsion morphology and types with the platelet-like SNCs. [ABSTRACT FROM AUTHOR]

Dispersion states are vital for fibrous nanocelluloses to be used as reinforcements for polymers, which is highly dependent on geometry of nanocelluloses. Three types of nanocelluloses with various fiber aspect ratios were used to prepare target composite samples with poly(β -hydroxybutyrate) in this work. Viscoelasticity/elastoplasticity were used as probes to detect the flexibility-morphology relations of nanocelluloses in polymer. Cellulose nanocrystals (aspect ratio = 8) were rigid in polymer, retaining their rod-like shape, whereas bacterial celluloses (aspect ratio = 600) fully flexible, forming closely networked structure, and cellulose nanofibers (aspect ratio = 70) semi-flexible, dispersing as loosely flocculated clusters. Owing to these differences, the viscoelastic flow and elastoplastic deformation of three kinds of composites differed from one another. The strain-scaling and hysteresis work-scaling behaviors were then used to establish relaxation scale-structure correlations of target samples. This work provides interesting information around regulating the dispersion of nanocelluloses in polymer composites by tailoring aspect ratios of nanocelluloses. • Percolation thresholds of composites reduce with increased aspect ratios of fibers. • Stress-scaling is independent on the geometry or size of fibers during start-up flow. • Hysteresis energy-scaling is independent on the loading and aspect ratios of fibers. [ABSTRACT FROM AUTHOR]

• Three types of nanocelluloses show different fiber flexibility in the emulsions. • Flexible BC nearly has no emulsifying capacity to oil- in -water system. • Semi-flexible CNF shows better emulsifying capacity than rigid CNC. • Depletion stabilization results in the formation of droplet cluster structure. • Steric hindrance plays more important role in the CNF-stabilized emulsion. Three types of nanocelluloses, including bacterial cellulose (BC), cellulose nanofiber (CNF) and cellulose nanocrystal (CNC), were used to prepare oil- in -water Pickering emulsions with the objective to disclose the effect of fiber flexibility on emulsification. In aqueous suspensions, the shortest CNC is rigid, while the longest BC fully flexible, which result in large difference in their dilute- to -semi-dilute concentrations, and in the rheological percolations. Thus, these cellulosic nanofibers play different roles during emulsification. Flexible BC nearly has no emulsifying capacity, whereas semi-flexible CNF and rigid CNC can be well used to stabilize emulsions. For the CNF-stabilized system, depletion effect is dominant, leading to the formation of droplet clusters easily, while for the CNC-stabilized one, repulsive effect plays more important role. Visible evidence regarding relaxation of long-term structure of droplets is further disclosed by dynamic rheology. This work proposes interesting views around tailoring morphology and viscoelasticity of Pickering emulsions by regulating fiber flexibility. [ABSTRACT FROM AUTHOR]