Rheological behavior was examined for aqueous suspensions of nano-cellulose (NC) fibers. The plain-NC and carboxymethyl NC (CM-NC) fibers of an average diameter of ≅ 20 nm and a length of > 1μm were used. These fibers had a curvy, multiple-branch (branch-on-branch) structure, and some of those branches were bundled into thicker trunks in particular for CM-NC, as revealed from TEM. Both plain-NC and CM-NC suspensions, having a low NC concentration of 0.62 wt%, exhibited essentially elastic behavior under small strain in the linear regime. This elastic behavior was attributable to elastic bending of the NC fibers that formed a mesh in the suspensions. Comparison of the elastic modulus data with the modified Doi-Kuzuu model prediction suggested that the curvy, multiple-branch structure of the NC fibers significantly increases effective contacts between the fibers thereby enhancing the elasticity, in particular for the plain-NC fiber having scarcer trunk portions compared to the CM-NC fiber. Corresponding to this origin of the enhanced elasticity, the suspensions exhibited significant nonlinearity under large amplitude oscillatory shear (LAOS) and steady shear flow. Namely, the modulus and steady state viscosity of the suspensions decreased roughly in proportion to reciprocal of the strain and shear rate, respectively, although strain-hardening was also noted for the CM-NC suspension under moderately small strains. This nonlinear decrease of the modulus and viscosity, phenomenologically classified as the yielding, would have resulted from strain/flow-induced slippage (decrease of the effective contacts) between the fibers.
The uniaxial tensile behavior of κ-carrageenan films with saccharides was examined to elucidate the plasticizing effect of saccharides. Five saccharides as well as two alditols for comparison were used. The films were tested after removal of water as far as possible. The tensile modulus (E) decreased as the fraction of the saccharides increased, suggesting apparent plasticizing effect of the saccharides. The plasticizing effect was discussed referring the glass transition temperature (Tg) of the saccharides, because the saccharides were considered in the “supercooled” liquid state even below the melting point. Since the saccharides except the monosaccharides showed plasticizing effect similar to that of glycerol both above and below Tg, namely regardless of whether the saccharide was in the solid state or in the liquid state, a blending law of the moduli was proposed to explain the decrease in E. This means that these additives including glycerol are not real plasticizers in the conventional sense of the word. On the other hand, the monosaccharides glucose and fructose acted as better plasticizers at 100 °C. The additional factor was attributed to the solubility of these saccharides to the matrix κ-carrageenan, indicating the real plasticizing effect of the monosaccharides. Interestingly, this factor for fructose disappeared at 29 °C, still above Tg.
Effect of uniaxial strain on optical anisotropy for a cellulose triacetate (CTA) film was investigated by simultaneous measurements of stress and birefringence. Strain rates at hot-stretching process hardly affected orientation birefringence of CTA, although stress level increased with increasing the strain rate. Furthermore, birefringence was found to decrease only slightly after the cessation of the hot-stretching, i.e., stress relaxation process, beyond its glass transition temperature. Wide-angle X-ray diffraction patterns revealed that the orientation of crystals in the stretched film was not relaxed during the stress relaxation process. These results indicate that the orientation birefringence of CTA is mainly determined by the orientation of crystals.
In our previous paper, we reported that the cellulose nano-fiber (CNF) treated with Cardo material (BPFG), referred to as B-CNF, dispersed finely in an organic solvent as well as in polylactic acid (PLA) matrix, and the mechanical properties (especially at the temperature region above the grass transition temperature, Tg) of PLA/CNF composites were improved with the amount of B-CNF content (0.3, 0.5, and 1.1 wt%) in the composites. However, the degree of substitution of BPFG on CNF was only 8.6 wt% (18.6 mmol%) and the effect of the substitution of BPFG was not determined. In this study, we prepared several B-CNF with various degrees of substitution to BPFG and examined the effect of the substitution on the mechanical properties of PLA/B-CNF composites. Moreover, we also prepared the CNF treated by Bisphenol-A diglycidyl ether (BPAG), which does not have Cardo structure in its chemical structure, and determined the effect of the difference in chemical structure (with/without Cardo structure) on their mechanical properties at high temperature. Master batches were prepared by mixing CNF and PLA in an organic solvent and then they were further blended with PLA in a kneader at 150 °C. TEM observation revealed that both of the CNF treated with BPFG (B-CNF) and that treated with BPAG (BisA-CNF) dispersed homogeneously in the PLA. The storage moduli of B-CNF/PLA composites above Tg increased with the degree of BPFG substitution. The storage moduli of B-CNF/PLA and BisA-CNF/PLA with similar degree of substitution were about 5 times higher than that of neat PLA above Tg, and the modulus of B-CNF/PLA was slightly higher than that of BisA-CNF/PLA. Moreover, the peak intensity of tanδ of B-CNF/PLA and BisA-CNF/PLA were lower than that of neat PLA, and the intensity of tanδ of B-CNF/PLA was lower than that of BisA-CNF/PLA. These results suggest that the molecular motion of PLA was restricted by the presence of B-CNF and BisA-CNF because the interaction between PLA and B-CNF or BisA-CNF is much stronger than that in untreated CNF/PLA. Furthermore, B-CNF improved the mechanical properties of the composite more effectively than BisA-CNF for the PLA composite above Tg. From the result of Coefficient of Thermal Expansion (CTE) measurements, we estimated the volume fraction of PLA phase interacting with B-CNF and BisA-CNF by means of the “mixtures rule”. As a result, it was found that the PLA phase interacting with of B-CNF/PLA is larger than that in BisA-CNF/PLA. This may be one of the factor which caused the difference in their mechanical properties of B-CNF/PLA and BisA-CNF/PLA. Finally, CTE measurements revealed that there is a strong relation between the intensity of tanδ at Tg and the volume fraction of the PLA interacting phase in PLA matrix.
Bread can be prepared using only rice flour if the viscoelastic properties of the rice batter are appropriate. These properties depend on the line of rice due to differences in the amylose content and the architecture of the amylopectin between rice lines; consequently, the rheology of rice batter can be changed by using flour made from various rice lines. In the present study, we used the three mutant rice lines, namely, Nipponbare, e1, and #4019, which have different amylopectin architectures and different amylose contents. The baking qualities of breads made from these rice lines were characterized by expansion ratio and bubble size. Cross-sectional images of the baked breads were also examined. For rheological assessment, we measured the storage modulus G' and the loss modulus G" of batter made using flour from each line. The best baking qualities was obtained using line e1. A molecular model of the swelling of amylopectin is presented to explain the storage modulus results.
Cellulose nanofibers (CNFs) have many useful properties, including high strength and low thermal expansion. They are environmentally friendly, renewable, safe, and biodegradable. The focus of this study was to develop lightweight thermoplastic polymer composites with good mechanical properties by incorporating CNFs pretreated with a cationic reagent. Polyamide 11 (PA11) was mixed with the surface-treated CNFs using a twin-screw extruder and the resulting pellets were injection molded. Four different cationic reagents were used to modify the hydroxyl groups on the CNF surface, which resulted in better dispersion of the CNFs in the composites owing to reduced hydrogen bonding between the CNFs. The best cationic reagent for the preparation of CNF-reinforced composites was poly (N-methyl diallyl amine)-epichlorohydrin. The polymer consisted of repeating cationic quaternary ammonium salt units and was grafted with epichlorohydrin, which contained a reactive epoxy group. The molten viscosity of the composite prepared using poly (N-methyl diallyl amine)-epichlorohydrin was the highest of all the prepared composites. The high viscosity was attributed to the dispersed state of the CNFs in the composites and interfacial interactions between CNF and the PA11 matrix. The mechanical properties, fracture aspect, and thermal properties of the composites were strongly related to the molten viscosity. Cationic pretreatment of the surface of the CNFs enhanced the dispersion of the fibers and significantly improved the mechanical and thermal properties of the composites.
We investigated the structure and viscoelastic properties of ternary gels of curdlan in mixed solvents of dimethyl sulfoxide (DMSO) and water as a function of the weight fraction of water φw to reveal the gelation mechanism. Dynamic viscoelasticity measurements demonstrated that the gelation begins to be induced by the addition of 1 wt % of water (i.e., φw = 0.01) and that the plateau modulus increases with increasing φw in the φw range up to 0.15. Spin-lattice relaxation time T1 measurements using 1H-NMR technique revealed that there exist at least two different types of water with different T1 values in the ternary gel. By comparing those T1 values with that of the water proton in the mixed solvent, the water with shorter T1 (Water-1) was ascribed to the water molecules dispersed in the mixed solvent contained in the ternary gel. On the other hand, the water with the longer T1 (Water-2) was presumed to exist under the situation where water-water interaction is dominant. We hypothesized that Water-2 is localized around the vicinity of cross-linking points and is more relevant to the formation of cross-linking points. Almost a linear relation was observed between the plateau modulus Gc and the weight fraction of Water-2 φWater-2, indicating that the number of cross-linking points increases in proportion to φWater-2 in a constant volume of the ternary gel. Further, the distance between neighboring cross-linking points and the size of the region of Water-2 (i.e., equivalent spherical radius) was estimated at 15-9.3 nm and 2.1-2.5 nm depending on φWater-2, respectively. All these results support the validity of our hypothesis about Water-2.
We have established a new system to measure viscoelastic behavior for liquid, which shows too low viscoelasticity to be measured precisely with conventional mechanical rheometers, using modified dynamic light scattering (DLS) techniques. The DLS viscoelastic measurements were carried out for dilute aqueous solutions of chemically modified cellulose ethers (ChMC(1.9-0.25-Mw /103)) with the degree of substitution by methoxy groups of 1.9, that by hydroxypropoxy groups of 0.25 and the nominal weight average molecular weight of Mw = 75000, 300000 and 380000 containing a small amount of poly(styrene) latex (PL) beads with the radius of 0.05 μm. The dependence of creep compliance (J(t)) on time (t) was evaluated from the auto-correlation function of fluctuating scattered light intensity because of Brownian motions of dispersed PL beads. The viscoelasticity obtained from J(t) data revealed that ChMC(1.9-0.25-Mw /103)s do not behave as flexible polymer chains, but rigid rod-like molecules with the length of 130, 320 and 360 nm, respectively.
Curdlan gels were prepared by dialyzing its alkaline solutions in HCl solutions, and its optical anisotropy and molecular orientation in the gel were investigated by birefringence and small-angle X-ray scattering (SAXS). The observed asymmetrical two-dimensional SAXS pattern indicated that the orientation of polymer chains was perpendicular to the inflow of H+ in the gelation process. The birefringence results suggested that phase-separated domains were elongated along the molecular orientation. When the gel was dried, inversion of the birefringence sign was observed. This was attributed to the competing effects of the form birefringence due to the deformation of the domains and the intrinsic birefringence due to the molecular orientation.
Dissolution conditions of cotton, linter pulp, avicel, and nadelholz sulfur pulp (NSP) into 1-butyl-3-methylimidazolium chloride (BmimCl), 1-allyl-3-methylimidazolium chloride (AmimCl), and 1-ethyl-3-methylimidazolium acetate (EmimAc) are examined to obtain colorless and high viscosity uniform solutions. The tested dissolution conditions are inadequate for EmimAc. Degradation of cellulose was suppressed by choosing adequate dissolution temperatures for BmimCl (130 °C) and AmimCl (110 °C) solutions at depressed conditions without mechanical stirring, though it was difficult to obtain colorless uniform solutions for avicel and linter. Dynamic viscoelastic properties of uniform solutions of cotton and NSP in BmimCl and AmimCl showed typical terminal region behavior. Lowering the bubble burst speed in the early stage of dissolution by keeping at 0.1 atm was effective compared to high vacuum. Moreover, preheating at lower temperature resulted in more mild bubble burst and the higher relative viscosity, denoting that the molecular weight of cellulose change with the swelling condition and bubble burst speed by changing the temperature and pressure.
Stress-strain relations of a cured epoxy network subjected to uniaxial stretching during which the tensile rate changed stepwise were observed at a temperature below the glass transition. When the tensile rate was increased or decreased to a prearranged identical value, the stress-strain curves after the tensile rate change finally coincided with each other in the post-yield strain-hardening region, after showing transient variations. This behavior was observed independently of the amount of strain where the strain rate changed. Thus, the stress-strain relation in the strain-hardening region at a given strain rate did not depend on preceding strain rate history. Since the strain-hardening behavior in this system is ascribable to an increase of the rubbery stress originated from the chain orientation, the strain-hardening behavior independent of strain rate history means that the intensity of glassy stress, which is another stress component of glassy polymers, is uniquely determined only by the strain rate at the moment. Thus, the glassy stress in the strain range well beyond the yield point proved to be a steady flow stress in the network polymer.
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