The dilute solution properties of a high-charge-density cationic cellulose ether (KG-30M) at different pH values were studied with a laser scattering instrument, from the hydrodynamic radius (Rh) at different angles, and the root mean square radius of rotation Rg The ratio to Rh infers that its shape is irregular but close to spherical. Then, with the help of rheometer, three concentrated solutions of cationic cellulose ethers with different charge densities were studied in detail, and the influence of concentration, pH value and its own charge density on its rheological properties were discussed. As the concentration increased, Newton’s exponent first decreased and then decreased. Fluctuation or even rebound occurs, and thixotropic behavior occurs at 3% (mass fraction). A moderate charge density is beneficial to obtain a higher zero-shear viscosity, and pH has little effect on its viscosity.
Key words: cationic cellulose ether; morphology; zero shear viscosity; rheology
Cellulose derivatives and their modified functional polymers have been widely used in the fields of physiological and sanitary products, petrochemicals, medicine, food, personal care products, packaging, etc. Water-soluble cationic cellulose ether (CCE) is due to its With strong thickening ability, it is widely used in daily chemicals, especially shampoos, and can improve the combability of hair after shampooing. At the same time, because of its good compatibility, it can be used in two-in-one and all-in-one shampoos. It also has a good application prospect and has attracted the attention of various countries. It has been reported in the literature that cellulose derivative solutions exhibit behaviors such as Newtonian fluid, pseudoplastic fluid, thixotropic fluid and viscoelastic fluid with the increase of concentration, but the morphology, rheology and influencing factors of cationic cellulose ether in aqueous solution There are few research reports. This paper focuses on the rheological behavior of quaternary ammonium modified cellulose aqueous solution, in order to provide a reference for practical application.
1. Experimental part
1.1 Raw materials
Cationic cellulose ether (KG-30M, JR-30M, LR-30M); Canada Dow Chemical Company product, provided by Procter & Gamble Company Kobe R&D Center in Japan, measured by Vario EL elemental analyzer (German Elemental Company), the sample The nitrogen content is 2.7%, 1.8%, 1.0% respectively (the charge density is 1.9 Meq/g, 1.25 Meq/g, 0.7 Meq/g respectively), and it is tested by German ALV-5000E laser light Scattering instrument (LLS) measured its weight average molecular weight is about 1.64×106g/mol.
1.2 Solution preparation
The sample was purified by filtration, dialysis and freeze-drying. Weigh a series of three quantitative samples respectively, and add standard buffer solution with pH 4.00, 6.86, 9.18 to prepare the required concentration. In order to ensure that the samples were fully dissolved, all sample solutions were placed on a magnetic stirrer for 48 hours before testing.
1.3 Light scattering measurement
Use LLS to measure the weight-average molecular weight of the sample in dilute aqueous solution,, the hydrodynamic radius and the root mean square radius of rotation when the second Villi coefficient and different angles,), and infer that this cationic cellulose ether is in the aqueous solution by its ratio status.
1.4 Viscosity measurement and rheological investigation
The concentrated CCE solution was studied by Brookfield RVDV-III+ rheometer, and the influence of concentration, charge density and pH value on rheological properties such as sample viscosity was investigated. At higher concentrations, it is necessary to investigate its thixotropy.
2. Results and discussion
2.1 Research on Light Scattering
Due to its special molecular structure, it is difficult to exist in the form of a single molecule even in a good solvent, but in the form of certain stable micelles, clusters or associations.
When the dilute aqueous solution (~o.1%) of CCE was observed with a polarizing microscope, under the background of the black cross orthogonal field, “star” bright spots and bright bars appeared. It is further characterized by light scattering, the dynamic hydrodynamic radius at different pH and angles, the root mean square radius of rotation and the second Villi coefficient obtained from the Berry diagram are listed in Tab. 1. The distribution graph of the hydrodynamic radius function obtained at a concentration of 10-5 is mainly a single peak, but the distribution is very wide (Fig. 1), indicating that there are molecular-level associations and large aggregates in the system; There are changes, and the Rg/Rb values are all around 0.775, indicating that the shape of CCE in solution is close to spherical, but not regular enough. The effect of pH on Rb and Rg is not obvious. The counterion in the buffer solution interacts with CCE to shield the charge on its side chain and make it shrink, but the difference varies with the type of counterion. Light scattering measurement of charged polymers is susceptible to long-range force interaction and external interference, so there are certain errors and limitations in LLS characterization. When the mass fraction is greater than 0.02%, there are mostly inseparable double peaks or even multiple peaks in the Rh distribution diagram. As the concentration increases, Rh also increases, indicating that more macromolecules are associated or even aggregated. When Cao et al. used light scattering to study the copolymer of carboxymethyl cellulose and surface-active macromers, there were also inseparable double peaks, one of which was between 30nm and 100nm, representing the formation of micelles at the molecular level, and the other The peak Rh is relatively large, which is considered to be an aggregate, which is similar to the results determined in this paper.
2.2 Research on rheological behavior
2.2.1 Effect of concentration: Measure the apparent viscosity of KG-30M solutions with different concentrations at different shear rates, and according to the logarithmic form of the power law equation proposed by Ostwald-Dewaele, when the mass fraction does not exceed 0.7% , and a series of straight lines with linear correlation coefficients greater than 0.99 were obtained. And as the concentration increases, the value of Newton’s exponent n decreases (all less than 1), showing an obvious pseudoplastic fluid. Driven by shear force, the macromolecular chains begin to untangle and orient, so the viscosity decreases. When the mass fraction is greater than 0.7%, the linear correlation coefficient of the obtained straight line decreases (about 0.98), and n begins to fluctuate or even rise with the increase of concentration; when the mass fraction reaches 3% (Fig. 2), the table The apparent viscosity first increases and then decreases with the increase of the shear rate. This series of phenomena is different from the reports of other anionic and cationic polymer solutions. The n value rises, that is, the non-Newtonian property is weakened; Newtonian fluid is a viscous liquid, and intermolecular slippage occurs under the action of shear stress, and it cannot be recovered; non-Newtonian fluid contains a recoverable elastic part and an unrecoverable viscous part. Under the action of shear stress, the irreversible slip between molecules occurs, and at the same time, because the macromolecules are stretched and oriented with the shear, a recoverable elastic part is formed. When the external force is removed, the macromolecules tend to return to the original curled form, so The value of n goes up. The concentration continues to increase to form a network structure. When the shear stress is small, it will not be destroyed, and only elastic deformation will occur. At this time, the elasticity will be relatively enhanced, the viscosity will be weakened, and the value of n will decrease; while the shear stress is gradually increasing during the measurement process, so n The value fluctuates. When the mass fraction reaches 3%, the apparent viscosity first increases and then decreases, because the small shear promotes the collision of macromolecules to form large aggregates, so the viscosity rises, and the shear stress continues to break the aggregates. , the viscosity will decrease again.
In the investigation of thixotropy, set the speed (r/min) to reach the desired y, increase the speed at regular intervals until it reaches the set value, and then quickly drop from the maximum speed back to the initial value to obtain the corresponding The shear stress, its relationship with the shear rate is shown in Fig. 3. When the mass fraction is less than 2.5%, the upward curve and the downward curve completely overlap, but when the mass fraction is 3%, the two lines no longer overlap, and the downward line lags behind, indicating thixotropy.
The time dependence of shear stress is known as rheological resistance. Rheological resistance is a characteristic behavior of viscoelastic liquids and liquids with thixotropic structures. It is found that the larger y is at the same mass fraction, the faster r reaches equilibrium, and the time dependence is smaller; at a lower mass fraction (<2%), CCE does not show rheological resistance. When the mass fraction increases to 2.5%, it shows a strong time dependence (Fig. 4), and it takes about 10 minutes to reach equilibrium, while at 3.0%, the equilibrium time takes 50 minutes. The good thixotropy of the system has conducive to practical application.
2.2.2 The effect of charge density: the logarithmic form of the Spencer-Dillon empirical formula is selected, in which the zero-cut viscosity, b is constant at the same concentration and different temperature, and increases with the increase of concentration at the same temperature. According to the power law equation adopted by Onogi in 1966, M is the relative molecular mass of the polymer, A and B are constants, and c is the mass fraction (%). Fig． 5 The three curves have obvious inflection points around 0.6%, that is, there is a critical mass fraction. More than 0.6%, the zero-shear viscosity increases rapidly with the increase of concentration C. The curves of the three samples with different charge densities are very close. In contrast, when the mass fraction is between 0.2% and 0.8%, the zero-cut viscosity of the LR sample with the smallest charge density is the largest, because the hydrogen bond association requires a certain contact. Therefore, the charge density is closely related to whether the macromolecules can be arranged in an orderly and compact manner; through DSC testing, it is found that LR has a weak crystallization peak, indicating a suitable charge density, and the zero-shear viscosity is higher at the same concentration. When the mass fraction is less than 0.2%, LR is the smallest, because in dilute solution, macromolecules with low charge density are more likely to form coil orientation, so the zero-shear viscosity is low. This has a good guiding significance in terms of thickening performance.
2.2.3 pH effect: Fig. 6 is the result measured at different pH within the range of 0.05% to 2.5% mass fraction. There is an inflection point around 0.45%, but the three curves almost overlap, indicating that pH has no obvious effect on zero-shear viscosity, which is quite different from the sensitivity of anionic cellulose ether to pH.
3. Conclusion
The KG-30M dilute aqueous solution is studied by LLS, and the hydrodynamic radius distribution obtained is a single peak. From the angle dependence and Rg/Rb ratio, it can be inferred that its shape is close to spherical, but not regular enough. For the CCE solutions with three charge densities, the viscosity increases with the increase of the concentration, but the Newton’s hunting number n first decreases, then fluctuates and even rises; pH has little effect on the viscosity, and a moderate charge density can obtain a higher viscosity .