Rheological Properties and Thermal Gelation Behavior of Hydroxypropyl Methyl Cellulose

Abstract: The rheological properties of four hydroxy propyl methyl cellulose (HPMC) solution samples at different concentrations were tested by rotational viscometer, and the thermal gelation effect of HPMC samples was studied by dynamic rotational rheometer. The results show that the relative molecular mass of HPMC has a significant effect on the rheological properties, while the effect of the substituents is not obvious; at the same time, the mathematical model relationship between the fluid behavior index and the concentration is obtained. The substituents have a significant effect on the thermal gelation effect, while for the sample series with different degrees of substitution, the relative molecular mass has different influence trends.

Key words:hydroxypropyl methylcellulose; rheology; gel

Cellulose ether is a modified cellulose derivative, which is widely used in medicine, building materials and synthetic resin industries. Studying the rheological properties of cellulose ether solutions is of great significance for its application, and it is also a hot spot for researchers. Cellulose ether has the property of reversible thermal gelation, and its solution will undergo a sol-to-gel transition during the heating process, and will return to the solution state after lowering the temperature. Therefore, cellulose ether hydrogels are widely used in tissue engineering and drug sustained release in the medical field. Methylcellulose is one of the most common cellulose ethers, and its thermogelling properties have been extensively studied. Hydroxypropylmethylcellulose (HPMC) is also an important cellulose ether, but there are few studies on its rheological properties and thermal gel properties. Therefore, this paper systematically studies the relative molecular weight and two substitutions The effect of groups (methoxyl and hydroxypropyl) on the rheological properties and thermal gelation of HPMC.

1. Experiment

1.1 Raw materials and sample preparation

Hydroxypropylmethylcellulose (HPMC) samples were provided by KIMA CHEMICAL CO., LTD without any treatment before use. The degree of substitution of HF series samples is: 27.5% for methoxy group; 2.0% for hydroxypropyl group. The degree of substitution of HK series samples is: 21.5% methoxyl group: 8.0% hydroxypropyl group. Two kinds of samples with relative molecular mass were used in the experiment, and the viscosities of their 2% aqueous solutions were 25 Pa·S and 75 Pa·8, respectively.

Add HPMC powder into deionized water at 60°C and stir evenly. Cellulose ether does not dissolve above its lower critical solution temperature. At this time, the solution is left to stand for a while, and after cooling to room temperature, it is stirred until a homogeneous solution is formed. Prepare solution samples with different mass concentrations of 0.5-10 g/L.

1.2 Testing

1.2.1 Viscosity test

Viscosity was measured on a Brookfield rotational viscometer. Add solution samples of different mass concentrations into concentric cylinders, stabilize for about 2 minutes, and test at a constant temperature of 30±0.1°C. The shear rate is controlled in the range of 0-2 000 S-1.

1.2.2 Dynamic rheological test

Four solutions of 10 g/L HK25M, HK75M, HF25M and HF75M were prepared, and the dynamic rheological tests were carried out on the ARES-RFS rotational rheometer. The samples were placed in a 34 mm diameter cylinder and tested in temperature sweep mode. The solutions were tested for storage and loss modulus during thermal cycling of heating at 20-80°C and cooling at 80-20°C. The heating and cooling rates are both 1°C/mil, and the frequency is 1rad/s.

2. Results and Discussion

2.1 Effect of relative molecular mass and degree of substitution on the rheological properties of solutions

From the rheological curves of shear stress (f) versus shear rate (y) in the viscosity test results, it can be seen that the four HPMC solutions show similar changes when the mass concentration increases. At low mass concentration (0.5---1 g/L), the rheological curve of shear stress versus shear rate is very linear, showing obvious Newtonian fluid characteristics. With the increase of mass concentration, the rheological curve shows the phenomenon of shear thinning, and the solution presents pseudoplastic fluid characteristics at this time.

Comparing the relative parameters of the rheological curves of different mass concentrations of HPMC solution and the fluid behavior index in the fitting degree to the mass concentration basic diagram, it can be found that there is a certain relationship between the two. From the influence of degree of substitution and relative molecular mass on the relationship between fluid behavior index and mass concentration, it can be seen that under the same degree of substitution, the influence of relative molecular mass on fluid behavior index is obvious. When the degree of substitution is the same, samples with high molecular weight have a smaller fluid behavior index than samples with low molecular weight, that is, the pseudoplastic characteristics of the fluid are more obvious. This is because the higher the relative molecular mass, the longer the chain of the molecule, the more obvious the distortion of the macromolecule, the more entanglements between the molecules, and the more complex conformation. The phenomenon of thinning is more pronounced and therefore more pseudoplastic, i.e. with lower fluid behavior.

The degree of substitution refers to the number of hydroxyl groups substituted by alcohol groups on each anhydroglucose unit in the cellulose molecular chain. The substituents have an important influence on the dissolution and gelation process of cellulose ether. HPMC has Methoxy and hydroxypropyl are two substituent groups. From the influence of degree of substitution and relative molecular mass on the relationship between fluid behavior index and mass concentration, it can be seen that when the relative molecular mass is the same, except for a few points, the fluid behavior index of HK series samples with high hydroxypropyl content is lower. This may be due to the large volume of the hydroxypropyl substituent group. At the same molecular mass level, the relative molecular mass of the sample with high hydroxypropyl content is slightly higher than that of another series of samples, so this effect is not very obvious.

2.2 Effect of relative molecular mass and degree of substitution on gel temperature

The mechanism of cellulose ether dissolution and gelation is explained by the widely accepted theory of "cagelike structure". When the solution of HPMC is at low temperature, there are hydrogen bonds between the hydrophilic groups on the macromolecules and the water molecules, which are surrounded by water molecules to form a cage structure. The heat applied by the temperature rise will break the hydrogen bond between the water molecule and the HPMC molecule, the cage-like supramolecular structure will be destroyed, and the water molecule will be released from the bond of the hydrogen bond to become a free water molecule, while the large HPMC The hydrophobic methoxy groups on the molecular chain are exposed, which makes hydrophobic association possible. If the methoxy groups on the same molecular chain are hydrophobically combined, this intramolecular interaction will make the whole molecule appear coiled. However, the increase in temperature will intensify the motion of the chain segment, the hydrophobic interaction in the molecule will be unstable, and the molecular chain will change from a coiled state to an extended state. At this time, the hydrophobic interaction between molecules begins to dominate. When the temperature gradually rises, more and more hydrogen bonds are broken, and more and more cellulose ether molecules are separated from the cage structure, and the macromolecules that are closer to each other gather together through hydrophobic interactions to form A hydrophobic aggregate. With a further increase in temperature, eventually all hydrogen bonds are broken, and its hydrophobic association reaches a maximum, increasing the number and size of hydrophobic aggregates. During this process, HPMC becomes progressively insoluble and eventually completely insoluble in water. After that, different hydrophobic aggregates begin to combine through hydrophobic interactions. When the temperature rises to the gel point, the three-dimensional network structure formed by the hydrophobic aggregates fills the system, which appears as the formation of a gel macroscopically.

The relationship between storage modulus and loss modulus and temperature of HPMC (HF25M) solution during heating and cooling cycle reflects the thermoreversible phenomenon of gelation process of HF25M HPMC solution during heating and cooling. When heated, both G' and G" decrease with the increase of temperature, G' is greater than G'', and the solution exhibits a viscoelastic behavior of an ordinary liquid. Here, a traditional determination of the gel transition is used Point method, that is, the intersection point of G' and G'' is defined as the gel point. For 10 g/L HPMC (HF25M) solution, when the temperature rises to about 66 ° C, thermal gel begins to precipitate, G 'less than G', indicating that the viscoelastic behavior of solids plays a dominant role at this time.

The gelation of many cellulose ethers has thermal reversibility, but the process of gelation during heating and degelation during cooling is not the same. When cooling, the intersection point of G' and G" appears at about 42°C. This indicates that the formation of the gel network structure requires a higher temperature (66°C) during heating, and the large-scale deconstruction of the gel network during cooling It occurs at a lower temperature (42°C). After that, the macromolecular complex formed by the hydrophobic interaction also disintegrates, and the HPMC molecules regain their freedom, and form hydrogen bonds with water molecules to dissolve in water and return to the solution state.

The effect of the degree of substitution on the gelation process of HPMC shows that the degree of substitution of HPMC has a significant impact on the gelation temperature. Whether it is high molecular weight or low molecular weight, the gelation temperature of the HK series is higher than that of the HF series. This is due to the higher degree of methoxyl substitution and lower degree of hydroxypropyl substitution of samples in the HF series. Hydroxypropyl is a kind of hydrophilic group, its existence is conducive to the formation of hydrogen bonds between macromolecules and water molecules, and a lower degree of substitution of hydroxypropyl means less hydrogen bonds are formed, which requires less Small energy destroys the "cage" structure between macromolecules and water molecules, and macromolecules are released from the shackles of water molecules more easily. As a hydrophobic group, the methoxy group is conducive to the hydrophobic interaction between macromolecules. A higher degree of methoxy substitution makes it easier to form hydrophobic aggregates between molecular chains, so the gel network can be formed at a lower temperature.

From the influence of relative molecular mass on the gelation process of HPMC, it can be seen that for two different series of samples, the influence of relative molecular mass on the gelation temperature is different. For the HF series, the gelation temperature of the high molecular mass samples is lower than that of the low molecular mass samples: while for the HK series, this trend is just the opposite. This result is possible considering that the two substituent groups in HPMC have opposite effects on gelation. Because the high relative molecular weight indicates that there are more substituent groups on the macromolecular chain at the same time, for the HF series, the promotion effect of methoxy group on gelation is greater than the hindering effect of hydroxypropyl group on it, so it contains The higher molecular weight HF75M with more methoxy groups has a lower gel temperature. On the contrary, the hydroxypropyl group plays a major role in the HK series, so the gel temperature of HK75M containing more hydroxypropyl groups is relatively higher.

3. Conclusion

(1) The relative molecular weight has a significant impact on the rheological properties of HPMC solutions. The samples with high molecular weight have more obvious pseudoplastic fluid properties, and the samples with high hydroxypropyl content have a smaller fluid behavior index. The obtained mathematical model shows that there is a consistent mathematical relationship between the fluid behavior index and the mass concentration for the four samples.

(2) The reversibility of HPMC thermogel is incomplete, and its gelation process and degelation process are not completely consistent. Substituents have a significant effect on the thermal gelation temperature, and the HF series with high methoxyl content and low hydroxypropyl content tend to form gels at lower temperatures. The influence of the relative molecular weight is affected by the degree of substitution. For the HF series, the influence of the methoxy substituent group is dominant, and the samples with high molecular weight are easy to form gels, while the trend of the HK series is opposite.