Research Background
As a natural, abundant and renewable resource, cellulose encounters great challenges in practical applications due to its non-melting and limited solubility properties. The high crystallinity and high-density hydrogen bonds in the cellulose structure make it degrade but not melt during the possession process, and insoluble in water and most organic solvents. Their derivatives are produced by the esterification and etherification of the hydroxyl groups on the anhydroglucose units in the polymer chain, and will exhibit some different properties compared with natural cellulose. The etherification reaction of cellulose can generate many water-soluble cellulose ethers, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), which are widely used in food, cosmetics, in pharmaceuticals and medicine. Water-soluble CE can form hydrogen-bonded polymers with polycarboxylic acids and polyphenols.
Layer-by-layer assembly (LBL) is an effective method for preparing polymer composite thin films. The following mainly describes the LBL assembly of three different CEs of HEC, MC and HPC with PAA, compares their assembly behavior, and analyzes the influence of substituents on LBL assembly. Investigate the effect of pH on film thickness, and the different differences of pH on film formation and dissolution, and develop the water absorption properties of CE/PAA.
Experimental Materials:
Polyacrylic acid (PAA, Mw = 450,000). The viscosity of 2wt.% aqueous solution of hydroxyethylcellulose (HEC) is 300 mPa·s, and the degree of substitution is 2.5. Methylcellulose (MC, a 2wt.% aqueous solution with a viscosity of 400 mPa·s and a degree of substitution of 1.8). Hydroxypropyl cellulose (HPC, a 2wt.% aqueous solution with a viscosity of 400 mPa·s and a degree of substitution of 2.5).
Film preparation:
Prepared by liquid crystal layer assembly on silicon at 25°C. The treatment method of the slide matrix is as follows: soak in acidic solution (H2SO4/H2O2, 7/3Vol/VOL) for 30min, then rinse with deionized water several times until the pH becomes neutral, and finally dry with pure nitrogen. LBL assembly is performed using automatic machinery. The substrate was alternately soaked in CE solution (0.2 mg/mL) and PAA solution (0.2 mg/mL), each solution was soaked for 4 min. Three rinse soaks of 1 min each in deionized water were performed between each solution soak to remove loosely attached polymer. The pH values of the assembly solution and the rinsing solution were both adjusted to pH 2.0. The as-prepared films are denoted as (CE/PAA)n, where n denotes the assembly cycle. (HEC/PAA)40, (MC/PAA)30 and (HPC/PAA)30 were mainly prepared.
Film Characterization:
Near-normal reflectance spectra were recorded and analyzed with NanoCalc-XR Ocean Optics, and the thickness of films deposited on silicon was measured. With a blank silicon substrate as the background, the FT-IR spectrum of the thin film on the silicon substrate was collected on a Nicolet 8700 infrared spectrometer.
Hydrogen bond interactions between PAA and CEs:
Assembly of HEC, MC and HPC with PAA into LBL films. The infrared spectra of HEC/PAA, MC/PAA and HPC/PAA are shown in the figure. The strong IR signals of PAA and CES can be clearly observed in the IR spectra of HEC/PAA, MC/PAA and HPC/PAA. FT-IR spectroscopy can analyze the hydrogen bond complexation between PAA and CES by monitoring the shift of characteristic absorption bands. The hydrogen bonding between CES and PAA mainly occurs between the hydroxyl oxygen of CES and the COOH group of PAA. After the hydrogen bond is formed, the stretching peak red shifts to the low frequency direction.
A peak of 1710 cm-1 was observed for pure PAA powder. When polyacrylamide was assembled into films with different CEs, the peaks of HEC/PAA, MC/PAA and MPC/PAA films were located at 1718 cm-1, 1720 cm-1 and 1724 cm-1, respectively. Compared with pure PAA powder, the peak lengths of HPC/PAA, MC/PAA and HEC/PAA films shifted by 14, 10 and 8 cm−1, respectively. The hydrogen bond between the ether oxygen and COOH interrupts the hydrogen bond between the COOH groups. The more hydrogen bonds formed between PAA and CE, the greater the peak shift of CE/PAA in IR spectra. HPC has the highest degree of hydrogen bond complexation, PAA and MC are in the middle, and HEC is the lowest.
Growth behavior of composite films of PAA and CEs:
The film-forming behavior of PAA and CEs during LBL assembly was investigated using QCM and spectral interferometry. QCM is effective for monitoring film growth in situ during the first few assembly cycles. Spectral interferometers are suitable for films grown over 10 cycles.
The HEC/PAA film showed a linear growth throughout the LBL assembly process, while the MC/PAA and HPC/PAA films showed an exponential growth in the early stages of assembly and then transformed into a linear growth. In the linear growth region, the higher the degree of complexation, the greater the thickness growth per assembly cycle.
Effect of solution pH on film growth:
The pH value of the solution affects the growth of the hydrogen bonded polymer composite film. As a weak polyelectrolyte, PAA will be ionized and negatively charged as the pH of the solution increases, thereby inhibiting hydrogen bond association. When the degree of ionization of PAA reached a certain level, PAA could not assemble into a film with hydrogen bond acceptors in LBL.
The film thickness decreased with the increase of solution pH, and the film thickness decreased suddenly at pH2.5 HPC/PAA and pH3.0-3.5 HPC/PAA. The critical point of HPC/PAA is about pH 3.5, while that of HEC/PAA is about 3.0. This means that when the pH of the assembly solution is higher than 3.5, the HPC/PAA film cannot be formed, and when the pH of the solution is higher than 3.0, the HEC/PAA film cannot be formed. Due to the higher degree of hydrogen bond complexation of HPC/PAA membrane, the critical pH value of HPC/PAA membrane is higher than that of HEC/PAA membrane. In salt-free solution, the critical pH values of the complexes formed by HEC/PAA, MC/PAA and HPC/PAA were about 2.9, 3.2 and 3.7, respectively. The critical pH of HPC/PAA is higher than that of HEC/PAA, which is consistent with that of LBL membrane.
Water absorption performance of CE/ PAA membrane:
CES is rich in hydroxyl groups so that it has good water absorption and water retention. Taking HEC/PAA membrane as an example, the adsorption capacity of hydrogen-bonded CE/PAA membrane to water in the environment was studied. Characterized by spectral interferometry, the film thickness increases as the film absorbs water. It was placed in an environment with adjustable humidity at 25°C for 24 hours to achieve water absorption equilibrium. The films were dried in a vacuum oven (40 °C) for 24 h to completely remove the moisture.
As the humidity increases, the film thickens. In the low humidity area of 30%-50%, the thickness growth is relatively slow. When the humidity exceeds 50%, the thickness grows rapidly. Compared with the hydrogen-bonded PVPON/PAA membrane, the HEC/PAA membrane can absorb more water from the environment. Under the condition of relative humidity of 70% (25°C), the thickening range of PVPON/PAA film is about 4%, while that of HEC/PAA film is as high as about 18%. The results showed that although a certain amount of OH groups in the HEC/PAA system participated in the formation of hydrogen bonds, there were still a considerable number of OH groups interacting with water in the environment. Therefore, the HEC/PAA system has good water absorption properties.
in conclusion
(1) The HPC/PAA system with the highest hydrogen bonding degree of CE and PAA has the fastest growth among them, MC/PAA is in the middle, and HEC/PAA is the lowest.
(2) The HEC/PAA film showed a linear growth mode throughout the preparation process, while the other two films MC/PAA and HPC/PAA showed an exponential growth in the first few cycles, and then transformed into a linear growth mode.
(3) The growth of CE/PAA film has a strong dependence on the solution pH. When the solution pH is higher than its critical point, PAA and CE cannot assemble into a film. The assembled CE/PAA membrane was soluble in high pH solutions.
(4) Since the CE/PAA film is rich in OH and COOH, heat treatment makes it cross-linked. The cross-linked CE/PAA membrane has good stability and is insoluble in high pH solutions.
(5) The CE/PAA film has good adsorption capacity for water in the environment.
Post time: Feb-18-2023