已传文件：photo/1766544328.png
To achieve the target concentration of cross-linking between boric acid and polyvinyl alcohol (PVA), the key lies in precisely controlling the cross-linking density to obtain the desired material properties (such as gel strength, swelling rate, mechanical properties).
Key steps and control methods for achieving the target concentration
1. Precisely define the target parameters
Target concentration: Usually refers to the final concentration of boric acid in the final gel or solution.
Boric acid parameters: Type (boric acid or borax, which hydrolyzes in water to borate ions), pH value (boric acid is more likely to form tetrahedral borate ions in an alkaline environment, resulting in higher cross-linking efficiency).
2. Precise solution preparation
Prepare the mother solutions separately: PVA mother solution: Dissolve PVA powder in deionized water at high temperature (90-95°C) and stir thoroughly until clear and transparent. Cool down and store. The concentration should be precisely weighed.
Boric acid mother solution: Dissolve boric acid in deionized water or a specific pH buffer solution. It is strongly recommended to use a pH buffer solution (such as phosphate or carbonate buffer solution) to stabilize the pH of the system, which is crucial for reproducibility.
3. Key control parameters and operations
pH value control (the most critical parameter): The optimal pH range for boric acid-PVA cross-linking is usually between 8-10. Within this range, boric acid mainly exists in the form of [B(OH)₄]⁻, which efficiently coordinates with the 1,3-diol structure of PVA.
Monitor and adjust the pH of the boric acid solution or the mixed system using a pH meter.
It is essential to use a buffer solution; otherwise, the pH will drift during the reaction.
4. Mixing sequence and method:
Recommended sequence: Slowly add the boric acid solution to the well-stirred PVA solution to ensure uniform cross-linking and avoid local excessive cross-linking to form lumps.
Temperature: Usually mixed at room temperature. Temperature will affect the reaction rate and gel time, and it is necessary to maintain consistency.
Cross-linking time: After mixing, the system may take several seconds to several minutes to form a gel. The gel time is also an indirect indicator for evaluating the cross-linking concentration. The target concentration should correspond to a reproducible gel time.
