Polyvinyl alcohol (PVA) is a polymer with polyhydroxy structure and has many excellent properties such as excellent water solubility, mechanical properties, gas barrier properties and biodegradability. However, the polyhydroxy structure of PVA easily forms hydrogen bonds between the intramolecular chain and the molecular chain, so that the melting point of PVA is very close to its decomposition temperature, which makes thermoforming processing very difficult.

At present, the molding of PVA is mainly based on aqueous solution processing, but PVA needs to be dissolved at high temperature, and the energy consumption is large, and the molding cycle is long, which greatly limits the application of PVA. Many kinds of modification methods are used at home and abroad to realize the thermoplastic processing of PVA, such as using the hydroxyl groups on the molecular chain of glycerin and PVA, weakening the hydrogen bonding of PVA and lowering the melting point of PVA, but glycerol easily precipitates from the PVA matrix, affecting the material properties. Chemical modification is to use the high reactivity of PVA hydroxyl group to introduce thermal stability functional groups on the PVA molecular chain, destroy the PVA molecular chain regularity, weaken the hydrogen bonding between PVA molecular chains, and finally realize the thermoplastic processing of PVA, but eventually it will be difficult to maintain the inherent properties of the PVA itself.

Some researchers have prepared TRIS modified polyvinyl alcohol (PVA-COOTRIS) by carboxylating the PVA hydroxyl group and then amide reacting with TRIS. TRIS is a polyhydroxy compound, which introduces its polyhydroxy structure into the PVA side chain, and can reduce the damage of PVA hydrogen bond caused by modification, thereby changing the regular structure of PVA molecular chain and reducing the crystallinity and melting point of PVA, to improve its thermal stability, broaden the thermal processing window of PVA, and realize the thermoplastic processing of PVA.

The specific operations are as follows:

(5) After the reaction was completed, the reactant was poured into acetone and precipitated, and washed with acetone for three times, filtered, and dried under vacuum at 60°C for 24 hours to obtain a PVA-COO-0.05 TRIS product in a yield of 90%.

Infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical thermal analysis and mechanical properties testing were used to study the effect of the introduction of TRIS on the structure and properties of PVA.

The introduction of TRIS (CAS 77-86-1) changes the molecular structure of PVA, and the introduced functional group can interact with the hydroxyl group of PVA, which weakens the hydrogen bonding of PVA itself. The introduction of TRIS can destroy the crystal structure of PVA, reduce the crystallinity and melting point of PVA, increase the decomposition temperature, obtain a wide melt processing window, and realize the thermoplastic processing of PVA without adding any plasticizer. The mechanical properties of PVA after TRIS modification were improved, the tensile strength was decreased, and the elongation at break was improved.

Reference

Fayong Li，Hesheng Xia，Preparation and Properties of Poly( vinyl alcohol) modified with Tri-hydroxymethyl Aminomethane. Polymer Materials Science and Engineering, 2018, 34, 7.