The properties of polyvinyl alcohol (PVA) are determined by its degree of polymerization (DP-often corresponding to molecular weight) and its degree of hydrolysis (DH-expressed as a molar percentage). Commercial PVA grades are typically designated by a combination of these two parameters; for instance, "PVA 17-88" indicates an average degree of polymerization of approximately 1,700 and a degree of hydrolysis of 88%.
1. Structure and Classification
Degree of Polymerization (DP): Classified into several grades: low (20,000–35,000), medium (120,000–150,000), high (170,000–220,000), and ultra-high (250,000–300,000). As the degree of polymerization increases, the solution viscosity, film-forming strength, and solvent resistance improve, while solubility decreases.
Degree of Hydrolysis (DH): Typical specifications include three main levels: 78%, 88%, and 98%. Grades with a hydrolysis degree between 78% and 89% are termed "partially hydrolyzed" and are soluble in cold water; grades exceeding 98% are considered "fully hydrolyzed" and require heating to temperatures above 85°C to dissolve . The degree of hydrolysis is a critical factor influencing the application performance of PVA; for example, high-hydrolysis PVA is frequently utilized in the fabrication of films and fibers.
2. Physical Properties
Thermal Properties: The melting point is approximately 230°C. Above 100°C, PVA gradually undergoes discoloration and embrittlement; between 160°C and 170°C, dehydration and etherification occur, resulting in a loss of water solubility; decomposition begins at 200°C; and above 250°C, polymers containing conjugated double bonds are formed. Recent studies have also explored the effects of microwave irradiation and temperature on the structural characteristics of PVA.
Optical and Electrical Properties: The refractive index ranges from 1.49 to 1.52. PVA exhibits high electrical resistivity, typically falling within the range of (3.1–3.8) × 10⁷ Ω·cm. By incorporating nanocomposites through doping, the optical bandgap as well as the linear and nonlinear optical properties of PVA can be tuned, making it suitable for various optoelectronic applications.
Solution Characteristics: The properties of aqueous PVA solutions are heavily influenced by the specific grade used. Solutions of high-hydrolysis grades (e.g., PVA 17-99) are prone to gelation at low temperatures. Furthermore, PVA solutions are highly sensitive to the presence of borax and boric acid; even trace amounts of these substances can trigger irreversible gelation.
3. Chemical Properties
The molecular chains of polyvinyl alcohol are rich in hydroxyl groups, exhibiting the typical reactivity of polyols; thus, they can undergo reactions such as esterification, etherification, and acetalization (e.g., the preparation of polyvinyl butyral [PVB] and polyvinyl formal [PVFM]). For instance, the preparation of carboxyl-modified PVA involves the copolymerization of vinyl acetate with acrylic acid, followed by alcoholysis, to yield a PVA product characterized by a high degree of alcoholysis, yet low crystallinity and high water solubility. PVA possesses good light resistance and is not prone to aging. It is combustible and emits a distinctive odor when burning.