Preparation Of Polyvinyl Alcohol/Metal Salt Composite Films And Study Of Coordination Mechanism And Properties

Polymer/metal ion complexes are an important category of metal-organics complexes with general applications in many fields such as catalysis,light-emitting devices and sensing materials.The formation of polymer/metal ion complexes is based on the interaction of metal ions and polar groups（-OH,-NH₂,-COOH,etc.）in polymers.Polyvinyl alcohol（PVA）is a kind of polymer with good water solubility,film formation and biodegradability,and contains a large number of hydroxyl groups（-OH）in the molecule.Multiple hydroxyl groups（-OH）in the molecules can be coordinated with transition metal ions to form polymer/metal ion complexes.The interaction between polymers and metal ions plays an important role in biomolecules,metal-supramolecular polymers and polymer/salt complex systems.However,detailed mechanism about the coordination bonds in polymer/metal inorganic salt complexes in the solvent-free state is uncertain.Studies on the distinction between coordinate covalent bonds and ion-dipole interactions are rather lacking.In this paper,PVA/metal inorganic salt composite films with different metal inorganic salt contents were manufactured by solvent casting.In addition,the compatibility of metal inorganic salts in the PVA matrix is systematically studied,as well as its effects on the thermal stability of composite film,crystallinity and refractive index（RI）.We have combined a variety of experimental methods to confirm that there are different interaction mechanisms between metal ions and PVA at different salt concentrations:at low salt concentrations,the interaction is in the form of coordination covalent bonds;at high salt concentrations,the interaction takes the form of ion-dipole physical interaction.The thesis is mainly divided into three sections:（1）Study on the physical properties and preparation of PVA/anhydrous Cu SO4 composite films:The dispersion of different contents of Cu SO₄ in the PVA matrix was systematically investigated,and the effects of different contents of Cu SO₄on the properties of the PVA/Cu SO₄ composite film such as initial thermal degradation temperature(T_d,i),RI,crystallinity（X_c）and glass transition temperature（T_g）were analyzied.The results showed that T_d,i and RI did not always increase with the increase of Cu SO₄ content.The maximum values of both T_d,i and RI were observed at Cu SO₄/PVA mass ratio of 1%,and the crystallinity appeared the lowest value.Compared to pure PVA,the melting point（T_m）of the PVA-Cu SO₄-1%composite film decreased slightly and T_d,i increased substantially,effectively increasing the temperature window between T_m and T_d,i,thus facilitating the theoretical basis for the thermal processing of PVA.（2）Study on the interaction mechanism between anhydrous Cu SO4 and PVA:When Cu SO₄ was introduced into the PVA matrix and the Cu SO₄/PVA mass fraction was lower than 1%and higher than 1%,the Cu²⁺-PVA interactions of the composites were different.When the mass fraction of Cu SO₄/PVA was lower than 1%,the composite films formed more stable coordination covalent bonds;while the ionic dipole physical interactions dominated when the mass fraction was higher than 1%.In addition,results of X-ray photoelectron spectroscopy（XPS）,near edge X-ray absorption fine structure（NEXAFS）and ultraviolet and visible light absorption spectrum（UV–vis）confirm the different bonding states of Cu（II）ions below 1%mass fraction and above1%mass fraction.Overall,the distinction between coordination covalent bonding and ionic dipole interactions enriches the theoretical understanding of PVA-Cu²⁺interactions and paves the way for the development of advanced polymer/metal salt composites through the formation of coordination covalent bonds.（3）Study on universality verification and application of PVA coordination bond with metal ions:A better understanding of the interactions between PVA and metal ions can help to develop PVA/metal salt composites with excellent properties.Based on the coordination interactions between metal ions and PVA,the effects of different metal inorganic salts with 1%mass fraction on the thermal stability of PVA matrix were investigated,as well as the crystal structure and thermal processing properties of PVA by different contents of Cu SO₄.The results showed that the enhancement effect of different metal inorganic salts on the T_d,i of PVA can be divided into three categories,i.e.,no effect:Mn Cl₂,Mg Cl₂ and Mg SO₄;enhancement of 10-20°C:Cu（NO₃）₂ and Co（NO₃）₂;enhancement of 50-100°C:Fe Cl₃,Al（NO₃）₃,Cu Cl₂ and Cu SO₄.1%Mass fraction Cu SO₄ forms the most coordination bonds with PVA,which can realize PVA thermoplastic molding process and enhance the water resistance performance.The interaction between Cu²⁺and the hydroxyl group on PVA has a great influence on the crystalline morphology of the polymer,and different contents of Cu SO₄ can induce the formation of different crystalline morphology of PVA.