Solubility PVAC

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

This principle is applied not only to the PVA-PVAc composites but to other polymer composites. The composite structure does not always need to be porous but may be powders and gels designed for the wettability by solvents and the extension of the surface area in soluble polymers. From this point-of-view, the present work sheds a new light on the research on composite materials related to graft polymers and copolymers. 

The first synthetic polymers to be used as paint varnishes were acrylic and vinylic resins. Poly(vinyl acetate) (PVAc), commercialized under the name Mowilith by Hoechst and Vinylite by Union Carbide, has been used in conservation as an adhesive since 1932 and in 1937 it was proposed as a picture varnish by Stout and Cross [63]. PVAc was soon rejected as a varnish because, despite its light stability and good solubility in organic solvents, it demonstrated poor optical properties in terms of colour saturation and the tendency to pick up dirt due to its low glass transition temperature. 

The differences in time-dependent adsorption behavior between 99% PVAC at 25° and 50°C demonstrate the influence of intra- and intermolecular hydrogen bonding in the adsorption process. The limiting surface pressure of the hydrophobic water-soluble polymer appears to be 33 mN/m, approximately 7 mN/m below that of commonly used surfactants. The rate of attainment of equilibrium surface pressure values is faster if there is uniformity of the hydrophobic segments among the repeating units of the macromolecule. 

PVAc is used in adhesives and coatings and is hydrolyzed producing water-soluble PVA (Equation 6.44). The PVA may be reacted with butyraldehyde to produce poly(vinyl butyral) used as the inner lining of safety glass. 

Esters such as PVAc may be hydrolyzed producing alcohols such as PVA (16.1), which will have the same DP as the ester. In fact, PVAc does not totally hydrolyze, but with reasonable effort the extent of hydrolysis is greater than 90%. Since PVAc is not water-soluble, but PVA, the extent of water solubility is dependent on the extent of hydrolysis. 

To date, PVA is only accessibly via removal of the acetyl groups from the precursor polymer PVAc. The highly acetylated polymer is not soluble in water. By generating OH groups via removal of the acetyl residues from the polymer backbone, the interaction with water solvent molecules becomes more favourable and the tendency to dissolve in water increases. 

The effect of structural regularity on properties of polymers may be illustrated by the hydrolytic products of polyvinyl acetate (PVAc). PVAc is insoluble in water, but because of the presence of polar hydroxyl groups, partially hydrolyzed PVAc is soluble in water. 

PVAc has a specific gravity of 1.2 and an index of refraction of 1.47. It has a solubility parameter of 9.5 H and is soluble in liquids with similar solubility parameter values, such as benzene, chloroform, and acetone ... 

It is customary to stop the hydrolysis of PVAc before all the acetyl groups are removed. Thus the commercial product, with a degree of hydrolysis of about 88%, is readily soluble in water but is resistant to less polar solvents, such as benzoie and gasoline. PVA fibers (Kuralon) are strong and insoluble in water because of a surface treatment with formaldehyde which reacts with the surface hydroxyl groups to produce polyvinyl formal on the polymer surface. 

These products are produced by the reaction of partially hydrolyzed PVAc with aldehydes. The acetal rings on these random amorphous polymer chains restrict flexibility and increase the heat deflection temperature to a value higher than that of PVAc. The heat deflection temperature of polyvinyl formal is about 90 C and is dependent on the specific composition of this complex polymer. Because of the presence of residual hydroxyl groups, commercial polyvinyl formal has a water absorption of about 1%. Polyvinyl formal has a Tg of 10S . It has a solubility parameter of about 10 H and is soluble in solvents with similar solubility parameters, such as acetone. 

Grafting and Stabilizers. The degree of grafting of poly(vinyl acetate) (PVAc.) on poly(vinyl alcohol) (PVA) and other stabilizers during emulsion polymerization strongly affects latex properties such as viscosity, rheology, and polymer solubility. 

We have studied the dispersibility of several pure PVAc-styrene graft copolymers with one PS branch in various selective solvents mainly at room temperature5. The experiment was done with two kinds of dried samples one was recovered from a tetrahydrofuran solution by pouring it into water and the other from a benzene solution which was poured into n-hexane. Let us refer to the former sample as A and the latter sample as B. Due to the difference in solubility of each polymer sequence in those solvents, sample A is supposed to have approximately such a microstructure that PVAc chains are extended and PS chains collapsed, while sample B has the inverse structure. A similar tendency was also pointed out by Merrett12. The results are summarized in Table 2. 

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