Chemistry

Polyvinyl alcohol

Polyvinyl alcohol


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Area of ​​Expertise - Macromolecular Chemistry

Polyvinyl alcohol (common abbreviations are PVOH, PVA or PVAL) is a water-soluble polymer which is produced from polyvinyl acetate by hydrolysis (as a polymer-analogous reaction). A direct synthesis is not possible because vinyl alcohol is not stable and is immediately converted to acetaldehyde by tautomerism.

Polyvinyl alcohol has excellent film-forming, emulsifying and adhesive properties. It is resistant to oils, fats and solvents. It is used primarily in the packaging sector (carbon dioxide barrier layer in PET bottles, water-soluble films for the manufacture of packaging bags) but also as an adhesive and thickener in latex lacquers, hair sprays, shampoos and adhesives. PVOH is odorless and non-toxic.

Learning units in which the term is dealt with

Polymer-analogous reactions45 min.

ChemistryMacromolecular ChemistryPolymers

Polymer-analogous reactions are reactions on macromolecules in which the chemical composition and thus the properties of a polymer are changed while maintaining the degree of polymerization. The polymer-analogous reactions are explained in more detail using the example of polyvinyl alcohol, the modification of cellulose and the peptide synthesis according to Merrifield. Particular attention is paid to the delay and acceleration of the reaction and cyclization reactions.


Polyvinyl alcohol

Polyvinyl alcohol (Abbreviation PVAL or PVOH) is a thermoplastic material that is usually produced as a white to yellowish powder by saponification (hydrolysis) of polyvinyl acetate (PVAC). The direct synthetic route is not possible. PVAL is resistant to almost all anhydrous organic solvents. [3] Partially saponified types of PVAL with approx. 13% PVAC content are readily soluble in water. The water solubility decreases with increasing degree of saponification. [4]


Structure and properties

Chemical structure

Similar to polyvinyl acetate, the head-to-tail arrangement of the monomers predominates in polyvinyl alcohol. The content of building blocks in a head-to-head arrangement is below 1 to 2%. The proportion of these proportions has a great influence on the physical properties of the polymer, such as its solubility in water. Polyvinyl alcohol is usually slightly branched due to chain transfers during the synthesis of polyvinyl acetate. The degree of polymerization is about 500 to 2500.

The degree of hydrolysis of the technically relevant types varies between 70 and 100 mol% depending on the intended use. If only partially saponified, the acetyl groups can be distributed randomly or in blocks in the polymer, depending on the process. The distribution of these acetyl groups influences important properties such as the melting point, the surface tension of aqueous solutions or protective colloid properties.

Polyvinyl alcohol, obtained from polyvinyl acetate, is an atactic plastic. However, it still has crystalline areas via the hydroxyl groups. The structure and history, i.e. branching, degree of hydrolysis, distribution of the acetyl groups, have an influence on this crystallinity of the polymer. The higher the degree of hydrolysis, the better the crystallizability. By heat treatment of fully saponified products, the crystallinity can be increased, which in turn reduces the water solubility. The higher the proportion of acetyl groups, the weaker the formation of crystalline zones.

Physical Properties

Polyvinyl alcohol is excellent layer-forming, emulsifying and adhesive. It has high tensile strength and flexibility. These properties depend on the humidity, as the plastic absorbs water. Water acts as a plasticizer, PVOH (PVAL) loses its tensile strength at high humidity, but gains elasticity. The melting point is 230-160 ° C, the glass transition temperature 85-160 ° C for completely hydrolyzed forms. The ceiling temperature is 200 & # 160 ° C. The temperature information varies with the molecular weight.

Chemical properties

It is resistant to oils, fats and organic solvents.


Polyvinyl alcohol ⋅ polymer type

5 Source: [Zietsch (1998)] Protection of powdery paint layer areas, paint layer deposition and connection with a 1-10% solution by spraying or by applying a brush. [Natural stone monitoring (2011)] Occasional laying down and selective securing of treated blocks of paint. [Natural stone monitoring (2011)] Occasional laying down and selective securing of treated blocks of paint. [Natural stone monitoring (2011)] Occasional laying down and selective securing of treated blocks of paint. [Natural stone monitoring (2011)] Occasional laying down and selective securing of treated blocks of paint. [Natural stone monitoring (2011)] Occasional laying down and selective securing of treated blocks of paint. [Natural stone monitoring (2011)]

6 Source: [Lehmann (2003)] Source: [Gramsdorff (2002)] Source: [Interview] Source: [Jägers (1984)] Source: [Jägers (1984)] Source: [Jägers (1984)] Source: [Jägers (1984)] Source: [Jägers (1984)] To consolidate loosened, powdery paint, it was sprayed on over the entire area. [Lehmann (2003)]

8 Partial pre-fixation (before partial consolidation) of some areas of the ship portals with a 10% solution in water. [Natural stone monitoring (2011)] Partial pre-fixation (before partial consolidation) of some areas of the console figures with a 10% solution in water. [Natural stone monitoring (2011)]


Polyvinyl Alcohol as a Biocompatible Alternative for the Passivation of Gold Nanorods

The functionalization of gold nanorods (GNRs) with polymers is essential for both their colloidal stability and biocompatibility. However, a bilayer of the toxic cationic surfactant cetyl trimethylammonium bromide (CTAB) adsorbed on the nanorods complicates this process. In, we report on a strategy for the biocompatible functionalization of GNRs with a hydrophobic polymeric precursor, polyvinyl acetate, which is then transformed into its hydrophilic analogue, polyvinyl alcohol. This polymer was chosen due to its well ‐ established biocompatibility, tunable “stealth” properties, tunable hydrophobicity, and high degree of functionality. The biocompatibility of the functionalized GNRs was tested by exposing them to primary human blood monocyte derived macrophages the advantages of tunable hydrophobicity were demonstrated with the long ‐ term stable encapsulation of a model hydrophobic drug molecule.

Polymer grafting: Gold nanorods can be functionalized with polyvinyl acetate, which hydrolyzes to polyvinyl alcohol. An aqueous dispersion of the resulting colloidally stable and nontoxic nanorods was tested by exposure to primary human blood monocyte derived macrophages.


Properties

The bond between fluorine and carbon is much stronger than that between chlorine and carbon in PVC, which is why polyvinyl fluoride is more chemically and physically stable than polyvinyl chloride. It can be used over a temperature range of −70 & # 160 ° C to around 110 & # 160 ° C, although higher temperatures (short-term) are also possible. It is dirt-repellent and resistant to many chemicals and plasticizers and impermeable to fats and oils. In addition, it has good weather resistance and is transparent. & # 914 & # 93

From a wavelength of around 300–400 nm and well into the infrared range, polyvinyl fluoride is permeable to light. Refractive index nD. 20 = 1.45. Flammability: slowly burning. & # 912 & # 93


Chemistry

Mixing liquids

Chemistry speaks of volume contraction (volume reduction) when, when mixing several liquids to form a solution, the total volume is smaller than the sum of the volumes of the individual components. The volume difference is called excess volume & # 160 $ ​​V ^ E $ and is negative when the volume contracts. In many solutions it is very little or practically nonexistent.

However, this effect is noticeable with a mixture of alcohol and water. For example, if you mix 48 160ml water with 52 160ml ethanol, the result is a total volume of 96.3 instead of 100 160ml. The reduction in volume depends non-linearly on the mixing ratio (see figure), which is why the alcohol content of the mixture cannot be determined by measuring the volume, but only by measuring the density or boiling point.

The cause of the volume contraction is the development of additional binding forces (formation of hydrogen bonds between the molecules), which means that they take up less space. In oceanography there is a similar, albeit smaller, effect because of the salinity in seawater.

There are also liquids which, when mixed, lead to volume dilatation, i.e. an increase in volume. In this case the excess volume is positive.

Influence on salary information

If the content of a component in a solution is given as a volume concentration $ = frac<>>><>>> $, the volume contraction is taken into account.

If, on the other hand, the volume contraction is to be disregarded, the content is preferably used as a volume fraction $ = frac<>>> < sum <>>>> $ specified.


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