Dissolution of polymer
Characteristics and thermodynamic interpretation of the dissolution process
（1）Characteristics of polymer dissolution process
The dissolution process of polymer is the process of polymer macromolecules under the action of solvent molecules, so that the interaction between macromolecules is constantly weakened, and then evenly mixed with solvent molecules until they become a dispersed homogeneous system.
The dissolution process of polymers is generally divided into two stages:first swelling then dissolution.First the solvent molecules into the outer layer of the polymer,and gradually from the outer layer into the inner layer,so that the polymer volume expansion,called the expansion stage;then,the macromolecules gradually enter the solvent until they from a uniforn solution and each complete dissolution,
Crosslinked polymers only swell,not dissolution,the higher the polymer branched, the lower the solubility.
The dissolution of crystalline polymers is first melting and then dissolution.Because the molecular arrangement of crystalline polymer is orderly, stacked tightly, molecules It is very difficult for solvent molecules to penetrate into the polymer because of this interactional force, the dissolution of crystalline polymers is much more difficult than that of amorphous polymers.
The higher the polarity of the polymer, the easier it is to dissolve,Non-polar crystalline polymers are difficult to dissolve at room temperature, generally need to rise high temperature, or even near the melting point, the crystalline state can be dissolved after the transition to amorphous state, such as fully isostatic polypropylene in dech choi 135°C can dissolve. Polar crystalline polymers are dissolved in polar solvents at room temperature , such as polyamide soluble in methyl phenol, 40% sulfuric acid, phenol - glacial acetic acid mixed solvent; Polyvinyl alcohol is soluble in water, ethanol, etc.
Polymers have a low rate of dissolution.The dissolution rate of polymers is much slower than the dissolution process of small molecules, which generally takes several hours or even days. Because of the huge size difference between polymer and solvent molecules, the speed of their molecular movement is different by order of magnitude. Therefore, when the polymer and solvent are mixed, the solvent molecules penetrate into the polymer quickly, while the polymer diffuses very slowly to the solvent.