Because near-complete hydrolysis leaves a regular chain of hydroxyls that hydrogen-bond strongly to one another, so the polymer develops substantial crystalline regions. Water cannot penetrate those regions at room temperature, and heating to around 90 degrees Celsius is needed to disrupt them and bring the polymer into solution.
2
What does the higher crystallinity give in return?
Substantially better film properties. Films cast from PVA 99 are stronger in tension, tougher, less permeable to oxygen and markedly more resistant to oils, greases and organic solvents than films from the partially hydrolysed grade, because a solvent cannot penetrate the crystalline regions. That is why it is used for barrier and release films.
3
Why is PVA 99 used in polarising film?
Because the polymer chains can be stretched into alignment and will hold that orientation, and iodine or dye complexed onto the aligned chains then absorbs light in one plane. That produces the polarising layer at the heart of every LCD panel, and it requires the chain regularity and film strength that only the fully hydrolysed grade provides.
4
Can PVA 88 and PVA 99 be substituted for one another?
Generally not. They differ fundamentally in dissolution temperature, film strength and solvent resistance, and an application chosen for one will usually fail with the other. Degree of hydrolysis should be treated as a primary specification, alongside viscosity, rather than as a minor grade variation.
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