What is happening at a molecular level when something gets folded, creased, or wrinkled? Why can some things get uncreased but others not?
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Virtually nothing happens at the molecular level (like, chemical bonds and stuff) except for crystals or other things which form one honkin big single molecule.
Anything that can fold happens at a layer above that, how various strands of molecules move about each other. Folding and bending some plastics causes that white color distortion and you can’t really just fold it back because you’ve disturbed all the polymer chains. Imagine a bunch of hair all combed down. Someone comes and ruffles the hairdo and while you can pat it back down, without a comb it’s going to be a jumbled mess. Other things like…. cloth, you just fold back and it’s fine because all the strands move about plenty. Imagine “folding” a bunch of water. It unfolds really easy because it’s all wet and doesn’t keep it’s order anyway.
Or consider a sheet of metal. Bending it will smear all the little crystal structures in it. Bending it back doesn’t change how they’ve all been stretched. Like stretching out a sweater in a direction makes the weave shift. These were molecules in a certain pattern and you jumbled that pattern. Unjumbling it is harder then moving it back. But yeah, the “weave of the fabric” is a good analogy for how stuff is held together. But this is more mechanical than chemical or molecular.
There are two kinds of deformation a material can undergo. Plastic deformation is permanent. Elastic deformation is not. Any kind of deformation either stresses or breaks the chemical bonds that hold it together.
Bending something puts the material close to the inside in compression, and the outer material in tension. Each material has a maximum amount of tension or compression it can endure before experiencing irreversible, plastic deformation. That’s when it either breaks or a crease forms.
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