why does hot water deform plastic bottles?
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materials of the plastic bottles are long chain and branched polymers. The internal atomic or molecular structure of such materials is in a metastable state where the molecules are in locally stretched and locked-in configurations. Such metastable structures result from the method of preparation of the bottles where a lump of molten plastic is forced into the shape and size of the desired bottle and cooled rapidly. This freezes the disordered entanglement of the polymer molecules along with the associated local strains and forces.
The materials with stable structures have no local internal forces on the molecules and the internal energy is at the global minimum. In such systems, any small imposed change in local structure is immediately relieved and the original structure, shape and size, is restored.
On the other hand, in the materials in metastable state, like the glasses, polymers and plastics, the internal energy of such materials is not at the minimum and there are locked-in strains which, in turn, are associated with local forces on the molecules. Structure of such materials is not unique, but different possible structures are separated by higher energy configurations, often termed as energy barriers. If the molecules acquire sufficient energy to overcome the barriers, they move to more stable configurations by relaxing the forces and relieving the strains. This also enables the system to minimize its energy. Minimization of energy is an universal tendency of all physical systems.
The materials with stable structures have no local internal forces on the molecules and the internal energy is at the global minimum. In such systems, any small imposed change in local structure is immediately relieved and the original structure, shape and size, is restored.
On the other hand, in the materials in metastable state, like the glasses, polymers and plastics, the internal energy of such materials is not at the minimum and there are locked-in strains which, in turn, are associated with local forces on the molecules. Structure of such materials is not unique, but different possible structures are separated by higher energy configurations, often termed as energy barriers. If the molecules acquire sufficient energy to overcome the barriers, they move to more stable configurations by relaxing the forces and relieving the strains. This also enables the system to minimize its energy. Minimization of energy is an universal tendency of all physical systems.
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