Final answer:
Plastic bottles made from PET shrink and distort when filled with hot water due to reaching temperatures between the glass transition temperature (Tg) and melting point, causing the polymer chains to gain mobility and the material to become flexible and deformable.
Step-by-step explanation:
When you put hot water into a plastic bottle, particularly one made of PET (polyethylene terephthalate), the bottle shrinks and becomes distorted due to the response of the plastic to increased temperature. This occurrence relates to the glass transition temperature (Tg) of the material. PET has a Tg that is a definite range where the material transitions from a hard, glassy state to a soft, rubbery state. When the temperature of the PET bottle is raised to a value between its Tg and its melting point, the thermal kinetic energy allows the polymer chains to gain enough mobility to move past one another, effectively making the material more flexible and more easily deformable.
Regarding the free volume concept, as the temperature increases, the polymer chains move more freely and occupy more space, which increases the free volume. However, if the internal or external pressure is high, coupled with the increased temperature within Tg and melting point, the material may not necessarily expand. It might contract or distort as the bottle attempts to relieve the stress, leading to shrinkage or distortion of the shape. This is counter-intuitive to usual behavior of solids expanding with heat due to increased kinetic energy, but is specific to the nature of polymers and the temperatures involved.
Related Examples
- When you heat a metal peg and a metal block for fitting, you would heat the block and not the peg. The block expands more due to the heat, while the peg remains smaller, allowing for easier insertion. When it cools, a tight fit is achieved.
- Running hot water over a metal lid covering a glass jar helps in opening it because the metal lid expands more than the glass due to higher thermal expansion coefficient.
- The breaking of a glass full of vinegar upon warming is due to the vinegar expanding more than the glass, increasing the pressure excessively if there's no air pocket to allow for expansion.