When a rubber band is stretched and held for some time, it undergoes a process called creep. Creep is the gradual deformation of a material under a constant load or stress. In the case of a rubber band, when it is stretched and held, the long polymer chains within the rubber band start to align and straighten out.
Initially, the rubber band stretches easily, but as time passes, the polymer chains undergo a process called stress relaxation. This means that the rubber band gradually loses some of its elasticity and the force required to maintain the stretched position decreases. This is why a rubber band that has been stretched and held for a while feels less resistant to further stretching compared to a fresh rubber band.
The extent of the creep and the rate at which it occurs depend on various factors such as the temperature, the quality of the rubber, and the magnitude of the applied force. Higher temperatures generally accelerate the creep process, causing the rubber band to deform more quickly.
It's important to note that if a rubber band is stretched beyond its elastic limit, it may undergo permanent deformation and lose its ability to return to its original shape when the stress is removed.