The movement of rubber molecular chains has multiple characteristics. Long chain molecules are composed of many repeating units. They have a connection to the side groups, some to the branches, and even the segments. In general, the molecular motion of rubber is not on the scale of the molecule as a whole, but in the form of a segment. The deformation caused by the motion based on the segment is irreversible, so when the temperature changes, the physical state of the rubber also changes. During the process from low to high temperature, the physical state of the rubber goes from the glass state to the high elastic state to the viscous flow state, thus undergoing a three-state.
(1) Glass state
At low temperatures, the molecular motion of the rubber is weakened to the point where it cannot excite the motion of the segments. In other words, in a “frozen state”, it is impossible to switch from one conformation to another, and its mechanical properties are like glass, hence the name. At this temperature point, the rubber changes from a viscoelastic body to a brittle glassy state. This transformation can be characterized by basic thermodynamic properties (such as heat capacity, coefficient of thermal expansion) and temperatures at which sudden changes occur. This temperature is called the “glass transition temperature” and the symbol Tg, which is the turning point temperature at which the rubber transitions from a high elastic state to a glass state. Different gel types each have a specific glass transition temperature, which varies depending on the molecular structure of the polymer, but the distribution range is narrow.
(2) High elasticity
High elastic state is the most important sign representing the use characteristics of rubber (different from other materials), which makes rubber feets can produce large deformation under the action of small external force, and the elongation can reach 500%~1000%; When the external force is released, it can be restored to its original size and shape, and the permanent deformation left behind is negligible. The elastic modulus of the rubber feet rises as the temperature rises and exotherms when stretched. When the temperature rises to a certain point, the segment is excited to rotate around the single bond of the main chain, constantly changing the conformation. The high elasticity and high adaptability of rubber come from this.
(3) Viscous flow state
When the outside temperature rises and the free flow of the macromolecule can be deformed as a whole, a relative displacement occurs between the macromolecular chains. This is because the macromolecules are not cross-linked at this time, so once they are subjected to external force, viscous flow will occur. Moreover, even if the external force is canceled, it is impossible to recover from deformation, which is an irreversible behavior. The important significance of the viscous flow state is that it provides an indispensable condition for processing. For example, when performing rubber mixing, calendering, extrusion or pressure vulcanization, the basic requirement of flow is indispensable.
In summary, from the application point of view, the glass transition temperature is the starting point of the rubber working temperature. When the rubber is at the ambient temperature below Tg, the rubber in the glass state loses the application significance of the elastomer, let alone processing. The high elastic state is the state in which the rubber exerts its elastic function, and exhibits a series of elastomeric properties such as toughness, high resilience, high wear resistance, high recovery, and tear resistance. The viscous flow state is also indispensable for the production of rubber feet.