Amorphous polymers do not have a crystallization temperature (Tm) like semi-crystalline polymers and have only a glass transition temperature (Tg). This has a decisive influence on the behavior of shape-shape-memory polymer systems.
A crystalline copolymer system alone can result in the crosslinker-treated copolymer losing its crystallinity and becoming practically amorphous.
An amorphous polymer depends on the level of crosslinking or the degree of polymerization to exhibit this effect. In the case of poly(norbornene), which is a linear, amorphous polymer, with a content of 70 to 80% of trans bonds in commercial products, molecular mass of approximately 3x106 g mol and Tg of approximately 35 to 45°C. Because it achieves an unusually high degree of polymerization, chain entanglements can be relied upon as anchor points to achieve the thermally induced shape-memory effect. Therefore, this polymer relies solely on physical anchor points. When heated up to Tg, the material abruptly changes from a rigid state to a tapered state (softens). To achieve the effect, the shape must be changed rapidly to avoid rearrangement of the segments of the polymer chains and immediately cool the material also very rapidly below Tg. Reheating the material back to Tg will show the recovery of the original shape.