Thermoplastic are all plastic marterials that can be melted and cooled repeatedly.
The thermoplastic polymers can be viewed as clusters of long single molecules held together by forces of attraction, if it warms up, these forces are overcome and the molecules move in relation to one another (softening). If you simply increase the temperature, polymers get to a viscous state and can be formed. Each time you repeat the operation of heating and molding the material loses some of its characteristics.
The individual molecules of a thermoplastic polymer should not be imagined as filaments arranged in an entirely random way but may have the most regular areas; the individual polymer molecules may have a degree of crystallinity.
More precisely, in the structure of a polymer there will be generally crystalline areas, with an orderly structure, and areas that are not crystal clear and are called amorphous. The amorphous state can be in turn of glassy or rubbery type , depending on the temperature (glass transition temperature).
No polymer is completely crystalline. A polymer with a high percentage of crystallinity is very fragile, and the amorphous regions give the material the ability to bend without breaking.