Polymer behaviour during cooling and resulting fibre structure

3 Polymer behaviour during cooling and resulting fibre structure 

Configuration is definitively determined by the arrangement of the chemical groups, and can be modified for a given polymer only by breakage of the chemical bonds. The conformation of the chains is controlled by interactions 

During the cooling of a semi-crystalline thermoplastic polymer, the macromolecules are organized around a germ of nucleation. At a particular temperature of thermodynamic balance between crystal and liquid (Tm°), the molar free enthalpies are equal in the solid and liquid phases. Crystallization is theoretically possible as soon as the solid free enthalpy 

During the spinning process, the polymer is subjected to an intense flow and to strong shear stresses, which can appreciably modify crystallization behaviour. The various studies available show an acceleration of crystallization under flow (Somani et al, 2000). During shear tests carried out before the isothermal crystallization of polypropylene, Lagasse and Maxwell (1976) highlighted the existence of a critical shear rate, beyond which the induction time decreases when the shear rate increases, the time before crystallization decreases under these conditions. However, certain studies (Vleeshouwers and Meijer, 1996) show that the use of the induction time as a parameter for the measurement of crystallization acceleration is not necessarily relevant. 

5 Representation of crystalline lamellae in polyethylene (Keller and Kolnaar, 1997). 

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