Spherulites

 

  An aggregation of crystallites as a spherical cluster, consisting of fibrillar crystalline lamellae (the fibril) radiating from the centre of the spherulite, or arising as branches.

 

  Spherulites are composed of lamellar ribbons that grow outward : 

          1) From a single (heterogeneous) nucleus,

          2) From a group of lamellas similar to that in a hedrite, or

          3) From a quadrite, a crosshatched lamellar array.

 

Fig. 12. A single spherulite growing in isotactic polystyrene. Optical micrograph taken in polarized light.

 

 

   Growing steps of a single spherulite

  After sufficient supercooling of a relatively concentrated solution or of a melt, spherulites can be grown at constant or slowly decreasing temperature.

Fig. 13. A schematic representation of the growing steps of a single spherulite.

 

  When the crystallization temperature is raised :

       - branching diminishes

       - the primary lamellas increase in average thickness

       - additional lamellas fill in the  spherulite structures at a later time

 

  At high pressure (4-5 kilobars) & temperature :

     spherulites with lamella of greater thickness and length than those obtained at 1 atm.

 

 

  Three crystallization regimes

     Regime 1 :   - at low degrees of supercooling (high crystallization temperatures)

                         - nucleation events are rare

                         - crystal growth is the main process

 

     Regime 2 :   - at lower crystallization temperatures (higher degrees of supercooling)

                         - nucleation is competitive with crystal growth

 

     Regime 3 :   - at still higher degrees of supercooling

                         - nucleation occurs at a greater frequency than crystal growth

                         - nucleation is the dominant process

 

   Polyethylene spherulites

 

Fig. 14. Crystal structure of polyethylene. Layers similar to that shown in the diagram are edge-on in the photo, being arranged in a radiating pattern to form the prominent crystalline features called spherulites.

 

  Three-dimensional view of spherulites - isotatic polypropylene

 

Fig. 15. Scanning electron micrographs of a fracture surface (liquid N2) of isotatic polypropylene crystallized isothermally from the melt in a glass tube at 110¡É for 5 hr after cooling from 200¡É : A. spherulites, B. spherulite surface.

 

 

Fig. 16. Scanning electron micrograph of the center of a polyethylene spherulite ion etched at 500 V(ac), 3 ma for 1hr in air.

 

  Part of an extended chain polyethylene spherulite

Fig. 17. Transmission electron micrograph of a fracture surface replica of an extruded chain polyethylene spherulite crystallized at 215-225 ¡É and 4.8 kb for 10-20 hr and cooled to room temperature under pressure.