Semicrystalline Polymers

Polymers can be classified into different types, from different perspectives. One way to classify them is based on the cross-linking degree, and has the following types:

  • thermoplastics
  • rubbers (elastomers)
  • thermosets

Thermoplastics, e.g., polyethylene and polypropylene (see last tutorial), can further be classified into amorphous and semicrystalline (with ordered crystalline phase) polymers, depending on the chain arrangement. In this tutorial, the basic deformation behaviors of semicrystalline polymers are summarized.

Basic Properties and Deformation Behavior

Semicrystaline polymers have two phases: the amorphous and crystalline phase. The crystalline phase has the following typical microstructures [2]:

  • normally 10% ~ 50% crystallinity (kinetically controlled)
  • crystals in 10 ~ 50 nm size
  • lamellae: thickness ~ 10 nm, lateral size ~ micrometer
  • lamellae forms spherulites in a larger scale

Macroscopic deformation behavior:

  1. linear elastic until elastic limit (yielding point)
  2. strain softening
  3. strain hardening
  4. permanent set upon unloading

The macroscopic observations correspond to the microstructure evolution [1]:

  1. the onset of isolated slip process
  2. a change into a collective activity of the slips
  3. the beginning of crystallite fragmentation
  4. chain disentanglement (finite irreversible deformation)

“The crystallinity is seen to increase the stresses at these four points while the critical strains at the four points are somewhat constant” [1]. “A granular substructure can be perceived as the basic structural feature for the crystalline lamellae” [1]. A semicrystalline polymer can be viewed as a “interpenetrated network by interlocked lamellae and the entangled amorphous phase” [2].

The crystalline phase has the following mechanical properties:

  • hard (stability of the crystals)
  • plastic flow (slip and fragmentation)
  • intralamellar slipping (small deformation)
  • deformation induced reoriented crystals (chain stretch induced melting and recrystallization)
  • tie molecule is less important than the disentanglement in the hardening regime [2]

Computer Simulations of Semicrystalline polymers

Approaches to study semicrystalline polymers (to a later tutorial):

  • macroscopic constitutive modeling (computational mechanics modeling at the continuum level, e.g., by FEM modeling)
  • microscopic crystalline structural evolution (microstructure evolution at the nanoscale level, e.g., by MD simulations)

References

  1. “Network Stretching, Slip Processes, and Fragmentation of Crystallites during Uniaxial Drawing of Polyethylene and Related Copolymers. A Comparative Study”. Macromolecules 32.13 (1999): 4390-4403.
  2. “Role of the Entangled Amorphous Network in Tensile Deformation of Semicrystalline Polymers”. Physical review letters 91.9 (2003): 095502.