Converting Magazine

Eldridge M. Mount

Impact of crystal orientation on film properties
August 4, 2008

In recent postings, we've been exploring the crystalline and amorphous orientation in stretched polymer films and how it is the amorphous chains which add strength to the film and carry the mechanical forces on the film. So, what is the impact of the crystal orientation?

First, recall that the orientation of the crystals proceeds in a manner to align the C-axis of the crystals in the direction of stretching. The C-axis is along the chain length of the polymer so all properties that are enhanced by the continuity of the chain, as opposed to the direction perpendicular to the chains, will be higher. Aside from the tensile properties, all forms of energy will propagate down the chain better than perpendicular to the chain, due to the covalent bonding of the backbone atoms, as opposed to the lateral, interchain, non-covalent molecular coupling.

Therefore, the transfer of thermal energy will be higher in the direction of C-axis orientation, giving higher thermal conductivity along the chain than perpendicular to it. The transfer of mechanical pulses will be faster along the chain than perpendicular to them (Sound speed is higher along the chain) giving a higher “sonic modulus.”  The speed of light will be retarded more along the chain (higher refractive index) due to higher electron density along the covalent bonds. 

Many of these enhanced properties from the crystal alignment can be used to measure the relative changes in molecular orientation of the crystals in the film.

Posted by Eldridge M. Mount on August 4, 2008 | Comments (1)