I have received the following very interesting question: Does tensile strength and orientation level move in a positive linear direction, and what if any forces could cause one to increase without impacting the other? For example: Can orientation increase with tensile properties being static?
Reply: In the broad sense orientation and tensile strength will correlate with each other and as the orientation level increases the tensile properties will generally increase. But this is generally true for the amorphous phase orientation which is different from the crystalline orientation. So in an amorphous polymer there is a direct relationship between orientation and tensile properties. There are also relationships between orientation and optical properties which can also be related to the tensile properties (birefringence). So if we are orienting a polymer and modifying the amorphous phase the tensile properties will track together.
However, when we orient a semicrystalline polymer at the start of the orientation the crystals are randomly oriented and at low elongations (below the yield point) the crystals are rotating in the direction of the orientation. At this point if you were measuring crystalline orientation you would see an apparent increase in chain alignment as the crystals rotate even though the amorphous phase may not have been appreciably stretched (because the crystals are stiffening the amorphous matrix) and therefore not increase in the amorphous orientation.
But crystals do not add to strength like the oriented amorphous phase (which carries the load) but crystals do add to stiffness (modulus). However once the crystals have rotated the remaining orientation changes are due solely to the orientation of amorphous phase and the strength increases with orientation.
So in general the orientation and tensile properties will track together, and this is due to changes in the amorphous phase molecular orientation. But the total orientation is due to the combination of the crystalline orientation and the amorphous orientation. So if you were tracking total orientation at low extension rates you might detect an apparent increase in total orientation as the crystals rotate but before the strength has increased much. But if you were measuring the amorphous orientation you would not see the orientation change without the tensile property change, they would track each other. If you only measured crystalline orientation you would see no correlation between tensile strength and crystalline orientation, because once the crystals rotate the crystal orientation cannot increase further but the polymer will get stronger as the amorphous phase orientates.
Hope this is clear; the answer depends on what orientation measure you are using. I will try and give some examples form the literature shortly.