Dear Brian,

i have two reflections:

Have you had a look at the complex and loss moduli to check how they compare to your tensile data?

I guess that you applied a contant stress with a superimposed harmonic stress (or you might have run it in ctrl-strain mode). How did that constant load affect your samples? Did they flow?

Your case is a really interesting phenomenon. As you have mentioned, it is generally and normally expected that the Young's modulus and Storage modulus show the same trend, increase or decrease, depending on the microstructural variations. But, one should be aware of the responses of a specimen below Tg and above Tg. I think, your case could be related to the duality in relaxation response of specimen against two highly different deformations during the uniaxial tensile (probably very high rate of deformation) and sinusoidal modes (relatively much lower rate). Could you please share the cross-head speed in tensile test and temperature(s) where you compared the modulus values?

Ali,

A cross head speed of 50mm/min was used during tensile testing and performed at room temp (21°C). The frequency used during DMA was 1Hz, with an amplitude of 15µm. The storage modulus and Young’s modulus were of the same order of magnitude ~2500MPa on the pre-annealed tubing.

You raise an interesting point; I would expect the higher strain rate associated with uniaxial tensile testing to result in a higher yield stress and thus tensile modulus. I ran an experiment on DMA, with a 1, 10 and 100hz frequency and saw the storage modulus increase with increasing strain rate (frequency). However, when I ran further experiments with lower cross head speeds (10, 5, 2.5 and 1 mm/min), the Modulus increased slightly by 14% until it got to 5mm/min, where it then started to reduce again at 2.5 and 1mm/min.

However, I’m still not sure why the relationship / trend has changed post annealing.

Andres,

There wasn’t a significant change in the loss modulus and tan delta results, and probably a little more variation than seen in the storage modulus, so hard to draw any concrete conclusions.

The test applied a constant stress. I can’t recall if there was any significant flow, nothing too notable anyway.

Thanks

Brian

Have you checked for degradation? PLA and PET decgrades significanlty when not dried. Any smalle molecule can induce further nucleation and crystalizaiton during annealing. The strenght depends on % crystalinity as well as Mn. So I supsect degradaiton. Check MFR before and after annealing.

Thanks for your response Amit.

I'm not so concerned with the loss of mechanical properties or molecular weight post annealing, as you mention that is quite common in polyesters. I'm just not sure why the Storage modulus in DMA would show a different post annealing trend to the Young's modulus in uniaxial tensile testing.

Thanks

Brian

Dear Brian, Once faced with unexpected results I always start by looking at the experimental settings. You mention a 15 micron deformation amplitude. What is the corresponding strain amplitude? Could it possibly be that you are pushing the DMA instrument load sensitivity limits? How high above the noise level was your load amplitude?

Could it be the speed of testing in uniaxial tensile vs. DMA test? Are you comparing at similar strain rates in linear region?

Dear Brian, The annealing effect is to generate chains relaxation that would increase lamellae thickness in the crystalline phase of PLA. The effect of annealing on PLA microstructure has been studied by Yu et al. They found out that annealing above Tg increased crystallinity because of polymer chain relaxation, which resulted in an increase in both strength and toughness. So, I think the increase in storage modulus post-annealing would be expected. Regarding Young modulus, I think the strain rate 50mm/min is too high to be comparable with storage modulus data. At high strain rate, there is insufficient time for the material to respond to stress with large-scale viscoelastic deformation or yielding. Therefore, I suggest to repeat the tensile test with low strain rate before and after annealing, if you would like to compare storage modulus and Young Modulus.