This may be as result of NPs agglomeration. Agglomerated nanoparticles decrease the polymeric network strength that can lead to decrease in Tg, as well as tensile characterizations. If you are using surfactant, you may need to increase the intensity of sonication or choose/change your surfactant. If not, you'd better do. hope it helped.

Also, in the case the crystallinity of your network increases by addition of NPs, it probably results in decreasing Tg.

If the nanoparticles are dispersed and small, they can interfere with the radius of gyration of a polymer chain, increase the free volume, thus acting like a plasticizer. However this type of dispersion is rare. See Kalika et al. and Freeman et al. for some really nice work in such systems. Other reasons for a Tg decrease however could be that some impurity (Fe or Ti) in the filler is decreasing the molecular weight of the polymer during processing. Another reason is that you are working with a crosslinked system and the nanoparticles and/or their surfactants are interferring with the extent of crosslinking in the system.

This paper may help explaining the effect of NP on free volume:

http://hawkergroup.mrl.ucsb.edu/sites/default/files/publications/2005_effect-of-ideal-organic-nanoparticles.pdf

Sorry Mr. Hadi Hezaveh; What is the effect of crystallinity on the glass transition temperature?

Mr. Mahmood....Polymer chains that are in a crystal cannot move. The backbone cannot move. Therefore the part of the chain that is in the crystal does not have a Tg. Only the amorphous part can have a Tg. Nanoparticles can affect the crystallinity though, which is a separate question. Nanoparticles such as montmorillonite are known to affect the type of crystal for nylon 6,6 for example.

When crytal part of your network increases the amorphous part (that is related to Tg) decreases.

Dear Mahmood

The usually accepted theory in the subject of Tg nanocomposites is that this property increase as result of the chain mobility is extremaly confined due to the interaction polymer - nanoparticles.

The previos art content in this paper is good

www.postech.ac.kr/mse/polymer/polyphys/PDF/20090805.pdf

This is an interesting prediction and I appreciate adding the paper to my files! However, it only suggests that the Tg should go up, consistent with the idea that the polymer near the surface of nanoparticles is attracted to it and confined by it, and that for a small volume percent of nanoparticles the amount of polymer confined is a large volume percent owing to surface area. This prediction also assumes that the system will act as if it was one phase. For Kalika's work (Comer, Heilman, Kalika, Polymer 51 (2010) 5245-5254.) we see the more expected result of two "phases" being formed, one confined and one bulk, each with its own Tg, and the Tgs approaching each other as the amount of filler is increased. As with all things nano, however, there is more than one mechanism for affecting the Tg possible. When the filler does not have an interaction with the polymer, one might expect interference with the free volume. In the Freeman work (Merkel, Freeman, et al. Chemistry of Materials 15 (2003) 109-123) we see no measurements of Tg, but instead positron annihilation measurements of free volume, which is directly related to Tg. The free volume goes up with filler content. What these authors did instead of Tg was measure permeability to a gas which went up dramatically with increase in impermeabile filler concentration. Not what you would expect a filler to do. (They did control for void volume.)So you can get confinement OR increased free volume. As the question asked why the Tg decreased, only an increase in void volume would resolve this. Both things can happen and in any given system its just a matter of which is the dominate effect. Of course, as mentioned before, if the nanoparticle or its surfactant creates chemical changes, then we have to invoke other explanations for changes in the Tg.

Dear All ,

Thank you for your useful comments,

I found Tg for polymer-GNP compsoites is lower than polymer matrix by 5 C on the DSC but On the DMA there is no signifcant difference btween the nanocompsoites and PET. Could you please advise me if there is any suggestion or comments ###

thanks

The glass transition temperatures measured by DSC were lower than the glass transition temperatures measured by DMTA. This result can be due to a fact that

the glass transition temperature of a polymeric material is a kinetic parameter which is affected by the time scale of the measurements.

The difference in the glass transition temperature of the samples in DSC analysis is much more pronounced.

Thank u for your comment.

I understand that tg in dcs is lower than in DMTa ,

But I am comparing with pure polymer.

I think two machine should give sane trend , decreasing or increasing.

Thanks

Hi, do you have any idea about the effect of length and number of chains on glass transition temperature,if any one can have such material please send me

The effect of shear stress of compounder during blending of polymer and filler causes molecular weight reduction. According to the Flory-Fox equation

( Tg= Tg∞ - K/Mn ), molecular weight reduction result in decline in Tg.

Dear Azhar Ashraaf Khan

The relation between length or number of monomer or molecular weight on glass transition temperature is described according to the Flory-Fox equation Tg= Tg∞ - K/Mn.

Tg∞: the maximum glass transition temperature of the polymer with an infinitive molecular weight

K: an empirical constant relating to the free volume of the end group in the polymer chains

Dear Mina karimi,

thanks for your clarification