Simulation Study On The Glass Transition Temperature Of Polymer Nanocomposites

The glass transition temperature(Tg)and melting temperature(Tm)of polymer nanocomposites(PNCs)are two important issues in polymer chemistry,polymer physics and soft matter physics.Factors like polymer chain length,polymer stiffness,and other properties(size,mass,interaction,etc.)of nanoparticles(NPs),can affect Tg and Tm of PNCs.In this dissertation,we carried out molecular dynamics(MD)simulations to unveil the effect of volume fraction of NPs(fNP),size of NPs(σNP),and bending modulus(ke)of polymer on the glass transition temperature of PNCs.We also study the effect of loading of NPs on melting temperature of PNCs at large kθ.This work provides a deep understanding of the glass transition temperature and melting temperature of polymer nanocomposites and will help in producing good quality polymer nanocomposites for advanced level applications such as thermal sensors,drug delivery systems and support materials.Our study is divided into three main parts.(1)The effect of volume fraction of nanoparticles on the glass transition temperature of polymer nanocomposites.The glass transition temperature Tg is estimated from the variation of system volume with temperature and the temperature-dependent diffusion of polymer described by Vogel-Fulcher-Tammann law.Results show that Tg can be regulated by changing fNP.A novel shift in Tg is observed,that is,Tg increases with fNP at fNP<fNP*(where fNP*is the volume fraction corresponding to maximum Tg)while it decreases with increasing fNP at fNP>fNP*The basic mechanism behind the novel shift in Tg is the competition between the attraction of nanoparticles towards polymer chains and the fast diffusion of nanoparticles.The increase in Tg at low fNP is due to the attraction of nanoparticles whereas the decrease in Tg at high fNP is attributed to the fast diffusion of nanoparticles.The diffusion of polymer above Tg is also investigated.The diffusion of polymer decreases with increasing fNP below fNP*and it increases with fNP above fNP*,in agreement with the variation of Tg.(2)The effect of size of nanoparticles on the glass transition temperature of polymer nanocomposites.The variation of Tg with σNP shows two distinct behaviours for PNCs at low and high volume fractions of NPs.At low fNP,Tg decays almost exponentially with σNP,whereas at high fNP,Tg shows a complex behaviour:it initially increases and then decreases with increasingσNP.The decrease in Tg with σNP is due to the significant decrease of adsorbed polymer monomers while the increase in Tg with σNP is attributed to the slower motion of larger nanoparticles.We have also investigated the diffusion and relaxation of polymer chains at a temperature above Tg for both low and high fNPs as a function of σNP.The diffusion constant and relaxation time of polymer chains in respective polymer nanocomposites are also consistent with Tg traced trend.(3)The effect of bending modulus of polymer on the glass transition temperature and melting temperature of polymer nanocomposites.The state of neat polymer bulks as well as PNCs at low temperature is dependent on the bending modulus of polymer(kθ).At small ke neat polymer bulks and PNCs are in an amorphous state with a glass transition at low temperature,whereas at large kθ both systems freeze into a crystal below the melting temperature.For a neat polymer system,the glass transition temperature increases with kθ,meanwhile melting temperature does not change obviously with kθ.However,for PNCs,the glass transition temperature and melting temperature change differently with increasing fNP.The increment of Tg with fNP is obvious at small kθ and it becomes less and less obvious with increasing kθ.Whereas Tm decreases obviously with increasing fNP.We demonstrate that the interplay between fraction of monomers contacting with NPs(fcontact)and the mean square end-to-end distance of polymer(<R2>)is responsible for these unique behaviours.