Molecular Dynamics Simulation Study On The Glass Transition Behavior Of Polymers

The glass transition is an inherent property of polymers, which directly affectsthe processing and mechanical properties of materials. Understanding the nature ofthe glass transition can provide researchers with a reliable scientific basis duringdesigning and optimizing performance of amorphous polymer materials. Inaddition, the motivation to investigate the glass transition behavior of confinedpolymers firstly lies in its significant practical importance related to controlling theproperties of materials confined to nanoscale geometries for various applications inchemistry, medicine and nanotechnology. Besides, it is also interesting from thetheoretical side since it may provide us a better understanding of the mechanism ofglass transition in bulk. The main contents are as follows:(1) Through simulating a chemically realistic chain (CRC) model and aquasi-freely rotating chain (QFRC) model of polystyrene (PS), we study the effectof internal rotational barriers on their static and dynamics properties and focus onthe relation between glass transition and chain conformation. Our resultsdemonstrate that internal rotational barriers not only slow down the conformationalrelaxation on the whole, but also induce stronger temperature dependence ofdihedral characteristic relaxation time. In addition, by comparing the curves ofmean-square displacement (MSD) and torsional autocorrelation function (TACF),we build a relation among the cage effect, the dynamic heterogeneity and theconformational relaxation on the time scales of α-and β-relaxation processes.Moreover, we find the non-ergodic of chain conformation is closely related to theglass transition phenomenon.(2) We study the influence of free surface, polymer-substrate interactionstrength, grafting density, and chain length on the glass transition behavior ofpolymer brushes. The dynamics analysis demonstrates the free surface and weak attractive substrate lead to a relatively enhanced monomer mobility, which further results in the decrease of local Tg. In contrast, the strong attractive substrate leads to higher Tg. In addition, the grafting sites reduce the monomer mobility and the higher grafting density becomes a key factor that affects the glass transition temperature Tg. Moreover, the grafting density and chain length have a combined effect on the brush thickness that affects the whole dynamics behavior. The spatial dependence of density and relaxation time of polymer brushes stresses the importance of long range cooperative motion in glass formers when approaches the glass transition temperature.(3) We investigate the static and the dynamic properties of polymer chains confined in both hard and soft nanopores. The influences of interaction strength, confinement size and the mobility of boundary (hard versus soft confinement) on the glass transition behavior of confined polymers are studies in detail. With strong attractive interaction, mobile boundary weaks obvious density oscillations and preferential bond orientations near the wall. In addition, the mobile boundary is a critical factor in determining the shift of Tg:It accelerates the structural relaxation of nearby monomers and leads to a lower Tg.The soft confinement effect is more obvious for nanopores with strong interaction than those with weak interaction. Moreover, the relation between Tg and confinement size demonstrates the Tg change of confined polymers is mainly controlled by the surface effects originated from the polymere-wall interaction and the mobility of nanopore boundary.(4) Through changing side-group stiffness, we obtain polymers with different fragility and investigate the effect of confinement on their glass transition temperature Tg. We find that there is a reduction of Tg for polymers in free-standing film as the film thickness decreases, and the polymer with stiffer side chains exhibits much more pronounced Tg variation in confinement than that with relatively flexible side chains. Our string analysis demonstrates that the magnitude of Tg variation for polymers in confinement can not be explained merely on the size of cooperatively rearranging regions, the primary effect of changing side-group stiffness is to alter the activation barrier for rearrangement, rather than string size. We clarify that free-surface perturbation is the primary factor in determining the magnitude of Tg variation for polymers in confinement:It is more significant for polymers having higher Tg and results in much more pronounced reduction of surface Tg and then the overall Tg of the polymers.(5) We construct polyethylene (PE) models with different degrees of branching (DBs) to investigate the influence of topology on the glass transition behavior. We find that Tg is increasing with DB, which can be ascribed to the difference of free volumes in PE systems with different DBs.