Molecular Dynamics Simulations of Dynamics in Polymer Nanocomposites

Author:Li Shu Jia

Supervisor:qian hu jun


Degree Year:2019





Polymer nanocomposites(PNCs)have received extensive research attentions over the past three decades.This is because nanoparticles(NPs)not only can improve original properties,but also can impart many new mechanical,optical,and electrical properties to polymers.All these changes make the application of polymers more extensive.Understanding how NPs enhance polymer properties and better designing PNC materials,require a deep understanding of the dynamics in PNCs.For example,understanding the dynamics of nanoparticles(NP)can help solve the outstanding problem in PNCs:how to make NP better dispersed.In addition,since polymers are usually in a glassy state,and polymers are often prepared and studied in the form of films in experiments and industries,the researches on the glass transition phenomenon of PNC films have a strong application background.The nanoconfinement effects caused by NP and film thickness are both derived from interfacial effects.Interfacial effects have important influences on glass transition.For instance,the size of the area affected by interface effects is closely related to the important concept in glass transition theories,the sizes of cooperatively rearranging regions.Therefore,studying the glass transition of PNC film also has important theoretical value.An important theory in glass transition researches is dynamic heterogeneity(DH).Under the nanoconfinement of PNC film,what new characteristics DH will present?This problem is not only critical for understanding the phenomenon of glass transition,but also has important guiding value for the application of PNC film.Therefore,the content of this dissertation is as follows:1.Translational and rotational dynamics of an ultra-thin nanorod probe particle in linear polymer melts.The study of NP dynamics in polymer melts not only contributes to the development of basic theories,but also has important values for practical applications such as new material design and drug transportation in cells.Previous studies have focused on spherical NPs,but the applications of nanorods(NR)have become increasingly widespread,and relevant researches are urgently needed.Because of its shape anisotropy,NR dynamics is anisotropy also,making PNC materials containing NR have many new properties.To investigate the dynamics of an ultrathin NR in polymer melts,we performed coarse-grained molecular dynamics simulations.We have found that the translational dynamics of an ultrathin NR,whose diameter is equal to the diameter of a polymer monomer,does not depend on the chain length of the polymer in entangled polymers.This is the first simulation work which verifies de Gennes’s theory predictions.When NR length is larger than the tube diameter of entangled polymers,the number of entangled polymer segments affects NR rotational dynamics,so the NR having a length twice the tube diameter has a weak dependence on the chain length of the polymer.In non-entangled polymers,the dependence of NR dynamics on polymer chain length is related to the length of NR.Our results show that NR dynamics depends on the relative size of the typical sizes(diameter and length)of NRs and the typical sizes of polymers(radius of gyration,tube diameter).These results provide a theoretical basis for better applications of NR in PNC material design.2.A simulation study on the glass transition behavior and relevant segmental dynamics in free-standing polymer nanocomposite films.The properties of a freestanding PNC film are simultaneously influenced by the confinement effects from the air-polymerization interface(free surface)and NPs.In this work we used molecular dynamics simulations to study how the attractions between NPs and polymer monomers affect the glass transition and related dynamics.The results show that the magnitude of the attractions will influence the position of NPs in the film.When the attractions between NPs and polymer monomers are weak,NPs are distributed at the free surface.When the attractions are strong,NPs are dispersed in the middle region of the film.The glass transition temperature Tg of the former film system is the same as the pure polymer bulk system without NPs.This is because NPs at the surface region“cap”and suppress the nano-confinement effects from the free surface.Specifically,the motions of polymer beads in the surface region are slowed down,the difference in dynamics of the different layers is reduced,and the degree of dynamic anisotropy is weakened.All these changes make the dynamics of the PNC film closer to the dynamics of a bulk system,eventually leading to its gT revert to the pure bulk value.Our results provide a clear and direct microscopic picture for lots of experimentally observed phenomena,in which nanoconfinement effects are inhibited by surface coverings.For a thin film system with strong attractive NPs,the strong attractions from NPs in the middle region of the film make polymer monomers in the film middle region more closely packed,leading to a rather slow dynamics,thereby increasing gT of the system.In addition,because NPs are in the middle region,the difference in dynamics of the different layers of the film is increased,and the surface region of the film has a stronger dynamic anisotropy than that in the pure polymer film.These results will help people better understand and design PNC films.3.A comparative study on the dynamic heterogeneity of supercooled polymers under nanoconfinement.Dynamic heterogeneity(DH)is a typical feature of glass transition and a key to understanding glass transition.We compared DH of pure polymers and PNCs in the supercooled state in free-standing and bulk state.Our results indicate that there are two competing factors in the film that affect DH of a film:dynamic gradient,faster average dynamics than bulk.Dynamic gradient is the difference in dynamics along the film thickness direction.It,as a static spatial dynamic fluctuation,makes non-Gaussian parameterα2,which describes the difference between the displacement distribution and the ideal Gaussian distribution,have a larger value in the film than the corresponding bulk system.Dynamic gradient also tends to increase the sizes of cooperatively rearranging regions(such as string),but its effect on string is less than the effect onα2.In thinner films,the contribution of dynamic gradient to increasing the string size is exceeded by the contribution of the faster average dynamics to reducing the string size,therefore the string size in the thinner film is smaller than the string size in the corresponding bulk system.While forα2,the influence from dynamic gradient is always dominant.Since dynamic gradient does not contribute to the dynamic fluctuations in temporal scales,the characteristic times of DH(such as the time of the peak ofα2 and string)and the ratio of the persistent time to the exchange time are dominated by faster dynamics.Therefore,these physical quantities have smaller values in a film than those in a bulk.Our results show that three common methods for describing DH:the dynamic distribution(such asα2),the dynamic cooperation(such as string),and the dynamic exchange(such as the ratio of the persistent time to the exchange time),have distinct emphasis on spatial and temporal fluctuations of DH.To obtain a complete physical picture of DH under nanoconfinement,it is necessary to use these three methods together.Our results highlight the unique contribution of dynamic gradient to DH and enhance the understanding of glassy dynamics.