Yijing Nie

Intrinsic correlations between dynamic heterogeneity and conformational transition in polymers during glass transition

We performed dynamic Monte Carlo simulation to investigate the micro-structural evolutions of polymers during glass transition. A new parameter, probability of segment movement, was proposed to probe the heterogeneity of local segment dynamics. A microscopic picture of spatial distribution of dynamic heterogeneity was obtained. A conformational transition was also detected. Further analysis demonstrated the existence of intrinsic links between the two phenomena. Compared with chain segments with gauche-conformation, segments with trans-conformation were packed more closely, and thus easier to be frozen. This difference in segmental mobility between the gauche- and trans-conformations results in the emergence of dynamic heterogeneity. Our simulation results reveal the underlying mechanism controlling the dynamic heterogeneity during glass transition from the viewpoint of local conformational changes.

Structural characteristics of a cooperatively rearranging region during the glass transition of a polymer system

The correlations between local ordered structures and cooperative motion were investigated by dynamic Monte Carlo (MC) simulation. The fraction of trans-conformation increases with the decrease of temperature, indicating the occurrence of a conformational transition from gauche- to trans-conformations. Due to the relatively high degree of close-packing, the trans-conformations are inclined to form local order. Furthermore, all the segments in the polymer system can be divided into two types: the ordered and the disordered ones. Compared with the ordered segments, the disordered segments have more neighboring vacancy sites, and thus move faster and randomly. Correspondingly, the segments in the local order have fewer neighboring vacancy sites, and exhibit lower mobility, which could only move cooperatively along the parallel direction. Those findings suggest that the cooperatively rearranging regions proposed by Adam and Gibbs contain more local ordered structures.

Relaxation and Crystallization of Oriented Polymer Melts with Anisotropic Filler Networks

The coexistence of nanofillers and shear flow can influence crystallization of polymer melts. However, the microscopic mechanism of the effect is not completely revealed yet. Thus, dynamic Monte Carlo simulations were used to study the effect of the filler networks formed by one-dimensional nanofillers on relaxation and crystallization of oriented polymer melts. The filler networks restrict the relaxation of oriented polymers and impose confinement effect on the chains inside the filler networks, resulting in higher orientation and lower conformational entropy of the inside chains compared to those of the outside chains. Thus, the confined inside chains have stronger crystallizability. During crystallization, the confined chains are nucleated on the filler surface and then form nanohybrid shish-kebab structures. Furthermore, the effect of fillers and chain orientation closely depends on some factors, such as polymer-filler interaction, filler content, and filler spacing. Our simulation results are consistent with some experimental findings. Thus, these results can provide new insights into the mechanism of crystallization of filled polymers and also guide researchers to develop new polymer nanocomposites with high performance.

Segmental dynamics in interfacial region of composite materials

In this work, we preformed dynamic Monte Carlo simulations to investigate the interfacial behaviors of polymers with two-dimensional filler. It was found that both the distributions of local segmental mobility and local glass transition temperatures in interfacial regions are controlled by interfacial interaction. For the system without interfacial interaction, the segments near interface have stronger mobility than those in bulk, due to the lower segment density. If the interfacial interaction is weakly attractive, there is no obvious difference of mobility between the interfacial and internal segments and no bound polymer exists near the interface. If the attractive interfacial interaction is strong, a gradient of local glass transition temperatures was observed, which demonstrates the presence of bound polymers in many layers with different segmental mobility. These findings could be used to explain the various experimental results about the segmental dynamics in the interfacial regions of polymer nanocomposites.Graphical abstract

Preparation of a novel magnetic and thermo-responsive composite and its application in drug release

A novel hyperbranched multi-arms star block copolymer with magnetic oriented target and thermo-responsive characters was synthesized. The atom transfer radical polymerization initiators were anchored on the surfaces of magnetic mesoporous silica by the chemical reaction of surface-modified amides with α-bromoisobutyryl bromide. The composites (FexOy@SBA-15@HPCMS-g-PNIPAAm) with magnetic mesoporous silica nanoparticles as core and hyperbranched star block copolymers as arms were prepared by iron(III)-mediated surface-initiated polymerization with FeCl3·6H2O as catalyst, PPh3 as ligand, ascorbic acid as reducing agent, N,N-dimethylformamide as solvent, and 4-chloromethylstyrene (CMS) and N-isopropylacrylaminde (NIPAAm) as monomer, respectively. The resultant materials were characterized by Fourier transform infrared, thermogravimetric analysis, N2 adsorption/desorption measurements, scanning electron microscopy, transmission electron micrographs, differential scanning calorimeter, and vibrating sample magnetometer. With aspirin as model drug, the drug release behavior of the hybrid materials was investigated in detail. Due to both the thermo-responsive and the magnetic property, the product may be used as a drug delivery system aiming at a cancer treatment.Graphical abstract

Examining the effect of hydroxyl groups on the thermal properties of polybenzoxazines: using molecular design and Monte Carlo simulation

The influence of methylol and phenolic hydroxyl on the thermal properties of polybenzoxazines has been studied using two monofunctional benzoxazine monomers synthesized from para methylol-/ethyl- phenol, aniline and paraformaldehyde. The chemical structures of the synthesized monomers are confirmed by 1H nuclear magnetic resonance (NMR), 13C NMR and Fourier transform infrared spectroscopy (FT-IR). Polymerizations are monitored by differential scanning calorimetry (DSC). The glass transition temperature (Tg) of each polybenzoxazine is measured by DSC as well as dynamic mechanical analysis (DMA), indicating the greatly increased Tg via incorporation of methylol functionality into benzoxazine moiety. Monte Carlo simulations are also applied to further investigate the underlying structure-property relationship between intermolecular hydrogen-bonding network originating from different types of hydroxyl groups and thermal properties of polybenzoxazines. The agreement between the experimental and simulation results provide us with a fundamental understanding of the designing roles in highly thermally stable polybenzoxazines.

One-dimensional nanofiller induced crystallization in random copolymers studied by dynamic Monte Carlo simulations

ABSTRACT Understanding crystallization mechanism of random copolymers is beneficial for the design and development of new polymeric materials. We performed dynamic Monte Carlo simulations to investigate the crystallization behaviours of random copolymers in polymer solutions induced by one-dimensional nanofiller. The effects of comonomer content on crystallinity and crystalline morphology were studied in detail. In the copolymers with relatively low comonomer contents, the final absolute crystallinity is independent of comonomer contents, while in the copolymers with relatively high comonomer contents the final absolute crystallinity decreases with increasing comonomer contents. In addition, the one-dimensional nanofiller can induce the formation of nanohybrid shish-kebab structures in the copolymers with low comonomer contents. However, in the copolymer with very high comonomer content, the presence of a large number of comonomers with no ability of parallel arrangements hinders the formation of nanohybrid shish-kebab structures.

Effect of interface on bulk polymer: control of glass transition temperature of rubber

In current paper, we demonstrated that molecular dynamics and glass transition of rubber can be controlled by constructing attractive interface between rubber matrix and fillers. Based on a combination of experiments and molecular simulations, it was revealed that interfacial segmental mobility was reduced and glass transition temperatures (Tgs) of epoxidized natural rubber (ENR) were significantly improved due to in situ polymerization of zinc dimethacrylate (ZDMA). During curing, ZDMA polymerizes in rubber matrix, resulting in the appearance of nanodispersion phases of poly-ZDMA (PZDMA). It was demonstrated that coordination interaction exists between epoxy groups and PZDMA in interfacial regions. Furthermore, using dynamic Monte Carlo simulations, it was observed that the interfacial regions that have highest content of epoxy groups exhibit lowest segmental mobility. Then, the increase of ZDMA content leads to the rise of the fraction of absorbed interfacial segments, and thus the Tgs of filled rubbers are improved.