Combing experimental methods and molecular simulations to study self-healing behaviors of polyurethane elastomers containing multiple hydrogen bond networks and flexible blocks.

The preparation of polymers with high self-healing ability is conducive to environmental protection and resource conservation. In the present work, two kinds of polyurethane (PU) elastomers were prepared: the one containing flexible end blocks (polypropylene glycol) and the other containing flexible end blocks and 2-ureido-4[1H]-pyrimidinone (UPy) groups that can form reversible quadruple hydrogen bonds. Both of the two PU elastomers have self-healing ability. At low temperatures the PU without UPy groups exhibits stronger self-healing ability, while at high temperatures the PU with UPy groups has better self-healing function. The difference can be attributed to the combined effect of segmental mobility and reversible network strength. Based on molecular simulations, we further observed that the self-healing behaviors are affected by four factors: healing temperature, reversible interaction strength, reversible interaction site density and segment diffusion ability.

Studying the effects of carbon nanotube contents on stretch-induced crystallization behavior of polyethylene/carbon nanotube nanocomposites using molecular dynamics simulations.

In the present work, we used molecular dynamics simulations to study the effects of carbon nanotube (CNT) contents on stretch-induced crystallization behavior in CNT filled polyethylene systems. During high-temperature stretching, the stretching is responsible for the orientation of CNTs, which then facilitates segment orientation and conformational transition from the gauche-conformation into the trans-conformation in interfacial regions. The systems with higher CNT contents have a higher degree of orientation and higher contents of trans-conformation during stretching, resulting in the formation of more precursors. During subsequent crystallization, the initial crystallization rate increases with the increase of the CNT content due to the increase in precursor contents in interfacial regions. However, after the CNT content exceeds a certain value, a filler network would be formed by CNTs, which can restrict chain movements and then lead to a decrease in the overall crystallization rate in the systems with high CNT contents.

Molecular simulation of polymer crystallization under chain and space confinement.

Polymer crystallization under chain and space confinements is studied by Monte Carlo simulation. The simulation results show that the crystallinity and melting temperature of confined systems increase with the increase of free chain content. Furthermore, the crystallinity and melting temperature of confined systems with larger lateral size are higher than those with smaller lateral size. These findings are in good agreement with the conclusions obtained in some experiments. An important phenomenon that cannot be observed in experiments has been confirmed, that is, the tethering point can be used as the nucleation site. For the confined polymer system with the lateral size of 8 lattice points, with the increase of free chain content, the surface free energy of the nuclei and the diffusion activation energy of the chains decrease due to the combined effects of chain conformation size and chain movement ability, which leads to the enhancement of the nucleation ability of polymers. However, for the confined polymer system with lateral size of 12 lattice points, with the increase of free chain content, the nucleation sites decrease and the critical free energy barrier increases, which are not conducive to nucleation. Moreover, the existence of interfacial interactions can also significantly change the crystallization of confined polymers. Our results indicate the crystallization kinetics of the confined polymer from a microscopic point of view.

Monte Carlo simulations of stereocomplex formation in multiblock copolymers.

Currently, controlling the formation of stereocomplex crystallites (SCs) in enantiomeric PLA blends is a research hotspot. In the present work, we performed dynamic Monte Carlo simulations to study the formation mechanism of SCs in multiblock copolymers. The effects of block number and crystallization temperature on SC formation were revealed. The relative size of block length and crystal thickness is an important factor. In the multiblock copolymers with block length longer than crystal thickness, both the increases of crystallization temperature and block number lead to the increase of SC content attributed to the relatively high degree of supercooling for SC formation and the improved local miscibility between different blocks, respectively. In the multiblock copolymers with block length equal to crystal thickness, each block can just form one crystalline stem, and then different blocks can be more easily alternately parallel-packed during SC formation. The system thus reaches the upper limit of the ability to form SCs. Therefore, both the further increases in crystallization temperature and block number can no longer cause the enhancement in SC formation.

Stereocomplex formation in mixed polymers filled with two-dimensional nanofillers.

The presence of nanofillers, such as graphene, can effectively promote stereocomplex formation in poly(l-lactide)/poly(d-lactide) blends. However, the detailed microscopic mechanism of the improved formation of stereocomplex crystallites (SCs) in filled polylactides is still unclear. Therefore, we performed dynamic Monte Carlo simulations to reveal the underlying mechanism of the effect of two-dimensional nanofillers on the formation of SCs in polymer blends. It was observed that the nanofillers can lead to the occurrence of heterogeneous nucleation of mixed polymer chains. On one hand, the miscibility of mixed polymers is improved in the interfacial regions, attributed to attractive interactions between polymers and nanofillers. On the other hand, for the heterogeneous nucleation process, chains prefer to crystallize by means of intermolecular packing alignment. Both phenomena contribute to the promotion of SCs.

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 (T g) of each polybenzoxazine is measured by DSC as well as dynamic mechanical analysis (DMA), indicating the greatly increased T g 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.

Controllability of Polymer Crystal Orientation Using Heterogeneous Nucleation of Deformed Polymer Loops Grafted on Two-Dimensional Nanofiller.

Nowadays, it is a research hotspot to realize the controllability of polymer crystal structure in polymer nanocomposites. However, polymer crystals induced by two-dimensional filler always exhibit random orientation, which somewhat limit the improvement of physical properties of polymer materials. In the current paper, dynamic Monte Carlo simulations were performed to explore the methods preparing crystals with uniform orientation. Heterogeneous nucleation of deformed polymer loops grafted on two-dimensional filler can induce the appearance of a special nanohybrid shish-kebab (NHSK) structure, in which the two-dimensional filler acts as "shish" and induces the formation of crystals with uniform orientation. The grafted deformed chains are first heterogeneously nucleated on filler surface, and then free chains participate in crystallization, resulting in the formation of the NHSK structure. The NHSK structure can only be formed in the systems with high interfacial interactions at high temperatures or moderate interfacial interactions at moderate temperatures or low interfacial interactions at low temperatures. The method proposed here can be used to achieve the controllability of polymer crystal orientation in experiments.

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.

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.

Thermodynamics of strain-induced crystallization of random copolymers.

Industrial semi-crystalline polymers contain various kinds of sequence defects, which behave like non-crystallizable comonomer units on random copolymers. We performed dynamic Monte Carlo simulations of strain-induced crystallization of random copolymers with various contents of comonomers at high temperatures. We observed that the onset strains of crystallization shift up with the increase of comonomer contents and temperatures. The behaviors can be predicted well by a combination of Flory's theories on the melting-point shifting-down of random copolymers and on the melting-point shifting-up of strain-induced crystallization. Our thermodynamic results are fundamentally important for us to understand the rubber strain-hardening, the plastic molding, the film stretching as well as the fiber spinning.

Using two-dimensional correlation dynamic mechanical spectroscopy to detect different modes of molecular motions in the glass-rubber transition region in polyisobutylene.

In this Article, we report the first study of the molecular dynamics in the glass-rubber transition region in polyisobutylene by 2D correlation dynamic mechanical spectroscopy (2DC-DMS). With the help of the high resolution and high sensitivity of the technique, the sub-Rouse modes are independently separated from the Rouse modes and local segmental motion (LSM). According to the positions and widths of autopeaks of three modes of molecular motions, the loss tangent peak is resolved into three peaks by nonlinear fitting method. Moreover, the glass-rubber transition region is divided into three regions. 2DC-DMS has been demonstrated to be a powerful tool for studying the molecular motions with different time/length scales.

Large-scale orientation in a vulcanized stretched natural rubber network: proved by in situ synchrotron X-ray diffraction characterization.

In situ studies of strain-induced crystallization in unfilled and multiwalled carbon nanotube (MWCNT)-filled natural rubber (NR) were carried out by using synchrotron wide-angle X-ray diffraction (WAXD). Synchrotron WAXD results indicate that more nuclei appear in the MWCNT-filled NR sample, leading to higher crystallinity, lower onset strain of crystallization, and remarkable enhancement in tensile strength. During deformation, despite the amorphous chains remaining in isotropic orientation, the domains of larger scale (10-100 nm) with high network chain density in the NR matrix are oriented. The MWCNTs induce significant variation of this orientational process, and it is monitored by the stearic acid (SA) crystallites, which are effective nanoprobes of the amorphous phase. The results indicate that a small amount of MWCNTs and SA crystallites can be used as new tools to analyze the microstructural orientation of NR during deformation. The results also yield new insight into the strain-induced crystallization mechanism.