Silyl‐Terminated Ethylene‐co‐Norbornene Copolymers by Organotitanium‐Based Catalysts

Ethylene (E) and norbornene (N) were copolymerized in the presence of PhSiH₃ as chain‐transfer agent with [Ti(η⁵:η¹‐C₅Me₄SiMe₂NBu^t)(η¹‐Me)₂] precatalyst combined with [Ph₃C][B(C₆F₅)₄]. The silane was introduced at chain‐ends of E‐co‐N copolymers with concomitant reinitiation of the growing polymer chain. The concentrations of the silane and polymer molecular weight are inversely correlated. The characteristic signals of  SiH₂Ph chain‐ends were observed by ¹H NMR. The Si heteroatom is predominantly adjacent to ethylene units in E‐co‐N copolymers with high N content.

     

Temperature and Pressure Effects on Local Structure and Chain Packing in cis‐1,4‐Polybutadiene from Detailed Molecular Dynamics Simulations

Summary: We present results for the temperature and pressure dependence of local structure and chain packing in cis‐1,4‐polybutadiene (cis‐1,4‐PB) from detailed molecular dynamics (MD) simulations with a united‐atom model. The simulations have been executed in the NPT statistical ensemble with a parallel, multiple time step MD algorithm, which allowed us to access simulation times up to 1 µs. Because of this, a 32 chain C₁₂₈ cis‐1,4‐PB system was successfully simulated over a wide range of temperature (from 430 to 195 K) and pressure (from 1 atm to 3 kbar) conditions. Simulation predictions are reported for the temperature and pressure dependence of the: (a) density; (b) chain characteristic ratio, C_n; (c) intermolecular pair distribution function, g(r), static structure factor, S(q), and first peak position, Q_max, in the S(q) pattern; (d) free volume around each monomer unit along a chain for the simulated polymer system. These were thoroughly compared against available experimental data. One of the most important findings of this work is that the component of the S(q) vs. q plot representing intramolecular contributions in a fully deuterated cis‐1,4‐PB sample exhibits a monotonic decrease with q which remains completely unaffected by the pressure. In contrast, the intermolecular contribution exhibits a distinct peak (at around 1.4 Å⁻¹) whose position shifts towards higher q values as the pressure is raised, accompanied by a decrease in its intensity.

3D view of the simulation box containing 32 chains of C₁₂₈ cis‐1,4‐polybutadiene at density ρ = 0.849 g · cm⁻³ and the conformation of a single C₁₂₈ cis‐1,4‐PB chain fully unwrapped in space.     

Solvent‐Dependent Structure Formation in Drying P3HT:PCBM Films Studied by Molecular Dynamics Simulations

Phase separation in the donor‐acceptor blend poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) during evaporation of a solvent using coarse‐grained molecular dynamics simulations is studied here. To this end, an equilibrated P3HT:PCBM:solvent mixture is placed in an elongated simulation box, after which solvent molecules are removed at regular time intervals from a region above the film. Three often‐used solvents are considered: chloroform (CFM), chlorobenzene (CLB), and orthodichlorobenzene (oDCB). The coarse‐grained solvent–solvent interaction parameters are tuned to reproduce the atmospheric boiling temperatures, while the PCBM–solvent interaction parameters are tuned to reproduce the PCBM solubilities. Other parameters are taken from the literature. During evaporation, the formation of a crust that is depleted of solvent, in which aggregation of P3HT and PCBM occurs, is observed. In agreement with experiment, the top region of the dry film is rich in PCBM for the cases of CLB and oDCB, and rich in P3HT for the case of CFM, while the very top layer of the film is always rich in P3HT. This vertical separation is ascribed to a competition between the tendency of P3HT to move to the surface due to its low surface energy and the different tendencies of PCBM to be dragged along to the surface by the evaporating solvent depending on its solubility. Also in agreement with experiment, the P3HT–PCBM interface area is larger for CLB and oDCB than for CFM. For CLB and oDCB, an indication for a spinodal P3HT–PCBM decomposition starting from the top and bottom surface is found, whereas for CFM the phase separation appears to be initiated in the bulk of the film.

     

Ethylene–Norbornene Copolymerization by Carbon Nanotube‐Supported Metallocene Catalysis: Generation of High‐Performance Polyolefinic Nanocomposites

Homogeneous surface coating of multi‐walled carbon nanotubes is achieved for the first time by in situ copolymerization of ethylene (E) and 2‐norbornene (N) as catalyzed directly from the nanotube surface previously treated by a highly active metallocene‐based complex, i.e., rac‐Et(Ind)₂ZrCl₂/MMAO‐3A. The copolymerization reaction allows for the destructuration of the native nanotube bundles, which upon further melt blending with an ethylene–vinyl acetate copolymer (27 wt.‐% vinyl acetate) matrix, leads to high‐performance polyolefinic nanocomposites. The microstructural analysis of the surface‐coating copolymer was carried out by ¹³C NMR spectroscopy and allowed determination of the actual N content incorporated along the chains. Depending on the experimental conditions used (e.g., E pressure, solvent, feed N concentration) the relative quantity of E–N copolymer can be tuned, as well as the N content in the formed copolymers and accordingly their glass transition temperature.

     

Crack Formation and Propagation in Molecular Dynamics Simulations of Polymer Liquid Crystals

In recent papers we have used statistical mechanics to predict multiple phase formation in polymer liquid crystals (PLCs).^[1, 2] Now we have performed molecular dynamics simulations of PLC copolymers as materials consisting of LC islands in flexible matrices. A method for creating such materials on a computer is described. The overall concentration of the LC units, island size, and spatial distribution of the islands (random, in rows, and evenly distributed throughout the material) were varied. Crack formation and propagation as a function of these parameters were investigated. The local concentration of LC units in each chain has been defined. We found that the probability of a crack initiation site goes symbiotically with the local LC concentration. The first small crack is sometimes a part of the path through which the material breaks, however, although several small cracks may evolve at first, some of these never evolve into larger cracks since crack arrest occurs. The results can be used for creation of real materials with improved mechanical properties.     

Preparation of Polar Ethylene–Norbornene Copolymers by Metallocene Terpolymerization with Triisobutylaluminium‐Protected But‐3‐en‐1‐ol

But‐3‐en‐1‐ol has been pre‐protected by triisobutylaluminium and terpolymerized with ethylene and norbornene by rac‐[Et(Ind)₂]ZrCl₂/MAO catalysts. The strong polarity of diisobutyl(but‐3‐en‐1‐oxy)aluminum causes a slight reduction in the catalyst activity and yields a small fraction of crystallinity. The but‐3‐en‐1‐ol content in the terpolymer is as high as 3.2% and can be readily adjusted by varying the reaction conditions. When the norbornene/ethylene ratio is over 10, the norbornene incorporation efficiency is not affected by the polar monomer and is close to that of the copolymerization. Similar to the ethylene/norbornene copolymers, the thermal properties of the obtained terpolymers are mainly determined by their norbornene contents.

     

Effect of Hydrogen Bonds on Structures and Glass Transition Temperatures of Maleimide–Isobutene Alternating Copolymers: Molecular Dynamics Simulation Study

The objective of this paper is to use molecular dynamics (MD) simulation to study the influence of hydrogen bonding on the structure and glass transition temperature (T_g) of maleimide–isobutene alternating copolymers (poly(RMI‐alt‐IB)), a promising material used in the optoelectronics industry. The T_g obtained by MD simulation shows that the incorporation of hydrogen bonding increased the T_g for 48 K. The static and dynamic properties of poly(RMI‐alt‐IB)s are examined in this study. All the results prove that intermolecular CO…H O is the main hydrogen bond in the copolymers, while negligible intramolecular hydrogen bonding is observed. The segmental mobility and chain mobility are decreased because of the existance of the hydrogen bonds.

     

Synthesis of Ultrahigh‐Molecular‐Weight Ethylene‐1‐Hexene Copolymers with High Hexene Content via Living Polymerization with Fluorinated Bis(phenoxy–imine) Titanium(IV)

The fluorinated FI–Ti catalyst bis[N‐(3‐propylsalicylidene)‐pentafluoroanilinato] titanium(IV) dichloride (PFI) combined with dried methylaluminoxane (dMAO) is investigated for ethylene/1‐hexene copolymerization at 50 °C under atmospheric pressure. The reaction shows good livingness and has a high activity at high [H]/[E] molar ratios up to 14. Ultrahigh molecular weight (>1.4 × 10⁶ g mol⁻¹) copolymers with high 1‐hexene content (>25 mol%) are prepared. Kinetic parameters of the copolymerization with PFI are determined. The first‐order Markov statistics applies and the product of the reactivity ratios r₁r₂ is close to 1, giving random unit distributions.

     

Influence of Cross‐Linking Density on the Glass Transition and Structure of Chemically Cross‐Linked PVA: A Molecular Dynamics Study

Mechanical properties and glass transitions of cross‐linked polymer networks depend strongly on both the network topology and cross‐linking density. A model is developed using a dynamic cross‐linking approach based on a cutoff distance criterion followed by a high‐temperature annealing procedure. The analysis focused on on the influence of cross‐linking degree on chain packing and hydrogen‐bond structure and on the roles played by various energy components in the glass transition process. T_g was calculated using two different methods; (i) from the intersection of lines drawn through points in a plot of specific volume versus temperature and (ii) from plots of different molecular energy components as a function of temperature.

     

Cyclopentadienyl Nickel and Palladium Complexes/Activator System for the Vinyl‐Type Copolymerization of Norbornene with Norbornene Carboxylic Acid Esters: Control of Polymer Solubility and Glass Transition Temperature

Summary: In non‐polar solvents such as toluene, Cp‐Ni and ‐Pd complexes (Cp = η⁵‐C₅H₅) with appropriate activators have been found to induce the addition polymerization of norbornene in excellent yields, for example (Cp)Pd(allyl)/[Ph₃C][B(C₆F₅)₄] gave 105 120 kg‐polymer/cat‐mol · h at room temperature. While the Cp‐Pd system was not suitable in the presence of ester‐substituted norbornenes, the Cp‐Ni system, for example (Cp)Ni(Cl)(PPh₃)/AlMe₃/B(C₆F₅)₃ can copolymerize norbornene with 5‐norbornene‐2‐carboxylic acid methyl ester in toluene to give high yields (up to 68% in 2 h at room temperature) of copolymer with variable contents of the methyl ester monomer unit (17.4–60.7 mol‐%). These copolymers have high molecular weights ( = 234 100–109 500) and narrow molecular weight distributions ( = 1.78–1.89). In addition, they are soluble in common organic solvents giving flexible and transparent films on casting, that show very high T_g in the range of 352.8 to 316.0 °C. The same Ni‐catalyst system can also copolymerize norbornene derivatives with bulky substituents, i.e., 2‐butyl‐5‐norbornene and 5‐norbornene‐2‐carboxylic acid butyl ester. The T_g of these copolymers are lower (294.9–267.3 °C) than the methyl ester‐based copolymers, demonstrating that the T_g of the polynorbornene copolymer film can be tailored simply by changing the alkyl group of the monomer to within the range of 352 to 267 °C.

Figure showing the addition polymerization of norbornene using Cp‐Ni complex with appropriate activators.     

Molecular Dynamics Simulation of Glass Transition Behavior of Polyimide Ensemble

The prediction of glass transition temperature from chemical structure has a great significance to select and design new high-properties materials. However, for the estimation and correlation methods, the deficiency of parameters for newer groups will lead to invalidity of Tg prediction or greater deviation from experiment. In the present work, we predicted Tg for a polyimide (PI) ensemble with rigid moieties, and analyzed structural factor that regards to the rotation barrier of the bridging…     

Temperature‐induced Molecular Chain Motions of Styrenic Triblock Copolymers Studied by Intrinsic Fluorescence Spectra

The temperature‐induced molecular chain motions of styrenic triblock copolymers (SBC), i.e. polystyrene‐block‐polybutadiene‐block‐polystyrene (SBS) and polystyrene‐block‐poly(ethylene‐co‐1‐butene)‐block‐polystyrene (SEBS), were studied by intrinsic fluorescence method. For SBS, the glass transition temperatures (T_gs) of B block and S block obtained by intrinsic fluorescence method were in good agreement with differential scanning calorimetry measurements (DSC). In the case of SEBS, an isoemission point was observed at about 310 nm at elevated temperatures, suggesting the slight conversion between the monomer and excimer emission. On this basis, the molecular chain motion of SEBS was monitored by both fluorescence intensity and excimer/monomer fluorescence ratio. Besides the T_gs of S block and EB blocks, a melting point (T_m) of weak crystalline in EB block was unambiguously determined by intrinsic fluorescence. Furthermore, it was found that the melting process directly led to the slight loosening of PS segments in interface and consequently the reduction of the amount of excimer. A reasonable mechanism was proposed to describe the molecular chain movements and phase transitions of SEBS upon heating. Moreover, the influence of temperature on the apparent activation energy of non‐radiative process (E_a^T) around T_g of S block was much stronger than that around T_g of B or EB blocks.     

Order and Phase Behavior of a Cylinder Forming Diblock Copolymers and Nano‐Particles Mixture in Confinement: A Molecular Dynamics Study

We study a coarse grained model of cylinder forming diblock copolymers and nano‐particles (NPs) mixture confined between Lennard–Jones hard walls. Two models for non‐selective interactions between monomers and NPs are applied. In the case of purely repulsive interactions between NPs and monomers (athermal case) strong segregation of NPs at the film surfaces and the formation of droplets of particles inside the copolymer film can be observed. For weakly attractive interactions between NPs and monomers (thermal case) formation of droplets of particles disappears and segregation on the film surfaces depend on temperature. The uptake of NPs by the copolymer film in the thermal case displays a non‐monotonic dependence on temperature which can be qualitatively explained by a mean‐field model. In both cases of non‐selective interactions NPs are preferentially localized at the interface between the microphase domains.

     

Following the Crystallization Process of Polyethylene Single Chain by Molecular Dynamics: The Role of Lateral Chain Defects

Summary: Langevin molecular dynamics (LMD) simulations have been performed in order to understand the role of the short chain branches (SCB) on the formation of ordered domains by cooling ethylene/α-olefins single chain models. Different long single-chain models (C₂₀₀₀) with 0, 5 and 10 branches each 1000 carbons were selected. The branches were randomly distributed along the backbone chain. Furthermore, C₁ (methyl) and C₄ (butyl) branches were taken into account. These models mimic the molecular architecture of ethylene/1-butene and ethylene/1-hexene random copolymers. The simulations are performed according to the following protocol: 20 random chain conformations for each model were equilibrated at high temperature (T* = 13.3) and then they were cooled in steps of 0.45 until the final temperature (T* = 6.2) by running a total of 35 × 10⁶ LMD steps. The distribution peaks of crystallization for each model were calculated by differentiating the global order parameter with respect to the temperature. The Tc* (crystallization temperature) decrease as the number of branches increases as it is experimentally observed. The formation of order in the copolymers is affected by the type and amount of the SCB in the backbone of the polymer chain. The stem lenght and crystallization fraction (α) were defined using the local-bond order parameter. Both parameters decrease as the number of branches increase. In all cases here shown, the C₄ branches are excluded from the ordered domains. However, we have observed that the methyl branch can be incorporated into the ordered regions. These facts satisfactorily agree with experimental data available in the literature.     

HTPB/增塑剂玻璃化转变温度及力学性能的分子动力学模拟

为了预测高分子粘结剂端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性及HTPB/增塑剂共混物的玻璃化转变温度(Tg)和力学性能,在COMPASS力场条件下采用分子动力学(MD)模拟方法对相容体系(HTPB-DOS)和不相容体系(HTPB-NG)进行了研究.结果表明,通过比较溶度参数差值(Δδ)的大小可以预测HTPB与增塑剂的相容性,即HTPB与DOS属于相容体系,而HTPB与NG不相容.通过温度-比容曲线可以得到HTPB、HTPB/DOS与HTPB/NG的Tg分别为197.54,176.30和200.03K.力学性能分析结果表明,添加DOS增塑剂后使HTPB的弹性模量(E),体积模量(K)和剪切模量(G)下降,材料刚性减弱,柔性增强,力学性能得到改善.本模拟方法可以作为预测聚合物/增塑剂共混物性能的有利工具,也可以为固体推进剂和高聚物粘结炸药的配方设计提供理论指导.     

Atomistic Characterisation of Li+ Mobility and Conductivity in Li7−xPS6−xIx Argyrodites from Molecular Dynamics Simulations,Solid‐State NMR,and Impedance Spectroscopy

The atomistic mechanisms of Li⁺ ion mobility/conductivity in Li_7?xPS_6?xI_x argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid–solid phase transition, which was characterised by low‐temperature differential scanning calorimetry, ⁷Li and ¹²⁷I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177±2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li⁺ ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS₄ tetrahedra. From connectivity analyses and free‐energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X‐ray diffraction experiments indicates a homogeneity range for Li_7?xPS_6?xI_x with 0.97≤x≤1.00. Within this range, molecular dynamics simulations predict Li⁺ conductivity at ambient conditions to vary considerably.     

Determining the Local Shear Viscosity of a Lipid Bilayer System by Reverse Non‐Equilibrium Molecular Dynamics Simulations

The parallel shear viscosity of a dipalmitoylphosphatidylcholine (DPPC) bilayer system is studied by reverse non‐equilibrium molecular dynamics simulations (RNEMD) with two different united‐atom force fields. The results are related to diffusion coefficients and structural distributions obtained by equilibrium molecular simulations. We investigate technical issues of the algorithm in the bilayer setup, namely, the dependence of the velocity profiles on the imposed flux and the influence of the thermostat on the calculated shear viscosity. We introduce the concept of local shear viscosity and investigate its dependence on the slip velocity of the monolayers and the particle density at the headgroup–water interface and the tail–tail interface. With this we demonstrate that the lipid bilayer is more viscous than the surrounding water phase, and that slip takes place near the headgroup region and in the centre of the bilayer where the alkyl tails meet. We also quantify the apparent increase in viscosity of the water molecules entangled at the water–headgroup interface.     

共聚单体乙烯对结晶性无规共聚聚丙烯平台模量的影响

采用齐格勒－纳塔催化剂制备了具有宽分子量分布和超高分子量的无规共聚聚丙烯(UHPPR)。利用平行板流变仪研究了UHPPR在180, 200 and 220℃的粘弹行为,发现UHPPR 180˚C和200˚C的损耗模量G″(ω)曲线,分别在频率为38.10rad/s 和 84.70rad/s处出现最大值峰。从而可以利用特定温度下的损耗模量曲线G″(ω),计算出具有宽分子量分布的结晶性超高分子量无规共聚聚丙烯(UHPPR)的平台模量(G_N⁰),所得UHPPR的平台模量(G_N⁰)在180 ℃和 200 ℃分别为4.51×10⁵ Pa和3.67×10⁵ Pa。这一结果表明,平台模量随乙烯含量的增加而提高,与分子量无关。