两端系留纳米粒子聚合物的分子动力学模拟

两端系留纳米粒子聚合物是研究末端对聚合物链弛豫行为影响的优选分子模型.本文构建了两端系留纳米粒子聚合物模型,运用粗粒化分子动力学方法研究了两端系留纳米粒子聚合物的特征温度和弛豫行为,探讨了纳米粒子半径和聚合物链长对玻璃化转变温度、结晶温度和介电性能的影响.研究表明,聚合物两端纳米粒子的存在可延缓聚合物链的弛豫并促进结晶,使两端系留纳米粒子聚合物的玻璃化转变温度和结晶温度均增加.研究结果与相关的实验报道吻合,可加深对两端系留纳米粒子聚合物结构和性能的理解.     

超级电容器用聚吡咯纳米粒子的比容量衰减机理研究

基于化学氧化聚合法制备了粒径约为40nm的掺杂对甲苯磺酸(TSA)的聚吡咯(PPy)复合纳米粒子.采用XRD、EDS、SEM和TEM研究了产物的结构和形貌特征,并用循环伏安、恒流充放电等方法测试了纳米粒子的电化学性能.结果表明PPy/TSA具有良好的快速充放电能力,在10 mA/cm2时比容量达到268Fg-1,但在1000次循环后发现其比容量下降了25.5%.用CV测试了PPy/TSA电极在第一次循环和第1000次循环的伏安曲线图,发现经过1000次循环后PPy/TSA电极的氧化峰逐渐消失,并且非法拉第电流减小.交流阻抗测试表明,PPy/TSA纳米粒子在反复充放电循环后,其界面电荷传递电阻增加,高频段双层电容值降低,但低频段的赝电容值却有所增加.通过比表面积测量发现,聚合物链的反复氧化-还原过程还可导致平均孔径减小、比表面积下降.红外和拉曼光谱也证实PPy纳米粒子在充放电过程中会发生降解,生成一种含氧的醌式结构,这表明聚合物电极材料比容量的下降可能与PPy主链的不可逆氧化和结构碎片化有关.     

La_(2-x)Pr_xNiO_(4+δ)体系低频内耗研究

利用内耗技术对La2-xPrxNi O4+δ(0≤x≤2.0)体系额外氧的状态进行了深入研究。La2-xPrxNi O4+δ(0≤x≤2.0)系列样品的XRD数据拟合计算得出:当x小于等于0.75时,体系为四方结构,而当x大于0.75时,体系转变为正交结构。通过对体系低频内耗和模量的测量研究发现,当0≤x≤0.75时,样品只存在一个弛豫内耗峰(P1峰),其峰值随x值增大而增大,与额外氧的浓度成正相关;当1.0≤x≤2.0时,样品中存在两个弛豫内耗峰,分析研究表明:高温侧内耗峰P1对应着前面发现的内耗单峰,它是由ab面额外氧对的取向弛豫运动引起的,在这里该内耗峰峰值随x值增大而减小,显然是受到额外氧的三维有序结构形成的影响。低温侧内耗峰峰P2可能与额外氧对的新的组态运动方式有关。     

纳米SiO2对火焰喷涂聚酰胺12涂层非等温结晶与熔融行为的影响

采用火焰喷涂法制备了聚酰胺12/纳米SiO2(PA12/nano-SiO2)复合涂层,利用差示扫描量热法(DSC)对复合涂层的非等温结晶过程、熔融行为进行了分析.结果表明:纳米SiO2粒子的加入不仅能提高复合涂层的结晶度和结晶温度,而且使复合涂层的熔融峰温度高于纯PA12涂层,说明纳米SiO2粒子具有明显的成核剂作用,它能诱导聚酰胺大分子结晶,提高其结晶能力、结晶速率、结晶的完善程度及结晶度,并能改善涂层的力学、耐老化及耐摩擦磨损性能.     

三元乙丙橡胶玻璃化转变温度以上的Sub-Rouse弛豫峰研究

采用动态力学分析法(DMA)研究了三元乙丙橡胶(EPDM)的链段弛豫行为.结果表明随着温度的升高,EPDM样品依次出现了3个内耗弛豫峰,分别是玻璃化转变(α转变)、sub-Rouse模式转变(α'转变)和Rouse模式转变(α″转变).通过分析频率谱中弛豫时间、弛豫强度、耦合参数和形状因子分别随温度的变化,发现在温度为384 K处,sub-Rouse模式转变存在动态转折点TB.而且,转折点的弛豫时间τα'(TB)大约是0.11 s,这与文献报道的PS及PVAc/PEO共混物(10(-0.5±0.5)s)等材料的弛豫时间非常接近,表明该转折点是在特定的弛豫时间产生的.基于链段耦合模型,我们提出此转折点是由低于TB温度时分子间耦合增强引起的.     

层层组装法制备核壳SiO_2/Pt粒子及其CO电氧化和原位电化学FTIR光谱

采用层层组装法合成了核壳SiO2/Pt粒子,用电化学循环伏安法(CV)和原位电化学傅里叶变换红外(FTIR)光谱研究了SiO2/Pt粒子对CO分子的氧化和吸附行为.透射电子显微研究表明:包覆SiO2的Pt壳是由团聚的Pt纳米微粒构成,其平均厚度大约为26nm.CO在SiO2/Pt粒子修饰的玻碳(GC)电极上的主氧化峰为0.49V(vsSCE),表现出比本体Pt金属好的催化性能.电化学FTIR光谱研究发现:线性CO在SiO2/Pt粒子上的IR吸收带的方向发生倒反,而且在不同的研究电位下,每个吸收带劈裂为两个间隔约为14cm-1的吸收带,这种劈裂现象在饱和吸附CO的Pt金属表面上是很难观察到的.这些异常的红外吸收现象可能是由SiO2/Pt粒子的结构效应导致的.     

纳米粒子尺寸效应引起的内层电子结合能位移

用XPS测定了氩离子溅射前后的Pt-PVP纳米粒子和TiO2、ZnO及SiO2纳米粒子的内层电子结合能, 并与其对应的体材料进行了比较. 结果表明, Ar+溅射前Pt-PVP的Pt 4f结合能比体材料Pt的稍低, 但Ar+溅射后由于PVP包覆层被除去, 裸露的Pt 纳米粒子的结合能明显高于Pt体材料. 与非纳米氧化物比较, 纳米氧化物TiO2、ZnO 和SiO2的内层电子结合能也向高结合能方向位移, 其位移大小的顺序为TiO2相似文献   

表面接枝改性纳米二氧化硅填充聚丙烯的结晶行为

应用差示扫描量热方法研究了纳米二氧化硅 (SiO2 )及其表面接枝改性对聚丙烯 (PP)结晶过程、等温与非等温结晶动力学的影响 ,并研究了上述等温结晶的熔融行为和平衡熔点 .研究发现纳米SiO2 具有明显的异相成核效应 ,能够提高PP的结晶温度、熔融温度、结晶度和结晶速率 ,但降低聚丙烯结晶的完善程度 .粒子的表面接枝处理 ,因改善了粒子与基体的亲和性而有利于粒子成核效应的提高 ,而且此效应尚与粒子的分散相关     

二氧化硅-聚丙烯腈核-壳结构复合纳米粒子的制备与表征

以甲基丙烯酸-3-(三甲氧基硅基)丙酯改性的SiO2纳米粒子为种子,采用乳液聚合法制备了粒径分布较窄的SiO2-聚丙烯腈(SiO2-PAN)核-壳结构复合纳米粒子。采用动态光散射、傅里叶红外光谱、透射电镜和扫描电镜表征了复合纳米粒子的粒径及分布、组成、形态和结构,并研究了表面活性剂的加入方式、反应温度及交联剂的引入对制备SiO2-PAN复合纳米粒子的影响。结果表明:SiO2-PAN复合纳米粒子为核-壳结构。采用半连续加入表面活性剂的方法,可以成功抑制乳液聚合中次级粒子的生成。通过增加表面活性剂的初始加入量、加快表面活性剂的补加速率,或降低反应温度,可使SiO2-PAN复合纳米粒子的粒径变小。反应温度的降低以及交联剂的引入使SiO2-PAN复合纳米粒子的表面变得平滑。     

VDF（81）／TeFE（19）共聚物的低温低频介电弛豫

用经过改进的介电弛豫谱仪测得不同结晶度的铁电共聚物ＶＤＦ（８１）／ＴｅＦＥ（１９）在－１２０－４０℃、１０－２－１０４ＨＺ范围内的复介电常数．低温介电弛豫过程显示室温以下共聚物的频率谱由低频和高频两部分叠合而成．低频部分符合ＷＬＦ方程，由非晶区被冻结分子链段的微布朗运动贡献；高频部分遵从Ａｒｒｈｅｎｉｕｓ规律，由晶区和非晶区分子链段较小尺度的局域运动产生．结果说明共聚物的玻璃化转变温度是－５２℃，局域弛豫活化能为３７．７ｋＪ／ｍｏｌ．     

Dielectric relaxation and molecular motion in poly(vinylidene fluoride)

The dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 10 Hz to 100 kHz at temperatures between ?196 and 150°C. Three dielectric relaxations were observed: the α relaxation occurred near 130°C, the β near 0°C, and the γ near ?30°C at 100 kHz. In the α relaxation the magnitude of loss peak and the relaxation times increased not only with increasing lamellar thickness, but also with decrease of crystal defects in the crystalline regions. In the light of the above results, the α relaxation was attributed to the molecular motion in the crystalline regions which was related to the lamellar thickness and crystal defects in the crystalline phase. In the β relaxation, the magnitude of the loss peak increased with the amount of amorphous material. The relaxation times were independent of the crystal structure and the degree of crystallinity, but increased slightly with orientation of the molecular chains by drawing. The β relaxation was ascribed to the micro-Brownian motions of main chains in the amorphous regions. The Arrhenius plots were of the so-called WLF type, and the “freezing point” of the molecular motion was about ?80°C. The Cole-Cole distribution parameter of the relaxation time α increased almost linearly with decreasing temperature in the temperature range of the experiment. The γ relaxation was attributed to local molecular motions in the amorphous regions.     

Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites

Dynamic mechanical–thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) were employed to investigate in detail glass transition and polymer dynamics in silver/poly(methyl methacrylate) (Ag/PMMA) nanocomposites. The nanocomposites were prepared by radical polymerization of MMA in the presence of surface modified Ag nanoparticles with a mean diameter of 5.6 nm dispersed in chloroform. The fraction of Ag nanoparticles in the final materials was varied between 0 and 0.5 wt%, the latter corresponding to 0.055 vol%. The results show that the nanoparticles have practically no effect on the time scale of the secondary β and γ relaxations, whereas the magnitude of both increases slightly but systematically with increasing filler content. The segmental α relaxation, associated with the glass transition, becomes systematically faster and stronger in the nanocomposites. The glass transition temperature T_g decreases with increasing filler content of the nanocomposites up to about 10 °C, in good correlation by the four techniques employed. Finally, the elastic modulus decreases slightly but systematically in the nanocomposites, both in the glassy and in the rubbery state. The results are explained in terms of plasticization of the PMMA matrix, due to constraints imposed to packing of the chains by the Ag nanoparticles, and at the same time, of the absence of strong polymer–filler interactions, due to the surface modification of the Ag nanoparticles by oleylamine at the stage of preparation.     

Changes of the Molecular Mobility of Poly(<Emphasis Type="Italic">ε</Emphasis>-caprolactone) upon Drawing,Studied by Dielectric Relaxation Spectroscopy

Dielectric relaxation spectroscopy (DRS) of poly(ε-caprolactone) with different draw ratios showed that the mobility of polymer chains in the amorphous part decreases as the draw ratio increases.The activation energy of the α process,which corresponds to the dynamic glass transition,increases upon drawing.The enlarged gap between the activation energies of the αprocess and the β process results in a change of continuity at the crossover between the high temperature a process and the α and β processes.At low drawing ratios the a process connects with the βprocess,while at the highest drawing ratio in our measurements,the a process is continuous with the a process.This is consistent with X-ray diffraction results that indicate that upon drawing the polymer chains in the amorphous part align and densify upon drawing.As the draw ratio increases,the α relaxation broadens and decreases its intensity,indicating an increasing heterogeneity.We observed slope changes in the α traces,when the temperature decreases below that at which τα ≈ 1 s.This may indicate the glass transition from the ‘rubbery’ state to the non-equilibrium glassy state.     

Mechanical relaxations in some ionene polymers. I. Effect of Ion concentration

Dynamic mechanical properties of m,n-ionenes, the structure of which are shown in Figure 1, were examined by torsional braid analysis. Three relaxations designated as α,β and γ were found. The α relaxation, ascribed to the primary relaxation due to an amorphous phase, was observed at 70–130°C, the temperature increasing with an increase of the ion concentration along the polymer chains. The β relaxation at around 0°C was related to the ionic portions of the polymers. The γ relaxation at around–120°C was a so-called local mode relaxation. The γ relaxation peak was split into two peaks in the very slowly cooled 12,10-ionene sample and the formation of an inhomogeneous structure in the amorphous phase is proposed.     

Isothermal crystallization kinetics and melting behavior of poly(ethylene terephthalate)/barite nanocomposites

Poly(ethylene terephthalate) (PET)/Barite nanocomposites were prepared by direct melt compounding. The effects of PET‐Barite interfacial interaction on the dynamic mechanical properties and crystallization were investigated by DMA and DSC. The results showed that Barite can act as a nucleating agent and the nucleation activity can be increased when the Barite was surface‐modified (SABarite). SABarite nanoparticles induced preferential lamellae orientation because of the strong interfacial interaction between PET chains and SABarite nanoparticles, which was not the case in Barite filled PET as determined by WAXD. For PET/Barite nanocomposites, the Avrami exponent n increased with increasing crystallization temperature. Although at the same crystallization temperature, the n value will decrease with increasing SABarite content, indicating of the enhancement of the nucleation activity. Avrami analyses suggest that the nucleation mechanism is different. The activation energy determined from Arrhenius equation reduced dramatically for PET/SABarite nanocomposite, confirming the strong interfacial interaction between PET chains and SABarite nanoparticles can reduce the crystallization free energy barrier for nucleus formation. In the DSC scan after isothermal crystallization process, double melting behavior was found. And the double endotherms could be attributed to the melting of recrystallized less perfect crystallites or the secondary lamellae produced during different crystallization processes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 655–668, 2009     

Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α₁ and α₂ relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α₁ relaxation to α₂ relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α₁ and α₂ relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α₁ relaxation cannot be observed, the α₂ and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.     

Thermal degradation kinetics of PET/SWCNTs nanocomposites prepared by the in situ polymerization

The decomposition and thermal behavior of poly(ethylene terephthalate) (PET)/carbon nanotubes (CNTs) nanocomposites were studied using thermogravimetric (TG) analysis in air atmosphere. A series of PET/single-walled CNTs (SWCNTs) materials of varying nanoparticles concentration were prepared using the in situ polymerization technique. Transmission electron microscopy and scanning electron microscopy micrographs verified that the dispersion of the SWCNTs in the PET matrix was homogeneous, while some relatively small aggregates co-existed at higher filler concentration. Two-stage decomposition was observed in the experiments. During first stage, strong chemical bonds are broken, i.e., aliphatic bonds and benzyl ring containing molecules decompose into small molecules in the gaseous phase. During second stage, when temperature is higher, the remaining nanotubes along with the residues of the first stage are burned. Kissinger and Coats–Redfern (5, 10, 20, 50 K min^?1) methods were applied to TG data to obtain kinetic parameters (activation energy, Arrhenius constant at 600 K and A factor) and Criado method to kinetics model analysis. In this kinetic model, energy activation is increasing with the increase of nanotubes concentration.     

Nuclear magnetic relaxation and molecular motion in poly(vinylidine fluoride)

Pulsed NMR T₁, T₂, and T_1ρ measurements are reported for poly(vinylidine fluoride) (PVF₂). The results demonstrate clearly the presence of four relaxation processes, three amorphous and one crystalline. The α relaxation is undoubtedly a crystalline one, while β and γ are both amorphous, in agreement with earlier conclusions from dielectric and dynamic mechanical measurements. The fourth relaxation (β′) observed initially in the mechanical measurements of Kakutani, but undetected in dielectric experiments, has been confirmed in our results and the process is described by an activation energy of 15.1 kcl/mole. Motion of folds on the surface of crystal lamellae is deemed to be the responsible mechanism for the β′ relaxation. Two models have been considered in the interpretation of the α process; rotation of crystalline chains in the vicinity of defects and rotational oscillation of restricted amplitude of all crystalline chains about the main chain axes. Rotation of amorphous chains is a possible mechanism for the γ process while motions of a general nature are responsible for the β relaxation. Our experimental results again indicate that spin diffusion plays an important role in the overall NMR response of the polymer.     

拉伸热历史对取向聚对苯二甲酸乙二酯膜在热处理过程中收缩和伸长的影响

弄清取向非晶态聚合物在热处理过程中的收缩和伸长的变化规律,以及所对应的结构变化,有较大的实用意义和科学意义。 对于取向聚对苯二甲酸乙二酯的热收缩和热伸长已有很多研究,但对于拉伸热历史对取向PET在热处理过程中的尺寸变化的影响尚缺乏系统的研究。在热拉伸的过程中发生分子链的取向、热弛豫和结晶三个相互竞争的过程。因此,改变拉伸条件可以得到具有各种不同取向和结晶的PET试样。当非晶态PET膜片在80-105℃以较低     

Dielectric relaxations and electrical conductivities of poly(alkyl methacrylates) under high pressure

Dielectric properties of four methacrylate polymers (methyl, ethyl, n-butyl and n-octyl) were studied in the frequency range 0.0001 cps–300 kcps at temperatures above and below the glass transition temperature and at various pressures up to 2500 atm. At temperatures well above T_g a single relaxation peak (α′ peak) was observed in the case of the higher n-alkyl methacrylates. However, this peak was split into two peaks, α and β, with decrease in temperature or increase in pressure. The molecular motions corresponding to the α and the β relaxation processes are the micro-Brownian motions of amorphous main chains and of flexible side chains, respectively. From the temperature and the pressure dependence of the average dielectric relaxation time of these polymers the single relaxation process (the α′ process) was attributed to the micro-Brownian motion of the main chain coupled with that of the side chain. The effects of temperature and pressure on the d.c. conductivity of these polymers were also studied.