利用大幅振荡剪切研究Laponite 凝胶的流变性质

通过大幅振荡剪切(LAOS)流变学方法, 研究了NaCl浓度对Laponite悬浮体系的结构及非线性黏弹性的影响. 在线性黏弹性区, Laponite体系的储能模量G′随着NaCl浓度的增加而逐渐增大. 体系的非线性黏弹性用响应应力的Fourier变换三次谐波的相对振幅I_3/1与Lissajous曲线的定量参数G_M, G_L, η_M和η_L描述. 当NaCl浓度较低时, I_3/1随应变振幅γ₀的增加而缓慢增加; 当NaCl浓度较高时, I_3/1随γ₀的增加迅速增大, 达到平台值. 不同NaCl浓度试样的G_M和G_L随γ₀变化的曲线区别不大, 但η_M和η_L随γ₀变化曲线在非线性区域出现峰值且区别较大. NaCl浓度越高, η_M和η_L出现峰值的γ₀越小, 峰值越大. 结果表明, Laponite凝胶的非线性黏弹性与凝胶网络结构有关, 随着NaCl浓度的增加, 粒子间的静电相互作用距离缩短, 粒子间距减小, 形成了更紧密的网络结构. 但这种网络在较大的应变下很容易被破坏, 出现非线性黏弹性.     

茂金属聚乙烯分子链流变学模型研究进展

茂金属聚乙烯的分子链长支化结构对其流变特性有重要影响，长支链可以使熔体弹性和剪切变稀效应明显增强。本文介绍了茂金属聚乙烯分子链模型建立的理论依据，从流变学角度综述了近年来茂金属聚乙烯分子链结构模型的研究进展，阐述了有关的长链支化结构模型及其与流变性能的关系。     

长链支化聚乳酸的多重松弛行为

在辐照法制备长链支化聚乳酸(LCB-PLA)的基础上,采用凝胶渗透色谱-多角度激光光散射联用(SEC-MALLS)表征了LCB-PLA的支化结构,利用动态流变学方法考察了PLA的黏弹松弛行为,计算得到了线型及支化PLA在较宽时间范围内完整的加权松弛时间谱.结果表明,由于长支链的引入及支链长度的增加,导致LCB-PLA松弛时间谱加宽,松弛时间增长,并呈现多重松弛行为.提出了一种计算长链支化聚合物支链长度的方法,可以定量表征LCB-PLA的支链长度以及长支链的分子量.     

基于Ziegler-Natta催化剂和ω-烯烃基甲基二氯硅烷共聚-水解化学制备长链支化高密度聚乙烯

由Ziegler-Natta催化剂所制备的高密度聚乙烯(HDPE)不含有长链支化结构,导致其加工应用性能受限.如何在Ziegler-Natta催化剂所制备的HDPE中引入长链支化结构,是烯烃聚合研究面临的长期挑战之一.本文报道通过ω-烯烃基甲基二氯硅烷共聚-水解化学,实现以Ziegler-Natta催化剂制备H型长链支化高密度聚乙烯(LCB-HDPE).以5-己烯基甲基二氯硅烷作为长链支化助剂,首先在Ziegerl-Natta催化剂催化的乙烯聚合中实现其与乙烯共聚,在聚乙烯分子链侧基上引入反应性二氯硅烷基团;聚合反应结束后对聚合物进行水处理,使近邻聚合物链上的二氯硅烷基团发生水解缩合反应,即制备H型LCB-HDPE.结果表明,Ziegler-Natta催化剂与ω-烯烃基甲基二氯硅烷共聚-水解化学的结合可实现LCB-HDPE的制备,长链支化密度可达0.15/1000C,而催化剂效率基本不受影响.所合成的LCB-HDPE的熔体流变学响应明显,随长链支化密度增加,熔体弹性增大,零切黏度提高,熔体强度和拉伸应变强化效应不断增强.同时,LCB-HDPE保持了高结晶性能,其熔点和结晶温度与线性聚乙...     

稳态荧光探针法研究复配乳化剂的聚集行为

采用荧光探针技术,根据芘的第一振动峰I₁(373 nm)与第三振动峰I₃(384 nm)荧光强度的比值(I₁/I₃)随乳化剂浓度的变化,对十二烷基磺酸钠(AS)与壬基酚聚氧乙烯醚(Oπ-10)复配乳化剂的CMC值进行了测定,并与十二烷基磺酸钠(AS),壬基酚聚氧乙烯醚(Oπ-10)的CMC值进行了比较,结果表明复配乳化剂的CMC介于两者之间.以二苯甲酮为猝灭剂,用稳态荧光法测定了不同比例复配乳化剂的胶束聚集数,实验结果表明,复配乳化剂浓度为4—9倍CMC时,测定的胶束聚集数随复配乳化剂浓度的增大而线性增大;且Oπ-10:AS<2:1时,随着复合乳化剂中Oπ-10比例的增大,复配乳化剂胶束聚集数增大.利用芘的I₁/I₃值,结合胶束微环境的极性变化规律,探讨了复配乳化剂的聚集行为对硅丙乳液聚合物的影响.     

新型染料敏化体系──荧光素染料碘鎓盐的光谱研究

本文合成了一系列荧光素类染料碘盐,并研究了其光谱及光化学性质。结果表明,通过改变结构,可改变它们的吸收光谱。提出可用I_Ⅰ/I_Ⅱ值来表征这类化合物在溶剂中的相对解离程度。随溶剂极性,染料母核C-6位周围(C-7,C-8位)空间位阻增大和染料母核上吸电子取代基增多,I_Ⅰ/I_Ⅱ值增大,它们的相对解离程度升高,光谱性质显着变化,光化学反应速度下降。在极性和非极性溶剂中,浓度对光谱和光化学性质具有不同的影响。显示这类化合物除可用作可见光敏化聚合和交联的光引发体系外,还可能用作探针探测周围环境的变化。     

α-烯烃基甲基二氯硅烷调控丙烯多相共聚制备高熔体强度高抗冲聚丙烯

作为最重要的热塑性高分子材料之一,聚丙烯在材料力学性能上的缺陷主要表现为冲击韧性尤其是低温韧性差,在熔体加工中的缺陷主要表现为熔体强度低,如何综合改善这两方面的性能是丙烯聚合和聚丙烯结构设计研究的重要问题.本文利用α-烯烃基甲基二氯硅烷调控丙烯多相共聚,在聚合反应完成后通过对聚合物进行水解处理,在共聚物中产生长链支化结构,制备了同时具有高熔体强度和高冲击韧性的新型聚丙烯.共聚物的凝胶渗透色谱(GPC)和熔体流变学测试结果均表明长链支化结构的存在,而试样断面扫描电镜(SEM)则清楚给出其以聚丙烯为基体和以乙丙无规共聚物橡胶(EPR)为分散相的相分离形态.共聚物在拉伸流变测试中表现出高熔体强度和显著的应变强化效应,在力学性能测试中显现出高缺口冲击强度.     

高分子材料支化结构与流变性的联系串讲北大核心CSCD

高分子链结构为直链型或支化型以及高分子支链的形态、多寡和长度对材料的剪切黏度、黏流活化能、熔体破裂现象等流变性能有显著影响,且与高分子材料成型加工行为密不可分。本文以串讲方式讨论了高分子材料支化结构与流变性能之间的联系。理解、串并、归纳和总结高分子支化结构与流变性之间的联系有利于使学生更加深刻领会《聚合物流变学》课程中高分子材料结构与性能间的关系,串联高分子物理、聚合物流变学、高分子材料成型加工系列专业课程知识。     

高分子材料支化结构与流变性的联系串讲

高分子链结构为直链型或支化型以及高分子支链的形态、多寡和长度对材料的剪切黏度、黏流活化能、熔体破裂现象等流变性能有显著影响,且与高分子材料成型加工行为密不可分。本文以串讲方式讨论了高分子材料支化结构与流变性能之间的联系。理解、串并、归纳和总结高分子支化结构与流变性之间的联系有利于使学生更加深刻领会《聚合物流变学》课程中高分子材料结构与性能间的关系,串联高分子物理、聚合物流变学、高分子材料成型加工系列专业课程知识。     

聚合物长支链的流变学表征方法

在聚合物中引入长支链能够显著地提高熔体强度,改善聚合物在拉伸流场中的加工性能,因此对长支链的表征显得十分重要。流变学被证实是一种非常有效且敏感的表征手段,近来受到普遍关注。本文总结了近年来国内外关于长支链聚合物流变学表征研究的最新进展,比较了不同类型流场下对长支链的流变学判断方法,尽量结合高分子链段分子结构解释了各种表征现象的含义,阐述了借助流变学对大分子拓扑结构的定性表征以及对长支链含量的定量表征方面的应用。     

Structure and shear orientation of a side chain liquid crystal polymer studied by small angle X-ray scattering

We have studied the structure and shear flow behaviour of a side chain liquid crystal polymer, namely P₁₂, in the smectic (SmA_d) phase. Simultaneous oscillatory shear and small angle X-ray scattering experiments showed that, as expected, the shearing of P₁₂ at high frequency and large strain amplitudes leads to the alignment of the smectic layer normals parallel to the neutral direction. It was also found that, on erasing the memory of the prior shear alignment through melting, P₁₂ can unexpectedly orient with the smectic layers perpendicular to the shear flow direction (b-configuration). We speculate that the b-configuration is adopted due to a counter-balance between the rather long spacer in P₁₂ which decouples mesogens from the backbone, and the oscillatory character of the shear applied to P₁₂.     

Temperature dependence of positron annihilation lifetime spectra for polyethylene: positron irradiation effects

Lifetime spectra for linear and branched polyethylene have been measured as a function of temperature. The longest lifetime, τ₃, and its intensity, I₃, are traced over a temperature range of 105–370 K. The lifetime decreases with decreasing temperature, and upon reheating the plot is retraced. I₃ has a minimum near 265 K with considerable hysteresis in the cooling/heating cycle. At the lower temperatures the increase in I₃ is attributed to source irradiation.     

Electric field effect on positronium formation in gamma-irradiated polypropylene and polyethylene

The effect of an external electric field (up to 48 kV cm⁻¹) on positronium (Ps) formation in γ-irradiated polypropylene and polyethylene has been studied by means of positron lifetime spectroscopy. The application of an electric field to unirradiated polypropylene and polyethylene considerably decreased the Ps formation. For example, the intensity of the longest-lived component (I₃) for one of the polypropylene samples was reduced from 26% to 18% by a field of 48 kV cm⁻¹. It was found that the electric field effect for the irradiated samples becomes weaker when increasing the irradiation dose. This result is discussed on the basis of the spur reaction model of Ps formation.     

Molecular and structural analysis of a triepoxide‐modified poly(ethylene terephthalate) from rheological data

The effects of reactive melt modification of poly(ethylene terephthalate) (PET) with a multifunctional epoxide—triglycidyl isocyanurate—that lead to chain extension/branching and formation of gel‐like structures were rheologically characterized. The storage and loss moduli and the complex viscosity of the modified PET were larger than those of the unmodified PET. The elastic or solidlike behavior of PET was enhanced after reactive modification as a result of chain extension/branching. Modified Cole–Cole plots revealed that the modified resins show higher elasticity than the unmodified one. Reactive modification characterized by the presence of long‐chain branching resulted in a wider molecular weight distribution. The effects of mixing temperature and the concentration of modifier corresponding to different stoichiometries were investigated. Higher amounts of modifier resulted in a polymeric structure near the sol–gel transition point whose linear viscoelastic properties obey scaling law. The relaxation spectrum was calculated from the oscillatory shear data by Tschoegl equations. Certain modified resins have rheological properties suitable for process operations such as extrusion foaming, blow molding, and thermoforming. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 958–969, 2003     

Rheology of multi‐walled carbon nanotube/poly(butylene terephthalate) composites

Multi‐walled carbon nanotube/Poly(butylene terephthalate) nanocomposites (PCTs) were prepared by melt compounding. The microstructure of PCTs was investigated using transmission electron micrographs and Fourier transform infra‐red spectrometer. The linear and nonlinear as well as transient rheological properties of PCTs were characterized by the parallel plate rheometer. The results reveal that the surface modification can improve the dispersion state of nanotube in matrix. PCTs present a low percolation threshold of about 1–2 wt % in contrast to that of Poly‐(butylene terephthalate)/clay nanocomposites. The network structure is very sensitive to both the quiescent and large amplitude oscillatory shear deformation, and is also to the temperature, which makes the principle of time‐temperature superposition (TTS) be valid on PCTs only in a very restricted temperature range. The stress overshoots to the reverse flow are strongly dependent on both the rest time and shear rate but show a strain‐scaling response to the startup of steady shear, indicating that the broken network can reorganize even under quiescent condition. The nanotube may experience the long‐range, more or less order during annealing process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2239–2251, 2007     

Distinguishing Linear from Star‐Branched Polystyrene Solutions with Fourier‐Transform Rheology

Summary: Fourier‐Transform rheology (FT rheology) was used to study the influence of the degree of branching on the nonlinear relaxation behaviour of polystyrene solutions. The results were compared with those obtained under oscillatory shear and step‐shear conditions. The different topologies could be distinguished using FT rheology where the other rheological measurements failed. Significant differences occurred under large amplitude oscillatory shear (LAOS) conditions as particularly reflected in the phase difference of the third harmonic, Φ₃, which could be related to strain‐softening and strain‐hardening behaviour. Currently, this work is extended towards different topologies in polyolefins (e.g. long chain branched).

Phase difference Φ₃ as a function of the Deborah number De at γ₀ = 2 for the polystyrene solutions measured at temperatures from 295 to 350.5 K.     

Relationship between kováts retention indices and molecular connectivity indices of tetralones, coumarins and structurally related compounds

A study was undertaken to test the ability of several molecular connectivity indices to predict the retention indices (I) of tetralones, coumarins and structurally related compounds determined on OV-17 and Apiezon L as stationary phases. The regression analyses with I_OV-17 showed that a two-variable linear regression equation with ²χ and ⁴χ^v_pc gives the best correlation coefficient, suggesting that retention depends basically on branching and the presence and number of adjacent atoms, and secondarily on unsaturations and the number and orientation of substituents. I_ApL gives the best correlation with a two-variable linear regression equation with ¹χ and ³χ^v_p indicating that on this stationary phase retention depends basically on the presence and number of adjacent atoms, and secondarily on unsaturation, branching of adjacent atoms and the presence of heteroaroms. ΔI (ΔI = I_OV-17 - I_ApL), according to the different polarities of both phases, considered to be a measure of the polar forces in retention, does not give a good correlation.     

氧化石墨烯对聚乙烯醇/硼酸水凝胶流变性能的影响

研究了氧化石墨烯(GO)对聚乙烯醇(PVA)/硼酸(borate)水凝胶线性及非线性流变性能的影响。 通过扫描电子显微镜、硼谱核磁共振波谱以及流变研究了水凝胶的流变性能。 结果表明,GO质量浓度在稀溶液区时,GO片层与PVA链间通过硼酸根离子形成了具有弹性活性的缔合点,有效地提高了水凝胶的平台模量、松弛时间和零切粘度;当GO进一步增加到亚浓溶液区,部分的交联剂被GO的团聚体捕获并处于非弹性活性的缔合状态,处于有效缔合状态的交联剂变少,导致平台模量、松弛时间和零切粘度降低。 稳态剪切测试下,样品在剪切增稠区的粘度增加随着GO添加量的增加明显加强,这与剪切场下取向的氧化石墨烯片层参与网络结构的重排有关。     

Melt rheology of polypropylene containing small amounts of high molecular weight chain. I. Shear flow

Large enhancements of the melt strength of polypropylene (PP) were achieved by the introduction of high molecular weight polyethylene (PE) into PP. The viscoelastic properties of the high‐melt‐strength PP melts under shear flow were investigated. It was found that the rheological properties of the high‐melt‐strength PP were distinctly different from those of conventional PP. The elastic response at low frequencies was significantly enhanced in comparison with the conventional PP, implying a presence of a long relaxation time mode that was not revealed in conventional PP. In step‐shear measurements, the fast and slow relaxation processes that characterized the linear viscoelastic properties were observed also for nonlinear relaxation moduli. The dependence of the damping for the slow process of the high‐melt‐strength PP on shear strain was much weaker than that of the fast process. These rheological behaviors characterizing the long relaxation time mode were further enhanced with the increasing concentration of high molecular mass PE. The unusual shear rheological behaviors were discussed in view of the role of high molecular weight PE as a long relaxation time mode within PP. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2692–2704, 2001