分子量分布对双烯聚合物粘弹性能的影响——Ⅱ.顺-1,4-聚异戊二烯的粘弹性能对分子量及其分布的依赖关系

本文利用凝胶渗透色谱和应力松弛方法研究了顺-1,4-聚异戊二烯的粘弹性能对分子量及分布的依赖性。实验结果表明,生胶的松弛模量和最长松弛时间与分子量和分子量分布都有依赖关系,主要是改变τ_m=KM_w~6关系中的K值,对β值的影响甚小。由于本体聚合物的链缠结而导致的非牛顿效应使生胶的τ_m与M_w的关系偏离3.4法则,这可用缠结网络密度来校正。探讨了生胶的应力松弛过程是橡胶分子链的滑移和解缠结兼有的两种运动。低分子量级份对链缠结网络有显著影响,起稀释剂作用,使网络的临界缠结分子量增高。     

分子量分布对双烯聚合物粘弹性能的影响—Ⅱ.顺-1,4-聚异戊二烯的粘弹性能对分子量及其分布的依赖关系

本文利用凝胶渗透色谱和应力松弛方法研究了顺-1,4-聚异戊二烯的粘弹性能对分子量及分布的依赖性。实验结果表明,生胶的松弛模量和最长松弛时间与分子量和分子量分布都有依赖关系,主要是改变τ_m=KM_w^β关系中的K值,对β值的影响甚小。由于本体聚合物的链缠结而导致的非牛顿效应使生胶的τ_m与KM_w的关系偏离3.4法则,这可用缠结网络密度来校正。探讨了生胶的应力松弛过程是橡胶分子链的滑移和解缠结兼有的两种运动。低分子量级份对链缠结网络有显著影响,起稀释剂作用,使网络的临界缠结分子量增高。     

MMA接枝改性PVC/CaCO3纳米复合材料的力学性能

采用熔融共混法制备PMMA接枝改性纳米CaCO3增韧PVC（PVC/CaCO3）复合材料,并研究了复合材料的力学性能.结果表明,通过表面PMMA的接枝改性,可以显著提高纳米CaCO3增韧聚氯乙烯复合材料的拉伸强度和拉伸模量,在纳米CaCO3颗粒表面PMMA包覆层厚度为2nm时,复合材料的拉伸强度和拉伸模量达到极大值.对比于未处理纳米CaCO3和钛酸酯偶联剂处理纳米CaCO3,PMMA接枝改性纳米CaCO3增韧PVC复合材料的拉伸强度得到较大幅度提高.SEM显示,经过PMMA接枝改性后的碳酸钙在PVC基体中分散均匀,与基体界面结合良好.     

长碳链聚醚酰胺弹性纤维开发及其弹性机理

以长碳链聚酰胺弹性体为原料,通过熔融纺丝技术制备了高性能化新型长碳链聚醚酰胺(LPAE)弹性纤维.该弹性体是以基于生物来源单体的长碳链聚酰胺为硬段,以聚醚为软段,其弹性可通过调节软硬段比例有效调控.测试结果表明,与目前市场上应用广泛的氨纶莱卡(LYCRA)相比,软段含量较高的LPAE纤维具有高断裂伸长,低初始模量的特点;在200%伸长范围内,其弹性回复率与氨纶相当,耐热性优于氨纶.分析得知,LPAE纤维的高弹性源于硬段聚酰胺存在强氢键相互作用且结晶度高,同时硬段充当物理交联点;软段具有良好的柔性,可以发生大变形,这种软硬段交替的嵌段分子链结构形成三维网络.大应变下,LPAE纤维弹性回复率降低是由分子链滑移及软段拉伸诱导结晶共同造成的.     

几种高聚物拉伸过程中的声发射现象

研究了４种塑料ＰＳ、ＰＯＭ、ＡＢＳ和ＰＣ拉伸过程中的声发射现象，发现声发射信号主要产生在屈服前和断裂时，说明高聚物的声发射信号主查由于分子链的解缠结，晶体的滑移裂纹的亚临界扩展和分子链的断裂等诸因素产生的，ＡＢＳ拉伸时大面积银纹的生成也可能是信号源，拉伸应变速率，材料的结构和不同的屈服过程等对塑料的拉伸声发射规律有较大影响。     

炭黑填充聚苯乙烯熔体流变行为

研究了炭黑(CB)填充聚苯乙烯(PS)熔体的稳态和动态流变行为. CB/PS复合体系在CB体积分数φ=0.06时发生逾渗转变. 结果表明, 低应变区熔体模量降低主要归因于粒子-粒子及粒子-高分子间作用力的破坏, 高应变下模量的急剧下降则主要与高分子链间解缠结有关. 采用“两相”模型拟合线性动态流变行为, 发现应变放大因子A_f(φ)、填充相模量及松弛指数与温度有关. A_f(φ)~φ关系符合Guth方程和扩散控制的粒子簇聚集模型. “粒子相”形状参数与聚集体分维度均随温度升高而有所降低, 说明CB粒子聚集体因团聚而趋于各向同性, 应变放大效应减弱. “粒子相”特征模量G'_f1(φ)和G"_f0(φ)与φ关系满足标度律. 当φ > 0.06时, G'_f1(φ)和G"_f0(φ)及其标度指数均随温度升高而明显降低, 其G'_f1(φ)变化幅度略大于G"_f0(φ), 说明“粒子相”弹性与黏性组分具有不同的温度依赖性. 随着温度升高, 扩散控制的CB粒子团聚过程加快, 应变放大效应减弱.     

新型聚酯增塑剂的合成及增塑聚氯乙烯性能

以生物质化工产品衣康酸加氢产物2-甲基丁二酸为原料,采用熔融缩聚法制备了一种新型聚酯增塑剂聚2-甲基丁二酸1,3-丙二醇酯,采用红外光谱(FTIR)和核磁共振(1H NMR)对聚酯的结构进行了表征.通过扫描电镜(SEM)、动态力学分析(DMA)、拉伸测试对该聚酯增塑聚氯乙烯(PVC)的增塑效果进行的表征结果表明,该聚酯与PVC相容性良好,可大大改善PVC材料的硬度,增加其断裂伸长率,降低PVC的玻璃化转变温度、拉伸强度及拉伸模量.与小分子增塑剂邻苯二甲酸二辛酯(DOP)相比,合成的新型聚酯增塑剂具有优异的耐抽出性、耐挥发性和耐迁移性,可提高PVC材料的使用寿命.     

高斯链网络模型与橡胶交联网络的差异

“橡胶弹性”是《高分子物理》中联系长链分子构象统计和高分子材料力学性能的重要章节之一。现行《高分子物理》教科书中有关内容多以Rehner-Flory“四链模型”为基础,从Gaussian单链熵、单链熵加和恒体积仿射形变角度推导Gaussian链网络构象熵和收缩力,而很少提及Gaussian链网络模型化过程所涉及的诸多近似,更不提及有关学术争议。基于现行《高分子物理》教程,不少学生认为硫化胶具有类似Gaussian链网络的交联结构,不少技术人员基于所谓理想网络结构来设计所谓大应变可逆变形柔性电子等前瞻性器件,在理论和实践上走了不少弯路,得出不少误导性结论。本文回顾Gaussian链网络模型化所涉及的基本理论、近似和争议,以便让读者认识到Gaussian链网络模型仅是具有一系列约束条件的“模型”,而与橡胶交联结构相差甚远。     

聚苯基单醚喹噁啉薄膜的形变机理

非晶聚合物塑性变形机理主要包括银纹化和剪切屈服[1 ,2 ] .银纹化是链段局部排列疏松区域或缺陷在膨胀应力作用下成为银纹核 ,引发银纹 ,银纹 本体界面应变软化 ,银纹微纤拉伸的应变硬化过程 ,使得聚合物银纹微纤沿拉伸方向取向 ,伴随这一过程聚合物的体积增大[3] .剪切屈服是分子链沿拉伸方向的流动以及分子链间的滑移过程 ,这一过程使聚合物形状改变而体积不变 .聚合物的形变机理与聚合物的内在性质如临界缠结分子量 ,缠结密度或硬度等有关[4] .聚苯基单醚喹啉是一种高性能的芳杂环聚合物 ,它的玻璃化转变温度是 2 98℃ ,它具有耐高温…     

链缠结对聚合物结晶行为的影响

链缠结是聚合物分子链相互作用的一种形式 ,它主要影响分子链的长程运动 .自链缠结的概念被提出以来 ,人们对聚合物粘弹性、流变行为和网络平衡力学等进行了理论研究和实验验证 .然而 ,在聚合物结晶领域 ,链缠结对结晶过程的影响一直存在着很大的争论 .Flory等 [1] 认为 ,聚合物结晶时分子链根本没有足够的时间进行构象调整 ,分子链进入晶格后 ,使大量的缠结链段被挤入非晶区 ,并由此建立了聚合物结晶“插线板”模型 .Hoffman等 [2 ]根据单根分子链从过冷熔体“卷饶”到晶体前沿所需的时间进行估算 ,结果比 Flory预言的快约 3～ 5个数量级…     

Inverse temperature dependence of strain hardening in ultrahigh molecular weight polyethylene: role of lamellar coupling and entanglement density

Irradiation of ultrahigh molecular weight polyethylene (UHMWPE) with a dose of 150 kGy by an electron beam can effectively increase the entanglement density in the amorphous phase and has little influence on the properties of the crystalline phase, which provides examples to comparatively investigate the role of lamellar coupling and entanglement density in determining the strain-hardening effect in semicrystalline polymers. The strain-hardening modulus, deduced from the Haward plots of true stress-strain curves, is inversely temperature-dependent and has a sharp transition around 65 degrees C that corresponds to the mechanical alphaI-process of the crystalline phase for both nonirradiated and irradiated samples, irrespective of the entanglement density in the amorphous phase. Lamellar coupling takes more effect in determining the strain-hardening behavior before the mechanical alphaI-process is activated. With further increasing temperature, lamellar coupling becomes weaker and the role of the entangled amorphous phase is gradually presented. However, the same temperature dependence of the strain-hardening modulus in both nonirradiated and irradiated samples indicates that the strain-hardening behavior in semicrystalline polymer is mostly determined by lamellar coupling rather than by entanglement density.     

REINFORCEMENT OF POLYDIMETHYLSILOXANE NETWORKS BY NANOCALCIUM CARBONATE

Although a number of investigations have been devoted to the analysis of silica or carbon black filled elastomer networks, little work has been done on the reinforcement of CaCO3 filled elastomer network. In this work, the reinforcement of polydimethylsiloxane (PDMS) network by using CaCO3 nano-particles was investigated. We have found a simultaneous increase of tensile strength, modulus and elongation with the increase in nano-CaCO3 content, which suggests that nano-CaCO3 particles can indeed be used as a reinforcing agent, just like silica or carbon black. Interestingly, the tensile strength,modulus and elongation were seen to leave off for the first time when the content of nano-CaCO3 particles reaches to 80%.PDMS also showed an enhanced elastic modulus and storage modulus with the increase in nano-CaCO3 content, particularly for samples with high nano-CaCO3 content. SEM was used to investigate the dispersion of the filler in PDMS matrix. A better dispersion was found for samples with high nano-CaCO3 content. A great increase of viscosity was found for samples with higher filler content, which is considered to be the reason for the good dispersion thus the reinforcement, because high viscosity will be helpful for breaking the agglomerates of fillers into small size particles under effect of shear. Our work provides a new way for the reinforcement of elastomer by using an adequate amount of nano-CaCO3 particles instead of as mall quantity of silica, which is not only economically cheap but also very effective.     

Effect of chain entanglements on plastic deformation behavior of ultra‐high molecular weight polyethylene

Samples of ultra‐high molecular weight polyethylene, in which the chain topology within the amorphous component was altered using two‐stage processing, including crystallization at high pressure in the first step, were produced and their deformation behavior in the plane‐strain compression was studied. Deformation and recovery experiments demonstrated that the state of the molecular network governed by entanglement density is one of the primary parameters controlling the response of the material on the imposed strain, especially at moderate and high strains. Any change in the concentration of entanglements markedly influences the shape of the true stress–true strain curve. The strain hardening modulus decreases while the onset of strain hardening increases with a decrease of the entanglement density within the amorphous component. Density of entanglements also influences the amount of rubber‐like recoverable deformation and permanent plastic flow. In material of the reduced concentration of entanglements permanent flow appears easier and sets in earlier than in the material with a higher entanglement density, becoming a favorable deformation mechanism at moderate strains. As a result, strong strain hardening is postponed to higher strain when compared with the samples of equilibrium entanglement density. In the samples of an increased entanglement density the molecular network becomes stiffer, with a reduced ability of strain induced disentangling of chains. Consequently, there is a less permanent flow and strain hardening begins earlier than in the reference material of an unaltered chain topology. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 276–285, 2010     

Effect of carbon nanofibers on mechanical and electrical behaviors of acrylonitrile‐butadiene rubber composites

Recently, carbon nanofibers have become an innovative reinforcing filler that has drawn increased attention from researchers. In this work, the reinforcement of acrylonitrile butadiene rubber (NBR) with carbon nanofibers (CNFs) was studied to determine the potential of carbon nanofibers as reinforcing filler in rubber technology. Furthermore, the performance of NBR compounds filled with carbon nanofibers was compared with the composites containing carbon black characterized by spherical particle type. Filler dispersion in elastomer matrix plays an essential role in polymer reinforcement, so we also analyzed the influence of dispersing agents on the performance of NBR composites. We applied several types of dispersing agents: anionic, cationic, nonionic, and ionic liquids. The fillers were characterized by dibutylphtalate absorption analysis, aggregate size, and rheological properties of filler suspensions. The vulcanization kinetics of rubber compounds, crosslink density, mechanical properties, hysteresis, and conductive properties of vulcanizates were also investigated. Moreover, scanning electron microscopy images were used to determine the filler dispersion in the elastomer matrix. The incorporation of the carbon nanofibers has a superior influence on the tensile strength of NBR compared with the samples containing carbon black. It was observed that addition of studied dispersing agents affected the performance of NBR/CNF and NBR/carbon black materials. Especially, the application of nonylphenyl poly(ethylene glycol) ether and 1‐butyl‐3‐methylimidazolium tetrafluoroborate contributed to enhanced mechanical properties and electrical conductivity of NBR/CNF composites.     

High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black

Rubber composites with very high moduli at low elongation, high elongation at break and high ultimate breaking strength have been developed. The matrix was acrylonitrile butadiene rubber (NBR) and the hybrid (fibrous and particulate) reinforcements were short, fine pineapple leaf fiber (PALF) and carbon black. The amount of PALF was fixed at 10 parts (by weight) per hundred of rubber (phr) while that of carbon black was varied from 0 to 30 phr. Uniaxial NBR composites were prepared. Tensile strength, elongation at break, modulus and tear strength of the hybrid composites were characterized in both longitudinal (parallel to the fiber axis) and transverse (perpendicular to the fiber axis) directions. The addition of carbon black causes the slope of the early part of the stress–strain curve to increase and also extends breaking to greater strains. At carbon black contents of 20 phr and above, the stress–strain relation displays an upturn at high elongations, providing greater ultimate strength. Comparison with the usual carbon black filled rubber shows that the composite behavior at low strains is determined by the PALF, and at high strains by the carbon black. This high performance PALF-carbon black reinforced NBR shows great promise for engineering applications.     

Preparation and properties of gluten/calcium carbonate composites

Environment friendly thermosetting composites were prepared by blending wheat gluten （WG） as matrix, calcium carbonate （CaCO3） as filler and glycerol as plasticizer followed by compression molding the mixture at 120 ℃ to crosslink the WG matrix. Morphology observation showed that the CaCO3 particles were finely dispersed in matrix. Incorporation of CaCO3 up to 10 wt% into the composites caused Young＇s modulus and tensile strength to increase markedly. On the other hand, the moisture absorption and elongation at break decreased slightly.     

Structure-property relationship of highly crosslinked rubber-iron oxide composite based on chloroprene rubber (CR) as well as on nitrile rubber (NBR); a comparative study using different models

Abstract

Highly crosslinked elastomer-iron oxide composite for grinding as well as for polishing application. With the recent introduction of organic acid-based coolants in polishing applications, the designed composites should have good resistance to oils. This investigation reports the preparation and properties of high crosslinked elastomer-iron oxide composites based on Chloroprene Rubber (CR) as well as on Nitrile Rubber (NBR) as main elastomer matrix and their comparative study. In NBR system, a small amount of natural rubber (NR) was used to improve the abrasion resistance. The crosslink density (CLD) was determined from the plateau modulus in DMA using Nielsen’s model. CLD was also determined based on the equilibrium-swelling ratio by using Flory-Rehner model. The CLD at lower cure time estimated by both methods was substantially different. Nevertheless, it converges to a common value at highly crosslinked state. Highly crosslinked CR shows 500% higher modulus at high temperature when compared to the NBR system.

Highly crosslinked elastomeric composites based on Chloroprene Rubber (CR) as well as Nitrile Rubber (NBR) with high iron oxide content were prepared. This investigation gives insights into the fabrication of composites and evaluates the network structure of highly crosslinked composites. Different models were used to characterize the elastomeric network structures in the composites.     

A time-integrated estimate of the entanglement mass in polymer melts in agreement with the one determined by time-resolved measurements

We make a critical examination of how the entanglement molecular mass Me is determined from various measurable quantities. We are guided by reptation theory, where it is assumed that characteristic relaxations abruptly change and become equal to those of a chain moving in a Gaussian tube, as soon as the corresponding length scales surpass the tube diameter d or similarly as soon as the corresponding mass surpasses a critical value. Taking this critical mass as a definition of the "reptational" entanglement mass, we observe that all methods based on time-resolved quantities, such as the single-chain dynamic structure factor S(q,t) and the zero-shear relaxation modulus G(t), give the same result. We observe that such a value differs, beyond error bars, from that obtained from the plateau modulus, which is a time-integrated quantity. We have investigated an alternative definition of entanglement mass in terms of time-integrated quantities and observe that the value of this specific entanglement mass is consistent with that obtained from the time-resolved observables. We comment on possible reasons for the plateau modulus discrepancy.     

Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50–300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles (‘holes’). The value of the critical matrix ligament thickness (ID_c) uniquely depends on the molecular structure: at an increasing network density, ID_c increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency.