【作者】 赵丽；

【导师】 郑强； 杨红梅；

【作者基本信息】 浙江大学 ， 材料加工工程， 2011， 博士

【摘要】 气相纳米碳纤维(VGCF)具有优异的力学、导电、导热性能,可与碳管媲美,是一种极具潜力的增强体与导电填料。VGCF填充聚合物导电复合材料的性能与VGCF在基体中的分散及其形成的聚集体、网络结构密切相关。论文以VGCF填充聚苯乙烯(PS)、聚丙烯(PP)为模型体系,考察原料参数、加工工艺、外场作用等因素对其结构、导电性能与流变行为的影响,以揭示VGCF填充体系黏弹行为的本质,建立结构-导电性能-流变行为间的关联,并考察外场作用下VGCF结构的演化机制。采用两相模型描述PS/VGCF复合体系的线性动态流变参数,将填充体系的复数模量分解为基体相、填料相各自独立的贡献。研究结果表明,VGCF长径比越大,越易在基体相中相互搭接形成粒子网络结构,导致应变放大因子(Af)、填料相模量增大,填料相模量的频率依赖性降低。PS/VGCF体系的Payne效应随VGCF体积分数(φ)增大而变得更加明显,随增容剂苯乙烯-马来酸酐共聚物(SMA)的加入而减弱。大应变(γ)区域,考虑应变放大效应,复合材料的储能模量(G’)～γ与损耗模量(G’)～γ曲线均可平移叠加至PS的相关曲线。这表明填充体系大应变区域的Payne效应取决于基体分子链的解缠结。导电-流变同步测试与透射电镜(TEM)观测发现,VGCF在剪切作用下发生取向,使得VGCF间搭接点破坏、间距增大、表面PS链可能发生解吸附,导致小γ区域Payne效应的形成。填充体系的非线性松弛行为符合时间-应变可分离原则。φ≤8vol%时,填充体系以基体PS的松弛为主；φ≥12vol%时,填充体系还体现出明显的填料相松弛行为。增容剂SMA基本不影响体系的非线性松弛行为。导电-流变同步测试结果表明,剪切作用下VGCF网络结构的变化强烈依赖于作用模式：大γ振荡剪切造成VGCF网络结构崩溃；瞬态剪切仅造成小范围VGCF间搭接点的破坏。热处理、PS冷却固化均可促进VGCF网络结构的回复。就不同填充体系受剪切后的回复性而言,VGCF明显弱于炭黑等近似球形粒子。在PP/VGCF复合体系中,PP基体结晶(等温或非等温)首先造成渗流网络破坏,结晶程度超过一定程度后,VGCF在PP无定形区域聚集,重新形成渗流网络。在PP结晶过程中,VGCF网络的破坏和回复过程与VGCF尺寸、φ密切相关。VGCF直径越大、长径比越小、φ越低,VGCF网络对外场刺激的响应越强。熔融混合工艺(记为混合温度-转速-时间)对VGCF有显著破坏作用,即使采用短时间弱剪切工艺(如190℃-30rpm-2.5min), PS基体中VGCF长度也由原料的35μm降至约15μm。调控熔融混合工艺可实现VGCF在PS基体中分布结构的可控。剪切作用弱于190℃-30rpm-10min、190℃-60rpm-1 Omin时,复合体系内部可见明显的VGCF团聚体结构,VGCF分散性差。在混合加工过程中,转速越高、混合时间越长,VGCF在基体中的分散性越好,填料相模量越低,复合体系导电性越差。VGCF填充体系的稳态流变行为具有明显的φ、VGCF尺寸依赖性。低剪切速率下,VGCF填充体系在φ(φ=2 vol%)较低时既呈现剪切变稀现象。φ越高、VGCF直径越小、长径比越大,剪切变稀现象越明显。高剪切速率下,添加VGCF可显著提高VGCF填充体系的粘流活化能。VGCF的加入可降低填充体系的熔体弹性,明显抑制挤出胀大效应。φ越高、VGCF直径越小、长径比越大,VGCF降低熔体弹性、抑制挤出胀大效应的作用越强。更多还原

【Abstract】 As one of the most important novel fillers, vapour grown carbon nanofibers (VGCFs), have extraordinary mechanical, thermal and electrical properties similar to carbon nanotube (CNT) and have been used as one of the most potential reinforcing materials and conductive fillers. The final performances of polymer/VGCF composites are always related to the dispersion, agglomeration and network formation of VGCF in the matrix.In this thesis, VGCF-filled polystyrene (PS) or polypropylene (PP) have been selected as research models. Influences of raw materials, mixing conditions and external fields on filler-phase structures, conductivity and rheological behaviors of the composites have been investigated in order to probe mechanisms of linear or nonlinear rheologies for VGCF-filled polymer, and detect response of VGCF network to actions of external fields, and establish relationship among VGCF-phase structure, conductivity and rheological behavior.The linear rheological behavior of VGCF-filled PS can be well fitted using the two phase model thus the global complex modulus of VGCF-filled polymers can be divided into a certain strain amplification e(?)ect and a structural contribution of the filler phase. These composites with high level of VGCF entanglement and contact have high values of strain amplification factor (Af) as well as characteristic moduli of the filler phase.Effect of volume fraction (φ) and aspect ratio of VGCF and coupling agent styrene-maleic anhydride copolymer (SMA) on nonlinear behaviors of VGCF-filled PS was investigated. The results indicate that the strain-softening strength increases with increasing (p and aspect ratio of VGCF, and decreases in case of SMA adding. Using Af arising from hydrodynamic effect as vertical and horizontal shifting factors, curves of the dynamic storage modulus (G) and the dynamic loss modulus (G") as a function of y for the composites can be superposed on those of pure matrix in large y region, suggesting that the matrix provides the main contribution to strain softening in large y region. Significant deflection from the master curves can be observed in small y region and the deflection becomes more marked at higher (p. Electrical resistance (R) tested as a function of y and TEM micrographs provide direct evidences for breakdown of filler-filler contact by y perturbation. Moreover, fiber slippage and orientation maybe accompanied by debonding of PS chain from the fiber surface, result in strain softening in small y region.Nonlinear relaxation behaviors of PS/VGCF composites follow time-strain separation principle. Whenφ≤8 vol%, composites show one relaxation behavior mainly arising from the matrix. When (φ≥12 vol%, composites exhibit another long-time relaxation due to filler network. Addition of SMA has no effect on nonlinear relaxation of composites.The evolution of VGCF network under shearing, heating or solidification of matrix was studied by using simultaneous measurements of G’and R. The results show that shearing destroys the percolation network and the destruction degree depends on the shearing mode. The VGCF network collapses under 100% oscillation strain shear and while it is just partly destructed under 1 s-1 steady shear. Moreover, thermal treatment and solidification of PS affect VGCF-network reformation. In PP/VGCF composites, crystallization of PP destroys the VGCF network firstly. When crystallinity is up to a certain value, VGCFs gather in the amorphous region to form a percolation network, and the shrink in volume upon cooling causes VGCF to get closer together.The processing-microstructure-property relationship for PS/VGCF composites was investigated. A remarked decrease in fiber length from 35 to 15μm was observed after mixing at low-shear condition (190℃-30rpm-2.5min). Composites with different dispersion levels of VGCF can be formulated by changing the mixing conditions. At low rate and short time mixing conditions, the composites have high Af values, characteristic moduli of the filler phase as well as electrical conductivity due to high entanglement level of VGCF.The effect ofφand VGCF size on viscosity and die-well ratio of VGCF filled polymers was studied through steady rheology measurement. Composites, containing 2 vol% VGCF exhibit shear thinning even at low shear rate and the shear thinning became more significant with increasingφand aspect ratio of VGCF. Addition of VGCF into polymer may increase flow activation energy and decrease primary normal stress difference (N1) and die-well ratio.It is noted that these effects were more significant with increasing (p and aspect ratio of VGCF.更多还原