增塑化学微交联聚氯乙烯中化学交联和物理交联的协同作用

由氯乙烯／ 邻苯二甲酸二烯丙基酯（ＶＣ／ＤＡＰ） 悬浮共聚合成了化学微交联聚氯乙烯（ＰＶＣ） 树脂,并进行增塑加工．共聚得到的化学交联ＰＶＣ 具有溶胶／ 凝胶分配特性,交联密度较低;化学交联ＰＶＣ 的溶胶和凝胶均存在分子链缠结作用,尤其当凝胶含量较高时,物理缠结对凝胶交联密度有较大贡献．化学交联对增塑ＰＶＣ 结晶性的影响较小,因此在增塑化学微交联ＰＶＣ 中同时存在化学交联网络和以分子链物理缠结点和微晶为交联点的物理交联网络,两者协同影响增塑ＰＶＣ 材料的性能．     

聚丙烯酰胺/水溶性酚醛交联反应机理的模拟

采用密度泛函、分子力学和分子动力学等方法从交联反应热力学、动力学等方面研究了聚丙烯酰胺/水溶性酚醛体系的交联机理. 以丙酰胺(PA)和(2,4,6)-三羟甲基苯酚(THP)为模型化合物, 采用密度泛函方法分别计算了体系中所有可能发生反应的吉布斯自由能和反应能垒. 研究结果表明, PA/THP共缩合与THP自缩合反应在热力学上都具有自发性. 其中, PA/THP邻位共缩合反应的能垒最小, 仅64.54 kJ/mol, 因此该反应的活性最大. 构建了聚丙烯酰胺/THP体系的交联网络模型, 模拟计算得到参予交联反应的-CONH₂为其总量的60%. 对交联产物进行X射线光电子能谱测试, 结果表明, 交联产物中仲胺含量与伯胺含量之比约为1.5∶1, 从而证实了模拟结果.     

基于多肽结构的聚合物水凝胶

多肽由于具有良好的生物相容性和生物可降解性、生物活性以及自组装特性, 近年来受到了广泛的关注。将多肽自组装特性引入到聚合物中,可赋予聚合物形成凝胶性并对凝胶网络分子结构做出一定控制,进而使凝胶具有如环境响应、力学可调等结构控制性能;将特殊功能性多肽引入到化学交联的聚合物凝胶网络中,可赋予水凝胶生物功能性,如细胞黏附、酶降解、抗菌等;将多肽的凝胶网络构建、结构控制作用以及功能性同时引入获得的物理/化学双重交联凝胶不仅赋予水凝胶一定的功能性,且多肽自组装贡献的物理交联结构还能对化学交联凝胶网络起增强作用。本文综述了基于多肽自组装的物理交联聚合物水凝胶、多肽功能化的化学交联聚合物水凝胶以及基于多肽的物理/化学双重交联的聚合物水凝胶,并展望了这些水凝胶的发展前景。     

四氯化碳存在下大孔苯乙烯-二乙烯苯共聚物的后交联反应

对在四氯化碳存在下 ,大孔St DVB共聚物以三氯化铝为催化剂的后交联反应进行了研究 .结果表明 ,起始共聚物的合成条件 ,包括交联剂DVB的用量 ,致孔剂的性质及其与单体的配比 ,对后交联产物的孔结构有重要影响 .以良溶剂甲苯为致孔剂及采用较高的DVB用量 ,所合成的起始共聚物经后交联后 ,比表面积很容易达到或超过 10 0 0m2 /g .一般情况下 ,后交联产物的孔容增加 ,孔径减小 ,孔结构稳定性明显提高 .当起始共聚物的DVB含量较低时 ,后交联反应以四氯化碳的参与为主 ;当DVB含量较高时 ,后交联反应以起始共聚物中固有的悬挂双键的参与为主     

双向形状记忆结晶聚合物及其复合材料的研究进展

形状记忆聚合物是一种典型的智能材料,具有质轻、形变量大、可对多种刺激进行响应等优点.根据形状记忆过程的可逆性进行分类,形状记忆效应可以分为2种:单向与双向形状记忆效应.与不可逆的单向形状记忆过程相比,双向形状记忆过程是可逆的,样品不需要使用者进行再次变形,就可以在原始形状与临时形状之间进行可逆转换,因此其具有极高的实用价值与广阔的应用前景,受到各国研究人员的广泛关注,成为当前的研究热点之一.本文总结了近年来所研究的双向形状记忆结晶聚合物及其复合材料,包括恒外力条件下(外力≠0)的准双向形状记忆结晶聚合物,无外力条件下的双向形状记忆结晶聚合物及其复合材料.具体来说,前者包括在恒外力作用下的化学或物理交联的结晶聚合物.后者包括双层或核-壳聚合物复合材料、由分步交联得到的双网络交联结晶聚合物、化学交联的双组分结晶聚合物、具有较宽熔融转变的化学交联结晶聚合物与物理交联的结晶聚合物.重点关注了这些材料的制备方法、影响因素及相应的双向形状记忆机理,并对其研究前景进行了展望.     

高分子黏弹性的经典唯象模型

高分子流体的黏弹性质是高分子科学和工程中一个非常重要的研究领域.与简单黏性液体和弹性固体不同,外场作用下缠结高分子流体呈现出复杂的黏弹性行为,例如应力不仅仅与应变幅度或应变速率有关,还与整个形变历史相关.近半个世纪以来,人们建立了很多描述这些复杂黏弹性质的模型和理论,其中一类是基于连续性介质力学原理的唯象模型,例如:Maxwell模型、Voigt-Kelvin模型和在时空上所有参数为常量的连续性模型;另一类是瞬态网络模型,该模型把缠结点考虑成瞬态交联点,高分子链看成珠簧链;还有一类是微观分子理论,其中最著名的是"管子模型".本文首先介绍缠结高分子流体的线性黏弹性响应和Boltzmann叠加原理的基本概念,然后,重点综述描述高分子黏弹性质仍非常有实际应用价值的3个经典唯象模型,包括Maxwell模型、Voigt-Kelvin模型和瞬态网络模型,特别是这些理论的详细推导和存在的主要问题.关于高分子黏弹性的微观理论将在其它综述中单独介绍.     

基于超支化聚合物两性水凝胶的制备及性能

通过N-丙烯酰-1,2-乙二胺盐酸盐(ADE)的Michael加成反应制备阳离子超支化低聚物聚N-丙烯酰-1,2-乙二胺盐酸盐(HADE),以HADE为大分子单体,以丙烯酰胺(AAm)和丙烯酸(AAc)为单体,在无需外加有机交联剂的条件下制备具有高机械强度的两性聚电解质水凝胶(HAH凝胶).结果表明,HAH凝胶可以被压缩超过99%的形变而不断裂,压缩强度高达61.2 MPa;HAH凝胶的断裂伸长率和断裂强度分别达到1700%和70.2 k Pa.由于HADE末端伯胺基与强氧化引发剂通过氧化还原反应生成胺自由基和自身结构中的双键同时参与聚合反应,因而为凝胶网络形成提供了必要的化学交联作用.同时HADE结构中胺基正电荷与AAc的羧基负电荷之间的离子交联也为凝胶网络提供了物理交联作用.2种交联作用的协同作用是HAH凝胶具有良好机械性能的根本原因.     

聚电解质基的韧性双网络水凝胶纤维

在聚电解质网络中引入氢键作用或共价键作用,分别形成物理-物理交联双网络和物理-化学交联双网络水凝胶纤维,纤维表面经水分蒸发形成类蜘蛛丝样的核-壳纤维结构。这种聚电解质基的水凝胶纤维展现了114.5MPa的高断裂强度、41.73MJ/m~3的高韧性、90%的高阻尼以及湿度响应的超收缩行为。     

EPR-g-PS热塑弹性体结构与性能的关系

通过聚苯乙烯大分子单体与乙烯、丙烯共聚得到的乙丙橡胶(EPR)为主干、聚苯乙烯(PS)为支链的接枝共聚物EPR-g-PS,在无化学交联的情况下,模压样品具有与三元乙丙硫化胶可比拟的强度(PS=37.5％,M_(300％)=113公斤/厘米~2,抗张强度=198公斤/厘米~2,伸长率=705％)。聚苯乙烯含量、平均支链数目和成型条件是影响接枝共聚物力学性能的主要因素。聚苯乙烯微区除了以热塑交联网络起到物理交联作用外,在应力作用下本身还发生非弹性形变,起到补强填料的作用,使EPR-g-PS表现出热塑弹性体的性质。     

基于遥爪型大分子交联剂的动态化学交联水凝胶的黏弹性质

以壳聚糖为主链,以双端苯醛基聚乙二醇(DF-PEG)为交联剂,以希夫碱为动态交联键制备了动态化学交联水凝胶,通过改变大分子交联剂的分子量和浓度调控凝胶网络结构。以旋转流变仪的动态频率扫描和稳态剪切为主要手段,研究了凝胶结构对凝胶模量、松弛时间、剪切增稠程度的影响。结果表明:DF-PEG的分子量和浓度会影响凝胶网络内弹性活性链的密度从而影响模量、松弛时间,而凝胶剪切增稠程度与弹性活性链密度密切相关。     

Elastic properties of crosslinked poly(vinyl alcohol) gels: Network topology

Current network theory exhibits inconsistencies which show up particularly clearly in deformation of networks prepared by crosslinking a polymer in solution. A check of theory can be obtained if one knows precisely the number of crosslinks in the network and if a range of deformations is applied to the network. In an effort to explore this problem we have examined the relation of shear modulus to crosslink density, primary molecular weight, and polymer concentration for a series of poly(vinyl alcohol) gels at low to intermediate concentrations. Aqueous poly(vinyl alcohol) solutions were crosslinked to form infinite networks using terephthalaldehyde. We find a large discrepancy with these poly(vinyl alcohol) gels between measured shear modulus and that calculated from classical elasticity theory assuming quantitative reaction of crosslinking. The ratio of measured to calculated modulus is independent of crosslink density for a given primary molecular weight and concentration. It shows linear dependence on polymer concentration prior to crosslinking and extrapolates to a critical concentration which is consistent with the effective sizes of the polymer molecules.     

Influence of crosslinking on physical properties of low density polyethylene

Influence of crosslinking on physical properties of low density polyethylene was studied.The results indicated that,at low degrees of crosslinking,the network hardly affects the crystallinity,elastic modulus(E) and yielding stress while it improves the tensile strength and strain at break simultaneously.Tensile strength reaches a maximum of about 24 MPa at 1.5 phr dicumyl peroxide(DCP) then decreases to a constant value of about 18 MPa due to decrease of crystallinity.E reaches its maximum at 0.5 phr DCP corresponding to gel fraction of about 75%without marked change in crystallinity.The crosslinked polyethylene exhibits two yielding processes,and both yielding stresses approximately linearly depend on crystallinity.     

Properties of poly(vinyl chloride) crosslinked by new difunctional reagents

Alkaline and alkaline earth salts of either dimercaptans obtained by duplication of o-mercaptobenzoic acid with α, ω-diols or of o-mercaptobenzoic acid have been synthesized. Their efficiency has been studied with respect to induction time before change in melt viscosity and crosslinking rate assessed with Haake plasticorder equipped with a Rheomix 600 internal mixer. Then efficiency with respect to crosslinking was also characterized by the assessment of the insoluble fraction in tetrahydrofuran. Mechanical properties of uncrosslinked and crosslinked PVC were compared at different temperature through either static tests with high strain as creep and tensile tests or dynamic test in the elastic domain. Except for creep resistance, crosslinking does not improve mechanical properties of plasticized PVC at temperature lower than 80°C if insoluble fraction in tetrahydrofuran is lower than 100 % by weight. Because chemical crosslinking leads to the existence of two interpenetrated networks, a physical one and a covalent one, the temporary physical network governs the properties until the melting of ordered domains, whereas the covalent network governs the properties if temperature rises above the melting temperature of the crystalline populations. To improve mechanical properties of crosslinked PVC at room temperature, the crosslinking density of the covalent crosslinks must be higher than the density of physical crosslinks. Such a situation is reached if insoluble fraction in tetrahydrofuran becomes 100 % by weight.     

Tightening of gelatin chemically crosslinked networks assisted by physical gelation

Developing the use of polymers from renewable sources to build hydrogels with tailored mechanical properties has become an increasing focus of research. The impact of the thermo‐reversible physical networks of gelatin (arising from the formation of triple‐helices) on the structure formation of a chemical network, obtained by crosslinking with glutaraldehyde (a non‐catalytic crosslinker), was studied using optical rotation, oscillatory rheology, and large strain mechanical deformation. We observed a direct correlation between the storage shear modulus of the chemical network grown in the gel state (i.e., simultaneously with the physical network) and the amount of gelatin residues in the triple‐helix conformation (χ). Since χ is directly affected by temperature, the value of the storage modulus is also sensitive to changes in the temperature of gel formation. χ values as low as 12% lead to an increase of the shear storage modulus of the crosslinked gel by a factor of 2.7, when compared to a chemical network obtained in the sol state (i.e., in the absence of a physical network). Our results show that the physical network acts as a template, which leads to a greater density of the chemical crosslinks and a corresponding higher elastic modulus, beyond what is otherwise achieved in the absence of a physical network. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1850–1858     

Moduli of model elastomeric networks prepared from polymer chains having a high plateau modulus,and their interpretation in terms of the constrained-chain model

High-molecular weight polybutadiene chains having approximately 47% cis-1,4 units and 45% trans-1,4 units were crosslinked through their carbon-carbon double bonds using p-bis(dimethylsilyl) benzene as crosslinking agent and chloroplatinic acid as catalyst. This particular polymer was chosen because the high plateau modulus it exhibits in the un-crosslinked state is taken to indicate large numbers of chain entanglements, and stress–strain measurements on such networks have frequently been interpreted with the assumption that the trapping of such entanglements during crosslinking should contribute significantly to their modull. It is shown in the present investigation that such results are equally well interpreted in terms of the new constrained-chain theory of rubbery elasticity. © 1993 John Wiley & Sons, Inc.     

Effects of synthesis-solvent on swelling and elastic properties of poly(N-isopropylacrylamide) hydrogels

Thermosensitive N-isopropylacrylamide (NIPA) hydrogels were synthesized by a free radical copolymerization with N,N′-methylenebisacrylamide (MBAA) in four solvents: water, ethanol, acetone and N,N-dimethylformamide. The swelling and elastic properties of the hydrogels were affected by the synthesis-solvents; the hydrogels (e.g. NIPA/MBAA = 1000/50 mol/m³-pre-gel solution) synthesized in water have smaller swelling volume and larger shear modulus at 10 °C than those synthesized in amphiphilic solvents. The network structure of hydrogels was estimated in terms of the conversion and two sorts of effective crosslinking density based on the Flory theory and the concentration of crosslinker. The hydrogels synthesized in water can have the microscopic inhomogeneous network arising from the entanglement of polymer chains, while the hydrogels synthesized in amphiphilic solvents can have the homogeneous network arising from the polymer concentration lower than the pre-gel solution and can be similar in network structure to the lightly crosslinked hydrogel synthesized in water.     

Influence of the crosslink network structure on stress‐relaxation behavior: Viscoelastic modeling of the compression set experiment

A viscoelastic approach of the compression set test is addressed in this work. This test measures the ability of rubber compounds to retain elastic properties after prolonged action of compressive stresses. Elastic properties were tested by recording the normal stress under a constant deformation of 25% with a laboratory rheometer. Considering the Boltzmann superposition principle, compression set data were modeled from the relaxation of Young's modulus, described by a Maxwell spectrum plus a constant E_∞ defining the elastic properties at the long times. This approach was developed with the copolymer of ethylene and vinyl acetate (EVA) networks crosslinked by radical chemistry and by an exchange reaction between acetate groups and silane compounds as crosslinking agents. Regarding the recovery of the elastic properties, radical chemistry provided better results than the exchange reaction for the identical crosslinking density of the network. Then, the Curro–Pincus molecular approach was developed to understand the influence of the microstructure of the EVA network on the elastic properties. The difference of the elastic properties between the two networks crosslinked by two different chemistry means was accounted for by considering the probability of having a dangling end of n units for a random crosslinking process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1779–1790, 2003     

Entanglement effects in model polymer networks

Many fundamental questions for the understanding of polymer networks are more suitably addressed by current computer simulations than by experiments. Details of the microscopic topology, such as the elastically active cluster or loop entanglements, can be identified as well as controlled. In particular, it is possible to isolate and quantify their effects on macroscopic observables such as the elastic modulus. The constraints due to connectivity and conserved topology are more clearly present for networks than for melts. Already for strand lengths between crosslinks which are relatively short, the effect of the conserved topology is important. The mode relaxation in a network is significantly different from that of a melt. For weakly crosslinked systems the melt entanglement length is the relevant scaling parameter. The elastic modulus of a long chain network under ideal conditions reaches an asymptotic value which is about 2.2 times smaller than the prediction of an affine model for a network made of strands of the melt entanglement length. An analysis of the stress reveals that in the linear regime the contribution from the excluded volume is dominant compared to that from the connectivity along the strands. For larger elongations, however, the non-linear elastic response is dominated by the chemically and topologically shortest paths through the system, where chemical crosslinks and topological entanglements between meshes of the network play a similarly important role.     

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Polymer modification through silane grafting and its subsequent crosslinking allows the rheological properties of a polymer to be tuned from those of a viscous melt to those of a crosslinked elastic network. In this study, a metallocene polyolefin resin is grafted with vinyl trimethoxy silane (VTMS) using dicumyl peroxide (DCP) as the initiator and is subsequently crosslinked in an oxidative environment. Dynamic rheological experiments are conducted to elucidate the effects of DCP and VTMS concentrations on the grafting and ensuing crosslinking processes. We find that the addition of VTMS alone to the polymer produces no grafting. In contrast, the presence of DCP by itself leads to direct crosslinking between polymer chains as suggested by an increase in elastic modulus and complex viscosity. Samples containing both DCP and VTMS undergo silane grafting, with the extent of grafting increasing with increasing DCP concentration. This conclusion is borne out by both rheological and Fourier transform infrared measurements. The grafted samples undergo silane crosslinking only in an oxidative environment and at temperatures equal to or greater than 190 °C. During crosslinking, the samples undergo a transition from a viscous melt with frequency‐dependent moduli to a gel exhibiting frequency‐independent moduli with the elastic modulus exceeding the viscous modulus. However, the kinetics of crosslinking and the extent of the modulus increase are a function of the DCP concentration, with both exhibiting a maximum at a specific DCP and VTMS combination. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2468–2479, 2000     

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl/divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.