【作者】 曹健；

【导师】 任显诚； 徐云祥；

【作者基本信息】 四川大学 ， 高分子科学与工程， 2023， 博士

【摘要】 橡胶因具有独特的高熵弹性被广泛应用于如轮胎、减震器和密封圈等诸多领域。橡胶中综合性能最优异的是天然橡胶（NR）,但其产量受限于地理环境。我国的NR自给率不足15%,远低于国际安全线（30%）。因此,通过开发具有优异综合性能的合成橡胶来替代天然橡胶是一个重要课题。因为具有与NR相似的橡胶烃结构,顺式1,4-聚异戊二烯（PIP）被认为是NR的最佳替代品,但其生胶及硫化胶的性能均弱于NR。因此,亟需制备高性能聚异戊二烯橡胶。此外,在橡胶网络中引入共价自适应网络（CANs）或制备vitrimer可以使其具有可回收和自愈合等功能。但是由于化学键本征的动态性,含有CANs的橡胶容易发生蠕变,导致其尺寸稳定性变差,如何提高可回收橡胶的抗蠕变性能,也是拓展其功能和使用范围的关键问题之一。为实现上述目标,需要针对不同性能对聚异戊二烯的分子链结构及其交联网络进行再设计。本论文从分子链结构调控出发,将不同的极性基团引入至聚异戊二烯分子链中并改变其序列（端基嵌段和无规,其中端基嵌段长度占聚合物主链的比例<1%）,进一步调整端基或链中交联方式获得不同特性的聚合物网络结构,从而制备高强韧、耐疲劳、抗蠕变或可回收等性质各异的高性能聚异戊二烯橡胶并研究其机理。主要研究内容如下:1.制备了含有极性甲基丙烯酸酯（MMA）基团的功能化聚异戊二烯作为模型聚合物,研究了MMA的序列（嵌段端基和无规）及含量对聚合物生胶及过氧化二异丙苯（DCP）交联橡胶性能的影响。当MMA位于聚合物分子链端基时,其更易形成聚集体,且聚集体的稳定性及其相互作用随着MMA含量的增加而增加。因此,生胶的力学性能随着MMA含量的增加而提高。样品使用DCP交联后,MMA中的双键会参与交联反应,从而提高聚合物交联密度并形成原位纳米相域。而且,交联网络的交联密度和纳米相域的模量及含量均随着MMA含量的增加而增加,因此可以实现聚合物的增强增韧。而当MMA含量较高（0.77mol%）时,因交联密度和纳米相域模量过大,其会发生脆性断裂,性能发生大幅度降低。因此,B-PIP-MMA-0.56%-DCP具有最好的力学性能,其拉伸强度为12.2 MPa、韧性为19.5 MJ/m³,表现出明显的增强增韧作用。对于无规聚合物,MMA在聚合物分子链中是无规分布的,其生胶和交联橡胶的性能均低于相同MMA含量的嵌段端基聚合物。2.为了制备具有高强、高韧和耐疲劳性能优异的硫磺硫化聚异戊二烯,在上一部分工作基础上,本部分选择PIP、B-PIP-MMA-0.56%和R-PIP-MMA-0.58%生胶,使用硫磺硫化体系制备了硫化橡胶PIP、B-PIP-MMA和R-PIP-MMA。MMA基团会聚集并参与硫化过程,不仅可以降低硫化网络的缺陷,还可以提高网络的本征缠结密度并形成高缠结区域。由于具有较低的网络缺陷,缠结区域在应力作用下会使“隐藏链”释放并促进分子链的取向结晶,因此,B-PIP-MMA的拉伸强度、断裂伸长率和韧性分别达到了26.2 MPa、885%和57.4 MJ/m³,优于PIP和R-PIP-MMA。三个样品分别压缩耐疲劳50万次后,B-PIP-MMA和R-PIP-MMA的动刚度可以分别保持在其初始值的97%和93%以上,而PIP则仅为75%且表现出明显的疲劳失效。这主要是由于端基MMA参与交联导致其附近的分子链产生更多的缠结,在疲劳过程中这些缠结可以通过滑移分散应力,从而提高其耐疲劳性能。本部分工作代表了一种通过简单的一步硫化方式制备端基交联和缠结网络,实现聚合物高性能化的策略。3.改善vitrimer弹性体在使用温度下的抗蠕变性能的关键是控制和调节其本征的动态特性。因此,深入研究分子变量对动态性能的影响对vitrimer的设计和应用很有帮助。交换单元分布是调整vitrimer动态特性的分子变量之一,但其对vitrimer网络动态行为的影响没有被系统研究过。本部分,我们制备了一系列含有乙酰乙酸酯（AA）的聚合物PIP-AA-n,其中AA分别分布在分子链末端和链中（n为嵌段部分的比例,分别0%、25%、50%和75%）,并使用二胺交联得到交换单元分布不同的交联网络PIP-VU-n。之后,研究了交换单元分布和动态行为之间的定量关系。因为交换单元分布的不均匀性,网络中同时存在低交联和高交联区域。由于高和低交联区域之间的协同效应,所形成网络的松弛时间随n先增加后降低,PIP-VU-50%具有最长的松弛时间。同时,所有样品的松弛时间都很短,可以保证其在高温下的加工。更重要的是,控制动力学的两个重要的内在参数,活化能（E_a）和拓扑学冻结转变温度（T_v）和n均具有线性关系。由于E_a和T_v随n的增加而增加,导致聚合物的蠕变应变和蠕变速率随n的增加而降低,抗蠕变性能得到提高。这项工作提供了一个通过调整交换单元分布来控制动力学,从而提高抗蠕变性能的策略。而且,所提出的策略和所产生的定量模型可能应用于其他的vitrimer,可以利用它来制备具有精确网络设计的理想vitrimer,以满足各种应用的需要。4.本部分工作中,制备了交联键主要为双硫键和多硫键的硫化橡胶,并引入催化剂铜离子(Cu²⁺)催化二硫交换反应,制备了可回收的硫磺硫化聚异戊二烯橡胶网络。具体地说,在聚异戊二烯链中引入了端基羟基和吡啶基,通过配位作用降低Cu²⁺在使用温度下的催化活性,从而提高抗蠕变性。虽然所得到的T_v较低,但其即使在100°C的使用温度下也表现出卓越的抗蠕变性,而且其在高温下网络重排能力没有受到影响。此外,回收的样品显示出优异的力学性能,拉伸强度超过7 MPa,断裂伸长率超过600%,这明显高于文献中报道的大多数含有动态共价网络的聚二烯橡胶体系。这项工作为设计使用工业硫磺硫化工艺制备具有增强的抗蠕变性、足够的拉伸强度和优异的弹性的再生橡胶提供了一个策略。更多还原

【Abstract】 Rubber is widely used in many applications such as tires,shock absorbers and seals,etc.,due to its unique high entropy and elasticity.Natural rubber（NR）has excellent comprehensive properties,but its production is geographically limited.The self-sufficiency rate of NR in China is less than 15%,which is much lower than the internationally recognized security line of 30%.Therefore,it is an important issue to develop high-performance synthetic rubber to replace NR.Due to the similar chemical composition and stereoregularity to NR,cis-1,4-polyisoprene（PIP）is considered to be the best alternative to NR.However,the performance of both PIP raw rubber and vulcanized rubber were inferior to that of NR.Therefore,there is an urgent need to prepare high-performance polyisoprene with properties matching or superior to NR.In addition,the introduction of covalent adaptive networks（CANs）into rubber networks or the preparation of vitrimer could endow their functions with recyclability and self-healing.However,due to the instinct dynamic nature of the chemical bonding and the high temperature in use condition,rubbers with CANs were susceptible to creep,causing poor dimensional stability which limits their applications.Therefore,rubber with excellent creep resistance and recyclability could be developed.To achieve these goals,the molecular chain structure and crosslinking networks of polyisoprene need to be designed for different properties.In this thesis,different polar groups were introduced into the polyisoprene chains.And the sequence（random and terminal block,the content of terminal block were<1 mol%）and content was altered,thus resulting in various crosslinking structures with terminal or in-chain crosslinking.As a result,the high-performance polyisoprene rubber with different high strength and toughness,fatigue resistance,recyclability or creep resistance,and the corresponding mechanisms were studied.The main studies are as follows:1.Functionalized polyisoprene with polar MMA groups were prepared as model polymers to study the effect of the sequence（terminal and random）and content of MMA on the properties of the polymer raw rubber and dicumyl peroxide（DCP）crosslinked rubber.For terminally functionalized polymer,polymer chains containing MMA were preferred to form aggregates and the stability of the aggregates and the interactions increase with increasing MMA content.As a result,the mechanical properties of the raw rubber improved with increasing MMA content.The double bonds in the MMA could participate in crosslinking reaction,thus increasing the crosslinking density and forming nanophase domains.Furthermore,the crosslinking density of the network and the modulus and content of the nanophase domains increase with the increase of MMA content,thus resulting in improved strength and toughness of the polymer.However,brittle fracture occurred at high MMA content（0.77 mol%）,due to the high crosslinking density and modulus of the nanophase domains.Therefore,B-PIP-MMA-0.56%-DCP exhibited the best mechanical properties with a tensile strength of 12.2 MPa and a toughness of 19.5 MJ/m³,showing a significantly enhanced strength and toughness.For the random polymers,MMA was randomly distributed in the polymer molecular chain and therefore its performance in both raw rubber and cross-linked rubber were lower than that of the terminal polymers with the same MMA content.2.To prepare sulfur vulcanized polyisoprene with high strength,high toughness and excellent fatigue resistance,PIP,B-PIP-MMA-0.56%and R-PIP-MMA-0.58%were selected and vulcanized with sulfur system,which were named PIP,B-PIP-MMA and R-PIP-MMA.The MMA groups could aggregate and participate in the vulcanization process,resulting in the formation of networks with reduced defects,increased entanglement density and nanophase domains.Due to the low network defects,the nanophase domains release the"hidden chains"under stress and promote molecular chain orientation and crystallization.The tensile strength,elongation at break and toughness of B-PIP-MMA reached 26.2 MPa,885%and 57.4 MJ/m³ respectively,which were better than those of PIP and R-PIP-MMA.The dynamic stiffness of B-PIP-MMA and R-PIP-MMA could be maintained at 95%and 93%of their initial values respectively,while PIP is only 75%and shows significant fatigue failure after500000 compression fatigue cycles.This is mainly due to the more entanglement of the molecular chains originating from the endgroups MMA,which could disperse the stress and energy by slipping during fatigue and thus improve its fatigue resistance.This part of the work represents a strategy for achieving high polymer performance through the preparation of endgroup crosslinked and entangled networks in a simple one-step vulcanization process.3.The key to improving the creep resistance at service temperature is controlling and regulating the instinct dynamic properties.Therefore,an in-depth study of the effect of molecular variables on dynamic properties was useful for the design and application of vitrimers.Exchangeable units distribution is one of the major molecular variables to tune the dynamics,but it has hardly been studied.Herein,a series of block copolymers PIP-AA-n with a block distribution in the terminal segment and a random distribution in the back segment（n is the proportion of block fraction,0%,25%,50%and 75%,respectively）were prepared.And the crosslinked networks PIP-VU-n with different degrees of exchangeable units distribution（n）were obtained by crosslinking with diamine.Then the quantitative relationship between exchangeable units distribution and dynamic behavior was investigated.Due to the uneven exchangeable units distribution,the crosslink-rich and crosslink-poor regions existed simultaneously in networks.The relaxation time of resulting networks increased and then decreased with n due to the synergy effect,and PIP-VU-50%showed the longest relaxation time.At the same time,the relaxation times for all samples were short enough to reprocess at elevated temperatures.What is more,two important intrinsic parameters to control the dynamics,the activation energy（E_a）and topology freezing transition temperature（T_v）showed strong dependence on n.And two quantitative linear models could be used to describe their relationships.Owning to the increased E_a and T_vwith n,the creep strains and creep rates decreased with n,revealing the improved creep resistance.This work provides a strategy to control the dynamics by adjusting the exchangeable units distribution thus enhancing creep resistance.And the proposed strategy and resulting quantitative models might be applied to other vitrimers,which could be utilized to prepare desired vitrimers with exact network designs for various applications.4.It is of great importance to achieve the recyclability of sulfur-cured rubbers with improved creep resistance.Herein,the recyclable sulfur-cured polyisoprene rubber network was prepared by introducing a catalyst copper ion(Cu²⁺)to catalyze the disulfide metathesis reaction of inherent disulfide and polysulfide bonds.Then,the terminal hydroxyl and pyridyl groups were introduced into the polyisoprene chain to reduce the catalytic activity of Cu²⁺at service temperature by coordinate interactions,thus improving the creep resistance.The resulting networks exhibited superior creep resistance even at a service temperature of 100°C,although they had lower T_vs,while the corresponding network rearrangement ability at elevated temperatures was not affected.Moreover,the recycled samples displayed excellent mechanical properties with tensile strengths exceeding 7 MPa and elongation at break over 600%,which were significantly higher than those of most reported polydiene rubber systems with dynamic covalent polymer networks（DCPNs）in the literature.This work provides a strategy for designing recycled rubbers with enhanced creep resistance,sufficient tensile strength,and superior stretchability using industrial sulfur-vulcanized processes.更多还原