Study On Model Of Rubber Elasticity On The Basis Of The Experimental Results Of Single Molecule Mechanics

As one of the four major industrial raw materials,natural rubber has a wide range of applications and benefits the humankind from many aspects.Natural rubber has many special and excellent mechanical properties,such as the long-range reversible elasticity,large elongation at break,and so on.The mechanical properties of rubber have attracted extensive attention of researchers for a long time.Early studies have shown that rubber is consisted of a huge crosslinked network.The mechanical properties of bulk natural rubber are strongly associated with the microscopic/molecular structure of the crosslinked network.To understand the relationship between them,researchers represented by Flory have proposed several classical rubber elasticity models(i.e.,statistical models),which lays a theoretical foundation for the progress of rubber industry.To simplify the complex problems,only the core elements in the network structure of rubber are retained in these statistical models.There are two main approximate treatments:(1)Only PI chains are considered as load-carrying chains that contribute to the overall elasticity of the natural rubber,while the contribution of polysulfide chains is ignored.(2)Only the entropic elasticity of the polymer chain is considered in the mechanical deformation of natural rubber,while the enthalpic elasticity is neglected.Due to these over-simplified approaches,these traditional models can not precisely describle the mechanical properties of bulk natural rubber.For example,the fitting accuracy of these models are largely limited;these models fail to describe the real structure of the crosslinked network,hence cannot be used to analyze the effects of some parameters of the crosslinked network like the type of crosslinks.To deeply understand the relationship between the microscopic structure of the crosslinked network and the mechanical properties of bulk rubber,it is necessary to further develop a more accurate and reasonable statistical model,which is helpful to design the rubbers with high performance.In this thesis,the traditional statistical model(the Gaussian statistical model)is modified and improved.Firstly,the real crosslinked network of natural rubber and the ideal crosslinked network of the statistical models are compared,and the unreasonableness of the ideal crosslinked network is analyzed.Secondly,we measure the single-chain mechanical properties of the two principal chains(PI and polysulfide chain)in the crosslinked network by using single-molecule atomic force microscopy(AFM).Then,we discuss whether the single-chain mechanics obtained by single-molecule AFM is appropriate to the elastic models of natural rubber.Again,according to the results of single-molecule AFM experiments,we further improve the Gaussian statistical model.Finally,we study the fitting performance of the modified model for experimental data and the simulation performance for the parameters of the crosslinked network in detail.With these studies,the following conclusions are obtained:(1)In this thesis,the single-chain elasticities of cis-polyisoprene(PI)and polysulfide in nonane are measured by single-molecule AFM.Then,the results of these measurements are verified by the modified freely jointed-chain(QM-FJC)model.Here,the Kuhn lengths and the single-chain elasticities of the two molecules are also obtained,which could be appropriate to the elastic models for natural rubber.(2)The newly obtained single-chain elasticity(the real single-chain elasticity)of PI is compared with the entropic elasticity of this polymer adopted in traditional statistical models.The result indicates that the two elasticities differ largely in the whole force region.When the new Kuhn length obtained in the current study(0.46 nm)is adopted,the deviation between the real single-chain elasticity and the entropic elasticity in the region of F < 62 p N vanishes.Thus,the deviation in the entropic region is attributed to the variation of Kuhn length.Here,62 p N is the critical point for PI,from which the effect of enthalpic elasticity becomes significant.Considering the critical point is rather low,the enthalpic elasticity of PI cannot be ignored.Therefore,the statistical model needs to be modified by the newly obtained singlechain elasticity of PI from both the entropic and enthalpic aspects.(3)In this thesis,a new model(TCQMG model)is proposed on the basis of the Gaussian model,by introducing the elasticities of the two principal chains(S segment and PI chain)obtained from experiments.Thus,a connection between the mechanical properties of bulk natural rubbers and the mechanical properties of the individual load-carrying chains in crosslinked networks is established.The TCQMG model can be used to simulate the mechanical properties of natural rubber in various deformation states,and the model is superior to the 8-chain model(and all other classic statistical models)in this aspect.Further analysis shows that the fitting performance of the TCQMG model is improved by the introduction of real single-chain mechanics.This result indicates that the mechanical properties of rubber are originated from the variation of both entropy and enthalpy.(4)The traditional model(8-chain model)can usually be used to simulate the effects of network chain density and network chain length on the mechanical properties of natural rubber.Due to the introduction of S segments,the TCQMG model can also be used to study the effects of other network parameters(such as the sulfur content,the type of crosslinks and the length of S segments)on the mechanical properties of natural rubber,which can hardly be realized by the traditional models.Therefore,the TCQMG model has more advantages in the study of the network parameters.The simulation results of the TCQMG model for these parameters with a good agreement to the experimental results.These simulation results are helpful to understand the relationship between the microscopic structure of the crosslinked network and the mechanical properties of bulk rubber.In addition,the model can predict the mechanical properties of natural rubber in the case of the formation of various types of crosslinks.(5)The network structure of natural rubber is similar to those of other vulcanized rubbers(such as butadiene rubber and styrene-butadiene rubber).Therefore,by integrating the singlechain mechanics of the polymer components of those rubbers obtained from single-molecule experiments into the TCQMG model,the model can be used as a general model for various rubbers.Furthermore,this model also provides a solid theoretical framework for other materials with a crosslinked network like hydrogels.