半晶态聚合物拉伸变形的微观机理

MICROSTRUCTURE EVOLUTION DURING TENSION DEFORMATION OF SEMI-CRYSTALLINE POLYMER

应用大规模分子动力学方法,采用粗粒化聚乙烯醇模型,模拟了晶区与非晶区随机交杂的半晶态聚合物模型系统,研究了半晶态聚合物在单轴拉伸变形过程中的应力-应变行为和微观结构演变.应力-应变曲线表现出4个典型变形阶段:弹性变形、屈服、应变软化和应变强化.在拉伸变形过程中,主要存在晶区折叠链之间的滑移、晶区破坏、非晶区的解缠结,以及分子链沿拉伸方向重新取向等4种主要的微结构演变形式.在屈服点附近,晶区分子链之间排列紧密程度减小而发生滑移,之后晶区变化需要的应力变小,从而形成应变软化现象.随着应变的增大,经各分子链段协同作用使非晶区分子链的解缠结和重新取向行为扩展到相对宏观尺度,导致拉伸应力增大而形成应变强化现象.

Molecular dynamics simulation with coarse-grained model of polyvinyl alcohol was used to investigate the structure of semicrystalline polymer through melt-cooling process. The relationship between microstructure and macroscopic mechanical behavior was then investigated to reveal the microscopic mechanism of semicrystalline polymer during uniaxial tension. The stress-strain behavior comprised elastic stage, yield stage, strain softening stage and strain hardening stage. Several important structural evolution forms were investigated： reorientation of molecular chains, slipping of PVA molecules in crystalline region, disturbed crystalline region（crystal to amorphous） and disentanglement of PVA molecules in the amorphous region. The stress-strain behaviors in the crystal region and amorphous region were investigated respectively. The stress varied in two regions during uniaxial tension, which mainly caused by various microstructure evolution in different stages. In the elastic stage, the main microstructure evolution was the reorientation of molecular chains. In the strain softening stage, the slipping behavior of folded chains in the crystalline region and the disentanglement of the PVA molecules in the amorphous region were the main structural evolution forms. The stress in the crystal region in this stage was larger than that in the amorphous region, because keeping slipping behavior of folded chains in crystalline region was harder than to deform in the amorphous region. In the strain hardening stage, the deformation of amorphous region wasmore difficult than crystal slipping, in other words, the disentanglement of PVA molecules need more energy. The stress in this stage increased which mainly led to the mechanical behavior of strain hardening. In conclusion, the coordinated microstructure evolution contributed to macroscopic mechanical behavior during tension in spite of the variation of the main microstructure evolutions in different stages.