【作者】 林渊；

【导师】 范毓润；

【作者基本信息】 浙江大学 ， 机械电子工程， 2012， 博士

【摘要】 由于高聚物结晶在工艺和实验中往往处在与金属基质接触的状态,基质的性质将会影响其结晶过程。本文以流变实验为主要的实验手段,结合差示扫描量热仪(DSC)测量、偏光显微镜观察,表面能测量技术等为辅助实验方法,研究了金属基质对高聚物材料的静态结晶和剪切诱导结晶的影响。研究发现,无论是采用高密度聚乙烯(HDPE)还是等规聚丙烯(iPP),样品在铝和不锈钢基质中的静态结晶的速度都是不相同的,在铝基质中的结晶速率均快于在不锈钢基质中的结晶速率,且随着等温结晶温度的升高,结晶速率的差别增大。在HDPE的结晶实验中我们发现,样品在不锈钢基质中等温结晶的样品所能达到的结晶程度低于在铝基质上结晶的样品,此外,HDPE在不锈钢基质上结晶时,达到液固转变点所需的时间要长于铝基质,且达到转变点时出现的屈服模量值也低于在铝板上结晶的HDPE样品。因此,样品在铝和不锈钢基质中的结晶机理可能不相同。在对iPP的等温结晶传热仿真计算中,发现基质的导热能力并非造成结晶速率区别的主要因素。实验发现,铝和不锈钢基质使熔体在靠近基质壁面处发生了异相成核,晶核在结晶过程的早期生成并生长,形成横晶并深入熔体内部(在1300C时横晶层厚度约为0.2mm),从而影响整个iPP样品的结晶。由于不锈钢和铝基质的表面自由能不同,造成的界面处熔体的异相成核能力不同(铝基质的表面自由能高于不锈钢基质在铝基, iPP/铝基质的界面成核能力较强),从而导致了iPP样品在铝和不锈钢基质中的整体结晶速率的区别。实验中,我们还发现,在DSC的等温结晶过程中,由于采用的铝制坩埚的表面能较高, iPP的薄片样品受界面结晶影响,得到的结晶曲线主要反映的是界面结晶。在iPP的剪切诱导结晶实验中,我们发现,样品在铝和不锈钢基质上的结晶速率也不相同,在铝基质上的结晶速率大于不锈钢基质。iPP过冷熔体流变曲线的间距依赖现象表明,在等温结晶发生前,基质对iPP过冷熔体进行的预剪切过程中易发生壁面滑移现象,滑移程度和基质的表面自由能密切相关。样品在表面自由能较高的铝基质上的滑移程度较小,因此在相同表观剪切速率下熔体的实际剪切速率较大,剪切所形成的晶核密度较大,从而使得结晶速率高于不锈钢基质中的结晶速率。通过偏光显微镜观测实验,我们得到了iPP在铝基质上的实际剪切功和晶核密度的关系。实验结果表明,iPP样品的晶核密度随剪切功的增加而增大,在低剪切速率下,这种关系和剪切速率无关,但在高剪切速率下,剪切功相同时,剪切速率越大,样品剪切诱导所生成的晶核密度越大。更多还原

【Abstract】 Concerning the fact that polymers often crystallize in contact with metal substrates in industrial production and experiment, the substrates effect on both static isothermal crystallization and shear induced crystallization of polymers is studied using rotational rheometer as well as polarized optical observation, differential scanning calorimetry measurements (DSC), et al. It is found that the static crystallization rate of either high density polyethylene (HDPE) or isotatic polypropylene (iPP) in contact with aluminum substrates is different with the sample in contact with stainless steel substrates, and samples in aluminum substrates are crystallizing faster than those in steel substrates. This difference becomes bigger at higher crystallization temperatures. In HDPE crystallization experiments, it is found that the degree of crystallinity of HDPE is higher in aluminum substrates than in steel substrates. and the time spent for reaching the Liquid/Solid-transformation point is shorter for aluminum plates; also. there is variety in yield value of storage modulus at the transformation point between these substrates. Therefore, it is indicated that the mechanism of crystallization of HDPE in aluminum and steel substrates should be different. The numerical simulation considering uniform bulk phase transition and latent heat conduction in isothermal crystallization of iPP shows that the substrate’s ability to remove the latent heat is not the dominant factor to cause the difference in crystallization rate. Transcrystallization zone, in which the heterogeneous nucleus density is controlled by the surface energy of substrate, was observed to grow towards the bulk with the thickness of about0.2mm for iPP to affect the global crystallization behavior, with the surface energy of aluminum higher than stainless steel. As a consequence, the DSC measurements mainly gives the crystallization curves of surface crystallization rather than the bulk crystallization due to the large surface-to-volume ratio of the specimen and the aluminum pan used which is a high surface energy substrate.The crystallization rate difference in aluminum and steel substrates is also observed in shear induced crystallization of iPP. Boundary slip at the interface between iPP and substrate, which also strongly depends on the surface energy of substrates, is found during the pre-shearing of supercooled iPP melt according to the gap dependence of rheological curves. With slip length smaller in aluminum substrates than in steel substrates, the real shear rate exerted to the melt is higher in aluminum substrates upon the same apparent shear rate reported by the rheometer. As a result, the shear induced nucleus desnsity is higher for iPP melts in aluminum plates, and so is the crystallization rate. With polarized optical observation, change of shear induced nucleus density of iPP melts with shear work applied is obtained in our experiments, which shows that the nucleus density of iPP melts is higher with higher shear works. However, at higher shear rates, the nucleus density also depends on the shear rates applied.更多还原