Mechanism Of Metal Substrate Effect On The Process Of Crystalline Polymers

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.