Investigations On The Crystal Structure Transition And Properties Of Isotactic Polybutene-1/CaCO₃ Nano Particles Composite Material

Polymorphic isotactic polybutene-1（iPB-1）in form I has good mechanical performances such as creep resistance at high temperatures,but iPB-1 usually crystallizes first into metastable form II during melt processing and molding.Form II spontaneously transits into stable form I and brings volume shrinkages,which seriously restricts the industrial applications of iPB-1,and the mechanism of II-I transition is not fully understood.In this thesis,the CaCO₃ modified by stearic acid was melt-blended into iPB-1 with different molecular weights,and the composites were studied by differential scanning calorimetry（DSC）,polarized light microscope（POM）,dynamic rheology,and mechanical testing.The effects of CaCO₃ on the melting and crystallization behaviors,II-I transition behaviors and mechanical properties of iPB-1are focused on.The main results are as follows:（1）After CaCO₃ addition,the crystallization temperature T_c,peak obtained from the DSC cooling curves of iPB-1 with different molecular weights decreased mainly due to the reduced spherulite radial growth rate as well as the fixed the nucleation density of iPB-1 melt crystallization.Further examination also revealed the fixed equilibrium melting point of form II.It indicated that the melting and crystallization of iPB-1 into form II is only kinetically inhibited by CaCO₃ addition.Meanwhile,CaCO₃ addition mainly inhibited the entire II-I transition process by inhibiting the II-I transition nucleation process of iPB-1 but without significant influences on the II-I transition growth process.The DSC results showed that the crystallinity of form I after the II-I transition was lower than the crystallinity of the corresponding as-crystallized form II and the difference varied with the increasing CaCO₃ content,which contradicts to the traditional solid-solid II-I transition viewpoint.（2）Further investigation on the direct form I formation revealed that the CaCO₃addition significantly reduced the amount of the direct crystallized form I,especially for iPB-1 with low molecular weight.It can be proposed that after iPB-1 melt crystallization into form II,the form I directly generated in the amorphous phase can trigger the II-I transition of the adjacent form II first.Then it started the entire II-I transition.The CaCO₃ addition mainly inhibits the direct form I formation and thus the II-I transition nucleation process,which can provide a way to understand the II-I transition mechanism of iPB-1.（3）The CaCO₃ nanoparticles are better dispersed in the iPB-1 with relatively low molecular weight,resulting in the enhanced mechanical and dynamic rheological performances of the composites.The CaCO₃ addition increased the glass transition temperature T_g of iPB-1 with the two different molecular weights.The inhibited iPB-1chain mobility was consistent with the suppression of the melting and melt crystallization behaviors of iPB-1 and the variations of the mechanical and rheological properties of the compostes with the CaCO₃content.