Study On Phase Transformation,Structural Controlling, And Corresponding Mechanical Properties Of β-Polypropylene

Isotactic polypropylene (iPP) has been widely used in many fields because of its great combination properties and good price-cost. As a typical polymorphic material, it contains several crystal modifications including monoclinic α-phase, the trigonal β-phase, the orthorhombic y-phase, and smectic mesophase (intermediate state between ordered and amorphous phase), among them, the α-phase (α-iPP) and β-phase (β-iPP) are the most popular ones. β-iPP with unique crystal structure, whose lamellae are almost parallel to each other, allows the initiation and propagation of plastic deformation more easily and then enhances the energy dissipating processing. Specifically, it is proposed that the enhanced toughness of β-iPP can be attributed to a stress-induced transformation from β-iPP to α-iPP at the root of a growing crack, since the exothermic characteristic of this transformation would actually lead crack tip blunting and subsequent yielding is favored. However, the effects of other components or the crystalline morphology of β-iPP on this phase transformation have been ignored, and the transformation mechanisms are still full of controversy, too.Based on this, a highly effective β-phase nucleating agent (P-NA) was introduced to achieve numerous formation of P-iPP, and then the crystalline micro-structure evolution as well as phase transformation during uniaxial tensile deformation was characterized. Considering about the effects of crystalline morphology and elastomer on the crystalline transformation of P-iPP to a-iPP, the phase transformation process controlling and succedent contributions to synergistic toughening effect of β-NA and elastomer in improving the fracture resistance of iPP were systematically studied. Also, the compounded nucleating agents (NAs) were introduced into elastomer (EPDM) modified iPP to control the crystalline structures of iPP matrix. Combined with the synergistic toughening effect of β-NA and EPDM on iPP, the combination mechanical properties of iPP/EPDM/NAs blends are controlled in a wide range by adjusting the mass fraction content of β-NA and a-NA in NAs. Furthermore, the effects of β-NA on the crystallization process, melting behaviour and mechanical properties of a new polypropylene, namely impact-resistant polypropylene copolymer (IPC), were analyzed. The results are listed as follows:1、The critical (3-NA content (0.05wt%) is ready to form a well-developed β-spherulites, while the supercritically (0.2wt%) nucleated iPP shows bundle-liked morphology without distinctly developed spherulites. SEM results show that, compared with the bundle-liked β-iPP, the well-developed β-spherulites are most likely to suffer inter-lamellae slipping or/and separation other than intra-lamellae slipping and shear, leading to a slower phase transformation rate of P-iPP to a-iPP during the tensile deformation and more P-iPP maintains and participats in the further deformation at relatively high strain. This is believed to be responsible for the significantly improved ductility of well-developed β-spherulites. Moreover, the formation of multiple small "crack bends" in well-developed β-spherulites is thought to be an important way for energy dissipation during the tensile deformation and thus leading higher toughness.2、The presence of elastomer particles (POE) prevents the phase transformation of β-iPP to a-iPP during tensile deformation, and this preventing effect enlarges with increased POE content. The reasons could be concluded as follows:Firstly, POE particles distribute in the amorphous region among the inter-lamellae structure, which prevents the intra-lamellae slipping and shearing and finally prevents the β-iPP to a-iPP transformation process of matrix. Secondly, the β-iPP to a-iPP transformation occurs mainly in the necking region during the elongation process. Elastomer makes the specimen more flexible compared to the specimens without elastomer. During the deformation process, the necking phenomenon of the former specimen is more invisible compared to the latter specimens. Thus, larger local strain is needed to invoke the phase transformation for the specimens with POE. POE as a role of preventing the phase transformation during the deformation is thought to be an important mechanism for synergistic toughening effect of β-NA and elastomer in improving the fracture resistance of iPP.3、The iPP matrix is well controlled through adjusting the mass fraction of compounded nucleating agents (NAs). Consequently, the mechanical properties of elastormer (EPDM) modified iPP/NAs blends are easily controlled in a wide range by making good use of the synergistic toughening effect of NA and EPDM on iPP. Specifically, the simultaneous improved stiffness, fracture resistance, and heating distortion temperature of the blends can be achieved accordingly in some compositions. And the direct relationship between mechanical properties and the relative fraction of β-iPP(Kβ) is established. Moreover, a new concept named synergistic toughening efficiency (fEPDM-NAs) is defined to quantificationally describe the synergistic toughening effect of NA and elactormer in improving the properties of iPP. The results show that the sample with higher fEPDM-NAs does not mean better toughness. But, at moderate Kβ, good combination properties of iPP/EPDM/NAs blends with proper fEPDM-NAs can be achieved.4、The dispersed phase was well distributed inside and between crystals or lamellae in IPC, which is ready to destroy the integrality and continuity of the matrix crystal structure of IPC to some extent, and easily leading to reorganization of crystal structure during heating process. β-NA nucleated IPC samples exhibited multiple melting behaviors during the heating process, these were related to the comprehensive function of β-and/or α-form crystal reorganization and a-form crystal recrystallization from molten β-form crystal. Combining the differential scanning calorimetry (DSC) analysis with in-situ polarized light microscope (PLM) observation, all the fusions were clearly located to the correspond crystal structure in IPC.