Orientation Induced Crystallization Of IPP And Preparation Of Microporous Membrane From Oriented Film By Stretching

Isotactic polypropylene (iPP) is one of the semicrystalline polymersexhibiting pronounced polymorphisms and morphologies depending onboth the chemical characters of the materials and their preparationconditions. At least three crystalline modifications, i.e., the monoclinic-form, the trigonal-form and the orthorhombic γ-form, have beenreported. It was well documented that the-iPP crystals arethermodynamically most stable and commercial grade iPP crystallizesfrom bulk most generally in its-form under traditional industryprocessing conditions. The metastable-iPP exhibits some excellentperformance characteristics such as low crystal density, meltingtemperature, fusion enthalpy, the remarkably improved elongation atbreak and impact strength, which make it useful for many industrialapplications. It can, however, only be obtained through some kinds ofspecial procedures, such as crystallization in thermal gradient,crystallization of the sheared or elongated melts, or crystallization at thepresence of nucleation agents. The formation mechanism of the-iPP crystals is, however, not clear.The deformation of-form polypropylene has been an interestingsubject of research because transformation from to or to smectic formwith unusual micropore formation can be obserIsotactic polypropylene(iPP) is one of the semicrystalline polymers exhibiting pronouncedpolymorphisms and morphologies depending on both the chemicalcharacters of the materials and their preparation conditions. At least threecrystalline modifications, i.e., the monoclinic-form, the trigonal-formand the orthorhombic γ-form, have been reported. It was welldocumented that the-iPP crystals are thermodynamically most stableand commercial grade iPP crystallizes from bulk most generally in its-form under traditional industry processing conditions. The metastable-iPP exhibits some excellent performance characteristics such as lowcrystal density, melting temperature, fusion enthalpy, the remarkablyimproved elongation at break and impact strength, which make it usefulfor many industrial applications. It can, however, only be obtainedthrough some kinds of special procedures, such as crystallization inthermal gradient, crystallization of the sheared or elongated melts, orcrystallization at the presence of nucleation agents. The formationmechanism of the-iPP crystals is, however, not clear. The deformationof-form polypropylene has been an interesting subject of researchved inthe process of deformation. So it can be used to prepare gas exchange membrane, filter membrane or seperator of lithium-ion batteries.In our reasearch group, by introducing fibers into supercooledmolten iPP matrices, homogeneous and heterogeneous matix-fibercomposites have been prepared, and the-iPP cylindritic crystals wasproduced. It can be seen that the different origins of the β-iPP cylindritesin those composites by comparing the melt recrystallization behaviors ofthe-iPP crystals. The oriented molecular chain in melt plays a veryimportant role in the subsequent-iPP crystallization.The best preparation condition of-iPP cylindritic crystals can beobtained depending on the formation mechanism and crystallizationcondition. Microporous membranes obtained from fibers induced-formpolypropylene films by stretching. The micropores are formedhomogeneously for the different strain of oriented-iPP crystalline andamorphous phases. The microporous membrane had pores around50-100nm, porosity of40~50%and the similar structure with the PEmicroporous of Celgard(USA). The formation of the β-iPP is related tothe melting, or at least partial melting of the iPP fibers.The machenicalproperties of microporous membrane can improved greatly for partialmelting of the iPP fibers.The porous membrane was used as Lithium-ion battery separators totest electric properties. It was observed that ionic conductivity,electrochemical stability window and charge-discharge cycle all meet the requirment of battery, and are better than domestic microporousmembrane.On the base of above experiments, we compared the mechanicalproperties of homogeneous iPP fiber–matrix composites andheterogeneous polyamide (PA)–iPP fiber–matrix composites. It wasfound that the mechanical properties of those composites such an thetensile modulus and tensile strength all increased significantly. Theelongation at break of iPP fibers/β-PP self-reinforced compositessignificantly increased with preparation temperature elevated which isdue to the surface crystallization contributes to the better interfaceadhesion and provides better bonding for iPP fibers/PP composites,while PA fibers/PP composites still exhibit a brittle fracture for the weakinterfacial adhesion between PA fibers and PP matrix.The preliminary study on melt recrystallization behaviors of the-iPP crystals induced by β-nucleating agent. A new type of-modification has been developed by self-seeding process.The growthprocess of these crystals is just like “photographic development process.”Crystalline phase transformation and the memory effect caused by localorder was observed during the melting and annealing process. A hightemperature is sufficient to destroy the local order and the β-nucleatingagent efficiently induces formation of β-form.