βα-growth Transition Of Isotactic Polypropylene Induced By Stepwise Crystallization

Isotactic polypropylene is one of the most important semi-crystallinepolymers with pronounced polymorphisms and morphologies, which exists indifferent crystal modifications with monoclinic (α), trigonal (β), orthorhombic (γ) et al. The α form is the most common and stable modification, whereastrigonal β form is a metastable phase. The transformation between β and αphase is a characteristic feature of isotactic polypropylene. In this paper, hightemperature growth transition induced by stepwise crystallization of β cylinder,β spherulites, β hexagonites was studied. Besides, crystalline structure andlamellar morphology of β spherulites experience low temperature growthtransition was also studied.1. β cylinder was initiated by introducing the iPP fiber into the iPPsupercooled melt. Stepwise crystallization at135oC and151oC was used toinitiate a βα growth transition on β cylinder. The crystalline structure andmorphologies was studied by POM. SEM was used to watch the lamellarmorphologies of the two phases at transition point. Results show that fan-shaped α crystals was induced at the frontier of β cylinder and the cylinderthat has not covered by α crystals continues to grown in β form.2. Pimelic calcium, a highly selective β nucleator, was obtained byreaction of pimelic acid and calcium stearate. iPP was melt-blended withpimelic calcium and then melt compressed to thin films with a thickness ofabout10-20μm. Stepwise crystallization was utilized to induce βα growthtransition on β spherulite. In situ POM investigation was used to study thegrowth of β and α spherulites during stepwise crystallization, and growthkinetics of two phases was discussed. Resulits show that after temperaturejumps to151oC, β shperulites still grow in β form. After about20or40min, αcrystal was found to initiate growth. SEM was used to investigate the lamellarbehavior of the two phases. Results show that the transition behavior of βspherulite is a lot alike that of β cylinder.3. The iPP/pimelic calcium thin film was prepared by solution casting;the film thickness is about0.5-1μm. The Polarized Optical Microscopy wasused to investigate the βα growth transition of β hexagionte and the factorsthat affect the βα growth transition, such as crystallization temperature, timeand thickness of the film. Results show that the frequency of βα growthtransition was increased with the crystallization temperature. The geometrysize and amount of α crystals was increased with crystallization time. Theamount of α crystals was found in direct proportion to film thickness, which due to the fact that β hexagonties grown in thicker film had more growth front,thereby provide more possible transition site. Scanning Electron Microscopywas used to investigate the lamellar morphologies of β hexagonties and αcrystals. α lamellae with cross-hatched structure was first formed. With thegrowth of lamellae, the radial lamellae became dominant and tangentiallamellae tended to disappear. The lamellar morphology of α spheruliteisothermally crystallized at151oC was investigated to dicuss the lamellarbehavior of β hexagonite. It is believed that the lamellar structure of α crystalat the βα transition point is same like the α spherulite, i.e., the cross-hatchedstructure was first formed as nucleation site, which means βα transitionbehavior was a nucleation process.4. Low temperature growth transition was performed on β spherulites.Polarized Optical Microscopy and Scanning Electron Microscopy were usedto investigate the crystalline structure and lamellar structure. Results showthat α crystals was induced during the crystallization. The flat-on and edge-onlamellae continue to grow in original lamellae; however the crystalline hasbeen changed into α form during the low temperature growth transition.