Crystallization Behavior and Crystal Morphology of Linear/Long Chain Branching Polypropylene Blends

Abstract

Strain hardening and crystallization behaviors, crystal structure and morphology of linear/long-chain-branching polypropylene blends (LPP/LCBPP) was studied by means of Rheotens, DSC, XRD, and POM. Upon incorporation of LCBPP into LPP, the melt strength and the strain rate durability of the latter were obviously enhanced, the nucleation and overall crystallization rates were increased and crystallization temperature elevated. At some middle fraction of LCBPP, the most remarkably effects were observed. The content of the branched architecture and the entanglement density were considered responsible for these influences.KEY WORDS: Long Chain Branching Polypropylene / Linear Polypropylene / Crystallization Behavior / Crystal Morphology / Commodity isotactic PP possesses many desirable physical properties such as good resistance to chemicals, high stiffness, high rigidity, high service temperature, and good temperature stability compared to other polyolefines. However, the lack of melt elasticity strain hardening behavior has limited its applications in extrusion foaming, extrusion coating, blow molding, and thermoforming, where the extensional flow dominates. [1][2][3][4] In order to improve the melt strength of linear isotactic PP (LPP), many efforts were made aiming at enhancing the strain hardening of LPP melt in extensional flow. Crosslinking, broading the molecular weight distribution, blending with other polyolefin were tried in the early researches. [5][6][7] Since 1990s, the introduction of long chain branching onto the backbone of LPP has been considered an effective approach to achieve the high melt strength. 8,9 A wide variety of methods were employed to achieve the grafting of long-chain side chains onto LPP, including electron-beam irradiation 10-13 and reactive extrusion with a multifunctional monomer and peroxide. 4,[14][15][16][17][18] Several commercial long chain branching polypropylene (LCBPP) grades were also developed. 19 However, LCBPP is more expensive than LPP, for this reason, to blend LPP with LCBPP constituted a practical way to generate a PP material with higher melt strength. The LCBPP introduced not only branching points and long side chains, but also more free volumes into the system. How such factors influence the crystallization and melting behavior and the properties of the melt remained interesting problems. In this work, blends of commercial LPP and LCBPP were prepared and the aforementioned effects were explored. EXPERIMENTAL MaterialsLinear PP (T36F) was obtained from Qilu Petrochemical Co. Ltd. Long chain branching PP (PF814) was purchased from Basell Co. Ltd. The molecular weight and polydispersity of the PPs were shown in Table I. Antioxidant Irganox B225 was provided by Ciba Specialty Chemicals. Preparation of the BlendsSeven blends of T36F/PF814 (LPP/LCBPP) with weight ratios of 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 0/100 were melt extruded and pelletized using a co-rotating twin screw extruder (WP-30, former WP company, Germany) at 100 rpm. The blends were stabilized ...

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