Joint effects of long-chain branching and specific nucleation on morphology and thermal properties of polypropylene blends

IntroductionIsotactic polypropylene (PP) possesses a prominent position among semi-crystalline polymers, which is mainly a result of its structural behavior.It implies a good mechanical performance in engineering applications.However, commercial production of polypropylene with Ziegler-Natta or metallocene catalysts results in highly linear chains, relatively low melt strength, and extensional viscosity.These characteristics bring drawbacks for processes that require high melt strength, such as foaming, blown film processing, blow molding, and thermoforming [1][2][3][4].The use of long-chain branched polypropylene (LCB-PP), recently developed and commercially available, prevents these problems and has opened up a wider field of applications.Long-chain branches are commonly introduced to linear polypropylene via direct synthesis using metallocene catalysis [5,6], electron beam, or gamma irradiation [7,8], and post reactor chemical modification [9,10].However, a higher price of LCB-PP has complicated their widespread use in industry.Thus, production of blends of linear isotactic polypropylene and LCB-PP can provide satisfactory properties at a reasonable price [11,12].Several crystallographic modifications have been recognized in PP -α-, β-, γ-phases, [13,14], and the mesomorphic 'smectic' phase, which is formed during high undercooling instead of a crystalline phase [14,15].From a practical point of view, monoclinic α-and trigonal β-phases have relevance [16,17].The β-phase is thermodynamically metastable and difficult to obtain under normal processing conditions.Specific nucleation caused by an addition of heterogeneous β-nucleating agent is an efficient way of 952 Joint effects of long-chain branching and specific nucleation on morphology and thermal properties of polypropylene blends