Crystallization kinetics, crystalline structures and properties of PB/PP blends regulated by poly(butene-block-propylene) copolymers

Blending isotactic polybutene-1 (PB) with isotactic polypropylene (PP) is a facial strategy to improve the mechanical properties and promote polymorphous transformation of PB. As the immiscible polymer pairs with strong crystallizability, the interactions between PP and PB in the melt state influence the subsequent processes like phase separation and crystallization, and then determine the crystalline structures and properties of the blends. Herein, two kinds of poly(butene-block-propylene) (PB-b-PP) copolymers with different sequence structures fractionated from PB/PP in-reactor alloy using temperature rising elution fractionation were utilized to compatibilize the PB/PP (80/20) blend. The phase morphologies, crystallization kinetics of PP and PB phases, and crystalline structures in PB/PP/PB-b-PP blends were characterized by phase contrast optical microscopy (PCOM), differential scanning calorimeter (DSC), and small-angle X-ray scattering (SAXS). The PB-b-PP copolymer with longer PP blocks (A) showed more satisfied compatibilization for PB/PP blends and PB/PP/PB-b-PP2.5 blend with 2.5 wt% copolymer as compatibilizer exhibited the most stable phase morphology with the smallest PP domains in melt state at 200 degrees C. During isothermal crystallization processes of the PB/PP/PB-b-PP blends at 135 degrees C for PP crystallization and then 90 degrees C for PB crystallization, the crystallization rate including nucleation rate and nucleation density, and crystallinity of PP and PB (form II) increased, and achieved the largest when the copolymer concentration was 2.5 wt% in PB/PP/PB-b-PP blend. The copolymer A significantly accelerated crystallization rate of PP compared with that of copolymer B. The PB/PP/A-2.5 blend with the largest lamellae thickness and highest crystallinity of PB form I, combined with the smallest spherulites, exhibited the highest modulus, tensile strength and toughness. The compatibilizing mechanism and roles of PB-b-PP copolymers for PB/PP blends were discussed to understand the physical and mechanical behaviors of the blends.

Automated summary

Blending isotactic polybutene-1 (PB) with isotactic polypropylene (PP) is an effective strategy to improve the mechanical properties and promote polymorphous transformation of PB. In this study, it was found that adding 2.5 wt% of PB-b-PP copolymer as a compatibilizer in PB/PP/PB-b-PP blends can achieve the most stable phase morphology and enhance the crystallization rate and crystallinity of PB and PP. Copolymer A is more efficient in accelerating the crystallization rate of PP compared to copolymer B, leading to significantly improved mechanical properties of the blends.