Effect of melt miscibility, polymorphism, and crystal morphology on tensile deformation of blends of isotactic polypropylene and propylene‐1‐hexene random copolymers

Abstract

The ductile behavior of isotactic polypropylene (iPP) can be effectively increased by blending with small contents (<25%) of a random propylene 1-hexene copolymer (PH). In this work, we have studied the uniaxial tensile deformation of binary blends of iPP with PH copolymers with 11 or 21 mol% 1-hexene. Blends iPP/PH11 are melt-miscible in the whole range of composition while iPP/PH21 blends are meltimmiscible, but partially compatible. On cooling, the lamellar morphology of each type of blend differs accordingly, and impacts their mechanical deformation. Miscible iPP/PH11 blends develop inter-mixed monoclinic lamellar stacks interconnected by tie molecules of iPP and PH admixed in the amorphous phase. During deformation, the monoclinic crystals transform to oriented mesophase at low strains due to effective stress transfer through the interconnected topology. These blends display the largest strain (~800%) and low recovery. Conversely, immiscible iPP/PH21 develop a coarser morphology of monoclinic and trigonal crystallites in the iPP-rich and PH21-rich domains, respectively. Less effective stress transfer associated with the coarse iPP/PH21 morphology leads to a delayed onset of orientation and a less effective monoclinic-mesophase transformation. The PH21 trigonal crystals of the blend orient but do not undergo polymorphic transformation. At high elongations fibrillar strain-induced trigonal crystals, provide a network of stable physical junction points that relax to the random orientation upon removal of the load, thus enhancing the elastic recovery of iPP/PH21 blends. K E Y W O R D S iPP copolymers, iPP crystallization, iPP mechanical properties, iPP polymorphism 1 | INTRODUCTION The polymorphic behavior, semicrystalline structure, and mechanical properties of isotactic poly(propylene) (iPP) change when defects or a comonomer are incorporated to the chain. The addition of a suitable content of comonomer to the iPP chain enables polypropylene materials with a wide spectrum of properties ranging from rigid thermoplastics to elasto-plastomers or to elastomers. [1]Mixtures of iPP and propylene-1-alkene thermoplastic elastomers, form both miscible, and immiscible (but compatible) blends. [2][3][4] In binary blends of random atactic propylene-butene (PB) copolymers analyzed by SANS, the melt-miscibility increased with increasing difference in content of 1-butene. [5] This effect was attributed to a negative enthalpy interaction between propylene and 1-butene monomer units and to attractive entropic contributions. Based on such results, it was hypothesized that the branches longer than ethyl,

Effect of hydrophobic nano‐silica/β‐nucleating agent on the crystallization behavior and mechanical properties of polypropylene random copolymers

“…PP‐R has a better pressure resistance and higher heat resistance than polypropylene at room temperature. Therefore, PP‐R material is a satisfactory pipeline transportation product 12,13 and can be used for water delivery to houses at room temperature or slightly lower temperature than room temperature. In recent years, with the rapid progress of the entire society, the consumption of PP‐R is also increasing 9,10 .…”