Abstract
The effects of graphene nanoplatelets (GNPs) on the nucleation of the β-polymorph of polypropylene (PP) were studied when melt-mixed at loadings of 0.1–5 wt % using a laboratory scale twin-screw (conical) extruder and a twin-screw (parallel) extruder with L/D = 40. At low GNP loadings (i.e., ≤0.3 wt %), the mixing efficiency of the extruder used correlated with the β-nucleating activity of GNPs for PP. GNP agglomeration at low loadings (<0.5 wt %) resulted in an increase in the β-phase fraction (Kβ) of PP, as determined from X-ray diffraction measurements, up to 37% at 0.1 wt % GNPs for composites prepared using a laboratory scale twin-screw (conical) extruder. The level of GNP dispersion and distribution was better when the composites were prepared using a 16-mm twin-screw (parallel) extruder, giving a Kβ increase of 24% upon addition of 0.1 wt % GNPs to PP. For GNP loadings >0.5 wt %, the level of GNP dispersion in PP did not influence the growth of β-crystals, where Kβ reached a value of 24%, regardless of the type of extruder used. From differential scanning calorimetry (DSC) measurements, the addition of GNPs to PP increased the crystallization temperature (Tc) of PP by 14 °C and 10 °C for the laboratory scale extruder and 16-mm extruder, respectively, confirming the nucleation of PP by GNPs. The degree of crystallinity (Xc%) of PP increased slightly at low GNP additions (≤0.3 wt %), but then decreased with increasing GNP content.
Highlights
The microstructure of polymer polymorphs, in terms of crystalline content, crystallite type, and size formed upon cooling, is widely influenced by processing conditions as well as the presence of additives [1]
165.5 ◦C, for a graphene nanoplatelets (GNPs) loading of 1 wt % for the samples prepared with the laboratory scale extruder and 0.1 wt % and 0.3 wt % for the ones prepared with the twin-screw extruder. These results suggest a change in PP polymorphism upon filler incorporation, as previously confirmed by X-ray diffraction (XRD) experiments
GNPs readily nucleated the β-polymorph of PP regardless of the extruder type used to prepare their composites (i.e., laboratory scale twin-screw extruder and L/D = 16 twin-screw extruder; see XRD)
Summary
The microstructure of polymer polymorphs, in terms of crystalline content, crystallite type, and size (i.e., packing geometries) formed upon cooling, is widely influenced by processing conditions as well as the presence of additives [1]. Isotactic polypropylene (PP) (i-PP) is a polymorph thermoplastic polymer with chains arranged in a helical conformation. The polymorphism of i-PP is derived from the different crystalline geometries (unit cells) in which the helices pack, namely monoclinic (α), trigonal (β), and triclinic (γ). A metastable smectic (δ) phase consisting of helices with a highly disordered arrangement can be obtained by quenching molten i-PP below 0 ◦C [2,3]. The α-form is the most common and stable polymorphic phase of i-PP. The mechanical properties of the β-form are associated with its peculiar broad lamellae morphology.
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