Impact of order of catalytic ethylene and propylene polymerization on nanometer scale isotactic polypropylene‐polyethylene blend morphology in nascent heterophasic granules revealed by low voltage scanning electron microscopy and scanning transmission electron microscopy

Abstract

AbstractNew heterophasic granules comprised of two high molecular weight semi‐crystalline components ‐ polyethylene (PE) and isotactic polypropylene (iPP) ‐ are prepared from a MgCl2 supported Zeigler‐Natta catalyst. The impact of order of monomer addition on granule morphology and extent of mixing of PE and iPP in these monomer‐to‐polymer scenarios are examined. In one polymerization propylene is followed by ethylene (iPP‐PE; 1), and in the second ethylene is followed by propylene (PE‐iPP; 2). Detailed morphological assessments were carried out on RuO4 stained sections of the nascent granules of these heterophasic products using low voltage scanning electron microscopy (LVSEM) and scanning transmission electron microscopy (STEM). In these side‐by‐side granule morphology comparisons, LVSEM, which has not been used for granule examinations, has proved to be invaluable, typically providing insights into phase morphology that STEM does not. Both polymerizations 1 and 2 result in intimate mixing of PE and iPP on the tens to hundreds of nanometer scale. However, morphological outcomes for 1 and 2 are markedly different. In the case of 1, comprising 31 wt% iPP and 69 wt% of PE (the second‐grown phase), a reticulated mesh network of PE is readily observed coursing through much of the first produced iPP globular and sub‐globular matrix. The PE fibrils that comprise this network are remarkably uniform in diameter; approximately 100–150 nm. This aspect of the morphology is consistent with a PE sheath encapsulating both iPP globules and sub‐globules commonly found in iPP homopolymer granules made with MgCl2 catalysts. This morphology is similar to that of the most extensively studied heterophasic granules, classic iPP‐ethylene‐propylene rubber (EPR) impact copolymer (ICP), where the second grown EPR fills in space between and around the iPP globules and sub‐globules. Thus, the dominant morphology of 1 is consistent with the often‐called pore‐filling model. Using the same catalyst and arriving at a similar global composition (65 wt% PE and 35 wt% iPP), sample 2 is produced through the reverse order of monomer addition. The dominant morphology for 2 cannot be explained by the pore‐filling model. Instead, LVSEM most clearly shows that the second grown iPP phase, is finely dispersed throughout the PE matrix (not outside it) as irregularly shaped inclusions that are nominally 100 to 400 nm in diameter, so the dominant morphology in 2 is consistent with the proposed but not previously observed core‐shell model. The STEM results for 2 also reveal the presence of primitive PE spherulites and organized PE lamellae in the granules; structures not found in 1 and apparently not reported before in polyethylene homopolymer granules. These structures form likely from heat released during ethylene polymerization in the first step in 2. In LVSEM, the PE and iPP phases provide different amplitude contrast (grayscale) that is useful for calibration. In the LVSEM images of both samples 1 and 2, non‐binary strong grayscale gradients are observed in many regions in the micrographs. These suggest, but do not yet prove, fine dispersion of some PE in the first formed iPP (1) as well as a fine dispersion, perhaps on the ~10 nm scale, of some iPP in the first formed PE (2). So even granules of 1 appear to have some hybrid quality; primarily pore‐filling, with some core‐shell result as well. Finally, in STEM images of both 1 and 2 well‐formed, long PE lamella provide occasional marked contrast (especially at boundaries) with nascent iPP, which exists in a microparacrystalline state (short, poorly formed crystals). Regardless of the morphological complexity, the exceedingly fine mixing of the two semi‐crystalline phases accomplished during dilation of the granules to accommodate each second produced crystallizable polymer offers the potential for utility of these heterophasic granules in materials design (grafting and cross‐coupling, for example).