Graphite-LDH hybrid supported zirconocene for ethylene polymerization: Influence of the support on the crystallization kinetics and thermal stability of polyethylene

Highlights

• Polyethylene (PE) was synthesized by using low crystallinity graphite-NiAl LDH (G/LDH) supported zirconocene catalyst.

• The presence of G/LDH nucleated the PE crystallization and shifted the T_onset to a higher value.

• The degradation mechanism was studied; neat PE followed phase boundary-controlled mechanism (R2, R3).

• SZ-PE initially followed R2, R3 mechanism and then diffusion-limited mechanism at a higher temperature.

Abstract

A hybrid nanomaterial, low crystallinity graphite-layered double hydroxide, was used as a support for zirconocene catalyst. The hybrid nanomaterial graphite-NiAl layered double hydroxides (G/LDH) were synthesized by the co-precipitation method. The synthesized nanomaterial was used as a support for zirconocene catalyst. The polymerization reactions were carried out for ethylene polymerization. The synthesized polyethylene (PE) was analyzed by using differential scanning calorimetry (DSC), Crystallization analysis & fractionation (CRYSTAF), and thermogravimetric analysis for its thermal and microstructural characteristics. The crystallization kinetics were studied by the Ozawa and combined Ozawa and Avrami models. It was found that the presence of G/LDH from the catalyst support nucleated the PE crystallization and shifted the crystallization onset temperature to a higher value. However, the overall crystallization rate was slowed by the presence of the nanomaterial due to growth impingement. Moreover, the PE synthesized by G/LDH supported catalyst possessed higher thermal stability than PE synthesized by unsupported zirconocene catalyst. The integral isoconversional method was used to evaluate the activation energy of thermal degradation and crystallization kinetics. The degradation mechanism was validated by the application of the integral master plot technique. The degradation mechanism of neat PE resembled phase boundary controlled mechanism second and third-order, i.e. (R2, R3), while PE synthesized by G/LDH supported catalyst had a shift in degradation mechanism from (R2, R3) to a diffusion-limited mechanism at the later stages of degradation.

Introduction

Recently, there has been a remarkable growth in the application of nanomaterials and hybrid nanomaterials for catalysis. One such area is nanomaterials' use as support for metallocene catalyst for olefin polymerization (Kaminsky, 1996; Busico and Cipullo, 2001). Metallocene catalyst provides the possibility to synthesize stereo-regular polymers with a predefined structure. Despite their several attractive properties, there is still a need to improve its specific characteristics such as mechanical strength, electrical conductivity, degradability, lower gas permeability, etc. for targeted applications (dos Ouros et al., 2014). Such desired improvements are usually achieved by preparing composites of polymers with other nanomaterials. These composites can be tuned for specific applications like energy storage applications, sensors, drug delivery, water purification, and food packaging (Youssef, 2013; Kavitha et al., 2014; Zhang et al., 2015). Polymer composites are generally prepared by (a) melt blending, (b) solution casting, and (c) in-situ polymerization methods. Amongst these routes, in-situ polymerization provides better filler dispersion within the polymer matrix due to exfoliation (dos Ouros et al., 2014). However, there are numerous nanomaterials, each one having specific characteristics. Some of the attained special research interests are graphene and carbon nanotubes (CNTs) due to their exceptional mechanical, electrical, thermal properties, and chemical functionalization capability. The nanocomposite of polyolefin with carbon-based nanomaterials have been extensively reported in the literature (Han et al., 1995; Trujillo et al., 2007; Kaminsky et al., 2008; Stürzel et al., 2012; Daud et al., 2015; Buffet et al., 2015a, Buffet et al., 2015b; Shehzad et al., 2016). The critical aspect of these nanomaterials related to this work is that these nanomaterials affect the catalytic activity, molecular weight (MWD), MWD distribution, and polymer morphology through in-situ polymerization. Similarly, clay-based nanomaterials, such as layered double hydroxides have also been investigated due to low cost, abundance, and essential characteristics such as flame retardancy (Zhao et al., 2012). LDH are hydrotalcite type compounds possessing a brucite like structure. The structure of LDH is composed of M²⁺ coordinated to six hydroxide anions and the octahedra share the edges building infinite two-dimensional layers. These materials have been explored widely in catalysis, adsorption, filtration, and reinforcement filler for polymers (Daud et al., 2018). As the LDH nanomaterials provide the flexibility to disperse various anions between its layers and vary the cation ratio, it can be fine-tuned for various catalysis applications. There are, however, some challenges with LDH and other nanomaterials during their utilization as support for catalysis. For example, the most common issue is the restacking of LDH layers. Likewise, the agglomeration of graphene also needs to be addressed to exploit its full potential. These issues can be resolved by hybridizing LDH and graphene (graphene-LDH nanocomposites) to achieve enhanced complementary properties (Zhao et al., 2012; Daud et al., 2016). The presence of graphene prevents the restacking of LDH and improves its stability and catalytic properties (Daud et al., 2016). Such graphene-LDH hybrids can also be used to support the metallocene catalyst for olefin polymerization. Some research studies have been reported in the literature which discussed the use of LDH as support for metallocene and Ziegler Natta catalyst (He and Zhang, 2007; Zhang et al., 2008; Xu et al., 2011; Buffet et al., 2015a, Buffet et al., 2015b; Buffet et al., 2016). These studies reported that the catalytic activity, polymer morphology, and molecular weight of polymer were influenced by LDH support depending upon the metal cations, anion, and other chemical treatment of LDH nanomaterials. However, studies of the low crystallinity graphite-LDH hybrid have not been reported so far.

Polyethylene synthesized by using a supported catalyst contains the impurities of the support. These nanomaterials influence the crystallization of PE, which consequently influences its physical and chemical properties. PE being a semicrystalline polymer possesses enough long-chain molecules and crystallizes under suitable conditions. The transformation from the melt or disordered state to crystalline form is a necessary process that influences the polymer's structure and morphology, which then dictates its thermodynamics, spectroscopic, physical, and mechanical properties (Mandelkern, 2004). Thus, understanding the crystallization kinetics and nanomaterials' effect on the nucleation and growth is of keen importance. Moreover, non-isothermal crystallization kinetic studies help the polymers' industrial processing because it occurs under non-isothermal conditions. Numerous studies have been reported in the literature about the crystallization kinetics of PE and other semicrystalline polymers (Mandelkern, 2004; Run et al., 2005; Shehzad et al., 2014). However, studies on in-situ polymerized PE/G-LDH nanocomposites have not been reported yet.

This work aims to present the non-isothermal crystallization and degradation kinetics of polyethylene synthesized by low crystallinity graphite-NiAl LDH supported zirconocene catalyst. The resulting polymer was then characterized for its thermal and microstructural characteristics. We employed well-known kinetics models to predict polymer transformation from melt state by utilizing differential scanning calorimetry (DSC). Two well-known kinetic models, i.e., the Ozawa and Mo models were used for crystallization kinetics. At the same time, the integral isoconversional method was applied to assess the activation energy of nanocomposites. Moreover, the thermal stability and degradation kinetics were studied by thermogravimetric analysis. The master plots technique was employed to get insights into the degradation mechanism. The catalytic activity's details, characterization of the catalyst, support, and synthesized polymers will be reported in another article.