Supramolecular and adaptable covalent polyethylene networks

Abstract

Polyethylene (PE) is the most widely used commodity thermoplastic due to its good solvent resistance, excellent flexibility, low cost and ease of processing. It finds application in common objects like plastic bags, packaging, automotive, medicine, aerospace and electronics.1, 2 However, its use is limited by low melting point, stress cracking and poor wear resistance. To overcome those issues and expand its applications, crosslinking of polyethylene is used. Crosslinking forms a high molecular weight network, which improves impact strength, stress cracking resistance, creep and abrasion resistance without altering significantly tensile strength and density.3, 4 Despite the numerous advantages of crosslinked polyethylene (PEX) and its wide applications, recycling is a major drawback. Because of the crosslinking, PEX does not flow after melting and cannot be reprocessed like a thermoplastic. Most of the PEX waste is currently landfilled or incinerated which is a major problem both for the environment as well as for a recovery of valuable materials.5, 6 PE network that could be de-crosslinked at will or crosslinked using dynamic covalent crosslinkers would be easy to process and recycle while keeping benefits of PEX. The objective of this thesis is to explore properties and potential applications of PE crosslinked using physical or dynamic covalent crosslinkers. Chapter 2 is focused on ureidopyrimidinone (UPy) functionalized polyethylene with enhanced mechanical properties. UPy was readily introduced into various PEs bearing hydroxy groups by solution grafting, affording physically crosslinked PE via supramolecular interactions. Utilizing low melting UPy where its methyl group is substituted with the isopropyl one (isopropyl UPy, IPR UPy), reactive extrusion process was developed that allowed to significantly shorten the reaction time and eliminate the use of solvents and catalysts. Chemical structures were confirmed by Fourier transform infrared spectroscopy (FT IR) and proton nuclear magnetic resonance (1H NMR). Differential scanning calorimetry (DSC), rheology and dynamical mechanical thermal analysis (DMTA) were employed to investigate thermal stability of the obtained polymers and revealed that UPy functionalized PE can be safely processed using techniques like compression molding and extrusion below 150 °C. Introduction of UPy significantly improved mechanical properties and altered rheology showing that quadruple hydrogen bonding interactions are present both in the solid state and in the PE melt up to 150 °C after which UPy starts to degrade before it can dissociate. The development of pyrene grafted polyethylene as a strain detector is described in Chapter 3. High density and very low density polyethylene graft maleic anhydride (HDPE MAH and VLDPE MAH respectively) functionalized with 1 aminopyrene (AP) were prepared via reactive extrusion. The resulting strain-reporting PE retains similar mechanical, thermal and rheological properties to that of the starting PE MAH materials. Fluorescent emission spectroscopy revealed pronounced changes in fluorescent behavior under stress due to the breakup of the pyrene excimers. For HDPE MAH AP this change was very sudden with a clear drop of excimer content (IE/IM) of around 50 % due to necking of the material stretched above 50 % strain. In contrast, VLDPE MAH AP showed no necking and a linear decrease of IE/IM ratio down to around 30 % when elongation up to 1100 % strain was reached while HDPE MAH AP broke after 200 % strain. Oxygen and water vapor barrier properties of PE HEMA functionalized with UPy (ureidopyrimidinone) or MIP (1 methylisocyanatopyrene) are investigated in Chapter 4. Functionalization of PE HEMA1 with 2.2 mol% of UPy decreased OP (oxygen permeability) by about 30 % at 23 °C and 0 % RH (relative humidity) and about 25 % at 38 °C and 50 % RH and WVP (water vapor permeability) by about 25 %. When 1.2 mol% of MIP was introduced into PE HEMA1, OP was decreased by about 35 % at 23 °C and 0 % RH and about 30 % at 38 °C and 50 % RH and WVP by about 40 %. Despite the achieved improvements the samples did not performed as well as the commercially available EVOH based multilayer structure used as a reference. Synthesis and characterization of polyethylene silyl ether vitrimers is discussed in Chapter 5. PE was dynamically crosslinked directly via reactive extrusion using commercially available N,N' Bis[3 (trimethoxysilyl)propyl] ethylenediamine (TMSPEDA). This fast and efficient process allowed to produce PE vitrimers without any synthetic effort or use of any solvent which makes it environmentally friendly and easy to upscale. Dynamic crosslinking transformed thermoplastic PE into an elastic solid with greatly improved melt strength as revealed by DMTA and rheology. Mechanical properties could be tuned by varying the amount of TMSPEDA crosslinker. All prepared vitrimers were insoluble in xylene and were not affected by moisture, demonstrating crosslinked character and excellent solvent and hydrolysis resistance. Despite the crosslink nature dynamic silyl ether exchange enabled processability and recyclability of this system.

References 1. Kaminsky, W., Polyolefins: 50 Years after Ziegler and Natta Ii. Springer: 2013. 2. Vasile, C., Handbook of Polyolefins. CRC Press: 2000. 3. Ghosh, P., Polymer Science and Technology. Tata McGraw-Hill Education: 1990. 4. Platzer, N., Encyclopedia of Polymer Science and Engineering. Wiley: 1986. 5. Shang, L.; Wang, S.; Zhang, Y.; Zhang, Y. Pyrolyzed Wax from Recycled Cross-Linked Polyethylene as Warm Mix Asphalt (Wma) Additive for Sbs Modified Asphalt. Constr. Build. Mater. 2011, 25, 886-891. 6. Lee, H.-s.; Jeong, J. H.; Cho, H.-K.; Koo, C. M.; Hong, S. M.; Kim, H.; Lee, Y.-W. A Kinetic Study of the Decross-Linking of Cross-Linked Polyethylene in Supercritical Methanol. Polym. Degrad. Stab. 2008, 93, 2084-2088.