Dynamic polyolefin networks

Abstract

Conventional thermoplastics such as polyethylene (PE) and polypropylene (PP) are known for being easily processable, lightweight, robust, and low-cost materials. Some of their flaws include low wear, chemical, and solvent resistance. To overcome these flaws, it is common practice to cross-link these materials by means of irradiation, peroxide, or other chemical methods. While significantly improving the material properties, this presents the inherent downside that these cross-linked thermosets are no longer processable. To circumvent this issue, dynamic cross-links can be introduced, which either adapt or dissociate upon applying a stress. The objective of this thesis was to first evaluate existing tools for the dynamic cross-linking of polymeric materials and, where possible, improve or provide new tools suitable to obtain adaptable polyolefin networks. Then, the implications of these dynamic cross-links for the material properties were elucidated, analyzing the dynamic mechanical and relaxation behavior, as well as investigating how these new properties relate to real world applications. To achieve this, dynamic cross-links based on multiple hydrogen-bonding units, aromatic stacking of cavitands, and a sub class of covalent adaptable networks (CANs), referred to as vitrimers, have been studied. To provide additional tools for the dynamic cross-linking of polymers, a new multiple hydrogen bonding motif capable of reversible association was developed and described. This motif, namely 1-(7-oxo-7,8-dihydro-1,8-naphtiridin-2-yl)urea (ODIN), is readily synthesized in two steps. X-ray crystallography revealed that its dimer is comprised of two regular and two bifurcated hydrogen bonds, resulting in a total of six hydrogen bonds. By alternating the arrangement of donor (D) and acceptor (A) moieties, a motif with a greater number of hydrogen bonds than the fields most commonly used motif ureidopyrimidone (UPy), but with a lower association constant (Ka), can be obtained. Thereby, ODIN combines the directionality of multiple hydrogen bonding that provide increased resistance towards water with a relatively weak Ka of 4·104 M-1 (in CHCl3). The weak Ka presents advantages when these moieties are embedded into apolar polymers, because the apolar polymer matrix stabilizes the dimer. For an inherently stronger motif (higher Ka), this could lead to processing issues. To demonstrate the potential of reversible cross-linking with this new multiple hydrogen bonding motif, liquid tri-functional Jeffamine T3000 bearing pendant amines was reacted with an isocyanate functionalized ODIN derivative, resulting in the moldable suprapolymer JeffODIN. The specific viscosity (sp) of this suprapolymer dissolved in chloroform increased significantly faster with concentration compared to the non-functional oligomeric chains. The reversible nature of this cross-linking was apparent from variable temperature infrared spectroscopy (VTIR), based on the reduced intensity and shift of bands involved in hydrogen bonding. The sextuple hydrogen bonding motif ODIN is used for the dynamic cross-linking of PE. By reacting ODIN bearing an aliphatic isocyanate with poly (ethylene-co-(2-hydroxethyl methacrylate)) (PE-HEMA), multiple hydrogen bonding array functionalized PE with an ODIN content ranging from 1 to 6 mol% were obtained. Compared to the pristine material, the obtained PE-HEMA-ODIN had increased tensile strength, stiffness, gas and water barrier properties, and a remarkably increased melt strength evident from the observed plateau modulus after melting. The formation of a network structure was revealed by a significant increase in glass transition (Tg) and shear and tensile storage moduli. The apolar PE matrix was found to stabilize the new hydrogen bonding motif, because in contrast to a supramolecular polymer, no substantial dissociation of hydrogen bonds was observed up to 175 °C. The adaptable nature of the hydrogen bonding system was revealed via stress relaxation measurements and compression molding of thin films and sample strips. In comparison to UPy functionalized polymers, differential scanning calorimetry (DSC) cycling and isothermal dynamic mechanical analysis (DMA) measurements found that the thermal degradation of ODIN polymers occurred at 190 °C, which is about 40 °C later than the temperature of degradation determined for UPy polymers. At these temperatures the degradation mechanism involves a permanent cross-linking resulting in an increased storage modulus (tension) (E’) during an isothermal DMA measurement performed at 190 °C. Due to the increased thermal stability of ODIN the concept could be extended to maleic anhydride (MA) functional PP, a polymer that usually melts above the degradation temperature of the UPy motif. An alternative method to reversibly cross-link PE was investigated, relying on the solvophobic reversible association of a quinoxaline cavitand in kite conformation with a Ka of 5 – 8.5·104 M-1 (in CH2Cl2). First, the cavitand was functionalized with an isocyanate group at the lower rim so that functionalization of a hydroxy or amine containing polymer can be readily achieved. This functional cavitand was reacted with PE-HEMA, and the resulting polymer was characterized by 1H-NMR, DSC, and FTIR. Films with varying amounts of cavitand, obtained by solution casting and slow evaporation, were tested for their oxygen barrier, which was found lacking due to a severely reduced crystallinity and increased free volume because of introducing the bulky group. Most significantly, the isocyanate functional cavitand provides a tool to easily introduce cavitands into polymers, or to attach further functionality. Vitrimers are an emerging class of permanently cross-linked materials, that are dynamic because these cross-links can undergo thermally triggered exchange reactions. Therefore, the network density is constant, but the materials are still malleable when the exchange reactions are fast enough. The introduction of exchangeable cross-links based on vinylogous urethanes and disulfides in PE is described, both in solution and in the melt. This is achieved by employing a suitable macromer precursor with pendant acetoacetate (ACAC) or MA groups together with multifunctional amine cross-linkers. The focus was laid on reactive extrusion to demonstrate an industrially relevant, easy to up-scale pathway to PE vitrimers. An importance was placed on the varying amount of cross-links per chain (LPC), which could be controlled by varying the number average molecular weight (Mn) and mol% of ACAC of the precursor polymer. Only after enough LPC to form a network structure were employed (>1.9 LPC), materials were insoluble with a maximum gel fraction of 65.3±4.4 %, and relaxation was controlled by the exchange kinetics following an Arrhenius type dependence on temperature. Compared to vinylogous urethane vitrimers based on short chain oligomers, an almost twice as large activation energy (Ea) of 108±2 kJ·mol-1 was determined due to the reduced density of reactive groups in the polymer matrix. By repeated tensile tests and extrusion of the material, recyclability of up to four times could be demonstrated for a network with 2.6 LPC. In closing, three methods to dynamically cross-link polyolefins were described, namely multiple hydrogen bonding, solvophobic interactions, and dynamic exchange reactions. Using a functional cavitand to obtain these systems appears to be the least appealing due to the synthetic effort involved and substantial size of the functional motif. In fact, the synthetic effort stopped an exhaustive investigation into the mechanical implications of the functional polymers simply due to the lack of enough material. Multiple hydrogen bonding and exchangeable cross-links were both effective tools to develop dynamic PE networks. The increased thermal stability and melt strength of these functional PE is useful in applications where high temperatures are used. Also, simply extruding materials with dynamic cross-links eliminates the need for "after the fact" static cross-linking. Implementing the vitrimer technology into existing production may be readily done. First, the precursor co-polymer can be prepared by catalyst free radical polymerization using methods employed to produce PE. Second, the synthesis of the vitrimer requires only one step that can be carried out inside the extruder, so that direct injection molding after synthesis is an option, combining processability of thermoplastics with thermoset resilience.