Long Term Evolution of Morphology, Melting and Crystal-Crystal Transitions Facilitated by Dynamic Bond Exchange in Ethylene Dynamic Networks

Dynamic polymer networks with topology conserving exchange reactions (vitrimers) have emerged as a promising platform for sustainable and reprocessable materials. A major focus thus far has been to understand how the exchange kinetics, crosslink density, and polymer backbone chemistry control the stress relaxation and reprocessability of vitrimers. In contrast, an understanding of crystallization phenomena, particularly how bond exchange affects kinetics, final morphology and melting temperatures (T_m) in vitrimers is currently lacking. With the introduction of dynamic bonds in a network, an additional timescale is generated which may facilitate crystal formation by allowing smaller strands to rearrange rather than an entire polymer chain which can potentially facilitate crystal perfection and growth. While some prior work has focused on semi-crystalline vitrimers based on polyethylene and poly(lactic acid), they did not investigate any temporal evolution of crystallinity or morphology due to dynamic bond rearrangements within the matrix. Knowledge of how dynamic bonds impact crystallization will be critical to the development of new polymers which are easier to recycle and reprocess while still retaining desirable properties. Another key factor in determining crystal structure and melting is the presence of precise motifs. For example, the melting temperatures of telechelic alkanes show a pronounced odd-even effect depending on the number of carbons between functional groups. In polymers, precise polyethylenes and polyacetals have been made as linear polymers, including materials with periodic ionic groups which crystallize and show enhanced conductivity. Precise permanent networks with alkane chain linkers have shown odd-even effects on the glass transition temperature and ionic conductivity. All of these studies point to the potentially critical role of precision on crystallization, which has not been investigated in vitrimers.<br/>Precise ethylene vitrimers with 6 – 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange and precision on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures (T_m) of 25-30 K is observed with isothermal crystallization over 30 days. Both C10 and C12 networks initially form hexagonal crystals, while the C10 network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C8), orthorhombic (C10), and hexagonal (C12) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomer, followed by repolymerization while preserving the mechanical properties and melting temperatures.