Over de Competitie tussen Kristallisatie en Diels-Alder-Verknoping in Dynamische Covalente Netwerken

Abstract

Additive manufacturing, and fused filament fabrication (FFF) in particular, has rapidly expanded beyond prototyping into structural applications in aerospace, healthcare, and soft robotics. Yet a fundamental limitation persists, namely that the layer-by-layer deposition process inherently creates weak interlayer interfaces due to incomplete chain diffusion and bonding across adjacent layers. In conventional semi-crystalline thermoplastics, crystallization rapidly arrests chain mobility near the interface before sufficient molecular integration can occur, producing anisotropic and mechanically inferior parts. Dynamic covalent networks (DCNs) based on the thermoreversible Diels-Alder (DA) reaction offer a compelling solution, as their reversible covalent bonds can stitch adjacent layers together while still permitting reprocessing. Incorporating a crystallizable backbone further confers the rapid solidification needed for shape fidelity during printing, as well as the mechanical stiffness and strength associated with the semi-crystalline microstructure. However, this dual solidification character introduces a poorly understood kinetic competition between chemical crosslinking and physical crystallization. The relative timing, extent, and spatial outcome of these two concurrent processes fundamentally govern the microstructure, mechanical properties, and interlayer adhesion of printed parts. Without mechanistic insight into this competition, rational design of processing conditions remains impossible. This dissertation addresses that gap systematically, ranging from molecular architecture to bulk solidification to processing-relevant interfacial bonding. Additionally, it also provides both the experimental tools and foundation for predictive models needed to exploit these materials in advanced manufacturing. Chapter 2 establishes the fundamental competition between crosslinking and crystallization by systematically varying the precursor functionality of poly(e-caprolactone) (PCL)-based DCNs across linear 2-functional, star-shaped 4-functional, and 6-functional architectures. Precursor functionality was found to govern both the resulting network topology and the timescale to reach gelation relative to crystallization, rather than the underlying Diels-Alder reaction kinetics: the 2-functional material undergoes reversible chain extension without ever forming a percolating network, the 4-functional DCN reaches its gel point on a timescale far slower than crystallization, and the 6-functional DCN, owing to its lower theoretical gel-point conversion, reaches gelation on a timescale comparable to crystallization. Additionally, the Diels-Alder crosslinking in these polymer networks is shown to consistently restrict chain mobility across all functionalities, manifesting as lower crystallization temperatures, broader crystallization peaks, reduced overall crystallinity, and smaller and more irregular crystal morphologies compared to decrosslinked counterparts. Equilibrium melting temperatures are raised across all multifunctional crosslinked networks, reflecting a universal constraint on chain positional freedom irrespective of network architecture. Furthermore, decoupling the nucleation from the crystal growth process reveals distinct functionality-dependent effects where star-shaped architectures suppress nucleation density due to steric constraints imposed by the network junction points upon crosslinking. In contrast, the bonded linear 2-functional system shows enhanced nucleation, which is attributed to molecular weight effects from chain extension, although crystal growth remains hindered in both cases. Together, these findings establish how precursor functionality and crosslinking state govern the balance between network formation and crystallization, providing a foundation for tailoring microstructure and mechanical properties through molecular design. Chapter 2 revealed the competition between the two solidification processes but could only investigate them separately and under indirect conditions. Chapter 3 addresses this limitation by developing and validating a hyphenated rheo-Raman methodology that simultaneously tracks the macroscopic viscoelastic evolution and the molecular-level configurational changes during crosslinking and crystallization within a single controlled sample. Oscillatory rheology monitors the mechanical response, while in situ Raman spectroscopy resolves DA conversion and PCL crystallinity in real time, via furan depletion and carbonyl peak evolution, respectively. The results uncover a clear bidirectional coupling between the two solidification mechanisms: pre-existing crosslinks restrict PCL chain mobility and thereby hinder crystallization, while progressing crystallization in turn sequesters reactive groups in the crystalline phase and limits further network formation. The applied temperature profile and cooling rate emerge as the decisive parameters controlling the relative timing and extent of both processes, with slow cooling allowing substantial crosslinking to form before crystallization onset, and fast cooling favoring a CrysCross sequence in which crystallization largely precedes network formation. This methodology addresses a critical experimental limitation, which is the inability to capture both mechanisms simultaneously under identical conditions. It also provides an experimental platform for directly linking molecular-level transformations to bulk mechanical behavior in any thermoresponsive dynamic polymer network. Building on the bulk characterization of Chapters 2 and 3, Chapter 4 translates these findings to the processing-relevant scale of fused filament fabrication using 4- and 6-functional DCNs as printing materials. A general thermal processing window for FFF of semi-crystalline DCNs is defined, bounded below by the glass transition temperature and above by side-reaction-inducing temperatures. The solidification pathway upon deposition, controlled by the combination of print and plate temperatures, is identified as the primary determinant of both bulk mechanical performance and interlayer adhesion. A key finding is that total DA conversion remains comparably high across all printing conditions, yet mechanical performance differs dramatically depending on whether the dynamic network forms before crystallization (CrossCrys) or after (CrysCross). The CrossCrys pathway is assumed to establish inter-lamellar covalent bridges in the unconstrained melt, enabling ductile, near-isotropic mechanical behavior through dynamic bond exchange. The CrysCross pathway likely confines reactive groups to narrow inter-lamellar domains before crosslinking can propagate, producing a chemically percolating but mechanically ineffective intra-lamellar network that fails in a brittle fashion despite equivalent DA conversion. The 6-functional CrossCrys combination emerges as the optimal strategy resulting in a combination of near-unity isotropy ratios with strong absolute mechanical performance by effectively erasing the layer-by-layer signature of the FFF process through inter-lamellar covalent stitching. Chapter 5 develops integrated structure-property relations that links the microscopic DA reaction and PCL crystallization kinetics to the macroscopic viscoelastic response and serves as a foundation for a global modeling framework enabling the prediction of rheological behavior under the complex, non-isothermal conditions of additive manufacturing. The DA reaction kinetics are described by a global single-adduct Arrhenius model, with activation energies consistent with the furan-maleimide polymer literature. Deviations at intermediate temperatures, attributed to endo/exo isomerization, highlight the sensitivity of the DA reaction to the local chemical environment within the semi-crystalline PCL matrix. The rheological response is modeled using Generalized Effective Medium (GEM) models that capture the two possible solidification mechanisms. A gelled network formation, located at DA conversions of 58% and 45% for the 4- and 6-functional systems respectively, marks the transition from a viscous melt to a viscoelastic solid, while PCL crystallization then superimposes a second solidification event that occurs simultaneously with the Diels-Alder reaction. The instantaneous extent of DA crosslinking, whether in a pre-gel or post-gel state, directly dictates the resulting crystallite size, lamellar organization, and the crystallinity threshold required to establish a mechanically percolating crystal network. By coupling the kinetic reaction model with the GEM rheological model, a foundation for a global modeling framework is given that enables prediction of viscosity and modulus evolution under arbitrary processing temperature profiles, providing a physically grounded tool for the rational design of processing cycles that balance thermal thinning, chemical crosslinking, and physical crystallization. In summary, this research provides a fundamental basis for understanding the concurrent chemical and physical solidification behavior of semi-crystalline dynamic covalent networks under complex thermal conditions. By mapping how network topology dictates final part performance, this work offers crucial insights into eliminating the mechanical anisotropy inherent to fused filament fabrication. Ultimately, the developed experimental methodologies and theoretical frameworks establish a robust foundation for the rational design and processing of high-performance, reprocessable polymers in advanced manufacturing.