Structural and Dynamical Insights into the Formation Process of a Cross-Linked Polymer Network

Many modern adhesives, sealants, and coatings rely on the controlled transition from a liquid to a solid state by forming a three-dimensional cross-linked polymer network, often referred to as curing. The curing process, which is initiated by the mixing of reactive components or an external trigger, defines the structure of a network and further controls the final mechanical properties of cured materials. However, the curing mechanism is not fully understood yet due to the lack of experimental tools capable of directly probing the structure and dynamics of a network over relevant time- and length scales.

We studied the curing process of a commercial two-component methyl methacrylate adhesive using in-operando X-ray photon correlation spectroscopy (XPCS), a method that closely probes the target manufacturing environment of the adhesive. We further integrated with other methods, such as TEM, DSC and rheology, to establish the structure−dynamics−process−property relationship. The results reveal four distinct stages in the curing process based on cure time (or “aging time”, t_age): (i) t_age < 60 sec: formation of nanodomains within a liquid matrix, (ii) 1 min < t_age < gel point ≈ 6 min: increase in nanodomain size and dynamics, (iii) t_age > gel point: interconnection of nanodomains to form a network structure, and (iv) t_age > 12 min: vitrification. Our findings at multiple spatial and temporal scales provide valuable insights for designing thermosets for various applications.