Structural and Dynamical Insights into the Formation Process of a Cross-Linked Polymer Network in Acrylic Adhesives During Thermal Curing

Journal Article · Thu Jul 24 00:00:00 EDT 2025 · Macromolecules

DOI:https://doi.org/10.1021/acs.macromol.5c01222· OSTI ID:2587072

Tsapatsaris, Leonidas ^[1]; Wiegart, Lutz ^[2]; Petrash, Stanislas ^[3]; Ribbe, Alexander ^[4]; Cash, Jeff ^[5]; Engels, Thomas ^[6]; Endoh, Maya ^[1]; Koga, Tadanori ^[1]

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1. Stony Brook Univ., NY (United States)
2. Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)
3. Adhesive Technologies; Henkel Corporation
4. Univ. of Massachusetts, Amherst, MA (United States)
5. Henkel Corporation, Bridgewater, NJ (United States)
6. Henkel AG&Co. KGaA, Duesseldorf (Germany)

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. Here, in this paper, we report the curing process of a commercial two-component methyl methacrylate (MMA) adhesive using in operando X-ray photon correlation spectroscopy (XPCS), a method that closely simulates the target manufacturing environment of the adhesive. The results are then integrated with those obtained by rheology, differential scanning calorimetry (DSC), and transmission electron microscopy to establish the structure–dynamics–process–property relationship. The XPCS results identify four distinct stages in the curing process after the mixing, extrusion, and deposition of the acrylic adhesive: (i) At a cure time (or “aging time”, t_age) of less than 1 min, nanodomains of polymerized MMA are formed within a liquid monomeric MMA matrix. The average size is several nm and remains constant over t_age, while the dynamics of the nanodomains are slowed down with t_age due to an increase in the viscosity of the MMA matrix. (ii) After t_age > 1 min, the size of the nanodomains increases with t_age until the gel point (= 6.3 min after mixing as determined by rheology). The dynamics of the nanodomains also increase due to the heat generated by the exothermic reaction. (iii) At the gel point, the nanodomains begin to interconnect each other, resulting in a network structure with a characteristic length of about 100 nm. This characteristic network size does not change for the rest of the curing process up to t_age = 500 min. The dynamics of the network structure, however, show a rapid slowing down with t_age up to t_age ≈ 12 min, corresponding to the onset of vitrification (as determined by rheology). (iv) At t_age > 12 min, when the DSC and rheology data can no longer provide meaningful information, the XPCS data show a further slowing down of the network dynamics associated with vitrification. Our results provide rich and complex insights into the physics and material design of thermosets in commercially relevant processes, which are essential for future industrial applications.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); National Science Foundation (NSF)
• Grant/Contract Number: SC0012704
• OSTI ID: 2587072
• Report Number(s): BNL--228578-2025-JAAM
• Journal Information: Macromolecules, Journal Name: Macromolecules Journal Issue: 15 Vol. 58; ISSN 1520-5835; ISSN 0024-9297
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE