Tzu-Chi Kuo

Two-component polyurethane (PU) adhesives are widely used in transportation, construction, and packaging due to their tunable strength and cost-effectiveness. In automotive, aerospace, and industrial applications, these adhesives must maintain good performance across typical usage temperatures (−40 to 80 °C). However, many commercial PU adhesives lose adhesion at elevated temperatures. This study uses sum frequency generation (SFG) vibrational spectroscopy, supported by Fourier transform infrared spectroscopy (FTIR) and adhesion testing, to investigate molecular-level mechanisms of such loss of PU adhesion to alumina and nylon. At the PU/nylon interface, adhesion loss correlates with reduced SFG signals from nylon C═O groups, suggesting interfacial disordering. In contrast, the PU/alumina interface shows no spectral change, indicating that adhesion loss likely stems from weakened van der Waals interactions. These findings offer insight into temperature-induced adhesion failure at buried PU interfaces, providing knowledge for developing PU adhesives with improved performance.

Polymer thin films are widely used in both industries and everyday life. The performance of thin films is improved by using multiple functional layers through either lamination with adhesives or multilayer coextrusions. Further machine direction stretching to achieve oriented (MDO) films or both machine direction and cross-machine direction stretching to achieve biaxially oriented (BO) films can continually enhance their performance. Polymer multilayer thin films can degrade upon stretching for certain uses due to the interlayer adhesion loss. In this research, we investigated the buried nylon/maleic anhydride (MAH)-functionalized ethylene octene (EO) copolymer (MAHfEO) interface in a multilayer thin film made from a coextrusion blowing process and followed with and without machine direction (MD) stretching. Sum frequency generation (SFG) vibrational spectroscopy was applied to study the chemical groups related to adhesion at the buried interface. The SFG signal of the unstretched film indicates weaker polymer chain entanglement and more ordered functional groups at the nylon/MAHfEO interface compared to the previously studied EVOH/MAHfEO interface in EVOH films. The signal increase from the unstretched to the 3× stretched films shows that reduced entanglement leads to adhesion loss. The comparatively small increase in signals from the 3× to the 6× stretched films indicates that the influence of entanglement loss due to the stretching becomes less significant for adhesion loss, while the chemical bond formed by the reaction at the buried interface plays a role in determining adhesion. Analysis of SFG spectra collected from orthogonal sample directions showed that the azimuthal orientation of interface functional groups was slightly reoriented toward the stretching direction. The research further explores the relationship between the polymer chain structure and its macromolecular properties, providing insights for rational material design and performance improvements.

Polymer multilayer films with functional tie layers are used in a variety of applications. The layer adhesion in these multilayer films is known to degrade upon stretching of the multilayer structure, which is required for certain applications. Although the loss of adhesion strength is empirically observed, the molecular basis or mechanism of this adhesion loss is unknown. In this research, we investigated buried interfaces of polymer multilayer thin films with and without stretching, in situ, to understand the reason for adhesion loss using sum frequency generation (SFG) vibrational spectroscopy. The unstretched film exhibits no SFG signals in the C═O stretching frequency region at the ethylene-vinyl alcohol copolymer/maleic anhydride (MAH) grafted polyethylene interface, indicating disordered interfacial C═O groups. This is because the C═O groups are randomly orientated at the interface due to the interfacial diffusion and interfacial chain entanglement of the two polymer materials, leading to a "diffused" interface instead of a distinct or sharp interface. For the 3× and 6× stretched films, SFG C═O stretching signals could be easily detected from the buried interfaces. Analysis of the SFG spectra collected from orthogonal directions showed that the azimuthal orientations of interfacial C═O groups were randomly distributed without a preferred alignment along the stretching direction. Further analysis of the SFG results suggests that the loss of physical entanglement at the interface, rather than interfacial chemical bond breaking, was the reason for the adhesion decrease in stretched films. This research provides important insights into future polymer thin-film material design and performance improvements.

The curing and adhesion properties of many silicone adhesives are mediated by organometallic catalysts (e.g., titanates, zirconates, or aluminates) and silane cross-linker and coupling agents in the formulation. However, how these catalysts and cross-linkers behave at the interface to enhance adhesion is not well established. In this study, sum frequency generation (SFG) vibrational spectroscopy was utilized to investigate the behaviors of titanium catalysts and silane cross-linkers at buried interfaces. When a titanium catalyst (titanium diisopropoxy-bis ethylacetoacetate (TDIDE)) was mixed with methyltrimethoxysilane (MTMS), a new SFG signal centered at ∼2815 cm^-1 emerges, especially in the formulations where there were substantially more MTMS than titanium catalysts. At the same time, methoxy signals (∼2845 cm^-1) were found to increase in intensity as more MTMS was present in the sample. In contrast, mixtures that had more titanium catalysts than MTMS did not show the ∼2815 cm^-1 peak or the strong methoxy signals. It is concluded that the ∼2815 cm^-1 peak is attributed to the C-H stretching mode of a Ti-OCH₃ moiety, an intermediate species formed by the reaction between MTMS and titanium catalyst that results in ligand exchange. SFG experiments were also conducted with TDIDE-MTMS mixtures incorporated into PDMS. Similar interfacial behaviors of TDIDE and MTMS could be observed at the interface. To the best of our knowledge, this is the first time that the mechanism has been elucidated as to the molecular interactions that occur between titanium catalyst and silane in silicone adhesive formulation at buried interface.

Self-assembled monolayers (SAMs) are highly ordered thin coatings that provide a variety of excellent properties to the material surface. They bestow unique properties to the material such as hydrophobicity, lubrication, and barrier performance but are challenging to investigate due to their low concentrations. In this study, we investigate the SAM formation of amide wax components that segregate from the carboxylic acid-functionalized polyethylene coating bulk and self-assemble at the surface. We applied sum frequency generation (SFG) vibrational spectroscopy to study the SAM formations and the resulting structures of segregated surface waxes as a function of the wax/polymer ratio, polymer matrix formulation, and wax molecule types. It was found that the type of wax molecules, the wax concentration, and the composition of the polymer matrix strongly influence the quality of the formed SAM. Increasing the heat treatment temperature had positive effects on the waxes that were weakly trapped by the bulk matrices but did not impact the waxes interacting strongly with the polymer matrix.

Silicone sealants and adhesives are extensively used in construction, automotive, industrial, and electronic applications because they exhibit excellent mechanical properties, strong adhesion, and good weather resistance. Room-temperature vulcanized (RTV) silicones develop good adhesion to many substrates and do not require heat for curing, which leads to flexible use in many applications. Although it is known that various factors such as relative humidity and temperature affect the curing of the RTV silicone adhesives, the interfacial chemistry that occurs during the curing process is still poorly understood but critical for success in adhesive applications. To address this, sum frequency generation (SFG) vibrational spectroscopy was used to probe the molecular details of the buried interface of the RTV silicone adhesive in situ. Time-dependent SFG experiments were conducted on two polydimethylsiloxane (PDMS) matrices, at three humidity levels, and with two kinds of silica surfaces to investigate the behavior of the methoxy groups at the interface and the impact of environmental conditions on the adhesion mechanism. It was found that both the methoxy groups from methyltrimethoxysilane (MTMS) and methoxy-terminated PDMS could segregate to the interface. The diffusion of MTMS and bulk rearrangement of methoxy-terminated PDMS lead to the segregation and ordering of methoxy groups at the interface. After comparing eight samples cured under different environmental conditions, the reactions of the interfacial methoxy groups were found to be facilitated by both the surface water on silica and moisture from the environment. The silylation treatment on the silica slows the reactions of the interfacial methoxy groups, while the high environmental humidity accelerates the consumption of the interfacial methoxy groups. These findings provide insightful information about the adhesion mechanism of RTV silicone adhesives and accelerate new product development.

Sum frequency generation vibrational spectroscopy (SFG) was applied to characterize the interfacial adhesion chemistry at several buried polymer interfaces in both model systems and blown multilayer films. Anhydride/acid modified polyolefins are used as tie layers to bond dissimilar polymers in multilayer barrier structures. In these films, the interfacial reactions between the barrier polymers, such as ethylene vinyl alcohol (EVOH) or nylon, and the grafted anhydrides/acids provide covalent linkages that enhance adhesion. However, the bonding strengths vary for different polymer–tie layer combinations. Here, using SFG, we aim to provide a systematic study on four common polymer–tie interfaces, including EVOH/polypropylene–tie, EVOH/polyethylene–tie, nylon/polypropylene–tie, and nylon/polyethylene–tie, to understand how the adhesion chemistry varies and its impact on the measured adhesion. Our SFG studies suggest that adhesion enhancement is driven by a combination of reaction kinetics and the interfacial enrichment of the anhydride/acid, resulting in stronger adhesion in the case of nylon. This observation matches well with the higher adhesion observed in the nylon/tie systems in both lap shear and peel test measurements. In addition, in the polypropylene–tie systems, grafted oligomers due to chain scission may migrate to the interface, affecting the adhesion. These by-products can react or interfere with the barrier–tie chemistry, resulting in reduced adhesion strength in the polypropylene–tie system.

Maleic anhydride (MAH)-modified polymers are used as tie layers for binding dissimilar polymers in multilayer polymer films. The MAH chemistry which promotes adhesion is well characterized in the bulk; however, only recently has the interfacial chemistry been studied. Sum frequency generation vibrational spectroscopy (SFG) is an interfacial spectroscopy technique which provides detailed information on interfacial chemical reactions, species, and molecular orientations and has been essential for characterizing the MAH chemistry in both nylon and ethyl vinyl alcohol copolymer (EVOH) model systems and coextruded multilayer films. Here, we further characterize the interfacial chemistry between MAH-modified polyethylene tie layers and both EVOH and nylon by investigating the model systems over a range of MAH concentrations. We can detect the interfacial chemical reaction products between MAH and the barrier layer at MAH concentrations of ≥0.022 wt % for nylon and ≥0.077 wt % for EVOH. Additionally, from the concentration-dependent reaction reactant/product SFG peak positions and the product imide or ester/acid C═O group tilt angles extracted from the polarization-dependent SFG spectra, we quantitatively observe concentration-dependent changes to both the interfacial chemistry and interfacial structure. The interfacial chemistry and molecular orientation as a function of MAH concentration are well correlated with the adhesion strength, providing important quantitative information for the future design of MAH-modified tie layers for a variety of important applications.

Silicone elastomers are widely used in many industrial applications, including coatings, adhesives, and sealants. Room-temperature vulcanized (RTV) silicone, a major subcategory of silicone elastomers, undergoes molecular structural transformations during condensation curing, which affect their mechanical, thermal, and chemical properties. The role of reactive hydroxyl (-OH) groups in the curing reaction of RTV silicone is crucial but not well understood, particularly when multiple sources of hydroxyl groups are present in a formulated product. This work aims to elucidate the interfacial molecular structural changes and origins of interfacial reactive hydroxyl groups in RTV silicone during curing, focusing on the methoxy groups at interfaces and their relationship to adhesion. Sum frequency generation (SFG) vibrational spectroscopy is an in situ nondestructive technique used in this study to investigate the interfacial molecular structure of select RTV formulations at the buried interface at different levels of cure. The primary sources of hydroxyl groups required for interfacial reactions in the initial curing stage are found to be those on the substrate surface rather than those from the ingress of ambient moisture. The silylation treatment of silica substrates eliminates interfacial hydroxyl groups, which greatly impact the silicone interfacial behavior and properties (e.g., adhesion). This study establishes the correlation between interfacial molecular structural changes in RTV silicones and their effect on adhesion strength. It also highlights the power of SFG spectroscopy as a unique tool for studying chemical and structural changes at RTV silicone/substrate interface in situ and in real time during curing. This work provides valuable insights into the interfacial chemistry of RTV silicone and its implications for material performance and application development, aiding in the development of improved silicone adhesives.