Structures and Adhesion Properties at Polyethylene/Silica and Polyethylene/Nylon Interfaces

The molecular structures of buried interfaces of maleic anhydride grafted and ungrafted polyethylene films with silica and nylon surfaces were studied in situ using sum-frequency generation (SFG) vibrational spectroscopy. Grafting maleic anhydride to polyethylene altered the molecular structures at buried interfaces, including changing the orientation of polymer methylene groups and resulting in the presence of CO groups at silica interfaces. These molecular level changes are correlated with enhanced adhesion properties, with ordered CO groups and in-plane orientation of the methylene groups associated with higher levels of adhesion. While improved adhesion was observed for grafted polyethylene at the nylon interface, no CO groups were detected at the interface using SFG, for films thermally treated at 185 °C. In this case, either no CO groups are present at the interface or they are disordered; the latter explanation is more likely, considering the observed improvement in adhesion

In Situ Observation of Chemical Reactions at Buried Solid/Solid Interfaces in Coextruded Multilayer Polymer Films

The characterization of chemical reactions at the buried interface is critical to understand interfacial molecular interactions and improve interfacial properties like adhesion. Interface-sensitive sum frequency generation (SFG) vibrational spectroscopy can probe the buried interface in situ nondestructively. While SFG has been used to study many model polymer interfaces, it has never been applied to study multilayer polymer films produced on commercial coextrusion lines. Here, we apply SFG to elucidate the molecular details of chemical reactions at the buried interface in multilayer cast films consisting of maleic anhydride (MAH)-modified Tie layers promoting the adhesion between polyamide and polyethylene. We demonstrated the utility of SFG to identify the reaction products from the interfacial reaction between MAH and polyamide with varying MAH concentrations and to monitor changes of the interfacial molecular orientation. The developed approach is generally applicable to probe chemical reactions and molecular interactions at buried interfaces in multilayer polymer films

Elucidating the Changes in Molecular Structure at the Buried Interface of RTV Silicone Elastomers during Curing

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