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.

Probing Covalent Interactions at a Silicone Adhesive/Nylon Interface.

Covalent bonding is one of the most robust forms of intramolecular interaction between adhesives and substrates. In contrast to most noncovalent interactions, covalent bonds can significantly enhance both the interfacial strength and durability. To utilize the advantages of covalent bonding, specific chemical reactions are designed to occur at interfaces. However, interfacial reactions are difficult to probe in situ, particularly at the buried interfaces found in well-bonded adhesive joints. In this work, sum frequency generational (SFG) vibrational spectroscopy was used to directly examine and analyze the interfacial chemical reactions and related molecular changes at buried nylon/silicone elastomer interfaces. For self-priming elastomeric silicone adhesives, silane coupling agents have been extensively used as adhesion promoters. Here with SFG, the interfacial chemical reactions between nylon and two alkoxysilane adhesion promoters with varied functionalities (maleic anhydride (MAH) and epoxy) formulated into the silicone were observed and investigated. Evidence of reactions between the organofunctional group of each silane and reactive groups on the polyamide was found at the buried interface between the cured silicone elastomer and nylon. The adhesion strength at the nylon/cured silicone interfaces was substantially enhanced with both silane additives. SFG results elucidated the mechanisms of organo-silane adhesion promotion for silicone at the molecular level. The ability to probe and analyze detailed interfacial reactions at buried nylon/silicone interfaces demonstrated that SFG is a powerful analytical technique to aid the design and optimization of materials with desired interfacial properties.

Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface.

Silicone adhesives are widely used in many important applications in aviation, automotive, construction, and electronics industries. The mixture of (3-glycidoxypropyl)trimethoxysilane (γ-GPS) and hydroxy-terminated dimethyl methylvinyl co-siloxanol (DMMVS) has been widely used as an adhesion promoter in silicone elastomers to enhance the adhesion between silicone and other materials including polymers. The interfacial molecular structures of silicone elastomers and the adhesion promotion mechanisms of a γ-GPS-DMMVS mixture in silicone without a filler or an adhesion catalyst (AC) have been extensively investigated using sum frequency generation (SFG) vibrational spectroscopy previously. In this research, SFG was applied to study interfacial structures of silicone elastomeric adhesives in the presence of a silica filler and/or a zirconium(IV) acetylacetonate adhesion catalyst at the silicone/polyethylene terephthalate (PET) interface in situ nondestructively to understand their individual and synergy effects. The interfacial structures obtained from the SFG study were correlated to the adhesion behavior to PET. The interfacial reactions of methoxy and epoxy groups of the adhesion promoter were found to play significant roles in enhancing the interfacial adhesion of the buried interface. This research provides an in-depth molecular-level understanding on the effects of a filler and an adhesion catalyst on the interfacial behavior of the adhesion promotion system for silicone elastomers as well as the related impact on adhesion.

Tough, Transparent, Photocurable Hybrid Elastomers.

We investigated polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA) interpenetrating polymer networks (IPNs) by both sequential and simultaneous syntheses. In the sequential IPN, the PDMS network was first thermally cured after which methyl methacrylate was swelled in and UV photopolymerized in situ. The simultaneous IPN consists of a one-pot, single-step UV cure of both components. Pure shear fracture and tensile tests were used to extract the Young's modulus, critical fracture strain, and fracture energy of the materials at varying PMMA fractions (up to 50 wt %). At high PMMA fractions, a maximum increase in Young's modulus (42×) and fracture energy (21×) was observed with little sacrifice in the optical properties and the extensibility of notched samples. The Krieger-Dougherty model for particle reinforcement was fit to the modulus data as a function of the PMMA fraction and showed good agreement. The optical properties and microstructure of the IPNs were investigated by UV-visible light transmission, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). As the weight fraction of PMMA increased, the simultaneous IPN became less transparent, while the sequential material showed the opposite trend. In the sequential IPN, the minority phase size decreased with increasing PMMA fraction, while it was constant for the simultaneous IPN. Therefore, it was concluded that the sequential IPN transparency is controlled by the size of the PMMA domains, but the simultaneous IPN transparency is controlled by the PMMA fraction. SAXS and AFM also showed evidence of bicontinuous network formation in the simultaneous IPN, which may affect the optical and mechanical properties.

Applications of Fast, Facile, Radiation-Free Radical Polymerization Techniques Enabled by Room Temperature Alkylborane Chemistry.

Fast, robust, and scalable techniques for covalent materials assembly are shown to be enabled by variants of a simple mixing-induced free radical initiation scheme broadly termed room-temperature alkylborane (RTA) chemistry. Unique process versatility, speed of reaction, high conversion, and structural control at ambient conditions occur by exploiting air-stable alkylborane-amine complexes that rapidly initiate upon mixing with common amine-reactive decomplexing agents such as carboxylic acid compounds. Three diverse application examples are presented, illustrating facile ambient routes to covalent assembly varying in length scale: (1) copolymers with controllable pressure-sensitive adhesive properties, (2) hydrophilically modified silicone microparticles from heterophase reactions, and (3) UV-free inkjet printable materials suitable for thick-textured patterning and printing, all conducted in open air with no radiation or atmospheric control. These examples demonstrate that this simple "bucket chemistry" can create intriguing degrees of freedom for polymerization, cross-linking and covalent macromolecular assembly with controllable structure and properties, suggesting further opportunities for both fundamental mechanistic investigation and application to a range of old and new materials assembly problems across length scales.

Understanding molecular structures of silanes at buried polymer interfaces using sum frequency generation vibrational spectroscopy and relating interfacial structures to polymer adhesion.

The use of silane adhesion promoters to improve adhesion of elastomeric materials to polymers has become increasingly common in many industrial applications. However, little is understood about the molecular-level mechanisms of how adhesion promoters enhance adhesion. Here, sum frequency generation (SFG) vibrational spectroscopy was used to probe the buried interface between poly(ethylene terephthalate) (PET) and (3-glycidoxypropyl)trimethoxysilane (gamma-GPS), and the interface between PET and a mixture of gamma-GPS and a methylvinylsiloxanol (MVS), a known adhesion-promoting mixture. Furthermore, the interfaces between PET and uncured silicone with incorporated silane or silane mixture and the interfaces between PET and cured silicone with incorporated silane or silane mixture were studied. The gamma-GPS methoxy groups were found to order at the polymer interface and the presence of MVS increased the interfacial segregation and/or order of gamma-GPS. For comparison, two other silanes, N-octadecyltrimethoxysilane (OTMS) and (tridecafluoro-1,1,2,2-tetrahydroctyl)trimethoxysilane (TDFTMS), as well as their mixtures with MVS were also studied at the various interfaces, and were found to exhibit different interfacial behaviors than gamma-GPS and the known silane adhesion-promoting mixture of gamma-GPS and MVS. Further, X-ray photoelectron spectroscopy (XPS) was used to investigate the exposed PET surfaces resulting from peeling the PET/cured silicone elastomer with TDFTMS and with the TDFTMS/MVS mixture interfaces, and it was shown that the fluorinated silane does segregate to the polymer interface. When correlated to adhesion testing results, it is inferred that segregation and ordering of the silane methoxy groups at the polymer/silane and polymer/silicone elastomer interfaces is crucial for adhesion promotion in this system.

Diffusion of one or more components of a silane adhesion-promoting mixture into poly(methyl methacrylate).

The surface-sensitive technique of sum frequency generation (SFG) vibrational spectroscopy has been applied to study the buried interfaces between different polymers including deuterated polystyrene (d-PS) and deuterated poly(methyl methacrylate) (d-PMMA) and a two-component silane adhesion-promoting mixture (SAPM) composed of (3-glycidoxypropyl)trimethoxysilane (gamma-GPS) and a methylvinylsiloxanol (MVS). Because of the dissolution of d-PS, no SFG CH stretching signals could be collected from the d-PS/gamma-GPS interface, and SFG signals collected from the d-PS/SAPM interface gradually disappeared over time. SFG results also showed that gamma-GPS can diffuse through the d-PMMA film. The diffusion of gamma-GPS through the d-PMMA film was confirmed by SFG studies on the interface between gamma-GPS and a d-PMMA/PS two-polymer layer system. Initially the SFG signal from the PS layer was detected. However, after gamma-GPS diffused through the d-PMMA film, the PS film was dissolved by the silane, and thus the SFG signal from PS was lost. Similar experiments have been carried out at the interface between the SAPM and the d-PMMA/PS two-polymer layer system and it was found that the diffusion time of the gamma-GPS in the SAPM through the d-PMMA film was significantly longer. These results were much different to those from previous SFG studies on the analogous PET interfaces and appear consistent with differences in solubility parameters calculated for these systems.

Sum frequency generation vibrational spectroscopic studies on a silane adhesion-promoting mixture at a polymer interface.

Sum frequency generation (SFG) vibrational spectroscopy was used to probe the interface between poly(ethylene terephthalate) with deuterated ethylene glycol subunits (d4-PET) and a silane adhesion-promoting mixture (SAPM) comprised of (3-glycidoxypropyl)trimethoxysilane (gamma-GPS) and a methylvinylsiloxanol (MVS). Such a mixture has been found to improve the adhesion of an addition-curing silicone elastomer to a range of plastic and metal substrates. Our results demonstrated that at the interface between d4-PET and a SAPM with a gamma-GPS/MVS ratio of 1:1 (w/w), the silane molecules not only segregated to the interface but also the methoxy headgroups likely adopted a greater net orientational order along the surface normal than at the d4-PET/gamma-GPS interface. The effects of varying the silane/siloxane ratio and using different siloxane oligomers on interfacial structures were also examined. This study provides unique molecular-level insights into the prerequisite conditions for adhesion of curable silicone adhesives.

Demonstrating the feasibility of monitoring the molecular-level structures of moving polymer/silane interfaces during silane diffusion using SFG.

In this paper, the feasibility of monitoring molecular structures at a moving polymer/liquid interface by sum frequency generation (SFG) vibrational spectroscopy has been demonstrated. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AATM, NH2(CH2)2NH(CH2)3Si(OCH3)3) has been brought into contact with a deuterated poly(methyl methacrylate) (d-PMMA) film, and the interfacial silane structure has been monitored using SFG. Upon initial contact, the SFG spectra can be detected, but as time progresses, the spectral intensity changes and finally disappears. Additional experiments indicate that these silane molecules can diffuse into the polymer film and the detected SFG signals are actually from the moving polymer/silane interface. Our results show that the molecular order of the polymer/silane interface exists during the entire diffusion process and is lost when the silane molecules traverse through the thickness of the d-PMMA film. The loss of the SFG signal is due to the formation of a new disordered substrate/silane interface, which contributes no detectable SFG signal. The kinetics of the diffusion of the silane into the polymer have been deduced from the time-dependent SFG signals detected from the AATM molecules as they diffuse through polymer films of different thickness.

Sum frequency generation studies at poly(ethylene terephthalate)/silane interfaces: hydrogen bond formation and molecular conformation determination.

To better understand the effects of interfacial molecular orientation on adhesion to plastics, the interfaces between poly(ethylene terephthalate) (PET) and different silane coupling agents were probed using sum frequency generation (SFG) vibrational spectroscopy. The polymer/air interface was dominated by the ester carbonyl, methylene, and phenyl groups. Upon contacting the PET film with the amino-functional silane 3-aminopropyltrimethoxysilane (ATMS), the ester carbonyl stretch shifted to a lower energy indicating the formation of hydrogen bonds between the polymer surface and the silane molecules. This shift was not observed when silanes that contained no hydrogen bond donors, such as (3-glycidoxypropyl)-trimethoxysilane and n-butyltrimethoxysilane, were placed into contact with the PET surface. Further evidence of silane ordering at the interface was observed as vibrational peaks attributed to the C-H stretching of the silane methoxy headgroups dominated the PET/silane spectra. It was determined that the conformation of the ATMS molecules at the interface was such that the amino endgroups were oriented toward the interface while the methoxy headgroups were directed toward the silane bulk.