Spectroscopically Detecting Molecular-Level Bonding Formation between an Epoxy Formula and Steel

The formation of the interfacial adhesion between an epoxy adhesive and a substrate was normally accompanied by the epoxy curing process on the substrate. Although the debate on the formation mechanism of the interfacial adhesion is still ongoing, this issue can causally be resolved by studying the interfacial structural formation between the epoxy adhesive and the substrate. Herein, to reveal the interfacial structural formation of a representative formula composed of epoxy (digylcidyl ether of biphenyl A, DGEBA) and amine hardener (2,2'-(ethylenedioxy) diethylamine, EDDA) with the steel substrate upon curing and postcuring treatments, sum-frequency generation (SFG) vibrational spectroscopy with a sandwiched transparent window/epoxy adhesive/steel setup was applied to detect and track the buried molecular-level structures at the epoxy adhesive/steel interface. An X-ray photoelectron spectroscopic (XPS) experiment was performed to probe the intentionally exposed interface to disclose the occurring interfacial chemical reaction. The reaction between the epoxy groups and the steel-surface OH groups and the molecular reconstruction of interfacial epoxy methyl groups upon curing and postcuring steps were confirmed. The latter also indirectly indicated the formation of the additional hydrogen bonding and the former bonding reaction at the interface. The above two spectroscopic experimental results matched up with the further examination of the adhesion strength. Therefore, this work elucidates the formation of the interfacial bonding between the epoxy formula and the steel substrate upon curing and postcuring treatments at the molecular level, thus providing an in-depth insight into the origin of the interfacial adhesion.

Molecular-Level Correlation between Spectral Evidence and Interfacial Bonding Formation for Epoxy Adhesives on Solid Substrates

Interfacial bonding strength of an epoxy-based adhesive depends on the interfacial interaction between the adhesive and the substrate. Normally, the curing process at the interface accompanied by the interfacial bonding formation is different from that in the bulk, and it is still a big challenge to probe the interfacial bonding formation at a molecular level. In this study, to trace the interfacial structural evolution of a representative formula of epoxy (digylcidyl ether of biphenyl A, DGEBA) and amine hardener [1,2-bis(2-aminoethoxy)ethane, EDDA] with the sapphire and silica substrates upon curing and post-curing steps, sum frequency generation (SFG) vibrational spectroscopy is employed to detect the molecular-level interfacial structural information. For the sapphire substrate, upon curing, backbone methylene (CH₂) stretching signals decrease, indicating the formation of a rigid chain network structure and thus losing the local methylene order, while vibrational signals of the sapphire surface hydroxyl (OH) groups (including hydrogen-bonded and unbonded) increase significantly, indicating the formation of a strong hydrogen-bonding and polar interaction between the epoxy adhesive and the sapphire surface. Upon post-curing, increased backbone CH₂ signals and decreased sapphire OH signals suggest interfacial chemical bonding formation due to the reaction between the epoxy rings and the sapphire surface OH groups. Orientation analysis confirms the enhanced ordering of the sapphire surface OH groups upon curing and post-curing, in comparison to the uncured epoxy formula. As for the fused silica, weak vibrational signals of the methylene (CH₂) and methyl (CH₃) groups are observed before curing, while both of them increase slightly for the cured and post-cured epoxy formulae, suggesting relatively less hydrophilic nature of the silica surface compared to that of the sapphire surface, also evidenced by the very weak OH signals upon curing and post-curing. Further measurement on the adhesion strength matches up with the above spectroscopic experimental results, substantiating the correlation between the macroscopic bonding strength of the epoxy adhesive and the microscopic molecular-level structure.

Passively immobilized cyanobacteria Nostoc species BB 92.2 in a moving bed photobioreactor (MBPBR): design, cultivation and characterization

The cyanobacterium Nostoc sp. BB 92.3. had shown antibacterial activity. A cultivation as biofilm, a self-forming matrix of cells and extracellular polymeric substances, increased the antibacterial effect. A new photobioreactor system was developed that allows a surface-associated cultivation of Nostoc sp. as biofilm. High-density polyethylene carriers operated as a moving bed were selected as surface for biomass immobilization. This system, well established in heterotrophic wastewater treatment, was for the first time used for phototrophic biofilms. The aim was a cultivation on a large scale without inhibiting growth while maximizing immobilization. Cultivation in a small photobioreactor (1.5 L) with different volumetric filling degrees of carriers (13.4-53.8 %) in a batch process achieved immobilization rates of 70-85 % and growth was similar to a no-carrier-control. In a larger photobioreactor (65-liter) essentially all of the biomass was immobilized on the carriers and the space-time yield of biomass (0.018 g_{cell dry weight} L^-1 day^-1 ) was competitive compared to phototrophic biofilm cultivations from literature. The use of carriers increased the gas exchange in the reactor by a factor of 2.5-3, but doubled the mixing time. Enriched gassing with carbon dioxide resulted in a short-term increase in growth rate, but unexpectedly it also adversely changed the growth morphology. This article is protected by copyright. All rights reserved.

Influence of the interfacial molecular structures of quaternary ammonium-type poly(ionic liquid) brushes on their antibacterial properties

Alkyl chains of C4 are more active in killing bacteria than C16 due to their orderly extension toward PBS solution.

Surface hydration for antifouling and bio-adhesion

Antifouling properties of materials play crucial roles in many important applications such as biomedical implants, marine antifouling coatings, biosensing, and membranes for separation. Poly(ethylene glycol) (or PEG) containing polymers and zwitterionic polymers have been shown to be excellent antifouling materials. It is believed that their outstanding antifouling activity comes from their strong surface hydration. On the other hand, it is difficult to develop underwater glues, although adhesives with strong adhesion in a dry environment are widely available. This is related to dehydration, which is important for adhesion for many cases while water is the enemy of adhesion. In this research, we applied sum frequency generation (SFG) vibrational spectroscopy to investigate buried interfaces between mussel adhesive plaques and a variety of materials including antifouling polymers and control samples, supplemented by studies on marine animal (mussel) behavior and adhesion measurements. It was found that PEG containing polymers and zwitterionic polymers have very strong surface hydration in an aqueous environment, which is the key for their excellent antifouling performance. Because of the strong surface hydration, mussels do not settle on these surfaces even after binding to the surfaces with rubber bands. For control samples, SFG results indicate that their surface hydration is much weaker, and therefore mussels can generate adhesives to displace water to cause dehydration at the interface. Because of the dehydration, mussels can foul on the surfaces of these control materials. Our experiments also showed that if mussels were forced to deposit adhesives onto the PEG containing polymers and zwitterionic polymers, interfacial dehydration did not occur. However, even with the strong interfacial hydration, strong adhesion between mussel adhesives and antifouling polymer surfaces was detected, showing that under certain circumstances, interfacial water could enhance the interfacial bio-adhesion.

Antifouling properties of materials play crucial roles in many important applications such as biomedical implants, marine antifouling coatings, biosensing, and membranes for separation.

Suppressed Surface Reorganization in a High-Density Poly(methyl methacrylate) Brush

A high-density poly(methyl methacrylate) (PMMA) brush (σ = 0.77 chain/nm²) with a lower molecular weight distribution was prepared onto a silicon wafer by surface-initiated atom transfer radical polymerization. The surface of the PMMA brush chains was characterized upon the process of the environmental change, from air to water, using contact angle measurements in conjunction with sum-frequency generation spectroscopy. The surface structure and properties altered less with the changing environment from air to water for the PMMA brush than for a spin-coated film; that is, the extent of surface reorganization could be suppressed by grafting densely-packed chains onto a substrate. Also, the water penetration into the brush surface was inhibited because of the densely packed chain structure.

Probing Surface Hydration and Molecular Structure of Zwitterionic and Polyacrylamide Hydrogels

A hydrogel is a hydrophilic cross-linked polymer network which can contain a large amount of water. Hydrogels with distinguished interfacial physical toughness were analyzed for their potential application as antifouling coating materials, utilizing sum frequency generation (SFG) spectroscopy as the interfacial analytical technique. The surface structures of one sulfobetaine (SBMA) zwitterionic hydrogel (ZWHG) and two polysaccharide hydrogels (PHGs) were probed in air; their interfacial structures with silica were examined using SFG in water and protein solutions, respectively. Both ZWHG and PHGs interfaces in water were dominated by strongly hydrogen-bonded water molecules, but the bonding strength associated with ZWHG was much stronger. Although all hydrogels experienced interfacial change in the presence of protein solutions, after cleaning, the zwitterionic hydrogel interface recovered almost completely while the other two hydrogels were subject to irreversible protein adsorption. Additionally, orientational analysis of ZWHG methyl groups in water was conducted and related to the superior hydrogen-bonding strength of water molecules at the ZWHG interface. The interfacial structures of hydrogel materials probed by SFG can be correlated to their antifouling properties. This research highlighted the critical role that hydrogen-bonding strength of interfacial water molecules play for antifouling applications.

Absolute Orientations of Water Molecules at Zwitterionic Polymer Interfaces and Interfacial Dynamics after Salt Exposure

Nonfouling zwitterionic polymers have wide applications ranging from the naval industry to biomedical engineering. Strong hydration at polymer surfaces has been proven to be crucial to their nonfouling property, but the absolute orientations of water molecules on the polymers and the competition between water and salt binding have not been elucidated. In this work, the absolute orientations of water molecules on two zwitterionic polymer brushes, poly(carboxybetaine methacrylate) (pCBMA) and poly(sulfobetaine methacrylate) (pSBMA), were measured using regular and phase-sensitive sum frequency generation (SFG) vibrational spectroscopy. The pH-dependent studies in a pH range from 2 to 12 showed that at a pH of 7, the water absolute orientations are different on the pCBMS and pSBMA surfaces. Phase-sensitive SFG studies confirmed the results obtained from the pH-dependent measurements. Salt effects on the hydration of zwitterionic polymers were examined as a function of time, which indicated that the pCBMA surface and the associated interfacial water exhibit a slow restructuring process after salt binding (likely due to the strong binding of pCBMA with water), whereas the surface of pSBMA and the associated water have a fast change after salt binding.

Capsaicin-Inspired Thiol-Ene Terpolymer Networks Designed for Antibiofouling Coatings

Novel photocurable ternary polymer networks were prepared by incorporating N-(4-hydroxy-3-methoxybenzyl)-acrylamide (HMBA) into a cross-linked thiol-ene network based on poly(ethylene glycol)diacrylate (PEGDA) and (mercaptopropyl)methylsiloxane homopolymers (MSHP). The ternary network materials displayed bactericidal activity against Escherichia coli and Staphylococcus aureus and reduced the attachment of marine organism Phaeodactylum tricornutum. Extensive soaking of the polymer networks in aqueous solution indicated that no active antibacterial component leached out of the materials, and thus the ternary thiol-ene coating killed the bacteria by surface contact. The surface structures of the polymer networks with varied content ratios were studied by sum frequency generation (SFG) vibrational spectroscopy. The results demonstrated that the PDMS Si-CH₃ groups and mimic-capsaicine groups are predominantly present at the polymer-air interface of the coatings. Surface reorganization was apparent after polymers were placed in contact with D₂O: the hydrophobic PDMS Si-CH₃ groups left the surface and returned to the bulk of the polymer networks, and the hydrophilic PEG chains cover the polymer surfaces in D₂O. The capasaicine methoxy groups are able to segregate to the surface in an aqueous environment, depending upon the ratio of HMBA/PEGDA. SFG measurements in situ showed that the antibacterial HMBA chains, rather than the nonfouling PEG, played a dominant role in mediating the antibiofouling performance in this particular polymer system.

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

Sum frequency generation vibrational spectroscopy (SFG-VS) has become one of the most appealing technologies to characterize molecular structures at interfaces. In this focal point review, we focus on SFG-VS studies at buried polymer interfaces and review many of the recent publications in the field. We also cover the essential theoretical background of SFG-VS and discuss the experimental implementation of SFG-VS.

Maternal phthalate exposure during the first trimester and serum thyroid hormones in pregnant women and their newborns

Animal and human studies have suggested that phthalate alters thyroid hormone concentrations. This study investigated the associations between phthalate exposure during the first trimester and thyroid hormones in pregnant women and their newborns. Pregnant women were enrolled from the prospective Ma'anshan Birth Cohort study in China. A standard questionnaire was completed by the women at the first antenatal visit. Seven phthalate metabolites were measured in one-spot urine at enrolment (10.0 ± 2.1 gestational weeks), as were thyroid hormone levels in maternal and cord sera. Multivariable linear regression showed that 1-standard deviation (SD) increase in natural log (ln)-transformed mono(2-ethylhexyl) phthalate (MEHP) and mono(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP) was associated with 0.163 μg/dL (p = 0.001) and 0.173 μg/dL (p = 0.001) decreases in maternal total thyroxine (TT4). Both MEHP and MEHHP were negatively associated with maternal free thyroxine (FT4; β: -0.013, p < 0.001 and β: -0.011, p = 0.001, respectively) and positively associated with maternal thyroid-stimulating hormone (β: 0.101, p < 0.001; β: 0.132, p < 0.001, respectively). An inverse association was observed between monobenzyl phthalate and maternal TT4 and FT4. A 1-SD increase in ln-transformed monoethyl phthalate was inversely associated with maternal TT4 (β: -0.151, p = 0.002). By contrast, the concentrations of phthalate metabolites in urine were not associated with those of thyroid hormone in cord serum. Our analysis suggested that phthalate exposure during the first trimester disrupts maternal thyroid hormone levels.

Molecular level studies on interfacial hydration of zwitterionic and other antifouling polymers in situ

Antifouling polymers have wide applications in biomedical engineering and marine industry. Recently, zwitterionic materials have been reported as promising candidates for antifouling applications, while strong hydration is believed to be the key antifouling mechanism. Zwitterionic materials can be designed with various molecular structures, which affect their hydration and antifouling performance. Although strong hydration has been proposed to occur at the material surfaces, probing the solid material/water interfaces is challenging with traditional analytical techniques. Here in this review, we will review our studies on surface hydration of zwitterionic materials and other antifouling materials by using sum frequency generation (SFG) vibrational spectroscopy, which provides molecular understanding of the water structures at various material surfaces. The materials studied include zwitterionic polymer brushes with different molecular structures, amphiphilic polymers with zwitterionic groups, uncharged hydrophilic polymer brushes, amphiphilic polypeptoids, and widely used antifouling material poly(ethylene glycol). We will compare the differences among zwitterionic materials with various molecular structures as well as the differences between antifouling materials and fouling surfaces of control samples. We will also discuss the effects of pH and biological molecules like proteins on the surface hydration of the zwitterionic materials. Using SFG spectroscopy, we have measured the hydration layers of antifouling materials and found that strong hydrogen bonds are key to the formation of strong hydration layers preventing protein fouling at the polymer interfaces.

STATEMENT OF SIGNIFICANCE

Antifouling polymers have wide applications in biomedical engineering and marine industry. Recently, zwitterionic materials have been reported as promising candidates for antifouling applications, while strong hydration is believed to be the key antifouling mechanism. However, zwitterionic materials can be designed with various molecular structures, which affect their hydration and antifouling performance. Moreover, although strong hydration has been proposed to occur at the material surfaces, probing the solid material/water interfaces is challenging with traditional analytical techniques. Here in this manuscript, we will review our studies on surface hydration of zwitterionic materials and other antifouling materials by using sum frequency generation (SFG) vibrational spectroscopy, which provides molecular understanding of the water structures at various material surfaces. The materials studied include zwitterionic polymer brushes with different molecular structures, amphiphilic polymers with zwitterionic groups, uncharged hydrophilic polymer brushes, amphiphilic polypeptoids, and widely used antifouling material poly(ethylene glycol). We will compare the differences among zwitterionic materials with various molecular structures as well as the differences between antifouling materials and fouling surfaces of control samples. We will also discuss the effects of pH and biological molecules like proteins on the surface hydration of the zwitterionic materials. All the SFG results indicate that strongly hydrogen-bonded water at the materials' surfaces (strong surface hydration) is closely correlated to the good antifouling properties of the materials. This review will be widely interested by readers of Acta Biomaterialia and will impact many different research fields in chemistry, materials, engineering, and beyond.

Low-Volatility Model Demonstrates Humidity Affects Environmental Toxin Deposition on Plastics at a Molecular Level

Despite the ever-increasing prevalence of plastic debris and endocrine disrupting toxins in aquatic ecosystems, few studies describe their interactions in freshwater environments. We present a model system to investigate the deposition/desorption behaviors of low-volatility lake ecosystem toxins on microplastics in situ and in real time. Molecular interactions of gas-phase nonylphenols (NPs) with the surfaces of two common plastics, poly(styrene) and poly(ethylene terephthalate), were studied using quartz crystal microbalance and sum frequency generation vibrational spectroscopy. NP point sources were generated under two model environments: plastic on land and plastic on a freshwater surface. We found the headspace above calm water provides an excellent environment for NP deposition and demonstrate significant NP deposition on plastic within minutes at relevant concentrations. Further, NP deposits and orders differently on both plastics under humid versus dry environments. We attributed the unique deposition behaviors to surface energy changes from increased water content during the humid deposition. Lastly, nanograms of NP remained on microplastic surfaces hours after initial NP introduction and agitating conditions, illustrating feasibility for plastic-bound NPs to interact with biota and surrounding matter. Our model studies reveal important interactions between low-volatility environmental toxins and microplastics and hold potential to correlate the environmental fate of endocrine disrupting toxins in the Great Lakes with molecular behaviors.

Effect of Solvent on Surface Ordering of Poly(3-hexylthiophene) Thin Films

Enhancement of charge transport in organic polymer semiconductors is a crucial step in developing optimized devices. A variety of sample preparation conditions, such as film fabrication method, solvent species, and annealing, were found to influence the hole mobility of organic polymers. Despite the fact that many factors can influence their performance, it is believed that polymer surface ordering plays a key role in determining organic polymer function. Here, sum frequency generation (SFG) vibrational spectroscopy was used to nondestructively map the surface/interfacial ordering of poly(3-hexylthiophene) (P3HT) films prepared using different solvents; we believe that solvent interactions determine the degree of surface/interfacial ordering. Both X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM) were used to supplement SFG to systematically study bulk crystallinity and surface morphology. We conclude that SFG is a powerful tool to elucidate the surface/interfacial structural information on polymer semiconducting films. We demonstrate that the solvent composition used to prepare P3HT thin films influences the resulting film surface morphology, surface/interfacial ordering, and bulk crystallinity.

The molecular interfacial structure and plasticizer migration behavior of “green” plasticized poly(vinyl chloride)

Both oxygen and argon plasma treatment made TBAC–PVC surfaces hydrophilic, but that of argon enhanced the migration of TBAC to water.

In situ observation of water behavior at the surface and buried interface of a low-k dielectric film

Water adsorption in porous low-k dielectrics has become a significant challenge for both back-end-of-line integration and reliability. A simple method is proposed here to achieve in situ observation of water structure and water-induced structure changes at the poly(methyl silsesquioxane) (PMSQ) surface and the PMSQ/solid buried interface at the molecular level by combining sum frequency generation (SFG) vibrational spectroscopic and Fourier transform infrared (FTIR) spectroscopic studies. First, in situ SFG investigations of water uptake were performed to provide direct evidence that water diffuses predominantly along the PMSQ/solid interface rather than through the bulk. Furthermore, SFG experiments were conducted at the PMSQ/water interface to simulate water behavior at the pore inner surfaces for porous low-k materials. Water molecules were found to form strong hydrogen bonds at the PMSQ surface, while weak hydrogen bonding was observed in the bulk. However, both strongly and weakly hydrogen bonded water components were detected at the PMSQ/SiO2 buried interface. This suggests that the water structures at PMSQ/solid buried interfaces are also affected by the nature of solid substrate. Moreover, the orientation of the Si-CH3 groups at the buried interface was permanently changed by water adsorption, which might due to low flexibility of Si-CH3 groups at the buried interface. In brief, this study provides direct evidence that water molecules tend to strongly bond (chemisorbed) with low-k dielectric at pore inner surfaces and at the low-k/solid interface of porous low-k dielectrics. Therefore, water components at the surfaces, rather than the bulk, are likely more responsible for chemisorbed water related degradation of the interconnection layer. Although the method developed here was based on a model system study, we believe it should be applicable to a wide variety of low-k materials.