Spectroscopic Studies on the Molecular Structural Changes of Plastics and Plasticizers at Model Environmental Interfaces.

The mainstream introduction of plastics poly(vinyl chloride) (PVC) and poly(styrene) (PS) has been integral to the formation of modern urban infrastructures and the advancement of medical, electronic, and industrial technologies. Global annual production of phthalate-plasticized PVC and PS plastics exceeds tens of millions of tons. Thus, it is important to further understand how these plastics interact with surrounding matter at a molecular level in order to more accurately gauge their impacts on a wide variety of environments. This work develops new in situ and ex situ experimental platforms to study PVC, phthalate, and toxin molecules at air, water and silica interfaces with surface sensitive spectroscopic techniques sum frequency generation vibrational spectroscopy, x-ray photoelectron spectroscopy and secondary ion mass spectrometry. Studies on the molecular effects of heat and water contact on phthalate-plasticized PVC revealed that phthalates can exist on plastic surfaces even when small percentages of plasticizers are used, increased temperatures induce phthalate leaching in hours, and water contact induces permanent plastic surface restructuring and phthalate leaching. Leached phthalates transfer to new surfaces via water in minutes. The molecular mechanisms and effects of plasma and UV light treatments designed to reduce phthalate leaching and degrade phthalates, respectively, were determined. Air plasma treatment induces drastic surface-bound radical reactions with PVC including scission, chlorine removal, and minor crosslinking. UV light exposure, deemed a potentially efficient method of phthalate removal for disposed plastics, results in surface and bulk phthalate degradation and little PVC damage. Short wave UV/H2O2 treatments lead to competing radical reactions with PVC chains and increased molecular surface disorder. Long wave UV-based treatments induce very minor chemical reactions in and on phthalate-plasticized PVC. Lastly, studies on PS plastics interacting with gas-phase environmental toxins revealed that nonylphenols (NPs) deposit and order differently on PS plastics when the plastic is located above a calm water surface versus over land. NPs remain much more highly ordered on PS if originally deposited under humid conditions even after moving water and air exposure. Ultimately the information gained on the molecular behaviors of plastics aids in the future design and formulation of newer, more environmentally friendly plastics.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/113648/1/jhankett_1.pd

Evaluating UV/H 2 O 2 exposure as a DEHP degradation treatment for plasticized PVC

Millions of tons of plasticized poly(vinyl chloride) (PVC) materials are disposed every year. A biologically sustainable and green method for removal of toxic plasticizers from polymer systems after disposal is highly desired since plasticizers can leach out into the environment over decades. Here we compare the surface and bulk structural changes of DEHP‐plasticized PVC after two treatments intended to degrade bis‐2‐ethylhexyl phthalate (DEHP) in PVC plastic: short wave (254 nm) UV with and without the addition of 35 wt % H 2 O 2. Sum frequency generation vibrational spectroscopy (SFG) reveals the addition of aqueous H 2 O 2 decreases CH 3 signals on the surface of the films up to 8 h, due to increased molecular disorder and the removal of alkyl chains. Secondary ion mass spectrometry demonstrates that the degradation of DEHP after 8 h of reaction is similar with and without the use of H 2 O 2 . However, FTIR results reveal that the introduction of H 2 O 2 reduces bulk DEHP degradation and leads to competing radical chain scission reactions with PVC. Therefore, simple short wave UV exposure may be an effective means to degrade DEHP within and on PVC plastic and the addition of H 2 O 2 is only beneficial if additional degradation of PVC is needed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131 , 40649.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106825/1/app40649-sup-0001-suppinfo01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106825/2/app40649.pd

Molecular Level Understanding of Adhesion Mechanisms at the Epoxy/Polymer Interfaces

It is important to understand the buried interfacial structures containing epoxy underfills as such structures determine the interfacial adhesion properties. Weak adhesion or delamination at such interfaces leads to failure of microelectronic devices. Sum frequency generation (SFG) vibrational spectroscopy was used to examine buried interfaces at polymer/model epoxy and polymer/commercial epoxy resins (used as underfills in flip chip devices) at the molecular level. We investigated a model epoxy: bisphenol A digylcidyl ether (BADGE) at the interfaces of poly (ethylene terephthalate) (PET) before and after curing. Furthermore, small amounts of different silanes including (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-Aminopropyl)­trimethoxysilane (ATMS), Octadecyltrimethoxysilane (OTMS­(18C)), and Octyltrimethoxysilane (OTMS­(8C)) were mixed with BADGE. Silane influences on the polymer/epoxy interfacial structures were studied. SFG was also used to study molecular interfacial structures between polymers and two commercial epoxy resins. The interfacial structures probed by SFG were correlated to the adhesion strengths measured for corresponding interfaces. The results indicated that a small amount of silane molecules added to epoxy could substantially change the polymer/epoxy interfacial structure, greatly affecting the adhesion strength at the interface. It was found that ordered methyl groups at the interface lead to weak adhesion, and disordered interfaces lead to strong adhesion

Molecular Structural Changes of Plasticized PVC after UV Light Exposure

Plasticized poly­(vinyl chloride) (PVC) materials for industrial, medical, and household use are often intentionally exposed to UV light, though its impact on the molecular integrity and toxicity of the surface and bulk of PVC materials is still not well understood. This paper investigates the surface and bulk molecular changes of plasticized PVC films with 25, 10, or 0 wt % bis-2-ethylhexyl phthalate (DEHP) plasticizer after exposure to short wave (254 nm) or long wave (365 nm) UV light. Surface analytical techniques including sum frequency generation vibrational spectroscopy (SFG) revealed short wave UV exposure induced major molecular changes on the plasticized PVC surfaces, resulting in increased surface hydrophilicity and decreased CH₃ content with increasing exposure time. Additionally, it was deduced from multiple techniques that the surface and the bulk of the plastic exposed to short wave UV contained phthalic monoesters and phthalic acid formed from multistep radical reactions. In contrast, when exposed to long wave UV, molecular content and ordering on the surfaces of the plastic remained relatively unchanged and the introduction of DEHP in plastic helped protect PVC chains from degradation. Results from this study demonstrate short wave UV exposure will result in plastic surfaces containing phthalates and phthalate-related products accessible to contact by living organisms

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

Silane Effects on Adhesion Enhancement of 2K Polyurethane Adhesives

With excellent properties such as great flexibility, outstanding chemical resistance, and superb mechanical strength, two-part polyurethane (2K PU) adhesives have been widely applied in many applications, including those in transportation and construction. Despite the extensive use, their adhesion to nonpolar polymer substrates still needs to be improved and has been widely studied. The incorporation of silane molecules and the use of plasma treatment on substrate surfaces are two popular methods to increase the adhesion of 2K PU adhesives, but their detailed adhesion enhancement mechanisms are still largely unknown. In this research, sum frequency generation (SFG) vibrational spectroscopy was used to probe the influence of added or coated silanes on the interfacial structure at the buried polypropylene (PP)/2K PU adhesive interface in situ. How plasma treatment on PP could improve adhesion was also investigated. To achieve maximum adhesion, two methods to involve silanes were studied. In the first method, silanes were directly mixed with the 2K PU adhesive before use. In the second method, silane molecules were spin-coated onto the PP substrate before the PU adhesive applied. It was found that the first method could not improve the 2K PU adhesion to PP, while the second method could substantially enhance such adhesion. SFG studies demonstrated that with the second method silane molecules chemically reacted at the interface to connect PP and 2K PU adhesive to improve the adhesion. With the first method, silane molecules could not effectively diffuse to the interface to enhance adhesion. In this research, plasma treatment was also found to be a useful method to improve the adhesion of the 2K PU adhesive to nonpolar polymer materials

Directly Probing Molecular Ordering at the Buried Polymer/Metal Interface 2: Using P‑Polarized Input Beams

Previously, we developed several methods to use sum frequency generation (SFG) vibrational spectroscopy to probe buried polymer/metal interfaces <i>in situ</i> by depositing polymer films with different thicknesses on metal surfaces or sandwiching a polymer thin film between a metal surface and a fused silica window. In this study, we developed a new and easier method to directly probe the polymer/metal interface by collecting ppp SFG spectra using a poly­(ethyl methacrylate) (PEMA)/silver (Ag) interface as an example. We confirmed that for a thin polymer film on metal, the dominant SFG signals were contributed from the polymer surface in air and/or the polymer metal interface, while the contribution from the polymer bulk could be ignored. Previously, we showed that the ssp spectra were contributed by both the polymer/air and polymer/metal interfaces. Here we demonstrated that the SFG ppp spectra were dominated by signals from the buried polymer/metal interface from which the structural information on the buried interface can be deduced. This method to probe the buried polymer/metal interface via SFG is relatively simple compared to our previous sample preparation techniques and/or data analysis methods