Graduation Year

2022

Document Type

Thesis

Degree

M.S.M.S.E.

Degree Name

MS in Materials Science and Engineering (M.S.M.S.E)

Degree Granting Department

Engineering

Major Professor

Nurettin Sahiner, Ph.D.

Co-Major Professor

Venkat R. Bhethanabotla, Ph.D.

Committee Member

Ramesh S. Ayyala, M.D.

Keywords

surface free energy, contact angle, wettability, surface modification, functionalization

Abstract

In this thesis, low density polyethylene (LDPE) and polydimethylsiloxane (silicone/PDMS) were treated with oxygen (O2) , air and carbon tetrafluoride (CF4) low pressure 13.56 MHz radiofrequency plasma. An experimental setup was used to determine process power and irradiation time for improved modification, the validation was done by measuring the contact angle of the treated samples immediately after treatment. The modification used commercially available sheets to imitate its use in the industry and various applications. The treatment aims to increase/decrease the surface free energy, making it possible to modify the intrinsic inert materials without affecting the bulk. Increased wettability, adhesion, and widening the range of applications are some of the research objectives in this work. The characterization was done by contact angle (CA) measurements, surface free energy (SFE) calculations, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy – attenuated total reflectance (FT-IR – ATR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). During the contact angle measurements, it was shown that the surfaces of both polymers were fluorinated after being treated with CF4 plasma displaying slightly higher contact angle values, this was also supported by FT-IR and XPS analysis, however there was hardly any difference between the before- and after-treatment CA values. The CA of LDPE and silicone were lowered from 100° to around 60° when treated with O2 or air plasma gas. In addition to the contact angle that was measured with DI water, various measuring liquids were used to determine the effects of the treatment in different environments such as varying pH levels and NaCl concentrations. Furthermore, its protein adhesion properties were studied by measuring the contact angle by using different concentrations of bovine serum albumin (BSA). The results proved that the modified polymers could find potential use in biomedical applications, however the environment and how the treated polymers react to it have to be considered. The different plasma gas treatments resulted in a successful modification of the surface properties, where the contact angle increased for the CF4 treated specimen and decreased for the air and O2 plasma gas treated ones. The surface free energy calculations support the findings of the before mentioned contact angle measurements, increasing with the hydrophilic treatments, and decreasing with the hydrophobic treatment. Furthermore, aging experiments, where the SFE was measured over a time period of 168h showed, that the O2 plasma treated silicone's low SFE from 10 mN/m was increased to 75 mN/m, and maintained its hydrophilic modification for longer when stored in water. In contrast, the SFE of LDPE was unaffected by any storing circumstances and remained steady for 168 hours. The LDPE treated with CF4 plasma displayed the lowest value in SFE, which fell from 35 mN/m to roughly 15 mN/m. The XPS and FTIR – ATR results show that the treatment introduces various oxygen and fluorine containing functional groups onto the surface of the samples, which is the cause for the modified adhesion and wettability properties. The SEM and AFM analysis are used to study the etching effects of the treatment, showing that LDPE is more affected by the caused etching than the silicone samples. Additionally, LDPE and silicone were subjected to gas plasma treatments up to six times in succession with various combinations of gases in order to achieve more control over the range of CA values, i.e., 120° to 30° for LDPE and 120° to 13° for silicone. Lastly, some possible future research work was touched, namely the succeeding plasma gas treatment with multiple gases (H2 and CO2) as well as the increased attachment of foreign materials such as rose bengal and pyrene which can widen the range of applications even further.

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