An Investigation of the Stability of Hydrophilic Polymer Thin Films: Crystallinity, Hydrophilicity, and Electrostatic Charge

dc.contributorBroaders, Kyle
dc.contributorDay, Tori
dc.contributor.advisorChen, Wei
dc.contributor.authorAlvarez, Carolina
dc.description.abstractPolymer thin films are ubiquitous in everyday life, playing a role in fields such as electronics, optics, space science, aircrafts, defense, medicine, sensors, and biotechnology. Thin film stability is a property that describes whether a film forms a continuous layer or ruptures into a morphology often characterized by holes, polygons, or droplets. This latter process, termed dewetting, has traditionally been considered an obstacle to avoid, but more recently it has been harnessed as a tool for producing patterned thin films with features on the nanoscopic scale. Regardless of its desirability, an understanding of the mechanisms that underlie dewetting is critical for tailoring the morphologies of thin films to the specifications of their applications. The stability of a thin film to some extent depends on its thickness, and spin coating offers a convenient and affordable means for producing thin films of tunable thickness. However, existing models for predicting the film thickness of spin coated films are based primarily on nonpolar polymer thin films and often fail to accurately predict experimentally obtained results. Therefore, the goal of this independent study is to uncover the mechanisms of the polymer deposition and film formation processes for hydrophilic polymers and to work towards a model that predicts the thickness and stability of spin coated hydrophilic thin films. Various hydrophilic polymers were selected as the focus of this investigation on the basis of their crystallinity, degree of hydrophilicity, and polymer charge. Poly(vinyl alcohol) (PVOH) is a semicrystalline polymer that exists in various degrees of hydrolysis corresponding to varying hydrophilicities. PVOH 99%H (more hydrophilic and crystalline) and PVOH 88%H (more hydrophobic and less crystalline) were studied in this work. Additionally, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) are two amorphous polyelectrolytes whose pH-tunable electrostatic charge and lack of crystallinity offer an insightful point of comparison to the neutral and crystalline PVOH polymers. Poly(sodium 4-styrenesulfonate) (PSS) is a polyanion bearing a permanent negative charge that was selected for this investigation to serve as a pH-resistant point of comparison as well as a slightly more hydrophobic water soluble polymer due to its styrene moiety. Poly(vinyl pyrrolidone) (PVP) is a neutral and amorphous polymer, which makes it similar to PVOH except for its lack of crystallinity. This makes it possible to isolate the effects of crystallinity on the polymer deposition process. Building on the research of previous lab members, a model for thin film formation that decouples total film thickness into a spontaneously deposited (h1) layer and a spin deposited (h2) layer was investigated in order to probe the relative contributions of polymer-substrate and polymer-polymer interactions, respectively, on thin film total thickness and stability. Static adsorption experiments were used to obtain h1 values for each polymer. These were then subtracted from the total thickness of the spin coated film of the corresponding polymer to determine the thickness of the h2 layer at various spin rates. Film morphologies under atomic force microscopy (AFM) were used to determine the stability of spin coated films. This information, combined with h1 and h2 values, allowed for an analysis of how the film formation process results in stability or lack thereof in the context of each polymer’s combination of properties. PVOH 99%H, PVOH 88%H, and PVP spin coated on silicon wafer are stable systems, forming films that do not dewet at any spin rate. Meanwhile, PAA, PAH, PSS, and PAA- spin coated on a silicon substrate are metastable systems because they dewet at some or all spin rates. PVOH 88%H, PVP, and PAA- formed relatively thicker films at the highest spin rates, indicating the presence of a thicker h1 layer. In the case of PVOH 88%H and PVP, this was attributed to the hydrophobicity of the polymer. In the case of PAA-, this was attributed to the aggregates that formed upon titration of PAA with NaOH to produce PAA-. At the lower spin rates, PVOH 99%H and PVOH 88%H formed relatively thicker films, which is evidence of a thicker h2 layer. This was attributed to the crystallinity of the polymers and, in the case of PVOH 88%H which was the thickest film at the lowest spin rate, polymer hydrophobicity. PAA, PAH, PSS, and PAA- all yielded shallow spin curves that were thinner than the stable systems at all spin rates (except PVOH 99%H at the highest spin rate). This is indicative of both weak cohesive forces and weak adhesive forces. However, the dewetting observed for these four systems implies that the cohesive forces dominate over the adhesive forces. Only PAH and PSS had spin curve exponents near -0.5, predicted by the well-known Meyerhofer model. The other exponents ranged from -1.3 to -0.2. The h1 thicknesses obtained from static adsorption experiments did not align with what was expected based on the properties of each system and the spin coated thickness at the highest spin rates. Only PVP formed a significant h1 layer, and all polymers including PVP formed thinner h1 layers than anticipated. This discrepancy was attributed to the presence of a loosely bound layer within the h1 layer that was rinsed off during the rinse steps associated with the static adsorption procedure. Repeating static adsorption with fewer rinse steps using PAA and PAA- showed that reducing the number of rinse steps produced h1 layers more similar in thickness to the h1 thickness projected by spin coated film thickness at the highest spin rate. The insights achieved through this work lay the foundation for future efforts to identify an experimental procedure that can produce h1 layers of reliable thickness. This will allow for a validation of the proposed decoupled thickness model. Additionally, this work has continued the ongoing effort to probe the effects of crystallinity, hydrophobicity, and electrostatic charge on adhesive and cohesive forces within polymer thin films. However, future work is still needed to explore some of the nuances of these polymer systems, particularly the apparent absence of hydrogen bonding as a driving force for polymer adhesion to the silicon substrate.en_US
dc.subjectPolymer Thin Filmsen_US
dc.titleAn Investigation of the Stability of Hydrophilic Polymer Thin Films: Crystallinity, Hydrophilicity, and Electrostatic Chargeen_US
mhc.institutionMount Holyoke College


Original bundle
Now showing 1 - 1 of 1
Thumbnail Image
Alvarez Carolina Thesis Final Revised.pdf
17.15 MB
Adobe Portable Document Format