Sciences - Cleveland L3
Stability of Hydrophilic Polymer Thin Films: Crystallinity, Hydrophilicity, and Electrostatic Charge
Student Name: Carolina Alvarez
Project advisor: Wei Chen
Polymer 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 discontinuous morphologies. 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 nanoscopic features. Regardless of its desirability, an understanding of the mechanisms of dewetting is critical for tailoring the morphologies of thin films to the specifications of their applications.
The stability of a thin film often 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 experimental results. Therefore, the goal of this 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 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 lack of crystallinity and pH-tunable electrostatic charge 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 to serve as a pH-resistant point of comparison.
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 is investigated in order to probe the relative contributions of polymer-substrate and polymer-polymer interactions, respectively, to total film thickness and stability. Static adsorption experiments were used to obtain h1 values for each polymer. These were then subtracted from the total thickness 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 properties. This combination of information is anticipated to paint a larger picture of how properties such as crystallinity, hydrophobicity, and polymer charge inform the film formation process and stability, which can then be applied to the broader category of hydrophilic polymer thin films.
Oxidation-responsive modified cellulose-based materials
Student Name: Aaditi Chopade
Project Advisor: Dr. Kyle Broaders
Biopolymers are an attractive avenue for drug delivery applications due to their abundance, biocompatibility, sustainability, and flexible modification. Cellulose-based materials are especially appealing, but their development is often hampered by the poor solubility of their parent polymer. Hydrophobic groups can be reversibly grafted onto a biopolymer to make oxidation sensitive solubility switching materials. The modified cellulose was prepared through a solution- phase reaction with hydrophobic groups that became hydrophilic in the presence of reactive oxidative species. The modified polymer could be fashioned into particles or films, which were investigated to encapsulate hydrophobic cargo and release the payload under oxidizing conditions and therefore yielding a hydrogen peroxide responsive material. The new material may hold promise in drug-eluting depot formations like stents, sutures, or beads.
Spontaneous Adsorption and Desorption of Poly(vinyl alcohol) on Polydimethylsiloxane Substrates
Student name: Julia Griffin
Project advisor: Wei Chen
The adhesion and stability of hydrophilic polymer thin films prepared from aqueous solutions are of paramount interest in many applications, such as microfluidics, biomedicine, and anti-fouling coatings. However, there are few literature reports on hydrophilic polymer thin films due to the challenging nature of the destabilizing polar interactions during solvent evaporation.1 Despite these challenges, understanding and employing aqueous, nontoxic solutions to form hydrophilic polymer thin films are crucial to further this area of research, expand their applicability, and limit toxicity of materials. Previous studies have reported the spontaneous adsorption of poly(vinyl alcohol) (PVOH) onto some hydrophobic substrates from aqueous solution,2,3 as well as the subsequent desorption of polymers in water. 4,5 In this study, we investigated the spontaneous adsorption and subsequent desorption of PVOH on hydrophobic and flexible polydimethylsiloxane (PDMS) substrates. We worked with two PVOH polymers with different degrees of hydrolysis (different ratios of hydroxyl to acetate groups) and various PDMS molecular weights since substrate molecular weight and mobility have been shown to affect adsorption of macromolecules.6,7 Our results indicate that the characteristics of the PVOH-PDMS thin films in terms of thickness, morphology, and stability are mainly influenced by PDMS molecular weight and PVOH degree of hydrolysis. The adsorbed and desorbed PVOH films are discontinuous on all PDMS substrates. The extent of the PVOH film dewetting and desorption significantly increases on the substrates of the highest molecular weights. These results give insight to the stability of PVOH-PDMS thin films in water, as well as the multilayered structure of the adsorbed films.
Synthesizing Reactive Oxygen Species Sensitive Drug Delivery Systems
Student name: Isabela Haskell
Project advisor: Kyle Broaders
Microparticulate immunotherapy has the potential to provide a more efficient and effective tactic for delivering therapeutic payloads to their sites of activity. Microparticles can be designed to protect their payload until conditions for their degradation are met. Because of the highly-oxidizing environment in the lysosomal compartments of antigen-presenting cells, payload release in response to reactive oxygen species is especially appealing4 . Boronic esters have been widely used to trigger degradation in the presence of hydrogen peroxide, and pinacol is an especially common esterifying diol that is used1 . Recent studies have found that dextran polymer-based delivery vehicles modified with pinanediol are more stable and soluble in organic solvents than their pinacol counterparts3 . These delivery vehicles can be made multifunctional through additional modifications via click chemistry reactions to the boronic ester such as therapeutic or fluorescent molecules2 . However, in the past these modifications were added to the pinanediol before the whole complex was added onto dextran, thus all of the reactions required to add pinanediol onto dextran must be repeated for each new modification3 . Instead of modifying pinacol prior to the addition to dextran, we considered whether the modifications can instead be added to a dextran polymer with pinacol already attached (Pin-B-dex). Pinacol was successfully added to dextran, confirmed by H-NMR, and addition of pinanediol, a stable diol that reacts well with esters, confirmed that Pin-B-Dex can be modified. In order to add more functional modifications, fluorescence, or therapeutic molecules, the modification molecule of choice will be attached to nopol diol enabling its addition onto Pin-B-Dex. One such modification was synthesized through the addition of pyrenebutyric acid, a fluorescent molecule, onto nopol diol. Pin-B-Dex was successfully modified according to H-NMR; however, the small yield of the product suggests that improvements could be made in future syntheses.
Stability of Poly(vinyl alcohol) Thin Films on a Flexible Polydimethylsiloxane Surface
Student name: Nancy Jiang
Project advisor: Professor Wei Chen
Poly(vinyl alcohol) (PVOH) thin films were adsorbed or spin-coated on silicon-supported polydimethylsiloxane (PDMS) surfaces to test the stability of the films. PDMS of 2 kDa molecular weight (PDMS2k ), classified to have a medium molecular weight, is a flexible polymer. While the thickness of the spin-coated 88% hydrolyzed PVOH (PVOH88%H ) on PDMS2k follows the Meyerhofer's model of h (thin film thickness) ∝ ω (spin rate)−1/2 , the film thickness of 99% hydrolyzed PVOH (PVOH99%H ) on the substrate is independent of spin rate. This difference can be explained through their difference in crystallinity and the decoupled thickness model, which separates the film thickness into two components - one results from polymer-substrate interaction (h1) and the other stems from polymer-polymer interaction (h2). The h1 thickness is equivalent to the spontaneously adsorbed thickness. Annealing of PVOH-PDMS2k samples at 94 °C for 15 minutes further amplified the mobility of the PDMS chains. As a result, spin-coated film morphology transitions from smooth to dewetted with holes, proving that PVOH films on PDMS2k is a metastable system. PVOH99%H thin films also exhibit a transition from spinodal dewetting (nanoscopic) to diffusion-limited aggregation (DLA) (microscopic) features, dependent on the PDMS thickness. This research provides insights into the stability of spin-coated thin films, which can be applied to fabrication of devices such as electronic semiconductors and thin-film batteries.