(2bt) Penetration of Fluorescent Dye through Polymer Coatings

Polymer coatings are critical as protective barriers and it would be beneficial to understand how the polymer properties relate to the penetration of small molecules and their subsequent contamination of coatings. Towards this goal, we investigate how a model dye molecule contaminates block copolymer coatings. In a typical experiment, a drop of fluorescent Rhodamine B dye solution of known concentration is placed on a coating of micron-order thickness, while confocal microscopy is used to visualize the resultant fluorescence inside the coating over time. As a starting point, block copolymers are synthesized with polystyrene (PS) blocks and modifiable polydimethylsiloxane (PDMS) blocks. Using image analysis methods, we track the temporal and spatial fluorescence distribution in the coating. A semi-empirical model is developed to determine the rate of contamination; we demonstrate that dye penetration is related to the chemical makeup and configuration of the block copolymers. Our work shows that confocal microscopy can be a useful tool to visualize the transport of a fluorophore in space and time through a coating. Ultimately, we expect that a better understanding of how small molecules penetrate into polymers will help guide the design of more effective coatings for a wide range of applications.

Research Interests

[References listed in attached document]

Rheology of soft material surfaces

The choice of a material for an application is often reliant on its properties measured in their bulk state. However, it is widely known that soft materials exhibit significantly different properties at their surfaces than in the bulk. This “surface” region can be defined as a depth range of a few nm[1] to a few microns[2]. The properties that deviate include viscosity[3], morphology[2], glass transition temperature[4], and aging[5]. For example, the bulk modulus of a novel water-repellent polymer is a sub-optimal representation of its mechanical strength, since the region which is utilized for its superhydrophobicity, i.e., the surface, might have lower moduli values. Rheology is instrumental in polymer melt processing[6], additive manufacturing[7], tribology[8], and in the fabrication of thin films[9], coatings[10], and adhesives[11]. Accurate measurement of the rheology of soft surfaces is thus of vital importance. But it is challenging from a practical aspect, and the consensus regarding its results is not unified[12].

To that end, I intend to apply my skills acquired in my graduate and postdoctoral research to study the rheology of soft materials at their surfaces, where the surface and the bulk co-exist, with each phase being affected by the structure and dynamics of the other. It should be noted that surface rheology is different from the highly researched field of interfacial rheology, which deals with the deformation of thin films of colloids, suspensions, emulsions etc., where the bulk effects are non-existent or considered negligible[13]. My primary goals are to devise experimental and theoretical methods to (i) accurately measure the rheology of soft material surfaces and relate it to its microstructure, and (ii) address specific practical problems using these techniques.

The proposed projects are at the confluence of the technical expertise I have acquired during my research career – my doctoral thesis was on the processing[14] and rheology[15,16] of polyolefin melts. My postdoctoral work enabled me to transition from polymer melts to elastomers and made me proficient in microscale characterization, soft deformation[17], and imaging[18]. I will utilize these skills to direct my research group into better understanding the dynamic responses of soft material surfaces.

My action plan to approach my goals comprises of the following three topics:

1. Controlled deformation of soft material surfaces

Soft materials such as polymer melts and self-assembled block copolymers usually possess larger chain mobilities in the surface than in the bulk, which leads to the surface region having lower viscosity values than the bulk region. The papers which describe measurement of stress-strain behavior of polymer melts use AFM[3,19,20] and nanoindenters[21] which are rigid probes, and are limited to sharply monodisperse polystyrene melts at high temperatures. I intend to use a flexible probe with a variable diameter for the surface of an elastomer (like PDMS) and the resultant wetting ridge deformation will be measured in-situ by confocal microscopy (Fig. 1a). The probe will be attached to a pressure-controlled nozzle which will be able to increase or decrease its size. Control over the size and rate of the deforming probe would result in precise strains in only the polymer surface, which would induce surfaces stresses that can be calculated using the geometric method of Style et al[22]. This setup would be able to thus measure the time-dependent rheological response of a soft material. Ultimately this technique can be extended to characterize the surface rheology w.r.t. polymer species, molecular weight, crosslinking, branching etc.

2. Slip in LAOS of polymers

In commercial processing applications, polymers often undergo large, fast strains. Large amplitude oscillatory shear (LAOS) rheological analysis has proven to be a vital tool to study the flow in such circumstances[23,24]. However, just as in small amplitude flow, the phenomenon of wall slip is present in LAOS rheology as well[25]. When undetected and uncorrected, this results in inaccurate measurements of rheometric data. While there is a large body of work in understanding slip in small amplitude flow[26], to the best of my knowledge, examinations into the conditions of slip in LAOS rheology have been focused on its effect on the resultant nonlinear responses[27] and boundary conditions[28]. I seek to explore the phenomenon by measuring its effects by tuning the microstructure of the soft surface layers. This is due to the hypothesis that slip may be the result of either (i) the debonding of the soft material from the interface, or (ii) the disengagement of the cohesive layers near the surface[29]. By using block copolymers (such as PS-PI[30]), I can adjust the morphology and composition of the surface layers w.r.t. to the bulk (Fig. 1b). The nature of slip measured in LAOS experiments and analysis of these materials would shed light on how the surface dynamics of soft materials affect its dynamic interactions with solid substrates.

3. Stress-birefringence of wetting ridges

For many anisotropic materials, tensile forces on the surface, termed as surface stresses[31], are crucial in dictating instabilities[32], patterning[33], and hydrodynamics[34]. This quantity is distinct from surface tension, and they are only equal for isotropic materials and liquids. Recently, an elegant geometric method, which requires no a priori knowledge of the deformed material, was described to measure the surface stresses for elastomers[22]. However, this method is true for only small surface strains, and it is not known if would hold for other soft materials such as polymer melts, where the stress would relax using different kinetics. I propose a method which utilizes birefringence[35] to visualize the pattern of stress distribution in a wetting ridge[36] (Fig 1c). Elastomers like PDMS[37] and polymer melts such as PS[38] both demonstrate stress-birefringence patterns under strain. These patterns can then be related to the tensile stress value using SAOS rheology[39]. Initially, ex situ birefringence imaging would be performed on polymer melt wetting ridges that have been frozen to preserve their shape. For elastomeric materials, a setup capable of in situ birefringence, while applying simultaneous strain, will be developed in the long term. This study will assist in the understanding of the distribution of stresses on a deformed soft surface, and its related phenomena like the Shuttleworth effect[40].

Teaching Interests

My motivation to being a faculty member is the result of being continuously inspired by a long line of teachers, tutors, professors, and mentors. My graduate studies in India have emphasized the importance of being a teacher whose students can unquestionably rely on for knowledge and guidance. On the other hand, my postdoctoral research in the USA has shown me how to give students ample independence in work and thought whilst always being approachable. Thus, my teaching philosophy has been an amalgamation of these best characteristics. My objective with every student is to make them curious and fascinated with whatever subject matter we are dealing with.

Considering my education and research skills, I am qualified to instruct courses such as Polymer Physics, Polymer Engineering, Transport Phenomena, and Numerical Methods. My ideal lecture (for every course) begins with a brief recapitulation of the previous class, and then transitions onto the topic in hand. In this way, the students will always be cognizant of where the current topic fits in the overarching scheme of the coursework. I would enthusiastically encourage the asking of questions at every stage. With this common foundation, each course would be modified according to its need. These include components like the incorporation of laboratory work, industrial visits, computational tools and programming language tutorials, open book tests etc.

I have been fortunate to apply my paradigm of teaching and mentoring on several occasions: as a visiting professor at FLAME University, as a senior research fellow in CSIR-NCL, and as a postdoctoral researcher in the Universities of Cincinnati and Kentucky. I am continuously processing both the positive and negative feedback I receive, and this enables me to mold my research group and teaching techniques towards their ideals.