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Relaxation Behaviour of Patterned Composite Polymer Surfaces and Underlying Compensation Phenomenon

Bhadauriya, Sonal
http://rave.ohiolink.edu/etdc/view?acc_num=akron1577984063649952

Abstract Details

2019, Doctor of Philosophy, University of Akron, Polymer Engineering.
Imprinted polymer and polymer nanocomposite surfaces are ubiquitous in nature but how do such surfaces relax? What underlying fundamental mechanism governs the relaxation behaviour of such films? These are some of the pressing questions in the realm of flexible electronics, advanced lithography, nanodevices and biosensors where a decay in the stability of imprinted nanostructures can render them unusable. Therefore, tuning the stability of imprinted features in polymer thin films presents an important problem from both fundamental and application standpoint. The stability of imprinted nanostructures in homopolymer thin films has been extensively studied, using both ex-situ and in-situ procedures where the structural decay is governed by Laplace pressure induced viscous flow. Imprinted polymer nanocomposite films are ubiquitously present; from laminates to drag reducing coatings but the effect of additives in imparting structural stability is essentially unexplored. In the first section of this study, the nano-structure decay (also called “slumping”) in imprinted PMMA thin films (≈ 100 nm) containing clustered PMMA grafted titania nanoparticles (TiO2 PGNP) and well-dispersed PMMA grafted Silica nanoparticles (SiO2 PGNP) was investigated. It was demonstrated that the stability of the nanoimprinted pattern at temperature above the matrix’s glass transition temperature can be tuned by addition of these nanoparticles to the polymer matrix. This is the first experimental investigation of structural decay of a patterned nanocomposite film and it sheds light on an underlying entropy-enthalpy compensation (EEC) behaviour governing the decay process. EEC signifies a linear dependence between the activation parameters of the relaxation process and is routinely observed in the dynamics of many condensed materials such as molecular additives and glass-forming materials. Herein we found relaxation behaviour of nanoparticle filled imprinted polymer films illustrating similar compensation effect, which lead to slower decay kinetics of nanocomposite thin film above a certain compensation temperature compared to the neat polymer film. This characteristic temperature is conventionally called the entropy−enthalpy compensation (EEC) temperature, Tcomp and it was found to be ≈ 92 (±5) °C for TiO2 PGNP and 91 (±3) °C for SiO2 PGNP, a temperature in the vicinity of the matrix Tg ≈ 97 °C. An important implication of the compensation behaviour is that the stabilizing effect of the PGNP on the imprinted patterns should no longer exists if the slumping temperature is lower than Tcomp. It was seen that the filled imprinted pattern slumps more rapidly compared to the pure PMMA film at 80 °C (T < Tcomp), thereby reversing the stabilization effect. EEC observed in the relaxation dynamics of imprinted nanocomposite films was also found to be independent of the dispersion of the PGNP and is observed for both clustered (TiO2 PGNP) and well-dispersed (SiO2 PGNP) systems. The second part of the dissertation focuses on understanding the relaxation behaviour of patterned polymer surfaces with additives interacting strongly with the matrix. Unmodified bare SiO2 nanoparticle was used and it was found that such particles lead to a better pattern retention above the compensation temperature compared to the grafted nanoparticle system. A compensation effect was witnessed in the relaxation dynamics of this pattern relaxation process leading to a much robust evidence towards a compensation phenomenon underlying these dynamics. The compensation temperature was found to be in the vicinity of the glass transition of the matrix PMMA, Tcomp ≈ 95 °C, similar to as reported before. EEC was robustly observed for clustered polymer grafted titania nanoparticle, well-dispersed polymer grafted silica nanoparticle and well-dispersed unmodified silica nanoparticles. In the third part of this dissertation, in-situ heated atomic force microscopy (AFM) was combined with automated line-by-line spectral analysis to quantify the relaxation or decay of nanopatterned polymer films at temperatures above the glass transition. This approach enables assessment of pattern fidelity with a temporal resolution of 1 s, providing the necessary data density to confidently capture the short time relaxation data as compared to conventional off-line serial measurements. Specifically, the thermal decay of nanopatterned PMMA and PMMA composite films (containing unmodified and PMMA-grafted silica nanoparticles, SiO2 NP) of varying concentrations and film thicknesses was studied using this new approach. Features imprinted on neat PMMA films were seen to relax at least an order of magnitude faster than the filled films at decay temperatures above the glass transition of the PMMA matrix. It was also seen that patterned films with the lowest residual thickness filled with unmodified SiO2 NP decayed the slowest. The effect of additive was almost negligible in reinforcing the patterned features for films with the highest residual thickness. The in-situ pattern decay measurement and the subsequent line-by-line spectral analysis enabled the investigation of various parameters affecting the pattern decay such as the underlying residual thickness, type of additive system, and the exposure temperature in a timely and efficient manner. In the last study, to gain insights into the below Tcomp relaxation and to fully map out the compensation phenomenon, the relaxation behaviour of polymer wrinkles as a function of temperature was investigated. For the first time, a full mapped out transition of decay kinetics as a function of temperature (below and above the glass transition of the matrix) and additive concentration was experimentally observed by utilizing decay of polymeric surface wrinkles. EEC effect was observed for an athermal and a favorable interacting composite system, thereby ensuring the robustness of the observed phenomenon. As a consequence of this compensation effect, relaxation kinetics for composite wrinkled films is faster than the neat polymer film below the characteristic compensation temperature, Tcomp and slower above the Tcomp. EEC proves itself to be the underlying mechanism for patterned polymer decay, governing the kinetics of any polymeric surface with patterns. In summary, the addition of nanoparticles to imprinted and wrinkled PMMA films leads to a significant stabilization of nanoimprinted patterns (above Tcomp) and provide a simple strategy to control high temperature nanostructural integrity in patterned polymer films. The compensatory relaxation dynamics observed for imprinted films were also observed in relaxation dynamics of wrinkled polymer films. The observations presented in this dissertation point towards a common relaxation dynamics origin of the EEC behavior in patterned polymer thin film and nanocomposite systems. These results have significant implications in the field of organic conductors, optical devices to name a few. This study provides fundamental insights into how nanoparticles influence the relaxation dynamics of imprinted composites; thereby opening this field for many oncoming researchers working in the field of durable textured polymer nanocomposites materials.
Alamgir Karim, PhD (Advisor)
Sadhan C. Jana, PhD (Committee Member)
Weinan Xu, PhD (Committee Member)
Ali Dhinojwala, PhD (Committee Member)
Jutta Luettmer-Strathmann, PhD (Committee Member)
219 p.
Engineering; Materials Science; Physics; Polymers
Polymer Nanocomposite, Slumping, Decay, Wrinkling, Entropy-Enthalpy Compensation Effect, Lithography, Patterning, Textured Surfaces

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Citations

  • Bhadauriya, S. (2019). Relaxation Behaviour of Patterned Composite Polymer Surfaces and Underlying Compensation Phenomenon [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1577984063649952

    APA Style (7th edition)

  • Bhadauriya, Sonal. Relaxation Behaviour of Patterned Composite Polymer Surfaces and Underlying Compensation Phenomenon . 2019. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1577984063649952.

    MLA Style (8th edition)

  • Bhadauriya, Sonal. "Relaxation Behaviour of Patterned Composite Polymer Surfaces and Underlying Compensation Phenomenon ." Doctoral dissertation, University of Akron, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=akron1577984063649952

    Chicago Manual of Style (17th edition)

akron1577984063649952
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This open access ETD is published by University of Akron and OhioLINK.