MULTIFUNCTIONAL PARYLENE-C MICROFIBROUS THIN FILMS

Open Access
Author:
Chindam, Chandraprakash
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 07, 2017
Committee Members:
  • Akhlesh Lakhtakia, Dissertation Advisor
  • Osama O. Awadelkarim, Committee Chair
  • Akhlesh Lakhtakia, Committee Member
  • Michael T. Lanagan, Committee Member
  • M. Amanul Haque, Outside Member
Keywords:
  • microfibrous thin films
  • Parylene C
  • multifunctionality
  • microfibers
  • phononic crystals
  • photonic crystals
  • wettability
Abstract:
Towards sustainable development, multifunctional products have many advantageous over single-function products: reduction in the number of parts, raw material, assembly time, and cost involved in a product’s life cycle. My goal for this thesis was to demonstrate the multifunctionalities of Parylene-C microfibrous thin films. To achieve this goal, I chose Parylene C, a polymer, because the fabrication of periodic mediums of Parylene C in the form of microfibrous thin films (mFTFs) was already established. A mFTF is a parallel arrangement of identical micrometer-sized fibers of shapes cylindrical, chevronic, or helical. Furthermore, Parylene C had three existing functions: in medical-device industries as corrosion-resistive coatings, in electronic industries as electrically insulating coatings, and in biomedical research for tissue-culture substrates. As the functionalities of a material are dependent on the microstructure and physical properties, the investigation made for this thesis was two-fold: (1) Experimentally, I determined the wetting, mechanical, and dielectric properties of columnar mFTFs and examined the microstructural and molecular differences between bulk films and mFTFs. (2) Using physical properties of bulk film, I computationally determined the elastodynamic and determined the electromagnetic filtering capabilities of Parylene-C mFTFs. Several columnar mFTFs of Parylene C were fabricated by varying the monomer deposition angle. Following are the significant experimental findings: 1. Molecular and microstructural characteristics: The dependence of the microfiber inclination angle on the monomer deposition angle was classified into four regimes of two different types. X-ray diffraction experiments indicated that the columnar mFTFs contain three crystal planes not evident in bulk Parylene-C films and that the columnar mFTFs are less crystalline than bulk films. Infrared absorbance spectra revealed that the atomic bonding is the same in all columnar mFTFs and bulk films. The static hydrophobicity of columnar mFTFs was found to be anisotropic and can be maximized by a proper choice of monomer deposition angle. In contrast, the hydrophobicity of bulk film is isotropic. 2. Mechanical properties: Dynamic storage and loss moduli of columnar mFTFs were determined in the 1 to 80 Hz frequency range for temperatures between -40 deg C and 125 deg C in one of two orthogonal directions lying wholly in the substrate plane: either (i) normal or (ii) parallel to the morphologically significant plane of the mFTF. The storage and loss moduli for normal loading did not exceed their counterparts for parallel loading. All columnar mFTFs were found to be softer than a bulk film. In both bulk and columnar forms, Parylene C was found to be rheologically not simple. 3. Relative permittivity: The charge-storage and absorption properties measured for the columnar mFTFs in the 100 Hz--1 MHz frequency range over temperatures between -40 deg C and 125 deg C were lower than the bulk film. Internal surfaces of the columnar mFTFs were found to increase the charge-storage capacity. The lower charge-storage capability of columnar mFTFs suggests their possible applications as interlayer dielectrics. The frequency dependence of the relative permittivity of the columnar mFTFs was identified in terms of the Hashin--Shrtikmann model. The elastodynamic bandgaps of Parylene-C mFTFs as phononic crystals were computationally determined for the columnar, chevronic, and chiral mFTFs. Microfibers were arranged either on a square or a hexagonal lattice with the host medium as either water or air. Following are the significant findings: 1. All bandgaps were observed to lie in the 0.01--162.9 MHz regime. The upper limit of the frequency of bandgaps was the highest for the columnar mFTFs and the lowest for the chiral mFTFs. More bandgaps were found to exist when the host medium is water than air. The presence of complete bandgaps suggests their use as bulk-acoustic-wave and surface-acoustic-wave filters. The softness of the Parylene-C mFTFs makes them mechanically tunable, and their bandgaps can be exploited in multiband ultrasonic filters. 2. An investigation was made to demonstrate Parylene-C mFTFs as circular-polarization filters. The relative permittivity of bulk Parylene C was determined as a function of frequency between 15 THz and 149 THz. Potential application of chiral mFTFs as reflectors of thermal energy was identified. The circular Bragg regime for chiral mFTFs of Parylene C was identified as 31.8--35.2 THz, making them useful as circular-polarization band-rejection filters.