Nandula D. Wanasekara

Utilizing Peptidic Ordering in the Design of Hierarchical Polyurethane/Ureas

One of the key design components of nature is the utilization of hierarchical arrangements to fabricate materials with outstanding mechanical properties. Employing the concept of hierarchy, a new class of segmented polyurethane/ureas (PUUs) was synthesized containing either a peptidic, triblock soft segment, or an amorphous, nonpeptidic homoblock block soft segment with either an amorphous or a crystalline hard segment to investigate the effects of bioinspired, multiple levels of organization on thermal and mechanical properties. The peptidic soft segment was composed of poly(benzyl-l-glutamate)-block-poly(dimethylsiloxane)-block-poly(benzyl-l-glutamate) (PBLG-b-PDMS-b-PBLG), restricted to the β-sheet conformation by limiting the peptide segment length to <10 residues, whereas the amorphous soft segment was poly(dimethylsiloxane) (PDMS). The hard segment consisted of either 1,6-hexamethylene diisocyanate (crystalline) or isophorone diisocyanate (amorphous) and chain extended with 1,4-butanediol. Thermal and morphological characterization indicated microphase separation in these hierarchically assembled PUUs; furthermore, inclusion of the peptidic segment significantly increased the average long spacing between domains, whereas the peptide domain retained its β-sheet conformation regardless of the hard segment chemistry. Mechanical analysis revealed an enhanced dynamic modulus for the peptidic polymers over a broader temperature range as compared with the nonpeptidic PUUs as well as an over three-fold increase in tensile modulus. However, the elongation-at-break was dramatically reduced, which was attributed to a shift from a flexible, continuous domain morphology to a rigid, continuous matrix in which the peptide, in conjunction with the hard segment, acts as a stiff reinforcing element.

Influence of secondary structure and hydrogen-bonding arrangement on the mechanical properties of peptidic-polyurea hybrids

Bio-inspired materials design is an important strategy used in the fabrication of tunable and mechanically enhanced polymeric systems. An important aspect of bio-inspiration is to understand how components, such as hierarchy and self-assembly, affect the properties of the designed materials. In this investigation, we explore the use of polypeptide secondary structure and hydrogen bonding arrangement, in order to determine their effects on the thermal and mechanical properties of fully synthetic peptidic polyureas. Specifically, we incorporate either short β-sheet forming peptide blocks of poly(β-benzyl-l-aspartate)5 or poly(ε-carbobenzyloxy-l-lysine)5 or longer peptide blocks of poly(β-benzyl-l-aspartate)20 or poly(ε-carbobenzyloxy-l-lysine)20 as α-helix forming domains into non-chain extended polyureas based on 1,6-hexamethylene diisocyanate and poly(dimethysiloxane). Secondary structure was found to be influenced by the weight fraction of peptide, e.g. increasing peptide weight fractions increased sheet or helical ordering. Additionally, the polyurea microstructure was comprised of nanofibrils with a secondary structure dependent fiber width, attributed to the peptidic motif alignment within the nanothreads. Analysis of the thermomechanical and tensile response revealed multiple trends, such as increased toughness attributed to β-sheet ordering and increased modulus with increased peptide weight fraction. It is anticipated that this observed interplay between peptide organization and mechanics will be applicable to engineering and biomaterial development due to the simplicity of the synthetic protocol and the promising mechanical tunability guided by the peptide segment.

Tunable Mechanics in Electrospun Composites via Hierarchical Organization

Design strategies from nature provide vital clues for the development of synthetic materials with tunable mechanical properties. Employing the concept of hierarchy and controlled percolation, a new class of polymer nanocomposites containing a montmorillonite (MMT)-reinforced electrospun poly(vinyl alcohol) (PVA) filler embedded within a polymeric matrix of either poly(vinyl acetate) (PVAc) or ethylene oxide-epichlorohydrin copolymer (EO-EPI) were developed to achieve a tunable mechanical response upon exposure to specific stimuli. Mechanical response and switching times upon hydration were shown to be dependent on the weight-fraction of MMT in the PVA electrospun fibers and type of composite matrix. PVA/MMT.PVAc composite films retained excellent two-way switchability for all MMT fractions; however, the switching time upon hydration was decreased dramatically as the MMT content was increased due to the highly hydrophilic nature of MMT. Additionally, for the first time, significant two-way switchability of PVA/MMT.EO-EPI composites was achieved for higher weight fractions (12 wt %) of MMT. An extensive investigation into the effects of fiber diameter, crystallinity, and MMT content revealed that inherent rigidity of MMT platelets plays an important role in controlling the mechanical response of these hierarchical electrospun composites.

Thermal Properties of Thermochromic Asphalt Binders by Modulated Differential Scanning Calorimetry

Thermochromic materials are substances that can reversibly change their colors in response to temperature. These materials can be designed to have high solar reflectance at high temperature and low solar reflectance at low temperature. They are therefore potential new smart materials that can improve pavement thermal conditions. The innovation of this study was to develop thermochromic asphalt binders possessing dynamic solar reflectance in response to environmental temperature. The preliminary study indicated that the thermochromic binder coating achieved a temperature reduction as high as 6.4°C under the conditions of a typical summer day in Cleveland, Ohio. A comparison of measurements under cold conditions in Cleveland demonstrated that thermochromic asphalt binder could remain warmer than conventional asphalt binder. This attribute helps to improve the resistance of asphalt pavement to low- temperature cracking, delays ice formation on the road surface, and potentially improves the durability and safety of asphalt roads. The phase transition temperature and latent heat of thermochromic asphalt binders were characterized by standard differential scanning calorimetry (DSC). The specific heat capacity, thermal conductivity, and thermal diffusivity of thermochromic asphalt binders were determined by modulated DSC at temperatures ranging from –20°C to 50°C. Comparison measurements found that thermochromic asphalt binders presented higher specific heat capacity, lower thermal conductivity, and lower thermal diffusivity than conventional pure asphalt binders. Empirical models were proposed for the variations of specific heat capacity and thermal conductivity of various asphalt binders as a function of temperature. These models provided data for design and simulation of thermal responses of asphalt pavement incorporating thermochromic binder materials.