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Piezoelectricity of poly(α-amino acids)-based films and fibers

The long-term goal of this research is to fabricate versatile piezoelectric materials (PM) that can be integrated into small transducers and energy harvesting device. Poly(γ-benzyl α,L-glutamate) (PBLG) is a synthetic poly(α-amino acids) reported to possess the highest electric dipole moment among all organic molecules. Different from conventional ceramic based PM which rely on dipoles from highly ordered crystalline lattices, the PBLG’s dipole originates from the hydrogen bonds of helical back bone and is much more resilient to impurities. In addition, PBLG’s extreme solubility in organic solvents allows various chemical processes for attaining a broader range of shapes, thickness, and sizes compared to ceramic PMs typically produced by mechanical foundries.

We initially worked on fabrication of piezoelectric polymer films and disks composed of piezoelectric PBLG, and a matrix polymer, poly(methylmethacrylate) (PMMA) that governs the mechanical characteristics of the composite material. Both corona-discharge and contact charging in a designed mold were used to pole the PBLG within MMA solution and polymerize the PMMA matrix. Although we observed consistent and reversible piezoelectric response of d
33= 17-20 pC/N for films fabricated by corona discharge and 1-2 pC/N for polymer disk produced by contact charging, higher piezoelectricity was unachievable due to difficulty in poling MMA solution with high PBLG concentration.

Towards this end, we discovered for the first time that electrospinning can be used as a one step method to produce polar polymer fibers with electric dipoles permanently poled in the direction of the fiber axis, resulting in high non-linear optical (NLO) activity and thermally stable piezoelectricity. The PBLG’s macroscopic dipole which is pre-aligned in the direction of helical axis can couple synergistically with external electric field and the shear force during electrospinning. The electrospun fibers exhibited a d
33 piezoelectric coefficient of 25 pC/N, which did not deteriorate even after 24 h of thermal treatment at 100 °C. To the best of our knowledge, this is one of the highest thermally stable piezoelectric coefficients reported for poled polymers. Considering the versatility of the electrospinning process combined with the chemical diversity of α-helical poly(α-amino acids), fibers electrospun from PBLG and related molecules show great promise as new electro-optical and electro-mechanical materials for small sensors and energy harvesting/scavenging devices.

Related publications:
(13)  Ren K, West JE, and Yu SM (2014) “Planar microphone based on piezoelectric electrospunpoly(γ-benzyl-a,L-glutamate) nanofibers” Journal of the Acoustical Society of America, 135 (6), EL291. 
(12)  Yu SM (2012) “Piezoelectric devices: Squeezed virus produces electricity” Nature Nanotechnology 7, 343-344.
(11)  Ren K, Wilson WL, West JE, Zhang QM, and Yu SM (2012) “Piezoelectric property of hot pressed electrospun poly(gamma-benzyl-alpha, L-glutamate) fibers” Applied Physics A-MaterialsScience & Processing 107, 639-646.
(10)  Farrar D, Ren K, Cheng D, Kim S, Moon W, Wilson WL, West JE, and Yu SM (2011) “Permanent polarity and piezoelectricity of electrospun alpha-helical poly(alpha-amino acid) fibers” Advanced Materials 23, 3954-3958.
(9)    Hwang Y, Je Y, Farrar D, West JE, Yu SM and Moon W (2011) “Piezoelectric properties of polypeptide-PMMA molecular composites fabricated by contact charging” Polymer 52, 2723–2728.
(8)    Farrar D, West JW, Busch-Vishniac IJ, and Yu SM (2008) “Fabrication of polypeptide-based piezoelectric composite polymer film” Scripta Materialia 591051-1054.
(7)    Farrar D, Yu SM, West JE, Busch-Vishniac I, Biermann PJ, and Arvelo Juan I Jr. (2005) “New materials with piezoelectric properties” International Symposium on Electrets, 10-12.
(6)    Fukuto M, Heilmann RK, Pershan PS, Yu SJM, Soto CM, and Tirrell DA (2003) “Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An x-ray scattering study” Journal of Chemical Physics 119, 6253-6270.
(5)    Fukuto M, Heilmann RK, Pershan PS, Yu SM, Soto CM, and Tirrell DA (2002) “Confinement-induced order of tethered alkyl chains at the water/vapor interface” Physical Review E 66.
(4)    Yu SJM, and Tirrell DA (2000) “Thermal and structural properties of biologically derived monodisperse hairy-rod polymers” Biomacromolecules 1, 310-312.
(3)    Yu SJM, Soto CM, and Tirrell DA (2000) “Nanometer-scale smectic ordering of genetically engineered rodlike polymers: Synthesis and characterization of monodisperse derivatives of poly(gamma-benzyl alpha,L-glutamate)” Journal of the American Chemical Society 122, 6552-6559.
(2)   Fukuto M, Heilmann RK, Pershan PS, Yu SJM, Griffiths JA, and Tirrell DA (1999) “Structure of poly(gamma-benzyl-L-glutamate) monolayers at the gas-water interface: a brewster angle microscopy and x-ray scattering study” Journal of Chemical Physics 111, 9761-9777.
(1)    Yu SJM, Conticello VP, Zhang GH, Kayser C, Fournier MJ, Mason TL, and Tirrell DA (1997) “Smectic ordering in solutions and films of a rod-like polymer owing to monodispersity of chain length” Nature 389, 167-170.

Collaborators:
       Dr. James West                                 Electrical and Computer Engineering, Johns Hopkins University
       Dr. Thao (Vicky) Nguyen                Mechanical Engineering, Johns Hopkins University
       Dr. Wonkyu Moon                            Mechanical Engineering, Pohang University of Science and Technology, Korean
       Dr. William Wilson                            Materials Science and Engineering, University of Illinois at Urbana-Champaign