Direct observation of shear piezoelectricity in poly-l-lactic acid nanowires

Abstract

Piezoelectric polymers are capable of interconverting mechanical and electrical energy, and are therefore candidate materials for biomedical applications such as sensors, actuators, and energy harvesters. In particular, nanowires of these materials are attractive as they can be unclamped, flexible and sensitive to small vibrations. Poly-l-lactic acid (PLLA) nanowires have been investigated for their use in biological applications, but their piezoelectric properties have never been fully characterised, even though macroscopic films and fibres have been shown to exhibit shear piezoelectricity. This piezoelectric mode is particularly interesting for in vivo applications where shear forces are especially relevant, and is similar to what has been observed in natural materials such as bone and DNA. Here, using piezo-response force microscopy (PFM), we report the first direct observation of shear piezoelectricity in highly crystalline and oriented PLLA nanowires grown by a novel template-wetting method. Our results are validated using finite-element simulations and numerical analysis, which importantly and more generally allow for accurate interpretation of PFM signals in soft nanostructured materials. Our work opens up the possibility for the development of biocompatible and sustainable piezoelectric nanogenerators and sensors based on polymer nanowires.

Nanostructured polymer-based piezoelectric and triboelectric materials and devices for energy harvesting applications

“…This is quite unlike other piezoelectric polymers, such as PVDF, where the polar β phase is necessary for piezoelectricity. As in the case of PVDF and nylon-11, templateassisted growth of PLLA NWs was found to result in highly aligned and crystalline NWs, where the crystallinity could be precisely controlled via the infiltration temperature during the template wetting growth process [38]. Figure 3(b) shows XRD patterns of as-grown PLLA NWs within the template where only one significant reflection attributed to inter-chain scattering from (1 1 0) & (2 0 0) planes was observed, and PLLA NWs freed from the template where many more reflections were observed producing a pattern consistent with the α phase.…”

“…In the case of poly-L-lactic acid (PLLA), there are four known crystal phases (α′, α, β and γ) based around a helical conformation of the polymer chain. It is not well understood which of these crystal phase is required for piezoelectricity, if indeed any in particular [38]. Given the similarities between the crystal structures, it seems unlikely that only one specific crystal phase is responsible for the piezo electric properties.…”

Mechanical Energy Harvesting Performance of Ferroelectric Polymer Nanowires Grown via Template‐Wetting

“…Importantly, a lower value of χ NW for P(VDF‐TrFE) nanowires grown in PI templates relative to those grown in AAO templates is consistent with materials characterization that was carried out and detailed in previous work, where lower crystallinity was reported for nanowires grown in PI templates as compared to those grown in AAO templates. The FEM analysis described here can be easily extended to other template‐grown polymeric materials for energy harvesting applications.…”

“…Table 1s hows values of W NW S , W E ,a nd c NW determined from the model in conjunction with experimental results for AAOa nd PI devices.T he values of c NW for P(VDF-TrFE) were seen to be consistent for each device type with average values of 7.1 %a nd 6.4 %f or AAO-and PI template-based devices,r espectively.N oc lear frequency dependence was seen,w hich is expected for this device geometry [6] where the frequencies used are far from expected resonances and therefore this furtherv alidatest he deter-mined values. Importantly,al ower value of c NW for P(VDF-Tr FE) nanowires grown in PI templates relative to those grown in AAOt emplates is consistentw ith materials characterizationt hat was carried out and detailed in previous work, [26] where lower crystallinity was reportedf or nanowires grown in PI templates as compared to those grown in AAO templates.T he FEM analysis described here can be easily extendedt oo ther template-grown polymeric materials [8,41] for energy harvestingapplications. Interestingly,a verage values of c D for AAOand PI devices were found to be 0.10 %a nd 0.75 %, respectively,e ven thought he PI template-grown P(VDF-TrFE) nanowires showed al ower c NW as compared to AAOt emplate-grown nanowires.T his could be attributed to the higher stiffness of the AAOt emplate as comparedt ot he PI template,w hich meantt hat al arger fractiono ft he input mechanical energy was lost to deforming the AAOt emplatet han the PI template leading to overall lower device efficiency.I nb oth cases the nanogenerator device design resulted in c D being low compared to c NW ,w hich represents at heoretical limit of c D for ag iven material.…”

Robust Fabrication of Polymeric Nanowire with Anodic Aluminum Oxide Templates

“…In literature, several groups have successfully used anodized aluminum oxide (AAO) that under proper anodizing conditions contains very well structured nanopores as templates to fabricate nanoscale roughness [2][3][4][5][6][7][8][9]. With advances in polymer sciences and discovery of precursor wetting films [10], nanotubes and nanofibers have been successfully fabricated with several polymers [2][3][4][5][6][7][8][9][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] using methods often termed as "template synthesis" [31,32]. Detailed reviews can be found in [4,26,[28][29][30].…”