Article pubs.acs.org/Macromolecules
Direct Evidence of Polar Ordering and Investigation on Cytophilic Properties of Pyroelectrified Polymer Films by Optical Second Harmonic Generation Analysis Stefano Lettieri,*,† Romina Rega,† Deborah K. Pallotti,† Oriella Gennari,† Laura Mecozzi,† Pasqualino Maddalena,‡ Pietro Ferraro,† and Simonetta Grilli† †
National Research Council, Institute of Applied Sciences & Intelligent Systems (ISASI-CNR) “E. Caianiello”, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy ‡ Physics Department, Università degli Studi di Napoli “Federico II”, Via Cintia, 80126 Napoli, Italy S Supporting Information *
ABSTRACT: Pyroelectrification (PE) has been proposed as a low-cost electrode-free tool for orienting and aligning dipole molecules in polymer layers, by means of the electric field produced by a pyroelectric crystal under appropriate thermal stimulations. Probing and assessing the coherent polarization arrangement is of fundamental importance for the process control and optimization but is also a challenging task. In fact, the probing operation must be noninvasive and avoid any disarrangement the dipoles and thus without interfering with the resulting surface charges. Here we show that the PE-induced polar ordering can be probed in situ by an electrode-free analysis based on the measurement of the intensity of the second-harmonic optical wave generated by the polymer film. In fact, the results show a substantial enhancement of the second-order susceptibility of the polymer layer, caused by the PE-induced dipoles alignment. Moreover, thanks to this approach, it is demonstrated the ability of PE process to polarize polymer layers even in the “noncontact configuration”, i.e., when a dielectric spacer (glass substrate) is placed between the pyroelectric crystal and the polymer film.
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and corona poling,1 in which using an external voltage source is needed. Although these methods are quite well established, there do exist some challenging problems. For example, severe charge injection from metal electrodes often occurs in contact poling, resulting in a large current that causes dielectric breakdown of the films. Corona poling is usually performed under several kilovolts, and the homogeneity of the poling fields is difficult to be controlled. Moreover, this method is prone to surface damaging of the poled films due to the presence of various reactive species (e.g., ozone or nitrogen oxides) in the corona discharge.17,18 Recently, we developed the “pyroelectrification” (PE) of polymer films, able to produce both mono- and bipolar domains with oriented dipoles through an innovative electrodefree technique.19 The films polarization was evidenced a posteriori by analyzing polarization-related phenomena, including jet printing and cell patterning. These tests do not alter the polarization of the polymer but also do not probe in a direct manner the degree of polar ordering induced in the polymer layer. Conversely, other approaches based on the use of applied voltages allow the direct probing of the polar order but are
INTRODUCTION Some dielectric materials become permanently or almost permanently polarized after being exposed to an electric field, through the alignment of dipoles or by the accumulation of surface charge.1,2 Electrets are defined as materials that maintain a quasi-permanent electrical polarization, while ferroelectrics exhibit a permanent polarization that is easily reversed by an applied field. The switchable polarization of ferroelectrics has led their use for a variety of microelectronic devices including data storage, sensors, and actuators2 but also for emerging biotechnology applications.3,4 The intrinsic electric field in electrets has been used different applications, such as self-assembly of micro/nanoparticles,5 enhanced electron/hole separation in solar cells,6 sensors and energy harvesting devices,7−9 photocatalysis,10 and xerography.11 Moreover, vast potentialities in programming and patterning the electric charges through the use of ferroelectric substrates have been widely demonstrated recently by using simple and effective processes.12−16 Recent attention is being devoted to polymer-based electrets as promising materials for the development of low-cost technologies based on flexible thin films. Different techniques allow to set a polar orderi.e., induce a coherent alignment of molecular dipoles and hence a macroscopic polarizationin polymeric films. The most common ones are contact poling © XXXX American Chemical Society
Received: April 17, 2017 Revised: September 18, 2017
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DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Schematic view of the samples preparation. The reference sample R is an amorphous PMMA deposited by spin-coating on a glass coverslip. Sample A was prepared by “contact PE”; i.e., the PMMA film was spin-coated onto a lithium niobate (LN) crystal, next subjected to the PE process and successively detached from the LN and transferred on a glass coverslip. Sample B was prepared by “non-contact PE”; i.e., a PMMA film was spin-coated on a glass coverslip that was next placed onto the LN crystal for the PE treatment. In this procedure, no physical contact between LN and polymer film occurs.
invasive and can even perturb the electrical field to be measured.20,21 In this work, we report an in situ and direct evaluation of the PE-induce polar ordering through detection and analysis of optical second harmonic intensity orin short“second harmonic generation” (SHG) analysis. SHG is the nonlinear optical phenomenon consisting in the generation of an electromagnetic wave of frequency 2ω (“second-harmonic” or SH wave) caused by the interaction between a material medium and a laser beam (“fundamental beam”) of frequency ω. Symmetry considerations allow to demonstrate that this process takes place only in material regions where symmetry under a spatial inversion is broken,22 such as regions characterized by a coherent dipole alignment. SHG spectroscopy is highly sensitive to polar order. It allows repeating analyses over time as it does involve neither physical nor electrical contacts. Furthermore, it does not induce temperature increases that might misalign the molecular dipoles and reduce the overall electret polarization. Finally, it also allows a spatial mapping of the polarization distribution. Thanks to these peculiarities, SHG spectroscopy is a versatile tool for investigating processes that involve noncentrosymmetric structures or functional interfaces. Examples are represented by studies on formation of conduction channels and local charges in organic field-effect transistors,23−26 studies on adsorbates dynamics,27 identification of surface states,28−30 measurement of anisotropy in carrier mobility in aligned polymers,31 or determination of orientation of chiral threedimensional structures.32 These arguments underline that SHG can represent an ideal method for the analysis of polymeric electrets. Despite this it has not been extensively adopted so far to this aim. Here we demonstrate the reliability of the SHG technique in pinpointing the induction of polar order in PE-processed polymer films under both “contact PE” and “noncontact PE”, namely, when a dielectric layer (i.e., glass slide) is inserted between the pyroelectric crystal and the polymer film. More in
detail, we show that the PE-processed layers of poly(methyl methacrylate) (PMMA) exhibit a dramatic enhancement of their SHG efficiency, as clear consequence of the establishment of PE-induced polar order in the otherwise amorphous polymer. These results were correlated to the outcomes of tests of live cell adhesion onto glass slides functionalized by the same polymers. The experimental outcomes highlight the possibility to use polarized films deposited onto glass substrates without the need to peel them off from the pyroelectric crystal, opening the range of PE applications to all of those cases where charged polymers are desirable onto rigid supports.
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EXPERIMENTAL DETAILS
Samples Preparation. The PE process was accomplished by a slow heating of the polymer film onto a lithium niobate (LN) crystal up to a temperature exceeding the glass transition temperature (Tg) of the polymer, followed by a rapid cooling able to generate the pyroelectric field which orients the dipole molecules. Further details can be found in ref 19. The samples consisted of PMMA polymer layer (PMMA in anisole at 30% w/w) supported on (1 × 1) cm2 sized and 0.13 mm thickness glass coverslip, prepared according to the three different procedures illustrated in Figure 1. The sample R consisted of a PMMA layer spincoated on a glass coverslip that was not subjected to PE process and hence was used as a reference sample. In the case of sample A, the PMMA layer was spin-coated onto the LN crystal used for PE procedure (“contact PE”), peeled off from the LN crystal, and then placed onto the coverslip. Finally, for sample B the PMMA layer was spin-coated onto a coverslip that was next placed onto the LN crystal for “non-contact” PE process. In this latter case, no physical contact between LN and polymer film occurs, and the eventual PMMA electrification reveals that the electric field created by excess pyroelectric charges at the LN/glass interface is capable to orient the PMMA molecules even in the presence of a dielectric (glass) spacer. SHG Measurements. Polarization-resolved second harmonic generation (PR-SHG) analysis was performed by detecting the optical intensity of the two orthogonal SH components (p-polarized wave, parallel to the plane of incidence, and s-polarized wave, orthogonal to the plane of incidence) versus the polarization angle α of the linearly B
DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules polarized fundamental beam. The experiment was performed in reflection mode (angle of incidence θ of about 45°) by using a Nd:YAG mode-locked laser (λ = 1064 nm wavelength, 20 ps pulses duration, 10 Hz repetition rate) as fundamental beam. The relevant experimental geometry is represented in Figure 2, showing the
representation of Figure 2 and considering a medium whose symmetry C∞v (i.e., invariant for rotation along the z-axis and in which z and −z directions are not equivalent), the intensity of the p- and s-polarized SHG intensity (indicated as IP2ω and IS2ω) are the following (demonstration is reported in the Supporting Information): I2Pω(α) = (Iω sin θ )2 |a(sin α)2 + b|2
(1)
I2Sω(α) = (Iω sin θ )2 |c sin(2α)|2
(2)
In eqs 1 and 2, θ is the angle of incidence, Iω and α are the intensity and the polarization angle of the fundamental beam, and a, b, and c are coefficients that involve the non-null independent elements of the second-order dielectric susceptibility χ(2) tensor. As discussed in the Supporting Information, the only non-null component of the χ(2) tensor of a homogeneous planar distribution of oriented dipoles are χ(2) zzz, (2) χ(2) zxx, and χxxz, while the equivalence of the directions parallel to (2) the sample surface (xy-plane) fixes the conditions χ(2) zxx = χzyy (2) (2) (2) (2) and χxxz = χyyz = χxzx = χyzy . The coefficients b and c have a very (2) simple expression, i.e., b = χ(2) zxx and c = χxxz, while the coefficient a has a much more complicated expression that involves a linear combination of all of the three independent non-null terms of the χ(2) tensor. As mentioned in the Introduction, the second-order dielectric susceptibility of a material in which inversion symmetry holds is null. Hence, no “bulk” second-order polarization can exist in isotropic systems such as amorphous solids, liquids, or gases. A residual SHG signal is indeed present even in these cases, as the inversion symmetry is always broken at the surface of a material (“surface SHG”). However, such a surface SHG signal is generated only by surface monolayer of atoms/molecules, and hence its intensity is usually very low. PMMA molecules in sample R are randomly oriented; hence, the film is isotropic. Therefore, sample R is expected to exhibit no SH intensity, except for the residual surface SHG one. Differences between the SHG behavior of sample R and the PE-treated samples (A, B) should hence be attributed to the different degree of dipoles alignment (polar order) that breaks the inversion symmetry along the axis of dipole alignment. Each of the samples A and B was tested side by side with the reference sample R in two successive experimental runs (R + A and R + B), in order to minimize eventual systematic errors due to different environment conditions. The sample pairs were placed on the horizontal sample holder mounted on a manual translation stage and the SHG intensity at a fixed polarization configuration was measured by scanning the samples surface along a direction. The experimental results are shown in Figure 3, reporting the SHG intensity measured in p-in/p-out configuration (i.e., I2ω p (α = π/2)) in different positions of the PMMA films. It is immediately observed that the both A and B samples exhibit a sensible increase in the SHG signal with respect to untreated PMMA (sample R). As expected, a weak SHG signal was detected also for sample R, likely to be due to surface contribution. Importantly, the experimental SHG intensity for PE-processed PMMA films (A and B samples) was found to be up to 30 times larger than the one measured in the reference sample R. This marked difference represents a direct proof of the occurrence of dipole orientation caused by PE and leading to a bulk nonlinear polarization enhancing the otherwise weak SH intensity.
Figure 2. Experimental geometry for the PR-SHG experiments. The polarization of the fundamental beam is defined by the polarization angle α formed by the direction of the electric field Eω with the sample surface (xy-plane). The cases α = 0 and π/2 correspond to s- and ppolarized fundamental beam, respectively. The polarization angle is varied by rotating a half-wavelength retardation plate mounted on a computer-controlled rotation stage. The reflected fundamental beam is blocked by optical filters, while the SH beam is detected through a photomultiplier coupled with the exit slit of a monochromator. polarization angle α and the coordinates system we use here to describe the SHG signal. The sample surface lies in the xy-plane, while the z-axis is oriented along the normal to the sample surface and xz is the plane of incidence. The polarization angle α is defined as the angle formed by the xy-plane and the polarization direction of the fundamental beam. Hence, the cases α = 0 and α = π/2 correspond to s- and p-polarized fundamental beam, respectively. The polarization angle α was varied through a half-wave retardation plate mounted on a computer-controlled motorized rotation stage. The fundamental beam reflected by the sample and the undesired residual SH wave generated in the various optical elements composing the setup were filtered off by using sharp cutoff optical filters. The pand s-polarized components of the SH output beam was selected by rotating a half-wave retardation plate operating at SH wavelength (λ2ω = 532 nm) and placed before a fixed polarizer. To remove any eventual spurious signal, the output beam was spectrally filtered at the SH wavelength by a PC-controlled motorized monochromator. The SHG beam intensity was detected by a photomultiplier tube. Each experimental point was determined by averaging the SHG signal over 200 laser shots. SH-SY5Y Human Neuroblastoma Cell Lines. The SH-SY5Y cell line was purchased from ECACC (Sigma-Aldrich, Milan, Italy). They were routinely grown in Dulbecco’s modified Eagle’s medium (DMEM) containing 4.5 g/L D-glucose and supplemented with 2 mM L-glutamine, penicillin (100 units/mL), streptomycin (100 μg/ mL), and containing 20% (v/v) fetal bovine serum (FBS) (GIBCO, Gaithersburg, MD). For the cell culture experiments, the SH-SY5Y were detached by means of Trypsin/EDTA solution (Sigma, Milan, Italy), resuspended in DMEM-20% FBS and seeded at a concentration of 1.0 × 105 cells/mL on the PMMA membranes (immersed in DMEM medium at 37 °C for 1 h prior to use) and, then, incubated into conventional 30 mm diameter Petri dishes at 37 °C and in a saturated humidity atmosphere containing 95% air and 5% CO2. Cells were allowed to grow in DMEM-20% FBS on the different substrates for 24 h.
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RESULTS AND DISCUSSION The fundamental theory of SHG is reviewed in the Supporting Information, in which we show that using the axis C
DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Optical microscope images of SH-SY5Y cells after 24 h incubation on (a) reference sample R, (b) sample A (“contact-PE”), and (c) sample B (“non-contact PE”).
Figure 3 and confirms the fact that the PE process is effective also if performed in the “non-contact” mode. It is worth mentioning that a full understanding of the actual process that determine the cytophilic/cytophobic behavior of an electrified polymer requires considering several aspects that have not been discussed in the present work. While we merely used the cells distribution to compare PMMA layers poled by means of the two methods (contact vs noncontact), we also underline that the formation of a stable bulk polarization is not a necessary requisite to obtain a cytophilic surface. In fact, cell adhesion has been observed also on PE-treated polymeric films with centrosymmetric molecular constituents, such as polystyrene (PS).19 In this latter case, cell adhesion is likely to have been caused by the formation of a space charge layer. The latter might originate from mobile charges present in the melted polymer (e.g., impurity ions inherited from the solvent) which are localized on the bottom surface of polymeric film (i.e., the surface in electrostatic contact with the pyroelectric crystal of lithium niobate) when the pyroelectric field is formed, acting as neutralization charge and electrostatically affect the cellular environment. A well-known theorem states that the electrostatic potential caused by a bulk three-dimensional uniform polarization P is equivalent to the one generated by a surface charge σ = P·n (where n is the unit vector along the surface normal). Such an equivalence between surface charge and bulk polarization suggests that the polar ordering achieved in PMMA should elicit a cell behavior qualitatively similar to the one caused by the space charge layer (expected in PS due to accumulation of impurity ions). In turn, this equivalence may explain why cytophilicity is enhanced by PE treatments in both materials. This consideration is indeed supported by contact angle measurements that we performed on both PMMA and PS before and after the PE process. The results are reported in Figure 6, showing images of a water droplet on PMMA (top images) and PS (bottom images) before (left) and after (right)
Figure 3. SHG intensity measured in p-in/p-out configuration for different points of reference (unpoled) PMMA sample R (black circles) and pyroelectrified PMMA samples A (red circles) and B (blue circles).
It is worth underlining that samples A and B exhibited similar SHG intensities. This indicates that the PE process is effective also in noncontact configuration: i.e., the field produced by pyroelectric excess charges on LN surface is capable to orient the PMMA molecules even in the presence of a dielectric spacer (glass). This feature is very important, as it indicates the possibility to perform an in situ poling of molecular films previously deposited to insulating substrates, avoiding the peeling-off procedure. Polarization-resolved SHG intensities measured for sample A are shown in Figures 4a and 4b, reporting the s-polarized
Figure 4. Polar plot of the s-polarized (a) and p-polarized (b) SHG intensity from sample A. The best-fit curves (blue curves) are obtained using the eqs 2 and 1.
intensity IS2ω(α) and the p-polarized intensity IP2ω(α) of the SH wave, respectively. The blue curves represent the best-fit curves obtained by using the theoretical functions in eqs 1 and 2. As we can see, the theoretical behavior is very well reproduced experimentally, indicating that the induced orientation is homogeneous across the spot size of the laser. Equivalent results have been obtained for sample B (not reported here). To attain a further confirmation on the effectiveness of the PE process without direct contact with the LN crystal, we tested the PE-induced cytophilicity of PMMA films prepared under both “contact PE” and “noncontact PE” procedures. Human neuroblastoma cells (SH-SY5Y) were cultured in vitro on the three different simples R, A, and B. It is well-known that PMMA polymer is naturally cytophobic and requires specific functionalization treatments in order to promote cell adhesion. In fact, Figure 5a shows clearly the lack of cell adhesion on the pristine PMMA (i.e., on the unprocessed reference sample R). Conversely, both samples A (Figure 5b) and B (Figure 5c) clearly appear to favor the cell adhesion and spreading. More importantly, no substantial differences between them is recognized: this result is in agreement with those shown in
Figure 6. Images of water droplets and contact angle measured in the following conditions: (A) PMMA before PE process; (B) PMMA after PE process; (C) PS before PE process; (D) PS after PE process. D
DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules the PE process. Subsequent to the pyroelectrification, water contact angles θ decreased from 63° to 48° for PMMA and from 84° to 70° for PS, indicating that bulk polarization (PMMA) and surface space charge formation (PS) both lead to an improved hydrophilicity of the polymer surface, caused by the change in water droplet surface energy induced by its electrostatic interaction with the substrate.33,34 Finally, we note that free charges localized on a polymer surface would lead to a surface-type SHG that cannot be discerned from a bulk-like SHG caused by coherent ordering of polar groups. In fact, separation of the two contributes requires a phase-sensitive analysis (i.e., homodyne SHG detection)35,36 of the azimuthal-dependent SH intensity I(ϕ,2ω), which in our case is null due to the in-plane isotropy of the polymer films.
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ACKNOWLEDGMENTS
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REFERENCES
(1) Electrets; Sessler, G. M., Ed.; Topics in Applied Physics; Springer: Berlin, 1987; Vol. 33. (2) Setter, N.; Damjanovic, D.; Eng, L.; Fox, G.; Gevorgian, S.; Hong, S.; Kingon, A.; Kohlstedt, H.; Park, N. Y.; Stephenson, G. B.; Stolitchnov, I.; Taganstev, A. K.; Taylor, D. V.; Yamada, T.; Streiffer, S. Ferroelectric Thin Films: Review of Materials, Properties, and Applications. J. Appl. Phys. 2006, 100 (5), 051606. (3) Marchesano, V.; Gennari, O.; Mecozzi, L.; Grilli, S.; Ferraro, P. Effects of Lithium Niobate Polarization on Cell Adhesion and Morphology. ACS Appl. Mater. Interfaces 2015, 7 (32), 18113−18119. (4) Carville, N. C.; Collins, L.; Manzo, M.; Gallo, K.; Lukasz, B. I.; McKayed, K. K.; Simpson, J. C.; Rodriguez, B. J. Biocompatibility of Ferroelectric Lithium Niobate and the Influence of Polarization Charge on Osteoblast Proliferation and Function: Biocompatibility of Ferroelectric Lithium Niobate. J. Biomed. Mater. Res., Part A 2015, 103 (8), 2540−2548. (5) Jacobs, H. O. Submicrometer Patterning of Charge in Thin-Film Electrets. Science 2001, 291 (5509), 1763−1766. (6) Liu, F.; Wang, W.; Wang, L.; Yang, G. Ferroelectric-Semiconductor Photovoltaics: Non-PN Junction Solar Cells. Appl. Phys. Lett. 2014, 104 (10), 103907. (7) Beeby, S. P.; Tudor, M. J.; White, N. M. Energy Harvesting Vibration Sources for Microsystems Applications. Meas. Sci. Technol. 2006, 17 (12), R175−R195. (8) Bowen, C. R.; Kim, H. A.; Weaver, P. M.; Dunn, S. Piezoelectric and Ferroelectric Materials and Structures for Energy Harvesting Applications. Energy Environ. Sci. 2014, 7 (1), 25−44. (9) Peano, F.; Tambosso, T. Design and Optimization of a MEMS Electret-Based Capacitive Energy Scavenger. J. Microelectromech. Syst. 2005, 14 (3), 429−435. (10) Li, L.; Salvador, P. A.; Rohrer, G. S. Photocatalysts with Internal Electric Fields. Nanoscale 2014, 6 (1), 24−42. (11) Pai, D. M.; Springett, B. E. Physics of Electrophotography. Rev. Mod. Phys. 1993, 65 (1), 163−211. (12) Esseling, M.; Zaltron, A.; Sada, C.; Denz, C. Charge Sensor and Particle Trap Based on Z-Cut Lithium Niobate. Appl. Phys. Lett. 2013, 103 (6), 061115. (13) Gennari, O.; Grilli, S.; Coppola, S.; Pagliarulo, V.; Vespini, V.; Coppola, G.; Bhowmick, S.; Gioffré, M. A.; Gentile, G.; Ambrogi, V.; Cerruti, P.; Carfagna, C.; Ferraro, P. Spontaneous Assembly of Carbon-Based Chains in Polymer Matrixes through Surface Charge Templates. Langmuir 2013, 29 (50), 15503−15510. (14) Grilli, S.; Vespini, V.; Ferraro, P. Surface-Charge Lithography for Direct PDMS Micro-Patterning. Langmuir 2008, 24 (23), 13262− 13265. (15) Carrascosa, M.; García-Cabañes, A.; Jubera, M.; Ramiro, J. B.; Agulló-López, F. LiNbO 3: A Photovoltaic Substrate for Massive Parallel Manipulation and Patterning of Nano-Objects. Appl. Phys. Rev. 2015, 2 (4), 040605. (16) Chen, L.; Li, S.; Fan, B.; Yan, W.; Wang, D.; Shi, L.; Chen, H.; Ban, D.; Sun, S. Dielectrophoretic Behaviours of Microdroplet Sandwiched between LN Substrates. Sci. Rep. 2016, 6, 29166. (17) DeRose, C. T.; Enami, Y.; Loychik, C.; Norwood, R. A.; Mathine, D.; Fallahi, M.; Peyghambarian, N.; Luo, J. D.; Jen, A. K.-Y.;
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00794. SHG theory and demonstration of eqs 1 and 2 (PDF)
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R.R., O.G., L.M., P.F., and S.G. acknowledge the Italian Ministry of Research for financial support under “Futuro in Ricerca 2010” Program (Protocol RBFR10FKZH), “Progetto Operativo Nazionale” AquaSystem (Protocol 1719), and the Short-Term Program of CNR. S.L. and D.K.P. acknowledge the financial support from INFN-CNR national project (PREMIALE 2012) EOS, from Regione Campania (INTERFET project, L.R. n.5/02) and from the Italian Ministry of Foreign Affairs and International Cooperation (NANOGRAPH Project).
CONCLUSION We presented here an SHG-based technique for evaluating in situ the dipole alignment in pyroelectrified PMMA layers. Only recently we developed the PE process and demonstrated its potential applications for jet printing and cell patterning, but still a direct evaluation of the charge effect was desirable. The results demonstrate the significant enhancement of the SHG efficiency in the case of pyroelectrified PMMA films, thus proving the well alignment and stabilization of the polar order. More precisely, we show that the PE process drastically increases the efficiency of SHG signal up to a factor of about 30 while maintaining the isotropy along the sample surface that characterizes the original disordered polymeric film. Moreover, we show that the PE-induced orientation occurs even in a “noncontact” configuration, i.e., in the presence of a dielectric glass spacer between the polymer film and the pyroelectric crystal. This was demonstrated also by the ability of live cells to adhere onto glass slides functionalized with pyroelectrified PMMA. The ability of this SHG technique to map the polarization alignment in PE polymers through a contact- and electrode-free procedure represents a powerful noninvasive way to analyze the polar order degree inside the pyroelectrified polymer giving significant information about its electrical state. It can open the way to a deep investigation of the charge decay mechanisms in such pyroelectrified polymers. Moreover, this paves the way toward applications in which the electret is fabricated on an insulating substrate, avoiding its detachment from the pyroelectric crystal.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.L.). ORCID
Stefano Lettieri: 0000-0002-4471-3024 Pasqualino Maddalena: 0000-0001-5823-3331 Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b00794 Macromolecules XXXX, XXX, XXX−XXX
