TY - JOUR A1 - Thayumanasundaram, Savitha A1 - Raman Venkatesan, Thulasinath A1 - Ousset, Aymeric A1 - Van Hollebeke, Kim A1 - Aerts, Luc A1 - Wubbenhorst, Michael A1 - Van den Mooter, Guy T1 - Complementarity of mDSC, DMA, and DRS Techniques in the Study of T-g and Sub-T-g Transitions in Amorphous Solids BT - PVPVA, Indomethacin, and Amorphous Solid Dispersions Based on Indomethacin/PVPVA JF - Molecular pharmaceutics N2 - Recently, glasses, a subset of amorphous solids, have gained attention in various fields, such as polymer chemistry, optical fibers, and pharmaceuticals. One of their characteristic features, the glass transition temperature (T-g) which is absent in 100% crystalline materials, influences several material properties, such as free volume, enthalpy, viscosity, thermodynamic transitions, molecular motions, physical stability, mechanical properties, etc. In addition to T-g, there may be several other temperaturedependent transitions known as sub-T-g transitions (or beta-, gamma-, and delta-relaxations) which are identified by specific analytical techniques. The study of T-g and sub-T-g transitions occurring in amorphous solids has gained much attention because of its importance in understanding molecular kinetics, and it requires the combination of conventional and novel characterization techniques. In the present study, three different analytical techniques [modulated differential scanning calorimetry (mDSC), dynamic mechanical analysis (DMA), and dielectric relaxation spectroscopy (DRS)] were used to perform comprehensive qualitative/quantitative characterization of molecular relaxations, miscibility, and molecular interactions present in an amorphous polymer (PVPVA), a model drug (indomethacin, IND), and IND/PVPVA-based amorphous solid dispersions (ASDs). This is the first ever reported DMA study on PVPVA in its powder form, which avoids the contribution of solvent to the mechanical properties when a selfstanding polymer film is used. A good correlation between the techniques in determining the T-g value of PVPVA, IND, and IND/ PVPVA-based ASDs is established, and the negligible difference (within 10 degrees C) is attributed to the different material properties assessed in each technique. However, the overall T-g behavior, the decrease in T-g with increase in drug loading in ASDs, is universally observed in all the above-mentioned techniques, which reveals their complementarity. DMA and DRS techniques are used to study the different sub-T-g transitions present in PVPVA, amorphous IND, and IND/PVPVA-based ASDs because these transitions are normally too weak or too broad for mDSC to detect. For IND/PVPVA-based ASDs, both techniques show a shift of sub-T-g transitions (or secondary relaxation peaks) toward the high-temperature region from -140 to -45 degrees C. Thus, this paper outlines the usage of different solid-state characterization techniques in understanding the different molecular dynamics present in the polymer, drug, and their interactions in ASDs with the integrated information obtained from individual techniques. KW - amorphous solids KW - PVPVA KW - indomethacin KW - ASDs KW - dynamic mechanical KW - analysis KW - dielectric relaxation spectroscopy KW - sub-T-g relaxations KW - relaxation dynamics Y1 - 2022 U6 - https://doi.org/10.1021/acs.molpharmaceut.2c00123 SN - 1543-8384 SN - 1543-8392 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Wübbenhorst, Michael A1 - Gerhard, Reimund T1 - Structure-property relationships in three-phase relaxor-ferroelectric terpolymers JF - Ferroelectrics N2 - Poly(vinylidenefluoride-trifluoroethylene)-based (P(VDF-TrFE)-based) terpolymers represent a new class of electroactive polymer materials that are relaxor-ferroelectric (RF) polymers and that offer unique and attractive property combinations in comparison with conventional ferroelectric polymers. The RF state is achieved by introducing a fluorine-containing termonomer as a "defect" into the ferroelectric P(VDF-TrFE) copolymer, which reduces the interaction between the VDF/TrFE dipoles. The resulting terpolymer exhibits a low Curie transition temperature and small remanent and coercive fields yielding a slim hysteresis loop that is typical for RF materials. Though the macroscopic behavior is similar to RF ceramics, the mechanisms of relaxor ferroelectricity in semi-crystalline polymers are different and not fully understood yet. Structure-property relationships play an important role in RF terpolymers, as they govern the final RF properties. Hence, a review of important characteristics, previous studies and relevant developments of P(VDF-TrFE)-based terfluoropolymers with either chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) as the termonomer is deemed useful. The role of the termonomer and of its composition, as well as the effects of the processing conditions on the semi-crystalline structure which in turn affects the final RF properties are discussed in detail. In addition, the presence of noteworthy transition(s) in the mid-temperature range and the influence of preparation conditions on those transitions are reviewed. A better understanding of the fundamental aspects affecting the semi-crystalline structures will help to elucidate the nature of RF activity in VDF-based terpolymers and also help to further improve their applications-relevant electroactive properties. KW - Relaxor-ferroelectric (RF) fluoropolymers KW - structure-property KW - relationships KW - Curie transition KW - dielectric hysteresis KW - thermal KW - processing KW - mid-temperature transition(s) Y1 - 2022 U6 - https://doi.org/10.1080/00150193.2021.2014260 SN - 0015-0193 SN - 1563-5112 VL - 586 IS - 1 SP - 60 EP - 81 PB - Routledge, Taylor & Francis Group CY - Abingdon ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Smykalla, David A1 - Ploss, Bernd A1 - Wübbenhorst, Michael A1 - Gerhard, Reimund T1 - Non-linear dielectric spectroscopy for detecting and evaluating structure-property relations in a P(VDF-TrFE-CFE) relaxor-ferroelectric terpolymer JF - Applied physics : A, Materials science & processing N2 - Non-linear dielectric spectroscopy (NLDS) is employed as an effective tool to study relaxation processes and phase transitions of a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) relaxor-ferroelectric (R-F) terpolymer in detail. Measurements of the non-linear dielectric permittivity epsilon 2 ' reveal peaks at 30 and 80 degrees C that cannot be identified in conventional dielectric spectroscopy. By combining the results from NLDS experiments with those from other techniques such as thermally stimulated depolarization and dielectric-hysteresis studies, it is possible to explain the processes behind the additional peaks. The former peak, which is associated with the mid-temperature transition, is found in all other vinylidene fluoride-based polymers and may help to understand the non-zero epsilon 2 ' values that are detected on the paraelectric phase of the terpolymer. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 degrees C and is due to conduction and space-charge polarization as a result of the accumulation of real charges at the electrode-sample interface. KW - Non-linear dielectric spectroscopy KW - P(VDF-TrFE-CFE) KW - Relaxor-ferroelectric polymer KW - Dielectric hysteresis KW - Curie-transition KW - Mid-temperature transition Y1 - 2021 U6 - https://doi.org/10.1007/s00339-021-04876-0 SN - 0947-8396 SN - 1432-0630 VL - 127 IS - 10 PB - Springer CY - Berlin ; Heidelberg ; New York ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Smykalla, David A1 - Ploss, Bernd A1 - Wübbenhorst, Michael A1 - Gerhard, Reimund T1 - Tuning the relaxor-ferroelectric properties of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Terpolymer films by means of thermally induced micro- and nanostructures JF - Macromolecules : a publication of the American Chemical Society N2 - The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures. KW - Annealing (metallurgy) KW - Hysteresis KW - Insulators KW - Phase transitions KW - Polarization Y1 - 2022 U6 - https://doi.org/10.1021/acs.macromol.2c00302 SN - 0024-9297 SN - 1520-5835 VL - 55 IS - 13 SP - 5621 EP - 5635 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Gerhard, Reimund T1 - Influence of film stretching on crystalline phases and dielectric properties of a 70/30 mol% poly(vinylidenefluoride-tetrafluoroethylene) copolymer JF - Journal of advanced dielectrics N2 - Polyvinylidene fluoride (PVDF)-based copolymers with tetrafluoroethylene (P(VDF-TFE)), trifluoroethylene (P(VDF-TrFE)) or hexafluoropropylene (P(VDF-HFP)) are of strong interest due to the underlying fundamental mechanisms and the potential ferro-, pyro- and piezo-electrical applications. Their flexibility and their adaptability to various shapes are advantageous in comparison to inorganic ferroelectrics. Here, we study the influence of stretching temperature on the crystalline phases and the dielectric properties in P(VDF-TFE) films by means of Dielectric Relaxation Spectroscopy (DRS), Fourier-Transform InfraRed spectroscopy (FTIR), Wide-Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Especially, the effect of stretching and the influence of the temperature of stretching on the mid-temperature (T-mid) transition are studied in detail. The results show that stretching has a similar effect as that on PVDF, and we observe an increase in the fraction of ferroelectric beta-phase with a simultaneous increment in both melting point (T-m) and crystallinity (chi(c)) of the copolymer. While an increase in the stretching temperature does not have a profound impact on the amount of ferroelectric phase, the stability of the ferroelectric phase seems to improve - as seen in the reduction of the Full Width at Half Maximum (FWHM) of the WAXD peaks in both parallel and perpendicular directions to the molecular chain axis. The observation is also supported by the reduction of dissipation losses with an increase in stretching temperature - as seen in DRS measurements. Finally, both stretching itself and the temperature of stretching affect the various molecular processes taking place in the temperature range of the T-mid transition. KW - P(VDF-TFE) copolymer KW - dielectric properties KW - crystalline phases KW - film KW - stretching KW - mid-temperature transition Y1 - 2020 U6 - https://doi.org/10.1142/S2010135X2050023X SN - 2010-135X SN - 2010-1368 VL - 10 IS - 5 PB - World Scientific CY - Singapore ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Frübing, Peter A1 - Gerhard, Reimund T1 - Electrical polarization phenomena, dielectric relaxations and structural transitions in a relaxor-ferroelectric terpolymer investigated with electrical probing techniques JF - Materials research express N2 - Dielectric Relaxation Spectroscopy (DRS) and Thermally Stimulated Depolarization Current (TSDC) measurements were employed to study dielectric-relaxation processes, structural transitions and electric-polarization phenomena in poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films. Results from DRS confirm the existence of two separate dispersion regions related to a para-to-ferroelectric phase transition and to the glass transition. The dipolar TSDC peak correlates with the loss peak of the ? relaxation that represents the glass transition. The electric polarization calculated from the dipolar TSDC peak (glass transition) shows a non-linear electric-field dependence and saturates at high electric poling fields. As the observed behaviour is essentially the same as that of the electric polarization obtained from direct polarization-versus-electric-field hysteresis measurements, TSDC experiments are also suitable for studying the polarization in relaxor-ferroelectric polymers. A saturation polarization of 44 mC m(?2) was found for an electric field of 190 MV m(?1). KW - P(VDF-TrFE-CFE) terpolymer KW - relaxor-ferroelectric polymers KW - dielectric relaxation spectroscopy KW - thermally stimulated depolarization current KW - electrical polarization hysteresis Y1 - 2019 U6 - https://doi.org/10.1088/2053-1591/ab5352 SN - 2053-1591 VL - 6 IS - 12 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Frübing, Peter A1 - Gerhard, Reimund T1 - Relaxation processes and structural transitions in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor-ferroelectric terpolymers as seen in dielectric spectroscopy JF - IEEE transactions on dielectrics and electrical insulation N2 - Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation. KW - P(VDF-TrFE-CFE) terpolymer KW - relaxor-ferroelectric polymer KW - dielectric relaxation spectroscopy KW - Curie transition KW - differential scanning calorimetry Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007440 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 6 SP - 2229 EP - 2235 PB - Institut of Electrical and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gerhard, Reimund T1 - Origin of the mid-temperature transition in vinylidenefluoride-based ferro-, pyro- and piezoelectric homo-, co- and ter-polymers JF - Materials Research Express N2 - The existence of an intermediate transition between the glass and the Curie/melting temperatures in Poly(vinylidene fluoride) (PVDF) and some of its co- and ter-polymers has been reported by several authors. In spite (or because?) of various different explanations in the literature, the origins of the transition are still not clear. Here, we try to understand the extra transition in more detail and study it with thermal and dielectric methods on PVDF, on its co-polymers with trifluoroethylene (P(VDF-TrFE)) and tetrafluoroethylene (P(VDF-TFE)), and on its ter-polymer with trifluoroethylene and chlorofluoroethylene (P(VDF-TrFE-CFE). Based on interpretations from the literature and our experimental studies, we propose the new hypothesis that the intermediate transition should have several interrelated origins. Especially since the relevant range is not far above room temperature, better understanding and control of their properties may also have practical implications for the use of the respective polymer materials in devices. KW - mid-temperature transition KW - vinylidenefluoride (VDF)-based polymers KW - Differential Scanning Calorimetry (DSC) KW - Dielectric Relaxation Spectroscopy (DRS) KW - ferroelectric polymers KW - pyroelectric polymers KW - piezoelectric polymers Y1 - 2020 U6 - https://doi.org/10.1088/2053-1591/ab842c SN - 2053-1591 VL - 7 PB - IOP Publ. CY - Bristol ER - TY - JOUR A1 - Qiu, Xunlin A1 - Benjamin, Aravindan Joseph A1 - Raman Venkatesan, Thulasinath A1 - Schmidt, Georg C. A1 - Soler, Ricardo Alonso Quintana A1 - Panicker, Pramul Muraleedhara A1 - Gerhard, Reimund A1 - Hübler, Arved Carl T1 - Dielectric and electroacoustic assessment of screen-printed piezoelectric polymer layers as flexible transducers BT - influence of the electrode material JF - IEEE transactions on dielectrics and electrical insulation N2 - Here, piezoelectric transducers consisting of a P(VDF-TrFE) layer with either silver or PEDOT:PSS screen-printed electrodes are studied. The influence of electrodes on the dielectric and electroacoustic properties are studied in dielectric-spectroscopy and ferroelectric-hysteresis measurements. Only when both the bottom and the top electrodes are made of silver, the typical dielectric relaxation of the P(VDF-TrFE) layer is clearly observed. When one or two of the electrodes are of PEDOT:PSS, a Debye-like relaxation is present. Compared with silver electrodes, PEDOT:PSS electrodes allow for moderate self-healing. Consequently, samples with bottom and top PEDOT:PSS electrodes can be poled to saturation, while samples with silver electrodes can hardly be poled to saturation due to destructive electric breakdown. Acoustic transducer measurements show that silver electrodes facilitate higher and broader frequency operation, while PEDOT:PSS electrodes bring slightly lower total harmonic distortion. Overall, the acoustic performance shows no significant deviations between differently electroded samples so that silver electrodes do not offer any advantages for the transducers studied here due to their much higher tendency for destructive electric breakdown. KW - poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) KW - dielectric KW - spectroscopy KW - ferroelectricity and piezoelectricity in polymers KW - screen KW - printing KW - printed electroacoustic thin-film transducers Y1 - 2020 U6 - https://doi.org/10.1109/TDEI.2020.008864 SN - 1070-9878 SN - 1558-4135 SN - 0018-9367 VL - 27 IS - 5 SP - 1683 EP - 1690 PB - Institute of Electrical and Electronics Engineers CY - New York, NY ER -