Multifunctional electroactive electrospun nanofiber structures from water solution blends of PVA/ODA–MMT and poly(maleic acid-alt-acrylic acid): effects of Ag, organoclay, structural rearrangement and NaOH doping factors

Poly[(vinyl alcohol)_0.89-co-(vinyl acetate)_0.11] (PVA, 89% hydrolyzed, average M_w = 31.000–51.000 Da), octadecyl amine-montmorillonite (ODA–MMT, nanomer I.30E, nanocor Co.) with the following average parameters: content of ODA surfactant-intercalant 25%–30%, particle size 8−10 μm, bulk density 0.41 g cm⁻³ and crystallinity 52.8%, and silver nitrate (AgNO₃, 99.995%, melting point 202 °C with decomposition, d = 4.35 g cm⁻³) were purchased from Sigma-Aldrich. Poly(maleic acid-alt-acrylic acid) [poly(MAc-alt-AA)] with content of maleic acid unit = 47.17 mass%, glass-transition temperature (T_g) 62.5 °C and melting temperature (T_m) 137.5 °C, intrinsic viscosity (η_in) 1.25 dL g⁻¹ in dioxane at 25 °C, acid number 878 mg KOH g^–1 (by alkali titration), and electrical conductivity (σ) = 4.06 × 10⁻¹² S cm⁻¹ (by Keithley 6512 electrometer) was synthesized by complex-radical copolymerization method. All other solvents and reagents were analytical grade and used without purification.

The compositions, chemical structures and important assignments of these materials are given in table 1.

Table 1.  The compositions chemical structures and assignments of the materials used in electrospinning.

Materials composition	Chemical structure	Assignments
PVA		Reactive matrix polymer
Poly(maleic acid-alt-acrylic acid) 40 and 20 mass %		Reactive alternating partner copolymer-1
Ag-carrying poly((maleic acid-alt-acrylic acid) 40 and 20 mass %		Ag complex of reactive alternating partner copolymer-2
ODA-MMT clay 5.0 mass %	CH3-(CH2)17-NH2−MMT	Reactive organoclay-nanofiller
Silver nitrate	AgNO3	Precursor for in situ generation of Ag nanoparticles onto nanofibers
Intercalated PVA/ODA-MMT nanocomposite 60 and 80 mass%		Matrix intercalated nanocomposite

Poly(MAc-alt-AA) was synthesized by complex-radical copolymerization according to our previous published method [22]. Briefly, copolymerization of maleic anhydride with acrylic acid at 1:1 molar ratio was carried out in 1, 4-dioxane solution in the presence of benzoyl peroxide (0.1%) as an initiator at 70 °C under nitrogen flow. The copolymer was purified by twice prepcipitating from dioxane solution with n-hexane and washed with several portions of benzene and diethyl ether, and dried under vacuum at 50 °C to constant weight. This copolymer was easily dissolved in pure water with hydrolysis of anhydride unit and fully transferred to acid unit with regularly repeated structure as poly(maleic acid-alt-acryic acid). Each unit contained three carboxyl groups with strong intra- and intermacromolecular hydrogen bonding. Water solutions of poly(MAc-alt-AA) and its silver carrying complex were used in reactive electrospinning nanotechnology as partner copolymers.

Nanofibers from matrix/partner composites were prepared from blends of water dispersion of PVA/ODA–MMT as matrix polymer composite, water solutions of poly(MAc-alt-AA) and its Ag-carrying composition as partner polymers by electrospinning method using the following optimized parameters: concentration of polymer solutions of 9% w/v, high voltage of 24 kV, flow rate of 0.5 mL h⁻¹, and distance of 15–20 cm between the tip of the needle and the grounded collector. The prepared polymer solutions were put into a syringe and positive electrode of a high voltage power supply was mounted to needle of syringe whereas its negative electrode was attached on the collector. Then, high voltage was applied to polymer solution blends between needle and collector via syringe pump providing a constant flow rate. Randomly oriented nanofibers were obtained by collecting the fiber webs onto an aluminum foil fixed on stationary collector.

Fourier transform infrared (FTIR) spectra were recorded on a FTIR Nicolet 510 spectrometer in the range of 4000–400 cm^–1 with a resolution of 4 cm⁻¹. X-ray powder diffraction (XRD) patterns were performed with a PANANALYTICAL x-ray diffractometer equipped with a CuKα tube and Ni filter (λ = 1.5406 Å). XRD diffractograms were measured at 2θ angles in the range 1°–70°.

Surface morphology of nanofibers was examined using a scanning electron microscope (SEM, Zeiss Supra). All specimens were freeze-dried and coated with a thin layer of platin before testing. Thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses were performed at linear heating rate of 10 °C min^–1 under nitrogen flow using EXTRAR600 TG–DTA6300 and diamond DSC Perkin Elmer thermal analyzers. The thermal degradation temperature taking into account was the temperature at onset (T_d(onset)) and the temperature of maximum weight loss (T_d(max)). Electrical conductivity and thermal resistance of nanofiber samples as solid/colloidal electrolytes were measured by using a test chamber (JANIS VPF 100 kryostat). Current through thin fiber film was recorded with a Keithley 2400 current–voltage measurement system. Square-shaped samples (0.25 cm²) with four contact points at the corners were prepared to carry out conductivity measurements which were done according to the van der Pauw method. Experiments were conducted in temperature range of 20 °C–50 °C with 1 °C steps (Keithley 6487 Picoammeter/Voltage source) and pressure range of 50–800 torr. Sample temperature was monitored regularly by using a Pt 100 sensor close to the sample and measured with Lakeshore model 331-temperature controller with sensitivity of ±0 °C. All measurements were performed with a PC through a GPIB converter card.

The schematic representation of the formation of multifunctional nanofiber structures during electrospinng processing is given in figure 1. It was proposed that the following chemical and physical interfacial interactions occurred between PVA and the copolymer (figure 1(A)), the copolymer and ODA–MMT, the copolymer and Ag precursors and in situ generated AgNPs (figure 1(B)): (i) esterification/cross-linking of carboxyl groups of the copolymer with hydroxyl groups of PVA which accompanied with release of H₂O molecules (figures 1(A) and (C)); (ii) amidization of carboxyl groups with octadecyl amine intercalant, (iii) esterification/grafting carboxyl groups with ≡Si–OH at edges of MMT clay, and (iv) the formation of Ag-carrying polymer complexes and transformation of Ag precursors to reduced and stabilized AgNPs form onto nanofiber surface during electrospinning (figure 1(B)). In the presented study, thermal treatment were also applied to nanofibers at 25 °C–250 °C with heating rate of 10 °C min^–1 in nitrogen atmosphere under vacuum in order to evaluate changes in physical and chemical structure of NFCs. After annealing (figure 1(C)), rearranged structures were formed via decarboxylation of maleic acid units with a release of carbon dioxide (CO₂) and monoxide (CO) [28], and degrading release of acrylic monomer from copolymer chains [29]. The specific degradation mechanism of homo- and copolymers of maleic anhydride (MA) was also reported by our group [30, 31] and other researchers [32, 33]. Alternating copolymers of MA and their hydrolyzed derivatives exhibit specific thermal degradations which accompanied by the various structural rearrangements at temperature range around 120 °C–500 °C. The degradation mechanism of alternating copolymers of MA with vinyl acetate [34], isopropenyl acetate [35], allyl acetate [36], styrene [37], α-olefins [38] and trans-stilbene [39] were also investigated by many researchers in detail. According to these investigations, thermal decompositions occurred due to the elimination of volatile CO₂ and CO and lead the formation of regular structures with backbone double bonds conjugated to the C=O groups. It was suggested that the structural rearrangements of nanofiber structures can be play an important role in improving conductivity of nanofiber electrolytes due to enhancement of a combination of ion charged sites with chain conjugated double bonds. Thus, the formation of new double bond conjugated sites onto backbone chains is very important factor for enhancement main properties of nanofibers, especially their electrical and thermal behaviors. A new approach reported for the first time in the study can be used in developing polymer nanofiber electrolytes.

Zoom In Zoom Out Reset image size

The chemical structures of NFCs before (figure 2(a)) and after annealing (figure 2(b)) confirmed essentially structural changes indicating the formation of new absorption bands from rearranged fiber structures. FTIR spectra of fiber structures showed the following characteristic absorption bands (figure 2(a)): very broad peak at 3320 cm⁻¹ associated with −OH stretching from strong hydrogen bonding matrix PVA and the partner copolymer through physical interactions of hydroxyl and carboxyl groups. This peak significantly shifted to lower absorption region as compared with absorbtion band at 3438 cm⁻¹ [40] from free non-hydrogen bonding OH group, which is absence in spectra of nanofibers. Three absorption bands at 2960, 2920 and 2850 cm⁻¹ were related to C−H stretching in CH₃, CH₂ and CH groups from backbone chains of the matrix and partner polymers and octadecyl amine group of ODA–MMT. The presence of these groups was confirmed by the antisymmetric deformation bands at 1375 and 1337 cm⁻¹ and CH₂ rocking band at 705 cm⁻¹ in −(CH₂)_n− chain. The broad band around 2740–2350 cm⁻¹ can be attributed to ${{{\rm{NH}}}_{3}}^{+}-OOC-$ octadecyl amine/carboxyl hydrogen bonding complex in amine hydroholides [41].

Zoom In Zoom Out Reset image size

The bands at 1560 and 1407 cm⁻¹ associated with antisymmetric stretching −COO− in Ag-carrying polymer complexes. Strong fairly band at 1715 cm⁻¹ can be attributed to symmetric −C=O stretching from −COOH and −COOR (saturated ester) groups. The intensity of this peak significantly increased with increasing the partner copolymer fraction (NFC-6/4) due to enhancement of the fiber composite with carboxylic and ester groups. This absorption band indicated the occurrence of covalence cross-linking in the nanofibers via esterification of the carboxyl groups of the copolymer with the hydroxyl groups of PVA during electrospinning. Absorption band at 1255 cm⁻¹ was related to C−O−C antisymmetric stretching from ester links. The peaks at 1162 and 1047 cm⁻¹ can be attributed to −C−OH stretching and −C−O−OH secondary alcohol, respectively. The Si−O−Si band from organoclay appeared at 1080 cm⁻¹. The peak at 910 cm⁻¹ was related to C−O−H deformation.

FTIR spectra of annealed NFCs are given in figure 2(b). Comparative analysis of the samples before and after annealing indicated the following changes (appearance of new absorption bands, disappearance some bands, and decreases in peak intensities) which conformed the structural rearrangement via predominantly decarboxylation of anhydride units: (i) new peaks at 947 cm⁻¹ (backbone chain conjugated –CH=CH– band [41]), around 830 cm⁻¹ and 620 cm⁻¹; (ii) almost disappearance of broad band at 2740–2350 cm⁻¹ and strong peak at 910 cm⁻¹, significant decrease in intensity or disappearence of C–H stretching in CH and CH₂ groups; (iii) decreases in intensities, especially those in NFC-6/4, and shift of 3320 cm⁻¹ broad band to more high region (3333 cm⁻¹). These structural rearrangements which were related to primarily changes of backbone chain and side functional groups (–OH and –COOH) logically conformed with the above mentioned proposal which is schematically represented in figure 1.

XRD results which belong to the physical structures of NFCs are given in figure 3. NFC-8/2 and NFC-6/4 having visible weak crystalline peaks at clay region predominantly exhibited amorphous structures (figures 3(a) and (b)), which indicated the transfer of fiber structures to almost fully colloidal state after partially elimination of water molecules during electrospinning via dispersing, absorption and in situ physical and chemical processing in the matrix and the partner (copolymer) systems. The characteristic strong reflection peak of neat PVA around 2θ of 19°–21° angles [42] significantly decreased and shifted to the organoclay region. The nanofiber composite incorporated with silver nanoparticles (NFC-8/2-AgNPs) showed higher crystallinity and had more numbers of peak reflections at different angles due to physical interaction of AgNPs with carboxylate ions from the partner polymer (figure 3(c)). The formation of microparticles (1443.74 nm) at angle 2θ of 18.79° can be attributed to the water absorption of colloidal PVA. Observed peaks at angles 2θ of 29.80° and 53.80° can be attributed to the (111) and (200) planes [43] of in situ generated AgNPs with sizes of 30.25 and 70.36 nm onto nanofiber surfaces, respectively. It was proposed that the formation of the polymer nanofibers with colloidal amorphous structures during electrospinning of the water dispersed/solution blends of PVA/ODA–MMT and the alternating copolymer not only enhanced with carboxylate ions and their silver complexes, but also in situ generated of AgNPs would be take place an important role to improve the electrical properties (conductivity and resistance) of novel kinds of polymer nanofiber electrolytes.

Zoom In Zoom Out Reset image size

Figure 4 presents morphologies and diameter distributions of NFCs with or without the partner polymer. According to the images, all samples exhibited fine fiber morphology. Whereas PVA/ODA-MMT has average diameter of 276 nm (figure 4(a)), NFC-8/2 and NFC-6/4 exhibit relatively lower average diameter (201 nm and 206 nm, respectively) (figures 4(b) and (c)). This observed fact indicated that the alternating partner copolymer with regular hydrogen bonding carboxylate ions in side-chain essentially influenced the phase separation processing via in situ chemical and physical interfacial interactions during electrospinning. It was observed that the morphology, fiber diameters, and diameter distributions strongly depended on the fraction of the alternating partner copolymer in NFC blends which contains regular repeated functional units and their strong hydrogen bonded links via partner−COOH...OH−matrix polymer chains and partner−COOH...NH₂-R (from organoclay). Polymer-copolymer physical interactions easily transferred to covalence bonded structures through esterification and amidation in the phase separation processing during electrospinning.

Zoom In Zoom Out Reset image size

SEM analysis results of NFC-8/2 (annealed) and NFC-6/4 (annealed) after thermal treatment through chemical structural rearrangements via decarboxylation of maleic anhydride/acid units to form new conjugated double bond sites onto backbone chains of partner copolymer are given in figure 5. This structural phenomenon was confirmed by FTIR and thermal degradation analysis results given in figures 2 and 6, respectively, as well as schematically represented structural rearrangements in figure 1. Morphology parameters such as degree of cross-section density, fiber diameters, and diameter distributions essentially improved due to the effects of these rearrangements after annealing process.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

TGA and DTG thermograms of NFCs before and after annealing are given in figure 6. Chemical changes and degradations occurred at increasing temperature conditions. According to the TGA–DTG curves, all NFCs exhibited multi-step degradations via decarboxylation of carboxyl/anhydride units at a temperature range 75 °C–145 °C (exo-thermic peaks). Step infection point (SIP) around 179.20 °C–197.85 °C with lower areas for NFC-8/2 and NFC-6/4 was related to release of absorbed water molecules, CO₂ and CO from fiber composites. Intensity of this degradation peaks (dublet) increased in NFC-8/2-Ag sample due to transfer of silver salts to AgNPs. This in situ generated AgNPs significantly stabilized the following chain degradation which conformed to almost disappearance of broad peak at ∼435 °C. Two broadening peaks with the higher areas in NFC-8/2 and NFC-6/4 (figures 6(A) and (C)) were associated with the chain degradations of the partner copolymer.

For the full decarboxylation of both maleic and acrylic units it is necessary 38.91 mol% of released products (9.57 mol% H₂O, 26.02 mol% CO₂ and 14.50% CO) which was theoretically calculated as 100% degree of chemical degratative arrangement of the structure in backbone chains of alternating partner copolymer. It was experimentally found that the weight loss values for NFC-8/2 (5.82%), NFC-8/2-Ag (8.90%) and NFC-6/4 (15.71%) corresponded to the decarboxylation degree of 10.94 mol%, 16.74 mol% and 29.55 mol%, respectively. Agreeing with these data, partially degradative rearrangement (structure II in figure 1(C)) occurred during electrospinning process. The above mentioned mechanism of chemical degradative rearrangement was confirmed by fully disappearance of the described endo-termic paeks after the annealing process (figures 6(D)–(F)). The results estimated was also logically consistent with the changes of the structural factors observed in FTIR and SEM.

Figure 7 shows the temperature dependence of conductivity and Arrenious plot, and TGA–DTG and DSC curves of pristine alternating copolymer. TGA–DTG analyses showed multi-step degradations for the copolymer (figure 7(a)): via anhydridation of –COOH groups with release of water molecules at 50 °C–100 °C (1st step), decarboxylation of anhydride units with elimination of carbon mono- and di-oxides at 110 °C–220 °C (2nd step), and main chain degradations around 240 °C–300 °C (3rd step) and 390 °C–450 °C (4th step). DSC curve indicated that the alternating copolymer exhibited glass-transition (T_g) at 62.5 °C and melting (T_m) at 137.5 °C. The conductivity of the copolymer underwent a breaking or rearrangement due to increasing temperature (figure 7(b)). In fact, different temperature regions with different activation energies were involved. Above transition temperature, the conductivity of the copolymer increased so quickly that the activation energy (E_a = 1.52 eV) could almost be defined at lower temperature range from 27 °C to 45 °C (figure 7(c)).

Zoom In Zoom Out Reset image size

Current (A) versus voltage (V) plots at temperature range of 23 °C–70 °C were used to determine the thermal resistance and conductivity values of NFCs (figures 8(A)–(C)). Thermal resistance (R) of NFC-8/2 and NFC-6/4 increased with increasing temperature from 23 °C (R ∼ 10⁷ Ω) to 70 °C (R ∼ 10⁹ Ω) (figure 8(D)) and from 23 °C (R ∼ 0 Ω) to 42 °C (R ∼ 60 MΩ) (figure 8(E)), respectively. Conductivity (σ) of NFCs increased with decreasing temperature from 40 °C to 27 °C (figures 9(A)–(C)). The conductivity parameters depended on the loading partner copolymer and incorporated AgNPs: σ = 2.21 × 10⁻¹¹ S m⁻¹, 3.68 × 10⁻¹⁰ S m⁻¹and 2.42 × 10⁻¹⁰ S m⁻¹ for NFC-8/2, NFC-6/4 and NFC-8/2-AgNPs, respectively. Activation energies (E_a) of NFCs (1.45 eV for NFC-8/2, 1.70 eV for NFC-6/4 and 0.01 eV for NFC-8/2-AgNPs) were calculated at 22 °C–35 °C temperature range from Arrhenius plots of ln σ versus 1/T using well known equation (figures 9(D)–(F)) [44]:

$$\begin{eqnarray*}&&\sigma ={\sigma }_{0}\;\mathrm{exp}(-{E}_{{\rm{a}}}/kT),\end{eqnarray*}$$

where σ₀, E_a and k are conductivity prefactor, conductivity activation energy and Boltzmann's constant (k = 8.625 × 10⁻²⁰ eV K⁻¹), respectively. The following increase of temperature up to 65 °C had not significant effect on conductivity and activation energy.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Conductivity of NFCs also strongly depended on conduct times. Obtained results indicated that the conductivity of NFCs essentially improved with applied various conduct times from 0 min up to 4.5 h (figure 10). Maximum conductivity values of σ_dc = 5.0 × 10⁻⁸, 6.74 × 10⁻⁸ and 1.17 × 10⁻⁶ S m⁻¹ were observed for NFC-8/2, NFC-6/4 and NFC-8/2-AgNPs at conduct times of 5 min, 20 min and 5 min, respectively. The kinetic plots of conductivity versus time (figures 10(D)–(F)) showed that changes in conductivity continued up to 2 h for all NFCs, and then this dependence almost stabilized up to 4.5 h. Colloidal polymer nanofibers exhibited higher conductivity when compared with those for pristine PVA (1.25 × 10⁻¹⁵ S cm⁻¹) [17] and partner copolymer (5.0 × 10⁻¹¹ S cm⁻¹).

Zoom In Zoom Out Reset image size

After the annealing process and treatment with NaOH as doping agent, the electrical resistance and conductivity of NFCs (figure 11) visible increased as compared with NFC-8/2 and NFC-6/4. Absorption of NaOH solution (0.5 M) on the samples (0.005 ml onto 0.25 cm² surface of nanofiber film with thickness ∼10 μm) significantly increased conductivity parameters. This phenomenon observed can be explained by an increase in the ion-charged sites in the nanofiber structures via activation of carbonyl and double –CH=CH– conjugation onto main chains of rearranged macromolecules. SIP around 50 °C–60 °C observed in figure 11(E) was due to transfer of nanofiber structures to glass-transition state. The presence of NaOH caused the appearance of strong second peak. It was proposed that Na⁺ ions from ionized NaOH formed bridges between two –C=O and –CH=CH– groups onto rearranged polymer chains, and enhanced ion-charged sites, and therefore improved the conductivity of NFCs. Increase of temperature from 30 °C to 70 °C significantly decreased conductivity of NFC-8/2 (annealed) and NFC-8/2-annealed-NaOH (figures 11(B) and (F)). Temperature dependence of conductivity for NFC-6/4 (annealed)-NaOH predominantly decreased with an increase in the partner copolymer fraction as compared with that for NFC-8/2 (annealed)-NaOH. The conductivities of these nanofiber samples reached maximum level as ∼10⁻⁶ S m⁻¹ and ∼10⁻⁸ S m⁻¹ around 30 °C–35 °C. The annealing procedure and NaOH doping factors also dramatically increased conductivities of NFCs compared with those for the matrix PVA polymer.

Zoom In Zoom Out Reset image size

Activation energies of NFCs without and with NaOH doping were calculated as 0.041 eV for NFC-8/2 (annealed), 1.038 eV for NFC-8/2 (annealed)-NaOH, 0.037 eV for NFC-6/4 (annealed), and 0.071 eV for NFC-6/4 (annealed)-NaOH at 30 °C –70 °C temperature range. Thus, multifunctional rearranged structures and their NaOH physical modifications exhibited higher semiconductor properties.