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419The Polymer Society of Korea

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Macromolecular Research, Vol. 21, No. 4, pp 419-426 (2013)

Thermal Imidization Peculiarities of Electrospun BPDA-PDA/ODACopolyamic Acid Nanofibers

Laura Peciulyte*,1, Ramune Rutkaite1, Algirdas Zemaitaitis1, Milena Ignatova2, Iliya Rashkov2, and Nevena Manolova2

1Laboratory of Biopolymer Research, Faculty of Chemical Technology, Kaunas University of Technology,Radvilenu pl. 19, LT-50254 Kaunas, Lithuania

2Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev St bl. 103A, BG-1113 Sofia, Bulgaria

Received March 23, 2012; Revised May 23, 2012; Accepted May 26, 2012

Abstract: Copolyamic acid (coPAA) based on 4,4'-oxydianiline (ODA), p-phenylenediamine (PDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was synthesized in N,N-dimethylformamide (DMF). The preparationof continuous defect-free nanofibers from BPDA-PDA/ODA coPAA was achieved by electrospinning of its DMFsolution. The average fiber diameter significantly increased from 385 to 590 nm on increasing the total polymer con-centration of the spinning solutions from 5 to 7 wt%. The addition of dodecylethyldimethylammonium bromide(DEDAB) salt to the spinning solution resulted in the procurement of coPAA nanofibers with a much smaller (morethan 3 times) average diameter. The coPAA imidization process was investigated through FTIR spectroscopy. Thechemical composition and morphology of coPI nanofibers were assessed by X-ray photoelectron spectroscopy andscanning electron microscopy. Imidization under isothermal conditions proceeded faster in the first stage. Activationenergies in the first and second imidization stages were similar when DEDAB had been added into the electrospin-ning solution. Cylindrical or crimped defect-free nanofibers of BPDA-PDA/ODA copolyimide (coPI) were obtainedby the stepped thermal imidization of coPAA. The morphology of coPI nanofibers depends on the curing tempera-ture. The crimped coPI nanofibers were most probably due to the relief of residual stress when the curing tempera-ture was higher than the polymer glass transition temperature.

Keywords: electrospinning, BPDA-PDA/ODA copolyimide nanofibers, crimped nanofibers.

Introduction

Electrospinning is an attractive approach for the fabrica-tion of continuous nanofibers from a wide range of naturaland synthetic polymers for variety of applications, includingwound dressings, tissue engineering scaffolds, drug delivery,biosensors, membranes for ultrafine filtration, nanoelectronicsetc..1-8 Electrospun polymer materials are drawing a greatattention because of their unique properties such as highsurface-to-volume ratio, high porosity and diameters in thenano-scale. Aromatic polyimides (PIs) have been extensivelystudied and have found many technological applicationsdue to their high thermal stability, high mechanical proper-ties, good chemical resistance, and dielectric resistance in thebroad temperature interval.9-11 These advantageous proper-ties make the aromatic PIs promising candidates for prepar-ing the high-performance nanofibers by electrospinning.

Polyimide fibers with diameters in the nanometer scale

can be successfully produced by electrospinning of polyamicacid (PAA) followed by thermal curing.12-15 Generally, themechanical properties of the electrospun nanofibers areinfluenced by the structure of the polymer chain, polymermolar mass and fiber morphology. It has been reported thatthe partially aligned rigid homoPI nanofibrous mats have ahigh tensile strength and high axial tensile modulus, butpossess lower elongation at break, that corresponds to a lowertoughness and impact resistance.12 The tensile strength, modu-lus and elongation at break of the rigid homoPI nanofibersconsisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA) and p-phenylenediamine (PDA) residues (BPDA-PDA)12 are respectively 664 MPa, 15.3 GPa and 5%, comparedto 384 MPa, 11.5 GPa and 3.9% for rigid BPDA/biphenyl-amide (BPA) homoPI nanofibers.14 To enhance their compli-ance, the partially aligned PI nanofibers have been producedbased on flexible BPDA/BPA/4,4’-oxydianiline (ODA) coPI14

or flexible BPDA/2,2-bis[4-(4-aminophenoxy)phenyl]hexaflu-oropropane homoPI.16

Recently, partially aligned homoPI nanofibers based on

DOI 10.1007/s13233-013-1032-7

*Corresponding Author. E-mail: [email protected]

L. Peciulyte et al.

420 Macromol. Res., Vol. 21, No. 4, 2013

BPDA/ODA have been prepared and it has been found thatthermal curing resulted in fusion of nanofilaments at con-tacts.17 It has been reported that residual solvent and dode-cylethyldimethylammonium chloride additive have an effecton the fusion of nanofibers in bundles during thermal curingof PAA. However, systematic studies on the effect of thethermal treatment on the morphology of PI nanofibers arestill lacking.

This paper reports on the preparation of coPI nanofibersby electrospinning of BPDA-PDA/ODA coPAA followed bythermal imidization. The chemical composition and themorphology of the coPAA and coPI nanofibers were char-acterized by FTIR spectroscopy, X-ray photoelectron spec-troscopy (XPS), and scanning electron microscopy (SEM).The influence of thermal treatment on the morphology ofcoPI nanofibers was also studied.

Experimental

Materials. 4,4'-Oxydianiline (ODA), p-phenylenediamine(PDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA) (97%) were obtained from Balti Kaubad Ja Tee-nused AS (Estonia). The diamines were used as received,while the dianhydride (BPDA) was heated to 280 oC before thesynthesis. Dodecylethyldimethylammonium bromide (DEDAB)and N,N-dimethylformamide (DMF) (99.9%) were obtainedfrom Sigma-Aldrich and used as received.

coPAA Synthesis. coPAA was prepared by adding BPDAto a stirred solution of ODA and PDA in DMF at a tempera-ture of 15 oC. The content of the solids was calculated to be10 wt%. The reaction mixture was intensively stirred for 2-3 h. The degree of coPAA polymerization was characterizedby inherent viscosity. This parameter for all the samples was3 dL/g.

Preparation of coPAA Nanofibers. coPAA nanofibers wereprepared by electrospinning of coPAA solutions in dry DMF atpolymer concentration of 5 wt% (PAA5) or 7 wt% (PAA7).DEDAB-containing coPAA mats (PAA5-DEDAB and PAA7-DEDAB) were prepared by electrospinning of coPAA solu-tions containing DEDAB (0.15 wt% of coPAA content).The utilized home-made electrospinning set-up designedand developed by the team from the Laboratory of Bioac-tive Polymers, Institute of Polymers, Bulgarian Academy ofSciences, consisted of a high voltage power supply (gener-ating positive DC voltages from 30 to 50 kV); a pump fordelivering the spinning solution at a constant rate; a syringe(5 mL), equipped with a positively charged metal needlethrough which the spinning solution is delivered; an auxiliaryfocusing positively charged ring and a negatively chargedrotating aluminum drum collector with a diameter of 45mm. Electrospinning was performed at a flow rate of thespinning solution of 0.8 mL/h, at a constant value of theapplied voltage - 46 kV, a distance from the needle tip to therotating drum collector -25 cm and rotating speed of the col-

lector -1,100 rpm. In this way, uniform electrospun coPAAmats with size of 15×20 cm were prepared. The electrospunfibrous mats were placed under vacuum for 1 h at 100 oC toremove nonbonded solvent.

Prior to electrospinning, the dynamic viscosity of the BPDA-PDA/ODA coPAA spinning solutions was measured usinga REOTEST-2 (Germany), gradually increasing shear strainat 14 oC. The electrical resistance of the spinning solutionswas measured in an electrolytic cell equipped with rectan-gular sheet platinum electrodes having a surface area of 0.6cm2 and disposed at a distance of 2.0 cm. During the mea-surements short electric pulses with opposite direction wereapplied to the Pt electrodes in order to avoid accumulationof ionic charge and polarization effects in the vicinity ofelectrode surface. This allowed solution resistance in therange 20-2,000 kW to be measured with an accuracy of±3%. Calibration of the electrolytic cell was performed usingstandard solution of KCl (conductivity 140.8 mS/m) andconstant of the cell (Kcell) was determined. The conductivityof the spinning solutions (σ, µS/cm) was calculated fromthe following equation:

(1)

where ρ is the specific resistance of the solution (µΩ·cm),R-electrical resistance of the solution (µΩ).

Thermal Imidization. The coPAA fibers were dried andcured isothermally or by heating in stages (stepped imidiza-tion): at 130 ºC for 30 min, 160 ºC-30 min, 260 ºC-10 min,270 ºC-25 min, and 340 ºC-25 min. coPI nanofibers PI5,PI7, PI5-DEDAB and PI7-DEDAB were obtained by steppedimidization by using PAA5, PAA7, PAA5-DEDAB, and PAA7-DEDAB nanofibers, respectively.

Analysis. The conversion of coPAA to coPI was followedby means of FTIR spectra of the materials, taken with a Spec-trum X Perkin-Elmer FTIR spectrophotometer. The imidiza-tion degree was measured with FTIR, mainly on the basis ofthe Lambert-Beer Law and absorbance superposition.18

According to the Lambert-Beer Law, the absorbance of theabsorbent component is directly proportional to its concen-tration. Therefore, the absorbance of the film varies with thecontent of imide groups. Thus the degree of imidization wascalculated from the ratio of the intensity of imide absorptionband at 1776 cm-1 to the intensity of band at 1500 cm-1 in theFTIR spectra.19 The reference band was obtained from theintensity ratio of two absorption bands of a fully imidizedcoPAA sample.

The degree of imidization of partially imidized sampleswas calculated from the following equation:

(2)

where A is absorbance intensity, and subscripts t and ∞denote the bands of the sample and the reference, respec-

σ 1ρ--- 1

Kcell R⋅----------------= =

αtA1776/A1500( )t

A1776/A1500( )∞

------------------------------- 100×=

Thermal Imidization Peculiarities of Electrospun BPDA-PDA/ODA Copolyamic Acid Nanofibers

Macromol. Res., Vol. 21, No. 4, 2013 421

tively. The reference sample of 100% imidized coPI was pre-

pared by heating the coPAA nanofibers stepwise up to 340 oC.The 2 stage imidization of PAA nanofibers was expressed

by the first-order reaction.20 The imidization degree withtime, which was measured during isothermal imidization,was fitted by the following first-order rate equation:

(3)

where α is the imidization degree at time t, and k is the rateconstant. Rates of reactions compared between each otherby calculation the ratio of reaction rate constants in the firstand second stages

Activation energy Ea were determined from Arrheniusequation:

(4)

where A is the pre-exponential term, R is the gas constant,and T is the imidization temperature.

The morphology of the mats was evaluated by SEM. Forthis purpose, the electrospun coPAA and coPI nanofiberswere vacuum-coated with silver and analyzed using a scan-ning electron microscope FEI Quanta 200 FEG. The averagefiber diameter and the standard deviation were estimated interms of the criteria for complex evaluation of electrospunmats reported elsewhere21 by using Image J software program22

and measuring at least 30 fibers from each SEM image.The electrospun pristine coPAA and coPI mats were ana-

lyzed by X-ray photoelectron spectroscopy (XPS). The XPSmeasurements were carried out in the UHV chamber of anESCALAB-MkII (VG Scientific) electron spectrometer usingMgKα excitation with a total instrumental resolution of~1 eV. Energy calibration was performed, taking the C 1sline at 285 eV as a reference. Surface atomic concentrationswere evaluated using Scofield’s ionization cross-sectionswith no corrections for λ (the mean free path of photoelec-trons) and the analyzer transmission function. Experimentalvalues for element atomic percentage obtained from XPSanalysis are the averages of three independent measure-ments. Differential scanning calorimetry (DSC) analysis ofcoPI nanofibers was performed with a Q100 TA instruments(USA) thermal analyzer at a heating rate of 20 oC/min in anitrogen atmosphere.

Results and Discussion

Preparation and Characterization of coPAA Nanofibers.Many aromatic polyimides are insoluble in almost all sol-vents. Therefore, PI nanofibers are fabricated by electro-spinning of the PI precursor - PAA solution and then curingthe obtained PAA nanofibers to convert them into PI nanofi-bers. To obtain BPDA-PDA/ODA coPAA, the reaction of anaromatic dianhydride BPDA with aromatic diamines - PDA

and ODA was carried out to form a BPDA-PDA/ODAcoPAA soluble in dimethylformamide (Figure 1, stage 1):

coPAA spinning solutions were prepared by diluting thereaction mixtures with DMF to decrease the viscosity.

The morphology of electrospun nanofibers and their aver-age diameters are strongly influenced by the composition ofthe spinning solution, solution concentration, and appliedfield strength (AFS).3,23-26 Preliminary screening experimentson the electrospinning conditions performed varying theconcentration in the range from 7 to 5 wt%, the AFS from1.0 to 1.9 kV/cm, and the feeding rate from 0.8 to 2.0 mL/hshowed that cylindrical defect-free coPAA fibers with rela-tively narrow diameter distribution of the fibers were formed atpolymer concentration 5 wt%, feeding rate 0.8 mL/h, andAFS of 1.84 kV/cm (Figure 2). On increasing the total poly-

11 α–-----------⎝ ⎠

⎛ ⎞ln kt=

k Aexp Ea/RT–( )=

Figure 1. Formation of BPDA-PDA/ODA coPI.

Figure 2. SEM-micrographs of electrospun nanofibers preparedat different coPAA concentrations in DMF: (a) 5 wt%; (b) 5 wt% inthe presence of 0.15% DEDAB; (c) 7 wt%; (d) 7 wt% in the pres-ence of 0.15% DEDAB.

L. Peciulyte et al.


mer concentration of the spinning solutions from 5 to 7 wt%the average fiber diameter increased (Table I). This may beexplained by the increase in the viscosity of coPAA solu-tions (Table I). Indeed, for the solutions at concentrations of5 and 7 wt%, the dynamic viscosity values were 1,000 and6,600 mPa s, respectively. Moreover, on adding DEDAB tocoPAA solutions the average diameter of the nanofibersdecreased (Table I). The observed effect most probably dueto the increase in the conductivity of the solution when theorganic salt - DEDAB is added (Table I). The solution con-ductivity increased from 6.2 to 212.4 µS/cm in the case of5 wt% coPAA when the salt was added. Similar behaviorhas been observed in other systems1,4,15,27-29 and is explainedby the higher charge density on the surface of ejected jetduring spinning thus imposing higher elongation forces tothe jet.

The surface composition of the coPAA mats was analyzedby XPS. In the high resolution C1s spectrum of the coPAAmat (Figure 3(a)) four peaks were detected at 284.7 eV (-C-Hor -C-C-), 285.4 eV (-C-N, -C-O, -C-C(O), -C-C(N)), 286.6 eV(O=C-NH) and 288.8 (-COOH). The O1s signal (Figure 3(b))consisted of three peaks: at 531.2 eV, assigned to O=C-NH,at 532.1 eV, assigned to O=C-O and at 533.4 eV, attributedto -C-O-C-, as well as to -COOH, respectively. In the expandedN1s spectrum (Figure 3(c)) two peak components were iden-tified, assigned to O=C-NH at 400.0 and to O=C-NH2 at401.5 eV. Moreover, the atomic percentages of the elements(70.4% C, 23.7% O, and 5.9% N), experimentally deter-mined from the XPS peaks were close to the theoretical val-ues calculated from the chemical composition of the coPAAmats (68.8% C, 24.4% O, and 6.8% N).

Imidization of coPAA Nanofibers. coPAA nanofibers wereconverted into coPI by curing (Figure 1, stage 2). The con-version of coPAA into coPI was confirmed by FTIR spec-troscopy (Figure 4).

The bands at 3200-3300 cm-1 (NH stretching vibration ofthe -CONH- groups of coPAA, Figure 4(A)), 2800-3200cm-1 (OH stretching vibration of the -COOH groups ofcoPAA, Figure 4(A)), and bands at 1663 cm-1 (Amide I, Fig-ure 4(C)) and 1550 cm-1 (Amide II, Figure 4(D)) were miss-ing in the FTIR spectra of the coPI mats. Meanwhile, the

Table I. Dynamic Viscosity (η) and Conductivity (σ) of Spinning Solutions, and Average Fiber Diameter ( ) of ElectrospuncoPAA Fibers; SD-Standard Deviation

Electrospun Mats coPAA Concentration (%) Amount of DEDAB (%, w/w) η (mPa s) σ (µS/cm) d (nm) SD

PAA5 5 0 1,000 6.2 385 75

PAA5-DEDAB 5 0.15 900 212.4 115 20

PAA7 7 0 6,600 6.3 590 120

PAA7-DEDAB 7 0.15 5,800 162.3 330 70

d

Figure 3. XPS peak fittings for coPAA mats (C1s (a), O1s (b), and N1s (c)).

Figure 4. FTIR spectra of: coPAA (1) and coPI (2) (A-2900-3200 cm-1, B-1720 cm-1, C-1663 cm-1, D-1550 cm-1, E-1776 cm-1,F-1700cm-1, G-1380 cm-1, H-725 cm-1).



appearance of characteristic bands at 1776 cm-1 (C=O in theimide cycle, asymmetric stretching, Figure 4(E)), 1700 cm-1

(C=O in the imide cycle, symmetric stretching, Figure4(F)), 1380 cm-1 (C-N in the imide cycle, stretching, Figure4(G)) and 725 cm-1 (C=O in the imide cycle, bending, Fig-ure 4(H)) was registered.

The conditions of thermal imidization have a considerableeffect on the performance of the material. Thus, two waysof coPAA nanofiber curing were studied in this work. In thefirst one, imidization was performed under isothermal con-ditions. In the second one, stepped imidization was carriedout. The dependence of the coPAA imidization degree onthe conditions of sample curing in the stepped imidization isshown in Figure 5.

The increase in duration and temperature of curing pro-cess resulted in increase in the nanofiber imidization degree(Figure 5). Nanofiber samples were also imidized under iso-thermal conditions at 130, 160, and 190 oC. As shown inFigure 6, the nanofiber imidization degree increased with

increasing the curing temperature.The imidization degree up to 30% was obtained when the

imidization was performed at 130 oC. At a temperature of160 oC the number of imide groups was two times higherthan that at 130 oC (Figure 6). The imidization proceeded intwo stages. As it could be seen from the data presented inthe Table II in the first stage the imidization rate was up to50 times faster (ratio k1/k2). The faster reaction in the firststage may be attributed to the presence of residual solventDMF in the nanofibers, which can enable the mobility oflarge macromolecular segments. Moreover, the cyclizationproceeds at the highest speed when the o-carboxyamidegroup is at the optimum conformation.30 In the second stage,the mobility of macromolecules decreases when most of thesolvent has evaporated. The results showed that addition ofDEDAB to the coPAA solutions had no significant effect onthe imidization rate (Table II).

The activation energies (Ea) of imidization stages wereFigure 5. coPAA imidization degree dependence on sample cur-ing conditions in the stepped imidization.

Figure 6. The dependence of the coPAA imidization degree onthe curing duration, at a curing temperature: 1-130 oC; 2-160 oC;3-190 oC.

Table II. The Apparent Rate Constants and Activation Energy of coPAA Imidization

Sample Imidization Temperature(°C) k1×10-5 (s-1) r1

2 Activation Energy(kJ/mol) k2×10-5 (s-1) r2

2 Activation Energy (kJ/mol)

PAA5

130 6 0.9246

54.9

4 0.9121

35.8160 20 0.9967 2 0.9897

190 50 0.9894 1 0.9999

PAA5-DEDAB

130 6 0.9582

41.2

2 0.9438

39.2160 10 0.964 6 0.9891

190 30 0.9826 9 0.9988

PAA7

130 8 0.9903

40.9

3 0.9715

22.0160 10 0.9933 5 0.9899

190 40 0.9929 7 0.9987

PAA7-DEDAB

130 5 0.9231

54.1

1 0.9992

50.4160 20 0.9948 3 0.9933

190 40 0.9929 7 0.9986

L. Peciulyte et al.


calculated using the Arrhenius equation (Table II). Althoughthe reaction rates are different, activation energies of thefirst and the second imidization stages are similar whenDEDAB has been used in the electrospinning solutions.However, the activation energy of the first stage was higherup to 46% when nanofibers have been electrospun withoutDEDAB. It is well known that the residual DMF in the coPAAnanofibers forms a complex with the -COOH or -CONH- ofcoPAA31 as it is shown in Figure 7. The conditions ofdecomposition of the complex change when the DEDABcations, which is prone to form a complex with coPAA, isadded to the spinning solutions, and this most probablyinfluences both the energy of the process and the values ofactivation energies (Ea) of imidization.

A confirmation of the successful imidization of coPAAnanofibers was also obtained from the performed XPS anal-yses. Similar changes in the XPS spectra of coPI obtainedby using the two different imidization approaches, compared tothat of pristine coPAA mats, were registered. The expandedN1s spectrum of coPI mats has only one peak component at400.2 eV, assigned to imide -N(C=O)2 (Figure 8(c) and (f)). Con-siderable differences were observed in the C1s and O1s spec-tra of the coPI mats, as well (Figure 8(a),(b),(d), and (e)). Inthe expanded C1s spectrum the appearance of a new peak at288.5 eV assigned to -N(C=O)2 was observed (Figure 8(a) and(d)). The peak component at 286.6 eV assigned to O=C-NHdisappeared after imidization. The detailed O1s spectrumshows only two peak components at 532.0 eV characteristic of-C=O groups and at 533.6 eV typical of C-O-C groups (Fig-ure 8(b),(e)). As it is seen from Table III, the experimentally

Figure 7. Schematic representation of the complexation between-COOH or -CONH- of PAA and residual DMF in coPAA nanofi-bers.

Figure 8. XPS peak fittings for: coPAA mats after stepped imidization at different temperatures (C1s (a), O1s (b), and N1s (c)) and coPAAmats after imidization under isothermal conditions (190 oC) (C1s (d), O1s (e), and N1s (f)).

Table III. XPS Elemental Analysis (atomic percentage)

Electrospun Mats C1s (%) O1s (%) N1s (%)

coPAA 68.8c

70.4d24.4c

23.7d6.8c

5.9d

coPI after Imidizationa 74.6c

76.0d18.0c

17.5d7.4c

6.5d

coPI after Imidizationb 74.6c

76.5d18.0c

17.1d7.4c

6.4d

aStepped imidization was carried out at different temperatures. bIm-idization was carried out under isothermal condition (temperature190 oC). cTheoretical values, based on the chemical composition ofelectrospun mats. dExperimental values obtained from XPS analysis.



determined atomic percentages of the elements are close tothe theoretically calculated ones.

As it is seen from the SEM analyses (Figure 9), after ther-mal imidization (stepped or isothermal imidization) of coPAAmats the obtained coPI nanofibers remained defect-free andcylindrical. After the stepped imidization the average diam-eters of the coPI fibers (see Table IV) were close to that of

the pristine coPAA fibers (see Table I). Slightly lower diam-eter of treated nanofibers was obtained due to evaporationof residual solvent as well as splitting-off of water duringimidization.

However, in the case of complete stepped imidizationcrimping of nanofibers was observed (Figure 9(a)). Thisphenomenon is not manifested after two hours of isothermaltreatment at a temperature of 190 oC (Figure 9(b)).

It has been reported32 that the electrospun fibers crimpedwhen the operating temperature was greater than the glasstransition temperature of the polymer. In our case, the poly-mer glass transition temperature increases on increasing theimidization degree (Figure 10). To verify the proposed crimp

Figure 9. SEM-micrographs of coPI nanofibers: (a) after steppedimidization; (b) after 2 h of isothermal imidization at 190 oC.

Table IV. Average Fiber Diameter ( ) of coPI Fibers

Electrospun Mats d (nm)

PI5 360 ± 70

PI5-DEDAB 110 ± 15

PI7 540 ± 120

PI7-DEDAB 260 ± 35

dFigure 10. Influence of the imidization degree on the glass tran-sition temperature.

Figure 11. SEM-micrographs of electrospun coPAA nanofibers cured at 190 oC after additional curing at different temperatures: (a)190 oC; (b) 260 oC; (c) 290 oC; (d) 300 oC; (e) 320 oC; (f) 340 oC.

L. Peciulyte et al.


mechanism a PAA5 nanofiber sample after curing at 190 ºCfor 60 min (PAA5190) was cured additionally under isother-mal conditions. After curing PAA5190 nanofibers have imi-dization degree of 75.5% and a glass transition temperature(Tg) of 230 oC.

The morphology of PAA5190 nanofibers cured for 30 minat temperatures over Tg was characterized by SEM (Figure11). As it is seen from Figure 11(b), the nanofibers crimpafter curing at a temperature higher than Tg by 30 oC. Thecrimping is more strongly expressed at higher temperatures(Figure 11(c)-(f)). Similarly to the reported crimping in elec-trospun poly(lactide-co-caprolactone) fibers32 the crimping ofthe coPAA fibers may be attributed to the residual stress inthe fibers due to the fiber alignment in the process of elec-trospinning and the relief of stress by heating at tempera-tures higher than the glass-transition temperature.

Conclusions

BPDA-PDA/ODA coPI nanofibers were prepared by elec-trospinning of coPAA solutions of different concentrationsand with or without DEDAB addition followed by thermalimidization. The isothermal imidization of coPAA nanofi-bers proceeded in two stages: the first stage was up to 50times faster. This might be explained by the presence ofresidual plasticizing solvent in the nanofibers and the highersegmental mobility of macromolecules. The coPI nanofi-bers crimped when the curing temperature was higher thanthe polymer glass transition temperature and this was mostprobably due to the relief of residual stress resulting fromthe fiber alignment in the process of electrospinning.

Acknowledgment. The authors are grateful to the Researchcouncil of Lithuania and National Paying Agency under theMinistry of Agriculture for financial support of the project MT1131. Financial support from the Bulgarian National ScienceFund (Grant DCVP 02/2 (UNION)) is gratefully acknow-ledged.

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