ISSN 0965-545X, Polymer Science, Series A, 2018, Vol. 60, No. 4, pp. 483–490. © Pleiades Publishing, Ltd., 2018.Original Russian Text © V.V. Matrenichev, P.V. Popryadukhin, V.P. Sklizkova, V.M. Svetlichnyi, A.E. Kryukov, N.V. Smirnova, E.M. Ivan’kova, E.N. Popova, I.P. Dobrovol’skaya,V.E. Yudin, 2018, published in Vysokomolekulyarnye Soedineniya, Seriya A, 2018, Vol. 60, No. 4, pp. 296–303.

MEDICAL POLYMERS

Obtainment of Aromatic Polyimide Nanofibersand Materials on Their Basis for Cell Technologies

V. V. Matrenicheva,*, P. V. Popryadukhina,b, V. P. Sklizkovab, V. M. Svetlichnyib, A. E. Kryukovb,N. V. Smirnovab, E. M. Ivan’kovaa,b, E. N. Popovab, I. P. Dobrovol’skayaa,b, and V. E. Yudina,b

aPeter the Great St. Petersburg Polytechnic University, St. Petersburg, 195251 Russiab Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, 199004 Russia

*e-mail: acrossq@gmail.comReceived October 12, 2017;

Revised Manuscript Received December 19, 2017

Abstract—Nanofibers with a diameter of 100–300 nm are obtained by electroformation of solutions of poly-amide acid based on 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride and o-toluidine in an N,N-dimethyl-acetamide/benzene solvent mixture. Thermal treatment of nanofibrous polyamide acid material leads to theformation of nanofibers of aromatic polyimide with a diameter of 100–200 nm. The temperature of thebeginning of thermal decomposition of polyimide nanofibers in an argon atmosphere is 537°С. SEM imagesshow that the material based on aromatic polyimide nanofibers preserves its elastic properties even at the tem-perature of liquid nitrogen. The obtained material is characterized by the absence of cytotoxicity: humanfibroblasts cultivated on it are characterized by high proliferative activity.

DOI: 10.1134/S0965545X18040053

INTRODUCTION

Materials based on aromatic polyimides (PI) arecharacterized by high thermal stability, high strength,and elastic properties; they are stable against acid andradiation treatment [1]. The products made of PI pre-serve their operating characteristics at low temperature(even at the temperature of liquid nitrogen). PI are tra-ditionally used as binders in composite constructivematerials owing to these properties; PI films and fabricare used for the filtration of corrosive liquid and gas-eous media and as separating membranes in energystorage systems characterized by high capacitance [2–4]. Recently, PI have been used in biology and medi-cine, in particular, for the development of molecularlyimprinted materials and membranes for blood detoxi-fication [5, 6]. These polymers are used as matrices forcell technologies for the in vitro cultivation of hepato-cytes, fibroblasts, and keratinocytes [7, 8].

At present, tissue engineering can be consideredone of the promising fields of science. Data on theobtainment and investigation of properties of 1D, 2D,and 3D polymer matrices for tissue engineering aregiven in monograph [9]. It was shown that matricesbased on chitosan, polylactide, and aliphatic copoly-amide are characterized by biocompatibility and theabsence of cyto- and genotoxicity. Here the supramo-

lecular and microporous structure of these polymermatrices plays an important role in adhesion of stemand somatic cells and in the normal genesis of cell pro-cesses.

It was shown in [9–11] that polymer matrices basedon nanofibers obtained by electroformation are char-acterized by an optimal ratio between fiber diameterand pore size which provides adhesion of cells on thesurface of matrix and processing of exchange processesrequired for proliferation and cell mobility. The perse-verance of cell proliferative activity after long storageat low temperatures (especially in a liquid nitrogenatmosphere) is an important specific feature of thecells of living organisms.

The technologies of nanofiber formation frompolyvinyl alcohol (PVA), poly(ethylene oxide) (PEO),polyvinylpyrrolidone, aliphatic copolyamides, andother polymers [12–14] are well known. Materialsmade of such fibers are characterized by low density,high porosity, and moisture and gas permeability.Most of the polymers and materials on their basis arenot characterized by the required mechanical proper-ties (first of all, elasticity) at low temperature. This sig-nificantly impedes their usage for the creation of tissueengineering preparations composed of polymer matrixand stem or somatic cells. At the same time, it isknown that aromatic PI preserve their elastic proper-ties even at low temperature down to the temperature

483

484 MATRENICHEV et al.

of liquid nitrogen [1]. This fact makes polyimidespromising materials for cell technologies, in particu-lar, for containers for long-term storage of cell mate-rial.

Aromatic PI are usually obtained according to thetwo-stage technique. Polyamide acid (PAA) is

obtained in the first stage by polycondensation of aro-matic diamines and aromatic dianhydrides in aproticsolvents or their mixtures with various volatile solvents(benzene, toluene, tetrahydrofuran) [15]. PAA istransformed into PI in the second stage during theprocess of further cyclodehydration:

The process of imidization can be performed eitherby a thermal method or using chemical reagents [2].

Nanofibers were formed from solution of PAAobtained by polycondensation of pyromellitic dianhy-dride and oxydianiline in DMFA [16]. The method offormation of polyimide nanofibers by electroforma-tion of PAA in N,N-dimethylamide (DMAA) is alsowell known [17, 18].

The removal of high-boiling aprotic solvent fromPAA solutions during the electroformation of nanofi-bers is a significant problem of their obtainment.Blow-off with hot gas and other energy-consumingmethods, which also carry a significant environmentalload, are used. The residues of solvents in nanofibrousPAA material can cause cytotoxicity of material anddecrease proliferative activity of cells. It is known thatit is possible to completely remove the solvent duringthe process of thermal imidization of PAA-basedmaterial [1].

The goal of the present research was to obtain PInanofibrous material, to study its strength, deforma-tion, and thermal properties, and to examine the cyto-toxicity and proliferative activity of human fibroblastson a matrix made of PI nanofibers.

EXPERIMENTALPolyamide acid was obtained by polycondensation

of 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride(BP) and o-toluidine (TD) in DMAA. The PAA con-centration in solution was 12 wt %. Electroformationof nanofibers from PAA was performed according to

the following technique. Firstly, films 25–30 μm thickwere formed from PAA solution: PAA solution inDMAA was applied onto the substrate using split drawdie; the system was dried at 60°С for 12 h. Then thePAA film samples were dissolved in a DMAA–ben-zene mixture at a ratio of 40 : 60 v/v %, which is closeto the threshold of polymer precipitation. The con-centration of solutions was varied from 8 to 15 wt %.The prepared solutions were stirred at room tempera-ture for 24 h, then filtered and deaerated for 2 h atpressure Р = 0.1 atm. PAA solutions in mixed solventwere used for electroformation of nanofibers.

The rheological properties of solutions were stud-ied with Physica MCR-301 rheometer (Anton Paar,Austria) in the shear mode at 20°С. Surface tensioncoefficient σ was estimated using a DSA-30 tensiome-ter (Kruss, Germany) [19].

The electroformation process was performed at aNanon-01A facility (MECC Co., Japan). The voltagewas 24 kV, the distance between electrodes was125 mm, a metal plate served as receiver electrode,and the solution feed rate was 28 × 10–5 cm3/s.

The material based on PAA nanofibers was sub-jected to stepwise thermal treatment. The temperaturewas raised each time by 50°C within 100–420°С; everysample was kept in the heat chamber for 15 min. Mate-rials composed of PI nanofibers were obtained duringthermal cyclodehydration of PAA.

A Supra 55VP scanning electron microscope (CarlZeiss, Germany) was used for the investigation ofstructure of the materials. A layer of gold ~25 nm thick

C

C

C

C

O

OO

O

HN

HO

NH

OH

H3C CH3

n

−2n H2O, 400°C

C

C

C

C

O

OO

O

N N

H3C CH3

n

POLYMER SCIENCE, SERIES A Vol. 60 No. 4 2018

OBTAINMENT OF AROMATIC POLYIMIDE NANOFIBERS AND MATERIALS 485

Fig. 1. The dependence of effective viscosity η on defor-mation rate for PAA solutions with concentration of 8 (1)and 15 wt % (2).

0.1

10

1000

0.001

η, Pa s

0.1 10 1000 100 0000.001

1

2

.γ, s−1

γ�

was sprayed onto the material using an Eiko-IB3 ioncoater; the ion current was 6 mA and the interelec-trode voltage was 1.5 kV.

Thermogravimetric measurements of the sampleswere performed on a 209 F1 Iris analyzer(NETZSCH, Germany). The sample with weight of2–3 mg was placed in an open crucible (Al2O3) whichwas mounted in a holder. The measurements were per-formed at temperatures of 30–800°С at the heatingrate of 10 K/min in an argon atmosphere.

The IR spectra were registered with a Vertex-70spectrometer (Bruker) using single reflection ATRPike accessory (ZnSe 45° prism).

The study of mechanical properties of nanofibrousPI materials was performed under uniaxial extensionat room temperature. The samples characterized byworking section dimensions of 20 × 2 mm and thick-ness of 100–250 μm were tested using an Instron 5943universal tension testing machine. The rate of exten-sion was 10 mm/min. The mechanical characteristicswere determined by statistical averaging of measure-ments made for at least ten samples.

The investigation of cytotoxicity of PI nanofibrousmaterials was performed using fibroblasts of humanskin which were cultivated in DMEM medium (Bio-lot, Russia) with addition of 10% of HyClone fetalbovine serum (USA) under a 5% СО2 atmosphere at37°С. The cells were used after 4–6 cultivation pas-sages; the nutrient medium was changed every threedays. The proliferative activity of cells was studiedusing tetrazolium dye (3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl-tetrazolium bromide) according to theso-called MTT method [20].

RESULTS AND DISCUSSION

In the present research, the solutions of rigid chainpolyamide acid BP–TD in amide (DMAA) or mixedsolutions (DMAA/benzene) were used to obtainnanofibers by electroformation; the ratios of compo-nents of solvents and polymer concentration in thesolution were varied. It is known that PAA solutions inamide solvents are strongly associated [21]. The inves-tigations of the structure of PAA solutions by polarizedlight scattering [22] showed that the addition of ben-zene to PAA solution in DMAA leads to the rupture ofhydrogen bonds between solvent and functionalgroups of polymer and to the disruption of supramo-lecular structure and its ordering. This facilitates theextraction of solvents and accelerates the formation ofnanofibers during electroformation. It should be men-tioned that the solution is the most homogeneous andthe ordered regions exhibit the maximum size specifi-cally at DMAA : benzene ratio = 40 : 60 [23]. It is

POLYMER SCIENCE, SERIES A Vol. 60 No. 4 2018

known that the oriented structure of polymer provideshigh strength and elastic properties of fibers [24].

The investigation of rheological properties of BP–TD polyamide acid solutions in DMAA/benzene mix-ture was performed to determine the optimal proper-ties of electroformation of nanofibers from PAA.

The dependences of effective viscosity of PAAsolutions with concentration of 8 or 15 wt % on shearrate γ are given in Fig. 1. The effective viscosity signifi-cantly decreases at the increase in γ in the range of10‒3–1.0 s–1. The dependence of effective viscosity onshear rate is absent at γ higher than 1.0 s–1. The non-Newtonian dependences of the viscosity of polymersystems on the shear rate can be explained in terms ofseveral theories [25]. Most of them suppose reversibleor irreversible rupture of the self-similar structure ofpolymer solution and the formation of a new one. Thistransition is usually followed by the change in config-uration of macromolecules, which leads to significantchanges in the viscoelastic properties of polymer solu-tion, in particular, to the change in the dependence ofeffective viscosity on the shear rate. One can supposethat the decrease in value of PAA solution viscosity atthe increase in shear rate (Fig. 1) is related to the factthat PAA solutions exhibit viscoelastic behavior, whichis obviously related to the structurization of solutionowing to the interaction of PAA functional groups.The value of the liquid limit for 8 and 15 wt % solutionsis 0.03 and 0.07 Pa, respectively. These low valuesfacilitate the easy rupture of the initial physical net-work under shear deformation.

It should be mentioned that the value of surfacetension also significantly affects the process of electro-formation of nanofibers [10]. High surface tensionprevents the polymer stream from splitting intomicrostreams; it promotes the formation of sphericaldefects. At the same time, it is impossible to obtain a

486 MATRENICHEV et al.

Table 1. Values of surface tension of PAA solutions with dif-ferent concentration and copolyamide solution

Solution Concentration,wt %

Surface tension, 10–3 mN/m

PAA 8 27.6 ± 1.8

15 32.8 ± 1.2

Copolyamide 17 34.4 ± 1.1

16 31.2 ± 1.0

homogeneous structure of nanofibers on the receiverelectrode if the value of surface tension is too low. Thevalues of surface tension of PAA solutions character-ized by different concentration are given in Table 1.Thus, the value of surface tension increases as the PAAconcentration is raised. It should be mentioned thatthe hanging drop method used in the present researchgives the most detailed data on the behavior of thepolymer solution under the forces of surface tension,in particular, the process of polymer stream formationafter withdrawal from the cylindrical die. The mea-surement of equilibrium surface tension is the closestto the conditions of formation of nanofibers because itis known [26] that the change in shape of a drop whichis formed at the end of the supply electrode, its trans-formation into a stream, and further split into microst-reams take place during the electroformation processunder the action of an electric field. The efficiency of

PO

Fig. 2. SEM images of nanofibrous materials obtained from PAand the material based on BP–TD polyimide (d).

2 μm

2 μm

(а)

(c)

these processes significantly depends on the value ofsurface tension. The formation of nanofibrous struc-ture during the electroformation process takes placeboth under the influence of shear stress at passing ofsolution through the die and in the electric field at fur-ther stretching of the stream at its passing from anodeto cathode according to the trajectory described by theTaylor сone. As a result, an optimal structure of nano-fibers is formed.

The values of surface tension of aliphatic copoly-amide in an alcohol/water mixture [27] are given inTable 1 as a comparison. The electroformation ofnanofibers from this polymer is characterized by ahigh rate; defects are almost absent. Despite the dif-ferent nature of solutions, it can be considered that therange of values of surface tension which provides sta-ble electroformation would be the same for the forma-tion of nanofibers which are free from defects.

PAA solutions with concentration of 8, 14, and15 wt % in DMAA : benzene mixtures characterized by40 : 60 v/v % ratio were supplied through the die withcapillary diameter of 0.12 cm and feed rate Q =28 × 10–5 cm3/s into an electric field with intensityЕ = 176 kV/m. The calculation according to the for-mula [28] which estimates the correlation between theshear rate γ, volume feed rate of polymer Q, and dieradius R,

(1)γ =π 34 ,QR

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A solutions with concentration of 8 (a), 14 (b), and 15 wt % (c)

2 μm

2 μm(b)

(d)

OBTAINMENT OF AROMATIC POLYIMIDE NANOFIBERS AND MATERIALS 487

Fig. 3. IR spectra of the materials based on PAA (1) and PI (2).

1

2

35004000 3000 2500 2000 1500 1000 500ν, cm−1

showed that the value of γ is 0.1 s–1 at the formation ofnanofibers. The experimental data showed that theincrease in the value of γ at the increase in feed rate Qleads to a significant increase in imperfection of thematerial.

SEM images of nanofibrous materials obtained byelectroformation of PAA solutions characterized bydifferent concentration are given in Fig. 2. Sphericalparticles (300 nm–3 μm) are formed on the receiverelectrode at the formation of 8 wt % solution (Fig. 2a);nanofibers are absent. Both spherical particles andnanofibers with diameter of 50–80 nm are formed as

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Fig. 4. TGA curves of nanofibers made of BP–TD polyim-ide (1) and PAA (2).

100 300 500 700

40

60

80

100

Weight, %

T, °C

1

2

the concentration is raised to 14 wt % (Fig. 2b). Thestructure composed of nanofibers with diameter of100–300 nm is formed at PAA concentration of15 wt % (Fig. 2c). The study showed that this concen-tration is considered optimal for the obtainment ofnanofibers from PAA solution in DMAA/benzenemixture. Experiments demonstrated that the rate offormation of nanofibers should be significantlydecreased for the obtainment of nanofibrous struc-tures from solutions of higher concentration; thismakes the process non-value-added.

Materials based on PAA nanofibers were subjectedto stepwise thermal treatment, which caused the for-mation of macromolecules of aromatic PI. The resultsof IR study support this (Fig. 3). The elimination ofbonds at 3500–3100, 1660, and 1550 cm–1 (vibrationsof СООН and NH2) typical of PAA was observed afterheating; on the contrary, the bands at 1774, 1720, 1374,and 722 cm–1 typical of PI emerged.

TGA curves of the materials based on aromatic PIand PAA are given in Fig. 4. It is seen that nanofibrousmaterial obtained by the electroformation methodwith further thermal treatment is characterized by hightemperature of thermal decomposition (537°С). It isknown that the temperature of beginning of thermaldecomposition of aromatic polyimide (BP–TD) is530°С [1]. This indicates a rather complete passing ofchemical processes which resulted in the formation ofPI macromolecules. The TGA curve for the materialbased on PAA nanofibers exhibits a significant differ-

488 MATRENICHEV et al.

Table 2. Mechanical properties of the materials based on nanofibers made of BP–TD polyimide at 20°С

Materials Elastic modulus, MPa Ultimate strength, MPa Deformation before rupture, %

BP–TD polyimide 71.5 ± 8.3 7.0 ± 0.9 32.2 ± 8.5

Copolyamide 54.5 ± 7.1 7.1 ± 0.5 259.0 ± 15.0

ence from the TGA curve for PI. It is characterized bysignificant mass loss in the range of 100–250°С,which is usually related to the processes of chaincyclization and PAA transformation into PI [1].

On basis of results of IR and TGA studies, one canconclude that Fig. 2d shows a microphotograph ofporous material based on nanofibers from aromaticPI. This material is characterized by a homogeneousstructure; it does not contain spherical defects; thediameter of fibers is 100–200 nm.

The values of elastic modulus, ultimate strength,and specific elongation at rupture of the materialsbased on nanofibers from aromatic PI at room tem-perature are given in Table 2. The given characteristicsare close to the analogous values for materials based onnanofibers from other polymers, in particular, poly-coamides [27].

As has been mentioned, the application of materi-als based on nanofibers to cell technologies and cellengineering attracts special interest. The absence ofcytotoxicity is the main demand for the materialswhich can be used as matrices for cell technologies.The investigation of cytotoxicity of nanofibers of BP–TD polyimide by the MTT method showed that, afterfive days of cultivation, the density of cells fixed on thematerial under study was 1.5 × 103 cells/cm2, while onthe surface of a standard f lask this value was 2.3 ×103 cells/cm2. The minor difference between theobtained values is probably related to significant dif-ferences of the structure of the surface of nanofibrousmaterials compared to the surface of a standard f lask.It is obvious that, in the case of nanofibrous struc-tures, the area of the near-surface layer of highlyporous material which is able to interact with cells islower than the area of the nonporous smooth surface

PO

Fig. 5. SEM images of surface of material made of BP–TD pol

20 μm(а)

of a standard plate well. This fact can explain ratherlower density of cells on the surface of porous filmcompared to the density on the surface of a standardflask after five days of cultivation. Thus, one can con-clude that nanofibers based on BP–TD polyimide donot exhibit cytotoxicity.

Figure 5 shows SEM images of porous materialsbased on PI nanofibers after two and five days of fibro-blast cultivation. One can see that active proliferationof cells fixed on the material surface takes place in thecourse of two days. Cells are uniformly distributedalong the surface and occupy all its area. This factindicates high proliferation activity of cells on the sur-face of porous material composed of BP–TD polyim-ide nanowires. Thus, the developed materials can beused as matrices for cell technologies.

The preservation of elasticity at low temperatures isan important specific feature of materials based onBP–TD polyimide [1, 24]. The studies of character ofrupture of the obtained materials in a liquid nitrogenatmosphere prove this fact. SEM images of the endface of split of films based on nanofibers of copolyam-ide and BP–TD polyimide in liquid nitrogen are givenin Fig. 6a. A large amount of ends of nanofibers withdiameter of 800–900 nm is observed at rupture ofcopolyimide film. These values match the diameter ofthe analogous fibers given in [26], which indicates abrittle character of rupture of material composed ofcopolyamide nanofibers in a liquid nitrogen atmo-sphere. The ends of nanofibers are almost absent onthe end face of low-temperature split of BP–TD poly-imide material (Fig. 6b). Most of the nanofibers with

LYMER SCIENCE, SERIES A Vol. 60 No. 4 2018

yimide nanofibers after (a) two and (b) five days of cultivation.

20 μm(b)

OBTAINMENT OF AROMATIC POLYIMIDE NANOFIBERS AND MATERIALS 489

Fig. 6. SEM images of split materials based on nanofibersobtained from copolyamide (a) and BP–TD polyimide (b).

2 μm

1 μm

(а)

(b)

diameter about 200 nm exhibit different orientation.One can suppose that plastic deformation of nanofi-bers without brittle rupture takes place as a load isapplied to the sample.

The given data indicate that material based onnanofibers of BP–TD polyimide preserved elasticityat low values of temperature, including atmosphere ofliquid nitrogen; such behavior is different from thattypical of most of the materials based on aliphaticpolymers, including films made of CPA. This import-ant property of material made of PI nanofibers com-bined with high proliferative activity of human fibro-blasts and absence of cytotoxicity allows one to expectits application in cell technologies.

CONCLUSIONS

The study of the dependence of effective viscosityof PAA solutions based on different concentrations ofBP–TD in binary DMAA/benzene solvent on shearrate and investigation of surface tension made it possi-ble to elaborate a method to obtain nanofibers fromPAA solutions. IR spectroscopy and TGA showed thatthermal treatment of the obtained material leads to theformation of nanofibers based on aromatic PI; ther-mal decomposition of the latter starts at 537°С. Thematerials based on PI remain elastic at the tempera-

POLYMER SCIENCE, SERIES A Vol. 60 No. 4 2018

ture of liquid nitrogen; they are characterized byabsence of cytotoxicity because human fibroblasts cul-tivated on these materials exhibit high proliferativeactivity.

ACKNOWLEDGMENTS

This work was supported by the Russian ScienceFoundation (project no.14-33-00003).

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Translated by P. Vlasov

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