Radiation-chemical preparation of poly(vinyl alcohol) hydrogels

Anastasia V. Duflot, Natalia K. Kitaeva n, Vladimir R. DuflotBranch of JSC “Scientific and Research Institute of Physical Chemistry named after L.Ya. Karpov”, Kievskoye Shosse, Obninsk 249033, Kaluga Region, Russia

H I G H L I G H T S

� The synthesis and the properties of poly(vinyl alcohol) hydrogels were studied.� PVA was modified by glycidyl methacrylate before gamma cross-linking.� The modification results in decreasing of PVA cross-linking dose by 3 orders lower.� The gel fraction and water content of the hydrogels were measured.� A fraction of the “bound” water in hydrogels is independent of gel fraction content.

a r t i c l e i n f o

Article history:Received 11 December 2012Accepted 2 August 2014Available online 16 September 2014

Keywords:Poly(vinyl alcohol)Glycidyl methacrylateRadiation-chemical cross-linkingHydrogelDifferential scanning calorimetrySwelling

a b s t r a c t

This work reports the usage of method of radiation-chemical synthesis to prepare cross-linked hydrogelsfrom poly(vinyl alcohol) modified with glycidyl methacrylate. Synthesis kinetics of modified poly(vinylalcohol) and properties of hydrogels were studied. The gel fraction, swelling, mechanical properties, andwater content of the hydrogels were measured. It was found that gel fraction increases with increasingradiation dose, concentration of modified poly(vinyl alcohol), and reaches 60%. It was established bydifferential scanning calorimetry that a fraction of the “bound” water in hydrogels is 50–70% andindependent of gel fraction content. In addition to “bound” and “free” states, water in hydrogels is alsopresent in the intermediate state.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

To date, polymer hydrogels are among the most versatile andadvanced materials that are used in a number of application areasin medical field.

The researchers involved in the development of new biomater-ials pay attention to polymer hydrogels due to their hydrophilicnature, potential biocompatibility, excellent mechanical and elas-tic properties similar to those of living tissues, and many otherunique characteristics (Rosiak, 1995, 1999b; Shtilman et al., 1999;Shtilman, 2006a). The structure with interconnected pores pro-vides a high specific surface of hydrogels, makes functional groupsaccessible for ligand attachment, eliminates diffusion problemsunder sorption and desorption of substances of a wide range ofmolecular weights. A great variety of the unique propertiesconsiderably increases potential applications of these systems(Chen et al., 1999; Shtilman et al., 2006b, Artyukhov et al., 2011a,2011b).

Polymer gels (hydrogels) are divided into physical and chemical(Rosiak et al., 2003; Ulanski and Rosiak, 2004; Gulrez and Phillips,www.intechopen.com). Physical hydrogels are the most availablehydrogels, herewith they are usually thermally reversible. Physicalhydrogels are formed under coagulation and the following solcoalescence, temperature decrease or increase, concentration ofpolymer solutions or separation of a new dispersed phase fromsupersaturated solutions. Chemical hydrogels are non-reversibleand can be produced by condensation and polymerization of bi-and multifunctional monomers as well as cross-linking of linearmacromolecules by chemicals (chemical cross-linking), photoini-tiators (photochemical cross-linking), gamma radiation or accel-erated electrons (radiation cross-linking).

Numerous data on poly(vinyl alcohol) (PVA) cross-linking withusing low-molecular weight di- and multifunctional cross-linkers(formaldehyde, dicarboxylic acids and their functional derivatives,diisocyanates, etc.) can be found in the reference literature. Somearticles are concerned with usage of electron and gamma radiationfor PVA cross-linking (Ulanski et al., 1998; Bauer, 1972; Park et al.,2003, 2004).

The reference literature states that gamma radiation applica-tion for medical hydrogels synthesis, including PVA hydrogels, is

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journal homepage: www.elsevier.com/locate/radphyschem

Radiation Physics and Chemistry

http://dx.doi.org/10.1016/j.radphyschem.2014.08.0020969-806X/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ7 48439 74776; fax: þ7 48439 63911.E-mail address: kitaeva.natalia@gmail.com (N.K. Kitaeva).

Radiation Physics and Chemistry 107 (2015) 1–6

preferable as it allows eliminating residual substance initiatorsand other low-molecular impurities in hydrogels (Pikayev, 1987;Bauer, 1972; Peppas and Merrill, 1976; Rosiak, 2007; Mishra et al.,2007).

However, radiation exposure on PVA is not as unambiguous asfor the others carbon-chain polymers. Specifically, it is commonlyaccepted that in the absence of oxygen irradiation induces cross-linking and gel formation of PVA in an aqueous solution, whereasit results in PVA predominant degradation in solid state (Pikayev,1987). For example, papers (Wang et al., 2000; Mishra et al., 2007;Rosiak and Ulanski, 1999a) quote the results for irradiation cross-linking of PVA or PVA mixtures with other soluble polymers indeoxygenated aqueous solutions. The doses applied for the radia-tion cross-linking are rather high and vary from 5 to 20 kGyand more.

In recent times there have occurred research works describinguse of compounds containing both the functional group capable ofreacting with PVA hydroxyl groups and the double bonds capableof entering into polymerization under free-radical mechanism(Shtilman et al., 1999, 2006b, 2007; Artyukhov et al., 2011a,2011b). Such method of PVA hydrogels preparation consists oftwo stages: stage one is the modification of PVA and the additionof double bonds into the macromolecules; stage two is polymer-ization of these double bonds and the formation of three-dimensional network of PVA-based hydrogel.

In this case both relatively simple compounds and speciallysynthesized complex low-molecular compounds and oligomers,such as acryloyl chloride, glycidyl acrylate etc., are used as cross-linking modifiers. The cross-linking is performed only by sub-stance initiators.

In the literature there are no any data about usage of PVAmodified by such a method for gamma irradiation cross-linking.

However, irradiation cross-linking of PVA is substantially thesimplest and the cheapest method for hydrogels preparation andPVA modification (mPVA) which considerably reduces the dose ofgamma radiation required for the formation of a three-dimen-sional network structure.

Use of the proposed method will give an option to obtain cross-linked structures based on the modified PVA under the absorbeddoses less than 1 Gy; that is several orders lower than standarddoses for cross-linking of original PVA (5–20 kGy).

The aim of the present paper was (1) to study the radiation-chemical PVA hydrogel synthesis to prove that preliminary mod-ification of PVA with glycidyl methacrylate allows to decrease thegamma-radiation dose by 2–3 orders under comparable condi-tions, and (2) to evaluate the water state in synthesized hydrogels.

2. Experimental

2.1. Materials

PVA with a degree of polymerization 2.5�104 and saponifica-tion of 98.68 mol% were purchased from Plastpolymer, Yerevan(Armenia). Glycidyl methacrylate (GMA) was purchased fromSigma. GMA was purified by vacuum distillation. Dimethylsulf-oxide (DMSO) was used without further purification.

2.2. Preparation of modified PVA

GMA was chosen as a modifying agent. By analogy with theauthors of the articles (Shtilman et al., 2006b; Artyukhov et al.,2011a, 2011b) PVA was modified under the following procedure.

The synthesis of mPVA was carried out in DMSO with sulfuricacid as an accelerant at 80 1С, PVA concentration of 8.7 wt. % andGMA–PVA correlation of 1/12 mol/mol equivalent for 2.5 h.

On completion, the reaction system was cooled. The polymerwas precipitated in acetone. To separate the precipitated polymer,it was twice re-precipitated in acetone and twice filtered. Theformed polymer was dried in vacuum at 20–25 1C to constantweight.

The degree of hydroxyl group substitution in mPVA wasdetermined by the following procedure. Residual GMA and mPVAcontent was defined by the alkaline saponification of ester groupswith the subsequent titration of hydrochloric acid alkali excess.Simultaneously the similar procedure was used to analyze theinitial PVA to determine the residual acetate group content. Thedegree of hydroxyl group substitution in mPVA in mol% wascalculated by the difference in residual content of GMA andacetate groups.

2.3. Radiation cross-linking of modified PVA

Radiation cross-linking of mPVA was carried out in glassampoules under vacuum. Aqueous solution of mPVA was chargedinto the ampoule; the ampoule was evacuated by triple freezing inliquid nitrogen to remove oxygen and then defrosted by immer-sion into warm water at a constant pressure of (0.15–0.20) Pa.

Irradiation of mPVA was performed at 0 1C and 20 1C using a60Co gamma radiation source in gamma-ray unit K-200.

Gamma radiation dose rate equaled to 0.1 Gy s�1. The dosime-try procedure was performed with Fricke dosimeter: aqueoussolution of 1 mmol dm�3 ferrous sulfate, 1 mmol dm�3 sodiumchloride and 0.4 mol dm�3 sulfuric acid were exposed for fixedtime in the gamma camera of the facility, concentration of ferricions in the solution was determined spectrophotometrically (Holmand Berry, 1970), the gamma radiation dose rate was calculated.Spectra were recorded at wavelengths of 302 nm (SF-2000spectrophotometer).

2.4. Measurements

FTIR measurements were performed with Fourier IR spectro-meter, FSM 1202, in the range of 4000–400 cm�1.

The inherent viscosity of PVA solution at the 25 1С η� �

H2O25

� �

was determined from the results of the viscosity measurements ofthe solutions with the different PVA concentration at the 25 1Сwith Ubbelohde viscosimeter (Nesterov, 1984; Kireev, 1992).

The samples were extracted by water in a Soxhlet apparatus for24 h and further dried to a constant weight. The gel fraction wasthen calculated gravimetrically by using the following formula:

G¼ W=W0� �� 100;

where G is the gel fraction (%), W and W0 are the weight of thesample after and before extraction, respectively.

The samples were analyzed in the differential scanning calori-meter PT/10 DSC “Linseis”. The heating rate was 5 1C min�1.Samples of cross-linked mPVa swollen at equilibrium were usedfor DSC testing.

Physical and mechanical characteristics of the mPVA sampleswere measured at room temperature on a universal tensilestrength testing machine Zwick 0.05 at the loading rate of5 mm min�1. Test samples were cylinders (L/D ratio – �1.5,diameter – 10 mm) extracted from the glass vials after polymer-ization. Water content in the samples used for mechanical testingwas specified by the conditions of the radiation cross-linking.

3. Results and discussion

PVA hydrogels were prepared in two stages using gammaradiation: at first PVA was modified with GMA, and then mPVAwas exposed to gamma-radiation for cross-linking (Fig. 1).

A.V. Duflot et al. / Radiation Physics and Chemistry 107 (2015) 1–62

Analysis of mPVA was performed by the FTIR spectroscopy andby the determination of the degree of substitution of hydroxylgroup in PVA by GMA groups.

The presence of GMA links in the mPVA composition wasidentified by absorption bands in the FTIR spectra at the followingwave numbers (ν, cm�1): 1020, 1060, 1195 (simple ether group),950, 1660 (carbon–carbon double bond) and 1710 (double bond ofthe carbonyl group). The fragments of FTIR spectra of the initialPVA, GMA and mPVA displaying the presence of СQС and CQObonds in mPVA are shown on Fig. 2.

The measurement of the degree of hydroxyl group substitutionin mPVA showed that the hydroxyl group fraction had decreasedby 8.2–8.5 mol% during the PVA modification.

So that the side chains with multiple bonds, which areintroduced into the PVA chain by modification, can polymerizeby a radical mechanism on exposure to gamma-radiation inaqueous medium. Besides, only interaction of double СQС bondslocalized in different mPVA macromolecules leads to the forma-tion of three-dimensional cross-linked structures and therefore tothe formation of insoluble gel.

The formation of three-dimensional hydrogel was investigateddepending on mPVA concentration in the reaction mixture andtemperature of radiation cross-linking. The kinetic study of mPVAradiation cross-linking has proved that under these conditionspolymerization proceeds efficiently (Fig. 3). The product formed atthe earliest stage of polymerization can immobilize all the watercontained in a sample.

It is seen in Fig. 3 that the kinetic dependence of mPVAradiation cross-linking has a typical shape which is similar topolymerization of bi- and polyfunctional (meth)acrylic monomers.With increasing temperature the rate of gelation is naturallygrowing. However, a higher content of gel fraction is reached ata lower temperature. Irradiation of the initial PVA solution underthe same conditions (temperature, concentration and absorbeddose) does not lead to the formation of cross-linked structures.

It is hardly possible to reach the content of gel fraction higherthan 60% at acceptable irradiation time. It can be due to thecompetition between the reaction of intramolecular interaction ofdouble CQC bonds (i.e., the bonds localized in the same mPVAmacromolecule but not leading to the cross-linking) and thereaction of intermolecular interaction (of the bonds localized inthe different mPVA macromolecule and leading to the cross-linking).

Similar phenomena were observed in the studies of radiationeffects on PVA-polyvinyl pyrrolidone mixture (Razzak et al., 2001),cross-linking of poly(vinyl methyl ether) (Rosiak, 2002), and in thestudy of three-dimensional radical polymerization of dimethacry-lates in the presence of branched polymethacrylates (PMA)(Kurmaz and Ozhiganov, 2009, 2010). Thus, the results ofKurmaz and Ozhiganov (2009, 2010) works describing results ofsol-gel analysis and GPC analysis of sol-fractions in differentpolymerizing compositions convincingly prove a dual role of thebranched PMA with “suspended” double C¼C bonds, which

simultaneously act as reactive macromonomer and chemicallyinert additives, i.e. polymer filler. That is to say, some part of thebranched PMA macromolecules is chemically active, and the restremains inert in the process of three-dimensional radicalpolymerization.

According to Kurmaz and Ozhiganov (2009, 2010), it can becaused by sterically constrained interaction of the growing poly-mer radicals with “suspended” double CQC bonds and theirspatial dislocation in the branched PMA. These spatial constraintsare implemented even at high concentrations of dimethacrylatedouble bonds; while dimethacrylate acts simultaneously as asolvent and comonomer of the polymerization system.

In absence of comonomer in our case, the cross-linkinginvolves only double CQC bonds of GMA being part of mPVA.

Fig. 1. Scheme of PVA hydrogel synthesis.

Fig. 2. FTIR spectra of PVA (1), GMA (2) and mPVA (3).

Fig. 3. Kinetics of radiation cross-linking of mPVA. Temperature of cross-linking,1С: 1–0; 2–20. Dose rate – 0.1 Gy s�1.

A.V. Duflot et al. / Radiation Physics and Chemistry 107 (2015) 1–6 3

With increasing cross-linking yield the mPVA macromolecularchains mobility decreases and also the steric restriction for doubleCQC chains interaction appears. As a result the cross-linkingreaction is “arrested” and the high conversion of double CQCbonds can0t be reached.

The gel fraction in Fig. 4 is presented as a function of mPVAconcentration in the reaction system. The relationship is critical.A three-dimensional network (macrogel) is not formed in thereaction system at mPVA concentrations lower than �2.5 wt%.A sharp increase in the cross-linking is observed in a narrow rangeof concentrations (from 2.5 to 3.5 wt%). The gel fraction reachesabout 40% and changes slightly with further increase of mPVAconcentration in the initial solution. All the water contained in theinitial reaction system is completely immobilized in a three-dimensional network.

To confirm the fact that cross-linked structure is set up at theconcentration from 2.5 to 3.5 wt%, the concentration of polymercoil overlapping (Сn) was calculated. It is known (Semchikov,2003; Tager, 2007) that Cn� [η]E1 corresponds to the polymerconcentration at which the coils in the solution begin to overlap.The viscosity of PVA solutions in water was measured and it wasfound that the intrinsic viscosity of the PVA–H2O at 25 1С equalsto (0.33070.005) dl g�1. So the concentration of polymer coiloverlapping is E3%. This value is in good agreement with thegelation concentration determined experimentally.

On this basis the following model can be proposed for mPVApolymerization or cross-linking. Before Cn is reached, polymeriza-tion probably occurs with microgelation, but microgel cannot befixed by standard extraction methods. At the critical concentrationCn polymerization process occurs in the channels, consisting ofinterconnected and overlapped coils penetrating the entire sam-ple. Further growth of mPVA concentration only increases thenumber of channels and cross-linking density of the sample.

Water phase transitions were studied by the DSC method toestimate the degree of water bonding in synthetic hydrogels.Phase changes were determined based on increasing and loweringthe sample temperature. The peak of water crystallizationrecorded in the cooling thermograms is considerably shifted alongthe temperature axis to lower temperatures compared to thetemperature of pure water crystallization. On heating the samplespre-cooled in the calorimeter at 5 1Сmin�1, the peaks can be seenon the thermograms due to melting and then boiling and evapora-tion of water immobilized in a polymer gel.

Numerous studies focused on the structure of water in aqueoussolutions of polymers (Rowland, 1984; Yanul et al., 1998), and onthe water-absorbing or water-swelling polymer substances. Nowa-days it is commonly assumed that near the polymer segmentswater molecules behave differently from normal “bulk” or “block”water because of interaction with polymers (Rowland, 1984),and in some cases hydrated polymer complexes are formed

(Yanul et al., 1998). This abnormal water is often called “bound”,“non-freezing”, “hydrated”, “ordered”, etc. The number of abnor-mal water types most likely depends on the energy of watermolecule interaction with different polymer segments, as well ason the experimental methods used in the studies. The state ofwater in the polymers was mostly investigated with gravimetric,calorimetric, infrared, and dielectric measurements, NMR spectro-scopy, etc.

The temperatures of water crystallization (curve 1), melting(curve 2) and evaporation (curve 3) are shown in Fig. 5 as afunction of the cross-linked polymer content in the hydrogelsample, where water is weakly bound with PVA segments duringheating and cooling. These dependencies are almost similarbecause the temperatures do not change or weakly decline withincreasing gel fraction content. It should be stressed that crystal-lization temperature of the water immobilized in the hydrogel issignificantly lower compared to the “block” water. Sometimes adifference can reach 301.

As compared to melting and evaporation, water crystallizationin the hydrogel occurs at the higher rates (Fig. 6) and in thenarrow temperature range.

The dependence of free water on gel fraction is shown in Fig. 7.A fraction of “free” water was determined by comparing theenthalpy of water phase transition (crystallization, melting andevaporation) in the hydrogel with that of pure water taken fromhandbook (Chemical Encyclopedia, 1988). Depending on the phasetransition selected for measurements, a fraction of “free” watervaried from 30 to 50%. Respectively, from 50 to 70% of water in thehydrogel is firmly bound with mPVA segments due to hydrogenbonding with OH-groups, and is not shown on the heating–coolingthermograms. A fraction of the “bound” water in the hydrogeldoes not change with the content of the cross-linked polymer in

Fig. 4. Gel fraction as a function of mPVA concentration in the reaction system at0 1С and irradiation time 180 min. Fig. 5. Temperatures of water crystallization (1), melting (2) and evaporation (3) as

a function of gel fraction. Temperature of cross-linking – 20 1С; mPVA concentra-tion in a reaction mixture – 7.84%.

Fig. 6. Rates of water phase transition as a function of gel fraction: (1) melting;(2) evaporation; (3) crystallization. Temperature of cross-linking – 20 1С; mPVAconcentration in a reaction mixture – 7.84%.

A.V. Duflot et al. / Radiation Physics and Chemistry 107 (2015) 1–64

the range of 20–70%, which indicates a high capacity of the cross-linked mPVA.

The conclusion of a high capacity of the cross-linked mPVA isconfirmed by the DSC experiments on the mPVA samples atequilibrium swelling after extraction (Table 1). The swelling variesin the range (0.2–15)�103% with changing the content of gelfraction from 44 to 70%.

The fraction of the “bound” water is in the range 0.2–0.7depending on the phase transition selected for “free” watermeasurements, and is practically unchanged (Fig. 8).

Some discrepancy between the values of “bound” water incross-linked mPVA obtained in different experiments can be dueto the condition of the analyzed hydrogel samples. In hydrogelwith the water content specified by the conditions of cross-linkingthe fraction of the “bound” water amounted to 0.5–0.7. In atequilibrium swelling gel with the higher total content of water thefraction of “bound” water amounted to 0.2–0.7.

To discuss “abnormal” water content in terms of the polymerchemical structure, it is more reasonable to express the content inmolar units rather than in mass fractions. According to calcula-tions, about 15 molecules of “abnormal” or “bound” water accountfor one monomer unit.

A fraction of “free” water was determined by the melting andcrystallization enthalpy for all the compositions (any content of gelfraction) immediately after polymerization. The value derivedfrom the melting enthalpy is about 10% higher compared tocrystallization enthalpy. In other words, the DSC data prove thepresence of “intermediate” water between “free” and “bound”states. “Intermediate” water shows no signs of crystallization onthe cooling thermograms, but melts on heating. A fraction of the“intermediate” water increases four times in the same samples atequilibrium swelling after extraction (Fig. 9).

Thus, water in mPVA gels can be in a “free” state (weak inter-action of water molecules with PVA segments), an “intermediate”

state and a “bound” state (fixed water molecules due to stronghydrogen bonds with PVA segments). The effect of gel fraction ongel swelling in water is shown in Fig. 10.

Fig. 7. Content of free water as a function of gel fraction in the samples directlyafter radiation-chemical synthesis: (1) evaporation; (2) crystallization; (3) melting.Temperature of cross-linking – 20 1С; mPVA concentration in a reaction mixture– 7.84%.

Table 1Experimental data based on the DSC thermograms of the cross-linked mPVA at equilibrium swelling after extraction. Concentration of mPVA in the reaction system – 7.84wt %.

Gel fraction (%) Swelling in water (%) Тin, 1С Тmax, 1С ΔHm, J g�1 Wmax, mJ s�1 Тin, 1С Тmax, 1С Δcr, J g�1 Wmax, mJ s�1

Melting Crystallization43.2 1458.9 0.9 12.0 �255.7 �17.1 �16.2 �25.0 137.0 36.7

99.7 127.0 �665.4 �25.045.6 771.7 0.8 14.4 �286.3 �16.2 �19.3 �36.1 127.1 38.9

103.1 126.1 �569.3 �25.069.46 196.0 �0.8 7.1 �248.7 �12.3 �21.2 �26.0 152.4 36.9

98.9 114.1 �890.9 �24.3

Fig. 8. Swelling of cross-linked mPVA after extraction as a function of free water:(1) evaporation; (2) crystallization; (3) melting. Temperature of cross-linking –

20 1С; mPVA concentration in a reaction mixture – 7.84%.

Fig. 9. Water in the intermediate state as a function of gel fraction: (1) cross-linkedpolymer; (2) cross-linked polymer after equilibrium swelling. Temperature ofcross-linking – 20 1С; mPVA concentration in a reaction mixture – 7.84%.

Fig. 10. Swelling as a function of gel fraction. Temperature of cross-linking – 20 1С;mPVA concentration in a reaction mixture – 7.84%.

A.V. Duflot et al. / Radiation Physics and Chemistry 107 (2015) 1–6 5

It is seen in Fig. 10 that equilibrium swelling of hydrogelsstrongly depends on gel fraction, and decreases dramatically in therange from 40 to 50%. At this gel fraction the kinetic curve reacheslimit values (Fig. 3). Probably, the formation of three-dimensionalstructure is almost complete at this time, and swelling of thecross-linked mPVA becomes stabilized at �200%.

The samples of mPVA were tested for compression strength.A typical compression diagram of mPVA hydrogel is shown inFig. 11.

It is seen in Fig. 11 that the deformation curve consists of twosections. The first section corresponds to the capillary desaturationof aqueous phase from gel pore structure. On the second sectionthe deformation of the polymer structure happens.

The effective modules of elasticity of mPVA polymer networkwere calculated as a slope ratio of the deformation curve in thesecond section of the diagram (Shtilman et al., 2006b). It wasobtained that the modules of elasticity for mPVA samples irra-diated at 0 and 20 1C amount to 100 and 192 kPa, respectively.

A comparison of elastic modules of radiation cross-linkedmPVA with the data received from the mPVA cross-linking withchemical initiators (Shtilman et al., 2006b) shows that the usage ofradiation-chemical cross-linking allows to obtain the higher cross-linking density.

4. Conclusions

This paper presents the radiation-chemical method for cross-linked structures preparation based on the modified PVA underthe absorbed doses three orders lower than doses of the originalPVA cross-linking. Dependence of kinetic characteristics on poly-merization temperature and mPVA concentration in solution isstudied, model of mPVA cross-linking is proposed.

The study of mechanical properties and swelling ability ofobtained hydrogels demonstrated that bound water fraction isindependent of gel-fraction content and equals to 50–70%. It wasestablished that besides “bound” and “free” states, water inhydrogels is also present in the intermediate state. It was demon-strated that the usage of gamma radiation instead of chemicalinitiators for hydrogel synthesis from mPVA leads to preparationof hydrogels with better mechanical-and-physical properties.

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Fig. 11. Typical compression diagram of mPVA hydrogel. Temperature of cross-linking – 20 1С; mPVA concentration in a reaction mixture – 7.84%; irradiation time– 105 min.

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