ORIGINAL PAPER

Poly(vinyl alcohol) and poly(dimethylsiloxane)-basedinterpenetrating polymer networks via radical polymerisation

Elidiane Carvalho Coelho & Daniela Pereira dos Santos &

Kátia Jorge Ciuffi & Jefferson Luis Ferrari &Beatriz Alves Ferreira & Marco Antonio Schiavon

Received: 27 January 2014 /Accepted: 18 August 2014# Springer Science+Business Media Dordrecht 2014

Abstract In this paper, interpenetrating polymer networks(IPNs) based on poly(vinyl alcohol) (PVA) and poly(dimeth-ylsiloxane) with terminal vinyl groups (PDMS-vinyl) withboth hydrophilic and hydrophobic features were prepared byradical reticulation. Specifically, an aqueous PVA solutionwas mixed with an alcoholic PDMS-vinyl solution in thepresence of 2,2-dimethoxy-2-phenylacetophenone (DMPAP)and N’,N-methylenebisacrylamide (MBA) as initiating andreticulating agents, respectively, to obtain samples with thefollowing PVA/PDMS-vinyl (w/w) compositions: 100/0, 75/25, 50/50, and 25/75. Confirmation of the IPN formation wasdone by a set of techniques such as Fourier transform infraredspectroscopy (FTIR), X-ray diffraction (XRD), thermogravi-metric analysis (TG), differential scanning calorimetry (DSC),as well as theoretical calculations. The techniques usedshowed evidence of cross-linking reactions occurring in bothPVA and PDMS-vinyl. Swelling experiments showed a prop-er balance of the resulting IPNs in swelling polar and apolarsolvents, according to their composition and cross-linking.Tools of Theoretical Chemistry (Methods of Molecular Me-chanics and Quantum Mechanics) were employed for the firsttime to investigate the cross-linking reaction between PVAand MBA and corroborate the experimental results. The PVA/

PDMS IPNs showed a high capacity (up to 80wt.%) to absorbwater and may be characterised as hydrogels.

Keywords Polymer networks . Poly(vinyl alcohol) .

Poly(dimethylsiloxane) . Radical Polymerisation .

Interpenetrating polymer networks

Introduction

Interpenetrating polymer networks (IPNs) consist of two poly-meric components that are separately cross-linked into twointerpenetrated networks with no chemical bonds betweenthem. The great interest in IPNs is from the potential ofdesigning materials with a range of properties and, in mostcases, by the generation of a synergistic effect on one or moreof the properties [1–3].

The simultaneous cross-linking is the most frequent meth-od used to synthesise IPNs. It is formed by polymerising twodifferent monomers or by using two different polymer chainsalong with complementary cross-linking agent pairs togetherin one step [2]. In this procedure, the reactants (monomers orpolymers) are mixed in solution before reticulation takes place[1, 4]. The IPNs may result from the simultaneous indepen-dent polymerisation of two monomers or polymers, and theindividual polymeric networks interpenetrate without chemi-cally binding to each other [1].

There are numerous IPNs containing poly(dimethylsiloxane)(PDMS) prepared in a variety of synthetic polymers [5–9],showing microphase separation and multiple structures becauseof the inherent immiscibility of PDMS with most organicpolymers. Many of these IPNs have interesting and usefulproperties because of the high chain flexibility, low surfaceenergy, high thermal stability, and low Tg of the polysiloxanechains [10].

E. C. Coelho :D. P. dos Santos : J. L. Ferrari :M. A. Schiavon (*)Grupo de Pesquisa em Química de Materiais–(GPQM),Departamento de Ciências Naturais, Universidade Federal de SãoJoão del-Rei, Campus Dom Bosco, Praça Dom Helvécio, 74,36301-160 São João del-Rei, MG, Brazile-mail: schiavon@ufsj.edu.br

K. J. CiuffiUniversidade de Franca–UNIFRAN, 14404-600 Franca, SP, Brazil

B. A. FerreiraUniversidade Federal de São João del-Rei, Campus Centro-OesteDona Lindu–CCO, 35501-296 Divinópolis, MG, Brazil

J Polym Res (2014) 21:561DOI 10.1007/s10965-014-0561-x

PDMS is widely employed in microflow devices [11–14],microcontact printing technology [14, 15], biomedical appli-cations [16, 17], controlled drug release systems [18, 19], andmicrofluidic-based analytical chemistry [20, 21]. PDMS dis-plays properties that favour an array of applications, such as:high permeability, good optical transparency down 280 nm,high flexibility, non-toxicity, biocompatibility, chemical andthermal stability, low curing temperature, and moldability [2,22, 23]. Nevertheless, the highly hydrophobic character of aPDMS chain has limited its use in some fields [14, 24].Several approaches involving physical and chemical modifi-cations have also been made to enhance surface properties ofPDMS to obtain long-term stability of hydrophilic PDMSsurfaces [20, 23, 25].

On the other side, poly(vinyl alcohol) (PVA) has beenextensively investigated for industrial applications, since it isone of the major non-toxic, hydrophilic, water-soluble, bio-compatible synthetic polymers with a relatively simple chem-ical structure, excellent film forming capacity, and high tensilestrength and flexibility [26]. An aqueous PVA solution couldbe easily cross-linked by a variety of techniques to formhydrogels which show desirable physicochemical character-istics, like an elastic nature and high degree of swelling inaqueous medium, finding extensive applications in variousfields, including hydrogels [26–31]. The polymer, however,lacks adequate mechanical strength because of a highly hy-drophilic nature [26]. Therefore, a proper combination of bothPVA and PDMS into polymeric networks would enhance thepoor hydrophilicity of PDMS with a synergistic effect on theother physicochemical properties of both chains, as has beendemonstrated for PVA-PDMS copolymers [32–35] and PVA-PDMS semi-IPNs [24, 36, 37].

In a previous work, we synthesised semi-IPNs based onPVA and PDMS bearing terminal hydroxyl groups (PDMS-OH) via the sol–gel process. The PDMS chains reacted withtetraethylorthosilicate (TEOS) through hydrolysis and con-densation reactions in the presence of PVA, to form aninterpenetrated system consisting of PVA chains and PDMS-OH chains reticulated with TEOS [24]. Such semi-IPNs weresuccessfully used as support for Mn(Salen) bases used incatalytic reactions with no solvent as well as using hydrogenperoxide as a polar oxidant in a process called “green cataly-sis” [38]. However, by using the sol–gel process, only lowamounts of PVAwere incorporated into the PDMS matrix (upto 20 % wt), resulting in maximum water swelling of just14 wt.%. Therefore, we have investigated a new route toincrease the amount of PVA into the PDMS polymer withthe aim to increase its hydrophilic character. Taking this ideainto account, this paper describes the synthesis of polymernetworks based on PVA and PDMS with terminal vinylgroups (PDMS-vinyl) via radical polymerisation using N,N’-methylenebisacrylamide (MBA) and 2,2-dimethoxy-2-phenylacetophenone (DMPAP) as reticulating and initiating

agents, respectively, to obtain samples with the followingPVA/PDMS-vinyl (w/w) compositions: 100/0, 75/25, 50/50,and 25/75. Confirmation of the IPN formation was done by aset of techniques such as Fourier transform infrared spectros-copy (FTIR), X-ray diffraction (XRD), thermogravimetricanalysis (TG), and differential scanning calorimetry (DSC).Swelling experiments showed a proper balance of theresulting IPNs in swelling polar and apolar solvents, accord-ing to their composition and cross-linking. To the best of ourknowledge, we have employed for the first time Tools ofTheoretical Chemistry (Methods of Molecular Mechanicsand Quantum Mechanics) to investigate the cross-linkingreaction between PVA and MBA, which corroborate the ex-perimental results.

Experimental

Materials

The interpenetrating polymer networks, labelled polymericnetworks (POLN) from hereon in, were synthesised from thefollowing precursors: PDMS bearing terminal vinyl groups(PDMS-vinyl), molar mass (Mw) 25,000 g mol−1; PVA, withhydrolysis degree of 99.8 %, Mw ranging from 85,000 to124,000 g mol−1; N,N’-methylenebisacrylamide (MBA),purity = 99 %, Mw = 154.17 g mol−1, used as reticulatingagent; and 2,2-dimethoxy-2-phenylacetophenone (DMPAP),purity = 99 %, Mw = 256.30 g mol−1, used as initiator ofpolymer chain reticulation. All of the aforementioned reagentswere purchased from Sigma-Aldrich. Isopropanol P.A./A.C.S.99.5 % was acquired from Isofar and employed as a solventfor the reticulating and initiating agents. All of the compoundswere used without any prior treatment.

Polymer network synthesis

The PVA and PDMS-vinyl-based polymer networks weresynthesised according to an adapted procedure described byShin et al. [37]. The samples were prepared with each polymerin isolation, to investigate the curing process of the resultingmaterial. The PVA film was obtained by refluxing an aqueoussolution containing 10wt.% of the polymer at a temperature ofapproximately 85 °C under magnetic stirring. In another con-tainer, 5 mL of isopropanol was added to the reticulating andinitiating agents, which were kept under stirring at roomtemperature until complete dissolution. The resulting solutionwas added to the PVA solution, and the system was main-tained under reflux at 85 °C for 7 h. The final solution wasplaced on a Teflon® Petri Dish and dried at room temperature.

The polymer networks containing both polymers weresynthesised in a similar way by preparing an isopropanolsolution of PDMS-vinyl, DMPAP, and MBA and adding it

561, Page 2 of 11 J Polym Res (2014) 21:561

to PVA dissolved in water under constant magnetic stirringand heating at 85 °C. The resulting system was kept underreflux at 85 °C for 7 h. Then, the material was placed on aTeflon® Petri Dish for curing at room temperature. Finally, thesample was dried in an oven at 70 °C for 24 h. Five sampleswere obtained: POLN100 (100 % PVA); POLN75 (75 %PVA/25 % PDMS); POLN50 (50 % PVA/50 % PDMS);POLN25 (25 % PVA/75 % PDMS), and POLN0 (100 %PDMS-vinyl). All of the samples also contained DMPAP andMBA at the same ratios: 1 % mass and 0.5 % mol,respectively.

Swelling measurements

Polymer reticulation was investigated by sample swelling inaqueous and organic media. To this end, films measuring 1×1 cm2 were weighed and immersed in water (polar solvent) orhexane (apolar solvent) and left to stand for 72 h. Then, thesamples were dried in an oven (70 °C, under vacuum) for 48 hand weighed again.

Theoretical calculations

To investigate how the functional groups connected the poly-mer chains in the synthesised samples and to confirm theinfrared spectroscopy results, theoretical chemistry tools wereemployed. To this end, some possible products from thereaction between PVA and MBAwere proposed and submit-ted to structural optimisation by classic (molecular mechanics,AMBER force field) and quantum (semiempirical PM3 meth-od) mechanics methods in a vacuum at 298 K. Next, thevariation in the standard enthalpy of formation (ΔH0

f) of theproposed products, PVA, and MBAwere calculated using theprogram HyperChem 7.5 [39]. On the basis of the results, thetwo products with the lowest ΔH0

f, i.e. the most enthalpicallystable products, had the vibrational frequency of their bondscalculated, to confirm the structure with the minimum globalenergy and to obtain the thermodynamic properties of thereaction. These calculations were conducted at the quantummechanics/semiempirical PM3 level, and implemented in theprogram Gaussian 09 W (2009).

Physico-chemical characterisation

The precursors and reagents as well as the synthesised sam-ples were characterised by Fourier transform infrared spec-troscopy (FTIR) on a Spectrum GX PerkinElmer spectrome-ter. PVA and the synthesised samples were analysed by atten-uated total reflectance (ATR). PDMS-vinyl, in the fluid form,was analysed in KBr windows in the transmission mode; theother reagents were analysed in the transmission mode usingKBr pellets. All of the spectra were recorded between 4000and 650 cm−1, with a 2 cm−1 resolution and accumulation of

32 scans. To assess the changes in the crystallinity pattern ofthe materials, the samples were analysed by X-ray diffractionon a diffractometer, XRD6000, Shimadzu, using Cu Kα (λ=1.5418 Å) radiation, tension = 40 kV, current = 30 mA, and 2θranging between 10 and 80°. The thermal stability of thestarting materials and the polymer networks was analysed onthe thermal analyser TA model SDT Q600. Analysis wasconducted between 25 and 800 °C, at a heating rate of10 °C min−1, under constant nitrogen flow. The vitreoustransition temperatures (Tg) of the materials and polymernetworks were measured on the calorimeter Diamond DSCPerkinElmer. Once we are working with polymeric networkscontaining PDMS, the temperature varied between −180 and240 °C; the heating rate was 10 °C min−1. To construct thecurve, two scans were recorded: from 30 to 240 °C, followedby cooling to −180 °C, and from −180 to 240 °C. For eachrun, a 3-min isotherm was carried out for stabilisationpurposes.

Results and discussion

Polymer network (POLN) synthesis

Polymer dissolution under reflux followed by curingfurnished PVA films containing the reticulating agent (samplePOLN100) or not (sample PVA). PDMS (POLN0) did notafford a film in the presence of MBA and DMPAP, probablybecause reticulation reactions were not sufficient to sustain thePDMS chains.

POLN75, POLN50, and POLN25, synthesised ac-cording to the experimental procedure described in theexperimental section, were homogeneous, crack-free,and with no phase separation. POLN100 was the leastthick and the most transparent, which is typical of PVA films.The remaining samples resembled each other, except forPOLN25, which retained an oily surface even after the curingprocess. This layer may have resulted from PDMS-vinyl thatdid not react (reticulate) completely, reinforcing the hypothesisthat PDMS-vinyl did not undergo 100 % reticulation.

Bearing in mind that PVA and PDMS-vinyl display ahydrophilic and hydrophobic character, respectively, swellingtests to analyse sample mass variation in the presence of apolar and an apolar solvent helped evaluate film reticulation.Figure 1 shows the mass loss results in the presence of waterand hexane; it revealed that all of the samples underwent massloss after contact with the tested solvents.

In water, the samples underwent between 3 and 30 % massloss. PVA provided the largest mass loss; as expected, this filmdid not contain any reticulating agent, so its linear chains werefree to dissolve in water. POLN100 and POL25 also experi-enced significant mass loss in aqueous medium. Concerning

J Polym Res (2014) 21:561 Page 3 of 11, 561

POLN25, the low PVA content (25 % mass), as compared toPDMS (75 % mass), may have favoured PDMS-vinyl reticu-lation over PVA reticulation, which in turn may have facili-tated PVA solubilisation. As for POLN100, the degree ofreticulation may not have been large enough to avoid massloss in aqueous medium. Interestingly, the mass loss resultsachieved for POLN100 and PVA differed significantly, indi-cating that reticulating reactions between PVA and MBA didtake place. POLN50 and POLN75 underwent the smallestmass losses, evidencing that these samples had a higher de-gree of PVA reticulation. In hexane, all of the samples pre-sented about 5 % mass loss, attesting to PDMS reticulation.

To investigate the reticulation phenomenon, we immersedthe samples PVA and POLN100 in water for 6 months. PVAdissolved completely, whereas POLN100 became deformedbut did not dissolve. Therefore, in the conditions used here,the PVA chains reticulated in the presence of MBA andDMPAP.

Investigation of the synthesis mechanisms

Shin et al. [37] synthesised PVA and PDMS-based hydrogelsvia radical reticulation in the presence of DMPAP as aninitiator and MBA as a reticulating agent. Although DMPAPis considered to be a photoinitiator, the authors employed it asa thermal initiator. In another paper, Shin et al. [36] preparedPVA/PDMS- and chitosan-based semi-IPN polymer net-works, also using DMPAP as an initiator.

No initiator will ever achieve 100 % efficiency, becausereticulation also depends on whether the generated radicals

can diffuse towards the reaction site of the polymer dispersedin the solvent system. Acetophenone derivatives produceradicals via cleavage followed by a second fragmentation[40]. Indeed, DMPAP decomposes into the benzoyl and theketal benzoyl radicals; the latter decomposes into methylbenzoate and methyl radical [41, 42]. The formation of morethan one radical species makes the mechanism through whichinitiators act even more complex [41].

On the basis of the reticulation process, it was possible toinfer that network formation occurred not only via cleavage ofthe unsaturated bond of the reticulating agent MBA, but alsothrough hydrogen atom abstraction from the carbon atomsituated between the nitrogen atoms in MBA. Besides that, itwas evident that PVA also reticulated into a single tridimen-sional network; however, it was not clear how the bondsoriginated. In the present case, PVA contains functional hy-droxyl groups along its chain, which afforded tridimensionalnetworks in the presence of MBA and DMPAP. In principle,neither PDMS with terminal vinyl groups (PDMS-vinyl) norpolymers of this same class bearing other functional groups,like PDMS-hydroxy, can generate networks; they only growin length. Nevertheless, the copolymerisation or reticulationwith compounds exhibiting two active sites would favournetwork formation.

It is well known that PVA can reticulate via reaction of itshydroxyl groups with dialdehydes or dicarboxylic acids. Forexample, Ibrahim [43] employed gamma radiation to preparePVA films reticulated with MBA for use in textile dye absorp-tion. Although the synthesis was successful, the authors didnot suggest a mechanism for the reaction.

Fourier transform infrared spectroscopy (FTIR)

Infrared spectra of the polymer networks POLN100,POLN75, POLN50, and POLN25 and of the precursor poly-mers PVA and PDMS-vinyl are shown in Fig. 2. The spectrumof the PVA film presented the bands typical of PVA [24, 44],particularly the large and intense band between 3500 and3200 cm−1, ascribed to O-H bond stretching of the hydroxylgroups (I) located along the polymer chain; bands in the3000–2800 cm−1 assigned to C-H bond stretching (II); bandsat 1415 and 1333 cm−1 assigned to C-H bending (III and IV);band at 1085 cm−1 assigned to C-O-C stretching (V); togetherwith the band at 1141 cm−1, assigned to C-O bond stretching(VI) [44, 45]. The band at 1141 cm−1 also refers to the part ofthe chain where intramolecular hydrogen bonds arise betweentwo neighbouring OH groups located on the same side of thecarbon chain plane. The crystalline portion of the polymerchains affects this band at 1141 cm−1 [44]. Therefore, it ispossible to estimate the degree of crystallinity using the ratiobetween the intensity of this band (marked as VI in thespectrum of PVA) and that of the band at 1085 cm−1 (whichcorresponds to C-O-C bond stretching of the acetate groups

05101520

25

30

POLN25

POLN50

POLN75

POLN10

0WaterHexane

Wei

ght l

oss

(%)

PVA

Hexane

Water

Fig. 1 Mass loss results for the as-prepared polymer networks after 72 hin contact with aqueous or hexane solution in swelling tests

561, Page 4 of 11 J Polym Res (2014) 21:561

that remained in PVA) (marked as V in the spectrum of PVA)[44]. In this way, PVA afforded a degree of crystallinity of0.41 (or 41 %), which is similar to the value of approximately0.44 reported by Mansur [44]. X-ray diffraction (XRD) anal-ysis later confirmed the presence of crystalline domains inPVA (Fig. 5).

The infrared spectrum of PDMS-vinyl displayed the char-acteristic bands of PDMS. The main bands in the spectra are:bands in the 3000–2800 cm−1 assigned to C-H bondstretchings (VII); band at 1263 cm−1 assigned to in-planebending (scissoring) of CH3 and stretching of Si-CH3 bonds(VIII); bands at 1100–1000 cm−1 assigned to the stretching ofSi–O–Si (IX), and 805 cm−1 assigned to stretching of Si-Cbonds (X). In the spectrum only the bands relative to the vinylgroups did not appear (stretching of C = C bond at1600 cm−1), probably because the CH = CH2 terminal groups’content was low as compared to the large number of repetitiveunits constituting the macromolecule. Such difficulty in deter-mining terminal functional groups is common for polymers.Although MBA exhibits a complex spectrum, it was possibleto identify the main bands (not shown). The spectrum of theinitiator DMPAP also contained the typical bands of thecompound (not shown) [45].

Compared with the spectra of the precursors PVA andPDMS-vinyl, the spectra of the POLN samples presentedintermediate features (Fig. 2): they exhibited a less intenseor no band relative to the PVA O-H bond around 3248 cm−1

(attribution I), corroborating the hypothesis that PVA

underwent reticulation reactions. The region between 2900and 2941 cm−1 (attribution II) evidenced another difference:both polymers absorbed in this region, whereas the POLNsamples followed the profile of their major constituent. Be-cause this absorption corresponds to C-H bond stretching inboth PVA and PDMS, this band did not provide furtherinformation about the reticulation process.

Regarding the network crystallinity, the intensity of the bandat 1141 cm−1 diminished in the spectra of POLN100 andPOLN50 as compared to the spectrum of PVA; this bandcompletely disappeared in the spectra of the other samples. Theestimated degree of crystallinity of POLN100 and POLN50 was23 and 19%, respectively. This marked a reduction in crystallinedomains for POLN100 as compared to PVA (degree of crystal-linity = 41 %), evidencing that the PVA chains reticulated—thenumber of hydroxyl groups decreased, these groups interactedless, and the intersegment spaces between the chains increased,making packing difficult. Concerning POLN50, the presence ofPDMSmay have lowered the degree of crystallinity even further.

Theoretical calculations

Because (i) literature works have not described mechanismsfor the reaction between PVA and the reticulating agent MBA,and (ii) the infrared spectra did not provide definite conclu-sions, Theoretical Chemistry Tools should help to elucidatewhich products originated from the reticulation reaction. Toestablish which reactionmost probably occurred, we proposed

4000 3500 3000 2500 2000 1500 1000 500

POLN50

Tran

smitt

ance

(a.u

.)

POLN75

XIXVIIIVII

Wavenumber (cm-1)

PDMS

POLN100

VI

V

IVIII

IIPVAI

POLN25

Fig. 2 Infrared spectra of thepolymer networks POLN100,POLN75, POLN50, and POLN25and of the precursor polymers PVAand PDMS. The band assignmentsare presented in the text

J Polym Res (2014) 21:561 Page 5 of 11, 561

products for the reaction between PVA and MBA and identi-fied the most stable product. To conduct this calculation, wedelimited a region containing five monomer units in the PVAchain and assumed that it reacted with MBA to afford one ofthe six products illustrated in Fig. 3.

Initially, molecular mechanics (force field AMBER, invacuum, at 298 K) followed by quantum mechanics (PM3semiempirical method, in vacuum, at 298 K) helped to opti-mise the geometry of the molecules. Next, we determined thevariation in the standard enthalpy of formation of PVA(ΔHf°=−272.76 kcal mol−1), MBA (ΔHf°=−41.90 kcal mol−1),and the six proposed products (Fig. 3) with the aid of theprogram HyperChem 7.5 [39]. The preliminary results demon-strated that products 2 and 5 were the most stable from anenthalpic viewpoint (i.e. considering ΔHf°).

Thus, the reticulation reaction most probably occurredbetween an MBA nitrogen atom and a PVA carbon atom aftera hydroxyl group left the polymer (product 5), or between anMBA carbon atom and a PVA oxygen atom via hydrogenatom abstraction from the hydroxyl group (product 2). Quan-tum mechanics calculations were performed considering neu-tral species, since the approach including radical specieswould enhance significantly the computational costs, and therequired level of refinement for the quantum mechanics

calculations (ab initio calculations) would be significantlyhigher as well. Although the results of the proposed mecha-nism with neutral species were quite consistent with the ex-perimental infrared data, other possibilities cannot beexcluded.

To better understand the reaction, we calculated the vibra-tional frequencies of these pre-selected structures, to identifywhich structure had the minimum global energy and toobtain the reaction thermodynamic properties (programGaussian 09 W). The results corroborated the hypothesisthat product 5 (ΔHf°=−33.128 kcal mol−1) was themost stable: its reaction also had lower ΔHf° than product2 (ΔHf°=−20.858 kcal mol−1). Furthermore, the reactionleading to product 5 (ΔGf°=−12.677 kcal mol−1) was morespontaneous, because it presented more negative Gibbs freeenergy variation than product 2 (ΔGf°=−2.954 kcal mol−1).

On the basis of theoretical data, we compared the simulatedspectrum of a representative PVA oligomer with the spectrumrecorded for the PVA film. Table 1 lists the absorptionsobtained by means of the two methods. The theoretical vibra-tional frequencies agreed with the experimental data. Thedifferences between theoretical and experimental valuesstemmed from study conditions: theoretical calculations con-sidered the oligomer in a vacuum, whereas experimental data

NN

CH2

O

CH2

O

CH3CH3

OHOHOH

H H

Product 1 ∆Hf

º= -201.58 kcal mol

-1

NHNHCH2

OCH2

CH3CH3

OHOHOHOOH

Product 2 ∆Hf

º= -251.40 kcal mol

-1

NHNHCH2

OO

CH2

CH3CH3

OHOHOHOOH

Product 3 ∆Hf

º= -244.66 kcal mol

-1

OO

CH3CH3

OHOHOHO O

NN

CH2CH2

H H

Product 4 ∆Hf

º= -166.28 kcal mol

-1

CH3CH3

OHOHOHOH

N NHCH2

O OCH2

Product 5 ∆Hf

º= -254.57 kcal mol

-1

CH3CH3

OHOHOHN N

CH2 CH2

O O

Product 6 ∆Hf

º= -201.23 kcal mol

-1

Fig. 3 Possible products from thereaction between PVA and MBAand their respective ΔHf° value

561, Page 6 of 11 J Polym Res (2014) 21:561

concerned polymers in the condensed phase, which widenedthe bands relative to the calculated frequencies. Analogously,a comparison of the experimental spectrum of POLN100 withthe theoretical frequencies determined for products 2 and 5aided identification of the sample originating from the reactionbetween PVA and MBA (see Table 2).

The experimental frequencies achieved for POLN100 dif-fered from those calculated for products 2 and 5, for the samereasons discussed above in the case of PVA. Compared withproduct 2, the vibrations of product 5 agreed more closelywith the spectrum of POLN100. Therefore, in the employedexperimental conditions, the network emerged according tothe representation of product 5, originated from the reactionbetween PVA and MBA.

X-ray diffractometry (XRD)

In Fig. 4 are displayed the diffractograms of the PVA film andthe as-prepared POLN samples. All of the samples displayed a

peak at 2θ around 20°, corresponding to the (101) plane ofsemicrystalline PVA [46]. As expected, compared with theother samples, pure PVA presented a better-defined profileand more intense peaks. As mentioned previously, the crys-tallinity of reticulated PVA decreased when the hydroxylgroups reacted, reducing polymer packing and diminishingthe crystalline domains. The presence of PDMS in the samplesdid not affect sample crystallinity. Only in POLN50 did PVAexhibit lower crystallinity, probably because this was thecomposition that most favoured an interaction between PVAand PDMS.

Thermogravimetry (TG)

Figure 5 depicts the thermogravimetric curves of PVA andPOLN samples. PVA decomposition involved two main massloss steps. The first stage started at 200 °C—the polymerdehydrated and some volatile products emerged, producing aconjugated polyene. The second event occurred at approxi-mately 450 °C—polyene decomposed and carbides as well ashydrocarbides originated during the process [47]. These twomass loss steps happened for both PVA and POLN100; themaximum mass loss event temperatures took place at 270 and426 °C in the former case and at 280 and 438 °C in the lattercase. POLN100 was more thermally stable than pure PVA,corroborating reticulation reactions between PVA and MBA,albeit at a low degree. Besides, pure PVA presented a smallpeak around 100 °C, probably due to a loss of water moleculesadsorbed on the polymer surface.

The thermogravimetric curves registered for the POLNsamples revealed that increasing PDMS-vinyl content in thematerial improved thermal stability. This behaviour was ex-pected: polydimethylsiloxanes are thermally stable, because

Table 1 Comparison ofthe band positions oftheoretical and experi-mental PVA spectra

Band position / cm−1

Experimentalspectrum

Theoreticalspectrum

3246 3184

2941–2900 2961–2839

1412 1412

1323 1324

1135 1136

1087 1087

912 907

830 841

Table 2 Comparison of the band positions of experimental spectra ofPOLN100 and theoretical spectra of products II and V

Band position / cm−1

Experimental spectrum Theoretical spectrum

POLN100 Product II Product V

3230 3184 3184

2962 2961–2839/ 2959/ 2950 2960/ 2936

2914 2901 2915/ 2913

2852 2892 2855

1412 1412 1412

1327 1328/ 1320 1333/ 1316/ 1314

1091 1088/ 1087 1088/ 1080

1016 907 907

802 859/ 823 857/ 808

10 20 30 40 50 60 70 80

PVA

Inte

nsity

(a.u

.)

2

POLN 25POLN 50POLN 75POLN 100

Fig. 4 X-ray diffractograms of the as-synthesised polymer networks

J Polym Res (2014) 21:561 Page 7 of 11, 561

they contain a more ionic Si–O bond with higher energy(443.7 kJ mol−1). In general, PDMS undergoes significantthermal degradation above 400 °C [48]. Apart from the twomass loss steps mentioned above, a third mass loss eventaround 550 °C for POLN25 was observed. This loss probablycorresponded to degradation of PDMS chains, which existedin a high proportion in this sample.

Differential scanning calorimetry (DSC)

The DSC thermograms’ curves, heating and cooling curvesfrom DSC measurements are shown in Fig. 6 for PVA andPOLN25, and aided identification of the melting temperature(Tf), the crystallisation temperature (Tc), and the vitreoustransition temperature (Tg) of the samples.

For POLN25, the melting temperature lay between 214 and218 °C, close to the values described for PVA (between 220

and 267 °C). Compared with PVA, the POLN samples hadslightly lower Tf, except for POLN75. Therefore, the presenceof PDMS and/or a reticulating agent little impacted upon thePVA crystalline structure in the final material. In the sampleswith lower Tf, the PVA chains probably presented largermobility than the pure PVA film. This behaviour indicatedthat PVA underwent reticulation reactions via its hydroxylgroups, which diminished hydrogen bond interactions andcrystalline domain packing and facilitated melting [47].POLN75 Tf resembled that of pure PVA the most—asdiscussed earlier, the lower relative PVA content in this sam-ple led to less PVA reticulation in this network. Polymercrystallisation took place between 175 and 181 °C; PVApresented the highest Tc, as expected.

The Tg varied for all samples between 72 and 85 °C,agreeing with literature data (Tg, PVA=85 °C) [47]. Comparedwith PVA, POLN100 had lower Tg, at −72 °C (curve notshown). According to Krumova et al. [49], the introductionof reticulating agents influences not only crystallinity but alsospacings between the polymer chain segments in the amor-phous region, affecting Tg. Therefore, Tg helps to verify thepresence of cross-links in polymers.

0 100 200 300 400 500 6000

20

40

60

80

100

A

Wei

ght (

%)

Temperature (°C)

PVA POLN100

0 100 200 300 400 500 6000

20

40

60

80

100

Wei

ght /

%

Temperature (°C)

POLN100POLN75POLN50POLN25

B

Fig. 5 Thermogravimetric curves of (a) pure PVA film and POLN100and (b) POLN75, POLN50, and POLN25

-200 -100 0 100 200Temperature (°C)

Heating

Hea

t Flo

w /

a.u

PVA

Endo

CoolingTg

-200 -100 0 100 200

-43 -42 -41 -40 -39 -38

Hea

t Flo

w (a

.u.)

POLN25

Hea

t Flo

w (a

.u.)

Temperature (°C)

Endo Heating

CoolingTg

Temperature (ºC)

-39-42

Fig. 6 Heating and cooling curves from DSC measurements of PVA andPOLN25

561, Page 8 of 11 J Polym Res (2014) 21:561

The hydroxyl groups in PVA interact via hydrogen bonds,contributing to linear polymer rigidity. The presence of retic-ulating agents reduces the number of hydroxyls and decreaseshydrogen bond interactions, producing a less rigid polymerthan pure PVA. On the other hand, reticulation diminishessegmental mobility and increases rigidity. Therefore, reticula-tion impacts upon Tg in different ways [47]. Here, the effect offewer hydrogen bonds overcame the effect of reduced mobil-ity elicited by reticulation, as attested by the lower Tg verifiedfor the POLN samples. Again, POLN75 was the exception: itsTg did not differ from the PVA film Tg, reinforcing thehypothesis that PVA is little reticulated in this network.

PDMSmelting peaked at two points: at approximately −43and −36 °C [50]. This evidenced that two “types” of crystalemerged. POLN50 and POLN25 also presented the two en-dothermic peaks corresponding to melting of the PDMS crys-talline domains. In the inset box of the Fig. 6 both peaks canbe visualized at −42 and −39 °C for the POLN25 sample. Itwas not possible to detect these peaks for POLN75, probablybecause this sample presented low PDMS content. PDMS hadTg around −128 °C, observed in the cases of POLN50 andPOLN25 but not in the case of POLN75, which containedonly 25 % PDMS-vinyl.

Swelling experiments in water

The synthesised samples PVA, RPOL100, RPOL75,RPOL50, and RPOL25 were tested for swelling experimentsin water. The water swelling kinetics of the RPOL IPNstogether with PVA film, at room temperature, are displayedin Fig. 7. The PVA/PDMS IPNs showed a high capacity (up to80 wt.%) to absorb water and may be characterised ashydrogels. As expected, the PVA film showed a significant

swelling above the other samples due not only to its highhydrophilicity, but also to the fact that it did not undergocross-linking reactions. This allows their free OH groups tointeract with the water molecules from the solvent. Moreover,no cross-linking allows for greater mobility followed by anadjustment of the polymer chains with the water inlet.The sample RPOL100 also shows high swelling, since itis composed only of PVA. However, due to the cross-linked chains, their absorption capacity is somewhatreduced in comparison to PVA. It is also possible toobserve that RPOL75 and RPOL50 have presented al-most the same behaviour, showing that in this case, thedecrease of content of PVA did not necessarily lead to adecrease in swelling. Interestingly, the RPOL25 sampleshowed a higher swelling (close to the RPOL100),which may be due to the loss of some PDMS chainsthat were not fully cross-linked in this sample.

These results showed that IPNs based on PDMS/PVAprepared by radical polymerisation appear to be goodcandidates for utilisation as support for catalysts ingreen chemistry by using hydrogen peroxide as an ox-idant, since they are highly hydrophilic and have alsothe good stability due to the polysiloxane chains. Thepreparation of such materials, in which the catalyst isoccluded into these PDMS/PVA IPNs, is underway inour laboratory.

Conclusions

The synthesis of polymeric networks (POLN) was suc-cessfully carried out and the products can be assigned asinterpenetrating networks (IPNs). The techniquesemployed for characterisation of materials gave evidencethat both PDMS-vinyl as PVA underwent cross-linkingreactions with the synthetic procedure applied. Radicalpolymerisation allowed PVA to reticulate via binding ofthe MBA nitrogen atom to the PVA carbon linked tohydroxyl, with release of the latter group, as confirmedby theoretical studies. TG analysis revealed that lowerPVA content in the sample, that is, higher PDMS con-tent, furnished more thermally stable materials. Regard-ing the crystallinity of the samples, it was observed thatPDMS did not affect PVA crystallinity in a large exten-sion. POLN50, which had similar PVA and PDMS-vinylcontent, was an exception: PVA decreased, probablybecause the PVA and the PDMS-vinyl phases interactedmore strongly in this composition. The PVA/PDMS IPNsshowed a high capacity (up to 80 wt.%) to absorb waterand may be characterised as hydrogels, which are verysuitable for applications such as catalytic support.

0 1 2 3 4 50

20

40

60

80

100

Wat

er s

wel

ling

(%)

Time (h)

PVA POLN 100 POLN 75 POLN 50 POLN 25

Fig. 7 The water swelling kinetics of the POLN IPNs together with PVAfilm at room temperature. (Continuous lines are just a guide to the eyes)

J Polym Res (2014) 21:561 Page 9 of 11, 561

Acknowledgments This work was supported by Coordenação deAperfeiçoamento de Pessoal de Nível Superior–CAPES, ConselhoNacional de Desenvolvimento Científico e Tecnológico–CNPq andFundação de Amparo à Pesquisa de Minas Gerais–FAPEMIG.

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