Characterization of poly(vinyl alcohol)/poly(ethylene glycol) hydrogels

and PVA-derived hybrids by small-angle X-ray scattering

and FTIR spectroscopy

Herman S. Mansur*, Rodrigo L. Orefice, Alexandra A.P. Mansur

Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Rua Espırito Santo, 35/28 Andar, 30160030 Centro,

Belo Horizonte, MG, Brazil

Received 15 June 2004; received in revised form 12 August 2004; accepted 17 August 2004

Abstract

The purpose of this study is to develop novel poly(vinyl alcohol) (PVA)/poly(ethylene glycol) (PEG) hydrogel blends and PVA-derived

organic–inorganic hybrid materials and perform nanostructural characterizations. PVA and PEG hydrogels were prepared by dissolving the

polymer in aqueous solution, followed by addition of glutaraldehyde (GA) chemical crosslinker. Hybrids were synthesized by reacting PVA

in aqueous solution with tetraethoxysilane (TEOS). PVA/TEOS were also modified in the nanometer-scale by crosslinking with GA during

the synthesis reaction. Hydrogels and hybrids were characterized by using small-angle X-ray scattering synchrotron radiation (SAXS) and

Fourier transform infrared spectroscopy (FTIR). Thin film samples were prepared for SAXS experiments. SAXS results have indicated

different nano-ordered disperse phases for hydrogels made of PVA, PEG, PVA/GA, PVA/PEG. Also, PVA/TEOS and PVA/TEOS/GA

hybrids have indicated different X-ray scattering patterns. FTIR spectra have showed major vibration bands associated with organic–

inorganic chemical groups present in the hybrid nanocomposites PVA/TEOS and PVA/TEOS/GA. PVA/PEG hydrogels and PVA-derived

hybrid materials were successfully produced with GA crosslinking in nanometer-scale network.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Hybrids; Nanocomposite; Hydrogel

1. Introduction

Recently, the field of material science has witnessed the

emergence of both hydrogels and novel class of materials

called organic–inorganic hybrids. Hydrogels and hybrid

materials are of intensive interest in contemporary material

chemistry as these materials have potential applications in

biomedical devices, matrices for drug delivery systems,

carrier for cells immobilization, carrier for signaling

molecules, and bioseparation membranes [1–6]. The major

driving forces behind the intense activities in this area are

the new and different properties of these materials, which

the traditional composites and conventional materials do not

have. Hybrids would combine properties of organic

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.08.036

* Corresponding author. Tel.: C55-31-3238-1843; fax: C55-31-3238-

1815.

E-mail address: hmansur@demet.ufmg.br (H.S. Mansur).

polymers with ceramics. These different components can

be mixed at length scales ranging from nanometer to

micrometer, in virtually any ratio leading to the so-called

hybrid organic–inorganic materials. They are also termed as

‘ceramers’ and ‘ormosils’ (organically modified silicates) or

‘ormocers’ (organically modified ceramics), which are

normally nanocomposites [4]. The hybrids having such

combined characteristics of organic and inorganic sub-

stances promise new high performance or high functional

materials to fully exploit this technical opportunity with

benefits of the better of the two worlds. On the other hand,

hydrogels are three-dimensional, hydrophilic polymeric

networks capable of absorbing and retaining different

amounts of water or biological fluids. The networks are

insoluble due to the presence of chemical crosslinks

(junctions, tie-points) or physical crosslinks (crystallites,

entanglement), which permit hydrogels to be thermodyna-

mically compatible with water [7–9]. As a result, in

Polymer 45 (2004) 7193–7202

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H.S. Mansur et al. / Polymer 45 (2004) 7193–72027194

comparison to other synthetic materials, hydrogels resemble

nature living tissue closely in their physical properties due

to their high water contents and softness, which also

contribute to their biocompatibility and biodegradability.

PVA and PEG hydrogels have been widely explored as

water-soluble polymers for numerous biomedical and

pharmaceutical applications due to the advantages of non-

toxic, non-carcinogenic and bioadhesive properties [10,11].

A great variety of methods to establish crosslinking have

indeed been used to prepare hydrogels and organic–

inorganic nanocomposite systems. Water-soluble polymers

with hydroxyl groups (e.g. PVA and PEG) can be

chemically crosslinked with several reagents such as

glutaraldehyde, succinyl chloride among several others.

Fig. 1 shows an illustration of hydrogels produced by

physical and chemical crosslinking of polymer chains.

Chemical crosslinking is a highly versatile method to create

and modify polymers, where properties can be improved,

such as mechanical, thermal and chemical stability. Mostly

water-soluble polymers have been used as reagents that

would undergo physical or chemical crosslinking processes.

They can also be blended with other water-soluble polymers

and again undergo crosslinking process either physically or

chemically [10,11]. Polymer blends are produced by

physical mixing of two or more existing polymers. It is a

convenient route to develop new polymeric materials, which

combine the properties of more than one existing polymer.

This strategy is usually cheaper and less time-consuming

than the development of new monomers and/or new

polymerization routes. A wide range of material properties

can be obtained by merely changing the blend composition.

So, hydrogels based on PVA/PEG, with different cross-

linked nanostructure, create unique opportunities for con-

trolling biodegradability, pH sensitive drug carriers, and

Fig. 1. Illustrations of (a) water-soluble polymer chains; (b) hydrogel

chemically crosslinked network; (c) hydrogel physically crosslinked

network.

designing tissue engineering scaffolds. Despite of the

tremendous advances that have been made in these rapidly

growing fields some important challenges have yet to be

overcome. Most of these processes take place at nanometric

scale due to interactions between the material and the

biological system. Therefore, nanoscience and nanoengi-

neering will play a crucial role on understanding and

designing novel materials modified for specific functions.

The nanostructure of hydrogels and hybrids are very

complex and up to now are not properly understood. As a

result, the characterization of such polymeric hydrophilic

networks and hybrids in nano-order scale would allow

researchers to design new systems tailoring their properties

for different applications. In order to understand their

unusual properties, knowledge about their network is

generally required. Because of the nanometric and rather

disordered nature of precursors, intermediate materials and

final products, their structural characterization is a challenge

for material scientists. If the relevant structural features are

at a super-atomic level, from 1 to 100 nm, small-angle X-ray

scattering (SAXS) is the most broadly used technique

[10–12]. The SAXS synchrotron radiation provides statisti-

cal and overall information averaged in a volume in the

order of 1 mm3. SAXS beamlines in synchrotron radiation

laboratories provide very intense monochromatic X-ray

beams that make studies of weak scatterer materials possible

and, also, in situ analyses of structural transformations with

a high time resolution. Besides providing a high photon flux,

the nature of synchrotron radiation emission spectrum

allows one to use the effect of anomalous scattering for

many useful applications. Fourier transform infrared

spectroscopy (FTIR) can be performed in many cases

because, it is sensitive on changing the local chemical

environment, being an extremely useful complement for

scattering investigations.

In summary, poly(vinyl alcohol) and poly(ethylene glycol)

hydrogels were produced by glutaraldehyde crosslinking

reactions and polymer blending. Also, organic–inorganic

hybrids derived from poly(vinyl alcohol) and tetraethoxy-

silane were synthesized and crosslinked using glutaraldehyde

PVA/TEOS/GA. The PVA/PEG hydrogel networks and

PVA/TEOS hybrid matrices formed were characterized at

the nanosize level by SAXS and FTIR spectroscopy.

2. Experimental section

2.1. Hydrogels and hybrids synthesis

Poly(vinyl alcohol) (PVA-CPQ Chemical Industry,

Brazil) was obtained as a 90C% hydrolyzed powder with

!1% residual acetate groups and a reported average

molecular weight of 72,000 g/mol. Glutaraldehyde or 1,5-

pentanadial (Sigma-Aldrich) was obtained as a 25% (w/w)

aqueous solution. Poly(ethylene glycol) (PEG, MwZ1500 g/mol) denoted as PEG1500 was obtained from

H.S. Mansur et al. / Polymer 45 (2004) 7193–7202 7195

LabSynth, Brazil. Tetraethoxysilane Si(OC2H5)4 (TEOSO98%) was supplied by Sigma-Aldrich. 96-well polystyrene

microplates (Nunc MaxiSorp) were used as molds. Milli-Q

deionized water was used in all aqueous solutions

(18.0 MU).

PVA hydrogel was prepared by fully dissolving 5.0 g of

polymer powder without further purification in 100 ml of

Milli-Q deionized water, under magnetic stirring, at

temperature of 60G2 8C. PVA 5 wt% solution was let to

cool down to room temperature and the pH was corrected to

2.00G0.05 with 1.0 M HCl. PEG1500 hydrogel was

prepared by completely dissolving 5.0 g of polymer powder

without further purification in 100 ml of Milli-Q deionized

water, under magnetic stirring, at room temperature, and the

pH was corrected to 2.00G0.05 with 1.0 M HCl. PVA and

PEG solutions were cast into 96-well polystyrene micro-

plates, with 100 ml/well, where solidification has occurred

(24–72 h). Also, hydrogels systems based upon blending of

PVA and PEG polymers were produced by mixing equal

volumes of polymer precursors aqueous solutions (PVA/-

PEG-50/50 v/v%). PEG/PVA (50/50) solution was poured

into 96-well polystyrene microplate, where solidification

has occurred within 72 h. A schematic representation of

usual procedures to obtain hydrogels is summarized in Fig.

1. PEG or PVA water-soluble polymer chains (Fig. 1(a)),

hydrogel chemically crosslinked (e.g. glutaraldehyde) net-

work (Fig. 1(b)) and hydrogel physically crosslinked (e.g.

blending) network (Fig. 1(c)). Hybrids derived from

poly(vinyl alcohol) and tetraethoxysilane were synthesized

via aqueous routes. Under steady stirring, 5.0 ml of

tetraethoxysilane was gently added to previously prepared

PVA acid solution at temperature of 25G1 8C. PVA/TEOS

solution was poured into a 96-well polystyrene microplate,

with 100 ml/well, and allowed to solidify for 24–72 h.

Crosslinked hybrids were prepared by mixing 20.0 ml of

PVA/TEOS aqueous solution with 5.0 ml of glutaraldehyde.

The procedure was conducted under moderated stirring at

temperature of 25G1 8C. PVA/TEOS/GA solution was cast

into a 96-well polystyrene microplate, with 100 ml/well,

where gelation and solidification have occurred (24–72 h).

For SAXS experiments, PVA, PVA/GA and PVA/TEOS/

GA thin film samples were also prepared by spreading few

droplets of each solution onto microscopy glass slides,

allowing them to solidify for 24–72 h.

2.2. Hydrogels and hybrids characterization

2.2.1. FTIR spectroscopy

FTIR was used to characterize the presence of specific

chemical groups in the PVA/PEG hydrogels and hybrid

networks, reflecting the effectiveness of the developed

procedure for producing different nanostructured materials.

FTIR spectra were obtained within the range between 4000

and 400 cmK1 (Perkin–Elmer, Paragon 1000), using diffuse

reflectance spectroscopy method (DRIFTS-FTIR). Hybrids

were milled and mixed with dried KBr powder. Samples

were placed in a sampling cup and 32 scans were acquired at

2 cmK1 resolution with the subtraction of KBr background.

Samples of PVA and PEG were prepared by spreading few

droplets of the polymer aqueous solution onto ATR crystals

(attenuated total reflection Fourier transform infrared (ATR-

FTIR)) and let them dry for 24 h before FTIR

measurements.

2.2.2. Synchrotron small angle X-ray scattering analysis

The measurements of SAXS spectra were performed

using the SAS beam line of the National Synchrotron Light

Laboratory (LNLS, Campinas, Brazil). The photon beam

used in the LNLS SAXS beamline comes from one of the 12

bending magnets of the electron storage ring. The white

photon beam is extracted from the ring through a high-

vacuum path. After passing through a thin beryllium

window, the beam is monochromatized (lZ1.608 A) and

horizontally focused by a cylindrically bent and asymme-

trically cut silicon single crystal. The focus is located at the

detection plane. The reflection plane is (111), the asym-

metry angle equal to 108 in condensing mode, the energy

range is 6–12 keV (1–2 A) and the energy resolution:

(E/DE) is about 1000 for typical detector-to-sample distance

[5]. A set of slits defines the beam vertically. A position

sensitive X-ray detector (PSD) and a multichannel analyzer

were used to determine the SAXS intensity. The X-ray

scattering intensity, I(q), is experimentally determined as a

function of the scattering vector ‘q’ whose modulus is given

by qZ ð4p=lÞsinðqÞ; where l is the X-ray wavelength and q

being half the scattering angle. Each SAXS pattern

corresponds to a data collection time of 900 s. From the

experimental scattering intensity produced by all the studied

samples the parasitic scattering intensity produced by the

collimating slits was subtracted. All SAXS patterns were

corrected for the non-constant sensitivity of the PSD, for the

time varying intensity of the direct synchrotron beam and

for differences in sample thickness. Because of the normal-

ization procedure, the SAXS intensity was determined for

all samples in the same arbitrary units so that they can be

directly compared [13–15].

3. Results and discussion

Hydrogels were produced based on PVA and PEG via

aqueous route by polymer blending and glutaraldehyde

chemical crosslinking. We have also synthesized hybrids

samples via chemical reaction of organic polymer (PVA)

with silicon alcoxide (TEOS) and crosslinked by glutar-

aldehyde. TEOS hydrolysis and policondensation reactions

have occurred into poly(vinyl alcohol) acid aqueous

solution. Disc-like samples were produced with average

weight of 10G2 mg, 1.0 mm thick, and 5.0 mm diameter.

They were found to be optically transparent to visible light

and mechanically stable to be handled. A schematic

representation of the hybrid network based on

H.S. Mansur et al. / Polymer 45 (2004) 7193–72027196

PVA/TEOS/GA and typical hybrid discs are shown in Fig.

2(a) and (b), respectively.

3.1. FTIR spectroscopy characterization

We have used FTIR spectroscopy for the characteriz-

ation of PVA and PEG1500 hydrogels and hybrid materials.

Poly(ethylene glycol) or just PEG1500 samples with

molecule chemical structure (HO–CH2–(CH2–O–CH2)n–

CH2–OH) have exhibited important absorption bands from

FTIR spectroscopy measurements as shown in Fig. 3. It

was verified contributions associated with stretching of

ether groups [10–12,16], from 1050 to 1150 cmK1, with

maximum peak at nZ1150 cmK1. Characteristic alkyl (R–

CH2) stretching modes from nZ2850–3000 cmK1 were

observed [16]. Also, hydroxyl group contribution was

observed with absorption ranging from nZ3200–

3600 cmK1. It should be noted that the presence of

Fig. 2. Schematic representation of hybrid obtained based on poly(vinyl alc

nanostructured network formation (a); PVA/TEOS/GA hybrid produced with dis

hydrophilic and hydrophobic moieties in polyethylene

glycol chains (PEG1500) verified by FTIR spectroscopy

generally gives them an unique ability of to be soluble in

both aqueous and organic solvents. Such property is crucial

for drug delivery systems, where hydrophobic molecules

have to be conducted in living physiological conditions

[11]. As a consequence, PEG is widely used separated,

conjugated or blended with other polymers such as PVA.

In Fig. 4(a), FTIR spectrum of pure PVA reference sample

is shown. It clearly reveals the major peaks associated with

poly(vinyl alcohol). For instance, it can be observed C–H

broad alkyl stretching band (nZ2850–3000 cmK1) and

typical strong hydroxyl bands for free alcohol (nonbonded

–OH stretching band at nZ3600–3650 cmK1), and hydro-

gen bonded band (nZ3200–3570 cmK1) [7,8,16–19].

Intramolecular and intermolecular hydrogen bondings are

expected to occur among PVA chains due to high

hydrophilic forces. An import absorption peak was verified

ohol)/TEOS crosslinked with glutaraldehyde with an organic–inorganic

c-like shape and optically semi-transparent (b).

Fig. 3. FTIR spectra of poly(ethylene glycol). Hydroxyl vibration band

(left) and ether group absorption region (right). Diffuse reflectance mode

and KBr background.

H.S. Mansur et al. / Polymer 45 (2004) 7193–7202 7197

at a frequency of nZ1142 cmK1 (C–O, nZ1090–

1150 cmK1). According to the literature [7,8,17–19], this

vibrational band is mostly attributed to the crystallinity of

the PVA, related to carboxyl stretching band (C–O). Such

Fig. 4. FTIR spectra of (a) poly(vinyl alcohol) and (b) PVA wit

absorption band at nZ1142 cmK1 has been used as an

assessment tool of poly(vinyl alcohol) structure because it

is a semicrystalline synthetic polymer able to form some

domains depending on several process parameters [21].

FTIR spectrum of hybrid made of PVA/TEOS is showed in

Fig. 4(b). It can be observed that major vibration bands

(Si–O–Si, nZ1080 and 450 cmK1; Si–OH, nZ950 cmK1)

associated with polysiloxane (TEOS) reactions of hydroly-

sis and condensation added to PVA polymer solution. Also,

in the frequency range from 3000 to 3650 cmK1, mainly

related to hydroxyl groups [16], a broader band was noted

for PVA/TEOS hybrid spectrum (Fig. 4(b)) compared to

PVA (Fig. 4(a)). Such result is believed to be due to the

TEOS sol–gel reactions that have altered PVA chains tri-

dimensional structure. PVA molecular entanglements and

crystallinity depend on hydrophilic/hydrophobic force

balance. Hydrogen bonds play a crucial role in such

conformational arrangements, creating hydrophically

associated domains [20]. Therefore, introducing of Si–

OH and Si–O–Si through hydrolysis and condensation

reactions of TEOS has modified PVA semi-crystalline

structure. Such broad band observed on FTIR spectrum of

PVA/GA has also some contribution of physically and

chemically water incorporated during the hybrid synthesis.

These results have clearly indicated that an organic–

inorganic hybrid network was achieved based on PVA and

TEOS (Fig. 2(a)). In Fig. 5, FTIR spectra of hybrid

crosslinked PVA/TEOS/GA and PVA/GA are shown.

FTIR spectrum in Fig. 5(a) is associated with PVA

crosslinked by glutaraldehyde. It can be observed that

two important peaks at nZ2850 and 2750 cmK1 of C–H

stretching are related to aldehydes [16]. Also, strong band

h TEOS. Diffuse reflectance mode and KBr background.

Fig. 5. FTIR spectra of (a) PVA/GA; (b) PVA/TEOS/GA; (c) subtracted spectrum (a) and (b).

H.S. Mansur et al. / Polymer 45 (2004) 7193–72027198

from carbonyl group associated with aldehyde group was

verified (CaO at nZ1720–1740 cmK1). Infrared data in

Fig. 5(b) shows major vibration bands due to Si–O–Si

bonds (nZ1080 and 450 cmK1) and Si–OH bonds (nZ950 cmK1) from polysiloxanes reactions (TEOS) [4]. In

order to enlarge the peaks due to tetraethoxysilane, a

spectral subtraction involving the spectra of PVA/GA (Fig.

5(a)) and PVA/TEOS/GA (Fig. 5(b)) was performed and

the result is exhibited in Fig. 5(c). This result (Fig. 5(c))

shows peak at the inorganic region related to Si–O–Si

bonds (nZ1050 cmK1) and Si–OH bonds (nZ950 cmK1)

that are characteristic from TEOS derived materials [4].

Carbon–oxygen single bonds display stretching bands in

the region 1200–1100 cmK1. These bands are generally

strong and broad. In summary, FTIR spectra showed in

Fig. 5 have confirmed the formation of PVA/TEOS/GA

hybrids with network crosslinking. Glutaraldehyde (1,5-

pentadial) has acted as a crosslinker among polymer chains

of PVA and an organic–inorganic covalent binder. In Fig.

6(c) a schematic representation of hybrid crosslinked

nanostructure is shown. In order to establish crosslinking,

some physical–chemical conditions have to be applied, for

instance reactions occurring in low pH solution, where so-

called Schiff bases are formed [21]. Major proposed

chemical reactions are summarized in Fig. 6(a) and (b)

involving both hydroxyl functional groups from silanol and

from poly(vinyl alcohol). Organic–inorganic network of

hybrids were obtained according to the proposed reactions

of covalent bindings through PVA (Fig. 6(a)) hydroxyl

groups (C–OH) and silanol groups (Si–OH) from TEOS

(Fig. 6(a)) during the sol–gel processing in acid solution

(pHZ2.0). It should be pointed out that even though

hybrid nanostructures can also be reached by physical

entrapment of inorganic particles (SiO2) into polymer

crosslinked network (PVA/GA) [9,21,22], the chemical

route developed would give a truly nanostructured hybrid

organic–inorganic matrix. Therefore, FTIR spectra shown

in Figs. 4 and 5 have given strong evidence that the

experimental procedure developed in this work was

successful in obtaining and altering the organic–inorganic

structure of PVA, PVA/TEOS and PVA/TEOS/GA. In

addition to that, PVA/PEG hydrogel FTIR spectra showed

a most important chemical groups generally used to

characterize and evaluate polymer-blending procedures.

3.2. Synchrotron small-angle X-ray scattering study

SAXS technique from synchrotron radiation source was

used to evaluate PVA/PEG hydrogels networks and the

hybrid PVA/TEOS/GA structure at the nano-order level.

Thin films of PVA, PVA/GA and PVA/TEOS/GA deposited

onto microscope glass slides were investigated. All SAXS

results are presented as plots with experimental scattering

intensity, I(q), as a function of the modulus of the scattering

vector, q. SAXS is a particularly adequate technique to

study the structure of hybrid material because of their high

Fig. 6. (a) Chemical crosslinking of poly(vinyl alcohol) glutaraldehyde-mediated; (b) glutaraldehyde-mediated crosslinking of silanol groups; (c)

representation of hybrid nanostructured network of PVA/TEOS/GA.

H.S. Mansur et al. / Polymer 45 (2004) 7193–7202 7199

contrast in electronic densities between organic and

inorganic phases and nano-scaled structure [11,12]. In

order to analyze the SAXS results, a simple two-electron

density model was applied. Therefore, we should stress that

this model should be considered as a first approximation.

Fig. 7. SAXS spectra intensity, I(q), as a function of q: (a) PVA/TEOS/GA;

(b) PVA; (c) PVA/GA.

3.2.1. SAXS characterization of hybrids

SAXS curves have showed a quite different dependence

on vector q with the scattering intensity I(q) corresponding

to samples PVA, PVA/GA and PVA/TEOS/GA. Fig. 7

displays SAXS curves corresponding to three different

samples: Fig. 7(a) from hybrid PVA/TEOS/GA, Fig. 7(b) of

pure polymer PVA and Fig. 7(c) of PVA crosslinked with

glutaraldehyde. In Fig. 7(b), pure PVA sample has shown a

‘knee’ type curve, with a single peak with maximum located

at scattering vector qZ0.04 AK1. Such trend can be

explained by assuming a semi-crystalline structure of

PVA polymer sample. For the model of isolated domains

H.S. Mansur et al. / Polymer 45 (2004) 7193–72027200

embedded in a continuous matrix, the average distance

between domains, d, can be estimated by using the simple

equation (Eq. (1)) given by:

d Z 2p=qmax (1)

where, qmax is the modulus of the scattering vector

corresponding to the maximum of the SAXS intensity

function [12].

The average size of the domain is determined by

assuming spherical entities, with a radius R, forming a

compact arrangement. Based on Eq. (1), we would have an

average size of 15 nm for PVA nanocrystallites (qZ0.04 AK1; Fig. 7(b)). This result has strong correlation

with values reported in recent publications [1,3,6], where

the PVA crystals were found to be in the range from 7 to

20 nm, obtained by X-ray diffraction technique from PVA

based hydrogels. In spite of much research into the

microstructure and nanostructure of crystalline polymers

some details are still to be deeply investigated [20].

Hydrogen bonds are very important on stabilizing polymer

structures. Therefore, the number of hydrogen bonds present

in poly(vinyl alcohol) chain will be maximized, causing all

of them to occur in parallel sheets, with strong intermole-

cular forces, stabilizing polymer crystals [23,24]. The usual

form of such crystal domains is lamellar, occurring in thin

plates or sheets. These lamellar are typically 10–20 nm in

size. In summary, SAXS curve obtained for PVA sample

has clearly indicated the formation of nanocrystalline

domains with estimated average size of 15 nm. For that

reason, it is assumed that PVA network is made of a

crystalline phase embedded in a continuous amorphous

polymer matrix [22–24]. In Fig. 7(a), no evidence of

scattering q vector maximum peak was verified associated

with PVA/TEOS/GA hybrid sample. SAXS curve of PVA/

TEOS/GA sample (Fig. 7(a)) has presented a typical power-

law dependence on vector q [11,12] when compared to

curves of poly(vinyl alcohol) (Fig. 7(b)) and PVA modified

with GA (Fig. 7(c)). Such trend is assumed to be caused by

breaking most nano-ordered tri-dimensional organic struc-

tured previously found in the PVA sample. Due to siloxane

hydrolysis and policondensation reactions with PVA

aqueous solution, several new chemical covalent bonds

have been created (Si–C, Si–OH, Si–O–Si) reducing the

hydrogen bond formation between polymer chains. Briefly,

for PVA/TEOS/GA samples, we consider that the network

is basically composed of some multi-dispersed nanocrystal-

line PVA domains embedded into a continuous amorphous

organic–inorganic hybrid matrix. Besides that, such effect

on reducing the crystalline domains formation of PVA was

further observed by adding glutaraldehyde. The crosslinking

of polymer chains have occurred, causing less flexibility for

spatial conformational mobility. As a consequence of such

organic–inorganic nanostructure, no specific maximum

value for vector q of synchrotron radiation was detected

(Fig. 7(a)). The proposed hybrid organic–inorganic

chemical structure is shown in Fig. 6(c). The explanation

suggested in this work for the differences observed between

PVA and PVA/TEOS/GA hybrid samples were also

confirmed by crosslinking poly(vinyl alcohol) with GA.

SAXS curve shown in Fig. 7(c) clearly reveals a broader

band for the maximum value vector q, varying from 0.03 to

0.04 AK1, when compared to pure PVA (qZ0.04 AK1).

The addition of strong crosslinker agent (GA) has

‘hardened’ the PVA chain structure reducing the possibility

of hydrogen bonds formation. Therefore, less well-defined

nanocrystalline PVA domains were formed. In summary,

for PVA/GA samples, we consider that the network is

basically composed of some multi-dispersed nanocrystal-

line PVA domains embedded into a continuous amorphous

polymeric matrix. Another information about the domain

arrangement can be obtained from the width of SAXS peak

[5]. Similarly to the determination of the crystallite size in

polycrystals, a rough estimate of the average size of the

correlation domains, Lc, associated with the spatial

distribution of agglomerates, can be obtained by applying

Scherrer [5,11] equation (Eq. (2)):

Lc Z 4p=Dq (2)

where, Dq is the full width at half-maximum of the

correlation peak of the measured SAXS function.

Assuming DqZ0.01 AK1, vector q, varying from 0.03 to

0.04 AK1, we have calculated an average size LcZ120 nm

of distribution among nanocrystallites of PVA/GA. Again,

such results has confirmed the loss of crystallinity and nano-

ordered structure of PVA by crosslinking with GA.

3.2.2. SAXS characterization of hydrogels

Hydrogels based on polymer blending are quite complex

systems and have been investigated for decades [19].

Recently, the tri-dimensional distribution of semicrystalline

polymer constituents forming blended nanostructure was

studied by small-angle neutron scattering and SAXS [11,

12]. In the present study, we have investigated the

nanostructure of PVA/PEG1500 hydrogels through synchro-

tron radiation SAXS experiments. SAXS data collected for

PVA/PEG synthesized hydrogels are shown in Fig. 8. In Fig.

8, the important pattern alterations of SAXS curves from

pure PVA (Fig. 8(c)) compared to PVA/PEG blended

hydrogel (Fig. 8(b)) and PVA/PEG/GA chemically cross-

linked hydrogel (Fig. 8(a)) can be observed. The curves

showed noticeable maximum at low scattering vector q. It

can be noted that all scattering patterns have a single peak

with maximum located at approximately qZ0.04 AK1.

Such trend can be attributed to semicrystalline domains as

expected to be found on both PVA and PEG components.

However, the broadening of the q vector peaks was

obviously verified on PVA/PEG (Fig. 8(b)) and PVA/

PEG/GA (Fig. 8(a)) curves when compared to pure PVA

curve (Fig. 8(c)). Such behavior could be explained by

assuming that pure PVA has a spatial distribution between

Fig. 8. SAXS spectra intensity, I(q), as a function of q: (a) PVA/PEG/GA; (b) PVA/PEG; (c) PVA.

H.S. Mansur et al. / Polymer 45 (2004) 7193–7202 7201

crystalline domains embedded in amorphous polymeric

matrix. On the other hand, PVA blending with PEG1500 is

likely to have modified PVA such crystallinity and lamellae

packing. As reported in the literature, both PVA and PEG

have semicrystalline structures [7,10,11,17]. So, a hom-

ogenous blended system made of PVA and PEG chains is

expected to be found with an average contribution from both

polymer constituents. On a molecular level, the crystallites

of PVA and PEG can be described as a layered structure

[10–12,14,17]. A double layer of chains is held together by

hydrogen bonds while weaker van der Waals forces operate

between the double layers [10,11]. The folded chains of

PVA chains and PEG lead to crystallites, which are small,

ordered regions, in an unordered, amorphous polymer

matrix [10]. As a consequence, PEG1500 addition in PVA

would have caused a wider crystalline size distribution

resulting on the SAXS scattering pattern verified for

blended hydrogels (Fig. 8(a) and (b)). Chemical cross-

linking of PVA/PEG1500 with glutaraldehyde (Fig. 8(a))

seemed to have minor effect on altering the scattering

performance of PVA/PEG blend. Interestingly, as reported

in the literature [11], researchers found the formation of

crystallites during the dehydration and annealing of PVA

hydrogels has served as crosslinks in addition to the ones

formed through chemical reactions. Thus, the SAXS

scattering behavior presented by PVA/PEG and PVA/

PEG/GA were likely to be governed by average contribution

of blend components. That means, the overall scattering

patterns observed for PVA/PEG hydrogels are due key

factors such as lamellae size, spatial distribution, crystal-

linity in lamellar stacks and the degree of crystallinity

compared to amorphous polymer matrix, from contributions

of both PVA and PEG1500 components. Further investi-

gation based on density measurements, calorimetric

methods, and X-ray diffraction analysis would bring more

understanding on the PVA/PEG blending to nanometric

scale.

4. Conclusion

We have effectively produced PVA/PEG hydrogels and

chemically crosslinked with glutaraldehyde via aqueous

route. PVA/PEG hydrogel blends were properly character-

ized by using SAXS and FTIR spectroscopy techniques.

SAXS and FTIR spectroscopy characterizations have also

confirmed that hybrid organic–inorganic materials were

successfully obtained based on the combination of PVA and

TEOS with glutaraldehyde crosslinked nanometer-scale

network. In addition to that, SAXS synchrotron radiation

associated with FTIR spectroscopy have proven to be a

powerful tools for nanoscience investigation.

Acknowledgements

The authors acknowledge CNPq/FAPEMIG/CAPES for

financial support on this project. The authors are also

particularly grateful for the important contribution from

LNLS staff and for synchrotron SAXS facilities.

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