d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1709–1716

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Eugenol functionalized poly(acrylic acid) derivatives in theformation of glass-ionomer cements

Luis Rojoa, Blanca Vazqueza, J. San Romana, Sanjukta Debb,∗

a Institute of Polymer Science and Technology, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spainb King’s College London Dental Institute at the Guy’s King’s and St. Thomas’ Hospitals, Department of Biomaterials, Biomimetics &Biophotonics, Floor 17, Guy’s Tower, London Bridge, London SE1 9RT, UK

a r t i c l e i n f o

Article history:

Received 26 July 2007

Received in revised form

3 March 2008

Accepted 15 April 2008

Keywords:

Glass-ionomer cements

Eugenol derivatives

Copolymers of acrylic acid

a b s t r a c t

Eugenol possesses analgesic and anti-inflammatory properties with the ability to relieve

pain in irritated or diseased tooth pulp, thus, incorporating polymers with eugenol moieties

in dental cements is attractive. An acrylic derivative of eugenyl methacrylate (EgMA) was

copolymerized with acrylic acid (AA) using a radical initiator, to yield a water soluble copoly-

mer of acrylic acid and eugenyl methacrylate {p(AA-co-EgMA)}, which was then applied in

the formulation of glass-ionomer cements for potential application as dental cements. Three

concentrations of the p(AA-co-EgMA) copolymer in water were studied by, 30 wt%, 40 wt%

and 50 wt%, and used with different powder:liquid ratios to formulate the glass-ionomer

cements. The setting kinetics showed that both the concentration of the copolymer and the

powder:liquid ratio influenced the working and setting times. Thus, selected formulations

were used for further characterization of their mechanical properties, water uptake and fluo-

ride release, to optimize the cement formulation. The experimental glass-ionomer cements

exhibited physical and mechanical properties in compliance to ISO standard requirements

with the benefit of the initial pH being greater than the commercial formulation used as the

standard cement.

Furthermore, the presence of the eugenyl moieties bound to the polymer matrix was

advantageous with respect to moisture sensitivity and anti-bacterial properties.

emy

1

Emtattvii

relieve pain in irritated or diseased tooth pulp [2] antimicrobial

0d

© 2008 Acad

. Introduction

ugenol (4-allyl-2-methoxyphenol) has been used in dentalaterials since 19th century. It is currently used in combina-

ion with zinc oxide (ZOE) as temporary pulp capping agentsnd as filling materials in root canals, where eugenol func-ions as an obturation agent producing a soothing effect onhe pulp. These cements produce some adverse effects in

ivo probably due to the release of unreacted eugenol, which,n some concentrations, can produce tissue irritation andnduce inflammatory reactions over the oral mucous mem-

∗ Corresponding author. Tel.: +44 20 71881817; fax: +44 20 7188 1823.E-mail address: sanjukta.deb@kcl.ac.uk (S. Deb).

109-5641/$ – see front matter © 2008 Academy of Dental Materials. Puoi:10.1016/j.dental.2008.04.004

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

brane. The presence of free eugenol in the cements can alsocause a detrimental effect on the physical and mechani-cal properties of the overlying permanent dental compositeresins that cure mainly by free radical polymerization [1–3].However, literature reports indicate that there are numerousadvantages of eugenol such as pharmacological properties,analgesic and anti-inflammatory properties with the ability to

and anti-aggregating function [3], antipyretic activity [4] andanti-anaphylactic properties preventing mast cell degranula-tion [5]. In addition, eugenol can prevent lipidic peroxidation

blished by Elsevier Ltd. All rights reserved.

s 2 4

to set for 24 h and then stored in distilled water for 24 h priorto mechanical testing, fluoride release, water uptake and pHsurface measurements. Specimens were also aged for 6 weeksin distilled water at 37 ◦C for mechanical testing.

Table 1 – Experimental glass ionomer cementsformulated in this work

Liquid phase Solid phase Solid:liquid ratio

GC Fuji IX GP liquid GC Fuji IX GP Powder 3:1a,b

LP30 GC Fuji IX GP Powder2:1a

3:1a

4:1

LP40 GC Fuji IX GP Powder2:13:1a,b

2:1a,b

1710 d e n t a l m a t e r i a l

in the initial steps due to the presence of the phenolic group,which can scavenge free radicals [6].

Combining the properties of eugenol molecules with glass-ionomer cements is an interesting concept, especially as achemically bound entity, namely a methacrylate derivative,which is expected to imbibe anti-inflammatory effects andalter moisture sensitivity of the cement. Eugenyl methacrylate(EgMA) monomer [7] is a suitable molecule for introducing theeugenol activity in the glass ionomer formulations via copoly-merization with acrylic acid (AA). Glass ionomer cements(GICs) are self-hardening cements that are formed by the reac-tion of an ion leachable calcium fluoro aluminosilicate glasspowder and polyalkenoic acid. The setting involves neutral-ization of the acid groups available from the water-solublepolymer and the base, which is a calcium fluoro aluminosil-icate glass powder. The setting reaction occurs between thetwo phases, thus, it is heterogeneous and is sensitive to theparticle size of the glass, the concentration and the natureof the poly acid [8]. Conventional glass-ionomers suffer fromsome disadvantages such as a short working time, and aregenerally brittle in nature. Glass ionomers cements are alsosensitive to moisture in the early stages of placement and canlose matrix-forming ions in presence of excessive moistureor desiccation in case of patients who breathe through themouth. This work reports the synthesis of a new copolymerderived from acrylic acid and eugenol methacrylate, whichcan be potentially used in glass ionomer cement formulationswith improved properties in comparison with the conven-tional cements.

2. Materials and methods

2.1. Materials

Acrylic acid was purchased from Sigma–Aldrich and purifiedby distillation under reduced pressure. The monomer EgMAwas synthesized as reported previously [7]. 2,2′ Azobisisobu-tyronitrile (AIBN) (Merck) was recrystallized from methanol(mp 104 ◦C). The solvents ethanol (Scharlau) and acetone (SDS)were purified by standard procedures. Fuji IX was purchasedfrom Kent Dental, UK.

2.2. Synthesis and characterization of the copolymer

A mixture of acrylic acid and EgMA with a AA:EgMA feedmolar ratio of 80:20 was deoxygenated at room temperaturefor 15 min. The mixture was copolymerized at 60 ◦C in ethanol([M] = 1 mol/l), using AIBN (1 wt% with respect to monomers) asa radical initiator, for 24 h in order to obtain soluble products.Once the reaction time was over, the solvent was removedby flash distillation under reduced pressure and the copoly-mer was washed with acetone to remove residual monomers.Subsequently, the copolymer was dissolved in a minimumamount of water and lyophilized.

The copolymer was characterized by NMR spectroscopy

(Varian XLR-300 spectrometer) and ATR-FTIR spectroscopy(PerkinElmer Spectrum One). The molar fraction of eachmonomeric unit in the copolymer was determined from the1H NMR spectrum by using Eq. (1), where f(M1) is the molar

( 2 0 0 8 ) 1709–1716

fraction of M1 unit (acrylic acid), IM1 the integration of the sig-nal assigned to the carboxylic proton of the acrylic acid andIM2 is the integration of the signals assigned to aryl and allylprotons of the M2 unit (EgMA).

f (M1) = IM1 /N0H+

IM1 /N0H+ + IM2 /N0H+ (1)

The molecular weight of the copolymer was determinedby size exclusion chromatography (SEC). A solution was pre-pared by dissolving an appropriate amount of the polymer ina CH3CN/KNO3 buffer solution (0.1 M), to give a concentrationof 10 mg/ml, stirring it during 2 h to dissolve completely. Afterthat, the solution was then filtered through a 0.45 �m PTFEmembrane prior to the measurement. The equipment usedfor the determination was a GPC Waters 1515 Isocratic HPCLPump, with a precolumn and two columns Waters Ultrahy-drogelTM 500 and 250 maintained at 30 ◦C using a thermostat,and connected to a refractive index detector, Waters 2414.The solvent used was CH3CN/KNO3 buffer solution (0.1 M).The flow rate was 0.5 ml/min. Equipment was previously cali-brated with monodisperse poly(ethylene oxide) (PEO) samples(Waters).

2.3. Cement preparation

New GICs were prepared using different concentrations ofaqueous solutions of the synthesized copolymer as a liq-uid phase. Solutions with 30% (LP30), 40% (LP40) and 50% byweight (LP50) of the p(AA-co-EgMA) copolymer were preparedin distilled water and allowed to dissolve completely at roomtemperature. The powder component of the commercial GIC,FujiIX, was used as the basic glass in all cement formula-tions. Different powder:liquid ratios were employed and areshown in Table 1. The experimental GICs were prepared byhand mixing of the corresponding liquid phase with the GCFuji IX powder. The commercial GC Fuji IX GP cement was usedas a control formulation. The cement specimens were allowed

LP50 GC Fuji IX GP Powder 3:15:2

a Used for mechanical properties testing.b Used for water uptake and fluoride release.

4 ( 2

2

Cipsd5Tt

2

CwI5(paaw1utdo

ttwwtott

2

Hsttwloi

(wps

%

%

graphed and compared with the Fuji IX.

2.9. Statistical analysis

d e n t a l m a t e r i a l s 2

.4. Curing parameters

uring parameters were determined using a Wilson oscillat-ng rheometer [9]. A small amount of the cement dough waslaced between the plates of the rheometer and allowed toet. Working (WT) and setting (ST) times of each cement wereetermined by calculating the time taken to reach 95% and% of the initial amplitude of the oscillation, respectively.he values reported are the average of three determina-

ions.

.5. Mechanical properties

ompressive, diametral tensile and flexural strengths (FS)ere measured according to the ISO9917 standard [10]. An

nstron 1195 Universal testing machine was used with a00 N load cell. Specimens for the compressive strengthCS) and diametral tensile strength (DTS) tests were pre-ared by mixing sufficient cement, using a metal spatuland supplied mixing pad, to fill six metal cylinder moulds,ll 6 mm in height and 4 mm in diameter, which were pre-axed to prevent cement adhesion. A crosshead speed ofmm/min was applied in these tests. Samples for flex-ral strength in three-point bending were prepared byhe same method, but using bar moulds geometry (1 mmepth × 5 mm breadth × 50 mm length) and a crosshead speedf 0.1 mm/min.

The compressive strength was calculated from the equa-ion CS = P/�r2 (Eq. (2)), where P is the load at fracture and rhe radius of the sample cylinder. Diametral tensile strengthas determined from the relationship DTS = 2P/�dt (Eq. (3)),here d is the diameter and t the thickness, respectively, of

he cylinder. The flexural strength in three-point bending wasbtained using the expression FS = 3Pl/2bd2 (Eq. (4)), where l ishe distance between the two supports, b the breadth and dhe depth of the specimen.

.6. Water sorption

ydration studies were carried out by immersion of diskhaped specimens (5 mm diameter × 1 mm thickness) in dis-illed water at 37 ◦C. At appropriate times, the specimens wereaken out, the surface dried carefully with filter paper andeighed until equilibrium. The specimens after reaching equi-

ibrium were dried in the oven at 50 ◦C until constant weight, inrder to calculate the weight loss. Measurements were made

n triplicate and results averaged.The percentages of water sorption (%WS) and weight loss

%WL) were determined using Eqs. (5) and (6), respectively,here Wt is the weight at t time, Wf is the weight of the sam-le dried at 50 ◦C at time t, and W0 is the initial weight of thepecimen.

Wt − W0

WS =W0

× 100 (5)

WL = W0 − Wf

W0× 100 (6)

0 0 8 ) 1709–1716 1711

The early stages of diffusion-controlled uptake of water [11]in GICs are given by

Mt

M∞ = 2{

Dt

�l2

}1/2(7)

where Mt is the mass uptake at time t, M∞ is the equilibriumuptake, 2l is the thickness, and D is the diffusion coefficient.Diffusion coefficients can be evaluated from the slope valuesof the initial linear part of the reduced sorption curves. A plotof Mt/M∞ against t1/2 should provide a straight line with theslope, s, then given by

s = 2{

D

�l2

}1/2(8)

and the value of D can be evaluated from the slope value.

D = 2

{s2�l2

4

}(9)

2.7. Fluoride release and pH surface measurement

Specimen discs (5 mm diameter × 1 mm thickness) were usedin all the cases. Each specimen was immersed in an individ-ually capped polystyrene test tube containing 2 ml deionizedwater and stored at 37 ◦C. To avoid fluoride saturation of thesolution, the stored water was exchanged at different timesand collected for the fluoride measurement. pH was measuredon the last drop on the specimen surface before collecting witha standard pH electrode (BDH Glass+, Fisherbrand Hydrus 100)and the specimens were immersed in a new polystyrene tubewith 2 ml of fresh deionized water. An equal volume (2 ml)of buffer solution (TISAB I BDH limited Poole England) wasadded to each solution. The solution was stirred and fluo-ride concentrations were recorded in ppm using a selectivefluoride electrode (Cole Parmer 27502) connected to an ionanalyzer (OAKTON 510 ion series). Standard curves between1 ppm and 100 ppm F− were used to calibrate the electrode.Fluoride release and surface pH were measured over 14 daysand reported as pH and �g of F− per mg of material, respec-tively, as a function of time.

2.8. ESEM surface characterization

Fracture samples from mechanical testing were dried andsputter-coated with gold before examination under a ESEMapparatus (Philips XL 30) at an accelerating voltage of 15 keV.Surfaces of each of the experimental cements were micro-

Statistical analysis was performed using one-way analysis ofvariance (ANOVA) (p < 0.05). Values significantly different withrespect to the commercial GC Fuji IX GP glass ionomer, aremarked as * on the corresponding results.

1712 d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1709–1716

1

Fig. 2 – FTIR-ATR Spectra of the GC Fuji IX GC liquid phaseand the 50% by weight solution of the p(AA-co-EgMA)

Fig. 1 – H NMR spectrum of the acrylic copolymerp(AA-co-EgMA) (<10 wt%) in DMSO-d6 at 25 ◦C.

3. Results

3.1. Synthesis of the copolymer

The polymerization of the AA and EgMA was carried out asdescribed in Section 2 that yielded a white and amorphouspowder after isolation and purification. Fig. 1 shows the 1HNMR spectrum of the p(AA-co-EgMA) copolymer, where theresonance signals of the corresponding monomeric units canbe observed. The spectrum confirms that the polymerizationreaction took place through the acrylic and methacrylic groupsfor the AA and EgMA monomeric units, respectively.

The mole fraction of the monomers in the copolymer wascalculated by considering the resonance signal of the car-boxylic proton (1H; 11.5–13.5 ppm) of the acrylic acid unit, andthose assigned to the aryl protons (3H; 6.5–7.5 ppm) and allylprotons (3H; 4.8–6.4 ppm) of the EgMA. Applying Eq. (1), themolar fraction of EgMA in the copolymer chain was found tobe 0.13. This fact indicates that the macromolecular chainsconsist mainly of acrylic acid units with carboxylic protonsavailable for the characteristic acid–base reaction in the GICsformation, with 13% of pendant eugenol moieties.

Different aqueous solutions of the p(AA-co-EgMA) wereprepared and the FTIR-ATR spectra were recorded and com-pared with that of the liquid phase of GC Fuji IX. Fig. 2 shows

both spectra where the characteristic signals of the O–H (str-AA) at 3300 cm−1 (broad), C O (str-AA) at 1700 cm−1, C C (Aryl)at 1500 cm−1 and C–O (str-AA) at 1230 cm−1 can be observed.

Table 2 – Compressive strength (CS), diametral tensile strengthwith their standard deviations (S.D.), of the different GICs at 24

Liquid phase 24 h

Solid:liquid CS (MPa) (S.D.) DTS (MPa) (S.D.) FS (

GC FUJI IX GP 3:1 (com) 165.6 [10] 18.2 [2.5]LP50 2:1 136.4* [6,2] 14.6* [3.5]LP40 3:1 135.7* [14] 12.3* [2.5]

∗ Statistically different with respect to GC FUJI IX GP (p < 0.5).

copolymer (LP50) in water.

Average molecular weight of the p(AA-co-EgMA) copolymerwas determined by gel permeation chromatography using arefractive index detector. A progressive increase was observedin the molecular weight for the latter runs which can be pre-sumed to be the blocking of the active sites and hence thedecrease in absorption. This fact could indicate an overesti-mation on the molecular weights. The calculated molecularweight averages based on several runs were Mw = 156,000 andMn = 138,000, Mw/Mn = 1.1.

3.2. Cement preparation and curing parameters

The glass-ionomer cements using the liquid component pre-pared from the copolymer were formulated by mixing thepowder of a commercial formulation, namely Fuji IX. Thecements were mixed using similar techniques to those usedfor conventional GICs. GICs formulated are summarized inTable 1 and curing parameters were determined for all thecement compositions. It was observed that the setting times(ST) of the experimental cements showed an increase withincreasing copolymer solution concentration, however, theworking times (WT) were in the range of that of commercial

formulation. Fig. 3 shows setting and working times for LP30,LP40 and LP50 at solid:liquid ratios 2:1 (Fig. 3a) and 3:1 (Fig. 3b),respectively.

(DTS) and flexural strength (FS) at three-point bending,h and 6 weeks

6 weeks

MPa) (S.D.) CS (MPa) (S.D.) DTS (MPa) (S.D.) FS (MPa) (S.D.)

1.6 [0.5] 223.6 [0.7] 15.1 [1.2] 0.8 [0.14]1.9 [0.6] 140.1* [7.8] 14.0 [1.3] 1.8* [0.2]4.2* [1.0] 130.1* [9.6] 13.9 [1.7] 2.3* [0.3]

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1709–1716 1713

Fig. 3 – Setting and working times calculated from theWL

3

TtusGbdvIspcI

3

Wp

3.5. Fluoride release and pH surface measurement

All reported materials exhibited a similar pattern of fluoriderelease, characterized by a strong initial release in the first 2

ilson rheometer oscillograms (n = 6) for LP30, LP40 andP50 at solid:liquid ratios 2:1(a) and 3:1 (b).

.3. Mechanical properties

able 2 summarises average values and standard deviations ofhe compressive strength, diametral tensile strength and flex-ral strength values obtained for the specimens tested aftertored in distilled water for 24 h and 6 weeks. LP40 and LP50ICs presented CS values statically lower than those of Fuji IXut maintained over the 130 MPa required by the ISO 9917 stan-ards, after the 6 weeks of immersion in distilled water. DTSalues for LP40 and LP50 GICs present lower values than FujiX but non-statistical differences were observed after 6 weekstored in distilled water. However, flexural strength at three-oint bending was higher for both experimental GICs than theommercial one. Statistical differences with respect to the FujiX were shown after 24 h and 6 weeks stored in water.

.4. Water sorption

ater sorption (%WS) of the GICs was recorded at differenteriods of time during 5 weeks and the values plotted against

Fig. 4 – Water sorption characteristics of the experimentalGICs and commercial GC Fuji IX as a function of time.

time are shown in Fig. 4. Equilibrium was attained after 6 days,where the amount of water uptake was considered to be con-stant. At that time, the commercial GC Fuji IX showed a similarbehavior to LP40, with values of %WS of 2.4 and 2.7, respec-tively, these values being much higher than that obtained forLP50 (%WS = 1.7). Weight loss of the samples was determinedafter they had reached equilibrium uptake. Plots were foundto be linear at the early stages, before 60% of the water uptakewas reached. In order to determine the diffusion coefficientsthe water uptake curves were replotted using the data fromthe early stages of sorption and the slopes of the initial linearpart (Fig. 5) were used in Eq. (8) and the values are shown inTable 3.

Fig. 5 – Reduced sorption curves for experimental andcommercial GICs.

1714 d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1709–1716

Table 3 – Equilibrium water sorption (WS), weight loss (WL) and calculated slopes (s) of the reduced sorption curves ofthe experimental GICs and the commercial Fuji IX

GIC formulation WS (%) WL (%) S (s−1/2)

GC Fuji IX GP Liquid GC Fuji IX GP PowderLP40 GC Fuji IX GP PowderLP50 GC Fuji IX GP Powder

Fig. 6 – Fluoride release from experimental GICs in 12 days

period.

days (Fig. 6). Thereafter, the release rates decreased with timeuntil they reached an asymptotic tendency to the equilibriumover the initial first week.

pH was measured on the GIC surfaces that showed arelatively constant value, however, the pH values for theexperimental cements LP50 and LP 40 were statistically signif-icantly higher than FujiIX. The pH for FujiIX cement increased

with time but was lower in comparison to the experimentalcements over the measurement period Fig. 7.

Fig. 7 – pH surface values and standard deviations, of theFuji IX, LP40 and LP50 GICs as a function of time.

2.4 4.3 0.0422.7 6.0 0.0331.7 9.0 0.030

3.6. ESEM surface characterization

Scanning electron micrographs of the fracture surface of thecements Fuji IX, LP40 and LP50 are shown in Fig. 8, show-ing slight differences among them. The glass particles of thematrix can be distinguished by the presence of angular edgesof the areas as a result of the acid–base reaction.

4. Discussion

Current GICs have addressed several clinical challenges facedby the dentist such as adhesion to tooth tissue, aestheticsand durability to an extent. Fluoride release occurs in GICs,which is well documented as an anticariogenic agent that mayaffect bacterial metabolism under simulated in vitro cariogenicconditions. However, the reduction of the incidence of sec-ondary caries as a direct result of fluoride release from GICs areyet to be proven conclusively using prospective random clin-ical studies [12]. In this study we describe the developmentof GICs with chemically bound eugenol moieties that mayfurther enhance the bacteriostatic and therapeutic potentialof the cements. Eugenol is known for its pharmacologicaluse [2,3] and the incorporation of eugenol derivatives in newcement formulations, which maintain the physical, chemicaland biological requirements of GICs in addition to bacterio-static, analgesic and anti-inflammatory effects is of interest.In the experimental cements the eugenyl residue is cova-lently linked to the macromolecules, thus, these systems donot present the inhibitory effects characteristic of the phe-nol derivatives and therefore, they are compatible with thepolymerization of other resinous materials [13]. NMR analy-sis confirmed than the eugenol moieties are linked throughan ester bond and they could be hydrolyzed and delivered insmall amounts to the media, this active molecule in long timeperiods would reach concentrations below the cytotoxic ones[14].

It has been reported that EgMAs derivatives yield cross-linked materials [7] due to the participation of the allylicdouble bound in the polymerization. Under the experimentalconditions applied in this work, the allylic double bonds didnot participate in the polymerization remaining free to do soin potential postcross-linking reactions such as light activatedpolymerization. This would promote some extra structuralstabilization and a continuous interphase with other mate-rials, consequently with higher homogeneity. Thus, thesecements are being investigated as light cured GICs atpresent.

Data reported in this work so far show that the exper-imental GIC cements, LP40 3:1 and LP50 2:1 have curingparameters, which comply with the standards and similarto other commercial formulations. The first stage of setting

d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1709–1716 1715

F re sL

goofiwocudmdtTtbastpttwbaltpcbdiG

[u

ig. 8 – Scanning electron micrographs of compressive fractuP40 (b) and LP50 (c).

enerally regarded as the time at which the cement hardens,ccurs within the first 15 min from mixing, whereas the sec-nd stage is more complex wherein maturation occurs. Therst stages of the setting reaction are sensitive to water uptakehile the latter stage is more susceptible to dehydration, bothf which restrict the full potential of the GICs [15]. The modifi-ation of the ionic polymer with the inclusion of a hydrophobicnit is expected to have an effect on both hydration andehydration events. In controlling the properties through theodification of the ionic polymer, the synthesized eugenyl

erivative copolymer showed that the copolymer composi-ion and molecular weight did influence the setting kinetics.he amount of acrylic acid units on the copolymer influenced

he rate of reaction and as expected, the less number of car-oxylic acid present in the new eugenol copolymers exhibitedretarding of the reaction with the basis glass in compari-

on with the commercial formulation. It is interesting to notehat despite the high-molecular weight determined for the(AA-co-EgMA) copolymer, the setting times were a bit higherhan that of the polyacid employed commercially. It implieshat high molecular weights of the polymers may still be usedithout detrimental effects on the setting times as reported

y other authors [16]. The concentration of the copolymerlso influenced the rate of reaction as expected. In formu-ating the liquid phase, both concentration and viscosity ofhe phase are important and a suitable balance is required torovide optimum viscosity and properties. In general, higheroncentrations of polymers yield higher cement strengthsut also increase viscosities and so, they lead to handlingifficulties. A higher concentration of the copolymer exhib-

ted shorter working times as is observed with experimental

ICs.

Water acts as a plasticizer in the bulk polymer structure17]. Slopes of the initial parts of the sorption curves and waterptake curves show that water was absorbed more rapidly and

urfaces (a) and diametral fracture surfaces (b) for Fuji IX (a),

in higher amounts in commercial GIC than the experimentalLP40 and LP50. The eugenyl residue increases the fraction ofthe volume surrounding the macromolecules providing moreflexible materials [18]. This balance makes experimental glassionomers materials with a marked increase in the flexuralstrength values keeping the other mechanical properties overthe standard requirements. As the net amounts of moistureuptake is lower, less contraction volumetric change can beexpected [19] thus, lowering the risk of failure in the eugenylGICs derivatives.

The incorporation of the eugenol molecules in thepoly(acrylic acid) copolymer results in the net decrease ofthe amount of −COOH groups in the matrix that retain thefluoride ions. Fluoride release is controlled by the hydra-tion of the matrix, pH of the leachables and amounts ofCOOH groups. Thus, it is difficult to make a direct compar-ison of the values of F− with the control group. Hence adirect comparison was not carried out but the data from thecumulative fluoride release confirmed that the experimen-tal cements had comparable release. Furthermore, as the pHvariation in the experimental cements is different than thecontrol cement, it is also expected to influence the releaseof fluoride. The higher release of fluoride during the initialstages is expected to improve the anticariogenic effects [20].The release of fluoride and the presence of the eugenyl moi-eties, that can act as an immobilized bacteriostatic group[20,21] will improve the overall bacteriostatic properties of thecement.

The surface pH values of the experimental cements werehigher than the control FujiIX cement throughout the mea-surement period, which may be due to the lower net amount

of −COOH groups in the copolymer and essentially thesebeing neutralized in the early stages of the reaction. The risein pH was more evident in the FujiIX cement, whereas theexperimental cements were fairly constant. The higher pH

s 2 4

r

[21] Brook IM, Hatton PV. Glass-ionomers: bioactive implant

1716 d e n t a l m a t e r i a l

is expected to reduce irritation of the tooth pulp. Thus, thehigher pH and increase in fluoride release with the propertiesof eugenol may impart improved tissue response, however, thebactericidal effects need.

Scanning electron micrographs of the cements wererecorded after fracture. The glass particles could be clearlydistinguished from the matrix from the presence of the angu-lar edges of the areas in the micrographs of Fuji IX (Fig. 8).The edges of the glass particle appear to be eroded as aresult of degradation of the acid–base reaction. In general,the microstructure shows that partially degraded glass par-ticles are embedded in a matrix of calcium and aluminiumpolyalkenoates and sheathed in a layer of siliceous gel proba-bly formed on the boundaries of the glass particle. Hatton andBrook [21,22] have earlier confirmed this model for conven-tional GICs.

5. Conclusion

The viability of a novel acrylic copolymer with immobilizedeugenol moieties to be used in GICs has been described.Aqueous solutions of the p(AA-co-EgMA) copolymer were ableto react with aluminosilicate glasses to form polyalkenoatecements. The cement formed with the 40 wt% and 50 wt%aqueous solutions compared with conventional GICs showedgood parameters according with the standards to be used asdental water-based cements. The longer setting times suggesta slower rate of the cement formation but the hardening isproduced in a period of time that allows the manipulation ofthe glass-ionomer formulation and its application adequately,with good mechanical properties of the hardened compositeand excellent homogeneity. Finally, p(AA-co-EgMA) copolymershowed improved properties in terms of its long period anti-cariogenic properties as well as the activity of the eugenylmoieties.

Acknowledgment

Financial support from the Comision Interministerial deCiencia y Tecnologıa, CICYT (MAT2004-01654) is gratefullyacknowledged.

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( 2 0 0 8 ) 1709–1716

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