ARTICLE IN PRESS
Biomaterials 26 (2005) 6449–6459
www.elsevier.com/locate/biomaterials
Effects of resin hydrophilicity on water sorption and
changes in modulus of elasticity
Shuichi Itoa, Masanori Hashimotob, Bakul Wadgaonkarc, Nadia Svizerod, Ricardo
M. Carvalhoe, Cynthia Yiuf, Frederick A. Rueggebergg, Stephen Foulgerh, Takashi Saitoa,
Yoshihiro Nishitanii, Masahiro Yoshiyamai, Franklin R. Tayf, David H. Pashleyc,
a
Department of Operative Dentistry and Endodontology, School of Dentistry, Health Sciences University of Hokkaido, Sapporo, Japan
b
Division of Pediatric Dentistry, Hokkaido University, Graduate School of Dental Medicine, Sapporo, Japan
c
Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA, USA
d
Department of Operative Dentistry, School of Dentistry, University of Sagrado Caracao, Bauru, SP, Brazil
e
Department of Restorative Dentistry and Endodontics, Bauru School of Dentistry, University São Paulo, Bauru, SP, Brazil
f
Pediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Pokfulam, Hong Kong, SAR, China
g
Department of Oral Rehabilitation, School of Dentistry, Medical College of Georgia, Augusta, GA, USA
h
School of Materials Science and Engineering, Clemson University, Clemson, South Carolina, USA
i
Department of Operative Dentistry, School of Dentistry, Okayama University, Okayama, Japan
Received 25 February 2005; accepted 17 April 2005
Available online 8 June 2005
Abstract
As acidic monomers of self-etching adhesives are incorporated into dental adhesives at high concentrations, the adhesive becomes
more hydrophilic. Water sorption by polymers causes plasticization and lowers mechanical properties. The purpose of this study was
to compare the water sorption and modulus of elasticity (E) of five experimental neat resins (EX) of increasing hydrophilicity, as
ranked by their Hoy’s solubility parameters and five commercial resins.
Methods: After measuring the initial modulus of all resin disks by biaxial flexure, half the specimens were stored in hexadecane and
the rest were stored in water. Repeated measurements of stiffness were made for 3 days. Water sorption and solubility measurements
were made in a parallel experiment.
Results: None of the specimens stored in oil showed any significant decrease in modulus. All resins stored in water exhibited a
time-dependent decrease in modulus that was proportional to their degree of water sorption. Water sorption of EX was
proportional to Hoy’s solubility parameter for polar forces (dp ) with increasing polarity resulting in higher sorption. The least
hydrophilic resin absorbed 0.55 wt% water and showed a 15% decrease in modulus after 3 days. The most hydrophilic experimental
resin absorbed 12.8 wt% water and showed a 73% modulus decrease during the same period. The commercial resins absorbed
between 5% and 12% water that was associated with a 19–42% reduction in modulus over 3 days.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Bis-phenol a derivative; Hydrophilicity; Hydroxyethyl methacrylate; Elasticity; Water
1. Introduction
Corresponding author. Tel.: +1 706 721 2033;
fax: +1 706 721 6252.
E-mail address: dpashley@mail.mcg.edu (D.H. Pashley).
0142-9612/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biomaterials.2005.04.052
As dentine adhesives increase in hydrophilicity, they
can absorb considerably more water [1,2] than the
original enamel adhesives or pit-and-fissure sealants that
are composed of comparatively more hydrophobic
resins [3]. Because of small molecular size and high
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
molar concentration of water, it can penetrate into
nanometer-size free volume spaces between polymer
chains [4,5], or cluster around functional groups that are
capable of hydrogen bonding [6,7]. The major effect of
water on polymer matrices is a depression of glasstransition temperature (Tg), that results in a decrease in
thermal stability and polymer plasticization. These
changes occur by different mechanisms, depending on
the level of interaction of sorbed water molecules with
the polymer matrix. Water sorption may deteriorate
polymer mechanical properties, such as the modulus of
elasticity, yield strength and produce changes in yield/
deformation mechanisms. Sorbed water may also result
in hygrothermal degradation during aging (such as the
formation of swelling stresses, microcrack and craze
formation), degradation of the matrix/fiber or matrix/
filler interfaces, and polymer chain scission through
hydrolytic cleavage [8–10]. It is thus anticipated that if
the adhesive layer coupling resin composite to hybridized dentine becomes less stiff over time due to water
sorption, it may adversely affect stress distribution
across the bonded interface possibly resulting in
debonding under repeated loading.
Water sorption within polymer matrices created by
contemporary hydrophilic dentine adhesives is not
always uniform. Using ammoniacal silver nitrate to
trace the distribution of water sorbed, Tay et al. have
shown both uniform and non-uniform water uptake into
commercial adhesive resins [11–13]. Uniform absorption
was seen as isolated individual silver grains, while the
non-uniform type formed linear, branched water-filled
channels [12]. When dentine bonded with adhesive resins
was stored in water for 12 months, the distribution of
absorbed water increased dramatically [13]. Clearly,
water sorption by dental resins and the hybrid layer is
more complex than expected [14].
Data on the effect of water uptake on the mechanical
properties of polymers has been based mostly on epoxybased systems. However, little is known of the water
sorption characteristics in hydrophilic methacrylatebased resins that are commercially employed as adhesives for bonding to hydrophilic substrates such as
dentine. Hydrophilic and/or ionic resin monomers are
incorporated in most contemporary dentine adhesives to
enable them to bond to intrinsically wet dentine
substrates. In order for self-etching primers and
adhesives to diffuse through smear layers and demineralize the underlying dentine, they are rendered more
acidic by increasing the concentration of ionic or acidic
monomers. Moreover, the correlation between resin
hydrophilicity and water sorption has not been convincing in previous studies as a systematic method for
calculating the polarity of the resin systems has not been
utilized [5,10,15,16].
The purpose of this work was to study the effects of
water sorption on the elastic modulus of a series of
unfilled resins of known composition as well as dental
resins sold commercially. These resins included both
hydrophobic and hydrophilic monomers, from which
the relative hydrophilicity could be determined and
expressed in terms of Hoy’s solubility parameters. It was
hypothesized that resins having higher Hoy’s solubility
parameter for polar forces result in (1) higher water
sorption values and (2) decreased elastic modulus.
2. Materials and methods
The composition of the five experimental neat dental resin
blends and their Hoy’s solubility parameters are listed in
Table 1 along with those of five commercial resins, and the
monomer structures are shown in Fig. 1. Hoy’s solubility
parameters (d) were calculated by summing the molar
attractive constants of each repeating functional group in the
polymers according to the method of Van Krevelen [17] and
Barton [18]. These intermolecular attractive forces can be
categorized as either polar forces (dp ), hydrogen bonding
forces (dh ), or dispersive forces (dd ). The square root of the
sum of these squares of forces yields the total cohesive energy
density (dt ) that is equivalent to Hildebrand’s solubility
parameter [8]. The dt values are included in Table 1 for those
who prefer to use the Hildebrand solubility parameter.
The experimental resins were formulated by Bisco Dental
Products Co. (Schaumburg, IL, USA). The commercial resins
were Excite (Ivoclar/Vivadent, Schaan, Liechtenstein), Scotchbond Multi-Purpose adhesive (MP, 3M-ESPE, St. Paul, MN,
USA), One-Up Bond F (Tokuyama Corp., Tokyo, Japan),
Xeno III (L.D. Caulk, Milford, DE, USA) and Clearfil SE
Bond (Kuraray Medical Inc., Osaka, Japan).
2.1. Specimen preparation
The experimental comonomer mixtures were used as neat
solutions. They were placed in wells made in teflon molds to
form disks 6.070.1 mm in diameter and 0.570.02 mm thick.
The surface of the comonomers was covered with a glass cover
slip to exclude atmospheric oxygen, forming a flat surface, and
the resin was light-cured for 30 s using a dental curing light
(VIP unit, Bisco, Schaumburg, IL, USA) operated at 600 mW/
cm2 with the tip held 1 mm from the cover slip. After removing
the disk from the mold, a similar light exposure was applied to
the lower disk surface. Specimen dimensions to the nearest
0.01 mm were measured using a digital micrometer. Ten resin
disks were made for each of the five experimental resin blends
and for the five commercial resins. Two of the commercial
adhesives (Clearfil SE Bond and Scotchbond Multi-Purpose)
were solvent-free and were used as supplied. One-Up Bond F
contains bound water that cannot be evaporated (unpublished
observation). It was used as dispensed. Excite contains only
20% ethanol/water while Xeno III contains 30% ethanol/
water. These solvents were evaporated with an air syringe just
prior to disk formation. The polymerized disks were stored dry
for 24 h at 23 1C and then placed over anhydrous calcium
sulfate (Drierite, Fisher Scientific, Atlanta, GA) for another
24 h at 23 1C prior to obtaining initial dry weights.
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6451
Table 1
Composition and Hoy’s solubility parameters of resin blends used
Resin blend
Composition
Wt%
Hoy’s solubility parameters (J/cm3)1/2
dd
dp
dh
dt
1
E-BisGMA
TEGDMA
CQ
EDMAB
70.00
28.75
0.25
1.00
15.4
10.2
4.5
19.0
2
BisGMA
TEGDMA
CQ
EDMAB
70.00
28.75
0.25
1.00
14.2
12.1
9.5
21.1
3
BisGMA
HEMA
CQ
EDMAB
70.00
28.75
0.25
1.00
14.0
13.1
10.8
22.0
4
BisGMA
TEGDMA
TCDM
CQ
DMABA
40.00
28.75
30.00
0.25
1.00
14.3
12.2
8.5
20.7
5
BisGMA
HEMA
2MP
CQ
EDMAB
40.00
28.75
30.00
0.25
1.00
14.5
13.7
9.7
22.3
Excite
SBMP
SE bond
One-Up bond
Xeno III
BisGMA/HEMA/MA-154
BisGMA/HEMA/CQ/DPHF
BisGMA/HEMA/MDP/CQ
E-BisGMA/MAC-10/HEMA
UDMA/HEMA/Pyro-EMA-SK
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Abbreviations:- 2 MP: Bis[2-(methacryloyloxy)ethyl] phosphate; Bis-GMA: bisphenol A diglycidyl ether dimethacrylate; Bis-GMA-E: ethoxylated
bisphenol A diglycidyl ether dimethacrylate; CQ: camphorquinone; EDMAB: ethyl N,N-dimethyl-4-aminobenzoate; HEMA: 2-hydroxylethyl
methacrylate; DMABA: dimethylaminobenzoic acid; TEGDMA: triethylene-glycol dimethacrylate; TCDM: di(hydroxyethylmethacrylate) ester of 5(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride; MDP-10: 10-methacryloyloxydecamethylene phosphoric acid;
MA-154: acryloyloxyethyl phosphoric acid; UDMA: dimethacryloyloxyethyl 2,2,4(3,3,5)-trimethylhexamethylene dicarbamate; MAC-10: 10methacryloyloxydecamethylene malonic acid; dd : dispersion component; dp : polar component; dh : hydrogen bonding component; dt : total cohesive
energy density value.
2.2. Monomer conversion
To ascertain that the increase in water sorption of the more
hydrophilic resin blends was not caused by their lower extent
of cure, the monomer conversion of the experimental
comonomer formulations was measured using infrared spectroscopy, according to the method of Rueggeberg et al. [19].
Briefly, cured specimens were prepared by curing thin films of
the five resins. A 6 mm hole was punched in a piece of singlesided Scotchtape, that was placed directly over the 2 2 mm
diamond crystal of a horizontal diamond attenuated total
reflectance unit (Golden Gate-SPECAC, Inc., Woodstock,
GA). As the tape was 5071 mm thick, the hole provided a
convenient ‘‘well’’ into which was placed 5 mL of each neat
resin blend. The fluid was covered with a thin Mylar film
which, in turn, was covered by a glass slide. The resin was
cured as described above, but using a 120 s exposure. Multiple
scans were made before and after light exposure at 2 cm1
resolution between 1680 and 1550 cm1 at a rate of one scan/s
for 305 s, using an Fourier Transform Infrared spectrophotometer (F75-40, Bio-Rad Laboratories, Cambridge,
MA, USA). The degree of conversion was calculated using
changes in the molar ratios (represented as peak absorbance
height) of aliphatic (1636 cm–1)/aromatic (1608 cm–1) carbon
double bonds on the cured (C) and uncured (U) states.
Conversion was calculated by using the following equation
[19]:
% Conversion ¼ ð1 C=UÞ 100.
(1)
2.3. Modulus of elasticity (modulus)
The polymerized specimens in each group (Section 2.1) were
stored in air for 24 h. The modulus of each specimen was
measured in biaxial flexure using a universal testing machine
(Model 5844, Instron, Canton, MA, USA) by load–displace-
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H2C
CH3
CH3
CH3
C
CH3
COOCH2CHCH2O
C
C
C
CH2CH2
O
C
OCH2CHCH2OOCH2
CH3
OH
H2C
CH2
OH
O
OH
HEMA
BisGMA
O
CH3
H2C
C
CH3
CH3
O CH2CH2
O
C
n
CH3
CH3
O
CH2CH2
C
O
CH2
H2C
n
HO
C
COOCH2CH2OOC
CH
CH3
CH3
COOCH2OOCHCHNHCH2
C
C
H2
C
H2
C
CH3
OH
C
CH2
COOCH2CH2OC
TCDM
CH3
C
C
CH2
E-BisGMA
H2C
O
CH3
C
NHCOOCH2
H2
C
CH3
CH2
CH2
C
CH3
O
C O
CH2CH2
O
P
O
CH2CH2
O C
C
CH2
OOC
OH
O
O
CH3
2MP
UDMA
CH3
CH3
O
CH2OCC
CCH2O(CH2)2
P
OH
O
OH
H2C
C
COOH
COO(CH2)10
CH
H2C
CH3
CH3
C
C
C
O
O
(CH2CH2O)
3
O
CH2
C
O
COOH
MA -154
MAC -10
TEGDMA
Fig. 1. Chemical structure of the six methacrylate monomers used in this work. Abbreviations: BisGMA ¼ 2,2-bis(4,2-hydroxy-3-methacryloyloxy
propoxyphenyl) propane; E-BisGMA ¼ ethoxylated BisGMA; TCDM ¼ dihydroxy ester of 3a,4,5,7a-tetrahydro-7-methyl-5 (tetrahydro-2,5-dioxo3-furanyl)-1,3-isobenzofurandione; 2MP ¼ Bis[(2-methacryloyl)ethyl] phosphate; HEMA ¼ hydroxyethyl methacrylate; TEGDMA: tri-ethyleneglycol dimethacrylate. MAC-10 ¼ 10-methacryloyloxydecamethylene malonic acid; UDMA ¼ urethane dimethacrylate; MA-154 ¼ 2-[4-(dihydroxyphosphoryl)-2-oxabutyl) acrylate.
ment. After determining the linear portion of the load–displacement curve at 1 mm/min, all subsequent modulus measurements were made by recording the load that developed when
the specimen was displaced vertically by 0.125 mm. The
modulus of elasticity (E) was calculated assuming the
configuration of a circumferentially supported resin disk
(Fig. 2) by using a modification of the method of Kirstein
et al. [20],
As each specimen was subjected to eight biaxial flexure
measurements, a separate set of disks was prepared from resin
#5 (the most hydrophilic resin) to determine if less frequent
testing affected the results. This set of resin disks was
incubated for the same period but was only flexed twice: once
at the beginning and again at the end of the incubation time.
2.4. Water sorption
KLa2
E¼
,
yt3
(2)
where E is the modulus of elasticity (MPa), K is the load
support constant, L is the applied load (N), a is the disk radius
(mm), y is the vertical displacement (mm) and t is the disk
thickness (mm). The disks were then placed in separate capped
bottles containing either 10 ml of hexadecane (Fisher Scientific,
Atlanta, GA) or deionized water at 25 1C. Modulus measurements were repeated for each specimen after 15, 30, 60, 180,
720 min, and at 1 and 3 days of incubation. At each time
period, the disk was removed from the medium, tested while
still moist, and returned to its storage medium after only a few
minutes.
Water sorption was measured using a separate set of resin
disks made exactly as those used for measuring modulus
following the method outlined in ISO 4049 (12–1998).
However, disk diameters were 6 mm instead of 15 mm, to
match the dimensions of specimens used for testing modulus.
Water sorption (expressed as weight %) was calculated as the
difference between the dry mass of the resin disk before water
sorption and after each immersion time until reaching a
plateau (i.e. maximal water sorption). Solubility was calculated
as the difference in dry mass before immersion and after
reaching the water sorption plateau, following drying in a
sealed chamber filled with anhydrous calcium sulfate [21]. Disk
volume was determined by measuring diameter and thickness
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
Direction of piston movement
φ2.25
Lip restricting
circumferential
deflection
Test sample
φ 6.0
φ1.58
Self leveling
half bearing
φ6.98
φ5.16
Hollow well into
which specimen
rests
6453
fluorescent light to facilitate reduction of the diamine silver
ions into metallic silver particles. Cross-sections (1 mm thick)
were cut from each disk using a diamond-impregnated copper
disk (Isomet Saw, Buehler Ltd., Lake Bluff, IL). Each crosssection was embedded in Epon 812 (Electron Microscopy
Sciences, Haffield, PA) as previously described [12]. After
curing the embedding epoxy, the methacrylate resin disks (i.e.
0.5 mm thick) supported on both sides with epoxy resin were
cut into 90 nm thick sections using an ultramicrotome (Reichert Ultracut S, Leica, Vienna, Austria). The ultrathin sections
were examined unstained, using transmission electron microscopy (CM100, Philips, Eindhoven, The Netherlands) operating at 80 kV.
3. Results
3.1. Monomer conversion
Fig. 2. Schematic showing the resin disk supported circumferentially
and loaded in the center.
before and after water exposure. Dry weight measurements
were followed daily for 10 days. The values (%) for water
sorption (WS) and solubility (SL) were calculated as
WS ¼ M 2 M 1 =V ,
(3)
SL ¼ M 1 M 3 =V ,
(4)
where M 1 is the initial dry constant mass (mg) before water
immersion; M 2 is the mass (mg) after water immersion; M 3 is
the mass (mg) after drying specimens that had reached their
maximum water sorption and V is the specimen volume in
mm3. Net water uptake was calculated as the sum of water
sorption and solubility [3].
2.5. Statistical analyses
One-way ANOVA was used among test values of each
parameter. If differences were found, pair-wise testing was
performed using the Student Newman–Kuels multiple comparison test. Regression analyses were used to examine the
relationship between water sorption and the different Hoy’s
solubility parameters of the experimental resins, and between
changes in the resin modulus and water sorption. For all
analyses, statistical significance was pre-set at a ¼ 0:05.
2.6. Transmission electron microscopy
Two additional resin disks from each group were prepared
as outlined in Section 2.1 above. The discs were immersed for 3
days in an aqueous tracer solution of 50 wt% ammoniacal
silver nitrate (pH ¼ 9.5) [11]. After immersion, the silverimpregnated resin disks were rinsed thoroughly in distilled
water and placed in a photodeveloping solution (Developer D76, Eastman Kodak, Rochester, NY) for 8 h with exposure to
Conversion values for the experimental resins 1–5
ranged from a low of 58% (74.9) for resin 1 to a high of
71.6% (72.5) for resin 5 (Table 2). Conversion values
for the commercial resins ranged from 68.4 (70.8) for
MP to 97.4 (0.9) for Excite. There was no significant
correlation between percent conversion and modulus
prior to water sorption (Table 3).
3.2. Water sorption and solubility changes over time
When mass gain (i.e. water sorption) and mass loss
(i.e. solubility) of disks made from the experimental
resins 1–5 were plotted against time, the lowest water
sorption (0.55 wt%) was found in resin 1 followed by
resins 2, 4 and 3 with resin 5 showing the highest value,
12.8% (Fig. 3). The time required to reach maximum
water sorption was different among the five resin blends,
being the shortest for resin 5, followed by resin 1, 3, 4
and 2 (Fig. 3). Solubility values for resins 1–4 ranged
from 0.5–1%, while that of resin 5 was about 4%
(Table 4). The water sorption of the commercial resins
varied from a low of 5% for SE Bond to a high of 11.6%
for Xeno III (Fig. 4). Their solubility ranged from a low
of 1.1% for SE Bond to 9.3% for Xeno III (Fig. 5).
Since water sorption and solubility occur simultaneously
[3], they were added together to provide an estimate of
the total water uptake. These values varied from 1.1%
to 20.9% among the resin blends (Table 4).
When the maximum water sorption for the experimental resins 1–5 were plotted against their Hoy’s
solubility parameters, the best correlation was found
using dp , the Hoy’s solubility parameter for polar forces
(R2 ¼ 0:93; po0:01; Fig. 5—solid line). Analysis
of this relationship using the other solubility parameter
(dd , dh ) also showed significant correlation to maximum
water sorption, but with lower R2 values (dh ¼ 0:88,
dd ¼ 0:80).
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3.3. Modulus change over time
higher than the experimental resins. The moduli of SE
Bond, MP and Excite fell 21–25% when incubated in
water for 3 days. Somewhat larger decreases in moduli
were seen in One-Up Bond and Xeno III (39 and 42%,
respectively, Table 3).
When the resins were incubated in water, they all
showed a time-dependent final modulus decrease. The
modulus of resin 1 and 2 decreased 15% (Table 3). This
decrease in stiffness was significantly less (po0:05) than
the 26% and 29% decrease recorded for resins 3 and 4,
respectively. The modulus of resin 5 decreased 73% in 3
days and was significantly lower than all the other resin
groups (Table 3). The commercial resins showed
decreases in modulus of 21–42% over 3 days of
incubation in water.
When mean modulus decrease of the experimental
resins was plotted against the Hoy’s solubility parameter
for polarity (dp ) (Fig. 3—dotted line), a significant
(p ¼ 0:05), positive correlation was found (R2 ¼ 0:74).
Lower R2 values were obtained (dh ¼ 0:71, dd ¼ 0:44)
in the others between modulus and solubility
parameters.
A separate experiment in which disks prepared from
resin 5 were flexed only at the beginning and after 3 days
of water immersion, revealed no significant difference in
modulus values between these groups and those undergoing the conventional flexing test (p40:05). Thus, as a
factor, repeated flexing of the disks did not influence
water sorption (data not shown).
Modulus before water sorption was highest for
experimental resin 2, followed by 3, 4, 5 and 1, in
decreasing magnitude (Table 3). When resins 3 and 4
were incubated in hexadecane, they exhibited a slight
time-dependent increase in final modulus, but were not
statistically different from other hexadecane-stored
specimens. Resins 1, 2 and 5 showed a slight decrease
in modulus; however, the differences among the five
groups were not statistically significant (p40:05). The
commercial resins had initial moduli that were generally
Table 2
Percent conversion of the experimental resin blends
Resin blend
Conversion (%)
1
2
3
4
5
SE Bond adhesive
One-Up Bond F
Scotchbond MP adhesive
Xeno III
Excite
58.074.9a
61.170.7a
66.971.1b
64.970.8a,b
71.672.5c
82.470.6e
79.970.8d
68.470.8b
77.370.2d
97.470.9f
Mean7SD (n ¼ 5) percent conversion. Resin composition presented
in Table 1. Groups identified by different superscript letters are
significantly different (po0:05).
Table 3
Changes in modulus of elasticity (E) MPa, of experimental and commercial resins during storage in water vs. hexadane
Resin
Water storage
Initial E
1
2
3
4
5
Excite
SE Bond
MP
One-Up
Xeno III
2300
(898)
4300
(401)
4249
(213)
3351
(288)
3153
(122)
3352
(163)
5274
(458)
5571
(747)
4559
(235)
4239
(534)
Hexadecane storage
Final E
1942
(729)
3648
(362)
3126
(54)
2355
(216)
865
(77)
2627
(105)
4289
(209)
4419
(676)
2693
(291)
2447
(321)
% change
b
15%
(6)
15%b
(4)
26%c
(4)
29%c
(8)
73%e
(3)
25%c
(3)
21%c
(3)
21%c
(7)
39%d
(7)
42%d
(5)
Initial E
Final E
% Change
2395
(416)
4036
(310)
3956
(141)
3651
(225)
3410
(367)
3124
(435)
5079
(483)
6385
(679)
4420
(338)
3714
(279)
2344
(425)
4002
(182)
4172
(190)
3690
(103)
3185
(67)
3454
(284)
5653
(357)
6196
(567)
4383
(402)
3918
(139)
1%a
(5)
4%a
(9)
6%a
(3)
4%a
(2)
2%a
(4)
5%a
(10)
13%a
(7)
3%a
(7)
0%a
(13)
9%a
(7)
Values are mean (SD) of n ¼ 5. Measured initial and final values at time t ¼ 0 and 3 days, respectively. Groups identified by different superscript
letters are significantly different (po0:05). Composition of resin 1–5 are given in Table 1.
ARTICLE IN PRESS
Sorption
6455
15
Resin 5
10
Resin 3
Resin 4
5
Resin 2
Resin 1
0
-5
Solubility
% water sorption/solubility
S. Ito et al. / Biomaterials 26 (2005) 6449–6459
-10
0
20
40
60
80
100
120
140
160
180
time, min(1/2)
Fig. 3. Water sorption (% wt gain) and solubility (% wt loss) of
experimental resins 1–5. Negative values indicate loss of dry weight
(solubility).
Table 4
Water sorption/solubility of resin blends determined by weight change
Resin
Water
sorption (%)
Solubility
(%)
Net water
uptake (%)a
1
2
3
4
5
SE bond
MP
Excite
One-Up
Xeno III
0.5570.08a
2.6270.12b
5.5870.03d
4.5970.06c
12.8370.27f
4.9570.01c
6.6470.03d
8.0270.25e
8.1470.27e
11.6372.13f
0.5670.06a
0.8570.15b
0.9870.07c
1.1070.15c
3.9370.37d
1.1470.05c
+1.4370.12c
+11.2370.25g
4.3770.05e
9.3071.51f
1.11
3.47
6.56
5.69
16.76
6.09
8.07
19.25
12.51
20.93
Sorption
15
3.4. Correlation between water sorption and modulus
reduction after water sorption
When reduction in modulus with time was plotted
against the maximum percent water sorption of each
resin, significant inverse correlations were obtained
(Fig. 6). The initial modulus was lowest in resin 1
and decreased 15% over the 3 days of water immersion.
The modulus of resin 5 was intermediate, but
decreased the most (i.e. 73%) of all resins during water
immersion. Resin moduli at 2% water sorption were
compared to determine the effect of a constant
water sorption value. These results are shown in the
insert in Fig. 6 and reveal that the modulus of resin
2 fell 16% at this sorption value, while that of resin 3 fell
18%. The moduli of resins 4 and 5 decreased 22%
and 24%, respectively at the same 2% water sorption
value.
10
3.5. Transmission electron microscopy
5
SE Bond
0
MP
-5
Solubility
% water sorption/solubility
Values are mean7SD (n ¼ 5), percent change in weight. Groups
having different superscript letters are significantly different (po0:05).
a
Net water uptake is the sum of water sorption and solubility.
Fig. 5. Regression line for percent water sorption and percent change
in modulus of elasticity after 3 days of experimental resins 1–5 vs. their
Hoy’s solubility parameter for polar forces (dp ) of those resins.
One-Up Bond
Xeno
Excite
-10
-15
0
20
40
60
80
100
120
140
160
time, min(1/2)
Fig. 4. Water sorption (% wt gain) and solubility (% wt loss) of five
commercial resins. Negative values indicate loss of dry weight
(solubility). SE Bond ¼ Clearfil SE Bond adhesive (Kuraray Medical,
Inc., Osaka, Japan); MP ¼ Scotchbond Multi-Purpose Plus adhesive
(3M-ESPE, St. Paul, MN, USA); One-Up Bond ¼ One-Up Bond F
(Tokuyama Dental Corp., Ibarak, Japan); Xeno ¼ Xeno III (Caulk/
Dentsply, Milford, DE, USA); Excite ¼ Excite (Ivoclar/Vivadent,
Schaan, Liechtenstein).
Resins 1–3 showed only uniform, spot-like distributions of silver uptake throughout their bulk (Fig. 6A–C)
that decreased in size from the periphery toward the
center of the resin disks. These segregated silver grains
could only be seen at very high magnifications
(430,000 ) for resins 1–3, but could clearly be
identified at lower magnifications (11,000–14,000 )
for resins 4 and 5. In addition, resins 4 (Fig. 6D)
and 5 (Fig. 6E) exhibited surface zones that were
characterized by the presence of branched, silver-filled
fissures that propagated from the disk surface to a
depth of 2–3 mm. This surface crazing was observed
consistently for the most hydrophilic resin 5 and was
identified in 50% of the specimens examined in resin 4
(Fig. 7).
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
Fig. 6. Changes in modulus of elasticity of resins 1–5 as a function of
percent water sorption. The dotted vertical line compares the changes
in stiffness of resins 2–5 at 2% water sorption. The values are listed in
the insert table.
4. Discussion
The results of this study support the hypotheses that
(1) methacrylate resins having higher Hoy’s solubility
parameters for polar forces have higher water sorption
values and (2) lower moduli of elasticity, than resins
with lower Hoy’s solubility parameters for polar forces.
The results of water sorption and decreases in
modulus of the commercial resins was similar to those
of the experimental resins 3–5. However, as their exact
composition is unknown it was impossible to determine
their relative hydrophilicity based on the solubility
parameters of the respective components.
The results clearly show that decreases in resin
modulus (i.e. modulus of elasticity, E) of these unfilled
copolymers were proportional to their extent of water
sorption (Fig. 3). Both the magnitude of the water
sorption and the decline in modulus is directly related to
the Hoy’s solubility parameter for polar forces (dp )
(Fig. 5). That is, the more polar the resin, the higher the
water sorption and the greater the reduction in modulus
[5]. The polar functional groups in the model resins used
(Fig. 1) included OH groups in Bis-GMA and HEMA,
carboxyl groups in TCDM, and phosphate groups in 2MP. In the current study, resin polarity was found to be
the major determinant of equilibrium water uptake. Less
polar resins, such as resin 1, absorbed very little water
(Table 4) compared to the more polar species. Water
attracted to polar groups forms hydrogen bonds [6]
resulting in ‘‘bound water’’ and is responsible for
plasticizing polymers [22,23]. The increase in discrete,
spot-like distribution of silver seen in the TEMs of resins
1–5 (Fig. 6) correlates well with the water sorption data
of the resins. The highest solubility was seen in resin 5
which also exhibited the most crazing (Fig. 6E).
These results confirm the previous report of the effects
of water storage on the ultimate tensile strength (UTS)
of these same resin systems [24]. In that study, UTS
reduction of specimens stored in water were significantly
correlated to their Hoy’s solubility parameter for
hydrogen bonding forces (dh ). In the current study,
reduction in modulus correlated better with the solubility parameter for polar forces (dp ). In the UTS study,
the specimens were 0.9 mm thick and reached their
lowest values in less than 30 days. In the current work,
the resin disks were only 0.5 mm thick and the
experiment was stopped after 3 days due to the extreme
reduction in the modulus of resin 5.
Water sorption in the five experimental resin blends
varied from 0.6 to 12.8 wt%, depending upon their
degree of hydrophilicity (the greater the hydrophilicity,
the greater the sorption) (Table 4). Sideridou et al. [25]
recently reported that the water sorption at equilibrium
for poly(Bis-EMA), poly(Bis-GMA), poly(UDMA),
and poly(TEGDMA) were 1.8, 2.6, 2.9 and 6.3 wt%,
respectively. The authors attributed the higher sorption
in poly(Bis-GMA) and poly(UDMA) to the higher
cohesive energy density of the hydroxyl groups
(2980 J cm3) and the urethane groups (1425 J cm3)
compared with those ether groups (881 J cm3). In
addition, the high water sorption in poly(TEGDMA)
was attributed to its low modulus before water sorption,
with the increased flexibility of the polymer chains
permitting a greater degree of swelling after water
sorption [25].
Although resin blends containing Bis-GMA and
TEDGMA are considered relatively hydrophilic showing appreciable water sorption and solubility characteristics [26], they are substantially less hydrophilic when
compared with the ionic resin monomers with carboxylic (resin 4 and One-Up) or phosphate groups (resin
5, Excite, SE Bond, Xeno III, Fig. 1). Thus, it should not
be surprising that much higher water sorption values
were observed in these resin blends. It is important to
note that the increase in water sorption of these
hydrophilic resin blends was not caused by reduced
conversion or cross-linking [27] within the resin matrices. Indeed, they exhibited similar or higher conversion values when compared with resin blends 1 and 2.
Likewise, the significantly greater reduction in modulus
for resins 3–5 and One-Up and Xeno III after water
sorption is not related to the relatively low initial
modulus of the resin blends [28]. Rather, the increase in
water sorption associated with the more hydrophilic
resin blends is likely to be associated with an increase in
bound water within the polymer matrix. With the use of
FTIR spectroscopy, three molecular water species have
been identified within epoxy resin matrices after water
sorption [29]. The S0 species [29] or the portion of
absorbed water that freezes at 0 1C [22], is thought to be
associated with bulk water that occupies the free volume
of the polymer matrix. The Sl species [29], or the portion
of water that freezes below 0 1C [22], represents either
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
6457
Fig. 7. Transmission electron micrographs of resin disks made from resin 1–5 after 3 days of immersion in 50 wt% silver nitrate. Micrographs 7A–C
were made at 30,000 magnification, while 7D was made at 11,000 and 7E at 14,000X. A. Resin 1 showed little uptake of silver. B. The individual
spot-like silver deposits (open arrowhead) seen in resin 2–5 are thought to represent uniformly distributed water uptake. C. These spot-like silver
deposits increased in density and size, comparing resins 1 vs. 3. D. Resin 4 revealed an outer heavy silver deposit on the surface that is thought to
represent surface erosion of resin and linear arrays of silver penetrating 901 to the surface. E. Resin 5 showed the most surface crazing. The 3 mm zone
beneath the surface (between the open arrows) includes a series of faint vertical linear channels that branch (pointer). E ¼ epoxy embedding resin.
They represent crazing that can facilitate further water permeation.
self-associated dimers, or water molecules with weak
hydrogen bonding along the secondary hydration shells.
Conversely, the S 2 molecular species [29], or the nonfreezable portion of absorbed water molecules, are
firmly bound to polar sites along the polymer network
and exhibit high plasticizing efficiency. Hydrogen
bonding between water molecules and polar hydroxyl,
carboxylate, or phosphate groups of polymer networks
[6,10,22] disrupt interchain hydrogen bonding, altering
the molecular structure and increasing the segmental
mobility of polymer chain segments. These changes are
reflected by reduction in the mechanical properties and
decline in the glass transition temperatures of the
polymerized resins [30]. Further studies should be
performed to examine the effect of resin hydrophilicity
on the lowering of glass transition temperatures of these
experimental methacrylate-based resin blends after
water sorption.
It is worth comparing the water sorption data derived
from the experimental resin blends with those previously
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
reported using commercial adhesive systems. One of the
most hydrophobic commercial dentine adhesives contains 5 vol% 4-methacryloxyethyl trimellitic anhydride
(4-META) in 95 vol% methyl methacrylate. Unemori et
al. [3] reported that this material had a water sorption of
0.4 wt% at 37 1C. Thus, by comparison, the experimental resin 1 used in the current study, that only
absorbed 0.6 wt% of water, can be considered a
hydrophobic adhesive. Burrow et al. [2] reported that
the adhesive resins in the products Universal Bond (L.D.
Caulk, Milford, DE, USA), All Bond 2 (Bisco,
Schaumburg, IL, USA) and Clearfil Liner Bond II
(Kuraray Medical, Tokyo, Japan) had maximum water
sorption values of 2.0, 3.9, 4.8 and 5.5%, respectively.
These values are similar to the net water uptake
observed in resins 2–4. Conversely, experimental resin
5 exhibited a water sorption value that is much higher
(ca. 16.8%, Table 4), and thus should be regarded as
very hydrophilic. Such a resin is comparable to some of
the very acidic, self-etching adhesive products that
contain dicarboxylic (One-Up Bond, Fig. 1) or phosphate functional groups such as Xeno III (Dentsply
DeTrey, Konstanz, Germany). Xeno III contains a large
diphosphate methacrylate derivative called tetra-methacryl-ethyl pyrophosphate that hydrolyzes in the presence
of water to a product that is identical to 2 MP used in
resin 5 (Fig. 1). Thus, resin 5 and Xeno III are similar
although not identical because Xeno III contains
additional monomers.
When specimens of resin 5 that had absorbed water
for 3 days were placed in 0% RH chambers for 6 h, their
moduli returned to original values (data not shown).
This finding indicates that the absorbed water was
loosely bound, and that the decrease in modulus was
due to plasticization by water [6] rather than solubilization of resin (Table 4). Even so, immersion of resins 4
and 5 in ammoniacal silver nitrate for the same period
revealed the presence of abnormal water channels that
appeared along the disk periphery (Fig. 6). These
surface defects may be interpreted as surface crazing
[31,32]. It has been shown that moisture can also cause
structural damage by inducing microcavities or crazes in
polymeric materials [33,34]. The formation of this
structural damage can further accelerate water uptake
[31,35], generate internal swelling stresses [8], or form
chain scission via hydrolysis of ester bonds [4,29]. These
irreversible water–resin interactions may be more
pronounced when the polymers are allowed to undergo
repeated sorption/desorption cycles [10]
Using neat resin blends with known, and varying
hydrophilicities to simulate non-solvented resin mixtures
employed in various forms of commercial enamel and
dentine adhesives, this study showed that water sorption
and modulus reduction after water sorption are directly
correlated with the polar attributes of the solubility
parameters of these resins. In particular, the extensive
amount of water sorption and solubility that was
coupled with large reduction in resin modulus after
water sorption for the most hydrophilic resin blend is a
cause of concern, as the concentration of acidic resin
monomers utilized in this resin blend is similar to those
employed in contemporary aggressive self-etching dentine adhesives. Formulation compromises are made
when creating these self-etching adhesives. The inclusion
of relatively high concentrations of acidic monomers
and water, to permit ionization of those monomers and
solubilization of calcium and phosphate, makes these
polymers very hydrophilic. The advantages that these
systems provide during bonding may be compromised
by relatively large subsequent water sorption behavior,
that lowers the stiffness of adhesive layer which couples
resin composites to dentine. This change may result in
poor load transfer across the bonded interface over
time, leading to catastrophic joint failure.
It must be mentioned that the results from this study
were generated under conditions that were far removed
from clinical practice. Clinically, adhesive resin films are
protected from water by the underlying dentine and the
overlying resin composite that would greatly restrict free
water diffusion. The use of neat resins instead of
solvated resins, on the other hand, represents idealized
conditions in which these resins are polymerized or
challenged [36]. It is anticipated that the use of solvated
resins will create non-homogenous regions with uncontrollable voids, or increased polymer chain mobility,
making these polymers more susceptible to water
sorption. The inclusion of fillers in commercial adhesives
may also alter their water sorption characteristics [35]
and the subsequent changes in mechanical properties
after water sorption. Clearly, long-term studies on the
durability of resin–dentine bonds are needed to provide
a more realistic evaluation of the longevity of bonds
made with new, more hydrophilic self-etch adhesives.
5. Conclusion
This study demonstrated that water sorption by
methacrylate-based neat resins is positively correlated
with their polarity as defined by their Hoy’s solubility
parameters for polar forces (dp ). As both water sorption
and dp values increased, the modulus of elasticity of the
resins decreased significantly, to very low (ca. 0.8 GPa)
values.
Acknowledgements
The authors thank Bisco Dental Products Co.
(Schammberg, IL) for the preparation of the five
experimental resin blends employed in this study, and
Ivoclar/Vivadent (Schaan, Liechtenstein), 3M ESPE (St.
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S. Ito et al. / Biomaterials 26 (2005) 6449–6459
Louis, MO, USA), Tokuyama Corp. (Tokyo, Japan),
Kuraray Medical Inc. (Osaka, Japan), L.D. Caulk
(Milford, DE, USA) for their generous donations of
their dentin bonding systems. This work was supported
by Grants R01 DE04911 and R01 DE15306 from the
National Institute of Dental Research (P.I. David
Pashley), by Grant 10204604/07840/08004/324/01, Faculty of Dentistry, the University of Hong Kong, and by
Grant 474226/03-4 from CNPq, Brazil. The authors are
grateful to Michelle Barnes for secretarial support.
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