J. Biomedical Science and Engineering, 2010, 3, 1050-1060 JBiSE
doi:10.4236/jbise.2010.311136 Published Online November 2010 (http://www.SciRP.org/journal/jbise/).
Published Online November 2010 in SciRes. http://www.scirp.org/journal/jbise
Synthesis and application of a novel star-hyperbranched
poly(acrylic acid) for improved dental restoratives
Jun Zhao, Yiming Weng, Dong Xie*
Department of Biomedical Engineering, Purdue School of Engineering and Technology, Indiana University-Purdue University at
Indianapolis, Indianapolis, USA.
Received 15 September 2010; revised 27 September 2010; accepted 30 September 2010.
ABSTRACT
A new star-hyperbranched poly(acrylic acid) has
been synthesized and incorporated into dental glass-
ionomer cement for enhanced mechanical strengths.
The effects of arm number and branching on viscos-
ity of the polymer aqueous solution and mechanical
strengths of the formed experimental cement were
evaluated. It was found that the higher the arm
number and the more the branching, the lower the
viscosity of the polymer solution as well as the me-
chanical strengths of the formed cement. It was also
found that the experimental cement exhibited sig-
nificantly higher mechanical strengths than commer-
cial Fuji II LC. The experimental cement was 51% in
CS, 55% in compressive modulus, 118% in DTS,
82% in FS, 18% in FT and 85% in KHN higher than
Fuji II LC. The experimental cement was only 6.7%
of abrasive and 10% of attritional wear depths of
Fuji II LC in each wear cycle. It appears that this
novel experimental cement is a clinically attractive
dental restorative and may potentially be used for
high-wear and high-stress-bearing site restorations.
Keywords: Star-Hyperbranched Poly(Acrylic Acid);
Light-Cured Glass-Ionomer Cement; Atom-Transfer Ra-
dical Polymerization; Mechanical Strength
1. INTRODUCTION
There are three major dental filling restoratives includ-
ing dental amalgam, composite resins and glass-ionomer
cements. Glass-ionomer cements are one of the most
promising restoratives in dentistry [1]. Since their inven-
tion, these cements have been successfully applied in
dentistry for almost 30 years [1-4]. The success of these
cements is attributed to the facts that they are known for
their unique properties such as direct adhesion to tooth
structure and base metals [5,6], anticariogenic properties
due to release of fluoride [7], thermal compatibility with
tooth enamel and dentin because of low coefficients of
thermal expansion similar to that of tooth structure [8],
minimized microleakage at the tooth-enamel interface
due to low shrinkage [8], and low cytotoxicity [9,10].
An acid-base reaction between calcium and/or alumi-
num cations released from a reactive glass and carboxyl
anions pendent on polyacid describes the setting and
adhesion mechanism of GICs [2,11]. Despite numerous
advantages of GICs, brittleness, low tensile and flexural
strengths have limited the current GICs for use only at
certain low stress-bearing sites such as Class III and
Class V cavities [1,2]. Much effort has been made to
improve the mechanical strengths of GICs [1,4,11] and
the focus has been mainly on improvement of polymer
backbone or matrix [1,4,11,12-18]. Briefly two main
strategies have been applied. One is to incorporate hy-
drophobic pendent (meth)acrylate moieties onto the
polyacid backbone in GIC to make it become light- or
redox-initiated resin-modified GIC (RMGIC) [12-15,17]
and the other is to directly increase molecular weight
(MW) of the polyacid [16-18]. As a result, the former
has shown significantly improved tensile and flexural
strengths as well as handling properties [12-15,17]. The
strategy of increasing MW of the polyacid by either in-
troducing amino acid derivatives or N-vinylpyrrolidone
has also shown enhanced mechanical strengths [16-18];
however, the working properties were somehow de-
creased because strong chain entanglements formed in
these high MW linear polyacids resulted in an increased
solution viscosity [16,17]. It is known that viscosity is
inversely proportional to MW of a polymer and a poly-
mer with high MW often show both high mechanical
strengths and viscosity [7,8]. So far, all the polyacids
used in commercial GIC formulations have been linear
polymers and using high MW of these linear polyacids
has been limited due to the viscosity issue.
Polymers with star, hyperbranched or dendritic shapes
often demonstrate low solution or melt viscosity because

J. Zhao et al. / J. Biomedical Science and Engineering 3 (2010) 1050-1060
1051
these molecular structures behave similar to a so lution of
hard spheres and exhibit limited chain entanglements,
which is beneficial to polymer processing [19,20]. Re-
cently, we have developed a light-curable glass-ionomer
system composed of the 4-arm star polymer [21]. The
polymer was synthesized via an advanced polymeriza-
tion technique - atom-transfer radical polymerization
(ATRP). The formed GIC system has no monomer in it.
Because of this unique nature, the system has demon-
strated substantially higher mechanical strengths as
compared to Fuji II LC [21,22]. The main purpose of
using star-shape polymer was to improve the mechanical
strengths of the current GICs by altering the molecular
architectures of the polymers. The strategy has been
found valid [21,22].
This paper reports the synthesis and characterization
of a new star-hyperbranched poly(acrylic acid) or poly
(AA), use of the polymer to formulate the cements with
glass fillers, and evaluation of the mechanical strengths
of the formed cements.
2. MATERIALS AND METHODS
2.1. Materials
2-Hydroxylethylacrylate (HEA), 2-bromoisobutyryl bro-
mide (BIBB), pentaerythritol, 1,1,1-tris-(hydroxymethyl)-
propane, dipentaerythritol, triethylamine (TEA), pyri-
dine, CuBr, N,N,N',N'',N''-pentamethyldiethylenetriamine
(PMDETA), dl-camphoroquinone (CQ), 2-(dimethyl-
amino) ethyl methacrylate (DMAEMA), tert-butyl acry-
late (t-BA), glycidyl methacrylate (GM), hydrochloric
acid (37%), diethyl ether, dioxane, N,N-dimethylfor-
mamide (DMF) and tetrahydrofuran (THF) were used as
received from VWR International Inc (Bristol, CT)
without further purifications. Fuji II LC cement and Fuj i
II LC glass powders were used as received from GC
America Inc (Alsip, IL).
2.2. Synthesis of the 2-(2-Bromoisobutyryloxy)
Ethyl Acrylate (BIEA) Initiator
The initiator BIEA was synthesized as shown below: to a
flask containing HEA (9.7 mmol), TEA (10.7 mmol) and
THF (15 ml), a solution of BIBB (10.2 mmol) in THF
(25 ml) was added dropwise to keep the temperature
below 5oC with the help of an ice-water bath. The reac-
tion was run at room temperature for additional 4 h be-
fore the formed precipitates were filtered. The filtrate
was then concentrated under a reduced pressure to afford
a yellowish oil with a yield of 74%. The synthesis
scheme is shown in Figure 1.
2.3. Synthesis of the 4-Arm Pentaerythritol
Tetrakis (2-Bromoisobutyrate) Initiator
The 4-arm initiator was synthesized following the pub-
lished procedures [21]. Briefly, to a reactor charged with
TEA (10 ml), pentaerythritol (11.0 mmol) and THF (20
ml), a solution of BIBB (81.0 mmol) in THF (25 ml)
was added dropwise with stirring at room temperature.
After addition was completed, additional 1 h was added
to complete the reaction. The solution was washed with
5% NaOH and 1% HCl, followed by extracting with
ethyl acetate. The extract was dried with anhydrous
MgSO4, concentrated in vacuo and crystallized. The
final product was re-crystallized from diethyl ether. The
3-arm and 6-arm initiators were synthesized in a similar
way as described above except that 1,1,1-tris-(hydro-
xymethyl)-propane and dipentaerythritol were used as a
core instead.
2.4. Synthesis of the GM-Tethered
Star-Hyperbranched Poly(AA)
The GM-tethered star-hyperbranched poly(AA) was
synthesized via three steps: synthesis of star-hyper-
branched poly(t-BA) via ATRP, conversion of poly(t-BA)
to poly(AA) and tethering of GM onto poly(AA). For
synthesis of poly(t-BA), to a flask containing dioxane, a
mixture of BIEA, 4-arm initiator, PMDETA (ligand) and
t-BA was added in a predetermined ratio. After the above
solution was degassed and nitrogen-purged via three
freeze-thaw cycles, CuBr (catalyst) was incorporated.
The solution was then heated to 120°C to initiate the
ATRP [23]. The proton nuclear magnetic resonance
(1HNMR) spectrometer was used to monitor the reaction.
After the polymerization was complete, the poly(t-BA)
polymer was precipitated from water. CuBr and
PMDEMA were removed by re-precipitated from diox-
ane/water. For conversion of poly(t-BA) to poly(AA),
the poly(t-BA) polymer was hydrolyzed in a mixture
containing dioxane and HCl (37%) under refluxed con-
dition for 18 h [24]. The formed poly(AA) was dialyzed
against water until the pH became neutral. The purified
star-hyperbranched poly(AA) was obtained through
freeze-drying. For GM tethering [21,22], to a flask con-
taining the star-hyperbranched poly(AA) and DMF, a
mixture of GM, DMF, and pyridine (1% of GM, by
weight) was added dropwise. Under a nitrogen blanket,
the reaction was run at 60oC for 5 h and then kept at
room temperature overnight. The polymer tethered with
GM was recovered by precipitation from diethyl ether,
followed by drying in a vacuum oven at room tempera-
ture. The overall synthesis scheme is shown in Figure 1.
2.5. Characterization
The synthesized initiator and polymers were characterized
by 1HNMR spectro scop y using a 500 MHz Bruk er NMR
spectrometer (Bruker Avance II, Bruker BioSpin Corpo-
ration, Billerica, MA). The deuterated methyl sulfoxide

J. Zhao et al. / J. Biomedical Science and Engineering 3 (20 10) 1050-1060
1052
Figure 1. Schematic diagrams for preparation of the GM-tethered star-hyperbranched poly(AA): (1)
synthesis of the 4-arm BIBB; (2) synthesis of BIEA initiators; (3) synthesis of poly(t-BA) via ATRP;
(4) hydrolysis of poly(t-BA); (5) GM tethering.
(d-DMSO) and chloroform (CDCl3) were used as sol-
vents. The molecular weight (MW) and molecular
weight distribution (MWD) of the synthesized poly
(t-BA)s were determined in THF via a Waters GPC unit
(Waters Corp., Milford, MA) with standard GPC tech-
niques, using a polystyr ene standard.
The viscosities of the liquids formulated with the
GM-tethered star-hyperbranched poly(AA)s and distilled
water were determined at 23°C using a programmable
cone/plate viscometer (RVDV-II + CP, Brookfield Eng.
Lab. Inc., Middleboro, MA).
The fracture and wear surfaces of the selected speci-

J. Zhao et al. / J. Biomedical Science and Engineering 3 (2010) 1050-1060
1053
mens from the FS, FT and wear tests were observed at a
magnification of 1,500× using a scanning electron mi-
croscope (Model JSM 5310, JOEL Ltd, Tokyo, Japan).
The specimens were vacuum sputter-coated with gold-
palladium (Au-Pd), and a vacuum was used to dehydrate
the coated specimens before SEM analysis.
2.6. Sample Preparation
The experimental cements were formulated with a two-
component system (liquid and powder) [22]. The liquid
was formulated with the GM-tethered polymer, water
(polymer/water (P/W) ratio = 70/30, by weight), CQ
(photo-initiator, 0.9%, by weight) and DMAEDA (acti-
vator, 1.8%). Fuji II LC glass powder was used to for-
mulate the experimental cements with a powder/liquid
ratio of 2.7 unless specified. Fuji II LC cement was used
as control and prepared per manufacturer’s instruction
where the P/L ratio = 3.2.
Specimens were fabricated at room temperature ac-
cording to the published protocols [21,22]. Briefly, the
specimens were prepared for different tests following the
geometries below: 1) cylindrical specimens (4 mm in
diameter × 8 mm in length) for compressive strength (CS);
2) disk specimens (4 mm in diameter × 2 mm in thick-
ness) for diametral tensile strength (DTS); 3) rectangular
specimens (3 mm in width × 3 mm in th ickn ess × 25 mm
in length) for flexural strength (FS); 4) rectangular
specimens (4 mm in width × 2 mm in th ickn ess × 20 mm
in length), fitted with a sharp blade for generating 2-mm-
long notch, for fracture toughness (FT) [25]; 5) disk
specimens (4 mm in diameter × 2 mm in height), where
the smooth surface at the diametral side was generated
by pressing the cement against a microscopic slide be-
fore setting, for Knoop hardness; and 6) rectangular
specimens (4 mm in width × 2 mm in th ickn ess × 10 mm
in length) for wear tests. All the specimens were exposed
to blue light (EXAKT 520 Blue Light Polymerization
Unit, GmbH, Germany) for 2 min, followed by condi-
tioning at 37oC in 100% humidity for 15 min and then in
distilled water for 24 h prior to testing.
2.7. Evaluation
CS, DTS, FS and FT tests were performed on a screw-
driven mechanical tester (QTest QT/10, MTS Systems
Corp., Eden Prairie, MN), with a crosshead speed of 1
mm/min. The FS and FT tests were performed in three-
point bending, with a span of 20 mm and 16 mm, respec-
tively, between supports. The sample sizes were n = 6-8
for each test. CS was calculated using an equation of CS
= P/r2, where P = th e load at f ract ure and r = the radius
of the cylinder. DTS was determined from the relation-
ship DTS = 2P/dt, where P = the load at fracture, d =
the diameter of the cylinder and t = the thickness of the
cylinder. FS was obtained using the expression FS =
3Pl/2bd2, where P = the load at fracture, l = the distance
between the two supports, b = the breadth of the speci-
men, and d = the depth of the specimen. FT was calcu-
lated from the equation

IC PS
K
faW
BW

, where KIC
= the index for FT, P = the load at fracture, S = the dis-
tance between supports, a = the length of notch, B = the
thickness, and W = the width of specimen. The f is a
function of (a/W), as shown below [25]:
 


0.5 2
1.5
31.9912.15 3.932.7
21 21
xxx xx
fx xx






The hardness test was performed on a micro-hardness
tester (LM-100, LECO Corporation, MI) using a dia-
mond indenter with 25 g load and 30 s dwell time [26].
Knoop hardness number (KHN) was averaged from six
readings for each sample.
The wear test was conducted using the Oregon Health
Science University (OHSU) oral wear simulator (Proto-
tech, Portland, OR) employing ceramic antagonists to
produce both abrasive and attritional wear [27,28]. The
test was performed following the published procedures
[29] with a slight modification. Briefly, after polishing
with sand paper, the specimen embedded in the mold
was tightened into an individual wear chamber, followed
by the addition of a food like slurry consisting of 1.0 g
ground poppy seed, 0.5 g PMMA powder and 5 ml dis-
tilled water. The abrasion force was set at 20 N and the
attrition force at 90 N. The specimen was subject to
70,000 wear cycles at a frequency of 1 Hz. The worn
specimen was analyzed using an optical surface pro-
filometer (Surftronic 3+, Taylor Hobson Ltd, Leicester,
England) [29]. Both abrasive and attritional wear depths
were obtained per manufacturer’s recommendation, av-
eraging from three traces. Four specimens were tested to
obtain a mean wear value for each material or formula-
tion.
2.8. Statistical Analysis
One-way analysis of variance (ANOVA) with the post
hoc Tukey-Kramer multiple-range test was used to de-
termine significant differences of the measured proper-
ties among the materials in each group. A level of α =
0.05 was used for statistical significance.
3. RESULTS
3.1. Synthesis and Characterization
3.1.1. Characterization of the Sy nt hesiz e d Ini tiators
and Polymers
Figure 2 shows the 1HNMR spec tra of t- BA, 4-arm B IBB,

J. Zhao et al. / J. Biomedical Science and Engineering 3 (20 10) 1050-1060
1054
Figure 2. 1HNMR spectra: (a) t-BA, (b) 4-arm BIBB, (c)
BIEA, (d) poly(t-BA), (e) poly(AA) and (f) GM-tethered
poly(AA).
BIEA, poly(t-BA), poly(AA) and GM-tethered poly(AA).
The chemical shifts (ppm) were found and listed below:
(a) t-BA: 1.50 (-CH3, 9 H), 5.68 (=CH2, 1 H), 6.00 (=
CHCO-, 1 H) and 6.27 (=CH2, 1 H); (b) 4-arm BIBB:
1.93 (-C(CH3)2, 24 H) and 4.32 (CCH2O, 8 H); (c) BIEA:
1.86 (-CH3, 6 H), 4.36 (-O CH2CH2O-, 4 H), 5.82 (=CH2,
1 H), 6.08 (=CHCO-, 1 H) and 6.36 (=CH2, 1 H); (d)
poly(t-BA): 1.38 (-CH3), 1.78 (-CH2-) and 2.15 (-CHCO-);
(e) poly(AA): 1.51 (-CH3), 2.36 (-CH2-), 3.37 (-CHCO-)
and 12.24 (-COOH); and (f) GM-tethered poly(AA):
1.50 (-CH3), 2.25 (-CH2-), 3.25 (OH), 3.35 (-CHCO-),
3.80-4.15 (-OCH2-) 5.67 (CH2=), 6.06 (CH2=) and 12.22
(-COOH).
3.1.2. P olymerization Kinetics
The ATRP polymerization kinetics of the star-hyper-
branched poly(t-BA) was studied using 1HNMR. After
the reaction was initiated, aliquots were retrieved from
the reaction system at different time intervals, dissolved
in CDCl3 and measured with 1HNMR immediately. Fig-
ure 3 shows a kinetic plot of monomer to polymer con-
version versus time and a semi-logarithmic plot of ln
([M]0/[M]) versus time, where [M]0 = the initial concen-
tration of the monomer and [M] = the monomer concen-
tration at any time. The conversion was calculated by
comparison of the peak integrations between 6.27 (HC =
C) and 1.2-1.6 ppm (-CH3). The values of ln ([M]0/[M])
were obtained from ln [1/(1-conversion%)]. Two stages
were found from the plot of ln ([M]0/[M]) versus time: a
linear plot with 0.484 (slope) and 0.943 (R2) within 3 h
after the reaction was initiated and a d eviated plot with a
steeper slope after 3 h. The conversion corresponding to
the linear plot at 3 h was 78%. The conversion reached
98.7% at 4 h, indicating that the reaction was accelerated
Figure 3. Conversion and kinetic plot of the star-hyper-
branched poly(t-BA) derived from 1HNMR spectra: 4-star/
BIEA = 1:4 (by mole), (4-arm/BIEA)/t-BA = 1% (by mole),
t-BA/dioxane = 50/50 (by weight) and reaction temerature =
120°C.
after 3 h.
3.1.3. Effects of the Arm Number and Branching on
MW, MWD and Viscosity
The measured MW and MW distribution (MWD) of the
synthesized star-hyperbranched poly(t-BA) and the vis-
cosities of the corresponding polymer aqueous solutions
are shown in Table 1. Increasing the arm number in-
creased the MWD but decreased the solution viscosity.
Increasing the branching increased the MWD but de-
creased the solution viscosity. In other words, both star
arm number and branching increased the MW distribu-
tion of the synthesized poly(t-BA) but decreased the
viscosity of the poly(AA) aqueous solution, indicating
Ta b le 1. Mn, Mw, MWD and solution viscosity of the synthe-
sized polymers1.
M
n (Dalton) Mw (Dalton) MWD Viscosity (cP × 10-3)4
Effect of arm number2
316630 55212 3.32 9.27
417164 49089 2.86 6.11
616725 91988 5.54 4.65
Effect of branching3
L17164 49089 2.86 6.11
M12274 46150 3.76 3.21
H10575 44204 4.18 1.90
1Mn (number average MW), Mw (weight average MW) and MWD (MW
distribution) were measured by GPC; 2The 3, 4 and 6 stands for star arm
number, respectively; 3L, M and H represents the BIEB used in low, me-
dium and high level; 4Viscosity of the GM-tethered poly(AA) in water
(polyme r: water = 60:40, by wei g ht ) was determined at 2 3oC.

J. Zhao et al. / J. Biomedical Science and Engineering 3 (2010) 1050-1060
1055
that more star arm number and more branching in the
polymer can reduce the solution viscosity.
3.2. Evaluation
Table 2 shows the effects of the arm number and
branching on CS, DTS, FS and compressive modulus of
the experimental cements. There is a trend that increas-
ing the arm number and branching decreased CS, DTS,
FS and modulus, although most of the values in each
category were not statistically different from one another.
Table 3 shows the effects of the arm number and branch-
ing on KHN, FT, abrasion and attrition of the experi-
mental cements. There is also a trend that increasing the
arm number and branching decreased KHN, FT, abra-
sion resistance and attrition resistance, although some of
the values in each category were not statistically differ-
ent from one another. Figure 4 shows the effect of aging
in water on CS, DTS and compressive modulus of the
experimental cements. Significant increases were found
in CS (253.6 to 320.2 MPa), DTS (60.8 to 67.9 MPa)
and modulus (5.76 to 8.27 GPa) from 1 hour to 1 day.
There were no statistically significant changes found
among CS, DTS and modulus, although there is an in-
creasing trend for all the strengths with the time.
Table 4 shows the mean values of CS, modulus, DTS,
FS, FT, KHN, abrasion and attrition of the experimental
cement (EXPGIC) versus commercial Fuji II LC cement.
Apparently, EXPGIC exhibited significantly higher val-
ues than Fuji II LC in all the measured mechanical prop-
erties (p < 0.05). Briefly, EXPGIC was 51% in CS, 55%
Table 2. Effects of arm number and branching on CS, DTS, FS
and modulus of the cements1.
CS [MPa] DTS [ MPa] FS [MPa] Modulus [GPa]2
Effect of arm number
3 320.2 (9.4)a, 3 72.6 (3.6)c 114.2 (14.1)f 8.27 (0.10)h
4 301.7 (9.4)a, b 67.9 (2.7)c, d 101.4 (7.6) f, g 7.95 (0.14)h, i
6 286.3 (8.9)b 58.4 (3.8)d, e 92.4 (11.1)g 7.56 (0.22)i
Effect of branching
L 301.7 (9.4)a, b 67.9 (2.7)c, d 101.4 (7.6)f, g 8.27 (0.10)h
M 285.9 (9.5)b 58.8 (3.6)d, e 88.4 (9.1)g 7.82 (0.14)i
H 257.8 (10) 49.4 (3.4)e 89.1 (15)g 7.49 (0.27)i
1Polymer = GM-tethered star-hyperbranched poly(AA), (4-star/BIEA)/t-BA
= 1% (by mole), GM grafting ratio = 50% (by mole), P/W ratio = 70/30 (by
weight), P/L ratio = 2.7 (by weight). 2Modulus = compressive modulus;
3Entries are mean values with standard deviations in parentheses and the
mean values with the same letter in each category were not significantly
different (p > 0.05). Specimens were conditioned in distilled water at 37°C
for 24 h pri or to testing.
Table 3. Effects of arm number and branching on KHN, FT,
abrasion and attrition of the cements.
KHN FT [MPa·m0.5] Abrasion
[nm·cycle-1] Attrition [nm·cycle-1]
Effect of arm number
358.9 (3.5)a, 11.05 (0.06)c 0.41 (0.12) 0.71 (0.06)e
458.5 (0.6)a 1.11 (0.18)c 0.26 (0.05)d 0.73 (0.20)e
651.4 (4.3)b 1.06 (0.13)c 0.26 (0.07)d 1.29 (0.32)f
Effect of branching
L58.5 (0.6)a 1.11 (0.18)c 0.26 (0.05)d 0.73 (0.20)e
M49.2 (1.4)b 1.11 (0.22)c 0.32 (0.06) 0.92 (0.15)
H50.2 (1.4)b 1.08 (0.13)c 0.56 (0.18) 1.31 (0.30)f
1Entries are mean values with standard deviations in parentheses and the
mean values with the same letter in each category were not significantly
different (p > 0.05). Specimens were conditioned in distilled water at 37°C
for 24 h pri or to testing.
Figure 4. Strength change of the cement in the course of aging
in water: polymer = GM-tethered star-hyperbranched poly(AA),
4-arm/BIEA = 1:4 (by mole), (4-star/BIEA)/t-BA = 1% (by
mole), GM grafting ratio = 50% (by mole), P/W ratio = 70/30
(by weight), P/L ratio = 2.7 (by weight). Specimens were con-
ditioned in distilled water at 37°C prior to testing.
in compressive modulus, 118% in DTS, 82% in FS, 18%
in FT and 85% in KHN higher th an Fuji II LC. For wear
test, EXPGIC was only 6.7% of abrasive and 10% of
attritional wear depths of Fuji II LC in each wear cycle.
Figures 5(a-f) illustrates representative regions of the
fracture surfaces after the FS ((a) and (b)) and FT ((c)
and (d)) tests, and the wear surfaces after the attrition ((e)
and (f)) test, respectively, using a standardized magnifi-
cation of 1500×. The crack in the microstructure was
caused by dehydration during preparation for SEM
analysis [26]. In the case of Figures 5(a), (b), (c), (d), it
is apparent that the fracture surfaces with EXPGIC ((b)
and (d)) look denser, more integrated and more rugged

J. Zhao et al. / J. Biomedical Science and Engineering 3 (20 10) 1050-1060
1056
(a) (b)
(c) (d)
(e) (f)
Figure 5. Fracture and wear surface photomicrographs at a magnification of 1500×: (a), (b) and (c) = fracture surface from FS, frac-
ture surface from FT, attrition wear surface from wear tests, respectively, for Fuji II LC. (d), (e) and (f) = fracture surface from FS,
fracture surface from FT, attrition wear surface from wear tests, respectively, for EXPGIC. Specimens were conditioned in distilled
water at 37°C for 24 h prior to testing.

J. Zhao et al. / J. Biomedical Science and Engineering 3 (2010) 1050-1060
1057
Ta b l e 4 . Mechanical property comparison between Fuji II LC
and EXPGIC.
Property Fuji II LC EXPGIC1
CS [MPa] 212.7 (15)2 320.2 (9.4)
Modulus [GPa] 5.33 (0.09) 8.27 (0.1)
DTS [MPa] 31.2 (2.2) 67.9 (2.7)
FS [MPa] 55.8 (4.1) 101.4 (7.6)
FT [MPa·m0.5] 0.94 (0.01) 1.11 (0.18)
KHN 31.7 (1.0) 58.5 (0.6)
Abrasion [nm·cycle-1] 3.90 (0.81) 0.26 (0.05)
Attrition [nm·cycle-1] 7.21 (1.99) 0.73 (0.20)
1Polymer = GM-tethered star-hyperbranched poly(AA), 4-star/BIEA = 1:4
(by mole), (4-star/BIEA)/t-BA = 1% (by mole), GM grafting ratio = 50%
(by mole), P/W ratio = 70/30 (by weight), P/L ratio = 2.7 (by weight);
2Entries are mean values with standard deviations in parentheses and all the
mean values in each category were significantly different (p < 0.05).
Specimens were conditioned in distilled water at 37°C for 24 h prior to
testing.
than those with Fuji II LC ((a) and (c)). The rugged and
highly integrated fragments suggest that the experimen-
tal cement was a tougher material than Fuji II LC. In
contrast, loosely bonded fragments are observed in Fuji
II LC. On the other hand, more big pores or voids are
observed in Fuji II LC ((a) and (c)), indicating more air
bubbles trapped during the cement preparation. For attri-
tion ((e) and (f)), Fuji II LC (e) showed many crashed
filler particles on a much rougher surface compared to
EXPGIC (f). The wear surface of EXPGIC (f) looks
much smoother than that for Fuji II LC.
4. DISCUSSION
4.1. Synthesis and Characterization
All the chemical shifts in Figure 2 confirmed the struc-
tures of the synthesized initiators and polymers. The
characteristic chemical shifts at 3.25 (OH), 5.67 (CH2=)
and 6.06 (CH2=) identified the difference between the
star-hyperbranched poly(AA) and GM-tethered star-hy-
perbranched poly(AA).
Regarding the synthesis of the star-hyperbranched
polymer via ATRP, it is known that ATRP reaction gen-
erally exhibits the first-order kinetics due to persistent
radical effect [30]. As shown in Figure 3, ln([M]0/[M])
increased linearly with time until the monomer conver-
sion reached 78%, indicating that the reaction followed
the first-order kinetics at the early stage of polymeriza-
tion. However, after 3 h the plot deviated more from
linearity showing an accelerated fashion. The accelera-
tion may be attributed to the reason that at a higher con-
version the acrylate groups were located at the end of
extended polymer chains, which may limit their mobility
and thus reduce their reactivity with the propagating
radicals. In addition, the solution viscosity was found to
be significantly high at the later stage. These two reasons
might lead to reduction of the termination constant and
auto acceleration of the polymerization. As a result, the
conversions wer e 98.7% at 4 h and 99.7% at 5 h, respec-
tively.
We have also studied the effects of arm number and
branching on MW, WMD of the polymer and viscosity
of the corresponding poly(AA) aqueous solution (see
Table 1). From Table 1, the higher the star arm number,
the lower the viscosity. This is logical because the 6-arm
star polymer is more like a sphere as compared to 3- and
4-arm star polymers. For the effect of the branching, we
synthesized the star-hyperbranched polymers by chang-
ing the concentration of BIEA while keeping the star-
shape initiator constant. It was found that increasing
BIEA or branching significantly decreased the viscosity
of the polymer aqueous solution. It has been noticed that
polymers with star, hyperbranched or dendritic shapes
often demonstrate low solution or melt viscosity because
these molecular structures behave similar to a so lution of
hard spheres and exhibit limited chain entanglements,
which is beneficial to polymer processing [19,20]. In
dental clinics, it is known that mixing a dental cement
requires a workable solution viscosity for the polymer
solution. Relatively low so lution viscosity fav ors cement
mixing clinically and reduces the probability of forming
flaws or defects, thus enhancing the mechanical strength
[2,4,8]. Therefore, without compromising the mechani-
cal strengths a polymer solution with a low viscosity
would be favorable to d e ntal clinics. The resu lts in Table
1 support that branching does decrease the viscosity.
The results in Tables 2 and 3 show that there is a
trend that increasing either arm number or branching
decreased the mechanical strengths, although the de-
crease was not statistically significant. This can be at-
tributed to the fact that all the initiators we have used in
this study are mainly composed of hydrocarbons and
esters. None of them contain functional groups such as a
carboxyl group for strength enhancement. The bulky
hydrophobic initiator cores do not contribute any strength
enhancement to the cement system. Therefore, the more
the initiator in the system, the lower the mechanical
strength is. Fortunately, these cores did not affect the
strength significantly.
It is well known that GICs increase their strengths in
water with time due to constant salt-bridge formations
[3,31]. To demonstrate the unique nature of GICs in our
new cement system, we measured the CS, DTS and
compressive modulus values of the experimental cement

J. Zhao et al. / J. Biomedical Science and Engineering 3 (20 10) 1050-1060
1058
after conditioning in water for 1 h, 1 day, 1 week and 1
month (see Figure 4). The experimental cement showed
a significant increase (26% in CS, 12% in DTS and 44%
in modulus) from 1 hour to 1 day. After that a slower
increase was notic ed.
Finally, we measured the mechanical properties of
commercial Fuji II LC cement and compared them with
those for the experimental cement. Table 4 shows all th e
measured mechanical properties of the optimal experi-
mental cement (EXPGIC) vs. Fuji II LC. EXPGIC
showed significantly higher mechanical strengths in-
cluding CS, compressive modulus, DTS, FS, FT, KHN,
abrasion and attrition than Fuji II LC. The higher me-
chanical strengths exhibited by EXPGIC can be attrib-
uted to the nature of this unique experimental cement
system. As mentioned in the section of Materials and
Methods , EXPGIC w as compo sed of st ar-hyperbr anched
poly(AA) polymer, water and initiators. There were no
any low MW comonomers in it. Essentially this is a
monomer-free cement system. The polymer aqueous
liquid contains highly concentrated GM-tethered star-
hyperbranched poly(AA), which not only provides a
large quantity of carboxyl groups for salt-bridge forma-
tion but also a substantial amount of carbon-carbon dou-
ble bond (methacrylate) for covalent crosslinks. In con-
trast, in addition to linear poly(AA) and water, Fuji II
LC contains a substantial amount of HEMA (2-hydro-
xylethyl methacrylate, a low MW monomer) and other
low MW methacrylate or dimethacrylate comonomers
[13]. These low MW monomers and oligomers led to a
lower strength. The fracture and wear surface photomi-
crographs from SEM (Figure 5) strongly supported the
mechanical strength differences between the two ce-
ments.
5. CONCLUSIONS
This study developed a new glass-ionomer cement sys-
tem composed of the newly synthesized star-hyper-
branched poly(AA). The star-hyperbranched poly(AA)
was synthesized via ATRP. The effects of arm number
and branching on viscosity, CS, DTS and compressive
modulus of the formed polymers and cements were
evaluated. The results showed that both arm number and
branching showed significant effects on viscosity and
mechanical strengths. It was found that the more the arm
number and branching that the polymer has, the lower
the viscosity of the polymer aqueous solution as well as
the mechanical strengths of the formed cement. Within
the limitations of this study, the experimental cement
formulated with the newly synthesized star-hyper-
branched polymer exhibited significantly higher me-
chanical strengths than commercial Fuji II LC. The ex-
perimental cement was 51% in CS, 55% in compressive
modulus, 118% in DTS, 82% in FS, 18% in FT and 85%
in KHN higher than Fuji II LC. The experimental ce-
ment was only 6.7% of abrasive and 10% of attritional
wear depths of Fuji II LC in each wear cycle. It appears
that this novel experimental cement is a clinically attrac-
tive dental restorative and may potentially be used for
high-wear and high-stress-bearing site restorations. Fu-
ture studies will include bonding to tooth and in vitro
biocompatibility tests.
6. ACKNOWLEDGEMENTS
This work was sponsored by NIH challe nge grant (RC1) DE020614.
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LIST OF THE ABBREVIATIONS
COMMONLY SHOWN IN THE PAPER:
GIC–glass-ionomer cement;
CS–compressive strength;
DTS–diametral tensile strength;
FS–flexur a l s trength;
KHN–Knoop h a rd ness number;
FT–fracture toughness;
ATRP–atom-transfer radical pol y merization;
MW–molecular weight;
MWD = molecular weight distribution;
EXPGIC–experimental glass-ionomer cement;
AA–acrylic acid;
GM–glycidyl methacrylate;
BIEA–2-(2-bromoisobutyryloxy) ethyl acrylate;
BIBB–2-bromoisobutyryl bromide;
t-BA–tert-butyl acrylate.