Synthesis and properties of liquid crystalline urethane methacrylates for dental composite applications

Article abstract of DOI:10.1002/pola.24693

Three novel liquid crystalline methacrylates have been synthesized and characterized to be tested as comonomers in light-curing dental resin-based composites. The selected formulations consist of an alkylammonium or cholesteryl urethane methacrylate and 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl] propane (BisGMA) or a BisGMA derivate modified with urethane methacrylate groups, further diluted with triethyleneglycol dimethacrylate (TEGDMA) and reinforced with 70% filler (zirconium silicate nanopowder, silanized filler). This study addresses the relationships between the LC monomer structure, photopolymerization rates (by differential scanning photo calorimetry), and specific properties of the dental resin composites (volumetric shrinkage, water sorption, water solubility, and hydrophobicity). The investigation of LC properties by differential scanning calorimetry and polarizing microscopy indicated that the LC mesophase is stable to room temperature (cationic monomers) or at 40 °C (cholesteryl methacrylate). It was found that the polymerization rate for LC urethane methacrylates used in combination with BisGMA/TEGDMA (0.122-0.136 s^-1) is higher than that of the mesogenic monomers alone (0.085-0.107 s^-1). The structures of the urethane monomers and, consequently, the viscosity of the comonomer mixture influence both the rate and the degree of conversion (44.8-67.5 %) of the photopolymerization process. Polymerization shrinkage measured by pycnometry showed lower values for LC monomers (3.25-3.43 vol %) comparatively with the monomer mixture (5.19-6.65 vol %). Preliminarily, the effect of ammonium groups from two resin composites incorporating alkylammonium structures (4.5 wt %) was tested on Streptococcus mutans, and distinct zone of inhibition was observed.

Full text of DOI:10.1002/pola.24693

Synthesis and Properties of Liquid Crystalline Urethane Methacrylates
for Dental Composite Applications
TINCA BURUIANA,1 VIOLETA MELINTE,1 GEORGE COSTIN,2 EMIL C. BURUIANA1
1Petru Poni Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, 700487 Iasi, Romania
2Faculty of Dentistry, Apollonia University, Iasi, Romania
Received 14 January 2011; accepted 24 March 2011
DOI: 10.1002/pola.24693
Published online 25 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Three novel liquid crystalline methacrylates have
been synthesized and characterized to be tested as comonomers
in light-curing dental resin-based composites. The selected for-
mulations consist of an alkylammonium or cholesteryl urethane
methacrylate and 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)-
phenyl]propane (BisGMA) or a BisGMA derivate modified with
urethane methacrylate groups, further diluted with triethylenegly-
col dimethacrylate (TEGDMA) and reinforced with 70% filler (zir-
conium silicate nanopowder, silanized filler). This study
addresses the relationships between the LC monomer structure,
photopolymerization rates (by differential scanning photo calo-
rimetry), and specific properties of the dental resin composites
(volumetric shrinkage, water sorption, water solubility, and
hydrophobicity). The investigation of LC properties by differential
scanning calorimetry and polarizing microscopy indicated that
the LC mesophase is stable to room temperature (cationic mono-
mers) or at 40 ꢀC (cholesteryl methacrylate). It was found that the
polymerization rate for LC urethane methacrylates used in combi-
nation with BisGMA/TEGDMA (0.122–0.136 sꢁ1) is higher than
that of the mesogenic monomers alone (0.085–0.107 sꢁ1). The
structures of the urethane monomers and, consequently, the vis-
cosity of the comonomer mixture influence both the rate and the
degree of conversion (44.8–67.5 %) of the photopolymerization
process. Polymerization shrinkage measured by pycnometry
showed lower values for LC monomers (3.25–3.43 vol %) compa-
ratively with the monomer mixture (5.19–6.65 vol %). Preliminar-
ily, the effect of ammonium groups from two resin composites
incorporating alkylammonium structures (4.5 wt %) was tested
on Streptococcus mutans, and distinct zone of inhibition was
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observed.
2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 2615–2626, 2011
KEYWORDS: dental polymers; LC monomers; photopolymeriza-
tion; synthesis
INTRODUCTION Within the dental resin composites, the
monomer systems based on photopolymerizable dimethacry-
lates incorporating diglycidyl methacrylate of bisphenol
In tandem, the urethane dimethacrylates chemistry,20,21 often
used as a base resin, was explored continuously and even
exploited in the recently created composite resin systems
(Kalore/Venus Diamond) with small shrinkage.22 Significantly,
the use of monomers with high-molecular volume and multiple
functionality is an alternative way to minimize the polymeriza-
tion shrinkage.23,24 In this regard, our research is based on the
reactive acid functionalized urethane oligomers25,26 and nonacid
ones,27 tested in formulating the matrix for dental composites.
A
(BisGMA), 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-
2,4,4-trimethylhexane urethane (UDMA), and triethylenegly-
col dimethacrylate (TEGDMA) as diluent comonomer are
widely used as the organic phase in esthetic restorative
treatments.1,2 Despite of several practically validated advan-
tages, there are some deficiencies associated with the resin
matrix, such as high-volumetric shrinkage that accompanies
polymerization of the composites, incomplete conversion of
double bonds, and susceptibility to water sorption,3–5 all
with adverse effects on the properties of dental materials.6–8
To diminish these drawbacks, a change in monomer chemis-
try and its relationship with the filler seems to be an ideal
solution to the above-mentioned problems. Hence, extensive
efforts have been made to develop a new resin monomer
technology, involving BisGMA derivatives,9,10 fluorinated
monomers,11,12 oxa-spirocyclic monomers,13 dendrimers,14 and
ormocers,15,16 but excepting siloranes,17,18 the inconveniences
are not completely solved yet.19
Another strategy has focused on innovative monomers, such
as liquid crystal dimethacrylates28–31 that remains in a LC
state under processing conditions of the dental composites
and produce a lower decrease in volume on polymerization.
Specifically, 1,4-bis[4-(6-acryloyl-1-hexyloxy)-benzoyloxy]-2-t-
butyl benzene (C6) and its methacrylic analog have demon-
strated a volumetric shrinkage of about 2 vol %.30,32 Ritter
et al.28 and Norling et al.29 synthesized dimethacrylates con-
taining aromatic esters with or without alkoxy and/or cho-
lesteryl groups and showed that they could be used for this
purpose.
Correspondence to: E. C. Buruiana (E-mail: emilbur@icmpp.ro)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2615–2626 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of LC urethane methacrylate with choles-
teryl moiety (UMA-Chol).
are colorless and clear viscous liquids, whereas UMA-Chol is a
white powder, all derivatives being soluble in organic solvents
as acetone, chloroform, methylene chloride, tetrahydrofuran
(THF), and dimethyl sulfoxide, as well as in the photocurable
acrylic monomers encountered in commercial resin composites
(TEGDMA and BisGMA).
The structures of the synthesized monomers were confirmed
by 1H NMR and FTIR spectroscopies. Thus, for example, in
the 1H NMR spectrum of UMA-Q2 (Fig. 1) can be remarked
the presence of a peak centered at 6.7 ppm corresponding to
the NH urethane proton, whereas the unsaturated aliphatic
protons in trans- and cis-configurations relative to the CH3
unit from HEMA can be observed at 6.13 and 5.59 ppm,
respectively. Other chemical shifts were found in the spectral
region for methylene protons near to the ester-urethane
groups and the ester unit (4.56, 4.23, and 3.96 ppm) and
methylene and methyl protons in the neighborhood of the
quaternary ammonium group (3.6 and 3.46 ppm) as well as
the CH3 protons connected with the double bond (1.93 ppm).
The methylene and methyl protons from the aliphatic chain
SCHEME 1 Synthesis of LC urethane methacrylates (UMA-Q1
and UMA-Q2) bearing quaternary ammonium groups.
In this contribution, we present the synthesis and photopoly-
merization of three monomethacrylates with LC properties,
and the dual effect of the quaternary ammonium structure
on their properties including preliminary bacteriostatic activ-
ity of the resulting resin composites against Streptococcus
mutans. As already reported by others, innovation in restora-
tive dentistry has been directed toward producing materials
with ‘‘bioactive’’ functions to provide therapeutic effects.33–36
RESULTS AND DISCUSSION
Preparation and Structural Characterization
The synthesis of new polymerizable monomers which in a
single molecule to combine the liquid crystalline nature with
the bacteriostatic/bactericide action constitute a modern
approach for developing ‘‘bioactive’’ resin composites used in
dentistry, but not only. Quaternary ammonium is known for
its wide spectrum of antibacterial activity, and for this partic-
ularity it can be formulated in resin composites or adhesive
systems to prevent the initiation and progression of caries
adjacent to restorations.37–40 On the basis of this
information, in the first step, we prepared new urethane
methacrylates with quaternary ammonium groups 2-(metha-
cryloyloxyethylcarbamoyloxy)ethyl
dimethyldodecylammo-
nium bromide (UMA-Q1) and 2-(methacryloyloxyethylcar-
bamoyloxy)ethyl hexadecylammonium bromide (UMA-Q2)
(Scheme 1). Starting from cholesterol and 2-isocyanatoethyl
methacrylate, an urethane methacrylate bearing cholesteryl
fragment as mesogen, namely cholesteryl N-(2-methacryloylox-
yethyl) carbamate (UMA-Chol) was also obtained (Scheme 2).
The cationic urethane methacrylates (UMA-Q1 and UMA-Q2)
FIGURE 1 1H NMR spectrum of UMA-Q2 urethane methacrylate
in CDCl3.
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give the signals at 1.74, 1.25, and 0.88 ppm, respectively.
From the integral ratio of aliphatic protons versus unsatu-
rated protons (5.6–6.13 ppm), it can be appreciated that the
structure of the urethane methacrylate monomer is correctly
represented. Also, the FTIR spectra of urethane methacry-
lates UMA-Q1, UMA-Q2, and UMA-Chol confirmed the pro-
posed structures (urethane NH at 3500–3320 cmꢁ1, carbonyl
CO at 1717 cmꢁ1, and carbon–carbon double bond from the
methacrylate function at 1637 and 816 cmꢁ1).
A careful examination of the literature data regarding the
photopolymerization of liquid crystalline monomers capable
to reduce the polymerization shrinkage of a resin composite
suggested that it is necessary to understand if such struc-
tures could respond to this goal. Further analysis of the ther-
mal properties of the urethane methacrylates, which can
generate mesophase at low temperatures evaluated by
means of differential scanning calorimetry (DSC), showed
that the isotropization temperature of them was inferior to the
polymerization temperature of the methacrylic groups. For
example, for UMA-Q1, a single exothermic peak at ꢁ3.7 ꢀC
appeared on its cooling scan evidences the isotropic to the liq-
uid crystalline transition, as shown in Figure 2(a). On the
second heating run, the endothermic peak located at about
ꢀ
11.2 C is due to isotropization of the alkyl mesogen, suggest-
ing the mesophase formation on this temperature range.
DSC thermogram for UMA-Chol during the second heating
scan [Fig. 2(b)] displayed one endothermic peak at about
112 ꢀC, which represents the melting transition that proved
to be identical with the clearing point of the LC monomer.
Besides, no additional peaks could be discerned up to 125
ꢀC. On cooling scan, an exothermic peak located at about
ꢀ
101 C can be associated to the isotropic to LC phase transi-
tion, when the mesophase is formed, as confirmed the visual
observation in polarized light microscopy of the sample.
Moreover, by further cooling crystallization of UMA-Chol was
not detected, but the mesophase quality remains intact up to
30 ꢀC. According to Mulhaupt’s results, the suppression of
crystallization by a slightly branched structure of some choles-
terol-derivatized dimethacrylates would be responsible for the
creation of stabilized mesophases at ambient temperature,
offering the possibility to be polymerized under such condi-
tions (LC medium and low temperature).41 This indicates the
necessity of further investigation of the urethane choesteryl
dimethacrylate to evaluate its role in determining the LC char-
acteristics and its usefulness in dental composites.
Nevertheless, it is essential to examine if the incorporation
of a low quantity (15 wt %) of LC methacrylate into a dental
monomer mixture can preserve its liquid crystal nature in
the uncured components. In fact, the DSC thermograms [Fig.
2(c)] performed on the combination based on UMA-Q2,
BisGMA analog (BisGMA-1), and TEGDMA (F5, Table 1)
showed a well-defined endothermic peak (1.75 ꢀC) on the
repeated heating cycle accompanied by the formation of an
FIGURE 2 DSC heating and cooling traces for urethane methac-
rylate with alkylammonium groups UMA-Q1 (a), UMA-Chol (b),
and for the uncured formulation F5 containing 15 wt % UMA-
Q2, 49.25 wt % BisGMA-1, and 34.25 wt % TEGDMA (c); micro-
scopic texture of F5 collected under crossed polarizer at room
temperature (in set). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
ꢀ
exothermic peak (ꢁ7.73 C) on the cooling scan correspond-
ing to the mesophase. Taking into account that BisGMA-1
synthesized in our group is less viscous than the unmodified
SYNTHESIS AND PROPERTIES OF URETHANE METHACRYLATES, BURUIANA ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Composition (wt %) of Experimental Formulations (F)
of a formulation (F6) containing 15% reactive mesogen,
49.25% BisGMA-1, and 34.25% TEGDMA was also visualized
by POM [Fig. 3(d)], but a great variation of the liquid crystal
texture (30 ꢀC) was observed when compared with the LC
monomer before photopolymerization. On cooling, the smec-
tic-A structure remained relatively unchanged up to room
temperature (for a short time) suggesting the stable charac-
ter of the higher ordered mesophase.
F1
15
F2
F3
F4
15
F5
15
F6
UMA-Q1
UMA-Q2
UMA-Chol
BisGMA
15
15
15
49.25
34.25
49.25
34.25
49.25
BisGMA-1
TEGDMA
49.25
34.25
49.25
34.25
49.25
34.25
Photopolymerization
34.25
To establish the photoreactivity of the LC urethane methacry-
late monomers, the cure kinetics of UV-initiated photopoly-
merization of them taken alone or in combination with other
commercial monomers were determined by photoDSC, the
rates of polymerization, and the conversions being included
in Table 2. The photobehavior of these monomers was inves-
tigated in the presence of 1.5 wt % Irgacure 651, a typical
UV photoinitiator and in the absence of any filler. Under the
given photopolymerization conditions, the methacrylic struc-
tures are subjected to UV irradiation, and the polymerization
enthalpy was measured. Although the photopolymerization
behavior of BisGMA and TEGDMA is demonstrated in the lit-
erature,43,44 for a better correlation of data, we have per-
formed their photopolymerization in identical conditions as
those used for the LC urethane methacrylates, taking into
BisGMA, it can be appreciated that the transition tempera-
tures observed above may be due to the lower viscosity of
its organic phase. Therefore, there is a DSC proof for a dis-
tinct signal of the mesophase existence originating from the
ordering of the alkyl chain in the above uncured formulation,
and this is sustained by the polarizing optical microscopy
image attached to the latter figure (in set). The polarizing
optical microscopy (POM) observations recorded at room
temperature for the alkylammonium monomers (UMA-Q1
and UMA-Q2) and at 40 ꢀC for UMA-Chol [Fig. 3(a–c)]
revealed that these monomers seems to be characterized by
a smectic texture, for example, for other acrylates.42 To
obtain further evidence for UMA-Chol, the photomicrograph
FIGURE 3 Microscopic textures of urethane methacrylates taken under a crossed polarizer: (a) UMA-Q1 and (b) UMA-Q2 taken at
room temperature; (c) UMA-Chol at 40 ꢀC, and for the uncured formulation F6 containing 15 wt % UMA-Chol, 49.25 wt % BisGMA-
1, and 34.25 wt % TEGDMA at 30 ꢀC (d).
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TABLE 2 Photo-DSC Data of the Monomers and Formulations
irradiation is recorded for the less viscous urethane mono-
(Percent Standard Deviation is in Parentheses)
mer (e.g., DCUMA-Q1: 56.6% compared with DCTEGDMA:
75.6%), whereas a lower conversion is achieved for the
UMA-Chol monomer (DC: 15.4%), probably due to its tend-
ency to crystallize at room temperature. In case of the UMA-
Q2 (DC: 33.1%), it is possible that the mobility restrictions
imposed by vitrification reached at relatively low conversion
to be responsible for the obtained conversion. Compared with
the polymerization rate, the conversion degree [Fig. 4(b)]
attained of each monomer followed the order TEGDMA>UMA-
Q1>BisGMAꢂBisGMA-1>UMA-Q2> UMA-Chol. This result,
may, in part, be explained by the fact that between viscosity
and the degree of conversion there is an inverse relationship
only up to a certain level, where the decrease in viscosity gen-
erates a low polymerization conversion owing to the termina-
tion reaction controlled by diffusion. A similar behavior was
observed by Sideridou et al. during the free radical homopoly-
merization of BisGMA, UDMA, or TEGDMA, when UDMA
showed the highest polymerization rate, and TEGDMA had the
highest final conversion.45
Monomer/
Polymerization
Rate (sꢁ-1
Formulation
tmax (s)
)
DC (%)*
UMA-Q1
UMA-Q2
UMA-Chol
BisGMA
5.31 (0.61)
2.64 (0.81)
2.58 (0.99)
11.34 (0.24)
3 (0.51)
0.085 (0.0026)
0.107 (0.0058)
0.090 (0.0052)
0.056 (0.0076)
0.085 (0.0072)
0.057 (0.0034)
0.126 (0.0138)
57.9(1.64)
33.1 (2.23)
16.2 (3.35)
54.5 (3.79)
41.6 (2.04)
79.8 (1.83)
50.8 (3.21)
BisGMA-1
TEGDMA
15.06 (0.48)
2.88 (0.37)
BisGMA-1/
TEGDMA
F1
F2
F3
F4
F5
F6
2.94 (0.46)
3.12 (0.54)
2.94 (0.69)
3.72 (0.39)
3.18 (0.49)
3.06 (0.61)
0.128 (0.0087)
0.122 (0.0051)
0.136 (0.0045)
0.111 (0.0027)
0.122 (0.0031)
0.125 (0.0042)
57.7 (4.48)
51.5 (2.76)
48.8 (3.27)
67.5 (2.89)
61.1 (3.60)
56.7 (2.99)
We have also examined the influence of monomer composi-
tion (detailed in Table 1) on the essential parameters above
discussed. Photo-DSC experiments showed almost identical
photopolymerization profile [Fig. 5(a)] for the formulations
containing 15 wt % UMA-Q1, UMA-Q2, or UMA-Chol into a
mixture of BisGMA and TEGDMA (F1–F3), for which the
polymerization rates are relatively narrow (0.122–0.136
* After 5 min of irradiation.
account the fact that the conversion degree for methacrylic
function depend on different factors (light intensity, initiator
type and concentration, temperature). If the rate of polymer-
ization for urethane methacrylate monomers varied between
0.085–0.107 sꢁ1, for BisGMA or TEGDMA this was around
0.056 sꢁ1, while for BisGMA-1 the rate increased slightly
(0.085 sꢁ1). As could be seen in Figure 4(a), the polymeriza-
tion rates for the LC monomers are relatively high (UMA-
Q2>UMA-Chol>BisGMA-1ꢂUMA-Q1>BisGMA>TEGDMA),
though their polymerization is a diffusion-controlled process,
which causes a restricted movement of macroradicals owing
to the increased viscosity. Under these circumstances, the
highest degree of conversion after a reaction time of 60 s of
s
ꢁ1). It may be seen that the degree of conversion in these
combinations of monomers depends on the polymerizable
monomer structure and viscosity, the observed increase
being between 44.8 and 57.7%.
This type of behavior suggests that the copolymerization is
no longer diffusion-controlled because the TEGDMA has
given to the reacting media a higher mobility preventing
autoacceleration. In the case of formulations (F4–F6) that
contain LC urethane methacrylate and BisGMA or BisGMA-1,
FIGURE 4 Photo-DSC rate profiles of the methacrylic monomers used (a) and plots of their double bond conversion versus irradia-
tion time (b). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
SYNTHESIS AND PROPERTIES OF URETHANE METHACRYLATES, BURUIANA ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Rate of polymerization and conversions for the F1–F3 formulations and for the mixture BisGMA/TEGDMA (a) and (b), for
the F4–F6 formulations and BisGMA-1/TEGDMA. Photopolymerization was initiated with Irgacure 651. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
the photopolymerization processes occurred at inferior rates
of polymerization (0.111–0.125 sꢁ1) comparatively with F1–
F3, where there is TEGDMA as reactive diluent. It can be
remarked that the final conversions (56.7–67.5%) reached
after 5 min of irradiation are relatively high [Fig. 5(b)]. The
photopolymerization rate (Rp ¼ 0.126 sꢁ1) for the mixture
BisGMA-1/TEGDMA was similar to that found in the F4–F6
formulations, but the conversion degree (50.8%) was lower
than in the monomer combinations (F4 > F5 > F6 >
BisGMA-1/TEGDMA). Thus, it can appreciate that the formu-
lations containing BisGMA are more reactive than those with
modified BisGMA, whereas the final degree of conversion is
higher by using of BisGMA-1. Finally, it is important to note
that it was not possible to discriminate the photoinitiator
reactivity (Irgacure 651), which initiated copolymerization of
the urethane LC momomers in all investigated formulations
(F1–F6), as confirmed the time to reach the maximum poly-
merization heat, tmax (2.94–3.18 s). Because Irgacure 819 is
known for its better photoinitiating activity, it is necessary
to test it in our experiments. By replacement of Irgacure 651
with Irgacure 819 in three formulations (F4–F6), the latter
exhibited a similar activity expressed through the maximum
rate of polymerization and double bond conversion increased
with less than 10% (Fig. 6).
shrinkage attributed to the compactly packed molecular
arrangement, which at isotropization temperature produce
low-viscosity mixtures that allow a better handling of high-
molecular weight monomers.30 Hence, the polymerization
shrinkage of the monomer mixtures based on 70 wt % LC
methacrylate and 30 wt % TEGDMA, together with the
photoinitiator system 4-(dimethylamino)-phenylacetic acid
(DMPheAA, 1%)/camphorquinone (CQ, 0.5%) was measured,
the obtained values being given in Figure 7. The low poly-
merization shrinkage found for UMA-Q1 (PS ¼ 3.43 vol %),
UMA-Q2 (PS ¼ 3.25 vol %), and UMA-Chol (PS ¼ 3.29
vol %) suggested that these values are acceptable because
each combination contains 30 wt % TEGDMA, whose poly-
merization shrinkage is of 12.32 vol %. Higher amount of
BisGMA besides TEGDMA (70/30 wt %) reduced the volumet-
ric shrinkage at 7.04 vol %.47 Following then the influence of
liquid crystalline nature on the shrinkage of polymerization, it
is clear that for all formulations the values (5.19 ꢃ 6.65
On changing the irradiation source (UV–vis light), photoini-
tiator (CQ/amine), and measuring method (Fourier trans-
form infrared spectroscopy), the degree of conversion deter-
mined through FTIR analysis (unshown here) for F4 (DC:
65%) is comparable with that above commented. At this
point, we should neither exclude the possibility that a post-
curing (dark postcure) to occur for such composites within
the first 24 h.46 Indeed, in FTIR evaluation after 24 h, the
formulation F4 irradiated with UV–vis light had an increase
in conversion of about 20%.
FIGURE 6 Rate of polymerization and conversions versus time
for the F4–F6 formulations. Photopolymerization was initiated
with Irgacure 819. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
Polymerization Shrinkage
It is well known that one of the most important features of
the polymerizable LC materials is their low polymerization
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filler (zirconium silicate nanopowder or silanized filler). The
analysis of the data illustrated in Figure 8(a) showed that
the water sorption values varied from 1.23 to 1.98% for all
resin composites (F1–F6), and water solubility values are in
the interval 0.18–0.35 wt %. Therefore, the hydrophilic cati-
onic groups from UMA-Q1 and UMA-Q2 increased the water
sorption values (F1, F2, F4, and F5), when compared with
the samples containing the UMA-Chol (F3 and F6), all these
values being comparable with literature data.50 Water sorp-
tion (1.04 wt %) and water solubility (0.14 wt %) were
improved in the case of F4-Sil formulation with silanized fil-
ler, comparatively with the formulations F4–F6 incorporating
BisGMA-1, for which a diminution of adsorbed water with
about 0.1–0.2% was found. In the series of urethane metha-
crylates, the highest percent of water solubility was deter-
mined for the composites with UMA-Chol monomer (F3 and
F6), probably due to the structural effect exercised of this
voluminous monomer in the photocrosslinked matrix.
Additionally, for such resin composites, the water contact
angle values give information on the wetting properties of
the surfaces and not only, because the hydrophilic functional
groups (hydroxyl, urethane, carboxyl, ester, and quaternary
ammonium) have impact on their behavior, for instance cell
attachment or proliferation.51 Herein, water contact angles of
the formulations (F1–F6) in air were measured using the
sessile droplet method. As shown in Figure 8(b), the water
contact angles of the composites based on BisGMA analog
(F4: 80.4ꢀ; F5: 75.5ꢀ; F6: 83.9ꢀ) increased with about 10–
15ꢀ when compared with those determined for composites
with BisGMA (F1: 66.3ꢀ; F2: 61.6ꢀ; F3: 73ꢀ). These results
suggest that the resin surfaces became less wettable by the
incorporation of BisGMA-1, observation attributable to the
decrease number of hydroxyl moieties remained after modifi-
cation with methacrylate photopolymerizable groups. The
incorporation of LC structures in these composites is
reflected through an increased hydrophobicity for the sam-
ples with urethane cholesteryl methacrylate (F3 and F6).
The combined presence of silanized silicate and LC mono-
FIGURE 7 Plot graph indicating the volumetric polymerization
shrinkage of urethane methacrylates containing 30% TEGDMA
(*) and for some formulations. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
vol %) found are comparable with the literature data, though
the amount of urethane methacrylate included in each sample
is small (15 wt %). By replacing the BisGMA with BisGMA-1,
the polymerization shrinkage decreased with almost 1 vol %.
Surface Analysis and Testing of Surface
Water sorption and water solubility are two processes, which
take place simultaneously, when the composite resins are in
contact with water and oral fluids and are key elements in
assessing their further performances, because the chemical
stability in a wet environment is crucial to guarantee the via-
bility of these materials. Generally, water sorption is diffu-
sion-controlled process and depends on the hydrophilicity48
and crosslinking degree of the cured resins.49 Water sorption
and water solubility measurements were performed in dis-
tilled water at 37 ꢀC, on samples containing additional 70%
mer/BisGMA-1/TEGDMA (F4-Sil) generated
a
modest
FIGURE 8 Percent of water sorption and water solubility (a) and contact angle values (b) for the resin composites. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
SYNTHESIS AND PROPERTIES OF URETHANE METHACRYLATES, BURUIANA ET AL.
2621

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 9 Bacteriostatic testing of the resin composites incorporating UMA-Q1 (a) and UMA-Q2 (b) against S. mutans. Addition-
ally, the inhibition spot for UMA-Q2-incorporated composite after the washing process of the disk was significantly reduced (b, in
set). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
increase of the water contact angle of about 1.5ꢀ, most prob-
EXPERIMENTAL
ably due to an incomplete silanization of the filler.
Materials
The commercial monomers used in this study are BisGMA
and TEGDMA (Sigma-Aldrich Chemical). 2-Dimethylaminoe-
thanol, 1-bromododecane, 1-bromohexadecane, cholesterol,
2-isocyanatoethyl methacrylate, and dibutyltin dilaurate were
purchased from Sigma Aldrich Chemical and used without
further purification. The initiators used were Irgacure 651,
Irgacure 819, camphorquinone (CQ), and 4-(dimethylamino)-
phenylacetic acid (DMPheAA; from Sigma-Aldrich Chemical).
The filler zirconium silicate nanopowder was purchased
from Sigma-Aldrich Chemical and has an average particle
diameter < 25 nm (XRD) as specified by the producer.
The filler was silanized by using the silane coupling agent
3-methacryloyloxypropyltrimethoxysilane (MPTS) from Sigma
Aldrich Chemical. For silanization, the zirconium silicate was
mixed with 1.0 wt % water and 5.0 wt % MPTS at room tem-
perature over a period of 2 h, the modified filler being subse-
quently dried at 50 ꢀC for 4 days, according to the method
from the literature.15
Preliminarily, the bacteriostatic susceptibility testing of two
composites (F4 and F5) was evaluated against Streptococcus
mutans, commonly found in dental field.52 The incidence of
quaternary ammonium groups on the resin composite sur-
face may offer an explanation of the bacteriostatic effect
observed by the incorporation of a low quantity (4.5 wt %)
of UMA-Q1 (F4) or UMA-Q2 (F5). In Figure 9, the images col-
lected indicate the size of a clear halo produced around the
hybrid composite films in the standard diffusion assay.
Therefore, the composite surface bearing dodecylammonium
groups (UMA-Q1) directly exposed to the medium is very
active in inhibiting bacterial growth, the diameter of inhibi-
tion zone being around 24 mm [Fig. 9(a)]. Compared with
UMA-Q1, the incorporation of the second monomer with a
long chain (C16) in the composite F5 was less efficient in
terms of bacteriostatic activity [Fig. 9(b)]. At first glance, this
observation would indicate that a certain part of the antimi-
crobial active monomer has not been covalently incorporated
in the cured composite and is diffusing from the correspond-
ing surface into the culture medium. Therefore, a larger zone
of microbial growth inhibition may be related to the fact the
UMA-Q1 diffuses more easily in the surrounding agar medium
killing microbe by a release mechanism of the cationic mono-
mer with lower viscosity. The same experiment performed
with the F5 disk after 2 h of washing with bidistilled water to
remove nonpolymerized monomer revealed the formation of a
small inhibition area of 1–2 mm around the composite film
[Fig. 9(b), in set]. This finding suggests that the release behav-
ior of soluble ammonium urethane acrylates together with the
persistency of the observed effect should be further examined,
because a long-term release of the antimicrobial agent is
essential. In agreement with the literature data,53 it seems
that the dental resin composites containing urethane metha-
crylates with quaternary ammonium groups could contribute
to the reduction of S. mutans bacteria growing, considered
responsible in the caries formation process.
Intermediates and Monomers Synthesis
For the preparation of the quaternary ammonium intermedi-
ates dodecyl(2-hydroxyethyl) dimethylammonium bromide
(Q1) and hexadecyl(2-hydroxyethyl) dimethylammonium bro-
mide (Q2) was used the same method, reason for that the
synthetic details given below are referring for Q1. Thus, 5 g
(56 mmol) 2-dimethylaminoethanol was treated with 17 mL
(56 mmol) 1-bromododecane in anhydrous acetone. The
reaction was carried out at reflux for 12 h, and then the ace-
tone was removed by distillation until the volꢀume reduced
to half. The remaining mixture was cooled at 5 C for several
hours, and the obtained product was isolated by filtration
and washed with ethyl ether.
Urethane methacrylate derivatives with quaternary ammo-
nium or cholesteryl moieties were synthesized starting from
30 mmol (10.14 g Q1; 11.82 g Q2; and 12.2 g cholesterol,
respectively) hydroxyl-capped derivatives dissolved in
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ARTICLE
SCHEME 3 Structure of the BisGMA modified with urethane methacrylic groups (BisGMA-1).
anhydrous THF and 4.33 mL (30 mmol) 2-isocyanatoethyl
UMA-Q2
methacrylate was added, the mixture being stirred at 40 ꢀC
for 24 h in the presence of dibutyltin dilaurate. The course
of the reaction was followed through the infrared absorption
of the isocyanate stretching band (2260 cmꢁ1). For removing
catalyst, the monomer solution in methylene chloride was
stirred with 2% HCl solution (4:1 v/v) saturated with salt
(NaCl) at room temperature. After separation of organic
phase and the evaporation of the solvent, the urethane meth-
acrylates UMA-Q1 and UMA-Q2 were collected as colorless
viscous liquids, and the cholesteryl urethane methacrylate
(UMA-Chol) as a white powder.
Yield: 16.1 g (98%). FTIR (KBr, cmꢁ1): 3376 (free NH); 3219
(bonded NH); 2854–2954 (CAH); 1719 (CO); 1638 and 815
(CH¼¼CA); 1535 (amide II); 1255 and 1166 (CAO). Anal.
Calcd for C27H53BrN2O4: C, 59.00; H, 9.72; N, 5.10. Found: C,
59.28; H, 9.87; N, 4.92. g ¼ 1.8 Pa s.
UMA-Chol
Yield: 15.4 g (95%). 1H NMR (CDCl3, d ppm): 6.7 (t, 1H,
NH); 6.13 (s, 1H, CH2¼¼C in trans position relative to CH3
unit); 5.6 (s, 1H, CH2¼¼C in cis position relative to CH3 unit);
5.37 (m, 1H, CH¼¼ from cholesteryl); 4.24 (m, 2H,
COOACH2ACH2ANH); 3.48 (m, 2H, COOACH2ACH2ANH);
1.95 (s, 3H, CH2¼¼CACH3); 2.32-0.66 (m, 2H, methyl and
methyn protons from cholesteryl). FTIR (KBr, cmꢁ1): 3365–
3257 (NH involved in the formation of hydrogen bonding);
2854–2956 (CAH); 1720 (CO); 1638 and 815 (CH¼¼CA);
1540 (amide II); 1257 and 1167 (CAO). Anal. Calcd for
Q1
Yield: 16.3 g (86%). 1H NMR (CDCl3, d ppm): 5.0 (1H, OH);
4.14 (m, 2H, HOACH2); 3.76 (m, 2H, HOACH2ACH2ANþ);
3.55 (m, 2H, CH2ANþ); 3.38 (m, 6H, CH3ANþ); 1.75 (m, 2H,
CH2ACH2ANþ); 1.25 (m, 18H, ACH2); 0.88 (m, 3H,
ACH2ACH3). IR (KBr, cmꢁ1): 3243 (OH); 2850–2953 (CAH);
1470 (CH2); 1080 (CANþ); 720 (CAH). Anal. Calcd for
C35H59NO4: C, 75.36; H, 10.66; N, 2.51. Found: C, 75.51;
ꢀ
H,10.89; N, 2.35. Mp: 116–118 C.
C
16H36BrNO: C, 56.79; H, 10.72; N, 4.14. Found: C, 57.12; H,
ꢀ
The synthesis and characterization of BisGMA functionalized
with urethane methacrylate groups (BisGMA-1) was described
in a paper submitted to publication, and the structure of
BisGMA is depicted in Scheme 3. Shortly, 5 g (10 mmol) of
BisGMA was dissolved in 20-mL THF, and 2.53-mL (18 mmol)
2-isocyanatoethylꢀ methacrylate was added, and the reaction
conducted at 40 C for 24 h. The consumption of isocyanate
groups was determined by FTIR spectroscopy. The procedure
for removing catalyst was the same as for LC monomers
except the brine solution. The final product (BisGMA-1) was
collected by solvent evaporation as a colorless viscous liquid.
10.86; N, 3.87. Mp: 193 C.
Q2
Yield: 17.5 g (79%). 1H NMR (CDCl3, d ppm): 5.0 (1H, OH);
4.12 (m, 2H, HOACH2); 3.76 (m, 2H, HOACH2ACH2ANþ);
3.56 (m, 2H, CH2ANþ); 3.38 (m, 6H, CH3ANþ); 1.75 (m, 2H,
CH2ACH2ANþ); 1.25 (m, 26H, ACH2); 0.88 (m, 3H,
ACH2ACH3). IR (KBr, cmꢁ1): 3243 (OH); 2850–2953 (CAH);
1470 (CH2); 1080 (CANþ); 720 (CAH). Anal. Calcd for
C
20H44BrNO: C, 60.89; H, 11.24; N, 3.55. Found: C, 61.13; H,
ꢀ
11.55; N, 3.36. Mp:203 C.
UMA-Q1
BisGMA-1
Yield: 14.3 g (97%). 1H NMR (CDCl3, d ppm): 6.8 (t, 1H,
NH); 6.13 (s, 1H, CH2¼¼C in trans position relative to CH3
unit); 5.58 (s, 1H, CH2¼¼C in cis position relative to CH3
unit); 4.57 (m, 2H, NHACOOACH2ACH2); 4.24 (m, 2H,
COOACH2ACH2ANH); 3.96 (m, 2H, COOACH2ACH2ANH);
3.6 (m, 2H, COOACH2ACH2ANþ); 3.44 (m, 8H, CH3ANþ and
CH2ACH2ANþ); 1.93 (s, 3H, CH2¼¼CACH3); 1.75 (m, 2H,
CH2ACH2ANþ); 1.25 (m, 18H, ACH2); 0.88 (m, 3H,
ACH2ACH3). FTIR (KBr, cmꢁ1): 3411 (free NH); 3231
(bonded NH); 2854–2956 (CAH); 1721 (CO); 1638 and 815
(CH¼¼CA); 1538 (amide II); 1257 and 1167 (CAO). Anal.
Calcd for C23H45BrN2O4: C, 55.97; H, 9.19; N, 5.68. Found: C,
55.82; H, 9.42; N, 5.37. g ¼ 0.6 Pa s.
Yield: 7.3 g (95 %). 1H NMR (CDCl3, d ppm): 7.11 and
6.81 (d, 8H, aromatic); 6.1 (d, 3.6H, CH2¼¼C in trans position
relative to CH3 unit); 5.57 (s, 3.6H, CH2¼¼C in cis position
relative to CH3 unit); 5.38–5.32 (2H, NH); 4.0–4.4 (m,
13.6H, CH2AOACO, CHAOACO and AOACH2ACHA); 3.5
(m, 3.6H, CH2ANHACO); 1.94 (d, 11.4H, CH2¼¼CACH3); 1.62
(m, 6H, CH3AC).
The content of urethane methacrylate structures from
BisGMA-1 was calculated by the comparison of the integral of
the aromatic protons to that of unsaturated protons. Accord-
ingly, it was found that the total number of methacrylate
groups in BisGMA-1 is about 3.8 unsaturated units/molecule,
SYNTHESIS AND PROPERTIES OF URETHANE METHACRYLATES, BURUIANA ET AL.
2623

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
and this corresponds to a functionalization degree of the
hydroxyl groups with urethane methacrylate groups of 90 %.
cylinders (ꢄ0.2 g in weight) obtained after photopolymeriza-
tion under UV/visible irradiation for 60 s on each side were
placed in a density bottꢀle of 20 cm3 volume containing dis-
IR (KBr, cmꢁ1): 3376 (NH); 2929–2962 (C–H); 1720 (C¼¼O);
1637 and 815 (CH2¼¼C); 1608 (aromatic); 1510 (amide II);
1169 (CAO). g ¼ 6.9 Pa s.
tilled water (qH O at 25 C ¼ 0.99707 g/cm3). A Partner bal-
2
ance (accuracy ¼ 60.000001 g) was used. Three successive
measurements for all tested composites were performed.
The polymerization shrinkage was determined separately for
each specimen using the eq 1:
Characterization
The intermediates and monomers structures were verified
by 1H NMR and FTIR spectroscopy using a Bruker Avance
DRX 400 spectrometer and a Bruker Vertex 70 FTIR spectro-
photometer, respectively. Elemental analysis (C, H, N) was
carried out by means of a CHNS 2400 II Perkin Elmer ana-
lyzer. Viscosity measurements of the urethane methacrylates
were determined with a Brookfield cone and plate viscome-
ter at room temperature. The test was run at spindle speeds
of 6 and 12 rpm, and the viscosity readings obtained were
recorded and expressed as Pascal second (Pa s). Thermal
transitions temperatures were determined by DSC (Pyris
Diamond DSC, Perkin Elmer, USA). The scan was run at heat-
ing and cooling rates of 15 ꢀC minꢁ1, under a nitrogen
atmosphere. Sample sizes of 10–11 mg were used, and the
inception of endotherms was taken as the transition temper-
ature. Optical investigations were carried out on an optical
microscope for reflected light, polarization, fluorescence, and
thermosystem (Leica DM2500 M) equipped with a heating
stage (Mettler Toledo FP82HT). Differential scanning photo-
dcured ꢁ duncured
Polymerization shrinkage ¼
ꢅ 100
(1)
dcured
where, d is the density of the formulation.
The filled resin composites for contact angle, water sorption,
water solubility determinations, and antimicrobial tests were
prepared by using 30 wt % organic matrix and 70 wt %
inorganic filler. The organic resin matrix consisted of a LC
monomer:BisGMA(BisGMA-1):TEGDMA mixture (15:49.25:
34.25, wt/wt) and CQ/DMPheAA as the initiator system. The
static water contact angle measurements were made on disc-
shaped specimens (15 6 0.1 mm diameter, 1 6 0.1 mm
thickness) using goniometer KSV Cam 200. Droplets (2 lL)
of double-distilled water were placed on the disc specimen
surface, the average contact angle being calculated starting
from at least 10 separate measurements. The water sorption
and water solubility were determined57 by preparing four
disk specimens of reduced dimensions (15 6 0.1 mm diame-
ter, 1 6 0.1 mm thickness) for each group of mixtures, using
a Teflon split ring mould between two glass plates covered
with polyethylene film. The composites were preconditioned
over a desiccant containing calcium sulfate at 37 ꢀC until
their weight remain constant (initial weight m1). Further,
calorimetry studies were performed on
a DuPont 930
apparatus with a double heat differential calorimeter 912,
calibrated with indium metal standard according to the liter-
ature.54
A standard high-pressure mercury lamp with
4.5 mW/cm2 light intensity was used for irradiating the sam-
ple (1.5 6 0.5 mg) in the presence of Irgacure 651 or Irga-
cure 819 as the initiator (1.5 wt %), using DSC DuPont
standard pans. The measurements were performed in an iso-
thermal mode under the ambient atmosphere, and irradia-
tion started after 1 min of equilibration. The heat flux as a
function of reaction time was monitored using DSC under
isothermal conditions, both the rate of polymerization and
conversion being calculated as a function of time.43 Pho-
toDSC data were analyzed using the one-way ANOVA test.
For the determination of the conversion degree by FTIR,
a mixture of 15% UMA-Q1, 49.25% BisGMA-1, 34.25%
TEGDMA, and camphorquinone (CQ, 0.5%)/4-(dimethyla-
mino)-phenylacetic acid (DMPheAA, 1%) as the photoinitia-
tor was homogenized and manually coated on the KBr plate.
The UV/visible irradiation were performed with a dental de-
vice (LA 500, Blue Light, k ¼ 400–500 nm, LA 500, Blue
Light, k ¼ 400–500 nm, intensity of light 14 mW/cm2), and
the FTIR absorption spectra were measured as a function of
irradiation time, and the degree of conversion was deter-
mined from the differences appeared in the C¼¼C stretching
ꢀ
specimens were placed in distilled water at 37 C for differ-
ent time intervals and then removed from the water, lightly
blotted with a paper to eliminate the surface-adherent water,
and weighed. After 3 weeks (m2), specimens were placed
into a desiccator with calcium sulphate and dried at 37 ꢀC
until their weight was constant again (m3). The water solu-
bility for each sample was determined using the eq 2:
m1 ꢁ m3
Water solubilityðwt %Þ ¼
ꢅ 100
(2)
m1
whereas, the water sorption was calculated using eq 3:
m2 ꢁ m3
Water sorptionðwt %Þ ¼
ꢅ 100
(3)
m1
For antimicrobial testing of the resin composites containing
the quaternary ammonium methacrylates (4.5 wt %), the
agar disc diffusion method was used. The bacterium (Strep-
tococcus mutans ATCC 35688) was subcultured to nutrient
agar and incubated overnight at 37 ꢀC. Then, the cells were
dispersed in the same medium to reach 104–105 CFU/ml.
The agar plates were streaked with a sterile swab moistened
with the bacterial suspension. The composite films (diame-
ter: 8 mm, thickness: 0.15 mm) previously sterilized by irra-
diation also were placed over the surface of the agar plates
vibration at 1637 cmꢁ1 55
The polymerization shrinkage of
.
cured and uncured specimens tested in the present study
was determined by the density bottle method, at room tem-
perature.56 Photocurable pastes for volumetric shrinkage
measurement were formulated using a weight ratio of ure-
thane monomer:BisGMA (BisGMA-1):TEGDMA:CQ:DMPheAA
of 15:49.25:34.25:0.5:1. The specimens in the form of small
2624
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ARTICLE
10 Khatri, C. A.; Stansbury, J. W.; Schultheisz, C. R.; Antonucci,
and then incubated. All the test plates were incubated over-
night at 37 ꢀC. Bacteriostatic effect was determined by the
ability of the material to inhibit bacterial growth on the agar
surface in contact with it. The reactions of the microorgan-
isms with the resin composites were assessed by measuring
the size of the inhibitory zone.
J. M. Dent Mater 2003, 19, 584–588.
11 Prakki, A.; Tallury, P.; Mondelli, R. F. L.; Kalachandra, S.
Dent Mater 2007, 23, 1199–1204.
12 Galli, G.; Gasperetti, S.; Bertolucci, M.; Gallot, B.; Chiellini.
F. Macromol Rapid Commun 2002, 23, 814–818.
CONCLUSIONS
13 Chappelow, C. C.; Pinzino, C. S.; Chen, S. S.; Jeang, L.; Eick,
J. D. J Appl Polym Sci 2007, 103, 336–344.
In summary, urethane methacrylates containing alkylammo-
nium (C12, C16) or cholesteryl mesogens were synthesized,
characterized, and formulated in dental resin composites.
Incorporation of a low quantity of LC monomer (15 wt %)
into a combination of dental monomers is capable to retain
its LC nature in the uncured formulations. Aspects concern-
ing their photoreactivity showed that the degree of conver-
sion increased by using of LC monomer and BisGMA modi-
fied with urethane-methacrylate functionalities (90%) into a
formulation based on these monomers and TEGDMA. For the
same formulations, the polymerization shrinkage and water
sorption values decreased. Composite prepared by mixing of
LC monomer (UMA-Q1), Bis-GMA-1, and TEGDMA with silan-
ized zirconium silicate presented lower water sorption/
solubility values, and a small increase of hydrophobicity. The
polymer resin composites prepared from suitable formula-
tions containing a cationic methacrylate provided bacterio-
static activity against S. mutans, especially that based on LC
methacrylate with dodecylammonium groups (4.5 wt %).
14 Matinlinna, J. P.; Lassila, L. V. J.; Kangasniemi, I.; Yli-Urpo,
A.; Vallittu, P. K. Dent Mater 2005, 21, 287–296.
15 Moszner, N.; Gianasmidis, A.; Klapdohr, S.; Fischer, U. K.;
Rheinberger, V. Dent Mater 2008, 24, 851–856.
16 Gungor, A.; Kayaman-Apohan, N.; Mert, A.; Kahraman, M.
¨
¨
V. J Polym Res 2008, 15, 389–395.
17 Ernst, C. P.; Galler, P.; Willershausen, B.; Haller, B. Dent
Mater 2008, 24, 319–327.
18 Weinmann, W.; Thalacker, C.; Guggenberger, R. Dent Mater
2005, 21, 68–74.
19 Kleverlaan, C. J.; Feilzer, A. J. Dent Mater 2005, 21,
1150–1157.
20 Moszner, N.; Fischer, U. K.; Angermann, J.; Rheinberger, V.
Dent Mater 2008, 24, 694–699.
21 Kerby, R. E.; Knobloch, L. A.; Schricker, S.; Gregg, B. Dent
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22 Ferracane, J. L. Dent Mater 2011, 27, 29–38.
The authors (T. Buruiana and V. Melinte) gratefully acknowl-
edge the financial support of the European Social Fund—‘‘Cris-
tofor I. Simionescu’’ Postdoctoral Fellowship Programme (ID
POSDRU/89/1.5/S/55216), Sectorial Operational Programme
Human Resources Development 2007–2013. The authors
thank the PRAXIX Center for the antimicrobial test and contri-
butions through many discussions.
23 Kim, J.-G.; Chung, C.-M. Biomaterials 2003, 24, 3845–3851.
24 Podgo´ rski, M. Dent Mater 2010, 26, 188–194.
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26 Buruiana, T.; Melinte, V.; Stroea, L.; Buruiana, E. C. Polym J
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