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Journal of Macromolecular Science, PartA: Pure and Applied ChemistryPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lmsa20
Preparation and Characterizationof Hydroxyapatite ImpregnatedSemi‐interpenetrating PolymerNetworks (IPNs) of Polyvinyl Alcohol andPoly(Acrylamide‐co‐acrylic Acid)A. K. Bajpai a & Raghvendra Singh aa Bose Memorial Research Laboratory, Department of Chemistry ,Government Autonomous Science College , Jabalpur (M.P.), 482001,IndiaPublished online: 22 Aug 2007.
To cite this article: A. K. Bajpai & Raghvendra Singh (2004) Preparation and Characterization ofHydroxyapatite Impregnated Semi‐interpenetrating Polymer Networks (IPNs) of Polyvinyl Alcoholand Poly(Acrylamide‐co‐acrylic Acid), Journal of Macromolecular Science, Part A: Pure and AppliedChemistry, 41:10, 1135-1159, DOI: 10.1081/MA-200026560
To link to this article: http://dx.doi.org/10.1081/MA-200026560
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Preparation and Characterization ofHydroxyapatite Impregnated
Semi-interpenetrating Polymer Networks(IPNs) of Polyvinyl Alcohol andPoly(Acrylamide-co-acrylic Acid)
A. K. Bajpai* and Raghvendra Singh
Bose Memorial Research Laboratory, Department of Chemistry,
Government Autonomous Science College, Jabalpur (M.P.), India
ABSTRACT
A semi-interpenetrating polymer network (IPN) of poly(vinyl alcohol) and poly(acryl-
amide-co-acrylic acid) was prepared, and the formation of calcium hydroxyapatite
(HAP) on/in the polymeric network was studied by a novel alternate soaking
process. The prepared HAP–IPN matrix was characterized by IR, x-ray diffraction
(XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA),
and scanning electron micrograph (SEM) analyses, and structural parameters of the
IPN such as average molecular weight between crosslinks (Mc), crosslink density
(q), and number of elastically effective chains (Ve) were calculated from water
uptake measurements. The swelling of the IPN in phosphate and calcium solutions
and the amount of HAP formed were investigated as a function of the chemical
architecture of the polymeric matrix. The influence of number of reaction cycles and
1135
DOI: 10.1081/MA-200026560 1060-1325 (Print); 1520-5738 (Online)
Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: A. K. Bajpai, Bose Memorial Research Laboratory, Department of Chemistry,
Government Autonomous Science College, Jabalpur (M.P.) 482001, India; E-mail: akbajpailab@
yahoo.co.in.
JOURNAL OF MACROMOLECULAR SCIENCEw
Part A—Pure and Applied Chemistry
Vol. A41, No. 10, pp. 1135–1159, 2004
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temperature of the swelling medium on the amount of HAP formed within the IPN were
also studied.
Key Words: Polyvinyl alcohol; IPN; Hydroxyapatite; Characterization; Swelling.
INTRODUCTION
The recent past has witnessed a great concern over exploring the possibility for using
polymers as substrates for the selective deposition of sparingly soluble salts, as it is of
paramount importance from the point of view of both modeling of processes related to
biomineralization[1] and for the design and development of novel biomaterials.[2] In the
biomedical field, the synthesis of hydroxyapatite (HAP)/polymer composite materials is
of great interest for the development of biomaterials suitable to repair the skeletal
system.[3 – 5] HAP, Ca10(PO4)6(OH)2, is the main inorganic component of human bones
and teeth, and possesses significant bioactive and osteoconductive properties.[6] There
are several reasons to consider HAP powder as an appropriate reinforcement for
organic polymers, as HAP-filled composites using organic polymers have been largely
used as bone cements,[7] dental implants,[8] bone replacement materials,[9,10] etc. More-
over, polymer–HAP composites have also been attempted in hard tissue engineering in
order to substitute large bone defects.[11] One of the main advantages of HAP/polymer
composites with respect to HAP biomaterials is the possibility to modulate biodegrada-
bility, bioactivity, and mechanical properties through variations in compositions. Further-
more, the presence of the polymer could improve the interfacial bonding of the composite
with bone tissue.[12]
Several strategies have been developed to produce apatite crystal deposition on a
substrate such as thermal sprayed and plasma sprayed coating,[13] precipitation,[14] and
soaking in simulated body fluid.[15] All these methods are basically aimed at developing
a novel material that mimics bone structure. In previous studies, hydrogel–apatite compo-
sites were prepared by a biomimetic process which was developed by Kokubo and
coworkers.[16] This process could effectively produce apatite on the surface of various
kinds of matrix; however, it takes a lengthy time to form a large amount of apatite
on/in hydrogels using this process.[17]
Very recently, a novel technique has been developed[18] that involves an alternate
soaking process with great potential to form calcium phosphate on/in biomaterials over
a short period of time. The method has been frequently employed in apatite–polymer
composite formation.[19]
Polyvinyl alcohol (PVA) is a water soluble, non-toxic, non-immunogenic polymer
with a remarkable film forming property. It has been widely used as a basic material
for a variety of biomedical applications such as contact lenses material, skin replacement,
artificial cartilage replacement,[20] etc. Thus, realizing the vital role of PVA polymers in
biomedical engineering, the present investigation aims at preparing apatite–polymer com-
posites by alternate soaking process of a semi-interpenetrating polymer network (IPN) of
PVA and poly(acrylamide-co-acrylic acid). Polymers of acrylamide (AM) and acrylic acid
(AA) are well-known hydrophilic polymers and have been greatly used in making water-
sorption materials. Moreover, polymers of AA also show a pH dependent swelling, which
permits their use as pH sensitive biomaterials.[21]
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EXPERIMENTAL
Materials
PVA (hot processed, M.Wt. 40,000, degree of hydrolysis 98.6%) was obtained from
Burgoyne Burbidges & Co. (Mumbai, India) and used without further purification. AM
(Research Lab., Pune, India) was crystallized twice from methanol (Guaranteed
Reagent Grade) and dried under vacuum over anhydrous silica for a week. AA was
freed from the inhibitor by vacuum distillation and collecting the middle fraction. N,N0-
Methylene bisacrylamide (MBA) (Central Drug House, Mumbai, India) and potassium
persulfate (KPS) (Loba Chemie, India) employed as crosslinking agent and polymeriz-
ation initiator, respectively, were used as received. Na2HPO4, CaCl2, and Tris(hydroxy
methyl) aminomethane (Tris) were purchased from E. Merck (India) and used as received.
Triple distilled water was used throughout the experiments.
Preparation of IPN Hydrogel
An IPN may be defined as an intimate mixture of two or more polymers when
at least one monomer has been polymerized in the immediate presence of another
preformed polymer.[22] In the present work, the polymerization was carried out by a
free radical polymerization method as reported in the literature.[23] In a typical exper-
iment, 3.0 g PVA, 14.0 mM AM, 14.5 mM AA, 0.129 mM MBA, 0.094 mM KPS, and
25 mL distilled water were added in a petri dish (diam. 200, Corning). The mixture
was homogenized by manual mixing, and kept at 708C in an oven for 3 hr. The prepared
IPN was allowed to swell in water for 72 hr so that the unreacted chemicals were leached
out and the gel becomes pure. The swollen gel was cut into small circular discs 4 mm in
diameter and dried at room temperature for a week. The dry discs were stored in airtight
polyethylene bags.
IR Spectra
The IR spectral analysis was carried out on a Perkin–Elmer spectrophotometer
(FTIR, Paragon 1000).
X-Ray Diffraction Studies
X-ray diffraction (XRD) patterns of HAP impregnated IPNs were recorded on a
Philips (Holland) automated x-ray powder diffractometer. For XRD study, the dried
IPN sample was placed on the glass slide specimen holder and exposed to x-rays in vertical
goniometer assembly. The scan was taken between 108 and 658 with a scanning speed of
2.48min21. The operating target voltage was 35 kV, tube current was 20 mA, and radi-
ations used were Fe Ka (l ¼ 0.193 nm).
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Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was carried out using a Perkin–Elmer Thermal
Analyzer TGS-4 in N2 atmosphere at a heating rate of 108C min21 up to 9008C. The
sample weights were in the range 5–10 mg.
Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was carried out on a Mettler DSC instrument
in N2 atmosphere at a heating rate of 108C min21.
Scanning Electron Micrograph
Scanning electron micrographs (SEMs) of IPN and apatite impregnated IPN were
recorded on a SEM (JEOL ISM 5200).
Swelling Experiments
The extent of swelling was determined by a conventional gravimetric procedure as
described in our earlier communications.[24,25] In a typical experiment, preweighed pieces
of IPNs were allowed to swell in triple distilled water, P, and Ca solutions, respectively,
for a predetermined time period, taken out at definite time intervals, pressed gently
between two filter papers to remove excess of solvent, and finally weighed in a sensitive
balance. The degree of swelling was expressed in terms of the swelling ratio as given below:
Swelling ratio ¼Weight of the swollen gel
Weight of the dry gelð1Þ
HAP Formation on/in Swollen IPN
The HAP formation on/in the IPN was achieved by an alternating soaking process as
described elsewhere[18] and has been shown in Fig. 1. In brief, a preweighed piece of IPN
was first immersed in 25 mL CaCl2 (200 mM)/Tris–HCl (pH 7.4) aqueous solution (Ca
solution) at 278C for 2 hr, followed by rinsing at 378C with distilled water. After
rinsing, the swollen IPN was allowed to swell in Na2HPO4 (120 mM) aqueous solution
(P solution) at 378C for 2 hr. Through these two steps, the formation of apatite on/in
the IPN was done by alternately soaking the IPN in these solutions. A photograph of
IPN with apatite formation is shown in Fig. 2. The amount of apatite formed on/in IPN
was calculated by the following equation:
Apatite formed ¼Wap �Wo
Wo
ð2Þ
where apatite formed is the weight of apatite formed in mg g21 gel, Wap is the weight of the
PVA–IPN–apatite in a dry state, and Wo is the weight of PVA–IPN in a dry state.
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Figure 1. Schematic presentation of experimental procedures for obtaining HAP.
Figure 2. A photograph depicting the formation of HAP on/in the IPN.
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RESULTS AND DISCUSSION
Characterization of HAP–IPN Matrix
IR Spectral Analysis
The IR spectra of PVA–IPN is shown in Fig. 3(a). The spectra clearly mark the
presence of PVA as evident from a broad band appeared at 3117–3575 cm21 (due to
H-bonded hydroxyls); methylene group at 2963 cm21 (due to asymmetrical stretching,
i.e., y asCH2) and at 2786 cm21 (symmetrical stretching, y sCH2); amide group at
3575 cm21 (N–H stretching); and carboxyl groups at 1759 cm21 (C55O stretching
band), 1599 cm21 (asymmetrical stretching of COO2), and 1414 cm21 (due to symmetri-
cal stretching of –COO2). The spectra also show a prominent band at 2172 cm21, which
could be attributed to the presence of –C–N group of crosslinking agent, i.e., MBA.
Thus, the IR spectra of the IPN confirm the presence of PVA, AM, AA, and MBA.
The IR spectra of HAP impregnated IPN is also depicted in Fig. 3(b), which clearly
confirms the formations of HAP on/in the IPN. The observed bands at 1030, 690, 664, and
560 cm21 corresponding to P–O asymmetrical and symmetrical stretching vibrations and
O–P–O bending vibrations of the phosphate groups[26] and at 630 cm21 corresponding to
rotating vibrations of OH groups[27] of HAP clearly identify the substance formed as HAP.
Network Studies
One of the most important structural parameters characterizing a crosslinked polymer
is the average molecular mass between crosslinks, which is directly related to the crosslink
Figure 3. The IR spectra of the IPNs (a) without apatite and (b) with apatite.
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density. The magnitude of Mc significantly affects the physical and mechanical properties
of crosslinked polymers, and its determination has great practical significance. Equili-
brium swelling is widely used to determine Mc. Early research by Flory and Rehner
laid the foundation for the analysis of equilibrium swelling. According to the theory of
Flory and Rehner, for a perfect network:
Mc ¼�V1dpðV
1=3s � Vs=2Þ
lnð1� VsÞ þ Vs þ xV2s s
ð3Þ
where Mc is the number average molar mass of the chain between crosslinks: V1 is the
molar volume (mL mol21); dp is the polymer density (g mL21); Vs is the volume fraction
of polymer in the swollen gel; and x is the Flory–Huggins interaction parameter between
solvent and polymer.
The swelling ratio is equal to 1/Vs. Here, the crosslink density q is defined as the mole
fraction of crosslinked units.
q ¼Mo
Mc
ð4Þ
where Mo is the molar mass of the repeating unit.
Other authors defined a crosslink density, Ve, as the number of elastically effective
chains totally included in a perfect network per unit volume being simply related to q since:
Ve ¼dpNA
Mc
ð5Þ
where NA is the Avogadro number. Then:
Ve ¼dpNAq
Mc
ð6Þ
Since the hydrogel is a copolymer structure, the molar mass of the polymer repeat
unit, Mo, can be calculated using the following equation:
Mo ¼nAMMAM þ nAAMAA
nAM þ nAA
ð7Þ
where nAM and nAA are the number of moles of AM and AA, whereas MAM and MAA being
the molar masses of the two monomers, respectively.
In a similar way, the value of polymer–solvent interaction parameter may also be
calculated, i.e.,
x ¼nAMxAM þ nAAxAA
nAM þ nAA
ð8Þ
The value of V1, xAM, and xAA were taken from the literature.[28,29] The values of
Mc, q, and Ve calculated for varying IPN compositions are summarized in Table 1,
which clearly reflects how the average molecular weight and crosslink density vary
with the chemical architecture of the IPNs.
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XRD Studies
The formation of HAP on/in the swollen IPN was confirmed by XRD studies as
shown in Fig. 4. The results clearly indicate the occurrence of broad peaks around 268,31.68, 32.28, and 32.98, which are assigned to the main peaks of the HAP crystals.[30] It
is also notable from the spectra that the HAP impregnated in the IPN is less crystalline
in comparison with the commercial grade HAP spectra as evident from the broad
nature of the HAP–IPN peaks against the sharp peaks of commercial grade pure HAP.
DSC Studies
Thermal characterization of the prepared IPN and HAP impregnated IPN has been
performed by constructing their thermograms as shown in Fig. 5(a) and (b), respectively.
The thermogram shown in curve (a) clearly presents combined thermal features of IPN
components, i.e., PVA, polyacrylamide (PAM), and polyacrylic acid (PAA), respectively.
A broad curve obtained between the temperatures 508C and 1508C contains minor
endothermic peaks at 658C, 958C, and 1208C, which may be assigned to glass transition
temperatures of PVA, PAM, and PAA, respectively. Two major endotherms appearing
at 2058C and 2168C may be attributed to melting of PAM chains and PVA contained in
the IPN, respectively. The thermogram also displays endotherms at a higher temperature
side and they may be attributed to degradation of constituent polymers. For example, a
minor endotherm at 3108C implies for the onset of PVA degradation, whereas a relatively
sharp endotherm at 3758C suggests for the decomposition of side groups of PVA. Simi-
larly, endotherm at 3458C may be attributed to excessive thermal degradation of PAM.
Thus, beyond 3008C the polymer matrix undergoes breakdown of macromolecular chains.
Table 1. Data showing the network parameters of IPNs of varying composition.
S. no.
PVA
(g)
AM
(mM)
AA
(mM)
MBA
(mM) Mc � 1023 qe � 102 Ve � 10220
1 2.0 14.0 14.5 0.129 1.24 5.86 7.2
2 2.5 14.0 14.5 0.129 2.63 2.75 3.42
3 3.0 14.0 14.5 0.129 11.6 0.62 0.775
4 3.5 14.0 14.5 0.129 0.07 8.29 10.2
5 3.0 7.0 14.5 0.129 8.23 0.87 1.09
6 3.0 14.0 14.5 0.129 11.65 0.62 0.775
7 3.0 21.1 14.5 0.129 5.23 1.36 1.72
8 3.0 28.1 14.5 0.129 3.69 1.93 2.44
9 3.0 14.0 7.2 0.129 10.120 0.718 0.892
10 3.0 14.0 14.5 0.129 11.65 0.62 0.775
11 3.0 14.0 21.8 0.129 2.07 3.45 4.356
12 3.0 14.0 29.1 0.129 1.006 7.12 8.97
13 3.0 14.0 14.5 0.129 11.65 0.62 0.775
14 3.0 14.0 14.5 0.194 5.14 1.41 1.755
15 3.0 14.0 14.5 0.259 1.38 5.25 6.52
16 3.0 14.0 14.5 0.320 0.66 7.82 9.20
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The thermogram depicted in curve (b) indicates a broad endotherm up to about 3508C,
which is indicative of enhanced thermal stability of the IPN. A major exothermic peak at
5198C implies for decomposition of HAP in the matrix. The obtained higher value of
enthalpy of fusion (4.078 kJ/q) clearly suggests for a crystalline nature of the impregnated
HAP.
Thermogravimetric Analysis
In order to assess the thermal stability of HAP–IPN composite thermograms were
constructed as shown in Fig. 6. It is clear from the TG–DTG curves that onset of two
major decomposition occurs at 3108C and 5198C, which may be assigned to polymer
and HAP decompositions, respectively. Since the IPN contains the major proportion of
PVA (% w/w), its decomposition mainly involves dehydration accompanied by the for-
mation of some volatile products. The residue may contain predominantly polymers
with conjugated unsaturated structures.[31] The second decomposition at 5198C may be
majorly attributed to the conversion of the HPO422 present on the HAP surface to
pyrophosphate and water release[32] as given below:
2HPO2� ! P2O4�7 þ H2O
Thus, the peak observed at 5198C also provides additional evidence for the presence
of HAP in the IPN.
Figure 4. XRD patterns of (a) pure HAP and (b) HAP–IPN matrix.
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SEM Studies
SEMs shown in Fig. 7 clearly reveal the needle shape crystal structure of HAP. As can
be seen, most of the crystals are aggregated in clusters while the few isolated ones appear
very small.
Swelling in P and Ca Solutions
In order to determine the time period for equilibrium swelling of the IPN in P and Ca
solutions, preweighed pieces of IPNs were allowed to swell in the two solutions and the
weights of swollen IPNs were recorded after definite time intervals. The results are
shown in Fig. 8, which clearly reveal that the swelling ratio increases with time and
after 2 hr it attains a constant value. The results also indicate that the IPN swells much
Figure 5. DSC curves of (a) IPN and (b) HAP–IPN matrix.
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greater in P solution than in Ca solution. The observed greater swelling in P solution could
be attributed to the fact that P solution contains negatively charged phosphate ions and
when the ions diffuse into the IPN, they interact with the carboxylate ions (–COO2) of
PAA segment of the IPN and, thus, cause repulsion among the network chains. This
obviously results in a relaxation of network chains, which consequently imbibe greater
amount of solution and, therefore, display larger swelling.
In the case of Ca solution, the positively charged Ca2þ ions diffuse into the IPN and
interact with carboxylate ions of PAA, thus undoing the anionic nature of the network
chains. This obviously results in shrinkage of the IPN and, therefore, a lower degree of
sorption of Ca solution is noticed.
It is worth mentioning here that the order of swelling of IPN in the P and Ca solution
was as follows: first the IPN was swollen in P and then in Ca solution. The reason is quite
obvious now as the swelling results clearly imply that a greater swelling in P loads greater
number of phosphate ions in the IPN, which may favorably react with the Ca2þ ions when
the IPN swells in Ca solution. The following reaction may occur
10CaCl2 þ 6Na2HPO4 þ 2H2O! Ca10ðPO4Þ6ðOHÞ2 þ 12NaClþ 8HCl
Effect of Composition of IPN on Swelling
The significance of swelling process in apatite formation is well recognized. Thus, it is
essential to analyze how the chemical architecture of the IPN affects the extent of swelling
in P and Ca solutions, respectively.
Figure 6. TGA plots of HAP–IPN matrix.
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Effect of PVA
PVA is a hydrophilic polymer and its increasing amount in the feed composition is
expected to enhance the hydrophilicity of the network and subsequently the degree of
swelling also. In the present study, the effect of PVA on the swelling ratio of the
IPNs has been investigated by varying the concentration of PVA in the range 2.0 –
3.5 g. The results are shown in Fig. 9, which imply that the swelling ratio increases
with increase in PVA concentration from 2.0 to 3.0 g, while a drastic fall is noticed
beyond 3.0 g of PVA content. The observed results may be explained by the fact that
initially due to an increase in hydrophilicity of the IPN, a greater number of water
molecules are attracted and, therefore, the IPN swells. However, beyond an optimum
concentration of PVA (3.0 g in the present case) the IPN becomes so dense that the phos-
phate ions are hindered from entering into the gel and as a result the swelling ratio
decreases. The observed results are also supported by the crosslink density data as sum-
marized in Table 1. A similar explanation may also be given to explain the swelling of
the IPN in Ca solution.
Figure 7. SEM of HAP–IPN matrix.
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Effect of AM
AM is a hydrophilic and non-ionic monomer. When the concentration of AM is varied
in the range 7.0–28.1 mM, the swelling ratio appreciably increases up to 14.0 mM of AM
and beyond it a sudden fall in the swelling ratio is noticed. The results depicted in Fig. 10
may be attributed to the reason that with increasing AM in the IPN, the length of PAM
segments increases in the IPN, thus creating wide voids in the IPN. This obviously facilitates
the passage of phosphate and calcium ions into the gel and the swelling ratio increases.
However, beyond 14.0 mM of AM content the network chain density becomes so high
that the chain relaxation becomes unlikely and the phosphate ions do not get into the gel
easily. This obviously results in a suppressed swelling ratio of the IPN.
Effect of AA
The swelling ratio (Q) of a hydrogel can best be described by Flory’s swelling theory
as given below:
Q5=3 ¼½ði=2 VuS1=2Þ þ ð1=2� X1Þ=V1�
1=2
Ve=Vo
ð9Þ
Figure 8. Swelling ratio (SR) vs. time curves for the swelling of the IPN in (W) calcium and
(†) phosphate solutions for a definite composition of IPN. [PVA] ¼ 3.0 g, [AM] ¼ 14.0 mM,
[AA] ¼ 14.5 mM, [MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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where i/Vu is the concentration of fixed charge referred to the swollen network; S, the
ionic concentration in the external solution; (1/2 2 X1)/V1, the affinity of the hydrogel
with water; and Ve/Vo is the crosslinked density of the hydrogel. Q has a relation to
the ionic osmotic pressure, crosslinked density, and affinity of the hydrogel with water.
As one of the monomers of the IPN is ionic (AA), an increase in its feed concentration
will certainly result in an increase in i/Vu term and as a consequence the swelling
ratio will increase.
In the present study the effect of AA on the swelling ratio of the IPN has been inves-
tigated by varying the AA in the concentration range 7.2–29.1 mM in the feed mixture of
the IPN. The results are shown in Fig. 11, which indicate that the swelling ratio increases
with increase in AA in the hydrogel up to 14.5 mM, while a drastic fall is noted beyond
14.5 mM of AA. The observed findings may be explained by the fact that on increasing
AA content in the IPN, the IPN acquires increasing anionic charge due to dissociable
–COOH groups, which results in expansion of the network due to inter-chain repulsions
between –COO2 charges. Obviously, in the expanded IPN, more and more P and Ca solu-
tions will be imbibed. However, beyond 14.5 mM of AA, much greater negatively charged
Figure 9. Effect of PVA content in the IPNs on their swelling ratio in (W) calcium and (†) phos-
phate solutions for a given composition of the IPN. [AM] ¼ 14.0 mM, [AA] ¼ 14.5 mM,
[MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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network chains repel phosphate ions from entering into the IPN and the swelling ratio
decreases. In the case of Ca solution, the positively charged Ca2þ ions will bind to the car-
boxylate ions of the AA and, therefore, the IPN undergoes a shrinkage as their negative
charges are nullified.
Effect of Crosslinker
One of the effective means of modifying sorption behavior of a polymeric matrix is to
vary the crosslink density of the network by incorporating varying amounts of crosslinker
in the feed mixture of the gel. In the present study, this has been achieved by employing
varying concentration of the crosslinker MBA in the feed mixture in the range 0.064–
0.259 mM. The results are depicted in Fig. 12, which indicate that the swelling ratio
constantly decreases with increase in concentration of crosslinking agent. The results
are quite obvious and can be explained by the fact that an increased number of crosslink
Figure 10. Effect of AM content in the IPNs on their swelling ratio in (W) calcium and (†) phos-
phate solutions for a given composition of the IPN. [PVA] ¼ 3.0 g, [AA] ¼ 14.5 mM,
[MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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points results in a decrease in diffusivity of phosphate and calcium ions and water
molecules, which, in turn, lowers the swelling ratio.
Some authors[33] have reported that the introduction of crosslinker into the gel tends
to increase the glass transition temperature (Tg) of the gel, which results in a restrained
segmental mobility of macromolecular chains and, therefore, lowers the swelling ratio
of hydrogel.
Effect of IPN Composition on Apatite Formation
It is now a well-recognized fact that the apatite formation on/in a polymeric matrix is
basically determined by the extent of swelling of the matrix in the P and Ca solutions.
Since the swelling of the hydrogel is a function of the chemical architecture of the gel,
obviously the amount of apatite formed on/in the gel will also depend on the chemical
constitution of the gel.
Figure 11. Effect of AA content in the IPNs on their swelling ratio in (W) calcium and (†) phos-
phate solutions for a given composition of the IPN. [PVA] ¼ 3.0 g, [AM] ¼ 14.0 mM,
[MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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When the concentration of PVA varies in the range 2.0–3.5 g, the amount of apatite
formed also varies. As shown in Table 2, the amount of apatite formed increases up to 3.0 g
of PVA content, while beyond it a decrease in apatite formed is noticed. The results are
quite obvious and may be explained by the fact that the swelling of the IPN in the both
P and Ca solutions also increases in the range 2.0–3.0 g and a fall is observed beyond
3.0 g of PVA. The obtained results also confirm the dependence of apatite formation on
the extent of swelling.
In the case of variation in AM in the range 7.0–28.1 mM, the amount of apatite
formed increases with an increase in AA concentration in the range 7.0–14.0 mM,
while beyond 14.0 mM a decrease in apatite formed is noticed. The observed results are
summarized in Table 2 and may be explained on the basis of the fact that the IPNs also
display a corresponding variation in their swelling behavior, which accordingly affects
the apatite formation on/in the IPN. The variation in swelling ratio of the IPN with
AM concentration has already been explained earlier.
In the same way, the variation in AA and crosslinker concentrations in the range 7.2–
29.1 mM and 0.064–0.259 mM, respectively, has also been found to influence the amount
Figure 12. Effect of crosslinker (MBA) content in the IPNs on their swelling ratio in (W) calcium
and (†) phosphate solutions for a given composition of the IPN. [PVA] ¼ 3.0 g, [AM] ¼ 14.0 mM,
[AA] ¼ 14.5 mM, and temperature ¼ 278C + 0.28C.
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of apatite formed. The results are summarized in Table 2, which may again be explained
on the basis of the swelling-response of the IPNs to the concentration variation of AA and
crosslinking agent, respectively.
Effect of Reaction Cycles
The amount of apatite formed should progressively increase with increase in number
of reaction cycles which allow P and Ca solutions to interact on/in the IPN. However, a
significant observation has also been noted that the number of reaction cycles when the
apatite formation attains a limiting value greatly depends on the chemical composition
of the IPN. The forthcoming para discusses the influence of chemical composition of
the IPNs on the number of cycles required for optimum apatite formation. As shown in
Fig. 13 the amount of apatite formed increases with number of reaction cycles for IPNs
of varying PVA content. This is quite obvious also as increase in number of cycles facili-
tates increase in diffusion of phosphate and calcium ions into the IPN and, therefore, the
apatite formation increases. However, it is also revealed by the figure that with increase in
PVA in the IPN, the number of reaction cycles goes on decreasing at which an optimum
apatite is formed on/in the IPN. The figure indicates that at lowest PVA content (2.0 g) the
apatite formation completes in V cycles, whereas at the highest concentration (3.5 g) of
PVA, a complete apatite formation is noticed after II cycles only. The reason for the
observed results may be attributed to the fact that with increase in the PVA content, the
network acquires increase in hydrophilicity which provides a favorable environment for
interaction between the phosphate and calcium ions. This obviously results in a speedy
apatite formation.
Table 2. Data showing the effect of chemical composition of the IPN on the amount of apatite
formed.
S. no.
PVA
(g)
AM
(mM)
AA
(mM)
MBA
(mM)
Apatite formed
(mg g21 IPN)
1 2.0 14.0 14.5 0.129 390
2 2.5 14.0 14.5 0.129 520
3 3.0 14.0 14.5 0.129 600
4 3.5 14.0 14.5 0.129 400
5 3.0 7.0 14.5 0.129 150
6 3.0 14.0 14.5 0.129 600
7 3.0 21.1 14.5 0.129 350
8 3.0 28.1 14.5 0.129 330
9 3.0 14.0 7.2 0.129 340
10 3.0 14.0 14.5 0.129 600
11 3.0 14.0 21.8 0.129 320
12 3.0 14.0 29.1 0.129 280
13 3.0 14.0 14.5 0.129 600
14 3.0 14.0 14.5 0.194 330
15 3.0 14.0 14.5 0.259 260
16 3.0 14.0 14.5 0.320 180
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In the case of AM also, a similar trend is noted as shown in Fig. 14. The results
clearly imply that the amount of apatite formed increases with increase in number of
reaction cycles. However, it is also found that with increase in AM content in the
IPN, the number of reaction cycles for optimum apatite formation decreases. The
observed results may again be explained on the basis of the increasing hydrophilicity of
the network.
The effect of AA on the number of reaction cycles for optimum apatite formation is
depicted in Fig. 15. Although AA is also a hydrophilic ionic monomer, however, a
reverse trend has been observed, i.e., with increase in AA content in the IPN, the number
of reaction cycles increases when complete apatite formation occurs. The results may be
explained by the fact that increase in AA content in the IPNs produces greater inter-ionic
repulsions among the network chains and thus widens the voids in the network, which
allow more and more phosphate and calcium ions to get into the network. However,
because of the electrical repulsion between phosphate ions and network charges the phos-
phate ions enter into the gel and produce apatite with great difficulty. Thus, greater
number of reaction cycles will be needed for optimum apatite formation.
Figure 13. Variation in the amount of HAP formed with number of reaction cycles for varying
amounts of PVA content in the IPN for a given composition of the IPN. [PVA] ¼ 3.0 g,
[AA] ¼ 14.5 mM, [MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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In the case of increasing the amount of crosslinking agent in the IPN, quite different
type of results are found as shown in Fig. 16. It is clear from the observed results that
apatite formation goes on increasing with increase in number of reaction cycles at
higher crosslinker content, i.e., greater than 0.064 mM of MBA. The observed results
may be due to the fact that increasing crosslinker content enhances the crosslink
density of the IPNs and they display a slow swelling rate in P and Ca solutions. Thus,
optimum apatite formation is expected after greater number of reaction cycles.
Effect of Temperature
Temperature plays an important role in regulating the amount of apatite formation as
this not only affects the degree of swelling in P and Ca solutions but also influences the
reaction of apatite formation. In the present study, the effect of temperature on the yield
of apatite formation has been investigated by varying the temperature in the range 15–
508C. The results are shown in Fig. 17, which reveal that the amount of apatite formed
Figure 14. Effect of number of reaction cycles on the amount of HAP formed on/in the IPNs con-
taining varying amounts of AM at fixed composition of the IPN. [PVA] ¼ 3.0 g, [AA] ¼ 14.5 mM,
[MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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increases in the range 15–278C, while beyond 278C a fall is noticed. The results could be
attributed to the fact that when temperature increases from 158C to 278C, the extent of
swelling of the IPNs in P and Ca solution also increases because of enhanced rates of dif-
fusion of phosphate and calcium ions into the matrix and subsequent relaxation of network
chains. Both of these processes favor apatite formation on/in the IPN. However, beyond
278C, apatite formation decreases due to the reason that at higher temperature degree of
swelling may decrease due to breaking of hydrogen bonds between water molecules
and network chains.
CONCLUSION
A semi-IPN composed of PVA and crosslinked poly(acrylamide-co-acrylic acid)
provides favorable environment within its matrix for apatite formation by the alternate
soaking process. The HAP impregnated semi-IPN when examined by IR spectroscopy
clearly presents evidences of apatite formation within the polymer matrix. The apatite for-
mation within the matrix is also confirmed by XRD studies, which indicate less crystalline
Figure 15. Effect of number of reaction cycles on the amount of HAP formed on/in the IPNs
containing varying amounts of AA at fixed composition of the IPN. [PVA] ¼ 3.0 g, [AM] ¼
14.0 mM, [MBA] ¼ 0.129 mM, and temperature ¼ 278C + 0.28C.
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nature of formed apatite. It is also revealed by thermal investigations that apatite formation
results in an enhancement of thermal stability of the polymer matrix. The SEM studies
clarify morphology of the apatite impregnated matrix and reveal that the needle shaped
crystals of apatite get scattered over the IPN surface.
The IPNs display greater swelling in P solution than in Ca solution. The degree of
swelling in both the P and Ca solutions is initially found to increase with increase in
concentrations of PVA, AM, AA, and MBA solutions and after an optimum concentration
it decreases. The amount of apatite formed is influenced by the chemical composition
of the IPN in a similar way as the extent of swelling is affected by variation in the
composition of the semi-IPN. Thus, the apatite formation is directly regulated by the
extent of swelling of the polymer matrix.
The apatite formed is found to increase with increase in number of reaction cycles and
the number of cycle at which optimum apatite formation occurs is also affected by the
chemical composition of IPN. It is found that on varying PVA and AM, the number of
reaction cycles for optimum apatite formation decreases with increase in concentration,
while with AA a just reverse trend is noticed. In the case of addition of higher
Figure 16. Effect of number of reaction cycles on the amount of hydroxy-apatite formed on/in
the IPNs containing varying amounts of crosslinker (MBA) at fixed composition of the IPN.
[PVA] ¼ 3.0 g, [AM] ¼ 14.0 mM, [AA] ¼ 14.5 mM, and temperature ¼ 278C + 0.28C.
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concentration of crosslinker (MBA) the apatite formation continues with reaction cycles.
The amount of apatite formed is also found to increase with increase in temperature up to
278C, while a decrease is noticed beyond it.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the Indian Institute of Technology, Mumbai,
India and Central Drug Research Institute, Lucknow, India for carrying out IR, DSC,
TGA, XRD, and SEM analyses.
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Received December 2003
Accepted April 2004
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