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polyvinyl alcohol supramolecular hydrogels
Journal Pre-proof Freeze-thawed humic acid/polyvinyl alcohol supramolecular hydrogels Mohammad Sirousazar, Paria Khodamoradi
PII:
S2352-4928(19)30300-9
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100719
Reference:
MTCOMM 100719
To appear in:
Materials Today Communications
Received Date:
8 June 2019
Revised Date:
20 October 2019
Accepted Date:
23 October 2019
Please cite this article as: Sirousazar M, Khodamoradi P, Freeze-thawed humic acid/polyvinyl alcohol supramolecular hydrogels, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100719
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Freeze-thawed
humic
acid/polyvinyl
alcohol
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Mohammad Sirousazar a,, Paria Khodamoradi b
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supramolecular hydrogels
Faculty of Chemical Engineering, Urmia University of Technology, Urmia, Iran
b
Faculty of Chemical Engineering, Ahar Branch, Islamic Azad University, Ahar, Iran
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a
Corresponding author. E-mail address: [email protected] (M. Sirousazar)
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Highlights:
Freeze-thawed humic acid/polyvinyl alcohol supramolecular hydrogels were prepared.
The structural, physical, mechanical and thermal properties of the supramolecular hydrogels
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were studied.
The humic acid nanoparticles aggregated the polyvinyl alcohol chains.
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The supramolecular hydrogels had a club-shaped morphology.
The characteristics of supramolecular hydrogels were under the influence of humic acid content.
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ABSTRACT Novel supramolecular hydrogels were prepared on the basis of polyvinyl alcohol loaded with different amounts of humic acid nanoparticles via a freezing-thawing method. The results showed the humic acid nanoparticles act as functional sites to aggregate the polyvinyl alcohol chains and result in the formation of three-dimensional supramolecular networks. It was shown that the humic acid nanoparticles can interact with the polyvinyl alcohol chains and
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interconnect the polymeric chains as functional crosslinkers. The results showed up to 3.4% increase in the gel fraction value of polyvinyl alcohol hydrogel by incorporating humic acid nanoparticles. In addition, 2.5 and 5.7 °C increases in the glass transition temperature and the
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melting point of polyvinyl alcohol hydrogel were observed by incorporating 9 wt.% of humic acid, respectively. By incorporating the mentioned level of humic acid, up to 87 and 26%
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increases were observed in the storage modulus at glassy state and crystallinity of polyvinyl
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alcohol hydrogel.
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Keywords: Supramolecular hydrogel; Humic acid; Polyvinyl alcohol; Freezing-thawing
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1. Introduction Hydrogels are three-dimensional, hydrophilic and crosslinked networks that contain large volumes of water or aqueous solutions because of the surface tension or capillary effect. They display thermodynamic compatibility with water which allows them to swell in water or aqueous media [1-4]. The ability of hydrogels to absorb water (swelling) is due to the presence of hydrophilic groups, such as -OH, -CONH-, -CONH2, -COOH and -SO3H in their structure
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[5]. They are insoluble in aqueous solutions due to the presence of crosslinking points or zones in their three-dimensional networks. Hydrogels can be divided into two major categories based on the type of the nature of crosslinking, including the chemical and physical (or
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supramolecular) hydrogels [6]. The chemical hydrogels are formed by the creation of permanent chemical crosslinking among the polymer chains via chemical covalent bonding.
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Chemically crosslinking of hydrogels via permanent covalent bonds causes the hydrogels to be
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brittle in practical applications [7]. In supramolecular hydrogels, as a novel class of noncovalently crosslinked polymeric networks, the crosslinking is created by exploiting non-
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covalent interactions between polymeric chains [8]. Unlike traditional chemistry which relies on covalent interactions, supramolecular chemistry
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focuses on weaker and reversible non-covalent interactions between molecules [7]. A variety of non-covalent interactions, such as hydrogen bonding, ionic and associative interactions,
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metal coordination, host-guest interactions, aromatic stacking and electrostatic interactions have been successfully employed as driving forces for crosslinking of polymeric chains and creation of entangled and crosslinked networks in supramolecular hydrogels [9-12]. The supramolecular crosslinking may reduce the structural flexibility of polymeric hydrogels and alter their properties and performances. The microstructural characteristics, such as the crosslinking density and the distribution of crosslinks play crucial roles in determining the
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macroscopic properties and functions of the resultant supramolecular hydrogels [13]. Compared to the chemically crosslinked hydrogels, the supramolecular hydrogels offer attractive advantages because they enable the introduction of different physical properties, simpler syntheses, multi-functionality and increased efficacy [11]. Supramolecular hydrogels have found different applications in various practical fields, such as adsorption [14], biocatalysis [15], wound and burn healing [16,17], drug delivery [18,19] and tissue engineering [20].
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Humic acid (HA) is an organic material distributed naturally in terrestrial soil, natural water and sediments resulting from the decay of vegetable and natural residues [21]. HA is one of the major components of humic substances that contains both hydrophilic and hydrophobic
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groups, such as carboxyl, phenolic and hydroxyl groups connected to a skeleton of aliphatic or aromatic units [22,23]. One of the major sources of HA is leonardite. Leonardite, as the
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oxidized form of lignite, is a completely natural and organic substance, which contains a
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significant amount of humic substances, such as HA, fulvic acid and humins [24]. Leonardite is rich in organic matter (50-75%) and its HA content could vary between 30 to 80% [25,26]. Alkaline leaching is the conventional method utilized in the extraction of HA from leonardite.
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In extracting HA from leonardite, NaOH, KOH, NH3 and Na4P2O7 are generally used as alkaline solvents [24]. The elemental compositions of HA extracted from leonardite have been
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measured as: C% 53.78; H% 3.35, N% 2.09 and O% 40.0 [27].
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HA has been previously utilized in some limited researches for the production of polymeric blends and composites. The composites on the basis of polyaniline and HA were prepared by adding different amounts of HA into the chemical polymerization process of aniline monomers and the prepared composites were used for adsorption of Hg(II) ions and Cr(VI) anions [28,29]. In another research, HA was utilized to fabricate a polymer hydrogel composed of alginate/HA/Fe-aminoclay. The prepared hydrogel was successfully utilized as a potential
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adsorbent for the removal of radioactive strontium ions from aqueous solutions [30]. Recently, a new chitosan-HA-zerovalent iron nanocomposite was prepared and the prepared nanocomposite was examined for nitrate ion reduction in water. HA was used for intramolecular crosslinking of the chitosan linear chains to increase the active sites on the chitosan. Using the prepared HA-based nanocomposite, the reduction of nitrate in water was observed to be highly effective [31]. Blends based on the polyvinyl alcohol (PVA) doped with HA were prepared by exposing to the gamma irradiation. The ability of HA to stabilize the
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PVA molecular structure during the intensive gamma irradiation exposition was investigated and it was shown that the presence of HA slightly stabilized the PVA sample [32]. Recently, HA-embedded chitosan-PVA smart pH-sensitive hydrogels were synthesized using
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glutaraldehyde as a crosslinker. The triple synergistic effects of HA, chitosan and PVA onto the dynamic swelling behavior of the prepared pH-sensitive hydrogels were experimentally
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studied. It was concluded that the HA-embedded hydrogels can be tuned to serve as pH-
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sensitive smart materials and resulting pH-sensitive hydrogels can be used for biotechnological and drug delivery applications [33].
In this work, HA nanoparticles were extracted from leonardite and utilized to prepare novel
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supramolecular hydrogels. The supramolecular hydrogels were prepared on the basis of PVA, loaded with various content of HA, via a cyclic freezing-thawing method. The effects of HA
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on the structural, physical, thermal and mechanical properties of the prepared HA/PVA
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supramolecular hydrogels were experimentally studied.
2. Experimental 2.1. Materials PVA having an average number molecular weight ( M n ) of 74,800 and degree of hydrolysis greater than 98% was purchased from Nippon Gohsei, Japan. Leonardite, as the natural source
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of HA, was obtained from Mineral Tarim, Turkey. NaOH and HCl were purchased from Merck, Germany. Double distilled water (DDW) was used in the preparation of all aqueous solutions.
2.2. Extraction of HA nanoparticles from leonardite HA nanoparticles were extracted from leonardite by removing humin and fulvic acid using NaOH and HCl aqueous solutions, respectively on the basis of the previously reported
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procedure [27,34]. Briefly, 1 g of leonardite was added to 50 ml of 0.1 M NaOH solution and then the resulting solution was stirred at 700 rpm for 24 h at room temperature. The solution was then centrifuged at 7000 rpm for 10 min and subsequently, the supernatant was collected
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and the remaining precipitates were discarded. The obtained solution was then titrated by 40 mL of 0.1 M HCl solution and stirred at 400 rpm for 1 h. The resulting solution was then
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centrifuged at 7000 rpm for 10 min. After centrifugation, the obtained precipitates were
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separated and washed three times with deionized water and then dried in an oven at 40 °C. Finally, the obtained HA nanoparticles were ground into a fine powder with nearly a
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homogenous particle size and used in the production of HA/PVA supramolecular hydrogels.
2.3. Preparation of HA/PVA supramolecular hydrogels
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To prepare each HA/PVA supramolecular hydrogel, firstly, a predetermined quantity of HA
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nanoparticles was weighed and ultrasonicated in DDW for 4 h to obtain a uniform dispersion. Next, a certain amount of PVA was gradually added to the HA dispersion at 90 °C (using a water bath) with a stirring speed of 200 rpm for 4 h. Then, the obtained uniform HA/PVA dispersions were poured into pre-designed plastic molds and subjected to the freezing-thawing cyclic process. Initially, the molds were kept in a freezer at -15 °C for 24 h to perform the freezing stage and induce the crystallization of PVA chains. After the freezing stage, the molds
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were placed at room temperature for 24 h to thaw. In order to enhance the crystallization of PVA chains and also to increase the crosslinking content and network stability [35], the freezing-thawing process was repeated three times for each sample to create the final supramolecular hydrogel samples. All prepared HA/PVA supramolecular hydrogels had 10 wt.% of PVA (based on the weight of the total sample, i.e. wet basis) and 0, 3, 6, 9 and 12 wt.% of HA (based on the weight of PVA and HA in the sample, i.e. dry basis). The prepared supramolecular hydrogels were named as SHx, which x shows the weight percentage of HA in
supramolecular hydrogels were summarized in Table 1.
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2.4. Characterization
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the completely dried sample. The designations and detailed compositions of the prepared
The structural and morphological characteristics of the prepared supramolecular hydrogels
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were studied using Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry
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(XRD), transmission electron microscopy (TEM) and also scanning electron microscopy (SEM).
The characteristic functional groups of the HA, pure PVA (SH0) and typical HA/PVA
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supramolecular hydrogels loaded with 6 and 9 wt.% HA (i.e., SH6 and SH9) were analyzed by FTIR. Each sample was completely dried under vacuum and then grounded with KBr powder
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and compressed into a disc. The characterizations were performed using a Thermo Nicolet
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(FTIR NEXUS-670, USA) spectroscope. Spectra were collected in the range 650-4000 cm-1 with 2 cm-1 spectral resolution and 20 scans were performed for each sample. The XRD was conducted on all HA-loaded supramolecular hydrogels and also on the pristine HA and HA-free hydrogel (SH0), as references. The XRD experiments were conducted with a diffractometer (Siemens D5000, Germany) equipped with monochromatic CuKα radiation (λ= 0.154 nm, 40 kV and 30 mA) at room temperature. Data were collected over the 2θ range of 2-
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60° at a scanning speed of 2 °/min. Prior to the test, all samples were completely dried in a vacuum oven. The particle size distribution of the HA was determined by the dynamic light scattering (DLS) technique. A dilute dispersion of HA in DDW was prepared and ultrasonicated before the DLS analysis. The DLS analysis was conducted on a Horiba SZ-100 nanoparticle analyzer (Germany) equipped with a 532 nm diode-pumped solid state (DPSS) laser at a scattering angle of 173 º, operated at a temperature of 25 °C.
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The morphology of a typical supramolecular hydrogel (SH6) was observed by TEM. The sample was completely dried under vacuum and then embedded in an epoxy matrix prior to observation. Then the sample was frozen in liquid nitrogen and cut to 70-100 nm thickness
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section, perpendicular to the film, using a diamond knife. Finally, the sample was placed onto 400 mesh copper grid and the observation was performed using a transmission electron
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microscope (EM 208 S, Philips, Netherlands) with an acceleration voltage of 100 kV.
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The particle size of HA and also the fracture surface morphologies of three typical samples, i.e. SH0, SH6 and SH12, were investigated using SEM. The supramolecular hydrogels were broken after freezing in liquid nitrogen and the cross sections coated with a thin layer of gold.
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The SEM images were taken of the fracture cross sections of the supramolecular hydrogels. The observations were carried out using a KYKY-EM3200 (KYKY Technology Development
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Ltd., China) scanning electron microscope.
2.5. Gel fraction measurement The gel fraction values of supramolecular hydrogels, as criteria of the contents of crosslinked PVA chains in three-dimensional networks of prepared samples, were measured by extraction of uncrosslinked polymer chains from the sample and comparing it with the non-extracted sample, using a gravimetric technique. For this purpose, an identical piece of each sample (ca.
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1g) was chosen and dried under vacuum condition. Each drying sample was weighed at regular time intervals and drying continued until the mass of the dried sample showed a constant mass (mi). Then, each dried sample was immersed into the excess amount of DDW to extract uncrosslinked species. The extraction process was continued for a week and the extraction solution was occasionally replaced with fresh DDW. Finally, the extracted sample was removed from DDW and dried under vacuum until the mass of the dried sample showed a constant value (mf).
Gel Fraction
mf mi
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The gel fraction value of each sample was calculated using the below equation [36]: 100
-p
(1)
The gel fraction experiment was repeated three times for each supramolecular hydrogel and the
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average value was reported.
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2.6. Thermal and mechanical experiments
The thermal and mechanical-thermal properties of the prepared HA/PVA supramolecular
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hydrogels were investigated using differential scanning calorimetry (DSC) and dynamic mechanical-thermal analysis (DMTA). DSC was carried out on three typical samples (SH0,
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SH6 and SH9) using a DSC-Netzsch-200-F3 apparatus. The samples were completely dried in a vacuum oven prior to analysis. The analysis was performed at a scan rate of 10 °C/min for
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the temperature range of -40 to 230 °C and continuous nitrogen flow was employed. The dynamic mechanical-thermal and viscoelastic properties of SH0, SH6 and SH9 samples were also studied. A dried strip of each sample (30 mm long, 10 mm wide and 0.1 mm thickness) was analyzed using a DMA 242-C (Netzsch, Germany) dynamic mechanical analyzer in the tension mode. The scans were performed at a frequency of 1 Hz and a ramp of 5 °C/min from -80 to 140 °C. 10
2.7. Swelling test The abilities of the prepared supramolecular hydrogels in absorbing water molecules from aqueous solutions were experimentally determined using the swelling test. Three pieces of each sample (each of nearly 1 g) were selected and immersed into the excess amount of DDW for a week to extract and remove the uncrosslinked species. Then, the extracted samples were dried under vacuum condition until the mass of each sample fixed in constant value (m0). Then each
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piece of the dried samples was separately immersed in 500 cc of DDW. Each immersed sample was taken out of the DDW at predefined time intervals, the outer surface of the sample was wiped using the absorbing paper and the swollen sample was then weighed (mt). The swelling
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ratio at any swelling time (q(t)), as a quantitative measure of the ability of sample in water absorption, was calculated using the following equation [37]: mt m0
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q(t )
(2)
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The swelling test was continued for each sample to reach the equilibrium swelling condition, where the mass of the swollen sample showed a constant value (m∞). The swelling ratio at
m m0
(3)
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q ( )
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equilibrium condition (i.e., q(∞)) was calculated for each sample using the below equation:
The mean value resulting from three individual tests for each sample was reported as the final
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value of the swelling ratio. The swelling test was carried out for each sample at three different swelling temperatures of 25, 37 and 55 °C, to investigate the effect of the temperature of swelling medium on swelling ratios of samples.
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3. Results and Discussion 3.1. Structural properties FTIR was used to demonstrate the characteristic peaks of HA/PVA supramolecular hydrogels and the possible interactions developed between the HA and PVA functional groups. The FTIR spectra of pristine HA, pure PVA hydrogel (SH0) and SH6 and SH9 samples in the spectral scale of 650 to 4000 cm-1, along with the schematic molecular structures of the HA [38] and
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PVA were shown in Fig. 1. The typical bands of pure PVA hydrogel, including C–O stretching band at 1108 cm-1, C–C stretching band at 1459 cm-1, C–H stretching band at 2952 cm-1 and a broad O−H stretching band centered in the vicinity of 3360 cm-1 were observed [39]. The O−H
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stretching band of PVA hydrogel was broadened because of the crystallization of the PVA chains and the creation of hydrogen bonding among the hydroxyl groups of PVA chains during
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the freezing-thawing process. Fig. 1 also demonstrates common characteristic bands for HA at
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1612, 1704, 2910 and 3390 cm-1. The band at 1612 cm-1 refers to the C=C stretching vibration [40]. The band at 1704 cm-1 is related to the vibrations of C=O stretches from the carboxyl
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group with trace amounts of ketones [41,42]. The band at 2910 cm-1 is attributed to aliphatic C–H asymmetric stretching vibration mode in methyl and/or methylene groups. The band at
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3390 cm-1 corresponds to the stretching vibration of the O–H in the carboxyl group as well as alcoholic and phenolic hydroxyl groups. The band at this wavenumber is broad because of the
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presence of intensive hydrogen bonding involving these groups [43]. Similar typical peaks of reference materials (HA and PVA) can be observed in the FTIR spectra correspond to the SH6 and SH9 supramolecular hydrogels. However the spectra of the SH6 and SH9 samples show that the band related to the O−H groups of HA has been merged with the broad O−H band of PVA and resulted in a new band by shifting to lower wavenumbers centered at ca. 3226 and 3272 cm-1, respectively. Based on the FTIR results, it can prospect that by adding HA to PVA,
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the Keesom dipole-dipole interaction [44] in the form of the hydrogen bonding was developed among the hydroxyl group of the PVA chains and hydroxyl and carboxyl groups of the HA. It can be inferred that by incorporating the HA to the PVA hydrogel, some additional interactions, rather than the interactions (hydrogen bonding) created among the PVA chains due to crystallization during the freezing-thawing process, were developed between functional groups of HA and PVA chains. In other words, the results proved the supramolecular structure of the HA/PVA hydrogels and indicated the crosslinking role of the HA in the HA/PVA
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supramolecular hydrogels. The results imply, in addition to the crystallization of PVA chains during the freezing-thawing process, the HA nanoparticles act as active sites to interact with the PVA functional hydroxyl groups, aggregate the PVA chains and create the supramolecular
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structure in the HA/PVA hydrogels. The mechanism of the PVA chain crosslinking and the supramolecular hydrogel formation was schematically illustrated in Fig. 2.
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The gel fraction values of the HA/PVA supramolecular hydrogels versus the loading level of
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incorporated HA were shown in Fig. 3. The results showed that all HA-loaded hydrogels have higher gel fraction values compared with the HA-free sample (SH0) and the gel fraction was increased by increasing the loading level of HA in supramolecular hydrogels up to 9 wt.%.
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This means that more PVA chains were crosslinked in the presence of HA nanoparticles. The results showed that up to 3.4% increase in the gel fraction of pure PVA hydrogel could be
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achieved by incorporating 9 wt.% of HA into the hydrogel matrix. The increased gel fraction
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values of HA-loaded hydrogels are in agreement with the FTIR results and could be related to the development of interactions between the functional groups of the PVA chains and HA particles. The gel fraction results confirmed the crosslinking role of HA in the prepared supramolecular hydrogels. The decreasing trend of gel fraction value for HA loading level higher than 9 wt.% may be due to the possible agglomeration of the HA particles in SH12 supramolecular hydrogel due to the high loading level of HA. It is obvious that by
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agglomeration of the HA particles, the surface area of the particles is decreased and the quantity of the created hydrogen bonding between the PVA chains and HA particles declined and as a result, the crosslinking content and gel fraction value of the SH hydrogel is decreased. Fig. 4 shows the XRD patterns of HA, pure PVA hydrogel (SH0) and HA/PVA supramolecular hydrogels containing 3, 6, 9 and 12 wt.% of HA (SH3, SH6, SH9 and SH12). The XRD pattern of HA showed two broad diffraction peaks around 11.6 and 25.7° exhibiting its almost amorphous nature [40,43,45]. However, two peaks at ca. 31.7 and 45.6° were also observed in
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the X-ray diffractogram of HA which may be attributed to the presence of inorganic compounds, such as gypsum (CaSO4.2H2O) [46] and pyrite (FeS2) [47], as two main constituents being existed in the pristine leonardite. Pure PVA hydrogel i.e., SH0, exhibited a
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semi-crystalline structure with two broad characteristic diffraction peaks around 13 and 20° [48,49]. In the XRD patterns of the HA/PVA supramolecular hydrogels, only the characteristic
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diffraction peak of PVA was seen, whereas no characteristic peaks of HA were observed. It
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may be because the additive amount of HA is small, and its diffraction is very weak compared to PVA. Similar peaks observed for pure PVA hydrogel around 13 and 20° were also observed for each HA-loaded supramolecular hydrogels at nearly the same angles. Moreover, it is clearly
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seen that the characteristic peaks intensities of PVA for SH3, SH6, SH9 and SH12 samples have been increased compared with that for SH0 sample. This indicates that the degree of
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crystallinity of PVA chains in the HA/PVA hydrogels were increased with an increase in the
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HA content. The XRD results were in accordance with the FTIR and gel fraction results indicating the role of HA particles as crosslinkers which can increase the crosslinking intensity of PVA chains in HA/PVA supramolecular hydrogels and promote the tendency of PVA chains to be arranged in a more crystalline structure.
3.2. Morphological observations
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The scanning electron micrograph of HA extracted from leonardite was shown in Fig. 5a. As seen, most of the particles were in the nanometric range in size, with the tendency to aggregate to each other. Fig. 6 represents the particle size distribution histogram of the HA particles obtained by the DLS analysis. The histogram showed a mean particle size of 116.5±36.7 nm for HA particles, which is in accordance with the SEM observation. TEM observation was utilized to get information about the dispersion level of the HA nanoparticles inside the PVA hydrogel matrix. The morphology of a typical supramolecular
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hydrogel (SH6) examined by TEM was shown in Fig. 5b. The dark area represents the HA particles and the gray-white area represents the polymer matrix. TEM image exhibited a fair dispersion of the HA nanoparticles inside the PVA matrix. However, some agglomerated sites
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of HA particles could be also observed in the TEM image.
The fracture surface morphologies of the prepared HA/PVA supramolecular hydrogels were
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observed by SEM. The SEM micrographs of cryogenic fracture surfaces of typical HA/PVA
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supramolecular hydrogels containing 0, 6 and 12 wt.% of HA, with two magnifications of each, were illustrated in Fig. 7. As seen, pure PVA hydrogel had a sponge-shaped porous morphology with large pore sizes (Fig. 7a). However, the HA-loaded supramolecular hydrogels displayed
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a compact club-shaped morphology with reduced pore sizes compared with the HA-free sample. The creation of a club-shaped morphology in HA/PVA supramolecular hydrogels was
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due to the presence of HA nanoparticles as the crosslinking agent inside the three-dimensional
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network of hydrogels. A similar club-shaped morphology for polyaniline polymer matrix loaded with the HA particles has been previously reported [29].
3.3. Thermal characteristics
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The thermograms resulted from DSC analysis for SH0, SH6 and SH9 samples in the temperature range of -40 to 230 °C were demonstrated in Fig. 8. All samples exhibited an almost identical thermal behavior owing endothermic peaks above 215 °C, corresponding to their melting points (Tm). The important thermal characteristics of samples, including the Tm, enthalpy of fusion (∆Hf) and relative crystallinity percentage (Xc) were extracted from the obtained DSC curves and listed in Table 2. The Xc values were calculated using the following equation [50]: H f (1 H )H f
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(4)
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XC
where, ωH is the weight fraction of HA in dried supramolecular hydrogel and H f is the
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theoretical enthalpy of fusion for a 100% crystalline PVA, which is equal to 138.6 J/g [51].
The results showed the Tm of pure PVA hydrogel was increased by 2.4 °C by incorporating 6
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wt.% of HA. However, nearly 5.7 °C increment in the Tm of pure PVA hydrogel was observed
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by adding 9 wt.% of HA. The increased melting point of HA-loaded supramolecular hydrogels could be related to their increased gel fraction values and developed more crosslinking zones inside the supramolecular hydrogels due to the presence of HA nanoparticles. Table 2 shows
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that by incorporating 6 and 9 wt.% of HA into the PVA hydrogel, the crystallinity percentages were increased by ca. 14.5 and 26.4%, respectively. The obtained results for crystallinity were
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in agreement with the XRD results and implied that the HA could actually increase the
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crystallization of PVA chains in the HA/PVA supramolecular hydrogels. It can be deduced that the HA nanoparticles can act as the starting points for PVA crystallization during the freezingthawing process and cause some increments in the crystallinity of HA/PVA supramolecular hydrogels. Furthermore, samples demonstrated broad and shoulder-like endothermic peaks around 40-55 °C, which may be attributed to the glassy-rubbery phase transition of samples. Since the peaks
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were very weak and broad, the obtained glass transition temperatures (Tg) from DSC curves for examined samples would not be accurate and trustable. The exact values of Tg will be reported based on the DMTA results, in the following section.
3.4. Dynamic mechanical-thermal behavior Fig. 9 shows the DMTA results (i.e. storage modulus (E') and tan curves versus temperature) for HA-free hydrogel and supramolecular hydrogels loaded with 6 and 9 wt.% of HA. A similar
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trend was observed for E' and tan curves of all samples by increasing the temperature. At temperatures below Tg, E' had a relatively high magnitude and was gradually decreased by increasing the temperature. In this temperature range, the hydrogels were in the glassy state
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and thus they showed a very rigid nature with a high E' value. As the temperature reached around Tg, the value of E' declined sharply due to the glassy-rubbery phase transition of the
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hydrogel and as a result, a peak appeared in the tan curve. Following the glassy-rubbery
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phase transition, the hydrogel was transmitted into the rubbery state and the storage modulus decreased slightly with temperature rise.
The results showed that the storage modulus of HA-loaded hydrogels was always higher than
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that of the HA-free sample. Furthermore, the E' value was increased by increasing the HA content in the supramolecular hydrogels. For instance, the E' of the SH6 and SH9
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supramolecular hydrogels at the glassy state were, on average, 24 and 87% higher than that of
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the HA-free hydrogel, respectively. The higher storage moduli of HA-loaded supramolecular hydrogels showed the reinforcing ability of HA nanoparticles in the matrix of the PVA hydrogel. This could be attributed to the presence of HA nanoparticles in the three-dimensional networks of HA-loaded supramolecular hydrogels and their positive roles in increasing the gel fraction values and the creation of more interconnected networks compared with the pure PVA hydrogel.
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The Tg values of the examined samples were extracted from the tan curves as: 63.2, 64.5 and 65.7 °C for SH0, SH6 and SH9 samples, respectively. The increased Tg values in HA-loaded supramolecular hydrogels were due to the presence of HA nanoparticles in their structure and the increased gel fraction values. It is obvious that in a hydrogel loaded with a higher content of HA, the number of crosslinked PVA chains is increased and shorter sub-chains are created inside the hydrogel network. At this condition, the mobility and the freely movement of the PVA sub-chains are restricted and as a result, the glassy-rubbery phase transition of hydrogels
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occurs at higher temperatures and the Tg values are increased. The obtained DMTA results demonstrated the effective role of HA in enhancing the mechanical and thermal properties of
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the HA/PVA supramolecular hydrogels.
3.5. Swelling behavior
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The swelling kinetics of HA/PVA supramolecular hydrogels in the form of the q(t) curves
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against the swelling time, at the swelling temperatures of 25, 37 and 55 °C, were shown in Fig. 10. All supramolecular hydrogels demonstrated an identical swelling trend at all the swelling temperatures and their q(t) values increased with increasing the swelling time. The slope of the
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swelling curve of each sample was higher and the swelling process happened faster in the early stages of the swelling process and the sample reached an almost stable swelling ratio
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(equilibrium swelling condition) by increasing the swelling time.
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The results showed that the time required to attain a certain level of q(t) depends on the quantity of HA added to the supramolecular hydrogel. It can be seen that in most cases, the supramolecular hydrogels loaded with higher HA contents demonstrated nearly a slower and more prolonged swelling to reach a given degree of swelling. For example, at the swelling temperature of 37 °C, the required time to reach the q(t) of 2.5 was c.a. 300 and 720 min for the SH3 and SH12 samples, respectively. This phenomenon could be explained on the basis of
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the slower and more restricted relaxation of PVA chains during the swelling process in the supramolecular hydrogels loaded with a higher amount of HA. Furthermore, restricted mass transfer of the water molecules inside the supramolecular hydrogels containing a higher content of HA, due to their more interconnected and entangled networks with smaller pore sizes, may be recognized as another reason in this respect. The effects of HA content and swelling temperature on the q(∞) of the supramolecular hydrogels were exhibited in Fig. 11. The results showed that the q(∞) could be increased by
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either decreasing the incorporated HA loading level or by increasing the swelling temperature in most cases. For instance, the q(∞) of supramolecular hydrogel containing 3 wt.% of HA at 37 °C was 3.4, while this value for the sample containing 12 wt.% of HA at the same swelling
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temperature was 3. In addition, the results showed that the q(∞) value of the SH6 sample increased from 3.4 to 4.2 by increasing the swelling temperature from 25 to 55 °C. The
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decreasing trend of swelling ratios by increasing the content of incorporated HA in HA/PVA
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supramolecular hydrogels could be related to their higher gel fraction values, more entangled structures, less pore sizes and more tortuous paths for the diffusion of water molecules during the swelling process. The positive effect of swelling temperature on the swelling ratio of
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supramolecular hydrogels could be attributed to the increased chain flexibility of PVA and the
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increased diffusion coefficient of water molecules at a higher swelling temperature.
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4. Conclusions
Supramolecular hydrogels on the basis of HA and PVA were prepared using a freezing-thawing cyclic technique. The results showed that the HA nanoparticles acted as functional sites to aggregate the PVA chains and resulted in the formation of three-dimensional supramolecular networks. Based on the FTIR results, the development of interactions between the functional groups of HA nanoparticles and PVA chains in the form of the hydrogen bonding was proved.
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The gel fraction test showed the role of HA nanoparticles as crosslinking agents in the network of HA/PVA supramolecular hydrogels and an increase in the gel fraction value of pure PVA hydrogel was observed by incorporating HA into the PVA matrix. The XRD results indicated that the degree of crystallinity of PVA chains in the HA/PVA supramolecular hydrogels was increased with an increase in the HA content. The morphology of HA/PVA supramolecular hydrogels and the dispersion level of HA in PVA matrix were observed by TEM and SEM techniques. It was observed that the HA particles were mostly in the nanometric range in size
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and the sponge-shaped morphology of pure PVA hydrogel changed to a club-shaped morphology by incorporating HA nanoparticles. Based on the DSC and DMTA results it was found that the thermal and mechanical properties of HA/PVA supramolecular hydrogels were
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strongly under the influence of the loading level of incorporated HA into the supramolecular hydrogel. It was shown that the glass transition temperature, melting point, crystallinity and
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storage modulus of PVA hydrogels were increased by incorporating HA nanoparticles and
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further increases were observed by increasing the HA content. According to the results obtained from the swelling test, an almost decreasing trend was observed for swelling ratios of HA/PVA supramolecular hydrogels by increasing the HA loading level. However, the swelling
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of supramolecular hydrogels tended to increase by increasing the temperature of the swelling medium. The prepared HA/PVA supramolecular hydrogels are promising materials to be used
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in practical applications, for example as artificial organs and wound dressings in the biomedical
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field and also as absorbent materials in water purification systems and oil recovery applications, due to their improved structural, physical, thermal and mechanical properties. Furthermore, they may be used as potential materials in practical applications where a controlled and prolonged swelling and diffusion are required, for instance as drug carriers in designing the prolonged and controlled drug delivery systems and also as absorbent gels in prolonged irrigation applications. Finally, it was concluded that the loading level of HA
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nanoparticles is a key factor affecting the structural, physical, thermal and mechanical properties of the HA/PVA supramolecular hydrogels and the desired sample would be produced for designed applications by manipulating the content of incorporated HA nanoparticles into the supramolecular hydrogel matrix. Conflict of interest:
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There is no conflict of interest.
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Figures Captions:
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Matter 7 (2011) 2373-2379.
Fig. 1. (a) Molecular structures of the HA [38] and PVA and (b) FTIR spectra of pristine HA
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and SH0, SH6 and SH9 supramolecular hydrogels.
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Fig. 2. The mechanism of crosslinking and the HA/PVA supramolecular hydrogel formation.
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Fig. 3. Gel fraction values of supramolecular hydrogels versus the HA loading level.
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Fig. 4. XRD profiles of pristine HA and HA/PVA supramolecular hydrogels.
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Fig. 5. (a) SEM micrograph of HA extracted from leonardite and (b) TEM image of the SH6
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supramolecular hydrogel.
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Fig. 6. Particle size distribution histogram of the HA obtained from the DLS analysis.
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magnifications of 2500X and 10000.
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Fig. 7. SEM images of the samples (a) SH0, (b) SH6 and (c) SH12 with two different
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Fig. 8. DSC thermograms of SH0, SH6 and SH9 supramolecular hydrogels.
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Fig. 9. Plots of (a) storage modulus and (b) tan versus temperature for SH0, SH6 and SH9
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supramolecular hydrogels.
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Fig. 10. Swelling kinetics curves of supramolecular hydrogels at the swelling temperature of (a) 25, (b) 37 and (c) 55 °C.
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ro of -p re lP na ur Jo Fig. 11. Equilibrium swelling ratios of supramolecular hydrogels at the swelling temperature of (a) 25, (b) 37 and (c) 55 °C.
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ro of -p re lP na ur Jo Table 1. Designations and chemical compositions of the prepared HA/PVA supramolecular hydrogels.
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Sample
PVA
DDW
designation
(wt.%)
(wt.%)
SH0
10
SH3
HA (wt.%)
Dry basis
90
0
0
10
89.69
0.31
3
SH6
10
89.36
0.64
6
SH9
10
89.01
0.99
9
SH12
10
88.64
1.36
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Wet basis
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Table 2. Thermal properties of typical HA/PVA supramolecular hydrogels.
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Tm
∆Hf
Xc
Designation
(°C)
(J/g)
(%)
SH0
215.6
54.8
39.5
SH6
218.0
58.9
45.2
SH9
221.3
63.0
49.9
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Sample
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PII:
S2352-4928(19)30300-9
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100719
Reference:
MTCOMM 100719
To appear in:
Materials Today Communications
Received Date:
8 June 2019
Revised Date:
20 October 2019
Accepted Date:
23 October 2019
Please cite this article as: Sirousazar M, Khodamoradi P, Freeze-thawed humic acid/polyvinyl alcohol supramolecular hydrogels, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100719
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Freeze-thawed
humic
acid/polyvinyl
alcohol
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Mohammad Sirousazar a,, Paria Khodamoradi b
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supramolecular hydrogels
Faculty of Chemical Engineering, Urmia University of Technology, Urmia, Iran
b
Faculty of Chemical Engineering, Ahar Branch, Islamic Azad University, Ahar, Iran
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a
Corresponding author. E-mail address: [email protected] (M. Sirousazar)
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Highlights:
Freeze-thawed humic acid/polyvinyl alcohol supramolecular hydrogels were prepared.
The structural, physical, mechanical and thermal properties of the supramolecular hydrogels
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were studied.
The humic acid nanoparticles aggregated the polyvinyl alcohol chains.
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The supramolecular hydrogels had a club-shaped morphology.
The characteristics of supramolecular hydrogels were under the influence of humic acid content.
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ABSTRACT Novel supramolecular hydrogels were prepared on the basis of polyvinyl alcohol loaded with different amounts of humic acid nanoparticles via a freezing-thawing method. The results showed the humic acid nanoparticles act as functional sites to aggregate the polyvinyl alcohol chains and result in the formation of three-dimensional supramolecular networks. It was shown that the humic acid nanoparticles can interact with the polyvinyl alcohol chains and
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interconnect the polymeric chains as functional crosslinkers. The results showed up to 3.4% increase in the gel fraction value of polyvinyl alcohol hydrogel by incorporating humic acid nanoparticles. In addition, 2.5 and 5.7 °C increases in the glass transition temperature and the
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melting point of polyvinyl alcohol hydrogel were observed by incorporating 9 wt.% of humic acid, respectively. By incorporating the mentioned level of humic acid, up to 87 and 26%
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increases were observed in the storage modulus at glassy state and crystallinity of polyvinyl
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alcohol hydrogel.
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Keywords: Supramolecular hydrogel; Humic acid; Polyvinyl alcohol; Freezing-thawing
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1. Introduction Hydrogels are three-dimensional, hydrophilic and crosslinked networks that contain large volumes of water or aqueous solutions because of the surface tension or capillary effect. They display thermodynamic compatibility with water which allows them to swell in water or aqueous media [1-4]. The ability of hydrogels to absorb water (swelling) is due to the presence of hydrophilic groups, such as -OH, -CONH-, -CONH2, -COOH and -SO3H in their structure
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[5]. They are insoluble in aqueous solutions due to the presence of crosslinking points or zones in their three-dimensional networks. Hydrogels can be divided into two major categories based on the type of the nature of crosslinking, including the chemical and physical (or
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supramolecular) hydrogels [6]. The chemical hydrogels are formed by the creation of permanent chemical crosslinking among the polymer chains via chemical covalent bonding.
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Chemically crosslinking of hydrogels via permanent covalent bonds causes the hydrogels to be
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brittle in practical applications [7]. In supramolecular hydrogels, as a novel class of noncovalently crosslinked polymeric networks, the crosslinking is created by exploiting non-
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covalent interactions between polymeric chains [8]. Unlike traditional chemistry which relies on covalent interactions, supramolecular chemistry
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focuses on weaker and reversible non-covalent interactions between molecules [7]. A variety of non-covalent interactions, such as hydrogen bonding, ionic and associative interactions,
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metal coordination, host-guest interactions, aromatic stacking and electrostatic interactions have been successfully employed as driving forces for crosslinking of polymeric chains and creation of entangled and crosslinked networks in supramolecular hydrogels [9-12]. The supramolecular crosslinking may reduce the structural flexibility of polymeric hydrogels and alter their properties and performances. The microstructural characteristics, such as the crosslinking density and the distribution of crosslinks play crucial roles in determining the
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macroscopic properties and functions of the resultant supramolecular hydrogels [13]. Compared to the chemically crosslinked hydrogels, the supramolecular hydrogels offer attractive advantages because they enable the introduction of different physical properties, simpler syntheses, multi-functionality and increased efficacy [11]. Supramolecular hydrogels have found different applications in various practical fields, such as adsorption [14], biocatalysis [15], wound and burn healing [16,17], drug delivery [18,19] and tissue engineering [20].
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Humic acid (HA) is an organic material distributed naturally in terrestrial soil, natural water and sediments resulting from the decay of vegetable and natural residues [21]. HA is one of the major components of humic substances that contains both hydrophilic and hydrophobic
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groups, such as carboxyl, phenolic and hydroxyl groups connected to a skeleton of aliphatic or aromatic units [22,23]. One of the major sources of HA is leonardite. Leonardite, as the
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oxidized form of lignite, is a completely natural and organic substance, which contains a
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significant amount of humic substances, such as HA, fulvic acid and humins [24]. Leonardite is rich in organic matter (50-75%) and its HA content could vary between 30 to 80% [25,26]. Alkaline leaching is the conventional method utilized in the extraction of HA from leonardite.
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In extracting HA from leonardite, NaOH, KOH, NH3 and Na4P2O7 are generally used as alkaline solvents [24]. The elemental compositions of HA extracted from leonardite have been
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measured as: C% 53.78; H% 3.35, N% 2.09 and O% 40.0 [27].
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HA has been previously utilized in some limited researches for the production of polymeric blends and composites. The composites on the basis of polyaniline and HA were prepared by adding different amounts of HA into the chemical polymerization process of aniline monomers and the prepared composites were used for adsorption of Hg(II) ions and Cr(VI) anions [28,29]. In another research, HA was utilized to fabricate a polymer hydrogel composed of alginate/HA/Fe-aminoclay. The prepared hydrogel was successfully utilized as a potential
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adsorbent for the removal of radioactive strontium ions from aqueous solutions [30]. Recently, a new chitosan-HA-zerovalent iron nanocomposite was prepared and the prepared nanocomposite was examined for nitrate ion reduction in water. HA was used for intramolecular crosslinking of the chitosan linear chains to increase the active sites on the chitosan. Using the prepared HA-based nanocomposite, the reduction of nitrate in water was observed to be highly effective [31]. Blends based on the polyvinyl alcohol (PVA) doped with HA were prepared by exposing to the gamma irradiation. The ability of HA to stabilize the
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PVA molecular structure during the intensive gamma irradiation exposition was investigated and it was shown that the presence of HA slightly stabilized the PVA sample [32]. Recently, HA-embedded chitosan-PVA smart pH-sensitive hydrogels were synthesized using
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glutaraldehyde as a crosslinker. The triple synergistic effects of HA, chitosan and PVA onto the dynamic swelling behavior of the prepared pH-sensitive hydrogels were experimentally
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studied. It was concluded that the HA-embedded hydrogels can be tuned to serve as pH-
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sensitive smart materials and resulting pH-sensitive hydrogels can be used for biotechnological and drug delivery applications [33].
In this work, HA nanoparticles were extracted from leonardite and utilized to prepare novel
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supramolecular hydrogels. The supramolecular hydrogels were prepared on the basis of PVA, loaded with various content of HA, via a cyclic freezing-thawing method. The effects of HA
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on the structural, physical, thermal and mechanical properties of the prepared HA/PVA
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supramolecular hydrogels were experimentally studied.
2. Experimental 2.1. Materials PVA having an average number molecular weight ( M n ) of 74,800 and degree of hydrolysis greater than 98% was purchased from Nippon Gohsei, Japan. Leonardite, as the natural source
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of HA, was obtained from Mineral Tarim, Turkey. NaOH and HCl were purchased from Merck, Germany. Double distilled water (DDW) was used in the preparation of all aqueous solutions.
2.2. Extraction of HA nanoparticles from leonardite HA nanoparticles were extracted from leonardite by removing humin and fulvic acid using NaOH and HCl aqueous solutions, respectively on the basis of the previously reported
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procedure [27,34]. Briefly, 1 g of leonardite was added to 50 ml of 0.1 M NaOH solution and then the resulting solution was stirred at 700 rpm for 24 h at room temperature. The solution was then centrifuged at 7000 rpm for 10 min and subsequently, the supernatant was collected
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and the remaining precipitates were discarded. The obtained solution was then titrated by 40 mL of 0.1 M HCl solution and stirred at 400 rpm for 1 h. The resulting solution was then
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centrifuged at 7000 rpm for 10 min. After centrifugation, the obtained precipitates were
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separated and washed three times with deionized water and then dried in an oven at 40 °C. Finally, the obtained HA nanoparticles were ground into a fine powder with nearly a
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homogenous particle size and used in the production of HA/PVA supramolecular hydrogels.
2.3. Preparation of HA/PVA supramolecular hydrogels
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To prepare each HA/PVA supramolecular hydrogel, firstly, a predetermined quantity of HA
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nanoparticles was weighed and ultrasonicated in DDW for 4 h to obtain a uniform dispersion. Next, a certain amount of PVA was gradually added to the HA dispersion at 90 °C (using a water bath) with a stirring speed of 200 rpm for 4 h. Then, the obtained uniform HA/PVA dispersions were poured into pre-designed plastic molds and subjected to the freezing-thawing cyclic process. Initially, the molds were kept in a freezer at -15 °C for 24 h to perform the freezing stage and induce the crystallization of PVA chains. After the freezing stage, the molds
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were placed at room temperature for 24 h to thaw. In order to enhance the crystallization of PVA chains and also to increase the crosslinking content and network stability [35], the freezing-thawing process was repeated three times for each sample to create the final supramolecular hydrogel samples. All prepared HA/PVA supramolecular hydrogels had 10 wt.% of PVA (based on the weight of the total sample, i.e. wet basis) and 0, 3, 6, 9 and 12 wt.% of HA (based on the weight of PVA and HA in the sample, i.e. dry basis). The prepared supramolecular hydrogels were named as SHx, which x shows the weight percentage of HA in
supramolecular hydrogels were summarized in Table 1.
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2.4. Characterization
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the completely dried sample. The designations and detailed compositions of the prepared
The structural and morphological characteristics of the prepared supramolecular hydrogels
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were studied using Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry
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(XRD), transmission electron microscopy (TEM) and also scanning electron microscopy (SEM).
The characteristic functional groups of the HA, pure PVA (SH0) and typical HA/PVA
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supramolecular hydrogels loaded with 6 and 9 wt.% HA (i.e., SH6 and SH9) were analyzed by FTIR. Each sample was completely dried under vacuum and then grounded with KBr powder
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and compressed into a disc. The characterizations were performed using a Thermo Nicolet
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(FTIR NEXUS-670, USA) spectroscope. Spectra were collected in the range 650-4000 cm-1 with 2 cm-1 spectral resolution and 20 scans were performed for each sample. The XRD was conducted on all HA-loaded supramolecular hydrogels and also on the pristine HA and HA-free hydrogel (SH0), as references. The XRD experiments were conducted with a diffractometer (Siemens D5000, Germany) equipped with monochromatic CuKα radiation (λ= 0.154 nm, 40 kV and 30 mA) at room temperature. Data were collected over the 2θ range of 2-
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60° at a scanning speed of 2 °/min. Prior to the test, all samples were completely dried in a vacuum oven. The particle size distribution of the HA was determined by the dynamic light scattering (DLS) technique. A dilute dispersion of HA in DDW was prepared and ultrasonicated before the DLS analysis. The DLS analysis was conducted on a Horiba SZ-100 nanoparticle analyzer (Germany) equipped with a 532 nm diode-pumped solid state (DPSS) laser at a scattering angle of 173 º, operated at a temperature of 25 °C.
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The morphology of a typical supramolecular hydrogel (SH6) was observed by TEM. The sample was completely dried under vacuum and then embedded in an epoxy matrix prior to observation. Then the sample was frozen in liquid nitrogen and cut to 70-100 nm thickness
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section, perpendicular to the film, using a diamond knife. Finally, the sample was placed onto 400 mesh copper grid and the observation was performed using a transmission electron
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microscope (EM 208 S, Philips, Netherlands) with an acceleration voltage of 100 kV.
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The particle size of HA and also the fracture surface morphologies of three typical samples, i.e. SH0, SH6 and SH12, were investigated using SEM. The supramolecular hydrogels were broken after freezing in liquid nitrogen and the cross sections coated with a thin layer of gold.
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The SEM images were taken of the fracture cross sections of the supramolecular hydrogels. The observations were carried out using a KYKY-EM3200 (KYKY Technology Development
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Ltd., China) scanning electron microscope.
2.5. Gel fraction measurement The gel fraction values of supramolecular hydrogels, as criteria of the contents of crosslinked PVA chains in three-dimensional networks of prepared samples, were measured by extraction of uncrosslinked polymer chains from the sample and comparing it with the non-extracted sample, using a gravimetric technique. For this purpose, an identical piece of each sample (ca.
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1g) was chosen and dried under vacuum condition. Each drying sample was weighed at regular time intervals and drying continued until the mass of the dried sample showed a constant mass (mi). Then, each dried sample was immersed into the excess amount of DDW to extract uncrosslinked species. The extraction process was continued for a week and the extraction solution was occasionally replaced with fresh DDW. Finally, the extracted sample was removed from DDW and dried under vacuum until the mass of the dried sample showed a constant value (mf).
Gel Fraction
mf mi
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The gel fraction value of each sample was calculated using the below equation [36]: 100
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(1)
The gel fraction experiment was repeated three times for each supramolecular hydrogel and the
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average value was reported.
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2.6. Thermal and mechanical experiments
The thermal and mechanical-thermal properties of the prepared HA/PVA supramolecular
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hydrogels were investigated using differential scanning calorimetry (DSC) and dynamic mechanical-thermal analysis (DMTA). DSC was carried out on three typical samples (SH0,
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SH6 and SH9) using a DSC-Netzsch-200-F3 apparatus. The samples were completely dried in a vacuum oven prior to analysis. The analysis was performed at a scan rate of 10 °C/min for
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the temperature range of -40 to 230 °C and continuous nitrogen flow was employed. The dynamic mechanical-thermal and viscoelastic properties of SH0, SH6 and SH9 samples were also studied. A dried strip of each sample (30 mm long, 10 mm wide and 0.1 mm thickness) was analyzed using a DMA 242-C (Netzsch, Germany) dynamic mechanical analyzer in the tension mode. The scans were performed at a frequency of 1 Hz and a ramp of 5 °C/min from -80 to 140 °C. 10
2.7. Swelling test The abilities of the prepared supramolecular hydrogels in absorbing water molecules from aqueous solutions were experimentally determined using the swelling test. Three pieces of each sample (each of nearly 1 g) were selected and immersed into the excess amount of DDW for a week to extract and remove the uncrosslinked species. Then, the extracted samples were dried under vacuum condition until the mass of each sample fixed in constant value (m0). Then each
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piece of the dried samples was separately immersed in 500 cc of DDW. Each immersed sample was taken out of the DDW at predefined time intervals, the outer surface of the sample was wiped using the absorbing paper and the swollen sample was then weighed (mt). The swelling
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ratio at any swelling time (q(t)), as a quantitative measure of the ability of sample in water absorption, was calculated using the following equation [37]: mt m0
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q(t )
(2)
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The swelling test was continued for each sample to reach the equilibrium swelling condition, where the mass of the swollen sample showed a constant value (m∞). The swelling ratio at
m m0
(3)
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q ( )
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equilibrium condition (i.e., q(∞)) was calculated for each sample using the below equation:
The mean value resulting from three individual tests for each sample was reported as the final
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value of the swelling ratio. The swelling test was carried out for each sample at three different swelling temperatures of 25, 37 and 55 °C, to investigate the effect of the temperature of swelling medium on swelling ratios of samples.
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3. Results and Discussion 3.1. Structural properties FTIR was used to demonstrate the characteristic peaks of HA/PVA supramolecular hydrogels and the possible interactions developed between the HA and PVA functional groups. The FTIR spectra of pristine HA, pure PVA hydrogel (SH0) and SH6 and SH9 samples in the spectral scale of 650 to 4000 cm-1, along with the schematic molecular structures of the HA [38] and
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PVA were shown in Fig. 1. The typical bands of pure PVA hydrogel, including C–O stretching band at 1108 cm-1, C–C stretching band at 1459 cm-1, C–H stretching band at 2952 cm-1 and a broad O−H stretching band centered in the vicinity of 3360 cm-1 were observed [39]. The O−H
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stretching band of PVA hydrogel was broadened because of the crystallization of the PVA chains and the creation of hydrogen bonding among the hydroxyl groups of PVA chains during
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the freezing-thawing process. Fig. 1 also demonstrates common characteristic bands for HA at
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1612, 1704, 2910 and 3390 cm-1. The band at 1612 cm-1 refers to the C=C stretching vibration [40]. The band at 1704 cm-1 is related to the vibrations of C=O stretches from the carboxyl
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group with trace amounts of ketones [41,42]. The band at 2910 cm-1 is attributed to aliphatic C–H asymmetric stretching vibration mode in methyl and/or methylene groups. The band at
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3390 cm-1 corresponds to the stretching vibration of the O–H in the carboxyl group as well as alcoholic and phenolic hydroxyl groups. The band at this wavenumber is broad because of the
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presence of intensive hydrogen bonding involving these groups [43]. Similar typical peaks of reference materials (HA and PVA) can be observed in the FTIR spectra correspond to the SH6 and SH9 supramolecular hydrogels. However the spectra of the SH6 and SH9 samples show that the band related to the O−H groups of HA has been merged with the broad O−H band of PVA and resulted in a new band by shifting to lower wavenumbers centered at ca. 3226 and 3272 cm-1, respectively. Based on the FTIR results, it can prospect that by adding HA to PVA,
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the Keesom dipole-dipole interaction [44] in the form of the hydrogen bonding was developed among the hydroxyl group of the PVA chains and hydroxyl and carboxyl groups of the HA. It can be inferred that by incorporating the HA to the PVA hydrogel, some additional interactions, rather than the interactions (hydrogen bonding) created among the PVA chains due to crystallization during the freezing-thawing process, were developed between functional groups of HA and PVA chains. In other words, the results proved the supramolecular structure of the HA/PVA hydrogels and indicated the crosslinking role of the HA in the HA/PVA
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supramolecular hydrogels. The results imply, in addition to the crystallization of PVA chains during the freezing-thawing process, the HA nanoparticles act as active sites to interact with the PVA functional hydroxyl groups, aggregate the PVA chains and create the supramolecular
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structure in the HA/PVA hydrogels. The mechanism of the PVA chain crosslinking and the supramolecular hydrogel formation was schematically illustrated in Fig. 2.
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The gel fraction values of the HA/PVA supramolecular hydrogels versus the loading level of
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incorporated HA were shown in Fig. 3. The results showed that all HA-loaded hydrogels have higher gel fraction values compared with the HA-free sample (SH0) and the gel fraction was increased by increasing the loading level of HA in supramolecular hydrogels up to 9 wt.%.
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This means that more PVA chains were crosslinked in the presence of HA nanoparticles. The results showed that up to 3.4% increase in the gel fraction of pure PVA hydrogel could be
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achieved by incorporating 9 wt.% of HA into the hydrogel matrix. The increased gel fraction
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values of HA-loaded hydrogels are in agreement with the FTIR results and could be related to the development of interactions between the functional groups of the PVA chains and HA particles. The gel fraction results confirmed the crosslinking role of HA in the prepared supramolecular hydrogels. The decreasing trend of gel fraction value for HA loading level higher than 9 wt.% may be due to the possible agglomeration of the HA particles in SH12 supramolecular hydrogel due to the high loading level of HA. It is obvious that by
13
agglomeration of the HA particles, the surface area of the particles is decreased and the quantity of the created hydrogen bonding between the PVA chains and HA particles declined and as a result, the crosslinking content and gel fraction value of the SH hydrogel is decreased. Fig. 4 shows the XRD patterns of HA, pure PVA hydrogel (SH0) and HA/PVA supramolecular hydrogels containing 3, 6, 9 and 12 wt.% of HA (SH3, SH6, SH9 and SH12). The XRD pattern of HA showed two broad diffraction peaks around 11.6 and 25.7° exhibiting its almost amorphous nature [40,43,45]. However, two peaks at ca. 31.7 and 45.6° were also observed in
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the X-ray diffractogram of HA which may be attributed to the presence of inorganic compounds, such as gypsum (CaSO4.2H2O) [46] and pyrite (FeS2) [47], as two main constituents being existed in the pristine leonardite. Pure PVA hydrogel i.e., SH0, exhibited a
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semi-crystalline structure with two broad characteristic diffraction peaks around 13 and 20° [48,49]. In the XRD patterns of the HA/PVA supramolecular hydrogels, only the characteristic
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diffraction peak of PVA was seen, whereas no characteristic peaks of HA were observed. It
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may be because the additive amount of HA is small, and its diffraction is very weak compared to PVA. Similar peaks observed for pure PVA hydrogel around 13 and 20° were also observed for each HA-loaded supramolecular hydrogels at nearly the same angles. Moreover, it is clearly
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seen that the characteristic peaks intensities of PVA for SH3, SH6, SH9 and SH12 samples have been increased compared with that for SH0 sample. This indicates that the degree of
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crystallinity of PVA chains in the HA/PVA hydrogels were increased with an increase in the
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HA content. The XRD results were in accordance with the FTIR and gel fraction results indicating the role of HA particles as crosslinkers which can increase the crosslinking intensity of PVA chains in HA/PVA supramolecular hydrogels and promote the tendency of PVA chains to be arranged in a more crystalline structure.
3.2. Morphological observations
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The scanning electron micrograph of HA extracted from leonardite was shown in Fig. 5a. As seen, most of the particles were in the nanometric range in size, with the tendency to aggregate to each other. Fig. 6 represents the particle size distribution histogram of the HA particles obtained by the DLS analysis. The histogram showed a mean particle size of 116.5±36.7 nm for HA particles, which is in accordance with the SEM observation. TEM observation was utilized to get information about the dispersion level of the HA nanoparticles inside the PVA hydrogel matrix. The morphology of a typical supramolecular
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hydrogel (SH6) examined by TEM was shown in Fig. 5b. The dark area represents the HA particles and the gray-white area represents the polymer matrix. TEM image exhibited a fair dispersion of the HA nanoparticles inside the PVA matrix. However, some agglomerated sites
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of HA particles could be also observed in the TEM image.
The fracture surface morphologies of the prepared HA/PVA supramolecular hydrogels were
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observed by SEM. The SEM micrographs of cryogenic fracture surfaces of typical HA/PVA
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supramolecular hydrogels containing 0, 6 and 12 wt.% of HA, with two magnifications of each, were illustrated in Fig. 7. As seen, pure PVA hydrogel had a sponge-shaped porous morphology with large pore sizes (Fig. 7a). However, the HA-loaded supramolecular hydrogels displayed
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a compact club-shaped morphology with reduced pore sizes compared with the HA-free sample. The creation of a club-shaped morphology in HA/PVA supramolecular hydrogels was
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due to the presence of HA nanoparticles as the crosslinking agent inside the three-dimensional
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network of hydrogels. A similar club-shaped morphology for polyaniline polymer matrix loaded with the HA particles has been previously reported [29].
3.3. Thermal characteristics
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The thermograms resulted from DSC analysis for SH0, SH6 and SH9 samples in the temperature range of -40 to 230 °C were demonstrated in Fig. 8. All samples exhibited an almost identical thermal behavior owing endothermic peaks above 215 °C, corresponding to their melting points (Tm). The important thermal characteristics of samples, including the Tm, enthalpy of fusion (∆Hf) and relative crystallinity percentage (Xc) were extracted from the obtained DSC curves and listed in Table 2. The Xc values were calculated using the following equation [50]: H f (1 H )H f
100
(4)
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XC
where, ωH is the weight fraction of HA in dried supramolecular hydrogel and H f is the
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theoretical enthalpy of fusion for a 100% crystalline PVA, which is equal to 138.6 J/g [51].
The results showed the Tm of pure PVA hydrogel was increased by 2.4 °C by incorporating 6
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wt.% of HA. However, nearly 5.7 °C increment in the Tm of pure PVA hydrogel was observed
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by adding 9 wt.% of HA. The increased melting point of HA-loaded supramolecular hydrogels could be related to their increased gel fraction values and developed more crosslinking zones inside the supramolecular hydrogels due to the presence of HA nanoparticles. Table 2 shows
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that by incorporating 6 and 9 wt.% of HA into the PVA hydrogel, the crystallinity percentages were increased by ca. 14.5 and 26.4%, respectively. The obtained results for crystallinity were
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in agreement with the XRD results and implied that the HA could actually increase the
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crystallization of PVA chains in the HA/PVA supramolecular hydrogels. It can be deduced that the HA nanoparticles can act as the starting points for PVA crystallization during the freezingthawing process and cause some increments in the crystallinity of HA/PVA supramolecular hydrogels. Furthermore, samples demonstrated broad and shoulder-like endothermic peaks around 40-55 °C, which may be attributed to the glassy-rubbery phase transition of samples. Since the peaks
16
were very weak and broad, the obtained glass transition temperatures (Tg) from DSC curves for examined samples would not be accurate and trustable. The exact values of Tg will be reported based on the DMTA results, in the following section.
3.4. Dynamic mechanical-thermal behavior Fig. 9 shows the DMTA results (i.e. storage modulus (E') and tan curves versus temperature) for HA-free hydrogel and supramolecular hydrogels loaded with 6 and 9 wt.% of HA. A similar
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trend was observed for E' and tan curves of all samples by increasing the temperature. At temperatures below Tg, E' had a relatively high magnitude and was gradually decreased by increasing the temperature. In this temperature range, the hydrogels were in the glassy state
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and thus they showed a very rigid nature with a high E' value. As the temperature reached around Tg, the value of E' declined sharply due to the glassy-rubbery phase transition of the
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hydrogel and as a result, a peak appeared in the tan curve. Following the glassy-rubbery
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phase transition, the hydrogel was transmitted into the rubbery state and the storage modulus decreased slightly with temperature rise.
The results showed that the storage modulus of HA-loaded hydrogels was always higher than
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that of the HA-free sample. Furthermore, the E' value was increased by increasing the HA content in the supramolecular hydrogels. For instance, the E' of the SH6 and SH9
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supramolecular hydrogels at the glassy state were, on average, 24 and 87% higher than that of
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the HA-free hydrogel, respectively. The higher storage moduli of HA-loaded supramolecular hydrogels showed the reinforcing ability of HA nanoparticles in the matrix of the PVA hydrogel. This could be attributed to the presence of HA nanoparticles in the three-dimensional networks of HA-loaded supramolecular hydrogels and their positive roles in increasing the gel fraction values and the creation of more interconnected networks compared with the pure PVA hydrogel.
17
The Tg values of the examined samples were extracted from the tan curves as: 63.2, 64.5 and 65.7 °C for SH0, SH6 and SH9 samples, respectively. The increased Tg values in HA-loaded supramolecular hydrogels were due to the presence of HA nanoparticles in their structure and the increased gel fraction values. It is obvious that in a hydrogel loaded with a higher content of HA, the number of crosslinked PVA chains is increased and shorter sub-chains are created inside the hydrogel network. At this condition, the mobility and the freely movement of the PVA sub-chains are restricted and as a result, the glassy-rubbery phase transition of hydrogels
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occurs at higher temperatures and the Tg values are increased. The obtained DMTA results demonstrated the effective role of HA in enhancing the mechanical and thermal properties of
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the HA/PVA supramolecular hydrogels.
3.5. Swelling behavior
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The swelling kinetics of HA/PVA supramolecular hydrogels in the form of the q(t) curves
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against the swelling time, at the swelling temperatures of 25, 37 and 55 °C, were shown in Fig. 10. All supramolecular hydrogels demonstrated an identical swelling trend at all the swelling temperatures and their q(t) values increased with increasing the swelling time. The slope of the
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swelling curve of each sample was higher and the swelling process happened faster in the early stages of the swelling process and the sample reached an almost stable swelling ratio
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(equilibrium swelling condition) by increasing the swelling time.
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The results showed that the time required to attain a certain level of q(t) depends on the quantity of HA added to the supramolecular hydrogel. It can be seen that in most cases, the supramolecular hydrogels loaded with higher HA contents demonstrated nearly a slower and more prolonged swelling to reach a given degree of swelling. For example, at the swelling temperature of 37 °C, the required time to reach the q(t) of 2.5 was c.a. 300 and 720 min for the SH3 and SH12 samples, respectively. This phenomenon could be explained on the basis of
18
the slower and more restricted relaxation of PVA chains during the swelling process in the supramolecular hydrogels loaded with a higher amount of HA. Furthermore, restricted mass transfer of the water molecules inside the supramolecular hydrogels containing a higher content of HA, due to their more interconnected and entangled networks with smaller pore sizes, may be recognized as another reason in this respect. The effects of HA content and swelling temperature on the q(∞) of the supramolecular hydrogels were exhibited in Fig. 11. The results showed that the q(∞) could be increased by
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either decreasing the incorporated HA loading level or by increasing the swelling temperature in most cases. For instance, the q(∞) of supramolecular hydrogel containing 3 wt.% of HA at 37 °C was 3.4, while this value for the sample containing 12 wt.% of HA at the same swelling
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temperature was 3. In addition, the results showed that the q(∞) value of the SH6 sample increased from 3.4 to 4.2 by increasing the swelling temperature from 25 to 55 °C. The
re
decreasing trend of swelling ratios by increasing the content of incorporated HA in HA/PVA
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supramolecular hydrogels could be related to their higher gel fraction values, more entangled structures, less pore sizes and more tortuous paths for the diffusion of water molecules during the swelling process. The positive effect of swelling temperature on the swelling ratio of
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supramolecular hydrogels could be attributed to the increased chain flexibility of PVA and the
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increased diffusion coefficient of water molecules at a higher swelling temperature.
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4. Conclusions
Supramolecular hydrogels on the basis of HA and PVA were prepared using a freezing-thawing cyclic technique. The results showed that the HA nanoparticles acted as functional sites to aggregate the PVA chains and resulted in the formation of three-dimensional supramolecular networks. Based on the FTIR results, the development of interactions between the functional groups of HA nanoparticles and PVA chains in the form of the hydrogen bonding was proved.
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The gel fraction test showed the role of HA nanoparticles as crosslinking agents in the network of HA/PVA supramolecular hydrogels and an increase in the gel fraction value of pure PVA hydrogel was observed by incorporating HA into the PVA matrix. The XRD results indicated that the degree of crystallinity of PVA chains in the HA/PVA supramolecular hydrogels was increased with an increase in the HA content. The morphology of HA/PVA supramolecular hydrogels and the dispersion level of HA in PVA matrix were observed by TEM and SEM techniques. It was observed that the HA particles were mostly in the nanometric range in size
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and the sponge-shaped morphology of pure PVA hydrogel changed to a club-shaped morphology by incorporating HA nanoparticles. Based on the DSC and DMTA results it was found that the thermal and mechanical properties of HA/PVA supramolecular hydrogels were
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strongly under the influence of the loading level of incorporated HA into the supramolecular hydrogel. It was shown that the glass transition temperature, melting point, crystallinity and
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storage modulus of PVA hydrogels were increased by incorporating HA nanoparticles and
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further increases were observed by increasing the HA content. According to the results obtained from the swelling test, an almost decreasing trend was observed for swelling ratios of HA/PVA supramolecular hydrogels by increasing the HA loading level. However, the swelling
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of supramolecular hydrogels tended to increase by increasing the temperature of the swelling medium. The prepared HA/PVA supramolecular hydrogels are promising materials to be used
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in practical applications, for example as artificial organs and wound dressings in the biomedical
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field and also as absorbent materials in water purification systems and oil recovery applications, due to their improved structural, physical, thermal and mechanical properties. Furthermore, they may be used as potential materials in practical applications where a controlled and prolonged swelling and diffusion are required, for instance as drug carriers in designing the prolonged and controlled drug delivery systems and also as absorbent gels in prolonged irrigation applications. Finally, it was concluded that the loading level of HA
20
nanoparticles is a key factor affecting the structural, physical, thermal and mechanical properties of the HA/PVA supramolecular hydrogels and the desired sample would be produced for designed applications by manipulating the content of incorporated HA nanoparticles into the supramolecular hydrogel matrix. Conflict of interest:
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There is no conflict of interest.
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Figures Captions:
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Matter 7 (2011) 2373-2379.
Fig. 1. (a) Molecular structures of the HA [38] and PVA and (b) FTIR spectra of pristine HA
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and SH0, SH6 and SH9 supramolecular hydrogels.
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Fig. 2. The mechanism of crosslinking and the HA/PVA supramolecular hydrogel formation.
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Fig. 3. Gel fraction values of supramolecular hydrogels versus the HA loading level.
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Fig. 4. XRD profiles of pristine HA and HA/PVA supramolecular hydrogels.
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Fig. 5. (a) SEM micrograph of HA extracted from leonardite and (b) TEM image of the SH6
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supramolecular hydrogel.
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Fig. 6. Particle size distribution histogram of the HA obtained from the DLS analysis.
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magnifications of 2500X and 10000.
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Fig. 7. SEM images of the samples (a) SH0, (b) SH6 and (c) SH12 with two different
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Fig. 8. DSC thermograms of SH0, SH6 and SH9 supramolecular hydrogels.
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Fig. 9. Plots of (a) storage modulus and (b) tan versus temperature for SH0, SH6 and SH9
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supramolecular hydrogels.
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Fig. 10. Swelling kinetics curves of supramolecular hydrogels at the swelling temperature of (a) 25, (b) 37 and (c) 55 °C.
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ro of -p re lP na ur Jo Fig. 11. Equilibrium swelling ratios of supramolecular hydrogels at the swelling temperature of (a) 25, (b) 37 and (c) 55 °C.
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ro of -p re lP na ur Jo Table 1. Designations and chemical compositions of the prepared HA/PVA supramolecular hydrogels.
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Sample
PVA
DDW
designation
(wt.%)
(wt.%)
SH0
10
SH3
HA (wt.%)
Dry basis
90
0
0
10
89.69
0.31
3
SH6
10
89.36
0.64
6
SH9
10
89.01
0.99
9
SH12
10
88.64
1.36
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Wet basis
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12
Table 2. Thermal properties of typical HA/PVA supramolecular hydrogels.
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Tm
∆Hf
Xc
Designation
(°C)
(J/g)
(%)
SH0
215.6
54.8
39.5
SH6
218.0
58.9
45.2
SH9
221.3
63.0
49.9
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Sample
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