Hybrid superabsorbent polymer networks (SAPs) encapsulated with SiO2 for structural applications

In this work, materials that as additives in cement promote selfsealing/healing properties by the gradual release of water they absorb were synthesized, characterized and evaluated. Specifically, hybrid SAPs that absorb high ammounts of water encapsulated with SiO2 that facilitates their incorporation in the matrix since it improves their chemical affinity were investigated. The structure and morphology of the fabricated SAPs were characterized analytically and confirmed the synthesis of P(MAA-coEGDMA)@SiO2 nanocomposite. Its particle size is expected to reduce the size of the pores formed due to the absorbing/desorbing water process during the mixing and curing of cement. Moreover, the water absorbance of the above mentioned material as well as its ability to maintain its original structure during subsequent cycles of absorbing/desorbing water from different mediums and specifically from distilled water (DW) and cement slurry filtrate (CS) were evaluated. CS was chosen to mimic the cementitious environment considering the presence of various ions and its pH value (~ 12). The results revealed that the absorption ratio of P(MAAco-EGDMA)@SiO2 in DW and CS was higher than 1500 wt.% its original dry weight, while SEM pictures proved that the hybrid SAPs maintained their structure after the water absorption tests.


Introduction
Superabsorbent polymers (SAPs) are partially crosslinked, three-dimensional (3D) polymer networks which can retain water from aqueous solutions in huge amounts due to their hydrophilic nature.The water retained by SAPs can be up to thousands of times their own dry weight [1][2][3].Their hydrophilic nature is owed to hydrophilic groups incorporated in the polymer structure.The swollen state of SAPs is known as hydrogels which can also demonstrate a reversible behaviour by de-swelling.Hydrogels maintain the initial 3D structure of SAPs and are not dissolved in water basically due to their crosslinking [4,5].Hydrogels are widely used in hygienic products, agriculture, drug delivery systems, sealing, pharmaceuticals, biomedical applications, tissue engineering, biosensors etc [5].Both swelling and deswelling behaviour of hydrogels highly depend on their architecture.Therefore, several modifications on the design of the polymeric molecules have been investigated and the corresponding results have been reported [4].For instance, rapid kinetics are favoured by decreased molecule size and the incorporation of micro porosity on the polymer surface increases the contact area between the hydrogel and the solvent [4].
Recently, interesting applications of hydrogels in construction and more specifically in cementitious materials have been investigated.SAPs in concrete reduce autogenous shrinkage and form a novel approach to limit cracking.This effect is realized through the gradual release of water during cement setting and hardening thus promoting a self-curing functionality through CaCO3 precipitation [6,7].Nevertheless, the lack of adhesion between the phases of the cement additives and the SAPs could result in failure of the interface.Moreover, one of the greatest challenges regarding the SAPs/cement composites is the process which is followed for their incorporation in the concrete mix as well as the decrement in the mixture's workability.More specifically, the SAPs should retain their ability to absorb and release water after and during a crack formation.Consequently, a selfhealing/sealing effect will also ideally take place during water absorption and swelling of the cross-linked polymer network.Additionally, the absorbing/desobing behaviour of SAPs during and after the mixing procedure, respectively, is responsible for the macro-pores formation in the produced specimens and the deterioration of their mechanical properties.The motivation of the present work is to counterbalance the abovementioned drawbacks by the introduction of materials with increased compatibility with the cement-based matrix that will exhibit high water absorption capacity and keep their morphology and structure in conditions that mimick the cement environment [3,6,7].
The aim of this work is to fabricate SAPs in the sub-micron to nano-scale, of enhanced chemical affinity to cement that reveal chemical stability, high water absorbency and reversible absorption/desorption behaviour in ionic solutions, all properties relevant to the SAPs expected behaviour as additives in cement admixtures to promote self-sealng/healing and reduce autogeneous shrinkage during and after mixing of cement [8].
In this study, hybrid organic core-inorganic shell SAPs were synthesized by the copolymerization of Methacrylic Acid (MAA) with Ethylene Glycol Dimethacrylate (EGDMA), followed by chemical modification of the carboxyl groups with NaOH thus enhancing SAPs hydrophilicity, while the subsequent encapsulation of the modified copolymer with SiO2 inorganic shell led to the production of P(MAA-co-EGDMA)@SiO2 nanocomposites.The SAPs encapsulated with the SiO2 inorganic shell showed immense improval in water absorbency in both distilled water and cement slurry filtrate (ionic solution that mimics the conditions in cement) and at the same time proved to maintain their structure after absorbion/desorption tests in both mediums as shown in the corresponding SEM images.Moreover, they are expected to reveal enhanced chemical compatibility with cement due to the SiO2 shell and to reduce the size of the pores formed in cement during and after the mixing procedure because of the absorbing/desobing behaviour of SAPs, due to their small size and narrow size distribution.

Synthesis of hybrid super absorbent polymer networks, SAPs
The synthesis of the hybrid SAPs is consisted of three steps: (i) preparation of the SAPs based on the copolymerization of MAA with EGDMA, (ii) ionization of the synthesized SAPs and (iii) encapsulation of the synthesized SAPs in SiO2 shell.(i) Preparation of the SAPs P(MAA-co-EGDMA) sub-micron spheres were prepared by radical polymerization in ACN.The polymerization took place in a 1 L-reactor equipped with a condenser under inert atmosphere (nitrogen, N2, flow) at 80 o C with constant and vigorous stirring.ACN (800 mL) was added in the reactor vessel and vigorous stirring began.The reaction proceeded under N2 flow for about half an hour to discard oxygen remains from the reaction vessel when at the same time temperature was set at 80 o C. When the temperature reached the set point, first the distilled MAA (21 mL) and then EGDMA (2.7 mL) were added in the reactor dropwise.Finally, the initiator, KPS (0.7 g) which was previously dissolved in a small amount of ACN (~ 5 mL) using an ultrasonic bath was supplied in the reaction mixture in the same manner and the polymerization reaction continued overnight.The polymer was centrifuged at 7000 rpm for 10 minutes and then washed with ACN twice.P(MAA-co-EGDMA) spheres were stored in paste form in ACN.(ii) Ionization of the synthesized SAPs Around 12 g of P(MAA-co EGDMA) paste were dispersed in 600 mL ACN in a big beaker under magnetic stirring.2.44 g of NaOH was then dissolved in 2-3 mL of distilled water in an ultrasonic bath and was added to the PMAA solution.The reaction continued overnight and the product was centrifuged at 7000 rpm for 10 minutes and then washed with ACN twice.The ionized P(MAA-co-EGDMA) spheres (P(MAA-co-EGDMA)-NaOH) were stored as a paste in ACN.(iii) encapsulation of the synthesized SAPs in SiO2 shell P(MAA-co-EGDMA)-NaOH paste (6.5 -7 g) was dissolved in approximately 100 mL ACN using an ultrasonic bath and then the solution was supplied in the rest of the ACN (900 mL) which was placed in a big beaker under vigorous stirring.The mixture was left under stirring overnight in order to disperse any agglomerates of the P(MAA-co-EGDMA)-NaOH spheres.TEOS (42 mL) was added in the mixture dropwise and the reaction continued overnight.The product (P(MAA-co-EGDMA)@SiO2) was received after centrifugation at 5000 rpm for 5 minutes / washing with ACN twice and stored as a dry pulverized powder.

Instrumental Analysis
SEM (Scanning Electron Microscopy) was utilized to characterize the structure, shape and size of the SAPs prior and after swelling.SEM characterization was performed using a Phillips Quanta Inspect (FEI Company) microscope, QUANTA 200 with W (tungsten) filament 25 kV equipped with EDAX GENESIS (AMETEX PROCESS & ANALYTICAL INSTRUMENTS).All the samples were coated with gold before SEM observation.Thermogravimetric analysis (TGA) of synthesized hydrogels were performed using Polymer Laboratories systems at a heating rate of 20 C/min under nitrogen atmosphere.
Additional data concerning the structure of the produced SAPs and the composition of their surface coating were obtained by TEM (Transmission Electron Microscopy) observations which were carried out in a JEM2000 FX (200KV, resolution 0.28 nm).
The functional groups of the specimens were characterized by FTIR (equipped with an Attenuated Total Reflectance module with a diamond crystal)) using an Agilent Cary 630 spectrometer.FTIR spectroscopy measurements were performed on as-produced vacuum dried pulverized specimens.
Thermogravimetric analysis (TGA) of the synthesized hydrogels was performed using NETZSCH STA 449 FS Jupiter system at a heating rate of 5 o C/min in nitrogen atmosphere in the range of 20 to 600 o C.

Absorption ratio measurements
To determine absorption ratio (AR) of SAPs (gg -1 ) weighed amounts of vacuum dried pulverised SAPs (M1) and excess of distilled water (DW) and cement slurry filtrate (CS) were added in eppendorfs.CS was produced to mimic the environment inside the concrete (high pH and presence of ions).Specifically, a cement solution was prepared by mixing 10 g cement (CEM I 42.5R, Lafarge) with 100 mL tap water.The cement slurry was first mixed for 1 h and then was filtered.The CS pH value was determined by pH indicator paper around 12 [9].
At specific time intervals (5, 10, 15, 20, 30, 60 minutes and one day) the SAPs were centrifuged at 15.000 rpm for 5 minutes, the supernatant liquid was rejected and the SAPs were weighed again (M2).The absorption ratio (AR%) was calculated using the following equation.Creation of the hydrogel was in all cases instantaneous.

Instrumental Analysis
SEM: Figure 1 shows SEM images of the as-synthesized P(MAA-co-EGDMA) (1a), P(MAA-co-EGDMA)-NaOH (1b) and P(MAA-co-EGDMA)@SiO2 (1c).From these images it is observed that the synthesized SAPs have a uniform structure.They are spherical and their size lies in the submicron area, while they maintain their shape and size during all the synthesis steps (ionization and encapsulation in the SiO2 shell).Moreover, only the hybrid organic core -inorganic shell SAPs, P(MAA-co-EGDMA)@SiO2, maintains its structure (shape and size) after the water absorption tests.
According to SEM-EDAX analysis only the submicron spheres of P(MAA-co-EGDMA)@SiO2 contain silica, while oxygen and carbon concentration is reduced from P(MAA-co-EGDMA) to P(MAA-co-EGDMA)@SiO2 thus proving the successful encapsulation of the polymeric core in the inorganic shell.These results are presented in Table 1.
Table 1.SEM-EDAX analysis for carbon, oxygen and silicon in P(MAA-co-EGDMA) and P(MAAco-EGDMA)@SiO2 samples.TEM: In Figures 3-5 HRTEM images of the P(MAA-co-EGDMA)@SiO2 sample are presented.It can be seen that in this sample nanoparticles reveal an amorphous corecrystalline shell structure where the 0.33 nm d-spacing of the {10-11} planes of hexagonal α-SiO2 (bulk value 0.334 nm) is clearly resolved.In the nanoparticles shown in Figures 3-5 the width of the shell is of the order of 10 to 13 nm, that corresponds to nearly 25-35 {10-11} SiO2 planes.The FTIR spectra for the as-prepared, the ionized and the encapsulated SAPs are presented in Fig. 6.These spectra confirmed the formation of the inorganic cell on the polymeric core.More specifically, the spectra of only the modified spheres (P(MAA-co-EGDMA)-NaOH) and the encapsulated spheres (P(MAA-co-EGDMA)@SiO2) revealed a broad band at 3200 -3400 cm -1 , which is attributed to -OH stretching vibration due to ambient water absorbance as these molecules compared to the original P(MAA-co-EGDMA) sub-micron spheres have increased hydrophilicity.Moreover, the peaks at 1046, 782 and 445 cm -1 are assigned to amorphous Si-O-Si absorbance vibrations [10,11].

Sample
The FTIR spectra of P(MAA-co-EGDMA) and P(MAA-co-EGDMA)-NaOH displayed the characteristic absorption band of C=O vibration at 1730 cm -1 which are attributed to the carbonyl groups in the MMA component [12].Additionally, the absorption peak at 1152 cm −1 was ascribed to the asymmetrical stretching of C-O in the MMA units.Transmitance (a.u.) Wavenumber (cm -1 ) P(MAA-co-EGDMA) P(MAA-co-EGDMA)-NaOH P(MAA-co-EGDMA)@SiO2 Fig. 6.FTIR spectra of P(MAA-co-EGDMA), P(MAA-co-EGDMA)-NaOH and P(MAA-co-EGDMA)@SiO2 The TGA of the P(MAA-co-EGDMA)@SiO2 sample is illustrated in Figure 7.The first degradation process (30-180 o C, 19 wt.%) is related mainly to the loss of physical absorbed water molecules through the formation of intra-and intermolecular anhydride links and acetonitrile (derived from the synthetic procedure).The second degradation step (180-300 o C, 3 wt.%) is assigned to the decarboxylation of a fraction of the -COOH groups by which CO2 is formed.The third and more pronounced weight loss (300-500 o C, 44 wt.%), is ascribed to the complete polymer decomposition.The residual mass at 550 o C corresponds to the SiO2 inorganic shell and is equal to approximately 33 wt.% of the material initial mass.

Absorption ratio measurements
The SAPs AR is mainly related to the properties of the external solution such as presence of ions, charge number, ionic strength and pH value, as well as the SAPs' nature (elasticity of the network, presence of hydrophilic functional groups, and extent of crosslinking).More specifically, it is observed that SAPs AR decreases significantly in salt solutions compared to the corresponding values in distilled water, while this behaviour is mainly attributed to a "charge screening effect" of the additional ions and "ionic crosslinking" at the surface of particles in salt solutions of multivalent ions [1].
In this work, the absorption ratio (AR) of P(MAA-co-EGDMA), P(MAA-co-EGDMA)-NaOH and P(MAA-co-EGDMA)@SiO2 was studied in two different mediums: distilled water, DW, (pH ~ 7) and cement slurry filtrate, CS, (pH ~ 12), which is an ionic medium that mimics the conditions in the cement environment.The AR of all SAPs is shown in Figure 8.All SAPs synthesized, P(MAA-co-EGDMA), P(MAA-co-EGDMA)-NaOH and P(MAA-co-EGDMA)@SiO2, absorbed great amounts of water and specifically 515 g, 2880 g and 3900 g per g of dry material respectively (maximum AR values are noted for each case) thus forming hydrogels.The absorption capacity of these SAPs in DW and CS followed a similar trend as the amount of water absorbed was higher in the case of DW than in the case of CS for all SAPs, a behaviour which is in accordance with literature [1].
Nevertheless, significant differences were observed among the three different SAPs.In both DW and CS the highest AR value was obtained by P(MAA-co-EGDMA)@SiO2 and reached 3900 gg -1 and 1920 gg -1 , respectively, while the corresponding values for P(MAAco-EGDMA) was determined at 515 and 511 gg -1 , respectively.