Structure and thermochemical study of strontium sodium phosphate glasses

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Structure and thermochemical study of strontium sodium phosphate glasses

Mohamed Atef Cherbib

^a

, Saida Krimi

^b

, Abdelaziz El Jazouli

^c

, Ismail Khattech

^a,

⁎ , Lionel Montagne

^d

, Bertrand Revel

^d

, Mohamed Jemal

^a

aUniversité de Tunis El Manar, Faculty of Science, Chemistry Department, Laboratory of Materials Crystal Chemistry and Applied Thermodynamics LR15SE01, Tunisia

bLPCMI, Faculté des Sciences Aïn Chok, UH2C, Casablanca, Morocco

cLCMS, URAC 17, Faculté des Sciences Ben M'Sik, UH2MC, Casablanca, Morocco

dUniv. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France

a b s t r a c t a r t i c l e i n f o

Article history:

Received 18 February 2016

Received in revised form 29 April 2016 Accepted 9 May 2016

Available online xxxx

Phosphate glasses having the general formula (100-x) NaPO3-x SrO with an O/P ratio varying between 3 and 3.25 were synthesized by melt quenching technique. The glasses are studied in order to determine the inﬂuence of SrO addition on the structural and physical-chemical properties. Density, glass transition and crystallization temperatures increase with SrO content, showing the shrinking and reticulation of phosphate network. The chemical durability of the glass series is enhanced by the substitution of Na2O and P2O5by SrO. Fourier Transform Infrared Spectroscopy and ³¹P Magic Angle Spinning Nuclear Magnetic Resonance spectroscopies revealed the depolymerisation of the glass network by the decrease of middle chain units to create end chain units species.

Calorimetric study of the dissolution of glasses in acid solution shows a decrease of the dissolution enthalpy when the SrO is added.

Keywords:

Glass structure Strontium phosphate glass Glass thermochemistry Chemical durability

1. Introduction

Phosphate glasses have attracted attention initially for their optical properties[1,2], but their low chemical durability constitutes a problem for their use in common applications. This results from the open structure of phosphate glasses when compared to silica glasses[3,4]and the solubility can be tailored to create bioactive materials for bone re- pair and tissue engineering[5,6]. Theﬁrst generation of bioactive glasses contains silica[7]but the lack of clinical data about the long- term effect of silica in vivo induces the development of a new generation of bioactive materials[8]. Alkali-earth phosphate glasses are good candidates for these applications, since they can be dissolved in vivo without releasing toxic elements[6].

Strontium oxide enhances bone regeneration and also provides a radio-opacity for the implants[9–11]. Thus, because of the importance of strontium oxide in bone healing and the similarity of phosphate glasses with the mineral part of bones, the study of strontium phosphate glasses seems instructive to point out the inﬂuence of SrO on the physical and structural properties of phosphate glasses.

This work deals with the synthesis of glasses with the formula (100- x) NaPO3-x SrO with x ranging from 0 to 20% molar. Density, differential

scanning calorimetry and chemical durability measurements were carried out on these compounds, together with structural investigations by Fourier Transform Infrared and³¹P Magic Angle Spinning Nuclear Magnetic Resonance spectroscopies. Microcalorimetry was also used to determine the effect of SrO on the thermochemical properties of the glasses.

2. Experimental

2.1. Glass preparation

Strontium carbonate SrCO3(Sigma Aldrich) and sodium phosphate monobasic monohydrate NaH2PO4·H2O (Sigma Aldrich), with high pu- rity (99%), were used as a starting materials for the synthesis. The suit- able proportions of the reagents were mixed and heated in a platinum crucible at 400 °C during 3 h then at 600 °C for 3 h in order to eliminate water and initiate the condensation of the phosphate groups. The melt was obtained between 750 °C and 1100 °C for composition varying from NaPO3to 80 NaPO3-20 SrO mole%, respectively. The batch was then poured into a set of four graphite crucibles and the four disks obtained for each glass composition were annealed at 250 °C for 3 h in order to remove strains from the glasses andﬁnally kept in the desicca- tor prior to use. In the present manuscript, the glasses are labelled according to the strontium oxide amount.

Journal of Non-Crystalline Solids 447 (2016) 59–65

⁎ Corresponding author.

Contents lists available atScienceDirect

Journal of Non-Crystalline Solids

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j n o n c r y s o l

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2.2. X ray powder diffraction (XRPD) and inductively coupled plasma analysis (ICP)

The amorphous state of the glasses was checked by X-ray powder diffraction. Diffractogramms were recorded on a D8 Bruker diffractom- eter using Cu Kαradiation and the data were collected between 10 and 70° (expressed in 2 ).

The nominal composition of the glasses was measured by Inductive- ly Coupled Plasma analysis using a Jubin Yvon Ultima C spectrometer.

For that purpose, the glasses were dissolved in a 7.5 wt% HCl solution previous to their ICP analysis.

2.3. Density measurements and thermal analysis

The density of the glasses was obtained by Archimedes' method using diethyl orthophthalate as immersion liquid. The accuracy of the density measurements is about ±0.03 g/cm³. The molar volume was calculated from the density and the molar mass and its random error was retrieved considering the accuracy of the density measurements.

Glass transition (Tg) and crystallization (Tc) temperatures of the glasses were measured by Differential Scanning Calorimetry using a Setaram DSC 131 evo, with a heating rate of 10 °C per minute, under ni- trogen atmosphere. The accuracy of the measurements is ±5 °C.

2.4. Chemical durability

The chemical durability of the glasses was characterized considering the complete dissolution time and the initial dissolution rate per surface unit determined at room temperature and using distilled water as a leaching solution. The water volume (V) was adjusted as a function of the surface of the glass disks (S) according to the ratio (S/V = 0.04 cm⁻¹). The pristine samples mass was between 1 g and 3 g (±0.1 mg). After the test, the glasses were dried with acetone and the weight loss was measured with a precision balance. Weight loss measurements were carried out until the complete dissolution of the glass disks. The initial dissolution rate per surface unit (mg/cm²/min) of the samples was deduced from a polynomialﬁt of the plot of the weight loss variation (g) as a function of the leaching time t (min). The deriva- tive of theﬁt function at t = 0 per sample surface is taken as the initial dissolution rate. The accuracy of the weight loss measurement is

±0.1 mg and the random error on the initial dissolution rate was calculated considering the errors of the weight loss and surface measurements.

2.5. Spectroscopic investigations

Fourier Transform Infrared (FTIR) Spectra were recorded on a Shimatzu FTIR 8400 S spectrometer in the frequency range 400– 4000 cm⁻¹. 9 mg of the glass powder were mixed with 300 mg of spectroscopic grade KBr and pressed into pellet for the analysis.

31P Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectra were obtained using a Bruker spectrometer operating at 9.4 T.

The spectra were recorded in 8 scans with a spinning speed of 12.5 KHz, a pulse length of 45° and a relaxation decay of 120 s, which was veriﬁed to be long enough to enable recording of quantitative spectra. The³¹P chemical shifts are expressed in ppm relative to a 85%

weight phosphoric acid solution.

2.6. Thermochemical study

The heat of dissolution of the glasses was measured at 298 K using a C-80 Setaram microcalorimeter. The latter is provided with two reversal mixing cells surrounded by thermopiles for the detection of the heat ﬂow. 20 mg of the glass samples to be dissolved were introduced in the lower compartment of the reaction cell and separated with a mov- able cover from the upper compartment, which contained 4.5 mL of 4.5 wt% H3PO4solution. The reactants were mixed by the reversal me- chanical system of the device[12]. The precision of the microcalorimeter was checked by standard dissolution reactions[12,13]and the accuracy on those measurements was calculated using a statistical Fig. 1.Diffractogramms of the glasses with the formula (100-x) NaPO3-x SrO.

Table 1

Nominal and analyzed compositions obtained by ICP analysis of the glasses with the formula (100-x) NaPO3-x SrO.

SrO

nominal/analyzed Na2O

nominal/analyzed P2O5

nominal/analyzed Glass label

0/0 50/50.5 50/49.5 NaPO3

5/3.4 47.5/48.6 47.5/48.0 5 SrO

10/7.0 45/46.0 45/47.0 10 SrO

15/13.0 42.5/43.3 42.5/43.7 15 SrO

20/18.0 40/41.5 40/40.5 20 SrO

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model proposed by Pattengill[14]. The random error of the heat of dissolution was calculated consideringﬁve integrations of the obtained raw signal of the dissolution.

3. Results

3.1. XRPD and ICP analysis

Fig.1shows the diffractogramms of the glasses with the formula (100-x) NaPO3-x SrO. In the studied range of compositions the amorphous state is kept until 20% SrO.Table 1presents the analyzed and theoretical amounts of SrO, Na2O and P2O5. The gap between these compositions is 3% at most.

3.2. Density measurement and thermal analysis

The density and molar volume variations of the glasses are listed in Table 2and plotted inFig. 2. The density increases from 2.48 g/cm³for NaPO₃to 2.80 g/cm³for the glass labelled 20 SrO (80 NaPO₃-20 SrO).

The molar volume decreases from 41.16 cm³/mol for sodium metaphosphate to 36.56 cm³/mol for the 20 SrO glass.

Table 2andFig.3show the glass transition Tg and crystallization Tc temperatures variations versus glasses composition. The glass transition increases linearly from 297 °C for NaPO3to 323 °C for 20 SrO glass (80 NaPO3-20 SrO). The crystallization temperature increases regularly from 371 °C for sodium metaphosphate glass to 434 °C for the glass 15 SrO, then decreases to 412 °C for the 20 SrO glass.

Fig.4shows as examples the weight loss versus leaching time for the glasses labelled 5 SrO and 10 SrO andFig.5gathers the variation of the complete dissolution time and initial dissolution rate of the glasses versus SrO content. The complete dissolution time for 5 SrO and 10 SrO rises from 4.5 h to 21 h respectively. The complete dissolution time increases from 2 h for the sodium metaphosphate glass to 412 h for the 20 SrO glass. The initial dissolution rate decreases from 3.28 mg/cm²/min for NaPO3to 0.008 mg/cm²/min for the 20 SrO glass (Table 2).

3.4. Spectroscopic investigation 3.4.1. FTIR spectroscopy

Fig. 6shows the evolution of the FTIR Spectra of the glasses as SrO is added. Two regions are observed, theﬁrst one is located between 400 cm⁻¹ and 930 cm⁻¹where the bands at 500, 700, 780 and 850 cm⁻¹are unchanged. The second region between 950 cm⁻¹and 1350 cm⁻¹exhibits four bands at 1040, 1090, 1160 and 1280 cm⁻¹ whose intensities are dependent of the glass composition.

3.4.2. Solid state nuclear magnetic resonance

Fig.7presents the isotropic chemical shifts of the studied compositions. Theﬁrst resonance, between−19 and−17 ppm decreases with increasing strontium oxide and the second resonance located between 3 to−2 ppm increases with the SrO content in the glasses.

Table 2

Density, molar volume, glass transition temperature, crystallization temperature, complete dissolution time, initial dissolution rate and enthalpy of dissolution of the (100-x) NaPO3-x SrO glasses.

Glass compositions Density (g/cm³)

±0.03

VM(cm³/mol) Tg (°C)

±5

Tc (°C)

±5

Complete dissolution time (h) Initial dissolution rate (mg/cm²/min) Enthalpy of dissolution (kJ/mol)

NaPO3 2.48 41.11 ± 0.5 297 371 2 3.28 ± 0.1 5.24 ± 0.04

95 NaPO3-5 SrO 2.55 40.02 ± 0.5 300 384 4, 5 3.08 ± 0.1 4.17 ± 0.08

90 NaPO3-10 SrO 2.67 38.25 ± 0.4 312 407 21 0.251 ± 0.2 3.71 ± 0.03

85 NaPO3-15 SrO 2.74 37.31 ± 0.4 316 434 192 0.154 ± 0.2 2.97 ± 0.09

80 NaPO3-20 SrO 2.8 36.54 ± 0.4 323 412 412 0.008 ± 0.003 2.07 ± 0.04

Fig. 2.Density and molar volume variation of the (100-x) NaPO3- x SrO glasses. The lines are drawn as guide to the eyes.

M.A. Cherbib et al. / Journal of Non-Crystalline Solids 447 (2016) 59–65 61

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3.5. Microcalorimetric investigations

Fig.8shows the variation of the molar heat dissolution of the phosphate glasses versus SrO content in 4.5% weight H3PO4solution. Phos- phoric acid was used to prevent interactions between the dissolved species from the glasses and to have comparable results with other glass systems[24–26]. The heat of dissolution decreases from 5.24 kJ/

mol for sodium metaphosphate glass to 2.12 kJ/mol for 20SrO- 80NaPO3glass composition (Table 2).

4. Discussion

4.1. XRPD and ICP analysis

The XRPD patterns of the glasses reveal that the amorphous state is maintained up to 20% of SrO in the glasses. Above this content, the glass crystallizes readily after the quenching. This could result from the decrease in the viscosity of the melt when the amount of P₂O₅decreases [4], inducing a greater facility for the different species to move in the

melt and crystallize. The nominal and analyzed compositions of the glasses are shown inTable 1. The difference between the experimental and theoretical amounts of strontium could appear from the interfer- ence in the ICP analysis causing the detection of lower amount of SrO.

The diminution in SrO content induces an increase in the amounts of the other oxides.

4.2. Density measurement and thermal analysis

The increase of the density of the glasses is explained by the higher molar mass of strontium oxide compared to NaPO3. The decrease in the molar volume of the glasses is related to the larger cation potential (Z/a)[15]of Sr²⁺compared to that of Na⁺, 1.7 Å⁻¹and 0.95 Å⁻¹, respectively creating a more compact glass network.

The variation in the Tg values suggests an increase of the rigidity of the phosphate network with increasing SrO content. SrO and Na2O are both classiﬁed as a modiﬁer oxides but Sr²⁺connects two phosphate sites compared to one site for Na⁺[4]. The variation of the crystallization temperature could be explained by the greater crystallization Fig. 3.Glass transition temperature (Tg) and crystallization temperature (Tc) variation of the (100-x) NaPO3-x SrO glasses. The lines are drawn as guide to the eyes.

Fig. 4.Weight loss of the 5 SrO and 10 SrO glasses versus leaching time. The lines are drawn as guide to the eyes.

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tendency of the 20 SrO glass. Indeed, the latter composition delimits the glass forming region and thus crystallizes more easily.

As expected, from the results of chemical durability tests, the higher the SrO content is, the higher is the chemical durability of the glasses.

The increase in the initial dissolution rate from 3.08 mg/cm²/min to 0.251 mg/cm²/min for 5 SrO and 10 SrO glasses respectively, indicates that up to 5 mol%, the strontium oxide has a efﬁcient effect on the chemical durability of the glasses. These results could be explained by the increase in the cross links between the phosphate units when the strontium oxide is added and thus conﬁrming its reticulation effect.

The phosphate glasses dissolution begins by the hydration process [16]. This reaction consists in the diffusion of water into the glass surface and the migration of entire phosphate chains into the solution. It seems that the largeﬁeld strength of Sr²⁺increases gradually the reticulation of the phosphate network and creates a less sensitive P\\O\\Sr bonds

toward hydration than the P\\O\\Na and P\\O\\P bonds and therefore enhance the chemical durability of the glasses[4].

4.4. Spectroscopic investigation 4.4.1. FTIR spectroscopy

Phosphate sites are classiﬁed using the Qⁿterminology established by Van Wazer[17], with n is the number of bridging oxygen atoms per phosphate tetrahedron. The distribution of the Qⁿgroups is dependent on the oxygen to phosphorus ratio (O/P). In the studied compositions of glasses, the ratio O/P is between 3 and 3.25 thus, Q²and Q¹ are expected to be predominant in the phosphate network.

In theﬁrst region of the FTIR Spectra of the glasses, the bands are assigned to the bending of the P\\O\\P linkage at 500 cm⁻¹. The sym- metricυsand asymmetricυasstretching modes of the P\\O\\P linkage appear at about 700–780 cm⁻¹and 850 cm⁻¹respectively[18]. The second region between 950 cm⁻¹and 1350 cm⁻¹exhibits two bands at about 1040 cm⁻¹and 1090 cm⁻¹which increase when the strontium oxide is added. These bands are related to the symmetricυs and Fig. 5.Initial dissolution rate and complete dissolution time of the (100-x) NaPO3-x SrO glasses. The lines are drawn as guide to the eyes.

Fig. 6.FTIR spectra of the (100-x) NaPO3-x SrO glasses.

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asymmetricυasstretching modes of the end chains units or Q¹species, respectively. The bands at 1160 and 1280 cm⁻¹decrease with the SrO content. These latter are assigned to the symmetricυsand asymmetric υasstretching modes of Q²metaphosphate species[18–20]. These spec- tral changes can be attributed to the depolymerisation of the phosphate network. For x = 0, the structure consists in quasi inﬁnite chain of PO4, which evolves into smaller chains throughout the addition of SrO.

4.4.2. Solid state nuclear magnetic resonance

Theﬁrst resonance in the³¹P MAS-NMR spectra, located between

−19 and−17 ppm, is assigned to Q²sites. The second resonance from 3 to−2 ppm, is related to the end chain units Q¹. These results conﬁrm the depolymerisation of the phosphate network when the strontium oxide is added[9,21].

The shielding of the Q²species is relatively constant but the isotropic chemical shift of Q¹species decreases through the depolymerisation process. These results suggest a different cation distribution around the Q¹and Q²species and a connexion of the Sr²⁺with the Q¹whereas the Na⁺seems to remain linked to the Q²species[21].

The proportions of Q²and Q¹species were obtained from the integration of the surface areas of the peaks, they are close to those

calculated using Van Wazer equations and reported inTable 3together with the average chain length[3,17]. The Q¹proportion rises progres- sively and the average chain length decreases with increasing SrO content.

The proportion of the Qⁿspecies and the average chain length in the glasses affect the durability of phosphate glasses[4]. Indeed, the larger the proportion of Q¹groups, the larger is the dissolution time of the glasses. The double effect of the depolymerization of the phosphate network and the linking of the strontium oxide to the end chain units on the dissolution behaviour of the glasses had been also reported previously[22,23].

4.5. Microcalorimetric investigations

The variation of the heat of dissolution of the glasses when increasing SrO content is related to the decrease in the average chain length of polyphosphate species when the strontium oxide is added. This decline of the heat of dissolution was also observed previously for other glass systems having MgO, ZnO and MnO as intermediate oxides[24–26], but its value is lower compared to the glass containing SrO. This result Fig. 7.³¹P MAS NMR spectra of the (100-x) NaPO3-x SrO glasses.

Fig. 8.Variation of the molar dissolution heat of the glasses of the (100-x) NaPO3-x SrO glasses. The lines are drawn as guide to the eyes.

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could be related to the lower charge to size ratio of Sr²⁺compared to Zn²⁺, Mg²⁺and Mn²⁺.

5. Conclusion

This work highlights the effect of SrO addition on some properties of a series of phosphate glasses with the formula (100-x) NaPO3-x SrO. The higherﬁeld strength of Sr²⁺results in the shrinking of the phosphate network. The substitution of the P\\O\\Na and P\\O\\P bonds by the P\\O\\Sr bonds increases the reticulation and enhances the chemical durability of the glasses. The structural investigations indicated the depolymerisation of the metaphosphate chains into shorter polyphosphate species as the SrO content increases and a different cation distribution around the Q¹and Q²species. The decrease in the heat of dissolution in acid solution shows a dependence of this quantity with decreasing average chain length.

Acknowledgments

The authors want to thank Professor Latifa Bargaoui from INSAT for the DSC measurement. The authors also thank the Chevreul Institute (FR 2638) for its help in the development of this work. Chevreul Insti- tute is supported by the French“Ministère de l'Enseignement Supérieur et de la Recherche”, the“Région Nord-Pas de Calais”and the“Fonds Européen de Développement des Régions”.

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Table 3

Q²and Q¹proportion calculated from integration of the surface area of the peaks compared to the proportion and the average chain length calculated according to Van Wazer equations for the series (100-x) NaPO3-x SrO.

Glass composition Q²proportion calculated Q¹proportion calculated Q²proportion from[3,17] Q¹proportion from[3,17] Average chain length from[3,17]

95 NaPO3-5 SrO 89 11 86 14 19

90 NaPO3-10 SrO 78 22 73 27 4.5

85 NaPO3-15 SrO 65 35 60 40 2.83

80 NaPO3-20 SrO 50 50 48 52 1.5