Journal ofMaterials Chemistry B

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aSchool of Chemical and Biomolecular Engin

2006, NSW, Australia. E-mail: fariba.dehghbUniversity of New South Wales, Mark Wai

UNSW, Sydney, 2052, NSW, AustraliacFaculty of Pharmacy, The University of Syd

† Electronic supplementary informa10.1039/c2tb00251e

Cite this: J. Mater. Chem. B, 2013, 1,1835

Received 15th October 2012Accepted 30th November 2012

DOI: 10.1039/c2tb00251e

www.rsc.org/MaterialsB

This journal is ª The Royal Society of

Molecular interactions in coupled PMMA–bioglasshybrid networks†

Roya Ravarian,*a Hua Wei,a Aditya Rawal,b James Hook,b Wojciech Chrzanowskic

and Fariba Dehghani*a

Organic–inorganic hybrids were produced by chemical bonding of poly(methyl methacrylate) (PMMA) and

bioglass using the silane coupling agent (3-trimethoxysilyl propyl methacrylate (MPMA)). The effects of

MPMA : PMMA molar ratio and volume fraction of polymer solution on the characteristics of hybrids

were assessed. 1H, 13C and 29Si solid state NMR analyses were used to investigate the structural features

at molecular level to better understand the impact of coupling agent on the network structure of

hybrids. Tn peaks in 29Si CPMAS NMR spectra were only observed for hybrids and not for physical

mixtures, which confirmed the presence of Si–C covalent bonds and formation of class II hybrid.

Compared to physical mixture, the number of Q4 species also showed a 20% reduction for the highly

coupled hybrid containing 0.1 M ratio of MPMA : PMMA and 60 vol% polymer solution. These analyses

elucidate the integration of PMMA chains into bioglass structure in nano-scale. Additionally, presence of

peaks at 1030 cm�1 (Si–O–Si) and 1110 cm�1 (Si–O–C) in Fourier transform infrared (FTIR) spectra

confirmed the formation of covalent bonds in hybrid components. These results showed molecular

interaction between PMMA and silica and shed light on the enhancement of physical properties of

hybrids. For instance, thermal gravimetric analysis (TGA) of the hybrid showed significant enhancement

in thermal resistivity as compared to the neat PMMA. The composition of PMMA–bioglass hybrids was

tuned to provide fast gelation of ca. two hours yielding homogenous and transparent materials with

unique thermal stability, functionality and broad applications, particularly as bone implants.

1 Introduction

Organic–inorganic hybrids are amongst the most promisingmaterials since their physico-chemical properties can be readilytuned by judicious formulation. They provide an opportunity toexploit properties of both organic and inorganic frameworks aswell as manipulate their functional characteristics by varyingparameters such as composition, procedure, coupling and thedispersion of components in the matrix. The critical aspect isthat the uniform distribution provided by chemical couplingbetween organic and inorganic components in hybrids yieldsunique properties that are not achievable in conventionalcomposites or even nanocomposites. Hybrids as a new class ofmaterials have recently attracted considerable attention due totheir broad applications in versatile technologies such as optics,electronics, mechanics, membranes, protective coatings, catal-ysis, sensors and biomaterials.1–4

eering, The University of Sydney, Sydney,

ani@sydney.edu.au; Tel: +61 293514794

nwright Analytical Centre, NMR facility,

ney, Sydney, 2006, NSW, Australia

tion (ESI) available. See DOI:

Chemistry 2013

The molecular bonding between organic and inorganiccompounds in hybrids creates materials that exhibit synergis-tically enhanced properties compared to their physicalmixtures. The organic and inorganic components are mixed indomain sizes of nanometers and the molecular interactionsbetween the phases lead to the formation of class I and IIhybrids. Class I hybrids includes a mixture in which there is aweak interaction between organic and inorganic compoundssuch as van der Waals or hydrogen bonds forces, while class IIinvolves hybrids with covalent or iono-covalent bonds, whichimpact the nal material properties.1,4

Sol–gel synthesis is an attractive method for the productionof organic–inorganic hybrids due to the low processingtemperature. In particular, this technique allows the incorpo-ration of organic compounds into the inorganic structurewithout thermal degradation.5 Organic molecules are mixedwith metal alkoxides or/and organosilanes which undergohydrolysis and condensation reactions and trap and/or bond tothe organic components in the gel structure.2 As an example,the hydrolysis and condensation reactions of tetraethyl ortho-silicate (TEOS), a common precursor for the fabrication of silicanetwork, are shown in eqn (1)–(3) (ref. 6) and illustrate theformation of Si–O–Si network. The organosilanes couplingagents such as 3-(trimethoxysilyl)propyl methacrylate (MPMA)

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bond to both organic and inorganic molecular structures due tothe existence of appropriate moieties.7 Organic molecules suchas poly(methyl methacrylate) can be copolymerized with MPMAand are covalently bonded to the silica network structure by sol–gel method. The molecular structure of MPMA, functionalizedPMMA and the PMMA–silica class II hybrid are shown in Fig. 1.

Hydrolysis:

^Si(OR) + H2O 4 ^Si(OH) + ROH (1)

Condensation:

^Si(OR) + (OH)Si^ 4 ^Si–O–Si^ + ROH (2)

^Si(OH) + (OH)Si^ 4 ^Si–O–Si^ + H2O (3)

Poly(methyl methacrylate) (PMMA)–silica hybrids havebroad applications in optics,8 mechanics,8–10 electronics,10,11

bone cements12,13 and dentistry.12,14,15 PMMA is a thermoplasticpolymer,10 known as organic glass16 that shows favorablemechanical properties.9 This polymer is used for orthopaedicimplants; however, lack of bioactivity limits its applications.Bioglass, on the other hand, is considered as an inorganic glasswith brittle but hard characteristics that possesses bioactivity.8

Therefore, PMMA and bioglass combinations have the advan-tages of both compounds. For example, increasing theconcentration of silica by 20% resulted in the 20 �C enhance-ment of decomposition temperature of PMMA–silica hybridmaterial.10 Furthermore, embedding PMMA into the bioactivesilica structure improves the mechanical properties andreduces the brittleness to fabricate a favorable material forbiomedical applications such as bone implants.9,12

PMMA–silica hybrids that have been fabricated by sol–gelmethods are listed in Table 1. Chang et al.10 observed thatelevating the silica fraction in these hybrids enhanced the onsetthermal decomposition temperature. They, also, demonstrated

Fig. 1 The schematic molecular structure of (a) 3-trimethoxysilylpropyl methacrylabioglass network (class II hybrid).

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that the nature of silica had negligible effect on this property.10

Wei et al.16 found that MPMA and silica composition hadsignicant impacts on the thermal decomposition and hard-ness of hybrids. Increasing the amount of silica and elevatingthe ratio of MPMA in polymer enhanced the onset thermaldecomposition from 230 �C to 370 �C. Other studies assessedthe effect of silica content on the characteristics of these hybridsin macro level and measured their surface and mechanicalproperties.8,9,11,12,17,18 However, to the best of our knowledge, theeffect of composition of coupling agent (MPMA) on the char-acteristics of hybrid has not yet been thoroughly investigated atthe molecular scale, which is the focus of this study.

It is anticipated that the composition of hybrid material,absence or presence of coupling between organic and inorganiccomponents have a signicant impact on the molecular struc-ture, hence, macroscopic characteristics such as thermalbehavior. Therefore, the specic aim of this study was toinvestigate the effect of composition of functional groups on themolecular structure of PMMA–bioglass hybrid with a view toimprove their physical properties. Analytical methods such asFourier Transform Infrared (FTIR), 1H, 13C and 29Si solid stateNMR spectroscopy were used to study the microstructure ofhybrids at the molecular level. We assessed the nano-scaleinteraction between PMMA and bioglass using two dimensionalsolid state NMR analytical technique. The results of theseanalyses can be used to determine the effect of molecularinteraction on the physical properties and to optimize thecomposition of hybrid for preventing phase separation toobtain superior product. In this study, TEOS with four func-tional groups was selected as the silica source and PMMA–bioglass hybrid was fabricated by sol–gel method. The compo-sition of hybrid was optimized for chemical coupling betweenPMMA and silica to yield a molecular structure with uniformdistribution and nano-scale interaction between thecomponents.

te (MPMA), (b) copolymer of PMMA-co-MPMA, and (c) copolymer coupled to the

This journal is ª The Royal Society of Chemistry 2013

Table 1 The list of PMMA–silica hybrids synthesized, processing conditions, characterizations, and applications

Variables T (�C), period Macrostructure Analysis Application

Type of silica source(tetramethoxysilane,methyltrimethoxysilane,phenyltriethoxysilane)10

25, 24 h; 60, 3 h; 100, 3 h;150, 24 h; under vacuum

Film FTIR, 13C and 29Si NMR (CP/MAS), DSC, TGA

—

MPMA, drying condition8 80 or 90, 3 or 6 h Coated on PMMA andsilicon substrate

FTIR, nanoindentation,hardness, AFM

Electrics

TEOS/MMA ratio, water9 60–70, 4 days; 75–100, 3days;

Monoliths FTIR, thermal diffusivity,hardness, indentation

Electrics

MPMA, TEOS16 25, 2 months; 60, 6 days; 200,4 h

Monoliths TGA, density, hardness Dental

Content of silica18 25, 1 week; 25, 2 days (undervacuum)

Monoliths Cell-interaction behavior Bone cement

Silica17 25, 1 week; 30, 48 h (undervacuum)

SEM, FTIR, XRD, inductivelycoupled plasma atomicemission spectroscopy

Bone cement

Type of silica source(dimethyldiethoxysilane andTEOS)12

25, 1 week; 30, 48 h (undervacuum)

Monoliths SEM, FTIR, XRD, hardness Bone cement and dental

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2 Materials and methods2.1 Materials

Precursors required for synthesis of PMMA-co-MPMA copoly-mer including MPMA, azobisisobutyronitrile (AIBN) and N,N0-dimethylformamide (DMF) were purchased from Sigma-Aldrichand used as received. Methyl methacrylate (MMA) purchasedfrom Sigma was used aer distillation under reduced pressure.Hydrochloric acid (HCl; Merck), tetraethyl orthosilicate (TEOS;Sigma-Aldrich), calcium chloride dihydrate (CaCl2$2H2O (CC);Ajax Finechem Pty Ltd), tetrahydrofuran (THF; Merck) anddeionized water were used for the fabrication of inorganicsolution and hybrid.

2.2 Preparation of inorganic solution

TEOS was mixed with deionized water and HCl and stirred for30 minutes followed by addition of calcium chloride dihydrateand stirring for another 30 minutes to yield a homogenouslymixed solution. The precursors were used with the molar ratioof TEOS : water : HCl : CC ¼ 1 : 8 : 0.01 : 0.2 and the solutionwas referred to as sol(A).

2.3 Preparation of organic solution

Poly (methyl methacrylate)-co-3-(trimethoxysilyl)propyl meth-acrylate (PMMA-co-MPMA) copolymer was synthesized by freeradical polymerization technique using AIBN as an initiator.Precursors were mixed in a Schlenk ask (MMA : AIBN ¼ 200(mol ratio), DMF (20 mL)) and degassed by three freeze–pump–thaw cycles. Polymerization was conducted in oil bath at 70 �Cfor 12 hours. The solution was then cooled down at roomtemperature and polymer was puried by precipitation indiethyl ether followed by ltration and drying in vacuum for aperiod of 2 days. The yield in each run was �1 g (70%). PMMA-co-MPMA was dissolved in THF with a concentration of 10 wt%and the solution was labeled as sol(B).

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2.4 Hybrid formation

Different volume ratios of sol(A) and sol(B) were prepared andmixed by magnetic stirrer for one hour at room temperature toform a uniform mixture. The solutions were then kept sealed atambient temperature prior to gel formation followed by dryingunsealed for 7 days at the same conditions. The samples wereincubated at 37 �C for 7 days and nally the monoliths werevacuum dried for 2 days at 40 �C. This drying procedure wasconducted to preserve the monolith structure during solventevaporation and also to remove residues of solvents.

2.5 Characterization

2.5.1 1H and 13C nuclear magnetic resonance (NMR). Thesynthesized polymer was characterized by 1H and 13C NMR on aBruker Ultra Shield Avance (400 MHz) spectrometer usingCDCl3 as the solvents. The concentration of the samples was 20g L�1.

2.5.2 1H, 13C and 29Si solid state NMR. Prepared gel wascharacterized by solid state NMR analyses to determine thestructure of solid network. All the NMR experiments werecarried out on a Bruker Biospin Avance III solids-300 MHz andAvance II standard bore, 700 MHz spectrometers (Bruker-Bio-spin, Rheinstetten, Germany). A Bruker 4 mm double resonancemagic-angle spinning (MAS) probehead was used with magic-angle spinning (MAS) frequencies of 5 kHz for 1H–29Si HetCorand 12 kHz for other experiments. The 90� pulse length was 4 msfor 1H and 4.5 ms for 29Si and 13C nuclei.

A 300 s recycle delay was used in direct-polarization 29SiNMR experiments. The SPINAL-64 1H–29Si and 1H–13C hetero-nuclear decoupling strength was 80 kHz during 29Si and 13Cdetection. Hartman–Hahn cross-polarization (HHCP) wasemployed for experiments requiring polarization transfer from1H to 13C or 29Si with a contact times of 1 ms and 4 ms,respectively. 1H–29Si and 1H–13C HetCor experiments werecarried out with frequency-switched Lee-Goldburg (FSLG) for

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Fig. 2 400 MHz 1H NMR spectra of (a) PCont, (b) PLow, (c) PCont expansion in theregion of 3.5–4.1 ppm, and (d) PLow expansion in the region of 3.5–4.1 ppm inCDCl3.

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1H–1H homonuclear decoupling and 1H chemical shis werescaled by 0.571 accordingly during data processing. We alsoattempted to use 160 t1 increments of 51 ms. The glycine 13C COpeak at 176 ppm, the Kaoline 29Si (ref. 19) peak at �91 ppm andhydroxyapatite 1H peak at 0.2 ppm (ref. 20) were used to cali-brate the 13C, 29Si and 1H chemical shis, respectively. The neatbioglass, PMMA and hybrids were dried, powdered and ca. 60mg were packed into the NMR rotors for measurement.

2.5.3 Gel permeation chromatography (GPC). Synthesizedcopolymers were characterized for the weight average molecularweight (Mw), number average molecular weight (Mn) and poly-dispersity index (PDI) (Mw/Mn) by GPC with a Shimadzu Prom-inence HPLC (Kyoto, Japan) equipped with a GPC column (JordiGel DVB mixed bed, Alltech) diode array detector (SPD-M20A,Shimadzu) and refractive index detector (RID-10A, Shimadzu).The copolymers were dissolved at a concentration of 1 mg mL�1

in HPLC grade THF and 10 vol% DMF. The mobile phase was10 vol% DMF in THF and delivered at 1 mL min�1 through thecolumn that was kept constant at 50 �C. The GPC system wascalibrated with PEG standards and the relative molecularweight of polymers was determined by comparing the retentiontimes to that of standards using Waters Empower soware.

2.5.4 Attenuated total reection Fourier transform infrared(ATR-FTIR). The functional groups of samples were analyzed byATR-FTIR spectroscopy (FTIR; Nicolet 6700, Thermo FisherScientic Inc.). Synthesized polymers and hybrid samples werecharacterized for their molecular structures at the scan speed of32 scans per min.

2.5.5 Scanning electron microscopy (SEM). The surfacemicrostructures and phases formed on the specimens werescanned by eld emission scanning electron microscopy (FE-SEM; Zeiss ULTRA plus). Samples were mounted on aluminiumstubs using conductive carbon paint, then gold coated by usingEmitech K7550X instrument prior to SEM analysis.

2.5.6 Thermogravimetric analysis. Thermogravimetricanalysis (TGA, TA Q-500) was carried out to study the thermaldecomposition of samples. Analyses were conducted over thetemperature range of 30 �C to 600 �C at heating rate of 20 �Cmin�1 under nitrogen atmosphere.

2.5.7 Differential scanning calorimetric (DSC) measure-ment. The thermal property of polymers was analyzed by DSC(TA Q-1000, USA). The samples had an average weight of 4 mgand were heated from room temperature to 400 �C underconstant nitrogen ow (50 mL min�1). Samples were sealed inthe aluminum pans and heated at a rate of 10 �C min�1. Stan-dard aluminum sample pans were used as the reference.

Table 2 Prepared copolymers of PMMA-co-MPMAa

CodeMPMA : PMMA(mol ratio)

Molecular weight(Mn) (Da)

Polydindex

PCont 0 17 025 1.89PLow 0.004 19 136 1.87PMed 0.02 18 480 2.35PHigh 0.1 21 721 1.77

a Mw: weight average molecular weight; Mn: number average molecular w

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3 Result and discussion3.1 Synthesis of PMMA-co-MPMA

PMMA-co-MPMA copolymers were synthesized withMPMA : PMMAmolar ratios of 0, 0.004, 0.02 and 0.1 which werecoded as control polymer (PCont), polymers with low (PLow),medium (PMed) and high (PHigh) ratio of functional groups,respectively (Table 2). The suffix denes the ratio of function-alization of a copolymer that corresponds to the ability forcovalent bond formation with bioglass.

1H NMR analysis was used to characterize the molecularstructure of synthesized copolymers. Fig. 2(a and b) shows the1H NMR results of PCont and PLow in deuteriochloroform(CDCl3). The presence of peaks at 3.59 ppm (a, –OCH3), 1.014ppm (b, –CH2) and 1.621 (c, –CCH3) in 1H NMR spectrumconrmed the complete synthesis of PCont (PMMA). The occur-rence of peak at 3.92 ppm (d, –(COOCH2)) in Fig. 2(b)

ispersity(Mw/Mn)

Functional groups(%)

Functional groups(mol per 1 g copolymer)

0 00.2 19.84 � 10�6

0.4 39.4 � 10�6

0.8 77.73 � 10�6

eight.

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demonstrated the successful copolymerization of MPMA withMMA even at the lowest molar ratio of MPMA : PMMA ¼ 0.004(PLow). The absence and presence of peaks that are associatedwith MPMA in PCont and PLow were conrmed on NMR spectraand shown in Fig. 2(c and d). The 1H NMR spectra of PMed andPHigh showed the same characteristic peaks of MMA (3.59 ppm)and MPMA (3.92 ppm). The molecular weights of the synthe-sized copolymers, percentage of functional groups and numberof moles of these groups per 1 g of each copolymer werecalculated and listed in Table 2. 13C NMR spectrum of PMMA-co-MPMA copolymer synthesized in this study is provided inFig. S1 (ESI†).

Molecular structures of synthesized copolymers of PMMA-co-MPMA with different ratios of MPMA were conrmed by FTIRanalysis. As depicted in Fig. 3, bonds at 1722 cm�1 and 1452cm�1 were assigned to a C]O symmetric stretching and CH2

bending, respectively.10 The peaks at 2944 cm�1 and 2983 cm�1

were assigned to the stretching mode of a C–H bond.12,21,22 Thepeak observed at 1070 cm�1 was attributed to the Si–O–Cbending23 and C–O vibrations were detected at 1150 cm�1.22 Itwas observed that by increasing the MPMA : PMMA molar ratio

Fig. 3 FTIR results of (a) PCont, (b) PLow, (c) PMed and (d) PHigh.

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from PLow to PHigh, the Si–O–C peak at 1070 cm�1 was increased,which was in agreement with the 1H NMR results conrmingthe contribution from higher content of MPMA to the entirecopolymers structure.

3.2 Synthesis of organic–inorganic hybrids

Bioglass (A) with composition of TEOS : water : HCl : CC ¼1 : 8 : 0.01 : 0.2 and polymer (B) solutions in THF (10 wt%) weremixed in three different volumetric ratios of sol(B) : sol(A) ¼40 : 60, 60 : 40 and 80 : 20 that was followed by co-condensationof the silanol groups of organic and inorganic components. Theresultant solutions were coded as H40, H60 and H80 representingthe volumetric ratios of polymer to bioglass solutions. As anexample, HCont40 is the hybrid sample prepared from PContpolymer and sol(B) : sol(A) ¼ 40 : 60. These hybrids were visu-ally examined to determine the effect of composition of (poly-mer : bioglass solutions) and the fraction of MPMA on thetransparency, phase separation and gelation behavior ofsamples. Transparency is an indicative parameter for theabsence of phase separation below the scale of 400 nm.15

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Table 3 The effect of composition of hybrid samples on gelation time and optical property

Sample codeMPMA : PMMA(mol ratio) Sol(A) (vol%) Sol(B) (vol%) Phase formation Optical status Gelationa (days)

Bioglass — 100 0 OP T 5HCont80 0 20 80 OP NT NGHCont60 0 40 60 OP NT NGHCont40 0 60 40 TP NT NGHLow80 0.004 20 80 OP NT NGHLow60 0.004 40 60 OP NT NGHLow40 0.004 60 40 TP NT NGHMed80 0.02 20 80 OP T NGHMed60 0.02 40 60 OP NT NGHMed40 0.02 60 40 TP NT NGHHigh80 0.1 20 80 OP T 0.08 (¼ �2 h)HHigh60 0.1 40 60 OP T 0.2 (¼ �5 h)HHigh40 0.1 60 40 OP T 20

a Gelation time aer one hour mixing; OP: one phase formation; T: transparent; NT: not transparent; NG: no gel formation before 20 days; TP: twophase formation.

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Ratio of MPMA : PMMA played a critical role on gelation andformation of transparent products. The samples that had theirtransparency and gelation time examined are listed in Table 3.It was observed that by increasing the molar ratio of MPMA inthe copolymers structure, rmer and more transparent mono-liths were formed; however, at low MPMA : PMMA molar ratio(e.g. <0.1) samples were less transparent and gelation time wasgreater than 20 days at ambient temperature (Fig. 4(a and b)).

The composition of bioglass was another factor that had asignicant impact on phase separation, transparency andgelation time of hybrids. As shown in Fig. 4, one-phase solutionwas acquired for HCont60 and HHigh60, however, when using highcomposition of bioglass (e.g. HCont40 > 50 vol%), two separatephases were formed due to the agglomeration of polymer chains(Fig. 4c). The functionalisation of PMMA with MPMA hadsignicant impact on increasing the concentration of bioglassthat can be used for hybrid formation with no phase separation(e.g. polymer agglomeration). For example, at lowMPMA : PMMA molar ratio, the hydrophobic effect of PMMAinhibits covalent bonding and creation of class II hybrid withuniform distribution of the phases. Therefore, two separatezones of polymer and bioglass were formed for HCont40 samples.However, the same composition of bioglass could be used infabrication of class II hybrid (HHigh40) with no phase separationat high MPMA concentration (MPMA : PMMA ¼ 0.1). Further-more, it was observed that the gelation time of HHigh sampleswas tuned by varying the composition of bioglass. The gelationtime of hybrid material was decreased from 20 days to 2 h at

Fig. 4 Optical images of hybrid samples of (a) HCont60, (b) HHigh60 and (c) HCont40.

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room temperature when the composition of polymer solutionwas increased from 40 vol% to 80 vol%.

The microhardness identication number for hybrid samplecontaining highest MPMA molar ratio (HHigh80) was more than15 times higher than pure bioglass. This result shows thataddition of covalent bonding between organic–inorganiccompounds not only accelerates the gel formation, but alsodramatically enhances the mechanical properties of hybrids.

3.3 Molecular scale analyses13C CP-MAS NMR results of the PCont, PHigh and the HHigh60

samples are shown in Fig. 5. The spectra clearly demonstratethe 13C NMR peaks of PMMA scaffolding material. Broad yetwell resolved C]O, OCH3, CH2 and CH3 peaks of PMMA

Fig. 5 13C CP-MAS NMR spectra of (a) neat PMMA (PCont), (b) PMMA with highcoupling agent (PHigh) and (c) HHigh60 hybrid acquired with high power 1Hdecoupling. The spinning side bands are marked with and asterisk.

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backbone are visible. The broadening of 13C NMR peaks in thesolid state can be directly assigned to the effects of conforma-tional disorder within the amorphous PMMA polymer. Inaddition, in the case of functionalized PMMA in PHigh, the –O–CH2– peaks of coupling agent MPMA chains were also visible at67 ppm. We also observed the increased broadening of back-bone –CH2 signal in the HHigh60 hybrid material. It is hypothe-sized that this effect may be due to polymer chain interactionwith the relatively rigid bioglass components that underpinsbioglass–polymer hybrid formation interaction in nano-scalelevel.

The schematic illustration of molecular structure ofproposed hybrid is shown in Fig. 6(a). Q and T are parametersthat describe the hybrid network connectivity. Qn shows asilicon bonds to n other silicons via “bridging” oxygen atoms,and the T sites indicate a silicon atom bonds to one carbonatom (Si–C) and to other silicons.24 For example, T3 shows that asilicon atom bonds to three silicon and one carbon atom. Threemain peaks are observed at �110, �100 and �90 ppm in thequalitative 29Si CPMAS NMR in Fig. 6(b), that correspond to theQ4, Q3 and Q2 species, respectively. Tn sites are only visible inthe HHigh60 and HHigh80 hybrids at �50, �60 and �65 ppm. Thepresence of these peaks conrmed the formation of Si–C bonds,hence demonstrated the contribution of polymer into the bio-glass network structure. Therefore, it can be concluded that atleast a molar ratio of MPMA : PMMA ¼ 0.1 is required forfabrication of PMMA–bioglass class II hybrid. It is, therefore,anticipated that in HHigh materials, there is a high degree ofinteraction at molecular level between PMMA and bioglass (e.g.nano-scale interaction).

Directly polarized, single-pulse 29Si NMR spectra with high-power 1H decoupling (HPDMAS) are shown in Fig. 7. Theresulting 29Si peaks were deconvoluted to quantify the bioglassstructure in the composites using the DMt soware.25 Incomparison to the CPMAS spectra (Fig. 6(b)), the 29Si single-pulse spectra exhibited a higher intensity of Q4 sites, which was

Fig. 6 (a) The simplified schematic structure of hybrid in the absence of Ca2+, (b)

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due to the nature of two experiments. In CPMAS, only those 29Sispecies at close proximity of ca. 0.5 nm from 1H sites were cross-polarized and yielded a NMR signal. Consequently, the mostcondensed Q4 silicon sites, which are the furthest away from 1Hspecies, are under-represented in the CPMAS spectrum.However in the single-pulse experiment, 29Si NMR signal wasnot dependent on proximity to 1H species, therefore, yielded thefull quantitative signal of all the 29Si species.

The deconvolution of spectra in Fig. 7(a) yielded the relativepopulation of different silicon Q sites in the materials, whichare tabulated in Fig. 7(b). Q4 sites are indicative of the degree ofcondensation of bioglass within the hybrids, therefore, themicro-architecture of inorganic phase in these materials. As canbe seen from Fig. 7(b), in the neat bioglass and hybrids in theabsence of coupling agents (bioglass and HCont60), the relativeamount of Q4 species was ca. 15–22% higher as compared tohybrids with high coupling agent (HHigh60 and HHigh80), whilehybrids with low and medium coupling agents (HLow60 andHMed60) possessed intermediate concentrations of Q4 species.These data demonstrate that the coupling agent has a directimpact on the condensation reaction of silicate framework. Thelower populations of Q4 species in the HHigh60 and HHigh80

hybrid materials demonstrate smaller domain sizes of bioglassphase and potentially a more open network structure ascompared to the HLow, HCont and neat bioglass samples. Wehypothesize that while the coupling agent acts as a site for thenucleation of bioglass condensation, the presence of PMMApolymer chains serves to hinder the formation of very largedomains of bioglass. This mechanism is anticipated to yield astructure with closely interacting polymer and bioglass phaseson the nanometer scale.

We measured 2D 1H–13C and 1H–29Si HetCor spectra ofHHigh60 material to conclusively identify the nano-scale inter-action between the polymer and bioglass phases. In these 2DNMR spectra, the 1H chemical shis correlating to 13C/29Sichemical shis were spread in a 2D map, with the intensities

29Si CPMAS NMR spectra of hybrid samples.

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Fig. 7 (a) Directly polarized 29Si HPDMAS NMR of PMMA–bioglass hybrids. The acquired spectra are shown in bold while deconvoluted peaks fits are shown as thinlines, (b) Relative populations for the different Q sites in the hybrid materials.

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plotted as contour levels as shown in Fig. 8. The 2D 1H–13CHetCor was acquired with a short contact time of 100 ms ofcross-polarization transfer from the 1H to the 13C nuclei. As aresult this spectrum primarily shows the localized correlationsof 13C species to their directly bonded 1H species. In the caseof 2D 1H–29Si HetCor, the spectrum was acquired with arelatively long contact time of 4 ms of cross polarizationtransfer. This means that in addition to the two-bond Si–O–Hcorrelation, this spectrum also shows correlations to protonswithin ca. 1 nm proximity. As expected from the nature ofbioglass, we observed strong 29Si correlations to 1H species atca. 0.5 ppm and 4.5 ppm. These signals were assigned toisolated silanol OH species and weakly adsorbed H2O moie-ties.26 Additionally, a strong correlation of 1H species at ca. 2.5ppm corresponding to the methylene protons of PMMA withthe bioglass 29Si was observed. Considering the fact that thehybrid material was not isotopically labelled and that NMR isan inherently insensitive technique, the observation ofmolecular correlation between the PMMA and the bioglass byNMR implies close molecular contact between a signicantfraction of the PMMA molecules and the bioglass.27 Thisconrms the formation of a nano-scale composite in thehybrid material.

Fig. 8 2D 1H–13C and 1H–29Si HetCor HHigh60. The 1D 1H projections are plotted to

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FTIR was also used to investigate the molecular structure ofthe prepared hybrids. The FTIR spectra of neat bioglass, PMMA(PCont), bioglass–PMMA hybrid (HHigh60) and physical mixture(HCont60) are shown in Fig. 9. The peaks at 1030–1040 cm�1, 930cm�1 and 790 cm�1 corresponded to Si–O–Si network structureand were detected only for bioglass and HHigh60. These peakswere not observed in HCont60 due to the lack of gel formation inthis sample.28,29 The peaks at 1625 cm�1 and 3370 cm�1 werecorresponded to the presence of CO3

2� and OH groups suppliedfrom bioglass composition, respectively.7,30

The absence of C]O peak at 1722 cm�1 and the presence ofsmall Si–O–C peak at 1110 cm�1 in the FTIR spectra of HCont60

conrmed the formation of weak van der Waals force betweenthe oxygen in PMMA (C]O) and silicon ion owing to highelectropositivity of Si4+ ions. This intermolecular interactionresulted in the formation of class I hybrid of PMMA and bio-glass by sol–gel method. In this method, components weremixed in the form of liquid, hence, the steric effect wasdecreased and the likelihood of inducing van der Waals forcesbetween molecules was increased. On the other hand, thepresence of C]O in the HHigh60 hybrids showed that the siliconions were contributed in Si–O–Si covalent bonds.31 The FTIRresults showed that the amount of MPMA coupling agent played

the left of the 2D spectra while the 13C and 29Si projections are plotted at the top.

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Fig. 9 FTIR spectra for (a) HCont60, (b) HHigh60, (c) PCont, and (d) bioglass.

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a critical role on the molecular interactions of compounds fromweak to strong bonding that may result in the production ofmaterials with unique physical properties such as morpholog-ical features and thermal stability.

3.4 Morphology

The surface morphology of HCont60 and HHigh60 hybrids wasinvestigated by scanning electron microscope (SEM). As shownin Fig. 10, HCont60 morphology was inhomogeneous and thephase separation between the bioglass and polymer wasvisible on the micron scale. However, in the chemicallycoupled hybrid (HHigh60) a homogenous and smooth surface

Fig. 10 SEM images of (a) HCont60 and (b) HHigh60.

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was detected without any indication of large scale phase sepa-ration. These images conrmed that phase separation wasavoided by formation of covalent bonding between PMMA andbioglass.

3.5 Thermal behavior

Thermal behavior of synthesized material is an importantcharacteristic from the aspect of their application especiallywhen high temperature process is required for example insterilization processes for bioimplants. It was anticipated thatnano-scale interaction between bioglass and polymer wouldsignicantly impact the characteristics of hybrid material. Asshown in Fig. 11(a), the TGA prole of neat PMMA-co-MPMAcopolymer showed three distinct peaks at 165 �C, 270 �C and360 �C. These peaks are attributed to the presence of head-to-head linkages, end-chain unsaturation and random scissionwithin the polymer chain, respectively.32,33 The polymer wasdegraded completely by 400 �C due to the breakdown of itsbackbone structure.

The TGA prole of bioglass depicted in Fig. 11(b) shows asingle peak at 100 �C, which is due to the loss of water. A steadystate prole was observed for bioglass up to 600 �C because ofthermal stability of silica structure. Wei et al.,16 also, reportedthe same trend for the TGA prole of bioglass derived fromTEOS by sol–gel reaction.

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Fig. 11 TGA (left) and DTG (right) curves of (a) PMed, (b) bioglass, (c) HCont60, (d) HMed60, (e) HHigh60.

Table 4 Characteristic peaks of the thermal decomposition of samples and theirSiO2 content

PMed Bioglass HCont60 HMed60 HHigh60

SiO2 wt% 0 80 57 46 461st peak (�C) 165 — 216 218 —2nd peak (�C) 270 — 284 291 249OTDT (�C) 360 — 381 390 400

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The onset thermal decomposition temperature (OTDT) ofHCont60 was increased compared to pure polymer due to theaddition of silica, as listed in Table 4. Formation of covalentbonding between organic and inorganic components had thesame impact on thermal characteristics of HMed60 and HHigh60

hybrids. The degradation peaks were at 360 �C, 381 �C, 390 �C,and 400 �C for PMed, HCont60, HMed60, HHigh60, respectively. Thusthe OTDT of these samples was tuned to higher temperatures byincreasing the MPMA composition which greatly impacted themolecular structure and level of chemical interaction. The levelof covalent bonding possessed a signicant impact on thermalbehavior of products as shown in the deconvoluted thermalgravimetry (DTG) curve of the HHigh60 (Fig. 11). HCont60 andHMed60 possessed three major distinct weight losses in TGA andDTG curves, which were in agreement with the degradationprole of synthesized polymer; however, for HHigh60 sample,weight losses were detected only at two distinct temperatures(249 �C and 400 �C). This phenomenon occurred due to thepresence of strong covalent bonds between PMMA chains andsilica structure that inhibited thermal degradation within therange of 200 �C to 300 �C. OTDT of HHigh60 was also higher thanother hybrid products with lower levels of coupling such asHMed60 and HLow60.

The thermal stability of optimum hybrid sample (HHigh60)was paramount compared with data reported in previousstudies for PMMA–bioglass hybrids.15,34–38 The effect of compo-sition of MPMA and silica content on thermal degradationproperties of PMMA–silica hybrids were studied by Wei et al.16

They observed similar impact of silica content and MPMAcomposition on thermal stability. However, the maximum

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thermal stability achieved in their study was 340 �C thatappeared for the hybrid with similar MPMA : PMMAmolar ratioand silica content used in our study.16 The lower OTDT in theirstudy might be attributed to fewer covalent bonds betweenPMMA and silica.

Degradation prole of the hybrid was also inuenced bypresence of covalent bonding. A similar trend was observed forthe degradation prole of HCont60 (Fig. 11(c)) and polymer (PMed)(Fig. 11(a)) at temperatures below OTDT of the polymer(i.e. 360 �C). In this range of temperature, the degradation ofHCont60 was greater than HHigh60; however, the amount ofsilica maintained at 600 �C for HCont60 was higher than HHigh60.These results demonstrate that the polymer and silica inphysical mixture (HCont60) were degraded separately;however, in class II hybrid (HHigh60) degradation was uniformdue to covalent bonding between PMMA and silica. Theseresults were in agreement with the data obtained by differentialscanning calorimetric analysis (DSC) which is presented inFig. S2 (ESI†).

4 Conclusions

The structure of PMMA-co-MPMA–bioglass hybrids was studiedat the molecular level using various techniques such as 13C, 29Siand 1H NMR (1D and 2D), FTIR, TGA and DSC analyses. Theresults of these analyses corroborated the covalent bondingbetween these organic and inorganic compounds as a result ofMPMA. A more open network was obtained by increasing themolar ratio of MPMA into the structure. The compositions ofMPMA and the ratio of polymer to bioglass solutions weremodied to decrease the gelation time from days to only twohours at room temperature in the absence of any initiator. Atransparent and rm composite structure was produced withuniform distribution of the compounds at optimum composi-tion when using high amount of MPMA and less than 40%bioglass. In addition, the thermal stability of hybrid at optimumcomposition was dramatically higher than the value reported inprevious studies underlining the superior physical characteris-tics of material produced in this study. This material is expectedto have broad applications, such as gate dielectric, optics, and

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coating as well as biomedical applications including dental andbone replacement.

Acknowledgements

Authors acknowledge the nancial support from AustralianResearch Council. The rst author is also grateful for thetechnical support of Ms Elizabeth Dobrinsky, her colleagues MrAli Fathi and Mr Ali Negahi Shirazi.

Notes and references

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