EUROPEAN
European Polymer Journal 43 (2007) 4103–4113
www.elsevier.com/locate/europolj
POLYMERJOURNAL
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Macromolecular Nanotechnology
New organic–inorganic nanohybrids via ring openingpolymerization of (di)lactones initiated
by functionalized polyhedral oligomeric silsesquioxane
Anne-Lise Goffin a, Emmanuel Duquesne b, Sebastien Moins a,Michael Alexandre b, Philippe Dubois a,b,*
a Laboratory of Polymeric and Composite Materials, University of Mons-Hainaut, Place de Parc 20, 7000 Mons, Belgiumb Materia Nova Asbl, Parc Initialis, Avenue Copernic 1, 7000 Mons, Belgium
Received 4 June 2007; received in revised form 21 July 2007; accepted 22 July 2007Available online 10 August 2007
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Abstract
The synthesis of new organic–inorganic materials were investigated. Poly(e-caprolactone) and poly(L, L-lactide) cova-lently end-capped by a polyhedral oligomeric silsesquioxane (POSS) moiety, leading to new nanohybrid materials, weresuccessfully prepared by coordination–insertion ring opening polymerization (ROP) of e-caprolactone (e-CL) and L,L-lac-tide (L,L-LA) respectively. The reaction was initiated from the primary amine available on aminopropylheptakis(isobu-tyl)POSS nanoparticles and catalyzed by tin(II) 2-ethylhexanoate (tin octoate, Sn(oct)2). The covalent grafting of thepolyester chains onto the POSS ‘‘nanocages’’ was assessed by selective fractionation experiment, Fourier transform infra-red spectroscopy (FTIR) and further confirmed by proton nuclear magnetic resonance (1H NMR). The polymerizationproved to be well controlled as attested by the quite narrow polydispersity indices as determined by size exclusion chro-matography (SEC). Furthermore, well-defined semi-crystalline poly(e-caprolactone)-b-polylactide block copolymer carry-ing such POSS nanoparticle at one end was successfully synthesized attesting for the very efficient control over thepolymerization reaction.� 2007 Elsevier Ltd. All rights reserved.
Keywords: Nanohybrids; Nanocomposites; POSS; Polyesters; Ring opening polymerization
0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved
doi:10.1016/j.eurpolymj.2007.07.041
* Corresponding author. Address: Laboratory of Polymericand Composite Materials, University of Mons-Hainaut, Place deParc 20, 7000 Mons, Belgium. Tel.: +32 65373483; fax: +3265373484.
E-mail address: [email protected] (P. Dubois).
1. Introduction
Since the emergence of nanotechnologies, astrong demand exists to continuously improve theproperties of the so-called nanomaterials. Thus,the introduction of nano-sized inorganic reinforce-ments has appeared to be a very efficient way tomodify polymer-based materials. By means of agood choice of the blended partners and an efficient
.
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compounding process, this technique can lead to theproduction of new materials with improved proper-ties. Indeed, it is now well known that the introduc-tion of rigid filler into thermoplastic or thermosetmatrices may lead to some improvement of theirmechanical, thermal, barrier, electrical or fire resis-tance properties [1–4], depending on the nature ofthe chosen filler. Compared to micrometric size fill-ers for which a high amount of filler content isrequired for a real improvement of the compositeproperties, only a few percents of nanofillers (1–5 wt%) are usually sufficient to obtain a nanocom-posite showing the same behaviors, if not better [5].
Among all the nanofillers, layered silicates havebeen indubitably the most reported in the literature[6–8]. Even if they have drawn less attention thanlayered silicates, polyhedral oligomeric silsesquiox-ane (POSS) have been more and more studied inthe past decade [9,10]. Indeed, they have proved toinfluence the thermal, mechanical [5] and particu-larly fire resistance [2,3,11] properties of polymermaterials. Typically POSS nanoparticle is a 3D cagelike siloxane structure surrounded by eight organicR groups (RSiO1.5). These organic groups can behydrocarbon species, which allow compatibilizationand solubility of the particles into polymer matrices,or they can embody a large range of functionalgroups suitable for the preparation of nanohybridsby copolymerization or grafting [12]. The struc-ture–property relationship of polymer incorporatingPOSS has been recently reviewed [9,13] and interest-ingly, several recent publications reported on theuse of these new nanomaterials in many differentapplications [5,14–21] including the potential ofPOSS-based nanocomposites in the biomedical field[22,23]. However, as depicted in the literature, inmost of the cases, such nanofillers tend to aggregatewhen they are simply melt blended within polymermatrices, mainly due to the existence of numerousnanofiller–nanofiller interactions [24,25]. In orderto improve the dispersion, the nanofiller and thematrix need to be compatibilized. For instance,the nanofiller can be organo-modified in order topromote more favorable nanofiller–polymer interac-tions, leading to a reduction of the interfacialenergy. The creation of a strong chemical bondbetween the filler and the polymer matrix allowsfor a complementary enhancement of the materialproperties compared to the simple blends. Indeed,the presence of a covalent bond can improve theinterfacial adhesion between the organic and theinorganic part of the composition, leading conse-
quently to a material with better mechanical proper-ties for instance, as nicely reviewed by Pittman in2001 [26].
The synthesis of POSS-based nanohybrids, i.e.,polymer grafted POSS nanoparticles has drawn alot of attention over the last decade. For instance,Mather and et al. reported the preparation ofamphiphilic telechelic incorporating POSS copoly-mers. Indeed, a, x diol end-capped PEG homopoly-mers of various molecular weights were reacted withmonoisocyanate functionalized POSS nanoparticles[27]. They evidenced a strong modification of thethermal and morphological properties of the amphi-philic telechelics by controlling the hydrophilic–hydrophobic balance. Later, the group of Coughlinhas used the same monoisocyanate–POSS to synthe-size hemi-telechelic polystyrene–POSS copolymer[28]. Authors have developed a synthetic protocolthat can act as an experimental model to probethe ordering or aggregation behavior of inorganicnanoparticles within polymeric matrices.
In this contribution, we report on the synthesis ofwell-defined POSS–poly(e-caprolactone) (POSS–PCL) and POSS–poly(L,L-lactide) (POSS–PLA)nanohybrids prepared by ring opening polymeriza-tion (ROP) of e-caprolactone (CL) and L,L-lactide(LA) catalyzed by tin(II) 2-ethylhexanoate (tin octo-ate, Sn(oct)2). Purposely, the ROP is initiated by theprimary amine group made available onto anaminopropylheptakis(isobutyl)POSS (POSS-NH2).PCL and PLA have been chosen as the organiccounterpart of these novel nanohybrids owing totheir biodegradability and biocompatibility, whichcan be consequently useful as environmentallyfriendly material or for biomedical applications.Moreover, PCL is well known to be miscible or atleast mechanically compatible with a large varietyof polymers, e.g., SAN, PVC, chlorinated polyethyl-ene. . . [29]. Therefore, the as-synthesized POSS–PCL nanohybrids could then be used as surface-compatibilized POSS for further dispersion in poly-meric matrices known to be miscible (or simplymechanically compatible) with the PCL segment.It is worth pointing out that very recently Zhenget al. have interestingly reported about the synthesisof octaarmed star PCL with a POSS core. Accord-ingly a octa(3-hydroxypropyl)POSS was consideredas an octafunctional initiator for promoting the CLring opening polymerization [32]. Even morerecently (while this contribution was submitted forpublication), Ni and Zheng extrapolated the CLpolymerization using 3-hydroxypropylheptaphenyl
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POSS as a monofunctional intiator and allowing forsupramolecular inclusion complexes between theresulting POSS-b-PCL and alpha-cyclodextrin [33].
2. Experimental section
2.1. Materials
e-Caprolactone (CL) (99%, Acros) was dried 48 hover calcium hydride and distilled under reducedpressure. Toluene (p.a., Labscan) was dried overcalcium hydride and distilled. L,L-Lactide (LA)(99%, Galactic) was recristallized in toluene at0 �C and dried under vacuum at ambient tempera-ture. Tin(II) ethylhexanoate (Sn(Oct)2) (95%,Aldrich) was used as received without further puri-fication. Aminopropylheptakis(isobutyl)POSS(POSS-NH2, 97%) was purchased from HybridPlastics. Note that the purity (i.e., amino functional-ization) of the received sample was actually esti-mated to 83% by multinuclear NMR, theimpurities proved to be ‘‘open’’ POSS deprived ofamino-functionalization. Since such impurities didnot participate in the polymerization reaction (andthe resulting polyester grafting) and furthermorethey were readily eliminated at the end of the poly-merization by simple selective precipitation step (seepolymerization procedure hereafter), the POSS-NH2 sample was used as received.
2.2. Polymerization procedure
Typically, 0.2 g (nNH2= 2.26 · 10�4 mol) of
aminopropylheptakis(isobutyl)POSS (POSS-NH2)was introduced under nitrogen flow in a two-neckflask equipped with a three-way stopcock and amagnetic stirring bar. Then, toluene (7 mL),Sn(Oct)2 (0.2 mL of a 0.09 M toluene solution)and e-caprolactone (VCL = 2 mL; mCL = 2.06 g;nCL = 0.018 mol) were added successively undernitrogen using previously flame-dried syringes. Themixture was then heated at 100 �C under stirring.After the desired reaction time, the polymerizationwas stopped by adding of a few drops of dilutedhydrochloric acid (1 M). The POSS end-cappedPCL was recovered from heptane precipitation, fil-tered and dried until constant weight at 40 �C undervacuum. As far as L,L-lactide (LA) (co)polymeriza-tion was concerned, a determined amount of puri-fied LA (previously dissolved in hot dry toluene)was added to the reaction medium (after CL poly-merization in the case of POSS-P(CL-b-LA) synthe-
sis) and the (co)polymerization allowed to proceedat 100 �C (80 �C for POSS-b-PLA) before additionof a few drops of diluted hydrochloric acid(0.1 M) and further precipitation, filtration and dry-ing as reported above.
2.3. Characterization
Proton and carbon nuclear magnetic resonance(1H and 13C NMR) spectra were recorded on solu-tions prepared in CDCl3 using a Bruker AMX-300apparatus at a frequency of 300 MHz. Fouriertransform infrared (FTIR) spectra were recordedusing a BIO-RAD Excalibur spectrometer equippedwith an ATR Harrick Split PeaTM. Size exclusionchromatography (SEC) was performed in THF(sample concentration: 1 wt%) at 35 �C using apolymer laboratories (PL) liquid chromatographequipped with a PL-DG802 degazer, an isocraticHPLC pump LC1120 (flow rate: 1 mL/min), aBasic-Marathon Autosampler, a PL-RI refractiveindex detector and four columns: a guard columnPLgel 10 lm (50 · 7.5 mm) and three columnsPLgel mixed-B 10 lm (300 · 7.5 mm). Molecularweight and molecular weight distribution were cal-culated by reference to a calibration curve built upby using polystyrene standards. The thermal behav-ior of the copolymer was analyzed by differentialscanning calorimetry (DSC) using a DSC Q100from TA Instrument at a heating and cooling ratesof 10 K/min under nitrogen flow.
3. Results and discussion
3.1. Synthesis of POSS–PCL nanohybrids
In a first series of experiments, the polymeriza-tion/grafting of polyesters chains has been per-formed using commercially available POSS-NH2
nanofillers (from Hybrid Plastics) in toluene in a rel-atively diluted medium ([CL]0 = 2 M) at high tem-perature (T = 100 �C). The monomer conversionhas been recorded in function of time and themolecular parameters of the as-synthesized POSSend-capped PCL chains characterized by 1H NMRand SEC. The experimental conditions and resultsare reported in Table 1. The mechanism of theROP of cyclic esters initiated by tin octoate and pri-mary amines has been deeply studied and recentlyreported in the literature [30]. Accordingly, thepolymerization reaction proved to take placethrough a coordination–insertion mechanism
Table 1Characterization of the POSS–PCL and POSS–PLA nanohybrids as synthesized by ROP of CL and L,L-LA, respectively
Entry Monomer Time (h) Conv.a (%) DPthb DPNMR
c Mn SECd (g/mol) Mw/Mn
d
1 CL 2 24 23 40 6500 1.222 CL 3 42 40 50 9400 1.173 CL 4 86 83 85 14,300 1.274 CL 5 81 78 85 14,800 1.315 CL 6 89 85 75 13,700 1.336 CL 7 99 95 100 18,600 1.38
7 LA 1 53 48 42 5000 1.078 LA 2 71 64 57 6700 1.089 LA 3 79 71 66 7900 1.0810 LA 4 86 78 72 8200 1.0911 LA 5 90 82 75 8300 1.09
([CL]0[NH2]0 = 95; [NH2]0/[Sn(oct)2]0 = 2.5; [CL]0 = 2 M; T = 100 �C) ([LA]0/[NH2]0 = 90; [NH2]0/[Sn(oct)2]0 = 2.5; [LA]0 = 1 M;T = 80 �C).
a As measured by gravimetry.b DPth = ([Monomer]0/[NH2]0) · conv.c As measured by 1H NMR in CDCl3; DP(PCL)NMR = [(Id/2)/(Ih,f+g/58)] (cf. Fig. 1). DP(PLA)NMR = [(Ia)/(Ih,f+g/58)] (cf. Fig. 5).d As measured by SEC (relative to polystyrene standards).
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involving the O-acyl cleavage of the cyclicmonomer.
After one hour of polymerization time, no poly-mer is recovered from tentative precipitation in hep-tane. This observation can be explained by thesolubility of POSS-NH2 in this solvent. Indeed,the POSS grafted with short polyester oligomer
Fig. 1. 1H NMR spectrum of the POSS-NH2 and a PO
are soluble enough to lead to a partial fractionationof the sample. After 2 h, the grafted PCL chain getssufficiently long to promote the POSS–PCL nano-hybrid precipitation allowing the monomer conver-sion measurement. A typical 1H NMR spectrum isdepicted in Fig. 1. This spectrum attests for thecovalent grafting of the PCL chains onto the POSS
SS–PCL nanohybrid in CDCl3 (entry 2, Table 1).
A.-L. Goffin et al. / European Polymer Journal 43 (2007) 4103–4113 4107
cages by the quantitative shift of the signal –CH2–
NH2 from 2.6 ppm (signal i in Fig. 1) to a new signal–CH2–NH–C(O)– at 3.2 ppm (signal i 0 in Fig. 1).Moreover, one can note the presence of a signalcharacteristic of an amide proton around 5.5 ppm.These observations have been confirmed by FTIRanalysis. Indeed, the FTIR spectrum in Fig. 2. evi-dences the presence of both POSS and PCL by theappearance of the siloxane and carbonyl character-istic absorption bands at 1099 and 1720 cm�1,respectively. Moreover, the presence of a weak bandaround 1648 cm�1 characteristic of an amide link-
2200270032003700
Wavenumber (cm
2940 cm-1 : -CH
CH2 NH
1648
Fig. 2. FTIR spectrum of a POSS–PC
15 15.5 16 16.5 17elution v
Fig. 3. Size exclusion chromatograms of POSS–PCL n
age is again in favor for the covalent grafting ofthe polyester chains.
In Table 1, one can note a gradual increase of themonomer conversion in function of reaction time.This evolution goes with a continuous growth ofthe grafted polyester chains as attested by the poly-merization degree (DP) measured by 1H NMR.Interestingly enough, this chain growth is clearlyevidenced by SEC (Fig. 3). Fig. 3 displays the evo-lution of the SEC signal towards lower elution vol-umes, i.e., higher molecular weights, with increasingmonomer conversion. Moreover, a good control
70
75
80
85
90
95
100
70012001700
-1)%
tra
nsm
itta
nce
1720 cm-1: -C=O
1099 cm-1 : -Si-O
CO
cm-1 :
L nanohybrid (entry 2, Table 1).
17.5 18 18.5 19 19.5olume
1h
2h
3h
4h
5h
6h
7h
POSS
anohybrids (effect of reaction time, see Table 1).
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over the polymerization reaction can be observed asattested by the rather narrow molecular weight dis-tributions (Mw/Mn = 1.2�1.4), at least for such aparticular inorganic initiator.
3.2. Synthesis of POSS–PLA nanohybrids
Concerning the polymerization of L,L-LA, itcomes out from Table 1 that the reaction kineticsis enhanced compared to CL. Indeed, even in amore diluted medium than for ROP of e-caprolac-tone ([LA]0 = 1 M) as well as and a lower tempera-ture (80 �C against 100 �C for CL), the monomerconversion reaches ca. 50% after only one hour ofreaction. However, in spite of this dramatic increaseof the polymerization rate, the control over themolecular parameters of the synthesized POSS–PLA copolymer remains effective. Indeed an excel-lent control is observed all along the polymerizationreaction as attested by the experimental DP’s (mea-sured by 1H NMR) close to the theoretical values aswell as the remarkably narrow polydispersity indi-ces (Mw/Mn < 1.1). This is clearly evidenced in theSEC chromatograms where the total shift of the sig-nal towards higher molecular weight (without anyremaining traces of homopolymer) can be observedwhile maintaining very narrow molecular weightdistribution (Fig. 4). The composition of the copoly-mers has been attested by 1H NMR and is depicted
13 14 15 16elution vo
Fig. 4. Size exclusion chromatograms of POSS–PLA n
in Fig. 4. The characteristic signals of the PLA seg-ments can be assigned at 1.6 and 5.2 ppm for theprotons –C(O)– CH(CH3)– O– and –C(O)–CH(CH3)– O–, respectively. Moreover, one cannote the presence of the signal CH2–NH–C(O)– at3.3 ppm (signal i 0 in Fig. 5) as well as the character-istic signal of an amide proton around 5.5 ppm.Once again, these observations have been confirmedby FTIR (Fig. 6). The band characteristic of thesiloxane bonds appears around 1095 cm�1 and thecarbonyl band around 1720 cm�1. Finally, the weakband around 1680 cm�1 can be assessed to an amidelinkage attesting the covalent grafting of the poly-lactide chains.
3.3. Synthesis of POSS-P(CL-b-LA) block
copolymer
The POSS-NH2 proved to be very efficient asinitiator for the ROP of e-CL as well as L,L-LA cat-alyzed by tin octoate. Accordingly, a novel POSS-end functionalized diblock copolyester has beenstudied by the introduction of l,L-lactide in the poly-merization medium right after POSS–PCL synthe-sis. That leads not only to the synthesis of POSS-P(CL-b-LA) block copolymer but gives some moreevidence for the effective controlled character ofthe ROP of lactones as initiated from POSS-NH2.In order to prepare such diblock copolyesters by
17 18 19lume (ml)
1h2h3h4h5hPOSS
anohybrids (effect of reaction time, see Table 1).
Fig. 5. 1H NMR spectrum of POSS–PLA nanohybrid in CDCl3 (entry 7, Table 1).
Fig. 6. FTIR spectrum of a POSS–PLA nanohybrid (entry 7, Table 1).
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sequential polymerization, CL was first polymerizedfor 7 h to ensure almost complete conversion. Analiquot was picked out for characterization just
before adding L,L-lactide in the reaction medium.The synthesis strategy is depicted in Scheme 1. After30 h of reaction time, the copolymer was recovered
O
O
POSS NH2 POSS NH C CH2 CH2 CH2 CH2 CH2 O
O
n Sn oct
POSS NH C CH2 CH2 CH2 CH2 CH2 O
O
n C CH
CH3
OO
H2
OO
O
OCH3
CH3
1. Sn(oct)2
tolueneT=100°C
n+t = 7h
2. mtoluene
T=100°C
3. H3O+
t = 23h
LA
m
Scheme 1. Synthesis of POSS-P(CL-b-LA) block copolymer.
14 15 16 17 18elution volume
POSS-PCL NanohybridMw/Mn = 1.2 Mn = 14.000
POSS-P(CL-b-LA) CopolymerMw/Mn = 1.5 Mn = 38.000
Fig. 7. SEC traces of POSS–PCL nanohybrid and POSS-P(CL-b-LA) block copolymer (see Table 2).
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by precipitation from heptane to remove unreactedCL, in any, and non-functionalized POSS. Then, thecopolymer was dissolved in THF and precipitatedfrom methanol at 0 �C to remove residual lactide(soluble in methanol while the copolyester is insolu-ble). The lactide conversion of 86% has been deter-mined by gravimetry. The as-synthesized copolymerhas then been characterized by SEC and NMR.Table 2 shows the experimental conditions andmolecular parameters of the recovered POSS-P(CL-b-LA) block copolyester. The controlledbehavior of the copolymerization reaction is clearlyattested by SEC. Fig. 7 gives evidence for the com-plete shift of the POSS–PCL nanohybrid signalthrough higher molar masses after lactide copoly-merization, and the appearance of a monomodalsignal corresponding to the copolymer. No termina-tion side-reaction can be detected since there is notrace of POSS–PCL species in the chromatogram.1H and 13C NMR analyses were performed on therecovered polymeric chains in order to determinethe copolymer structure. Fig. 8 presents the 1HNMR spectrum of the POSS–PCL nanohybrid
Table 2Characterization of the POSS-P(CL-b-LA) copolymer as synthesized b
Conv. (%) DPthc DP
CL a LA b CL LA CL
�100 86 96 258 94
[CL]0/[NH2]0 = 96; [LA]0/[NH2]0 = 300); [NH2]0/[Sn(oct)2]0 = 2.5; T =a Estimated based on previous experiments (see Table 1).b Determined by gravimetry.c DPth monomer = ([M]0/[NH2]0) · conv. where M = CL or LA.d As measured by 1H NMR in CDCl3; DPNMR PCL = [(Id/2)/(Ih,f+g/5e As measured by SEC (relative to polystyrene standards).
and the corresponding POSS-P(CL-b-LA) blockcopolymer. The characteristic signals of POSS,PCL and PLA segments have been assigned andthe composition can be determined from the ratio
y ROP
NMRd Mw/Mn
e
LA POSS-CL POSS-P(CL-b-LA)
336 1.2 1.5
100 �C.
8)] and DPNMR PLA = [(Im)/(Ih,f/58)] (cf. Fig. 8).
Fig. 8. 1H NMR spectra of POSS–PCL nanohybrid and POSS-P(CL-b-LA) block copolymer.
3
(ppm)
0102030405060708090100110120130140150160170180190
c
h
f b d
i
a
g
e
(ppm)161691701711721731741756
NH C CH2 CH2 CH2 CH2 CH2 O
O
n C CH
CH3
OO
H2POSSa
b c d e hm
i
f
g
180 160 140 120 100 80 60 40 20 0 ppm
Fig. 9. 13C NMR spectrum of POSS-P(CL-b-LA) block copolymer in CDCl3.
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Fig. 10. DSC curve of POSS-P(CL-b-LA) block copolymer (see Table 2).
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of their respective intensities as referenced in Table2. The 1H NMR spectrum highlights a good agree-ment between the relative comonomer experimentalcomposition and the expected values based on thestarting composition and the respective monomerconversion (see Table 2). It is worth noting thatthe small difference between theoretical and experi-mental DP as well as the polydispersity index forsuch a ROP are quite acceptable for conversionsnear 100% (see Table 1 for the sake of comparison).A 13C NMR analysis, by focusing on the carbonylregion of the spectrum, brought additional evidencefor the diblock structure of the copolymer and con-firms that no side-reactions, such as transesterifica-tion, have occurred (Fig. 9). Indeed, only twotypes of carbonyl groups are observed (CL:174 ppm and LA: 170 ppm) without any signalsdetectable in-between these two main resonancepeaks. Such intermediate signals would have beenthe signature of some more randomly distributedcomonomers resulting from transesterification reac-tions [31]. The POSS-P(CL-b-LA) diblock copoly-mer has been further analyzed by DSC.Interestingly enough, both blocks can crystallizeand the melting endotherms of each block areclearly observed (Fig. 10). On one hand, a crystalli-zation exotherm is centered on 94 �C followed bythe melting endotherm at 161 �C typical of thePLA segment. On the other hand, a more complextransition is observed at ca. 47 �C, more likelysuperposing the Tg of the PLA block (around 50–55 �C) and the melting endotherm of the PCL block(around 50–60 �C).
4. Conclusion
In this work, novel organic–inorganic nanohy-brids were synthesized by grafting polyesters chainsfrom amino-functionalized POSS nanocages. Suchnanohybrids were obtained via ROP of lactones ini-tiated by the primary amine groups available ontothe inorganic nanofillers and catalyzed by tin octo-ate. Firstly, the ROP of e-CL and L,L-LA wereinvestigated. The formation of a covalent amide linkbetween the nanofiller and the growing polyesterchains was evidenced by fractionation experiment,1H NMR and FTIR. Then, the growth of the poly-mer chains has been followed in function of time.This study has evidenced a very good correlationwith the theoretical values. Moreover, the molecularweight distribution remained narrow, attesting for agood control over the polymerization. Finally, thesynthesis of a POSS-P(CL-b-LA) block copolymerhas been carried out. 1H NMR analysis gives creditfor the control achievable on the copolymer compo-sition. The quantitative shift of the POSS–PCLnanohybrid SEC trace towards higher molar massesand the observation of a monomodal signal attestfor the absence of termination side-reactions. Theabsence of transesterification reactions has beenconfirmed by 13C NMR. Indeed, only two carbonylsignals respectively assigned to the ester functions ofthe CL and LA were detected. Interestingly enough,these POSS–PCL nanohybrids can be considered as‘‘compatibilized’’ nanofillers to be finely dispersedin either PCL or any polymeric matrices known tobe miscible (or at least mechanically compatible)
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with the PCL chains covalently bonded onto theinorganic nanocages. These results will be the topicof a forthcoming paper.
Acknowledgements
Authors are grateful to the ‘Region Wallonne’and European Community (FSE, FEDER) in theframe of Objectif-1 and Phasing Out financing ofMateria Nova. This research was carried out inthe frame of the STReP European Research pro-gram ‘‘NANOFIRE’’ (Contract No. 505637) fromthe 6th Framework Program. This research was par-tially performed in the frame of SSTC-PAI 6/27project of the Belgian Federal Science Policy Office.
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