Transcript
Page 1: In situ photocrosslinkable nanohybrids based on poly(ε-caprolactone fumarate)/polyhedral oligomeric silsesquioxane: synthesis and characterization

ORIGINAL PAPER

In situ photocrosslinkable nanohybrids basedon poly(ε-caprolactone fumarate)/polyhedral oligomericsilsesquioxane: synthesis and characterization

Seyed Amin Mirmohammadi & Mohammad Imani &Hiroshi Uyama & Mohammad Atai

Received: 1 July 2013 /Accepted: 6 October 2013 /Published online: 3 November 2013# Springer Science+Business Media Dordrecht 2013

Abstrac t Two types of b iodegradable , in s i tuphotocrosslinkable macromers were synthesized consisting alinear copolyester i.e., PCLF based on fumaric acid (FA) andpoly(ε-caprolactone diol) (PCL diol), and its nanohybrid coun-terpart (POSS-PCLF) composed of FA, PCL diol and polyhe-dral oligomeric silsesquioxane (POSS). Chemical structure ofthe macromers was characterized by 1HNMR and FTIR spec-troscopy. The synthesized macromers were photocrosslinkedby visible light irradiation in the presence of camphorquinoneand N,N’-dimethylaminoethyl methacrylate as a photoinitiatorand accelerator, respectively. Photocrosslinking characteristicsof the compositions were investigated as a function of the initialPCL diol molecular weight, presence of POSS nanoparticles orN-vinylpyrrolidone (NVP, as a reactive diluent) by tracingdegree of conversion by FTIR spectroscopy, equilibrium swell-ing studies. Thermal and dynamical properties of themacromers and the resulting networks were studied by TGA,DSC and DMA techniques pre- and post-photocrosslinking. Incontrast to the self-crosslinkability of the macromers, thecrosslinking reaction was promoted more efficiently in thepresence of NVP as a reactive diluent. This scrutiny showedthat there is an optimum point for POSS concentration to obtainthe maximum improvement in the network properties.

Keywords Nanohybrids . Fumarate-basedmacromers .

Polyhedral Oligomeric Silsesquioxane (POSS) .

Photocrosslinking . In situ forming . Visible light

Introduction

During the past few decades, researchers in the area of tissueengineering have done all of their best to discover novelmethods and materials for renewing the injured tissues. De-livery of cells, using a carrier, to the injured tissues has beenreported as one of the successful strategies to attain this goal[1–6]. It is noteworthy that the selected cell carrier should bebiocompatible with no degradation product possessing toxiceffect on the body tissues. The material should also exhibitgood mechanical properties with clinically relevant handlingproperties [7–11].

To provide more convenience to patients than using theconventional tissue scaffolds, injectable and in situcrosslinkable materials have been recently developed. So,several unsaturated fumarate-based polymers like polyethyl-ene glycol fumarate (PEGF), poly(ε -caprolactone fumarate)(PCLF) and polypropylene fumarate (PPF) have been exam-ined for tissue engineering applications until now. Some ofthese fumarate-based macromers may exhibit self-crosslinkable properties and can be cured in different waysusing thermal, redox or photo-initiated crosslinking capability.The named polyesters biodegrade to biocompatible residuesby hydrolysis to the corresponding constituent monomers thatare easily excreted from human body hence, they have recent-ly attracted the scientific community interest to develop im-plantable tissue engineering scaffolds or drug delivery carriersas implantable devices. Thus, injectable and in situcrosslinkable polyesters with a wide range of mechanicaland biodegradation properties have been reported to havemore potential for some special purposes like drug deliveryapplications [9, 12–15].

Using inorganic nanoparticles such as hydroxyapatite (HA)or polyhedral oligomeric silsesquioxane (POSS) to modify un-saturated aliphatic polyesters are reported to yield in enhancedmechanical properties, thermal stability, biocompatibility profile

S. A. Mirmohammadi :H. UyamaDepartment of Applied Chemistry, Graduate School of Engineering,Suita, Osaka 565-0871, Japan

S. A. Mirmohammadi (*) :M. Imani (*) :M. AtaiNovel Drug Delivery Systems Department, Iran Polymer andPetrochemical Institute, P.O.Box 14965/115, Tehran, Irane-mail: [email protected]: [email protected]

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[16–21]. The recent investigations showed that POSS, anorganosilicon oligomer existing as a nanoscale hollow cage witha generic empirical formula (RSiO1.5)n and 1–3 nm in diameter,can be incorporated into polymers as a building block to providehybrid organic–inorganic nanocomposites [22–33]. Further-more, the POSS-based materials have been reported to be non-toxic and cytocompatible [34, 35]. POSS nanoparticles canimprove biostability ofmaterials due to their hydrophobic natureand create an inert porous network which is more resistant tohydrolysis for tissue engineering applications. These nanocages,due to the presence of hydrophobic forces and intermolecularinteractions between themselves, can create physical network inthe polymer matrix providing some POSS crystalline aggregateswith globular or spherical morphology [36–42]. For instance,Yaseen et al. synthesized a novel flexible nanohybrid by graftingPOSS nanocage to poly(carbonate-urea)urethane via the trans-cyclohexanediol substituent group [38]. Bai et al. also preparedtwo uniform inorganic–organic core-shell hybrid latex particlesbased on POSS/polystyrene-butylacrylate-fluorinated acrylate[36] and POSS/polyacrylonitrile/polydimethylsiloxane [37] byseeded emulsion polymerization with POSS as the core.

The purpose of this study is to synthesize a new in situphoto-crosslinkable nanohybrid, based on PCL fumarate(PCLF) and POSS nanocages containing various contents ofthe nanofiller capable to form chemical network. The authorshave been previously focused on developing drug elutingbiomedical devices including cochlear implant electrodes[43], bone cement and recently stents [44]. It is expected toobserve high endothelialization potential from thesenanohybridmaterials when coated on endovascular biomedicaldevices. Self crosslinkable character of these unsaturatedmacromers was investigated and compared with PCLF. NVP,a reactive diluent, was also examined to provide highcrosslinking ratio. The NVP monomer is commonly used asa reactive diluent in ultraviolet (UV), electron beam and visiblelight curing of polymers applied as inks, coatings or adhesives.These three dimensional organic–inorganic nanohybrids canbe used as suitable candidates in tissue engineering applica-tions. The density of physical networks (formed by POSSnanoparticles) and chemical networks (formed by radical po-lymerization of C = C double bonds) can affect some importantproperties of the crosslinked nanohybrids especially the me-chanical ones as reported here in this research. To the best ofour knowledge, synthesis and characterization of thesemacromers are not reported elsewhere.

Experimental

Materials

Trans-cyclohexanediol isobutyl POSS (POSS diol) was pur-chased from Hybrid Plastics Co., (Hattiesburg, USA) and used

as received. Tetrahydrofuran (THF) (Merck Chemicals Co.,Darmstadt, Germany) was dried by adding sodium wire for48 h and consequent distillation under reduced pressure. ε-Caprolactone (ε-CL) monomer (99 %, Fluka, Munchen, Ger-many) and ethylene glycol (EG) (Merck, Germany) were driedover calcium hydride for 48 h and distilled under reducedpressure. Potassium (synthesis grade) was purchased fromMerck, Germany. N-vinylpyrrolidone (NVP), camphorquinone(CQ), calcium hydride, fumaryl chloride (FuCl), potassiumcarbonate (K2CO3), N,N’ -dimethylaminoethyl methacrylate(DMAEMA), methanol, dichloromethane (DCM), n-hexane,toluene, chloroform (CHCl3) and acetone were purchased fromWako Pure Chemical Industries Ltd., Osaka, Japan. FuCl waspurified by distillation at 161 °C under ambient pressure. An-hydrous potassium carbonate (K2CO3) was grinded into a finepowder before application and kept desiccated before any fur-ther application. Anhydrous DCM was obtained by distillationafter reflux for 1 h in the presence of calcium hydride. All otherreagents were used as received without further purification.

Methods

Synthesis of linear OH-PCL-EG-PCL-OHand OH-PCL-POSS-PCL-OH nanohybrid copolyester

In this research, POSS diol nanoparticles and EG were sepa-rately used as initiators for anionic ring opening polymeriza-tion (AROP) of ε -CL to obtain HO-PCL-POSS-PCL-OH(POSS-PCL diol) and HO-PCL-EG-PCL-OH (PCL diol) co-polymers. In a typical batch, EG or POSS diol (0.16 mmole)was dissolved in freshly double-distilled and dried THF(7 mL) charged into a two-necked glass reactor. The mixturewas kept stirred for 5 h in an oil bath at 63 °C after addingpotassium metal wire. After extracting the residual potassiumparticles, a definite amount of ε -CL monomer (withmonomer/initiator molar ratio of 100, 151 and 202) was addedto the reaction flask. Upon completion of the reaction, themixture was poured into a large volume of deionized water.The precipitate was purified two times by solvent (THF)/non-solvent (n -hexane) addition method and dried at 40 °C invacuo (EYELAVos-201SD, Tokyo, Japan). The characteris-tics of synthesized macromers were summarized in Table 1.

Synthesis of PCLF and POSS-PCLF macromers

The procedure of PCLF and POSS-PCLF macromers synthe-sis is adopted from the works of Sharifi et al., with minormodifications as illustrated in Scheme 1 [14]. The purifiedFuCl and PCL diol (or POSS-PCL diol) were reacted in 1:0.99molar ratios. To this end, the dried PCL diol or POSS-PCLdiol macromers were dissolved in 100 mL of anhydrousdichloromethane (DCM) and charged into a reaction flaskequipped with magnetic stirrer. Potassium carbonate powder

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was added to the reaction mixture in 5:1 molar ratio respectiveto the diol macromer. The FuCl was separately dissolved in50 mL anhydrous DCM and added dropwise to stirredmacromer/DCM/K2CO3 suspension while the reaction mix-ture was maintained at boiling temperature of DCM under

reflux condensation under nitrogen atmosphere. After addingall the FuCl-DCM solution (30 min) the reaction mixture wasallowed to stir for an additional 12 h. Upon completion of thereaction, the organic phase was washed with water and cen-trifuged at 6,000 rpm for 45 min (hamic CF 15RX, Tokyo,Japan) to remove all particulate matters also aqueous solubleresidues like K2CO3. The solvent was then removed byrotovaporation and the residue was dissolved in smallamounts of acetone/methanol (90 %V/V) mixture then anappropriate amount of water (as a non-solvent) was added toprecipitate the macromers. The product was re-dissolved inTHF and dropped into an excess volume of n-hexane to bepurified. The purification stages were repeated several timesto obtain a pure, transparent mass of polymer. The resultingmacromers were vacuum-dried at 35 °C for 24 h and storedat −20 °C for further use.

Synthesis of photocrosslinked networks

The macromers were crosslinked by visible light irradiation atblue region (λ =430–500 nm) in the presence of CQ andDMAEMA as a photoinitiator-accelerator system (Scheme 1).Therefore, an appropriate amount of melted macromer (T =60 °C) was mixed with NVP (10 %w/w) on a glass slide by aspatula. DMAEMA (2%w/w to macromer) and a correspond-ing amount of CQ (1:1 w/w to DMAEMA) was added to themixture. The specimens were made as thin films in the sameway as FTIR samples were prepared (see “Fourier-transforminfrared (FTIR) spectroscopy” section). The samples werethen cured for 50, 75 and 100 s using a blue light source withan irradiation circa of 2,000 mW.cm-2 (G-Light Prima-II; GCCo., Tokyo, Japan) at 60 °C. To investigate self crosslinkingproperty of the macromers, the same procedure was also

Table 1 Synthesis medium compositions alongwith the molecular weight, thermal and dynamical characteristics of the linear diol macromers includingdiol and fumarate-based compounds

Macromer type Samplecode

Initiator Initiator/monomermolar ratio

Initiatorcontent(%w/w)

Mn (g.mol−1) PDI Tga (°C) Tm (°C) ΔHm (J/g) Crystallinity

(%)Td (°C) Tanδ

PCL diol LPCL1 EG 100 0.56 17830 1.73 −66.7 54.4 91.7 65.97 363.2 6.83

LPCL2 151 0.37 23980 1.43 −67.3 53.3 86.4 62.16 360.0 6.29

LPCL3 202 0.28 31770 1.62 −68.7 53.1 74.2 53.39 361.4 6.18

POSS-PCL diol PPCL1 POSS diol 100 8.33 18850 2.18 −73.8 51.6 59.3 42.66 403.7 7.12

PPCL2 151 5.52 27130 1.78 −73.3 51.8 60.2 43.31 399.5 7.73

PPCL3 202 4.13 34130 1.87 −69.0 53.7 69.2 49.78 394.1 8.56

PCLF LPCLF1 EG 100 0.56 33290 2.76 −60.6 51.7 82.4 59.24 378.7 7.27

LPCLF2 151 0.37 43800 2.34 −62.5 51.2 80.1 57.50 371.7 6.85

LPCLF3 202 0.28 57550 2.35 −64.0 49.4 68.0. 48.82 374.4 6.93

POSS-PCLF PPCLF1 POSS diol 100 8.33 35250 2.67 −68.4 51.0 54.2 38.91 409.5 8.07

PPCLF2 151 5.52 48430 2.16 −67.5 51.2 57.7 41.42 397.2 7.17

PPCLF3 202 4.13 60100 2.37 −65.7 52.6 64.8 46.52 401.1.6 7.67

a Obtained from DSC experiments

Scheme 1 Synthesis of fumarate-based PCL-POSS macromers andschematic representation of its crosslinking reaction

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applied without adding NVP to the samples. In this case, theinitiator and accelerator were only dissolved in the moltenmacromers.

Characterizations

Nuclear magnetic resonance (NMR)

Proton NMR (1HNMR) spectra were recorded on a BrukerDPX400 spectrometer (Bruker, Germany) at room tempera-ture. CDCl3 was used as a solvent and chemical shifts wererecorded in ppm from the signal of tetramethylsilane.

Fourier-transform infrared (FTIR) spectroscopy

FTIR spectra (4,000–400 cm−1) were acquired on a ThermoScientific, NICOLET IS5 spectrophotometer, Madison, USA.The samples were placed between two polyethylene films,pressed to form a very thin film and the absorbance peaks ofuncured samples were recorded. The specimens were exam-ined at 4 cm−1 resolution and 32 scans at room temperature.

Gel permeation chromatography (GPC)

GPC analysis is carried out by using a Tosoh GPC-8020 appa-ratus equipped with refractive index (RI) and UV detectors. Theinstrument was equipped by TSKgel GMHHR-L and GMHHR-M columns. Chloroform (LC grade) was run as an eluent at aflow rate of 1.0 mL.min−1 at 40 °C through the column.

Thermal behavior and mechanical properties

Thermal behavior of the synthesized macromers (PCL diol,POSS-PCL nanohybrids and their corresponding copolymerswith FuCl) was studied by differential scanning calorimetry(DSC) and thermogravimetric analysis (TGA) measurementsusing a DSC 6020 and a TG/DTA 7200 (Seiko InstrumentsInc., Tokyo, Japan) instruments, respectively. Thermal stabil-ity of the crosslinked samples was studied by TGA. All of themeasurements (DSC and TGA) were performed at the heatingrate of 10 °C.min−1 under nitrogen atmosphere. Degree ofcrystallinity of the samples was calculated using Eq. 1 asfollows:

Crystallinity %ð Þ ¼ ΔHm=ΔHm○½ � � 100 ð1Þ

where, ΔHm stands for the heat of melting determined byintegrating the area (J.g−1) under the DSC melting signalobtained from the second heating cycle. To eliminate thethermal history, the specimens were heated from room tem-perature to 100 °C (first heating cycle), then cooled to −200 °Cand for the second heating cycle, they were heated again to100 °C. The time for fixing the temperature at the end of each

cycle was 5 min. The term ΔHm° is a reference value(139.3 J.g−1 for PCL) and represents the heat of melting ifthe copolymer was 100 % crystalline [45].

All samples (40×10×0.2 mm) including crosslinked ornon-crosslinked macromers were also examined by dynamicmechanical analysis (DMA) at a constant frequency of 1 Hzand a temperature range from −100 to 40 °C, using a SeikoEXSTAR6000 instrument (Seiko Instruments Inc., Tokyo,Japan) in tensile mode.

Degree of conversion (DC%)

The degree of conversion (DC%) of the fumarate-basedmacromers crosslinked in the presence and absence of NVPwere measured using FTIR spectroscopy. The samples wereplaced between two polyethylene films, pressed to form a verythin film and the absorbance peaks of uncured samples wererecorded. The samples were then light cured for the specifiedtime periods (see description under “Synthesis ofphotocrosslinked networks” section) and the peaks were col-lected for cured samples. Degree of conversion was deter-mined by the ratio of absorbance intensities of aliphatic C = Cbonds (at 1,640 cm−1) against two internal reference peaks ofcarbonyl (C = O) bonds of macromer (at 1,720 cm−1) andasymmetric bending of CH2 units of PCL backbone at730 cm−1 before and after curing. The degree of conversionwas then calculated as follows:

DC% ¼ 1−

1640 cm−1

Reference Peak cm−1

� �Peak area after curing

1640 cm−1

Reference Peak cm−1

� �Peak area before curing

0BBB@

1CCCA� 100

ð2ÞDC% of the each specimen (n =3) was calculated consid-

ering two aforementioned reference peaks and the averagevalues were reported [15].

Equilibrium swelling studies

A sol fraction extraction study was performed on thephotocrosslinked specimens. The specimens were accuratelyweighed (W1) and placed into 50 mL of THF under gentleshacking to swell and extract the unreacted ingredients like themacromers, NVP monomers, polyvinyl pyrrolidone (PVP)homopolymers and so on. After 72 h, the specimens wereremoved from solvent and immediately weighted (W2) thendried to constant weight at 40 °C (W3). Swelling data wereused to calculate the equilibrium swelling ratio and sol frac-tion percent for each formulation using the following equa-tions [13, 15]:

Swelling ratio% ¼ W 2−W 3

W 3� 100 ð3Þ

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Soft fraction% ¼ W 1−W 3

W 1� 100 ð4Þ

Gel yield GYð Þ% ¼ 1−W 1−W 3

W 1

� �� 100 ð5Þ

Crosslink density of the networks was determined usingequilibrium swelling data according to the Flory–Rehnerequation [14, 46]. The equilibrium degree of swelling (Q ),which is the reciprocal of volume fraction of polymer inswollen state (Vp), was calculated based on the swelling dataof the samples in various solvents using the following equa-tions:

Vp ¼ Wp=dpWp=dp þWs=ds

ð6Þ

Q ¼ 1

Vpð7Þ

where, Wp, Ws, dp and ds stand for dry weight of polymer,weight of solvent taken up at equilibrium swelling, polymerdensity and solvent density, respectively. In order to determinethe values for solubility parameter (SP) of the photocrosslinkedsamples, Q was plotted against SP (δ) for different solvents.From the plot it was found that THF has maximum values ofQfor the photocrosslinked samples. Average molecular weightsbetween crosslinks (Mc) were determined in THF as a solventat 25 °C according to the Flory–Rehner equation:

Mc ¼ −υsdp V

1=3p −Vp=2

� �

ln 1−Vp

� �þ Vp þ χ12V2p

ð8Þ

where, υs and χ12 are molar volume of solvent and polymer–solvent interaction parameter, respectively. The polymer–sol-vent interaction parameter was calculated using Eq. 9:

χ12 ¼Vs

RTδp−δs� �2 þ 0:34 ð9Þ

In which δp and δ s stand for solubility parameter of thepolymer and solvent (i.e., THF) pair, respectively. The R , Tand Mc parameters are ideal gas constant, absolute tempera-ture and average molecular weight between crosslinks,respectively.

Statistical analysis

Statistical analyses were performed using MiniTab 15 soft-ware. Single factor analysis of variance (ANOVA)was used toassess the statistical significance of the results. Differenceswere considered statistically significant when the p value wasless than 0.05.

Results and discussion

Linear PCL diols and POSS-PCL diol nanohybrids

The mechanism of AROP of ε -CL, initiated by activated EGand POSS diol species, and characterization of the resultingcopolymers including linear PCL diols and POSS-PCL diolnanohybrids have been previously reported elsewhere by thesame team [47, 48]. This was done by tracing the changesobserved in the mean molecular weight and molecularweight distribution of the deconvoluted fractions detect-ed in the gel permeation chromatograms (GPC) of thesynthesized polymers. Chemical structure of the macromerswas also confirmed by proton and carbon nuclear magneticresonance (1HNMR and 13CNMR) spectroscopy. GPC char-acterization of the synthesized macromers, using EG or POSSdiol as the central block of the triblock copolymer, is tabulatedin Table 1.

Linear PCLF and POSS-PCLF macromers

Structural characteristics

Diol macromers (PCL diol and POSS-PCL diol) were con-verted to their corresponding unsaturated, fumarate-based de-rivatives (PCLF and POSS-PCLF) by conducting a directpolycondensation reaction between them and acyl halide de-rivative of fumaric acid i.e., FuCl in the presence of K2CO3 asa proton scavenger. Adjusting the molar ratio of the diolmacromers and FuCl resulted in unsaturated derivatives end-ing with carboxylic acid end functional groups as shown in thefirst part of Scheme 1.

In polyesterification reactions, acyl halides possess highreactivity toward diols comparing with their carboxylic acidscounterparts hence a higher rate of conversion may be obtain-ed than direct polycondensation reactions. However, the reac-tion yields in acid side products e.g., HCl in this case whichnegatively affects the esterification reaction in turn. To resolvethis limitation, ranges of compounds (mostly alkalis and silentalkalis) are usually added to the reaction mixture as protonscavengers to neutralize the acidulous byproducts. Mostly,tertiary amines like triethylamine, dimethylaminopyridine orpyridine itself were used as a proton scavenger to increase thedegree of macromers unsaturation however; these amineswere reported to produce macromers having a dark browncolor as the final reaction product [12, 15]. Such dark mass ofpolymer is unable to be photocrosslinked effectively by visi-ble light irradiation [13–15]. So, K2CO3 was used in this studyto synthesize transparent light yellow colored unsaturatedmacromers with improved photocuring capability accordingto our previous works in the same field [15].

The 1HNMR and FTIR results of the obtained macromersare as follows:

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1HNMR: δ 1.27, 1.42, 1.67, 2.33 and 4.08 (methyleneprotons of PCL backbone), δ 4.23 (methylene protons ofPCL backbone close to ester bonds with fumarate blocks), δ4.30 (methylene protons of central EG block), δ 6.89 and 7.06(protons of fumarate group of PCLF in middle and end of thecopolymer chain, respectively) and δ 10.63 (protons of car-boxyl groups in the ends of chain). In addition of the men-tioned peaks, δ 0.62 and 0.98 (methylene and methyl protonsof POSS) and δ 4.50 (protons of cyclohexyl substituentgroups attached to C–O bonds) were observed for POSS-PCL macromers.

Assignment of protons belonging to carboxylic acid endfunctional groups in the synthesized unsaturated macromers issomewhat difficult. The problem probably arises from the factthat the position of these protons varies dramatically depend-ing on their surrounding chemical environment e.g., type ofthe solvent(s) used also concentration and purity of themacromer which in turn highly depends on whether themacromer is totally dried or not [15]. The large chemical shiftsobserved for hydroxyl protons are due to variation in hydro-gen bonding extent. Since these protons are engaged in hy-drogen bonding, the more the proton gets deshielded, thehigher will be its chemical shift. Although existence of car-boxyl groups in the end of chains is clear in the Fig. 1, twodistinct peaks for protons of fumarate groups were detected atδ 6.89 and 7.06 are showing that the synthesized macromersare terminated by carboxylic acid functional groups.

FTIR results: 2,940 cm−1 and 2,860 cm−1 (C–H stretching),1,720 cm−1 (C = O stretching of ester functional groups), 1,640 cm−1 (fumarate backbone double bonds) and 1,160 cm−1

(C–O–C stretching). In addition to the mentioned signals, apeak was appeared at 1,090 cm−1 which was attributed toPOSS (Si–O–Si stretching) blocks in the copolymer structure(Fig. 2).

More characteristics of the fumarate-based macromersused throughout this research are summarized in Table 1.The molecular weights of the primary diol macromers as wellas the resulting fumarate-based counterparts were determinedby GPC as shown in this table. The resulting fumarate-basedlinear macromers are about 45–60 % more polydispersed in

terms of molecular weight distribution than their starting PCLdiol counterparts, but for POSS-PCLFs these percentages waslower in the range of 21 to 27 %. It shows that in thispolycondensation reaction, joining of the diol macromers witha huge pendant groups together will be resulting in narrowerpolydispersity indexes (PDI). In other words, due to existenceof a huge pendant group in the polymer backbone, lowerirregularities are observed after reaction while quite the oppo-site is the case of linear PCLFs. A closer look at GPC results inTable 1 shows that ratio of number average molecular weight(Mn) of the unsaturated macromers to Mn of their correspond-ing primary macromers decreases with increasing inMn of thestarting macromers. These ratios are 1.88, 1.83 and 1.81 forlinear PCLF macromers and, 1.87, 1.79 and 1.76 formacromers with POSS pendant groups, respectively. Al-though there is no significant difference between these values(p >0.05), but this phenomenon maybe attributed to lowerreactivity and mobility of OH end groups in higher molecularweights (due to higher viscosity) of diol macromers [49]. Theeffect of POSS huge pendant group was also observed in thiscase which is resulting in lower values. On the other hand, thesteric hindrance effect of POSS cage and formation of POSSaggregates [50, 51] reduce mobility and reactivity of OH endgroups in POSS-PCL diol macromers. The calculated ratiosyield the degree of oligomerization ≈3 for all of the examinedmacromers. This means that each fumarate based macromerhas, in average, one and two unsaturated C = C bonds in themiddle part and two ends of the chain, respectively. Existenceof fumarate groups in the macromer backbone has previouslybeen proved by H1NMR and IR spectroscopy findings.

Thermal characteristics

Melting temperature (Tm), heat of fusion (ΔHm), and percent-age of crystallinity of the initial diol macromers their counter-part fumarate-containing ones are summarized in Table 1along with their composition according to feed ratios of theEG or POSS diol initiators in the reaction mixture. The crys-tallinity percentage of the samples was calculated using Eq. 1.Both Tm and crystallinity percentage of the fumarate-basedmacromers (synthesized from different diol precursors)showed lower values comparing with their correspondinginitial diols. The crystallinity of all macromers decreasedFig. 1 H1NMR spectroscopy of POSS-PCLF macromer

Fig. 2 FTIR spectroscopy of POSS-PCLF macromer

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significantly by 2–6 % after copolymerization with FuClprobably due to the presence of fumarate units consisting ofrigid C = C bonds in trans position, which may have causeddisruption in crystal formation.

Crystallinity of the linear diol macromers (PCL diols) andtheir resulting fumarate derivatives (PCLFs) along with theircorresponding Tg and Tm values significantly decreased withincreasing the molecular weight due to the chain entangle-ments and subsequently reduction in chain folding and for-mation of ordered chain regions [52–55]. However, contraryresults were obtained for POSS-PCL diols and POSS-PCLFmacromers (Table 1). It is obvious that the lower crystallinity,resulting in a higher mobility of polymer chains, is the maincause of decreasing Tg, Tm and ΔHm values [56–58]. Thisfacilitation of chain movements in the polymer matrix hasbeen enhanced by chemical incorporation of POSS nanocageas a pendant group to the polymer backbone for POSS-PCLnanohybrids. Existence of a POSS nanocage in the middle ofpolymer backbone as a co-monomer prevents the formation ofwell-ordered chain regions and chain folding, and consequent-ly is against of formation of crystalline structures. Thus, thecrystallinity, Tm and Tg of POSS-PCLs and their fumaratebased macromers increased with decreasing in POSS contentin the copolymer matrix.

As Mn increases, Tg will increase according to Fox-Floryequation [15]:

Tg ¼ Tg∞−A

Mnð10Þ

Tg∞ and A were calculated from the intercept and slope ofthe linear plot of Tg vs. 1/Mn, respectively. Using the reportedMn values reported in Table 1, the extrapolated glass transi-tion temperatures (Tg∞) for PCL diol, PCLF, POSS-PCL dioland POSS-PCLF were calculated as −70.96, −68.65, −65.08and −62.05 °C, respectively. The A values i.e., a constant werealso equal to −78,691, −268,246, 175,015 and 214,920 g.mol−1, respectively. It can be concluded from Fox-Flory equation that when parameter A is negative e.g., forPCL diols and PCLFs, the Tg values will be higher than Tg∞

andwill be lower than Tg∞ for macromers with positive valuesofA (like what observed for POSS-PCL diol and POSS-PCLFmacromers).

Each caprolactone unit has four free rotating carbon–carbonsingle bonds, which provide macromers with flexible chains.However, the addition of fumarate groups along the macromerschain comes in the way of free rotation around C–C bonds ofthe backbone chain and hence hinders the chain mobility,resulting in an increase of around 7–10 % and 2–12 % in Tg

values over PCL diols and POSS-PCL diols, respectively.TGAwas also performed to determine thermal stability of

the macromers and the results showed one single degradationstep for all samples. The temperature at maximum degradation

rate (Td) for each sample was obtained from differential TGA(DTGA) and collected in Table 1. As shown in this tableincorporation of POSS nanoparticle, due to its inorganic na-ture, increases Td and the higher the POSS content, the higherthe Td. But, there is no significant difference between Td ofthe macromers with and without fumarate blocks (p >0.05). Itshows that addition of fumarate comonomers to PCL diol andPOSS-PCL diol macromer chains has no remarkable effect ontheir thermal stabilities.

Crosslinked PCLF and POSS-PCLF networks

Crosslinked network characteristics

The CQ/amine photoinitiator system is widely used for gen-erating free radicals for curing of biomedical restoration ma-terials like dental materials [15]. Using this photoinitiationsystem, the linear unsaturated macromers were converted tothe crosslinked networks. The networks were characterized bystudying their swelling behavior in their thermodynamicallyfavorable solvent. Figure 3 shows a plot of equilibrium degreeof swelling (Q ) of the photocrosslinked LPCL2 (XLPCL2)and PPCL2 (XPPCL2) (see Table 2) specimens against solu-bility parameter (SP ) of some of the tested solvents i.e.,toluene (δ =8.91 (cal.cm−3)0.5), chloroform (δ =9.21(cal.cm−3)0.5), THF (δ =9.52 (cal.cm−3)0.5), acetone (δ =9.77(cal.cm−3)0.5) and DCM (δ =9.93 (cal.cm−3)0.5) (http://cool.conservation-us.org/byauth/burke/solpar/solpar2.html). It isnoteworthy that the samples were photocured for 100 s in thepresence of NVP and the solvents were selected based on thePCL solubility parameter i.e., ≈ 9.63 (cal.cm−3)0.5) [59].According to the results, THF can be chosen as the bestsolvent for the macromers to obtain the maximum equilibriumswelling ratio for the photocrosslinked macromers. Hence, THFwas used to determine sol fraction, equilibrium swelling ratioandMc values of the samples throughout this report.

Although the neat fumarate-based macromers possess thecapability of being crosslinked, the curing reaction is very

Fig. 3 Equilibrium degree of swelling (Q) of the crosslinked PCLF andPOSS-PCLF networks against Hildebrand solubility parameter of someof the tested solvents

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slow and results in a putty-like material, even after 100 s oflight irradiation [14, 15]. This behavior is probably due to thesteric hindrances imposed by the fumarate double bonds (po-sitioned with their arms as trans isomer along with the PCLFbackbone) which partially restrict the movement of themacroradicals produced by the PCLF chains. As shown inFig. 4a, although there is no considerable difference between

DC% of the PCLFs in a similar curing time (p >0.05), but solfraction (Fig. 5a), equilibrium swelling ratio (Fig. 6a) andMc

values (Fig. 7a) increase with an increase in the molecularweight of the initial diol macromer. On the other hand, withdecrease in the crosslinkable C = C double bonds along thePCLF chains, sol fraction, swelling ratio and Mc values havebeen increased. From these figures it is clear that the more theexposure time, the higher the DC% and GY (Table 2) andsubsequently lower sol fraction, swelling ratio andMc valueswere observed. Also, statistical analyses show that there aresignificant differences between the results acquired from dif-ferent curing times (p <0.05). It can be concluded that the bestcuring time (in the range of values studied here) was 100 s forthese samples, resulting in highest C = C double boundconversion.

Beside mentioned reasons, existence of POSS nanocages isanother important parameter which can affect the properties ofultimate crosslinked nanohybrids. A huge pendant group likePOSS nanocage probably causes to a significant hindranceeffect and lowers chain activity of the macroradicals formedduring the crosslinking process of POSS-PCLF macromers.Additionally, due to the hydrophobic forces and intermolecularinteraction between POSS nanocages, they tend to gather veryclosely to create globular structures, even in solution phase [39].Formation of these vesicular structures (like some small buck-lers) maybe prevent the exposure of C = C double bond to theblue light and subsequently lower DC, GY and, higher solfraction, swelling ration and Mc values were obtained in

Table 2 Selected network characteristics of the crosslinked PCLF andPOSS-PCLF macromers after curing by blue light irradiation for 100 s

Sample code NVPcontent (%)

Tga (°C) Td (°C) Gel yield

(%)Tanδ

XLPCL1 10 −62.4 378.6 83.8 9.17

0 −60.1 374.1 53.3 6.64

XLPCL2 10 −64.4 369.2 84.2 8.80

0 −63.8 373.7 56.9 6.41

XLPCL3 10 −64.8 375.4 82.7 8.92

0 −62.3 376.5 57.6 6.14

XPPCL1 10 −63.7 401.3 84.0 8.73

0 −67.0 406.2 49.8 5.49

XPPCL2 10 −63.3 394.5 84.1 9.27

0 −65.4 403.4 52.1 6.86

XPPCL3 10 −63.5 398.5 82.5 9.05

0 −62.7 401.0 54.4 6.34

a Obtained from DSC experiments

Fig. 4 DC% of the crosslinked samples, a) self crosslinking and b) inpresence of NVP

Fig. 5 Sol fraction of the crosslinked samples, a) self crosslinking and b)in presence of NVP

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comparison with macromers without POSS nanoparticles. Sta-tistical analyses show significant differences (p<0.05) for Mc

and swelling ratio between crosslinked macromers with and

without POSS nanoparticles. But, the differences for GY, solfraction and DC are not remarkable (p>0.05).

It is noticeable that with increasing in the molecular weightof initial diol macromer and decrease in POSS content, lowerhindrance effects and lower vesicular structures were formed[39, 47] and consequently may result in higher DC, GYand animproved sol fraction and equilibrium swelling ratio. Anotherpoint which can be observed from these figures is that the Mc

values, DC, GY, sol fraction and swelling ratio of crosslinkedPOSS-PCLFmacromers with higher molecular weight, due tolower POSS content, have been got very close to the men-tioned properties for PCLFs cured at the same curing time. Inother words, the longer arms length of the primary POSS-PCLdiol chain, the better the melioration in properties. Like theprevious part, the best results were also obtained from 100 sexposure time of blue light.

By adding the NVP monomer, DC% (Fig. 4b) and GY(Table 2) increase and consequently the sol fraction (Fig. 5b),swelling ration (Fig. 6b) and Mc values (Fig. 7b) of systemwill be decreased in different curing times. These results aredue to the bridging reactions between short growing chains(produced by the reaction of primary radicals with NVPmonomers) and fumarate functional groups on the macromerchains. This mechanism incorporates additional fumarateunits in the unsaturated macromers network and increasesthe compactness of the crosslinked fumarate basedmacromers/NVP networks due to conversion of more inter-molecular van der Waals forces to the covalent single bonds.During the free radical copolymerization of unsaturatedmacromers and NVP, crosslinking results to a progressivelygrowing three-dimensional network. The radicals formed afterthe initiation react with a NVP monomer or a fumarate unit ofthe macromers to form the growing primary macroradicals.Since unsaturated macromer is less mobile than NVP mono-mer due to its molecular size and functionality, the growingprimary macroradicals react with NVP monomers to formshort growing chains. These short growing chains react withthe other fumarate units to form crosslinks between the PCLFchains. This reaction becomes diffusion-controlled near andafter gel point since the mobility of free radicals and reactantswill be impeded by the network structure [14, 15]. It is clearaccording to statistical analyses that increase in curing time byblue light, due to better and completion of crosslinking pro-cess, considerably improves the ultimate properties ofcrosslinked samples (p <0.05).

The obtained data also support this fact that, like selfcrosslinking process of POSS-PCLF macromers, existence ofPOSS nanoparticle prevents to achieve an ideal crosslinkingreaction in all curing times. The statistical analyses show thesame results for Mc values and swelling ratio (p <0.05), andGY, sol fraction and DC (p >0.05) between macromers withand without POSS nanoparticles. For example, the obtainedresults of XPPCLF1 which has the highest amounts of POSS

Fig. 6 Swelling ratio of crosslinked samples, a) self crosslinking and b)in presence of NVP

Fig. 7 Mc values of crosslinked samples, a) self crosslinking and b) inpresence of NVP

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(8.33 %) are not good as expected even in 100 s of curing time.But in presence of NVP, the ultimate properties of specimensare much better than the self-crosslinked macromers and inhigher molecular weights, owing to lower POSS content, theoutcomes have been improved.

It can be concluded that linear PCLFs and specially POSS-PCLF macromers have no admissible potential to use as self-crosslinkable macromers in various applications. But, betterresults for self-crosslinkability of the linear PCLFs with muchlower molecular weights have been reported in the literature(≤2,000 g.mol−1) [14, 15, 49, 60]. Therefore, a diluent mono-mer (like NVP) must be served as both viscosity modifier andalso co-participating in crosslinking reactions for linear highmolecular weight PCLFs and especially POSS-PCLFnanohybrids. The macromers crosslinked for 100 s of curingtime, because of the maximum improvement in sample prop-erties, were chosen for more characterizations.

Thermal and mechanical behavior

Values of Tg for the photocrosslinked samples were deter-mined using DSC thermograms and collected in Table 2.Statistical analysis elucidates no dramatic difference betweenTg values (p >0.05), and also between these values and Tg

values of the linear fumarate-based samples shown in Table 1(p >0.05). These results show no substantial influence on Tg

of the specimens after formation of the 3D network structureup to this level. This phenomenon may be accounted for lowdegree of oligomerization (existence of only three unsaturatedcrosslinkable C = C bonds along the macromer chain) whichresults in highMc and mesh size values, and consequently nosignificant changes in Tg after crosslinking process.

This phenomenon can be explained from other viewpoint:glass transition temperature is a reversible transition in theamorphous materials, or in amorphous regions within semi-crystalline materials from a hard and relatively brittle state intoa rubbery-like state. Tg of polymers is often expressed as thetemperature at which the Gibbs free energy is such that theactivation energy for the cooperative movement of 50 or soelements of the polymer is exceeded and these movementsdecrease in the higher crystalline state. It can be concludedthat crystallinity percentage of ultimate crosslinked samplesreduces during crosslinking process in one hand which canfacilitate the chains mobility in turn, and subsequently resultsin lower values for Tg. But, on the other hand, crosslinkingprocess creates chemical bonds between macromer chainswhich can prevent better mobility of the ultimate specimens.So, crosslinking process has an interactive dual effect on Tg

values.The improving effect of inorganic POSS nanoparticles on

the heat resistance property of polymers, especially PCL, wasconfirmed before in the literature [36] and also our recent work[48]. Thermal stability profiles of all macromers crosslinked by

blue light irradiation for 100 s (without NVP) were also inves-tigated. The results showed similar pattern consisting of onesingle degradation stage for uncrosslinked fumarate-basedmacromers. As shown in Tables 1 and 2, there is no significantdifference between Td of the crosslinked and uncrosslinkedspecimens. On the other hand, the p values obtained fromstatistical analyses for Td of both linear fumarate-basedmacromers (PCLFs and crosslinked PCLFs) and nanohybrids(POSS-PCLF and crosslinked POSS-PCLF macromers) werehigher than 0.05. However, in the presence of NVP, anotherdegradation step was observed for all samples at temperatureabout 235 °C. This small weight loss (about 7 %) is possiblyattributed to degradation of NVP moieties trapped betweenmacromer chains with or without formation of chemical bonds.In this case, like the crosslinked samples without NVP, there isno considerable difference between Td of specimens (p >0.05)for the linear samples and nanohybrids. These results indicatethat the changes in molecular weights and also conversion offumarate-based linear macromer chains to three-dimensionalnetworks (due to low degree of oligomerization) have nonotable effect on thermal stability of samples. This phenome-non was also reported by Wang et al. for self-crosslinkablePCLF with different molecular weights [60].

As mentioned before, DMA study of PCLFs and POSS-PCLF specimens was performed in our recent work. Accord-ing to the results, it was proved that addition of POSS nano-particles to PCL chain can improve mechanical properties[48]. Storage and loss moduli, and tanδ of the crosslinked

Fig. 8 DMA results of crosslinked and uncrosslinked macromers at20 °C, a) storage and b) loss moduli

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and uncrosslinked macromers were also examined byDMA at20 °C and shown in Fig. 8 and, Tables 1 and 2, respectively.As shown in Fig. 8a and b and Table 1, there are no significantdifferences between storage and loss moduli, and tanδ of thestarting diol macromers and fumarate-based specimens (withand without POSS nanoparticles) (p >0.05). It means that inthis case, addition of fumarate comonomers to the linear PCLdiols and POSS-PCL macromers and subsequent increase inthe molecular weight has no noticeable effect on mechanicalproperties. But, Fig. 8a and b and Table 2 show remarkableeffect of crosslinking process on the storage and loss moduli,and tanδ (p <0.05) of the samples, respectively which mayattributed to formation of 3D network. The highest enhance-ments in the mechanical properties observed for the highestlevel of crosslinking density in presence of NVP. This phe-nomenon can prove the formation of networks possessinghigher crosslinking density in comparison to the self-crosslinked macromers. More scrutiny of Fig. 8 and Table 2demonstrate that existence of POSS nanoparticles has a inter-active dual effect on mechanical properties. On one hand,POSS nanocages, due to their inorganic natures, can improvemechanical properties of PLC matrix which are observed forXPPCL2 and XPPCL3 samples in comparison to XLPCL2and XLPCL3, respectively. On the other hand, due to theformation of more vesicular structures and hindrance effectof POSS nanocages in higher POSS concentration,crosslinking density has been reduced which is observed forXPPCL1 in comparison to XLPCL1. Thus, the most appro-priate concentration of POSS nanoparticle which has themaximum effect on mechanical properties as a nanofillerand minimum suppressive effect on crosslinking density is5.52 % which is related to XPPCL2 (See Fig. 8 and Table 2).

For PCL diol and PCLF specimens, less improvement inthe mechanical properties are expected due to lower crystal-linity percentages after increasing in the Mn of linearmacromers however, higher Mn values may play a moreeffective role to improve mechanical properties. Fornanohybrids (POSS-PCL diols and POSS-PCLFs), increasingin POSS content plays the same role against increasing in bothcrystallinity and Mn. On the other hand, although crystallinitypercentages increase with increasing in Mn of these samples,but POSS contents decrease which causes less improvementin the mechanical properties.

Conclusion

Unsaturated PCLF and POSS-PCLF specimens were synthe-sized via polycondensation reaction between fumaryl chlorideand, PCL diol and POSS-PCL diol in the presence of potas-sium carbonate. The effect of molecular weight of macromers,POSS content and NVP (as a reactive diluent) were investi-gated. Results revealed that although neat PCLF and POSS-

PCLF were photopolymerized, but they showed putty likebehavior after 100 s of blue light irradiation and a very lowdegree of conversion and GY, and very high sol fraction,swelling ratio and Mc value, especially for crosslinkedPOSS-PCLF macromers. Adding about 10 % of NVP mono-mers caused a dramatic increase in their degree of conversionand GY, and considerable decrease in their sol fraction, swell-ing ratio and Mc value. Comparison between the results forPCLFs and POSS-PCLF macromers depicts that existence ofPOSS nanoparticles, due to formation of vesicular struc-ture and their hindrance effect, prevents better and com-plete carbon-carbon double bond conversion in thecrosslinking process by visible light irradiation. But, thereis an optimum point of POSS concentration in the PCLFmatrix which shows the maximum enhancement for ultimatecrosslinked nanohybrid, especially in mechanical and thermalproperties.

Acknowledgments The authors appreciate Dr. Muneo Tanaka, thehead of Tanaka Dental Clinic, for his kind helps in the use of blue lightdevice (LED).

References

1. Temenoff JS, Park H, Jabbari E, ConwayDE, Sheffield TL, AmbroseCG, Mikos AG (2003) Thermally cross-linked oligo(poly(ethyleneglycol) fumarate) hydrogels support osteogenic differentiation ofencapsulated marrow stromal cells in vitro. Biomacromolecules5(1):5–10. doi:10.1021/bm030067p

2. Sarvestani AS, He X, Jabbari E (2007) Viscoelastic characterizationand modeling of gelation kinetics of injectable in situ cross-linkablepoly(lactide-co-ethylene oxide-co-fumarate) hydrogels.Biomacromolecules 8(2):406–415. doi:10.1021/bm060648p

3. Goonoo N, Bhaw-Luximon A, Bowlin GL, Jhurry D (2013) Anassessment of biopolymer- and synthetic polymer-based scaffoldsfor bone and vascular tissue engineering. Polym Int 62(4):523–533.doi:10.1002/pi.4474

4. Sell S, Barnes C, Smith M, McClure M, Madurantakam P, Grant J,McManus M, Bowlin G (2007) Extracellular matrix regenerated:tissue engineering via electrospun biomimetic nanofibers. PolymInt 56(11):1349–1360. doi:10.1002/pi.2344

5. Djordjevic I, Choudhury NR, Dutta NK, Kumar S (2013)Poly[octanediol-co-(citric acid)-co-(sebacic acid)] elastomers: novelbio-elastomers for tissue engineering. Polym Int 60(3):333–343. doi:10.1002/pi.2996

6. Ngadaonye J, Geever L, Killion J, Higginbotham C (2013)Development of novel chitosan-poly(N, N-diethylacrylamide) IPNfilms for potential wound dressing and biomedical applications. JPolym Res 20(7):1–13. doi:10.1007/s10965-013-0161-1

7. Sarvestani AS, Jabbari E (2006) Modeling and experimental investi-gation of rheological properties of injectable poly(lactide ethyleneoxide fumarate)/hydroxyapatite nanocomposites. Biomacromolecules7(5):1573–1580. doi:10.1021/bm050958s

8. Jabbari E,Wang S, Lu L, Gruetzmacher JA, Ameenuddin S, HefferanTE, Currier BL, Windebank AJ, Yaszemski MJ (2005) Synthesis,material properties, and biocompatibility of a novel self-cross-linkable poly(caprolactone fumarate) as an injectable tissue engineer-ing scaffold. Biomacromolecules 6(5):2503–2511. doi:10.1021/bm050206y

J Polym Res (2013) 20:297 Page 11 of 13, 297

Page 12: In situ photocrosslinkable nanohybrids based on poly(ε-caprolactone fumarate)/polyhedral oligomeric silsesquioxane: synthesis and characterization

9. Sharifi S, Blanquer SBG, van Kooten TG, Grijpma DW(2012) Biodegradable nanocomposite hydrogel structures withenhanced mechanical properties prepared by photo-crosslinkingsolutions of poly(trimethylene carbonate)–poly(ethylene glycol)–poly(trimethylene carbonate) macromonomers and nanoclayparticles. Acta Biomater 8(12):4233–4243. doi:10.1016/j.actbio.2012.09.014

10. Martina M, Hutmacher DW (2007) Biodegradable polymers appliedin tissue engineering research: a review. Polym Int 56(2):145–157.doi:10.1002/pi.2108

11. Camci-Unal G, Alemdar N, Annabi N, Khademhosseini A (2013)Oxygen-releasing biomaterials for tissue engineering. Polym Int62(6):843–848. doi:10.1002/pi.4502

12. Shafieyan Y, Sharifi S, Imani M, Shokrgozar MA, Aboudzadeh N,Atai M (2011) A biocompatible composite based on poly(ε-caprolactone fumarate) and hydroxyapatite. Polym Adv Technol22(12):2182–2190. doi:10.1002/pat.1743

13. Doulabi ASH, Mirzadeh H, Imani M, Sharifi S, Atai M, Mehdipour-Ataei S (2008) Synthesis and preparation of biodegradable andvisible light crosslinkable unsaturated fumarate-based networks forbiomedical applications. Polym Adv Technol 19(9):1199–1208. doi:10.1002/pat.1112

14. Sharifi S,Mirzadeh H, ImaniM, Rong Z, Jamshidi A, Shokrgozar M,Atai M, Roohpour N (2009) Injectable in situ forming drug deliverysystem based on poly(ε-caprolactone fumarate) for tamoxifen citratedelivery: gelation characteristics, in vitro drug release and anti-cancerevaluation. Acta Biomater 5(6):1966–1978. doi:10.1016/j.actbio.2009.02.004

15. Sharifi S, Mirzadeh H, Imani M, Atai M, Ziaee F (2008)Photopolymerization and shrinkage kinetics of in situ crosslinkableN-vinyl-pyrrolidone/poly(ε-caprolactone fumarate) networks. JBiomed Mater Res A 84A(2):545–556. doi:10.1002/jbm.a.31384

16. Kuo S-W, Chang F-C (2011) POSS related polymer nanocomposites.Prog Polym Sci 36(12):1649–1696

17. Wu J, Mather PT (2009) POSS polymers: physical properties andbiomaterials applications. Polym Rev 49(1):25–63. doi:10.1080/15583720802656237

18. Sharifi S, Shafieyan Y, Mirzadeh H, Bagheri-Khoulenjani S, RabieeSM, Imani M, Atai M, Shokrgozar MA, Hatampoor A (2011)Hydroxyapatite scaffolds infiltrated with thermally crosslinkedpolycaprolactone fumarate and polycaprolactone itaconate. JBiomed Mater Res A 98A(2):257–267. doi:10.1002/jbm.a.33108

19. Amirian M, Nabipour Chakoli A, Sui J, Cai W (2012) Enhancedshape memory effect of poly(L-lactide-co-μ-caprolactone) biode-gradable copolymer reinforced with functionalized MWCNTs. JPolym Res 19(2):1–10. doi:10.1007/s10965-011-9777-1, C7–9777

20. Pielichowska K (2012) The influence of molecular weight on theproperties of polyacetal/hydroxyapatite nanocomposites. Part 2. Invitro assessment. J Polym Res 19(2):1–10. doi:10.1007/s10965-011-9788-y, C7–9788

21. Shi Y, Huang G, LiuY, QuY, ZhangD, Dang Y (2013) Synthesis andthermal properties of novel room temperature vulcanized (RTV)silicone rubber containing POSS units in polysioxane main chains.J Polym Res 20(9):1–11. doi:10.1007/s10965-013-0245-y

22. Chen J, Loo LS, Wang K (2011) Enhanced mechanical properties ofnovel chitosan nanocomposite fibers. Carbohydr Polym 86(3):1151–1156

23. Fox DM, Lee J, Zammarano M, Katsoulis D, Eldred DV, HaverhalsLM, Trulove PC, De Long HC, Gilman JW (2012) Char-formingbehavior of nanofibrillated cellulose treated with glycidyl phenylPOSS. Carbohydr Polym 88(3):847–858

24. Goffin A-L, Duquesne E, Raquez J-M, Miltner HE, Ke X, AlexandreM, Van Tendeloo G, Van Mele B, Dubois P (2010) From polyestergrafting onto POSS nanocage by ring-opening polymerization tohigh performance polyester/POSS nanocomposites. J Mater Chem20(42):9415–9422

25. Guo Y-L, Wang W, Otaigbe JU (2010) Biocompatibility of syntheticpoly(ester urethane)/polyhedral oligomeric silsesquioxane matriceswith embryonic stem cell proliferation and differentiation. J TissueEng Regen Med 4(7):553–564. doi:10.1002/term.272

26. Silva R, Salles C, Mauler R, Oliveira R (2010) Investigation of thethermal, mechanical and morphological properties of poly(vinylchloride)/polyhedral oligomeric silsesquioxane nanocomposites.Polym Int 59(9):1221–1225. doi:10.1002/pi.2851

27. Govindaraj B, Sundararajan P, Sarojadevi M (2012) Synthesis andcharacterization of polyimide/polyhedral oligomeric silsesquioxanenanocomposites containing quinolyl moiety. Polym Int 61(8):1344–1352. doi:10.1002/pi.3195

28. Lee KS, Chang Y-W (2013) Thermal and mechanical properties ofpoly(ε-caprolactone)/polyhedral oligomeric silsesquioxane nano-composites. Polym Int 62(1):64–70. doi:10.1002/pi.4309

29. Wang W, Ding W, Yu J, Fei M, Tang J (2012) Synthesis andcharacterization of a novel POSS/PS composite via ATRP ofbranched functionalized POSS. J Polym Res 19(9):1–6. doi:10.1007/s10965-012-9948-8, C7–9948

30. Xu H, Kuo S-W, Huang C-F, Chang F-C (2002) Poly(acetoxystyrene-co-isobutylstyryl POSS) nanocomposites: characterization and molec-ular interaction. J Polym Res 9(4):239–244. doi:10.1023/a:1021303818406

31. Song X, Zhou S, Wang Y, KangW, Cheng B (2011) Mechanical andelectret properties of polypropylene unwoven fabrics reinforced withPOSS for electret filter materials. J Polym Res 19(1):1–8. doi:10.1007/s10965-011-9812-2, C7–9812

32. Gao J-g, Li S-r, Kong D-j (2011) Reaction kinetics and physical proper-ties of unsaturated polyester modified with methylacyloxylpropyl-POSS.J Polym Res 18(4):621–626. doi:10.1007/s10965-010-9456-7

33. Fei M, Jin B, WangW, Liu L (2010) Synthesis and characterization ofAB block copolymers based on polyhedral oligomeric silsesquioxane.J Polym Res 17(1):19–23. doi:10.1007/s10965-009-9285-8

34. Wang D, Fredericks PM, Haddad A, Hill DJT, Rasoul F, WhittakerAK (2011) Hydrolytic degradation of POSS–PEG–lactide hybridhydrogels. Polym Degrad Stab 96(1):123–130

35. Sheikh FA, Barakat NAM, Kim B-S, Aryal S, Khil M-S, Kim H-Y(2009) Self-assembled amphiphilic polyhedral oligosilsesquioxane(POSS) grafted poly(vinyl alcohol) (PVA) nanoparticles. Mater SciEng C 29(3):869–876

36. Bai R, Qiu T, Duan M, Ma G, He L, Li X (2012) Synthesis andcharacterization of core–shell polysilsesquioxane-poly(styrene-butylacrylate-fluorinated acrylate) hybrid latex particles. Colloids Surf APhysicochem Eng Asp 396(0):251–257

37. Bai R, Qiu T, Han F, He L, Li X (2012) Preparation and character-ization of inorganic–organic trilayer core–shell polysilsesquioxane/polyacrylate/polydimethylsiloxane hybrid latex particles. Appl SurfSci 258(19):7683–7688

38. Yaseen M, Zhao X, Freund A, Seifalian AM, Lu JR (2010) Surfacestructural conformations of fibrinogen polypeptides for improvedbiocompatibility. Biomaterials 31(14):3781–3792

39. Lahooti-Fard F, Imani M, Yousefi A (2013) Formation of vesicularstructures by a mono-tethered polyhedral oligomeric silsesquioxaneamphiphilic diacid derivative in a solvent mixture. J Iran Chem Soc10(2):229–236. doi:10.1007/s13738-012-0145-9

40. Fu BX, Hsiao BS, White H, Rafailovich M, Mather PT, Jeon HG,Phillips S, Lichtenhan J, Schwab J (2000) Nanoscale reinforcementof polyhedral oligomeric silsesquioxane (POSS) in polyurethaneelastomer. Polym Int 49(5):437–440. doi:10.1002/(sici)1097-0126(200005)49:5<437::aid-pi239>3.0.co;2-1

41 . Madhavan K, Gnanaseka r an D, Reddy BR (2011 )Poly(dimethylsiloxane-urethane) membranes: effect of linear silox-ane chain and caged silsesquioxane on gas transport properties. JPolym Res 18(6):1851–1861. doi:10.1007/s10965-011-9592-8

42. Zhang X, Yan C, Fang S, Zhang C, Jia T, Zhang Y (2010) Awater-soluble organic–inorganic hybrid material based on polyhedral

297, Page 12 of 13 J Polym Res (2013) 20:297

Page 13: In situ photocrosslinkable nanohybrids based on poly(ε-caprolactone fumarate)/polyhedral oligomeric silsesquioxane: synthesis and characterization

oligomeric silsesquioxane and polyvinyl alcohol. J Polym Res 17(5):631–638. doi:10.1007/s10965-009-9351-2

43. Farahmand Ghavi F, Mirzadeh H, Imani M, Jolly C, Farhadi M(2010) Corticosteroid-releasing cochlear implant: a novel hybrid ofbiomaterial and drug delivery system. J Biomed Mater Res B ApplBiomater 94(2):388–398

44. Latifi A, Imani M, Khorasani MT, Joupari MD (2013)Electrochemical and chemical methods for improving surface char-acteristics of 316L stainless steel for biomedical applications. SurfCoat Technol 221(0):1–12. doi:10.1016/j.surfcoat.2013.01.020

45. Jenkins MJ, Harrison KL (2006) The effect of molecular weight onthe crystallization kinetics of polycaprolactone. Polym Adv Technol17(6):474–478. doi:10.1002/pat.733

46. Sen M, Yakar A, Güven O (1999) Determination of average molec-ular weight between cross-links (Mc) from swelling behaviours ofdiprotic acid-containing hydrogels. Polymer 40(11):2969–2974. doi:10.1016/S0032-3861(98)00251-1

47. Mirmohammadi SA, Imani M, Uyama H, Atai M, Teimouri MB,Bahri-Lale N. The effects of solvent and initiator on anionic ringopening polymerization of -caprolactone: synthesis and characteri-zation. Polym Int:n/a–n/a. doi:10.1002/pi.4531

48. Mirmohammadi SA, Imani M, Uyama H, Atai M (‘Jast Accepted’)Hybrid organic–inorganic nanocomposites based on poly(ε-caprolactone)/polyhedral oligomeric silsesquioxane: synthesis andin vitro evaluations. Int J Polym Mater Polym Biomater

49. Hashemi Doulabi AS, Sharifi S, Mirzadeh H, Imani M, Atai M,Mehdipour-Ataei S (2008) Erratum: synthesis and preparation ofbiodegradable and visible light crosslinkable unsaturated fumarate-based networks for biomedical applications. Polym Adv Technol19(9):2–2. doi:10.1002/pat.1240

50. Lee KM, Knight PT, Chung T, Mather PT (2008) Polycaprolactone–POSS chemical/physical double networks. Macromolecules 41(13):4730–4738. doi:10.1021/ma800586b

51. Zheng Y, Wang L, Zheng S (2012) Synthesis and characterization ofheptaphenyl polyhedral oligomeric silsesquioxane-capped poly(N-

isopropylacrylamide)s. Eur Polym J 48(5):945–955. doi:10.1016/j.eurpolymj.2012.03.007

52. Peponi L, Navarro-Baena I, Báez JE, Kenny JM, Marcos-FernándezA (2012) Effect of the molecular weight on the crystallinity of PCL-b-PLLA di-block copolymers. Polymer 53(21):4561–4568

53. Chen X, Hou G, Chen Y, Yang K, Dong Y, Zhou H (2007) Effect ofmolecular weight on crystallization, melting behavior and morphol-ogy of poly(trimethylene terephalate). Polym Test 26(2):144–153

54. Luo S, Grubb DT, Netravali AN (2002) The effect of molecularweight on the lamellar structure, thermal and mechanical propertiesof poly(hydroxybutyrate-co-hydroxyvalerates). Polymer 43(15):4159–4166

55. Grosvenor MP, Staniforth JN (1996) The effect of molecular weighton the rheological and tensile properties of poly( -caprolactone). Int JPharm 135(1–2):103–109

56. Koleske JV, Lundberg RD (1969) Lactone polymers. I. Glass transi-tion temperature of poly-ε-caprolactone by means on compatiblepolymer mixtures. J Polym Sci A 2 Polym Chem 7(5):795–807.doi:10.1002/pol.1969.160070505

57. Mizuno A, Mitsuiki M, Motoki M (1998) Effect of crystallinity onthe glass transition temperature of starch. J Agric Food Chem 46(1):98–103. doi:10.1021/jf970612b

58. Willbourn AH (1958) The glass transition in polymers with the(CH2) group. Trans Faraday Soc 54(0):717–729. doi:10.1039/tf9585400717

59. Bordes C, Fréville V, Ruffin E, Marote P, Gauvrit JY, Briançon S,Lantéri P (2010) Determination of poly(ɛ-caprolactone) solubilityparameters: application to solvent substitution in a microencapsula-tion process. Int J Pharm 383(1–2):236–243. doi:10.1016/j.ijpharm.2009.09.023

60. Wang S, Yaszemski MJ, Gruetzmacher JA, Lu L (2008)Photo-crosslinked poly(ɛ-caprolactone fumarate) networks:roles of crystallinity and crosslinking density in determiningmechanical properties. Polymer 49(26):5692–5699. doi:10.1016/j.polymer.2008.10.021

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