melting and crystallization behavior of polyhedral oligomeric silsesquioxane-capped...

Melting and Crystallization Behavior of PolyhedralOligomeric Silsesquioxane-Capped Poly(e-caprolactone)

YONG NI, SIXUN ZHENG

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of China

Received 11 January 2007; accepted 10 April 2007DOI: 10.1002/polb.21222Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: In this study, we investigated the melting and crystallization behavior ofpolyhedral oligomeric silsesquioxane (POSS)-capped poly(e-caprolactone) PCL withvarious lengths of PCL chains by means of X-ray diffraction and differential scanningcalorimetry. This organic–inorganic macromolecule possesses a tadpole-like structurein which the bulky POSS cage is the ‘‘head’’ whereas PCL chain the ‘‘tail’’. The novelorganic–inorganic association result in the significant alterations in the melting andcrystallization behavior of PCL. The POSS-terminated PCL displayed the enhancedequilibrium melting points compared to the control PCL. Both the overall crystalliza-tion rate and the spherulitic growth rate of the POSS-terminated PCLs increasedwith increasing the concentration of POSS (or with decreasing length of PCL chainin the hybrids). The analysis of Avrami equation shows that the crystallization of thePOSS-terminated PCL preferentially followed the mechanism of spherulitic growthwith instantaneous nuclei. It is found that the folding free energy of surface for thePOSS-terminated PCLs decreased with increasing the concentration of POSS. It isfound that the folding free energy of surface for the POSS-terminated PCLsdecreased with increasing the concentration of POSS. VVC 2007 Wiley Periodicals, Inc.

J Polym Sci Part B: Polym Phys 45: 2201–2214, 2007

Keywords: amphiphiles; composites; crystal structures; crystallization; differentialscanning calorimetry; melting point; polyesters; ring-opening polymerization; poly(e-caprolactone); polyhedral oligomeric silsesquioxanes

INTRODUCTION

Incorporating inorganic or organometallic seg-ments to organic polymers to afford organic–inorganic hybrid materials continues to be adriving force for the development of new materi-als.1–6 The favorable combination of propertiesresulting from the synergism between organicand inorganic components has led in recentyears to an increased effort of research on struc-ture and properties of this class of hybrid mate-rials. Polyhedral oligomeric silsesquioxanes

(POSS) are a class of versatile building blocksfor preparing inorganic–organic hybrid materi-als with designed properties.7–18 A typical POSSmolecule possesses the structure of cube-octa-meric frameworks represented by the formula(R8Si8O12) with an inorganic silica-like core(Si8O12) (� 0.53 nm in diameter) surrounded byeight organic corner groups, one or more ofwhich is reactive (Scheme 1). The polymers con-taining POSS are becoming the focus of manystudies because of their excellent comprehensiveproperties. POSS molecules can be incorporatedinto organic polymers via physical blending19–22

and chemical reaction.7–18 During the past year,there have been extensive studies on the modi-fication of POSS on thermal and mechanical

Correspondence to: S. Zheng (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 2201–2214 (2007)VVC 2007 Wiley Periodicals, Inc.

2201

properties of organic polymers. Nonetheless, theeffects of POSS on crystallization behavior ofpolymers were only occasionally involved.21,22

Hsiao and coworkers21 and Joshi and Butola 22

reported that the physical inclusion of a smallamount of POSS has profound effects on crystal-lization behavior of polypropylene (PP) and poly-ethylene (PE). It was identified that the inclu-sion of a small amount of POSS can significantlyaccelerate completion of crystallization processsince the POSS acted as the nucleating agentsof PP and PE. Nonetheless, the influence ofchemically-included POSS on melting and crys-tallization behavior of polymers remains largelyunexplored although chemical reactions havebeen proved to be efficient to promote the inter-component interactions between POSS and poly-mer chains. In the previous work,23 we investi-gated the synthesis and crystallization behaviorof an octa-armed star-shaped PCL with POSScore. In this work, we designed and synthesizeda tadpole-like polymer in which the bulky POSScage is the ‘‘head’’ whereas PCL chain the ‘‘tail’’,to investigate the melting and crystallizationbehavior of a POSS-capped PCL. The POSS-ter-minated polymers are a class of novel amphi-philic macromolecules because of the presence oforganic polymer chain and inorganic portion(viz. silsesquioxane cage). The formation of thenovel organic–inorganic macromolecular topol-ogy could lead to significant alteration in struc-ture and properties of polymers.

Poly(e-caprolactone) (PCL) is an important ali-phatic polyester. Owing to its degradability andbiocompatibility, this polymer represents aninteresting candidate for environmentally benignpacking materials and for biomedical applica-tions.24–29 The PCL-terminated POSS is a novelorganic–inorganic amphiphilic macromolecule,which could display the structural feature ofblock copolymers. On the one hand, the presence

of bulky POSS cages at one chain end constitutesa confinement to crystallization of PCL chains.On the other hand, the terminal POSS cagescould act as the nucleating agent of crystalliza-tion for PCL. The converse trends motivate toinvestigate the melting and crystallization be-havior of the PCL-POSS amphiphile. To the bestof our knowledge, there are no precedent studieson crystallization of POSS-capped PCL. It shouldbe pointed out that the topology of the POSS-capped PCL is quite different from that of theocta-armed star PCL with POSS core reportedpreviously,23 which allows the investigation ofthe effect of the bulky terminal groups (viz.POSS) on the melting and crystallization behav-ior of PCL. To this end, a 3-hydroxypropylhepta-phenyl POSS [(C6H5)7(HOC3H6)Si8O12] was usedas the initiator to prepare a series of POSS-ter-minated PCL with various molecular weights viaring-opening polymerization (ROP) catalyzedwith stannous (II) octanoate [Sn(Oct)2]. The crys-tallization and melting behavior of the hybridPCLs was addressed on the basis of differentialscanning calorimetry (DSC).

EXPERIMENTAL

Materials

Phenyltrimethoxysilane [C6H5Si(OCH3)3, 98%]was supplied by Zhejiang Chem-Tech, Zhejiang,China. 3-Chloropropyltrichlorosilane [ClC3H6

SiCl3, 97%] was purchased from Gelest, USAand used as received. The monomer, e-caprolac-tone (CL) was purchased from Fluka, Germanyand it was distilled over CaH2 under decreasedpressure prior to use. A control PCL wasobtained from Solvay Chemical, UK and it has aquoted molecular weight of Mn ¼ 50,000 andMw ¼ 65,000. Silver nitrate (AgNO3), sodiumhydroxide (NaOH), and stannous (II) octanoate[Sn(Oct)2] are of analytically pure grade andwere purchased from Shanghai Reagent, China.The solvents such as tetrahydrofuran (THF),dichloromethane (CH2Cl2), ethanol, petroleumether (distillation range: 60–90 8C) were obtainedfrom Shanghai Reagent, Shanghai, China. Beforeuse, THF was dried by distillation over Na andstored in a sealed vessel with the molecularsieve of 4 A.

3-Hydroxypropylheptaphenyl POSS was usedas the initiator of the ROP of CL to prepare thePOSS-capped PCL. The preparation of 3-hydroxy-

Scheme 1. Structure of POSS.

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Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

propylheptaphenyl POSS was detailed in theprevious work,30 which is briefly outlined here(Scheme 2). In the synthesis, phenyltrimethoxy-silane [C6H5Si(OCH3)3] was used as a startingcompound, the hydrolysis of which in thepresence of NaOH afforded heptaphenyltricyclo-heptasiloxane trisodium silanolate [Na3O12Si7(C6H5)7]. Na3O12Si7(C6H5)7 was employed to pre-pare 3-chloropropylheptaphenyl POSS [(ClCH2

CH2CH2)(C6H5)7Si8O12] via a corner-cappingreaction with 3-chloropropyltrichlorosilane. Toobtain 3-hydroxypropylheptaphenyl POSS, a nu-cleophilic substitution of chlorine atom in 3-chloropropylheptaphenyl POSS by a hydroxylwas carried out in the presence of fresh silveroxide (Ag2O).

FTIR (cm�1, KBr window): 3075, 1596, 1430,1135 � 1090 (Si��Ph), 1090 � 1000 (Si��O��Si),2956 (��CH3), 2934 (��CH2), 3440 (O��H). 1HNMR (ppm, CDCl3): 7.72 � 7.79 (m, 14.2H, pro-tons of aromatic rings); 7.33 � 7.51 (m, 21.2H,protons of aromatic rings); 3.52 (t, SiCH2CH2-

CH2OH, 2.0H); 1.99 (m, SiCH2CH2CH2OH,2.0H); 1.00 (m, SiCH2CH2CH2OH, 2.0H). 13CNMR (ppm, CDCl3): 9.75 (SiCH2CH2CH2OH);26.53 (SiCH2CH2CH2OH); 47.41 (SiCH2CH2-

CH2OH); 128.12, 128.20, 130.38, 130.50, 131.06,131.14, 134.38, 134.44 (carbons of aromaticrings). 29Si NMR (ppm): �64.06, �77.02.

Preparation of POSS-TerminatedPoly(e-caprolactone)

The ROP of CL catalyzed by stannous (II) octa-noate [Sn(Oct)2] was employed to prepare thePCL terminated with POSS using 3-hydroxypro-pylheptaphenyl POSS as the initiator. The ROPwas carried out to synthesize the POSS-termi-nated PCLs on a standard Schlenk line system.In a typical experiment, to a predried flask con-nected to the Schlenk line 3-hydroxypropylhep-taphenyl POSS [(C6H5)7(HOCH2CH2CH2)Si8O12](2.00 g, 1.97 mmol), CL (2.92 g, 25.64 mmol)were charged and a catalytic amount of Sn(Oct)2

was added. The reactive mixture was degassedvia three pump-thaw-freeze cycles and thenimmersed in a thermostated oil bath at 110 8Cfor 24 h. The crude products were dissolvedwith CH2Cl2 and precipitated using an excessiveamount of petroleum ether. This procedure wasrepeated for three times to remove any unreactedmonomer. The precipitates were filtered anddried in vacuo at 40 8C for 48 h to give 4.797 gpolymer with a yield of 97.5 wt %.

1H NMR (ppm, CDCl3): 7.72 � 7.79 (m, protonsof aromatic rings); 7.33 � 7.51 (m, protons of ar-omatic rings); 4.12 (t, SiCH2CH2CH2��PCL);1.99 (m, SiCH2CH2CH2��PCL); 1.00 (t, SiCH2

CH2CH2��PCL); 4.02 � 4.10 [t, ��OCO(CH2)4

CH2]; 3.61�3.68 (m, CH2OH, terminal hydroxy-

Scheme 2. Synthesis of POSS-terminated PCL.

MELTING AND CRYSTALLIZATION OF POSS-CAPPED PCL 2203


methylene group); 2.23 � 2.40 [t, ��OCOCH2

(CH2)4]; 1.54 � 1.70 [m, ��OCOCH2CH2CH2CH2

CH2]; 1.30 � 1.46, [m, ��OCOCH2CH2CH2CH2

CH2].

Measurement and Techniques

Nuclear Magnetic Resonance Spectroscopy

The 1H NMR, 13C NMR measurements werecarried out on a Varian Mercury Plus 400 MHzNMR spectrometer at 25 8C. The samples weredissolved with deuterated CDCl3 and the solu-tions were measured with tetramethylsilane(TMS) as an internal reference.

Fourier Transform Infrared Spectroscopy

The Fourier Transform infrared spectroscopy(FTIR) measurements were conducted on a Per-kinElmer Paragon 1000 Fourier transform spec-trometer at room temperature (25 8C). The sam-ple films were prepared by dissolving the poly-mers with CHCl3 (5 wt %) and the solutionswere cast onto KBr windows. The residual sol-vent was removed in a vacuum oven at 60 8C for2 h. All the specimens were sufficiently thin tobe within a range where the Beer–Lambert lawis obeyed. In all cases 64 scans at a resolution of2 cm�1 were used to record the spectra.

Gel Permeation Chromatography

The molecular weights of polymers were meas-ured on a PerkinElmer 200 gel permeation chro-matography (GPC) instrument with a PL mixed-B10m column. Polystyrene was used as thestandard, and THF was used as the eluent at aflow rate of 1 mL/min.

Wide Angle X-ray Diffraction

The wide-angle X-ray diffraction (XRD) experi-ments were carried out on a Shimadzu XRD-6000 X-ray diffractometer with CuKa (k ¼ 0.154nm) irradiation at 40 kV and 30 mA using a Nifilter. Data were recorded in the range of 2h¼ 5–408 at the scanning rate and the step sizeof 4.08/min and 0.028, respectively.

Differential Scanning Calorimetry

The calorimetric measurements were performedon a PerkinElmer Pyris 1 DSC in a dry nitrogenatmosphere. The instrument was calibrated

with standard Indium. The samples were firstlyheated up to 100 8C at the heating rate of 20 8C/min and were hold at this temperature for5 min to melt the crystals, followed by quench-ing to �70 8C. The second heating scans werecarried out from �70 to 100 8C at the heatingrate of 20 8C/min and the thermograms wererecorded. After that, the samples were cooledfrom 100 to �70 8C at the cooling rate of 10 8C/min and the cooling DSC curves were recorded.Crystallization temperatures (Tc) and meltingtemperatures (Tm) were taken as the tempera-tures at minimum and maximum of both endo-thermic and exothermic peaks, respectively.

Equilibrium melting points were measured bymeans of DSC. The samples were hold at 100 8Cfor 3 min to erase the thermal history, and thenquenched to the desired temperature for isother-mal crystallization toward completion. To mea-sure the melting temperatures (Tm’s) of the sam-ples, the crystallized samples were heated at aheating rate of 10 8C/min and the Tm’s weretaken as the temperatures at which the crystalswere totally molten. The overall crystallizationkinetics from the melts was analyzed by DSC.The samples were molten at 100 8C for 5 min toremove the thermal history, and then quenchedto the desired temperatures at which the iso-thermal crystallization was allowed to carry outto completion, recording the isothermal thermo-graphs. The conversion of crystallization, X(t),as a function of crystallization time (t) wasdetermined using the following equation:

XðtÞ ¼Zt

0

dH

dt

8>: 9>;dt

,Z10

dH

dt

8>: 9>;dt ð1Þ

where the integral in the numerator is the en-thalpy generated at time t and the integral inthe denominator is the total enthalpy of crystal-lization at t ¼ ?. The observed melting temper-ature (Tm) of the isothermally crystallized sam-ples were measured by heating the samplesfrom Tc up to 100 8C at a heating rate of 10 8C/min and the Tm’s were taken as the tempera-tures at which the crystals were totally molten.

RESULTS AND DISCUSSION

Synthesis and Characterization of Polymers

The synthetic route of the organic–inorganicPCL end-capped with POSS is described in

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Scheme 2. 3-Hydroxypropylheptaphenyl POSS[(HOCH2CH2CH2)(C6H5)7Si8O12]30 was used asthe initiator and the ROP of CL was catalyzedby stannous (II) octanoate [Sn(Oct)2] and wascarried out at 110 8C for 24 h. By controlling themolar ratios of 3-hydroxypropylheptaphenyl

POSS to CL, the POSS-terminated PCL sampleswith various molecular weights were obtained.Figure 1 shows the FTIR spectra of 3-hydroxy-propylheptaphenyl POSS and POSSPCL1. 3-Hydroxypropylheptaphenyl POSS is character-ized by the bands at 1134 and 3500 cm�1,respectively. The band at 1134 cm�1 is ascribedto the stretching vibration of Si��O��Si bonds inPOSS groups whereas the bands at about 3500cm�1 to the stretching vibration of hydroxylgroups of the POSS. The hydroxyl bands arecomposed of two components at 3640 and 3444cm�1, respectively. The former is assigned tofree hydroxyl group whereas the latter to theself-associated hydroxyl groups. For POSSPCL1,it is seen that the stretching vibration band ofhydroxyl groups in the range of 3200–3500 cm�1

is too weak to be detected because of the forma-tion of long PCL chain. The appearance of car-bonyl stretching vibration band at 1725 cm�1 to-gether with the detection of Si��O��Si band inthe range of 1090–1000 cm�1 indicate that thesyntheses of POSS-terminated PCLs were suc-cessfully obtained. Shown in Figure 2 are the1H NMR spectra of 3-hydroxypropylheptaphenylPOSS, 3-trimethylsiloxyl propylheptaphenylPOSS and POSSPCL1 respectively. The 3-trime-thylsiloxyl propylheptaphenyl POSS was pre-pared via the reaction between 3-trimethylsi-loxyl propylheptaphenyl POSS and methyltri-

Figure 1. The FTIR spectra: (A) 3-hydroxypropyl-heptaphenyl POSS; (B) POSSPCL1.

Figure 2. The 1H NMR spectra: (A) 3-hydroxypropylheptaphenyl POSS, (B) 3-tri-methylsiloxyl propylheptaphenyl POSS, and (C) POSSPCL1.



chlorosilane,30 to confirm the preparation of 3-trimethylsiloxyl propylheptaphenyl POSS. Interm of the ratio of the integration intensity ofthe methyl proton at 0.09 ppm to that of aro-matic ring proton, it is judged that 3-trimethyl-siloxyl propylheptaphenyl POSS was success-fully obtained. For POSSPCL1, the resonance of3-hydroxypropylheptaphenyl POSS was discerni-ble in the 1H NMR spectrum except for the reso-nance assignable to the proton of PCL chain,suggesting that this sample combines the struc-tural features of 3-hydroxypropylheptaphenylPOSS and PCL. The as-prepared polymers weresubjected to GPC measurements. The results ofpolymerization are summarized in Table 1 andthe plots of molecular weights and molecularweight distribution as functions of the molar ra-tio of CL to 3-hydroxypropylheptaphenyl POSSwere presented in Figure 3. It is seen thatunder the present condition of polymerization,the conversion of the monomer was performedto completion, suggesting that 3-hydroxypropyl-heptaphenyl POSS is an efficient initiator. Theobservation that the experimental molecularweights were in a good agreement with the the-oretical values calculated according to the molarratios of 3-hydroxypropylheptaphenyl POSS toCL indicates that the molecular weight of thePOSS-ended PCL can be controlled, that is thepolymerization is in a living fashion.

The wide-angle X-ray diffraction (WAXRD)measurements between 58 and 408 were carriedout to examine the crystalline structure of thePOSS-terminated PCL samples. Representa-

tively shown in Figure 4 is the diffraction pat-tern of POSSPCL1. For comparison, the XRDprofile of control PCL was also incorporated inthis figure. The diffraction peak positions of con-trol PCL agree well with the values reported inliterature with unit cell (orthorhombic) parame-ters to be a ¼ 7.47 A, b ¼ 4.98 A, c ¼ 17.05 A.31

The intense diffraction peaks located at 2h¼ 21.398, 22.008, 23.708, and 43.998 wasassigned to the (110), (111), and (200) reflectionsof PCL.32 For POSSPCL1, the diffraction peaksare found to be 2h ¼ 7.288, 21.408, 22.038, and

Table 1. Results of Polymerization for POSS-EndPCL with Different Molecular Weights

Entry [M]/[I]a Mn Mn,theob Mw/Mn

c Yield (%)

POSSPCL1 13/1 2490d 2430 – 97.5POSSPCL2 27/1 3860d 3970 – 97.1POSSPCL3 80/1 9880d 9760 – 96.3POSSPCL4 135/1 16400c 15880 1.30 96.8POSSPCL5 350/1 40940c 39440 1.24 96.4

a M ¼ CL, I ¼ initiator (viz. 3-hydroxypropylheptaphenylPOSS).

b Mn,theo ¼ ([M]/[I] 3 MCL 1 MI) 3 Yield, Mn,theo denotesthe theoretical number–average molecular weight of PCL.

c Weight-average molecular weight (Mw) and number-av-erage molecular weight (Mn) are determined by GPC.

d Mn,NMR was determined on the basis of the integral ra-tio of the methylene signal on the polymer backbone (��CH2,2.23 � 2.40 ppm) and the signal on the primary hydroxymethylene end group (HOCH2, 3.61 � 3.68 ppm).

Figure 3. The plots of molecular weight and molec-ular weight distribution as functions of molar ratio ofCL to 3-hydroxypropylheptaphenyl POSS. The opensymbols represent the data of molecular weight deter-mined by 1H NMR spectroscopy whereas the filledsymbols represent the data determined by GPC.

Figure 4. The XRD curves: (A) Control PCL; (B)POSSPCL1.

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23.758, respectively, which are close to those ofthe control PCL. However, careful comparisonshows that the diffraction peaks of the POSS-terminated PCL were significantly shifted to thehigher values of 2h. This observation suggeststhat the crystal structure of the POSS-termi-nated PCL were changed to some extent. It isplausible to propose that during crystallizationthe bulky POSS blocks cannot be rejected fromthe front of crystal growths and the presence ofthe bulky POSS cages could alter the structureof PCL crystals. It should be pointed out thatthere was a new diffraction peaks at 2h ¼ 7.288for the POSS-terminated PCL. The new diffrac-tion peak could be related to the POSS blocks.Because of the immiscibility between PCLchains and POSS portion, the terminal POSSblocks could be self-assembled into microdo-mains, which could display the similar diffrac-tion profile as octaphenyl POSS.33 It should bepointed out that the XRD curves (not shown forbrevity) of the POSS-terminated PCL with thehigher molecular weights (e.g., POSSPCL4 andPOSSPCL5) have been close to that of the con-trol PCL.

All the POSS-terminated PCL samples weresubjected to the DSC experiments to investigatethe melting behavior of the organic–inorganicPCL. The samples were heated up to 100 8C tofuse the crystals and then quenched to 70 8C forthe second DSC scans. The heating DSC ther-mograms of the control and POSS-terminatedPCL are shown in Figure 5. It is seen that allthe POSS-terminated PCL samples displayedthe melting transitions at about 55 8C, whichare lower than that of the control PCL exceptfor POSSPCL5. It is noted that the melting tem-peratures decreased with increasing the percent-age of POSS in the samples. In another word,the melting temperatures of POSS-terminatedPCL increased with increasing the length ofPCL chains. The melting temperature of thesample with the highest molecular weight (i.e.Mn ¼ 40,000) is quite close to that of controlPCL. In addition the dual melting transitionswere observed for the quenched samples of thePOSS-terminated PCL, at the commonly usedheating rate, 20 8C/min (see Fig. 5). The dualmelting transition of PCL is attributed to melt-ing of the initial crystals followed by recrystalli-zation and final melting of the crystals grownduring the heating scan.34 The fact that thedual melting transitions for the POSS-termi-nated PCL samples occurred even during the

fast heating process implies that the crystalliza-tion rearrangement was slowed down because ofthe presence of POSS portion in the amphiphilicpolymers. From Figure 5, the overall crystallin-ity of the POSS-terminated PCL samples can becalculated from the heating DSC curves usingthe following equation:

Xc ¼DHf

DH0f

3 100% ð2Þ

where DHf is the fusion enthalpy of the re-scanned samples after quenching from the meltsand DHf

0 is the fusion enthalpy of the perfectlycrystallized PCL and equals 136 J/g.35 Figure 6shows the plots of the crystallinity of PCL chainin the POSS-terminated PCL as functions of mo-lecular weights. It is of interest to note that forthe POSS-terminated PCL with the shorter PCLchains (viz. POSSPCL1 through POSSPCL4),the crystallinity of PCL chains was significantlyhigher than that of control PCL. While the chainof PCL is sufficiently long (viz. POSSPCL5), thecrystallinity of the POSS-terminated PCL is closeto that of the control PCL. This observation indi-cates that the crystallinity of PCL chains wassignificantly affected by the presence of POSSterminal groups.

Figure 5. The heating DSC thermograms of controland POSS-terminated PCL with variable molecularweights (second scans). (A) control PCL, (B) POSSPCL5,(C) POSSPCL4, (D) POSSPCL3, (E) POSSPCL2, and(F) POSSPCL1.



The nonisothermal crystallization of thePOSS-terminated PCL can be investigated bymeans of cooling DSC scans. Shown in Figure 7are the cooling DSC curves of the control andPOSS-terminated PCL samples. It is noted thatthe crystallization temperatures (Tc’s) of thePOSS-terminated PCL samples with fairly highmolecular weights (e.g., POSSPCL3, POSSPCL4,and POSSPCL5) are much higher than that ofthe control PCL whereas the samples with thelower molecular weight exhibited the valueslower than that of control PCL. It is noted thatthe Tc’s decreased with increasing the percent-age of POSS in the samples. The enhanced crys-tallization temperatures indicate that the crys-tallization of PCL chains in the POSS-contain-ing samples is faster than in the control PCL. Itis proposed that there could be two competitivefactors to affect the crystallization behavior ofPCL chains depending on the length of PCLchains (or POSS contents). The first one isrelated to the macromolecular topological effect.For the POSS-terminated PCLs, one of crystal-line PCL chain ends was covalently bonded withthe nanosized cubic silsesquioxane cage throughan ester linkage. The presence of bulky POSSportion could constitute an obstacle to crystalli-zation arrangement, which has been evidencedby the appearance of the dual melting transitionin the heating DSC scan, and thus reduce therate of crystallization.21 The second effect could

be related to the heterogeneous nucleating effectof POSS portion. It has been shown that for theocta-armed star-shaped PCL with POSS core,the inorganic octameric cubic POSS core couldact as the heterogeneous nucleating agent toaccelerate the crystallization of PCL. The firstfactor could dominates with the samples withthe lower molecular weights, which was evi-denced the depression in melting points in termsof the heating DSC scans. The second factor isimportant to affect the crystallization of thePOSS-terminated PCL samples with the longerPCL chain if the effect of heterogeneous nucleat-ing is also applicable to the POSS-terminatedPCL. It is necessary to know the mechanism ofnucleating by investigating the crystallizationkinetics of crystallization.

The equilibrium melting points of the controlPCL and the POSS-terminated PCL were deter-mined by means of DSC. Shown in Figure 8 arethe plots of T0

m as a function of crystallizationtemperature (Tc) for the control PCL and thePOSS-end PCLs in a wide range of undercooling.The equilibrium melting points (Tm

o ) are deter-mined by extrapolation to the lines of Tm � Tc

according to the Hoffman-Weeks equation:36,37

T0m ¼ UTc þ ð1 � UÞT0

m ð3Þ

Figure 6. The plot of crystallinity of POSS-termi-nated PCL as a function of molecular weight.

Figure 7. The cooling DSC thermograms of the lin-ear and POSS-terminated PCL at the rate of �10 8C/min. (A) control PCL, (B) POSSPCL5, (C) POSSPCL4,(D) POSSPCL3, (E) POSSPCL2, and (F) POSSPCL1.

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where F is a stability parameter and Tm the appa-rent melting temperature. The value of F isbetween 0 (Tm ¼ Tm

o for all Tc, in the case of moststable crystal) and 1 (Tm ¼ Tc in the case of inher-ently unstable crystal). The F parameters can beobtained from the slope of the straight linesand the values of Tm

o were determined by extra-polating the least-squares fit lines of the experi-mental data according to eq 3 to intersect the lineof Tm ¼ Tc as shown in Figure 8. The Tm

o togetherwith F parameters are summarized in Table 2.The value of F was 0.177 for the control PCLwhereas the values of 0.356 � 0.387 were obtainedfor these POSS-terminated PCLs, implying thatthe crystals of POSS-terminated PCL are less sta-ble than that of the control PCL. In terms of thestability parameter, it is plausible to propose thatthe presence of bulky POSS at chain endsdecreased the stability of the PCL crystals. Therecould be two factors to affect the stability of crys-tals. On the one hand, the chain length of PCLfractions in the POSS-terminated PCL is shorterthan that of the control PCL and the short PCLchains could be difficult to develop stable crystals.On the other hand, the bulky POSS affect thestructure of PCL crystals as shown by XRDresults. The effect is more pronounced for thePOSS-terminated PCL with shorter PCL chains.Nonetheless, it is accepted that the PCL crystalsin the POSS-containing polymers are still fairly

stable. It is noted that except for the sample withshortest PCL chains (viz. POSSPCL1) all otherPOSS-terminated PCLs displayed the enhancedequilibrium melting points (Tm

o ) compared to thecontrol PCL. The values of Tm

o were increasedwith increasing the length of PCL chains. Themelting behavior of the POSS-terminated PCLs isin a marked contrast to that of cage-like fullerene(C60)-terminated poly(ethylene oxide).38 Theenhancement of equilibrium melting points couldbe related to the changes of crystal structureresulting from the presence of POSS terminalgroups. In addition, the increased Tm

o for thePOSS-terminated PCLs could be ascribed to thepositional restriction imposed on the PCL chainsby POSS portion, which reduces the melt entropyof PCL portion with respect to their linear ana-logues. It should be pointed out that the decreasedmelting points for the samples (e.g., POSSPCL1)could be dominantly ascribed to its shortest PCLchain (c.a. Mn ¼ 1500).

Isothermal Crystallization Kinetics

Analysis of Avrami Equation

The conversion of crystallization as functions oftime was representatively shown in Figure 9 forPOSSPCL3. It can be seen that with increasingcrystallization temperature, the characteristicsigmoid X(t) � t curves are significantly shiftedright side along the time axis, suggesting thatcrystallization processes become progressivelyslow. Therefore, nucleation is a rate-controllingstep under the present conditions of crystalliza-tion. From the curves, the times of half-crystalli-zation t1/2, defined as the time required for halfthe crystallinity to develop were obtained. Theplots of t1/2 as functions of Tc for various sam-ples were shown in Figure 10. For the sam-ples with longer PCL chains, it is noted that ata given Tc, the overall crystallization rates

Figure 8. Hoffman–Weeks plots for the determina-tion of equilibrium melting points of control PCL andPOSS-terminated PCL.

Table 2. Equilibrium Melting Points (Tmo ) and

Stability Parameters (F)

Samples Tmo (8C) F

POSSPCL1 59.05 0.361POSSPCL2 64.13 0.358POSSPCL3 63.24 0.387POSSPCL4 66.13 0.356POSSPCL5 67.05 0.360Linear PCL 61.95 0.177



decreased with increasing the length of PCLchains. In another word, the overall crystalliza-tion rates increased with the percentage ofPOSS in the samples. This result could berelated to the change in nucleation rate (ormechanism) for the POSS-terminated PCL sam-ples that is the POSS portion could serve as aheterogeneous nucleating agent to accelerate

the rate of nucleation. Nonetheless, it is seenthat the samples the shorter PCL chains (viz.POSSPCL1, POSSPCL2) displayed lower overallrates of crystallization than the samples withthe longer PCL chains (viz. POSSPCL3,POSSPCL4, POSSPCL5) at given Tc. This obser-vation could be interpreted on the basis of thefollowing two factors. On the one hand, theshort chains are difficult to develop the crystalsof PCL. On the other hand, the restriction of thebulky POSS cage on the crystallization of PCLis significantly pronounced in these samplesalthough the POSS end groups could act as theheterogeneous nucleating agent to promote thecrystallization of PCL.

The kinetics of isothermal crystallization ofthe POSS-terminated PCL was analyzed usingAvrami equation39:

Xt ¼ 1 � expð�Kn tnÞ ð4Þ

where Kn is the overall kinetic rate constantand n is Avrami index that depends on the typeof nucleation and on the geometry of growingcrystals. The values of Kn and n were obtainedfrom the intercept and the slope of the linearplots of log [�ln(1 � X(t))] as a function of log t(Fig. 11). The experimental results are well coin-cident with the linear relation for the early partof transformation. Generally, Avrami equation isusually only valid at low conversion of crystalli-

Figure 10. The plots of t1/2 as functions of Tc forvarious samples.

Figure 11. The linear plots of log [�ln(1 � X(t))] asa function of log t.

Figure 9. The representative plots of the conversionof crystallization as functions of time for a POSS-endPCL (viz. POSSPCL3).

2210 NI AND ZHENG


zation as long as the impingement is not seri-ous.34,40,41 The values of Kn, n, t1/2, and T 0

m aresummarized in Table 3. Theoretically, the valueof n ¼ 4 can result from spherulitic grow fromsporadic nuclei whereas n ¼ 3 can be attachedto the spherulitic growth of instantaneousnuclei. For the control PCL, the n valuesobtained were in the vicinity of 4.0, suggesting athree-dimensional spherulitic growth of sporadicnuclei. For the POSS-terminated PCL, it isnoted that the Avrami indices were reduced toabout three. This observation suggests that thecrystallization process may be the spheruliticgrowth of instantaneous nuclei. It is of interestto note that the result of the POSS-terminatedPCL is different from that obtained from theocta-armed star-shaped PCL with POSS core.23

It is plausible to propose that this result isrelated to the topological structure of the POSS-containing PCL. In the system of octa-armed

star-shaped PCL with POSS core, the POSScages were homogenously dispersed into the ma-trix of PCL as the centers of star-shaped struc-tures and the effect of heterogeneous nucleatingcould dominate the process of crystallization. Incontrast, the POSS-terminated PCL possesses atadpole-like structure in which the bulky POSScage is the ‘‘head’’ whereas PCL chain the ‘‘tail’’.The organic–inorganic macromolecule is a typi-cal amphiphile. Because of the immiscibilitybetween PCL chains and POSS groups, the ter-minal POSS groups could be self-assembledinto the nanodomains, which are evidenced bythe appearance of the diffraction peaks at 2h¼ 7.288, assignable to POSS aggregates in XRDprofiles (See Fig. 4). The POSS nanodomainscould either increase volume or provide chainconfinement to suppress the spherulitic growthof sporadic nuclei.

Overall Kinetic Rate Constants (Kn)

The spherulitic growth kinetics of the POSS-terminated PCL samples was analyzed in termsof Hoffman–Lauritzen crystallization kinetictheory.42–44 The dependence of the growth rateG on crystallization temperature Tc and onundercooling (DT ¼ T0

m � Tc) is expressed withthe following equation45:

G ¼ U2G0 exp �DF�

RTc

� �exp � Kg

fTcDTþ 2ruT

0m lnU2

b0DHmDT

� �ð5Þ

f ¼ 2Tc

T0m þ Tc

ð6Þ

Kg ¼ Zb0rrcT0m

kDH0f

ð7Þ

where G0 is a pre-exponential factor, generallyassumed to be a constant or proportional to Tc;DF* represents the sum of the activation ener-gies for the chain motion in the melt. R is thegas constant and k is the Boltzmann constant.F2 is the volume fraction of crystalline polymer.The quantity f is the correction factor for the en-thalpy of fusion. Kg is the nucleation factor; b0

the layer thickness; DHf0 ¼ 136 J/g35 is the theo-

retical enthalpy of fusion per unit mass; r andre, the surface energies of lateral and extremitysurfaces, respectively; and Z, a coefficient that

Table 3. Values of t1/2, Kn, n, and T0m at Various Tc’s

PCL Tc (K)T 0

m

(K)t1/2

(min)Kn

(min�n) n

Linear PCL 304 329.5 1.34 3.66 3 10�2 4.1305 329.7 2.56 1.64 3 10�3 4.0306 329.9 3.74 2.47 3 10�4 3.9307 330.0 4.92 1.65 3 10�3 3.8308 330.2 6.26 4.19 3 10�5 4.2

POSSPCL1 304 322.3 1.50 6.06 3 10�1 3.5306 322.8 2.50 1.76 3 10�1 3.4307 322.6 3.12 5.22 3 10�2 3.5308 323.1 4.15 5.09 3 10�2 3.8309 323.6 6.25 3.05 3 10�2 3.7

POSSPCL2 307 326.1 1.15 9.28 3 10�1 3.1309 327.1 2.32 3.01 3 10�1 3.2311 327.6 4.82 8.73 3 10�2 3.0313 328.3 10.15 3.28 3 10�2 3.3314 329.0 12.71 5.86 3 10�3 3.1

POSSPCL3 313 327.1 2.00 2.87 3 10�1 2.8314 327.6 2.60 9.76 3 10�2 2.6315 327.8 4.32 2.28 3 10�2 2.6316 328.3 6.30 6.59 3 10�3 2.9317 328.9 9.20 1.06 3 10�3 3.0

POSSPCL4 313 329.9 0.94 1.35 3 10�3 3.5314 330.4 2.22 4.95 3 10�1 3.2315 330.5 3.02 6.20 3 10�2 3.1316 331.0 5.12 1.55 3 10�2 3.2317 331.5 7.90 6.97 3 10�3 3.2

POSSPCL5 311 329.9 1.36 5.58 3 10�1 2.4313 330.4 2.78 6.84 3 10�2 2.4315 330.9 5.98 9.29 3 10�3 2.4316 331.4 8.70 2.29 3 10�3 2.7317 331.9 10.72 9.90 3 10�4 2.8



depends on the growth regime, Z ¼ 4 in regimesI and III and Z ¼ 2 in regime II.46,47

The transport term DF* in eq 5 may be esti-mated with a satisfactory precision using WLFequation48:

DF� ¼ C1Tc

C2 þ Tc � Tgð8Þ

where C1 and C2 are constants (generallyassumed as 4120 cal/J and 51.5 K, respectively)and Tg the glass transition temperature.

For the overall crystallization rate, we usedG ¼ CK

1=nn , where C is a constant. Assuming

r ¼ 0:1b0DH0f

49 and taking into account the re-lations (6) and (7), the above expression may bereorganized as:

f ðKnÞ ¼1

nlnKn � lnU2 þ

C1

RðC2 þ Tc � TgÞ

� 0:2T0m logU2

DT¼ lnA0 �

Kg

f TcDTð9Þ

By plotting the quantity f(Kn) against 1f TcDT

,the linear plots are shown in Figure 12 for thevarious samples. For all samples, single slopesare evident in the range of Tc investigated, indi-cating single regime. Herewith, regime IIgrowth kinetics was employed to estimate thefolding free energy of surface (re). From the

slopes and the intercepts of these lines, valuesof the free surface energy of folding re and pre-exponential factors A0 were calculated by usingb0 ¼ 4.38 3 10�8 cm. F2 can be calculated bythe densities (dPCL ¼ 1.146 g/mL, dPOSS ¼ 1.27g/mL33) and molecular weight of the POSS-ter-minated PCLs. The results are summarized inTable 4. Figure 13 presents the plot of the val-ues of re as a function of the molecular weightof the POSS-terminated PCL. The values of re

¼ 0.034–0.109 J/m2 were obtained, which are in-termediate between the lower than the values(0.027 J/m2) obtained by Ong and Price40 andthe higher values (0.112 J/m2) obtained by Gou-let and Prud’homme.50 It is seen that the valuesof re decreased with increasing the content ofPOSS, (or with decreasing the molecularweights of the POSS-terminated PCL samples).This observation could be interpreted in terms

Figure 12. The plots of the quantity f(Kn) against1

f TcDT.

Table 4. Values of Ao, Kg, and re

PCLs Ao Kg (K2) re (J/m2)

POSSPCL1 2.12 3 109 4.52 3 104 4.90 3 10�2

POSSPCL2 6.83 3 109 6.65 3 104 7.10 3 10�2

POSSPCL3 6.25 3 1010 7.93 3 104 8.48 3 10�2

POSSPCL4 4.91 3 1011 9.46 3 104 10.04 3 10�2

POSSPCL5 2.00 3 1011 10.30 3 104 10.90 3 10�2

Control PCL 1.79 3 1010 6.22 3 104 5.68 3 10�2

Figure 13. The plots of the values of re as a func-tion of the molecular weight of control PCL andPOSS-terminated PCL.

2212 NI AND ZHENG


of the combination of the surface enthalpy andentropy of folding (re ¼ He � TSe).

37 It isexpected that in the POSS-terminated PCLs,the covalent attachment of PCL chain to cubicPOSS through ester linkages could restrict thepositional freedom and thus the entropy of fold-ing (Se) will be decreased with respect to that ofcontrol PCL. It is plausible to propose that theentropy of folding (Se) is not favorable for thedecrease in the folding free energy of surface(re) for the POSS-end PCLs. Therefore; theenthalpic contribution (He) to re might be signif-icant to affect the folding free energy of surface.In the present case, the change in folding en-thalpy could be associated with the change ofcrystalline structure posed by the inorganicPOSS fraction, which was evidenced by theresults of XRD. It is noted that the values of re

increase with increasing the molecular weight ofPCL chain, implying that effect of POSS portionis not so significant for the POSS-terminatedPCLs with the sufficiently high molecularweights.

CONCLUSIONS

In this study, we investigated the melting andcrystallization behavior of POSS-terminatedPCLs with various lengths of PCL chains bymeans of XRD and DSC. The XRD results indi-cate that the incorporation of POSS portiongave rise to the alteration in PCL crystal struc-ture. The POSS-terminated PCLs displayed theincreased equilibrium melting points comparedto the control PCL. Both the overall crystalliza-tion rate and the spherulitic growth rate of thePOSS-terminated PCLs increased with increas-ing the concentration of POSS (or with decreas-ing length of PCL chain in the hybrids). Theanalysis of Avrami equation shows that the crys-tallization of the POSS-terminated PCL prefer-entially followed the spherulitic growth of in-stantaneous nuclei. It is found that the foldingfree energy of surface for the POSS-terminatedPCLs decreased with increasing the concentra-tion of POSS.

This financial support from Shanghai Science andTechnology Commission, China under a key project(No. 02DJ14048) was acknowledged. The authors alsowould like to acknowledge Natural Science Founda-tion of China (Project No. 20474038 and 50390090)

and Shanghai Educational Development Foundation,China under an Award (2004-SG-18) to ‘‘ShuguangScholar’’ for the partial support.

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melting and crystallization behavior of polyhedral oligomeric silsesquioxane-capped...

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