European Polymer Journal 46 (2010) 24–33

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

A facile approach to preparation of long-chain-branched poly(p-dioxanone)

Feng Chen a, Si-Chong Chen a,*, Ke-Ke Yang a,b, Xiu-Li Wang a, Yu-Zhong Wang a,b,*

a Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), College of Chemistry, Sichuan University, Chengdu 610064, Chinab State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

Article history:Received 4 April 2009Received in revised form 31 July 2009Accepted 9 September 2009Available online 12 September 2009

Keywords:Long-chain-branchPoly(p-dioxanone)Chain-extendingRheology

0014-3057/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.09.008

* Corresponding authors.E-mail addresses: chensichong@scu.edu.cn (S.-C.

edu.cn (Y.-Z. Wang).

a b s t r a c t

Long-chain-branched poly(p-dioxanone)s (LCB-PPDOs) with different branch densitieswere prepared via the chain-extending reaction of hydroxyl group terminated linear bi-functional PPDO (2a-PPDO) and star-like tri-functional PPDO (3a-PPDO) prepolymers,which were synthesized by the ring-opening polymerization of p-dioxanone (PDO) using1,4-butanediol (BD) and trimethylolpropane (TMP) as multi-functional initiators, respec-tively. The undesirable gelation was successfully depressed by adjusting the chain lengthand feed ratio of prepolymers. The average molecular weight between branch points(Mb) and the average number of branch per 100,000 g/mol (Bn) of LCB-PPDOs were calcu-lated from the 1H NMR spectra. The average number of branch ranged from 0 to 6.72branch points per 100,000 g/mol, and the number-average molecular weights betweenbranch points ranged from 6900 to 20,500 g/mol. The results of differential scanning calo-rimetry (DSC) showed that the crystallization behavior of LCB-PPDOs was changed evi-dently with the branch density. Small-amplitude dynamic oscillatory rheometer wasused to investigate the rheological properties of the melts of LCB-PPDO including zero-shear viscosity, storage modulus, relaxation times and loss angle, which largely dependedon the branch density and length of LCB-PPDOs. Therefore, the rheological behaviors ofPPDO can be well-controlled via synthesizing LCB-PPDOs with the desired architectures.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

As a biodegradable polyether ester, poly(p-dioxanone)(PPDO) has attracted many attentions because of its specialcharacters. The coexistence of ester and ether bonds in poly-mer chains endues PPDO with both excellent biodegradabil-ity and flexibility [1]. Compared to other well reportedbiodegradable aliphatic polyesters, such as polylactic acid(PLA), polycaprolactone (PCL), poly(b-hydroxybutyrate)(PHB), PPDO not only have good mechanical strength (ten-sile strength > 30 MPa), but also have excellent flexibility(elongation at break > 300%) and proper melting tempera-ture (110 �C). The PPDO has already been used in some bio-medical applications such as suture, bone repair device andmay have potential applications in peroral or parenteral

. All rights reserved.

Chen), yzwang@scu.

drug delivery systems. With the decrease of its monomercost, PPDO also may be used in universal uses, such as films,molded products, laminates, foams, non-woven material,adhesives and coatings [1–3]. However, the linear PPDO,which has relatively low melt strength, is very hard to ther-moform in common thermoplastic processing, especiallyfoaming and blow molding.

Introducing long-chain branches to linear polymer is aneffective approach to increase the melt strength [4–6]. Along-chain-branch (LCB) is long enough to entangle withother chains in melt and concentrated solutions, therebyaltering the flow properties. The LCB polymers were foundto exhibit the following properties: lower Newtonian viscos-ity and stronger shear thinning than a linear polymer ofequal molecular weight, strong melt elasticity expressed bythe first normal stress difference and storage modulus, andenhanced strain hardening under elongational flow [7,8].

Various types of branched polyesters such as star-branched, comb-like, randomly branched and super-branched

Scheme 1. Synthesis of LCB-PPDO.

F. Chen et al. / European Polymer Journal 46 (2010) 24–33 25

polyesters, were prepared using different polymerizationtechniques [4,9–12]. The strategies of preparation of theseLCB-polyesters were based on the co-polycondensationof multi-functional (>2) monomer with bi-functional mono-mer. Hess et al. [11] changed the level of random branchingin PET by adding different levels of trimethylolpropane(TMP) to the melt polymerization, and the branched PETpossessed a lower zero-shear viscosity compared to linearchains of equal Mw. Kim et al. [12] prepared long branchedpolybutylene succinate (PBS) by introducing a branchingagent, trimethylol propane (TMP), to the polycondensationsystem of succinic acid and 1,4-butanediol, and observed anincreased shear-thinning extent and decreased loss tangentas the level of tri-functional branching agent was increased.For those polyesters, such as PET and PBS, prepared by poly-condensation reactions, the LCB structure is relatively easy toobtain by introducing a certain amount of multi-functionalmonomer.

However, to the best of our knowledge, no attemptabout the preparation of those LCB-aliphatic polyesters ob-tained by ring-opening polymerization of lactone or lac-tide, such as PCL, PLA and PPDO, was reported, probablybecause it was very hard to obtain the multi-functionalcyclic monomer for ring-opening polymerization.

The aim of this work is to synthesize long-chain-branched poly(p-dioxanone) (LCB-PPDO) so as to improvethe rheological and processing properties of PPDO. Arelatively facile synthesis strategy was adopted to prepareLCB-PPDO. Firstly, trimethylolpropane (TMP) was used asinitiator and SnOct2 as a catalyst to initiate the ring-opening polymerization of PDO, given the star-shapedtri-hydroxyl-terminated poly(p-dioxanone) (3a-PPDO)prepolymer [13]. Then LCB-PPDOs with different branchdensity were synthesized via the reaction between the ter-minated hydroxyl groups of prepolymer using hexameth-ylene diisocyanate (HDI) as the chain-extending agent,which has the low toxicity of its degradation productsand high reactive activation for –OH groups [14]. Com-pared to those traditional methods preparing branchedpolyesters, the coupling reaction of functional terminatedprepolymer could produce LCB polymer with relativelyshort time and high efficiency. Moreover, the molecularstructure of LCB polymer could also be easily controlledby adjusting the chain length of prepolymers. To depressthe probability of gelation, linear bi-hydroxyl-terminatedPPDO prepolymer was introduced to the chain-extendingreaction. The influence of the molecular topology on therheological and crystallization behaviors of the LCB-PPDOswas discussed.

2. Experimental

2.1. Materials

The monomer of PPDO, p-dioxanone, was provided bythe Pilot Plant of the Center for Degradable and Flame-Retardant Polymeric Materials (Chengdu, China) andpurified by vacuum distillation from CaH2. SnOct2 was pur-chased from Sigma Aldrich (USA) and used as received. 1,4-Butanediol (BD) was purchased from Zhiyuan ChemicalCompany (Tianjin, China) and used after vacuum distilla-

tion for three times. Trimethylolpropane (TMP) was pur-chased from Sinopharm Chemical Reagent Co, Ltd. andused without further purification. Hexamethylene diisocy-anate (HDI) was purchased from Tokyo Chemical IndustryCo. and used without further purification.

2.2. Preparation of 2a-PPDO prepolymer and 3a-PPDOprepolymer

The polymerization of PDO was performed in a flame-dried and argon-purged glass reactor with magnetic stir-ring. Firstly, a certain amount of TMP was put into differentreactors, into which BD was injected with a syringe. Then,the PDO monomer was injected into the reactors after re-peated evacuation and purging with argon several times,and the reactors were immersed into an oil bath at 80 �Cwith stirring for 30 min. The polymerization was conductedfor 48 h after certain amount of the SnOct2/toluene solutionwas injected into the reactors. The reactors were rapidlycooled to room temperature via ice cooling after reactionand the product was shattered with crushing machine.

2.3. Preparation of LCB-PPDO via chain extending

Scheme 1 illustrates the reaction mechanism of formingLCB-PPDO polymers via the reaction between the termi-nated hydroxyl group of prepolymers and NCO group ofHDI. In this work, three pairs of the prepolymers with dif-ferent lengths of chains were used (Table 1). 3a-PPDO pre-polymer (10 g) and 2a-PPDO prepolymer (10 g) were putinto a dried glass reactor. The reactor was rigorously driedby evacuating and purging with argon several times priorto immerse into a temperature-adjusted silicone oil bath.HDI was injected with a syringe while stirring after theprepolymers were melted completely and commixed uni-formity. The reaction was allowed to proceed for 1 h underargon until the intrinsic viscosity were not raised and thenrapidly cooled to room temperature via ice cooling afterreaction. The crude products were grinding milled to pow-der and then soxhlet-extracted by using aceton (containing1% of ethanol) as solvent for more than 48 h, followed bydrying in 50 �C vacuum oven until constant weight

Table 1Preparation condition and the molecular weight of the PPDO prepolymer.

Sample PDO/OH(mol/mol)

OH/SnOct2

(mol/mol)Yield (%) Mn

a

(�103 g/mol)Mw

a

(�103 g/mol)MWDa Dp

b M0nc

(�103 g/mol)Mn00d

(�103 g/mol)

2a-20 20/1 500/1 85.4 6.3 8.2 1.30 37 3.9 4.33a-20 20/1 500/1 87.8 10.4 14.3 1.37 47 5.0 6.32a-40 40/1 500/1 89.2 13.3 18.4 1.39 81 8.3 8.33a-40 40/1 500/1 88.9 16.3 22.0 1.35 90 9.3 12.42a-60 60/1 500/1 89.0 16.0 22.1 1.39 97 10.0 12.33a-60 60/1 500/1 91.1 24.3 34.2 1.40 156 16.0 18.5

a Measured by GPC.b The degree of polymerization of prepolymers, calculated by 1H NMR: for 2a-PPDO: Dp = 2Ib/Id, Ib; for 3a-PPDO: Dp = 3Ib/2If.c The M0n was calculated by 1H NMR as follows: for 2a-PPDO: M0n = Dp � 102 + 90; for 3a-PPDO: M0n = Dp � 102 + 134.d The M00n is the theoretical molecular weight of prepolymer calculated by the feed ratio of PDO/initiators.

26 F. Chen et al. / European Polymer Journal 46 (2010) 24–33

reached. The unreacted –NCO groups were then removedor terminated before the characterization.

2.4. Characterization and test

The number-average molecular weight (Mn) and molec-ular weight distribution (MWD, Mw/Mn) of prepolymerswere determined by gel permeation chromatography(GPC) (Waters 1515 isocratic HPLC pump, Waters 717 plusautosampler, Waters 2414 refractive index detector) at30 �C with a calibration curve constructed using polysty-rene (PSt) standards. Chloroform was used as solvent andeluent with a flow rate of 1.0 mL/min. The intrinsic viscos-ity [g] was measured in phenol/1,1,2,2-tetrachloroethane(1:1 v/v) solution using an Ubbelohde viscometer main-tained at 30 �C. 1H NMR spectra (400 MHz) and 13C NMRspectra (100 MHz) were recorded by a Varian Inova 400NMR Spectrometer using deuterated chloroform (CDCl3)for prepolymer and deuterated dimethyl sulfoxide (d6-DMSO) for LCB-PPDO as solvent. All the samples werepacked with filter paper and then Soxhlet extracted in boil-ing acetone for 48 h before test. DSC measurement was per-formed with TA Q200 (TA instruments, USA) in sealedaluminum pan under nitrogen atmosphere. The sampleswere cooled to �50 �C from room temperature, and thenheated to 140 �C at a heating rate of 10 �C/min, kept for5 min to eliminate the thermal history, then cooled to�50 �C at a cooling rate of 10 �C/min. The samples wereagain heated up to 140 �C at the same rate. X-ray diffractionpatterns were taken on a Philips X’Pert X-ray diffractometerwith Cu Ka radiation. Rheological measurements were per-formed on a rotational rheometer, Gemini 200HR (BohlinInstruments, UK) using a heated stream of nitrogen gasfor temperature control. All measurements were performedunder a nitrogen atmosphere, and 25 mm parallel plateswith gap slightly exceeding 1 mm were used. Samples wereformed into 25 mm disks by compression-molding. Small-amplitude oscillatory shear was performed in the fre-quency range of 0.01–100 rad/s at 120 �C.

3. Results and discussion

3.1. Preparation and characterization of PPDO prepolymers

The prepolymers, hydroxyl-terminated linear bi-func-tional PPDO (2a-PPDO) and star-shaped tri-functional

PPDO (3a-PPDO) prepolymers, were prepared by using1,4-butanediol (BD) and trimethylolpropane (TMP) as aninitiator, respectively, and SnOct2 as a catalyst. Hydroxylgroups reacted with catalyst, stannous octate (Sn(Oct)2),to co-initiate the ring-opening polymerization of PDOmonomer. Therefore, the length of chain can be controlledby changing the molar ratios of PDO/initiator [15]. All thesamples were prepared at 150 �C for 1 h. The molecularcharacteristics of prepolymers are listed in Table 1. Theresults clearly indicated that the molecular weights in-creased with the feed molar ratio of PDO to initiators.

1H NMR (400 MHz, CDCl3) was used to learn the detailedinformation on the microstructure of 2a-PPDO and 3a-PPDO. Resonances located at dHa 4.1 ppm, dHb 3.8 ppm,dHc 4.2 ppm were assigned to the three methylene hydrogenatoms of PPDO, respectively. The signals occurring at dHh

1.7 ppm and dHq 4.05 ppm were assigned to the methyleneprotons of BD in 2a-PPDO. The signal occurring at dHf

0.88 ppm was assigned to the methyl protons of reactedTMP in 3a-PPDO. The signals of 3a-PPDO at dHg 1.5 ppm,dHe 4.07 ppm and dHe’ 3.45 ppm were assigned to the meth-ene groups connected with methyl, ester, and hydroxylgroups, respectively (Fig. 1). The resonance at dHe’

3.45 ppm suggested that the uncompleted esterification ofhydroxyl groups of TMP.

The number-average molecular weights of the prepoly-mers derived from different methods were listed in Table1. It was found that the Mn

0 calculated by NMR was lessthan theoretical value (Mn

00) among all the different ratiosof PDO/OH, which was caused by unreacted monomer.The Mn obtained from GPC measurement was the highestamong three different methods. Similar trend was reportedfor some other aliphatic polyester such as PLLA and PBS[16].

3.2. Synthesis of LCB-PPDOs

The detail approach of synthesis of LCB-PPDO was de-scribed above and outlined in Scheme 1. All the sampleswere prepared at 150 �C for 60 min and HDI was used asthe chain-extending agent. Since the resulted long-chainbranch structure could be controlled by adjusting themolecular weight of prepolymers [17], three pairs of theprepolymers with different chain lengths were used forpreparing LCB-PPDO with different molecular structure.Theoretically, the chain-extending reaction of prepolymer

Fig. 1. 1H NMR spectra of prepolymer of 2a-PPDO (a) and 3a-PPDO (b).

F. Chen et al. / European Polymer Journal 46 (2010) 24–33 27

with tri-functional terminal groups may result in gelationof final products [18]. The probability of gelation dependsdirectly on the concentration of the tri-functional terminalgroups in the reaction system. Therefore, to avoid the gela-tion by decreasing the concentration of tri-functional ter-minal groups, bi-functional prepolymer 2a-PPDO withdifferent molecular weight and contents were added. Therelationship between intrinsic viscosity [g] of productsand the feed ratios of –NCO/–OH were also studied.Although the molecular weight can not be calculated fromthe data of intrinsic viscosity due to the existence of LCBstructure, it was still a good way to compare the magni-tude of molecular weight among them. And the numberof terminated hydroxyl groups of prepolymers was equal

to the number of hydroxyl groups of initiators. The prod-ucts were dissolved in phenol/1,1,2,2,-tetrachloroethane(1:1/v:v) to determine whether the gelation occurred.The results are presented in Fig. 2.

For those prepolymer with high molecular weight, nogelation was observed in the whole range of –NCO/–OH ra-tio. The intrinsic viscosities of LCB-PPDOs increased withthe increase of –NCO/–OH ratio firstly, but after reachedthe maximum value, when the –NCO/–OH ratio wasslightly higher than the 1:1, the intrinsic viscosity of LCB-40s and LCB-60s decreased with the addition of moreHDI. These results are in good accordance with those of re-ported for chain-extending reaction of linear PPDO [15].The excess HDI may act as terminated agent, which de-

Fig. 2. The influence of feed ratio of HDI and prepolymer on the chain-extending reaction.

28 F. Chen et al. / European Polymer Journal 46 (2010) 24–33

creased the concentration of hydroxyl groups. Forbranched polymer, not only the molecular weight but alsothe branch density has important influence on the intrinsicviscosity ([g]). The branched polymer had lower [g] thanthe linear polymer with similar molecular weight, and thisdeviation increased with the branch density [17]. In thiswork, because of the introduction of linear bi-functionalprepolymer 2a-PPDO, the branch density of the resultingproducts was relatively low compared to those of commonbranched polymer. The increase in [g] of LCB-PPDO attrib-uted to the increase of molecular weight overwhelm thedecrease of [g] attributed to increase of branch density.Therefore, the [g] of LCB-PPDO showed similar changingtrend as those of chain-extending reaction of linear bi-functional polymers using isocyanate as extender [15].

For prepolymer with low molecular weight (2a-20/3a-20), gelation was observed when the HDI/OH ratio higherthan 1:1. When more 2a-PPDO was added, the minimum–NCO/–OH ratio at which the gelation occurred also in-creased obviously. This phenomenon was attributed tothe changes in concentration of tri-functional terminal hy-droxyl groups. The prepolymer with shorter chain lengthor higher content of 3a-PPDO have higher concentrationof tri- functional terminal hydroxyl groups and thereforehave higher probability of gelation. Therefore, it is an effec-tive way to avoid the gelation by adjusting the concentra-tion of tri-functional terminal hydroxyl groups.

3.3. Molecular structure of LCB-PPDO

The 1H NMR (DMSO, 400 MHz) spectroscopy of LCB-PPDO was shown in Fig. 3(a). Besides the three typical sig-nals ascribed to PPDO units (dHa 4.16 ppm, dHb 3.76 ppm,dHc 4.21 ppm) and the signals ascribed to TMP (dHf 0.88)and BD (dHh 1.68) units, the weak single peaks at7.18 ppm (dHm) and 1.26 ppm, 1.43 ppm, 2.98 ppm are as-cribed to the amide –NH groups and methylene protons ofHDI (Fig. 3a), suggesting that the prepolymers were suc-cessfully coupled by HDI. Weak resonances of PDO mono-

mer were found in the NMR spectra of LCB-PPDO,suggesting that a trace amount of PDO monomer wasformed by the unzipping reaction of PPDO [19] duringthe chain-extending reaction at 150 �C. Since the amountof monomer was relatively small, it did not show obviousimpact on the properties of the samples. 13C NMR spec-trum of LCB-PPDO was shown in Fig. 3(b). The peak at68.9, 72.6, 74.4 and 175.4 ppm are assigned to carbons ofmethene groups and carbonyl group PPDO, respectively.The resonance of the isocyanate group which typically oc-curred at about 121 ppm was not observed, suggested thatthe unreacted –NCO groups had been reacted or removedafter the purification.

The average molecular weight between branch pointsand the average number of branch per 100,000 g/mol (Bn)of LCB-PPDO can be calculated from 1H NMR spectra by fol-lowing equations and listed in Table 2.

The intensity of the methyl groups (I0.88) and methinegroups (I1.67) can be used to calculate the molar ratio(Mr) of 3a-PPDO to 2a-PPDO in the products by the follow-ing equation:

Mr ¼n2a

n3a¼ 3� I1:67

4� I0:88ð1Þ

where I1.67 is the intensity of methene groups of BD andI0.88 is the intensity of methyl groups of TMP, n3a and n2a

are the average numbers of 3a-PPDO and 2a-PPDO seg-ments per LCB-PPDO molecule, respectively.

The average molecular weight between branch points(Mb) can be expressed by following:

Mb ¼23�M3aþMr �M2a ð2Þ

where M3a and M2a are the number-average molecularweights of 3a-PPDO and of 2a-PPDO, respectively.

The average number of branch per 100,000 g/mol (Bn)can be calculated by following equation [1]:

Bn ¼100;000

Mb� 1

2ð3Þ

A series of LCB-PPDOs with similar molecular weight butdifferent branch densities were selected for investigatingthe influence of molecular structure on the characteristicsof LCB-PPDO. The factors of molecular structure, such asthe average number of branch per 100,000 g/mol (Bn), theaverage molecular weight between branch points, whichderived from the equations hereinbefore, of those LCB-PPDOs are all listed in Table 2.

3.4. Thermal and crystalline behaviors of LCB-PPDO polymers

The thermal behavior and crystallization of LCB-PPDOwere studied via DSC. Clearly, the LCB structures includeMb and branch density does have a significant influenceon the thermal behavior and the crystallinity. The DSCcurves of LCB-PPDOs and linear PPDO during the first heat-ing scan and cooling scan and the subsequent reheatingscan are shown in Fig. 4(a)–(c), respectively. The detailedthermal characteristics, such as glass transition tempera-ture (Tg), crystallization temperatures (Tc), melting tem-perature (Tm), crystallization enthalpy during the cooling

Fig. 3. (a) 1H NMR spectra and (b) 13C NMR spectra of LCB-PPDO.

F. Chen et al. / European Polymer Journal 46 (2010) 24–33 29

Table 2Molecular structure and rheological characteristics of LCB-PPDOs.

Sample Mb (g/mol) Mr Bn g0, 120 �C (Pa s) na k (s)

LCB-20 8100 1.25 6.9 502 0.32 5.9LCB-40 14,000 0.96 3.8 422 0.43 2.3LCB-60 23,800 1.32 2.0 399 0.39 0.85PPDO – – 0 197 0.99 0.15

a Shear-thinning index.

30 F. Chen et al. / European Polymer Journal 46 (2010) 24–33

scan (DHc1), the second crystallization enthalpy during theheating scan (DHc2) and the enthalpies of fusion (DHm) areall listed in Table 3.

Fig. 4. DSC patterns of LCB-PPDO and linear PPDO: (a) the first h

Table 3Thermal characteristics of LCBPPDO and Linear PPDO from DSC patterns.

Sample The first heating scan The cooling scan

Tg (�C) Tm (�C) DHm (J/g) Xc (%) Tg (�C) Tc (�C) DHc1

LCB-20 �19.7 90.5 55.8 39.6 �20.7 – –LCB-40 �23.5 96.5 60.3 42.8 �23.9 – –LCB-60 �18.9 100.7 69.2 49.1 �18.3 35.8 0.8PPDO �15.0 104.6 73.0 51.8 �16.4 39.3 19.2

As shown in Fig. 4(b), unlike linear PPDO, LCB-PPDOshave no crystallization observed during cooling. In the sub-sequent heating scan, it can be seen that the crystallizationtemperatures (Tc) increase evidently with branch density.The LCB structure and the presence of heterogeneous seg-ments such as TMP, BD and amide in LCB-PPDO chains maydisturb the crystallization of PPDO segments.

In order to further explain the phenomena, we intro-duce the conception of relative crystallinity (Xc), whichwas calculated from the first and the second heating scanusing the following equations:

Xc ¼DHm

DH0� 100% ð4Þ

eating scan; (b) cooling scan; (c) the second heating scan.

The second heating scan

(J/g) Tg (�C) Tc (�C) DHc2 (J/g) Tm (�C) DHm (J/g) Xc (%)

�18.5 59.0 11.7 89.9 13.4 9.5�20.8 44.5 49.2 97.3 52.9 37.5�16.2 42.4 50.3 101.1 61.9 43.9�14.2 34.9 34.4 104.3 71.5 50.7

F. Chen et al. / European Polymer Journal 46 (2010) 24–33 31

where DHm is the specific enthalpy of melting for the firstand the second heating scan and DH0 is the melt enthalpyof the hypothetically 100% crystalline PPDO (DH0 = 141 J/g)[20]. The detailed data were listed in Table 3. It is docu-mented that a high degree of branching in a polymer back-bone provides lower crystallinity, for the case of symmetricchains that easily crystallize, than a linear polymer withequal molecular weight [21]. The Xc1 of LCB-PPDOs fromthe first heating scan is lower than that of linear PPDO,and is decreased with the increase of branch density,indicating that the presence of branching structure andheterogeneous segments may frustrate the crystallizationprocess of PPDO segments and therefore decrease the de-gree of crystallinity even after the consummate crystalliza-tion process. Moreover, during the second heating scan, thecrystallinity of LCB-PPDOs was lower than that during thefirst heating scan, and the difference between two heatingscans decreased with the increase of branch density. Theintroduction of heterogenous molecular segments frominitiator and chain extender may also decrease the crystal-lization rate of PPDO segments and the samples could notcrystallize completely during the cooling and followingsecond heating scan with 10 �C of heating and cooling rate[15]. The LCB-PPDO with higher branch density has highercontent of heterogeneous molecular segments from BD,TMP, and HDI, and therefore resulted in a much smallerdegree of crystallinity than those of linear PPDO andLCB-PPDO with lower branch density during the secondheating scan.

The XRD patterns of linear PPDO and LCB-PPDOs areshown in Fig. 5. Obvious reflections are observed at2h = 22�, 24� and 29�, assigned to 210, 020 and 310 planes,respectively [22], It is clear that the XRD profiles ofLCB-PPDOs are almost identical as those of linear PPDO,indicated that the crystallization was decided by PPDO seg-ments, and the introduction of LCB structure and heteroge-neous segments did not disturb the unit-cell dimensions ofPPDO segments. This result is in good accordance with thoseof star-branched PPDO [13] and linear chain-extended PPDOusing isocyanate as extender [15] reported before.

Fig. 5. XRD curves of the PPDO and LCB-PPDOs.

3.5. Rheological properties of LCB-PPDOs

The Small-Amplitude Dynamic Oscillatory Shear mea-surement was used to study the rheological properties ofLCB-PPDOs. The shear rheological behavior of macromole-cules is very sensitive to the topological structure. Thepresence of very small amounts of LCB can influence thezero-shear viscosity (g0), the complex viscosity (g*) andthe degree of shear thinning, as compared to the linearpolymers with similar molecular weight [23–25]. Fig. 6showed the complex viscosities of the LCB-PPDOs and Lin-ear PPDO. From this figure, it was readily apparent that thebranch density had a significant influence on the complexviscosity over the entire frequency range studied. Theshear thinning of LCB-PPDOs started at lower frequencythan that of linear PPDO. Unlike linear PPDO, no obviousNewonian-plateau was observed for LCB-PPDOs at low fre-quency which can be attributed to the long relaxationtimes of branched polymers. The branches of LCB-PPDOsmay form sufficient entanglements with each other andthe number of entanglements increased with branch den-sity. Therefore, LCB-PPDOs exhibit a higher complex vis-cosity than linear PPDO with similar molecular weight,and the complex viscosities of LCB-PPDOs increased withthe branch density at low frequency. While at high fre-quency, the LCB-PPDOs exhibited a lower viscosity thanlinear PPDO owning to a higher degree of shear thinning.Furthermore, the extent of shear thinning was increasedwith branch density of the LCB-PPDOs. The viscosity ofthe samples could be fitted by the Cross equation [26],given by Eq. (5):

g�ðxÞ ¼ g0

1þ ðkxÞnð5Þ

where k is relaxation time, g0 is zero-shear viscosity, and n isshear-thinning index, which indicates the degree of non-Newonian behavior. These values are all listed in Table 2.LCB-PPDOs exhibit much higher zero-shear viscosity, higherrelaxation time and lower shear-thinning index comparingto those of linear PPDO. In addition, the g0 of the LCB-PPDOsand relaxation time increased while the shear-thinning in-

Fig. 6. Complex viscosities versus angle frequency of the LCB-PPDOs andPPDO at 120 �C.

Fig. 7. Storage modulus of the LCB-PPDOs and PPDO at 120 �C. Fig. 8. Master curves of storage and loss modulus of LCB-20 and PPDO at120 �C.

Fig. 9. Tan d versus angle frequent for the LCB-PPDOs and PPDO at 120 �C.

32 F. Chen et al. / European Polymer Journal 46 (2010) 24–33

dex decreased with the increase of the branch density. Sincethose LCB-PPDOs used for rheological characterization havesimilar molecular weight, the changes in LCB structureshould be the main reason for such results. Similar phenom-ena were also observed for other LCB polymers [25,27–30].

Besides zero-shear viscosity, other properties such asthe storage and loss moduli (G0, G00) were also influencedby LCB [24,30–32]. Fig. 7 showed the storage modulus G’plotted as a function of angular frequencies at a referencetemperature of 120 �C for LCB-PPDOs and linear PPDO.The G’ of linear PPDO increased with frequency and theslope of G0 approached 2 at low frequency. Whereas LCB-PPDOs were observed to have broadened G0 with slopesonly approached 1.3 at low frequencies measured, and de-creased with the increase of branch density, indicating therelaxation times increased with the increase of the branchdensity of the polymers. LCB added a mode of relaxation atlow frequency, which was not present in the linear PPDO.The master curves of storage and loss modulus of LCB-20and linear PPDO were shown in Fig. 8 with the remaindersomitted to prevent crowding. At lower frequencies, linearPPDO followed the well-known frequency dependence,which was usually observed for linear polymers, i.e., G0

and G00 became proportional to x2 and x, respectively. Itwas apparent that LCB-20 had significantly higher valuesof G0 and G00 than linear PPDO at low frequencies, whileat the high frequencies xc, a crossover between G0 andG00 curves of LCB-20 was observed. Moreover, no propor-tionality of G0 and G00 to x2 and x was observed for LCB-PPDOs at the lowest frequencies measured, which con-firmed the presence of long-chain-branched structure.The relaxation time determined from s = 1/xc, where xc

is the frequency at which the G0 and G00 curves cross eachother, is in good accordance with that of calculated fromEq. (5).

The frequency independency of loss tangent tan(d) � G00/G0 has been widely used for examining LCB poly-mers [24,25,33]. Fig. 9 indicates tan d as a function of fre-quencies for LCB-PPDOs and linear PPDO. For PPDO, tan dincreased intensively with the increase of frequency, whichwas a typical terminal behavior of liquid-like material. ForLCB-PPDOs, tan d was lower than that of linear PPDO at the

same frequency; moreover, it decreased with the increaseof branch density of LCB-PPDOs. As mentioned previously,the relaxation time was determined by xc, a frequency atwhich the G0 and G00 curves cross each other, and tan (d)is equal to 1 at the crossover. Because tan d decreased withthe increase of branch density of LCB-PPDOs at the samefrequency, xc decreased with the increase of branch den-sity, indicating the relaxation take place at a lower fre-quency for high branch density LCB-PPDO.

4. Conclusions

PPDO’s with long-chain branches (LCB-PPDOs) havebeen successfully synthesized via the reaction betweenthe terminated hydroxyl groups of linear bi-functionaland star-shape tri-functional PPDO prepolymers usingHDI as the chain-extending agent. The average number ofbranch per 100,000 g/mol (Bn) and branch length of LCB-PPDOs can be controlled by changing the chain length ofprepolymers, and range from 0 to 6.72 branch points per100,000 g/mol and from 6900 g/mol to 20,500 g/mol,respectively, according to the results calculated from 1HNMR. The undesired gelation reaction could be also

F. Chen et al. / European Polymer Journal 46 (2010) 24–33 33

depressed successfully by adjusting the chain length andcontent of tri-functional prepolymers. The crystalline tem-perature Tc of LCB-PPDOs increases, and the relative crys-tallinity Xc and crystallization rates decrease evidentlywith the increase of branch density, but the crystallinestructure has not been disturbed according to the resultsinvestigated by DSC and XRD technique. Small-amplitudeoscillatory shear experiments show that the LCB structureshave obvious influence on the rheological properties ofPPDO. As the branch density of PPDO increases, zero-shearviscosity g0, G0 at low frequency and relaxation times kincrease, whilst the shear-thinning index n and loss angledecrease, which are in accordance with those of represen-tative LCB polymers. Therefore, the rheological behaviorsof PPDO can be well-controlled via synthesizing LCB-PPDOs with the desired architectures.

Acknowledgement

This work was supported financially by the NationalNatural Science Foundation of China (No. 20874065,50525309), the Research Fund for the Doctoral Programof Ministry of Education of China (No. 200806100012)and the National Key Technology R&D Program (2007BAE28B06). The Analytical and Testing Center of Sichuan Uni-versity provided NMR analysis.

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