Morphology and Crystalline Structure of Poly(ε-Caprolactone) Nanofiber via Porous Aluminium Oxide...

Morphology and Crystalline Structure of

Poly(e-Caprolactone) Nanofiber via PorousAluminium Oxide Template

Yang Chen,1,2 Lina Zhang,*1 Xiaoying Lu,2 Ning Zhao,2 Jian Xu*2

1Department of Chemistry, Wuhan University, Wuhan 430072, ChinaFax: þ86-27-68754067; E-mail: [email protected]

2State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences (ICCAS)

Received: March 23, 2006; Revised: May 22, 2006; Accepted: June 26, 2006; DOI: 10.1002/mame.200600134

Keywords: crystalline structure; melting behavior; morphology; nanofiber; poly(e-caprolactone)

Introduction

In recent years, the production and control of fibers at the

submicron or nanometer scale have been attracted much

attention.[1] Due to their very high aspect ratio and superior

mechanical properties, a large numbers of efforts have been

seen in using nanofibers for a variety of applications including

drug delivery, tissue engineering, conductive nanowires,

nanosensors, biochemical protective clothes and wound

dressing for military, electromagnetic interference (EMI)

shielding and super-hydrophobic surfaces.[2–15] The current

production methods of nanofibers are primarily electrospin-

ning, phase separation, self-assembly and template-guided

processes.[16–22] It is worth noting that a simple technique for

the fabricationof polymernanotubeswith amonodisperse size

distribution and uniformorientationvia awetting processwith

nano-porous membranes as templates was developed.[23]

Subsequently, Feng et al. reported a novel way to prepare

Summary: Poly(e-caprolactone) (PCL) nanofibers with adimension of about 150 nm were successfully fabricated byusing a process of extruding PCL solution via a porousaluminium oxide template and then solidifying in methanol.The morphology, melting behavior and crystalline structureof the nanofibers were investigated by using scanning elect-ron microscopy (SEM), differential scanning calorimetry(DSC) andX-ray diffraction (XRD). The results revealed thatthe weight-average molecular weight (Mw) of PCL hardlyinfluenced the morphology of the nanofibers. However, themelting temperature (Tm) of the PCL crystalline increasedslightly from 55.4 to 57.5 8C with an increase in Mw. Theaccessional pressure and the presence of the porous templateplayed an important role in the improvement of the orienta-tion and crystallization structures of the polymer chainswhenthey were passing through the nano-scale porous channel,leading to the conglomeration of the fiber and themuch largerdiameter than those from the pressure-induced extrusionprocess. Furthermore, comparing the processes with andwithout accessional pressure, the crystallinity of the nano-fibers obtained under 0.2 MPa pressure increased, and thediffraction for the (001) lattice plane occurred.

SEM image of PCL nanofibers extruded via a porousaluminium oxide template with the aid of pressure.

Macromol. Mater. Eng. 2006, 291, 1098–1103 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1098 DOI: 10.1002/mame.200600134 Full Paper

aligned polyacrylonitrile nanofibers by only extrusion of the

PAN precursor solution through an anodic aluminium oxide

(AAO) template and then into the solidifying solution under

pressure.[13] These achievements provide the possibility to

investigate the effects of the pressure and porous template on

the melting behavior and crystal structure of polymer nano-

fibers. The fabricationmethodwithAAO templates also could

be a premise to further investigate the nanofibers with more

complex interactions and molecular structures fabricated via

template-aided, electrospinning, phase separation or other

methods. Poly(e-caprolactone) (PCL) is a semicrystalline,

biodegradable and biocompatible polymer, and the biode-

gradable nanofibers prepared from PCL and poly(lactic acid)

(PLA) have been widely studied as supporting scaffold, cont-

rolled release drug delivery and suture application.[24–26] As is

well-known, physicochemical properties, such as crystalli-

zation, melting and glass transition behavior largely influence

the function and application of a polymer materials. Recently,

much fundamental research has focused on the crystallization

and melting of nano-scale polymer domains in physical

or chemical confinement states.[27,28] However, studies of

the crystallization and melting behaviors of the template-

dependedPCLnanofibers havebeen scarcely reported, despite

their significance for controlling the properties and specifying

the use area of those fibers.

In this work, we used an aluminium oxide template for

the fabrication of the PCL nanofibers with a self-made

instrument. The morphology, melting behavior and crys-

talline structure of the nanofibers were detected by using

scanning electron microscopy (SEM), differential scanning

calorimetry (DSC) andX-ray diffraction (XRD). The object

of this paper was to discuss the effects of molecular weight,

pressure and template on the structure and the properties of

the PCL nanofibers.

Experimental Section

Materials

Aluminium oxide templates with a pore diameter of 200 nmwere purchased from Whatman International Ltd (Maidstone,England). Commercially obtained e-caprolactone (e-CL) witha purity of 99% (Sigma-AldrichCo.,Milwaukee,WI)was usedwithout further purification. Stannous octoate (Sigma-AldrichCo., Milwaukee, WI) was distilled under vacuum and dis-solved in freshly dried toluene prior to use. Methylene chlo-ride, tetrahydrofuran and methanol were of analytical grade,and supplied by Shanghai Chemical Co. (Shanghai, China).

Synthesis of Poly(e-Caprolactone) (PCL)

Microwave-assistedpolymerizationwasperformed in a2.45GHzmicrowave oven according to themethod by Liao et al.[29] withminor modifications. A mixture of e-CL with a stannous octo-ate content of 0.1% in a vacuum-sealed ampule (50 Pa) wasirradiated at a microwave power of 680W for a predetermined

period of time (5, 8 or 10 min). After the sample was quenchedin an ice-water bath, the crude product was dissolved inmethylene chloride and precipitated by cold methanol. Theprecipitate was filtrated and vacuum-dried at ambient tem-perature to obtain poly(e-caprolactone). The weight-averagemolecular weight (Mw) of PCL with different irradiating timeswas estimated by size exclusion chromatography combinedwith laser light scattering (SEC-LLS)was carried out on a laserphotometer (l¼ 633 nm; DAWN1DSP, Wyatt TechnologyCo., St. Barbara, USA) combined with a P100 pump (ThermoSeparation Products, San Jose, USA) equippedwith TSK-GELG4000H6 column (7.5 mm �300 mm) at 25 8C. A spectrasystem detector (RI-150, TSP, USA) was simultaneouslyconnected. The eluent was tetrahydrofuran (THF) with a flowrate of 1.0 mL �min�1. All the solutions having a polymerconcentration from 1.0� 10�3 to 2.0� 10�3 g �mL�1 werefiltered first with a sand filter, and followed by a 0.45 mm filter(Whatman International Ltd, Maidstone, England), then keptin sealed glass bottles before being injected into the SECcolumn. The refractive index increment (dn/dc) of SBS inTHF at 25 8C was determined by the optilab refractometer(DAWN1DSP, Wyatt Technology Co., USA) at 633 nm to be0.079 mL � g�1. Astra (Version 4.70.07) software was utilizedfor the data acquisition and analysis. The SEC of PCL areshown in Figure 1. The Mw values of PCL with differentpolymerization time (10, 8 or 5 min) are 6.6� 104, 4.9� 104

and 3.6� 104, respectively, with the polydispersity (Mw=Mn)index of 1.6, 2.1 and 1.4.

Preparation of PCL Nanofibers

A porous aluminium oxide template with a pore diameter of200 nmwas placed on the bottomof the glass tube.Anumber ofholeswere bored in the bottomof the glass tubewith a diameterof about 2 mm, and the glass tube with the template was fixedon a suction flask containing about 200 mL methanol. Thesuction flask was put in an ice bath. The PCL nanofibers werefabricated by the following process: Firstly, PCLwas dissolvedin THF with stirring at ambient temperature for 4 h to obtain a3 wt.-% precursor solution. Secondly, the clear PCL solution

Figure 1. Size exclusive chromatograms of PCL with differentmolecular weights.

Morphology and Crystalline Structure of Poly(e-Caprolactone) Nanofiber via Porous Aluminium Oxide Template 1099

Macromol. Mater. Eng. 2006, 291, 1098–1103 www.mme-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

was transferred into a separating funnel. The flux of theprecursor PCL solution was controlled by the valve of thefunnel and kept at a available value of about 2 mL �min�1.Thirdly, the water pump was applied to supply a pressure of0.2 MPa in order to extrude the precursor into the methanolsolidifying solution. Thus, PCL nanofibers were formed in thesolidifying solution. Finally, the solidified PCL nanofiberswere collected through a filtration process. The completelysolidified surfacewaswashedwith distilledwater several timesand then dried under vacuum for 48 h. The fibers from the PCLwithMw of 6.6� 104, 4.9� 104 or 3.6� 104 were named F-1,F-2 and F-3, respectively. To investigate the effects of thepressure and template on the morphology and meltingbehavior, the PCL particles extruded without the aid ofpressure were named F-1-N, and those obtained without usingthe Al2O3 template were named F-1-P.

Characterization

Scanning electron microscopy (SEM) was carried out on aHitachi S-570 (Japan) scanning electron microscope. Thesheets in the dry state were frozen in liquid nitrogen, snappedimmediately, and then vacuum-dried. The fracture surfaces(the cross sections) of the sheets were sputtered with gold, andthen observed and photographed. Differential scanning calo-rimetry (DSC) was performed on a Netzsch DSC-204 dif-ferential scanning calorimeter (Netzsch Co., Germany), whichwas calibrated with an indium standard. The sample (about13 mg) was placed in a pressure-tight aluminium capsule, andscanned from �50 to 150 8C under nitrogen atmosphere withthe heating rate of 20 8C �min�1. The XRD patterns wereobtained from a D8 Advance diffractometer (Bruker, USA)

equipped with a Cu Ka radiation source (l¼ 0.154 nm). Thediffraction data were collected from 2y¼ 1� 408 in a fixedtime mode with a step interval of 0.18.

Results and Discussion

The SEC chromatograms of PCL with differentMw in THF

solution are shown in Figure 1. The narrow peaks at dif-

ferent elution volumes indicated that the molecular weight

and the polydispersity of PCL have been controlled.[29]

These results indicated that PCL with different molecular

weights were successfully synthesized by controlling the

microwave irradiation time.

Figure 2 shows the SEM images of the aluminium oxide

templates, the PCL nanofibers of F-1, F-2, F-3 and the

PCL particles without the aid of pressure or template. The

porous structure of the anodic aluminium oxide template is

maintained through the extruding process, as shown in

Figure 2(a). In this case, the micro-pore with an average

diameter of about 200 nm could provide the nano-scale

channels for extruding of the PCL solution. The fibers with

a diameter in about 150 nm are observed in Figure 2(b–d),

indicating that the PCL nanofibers have been obtained via

the porous template with the aid of pressure. The smaller

dimension of the fibers than that of the channel on the

template could be explained by the leaving of the solvent

and the shrinkage of the PCL during the solidified process.

On the whole, the molecular weight variation in a range

from 3.6� 104 to 6.6� 104 makes almost no influence

on the morphology of the PCL nanofibers. Without the

Figure 2. SEM images of: (a) the aluminium oxide templates, (b) F-1, (c), F-2 and (d) F-3 nanofibers,as well as the PCL particles: (e) without the aid of pressure and (f) without template.

1100 Y. Chen, L. Zhang, X. Lu, N. Zhao, J. Xu


presence of pressure, the solidified PCL deposits show a

fiber-like shape as shown in Figure 2(e). The diameter of

these is much larger than the nanofibers from the pressure-

inducing extrusion. This could be assigned to the

conglomeration of the PCL micro drops. The loss of

the pressure leads to the absence of the driving force for

the continuous flowing of PCL solution through the nano-

scale channel. The PCL solution dropping directly into the

solidifying solution without the orientation of template and

pressure forms congregated grains. TheSEMmicrograph of

the PCL grains is shown in Figure 2(f). On the basis of

morphology, it is believed that PCL nanofibers could be

prepared by way of extruding from the template under a

certain pressure, with the template and pressure having

great influence on the morphology and structure of the

nanofibers.

Figure 3 presents theDSC thermograms of the nanofibers

from the PCL with different molecular weight. It is clear

that endothermic peak belonging to the melting of PCL

crystal. The melting temperature (Tm) is elevated from 55.4

to 57.5 8C when the molecular weight increases from

3.6� 104 to 6.6� 104. The melting enthalpy calculated

from the area of the melting peak increases from 49.4 to

53.5 J � g�1, that is to say, the crystallinity of the PCL nano-

fibers slightly increases with the increase of the molecular

weight of the PCL. These results indicate that increasing

molecular weight could help to the crystallization of PCL at

nano-scale. The crystalline structure of the PCL nanofibers

was detected with XRD and the results are shown in

Figure 4. There are two obvious peaks at a 2y of 21.4 and

23.98. The former one is assigned to the diffraction of the

(110) lattice plane, whereas the latter is attributed to the

(200) plane.[30] Interestingly, a weak diffraction peak

occurs in the low 2y range from 2.36 to 2.908. This dif-

fraction is attributed to the (001) lattice plane of the PCL

crystal. Normally, the (001) lattice plane is assigned to the

periodicity of the chain direction. So this phenomenon

suggests that the extrusion of the PCL solution through the

nano-scale channel is beneficial for the orientation of

polymer chains in the nano-fibers. However, the (001)

spacing values of 30.4� 37.4 A calculated with the Bragg

function are larger than that of the poly(o-pentadecalac-tone) (PPDL) crystal. This indicates the stacking structures

of the PCL nanofibers in this case are looser than those of

conditioned PPDL samples.[31]

Figure 5 shows the effect of the extruding pressure upon

the melting behavior of the PCL fibers. It can be clearly

observed that the crystallinity of the PCL fibers through the

same process but without the aid of pressure is much lower

than those with pressure driving, which is depicted as the

decrease of melting enthalpy and the crystallization peak at

Figure 3. DSC thermograms of PCL nanofibers with differentmolecular weight (F-1, F-2 and F-3).

Figure 4. XRD patterns of crystalline structure of PCL nano-fibers with different molecular weight (F-1, F-2 and F-3) at2y¼ 1� 408.

Figure 5. DSC thermograms of PCL nanofibers with (F-1) orwithout (F-1-N) the aid of pressure.



54 8C prior to the melting peak at 57 8C during heating.

Under the fabrication condition mentioned above, the PCL

should crystallize in the period from the extrusion to the

solidified condition, because there is no accessional condi-

tioning for the crystallization of the PCL fiber before the

DSC testing. So it is believed that the pressure plays a key

role in the orientation and crystallization of the PCL

nanofibers during such a fast extrusion and solidified pro-

cess. The XRD patterns of F-1 and F-1-N are depicted in

Figure 6, showing the effect of pressure upon the crystalline

structure of the nanofibers. In the wide angle range, the

diffractions at 21.4 and 23.98 for the PCL samples with or

without pressure driving are similar, that is, the pressure has

no significant influence on the structure of the (110) and

(200) lattice plane of PCL crystalline. However, the weak

diffraction at the small 2y range disappears when the PCL

solution was extruded without the assistance of pressure.

This result suggests that the main contribution of the pres-

sure to the crystallization of the PCL fibers is to extend and

orientate the polymer chains toward the chain direction

during their passing through the nano-scale channel on the

porous template.[32] The observations from DSC and XRD

for the F-1 and F-1-N samples meet agreement with the

results from SEM.

Figure 7 shows themelting behavior recorded byDSC for

the grain-like PCL particles obtained without the porous

template. We can observe that the Tm of the PCL crystalline

is hardly changed, whereas the value of melting enthalpy of

the nanofibers (F-1) is much larger than those grain-like

particles (F-1-P). The crystallinity of the grains estimated

by the area of the melting peak is much lower than those

nanofibers. This further confirms that the template with

nano-scale channels and the accessional pressure are res-

ponsible for the orientation and shaping of the nanofibers.

Conclusion

Poly(e-caprolactone) (PCL) nanofibers with a dimension

of about 150 nm have been successfully fabricated via a

process of extruding PCL solution under 0.2 MPa pressure

through a porous aluminium oxide template and then

solidifying in methanol. The morphology of the nanofibers

hardly changed as the molecular weight of PCL increased.

However, when the accessional pressure was absent, the

dimension of the fiber became larger as a result of the

conglomeration of the PCL micro drops from the channels

of template. The increase of the molecular weight of

the PCL slightly elevated the melting temperature (Tm) of

the nanofibers. The results from XRD revealed that the

accessional pressure played a key role in the orientation and

the crystallization of the polymer chains. On the whole, the

accessional pressure and the aluminium oxide template

with a pore dimension of 200 nm were responsible for the

morphology, melting behavior and crystalline structure of

the PCL nanofibers.

Acknowledgements: The authors thank the NSFC (No.50373049, 50425312, 50521302) and CAS Innovation Projectfor financial support.

[1] Y. Z. Zhang, Z. M. Huang, X. J. Xu, C. T. Lim,S. Ramakrishna, Chem. Mater. 2004, 16, 3406.

[2] X. Zong, K. Kim, D. Fang, S. Ran, B. S. Hsiao, B. Chu,Polymer 2002, 43, 4403.

[3] E. R. Kenawy, G. L. Bowlin, K. Mansfield, J. Layman,D. G. Simpson, E. H. Sanders, G. E. Wnek, J. ControlledRelease 2002, 81, 57.

[4] J. Zeng, X. Xu, X. Chen, Q. Liang, X. Bian, L. Yang, X. Jing,J. Controlled Release 2003, 92, 227.

[5] W. J. Li, C. T. Laurencin, E. J. Caterson, R. S. Tuan, F. K. Ko,J. Biomed. Mater. Res. 2002, 60, 613.

Figure 6. XRD patterns of crystalline structure of PCL depositswith (F-1) or without (F-1-N) the aid of pressure at 2y¼ 1� 408.

Figure 7. DSC thermograms of PCL nanofibers with (F-1) orwithout (F-1-P) the aid of the template.

1102 Y. Chen, L. Zhang, X. Lu, N. Zhao, J. Xu


[6] H. Yoshimoto, Y. M. Shin, H. Terai, J. P. Vacanti,Biomaterials 2003, 24, 2077.

[7] C. Y. Xu, R. Inai, M. Kotaki, S. Ramakrishna, Biomaterials2004, 25, 877.

[8] A. G. MacDiarmid, W. E. Jones, I. D. Norris, J. Gao, A. T.Johnson, N. J. Pinto, J. Hone, B. Han, F. K. Ko, H. Okuzaki,M. Llaguno, Synth. Met. 2001, 119, 27.

[9] C.Drew,X. Liu, D. Ziegler, X.Wang, F. F. Bruno, J.Whitten,L. A. Samuelson, J. Kumar, Nano Lett. 2003, 3, 143.

[10] M. S. Khil, D. I. Cha, H. Y. Kim, I. S. Kim, N. Bhattarai, J.Biomed. Mater. Res. Part B: Appl. Biomater. 2003, 67B, 675.

[11] H. Schreuder-Gibson, P. Gibson, K. Senecal, M. Sennett,J. Walker, W. Yeomans, D. Ziegler, P. P. Tsai, J. Adv. Mater.2002, 34, 44.

[12] X. Wang, C. Drew, S.-H. Lee, K. J. Senecal, J. Kuma, L. A.Samuelson, Nano Lett. 2002, 2, 1273.

[13] L. Feng, S. H. Li, H. J. Li, J. Zhai, Y. L. Song, L. Jiang, D. B.Zhu, Angew. Chem. Int. Ed. 2002, 41, 1221.

[14] L. Feng, Y. L. Song, J. Zhai, B. Q. Liu, J. Xu, L. Jiang, D. B.Zhu, Angew. Chem. Int. Ed. 2003, 42, 800.

[15] B. O. Lee, W. J. Woo, M. S. Kim, Macromol. Mater. Eng.2001, 286, 114.

[16] A. Frenot, I. S. Chronakis,Curr. Opin. Colloid Interface Sci.2003, 8, 64.

[17] C. Y. Chen, J. W. Wang, M. H. Hon,Macromol. Mater. Eng.2006, 291, 123.

[18] P. Supaphol, C. Mit-uppatham, M. Nithitanakul,Macromol.Mater. Eng. 2005, 290, 933.

[19] C.M. Hsu, S. Shivkumar,Macromol. Mater. Eng. 2004, 289,334.

[20] J. A. Matthews, G. E. Wnek, D. G. Simpson, G. L. Bowlin,Biomacromolecules 2002, 3, 232.

[21] C. G. Wu, T. Bein, Science 1994, 264, 1757.[22] C. R. Martin, Science 1994, 266, 1961.[23] M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn,

K. Nielsch, J. Schilling, J. Choi, U. Go, Science 2002, 296,1997.

[24] A. J. Bomb, J. Kost, D. M. Wiseman, ‘‘Handbook of bio-degradable polymer’’, Hardwood Academic Publisher,1997.

[25] Y. Z. Zhang, J. Venugopal, Z. M. Huang, C. T. Lim, S.Ramakrishna, Biomacromolecules 2005, 6, 2583.

[26] S. Lee, B. Kim, S. Kim, S. Kang, Y. Kim,Macromol. Biosci.2004, 4, 802.

[27] R. Ho, Y. Chiang, C. Lin, B. Huang,Macromolecules 2005,38, 4769.

[28] Q. Guo, R. Thomann, W. Gronski, R. Staneva, R. Ivanova,B. Stuhn,Macromolecules 2003, 36, 3635.

[29] L. Q. Liao, L. J. Liu, C. Zhang, F. He, R. X. Zhuo, K. Wan,J Polym Sci Part A: Polym. Chem. 2002, 40, 1749.

[30] M. Kakudo, N. Kasai, ‘‘X-Ray Diffraction by Polymers’’,American Elsevier Publishing, New York 1972.

[31] G.Ceccorulli,M. Scandola, A.Kumar, B.Kalra, R.A.Bross,Biomacromolecules 2005, 6, 902.

[32] M. Rubinstein, R. H. Colby, ‘‘Polymer Physics’’, OxfordUniversity Press, 2001.



Morphology and Crystalline Structure of Poly(ε-Caprolactone) Nanofiber via Porous Aluminium Oxide...

Documents

Transcript of Morphology and Crystalline Structure of Poly(ε-Caprolactone) Nanofiber via Porous Aluminium Oxide...