364

Full Paper

DOI: 10.1002/mabi.200600017

Summary: Fibrous membranes with a fiber diameter ranging from 80 to 800 nm are prepared from polyacrylonitrile and poly[acrylonitrile-co-(N-vinyl-2-pyrrolidone)] by the electrospinning process. The parameters can be controlled to fabricate fibrous membranes with similar fiber diameters (between 600 and 800 nm) for further studies on the swelling behaviors and water states. Water swelling experiments indicate that the fibrous membrane has a great capacity for water sorption, which reaches a maximum in a few minutes because of its extremely high porosity. Furthermore, a remarkable overshoot occurs as a result of polymer chain relaxation and the non-compact structure of the fibrous membranes. Contrary to the dense membrane, the equilibrium water content in the fibrous membrane decreases with the content of hydrophilic NVP though the maximum is almost the same. Results from DSC experiments demonstrate that only non-freezable bound water and free water can be distinguished in the fibrous membrane. On the basis of the results of water swelling and DSC experiments, it is concluded that the specific behaviors of the fibrous membranes are induced by the noncompact and pore-fiber discontinuous structure, which is different from either dense membranes or hydrogels.

DSC curves of fully swollen electrospun fibrous membranes and of fully swollen dense membranes with different NVP contents.

Fibrous Membranes Electrospinning from Acrylonitrile-Based Polymers: Specific Absorption Behaviors and States of Water Ling-Shu Wan, Zhi-Kang Xu,* Hong-Liang Jiang Institute of Polymer Science, and Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, P. R. China Fax: þþ86 571 8795 1773; E-mail: [email protected]

Received: January 27, 2006; Revised: March 17, 2006; Accepted: March 17, 2006; DOI: 10.1002/mabi.200600017 Keywords: copolymers; electrospinning; fibers; fibrous membrane; polyacrylonitrile; swelling; water state

Introduction Our previous work has confirmed the good biocompatibility of the copolymer of acrylonitrile with N-vinyl-2-pyrrolidone (NVP) and the swelling behaviors of the corresponding dense membranes have been reported as well as the water states examined by differential scanning calorimetry (DSC).[1] Recently, membranes made up of nanofibers have received considerable interest. This is attributed to their amazing characteristics such as very large surface area to volume ratio and superior mechanical properties compared with traditional membranes. Electrospinning is a simple

Macromol. Biosci. 2006, 6, 364–372

and versatile method to generate nano- to submicrometer fibers. Many researchers have shown great interest in the preparation and utilization of nanofibers by the electrospinning process which has led to many outstanding improvements. Xia and other researchers have reviewed the main progress of electrospinning over the past few years.[2–4] Most recently, some polyacrylonitrile (PAN)-based electrospun fibrous materials have been reported, especially those embedded with carbon nanotubes.[5–14] There is no doubt that electrospun fibrous membranes can be used for biomedical purposes or biological

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fibrous Membranes Electrospinning from Acrylonitrile-Based Polymers: . . .

catalysis/separation.[15–17] However, the behaviors of biological molecules adsorbed or coupled on this type of membrane must be carefully studied, because the nanostructure may endow the fibrous membrane with unfamiliar properties. Biological molecules such as enzymes often function in aqueous solution, therefore, swelling with water of the electrospun fibrous membrane is important for the adsorption and function of these molecules. Moreover, many polymers that provide a biocompatible or friendly microenvironment are hydrophilic or even water soluble.[18–22] Therefore, to clarify the possible effects of the swelling behaviors of an electrospun fibrous membrane is very interesting. Up to now, most concerns has been focused on the swelling behaviors of hydrogels. As a kind of material with extremely high porosity, however, few results concerning the swelling behaviors of electrospun fibrous membranes have been reported.[23] Besides the swelling behaviors, understanding the influence of water states in the membrane is an attractive and challenging work.[24–26] Water in a polymer membrane can be classified into non-freezable and freezable forms, or more specifically, non-freezable bound water, freezable bound water, and free water. However, the number of water types detected in a membrane depends on the technique used. NMR spectroscopy,[27] DSC,[28–32] and IR spectroscopy[25,33–35] have been reported to probe the water states in swollen polymers. Among them, DSC has been extensively used to study phase transition behavior, heat capacity, and pore size distribution by examining the water in polymers.[36–40] It is well known that PAN shows many advantages and has been widely used as membrane materials.[18,41–45] According to our previous work and other reports,[1,20,46] poly[acrylonitrile-co-(N-vinyl-2-pyrrolidone)] (PANCNVP) is of excellent biocompatibility and shows some applications in blood-contacting devices and enzyme immobilization. Since the dense membranes have been studied previously, herein, the states of water in the corresponding fibrous membranes as well as the swelling behaviors are investigated. Membranes with a fiber diameter between 80 and 800 nm are electrospun from biocompatible PANCNVPs. Swelling behaviors of the resultant membranes in water are carefully studied and the water states are also examined by DSC.

365

Table 1. Molecular weight and NVP content of the polymers used for electrospinning. Polymers

PAN PANCNVP7 PANCNVP15 PANCNVP22 PANCNVP31

[Z]

M v a) (104)

NVPb)

mL  g
1

g  mol
1

wt.-%

116.4 121.3 137.6 153.8 143.5

5.00 5.30 6.20 7.20 6.60

0 6.9 14.9 21.7 30.6

a)

Viscosity-average molecular weight of the polymer was 0:768 calculated from the relationship of [Z] ¼ 2.865  10
2 M v in which the intrinsic viscosity was measured in DMSO at 30 8C. b) The content of NVP in the polymer was calculated from 1 H NMR spectroscopy.

syringe, a blunt-end needle (12#, inner diameter is 1.2 mm), a ground electrode (tinfoil sheet on a flat glass), and a high voltage power supply (GDW-a, Tianjin Dongwen Highvoltage Power Supply Plant, China) with a low current output. A positive voltage (10 kV) was applied to the polymer solution with the distance between the syringe tip and the collector surface being ca. 15 cm for PANCNVP solutions, respectively. PAN or a PANCNVP homogeneous solution (8–16 wt.-% for PAN and 12 wt.-% for PANCNVP) was prepared by dissolution in N,N-dimethylformamide (DMF) at about 60 8C with stirring for 6 h and was electrospun after air bubbles were removed completely. The flow rate of the solution was kept at 1 mL h
1 by a microinfusion pump (WZ-50C2, Zhejiang University Medical Instrument Co., Ltd., China). It usually took at least 2 h to obtain a usable membrane and the resultant membrane was dried under vacuum at 60 8C before use.

Membrane Characterization Field-emission scanning electron microscopy (FESEM, Sirion-100, FEI, USA) was applied to study the morphologies of the fibrous membrane. Before analysis, the samples were sputtered with gold using an Ion sputter JFC-1100. Static water contact angles of the membranes were measured by the sessile drop method at room temperature with a contact angle goniometer (Dataphysics, OCA20, Germany) equipped with video camera. At least ten measurements of different water drops were averaged to get a reliable value. Dynamic contact angle, the dependence of contact angle on time, was also recorded every 2 s over 40 min.

Experimental Part Sample Preparation

Swelling Behavior Determination

PAN and PANCNVP were synthesized in our laboratory by solution polymerization methods. Details for the characterization of the copolymers were described in our previous paper[46] and typical results are summarized in Table 1. The electrospinning set-up utilized in this study is similar to that reported by Fujihara et al.,[47] which consists of a plastic

Dry membrane was cut into pieces of 4  4 cm2 and weighed accurately. These membranes were then immersed into deionized water at a designated temperature and left until equilibrium was achieved. The equilibrium water content, WE, was calculated by gravimetry according to the following equation:

Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

366

L.-S. Wan, Z.-K. Xu, H.-L. Jiang

WE ¼ ðW
W0 Þ=W

ð1Þ

where W is the weight of the fully swollen membrane and W0 is the weight of the dry membrane. The swelling behavior was followed by measuring the weight gain with the time of immersion after wiping the surface with filter papers. Water uptake Wc(t) at time t was also obtained according to the following equation:

Wc ðtÞ ¼ ðWt
W0 Þ=W0

DSC conducted on a STA409PC thermal analysis system was used to examine the states of water in the swollen membranes. The samples sealed in aluminum pans were cooled to
60 8C and then heated to 25 8C at a heating rate of 5 8C min
1 under a nitrogen gas flow. Samples of PANCNVP with different water contents were prepared by water evaporation at room temperature of the membranes previously swollen to equilibrium.

ð2Þ

where Wt is the weight of the membrane at time t. The behavior of water evaporation of the fully swollen membranes was measured gravimetrically at 25 8C (relative humidity is 50%) after the corresponding equilibrium water content WE had been determined. The membranes were exposed to the air and the weight changes were recorded at regular time intervals. Water retention Wr is defined as:

Wr ¼ ðWt
W0 Þ=ðWE
W0 Þ

DSC Measurement

ð3Þ

Results and Discussion Preparation and Characterization of the Fibrous Membranes Table 1 shows the molecular weight and the composition of the polymers used for membrane fabrication. The copolymers are analyzed by 1H NMR spectroscopy to measure the content of NVP and then the polymers are denoted as PAN, PANCNVP7, PANCNVP15, PANCNVP22, and

Figure 1. Typical FESEM micrographs of membranes electrospun from a) 8, b) 10, c) 12, and d) 16 wt.-% PAN. (50 000 for (a–c) and 10 000 for (d)) Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fibrous Membranes Electrospinning from Acrylonitrile-Based Polymers: . . .

PANCNVP31. Electrospinning parameters for fabricating the fibrous membranes are optimized by evaluating the effects of polymer concentration, molecular weight, voltage applied, and tip-to-collector distance. The solution concentration has a great effect on the fiber diameter (for beaded fibers, which is defined as the diameter of thread between beads). As shown in Figure 1, fibers with uniformly dispersed size can be produced from PAN solution with a concentration ranging from 8 to 16 wt.-%. Fibers with a diameter of about 80 nm are obtained from 8 wt.-% PAN while the diameter increases sharply to about 700 nm for 16 wt.-% PAN. It can be seen from Figure 2 that fibers with a diameter down to about 200 nm can be produced from PANCNVP7, which possesses the lowest molecular weight among the studied copolymers. The average diameters of fibers are summarized in Figure 3. Bead-free fibrous membranes can only be obtained from those copolymers with relatively high molecular weights, i.e., PANCNVP22 and PANCNVP31, though the introduction of NVP seems to show some effect on the morphologies of the fibrous membranes. However, beads in the membrane

367

can be avoided by increasing the solution concentration. Bead-free fibers with a diameter of 600–800 nm are produced by increasing the solution concentration from 12 to 16 wt.-% for PANCNVP7 and PANCNVP15. Compared with polymer concentration and molecular weight, on the other hand, the applied voltage and the tip-to-collector distance exhibit less influence on the fiber diameter. As proposed by Huang et al.,[48] both beads on the fibers and too large a diameter will decrease the mechanical strength of the fibrous membranes. Therefore, for both PAN and the copolymers, bead-free fibrous membranes with a fiber diameter between 600 and 800 nm are collected for further studies. Static water contact angles of the fibrous membranes and found to be about 1308, while those of the corresponding dense membranes were only about 708. It seems that the contact angles have nothing to do with the chemical composition of the polymers because of the high porosity and the special structure of the fibrous membranes. Similarly, superhydrophobic electrospun fibrous membranes have been reported.[6,49] Figure 4 shows the dynamic

Figure 2. Typical FESEM micrographs of membranes electrospun from PANCNVPs with the following content of NVP: a) 7, b) 15, c) 22, and d) 31 wt.-%. (10 000). Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

368

L.-S. Wan, Z.-K. Xu, H.-L. Jiang

Swelling Behaviors of the Fibrous Membranes

Figure 3. Average diameters of fibers for membranes electrospun from (1) 8, (2) 10, (3) 12, and (4) 16 wt.-% PAN and 12 wt.-% PANCNVPs with the following content of NVP: (5) 7, (6) 15, (7) 22, and (8) 31 wt.-%.

water contact angles on the fibrous PAN and PANCNVP membranes. The PANCNVP membrane from a 16 wt.-% solution shows the same behavior as the PAN membrane. For the fibrous membrane fabricated from a low concentration of PANCNVP, an especially thin membrane (i.e., the membrane is collected for a short time, such as 60 min), a water drop on the membrane surface is unstable and tends to quickly penetrate into the membrane. This means the extraordinary structure might govern the properties of the electrospun fibrous membrane.

As we know, the swelling of polymer gels takes place mostly because of the hydrophilic groups, for example, the hydrogen-bonding ability is very important.[50] For the dense PANCNVP membranes, results reported previously[1] show that the WE increases remarkably with the content of NVP in polymers because of the hydrophilicity of NVP units.[28] Furthermore, the value of Wc increases slowly and then reaches an equilibrium after the membrane is immersed into water for 120 to 180 min. It is regular and reasonable because the relatively denser structure of the dense membrane hinders the diffusion of water molecules. On the other hand, the fibrous membranes show different swelling behaviors compared with the corresponding dense membranes. Figure 5 shows the swelling kinetics of the fibrous membranes. It is clear that the ability to adsorb water of the fibrous membranes is very large, as indicated by the WE data shown in Table 2. It seems that water molecules diffuse into the fibrous membrane as soon as the membrane is fully dipped into water. The value of Wc of every fibrous membrane shows a maximum at a very short time (10 min or less) and then decreases slowly.[23] It is noticeable that the maximum is approximately the same for each fibrous membrane, which might result from the fiber diameter of each sample being almost the same. Interestingly, in comparison with the dense membranes, an obvious overshoot can be found for all fibrous membranes. Unlike the denser structure of dense membranes, the fibrous membranes are highly porous. Thus the diffusion of water molecules into the membrane is very

Figure 4. Dynamic water contact angles of membranes electrospun from 16 wt.-% PANCNVP15 (line a); 16 wt.-% PAN (line b), and 6 wt.-% PANCNVP15 (black square). The inset shows the results of 6 wt.-% PANCNVP15 within 22 s. Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fibrous Membranes Electrospinning from Acrylonitrile-Based Polymers: . . .

369

Figure 5. Sorption isotherms of electrospun fibrous membranes with different NVP contents at 37 8C. (~) PAN, (&) PANCNVP15, and (&) PANCNVP22.

easy. Generally, the WE of a polymer gel is determined by the content of hydrophilic components, the cross-linking density, and some other factors, i.e., the swelling is mainly affected by the interactions between water molecules and the hydrophilic groups and the free volume. The free volume (strictly porosity should be used here) of the fibrous membrane is extremely large, which should govern the swelling behaviors. This point is supported by the fact that the maximum of Wc is almost the same for the three fibrous membranes. After the penetration of water molecules into the fibrous membrane, the effect of the relaxation of polymer chains to their equilibrium state in the presence of water molecules is more marked than that for a dense membrane or cross-linked hydrogel. This process results in an expulsion of some water molecules from the fibrous membrane, thus overshoot takes place markedly. The dense membranes do not display any significant overshoot behavior, which indicates a tighter structure and a confined chain relaxation. It is obvious that the fibrous membrane shows an non-compact structure, and the process of overshoot is expected to be more pronounced for mem-

PAN PANCNVP15 PANCNVP22

Water States Examined by DSC

WE

Wf

Wnf

0.828 0.772 0.674

0.812 0.666 0.526

0.004 0.117 0.145

The study of the structure and chemical behavior of water is highlighted as one of the breakthroughs of the year of 2004 by Science magazine.[55] Undoubtedly, the interactions between water molecules and polymer chains are also very important for the properties of polymers.[33–35,56] DSC is one of the most common methods used to probe the distribution of different types of water in swollen polymers, which monitors the heat capacity associated with the phase transition induced by temperature changes. With few exceptions, DSC is insensitive to the thermal events of water below
70 8C, for the change in heat capacity is too small to be detected. The samples are cooled to about
70 8C and the heating to room temperature is recorded.

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 2. Contents of equilibrium water (WE), freezable water (Wf) and non-freezable bound water (Wnf) in the studied membranes at 37 8C. Samples

branes with high NVP contents.[51] Therefore, an obvious overshoot behavior is observed and the WE decreases with the NVP content for the fibrous membranes. Overshoot for hydrogels has been reported by several researchers[52–54] and the structure of the fibrous membrane seems to be responsible for its specific swelling behaviors. Water retentions of the fully swollen membranes are also measured in the air at room temperature, as shown in Figure 6. Water retention increases with the NVP content though the equilibrium water retentions in the fibrous membranes were very low because of the ease of evaporation. Moreover, the evaporation of water is faster for a membrane with lower content of hydrophilic NVP, which might be associated with the interaction between water molecules with the hydrophilic moieties.

Macromol. Biosci. 2006, 6, 364–372

370

L.-S. Wan, Z.-K. Xu, H.-L. Jiang

Figure 6. Water retentions of electrospun fibrous membranes with different NVP contents in the air at room temperature. (~) PAN, (&) PANCNVP15, and (&) PANCNVP22.

Figure 7 shows the DSC curves of the fully swollen membranes and a melting peak can be found for each sample, which originates from the freezable water. The shape of the melting peak is influenced by several factors, such as the heating rate during DSC analysis and the water content in the membrane. Some researchers report that two distinct melting peaks induced by free water and freezable

bound water can be seen in DSC curves at around 0 and
10 8C.[28,30] A melting peak with a shoulder has also been reported by Tanaka and co-workers[25,26] Sharp peaks at about 2 8C are found for the fibrous membranes (Figure 7) while a visible shoulder appears for the corresponding dense membrane (see ref. [1]). It means that freezable bound water does not exist in the fibrous membranes at all or the

Figure 7. DSC curves of fully swollen electrospun fibrous membranes with different NVP contents as indicated in the diagram.

Figure 8. DSC curves of electrospun fibrous PANCNVP22 membrane with different water contents as indicated in the diagram. Here WE denotes the water content in the membrane after evaporating in the air.

Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fibrous Membranes Electrospinning from Acrylonitrile-Based Polymers: . . .

amount of this type of water is very small. It is acceptable that only free and non-freezable bound water exists in the fibrous membranes, because the ‘hole’ of the fibrous membranes is very large and the water molecules can freely move between fibers except for those tightly bound to the polymer chain (such as to the carbonyl group in the NVP unit). For the dense membranes, however, the free volume confines the movements of the water molecules to some extent. Furthermore, the content of non-freezable water Wnf can be calculated from Equation (4) by assuming the enthalpy change of the melting of the freezable water equals that of pure bulk water at any temperature:[30,57,58] Wnf ¼ WE
ðWf
Wfb Þ ¼ WE
Qendo =Qf

ð4Þ

where Wf, Wfb, and Wnf are contents of free water, freezable bound water, and non-freezable bound water in the swollen membranes, respectively. Qendo (J  g
1) is the heat that is from the area of the DSC curve divided by the weight of swollen membrane, while Qf (334 J  g
1) is the phase transition heat of pure bulk ice reported in the literature. As shown in Table 2, the content of Wnf increases with the NVP content for both fibrous and dense membranes. On the other hand, the amount of Wf (including freezable free water and freezable bound water) of the fibrous membrane is very large and decreases with the NVP content. Different from a dense membrane and cross-linked hydrogel, the fibrous membrane has a pore-fiber discontinuous structure. Thus, a majority of Wf is freezable free water. However, the increase of the Wf of a dense membrane with the NVP content can be attributed to the introduction of hydrophilic NVP units to some extent, as in the case of the swelling of hydrogels. This point is well supported by the discrimination of the curve shape in Figure 7. In addition, DSC curves of a PANCNVP22 fibrous membrane (Figure 8) and a PANCNVP31 dense membrane (see ref. [1]) with different water contents are recorded to ascertain the non-freezable bound water content. No melting peak is found for the fibrous membrane with WE ¼ 0.094 (WE ¼ 0.153 for the dense membrane), while distinct melting peaks appear for those with a WE of more than 0.429 (0.343 for the dense membrane).

Conclusion PAN-based fibrous membranes are prepared by the electrospinning process and the swelling behaviors of the resultant membranes are carefully studied. Different from the swelling of the dense membrane, water molecules enter the fibrous membrane rapidly after the membrane is fully dipped into water. The maximum water uptake is achieved in a very short time and is almost the same for each fibrous membrane. An obvious overshoot can then be observed. Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

371

Furthermore, the equilibrium water content decreases with the NVP content in the copolymer, which might be a result of the relaxation of polymer chains and, in particular, the non-compact structure of the fibrous membrane. Results from DSC experiments confirm that a large amount of freezable free water exists in the fibrous membranes, though the content of non-freezable bound water increases with the NVP content. Effects of the water contents and the water states on the adsorption and activity of enzymes, which requires essential water to function better in organic media, are now being examined in our lab.

Acknowledgements: Financial support from the National Natural Science Foundation of China (Grant no. 50273032) is gratefully acknowledged.

[1] L.-S. Wan, Z.-K. Xu, X.-J. Huang, Z.-G. Wang, P. Ye, Macromol. Biosci. 2005, 5, 229. [2] D. Li, Y. N. Xia, Adv. Mater. 2004, 16, 1151. [3] Z. M. Huang, Y. Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. 2003, 63, 2223. [4] L. A. Smith, P. X. Ma, Colloids Surf. B 2004, 39, 125. [5] C. Drew, X. Liu, D. Ziegler, X. Y. Wang, F. F. Bruno, J. Whitten, L. A. Samuelson, J. Kumar, Nano Lett. 2003, 3, 143. [6] K. Acatay, E. Simsek, C. Ow-Yang, Y. Z. Menceloglu, Angew. Chem. Int. Ed. 2004, 43, 5210. [7] S. F. Fennessey, R. J. Farris, Polymer 2004, 45, 4217. [8] X. H. Qin, Y. Q. Wan, J. H. He, J. Zhang, J. Y. Yu, S. Y. Wang, Polymer 2004, 45, 6409. [9] S. Y. Gu, Q. L. Wu, J. Ren, G. J. Vancso, Macromol. Rapid Commun. 2005, 26, 716. [10] Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin, E. Zussman, Langmuir 2003, 19, 7012. [11] J. J. Ge, H. Q. Hou, Q. Li, M. J. Graham, A. Greiner, D. H. Reneker, F. W. Harris, S. Z. D. Cheng, J. Am. Chem. Soc. 2004, 126, 15754. [12] H. Q. Hou, D. H. Reneker, Adv. Mater. 2004, 16, 69. [13] C. Kim, S. H. Park, J. K. Cho, D. Y. Lee, T. J. Park, W. J. Lee, K. S. Yang, J. Raman Spectrosc. 2004, 35, 928. [14] H. Q. Hou, J. J. Ge, J. Zeng, Q. Li, D. H. Reneker, A. Greiner, S. Z. D. Cheng, Chem. Mater. 2005, 17, 967. [15] H. F. Jia, G. Y. Zhu, B. Vugrinovich, W. Kataphinan, D. H. Reneker, P. Wang, Biotechnol. Prog. 2002, 18, 1027. [16] J. B. Xie, Y. L. Hsieh, J. Mater. Sci. 2003, 38, 2125. [17] H. Chen, Y. L. Hsieh, Biotechnol. Bioeng. 2005, 90, 405. [18] T. Groth, B. Seifert, G. Malsch, W. Albrecht, D. Paul, A. Kostadinova, N. Krasteva, G. Altankov, J. Biomed. Mater. Res. 2002, 61, 290. [19] B. Kaeselev, J. Pieracci, G. Belfort, J. Membr. Sci. 2001, 194, 245. [20] N. Krasteva, U. Harms, W. Albrecht, B. Seifert, M. Hopp, G. Altankov, T. Groth, Biomaterials 2002, 23, 2467. [21] K. Ishihara, K. Fukumoto, Y. Iwasaki, N. Nakabayashi, Biomaterials 1999, 20, 1553. [22] H. Petersen, P. M. Fechner, D. Fischer, T. Kissel, Macromolecules 2002, 35, 6867. [23] L. M. Hansen, D. J. Smith, D. H. Reneker, W. Kataphinan, J. Appl. Polym. Sci. 2005, 95, 427. ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

372

L.-S. Wan, Z.-K. Xu, H.-L. Jiang

[24] K. Ishihara, H. Nomura, T. Mihara, K. Kurita, Y. Iwasaki, N. Nakabayashi, J. Biomed. Mater. Res. 1998, 39, 323. [25] M. Ide, T. Mori, K. Ichikawa, H. Kitano, M. Tanaka, A. Mochizuki, H. Oshiyama, W. Mizuno, Langmuir 2003, 19, 429. [26] M. Tanaka, A. Mochizuki, J. Biomed. Mater. Res. A 2004, 68A, 684. [27] W. G. Liu, K. D. Yao, Polymer 2001, 42, 3943. [28] M. B. Ahmad, M. B. Huglin, Polym. Int. 1994, 33, 273. [29] Z. H. Ping, Q. T. Nguyen, S. M. Chen, J. Q. Zhou, Y. D. Ding, Polymer 2001, 42, 8461. [30] S. J. Kim, C. K. Lee, S. I. Kim, J. Appl. Polym. Sci. 2004, 92, 1467. [31] J. C. Galin, M. Galin, J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 1103. [32] Y. L. Guan, L. Shao, J. Liu, K. D. Yao, J. Appl. Polym. Sci. 1996, 62, 1253. [33] S. Cotugno, G. Mensitieri, P. Musto, L. Sanguigno, Macromolecules 2005, 38, 801. [34] M. J. Liu, P. Y. Wu, Y. F. Ding, G. Chen, S. J. Li, Macromolecules 2002, 35, 5500. [35] L. N. Li, S. Y. Zhang, Y. H. Chen, M. J. Liu, Y. F. Ding, X. W. Luo, Z. Pu, W. F. Zhou, S. J. Li, Chem. Mater. 2005, 17, 839. [36] V. J. Mcbrierty, F. X. Quinn, C. Keely, A. C. Wilson, G. D. Friends, Macromolecules 1992, 25, 4281. [37] M. Tanaka, A. Mochizuki, N. Ishii, T. Motomura, T. Hatakeyama, Biomacromolecules 2002, 3, 36. [38] B. Vazquez, J. S. Roman, C. Peniche, M. E. Cohen, Macromolecules 1997, 30, 8440. [39] F. X. Quinn, E. Kampff, G. Smyth, V. J. Mcbrierty, Macromolecules 1988, 21, 3191. [40] J. P. Cohen-Addad, A. Prunelet, J. P. Bazile, A. Buda, M. Thomas, Biomaterials 2003, 24, 173.

Macromol. Biosci. 2006, 6, 364–372

www.mbs-journal.de

[41] W. C. Lin, T. Y. Liu, M. C. Yang, Biomaterials 2004, 25, 1947. [42] T. Godjevargova, V. Konsulov, A. Dimov, J. Membr. Sci. 1999, 152, 235. [43] P. Ye, Z.-K. Xu, A.-F. Che, J. Wu, P. Seta, Biomaterials 2005, 26, 6394. [44] F.-Q. Nie, Z.-K. Xu, X.-J. Huang, P. Ye, J. Wu, Langmuir 2003, 19, 9889. [45] Z.-K. Xu, R.-Q. Kou, Z.-M. Liu, F.-Q. Nie, Y.-Y. Xu, Macromolecules 2003, 36, 2441. [46] L.-S. Wan, Z.-K. Xu, X.-J. Huang, Z.-G. Wang, J.-L. Wang, Polymer 2005, 46, 7715. [47] K. Fujihara, M. Kotaki, S. Ramakrishna, Biomaterials 2005, 26, 4139. [48] Z. M. Huang, Y. Z. Zhang, S. Ramakrishna, C. T. Lim, Polymer 2004, 45, 5361. [49] M. L. Ma, R. M. Hill, J. L. Lowery, S. V. Fridrikh, G. C. Rutledge, Langmuir 2005, 21, 5549. [50] E. Diez-Pena, I. Quijada-Garrido, J. M. Barrales-Rienda, Macromolecules 2002, 35, 8882. [51] P. Ghi, D. J. T. Hill, A. K. Whittaker, J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1939. [52] C. M. Hassan, N. A. Peppas, Macromolecules 2000, 33, 2472. [53] E. Diez-Pena, I. Quijada-Garrido, J. M. Barrales-Rienda, Macromolecules 2003, 36, 2475. [54] K. Itano, J. Y. Choi, M. F. Rubner, Macromolecules 2005, 38, 3450. [55] G. Images, J. Banfield, Science 2004, 306, 2013. [56] T. Tsuruta, Adv. Polym. Sci. 1996, 126, 1. [57] A. Higuchi, T. Iijima, Polymer 1985, 26, 1207. [58] A. Higuchi, T. Iijima, Polymer 1985, 26, 1833.

ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim