Confined crystallization of POM in the CA-nanotubes fabricated by coaxial electrospinning

European Polymer Journal xxx (2013) xxx–xxx

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Macromolecular Nanotechnology

Confined crystallization of POM in the CA-nanotubes fabricated by coaxial electrospinning Hongjun Luo a,b, Yong Huang a,c,d,⇑, Dongshan Wang a a

Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China University of Chinese Academy of Sciences, Beijing 100049, China c National Engineering Research Center for Engineering Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China d Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b

a r t i c l e

i n f o

Article history: Received 29 July 2012 Received in revised form 26 February 2013 Accepted 28 February 2013 Available online xxxx Keywords: Coaxial electrospinning Core–sheath Confined crystallization Polyoxymethylene (POM) Cellulose acetate (CA)

a b s t r a c t Polyoxymethylene (POM)/cellulose acetate (CA) core–sheath ultrafine fibers were fabricated by coaxial electrospinning and used to study crystallization in a confined, long cylindrical space. Scanning electron microscopy and transmission electron microscopy revealed that the core–sheath structured ultrafine morphology was presented for the fibers with the average sheath diameter from 679 to 1495 nm and core diameter from 148 to 1000 nm which increased with increasing the concentration of the POM solution. The nonisothermal crystallization study indicated that the confinements sharply reduced the crystallinity and the crystallization temperature of the POM. The Avrami index derived from isothermal crystallization was also decreased with decreasing the dimension of the confined space. However, Wide angle X-ray diffraction showed the crystal size of the POM was decreased distinctly only when the dimension of the confinement was about 150 nm and increased with increasing the dimension of the confinement. It was suggested that both the mode of the crystal growth and the crystallinity will be affected by the cylindrical confinement during POM crystallization. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Crystallization is very important in polymer physics and the bulk crystallization of semi-crystalline polymers has been widely studied since 1950s [1]. As long as the boom of nanotechnologies during the past few decades, the investigation of polymer confined crystallization behaviors is a new important area in polymer physics due to the potential applications as well as scientific interest [2,3]. Several different physical approaches have been successfully applied to investigate crystallization of some polymers under

⇑ Corresponding author at: National Engineering Research Center for Engineering Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel.: +8610 82543478. E-mail address: [email protected] (Y. Huang).

one-, two- or three-dimensional (1D, 2D or 3D) confinements, such as the ultrafine polymer films fabricated by solution spin-coating method [4,5], the multi-layers of polymer/polymer films with hundreds or thousands of alternating layers prepared via melt layer-multiplying coextrusion [6–8], the polymer nano-cylinder confined in organic/inorganic templates [9–13]. Polymer–inorganic/organic hybrids synthesized by the sol–gel technique [14,15]. It is well known that polyoxymethylene (POM) is one of the typical engineering plastics with high crystallinity. The bulk crystallization of POM has been widely studied in the past three decades [16–25], but there is hardly any reference about confined crystallization of POM. The reason for that may be no appreciate method for fabrication of POM confinements being found so far, because the poor thermal stability of POM makes it difficult to fabricate multi-layers films, the high volatility of solvents for POM

0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.02.037

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(trifluoroacetic acid, TFA and hexafluoroisopropanol, HFI) makes it unable to obtained ultrathin films and the strong corrosivity of these solvents to metal templates makes it unfeasible to obtain nanorods. Fortunately, electrospinning (ES) is an efficient method for fabricating continuous ultrafine fibers and can be utilized to control the crystalline structure of POM [26,27]. However, the conventional ES fibers are still not appropriate for confined crystallization because of the aggregation of the fibers at melt state. The coaxial ES method, which is an improvement based on the conventional ES technique, is an easy, efficient, and robust method for making continuous core–sheath ultrafine fibers by using two different fluids [28–32]. Therefore, it may provide an efficient method for making stable confinements for semi-crystalline polymers if a crystalline polymer is made as the core and the other polymer as the sheath. A prevalent viewpoint has been achieved that a hexagonal POM crystal consisting of chains in helical conformation is common [33,34]. Another metastable form of the orthorhombic POM crystals can be found under some specific conditions [20,35]. In hexagonal POM crystals, the typical crystalline morphologies are either folded chain crystals (FCC), being always found in the crystalline growing from the dilute solution, or extended chain crystals (ECC), forming in the cationic polymerization [17,20,35]. The FCC and the ECC of the POM coexist in most cases [17,23,34,36], such as crystallization from melts. Fourier transform infrared spectroscopy (FTIR) has been proved as one of the most powerful methods for investigation the conformation of the POM among the various techniques because of its uniqueness and high sensitivity to changes in the chain’s conformation [36–38]. The ECC and FCC structures give the remarkably different FTIR spectra, although they show essentially the same X-ray diffraction pattern. Moreover, the FTIR spectra show dramatic differences from crystalline to melt state, and thereby the FTIR can be utilized to study the thermal behaviors of crystalline POM at various temperatures. In our previous work, the coaxial electrospinning had successively been utilized to study the confined crystallization of poly(ethylene glycol) (PEG) in cellulose acetate (CA) nanotubes by the DSC method. That work indicated the confinements had strong effects on the crystallization of PEG that the smaller of the confinements were, the lower crystallinity of PEG from re-melt in CA nanotubes. However, the thermal behavior could not be detected by FTIR as the chain conformation of PEG is not very sensitive to the physical states [39]. Seeing that the crystal structure of POM is sensitive to the FTIR as shown above, herein, the coaxial ES technique was also used to fabricate CA/POM core–sheath (C–S) ultrafine fibers with different core dimensions, in which the sheath was CA and the core was POM. The confined crystallization behavior of the POM using the obtained C–S ultrafine fibers as subjects was investigated by both DSC and FTIR, and the crystalline structure as well as its relationship with the thermal stability of the POM, formed in the confinement, was then discussed. This work would initiate a new method to study and control the POM morphology under confined conditions.

2. Experimental section 2.1. Materials CA (Mn = 50,000 g/mol) were obtained from Aldrich. Polyoxymethylene (POM, F30-03) was supplied by PTM Engineering Plastics (Nantong) Co., Ltd. 1,1,1,3,3,3-hexafluoro-2-propanol (HFP, technical grade, DuPont) was purchased from Guangzhou Chemical Agent Company, China. All chemicals were used as received without further purification. HFP is the solvent for both of the sheath solution and the core solution. The sheath CA solution with concentration of 9 wt% was prepared by dissolving 4.50 g of CA in 45.50 g HFP, while the core POM solutions with concentration of 1, 4, 7 and 10 wt% were prepared using HFP as solvent as well. 2.2. Coaxial electrospinning The setup for coaxial electrospinning was shown in Fig. 1. The coaxial electrospinning (ES) process was performed by using 5 and 10 mL syringe to deliver the core solution and the sheath solution, respectively. The syringe for core solution was directly connected to the inner needle of the coaxial needles, which were connected to a high DC voltage supply (HB-Z503-4AC, Tian Jin Heng Bo High Voltage Power Supply Plant) and the other one for the sheath solution was combined to the outer needle by a piece of silica tube with a diameter of 1 mm. The feed rate was set as 3 mL/h according to the core component, and the feed rate of the sheath component was 5.55 mL/h calculated from that of the core component by multiplying the sheath/core feed ratio (which was determined by inner diameter square ratio of the syringes). The distance between the tip and the grounded rotate collector was kept constant at 15 cm. Two concentric needles (connected to a high voltage supply) were home-made with the types of 26G and 17G for the core solution and the sheath solution, respectively. All the coaxial ES experiments were performed under the condition of the temperature controlled at 26 °C and the ambient humidity kept for 60%. The samples obtained from the coaxial ES were CA9 wt%/ POM1 wt%, CA9 wt%/POM4 wt% and CA9 wt%/POM7 wt%, which were shortened as C9O1, C9O4 and C9O7,

Fig. 1. The scheme of coaxial electrospinning device and tip structure of coaxial needle tip.

Please cite this article in press as: Luo H et al. Confined crystallization of POM in the CA-nanotubes fabricated by coaxial electrospinning. Eur Polym J (2013), http://dx.doi.org/10.1016/j.eurpolymj.2013.02.037

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H. Luo et al. / European Polymer Journal xxx (2013) xxx–xxx Table 1 Parameters of coaxial electrospinning.

a b c d

Sample

Concentration (wt%)

Contenta (wt%)

Feed rateb (ml/h)

Ratioc (RS/C)

TCDd (cm)

Voltage (kV)

C9O1 C9O4 C9O7 CA9 POM10

1 4 7 9 10

5.58 18.08 28.35 0 100

3 3 3 3 3

1.85 1.85 1.85 – –

15 15 15 15 15

20.5 22.0 23.0 19.5 18.0

The The The The

theoretical content of POM in the electrospun fibers. feed rate of the core solution. feed ratio of sheath/core solution, RS/C. need tip to collector distance, TCD.

respectively. Before the coaxial ES process, the solutions CA9 wt% and POM10 wt% were individually electrospun, and the obtained samples were labeled as CA9 and POM10, respectively. Table 1 lists the parameters of coaxial ES for each sample.

POM was analyzed using the Avrami theory according to the isothermal crystallization exotherm. The degree of crystallinity (Xm) of POM was calculated from the heat of fusion of the second heating run according to the following equations:

2.3. Characterization

X m ¼ DHf =ðx  DHhf Þ  100%

2.3.1. Microscopy Morphologies of the fiber were observed by scanning electron microscopy (SEM) (S-4300, HITACHI) and transmission electron microscopy (TEM) (JEM-100CXII, JEOL). SEM was conducted with the operating voltage of 10 kV after all the samples were sputter-coated by a E-1010 Ion sputter coater with Au/Pd in the thickness of about 100 Å. The operating voltage was 80 kV in the TEM observations after the electrospun fibers were directly deposited on Cu grids coated with a layer of carbon film. 2.3.2. Thermal analysis The thermal stability of the core–sheath fibers was studied by Thermal Gravity Analysis (TGA, TGAQ50, TA) in order to investigate whether the crystal structure and size would affect the pyrolysis temperature of POM. The samples were heated from 50 °C to 500 °C at the heating rate of 10 °C/min under the protection of nitrogen (N2) atmosphere. Thermal analysis was carried out by using differential scanning calorimetry (DSC, DSC 1, Mettler Toledo) with Julabo FT100 cooler (80 °C). For non-isothermal crystallization, the samples were firstly heated to 205 °C at 20 °C/ min and kept at 205 °C for 5 min to erase the thermal history. Subsequently, they were cooled to 50 °C at the rate of 10 °C/min and kept for 5 min, and then reheated to 200 °C at a rate of 20 °C/min. The maximum of the melting endotherm of the second heating run was used to determine the melting temperature (Tm), whereas the onset and maximum of the crystallization exotherm were used to determine the onset crystallization temperature (Toc) and the crystallization temperature (Tc), respectively. For isothermal crystallization analysis, the samples were also heated to 205 °C at 20 °C/min and kept at 205 °C for 5 min to erase the thermal history firstly. Subsequently, they were cooled to corresponding temperature quickly for isothermal crystallization and after equilibrated at the crystallization temperature for 5 min, they were reheated to 200 °C at a rate of 20 °C/min. The isothermal crystallization kinetic of

ð1Þ

where DHf is the apparent heat of fusion per gram of the core–sheath fibers, DHhf is the thermodynamic heat of fusion per gram of completely crystalline POM and was assumed to be 317.9 J/g [26,40]. x is the weight fraction of POM in the composite fibers which can theoretically calculated according to the following equation:

x¼

wPOM  100% wPOM þ wCA  RS=C

ð2Þ

where wPOM is the mass of POM in unit volume of POM solution, wCA is the mass of CA in unit volume of CA solution, and Rs/c is the feed ratio of sheath/core solution (e.g. CA/POM solution). The actual values of these parameters are shown in Table 1. 2.3.3. Spectrum characterization Fourier transform infrared spectrums of the ultrafine fibers were recorded by infrared spectrograph (FTIR, 6700FTIR, Nicolet) at frequencies from 4000 to 400 cm1 with a resolution of 4 cm1 under air atmosphere. The samples for the FTIR investigation were the nonwoven fabric consisting of hundreds of fibers. The nonwoven with preferred thickness of 10–20 lm was sandwiched between two small KBr (/1.3 cm) windows to prevent the hightemperature flow and finally placed into a homemade in situ pool (programmable heating and cooling device). The nonwoven fabric was first heated from 35 °C to 190 °C at the rate of 20 °C/min and equilibrated at 190 °C for 5 min. At the temperature of 190 °C, the POM in the nonwoven fabric was almost completely molten. After that, the nonwoven fabric was cooled from 190 °C to 35 °C at the speed of 5 °C/min. At the same time, the IR spectra were collected one time every 2.5 °C from 190 °C to 35 °C. 2.3.4. X-ray Diffraction The crystalline structure of POM confined in the core–sheath fibers was checked out by wide angle X-ray diffraction (WAXD) instrument (X-D8 focus, Bruker). The

Please cite this article in press as: Luo H et al. Confined crystallization of POM in the CA-nanotubes fabricated by coaxial electrospinning. Eur Polym J (2013), http://dx.doi.org/10.1016/j.eurpolymj.2013.02.037

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H. Luo et al. / European Polymer Journal xxx (2013) xxx–xxx

Fig. 2. SEM images of CA9 (a) and POM10 (b) electrospun fibers from CA solution (9 wt%) and POM solution (10 wt%), respectively.

apparent crystallite size (ACS) was estimated using the Scherrer equation [33]:

3. Results and discussion 3.1. Fabrication of stable cylindrical confinements for POM

k ACS ¼ ðD2hÞ cosðhÞ

ð3Þ

where k is the wavelength of X-ray, D2h is the integral breadth (in radians) of the crystalline peak and h is onehalf of the scattering angle h.

Before the coaxial ES process, the spinning ability of the CA and the POM solution was checked by single ES, respectively and Fig. 2 shows the SEM images of CA9 and POM10. The CA9 fibers are uniform and have some grooves on the surface, while the POM10 fibers are anomalous and joint to

Fig. 3. The SEM images of the as-spun fibers (a–c) and treated fibers (a0 –c0 ). The a–c respects the as-spun C9O1, C9O4 and C9O7, respectively; the a0 –c0 corresponding to a–c respects the as-spun fibers treated by embedded in epoxy resins, fractured in liquid nitrogen and selectively etched of the CA in acetone.

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Fig. 4. SEM images of the electrospun fiber of (a) POM10, (b) C9O1, (c) C9O4 and (d) C9O7.

Fig. 5. TEM images of as-spun C9O7 (a) fibers and etched C9O1 (b), C9O4 (c) and C9O7 (d) fibers.

each other thereby forming spider web structure. It is believed that the CA 9 wt% solution is better to form fibers by electrospinning than the POM 10 wt% solution. For the coaxially electrospun CA/POM fibers, the fibers of C9O1, C9O4 and C9O7 are uniform and bead-free (Fig. 3a–c), of which the dimension is increased gradually with increasing the concentration of the POM solution.

In order to investigate the variation of the morphology and dimension of the fibers, the as-spun fibers (POM10, C9O1, C9O4 and C9O7) were thermally treated at 195 °C for one hour and then naturally cooled to the room temperature. Fig. 4 shows the SEM images of the post-treated ultrafine fibers. It can be seen that the POM10 fibers aggregated together to form a film with the disappearance of the

Please cite this article in press as: Luo H et al. Confined crystallization of POM in the CA-nanotubes fabricated by coaxial electrospinning. Eur Polym J (2013), http://dx.doi.org/10.1016/j.eurpolymj.2013.02.037

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Fig. 6. TGA and DTGA curves of CA, POM and CA/POM fibers.

Table 2 Thermal parameters of CA, POM and their complicate fibers. Samples

Tonset (°C)

Tmax1 (°C)

Tmax2 (°C)

Charred residue (500 °C) (%)

CA POM bulk POM 10 wt% C9O1 C9O4 C9O7

340.9 313.1 359.6

364.7 338.8 399.9

– 402.2 –

13.3 0.8 0.7

249.8 255.1 311.2

263.9 272.1 340.0

366.9 368.1 362.9

11.1 9.5 9.5

fibrous morphology after the thermal treatment (Fig. 4a), while the fibers of C901, C904 and C907 reserved the fibrous morphology without any aggregation (Fig. 4b–d). It is suggested that the core–sheath fibers had good dimension stability and the POM were separated to each other by CA sheath and no aggregation of the POM occurred when the POM was at melt state with the protection of the CA sheath.

CA is solvable but POM is insoluble in acetone, so the POM core fibrils could be indirectly observed after removing the CA sheath of the fiber to exposing the POM fibrils. Firstly, the core–sheath CA/POM fibers were embedded in epoxy resins using the amine as the solidified agent; and then, the solidified epoxy resins with the fibers was brittlely ruptured in liquid nitrogen after the epoxy resins being solidified and finally, the sample was dipped in acetone for 24 h to wash away the CA component. The morphology of the acetone treated brittle failure surface was observed by SEM. The SEM micrographs are shown in Fig. 3a0 –c0 and a00 –c00 . It can be seen that there are numerous pores on the fractured surface for sample C9O1 (Fig. 3a0 ), while there are both pores and fibrils growing out from some pores on the fractured surface for sample C9O4 and C9O7 (Fig. 3b0 and c0 ), which can be more clearly seen in the Fig. 3b00 and c00 . It is confirmed from Fig. 3 that the POM forms the fibrils in the central of the core–sheath fibers. But the POM fibrils are too fine to remain in the pores for sample C9O1 (Fig. 3a0 ). In order to further confirm the existence of the POM fibrils, the as-spun fibers were cut less than 1 mm and put in acetone for 24 h. Then residual fibers were collected and re-dispersed in acetone for another 24 h. These processes were repeated three times to make sure that the CA component of the fiber was completely washed off. The final fibers were observed by TEM. The morphologies of the final POM fibrils of C9O1, C9O4 and C9O7 are shown in Fig. 5b–d, respectively. The POM fibrils can be clearly observed in Fig. 5 and it can be found that the diameter of the fibrils is increased from C9O1 to C9O7, which means that the dimension of the POM core was increased with increasing the concentration of the POM solution. The results of the SEM and TEM observations confirm that the coaxial ES fibers were core–sheath structured and the POM as the core was covered by the CA sheath. The real dimensions of the whole fibers and the POM fibrils are listed in Table 3 and it can be seen that the diameter of the whole fibers and the POM fibrils increased with increasing the concentration of the POM solution. It is suggested that the dimension of the POM confined in the CA tube (the CA sheath) can be controlled by tuning the concentration of the POM solution. Therefore, the stable core–sheath ultrafine fibers provide ideal confinements for the POM confined crystallization.

3.2. Thermal stability of POM with different structure The TGA and DTGA curves of the CA, POM10, bulk POM and CA/POM fibers are shown in Fig. 6. Table 2 lists the data corresponding to the onset temperature of weight loss

Table 3 thermal properties of electrospun CA/POM C–S ultrafine fibers and POM bulk with different confinements. Sample

C9O1 C9O4 C9O7 POM

Dimension (nm) Sheath

Core

679 750 1496 –

148 319 1000 –

Tc (°C)

Tm (°C)

Xm (%)

Ttransition (°C)

130.28 139.61 142.85 147.96

165.74 166.98 172.54 174.43

47.47 55.85 68.81 50.64

152.5–145.0 160.0–152.5 167.5–160.0 167.5–162.5

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thermal stability between bulk POM and as-spun POM10 fibers may result from the crystal structure diversity, because more energy needs for melting of the ECC and deorientation of the extended molecular chains. The bulk POM consists of the FCC and ECC, while as-spun POM fibers may be mainly composed by the ECC structure, which would be proved by FTIR result below. As for CA/POM core–sheath fibers, the first step of the degradation occurs at the temperature between 240 and 350 °C and corresponds to the thermal degradation of POM molecular chains. The second step takes place from 350 to 400 °C and the Tmax2 keeps almost the same, which should be assigned to the thermal degradation of CA molecular chains. Meanwhile, it can be found from the TGA curves and Table 2 that the onset temperature of weight loss and Tmax1 of the fibers increased with increasing the POM content. This means that the dimensions have strong effects on the thermal stabilities of the POM and the thermal stability of the POM is decreased with decreasing the diameter of the fibers. 3.3. Nonisothermal crystallization of the POM in the cylindrical confinements

Fig. 7. The DSC curves of POM and C9O core–sheath ultrafine fibers, (a) represents the cooling curves and (b) represents the 2nd heating curves.

Fig. 8. The FTIR of CA, POM and CA/POM C–S ultrafine fibers.

by TGA measurements, the temperature of maximum weight loss obtained from the DTGA curves and the residue at 500 °C. It can be seen from TGA and DTGA curves that there are two-step thermal degradation processes for bulk POM and CA/POM composites, while there is only one step for CA and POM10 fibers. As for bulk POM, the peak of the first step occurs at the temperature of 338.80 °C and that of the second step occurs at the temperature of 402.23 °C which is close to the temperature (399.90 °C) of the maximum weight loss of POM10 fibers. The difference of

Fig. 7 shows the DSC curves of the CA/POM core–sheath ultrafine fibers as well as the bulk POM. Before the study of crystallization and re-melt, the samples were heated to 205 °C at 20 °C/min to erase the heat history. It can be seen from Fig. 7 that the crystallization temperature of the POM decreased with decreasing the confined geometry (Fig. 7a). The crystallization temperature (Tc) of the bulk POM was about 149 °C, while that of the C9O1 was only about 127 °C when the average confined geometry of the POM was 150 nm. Certainly, it should be indicated that there are two fractionated peaks in the cooling curve of the C9O1, in which the peak at higher temperature is closed to that of the sample C9O4. It is possible that there are small parts of the POM in C9O1 fibers whose confined dimension is as large as the average confined dimension of the C9O4. The re-melting behavior is shown in Fig. 7b. The peaks shift from the low temperature to the high temperature as increasing the dimension of the confinement. Moreover, the melt peak becomes broad when the confined dimension is increased, which indicates that the crystalline structure is more uniform in the smaller confined geometry. The thermal properties of the POM calculated from Fig. 7 are listed in Table 3. It can be suggested from the non-isothermal properties of the POM in the confinement that the Tc and Tm of the POM are strongly depressed by the confinement dimension, while the crystallinity is affected in a different way because the Xm of the confined POM is lower than that of the unconfined POM10 only when the dimension of confinement is below 150 nm (the former is 47.47%, the latter is 50.64%). When the dimension of the confined space increased to over 319 nm, the crystallinity of POM was larger than that of the unconfined POM (over 55.85%, see in Table 3). It is proposed that the crystallization in the POM without confinement is isotropic, which results in more amorphous area between the crystallites, while the crystallization of the POM in one dimensional confined space is anisotropic

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Table 4 Assignments of POM FTIR bands. Wavenumber (cm1)

Crystal morphology

Species

Assignments

1237 1135 1095 933 904 632

ECC FCC ECC ECC ECC ECC

E1 A2 A2 and E1 E1 A2 E1

COC bending + CH2 rocking COC asymmetric stretching + skeletal stretching (COC asymmetry) Skeletal stretching (COC asymmetry) + CH2 rocking Skeletal stretching (COC symmetry) + CH2 rocking Skeletal stretching (COC asymmetry) + CH2 rocking OCO bending

and FCC and FCC and FCC and FCC

Fig. 9. The FTIR spectra of the POM (a and a0 ) and CA (b and b0 ) electrospun fibers at variational temperatures from 190 °C to 35 °C.

and the crystal growth is prior in the direction along fibers, which results in the high crystallinity. Fig. 8 shows the static FTIR spectra of as-spun CA/POM C–S ultrafine fibers as well as pure CA and POM fibers. Assignments of POM FTIR bands have been confirmed in several Ref. [36–38], which are listed in Table 4. Especially, the shoulder peak at 1137 cm1 is related to the FCC and the peak at 904 is associated with the ECC. From Fig. 8, it can be seen that the intensity of bands at 1097, 936, 904 and 632 cm1 is gradually increased with increasing the POM content in the CA/POM core–sheath fibers, while the shoulder peak at 1137 cm1 is not distinct and maybe result from the concealment of the bands of CA and week FCC crystal in the fibers caused by POM with large extension during ES. However, the shoulder peak at 1137 cm1 shows different extent of variation at various temperatures and refers the growth of the FCC POM, which will be discussed in the follow. The FTIR was used to monitor the conformation variation of the unconfined POM and confined POM during crystallizing. The FTIR spectra of the POM and of the CA are

shown in Fig. 9 and from Fig. 9 it can be found that the spectra of the POM are greatly changed during the variation of the temperature (Fig. 9a and a0 ) but the spectra of the CA are almost unchanged (Fig. 9b and b0 ). Moreover, the intensity of the peak at 904 cm1 is decreased from the as-spun POM to recrystallized POM. It is suggested that the FTIR spectra of the POM can be used to monitor the variation of the chain conformation during the crystallization and the transformation of the crystal structure. For the unconfined POM, at high temperature, i.e., at melt state, there are only three peaks at 1215, 1109, and 925 cm1 in the region 1260–600 cm1, while during the crystallization with decreasing the temperature, the peak at 1215 cm1 disappears and a new one at 1237 cm1 gradually appears. At the same time, the peak at 1109 cm1 is gradually split into a shoulder peak at 1130 cm1 and a narrow peak at 1095 cm1, and the peak at 925 cm1 is fractionized into two peaks at 932 and 906 cm1. A new peak at 632 cm1 also appears. Additionally, the onset temperature of the changes is about 167.5 °C and the intensity of the peaks increases with decreasing the

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Fig. 10. The FTIR spectra of the CA/POM C–S ultrafine fibers at various temperatures from 190 °C to 35 °C, where (a and a0 ), (b and b0 ) and (c and c0 ) is for C9O1, C9O4 and C9O7, respectively.

temperature. Compared the FTIR spectrum of the as-spun POM with that of the recrystallization POM at 35 °C, it could conclude that the crystallinity of the recrystalline POM was lower than the as-spun POM. The FTIR spectra of the confined POM during the crystallization are shown in Fig. 10 and it can be found that the spectra of confined POM (Fig. 10) in the core–sheath fibers also show the similar changes with the variation of the temperature as the unconfined POM. It is necessary to illuminate that the variation of the bands in the FTIR spectra becomes more distinct as the result of the POM content increasing. Fig. 11 shows the variation of the intensity of the bands at 1130, 1097, 937, 906 and 632 cm1 for the POM during the crystallization in bulk and in confinement by CA sheath. It can be observed from Fig. 11 that there is distinct transition of the conformation of the POM chains. The temperature of the conformation transition for POM chains shifts from low to high temperature region for samples from C9O1 to bulk POM. It means that the transition temperature of the POM chain’s conformation is decreased with decreasing the dimension of the confinement by CA sheath. It is indicated that the confinement dimension strongly influenced the macromolecule array. In large confine geometry

or bulk condition, the POM chains could form FCC or ECC crystalline at high temperature, but in low confined condition, the POM chains would be slowly arranged at low temperature. On the other hand, the intensity of the band at 1130 cm1 shows transition at relative high temperatures, which indicates the formation of the FCC would became steady soon, while the intensity of the bands at the other four bands is increased steadily after the transition, which means the formation of the ECC was long-time. As the assignments of bands, the band at 1130 cm1 belongs to the FCC, the band at 906 cm1 is associated to the ECC and the bands at 1097, 937 and 632 cm1 are contributed by both the FCC and ECC. Comparing the variation of the intensity of the peaks at 1130 and 906 cm1 for as-spun POM and recrystallized POM, it can be found the intensity of the peak at 1130 cm1 increases for all samples after recrystallization, while the intensity of the peak at 906 cm1 decreases for C9O1, C9O4 and POM10 and increases a little for C9O7. Moreover, the intensity of the peak at 931 cm1 is stronger than that at 906 cm1. Therefore, it can be inferred that the FCC forms at high temperature and becomes steady shortly, while the formation of the ECC is slow and need a long period at the same

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H. Luo et al. / European Polymer Journal xxx (2013) xxx–xxx

Fig. 11. The relationship between the intensity of bands and temperatures of the fiber (a) C9O1, (b) C9O4, (c) C9O7 and (d) POM fiber.

Table 5 The parameters of WAXD patterns. Samples

C9O1 C9O4 C9O7 POM

Fig. 12. WAXD patterns of POM, CA and composite fibers of as-spun and after recrystallization.

As-spun

Recrystallization

Angle (°)

FWHM

ACS

Angle (°)

FWHM

ACS

23.06 23.00 23.00 23.00

0.667 0.624 0.664 0.699

11.4 13.2 13.1 13.0

23.05 23.00 22.96 22.99

0.576 0.544 0.524 0.532

14.3 15.2 15.8 15.7

temperature. Also, the FTIR results indicate that the FCC and ECC coexist in the POM crystallized in both bulk and the confinement. Moreover, the FTIR results indicate the crystallinity of the unconfined and the confined POM changed in different ways, that is, the unconfined the POM depressed its crystallinity while the confined POM enhanced their crystallinity but weakened the ECC structure. The major reason may be the deorientation of molecular chains of the unconfined and the confined in different levers and the molecular chains could arrange themselves into more perfect crystal in suitable confined condition than in very low confinements or in unconfined condition. Fig. 12 shows WAXD patterns of POM and CA fibers as well as the core–sheath fibers with different confinement space. It can be seen from Fig. 12 that there is a major peak at 2h = 23.0° (the reflection at 100 of the POM) for all samples except the CA, which indicates that the crystal structure is the same for POM crystallized both in the confinement and without confinement. However, the intensity of the diffraction peak is reduced with decreasing the confinement space, which also confirms the decrease of the crystallinity with decreasing the dimension of the

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confinement space. The apparent crystallite size (ACS) of POM calculated from the WAXD data is listed in Table 5 and obviously, the ACS of the POM under 150 nm confinement space in the as-spun fibers is smaller than that under 320 nm confinement space and bulk POM. After recrystallization, the ACS for all the samples increases to be larger than 14.9 nm and also the ACS of the POM confined in 150 nm confinement space is the smallest. These results indicate that the POM crystallization with free of the confinement is more perfect than that in the confinement space. It can be assumed that the movement of POM chains in the confinement space by CA sheath is restricted, which affects the growth of POM crystals and the size of the POM crystals will be smaller than that in the bulk crystallization. The WAXD results also well support the DSC results since the lamella thickness of POM is proportional to the Tm of POM. 3.4. Isothermal crystallization kinetics The Avrami equation can describe well the crystallization kinetics of polymers under isothermal conditions for various modes of nucleation and growth. The general form of the Avrami equation is

X t ¼ 1  expðKðTÞtn Þ

ð4Þ

where Xt is the relative crystallinity at crystallization time t, K(T) is a rate constant related to nucleation and growth parameters, and n is a constant depending on the mechanism of nucleation and the geometry of crystal growth. It is known that Avrami index n represents the patterns of crystal growth in isothermal crystallization. In

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heterogeneous nucleation, n = 3, 2, and 1 refer to three-, two-, and one-dimensional forms of growth, representing spheres, discs (lamellae), and rods crystals [41]. Xt can be calculated according to the following equation:

Xt ¼

Rt ðdH=dtÞdt Qt 0 ¼ R1 Q1 ðdH=dtÞdt 0

ð5Þ

where Qt and Q/ are the integrated heat flows generated at time t and infinite time, respectively, and dH/dt is the heat flow rate. The development of Xt as a function of time for bulk POM and CA/POM C–S fibers is shown in Fig. 13 for various isothermal crystallization temperatures (Tc,i). The general form of the Avrami equation can be rewritten to Eq. (6) as follow:

log½ lnð1  X t Þ ¼ n log t þ log KðTÞ

ð6Þ

Fig. 14 shows the Avrami plots of log[ln(1  Xt] versus log t at a selective isothermal crystallization temperature over a range of Xt from 3% to 100%. The linear part of the plots (where Xt typically ranges from 3% to 35%) represents the stage of primary crystallization, that is, the formation of nuclei of the critical size and their subsequent growth [41,42], and can be fitted by a straight line with R > 0.996. The Avrami index n of bulk POM is 1.58 and decreases to 1.34–1.28 for the confined POM in the composite fibers, which is all between 1 (rods crystals) and 2 (discs). It has been reported that the rods crystals (also be called as needle-like crystals) of POM were the ECC and another crystalline morphologies were FCC [17,24,37]. It is suggested that a hybrid structure of FCC and ECC has already formed on the initial stage of POM crystallization at cooling from the melts, which is consistent with the FTIR results. Moreover,

Fig. 13. Development of relative crystallinity Xt with time of (a) bulk POM and POM/CA C–S fibers (b – C9O1, c – C9O4 and d – C9O7) at various Tc,i.

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H. Luo et al. / European Polymer Journal xxx (2013) xxx–xxx

References

Fig. 14. Avrami plots of bulk POM as well as POM/CA C–S fibers at a selective Tc,i From a graphic representation of log[ln(1  Xt] versus log t, the constants n and K(T) can be obtained.

the decrease of the Avrami index n of POM should be attributed to either the nucleating effect of CA surface or more formation of the ECC fraction as POM confined in the tubular geometry, but the factors cannot be exactly determined. From the isothermal crystallization behavior of POM, it can be included that the formation of rod crystal is preferred in one dimensional confined POM and the formation of the lamella or the spherulite is the tendency in the unconfined POM. 4. Conclusions The ultrafine POM/CA coaxial fibers were fabricated by coaxial electrospinning technique. The SEM and TEM observations showed that the POM fibril was embedded in the CA tubule and the whole fiber formed a core–sheath structure. The average diameter of POM fibrils was increased from 150 to 1000 nm with increasing the concentration of the POM solution during coaxial ES. The DSC measurements indicated the crystallization temperature of POM was sharply reduced from 149 to 127 °C as the confined geometry decreasing, while the melt temperature of the POM was slightly affected and the crystallinity of POM decreased as the confined dimension becoming smaller. It was found that the formation of the FCC was prior to that of and the FCC and the ECC coexisted in all the samples crystallized in the confinement and free of the confinement. Isothermal crystallization kinetic indicated that the rod crystals were formed in both bulk POM and confined POM and the Avrami index was decreased from 1.58 for the bulk POM to 1.32 for the confined POM. The crystalline structure was not affected much by the confinement during the crystallization of the POM but the crystallite size of the POM was decreased with decreasing the dimension of the cylindrical confinement. Acknowledgments The financial supports by National Natural Science Foundation of China (Nos. 50821082 and 51003124) were greatly appreciated.

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