Figure 1 shows the schematics of the electrospinning setup for collecting the PBT/iPP fibers by using two spinnerets to simultaneously spin the PBT/TFA solution and the iPP/o-DCB solution. Since iPP cannot dissolve in o-DCB at room temperature, high temperature electrospinning should be applied to avoid the clogging of the spin-line at the Taylor cone. Thus, a heating jacket with the circulating silicone oil at a temperature of ca. 130 oC was used to maintain the iPP/o-DCB solution at a temperature sufficiently higher than the gel temperature of the solution. The temperature of the Taylor cone was measured to be 69.5 oC. The concentrations of the iPP/o-DCB solution and PBT/TFA solution were 8 and 10 wt%, respectively. A continuous process for electrospinning was reached by using the applied voltages of 8.3 and 10.3 kV for the iPP solution and PBT solution, respectively, at a fixed tip-to-collector distance of 7 cm for both solutions. By controlling the flow rate, the compositions of the PBT/iPP fibers on the collector can be determined. In this preliminary study, we applied the flow rates of 1 mL/h for both solutions, and the fiber composition was theoretically derived to be PBT/iPP= 58.3/41.7.
In order to obtain a uniform dispersion of PBT fibers in the iPP fibers, a rotating long rod with a diameter of 1.012 cm was used a grounded collector during electrospinning. The rotation speed of the rod was controlled to be 500 rpm, giving rise to a fiber collecting speed of 26.5 cm/s. To be a meaningful comparison, single electrospinning of the iPP/o-DCB solution was first performed to obtain the random iPP fiber membranes on the rod collectors. After changing the rod collector, single electrospinning of the PBT/TFA solution was followed to obtain the neat PBT fiber membranes. Afterwards, co-electrospinning of the iPP solution and PBT solution was conducted to receive the PBT/iPP fiber membranes for comparison. Figure 2 shows the SEM images of neat iPP, neat PBT and PBT/iPP fibers collected in this manner.
Based on the statistical analyses of 500 measurements, PBT fibers with an average diameter of 271 nm can be produced at a tip-to-collector distance (H) of 7 cm. On the other hand, slightly wet iPP fibers are produced at H= 7 cm; the average width was
1531 nm. For the PBT/iPP fibers, the average width is ca. 1157 nm.
Figure 3 shows the cooling curves of the (a) neat iPP and (b) PBT/iPP fibers at different rates after holding at 200 oC for 3 min. At 200 oC, the iPP fibers were completely melted but the PBT fibers remained intact since the melting temperatures of iPP fiber and PBT fibers were 160, and 222 oC, respectively. During cooling, iPP underwent the melt crystallization so that an exothermic peak was detected. The peak temperature (Tc) and crystallization enthalpy (Hc) were measured; the corresponding values are plotted as a function of cooling rate (R), as shown in Figures 4 and 5, respectively. It is found that Tc decreases with increasing R for both neat iPP and PBT/iPP fibers. With adding PBT fibers in the iPP matrix, the Tc is about 14 oC higher than that of the neat iPP fibers, regardless of the cooling rate applied. It indicates that the presence of the PBT fibers can enhance the crystallization of iPP. Since the content of iPP in the blend fibers was reduced, compared to the neat iPP sample, the value of Hc was decreased. Provided that the values of Hc were used to calculate the iPP content, the weight percentage of iPP in the blend sample is ca. 56%, which is higher than the theoretical calculated value of 42 wt%.
Figure 6 shows the melting curves at a heating rate of 10 oC/min for the (a) neat iPP fibers and (b) PBT/iPP fibers after cooling from 200 oC at different R. For the neat iPP fibers, two melting peaks are observed for samples cooled at high cooling rate;
they are denoted by Tm1, and Tm2, respectively. Double melting behavior of iPP is often reported in the literature; they are attributed to the melting of the subsidiary (daughter) lamellae and the main (mother) lamellae since only -form iPP is detected by WAXD. For the PBT/iPP fibers, single melting peak is observed regardless of the cooling rate applied. The melting enthalpy (Hm) was also determined for comparison to reveal the effect of cooling rate. The cooling rate dependence of Tm and Hm for the neat iPP and PBT/iPP fibers are shown in Figures 7 and 8, respectively. For the blend fibers, the melting temperature of the iPP component is in between Tm1 and Tm2
of the neat iPP fibers at a fixed cooling rate of R. Provided that the values of Hm
were used to calculate the iPP content, the weight percentage of iPP in the blend sample is ca. 57%.
To reveal the effect of PBT fibers on the developed crystal modification of iPP, WAXD experiments were carried out on the PBT/iPP fibers; the 2D WAXD pattern is shown in Figure 9. Also included are the 2D WAXD patterns of the neat iPP fibers and neat PBT fibers; they are all crystallized at a cooling rate of 60 oC/min from 200
oC after being held for 3 min. Figure 10 shows the radial averaged intensity profiles for the neat iPP, neat PBT and PBT/iPP fibers. After peak separation, the ratio of the integrated area of the crystalline reflections to the total reflection area was determined to represent the sample crystallinity. The derived crystallinity for the neat iPP and neat
PBT fibers are 48.8% and 58.1%, respectively. It is important to note that only -form iPP is detected in the PBT/iPP fibers; it implies that PBT only induces the -form iPP at its fiber surface. In other words, PBT fibers can act as a good -form nucleating agent for iPP matrix.
Figure 1. Schematics of co-electrospinning setup used to prepare PBT/iPP fiber mixtures for the subsequent crystallization study. The rotating roller is used to attain a good dispersion of PBT fibers and iPP fibers. The angle between the two spinnerets is 120 o.
Figure 2. SEM images of the as-spun fibers and the corresponding histogram of fiber diameter distribution, (a) PBT fibers, (b) iPP fibers, and (c) PBT/iPP fibers.
Figure 3. DSC cooling curves of (a) iPP fiber, (b) PBT/iPP fibers at different cooling rates R after being held at 200 oC for 3 min.
(a) (b)
Figure 4. Effect of cooling rate on the crystallization temperature Tc.
Figure 5. Effect of cooling rate on the crystallization enthalpy Hc.
Figure 6. DSC heating traces of (a) neat iPP fibers and (b) PBT/iPP fibers after nonisothermal crystallization at different cooling rates R. The heating rate is 10
oC/min.
Figure 7. Effect of cooling rate on the melting temperature Tm.
Figure 8. Effect of cooling rate on the melting enthalpy Hm.
(a) (b)
Figure 9. 2D WAXD patterns of the (a) neat PBT fibers, (b) neat iPP fibers, and (c) PBT/iPP fibers after a cooling rate of 60 oC/min from 200 oC.
Figure 10. WAXD intensity profiles of the neat iPP, neat PBT, and PBT/iPP fibers after a cooling rate of 60 oC/min from 200 oC.