Master batch of core/shell iPS/iPP fibers prepared from electrospinning Preparation of shell solution: 1 wt% iPP/o-DCB
1. 將油浴槽預熱 120 oC、高溫烘箱 100 oC、循環油浴槽 150 oC。
Preparation of core solution: 12 wt% iPS/o-DCB 1. 油浴槽預熱至 160 oC。 Tip-to collector distance H： 7 cm
Flow rate Q： (core solution) 12 wt% iPS/o-DCB 1.0 mL/h (shell solution) 1 wt% iPP/o-DCB 0.5 mL/h Applied voltage at the spinneret V： (top) 15.7 kV
Applied voltage at the collector: (bottom) -1.0 kV IR emitter：溫度設定= 700 oC, 與針距離= 12 cm
The schematics of the coaxial electrospinning is shown in Figure 1. It is noted that the temperature control of the iPP solution is the most important factor for this coaxial electrospinning experiment. This is because that iPP cannot be dissolved in o-DCB solvent at room temperature. Elevated temperatures at ca. 100 oC is generally required to dissolve the iPP powders to form a one-phase solution.
Depending upon the iPP concentration, the one-phase solution will form either gel or precipitates as the solution temperature is gradually reduced. The higher the iPP concentration in the solution, the lower the temperature at which the one-phase solution undergoes phase transition. This transition temperature is important according to our DSC study (Figure 2). To avoid the interruption of electrospinning during long time experiments (ca. 8 hrs), we chose the 1 wt% iPP/o-DCB solution as our model solution for electrospinning; this is because the transition temperature is a little high at ~70 oC so that our present setup can provide the sufficiently high temperature by heating jacket and IR emitter to ensure the continuous process for
obtaining the core/shell fibers iPS/iPP. The circulating silicone oil is maintained at 150 oC to avoid the precipitation (gel) of the iPP/o-DCB solution during processing.
Prior to electrospinning, it is important to obtain the gel (precipitating) temperature of the iPP/o-DCB solution. To this end, DSC cooling traces of iPP solutions with various concentrations from 100oC to 30oC are shown in Figure 2 (left), with different cooling rates. For the 8 wt% iPP solution cooled at a rate of 20 oC/min, an exothermic peak associated with the gelation is seen at ~68oC. As the cooling rate is reduced, the gel temperature is gradually increased. The melting temperature of the gels developed at different cooling rates is determined from the DSC melting traces at a constant heating rate of 10 oC/min, as shown in the right figure. It is noted that the exothermic enthalpy associated with the gelation during solution cooling is also recorded and is normalized with the iPP content to derive the H’gel, which is the indicator the amount of iPP involved in the gelation (precipitation). These values are displayed in Table 1. As expected, as the iPP content is decreased from 8 to 5 wt%, more iPP gel content evolves at a lower temperature of ~71oC. Thus, in order to perform the electrospinning of 1 wt% iPP solution, a solution temperature of 70 oC is sufficient to prevent the clogging phenomenon at the needle spinneret.
SEM images of the collected fibers are shown in Figure 4, and its fiber diameter distribution is also displaced from the measurement of 500 fibers. As can be seen that the obtained fibers are not perfect since some spherical particles are still seen; it implies the difficulty for a continuous electrospinning for 8 hr in order to obtain a sufficient amount (~ 1 gram) of master-batched fibers for later compounding to prepare the iPP composites. Nevertheless, the majority of the core/shell fibers are round fibers with an average diameter of 228 65 nm.
Since the concentrations and flow rates of the core solution of iPS and shell solution of iPP are known, the composition of the cire/shell fibers is estimated to be 90.86 wt% of the iPS. Thus, this fiber member containing high amount of iPS fiber is used as a master batch to prepare the composites by using a microcompounder (Figure 5) to proceed the melt blending. The advantage of the melt blending is the intensive high shear stresses induced in the compounder through twin-screw extruder so that the entangled iPS fibers can be separated each other to disperse more uniformly in the iPP melt matrix.
We can envision that the shell component of iPP is completely miscible with the melt matrix of iPP. Thus it is easier for the iPP chains in the melt matrix to diffuse into the gap in between the core/shell fibers to facilitate better dispersion of iPS in the iPP matrix in this manner via the intensive shear mixing in the microcompounder.
The processing parameters for the melt blending are: 180 oC for 2 min mixing with a rotating screw of 100 rpm. The iPP composites filled with 0, 0.1, 0.5, 1, and 5 wt%
iPS fibers are prepared as shown in Figure 6 for further investigation.
Effects of cooling rate on the -form formation in the iPP composites
Samples filled with different concentrations (0, 0.1, 0.5, 1.0 and 5.0 wt%) of iPS fibers were cooled from 200 °C to 30 °C at different cooling rates R, followed immediately by heating trace at 10 °C/min. Figure 7 shows the DSC cooling traces of the neat iPP (left) at different cooling rates of 1, 2, 5, 10 and 20 oC/min, and the subsequent heating traces at a heating rate of 10 oC/min (right). At a high cooling rate of 20 oC/min, the crystallization temperature is ~110 oC, and the two melting temperatures are observed at 158 and 164 oC, respectively. As the cooling is decreased, the crystallization temperature is increased and the only one melting temperature is detected. It implies that more perfect iPP crystals are developed at a low cooling rate and its melting temperature is increased to high temperature.
In contrast, the iPP composites filled with 5 wt% iPS fiber exhibit different behavior as shown in Figure 8, whereas the DSC cooling curves at different cooling rate is displayed in the left, and the corresponding melting curves of the crystallized samples are shown in the right. Compared with the neat iPP in Figure 7, the crystallization curves of iPP composites during cooling is not much different, whereas the melting curves exhibit four melting peaks. It implies that the presence of iPS fibers in the iPP matrix significantly alters the crystalline structures of iPP due to transcrystallization. To reveal the effect of iPS fiber on the crystallization kinetics of iPP, the cooling curves of composites are shown in Figure 9 at a rate of 5 oC/min in the left and at 20 oC/min in the left, respectively. They all show that addition of 0.1 and 0.5 wt% iPS fiber apparently retards the crystallization of iPP matrix, compared to the neat iPP sample. A higher content of iPS fibers (1 and 5 wt%) may lead to less retardation of iPP crystallization. It seems that in the composites the presence of iPS fibers reduces the rate of dynamic (nonisothermal) crystallization of iPP regardless of the fiber content. However, the benefit of fiber addition in the composites is the formation of -form crystals of iPP, which possess a higher toughness than the -form crystals.
Figure 10 shows the melting endotherms of the neat iPP and composites containing
iPS fibers. For the neat iPP cooled at 20 °C/min, the melting endotherm exhibited two melting peaks at 156.9 °C and 162.8 °C; the double melting behavior is associated with the -form crystals because only the -related crystalline reflections are detected by WAXD. The peak temperatures are denoted by Tm1, for the lower endotherm and by Tm2, for the higher endotherm. With decreasing R, Tm1, shifts to a higher temperature, but Tm2, gradually disappears. These results indicate that Tm1, is related to the melting of the originally crystallized iPP, and Tm2, is associated with the melting of reorganized/recrystallized iPP after partial crystal melting.
The iPP composite containing 5 wt% iPS fibers exhibites four apparent melting related to the reorganization/recrystallization of the originally-crystallized -phase after partial crystal melting.
For further verification, the melting behavior of the 20 °C/min-cooled composites was directly observed under polarized optical microscopy (POM) to confirm this phenomenon. The corresponding POM images at 130 °C and 155 °C before and after
-crystal melting are examined. Highly birefringent -form crystals are evidently identified at T < Tm1, prior to crystal melting. At Tm1, < T < Tm2,, the remaining
-form crystals remain space-filling and exhibit a reduced birefringence. At higher temperatures of Tm2, < T < Tm1,, all the -form crystals disappear to form amorphous dark regions, and the less birefringent -form iPP remained. These POM images clearly demonstrate that two -form crystallites with different birefringence/
perfection were produced during non-isothermal crystallization. Moreover, the -to-
crystal transformation during heating was not observed. Thus, the endotherm shown in Figure 10 involves the respective melting of both the - and -form with different crystal perfections.
The content of -form crystals developed in the composite can be strictly derived provided that the complex melting endotherm can be reasonably deconvoluted to account for the phase transitions. For a qualitative comparison, a rough estimate of
the melting enthalpy associated with the -form crystals ΔH was carried out by simple integration of the melting curve above the baseline. In addition, WAXD curves for samples crystallized at different cooling rates also indicate that the content of b-for crystals is increased with increasing iPS-fiber content, which is consistent with the DSC results.
Conclusion:
1. Core/shell iPS/iPP nenofibers have been successfully obtained from the coaxial electrospinning by carefully adjusting the processing parameters, especially the solution temperature of iPP to avoid its precipitation.
2. Using the core/shell fibers as the master batch, iPP composites with different iPS- fiber concentrations (0.1, 0.5, 1 and 5 wt%) are readily prepared by melt blending via microcompounder.
3. Electrospun iPS fibers are found to be dual-nucleating agent for iPP to produce both the and phases.
4. Given their high specific surface area, electrospun fibers with submicron diameters are ideal candidates as good NG and reinforced filler for the semicrystalline polymers, such as iPP, used in this work. Electrospun iPS fibers are dual-NG for iPP to produce both and phases. The relative proportions of - and -phases depend on the cooling rate. The -form content increases with increasing cooling rates.
Figure 1. Experimental setup used for the coaxial electrospinning of the iPP/o-DCB solution (shell, yellow) and iPS/o-DCB solution (core, blue color) to prepare the core/shell iPS/iPP nnaofibers. To avoid the precipitation of iPP/o-DCB solution, a heating jacket with circulating silicone oil at 150 oC is applied to keep the solution at
high temperature. Due to the fin-effect of the spinneret, the temperature drop of the iPP solution during flow from the bath to the needle tip is significant so that additional IR emitter is required to maintain the environmental temperature at high temperature of 70 oC, to avoid the solution clogging at the needle end to interrupt the electrospinning. The distance between the IR emitter and the spinneret is about 12 cm and the temperature setting for the IR emitter is 700 oC. By these parameters, continuous electrospinning is feasibly performed for hours to prepare the core/shell iPS/iPP fibers on the collector. It is noted that to control the flight of the charged jet (fibers) the collector is connected to a negative potential, which is in contrast with the spinneret connected by the positive potential. By controlling the concentrations of iPP (s) and iPS (c) solutions and the flowrates of the core solution (Qc) and the shell solution (Qs), the composition of the collected fibers can be obtained.
Figure 2. Determination of the gel (precipitation) temperature Tgel of the 8 wt%
iPP/o-DCB solution at different cooling rates (left), and the subsequent heating traces at a rate of 10 oC/min to determine the melting temperature of the iPP gel (right).
Figure 3. Plot of Tgel versus cooling rate and the extrapolated temperature at a zero cooling rate gives the thermodynamic gel temperature (left), and the plot of exothermic enthalpy during gel formation versus cooling rate (right).
Table 1. Thermal properties of iPP/o-DCB solution measured from the DSC curves.
Note: addition of salt is necessary for electrospinning in order to increase the solution conductivity, otherwise the neat iPP/o-DCB solution is not feasibly electrospun since the electric conductivity is too low. Hgel is the measured value, while H’gel is obtained by normalization with the iPP weight content. It is also noted that the melting temperature Tm,gel of the developed gel during cooling is much higher than the gel temperature Tgel.
Figure 4. SEM images of the as-spun core/shell iPS/iPP fibers and the histogram of the fiber diameter distribution.
Figure 5. Microcompounder (Xplore MC-15) used to prepare the iPP composites filled with different concentration of iPS nanofibers (0, 0.1, 0.5, 1, 5 wt%) by mixing the appropriate amount of master batch obtained from the coaxial electrospinning and the excess amount of raw iPP powders. The capacity of the microcompounder is 填料
容量：5 ~ 15 mL, 最高升溫：400 oC, 預熱 20~240 oC 約 20 min, 螺桿轉速：5 ~ 250 rpm).
Figure 6. iPP composites with iPS fiber concentration of 0, 0.1, 0.5, 1 and 5 wt% are prepared after melt blending via microcompounder.
Figure 7. DSC tracing curves of neat iPP during cooling from melt at 200 oC at different cooling rates (left), and the subsequent heating from 30 oC at a constant heating rate of 10 oC/min (right).
Figure 8. M DSC tracing curves of iPP composites filled with 5 wt% iPS fibers during cooling from melt at 200 oC at different cooling rates (left), and the subsequent heating from 30 oC at a constant heating rate of 10 oC/min (right).
Figure 9. DSC cooling curves of iPP composites filled with different amounts of iPS fibers during cooling from melt at 200 oC at a constant cooling rates of 5 oC/min and 20 oC/min (right).
Figure 10. DSC melting curves with a heating rate of 10 oC/min for iPP composites filled with different amounts of iPS fibers after cooling with different rates (-1, -2, -5, -10 and -20 oC/min) from 200 oC melt.