Abstract. Introduction. Experimental. Materials. SPE ANTEC Anaheim 2017 / 2449

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1 Effect of Multi-walled Carbon Nanotubes on poly(ε-caprolactone) foaming behavior Tong Liu, Yu-Chen Wu, Qing-Lin He, Meng-Meng Wang, Tai-Rong Kuang, Xiang-Fang Peng National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou , China. Abstract In this paper, we investigated the effect of multiwalled carbon nanotubes (MWCNT) on the foaming behavior of poly(ε-caprolactone) (PCL). The PCL/MWCNT nanocomposites prepared using HAAKE Rheometer, and the resulted composites were subsequently foamed using supercritical carbon dioxide (Sc-CO 2 ) foaming technology. Results showed that the involvement of MWCNT promoted crystallization of PCL matrix and improved the crystallinity of PCL matrix, which is attributed to the enhancement of melt strength. Morphological analysis presented that the MWCNT was well-dispersion in the PCL matrix at low loading. In Sc- CO 2 foaming process, the addition of MWCNTs led to higher cell densities, smaller cell sizes and uniform cell morphology in the composite foams. The results indicated that the MWCNT nanoparticle acted as a heterogeneous nucleation agent in the PCL matrix, and provided more nucleation site during the foaming process. Introduction In the past decade, bio-based polymers such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and poly(ε-caprolactone) (PCL) have been received great attentions as renewable plastics because of their biodegradability for energy and environment problems. PCL, as a semicrystalline thermoplastic polyester with considerable mechanical properties, good processability, excellent biodegradability, and low toxicity, has been widely used in agricultural, industrial, and biomedical fields, such as packaging, drug delivery, biological tissue scaffold, etc. [1-7] Polymeric foams have been attracting attentions from both academic and industrial areas for the past few decades because of their high impact strength, moderate specific strength and modulus, low density and thermal conductivity. However, PCL has some limitations in the application of foaming technique due to the relatively low melt strength [8, 9]. To overcome the poor foaming property of PCL and enlarge its application range, numerous efforts have been made to increase the melt strength of PCL matrix and thus improve the foaming property. Very recently, blending PCL with natural polymers (e.g., soy protein, cellulose whiskers and so on) or with nanofillers (e.g., carbon nanotube, carbon nanofiber, reduced graphene oxide, hydroxyapatite (HA) nanoparticles and so on) have been proven to be an effective way to solve the issues.[10-17] Mi et al. studied the effect of cellulose nanocrystals (CNC) on PCL injection foaming, and they found that CNC can act as a thickener which enhanced the melt strengthen of PCL and protected the cell wall from breaking.[10] Salerno et al. reported that hydroxyapatite (HA) nanoparticles also effectively improved the foaming performance of PCL.[11] A recently result reported by Kuang et al. revealed that the addition of MWCNT not only reduced the cell size and improved the cell density of PLA based foams but also enhanced the electrical conductivity and EMI shielding effectiveness due to the existence of porous structure.[17] However, to the best of our knowledge, there are few reports about the fabrication of PCL/MWCNT composite foams with micro-pores structure using supercritical carbon dioxide as blowing agent. In this work, the blends of PCL/MWCNT nanocomposites were prepared by HAAKE Rheometer. Base on the well distribution of MWCNT in PCL matrix, we further foamed the resulted composites by injection of supercritical carbon dioxide in a high-pressure vessel. A series of characterization, such as scanning electron microscope (SEM), Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), were performed to analyze the properties of different composites and their corresponding foams, and further discussed the effects of MWCNT content on the foaming behavior of PCL/MWCNT foams. Materials Experimental The PCL (CAPA 6500, M n = 50,000) used in this study was purchased from Perstorp UK Ltd. It has a specific gravity of 1.10 g/cm 3. Its glass transition and melting temperatures are -60 and C, respectively. The purified multi-walled carbon nanotubes (Purity 98%) with average outside diameter of 10.0 nm, inside diameter of 4.5 nm, and length of 4 µm, were supplied by Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. SPE ANTEC Anaheim 2017 / 2449

2 PCL/MWCNT Composites Preparation Before melt compounding, PCL and MWCNT were dried in an oven at 40 and 120 for 6h, respectively. Then PCL and MWCNT were mixed with different MWCNT content of 0.5%, 1% and 3% using a HAAKE Rheometer at 80 and 80 rpm for 10 min. For property comparison, the neat PCL were also processed at the same conditions. The composites were dried in an oven at 40 for 2h, and then were compression molded into thin sheets with thickness of 1mm using a plate vulcanizing machine (KS100HR, Dongguan Kesheng Industrial Co., Ltd, China) under 80 C for 360 s. Batch Foaming A similar batch foaming method have mentioned in our previous paper.[14] The detailed foaming procedure was as follows: the molded sheets were cut into small specimens of 10 mm 10 mm 1 mm, and then placed into a high-pressure vessel to foam; Figure 1 illustrates the batch foaming procedures, the vessel was heated to saturation temperature (T s ) of 40 C within 2 min, then compressed CO 2 to saturation pressure (P s ) of 20 MPa; the system was equilibrated at T s and P s for desired saturation time (t s, 1 h) to ensure CO 2 saturation; the vessel temperature was cooled to foaming temperature (T f ) of 15 C; the vessel was depressurized to atmosphere pressure by rapidly pressure releasing, and cooled to 5 C with circulating cooling system to maintain the foamed structure. Figure 1. Schematic diagram of the pressure and temperature evolution for batch foaming process. Scanning Electron Microscopy (SEM) The phase morphology of the PCL/MWCNT composites and the cell morphology of the foamed samples was observed by a SEM (Nano 430, FEI, U.S.). The composites and foamed samples were fractured in liquid nitrogen and sputtered with gold for scanning. The cell density and the average cell diameter were measured by Image Pro Plus software. The cell density N f (cells/cm 3 ) of foamed samples was calculated using equation (1) [9] 3 æ n ö2 Nf = ç è A ø (1) where n is the number of cells in the image and A is the area of the image. Differential Scanning Calorimetry (DSC) The thermal properties of PCL/MWCNT composites were conducted by DSC 204C (Netzsch, Germany). Put the specimens about 5mg in the aluminum crucibles for DSC testing under nitrogen atmosphere to protect the specimens from oxidation degradation. The samples were heated from 0 C to 100 C at a heating rate of 10 C /min and isothermal for 3 min, followed by cooling to 0 C at a cooling rate of 10 C /min and and isothermal for 3 min and then heated again to 100 C at a heating rate of 10 C /min.. Thermogravimetric Analysis (TGA) TGA was performed using TGA 209F3 IR thermal gravimetric analyzer (Netzsch, Germany) under a nitrogen atmosphere to study the thermal stability of neat PCL and their composites. Approximately 5 mg specimens were heated from 30 C to 600 C at the heating rate of 10 C /min. Results and Discussion Phase morphology of PCL/MWCNT Composites Figure 2 shows the SEM images of fracture surface for the PCL/MWCNT blends with various MWCNT content. Obviously, neat PCL shows homogeneous phase while a typical two-phase morphology was observed in the PCL/MWCNT composites with MWCNT contents range from 0.5 wt% to 3 wt%, which suggest the PCL and MWCNT are immiscible. And the number of the white dots (represent MWCNT phase) increased remarkably as the MWCNT content increasing from 0.5 wt% to 3 wt%. However, the size of white dots increased when the MWCNT content increased from 1% to 3 wt% because of the aggregation of the MWCNT phase during the mixing process. In general, the filler has a uniformly dispersion in the matrix, which would help enhance the foaming performance of PCL. SPE ANTEC Anaheim 2017 / 2450

3 Fig. 2. SEM micrographs of PCL and PCL/MWCNT nanocomposites: (a) neat PCL; (b) PCL/0.5 wt% MWCNT; (c) PCL/1 wt% MWCNT; (d) PCL/3 wt% MWCNT. Thermodynamic properties of PCL/MWCNT blends The DSC results of neat PCL and PCL/MWCNT nanocomposites are shown in Figure 3 and Table 1. As shown in Figure 3 and Table 1, T m of composites only have a small improvement compared to neat PCL with the addition of MWCNT. However, the crystallization temperature (T c ) of the composites were increased distinctly with the addition of MWCNT. For example, Neat PCL showed a crystallization temperature (T c ) and a melting temperature (T m ) of 30.1 C and 57.0 C, respectively. When the composite containing 3 wt% MWCNT, crystallization temperature and melting temperature are increased to 40.9 C and 59.6 C, respectively. It revealed that the addition of MWCNT enhanced the crystallization of PCL due to the effect of heterogeneous nucleation. Accordingly, more perfect crystal can be formed due to the increase of T c, and thereby improved T m. As a result, it suggested that the addition of WMCNT can play a role as the nucleating agents of PCL to enhance the ability of crystallization of PCL/MWCNT composites. The TG curves of pure PCL and PCL/MWCNT composites are shown in Figure 4. From the curves, the addition of MWCNT can make a slight enhance of the degradation temperature, with the content of MWCNT increased, the degradation temperature has a slight improvement. Furthermore, the mass of residuals was increased obviously as the MWCNT content increase, which was mainly attribute to the decomposition of MWCNT. The result illustrated that the addition of MWCNT can improve the thermal stability of the PCL. Figure 3. DSC curves of PCL/MWCNT nanocomposites: (a) cooling and (b) second heating scans. Table 1.DSC data for neat PLA and PCL/MWCNT blends. Samples First cooling Second heating Tc ( C) Tm ( C) ΔHm (J/g) Xc (%) Neat PCL % MWCNT % MWCNT % MWCNT Figure 4. TG curves of different PCL/MWCNT blends. SPE ANTEC Anaheim 2017 / 2451

4 Foaming behavior of PCL/MWCNT composites In order to investigate the effect of filler on PCL foaming behavior, supercritical carbon dioxide was used as a physical blowing agent. We investigated the foaming behavior of PCL/MWCNT composites under the foaming pressure of 20 MPa, and the foaming temperature was set at 15 C for all samples. The cell morphologies and cell diameter distribution of PCL and PCL/MWCNT foams with different MWCNT content were shown in Figure 5 and Figure 6, respectively. And the corresponding statistical data was plotted in Figure 7. From the pictures of SEM (Figure 5 and Figure 6), neat PCL and all three composites formed uniform cell structure. According to the statistical results, the cell size increased and cell density decreased remarkably as the MWCNT content increased. This was because of the well-dispersed MWCNT in the PCL matrix, which acted as heterogeneous nucleation sites during foaming. As we all know, cell nucleation and cell growth are two competing factors, the addition of MWCNT increased the amount of nucleation sites, thereby reduced cell size and increase cell density. For example, when the MWCNT content was reached 3 wt%, the average cell diameter was reduced 73.2%, and the cell density increased by 45 times compared with neat PCL under the same foaming conditions. Figure 6. Cell diameter distribution of PCL and PCL/MWCNT foams with different MWCNT content: (a) neat PCL; (b) PCL/0.5 wt% MWCNT; (c) PCL/1 wt% MWCNT; (d) PCL/3 wt% MWCNT. Figure 7. Effect of MWCNT content on the cell size and cell densities. Conclusions Figure 5. Morphology of PCL and PCL/MWCNT foams with different MWCNT content: (a) neat PCL; (b) PCL/0.5 wt% MWCNT; (c) PCL/1 wt% MWCNT; (d) PCL/3 wt% MWCNT. PCL/MWCNT nanocomposites were prepared by melt blending. SEM shown that MWCNT have a good distribution in PCL. Thermal testing results indicated that the addition of MWCNT enhanced the crystallization ability due to the nucleation effect of MWCNT. The PCL/MWCNT nanocomposite foams were prepared by batch foaming method using supercritical carbon dioxide. SEM results revealed that not only the neat but also all composites foamed uniform cell structure. Moreover, the addition of MWCNT act as the nucleation sites when foaming progress that reduced the cell diameter and improved the cell density. The result foams can be considered as lightweight degradable conductive material or electromagnetic shielding material. SPE ANTEC Anaheim 2017 / 2452

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