The interest of eco-friendly polymer nanocomposites is continuously growing because of the increased concern about environmental pollution. Polycaprolactone (PCL) is one of the extensively used bio-degradable polymers. The advantage of this biopolymer is further enhanced by the addition of nanofillers and developing nanocomposites. CNT is one of the best matching nanofillers, inclusion of CNT in PCL matrix increase the use of PCL in diverse areas. In this work PCL/CNT composites prepared by electrospinning technique, inorder to avoid CNT agglomeration, acid functionalized CNT (f-CNT) is used. Morphological properties of the electrospun nanofiber were studied by SEM and the interaction between PCL and f-CNT was studied by FT-IR. DSC and optical microscopic studies reveals the influence of f-CNT’s in enhances the crystallinity, and reduces the lamellar thickness and spherulite growth. H-bonded interaction between PCL and f-CNT enhances the physical properties of bio-degradable PCL and there by enhances its commercial application.
Key words: PCL/f-CNT; electrospinning; thermal properties; crystallinity.
Biodegradable polymer nanocomposites gained much attention in recent years because of their environmental friendly and non hazardous nature to the ecosystem. Polycaprolactone (PCL) is one of the widely used biodegradable polymer in various fields such as biomedical, automotive, packaging and energy harvesting etc . Degradation of PCL occurs through the hydrolysis of its ester linkage, this enhances the usage of this polymer in diverse areas. The widespread applicability of PCL is not only because of its biodegradable nature but also its good resistance towards water, oil, solvents etc., low melting point, low viscosity, and easy processibility. Among the PCL nanocomposites PCL/CNT nanocomposites attracted the attention of research community because of its excellent properties and special structure of CNT. Low density, high aspect ratio, and high specific surface area make it in to a more versatile filler . The incorporation of very low concentration of CNT will dramatically improves the tensile strength, modulus, fracture toughness, thermal and electrical conductivity, sensing ability etc of the polymer , .The role of MWCNT on enhancing the degradation temperature of PCL was studied by Chen et al., the authors reported that low concentration of MWCNT also shows significant improvement in the degradation temperature of PCL3. PCL/CNT nanocomposites also finds application in biomedical field, Niezabitowska et al. prepared PCL/CNT nanocomposites and used for drug delivery4. The property of carbon nanotubes and nanofibers on improving the mechanical, thermal and gas barrier properties of PCL was reported by Garcia et al.1. Agglomeration of CNT’s is one of the major problems faced during the processing and fabrication of polymer/CNT nanocomposites. This agglomeration is due to the strong interfacial interaction between will lead to reduce the physical properties (mechanical and electrical properties) of the matrix polymer. Functionalization is an effective method to enhance CNT polymer interaction, dispersion and alignment of CNT’s. This will also improves the interfacial interaction between polymer and CNT; result in the formation of nanocomposites with enhanced physical and electrical properties.
The present work uses acid functionalized (f-CNT), the main objective of the work is the fabrication of electrospun PCL/CNT nanocomposites and to study the effect of f-CNT on the thermal and crystallization behavior of PCL.
2. Materials and methods
Polycaprolactone (PCL) having Mw. 70,000-90,000 purchased from Sigma Aldrich (St. Louis, USA), Chloroform (molecular weight 119.38 g/mol, density 1.48 g/mol) obtained from Merck; Acid functionalized carbon nanotubes (f-CNT) was obtained from Sigma Aldrich (St. Louis, USA).
2.2. Preparation of electrospun PCL/f-CNT nanofibers
The PCL/f-CNT electrospun membranes were prepared by 10wt% of PCL in chloroform and stirred overnight to get homogenous solution , . To this solution, different amounts (0.25, 0.50, and 1.0 wt%) of f-CNT were added.The solution were sonicated for 5 minutes with a pulse rate of 5 sec on/ 5 sec off, at an amplitude of 35%. The resulted solution was taken in a 10 ml syringe and it was fixed in the syringe pump. Electrospinning Was carried out at room temperature, a thin aluminium foil was used as the substrate for deposition of fibers. The working distance was kept at 15 cm from the needle to the collector and flow rate was maintained as 1 ml/hr for all the experiments. The applied voltage for electrospinning process was 15 kV. The resultant nanofiber membranes were dried at 60?C for at least 24 hours to remove residual solvents . A schematic representation of electrospinning is shown in scheme 1.
Scheme.1. Preparation of electrospun PCL/f-CNT nanofibers
2.3. Characterization techniques
2.3.1. Scanning Electron Microscopy
The morphology of the nanocomposite was studied by using JEOL- scanning electron microscope. Inorder to make the polymer sample conducting, the samples were sputtered using platinum prior to the analysis.
2.3.2. Fourier transform infrared spectroscopy
FT-IR studies of electrospun PCL/f-CNT nanocomposites were performed using Perkin-Elmer Fourier transform infrared spectrometer.
2.3.3. Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) analysis was performed using Perkin Elmer Diamond DSC. 2.25 mg of the samples were heated from room temperature to 100oC at a heating rate of 10oC/min. the influence of nanofillers on crystallization temperature was studies by cooling the samples from 100oC to room temperature at a cooling rate of 10oC/min. the percentage crystallinity of the samples were calculated using the equation ,
where is the enthalpy of fusion of the polymer, is the slandered enthalpy of fusion of 100% crystalline polymer, wt% Nps is the weight percentage of nanoparticles in the polymer matrix. Enthalpy of fusion of 100% crystalline PCL was taken as 157J/g, inorder to calculate the percentage crystallinity of the polymer. The lamellar thickness ( ) of PCL was measured by Gibbs- Thomson equation ,
where is the slandered enthalpy of fusion of 100% crystalline polymer is the fold surface free energy of the polymer, is the melting temperature and is the equilibrium melting temperature of the polymer. The following literature values of PCL was used for the calculation , =157J/g, =4.32×10-2 J/m2 and =342.2K
2.3.4. Polarized optical microscopy
The effect of f-CNT’s on the spherulite size was studied by Polarized optical microscopic (POM) studies (Leica DMLP microscope equipped with DFC 295 camera). The samples were melted by heating up to 100oC and slowly cooled and the crystallization studies weere performed at a constant temperature of 42oC and the images are captures at a fixed time intervals.
3. Results and discussion
3.1. Scanning Electron Microscopy
Morphology of the nanocomposites was studied by SEM analysis. Figure.1. Shows the SEM images of PCL and PCL/f-CNT nanofibers. From the images it was clear that the neat PCL forms bead less fine fibers and on addition of functionalized CNT’s creates some beads the average fiber diameter decreases. The reduction in fiber diameter may be attributed to the alignment of f-CNT along the fiber direction during spinning . The 3D surface plot was obtained from SEM images using image J software is shown in Figure.2. The surface plot mainly depends on the surface roughness, fiber distribution and fiber diameter.
Figure.1. SEM images of (a) Neat PCL, (b) ) PCL/0.25% f-CNT, (c) PCL/0.5% f-CNT, (d) PCL/1.0% f-CNT electrospun fibers
Figure.2. Surface plot (a) Neat PCL, (b) ) PCL/0.25% f-CNT, (c) PCL/0.5% f-CNT, (d) PCL/1.0% f-CNT electrospun fibers obtained from SEM images using image J software