FABRICATION OF HIGH STRENGTH AND TOUGHENED BIODEGRADABLE ELECTROSPUN FIBERS: POLY(LACTIC ACID)/BIOMAX BLENDS

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1 FABRICATION OF HIGH STRENGTH AND TOUGHENED BIODEGRADABLE ELECTROSPUN FIBERS: POLY(LACTIC ACID)/BIOMAX BLENDS Sheikh Rasel, Remon Pop-Iliev, and Ghaus Rizvi, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North Oshawa, Ontario, Canada L1H 7K4 Abstract The objective of this study is to prepare a toughened and strengthened electrospun fibrous biodegradable poly(lactic acid) (PLA) mat blended with Biomax, an ethylene copolymer designed to modify PLA to improve toughness properties, using electrospinning. Morphological, thermal, mechanical, and thermomechanical properties of PLA/Biomax blends were investigated. Morphological findings indicated that the electrospun PLA/Biomax fibers were uniform and smooth with an average diameter of µm when Biomax contents below 2 % (w/v). The addition of 1 % of Biomax improved both thermal stability and mechanical properties of PLA/Biomax fibrous mats. PLA/Biomax mats with 1 % (w/v) of Biomax exhibited the maximum tensile strength of 4.6 MPa and tensile modulus of 103 MPa showing 64.3 % and 101 % improvement; as compared to neat PLA values of 2.83 MPa and 51 MPa, respectively. Furthermore, the presence of 1 % Biomax into electrospun PLA fiber mats improved the storage modulus by % compared to PLA fiber mat (41.5 MPa). Strong and toughened PLA/Biomax biodegradable fibrous mat might be potentially suitable to be used in packaging, filtration, reinforcement of composite, etc. Introduction Poly(lactic acid) (PLA) is well-known biodegradable thermoplastic polymer derived from 100 % renewable biomass resources, such as corn starch, sugarcane, potatoes, etc. Over the last few decades, PLA has been extensively studied in the field of biomedical, tissue engineering, and packaging materials [1, 2] because of their biocompatibility with living organisms, degradation properties, and ease of processing. It can be transformed into fibers by electrospinning to obtain desired functional properties. Electrospinning has proven to be an efficient, versatile and straightforward fabrication method that can be used to produce thin fibers from a rich variety of materials including polymers, composites, and ceramics [3]. The resulting electrospun fibers, having diameter in the micro- to nanometer ranges, carry outstanding characteristics, such as high specific surface area, flexibility in surface functionality, and superior mechanical properties compared with any other forms of the material [4]. Various potential applications including filtration, packaging, protective clothing, tissue engineering, and nanocomposites, of electrospun fibrous mats composed of PLA nanofibers are investigated [5, 6]. However, some limitations, including poor mechanical properties, low thermal stability, and low hydrophilicity of electrospun PLA mats, restrict its application in many fields [7]. The most effective methods for enhancing the mechanical properties of bulk PLA materials are: plasticization, blending with other materials, incorporating micro/nano fillers and copolymerization of lactic acid with a comonomer. Various kinds of fillers such as nanoclays, cellulose nanocrystals, carbon nanotubes, and graphene oxide have been employed to improve the thermal and mechanical properties of PLA [8, 9]. Shi et al. [7] reported that by the addition of cellulose nanocrystals (CNCs) up to 5 wt% within PLA electrospun fibers, the thermal stability and mechanical properties improved effectively. Yoon et al. [9] studied with poly(d, L-lacticco-glycolic acid) (PLGA) and graphene oxide (GO) nanosheets and showed that adding 2 wt% GO increased storage and loss moduli significantly. Ramdhanie et al. [10] demonstrated the morphological, thermal, and mechanical properties of electrospun blends of poly(lactic acid) (PLA) and poly(glycolic acid) (PGA) for potential use of biomedical applications. They showed that blended electrospun fibers with a diameter range of 0.9 to 1.3 µm improved the mechanical properties. Structural, mechanical, and thermal properties of electrospun PLA/polyaniline blend fibers were evaluated by Picciani et al. [11]. Biomax strong 120 is an ethylene copolymer considered to modify PLA in order to improve toughness properties in packaging and industrial applications. Biomax strong 120 is approved for food packaging application by US Food and Drug Administration (FDA) [12]. It is claimed that the addition of small amount of Biomax strong 120 to PLA results in a significant improvement in flexibility, impact strength, machinability, and reduce brittleness. Afrifah et al. [13] studied the effectiveness of an ethylene/acrylate copolymer in toughening PLA prepared by melt blending process. They reported that impact strength of the blends effectively improved by the impact modifier. The ductility, elongation at break, and energy to break increase SPE ANTEC Anaheim 2017 / 2091

2 significantly, although tensile strength and modulus dropped with increase impact modifier content. The aim of this paper is to improve the mechanical properties such as tensile strength, stiffness, and toughness of pristine electrospun PLA fibers by adding Biomax, an impact modifier of PLA, for packaging, filtration, and reinforcement of composites applications. Morphological, thermal, mechanical, and thermomechanical properties of PLA/Biomax fibrous mats prepared by electrospinning were investigated. Materials Experimental Methods Poly(lactic acid), PLA 6202D ( melt index of g/10 min at 210 o C) was kindly supplied by NatureWorks, USA and Biomax strong 120 was obtained from DuPont, USA. The melt flow rate (190 o C / 2.16 kg) and melting temperature of Biomax strong 120 were 12 g/10 min and 72 o C, respectively. Chloroform and N, N- Dimethylformamide (DMF) were purchased from Sigma- Aldrich, Canada. All solvents were of analytical grade and used as received. Electrospinning Setting PLA and PLA/Biomax solutions (15 % w/v) were prepared using a mixer of chloroform and DMF at a ratio of 8:2 by continuous magnetic stirring for four hours using a hot plate at 50 o C. To obtain a homogeneous solution, stirring was continued for twelve hours at room temperature. Then, the solution was loaded into 5 ml plastic syringe which was pumped through the needle tip (0.51 mm inner diameter) with the flow rate of 500 µl/h. A 10 KV high voltage was applied to the needle tip and the collector electrodes which were kept a 7 cm apart. The electrospun fibers were collected on a rotating drum with 1000 rpm. The electrospun fibrous scaffolds were dried at 40 o C for 12 h. Characterization Scanning Electron Microscopy (SEM) The surface morphologies of obtained electrospun fibers were observed under an environment scanning electron microscopy (Quanta FEG 250 ESEM). Thermogravimetric Analyzer (TGA) The thermal stability of electrospun fibrous mats was evaluated by thermogravimetric analyzer Q50 (TA instruments) from room temperature to 600 o C at a heating rate of 10 o C min -1 under nitrogen atmosphere. Approximately 5.5 mg of nanofibrous mats were used for each test. Differential Scanning Calorimeters (DSC) The thermal properties of fibrous mats such as, glass transition temperature (T g ), melting temperature (T m ), and degree of crystallinity (χ c ), were investigated with the help of differential scanning calorimeters Q20 (TA instruments) with a sample weight of 3 to 6 mg under a nitrogen atmosphere. The specimens were heated from room temperature to 220 o C at a heating rate of 10 o C min - 1 and held for 5 minutes. Then, the specimens cooled at a rate of 10 o C min -1 up to 40 o C. Finally, the thermal data was collected by reheating the sample from 40 o C to 220 o C at 10 o C min -1. Dynamic Mechanical Analysis (DMA) The tensile strength tests and the dynamic mechanical thermal analysis (DMTA) of fibrous PLA and PLA/Biomax mats were performed using dynamic mechanical analysis (DMA) Q800 (TA instruments). Samples were cut using razor blade with a width of 6.5 mm; thickness were measured under optical microscope and gauge lengths of the sample were evaluated by the instrument directly. The average thickness measured for all the electrospun mats were in the range of 0.09 to 0.18 mm. As the samples were soft and flexible, therefore, both ends of tested sample were covered with the scotch tape to distribute the load uniformly along the sample. The tensile strength tests were conducted at room temperature (30 o C) with a force ramp of 0.5 N min -1. The sample was loaded to the film tension clamp where the maximum force was set at 18 N with an initial preload force of N. The data reported are the average of five observations. From the test results, Young s modulus (E), tensile strength (σ t ), and elongation at break (ε b ) were evaluated. The DMTA experiments were conducted in tension mode at a constant frequency of 1 Hz and temperature gradually raised with a rate of 3 o C/min from room temperature (30 o C) to 120 o C. Surface Morphology Results and Discussion Figure 1 illustrations the surface morphologies of PLA and PLA/Biomax blends. Observing these SEM images, it is seen that the electrospun PLA fibers were uniform with no beads having diameters of 1.4 ± 0.28 µm (Figure 1a). The diameters of the electrospun fibers had no significant difference with increasing concentration of Biomax up to 2 % (w/v) as compared to PLA fibers. The average fiber diameter of PLA/Biomax-2 was measured 1.5 ± 0.24 µm (Figure 1b) for adding 2 % (w/v) of Biomax. The electrospun fibers became non-uniform with SPE ANTEC Anaheim 2017 / 2092

3 bimodal distributions when concentrations of Biomax were above 2 %. The distributions of PLA/Biomax-3 and PLA/Biomax-4 fiber diameters were calculated 1.4 ± 0.86 µm and 1.6 ± 1.02 µm, respectively (Figure 1c and 1d). Major factors that control the diameter of the fibers are: concentration of polymer solution, high voltage, feed rate, and distance between needle and collector. Since all the processing parameters of electrospinning were same except solution concentration, therefore, it is emphasized that the solution concentration of PLA/Biomax plays a significant role in the formation of fiber morphology. Tan et al. [14] reported similar findings by studying the effects of processing parameters on the surface morphology of electrospun PLA fibers and showed that polymer concentration, electrical conductivity of solvents and molecular weight of polymers are the most dominant parameters to control the morphology of electrospun fibers. exhibited a T c at 90.2 o C which is very close to neat PLA mat. The crystallinity percentage (χ c ) of PLA and PLA/Biomax blends was determined by the following equation: DH m (%) = 100 o DH w c (1) m Where, ΔH m is the melting enthalpy (J/g), ΔH o m is the melting enthalpy of the 100 % crystalline PLA (93.7 J/g), and w is the weight fraction of PLA in the blends [15]. From Table 1, it can be seen that there is no noticeable changes in the crystallinity and melting temperature of PLA and its blends with Biomax. The neat PLA and PLA/Biomax-4 electrospun mats presented 40.5 and 42.7 percentage of crystallinity. The endothermic picks of melting in PLA mat and in mixed PLA/Biomax mats occurred at 166 ± 1 o C. By studying melting temperature and crystallinity, the miscibility of a polymer blend can be identified [10]. In general, if one of the components in the polymer blends is crystalline, then the miscible component restricts to form crystalline structure. And thus, T m and crystallinity of polymer blend drop significantly with the addition of blended component. The DSC results (Figure 2 and Table 1) show that there was no significant transformation in the melting temperature and percentage of crystallinity of blended PLA/Biomax mats as compared to neat PLA mat. These findings indicate PLA and Biomax are immiscible. Hence, the electrospun fibers are phase separated structures. Figure 1. SEM images of PLA (a), PLA/Biomax-2 (b), PLA/Biomax-3 (c), and PLA/Biomax-4 (d) electrospun fibers. Thermal Properties Figure 2 shows the first heating DSC curves of PLA/Biomax fibrous mats containing 0 to 4% (w/v) of Biomax. The glass transition temperature (T g ), the peak crystallization temperature (T c ), melting temperature (T m ) and crystallinity percentage are listed in Table 1. The T g value slightly shifted from 64.1 o C (PLA) to 60.0 o C (PLA/Biomax-3). It is likely because that the PLA crystal phases were restricted due to plasticization []. There is a certain temperature above T g where the polymer chains have the mobility and become highly ordered by an exothermic reaction known as crystallization or cold crystallization temperature (T c ). All electrospun samples in this study showed a crystallization temperature (Figure 2) and a subsequent melting pick. Electrospun PLA and PLA/Biomax-1 blend fibrous mats demonstrated a Tc of 89.6 o C and 82.5 o C, respectively. Blend PLA/Biomax-2 Figure 2. DSC thermograms of PLA and PLA/Biomax fibrous mats with different Biomax contents. Table 1. Thermal characteristics of PLA and PLA/Biomax blends. Sample T g T c T m ΔH m (J/g) Χ c (%) SPE ANTEC Anaheim 2017 / 2093

4 PLA PLA/Biomax PLA/Biomax PLA/Biomax PLA/Biomax TG and differential TG thermograms of PLA and blended PLA/Biomax mats are presented in Figure 3a and b, respectively. Thermal parameters including T onset which is the initial weight loss temperature, the maximum degradation temperature (T max ) which is the highest thermal degradation temperature, and char yield (%) which is residue are summarized in Table 2. Thermal degradation of PLA and PLA/Biomax blends takes place in a single weight loss step. T onset and T max of PLA were at o C and o C, respectively, while PLA/Biomax-3 mats exhibited the highest T onset and T max values of o C and o C, respectively indicating improvement of heat resistance. However, the addition of 4 % of Biomax, the T onset and T max dropped to o C and o C. The amounts of char yield of PLA/Biomax blends were slightly higher than that of PLA. Figure 3. TG (a), and DTG (b) curves of PLA and PLA/Biomax fibrous mats. Table 2. Summary of TGA and DTGA of PLA and PLA/Biomax blends Sample T onset T max Char Yeild (%) PLA PLA/Biomax PLA/Biomax PLA/Biomax PLA/Biomax Mechanical Properties A typical stress-strain curve, the tensile strength (MPa), the tensile modulus (MPa), and elongation at break (%) of fibrous PLA and its Biomax blends mats are presented in Figures 4a-d, respectively. It is observed that the addition of 1% Biomax into PLA produced significant changes in the mechanical properties. The tensile strength of PLA/Biomax-1 mats was improved by 64.3 % upon mixing of 1 % Biomax as compare to PLA mats (2.8 MPa). Similar trend was noticed in the tensile modulus of PLA/Biomax-1 mats. In this case, the tensile modulus improved by 98.4 % from 51.2 MPa (PLA) to MPa. The elongation at break of PLA and PLA/Biomax-1 were 84.3 % and 73.4 %, respectively. An increase of tensile strength and modulus was contributed by the strong interfacial bonds between PLA and Biomax phases, which mitigated the stress concentration on the mats while load is applied. However, the mechanical properties of blended mats dropped gradually with further addition of Biomax. The tensile strength of PLA/Biomax blends with 2, 3, and 4 % of Biomax were 3.4 MPa, 3.6 MPa, and 1.5 MPa, respectively. The tensile modulus also followed the similar trend of the tensile strength. Stiffness of PLA/Biomax blends reduced from 67.5 MPa to 32 MPa with blending 2 to 4 % of Biomax. The elongation at break of blended mats also reduced when Biomax contents were above 2%. The phenomena of reduction of mechanical properties of blended fibrous mats can be explained by studying SEM images (Figure 1) and thermal studies (Table 1). The addition of Biomax percentage changed the electrospun fibers morphologies from uniform to bimodal. Furthermore, the thermal studies indicated that PLA and Biomax are immiscible polymers which caused phase separations. Both bimodal fibers morphologies and poor interaction between two blended polymers might be responsible for consequently reduction of blended mats properties. The thermomechanical properties including dynamic storage modulus (E ), and tan δ as a function of temperature are shown in Figure 5. It is observed from Figure 5a that the storage modulus increased significantly with the increasing up to 3% (w/v) contents of Biomax. Storage modulus improved by % from 41.5 MPa (PLA) to 86.1 MPA, when the content of Biomax increased from 0 to 1% (w/v) at 40 o C. Likewise, 85 % and 99 % improvement of storage modulus were observed SPE ANTEC Anaheim 2017 / 2094

5 in PLA/Biomax-2 (76.8 MPa) and PLA/Biomax-3 (82.5 MPa) mats, respectively, compared to PLA mat. The E values remain almost steady in the elastomeric region for all the samples. This may be associated with the strong interaction between blended polymers. However, the lower E value (37.8 MPa) was seen in PLA/Biomax-4 mats when loading 4 % (w/v) of Biomax, compared to PLA mat, indicating an increase in the flexibility of blended polymers. The dropped in E values for all the samples occurred at around glass transition region (in o C range). This can be attributed by the increment of molecular chain mobility of the polymers. Figure 5. Storage modulus (a) and loss factor (tan δ) (b), of PLA and PLA/Biomax blends as a function of temperature. Conclusions Figure 4. Typical tensile stress-strain curve (a), the tensile strength (MPa) (b), the tensile modulus (MPa) (c), and elongation at break (%) (d) of PLA nanofibers mat and its Biomax blends. Tan δ is defined as the ratio of loss modulus and storage modulus, which is related to the impact resistance of the material. In general, damping of blended materials is influenced by the interaction between the polymers and the void contents [9]. Figure 5b shows the damping properties (tan δ) of PLA and blended PLA/Biomax mats as a function of temperature. It is observed that the addition of Biomax caused reduction the area under the Tan δ curve, hence, reducing the damping capability of blended fibrous mats. Tan δ pick heights for blended mats were lower than that of PLA mat indicating a good interaction between blended polymers [9]. The peaks of the Tan δ curves show the glass transition temperature, lay in the o C range. This paper reports on the preparation of PLA and its Biomax blends ultrafine fibers by using electrospinning and their characterizations such as, morphological, thermal, mechanical, and thermomechanical properties. SEM results revealed that electrospun PLA and PLA/Biomax (up to 2 % of Biomax) fibers were uniform with an average diameter of µm whereas, nonuniform and bimodal distribution of PLA/Biomax fibers were produced with the addition of Biomax contents above 2 % (w/v). Thermal studies show that the heat resistance slightly improved by 20 o C in PLA/Biomax-1 and PLA/Biomax-3 fibrous mats compared to neat PLA mat. The tensile strength of PLA/Biomax fibrous mats increased by 64.3 % from 2.83 MPa (PLA) to 4.6 MPa with the addition of 1 % Biomax contents and gradually decreased with further increasing the Biomax contents. Likewise, the presence of 1 % Biomax within electrospun fiber mats improved the tensile modulus by about twice over that of PLA fiber mat. As compared to PLA mat (41.5 MPa), a significant improvement of storage modulus of %, 85 % and 99 % were reported in PLA/Biomax-1 (86.1 MPa), PLA/Biomax-2 (76.8 MPa) and PLA/Biomax-3 (82.5 MPa) fibrous mats, respectively, indicating a strong interaction between blended polymers. Acknowledgement SPE ANTEC Anaheim 2017 / 2095

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