Article pubs.acs.org/IECR
In-Situ Preparation of Poly(butylene succinate-co-butylene fumarate)/Hydroxyapatite Nanocomposite Sogol Naghavi Sheikholeslami, Mehdi Rafizadeh,* Faramarz Afshar Taromi, and Hadi Shirali Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran ABSTRACT: A three-step in situ polymerzition method was used to synthesize short-segmented block copolymers of poly(butylene succinate-co-butylene fumarate)/nano hydroxyapatite nanocomposites. The nanocomposites were characterized by 1H NMR and Fourier transform infrared spectroscopy. The bond formed between nanohydroxyapatite and polymer led to a good particle dispersion which was studied using scanning electron microscopy (SEM). The thermal and crystallization properties of nanocomposites were calculated via differential scanning calorimetry and X-ray diffraction, indicating the formation of the complete crystals with two different sizes. Producing nanofiber from nanocomposites was done using the electrospinning method. According to SEM results, nanofibers were continuous and bead-free and those produced from nanocomposites had larger fiber diameter compared to the neat copolyester. In contrast to tensile strength, elastic modulus, hydrolytic degradability, and hydrophilicity of the resulted nanofiber which were increased with the nanoparticle, the elongation at break was slightly decreased.
1. INTRODUCTION In recent years, fabrication of biodegradable polymers for medical applications such as drug delivery, prosthesis, and tissue engineering has attracted a great deal of scientific interest.1 Hence, several approaches such as mixing, copolymerization, and application of an inorganic material as the filler or biomimetic, have been used to improve the thermomechanical and biocompatibility properties of such polymers.2,3 To design a bone mimicking nanocomposite, one should take the bone structure into account, which is composed of two major nanoscale phases of minerals and proteins. Bone flexibility and strength are provided by proteins, whereas toughness and rigidity are achieved via minerals.4 Aliphatic polyesters are wellknown biodegradable polymers as they decompose into materials which are soluble in water and nontoxic.5−8 Good thermomechanical properties of aliphatic polyesters such as poly(butylene succinate) (PBS) leads to their use for many applications.9 Furthermore, making use of unsaturated aliphatic polyesters with biocompatibility and biodegradability properties which can cross-link to form a 3D scaffold, is an approach to achieve improved properties.10 So far, several unsaturated aliphatic comonomers such as fumaric acid,11 maleic acid,12 itaconic acid,13,14 and ricinoleic acid15 have been used to synthesize PBS copolyester. We have recently synthesized poly(butylene succinate-co-butylene fumarate) short-segmented block copolyester via the three-step polycondensation method.16 Besides copolymerization, the in situ process of synthesizing nanocomposites will lead to a higher dispersion of inorganic filler in the organic matrix, resulting in enhanced mechanical properties required in many medical applications.17,18 Different organic and inorganic nanoparticles have © XXXX American Chemical Society
been used to in situ polymerize PBS nanocomposite including silica,19 titanium oxide,20 graphene oxide,21 organo-montmorillonite,22 and a novel organo-modified layered double hydroxide.23 Among different inorganic nanoparticles, hydroxyapatite (HA) exhibits excellent biocompatibility and low degradation rate. Nevertheless, difficulties in shaping HA along with its fragility have limited its applications. Furthermore, because of the high surface energy of HA nanoparticles which make them agglomerate, it is difficult to disperse them in the matrix.24 Thus, the most appropriate way to prepare a nanocomposite with well dispersed HA nanoparticles is the in situ method.10 In previous studies, the 3D scaffolds have been fabricated using different methods among which electrospinning has been shown to be one of the most efficient ones.25 Nano- or microfibrous scaffolds are capable of mimicking the structure and morphology of the bone extracellular matrix. It is necessary to produce a well-dispersed nanohydroxyapatite (nHA) nanocomposite before electrospinning, as size, shape, and distribution of the filler clusters as well as the preparation method affect the properties of nanocomposites. In our previous work, we studied the properties improvement of poly(butylene succinate) via copolymerizing with ethylene terephthalate (ET) which is an aromatic comonomer. Using 10 mol % of ET increased the mechanical properties of the copolymer due to its crystallinity and hard segments. But Received: Revised: Accepted: Published: A
May 31, 2017 August 7, 2017 August 23, 2017 August 23, 2017 DOI: 10.1021/acs.iecr.7b02245 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
viscosity of nanocomposites. Samples were dissolved in chloroform, and the test was performed at 25 ± 0.1 °C. The number-average molecular weight (Mn) was calculated using the following equation:28
further amount of comonomer caused a decrease in degradability.26 It was found that preparation of nanocomposite via in situ polymerization with nHA nanoparticles led to further improvement of degradability of the mentioned copolyester.27 But it seems that using a new biodegradable unsaturated comonomer in PBS copolymerization can improve the mechanical properties and give sites for further reactions such as functionalization or cross-linking. Therefore, the overall purpose of this study in the first step was in situ polymerization, characterization, and investigation of thermomechanical properties and biodegradability of a novel short-segmented block poly(butylene succinate-cobutylene fumarate)/nano hydroxyapatite nanocomposite (PBS-BF-nHA). The second step was to fabricate 3D nanofiber scaffolds through the electrospinning method, and the effects of changing electrospinning parameters were also investigated. PBS-BF-nHA nanocomposite can be cross-linked to form a three-dimensional scaffold for applications such as bone tissue engineering. However, the present work has focused on nanocomposite synthesis and fiber preparation rather than scaffold evaluation.
M̅ n = 3.29 × 104 × [η]1.54
(1)
A Varian Mercury-300 instrument (Varian, Palo Alto, CA) was used to record proton nuclear magnetic resonance (1H NMR) spectra. CDCl3 was used as the solvent to lock the spectrometer, and tetramethylsilane (TMS) was employed as the reference. A Nexus 670 spectrophotometer (Nicolet Co., Waltham, MA) was used to record Fourier transform infrared spectra (FTIR) at room temperature. One milligram of products was mixed with 100 mg of KBr to produce the samples. Samples were vacuum-coated with gold in a Denton Desk II sputter coater (Moorestown, NJ) and examined using a VEGA3 SBU variable pressure device (Tescan, Czech Republic) at a 20 kV accelerating voltage to record scanning electron microscope images (SEM). To measure the contact angles (CA) of the nanocomposites, an OCA-15 plus contact angle microscope (DataPhysics Instruments GmbH, Filderstadcity, Germany) with SCA20 software was used at room temperature. The average value between six measurements of CA of the right and left sides of distilled water drops was reported. 2.4. Thermal Properties. A Mettler-Toledo 822e instrument (Columbus, OH) was used to perform differential scanning calorimetry (DSC) analysis; 4−10 mg of sample was melted at 160 °C for 2 min to remove thermal history. Then, DSC thermograms were obtained by cooling the sample to −50 °C and then heating again to 160 °C with cooling and heating rates of 10 °C/min. The X-ray diffraction analysis (XRD, Equinox 3000) was carried out with tube voltage, tube current, and CuKα1 irradiation of 40 kV, 30 mA, and λ = 0.1541874 nm, respectively. The test was performed on hot-pressed samples (thermoformed at 120 °C and cooled at room temperature) for 2θ in the range of 4°−120°. 2.5. Fiber Matrix Fabrication. HFIP as a solvent and a proper amount of PBS-BF-nHA were used to obtain homogeneous solutions of nanocomposites. The solutions were stirred for 8 h. Fabrication of nonwoven fiber matrices was done via a horizontal electrospinning setup at room temperature and pressure. The setup consisted of a 1 mL glass syringe (with a needle tip diameter of 0.51 mm), a syringe pump (New Era Pump System, NE1000, USA), a flat collector covered with an aluminum sheet and connected to the ground electrode, and a high voltage power supply (Gamma High Voltage Research, RR60, USA). To investigate the effects of electrospinning parameters on minimizing the fiber diameter, electrospinning solution concentration (C), applied voltage (V), and needle to collector distance (D) parameters were changed in the ranges of 4−10 wt %, 16−22 kV, and 6−9 cm, respectively, with an injection rate (Q) of 400 μL/min. The resulted fibers were vacuum-dried for 8 h at the temperature of 37 °C. 2.6. Fiber Matrix Characterization. The morphology of nonwoven fiber matrices was studied by scanning electron microscope (SEM), which is described previously. In each image, the average fiber diameter was measured with the help of ImageJ software. Determination of the elemental composition and of the Ca/P ratio of each sample was done by an energy-
2. EXPERIMENTAL SECTION 2.1. Materials. Fumaric acid (FA), succinic acid (SA), and 1,4-butanediol (1,4-BDO), were received from Daejung Co., Korea. Nanohydroxyapatite (nHA) with a diameter of 100 nm was purchased from DK-nano Co, China. Hydroquinone (HQ) as the inhibitor, titanium(IV) butoxide (TBT), as the polycondensation catalyst, polyphosphoric acid (PPA) as the thermal stabilizer, and chloroform as the solvent for intrinsic viscosity measurements were all received from Merck Co., Darmstadt, Germany. 2.2. Synthesis of Nanocomposites. Short-segmented block copolymers of poly(butylene succinate-co-butylene fumarate)/nanohydroxyapatite with 20 mol % of butylene fumarate, were synthesized using a three-step procedure. First, 1 mol of SA was mixed with 1.7 mol of 1,4-BDO in the reactor at 140 °C and 3.5 bar for 30 min. Subsequently, the esterification step was started by increasing the temperature to 190 °C, and continued (for about 105 min) until no more water could be collected. The progress of the reaction was evaluated by cooling, gathering, and weighing the produced water vapor. At the end of the esterification step, TBT, PPA, and nanohydroxyapatite with 1, 2.5, and 4 wt % were added as the catalyst, thermal stabilizer, and nanoparticle, respectively, and mixed in the reactor for 10 min at 200 °C and 2 bar. Thereafter, the in situ polycondensation step was started by increasing the temperature to 241 °C and reducing the pressure to vacuum (
