Article Journal of Nanoscience and Nanotechnology Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America Vol 15, 1–10, 2015 www.aspbs.com/jnn Effects of Porogen on Structure and Properties of Poly Lactic Acid/Hydroxyapatite Nanocomposites (PLA/HAp) Dinh Thi Mai Thanh1 ∗ , Pham Thi Thu Trang1 , Nguyen Thi Thom1, Nguyen Thu Phuong1 , Pham Thi Nam1 , Nguyen Thi Thu Trang1 , Jun Seo-Park2 , and Thai Hoang1 Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Hanoi, Vietnam Department of Chemical Engineering Hankyong National University, 327 Jungang-ro, Anseong-si, Gyeonggi-do, 456-749, Korea PLA/md-HAp/PEO porous nanocomposites for applications in bone engineering from poly lactic acid (PLA) incorporated with different NH4 HCO3 porogen content were prepared by solvent casting method The porosity, morphology and mechanical properties of the nanocomposites were determined The obtained results showed that the porosity of the nanocomposites increases from 10 to 49% with the increase of NH4 HCO3 porogen content from 0–30 wt% However, their Young’s modulus decreased 78% in comparison with the nanocomposite without using NH4 HCO3 porogen The bioactivity of the nanocomposite with 20 wt% NH4 HCO3 porogen was evaluated by examining the formation of hydroxyapatite (HAp) on its surface when being immersed in simulated body fluids (SBF) solution The in vitro degradation behavior of the nanocomposites immersed in the SBF solution at 37 C was systematically monitored at different time periods of 1, 3, 7, 14, 21 and 28 days SEM images showed the formation of hydroxyapatite on the surface of the nanocomposite after immersion day in the SBF solution The measurements of weight loss, pH solution, and XRD of the samples indicated that PLA/md-HAp/PEO nanocomposite without NH4 HCO3 porogen was degraded more slowly than the PLA/md-HAp/PEO nanocomposite with 20 wt% NH4 HCO3 porogen Keywords: Hydroxyapatite (HAp), Modified Doped Hydroxyapatite (md-HAp), Poly Lactic Acid (PLA), Porogen, PLA/HAp Nanocomposite, Solvent Casting Method, Simulated Body Fluids (SBF) INTRODUCTION Hydroxyapatite (Ca10 (PO4 (OH)2 , HAp) has been recognized as a promising bone substitute thanks to its chemical and biological similarities to the mineral phase of the native bones This bioceramic has been used for several years for medical applications.1 However, HAp being synthesized artificially did not have mechanical properties which are necessary for applying in bone implants One of the solutions to solve the above problem is to develop biocomposites such as HAp/metal, HAp/polymer3–5 which have been widely used in medicine and stomatology for the repair of bone tissue HAp/polymer composite has more advantages than original HAp or neat polymer.6 Polymer phase is able to have the same chemical composition as the polymer in bone tissue (collagen) but it could be synthesized as well.2 5–10 So far, special attentions have ∗ Author to whom correspondence should be addressed J Nanosci Nanotechnol 2015, Vol 15, No xx been paid to biodegradable polymer applied in surgery and bio-medicine in general Poly( -hydroxyesters) such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their copolymers have been widely used to fabricate different kinds of scaffolds in tissue engineering because of their good biodegradability, bio-compatibility and feasibility.11–17 However, there are few problems when using these polymers for tissue engineering in practice One of the limitations of these polymers is the lack of bioactivity so that the new bone tissue cannot bond to the polymer surface tightly when they are applied for the bone tissue engineering.4 Another problem is their high hydrophobicity.18 A previous study had shown that the adhesion rate of human endothelial cells on PLA is much lower than on the polystyrene The reason is the contact angle of PLA (71 ) is higher than that of the polystyrene (35 ).19 Recently, nanocomposite of nano HAp and PLA (PLA/HAp) has attracted much attention from researchers 1533-4880/2015/15/001/010 doi:10.1166/jnn.2015.12032 Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) because of their ability in replacing the metal and alloy implants Compared with HAp in the micron range, the nano-HAp has a larger surface area which exhibits enhanced mechanical properties due to the strong hydrogen bonding interactions between the nano-HAp and PLA.20 21 The dispersion of HAp in the PLA matrix is one of critical factors determining the properties of the PLA/HAp nanocomposite There are many methods to fabricate this composite such as emulsion method, melt mixing, high pressure processing, electrospinning, solvent casting method which have their own advantages and disadvantages.22–24 The solvent casting method is the facility of preparation and operation without any specialized equipment Fabrication of PLA/HAp nanocomposite by the solvent casting method has been developed by many researchers.25–27 In order to be applied in bone implant, PLA/HAp nanocomposite needs to have compatibly mechanical stability, mechanical strength and highly open porous structure which are necessary to develop tissue fluids; the size and distribution of pore should be suitable for cell in-growth.28–30 Several techniques have been developed to fabricate porosity materials, including porogen leaching,31–35 gas expansion,36 emulsion freezedrying,37 thermally induced phase separation38–41 and 3Dprinting,42 43 etc Compared with other techniques, the porogen leaching technique controls pore structure easily and has been well established in the preparation of porous nanocomposite Xu et al fabricated composite scaffolds for application in bone engineering from poly(D,L-lactide) (PDLLA) incorporate with different proportional bioactive wollastonite powders through a salt-leaching method, using NH4 HCO3 as porogen.44 In vitro bioactivity of PLA/HAp nanocomposites can be evaluated by immersing the material in saline,45 phosphate buffered saline (PBS)46 and the simulated body fluids (SBF).34 47 48 In this study, the porous PLA/HAp nanocomposites with different contents of NH4 HCO3 porogen were prepared by the solvent casting method The characterization, properties including IR spectra, water contact angle, tensile property, porosity morphology and phase structure of the nanocomposites were investigated The formation of HAp on the surface of the nanocomposites immersed in the SBF solution and their weight change were also discussed MATERIALS AND METHODS 2.1 Materials Poly lactic acid (PLA) was provided by Nature Works-USA (weight-average molecular weight Mw = 250 105 g/mol, density d = 24 g/cm3 ) Poly(ethylene oxide) (PEO) was provided by Sigma Aldrich (average molecular weight Mw = 105 g/mol) Calcium nitrate tetrahydrate (Ca(NO3 · 4H2 O, M = 236 15 g/mol, 99% pure), magnesium nitrate hexahydrate (Mg(NO3 · 6H2 O, Thanh et al M = 256 41 g/mol, 99% pure), zinc nitrate hexahydrate (Zn(NO3 · 6H2 O, M = 297 49 g/mol, 99% pure), diammonium hydrogen phosphate ((NH4 HPO4 , M = 132 06 g/mol, 99% pure), ammonium bicarbonate (NH4 HCO3 , M = 79 06 g/mol), tetrahydrofuran (THF, C4 H8 O, M = 74 12 g/mol, 95.5% pure), lactic acid (C3 H6 O3 , M = 90 08 g/mol, 85.5–90% pure), xylene (C8 H10 , M = 106 17 g/mol, 99% pure) were purity materials of China 2.2 Preparation of Doped HAp The nano-spherical HAp powder doped with magnesium and zinc (d-HAp: 13–22 nm) was synthesized by the chemical precipitation method at room temperature The (NH4 HPO4 aqueous solution was added drop by drop into [0.4 M Ca(NO3 · 4H2 O, 0.05 M Mg(NO3 · 6H2 O, 0.05 M Zn(NO3 · 6H2 O] aqueous solution (the ratio Ca/Mg/Zn of 9/0.5/0.5) at a rate of ml · min−1 during h under strong stirring (750 rpm) The M/P ratio was 1.67 (M = Ca, Mg, Zn) The pH of the mixture solution was adjusted to 10 by adding NH4 OH solution The process was performed within h by stirring, then within 24 h without stirring at room temperature The precipitate was washed for several times with distilled water to pH The obtained doped-HAp powder (d-HAp) was dried at 80 C for 48 h 2.3 Preparation of Modified Doped HAp The reaction system was prepared as following: 20 g of d-HAp powder was dispersed in 70 ml THF via stirring, heating to 65 C Lactic acid (LA) was added drop by drop into the above reaction mixture system for 30 minutes (d-HAp/LA = 1/2 wt/wt) and then 180 ml of xylene was added The resulted suspension was heated to 150 C and stirred for h Then, the modified doped HAp (note md-HAp) was obtained through filtering and being washed with ethylene ether for several times to remove the adsorbed solvent on md-HAp 2.4 Fabrication of PLA/md-HAp/PEO Nanocomposites The PLA/md-HAp/PEO nanocomposites were made by the solvent casting method The md-HAp, PEO and NH4 HCO3 powders were dispersed in 30 ml dichloromethane (DCM) by stirring in 30 minutes Ammonium bicarbonate salt (NH4 HCO3 ) was used as a porogen at different contents (0, 3, 7, 10, 20 and 30 wt%) The PLA was dissolved in 70 ml DCM in 30 minutes And then, combining two above mixtures together by stirring (110 rpm) during h, to form a gel paste mixture The gel paste mixture was then put into a die (4 × cm) and compressed at a pressure of 10 MPa for minutes at room temperature After that, the above die was put into vacuum and dried at room temperature for 24 h, and then continuously dried at 80 C within 24 h to remove the porogen J Nanosci Nanotechnol 15, 1–10, 2015 Thanh et al Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) –C=O % = m2 − m1 / m3 + m2 − m4 · 100 3500 3000 2500 2000 1500 610 563 1055 1465 1767 PLA/HAp/PEO 1000 500 Wave number (cm–1) Figure FT-IR spectra of PLA, md-HAp and PLA/md-HAp/PEO (70/30/5 wt/wt/wt) nanocomposite 2.5 Porosity of PLA/md-HAp/PEO Nanocomposite Porosity of the porous material was determined by the Archimedes’ method with an absolute ethanol as the immersion medium The specimens were dried at 80 C within h before being tested The dried sample was weighed as m1 All the air in specimens were removed by a vaccum pump After that, the specimens were totally submerged in the absolute ethanol The liquid saturated Figure specimen was weighed as m2 A pycnometer filled with ethanol was weighed as m3 Then, the liquid-saturated sample was put in filled pycnometer, m4 is the weight of the liquid-saturated sample after taken out of the liquid The open porosity obtained by: OH OH 4000 PO43– 1061 –CH 1457 1761 md–HAp 3002 2955 Transmittance (%) 2995 2930 PLA 2.6 Test In Vitro The in vitro degradation properties of the samples were evaluated in the simulated body fluids (SBF) In order to prepare litre of the SBF solution, g NaCl; 0.35 g NaHCO3 ; 0.4 g KCl; 0.48 g Na2 HPO4 · 2H2 O; 0.1 g MgCl2 · 6H2 O; 0.18 g CaCl2 · 2H2 O; 0.06 g KH2 PO4 ; 0.1 g MgSO4 · 7H2 O and g glucoza were dissolved in distilled water The pH of the SBF solution is 7.4 (this value is in the pH range of the human body fluids pH = 7.35–7.45).49–51 The samples of PLA/md-HAp with and without NH4 HCO3 were immersed in the cell containing 40 ml SBF, and kept at 37 C, during different immersion times: 1, 3, 7, 14, 21 and 28 days These samples were gently rinsed with distilled water before being dried within 24 h at 80 C The measurement of weight loss, pH and SEM images of these samples were determined The mass of PLA/md-HAp/PEO nanocomposites with and without porogen were determined by Precisa XR 205 SM-DR analysis balance The pH value of the SBF solution was measured by using pH3110 Meter (a) (b) (c) (d) SEM images of nanocomposites with the different PLA/md-HAp ratios: (a) 80/20, (b) 70/30, (c) 60/40 and (d) 50/50 (wt/wt) J Nanosci Nanotechnol 15, 1–10, 2015 Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) (a) 2000 (b) 55 PLA 50 Tensile strength / MPa E Modulus / MPa 1800 1600 1400 1200 1000 800 600 80/20 70/30 60/40 400 200 Thanh et al 50/50 PLA 45 40 35 30 25 80/20 20 70/30 60/40 15 50/50 10 0 Figure The mechanical properties: (a) Young’s modulus and (b) tensile strength of PLA and PLA/md-HAp/PEO nanocomposites with the different ratios of PLA/md-HAp 2.7 FT-IR FT-IR spectra analysis for PLA, md-HAp and PLA/mdHAp/PEO nanocomposite is used to determine characteristic groups of their molecules The FTIR spectra of the samples were recorded by using Nicolet/Nexus 670 Spectrometer (USA) at room temperature by averaging 16 scans with a resolution of cm−1 in transmission mode by using KBr pellet method The FT-IR spectra were recorded in the wave numbers range from 400 to 4000 cm−1 2.8 Scanning Electron Microscopy (SEM) The surface of PLA/md-HAp/PEO nanocomposites was examined by using Hitachi S-4800 Scanning Electron Microscope (SEM) 2.9 X-ray Diffraction The phase structure of PLA/md-HAp/PEO with and without NH4 HCO3 porogen after immersion days in the SBF solution were analyzed by X-ray Diffraction (XRD) (Siemens D5000 Diffractometer, CuK radiation ( = 54056 Å) with step angle of 0.030 , scanning rate of 0.04285 s−1 , and degree in range of 10–60 2.10 Mechanical Properties The mechanical properties (Young’s modulus and tensile strength) of PLA, PLA/md-HAp/PEO nanocomposites with and without porogen were measured by using a Zwick-Tensile Tester at room temperature with crosshead speed of 100 mm/min, the dumbbell shaped specimens and the measurements were carried out according to ASTM D638 2.11 Hydrophilicity or Hydrophobicity Determination The hydrophilicity or hydrophobicity of PLA and PLA/md-HAp/PEO nanocomposites with and without NH4 HCO3 porogen were evaluated through the measurement of water contact angles Each determination was obtained by averaging the results of five measurements Water contact angle measurements were performed by using a SEO Phoenix 150 Contact Angle Analyzer RESULTS AND DISCUSSION 3.1 Influence of md-HAp Content on the Morphology and Mechanical Properties of PLA/md-HAp Nanocomposites The Figure presented the FT-IR spectra of PLA, md-HAp and PLA/md-HAp/PEO nanocomposite (70/30/5 wt/wt/wt) All characteristic peaks of md-HAp (PO3− , OH− , CO2− O) were appeared in PLA/md3 ) and PLA (C HAp/PEO nanocomposite: (i) characteristic peaks of md-HAp (PO3− ) at 560, 607, 1061 cm−1 moved back to 563, 610, 1095 cm−1 in the nanocomposite The –CH vibration peaks in PLA (1457 cm−1 ) and in the nanocomposite (1465 cm−1 ) also shifted It indicates the molecular interaction between mdHAp and PLA in the nanocomposite (ii) In the nanocomposite, the vibration of the liaison –C O of neat PLA at 1761 cm−1 shifted to 1767 cm−1 This movement may be attributed to the formation of hydrogen bonding between the –OH of md-HAp and –C O of PLA Scanning electron microscopy (SEM) was used to observe the surface morphology of PLA/md-HAp/PEO nanocomposites with using wt% of PEO and the different ratios of PLA/md-HAp: 80/20, 70/30, 60/40 and 50/50 (wt/wt) (Fig 2) The content of HAp plays an important Table I The variation of porosity of PLA/md-HAp/PEO nanocomposites versus NH4 HCO3 porogen content Porogen content (wt%) 10 20 30 Porosity (%) 10 12 18 33 39 49 J Nanosci Nanotechnol 15, 1–10, 2015 Thanh et al Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) role in controlling the morphology of PLA/md-HAp/PEO nanocomposites With 20 wt% and 30 wt% of md-HAp, md-HAp powder was dispersed more regularly in PLA matrix Higher amounts of md-HAp (40 and 50 wt%) might cause the aggregation of md-HAp particles in PLA However, in order to apply in bone implants, the large content of HAp is good for biocompatibility, therefore, 30 wt% of md-HAp has been chosen for following studies The Young’s modulus of PLA/md-HAp/PEO nanocomposites decreased with the increase of md-HAp content (Fig 3) The Young’s modulus was 1806 ± 51 MPa with neat PLA sample; while the Young’s modulus of the nanocomposite dropped to the value of 593 ± 52 MPa with 20 wt% md-HAp added (down more than 67%) When the md-HAp content is 50%, the Young’s modulus of the nanocomposite was only 115 ± 42 MPa (a decrease of over 93%) The tensile strength of the nanocomposites was deduced similarly to the Young’s modulus 3.2 Influence of Porogen Content on the Porosity, Morphology and Mechanical Properties of PLA/md-HAp/PEO Nanocomposites The content of the porogen (NH4 HCO3 ) influenced on the porosity of the PLA/md-HAp/PEO nanocomposites As seen in Table I, the open porosity of the nanocomposites increased with the increase of NH4 HCO3 porogen (a) (b) (c) (d) (e) (f) Figure SEM images of PLA/md-HAp/PEO nanocomposites with NH4 HCO3 different porogen content: (a) wt%, (b) wt%, (c) wt%, (d) 10 wt%, (e) 20 wt% and (f) 30 wt% J Nanosci Nanotechnol 15, 1–10, 2015 Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) (b) 24 (a) 800 PLA/HAp/PEO/NH4HCO3 70/30/5/x wt/wt E Modulus / MPa 0% 500 3% 7% 10 % 20 % 400 300 200 30 % 100 PLA/HAp/PEO/NH4HCO3 70/30/5/x wt/wt 22 Tensile strength / MPa 700 600 Thanh et al 20 0% 3% 18 7% 10 % 16 20 % 14 12 30 % 10 Figure The (a) Young’s modulus and (b) tensile strength of PLA/md-HAp/PEO nanocomposites without and with 3, 7, 10, 20 and 30 wt% NH4 HCO3 porogen content content The open porosity was only 12% when the porogen content was wt%, while it reached 49% at 30 wt% of porogen content Without using NH4 HCO3 in the nanocomposites, the porosity of the nanocomposite was 10% because md-HAp nano powder itself also has the ability to increase the porosity of the nanocomposites.53 During the fabrication of nanocomposite, NH4 HCO3 molecules were uniformly distributed in the samples At 80 C, NH4 HCO3 was degraded to form air pores with small size (Fig 3) When drying at 80 C within 24 h, NH4 HCO3 in the nanocomposite was decomposed to form CO2 and NH3 gas (Fig 3) With high amounts of the porogen (20, 30 wt%), a part of generated gas was compressed inside of the nanocomposite and a rest generated gas was able to release out the surface to form high porosity of the nanocomposite However, the high porosity of the nanocomposite was able to destroy the structure in size and reduced tensile properties of the nanocomposites In the case of low porogen content (3, 7%), the generated gas still exist mainly in the nanocomposite by compressing and only a little generated gas was able to release out The SEM images of the nanocomposites with different contents of NH4 HCO3 porogen were shown in Figure In absence of NH4 HCO3 porogen, the PLA/md-HAp/PEO nanocomposites still have porous structure (Fig 4(a)) The porosity of this nanocomposite was nearly constant at low content of NH4 HCO3 porogen (3 or wt%) but it increased significantly when the NH4 HCO3 porogen content was up to 10, 20, 30 wt% In the nanocomposite, HAp interacts with PLA by hydrogen bonds and NH4 HCO3 porogen with low and high content was dispersed in the nanocomposite When drying the nanocomposite at 80 C within 24 h, NH4 HCO3 was decomposed to form CO2 and NH3 and pore size of the nanocomposite changed from small to high depending on NH4 HCO3 porogen content as above explained The effect of NH4 HCO3 porogen content on mechanical properties of the nanocomposites was also studied As seen in Figure 5, Young’s modulus and tensile strength of the nanocomposite decreased when porogen content increased For the samples without and with low porogen content (3 or wt%), the Young’s modulus and tensile strength changed not much, in agreement with determination results of the porosity of the nanocomposites The Young’s modulus of the nanocomposites decreased from 549 ± 54 MPa (sample without porogen) to 421 ± 49 and 400 ± 50 MPa for the sample having the porogen content of 10 and 20 wt%, respectively Specially, with the nanocomposite using 30 wt% porogen content, the Young’s modulus of the nanocomposites was only 120 ± 39 MPa, which decreased about 78% compared with PLA/md-HAp/PEO nanocomposite without porogen Therefore, component ratio of PLA/md-HAp/PEO = 70/30/5 with 20 wt% NH4 HCO3 porogen content was chosen to test in vitro bioactivity of the nanocomposite in the simulated body fluids (SBF) solution The hydrophilicity or hydrophobicity of PLA and PLA/md-HAp/PEO nanocomposites with and without porogen were evaluated by measuring the water contact angle (Table II) Table II demonstrated the measurement results of water contact angle of surfaces of neat PLA and PLA/mdHAp/PEO nanocomposites with and without 20 wt% NH4 HCO3 porogen The water contact angle of neat PLA is 83.1 ± 2.9 ,3 and its high value shows that PLA is a hydrophobic polymer PLA/md-HAp/PEO (70/30/5) nanocomposite has water contact angle of 63.7 which is lower than that of neat PLA because md-HAp powder is hydrophilic and it also increased the porosity of the nanocomposite.9 In the presence of 20 wt% Table II Water contact angle of PLA, PLA/md-HAp/PEO and PLA/md-HAp/PEO nanocomposites with 20 wt% NH4 HCO3 porogen Samples Water contact angles (degrees) PLA PLA/md-HAp/PEO (70/30/5) PLA/md-HAp/PEO (70/30/5) with 20 wt% NH4 HCO3 83 ± 63 ± 50 ± J Nanosci Nanotechnol 15, 1–10, 2015 Thanh et al Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) (1) (3) Water contact angle images of (1) PLA, PLA/md-HAp/PEO nanocomposites (2) without and (3) with 20 wt% NH4 HCO3 porogen NH4 HCO3 porogen, water contact angle of the nanocomposite decreased to 50.6 compared to nanocomposite without porogen (63.7 ) due to the increase of the porosity of the nanocomposite (Fig 6) This result indicated that the incorporation of HAp and NH4 HCO3 into hydrophobic polymers is a feasible approach to improve the hydrophilicity of the hydrophobic polymer 3.3 In Vitro Bioactivity of PLA/md-HAp/PEO Nanocomposites With and Without 20 wt% NH4 HCO3 Porogen in Simulated Body Fluids (SBF) Solution The in vitro degradation of PLA as well as the formation of HAp on/in PLA/md-HAp/PEO nanocomposites with and without 20 wt% NH4 HCO3 porogen into the SBF solution were evaluated by the variation of the pH of the SBF solution When nanocomposites were immersed into the SBF solution, there are two processes occurring simultaneously: the first process is hydrolysis of PLA expressed by two Eqs (1) and (2) to generate acid lactic, and release H+ ion; the second process is the formation of HAp, which consumes OH− ion Both of processes reduced pH of the SBF solution The formation of HAp can be explained as following: the hydrolysis of PLA released H+ ion, leading to the dissolution of HAp The calcium ions dissolved from the HAp increased the calcium ion concentration in the surrounding SBF, which was already supersaturated with respect to apatite; and the nancomposite surfaces provided favorable sites for apatite nucleation As a result of SEM, a large number of apatite nuclei formed on nanocomposite surfaces, grew spontaneously, and consumed the calcium and phosphate ions from the surrounding fluid.54 Ka RCOOH −→ RCOO− + H+ (1) time, at 37 C The pH value of the solution before soaking nanocomposites is 7.4 During the immersion time, the pH of the SBF solution containing PLA/md-HAp/PEO nanocomposites with and without NH4 HCO3 porogen decreased but the pH of the SBF solution containing PLA/md-HAp/PEO nanocomposite with 20 wt% NH4 HCO3 porogen decreased more strongly (39%) because PLA/md-HAp/PEO nanocomposite with NH4 HCO3 porogen (39%) has higher porosity than PLA/md-HAp/PEO nanocomposite without porogen (10%) Therefore, water molecules easily permeate into PLA/md-HAp/PEO nanocomposite with NH4 HCO3 porogen and the contact surface area of the nanocomposite with the SBF solution become higher The variation of weight of PLA/md-HAp/PEO nanocomposites with and without NH4 HCO3 porogen during immersion time was displayed in Figure The weight of the above nanocomposites decreased strongly after and immersion days It indicated that the decomposition of PLA in the nanocomposites happened strongly than the formation of HAp crystals And then, the weight of the nanocomposites increased continuously with 28 immersion days It is clear that the formation of HAp crystals on/in the nanocomposites increased significantly This can be explained by the formation HAp crystals 7.4 7.2 7.0 pH Figure (2) 6.8 6.6 6.4 6.2 (2) − 10Ca2+ + 6HPO2− + 8OH −→ Ca10 PO4 OH + 6H2 O (3) Figure showed the pH values of the SBF solution when immersing nanocomposites at different immersion J Nanosci Nanotechnol 15, 1–10, 2015 6.0 12 15 18 21 24 27 30 Time (day) Figure The pH variation of SBF solution according to immersion time of PLA/md-HAp/PEO nanocomposites (1) with and (2) without 20 wt% NH4 HCO3 porogen Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) 0.0010 0.0005 ∆m(g) 0.0000 –0.0005 –0.0010 –0.0015 –0.0020 –0.0025 12 15 18 21 24 27 30 Time (day) Figure The variation of weight of PLA/md-HAp/PEO nanocomposites (1) with and (2) without NH4 HCO3 porogen according to immersion time in SBF solution on/in the pore that will prevent hydrolysis process of PLA in the SBF solution Figure displayed images of PLA/md-HAp/PEO nanocomposites with 20% NH4 HCO3 which was immersed in the SBF solution during 0, 1, 3, 7, 14, 21 Thanh et al and 28 days The sample after immersion day appeared HAp nucleation crystals After or immersion days, HAp crystals grew with higher density The surface of the nanocomposites nearly covered fully with HAp crystals after 14, 21 or 28 immersion days in the SBF solution Specially, with the sample immersed during 28 days in the SBF solution, HAp crystals grew up to form a thicker block and it showed the degradation of PLA in the nanocomposite Figure 10 performed the XRD patterns of PLA/mdHAp/PEO nanocomposites before being immersed in the SBF solution; PLA/md-HAp/PEO without and with NH4 HCO3 porogen after immersion days in the SBF solution The XRD pattern of PLA/md-HAp/PEO nanocomposite before being immersed in the SBF solution expressed that PLA in the nanocomposite is a semicrystalline polymer (Fig 10(1)) Besides that, in the XRD patterns, there were two characteristic peaks of HAp at degree = 25,84 and 31,93 The diameter of HAp crystals in PLA/HAp nanocomposite based on the Scherrer equation at 25.84 is 19.87 nm The XRD patterns of PLA/md-HAp/PEO nanocomposites with and without NH4 HCO3 porogen after immersion days (Fig 10(2) and 10(3)) performed the appearance (a) (b) (c) (d) (e) (f) (g) Figure SEM images of PLA/md-HAp/PEO (70/30/5) nanocomposites with 20 wt% NH4 HCO3 porogen at the different immersion times in SBF solution: (a) 0, (b) 1, (c) 3, (d) 7, (e) 14, (f) 21 and (g) 28 immersion days J Nanosci Nanotechnol 15, 1–10, 2015 Thanh et al and the degradation of PLA/md-HAp/PEO nanocomposites with and without NH4 HCO3 porogen in the SBF solution showed the formation of the HAp on the surface of the nanocomposites and the hydrolysis process of PLA after being immersed in the SBF solution These porous nanocomposites are promising potential applications for bone implant 16.46 600 500 Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) 200 32.14 300 25.89 18.99 400 600 500 16.55 400 200 25.89 300 31.97 18.99 Acknowledgments: The authors gratefully acknowledge the Ministry of Science and Technology of Vietnam for financial support through the Bilateral Project Vietnam—Korea number 49/2012/HD-NDT 100 25.89 200 180 160 140 120 100 80 60 40 20 31.93 Intensity (au) 100 References and Notes 10 20 30 40 50 60 2θ (degree) Figure 10 XRD patterns: (1) PLA/md-HAp/PEO nanocomposite before immersing, PLA/md-HAp/PEO (2) without and (3) with NH4 HCO3 porogen after immersion days in SBF solution of characteristic peaks for crystal structure of PLA at degree were about 16,5 and 18,9 55 After immersion days in the SBF solution, PLA amorphous part in the nanocomposites was hydrolysed and PLA crystal part remained And two characteristic peaks of HAp at about degree = 25,89 and 31,94 were also shown in these patterns However the intensity of the characteristic peaks of PLA crystal in PLA/md-HAp/PEO nanocomposite with NH4 HCO3 porogen was higher than that in the nanocomposite without NH4 HCO3 porogen This was able to be explained as following: PLA/md-HAp/PEO nanocomposite with 20 wt% NH4 HCO3 porogen was more porous than PLA/md-HAp/PEO nanocomposite (Fig 4), so amorphous PLA part was hydrolysed more strongly, crystal PLA dominated and the formation of HAp became easily The formation of HAp after being immersed in the SBF was exhibited by the intensity of the characteristic peaks of HAp in the nanocomposite which was arranged as following order: PLA/md-HAp/PEO before being immersed < PLA/md-HAp/PEO without NH4 HCO3 porogen after immersion days < PLA/md-HAp/PEO with 20 wt% NH4 HCO3 porogen after immersion days CONCLUSION PLA/md-HAp/PEO porous nanocomposites using NH4 HCO3 porogen was prepared by the 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15, 1–10, 2015 ... Effects of Porogen on Structure and Properties of PLA/HAp Nanocomposites (PLA/HAp) (1) (3) Water contact angle images of (1) PLA, PLA/md-HAp/PEO nanocomposites (2) without and (3) with 20 wt% NH4 HCO3... processes occurring simultaneously: the first process is hydrolysis of PLA expressed by two Eqs (1) and (2) to generate acid lactic, and release H+ ion; the second process is the formation of... and consumed the calcium and phosphate ions from the surrounding fluid.54 Ka RCOOH −→ RCOO− + H+ (1) time, at 37 C The pH value of the solution before soaking nanocomposites is 7.4 During the immersion