polymers Article
Functionalization of Graphene Oxide with Low Molecular Weight Poly (Lactic Acid) Mingwei Yuan 1,2 , Yike Chen 3 , Minglong Yuan 3 , Hongli Li 3 , Xiansong Xia 1,2 and Chengdong Xiong 1, * 1 2 3
*
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China;
[email protected] (M.Y.);
[email protected] (X.X.) University of Chinese Academy of Sciences, Beijing 100049, China Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University, Kunming 650500, China;
[email protected] (Y.C.);
[email protected] (M.Y.);
[email protected] (H.L.) Correspondence:
[email protected]; Tel.: +86-28-8521-4764
Received: 27 December 2017; Accepted: 4 February 2018; Published: 12 February 2018
Abstract: In this paper, the hydroxyl groups on the surface of graphene oxide (GO) were used to initiate the ring-opening polymerization of a lactic acid O-carboxyanhydride. GO grafted with poly (L-lactic acid) molecular chains (GO-g-PLLA) was prepared. Lactic acid O-carboxyanhydride has a higher polymerization activity under mild polymerization conditions. Thus, the functionalization of the polymer chains and obtaining poly (lactic acid) (PLLA) was easily achieved by ring-opening polymerization with 4-dimethylaminopyridine (DMAP) as the catalyst. The results showed that with this method, PLLA can be rapidly grafted to the surface of GO in one step. As a result, the chemical structure of the GO surface was altered, improving its dispersion in organic solvents and in a PLLA matrix, as well as its bonding strength with the PLLA interface. We then prepared GO/PLLA and PLLA/GO-g-PLLA composite materials and investigated the differences in their interfacial properties and mechanical properties. GO-g-PLLA exhibited excellent dispersion in the PLLA matrix and formed excellent interfacial bonds with PLLA through mechanical interlocking, demonstrating a significant enhancement effect compared to PLLA. The water vapor and oxygen permeabilities of the GO-g-PLLA/PLLA composite decreased by 19% and 29%, respectively. Keywords: GO-g-PLLA; composite materials; water vapor; oxygen permeabilities
1. Introduction Poly (lactic acid) (PLLA) is receiving considerable attention for conventional uses, such as a packaging material, for the production of agricultural film, and more recently, as composites for technical applications [1]. However, its wider application has been limited by its relatively slow crystallization rate, poor gas barrier performance, and poor flexibility [1]. To overcome these challenges, various methods (such as Copolymerization and blending) have been used to enhance the comprehensive performance of poly L-lactide (PLLA). The degree of crystallinity and mechanical properties can be changed by blending. Copolymerization can change the performance of many aspects, mainly based on the properties of the graft molecules. For example, a rigid molecule can improve the strength of the copolymer, while the flexible molecule can enhance the tensile properties of the copolymer [1,2]. Graphene and graphene oxide (GO) have been used in PLLA nanocomposites, and graphite or GO can increase the crystallinity and the heat resistant temperature of the nanocomposite [3–7]. In a study of graphene and GO-modified polylactic acid, the main method used was to directly blend graphene with polylactic acid. Feng et al. developed a versatile method by grafting polymers on GO to enhance the properties of nanocomposites [8,9]. Chaobin et al. reported Polymers 2018, 10, 177; doi:10.3390/polym10020177
www.mdpi.com/journal/polymers
Polymers 2018, 10, 177
2 of 16
a new type of PLLA-GO nanocomposite and showed that a stereocomplex crystal could be formed between PLLA and GO-g-PDLA. The incorporation of GO nanofillers lead to a lower crystallization activation energy of the stereocomplex and a higher crystallinity in solution casting samples [10]. The PLLA-GO nanocomposites were prepared by blending commercial PLLA with GO-g-PDLA, in which GO-g-PDLA was synthesized via ring-opening polymerization using modified GO as the initiator. According to the above literature, the crystallinity and heat resistance of PLLA can be significantly increased by modifying PLLA with graphene and GO. Improving the gas barrier performance and flexibility of PLLA for the purpose of packaging and other uses is also necessary. Increasing the compatibility of nano polylactic acid composites and the adhesion force of the interface will improve their gas barrier property and their flexibility. Based on the “like dissolves like” principle, because inorganic nanoparticles are structurally different from polymers, modifying the structure of inorganic nanoparticles to improve their compatibility with polymers is essential. Albertsson et al. [11–14] used PLLA stereocomplex (SC) particles with a diameter of 300–500 nm to modify PLLA, and this initiated the interfacial crystallization of PLLA, resulting in a clear improvement in compatibility. In particular, the modification of PLLA with GO-PLLA SC composite particles significantly improved the thermal resistance and barrier properties of the material [12–14]. Based on the above analysis and the “like dissolves like” principle, this study aimed to prepare high barrier nanomaterial-PLLA membrane materials by modifying PLLA with structurally similar nanomaterials that were functionalized with low molecular weight PLLA. This study provides a new thought process and method to solve the key problem of nanomaterial-PLLA composite materials, and to lay a theoretical foundation for next-generation green food packaging materials. 2. Experimental Methods 2.1. Materials 2.1.1. Preparation of Raw Materials Lactic acid O-carboxyanhydride (LacOCA) was prepared according to the literature [15–18]. The L-lactic aqueous solution was provided by Henan Jindan. Lactic Acid, LLC, lithium hydroxide (LiOH), ethanol, tert-butyl methyl ether, triphosgene, and tetrahydrofuran were obtained from the Chengdukelong chemical reagent PLLAnt, all with a purity of approximately 99 wt %. GO was provided by Suzhou Carbon Abundance Graphene Science and Technology Co., Ltd., Suzhou, China, with a purity of ~99 wt %, a thickness of 0.6–1.0 nm, a sheet diameter of 0.5–5 µm, with 1–2 layers, and a specific surface area of 1000–1217 m2 /g. The PLLA was thermal film-grade PLLA from NatureWorks, LLC. 2.1.2. Preparation of GO-g-PLLA Tetrahydrofuran (THF) (150 mL) and 0.067 g of GO (lactic acid O-carboxyanhydride of 0.5/100) were added to a 250-mL single-neck flask and sonicated for 1 h to evenly disperse the GO in the THF. LacOCA (14 g) was added to the above solution at room temperature and stirred until dissolved. After the solution became clear, 0.046 g of 4-dimethylaminopyridine (DMAP) (1/300 of the amount of LacOCA) was added and stirred at room temperature overnight until no bubbles were present. Once the reaction was complete, the solution was concentrated and dried at 30 ◦ C. Then, chloroform was used to dissolve the polymerization products. At room temperature for 12 h, the insoluble substances were removed by centrifugation. The filtrate of homogeneous phase was precipitated by excessive ethanol to achieve solidity. The solids were filtered and dried under vacuum at 60 ◦ C for 8 h. The product gained was GO-g-PLLA (Scheme 1). As a contrast, PLA and GO are also dissolved in chloroform, and at room temperature for 12 h, the insoluble substances were removed by centrifugation. The filtrate of homogeneous phase was precipitated by excessive ethanol to achieve solidity. The solids were filtered and dried under vacuum at 60 ◦ C for 8 h. The product gained was not GO/PLLA
Polymers 2018, 10, 177 Polymers 2018, 10, x FOR PEER REVIEW
3 of 16 3 of 16
not GO/PLLA blend compound, it was after PLA.centrifugation Filtration afterwas centrifugation was dried using freezeblend compound, it was PLA. Filtration dried using freeze-drying method to drying method to obtain GO. obtain GO.
Scheme 1. Synthetic of graphene graphene oxide oxide (GO) (GO) grafted grafted with with poly poly LL-lactic -lactic acid acid molecular molecular chains chains Scheme 1. Synthetic route route of (GO-g-PLLA). (GO-g-PLLA).
2.1.3. 2.1.3. Preparation Preparation of of GO-g-PLLA/PLLA GO-g-PLLA/PLLAComposite Composite Material Material A of GO-g-PLLA (4, 8, or g) was mixed g of PLLA. (2000 A specific specificamount amount of GO-g-PLLA (4,128, or 12 g) with was 800 mixed with Chloroform 800 g of PLLA. mL) was then added and mechanically stirred to dissolve the mixture. After the solution became Chloroform (2000 mL) was then added and mechanically stirred to dissolve the mixture. After the clear, thebecame solutionclear, was the sonicated forwas 2 h.sonicated Then, 5000 ethanol wasmL added to the was solution to solution solution formL 2 h.of Then, 5000 of ethanol added precipitate a gray-white solid athat was vacuum-dried at 60vacuum-dried °C for 8 h to at obtain GO-g-PLLA/PLLA to the solution to precipitate gray-white solid that was 60 ◦ C for 8 h to obtain composites containing 0.5%, 1%, and 1.5% GO-g-PLLA. GO-g-PLLA/PLLA composites containing 0.5%, 1%, and 1.5% GO-g-PLLA. 2.1.4. 2.1.4. Instruments Instruments and and Characterization Characterization Nuclear Nuclear magnetic magnetic resonance resonance (NMR), (NMR), infrared infrared (IR), (IR), TGA, TGA, and and X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) were wereconducted conductedtotodetermine determine changes in the chemical characteristics ofand GOGO-g-PLLA. and GO-g(XPS) thethe changes in the chemical characteristics of GO ◦ 1 H NMR PLLA. NMR spectra were recorded with Bruker 400 MHz spectrometers at 25 °C. Chemical for NMR spectra were recorded with Bruker 400 MHz spectrometers at 25 C. Chemical shifts for shifts 1H NMR spectra were referenced internally using the residual solvent resonances and spectra were referenced internally using the residual solvent resonances and tetramethylsilane (TMS) tetramethylsilane (TMS) The as ansolvent internal reference. solvent waschloroform, used in the and deuterium chloroform, as an internal reference. was used in The the deuterium with a solubility of and with a solubility of 10 mg/ml. the dissolution was not precipitation. it was limpid. The molecular 10 mg/mL. the dissolution was not precipitation. it was limpid. The molecular weight of free PLLA weight of free PLLA determinedchromatography using gel permeation (GPC) 40 °C with usinga was determined usingwas gel permeation (GPC) chromatography at 40 ◦ C using THF as theateluent THF as the eluent with a flow rate of 1 mL/min. The system equipped with a Waters 515 pump and flow rate of 1 mL/min. The system equipped with a Waters 515 pump and a Waters 2414 refractive − 1 aindex Waters 2414 refractive index detector. IR (Bruker Tensor 37) spectra were recorded from 600 to 4000 detector. IR (Bruker Tensor 37) spectra were recorded from 600 to 4000 cm with a resolution −1 aand cm with resolution of 2TGA cm−1curves and 32were scans. TGA curves obtained from a Mettler SDTA851e. of 2−1 cm 32 scans. obtained fromwere a Mettler SDTA851e. The samples were ◦ ◦ The samples were heated from 25 to 800 °C at a rate of 10 °C/min in an aluminum crucible under 50 heated from 25 to 800 C at a rate of 10 C/min in an aluminum crucible under 50 mL/min of nitrogen mL/min of nitrogen purging.were XPSperformed measurements performed using a PHI5000 Versaprobe-II purging. XPS measurements usingwere a PHI5000 Versaprobe-II System Auger electron System Auger electron spectrometer (Physical Company, U.S.) equipped with a spectrometer (Physical Electronics Company, U.S.) Electronics equipped with a hemispherical electron analyzer hemispherical electron analyzer and a scanning monochromatic AlKa (hm = 1486.6 eV) X-ray source. and a scanning monochromatic AlKa (hm = 1486.6 eV) X-ray source. The particle size distributions The particle size distributions GO and GO-g-PLLA the chloroform using S3500SI of GO and GO-g-PLLA in theofchloroform were testedinusing a S3500SI were laser tested particle sizeaanalyzer laser particle size analyzer system (Micortrac Co. Ltd., Norristown, PA, USA) with an equivalent system (Micortrac Co. Ltd., Norristown, PA, USA) with an equivalent sphere model and a measuring sphere a measuring rangecharacterize of 0.01 to 2800 µm. To further characterize the dispersions of range ofmodel 0.01 toand 2800 µm. To further the dispersions of GO and GO-g-PLLA before and GO GO-g-PLLA beforewe and after grafting in solvents, we performed dynamic scattering to afterand grafting in solvents, performed dynamic light scattering to measure thelight particle size and measure the particle size and distribution. The solutions had a concentration of 1 mg/mL. Each distribution. The solutions had a concentration of 1 mg/mL. Each solution was sonicated for 2 h after solution was h after dissolving the particles and immediately tested. dissolving thesonicated particles for and2 immediately tested. The differential The crystallization crystallization behaviors behaviors of of PLLA PLLA and and nanocomposites nanocomposites were were measured measured by by aa differential scanning calorimeter (DSC) (214, Netzsch, Selb, Germany) in a nitrogen flow (50 mL/min) with a scanning calorimeter (DSC) (214, Netzsch, Selb, Germany) in a nitrogen flow (50 mL/min) with ◦ ◦ heating over a heatingrate rateofof1010°C/min C/min overa atemperature temperaturerange rangeofof2020toto200 200°C. C. Flexural Flexural tests tests of of the the PLLA PLLA composites were determined by a CMT4104 universal tester (Meites Industrial Systems (Wuhu, composites were determined by a CMT4104 universal tester (Meites Industrial Systems Co., Ltd. China) China)). Co., Ltd.). length, width, and thickness of the were sample were andrespectively, 1.10 mm, (Wuhu, TheThe length, width, and thickness of the sample 50, 15 and50, 1.1015mm, respectively, to GB/T 1040.3-2006 at aspeed crosshead of 50 mm/min. Five according to according GB/T 1040.3-2006 standard, atstandard, a crosshead of 50speed mm/min. Five different different measurements were recorded for each sample. The Water Vapor Permeability (WVP) of the measurements were recorded for each sample. The Water Vapor Permeability (WVP) of the films was films was gravimetrically determined according to the ASTM E96-95 standard. The films had a gravimetrically determined according to the ASTM E96-95 standard. The films had a thickness of thickness of 0.04 ± 0.005 mm and were obtained by casting. The screw insertion temperatures were 160, 175, 175, and 170 °C. The PO2 was tested on an Oxy Sense5250I oxygen analyzer.
Polymers 2018, 10, 177
4 of 16
0.04 ± 0.005 mm and were obtained by casting. The screw insertion temperatures were 160, 175, 175, and 170 ◦ C. The PO2 was tested on an Oxy Sense5250I oxygen analyzer. Polymersand 2018,Discussion 10, x FOR PEER REVIEW 3. Results
4 of 16
3. Results and Discussion 3.1. Characterization of GO-g-PLLA
Chaobin and Inoue reported the synthesis of GO-g-PLLA through the ring-opening 3.1. Characterization of GO-g-PLLA polymerization of lactide monomers, initiated by the grafted OH groups on GO and under the Chaobin and Inoue reported the synthesis of GO-g-PLLA through the ring-opening catalysis of Sn(Oct)2 . The synthesis temperature was 120 ◦ C and a metal catalyst was used [10,15]. polymerization of lactide monomers, initiated by the grafted OH groups on GO and under the Considering biomedical food potential of PLLAs, focused onwas metal-free synthetic 2. Theand catalysisthe of Sn(Oct) synthesis temperature was 120studies °C and have a metal catalyst used [10,15]. methods under mild conditions. Bourissou et al. reported the metal-free synthesis of PLLA Considering the biomedical and food potential of PLLAs, studies have focused on metal-free by ring-opening of LacOCA. LacOCA exhibited remarkable reactivity compared to lactide. synthetic polymerization methods under mild conditions. Bourissou et al. reported the metal-free synthesis of PLLA of LacOCA. LacOCApolydispersities exhibited remarkable PLLAbyofring-opening controlled polymerization molecular weights and narrow are reactivity typically compared obtained tounder PLLA of controlled molecular weights narrow[16–19]. polydispersities are studies typicallydemonstrated obtained mild lactide. conditions using DMAP and various proticand initiators The above under mild conditions using DMAP and various protic initiators [16–19]. The above studiesin the the ring-open polymerization of lactide or LacOCA. When there are hydroxyl compounds demonstrated the ring-open polymerization of lactide or LacOCA. When there are hydroxyl polymerization system, after polymerization, these hydroxyl compounds are linked to PLLA chains compounds in the polymerization system, after polymerization, these hydroxyl compounds are through chemical bonds to achieve the modification of PLLA [10,17–19]. In this study, we use metal-free linked to PLLA chains through chemical bonds to achieve the modification of PLLA [10,17–19]. In synthetic methods to synthesize GO-g-PLLA by ring-opening polymerization of LacOCA, as shown in this study, we use metal-free synthetic methods to synthesize GO-g-PLLA by ring-opening Figure 1. polymerization of LacOCA, as shown in Figure 1.
Figure 1. The 400 MHz 1H nuclear magnetic resonance (NMR) spectrum of the GO-g-PLLA.
Figure 1. The 400 MHz 1 H nuclear magnetic resonance (NMR) spectrum of the GO-g-PLLA.
In our experiment with the preparation of GO-g-PLLA, the polymerization was completed. Then
In our experiment with the preparation of GO-g-PLLA, the12polymerization completed. chloroform was used to dissolve the polymerization products. After h of placement, wewas found that chloroform solution homogeneous. No insoluble substance found byofcentrifugation. Thefound Then the chloroform was used was to dissolve the polymerization products.was After 12 h placement, we filtrate of homogeneous phase was precipitated by excessive ethanol to achieve solidity. The solids that the chloroform solution was homogeneous. No insoluble substance was found by centrifugation. were filtered and dried phase under was vacuum to obtainby GO-g-PLLA. In the comparison experiment The filtrate of homogeneous precipitated excessive ethanol to achieve solidity. Theofsolids preparing GO/PLLA blend compound, we found that the chloroform dissolved in GO/PLLA after 12 were filtered and dried under vacuum to obtain GO-g-PLLA. In the comparison experiment of h statics was divided into two phases. Because GO is insoluble in chloroform, it will be precipitated from the solution. After centrifuge separation, the insoluble substance obtained is pure GO. The
Polymers 2018, 10, 177
5 of 16
preparing GO/PLLA we found that the chloroform dissolved in GO/PLLA after 125 of h 16 Polymers 2018, 10, x FORblend PEER compound, REVIEW statics was divided into two phases. Because GO is insoluble in chloroform, it will be precipitated from dissolved in chloroform were found to be pureobtained PLA after the same process. The two thecompounds solution. After centrifuge separation, the insoluble substance is pure GO. The compounds experiments also showed that free GO could separated from GO-g-PLLA or PLLA by chloroform dissolved in chloroform were found to be purebe PLA after the same process. The two experiments dissolution andfree centrifugation. GO/PLLAfrom blend compoundorisPLLA to beby prepared, the dissolution GO and PLA also showed that GO could beIfseparated GO-g-PLLA chloroform cannot be centrifuged after mixing in chloroform; can be obtained direct and centrifugation. If GO/PLLA blend compoundGO/PLLA is to be prepared, the GOby and PLAfreeze-drying. cannot be To confirm thatinthe GO-g-PLLA has acan PLA structure performed magnetic centrifuged after mixing chloroform; GO/PLLA be obtained by unit, direct we freeze-drying. characterization GO-g-PLLA From Figure 1, the characteristic peaks of PLLA To confirm thaton thethe GO-g-PLLA has apolymers. PLA structure unit, we performed magnetic characterization at 1.5 and 5.1 ppm,From which agree1,with data reported peaks in the of literature [20–25] and belong onappeared the GO-g-PLLA polymers. Figure the characteristic PLLA appeared at 1.5 and to characteristic proton for the methine (a) and methyl (b)and groups on to thethe PLLA chain. From 5.1the ppm, which agree withpeaks data reported in the literature [20–25] belong characteristic the NMR of the GO-g-PLLA, 4.35 corresponds to the methine −CH− located at proton peaksspectrum for the methine (a) and methyl (b)ppm groups on the PLLA chain. Fromgroup the NMR spectrum of the PLLA which is adjacent the terminal groups of PLLA. finding of the end GO-g-PLLA, 4.35chain, ppm corresponds to the to methine group hydroxyl –CH– located at the end of This the PLLA is consistent with thattoofthe Sun and Hehydroxyl [10], in which was prepared by the ring-opening chain, which is adjacent terminal groupsGO-g-PLLA of PLLA. This finding is consistent with that of polymerization Sun and He [10],ofinlactide. which GO-g-PLLA was prepared by the ring-opening polymerization of lactide. confirm that GO-g-PLLA a GO structure unit, performed XRD GO-g-PLLA ToTo confirm that thethe GO-g-PLLA hashas a GO structure unit, wewe performed XRD onon thethe GO-g-PLLA polymers. Figure 2 shows the XRD patterns of pure GO, PLLA, and GO-g-PLLA composite. seen, polymers. Figure 2 shows the XRD patterns of pure GO, PLLA, and GO-g-PLLA composite. AsAs seen, ◦ ◦ most intense diffraction peak of PLLA = 16.6°, the diffraction 2θ =. 10.5°. thethe most intense diffraction peak of PLLA at 2θat= 2θ 16.6 , the diffraction peak peak of GOofatGO 2θ =at10.5 For theFor the GO-g-PLLA composite, the location and shape of diffraction the diffraction peaks of the three samples were GO-g-PLLA composite, the location and shape of the peaks of the three samples were similar to those of pure PLLA and GO. As the sample was dissolved by chloroform and separated similar to those of pure PLLA and GO. As the sample was dissolved and separatedby show that that the thePLLA PLLAmolecular molecularchains chainswere weresuccessfully successfully grafted bycentrifuge, centrifuge, the the results thus show grafted to to thethe surface of GO. surface of GO.
Figure 2. The XRD patterns of pure GO , PLLA, GO-g-PLLA. Figure 2. The XRD patterns of pure GO, PLLA, andand GO-g-PLLA.
In order to further confirm that the GO-g-PLLA has a GO structure unit, we performed UV/Vis In order to further confirm that the GO-g-PLLA has a GO structure unit, we performed UV/Vis characterization on the GO-g-PLLA polymers. Figure 3 shows the UV/Vis spectra of PLLA, GO, GOcharacterization on the GO-g-PLLA polymers. Figure 3 shows the UV/Vis spectra of PLLA, GO, g-PLLA, and GO/PLLA. The UV/Vis spectrum of PLLA, GO-g-PLLA, and GO/PLLA was observed in GO-g-PLLA, and GO/PLLA. The UV/Vis spectrum of PLLA, GO-g-PLLA, and GO/PLLA was the DCM solution, while those of PLLA, GO-g-PLLA, and GO/PLLA were in the solution. GO-gobserved in the DCM solution, while those of PLLA, GO-g-PLLA, and GO/PLLA were in the PLLA shows very broad absorption with continuously decreasing intensity ranged from 220 to 330 solution. GO-g-PLLA shows very broad absorption with continuously decreasing intensity ranged nm. On the other hand, PLLA and GO-g-PLLA shows the absorption in the range from 250 to 330 nm, from 220 to 330 nm. On the other hand, PLLA and GO-g-PLLA shows the absorption in the range and no absorption peak is observed in the 330 to 800 nm range. Further, PLLA, GO-g-PLLA, and GO/PLLA shows characteristic peaks in the wave length region shorter than 250 nm, while no evident absorption in the higher wave length region. In the absorption spectrum in the range from 220 to 320 nm, the GO-g-PLLA shows absorption with special features characteristics for both PLLA and GO, indirectly indicating that the PLLA chain was grafted onto the surface of GO [15].
Polymers 2018, 10, 177
6 of 16
from 250 to 330 nm, and no absorption peak is observed in the 330 to 800 nm range. Further, PLLA, GO-g-PLLA, and GO/PLLA shows characteristic peaks in the wave length region shorter than 250 nm, while no evident absorption in the higher wave length region. In the absorption spectrum in the range from 220 to 320 nm, the GO-g-PLLA shows absorption with special features characteristics for both PLLA and2018, GO,10,indirectly indicating Polymers x FOR PEER REVIEW that the PLLA chain was grafted onto the surface of GO [15]. 6 of 16
Figure 3. UV/Vis spectra GO , PLLA, GO-g-PLLA, and GO/PLLA. Figure 3. UV/Vis spectra of of GO, PLLA, GO-g-PLLA, and GO/PLLA.
3.2. Determination of Molecular Weight by Gel Permeation Chromatography (GPC) 3.2. Determination of Molecular Weight by Gel Permeation Chromatography (GPC) We estimated the grafting by testing the molecular weight of the free PLLA generated in the We estimated the grafting by testing the molecular weight of the free PLLA generated in the reaction of the molecular length of PLLA to the graphene surface [26,27]. The GO-g-PLLA polymer reaction of the molecular length of PLLA to the graphene surface [26,27]. The GO-g-PLLA polymer had sharp, unimodal distributions, indicating that GO had completely copolymerized with lactide had sharp, unimodal distributions, indicating that GO had completely copolymerized with lactide and and that no lactide homopolymerization had occurred. The average molecular weight was 15,000 that no lactide homopolymerization had occurred. The average molecular weight was 15,000 g/mol, g/mol, and the polydispersity coefficient was 1.09. and the polydispersity coefficient was 1.09. Infrared (IR) Spectroscopy 3.3.3.3. Infrared (IR) Spectroscopy Figure 4 shows Fourier transform (FTIR) spectra polymers, which typical Figure 4 shows thethe Fourier transform IRIR (FTIR) spectra of of thethe polymers, in in which thethe typical polyester absorption peaks appeared. In addition, the PLLA, GO, and GO-g-PLLA IR measurements polyester absorption peaks appeared. In addition, the PLLA, GO, and GO-g-PLLA IR measurements detected changes chemical functional groups upon grafting, shown Figure The peaks detected changes in in thethe chemical functional groups upon grafting, as as shown in in Figure 4. 4. The peaks −1 belong to the O–H stretching peaks in PLLA, GO, and GO-g-PLLA, − 1 at 3447, 3440, and 3443 cm at 3447, 3440, and 3443 cm belong to the O–H stretching peaks in PLLA, GO, and GO-g-PLLA, −1 −1 respectively. The peak at 1758 corresponds the stretching vibration peak C=O ester respectively. The peak at 1758 cmcm corresponds toto the stretching vibration peak of of C=O in in thethe ester bond PLLA chain (Figure 4a,d) [28,29]. significantly stronger C=O stretching vibration peak bond in in thethe PLLA chain (Figure 4a,d) [28,29]. AA significantly stronger C=O stretching vibration peak −1 for GO-g-PLLA, which was likely caused by the grafting of the PLLA molecular − 1 appeared at 1758 cm appeared at 1758 cm for GO-g-PLLA, which was likely caused by the grafting of the PLLA molecular chains ontothe thesurface surfaceofofGO. GO. Therefore, Therefore, the of of GO initiated thethe ringchains onto the hydroxyl hydroxylgroups groupson onthe thesurface surface GO initiated opening of the lactic acid O-carboxyanhydride and the esterification with carboxyl groups [30]. The ring-opening of the lactic acid O-carboxyanhydride and the esterification with carboxyl groups [30]. GO/PLLA blends were also tested by infrared contrast, and the GO/PLLA curves of the blends were The GO/PLLA blends were also tested by infrared contrast, and the GO/PLLA curves of the blends found to to be be different from (Figure 4d). 4d). Figure Figure4d 4disisconsistent consistentwith with were found different fromthose thoseofofthe the copolymer copolymer (Figure thethe findings of Sun and He [10], in which GO-g-PLLA was prepared by the ring-opening polymerization findings of Sun and He [10], in which GO-g-PLLA was prepared by the ring-opening polymerization of lactide. Furthermore, the characteristic peaks of PLLA, including the stretching vibration of C– CH3, the bending vibration of –CH3, and the asymmetric bending vibration of –CH3, appeared at 1093, 1185, and 1457 cm−l, respectively [15,28,31–33], in the spectrum of GO-g-PLLA. The results thus show that the PLLA molecular chains were successfully grafted onto the surface of GO.
Polymers 2018, 10, 177
7 of 16
of lactide. Furthermore, the characteristic peaks of PLLA, including the stretching vibration of C–CH3 , the bending vibration of –CH3 , and the asymmetric bending vibration of –CH3 , appeared at 1093, 1185, and 1457 cm−l , respectively [15,28,31–33], in the spectrum of GO-g-PLLA. The results thus show that the PLLA molecular chains were successfully grafted onto the surface of GO. Polymers 2018, 10, x FOR PEER REVIEW 7 of 16
Figure transform IR IR (FTIR) (FTIR) spectra spectraof of(a) (a)PLLA, PLLA,(b) (b)GO, GO,(c) (c)GO/PLLA GO/PLLA, ,and (d) GO-g-PLLA. GO-g-PLLA Figure 4. 4. Fourier Fourier transform and (d)
3.4. Thermogravimetric Analysis (TGA) 3.4. Thermogravimetric Analysis (TGA) From Figure 5, the weight loss of GO appeared in the temperature range of 50 ◦to 200 °C. The From Figure 5, the weight loss of GO appeared in the temperature range of 50 to 200 C. The weight weight loss at◦50–100 °C was due to the evaporation of physical water in GO, and the weight loss◦at loss at 50–100 C was due to the evaporation of physical water in GO, and the weight loss at 100–200 C 100–200 °C was caused by the decomposition of hydroxyl and carboxyl groups on the surface of GO was caused by the decomposition of hydroxyl and carboxyl groups on the surface of GO to carbon to carbon monoxide (CO), carbon dioxide (CO2), and water vapor [34–36]. The weight loss of GO-gmonoxide (CO), carbon dioxide (CO2 ), and water vapor [34–36]. The weight loss of GO-g-PLLA PLLA appeared at 220–380 °C, which can be divided into two regions. The first region that occurred appeared at 220–380 ◦ C, which can be divided into two regions. The first region that occurred at at 220–280 °C was attributed to the degradation of the residual oxygen-containing functional groups 220–280 ◦ C was attributed to the degradation of the residual oxygen-containing functional groups on GO; the other region at 280–380 ◦°C was caused by the degradation of PLLA grafted to the surface on GO; the other region at 280–380 C was caused by the degradation of PLLA grafted to the surface of GO [26,34,35]. The PLLA weight loss mainly appeared at 340–380 °C. Based on the literature and of GO [26,34,35]. The PLLA weight loss mainly appeared at 340–380 ◦ C. Based on the literature and according to the TGA curves, the grafting ratio of PLLA to the surface of GO was approximately 60.2 according to the TGA curves, the grafting ratio of PLLA to the surface of GO was approximately wt % [37] for the in-situ ring-opening polymerization of the lactic acid O-carboxyanhydride. The 60.2 wt % [37] for the in-situ ring-opening polymerization of the lactic acid O-carboxyanhydride. weight loss of GO-g-PLLA was between those of GO and PLLA. We also performed a TG analysis on The weight loss of GO-g-PLLA was between those of GO and PLLA. We also performed a TG analysis GO/PLLA blends, and the results showed that a significant difference existed compared to the on GO/PLLA blends, and the results showed that a significant difference existed compared to the copolymer of GO-g-PLLA. The results thus show that the PLLA molecular chains were successfully copolymer of GO-g-PLLA. The results thus show that the PLLA molecular chains were successfully grafted to the surface of GO. grafted to the surface of GO. 3.5. (XPS) 3.5. X-ray X-ray Photoelectron Photoelectron Spectroscopy Spectroscopy (XPS) To further characterize characterize the the changes changes in in the the chemical groups on on GO, To further chemical functional functional groups GO, we we performed performed XPS XPS before and after grafting. The C 1s peaks before and after grafting are shown in Figure 6. The areas before and after grafting. The C 1s peaks before and after grafting are shown in Figure 6. The areas of C=C/C–C peak (283.38 eV)eV) waswas 26.1143%, the of the the peaks peaks for forGO GObefore beforegrafting graftingwere wereasasfollows: follows:the the C=C/C–C peak (283.38 26.1143%, C–OH peakpeak (284.21 eV) was the C–O eV) was 37.8132%, the C=O peak (286.47 the C–OH (284.21 eV) 16.7404%, was 16.7404%, the peak C–O (285.75 peak (285.75 eV) was 37.8132%, the C=O peak eV) was 13.7880%, and the O–C=O peak (287.41 eV) was 5.5441% [38,39]. The C 1s peaks after grafting (286.47 eV) was 13.7880%, and the O–C=O peak (287.41 eV) was 5.5441% [38,39]. The C 1s peaks are shown in Figure 4b. The peak area of –OH for GO changed to 5.5441%, exhibiting a substantial decrease. The peak areas of C–H and O–C=O substantially increased to 17.2537% and 16.8664%, respectively [36]. The results show that, as the initiator, the hydroxyl groups (–OH) on the surface of GO initiated the ring-opening polymerization of LacOCA with the assistance of a catalyst, thus grafting the polymer to the surface of GO [40]. As the reaction progressed, the surface of GO was
Polymers 2018, 10, 177
8 of 16
after grafting are shown in Figure 4b. The peak area of –OH for GO changed to 5.5441%, exhibiting a substantial decrease. The peak areas of C–H and O–C=O substantially increased to 17.2537% and 16.8664%, respectively [36]. The results show that, as the initiator, the hydroxyl groups (–OH) on the surface initiated the ring-opening polymerization of LacOCA with the assistance of a catalyst,8 of 16 Polymers 2018,of 10,GO x FOR PEER REVIEW thus grafting the polymer to the surface of GO [40]. As the reaction progressed, the surface of GO was almost ratio completely covered by a2.16 layerfor of PLLA The testThis results show thatPLLA the carbon to oxygen decreased from GO tomolecular 1.67 for chains. GO-g-PLLA. is because contains Polymers 2018, 10, x FOR PEER REVIEW 8 of 16 to oxygen ratio decreased from 2.16 for GO to 1.67 for GO-g-PLLA. This is because PLLA contains more oxygen that GO, which agrees with the reports in the literature [37]. more oxygen that GO, which agrees with the reports in the literature [37]. to oxygen ratio decreased from 2.16 for GO to 1.67 for GO-g-PLLA. This is because PLLA contains more oxygen that GO, which agrees with the reports in the literature [37].
Figure 5.5.TGA curves ofof(a) (a) GO, (b) PLLA, (c) GO-g-PLLA, and (d) GO/PLLA. Figure 5.TGA TGA curvesof (a)GO, GO,(b) (b)PLLA, PLLA, GO-g-PLLA, and (d)GO/PLLA. GO/PLLA. Figure curves (c)(c) GO-g-PLLA, and (d)
Figure 6. 6. X-ray photoelectron spectroscopy Figure X-ray photoelectron spectroscopy(XPS) (XPS)curves curvesofof(a)(a)GO GOand and(b) (b)GO-g-PLLA. GO-g-PLLA.
Figure 6. X-ray photoelectron spectroscopy (XPS) curves of (a) GO and (b) GO-g-PLLA. 3.6. Characterization of Physical Properties
3.6. Characterization of Physical the Properties To further characterize dispersions of GO and GO-g-PLLA before and after grafting in solvents, we used dynamic light scattering to measure the particle size and distribution. The solutions To further characterize the dispersions of GO and GO-g-PLLA before and after grafting in had a concentration of 1 mg/mL. Each solution was sonicated for 2 h after dissolving the particles and solvents, we used dynamic light scattering to measure the particle size and distribution. The solutions immediately tested. The results (Figure 7) show that the particle diameters of GO and were 108.5 µm hadinaHconcentration of 1 mg/mL. Each solution was sonicated for 2 h after dissolving the particles and 2O and 163.8 µm in chloroform. Thus, the dispersion of GO in chloroform was significantly better immediately tested. results (Figure 7) show that particle diameters offew GOlayers and were 108.5 µm than in water. The The results indicate that GO existed as the a stable single layer or as in water in H 2O and 163.8of µmthe in chloroform. Thus, the dispersion of GOthe in chloroform was significantly [41]. Because strong van der Waals’ force between GO nanosheets, GO tends to better agglomerate. Therefore, particle size size distribution of GO layer in solvents can layers reflect in its water than in water. The results the indicate that GOand existed as a stable single or as few dispersion solvents [42]. Thevan particle of GO-g-PLLA dissolvedthe in water 81.8 µm, exhibiting [41]. Becausein of the strong dersize Waals’ force between GO was nanosheets, GO tends to a very wide particle size dispersion. The results show that after the ring-opening polymerization of agglomerate. Therefore, the particle size and size distribution of GO in solvents can reflect its
Polymers 2018, 10, 177
9 of 16
3.6. Characterization of Physical Properties To further characterize the dispersions of GO and GO-g-PLLA before and after grafting in solvents, we used dynamic light scattering to measure the particle size and distribution. The solutions had a concentration of 1 mg/mL. Each solution was sonicated for 2 h after dissolving the particles and immediately tested. The results (Figure 7) show that the particle diameters of GO and were 108.5 µm in H2 O and 163.8 µm in chloroform. Thus, the dispersion of GO in chloroform was significantly better than in water. The results indicate that GO existed as a stable single layer or as few layers in water [41]. Because of the strong van der Waals’ force between the GO nanosheets, GO tends to agglomerate. Therefore, the particle size and size distribution of GO in solvents can reflect its dispersion in solvents [42]. The particle size of GO-g-PLLA dissolved in water was 81.8 µm, exhibiting a very wide particle size dispersion. The results show that after the ring-opening polymerization of lactic acid O-carboxyanhydride, the hydrophobic PLLA was successfully grafted onto the surface of GO, thus making GO-g-PLLA a hydrophobic material, exPLLAining its poor dispersion in Polymers 2018, 10, x FOR PEER REVIEW 9 of 16 water. A particle size of 30.59 µm in chloroform indicated that the dispersion of GO-g-PLLA in chloroform was substantially better than that in water. After grafting, GO was transformed from a into a hydrophobic material. Therefore, improving the dispersion of GO in chloroform is possible by hydrophilic into a hydrophobic material. Therefore, improving the dispersion of GO in chloroform is obtaining through through the ring-opening polymerization ofoflactic acid Opossiblefunctional by obtainingGO-g-PLLA functional GO-g-PLLA the ring-opening polymerization lactic acid carboxyanhydride initiated by the hydroxyl groups O-carboxyanhydride initiated by the hydroxyl groupson onthe theGO GO surface. surface.
Figure Lasersize sizeand and shape shape analysis GO-g-PLLA. Figure 7. 7.Laser analysisofofGO GOand and GO-g-PLLA.
To further confirm the andGO-g-PLLA GO-g-PLLAinin chloroform, prepared 0.5 mg/mL To further confirm thesolubility solubilityof of GO GO and chloroform, we we prepared 0.5 mg/mL GO-g-PLLA solutionsininchloroform. chloroform. After After the sonicated for for 2h2 and left left GO GO andand GO-g-PLLA solutions the solutions solutionswere were sonicated h and standing for 12 h (Figure 8), the solubility was investigated. All the GO precipitated to the bottom of of standing for 12 h (Figure 8), the solubility was investigated. All the GO precipitated to the bottom the chloroform after solutionwas wassonicated sonicated and GO-g-PLLA waswas still still evenly the chloroform after thethe solution andleft leftfor for1212h,h,whereas whereas GO-g-PLLA evenly dispersed in the chloroform. Similarly, when chloroform was used to dissolve GO and PLLA blends, dispersed in the chloroform. Similarly, when chloroform was used to dissolve GO and PLLA blends, 12 h later, GO was precipitated, because the dispersion of GO in chloroform is poor. These different 12 h later, GO was precipitated, because the dispersion of GO in chloroform is poor. These different behaviors indicate that the PLLA molecular chains grafted to the GO surface very strongly interacted behaviors indicate that the PLLA molecular chains grafted to the GO surface very strongly interacted with the solvent, thus increasing the dispersion of GO-g-PLLA in chloroform [37]. with the solvent, thus increasing the dispersion of GO-g-PLLA in chloroform [37].
standing for 12 h (Figure 8), the solubility was investigated. All the GO precipitated to the bottom of the chloroform after the solution was sonicated and left for 12 h, whereas GO-g-PLLA was still evenly dispersed in the chloroform. Similarly, when chloroform was used to dissolve GO and PLLA blends, 12 h later, GO was precipitated, because the dispersion of GO in chloroform is poor. These different behaviors indicate Polymers 2018, 10, 177 that the PLLA molecular chains grafted to the GO surface very strongly interacted 10 of 16 with the solvent, thus increasing the dispersion of GO-g-PLLA in chloroform [37].
Figure 8. Dispersions of GO and GO-g-PLLA in chloroform.
Figure 8. Dispersions of GO and GO-g-PLLA in chloroform. Polymers 2018, 10, x FOR PEER REVIEW
10 of 16
3.7. Differential Differential Scanning Scanning Calorimetry Calorimetry (DSC) (DSC) of PLLA and Its Composites 3.7. From the the DSC DSC curves curves in in Figure Figure 99 and and Table Table 1, the the glass glass transition transition (Tg) (Tg) and and melting melting (Tm) (Tm) From ◦ C, respectively. Tg and Tm increased to 68 and 168 ◦ C, temperatures of of pure pure PLLA PLLA are are 63 63 and and 167.8 167.8 °C, temperatures respectively. Tg and Tm increased to 68 and 168 °C, ◦ C after adding 1% and 1.5% respectively, after afteradding adding0.5% 0.5% GO-g-PLLA. increased to °C 170after respectively, GO-g-PLLA. TmTm increased to 170 adding 1% and 1.5% GO-gGO-g-PLLA. However, not increase markedly with further increases in GO-g-PLLA PLLA. However, Tg didTg notdid increase markedly with further increases in GO-g-PLLA content.content. When WhenGO-g-PLLA 1.5% GO-g-PLLA was added, Tg decreased 62 ◦The C. The increases andTm Tm for for the the 1.5% was added, Tg decreased to 62to °C. increases in in TgTgand nanocomposite were likely due to the increase in the interaction between PLLA and GO and its nanocomposite were likely due to the increase in the interaction between PLLA and GO and its derivatives. The The mechanical mechanical interlocking, interlocking, hydrogen hydrogen bonding, and/or and/orelectrostatic electrostaticforces forces restrict restrict the the derivatives. movement between between the the polymer polymer chains chains [43,44]. [43,44]. Therefore, Therefore, the the composite composite with with GO-g-PLLA GO-g-PLLA exhibited exhibited movement better thermal thermal stability and a higher interfacial adhesion strength than pure PLLA. better
Figure curves of of PLLA PLLA and and GO-g-PLLA/PLLA. GO-g-PLLA/PLLA. Figure 9. 9. Differential Differential scanning scanning calorimetry calorimetry (DSC) (DSC) curves Table 1. The glass transition temperature (Tg), melting temperature (Tm), enthalpy of melting (ΔHm), and degree of crystallinity (Xc) of PLLA and GO-g-PLLA/PLLA.
Sample PLLA
Tg (°C) 63
Tm (°C) 167.8
ΔHm (J/g) 23.84
Xc (%) 25.6
Polymers 2018, 10, 177
11 of 16
Table 1. The glass transition temperature (Tg), melting temperature (Tm), enthalpy of melting (∆Hm), and degree of crystallinity (Xc) of PLLA and GO-g-PLLA/PLLA. Sample
Tg (◦ C)
Tm (◦ C)
∆Hm (J/g)
Xc (%)
PLLA GO-g-PLLA/PLLA (0.5%) GO-g-PLLA/PLLA (1%) GO-g-PLLA/PLLA (1.5%)
63 68 68 62
167.8 168 170 170
23.84 35.67 39.51 37.13
25.6 38.3 42.4 39.8
In addition, DSC data showed that the addition of the GO-g-PLLA composite improved the crystallization rate of PLLA, as shown in Table 1, in which ∆Hm (J/g) and Xc represent the enthalpy of melting and degree of crystallinity, respectively. Thus, with the addition of 0.5% GO-g-PLLA, Xc increased from 25.6% to 38.3% and continued to increase with increasing amounts of GO-g-PLLA composite. 3.8. Characterization of the Mechanical Properties of PLLA and Its Composite Polymers 2018, 10, x FOR PEER REVIEW
11 of 16
Figure 10 shows the tensile strength and breaking strain of PLLA, GO/PLLA, 3.8. Characterization ofThe the Mechanical of PLLA and Its that Composite and GO-g-PLLA/PLLA. tensile Properties test results show the tensile strength and breaking shows the and tensile strength and breaking strainincreased of PLLA, GO/PLLA, and and GO-g-5.64% after strain of PLLAFigure were10 47.97 MPa 4.44%, respectively, which to 51.09 MPa PLLA/PLLA. The tensile test results show that the tensile strength and breaking strain of PLLA were adding GO and further increased to 71.26 MPa and 6.17% after adding the GO-g-PLLA composite. 47.97 MPa and 4.44%, respectively, which increased to 51.09 MPa and 5.64% after adding GO and Both copolymers and blends enhance the mechanical properties of PLLA. The GO-g-PLLA composite further increased to 71.26 MPa and 6.17% after adding the GO-g-PLLA composite. Both copolymers exhibited and increased tensile strength and breaking strain of The 48.6% and 39%, respectively. The results blends enhance the mechanical properties of PLLA. GO-g-PLLA composite exhibited increased tensile strength and breaking strain of 48.6% and 39%, respectively. The results confirm confirm those of previous reports because of the high specific surface area and high elastic modulus. those of previous reports because of the high specific surface area and high elastic modulus. After After dispersion in polymers, graphene and its derivatives can bear loads and thus considerably dispersion in polymers, graphene and its derivatives can bear loads and thus considerably enhance enhance the mechanical properties of a polymer the mechanical properties of a polymer [45–47]. [45–47].
Figure 10. Stress-strain curves of PLLA, GO/PLLA, and GO-g-PLLA/PLLA.
Figure 10. Stress-strain curves of PLLA, GO/PLLA, and GO-g-PLLA/PLLA. 3.9. Characterization of the Morphology of PLLA and Its Composite
3.9. Characterization of the Morphology of GO PLLA and Its Composite To investigate the dispersion of in the composite and the morphology of the fracture surface, we used scanning electron microscopy (SEM) to characterize the fracture surface of PLLA and GO-gTo investigate the dispersion of GO in the composite and the morphology of the fracture surface, PLLA/PLLA, as shown in Figure 11. A large quantity of evident undulations appeared at the fracture we used scanning electron to characterize the fracture surface surface of PLLA (Figuremicroscopy 11a). In contrast,(SEM) for GO-g-PLLA/PLLA (Figure 11b), the fracture surfaceof wasPLLA and GO-g-PLLA/PLLA, as shown in of Figure A large evident undulations appeared at coarser and denser than that PLLA, 11. indicating that quantity the energy of required for fracturing is higher than that for pure PLLA. In addition, the fracture of the composite was different from the brittle the fracture surface of PLLA (Figure 11a). In contrast, for GO-g-PLLA/PLLA (Figure 11b), the fracture fracture of pure PLLA. Conversely, there was no clear agglomeration of graphene on the fracture surface was coarser and denser than that of PLLA, indicating that the energy required for fracturing surface, further indicating that GO-g-PLLA was evenly dispersed in PLLA.
Polymers 2018, 10, 177
12 of 16
is higher than that for pure PLLA. In addition, the fracture of the composite was different from the brittle fracture of pure PLLA. Conversely, there was no clear agglomeration of graphene on the fracture surface, further that GO-g-PLLA was evenly dispersed in PLLA. Polymers 2018, 10, x indicating FOR PEER REVIEW 12 of 16
Polymers 2018, 10, x FOR PEER REVIEW
12 of 16
Figure 11. electron microscopy (SEM) of the fracture surfaces of PLLA and GO-gFigure 11. Scanning Scanning electron microscopy (SEM) of tensile the tensile fracture surfaces of PLLA and PLLA/PLLA. GO-g-PLLA/PLLA. Figure 11. Scanning electron microscopy (SEM) of the tensile fracture surfaces of PLLA and GO-g-
3.10. Barrier Tests 3.10. BarrierPLLA/PLLA. Tests
3.10.1. Water Vapor Permeability (WVP) (WVP) Test Test 3.10. Barrier Tests 3.10.1. Water Vapor Permeability WVP is the the most important barrier property formaterials. materials.The The WVP values pure PLLA WVP is most important barrier property WVP values of of pure PLLA andand its 3.10.1. Water Vapor Permeability (WVP) Test for its nanocomposite films are shown in Figure 12. films The films had a thickness of±0.04 ± 0.005 mmwere and nanocomposite films are shown in Figure 12. The had a thickness of 0.04 0.005 mm and WVP is the most important barrier property for materials. The WVP values of pure PLLA and ◦ C.170 were obtained by casting. Thearescrew insertion were 160, 175, and °C. The of WVP obtained casting. Thefilms screw insertion werehad 160, 175, and 170 The WVP the itsby nanocomposite shown in temperatures Figuretemperatures 12. The films a 175, thickness of175, 0.04 ± 0.005 mm and –7 g·m/m 2·s·pa. The WVP decreased to 3.797 × − –7 g·m/m22·s·pa − 7 2 7 of the pure PLLA thin film was 4.688 × 10 10 werethin obtained by casting. screwginsertion were 160, 175, 175, and 170 pure PLLA film was 4.688 The × 10 ·m/m ·stemperatures ·pa. The WVP decreased to 3.797 ×°C. 10 ThegWVP ·m/m ·s·pa 2·s·pa. The WVP decreased to 3.797 × 10–7 g·m/m2·s·pa of the 0.5% pure thin film was 10–7 g·m/m after adding adding 0.5%PLLA GO-g-PLLA, and4.688 this×value value 19% lowerthan thanthat thatofof pure PLLA. Further decreases after GO-g-PLLA, and this isis19% lower pure PLLA. Further decreases in after adding 0.5% GO-g-PLLA, and this value is or 19% lower than that of pure PLLA. Further decreases GO-gin WVP were not pronounced after adding 1% 1.5% GO-g-PLLA, because with additional WVP were not pronounced after adding 1% or 1.5% GO-g-PLLA, because with additional GO-g-PLLA, in WVP were not pronounced after adding 1% or 1.5% GO-g-PLLA, because with additional GO-gPLLA, the large amount of hydrophilic oxygen-containing functional GO surface can the large amount of hydrophilic oxygen-containing functional groupsgroups on the on GOthe surface can easily PLLA, the large amount of hydrophilic oxygen-containing functional groups on the GO surface can easily adsorb water, thus increasing thethecomposite absorption capacity ofvapor. water vapor. adsorb water, thus increasing the composite absorption capacity of water easily adsorb water, thus increasing composite absorption capacity of water vapor.
Figure 12. PLLA and GO-g-PLLA/PLLA Water Vapor Permeability (WVP) results.
Figure 12. PLLA and GO-g-PLLA/PLLA Water Vapor Permeability (WVP) results.
Figure 12. PLLA and GO-g-PLLA/PLLA Water Vapor Permeability (WVP) results.
Polymers 2018, 10, 177
13 of 16
Polymers 2018, 10, x FOR PEER REVIEW 3.10.2. Oxygen Permeability (PO 2 ) Test
13 of 16
The permeability was calculated using Equation (1). The test results are shown in Figure 13. 3.10.2. Oxygen Permeability (PO2) Test The PO2 of PLLA thin film was 3.013 (cm3 /(24 h × m2 ) × (cm/bar). The oxygen barrier capacity The permeability was calculated using Equation (1). The test results are shown in Figure 13. The 3 /(24 h × m2 ) × (cm/bar) when 0.5 wt % GO-g-PLLA was added, which is a decreased PO2to of 2.135 PLLA (cm thin film was 3.013 (cm3/(24 h × m2) × (cm/bar). The oxygen barrier capacity decreased 29.14% to reduction. When 2 wt GO-g-PLLA was0.5 added, PO2 of thewas composite thin film 3 2) × 2.135 (cm /(24 h × m% (cm/bar) when wt %the GO-g-PLLA added, which is adecreased 29.14% 2 ) × (cm/bar), a reduction of 46.53%. In addition, when the amount of to 1.611reduction. (cm3 /(24When h ×2m wt % GO-g-PLLA was added, the PO2 of the composite thin film decreased to 1.611 GO-g-PLLA exceeded the decrease in PO2ofwas not pronounced. (cm3/(24 h × m2)2%, × (cm/bar), a reduction 46.53%. In addition, when the amount of GO-g-PLLA exceeded 2%, the decrease in PO2 was not pronounced. 2 Permeability = OTR × (thickness/∆P) = [cm3 /(m × 24 h)] × (cm/bar) 3 2
Permeability = OTR × (thickness/ΔP) = [cm /(m × 24 h)] × (cm/bar)
(1)
(1)
3.0
3 2 [(cm /(24h×m )]×(cm/bar)
2.5
2.0
1.5
1.0
0.5
0.0 L PL
A
/PL LA L P -gGO
) 5% (0. A L GO
LL g-P
PL A/
) (1% A L
PL A/ L PL -gGO
) % 1.5 ( LA GO
LL g-P
PL A/
) (2% A L
Figure 13. PLLA and GO-g-PLLA/PLLA oxygen permeability (PO2) results.
Figure 13. PLLA and GO-g-PLLA/PLLA oxygen permeability (PO2 ) results.
4. Conclusions
4. Conclusions
We used hydroxyl groups on the surface of GO to initiate the ring-opening polymerization of
O-carboxyanhydride and successfully PLLA the surface polymerization of GO surface to of Welactic usedacid hydroxyl groups on the surface of GOgrafted to initiate theonto ring-opening obtain the GO-g-PLLA composite. Compared to studies in onto the literature, thisofmethod is metal-free lactic acid O-carboxyanhydride and successfully grafted PLLA the surface GO surface to obtain and under mild conditions (25 °C). For PLLA with graphene and GO composite materials, the the GO-g-PLLA composite. Compared to studies in the literature, this method is metal-free and under literature focuses on the study of the crystallinity and heat resistance of PLLA. Our study shows that ◦ mild conditions (25 C). For PLLA with graphene and GO composite materials, the literature focuses the functional materials prepared by this method largely improve the compatibility and the tensile on the study of the crystallinity and heat resistance of PLLA. Our study shows that the functional and barrier properties of the resulting composite materials. When 0.5 wt % of GO-g-PLLA was added, materials by this largely strength improveofthe andcomposite the tensile and barrier theprepared tensile strength andmethod tensile fracture the compatibility GO-g-PLLA/PLLA increased by properties of the resulting composite materials. When 0.5 wt % of GO-g-PLLA was added, tensile 48.6% and 39%, respectively, compared to those of PLLA, whereas the water vapor andthe oxygen strengthpermeabilities and tensile fracture strength of the GO-g-PLLA/PLLA increased by 48.6% and of the GO-g-PLLA/PLLA composite decreasedcomposite by 19% and 29%, respectively, 39%, respectively, to those of PLLA, theperformance water vapor oxygen compared tocompared the neat polymer. Improving thewhereas gas barrier andand flexibility of permeabilities PLLA for the purpose of packaging and other uses willby help toand improve the range of its uses in to practical of the GO-g-PLLA/PLLA composite decreased 19% 29%, respectively, compared the neat applications. polymer. Improving the gas barrier performance and flexibility of PLLA for the purpose of packaging and other uses will help to improve the range of its uses in practical applications.
Polymers 2018, 10, 177
14 of 16
Acknowledgments: This work was supported by the National Natural Science Foundation of China (Project Nos. 31460247, 81460542, 81760644), the Biodegradable Materials Innovative Research Team (in Science and Technology) at the University of Yunnan Province and the Innovation Team Based on Research and Application of Biological Functional Materials of Yunnan Minzu University (2017HC034). Author Contributions: This paper was accomplished based on the collaborative work of the authors. Mingwei Yuan performed the experiments, analyzed the data, interpreted the experimental results, and wrote the paper. Chengdong Xiong and Minglong Yuan supervised the entire research progress and contributed to the experimental design. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
4.
5. 6.
7. 8.
9.
10. 11. 12.
13. 14.
15. 16. 17.
Rasal, R.M.; Janorkar, A.V.; Hirt, D.E. Poly(lactic acid) modifications. Prog. Polym. Sci. 2010, 35, 338–356. [CrossRef] Anderson, K.S.; Schreck, K.M.; Hillmyer, M.A. Toughening polylactide. Polym. Rev. 2008, 48, 85–108. [CrossRef] Xu, J.-Z.; Chen, T.; Yang, C.-L.; Li, Z.-M.; Mao, Y.-M.; Zeng, B.-Q.; Hsiao, B.S. Isothermal crystallization of poly(L-lactide) induced by graphene nanosheets and carbon nanotubes: A comparative study. Macromolecules 2010, 43, 5000–5008. [CrossRef] Murariu, M.; Dechief, A.L.; Bonnaud, L.; Paint, Y.; Gallos, A.; Fontaine, G.; Bourbigot, S.; Dubois, P. The production and properties of polylactide composites filled with expanded graphite. Polym. Degrad. Stab. 2010, 95, 889–900. [CrossRef] Cao, Y.; Feng, J.; Wu, P. Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly(lactic acid) composites. Carbon 2010, 48, 3834–3839. [CrossRef] Wang, H.; Qiu, Z. Crystallization kinetics and morphology of biodegradable poly(L-lactic acid)/graphene oxide nanocomposites: Influences of graphene oxide loading and crystallization temperature. Thermochim. Acta 2012, 527, 40–46. [CrossRef] Wang, H.; Qiu, Z. Crystallization behaviors of biodegradable poly(L-lactic acid)/graphene oxide nanocomposites from the amorphous state. Thermochim. Acta 2011, 526, 229–236. [CrossRef] Liu, S.; Gordiichuk, P.; Wu, Z.-S.; Liu, Z.; Wei, W.; Wagner, M.; Mohamed-Noriega, N.; Wu, D.; Mai, Y.; Herrmann, A.; et al. Patterning two-dimensional free-standing surfaces with mesoporous conducting polymers. Nat. Commun. 2015, 6, 8817. [CrossRef] [PubMed] Liu, S.; Zhang, J.; Dong, R.; Gordiichuk, P.; Zhang, T.; Zhuang, X.; Mai, Y.; Liu, F.; Herrmann, A.; Feng, X. Two-dimensional mesoscale-ordered conducting polymers. Angew. Chem. Int. Ed. 2016, 55, 12516–12521. [CrossRef] [PubMed] Sun, Y.; He, C. Synthesis and stereocomplex crystallization of poly(lactide)-graphene oxide nanocomposites. ACS Macro Lett. 2012, 1, 709–713. [CrossRef] Arias, V.; Odelius, K.; Hoglund, A.; Albertsson, A.-C. Homocomposites of polylactide (PLLA) with induced interfacial stereocomplex crystallites. ACS Sustain. Chem. Eng. 2015, 3, 2220–2231. [CrossRef] [PubMed] Xu, H.; Wu, D.; Yang, X.; Xie, L.; Hakkarainen, M. Thermostable and impermeable “nano-barrier walls” constructed by poly(lactic acid) stereocomplex crystal decorated graphene oxide nanosheets. Macromolecules 2015, 48, 2127–2137. [CrossRef] Xu, H.; Xie, L.; Wu, D.; Hakkarainen, M. Immobilized graphene oxide nanosheets as thin but strong nanointerfaces in biocomposites. ACS Sustain. Chem. Eng. 2016, 4, 2211–2222. [CrossRef] Xu, H.; Feng, Z.-X.; Xie, L.; Hakkarainen, M. Graphene oxide-driven design of strong and flexible biopolymer barrier films: From smart crystallization control to affordable engineering. ACS Sustain. Chem. Eng. 2016, 4, 334–349. [CrossRef] Hua, L.; Kai, W.; Yang, J.; Inoue, Y. A new poly(L-lactide)-grafted graphite oxide composite: Facile synthesis, electrical properties and crystallization behaviors. Polym. Degrad. Stab. 2010, 95, 2619–2627. [CrossRef] Kricheldorf, H.R.; Jonte, J.M. New polymer syntheses. 8. Synthesis and polymerization of L-lactic acid O-carboxyanhydride (5-methyl-dioxolan-2,4-dione). Polym. Bull. 1983, 9, 276–283. Bonduelle, C.; Martin-Vaca, B.; Bourissou, D. Lipase-catalyzed ring-opening polymerization of the O-carboxylic anhydride derived from lactic acid. Biomacromolecules 2009, 10, 3069–3073. [CrossRef] [PubMed]
Polymers 2018, 10, 177
18.
19. 20.
21.
22.
23.
24. 25.
26. 27. 28. 29.
30.
31. 32.
33.
34.
35.
36.
15 of 16
Thillaye du Boullay, O.; Marchal, E.; Martin-Vaca, B.; Cossio, F.P.; Bourissou, D. An activated equivalent of lactide toward organocatalytic ring-opening polymerization. J. Am. Chem. Soc. 2006, 128, 16442–16443. [CrossRef] [PubMed] Chen, C.; Chen, L.; Chen, H.; Li, H.; Yuan, M. Synthesis and solubility of polylactide-co-poly(amino acid) random copolymer. Polym. Bull. 2013, 70, 1739–1751. [CrossRef] Yuan, M.L.; Li, X.H.; Xiong, C.D.; Deng, X.M. Polymerization of lactides and lactones—5. Ring-opening polymerization of epsilon-caprolactone and dl-lactide by rare earth 2-methylphenyl samarium. Eur. Polym. J. 1999, 35, 2131–2138. [CrossRef] Deng, X.M.; Yuan, M.L.; Xiong, C.D.; Li, X.H. Polymerization of lactides and lactones. II. Ring-opening polymerization of epsilon-caprolactone and DL-lactide by organoacid rare earth compounds. J. Appl. Polym. Sci. 1999, 71, 1941–1948. [CrossRef] Yuan, M.L.; Xiong, C.D.; Li, X.H.; Deng, X.M. Polymerization of lactides and lactones. III. Ring-opening polymerization of DL-lactide by the (eta(3)-c3h5)(2)sm(mu(2)-cl)(2)(mu(3)-cl)mg(tmed)(mu(2)-cl)mg(tmed) complex. J. Appl. Polym. Sci. 1999, 73, 2857–2862. [CrossRef] Deng, X.M.; Yuan, M.L.; Xiong, C.D.; Li, X.H. Polymerization of lactides and lactones. IV. Ring-opening polymerization of epsilon-caprolactone by rare earth phenyl compounds. J. Appl. Polym. Sci. 1999, 73, 1401–1408. [CrossRef] Deng, X.M.; Yuan, M.L.; Li, X.H.; Xiong, C.D. Polymerization of lactides and lactones vii. Ring-opening polymerization of lactide by rare earth phenyl compounds. Eur. Polym. J. 2000, 36, 1151–1156. [CrossRef] Yuan, M.L.; Wang, Y.H.; Li, X.H.; Xiong, C.D.; Deng, X.M. Polymerization of lactides and lactones. 10. Synthesis, characterization, and application of amino-terminated poly(ethylene glycol)-co-poly(epsilon-caprolactone) block copolymer. Macromolecules 2000, 33, 1613–1617. [CrossRef] Fang, M.; Wang, K.; Lu, H.; Yang, Y.; Nutt, S. Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites. J. Mater. Chem. 2009, 19, 7098–7105. [CrossRef] Wang, J.; Shi, Z.; Ge, Y.; Wang, Y.; Fan, J.; Yin, J. Functionalization of unzipped carbon nanotube via in situ polymerization for mechanical reinforcement of polymer. J. Mater. Chem. 2012, 22, 17663–17670. [CrossRef] Song, W.; Zheng, Z.; Tang, W.; Wang, X. A facile approach to covalently functionalized carbon nanotubes with biocompatible polymer. Polymer 2007, 48, 3658–3663. [CrossRef] Yoon, J.T.; Lee, S.C.; Jeong, Y.G. Effects of grafted chain length on mechanical and electrical properties of nanocomposites containing polylactide-grafted carbon nanotubes. Compos. Sci. Technol. 2010, 70, 776–782. [CrossRef] Goffin, A.-L.; Raquez, J.-M.; Duquesne, E.; Siqueira, G.; Habibi, Y.; Dufresne, A.; Dubois, P. From interfacial ring-opening polymerization to melt processing of cellulose nanowhisker-filled polylactide-based nanocomposites. Biomacromolecules 2011, 12, 2456–2465. [CrossRef] [PubMed] Yoon, J.T.; Jeong, Y.G.; Lee, S.C.; Min, B.G. Influences of poly(lactic acid)-grafted carbon nanotube on thermal, mechanical, and electrical properties of poly(lactic acid). Polym. Adv. Technol. 2009, 20, 631–638. [CrossRef] Amirian, M.; Chakoli, A.N.; Sui, J.H.; Cai, W. Enhanced mechanical and photoluminescence effect of poly(L-lactide) reinforced with functionalized multiwalled carbon nanotubes. Polym. Bull. 2012, 68, 1747–1763. [CrossRef] Olalde, B.; Aizpurua, J.M.; Garcia, A.; Bustero, I.; Obieta, I.; Jurado, M.J. Single-walled carbon nanotubes and multiwalled carbon nanotubes functionalized with poly(L-lactic acid): A comparative study. J. Phys. Chem. C 2008, 112, 10663–10667. [CrossRef] Chen, G.X.; Kim, H.S.; Park, B.H.; Yoon, J.S. Controlled functionalization of multiwalled carbon nanotubes with various molecular-weight poly(L-lactic acid). J. Phys. Chem. B 2005, 109, 22237–22243. [CrossRef] [PubMed] Chen, L.; Xu, Z.; Li, J.; Li, Y.; Shan, M.; Wang, C.; Wang, Z.; Guo, Q.; Liu, L.; Chen, G.; et al. A facile strategy to prepare functionalized graphene via intercalation, grafting and self-exfoliation of graphite oxide. J. Mater. Chem. 2012, 22, 13460–13463. [CrossRef] Yang, J.-H.; Lin, S.-H.; Lee, Y.-D. Preparation and characterization of poly(L -lactide)-graphene composites using the in situ ring-opening polymerization of PLLA with graphene as the initiator. J. Mater. Chem. 2012, 22, 10805–10815. [CrossRef]
Polymers 2018, 10, 177
37.
38.
39. 40. 41. 42.
43.
44. 45. 46.
47.
16 of 16
Li, W.; Xu, Z.; Chen, L.; Shan, M.; Tian, X.; Yang, C.; Lv, H.; Qian, X. A facile method to produce graphene oxide-g-poly(L-lactic acid) as an promising reinforcement for plla nanocomposites. Chem. Eng. J. 2014, 237, 291–299. [CrossRef] Stankovich, S.; Dikin, D.A.; Piner, R.D.; Kohlhaas, K.A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S.T.; Ruoff, R.S. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558–1565. [CrossRef] Dreyer, D.R.; Park, S.; Bielawski, C.W.; Ruoff, R.S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228–240. [CrossRef] [PubMed] Sun, Y.; He, C. Synthesis, stereocomplex crystallization, morphology and mechanical property of poly(lactide)-carbon nanotube nanocomposites. RSC Adv. 2013, 3, 2219–2226. [CrossRef] Shen, J.; Hu, Y.; Shi, M.; Lu, X.; Qin, C.; Li, C.; Ye, M. Fast and facile preparation of graphene oxide and reduced graphene oxide nanoPLLAtelets. Chem. Mater. 2009, 21, 3514–3520. [CrossRef] Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y. A facile one-step method to produce graphene-cds quantum dot nanocomposites as promising optoelectronic materials. Adv. Mater. 2010, 22, 103–106. [CrossRef] [PubMed] He, L.; Sun, J.; Wang, X.; Fan, X.; Zhao, Q.; Cai, L.; Song, R.; Ma, Z.; Huang, W. Unzipped multiwalled carbon nanotubes-incorporated poly(L-lactide) nanocomposites with enhanced interface and hydrolytic degradation. Mater. Chem. Phys. 2012, 134, 1059–1066. [CrossRef] Leszczynska, A.; Pielichowski, K. Application of thermal analysis methods for characterization of polymer/montmorillonite nanocomposites. J. Therm. Anal. Calorim. 2008, 93, 677–687. [CrossRef] Suk, J.W.; Piner, R.D.; An, J.; Ruoff, R.S. Mechanical properties of mono layer graphene oxide. ACS Nano 2010, 4, 6557–6564. [CrossRef] [PubMed] Ramanathan, T.; Abdala, A.A.; Stankovich, S.; Dikin, D.A.; Herrera-Alonso, M.; Piner, R.D.; Adamson, D.H.; Schniepp, H.C.; Chen, X.; Ruoff, R.S.; et al. Functionalized graphene sheets for polymer nanocomposites. Nat. Nanotechnol. 2008, 3, 327–331. [CrossRef] [PubMed] Sengupta, R.; Bhattacharya, M.; Bandyopadhyay, S.; Bhowmick, A.K. A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites. Prog. Polym. Sci. 2011, 36, 638–670. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
