New Stereocomplex PLA-Based Fibers: Effect of POSS on Polymer

Jul 1, 2014 - In this work, a novel approach for the functionalization of electrospun stereocomplex polylactide (sc-PLA)-based fibers, prepared from s...
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New Stereocomplex PLA-Based Fibers: Effect of POSS on Polymer Functionalization and Properties Orietta Monticelli,*,† Matilde Putti,† Lorenza Gardella,† Dario Cavallo,† Andrea Basso,† Mirko Prato,‡ and Simone Nitti‡ †

Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso, 31, 16146 Genova, Italy Nanochemistry Department, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

‡

S Supporting Information *

ABSTRACT: In this work, a novel approach for the functionalization of electrospun stereocomplex polylactide (sc-PLA)-based fibers, prepared from solutions containing equimolar amount of high-molecular-weight poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), was developed. The method, which consists in introducing functionalized polyhedral oligomeric silsesquioxanes (POSS) into the electrospinning solutions, was carried out by employing as a solvent system a 2:1 mixture of chloroform (CHCl3) and 1,1,1,3,3,3hexafluoro-2-propanol (HFIP), never applied in the production of sc-PLA fibers, which was found to promote POSS solubilization while simultaneously allowing to obtain an excellent fiber homogeneity. Indeed, the specific effect of the single components of the solvent mixture, CHCl3 and HFIP, on fiber structuring and morphology was evaluated. Conversely to the fiber morphology, which turned out to be significantly affected by the chosen electrospinning solvent, the PLA stereocomplexation, occurring upon subsequent annealing treatment at 100 °C (as evidenced by differential scanning calorimetry (DSC) and X-ray diffraction analyses), was found to be similar for fibers prepared starting from the different types of solvent. Unlike solution casting, electrospinning allows the exclusive formation of stereocomplex crystallites, simultaneously promoting a submicrometric dispersion of the silsesquioxanes, with the consequent fiber functionalization. In our work, two different kinds of POSSone characterized by hydroxyl groups (POSS−OH) and another one functionalized with an aminobearing molecule (POSS−NH2)were exploited to impart hydroxyl and amino functionalities to PLA based nanofibers, while preserving the capability of the polymer system to form a pure stereocomplex on subsequent annealing. In particular, it was found that the amino groups of the sc-PLA fibers functionalized with POSS−NH2, promote specific interactions with a metal precursor, i.e., PdCl2, which, as a result of a subsequent reduction, forms metal nanoclusters homogeneously dispersed on the fiber surface. The higher thermal and chemical resistance of the sc-PLA fibers with respect to those based solely on PLLA allowed to significantly broaden the applications of the catalytic system. Indeed, the sc-PLA/Pd fibers turned out to be very active in the Heck reaction, easily recoverable and reusable for multiple catalytic cycles.

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INTRODUCTION Recently, poly(L-lactide) (PLLA), a thermoplastic high modulus/high strength aliphatic polyester easily processed by conventional techniques,1−4 has attracted considerable attention due to both its derivation from renewable resources (such as corn starch) and biodegradability/compostability.5,6 Today, its main commercial uses are in the field of recyclable packaging as well as in textiles.7 Aside from the above exploitations, thanks to its biocompatibility, PLLA is widely employed in biomedical applications, such as sutures, orthopedic pins, and systems for drug delivery.8 Moreover, it has been shown that PLLA-based nanofibers, produced by electrospinning and characterized by high surface area, allow enhanced cell adhesion and proliferation, this finding further widening PLLA potential applications in the field of tissue engineering.9−12 It is known that the association of PLLA with its Denantiomer poly(D-lactide) (PDLA), results in the formation of © XXXX American Chemical Society

stereocomplex-type PLA (sc-PLA) whose thermal and mechanical properties surpass those of its homopolymer constituents.13,14 Indeed, pure stereocomplex crystallites have a melting temperature of 230 °C, which is 50 °C higher than that of PLLA and PDLA homocrystals. Moreover, it has been reported that stereocomplex-PLA is more stable against hydrolysis than PLLA.13 Despite the interest for the above promising material, the approaches commonly used to produce fibers, such as melt15 or solution spinning,16 allow only the formation of a product containing a mixture of both homo- and stereocomplex-crystallites. Furthermore, these processes require subsequent long-time annealing at elevated temperatures, as high as 180 °C, or drawing to high ratios, to get a consistent Received: March 12, 2014 Revised: June 25, 2014

A

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investigated. In spite of the interest for sc-PLA, to the best of our knowledge, no work on the formation and properties of nanostructured sc-PLA nanofibers has been reported so far. In particular, in this work we report, for the first time, on the development of nanofibers based on sc-PLA and polyhedral oligomeric silsesquioxanes (POSS). Indeed, POSS, which are organic−inorganic molecules32,33 (approximately 1−3 nm in diameter), were incorporated into different polymer nanofibers (cellulose acetate,34 polyvinylidene fluoride,35 sulfonated poly(arylene ether sulfone),36 poly(vinyl alcohol),37 poly(Nisopropylacrylamide),38 poly(butylene terephthalate),39 ethylene−propylene−diene rubber,40 and poly(styrene-co-maleic anhydride),41) with the aim to modify the material characteristics. In general, referring to the polymer/POSS electrospun fibers system, the silsesquioxane can (i) be dispersed at a nanometric level, thanks to the fast solvent evaporation occurring during the electrospinning process,34,35 (ii) improve the fiber mechanical35 and thermal properties,39 and (iii) confer functionality to the polymer matrix, thanks to the functional groups which are linked to the siliceous structure and that can be chosen on the basis of the specific application of the fibers. This latter aspect is extremely relevant, as the reactivity, given to the polymer matrix by the dispersion of functional-POSS, may allow the interaction or even the link of fibers with specific molecules, such as drugs and metal precursors, thus extending the applications of these materials. The potential advantage offered by this system is particularly evident if one compares for example electrospun sc-PLA fibers bringing tertiary amino groups, capable, as reported by Spasova et al.,42 to impart hemostatic and antibacterial properties to the stereocomplex fibrous material. In this system the functionalities were formed using diblock copolymers, namely polymers whose laboratory synthesis is costly and time-consuming. On this basis, in this work new electrospun fibers based on sc-PLA and functionalized POSS molecules have been prepared, paying particular attention to the specific effect of the electrospinning solvent on fiber morphology and structuring and to the role of the silsesquioxane on the final fiber properties.

fraction of stereocomplex-PLA. Electrospinning, namely a technique in which a charged polymer solution jet is continuously drawn from a fine spinneret, has been recently proved to be very effective in promoting PLA stereocomplexation, thanks to the orientation of polymer chains caused by the combined application of electrically induced shearing forces and rapid solidification. Similar as in the previous studies with mechanical drawing of the blend PLLA/ PDLA fibers during spinning or thermal treatment,15,16 it was expected that the extension of the polymer chains, raising the probability of interaction between PLLA and PDLA segments, would have resulted in enhanced formation and growth of stereocomplex crystallites. Indeed, in 2006, Tsuji et al.17 first reported on electrospun stereocomplex nanofibers, with the solution for electrospinning prepared by dissolving a solutioncast PLLA/PDLA blend film in chloroform. By applying a high voltage, they were able to obtain predominantly stereocomplex nanofibers, with a negligible small amount of homocrystals, even for high-molecular-weight PLLA/PDLA. However, in this case, the enhanced stereocomplexation observed during electrospinning could be also due−other than to the chain orientation caused by the imposed electric field−to the probable presence of stereocomplex nuclei (formed during solution casting for film preparation and insoluble in the electrospinning solution). Afterward, Ishii et al.18 obtained scPLA nanofibers, characterized by the unique presence of stereocomplex crystallites, by electrospinning a PLLA/PDLA solution containing equal amounts of PLLA and PDLA and by applying a following annealing treatment at 100 °C. While the as-spun nanofibers were completely amorphous, the annealed ones showed the (exclusive) presence of stereocomplex crystals, this finding demonstrating that, even though no crystalline order is developed during the electrospinning process, it allows the development of side-by-side alignment of PLLA and PDLA chains required to have stereocomplex crystals, which can then form in the subsequent annealing treatment. These sc-PLA nanofibers were found to have a slower degradation rate and to cause milder inflammatory reaction than PLLA nanofibers, as evaluated in vivo by subcutaneous implantation in rats.19 Moreover, Fundador et al.20 compared the enzyme degradation of sc-PLA nanofibers, prepared by means of electrospinning, with that of PLLA nanofibers, finding that PLLA nanofibers can be degraded by proteinase K more rapidly than sc-PLA nanofibers and that the annealing of these latter makes them completely resistant to the action of this enzyme. It is worth underlining that in the three above-mentioned works18−20 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was used as solvent system. More recently, Zhang et al.21 reported on highly stereocomplex aligned nanofibers, electrospun from a 50/50 PLLA/PDLA solution in a 60/40 dichloromethane/pyridine solvent mixture onto a high take-up velocity rotating collector. Although these works demonstrate that the employed solvent could influence the polymer structuring, a detailed investigation on this issue has not been reported so far. Moreover, also the specific effect of the solvent on nanofiber morphology has often been overlooked, the morphological analysis being extremely limited or completely absent in some of the above-mentioned works.17,21 In general, in order to further improve the characteristics of electrospun fibers, proper fillers have been added to the polymer matrix.22,23 In the case of PLA electrospun fibers, several kinds of fillers and nanofillers, such as glass,24 carbon nanotube,25−27 layered silicates,28−30 and silver,31 were

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MATERIALS AND METHOD

Materials. Poly(L-lactide) (PLLA), PLLA 1010 Synterra (average molecular weight 1 × 105), and poly(D-lactide) (PDLA), PDLA 1010 Synterra (average molecular weight 1 × 105), purchased from Purac (The Netherland) in powder form, were used as received. Aminopropyl heptaisobutyl POSS (from now on referred as POSS−NH2) (Figure 1S(a)) and trans-cyclohexanediolisobutyl POSS (from now on referred as POSS−OH) (Figure 1S(b)) were purchased from Hybrid Plastics (USA) as crystalline powders and used as received. The following solvents, all from Aldrich and used as received, were used for the electrospinning solution preparation: 1,1,1,3,3,3-hexafluoro-2propanol (HFIP), chloroform (CHCl3), dichloromethane (CH2Cl2), and pyridine. Fiber and Film Preparation. As shown in Table 1, equal amounts of PLLA and PDLA were dissolved in the different solvents to various final polymer concentration. POSS-based solutions were prepared by adding POSS−NH2 or POSS−OH to PLLA/PDLA solutions. The mixtures were stirred for 12 h at room temperature. Dense films with a thickness of ca. 200 μm were prepared by casting the above-reported solutions used for electrospinning on a glass slide with the help of a brass knife and evaporating the solvent in vacuum at 80 °C for 12 h. PLLA/PDLA and PLLA/PDLA-based POSS solutions were electrospun using a conventional electrospinning system. The solutions were loaded in a syringe (model Z314544, diameter B

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reduction was carried out using a methanol solution of NaBH4 (from Aldrich, 0.1 M) at room temperature for 2 h. After reduction, the fibers were washed extensively to remove the excess of the reducing agent and dried at 60 °C under vacuum for at least 24 h. Catalytic Activity Test. Catalysts based on palladium have provided some of the most effective and exciting transformations in organic chemistry. A wide range of coupling reactions are known, including the Heck, Suzuki and Sonagashira reactions, all of which are efficient and have excellent generality. Catalytic activity of sc-PLA/ POSS−NH2/Pd fibers was tested in a model Heck reaction between iodobenzene and styrene. Conversion was determined by analyzing the crude mixture by GC-MS (HP 5890 Series II), by comparison of the relative areas of the peaks of starting materials and product. On the other hand, visual inspection of the suspension after the reaction, as well as of the fibers after filtration (when possible) was use in a first instance to exclude conditions capable to damage the catalyst. The reactions were performed in parallel on sc-PLA/Pd fibers based on 2 and 5 wt % of % POSS−NH2. Under optimized conditions, 10 mg of fibers were dispersed in 500 μL of acetonitrile containing 5.5 μL of iodobenzene, 6.6 μL of styrene, and 11 μL or 110 μL of triethylamine (Et3N). Fibers were tested for their ability to maintain their catalytic properties after the Heck reaction; for this purpose, the same fibers were used three times under the same reaction conditions, and the outcome of the reaction was monitored as before. Characterization. To study the sample surface morphology, a Leica Stereoscan 440 scanning electron microscope was used. All the samples were thinly sputter-coated with carbon using a Polaron E5100 sputter coater. The fiber diameters and their distribution were measured using an image analyzer, with ImageJ 1.41 software. Differential scanning calorimetry (DSC) measurements were performed with a TC10A Mettler calorimeter calibrated with high purity indium and operating under a continuous flow of nitrogen. Samples having masses between 5 and 11 mg were heated from 25 to 250 °C at 10 °C/min. The degree of crystallinity was estimated by using a melting enthalpy of 146 J/g for a 100% crystalline stereocomplex-PLA.43 TGA measurements were performed using a Stare System Mettler thermobalance, under nitrogen flow, at a heating rate of 10 °C/min. Static wide-angle X-ray diffraction was carried out in reflection mode a Philips PW 1830 powder diffractometer (Ni-filtered Cu Ka radiation, k = 0.1542 nm). The crystalline phase content was quantitatively determined through deconvolution of the WAXD patterns using the software package Peak-Fit. The actual WAXD pattern was considered to be the superposition of the intensity of the reflections associated with both homo- and stereocomplex crystals, plus the amorphous halo. The amount of the crystalline phases was calculated as the ratio between the area of the corresponding peaks to the total area of the pattern. XPS characterization was performed with a Kratos Axis Ultra spectrometer using a monochromatic Al Kα source (10 mA, 15 kV). High resolution analyses were carried out with a pass energy of 40 eV. Binding energy scale has been charge corrected to have the well resolved C 1s peak of carboxylic groups (COO) of sc-PLA at 289 eV. Elemental analysis of the sc-PLA/POSS−NH2/Pd samples was carried out via inductively coupled plasma optical emission spectrometry (ICP-OES), performed on a iCAP 6300 DUO spectrometer (Thermo Fisher). A 6 mg sample of the fibers was dissolved in 0.5 mL of a concentrated HCl/HNO3 3:1(v/v) (Carlo Erba superpure grade) mixture and left overnight at room temperature, in order to completely disrupt any organic component. Afterward, Milli-Q grade water (18.3 M Ohm) was added (4.5 mL) to the sample. The solution was, then, filtered using 0.45 μm pore size filter. Palladium concentration was measured using the most sensitive palladium emission line (340.4 nm).

Table 1. Characteristics of the Prepared Electrospinning Solutions type of solvent CH2Cl2/pyridine (60/40) CH2Cl2/pyridine (60/40) CHCl3 CHCl3 CHCl3 HFIP HFIP/CHCl3 (1/2) HFIP/CHCl3 (1/2) HFIP/CHCl3 (1/2) HFIP/CHCl3 (1/2)

polymer concentration (% w/v)

type of POSS

POSS concentration

polymer solubility

5

−

−

±

10

−

−

−

8 10 12 12 12

− − − − −

− − − − −

+ ± − + +

12

POSS− OH POSS− NH2 POSS− NH2

2

+

2

+

5

+

12 12

d=11.6 mm, Aldrich Fortuna Optima) placed in the horizontal direction. A Gamma high-voltage research power supply (Model ES30P-5W) was used to charge the solution in the syringe with a positive DC voltage. The positive electrode was connected to the needle (diameter d = 0.45 mm) of the syringe and the negative electrode was attached to the grounded collector, an aluminum sheet wrapped on a glass cylinder (height 4 cm, diameter 14.5 cm). The distance between the tip and the collector was 20 cm. A syringe pump (Harvard Apparatus Model 44 programmable syringe pump) was used to feed the needle. The needle tip and the ground electrode were contained in a hollow plastic cylinder (height 30.5 cm, inner diameter 24 cm, and thickness 3.5 mm), internally coated with a polytetrafluoroethylene sheet (thickness 1 mm), which was supplied with a XS Instruments digital thermohygrometer (model UR100, accuracy ±3% RH and ±0.8 °C) as humidity and temperature sensor to monitor and control the ambient parameters (temperature around 21 °C). A glass Brooks rotameter was used to keep constant the air flow in the enclosed electrospinning space. The air flow was fed in the chamber at atmospheric pressure from an inlet placed behind the collector. In the case of the fibers prepared from CHCl3, electrospinning was performed by using solutions with a polymer concentration of 8% w/v, which allowed to solubilize PLLA and PDLA, while for HFIP and the mixture HFIP/CHCl3 the concentration of the solutions was chosen on the basis on the literature data.20 The influence of the electrospinning parameters on the fibers morphology was studied as follows. The influence of the relative humidity (RH) was evaluated by varying RH (from 0 to 50%) and maintaining the other electrospinning parameters (voltage tension (V), solution concentration, tip−collector distance, air flow rate and temperature) constant. Moreover, for each used RH, two different values of the voltage tension (15 and 20 kV) were applied. For each set of parameters, at least four mats were electrospun and the reproducibility of the experiments was established by analyzing the fiber morphology in different places (at least 5) of the mat surface by using an optical microscopy (Leika DMLP polarized optical microscope equipped with a 20× objective lens). The established conditions are solution flow rate 0.004 mL/min, tip-collector distance 20 cm, air flow rate 3.5 L/min, and temperature 21 °C. Both fibers and films were subjected to an annealing treatment at 100 °C for 4 h. Preparation of PLLA/PDLA/Pd and PLLA/PDLA/POSS−NH2/ Pd Fibers. Both PLLA/PDLA and PLLA/PDLA/POSS−NH2 fibers, preliminary subjected to the annealing treatment, were stirred in a methanol (from Aldrich) solution containing 0.01 wt % PdCl2 (from Aldrich) at room temperature for 48 h. The fibers/PdCl2 ratio was adjusted to 10/1 (w/w). The fibers were then filtered, washed several times with methanol and dried overnight at 60 °C. Palladium

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RESULTS AND DISCUSSION Study of the Influence of the Solvent on Morphology of PLLA/PDLA Electrospun Fibers. Solutions based on equal C

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Figure 1. SEM micrographs of PLLA/PDLA fibers prepared by applying the optimal electrospinning conditions and using the following solvents: (a) CHCl3, (b) HFIP, and (c) CHCl3/HFIP.

amounts of PLLA and PDLA and prepared by using chloroform (CHCl3), 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and dichloromethane (CH2Cl2)/pyridin (volume ratio of 60/40) as solvents, namely systems already exploited for the preparation of PLA stereocomplex fibers, were electrospun. Moreover, to promote POSS solubilization, and, as discussed later on, to create defect-free fibers, a new type of solvent mixture based on HFIP and CHCl3 (volume ratio of 1/2) was investigated for the first time. The most promising systems, namely those whose use enabled us to obtain homogeneous and bead-free fibers, were analyzed in detail, evaluating the influence of several parameters, such as voltage, polymer concentration and relative humidity (RH) on the morphology of the fibers. The solvent mixture, CH2Cl2/pyridin, already used by Zhanget al.,21 was discarded as the polymer pair used was scarcely soluble even at low concentrations (5% w/v). Also CHCl3 turned out not to be an appropriate solvent as it was not possible to select proper electrospinning conditions (polymer concentration, humidity, voltage, etc.) to obtain defect free fibers (Figure 1a). Indeed, in the case of the use of this solvent, the formation of a gel, related to the poor solubility of PLA stereocomplex, occurs at relatively low concentration (10% w/v) and it was not possible to further increase the amounts of the two polymers in the solution. Moreover, a comparison with the literature data is not possible as a detailed morphological characterization of the PLLA/PDLA fibers obtained by using this solvent is lacking. Conversely, in the case of the use of HFIP, the most appropriate electrospinning conditions which allowed to obtain bead-free and homogeneous fibers were selected (Figure 1b). In particular, the influence of both RH and V on fiber morphology was investigated, by applying the same concentration reported in the literature. A specific study of the above parameters was not given in previous works reporting on the use of HFIP in the electrospinning of stereocomplex-PLA. Figure 2S shows the images of the fibers prepared by varying RH from 10% to 50%. It is evident that, by increasing RH, the homogeneity of the fibers tends to decrease. This trend is in

agreement with that reported in the case of the electrospinng of PLLA. Indeed, this parameter, whose influence is not always investigated in the literature, was recently related to the hydrophobicity of the polymer matrix, hydrophobic polymers, such as PVDF,44 showing an opposite behavior with respect to that found for hydrophilic polymers. The photos of the fibers, obtained by varying V and maintaining RH constant, are given in Figure 2Sd. By increasing V from 15 to 20 kV, the fibers dimensions as well as their homogeneity tend to decrease. This effect is evident also by applying a voltage of 30 kV. It is worth underlining that the specific effect of the above parameter on the fiber morphology was investigated in several studies. Nevertheless, from the literature data it is not possible to deduce a general rule, the voltage effect depending on the polymer/solvent pair used. As previously mentioned, fibers were also prepared by using the mixture HFIP/CHCl3 (Figure 1c) and the effect of both V and RH on fiber morphology was studied. The morphological characterization demonstrated that the effect of the latter parameter is similar to that found in the case of fibers prepared from HFIP, that is an improvement of fiber homogeneity by decreasing RH (Figure 3S). Moreover, as far as the contribution of V is concerned, for this type of solvent mixture, the value, which allows to obtain defect-free fibers was found to be 20 kV (Figure 3Sd). Study of the Influence of the Solvent on Structuring of PLLA/PDLA Electrospun Fibers. Figure 2 shows the structural characterization of fiber as spun from various solvents. The wide-angle X-ray diffraction patterns clearly indicate that all the as-spun fibers are mainly in the amorphous state, containing only faint traces of PLA homocrystals.45 Fibers spun from chloroform exhibit slightly higher crystallinity, probably due to the fact that CHCl3 is a poorer solvent for PLA crystals than HFIP.13 DSC traces for the three samples are very similar (Figure 3): they all show an enthalpic relaxation peak, followed by a cold crystallization exotherm around 80 °C and, eventually, an D

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film, thus probably containing not-dissolved sterocomplex nuclei capable to promote stereocomplex formation. Obviously, this procedure requires a longer “two-steps” preparation of the electrospinning solution. WAXD data of the fibers annealed at 100 °C for 4 h are reported in Figure 4. As expected, the annealing treatment

Figure 2. X-ray diffraction patterns of fibers prepared from solutions based on (a) CHCl3, (b) HFIP, and (c) HFIP/CHCl3.

Figure 4. X-ray diffraction patterns of annealed fibers prepared from the following solvents: (a) CHCl3, (b) HFIP, and (c) HFIP/CHCl3.

allows the crystallization of the polymer, with the appearance of characteristic peaks for crystalline structures in X-ray patterns. Similarly to what already observed during continuous heating, the annealing process of electrospun fibers leads to the development of purely stereocomplex crystals, identified by diffraction peaks at 2θ values of 12, 21, and 24° (Figure 4).45 The stereocomplex content, as evaluated from the deconvolution of WAXD patterns, is around 50% for all the annealed samples, regardless of the solvent used for electrospinning. The minimal homocrystals content, produced during the electrospinning process and surviving the annealing treatment, could be quantified to be as low as few units percents (around 3% for the samples electrospun from HFIP and HFIP/CHCl3, around 7% for the sample electrospun from CHCl3), being practically negligible. The remaining complementary fraction can be assigned to the amorphous PLA phase. DSC curves (Figure 5) show the absence of any cold crystallization event on subsequent heating of annealed fibers

Figure 3. DSC traces of fibers prepared from the following solvent: (a) CHCl3, (b) HFIP, and (c) HFIP/CHCl3.

endotherm of fusion at about 230 °C ascribable to the melting of stereocomplex crystallites.45 No peak corresponding to the melting of homocrystals (170−180 °C)45 can be detected, with the exception of the fibers spun from CHCl3 which show a signal−though very weak−at about 170 °C (see the inset). Together with the information from X-ray diffraction, the differential scanning calorimetry results indicate that the as-spun fibers, which are mainly amorphous (as expected due to the fast evaporation of the solvent during the electrospinning process, which hampers macromolecules crystallization), undergo major changes during the dynamic heating at 10 °C/min, leading to stereocomplex formation. It is worth underlining that a characterization based on the DSC alone would seriously overestimate the stereocomplex content in the samples, due to a deceiving low enthalpy of cold crystallization. Indeed, stereocomplex crystallization during heating without an explicit cold crystallization peak has been observed for quenched PLLA/ PDLA blend films.46 Our results are perfectly in line with those reported by Ishii et al.18 and Fundador et al.20 and point out that, even though the as-spun fibers are predominantly amorphous, the electrospinning process is effective in promoting the association of PLLA and PDLA chains, which allows the exclusive formation of stereocomplex crystallites upon further thermal treatments. The difference with the results obtained by Tsuji et al.,17 who reported on stereocomplexation directly occurring during electrospinning from a chloroform solution, can be explained considering that their solution was prepared starting from a cast PLLA/PDLA blend

Figure 5. DSC traces of annealed fibers prepared from the following solvents: (a) CHCl3, (b) HFIP, and (c) HFIP/CHCl3.

and the presence of the only endotherm peak characteristic of stereocomplex−PLA, the melting peak corresponding to homocrystals being just a weak and broad bump. From the DSC traces a stereocomplex crystallinity higher than 40% can be estimated for all the nanofibers, thus supporting the WAXD results. E

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Figure 6. SEM micrographs of PLLA/PDLA fibers prepared by applying the optimal electrospinning conditions and containing: (a) POSS−NH2 and (b) POSS−OH.

This finding, that purely stereocomplex nanofibers can be obtained by means of electrospinning technique and subsequent annealing treatment at 100 °C, is in agreement with what observed for similar blends,18,20 as well as for systems constituted by high molar mass polylactide and diblock copolymers consisting of poly(L- or D-)lactide and poly(N,Ndimethylamino-2-ethyl methacrylate)42 or poly(butylene succinate) blocks.47 As a novelty, we were able to produce purely and highly stereocomplex-crystalline fibers, characterized by a regular morphology, by employing a 1:2 HFIP/CHCl3 mixture as a solvent, this being a cheaper alternative to the pure HFIP commonly employed, other than providing a good solvent system for POSS molecules to be used as nanofillers (see following section). The structure and thermal properties of films cast from the different solvents are shown for comparison in Figure 4S and 5S. Because of slower solvent evaporation, cast films exhibit higher crystallinity than electrospun fibers. Though, as expected on the basis of the high molecular weight of the employed polymers,48 all the samples crystallize, by solvent evaporation, in the PLA homocrystal form (α-phase), again with the exception of the film cast from chloroform, which, likely due to the lower critical concentration for stereocomplex crystallization, contains meaningful traces of stereocomplex crystallites. In accordance with the WAXD patterns, all the differential scanning calorimetry curves display the homocrystal melting peak at 170 °C, plus an additional high-temperature melting peak at about 230 °C, which must be related to a fraction of stereocomplex crystals which develops during heating, analogously to the situation reported for as-spun fibers. Annealing of the casted films at 100 °C for 4 h just leads to an increase in homocrystal crystallinity (Figure 6S and 7S), pointing out that, unlike what was observed for electrospun systems, such a thermal treatment is inadequate to obtain stereocomplex films. On the basis of the different thermal behavior of the electrospun fibers with respect to cast films, it is possible to infer, also for this new solvent mixture, a specific effect of the electrospinning on the polymer structuring. Indeed, the peculiar action of the applied electrical field together with the rapid polymer solidification, proper of the electrospinning technique, accounts for a stretching of the macromolecular chains, thus promoting the formation of streocomplex PLA.

This specific effect was found to be retained independent of the applied solvent system, regardless the stereocomplex solubility. Study of the Influence of POSS on Morphology of PLLA/PDLA Electrospun Fibers. Fibers based on PLLA and PDLA, prepared starting from solutions containing equimolar quantity of the two enantiomers and two different types of silsesquioxane molecules, POSS−OH or POSS−NH2, were prepared and characterized. As previously mentioned, the chosen solvent mixture, HFIP/CHCl3, proved to be an effective solvent for both the polymer pair (up to a concentration of 12%) and the POSS molecules. Also for these systems, the effect of V and RH on fiber morphology was investigated. Similar as observed for the previously described PLLA/PDLA fibers, the morphology of those containing both POSS−OH and POSS−NH2 gets worse by increasing RH, and the optimal voltage, which assures a fine dimensional homogeneity, was found to be 20 kV (Figure 8S). These results demonstrate that the presence of POSS does not modify the electrospinnability of the solutions. In addition, it was observed that the POSScontaining fibers prepared by applying the optimal electrospinning conditions possess a structural homogeneity which is even better than that of the fibers just based on PLLA/PDLA (Figure 9S). This phenomenon, already observed with other polymer matrices, such as PVDF35 and PBT,39 was attributed to the ability of silsesquioxanes to modify the surface tension of the electrospinning solution. Moreover, as demonstrated by SEM micrographs shown in Figure 6, no micrometer-sized POSS aggregates are present on the surface. In the light of this result, it is possible to conclude that this technique is capable of promoting the distribution of POSS at a submicrometric level in the system PLLA/PDLA, similar as in the case of other polymer matrices. Furthermore, it is worth underlining that the dispersion of POSS−NH2 and POSS−OH confers functionalization to the fibers. Indeed, comparing IR spectra of the neat PLLA/PDLA fibers with those of the fibers containing the silsesquioxanes, modifications of the band in the region between 3600 and 3400 cm−1, namely the interval of the wavenumbers characteristic of amino and hydroxyl groups, are visible in the three samples, thus supporting a likely functionalization of the polymer matrix with the above functional groups (Figure 10Sa). Moreover, XPS analysis on POSS−NH2-based PLLA/PDLA fibers further support the functionalization since it clearly shows the presence F

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of a N 1s peak centered at binding energy of (399.7 ± 0.2) eV, a value typical of NH2 groups (Figure 11Sb).49,50 Study of the Influence of POSS on Structuring of PLLA/PDLA Electrospun Fibers. Figure 7, parts A and B,

Figure 8. TG curves from 200 to 500 °C (in the inset from 300 to 400 °C) of sc-PLA, sc-PLA/POSS−OH and sc-PLA/POSS−NH2 fibers, in nitrogen.

PLLA and PDLA chains which decrease their mobility, thus retarding the thermal degradation of the film.14 The thermal decomposition of all the analyzed fibers was found to occur in one step in the temperature range between ca. 300 and 400 °C (see insert in Figure 8). Nevertheless, the presence of POSS in the fibers significantly affects the onset degradation temperature (Tonset), being Tonset of sc-PLA fibers 319 °C and those of the fibers containing POSS−OH and POSS−NH2 340 and 338 °C, respectively. Also the temperature corresponding to the maximum weight loss rate temperature (Tmax) is slightly shifted to higher temperatures in the fibers based on POSS (sc-PLA fibers Tmax =356 °C, scPLA/POSS−OH fibers Tmax =365 °C, sc-PLA/POSS−NH2 fibers Tmax = 362 °C). This behavior, already found with other polymer matrices52 was ascribed to the formation of a silica layer on the surface of the polymer hence serving as a barrier and limiting the degradation of the polymer. Indeed, as at the Tonset, the evaporation of both POSS−OH and POSS−NH2 is not completed (Figure 12S), the silsesquioxane can play a protection action toward the polymer. These results point out that not only the formation of stereocomplex can affect PLA degradation, as reported in the literature, but that also the presence of POSS, which in the case of electrospun nanofibers is homogeneously dispersed in the polymer matrix, can contribute to the increase of the polymer thermal stability. Study of the Influence of POSS on Properties of PLLA/ PDLA Electrospun Fibers. The study of the influence of POSS on material properties was concentrated on the fibers based on POSS−NH2. Nevertheless, it is possible to envisage also for sc-PLA/POSS−OH fibers potential applications in different fields. Indeed, the hydroxyl functionalities, conferred to the fibers by the addition of POSS−OH, might allow interactions with specific molecules, such as drugs, as well as modify the surface tension of the fibers. As far as sc-PLA/ POSS−NH2 fibers are concerned, they were applied as novel catalytic support. Indeed, the sc-PLA fibers based on 2 and 5 wt % of POSS−NH2 were contacted with a solution containing the metal precursor, PdCl2, which after a washing treatment underwent a reduction with NaBH4. By simply comparing the photos of the fibers (Figure 13S), it is already evident that the amount of linked metal is related to the concentration of silsesquioxane, being the fibers based on 5 wt % of POSS−NH2 darker (Figure 13Sa) that those containing a lower amount of POSS (Figure 13Sb). This preliminary evidence is supported by

Figure 7. (A) X-ray diffraction patterns of (a) annealed PLLA/PDLA/ POSS−NH2 fibers and (b) PLLA/PDLA/POSS−NH2 fibers. (B) Xray diffraction patterns of (a) annealed PLLA/PDLA/POSS−OH fibers and (b) PLLA/PDLA/POSS−OH fibers.

shows the WAXD patterns of as-spun (a) and annealed (b) fibers containing ammine and hydroxyl functionalized POSS, respectively. As for the neat samples (Figure 2), the as-spun samples loaded with POSS exhibit only a very poor homocrystal crystallinity. The presence of a small diffraction peak at 2θ value of around 8°, and associated with the presence of crystalline POSS,51 can be appreciated in the WAXD patterns of both POSS-containing samples. Upon annealing, a further increase of crystallinity occurs, with stereocomplex crystals being the only crystalline structure developing, analogously to the case of neat fibers. A stereocomplex content of about 50% was estimated by means of WAXD pattern deconvolution also in the case of the POSS-loaded annealed samples. This result highlights that silsesquioxane does not alter the polymer structuring. To summarize, the developed approach enabled us to prepare highly crystalline PLA fibers, stereocomplex-PLA being the neatly predominant crystal formsimultaneously containing homogeneously dispersed ammineand hydroxyl-bearing POSS molecules capable to confer functionalities to the electrospun fibers. As it will be shown in the following, the exclusive structuring of the polymer matrix into stereocomplextype PLA confers higher thermal stability to the material, further widening its end use applications, e.g., in heterogeneous catalysis. Study of the Influence of POSS on Thermal Stability of PLLA/PDLA Electrospun Fibers. The study of the influence of POSS on fiber thermal properties was completed by carrying out also thermal gravimetric analysis (TGA). Figure 8 compares TGA curves of neat sc-PLA with those of the systems containing POSS−NH2 and POSS−OH. First, it is worth underlining that in the literature it was demonstrated that the thermal stability of PLLA/PDLA blended films in the melt is enhanced compared with that of the pure PLLA and PDLA films, due to the peculiar strong interactions between G

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Figure 9. (a) FE-SEM micrograph of sc-PLA/POSS−NH2 (5 wt %)/Pd fibers and (b) histogram of the relative population as a function of Pd particle diameter.

were insoluble, as we wanted to test them as heterogeneous catalyst. For this reason our screening was limited to acetonitrile and N-methyl-2-pyrrolidone (NMP). As reactions were performed in sealed tubes, temperatures above the boiling point of the solvents could be used, and experiments were carried out alternatively at 80 and 120 °C. Conventional heating in an oil bath and microwave heating were investigated without a significant difference in performances. The base employed was always Et3N, as heterogeneous bases such as potassium carbonate, commonly employed in these reactions, could interfere with the final recovery of the fibers. Reactions were tested alternatively with a stoichiometric amount and with a large excess (10 equiv) of the amine. The screening of reaction conditions resulted in the choice of acetonitrile as the solvent and 80 °C as the optimum temperature. In fact, the use of NMP and/or higher temperatures caused complete dissolution of the fibers in the medium. It is relevant to point out that PLLA−POSS−NH2/Pd fibers, prepared by applying the same procedure adopted for development of sc-PLA/ POSS−NH2/Pd fibers, were found to be completely dissolved at 80 °C, thus demonstrating, as already mentioned, that the PLA stereocomplexation allows to widen the polymer applications. The last parameter that was investigated was the amount of base employed, and from this point of view the use of an excess of Et3N was found beneficial for the reaction kinetics. 5% POSS fibers, under these conditions, allowed complete conversion of starting materials into product in 72 h. This result is particularly remarkable if we take into consideration that PLLA/PDLA/ POSS−NH2 fibers contain only 0.1 wt % Pd (as measured by ICP-OES), and that this amount is sufficient to catalyze the reaction, probably due to the enhanced surface area. Under the same conditions 2% POSS fibers produced only 33% conversion. At this stage it was important to assess whether the catalytic activity was a result of the heterogeneous catalyst and/or of Pd particles leaked into the solution. Fibers recovered from the Heck coupling were therefore reused in additional catalytic cycles and the reaction outcome was again monitored by GC−

ICP-OES analysis, which reveals that the quantity of Pd retained by the neat sc-PLA is negligible, while that measured for the fibers based on 2 and 5 wt % of silsesquioxane is 0.06 and 0.1%, respectively. Indeed, it is possible to conclude that amino groups, related to the presence of the silsesquioxane in the POSS−NH2-based fibers promote specific interactions with PdCl2, allowing its deposition on the fiber surface.53 Figure 9 shows FE-SEM of sc-PLA/POSS−NH2 after the treatment with PdCl2 and reduction. The aggregates, formed on the surface of the fibers, appear to be uniformly distributed with an average diameter of 6 nm. The Pd cluster dimensions are similar to those obtainable using neat PLLA fibers54 as well as those reported for other polymeric supports (less than 4 nm on nylon55 between 2 and 4 nm on resins56 and hyperbranched aramids53,57). Catalytic activity of sc-PLA/POSS−NH2/Pd fibers was tested in a model Heck reaction between iodobenzene and styrene (Figure 10).

Figure 10. Scheme of Heck reaction.

In a first set of experiments optimal conditions were investigated: different solvents were tested at various temperatures in the presence of variable amounts of triethylamine (Et3N) as base. Parameters that were taken into consideration were % conversion of the starting materials into stilbene and appearance of the fibers recovered after the reaction. During these experiments magnetic stirring was found to be deleterious for the stability of the fibers, and therefore the reactions were left standing with seldom manual agitation. Although typical solvents for the Heck reaction are polar aprotic solvents, such as dimethylformamide (DMF), dimethylacetamide (DMA) or dimethyl sulfoxide (DMSO), our choice was limited to solvents where sc-PLA/POSS−NH2/Pd fibers H

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particular, the sc-PLA fibers containing POSS−NH2 were applied for the development of a novel heterogeneous catalytic system based on Pd. The sc-PLA/POSS−NH2/Pd system, which consists of sc-PLA fiber decorated with Pd nanocluster, was used in the Heck reaction at 80 °C, namely a temperature where the neat PLLA/Pd fibers54 were found to be completely dissolved. Conversely, the stereocomplexation renders the system stable at the above temperature and the sc-PLA/Pd fibers turned out to be very active in the Heck reaction, easily recovered and reusable.

MS. In order to assess more precisely partial loss of catalytic activity, it was decided to stop the reactions before complete conversion was achieved, and to compare the results obtained in the distinct cycles. Results are summarized in Table 2 and Table 2. Stilbene Conversion as a Function of the Reaction Parameters run

POSS−NH2 concentration (wt %)

time (h)

Et3N concn (equiv)

conversion (%)

1 1 1 1 1 1 1 1 2 2 2 3 2

2 2 5 5 2 2 5 5 2 2 5 5 5

24 72 24 72 24 72 24 72 72 72 72 72 72

1 1 1 1 10 10 10 10 1 10 1 1 10

10 22 11 27 9 33 18 100 0 0 17 15 4

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ASSOCIATED CONTENT

S Supporting Information *

Structure of POSS−NH2 and POSS−OH, photos of neat PLLA/PDLA fibers prepared from HFIP by applying different electrospinning conditions, photos of neat PLLA/PDLA fibers prepared from HFIP/CHCl3 by applying different electrospinning conditions, X-ray diffraction patterns of cast films prepared from CHCl3, HFIP, and HFIP/CHCl3, DSC traces of cast films prepared from CHCl3, HFIP, and HFIP/CHCl3, Xray diffraction patterns of annealed cast films prepared from CHCl3, HFIP, and HFIP/CHCl3, photos of neat PLLA/ PDLA/POSS−NH2 fibers prepared from HFIP/CHCl3 by applying different electrospinning conditions, histogram of PLLA/PDLA and PLLA/PDLA/POSS−NH2 fibers, FT-IR spectra of PLLA/PDLA/POSS−NH2, PLLA/PDLA/POSS− NH2, and PLLA/PDLA/POSS−OH fibers. XPS spectrum collected on POSS−NH2-based PLLA/PDLA fibers in the energy region typical for N 1s peaks TG curves of POSS−NH2 and POSS−OH. Photos of sc-PLA/POSS−NH2 (5 wt %), scPLA/POSS−NH 2 (2 wt %) and sc-PLA fibers after impregnation with PdCl2, washing and reduction with NaBH4.This material is available free of charge via the Internet at http://pubs.acs.org.

show that, although reactions performed with 10 eq Et3N resulted in faster conversions, after the first cycle the catalytic activity was almost completely lost, probably because of leakage of Pd into the solution. On the other hand, when 1 equiv of Et3N was used, a small decrease of catalytic activity was observed after the first cycle, but this remained unchanged between the two following cycles. This was attributed to partial loss of Pd during the first cycle, while the portion of Pd tightly bound to the fibers through the amine functionalities remained unchanged and maintained its catalytic activity throughout the various cycles. Indeed, ICP-OES characterization of the fibers demonstrates that the loss of Pd after the first catalytic cycle was between 50% and 60%, but no additional loss was observed after the subsequent cycles, in agreement with the experimental observations.58 Calculation of TOF after the third cycle gave a very good figure (2 h−1), comparable to the one obtained by Wai et al. (4 h−1) with carbon nanotube-supported Pd, and 1 order of magnitude higher than the one reported by the same authors for commercial 10% Pd/C (0.27 h−1).59

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AUTHOR INFORMATION

Corresponding Author

*(O.M.) Telephone: +39 010 3536196. Fax: +39 010 3538733. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Italian Ministry of Education and University through the 2010-2011 PRIN project (Grant No. 2010XLLNM3_005).

CONCLUSIONS In this work, novel nanostructured electrospun fibers based on stereocomplex polylactide (sc-PLA) were developed. The fibers, characterized by a significant structural homogeneity, were prepared from a solvent system, never applied in the preparation of sc-PLA fibers, which allowed to solubilize the polymer pair, composed of equimolar amounts of highmolecular-weight poly(L-lactide) PLLA and poly(D-lactide) PDLA, as well as the used nanofillers, namely polyhedral oligomeric silsesquioxanes (POSS). The exploitation of two types of functionalized POSS, one with hydroxyl groups linked to the siliceous cage (POSS−OH) and another one functionalized with an amino bearing molecule (POSS−NH2), confer amino and hydroxyl functionalities to the fibers, which showed a submicrometer dispersion of the silsesquioxane. Furthermore, the presence of POSS was found not to modify the polymer structuring but to ameliorate fiber morphology and resistance to thermal degradation. Clearly, the above properties extend the potential applications of the sc-PLA/POSS fibers. In

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