PCL immiscible blends

European Polymer Journal 58 (2014) 69–78

Contents lists available at ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Silsesquioxanes: Novel compatibilizing agents for tuning the microstructure and properties of PLA/PCL immiscible blends Orietta Monticelli a,⇑, Michela Calabrese a, Lorenza Gardella a, Alberto Fina b, Emilia Gioffredi b b

Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso, 31, 16146 Genova, Italy Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino-sede di Alessandria, viale Teresa Michel, 5, 15121 Alessandria, Italy

a r t i c l e

i n f o

Article history: Received 16 April 2014 Received in revised form 13 June 2014 Accepted 25 June 2014 Available online 3 July 2014 Keywords: POSS Polymer compatibilizer Nanocomposites PLA PCL

a b s t r a c t In this work, a novel and efficient approach for the compatibilization of poly(lactide) (PLA) and poly(e-caprolactone) (PCL) immiscible blends, consisting in the addition of ad hoc functionalized polyhedral oligomeric silsesquioxane (POSS) molecules, was investigated. To this purpose, different kinds of POSS were exploited: one without specific functionalities (named POSS-oib) and another one characterized by hydroxyl groups (named POSS-OH) potentially capable of making the silsesquioxane compatible with both the polymer matrices. Moreover, in order to improve the silsesquioxane adhesion to the components of the blend, an amino-functionalized POSS was subjected to the grafting of a poly(e-caprolactone)-b-poly(L-lactide) diblock copolymer (POSS-PCL-b-PLLA) by means of ring opening polymerization. The above silsesquioxanes were loaded into PLA/PCL blends through melt blending. SEM characterization showed that the POSS molecules are capable of modifying the blend morphology to different degrees. In particular, the synthesized POSS-PCL-b-PLLA nanohybrid proved very effective at ameliorating the compatibility of PLA/PCL blends by preventing the dispersed domains from coalescence, despite the small amount of silsesquioxane (2 wt.%), and consequently of diblock copolymer, added to PLA/PCL blend. The peculiar co-continous phase morphology, promoted at the interface by the presence of the above POSS, was found to enhance the mechanical properties of the blend, improving the elongation at break without reducing its Young’s modulus. Our study demonstrated that not only the microstructure but also the thermal properties of the blends were significantly affected by the presence of silsesquioxane molecules dispersed in the blend. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recently, expected shortage in fossil reserves and increased environmental concern have encouraged the development of biomaterials for several applications. Indeed, polymers derived from renewable resources are the frontrunner of the advances realized in this trend [1]. Among the wide range of bio-based and biodegradable polymers available, polylactide (PLA), a renewable and ⇑ Corresponding author. Tel.: +39 010 3536196; fax: +39 010 3538733. E-mail address: [email protected] (O. Monticelli). http://dx.doi.org/10.1016/j.eurpolymj.2014.06.021 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.

biodegradable polymer obtained from ring opening polymerization of lactide [2,3], is the most promising due to its excellent mechanical properties, comparable to those of conventional polymers such as polystyrene (PS) and polyethylene (PE), and its low cost [4]. However, brittleness, poor elongation at break, narrow processing window and low melt strength represent open challenges for the application of this polymer [5,6]. In particular, the brittleness of PLA restricts its use in many fields. Thus, to improve the brittleness of PLA, blending with a ductile biodegradable polymer such as poly(e-caprolactone) (PCL) has been widely used. However, PCL and PLA are thermodynamically incompatible

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with each other and can only form a multiphase structure in their blended system with poor interfacial adhesion, which restricts its further applications [7–9]. Considerable efforts have been made to enhance the compatibility between PLA and PCL by using generally known compatibilization strategies, such as the addition of polymeric compatibilizers and reactive compatibilization methods. In particular, diblock or multiblock copolymers, whose chemical nature is identical to that of the main components of the blends, were found to be particularly effective in improving the compatibility of PLA/PCL blends [10–13]. Another way to modify the interfacial properties of immiscible blends, developed more recently, is the addition of inorganic fillers, such as carbon black, organoclay and carbon nanotubes [14–21]. As far as PLA/PCL blends are concerned, the effect of carbon nanotubes (CNTs) on the morphology and properties of these systems was widely studied. Wu et al. [22] demonstrated that, in the case of PCL/PLA blends in which PCL was the matrix phase, the ternary systems containing carboxylic multi walled CNTs (MWCNTs) presented high improvement of the performances in terms of conductive, as compared with those of the neat PCL/PLA blend, because of the selective localization of MWCNTs both in the matrix PCL phase and at the interface. As far as the mechanical properties of the PCL/PLA/MWCNTs blends are concerned, the tensile yield strength was found to increase while the elongation at break decreased by the addition of MWCNTs to the blends. Indeed, the observed interface localization turned out to simultaneously bring reinforcement and compatibilization to the immiscible blend. Later on, the same authors studied also the dielectric loss and conductivity of the same PCL/ PLA blends containing MWCNTs [23]. More recently, the specific effect of the addition of acid-oxidized MWCNTs on the phase morphology and electrical conductivity was investigated for PLA/PCL/MWCNTs composites with various PCL compositions ranging from 5 to 90 wt.% [24]. The results showed that MWCNTs were selectively dispersed in the PCL phase, regardless of PCL phase domain size. Moreover, the selective localization of CNTs in the PLA/ PCL blend was compared with that of clay, finding out that the latter nanofiller was preferentially localized in the PLA phase and interface, while CNTs were mainly dispersed in the PCL phase and interface [25]. Indeed, the phase morphology turned out to be remarkably improved, because the appropriate localization of the nanofiller could prevent coalescence of the domains and favor the break-up of the droplets. As far as the properties of the blend based on clay are concerned, the presence of this nanofiller was found to enhance the material biodegradability and reduce oxygen permeability [26]. Another kind of filler used by Jain et al. [27] as compatibilizer agent of PLA/PCL blends was micro-talc. The addition of this filler decreases PLA domain size and voids in the matrix, resulting in significant improvement of oxygen and water barrier properties. However, the elongation at break of talc reinforced PLA/ PCL blends was found to decrease with increasing the talc loading. More recently, Goffin et al. [28] exploited also cellulose nanowhiskers (CNW), subjected to the surface grafting of

different polyesters, such as PCL, PLA as well as PCL-bPLA diblock copolymers, which were used as blend compatibilizers. It was demonstrated that (co)polyestergrafted CNW, especially CNW-g-PCL-b-PLA diblock copolymers, can tune the compatibility of PCL/PLA blends and their related microstructures. In this work, the specific effect of polyhedral oligomeric silsesquioxanes (POSS) molecules on the morphology and properties of PLA/PCL blends has been analyzed in detail for the first time. Indeed, POSS are organic/inorganic molecules, sizing approximately 1–3 nm, with general formula (RSiO1,5)n – where R can be an hydrogen atom or an organic group (such as alkyl, aryl or any of their derivatives) [29]. These hybrid molecules, which can either be easily incorporated into a polymer matrix by physical dispersion [30] or linked to polymer chains by direct polymerization (grafting) via the reactive side groups of the silsesquioxane molecule [31–33], were found to significantly improve the material mechanical properties [34,35], thermal behavior and flame retardancy [36]. The affinity of POSS for polymer matrices, as well as the possibility to vary the silsesquioxane functional groups, make these hybrid molecules potential candidates for the compatibilization of polymer systems. Moreover, with respect to the above mentioned nanofillers, the reactions to modify POSS, which can be performed in a solvent capable of solubilizing the silsesquioxane, are easier and more controllable. Nevertheless, to the best of our knowledge, this work represents the first systematic report on the influence of POSS on the morphology and properties of polymer blends. In particular, different kinds of silsesquioxane molecules, both characterized by functional groups potentially able to establish interactions with the polymer matrices and without specific functionalities, have been investigated as compatibilizers in PLA/PCL blend system. Moreover, a POSS bearing a PCL-bPLLA diblock copolymer was ad hoc synthesized and used, for the first time, as blend compatibilizer.

2. Experimental section 2.1. Materials Polylactide (PLA) is a commercial product from Nature Works Co. Ltd. USA (2002D, Mn = 100,000 g/mol) with a residual monomer content less than 0.3 wt.%. Poly(e-caprolactone) was obtained from Solvay Ltd. Belgium (CAPA 6500, Mn = 50,000 g/mol). Octaisobutyl POSS (referred to as POSS-oib in the following) and trans-cyclohexanediolisobutyl POSS (referred to as POSS-OH in the following) were purchased from Hybrid Plastics (USA) as crystalline powders and used as received. Chemical structures for POSSoib (M = 873.6 g/mol, melting temperature = 160 °C) and POSS-OH (M = 959.7 g/mol, melting temperature = 270 °C) are reported in Fig. 1S, while in Figs. 2S and 3S 1H NMR spectra are shown. For the preparation of POSS-PCL-b-PLLA: e-caprolactone (97%), purchased from Sigma–Aldrich, was distilled under reduced pressure; L-lactide (98%), kindly supplied by Purac, was recrystallized three times from anhydrous toluene and dried under vacuum at ambient temperature prior to use.

Anhydrous toluene (99.8%) and tin(II) 2-ethylhexanoate (Sn(Oct)2) (95%) were purchased from Sigma–Aldrich and used without further purification. Aminopropyl heptaisobutyl POSS (POSS-NH2 from now on) was purchased from Hybrid Plastics as crystalline powder and used as received. 2.2. Synthesis of POSS-PCL-b-PLLA The covalent grafting of the PCL-b-PLLA diblock copolymer onto the POSS molecules was obtained via ring-opening polymerization (ROP) of e-caprolactone and L-lactide catalyzed by Sn(Oct)2, exploiting as initiator the primary amino group of POSS-NH2 [37]. In the first step, a POSSended PCL pre-polymer was synthesized via ROP of e-caprolactone initiated by POSS-NH2; the reaction product was subjected to a double purification stage, so as to remove both unreacted e-caprolactone and unreacted POSS; the POSS-PCL pre-polymer, whose 1H NMR spectrum is shown in Fig. 4S, was then used as initiator in the second step ROP of L-lactide to give the desired POSS-PCL-b-PLLA nanohybrid. Briefly, 0.4 g of POSS-NH2 were introduced into a twoneck flask equipped with a magnetic stirrer, followed by 14 ml of toluene and 4 ml of e-caprolactone in sequence, all under nitrogen flow. The reaction vessel was then immersed into a thermostatically controlled oil bath (set at 100 °C) while under stirring. As soon as a homogeneous solution was obtained, Sn(Oct)2 in a toluene solution ([NH2]/[Sn(Oct)2] = 2.5) was added under nitrogen, and the reaction allowed to proceed for 7 h. After this time, the polymerization was stopped by quenching the reaction mixture into an ice bath, and the POSS-PCL pre-polymer was recovered by precipitation into an excess of cold methanol (aimed to remove the unreacted e-caprolactone monomer), followed by several washings with hexane (to remove unreacted POSS). The filtered product was then vacuum dried at 40 °C till constant weight. The as-synthesized POSS-PCL nanohybrid was characterized by means of 1H NMR spectroscopy, the mean degree of polymerization of PCL estimated to be around 70. In the subsequent step, 0.5 g of POSS-PCL were added under nitrogen to a predetermined amount of purified L-lactide previously introduced in the reactor together with toluene ([L-lactide] = 1 M) (the L-lactide to POSS-PCL molar ratio was adjusted so as to obtain a diblock copolymer having PCL and PLLA sequences of similar length). The mixture was heated at 100 °C under stirring until complete solubilization of both the reagents and Sn(Oct)2 in a toluene solution ([POSS-PCL]/[Sn(Oct)2] = 2.5) was added to the reaction medium. Then, the reaction was allowed to proceed at 100 °C for 23 h before quenching into an ice bath. The reaction product was recovered by precipitation from cold methanol (to remove residual lactide), filtered and vacuum dried at 40 °C till constant weight before its characterization and employment.

namely a laboratory internal mixer provided with a mechanical stirrer (Heidolth, type RZR1), which was connected to a vacuum line and evacuated for 30 min at room temperature. Then, the reactor was purged with argon for 30 min. The above operations were repeated at least three times, to be sure to avoid humidity to come in contact with the reagents. The reactor was placed in an aluminum block oven at 180 °C and, when the polymers were completely molten, POSS was added under inert atmosphere. At the applied temperature, which is generally used in the preparation of PLA/PCL blends, the systems are thermally stable, as verified by TGA measurements (results not shown). PLA/ PCL blends based on POSS were prepared by mixing the polymers and POSS (POSS-oib, POSS-OH and POSS-PCL-bPLLA), at a concentration of 2 wt.%, under inert atmosphere, using a mixing time of 10 min. In the case of the blend based on POSS-oib, a sample was also prepared by mixing PLA with a previously prepared PCL-POSS-oib system, using the same conditions as for the other blends. The latter composite was prepared by adding POSS-oib to PCL at 100 °C according to the above described procedure. Neat PLA/PCL blend was prepared and characterized under the same conditions, as reference material. Table 1 shows the characteristics of the prepared sample.

2.4. Characterization A Zeiss Supra 40 VP field emission scanning electron microscope equipped with a backscattered electron detector was used to examine the blend morphologies and Si distribution. The specimens were submerged in liquid nitrogen for 30 min and fractured cryogenically. All samples were thinly sputter-coated with carbon using a Polaron E5100 sputter coater. EDS analysis was accomplished by analyzing at least 20 spots (20 nm large) for each samples. Both POSS-PCL and POSS-PCL-PLLA were characterized by means of 1H NMR spectroscopy: 1H NMR spectra were recorded with a Varian NMR Mercury Plus apparatus at a frequency of 300 MHz in CDCl3 solutions containing TMS as internal standard. Differential scanning calorimetry was performed under a continuous nitrogen purge on a Mettler calorimetric apparatus, model DSC1 STARe System. Both calibrations of heat flow and temperature were based on a run in which one standard sample (indium) was heated through its melting point. The samples, having a mass between 2.5 and 6 mg, were heated from room temperature to 200 °C, then cooled down to room temperature and finally heated

Table 1 Characteristics of PLA/PCL and PLA/PCL/POSS blends.

2.3. Preparation of PLA/PCL and PLA/PCL/POSS blends Before accomplishing the blend preparation, both the polymer matrices were dried overnight at 60 °C. 70 wt.% of PLA and 30 wt.% of PCL were added to a glass reactor,

a

Sample name

POSS type

PLA70PCL30 PLA70PCL30POSS-oib PLA70(PCL30POSS-oib)a PLA70PCL30POSS-OH PLA70PCL30 POSS-PCL-b-PLLA

– POSS-oib POSS-oib POSS-OH POSS-PCL-b-PLLA

Blend prepared by adding the composite system PCL/POSS-oib to PLA.

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to 200 °C again. A scanning rate of 10 °C/min was used both on heating and cooling. Rheological properties of PLA/PCL/POSS composites were measured on a strain-control rheometer (Advanced Rheometric Expansion System, ARES, TA Instruments, USA) with parallel-plate geometry (diameter of 25 mm). The instrument was equipped with a convection oven in compressed gas to control the temperature. Before the measurements, the dried PLA/PCL/POSS composites were pressed at 180 °C into disks with thickness of 1 mm and diameter of 25 mm. The disk-like samples were further dried at 80 °C in vacuum for 8 h before the measurements. Oscillatory frequency sweeps ranging from 0.01 to 100 rad/ s with a fixed strain (chosen and selected for each sample in order to fall in the linear viscoelastic region) were performed in nitrogen atmosphere at 180 °C, to investigate the viscosity at the compounding temperature. After the sample loading, an approximate 5 min equilibrium time was applied prior to each frequency sweep. Oscillatory time sweeps were performed to ensure the dried composites were stable for at least 30 min at the testing temperature. The tensile properties of neat PLA/PCL blend and PLA/ PCL/POSS blends were determined at room temperature by an Instron Mechanical Tester (Instron 5565) at a crosshead speed of 50 mm/min using rectangular specimens with dimension of 10  25  0.5 mm. The reported property values represent an average of the results for tests run on six specimens, along with their experimental deviation. 3. Results and discussions Novel composite systems based on POSS, were prepared by adding to the molten blends, composed by PLA and PCL in the weight ratio of 70/30 (PLA70PCL30), two kinds of silsesquioxanes, one characterized by hydroxyl functionalities potentially compatible with both the polymer matrices (POSS-OH) and one without specific functional groups (POSS-oib). Furthermore, a PLLA-PCL block copolymer bearing POSS (POSS-PCL-b-PLLA) was ad hoc synthesized and added to the PLA70PCL30 blends. 3.1. Characterization of POSS-PCL-b-PLLA The successful synthesis of the PCL-b-PLLA diblock copolymer grafted onto the POSS molecules and its composition have been assessed by means of 1H NMR spectroscopy. Fig. 1 compares the 1H NMR spectrum of POSS-NH2 and that of the POSS-PCL-b-PLLA nanohybrid: the presence of the characteristic CH(2)–O–CO protons of PCL and PLLA can be easily recognized in the spectrum of POSS-PCL-bPLLA (signals d and m respectively), together with the absence of the resonance of the CH2–NH2 protons of POSS-NH2 at 2.6 ppm (signal i) which is shifted to 3.2 ppm (faint signal i0 ). Since this shift, which agrees with the formation of an amide linkage, can be observed also in the spectrum of the POSS-PCL pre-polymer (not shown), the covalent grafting of the PCL chains onto the

silsesquioxanes can be inferred. The PLLA block has then grown from the POSS-PCL chain. The composition of the PCL-b-PLLA diblock copolymer was determined by comparing the resonance of the CH(2)–O–CO protons of PCL and PLLA blocks (signals d and m respectively) with the integrated intensities due to the POSS protons (signals h, and f + g in Fig. 1). The mean degree of polymerization of the PCL and PLLA segments, as estimated by 1H NMR spectra, is 75 and 90 (in L-lactide units) respectively. The 1H NMR data thus confirmed the effective synthesis of a POSS end-capped PCL-b-PLLA diblock copolymer, having two homo-blocks of similar length. The hybrid molecule holds a melting temperature of 160 °C.

3.2. Blend morphological characterization Fig. 2 compares SEM micrographs of the neat PLA/PCL (PLA70PCL30) blend with those of the systems containing POSS. Clearly, the former sample (Fig. 2a) shows a typical seaisland morphology, where the discrete PCL spherical domains, with dimensions between 0.5 and 8 lm (Fig. 2Sa), are dispersed in the PLA matrix. Indeed, as reported in the literature [13], the viscosity of PCL at 180 °C is far lower than that of PLA, which is expected to favor a phase separation. In the case of the blend prepared by adding POSS-oib directly into the melt consisting of the two polymers (PLA70PCL30POSS-oib), no significant differences in the morphology (Fig. 2b), as well as in the PCL domain dimensions (Fig. 5Sb), can be noticed as compared to the neat blend. Moreover, some micrometric POSS aggregates were visible on the fractured surface (Fig. 2b, red1 circles). Conversely, by incorporating the above silsesquioxane into PCL through melt mixing and subsequently blending this PCL/ POSS-oib into PLA (Fig. 6S, PLA70(PCL30POSS-oib)), the dimensions of PCL domains in the final blend are decreased, these ranging between 0.5 and 3 lm (Fig. 5Sc). A refinement of blend morphology has often been observed for different blend systems with different added nanoparticles [14–21]. Several possible explanations have been proposed, including: (i) a reduction of the interfacial energy, (ii) the inhibition of coalescence due to the presence of a solid barrier located at the interface between the two polymers, (iii) the change in the viscosity ratio of the phases due to the uneven distribution of the filler, (iv) the immobilization of the dispersed drops by the creation of a physical network of particles when the concentration of solid is above the percolation threshold and (v) the strong interaction of polymer chains onto the solid particles inducing steric hinderance [38]. However, it is often difficult to demonstrate which effect is indeed controlling the morphology evolution, owing to the difficulties in the experimental measurement of the related parameters, such as the interfacial tension and particle segregation. In the present case, SEM–EDS analyses (Fig. 7Sa) demonstrated that in the sample PLA70(PCL30POSS-oib) POSS is preferentially localized in PCL domains, while in the case 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.

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Fig. 1. 1H NMR characterization of POSS-NH2 and POSS-PCL-b-PLLA.

(a)

(b)

(c)

(d)

10 µm Fig. 2. SEM micrographs of the blends: (a) PLA70PCL30, (b) PLA70PCL30POSS-oib, (c) PLA70PCL30POSS-OH and (d) PLA70PCL30POSS-PCL-b-PLLA.

of the other samples the silicon concentration, related to the presence of POSS, is constant throughout the surface. As an example in Fig. 8S, the EDS spectra of PLA70PCL30POSS-OH is given. The selective localization of POSS can be ascribed

both to the preparation method of the blend and to the difference in the rheological properties of the two polymers. Indeed, as POSS was first dispersed in PCL, hence polymer chains can easily embed POSS and may tend to retain it,

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similarly to previously reported absorption of polymers on multiwall carbon nanotubes [39], also taking into account that no large difference is expected in terms of interfacial tension between PLA/POSS and PCL/POSS. Furthermore, viscosity difference between PCL and PLA may also play a role. Indeed, it is likely that the mobility of the highly viscous PLA is insufficient to allow PLA chains to diffuse around and into POSS aggregates as easily as PCL chains, thus favoring the preferential location of the silsesquioxane in the lower viscosity polymer. It is worth to underline that also in other immiscible blend systems, the selective localization of nanofillers, such as carbon nanotubes (CNTs), was reported when large differences in the viscosities of the two blend components are present [22]. The influence of POSS on the final morphology of the blend may result either from its intrinsic interfacial activity or, more likely, from the increase of the PCL viscosity, which provides a kind of barrier to its coalescence, as mentioned above. Indeed, this hypothesis is supported by rheological measurements, carried out on PCL/POSS-oib system, which demonstrate that the dispersion of the silsesquioxane in the polymer leads to an increase in the polymer viscosity over the whole frequency range (results not shown). The morphology of the blend containing POSS-OH (PLA70PCL30POSS-OH), shown in Fig. 2c, is different from that based on POSS-oib, since PCL domains, which maintain the same distribution of those in the pristine blend, appear to be adherent to the matrix of PLA with a reduced extent of the voids. In order to interpret this phenomenon, it is important to underline that this kind of silsesquioxane – unlike the previously employed one – is completely molten at the processing temperature (Tm = 160 °C) and therefore its distribution/solubilization, in both the blend components, is made easier. Moreover, during cooling the silsesquioxane molecules can locate at the interface between the two polymers thanks to the POSS hydroxyl groups high affinity for both the phases, thus favoring the process of compatibilization. On the other hand, the compatibilization process cannot be considered complete, as holes remain in the composite blend and the PCL domain dimensions seem not to decrease. Fig. 2d shows the micrograph of the blend based on POSS carrying the block copolymer (POSS-PCL-b-PLLA). The morphology of the sample PLA70PCL30POSS-PCL-bPLLA completely changes with respect to those previously described. The loading of POSS-PCL-b-PLLA induces a fine miscibility of the blend components, thus giving rise to an almost homogenous microstructure. Indeed, it was reported that well-defined copolymers, whose chemical nature is identical to that of the main components, can act as emulsifying agents, reducing the coalescence effect by lowering the interfacial tension between the two components and finally leading to a well-dispersed morphology. However, it is almost impossible for all the added copolymer molecules to reach the interface during melt blending. Therefore, in most cases, a number of copolymer molecules preferentially form micelles in the immiscible polymers instead than locating at their interfaces [40]. Thus, in order to promote the emulsifying effect of the copolymers, it is necessary to add relevant amounts of

these molecules in the blends (from 2 to 20 wt.%) [10– 13], increasing the production costs and the risk of reducing the properties of the final material. It is of utmost relevance that, conversely to the PLA-PCL diblock classically employed as a compatibilizing agent of PLA/PCL blends, the use of the ad hoc synthesized nanohybrid allowed us to significantly reduce the amount of added copolymer, though significantly improving the morphology of the blend. In order to explain this peculiar behavior, the specific structure of the hybrid POSS-ended copolymer has to be taken into account. Indeed, the presence of the bulky silica cage of the silsesquioxane, directly linked to the macromolecular chain, may limit the aggregation of the copolymer, thus maximizing its effect as a compatibilizer on the interfacial tension of the blend components. 3.3. Blend thermal characterization The thermal properties of the prepared blends were studied by means of DSC. In Figs. 3 and 4, DSC traces, taken from the second heating and from cooling, respectively, for neat PLA/PCL and PLA/PCL/POSS blends are shown. DSC trace of PLA70PCL30 exhibits two independent melting peaks, the first one corresponding to PCL (at ca. 56 °C), the second one to PLA (at ca. 150 °C) (Fig. 3a). Indeed, differences in DSC traces of blend based on POSS are clearly observed, thus demonstrating that the composite systems hold distinctive behaviors. In the case of the blend containing POSS-oib (Fig. 3b), no significant differences with respect to the neat PLA/PCL blend have been observed. This finding supports the morphological characterization, which demonstrated a scarce influence of the silsesquioxane on the blend morphology. Conversely, the blend prepared by adding the binary system PCL/POSS-oib to PLA shows just small changes in the crystallization and melting of the PLA component, whereas significant differences are observed for PCL melting. Indeed, the presence of POSS seems to limit the crystallization of PCL (Fig. 4c), indirectly supporting the preferential localization of the silsesquioxane in this phase. The specific effect of POSS molecules on polymer crystallization was (a) (b)

Endo up

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(e)

30

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110

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Temperature (°C) Fig. 3. DSC traces of: (a) PLA70PCL30, (b) PLA70PCL30POSS-oib, (c) PLA70 (PCL30POSS-oib), (d) PLA70PCL30POSS-OH and (e) PLA70PCL30 POSS-PCL-bPLLA (from second heating cycle).

O. Monticelli et al. / European Polymer Journal 58 (2014) 69–78 (a) (b)

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180

150

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90

60

30

0

Temperature (°C) Fig. 4. DSC traces of: (a) PLA70PCL30, (b) PLA70PCL30POSS-oib, (c) PLA70 (PCL30POSS-oib), (d) PLA70PCL30POSS-OH and (e) PLA70PCL30 POSS-PCL-bPLLA (from cooling cycle).

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heating, may be related to a preferential localization in the PLA phase of the silsesquioxane, which is only partially at the interface of the discrete PCL domains, thus corroborating the morphological characterization. The DSC traces of blend based on POSS-PCL-b-PLLA, shown in Figs. 3e and 4e, present some modification in the PCL part. Indeed, the crystallinity of PCL significantly increases upon incorporation of POSS-PCL-b-PLLA, the total melting enthalpy increasing from 29 J/g in the neat blend to 50 J/g in the blend PLA70PCL30POSS-PCL-b-PLLA. We interpreted this finding in shed of the light of the peculiar structure of POSS-PCL-b-PLLA, whose siliceous cages, being linked to the diblock polymer chains, cannot homogenously disperse in the PCL microdomains, but are forced to form a sort of layer at their interface, thus being able to act as nucleating agents.

Fig. 5 shows the evolution of storage modulus (G0 ) as a function of frequency for the neat PLA/PCL blend and for the composite blend based on POSS-OH, POSS-oib and POSS-PCL-b-PLLA. For the sake of completeness, in Fig. 9S the curves of storage modulus (G0 ) versus loss modulus (G00 ) of the above samples are shown. The presence of POSS-oib, directly added in the blend (PLA70PCL30POSSoib), seems not to modify G0 of the mixture (results not shown). The shoulder presented on the modulus curve of all the blend samples can be attributed to the shape relaxation of the discrete PCL phases in the PLA matrix. Indeed, during small amplitude oscillatory shear tests, the total area of the interface and the interfacial energy are changing periodically with much longer relaxation, caused by the presence of interface, which leads to an additional transition on the modulus curves at low frequency region. Remarkably, the addition of the silsesquixanes to the blend leads to an increase of the low-frequency modulus, this effect being particular noticeable in the case of the introduction of POSS-PCL-b-PLLA. This phenomenon indicates that the blend based on POSS-PCL-b-PLLA shows higher interfacial elasticity than that of the neat blend

100000

G' (Pa)

10000 1000 100 10 1 0.01

0.1

1

10

Frequency (rad/sec) Fig. 5. Storage modulus (G0 ) versus frequency at 180 °C for PLA70PCL30 (), PLA70PCL30POSS-OH (j), PLA70(PCL30POSS-oib) (N) and PLA70PCL30POSS-PCL-bPLLA ().

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3.4. Rheological characterization widely investigated and shown to depend on various parameters, such as the nature of the silsesquioxane substituents and the level of POSS dispersion in the polymer matrix. In general, the presence of POSS as a pendant group of the backbone chain is likely to hinder the crystallization process [29]. Similarly, it was observed that also the simple physical dispersion of POSS into the polymer matrix can lead to a partial restrain of the polymer structuring. In particular, a reduction of the degree of crystallinity in a composite system based on PCL and POSS was observed by Lee and Chang [41], who interpreted this behavior by taking into account both the nanometric dispersion of the silsesquioxane and the hydrogen-bonding interactions occurring between the silanol funtionalities of POSS and the polymer chains. Based on the evidence that in our system the silsesquoxane causes a significant decrease of the PCL crystallinity, it is possible to hypothesize, this is due to the submicrometer dispersion and its relatively high concentration in the PCL domains. DSC traces of the blend containing POSS-OH is shown in Figs. 3d and 4d. In this case, no significant changes in the melting and crystallization peaks of PCL is observed, but in that of PLA. The reduced crystallization of PLA during

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25

30

Strain (%) Fig. 6. Stress–strain curves of the blends: (+) PLA70PCL30, (––) PLA70 (PCL30POSS-oib), (h) PLA70PCL30POSS-oib, () PLA70PCL30POSS-PCL-bPLLA.

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due to reduced domain size. Indeed, the emulsification caused by POSS seems to change the interfacial structure.

3.5. Blend mechanical properties Stress–strain curves of the blends are shown in Fig. 6, while in Table 2 the mechanical properties of the various samples are summarized. PLA is well known to be a glassy polymer at room temperature, showing a relatively high modulus (1200 MPa) and maximum strength (65 MPa) associated with a low deformation at break, namely about 9%. On the other hand, PCL has significantly lower stiffness and resistance, but exhibits a very high elongation at break (about 1200%). Thus, the mechanical properties of the blend are expected in an average between the two polymers, reflecting the volume ratio of the two phases, the morphology of the blend and the quality of the interfaces. Indeed, both modulus and strength for PLA70PCL30 are intermediate between those of PLA and PCL. On the other hand, elongation at break is not significantly changed as compared with PLA, evidencing no toughening due to the presence of PCL and thus confirming very low interaction at the PLA/PCL interface. Limited or no increases are observed in modulus and strength in the presence of the different POSS. However, comparing PLA70PCL30POSS-oib to PLA70(PCL30POSS-oib), it seems that the elastic modulus of the latter is slightly

Table 2 Mechanical measurement results. Sample code

E (MPa)

rmax

ebreak (%)

(MPa) PLA PCL PLA70PCL30 PLA70PCL30POSS-oib PLA70(PCL30POSS-oib) PLA70PCL30POSS-OH PLA70PCL30 POSS-PCL-bPLLA

1200 ± 40 220 ± 10 770 ± 20 820 ± 60 700 ± 40 830 ± 10 770 ± 10

65 ± 2 20 ± 8 38 ± 2 39 ± 3 34 ± 5 22 ± 5 36 ± 1

9±1 1200 ± 400 9±4 10 ± 2 7±1 6±1 23 ± 4

lower. This is likely reflecting the difference in morphology previously described; indeed, when POSS is dispersed as microparticles in the continuous phase (or in both phases), a weak reinforcement can be expected [42]. On the other hand, as in PLA70(PCL30POSS-oib) POSS is preferentially located in the separate phase (PCL) and no good interaction is obtained at the PLA/PCL interface, no significant reinforcement of the blend is expectable. Furthermore, a very interesting behavior was observed for PLA70PCL30POSSPCL-b-PLLA, in terms of about 150% increased elongation at break, corresponding to about 125% increase in toughness. It is worth to underline that the application of other kinds of nanofillers, such as multi walled carbon nanotubes (MWCNTs) [22] and talc [27], whose addition induces from one part conductive properties, in the case of blends based on MWCNTs and barrier properties for those containing talc, from the other it is accompanied by a decrease of the elongation at break. Indeed, in our system the dramatic enhancement of elongation at break is certainly related to a radical improvement of the compatibility of the two phases, which is further evidencing the effectiveness of POSS-PCL-b-PLLA as a compabilizer between PLA and PCL.

4. Conclusions This work has demonstrated the potentialities of POSS as novel compatibilizing system for immiscible blends, opening the way to an innovative exploitation of these hybrid molecules in the field of polymer materials. Indeed, the specific effect of POSS on the features of blends has been evidenced by studying a system of great application interest, consisting of poly(lactide) (PLA) (70 wt.%) and poly(e-caprolactone) (PCL) (30 wt.%). By investigating different kinds of silsesquioxanes, it was proved that the properly choice of the functional groups attached to the siliceous cage allow to modify the blend microstructures. While a POSS without specific functionalities allowed to decrease PCL domains, probably because of the effect of the above silsesquioxane on the system viscosity ratio, another one characterized by hydroxyl groups increased the adhesion between the polymer phases. The exploitation of an ad hoc synthesized POSS, carrying a poly(e-caprolactone)-b-poly(L-lactide) diblock copolymer, allowed to further improve the blend morphology giving rise to an almost homogenous microstructure. It is of utmost relevance that, conversely to the classical PLA-PCL diblock, applied as compatibilizing agents for PLA/PCL blends, the use of the synthesized hybrid molecule allowed to significantly reduce the amount of added copolymer, which is directly attached to the silicon cage, though significantly improving the morphology of the blend. This peculiar behavior connected to the use of POSS was ascribed to the presence of the bulky silica cage of the silsesquioxane, which, being directly linked to the macromolecular chain, might limit the aggregation of the copolymer, thus maximizing the effect of the compatibilizer on the interfacial tension of the blend components. The resultant co-continous phase morphology promoted at the interface by the presence of the above POSS was found to enhance the mechanical properties of the blend, improving the elongation at break without reducing its Young’s modulus.

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

Appendix A. Supplementary material Structure of POSS-oib and POSS-OH, PCL domain distribution in the blends, SEM micrograph of the blend PLA70PCL30POSS-oib, Si dispersion by SEM–EDS analysis of PLA70(PCL30POSS-oib), g* as a function of time for PCL and PCL containing POSS-oib. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2014.06.021.

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