Selective Localization of Nanoparticles to Enhance the Properties of PBS/PLA Nanocomposite Blown Films

The addition of nanoparticles could allow to upgrade the performances of biopolymers, making them fit to replace petroleum-based plastics. In order to maximize the sustainability benefits of bio-nanocomposites, it is fundamental to opportunely tune their bulk properties by acting on their nanostructure. In particular, in nanocomposite blends the final morphologies are highly dependent on the selective localization of the nanoparticles: mainly inside one of the polymeric phases and/or at the interface. In this work, nanocomposite PBS (polybutylene succinate)/PLA (polylactic acid) blown films were prepared with the aim of optimizing their mechanical and gas barrier properties by profiting from the multiple functions of nanofillers as reinforcements, compatibilizers and morphology-directors. PBS/PLA blends, at a constant polymers’ weight proportion (80/20), were prepared by twin-screw extrusion, adding a constant amount of a lamellar nanosilicate (Cloisite 30B), according to different mixing routes: (i) by first dispersing the nanofillers inside the PLA or PBS phase, followed by the compounding with the other resin; (ii) by direct mixing of all the three components. The nanocomposite systems showed a finer dispersion of the PLA phase compared to the neat PBS/PLA blend. While a good exfoliation degree of the nanosilicate was generally observed for all the hybrid samples, the different blending sequence of the nanofillers significantly affected their preferential localization. In particular, the barrier and mechanical performances of the hybrid film PBS/(PLA + C30B) were most positively affected by the selective localization of the nanoplatelets at PBS/PLA interface, since their compatibilizing action was effectively exploited. In fact, this latter sample exhibited an interesting increase (+ 29%) of the deformation at break and a significant improvement (+ 33%) of the oxygen barrier compared with the neat PBS/PLA blend.


Introduction
The negative environmental impact of petroleum-based polymers [1,2] has led to a growing interest in the use of bio-based and biodegradable plastics.However, to increase the use of these materials several problems still need to be solved.In fact, not only the production costs are higher than traditional plastics, but technological limits, in terms of processability and performance of bioplastics compared to conventional polymers, have not been completely overcome to date.
Based on the forecast to 2026 [3], the polylactic acid (PLA) and the polybutylene succinate (PBS) will be among the most requested bioplastics on the market, and PBS world production capacity will exceed that of PLA.These two polymers show complementary properties [4][5][6][7][8][9][10]: PLA is stiff and brittle, while PBS is tough and ductile, furthermore this latter displays a higher heat resistance, while the PLA is more transparent.Thus, blending these two biodegradable polymers could allow to expand their application fields.
In this regard, it is important to highlight that the flow and solid-state properties of polymer blends depend not only on the characteristics and relative percentage of the components, but also on the blend morphology and on the interfacial adhesion between the polymeric phases [11].These features are, in turn, strongly affected by the compatibility degree of the blend.
In particular, since the PBS/PLA system is immiscible, several authors have studied its compatibilization by different approaches which mainly consist of: the use of block copolymers [12,13] or chain extenders during a reactive extrusion [14,15], and by adding nanofillers [16][17][18][19][20][21][22][23].This latter strategy has interesting potentialities, as nanofillers show a lower specificity than traditional compatibilizers, which very often have to be prepared ad hoc for the polymer blend under consideration.Moreover, from an economic point of view, nanoparticles are generally less expensive than block copolymers.On the other hand, the nanofillers increase the mechanical and barrier performances of the corresponding nanocomposite systems [24,25].However, as it can be deduced from the studies reported in the literature on nanocomposite polymer blends, based on both traditional [26][27][28][29] and biodegradable [21,[30][31][32][33][34][35][36] materials, the final morphologies of these hybrid systems are highly dependent on the localization of the nanoparticles which, during the melt compounding process, can arrange inside one of the polymeric phases and/or at the interface.In this context, besides thermodynamics, in terms of the interfacial tensions between each polymer of the blend and the nanofillers, also kinetic parameters, such as the mixing time and the blending sequence, play a key role in determining the preferential localization of the nanoparticles [21,37,38] and, in turn, their function in blend's morphology refinement and stabilization, as well as compatibilizers [20, 21, 26-28, 30, 39, 40].
The experimental activity, carried out in this work, concerned the preparation of PBS/PLA blends by twinscrew extrusion followed by film blowing process.Blends composition was based on 80% by weight of PBS, since in several studies [5-7, 10, 41] a synergistic effect was discovered at this resins' proportion.Moreover, in order to obtain a fine dispersion of the minor phase inside the blend, the selection of the neat resins was based on their peculiar viscoelastic behavior.
Then, an organically modified lamellar nanosilicate (Cloisite 30B) was added to the PBS/PLA system, following different mixing routes: (a) by first dispersing the nanofillers inside the PLA or PBS phase and, then, mixing the obtained nanocomposite system with the other resin through two successive steps in the twin screw extruder, or (b) by mixing the two polymer phases and the silicate in a single step in the extruder.The neat and nanocomposite blends were, then, processed by means of a laboratory-scale film blowing apparatus, and the obtained films were characterized in terms of mechanical and oxygen barrier properties.
The aim of the work was to evaluate the effect of the blending sequence on the nanoparticles' dispersion and their selective localization, as well as on the polymer phases' morphology.Moreover, the correlations between the structure of the hybrid PBS/PLA systems and their bulk properties (rheological, mechanical and oxygen barrier) were deeply investigated.

Materials and Preparation of PBS/PLA Nanocomposite Blown Films
PBS/PLA nanocomposite blends, at a constant polymers' weight proportion (80/20) and nanofiller amount (2 wt% on the whole blend), were prepared using as polymeric phases a bio-based and biodegradable PBS grade (Bio PBS™ FZ91 by Mitsubishi Chemical) and a PLA grade at 4 wt% of isomer D (PLA-L96HH by Sulzer Chemtech), whose main characteristics are reported in Table 1.
The compounding was conducted using a twin-screw extruder (Dr.Collin GmbH -ZK 25-48D) with co-rotating intermeshing screws (D screw =25 mm, L/D = 42).A screw speed of 150 rpm and a temperature profile of 150-165-165-165-165-165-165-165 °C (from hopper to die) were used.Prior to processing, the materials (both the polymers and the nanoclay) were dried in a vacuum oven at 80 °C for 18 h, to prevent polymer degradation and bubble formation during extrusion.The nanocomposite PBS/PLA blends were prepared according to different mixing sequences, as schematized in Fig. 1.In one case the two polymers and the silicate were mixed in a single step in the extruder; in the other two cases the nanofillers were dispersed first inside the PLA or the PBS phase and then the obtained nanocomposite system was blended with the other resin through two successive steps in the twin screw extruder.The amounts of C30B added inside the PLA or PBS phase were chosen so that the weight% of the silicate, calculated on the whole blend, was 2% in all the specimens.
The strand of the extruded material was cooled in a water bath, at approx.15 °C, and a strand pelletizer (Dr.Collin GmbH CSG 171/2) allowed to produce pellets of suitable size for the characterization and further processing.
Blown films were prepared using a Gimac single-screw extruder (L/D = 24, D screw = 12 mm), equipped with an annular die (Dr.Collin GmbH, Munich -Germany) with D in = 30 mm, D ex = 32 mm.Screw speed was set at 35 rpm and the temperature profile was 190-175-160-150 °C (from hopper to die).The air pressure inside the tubular film and the draw up speed of the nip rolls were fixed in order to obtain stable conditions, which were achieved at a blow-up ratio (BUR) and draw ratio (DR) of 1.6 and 32 respectively.The films, collected under these conditions, had a thickness of about 22 ± 1.1 μm.

Characterization Techniques
The amount of silicate into each extruded nanocomposite system was measured by drying each sample at 100 °C for 18 h under vacuum and weighting it before and after placing in a furnace at 900 °C for 45 min in air.Each determination was repeated on five specimens to obtain statistical silicate loading values.The differences between the effective and the nominal loading levels resulted always lower than 10%.
Rheological experiments were conducted with a Rheometric Scientific rotational rheometer, ARES (Rheometric Scientific, New Castle, Delaware, USA) under a nitrogen atmosphere and after drying the samples at 80 °C for 18 h in a vacuum oven in order to avoid moisture induced degradation phenomena.Rheological measurements were carried out in oscillatory at 190 °C from 0.1 to 100 rad/s, using parallel plates with 25 mm diameter.To perform the dynamic tests within the linear viscoelastic region, a strain amplitude of 5% of was set.
FTIR measurements were conducted in ATR mode on the neat materials in the range of 4000-650 cm − 1 , using a Nexus ThermoNicolet spectrometer (Thermo Scientific, Waltham Massachusetts, USA).The spectra were collected with a resolution of 2 cm − 1 , co-adding 64 scans.
The scanning electron microscopy (SEM) analysis was carried out by means of a Zeiss EVO MA10 microscope (Carl Zeiss SMT AG, München-Hallbergmoos, Germany).The specimens were cryo-fractured in liquid nitrogen and PLA domains were etched with tetrahydrofuran at 50 °C [7], so to be able to observe the blends' morphology.In order to capture the images, the samples were sputter-coated with a 200-440 Å thick gold layer, using a Leica EMSCD005 metallization device.
Transmission electron microscopy (TEM) analysis of the blown films was conducted using a Philips EM 208 microscope (Philips, Eindhoven, The Netherlands) at voltage 80 kV and maximum resolution 1 nm, with different Conversely, the η* curve of PLA exhibits a larger Newtonian plateau, followed by a less pronounced shear-thinning behavior compared with PBS.Moreover, this latter has complex viscosity values higher than PLA in almost the whole frequency range analyzed, in fact, only for values of ω close to 100 rad/s the η* plots of the two resins overlap, resulting η* PLA /η* PBS ≅1.
From the comparison of the storage moduli G' of PBS and PLA (Fig. 2b), it can be observed that the polybutylene succinate has a more pronounced elastic behavior than the polylactic acid.
Regarding the complex viscosity of PBS/PLA blend (Fig. 2a), it shows values higher than both the neat resins in the whole frequency range analyzed, with greater deviations at low ω.In other words, the viscosity of the blend was significantly higher than the linear average of the components' viscosities, evidencing a good distribution/dispersion of PLA phase inside the PBS matrix.Another indirect information on the blend morphology, can be inferred from the graph of the storage modulus.At low frequencies, the G' curve of PBS/PLA blend shows a sort of "knee" (as highlighted by the arrow in Fig. 2b), while at higher frequencies it almost overlaps the PBS plot.This trend can be attributed to the elasticity effect due to interfacial tension, that is less evident at higher frequencies, at which the elasticity of the polymer dominates.The more marked this sort of "knee" is, the greater the elastic contribution due to the interface between the polymeric phases and, therefore, the finer the dispersion of the minor component.
In agreement with the rheological results, from the micrograph in Fig. 2c it is possible to appreciate the good dispersion of the PLA droplets inside the PBS matrix.
In order to investigate the extent of compatibility between PBS and PLA, FTIR analysis was carried out and the spectra for the blend PBS/PLA and its component resins are shown in Fig. 3.In Table 2 the main absorption bands are correlated with the vibrations of the characteristic functional groups of PBS and PLA [10].
In the spectrum of the blend, the absorption bands of both PBS and PLA can be observed.However, by analyzing in detail the peaks corresponding to the stretching of the carbonyl C = O (Fig. 3a), variations in the intensity of such bands are evident for the blend compared with the component resins.
In particular, the PBS spectrum show three absorption bands assigned to the stretching of the C = O group, which correspond to different structures of the resin: the band at 1735 cm − 1 is representative of the amorphous phase, the peak at 1720 cm − 1 of the oriented amorphous phase and the one at 1712 cm − 1 corresponds to the crystalline phase [10].In particular, the peak at 1712 cm − 1 is very pronounced, while the bands at 1720 cm − 1 and at 1735 cm − 1 are hardly magnification levels.The images were captured on sections located normal to the machine direction and prepared by cutting ultra-thin specimens with a Leica Utracut UCT microtome (Leica Microsystems GmbH, Vienna, Austria).
Oxygen permeability of the films was measured with a GTT permeability device (Brugger Feinmechanik GmbH, Munich) at 30 °C and RH = 10%, according to ASTM D1434 procedure.
Mechanical tests were carried out, following ASTM standard D882, by a CMT4000 Series dynamometer (SANS, Shenzhen, China), equipped with a load cell of 100 N. Rectangular samples were cut from the blown films along the machine direction and were tested with a grip distance of 30 mm and a crosshead speed of 3 mm/min to measure the Young's modulus and 300 mm/min to determine the stress and strain at break.At least ten specimens for each type of film were tested to assess the reproducibility of the results.

Characterization of the Neat Resins and PBS/PLA Blend
A PBS/PLA blend was prepared by twin-screw extrusion, using a weight ratio of the two resins constant and equal to 80/20, since according to previous results reported in literature [7], at this relative percentage, PBS and PLA show compatibility to some extent.The blend behavior in the molten state was analyzed by means of shear rheological tests in dynamic mode.Moreover, the blend's morphology was observed by a scanning electron microscope and infrared spectroscopic analysis.
It is worthy to point out that, in order to obtain a fine dispersion of the minor phase in the blend during the melt compounding process, the selection of the grade for each component resin has to be conducted taking into account their peculiar viscoelastic behavior.In particular, since the breaking of the dispersed phase into particles of smaller size is difficult or even impossible when its viscosity (η* d ) is much lower or greater than that of the matrix (η* m ) [11], the resins for the blend were selected so that their viscosity ratio η* d /η* m was close to 1, at the highest strain rates occurring during the processing [11].Furthermore, the chosen polymer matrix should be highly elastic, so that the coalescence of the dispersed particles can be strongly inhibited [11].
The graphs of the complex viscosity (η*) and storage modulus (G') for PBS/PLA blend are compared in Fig. 2a-b with the corresponding neat polymers.
The complex viscosity curve of PBS shows a shearthinning trend in most of the frequency (ω) range investigated, with a short Newtonian plateau at low frequencies.cooling thermograms for these samples are reported in Fig. 4a-b, respectively.
The PBS/PLA sample exhibits the melting peaks of both PBS and PLA, at the same temperatures (T m, PBS = 115 °C and T m, PLA = 155 °C) as the neat components.On the other hand, a significant reduction in the cold crystallization temperature (T cc, mix = 92 °C) of the PLA inside the blend is observed compared with the pure polymer (T cc, PLA = 112 °C), highlighting a plasticizing effect of the polybutylene succinate on PLA, in agreement with previous results reported in literature by other authors [5].
The calculation of the crystallinity degrees, based on the heating enthalpies of PBS and PLA inside the blend, would inevitably suffer from errors, since the PLA cold crystallization peak overlaps, to a certain extent, with the PBS melting peak (Fig. 4a).Therefore, the determination of the PBS crystallinity degree, when mixed with PLA, was carried out using the crystallization enthalpies from the cooling thermograms.With regard the PLA phase, it did not show any peak during the cooling scans, (Fig. 4b), since visible.On the other hand, PLA shows a single peak at 1746 cm − 1 , assigned to the stretching of the carbonyl and characteristic of the crystalline phase of the resin [10].
Looking at the wavenumbers' range, corresponding to the stretching of the C = O group for the PBS/PLA blend, the same peaks as the neat PBS can be observed: with the same intensity at 1712 cm − 1 , while the one at 1720 cm − 1 is more pronounced.Furthermore, another absorption band appears at 1756 cm − 1 , associated with the vibration of PLA's carbonyl and characteristic of the amorphous phase of this polymer.Thus, a slight interaction between PBS and PLA inside the blend is highlighted by the increase in the respective amorphous phases, suggesting that, at the selected weight proportion of the two resins, compatibility exists to some extent, so promoting a fine dispersion of the polylactide phase inside the PBS matrix.
The PBS/PLA blend and its components were also submitted to thermal characterization by differential scanning calorimetry (DSC) and, in particular, the first heating and

PBS/PLA Nanocomposite Systems: Effect of the Nanoparticles and their Selective Localization on Rheological Properties and Blends' Morphology
The nanocomposite PBS/PLA/C30B systems were prepared following different routes for the addition of the nanofiller (Fig. 1); the weight% of C30B, calculated on the whole blend, was about 2% in all the specimens.
The complex viscosity (Fig. 5a) and storage modulus plots (Fig. 5b) of the different nanocomposite systems are compared with the corresponding graphs for the neat PBS/ PLA blend.
All the nanocomposite systems show an increase of the viscoelastic parameters with respect to the unfilled blend, with more significant deviations of η* and G' values in the low frequency range.In particular, it is interesting to highlight that, even if the silicate content is similar (about 2 wt%), the rheological response of the nanocomposite hybrids respect to the neat blend is significantly different.More specifically, the (PBS + C30B)/PLA and (PBS/ PLA) + C30B samples show the most pronounced increases of η* and G'.In low frequencies region, the hybrid systems, also, exhibit a reduction of the G' slope compared to the unfilled blend (Table 3), indicating an increased solid-like behavior.
This rheological behavior must be correlated with both the nanofillers' distribution/dispersion degree and their it is characterized by a slow melt crystallization kinetics in non-isothermal conditions.However, the presence of the PLA phase inside the blend affects the PBS crystallization kinetics, in fact a decrease in both T c and the crystallinity degree can be observed.In particular, these values change from 94 °C to 52%, respectively, for the neat PBS, to 90 °C and 46% for the PBS phase within the blend, in agreement with the outcomes of the spectroscopic analysis (Fig. 3). the neat one, as it can be deduced from the SEM images in Fig. 6.
The PLA domains, of lighter color and ellipsoidal shape, can be also clearly distinguished from TEM images (Fig. 7) and result uniformly distributed inside the PBS matrix.In particular, PLA droplets dimensions are similar (160 ± 20 nm) for the PBS/(PLA + C30B) and (PBS/PLA) + C30B systems, while, in the case of the (PBS + C30B)/PLA sample, PLA domains show a larger mean size (240 ± 40 nm).Regarding the nanostructure of the hybrid samples, the different mixing routes influenced the preferential silicate lamellae localization inside the blends, as well as the nanofiller dispersion degree.Some recent papers [16,42] reported that the presence of the nanoparticles (C30B) at interface between the polymeric phases of a PBS/PLA blend is favored from a thermodynamic point of view, as predicted by the value of the wetting coefficient, that resulted equal preferential localization (in one polymeric phase and/or at the interface) within the blend, as well as the morphology of the polymeric phases.To this purpose, SEM and TEM analyses were carried out on both the neat and nanocomposite blends.
In particular, the presence of the silicate nanoplatelets inside the PBS/PLA blend determines a reduction in the PLA domains' size for all the hybrid systems respect to

Barrier and Mechanical Properties of the Blown Films
The neat and nanocomposite PBS/PLA blends were extruded by film blowing using a laboratory-scale apparatus and, for comparison purposes, films based on the neat PBS matrix were also prepared.The films were characterized by oxygen permeability tests, conducted at T = 23 °C and RH = 10%.
The results, expressed as permeability coefficient PO 2 are shown in Fig. 8.No significant differences can be observed between the oxygen barrier performance of the PBS film compared with the PBS/PLA blend.Regarding the nanocomposite PBS/ PLA blends with about 2% by weight of C30B, the permeability coefficient values differ significantly depending on the filler mixing sequence.In particular, the film (PBS/ PLA) + C30B shows a slightly lower permeability coefficient (-15%) respect to the one of the neat blend.On the other hand, the film (PBS + C30B)/PLA, shows the worst barrier properties, conversely to the sample PBS/(PLA + C30B).In fact, this latter exhibits the lowest oxygen permeability value than all the analyzed nanocomposite systems, with an increase in the barrier performance of about 33% compared to the unfilled PBS/PLA film.
The main factors that can affect the barrier properties of the hybrid films concern not only the blend morphology and the nanostructure obtained, but also the miscibility degree of the resins in the blend.In particular, in a recent study Scholes [43] demonstrated that the theoretical models, generally used to predict the permeability of immiscible polymer blends, underestimate the experimental results as they are based on the permeability values of the single bulk resins to ω a = 0.37 [16].TEM micrographs of all the nanocomposite systems (Fig. 7) confirm the presence of the silicate lamellae at the interface of the PLA domains, although clear differences can be observed depending on the filler mixing sequence.In particular, when the nanoparticles are previously melt compounded with the PLA phase (Fig. 6a, a'), during the subsequent mixing of this nanocomposite with the PBS, the silicate platelets migrate at the interface of the PLA droplets, completely covering this surface and forming intercalated structures in some areas.
On the other hand, with regard to the systems in which the cloisite is added to the PBS phase (Fig. 7b, b') or blended simultaneously with the two polymeric phases (Fig. 7c, c'), the silicate nanoparticles can be observed both at PBS/PLA interface and within the PBS matrix.In particular, the TEM images at higher magnification, clearly show that in the case of the (PBS + C30B)/PLA sample (Fig. 7b'), larger portions of the interfacial region are free from the nanofillers, compared to the (PBS/PLA) + C30B system (Fig. 7c').
On the other hand, a better distribution and dispersion degree of the nanofillers within the (PBS + C30B)/PLA and (PBS/PLA) + C30B systems can be observed compared to the PBS/(PLA + C30B) sample.These findings coherently correlate with the results of the dynamic rheological analysis (Fig. 4) that highlights for the first two nanocomposite systems, i.e. (PBS + C30B)/PLA and (PBS/PLA) + C30B, a higher increase of the viscoelastic parameters η* and G' in the low frequency region compared with the PBS/ (PLA + C30B) sample at the same nanofiller content.since the PBS/PLA 80/20 film is much stiffer than the neat PBS matrix, but as ductile as this latter.The good balance in the mechanical performances for the PBS/PLA blend can be attributed to the fine dispersion of the PLA phase inside the PBS matrix, as evidenced by the rheological results and the SEM image of the blend (Fig. 2).
The addition of 2wt% of cloisite in the PBS/PLA blend, according to the three mixing routes analyzed, differently affected the tensile mechanical behavior of the system, depending on the obtained polymer phases' morphologies and nanostructures, as already highlighted for the oxygen barrier properties of the same films.In particular, while there are no significant differences in the stiffness data between the neat PBS/PLA blend and the corresponding nanocomposite systems, the tensile properties at break of these latter depend on the filler addition sequence.Specifically, the film PBS/(PLA + C30B) shows the best performances: in fact, interesting increments in the mechanical properties at break are observed (+ 29% for the strain at break and + 21% for the stress at break) compared with the unfilled mixture.As for the barrier properties, also this result can be coherently attributed to the compatibilizing function of the nanofillers, since in this system they are preferentially located at interface between the two polymeric phases (Fig. 7a, a').This, in fact, facilitates the stress transfer through the PBS/PLA interface with consequent improvement of the mechanical ultimate properties of the blend.
For the systems (PBS + C30B)/PLA and (PBS/ PLA) + C30B, the silicate lamellae only partially cover the interfaces between the PLA domains and the PBS matrix (Fig. 7b, b'-c, c') and this affects the ductility of such samples.In particular, in perfect agreement with the oxygen permeability data, the film (PBS + C30B)/PLA shows a slight reduction (-13%) of the deformation at break compared with the neat blend, while the system (PBS/PLA) + C30B exhibits a small increase (+ 13%) of the same mechanical property.
Finally, it is worthy to point out that not only the nanofillers' dispersion degree and the preferential localization inside the blend, but also their influence on the crystallization behavior of the polymeric phases may play a key role on the films' barrier and mechanical properties.
To this purpose, DSC analysis was conducted on neat and nanocomposite PBS/PLA films.In particular, in Fig. 9 the thermograms relative to the first heating and cooling are reported.Moreover, in Table 5 the crystallization temperatures and enthalpies, as well as the crystallinity degrees for the nanocomposite blends are compared with the corresponding thermal parameters of the neat PBS/PLA blend.
These results did not evidence significant differences in the main thermal parameters of all the nanocomposites films compared with the neat PBS/PLA blend, further and do not consider the real morphology of the mixture.This latter, in fact, is "less compact" than bulk polymers due to the presence of a more or less extensive interfacial region between the polymeric components, which might significantly contribute to the free volume available for the passage of gases.
Based on the phase morphology and nanostructure obtained for each hybrid film, as evidenced by TEM images of the samples (Fig. 7), it is possible to coherently interpret their oxygen permeability data.In particular, the best barrier performance of PBS/(PLA + C30B) film can be attributed to the good dispersion of the PLA phase (whose mean dimension is ca.160 nm) and, above all, to the preferential localization of the nanofillers at PBS/PLA interface.In this case the compatibilizing action of the nanoparticles can be effectively exploited through the intercalation of the polymer chains, both of PBS and PLA, inside the gallery of the silicate lamellae [17].As a consequence, a reduction in the volume of micro-voids at the interface occurred, with a significant improvement in the barrier properties of the system.The system (PBS/PLA) + C30B, whose permeability is intermediate among the values of the other hybrid films, shows a dispersion degree of the PLA phase (Fig. 7a,  c) similar to the one of the sample PBS/(PLA + C30B), but the interface is only partially covered by the nanoparticles.Finally, the specimen (PBS + C30B)/PLA evidences the worst barrier performance since a greater dimension of the dispersed phase (ca.240 nm) can be observed and, moreover, larger portions of the interfacial region are free from the nanofillers respect to the system (PBS/PLA) + C30B.
The mechanical characterization of the films was carried out by tensile tests and Table 4 reports the main tensile properties of the produced films for comparison.
The PBS/PLA blend shows a pronounced increase in the elastic modulus (+ 90%), without significant variations in the values of the elongation at break compared with the neat PBS.Therefore, the addition of 20 wt% of PLA to PBS produced a marked improvement of the mechanical behavior particular, when the nanofillers were first added inside the PLA phase, they essentially locate at PBS/PLA interface.The peculiar characteristics of the produced hybrid blends, in terms of phase morphology and nanostructure, influenced in a different extent their rheological, barrier and mechanical properties.In particular, the rheological behavior of the nanocomposite systems resulted mostly affected by the dispersion degree of both the minor PLA phase and the nanofillers.In fact, the sample (PBS/PLA) + C30B showed the highest values of the viscoelastic properties (η* and G'), since it was characterized by a finer dispersion of PLA droplets and a better silicate exfoliation degree, compared to the other hybrid systems.Conversely, the gas permeability and the tensile strength mainly depend on the interfacial characteristics of the nanocomposite blends.Thus, the sample PBS/(PLA + C30B) showed the best oxygen barrier (+ 33%) and mechanical properties (+ 29% for the strain at break and + 21% for the stress at break) compared to the unfilled PBS/ PLA film, since the nanoparticles preferentially located at interface, so effectively exploiting their compatibilization action.
article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http://creativecommons. org/licenses/by/4.0/.

Fig. 1
Fig. 1 Scheme for the preparation of nanocomposite PBS/PLA blends according to different mixing routes

Fig. 2
Fig. 2 Comparison of the viscoelastic parameters plots for the blend PBS/PLA with the corresponding neat polymers: (a) complex viscosity curves; (b) storage modulus curves.(c) SEM images of PBS/PLA blend

Table 2 Fig. 3
Fig. 3 Comparison of FTIR spectra for PBS/PLA blend and its polymer components: (a) adsorption bands relative to the stretching of the carbonyl C = O

Fig. 5 Fig. 4 Fig. 7 Fig. 6
Fig. 5 Comparison of (a) the complex viscosity η* and (b) the storage modulus G' plots for the neat PBS/PLA blend and the nanocomposite PBS/ PLA/C30B systems

Fig. 8
Fig. 8 Oxygen permeability coefficients, measured at T = 23 °C and RH = 10%, for the neat PBS and PBS/PLA blend and the corresponding nanocomposite films

Table 1
Main physical properties of the polymers PBS and PLA used

Table 3
Slopes of storage modulus plots in the low frequency range for the neat PBS/PLA blend and the nanocomposite PBS/PLA/C30B

Table 4
Main tensile mechanical properties (E, Young's modulus; ε y , σ y , strain and stress at yield; ε b , σ b , strain and stress at break) of neat PBS and PBS/PLA films and the corresponding nanocomposite sys-