4.1. Morphology
TEM analysis was used to investigate the dispersion state of GnP nanoplatelets in the PBS composites. The micrographs are shown in
Figure 2 and micrographs taken with a lower magnification are presented in
Figure S1 (
Supplementary Materials). Similar dispersion level was found regardless the amount of GnP. Incorporation of GnP into the PBS matrix led to nanocomposites in which the dispersion of the nanofillers can be considered good enough knowing the initial particle/aggregate size (average of 25 µm, measured by Scanning Electron Microscopy) (
Figure S2). Some aggregates of hundreds of nanometers or more, consisting in graphene platelets as well as individual graphene nanosheets (indicated with arrows in
Figure 2) were observed within the PBS matrix. Moreover, the coexistence of small dispersed graphene layers and tactoids consisting of various layers could be a sign of good interactions of these GnP with the polymer. Similar results were reported by Fukushima et al. for the incorporation of expanded graphite into a PLA polymer matrix [
37].
Wide angle X-ray scattering analysis (WAXS) is a powerful tool for examining the structure of polymer nanocomposites. In our case, WAXS was firstly used to check the crystalline nature of the PBS matrix and secondly to determine interlayers distance of GnP. WAXS patterns of the neat matrix and different nanocomposites films are presented in
Figure 3A). For clarity, the patterns shown here were shifted vertically and intensity of GnP pattern was divided by 10. The WAXS patterns of the neat PBS and its nanocomposites samples were quite similar. This indicated that the crystalline matrix had the same crystal structure, an α-form of the PBS crystal. The α-form crystal displayed four main diffractions peaks at 19.6°, 21.9°, 22.7°and 29.0° corresponding to respectively (020), (021), (110) and (111) planes respectively [
38]. The presence of GnP did not drastically modify the crystalline structure of the PBS matrix. The diffractogram of GnP showed a very intense and narrow peak at 26.6 ° referring to X-ray reflection on the (002) planes of well-ordered graphite. The intensity of this peak increased as the amount of GnP in nanocomposites increased. This plane is generally used to evaluate the exfoliation quality of carbon-based fillers [
39]. Corresponding to the perpendicular plane to filler, Bragg’s Law can be used to calculate the
dspacing between two sheets of GnP. Higher value of
dspacing was obtained for lower value of 2
θ. In our case, from Bragg’s Law a value of
d(002) = 0.33 nm was obtained and corresponded to a graphite-type [
40,
41]. This seemed to be consistent with the visual aggregate seen on microscopy.
A deconvolution procedure was applied on the XRD patterns according to the position of the different peaks defined previously, using the opensource software Fityk. The diffraction pattern can be decomposed into a broad amorphous halo and peaks from α crystalline phase [
42]. The result of the deconvolution curves of the neat matrix as example is shown in
Figure 3B and allows a quantification of the crystallinity index (
Xc-WAXS). The obtain value of
Xc-WAXS for neat PBS matrix was equal to 49%. This value was in agreement to the value found by Phua et al. who also found a value of
Xc of 49% by WAXS measurement [
43]. A slight decrease of
Xc-WAXS from 49 to 47% was obtained with addition of GnP whatever the GnP loading. The slight decrease was attributed to the uncertainty of the measurement of the area of the (
) plane since it appears at almost the same 2
θ as the (002) plane of GnP. Overall, the crystallinity index of PBS measured by WAXS was considered to be unchanged after the addition of GnP. In order to confirm the results obtained by WAXS analysis and have more information relating to the crystalline morphology of the films and the chains mobility of the amorphous phase, these specimens were also analyzed by differential scanning calorimetry.
Chains mobility of the amorphous phase and crystalline structure of PBS matrix are discussed from DSC curves (
Figure 4). The values of glass transition temperature (
Tg) of neat matrix and different nanocomposites are listed in
Table 3. The obtained value for PBS was found as −35 °C. Bhatia et al. found a similar value of
Tg (around −34 °C) [
44]. After the incorporation of GnP, a small decrease of
Tg of the PBS matrix was observed and may be due to the reduction of molar mass as seen from SEC analysis. However, by considering uncertainty on each value, change in
Tg values were not significant enough to allow strong modification of polymer chains mobility in the amorphous phase. Similar phenomenon has been found by Goncalves et al. on PLA/GnP nanocomposites [
11].
Thermograms of the first heating run of PBS neat matrix and nanocomposites show three endothermic melting peaks, labeled as I, II, and III from low to high temperature. The first melting peak (
TmI) was measured as 35 °C and was not changed after addition of GnP. Makhatha et al. showed that the first endotherm (
TmI) was not observed when crystallization was performed under non-isothermal condition [
45]. In this case, the small endothermwas probably due to the thermal history of the nanocomposites and could be explained by an annealing process occurring at room temperature during the storage of the films since the molecular mobility is high at room temperature (
Tg = −35 °C). Indeed, this melting peak disappeared for the second heating run (
Figure S3). The double endothermic peaks phenomenon (II and III) have been largely discussed in the literature. Several models were proposed to explain the multiple melting behavior of thermoplastic semi-crystalline polymers, of which the most important one is the presence of melting, recrystallization, and remelting phenomena [
45,
46,
47]. According to this model, the first step corresponded to the melting and recrystallization of crystallites with lower thermal stability, followed by the melting of the crystallites with higher thermal stability formed through the recrystallization of the melting of the crystallites of the lower melting endotherms and those already present in the polymer. After the addition of GnP, no modification of
TmIII was observed whereas
TmII was slightly decreased. Moreover, the distinction between the melting peaks II and III was more apparent as the amount of GnP increased. This phenomenon of peak separation was observed by Makhatha et al. with an increase of the cooling rate [
45]. The estimated crystallinity index, measured with melting peaks II and III, of the neat PBS was 38% in the same range order that the values reported by Wang et al. and Pallathadka et al. [
22,
26]. No modification of
Xc-DSC was observed when GnP were added. Similar phenomenon has been found by Goncalves et al. on Polylactic acid (PLA)/GnP nanocomposites [
11] showing no significant decrease of
Xc in nanocomposites. It might be concluded GnP fillers had consequently no effect on the crystallization of the PBS matrix. Despite the difference in the absolute values obtained by WAXS and DSC methods, both
XC-WAXS and
XC-DSC exhibited the same tendency. As the crystalline lamellae are considered to be impermeable to small molecules, an improvement of barrier properties would not be input to a crystalline change of the PBS matrix in presence of the GnP.
The temperature of the maximum of the peak of crystallization (
Tc) of neat PBS was measured as 97 °C. The value of
Tc was consistent with those reported in the literature [
6,
28,
47]. Despite the fact that graphitic materials are known to act as nucleating agent when added in PBS [
17,
48,
49], in our case, an increase of GnP loading in nanocomposite led to decrease the temperature of crystallization.
Tonset, corresponding to the onset temperature of crystallization and
Tc were slightly decreased from 97 °C to 92 °C and 104 °C to 99 °C respectively (
Table 3) for the PBS/GnP2 which can be explained by an anti-nucleating effect of the nanofillers. Similar results have been described by Gomari et al. [
50] for Poly (ethylene oxide) (PEO)/GnP nanocomposites where an anti-nucleating effect of GnP was partially described and attributed to the ability of GnP to hinder the crystallization. The degree of supercooling (
) implying the crystallization rate [
50] remained unchanged with increasing GnP amount. The full width at the half height maximum of the crystallization peak (FWHM) is considered as an indication of the spherulites size distribution, so the smaller FWHM values demonstrate narrower size distribution [
50,
51]. A decrease in the FWHM value was obtained when GnP amount increased, indicating a decrease of the size distribution of crystallites. Bhattacharyya et al. showed on Polypropylene/Single Wall Carbon NanoTube (SNWT) nanocomposites, that this effect could be explained, at least partially, by an evenly distribution of heat on the polymer due to the higher thermal conductivity of the carbon nanotubes over the neat polymer [
51]. This phenomenon has been noticed in various studies [
51,
52] with carbon-based filler. This result seemed to be in agreement with the decrease of
TmII seen on first and second heating step (
Figure 4 and
Figure S3) that led to a visual separation between both endotherms.
4.2. Thermal and Mechanical Properties
Thermogravimetric mass loss curves of the GnP in a powder form are shown in
Figure 5. GnP degradation exhibited two mass losses. The first mass loss (around 4%) at 450 °C was attributed to the presence of small quantities of remaining compounds used during the GnP formation. The second mass loss at 650 °C was attributed to the oxidation followed by the degradation of the GnP. This mass loss is commonly seen on graphite-based materials on a dry state under oxidative atmosphere [
53].
The typical mass loss curves under air atmosphere for neat PBS matrix and its composites are presented in
Figure 5. Two steps were observed on the mass loss curve relative. The first one at about 360 °C and the second one at about 450 °C. Makhatha et al. also showed similar results on PBS [
45]. They attributed the first loss to high molecular weight chains decomposition into smaller chain fragments via an initial scissoring of the chain end, followed by the second one, attributed to the following degradation by thermal-oxidation into volatile small molecular products in the presence of dioxygen, of the previous degraded chains.
The amount of GnP in each nanocomposite was deduced from the mass residues measured at 560 °C on the nanocomposites and the neat matrix. This value of 560 °C was chosen because at this temperature both mass loss of PBS was already observed. It also should be noted that the mass loss of GnP at 560 °C was considered for the calculation of GnP loading. The experimental values of GnP loading in mass and volume are listed in
Table 4. The value of GnP density,
ρGnP, was taken as 2.22 g cm
−3 which is typical for graphitic materials [
54]. Lower values than expected ones were obtained because of difficulties faced in the masterbatch preparation due to the powdery form of GnP.
The thermal stability of samples was evaluated by the determination of the temperature of 5% mass loss (
), 50% mass loss (
), and 90% mass loss (
). The obtained values are listed in
Table 4.
values of nanocomposites were close compare to those obtained for the neat matrix. A small increase of
values should be noted, reflecting a slightly lower degradation rate at the end of the chain degradation phenomenon in presence of GnP.
DMA allows measurements of the response of a given material to an oscillatory deformation as a function of temperature.
Figure 6 shows the evolution as a function of the temperature of
G′,
G″ and tan
δ, for neat PBS and corresponding nanocomposites. At low temperature, PBS and its associated nanocomposites were in a glassy state. At the end of the glassy state plateau, a steep decrease on
G′ was observed revealing the apparent glass transition which was in agreement with those found by Makhatha et al. and Yue et al. [
45,
55]. The steep decrease was followed by a rubbery state described by a slow linear decrease of
G′. A shoulder around 40 °C was also observed and can be explained by the melting of crystallites corresponding to the
TmI peak underlined from DSC analysis. This shoulder disappeared for the second heating run (
Figure S4 in
Supplementary Materials). The incorporation of GnP did not affect significantly the glass transition temperature. In the same way, the obtained values of
G″ for the nanocomposites regardless the GnP amount were close to those obtained for the PBS matrix, indicating similar internal frictions. The presence of GnP did not lead to significant shift and broadening of the tan
δ for all nanocomposites compared to that of neat PBS. A maximum of tan
δ was observed around −16 °C, similar to that reported by Yue et al. [
55]. This observed behavior was in agreement to those observed from DSC analyses.
Usually, addition of nanofiller to a polymer matrix leads to an increase of stiffness accompanied with embrittlement. In our case, introduction of nanofillers, irrespective of the amount (except for PBS/GnP0.5) led to a small enhancement of stiffness (13%) and no change of stress at break. However, a small decrease of elongation at break was obtained as the amount of GnP increased. From this result, it might be concluded that the presence of GnP led to a slight reinforcing effect which can be probably attributed to a rather good dispersion of nanofillers in polymer matrix and also to the rather good interfacial adhesion between nanofillers and the polymer matrix.
4.3. Water Sorption
Sorption isotherm curves at 25 °C were obtained by plotting the mass gain at equilibrium (
M) as a function of the water activity (
aw), (
Figure 7A). All isotherm curves displayed a BET III shape according to the classification of Brunauer, Emmett, and Teller (BET) [
56]. It consists in a linear evolution of water uptake at low water activity (
aw 0.4) followed by a convex part at high activity (
aw > 0.5). The increase of water uptake at high activity is usually explained by the formation of water clusters [
57].
The obtained values of mass gain for neat PBS matrix were in the same order to those obtained by Charlon et al. [
3], who found for
aw = 0.9 a mass gain at equilibrium of 0.9%, compared to our obtained value (1.6%). The slight difference can be related to a difference of crystallinity index due to a different thermal history of the films. The water sorption capacity of PBS was in the same range order compared to different polyester polymers such as Polylactic acid (PLA) [
3,
58], Polycaprolactone (PCL) [
59] and Polyethylene Terephthalate (PET) [
60] which presented for
aw = 0.9 at
T = 25 °C a value of mass gain at equilibrium of 1%, 0.5% and 0.9% respectively. These obtained values of water mass gain for PBS highlighted a hydrophobic character regarding other polymers known as hydrophilic like plasticized starch [
61], chitosan [
29], or polyamide 6 (PA6) [
62], which presented for
aw = 0.9 at
T = 25 °C a value of mass gain at equilibrium of 50%, 45% and 12 % respectively.
In presence of GnP, whatever the amount, the mass gain at equilibrium were quite similar until aw ≤ 0.7, and slightly differed for aw > 0.7. Considering the crystalline part of PBS matrix and the amount of GnP which are both considered as impermeable to water molecules, the mass gain at equilibrium were calculated as a function of the amorphous phase of the polymer matrix using the crystalline index determined from DSC analysis. A single curve was obtained, so it can be concluded that the presence of GnP had no impact on the water sorption mechanism and the water sorption phenomenon occurred in the amorphous part of the PBS matrix. This further confirms a good interfacial adhesion between nanofillers and the polymer matrix.
To go further in the sorption mechanism understanding, the average number of water molecules sorbed in a single amorphous unit of polymer (
Ni), was calculated from the following equation:
where
Ma is the mass gain at equilibrium of PBS amorphous part,
MP and
Mw are respectively the molar mass of the studied polymer unit (
Mp = 172 g mol
−1) and the molar mass of water (
Mw = 18 g mol
−1).
The evolution of
Ni as a function of the water activity is presented in
Figure 7B. The obtain isotherm logically displayed the same shape than the isotherm presented in
Figure 7A. From this representation, it can be seen than for
aw = 0.9, there was one water molecule in average sorbed every 4 units of PBS in the amorphous phase.
To get a better understanding of the water sorption at the molecular scale, the isotherm curve of the neat PBS was modeled using the GAB equation, combined with the theory from Zimm and Lundberg.
The values of GAB parameters and mean relative percentage of deviation modulus (
MRD) are given in
Table 6. Firstly, examination of
MRD indicated that the model is convenient and allows an accurate description of the experimental sorption isotherm as shown by the theoretical curve plotted in
Figure 7B.
From Mm value, it could be deduced that for one amorphous PBS unit, 0.045 H2O molecules were in strong interactions, corresponding to an average of one water molecule every 22 amorphous units of PBS. It should be noted that the saturation of the polymer monolayer occurred for aw ≈ 0.4.
By using the theory of Zimm and Lundberg, it was possible to determine the
MCS values from the parameters deduced from GAB equation (Equation (10)). The plot of
MCS versus the water activity is represented in
Figure 8.
In
Figure 8A,
MCS values are close to unity at low water activity (below
aw = 0.4) and then increased at higher activities. Beyond
aw = 0.4, interactions for a water molecule to another sorbed molecule appeared and became preponderant leading to the progressive formation of water clusters. At the highest activity (
aw = 0.9), there was about 5 water molecules per cluster. This value was higher than that found for on PA6 [
63], chitosan [
29] and starch [
61] which were 3, 4 and 2 respectively. This higher size of
MCS for PBS compare to these others hydrophilic polymers can be explained by a lower affinity of water molecules to the PBS chain. As a consequence, at higher water activity, water molecules preferred to be in self-interactions that interact to the polymer chains.
The evolution of
MCS as a function of
Ni is presented in
Figure 8B. The value of MCS was constant and close to unity until a value of
Ni = 0.045 then linearly increased. The first part (
Ni < 0.045) corresponded to an individual distribution of the water molecules and the saturation of the monolayer and the second part (
Ni > 0.045) corresponded to the formation of water clusters. The water clusters size was proportional to the amount of sorbed water molecules.
The number of sorption sites per monomer unit in the amorphous phase which is defined as the ratio between the number of molecules sorbed per monomer unit in the amorphous phase to the mean cluster size (
Ni/
MCS) was also determined. The evolution of
Ni/MCS as a function of the water activity is represented in
Figure 8C. At low water activity, a linear increase of
Ni/MCS was observed. As explained previously, in that range of water activity an individual distribution of the water molecules occurred on the different monomer unit of PBS in the amorphous phase. This sorption mechanism was a Henry type sorption with a random absorption of the water molecules in the polymer. For higher water activity,
Ni/MCS reached a plateau due to the aggregate phenomenon. It can be concluded that the average number of sorption’s sites was statistically one every 22 monomer units of PBS in the amorphous phase. Sabard et al. [
64] and Blanchard et al. [
57] obtained one sorption site every 22 and 13 monomer units for water sorption for PA6 and EVOH respectively. These differences can be explained by a difference of affinity between water molecules and the respective polymer.
For all systems,
n values close to 0.5 were obtained for almost all activities (
Table S1 in
Supplementary Materials). The diffusion mechanism can be considered as Fickian. The water diffusion coefficient values (
D) were plotted as a function of water activity (
aw) in a semi-logarithmic scale,
Figure 9A. The obtained
D values of the neat PBS were in agreement with those reported in literature [
3].
Whatever the film composition, the diffusion rate was dependent on the amount of water molecules sorbed. Constant
D value was recorded up to
aw of 0.7, after that
D values decreased. This evolution of
D was in accordance with the sorption isotherm shape of curve. The constant value of
D was explained by the Henry’s sorption mode and the was attributed to the water clustering phenomenon [
65].
Generally, decrease of
D is observed when first water molecule clusters are formed. In our case, decrease of
D should have been observed from
aw = 0.5. This delay could be explained by a low amount of sorbed water molecules in the polymer associated to a low number of sorption’s sites per monomer unit in the amorphous phase. The decrease of
D was obtained when the
MCS was higher than 2 as shown in
Figure 9B.
The influence of introduced GnP nanofillers on water sorption kinetics was investigated and this effect could be evidenced in
Figure 9A. Whatever the nanofiller amount, in the whole range of water activity, the shape of the curve relative to the nanocomposites was similar compared to that of the neat matrix. A decrease of diffusivity was observed in the whole range of water activity. This decrease seemed to be slightly increased as the amount of GnP increased. As graphene act as impermeable obstacles, the diffusion rate became slower because the water molecules follow a more tortuous path to pass through the composite film.
The tortuosity factor can be expressed by the following equation:
where
D0 is the diffusion coefficient in the neat polymer,
D the diffusion coefficient in the composite and
τ is the tortuosity factor. The tortuosity factor is also defined by the ratio
where
d′ the length pathway of the permeant molecule in the polymer with impermeable barrier (filler, crystallite, etc) and
d the length pathway of the permeant molecule in the polymer without impermeable barrier. In this case, since the crystallinity index was not modified by addition of filler, the modification of the tortuosity was considered to be only due to the presence of GnP.
The tortuosity factor τ, was determined on the whole range of water activity. For a given system, τ was independent of the water activity. Taking uncertainties into account, the τ increased as the amount of GnP increased from 1.3 ± 0.2 to 2.0 ± 0.1 for 0.1 wt.% and 2 wt.%, respectively. This indicates that the diffusion mechanism resulted on a geometric type phenomenon.
4.4. Water and Dioxygen Permeability
Water permeability measurements were performed on neat matrix and different nanocomposites for water activity range from 0.5 to 1 at 25 °C. The evolution of the water permeability coefficient
as a function of the water activity is shown in
Figure 10A. The obtained value of
for neat PBS matrix at
aw = 1 was equal to 2518 Barrer, in good agreement with this reported in the same condition by Charlon et al. (2616 Barrer) [
3]. The obtained value was higher than those of common polyesters such as PLA, PET and PHBV 1957 Barrer [
58], 150 Barrer [
66] and 149 Barrer [
67], respectively. In the tested range of water activity,
increased linearly as the water activity increased. This result can be explained by the small plasticization effect of the PBS matrix due to the presence of water molecules sorbed by the polymer. Even with the low hydrophilic character of PBS, the amount of water molecules sorbed increased as seen previously from sorption analyses with the increase of water activity. Then, the presence of water molecules led to increase the mobility of the polymer chains resulting in an increase of water permeability coefficients.
In order to study more specifically the effect of the presence of GnP on the water permeability, the data of
Figure 10A are examined. The introduction of GnP led to an improvement of the barrier properties. The reduction of the permeability increased as the GnP amount increased within the PBS matrix. As reported in the literature, GnP are considered as impermeable fillers for small molecules [
68,
69]. Furthermore, as shown by the TEM micrographs (
Figure 2), homogenously dispersed small nanoplatelets within the matrix were observed leading to a significant increase of the gas pathway by a tortuosity effect. As generally observed, the tortuosity increased as the GnP amount increased in the matrix [
8,
12]. To discuss more specially the effect of the presence of GnP, the values of water relative permeability (
) which is defined as the ratio of the permeability coefficient of the nanocomposite on the permeability of its associated matrix were determined. The evolutions of
as a function of the water activity are shown in
Figure 10B.
In agreement with the previous discussion, decreased as the GnP amount increased in the whole range of water activity. Considering the uncertainty of measurements, seemed to be constant in the whole range of water activity. From this, it could be concluded that the increase of water sorbed molecules in the polymer did not lead to a plasticization effect of the polymer matrix great enough to minimize the contribution of interfaces between GnP fillers and the polymer matrix at high water activity.
Dioxygen permeability measurements were performed on neat matrix and different nanocomposites for water activity range from 0 to 0.9 at 25 °C. The evolution of the dioxygen permeability coefficient (
) as a function of the water activity is shown in
Figure 11A.
of neat PBS in the anhydrous state was equal to 0.135 Barrer. This value was in the same order with the one reported by Messin et al. who obtained a dioxygen permeability of 0.196 Barrer in the same experimental condition [
42]. This value was higher than common polyesters such as PET and PHBV which have water permeability of respectively, 0.09 Barrer [
66] and 0.031 Barrer [
67] but was slightly smaller than the value found on PLA of 0.23 Barrer [
58]. As in case of water permeability analysis, in the tested range of water activity,
increased linearly as the water activity increased and can be explained by a plasticization of the PBS matrix due to the presence of water molecules sorbed by the polymer which tended to decrease the cohesive density energy of the films. Similar behaviors have been reported in the literature by Tenn et al. on PLA [
58].
Here again, introduction of GnP led to an improvement of the dioxygen barrier properties. Improvement increased as the amount of GnP within the PBS matric increased. The evolution of the values of dioxygen relative permeability (
) as a function of the water activity are shown in
Figure 11B. Considering the uncertainty of measurements,
seemed to be constant in the whole range of water activity. As explained previously, the increase of water sorbed molecules in the polymer did not lead to a plasticization effect of the polymer matrix great enough to minimize the contribution of interfaces between GnP fillers and the polymer matrix at high water activity.
Different models, such as Nielsen [
70], Cussler-Aris [
71], Bharadwaj [
72], have been proposed in the literature to describe the tortuosity and as a consequence the improvement of barrier properties induced by the dispersion of impermeable nanoplatelets fillers in a polymer matrix. The Bharadwaj model gives a good understanding on the modeling of permeability on nanocomposites (Equation (13)) [
72].
where
Pr is the relative permeability,
α the aspect ratio,
φ is the volume fraction of impermeable phase and
S is the orientation of fillers in the nanocomposites. A value of
S = −0.5 showing a perpendicular orientation of fillers with the membrane surface, a value of
S = 0 a random orientation of fillers and a value of
S = 1 a parallel orientation of fillers with the membrane surface.
From Bharadwaj model, the mean aspect ratio was calculated and an example of obtained results is shown in
Figure 12. For both water and oxygen molecules, an average aspect ratio of 280 ± 50 was found considering the obtained permeability values for all water activity studied and the best fitting was obtained with a value of
S equal to 1, enhancing the parallel orientation of the film observed on
Figure 2. With a value of
S = 1, the Bharadwaj model becomes the Nielsen model [
70]:
This model gave the best but was not greatly fitting with our experimental data due an apparent threshold value of 1 wt.% GnP. After this value, the improvement was no longer efficient because of the aggregation of GnP. Graphite base filler are known to have low critical filler concentration of agglomeration, commonly under 1 wt.% [
73]. Similar phenomenon has been reported with graphite-based filler, with sometime an increase after the threshold value due agglomeration of the filler which create a connecting pathway of free volume at the interface filler/matrix which ease permeant molecules diffusion [
74].