Effect of Dicumyl Peroxide on a Poly(lactic acid) (PLA)/Poly(butylene succinate) (PBS)/Functionalized Chitosan-Based Nanobiocomposite for Packaging: A Reactive Extrusion Study.

Nanobiocomposites with balanced mechanical characteristics are fabricated from poly(lactic acid) (PLA)/poly(butylene succinate) (PBS)blend at a weight ratio of 80/20 in association with varying concentrations of functionalized chitosan (FCH) through reactive extrusion at a temperature of 185 °C. The combined effect of FCH and dicumyl peroxide (DCP) showed insignificant change in tensile strength with a remarkable increase in % elongation at break (∼45%) values. Addition of DCP also caused increase in the molecular weight (M w ∼ 22%) of the PLA/PBS/1DFCH nanobiocomposite, which is attributed to the cross-linking/branching effect of FCH on the polymers. The interfacial polymer-filler adhesion is also improved, which is observed from the field-emission scanning electron microscopy images of PLA/PBS/1DFCH. For PLA/PBS/1DFCH, the crystallization rate and nucleation density of PLA are increased because of cross-linked/branched structures are developed, which acted as nucleating sites. Therefore, the present work facilitates a simple extrusion processing with a combination of balanced thermal and mechanical properties, improved hydrophobicity (∼27%), and UV-C-blocking efficiency, which draw the possibility for the utilization of the ecofriendly nanobiocomposite in the packing of UV-sensitive materials on a commercial level.

Introduction

The recent trend is focused on consumption of bioplastics in developing a novel process for the fabrication of ecofriendly polymeric materials and for solving the fossil-oil crisis associated with conventional synthetic polymers.[1] Poly(lactic acid) (PLA), a biomass-based thermoplastic polyester, is generally produced from renewable resources (tapioca roots, sugar beet, and sugar cane) and is the most sustainable alternative to the conventional synthetic polymers compared with other biopolymers.[2] Among those bioplastics, PLA has drawn considerable interest due to its competitive mechanical properties, biocompatibility, and easier processibility.[3] However, the application of PLA on industrial scale is limited because of its brittle nature, high glass-transition temperature (Tg ∼ 55 °C), low crystallization rate, poor melt strength, and inferior barrier properties.[4]

From this view point, modification of PLA by copolymerization or physical blending and loading of a nanofiller or plasticizer is an economic and effective approach to overcome these limitations. The primary role of blending of PLA with flexible biopolymers that exhibit lower Tg values such as poly(butylene succinate) [PBS (Tg ∼ −32 °C)], poly(butylenes succinate-co-adipate) (Tg ∼ −45 °C), and polycaprolactone (Tg ∼ −60 °C) is to maintain the biodegradability with tuned Tg, which resolves the brittleness problem and finally improves the toughness.[5,6]

Melt polycondensation allows the synthesis of PBS by the reaction of 1,4-butanediol with succinic acid, which can also be derived using monomers from fossil feedstock or renewable bioresources.[7,8] PBS is also a bio-based polyester with high flexibility, impact strength, and chemical resistance, which is similar to polypropylene (PP). Thus, PBS has attracted much attention as a promising ecofriendly bioplastic and could be an appropriate choice to melt-mix it with PLA. Nevertheless, applications of PBS are limited because of its poor gas barrier, water vapor transmission rate, and melt viscosity (η) in the packaging sector.

In fact, PLA/PBS system is one of the best choices because the combination of two gives the desired properties of each polymer. One polymer has potential to tailor the mechanical characteristics (e.g., toughness, elongation at break) of the blend to control the physiological environment by varying the blend ratio, and the other polymer provides stability to the molecular properties of the two-component system.[6,9] Many research papers are reported about the PLA/PBS system in the literature, and it is claimed that the PLA/PBS (80/20) blend has shown better compatibility in comparison to that of other blend ratios.[10] However, the properties of this system are not significantly enhanced by melt processing probably due to poor interfacial adhesion and phase separation between PLA and PBS.

Chitosan (CH) is a cationic polysaccharide produced by deacetylation of chitin, which is an extremely recommended biopolymer for packaging applications due to its biodegradability; biocompatibility; excellent oxygen barrier; antioxidant, antifungal, and antimicrobial characteristics; and most importantly its film forming ability.[10] It is the second most abundant biopolymer in nature after cellulose and can be used as a polymer matrix as well as in the form of a biofiller. On the other hand, chitosan has poor dispersion in the PLA matrix due to its hydrophilic nature. According to Pal et al.,[10] hydrophobic functionalized chitosan (FCH) can solve the dispersion problem of chitosan in the hydrophobic PLA matrix. It is also worth mentioning that FCH has a great potential to be used as a bio-based filler in the food packaging application with enhanced barrier properties.

As mentioned above, PLA, PBS, and FCH are one of the most promising candidates and play vital roles to contribute toward the marketing of bioplastics designed for sustainable packaging. None of these three components can fulfill the demand for almost all structural materials in packaging application when used alone. For food packaging applications, barrier to UV light is also one of the essential characteristics to inhibit the decomposition of packed food stuff.[11] It is a well-known fact that UV light damages the vitamins and fatty acids found in food items. Therefore, this demand should be fulfilled for its benefits in the food packaging application.

The evolution of the nanobiocomposite with requisite melt strength, thermal performance, and stiffness–toughness balance is still a challenging task. In comparison with a binary system, a multicomponent polymer system usually shows more balanced performance in terms of physicochemical and thermal properties. Recently, investigation on the properties of multicomponent-based polymeric systems formed by the combination of three or more components has attracted the attention from both the commercial and the academic world.[12] Therefore, significant attention has to be paid for multicomponent-based polymeric systems at industrial scale.

To the best our knowledge, a PLA/PBS/FCH-based nanobiocomposite in the presence and absence of dicumyl peroxide (DCP) has not been evaluated so far. To consider the promising tailored characteristics of PLA and PBS, we investigated the influence of FCH and DCP (as a compatibilizing agent) on various analytical properties of the PLA/PBS system.

The reactive extrusion technique is utilized to prepare the nanobiocomposite with balanced performance, and different characteristics such as surface morphology, structural, thermal, and mechanical properties of the multicomponent system are evaluated to check its feasibility of production in a continuous manner. These complementary properties of PLA/PBS/FCH with addition of DCP are very important in tailoring the mechanical properties, melt strength and thermal performance.

Results and Discussion

Plausible Reaction Mechanism

The plausible reaction mechanism of the nanobiocomposites in the presence of DCP during the melt state under high shear action is illustrated in Scheme . When DCP is decomposed into free radicals at elevated temperature, it could facilitate the extraction of H’s from the PLA and PBS chains to generate free radicals on the backbone of PLA and PBS components, which is a result of propagation of the radical reaction to form a cross-linked/branched structure between PLA and PBS.[13,14] PLA generates free radicals on tertiary C atoms, which get readily stabilized during reactive extrusion.[4,14] It is well reported that PBS contains many secondary hydrogen (H) atoms and provides easy abstraction of hydrogen’s (H) by peroxide radicals in comparison with PLA. On the other hand, free radicals are formed on lactic acid oligomer (OLLA) chains attached to FCH. It is worth mentioning that there are possibilities of the formation of a strong covalent (C–C) chemical linkage via a combination of PLA/FCH, PBS/FCH, PLA/PBS, PLA/PLA, PBS/PBS, PLA/PBS/FCH, and PLA/FCH/PBS. This may result in the formation of more complex products, which include crosslinking/branching of PLA and PBS with FCH, and PLA-g-PBS copolymers, and the resulting reaction will be more complicated.[4,15,16] In a nutshell, it can be assessed that the cross-linked/branched structures in the nanobiocomposites play a vital role to amend the properties of the reactive nanobiocomposites in a controlled fashion. With the help of literature, we have put forwarded a plausible reaction mechanism shown in Figure .

Scheme 1

Generalized Schematic Illustration of the Possible Reaction Mechanism of PLA/PBS Blends in the Presence of FCH along with DCP during Reactive Extrusion

Figure 1

Plausible reaction mechanism of the PLA/PBS/FCH nanobiocomposite with DCP.

Fourier Transform Infrared (FTIR) Analysis

The FTIR–attenuated total reflectance (ATR) technique is used to identify the structural modification taking place during the chemical reactions among PLA, PBS, and FCH in the presence of DCP, as shown in Figure . The characteristic infrared bands of all of the samples are tabulated in Table . The representative characteristic peaks of PLA are as follows: 2996 (corresponding to the asymmetric CH3 stretching), 2946 (symmetric CH3 stretching), 2880 (C–H stretching), 1748 (C=O stretching), 1382 (symmetric bending of CH3), 1381–1358 (symmetric bending of CH3), 1452 (asymmetric bending of CH3), 1180 (asymmetric C–O–C asymmetric rocking CH3), 1128 (symmetric rocking CH3), 1080 (symmetric stretching C–O–C), 1041 (stretching C–CH3), 868 (stretching C–COO and also attributed to the amorphous region of the PLA phase), 954 (rocking CH3 + stretching C–C), and 753 cm–1 (bending C=O in PLA due to its crystalline phase).[17−19] However, PBS shows a band at 2880 cm–1 (−CH2 stretching) in the PLA/PBS blend. For PLA/PBS/1FCH spectra, the transmittance band at 1646 cm–1 also confirms the presence of amide linkage in FCH. It is noticed that the intensity of bands at 2996, 2946, and 2880 cm–1 is decreased, whereas the band at 2964 (−CH2 vibration) is more intense than that of the PLA/PBS blend. However, a new chemical bond is not formed after the addition of FCH into the PLA/PBS blend, which indicates merely an electrostatic interaction of FCH with the PLA/PBS system.[20]

Figure 2

Chemical structural analysis of the PLA/PBS blend and its ternary nanobiocomposite with or without DCP using FTIR in (a) full range (4000–700 cm–1), (b) 3500–2700 cm–1, (c) 1850–1600 cm–1, (d) 1680–1560 cm–1, and (e) 1500–700 cm–1 and (e′) enlarged image of PLA/PBS/1DFCH from 1500 to 700 cm–1.

Table 1

FTIR Peak Analysis for the PLA/PBS Blend and Its Ternary Nanobiocomposite with and without DCP

Wavenumber assignments	PLA/PBS	PLA/PBS/1FCH	PLA/PBS/1DFCH
asymmetric stretching C–H in CH3	2996	2996	2992
symmetric stretching C–H in CH3	2946		2945
CH2 vibration	2964	2964
stretching C–H in PLA + stretching CH2 in PBS	2880	2880
asymmetric stretching C–H in CH3			2918
symmetric stretching C–H in CH3			2851
stretching C(=O) in PLA	1748	1748	1746
stretching C(=O) in PBS			1713
doublet corresponding to CH2 twisting appeared in PBS			1338–1312
C–OH bending of PBS			917
stretching C–COO and also attributed to amorphous phase of PLA	868	868	866
formation of new C–C linkage			805
CH2 stretching	2964	2964

After addition of DCP into PLA/PBS/1FCH, we observe that the band at 2996 cm–1 is slightly shifted to 2992 cm–1 and the band at 1748 cm–1 is shifted to 1746 cm–1 due to change in structure. The peak at 2880 cm–1 is absent and two new peaks at 2918 cm–1 (asymmetric stretching CH3) and 2850 cm–1 (symmetric stretching CH3) are found. Further modification in the chain due to branching/cross-linking through the formation of a new band at 805 cm–1 (C–C bond) is detected, which influences the intensity of ester linkages.[21] While chain modification in PBS chains is confirmed through the presence of doublet attributed to twisting of CH2 at 1338–1312 cm–1, C=O group at 1712 cm–1 and band at 917 cm–1 assigned to C–OH bending in carboxylic group.[22−24] It may suggest the interaction of PLA, PBS, and FCH in the presence of DCP existed upon reactive modification.

Gel Content (%)

As mentioned in the schematics of the reaction mechanism, the PLA/PBS ternary nanobiocomposite system with the DCP content shows a complex type of reaction through the formation of a cross-linked/branched structure as well as a copolymer of PLA and PBS in the presence of DCP. The gel contents of PLA/PBS and PLA/PBS/FCH nanobiocomposites at various FCH loadings (1 and 3 wt %) with the addition of 1 phr DCP are determined on the basis of the extraction process. It is found that the gel fraction of the PLA/PBS/1DFCH nanobiocomposite is significantly improved in comparison to that of the PLA/PBS blend, as shown in Figure . The reason behind such high gel content is the development of cross-linking sites within all of the three components. At the fixed DCP amount, the gel fractions of PLA/PBS with FCH composites are approximately inversely proportional to the FCH quantity. It is observed that the calculated gel fraction is lower than 10% when the FCH content is 3 wt % in the presence of DCP. This is probably due to the reduction in cross-linking efficiency, which subsequently leads to the thermal decomposition of the polymeric system due to agglomeration of FCH at higher loading (3 wt %).[25]

Figure 3

Gel content (%) of PLA/PBS-based composites in the presence of DCP as a function of FCH content.

Gel Permeation Chromatography (GPC) Analysis

Figure a,b represents the distribution of molecular weight and polydispersity index (PDI) for PLA and PLA/PBS-based nanobiocomposite with or without DCP as a function of FCH content. Compared with the processed PLA, depletion in the Mw and Mn values of the PLA/PBS blend by ∼22 and ∼10%, respectively, indicates that it had experienced thermal degradation during its processing at elevated temperature. Interestingly, reactive extrusion of PLA/PBS/1DFCH nanobiocomposites shows an increase in both Mn and Mw by ∼16 and ∼51%, respectively, and this could be due to the formation of some cross-linking/branching sites, which is also referred to as an improvement in the value of gel content, as mentioned in the previous section. This is probably due to the fact that reactive processing helps to inhibit the thermal decomposition in the presence of 1 phr DCP. However, the PLA/PBS/3DFCH nanobiocomposite underwent a decrease in Mn and Mw with a slightly high PDI ∼ 2.4 and a noticeable reduction in the gel content supports the possible agglomeration.

Figure 4

(a) Number-average (Mn) and weight-average (Mw) molecular weight distribution and (b) polydispersity index (PDI) of PLA and PLA/PBS-based composites in the presence of DCP as a function of FCH content.

When FCH is added into PLA/PBS, it displays small reduction in values of Mn and Mw, as compared to those of the PLA/PBS blend. Therefore, it can be inferred that FCH has a minor effect on thermal decomposition of the PLA/PBS blend. After addition of DCP, the cross-linking and degradation occur simultaneously in the reactive modified nanobiocomposite, which enhanced both the high- and low-molecular-weight regions, resulting in the improvement of PDI (broadening molecular weight distribution) index. From Figure 2S, the presence of cross-linked/long-chain branched linkage is confirmed due to the appearance of a new shoulder peak in the side of the high-molecular-weight (Mw) population in the case of PLA/PBS/1DFCH. Also, this fact should be considered that the hydrodynamic volume is highly influenced by the chain topology and thus determination of molar mass distribution using only GPC is not a preferable technique.[21]

Field-Emission Scanning Electron Microscopy (FESEM) Analysis

As it is known that PLA and PBS are semicrystalline polymers, their thermal resistance and mechanical characteristics are mostly affected by two important factors, i.e., solid-state morphology and % crystallinity.[26] The fractured morphology of all of the nanobiocomposites as well as the PLA/PBS sample was observed through FESEM analysis. For the polymer blend, moderate interfacial adhesion between two phases is favorable to the enhancement of mechanical characteristics, particularly toughness, whereas too weak or too strong interfacial adhesions suppress the stress through interfacial debonding or induce complete phase separation between dispersed and matrix phases. FESEM micrographs of the fractured surfaces of PLA/PBS, PLA/PBS/DCP, and PLA/PBS/FCH with and without DCP are displayed in Figure a–d. It has been reported earlier that PLA and PBS are partially compatible to each other.[14] As seen in Figure a, PBS is poorly embedded in the PLA matrix and the interfacial adhesion between PLA and PBS phases is weak, as evidenced by the oval cavities found after the tensile deformation.[12,27] In case of the PLA/PBS reactive blend, no dispersed phase of PBS is noticed due to reduction in PBS size.[14,27] Moreover, the presence of voids between PLA and PBS is also observed. These could assist as stress concentrators during tensile stress, responsible to level off the mechanical properties such as tensile strength (TS) in Figure b.

Figure 5

FESEM micrographs for the tensile fractured surface of the PLA-based ternary system: (a) PLA/PBS blend, (b, b′) PLA/PBS/1DCP, (c, c′) PLA/PBS/FCH, and (d, d′) PLA/PBS/1DFCH.

For the PLA/PBS/1FCH nanobiocomposite, FCH is dispersed into the PLA/PBS phase, as displayed in Figure c. After the addition of 1 wt % FCH in PLA/PBS/DCP, the morphology of the nanobiocomposite is quite uniform, as clearly demonstrated in Figure d. Moreover, the FCH nanoparticles are well adhered to PLA and PBS phases in the presence of DCP and no voids are noticed after the tensile fracture, indicating improvement in the interfacial adhesion between PLA and PBS phases, which is again attributed to the development of a copolymer or crosslink network at the interface. Consequently, it could control the mechanical properties significantly. It also has the potential to prevent stress concentration when the external force is applied to the composite in tensile mode.[27] Therefore, synergistic influence of FCH along with DCP could enhance the compatibility of PLA and PBS domains in the studied nanobiocomposites.

Wide-Angle X-ray Diffraction (XRD) Analysis

The crystal structures of PLA/PBS ternary nanobiocomposites have been verified to observe the influence of cross-linking/branching by using the wide-angle XRD technique. From Figure 1S, it is clear that neat PLA has no peak at the set processing conditions; it confirms that the sample is amorphous in nature. However, diffraction peaks positioned at Bragg angles of 16.4° (200)/(110) and 18.6° (110) are assigned to the α′ phase crystal of PLA in PLA/PBS and their nanobiocomposites, as shown in Figure a.[26,28] This represents the orthorhombic crystal unit cell of PLA, which is confirmed by reflection pattern (200). The peaks appearing at 22.5° (110) are assigned to PBS. When 1 wt % FCH is added with DCP in the PLA/PBS blend, it shows an increase in the peak height at 16.4°, indicating the growth of α′-crystal of PLA. From Figure b, it is noticed that the addition of FCH into PLA/PBS gradually decreases the relative crystallinity (Xc) of PLA in the PLA/PBS ternary nanobiocomposite. However, Xc of the PLA/PBS/1DFCH nanobiocomposite is significantly increased up to 33%. Furthermore, it can be concluded that the incorporation of FCH with DCP in PLA/PBS not only improves the Xc of the nanobiocomposite but also contributes to the growth of α′ crystal of the PLA phase.

Figure 6

(a) Wide-angle XRD analysis and (b) crystallinity of PLA/PBS-based nanobiocomposites.

Differential Scanning Calorimetric (DSC) Analysis

DSC analysis is carried out to understand the crystallization and melting behavior of the fabricated PLA/PBS blend and its ternary nanobiocomposites. In the present investigation relating the thermal properties of nanobiocomposites, it is necessary to study the effect of FCH on the thermal parameters of the PLA/PBS system with or without DCP. The obtained second heating and cooling curves are depicted in Figure a,b, respectively; the corresponding melting and crystallization parameters are also mentioned in Figure a,b. For neat PLA, second heating scans show glass-transition temperature (Tg) at 60 °C and cold crystallization temperature (Tcc) at 115 °C. The crystallization (Tc) temperature is not identified during the first cooling scan (at 10 °C/min), which gives the confirmation that neat PLA shows a slow crystallization property for being amorphous in nature.[29] This finding is in agreement with the wide-angle XRD result also. Furthermore, neat PLA exhibits double melting peaks and can be explicated by a similar type of crystal structure that melted at various temperatures due to lamellar divergence.[30,31] For the PLA/PBS blend, the Tg and Tm of PLA are unchanged, indicating that both are thermodynamically incompatible.[31−33] However, broad Tcc of the PLA phase is shifted to a lower temperature as blended due to the dispersed molten PBS content or the presence of contamination in PBS. Therefore, it can function as a crystallization nucleus for the PLA system.[34] The Tm of PBS is found to be 110.9 °C, and Tccof the PLA component is found to be around 100.9 °C, which is attributed to the partial overlapping of the melting peak of PBS with the cold crystallization peak of PLA. On the other hand, mobility of PBS segments is strongly constrained by PLA chains and inhibited the PBS crystallization. Therefore, no peak is noticed for crystallization of PBS during the cooling run. For PLA/PBS-based ternary nanobiocomposites, the melting peak of PBS is not clearly observed because it is masked by the cold crystallization region of PLA. Subsequently, due to the confinement effect, the crystallization of PBS has been restricted, as mentioned before. Moreover, the presence of FCH shows no significant change in Tg and Tcc of PLA in the PLA/PBS-based ternary system in comparison with the PLA/PBS blend. Similarly, Tm of both PLA and PBS was not affected by incorporating FCH, but their melting enthalpy values are reduced with increasing FCH loading up to 3 wt % due to the reduction in the volume fraction of the polymer.[35]

Figure 7

DSC thermograms of neat PLA, PLA/PBS blend, and their nanocomposites with and without DCP: (a) second heating cycles and (b) first cooling cycles.

For PLA/PBS/1DFCH, it is noticed that Tg is not clearly visible and the relative temperature at this inflection (Tg) point is found to be 60 °C. The result shows that the melting peak related to the PBS phase becomes clear and the reduction in Tm,PLA is observed due to the generation of imperfect crystals of PLA. For DCP-treated PLA/PBS/1FCH, the decrease in the value of Tm,PLA is probably due to the broad molar mass distribution of the polymeric matrix by the developed cross-linking sites between the chains.[36] It is observed that the shapes of the melting endotherms slightly vary when 1 phr DCP content is added to PLA/PBS/FCH. These data revealed a decrease of ΔHm,PLA after incorporating DCP in PLA/PBS/FCH. This also exhibited a notable Tc in the absence of Tcc for the PLA phase, indicating an increment of the crystallization ability of the amorphous part of PLA from the melt state to cooling cycle. It probably explained from the interface due to phase separation where favorable nucleation sites formed for crystal growth.[12,25] Other researchers have reported that when the Tc of PLA is higher than 126 °C it crystallizes as an α-form.[37] At a higher loading of FCH (∼3 wt %), the decrease in Tc of PLA demonstrates that the unreacted parts of PLA chains have rearranged along with the essential energy to overcome the reaction barrier, and finally crystal is formed at a lower temperature.[38] These data show that the value of ΔHc is also increased, which allows the higher portion of unreacted PLA chains to undergo rearrangement to form a crystal.

The percentage crystallinity values of both PLA and PBS domains are determined, and their values in nanobiocomposites are summarized in Table . The PLA/PBS blend depicts percentage crystallinity (Xc) of 24.86% for PLA and 15.72% for the PBS phase. It could be considered that PBS is more efficient in assisting the crystallization of the PLA phase. However, the Xc (%) of PBS is slightly reduced in the presence of PLA content. It is attributed to the fact that crystallization of PBS is compromised by the PLA phase. Figure reports that with the addition of 1 wt % FCH in the PLA/PBS blend, Xc of PLA content is slightly enhanced, whereas Xc of PBS is decreased. It is found that the Xc of PLA is further reduced on incorporation of FCH especially at a higher loading (∼3 wt %), whereas crystallinity of the PBS phase remained unchanged. Conversely, FCH has a tendency to suppress the crystallization of the PBS phase. With the aid of 1 phr DCP content, the values of crystallinity of both PLA and PBS are increased from ∼27.15 to 49.01 and ∼0.6 to 4.15%, respectively, in the PLA/PBS/1DFCH nanobiocomposite. For the PLA/PBS/3DFCH nanobiocomposite, it is clear that the Xc of PBS content is not altered significantly, whereas the crystallinity of PLA is slightly reduced to 43.23%. This implies the low molecular segment mobility during crystallization.

Table 2

Thermal Stability Parameters and Percentage Crystallinity (% Xc) of PLA/PBS and Their Nanobiocomposite with or without DCP

TGA parameters						DSC parameters
samples	T10 (°C)	T50 (°C)	Tmax (°C)	T90 (°C)	wt loss (%) (up to 400 °C)	% Xc,PLA	% Xc,PBS
PLA/PBS	342	368	371	392	5.7	24	15
PLA/PBS/1FCH	340	366	369	385	4.0	27	0.6
PLA/PBS/3FCH	337	362	366	381	3.6	14	0.5
PLA/PBS/1DFCH	338	370	373	395	7.5	49	4
PLA/PBS/3DFCH	335	368	372	393	4.7	43	3

Thermogravimetric Analysis (TGA) Analysis

Figure a displays the TGA plots of PLA/PBS and PLA/PBS/FCH nanobiocomposites with or without DCP, and quantified parameters are also given in Table . All of the film samples show a two-stage degradation process, revealed by the first derivative thermograms (DTGs) as displayed in Figure b. As mentioned earlier, the incorporation of 1 wt % FCH into the PLA/PBS blend shows no significant reduction in molecular weight, as evidenced by GPC analysis, which is subsequently found to have less effect on thermal properties. For the PLA/PBS/1FCH nanobiocomposite, TGA results show that T10 (10% weight loss temperature), T50 (50% weight loss temperature), and Tmax (maximum degradation temperature) are varied insignificantly in comparison to those for the PLA/PBS blend, which may be due to enhanced compatibility between the PLA and PBS phases in the presence of 1 wt % FCH. However, T90 (90% weight loss temperature) is reduced by ∼6 °C after addition of FCH as compared to that for the PLA/PBS blend, which indicates that FCH promotes the degradation of the PLA/PBS blend at a higher temperature. However, the composite shows reduction in thermal parameters with a higher loading of FCH (3 wt %) in the PLA/PBS blend. This could be explained by the increase in the short OLLA chain attached with chitosan that gives the plasticization effect and finally supports the thermal decomposition of the polymer composite at a lower temperature.[10,36]T50 and T90 values for PLA/PBS/1DFCH are shifted to higher temperatures; however, no significant change in T10 is observed in comparison to that for the PLA/PBS/FCH nanobiocomposite. This suggests that the thermal stability is enhanced due to the development of some cross-linking/branching sites at the polymer interface, which control the decomposition process at a higher temperature.[38,39] The high value of the char yield of the PLA/PBS/1DFCH nanobiocomposite in the high-temperature region is crucial. This indicates that PLA/PBS/1DFCH has good thermal stability. However, in comparison with the PLA/PBS blend, T10 reduction might be associated with the polymer chain scission with the initiator (DCP) and Tm and T50 are merely affected.

Figure 8

(a) TGA and (b) DTG plots of the PLA-based ternary system with and without DCP.

Mechanical Properties

The ultimate tensile strength (TS) of neat PLA is 48 MPa, and elongation at break (% ε) is 8% due to its brittle nature. After addition of 20 wt % PBS, the elongation at break is enhanced up to 14%, whereas tensile modulus (TM) and strength have dropped. This finding confirms the improvement in ductility of PLA during the tensile test. The analysis of the mechanical properties is done to investigate the influence of FCH on the PLA/PBS blend with or without DCP, including the elongation at break (%), tensile strength (MPa), and modulus (GPa). Mechanical properties of all of the strips are displayed in Figure a,b. As expected, the elongation at break is gradually increased from 13 to 24% (approximately 45% increment) for the PLA/PBS/FCH nanobiocomposite due to the presence of low-molecular-weight FCH, which has plasticizing effect. These results indicate that the ductility is improved, which made it more stretchable in comparison with neat PLA. Meanwhile, the influence of FCH is detrimental to Young’s modulus and tensile strength. This could be ascribed to the shorter FCH segments that have the tendency to align faster than the long polymeric segments under the tensile run.[10] Therefore, the presence of shorter polymer chains reduced the Young modulus and strength, but the calculated tensile strength is lower in comparison to that of polyethylene (40 MPa).[40] However, on further addition of 3 wt % FCH in the PLA/PBS blend, elongation at break is decreased probably due to agglomeration of FCH but the tensile strength and modulus almost remain constant. In fact, this is probably due to the insufficient transfer of stress across the interphases of either PLA or PBS phase to the FCH due to their poor interphase adhesion.

Figure 9

Mechanical properties: (a) tensile strength and elongation at break (%) and (b) tensile modulus of the PLA-based ternary system with and without DCP.

When FCH is 1 wt % and DCP content is 1 phr, the PLA-based ternary nanobiocomposite exhibits balanced mechanical properties. For the PLA/PBS/1DFCH nanobiocomposite, the tensile strength and modulus are improved and reduction in elongation at break is noticed in comparison to that of the PLA/PBS/FCH nanobiocomposite. The improvement in the tensile strength and modulus can be assigned to the formation of some cross-linked/branched structure, which subsequently led to restrict the mobility of the polymer chains to dissipate energy under tensile load. However, the determined value of tensile strength is close to that for polyethylene (40 MPa) and elongation at break is still higher than that of the PLA/PBS blend. Hence, the PLA/PBS/1DFCH nanobiocomposite is very useful for the food packaging application. For the PLA/PBS/3DFCH nanobiocomposite, the grafting efficiency is also reduced due to agglomeration of FCH. This may contribute to the drop in the elongation at break, but the tensile strength and modulus are almost constant.

Contact Angle

A film utilized for the food packaging application should prevent the food items from humidity during transport to the consumption stage. The contact angle test is utilized to quantify the wettability of the solid substrate by depositing a liquid drop on its surface.[41] The estimated values of static contact angle indicate the relative wettability of PLA, PLA/PBS blend, and their nanobiocomposites, as shown in Figure . This contact angle is highly relying on the surface tension of the water droplet and surface energy of the examined film surface. The contact angles of PLA and PLA/PBS blend are 70 ± 2 and 80 ± 1°, respectively, which confirms that the PLA/PBS blend is more hydrophobic than neat PLA. It is a well-known fact that hydrophobic PBS shows weak affinity toward water molecules, which subsequently leads to enhance the water resistance of the polymer blend.[42] The addition of FCH into the PLA/PBS blend shows no remarkable change in the wettability of the PLA/PBS blend, which is due to the presence of less polar OLLA tails with low polarity tethered to the hydrophilic chitosan, and thereby maintaining hydrophobicity of the polymer nanobiocomposite film surface.[10,42] For the PLA/PBS/1DFCH nanobiocomposite, the static contact angle is enhanced up to ∼27° in comparison to that of neat PLA, which is probably due to the combined influence of DCP along with FCH. Furthermore, it can be suggested that nucleation of the polymer crystallite contributes to the reduction in polarity of chains, which leads to a higher static contact angle (89 ± 5°) of the nanobiocomposite film. Another possible reason of increased hydrophobicity of the PLA/PBS/1DFCH nanobiocomposite is probably film roughness.[43] Hence, it can be inferred from the contact angle data of all of the samples that the PLA/PBS/1DFCH nanobiocomposite shows the highest degree of hydrophobicity, which is a desirable touchstone in food packaging applications.

Figure 10

Determination of static contact angle (at 25 °C) to confirm hydrophobicity of PLA, PLA/PBS, and PLA/PBS/FCH with and without DCP.

UV Transmittance

The percentage transmission of PLA/PBS and PLA/PBS/FCH nanobiocomposites with or without DCP is displayed in Figure a. The PLA/PBS-based film is the most transparent, representing the maximum percentage of transmission in the visible region (400–800 nm). Further incorporation of FCH into the PLA/PBS blend provokes a remarkable reduction in the transparency of the nanobiocomposite in the visible and UV (400–200 nm) regions, which is probably due to its brownish color.[11] This effect is more pronounced with FCH along with DCP. Subsequently, at 275 nm, which is related to the UV-C region, the transmittance values of the PLA/PBS ternary nanobiocomposites decreased 10, 36, 49, and 62% with the incorporation of 1,3 wt % FCH without and with DCP in comparison with those for the PLA/PBS blend, as seen in Figure b. PLA/PBS/1FCH with DCP demonstrates the maximum UV-blocking influence (250–400 nm) with the lowest UV-C region. Therefore, this result suggests that the PLA/PBS/FCH nanobiocomposite with DCP has enhanced the UV barrier, which is an interesting characteristic for the UV-sensitive food packaging application.

Figure 11

UV–visible spectra (a) in full range (1100–200 nm) and (b) comparison of % transmittance at 700, 400, and 275 nm.

Polarized Optical Microscopy (POM) Analysis

POM investigation for the spherulite growth patterns developed in PLA/PBS, PLA/PBS/FCH, and PLA/PBS/1DFCH during isothermal crystallization at 126 and 140 °C is depicted in Figure a,b respectively. For all samples, maltese cross patterns for PLA spherulites have been observed[44] and the nucleation density is enhanced gradually with time. The POM result confirms that the growth of PBS spherulites is not observed at a temperature above Tc (80 °C) of PBS in the PLA-rich blend. This is probably due to the fact that PLA macromolecule chains in the PLA/PBS (80/20) blend strongly inhibited the crystallization of PBS chains.[45] In the case of the PLA/PBS blend, large and distinct spherulites are developed, which show the weak nucleation ability.[46] Moreover, the incorporation of FCH into the PLA/PBS blend could slightly improve the nucleation ability. PLA/PBS/1DFCH displays the strongest nucleation ability, and after 15 min, spherulites of PLA cover the bulk region. The effect and differences among the PLA/PBS blend and their nanobiocomposites in the presence or absence of DCP are clearly noticed after isothermal crystallization at 140 °C. The growths of spherulites (slope determined from the spherulite radius (μm) against the crystallization time (min)) and nucleation density (the total number of spherulites/mm2) are displayed in Figure c,d, respectively, for PLA/PBS, PLA/PBS/1FCH, and PLA/PBS/1DFCH at 140 °C. At 140 °C, the PLA/PBS/1DFCH nanobiocomposite represents the strong nucleation ability, which is due to the generation of the cross-linking/branching sites, which possibly function as nucleation sites.[47−49] Both DSC and POM analyses confirm that the crystallization of the PLA/PBS blend is accelerated by the presence of FCH with DCP. According to the studies of Kim et al.[25] and Ji et al.,[49] the DSC result confirmed that the developed cross-linked/branched structures during reactive blending may act as nucleation sites in the reactive system.

Figure 12

POM images of the PLA/PBS blend and its FCH-based nanobiocomposite with or without DCP crystallized isothermally at (a) 126 °C and (b) 140 °C (0–20 min). (c) Radius and growth rate of spherulites and (d) nucleation density (number of spherulites/mm2) at 140 °C.

Moreover, the growth rate of spherulites of PLA/PBS/1DFCH is inhibited (after 10 min) due to the entanglement of polymeric chains, which subsequently led to overlapping of high-density spherulites and finally deformation in their shapes.[47,50] It is found that the lowest crystal size and the highest nuclear density (maximum number of spherulites/mm2) are noticed for PLA/PBS/1DFCH. Figure 3Sa,b displays the dependency of the number of spherulites on time at 126 and 140 °C, respectively

Conclusions

The present investigation provides a simple industrially viable melt extrusion technique to fabricate a PLA/PBS/FCH-based ternary nanobiocomposite with or without DCP, and the structure modification is confirmed by FTIR analysis. The surface morphology study confirms that no PBS droplet is observed in the PLA continuous phase by incorporating DCP or 1 wt % FCH. Furthermore, good dispersion of FCH is observed for the PLA/PBS/1DFCH nanobiocomposite in both polymer domains, which helps in obtaining a balanced set of mechanical properties with improvised crystallization efficiency. Interestingly, Xc of PLA is enhanced 33 and 27%, as confirmed by wide-angle XRD and DSC, respectively, for PLA/PBS/1DFCH. Also, DSC investigation confirms that the crystallization rate of PLA is increased after the incorporation of DCP in the PLA/PBS/FCH nanobiocomposite, mostly because the developed cross-linked structure acts as nucleating sites. Furthermore, as evident by POM results, spherulites formed in the reactive nanobiocomposite are larger in number and smaller in size than those formed in the PLA/PBS blend and ternary nanobiocomposite (0% DCP). In addition, the nucleation density is also the highest, which is referred to the active participation of FCH in the presence of DCP. For PLA/PBS/1DFCH, the static contact angle is significantly increased (∼27°) as compared to that of PLA. The remarkable UV-C-blocking effect of the reactive modified nanobiocomposite may suggest a possible application toward packaging of UV-sensitive materials.

Experimental Section

Materials

Poly(l-lactic acid) (PLA) (grade: 4032D, 98.6 of d-lactic and 1.4 of l-lactic acid unit) with number-average molecular weight, Mn, ∼ 150 000 Da and polydispersity index, PDI, ∼ 1.3 was purchased from NatureWorks. Poly(butylene succinate) (grade: 1001MD) with Mn ∼ 88 000 Da and PDI ∼ 2.2 was supplied by Showa Highpolymer Singapore Pte. Ltd. (Japan). Dicumyl peroxide (DCP, purity >99.5%) was purchased from Sigma Aldrich (India) and used as a radical initiator. Whatmann filter paper (grade: 1) was used to extract the functionalized chitosan (FCH). Acetone was supplied by SISCO Research Laboratories (SRL chemicals, India). All of the chemicals have been consumed without any further purification.

One-Pot Microwave-Assisted Synthesis of FCH

FCH is produced using microwave-assisted synthesis as reported elsewhere.[10] Both l-lactic acid and chitosan are mixed in the ratio of 3.33:1 (w/w %) in a round-bottom flask (RBF) to perform the synthesis of functionalized chitosan. Lactic acid is mixed manually and completely soaked by hydrophilic chitosan to enhance the reaction performance. The soaked mixture is placed inside a microwave oven, and an inert atmosphere inside RBF is generated by purging N2 gas. The condenser and ice bath are connected with RBF and are placed outside the microwave in such a way that the generated byproducts can easily collect in the reservoir. The reaction termed as “condensation polymerization” is executed at 240 W and 110 °C for 30 min. The microwave is automatically stopped after the completion of reaction, and the N2 atmosphere is broken by closing the gas valve.

A low-molecular-weight oligomer chain (Mn and Mw values are 1400 and 3000 Da, respectively) has been attached with the chitosan backbone during the reaction, and finally, a dark brown viscous product is formed as per the mentioned reaction pathway in Scheme .

Scheme 2

Reaction Pathway for the Synthesis of Functionalized Chitosan

Thereafter, a fixed amount of FCH is dissolved in acetone (20 mL) and stirred at 1000 rpm for 6 h, followed by vacuum filtration. The supernatant has been discarded, and the remaining solution is collected for its use in further experimental work.

Preparation of PLA/PBS/DCP/FCH Nanobiocomposites via the Melt Extrusion Technique

PLA and PBS pellets are vacuum-dried at a temperature of 40 °C overnight to remove bound moisture. DCP (1 phr) is dissolved in acetone (10 mL) and sprayed on the PLA/PBS (80:20 wt %) blend.[9] The DCP-coated PLA/PBS blend is mixed with filtered FCH (1 and 3 wt %) and left at room temperature to remove the trapped acetone before processing, followed by drying in a vacuum oven at 60 °C. Then, PLA/PBS/FCH pellets with or without DCP are melt-mixed using a twin extruder (HAAKE MiniLab II, Thermo Fisher Scientific), and the processing temperature and screw speed are set as 185 °C and 60 rpm, respectively, for ∼5 min. Hereafter, PLA/PBS-based composites extruded with or without DCP having different loadings of FCH (1 and 3 wt %) are labeled as PLA/PBS (80/20), PLA/PBS/1FCH (80/20/1), PLA/PBS/3FCH (80/20/3), PLA/PBS/1DFCH (80/20/1/1), and PLA/PBS/3DFCH (80/20/1/3). Finally, the extruded strips are used for testing.

Characterization Techniques

Fourier Transform Infrared (FTIR) Study

IR spectra of samples are collected by Spectrum FTIR (Frontier, PerkinElmer). These are recorded in transmission mode with attenuated total reflectance (ATR) assembly. An extruded strip is placed on a ZnSe-based crystal surface in the wave number range 4000–600 cm–1 with 16 scans at a 4 cm–1 resolution for the analysis. A background spectrum is collected prior to analysis to compensate the influence of humidity and carbon dioxide present in air by subtracting spectra.

Gel Content (%)

High-performance liquid chromatography (HPLC)-grade chloroform is used for the removal of unreacted part because it is a good solvent for PLA, PBS, and FCH but not appropriate for cross-linked structure. The amount of gel from the extruded strips is obtained by washing it in excess chloroform for ∼12 h; after that, the suspension is vacuum-filtrated and gel is washed several times to remove the unreacted portion. Finally, the gel is collected and vacuum-dried at ∼40 °C to remove the chloroform. The gel fraction (%) is calculated using eq where W0 is the initial weight of extruded strips and Wgel is the final weight of dried gel obtained after extraction.

Gel Permeation Chromatography (GPC)

This is used for the analysis of samples to determine the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI). Samples (∼40 mg) are dissolved in HPLC-grade chloroform (∼2 mL) for 24 h and filtered prior to analysis.

Field-Emission Scanning Electron Microscopy (FESEM)

FESEM (ZEISS) is used to examine the fracture behavior of the extruded strips at an accelerating voltage of ∼2–5 kV. After tensile testing, cross sections of the film strips are mounted on a stub using carbon tape and sputter-coated with a thin layer of gold for ∼180 s under vacuum before analysis.

Wide-Angle X-ray Diffraction (XRD)

XRD measurements are carried out using a diffractometer (D8 Advance, Bruker, Germany) equipped with Ni-filtered Cu Kα radiation (λ = 0.1541 nm). The operating sources (receiving and diverging slits) are set at 40 kV and 40 mA at a scan rate of ∼2 s/step in the 2θ range of ∼3–50°. Film strips are annealed at 60 °C for 6 h prior to analysis. The diffractogram inbuilt software is used to deduct the peaks, and the crystallinity indexes (ICR) of polymer composites are determined using eq where Ac and Aa represent the area of crystalline and amorphous peaks, respectively.

Differential Scanning Calorimetry (DSC)

The calorimetric measurement of the sample is subjected to DSC (NETZSCH, Germany) equipped with a liquid nitrogen cylinder required for cooling. The sample is kept in a platinum crucible, heated from −50 to 200 °C at a scanning rate of 10 °C/min under a nitrogen (50 mL/min) atmosphere, and held in the isothermal state for 2 min at 200 °C to erase the thermal history. The sample is then cooled to −50 °C with the same scan rate to understand the crystallization behavior. Then, it is heated again to 200 °C with the scan rate as that in the first heating cycle. Indium is used to check the calibration according to the given standard before sample analysis. For calculating the percentage of crystallinity (%Xc), the used standard heats of fusion (ΔHm°) for pure PLA and PBS are 93.0 and 110.0 J/g, respectively.

Thermogravimetric Analysis (TGA)

Thermal stability of the PLA/PBS blend and its composites is calculated by TGA (4000 PerkinElmer) to acquire knowledge about its thermal behavior. The measurement is carried out in the temperature range of 30–600 °C at 10 °C/min under a N2 inert atmosphere.

Universal Testing Machine (UTM)

The mechanical properties (elongation at break, tensile strength, and modulus) of the strips are measured using UTM KIC 2-050-C (Kalpak Instruments, India) equipped with a load cell (500 N) at a speed of 5 mm/min. The dimensions of the specimens are 50 mm in length and 5 mm in width. Three specimens are tested for each sample, and the average value is reported with the standard deviation.

Contact Angle (CA) Analysis

Hydrophobicity quantification is performed using a CA analyzer (Krüss DSA25, Germany). The sessile drop method is used to measure the static contact angle by depositing a drop of Millipore water on the film substrate at various locations. The measurement of each sample (∼2 × 2 cm2) is repeated at least three times, and sufficient time is provided for the drop to settle before angle measurement.

UV–Visible Spectrophotometry

Optical transmittance of samples (25 × 15 mm2) is measured with a UV–visible spectrophotometer (Lambda 25, PerkinElmer) over the wavelength range 250–1100 nm. The transmittance spectrum is analyzed in the presence of air as a reference medium.

Polarized Optical Microscopy (POM)

POM analysis is carried out to investigate the growth rate of PLA crystals with a POM eclipse (LV100N POL, Nikon Co., Japan) instrument. The samples (∼0.3 mg) of PLA/PBS, PLA/PBS/FCH, and PLA/PBS/1DFCH are sandwiched between two glass slides to make a thin film at 190 °C. For POM study, all of the samples are first melted by a hot stage (Linkam TST350, Linkam Scientific Instruments, U.K.) at 185 °C with 50 °C/min and held for 3 minutes. After this, samples are cooled rapidly to distinct temperatures (i.e., 126 and 140 °C) with the same rate and placed at isothermal conditions for 20 min. These hold temperatures would detect the influence of DCP along with FCH on PLA crystals.