Cellulose Functionalized High Molecular Weight Stereocomplex

Research Article pubs.acs.org/journal/ascecg

Cellulose Functionalized High Molecular Weight Stereocomplex Polylactic Acid Biocomposite Films with Improved Gas Barrier, Thermomechanical Properties Arvind Gupta and Vimal Katiyar* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Kamrup, Assam 781039, India S Supporting Information *

ABSTRACT: This work presents a facile, solvent-free approach for the fabrication of PLA biocomposites, followed by melt extrusion process to prepare stereocomplex PLA films with excellent thermomechanical and gas barrier properties. The presence of stereocomplex crystallites improves the thermal properties of polylactic acid (PLA); however, the formation of stereocomplex crystallites is predominantly lesser compared to homocrystallites in case of high molecular weight poly(L-lactic acid) and poly(D-lactic acid) blend. Grafting of biofillers with polymer matrix chains may help in homogeneous dispersion and formation of stereocomplex crystallites. Henceforth, stereocomplex PLA was fabricated with cellulose microcrystals (CMC) as filler, after chemical modification by in situ ring opening polymerization of D-lactide. The stereocomplexation in the blend system was found to be enhanced by the extended molecular surface area provided by grafted CMC. As confirmed by morphological analysis, the modification of CMC drives the homogeneous dispersion into the matrix and reduction in the size of CMC in the range of ∼200 nm diameter. Increased melting temperature (∼209 °C) with no evidence of homocrystallites confirm the role of grafted CMC in the formation of stereocomplex crystallites by suppressing the development of homocrystals. The fraction of stereocomplex crystallites was found to be 100% when analyzed using X-ray analysis. The enhanced stereocomplexation in the composites resulted ∼96% improvement in the tensile strength in comparison to pristine PLLA/PDLA blend. Interestingly, the oxygen permeability and water vapor permeability were reduced by ∼25% and ∼35%. The improved thermomechanical properties of the biocomposites through enhanced stereocomplexation may comply with the requirement for high temperature engineering and packaging applications. KEYWORDS: Stereocomplex, Polylactic acid, Cellulose microcrystals, Grafted cellulose microcrystals

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Stereocomplexation in PLA was first introduced by Ikada and co-worker.6 They have reported increased melting temperature of the stereocomplex PLA by ∼50 °C in comparison to enantiomeric pure PLA. Stereocomplexation is the special type of crystalline arrangement of PLA chains that can be fabricated by solution or melt mixing of the two enantiomers, i.e., poly(Llactic acid) (PLLA) and poly(D-lactic acid) (PDLA). Formation of stereocomplex crystallites increase the chain packing, which ultimately leads to enhancement in the crystalline density of PLA which in turn results in the enhancement of the physical properties of the material such as thermal, barrier, mechanical, and so forth. It is also reported that, in the case of molar masses greater than 100 kDa, homocrystals evolve along with stereocomplex crystallites, which gives a negative impact to the properties of the end product such as decreasing the

INTRODUCTION Increasing concerns about the environment and human health1 have encouraged the scientific community to develop biobased materials for different applications. Among the various biodegradable polymers, polylactic acid (PLA) is considered to have a relatively higher barrier, thermal, and mechanical properties which make it a promising candidate for replacing the petroleum-based polymers.2 PLA is a biobased aliphatic polyester derived from lactic acid which is a fermented product of renewable resources such as corn, potato, or other glucoserich materials.3 Due to some limitations such as relatively low glass transition temperature, poor melt strength, low crystallization rate, relatively higher gas permeability, and so forth as compared to petroleum based polymers, PLA is not to date extensively being used in the engineering applications. In order to improve the properties of PLA, significant efforts have been made by the researchers worldwide and attention has been given to stereocomplexation and fabrication of biocomposite4,5 of PLA. © 2017 American Chemical Society

Received: April 7, 2017 Revised: June 5, 2017 Published: June 15, 2017 6835

DOI: 10.1021/acssuschemeng.7b01059 ACS Sustainable Chem. Eng. 2017, 5, 6835−6844

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ACS Sustainable Chemistry & Engineering

neously in the PLA matrix, and the PDLA chains grafted with CMC will provide the extended molecular surface area for the interaction with enantiomeric PLA, which may enhance the stereocomplexation in matrix. The mechanism of stereocomplexation in the presence of PDLA grafted CMC, and its effect on various properties of the fabricated biocomposites including mechanical, barrier, and thermal properties are discussed in detail.

melting temperature of PLA, and relatively poor mechanical and barrier properties. However, producing biocomposite using natural fillers such as clays,2,7 cellulose,8 cellulose nanocrystals,9 silk,10,11 chitosan,12,13 gums,14 sucrose palmitate,15 and so forth also enhance the thermal, barrier, and mechanical properties of PLA. Researchers are using these materials directly or after modification16,17 with polymer matrices to enhance the property.18 Among these materials, cellulose is the most abundant, naturally occurring, biobased, low cost material, and has relatively excellent chemical and physical properties.19 It is often observed that the reinforcing efficiency of fillers in polymer matrices is deeply affected by its uneven mixing and agglomeration.20−22 It is known that cellulose has a large number of hydroxyl functional groups present on its surface, and it can easily be modified.23 In-situ polymerization of monomer in the presence of fillers is found to be effective in order to make homogeneous mixture which leads to the improved properties of the materials.7,24−31 In case of PLA, hydroxyl functional groups present on the surface of cellulose molecules can initiate the polymerization of lactide and propagate on the surface of cellulose. Researchers have adopted the dual strategy, i.e., stereocomplexation and formation of biocomposite to form the stereocomplex PLA cellulose biocomposite. Jiang et al.32 have directly used cellulose nanocrystals as a nucleating agent in the PLLA/PDLA blend matrix, and they found improved crystallizability of the blend. Habibi et al.33 have studied the effect of stereocomplexation on cellulose PLA biocomposite. They synthesized the surface modified cellulose nanocrystal by ring opening polymerization of D-lactide in solution medium and melt blended with PLLA to form a stereocomplex. A similar process of the modification of cellulose has been utilized by Wu et al. and Miao et al.29,34 and stereocomplex had been reported either by solution or by melt blending method. Purnama and Kim35 have prepared the biocomposite of stereocomplex polylactide and cellulose nanowhiskers using supercritical fluid technology. They have grafted the cellulose nanowhiskers with PLLA or PDLA and mixed with each other in the ratio of 1:1, and treated the same in supercritical condition in the presence of carbon dioxide and dichloromethane. Other methods such as use of comb-shaped cellulose-g-poly(lactide) nanohybrids36 or cellulose acetate-gpoly(lactic acid) with comblike topology37 have also been employed by the researchers to fabricate the stereocomplex PLA cellulose biocomposite. It is commonly found in most of the research articles that grafting or modification of cellulose involves organic solvents, and the same is also used in the fabrication of stereocomplex PLA cellulose biocomposite. Several reports are available which deal with the crystallization kinetics of stereocomplexation in a PLA cellulose biocomposite. However, to the best of our knowledge, no literature has yet been reported emphasizing its real-time application oriented properties like packaging and high temperature engineering field. In this work, we present a simple and versatile method for chemical modification of cellulose microcrystals (CMC) and its subsequent use in the fabrication of stereocomplex PLA cellulose biocomposite. PDLA grafted CMC has been produced by in situ ring opening bulk polymerization of D-lactide in the presence of CMC and the same is used as a filler into the PLLA matrix. It is assumed that, due to the grafting of PDLA on the surface of CMC, modified CMC will be dispersed homoge-

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EXPERIMENTAL SECTION

Materials. L-lactic acid and D-lactic acid were procured from Purac, India and Musashino, Japan, respectively. Tin oxide (SnO), cellulose microcrystals (CMC), hexafluoroisopropanol (HFIP), and stannous octoate were purchased from Sigma-Aldrich. Toluene and chloroform were provided by Merck India. L-lactide and D-lactide were synthesized by a two-step polymerization and depolymerization procedure. L-lactic acid or D-lactic acid was dehydrated at 110 °C to get the oligomeric PLLA or PDLA, and it was depolymerized in the presence of tin oxide to obtain L-lactide or D-lactide after purification. All of the chemicals were used as-received without further purification. Preparation of PDLA Grafted CMC. PDLA grafted CMC with different CMC compositions (1, 5, and 10 wt %) were synthesized through in situ ring opening polymerization. In-situ polymerization was conducted in a glass ampule with magnetic stirring bar dipped in a oil bath. D-lactide and the required amount of CMC and catalyst tin octoate toluene solution were added to the ampule with a monomer and catalyst molar ratio of 2000:1. The ampule was purged with argon gas for 1 h to make it free from moisture and oxygen content. The vacuum was applied to the ampule for 2 h to eliminate the excess amount of toluene. Ampoule was sealed under vacuum conditions, and the oil temperature was raised to 105 °C. Mixing of monomer, CMC, and catalyst was done for 2 h after which the temperature was increased to 160 °C, and the ampule was left for two more hours for ring opening polymerization. The obtained PDLA grafted CMC was removed by breaking the ampule and named as PDLA-1%CMC, PDLA-5%CMC, and PDLA-10%CMC. PLLA and PDLA were also synthesized in the same fashion, and the molecular weights of the synthesized polymers are listed in Table 1. The granulated form of

Table 1. Molecular Weight and Specific Rotation of the Synthesized PLAa sample name

Mw (kDa)

PDI

[α]25589

PLLA PDLA PDLA-1%CMC PDLA-5%CMC PDLA-10%CMC

239 182 192 222 217

1.5 1.5 2.1 1.9 2.1

−157 156

a Mw: Weight-average molecular weight, PDI: Polydispersity index, [α]25589: Specific rotation

PLLA, PDLA, and composite were kept in a vacuum oven at 40 °C for 12 h to remove the residue monomer. During ROP, PDLA was also formed, which remained in the PDLA-CMC biocomposite. Preparation of Stereocomplex PLA CMC Biocomposite. In this procedure, PDLA-CMC and PLLA, with 1:1 a weight ratio, were mixed and extruded into the twin screw miniextruder (HAAKE MiniLab II from Thermo Scientific). The miniextruder was first heated to 210 °C for 30 min to get a uniform temperature, and the screw speed was maintained at 30 rpm. Initially, the neat PLLA was extruded to get a homogeneous temperature into the equipment. The mixture of PLLA and PDLA-CMC was introduced into the closed loop (recycle mode) for melt mixing of materials in the presence of an inert (N2 gas) environment. After 2 min, the loop was opened to receive the mixed melt into the injection molding cylinder maintained at 210 °C (HAAKE MiniJet Pro from Thermo Scientific) for the formation of dumbbell-shaped articles. The mold temperature was kept constant at 6836


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ACS Sustainable Chemistry & Engineering 90 °C. The stereocomplex PLA biocomposite with 0.5%, 2.5%, and 5% CMC content were denoted as sPLA-0.5%CMC, sPLA-2.5%CMC, and sPLA-5%CMC, respectively, while a neat stereocomplex PLA was denoted as sPLA. The obtained samples were annealed at 120 °C for 30 min before further analysis. Characterization. The FTIR spectra of the samples was measured by attenuated total reflection (ATR) mode in Frontier FT-IR spectrometer (PerkinElmer, U.S.A.) at room temperature. The spectra were recorded after 16 scans from wavenumber 4000 cm−1 to 650 cm−1. The purification of grafted CMC was done by dissolution followed by centrifugation process. Grafted CMC was dissolved into the chloroform (10 mg/mL) and centrifuged at 5000 rpm, and the solid residue was again dissolved into chloroform to remove the excess PLA or oligomeric chains and followed by centrifugation. This process was repeated four times, and complete removal of ungrafted PLA chains was confirmed by FTIR. The prepared biocomposite was directly analyzed, and the spectra were recorded. The molecular weight of the prepared materials were measured using the gel permeation chromatography (GPC) (Shimazdu, Japan) at 40 °C. HPLC grade chloroform was used as an eluent with a flow rate of 1.0 mL·min−1. The system was calibrated using the monodispersed polystyrene standard. Prepared biocomposite was dissolved into the chloroform with 5% HFIP content and filtered through 0.45 μm PTFE syringe filters before analysis. The percent grafting and percent conversion of CMC were determined using eqs 1 and 2,

percent grafting (%) =

W1 − Wo × 100 Wo

(1)

W1 × 100 Wo

(2)

percent conversion (%) =

where, Xc,sc and Xc,hc are the degree of crystallinity of stereocomplex and homo crystallites, respectively. Ac,sc and Ac,hc are the area for the stereocomplex, homo crystallites, respectively, and Aa is the area of amorphous phase. The oxygen gas transmission rate (OTR) for the biocomposites at different loading of CMC was determined using OX2/231 oxygen permeability tester (Labthink, China). Measurements were carried out at 15 °C, 23 °C, 35 °C, and 45 °C using oxygen gas of high purity (99.999%) on a film having an area of 50 cm2 following the ASTM D3985 standard. Pure oxygen (99.9%) at a pressure of 0.5 bar and a flow rate of 20 mL/min was maintained into the upper half of the sample chamber during analysis while nitrogen gas was continued into the lower half of the chamber. The chambers were purged for at least 6 h before measurement. The test was performed for at least 6 h so as to reach the steady state. Water vapor transmission rate (WVTR) was recorded using a PERMATRAN-W Model 1/50 (Mocon, U.S.A.) following ASTM standard E398-03. The relative humidity (RH) was fixed to 100% in the wet chamber and 10% in the dry chamber, yielding a driving force of 90% RH. The film of 50 cm2 area was analyzed at the atmospheric pressure and at a temperature of 37.8 ± 0.1 °C. The films for OTR and WVTR analysis were prepared by solution cast method. Sample obtained after melt extrusion was dissolved in chloroform with 5% v/v HFIP content and kept for stirring for 24 h at room temperature. The obtained solution was poured into a PTFE Petri dish and left for 24 h to allow the solvent to evaporate. The obtained polymer films were dried in a vacuum oven at 50 °C for 24 h. The tensile strength and the percentage elongation of the prepared samples (5 mm width, 2 mm thickness, and 50 mm gauge length) were measured using universal testing machine (KIC-2-050-C, Kalpak instruments and controls, India) equipped with 500 N load cell at a constant cross-head speed of 5 mm·min−1 in tensile mode. Five replicates of each sample were tested and the mean of the obtained results were reported along with standard deviation. Mechanical stability of the prepared biocomposites at higher temperatures with dynamic force application were measured using DMA (DMA 242 E model, NETZSCH GmbH) in the temperature range 25−160 °C at 2 °C·min−1 heating rate, 1 Hz frequency, and 10 μm displacement amplitude.

where Wo and W1 are the weight of CMC and weight of grafted CMC, respectively. The surface topography and morphology of CMC and other PLA composites were assessed using Atomic force microscopy (AFM) (Agilent, Model 5500 series) with silicon cantilever having a spring constant of 42 N/m at a resonance frequency of 320 kHz. The samples were prepared by drop casting followed by drying at room temperature for 48 h. AFM images were analyzed with the Gwyddion software (version 2.41). Field Emission Scanning Electron Microscope (FESEM) was used to examine the topography of the fractured surface of different samples placed on carbon tape. The samples were further coated in gold sputtering unit for 30 s and characterized using FESEM (Sigma, Zeiss, GmbH) at an accelerating voltage of 3−4 kV. The thermal behavior of the biocomposite was measured using the differential scanning calorimeter (DSC) (Phoenix DSC 204 F1 NETZSCH, GmbH) under nitrogen atmosphere. Five to 10 mg of sample was heated from 20 to 250 °C and kept under isothermal conditions at 250 °C for 2 min to get rid of the thermal history. The material was cooled to 20 °C with a 10 °C·min−1 cooling rate and then heated to 250 °C with same heating rate. X-ray diffraction (XRD) spectra were taken using Model-D8 Advance system diffractometer (Bruker, Germany) equipped with Cu−Kα radiation (λ = 0.1541 nm) as X-ray source operating (40 kV, 40 mA) at a scan rate of 3° per min in the 2θ range 5°−40°. The degree of crystallinity and fraction of stereocomplex crystallites (fsc) were determined using following the equations:

Xc,sc(%) =

Xc,hc(%) =

fsc (%) =

Ac,sc (Ac,sc + A a ) Ac,hc (Ac,hc + A a )

Xc,sc (Xc,hc + Xc,sc)

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RESULT AND DISCUSSIONS Synthesis of PDLA Graft CMC. FTIR is a powerful tool to examine the conformation of molecular functional groups and was employed to analyze the grafting of CMC with PLA molecule. Figure 1 shows the spectra of PLA, CMC, and grafted CMC. It can be seen from Figure 1 that the characteristic peaks for PLA are located at 1754 cm−1, 1453 cm−1, 1382 cm−1, 1362 cm−1, 1127 cm−1, and 868 cm−1, which corresponds to the carbonyl stretching, methyl bending, symmetric and asym-

× 100 (3) × 100 (4)

× 100 (5)

Figure 1. FTIR spectra of CMC, graft CMC, and PDLA. 6837


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Figure 2. AFM images of the dispersed CMC (a), PDLA-1%CMC (room temperature) (b), PDLA-1%CMC (annealed at 120 °C) (c), PDLA-10% CMC (annealed at 120 °C) (d), pristine PDLA (annealed at 120 °C) (e), pristine sPLA (annealed at 120 °C) (f), and sPLA-5%CMC (annealed at 120 °C) (g).

Figure 3. Cartoon for the modification of CMC and its demonstration by AFM images.

metric bending of −CH−, stretching of −C−O− and −C−C− backbone of PLA3, respectively. Peaks corresponding to stretching vibration of hydrogen bonded hydroxyl group and C−H symmetrical stretching in CMC are located at 3384 cm−1 and 2898 cm−1, respectively. The −CH2, −CH, and OH deformation was assigned to 1427 cm−1, 1373 cm−1, and 1317 cm −1 , respectively. Wavenumber 1210−920 cm −1 was attributed to the C−O stretching of the C−OH and C−O− C bonds. The peak at 894 cm−1 corresponds to the stretching and deformation of C−O−C, C−C−H, and C−C−O in

CMC.38 The spectra of grafted CMC was recorded after purification. It was observed that the spectrum of grafted CMC has one additional peak at 1754 cm−1 along with all the characteristic peaks of CMC. The presence of characteristic peak for carbonyl group, i.e., CO of PLA at 1754 cm−1 in spectra of grafted CMC shows the grafting of PLA chains on the surface of CMC. Peaks related to the methyl bending, bending of C−H, and stretching of CO in PLA chains were found to merge with peaks related to CMC. The broadness of the spectra at 3384 cm−1 was reduced as CMC was grafted with 6838


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ACS Sustainable Chemistry & Engineering PLA chains. Further, it was also seen that the 2 wt % suspension of the grafted CMC in chloroform was more stable than pristine CMC as shown in Figure S1 (Supporting Information, SI). The percent grafting and conversion of CMC was also calculated, which resulted in ∼65% CMC percent grafting and ∼165% percent conversion. Atomic force microscopy is a powerful technique to visualize the samples in micro and nanometer range. The AFM images of several samples are shown in Figure 2. Specimens were prepared by solution-drop cast method followed by drying. In case of pristine CMC, the particles of CMC have formed the percolated self-structure as shown in Figure 2a. This percolation network may be facilitated by the hydrogen bonding between the CMCs.39,40 The sample of PDLA-1% CMC was also cast on the coverslip in a similar way as that of a pristine CMC, and the same percolated structure was formed due to presence of CMCs as shown in Figure 2b. To understand the crystallization and spherulite formation by PDLA, the same sample was annealed at 120 °C for 1 h, and AFM images are shown in Figure 2c. It was found that the PDLA chains were crystallized on the surface of the CMC due to grafting which led to the epitaxial growth of PDLA chains on CMC (Figure 2e). With increasing CMC concentration, the void space between the CMC particles reduced as shown in Figure 2d. The growth of the spherulite on the surface of CMC confirms that PDLA chains have the affinity toward CMC which may be the result of grafting. From the above discussion, the grafting and its effect has been summarized in Figure 3, which shows that in situ synthesis of PDLA in the presence of CMC leads to the development of grafted CMC. This grafting helps to enhance the dispersion of CMC into the PLA polymeric matrix which ultimately affects properties of the final product. Stereocomplex Formation. Prepared grafted CMC was used as filler into the PLA matrix to examine its effect on the formation of stereocomplex crystallites. Calorimetry is the best suited analytical technique for the determination of stereocomplex crystallites in PLA. It is known that the homocrystals of PLA have melting temperatures less than ∼180 °C whereas, the stereocomplex crystallites are found to have the melting temperature above 200 °C.41 DSC analysis of the biocomposites has been shown in Figure 4a. It can be observed from that the thermogram of sPLA has three distinct types of endothermic peaks; the peaks at ∼152 °C and ∼180 °C which are related to the homocrystals of PLLA and PDLA may be in α and α′ crystallite form,42 whereas the third peak at ∼211 °C is related to the stereocomplex crystallites. However, the peaks related to the homocrystallites disappeared with the addition of the grafted CMC into the PLA matrix. It may so happen that CMC helped in the formation of stereocomplex crystallites while developing some hindrance in homocrystallization. In order to obtain the insights of the phenomenon, a detailed study was made through XRD and FTIR analysis of the samples which has been discussed in the subsequent sections. The DSC exotherm of samples in the cooling cycle are shown in Figure 4b. As shown, it was found that the crystallization peak shifted toward the higher temperature range from 116.2 °C to 144.1 °C. Increase in the crystallization temperature gives the evidence about the restriction of polymer chains due to presence of grafted CMC which needs more energy to be folded or form crystallites. At higher temperature, polymer chains have higher energy to easily be diffused from amorphous phase to crystalline phase and fold to form spherulites.

Figure 4. DSC thermogram of sPLA and sPLA-CMC biocomposite (heating cycle) (a) and cooling cycle (b).

Formation of stereocomplex crystallites was further analyzed by X-ray diffraction analysis, and the diffraction spectra are shown in Figure 5. It is known that PLA homocrystals crystallize (α form) in a pseudo-orthorhombic unit cell of dimensions a = 1.07 nm, b = 0.595, and c = 2.78 nm containing 103 helical structure (10 Å rise per 3 monomeric units). The diffraction peaks for PLA situated at 14.7°, 16.5°, and 18.9° correspond to (010), (200/110), and (203) crystal planes, respectively (XRD pattern of the CMC, PDLA, and PDLACMC biocomposites is shown in Figure S4), whereas the stereocomplex crystallites of PLA crystallized in a triclinic unit cell with dimensions a = 0.916 nm, b = 0.916 nm, c = 0.870 nm, α = β = 109.2°, and γ = 109.8° are found to have 31 helical structure (3 Å rise per monomeric units).43 The diffraction pattern at 11.8°, 20.6°, and 23.9° are related to stereocomplex crystallites and correspond to the (110), (300)/(030), and (220) lattice planes, respectively. It has been found that the intensity of diffraction pattern for the homocrystals was reduced as the content of grafted CMC increased to 2.5% and vanished in case of 5% grafted CMC content. Similarly, the intensity of the peaks related to the stereocomplex crystallites enhanced drastically with increase in the grafted CMC content (Figure 5a). It shows that the presence of grafted CMC in the matrix promotes the formation of stereocomplex crystallites and suppresses the development of the homocrystals. In terms of crystallinity, it has been seen that the overall crystallinity was found to be almost same, i.e., ∼55% and only the content of 6839


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which exist due to the presence of opposite helical structure of PLLA and PDLA.45 This hydrogen bonding was also confirmed by the red shift of crystalline sensitive peak of PLA from 1040 to 1038 cm−1 (Figure 6d) and the shift of methyl stretching vibration spectra from 2996 to 2993 cm−1 (Figure 6c). From the above discussion, it can be concluded that the grafted CMC complements the formation of stereocomplex crystallites, while suppressing the development of the homocrystallization. The schematic in Figure S2 can be made on the basis of above discussion, which shows the melt mixing of PDLA-CMC and PLLA and its cooling. During cooling, the melt was crystallized and led to the formation of stereocomplex crystallites depending on the content of grafted CMC. Barrier Properties. Effect of grafted CMC on stereocomplexation in PLA matrix and its ultimate effect on the barrier properties was estimated. It has already been discussed in the previous section that the homocrystals form a pseudo orthorhombic unit cell with a 103 helical (10 Å rise per 3 monomeric unit) structure, and stereocomplex crystallites form a triclinic unit cell with a 31 helical (3 Å rise per monomeric unit) structure. The lower length rise per unit of monomeric units for the stereocomplex crystallites suggest the compact nature of crystals, which may prevent the diffusion of gaseous molecules through it. Development of the biocomposite proved to be able to enhance the barrier properties such as oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of the PLA.13,14,46 Due to the compact crystalline nature of the stereocomplex crystallites, the surface of the PLA sheet or film gets modified, repelling water molecules and thus preventing the permeation. Similarly, the dense (the densities of the fabricated materials are shown in Table S1) structure of the stereocomplex crystallites reduce the diffusion of the oxygen molecules which enhance the oxygen barrier property of PLA. The present work references the inclusion of fillers into the PLLA/PDLA blend system that are not only efficient in improving the degree of stereocomplexation but also increase the inherent oxygen and water barrier property through entailing a tortuous path for the molecules.47 The water vapor permeability of different samples is shown in Figure S3. WVTR for sPLA was found to be 8.91 g·mm·m−2·day−1 and it was reduced by 34.8% to 5.81 g·mm·m−2·day−1 after addition of 2.5% grafted CMC. This reduction can be attributed to the film surface modification by the formation of stereocomplex crystallites. WVTR is highly dependent on the surface energy which may be enhanced by the formation of stereocomplex crystallites, and the dense packing of PLA chains also hinder the diffusion of water molecules. However, for sPLA-5%CMC, WVTR again increased to 7.07 g·mm·m−2·day−1, which may be due to the presence of high amount of grafted CMC. CMC, without modification, is recognized as a hydrophilic substance, and it may become hydrophobic after modification.48 However, increased content of grafted CMC may develop the diffusion pathway for the water molecules to permeate through it and may be accountable for increased WVTR. Enhanced stereocomplexation and the content of grafted CMC also affect oxygen permeability. The results of OTR are displayed in Figure 7, and it was found that the enhanced stereocomplexation and the presence of filler in the matrix affects the oxygen barrier properties. The OTR for sPLA was found to be 8.1 × 10−2 cm3·mm·m−2·day−1·kPa−1 was reduced by approximately 25% to 6.1 × 10−2 cm3·mm·m−2·day−1·kPa−1 when 5% grafted CMC was added to the matrix. The increased

Figure 5. X-ray diffraction of sPLA and sPLA-CMC biocomposite (a) and its degree of crystallinity (b).

homocrystals was reduced from 19.6% for sPLA to 0% for sPLA-5%CMC and crystallinity for stereocomplex crystallites increased from 32.4% to 54.6%. The fraction of stereocomplex crystallites in the matrix increased from 62.3% to 100%, as shown in Figure 5b, confirms that the presence of grafted CMC provides the extended enantiomeric molecular surface area for the interaction with opposite enantiomeric PLA chains. Formation of the stereocomplex crystallites in the PLA matrix was also confirmed by FTIR as shown in Figure 6. It is well-known that the FTIR spectra is sensitive to the crystalline form of PLA. Spectra of PLA and grafted CMC was compared to understand the effect of grafted CMC in PLA formation of stereocomplex crystallites. Complete spectra of all the samples are shown in Figure 6a, the characteristic peaks corresponding to PLA were already mentioned in the previous section. The characteristic peak for the stereocomplex 31 helical structure was located at 908 cm−1,44 increased intensity of the peak at 908 cm−1 in comparison to sPLA suggests the enhancement in the content of stereocomplex crystallites, qualitatively, as shown in Figure 6b. The stereocomplex crystal formation was the ultimate result of the interaction between opposite enantiomeric PLA chains or molecules. The establishment of hydrogen bond between carbonyl carbon of one enantiomeric PLA molecule and methyl hydrogen of opposite enantiomeric PLA molecule is responsible for the stereocomplexation in PLA 6840


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Figure 6. FTIR spectra of the sPLA and sPLA-CMC biocomposites.

Figure 7. Oxygen permeability of sPLA and sPLA-CMC biocomposite.

Figure 8. Ultimate tensile strength and percentage elongation of the sPLA and sPLA-CMC biocomposite.

crystalline domain in the matrix prevents the diffusion of oxygen molecules and also the presence of filler increases the tortuous path for the oxygen molecules. Mechanical Property. The effect of stereocomplexation and the presence of filler in the PLA matrix on mechanical properties were also examined. The ultimate tensile strength (UTS) and percentage elongation of the samples was shown in Figure 8. The UTS of sPLA without filler was found to be 29.2 MPa which was increased to 57.3 MPa (enhancement ∼96%) in the case of sPLA-5%CMC. Increment in the tensile strength

can be ascribed to the interaction of PLLA chains with the grafted CMC which provides extended arms of PDLA. The PLLA and PDLA chains form the stereocomplex crystallites, and CMC hinders the movement of the chains, which ultimately increases the resistance to mechanical pull. As reported, the modification of CMC and its use as a filler in the PLA matrix improves the tensile properties at the cost of the percentage elongation at break.49,50 However, in this study, the percentage elongation at break first increased from ∼1.9% for sPLA to ∼36% for sPLA-0.5%CMC, and this increase can be 6841


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Figure 9. FESEM images of the CMC particles (a), fractured surface of sPLA (b), sPLA-2.5%CMC (c), and sPLA-5%CMC (d) biocomposite.

surface of CMC confirms the interaction and affinity of PLA chains toward the filler. Stereocomplexation and presence of filler exert a substantial effect on thermomechanical properties of sPLA. The dynamic mechanical analysis (DMA) of the biocomposites suggests the enhancement in the thermomechanical properties. The plot for the storage modulus against temperature is shown in Figure 10. It was observed that the storage modulus of the composite was found to improve to more than 3500 MPa before glass transition in case of sPLA-5%CMC compared to sPLA. It was also confirmed that the composite was thermomechanically

the result of stretching of stereocomplex crystallites which was induced by the grafted CMC. Elongation was further reduced to ∼24% in the case of sPLA-5%CMC. The reduction in the elongation could be caused by the induced rigidity of the polymer chains by CMC, which was a direct reflection of the reinforcement. The increased content of CMC was found to segregate the polymer chains and reduce the mobility which ultimately affects the tensile strength and elongation. This enhancement in the tensile strength and percentage elongation was also the result of uniform dispersion of CMCs into the polymer matrix. The FESEM of CMCs and the fractured surface of sPLA and sPLA-CMC biocomposites was shown in the Figure 9, which shows homogeneous dispersion of CMC in the polymer matrix. From Figure 9, it can clearly be perceived that the size of the CMC was subsequently reduced during the melt processing of polymer composites. The average diameter of the CMC was found to be ∼8 μm (Figure 9a) which was reduced to ∼220 nm (Figure 9d) during melt processing which promotes the dispersion of the filler and results in the improvement of mechanical properties. The interfacial interaction of polymer with filler is also a significant parameter which has a significant impact on tensile strength and percentage elongation. The AFM study of the composite can give indirect information regarding the interaction of polymer and filler. As discussed in the previous section, comparison of AFM images of sPLA and sPLA-5% CMC is shown in Figure 2. In Figure 2f, the grains of grown spherulite of sPLA can be seen easily which were seen as percolated structures in the case of sPLA-5%CMC (Figure 2g). This percolated and channeled growth of the spherulite on the

Figure 10. Storage modulus of the sPLA and sPLA-CMC biocomposite. 6842


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ACS Sustainable Chemistry & Engineering stable until 140 °C in comparison to ∼100 °C of pristine sPLA. It was observed that the reduction in storage modulus after glass transition become significantly lesser for sPLA-5%CMC, which can be attributed to the enhanced stereocomplexation and the reinforcing action of the grafted CMC that reduced the mobility of the polymer chains significantly.

ments Facility (CIF) at Indian Institute of Technology Guwahati (IIT Guwahati), and India and Institute of Advanced Study in Science and Technology (IASST) Guwahati, Government of India for providing research and analytical facilities.

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(1) Rustagi, N.; Pradhan, S. K.; Singh, R. Public health impact of plastics: An overview. Indian Journal of Occupational and Environmental Medicine 2011, 15 (3), 100−103. (2) Raquez, J.-M.; Habibi, Y.; Murariu, M.; Dubois, P. Polylactide (PLA)-based nanocomposites. Prog. Polym. Sci. 2013, 38 (10−11), 1504−1542. (3) Garlotta, D. A Literature Review of Poly(Lactic Acid). J. Polym. Environ. 2001, 9 (2), 63−84. (4) Rhim, J.-W.; Park, H.-M.; Ha, C.-S. Bio-nanocomposites for food packaging applications. Prog. Polym. Sci. 2013, 38 (10−11), 1629− 1652. (5) Babu, R. P.; O’Connor, K.; Seeram, R. Current progress on biobased polymers and their future trends. Prog. Biomater. 2013, 2 (1), DOI 810.1186/2194-0517-2-8. (6) Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S. H. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules 1987, 20 (4), 904−906. (7) Katiyar, V.; Gerds, N.; Koch, C. B.; Risbo, J.; Hansen, H. C. B.; Plackett, D. Poly l-lactide-layered double hydroxide nanocomposites via in situ polymerization of l-lactide. Polym. Degrad. Stab. 2010, 95 (12), 2563−2573. (8) Murphy, C. A.; Collins, M. N. Microcrystalline cellulose reinforced polylactic acid biocomposite filaments for 3D printing. Polym. Compos. 2016, DOI: 10.1002/pc.24069. (9) Dhar, P.; Tarafder, D.; Kumar, A.; Katiyar, V. Effect of cellulose nanocrystal polymorphs on mechanical, barrier and thermal properties of poly(lactic acid) based bionanocomposites. RSC Adv. 2015, 5 (74), 60426−60440. (10) Cheung, H.-Y.; Lau, K.-T.; Pow, Y.-F.; Zhao, Y.-Q.; Hui, D. Biodegradation of a silkworm silk/PLA composite. Composites, Part B 2010, 41 (3), 223−228. (11) Tesfaye, M.; Patwa, R.; Kommadath, R.; Kotecha, P.; Katiyar, V. Silk nanocrystals stabilized melt extruded poly (lactic acid) nanocomposite films: Effect of recycling on thermal degradation kinetics and optimization studies. Thermochim. Acta 2016, 643, 41−52. (12) Li, L.; Ding, S.; Zhou, C. Preparation and degradation of PLA/ chitosan composite materials. J. Appl. Polym. Sci. 2004, 91 (1), 274− 277. (13) Pal, A. K.; Katiyar, V. Nanoamphiphilic Chitosan Dispersed Poly(lactic acid) Bionanocomposite Films with Improved Thermal, Mechanical, and Gas Barrier Properties. Biomacromolecules 2016, 17 (8), 2603−2618. (14) Tripathi, N.; Katiyar, V. PLA/functionalized-gum arabic based bionanocomposite films for high gas barrier applications. J. Appl. Polym. Sci. 2016, 133 (21), DOI 10.1002/app.43458. (15) Valapa, R. B.; Pugazhenthi, G.; Katiyar, V. Fabrication and characterization of sucrose palmitate reinforced poly(lactic acid) bionanocomposite films. J. Appl. Polym. Sci. 2015, 132 (3), DOI 10.1002/app.41320. (16) Mukherjee, T.; Sani, M.; Kao, N.; Gupta, R. K.; Quazi, N.; Bhattacharya, S. Improved dispersion of cellulose microcrystals in polylactic acid (PLA) based composites applying surface acetylation. Chem. Eng. Sci. 2013, 101 (0), 655−662. (17) Missoum, K.; Belgacem, M.; Bras, J. Nanofibrillated Cellulose Surface Modification: A Review. Materials 2013, 6 (5), 1745−1766. (18) Bondeson, D.; Oksman, K. Polylactic acid/cellulose whisker nanocomposites modified by polyvinyl alcohol. Composites, Part A 2007, 38 (12), 2486−2492. (19) Ummartyotin, S.; Manuspiya, H. A critical review on cellulose: From fundamental to an approach on sensor technology. Renewable Sustainable Energy Rev. 2015, 41, 402−412.

CONCLUSIONS A facile and hasty technique involving solvent-free composition has been adopted for the fabrication of stereocomplex PLA/ CMC biocomposites which showed outstanding thermal, mechanical, and barrier properties. The in situ modification of CMC with PDLA was found to be highly effective in enhancing the dispersion of CMC into the polymer matrix, as suggested by FESEM results. The homogeneously dispersed, grafted CMC provided an extended molecular surface area for the formation of stereocomplex crystallites, hindering the homocrystallization phenomenon. AFM analysis indicated a good degree of interaction between the polymer matrix and the grafted CMC, thereby leading to an improved state of filler dispersion within the polymer matrix, which directly influenced the development of stereocomplex crystallites in the resulting biocomposites. By the combined effect of enhanced stereocomplexation and the CMC fillers, the oxygen permeability and water vapor permeability were found to reduce by ∼25% and ∼35%, respectively. The improved storage modulus (∼3500 MPa) and the tensile strength (∼96%) over neat sPLA are a result of effective reinforcement of PLA as well as grafted CMC fillers, which are highly effective in enhancing the stereocomplexation over homocrystallization in the biocomposite. Hence, the involved solvent-free bulk polymerization technique for the modification of CMC and the versatile method of stereocomplex PLA biocomposite fabrication may turn out to be an effective way to enhance properties that can be related to applications both in high temperature engineering and packaging applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01059. Dispersion of CMC and grafted CMC in chloroform, cartoon for the formation of sPLA-CMC biocomposite, and its stereocomplexation, WVTR of the sPLA and sPLA-CMC, XRD spectra of CMC, PDLA, and PDLACMC and densities of PLLA, sPLA, and sPLA-CMC biocomposites (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.K.). ORCID

Vimal Katiyar: 0000-0003-4750-7653 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Centre of Excellence for Sustainable Polymers (CoE-SusPol) funded by the Department of Chemicals and Petrochemicals (DCPC), Central Instru6843


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Isolated from Microcrystalline Cellulose. Biomacromolecules 2009, 10 (4), 712−716. (40) Capadona, J. R.; Van Den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C. A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates. Nat. Nanotechnol. 2007, 2 (12), 765−769. (41) Wei, X.-F.; Bao, R.-Y.; Cao, Z.-Q.; Yang, W.; Xie, B.-H.; Yang, M.-B. Stereocomplex Crystallite Network in Asymmetric PLLA/PDLA Blends: Formation, Structure, and Confining Effect on the Crystallization Rate of Homocrystallites. Macromolecules 2014, 47 (4), 1439−1448. (42) Su, Z.; Li, Q.; Liu, Y.; Hu, G.-H.; Wu, C. Multiple melting behavior of poly(lactic acid) filled with modified carbon black. J. Polym. Sci., Part B: Polym. Phys. 2009, 47 (20), 1971−1980. (43) Tsuji, H. Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications. Macromol. Biosci. 2007, 7 (12), 1299−1299. (44) Zhang, J.; Sato, H.; Tsuji, H.; Noda, I.; Ozaki, Y. Infrared Spectroscopic Study of CH3···OC Interaction during Poly(l-lactide)/ Poly(d-lactide) Stereocomplex Formation. Macromolecules 2005, 38 (5), 1822−1828. (45) Brizzolara, D.; Cantow, H.-J.; Diederichs, K.; Keller, E.; Domb, A. J. Mechanism of the Stereocomplex Formation between Enantiomeric Poly(lactide)s. Macromolecules 1996, 29 (1), 191−197. (46) Park, S.-H.; Lee, H. S.; Choi, J. H.; Jeong, C. M.; Sung, M. H.; Park, H. J. Improvements in barrier properties of poly(lactic acid) films coated with chitosan or chitosan/clay nanocomposite. J. Appl. Polym. Sci. 2012, 125 (S1), E675−E680. (47) Cui, Y.; Kumar, S.; Rao Kona, B.; van Houcke, D. Gas barrier properties of polymer/clay nanocomposites. RSC Adv. 2015, 5 (78), 63669−63690. (48) Takács, E.; Wojnárovits, L.; Borsa, J.; Rácz, I. Hydrophilic/ hydrophobic character of grafted cellulose. Radiat. Phys. Chem. 2010, 79 (4), 467−470. (49) Dhar, P.; Tarafder, D.; Kumar, A.; Katiyar, V. Thermally recyclable polylactic acid/cellulose nanocrystal films through reactive extrusion process. Polymer 2016, 87, 268−282. (50) Shi, Y.; Wang, B. Mechanical properties of carbon fiber/ cellulose composite papers modified by hot-melting fibers. Prog. Nat. Sci. 2014, 24 (1), 56−60.

(20) Gregorova, A.; Machovsky, M.; Wimmer, R. Viscoelastic Properties of Mineral-Filled Poly(lactic acid) Composites. Int. J. Polym. Sci. 2012, 2012, 6. (21) Liu, H.; Webster, T. J. Mechanical properties of dispersed ceramic nanoparticles in polymer composites for orthopedic applications. Int. J. Nanomed. 2010, 5, 299−313. (22) Gwon, J.-G.; Cho, H.-J.; Chun, S.-J.; Lee, S.; Wu, Q.; Lee, S.-Y. Physiochemical, optical and mechanical properties of poly(lactic acid) nanocomposites filled with toluene diisocyanate grafted cellulose nanocrystals. RSC Adv. 2016, 6 (12), 9438−9445. (23) Kang, H.; Liu, R.; Huang, Y. Graft modification of cellulose: Methods, properties and applications. Polymer 2015, 70, A1−A16. (24) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Synthesis and properties of two-dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chem. Mater. 1993, 5 (12), 1694−1696. (25) Alzari, V.; Nuvoli, D.; Sanna, R.; Scognamillo, S.; Piccinini, M.; Kenny, J. M.; Malucelli, G.; Mariani, A. In situ production of high filler content graphene-based polymer nanocomposites by reactive processing. J. Mater. Chem. 2011, 21 (41), 16544−16549. (26) Imre, B.; Pukánszky, B. Compatibilization in bio-based and biodegradable polymer blends. Eur. Polym. J. 2013, 49 (6), 1215− 1233. (27) Nishida, M.; Tanaka, T.; Yamaguchi, T.; Suzuki, K.; Kanematsu, W. Intercalative polymerization of L-lactide with organically modified clay by a reactive extrusion method and instrumental analyses of the poly(lactic acid)/clay nanocomposites. J. Appl. Polym. Sci. 2012, 125 (S1), E681−E690. (28) Hamad, W. Y.; Miao, C. Nanocomposite biomaterials of nanocrystalline cellulose (ncc) and polylactic acid (pla). Google Patents: 2011. (29) Miao, C.; Hamad, W. Y. In-situ polymerized cellulose nanocrystals (CNC)poly(l-lactide) (PLLA) nanomaterials and applications in nanocomposite processing. Carbohydr. Polym. 2016, 153, 549−558. (30) Paul, M.-A.; Alexandre, M.; Degée, P.; Calberg, C.; Jérôme, R.; Dubois, P. Exfoliated Polylactide/Clay Nanocomposites by In-Situ Coordination−Insertion Polymerization. Macromol. Rapid Commun. 2003, 24 (9), 561−566. (31) Lönnberg, H.; Zhou, Q.; Brumer, H.; Teeri, T. T.; Malmström, E.; Hult, A. Grafting of Cellulose Fibers with Poly(ε-caprolactone) and Poly(l-lactic acid) via Ring-Opening Polymerization. Biomacromolecules 2006, 7 (7), 2178−2185. (32) Jiang, L.; Lv, P.; Ma, P.; Bai, H.; Dong, W.; Chen, M. Stereocomplexation kinetics of enantiomeric poly(l-lactide)/poly(dlactide) blends seeded by nanocrystalline cellulose. RSC Adv. 2015, 5 (87), 71115−71119. (33) Habibi, Y.; Aouadi, S.; Raquez, J.-M.; Dubois, P. Effects of interfacial stereocomplexation in cellulose nanocrystal-filled polylactide nanocomposites. Cellulose 2013, 20 (6), 2877−2885. (34) Wu, H.; Nagarajan, S.; Zhou, L.; Duan, Y.; Zhang, J. Synthesis and characterization of cellulose nanocrystal-graft-poly(d-lactide) and its nanocomposite with poly(l-lactide). Polymer 2016, 103, 365−375. (35) Purnama, P.; Kim, S. H. Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers. Polym. Degrad. Stab. 2014, 109, 430−435. (36) Ma, P.; Jiang, L.; Xu, P.; Dong, W.; Chen, M.; Lemstra, P. J. Rapid Stereocomplexation between Enantiomeric Comb-Shaped Cellulose-g-poly(l-lactide) Nanohybrids and Poly(d-lactide) from the Melt. Biomacromolecules 2015, 16 (11), 3723−3729. (37) Bao, J.; Han, L.; Shan, G.; Bao, Y.; Pan, P. Preferential Stereocomplex Crystallization in Enantiomeric Blends of Cellulose Acetate-g-Poly(lactic acid)s with Comblike Topology. J. Phys. Chem. B 2015, 119 (39), 12689−12698. (38) Mujica-Garcia, A.; Hooshmand, S.; Skrifvars, M.; Kenny, J. M.; Oksman, K.; Peponi, L. Poly(lactic acid) melt-spun fibers reinforced with functionalized cellulose nanocrystals. RSC Adv. 2016, 6 (11), 9221−9231. (39) Capadona, J. R.; Shanmuganathan, K.; Trittschuh, S.; Seidel, S.; Rowan, S. J.; Weder, C. Polymer Nanocomposites with Nanowhiskers 6844


Cellulose Functionalized High Molecular Weight Stereocomplex

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