Cellulose grafted aliphatic polyesters: Synthesis, characterization and biodegradation under controlled conditions in a laboratory test system

Journal Pre-proof Cellulose grafted aliphatic polyesters: synthesis, characterization and biodegradation under controlled conditions in a laboratory test system

Tabaght Fatima Ezahra, El Idrissi Abderrahmane, Bellaouchi Reda, Asehraou Abdeslam, Aqil Mohamed, El Barkany Soufian, Benarbia Abderrahim, Achalhi Nafea, Tahani Abdesselam PII:

S0022-2860(19)31691-6

DOI:

https://doi.org/10.1016/j.molstruc.2019.127582

Reference:

MOLSTR 127582

To appear in:

Journal of Molecular Structure

Received Date:

02 August 2019

Accepted Date:

12 December 2019

Please cite this article as: Tabaght Fatima Ezahra, El Idrissi Abderrahmane, Bellaouchi Reda, Asehraou Abdeslam, Aqil Mohamed, El Barkany Soufian, Benarbia Abderrahim, Achalhi Nafea, Tahani Abdesselam, Cellulose grafted aliphatic polyesters: synthesis, characterization and biodegradation under controlled conditions in a laboratory test system, Journal of Molecular Structure (2019), https://doi.org/10.1016/j.molstruc.2019.127582

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Cellulose grafted aliphatic polyesters: synthesis, characterization and biodegradation under controlled conditions in a laboratory test system Tabaght Fatima Ezahra1, El Idrissi Abderrahmane1, Bellaouchi Reda2, Asehraou Abdeslam2, Aqil Mohamed3, El Barkany Soufian4, Benarbia Abderrahim1, Achalhi Nafea1, Tahani Abdesselam5 1.

Department of chemistry, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco

2.

Department of biology, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco

3.

Materials Science and Nano-engineering, Mohammed VI Polytechnic University (UM6P), Lot 660 Hay Moulay Rachid, Ben Guerir, Morocco

4.

Department of Chemistry, Multidisciplinary Faculty, Mohamed first University, Nador, Morocco

5.

Department of Chemistry, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco

212 6 11 54 34 99 [email protected]

1

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1. Abstract Novel biodegradable cellulose derivatives were synthesized using ‘grafting onto’ process. The synthesis method involves three-steps using 1, 6-hexamethylene diisocyanate as a coupling agent. This method improves the adhesion with cellulose fibers and was conducted using simple experimental techniques. In the first stage, the poly( butylene adipate) (PBA) and poly( ethylene adipate) (PEA) polyesters were synthesized by direct polycondensation reactions. Secondly, these polyesters were grafted onto cellulose surface using a simple addition reaction between the hydroxyl groups of the cellulose and the isocyanate groups existing at the ends of the polyester chains (precursors). The structure and physical properties of the prepared polymers were characterized by FTIR, NMR and TGA/DTG analysis. This simple and efficient strategy of synthesizing cellulose grafted aliphatic polyesters will provide new opportunities to fabricate many other biodegradable polymer composites. Keywords: cellulose and its derivatives, biodegradation process, aliphatic polyesters

2. Introduction Great efforts are being focused into the development of biodegradable polymers from renewable resources to avoid problems associated with more conventional petrochemical feed stocks such as the limited availability in the future [1], increasing price [2] and environmental pollution caused by plastic waste [3–6]. Biodegradable polymers have recently received more attention because of their applications in environment protection, resources recycling, and their potential usage as green materials in packaging (mulching films, agricultural staples, coatings to protect seeds, chewing gums, cigarette filters, cartridges, cartridge wax and biomedical implants) [7–9]. The classification of biodegradable polymers as natural or synthetic materials depends on their origin. Biodegradable biopolymers can also be classified into three categories: (i): polymers derived from plant resources such as polysaccharides and proteins; (ii): polymers derived from bacterial origin such as poly( hydroxybutyrate) (PHB); (iii): polymers synthesized from agro-based resource monomers i.e. poly( lactic acid) [10]. The cellulose; one of these biodegradable biopolymers; is the most abundant and promising renewable resources. It is an environmentally friendly alternative to synthetic products derived from petroleum industry. Recently, modified cellulose has been used as reinforcement for various composites, due to its excellent mechanical and thermal performances. Cellulose is fully biodegradable under a wide variety of environmental conditions [11]. Due to their natural abundance and low cost, various processes of cellulose modifications have been studied; chemical modification such as halogenation, esterification, etherification, and oxidation [12] or blending [13].

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Journal Pre-proof Luan et al. have implemented a facile and highly efficient method for the synthesis of propionic anhydride and butyric anhydride cellulose esters. The derivatives with high degree of substitution (DS) values exhibited good solubility in some organic solvents [14]. Among all modification methods, the blending of cellulose and its derivatives with polyesters is the most used one in order to obtain fully biodegradable materials with improved mechanical properties [15–20]. Many studies on polyester-based biocomposites or natural fiber-reinforced polyester blends have been reported [21–30]. In fact, polyesters are considered the most competitive biodegradable polymers commercialized currently. In particular, Poly( alkylene dicarboxylates) are gaining increasing attention because they can be obtained from renewable resources and are characterized by a relatively low production cost, easy processability and good thermal and mechanical properties. The extensive application of such polymers could not only mitigate the negative effect of non-degradable plastics on the environment but also reduce the dependence on fossil resources [31]. The grafting of biodegradable polyesters to the cellulose chains enables obtaining new materials capable of overcoming some drawbacks often seen in the case of polysaccharides, such as poor long-term stability and mechanical properties, due to their hydrophilic nature [32]. These materials, based on renewable resources, can also be implemented in order to replace “standard” thermoplastic materials. Ye et al. [33] have elaborated composites based on cellulose nanocystals (CNCs) incorporated into poly( butylene adipate) (PBA) matrix. Yu et al. [34] modified CNCs by grafting poly( 3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) via homogeneous acylation reaction using toluene diisocyanate (TDI) as coupling agent and dibutyltin dilaurate as a catalyst. The CNCs were incorporated into PBA matrix to regulate its crystallization behavior and enzymatic degradation performance [33]. Cellulose-graft-polyesters can also be obtained by graft reactions such as the ones reported by Li et al. [35], Paquet et al. [36] and Fei et al. [37]. Moreover, Yan et al. have used an effective method for grafting L-lactide (LA) from cellulose by ring opening polymerization (ROP) in homogeneous medium [ionic liquid: 1-allyl-3-methylimidazolium chloride (AmimCl)] at 80 °C using 4-dimethylaminopyridine (DMAP) as an organic catalyst [38]. The structural and thermal properties of the grafted compounds were investigated and exhibited thermoplastic behavior. In addition, the copolymer having hydrophobic poly ( L-lactide) (PLLA) and hydrophilic (cellulose) segments shows an amphiphilic character in water and can be used in controlled drug release [39]. By the ROP, Lönnberg et al. have also successfully grafted the biopolymers polycaprolactone (PCL) and poly( L-lactic acid) (PLLA) from cellulose surfaces, then, the grafting efficiency was improved after activation of the cellulose surface [40]. Furthermore, Samain et al.

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Journal Pre-proof have prepared different oligoesters (poly( 3-hydroxyoctanoate-co-3-hydroxy-hexanoate) (PHO), PHB, poly( 3-hydroxybutyrate-co- hydroxyvalerate) (PHBHV) and PCL). These oligomers were further grafted onto cellulose films through direct esterification without any catalyst [41]. In addition, Goffin et al. have implemented an intelligent method that improves the compatibility between two biodegradable polyesters (PCL and polylactide (PLA)) by grafting of these polyester blends into cellulose nanowhiskers (CNW). The cellulose derivatives serve as suitable nanofillers to improve the compatibility of PCL/PLA blends and their microstructures [42]. The issue of preparing new composites having properties of both synthetic and natural compounds, is a topic of high interest for research. Novel hybrid materials (nanocomposites) based on poly( 2aminophenyl disulfide)/silica gel, polyaniline(PANI)/Al2O3, (PANI)/ZnO and polypyrrole (PPy)derived polymer/ZrO2 were prepared and characterized. These hybrid materials show great effects (interface of the nanoparticles, nature of the monomers, polymer structure,..) on the properties of the nanocomposites [43–46]. To improve interfacial adhesion between cellulose and PBA or PEA polyesters, the modification of cellulose was carried out using diisocyanate as coupling agent. Firstly, we report the synthesis of the PBA and PEA using reagents deriving from sustainable raw resources. These polyesters were characterized using different techniques. Secondly, PBA or PEA were inserted into cellulose using grafted onto technique that was performed in a single flask, therefore avoiding the use of extra chemicals (organic solvents and catalysts) and cleaning steps which lead into a more efficient and ecofriendly procedure. This method has some advantages such as the high reaction rates, the absence of elimination products and the chemical stability of the urethane moiety [47]. The products were characterized by FTIR, NMR, thermal analysis (DSC and TGA/TDA); and their solubility phenomena were reported. The biodegradation study of various samples was investigated according to ASTM, AFNOR, DIN and ISO standards. We report here the data on the preparation, the properties and the process of biodegradation of hybrid polymers containing in their main chains synthetic and natural units.

3. Experimental 3.1.

Materials

Cellulose (DPw ≈ 1400 and Mw ≈ 227200 g/mol) was extracted from Esparto "Stipa-tenacissima" of Eastern Morocco following the procedure developed by El Idrissi et al. [48]. It was dried in a vacuum oven at 90 °C for 48 h before using. Dibutyltin dilaurate (DBTL 95%) and paratoluene sulfonic acid (APTS) were used as catalysts and purchased from Sigma-Aldrich. Lithium chloride (LiCl 98%) was obtained from Riedsl-de Haën Company. N, N-dimethyl acetamide (DMAc), 1,6-

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Journal Pre-proof hexamethylene diisocyanate (HDI 98%), 1,4-butanediol, 1,2-ethylene glycol, phenyl isocyanate and adipic acid were purchased from Sigma-Aldrich. All other chemicals were of analytical grade and were purchased from Sigma-Aldrich too. Lixivia was recuperated from landfill site of Oujda city (Morocco).

3.2.

Measurements

3.2.1. FTIR analysis FTIR experiments were performed using Shimadzu Fourier Transform Infrared spectrometer FTIR8400S using KBr discs containing 2% of finely ground sample. Twenty scans were taken of each sample recorded from 4000 to 400 cm-1. The FTIR spectra were accumulated at a resolution of 4 cm−1. 3.2.2. NMR analysis 1H

NMR spectra of the synthesized material were recorded on a Bruker advance 250 MHz

spectrometer using deuterated dimethyl sulfoxide (DMSO) as solvent and tetramethylsilane TMS as an internal reference. The molecular weight (Mn) was calculated by 1H NMR end-group analysis according to the equation 1 and the results are summarized in table 1: (1)

𝑀𝑛 = 𝑀0 ∗ 𝑛 + 𝑀(𝑒𝑛𝑑 𝑔𝑟𝑜𝑢𝑝)

Where; Mn is molecular weight of the polyester, M0 is the molecular weight of repeating monomer units and n is the number of repeating monomers units. 𝑛 =

𝐼𝑟𝑒𝑝𝑒𝑎𝑡𝑖𝑛𝑔 𝑚𝑜𝑛𝑜𝑚𝑒𝑟 𝑢𝑛𝑖𝑡𝑠 𝑁𝑚 𝐼𝑒𝑛𝑑 𝑔𝑟𝑜𝑢𝑝 𝑝𝑟𝑜𝑡𝑜𝑛𝑠 𝑁𝑔

(2)

I repeating monomer units is the integration of the protons of the repetitive monomeric units, I end group protons is the integration of the protons of the end groups, Nm represent the number of protons in the repeating monomer units and Ng is the number of protons of the end group. We have also used the 1H NMR technique to estimate the degree of substitution (DS) of the cellulose derivatives prepared using the equations 3 [49] and the results are summarized in table 2: 𝑛𝐴𝑈𝐺 ∗ 𝐼𝑖

(3)

𝐷𝑆 = 𝑛𝑖 ∗ 𝐼

𝐴𝑈𝐺

Where; 𝐼𝑖 is the integration of the grafted group (i), 𝐼𝐴𝑈𝐺 is the integration of the cellulose skeleton, 𝑛𝐴𝑈𝐺represents the number of protons of cellulose skeleton (𝑛𝐴𝑈𝐺 = 7) and 𝑛𝑖 is the number of protons of the grafted group (i).

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Journal Pre-proof 3.2.3. Thermal analysis Test differential scanning calorimetry analysis (DSC) is performed by a TA DSC Q20 (United States). About 10 mg of each sample were placed in sealed aluminum capsules and were subjected to two scans from -40 to 200 °C with a rate of 10 °C/min under inert atmosphere. Thermal behaviors were determined using a Thermogravimetric Analyzer TGA, Q500 from TA instruments, at heating rate of 20 °C min−1 under a 50 mL min−1 nitrogen flow within the range of 25 to 600 °C. 3.2.4. X-ray diffraction (XRD) The X-ray diffraction (XRD) of the biopolymers was performed using a diffractometer system (Shimadzu XRD 6000) with Cu (λ = 0.154 Å). To estimate the state order in the obtained biopolymers, the crystallinity index (CrI) was calculated using equation (4) cited in the literature [50]: 𝐶𝑟𝐼 (%) =

𝐼002 ― 𝐼𝑎𝑚 𝐼002

(4)

∗ 100

Where; I002 is the maximum intensity (in arbitrary units) of the 002 lattice diffraction and Iam is the intensity of diffraction in the same units at 2θ = 18°. 3.2.5. Solvent Solubility Powder samples were placed in a small weighting bottle and dried in a vacuum oven at 70 °C. Then, each dried sample was placed in a bottle with an adequate solvent at room temperature. A transparent solution was obtained, otherwise the mixture was heated and maintained at 120 °C under stirring until the powder disappeared (visual monitoring). 3.2.6. Biodegradability test methods There are several different methods and techniques for determining the degradability of plastic materials. In this work, we have focused on visual observations and quantitative determination of microbial growth when polyester-grafted cellulose are submitted to biodegradation and used as a carbon source. This test was established in two steps. The first one was based on the visual observation of growth and spread of microorganisms (solid medium). The second test was based on the calculation of the biomass (g) resulting from the biodegradation of the products (liquid medium). This test was reinforced by the biodegradability following the standard test method ISO 14851.

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Journal Pre-proof Biodegradation under aerobic solid conditions: This test method, has been standardized and adopted basically in the same form by the American Society for Testing and Materials (ASTM G21-70, G22-76, G29-75), by the French Association for normalization (AFNOR X 41-514-81 and X41-517-69), by the German Institute of Standardization (DIN 53739) and by the International Organization for Standardization (ISO 846) [51]. The tested samples were placed on the surface of a mineral salt agar (MS) which is composed of monopotassium phosphate (KH2PO4: 0.7 g), potassium hydrogen phosphate (K2HPO4: 0.7 g), magnesium sulfate heptahydrate (MgSO4/7H2O: 0.7 g), ammonium nitrate (NH4NO3: 1 g), sodium chloride (NaCl: 0.005 g), ferrous sulfate heptahydrate (FeSO4/7H2O: 0.002 g), zinc sulfate heptahydrate (ZnSO4/7H2O: 0.002 g) and manganese Sulfate heptahydrate (MnSO4/7H2O: 0.001 g) dissolved in sufficient distilled water to make up 1000 ml with agar (Autoclaving at 121 °C for 20 min and pH between 6.0 and 6.5) in a Petri dish containing no additional carbon source. The test material and the agar medium were sprayed with a standardized mixed inoculum of lixivia. Petri dishes are incubated at a constant temperature for 21 to 28 days. The test material was then subjected to the visual assessment. Biodegradation under aerobic liquid conditions: The tests in a liquid medium were performed in an Erlenmeyer containing a mineral medium (ML) (same composition such as MS, but without agar). The tested samples were placed in ML medium which was contaminated with lixivia. The presence of microorganisms was confirmed by visual observation. ML medium contaminated with lixivia without a source of carbon was used as a blank test. The quantity of biomass was obtained after 5 days under stirring (150 rpm), filtration and drying. This test was reinforced by another one. The assessment of biodegradability was performed following the standard test method ISO 14851 [52]. Glass flasks with 600 mL capacity were used as test vessels. The flasks were filled until a final 400 mL volume and kept closed with glass caps. The liquid medium that was tested is the ‘‘standard test medium” described in ISO 14851 (Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium-method by measuring the oxygen demand in a closed respirometer), based on the following solutions: Solution A: KH2PO4 (8.5 g/L), K2HPO4 (21.75 g/L), Na2HPO4.2H2O (33.4 g/L), NH4Cl (0.5 g/L); Solution B: MgSO4.7H2O (22.5 g/L); Solution C: CaCl2.2H2O (36.4 g/L) and Solution D: FeCl3.6H2O (0.25 g/L). The test medium was prepared by adding solution A (10 mL), solutions B, C, and D (1 mL each) to about 500 mL of water and bringing the volume to 1000 mL with water. The inoculum solution was prepared as follows. A sample was drawn from the soil, diluted with water in order to reach final total solids concentration of 200 g/L, and aerated for 4 h. Each vessel was filled with 380 mL of test medium and inoculated with 20 mL of inoculum solution. Test or reference samples were added to

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Journal Pre-proof each vessel (50 mg), with the exception of the blanks. Two replicates were used for each material and for blanks. The tests were stopped when O2 consumption was no longer detectable, in all cases at least after three months. The Oxitop system used in the determination of biochemical oxygen demand (BOD) contains an individual number of reactors consisting of glass bottles with a carbon dioxide trap (sodium hydroxide) in the headspace. The bottles are supplied with a magnetic stirrer and sealed with a cap containing an electronic pressure indicator. Afterwards, each vessel was kept under aeration for 15 min to restore the original oxygen concentration of the liquid medium. The vessels were then closed and put back in incubation. The net BOD of the test material was calculated as the difference between oxygen consumption in the test and in the blank flasks using the equation (5): (6)

𝑛𝑒𝑡 𝐵𝑂𝐷 = 𝐵𝑂𝐷𝑡𝑚 - 𝐵𝑂𝐷𝑏

Where; BODtm is the total biochemical oxygen demand of the test material from one flask, BODb is the biochemical oxygen demand of the blanks (average of two flasks). The percentage of biodegradation Dtm is calculated as the ratio of the net biochemical oxygen demand to the total theoretical oxygen demand (total ThOD, referred to the amount of test material introduced originally in the flask) using the equation (6): 𝑛𝑒𝑡 𝐵𝑂𝐷

(7)

𝐷𝑡𝑚 = 𝑡𝑜𝑡𝑎𝑙 𝑇h𝑂𝐷 × 100

Where; Dtm is the Percentage of biodegradation, net BOD is the specific Biochemical Oxygen Demand (in mg O2/L), ThOD is the Theoretical Oxygen Demand (in mg O2/L). The ThOD of the polymer (CcHhOoNn)n, with a relative molecular mass Mr (per monomer), was calculated according to: 𝑇h𝑂𝐷 =

31.9988(𝑐 + 0.25(h - 3𝑛) - 0.5𝑜) 𝑀𝑟

(8)

The BOD of the cellulose and its derivatives was evaluated for a period of 28 days for Cell-HDIPBA and 22 days for Cell-HDI-PEA. The experiments were repeated twice. In both cases, after this period, oxygen consumption in respirometers was no longer detectable.

3.3.

Synthetic procedures

3.3.1. Synthesis of polyesters (PBA; PEA) All polyesters were prepared by a direct condensation polymerization method described previously in the literature [17,53]. Briefly, an appropriate amount of 1, 4-butanediol or 1,2-ethylene glycol, adipic acid and APTS as a catalyst (0.1% mol/number of moles of the diol) (table 1) were charged

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Journal Pre-proof into a three-necked round-bottom flask and 25 ml of toluene were added. The system was equipped with a mechanical stirrer, nitrogen inlet, Dean Stark and a condenser. The temperature was kept at 120 °C for 24 hours to remove water by azeotropie. When no more water was distilled out under normal vacuum, polycondensations under high vacuum was further carried out at high temperature (240 °C) and high pressure (0.2 MPa). The polyesters were isolated by precipitation in methanol, filtered and dried under vacuum at 60 °C. 3.3.2. Grafting method Firstly, 0.06 mmol of a poly(butylene adipate) (PBA) or poly(ethylene adipate) (PEA) reacted with 0.06 mmol of phenyl isocyanate (ph-NCO) to keep a single hydroxyl group at the end of the polyester chain named (ph-ROH); the ph-NCO was added dropwise to the polyester (PBA or PEA) and the system was kept under stirring in N2 gas atmosphere at 80 °C for 3 h. Secondly, in a three-necked round-bottom flask equipped with an addition funnel, thermometer, magnetic stirrer and a reflux condenser, 0.06 mmol of 1,6-HDI were introduced and an equal molar of the Ph-ROH with a relative amount of DBTL as a catalyst was placed in the funnel and was added dropwise to the reaction mixture. The system was kept under stirring under N2 gas atmosphere at 80 °C for 3 h. At the end, the isocyanate precursor was formed with a high yield (PhR’-NCO). Finally, without insulating the blocked isocyanate, a cellulose solution already prepared according to El Idrissi et al. [48] (0.06 mmol, 2 equivalents) was added to the mixture. This addition reaction was catalyzed by DBTL and kept under stirring at 80 °C for 3 h in N2 gas atmosphere until all NCO groups had completely reacted and their absorbance band in FTIR spectrum at 2270 cm-1 had completely disappeared. The resulted products (Cell-HDI-PBA and CellHDI-PEA) were precipitated in a mixture of methanol/water (1/3), filtered using a membrane filter, washed with distilled water and methanol and finally dried under vacuum at 80 °C [54].

4. Results and discussion 4.1.

Synthesis

The polyesters (PBA and PEA) were respectively prepared according to the procedure described in the literature [17,53], that is, by polycondensation reaction between adipic acid and 1,4-butanediol, or 1,2-ethylene glycol. The polyesters were isolated as white solids with high yield. The chemical reaction is shown in Fig. 1.

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Journal Pre-proof Esterification: O

O HO

HO

OH

R

APTS, N2

OH

O

120°C, 24h

R

O

n-x

O

O

Polycondensation: O

O O

R

P (0.2Pa), N2

O

n-x

O

240°C, 6h

O

R

O

n

O Where: R= CH2CH2, Polyestre is PEA R= CH2CH2CH2CH2, Polyestre is PBA

Figure 1: Synthesis of PBA and PEA.

The modification of the cellulose fibers surface is established in three stages. Firstly, the hydroxyl group of the polyesters (PBA or PEA) reacted with the isocyanate group of the ph-NCO (Figure 2a), following the procedure described in the experimental part, leading to a polyester with only one hydroxy group (OH) at the chain end (Figure 2a). This compound reacted in the second stage of this procedure with 1, 6-HDI in order to prepare precursors having an isocyanate group at their chain end (ph-R’-NCO) (Figure 2b). As expected, the FTIR spectrum displayed a strong peak at 2270 cm1,

characteristic of NCO group. Finally, in the last stage, a reaction takes place between the

remaining isocyanate groups of the precursors, and the hydroxy groups of the cellulose backbone using the grafting onto technique as described in the Figure 2c. Figure 2 summarizes the different steps of this grafting technique. This synthesis was homogeneously performed in one vessel. Chemical reactions of isocyanate bearing molecules with cellulose fibers have been the subject of numerous studies [38,39,55,56] because urethane formation presents many advantages such as: (i) Relatively high reaction rates, (ii) Absence of secondary products and (iii) Chemical stability of urethane moiety [57]. Furthermore, this grafting method does not lead to any residues or toxic wastes, because NCO group possesses high reactivity; therefore, some green chemistry principles are fulfilled. In the literature, biocomposites are usually synthesized in expensive solvents or following complex techniques. However in our case, the final product can be obtained by a simple and efficient strategy conducted in a simple one-step addition reaction, so the effect of toxicity decreases.

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Journal Pre-proof (a) O

OH R OH

C

DBTL, N2, 80°C

N

OH

R

O

NH O

(b)

H N

O

OCN

OH

R

H N

DBTL, N2, 80°C

NCO

O

O O

R

H N

O

NCO

O

OR'

OH

(c)

O

O OH

*

OH

Where R is PBA

*

H N

Ph

1400

R

O

DBTL, N2 80°C

NCO 3

*

O

O OR' OR'

* 1400

O

or PEA

O

n

O

R' is H or

O O

O

O O

O

H N

O n

O

O

H N

N H

O O

R

O

NH

Ph

O

Figure 2: Synthesis of (a) the product (ph-ROH), (b) the precursor (ph-R’-NCO) and (c) the cellulose grafted polyesters (Cell-HDI-PBA and Cell-HDI-PEA). Table 1: Molecular weight (Mn) measured by 1H NMR, the glass transition temperature (Tg) measured by DSC and the different molar ratios of monomers used in the preparation of the polyesters. Samples

n(adipic acid)

n(1,4-butanediol)

n(1,2-ethylene glycol)

Mn(g/mol)

Tg (°C)

Td (°C)

PBA

0.11 mol

0.12 mol

-

24000

-66.8

350

PEA

0.11 mol

-

0.12 mol

950

-46

360

Table 1 summarizes the Molecular weight, the glass transition temperature (Tg) and the different molar ratios of monomers used in the preparation of polyesters PBA and PEA. It can be seen that PBA had lower thermal stabilities compared to that of PEA. The glass transition temperature of PEA was higher than that of PBA. Finally, PBA has higher molecular weight than PEA.

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NMR analysis

The success of grafting different polyesters on the cellulose was confirmed by NMR analysis. Figures 3 and 4 show the 1H NMR spectra of the polyesters (PBA and PEA) and samples derived from their grafting on cellulose, respectively. The 1H NMR spectrum of PBA (Figure 3a) shows multiple peaks in the range 1.6 - 1.75 ppm attributed to the protons of the methylene -(CH2)2situated in the center of adipic acid and to the protons of methylene centered in the structure of 1,4butanediol respectively (b). In addition, the signal located at 2.3 ppm is attributed to methylene protons of adipic acid on α of the carbonyl of the ester group (-CH2-COO-) (a). Finally, the peak at 4.1 ppm is attributed to the methylene protons of the 1, 4-butanediol situated on the α position of the ester group (c) (-CH2-OCO-) (Figure 3a). The 1H NMR spectrum of PEA (Figure 3b) presents, in addition to the signals already attributed to the protons of adipic acid, the signal at 4.25 ppm assigned to the protons of 1,2-ethylene glycol (c). The protons of the methylene on the α of the hydroxyl group (-CH2OH) at the chain end (d) appeared at 3.73 ppm, and are used to determine the molecular weight of the PEA synthesized. These protons are used to determine the molecular weight of the synthesized PEA. These results are in agreement with those cited in the literature results [58].

Figure 3: 1H NMR spectra for (a) PBA and (b) PEA polyesters.

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Journal Pre-proof Modified celluloses (Cell-HDI-PBA and Cell-HDI-PEA) were characterized by

1H

NMR

spectroscopy (as shown in Fig. 4a and 4b). As in the literature [39,59,60], chemical shifts of hydrogen atoms of the glucose ring from native cellulose appeared between 3.10 and 5.80 ppm. After modification, a new chemical shift located between 1.20 ppm and 4.30 ppm appeared that corresponded to all the signals of the PBA and PEA whose attributions were already made in Figure 4. These peaks indicated that the modification of cellulose by aliphatic chain (HDI-PBA or HDIPEA) was achieved. These peaks are situated between 1 and 1.75 ppm and 2 and 3 ppm. Compared with cellulose grafted with PLA, different peaks appearing between 1, 1.75 and 4.2 ppm were attributed to the different protons of PLLA (-CH- / -CH3-) [39]. The protons of the phenyl group contained in the modified cellulose samples give signals in the area of 7 ppm: between 7.07 and 7.47 ppm for Cell-HDI-PBA and between 6.90 and 7.47 ppm for Cell-HDI-PEA. The proton of the urethane (-NHCO-O-) resulting from Ph-NCO gives a signal towards 9 ppm; while those from HDI show a peak at 5.8 ppm. These signals confirm that the grafting process was successful, and these results are in agreement with those in the literature [61].

Figure 4: 1H NMR spectra of (a) Cell-HDI-PBA and (b) Cell-HDI-PEA.

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FTIR analysis

Figure 5 shows the FTIR spectra of the polyesters (PBA and PEA), the neat cellulose and cellulose grafted with polyesters (Cell-HDI-PBA and Cell-HDI-PEA). The FTIR spectrum of cellulose (Fig. 5a), shows a large absorption band at 3400 cm-1 which is a characteristic of the stretching vibration of the hydroxyl groups. The band situated at 2899 cm-1 is associated with the stretching vibration of -CH-/-CH2- and the absorption band at 1647 cm-1 corresponds to naturally absorbed water. The spectrum also shows some absorption bands around 1431 cm-1 and 1373 cm-1 characterizing respectively the deformation vibrations of -CH- and -OH, respectively. Furthermore, the peak at 1058 cm-1 is assigned to the C-O-C stretching vibration, whereas the absorption band at 895 cm-1 is a characteristic of the β-glucidic bond [62]. For PBA and PEA (Fig. 5b and 5c), the C=O stretching vibration is represented by an intense peak around 1735 cm-1. The absorption bands appearing at 2900 and 2957 cm-1 are assigned to the –CH/-CH2- stretching bonds. Furthermore, the band around 1260 cm−1 is due to the stretching vibration of C-O bond and the absorption band of C-O-C bond is situated at 1170 cm−1. In Figures 5d and 5e, an additional strong absorption band that appeared around 1740 cm-1 on all the spectra of the cellulose derivative samples are attributed to the stretching vibration of carbonyl groups in PBA and PEA. It should be noted that the urethane group carbonyl absorption bands are overlapped by those of the carbonyl ester. In addition, the intensity of the peaks around 2860 and 2970 cm-1 increases due to the addition of -CH- / -CH2- groups derived from PBA, PEA and 1,6HDI. Absorption at 1647 cm-1, ascribed to the intramolecular stretching of H-O-H, has also been found to decrease as the grafting of PBA and PEA on the cellulose surface increases. Moreover, the absorption bands characterizing the NH and OH groups appeared around 3400 cm-1 and the absorption band around 1550 cm-1 is assigned to the bending vibration of NH. Finally, the stretching vibration band around 1600 cm-1 is associated with C=C band of phenyl group. The FTIR data showed that the cellulose and the polyesters reacted successfully. Similar results are reported in other works [33,56].

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Figure 5: FTIR spectra of (a) Cellulose, (b) PBA, (c) PEA, (d) Cell-HDI-PBA and (e) Cell-HDI-PEA.

4.4.

Thermal analysis

The DSC heating and cooling curves of PBA and PEA are shown in Fig. 6a and 6b respectively. The PBA reveals a pre-melting peak at Tpm = 50 °C, two melting peaks at Tf1 = 54 °C and Tf2 = 62.5 °C. The main crystallization peak is obtained at around TC = 27 °C. For PEA (Fig. 6b), the DSC analysis during the first scan shows a melting peak at Tm2 = 52 °C, the second scan shows a melting peak at Tm1 = 32 °C, also a peak of crystallization at TC = 12.5 °C. During the first heating there was a massive destruction of the polymer and the resulting molecules crystallized quickly. The existence of different crystalline structures makes the melting behavior complex for both PEA and PBA [63].

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Figure 6: DSC spectra of (a) PBA and (b) PEA.

The influence of the grafted PBA and PEA chains on the thermostability of the cellulose was investigated using TGA analysis. Figure 7 shows the variation of weight loss as a function of temperature for cellulose, PBA, PEA and cellulose-grafted polyesters (Cell-HDI-PBA and CellHDI-PEA). For PBA and PEA, in both TGA thermograms the weight loss takes place gradually. From the TGA curve of the PBA it can be seen that the degradation of the PBA was initiated at 350 °C and its complete degradation occurred at 400 °C, for the PEA, the degradation was initiated at 360 °C and its degradation ended around 380 °C. The PBA is slightly stable compared to the PEA. In contrast with our results, it is stated in the literature that PBA presents higher thermal stability than PEA [63]. The cellulose also shows the main stage of degradation towards 360 °C and constitutes approximately more than 90% of the its weight. The derivatives resulting from the grafting of the polyesters on the cellulose chains (Cell-HDI-PBA, Cell-HDI-PEA) have a low stability compared to the starting reaction compounds. In addition, for the derivatives Cell-HDI-PBA and Cell-HDI-PEA, their thermal stabilities show three slopes of weight loss. The step between 40 °C and 200 °C is related to the elimination of solvents especially residual water. The first weight loss between 210 °C and 300 °C corresponds to the detachment of polyester chains grafted onto the cellulose chains. The second stage of the weight loss represents the decomposition of the cellulose skeleton. The third step is located after 420 °C and may be due to the carbonization of the products to ash. Thus, it can be concluded that these cellulose derivatives show low stability compared to their primer analogues.

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Figure 7: TGA spectra of cellulose, PBA, PEA, Cell-HDI-PBA and Cell-HDI-PEA.

4.5.

X-ray diffraction

XRD results provide some important information about the grafting reactions and lattice of cellulose and its derivatives. The XRD patterns of the cellulose, PBA, PEA, Cell-HDI-PBA and Cell-HDI-PEA are presented in Figure 8. It can be seen that the diffractogram of cellulose has peaks at 2θ = 13.82°, 16.30°, 22.04° and 34.10°; assigned to the 101, 101, 002, and 040 diffraction planes respectively [64], justifying the typical cellulose I (Iα and Iβ) crystalline form [65]. The XRD patterns of PBA display some peaks around 2θ = 19.91°, 23.04° and 25.83° respectively. These results indicate that the diffraction patterns of PBA correspond to α-crystal form [66]. The crystals with α-form structure are formed at temperatures above 29 °C, while the crystals with β-form structure are formed at temperatures below 31 °C, and both of two structural crystals are formed simultaneously at 30±1 °C [67]. In addition, for PEA, the peaks are showing at 2θ = 20.51°, 21.64° and 24.69° respectively. The XRD patterns of Cell-HDI-PBA display some new peaks around 2θ = 16.64°, 19.03°, 22.03°, 23.70°, 24.08° and 25.85° respectively showing the presence of new crystal planes. In addition to Cell-HDI-PEA, the new peaks are situated at 2θ = 16.89°, 17.61°, 18.32°, 18.77°, 19.40°, 20.50°, 20.76°, 21.93°, 22.15°, 23.22°, 24.87° and 25.20° respectively. It can be noted that the grafting process is successful and that the crystalline morphology of cellulose has undergone changes.

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Figure 8: XRD spectra of (a) Cellulose, (b) PBA, (c) Cell-HDI-PBA, (d) PEA and (e) Cell-HDI-PEA.

The Crystallinity Index (CrI) decreased from 88% for cellulose to 64 % for Cell-HDI-PBA and 72 % for Cell-HDI-PEA respectively, indicating that the content of the crystalline regions decreased significantly during the grafting process. In addition, the crystallinity of PBA decreased from 96% to 64% as a result of grafting of PBA in to cellulose. Moreover, PEA lost its crystallinity from 95% to 72% when PEA was grafted in to cellulose also. This decrease is probably due to the difference in grafting percentages and the molecular structure of the aliphatic polyesters attached to the cellulose chain. In conclusion, the rupture of inter- and intra-chain interactions after modification have a direct influence on the crystalline state of the obtained materials. Table 2: Crystallinity index (CrI) of cellulose and its derivative samples.

4.6.

Sample

CrI (%)

Cellulose

88

PBA

96

Cell-HDI-PBA

64

PEA

95

Cell-HDI-PEA

72

Solvent Solubility

The solubility phenomenon of cellulose derivatives is an important factor for practical applications. The solubility parameter values are also very important data characterizing the miscibility of a system under the qualitative aspect [68]. Table 3 lists the solubility of the polyesters (PBA and PEA) and the cellulose derivatives (Cell-HDI-PBA and Cell-HDI-PEA) in various solvent systems at room temperature. 18

Journal Pre-proof Table 3: Solubility tests of polyesters and cellulose derivatives.

Polar protic solvents

Polar aprotic solvents

Non-polar aprotic solvents

Ethanol Methanol H2O Propane-2-ol DMSO DMAc DMF Acetone Acetonitrile Chloroform CH2Cl2 Toluene Hexane THF Diethyl ether

PBA

PEA

+ + ± + -

+ + ± + -

Cell-HDI-PBA (DS=0.75) + + + ± ± + ± -

Cell-HDI-PEA (DS=1.07) + + + + + ± ± -

+: Soluble, ±: Partly soluble, -: Insoluble

According to table 3, both polyesters (PBA and PEA) show a good solubility in aprotic polar solvents (DMSO, DMF and DMAc) and in the non-polar aprotic solvent CH2Cl2; however, no solubility was noted in protic polar solvent systems. The cellulose derivatives (Cell-HDI-PBA and Cell-HDI-PEA) show almost the same solubility phenomenon. They are soluble in DMSO, DMAc and DMF. Moreover, Cell-HDI-PEA shows more solubility in chloroform and dichloromethane and a partial solubility in toluene and THF at room temperature. Furthermore, the Cell-HDI-PBA derivative shows a good solubility in toluene and a partial solubility in chloroform, dichloromethane and THF. It is worth noticing that the cellulose is difficult to dissolve, it is insoluble in water and in typical organic solvents [69], but soluble in solvent systems such as ionic liquids [70] and DMAc/LiCl [71] etc. The functionalization of its hydroxyl groups is an efficient method of dissolving cellulose in typical organic solvents. These derivatives generally present a good solubility phenomenon in polar aprotic solvent systems and a low solubility phenomenon in nonpolar aprotic solvent systems at room temperature. These results suggested that grafting of polyesters onto cellulose improved the solubility of the derivatives. These results are in agreement with those of Yan et al. showing that the solubility of cellulose-g-poly( L-lactide) copolymers was improved compared to cellulose [55].

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Journal Pre-proof 4.7.

Biodegradability study

Biodegradation under aerobic solid conditions: The aerobic biodegradation experiments in solid conditions were validated by visual assessment (qualitative information).They show the amount of microorganisms growth on the surface of the material or clear areas due to the hydrolysis of the substrate by the enzymes. Therefore, thanks to the growth on the surface of the material in the Petri dish, the biodegradation course can be closely monitored and correlated with the disappearance of the specimens, confirming that the development of microorganisms is actually linked to the rate of biodegradation.

Figure 9: Evolution of biodegradation of Cellulose, Cell-HDI-PBA and Cell-HDI-PEA after 28 days of incubation in solid medium.

Figure 9 summarizes the evolution of biodegradation of the blank test, the cellulose, the Cell-HDIPBA and the Cell-HDI-PEA compounds under aerobic solid conditions. After 28 days of incubation, it was noted that cellulose, and its derivatives were largely colonized by the microorganisms. The cellulose, the Cell-HDI-PBA and the Cell-HDI-PEA samples started to degrade after 5 days and the blank test did not show any development of the microorganisms, indicating that the microorganisms that grew on the solid medium containing the cellulose and its derivatives were capable of degrading using the previous products as sole carbon source.

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Journal Pre-proof Biodegradation under aerobic liquid conditions: The aerobic biodegradation experiments in liquid conditions were validated by a calculation assessment. In the first stage and after incubation and stretching, the different samples show a modification (formation of biomass in liquid medium). The results clearly present an increase in weight of biomass produced by the biodegradation of cellulose and its derivatives. The data are summarized in table 4. The formation of this biomass is due to a growth of aerobic bacteria and their ability to assimilate the samples as the only carbon source. Table 4: Weight of biomass produced by the biodegradation of cellulose, Cell-HDI-PBA and Cell-HDI-PEA after 5 days of incubation and stretching.

Blank test Cellulose Cell-HDI-PBA Cell-HDI-PEA

P0(g)

P1(g)

P1-0(g)

1.7720 1.8500 1.8256 1.8496

2.1083 3.0146 2.7476 3.0154

0.3363 1.1646 0.9220 1.1658

P0(g) is the initial weight of the samples, P1(g) is the weight of the samples after 5 days of incubation and P1-0(g) is the weight of the biomass

Furthermore, this test was reinforced by the method named biochemical oxygen demand (BOD) using a closed respirometer. The BOD is the quantity of O2 indispensable for a biodegradation step and was determined in a closed respirometer (ISO 14851 (1999)) for 40 days. Biodegradability values were expressed as the amount of O2 consumed during the sample biodegradation by litre (mg O2/L). The amount of O2 consumed during biodegradation process (after correction with the blank test) was expressed as a percentage of the theoretical oxygen demand (ThOD). The ThOD is expressed as mass of O2 per mass of polymer, and was determined by calculating the amount of O2 necessary for aerobic mineralization of the cellulose derivatives, i.e., complete oxidation of Carbone to CO2.

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Figure 10: Biodegradation percentage of cellulose, Cell-HDI-PBA and Cell-HDI-PEA after 40 days of incubation at 25 °C in an aerobic liquid medium.

Biodegradation of cellulose and the polyester grafted cellulose samples (Cell-HDI-PEA and CellHDI-PBA) was evaluated by percentage and rate of the biodegradability. According to the curves plotted in Fig.10, it is interesting to note that the kinetics of degradation have a slight difference and show roughly the same rate of degradation. Two key steps occur in the polymer biodegradation process: firstly there is a depolymerization or chain cleavage step, and secondly there is a mineralization step. The first step normally occurs outside the organism due to the size of the polymeric chain and the insoluble nature of polymers in water medium. Once sufficiently small-size oligomeric or monomeric fragments are formed, they are transported into the enzyme where they are mineralized [72]. The biodegradation phenomena results, of the studied samples for 40 days of culture at 25 °C, are presented in Figure 10. The degradation kinetic for all the curves follows almost the same pace. The results show that, regardless of the cellulose and its derivatives, O2 consumption was noted. They also justify the multiplication and growth of microorganisms and their ability to use these compounds as a single source of carbon. It is worth noticing that the biodegradation (BD) curves are distinguished by a first phase (around 8 days), characterized by a low increase in BD values. This phase may be due to the adaptation of microorganisms to the new environment, by synthesizing enzymes able of degrading these samples.

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Journal Pre-proof The second phase noted on the BD curves is characterized by an exponential increase in biodegradation percentages for all samples. This exponential phase may be assigned to the growth phase of microorganisms, indicating the high and quick biodegradation process of the samples by

microorganisms. It can usually be modeled by an exponential equation of the form: 𝐵𝐷 = 𝐵𝐷0𝑒𝜇𝑡

(9)

𝑙𝑛 𝐵𝐷 = 𝜇(𝑡 - 𝑡0) + 𝑙𝑛 𝐵𝐷0

(10)

Where BD0 represents the biodegradation at t0, BD represents the biodegradation at t and μ represents the rate of biodegradation. The third phase of the biodegradation noted on the curves is characterized by a low increase in biodegradation values for all the samples. This phase may be attributed to the stationary phase of microbial growth or to the low concentration of nutrients, particularly the carbon source (biopolymers). From the results obtained in this work, the type of cellulose used is biodegraded over 72% after 40 days of incubation. The literature reported that cellulose after 40 days of incubation is biodegraded at least 60% in the compost [73]. It can be concluded that the cellulose is more degradable in liquid medium than in compost, probably the microorganisms in leachate are more diversified and numerous. The experiment results show that the Cell-HDI-PBA and Cell-HDI-PEA samples are biodegraded over 69% and 67% respectively. The experimental results suggested that the cellulose is more biodegradable than Cell-HDI-PBA and Cell-HDI-PEA samples having approximately the same biodegradation rate (69% and 67% respectively). Moreover, after the short adaptation period in which the microorganisms assimilate the samples as a carbon source by secreting the necessary enzymes, the BOD values increase exponentially. The bacteria grow through using the cellulose and its derivatives as food. Finally, a plateau phase is reached indicating that the biodegradation phenomena are complete. Table 5: Rate of biodegradation kinetic (μ) of the samples tested. Biopolymer

Rate of biodegradation (μ)

Cell

0.11

Cell-HDI-PBA

0.085

Cell-HDI-PEA

0.092

According to Table 5, the biodegradation rate order (μ) of the tested samples can be set up as follows: Cell-HDI-PBA
Journal Pre-proof copolymers, their hydrolytic degradation was conducted in phosphate buffered solution (PBS, pH 7.4) at 37 C and showed a rapid rate of degradation in comparison with cellulose and PLLA [55]. These variations in our results can be explained by the increase in the molecular weight (Mn) of the synthesized derivatives (Cell-HDI-PBA and Cell-HDI-PEA). This is one of the factors determining the biodegradation of polymers; a low molecular weight favors the biodegradation process [57]. Other parameters such as: the experimental conditions (pH, T(°C), CrI(%), etc.); nature of enzymes and bacteria can also be determinant factors affecting the biodegradation rates. The microbial attack can occur by involving esterase enzymes. Fungi associated with biopolymers are known for their esterase production capacity on various plant polysaccharides [74]. It has been already remarked that a positive result in a biodegradation test is a proof that these biopolymers are biodegradable. The rate of degradation is much dependent upon the biodegradation conditions (solid, liquid medium) and the molecular structure of the sample subjected to the biodegradation test.

5. Conclusions In this study, novel biodegradable cellulose derivatives were synthesized. The preparation of these materials has been established using ‘grafting onto’ process of polyesters (PBA and PEA) onto cellulose and carried out in simple experimental conditions. This synthesis procedure is an economic method conducted in one-pot, avoiding toxic and expensive reagents with absence of byproducts. The biocomposites obtained were characterized using different techniques. The novel derivatives show better solubility towards some solvent systems. Their biodegradation process was studied, and the development and the growth of microorganisms on the surface of the samples and in liquid medium were also justified. Consequently, these samples can be used as a source of carbon. These derivatives have biodegradability close to that of cellulose with a slight difference, therefore; they can replace non-biodegradable polymers in certain industrial applications such as: packaging, composites, etc. Acknowledgements This work was supported by CNRST under grant no: PPR/2015/17. We thank greatly CNRST for its support and the anonymous reviewers for their careful review and valuable suggestions on the manuscript. We thank also the Head of Chemical Department – F.S.O- for his helpful (local of physical measurements).

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Cellulose grafted aliphatic polyesters: synthesis, characterization and biodegradation under controlled conditions in a laboratory test system Tabaght Fatima Ezahra1, El Idrissi Abderrahmane1, Bellaouchi Reda2, Asehraou Abdeslam2, Aqil Mohamed3, El Barkany Soufian4, Benarbia Abderrahim1, Achalhi Nafea1, Tahani Abdesselam5 1. Department of chemistry, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco 2. Department of biology, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco 3. Materials Science and Nano-engineering, Mohammed VI Polytechnic University (UM6P), Lot 660 Hay Moulay Rachid, Ben Guerir, Morocco 4. Department of Chemistry, Multidisciplinary Faculty, Mohamed first University, Nador, Morocco 5. Department of Chemistry, Faculty of Sciences, Mohamed first University, Oujda 60000, Morocco 212 6 11 54 34 99 [email protected]

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Highlights: 

Aliphatic polyesters based on biopolymer;

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NMR, FTIR, XRD and TGA results indicated the grafting was successful;

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The procedure is an economic, a non-toxic, conducted in one-pot and pollution-free;



The samples have a good solubility and a great biodegradation phenomenon.

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