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Homogenous Synthesis of New C6 Regioselective Cellulose Acetate. Butyl hexane-1, 6-DiyldicarbamateBased on Esparto cellulose of. Morocco Oriental.

El Barkany & al./ Appl. J. Envir. Eng. Sci. 3 N°3(2017) 280-296

Homogenous Synthesis of New C6 Regioselective Cellulose Acetate Butyl hexane-1, 6-DiyldicarbamateBased on Esparto cellulose of Morocco Oriental S. El Barkany (a, *), I. Jilal (b), A. El Idrissi (c), M. Abou-Salama (a), A. Salhi (d), H. Amhamdi (e) (a)

(b)

Multidisciplinary Faculty of Nador, Mohamed 1st University, 60700 Nador –Morocco Laboratory of Solid, Mineral and Analytical Chemistry (LSMAC), Faculty of Sciences (FSO), Mohamed 1st University, 60000 Oujda – Morocco (c) Laboratory Applied chemistry and environmental (LCAE-URAC18), Faculty of Sciences of Oujda, Mohamed1st University, 60000 Oujda-Morocco (d) LCAE-URAC18, Faculty of Sciences of Oujda, Mohamed1st University, 60000 Oujda-Morocco (e) Laboratory of Physical Chemistry of the Natural Resources and Environment, Faculty of Sciences and Techniques in Al Hoceima, (Med I University), 32 003 Al Hoceima, Morocco

* Corresponding author. E-mail address: [email protected], Tel: (+212) 6 66 57 70 11; Fax: (+212) 5 36 50 06 03 Received 12 Jul 2017, Revised 2 Sep 2017, Accepted 17 Sep 2017

Abstract In this paper, we exploited the strong relation between the degree of substitution (DS) of the cellulosic derivatives and their physicochemical behavior, in particular the solubility. Thus, it is smart to change the problem of finding new and complex solvent system by an understanding the effect of cellulose acetate DS on the solubility process and to take it as a base for grafting new functionalities on the main chain of polymer to improve its physicochemical properties. In this context, we used cellulose acetate having a DS equal to 1.7 (DS1.7) as a base polymer. The value of 1.7 of the CA shows a good solubility in THF. The CA1.7 was synthesized from cellulose stemming from Esparto "Stipa tenacissima" of eastern Morocco by acetylation deacetylation process. This method proved the deacetylation of 50% of the C6-OH groups, which increases the reactivity of the hydroxyls liberated and consequently the ease of C6-regioselective grafting of the new functionalities in homogenous and organic medium (THF).C6 Regioselective Cellulose Acetate Butyl hexane-1,6-Diyldicarbamate was synthesized by grafting of urethane segments (Butyl 1,6-Diyldicarbamate) on cellulose acetate CA 1.7and

the resulted cellulose derivatives was characterized using different spectroscopic techniques

(FTIR, 1H NMR, 13C NMR, DEPT...). The influence of grafting behavior (DS value) on TGA/DTG thermograms and RXD diffractogramms was deeply investigated. NMR was used to study scarcely the regioselective grafting on CA in homogenous medium. The results of the structural analyzes showed that the grafting is regioselective on the hydroxyl groups of C6 position (OH-C6) and the DS of urethane groups was found around 0.7. Keywords: Esparto; Cellulose Acetate; Solubility Parameters; Regioselectivity; Cellulose Carbamates; Degree of substitution DS.

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1. Introduction Cellulose is the most abundant natural linear polymer and its isolation and purification represent one of the major areas of activity [1]. Thus, cellulose is the basis of modern industry in almost all of its sectors [2, 3]. However, it is useful to look for improvements to increase the scope of its applications, in particular, for friendly environmental application requirements, without leveraging its very important advantages such as the biodegradability, biocompatibility and the ability to be derived for the preparation of various useful products [4, 5]. Cellulose is practically very difficult to be used as chemical raw material due to its low solubility in common solvents and that is due to the hydrogen bonds between its very large units repeating (anhydroglucose) forming the cellulosic backbone. This phenomenon has been a stumbling block to its proper use and prevents the cellulose to be treatable by normal fusion technology (or solution). This handicap is usually overcome by chemical modifications and is completed generally in heterogeneous medium [6]. The dissolution step often proves tedious, expensive and results in a significant degradation of the macromolecular backbone of the cellulose. Some solvents have been reported, but they are rarely useful for a wide range of reactions synthesis because of their toxicity, their high cost, etc [7, 8], e.g. N, N dimethylacetamide (DMAc) containing lithium chloride (LiCl) is a solvent system commonly used to dissolve quickly polysaccharide materials [9-15]. In addition, an activation step often precedes the step of dissolving. The process of activation treatments accelerates the rate of dissolution causing swelling within and between crystallites, consequently decreasing the density of hydrogen bonds by increasing solvents accessibility [16, 17]. Gold, the gain of the solubility depends not only on a good activation, swelling or rupture of the hydrogen bonds, but can also be generated by the decrease of the molecular weight of the cellulosic material. Currently, a number of cellulose derivatives of, in particular, esters of cellulose are generally synthesized in heterogenic phase from anhydrides in the presence of catalyst or by condensation of acid chlorides on the cellulose [18]. However, problems such as poor distribution uniformity throughout hydroxyl groups, low yield and abundant formation of undesirable products has led to the development of new methods of acetylation enables to provide products with new features [19]. Among the solutions developed is achieving chemical conversions of cellulose under homogeneous conditions, but this method requires a suitable solvent systems [20-22]. To meet the environmental and ecological requirements, chemical modification was used as a means improvement of the physicochemical properties of the polymers, in particular, the solubility and biodegradability [23]. The solubility of the cellulose derivatives depends not only on the nature of grafted groups but also on the degree of substitution (DS) [24]. Thus, it is smart to change the problem of finding new and complex solvent system by an understanding the effect of cellulose acetate DS on the solubility process and to take it as a base for grafting new functionalities on the main chain of polymer to improve its physicochemical properties. In this paper, we rely on the results describing the solubility behavior of the cellulose Acetate published by El-Barkany et al. [24] .We used cellulose acetate having a DS equal to 1.7 (DS1.7) as a base polymer. The value of 1.7 was chosen because CA shows a good solubility in THF. The CA (DS ~ 1.7) was synthesized from cellulose stemming from Esparto "Stipa tenacissima" of eastern Morocco by acetylation deacetylation process (figure 1).

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Figure 1: Areas of the solubility of cellulose acetate according to DS [24] In this context, we propose a study to evaluate deeply the homogeneous regioselective grafting (in THF). The grafting of urethane segments (Butyl 1,6-Diyldicarbamate) on cellulose acetate (DSAC ~ 1.7) The resulted cellulose derivatives was characterized using different spectroscopic techniques (FTIR, NMR,..) and also by TGA/DTG and RXD. The influence of grafting behavior (DS value) on TGA/DTG thermograms and diffractogramms was deeply investigated. NMR was used to study scarcely the regioselective grafting on CA in homogenous medium. The results of the structural analyzes showed that the grafting is regioselective on the hydroxyl groups of C6 position (OH-C6) and the DS of urethane groups was fond around 0.7.

2. Experimental 2.1. Materials and reagents Unmodified cellulose was extracted in alkali medium from Esparto "Stipa tenacissima" of Eastern region of Morocco according to the method described by el Barkany et al. [25]. The Esparto plant was ground making particle size of about 500-900 microns. The resulting powder is dried at 60 °C for 16 h (or 5 hours at 105 °C) to constant weight. After removal of organic extractables, the powder was then treated with a 1N NaOH solution at 80 °C for 2 h under stirring. After each treatment, fibers were filtered and washed with bleach (NaClO) and distilled water until the sodium hydroxide was completely removed and washed again with ethanol and diethyl ether three times. The solid fraction rich in cellulose was again stirred at 80 °C for 4 h in an equal parts of acetate buffer solution, chlorite aqueous (1.7% in water) and distilled water. Sodium hydroxide (NaOH), potassium hydroxide (KOH) and acetic anhydride were purchased from Aldrich. Solvents such as toluene, ethanol (98%), diethyl ether, acetone and n-butanol (98%), are of high quality (for analysis)and were used as received from the commercial supplier without any further purification. 1, 6 hexamethylene diisocyanate (HDI) (98%) and Tetrahydrofurane (THF) were purchased from Merck company. THF was distilled by sodium metal. Dibutyltindilaurate (DBTL)

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(95%) was used as catalyst. All other chemicals are of analytical grade and were used without further purifications. 2.2. Preparation of cellulose acetate In the first stage, we prepared cellulose acetate with a maximum degree of substitution close to the value 3 (DS max ~ 2.9). 1 g of cellulose has been activated for 1 minute under stirring in a mixture of toluene (10ml)/acetic acid (5ml) (2/1: v/v), then 5 ml of acetic anhydride was gradually added to the reaction mixture and the perchloric acid was added as catalyst (two drops).After the complete addition of acetic anhydride, the reaction is maintained at room temperature for 15 min, the solid suspension (cellulose fibers) disappeared and the solution became clear and transparent. The cellulose acetate was then isolated by precipitation in distilled water, filtered under vacuum, washed widely with distilled water, dried in an oven at 40 °C to constant weight and kept for one week in the desiccator in the presence of P2O5. The CA1.7 was prepared by saponification process in 7.16 10-3 N KOH ethanolic solution. Thus, this normality value (0.2g KOH/50ml ethanol) was choosing according to the amount of hydroxyl groups to deacetylate. The saponification reaction was carried out in a glass flask equipped with magnetic stirring at room temperature for 24 h. Then, CA1.7 was filtered under vacuum and washed with ethanol and diethyl ether, dried at 40 °C until constant weight, then put it in a desiccator in the presence of P2O5 for one week. The new degree of substitution (DSCA) was determined by a complete deacetylation followed by a return volumetric dosage by 0.15 M acetic acid using phenolphthalein and the value of the degree of substitution (DS) was calculated using the equation (1):

Where; DSCA: degree of substitution of the acetyl group, MAGU: molar mass of the monomer cellulose (162 g mol-1), ΔnKOH: change in number of molar KOH before and after saponification, M acl: molar mass of acetic group (42.04 g mo -1). 2.3. Preparation of Cellulose Acetate carbamates-Urethane (CACU) After placing 1.15g of HDI and 10 ml of THF into a reactor, an equimolar amount of n-butyl alcohol in 10 mL of THF was added dropwise and the mixture is brought to stir for 2h. The precursor HDIOBt was used for the modification of CA1.7in THF. 1g of cellulose acetate (DS1.7) was weighed accurately and transferred with 30 ml of THF in a reactor, then heated for 30 min at 80 °C and a solution of the precursor (HDI-OBt/THF) with a catalytic amount of dibutyl-tin (DBTL) were added. The mixture was reheated at 80 °C for 2 h under stirring. In the end, the final product CA1.7-HDI-OBt was precipitated in diethyl ether, filtered and washed with diethyl ether before drying at 40 °C overnight. The product was finally ground to a fine powder and then characterized. 2.4. NMR method 1

H NMR is widely used in cellulose chemistry for the determination of DS. This method is based on

the integration of characteristic protons. The degree of substitution of the urethane segments

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DSUreth grafted on CA1.7 is calculated based on the integrations of the typical protons of signals. The equation (2) can be used to deduce the value of DSUreth:

Where; IAC is the integration of signals of methyl protons of the acetyl group and IUreth is the integration of signals corresponding to urethane proton (NHCOO). 2.5. Spectral analysis The chemical structures of the samples were evaluated by spectroscopic techniques FTIR, 1H NMR, 13C NMR and DEPT135. The experiments were performed using an infrared spectrometer Fourier transform type Shimadzu FTIR-8400S using KBr discs with finely ground of samples to 2%. Twenty measurements were taken for each sample; the range of number of recorded wave is 4000 to 400

cm-1. 1 H

NMR

and13C

NMR spectra were

recorded

on

an ADVANCE

Bruker

300

MHz spectrometer at 360 K using TMS as a reference standard and DMSO-d6 as solvent. Samples crystallinity was investigated by X-ray diffraction technique. The analysis were performed with a PAN analytical X'Pert Pro MPD Diffractometer using copper radiation CuKα (λ = 1.5418 A °).The voltage is 40 kV and the current operation is 30mA. All tests were carried out in the range 2θ = 5°-40°, height (pitch) 0.05 °/s. The samples were prepared in the form of pellets of ~ 0.25 g and were pressed granules in a mold diameter ~ 25 mm under a pressure of 50 MPa. The index of crystallinity (IC) was calculated using equation 3 [26]:

Where; Imin is the minimum intensity between 2θ ~ 18° and 19° and Imax is the maximum crystalline peak intensity between 2θ ~ 22° and 23°. The thermal behavior of the samples was performed using the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on Shimadzu DTG-60 simultaneous TG-DTA apparatus. The weights of the samples were between 8 and 12 mg, the interval of the temperature measurement was between room temperature and 500 °C with a rate of 10 °C.min-1 under nitrogen flow. The experimental results for describing the solubility were obtained by solvents tests, where, a spatula tip (some mg) of sample was added to ~ 3 ml of solvent. The samples were used as granular crushed.

3. Results and discussion 3.1. Synthesis of cellulose acetate (CA1.7) The low solubility in organic solvents, generally, is a major difficulty in the chemistry of polysaccharides, particularly in the case of cellulose and its derivatives in many applications. Thus, to overcome this drawbacks, and after a literature review of published data on the solubility of natural polymers in different solvents, it is essential to understand that the dissolving power depends on the polymer’s structures and the solvent nature. We have recently studied the solubility of cellulose

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acetate (CA) through a new theoretical approach taking into account the influence of the degree substitution (DS) values [24]. According to Flory-Huggins model and the molar volume approximation for polymeric systems [27], the system Solvent - Polymer is homogeneous for values χ SP less than 0.5, consequently, the polymer and the solvent are completely miscible if χ SP ≤ 0.5. This approximation can be taken as a good hypothesis to understand the strong relationship between the degree of polymerization and the solubility [27]. Cellulose acetate having a DS equal to 3 (DS3) was synthesized in a heterogeneous acid medium (acetic acid and toluene) by attacking the cellulose with acetic anhydride in the presence of perchloric acid as a catalyst (Figure 2):

Figure 2: Synthesis of cellulose acetate (DS AC ~ 2.9) The partial deacetylation is performed by saponification using an ethanolic solution (KOH/ethanol 7.16 10-3 N), the new degree of substitution (DSCA) was determined by a complete deacetylation and by a volumetric dosage. The CA (DS1.7) was purified by the dissolution-precipitation method in THFether. Spectral analysis Cellulose acetates (CA1.7) and unmodified cellulose FTIR are shown in figure 3. According to the figure 3, the FTIR of CA1.7 shows similar spectral characteristics and no new band absorption is observed after complete deacetylation AC0.1, indicating that no other chemical reactions occurred during deacetylation process.

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Figure 3: FTIR spectra of unmodified and acetylated cellulose (DS2.9and DS1.7) The cellulose acetylation is justified by the reduction of characteristic absorption bands of the hydroxyl groups (OH) and the appearance of three important new absorption bands characterizing acetyl ester groups. The absorption band located at 1749 cm-1 is assigned to the carbonyl stretching (C = O)ester, the methyl of the acetate group existed at about 1373 cm-1 (C-CH3) and that allocated to the group (C-O)ester appeared at around 1232 cm-1 [28, 29]. The absorption band located at 1647 cm1

corresponds to the bending mode of structural and naturally absorbed water [30].The absence of any

absorption band in the 1760-1840 cm-1 interval and around 1700 cm-1 confirms the absence of the residual acetic acid and acetic anhydride, indicating high purity of elaborated products. The absorption band around1047 cm-1 is attributed to the elongation of the connection (CO) cycle characteristic of glucopyranose [31]. Thus, the unchanged ratio of band intensities (CO)pyranose and that at 902 cm-1 assigned to the glycoside bonds β-D (1-4) between the glucose units that form the cellulosic skeleton, confirms that no degradation

chains take place during the acetylating reaction [32]. The non-

degradation of polymer chains may be due to the low concentrations of acid and alkaline solution used

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in the protocols. This suggestion can be confirmed by studying the change in the macromolecular weight during the stages of the reaction.

3.2. Synthesis of C6 Regioselective Cellulose Acetate Butyl hexane-1,6-Diyldicarbamate Polyurethanes formed a large family of materials and are characterized by a one common group; urethane group (-NHCOO-) [33]. They are generally obtained from the reaction between polyalcohol and polyisocyanate, the most commonly used method is the one-shot, where the direct mixture of reactants is followed by the simultaneous addition of a blowing agent, catalysts and other additives depending on the desired properties. The CO2 generating agent can be used to create foams inside material and a variety of additives may be added to the medium such as fillers stabilizers, crosslinking agents and chain extenders [34]. The most important applications of the polyurethanes can be summarized in the construction, transport and Shoes industry. In contrast, very interesting areas such as agricultural and medicinal applications have not been thoroughly studied until now. However, several efforts have been oriented in the sense of functionalization of various natural materials containing OH groups, such as polysaccharides [35], cashew nuts [36], etc. moreover, the literature reports very few research works in this field, this may be due to the major obstacle of the cellulose chemistry (solubility). In this context, a scope study on homogeneous grafting of urethane segments on cellulose acetate (CA 1.7) will be drawn (Figure 4):

Figure 4: Grafting reaction of a precursor terminated isocyanate group on cellulose acetate (CA1.7) The physicochemical properties of the cellulose derivatives are not only influenced by the size (DP), the nature of the substituent and the degree of substitution (DS), but also by the regioselectivity substitution and its distribution along the molecule skeleton [37]. Spectral analysis Generally, the formation of the urethane group is easily proved by the appearance and the identification of four characteristic absorption bands at1223 cm-1, 1533 cm-1, 1699 cm-1 and 3317 cm1

corresponding respectively to the elongation of the amide linkage ν (-HNCO-), deformation of the

bond δ (-NHCOO-) of carbamates, the elongation of the carbonyl group ν(-NHCOO-) and the urethane NH bond ν(-NHCOO-) [38, 39].

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Figure 5: FTIR spectra of CA1.7, precursor HDI-OBt and AC1.7 -HDI-OBt Figure 5 shows FTIR spectra ofCA1.7, precursor HDI-OBt and AC1.7 -HDI-OBt derivative. The precursor segment (HDI-OBt) was formed by reacting 1, 6-hexamethylene Di-Isocyanate (HDI) with Butanol (BtOH) which is reacted again in one pot with CA1.7 to give the final product AC1.7-HDIOBt. FTIR spectrum shows the appearance of anew absorption band corresponding to the elongation of the NH binding characteristic of urethanes of ν (-NHCOO-) around 3315 cm-1, the second characteristic band is located around 1690 cm-1 and it was attributed to the elongation of the carbonyl bond of urethanes ν (-NHCO O-) [40], while the typical band of the deformation of the bond NH urethanes δ(-NHCOO) is located at 1539 cm-1 [41]. However, the aliphatic di-urea produced during this reaction can be absorbed by hydrogen bonds on the surface ofAC1.7 -HDI-OBt fibers. this

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suggestion is illustrated by examining the FTIR spectrum of AC1.7 -HDI-OBt where the appearance of the characteristic absorption bands of carbonyl diurea ν(-NHCONH-) at 1627 cm-1 (amide I) and deformation of NH-urea δ(-NHCONH-) at 1573 cm-1 (amide II). The disappearance of the peak characterizing the isocyanate group (NCO) at 2276 cm-1 (overlooking on the precursor HDI-OBt spectrum) confirmed the total addition reaction between cellulose acetate AC1.7 and the precursor HDI-OBt. NMR spectra The distribution of acetyl groups on the three hydroxyl groups (OH) of AGU has a significant influence on the physicochemical properties of cellulose derivatives. The distribution of acetyl groups can be determined from the relative intensities of peaks at C6, C2 and C3 (figure 6a) characterizing to methyl acetyl groups grafted on primary OH(C6), and secondary OH (C2and C3) of the repeating unit. The figure 6a shows the 1H NMR spectrum ofCA1.7-appear between 0.9 and 1.5ppm, also the grafting of urethane segments is confirmed on the 1H NMR spectrum by the presence of two signals at 3.9 ppm and the peak at 2.9 ppm characterizing respectively the methylene proton of α-ester and α-urethane. Moreover, the proton of the urethane function (NH -COO) shows a signal peak at 7.0 ppm. The spectrum also indicated the presence of very small amounts of urea function (NH-CO-NH) whose protons give a signal peak at 5.74 ppm. However, the allophanates (ROCO) -NR'-CO-NHR'' may appear during the reaction at high temperatures beyond 100 ° C. Furthermore, the formation of allophanate compounds could be followed by the appearance of a characteristic peak at 10.1 ppm [42]. In this paper, the temperature of the reaction was maintained below 100 °C and no allophanate compound was detected.

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El Barkany & al./ Appl. J. Envir. Eng. Sci. 3 N°3(2017) 280-296 Figure 6a:1H NMR spectrum of AC1.7-HDI-OBt The partial deacetylation of cellulose acetate CA3, realized in this work, to prepare CA1.7 yielded to a specific distribution of acetyl groups, liberating about 50% of C6 hydroxyl groups and 75% of C3 hydroxyl groups. However, the primary C6-OH is the easiest one to graft. The value of the partial degree of substitution DSi of the acetyl moiety on the three OH groups was calculated from the integration of the acetyl protonpeaksusing1H NMR spectrum ofAC1.7 -HDI-OBt and the results are compared with those listed in the literature (Table 1).

DSAC 1.70 1.66 1.86 1.20

Table 1: degree of partial substitution of cellulose acetates obtained indifferent conditions DS2 DS3 DS6 The mode of reaction Ref. 0.87 0.32 0.52 Alkali deacetylation This work 0.63 0.53 0.50 Iodine-catalyzed acetylation [43] 0.70 0.62 0.55 Iodine-catalyzed acetylation [43] 0.60 0.31 0.29 acid-catalyzed deacetylation [44]

It is important to note that the three hydroxyl groups in the positions C2, C3 and C6have different reaction rates. The order of reactivity is high agreement with the reaction conditions of acetylation (or deacetylation) and the value of total degree of substitution obtained DS CA. In this work, after deacetylation of CA2.9 to reach a value of DSCA close to the DSCA~ 1.7, we observed a new specific distribution of acetyl groups in the order C2> C6> C3(Table 1). Although, in the homogeneous system solvation of cellulose(DMAc / LiCl), Marson and El Saud reported that the reactivity of acetylation is in the order C6> C2> C3 DSCA ~ 2.6 [45]. The similar order was observed in the case of cellulose acetylating realized in the 1,3-dimethyl-2-imidazolidinone (DMI) / LiCl as solvent system [46].While the cellulose acetate prepared in an ionic liquid such as chloride 1-allyl-3-imidazolium (AmimCl) with a DS (2.16), the order remains the same for position 6 and is reversed for positions 3 and 2 (C6> C3> C2) [47]. So far, no interpretation single has been advanced to explain this phenomenon (the difference in reactivity between the hydroxyl C3 and C2 positions).

The low steric hindrance of the hydroxyl group attached at C6-OH compared to the other groups C2OH and C3-OH is no doubt behind its high reactivity during an acetylation reaction except some reactions catalyzed by iodine [43]. Yet, for the DSCA values close to 1.5reached by the procedure of acid deacetylation, the distribution of acetyl groups is in the order C2> C3> C6 [44].

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Figure 6b: 13C NMR and DEPT135of AC1.7-HDI-OBt 13

C NMR spectrum of CA1.7-HDI-OBtderivative shows that the grafting of urethane segments on

cellulose acetate CA1.7 is confirmed by the occurrence of characteristic peaks situated between 14ppm and 63.7 ppm assigned to aliphatic carbons (-CH2- and -CH3) of the urethane segments. Indeed, the ester carbonyl signals (CO)ester were detected at 170.8 ppm (C6), 169.8 ppm (C3) and 169.49 ppm (C2), respectively. Also, the regioselectivity appears for urethanes segments grafted as well as a new signal assigned to the carbons of the urethane carbonyl(CO) urethane grafting on the C6 position(primary urethanes) is located at 158.5 ppm and that grafted on the C3 position (secondary urethane) was observed at 156.8 ppm (figure 6b).These results were confirmed by the DEPT spectra (figure 6b), the disappearance of the peaks in the region 150-175 ppm indicating that the signals observed in the spectrum 13 C NMR in this area corresponding to the quaternary carbons. On DEPT135 spectra we can similarly distinguish between primary, secondary and tertiary carbons by changing the orientation of the secondary ones. In addition, the typical chemical shift of isocyanate groups at 122.9 ppm is not

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observed on spectra, this indicates that the isocyanate groups have been completely consumed and are not present in the final product. X-ray diffraction The X-ray diffraction technique (XRD) was exploited to study the effect of the chemical modification reaction on the structure of unmodified cellulose fibers. XRD Spectra of unmodified cellulose fibers and CA1.7-HDI-OBt are shown in figure 7. The XRD diffractogramme of unmodified cellulose revealed the presence of four peaks corresponding to Bragg angles at 2θ ~ 15°, 16.3°, 22.6° and 34.5° respectively assigned to the (hkl) planes (101), (101), (002) and (040) derived from the crystalline polymorphic form of cellulose I [48].These characteristic peaks disappear in the diffractogramms of CA1.7and CA1.7-HDI-OBt, indicating that these modifications lead to significant changes in the crystalline density and in the order of the typical crystalline form of unmodified cellulose. The crystallinity index (Ic) was significantly decreased from 63.1% for cellulose fibers to 15% for the new cellulose derivative AC1.7-HDI-OBTindicating thereby the destruction of the crystalline order and consequently the increase in the concentration of amorphous zones.

Figure 7: X-ray diffraction patterns of the unmodified cellulose and CA1.7-HDI-OBt

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Furthermore, the XRD pattern of modified cellulose shows new peaks localized at the Bragg angles 2θ ~ 6.4°, 9.9°, 20.5° and 23.9° (figure 7). The new peaks indicate the structuring of the new lattice planes where the appearance of a novel crystalline order. This may be due to the self-construction of new crystalline area driven by hydrogen bonds between the NH groups of the grafted urethane. Thermal Analysis Generally, the thermal degradation of cellulose derivatives, such as cellulose esters, takes place in three stages. The first step is the removal of water and solvents, located between 50 °C and 150 °C, the second one is devoted to deacetylation at about 320 °C. Lastly, the thermal pyrolysis of cellulosic skeleton occurs at 370 °C [49]. The thermal stability depends on the molecular weight (Mw) and the crystallinity index (Ic) of the polymer. Moreover, the thermal properties of cellulose derivatives are sensitive to the degree of substitution (DS) changes, the nature of the grafted groups and the uniformity of distribution.

Figure 8a: Thermograms of unmodified cellulose and CA1.7-HDI-OBt

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Figure 8b: Derivative thermogravimetric signals (DTG) of unmodified cellulose and CA1.7-HDIOBt. Figures 8a and 8b show the DT-TGA curves for unmodified cellulose and AC1.7 –HDI-OBt. The endothermic event between room temperature 25 °C and 100 °C is attributed to the elimination of water amount adsorbed by the structure of polysaccharide. Unmodified cellulose present, at 291 °C, a wide endothermic peak attributed to thermal decomposition of the cellulosic backbone. The decomposition is also confirmed by the curve of the derivative of mass loss (DTG) illustrated on the figure 8b. Indeed, the strongest peak on the DTG diagram starts from 291 °C and has a maximum at about 330 °C. An important weight loss close to 70% was noted in the area of 291-370 °C what is attributed to the depolymerization of the cellulose polymer.

The DT-TGA thermograms of AC1.7-HDI-OBt present an endothermic corresponding to the adsorbed water around 100 °C. The apparition of the endothermic zero weight loss (constant weight) will be awarded to a structural phenomenon, in the case of polymers this is due to the mobility of urethane segments grafted. The material transforms from a glassy state to the rubbery state, this processing is associated with a characteristic temperature called the glass transition temperature Tg. However, the Tg is localized on the thermogram of CA1.7-HDI-OBt at around of 141 °C. At 252 °C the thermal behavior of CA1.7-HDI-OBt showed the first level of loss weight assigned to the decomposition of the urethane groups grafted on cellulose acetate the backbone. Deacetylation and thermal pyrolysis of cellulose macromolecular chains was observed between 280 °C and 400 °C.

4. Conclusion The modification of the cellulose fibers extracted from Esparto was successfully applied and the use of cellulose acetate as an intermediate product is an important alternative step allowing limiting the use of expensive and toxic solvents such as DMAc, pyridine, etc. However, the partial deacetylation

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allows the controlled release of active sites (hydroxyl groups), thus leading to a controlled regioselectivity grafting. Furthermore, X-ray diffraction technique shows that this step causes destruction of crystallinity order, consequently increases the proportion of the amorphous phase, and therefore a good accessibility of some chemical reactants such as urethane segments grafting, the modification becomes easy. The thermal study of CA1.7-HDI-OBt compounds shows a decrease in thermal stability due to degradation of grafted urethane groups. Eventually this method could lead the synthesis of plastic films from agricultural and forestry products. Acknowledgements This work was supported by CNRST under grant no: PPR/2015/17. We thank greatly the CNRST for its support and the anonymous reviewers for their careful review and valuable suggestions on the manuscript. References: 1. A. Pandey, CR. Soccol, P. Nigam, VT. Soccol. Bioresource technology 74 (2000) 69-80. 2. S. Richardson, L. Gorton. Analytica Chimica Acta 497 (2003) 27-65. 3. B. Focher, M. Palma, M. Canetti, G. Torri, C. Cosentino, G. Gastaldi. Industrial Crops and Products 13 (2001) 193-208. 4. A. Mohanty, M. Misra, G. Hinrichsen. Macromolecular materials and Engineering 276 (2000) 1-24. 5. A. Mohanty, A. Wibowo, M. Misra, L. Drzal. Composites Part A: applied science and manufacturing 35 (2004) 363-370. 6. G. Samaranayake, WG. Glasser. Carbohydrate polymers 22 (1993) 1-7. 7. CM. Buchanan, JA. Hyatt, DW. Lowman. Macromolecules 20 (1987) 2750-2754. 8. AJ. Stamm. Wood and cellulose science 1964. 9. CL. McCormick, PA. Callais. Polymer 28 (1987) 2317-2323. 10. YN. Kuo, J. Hong. Polymers for advanced technologies 16 (2005) 425-428. 11. J-F. Blachot, N. Brunet, P. Navard, J-Y. Cavaille. Rheologica acta 37 (1998) 107-114. 12. SM. Hudson, JA. Cuculo. Reviews in Macromolecular Chemistry 18 (1980) 1-82. 13. T. Bikova, A. Treimanis. Carbohydrate Polymers 48 (2002) 23-28. 14. T. Schult, T. Hjerde, OI. Optun, PJ. Kleppe, S. Moe. Cellulose 9 (2002) 149-158. 15. SC. Fox, KJ. Edgar. Cellulose 18 (2011) 1305-1314. 16. K. Huang, B. Wang, Y. Cao, H. Li, J. Wang, W. Lin, C. Mu, D. Liao. Journal of Agricultural and food Chemistry 59 (2011) 5376-5381. 17. A. Potthast, T. Rosenau, J. Sartori, H. Sixta, P. Kosma. Polymer 44 (2003) 7-17. 18. T. Heinze, A. Koschella, T. Liebert, V. Harabagiu, S. Coseri. The European Polysaccharide Network of Excellence (EPNOE): Springer; 2012. p. 283-327. 19. OA. El Seoud, H. Nawaz, EP. Arêas. Molecules 18 (2013) 1270-1313. 20. A. El‐Kafrawy. Journal of Applied Polymer Science 27 (1982) 2435-43. 21. L. Yan, T. Wei. Journal of Biomedical Science and Engineering 1 (2008) 37-43. 22. A. Striegel. Carbohydrate polymers 34 (1997) 267-274. 23. G. Samaranayake, WG. Glasser. Carbohydrate polymers 22 (1993) 79-86. 24. A. El Idrissi, S. El Barkany, H. Amhamdi, AK. Maaroufi, B. Hammouti. J Mater Environ Sci 3 (2012) 27085. 25. A. El Idrissi, S. El Barkany, H. Amhamdi, AK. Maaroufi. Journal of Applied Polymer Science 128 (2013) 537-548. 26. E. Samios, R. Dart, J. Dawkins. Polymer 38 (1997) 3045-3054. 27. S. El Barkany, A. El Idrissi, H. Amhamdi, A-K. Maaroufi. Cellulose and cellulose composites. Biochemistry Research Trends ed: Nova Science Publishers; 2015. p. 99-150. 28. T. Heinze, T. Liebert. Macromolecular Symposia: Wiley Online Library; 2004. p. 167-238. 29. R. Rajini, U. Venkateswarlu, C. Rose, T. Sastry. Journal of applied polymer science 82 (2001) 847-853.

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