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Studies concerning the accessibility of different allomorphic forms of cellulose

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Studies concerning the accessibility of different allomorphic forms of cellulose
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  Studies concerning the accessibility of differentallomorphic forms of cellulose Diana Ciolacu  • Luizildo Pitol-Filho  • Florin Ciolacu Received: 24 March 2010/Accepted: 12 November 2011/Published online: 19 November 2011   Springer Science+Business Media B.V. 2011 Abstract  The supramolecular architecture and themorphological structure of cellulose play an importantrole in its accessibility. In order to evaluate the effectof the crystalline form of organization on the acces-sibility, we selected cellulosic materials with signif-icant variations in the aforementioned characteristics.The assessment of the accessibility of cellulosicmaterials was performed experimentally through awater vapor sorption method. The kinetics and thethermodynamic parameters of water vapor sorptionprocess were determined, and a correlation betweenthe Flory–Huggins interaction parameters and thecrystallinity index was derived. We concluded that theallomorph involving the most accessible crystal sur-faces and amorphous regions was Cellulose II. Thecorrelation of the accessibility values with those of thecrystallinity index allowed us to evaluate the acces-sibility of the allomorphic forms of cellulose atdifferent crystallinity indexes. The obtained experi-mental data allowed us to quantify the accessibilityfor crystal surfaces and amorphous regions of thedifferent allomorphs in the order Cellulose II(38%) [ Cellulose I (24%) [ Cellulose III (10%). Keywords  Cellulose allomorphs    Water vaporregain    Accessibility    Mass transfer coefficient   Flory–Huggins interaction parameters List of symbols C Crystallinity, % F   Objective function  H   Air humidity, dimensionless K   Mass transfer coefficient, g (mg h) - 1  M   Molecular mass, g gmol - 1 n  Number of humidity conditions P  Partial pressure, atm Q  Amount of water adsorbed by the fibers, mg g - 1  R  Ideal gas constant t   Time, h T   Absolute temperature, K  V   Specific volume, cm 3 gmol - 1 Greek letters ø Volumetric fraction l  Chemical potential v  Flory–Huggins interaction parameter D. Ciolacu ( & )Laboratory of Physical Chemistry of Polymers,‘‘Petru Poni’’ Institute of Macromolecular Chemistry,Grigore Ghica Voda Alley, 41A, 700487 Iasi, Romaniae-mail: dciolacu@icmpp.ro; dciolacu02@yahoo.co.uk L. Pitol-FilhoFaculty of Engineering, Centro Universita´rio Cato´lica deSanta Catarina, Rua dos Imigrantes 500,Jaragua´ do Sul 89254-430, BrazilF. CiolacuFaculty of Chemical Engineering and EnvironmentalProtection, Natural and Synthetic Polymers Department,‘‘Gheorghe Asachi’’ Technical University of Iasi,Dimitrie Mangeron Bd., 71, 700050 Iasi, Romania  1 3 Cellulose (2012) 19:55–68DOI 10.1007/s10570-011-9620-1  Subscripts 0 Initialcel Celluloseeq Equilibrium concentrationexp Experimental Superscripts 0 Standard value for chemical potentialvap Saturation Introduction Cellulose, the main structural component of the cellwalls in plants, is an important raw material forindustry, whose role as a source of fuels andcommodity chemicals tends to increase in the future(Zhang and Lynd 2006). As one of the most unique and simplest polysac-charides molecules, the structure of cellulose isresponsible for the macroscopic properties of thepolymer, and has a remarkable and complex influenceon its chemical reactions. Such fact may be explainedby three hierarchical levels: the molecular level of asingle molecule, the supramolecular level concerningthe packing and aggregation of the molecules incrystals called microfibrils, and the morphologicallevel, i.e., the arrangement of microfibrils and inter-stitial voids in relation to the cell wall (Klemm et al.1998; Kra¨ssig 1993a). On the molecular level, cellu-lose is composed of linear chains of glucose unitslinked together by  b -1,4-glucosidic bonds. Thesechainsformwhisker-likecrystalswhich areassembledin a suprastructure.The crystalline structure of cellulose is highlyordered, with extensive hydrogen bonding betweenthe molecular chains. Cellulose can crystallize intoseveral different polymorphs. Natural cellulose (cel-lulose I) is the form found in nature and occurs in twoallomorphs, I a  and I b  (Nishiyama et al. 2002; Nishiy- ama et al. 2003). Cellulose II is the crystalline formthat emerges after regeneration from different mediaor mercerization with aqueous sodium hydroxide(Langan et al. 2001; Langan et al. 2005). Cellulose III I  and III II  are obtained from cellulose I and II,respectively, through liquid ammonia or organicamine treatment (Wada et al. 2004). Celluloses IV I and IV II  are a result of a thermal treatment of celluloseI and II, or could be obtained from the partialtransformation of cellulose III (Zugenmaier 2001). Amorphous cellulose can be prepared by severalmethods, such as ball-milling of cellulose, deacetyla-tion of cellulose acetate in anhydrous methanol, oreven by precipitation from a non-aqueous solventsystem into non-aqueous regeneration media (Kondo2005). Details about the arrangement of the chainsboth inside the crystals and between the crystals havebeen discussed extensively over the last years (Horii2000; Nishiyama et al. 2008; O ¨ztu¨rk et al. 2009).An important feature of cellulosic materials is theirtwo-phase morphology of crystalline (ordered) andamorphous (disordered) regions, which influence theaccessibility and the reactivity of the fibers.In order to evaluate the relative amount of crystal-line material in cellulose, crystallinity index (CrI)parameter is commonly used. Several techniquesare employed to determine the CrI, such as X-raydiffraction (XRD), solid-state  13 C NMR, Fouriertransform infrared spectroscopy (FTIR), Raman spec-troscopy. Although the results from these techniquesshow a good correlation, the values obtained can varysignificantly depending on the choice of instrumentand the data analysis technique (Park et al. 2009; Sathitsuksanoh et al. 2011). Recent works, recom-mend X-ray diffraction as the most appropriatetechnique to determine CrI (Thygesen et al. 2005; Park et al. 2010), however, using different dataanalysis techniques involving XRD, significant dif-ferent results were obtained.The physical properties of cellulose, as well as itschemical behavior, are strongly influenced by thearrangement of the cellulose molecules with respect toeach other and to the fiber axis. Thus, both crystallineand amorphous regions have been studied extensivelyin order to elucidate the micro- and macro-responsesof cellulose material to thermal, hydrothermal andchemical treatments (Ciolacu and Popa 2006; Ciolacuet al. 2006; Olaru et al. 2007). In any heterogeneous reaction involving cellulose, most reactants penetrateonly the amorphous regions and are located in thesedisordered areas and also on the surfaces of thecrystallites, leaving the bulk of the intracrystallinecellulose unaffected. To increase the chemical reac-tivity of cellulose, it is necessary to enhance theaccessibility of the crystalline regions through swell-ing or de-crystallization processes.The accessibility is indeed a structural parameterthat depends on the nature of the interactions that take 56 Cellulose (2012) 19:55–68  1 3  place between a reactant and the cellulosic substrate.Those interactions are influenced by the availability of the inner surface, by the supramolecular organizationand by the fibrillar architecture. The properties of cellulose depend directly on the moisture regain of thefibrous material and on the environmental moisture.The structural elements that are responsible for suchbehavior are the hydroxyl groups distributed along themacromolecular chains, and the compact network of hydrogen bonds, established through strong interac-tions between the chains. These networks grant, to thecellulosic substrate, a structure that permits a contin-uously dimensional modification in function of thevariations of the atmospheric conditions. The charac-teristics of morphological structure of cellulose fiberscondition the phenomena of moisture regain.One of the most known methods for the evaluationof the accessibility of cellulosic substrates is thesorption of water vapor, in controlled conditions of temperature and relative environmental moisture. Themain reason behind the choice of this method lies inthe fact that it is a simple technique that providesresults with a high degree of accuracy and reliability,allowing to compare simultaneous measurements of agreat number of samples, at a given temperature andrelative humidity.Several other methods have been developed tomeasure cellulose accessibility, including: the Bru-nauer–Emmett–Teller (BET) method based on thenitrogen adsorption isotherm, size exclusion chroma-tography, small-angle X-ray scattering (SAXS),microscopy, etc. (Hong et al. 2007). The choice of  the method strongly depends on the application. Forexample, for acid hydrolysis, accessibility based onvapor adsorption may be important. To investigate theprocess of enzymatic cellulose hydrolysis, cellulase-size exclusion chromatography may be a good indica-tor (Grethlein 1985). Recently, a technology has been developed to determine the cellulose accessibility tocellulase (CAC) based on adsorption of a non-hydro-lytic fusion protein containing a cellulose-bindingmodule and a green fluorescent protein (Hong et al.2007; Rollin et al. 2011) which clearly presents accessibility trends. Such a strategy allowed a com-parisonofCACvalues,whichshowedadecreaseintheorder: amorphous cellulose [ filter paper [ Avicel.The main contributions of the present work are thedescription and validation of a simple method toevaluate the percentage of participation of eachcrystalline form on the cellulose organization (cellu-lose I, II and III), as well as the explanation of thekinetic and thermodynamic behavior of the adsorptionprocess of water into cellulose allomorphs, and thedistinction of the differences that appear in thehydrophilic characters of allomorphic forms of cellulose. Experiments MaterialsMicrocrystalline cellulose, A (Avicel PH-101, Fluka)and spruce dissolving pulp, E (Extranier F, Rayonier,France) were used without further purification. Cottoncellulose, B (Arshad Enterprises, Pakistan) wasextracted in a Soxhlet extractor with ethanol andbenzene mixture (2:1), for 8 h, followed by boiling in1% aqueoussolution ofsodiumhydroxide for6 h. Thefinal product was washed with distilled water,immersed in 1% acetic acid, washed once more withwater, and air-dried. Without further processing, thesesamples are referred in this paper as AI, BI and EI,corresponding to the allomorph of native cellulose.Besides AI, BI and EI, two other allomorphs wereprepared: •  Cellulose II   (AII, BII and EII): the mercerizedcellulose with crystalline form of cellulose II wasobtained by soaking cellulose I in 17.5% NaOHduring 24 h, at 15   C, followed by washing itthoroughly with distilled water and air-dried; •  CelluloseIII  (AIII,BIIIandEIII):sampleswiththecrystalline form of cellulose III I  were prepared bysoaking cellulose I in organic amine (100%ethylenediamine) during 24 h, at room tempera-ture. The cellulose amine complex was washedwith anhydrous methanol and finally cellulose III I samples were air-dried.  Amorphous cellulose  (A amorph, B amorph and Eamorph) was obtained by dissolving cellulose solu-tions with SO 2 -diethylamine-dimethylsulphoxide(SO 2 -DEA-DMSO), with further regeneration byusing ethanol (Isogai and Atalla 1991). All other chemicals (Fluka) were used withoutpurification. Cellulose (2012) 19:55–68 57  1 3  Methods Water vapor regain  is a method to evaluate thehydrophilic/hydrophobic characteristics of fibers andis based on the determination of the quantity of waterthat the fibers can absorb and retain under strictlycontrolled conditions, as 65% relative humidity and25  ±  2   C (Standard DIN 1979). Three determina-tions were done for the water vapor adsorption and theaverage values were reported. In these three repeti-tions, standard deviations remained lower then 6 mgwater/g cellulose, for any point along the adsorptionisotherms. The water vapor uptake is expressed as aratio between the fiber weight, exposed to a standardatmosphere and the fiber weight when it is absolutelydry (Tappi Test Method 2008).  Degrees of polymerization  of cellulose (DP) weremeasured through the viscosity method in cupri-ethylenediamine (CED) solution (Da Silva Perez andHeiningen 2002; Standard ISO/FDIS 5351 2009).  X-ray diffraction method  X-Ray diffraction patterns of the samples werecollected on a RIGAKU RINT 2,500 apparatus,equipped with a transmission type goniometer usingnickel-filtered, CuK  a  radiation at 40 kV. The goniom-eter was scanned stepwise every 0.10   from 10   to 40  in the 2 h  range. To avoid preferred orientations, thecotton cellulose (B) and dissolving pulp samples(E) were cut into small pieces and then were placed ina glass capillary tube (Kondo et al. 2001). The diffraction patterns exhibited peaks which weredeconvoluted, with PeakFit 4.11 software, frombackground scattering by using Lorenzian functions,while the diffraction pattern of an artificially amor-phized sample was approximated to a Gaussian curve.The amorphous scattering curve was simultaneouslyfitted in the partially crystalline samples. The estima-tion ofthe crystallinity index(Cr.I.)was doneby usingthe Segal method, which consists in the estimation of the peaks intensity corresponding to crystalline andamorphous areas (Segal et al. 1959).  Accessibility of cellulose The accessibility of cellulose samples can be calcu-lated according to equation (Jeffries et al. 1968): Accessibility  ð % Þ ¼  100    MR17  ð 1 Þ where, MR—the content of moisture regain of thestudied sample, %; 17—the content of moisture regainof a completely amorphous sample, %.To determine the moisture regain, samples of cellulose allomorphs were exposed to standard atmo-sphere, at 25   C and 65% of relative humidity, during24 h (Standard DIN 1979). The moisture regain wascalculated by dividing the mass of absorbed water insample by the mass of the dry material (105   C, 4 h). Theoretical background The water vapor sorption in cellulose fibers may befitted to a kinetic model, described by Eq. 2 (Klemm et al. 1998): dQdt   ¼  K Q eq    Q   2 ð 2 Þ where  Q  is the amount of water uptake by the fibers inmg g - 1 , t  isthetimeinhoursand K  isthemasstransfercoefficient in g (mg h) - 1 .The subscript  eq  refers to the equilibrium concen-tration. The mass transfer coefficient is related to thediffusivity of water in the fibers (Seader and Henley2006), whereas the saturation concentration  Q eq  is themaximum sorption capacity, or the concentrationwhenthesystemreachesequilibrium.Bothparametersare dependent on temperature. Equation 2 was inte-grated to regress  Q eq  and K from the experimentaldata, as shown in Eq. 3:1 Q eq    Q ð t  Þ   1 Q eq    Q 0 ¼  K t     t  0 ð Þ ð 3 Þ InEq. 3thesubscript 0 referstotheinitialvalue,eitherfor the time or for the absorbed amount. The param-eters  K   and  Q eq  were regressed by fitting the exper-imental data to Eq. 3 and minimizing the objectivefunction  F   expressed by Eq. 4: F   ¼ X ð Q exp ð t  Þ    Q model ð t  Þ  Þ 2 ð 4 Þ In Eq. 4 the superscripts  exp  and  model  refer to theabsorbed concentration obtained experimentally andthroughsolutionofEq. 3,respectively.Equation 4isa typical objective function that takes into account the 58 Cellulose (2012) 19:55–68  1 3  square deviations between experimental values andthose obtained by the fitting equation.Once  Q eq  was obtained for each fiber, the volumefraction of water (ø) was calculated according toEq. 5, first as a function of the absorbed mass of water( m water  ) and later expressed in terms of   Q eq : U ¼ m water V  water   M  water  m water V  water   M  water  þ  m celV  cel  M  cel ¼ Q eqV  water   M  water  Q eqV  water   M  water  þ  V  cel  M  cel ð 5 Þ where  V   represents the specific volume in cm 3 gmol - 1 and  M   the molecular mass in g gmol - 1 .The subscripts water   and  cel  refer to water and cellulose, respec-tively. Methods to calculate the specific volume arebased on contribution rules and were discussedextensively on the literature (Fried 1995; Barton1991).The water absorbed into cellulose fibers (phase I) isin equilibrium with the water content in the air (phaseII), and the equality of the chemical potentials  l  isexpressed by Eq. 6: l water     l 0 water   RT    I ¼  l water     l 0 water   RT    II ð 6 Þ where the superscript  0  relates to the standard value,  R  is the ideal gas constant, and  T   is the temperature inKelvin.The chemical potential of the water in the cellulosefibers can be alternatively written as stated in Eq. 7: l water     l 0 water   RT    I ¼  ln u water   þ  1    u water  ð Þþ  v  1    u water  ð Þ 2 ð 7 Þ where  v  is the Flory–Huggins interaction parameterbetween the cellulose and the water (Fried 1995). Onthe other hand, the chemical potential of the water inthe vapor phase may be expressed by Eq. 8: l water     l 0 water   RT    II ¼  ln P water  P vapwater  ¼  ln  H   ð 8 Þ where  P  is the partial pressure of water, (and withsuperscript  vap  refers to its saturation pressure).Alternatively, the concept of relative humidity(  H  ) can be used.The samples were weighed at each experimentalconditiononceequilibriumwasassumedtobereached,and this mass variation was converted, for Flory–Huggins analysis purposes, into volume fraction of water in cellulose by applying Eq. 5. As Eqs. 6–8 show, usually for polymers, the Flory–Hugginsapproach establishes a relationship between the con-centration of the fluid phase (expressed in moisture, inthiscase)andtheconcentrationofthepolymericphase(expressed in volume fraction).It is possible to calculate the Flory–Hugginsinteraction parameter for a series of experiments withvarying humidity. In this case, the equilibrium equa-tion may be extended to:ln Y ni ¼ 1  H  i  ¼  ln Y ni ¼ 1 u i  þ X ni 1   u i ð Þ þ  v X ni 1    u i ð Þ 2 ð 9 Þ In Eq. 9  n  is the number of different humidityconditions. It is possible then to evaluate how thecrystallinity affects the Flory–Huggins interactionparameter. Results and discussion Water vapor sorption studiesThestructureofthedifferentcellulose allomorphswasassessed by the X-ray diffraction method. The studyrevealed that the modification in the crystallineorganization form of cellulose through differentchemical treatments definitely takes place, while thecrystallinity index decreases (Table 1). A comprehen-sive X-ray diffraction study of these cellulose allo-morphs was reported by the authors in previous papers(Ciolacu 2007; Ciolacu and Popa 2007; Ciolacu et al. Table 1  The crystalline index and degree of polymerization of the allomorphic forms of cellulosic samples, A, B and ESample CrI (%) DPAI 80 183AII 77 154AIII 60 169BI 71 3,078BII 60 2,048BIII 53 2,829EI 65 1,458EII 57 1,116EIII 48 1,179Cellulose (2012) 19:55–68 59  1 3
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