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Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

ORIGINAL PAPER

Glycidyl methacrylate derivatized xylan-rich hemicelluloses:synthesis and characterizations

Xinwen Peng ?Junli Ren ?Linxin Zhong ?

Runcang Sun ?Wenbin Shi ?Bojie Hu

Received:27October 2011/Accepted:25April 2012óSpringer Science+Business Media B.V.2012

Abstract In this paper,a novel type of hemicellu-losic derivative containing polymerizable double bonds (C=C)was synthesized by chemical reaction of xylan-rich hemicelluloses and glycidyl methacry-late (GMA)in dimethyl sulfoxide (DMSO)in pres-ence of catalysts.The chemical structure,reaction mechanism,and rheological properties of the deriva-tive were investigated by means of FT-IR,1H and 13C-NMR,DEPT 135NMR,GC–MS and rheometer.The in?uence of the reaction conditions including reaction time,temperature,catalysts,and the amount of reagents on the degree of substitution were investi-gated in detail.Results showed that the chemical reaction of xylan-rich hemicelluloses with GMA was transesteri?cation,which resulted in direct attachment of methacrylate (MA)groups to xylan-rich hemicel-luloses,instead of GMA.A maximum degree of substitution of 0.94could be achieved under the optimum reaction condition (40°C,1.8equiv of GMA to per xylose unit,36h,20%DMAP).Results from rheological analysis indicated that the aqueous solu-tions of the methacrylated xylan-rich hemicelluloses

(MAXH;0.5, 1.0and 2.0wt%)exhibited typical shear-thinning behavior in the range of shear rates tested.The viscoelasticity of MAXH solutions increased with the increasing concentration and DS.Keywords Xylan-rich hemicelluloses áGlycidyl methacrylate áTransesteri?cation áRheological properties

Introduction

Preparation of functional and biocompatible polymers and materials from biomacromolecules is an important way to settle a series of resources and environmental problems (Cunha and Gandini 2010;Sasso et al.2011;Mikkonen et al.2010).Hemicelluloses,which are economical,biocompatible,non-toxic,biodegradable,are considered to be the second most abundant renewable biopolymers.Currently research activities in the ?eld of hemicelluloses have been aimed at preparing functional biopolymers and biomaterials (Lindblad et al.2007;Edlund 2008;Goksu et al.2007;Pohjanlehto et al.2011),in which hemicelluloses-based hydrogels,?lms and coatings have spurred an increasing interest in the applications in food and medicinal industry including functional barriers and additives,wound dressings,and drug delivery because of their low cost,biodegradability,little health hazard,and low oxygen permeability (Lindblad et al.2001;

X.Peng áJ.Ren (&)áW.Shi áB.Hu

State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou,China

e-mail:renjunli@http://www.wendangku.net/doc/17416a60dd36a32d72758145.html

L.Zhong áR.Sun

Institute of Biomass Chemistry and Technology,Beijing Forestry University,Beijing,China

Cellulose

DOI 10.1007/s10570-012-9718-0

Miyazaki et al.2001;Grondahl et al.2004;Hansen and Plackett2008;Lin and Zhao2007).

Vinyl groups are the major polymerable groups which are commonly used in grafting for speci?c purpose and application.The presence of vinyl groups in polymer backbone or side chain allows themselves participating in the polymerization with polymerizable monomers or macromonomers containing carbon–carbon double bonds(Frechet et al.1995;Ishizu et al. 2000;Li and Armes2005;Hawker et al.2001). Therefore,introducing vinyl groups to hemicelluloses is an important strategy to further prepare novel biopolymers and biomaterials by the polymerization. In our previous work,maleic anhydrided hemicellu-loses containing carbon–carbon double bonds were prepared(Peng et al.2011a),which could be further polymerized with N-isopropylacrylamide to form hydrogels(Yang et al.2011).Moreover,glycidyl methacrylate(GMA)derivatization is another impor-tant approach to prepare the polymers containing polymerable vinyl groups,and have attracted attention in the preparation of functional polymers and materials (Guilherme et al.2005;Hebeish et al.1997;Ishizu and Mori2000;Morimoto et al.2005;Nagasaki et al.1997; Ohsedo et al.2004;Tsukahara et al.1990).Up to now, many GMA derivatized polysaccharides have been used to produce novel various hydrogels(Guilherme et al.2005;Reis et al.2003;Hedin et al.2010;Elizalde-Pena et al.2007;Vervoort et al.1997;Ferreira et al. 2000)by various polymerization(Frechet et al.1995; Ishizu et al.2000;Li and Armes2005;Hawker et al. 2001;Tsukahara et al.1990;Ishizu and Mori2000). Therefore,GMA derivatization can create opportuni-ties for the utilizations of xylan-rich hemicelluloses in preparation of biomaterials applied in biology,bio-medical science,and surface chemistry(Vervoort et al. 1997).

In this paper,we prepared a novel GMA derivatived biopolymer by transesteri?cation of xylan-rich hemi-celluloses with GMA in the presence of catalysts using dimethyl sulfoxide(DMSO)as the medium.The effect of reaction conditions on DS was discussed,and the chemical structure of GMA derivatives from xylan-rich hemicelluloses as well as reaction mechanism were investigated.The rheological behaviors of these resulting products were also determined by an AR2000 rheometer.This study provides a novel biomacromol-ecule-based precursor which can be easily changed into functional biopolymer and biomaterials by further polymerization with themselves and other monomers or macromonomers containing double bonds. Experiments

Materials

Xylan-rich hemicelluloses were isolated from Dend-rocalamus membranaceus Munro(Dm M)deligni?ed holocellulose using10%KOH at25°C for10h with a solid to liquid ratio of1:20(g/mL).The holocellulose was obtained by the deligni?cation of the extractive-free Dm M(40–60mesh)with sodium chlorite in acidic solution(pH3.7–4.0,adjusted by10%acetic acid)at 75°C for2h.The sugar analysis showed the following sugar composition(relative weight percent):82.3% xylose,9.6%arabinose,4.0%glucose,2.4%galact-ose,0.7%rhamnose,and0.8%mannose.Uronic acids,mainly4-O-methyl-D-glucuronic acid(MeG-lcA),were present in a noticeable amount(3.9%). Ethanol and DMSO were purchased from Guangzhou Chemical Reagent Factory,Guangzhou,China.Cata-lysts lipase(triacylglycerol acylhydrolase,EC3.1.1.3 was purchased from Leveking Bio-engineering Co., Ltd.,Shenzhen,China.Glycidyl methacrylate(GMA), 4-dimethyl-amino-pyridine(DMAP),N-bromosuccin-imide(NBS),triethylamine(TEA),and potassiumtert-butoxide(PB)were purchased from Aldrich Co.,Ltd., Shanghai,China.

Synthesis of GMA derivatized xylan-rich hemicelluloses

Firstly,0.66g of xylan-rich hemicelluloses(equal to 0.005mol of anhydroxylose in xylan-rich hemicellu-loses)was dissolved in DMSO(30mL)at95°C for 1.5h to guarantee the complete dissolution of xylan-rich hemicelluloses,followed by cooling to room temperature.Subsequently,catalyst was added to the mixture under stirring and the?nal solution was kept at40°C for30min.Required amounts of GMA was then added and the mixture was stirred for6,24,36, and48h at30,40,60,and80°C,respectively.After the required time,the reaction mixture was precipi-tated with150mL95%(w/w)ethanol,and then centrifuged at4,000rpm for20min.The precipitate was thoroughly washed with95%(w/w)ethanol three times.Finally,these products obtained were dissolved

Cellulose

in puri?ed water and freezed-dried at-50°C.The GMA derivatized xylan-rich hemicelluloses were prepared in duplicate,with4%standard error.As can be seen in Table1,26samples were synthesized by changing the reaction conditions.In this experi-ment,the molar ratios of GMA to Nx(xylose units in xylan-rich hemicelluloses)from3:2to3:1were achieved,and the amounts of catalysts(based on the dried xylan-rich hemicelluloses weight)were kept from5to20%.

FT-IR spectroscopy

FT-IR transmission spectra of xylan-rich hemicellu-loses and their derivatives were measured by using a Nicolet750spectrophotometer within the wave number ranging from400to4,000cm-1,and1.0%?nely ground sample was mixed with KBr to press a plate for measurement.

1H-NMR,13C-NMR and DEPT135NMR

The solution-state1H-NMR and13C-NMR spectra were obtained on a Bruker AVIII400MHz spectrom-eter operating in the FT mode at100.6MHz.The xylan-rich hemicellulosic sample(20mg for1H, 80mg for13C and DEPT135)was dissolved in 1mL D2O.The13C-NMR and DEPT135NMR spectrum was recorded at25°C after15,000scans. A30°pulse?ipping angle,a9.2l s pulse width,and a 1.36s acquisition time,and2s relaxation delay time were used.

Table1Effect of the reaction conditions on the DS of MAXH

TEA triethylamine,PB potassium tert-butoxide

a Molar ratio of GMA/Nx represents the molar ratio of GMA to anhydroxylose units in xylan-rich hemicelluloses

b DS was determined by percentages of C,H,O of product detected by elemental analysis Sample

number

Temperature/°C Molar ratio

of GMA/Nx a

Reaction

time/h

Catalyst/wt%dried

hemicelluloses

DS b

1303:22410%DMAP0.41 2403:22410%DMAP0.65 3503:22410%DMAP0.61 4603:22410%DMAP0.59 5803:22410%DMAP0.52 6403:202410%DMAP0.08 7403:52410%DMAP0.21 8406:52410%DMAP0.60 9403:22410%DMAP0.65 10409:52410%DMAP0.79 114012:52410%DMAP0.78 12403:12410%DMAP0.78 12409:5610%DMAP0.17 13409:51210%DMAP0.38 14409:52410%DMAP0.79 15409:53610%DMAP0.88 16409:54810%DMAP0.81 17409:53620%pyridine0.47 18409:53620%lipase0.32 19409:53620%TEA0.57 20409:53620%NBS0.54 21409:5365%PB0.41 22409:53610%PB0.49 23409:53620%PB0.59 24409:5365%DMAP0.71 25409:53610%DMAP0.88 26409:53620%DMAP0.94

Cellulose

Gas chromatography and mass spectrometry

The gas chromatography and mass spectrometry(GC–MS)analysis was used to detect the presence of glycidol(a possible by-product of the reaction of xylan-rich hemicelluloses with GMA).GC–MS was carried out with an Agilent5975C mass spectrometer and a7890A gas chromatograph with an INNOWAX capillary column(30m90.25mm i.d.,?lm thick-ness0.25l m).The carrier gas was helium.The column was kept at an initial temperature of50°C for 2min,and the temperature was raised at5°C/min to 200°C.The?ow rate was1mL/min and the mass range of35–500.

Determination of DS

The DS of the products was calculated by elemental analysis(Schwikal et al.2006).All samples of solid polymers were ground to powder and dried at60°C for24h before measurement.Carbon content in the substituted xylan-rich hemicellulosic samples was measured to determine the DS(Vaca-Garcia et al. 2001).The DS values were calculated as follow:

DS?C%?132à60 48à69?C%

where C%is the carbon content of product determined by elemental analysis.132and69are the molecular weights(g mol-1)of xylose unit in xylan-rich hemi-celluloses and the methacryloyl group.60and48are the total molecular weights(g mol-1)of carbon element in xylose unit and methacryloyl group, respectively.

Determination of molecular weights and molecular weight distribution

The molecular weights of XH and MAXH were determined by gel permeation chromatography(GPC) on a PL aquagel-OH50column(300mm97.7mm, Polymer Laboratories Ltd.),calibrated with PL pullu-lan polysaccharide standard(average peak molecular weights of783,12,200,100,000,1,600,000).Flow rate of0.5mL/min was maintained.The eluent was 0.02N NaCl in0.005M sodium phosphate buffer(pH 7.5).Detection was achieved with a Knauer differen-tial refractometer.The column oven was kept at30°C. XH and MAXH were dissolved with0.2N NaCl in 0.005M sodium phosphate buffer,pH7.5,at a concentration of0.1%.

Rheological properties

The dynamic rheological properties of hemicellulosic derivatives were measured between a40mm diameter steel parallel plate and a Peltier plate in an AR2000 rheometer(TA Instruments)at25°C.All samples were dissolved in water solution with a magnetic stirrer for30min at room temperature.The?ow curves were obtained in a Brook?eld DVIII instru-ment.A thin layer of paraf?n oil was applied on top of the sample to avoid evaporation.The values of the strain amplitude were checked to ensure that all oscillatory shear experiments were performed within the linear viscoelastic regime,where the dynamic elastic modulus(G0)and viscous modulus(G00)are independent of the strain amplitude.Flow data were collected over shear rates from10-2to103/s, frequency from10-1to102rad/s,respectively.All measurements were repeated twice.

Results and discussion

FT-IR spectra

The FT-IR spectra of xylan-rich hemicelluloses and their derivatives are illustrated in Figs.1and2.In Fig.1a,the bands at1,419,1,322,1,118,1,049and 897cm-1are associated with xylan-rich hemicellu-loses,in which two bands at1,118and1,049cm-1are typical of arabinoxylans(Fang et al.2000;Gupta et al. 1987).The band at1,166cm-1is assigned to C–O and C–O–C stretching with some contribution of OH bending mode.Moreover,an absorption band near 1,386cm-1was detected and it is due to the CH bending vibration present in xylan-rich hemicelluloses chemical structures.The band at1,075cm-1relates to the C–OH bending.The relatively strong absorption at around1,636cm-1indicated the characteristic of C=O mainly from4-O-methyl-glucuronic acid branches(Ge et al.2009).The bands at1,466,1,347 and1,247cm-1are assigned to CH2,OH,or CH bending.

Comparatively,in the spectrum of the GMA derivatized xylan-rich hemicellulosic sample26 (Fig.1b;Table1),the intense band at1,720cm-1is

Cellulose

attributed to C=O stretching frequency of conjugated ester groups (Guilherme et al.2005).The introduction of C=C groups from GMA into polysaccharide chains was con?rmed by the intense band at 813cm -1,indicative of the =CH out-of-plan bending.These results indicated that the chemical modi?cation occurred onto the isolated xylan-rich hemicelluloses.The FT-IR spectra of GMA derivatized xylan-rich hemicelluloses with different DS are also shown in Fig.2.The similar absorption band pro?le was observed in the spectra of GMA derivatized xylan-rich hemicelluloses.The enhancement in the intensity of absorbance at 1,720cm -1is presented with an increment in DS from 0.38(spectrum a)to 0.65(spectrum b),and to 0.71(spectrum c).

1

H and 13C NMR spectra

In 1H NMR spectra of isolated xylan-rich hemicellu-loses (Fig.3a),the signals at d 5.7and 4.3ppm are attributed to the anomeric protons of terminal a -L -arabinfuranosyl residues and 4-linked b -Xylp residues in xylan-rich hemicelluloses chains (Guilherme et al.2005;Peng et al.2010).A strong signal at d 4.7ppm is indicative of the residual solvent (D 2O).The signals at d 2.9–4.1ppm are originated from the equatorial proton and other protons of the Xylp residues (Geng et al.2006).In the spectrum of GMA derivatized xylan-rich hemicelluloses (Fig.3b),the signals at d 6.1and 5.7ppm are attributed to the protons at the double bond from the GMA (van Dijk-Wolthuis et al.1995).The signal appeared at d 1.9ppm is correlated to the methyl hydrogen (van Dijk-Wolthuis et al.1995).

In the 13C NMR spectrum of xylan-rich hemicel-luloses (Fig.4a),the main 1,4-linked b -D -xylp units are characterized by ?ve strong signals at d 102.6,75.6,73.5,72.7and 63.1ppm,which are assigned respectively to C-1,C-4,C-3,C-2and C-5positions of the b -D-xylp units.The signals at d 109.4,86.0,80.5,78.1and 61.3ppm correspond to C-1,C-4,C-2,C-3and C-5positions of the a -L -arabinofuranosyl residues linked to b -D -xylans,respectively (Sun et al.2002),while the signals at d 72.1and 70.0ppm are attributed to the C-2and C-5in MeGlcA residue.The glucose

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

5.7

4.7

4.1

3.73.43.1

2.9

6.15.7

5.0

1.9

4.3

4.4

(b)

(a)

3

2

1

H3

H1,H2

Fig.31H NMR spectra of xylan-rich hemicelluloses (spectrum a)and GMA derivatized xylan-rich hemicellulosic sample 26(spectrum b,DS of 0.94).R =xylan-rich hemicellulose-O–CO–or xylan-rich hemicellulose-O–CH(CH 2OH)–O–CO–in Scheme 1

Cellulose

residue in the xylan or degraded fragments of cellulose is identi?ed by two signals at d 81.3and 62.9ppm.By comparison of 13C NMR spectra of xylan-rich hemi-celluloses (spetrum a)and the GMA derivatized xylan-rich hemicellulosic sample 26(spectrum b)in Fig.4,it was possible to detect the presence of the signals attributed to the vinyl carbon of methacrylate groups (d 135,127ppm)and to the methyl carbon (d 17ppm;Guilherme et al.2005).These results indicated that the polymerizable C=C double bonds were introduced onto xylan-rich hemicelluloses.Reaction mechanisms of xylan-rich hemicelluloses with GMA

Although these results from FT-IR,1H NMR and 13C NMR con?rmed the chemical reaction of xylan-rich hemicelluloses with GMA,but the reaction mecha-nism is still not clear.According to van Dijk-Wolthuis (van Dijk-Wolthuis et al.1995;van Dijk-Wolthuis et al.1997),two different pathways could be consid-ered for the chemical reaction of xylan-rich hemicel-luloses and GMA,as shown in Scheme 1.In Scheme 1a,the GMA was proposed to react with xylan-rich hemicelluloses by opening the epoxy ring,and the reaction proceeded by a nucleophilic attack of a hydroxyl group of xylan-rich hemicelluloses at the methylene carbon of the epoxy group of GMA,which is a common reaction for an epoxide with an alcohol (van Dijk-Wolthuis et al.1995).In Scheme 1b,however,the GMA was introduced into the polysac-charide through transesteri?cation reaction (van Dijk-Wolthuis et al.1997).In order to well understand the reaction mechanism,the product was detected by DEPT135NMR.Next,the part of the reaction mixture was added to ethanol,and the precipitated product was removed by centrifugation.The supernatant was analyzed with GC–MS,the results were shown in Figs.5and 6.

In DEPT 135NMR spectra,positive signals indicate CH and CH 3groups,negative signals indicate CH 2groups,and no signals appear for quaternary carbons.Therefore,the carbon signals (especially some overlapping signals)obtained from 13C NMR spectra can be further distinguished.Carbon atoms of the possible products a (resulting from the pathway of Scheme 1a)and b (resulting from the pathway of Scheme 1b)are numbered in Fig.5.Obviously,the structure of possible products a and b in Fig.5can be distinguished by C8,C9,and C10signals in DEPT 135NMR spectra.The typical signals at d 101.6,76.3,73.6,and 72.7ppm are assigned to C1,C4,C3,C2of xylose units (Heinze and Koschella 2008;Chakraborty et al.2005).The signal at d 17.4ppm is assigned to CH 3(C7)of GMA,and the negative signals at d 62.9and 127.9ppm correspond to methylene groups of

102.6

75.6

73.5

72.7

63.1

61.358.9

109.4

86.0

80.5

81.3 78.1 70.0

177.1

168.3 XH

72.117.4

168.9

168.5

135.7

135.3 127.9

127.2

(b)

(a)

1

2 3

C1

C2 C3

Cellulose

xylose units (C5)and GMA (C6)(Koschella et al.2006).Obviously,the signals of carbon atoms of position 8,9,and 10in the model structure of possible product a (Fig.5)do not exist in the DEPT 135NMR spectra (Heinze and Koschella 2008),which indicated the presence of structure b (Fig.5),instead of structure a.In other words,the chemical reaction of xylan-rich hemicelluloses with GMA proceeded according to the pathway b in Scheme 1.

The chemical reaction which results in direct attachment of methacrylate esters to polysaccharide is transesteri?cation reaction,yielding methacrylated

xylan-rich hemicelluloses (MAXH)and the concom-itant formation of glycidol (Scheme 1b).In order to further con?rm the reaction mechanism,the presence of glycidol and its respective degradation products were identi?ed by the GC–MS,as shown in Fig.6.After 13.48min the peaks relate to m /z 73(1.0%,[M-H]?),m /z 56(2.6%,[M-H 2O]á?),base peak m /z 44(100%,[M-CH 2O]á?),43(89.4%,[M-CH 3O]á?),m /z 31(58.8%,[M-C 2H 3O]á?)and 29(42.4%,[M-C 2H 5O]á?;Reis et al.2003).This proves the presence of glycidol as by-product of the chemical reaction of xylan-rich hemicelluloses with GMA.These results demonstrated that methacrylation of xylan-rich hemicelluloses with GMA actually occurred via a transesteri?cation reaction,yielding MAXH with the methacryloyl group directly attaching to xylose unit (Scheme 1b).Similar results were also observed in the reactions of dextran (van Dijk-Wolthuis et al.1995;van Dijk-Wolthuis et al.1997),inulin (Vervoort et al.1997),galactomannan (Reis et al.2003),and cashew gum (Guilherme et al.2005)with GMA.The reaction mechanism is suggested as follow:a nucleophilic catalysis of DMAP with the formation of a methacry-loyl pyridinum salt intermediate followed by a general base catalysis (Vervoort et al.1997;van

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Dijk-Wolthuis

Scheme 1The proposed reaction mechanisms of xylan-rich hemicelluloses with GMA:a opening epoxy ring,b transesteri?cation reaction.R represents XH or substitute

1

43

2

7

5

6Fig.5DEPT 135NMR spectra of GMA derivatized xylan-rich hemicelluloses (sample 26with a DS of 0.94)

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Cellulose

et al.1997).Therefore,synthesis of GMA derivatized xylan-rich hemicelluloses through opening epoxy ring will not be successful in DMSO,which is attributed to the stabilization of glycidol anion through delocaliza-tion of the negative charge over the two oxygen atoms (van Dijk-Wolthuis et al.1997).The availability of polymerizable double bonds in MA will provide opportunities for synthesis of novel functional biopoly-mers and biomaterials by means of various polymer-ization technologies(Frechet et al.1995;Ishizu et al. 2000;Li and Armes2005;Hawker et al.2001; Tsukahara et al.1990;Ishizu and Mori2000).

Effect of the chemical reaction conditions

on the DS of MAXH

The extent of the chemical reaction of xylan-rich hemicelluloses with GMA was expressed by DS when the chemical reaction was carried out under different conditions.It is shown in Table1.An increase of reaction temperature from30to40°C led to an increment in the DS of the products from0.41to0.65. Thereafter DS decreased to0.52when the reaction temperature increased up to80°C.Reaction time shows a signi?cant in?uence on the reaction ef?ciency of transesteri?cation.An increase of reaction time from6to36h led to an increment in the DS of the products from0.17to0.88,thereafter DS decreased slightly when the reaction time increased to48h. Hence,attempts were made to carry out the chemical reaction for36h.

Table1also shows the relationship between the molar ratio of GMA to Nx(GMA/Nx)and the DS of the products.There was an increase in the DS with increasing the GMA/Nx molar ratio from3:20to9:5, and a maximum DS of0.85could be obtained at the GMA/Nx molar ratio of9:5.However,further increase in the GMA/GMA/Nx molar ratio(12:5–3:1)could not help to boost the reaction ef?ciency,which is indicated by a DS of0.78.

The in?uence of catalysts on transesteri?cation is also investigated in Table1.Among various catalysts, pyridine,PB,TEA,NBS and DMAP are effective catalysts and widely used in various reactions.Lipase also has been used in organic media for the catalysis of esteri?cation and transesteri?cation reactions(Weber et al.2001;Yadav and Devi2004).As shown in Table1,MAXH with a DS rang of0.35–0.94were achieved at the catalysts concentration of20wt%(based on the dried xylan-rich hemicelluloses weight). The catalysis effect of catalysts increased in this order: lipase\pyridine\NBS\TEA\PB\DMAP. Obviously,DMAP was the most effective catalyst in the transesteri?cation reaction of xylan-rich hemicel-luloses with GMA.Increasing the amount of DMAP from5to20wt%led to an increase of DS from0.71to 0.94.As compared with chemical catalysts,lipase showed low catalysis(MAXH with DS of0.35)in the condition given,which is possibly due to the unfavor-able reaction condition(e.g.,pH,temperature,solvent). Molecular weights and molecular weight distribution

The weight-average(M w)and number-average(M n) molecular weights as well as polydispersity(M w/M n) are listed in Table2.The M w of XH were42, 600g mmol-1,which were higher than that of MAXH with DS less than0.41(35,300g mmol-1),indicating that some degradation occurred to XH during dissolv-ing and reaction process.The depolymerization during reaction was also reported(Ren et al.2008).This may be due to that the glycosidic linkages were weakened in the process of homogeneous transesteri?cation reaction.In addition,the MAXH has a relatively low index of polydispersity(2.23–2.98)than the XH (3.01),which indicates that MAXH has a more uniform molecular weight distribution.

Table2The weight-average(M w)and number-average(M n) molecular weights as well as polydispersity(M w/M n)of XH and MAXH

Sample Total DS M w M n M w/M n

XH a–42,60014,100 3.01 10.4135,30012,600 2.80 20.6534,20014,100 2.43 60.0832,80011,000 2.98 70.2133,60011,800 2.85 110.7842,50017,000 2.50 130.3830,20011,200 2.70 150.8843,50018,200 2.39 260.9444,30019,900 2.23

a Native xylan-rich hemicelluloses without modi?cation

Cellulose

Rheological properties of MAXH

For better understanding the effect of GMA modi?-cation on the physic-chemical properties of the biopolymers and their possible applications,the rheological behaviors of MAXH were investigated in this study.Because MAXH with DS value above 0.21can well be dissolved in water and form a viscous solution,the rheological measurement was carried out in distilled water at concentrations of 0.5,1.0and 2.0%(w/w),and the results are shown in Figs.7,8,9.The viscosity of MAXH solutions decreased with the increasing shear rate,implying that the solutions

exhibited pseudoplastic or shear-thinning behavior in the range of tested shear rates due to the destroying of macromolecules network at higher shear rate.For concentrated solutions,an imposed deformation is applied to disrupt the entanglements while the new entanglements form at the same time to replace the disrupted ones.It was evident that viscosity of MAXH with higher concentration was higher than that with lower concentration (Fig.7).In our previous study (Peng et al.2011b ),we also found that the native xylan-rich hemicelluloses and carboxymethyl xylan-rich hemicelluloses (in alkaline solution)also exhib-ited shear-thinning behavior,which were commonly observed in other carbohydrate polymers (Torul and Arslan 2003;Hu et al.2007).At the same concentra-tion,MAXH with a DS of 0.94displayed a higher viscosity than that one with a DS of 0.41(Fig.7),which may be due to the better dissolution and stronger intermolecular interactions in aqueous solu-tion when more methacryloyl groups present in XH chain.

Elastic modulus (G 0)and viscous modulus (G 00)of solution re?ected the strength of elastic structure and viscous structure.Figure 8shows the frequency-dependent elastic modulus (G 0)and loss modulus (G 00)of the MAXH solutions at concentrations of 0.5,1.0and 2.0%.Both G 0and G 00increased with the increase of frequency.The elastic modulus G 0of MAXH (DS of 0.94)solutions (at concentrations of 0.5,1.0and 2.0%)were lower than loss modulus G 00at low frequencies,but went above G 00when the tested frequency was higher than 5rad/s.Therefore,MAXH

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Glycidyl methacrylate derivatized xylan-rich hemicelluloses- synthesis and characterizations

Cellulose

(DS of0.94)solutions showed a viscous behavior at low frequencies but an elastic behavior at higher frequencies.This implied that the rate of the formation of new entanglements is prominent over disruption of molecular entanglements at higher frequencies in MAXH aqueous solution(Lindblad et al.2005;Xu et al.2009).Figure9shows G0and G00as a function of frequency for MAXH solutions with different DS (0.41and0.94)at1.0%concentration.Both G0and G00of the MAXH with a DS of0.94were higher than those of MAXH with a DS of0.41at low frequencies (\11rad/s),indicating that MAXH with a DS of0.94 showed stronger intermolecular interactions in aque-ous solution at low frequencies,which was observed in GMA derivatized inulin(Vervoort et al.1999).This observation well agreed with the results obtained from the viscosity data in Fig.7.

Conclusions

In summary,a novel type of xylan-rich hemicellulosic derivative containing polymerizable double bonds (C=C)was synthesized by transesteri?cation reaction of xylan-rich hemicelluloses and glycidyl methacry-late in dimethyl sulfoxide,leading to the direct attachment of the methacryloyl groups to xylan-rich hemicelluloses,which was con?rmed by the presence of methacryloyl groups in the xylan-rich hemicellu-lose chains and glycidol in the reaction mixture solution.Under the optimum reaction condition(20 wt%DMAP,the GMA/Nx molar ratio of9:5,40°C for36h),the resulting product with a maximum DS value of0.94could be obtained.The MAXH showed shear-thinning performance and elasticity in aqueous solution(0.5,1.0and2.0wt%)at high frequency.The elastic modulus and loss modulus of MAXH solution increased with the increasing concentration and DS. This study provides a new biomacromolecular pre-cursor for the preparations of functional biopolymers and biomaterials from low-cost biomass.Further researches on the preparations of functional biopoly-mers and biomaterials(e.g.membrane,hydrogel, nanoparticle)for speci?c applications will be inter-esting and important.

Acknowledgments This work was supported by the grants from National Natural Science Foundation of China(No. 31070530and30930073),Foundation for the Author of National Excellent Doctoral Dissertation of China(No. 201169),Project of Guangzhou Science and Technology Plan (No.11C52080723/2011J4100065),Science and Technology project of Guangdong Province(No.2011B050400015),and the Fundamental Research Funds for the Central Universities(No. 2012ZZ0081),SCUT.

Appendix:Abbreviations

Full name Abbreviations

Xylan-rich hemicelluloses XH Glycidyl methacrylate GMA Methacrylate MA Methacrylated xylan-rich hemicelluloses MAXH Dendrocalamus membranaceus Munro Dm M

4-O-methyl-D-glucuronic acid MeGlcA Dimethyl sulfoxide DMSO

4-Dimethyl-amino-pyridine DMAP

N-bromosuccinimide NBS Triethylamine TEA Potassiumtert-butoxide PB

Xylose units in xylan-rich hemicelluloses Nx

Degree of substitution DS

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