A small-angle neutron scattering and rheology study of the composite of chitosan and gelatin

A small-angle neutron scattering and rheology study of the composite of chitosan and gelatin
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  Colloids and Surfaces B: Biointerfaces 70 (2009) 254–258 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces  journal homepage: A small-angle neutron scattering and rheology study of the compositeof chitosan and gelatin Yuan Wang a , Dong Qiu a , ∗ , Terence Cosgrove a , ∗∗ , Mark L. Denbow b a School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK  b St Michael’s Hospital, Bristol BS2 8EG, UK  a r t i c l e i n f o  Article history: Received 27 August 2008Receivedinrevisedform15 December 2008Accepted 22 December 2008Available online 31 December 2008 Keywords: SANSRheologyChitosanGelatin a b s t r a c t The composite chitosan/gelatin solutions and films formed from these solutions were studied by rheo-logical measurements, SANS and tensile tests. The relationship between the inter-molecule interactionswith microstructure, rheological behaviour of a solution and eventually the mechanical performance of formed films was established. It was found that the complex formed between chitosan and gelatin wasmainlythroughhydrogenbondbutthesizeofthestructurewasalsoaffectedbyelectrostaticrepulsions.The local structure (correlation length) and the global structure (large inhomogeneous structure size)in the composite solutions were found to be highly correlated to each other. It was also found that theinteractionsbetweenthesetwopolymersinsolutionwerecloselyrelatedtothemechanicalpropertiesof the formed films. This work will enable one to design films with desired mechanical properties throughthe combination of different polymers at optimum weight ratios.© 2008 Elsevier B.V. All rights reserved. 1. Introduction Chitosan is the  N  -deacetylated derivative of chitin, a naturallyabundant polysaccharide found in the exoskeleton of shellfish likeshrimpsorcrabs[1].Becauseofitsbiocompatibilityandeaseoffilmformation, chitosan has been extensively studied as a promisingmedicalmaterial[2].Thefeasibilitiesofchitosanbasedmaterialsas scaffoldsforthetissueengineering[3–6]andwounddressing[7–9] have been widely explored with some promising results. Owingto the amino groups, chitosan is ready to be chemically modifiedwithnewfunctionalgroups[10,11].Moreimportantly,chitosanhas an abundance of hydroxyl groups and is charged, which enablesit to form complexes with other molecules through electrostaticattractions or hydrogen bonds [12–17].Gelatin is another natural polymer, which is obtained by ther-maldenaturationorphysicalandchemicaldegradationofcollagen[18]. Due to their biocompatibility, low toxicity, biodegradabilityandadhesiveness,blendsofchitosanandgelatinhavebeenstudiedforpotentialusesinpharmaceuticalfields[19,20],tissueengineer- ingandwounddressing[21–23].Itisespeciallyinterestingthatthe compositefilmofchitosanwithgelatinshowsnon-linearmechani-calresponseontheirrelativeconcentration[17].Ithasalreadybeen ∗ Corresponding author. Current address: School of Physical Sciences, Universityof Kent at Canterbury, Canterbury CT2 7NH, Kent, UK. ∗∗ Corresponding author. E-mail addresses: (D. Qiu), Cosgrove). shown that carboxyl groups can interact with amino groups [24],whicharepresentinthesessystems.Inthisstudy,rheologicalmea-surements and small-angle neutron scattering (SANS) were usedto investigate the structures of various mixture solutions and ten-sile tests were used to measure the mechanical property of theformed films. (In this case, elongation was chosen since it is themost important aspect in application such as wound dressing.) 2. Experimental  2.1. Materials Gelatin was alkali-processed bovine ossein gelatin, kindly sup-pliedbyEastmanKodakCo.withanIEPofpH4.9andispolydisperse[25]. The high molecular weight fractions were removed by frac-tionalprecipitation;asaresultthissamplecontainsahighfractionof    -chain and is referred to as   -gelatin. This   -gelatin had anominal weight-average molecular weight of 100kDa. Chitosan(practicalgrade)waspurchasedfromSigma–AldrichCo.(UK)whichwas extracted from crab shells. It had a deacetylation percentagelargerthan85%,andaviscosity(Brookfield,1%solutionin1%aceticacid)higherthan200cP. dl -Lacticacid(85%solutioninwater)waspurchased from Lancaster Synthesis Ltd. (UK). All the materialswere used as received.  2.2. Sample preparation The stock gelatin solutions (1.5wt%) were prepared by adding3gofgelatininto197gof1wt%lacticacidsolution.Thesemixtures 0927-7765/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfb.2008.12.034  Y. Wang et al. / Colloids and Surfaces B: Biointerfaces 70 (2009) 254–258  255 werelefttoswellovernightandthendweltat50 ◦ Cfor1h.Thestockchitosansolutions(1.5wt%)werepreparedbyadding3gofchitosaninto 197g of 1wt% lactic acid solution and stirred overnight. Thecomposite chitosan/gelatin solutions were prepared by mixing theabove gelatin and chitosan stock solutions at different ratios andwere stirred for 2h at 50 ◦ C. The chitosan content in the compositesolution (chitosan%) was defined aschitosan% = Wc × 100Wc + Wg (1)where Wc and Wg are the weights of chitosan and gelatin stocksolutions, respectively. The samples for rheological measurementswere prepared in MilliQ water, while for SANS D 2 O was used toincreasethescatteringcontrastandminimizetheincoherentback-ground. When D 2 O was used, its mass ( M  D ) was converted to theequivalentH 2 Omass( M  H  )sothattheconcentrationofchitosanandgelatin equal to 1.5wt% in H 2 O M  H   = M  D  H   D (2)where   D  and   H   are the mass density of D 2 O (1.109gmL  − 1 ) andH 2 O (1.0gmL  − 1 ) respectively.The films were prepared by casting 30g solutions into PTFEmoulds, dried at 37 ◦ C and then neutralized by immersed in 1wt%NaOH solution for 45min. The samples were then rinsed withexcessive MilliQ water until the effluent pH 7 was reached. Theywere then dried at room temperature and soaked in MilliQ waterfor 12h before tensile measurements (swollen films).  2.3. Characterization The rheological properties of the chitosan/gelatin compositesolutions were measured at 25 ◦ C, 35 ◦ C, 45 ◦ C and 60 ◦ C by aBohlinCVORheometerwithacouettegeometry(C25)cup(27.5mmdiameter, bob 25mm diameter and height 37.5mm). Shear stresswas varied between 0.015Pa and 300Pa to give shear rates upto 1500s − 1 . The temperatures were kept stable with a circulatingwater bath. The samples were both measured with increasing anddecreasing shear rate.Thecompositechitosan/gelatinsolutionsinD 2 Oweremeasuredby SANS at chitosan contents (chitosan%) of 100%, 60%, 47%, 20%and 0. The SANS measurements were performed on D22 at the ILL,Grenoble,France.Allthesamplesweremeasuredattwogeometriesand the  Q   range was from 0.002Å − 1 to 0.2Å − 1 , in order to cover abroaderlengthscalefrompolymersegmentstopolymeraggregates(2  / Q  ).  Q   is the momentum transfer and is defined as Q   = 4   sin    2   (3)where    is the neutron wavelength (8Å in this study) and     is thescatteringangle.Thesamplesweremeasuredinrectangularquartscells with a path length of 2mm and all the measurements wereperformedat25 ◦ C.Overtwomillionneutroncountswerecollectedto get good statistics.Theelongationsoftheformedfilmsweremeasuredonatensilemachine (Hounsfield H5K-W) and the stretch speed was fixed at11.5mm/min. The films were cut into 4mm × 50mm stripes witha thickness of around 0.1mm. The results presented were meanvalues of six independent measurements on samples taken fromdifferent part of each film. Fig. 1.  Viscosity as a function of shear rate for the composite chitosan/gelatin solu-tions with different chitosan content at 25 ◦ C.   : 0%;   : 20%;  △ : 50%;  ▽ : 60%;  ♦ :100%. The solid lines are the fits to the Carreau Model (Eq. (4)). 3. Results and discussion  3.1. Rheological measurements Steady-staterheologicalmeasurementswereusedtocharacter-izethesecompositesolutions.Theexperimentswerecarriedoutatdifferent temperatures ranging from 25 ◦ C to 60 ◦ C. All the systemsshowed shear-thinning behaviour (only data from five representa-tivesamplesarepresentedforclarity),whichistypicalforpolymersolutionsoverthecriticaloverlapconcentration(Fig.1)[26,27].The samplesweremeasuredwithbothincreasinganddecreasingshearrates and the two datasets matched to each other reasonably well.Only both datasets for 100% chitosan are presented for clarity. Forother samples, only those with increasing shear rate are shown.The pure gelatin solution showed Newtonian behaviour over thewhole range of shear rate. Because of the low viscosity and the dif-ficulties in reaching a steady state at the low shear rate, results areonly presented from medium to high shear rate. In the high shearrate region, the increase in the viscosity might be a result of theflow instabilities (Taylor vortices) developed within the geometry[28]. These curves were fitted to Carreau Model [29] to obtain the zero-shear viscosity (  0 )  (   ) =  0 [1 + ( ˙ /  ˙   c  ) 2 ] n/ 2  (4)where  n  is an exponent describing the shear-thinning behaviourand ( ˙   c  ) is the critical shear rate. It can be seen that for all thesesamples the shear behaviour can be well described by this model.As expected, the zero-shear viscosity increases with the chitosancontent (Fig. 2).Viscosity decreases with temperature and typical datasets for60% chitosan solution are presented in Fig. 3. Similarly, the zero-shear viscosities can be obtained by fitting the data to Eq. (4). Thezero-shearviscositieshaveanArrheniustyperelationwithtemper-ature (Fig. 4)  0  =  A exp  − E  a RT    (5)where   0  is the zero-shear viscosity,  A  is a constant,  E  a  is theactivationenergy, R isthegasconstantand T  istheabsolutetemper-ature. The estimated values of activation energy,  E  a , are presentedin Table 1 and plotted in Fig. 5.  E  a  reflects the energy needed for  256  Y. Wang et al. / Colloids and Surfaces B: Biointerfaces 70 (2009) 254–258 Fig.2.  Zero-shearviscosityofthecompositechitosan/gelatinsolutionsasafunctionof the chitosan content at 25 ◦ C. Fig. 3.  Viscosity as a function of shear rate for the composite chitosan/gelatin solu-tions with a chitosan content of 60% at various temperatures.  : 25 ◦ C;  : 35 ◦ C; △ :45 ◦ C; ▽ : 60 ◦ C. The solid lines are the fits to the Carreau Model (Eq. (4)). Fig. 4.  Zero-shear viscosity as a function of temperature at different chitosan con-tents.   : 0%;   : 20%;  △ : 50%;  ▽ : 60%;  ♦ : 100%. The solid lines are the fits to theArrhenius type function (Eq. (5)).  Table 1 Parameters for the composite solutions and the films formed from them.Chitosan%100 60 50 47 20 0 E  a  (kJmol − 1 ) 35.3 37.4 38.2 – 29.7 2.6    (Å) 100.0 113.0 – 114.4 100.8 60.3 a  (Å) 647.0 583.5 – 595.4 638.4 775.2Elongation (%) 60.3 80.2 80.1 – 72.2 16.5 the movement of polymer units under shear which shows a clearmaximum at  ∼ 70% chitosan content (Fig. 5). It is known that chi-tosanandgelatincaninteractwitheachotherandformcomplexes[17],therefore,whenthetotalconcentrationofchitosanandgelatinwas fixed, we can expect that this interaction will be a functionof the product of the chitosan concentration (chitosan%) and thegelatin concentration (1-chitosan%) as is seen.  3.2. Small-angle neutron scattering measurements Thecompositechitosan/gelatinsolutionswithdifferentchitosancontents were measured by small-angle neutron scattering. It canbeseenthatthescatteringintensitydecaysvirtuallymonotonicallywith  Q   (Fig. 6), a behaviour more like a neutral flexible polymersolution rather than a polyelectrolyte solution [30–34]. This couldbe caused by the intra- or inter-molecular hydrogen bonds whichoffsettheelectrostaticinteractions.Ithasbeenshownthatthescat-teringfromasimilargelatininagoodsolventcanbefittedbythedeGennes model [35,36]. In this study, a much lower  Q   was availableand hence the contribution from any possible large scale inhomo-geneityalsoneedstobeconsidered;theSANSdatawerethenfittedwith the Lorentzian–Debye model (Fig. 6) [37,38]. I  ( Q  ) = I  a (0)(1 + a 2 Q  2 ) 2 + I    (0) / (1 +   2 Q  2 ) + I  inc   (6)where  I  ( Q  ) is the measured scattering intensity,  I  a (0) is the Debyeintensityatzero Q  , a istheDebyelengthwhichisthesizeofthelargeinhomogeneous structure,  I    (0) is the Lorentzian intensity at zero Q  ,     is the correlation length,  I  inc   is the  Q  -independent incoherentbackground.It can be seen that all the fits are acceptable over the wholeexperimental  Q   range. It is worth noting that the fit to the SANSdatafrom0%chitosanwaslesssatisfactoryata Q  ofaround0.01Å − 1 .One possible reason might be that the gelatin at this pH is chargedandmayshowsomepolyelectrolytebehaviourwhichisnotconsid-eredinthecurrentmodel.Asdiscussedabove,whenbothchitosan Fig.5.  E  a  andelongationasfunctionsofchitosancontent.  : E  a ;  :elongation.Thesolid lines are fits to a second order polynomial function.  Y. Wang et al. / Colloids and Surfaces B: Biointerfaces 70 (2009) 254–258  257 and gelatin were present, the hydrogen bond between them mightoffsetthepolyelectrolyteeffectandmakethepolymerchainsmoreneutral like.The correlation length (   ) and the size of the large scale inho-mogeneous structure ( a ) obtained from the fits are presented inTable 1 and plotted in Fig. 7. Similarly,     and  a  can be fitted to asecond order polynomial function and a peak at 62.4% and 61.4%of chitosan was found for each of them, respectively. It is inter-esting to see that both     and  a  peak at similar chitosan contents,thissuggeststhesetwostructuresatdifferentlengthscalesmaybedetermined by similar interactions. With 1% lactic acid by weight,both chitosan and gelatin are overall positively charged; therefore,the chance of them being associated through electrostatic attrac-tion would be low although not completely impossible (gelatinhas a broad range of p K  a  values) [39–42]. The main contributionfor the formation of complex should be inter-molecular hydrogenbond since both polymers have an abundance of amino, hydroxylorcarboxylicgroups.Unliketheelectrostaticattraction,thehydro-genbondrequiressomedegreeofstoichiometry;thisconsequentlyresults in the non-linear dependence of the complex structure onthechitosancontent.Aschitosanmoleculesbindtogelatinthroughhydrogen bonds, the charges brought in with chitosan will cause Fig. 6.  Scattering from the composite chitosan/gelatine solutions at 25 ◦ C.  : 100%chitosan;   : 60% chitosan;  △ : 47% chitosan;  ▽ : 20% chitosan;  ♦ : 100% gelatin. Thesolid lines are the fits to the Lorentzian–Debye model (Eq. (6)). Fig. 7.  The composition dependency of composite chitosan/gelatine solution struc-ture.  :Correlationlength,   ;  :largeinhomogeneitysize, a .Thesolidlinesarethefits to a second order polynomial function. the chitosan/gelatin complex to expand, and the local correlationlength starts to increase. After the maximum, the extra chitosanmolecules will cause the break-down of the complexes as electro-static repulsion exceeds hydrogen bond, and therefore collapsedpolymer coils are expected due to depletion effect (a decrease in    is then observed). Similarly, when the ratio of chitosan and gelatinmatches to the required amount, which is the minimum in thelargescalestructurelength( a ,squaresinFig.7),theoverallaffinitybetween chitosan and gelatin reaches the highest; other than thischitosan content, micro-phase separation may happen and conse-quently the large scale structure size increases. The coincidencebetween the maximum and minimum position of the correlationlength (   ) and the large scale structure size ( a ) is thus plausi-ble.  3.3. Film elongation The elongation of the films formed from the chitosan/gelatinsolutions were measured by the tensile tests in the swollen state.The reasons we chose the swollen states were: firstly, it was thepropertyoftheswollenfilmswhichisrelevantfortheirapplicationsonwounddressingsinbodyandsecondly,onlyinthesolvatedstatecan the electrostatic interactions be observed. It was found thatthe elongation had a peak at a chitosan content of around 69.0%by weight (the squares in Fig. 5). This value (69.0% by weight) isvery close to the one obtained for the activation energy (68.8%)from the rheological measurements; this is rather expected sincethey are both a reflection of how much the complex can endure adeformation. 4. Discussions Information from SANS, rheology and tensile tests all sug-gests that there is an optimal ratio of chitosan/gelatin to form thestrongest chitosan/gelatin complex. This has been found in a pre-vious study, where the interactions between these two polymerswere assumed to be via a polyelectrolyte complex [43]. However,inthisstudy,bothpolymerswereoverallpositivelycharged;hencethis type of interactions should not be the main contributor fortheformationofchitosan/gelatinecomplexbutthehydrogenbond.The rheological behaviour of a polymer solution is mainly deter-minedbyentanglementsandinteractionsbetweenpolymerchainsinthecontinuousmedia,therefore,thehydrogenbondingbetweengelatin and chitosan chains not only affects the structure of thechitosan/gelatin complex (    and  a  measured by SANS), but alsotherheologicalbehaviour( E  a  measuredfromrheologicalmeasure-ments) in solutions. The structure of chitosan/gelatin complex willaffect the micro structure of the films formed, so the interactionbetween chitosan and gelatin molecules can eventually affect themechanical properties of formed films. However, it is worth not-ing that there are two optimal chitosan ratios: the one is 68.8%in  E  a  and 69.0% in elongation; the other is 62.4% in     and 61.4%in  a . One reason for this difference might be the error during thesample preparation, but more likely, it could be a result of thechanges of the hydrogen bond between those molecules underapplied field during the measurements (shear in the rheologicalmeasurements and deformation in the tensile tests). Due to thespatial hindrance caused by the deformation, the hydrogen bondnumber may be reduced; therefore, more chitosan molecules areneeded to interact with gelatin molecules and the optimal chi-tosan content shifts to a higher value. This work has linked theinter-molecular interactions with the microstructure, the rheolog-ical behaviour of a solution and eventually the film performances,which will greatly assist the development of new film materi-als.  258  Y. Wang et al. / Colloids and Surfaces B: Biointerfaces 70 (2009) 254–258 5. Conclusions Thecompositechitosan/gelatinsolutionsandfilmsformedfromthese solutions were studied by rheological measurements, SANSand tensile tests. It was found that the complex formed betweenchitosan and gelatin was mainly through hydrogen bond but thesize of the structure was also affected by electrostatic repulsion.The local structure (correlation length) and the global structure(large inhomogeneous structure size) in the composite solutionswere found to be highly correlated to each other. It was also foundthattheinteractionsbetweenthesetwopolymersinsolutionwereclosely related to the mechanical properties of the formed films.This work will enable one to design films with desired mechan-ical properties through the combination of different polymers atoptimum weight ratios.  Acknowledgements We thank the ILL for providing neutron beam time. 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