Stimuli-Responsive Hydrogels Based on Polyglycerol...

Research ArticleStimuli-Responsive Hydrogels Based on PolyglycerolCrosslinked with Citric and Fatty Acids

Leidy C Solano-Delgado 1 Ceacutesar A Bravo-Sanabria 12

Carolina Ardila-Suaacuterez12 and Gustavo E Ramiacuterez-Caballero 12

1Grupo de Investigacion en Polımeros Escuela de Ingenierıa Quımica Universidad Industrial de Santander (UIS)Bucaramanga Colombia2Centro de Investigaciones en Catalisis Escuela de Ingenierıa Quımica Universidad Industrial de Santander (UIS)Bucaramanga Colombia

Correspondence should be addressed to Gustavo E Ramırez-Caballero gusramcauiseduco

Received 26 October 2017 Accepted 24 December 2017 Published 1 February 2018

Academic Editor Junshi Zhang

Copyright copy 2018 Leidy C Solano-Delgado et alThis is an open access article distributed under the Creative CommonsAttributionLicense which permits unrestricted use distribution and reproduction in anymedium provided the originalwork is properly cited

Polyglycerol-based hydrogels from biodegradable raw materials were synthesized by crosslinking reactions of polyglycerol withcitric and fatty acids Three hydrogels were studied varying molar ratios of crosslinking agent It was found that crosslinkamount type and size play a crucial role in swelling thermal mechanical and stimuli-responsive properties The hydrogelsabsorption capacity changed in response to temperature and pH external stimuli The hydrogel with the highest swelling capacityabsorbed more than 7 times its own weight at room temperature and pH 5 This material increased 14 times its own weight atpH 10 Creep-recovery tests were performed to study the effect of crosslinking agent on mechanical properties Deformationand percentage of recovery of synthesized hydrogels were obtained Formation of hydrogels was confirmed using FTIR andphysicochemical properties were analyzed by Scanning Electron Microscopy (SEM) Differential Scanning Calorimetric (DSC)and Dynamic Mechanical Analysis (DMA)This paper aims to give a contribution to biobased hydrogel knowledge from chemicalphysicochemical and mechanical point of view

1 Introduction

Hydrogels are polymeric crosslinked hydrophilic threendashdimensional networks that are not soluble in water but canabsorb large quantities of this molecule [1 2] Due to theirswelling ability hydrogels have been studied extensively fora variety of applications such as drug delivery [3ndash6] agri-cultural applications [7 8] removal of impurities in aqueoussolutions [9 10] biosensors [11] and spectrophotometricdetermination of drugs [12] In these applications swellingcapacity and thermal mechanical and stimuli-responsiveproperties are of most interest These properties are pre-scribed by the intrinsic properties of the main chain polymerand the crosslinking characteristics such as type amountand size of crosslinking molecules as well as environmentalconditions In this work synthesized hydrogels are based onrenewable materials the main chain polymer is polyglycerolobtained by glycerol polymerization [13] and citric and

fatty acids are the crosslinking molecules Polyglycerol has abiocompatible and flexible polyether backbone with a highnumber of hydrophilic functional groups which increasespolyglycerol versatility and enables the production of hydro-gels [14] Citric acid is a relatively small and multifunc-tional monomer with pendant functional groups that allowfuture ester bond-crosslink and hydrogen bonding [15 16]Fatty acids are relative long monomers with a carboxylicfunctional group that with chemical modification of double-double bond of unsaturated fatty acids allows the formationof crosslinked polymer structures [17] The present studyfocuses on the synthesis and characterization of novel poly-meric materials from biodegradable monomers that respondto pH and temperature stimuli The effect of type amountand size of crosslinking agents on thermal mechanical andstimuli-responsive properties was investigated

Previous studies have reported the synthesis of hy-drogels by crosslinking glycerol-derived polyglycerol with

HindawiInternational Journal of Polymer ScienceVolume 2018 Article ID 3267361 8 pageshttpsdoiorg10115520183267361

2 International Journal of Polymer Science

Table 1 Composition of fatty acids used as crosslinking agentCharacterization was made using a gas chromatography system(Agilent Technologies 6890 series) coupled to a FID detector usingan Agilent DB23 column and SUPELCO 38 FAMES as standards

Fatty acid Area percentage ()Palmitic 908Stearic 812Oleic 3214Linoleic 3631Linolenic 356Eicosenoic 406Others 671

poly(ethylene glycol) diglycidyl ether PEGDE [14] Theresulting hydrogels exhibited pH-dependent swelling behav-ior with a higher swelling capability at acidic pH valuecompared with swelling at neutral and basic pH values Sig-nificant research has been focused on materials that changetheir properties in response to external physical and chemicalstimuli such as pH electric field temperature and ionicstrength of the swelling agent due to acid or basic pendantfunctional groups present on the polymer backbone [18ndash21]Citric acid has been used as crosslinking agent for hydrogelsproduction for instance with poly(vinyl alcohol) [22] withvarying glycol unit (ethylene glycol diethylene glycol andtriethylene glycol) [23] and with cellulose [24] Fatty acidssuch as linoleic and oleic acid have been traditionally usedto improve mechanical properties and chemical resistance inpolymeric materials [25 26]

Formation of hydrogels was confirmed using FTIR Pen-dant acidic functional groups were identified in the polymernetwork which either accepts or releases protons as resultof changing external pH The calorimetric analysis was per-formed to study the effect of amount and size of crosslinkingagents on glass transition temperature Creep-recovery testswere conducted to elucidate structure-property relationshipin mechanical properties Hydrogels swelling behavior wasdetermined at different pH and temperatures Finally SEMmicrographs were taken to study morphological properties

2 Materials and Methods

21 Materials Glycerol (85) and Sulfuric acid (95) wereobtained from Merck Citric acid (99) is a commercialproduct of Suquin Ltda Bucaramanga Co The mixture offatty acids used in this studywas purchased from LaboratoriosLeon SA Bucaramanga Co Its composition is listed inTable 1

22 Experimental Procedure

221 Hydrogels Synthesis Hydrogels synthesis was carriedout in two steps using the same reaction system first steppolymerization reaction of glycerol to produce polyglyceroland second step crosslinking reaction of polyglycerol seeFigure 1 The reaction system consisted in a 50mL glass

Polymerization

Crosslinking

Branched polyglycerol

Fatty acids

Citric acid

OHOH

OH

OH

OH OH

OH

OH OH

OH

O

OOO

OO

O

O

O

O

OOO

O

OO

O O

O

OO

O

O

HO

HO

HO

HO

O

O

HO

HO

HOHO

HO

HOHOHO

HO

HO

HO

HO

HO

OOO

OHOH

HO

OH

OH

（2３4

（2

（2

C（3

C（3

Figure 1 Schematic representation of hydrogel synthesis First theglycerol polymerization and the subsequent addition of crosslinkingagents

reactor equipped with a nitrogen inlet catalyst feedingthermometer inlet and a distillation trap to continuouslyremove water from the reaction mixture The temperaturewas maintained at 160∘C using a temperature-controlledheating bath A vacuum pump was attached to the reactorthrough the condenser Condensation reactions were carriedout at pressure of 22 inHg 48wwof Sulfuric acidwas usedas a catalyst [13] The crosslinking agents were added to thereaction mass of polymerized glycerol just before reachingthe gel point without further addition of catalyst Thereaction proceeded until the hydrogel reached the gel point

Three different hydrogels were synthesized changingthe nature of crosslinking agent a hydrogel with citricacid as crosslinking agent a hydrogel with fatty acids ascrosslinking agent and a hydrogel with both citric and fattyacids as crosslinking agent Molar ratio between polyglycerolhydroxyl groups and crosslinking agents carboxyl groupsand molar composition of crosslinking agents of synthesizedhydrogels are reported in Table 2 After polymerization andcrosslinking process the hydrogelswerewashedwith distilledwater to remove catalyst and unreacted monomers

23 Characterization The synthesized hydrogels were testedto determinate their swelling behavior as a function of time indistilledwater at roomconditions Absorptionmeasurementswere also made at 35∘C 55∘C and 85∘C and pH 4 7 and10 to establish if the synthesized hydrogels have a responseto temperature and pH The absorption tests at different pH

International Journal of Polymer Science 3

Table 2 Samples compositions used for the hydrogels synthesis

Hydrogel

Molar ratios of OHgroups of polyglycerol(PG) COOH groups ofcrosslinking agents (CG)

Molar compositions ofCOOH groups of citricacid (CA) and fatty acids

(FFAs)PG-FFAs 1 005 100 FFAsPG-CA 1 045 100 CAPG-(CAFFAs) 1 05 90 CA-10 FFAs

were performed using Hanna Instruments buffer solutionsAll measurements were done in triplicate The hydrogelswater absorption was calculated using (1) [14]

119878 =119882119904 minus119882119889119882119889lowast 100 (1)

where 119882119889 is the dry hydrogel weight and 119882119904 is the swollenhydrogel weight

Fourier Transform Infrared Spectroscopy (FTIR) wasused to identify functional groups in synthesized hydro-gels The infrared spectra were obtained in transmittancemode in a Thermo Scientific spectrometer (Nicolet 1550FTIR) Glass transition temperatures were obtained usingDifferential ScanningCalorimetry (DSC) andmeasurementswere carried out on a DSC Discovery TA InstrumentsInc (USA) The samples were subjected to the followingthermal schedule first heating from minus80∘C to 180∘C at5∘Cmin to eliminate volatile substances and thermal historyof the materials second a cooling from 180∘C to minus90∘C at10∘Cmin third final heating from minus90∘C to 200∘C All scanswere performed under nitrogen purge gas of 50mLminScanning Electron Microscopy (SEM) measurements wereperformed to study themorphology of synthesized hydrogelsSamples were pretreated by being fully swollen in distilledwater then frozen in liquid nitrogen and lyophilized for72 h Freezendashdried hydrogels were consequently fractured formorphology visualizationThe uncoated environmental SEMimages were taken using a Quanta FEG 650 at accelera-tion voltages of 15 kV Creep-recovery tests of synthesizedhydrogels were tested using Dynamic Mechanical Analysis(DMA) and measurements were carried out on a Q800dynamic mechanical analyzer TA Instruments Inc (USA)The samples were swollen and tests were carried out in watermedium Treatments were compressed at 0007MPa during20min at three different temperatures after that the sampleswere recovered for 20min

3 Results and Discussion

31 FTIR Spectral Studies of Polymeric Hydrogels FourierTransform Infrared Spectroscopy (FTIR) analysis was con-ducted to synthesized hydrogels and compared with polyg-lycerol spectra see Figure 2 The hydrogels and polyglycerolspectra show hydroxyl group band from 3050 to 3600 cmminus1indicative of alcohol groups Hydrogels O-H stretch exhibitsa loss of intensity in comparison with polyglycerol as a conse-quence of crosslinking linkagesTheC-H stretching bandwas

Tran

smitt

ance

Wavenumber (cＧminus1)1000150020002500300035004000

OH CH

Polyglycerol

C=O

C=C

C-O

C-OC-OH

PG-(CA FFAs)

PG-CA

PG-FFAs

Figure 2 FTIR spectra of glycerol-derived polyglycerol andpolyglycerol-based hydrogels The main peaks associated with thestructures are highlighted

observed from 2800 to 3000 cmminus1 [27] Hydrogels with fattyacids as crosslinking agents treatment PG-(CA FFAs) andtreatment PG-FFAs show well-defined C-H stretching as aconsequence of hydrocarbon chains of fatty acids Absorptionin the range of 1700ndash1750 cmminus1 in hydrogels spectrum isrelated to C=O stretch of aliphatic esters produced by esteri-fication reactions between polyglycerol hydroxyl groups withcarboxylic groups present in citric acid and fatty acids In thespecific case of polyglycerol this absorption is due to acroleinformation [28] Absorption at 1650 cmminus1 is associated withC=C group of both unsaturated fatty acids and undesiredproducts such as acrolein

The spectra showed a peak at 1455 cmminus1 that correspondsto C-OH in-plane bending and CH2 bending [28] Thepeaks appearing at 1235ndash1290 cmminus1 in hydrogels spectrumare related to C-O vibration of unreacted carboxylic groupsThese pendant acidic functional groups will play a crucialrole in stimuli-responsive properties The C-O vibrationof unreacted carboxylic groups is highlighted in Figure 3It is observed that hydrogel PG-FFAs exhibits lower C-Ovibration intensity in comparison with hydrogels that havecitric acid as crosslinking agent This result is due to citricacid structure which has three carboxylic functional groupsand not all of them react in the crosslink reaction Finallyabsorption at 1000ndash1150 cmminus1 is related to C-O stretchingof the ether groups present in the polyglycerol backbone[14] and etherification reactions between polyglycerol andcrosslinking agents Polyglycerol hydroxyl groups can reactby etherification reactionswith citric acid hydroxyl group andwith fatty acids hydrated carbon-carbon double bonds

32 Thermal Properties Treatment PG-FFAs exhibits thelowest glass transition at 4076∘C (Figure 4) which is relatedto the lowest crosslink density and long chain size of fattyacids used as crosslinking agent Therefore less temperatureis needed to change the hydrogel state from glassy to rubbery


PG-(CA FFAs)PG-CAPG-FFAs


05

06

07

08

09

10

11

Tran

smitt

ance

()

Figure 3 Zoom of the FTIR spectra at 1235ndash1290 cmminus1 of the threesynthesized hydrogels The highlighted C-O vibration is related tothe unreacted carboxylic groups

On the other hand treatment PG-CA exhibits the highestglass transition at 5070∘C This result is due to highercrosslink density and citric acid short chain which gives lessflexibility to the material In the particular case of hydrogelPG-(CA FFAs) this material exhibits a glass transitiontemperature at 4287∘C which is between the two glasstransitions just described This result can be explained by thecombined features of each used crosslinking agent

33 Mechanical Properties The adequacy of hydrogels forspecific applications depends on their mechanical propertiesand response times [29] In this work mechanical prop-erties were evaluated by a creep-recovery test at differenttemperatures 35∘C 55∘C and 70∘C Tests were performedin water medium simulating a typical condition of hydrogelsin their applications Results are shown in Figure 5 Ingeneral all hydrogels have a small transition before a flatequilibrium region Hydrogels deformation and percentageof strain recovery (SR) at different temperatures dependstrongly on crosslink density size and type of crosslink agentPG-FFAs present the highest deformation at all temperaturesThis hydrogel has a slight variation of deformation at 35∘Cand 55∘C and a significant increase in deformation at 70∘Cdue to high molecular mobility at temperatures above glasstransition temperature Percentages of recovery of PG-FFAsare relatively low 3841 4145 and 2755 at 35∘C 55∘Cand 70∘C respectively High deformations and relatively lowpercentages of recovery of PG-FFAs hydrogel are due tolow crosslink density and long chains of fatty acids used toform the network High crosslink density and short chains ofcitric acid resulted in PG-CA hydrogels with relatively lowdeformation related to PG-FFA hydrogel PG-CA hydrogelpresented an increase in deformation and decrease in the

Temperature (∘C)200150100500minus50minus100

Cooling scan

Second heating scan

First heating scan

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)

(a)

Temperature (∘C)

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)

180160140120100806040200minus20minus40minus60minus80

5070∘C

4287∘C

4076∘C

PG-CA

PG-(CA FFAs)

PG-FFAs

(b)

Figure 4 (a) First and second heating scanDSC result for hydrogelssynthesized (b) Hydrogels glass transition temperatures found inthe second heating scan

percentage of recovery as temperature increases This resultmay be related to hydrogel chains mobility since glass tran-sition temperature is 50∘C Finally PG-(CA FFAs) hydrogelhas above 55∘C the lowest deformation and the highestpercentage of recovery of all hydrogels at this temperature

34 Swelling Properties The hydrogel with the highestswelling degree is PG-(CA FFAs) It absorbsmore than seventimes its ownweight see Figure 6This swelling behaviormaybe related to the contribution of both crosslinking agentsfatty acids long chains may contribute to large pore sizesand unreacted hydroxyl groups of citric acid in additionto unreacted polyglycerol hydroxyl groups increase wateraffinity Hydrogel PG-CA probably has smaller pore sizesfor holding water caused by citric acid short chains Finallyhydrogel PG-FFAS has only fatty acids that despite its long


Time (min)50403020100

Strain recovery(SR)

Plastic (irreversible)deformation

0

10

20

30

40

50

60

70

80St

rain

()

(a)

80

70

60

50

40

30

20

10

0

Stra

in (

)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

7190

4019

2871

S２ = 3841

S２ = 3987

S２ = 7217

T = 35∘C

(b)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

6966

4943

2714

S２ = 4145

S２ = 3059

S２ = 5095

T = 55∘C

0

10

20

30

40

50

60

70

80

Stra

in (

)

(c)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

8646

6263

2597

S２ = 2755

S２ = 2142

S２ = 4374

T = 70∘C

0

20

40

60

80

100

Stra

in (

)

(d)

Figure 5 Submersion-compression creep for treatments PG-FFAs PG-CA and PG-(CA FFAs) in DMA at 35∘C 55∘C and 70∘C

chain size its crosslinking degree is low holding limited watermolecules within its structure

341 Effect of pH on Swelling Behavior The swelling behaviorof hydrogels was studied after 60minutes of water absorptionat pH 4 5 7 and 10 From the results it was concluded thatthe hydrogels swelling behavior depends on external pH seeFigure 7The greater swelling capability of hydrogel PG-(CAFFAs) is observed when the pH value is 10 in which thehydrogel absorbs more than 137 times its own weight AtpH 4 and 7 the hydrogel also increases its swelling capabilityabsorbing 86 and 10 times their own weights respectively

As reported in other studies [20 30] an explanationfor these results is that at basic pH values the unreactedcarboxylic groups become ionized producing carboxylateions (RCOOminus) and H+ combines with OHminus from basicsolution forming H2O Negative charges of carboxylate ionscreate an electrostatic repulsion between the polymer chainsforcing the hydrogel to uncoil increasing free volumes in thehydrogel network

In the specific case of treatment PG-FFAs its swellingdegree at high pH is lower in comparison with the othertwo treatments because this material presents less unreactedcarboxylic groups see Figure 3 A different trend result was


PG-FFAsPG-CAPG-(CA FFAs)

5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()

Figure 6 Swelling behavior of treatments PG-FFAs (e) PG-CA(998771) and PG-(CA FFAs) (◼) Measurements were taken at 20∘C andpH 5 using (1)


4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()

Figure 7 pH-dependent swelling behavior of treatments PG-FFAs(e) PG-CA (998771) andPG-(CA FFAs) (◼)measured at 25∘C Swellingpercentage was calculated using (1)

obtained at pH 4 where hydrogels showed higher swellingbehavior than that at pH 5 this can be related to the attractionof H+ ions to hydroxyl and ether groups as reported before[14] Swelling results at different pH correlate with unreactedcarboxylic groups reported previously in FTIR results seeFigure 3 hydrogels with more free unreacted carboxylicgroups have a greater response to changes in pH

342 Effect of Temperature on Swelling Behavior Hydrogelsswelling behavior was studied after 60 minutes of waterabsorption at 20 35 55 and 85∘C (Figure 8) Results show thatat 35∘C the three hydrogels present the lowest swelling degreeAt this temperature all hydrogels are below glass transitiontemperature as shown in Figure 4 At temperatures above55∘C all hydrogels are above glass transition temperature Asa result hydrogels chain mobility increases water absorptionThe greater swelling capability is observed at 85∘C wherethe PG-(CA FFAs) hydrogel absorbs water 85 times its ownweight


20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()

Figure 8 The temperature-dependent swelling behavior of treat-ments PG-FFAs (e) PG-CA (998771) and PG-(CA FFAs) (◼) Swellingpercentage was calculated using (1)

35 Morphological Properties Morphology of hydrogel PG-(CA FFAs) is shown in Figure 9 It was found that the hydro-gel exhibits an uneven rough heterogeneous and slightlyporous structure The observed hydrogel pore heterogeneity(pore diameters from 2 to 62120583m) could be related to therandomness of crosslinking reactions between polyglyceroland citric-fatty acids which have different chain lengths andfunctional groups Furthermore the porous structure hasinterconnected pores forming open channels for capillaryabsorption of water see Figure 9(b)

This material looks like a sponge and its swelling processfollows the same principle keeping the water within itsstructure due to its free volumes This hydrogel may beconsidered as macroporous material according to IUPAC[31]

4 Conclusions

The effect of type amount and size of crosslinkingmoleculessuch as citric and fatty acids on thermalmechanical swellingand stimuli-responsive polyglycerol-based hydrogel proper-ties was studied The small citric acid molecule used ascrosslinking agent produced high crosslink density whichmanifested itself in higher glass transition temperature rela-tively low hydrogel deformation and higher amount of unre-acted carboxylic functional groups that propitiated stimuli-responsive properties On the other hand large fatty acidsmolecules used as crosslinking agent produced low crosslinkdensity which manifested itself in lower glass transition tem-perature higher hydrogen deformation and less unreactedcarboxylic functional groups It was concluded that it is possi-ble to tune hydrogel properties by the judicial combination ofcitric and fatty acids crosslinking agents The resulted hydro-gel combining both crosslink agents has an intermediate glasstransition temperature relative short deformation with highrecovery and unreacted carboxylic functional groups thatimproved stimuli-responsive properties in comparison withhydrogels with uncombined crosslinking agents


(a)

(a)

(b)

(b)

Figure 9 SEM micrographs of the hydrogel PG-(CA FFAs)

Conflicts of Interest

The authors declare that they do not have any conflicts ofinterest

Acknowledgments

This work was supported by Vicerrectorıa de Investigaciony Extension at Universidad Industrial de Santander (UIS)and funded by the agreement of cooperation (Code 9460)betweenUIS Corasfaltos and Colciencias Scanning ElectronMicroscopy (SEM)measurementswere supported by Labora-tory ofMicroscopy of Parque Tecnologico Guatiguara (PTG) -UISThe authors thankDr Alvaro Ramırez for reviewing thispaper

References

[1] A Słoniewska andB Pałys ldquoSupramolecular polyaniline hydro-gel as a support for ureaserdquo Electrochimica Acta vol 126 pp90ndash97 2014

[2] E S Dragan ldquoDesign and applications of interpenetratingpolymer network hydrogels A reviewrdquo Chemical EngineeringJournal vol 243 pp 572ndash590 2014

[3] L Bedouet F Pascale L Moine et al ldquoIntra-articular fateof degradable poly(ethyleneglycol)-hydrogel microspheres ascarriers for sustained drug deliveryrdquo International Journal ofPharmaceutics vol 456 no 2 pp 536ndash544 2013

[4] D S Jones G P Andrews D L Caldwell C Lorimer SP Gorman and C P McCoy ldquoNovel semi-interpenetratinghydrogel networks with enhanced mechanical properties andthermoresponsive engineered drug delivery designed as bioac-tive endotracheal tube biomaterialsrdquo European Journal of Phar-maceutics and Biopharmaceutics vol 82 no 3 pp 563ndash5712012

[5] Y Liang and K L Kiick ldquoHeparin-functionalized polymericbiomaterials in tissue engineering and drug delivery applica-tionsrdquo Acta Biomaterialia vol 10 no 4 pp 1588ndash1600 2014

[6] N K Singh and D S Lee ldquoIn situ gelling pH- and temperature-sensitive biodegradable block copolymer hydrogels for drug

deliveryrdquo Journal of Controlled Release vol 193 pp 214ndash2272014

[7] A I RaafatM Eid andM B El-Arnaouty ldquoRadiation synthesisof superabsorbent CMC based hydrogels for agriculture appli-cationsrdquo Nuclear Instruments and Methods in Physics ResearchSection B Beam Interactions withMaterials and Atoms vol 283pp 71ndash76 2012

[8] C Demitri F Scalera M Madaghiele A Sannino and AMaffezzoli ldquoPotential of cellulose-based superabsorbent hydro-gels as water reservoir in agriculturerdquo International Journal ofPolymer Science vol 2013 Article ID 435073 6 pages 2013

[9] X Ma Y Li WWang Q Ji and Y Xia ldquoTemperature-sensitivepoly(N-isopropylacrylamide)graphene oxide nanocompositehydrogels by in situ polymerization with improved swellingcapability andmechanical behaviorrdquo European Polymer Journalvol 49 no 2 pp 389ndash396 2013

[10] S Saber-Samandari S Saber-Samandari and M GazildquoCellulose-graft-polyacrylamidehydroxyapatite compositehydrogel with possible application in removal of Cu (II) ionsrdquoReactive and Functional Polymers vol 73 no 11 pp 1523ndash15302013

[11] T Endo R Ikeda Y Yanagida and T Hatsuzawa ldquoStimuli-responsive hydrogel-silver nanoparticles composite for devel-opment of localized surface plasmon resonance-based opticalbiosensorrdquo Analytica Chimica Acta vol 611 no 2 pp 205ndash2112008

[12] M Bahram F Hoseinzadeh K Farhadi M Saadat P Najafi-Moghaddam and A Afkhami ldquoSynthesis of gold nanoparticlesusing pH-sensitive hydrogel and its application for colorimet-ric determination of acetaminophen ascorbic acid and folicacidrdquo Colloids and Surfaces A Physicochemical and EngineeringAspects vol 441 pp 517ndash524 2014

[13] C Ardila-Suarez D Rojas-Avellaneda and G E Ramirez-Caballero ldquoEffect of Temperature and Catalyst Concentrationon Polyglycerol during Synthesisrdquo International Journal ofPolymer Science vol 2015 Article ID 910249 8 pages 2015

[14] S Salehpour C J Zuliani and M A Dube ldquoSynthesis ofnovel stimuli-responsive polyglycerol-based hydrogelsrdquo Euro-pean Journal of Lipid Science and Technology vol 114 no 1 pp92ndash99 2012

[15] R T Tran Y Zhang D Gyawali and J Yang ldquoRecent develop-ments on citric acid derived biodegradable elastomersrdquo Recent


Patents on Biomedical Engineering vol 2 no 3 pp 216ndash2272009

[16] D Gyawali P Nair Y Zhang et al ldquoCitric acid-derived insitu crosslinkable biodegradable polymers for cell deliveryrdquoBiomaterials vol 31 no 34 pp 9092ndash9105 2010

[17] L Fertier H Koleilat M Stemmelen et al ldquoThe use ofrenewable feedstock in UV-curable materials-A new age forpolymers and green chemistryrdquo Progress in Polymer Science vol38 no 6 pp 932ndash962 2013

[18] M Hamidi A Azadi and P Rafiei ldquoHydrogel nanoparticles indrug deliveryrdquo Advanced Drug Delivery Reviews vol 60 no 15pp 1638ndash1649 2008

[19] H Priya James R John A Alex and K Anoop ldquoSmartpolymers for the controlled delivery of drugs ndash a conciseoverviewrdquo Acta Pharmaceutica Sinica B (APSB) vol 4 no 2pp 120ndash127 2014

[20] K S De N R Aluru B Johnson W C Crone D J Beebe andJ Moore ldquoEquilibrium swelling and kinetics of pH-responsivehydrogels models experiments and simulationsrdquo Journal ofMicroelectromechanical Systems vol 11 no 5 pp 544ndash555 2002

[21] T Bartil M Bounekhel C Cedric and R Jeerome ldquoSwellingbehavior and release properties of pH-sensitive hydrogels basedon methacrylic derivativesrdquo Acta Pharmaceutica vol 57 no 3pp 301ndash314 2007

[22] Z Ajji ldquoPreparation of poly(vinyl alcohol) hydrogels containingcitric or succinic acid using gamma radiationrdquoRadiation Physicsand Chemistry vol 74 no 1 pp 36ndash41 2005

[23] D S Franklin and S Guhanathan ldquoSynthesis and characteriza-tion of citric acid-based pH-sensitive biopolymeric hydrogelsrdquoPolymer Bulletin vol 71 no 1 pp 93ndash110 2014

[24] C Demitri R Del Sole F Scalera et al ldquoNovel superabsorbentcellulose-based hydrogels crosslinked with citric acidrdquo Journalof Applied Polymer Science vol 110 no 4 pp 2453ndash2460 2008

[25] G Lligadas J C Ronda M Galia and V Cadiz ldquoOleic andundecylenic acids as renewable feedstocks in the synthesis ofpolyols and polyurethanesrdquo Polymer vol 2 no 4 pp 440ndash4532010

[26] M Moreno M Goikoetxea and M J Barandiaran ldquoBiobased-waterborne homopolymers from oleic acid derivativesrdquo Journalof Polymer Science Part A Polymer Chemistry vol 50 no 22pp 4628ndash4637 2012

[27] H S Mansur R L Orefice and A A P Mansur ldquoCharacteriza-tion of poly(vinyl alcohol)poly(ethylene glycol) hydrogels andPVA-derived hybrids by small-angle X-ray scattering and FTIRspectroscopyrdquo Polymer Journal vol 45 no 21 pp 7193ndash72022004

[28] S Salehpour Synthesis of stimuli-responsive hydrogels fromglycerol University of Ottawa 2012

[29] S Abdurrahmanoglu and O Okay ldquoRheological behavior ofpolymer-clay nanocomposite hydrogels Effect of nanoscaleinteractionsrdquo Journal of Applied Polymer Science vol 116 no 4pp 2328ndash2335 2010

[30] J Liu Q Li Y Su Q Yue and B Gao ldquoCharacterization andswelling-deswelling properties of wheat straw cellulose basedsemi-IPNs hydrogelrdquo Carbohydrate Polymers vol 107 no 1 pp232ndash240 2014

[31] M Thommes K Kaneko A V Neimark et al ldquoPhysisorptionof gases with special reference to the evaluation of surface areaand pore size distribution (IUPACTechnical Report)rdquo Pure andApplied Chemistry vol 87 no 9-10 pp 1051ndash1069 2015

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Table 1 Composition of fatty acids used as crosslinking agentCharacterization was made using a gas chromatography system(Agilent Technologies 6890 series) coupled to a FID detector usingan Agilent DB23 column and SUPELCO 38 FAMES as standards

Fatty acid Area percentage ()Palmitic 908Stearic 812Oleic 3214Linoleic 3631Linolenic 356Eicosenoic 406Others 671

poly(ethylene glycol) diglycidyl ether PEGDE [14] Theresulting hydrogels exhibited pH-dependent swelling behav-ior with a higher swelling capability at acidic pH valuecompared with swelling at neutral and basic pH values Sig-nificant research has been focused on materials that changetheir properties in response to external physical and chemicalstimuli such as pH electric field temperature and ionicstrength of the swelling agent due to acid or basic pendantfunctional groups present on the polymer backbone [18ndash21]Citric acid has been used as crosslinking agent for hydrogelsproduction for instance with poly(vinyl alcohol) [22] withvarying glycol unit (ethylene glycol diethylene glycol andtriethylene glycol) [23] and with cellulose [24] Fatty acidssuch as linoleic and oleic acid have been traditionally usedto improve mechanical properties and chemical resistance inpolymeric materials [25 26]

Formation of hydrogels was confirmed using FTIR Pen-dant acidic functional groups were identified in the polymernetwork which either accepts or releases protons as resultof changing external pH The calorimetric analysis was per-formed to study the effect of amount and size of crosslinkingagents on glass transition temperature Creep-recovery testswere conducted to elucidate structure-property relationshipin mechanical properties Hydrogels swelling behavior wasdetermined at different pH and temperatures Finally SEMmicrographs were taken to study morphological properties

2 Materials and Methods

21 Materials Glycerol (85) and Sulfuric acid (95) wereobtained from Merck Citric acid (99) is a commercialproduct of Suquin Ltda Bucaramanga Co The mixture offatty acids used in this studywas purchased from LaboratoriosLeon SA Bucaramanga Co Its composition is listed inTable 1

22 Experimental Procedure

221 Hydrogels Synthesis Hydrogels synthesis was carriedout in two steps using the same reaction system first steppolymerization reaction of glycerol to produce polyglyceroland second step crosslinking reaction of polyglycerol seeFigure 1 The reaction system consisted in a 50mL glass

Polymerization

Crosslinking

Branched polyglycerol

Fatty acids

Citric acid

OHOH

OH

OH

OH OH

OH

OH OH

OH

O

OOO

OO

O

O

O

O

OOO

O

OO

O O

O

OO

O

O

HO

HO

HO

HO

O

O

HO

HO

HOHO

HO

HOHOHO

HO

HO

HO

HO

HO

OOO

OHOH

HO

OH

OH

（2３4

（2

（2

C（3

C（3

Figure 1 Schematic representation of hydrogel synthesis First theglycerol polymerization and the subsequent addition of crosslinkingagents

reactor equipped with a nitrogen inlet catalyst feedingthermometer inlet and a distillation trap to continuouslyremove water from the reaction mixture The temperaturewas maintained at 160∘C using a temperature-controlledheating bath A vacuum pump was attached to the reactorthrough the condenser Condensation reactions were carriedout at pressure of 22 inHg 48wwof Sulfuric acidwas usedas a catalyst [13] The crosslinking agents were added to thereaction mass of polymerized glycerol just before reachingthe gel point without further addition of catalyst Thereaction proceeded until the hydrogel reached the gel point

Three different hydrogels were synthesized changingthe nature of crosslinking agent a hydrogel with citricacid as crosslinking agent a hydrogel with fatty acids ascrosslinking agent and a hydrogel with both citric and fattyacids as crosslinking agent Molar ratio between polyglycerolhydroxyl groups and crosslinking agents carboxyl groupsand molar composition of crosslinking agents of synthesizedhydrogels are reported in Table 2 After polymerization andcrosslinking process the hydrogelswerewashedwith distilledwater to remove catalyst and unreacted monomers

23 Characterization The synthesized hydrogels were testedto determinate their swelling behavior as a function of time indistilledwater at roomconditions Absorptionmeasurementswere also made at 35∘C 55∘C and 85∘C and pH 4 7 and10 to establish if the synthesized hydrogels have a responseto temperature and pH The absorption tests at different pH



Hydrogel





119878 =119882119904 minus119882119889119882119889lowast 100 (1)





Tran

smitt

ance


OH CH

Polyglycerol

C=O

C=C

C-O

C-OC-OH

PG-(CA FFAs)

PG-CA

PG-FFAs








05

06

07

08

09

10

11

Tran

smitt

ance

()





Cooling scan

Second heating scan

First heating scan

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)

(a)

Temperature (∘C)

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)


5070∘C

4287∘C

4076∘C

PG-CA

PG-(CA FFAs)

PG-FFAs

(b)





Time (min)50403020100

Strain recovery(SR)


0

10

20

30

40

50

60

70

80St

rain

()

(a)

80

70

60

50

40

30

20

10

0

Stra

in (

)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

7190

4019

2871

S２ = 3841

S２ = 3987

S２ = 7217

T = 35∘C

(b)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

6966

4943

2714

S２ = 4145

S２ = 3059

S２ = 5095

T = 55∘C

0

10

20

30

40

50

60

70

80

Stra

in (

)

(c)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

8646

6263

2597

S２ = 2755

S２ = 2142

S２ = 4374

T = 70∘C

0

20

40

60

80

100

Stra

in (

)

(d)








5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()



4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()





20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()




4 Conclusions



(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






Hydrogel





119878 =119882119904 minus119882119889119882119889lowast 100 (1)





Tran

smitt

ance


OH CH

Polyglycerol

C=O

C=C

C-O

C-OC-OH

PG-(CA FFAs)

PG-CA

PG-FFAs








05

06

07

08

09

10

11

Tran

smitt

ance

()





Cooling scan

Second heating scan

First heating scan

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)

(a)

Temperature (∘C)

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)


5070∘C

4287∘C

4076∘C

PG-CA

PG-(CA FFAs)

PG-FFAs

(b)





Time (min)50403020100

Strain recovery(SR)


0

10

20

30

40

50

60

70

80St

rain

()

(a)

80

70

60

50

40

30

20

10

0

Stra

in (

)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

7190

4019

2871

S２ = 3841

S２ = 3987

S２ = 7217

T = 35∘C

(b)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

6966

4943

2714

S２ = 4145

S２ = 3059

S２ = 5095

T = 55∘C

0

10

20

30

40

50

60

70

80

Stra

in (

)

(c)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

8646

6263

2597

S２ = 2755

S２ = 2142

S２ = 4374

T = 70∘C

0

20

40

60

80

100

Stra

in (

)

(d)








5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()



4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()





20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()




4 Conclusions



(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







05

06

07

08

09

10

11

Tran

smitt

ance

()





Cooling scan

Second heating scan

First heating scan

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)

(a)

Temperature (∘C)

minus04

minus03

minus02

minus01

00

01

02

03

Hea

t flow

nor

mal

ized

(Wg

)


5070∘C

4287∘C

4076∘C

PG-CA

PG-(CA FFAs)

PG-FFAs

(b)





Time (min)50403020100

Strain recovery(SR)


0

10

20

30

40

50

60

70

80St

rain

()

(a)

80

70

60

50

40

30

20

10

0

Stra

in (

)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

7190

4019

2871

S２ = 3841

S２ = 3987

S２ = 7217

T = 35∘C

(b)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

6966

4943

2714

S２ = 4145

S２ = 3059

S２ = 5095

T = 55∘C

0

10

20

30

40

50

60

70

80

Stra

in (

)

(c)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

8646

6263

2597

S２ = 2755

S２ = 2142

S２ = 4374

T = 70∘C

0

20

40

60

80

100

Stra

in (

)

(d)








5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()



4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()





20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()




4 Conclusions



(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





Time (min)50403020100

Strain recovery(SR)


0

10

20

30

40

50

60

70

80St

rain

()

(a)

80

70

60

50

40

30

20

10

0

Stra

in (

)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

7190

4019

2871

S２ = 3841

S２ = 3987

S２ = 7217

T = 35∘C

(b)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

6966

4943

2714

S２ = 4145

S２ = 3059

S２ = 5095

T = 55∘C

0

10

20

30

40

50

60

70

80

Stra

in (

)

(c)

Time (min)50403020100

PG-CAPG-(CA FFAs)

PG-FFAs

8646

6263

2597

S２ = 2755

S２ = 2142

S２ = 4374

T = 70∘C

0

20

40

60

80

100

Stra

in (

)

(d)








5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()



4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()





20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()




4 Conclusions



(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






5 10 15 20 25 30 35 40 45 50 550Time (min)

0

150

300

450

600

750

900

Swel

ling

()



4 5 6 7 8 9 10 113pH

0200400600800

1000120014001600

Swel

ling

()





20 25 30 35 40 45 50 55 60 65 70 75 80 85 9015Temperature (∘C)

0

200

400

600

800

1000

Swel

ling

()




4 Conclusions



(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





(a)

(a)

(b)

(b)




Acknowledgments


References





































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls
























Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




Stimuli-Responsive Hydrogels Based on Polyglycerol...

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Transcript of Stimuli-Responsive Hydrogels Based on Polyglycerol...