Syntheses and characterization of physically crosslinked hydrogels from dithiocarbamate-derived polyurethane macroiniferter

Syntheses and Characterization of Physically CrosslinkedHydrogels from Dithiocarbamate-Derived PolyurethaneMacroiniferter

ALPESH PATEL,1 KIBRET MEQUANINT1,2

1Department of Chemical and Biochemical Engineering, The University of Western Ontario, London,Ontario, N6A 5B9, Canada

2Graduate Program in Biomedical Engineering, The University of Western Ontario, London, Ontario,N6A 5B9, Canada

Received 8 April 2008; accepted 13 June 2008DOI: 10.1002/pola.22937Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A dithiocarbamate (DC)-based polyurethane macroiniferter (PUMI) wassynthesized and used to prepare physically crosslinked polyurethane-block-poly(acrylamide) (PU-b-PAAm) and polyurethane-block-poly(vinyl pyrrolidone) (PU-b-PVP) hydrogels. The success of the reactions has been confirmed by FTIR, 1H-NMR,and 13C-NMR Spectroscopy analyses. The number average molecular weight of theblock copolymers increased linearly with conversion and copolymerization time andthus followed a ‘‘living’’ radical mechanism. The water transport behavior of these poly-urethane-based hydrogels such as water uptake rate, equilibrium water content(EWC), transport number (n), characteristic diffusion rate constant (K), diffusioncoefficient (D), and pH effect on EWC has been investigated. The results revealedthat PU-b-PAAm hydrogels followed Fickian diffusion suggesting diffusion controlledswelling kinetics, whereas the PU-b-PVP hydrogels followed non-Fickian diffusionindicating that both diffusion and structural relaxation controlled the water trans-port. The PU-b-PAAm hydrogels showed higher swelling at both low and high pHthan at a neutral pH. This is attributed to protonation of the tertiary amines ofN,N0-diethyl-N,N0-bis(2-hydroxyethyl) thiuram disulfide (DHTD) at low pH and basehydrolysis of amide segments at high pH. In the thermogravimetric analysis; PUMI,PU-b-PVP and PU-b-PAAm have degraded in three distinct stages related to CS2 evo-lution, hard segment degradation, and soft segment degradation. VVC 2008 Wiley Periodi-

cals, Inc. J Polym Sci Part A: Polym Chem 46: 6272–6284, 2008

Keywords: dithiocarbamate; hydrogels; macroiniferter; physical crosslinking;polyurethanes; swelling

INTRODUCTION

Hydrogels are hydrophilic and crosslinked polymernetworks that can absorb plenty of water while

maintaining their dimensional stability. They arecomparable to many tissues because of their simi-larity in softness, elasticity, and slippery nature atthe interface.1 Because of these properties, hydro-gels are well suited as low-duty biomaterials forapplications such as drug delivery, cells encapsula-tion, soft tissue substitution, coatings for biomedi-cal devices, scaffolds for tissue engineering, andhemodialysis membranes.2–5 Because hydrogels

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 6272–6284 (2008)VVC 2008 Wiley Periodicals, Inc.

Correspondence to: K. Mequanint (E-mail: [email protected])

6272

have inherently poor mechanical properties,most studies in the past were conducted onthose which are chemically crosslinked. Chemi-cally crosslinked hydrogels, however, cannot beprocessed in organic solvents. Unreacted cross-linking agents and monomers that are trappedbecause of the high crosslink density couldleach out of the medical device, causing toxicityto tissues and cells. Because many hydrogelsfor biomedical applications are designed todeliver therapeutic agents, loading of biologi-cally active molecules before crosslinking haslimitations. This is because the crosslinkingprocess (thermally or photochemically) couldpotentially alter the biological activity.6 More-over, redissolving the polymer after the net-work is formed, for reshaping, is another issuerelated to chemically crosslinked hydrogels. Inthis regard, physically crosslinked hydrogelsoffer a clear advantage. The presence of crys-talline domains and hydrophobic/hydrophilicinteractions in physically crosslinked hydrogelsmay provide higher mechanical strength whileovercoming problems associated with chemi-cally crosslinked hydrogels.7

To synthesize physically crosslinked hydrogelswith well-organized crosslinked structures, con-trolled radical polymerization methods such asatom transfer radical polymerization,8,9 reversibleaddition-fragmentation chain transfer polymer-ization,10,11 and iniferter12,13 could be advanta-geous. The use of a macroiniferter technique tosynthesize polyurethane hydrogels is described inthis manuscript. This method is beneficialbecause a high purity of monomers is not critical.The term iniferter (initiator, transfer agent, andterminator), coined by Otsu and Yoshida, repre-sents a group of molecules that dissociate ther-mally or photochemically, initiate and propagatethe insertion of monomers, followed by primaryreversible radical terminations, and/or chaintransfers.14 This mechanism provides a means tosynthesize high–molecular-weight polymers withrelatively narrow molecular weight distribution.15

The term macroiniferter represents a polymericiniferter. DC-based iniferters have been studiedfor both thermally and photochemically inducedpolymerizations to synthesize block, star, graft,and/or functional polymers.16,17 For example, thethermal polymerization of methyl methacrylate(MMA) using DHTD showed that the reactionproceeded through ‘‘living’’ radical polymerizationand maintained its end-capped diol functional-ity.18 DHTD has also been studied as a photoi-

niferter to prepare polystyrene (PS).19 WhenDHTD is used as a photoiniferter, wave lengthsbetween 254 and 366 nm decompose the DC groupto generate radicals which initiate and propagatethe polymerization. These radicals can also bereversibly terminated to follow a controlled radi-cal polymerization process.20 In only one study,DHTD was incorporated into poly(ethylene glycol)(PEG) segments and used as a macroiniferter toprepare PEG-b-PMMA copolymers under irradia-tion with UV light. A linear increase in molecularweight of the block copolymers with reaction timewas observed indicating that the DC based macro-iniferter also followed the ‘‘living’’ radical poly-merization.21

Because of their excellent mechanical proper-ties, polyurethanes are attractive for many practi-cal applications.22,23 Previously, we reportedphysically crosslinked hydrogels using a tetra-phenyl ethane diol (TPED)-based polyurethanemacroiniferters for biomedical applications.24,25

These hydrogels were soluble in many organic sol-vents but only swelled in water. The mechanicalproperties of these hydrogels were also signifi-cantly higher than conventional hydrogels, andthe biological properties such as low proteinadsorption showed their potential biomedicalapplications. In this manuscript, we report anovel dithiocarbamate-based polyurethane macro-iniferter that was used to synthesize physicallycrosslinked polyurethane-based hydrogels. Thepolyurethane hydrogels were structurally andthermally characterized and examined for thekinetics of water transport.

EXPERIMENTAL

Materials

All chemicals were purchased from Sigma Aldrich(Milwaukee, WI). Poly(tetramethylene oxide) hav-ing molecular weight of 1000 g/mol (PTMO1000)was dried at 90 8C under a reduced pressure of200 mmHg. 4,40-Diphenylmethane diisocyanate(MDI) was melted and filtered to remove impur-ities. Acrylamide (AAm) was recrystallized twicefrom chloroform whereas N-vinyl pyrrolidone(NVP) was passed through an alumina column toremove the inhibitors. Methyl ethyl ketone(MEK) and dimethyl formamide (DMF) were dis-tilled at reduced pressure and the middle portionswere used. All other chemicals were used asreceived.

SYNTHESES OF PHYSICALLY CROSSLINKED HYDROGELS 6273

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Synthesis of N,N0-Diethyl-N,N0-bis(2-hydroxyethyl)thiuram Disulfide (DHTD)

2-Ethylaminoethanol (3.56 g, 0.04 mol) and trie-thylamine (8 mL) were dissolved in chloroform(40 mL) and the mixture was cooled below 10 8C.To this mixture, carbon disulfide (3.05 g, 0.04 mol)was added dropwise. After 3 h, iodine dissolved inchloroform was added dropwise, until the lightviolet color of iodine persisted. The product waswashed repeatedly with ice water to remove theaminohydroiodine. The mixture was dried overMgSO4 overnight, filtered, and the solvent wasevaporated under vacuum at ambient tempera-ture. The yellow viscous product was stored indark at 4 8C (Yield: 85%; Purity: 95%) and wascharacterized by FTIR, 1H-NMR, and 13C-NMRSpectroscopy (Scheme 1).

Synthesis of Segmented PolyurethaneMacroiniferter (PUMI)

PTMO1000 (10.00 g, 0.01 mol) and MDI (5.00 g,0.02 mol) were charged in a 500-mL glass reactorequipped with a heating element, a magnetic stir-rer, a condenser, and a nitrogen inlet. The reac-tion was carried out at 65 8C for 2.5 h. The reac-tion temperature was then reduced to 22 8C andDHTD (3.28 g, 0.01 mol) together with dibutyltindilaurate (DBTDL) (2% based on the isocyanatecontent) were added into the reaction mixture.MEK (80 mL) was added to reduce the viscosity ofthe reaction mixture. Samples were frequentlytaken for FTIR analyses and the chain extensionreaction was allowed to proceed until the peakrelated to the isocyanate (2265 cm�1) disappearedfrom the FTIR spectrum. The PUMI was precipi-tated using 10-fold excess cold methanol and driedat 30 8C in a vacuum oven. The PUMI was char-acterized using FTIR and 1H-NMR Spectroscopy.

Synthesis of Polyurethane-block-Poly(Acrylamide)Copolymer (PU-b-PAAm)

PUMI (1.50 g) and AAm (5.33 g, 0.075 mol) weredissolved into DMF (50 mL) and the mixture waspoured into a glass vial. The solution was purged

with nitrogen for 10 min. The glass vial was thenirradiated with UV light (model B100AP; UVP,Upland, CA) at 365 nm and a distance of 20 cm.After a designated reaction time, the glass vialwas quenched with an ice-salt mixture and thePU-b-PAAm was precipitated using 10-fold excesscold diethyl ether. The product was dried at 30 8Cin a vacuum oven. The residual monomer andhomo-PAAm have been extensively extracted indeionized (DI) water. Similar procedure was fol-lowed for different reaction times.

Synthesis of Polyurethane-block-Poly(Vinylpyrrolidone) Copolymer (PU-b-PVP)

A similar method, described for PU-b-PAAm, wasused to synthesize PU-b-PVP. But in this case, thePU-b-PVP was precipitated using 10-fold excesscold methanol. FTIR and 1H-NMR Spectroscopywere used to characterize the block copolymers.The detailed schematic diagram to synthesizePUMI, PU-b-PAAm, and PU-b-PVP is shown inScheme 2.

The sample nomenclature is based on thecopolymerization reaction time. For example, PU-b-PVP08 means the polyurethane-block-poly(vinylpyrrolidone) copolymer after an 8 h copolymeriza-tion time. For water transport studies, PUMI wasused as control.

Characterization Methods

Spectroscopic Analyses

Fourier Transform Infrared (FTIR) spectra wererecorded by using Bruker Vector 22 spectropho-tometer. Samples were directly mounted into thesample holder and 32 scans at 4 cm�1 resolutionwere collected for each specimen. Nuclear Mag-netic Resonance (1H-NMR and 13C-NMR) spectrawere recorded on a Varian1 INOVA 400 instru-ment operating at 400 MHz. CDCl3 was used as asolvent for DHTD whereas DMSO-d6 was used asa solvent for other products. ACD/2D NMR soft-ware was used for the integrations of observedpeaks.

Scheme 1. Syntheses of N,N0-diethyl-N,N0-bis(2-hydroxyethyl)thiuram disulfide(DHTD).

6274 PATEL AND MEQUANINT


Gel Permeation Chromatography (GPC)

The molecular weights were determined in DMFwith 0.1 M LiBr at 85 8C using a Waters 2695 sep-arations module equipped with a Waters 2414 dif-ferential refractometer and two PLgel 5 lmmixed-D (300 � 7.5 mm) columns from PolymerLaboratories. The calibration was performedusing polystyrene standards. The flow rate was1 mL/min, and Empower 2 software was used toexamine the eluted peaks.

Thermogravimetric Analysis (TGA)

TGA was carried out using a TA Instruments Q-series TGA Q 500 analyzer. The specimens, driedovernight at 50 8C in a vacuum oven, wereweighed in the range of 5 to 10 mg and heatedfrom 25 to 700 8C at a heating rate of 20 8C/minunder nitrogen. TA Instruments Universal Analy-sis 2000 software was used to analyze the data.

Equilibrium Water Content (EWC)and Swelling Kinetics

Polymer specimens (n ¼ 4) were cut into 5-mm di-ameter discs from 0.2-mm-thick cast films. To castthe films; chloroform, THF, and acetic acid wereused as solvents for PUMI, PU-b-PVP, and PU-b-PAAm, respectively. The specimens were driedovernight in a vacuum oven at 50 8C, cooled toroom temperature and weighed (Wd). They werethen swollen at ambient temperature (22 8C) indistilled water for 48 h. Swollen specimens weretaken out and blotted lightly using filter paper toremove the excess surface water. The hydratedspecimens were weighed (Wh) and EWC of thespecimens was calculated using,26

EWC ¼ Wh �Wd

Wh� 100 (1)

For the swelling kinetics study, specimens wereremoved at specified time intervals, blotted withfilter paper, and weighed. The water uptake of

Scheme 2. Syntheses of PUMI, polyurethane-block-poly(acrylamide) (PU-b-PAAm)and polyurethane-block-poly(vinyl pyrrolidone) (PU-b-PVP).



these hydrogels at specified time interval was cal-culated using eq 1 where Wh was the weight ofthe hydrogel specimen at that time interval. Todetermine the water transport mechanism intothe hydrogels, the water uptake data of each sam-ple were fitted into an empirical equation,27

f ¼ Wt=W1 ¼ Ktn (2)

where, f is the fractional water uptake at time t,Wt and W1 are the weights of the water absorbedin the hydrogels at time t and at equilibrium swel-ling, respectively. K is a characteristic rate con-stant that rely on the hydrogel structure, and n isa transport number that indicates whether diffu-sion and/or relaxation controls the swelling. Fromthe swelling data, the diffusion coefficient (D) forplanar geometry can be calculated using the equa-tion,28

Wt=W1 ¼ 16D

pL2

� �0:50

� t0:50 (3)

where L is the initial film thickness. The slope ofthe plot of Wt/W1 against t0.50 provides the valueof D.

pH-Dependent Swelling Studyon PU-b-PAAm Hydrogels

Britton-Robinson buffers of different pH were pre-pared using the method described by Caykaraet al.29 Briefly, phosphoric acid (2.70 mL), glacialacetic acid (2.30 mL), and boric acid (2.47 g) weredissolved in distilled water (1 L) and 10.00 MNaOH was added in required amount to get thebuffers with different pH. The PU-b-PAAm speci-mens (n ¼ 4) were swelled in different pH buffersfor 72 h and their EWC was calculated using eq 1.

RESULTS AND DISCUSSIONS

Syntheses of Macroiniferter and Block Copolymers

The iniferter method is a class of controlled radi-cal polymerization that could be used to synthe-size a variety of block copolymers, includinghydrogels with well-defined chain lengths thatare controlled easily. Thus, polymers and copoly-mers of desired molecular weight having low poly-dispersity index can be prepared using theiniferter technique. In the present study, DHTDwas used as a chain extender to synthesize novelDC-based PUMI. Such a PUMI was capable of

photochemically initiating the insertion of vinylmonomers and, thus allowed us to prepare physi-cally crosslinked PU-b-PVP and PU-b-PAAmhydrogels (Scheme 2).

The FTIR spectra of DHTD, PUMI, PU-b-PVP,and PU-b-PAAm xerogels are presented in Fig-ure 1. The peak related to the DHTD hydroxylstretching was observed at around 3410 cm�1.The peaks observed at 1487 cm�1, 1185 cm�1, and1040 cm�1 are assigned to CAN stretching, C¼¼Sstretching and S¼¼CAS stretching vibrationsof the DC group, respectively. The SAS stretchingof the DC group showed a strong peak at745 cm�1. The spectrum related to PUMI pre-pared from DHTD showed asymmetric H-bondedNAH stretching at around 3305 cm�1 and car-boxyl stretching at 1730 cm�1 related to the ure-thane group. It also showed a peak at 1217 cm�1

because of the mixed vibrations involving CANstretching along with NAH bending related to theurethane group. The peak corresponding to theCAOAC stretching of the PTMO segments wasobserved at 1105 cm�1. All these peaks were alsoobserved in the block copolymers. The PU-b-PVPspectrum showed a broad peak at 1730 cm�1

related to carboxyl stretching compared with thepeak observed in PUMI. In the PU-b-PAAm spec-trum, due to the presence of primary amide; sym-metric H-bonded NAH stretching was alsoobserved at 3180 cm�1 along with the asymmetricH-bonded NAH stretching at 3305 cm�1. Anasymmetric NH2 deformation of the polyacryl-amide block gives a further peak at 1665 cm�1.

Figure 2(a) shows the 1H-NMR spectrum ofthe DHTD, where the OH proton from DHTDappeared at 3.50 ppm. The protons associated to

Figure 1. FTIR spectra of DHTD, PUMI, PU-b-PVP,and PU-b-PAAm.



methyl and methylene groups attached to ANCS2

as well as the methylene protons attached to OHappeared at 1.10–1.30 ppm, 3.60–4.00 ppm, and4.35 ppm, respectively. In the 13C-NMR spectrumof the DHTD [Fig. 2(b)], the presence of the peakat 187 ppm related to the AC¼¼S further con-firmed the formation of a dithiocarbamate link-age. In the 1H-NMR spectrum of the PUMI,[Fig. 3(a)], the aromatic and aliphatic protonsfrom MDI are observed between 7.00 and 7.50ppm and 3.75 ppm, respectively. The aliphaticprotons related to PTMO, namely, CH2, OCH2,and CH2 attached to the urethane amide groupswere identified at 1.46 ppm, 3.34 ppm, and 4.03ppm, respectively. The urethane NH peak wasobserved at 9.48 ppm. In the 1H-NMR spectrumof the PU-b-PVP [Fig. 3(b)], additional peaksrelated to the methylene protons adjacent to theketone group of the NVP and adjacent to ACS2

from DHTD were also observed at 2.04 ppm and1.85 ppm, respectively. The methylene protonscorresponding to the tertiary amine of NVP andadjacent to aliphatic CH from NVP appeared at3.10 ppm and 1.30 ppm, respectively. In the 1H-NMR spectrum of the PU-b-PAAm [Fig. 3(c)],peak related to NH2 protons of the amide groupwere identified at 6.83 ppm. The aliphatic protonsrelated to the PAAm block, namely, methyleneprotons near the CH group and attached to theACS2 of DHTD were observed at 1.21 ppm and2.19 ppm, respectively.

Figure 4 shows the plot of the number averagemolecular weight (Mn) of PU-b-PVP xerogelsat different copolymerization times. A linear in-crease in molecular weight with conversion andreaction time indicates that the PUMI, having DCgroups incorporated into the polyurethane back-bone, initiated the copolymerization via a ‘‘living’’system. The polydispersity indices (PDI) of thepresent copolymers remained at around 1.45 �0.05 for the reaction times investigated (Table 1).These values are generally lower than reportedPDI for other DC-based iniferter systems.12,30,31

This could be attributed to the selective precipita-tion of the higher molecular weight polymer dur-ing the purification process, such that lower mo-lecular weight block copolymers and homopoly-mers remained in solution, decreasing thepolydispersity indices of the products. We haverecently demonstrated that selective precipitationcan, in fact, lead to remarkably low PDI for somepolycondensation polymers.32 From Figure 4 andTable 1, it is also evident that the change in themolecular weight of the xerogels with reactiontime is low. This was not anticipated but weattributed this to the hydrophilic nature of thepoly(vinyl pyrrolidone) block, which would affectthe overall hydrodynamic volumes of the poly-mers. For example, it has been shown that graft-ing of hydrophilic oligo(ethylene glycol)s ontohydrophobic polymers leads to an unexpecteddecrease in hydrodynamic volume and thus an

Figure 2. (a) 1H-NMR spectrum of DHTD and (b) 13C-NMR spectrum of DHTD.



apparent decrease in molecular weight.33 Becauseof the partial solubility in common GPC solvents,the molecular weights for PU-b-PAAm could notbe accurately determined.

Previous studies have shown that dithiocarba-mate derivatives can be used as an effective ini-

tiator for the controlled polymerization of differ-ent vinyl monomers.12,31,34 Literature data on thesynthesis of multiblock copolymers using poly-meric dithiocarbamate iniferter, however, isscarce.21,35 In this work we showed, for the firsttime, the use of PUMI chain extended with

Figure 3. 1H-NMR spectrum of (a) PUMI, (b) PU-b-PVP and (c) PU-b-PAAm.



DHTD as a novel method to prepare PU-b-PVPand PU-b-PAAm multiblock copolymers and, dem-onstrated the success of this approach. In addi-tion, the current polymers have significance asphysically crosslinked hydrogels from an applica-tion standpoint (see later section). The SAS link-ages of the DHTD are known to be responsible for

the linear increase of the molecular weight aswell as monomer conversion with time.18,36 Thus,during photopolymerization, the DC groups ofDHTD not only initiate the polymerization, butthey also possess excellent chain transfer proper-ties and actively involve in the reversible primaryradical termination. Owing to this reversible ter-mination, reported DHTD initiated homopolymerssuch as PS and PMMA were almost end-cappedwith hydroxyl groups18,36; that makes DHTD anexcellent vehicle to prepare desired macrodiols.

Figure 3. (Continued)

Figure 4. Average molecular weight of PU-b-PVPcopolymers as a function of copolymerization timeand conversion. Molecular weight increases linearlywith copolymerization time and conversion. The appa-rent low conversion is attributed to the poor yieldassociated with precipitation challenges.

Table 1. Molecular Weights of PU-b-PVPBlock Copolymers

SampleReactionTime (h) Mn � 10�4 Mw � 10�4 PDI

PUMI 0 3.45 4.85 1.41PU-b-PVP06 6 3.55 5.00 1.41PU-b-PVP08 8 3.58 5.01 1.40PU-b-PVP10 10 3.59 5.19 1.44PU-b-PVP12 12 3.67 5.18 1.41PU-b-PVP14 14 3.69 5.33 1.44PU-b-PVP24 24 3.83 5.68 1.48

Solvent: Dimethyl formamide; [PUMI]o ¼ 3.00 g/dL; [NVP]o¼ 1.50 mol/L.



Water Transport Propertiesof Polyurethane Hydrogels

Figure 5 shows the water transport behavior ofPU-b-PVP hydrogels at 22 8C as a function ofimmersion time. The DHTD-based PUMI, whichwas used as a control, showed a higher EWC com-pared with the previously reported TPED basedPUMI (8% vs 3%).25 The presence of the tertiaryamine in DHTD might be the reason for higherEWC in the control PUMI. For the block copoly-mer hydrogels, as the copolymerization time wasincreased, the water uptake as well as EWCincreased. An increase of hydrophilic segmentinto the copolymer, as indicated by an increase inmolecular weight (Fig. 4) increased the EWCwhich was expected. Kim and coworkers37 studiedpolyurethane-block-polyacrylic acid (PU-b-PAAc)using a TPED-based macroiniferter and showedthat the swelling of the block copolymersdecreased with reaction time. In that study, PEGhad been utilized as a soft segment and the netswelling of the block copolymers would be fromthe combined effect of PEG and PAAc segments,which decreased with increased copolymerizationtime. The water uptake and EWC for the PU-b-PAAm, presented in Figure 6, followed a similartrend. Hydrogels having an EWC up to 40% wereprepared from PU-b-PAAm copolymers, whereasfrom PU-b-PVP copolymers, hydrogels containing

an EWC up to 60% were prepared. In aqueousmedia, the PU-b-PVP hydrogels were swollenwithout any mass loss but dissolved into THF,DMF, and CHCl3 confirming that they are physi-cally crosslinked. PU-b-PAAm hydrogels werefound to be partially soluble in these solventswhile soluble in acetic acid.

The water transport properties of the currenthydrogels were determined by fitting the wateruptake data to eq 2 and, the results are listed inTable 2. For one dimensional planar geometry,the swelling is diffusion controlled if n ¼ 0.50(Case I or Fickian diffusion), where the rate ofstructural relaxation is faster than rate of diffu-sion. If n ¼ 1.00, (Case II diffusion) water trans-port is controlled by the rate of relaxation of thepolymer network. If the value of n lies between0.50 and 1.00, (anomalous or non-Fickian diffu-sion) diffusion as well as relaxation rates have aconsiderable effect on the swelling rate of thehydrogels. Except for PU-b-PAAm08, the trans-port numbers (n) for PU-b-PAAm hydrogels arearound 0.50, indicating that the PU-b-PAAmhydrogels followed Fickian diffusion. On the otherhand, the PU-b-PVP hydrogels exhibited trans-port number values between 0.53 and 0.57 andfollowed non-Fickian diffusion. Strong interchaininteraction due to hydrogen bonding betweenPVP segments and PU segments may have led to

Figure 5. Swelling isotherms of physically cross-linked PU-b-PVP hydrogels as a function of immer-sion time at 22 8C. PUMI (l); PU-b-PVP06 (*); PU-b-PVP08 (!); PU-b-PVP10 (~); PU-b-PVP12 (n); PU-b-PVP14 (h). All hydrogel isotherms show anincrease in water uptake with swelling time. More-over, water uptake increases with increase in copoly-merization time.

Figure 6. Swelling isotherms of physically cross-linked PU-b-PAAm hydrogels as a function of immer-sion time at 22 8C. PUMI (l); PU-b-PAAm04 (*);PU-b-PAAm06 (!); PU-b-PAAm08 (~); PU-b-PAAm10 (n); PU-b-PAAm12 (h). All hydrogel iso-therms show increase in water uptake with swellingtime. Moreover, water uptake increases with anincrease in copolymerization time.



a more compact structure and moved the value ofn towards the anomalous diffusion. We shouldpoint out that the accuracy of eq 2 is up to f ¼ Wt/W1�0.6.

The water diffusion coefficients of the pres-ent hydrogels were calculated using eq 3, andthe results are shown in Table 2. For both PU-b-PVP and PU-b-PAAm hydrogels, the diffusioncoefficients increased with copolymerizationtime. Because physical crosslinking points wereprovided by the hydrophobic polyurethane seg-ments, the role of hydrophilic segments is pri-marily for water transport through the polymernetwork. Therefore, as copolymerization timeincreased more hydrophilic monomer unitswere added and, these in turn increased theavailable volume fraction for the water diffu-sion into the network. In the PU-b-PVP hydro-gels, the calculated diffusion coefficients werein the range of 1.11 � 10�9 to 3.01 � 10�9 cm2/s�1.These values are one order of magnitude lowerthan reported values for chemically crosslinkedPVP-homopolymer.38 This difference is attrib-uted to the lower amounts of PVP incorporatedinto the hydrogels structure compared with thehomopolymer. The presence of the polyur-ethane segments that are hydrophobic couldrestrict water diffusion and ultimately reducethe diffusion coefficient values even for smallmolecules like water. The anomalous transportnumber presented in Table 2 indicates that dif-fusion is, in part, controlled by structuralrelaxation. Therefore, it is not surprising thatwe found lower diffusion coefficients for PU-b-PVP hydrogels. The inclusion of hydrophobicsegments into hydrophilic polymers is alsoknown to significantly reduce the diffusioncoefficients.39

On the other hand, the diffusion coefficients ofPU-b-PAAm vary from 4.30 � 10�9 to 4.70 � 10�7

cm2/s�1. By using Quasi-Elastic Light Scatteringmethod, Peters and Candau40 reported the waterdiffusion coefficient for chemically crosslinkedPAAm to be 4 � 10�7 cm2/s�1. Chemically cross-linked PMMA-co-PAAm hydrogels have also beenreported to have similar water diffusion coeffi-cient.41 The current PU-b-PAAm hydrogels hadcomparable water diffusion coefficients only after8 h of copolymerization. Between 0 h and 8 hcopolymerization time, the water diffusion coeffi-cients were very low presumably due to the slowpropagation rate of the iniferter system.13

Figure 7. Swelling isotherms of physically cross-linked PU-b-PAAm hydrogels at different pH at22 8C. PUMI (l); PU-b-PAAm04 (*); PU-b-PAAm06(!); PU-b-PAAm08 (~); PU-b-PAAm10 (n); PU-b-PAAm12 (h). All hydrogel isotherms show higherswelling at strong acidic and basic pH.

Table 2. Water Transport Properties of Physically Crosslinked PU-b-PAAm and PU-b-PVP Hydrogels

Sample n k � 102 D (cm2/s�1) � 107 D (cm2/s�1) � 109 EWC (%)

PUMI – – – – 08.30 � 1.02PU-b-PAAm04 0.50 1.38 0.043 – 25.67 � 2.32PU-b-PAAm06 0.51 3.23 0.471 – 34.26 � 2.46PU-b-PAAm08 0.57 4.52 1.104 – 34.64 � 0.67PU-b-PAAm10 0.51 7.72 3.172 – 38.80 � 1.16PU-b-PAAm12 0.49 10.95 4.705 – 40.06 � 2.25PU-b-PVP06 0.57 3.70 – 01.11 15.04 � 2.32PU-b-PVP08 0.56 1.06 – 01.89 35.67 � 0.62PU-b-PVP10 0.53 0.91 – 02.70 46.36 � 2.38PU-b-PVP12 0.53 0.95 – 03.76 50.74 � 2.96PU-b-PVP14 0.57 0.78 – 03.01 59.11 � 2.11



The Effect of pH on Swellingof PU-b-PAAm Hydrogels

The EWC study of PU-b-PAAm hydrogels at dif-ferent pH is shown in Figure 7, which revealsthat the PU-b-PAAm hydrogels have higher swel-ling in both acidic and basic media compared withthe neutral media. The strong hydrogen bondingbetween the tertiary amine groups of the DHTDsegments and the ether groups of the PTMO seg-

ments may have reduced the swelling betweenpH 4–11. In strong acidic conditions, the protona-tion of tertiary amines induce interchain repul-sions, which results in an expanded configurationand allows more water solvation. In strong basicconditions, the amide groups of polyacrylamidesegments can partially hydrolyze to acrylic acidand ionize to sodium acrylate.41 The ionic repul-sion between sodium ions expands the structurefurther and allows higher swelling of the matrix.The hydrolysis at higher pH was further con-firmed using FTIR Spectroscopy. The amide IIband at 1665 cm�1 almost disappeared while thecarboxylate ions stretching at 1450 cm�1

appeared at higher pH confirming the hydrolysis(Fig. 8).

Thermal Behavior of Macroiniferterand Block Copolymers

The TGA thermographs of the PUMI and theblock copolymers are presented in Figure 9. Con-trary to conventional polyurethanes, which areknown to degrade in two stages,42 Figure 9 showsthat the PUMI and the copolymer xerogelsdegraded in three distinctive stages. The initialdecomposition temperatures (Ti) and the maxi-mum decomposition temperatures (Tmax) of thepolymers are listed in Table 3. The thiuram disul-fide groups decomposed in the first stage and

Figure 8. FTIR spectra of physically crosslinkedPU-b-PAAm12 hydrogels at different pH. The spectrashow hydrolysis of PU-b-PAAm12 copolymer at higherpH.

Figure 9. TGA thermographs and related derivative thermographs of PUMI; PU-b-PVP and PU-b-PAAm. PUMI and block copolymers degraded in three distinct stages.



released CS2,35 whereas the second and third

stage weight losses are associated for the hardand soft segment decompositions, respectively.

CONCLUSIONS

Physically crosslinked polyurethane hydrogelswere synthesized using a novel dithiocarbamatebased PUMI. Spectroscopic studies confirmed thatthe block copolymers have been successfully pre-pared. A linear increase in molecular weight withcopolymerization time and conversion revealedthat the DC based PUMI follows the ‘‘living’’ radi-cal polymerization mechanism. The PDI alsoremained below 1.50. The water uptake studyshowed that PU-b-PAAm hydrogels having EWCup to 40% and PU-b-PVP hydrogels having EWCup to 60% were obtained. Water transport in poly-acrylamide based polyurethane hydrogels showedFickian diffusion whereas poly(vinyl pyrrolidone)based polyurethane hydrogels showed non-Fick-ian diffusion. PU-b-PAAm hydrogels interestinglyhad higher swelling behavior in strong acidic andbasic environments.

This work was supported by the Natural Sciences andEngineering Research Council of Canada (NSERC).

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Syntheses and characterization of physically crosslinked hydrogels from dithiocarbamate-derived polyurethane macroiniferter