Research Article

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Received: 30 July 2009, Revised: 19 October 2009, Accepted: 27 October 2009, Published online in Wiley Online Library: 2 December 2009

(wileyonlinelibrary.com) DOI: 10.1002/pat.1623

Fabrication of poly(acrylamide) hydrogelswith gradient crosslinking degree viaphotoinitiation of thick polymer system

Shurun Lia and Wantai Yanga*

In this paper, we report on the synthesis of poly(acry

Polym. Adv

lamide) (PAM) hydrogels by photoinitiation with a thick system.The hydrogels exhibited gradient crosslinking density along the light path. Thermogravimetric analyses (TGA) provedthe same effects. We investigated some factors affecting the swelling ratio of the hydrogels such as crosslinking agentconcentration, photoinitiator concentration, and monomer concentration. The as-prepared hydrogels might havesome potential applications in drug delivery systems and other function materials. Copyright � 2009 John Wiley &Sons, Ltd.

Keywords: gradient; hydrogels; photopolymerization; thick polymer system

* Correspondence to: W. Yang, State Key Laboratory of Chemical ResourceEngineering, College of Materials Science and Engineering, Beijing University ofChemical Technology, P.O. Box 37#, China.E-mail: yangwt@mail.buct.edu.cn

a Shurun Li, W. Yang

State Key Laboratory of Chemical Resource Engineering, College of Materials

Science and Engineering, Beijing University of Chemical Technology, Beijing

100029, China

Contract/grant sponsor: National Natural Science Foundation of China

(NSFC); contract/grant number: 50433040.

Contract/grant sponsor: National Natural Science Foundation of China

(NSFC); contract/grant number: 20374004.

Contract/grant sponsor: Polymer Chemistry and Physics, BMEC; contract/grant

number: XK 100100433.

INTRODUCTION

In the emerging technologies—aerospace, fast computers,biomaterials, and environmental sensors—functional gradientmaterials (FGM) with highly specialized functions are particularlyneeded. They have some intriguing properties (hardness, thermalconductivity, dielectric constant, etc.) that vary continuously fromone surface to the other. Gradient polymers are two-componentpolymeric systems in which the concentration of one componentvaries in a continuous way from one side to the other in systemswith plane-parallel geometry. The composition or structure is notmacroscopically homogeneous throughout the material, butvaries as a function of position. Such systems are generallyprepared by controlled polymerizations, polymers’ blending, anddiffusion polymerizations.[1–3]

Hydrogels are some three-dimensional network polymersknown to swell in aqueous solutions.[4–7] In the swollen state,these polymers become soft and rubbery, resembling a livingtissue and some possess excellent biocompatibility. Poly(acryl-amide) (PAM) has been used successfully as a basic material forthe preparation of hydrogel with good mechanical propertiesand high swelling degree. Hydrogels derived from PAM also havemany useful chemical and physical properties and have beeninvestigated for applications as smart polymers.[8] These appli-cations include immobilization of biocatalysts,[9] drug deliverysystems,[10–12] bioseparators,[13] and protein adsorption.[14]

In this paper, we describe a novel method to prepare PAMhydrogels with a gradient crosslinking density via photoinitiationof a thick system (about 10 cm). To date, photopolymerizationsare primarily restricted to the production of thin films andcoatings in the order of 100mm and below, because of theattenuation of light into the depth of the sample. A number ofinvestigators have recently reported theoretical descriptions ofthe photoinitiation of thick systems illuminated with monochro-matic or polychromatic light;[15–18] however, the applications ofthe photoinitiation of thick systems are scarce at present.

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In thick polymerization systems, light is strongly absorbed bythe photoinitiator, showing a significant light intensity gradientalong the light path. It leads to the different photoinitiaton rate ateach position, by which we could realize a gradient crosslinkingdensity in the thick hydrogels.

EXPERIMENTAL

Materials

Acrylamide (AM) and N,N’-methylenebisacrylamide (MBA) wereobtained from Beijing Chemical Reagents Company and usedwithout further purification. The photoinitiator 2-isopropylthio-xanthone (ITX) and co-initiator ethyl-4(dimethylamino)benzoate(EDAB) from H-UNIS Insight Co. Ltd were purified by recrystalliza-tion in ethanol.

Synthesis of gradient crosslinking hydrogels

Themonomer AM, ITX/EDAB (1:1mol/mol) as a photoinitiator andMBA as a crosslinking agent were dissolved in distilled water. The

009 John Wiley & Sons, Ltd.

Table 1. Mass composition of monomer, photoinitiator, and crosslinking agent in the feed solutions and correspondingabbreviations used for the hydrogels

Gel code AM (g) MBA (%)a ITX (g) EDAB (g) Water (ml)

S 2.0 0.01 0.005087 0.00386 20MBA01 1.6 0.01 0.005087 0.00386 20MBA05 1.6 0.05 0.005087 0.00386 20MBA10 1.6 0.10 0.005087 0.00386 20MBA50 1.6 0.50 0.005087 0.00386 20MBA100 1.6 1.00 0.005087 0.00386 20ITX-3 4.0 0.01 0.005087 0.00386 20ITX-4 4.0 0.01 0.0005087 0.000386 20AM8 1.6 0.05 0.005087 0.00386 20AM10 2.0 0.05 0.005087 0.00386 20AM15 3.0 0.05 0.005087 0.00386 20AM20 4.0 0.05 0.005087 0.00386 20

aMBA(%) equal to (mass of MBA/mass of AM)� 100.

Figure 1. Light intensity as a function of depth in the sample.

FABRICATION OF POLY(ACRYLAMIDE) HYDROGELS

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feed compositions in the initial mixtures are given in Table 1. Thesolutions thus prepared were placed in a cylindrical Pyrex reactor(2 cm in diameter and 10 cm in height). After nitrogen bubblingfor 30min, the reactor was sealed by biaxially orientedpolypropylene (BOPP) film and irradiated upright with a UVlight (8W low pressure mercury lamp: polychromatic but with thehighest intensity at 254 nm; the following UV intensity valueswere measured at 254 nm with Handy UV-B radiometer,Photoelectric Instrument Factory of Beijing Normal University,China) at room temperature for 5 hr. After the completion of thereaction, the hydrogels were cut into specimens of 2 cm long inturn from top to bottom and then immersed in excess of water atroom temperature for at least 72 hr to wash out the unreactedmaterials. The hydrogels were then dried at 508C under vacuumto constant weight; the monomer conversion was determinedgravimetrically.

Characterization

Thermogravimetric analyses (TGA) were performed on PAMhydrogels by using a TGS-2 microcalorimeter. TGA experimentswere performed with 4–5mg of the sample under a dynamicnitrogen atmosphere flowing at a rate of 45ml/min and at aheating rate of 108C/min. Swelling measurements were carriedout by immersing dry gel samples in water. The hydrogels wereallowed to equilibrate for 72 hr until a constant weight wasreached. The surface water was carefully wiped off beforeweighing and the swelling ratio (q) was recorded gravimetrically.

q¼ðweight of equilibrate gelÞðweight of dry gelÞ

RESULTS AND DISCUSSION

Characterization of photoinitiation for thick polymer system

In a photobleaching system, the light intensity gradient in thesample depends on both time and the depth beneath theilluminated surface. Initially, the initiator concentration is uniform,

Polym. Adv. Technol. 2011, 22 1442–1445 Copyright � 2009 John Wil

and the light intensity decreases exponentially with depth, asdescribed by Beer’s law (Figure 1). Immediately after illumination,the initiator is consumed at a rate proportional to the local lightintensity, thereby leading to an initiator concentration gradientwith the lowest concentration on the exposed surface. Thus, thelight intensity gradient and the initiator concentration gradientwill lead to a gradient crosslinking in thick PAM hydrogels.

Characterization of the gradient crosslinking hydrogels

For thick systems, a significant light intensity gradient may arisewhich leads to an inherently non-uniform initiation rate profilethat is exceptionally complex. It leads to the different photo-initiaton rates at each position. The concentration of free radicalsnear the light source is higher than the position far away. Thenthe total monomer conversions are decreased with the depth inthe thick system (Figure 2).Under TGA investigation, dry PAM hydrogels are stable up to

2858C, and decomposes above this temperature with the

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Figure 2. Monomer conversion as function of sample depth. Figure 4. Swelling ratio as function of sample depth.

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liberation of ammonia and formation of an imide group. Freewater is the primary volatile product below 2508C for PAMhydrogels.[19] The swelling capability of the PAM hydrogels isdirect to the crosslinking density, i.e. hydrogels with a lowercrosslinking density have higher swelling capability, while thosehydrogels with higher crosslinking density have lower swellingcapability. The weight loss percentage of PAM hydrogels up to2508C is plotted as a function of sample depth in Figure 3. It canbe seen that the weight loss percentage of each sample increaseswith the depth in the system. It further implies that thecrosslinking density in the thick hydrogels shows a gradientdecrease.The network crosslinking density of hydrogels strongly

influences their swelling properties. The lower crosslinkingdensity could absorb more water than the higher density, sothe swelling ratio highly depends on the crosslinking density ofhydrogels. We show in Figure 4 the swelling ratios of PAMhydrogels as a function of sample depth. It can be observed thatthe swelling ratio of each sample is increased with the depth in

Figure 3. Weight loss percentage up to 2508C as a function of sample

depth.

wileyonlinelibrary.com/journal/pat Copyright � 2009 John Wiley

the system. The hydrogel at the deep position of the thick systemhas the least crosslinking density and the one at the top positionhas the highest crosslinking density.From Figures 3 and 4, it is concluded that gradient crosslinking

density exists in the system under investigation. The gradientmonomer conversion in the thick system resulted from theconsiderable gradient light intensity. It also led to the gradientcrosslinking in the thick hydrogels. In addition, the polymerizationrate of the crosslinking agent decreased with the depth in thesystem. This also contributed to a gradient crosslinking density.

Effects of the crosslinking agent concentration

The effects of the varied crosslinking agent concentrations on theswelling ratio of the hydrogels are given in Figure 5. Itdemonstrates that the swelling ratio of the PAM hydrogelsincreases with the decrease in the crosslinking agent concen-tration. It is considered that higher crosslinking agent concen-tration leads to higher crosslinking density and lower swelling

Figure 5. Effects of the crosslinking concentration on the swelling ratio

of the hydrogels.

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Figure 6. Effects of photoinitiator concentration on the swelling ratio ofPAM hydrogels.

Figure 7. Effects of monomer concentration on the swelling ratio ofPAM hydrogels.

FABRICATION OF POLY(ACRYLAMIDE) HYDROGELS

ratio in the hydrogels. At a higher crosslinking agent concen-tration, the swelling ratio increases slowly with the sample depth.When the crosslinking agent concentration is 0.01%, the swellingratio of the resulting hydrogel increases rapidly with the depth. Inthe thick system, the two functionalities of the crosslinking agentundergo polymerization at varied rates. The light intensitydecreases with the depth, which results in decreased reactionrate of the crosslinking agent. Thus, the gradient crosslinkingdensity of the hydrogels is considerable as shown in Figure 5.

Effects of the photoinitiator concentration

The photoinitiator concentration plays a significant role inphotopolymerization. The effects of the different photoinitiatorconcentrations on the swelling ratio of the hydrogels areillustrated in Figure 6. It can be observed that the PAM hydrogelshave gradient crosslinking density in thick system, in whichphotopolymerization was carried out in the presence of aphotoinitiator of two different concentrations. Their swellingratios show a little difference. Nevertheless, the hydrogelsobtained with less photoinitiator showed a slightly higherswelling ratio. The PAM hydrogels photopolymerized with a lowerphotoinitiator concentration have longer chains than those usinghigher photoinitiator concentration. From Figure 6, we can seethat the photoinitiator concentrations exert slight influence in thethick system.

Effects of the monomer concentration

The effects of different monomer concentrations on the swellingratio of PAM hydrogels are shown in Figure 7. It can be seen thatthe swelling ratio of the PAM hydrogels increases with the sampledepth. It indicates that the gradient crosslinking density indeedexists in the hydrogels. From Figure 7, we can also observe thatthe swelling ratio increases with the decrease in monomerconcentration. At a lower monomer concentration, the rate ofcrosslinking reaction is low and thus the chains of PAM growlonger. Consequently, the PAM hydrogels prepared at lowermonomer concentration have higher swelling capability.

Polym. Adv. Technol. 2011, 22 1442–1445 Copyright � 2009 John Wil

CONCLUSIONS

The novel hydrogels with gradient crosslinking density have beenprepared by photopolymerization with a thick polymerizationsystem. The results of TGA and the swelling ratio of hydrogelsshowed that there is gradient crosslinking density in the thickpolymer system. The gradient crosslinking density results fromthe gradient monomer concentration in the system. The PAMhydrogels obtained at lower crosslinking agent concentrationand lower monomer concentrations have significant gradientcrosslinking density.

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