photopolymerization synthesis of poly(n-isopropylacrylamide) hydrogels

Photopolymerization Synthesis ofPoly(N-isopropylacrylamide) Hydrogels

HITOSHI KUBOTA, AKIRA FUKUDA

Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376, Japan

Received 10 July 1996; accepted 2 January 1997

ABSTRACT: Poly(N-isopropylacrylamide) (NIPAAm) gels were formed by photopoly-merization of NIPAAm in the absence of a crosslinker using a water solvent at 257C.Factors affecting formation were the wavelength region of irradiated light, the type ofphotoinitiators, and the concentrations of the photoinitiator and monomer. A high-pressure mercury lamp (400 W) was used as a light source. An NIPAAm concentrationof 10 wt % and irradiation time of 15 h was used for the photopolymerization. The gel(68% yield) was formed when the quartz glass system was used, but no gelation wasobserved for the Pyrex glass system that transmits light with l ú 290 nm. The gel(100% yield) was easily formed, even in the latter system, when 30 mmol/L of hydrogenperoxide and potassium persulfate were used as the photoinitiator. Water soluble photo-initiators such as ferric chloride and sodium anthraquinone-2,7-disulfonate were noteffective for the gel formation. Yield of the gel increased with increasing the potassiumpersulfate concentration (1–30 mmol/L), but it decreased when a high concentrationof hydrogen peroxide (60 mmol/L) was used. The gel yield increased with the NIPAAmconcentration (5–20 wt %). The degree of swelling of the resultant poly(NIPAAm)gels, which was measured by immersing the gels in water at various temperatures (0–507C) for 24 h, steeply decreased at about 307C with increasing temperature, exhibitinga temperature-responsive character. The gels swelled and shrank in water below andabove the temperature, respectively. The extent of the character depended on the con-centrations of hydrogen peroxide and monomer. The formation mechanism of the gelin the photopolymerization of NIPAAm using hydrogen peroxide photoinitiator wasdiscussed. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1313–1318, 1997

Key words: poly(N-isopropylacrylamide); hydrogel; photopolymerization; photoiniti-ator

INTRODUCTION shrink above the LCST. The hydrogel is usuallysynthesized by solution polymerization1–5 of NI-

Poly(N-isopropylacrylamide) (NIPAAm) is well PAAm using a redox initiator such as persulfates/known to exhibit a lower critical solution tempera- diamines in the presence of a crosslinker, N,N *-ture (LCST) at around 327C in aqueous solution; methylenebisacrylamide, at temperatures belowthat is, it dissolves in water below and precipi- the LCST.tates from solution above the LCST. Poly(NI- In a previous article6 we applied a radiation-PAAm) hydrogels1,2 with a crosslinked structure induced polymerization method for the synthesisare characterized by a temperature-responsive of poly(NIPAAm) hydrogels. The method wasnature in which they swell in water below and characterized by a simple and additive-free pro-

cess because no initiator and crosslinker existedin the polymerization system. Moreover, the de-Correspondence to: H. Kubota.

q 1997 John Wiley & Sons, Inc. CCC 0021-8995/97/071313-06 gree of crosslinking of the gel could be controlled

1313

4302/ 8EA4$$4302 06-18-97 08:26:30 polaa W: Poly Applied

1314 KUBOTA AND FUKUDA

Table I Effect of Wavelength Regioneasily by irradiation conditions. This method isof Irradiated Light on Formationespecially attractive for the synthesis of hydrogelsof Poly(NIPAAm) Gelsin the field of controlled drug derivery4,7–9 because

the hydrogels do not contain any residual chemi-Crosslinkedcal initiators. This article deals with synthesis ofPolymer (%)poly(NIPAAm) hydrogels by photopolymerization H2O2and their temperature-responsive characteristics. Irradiation Concentration Irradiation Irradiation

The gels could be synthesized by photopolymeri- System (mmol/L) Time: 7 h Time: 15 hzation of NIPAAm using a photoinitiator such ashydrogen peroxide (H2O2) in the absence of a cross- Quartz

system 0 66.8 67.9linker. Factors affecting formation of the poly(NI-PyrexPAAm) gels by photopolymerization were investi-

system 0 0 0gated in terms of the wavelength region of thePyrexirradiated light, the type of photoinitiators, and

system 30 96.2 98.4the concentrations of the photoinitiator andmonomer. [NIPAAm] Å 10 wt %, 257C.

EXPERIMENTAL the treatment, excess water on the gel was wipedoff by filter paper and then the gel was weighed.MaterialsThe swelling was calculated according to the fol-

NIPAAm, acrylamide, and methacrylamide were lowing equation:commercial products and were purified by recrys-tallization. H2O2, potassium persulfate (K2S2O8),

swelling (g/g) Å Ws 0 Wo

Woferric chloride, and sodium anthraquinone-2,7-disulfonate were used as the photoinitiator, all ofwhich were reagent grade and were used without where Wo and Ws denote weights of dried andfurther purification. swollen samples, respectively.

PhotopolymerizationRESULTSPhotopolymerization was carried out in a 3-mm

i.d. Pyrex glass tube containing an aqueous solu- Formation of Poly(NIPAAm) Gelstion of NIPAAm, in which known concentrations

The effect of the wavelength region of the irradi-of photoinitiator were dissolved under a nitrogenated light on the formation of the poly(NIPAAm)atmosphere. Irradiation with a high-pressuregel was examined using Pyrex and quartz glassmercury lamp (400 W) was performed at 257Ctubes (Pyrex and quartz systems, respectively),using a Riko rotary photochemical reactorand the results are shown in Table I. The Pyrex(RH400-10W), around which the glass tubes wereglass tube used in this study transmits light withrotated. After the polymerization the gels werel ú 290 nm. Photopolymerization of NIPAAm inremoved from the tubes and washed in deionizedthe quartz system was initiated even in the ab-water for 1 week. The gel fraction was measuredsence of photoinitiator to form poly(NIPAAm) gel.after Soxhlet-type extraction in ethanol for 24 hOn the other hand, no formation of the gel wasand drying the extracted gel under reduced pres-observed for the Pyrex system. It was found ac-sure to constant weight. Crosslinked polymer wascordingly that the light wavelength shorter thandefined as follows:about 290 nm was responsible for the formation of

crosslinked polymer (%) the poly(NIPAAm) gel. However, the mechanicalstrength of the gel was insufficient for the swell-

Å weight of ethanol-insoluble fractionweight of feed monomer

1 100 ing experiment. When H2O2 was used as a photo-initiator, the gel was formed in the Pyrex system,showing a higher yield than the quartz system.

Measurement of Swelling The resultant gel exhibited mechanical strengthsufficient for the swelling measurement. H2O2 isThe gel (10-mm length) was immersed in deion-

ized water at given temperatures for 24 h. After known to be decomposed by ultraviolet light,10–12


PHOTOPOLYMERIZATION SYNTHESIS OF NIPAAm HYDROGELS 1315

sition of H2O2 according to eq. (1) participate inthe termination of growing polymer radicals pro-duced through eqs. (2) and (4); and the reactionas well as the combination of primary radicals isemphasized in the system of higher H2O2 concen-trations, resulting in a decreased yield. The yieldincreased with increasing monomer concentration(Fig. 3), and a high yield of more than 95% wasobtained for the system of the monomer concen-trations beyond about 10 wt %.

Temperature-Responsive Characterof Poly(NIPAAm) Gels

Figures 4 and 5 show the effects of H2O2 andmonomer concentrations, respectively, on swell-ing of poly(NIPAAm) gels. The swelling steeplydecreased at about 307C with increasing tempera-ture, exhibiting the temperature-responsive char-acter of the poly(NIPAAm) gels. The gel swelledand shrank in water below and above the temper-Figure 1 Effect of K2S2O8 concentration on photoin-ature, respectively. The swelling at temperaturesduced formation of poly(NIPAAm) gels. [NIPAAm]below about 307C (Fig. 4) depended on the H2O2Å 10 wt %; irradiation at 257C and 15 h.concentration, decreasing in the following order:30 mmol/Lú 10 mmol/L ú 5 mmol/L É 1 mmol/L, although the level at higher temperatures wasand the resulting hydroxyl radicals initiate the

polymerization of vinyl monomers. nearly equal among the gels. This suggests thatthe degree of crosslinking is higher for the gelsThe type of photoinitiators was examined in

the Pyrex system using polymerization condi- prepared in the lower H2O2 concentration system.tions, such as the NIPAAm concentration of 10 wt% and irradiation time of 15 h at 257C. Poly(NI-PAAm) gel was formed in the system sensitizedwith K2S2O8 rather than the H2O2. Ferric chlo-ride13,14 and sodium anthraquinone-2,7-disulfo-nate,15,16 which are water soluble photoinitiatorsfor photopolymerization of vinyl monomers, werenot effective for the gel formation when concentra-tion ranges of 0.01–30 mmol/L were examined,although polymerization of NIPAAm proceeded.Figure 1 presents the effect of the K2S2O8 concen-tration on the gel formation. Gel yield increasedwith increasing K2S2O8 concentration, and 100%yield was observed when a 30 mmol/L K2S2O8 con-centration was used. With an H2O2-sensitized sys-tem (Fig. 2), on the other hand, an approximate100% yield of the gel was recorded for the systemof lower H2O2 concentrations compared to theK2S2O8-sensitized system. The phenomenon mayhave originated from a different decomposition ef-ficiency between the two photoinitiators under thepresent irradiation conditions. Moreover, the gelyield was greatly reduced in the 60 mmol/L H2O2 Figure 2 Effect of H2O2 concentration on photoin-concentration system. The supposition is that the duced formation of poly(NIPAAm) gels. [NIPAAm]

Å 10 wt %; irradiation at 257C and 15 h.hydroxyl radicals derived from the photodecompo-



Figure 5 Effect of monomer concentration on swell-Figure 3 Effect of monomer concentration on photo-ing of poly(NIPAAm) gels. Monomer concentration (wtinduced formation of poly(NIPAAm) gels. [H2O2] Å 10%): (s) 5, (l) 10, (n ) 15, (m) 20; [H2O2] Å 10 mmol/mmol/L; irradiation at 257C and 15 h.L; irradiation at 257C and 15 h.

The gel synthesized in the 60 mmol/L H2O2 con-using various concentrations of N,N*-methylene-centration had insufficient mechanical strengthbisacrylamide crosslinker. They observed that thethat was required for the swelling measurement.swelling ratio of the gel in the swollen region de-Inomata et al.17 examined the effect of a crosslink-creases as the degree of crosslinking of the geling point on the swelling behaviors of poly(NI-increases. As seen in Figure 5, the lower swellingPAAm) gels prepared by radical polymerizationlevel at temperatures below about 307C was af-forded for the gels prepared with the higher mono-mer concentrations, suggesting that higher mono-mer concentration is required for the formation ofa gel with a higher degree of crosslinking.

DISCUSSION

It is conceivable that photopolymerization of NI-PAAm in the presence of H2O2 photoinitiator pro-ceeds according to eqs. (1) – (4), where M, PH,and Pr denote NIPAAm monomer, poly(NI-PAAm), and its radicals, respectively.

H2O2 rhn

2rOH (1)

rOH / M r HOMr (2)

rOH / PH r Pr / H2O (3)Figure 4 Effect of H2O2 concentration on swelling of

Pr / M r PMr (4)poly(NIPAAm) gels. H2O2 concentration (mmol/L):(s ) 1, (l ) 5, (n ) 10, (m) 30; [NIPAAm] Å 10 wt %;irradiation at 257C and 15 h. Equation (1) is the formation of hydroxyl radicals


PHOTOPOLYMERIZATION SYNTHESIS OF NIPAAm HYDROGELS 1317

Scheme 1 The formation mechanism of the poly(NIPAAm) gel.

by photodecomposition of H2O2. Equation (2) amide. Accordingly, abstraction of a tertiary hy-drogen atom at following structure (a) position onleads to the formation of poly(NIPAAm). The hy-

droxyl radicals may also contribute to abstraction the poly(NIPAAm) substrate is important for thegel formation. Moreover, the abstraction reactionof the hydrogen atom from the poly(NIPAAm)

substrate produced in the polymerization system of another tertiary hydrogen atom at the (b) posi-tion may also participate in the gel formation.according to eq. (3). The resultant poly(NIPAAm)

radicals initiate polymerization of NIPAAm, that However, we found in electronic spin resonancemeasurements on g-ray irradiated poly(NI-is, initiation of the grafting reaction, which is indi-

cated by eq. (4). The reaction is supposed to leadto poly(NIPAAm) with a branched structure. Thepolymerization proceeds through repetition ofsuch reactions, resulting in gels with a cross-linked structure. With the system of high H2O2

concentration, it is conceivable that the termina-tion reaction of growing polymer radicals pro-duced according to eq. (4) by hydroxyl radicalsmay be emphasized to reduce gel yield and degreeof crosslinking. H2O2 can act as a photoinitiatorfor grafting18,19 of vinyl monomers on cellulose,where hydroxyl radicals produced by photodecom-position of H2O2 abstract a hydrogen atom fromthe substrate to yield cellulose radicals capable ofinitiating the grafting. However, the maximumgrafting was observed at an optimum concentra-tion of H2O2 and the formation of grafted polymerwas restricted by the use of H2O2 beyond the con-centration. The formation mechanism of poly(NI-PAAm) gel is schematically shown in Scheme 1.

To understand the radical formation on poly-(NIPAAm) leading to the crosslinked structure,the effect of the nature of the monomers was ex- Figure 6 Formation of gels in photopolymerizationamined using acrylamide and methacrylamide; of acrylamide and its derivatives. (s) NIPAAm, (l )the results are shown in Figure 6. Gel formation acrylamide, (n ) methacrylamide; [monomer] Å 10 wt

%; irradiation at 257C and 15 h.was observed for acrylamide but not for methacryl-



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photopolymerization synthesis of poly(n-isopropylacrylamide) hydrogels

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