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Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 55–60

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

abrication of complex microcapsules containingoly(allylamine)-g-poly(N-isopropylacrylamide)nd their thermal responsivity

eng Wang, Yi Zhu, Changyou Gao ∗

ey Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, and Department of Polymer Science and Engineering, Zhejiang University,o. 38, Zheda Road, Hangzhou 310027, China

r t i c l e i n f o

rticle history:eceived 29 May 2009eceived in revised form 25 July 2009

a b s t r a c t

Complex microcapsules containing thermo-responsive poly(allylamine)-g-poly(N-isopropylacrylamide)(PAH-g-PNIPAAm) were fabricated by adsorbing one layer of PAH-g-PNIPAAm onto CaCO3 microparticlesdoped with poly(styrene sulfonate) sodium salt (PSS), followed by dissolution of the CaCO3 particles. The

ccepted 28 July 2009vailable online 7 August 2009

eywords:icrocapsules

hermal responsiveoacervation

complex capsules were characterized by confocal microscopy, scanning electron microscopy, transmis-sion electron microscopy and atomic force microscopy. The existence of PAH-g-PNIPAAm in the complexmicrocapsules was confirmed by FTIR spectroscopy, and its thermo-responsive property was studied by1H NMR. The capsules showed different morphologies when dried at 25 and 50 ◦C, as well as enhancedstability at 40 ◦C in alkaline solution due to the additional hydrophobic interaction.

© 2009 Elsevier B.V. All rights reserved.

olyelectrolytes

. Introduction

Hollow microcapsules have been widely used in fields ofharmaceutics, cosmetic, food, textile, adhesive and agricultural

ndustry due to their isolating property, large inner volume andunable permeability [1–4]. To obtain the microcapsules with ver-atile structures and properties, new efforts are continuously triedo explore various techniques for fabrication. For example, by theayer-by-layer (LBL) assembly on sacrificial colloidal particles [5–7],

ultilayer microcapsules with ultrathin wall thickness and tun-ble wall structures and properties have been fabricated. Theyave shown potential applications in materials and life science asicroreactors, microcontainers, drug-delivery vehicles, protective

hells for cells or enzymes, transfection carriers for gene therapynd biosensors, etc. On the other hand, nowadays much attentions paid to smart materials too, which can respond to stimuli suchs pH, ionic strength, temperature, light, magnetic field and spe-ific biological substances, etc. [8–10]. For this context, the smarticrocapsules in particular thermo-responsive microcapsules are

articularly attractive, since their structures and properties can beasily mediated by temperature. This is of great help for many bio-elated applications such as controlled drug delivery.

∗ Corresponding author. Tel.: +86 571 87951108; fax: +86 571 87951108.E-mail address: cygao@mail.hz.zj.cn (C. Gao).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.07.051

Poly(N-isopropylacrylamide) (PNIPAAm) is a typical thermo-sensitive polymer whose lower critical solution temperature (LCST)is around 32–34 ◦C in water solution. Below the LCST, the poly-mer is soluble while above it the polymer is in a collapsed statethus becomes water insoluble. The phase transition is causedby the dehydration of the polymer chains above the LCST, atwhich the coiled polymer chains collapse into a globular con-formation due to the breakage of hydrogen bonds between theamide groups of PNIPAAm and water molecules [11]. Previouslywe have prepared a grafting copolymer of poly(allylamine)-graft-poly(N-isopropylacrylamide) (PAH-g-PNIPAAm), which has an LCSTat 34 ◦C, a temperature identical to that of the PNIPAAm regardlessof their grafting ratios [12–14].

Therefore, incorporation of the thermo-responsive polymersinto the multilayers is a typical strategy to obtain the thermo-responsive microcapsules. Indeed, PNIPAAm and other thermo-sensitive polymers have been directly assembled into LBL filmsand capsules by hydrogen bonding [15]. For example, Quinn andCaruso [16] prepared PNIPAAm/poly(acrylic acid) (PAA) multilayerthin films. The adsorption temperature of PNIPAAm significantlyaffects both the mass proportion of PNIPAAm in the film and thefilm surface morphology. On the other hand, much attention has

been paid to copolymers of PNIPAAm and polyelectrolytes such asPSS, PAH, PAA, poly(methacrylic acid) (PMA), poly(ethyleneimine)(PEI) and chitosan. Jaber and Schlenoff [17] reported ther-mally reversible modulation of ion transport through thermallyresponsive polyelectrolyte multilayers prepared from charged

56 F. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 55–60

(d) TE

p[tHtopdpsarr

Fig. 1. (a) CLSM, (b) optical transmission, (c) SEM and

oly(N-isopropylacrylamide) (PNIPAAm) copolymers. Kono et al.18] fabricated partly crosslinked PMA-PEI complex capsules con-aining the methacrylic acid-N-isopropylacrylamide copolymers.owever, this kind of capsules needs to be crosslinked and only

hose microcapsules prepared at temperatures higher than thatf the LCST have shown the thermal sensitivity. Glinel et al. [19]repared microcapsules by LBL deposition of oppositely chargediblock copolymers each containing PNIPAAm block onto colloidalarticles and subsequent decomposition of the cores. The capsulehrinkage in their size is accompanied by a decrease of the perme-

bility with increasing temperature, which is a result of structuralearrangement in the shells. However, this process is only partiallyeversible upon cooling.

Fig. 2. FTIR spectra of PSS/PAH-g-PNIPAAm and PSS/PAH complex capsules.

M images of PSS/PAH-g-PNIPAAm complex capsules.

The aim of this work is to fabricate complex microcapsules con-taining PNIPAAm and study their thermal responsivity. We havepreviously reported a facile route to fabricate complex polyelec-trolyte microcapsules via in situ coacervation [20,21]. In this workthis in situ coacervation method is applied to fabricate the thermo-responsive microcapsules by utilizing PAH-g-PNIPAAm as one ofthe polyelectrolytes. In this protocol, CaCO3 particles doped withPSS are used as the template, onto which only one single layer ofPAH-g-PNIPAAm is adsorbed. The capsule walls are formed duringthe core dissolution, in which the doped PSS is released and instan-taneously complexed with the adsorbed PAH-g-PNIPAAm to formthe shells eventually. The chemical structure and morphology ofthe complex hollow microcapsules are studied by Fourier trans-form infrared spectroscopy (FTIR), scanning electron microscopy(SEM) and transmission electron microscopy (TEM) and scanningforce microscopy (SFM). The capsules show better stability at 40 ◦Cthan at 20 ◦C in alkaline solution. To the best of our knowledge, thisis the first report on the effect of temperature on the morphologyand stability in alkaline solution of the microcapsules containingPNIPAAm.

2. Experiment

2.1. Materials and methods

Sodium poly(styrene sulfonate) (PSS, MW ∼ 70 kDa),poly(allylamine hydrochloride) (PAH, MW ∼ 70 kDa), rhodamine 6G(Rd6G), fluorescein sodium (FL) were obtained from Sigma–Aldrich.Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), sodium carbonate(Na2CO3), disodium ethylene diamine tetraacetate dihydrate(EDTA) were obtained from Shanghai Chemical Reagent Company,

China. All chemicals were used as-received. The water used in allexperiments was triple distilled.

PAH-g-PNIPAAm copolymers were synthesized by grafting car-boxylic end-capped PNIPAAm (PNIPAAm–COOH, Mw 2500) ontoPAH (Mw 65 kDa, Aldrich) chains as described previously [12]. The

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F. Wang et al. / Colloids and Surfaces A:

rafting degree of PNIPAAm was measured as 33% by elementalnalysis.

PSS-doped CaCO3 microparticles were prepared as describedreviously [20]. Briefly, 200 mg PSS was dissolved in 200 mL 0.05 Malcium nitrate solution, into which an equal volume of 0.05 Modium carbonate solution was rapidly poured at room temperaturender ultrasonication. 10 s later, the mixture was stored undis-urbed for 1 h and the precipitated CaCO3 particles were collectedy centrifugation and washed 3 times with water.

The PSS-doped CaCO3 microparticles (∼0.5% (w/v) in suspen-ion) were then incubated in 2 mg/mL PAH-g-PNIPAAm/0.5 M NaClolution for 30 min under shaking. The particles were separatedrom the solution by centrifugation at 500 × g for 5 min, followedy three washings with water. To obtain hollow capsules, the PAH--PNIPAAm coated microparticles were treated with 0.02 M EDTAolution for 30 min under gentle shaking. The capsules were sep-rated by centrifugation at 1500 × g for 5 min, followed by threeashings in fresh EDTA solution. Finally, the capsules were washedith water for 3 times.

Confocal laser scanning microscopy (CLSM) was employed toetect the size variation of the capsules in alkaline solution (pH 11,1.5 and 12) at 20 and 40 ◦C. The size variation of the capsules wasescribed by (D − D0)/D0 × 100%, where D and D0 are the averageiameters measured at the specific pH value and pH 7 and 20 ◦C,espectively.

.2. Instrumentation

1H NMR spectra were recorded by a Bruker 300 MHz spec-rometer employing D2O as solvent. FTIR spectra were acquiredn a Bruker FTIR spectrometer. A drop of capsule suspension wasropped onto a glass slide and air-dried. After sputtering withold, the samples were observed by a scanning electron micro-cope (SIRION-100 FEI). Copper grids sputtered with carbon films

ere used to support the microcapsules for transmission electronicroscopy (TEM) study. 10 �l of the microcapsule suspension was

ropped on the copper grid and air-dried. The measurement wasarried on a JEOL JEM-200CX TEM. A drop of sample suspensionas applied to freshly cleaved mica and air-dried for scanning

ig. 3. (a) SFM top-view image of PSS/PAH-g-PNIPAAm complex capsules dried at 25 ◦

ross-section line profile recorded along the line in (b).

ochem. Eng. Aspects 349 (2009) 55–60 57

force microscopy (SFM) study. The images were acquired by a SeikoInstruments scanning force microscope (SFM, SPI3800N Probe Sta-tion and SPA400 SPM Unit) in air at room temperature using atapping mode. CLSM images were taken with a Bio-Rad Radiance2100 confocal laser scanning microscope, equipped with a 100×oil immersion objective with a numerical aperture of 1.4. Rd6Gwas used to stain the microcapsules. The �-potentials of the CaCO3microparticles and the microcapsules were measured using a Zeta-sizer Nanoinstrument Nano Z. Each value was averaged from fiveparallel measurements.

3. Results and discussion

By adsorption of one layer of PAH-g-PNIPAAm on the PSS-dopedCaCO3 microparticles and subsequent core removal, PSS/PAH-g-PNIPAAm microcapsules were successfully acquired as shown inFig. 1. The capsules dispersed very well in water and no microscop-ically detectable breakages of the capsule walls were observed inboth the CLSM (Fig. 1(a)) and transmission (Fig. 1(b)) images. Thehollow nature of the resulted microcapsules was identified by SEM(Fig. 1(c)) and TEM (Fig. 1(d)) measurements, in which folds andcreases on the capsules could be similarly observed as that of thePSS/PAH capsules [20].

In the FTIR spectrum (Fig. 2) of PSS/PAH microcapsules, thearomatic ring stretch absorbance was found at ∼1600, 1510,and 1450 cm−1. The NH2 scissor bend was found at ∼1613 cm−1,which overlapped with the symmetric aromatic ring stretch at∼1600 cm−1. For the PSS/PAH-g-PNIPAAm microcapsules, the peakat 1651 cm−1 (amide I, C O stretching) overlapped with the NH2scissor bend peak and benzene symmetric ring stretch peak at∼1613 cm−1, and the peak at 1541 cm−1 (amide II, N–H bend-ing) overlapped with another aromatic ring stretch absorbance at1510 cm−1 found in the PSS/PAH microcapsules too. The antisym-metric bend of –CH3 at 1458 cm−1 overlapped with the ‘side way’

ring stretch absorbance of PSS at about 1450 cm−1, and two splittedmethyl umbrella bends were found at 1387 and 1367 cm−1 whichare the characteristic peaks of –C(CH3)2. The existence of amide andisopropyl groups confirms the successful incorporation of PAH-g-PNIPAAm into the complex microcapsules.

C, (b) an enlarged part of (a) indicated by the box and (c) its 3D image, and (d)

58 F. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 55–60

F t 50 ◦

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ig. 4. (a) SFM top-view image of PSS/PAH-g-PNIPAAm complex capsules dried aross-section line profile recorded along the line in (b).

AFM characterization revealed that the double walls have ahickness of 36 ± 6 nm, corresponding to a wall thickness of ∼18 nm,hich is almost the same as that of the PSS/PAH complex capsules

20,21]. The microcapsules obtained here are apparently thickerhan the same number of regular bilayer PAH/PSS (3–4 nm), whichs one of the advantages of the in situ coacervated method. By this

ay, the capsules are formed dynamically along with the dissolu-ion of the CaCO3 particles and release of the PSS. The rough surfacend pores of the CaCO3 particles adsorbs a large amount of PAH-g-NIPAAm, which combines with the released PSS to form the thickerapsule shells during the core dissolution.

Elemental analysis found that the molar ratio of PSS:PAH-g-NIPAAm in the microcapsules (1.1) was smaller than that of theSS:PAH microcapsules (1.3), revealing that the amount of PSS wasecreased while the amount of PAH-g-PNIPAAm was increased. This

s well consistent with the variation of the chemical structures of theolycations. In the PAH-g-PNIPAAm molecules, around 1/3 of the

onizable amine groups were substituted by the PNIPAAm chains,hus the ionic density of the polymers was significantly decreased.

oreover, the pendent PNIPAAm chains can block the nearby amine

ig. 5. 1H NMR spectra of PAH-g-PNIPAAm recorded at 25 and 50 ◦C, respectively.

C, (b) an enlarged part of (a) indicated by the box and (c) its 3D image, and (d)

groups from forming electrostatic binding with PSS. Consequently,more amount of PAH-g-PNIPAAm is required to compensate withthe same amount of PSS.

Figs. 3 and 4 compared the results of AFM characterization ofPSS/PAH-g-PNIPAAm complex capsules dried under 25 and 50 ◦C.Although the capsule size and wall thickness did not change obvi-ously, the surface morphology turned out to be quite different. Thecapsules dried at 25 ◦C (Fig. 3) show rougher surfaces with a root-mean-square roughness (RMS) of ∼5.8 nm, while those dried at50 ◦C (Fig. 4) was smoother (RMS ∼3.2 nm). Grains on those cap-sules dried at 25 ◦C were found to be 115 nm in diameter and 17 nmin height, respectively. Interstitials between the grains were clearlydetected. In contrast, much larger domain size (238 nm) was foundfor the capsules dried at 50 ◦C with a height of 11 nm, and deepinvagination between the grains were not observed. A comparablestudy revealed that the PSS/PAH complex capsules fabricated bythis protocol had a RMS of ∼5.8 nm, with a domain size of the orderof ca.100–200 nm regardless of the drying temperature.

Early investigation have found that besides the influence of cap-sule swelling during core removal, the lateral organization of themultilayers and their roughness depend mainly on polyelectrolyteaggregation and subsequent segregation of these complexes causedby evaporation of water [22]. Since there is little difference on sur-face texture of the PSS/PAH complex capsules dried at differenttemperatures, the surface texture difference for the PSS/PAH-g-PNIPAAm complex capsules dried at different temperatures can beonly attributed to the existence of PNIPAAm. It is known that thePNIPAAm chains become hydrophobic at higher temperature. Thehydrophobic interaction between the PNIPAAm chains can enhancethe interaction between the polymer chains and thereby lead to thehomogeneity of the capsules. Besides, dehydration of the polymerchains above the LCST will largely reduce water content in the cap-sule walls. This may contribute to the diminished segregation ofthe previously well-separated polyelectrolyte domains, resulting in

smoother surface texture but larger grain size.

The structure variation of PAH-g-PNIPAAm at 25 and 50 ◦C wasverified by 1H NMR (Fig. 5). At 25 ◦C, PAH-g-PNIPAAm was fullysolvated and signals associated with isopropyl groups (signals ‘a’and ‘b’) were clearly visible. Other peaks at ∼1.4 and 1.8 ppm are

F. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 55–60 59

PAAm

aP–sohsiphthpwFd

sPcitrWdeceWPrTdssoswsttbn2saats

Fig. 6. Alteration of diameters of (a) PSS/PAH and (b) PSS/PAH-g-PNI

ssigned to –CH2 and –CH groups on the backbone of both PAH andNIPAAm, and the peaks between 2.8 and 3.2 ppm are assigned toCH2 of PAH adjacent to –NH2 and –NH groups. An increase of theolution temperature to 50 ◦C resulted in complete disappearancef the signal ‘b’ and substantiate weakening of signal ‘a’. The overalligher chemical shifts of all the remaining resonance peaks at 50 ◦Chould be the result of alteration of the chemical environment. Its known that Above the LCST, the PNIPAAm chain adopts a com-act and collapsed conformation, which makes it difficult for theydrophobic isopropyl group to interact with water molecules andhereby accounts for the chemical shift of the peaks. On the otherand, existence of the hydrophilic PAH component inside the com-lex may increase the water content than that of the pure PNIPAAm,hich can explain the incomplete disappearance of the signal ‘a’ in

ig. 5. The PNIPAAm signal was regained when the temperatureropped to 25 ◦C, illustrating the reversible thermal responsivity.

Stability of the microcapsules in alkaline conditions was thentudied (Fig. 6). To better understand the contribution of theNIPAAm, firstly the PSS/PAH complex capsules (Fig. 6(a)) wereompared. At 20 ◦C, the average capsule diameter was onlyncreased about 5% when the pH value was increased from 7o 11, but rapidly increased 11% at pH 11.5. When the pH valueeached to 12, however, the capsule size was decreased −9%.

hen the pH value is above the pKa of PAH (8.4) [23], significanteprotonation of PAH shall occur. Since PSS is a strong poly-lectrolyte, the remaining negative charges on the capsule wallsause the polyelectrolytes to increase their mutual distance bylectrostatic repulsion [24], which causes the capsule swelling.

hen the pH value is higher than 11.5, the amino groups ofAH are deprotonated almost completely [25], leading to slowelease of PSS from the capsule walls by same charge repulsion.his was confirmed by UV–vis measurement, in which PSS wasetected in the supernatant of the microcapsules treated in pH 12olution. When the pH value is further increased, the quick disas-embly of the polyelectrolyte complex cause the fast dissolutionf the PAH/PSS polyelectrolyte capsules [26]. The PSS/PAH cap-ules showed a little diameter decrease in pH 7 solution (<−1%)hen the temperature was increased from 20 to 40 ◦C. Previous

tudy has showed that higher temperature facilitates the transitionowards a more coiled arrangement of the individual polyelec-rolyte molecules in multilayers and can accelerate the transientreakage of the electrostatic bonds. At pH 11, there was no sig-ificant difference between the extent of diameter variation at0 ◦C (5%) and 40 ◦C (2%). But at pH 11.5, while the capsules were

till swollen (11%) at 20 ◦C, they began to shrink at 40 ◦C (−5%);t pH 12, the extent of shrinking was much more prominentt 40 ◦C (−58%) than that at 20 ◦C (−9%). These results manifesthat higher temperature deteriorates the stability of the microcap-ules.

complex capsules in alkaline solutions at 20 and 40 ◦C, respectively.

As to PSS/PAH-g-PNIPAAm capsules (Fig. 6(b)), the extent ofdiameter variation at pH 11, 11.5, 12 were 10%, 10% and −53%,respectively at 20 ◦C, revealing that the PSS/PAH-g-PNIPAAm cap-sules have poorer stability in alkaline solution, especially at pH 12.This is attributed to the lower ionic density of the PAH-g-PNIPAAmand thereby the weaker interactions between the PSS/PAH-g-PNIPAAm pairs. When the temperature was increased to 40 ◦C, nodetectable diameter change occurred in pH 7 solution. Interestingly,at pH 11, 11.5 and 12, the extent of diameter variation became 2%,2% and −26%, respectively, manifesting an enhanced base stabil-ity at higher temperature. Incorporation of the PNIPAAm pendentchains can provide additional hydrophobic interaction above theLCST, which may maintain the macroscopic topology of the cap-sules even the electrostatic interaction at higher pH has beensignificantly weakened or destroyed. In this case, the PSS chainsshould be kinetically trapped within the enhanced crosslinked net-work through entanglements, which can either block the capsuleswelling induced by electrostatic repulsion or prevent quick leakageof PSS from the capsules.

4. Conclusion

In summary, complex capsules containing thermo-responsivePAH-g-PNIPAAm were successfully fabricated by in situ poly-electrolyte coacervation on PSS-doped CaCO3 particles. Thethermo-responsive property of PAH-g-PNIPAAm was studied by1H NMR, the incorporation of PAH-g-PNIPAAm into the complexmicrocapsules was confirmed by FTIR spectroscopy. This kind ofcapsules showed different morphologies when dried at 25 and 40 ◦Cand enhanced stability at 40 ◦C than that at 20 ◦C in alkaline solu-tion. With the advantages of time/materials saving and thermalsensitivity, this kind of microcapsules may find some applicationsin drug delivery and other biotechnology fields.

Acknowledgements

This study is financially supported by the Natural Science Foun-dation of China (20774084, 50873087) and the Major State BasicResearch Program of China (2005CB623902).

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