Materials Letters 90 (2013) 142–144

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Facile fabrication of novel polyhedral oligomeric silsesquioxane/carboxymethylcellulose hybrid hydrogel based on supermolecular interactions

Hongxia Liu a,b,n, Shaojun Huang a,b, Xiaojian Li a,b, Lihua Zhang a,b, Yuyuan Tan a,b,Chun Wei a,b, Jian Lv a,b

a Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, Chinab College of Material Science & Engineering, Guilin University of Technology, Guilin 541004, China

a r t i c l e i n f o

Article history:

Received 9 April 2012

Accepted 7 September 2012Available online 15 September 2012

Keywords:

Biomaterials

Nanoparticles

Hydrogen bond

Hybrid

Hydrogel

7X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2012.09.030

esponding author at: Guilin University of Te

& Engineering, Guilin 541004, China. Tel.: þ

6 773 5896672.

ail address: aozihx@yahoo.com.cn (H. Liu).

a b s t r a c t

Reactive octa-aminopropyl polyhedral oligomeric silsesquioxane (POSS-NH2) has a nano-sized structure

with a cubic silica core and octa-aminopropyl groups distributed at eight corners. Carboxymethyl cellulose

(CMC) is often used as a hydrogel polymer because of its water-solubility, renewability, biodegradability

and biocompatibility. Novel POSS/CMC hybrid hydrogel was fabricatied here by the aggregation of POSS-

NH2 nanoparticles and the formation of POSS-CMC hybrid block copolymers, which was driven by the

hydrogen bonding interactions between the amino groups of POSS-NH2 and the carboxyl groups of CMC.

D-(þ)-gluconic acid d-lactone (DGL) was used to alter the pH of the system which can further control the

formation of hydrogen-bonding interactions. Our method provides a facile and effective route to fabricate

novel hybrid hydrogel based on CMC or other cellulose derivatives.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogels are defined as three-dimensional polymer networksswollen by large amounts of water. They have wide applicationsin the fields of food, agriculture, water purification, drug delivery,tissue engineering, sensors and contact lenses, etc [1–8]. Carbox-ymethyl cellulose (CMC), a polyelectrolyte derived from naturalmaterials, has been extensively studied as a polymer source forhydrogels because of its water-solubility, renewability, biode-gradability and biocompatibility. Hydrogels or hybrid hydrogelsbased on CMC are usually fabricated by chemical or radiant cross-linking methods [9,10]. However, the involved cross-linkers to pre-pare these hydrogels, such as divinylsulfone (DVS), epichlorohydrin(ECH) and carbodiimide, are proved to be toxic or cytotoxic, whichsignificantly limit their applications in biological area. For thehydrogels prepared by radiant cross-linking, two competing pro-cesses, degradation and cross-linking, would simultaneously takeplace under high-energy irradiation, which results in hard controlof the resulted structure of hydrogel. Moreover, in some casesadditional complex instruments are needed during the fabrica-tion processes. Recently, Demitri et al. have reported a method toprepare hydroxy ethyl cellulose/CMC hydrogel with citric acid as across-linker, which can overcome the toxicity or cost issues [11].

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However, the preparation of hydrogel or hybrid hydrogel based onCMC in a more simple and effective way is still needed.

Polyhedral oligomeric silsesquioxane (POSS), depending on itsnanostructure and functional groups at eight corners, has attractedconsiderable attention in preparing well-defined amphiphilic diblockor triblock copolymers by living polymerization techniques and clickchemistry [12,13]. For example, POSS–PEG-POSS telechelics architec-ture can be used to build interpenetrating hybrid hydrogels withimpressive mechanical properties or fast thermo-response [13].

In this work, we developed a facile and effective method for thefabrication of POSS/CMC hybrid hydrogel by supermolecular inter-actions. First, POSS–CMC hybrid block copolymers were formedthrough hydrogen bonds between the amino groups of POSS-NH2

and the carboxyl groups of CMC. Subsequently, the POSS/CMC hybridhydrogel was obtained with the aggregated POSS-NH2 nanoparticlesas a cross-linker. This work provides a novel method for the fabri-cation of hybrid hydrogels based on CMC by physical cross-linking.POSS-NH2 can endow the hydrogels with special properties such asgood mechanical strength, high adsorption capacity for water anddyes. The hybrid hydrogels offer potential applications in drugrelease system, food, water purification and cosmetic.

2. Experimental part

Fabrication of POSS-NH2 nanoparticles: POSS-NH2 nanoparti-cles were synthesized as reported by Feher and Wyndham [14].The resulted POSS-NH2 nanoparticles have a uniform size of �4 nmas shown in Fig. 1.

H. Liu et al. / Materials Letters 90 (2013) 142–144 143

Formation of POSS/CMC hybrid hydrogels: 0.1 mL of POSS-NH2

aqueous dispersion (0.1 w/w %) at pH 11 was completely mixedwith 0.8 mL of CMC aqueous solution (2.5 w/w %). Then, 0.1 mL offresh DGL aqueous solution (0.1 mg/mL) was added to the abovemixture. After further shortly shaking and standing for 3 min,POSS/CMC hybrid hydrogels were gradually formed.

3. Results and discussion

The size of prepared POSS-NH2 nanoparticles is equivalent tothat of macromolecular chains. So, it is possible to form POSS–CMCblock copolymers by hydrogen bonding interactions between NH2

and COOH. The fabrication of POSS/CMC hybrid hydrogel is shownin Fig. 2. It should be pointed out that the pH of POSS-NH2 aqueousdispersion needs to be adjusted to above 11 before mixing withCMC to avoid the production of white POSS/CMC complex pre-cipitate due to the electrostatic interaction between the cationicNH3þ and the anionic COO� groups (as shown in Fig. 3b). In order to

form POSS/CMC hybrid hydrogel, DGL was used to slowly decreasethe pH of the mixture and induce the formation of POSS–CMC blockcopolymer through the hydrogen bonds between POSS-NH2 andCMC. Meanwhile, the aggregation of POSS-NH2 nanoparticles led tothe formation of a network with POSS nanoparticles as cross-linkers.Finally, the hybrid hydrogel composed of POSS-NH2 nanoparticlesand CMC was obtained (as shown in Fig. 3c).

Fig. 4a is the FT-IR spectra of POSS-NH2, CMC and POSS/CMChybrid hydrogel. The main hydrogen bonds in CMC include intra-

and inter-chain hydrogen bonds between O6H6 and CH2COOHand hydrogen bonds between water molecules and CMC [15].From the spectrum of CMC, we can see that the OH stretching

8.45nm

500nm

0 100 200 300 400 500 nm

7.5 6.5

5.5 4.5

3.5nm

Fig. 1. (a) AFM height image of the synthesized POSS-NH2 nanoparticles;

(b) height profile along the blue line in (a). (For interpretation of the references to

color in this figure legend, the reader is referred to the web version of this article.)

pH=11

pH= 6

POSS/H2O

CMC/water

DGL

Fig. 2. Schematic illustration of the fabric

range is in 3300–2500 cm�1, while the C¼O stretching peak isobserved in 1620–1550 cm�1 resulting from the inter-chainhydrogen bonds between O6H6 and CH2COOH and the hydrogenbonds between C¼O and water molecules [15]. The spectrum ofPOSS shows the characteristic peak of Si–O–Si at 1110 cm�1,which is a sharp peak different from that in random SiO2. 3200–2800 cm�1 corresponded to stretching of NH3

þ group originatedfrom intermolecular hydrogen bonds. However, in the spectrumof POSS–CMC hybrid hydrogel, the multiple peaks at 3200–2800 cm�1 disappeared, which means NH3

þ does not exist inPOSS–CMC hybrid hydrogel. The intensity of 1620–1550 cm�1

decreases and a new peak appears at 1730 cm�1 which correspondsto the stretching of COOH. It can be explained that the introduction ofPOSS-NH2 nanoparticles can possibly extend the distance betweenCMC macromolecules and weaken the hydrogen bonding and evenprevent the formation of hydrogen bonds between O6H6 andCH2COOH. Most of COOH groups in CMC did not form inter-chainhydrogen bonds, while intermolecular hydrogen bonds between –NH2 group in POSS-NH2 and partial –COOH group in CMC arepossibly formed. Fig. 4b shows the 1H solid-state magnetic anglespinning (MAS) NMR spectra of CMC and POSS/CMC hybrid hydrogel.Compared with CMC, the 1H MAS-NMR spectra of POSS/CMC hybridhydrogel became very wide, and the peak corresponding to theprotons of CMC shifts from d4.8 to d5.4. This obvious peak shift shouldbe attributed to the formation of hydrogen bonds between POSS andCMC in POSS/CMC hybrid hydrogel. Thus, we can deduce that POSS-NH2 nanoparticles can affect the inter-chain hydrogen bonds of CMCand form POSS–CMC block copolymer with CMC driven by hydrogen-bonding interaction between –NH2 group and –COOH group.

Fig. 5a and b is the AFM height and phase images of POSS/CMChybrid hydrogel, respectively. From Fig. 5a, we can observe that

pH=6~8

ation of POSS/CMC hybrid hydrogel.

Fig. 3. Photographs of the mixture of POSS aqueous dispersion (pH¼11) and CMC

aqueous solution (a), white POSS/CMC complex (b), and POSS/CMC hybrid

hydrogel (c).

Fig. 4. (a) FT-IR spectra of POSS-NH2, CMC and POSS/CMC hybrid hydrogel; (b) 1H-MAS-NMR spectra of CMC and POSS/CMC hybrid hydrogel.

500nm 500nm

90nm

Fig. 5. AFM height (a) and phase (b) images of POSS/CMC hybrid hydrogel.

H. Liu et al. / Materials Letters 90 (2013) 142–144144

the morphology of POSS-NH2 nanoparticles is different from thatshown in Fig. 1. As shown in the regions marked by dash circles,POSS-NH2 nanoparticles aggregate and result in a larger size.Moreover, two different phases could be obviously seen in Fig. 5b.The regions marked by dash circles corresponding to those inFig. 5a present the phase of POSS-NH2 nanoparticles which isdistinct from that around it. So, it can be proved that the POSS-NH2 nanoparticles have aggregated, and the nanoparticle aggre-gations can work as cross-linking agents during the formation ofPOSS–CMC hybrid hydrogel, which is consistent with our assump-tion shown in Fig. 2.

4. Conclusion

In this study, a novel POSS–CMC hybrid hydrogel was facilelyfabricated based on supermolecular interactions between POSS-NH2 and CMC. The method could also be utilized to fabricatehybrid hydrogels from other polymers with a similar structure toCMC or to fabricate double-network hybrid hydrogel employedCMC with other water-soluble polymers. The obtained hybridhydrogel containing POSS-NH2 should have improved mechanicalproperties, water uptake and dye adsorption capacity.

Acknowledgement

The authors would like to acknowledge the National NaturalScience Foundation of China (No. 51163003, No. 21204013), andGuangxi Natural Science Foundation (No. 2010GXNSFA013030,No. 2012GXNSFBA053153).

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