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Preparation and Properties of Water Soluble PAA PAS/SiO2HybridMaterialsKuo-Shien Huang a; Chiu Chun Lai b; Zong-Sian Lee a;Chien-Chung Hong aaDepartment of Polymer Materials, Kun Shan University, Yung Kang, Tainan, Taiwan bDepartment ofTextile Engineering, Chinese Culture University, Taipei, Taiwan

Online publication date: 13 April 2010

To cite this ArticleHuang, Kuo-Shien , Lai, Chiu Chun , Lee, Zong-Sian andHong, Chien-Chung(2010) 'Preparation andProperties of Water-Soluble PAA-PAS/SiO

2Hybrid Materials', Polymer-Plastics Technology and Engineering, 49: 5, 462

466

To link to this Article: DOI: 10.1080/03602550903532182URL: http://dx.doi.org/10.1080/03602550903532182

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Preparation and Properties of Water-Soluble PAAPAS/SiO2Hybrid Materials

Kuo-Shien Huang1, Chiu Chun Lai2, Zong-Sian Lee1, and Chien-Chung Hong1

1Department of Polymer Materials, Kun Shan University, Yung Kang, Tainan, Taiwan2Department of Textile Engineering, Chinese Culture University, Taipei, Taiwan

Copolymers with different weight ratios of AA/PAS (acrylicacid/poly(DL-aspartic acid)) were synthesized and blended withsol-gel precursors to prepare water-soluble PAAPAS/SiO2 inor-ganic/organic hybrid materials. The PAS polymer or its copolymerin PAAPAS/SiO2 formed hydrogen bonds with SiO2 and theamorphous structure of the hybrid material varied with the weight

ratio of PAA. The hybrid materials exhibited enhanced thermalresistance over the copolymer alone. All hybrid materials werewater-soluble and relatively insoluble in organic solvents.

Keywords Copolymerization; FTIR; Hybrid materials; NMR;Sol-gel

INTRODUCTION

The advancement in materials science, especially at thenanoscale, exhibits a broad influence over an array ofscientific disciplines. Nanometer-scale materials boastmany physical and chemical properties different from thoseof traditional materials, including higher tensile strength

and resilience[1,2]. A composite material consists of two ormore materials; when the dispersive grain diameter ofthe two constituents is within the nanometer range(1100 nm), the material may be referred to as a nanometercomposite material[3].

The sol-gel process has been widely used in the prep-aration of organic=inorganic hybrid materials. The basicprocess involves the hydrolysis of ester compounds ormetal alkoxides in an organic solvent by a catalytic amountof acid or base. When the resulting precursor sol is evenlydistributed, other elements, or dopants, may be added. Thesol is gradually gelated through a series of cross-linkingpolycondensation and agglomeration reactions. Depending

on the final application, the sol may be cast into a thin filmprior to complete gelation, or it may be dried and crushedinto a powder[47].

PAS (poly(DL-aspartic acid sodium)) and PAA (poly(acrylic acid)) are inexpensive, water-soluble, high-

molecular-weight materials. Previous reports haveindicated that PAA mixed with nanometer-scaledmaterials, such as fine grains of SiO2or TiO2, can increasethe strength of the polymer through the formation of aninorganic network structure, as well as form porous filmson the polymer surface to enhance water permeability[89].

In addition, PAS is a biodegradable and water-solublepolypeptide. Therefore, it can be easily applied as adispersing agent or raw material in detergents, biomedicine,and fabric processing applications[1013].

In this study, we examined the synthesis of water-solublecopolymers through the polymerization of different molarratios of AA and PAS. The copolymer was then blendedinto a precursor silica sol to make PAAPAS=SiO2hybridmaterials. The influence of polymer and copolymer compo-sition on the physical properties of the hybrid materialswas thoroughly investigated.

EXPERIMENTAL

MaterialsReagent grade acrylic acid (AA), ammonium peroxydi-

sulfate, hydrochloric acid, tetrahydrofuran, dimethylsulfoxide, and ethyl alcohol were supplied by Pharmaceuti-cal Corporation (Osaka, Japan). PAS (MW 3,000) wasobtained from Aldrich (Milwaukee, WI, USA). Isopropylalcohol (IPA), tetraethoxysilane (TEOS), and vinyl-triethoxysilane (VTES) were purchased from ACROS(Morris Plains, NJ, USA).

METHODS

Preparation of the PAAPAS Copolymer

A solution of AANa was prepared by neutralizing 60 gof AA with a NaOH solution (2.72g of NaOH dissolved in80 g of water), cooling to room temperature, and diluting1:1 with water. Five grams of PAS, 20 g of AANa, 2 gof VTES, 0.2 g of glycerin, and 0.3 g of ammonium persul-fate were placed into a single reaction vessel. Ingredientswere added one by one with the exception of the initiator.The reaction mixture was stirred, injected with helium for20 min, and heated to 40C.

Address correspondence to Kuo-Shien Huang, Department ofPolymer Materials, Kun Shan University, Yung Kang, Tainan,71003 Taiwan. E-mail: [email protected]

Polymer-Plastics Technology and Engineering, 49: 462466, 2010

Copyright# Taylor & Francis Group, LLC

ISSN: 0360-2559 print=1525-6111 online

DOI: 10.1080/03602550903532182

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At this point, the initiator was added and the polymer-ization proceeded for 1 h in a nitrogen atmosphere. Thetemperature was gradually increased to 70C, and the poly-merization process was allowed to continue for another 3 hto produce the PAAPAS copolymer. The incompletelyreacted oligomer and unreacted monomer were removedby ethanol extraction over 2 h. The resulting copolymer

was thoroughly dried, resulting in a fragmented solid.

Preparation of the PAAPAS/SiO2Complex Sol

TEOS was dissolved in 35mL of IPA and 3.6mL of0.05 N HNO3 and stirred for 60 min. The aforementionedcopolymer was mixed with the TEOS sol for 3 h at roomtemperature, thereby obtaining a mixed sol containingthe water-soluble PAAPAS=SiO2 complex. The complexsol was heated at 80C for 3 h in a vacuum oven to obtainthe hybrid material.

Analysis and Measurement

The molecular weight of the copolymers was measuredby gel permeation chromatography (GPC) in distilledwater at 20C. The Fourier transform infrared spectro-scopy=attenuated total reflection (FTIR=ATR) spectra ofthe samples were recorded with a Digilab FTS-200 spec-trometer equipped with a mercurycadmiumtelluride(MCT) detector (Bio-Rad, Hercules, CA, USA). A dia-mond crystal was used as the internal reflection element.Single beam spectra were the result of 64 scans, and thespectral resolution was 4 cm1.

13C- and 29Si-NMR spectra were measured with asolid-state Avance 400 nuclear magnetic resonance (NMR)spectrometer (Bruker Instruments, Billerica, MA, USA) at

100 MHz and 80MHz. Spectra were observed undercross-polarization with magic angle sample rotation andpower decoupling conditions using a 90 pulse and 4 s cycletime. X-ray powder diffraction (XRD) data were obtainedon a Bruker-AXS D8 equipped with parallel beam optics,Cu-target, scintillation counter, and rotating samplerchanger. Samples were run at 40kV, 100 mA, 260

theta=2theta, with a 0.01 step size and 5 s counting time.The surface morphology of the films was evaluated

under a scanning electron microscope (SEM; modelJSM 6400; JEOL, Tokyo, Japan). Samples were coatedwith gold to avoid surface charging. Thermal gravimetricanalyses (TGAs) of samples were measured using a

weight-loss analyzer (model 2200; DuPont, Wilmington,DE, USA). To determine the initial decompositiontemperature (IDT), a 510-mg sample was placed in analumina crucible and subjected to a thermal ramp from30 to 600C, at a heating rate of 20C min1 under anitrogen flow speed of 20 cc min1. Solubility measure-ments were conducted by stirring each sample individuallywith different solvents such as water, IPA, THF, DMSO,and ethanol.

RESULTS AND DISCUSSION

Molecular Weight of Copolymers

The molecular weight of each copolymer increased withthe weight percentage of AA, as shown in Table 1. This waslikely due to higher rates of AA homopolymerization andthe subsequent formation of long blocks of poly(AA),versus the heteropolymerization rate of AA with PAS.

FTIR

Figure 1 shows the FTIR spectra obtained for PAS, thecopolymer, and hybrid PAAPAS=SiO2 materials. Withthe exception of PAS, the other materials exhibitedclear >C=O absorption peaks at approximately 17101717 cm1, which correspond to the presence of AA. Allmaterials exhibited an absorption peak at 1402 cm1 dueto CH2C=O groups in PAS.

In addition, the copolymer spectra showed significantdifferences in the position of the OH, COONa, andCOOH absorption peaks relative to the corresponding

TABLE 1Molecular weight of PAA=PAS copolymers

Properties

Materials1 Mw Mn Mw=Mn

1=4 16,200 12,320 1.315

2=3 21,710 14,600 1.4872.5=2.5 24,050 15,950 1.5084=1 32,400 20,070 1.614

1Copolymers synthesized from different molar ratios of AA=PAS.

FIG. 1. FTIR spectra of copolymer and hybrid materials ((A) PAS,weight ratio of PAA=PAS: (B) 2=3 (TEOS 0%), (C) 2=3 (TEOS 2%),(D) 2=3 (TEOS 6%)).

PROPERTIES OF WATER-SOLUBLE PAA-PAS/SiO2 HYBRID MATERIALS 463


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peak positions in the hybrid material. The OH absorp-

tion peaks in the copolymer spectra shifted from3422 cm1 to 3262 or 3300cm1. The >C=O absorptionpeak also shifted to a lower frequency from 1717 cm1 toabout 1710 or 1700 cm1. These shifts suggest that hydro-gen bonds were formed between the COONa or COOHgroups in the copolymer and SiO2 groups in the sol-gelcomponent of the hybrid material.

With increasing weight ratios of PAS, as shown inFigure 2, the OH absorption peak shifted to between3000 and 3600 cm1. This was likely due to intramolecularhydrogen bonding. However, as discussed above, signifi-cant hydrogen bonding with the SiO2matrix was observedat higher proportions of PAA. Compared to the OH

absorption peak at between 3000 and 3600cm1

inFigure 2B or 2C, the >C=O absorption at 1710 cm1 inFigure 2D is not easily recognized.

NMR13

C-NMR spectra, obtained from all hybrid materialsdescribed herein, are presented in Figure 3. With increasinglevels of PAS, the peak at 120130 ppm became moreapparent, corresponding to the ring structure formed byintramolecular hydrogen bonding in PAS. Conversely, withincreasing proportions of PAA, the CO peak at approxi-mately 6065ppm increased in intensity. The addition ofTEOS had little influence on the >C=O peak in hybridmaterials. This implies that with the addition of 2% TEOS,the hydrogen-bonding capacity of the resulting SiO2 wassaturated by interactions with the copolymer.

29Si-NMR spectra of copolymer and hybrid materialsare shown in Figure 4. The peak corresponding to

SiOSi bonds in VTES shifted from 72.02 to80 ppm in Figure 4A, to between 61.33 and 77.76 ppmin Figure 4B, and further to between 77.53 and

FIG. 2. FTIR spectra of hybrid materials (weight ratio of AA=PAS:

(A) 1=4, (B) 2=3, (C) 2.5=2.5, (D) 4=1).

FIG. 3. 13C-NMR spectra of hybrid materials (weight ratio of AA=PAS:(A) 1=4 (2% TEOS), (B) 2.5=2.5 (2% TEOS), (C) 4=1 (2% TEOS), (D)2.5=2.5 (6% TEOS)).

FIG. 4. 29Si-NMR spectra of copolymer and hybrid materials (weightratio of AA=PAS: (A) 2=3 (0% TEOS), (B) 2=3 (2% TEOS), (C) 2=3

(6% TEOS)).

FIG. 5. X-ray diffraction data of copolymer and hybrid materials ((A)PAS, weight ratio of AA=PAS: (B) 1=4, (C) 2.5=2.5, (D) 4=1, (E)2.5=2.5 (hybrid materials, 2% TEOS)).

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81.73ppm in Figure 4C. Secondly, all there is the absorp-tion Feng of Q4(112 ppm) in Figure 4B and 4C, this pre-sents in the hydrolytic process for TEOS of special absorbpeak. These results suggest that the addition of more TEOSleads to a reduction in the Si content in VTES as a result ofhydrogen bonding between TEOS and the copolymer.

XRD and SEM

XRD data acquired on several different copolymers andhybrid materials are shown in Figure 5. With the exceptionof PAS, a diffraction peak was observed at 2h 9, corre-sponding to PAA. With increasing proportions of PAA,the diffraction peak at 2h 20 became more apparent.The addition of TEOS to produce the hybrid materialresulted in diffraction peaks at 31, 34, and 38, corre-sponding to diffraction from SiO2 generated during thesol-gel process. The little changes in the XRD data withincreasing proportions of AA indicate that the composition

of the copolymer, in addition to the presence of TEOS, wasnot influenced the amorphous of the hybrid materials.

SEM images of the hybrid materials are shown inFigure 6. The surface morphology varied with TEOS con-centration, with the surface of the copolymer alone beingnearly smooth. With the addition of 2% TEOS, minisculegrains of SiO2 appeared on the surface. At 6% TEOS, thegrains became more compact and showed evidence of con-densation due to increased SiO2 content and increasing

hydrophilicity of the hybrid material. The possibility existsthat water, released as a product of polymerization andsol-gel condensation reactions, may not have been com-pletely removed during drying.

TGA

Table 2 shows the thermal decomposition data obtainedon each copolymer and hybrid material. In the table, Arefers to the PAAPAS copolymer, B corresponds to the

FIG. 6. SEM images of hybrid materials (TEOS concentration: (A) 0%, (B) 2%, (C) 6%). Weight ratio of AA=PAS is 2=3.

TABLE 2Thermal properties of copolymer and hybrid materials

Properties

Samples T101 (C) First Td (C) Second Td (C) Chard yield2 (%)

PAA-PAS (A) 206 295 382 46.9PAA-PAS-VTES (B) 212 320 397 51.2PAA-PAS-VTES=SiO2 (C) 252 331 410 52.3

1Temperature of lost 10% weight.2Heated to 600C.

PROPERTIES OF WATER-SOLUBLE PAA-PAS/SiO2 HYBRID MATERIALS 465


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PAAPASVTES copolymer, and C refers to the PAAPASVTES=SiO2hybrid material. Relative to A, a greaterresidual mass was obtained with B due to the reactionbetween VTES and copolymer, and the incorporation ofSi into the copolymer structure. B also exhibited greaterthermal stability, with higher temperatures at 10% weightloss (T10), first stage Td, and second stage Td. This was

likely due to the formation of a silica network structurein the PAAPASVTES hybrid material. The greatest ther-mal resistance was observed for C and can be attributed tothe formation of an even larger silica network.

Solubility

Table 3 shows the solubility of PAAPAS=SiO2 hybridmaterials produced with different weight ratios of AANa=PAS and varying amounts of TEOS in different sol-vents. The PAAPAS=SiO2 hybrids were soluble in waterbut not in organic solvents. This was due primarily to thewater-solubility of PAANa, which is insoluble innonpolar-to-polar esters, ethylene glycols, and mostorganic solvents, including THF, IPA, and ethyl alco-hol[14]. Despite the tight mesh structure of the extensivesilica network in PAAPAS=SiO2hybrids, water-solubility

was maintained on account of the highly water-solublePAANa.

CONCLUSIONS

This report describes the preparation and physicalproperties of PAAPAS=SiO2 hybrid materials. FTIRanalyses showed that the PAS polymer or copolymer in

PAAPAS=SiO2 hybrid materials was subjected to hydro-gen bonding with SiO2. The amorphous structure of thePAAPAS copolymers were not varied considerably withincreasing weight ratios of PAA. The hybrid materials,when synthesized with appropriate concentrations ofTEOS, exhibited enhanced thermal resistance over thecopolymer alone. All hybrid materials described hereinwere water-soluble and relatively insoluble in organicsolvents.

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TABLE 3Solubility of hybrid materials

Weightratio of

AA=PAS

Conc.of TEOS

(%)

Solvents

H2O IPA THF DMSO Ethanol

1=4 0 1 2 1 2 4 6

2.5=2.5 0 1 2 4 6

4=1 0 1 2

4

6

1: soluble; 2: insoluble.

466 K.-S. HUANG ET AL.

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