Polyaspartic acid based superabsorbent polymers

European Polymer Journal 59 (2014) 363–376

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European Polymer Journal

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Review article

Polyaspartic acid based superabsorbent polymers

http://dx.doi.org/10.1016/j.eurpolymj.2014.07.0430014-3057/� 2014 Elsevier Ltd. All rights reserved.

Abbreviations: SAPs, superabsorbent polymers; PSI, polysuccinimide; PASP, polyaspartic acid; PASP-Na, polyaspartic acid sodium sadiaminobutane; DMF, dimethylformamide; DDA-PASP-Na, sodium dodecylamine modified polyaspartic acid; MW, molecular weight; IPN, interpepolymers; PNIPAAm, poly N-isopropylacrylamide; Semi-IPN, semi-interpenetrating polymers; CYS, cystamine.⇑ Corresponding author.

E-mail addresses: [email protected] (S. Sharma), [email protected] (A. Dua), [email protected] (A. Malik).

Shilpa Sharma, Amita Dua, Amita Malik ⇑Department of Chemistry, Dyal Singh College, University Of Delhi, Lodhi Road, New Delhi 110003, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 April 2014Received in revised form 12 July 2014Accepted 30 July 2014Available online 17 August 2014

Keywords:SuperabsorbentPolyaspartic acidCross-linkingInterpenetrating polymersSemi-interpenetrating polymersGrafted polymers

Superabsorbent polymers are widely used in many applications such as disposable diapers,feminine napkins, soil for agriculture and horticulture, gel actuators, water-blocking tapes,drug delivery systems, absorbent pads and other biomedical applications. Most of thesesuperabsorbents are non-biodegradable and thus increasing burden on the earth. Polymerscientist and chemists are looking for environmental friendly solutions. Polyaspartic acidpolymers have been reported to possess biodegradable properties. These polymers havebeen developed mainly as polyelectrolyte. However, this review compiles the work carriedon developing polyaspartic acid based superabsorbent polymers. The review covers syn-thetic methodology, characterization of these polymers by different techniques, differenttypes of polymer prepared using polyaspartic polymers which include co-polymers, graftedpolymers, interpenetrating and semi-interpenetrating polymers are covered. The biode-gradability studies carried out on the superabsorbent polymers are also discussed.

� 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction: absorbing, hydrogels & superabsorbent polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3642. Synthesis of polyaspartic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

2.1. Methodology of synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
2.1.1. Condensation product/dehydration to yield polysuccinimide and anhydro-polyaspartic acid . . . . . . . . . . . . . . 3672.1.2. Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3692.1.3. Precipitation of PASP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
2.2. Characterization of polyaspartic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3692.3. Properties of polyaspartic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

3. Superabsorbent polymers based on polyaspartic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
3.1. Co-polymers and cross-linked polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3703.2. Interpenetrating polymers & semi-interpenetrating polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3733.3. Grafted polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
lt; DAB,netrating

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364 S. Sharma et al. / European Polymer Journal 59 (2014) 363–376

4. Analysis of superabsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3745. Biodegradability of superabsorbent polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3746. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

1. Introduction: absorbing, hydrogels & superabsorbentpolymers

Polyaspartic acid (PASP) is a representative product innew-era of green chemistry. Polyaspartic acid (PASP), apoly (amino acid), is a promising water-soluble and biode-gradable polymer. Its biodegradability makes it particu-larly valuable from the point of view of environmentalacceptability and waste disposal. PASP is used widely asmineral scale inhibitor [such as CaSO4, BaSO4, CaCO3 andCa3 (PO4)2] in water treatment applications and as dispers-ing agents in detergents, paints and papermaking pro-cesses. Some studies indicate that as a scale inhibitor, thescale inhibiting capability of PASP is poorer than that ofphosphorus-containing inhibitor [1].

Polyaspartic acid polymers fall under the category ofwater soluble polymers with the property of biodegrad-ability in comparison to other conventional water solublepolymers. Water soluble polymers, such as poly(vinyl alco-hol), poly(ethylene glycol), and poly(acrylic acid), arewidely used in cosmetics, paper additives, dispersants,and detergent builders, but they are hardly recovered orcollected after use. Of concern is the diffusion and accumu-lation of such non-biodegradable water-soluble polymersin the earth’s environment after their release. Polymerswith carboxylic acid groups are one of the most importantwater-soluble polymers; e.g., poly (acrylic acid) has beenuse as detergent builders scale inhibitors and flocculantsand is directly released into the earth’s environment. How-ever, they are hardly biodegradable, except for their oligo-mers, which will possibly produce much damage to theenvironment [2–4].

Other applications include use of PASP as hydrogelmaterials. The hydrogel materials have been worked uponto prepare superabsorbent polymers. These superabsor-bent polymers (SAP) can absorb large amount of waterand the waste is hardly removed even under pressurebecause of its excellent water absorbing properties, SAPshave an unlimited number of applications. Hydrophilicnetworks that are responsive to some molecules, such asglucose or antigens can be used as biosensors as well asin drug systems, disposable sanitary products (for exam-ple, diapers, incontinence articles, feminine hygiene prod-ucts, airlaids and absorbent dressings), and in controlledrelease of drugs. Superabsorbent polymers have the abilityto sense environmental changes, like changes of pH, tem-perature, etc. [5,6]. In the field of medicine these havefound applications in the area of drug delivery systems[7], wound closure, healing products [7], scaffolds in tissueengineering and surgical implant devices [8–12].

To qualify as a superabsorbent, a dry material shouldspontaneously imbibe about twenty times or more of itsown weight of aqueous fluid. Moreover, the swollen mate-rial should retain its original shape, i.e. a swollen bead isstill recognizable as a bead, a swollen fiber as a fiber, anda swollen film as a film [13]. The ‘hydrogel’ resulting fromthe transformation of the dry superabsorbent must havesufficient physical integrity to resist flow and fusion withneighboring particles. When exposed to an excess of water,true superabsorbent hydrogel particles swell to their equi-librium volume and do not dissolve. This phenomenon isdue to the electrostatic repulsion between the charges onthe polymer chains and the difference in osmotic pressurebetween the inside and outside of the gels. The superabsor-bent polymers (SAP) are categorized as hydrogels whichcan absorb aqueous solutions via hydrogen bonding withthe water molecules. The important properties of superab-sorbent polymers are the swelling capacity and the elasticmodulus of the swollen cross-linked hydrogel. These twoproperties of the swollen cross-linked hydrogel are relatedto the cross-link density of the network modulus whichmeans that swelling capacity decreases with increasingcrosslink density.

These ultrahigh absorbing materials can imbibe de-ion-ized water as high as 1000–100,000% (10–1000 g/g)whereas the absorption capacity of common hydrogels isnot more than 100% (1 g/g). Visual and schematic illustra-tions of an acrylic – based anionic superabsorbent hydrogelin the dry and water swollen states are given in Fig. 1below [13].

The hygroscopic materials are usually categorizedinto two main classes based on the major mechanismsof water absorption, i.e., chemical and physical absorp-tions. Chemical absorbers (e.g., metal hydrides) catchwater via chemical reaction converting their entire nat-ure. Physical absorbers imbibe water via four mainmechanisms;

(i) reversible changes of their crystal structure (e.g., sil-ica gel and anhydrous inorganic salts);

(ii) physical entrapment of water via capillary forces intheir macro-porous structure (e.g., soft polyurethanesponge);

(iii) a combination of the mechanism (ii) and hydrationof functional groups (e.g., tissue paper);

(iv) the mechanism which may be anticipated by combi-nation of mechanisms of (ii) and (iii) and essentiallydissolution and thermodynamically favored expan-sion of the macromolecular chains limited bycross-linkages.

Fig. 1. Phenomena of super-absorbency [13]. {Column fitting image-2}.

Table 1Water absorptiveness of some common absorbent materials [13].

Absorbent materials Water absorbency (wt%)

Filter paper 180Tissue paper 400Soft polyurethane sponge 1050Wood pulp fluff 1200Cotton ball 1890Agricultural SAP 2020

S. Sharma et al. / European Polymer Journal 59 (2014) 363–376 365

Superabsorbent polymer materials (SAPs) fit in the lat-ter category, yet, they are organic materials with enormouscapability of water absorption. SAPs as hydrogels canabsorb and retain extraordinary large amounts of wateror aqueous solution relative to their own mass. The SAPparticle shape (granule, fiber, film, etc.) has to be basicallypreserved after water absorption and swelling, i.e., theswollen gel strength should be high enough to prevent aloosening, mushy, or slimy state. This is a major practicalfeature that discriminates SAPs from other hydrogels.

Traditional absorbent materials (such as tissue papersand polyurethane foams) unlike SAPs, will lose most oftheir absorbed water when they are squeezed. Table 1gives the water absorption in some of the common absor-bents but these lose their strength on water absorption.

Commercially, SAPs are majorly produced with acrylicacid as a key component, which are non-biodegradable[14]. Thus, SAP producers have shifted their attention to

the development of biodegradable SAPs in order to suitthe growing demand for biodegradable products.

Resembling the hydrogel family, the SAPs may be cate-gorized to four groups on the basis of presence or absenceof electrical charge located in the cross-linked chains:

(a) Non-ionic.(b) Ionic (including anionic and cationic).(c) Amphoteric electrolyte (ampholytic) containing

both acidic and basic groups.(d) Zwitterionic (polybetaines) containing both anionic

and cationic groups in each structural repeating unit[15].

For example, the majority of commercial SAP hydrogelsare anionic. However, according to the sources, SAPs areoften divided into two main classes; i.e., synthetic (petro-chemical-based) and natural. The latter can be divided intotwo main groups, i.e., the hydrogels based on polysaccha-rides and others based on polypeptides (proteins).

Polypeptides based SAPs have been explored to someextent till 2008. Commercially PASP based polymers havebeen studied and commercialized for antiscaling purpose.Soya based polymers have also been studied. Certain pat-ents have also been filed on developing superabsorbentpolymers which have been discussed subsequently.

Several reviews based on PASP have been publishedtaking care of different aspects [15–18]. Reviews on pol-yaspartic acid as a antiscaling agent and as a versatilechemical have been published [1,16]. Ichikawa and

Fig. 2. Synthesis of different polymers based on polyaspartic acid [22]. {Column fitting image-1.5}.

Fig. 3. Synthesis of low molecular weight PASP [23]. {Column fitting image-1.5}.



Nakajima have reviewed the super-absorptive materialsbased on polysaccharides and proteins [19]. Less workhas been reported with polypeptides [20]. This article hasbeen written to compile the work carried out on develop-ing superabsorbent polymers based on polyaspartic acid.

2. Synthesis of polyaspartic acid

Homo-poly (amino acids) of poly(aspartic acid)s,poly(L-lysine) and poly(c-glutamic acid)s have also beenemployed to prepare SAP materials [15]. In 1999, Rohmand Haas Company’s researchers reported lightly cross-linked high MW sodium polyaspartates with superabsorb-ing, pH- and electrolyte-responsive properties [21].

2.1. Methodology of synthesis

PASP is classified into linear or cross-linked structures.Commercial non-crosslinked PASP has a low molecularweight and is used as a crop growing accelerant, whereasthe cross-linked PASP and its derivatives are applied assuperabsorbent resin and as drug delivery vehicle. In1950, Donlar Company started to study linear polymer,the commercial application of cross-linked PASP isreported upon in detail. Modification of PASP has been car-ried out by co-polymerization, crosslinking and radiationpolymerization as shown Fig. 2 below [22].

The synthesis of polyaspartic acid (PASP) involves thefollowing steps:

(a) Condensation product/dehydration to yield polysuc-cinimide & anhydro-polyaspartic acid.

(b) Hydrolysis of the resulting product to yield Polyas-partic acid (PASP).

(c) Precipitation

2.1.1. Condensation product/dehydration to yieldpolysuccinimide and anhydro-polyaspartic acid

Number of methods for preparation of polyaspartic acidhas been disclosed in literature and patents.

US patent describes a method for making polyasparticacid from maleic anhydride and ammonia, by reactingthese constituents in a 1:1–1.5 M ratio by raising the tem-perature to 120–150 �C over a period of 4–6 h and main-taining it for 0–2 h. It has also been observed that

Table 2Reaction conditions for polycondensation of Aspartic acid [24].

S. No. Solvent Acid catalyst

1. Toluene 85% Phosphoric acid2. Mesitylene 85% Phosphoric acid3. DMF 85% Phosphoric acid4. Sulfolane 85% Phosphoric acid5. Toluene/sulfolane 85% Phosphoric acid6. Mesitylene/DMF 85% Phosphoric acid7. Mesitylene/NMP 85% Phosphoric acid8. Diethylbenzene/sulfolane 85% Phosphoric acid9. Mesitylene/sulfolane 85% Phosphoric acid

10. Trichloroacetic acid11. p-Toluene sulfonic acid12. Sulfuric acid

temperatures above 140 �C result in elimination of CO2

thus causing degradation of the material. The molecularweight is in the range 1000–4000 [23]. Fig. 3 gives thereactions involved.

Harada et al. have reported the preparation from poly-succinimide (PSI). PSI has been prepared with and withoutcatalyst from aspartic acid. The catalyst has been reportedto be phosphoric acid. The reaction temperatures men-tioned is 170 �C. The PSI prepared is further hydrolyzedwith alkali to prepare polyaspartic acid. No molecularweight range has been given [24].

Table 2 gives the results of acid catalyzed reaction ofpolycondensation of aspartic acid, by removal of waterfrom the reaction mixture. The reaction in sulfolane sys-tem was carried out by flashing nitrogen gas through thereaction mixture at 160 �C.

Another method to produce polyaspartic acid consistsof heating the aspartic acid in a fluidized bed reactor at221 �C for a period of 3–6h in a nitrogen atmosphere fol-lowed by alkaline hydrolysis [25].

Synthesis has also been reported by heating asparticacid to 200 �C in vacuum for a period of 120 h or boilingtetralin over a period of 100 h followed by alkaline hydro-lysis. Kovas showed that the thermal polymerization ofaspartic acid proceeds via polysuccinimide intermediate[26].

Polyaspartic acid has also been reported to be preparedfrom acid ammonium salts of malic, fumaric or maleic acidby dry distillation in the presence of nitric acid and hydro-chloric acid. Maleic anhydride and ammonia have beencondensed together to produce high molecular weightpolysuccinimide and polyaspartic acid by carrying outreactions above 150 �C [27].

Rotary driers, plate driers have been used to preparepolyaspartic acid from L-aspartic acid by heating at 440 �Fand 350 �F, respectively. The yield reported in case of latteris 80% [28].

US patent 5,296,578 covers the methodology to producepolysuccinimide and polyaspartic acid from maleic anhy-dride and ammonia by heating at 170 �C to yield 70% ormore of the polymer. High temperatures have been usedto carry out reaction between ammonia and fumaric acid,malic acid and maleic acid [29]. The reaction has reportedto have carried out at 190–350 �C for less than 4 h and at160 �C in twin screw extruder.

Reaction temperature (�C) Yield (%) Mw (Mw/Mn)

110–111 0 –164–166 77 24,800 (1.5)152–153 0 –158–159 89 19,000 (1.5)110–111 0 –150–152 59 12,900 (1.2)160–162 96 24,500 (1.6)176–178 96 49,300 (1.9)160–162 96 64,300 (1.9)164–166 0 –160–162 89 27,000 (1.5)160–162 96 27,900 (1.6)

Fig. 4. Polyaspartic acid and their derivatives. {Column fitting image-1.5}.

Table 3Parameters of PSI synthesis & properties of PASP-Na obtained by PSI hydrolysis [34].

Sample Flask volume Monomer:solvent weight ratio Non-solvent Yield of PSI (%) MW of PASPNa Polydispersity index of PASPNa

1 250 1:5 Acetone 45 6200 4.092 100 Acetone 60 18,100 2.173 Acetone:water 77 27,200 1.844 Methanol 67 14,100 2.675 Methanol:water 73 44,800 1.136 Isopropanol 69 40,400 1.227 Isopropanol:water 80 20,900 2.128 1:4 Methanol 88 13,700 2.239 1:3 99 44,800 1.14

10 1:2 96 15,300 2.22

MW – molecular weight corresponds to maximum of GPC peak.

Fig. 5. Different bonding in the polymer [16]. {Column fitting image-1}.

Fig. 6. NMR of PASP. {Column fitting image-1}.


Some of the reactions are discussed below in Fig. 4which can be used to prepare the polyaspartic acid andtheir derivatives. These reactions have been covered inthe Wikipedia.

D,L-Aspartic acid has also been reported to be preparedfrom maleic acid and ammonia at normal and at elevatedtemperature has been reported in German patentDD126075, followed by addition of N-formylacetamide at120–130 �C followed by HCl yielded the above product[30]. In place of HCl even metal hydroxides or ammoniumhydroxide can be used as reported by Boemke [31]. Themethods of producing D,L-aspartic acid have several disad-vantages. There is a need for more efficient methods.

Urea in its molten state can combine with malic, maleicand fumaric acid in a sealed tube for 12 h at 140–200 �C.This can be hydrolyzed with HCl [32].

Microwave radiations have also been reported to pre-pare PASP using maleic anhydride. This reaction involvesvarious steps. This includes hydrolysis of maleic anhydrideto yield maleic acid. This is further treated with ammoniaforming ammonium salts at 100 �C. This then forms anhy-dro-polyaspartic acid, on heating and removal of waterwhich on further hydrolysis gives PASP [33,34].

The molecular weight of the polymer has been deter-mined. The results are discussed in Table 3 below.


2.1.2. HydrolysisHydrolysis of the condensation product results in for-

mation of a-polyaspartic acid and b-polyaspartic acid. Bothalkali and acid can be used to carry out hydrolysis. Theresulting polymer contains not only D and L isomer, but alsoa and b peptide bonds in the main chain. The degree ofhydrolysis can be controlled by pH. The ratio of a and bbond in polyaspartic acid depends on the pH at whichhydrolysis is carried out. The lower the pH of the medium,the higher the a-bond is formed in the product. The ratio isunaffected by the ionic strength and temperature. Partialhydrolysis of polysuccinimde (PSI) can also be carriedout. This results in formation of sodium polyaspartate.The different bonding in the polymer is depicted belowFig. 5.

2.1.3. Precipitation of PASPThe developed sodium polyaspartate can be precipi-

tated as PASP which is uncross-linked and this can thenbe cross-linked with different compounds to achieve dif-ferent polymers. The precipitation is generally carriedusing methanol as mentioned in different papers for pre-paring the superabsorbent polymers [35,36]. Other sol-vents used for precipitation are acetone; acetone–water;isopropanol; isopropanol–water to mention a few [34].

2.2. Characterization of polyaspartic acid

The developed polymers have been characterized fortheir molecular weight using different techniques, viz. gelpermeation chromatography and viscosity [19,35]. Differ-ent molecular weights have been reported in differentpapers and patents.

Analysis of the polymers has been carried out usingInfrared spectroscopy, proton NMR and carbon13 NMR.Infrared spectroscopy has shown characteristic absorptionpeaks at 3300 cm�1, 3080 cm�1 for NH stretching,1710 cm�1 for CO carboxyl group, 1650 cm�1 for asparticacid and 1550 cm�1 for succinimide bond [16,19].

Naturally occurring PASP consists of linked aspartic acidunits up to a chain length of 50 subunits, these occur aspart of polyanionic proteins rich in aspartic acid and phos-phor-serine. In contrast to the linkage of aspartic acid innative materials, PASPs obtained by a thermal polymeriza-

Fig. 7. N-terminal group (1) and N-branching site (2) of polyaspartic acidproduced by thermal condensation [16]. {Column fitting image-1}.

tion process contain a- and b-linked moieties in a constantmolar ratio of 30:70 in a random distribution over thepolymer chain according to determinations by nuclearmagnetic resonance (NMR) spectroscopy [16,18].

The molecular structure of PASP has mainly beeninvestigated using spectroscopic methods [18]. In particu-lar, NMR was the most often used technique to obtaininformation about the structural composition of PASP.Alternative methods such as infrared spectroscopy alsoproved valuable in the analysis of PASP and/or itscopolymers.

PASP can be identified from its NMR spectrum (Fig. 6).Due to the polymer structure of PASP, there is absence ofsharp peaks, and the signals appear in the form of broadermultiplets. It is also possible to identify different copoly-mer units in the molecule. They were able to determinethe ratio of a- and b-amide units in the main chain by inte-grating the separated methylene signals in the proton-NMR spectrum.

13C NMR spectroscopy has also been used to provideinformation about the molecular structure of PASP. The ratioof a- and b-amide units using the methylene signals in the13C NMR spectrum, and then analyzed the amide bondsequence using the amide carbonyl signals. The distributionof the a- and b-bonds was observed to be random. 15N NMRspectroscopy was further used to obtain greater detail of thePASP structure, and especially the three-dimensional struc-ture of PASPs of different origin, is that of 15N NMR spectros-copy, both solid state and in solution. The biodegradation ofPASP is dependent upon its process of manufacture. It hasbeen suggested that the different biodegradation behaviorsof various PASPs might be assigned to the different tertiarystructures, and especially to branching sites within the poly-mer. By using 15N NMR spectroscopy of 15N-enriched modelcompounds, it was possible to visualize the N-terminalgroups and N-branching sites (Fig. 7). It could also be shownin this way that maleic imide terminal groups (1) and amidebranching sites (2) have an unfavorable influence on biodeg-radation [16]. The structure of 1 and 2 are given in Fig. 7.

Thermal analysis studies have also been carried out. Ithas been reported that PSI has no glass transition

Fig. 8. Different linkages possible in the polymers [16]. {Column fittingimage-1}.


temperature and decomposes at 424 �C. These studies havebeen reported by other authors also. Thermal stability ofPASP prepared by microwave radiation has also beenreported. The Tmax for PASP synthesized by conventionalmethod is 295 �C while that with microwave is 394 �Cwithout catalyst and 402 �C with catalyst [37].

2.3. Properties of polyaspartic acid

PASP is a water miscible polymer and possess biode-gradability as reported by different workers [1,16–18].These act as polyelectrolyte with anionic character due tothe presence of carboxylic acid groups. Naturally occurringPASP fragments consists of a,-linked L-aspartic acid. In con-trast, the repeating unit of synthetic polyaspartic acid mayexist in four different isomeric forms depending on the ste-reochemistry of starting material (D- and L-aspartic acid)and synthetic procedure leading to a and b links [16].These linkages are shown in Fig. 8 below.

3. Superabsorbent polymers based on polyaspartic acid

3.1. Co-polymers and cross-linked polymers

Homopolymers and co-polymers of amino-acids with awide range of properties from hydrophilic to hydrophobic,neutral to ionic and linear to random coil can be synthe-sized by modification of the above polymers.

The co-polymers of polyaspartic acid have found appli-cation as scale inhibitors, reverse osmosis membrane, indetergent and as inhibitors of dental plaque. Co-polymersof polyaspartic acid have been reported with polycarboxy-lic acid like citric acid, succinic acid, etc. [16]. Copolymer-ization of polyaspartic acid and lactic acid have beencarried out under microwave field using a D,L aspartic acidand racemic mixture of lactic acid in propylene carbonatesolution. The formation of this copolymer was carried outusing both techniques of bulk and solution polymerizationin the presence and in the absence of catalyst (o-phospho-ric acid). The presence of catalyst gave high yield of theproduct as compared to that obtained in the absence ofcatalyst [38,39].

PASP resin is prepared from PSI by chemical cross-link-ing methods. The maximum swelling ratio reported is983.3 g/g in patents [40–42].

The resin preparation involves different methodologiesfrom PSI:

Table 4Techniques for synthesis of PASP.

Preparation Technique Steps

By using organic solvents (cross-linking &hydrolysis of PSI)

PSI + DMF + water

PSI is hydrolyzed with NaOH

By using aqueous solvent (cross-linking &hydrolysis of PSI)

PSI + aqueous solvent

Cross-linked PSI ishydrolyzed with NaOH

(a) Crosslinking and hydrolysis or(b) Hydrolysis and then crosslinking.

Resin is dried and the procedure of drying also influ-ences the chemical structure of the resin. The absorbencyof the gel is affected. IR studies and SEM studies haveshown the differences caused due to drying. Lower cross-links produces better gel strength and absorption thanthe highly cross-linked structure [43].

Solvent free synthesis has been reported by Wang et al.Conventional synthesis of PSI involved use of DMF, DMSOwhich were then generated as waste. Combined cross-link-ing and hydrolysis has also been reported with taurinewhich makes the process environmentally and economi-cally attractive [44].

The techniques involved in the preparation of cross-linked polyaspartic acid are given in Table 4.

Zrinya et al. synthesized cross-linked polyaspartic acidusing two different cross-linking agents like diaminobu-tane (DAB) and cystamine (CYS). The formation of hydrogelexhibited different swelling properties depending on themolar ratio of each cross-linker. The cross-link densityaffects the swelling equilibrium which is explained onthe basis of thermodynamic concepts. The double bondcross-linked polyaspartic acid was synthesized by cross-linking the PSI chains using DAB and CYS. DAB resultedin stable cross-links, whereas CYS contain disulfide bonds,which was significantly weaker than C–C bonds. The cleav-age of disulfide linkages increased the volume of gel anddecreased the elastic modulus which was found to bestrongly dependent on the relative amounts of the twocross-linkers. The swelling increased by 50% after cleavageof the disulfide bonds [45].

Polyaspartic acid hydrogel have been produced bycross-linking reactions with functionalized derivative andde-functionalized derivative of PASP [45,46]. Gamma radi-ation has also been used for cross-linking [47]. The condi-tions and dosage used are discussed in Table 5 below.

PASP and polyethylene glycol diepoxide cross-linkedgel powder was prepared by thermal curing reaction fol-lowed by freeze drying [48].

Polyvinyl alcohol and L-aspartic acid copolymers (PVA-co-Asp), have been prepared by solution polycondensationprocedure using manganese acetate as a catalyst. The poly-mers prepared have been analyzed by IR and TGA tech-niques [49].

US patent 5,478,919 explains the synthesis of asparticacid copolymers. Copolycondensation of aspartic acid

Conditions Solvents Properties

Stirring for48 h

DMF + 1,6-hexamethylenediamine

Cross-linked PSIobtained

Vacuumdried

Precipitated by using methanol PASP hydrogel isobtained

Stirring for48 h

Aqueous solutions of dissolvedcross-linker & base

Cross-linked PSI

Vacuumdried

Precipitated by using methanol PASP hydrogel isobtained

Table 5Specific water content of poly(aspartic acid) hydrogels [47].

PASP conc. (wt/vol%) cb Irradiation dosage (kGy) Specific water content (g/g)b

pH = 3 pH = 7.5 pH = 13

5 32 NGa NG 10005 56 NG NG 32005 63 NG 1100 31005 100 NG 820 2500

10 55 NG NG NG10 63 NG 3400 NG

a NG = no gel formed.b Specific water content = weight of absorbed water/weight of dry hydrogel.

Table 6Characteristic of superabsorbent polymers developed by cross-linking followed by hydrolysis of PSI.

Steps Conditions Solvents Properties

Crosslinked PSI with hydrazine &Triethylamine

Heated to 50 �C + stirring DMF + crosslinker hydrazine + stirredfor 1 h

Crosslinked PSI obtainedDMF is removed by rotaryevaporator

PSI is hydrolyzed with NaOH at pH 10.8–11.0

Vacuum dried Solvent used for precipitation withmethanol

Gel obtained. 22.5 g/g ofsaline

PSI + 10%NaOH Vacuum dried Washed with methanol 19 g/g of salinePSI + NaOH powder + N2 gas Vacuum dried Washed with methanol 11 g/g of saline

Table 7Effect of Molecular weight of PSI on the characteristic of superabsorbent polymer [51,52].

MW ofPSI

Amines Hydrolysis Properties

38,000 Stirred in DMF & heated at 50 �C with hydrazine 10% NaOH 22.5 g/g of salineStirring overnight with terephthaloyl chloride in DMF

66,000 Stirred in DMF at RT for 2.5 h with hydrazine Solid obtained dried in vacuum treated at50 �C in water with 10% NaOH at pH 11

51 g/g of saline & 138 g/gwater

Terephthaloyl chloride in DMFHeated for 1.5 h at 26 �C. DMF removed under vacuum Treated with methanol

11,000 Hydrazine Treated at 50 �C in water with 10% NaOH Low molecular weight SAP.2 g/g of saline

Solution concentrated in vacuum &treated with methanol

96,000 DMF (20 parts) with lysine methyl ester (1.8 parts) stirred atRT for 5 h then kept for 50 h. Stirred for 2 h in methanol

Methanol + water + NaOH + dilute HCl 480 times water absorption& 61 times saline absorption

1,55,000 Lysine methyl ester + trimethyl amine Water-550 times & saline 63times

1,55,000 Lysine methyl ester + trimethyl amine Water + NaOH mixture in acetone Water-180 timesSaline-38 times

96,000 Lysine methyl ester dihydrochloride & monochloride Methanol + water + NaOH + HCl 860 times water & 70 timessaline

1,55,000 Lysine methyl ester dihydrochloride & monochloride 1050 times water & 72 timessaline

1,30,000 Hexamethylene diamine 880 times water & 70 timessaline

1,30,000 m-Xylene diamine 760 times water & 65 timessaline


precursors such as mono & diammonium maleate,maleamic acid etc., were carried out thermally with avariety of mono, di and multifunctional monomers con-taining amino, hydroxyl & carboxyl groups. The PSIcopolymers are then hydrolyzed by alkali to form PASP[50].

The cross-links have been introduced with amines indifferent patents. The cross-links of PSI have been reportedwith hydrazine, amines and also unsaturated ethyleneiccarboxylic compounds. The different results are tabulatedbelow in Tables 6 and 7 as studied in different patentsUS 6,072,024 and 58,89,072 [51,52].

Fig. 9. Cross-linking of PSI with Diamines [54]. {Column fitting image-1}.

Fig. 11. Reaction with allylamine as pendant group [57]. {Column fittingimage-1}.


The conditions have been varied and also studied.US patent 525,703 synthesized crosslinked polyanhyd-

roaspartic acid was formed by the reaction of PSI and tria-minononane. The crosslinked polymer was prepared byreacting PSI with triaminononane in a solvent or liquidmedium such as dimethylformamide (DMF) or water. ThisDMF medium is removed by distillation. The crosslinkedpolymers differ widely depending in the reaction of PSIand triaminononane. This means that the polymers having

Fig. 10. Cross-linking of PSI with natural am

different water solubility or swelling property is due to thevaried triaminononane and PSI molar ratio [53].

Zhao et al. has developed superabsorbent polymers bycrosslinking PSI with diamines and certain natural aminesfollowed by hydrolysis [54–56]. Figs. 9 and 10 give thereaction scheme for cross-links with diamines and naturalamines, respectively.

The effect salt absorption has been studied. The absorp-tion reduces with the increasing concentration of the salts.The kinetics of the absorption has been studied. In anotherstudy the effect of molecular weight of PSI on the absorp-tion characterization has been reported. Gel strength,effect of time of cross-linking and other reaction parame-ters has been reported.

ines [55]. {Column fitting image-2}.


Chang and Swift have also reported the formation ofthese polymers to be salt responsive and stimuli-respon-sive hydrogel. Different molecular weight of PSI were pre-pared and coupled with dicyclohexyl carbodiimide. Thesewere hydrolyzed and then treated with ethylene glycoldiglycidyl ether. The absorbency was comparative to thecommercial PASP [56].

In yet another paper the polyaspartate has been cross-linked after attaching a polymerization pendant group.Allyl amine has been used for this and then the polymeris cross-linked as reported by Satoshi et al. [57]. As the con-centration of all groups increases, the absorbencydecreases. Maximum absorbency of 430 g/g has beenobserved with 5.3% allylamine. The reaction for the sameis reported below Fig. 11.

3.2. Interpenetrating polymers & semi-interpenetratingpolymer

IPNs are conventionally defined as intimate combina-tion of two polymers, at least one of which is synthesizedor cross-linked in the immediate presence of the other[35]. This is typically done by immersing a pre-polymer-ized hydrogel into a solution of monomers and apolymerization initiator. IPN method can overcomethermodynamic incompatibility which occurs due to thepermanent interlocking of network segments and limitedphase separation can be obtained. The interlocked struc-tures of the cross-linked IPN components are believed toensure stability of the bulk and surface morphology [58].The main advantage of IPNs are relatively dense hydrogelmatrices can be produced with stiffer and toughermechanical properties, controllable physical propertiesand more efficient drug loading compared to conventionalhydrogels. Liu et al. have synthesized series of IPN hydro-gels to impart sensitiveness towards temperature and pHfluctuations [59]. The investigators have incorporated onepH sensitive polymer, polyaspartic acid into the PNIPAAmhydrogel system for improving its response rate to envi-ronmental temperature. The morphologies and thermalbehavior of the prepared IPN hydrogels were studied byboth SEM and DSC. The IPN hydrogels showed a large

Fig. 12. Reaction scheme for formation of semi-IP

and uneven porous network structure, without showingthe structure of PNIPAAm hydrogel. The swelling experi-ments reveals that IPN hydrogels exhibited much fastershrinking and re-swelling with respect to the compositionratio of the two network components. These fast respon-sive hydrogels foster potential applications in biomedicalfields.

If one polymer is linear and penetrates another cross-linked network without any other chemical bonds betweenthem, it is called a semi-inter penetrating network. Semi-IPNs can more effectively preserve rapid kinetic responserates to pH or temperature due to the absence of restrictinginterpenetrating elastic network, while still providing thebenefits like modified pore size & slow drug release [60].

Semi-interpenetrating have also been reported to beprepared by Zhao et al. [35] using acrylic acid and polyas-partic acid. The effects of salt, temperature and pH havebeen reported on these polymers. The cross-linker hasbeen reported to be N,N0-methylenebisacrylamide (NMBA)& N,N,N0,N0-tetramethylenebisacrylamides. The initiatorsused are persulphates.

The reaction scheme is discussed below in Fig. 12.In contrast to the acrylic acid based superabsorbent the

semi-IPN have not shown maximum swelling at pH 7 [16].The mechanism of absorption is discussed below (Fig. 13).

Semi-interpenetrating polymers of hyaluronic acid andpolyaspartic acid have been studied. It has been observedthat the superabsorbent character of PASP has been evi-denced by the maximum swelling degree reached at highvalue of pH [61].

3.3. Grafted polymers

Graft copolymer is a type of copolymer in which one ormore blocks of homopolymer are grafted as branches ontoa main chain, meaning it is a branched copolymer with oneor more side chains of a homopolymer attached to thebackbone of the main chain. Grafting reaction involvesthe copolymerization of a monomer onto the polymerbackbone – it originates from the formation of an activesite at a point on a polymer chain other than its end, fol-lowed by the exposure of this site to a monomer.

N polymer [35]. {Column fitting image-1.5}.

Fig. 13. Mechanism of absorption at different pH [35]. {Column fitting image-1.5}.


In the presence of cross-linker, graft copolymerizationformed three-dimensional polymeric networks, those thatswell quickly by absorbing a large amount of water. Mono-mer concentration, pH, and ionic strength are the factorswhich significantly influence the network formation madevia the free radical polymerization mechanism.

PASP polymers have also been prepared by grafting.Grafting of PASP has been carried out on hyaluronic acidfor medical application [62].

4. Analysis of superabsorbent

Different methods have been developed which include:absorption capacity: using Tea bag method; sieve method;centrifuge method; wicking capacity; absorbency underload; swelling rate; swelling gel strength; soluble fraction;ionic sensitivity; residual monomer and gel content. Thesehave been covered in detail by Zohuriaan Mehr et al. [13].

5. Biodegradability of superabsorbent polymers

Biodegradability describes a polymer that can bereduced to carbon dioxide, methane, water, and biomassunder biochemical action. Biodegradability may be con-trasted with the more generic term degradable. A degrad-able polymer undergoes decomposition or degradationunder unspecified environmental influences.

Some of the generalization that has been made aboutbiodegradability to chemical structure includes:

1. Naturally produced polymers biodegrade, and chemi-cally modified natural polymers may biodegrade,depending on the extent of modification.

2. Synthetic addition polymers with carbon–carbon back-bone do not biodegrade at molecular weights greaterthan about 500 g/mol.

3. Synthetic addition polymer with heteroatom in theirbackbone may biodegrade.

4. Synthetic step-growth or condensation polymers aregenerally biodegradable to a greater or lesser extent,depending on the chemical nature of the chain coupling,molecular weight, morphology and hydrophilicity.

5. Water solubility does not guarantee biodegradability.

Based on these generalities three approaches to achievetruly biodegradable bioabsorbents materials are apparent:modification of a superabsorbent to enhance its biodegrad-ability or modification of a biodegradable polymer (e.g.natural polymer) to enhance its superabsorbency. The firstapproach would entail incorporating some biochemicalcross-linker that degrades to form smaller chain polymers.The second approach would involve incorporating chargedfunctional group into a hydrophilic biopolymer to enhanceits absorbency, then cross-linking the polymer to achievedesired absorbance properties. These modifications mayimpact the biodegradability of the polymer. The thirdapproach of mixing biodegradable fillers with non-biode-gradable superabsorbents has also been attempted; how-ever these approaches have not led to a fullybiodegradable product.

Biodegradable polymers are a type of macromoleculeswhich are able to breakdown into smaller compounds orcompletely degraded in biologically active environments.The breakdown process is normally caused by microorgan-isms, however biodegradation can also occur throughhydrolysis and oxidation processes in biological environ-ment. Microorganisms of the soil are the main partieswhich contribute to majority of polymer degradation[19,54,56,63,64]. Hence, biodegradability of a polymerdepends on the types of biological enzymes and microor-ganisms present in the soil. Nevertheless, the specific pop-ulations desirable for a particular degradation aredependent upon numerous environmental factors in thesoil, such as soil moisture content, pH, soil organic matteretc. Moreover the environmental degradation is complexprocess. Chemical structure, molecular weight, morphol-ogy, crystallinity, Tg, hydrophilicity, and water uptake ofpolymer play an important role. Many degradation modeseither abiotic or biotic degradation can combine to degradethe polymer. Biodegradability of polyaspartic acid has beenstudied using activated sludge according to the method ofOECD301C. The copolymers of PASP have been reported tobe biodegradable. The biodegradability has been reportedto vary from 3.5% to 82% in PASP. The biodegradabilityhas been found to decrease on copolymerization with 3-amino-propionic acid. Better biodegradability has beenobserved in case of co-polymers with 6-amino-caproic

Fig. 14. Structure of the polymer [65]. {Column fitting image-1.5}.

Table 8MnO2-dispersion and biodegradable properties of sodium dodecylamine-modified poly(aspartic acid)s (DDA-PASP-Na) [65].

Sample Mole fraction of N-dodecyl aspartamideunits in PASP-Na (mol-%)a

Dispersion capacity of MnO2

(MnO2 g/g of polymer)bBiodegradability(removed TOC, %)c

DDA4-PASP-Na 4 2.3 16DDA14-PASP-Na 14 3.5 14DDA22-PASP-Na 22 6.2 20DDA32-PASP-Na 32 10.3 17DDA46-PASP-Na 46 5.7 21PASP-Nad 0 2.4 86PAA-Na 0 2.2 <1

a Determined by 1H NMR Spectra.b Using 0.05% polymer aqueous solution; 25 �C; time: 4 h.c Using activated sludge; temperature: 250 � C; time: 28 days.d Sodium poly(acrylate), Mw = 8000.


acid. Even polymerization with amine like dodecylaminereduces the biodegradability [65]. The structure of thepolymer is given below along with its biodegradability inFig. 14 and the results obtained are given in Table 8.

It was also found that sodium polyaspartate synthe-sized in the presence of phosphoric acid as a catalyst thepolymer is 100% biodegradable, whereas the same polymeris 70% degradable without catalyst. Takashi et al. studiedthe relationship of structure and biodegradability of vari-ous isomers of PASP was concluded that the chirality ofaspartic acid scarcely affected the biodegradability [65].

6. Conclusions

SAPs are one of the members of the smart hydrogels.Different class of superabsorbent polymers need to bedeveloped for varied applications. These hydrogels can bedeveloped using green technologies.

Acknowledgements

The authors are thankful to UGC for sponsoring thisproject and providing us an opportunity for carrying ourresearch in our field of interest. We are also thankful toour principle Dr. I.S. Bakshi for providing us timely support

and guidance for our research. We are also thankful to col-lege management for the required help.

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Polyaspartic acid based superabsorbent polymers