Rheological properties of reactive extrusion modified waxy starch and waxy starch-polyacrylamide copolymer gels

RESEARCH ARTICLE

Rheological properties of reactive extrusion modified waxystarch and waxy starch-polyacrylamide copolymer gels

Jingyuan Xu and Victoria L. Finkenstadt

US Department of Agriculture, National Center for Agricultural Utilization Research, Agricultural Research Service, Peoria, IL, USA

The rheological properties of modified waxy starch and waxy starch–polyacrylamide graftcopolymers prepared by reactive extrusion were investigated. Both materials can absorb hugeamount of water and form gels. The modified waxy starch and waxy starch–polyacrylamide graftcopolymer gels all exhibited viscoelastic solid properties. The waxy starch–polyacrylamide graftcopolymer gels with the concentration�10% showed weaker viscoelastic behaviors than those of thesame concentrations of modified waxy starch gels. However, at concentration �11.25%, the waxystarch–polyacrylamide graft copolymer gels displayed much stronger viscoelastic properties thanthose of the same concentrations of modified waxy starch gels. The analysis of modulus andconcentration dependence and stress relaxation measurements indicated that both modified waxystarch and waxy starch–polyacrylamide graft copolymer gels were physical gels meaning the cross-linkers between the molecules were physical junctions. The nonlinear steady shearing rheologicalproperties studies indicated that both modified waxy starch and waxy starch–polyacrylamide graftcopolymer gels exhibited shear thinning behavior, which can be well fitted with the power lawconstitutive equation. The function and behavior of the modified waxy starch and waxy starch–polyacrylamide graft copolymer gels suggest that these starch-based biomaterials should be potentialcandidates for applications in cosmetic gels, wound skin care materials, and agricultural products.

Received: September 6, 2012Revised: March 19, 2013

Accepted: March 20, 2013

Keywords:Graft copolymer / Polyacrylamide / Reactive extrusion / Starch / Viscoelastic properties

1 Introduction

Starch and modified starch materials have been widely usedin food and non-food products. Since the starch-based super-absorbents were developed by USDA scientists [1, 2], manyresearchers have been interested in these modified starchmaterials. Starch-based polymers can have similar functionalbehaviors as synthetic polymers, but have much betterenvironmental properties such as biodegradability. Variouspreparation of starch graft copolymers have been describedand reported [3]. One of the processing methods to prepare

modified starch-based polymers is reactive extrusion [4]. Thereactive extrusion is a process using a co-rotating twin screwextruder as a reactor to produce graft copolymers orcompatibilized immiscible polymer blends. The monomerA is polymerized through the extrusion in the presence of B,which can be either monomer or polymer. In the process, agraft or block copolymer of A and B as well as polymer Awere formed simultaneously. A variety of reactive extrusionmodified starches have been prepared and reported, such asesters [5, 6], ionic starches [7–9], oxidized starches [10],starch–polyester graft copolymers [11], and starch–polyacryl-amide (PAAm) graft copolymers [12–14]. The potentialutilizations of modified starch-based materials by reactiveextrusion may include cosmetic or personal care gels, skinwound healing dressings, and agricultural applications.

Willett and Finkenstadt [14] reported preparation ofPAAm graft copolymers using various starches and describedtheir graft parameters, solubility and absorbency. They usedacryliamide monomer with unmodified starches (corn, waxymaize, wheat, and potato) and cationic starches respectively

Correspondence: Dr. Jingyuan Xu, National Center for Agriculturaland Utilization Research, Agricultural Research Service, USDepartment of Agriculture, 1815 North University Street, Peoria,IL 61604, USAE-mail: [email protected]: þ1-309-681-6691

Abbreviations: PAAm, polyacryl-amide

DOI 10.1002/star.201200199Starch/Stärke 2013, 00, 1–7 1

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

to produce starch-PAAm graft copolymers by reactiveextrusion in a twin screw extruder using ammoniumpersulfate as initiator. They reported that the obtained PAAmgraft molecular weights were in a range of 317 000 and769 000; and the average conversion of monomer acrylamideto PAAm was about 89% that was independent of starchtype. In addition, they found that the modified starches andstarch-PAAm graft copolymers prepared by reactive extrusioncan absorb more than one hundred times their weight inwater and form gel-like materials. To identify the potentialapplications of these modified starch-based materials, weexplored the rheological behaviors of modified starches andstarch-PAAm graft copolymers prepared by reactive extru-sion. Both linear and nonlinear viscoelastic properties forthese materials are reported.

2 Materials and methods

2.1 Materials

Starch used in this work was waxy maize (Waxy 7350, A. E.Staley, Decatur Illinois), which is an unmodified corn starchwith essentially no amylose. Moisture content of the starch inambient conditions was approximately 10% measured bygravimetric weight loss. Acrylamide (Cytec, Inc.) was used assupplied as a 50% (wt%) aqueous solution. Reactive extrusionwas performed using a Werner & Pfleiderer ZSK 30 mm co-rotating twin screw extruder (Coperion Corporation, Ramsey,NJ). The condition and running parameters of the extruderwere described by Willett and Finkenstadt [14]. The starch/acrylamide ratio for producing graft copolymer was 1.5:1. Theextruder barrel consisted of 14 barrel sections, with eightindependent temperature control zones. Barrel temperatureset-points were 80/90/90/90/110/110/100/80°C. The totalfeed rate was 12.1 kg/h with water content of 50% (wt%).Liquids were fed using triple piston metering pumps (Eldex);rates were monitored by following weight loss of reservoircontainers. Ammonium persulfate 5% (wt%) solution wasfed at a rate to give 1% (wt%) based on the starch. Theextrudates were collected for approximately 15 s, yieldingsamples of approximately 50–60 g. The product of copolymercontained un-reacted starch, starch-graft-poly(acrylamide)copolymer (starch-g-PAAm), and polyacrylamide homopoly-mer (PAAm). Any remaining monomer was removed by thequenching process [14]. The extrudates were immediatelydispersed in ethanol with 0.5% (wt%) hydroquinone in ablender to stop any residual reaction. They were steepedovernight and then filtered and dried in a forced-air oven at105°C for 1 h. They were extracted in ethanol/water (30/70) toremove any residual monomer, then dried in a vacuum ovenat 105°C overnight. The samples provided for analysis werecoarse granules (1–3 mm or less). They were ground to finepowders and stored in air-tight glass jars for various analyses.

The modified waxy starch by the reactive extrusion wasprepared with the same procedure stated above without theacrylamide. In this work, the modified waxy starch by thereactive extrusion was named as RE-starch; and the product ofwaxy starch and acrylamide graft copolymer through thereactive extrusion was named as RE-starch-g-PAAm.

2.2 Measurements

A strain-controlled Rheometric ARES rheometer (TA Instru-ments, New Castle, DE) was used to perform the rheologystudies [15]. The 50-mm diameter cone and plate as well as25-mm parallel-plates geometries were adopted. The coneangle was 0.04 radians. The temperature was controlled at25 � 0.1°C by a water circulation system. The steady shearand stress relaxation experiments were also performed at25 � 0.1°C. Linear viscoelastic measurements were con-ducted for starch polymers. To ensure that all the measure-ments for the materials were made within the linear range forthe linear viscoelastic properties studies, the strain-sweepexperiments were conducted initially. An applied shear strainvalued in the linear range was adopted for the otherviscoelastic property measurements for the same material;fresh samples were used for each experiment. Linearviscoelasticity indicates that the measured parameters areindependent of applied shear strain. Small-amplitudeoscillatory shear experiments were conducted over afrequency (v) range of 0.1–500 rad/s, yielding the shearstorage (G0) and loss (G00) moduli. The storage modulusrepresents the non-dissipative component of mechanicalproperties. The elastic or “rubber-like” behavior is suggestedif the G0 spectrum is independent of frequency andgreater than the loss modulus over a certain range offrequency. The loss modulus represents the dissipativecomponent of the mechanical properties and is characteristicof viscous flow. The phase shift or phase angle (d) is definedby d ¼ tan�1(G00/G0), and indicates whether amaterial is solidwith perfect elasticity (d ¼ 0), or liquid with pure viscosity(d ¼ 90°), or something in between (0<d<90°). Stressrelaxation experiments measured the stress relaxation withthe time after thematerial is subject to a step increase in shearstrain. Nonlinear rheological studies were also conductedwith the same ARES instrument and the same geometrydescribed above. The steady shear measurements wereconducted with increasing shear rate step-wise in the range of0.1–500 s�1; the delay time was 10 s. Each measurement allabove was repeated at least three times with fresh samples.The relative errors were all within the range of �12%.

3 Results and discussion

Un-modified starches have low solubility in water and wouldnot gelatinize at room temperature. However, RE-starch and

2 J. Xu and V. L. Finkenstadt Starch/Stärke 2013, 00, 1–7


RE-starch-g-PAAm through reactive extrusion will absorbhundreds of times their weight in water and form gels atroom temperature. The DI water absorbency of RE-starch-g-PAAm produced by reactive extrusion is about 250 g/g [14].We investigated seven concentrations of RE-starch andRE-starch-g-PAAm gels from 5 to 17.5% (wt%) at 25°C.When the RE-starch mixed with water at room temperature, agel-like material was formed quickly. At 25°C, all measuredconcentrations of the RE-starch samples exhibited visco-elastic solid properties (Fig. 1). The linear rheologicalproperties of the RE-starch were concentration dependence.The higher of the concentration of the RE-starch was, thegreater its moduli would be (Fig. 1). At lower concentration of5%, the curve of the storage moduli (G0) for the RE-starch wasnearly frequency independent and higher than that of the lossmoduli (G00) within the measured frequencies. The curve ofthe loss moduli (G00) for the RE-starch was slightly frequencydependent especially at higher frequencies. These curveshapes were similar to those for rubber-like gel materials [16].The frequency-independence storagemodulus (G0) was about80 Pa for the 5% RE-starch gel. And the phase shifts for the5% RE-starch gel were in the range of 4.0–16.2° (Table 1). Atthe highest measured concentration of 17.5%, the RE-starchexhibited much stronger viscoelastic solid behavior than thatat lower concentrations. The shapes of both curves for thestoragemoduli (G0) and the loss moduli (G00) of the 17.5% RE-starch were similar to those of lower concentrations RE-starchgels; the G0 curve was nearly frequency independent and G00

curve was slightly frequency dependent. The frequency-independence storage modulus (G0) for 17.5% RE-starch gel

was about 1400 Pa (Fig. 1). And the phase shifts were in therange of 3.8–13.9° (Table 1). These results indicated that RE-starch gels exhibited viscoelastic solid properties and theirrheological behavior could be manipulated by the concentra-tion or water content. For reference, the G0 for the syntheticrubber can be 104 to 107 Pa depending on cross-linking, whiletheir phase shifts can be around 11°.

The RE-starch-g-PAAm that mixed with water at roomtemperature also formed gel-like materials. At 25°C, allmeasured concentrations of the RE-starch-g-PAAm samplesexhibited viscoelastic solid properties similar to RE-starchgels (Fig. 2). The linear rheological properties of the RE-

Figure 1. Linear viscoelastic properties of frequency sweepexperiment for the RE-starch. Storage modulus (G0) or lossmodulus (G00) as function of frequency at 25°C with 0.5% strain.Filled symbols: G0, opened symbols: G00. (*, �): 5% (wt%) RE-starch; (&, &): 10% (wt%) RE-starch, (~, ~): 17.5% (wt%) RE-starch.

Table 1. The linear viscoelastic properties of phase shift rangewithin the measured frequency range of 0.1–500 rad/sfor the RE-starch and RE-starch-g-PAAm gels

Material Phase shift range (°)

5% (wt%) RE-starch gel 4.0–16.27.5% (wt%) RE-starch gel 3.1–19.310% (wt%) RE-starch gel 3.8–18.811.25% (wt%) RE-starch gel 3.7–19.915% (wt%) RE-starch gel 4.4–15.917.5% (wt%) RE-starch gel 3.8–13.95% (wt%) RE-starch-g-PAAm gel 2.1–19.47.5% (wt%) RE-starch-g-PAAm gel 2.7–18.710% (wt%) RE-starch-g-PAAm gel 3.7–18.111.25% (wt%) RE-starch-g-PAAm gel 3.2–19.315% (wt%) RE-starch-g-PAAm gel 2.4–15.717.5% (wt%) RE-starch-g-PAAm gel 2.6–15.3

Figure 2. Linear viscoelastic properties of frequency sweepexperiment for the RE-starch-g-PAAm. Storage modulus (G0) orloss modulus (G00) as function of frequency at 25°C with 0.5%strain. Filled symbols: G0, opened symbols: G00. (*, �): 5% (wt%)RE-starch-g-PAAm, (&, &): 10% (wt%) RE-starch-g-PAAm,(~, ~): 11.25% (wt%) RE-starch-g-PAAm, (^, ◊): 17.5% (wt%) RE-starch-g-PAAm.

Starch/Stärke 2013, 00, 1–7 3


starch-g-PAAm gels were also concentration dependence.The higher the concentration of the RE-starch-g-PAAmwas, the greater its moduli would be (Fig. 2). When theconcentration �10% (wt%), the elastic or storage modulifor RE-starch-g-PAAm gels were lower and weaker thanthose for RE-starch gels. However, at the concentration�11.25% (wt%), the elastic or storage moduli for RE-starch-g-PAAm gels were higher and stronger than those for RE-starch gels (Figs. 1–3). At concentration of 10% (wt%), thefrequency-independence elastic modulus (G0) for RE-starch-g-PAAm gel was about 176 Pa, which was three times lessthan that for the 10% RE-starch gel. The phase shifts forthe 10% RE-starch-g-PAAm gel were in the range of 3.7–18.1° (Table 1), which were similar to those for the 10% RE-starch gel. However, at concentration of 11.25% (wt%), thefrequency-independence elastic modulus (G0) for RE-starch-g-PAAm gel was about 1000 Pa, which was much greaterthan that for the 11.25% RE-starch gel. The phase shifts forthe 11.25% RE-starch-g-PAAm gel were in the range of 3.2–19.3° (Table 1), which were similar to the phase shifts of the11.25% RE-starch gels. The frequency-independence G0 for17.5%RE-starch-g-PAAm gel was around 8000 Pa, which wasmore than five times greater than that for 17.5% RE-starchgel. The phase shifts for the 17.5% RE-starch-g-PAAm gelwere in the range of 2.6–15.3° (Table 1), which were similar tothose for the 17.5% RE-starch gel.

The trend of value of log(G0) versus log(C) followed powerlaw model described for physical gels [17]. The modulus ofRE-starch gel varied as C3.0; and the modulus of �10% RE-starch-g-PAAm gel varied as C2.3; while the modulus of�11.25% RE-starch-g-PAAm gel varied as C4.5 (Fig. 3). These

results supported the fact that both of themeasured RE-starchand RE-starch-g-PAAm gels were physical gels and themeasured concentrations were in the range of near criticalconcentration since their power index number were largerthan two. The power law index is dependent on the kineticsthat the gel formed [17]. The reason why the RE-starch-g-PAAm gels displayed dramatic property change within such asmall concentration shift (10–11.25%) is unclear. Thepossible explanation is that the molecular weight of RE-starch-g-PAAm is smaller than that of RE-starch; however,the RE-starch-g-PAAm possesses many short graft polymerbranches and un-reacted polyacrylamide homopolymer(PAAm) while the RE-starch does not. At lower concen-trations, these short graft polymer branches rarely havechances to interact with each other due to the large spaces,thus the RE-starch-g-PAAm gels exhibit weaker viscoelasticbehaviors than RE-starch gels due to the molecular weight.However, at higher concentrations, the spaces amongbranches become very small or even zero; there must bemany physical interactions not only among the short graftpolymer branches, but also among the short graft polymerbranches, un-reacted polyacrylamide homopolymer andlong starch chains. Therefore, these short graft copolymerbranches and un-reacted polyacrylamide homopolymermay play a strong physical cross-linking role at higherconcentrations.

The strain sweep measurements of three concentrationsof both RE-starch gels and RE-starch-g-PAAm gels arepresented in Fig. 4. The linear range of all measuredconcentrations of the samples was very small, below 1% of

Figure 3. The storage modulus (G0) values measured at 1 rad/s asfunction of concentration for the RE-starch and RE-starch-g-PAAmgels at 25°C. (*): RE-starch. (&): �10% (wt%) RE-starch-g-PAAm. (~): �11.25% (wt%) RE-starch-g-PAAm. The line is thefitting with the power law model.

Figure 4. Strain sweep experiment for the RE-starch and RE-starch-g-PAAm gels with 1 rad/s frequency at 25°C. (*): 5% (wt%) RE-starch-g-PAAm gel. (&): 10% (wt%) RE-starch-g-PAAm gel. (~):15% (wt%) RE-starch-g-PAAm gel. (�): 5% (wt%) RE-starchgel. (&): 10% (wt%) RE-starch gel. (~): 17.5% (wt%) RE-starchgel.



the shear strain, which indicated that the both RE-starch gelsand RE-starch-g-PAAm gels were networks so called “weak”gels [17]. A “weak” gel is cross-linked together by linkages thatare relatively stronger and more permanent than entangle-ments. While a gel formed by chain–chain entanglement isthe characteristics of a “strong” gel. However, a “weak” gel isextremely susceptible to disruption, while an entangled“strong” gel can have a linear range to 20% or more. Cross-linkers can be classified as covalent or physical junctions. Tofurther support the evidence that both RE-starch gels and RE-starch-g-PAAm gels are physical gels, we conducted stressrelaxation measurements. The stress relaxation experimentshowed that both RE-starch gels and RE-starch-g-PAAm gelshad long relaxation time. All measured materials relaxedless than a decade after 1000 s (Fig. 5). This resultindicated that both RE-starch and RE-starch-g-PAAm gelsare physically cross-linked gels, not a chemically cross-linked one even though they have long relaxation time.Because if a network is tightly cross-linked chemically, oneshould rarely see any relaxation and relaxation time shouldbe infinite. This result is also consistent with the aboveconclusion that both RE-starch and RE-starch-g-PAAm gelsare physical gels that the power law model predicted. Thelinear rheological properties that we investigated above forboth RE-starch and RE-starch-g-PAAm gels are similar tothose for some of the cosmetic gels [18, 19] and woundhealing materials [20–22]. For instance, the storage moduli ofcosmetic and pharmaceutical Carbopol gel are around 5–120 Pa, and phase shifts are in the range of 3–15° [19]. The

behavior of the gel could be manipulated and controlled byconcentration.

To better understand the processing behavior, the non-linear steady shear viscoelastic properties for both RE-starchand RE-starch-g-PAAm gels were studied. All of the studiedmaterials exhibited shear-thinning behavior over the entire

Figure 5. Stress relaxation measurements for the RE-starch andRE-starch-g-PAAm gels after being subject to a 0.5% strain at25°C. (*): 5% (wt%) RE-starch-g-PAAm gel. (&): 10% (wt%) RE-starch-g-PAAm gel. (~): 15% (wt%) RE-starch-g-PAAm gel. (�):5% (wt%) RE-starch gel. (&): 10% (wt%) RE-starch gel. (~):17.5% (wt%) RE-starch gel.

Figure 6. The nonlinear viscoelastic properties of the steady shearmeasurements for the RE-starch gels at the temperature of 25°C.Symbols are experiment results. Dashed lines are fitted with powerlaw model. (*): 5% (wt%) RE-starch gel. (&): 10% (wt%) RE-starch gel. (~): 17.5% (wt%) RE-starch gel.

Figure 7. The non-linear viscoelastic properties of the steady shearmeasurements for the RE-starch-g-PAAm gels at the temperatureof 25°C. Symbols are experiment results. Dashed lines arefitted with power law model. (*): 5% (wt%) RE-starch-g-PAAmgel. (&): 10% (wt%) RE-starch-g-PAAm gel. (~): 11.25% (wt%)RE-starch-g-PAAm gel. (^):17.5% (wt%) RE-starch-g-PAAmgel.



measured shear rates (Figs. 6 and 7). Shear-thinningrheological behavior can be characterized by a power lawconstitutive equation [23]. The power law equation may bewritten as

h ¼ K _gn�1 ð1Þwhere h is the shear viscosity, K is the front factor, _g is theshear rate, and n is the power law exponent. The value of nis less than one for material shear-thinning behavior. Weused Eq. (1) to fit shear-thinning viscosity for all measuredsamples. The experimental data were very well fitted by thepower law constitutive equation (Figs. 6 and 7). The results ofthe fits are summarized in Table 2. From Table 2, we can getthe impression that RE-starch and RE-starch-g-PAAm gelspossess almost the same power law exponent of �0.05 and�0.07, respectively. It is hard to explain the negative valuesof law exponent, n. However, the values that we obtainedhere are extremely close to zero. We might consider theywere zero. Therefore, RE-starch and RE-starch-g-PAAm gelsshould have very similar shear-thinning extent. These resultssuggested that RE-starch and RE-starch-g-PAAm gels shouldhave similar behavior during processing. There were somereports showing the negative n values. Padmanabhan andBhattacharya [24] reported that ground corn meal withdifferent moisture levels exhibited shear thinning with�0.193 � n � 0.811. The possible reasons that they sug-gested were molecular degradation of sample, viscousdissipation, fluid slip, etc. Fraiha et al. [25] also found thatthe mixture of ground corn and soybean grains exhibitedshear thinning behavior with negative n value at certaincondition. The possible reasons of negative n values forsamples in current work could be the viscous dissipation andfluid slip.

Examining the processing parameters during reactiveextrusion shows similar shear-thinning behavior inside theextruder for ungrafted and grafted waxy starch in terms of

specific mechanical energy. Specific mechanical energy is ameasure of the energy input into the polymer melt, and wascalculated from torque, pressure and output data afterconversion to shear rate and shear stress [26]. Pressure at thedie, however, did increase once polyacrylamide was graftedonto the waxy starch. This is consistent with the formation ofPAAm graft and PAAm homopolymer. The viscosities ofboth RE-starch and RE-starch-g-PAAm gels were concentra-tion dependent; the greater the concentration, the higher theviscosities (Figs. 6 and 7). The viscosities had a dramaticincrease between concentrations of 10 and 11.25% for RE-starch-g-PAAm gels, which is the same trend as the linearrheological behavior jumps between these concentrations.The nonlinear rheological properties that we investigatedabove for both RE-starch and RE-starch-g-PAAm gels are alsosimilar to those for some of the cosmetic gels [19, 27] andwound healing materials [20–22]. Thus, both linear andnonlinear viscoelastic behaviors studies in this work suggestthat RE-starch and RE-starch-g-PAAm gels should be goodcandidates for cosmetic and wound healing materials.

4 Conclusions

The linear and nonlinear rheological properties for reactiveextrusion modified waxy starch as well as modified waxystarch-g-PAAm graft copolymer gels were investigated. Bothmaterials exhibited viscoelastic solid behaviors; and theirproperties could be manipulated by concentration. Bothkinds of gels were physical gels, which followed the physicalgel power law model. The stress relaxation studies alsosupported that both materials were physical networks. Thelinear and nonlinear viscoelastic behaviors for modified waxystarch and modified waxy starch-g-PAAm graft copolymergels were similar to those of some cosmetic and woundhealing gels, which suggested that these modified starchmaterials be good candidates for application in cosmetic andskin wound healing products.

This work was financially supported by US Department ofAgriculture, Agricultural Research Service. Names are necessaryto report factually on available data; however, the USDA neitherguarantees nor warrants the standard of the product, and the useof the name by the USDA implies no approval of the product to theexclusion of others that may also be suitable.

The authors have declared no conflict of interest.

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Table 2. Power law model fitted parameters for the non-linearviscoelastic properties of the RE-starch and RE-starch-g-PAAm gels

Material K (Pa sn) n R2

5% (wt%) RE-starch gel 10.1 �0.05 0.997.5% (wt%) RE-starch gel 21.1 �0.05 0.9510% (wt%) RE-starch gel 31.3 �0.05 0.9911.25% (wt%) RE-starch gel 46.8 �0.05 0.9715% (wt%) RE-starch gel 172.3 �0.05 0.9817.5% (wt%) RE-starch gel 627.9 �0.05 0.965% (wt%) RE-starch-g-PAAm gel 1.4 �0.07 0.997.5% (wt%) RE-starch-g-PAAm gel 7.3 �0.07 0.9710% (wt%) RE-starch-g-PAAm gel 12.2 �0.07 0.9911.25% (wt%) RE-starch-g-PAAm gel 66.7 �0.07 0.9915% (wt%) RE-starch-g-PAAm gel 697.2 �0.07 0.9917.5% (wt%) RE-starch-g-PAAm gel 803.2 �0.07 0.99



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Rheological properties of reactive extrusion modified waxy starch and waxy starch-polyacrylamide copolymer gels