Evaluation of the Entanglement Molecular Weights of Maize Starches from Solution Rheological Measurements !

Cereal Chern. 75(3):360-364 The entanglement molecular weights of waxy maize (WM) and normal maize (NM) starches were calculated from solution rheological data. The viscoelastic behavior of both WM and NM starches were measured at several different concentrations and then shifted to produce a master curve for each of the materials. The theory of Doi and Edwards was used to calculate the plateau moduli from which values for the entanglement molecular weights for WM and NM starches were calculated. The entanglement molecular weights were 100 ± 15 kg/mol for WM starch and 96 ± 8 kg/mol for NM starch. These two values were within experimental error of one another and represent the entanglement molecular weight of Starch is a commodity polymer that is utilized today in a wide variety of commercial applications ranging from foods and adhesives to blends and composites. Increasingly, starch is being used in applications previously regarded as the realm of synthetic polymers because of the low cost and potential biodegradability of starch. In addition, starch is a renewable resource, and its potential to replace nonrenewable petrochemical-based polymers offers manufacturers a low-cost alternative to the increasing costs of many commercial synthetic polymeric materials. Starch consists of (1-4) linked a-D-glucopyranosyl units. Two major forms are found in natural starches: amylopectin and amylose. Amylopectin is a highly branched, very high molecular weight (typically 5,000-30,000 kg/mol) biopolymer. The branches in amylopectin are the result of the infrequent (1-6) bonds in the polymer. Amylose has a lower molecular weight (typically 20800 kg/mol) and is essentially a linear-chain polysaccharide. In the Unites States, the major source of starch is maize. Waxy maize (WM) starch consists of nearly pure amylopectin. Normal maize (NM) starch consists of 20-25% amylose and 80-75% amylopectin. High-amylose starches also are known. In its native form, starch granules are ::::10 JJ.m in diameter. These granules can be dispersed in aqueous media at temperatures of 60-80°C (Whistler et al 1984). Over the years, investigators have examined the rheological properties of starch in solution in either its native state (Steeneken 1989; Dintzis and Bagley 1995a; Dintzis et al 1995, 1996; Hansen et al 1991) or in transformed products (Evans and Haissman 1979, Eliasson 1986, Doublier et al 1987, Dintzis and Bagley 1995b). In addition, research was conducted on the rheological properties of starch during extrusion (Vergnes and Villemaire 1987; Vergnes et al 1987, 1993; Zheng and Wang 1994; Willett et al 1995; Della Valle et al 1996) and on the rheology and physical properties of starch and synthetic polymer blends (Seethamraju et al 1994, Bhattacharya et al 1995; Vaidya et al 1995; Yang et al 1996; Ram1 Biomaterials Processing Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, USDA, 1815 N. University Street, Peoria, IL 61604. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Corresponding author. E-mail: [email protected] Phone: 309/6816240. Fax: 309/681-6685 Publication no. C-1998-0408-01 R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1998. 360 CEREAL CHEMISTRY amylopectin, the major component of WM and NM starches. The entanglement degrees of polymerization for WM and NM starch, using a value of 162 g/mol for the monomer molecular weight of amylopectin, were 617 ± 92 and 592 ± 49, respectively. The values for the entanglement molecular weight and the entanglement degrees of polymerization for WM and NM starch were markedly higher than those quoted for many commercial polymers. This finding indicates that molecular weights of >1 million are required to produce starch-based materials with consistent physical properties. kumar et al 1997a,b). Rheological properties are important governing factors in control of the morphology developed during processing, including extrusion and jet-cooking processes. Despite the numerous applications for starch, many unanswered questions remain regarding its basic rheological and physical performance properties. Although several studies have examined the average molecular weight for cross-linked WM starch (GluckHirsh and Kokini 1997) and starch hydrogels (Kulicke et al 1989), an evaluation of the entanglement molecular weight of starch has yet to be reported. The entanglement molecular weight is an important rheological parameter that governs many of the flow and final state physical properties of polymers. The original concept of entanglement molecular weights for high molecular weight polymers was based on experimental observations of a discontinuity in the slope of the log viscosity versus log molecular weight curve. Below the entanglement molecular weight, the slope has a value of unity; above the entanglement molecular weight, the slope has a value of 3.4 (Klein 1987). Polymers produced at molecular weights below the entanglement molecular weight exhibit brittle fracture and, in general, have low tensile strengths. To obtain the optimal mechanical properties from a polymer, the molecular weight must be 8-10 times that of the entanglement molecular weight. In this study, solution rheology data were used to obtain a value for the entanglement molecular weight of high-amylopectin maize starches. MATERIALS AND METHODS Materials Two different starch materials were used in this study. NM starch (Buffalo 3401, CPC, Com Products Div., Summit-Argo, IL) consisted of 25% amylose and 75% amylopectin. WM starch (Amioca, American Maize Products, Hammond, IN) consisted of 98% amylopectin and 2% amylose. The dimethyl sulfoxide (DMSO) solvent was obtained from Aldrich Chemical Company (Milwaukee, WI). All the materials were used as received. Solution Preparation Solutions for the rheological measurements were prepared by adding the appropriate starch into a volumetric flask containing 10 mL of distilled water and stirring gently with a stirring bar at 30 rpm to wet and disperse the granules thoroughly. The stirring was continued and 60 mL of DMSO was added. The flask was then heated gradually for 20-30 min to 80°C, and held for 5 min at this temperature. As the starch dissolved, the dispersions which were originally opaque became clear with time. The solution was then RESULTS AND DISCUSSION to zero concentration. In Eqs. 1 and 2, Tl is the viscosity of the polymer solution, Tls is the viscosity of the solvent, and c is the concentration. cooled to room temperature and additional DMSO was added to bring the total final volume to 100 mL. Initially, solutions of the materials were made at a 5% (w/w) concentration. This stock solution was then diluted with DMSO and water (90: 10) to the desired concentrations. For this study, solutions were produced at concentrations ranging from 0.02-0.05 glmL. (3) (5) G'R == [::: rG' versus (OQc c**",2Q. [11] For WM starch, the measured intrinsic viscosity was 209 ± 4 mUg; for NM starch, the intrinsic viscosity was 178 ± 2 mL/g. Using Eqs. 3 and 4, the c* value is 0.014 glmL for WM starch and 0.017 g/mL for NM starch. The c** value is 0.048 glmL for WM starch and 0.056 glmL for NM starch. The highest concentration used in this study for both WM and NM starch was 0.05 glmL. Thus, the solutions studied are predominately in the semidilute regime, with only the highest concentration of the WM starch considered marginally concentrated. Oscillatory Shear Flow Experiments Small-amplitude oscillatory shear experiments at 25°C were conducted on WM and NM starch solutions. The data for WM obtained at 80°C were shifted to 25°C using the time-temperature superposition procedure of Williams et al (1955). The data for WM and NM starch were then shifted, with respect to concentration, using the method of corresponding states as described by Ferry (1980). In this approach, a master curve is constructed using shifts in time and concentration in analogy to time-temperature superposition. Care must be taken when applying this approach as the concentration dependence of the various viscoelastic functions varies from the plateau to the terminal regimes as well as from the semidilute to dilute regimes. Several researchers (Ferry 1980) have used the following expressions with success: the semidilute to concentrated solution behavior is designated as c** (Macosko 1994) and can be approximated crudely as: where Cl and C2 are the reference and shifted concentrations, respectively, and Q c is the concentration shift factor. Thus, a plot of G'R versus mac yields a master curve at the reference concentration. The reference concentration selected for both materials was 0.02 glmL, which will place the resulting master curves in the semidilute regime. The active shifting variable chosen was G'. The shifting was accomplished using software (Wavemetrics Igor Prosoftware, with an Apple Macintosh 8600/300 computer). Standard deviation of the fit was calculated as the difference between the log of the shifted storage moduli and the log of the reference storage moduli. The best overlap was chosen as the set of shift parameters that minimized the standard deviation. The results of the data shifts are illustrated in Figs. 1 and 2 for WM and NM starch, respectively. From Fig. 2, it is evident that the shifting routines applied to NM starch produced a master curve with more scatter in the data than they did for WM starch. This scatter might reflect the more complicated mixture of amylose and amylopectin that constitutes NM starch. (1) (2) '!'In(21 ) c 11 s versus concentration, or Rheological Measurements Rheological properties of each of the solutions were measured using a controlled-stress rheometer (CSe 500, CarriMed, Dorking, England) with a cone-and-plate fixture. All the rheological studies were conducted using a 4-cm diameter, 4° cone. The temperature of the sample was controlled using a Peltier plate which enabled the chamber of the rheometer to be controlled to within ±0.1°C. For each solution, a smaIl-amplitude oscillatory shear experiment was conducted using frequencies of O.OI-l0/sec. The oscillatory shear experiments were performed using a constant shear stress of 1,000 dyn/cm• Before the oscillatory shear experiments, a torque sweep was conducted on each sample to ensure that the applied shear stress was within the linear viscoelastic regime of the material. The shear storage and loss moduli (G' and Gil) were measured from the oscillatory experiments. Intrinsic Viscosity Measurements The intrinsic viscosities of the solutions were measured using a calibrated shear dilution viscometer (Cannon-Ubblohde series 150). For the solutions discussed in this work, the shear rates were all <lOO/sec. The measurements were performed in a circulating water bath at 25.0 ± 0.1°C. The stock solution was used for the first measurement and was then diluted in the viscometer. The resulting solution was mixed by gentle agitation before the next measurement. This process was continued until the flow time of the solution was within 5 sec of the flow time of the solvent. Flow times were measured a minimum of five times for each solution,. The intrinsic viscosity was obtained for each of the materials by extrapolating the specific viscosity and concentration (Tlsplc) where: Evaluation of Coil Overlap Concentration Stock starch solutions were diluted to concentrations of 0.020.05 glmL. This concentration range places the samples in the semidilute regime where coil overlap can affect the conformational dynamics of the coils in solution. The transition from dilute solution to semidilute solution behavior is designated as c* (Doi and Edwards 1988, Ferry 1980, Macosko 1994) and can be approximated by: