Also, the observed values were considerably higher than those calculated for pure PDMS. These results suggest expansion of the polymer coils owing to A - B interactions. I n toluene, the differences, as expressed by A, are small and represent only 6 to 12% of the calculated value. They are also independent of molecular weight and composition. I n M E K , however, the theta solvent for PDMS, values of A are up to 54c; greater than those calculated and are proportional to the mole fraction of polystyrene. Cyclohexane, a theta solvent for the PS segment, also caused expansion of the coil configuration, but t o a smaller extent than M E K . All these data show expansion of the predicted coil configuration caused by large contributions of A - B interactions. These findings cannot be reconciled with those of Dondos e t al. (1969), who, from a study of polystyrene-polymethylmethacrylate block copolymers, conclude that in dilute solution, in any solvent, the blocks are segregated and therefore, have distinct locations. The interpretation which most readily lends itself to our results is that the copolymers form randomly interpenetrating coils. In good solvents the coils are expanded by polymersolvent interactions and A - B interactions are a minimum, whereas, in a theta solvent for one of the blocks, repulsive A - B interactions contribute appreciably toward coil expansion. These large repulsive interactions are to be expected if the incompatibility of segment A with segment B is considered.
Literature Cited
Bohmer, B., Berek, D.. Eur. Poiym. J . . 6. 471 (1970). Bostick, E. E. (to General Electric Co.. Ltd.). U.S.Patent 3,483,270 (1969). Brandrup, J . , Immergut, E. H.. Eds.. “Polymer Handbook,” Interscience, New York, S.Y., 1966. Bushuk, W., Benoit, H., Can. J . Chem., 36, 1616 (1958). Davies, W. G., Elliott, B., Kendrick, T. C., (to Midland Silicones Ltd.), U. S.Patent 3,481,898 (1969). Dondos, A., Rempp, P., Benoit, H., Makromol. Chem., 130, 233 (1969). Gilman, H., J . Organometal. Chem., 2, 447 (1964). Greber, G., Balciunas, A., Makromol. Chem., 69, 193 (1963). Greber, G., Balciunas, A., ibid., 79, 149 (1964). Inagaki, H . , ibid., 86, 289 (1965). Krause, S.,J . Phys. Chem., 65, 1618 (1961). Milkovich, R. (to Shell Oil Co.), South African Patent Application 642,271 (1964). Milkovich, R . (to Shell Oil Co.), British Patent 1,000,090 (1965). Milkovich, R., Holden, G., Bishop, E. T., Hendricks, W. R., British Patent 1,035,873 (1966). Saam, J. C., Gordon, D. J., Lindsey, S., Macromolecules, 3, 1 (1970). Stockmayer, W . H., Fixman, M., J . Polym. Sci.,Part C, 1, 137 (1963). Urwin, J. R., Aust. J . Chem., 22, 1649 (1969). Utracki, L. A., Simha, R., Fetters, L . J.: J . Polym. Sci., Part A - 2 , 6, 2051 (1968).
Yellow D e n t Corn Starches Effects of Chlorine Oxidation David M. Hall‘, Eric Van Patten’, John L. Brown3, Grady R. Harmon, and Gordon H. N i x School of Engineering, Auburn Uniuersity, Auburn, Ala. 36830
C o r n starch is perhaps the major product used for the surface size application to paper and in warp sizing of textiles. The type of starch used is usually dictated by the desired solids content (add-on) and the viscosity of the paste desired. Preconverted starches, such as oxidized starches, can easily meet the low viscosity and free flowing characteristics a t high solids required for most paper and textile applications. The oxidized starches retain the granule structure and have the same general appearance under both the polarizing and ordinary light microscope as unmodified starch. They are insoluble in cold water and show the typical starch-iodine coloration upon exposure to iodine (Scallet and Sowell, 1967). In this study we are attempting t o correlate the surface T o whom correspondence should be addressed. address, American Maize-Products Co., Roby, Ind. 46326. Present address, Analytical Instrumentation Laboratories. Georgia Institute of Technology, Atlanta, Ga. 30332.
’ Present
structure of the starch with the cooking properties associated with the oxidized starch. Starch is a transparent spherulite. We have found that light diffraction through the starch can often give spurious images which are manifestations of internal details but which might appear as a surface structure. This can be convincingly seen by comparing the light micrographs with the images seen using the scanning electron microscope ( S E M ) . The starch granules, being nonconductive, must be shadowed with a metal coating; hence, only surface details are observed. From a comparison of the light microscope and SEM micrographs, some speculations concerning internal details are possible (Hall, 1968; Hall and Sayre, 1969, 1970a.b). We have extended the use of this instrument to study the changes in the surface structure of the starch upon oxidation. Since the starches were all treated identically in preparation for the scanning electron microscopical study, any observed difference in surface structure should be due only to the oxidation treatment. Chlorite oxidation of starch has been extensively studied. Some of the aspects Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
171
Oxidized corn starch has functional importance as a size material for textile and paper. The scanning electron microscope has been used to explore reasons for one defect in oxidized corn starch, namely, the loss in swelling and solubles of the starch highest in carboxyl content. We have used the percent alkaline hypochlorite treatment of the starches under study as an important quantitative guideline. The loss in swelling power appears to be due to a cementing of the outer surface of the granule either by a redeposition of solubilized starch fractions back onto the granule during drying or by some fusing-like action which the reaction or drying procedure promoted on the surface of the highly oxidized granule. Some speculations concerning the effects of alkaline hypochlorite on the iodine affinity, moisture sorption, and whiteness are given.
of the manufacture, uses, and physical and chemical properties have been reviewed (Scallet and Sowell, 1967). Roberts (1965), in a review of the nondegradative reactions of starch, including hypochlorite oxidation, states that no other starch reaction is more widely practiced, has been more extensively studied, and is less completely understood. Experimental
The preparation of the starch for scanning is the same as previously described (Hall and Sayre 1969, 1970a,b). The starch modifications and resultant analytical data reported were replicated three times and the results averaged. Oxidizing the Starch. The starch was oxidized using a modification of the procedure suggested by Hullinger (1964). All four samples (including the control) were treated in the same manner. Starch slurry (2000 grams) containing 35% dry solids common starch held in a suitable container equipped with continuous agitation was placed in a constant temperature (120" + 1.F) water bath. The p H of the slurry was adjusted to the range of 9.0-10.0 with sodium hydroxide and maintained in this range throughout the 4-hr chemical reaction. After initial p H adjustment, sufficient commercial sodium hypochlorite was added t o furnish the desired available chlorine required for the oxidation. In this series, 0.0, 1.5, 6.0, and 8.0% chlorine dosages were used. At the completion of the 4-hr reaction time, the oxidation was terminated by the addition of antichlorine. The pH was adjusted to 7.0 and the starch washed to remove excess salts. I t was then air-dried. At the low reaction temperature employed, little soluble carbohydrate would be expected to be removed during washing. Moisture Sorption. Moisture uptake of the granular starch was studied by placing starch samples in a moisturesaturated atmosphere a t room temperature (22" =t2.C) for one week following the procedures suggested by Sair and Fetzer (1944) and employing the suggestions of Leach
(1965). The moisture content of each sample was determined a t the end of this time period. One series had been air-dried to approximately lo? moisture while the other series had been dried in a vacuum oven (lOO°C, and 5 hr) prior to conditioning in the water-saturated atmosphere. Swelling Power and Solubility of Granular Starch. Swelling power and solubility were determined according to the procedure of Schoch (1964), in which a weighed starch sample is allowed to swell in water a t 85.C. The weights of both the swollen starch and the dissolved starch are then determined. Iodine Affinity. The iodine affinity of the starch was determined by Schoch's (1964) method for modified starches, in which a starch solution is potentiometrically titrated with a standard iodine solution. This method is not considered applicable to oxidized starches since a sharp inflection point is required to allow extrapolation of the linear portion of the curve back to intersect the zero axis. The starches employed with this work gave a sufficiently sharp inflection point to allow extrapolation; hence, the use of the calcium chloride method (Colburn and Schoch, 1964) was not necessary. Percent Carboxyl. The carboxyl content was determined by method C-22 (Standard Analytical Methods, Corn Industries Research Foundation, 1966). Hunter Whiteness. The whiteness measurements on the starch used in this study were determined by measuring light reflectance differences between the starch sample and a white porcelain disk. The porcelain disk was assigned the number of 100 for absolute whiteness. The instrument used was Hunterlab Model D40 Reflectometer for Whiteness. Viscosity. The viscosity was measured with a Brookfield Viscometer a t 20 rpm and 150" F. The dry solids concentration was not the same in each case. Where necessity dictated an increase in solids, the increase was kept in exact multiples so that a relative comparison could be made (Table I ) . Discussion
Table I. Effect of Chlorine Oxidation on Viscosity of Yellow Dent Corn Starch Viscosity
Treatment
sample no.
Carboxyl, %
1 2 3 4
0.0 0.13 0.58 0.73
172
Dry solids,
7 7 14 21
Yo
at 150" F, cP
5300 680 220 280
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
Since the starches in this study were oxidized under alkaline conditions, it is thought some depolymerization occurred and that keto groups and carbonyl groups are found in greater numbers than aldehyde groups (Roberts, 1965). I n this paper all oxidation products will be referred to as carboxyl groups since these would be expected to occur in the greatest number under the conditions used in this study (Schmorak and Lewin, 1963). Moisture Sorption. The capacity of starch granules to
sorb moisture is a function of the submicroscopic structure of the granules. Generally this depends upon the lateralorder distribution (disorder) and the number of sites (hydrogen bonds) available for attachment of the water to the substrate (Urquhart, 1959). In starch, the amount of disorder brought about by the reduced fit in branched chain (amylopectin) components is probably the reason for the high moisture regain of starches compared to cellulose. I t has been suggested that water is bound to starch as water of crystallization, bound or absorbed water, or as interstitial water (Ulmann and Schierbaum, 1958). The effects of chlorine oxidation on the moisture sorption of corn starch are shown in Table 11. When the starch is dried to only loc-; and then resaturated, the percent moisture sorbed increases with carboxyl content. This would be expected since carboxyl groups could have a wedging effect on the starch chains thereby possibly increasing interstitial water. This could also result in the production of new sites for moisture sorption on hydroxyl groups which might have been previously inaccessible due to interchain secondary bonding but destroying others. When the starch is oven-dried a t high temperatures and under vacuum, the rehydration does not show similar trends. The rehydration is much less than the starch rehydrated after air drying a t room temperature. As a speculation, when the chemically bound water is removed from starch under the influence of heat and vacuum, the starch chains, especially the molecular chains in the micellar regions, may form strong secondary forces which cannot be rehydrated. As a result, an increase in crystallinity or a t least the lateral order due to the strong interchain bonding may occur. Instead, all of the starch samples sorbed essentially the same amount of moisture. This would imply that the total number of sites in any of the starch samples had not changed and that the oven-dried starch had similar lateral order. Alternatively, carboxylation of the starch chains may have occurred only in regions of high disorder where the wedging action would be minimized. Severe drying in such a case might not greatly affect the highly absorbing noncrystalline and highly disordered components of the starch. The wedging action of the bulky carboxyl group may, in fact. maintain the highly disordered area by keeping the chains propped open. Only a few carboxyl groups may be needed to accomplish the wedging action; hence, some carboxyl groups could be wasted in more highly substituted starches. When the starch is gelatinized, however, this increased wedging action can promote lower setback and clarity of the cooled paste by interfering with the formation of intermolecular forces needed for retrogradation. One would expect that carboxyl groups, if they acted to wedge chains apart, would show an increase in moisture sorption similar to that observed for the air-dried starches. (Sair, 1967, has shown that rearrangement of starch does take place during heat treatment and speculates that some of the molecules or parts of them may rotate during such treatment.) Whistler and co-workers (1959) have shown that the physical properties of corn starch change during heat treatments. The int,ernal structure changes which occur during drying are not completely known; hence, only speculation concerning the effect of drying can he offered at present. I t is interesting, however, that the moisture content of the oven-dried starch is essentially constant over the carboxyl range covered in this study.
Table II. Effect of Chlorine Oxidation on Moisture Sorption of Yellow Dent Corn Starch Saturoted equilibrium moisture Sample no.
Carboxyl, %
After oir drying
After oven drying
1 2 3 4
0.0 0.13 0.58 0.73
19.4 19.8 20.0 20.4
15.8 16.0 15.6 15.8
Table 111. Effect of Chlorine Oxidation on Properties of Yellow Dent Corn Starches YO
Treatment sample
no.
1 (Control) 2 3 4
Chlorine treatment
0.0 1.5 6
8
O h
Hunter whiteness
Iodine affinity
Swelling power
Solu. bility
0.0 0.13 0.58 0.73
74.8 85.5 88.9 89.6
34 31 24 15
8.7 13.6 28.7 7.2
3.6 23.3 77.7 73.8
Carboxyl,
The difference in moisture sorption for the various amounts of oxidation points out the need t o check the moisture content of the grade of starch being utilized. Although the difference between the lowest oxidized and the highest oxidized starch is only 1% (1.0 lb of water 100 lb of starch), this may be significant when purchasing bulk quantities of starch, a practice which is increasing in the paper and textile industries. Hunter Whiteness. As would be expected, the whiteness of the starch increased with elevation of percent of chlorine (Table 111). The whiteness of the starch rapidly levels off a t about the 1.5% level and not much additional whiteness is obtained with increased oxidation treatment. As will be seen later, the high chlorine treatment seems to effect a smoothing out of the starch granular surface similar to a fusion or melting of the surface. The surface for the higher chlorine treatments appears to us to be slightly smoother than for the lower treatments. As a result, the small differences in whiteness between the 6 and 8'; chlorine level of treatment may be due only to surface effects and may not be real. Iodine Reactivity. The ability of the starch to complex with iodine is reduced as the degree of oxidation is increased. The average molecular chain length is quite likely reduced by oxidation since the viscosity of the cooked pastes is lowered. I t is generally accepted (Foster, 1966) that iodine uptake is dependent upon its ability to complex with the starch in helices of the amylose chains. If carboxyl groups do indeed cause wedging of starch chains then the helices may be destroyed, or, more likely the cornplexing ability which may depend upon a critical intraand interchain spacing may be reduced. Another possibility could be that bulky carboxyl groups formed by the oxidation of the starch would sterically prevent the iodine from entering into the helical coil; hence, the total amount of iodine complexed would be reduced. If large iodine atoms can enter into and complex within the helical chains, it appears likely that oxidation of some of the alcohol groups, particularly that of C6 on the glucose molecule, would be obtained when the smaller hypochlorite reagent molecules enter into and react a t sites along the helical coils swollen by the alkaline hypochlorite. Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
173
Figure 1. Unmodified yellow dent pearl corn starch composite of two micrographs 222OX
Figure 2. Y,ellow dent pearl corn storch tireated with 1.5% chlorine. Composite of 2 micrographs 2250X
Swelling Power and Solubility. In addition to formation of carboxyl groups which can act as wedges along the starch chains, it would he expected t h a t the alkaline hypochlorite would also hydrolyze some of the starch chains into smaller chain fragments (Roberts, 1965). I t is this starch hydrolysis which accounts for the high solids and low viscosity features of oxidized starches. The wedging action of the c a r b x y l groups would he expected to assist swelling since penetration of the water into t,he starch substrate during cooking would he enhanced. Increased swelling capacity would then allow the shorter hydrolyzed chain fragments easier diffusion from the substrate; hence, one would expect both an increase in swelling capacity and soluhles to result from an increase in carboxyl content. Table I11 shows this is apparently true up to 6% chlorine treatment. For the starch sample having the 8% chlorine treatment and highest carboxyl content, the swelling power is reduced to a value less than the untreated starch. The percentage of soluhles, while not reduced to that for the control, is less than that obtained a t the lower (6%) chlorine treatment. T o explain this apparently anomalous behavior, we subjected the starch samples to a scanning electron microscope analysis. The results are shown in Figures 1-4. For starches treated with up to the 6% chlorine, there is very little surface change as a result of the chlorine treatment. At the 6% treatment, some slight blistering of the surface is noted (Figure 31, but the surface characteristics of the majority of the granules apparently remain unchanged. At low chlorine treatments most of the effect of oxidation is on a microscale and involves chemical reactions with the starch chains with little effect upon the surface structure of the starch granules. At the 8% treatment level, there is some definite change to the surface of the starch granule. Whatever the cause, the starch granules have melted into each other. This could he due to gelatinization of the starch surfaces and to redeposition of swollen starch fractions back onto the granule surface during subsequent drying. The surfaces of the starch grains a t the higher chlorine treatment are somewhat smoother than those a t the lower 174
Ind. Eng. Chem. Prod. Res. Develop.,VoI. 10, No. 2, 1971
Figure 3. Yellow dent pearl corn starch treated with 6.0% chlorine. A composite of 2 micrographs 2250X
Figure 4. Yellow dent pearl corn starch treated with 8% chlorine. A composite of 2 micrographs 2380X
treatments; hence, the surface may have considerably less porosity. The possibility for cross-linking a t the surface cannot be entirely neglected. The net result is that the ability of the starch granule to swell is altered when compared to the cook-out of unmodified yellow dent corn starch. Some granules appear to he cemented to other granules. This cementing action could account for some, hut not all, of the loss of granule swelling. No method is presently available which would enable a chemical analysis of the granule surface or the internal starch architecture. The explanation for the canse of the loss of granule swelling must he based, a t present, purely upon speculation. It would appear that surface gelatinization during oxidation and subsequent drying of the starch surface could be a reasonable interpretation of the observed phenomena. One would expect that the higher chlorine treatment would result in more short chain fragments than for the lower chlorine treatments. This should increase the percent
solubles. If some granule surface change (a fusion or cementing action) has taken place. the rate of diffusion of the soluble fragment from the starch may have been reduced even though more soluble material was produced. For the test method employed. the time of treatment for starches in determining the percent solubles was the same for each starch sample. Given enough time for diffusion. the percent solubles may have increased. Alternatively. the chains a t the surface may have undergone some fusion and:or cross-linking and, as a result, not all of the chains could diffuse from the substrate. This outer surface change of the starch granule can affect functionality of the starch as a size material. During cooking it may be possible that this starch would not completely disperse a t plant cook temperatures. The resulting cooked-size slurry might then contain residues of starch surfaces which were not in true collodial dispersion. This would affect apparent viscosity and size flow property which in turn might affect the continuity of the deposited film if used as a coating binder. I t might also affect the film strength and bonding potential of the material as a warp size. although the total solids content might remain the same. Summary and Conclusions
The effect of starch oxidation is t o carboxylate some of' the glucose molecules of the starch chains and also to hydrolyze the chains so that :he starch undergoes maximum dispersion upon cooking. The oxidation of yellow dent corn in the preparation of oxidized starches apparently does not begin t o affect the surface structure of the starch until about the 6'c chlorine levels. This would imply that the oxidation a t iower chlorine treatment probably breaks down the chains only in the disordered regions of the starch. The breakdown might be assisted by the wedging action of carboxyl groups and the swelling action of the alkaline hypochlorite. Sufficient micellar and crystalline areas are still maintained throughout the reaction t o hold the granule structure intact. The surface of the granules must he particularly resistant since the swelling conditions in the test employed will allow the removal of the hydrolyzed soluble starch chains through diffusion from the granule. The SEM micrographs of starch show that a t high chlorine treatment, the surface undergoes some fusion. The starch surface is affected to the greatest extent since a considerable quantity of soluble fragments can still diffuse from the starch: if the entire granule underwent fusion. the amount of solubles which could be removed from the granule through swelling should have been reduced more than that observed. These results appear to support Whistler's (1959) concept of a case-hardened shell surrounding the corn starch granules. Such a shell or outer covering would retard swelling in the ummodified starch. Modification by osidation may increase the elastic nature of the outer surface through scission of some of the starch chains or through wedging apart of the strong intermolecular forces tending t o hold the surface and internal structure of the granule
intact. thereby promoting greater swelling. At high chlorine treatment, the outer surface is fused possibly through cross-linking or redeposition (retrogradation) of gelatinized starch during subsequent drying and is again made intractable. Swelling is reduced to below that of the untreated starch; however. starch chain fragments are apparently still able t o diffuse through the surface crosslinked network, or through breaks and fissures in the surface of the starch. Acknowledgment
The assistance of American Maize-Products Co. and their permission to publish the results of this work are gratefully acknowledged. Literature Cited
Colburn. C. R.. Schoch. T. J., "Methods in Carbohydrate Chemistry. Vol. IV. Starch," ed. by R . L. lvhistler. Academic. 1964. p 161. Corn Refiners Assoc., Fyashington, D . C., ..Standard Analytical Methods." 3rd ed., May 1966. Foster, F. F.. "Starch: Chemistry and Technology. 1'01. I; Fundamental Aspects." ed. by R. L. iyhistler and E. F. Paschall, Academic. 1965, p 383. Hall, David >I., "Proceedings of the 8th Annual Slashing Seminar." pp 120-33. Auburn University. Auburn. Ala.. Sept. 10-12. 1968. Hall. David NI., Sayre, ,Joseph G.. Text. Res. J . , 39 (111. 1044-53 (1969). Hall, David M., Sayre. Joseph G . , ibid., 40, 147-37 (1970a). Hall, David M.. Sayre, Joseph G.. ibid.. 236-66 (1970bj. Hullinger, C. L.. "Methods in Carbohydrate Chemistry, Vol. IV, Starch," ed. by R. L. Whistler. Academic. 1964, p 313. Leach, H. W., "Starch: Chemistry and Technology. Vol. I , Fundamental Aspects." ed. by R. L. Whistler and E. F . Paschall, Academic, 1963, p 290. Roberts, H. J.. ibid., p 475. Sair, L.. Cereal Chem., 44, 8-26 (1967). Sair. L.. Fetzer. W. R.. Ind. E n s . Chem.. 36. 200-208 (1944). Scallet. B. L.. Sowell. E. A,. "Starch: Chemistry and Technology. Vol. 11: Industrial Aspects." ed. by R. L. Whistler and E. F. Paschall. Academic. 1967. p 237. Schmorak, J., Lewin, M . , J . PolymerSci., A 1 , 2601 (1963). Schoch, T. J.. "Methods in Carbohydrate Chemistry. Vol. 4, Starch," ed. by R . L. Whistler, Academic. 1964. p p 106-8. Schoch, T. J.. ibid., p p 157-60. Ulmann. AI., Schierbaum. F.. Ernaehr.. 3. 6j (1958) (from Chem. Abst., 52, 19197. 1958). Urquhart. A. R.. "Recent Advances in t h e Chemistry of Cellulose and Starch." ed. by .J. Honeyman, Interscience. 1959. Whistler. R. L.. Goatlep. J. L., Spenser. W. \Y..Cerwl Chem., 36, 84-90 (19593.
RECEIVED for review March 2 3 , 1970 ACCEPTED ,January 1. 1971 This paper was presented in part at the 9th Annual Slashing Seminar. Auburn University. Auburn. Ala.. Sept. 9 -11. 1969. The authors thank ..\uburn L-niversity for a GratIT-in-.iid under which t h s study wax accomplished. \Ye further thank the .J. P.Stevens Co. for addtional support.
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