Physical Stabilization of Starch-Allylurea Blends by EB-Grafting: a

EB-Grafting: a Compositional and Structural Study. Aline Olivier ... Electron beam (EB) processing of fresh films with AU content above the solubility...
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Biomacromolecules 2000, 1, 282-289

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Physical Stabilization of Starch-Allylurea Blends by EB-Grafting: a Compositional and Structural Study Aline Olivier,† Fre´ de´ ric Cazaux,† Carole Gors,‡ and Xavier Coqueret*,† Laboratoire de Chimie Macromole´ culaire, UPRESA CNRS 8009, and Laboratoire de Dymamique et Structure des Mate´ riaux Mole´ culaires, UPRESA CNRS 8024, Universite´ des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France Received March 2, 2000; Revised Manuscript Received April 6, 2000

A structural and compositional study of thermoplastic blends prepared from native potato starch (NPS) and various amounts of allylurea (AU) was performed to gain a better understanding of possible radiochemical routes to physically stable materials. The blends, mixed at ca. 130 °C, were studied in the form of 150 µm-thick films. Upon aging at room temperature, the samples obtained from these blends exhibit macroscopic phase separation under the form of allylurea blooming at their surface. The maximal compatibility of allylurea in amorphized potato starch was assessed by gravimetry and compared to that of urea in mixtures with NPS prepared in similar conditions. Physical aging of the unstable blends was monitored for various initial AU contents. Electron beam (EB) processing of fresh films with AU content above the solubility limit was shown to prevent phase separation, essentially as a consequence of radiation grafting onto starch of the unsaturated additive. X-ray diffractometry was performed to control (i) the effective amorphization of starch upon mixing, (ii) the recrystallization of the incompatible AU fraction from untreated blends in the tetragonal form, and (iii) the retardation or the suppression of this phenomenon after EB processsing. The physical stability of the blends treated with a sufficient radiation dose (400-800 kGy) was confirmed by dynamic thermomechanical analysis of samples submitted to various hygrometric conditioning. Introduction Over the past few decades, there has been a great interest in developing biodegradable materials for replacing conventional thermoplastics. Starch-based materials are particularly attractive because they can be produced from a cheap, biodegradable, and annually available resource.1 To obtain thermoplastic starch (TPS) materials, native granular starch is amorphized at temperatures above 100 °C and plasticized with the aid of relatively low molecular weight additives such as water,2,3 glycerol,4,5 sorbitol,6,7 and urea.8,9 TPS processing takes place under controlled high pressure, temperature, and moisture by extrusion, compression molding, or injection molding. A major problem with TPS is the physical instability of the bulk material, caused by migration or phase separation of the plasticizer, as well as by retrogradation of the starch component.10-12 We have recently reported13 on a novel approach to this problem, by treating with accelerated electrons the blends of native potato starch (NPS) with N-allylurea (AU) that result from intimate mixing at 130 °C. AU was selected as a simple unsaturated homologue of urea, with expected similar destructurizing and plasticizing properties for starch and a poor aptitude to long chain polymerization in the amorphous blend. The preliminary investigation showed that AU propenyl group conversion obtained by * Author for correspondence. E-mail: [email protected]. † Laboratoire de Chimie Macromole ´ culaire, UPRESA CNRS 8009. ‡ Laboratoire de Dymamique et Structure des Mate ´ riaux Mole´culaires, UPRESA CNRS 8024.

electron beam (EB) irradiation was associated with macroscopic physical and mechanical stabilization. Discussing the nature of the various grafting and polymerization reactions that can take place under irradiation of the blends is beyond the scope of this paper. The idealized sketch of Scheme 1, though obviously oversimplified, gives an acceptable view for considering the present work. Owing to the potential technological interest of this method and to the complex physical and chemical transformations occurring at the various stages, a detailed study was undertaken to characterize starch-AU (SAU) blends and to gain a deeper insight into their physical behavior. In this context, the present paper focuses on the compositional and structural aspects of amorphization, phase separation, and blooming from blends before the EB treatment. It also examines the physical modifications of importance that follow radiation-induced AU immobilization. Experimental Section Materials. Native potato starch (NPS) was obtained from Roquette Fre`res (Lestrem, France). Allylurea (AU) was obtained from Aldrich. Melting point measurement and spectroscopic analyses (FTIR and 1H NMR) indicate that, apart from moisture, it can be essentially considered as a pure chemical. Urea was purchased from Acros with a purity of 99.5%. Sample Preparation. Thermoplastic starch-AU blends were prepared in an internal Brabender mixer at 130 °C with

10.1021/bm005527i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000

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Starch-Allylurea Blends Scheme 1. Simplified Representation of AU Radiation-Grafting onto Starch

a screw rotation speed of 50 rpm for 15 min. The blends were chopped into small cubes and quenched at -17 °C immediately after processing in order to avoid physical evolution. For the preparation of TPS films, the starting material was ground into a coarse powder and transformed by compression molding with a Darragon hydraulic press at 130 °C with a pressure of 10 MPa maintained during 5 min. The loss of water during this step is negligible, the water uptake measured after reconditioning the pressed sample at 58% RH being insignificant. After pressing, the films were stored at -17 °C until the moment of analysis. Characterization of the materials and EB processing of films were performed with samples reconditioned from the frozen stock just before use. Compositional Characterization of Blends. The moisture content in native starch and in the starch-AU blends was determined for samples initially equilibrated in closed chamber at 58% relative humidity (RH) for 5 days at 23 °C. The weight loss was measured gravimetrically, either after 2 weeks dehydration in a desiccator containing phosphorus pentoxide (P2O5) at 23 °C or by thermogravimetric analysis (Shimadzu TGA 51) measured in dry air flow at a constant end temperature of 120 °C (temperature rise from room temperature at 10 °C‚min-1). Flexibility of TPS Films. The flexibility of TPS films was assessed by means of a bending test. Film samples of thickness 150 µm were folded back around cylindrical rods of known radius. Starting from a 12 mm radius, the tests were carried on, successively, downward to 1 mm (1 mm step), until film cracking was observed. The smallest radius for which bending occurs without apparent damage is recorded for rating brittleness. X-ray Diffractometry. Measurements of diffracted intensities were recorded at ambient temperature with a diffractometer using a curved multidetector (INEL, CPS120), covering the scattering angle (2θ) range from 5 to 120°. The convergent X-ray beam was monochromatized by the (101h1) Bragg reflection of a curved quartz monochromator and operated at the Cu KR1 wavelength (λKR1 ) 1.54056 Å). Powdery NPS and AU samples were introduced into a Lindeman glass capillary of diameter 0.7 mm, and SAU films (ca 5 mm × 30 mm × 0.15 mm) were attached to a metallic frame. The samples were allowed to rotate around their long axis at the center of the goniometer, perpendicularly to the measurement plane. For kinetic monitoring of crystallization, the diagrams were obtained after a recording time of 1000 s. Analysis durations for physically stable samples were up to 3000 s (AU) or 60000 s (NPS, stable blends) in order to enhance the signal/noise ratio.

EB Processing. TPS samples containing AU were treated as films under the electron beam of an Electrocurtain CB150 generator operating at 175 kV. Current (4 mA) and conveyor speed were adjusted to yield a unit dose of 50 kGy per pass. Higher doses were obtained by increasing the number of passes in the chamber of the EB processor. The conversion of AU upon exposure to EB radiation was determined by FTIR monitoring of the disappearance of the peak at 1421 cm-1 assigned to the allylic double bond. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis was carried out on a Rheometrics RSA II solid analyzer working in the tension mode. The frequency was 1 Hz and the temperature was varied between -50 and +50 °C by steps of 5 °C. The maximum temperature was limited to 50 °C in order to avoid drying of the sample during testing. Nominal dimensions of the samples were 10 mm × 35 mm × 0.15 mm. The gap between the jaws of the rheometer was 22.1 mm. Results and Discussion Preparation of Starch-AU Blends. The blends were prepared in an internal mixer from the powdery potato starch and AU constituents premixed in the desired proportions. Transformation into molten thermoplastic materials proceeded within 15 min of mixing at temperatures ranging from 120 to 140 °C. Addition of small amounts of water facilitates processing by significantly lowering the melting temperature and melt viscosity of starch.2 The four blends named SAU20, SAU30, SAU40, and SAU50 were obtained from feed compositions including NPS, AU, and water in 100:20:10, 100:30:5, 100:40:5, and 100:50:5 parts (w/w/w), respectively. Addition to the dry mixture of five weight parts of water for 100 parts of NPS was sufficient if the AU feed was 30 parts and more. With the blend containing the lowest amount of AU (20 parts), the viscosity at 130 °C was too high to ensure adequate processing conditions. A larger amount of water was required in this case, yielding a material of higher initial moisture content. In the reference conditioning ambience we used for this study (23 °C, 58% RH), we indeed measured a water uptake with SAU30, -40, and -50 samples, whereas the SAU20 sample gave rise to water desorption (Figure 1). Equilibrium appears to be reached within a few hours, owing to the submillimetric thickness of the samples. Macroscopic Evidence of Demixing. Immediately after pressing the coarse TPS powder conditioned at 58% RH, the films were transparent and appeared visually to be homogeneous. This indicates that the granular structure was destroyed during processing. After 1 h of aging, the films

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Figure 1. Kinetics of hydration of TPS films stored in closed chamber at 23 °C and 58% RH: SAU50 (]), SAU40(O), SAU30 (4), and SAU20 (0). Table 1. Appearance of the TPS Films after Pressing and after Aging optical aspect

brittleness indexb

AU content pphdsa

initial

after 1 h

initial

after 350 h

23.5 35.3 47.1 58.8

transparent transparent transparent transparent

transparent opaque opaque opaque

0c 0c 0c 0c

0c 0c 1 9

a Expressed in g/100 g of dry starch. b Curvature radius for folding without apparent damage. c Folding backward with an angle of 180° without apparent damage.

Scheme 2. Folding Test for Measuring Film Flexibility

obtained from SAU30, -40, and -50 became opaque, AU molecules blooming at the sample surface. The phenomenon was not observed for the lowest amount of AU, indicating that the compatibility limit was between 24 and 35 parts per 100 parts of dry starch (pphds), under the given storage conditions (Table 1). The quantification of blend plasticization by means of standard thermomechanical or calorimetric analyses was perturbed by the physical instability of the samples. A less informative, but more reliable, evaluation of flexibility was preferred. We used a bending test performed on 150 µm thick films of each composition just after pressing or of each composition taken sequentially from the conditioning chamber. Each experiment can be rapidly performed without risking sample dehydration. It consists of folding the film around cylindrical rods of known diameter and determining the smallest curvature radius that does not induce visible damage, essentially by breaking (Scheme 2). The critical curvature radius before observing breaking can be used as a brittleness index for rating the TPS materials. The initial measurements show that the series containing 20-50 AU wt-parts can be folded as flexible sheets of conventional thermoplastic material (Table 1). Low molecular weight (LMW) molecules or salts with hydrogen bonding capability are expected to compete with water for interacting with starch anhydroglucose units (AGU).3 It is

Figure 2. Variations of brittleness index with aging time for TPS films stored in a closed chamber at 58% RH and 23 °C: SAU50 (]), SAU40 (O), SAU30 (4), and SAU20 (0).

generally considered that once a maximum number of AGU sites available for hydrogen bonding are interacting with the LMW compounds, the plasticizing effect is obtained as a consequence of the free-volume increase.14 However, this initial property of the SAU30, -40, and -50 films varies strongly upon prolonged storage in the conditioning chamber, whereas the SAU20 samples exhibit constant flexibility on the same time scale (Figure 2). The reduction of plasticizer content in thermoplastic starch, apparently caused by AU migration, results in higher brittleness indices very shortly after pressing the SAU50 films. The loss of flexibility appears with a longer delay, and proceeds with a lower rate, for SAU30 and -40 blends that include lower levels in AU. Taking into account the plascticizing effect of AU in starch, these two features are in qualitative agreement with the general kinetic behavior of unstable blends demixing under the control of local chain mobility.15 This phenomenon shall not be viewed as a rigidification by simple plasticizer depletion, since the SAU20 film have a brittleness index rated zero. Obviously, more complex transformations with compositional and structural heterogeneities are likely to take place. Compositional Analysis of Starch Blends. At the starting point of a structural investigation on blended solid materials, it is essential to have a precise knowledge of their chemical constitution. We have previously concluded from elemental and from spectroscopic analyses that thermal degradation of starch and AU was negligible in the processing conditions.13 The quantity presenting the greatest uncertainty is obviously the water content which has a strong influence on a number of physical properties exhibited by glucans.16 Recalling that the choice for AU as a radiation sensitive additive was based on the structural similarity with urea, a known destructurizing and plasticizing agent,8,9 the corresponding mixtures with urea were prepared and characterized for comparison. The data collected in Table 2 express the composition in weight parts for 100 parts of dry starch (pphds) or as a percentage of constituents in freshly prepared blend films equilibrated at 23 °C and 58% RH. The water content of NPS in these conditions is 17.7 pphds, in agreement with literature data for potato starch.17 The moisture content in

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Starch-Allylurea Blends Table 2. Plasticizer (AU or Urea) and Water Content Expressed in wt-parts per 100 parts of Dry Starch after Conditioning Freshly Prepared Samples at 58% RH and 23 °C blend name

starch content (wt, dry basis)

wt-amount of plasticizer

wt-amount of water

NPSa SAU20b,d SAU30b,d SAU40b,d SAU50b,d SU20c SU30c SU40c SU50c,d

100 (85.0%) 100 (74.5%) 100 (68.6%) 100 (63.6%) 100 (59.1%) 100 (70.2%) 100 (67.2%) 100 (62.2%) 100 (57.7%)

0 (0%) 23.5 (17.5%) 35.3 (24.3%) 47.1 (30.0%) 58.8 (34.8%) 29.4 (20.6%) 35.3 (23.7%) 47.1 (29.3%) 58.8 (33.9%)

17.7 (15.0%) 10.7 (8.0%) 10.4 (7.1%) 10.1 (6.4%) 10.3 (6.1%) 13.1 (9.2%) 13.6 (9.1%) 13.7 (8.5%) 14.6 (8.4%)

a Native potato starch. b Wt-parts of AU per hundred parts of native stock starch with 15% water content. c Wt-parts of urea per hundred parts of native stock starch with 15% water content. d Apparent signs of blooming 1 h after film preparation.

Figure 3. Weight amount of migrated AU from the SAU50 (]), SAU40 (O), SAU30 (4), and SAU20 (0) films stored in a closed chamber at 58% RH and 23 °C.

the blends was approximately 10.5 ( 0.5 pphds, independent of the AU feed in the four ternary mixtures SAU20, -30, -40, and -50. The determination of an invariant value of the water content in AU-starch blends would be expected in samples where external migration of excess AU is completed. Indeed, crystalline AU is not very hygroscopic as shown by moisture determination performed on the commercially available chemical (ca 0.25-0.85 wt % at 40-58% RH). The samples could then be depicted as blends with the equilibrium ternary composition surrounded by a moisture-free crust of excess AU. This view, which gives the observed compositional results, would only hold if the migration process was effectively achieved at the moment when moisture determination is made. Actually, though the signs of external AU migration are observed very soon after sample preparation, AU migration may proceed with a much different rate. The blooming at 58% RH was monitored by collecting the amount of AU crystal appearing, week after week, at the film surface (Figure 3). AU blooming appears as a slow process compared to water sorption and variation of the brittleness index. We can also assess from the plots of Figure 3 the maximal compatibility of AU (SAU) in amorphized starch, since internal demixing was not observed optically, and since no

significant additional blooming appears over the period of 3 months following the first 4 months of storage. In the reference conditions of this study, the value is SAU ) 25 ( 2 pphds. This is much less than for unsubstituted urea which possesses a structure allowing stronger interactions with the glucan, and gives blends without blooming for urea contents up to 53 ( 6 pphds (Table 2). This is in qualitative agreement with the solubility parameter of urea (δ ) 38.5 MPa0.5)18 much closer to that for starch (δ ) 45 MPa0.5)9 than the value for AU is, according to our estimate (δ ) 33-35 MPa0.5). Moreover, urea is less hydrophobic than AU, giving water contents at equilibrium ranging from 13 to 15 pphds (Table 2), higher than that in AU blends but still lower than in the native starch. Evidence that we provide on the physical instability over various time scales emphasizes the potential benefits of a modification process based on additive grafting. The rigidity increase that is observed with all samples, starts well before that significant transport at the sample surface is achieved in the case of SAU40 and SAU50 samples. This phenomenon can be interpreted in various ways that are not necessarily exclusive of the other: (i) change in the nature and amount of molecules H bonding to starch squeleton, (ii) phase separation decreasing the amount of plasticizer in the continuous phase including the polymer, and (iii) supramolecular organization and crystallization. A simple model can be used to describe the competition between AU and water for the AGU sites available for hydrogen bonding (Scheme 3). In a blend of initial AU content above its maximal equilibrium solubility, AU transport out of the pool of plasticizer molecules is expected to enhance the fraction of binding water molecules. Profound changes in intra- and intermolecular interactions are thus anticipated at this level from the demixing and migration of AU. Subsequent structural modifications may include single and double helix formation, as well as inclusion and supramolecular arrangements. Additionally, if microphase separation takes place very shortly after film preparation, the resulting heterogeneous material consisting in dispersed microdomains rich in AU, surrounded by starch blend poorer in AU plasticizer, would also show the rigidification we have measured. Though no evidence internal demixing could be observed in films by optical microscopy, X-ray diffraction (XRD) analysis will provide some interesting elements on this issue. Structural Characterization by X-ray Diffractometry. The starch used as starting material exhibits the wide-angle X-ray diffractogram of Figure 4, with the typical B-type pattern of potato starch granules.19 Using a standard procedure,20 we have determined from the contributions of the amorphous and crystalline fractions to the peak intensity at 2θ ) 17.1°, a crystallinity of 24 ( 1%. This value compares well with the 22% crystallinity determined by means of an extensive characterization of NPS.21 Since the blends were eventually physically unstable, the measurements were performed for a relatively short period of time (1000 s) to have a snapshot view of the blend structure. As a consequence, the diffractograms are somewhat noisy. The thermomechanical process applied during blend mixing induces amorphization of the native starch, as shown

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Scheme 3. Simplified Representation of the Material Balance Associated with Phase Separation and Its Influence on Starch Interactions with Compatible Plasticizersa

a

f(a) is an implicit function of equilibrium constants K1 and K2 and of blend composition.

by the diffractograms in parts a and c of Figure 5, obtained from freshly prepared SAU20 and SAU50 films, respectively. The initial B-type pattern, with a noticeable intense peak at 2θ ) 17.1°, is absent. Thus, snapshot records do not reveal strong crystalline organization. However, upon increasing the time of analysis (up to 60000 s), the films containing 20-50 parts of additive exhibit unambiguously V-type crystallinity, characterized by the peaks located at 2θ angles equal to 13.6 and 20.8°. This is exemplified by the diffractogram of Figure 5b obtained from a SAU20 film, which is not affected by any phaseseparated AU. The V-type pattern is generally observed when amylose crystals are produced in the form of inclusion complexes.22 A number of surfactants have been shown to induce this phenomenon, and AU obviously presents an amphiphilic structure. Such inclusion complexes appear to be formed during mixing and/or film processing of SAU blends. Nevertheless, one cannot exclude that this type of organization increases upon aging, even if precise quantification is uneasy. After 90 min-aging, the SAU20 sample gives a diffractogram similar to that of Figure 5a, but the SAU50 one

Figure 4. X-ray diffractogram of the native potato starch used for this study.

exhibits additional sharp peaks at 2θ values equal to 7.8, 22.2, 22.8, and 26.7° (Figure 5d). The appearance of these diffraction lines is approximately concomitant with the blooming phenomenon. A polycrystalline sample of commercial AU powder was submitted to X-ray diffraction analysis, but none of the expected four lines was observed in the obtained pattern (Figure 6a), although it was unambiguously established by spectroscopic characterization that the powder collected at the surface of the blooming films was pure AU.13 AU actually exhibits crystalline polymorphism.23 Besides the monoclinic form, AUmono, that is obtained from methanolic solutions, a tetragonal structure, AUtetra, is claimed to be obtainable from aqueous solutions. We were able to prepare the metastable allotropic form from the melt, placed under pressure, and submitted to forced cooling, in a procedure similar to that used for preparing the SAU films. We have recorded from this powder the diffractogram of Figure 6b exhibiting, among other, the four lines observed from the demixed blends. The position of the reflection lines

Figure 5. X-ray diffractograms of: (a) SAU20 immediately after pressing, (c) SAU50, immediately after pressing, and (b) SAU20 and (d) SAU50 after 1 h and 30 min of aging in a closed chamber at 58% RH and 23 °C.

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Figure 8. X-ray diffractograms of a SAU50 sample aged for 11 weeks and exhibiting strong blooming (a) and the sample after one (b) and two collections (c) of surface crystalline powder. Figure 6. X-ray diffractograms of the two allotropic forms of AU crystal: (a) monoclinic AU obtained commercially and (b) tetragonal AU after heating at 130 °C and cooling under pressure.

Table 3. Retardation of AU Demixing in SAU50 Films as a Function of EB Irradiation Dose radiation dose (kGy)

migrationa

induction timeb

AU conversionc

0 100 175 210 278 400 527 808

yes yes yes yes yes no no no

20 min 20 min 1 day 1 day 1 week 3-4 months >1 year >1 year

0 17 23 26 34 46 56 76

a Visual detection of blooming. b Time before detection of crystallinity by XRD at 2θ ) 7.8° with S/N ratio > 2. c Deduced from FTIR absorbance measurements at 928 cm-1.24

Figure 7. X-ray diffractograms of SAU50 after different time of aging in a closed chamber at 58% RH and 23 °C: Evolution of the peaks located at 2θ equal to 22.2 and 22.8°.

allow to calculate unambiguously the unit cell parameters of AUtetra.24 The three lines of highest intensity can be used as the signature of AU crystal for monitoring the phase separation process. For an SAU50 sample, the increase with aging time of the peaks located at 2θ ) 22.2 and 22.8° is shown in Figure 7. The location of phase separation in blends containing excess AU merits some discussion. We have already mentioned the increase in brittleness index that takes place in a much shorter time scale than macroscopic blooming. It was hypothesized that rapidly occurring demixing within the blend would give the observed mechanical effects. We have analyzed comparatively the X-ray diffractograms of one SAU50 blend film aged for 11 weeks and exhibiting strong AU blooming (Figure 8a). The intensity of AU sharp lines were only slightly reduced after careful cleaning of the film surface and immediate XRD analysis. This indicates that, in the absence of significant amounts of surface crystals, the transparent film contains crystallites of small size, resulting from internal microphase separation. Immobilization of AU by EB Processing. The method based on EB irradiation for preventing plasticizer migration from starch blends was presented in a preliminary paper.13 The report essentially contained macroscopic information on

the radiation-induced transformations. We have focused this part of the present structural investigation on films derived from the SAU50 starch blends, which contain the higher amount of incompatible AU, and exhibit fast blooming. The eventual detection of AU crystal blooming and the induction period preceding crystallization are indicated in Table 3 for the different doses of accelerated electrons. The corresponding AU conversion, assessed from FTIR spectroscopy, is also indicated. By virtue of radiation grafting and polymerization, AU migration is delayed for doses ranging between 100 and 400 kGy and completely prevented for doses above the latter value. The absence of AU blooming at intermediate conversion reveals the expected enhancement of compatibility upon covalent grafting. A series of X-ray diffractograms (Figure 9) recorded from SAU50 samples submitted to various EB doses and further aging for ca. 1 year indicates the various effects of physical stabilization induced by ionizing irradiation. Degradation of glucans under ionizing radiation is well-documented.25 It is expected that chain scission, depolymerization, and interchain bridging occur simultaneously,26 together with the reactions involving AU. The contribution of these effects to the general behavior of EB-modified SAU blends is under investigation. However, the present XRD results can be analyzed at this point on the basis of AU polymerization and grafting. Exposure to a 100 kGy dose does not prevent internal recrystallization. This confirms the visual inspection for blooming reported in Table 3. The sharper lines emerging from the diffractogram of Figure 9-a are actually assigned to the AUmono allotrope. The characteristic lines of AUtetra at

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reconditioned at 58% RH and finally stored for 1 week in a wet ambience (70% RH) and reconditioned at 58% RH. Despite this aging cycle, the thermomechanical spectra were very similar in shape and indicate temperatures for maximum tan δ in the same range (25 ( 4 °C). This stability contrasts with the large and fast variations of Tg in starch-glycerol blends that undergo retrogradation.4 Conclusion

Figure 9. X-ray diffractograms of SAU50 samples aged for ca. 1 year after exposure to 400 (a), 527 (b), and 808 kGy (c) EB dose.

Figure 10. Thermomechanical spectrum of an irradiated SAU50 film (dose ) 700 kGy) conditioned at 58% RH, after treatment (]), same film as previous plus 1 week aging in dry ambience (0), and same film as previous plus 1 week aging in wet ambience (4).

2θ ) 7.8, 22.2, and 22.8° are absent, but a number of emerging peaks are located at 2θ values where AUmono crystals give sharp and intense lines (11.9, 20.1, 21.2, 23, and 24°). This indicates ulterior physical evolution in phaseseparated samples. Higher doses are efficient for immobilizing AU, but a 527 kGy treatment does not prevent further crystallization of V-type (starred peaks in Figure 9b at about 13 and 20° for 2θ). This latter organization of the blended starch is essentially avoided for a 808 kGy dose. In addition, we cannot exclude, from the current data in hand, that various forms of V-type crystallinity appear in blends submitted to low radiation doses. On the basis of literature data,22 it is possible to distinguish VH type crystallinity, with peaks at 2θ ) 13.0 and 19.8°, from VA type with peaks at 2θ ) 13.4 and 20.8°. When starch blends are not affected by phase separated AU crystals, the peaks are observed at 2θ ) 13.6 and 20.8° in fresh samples, whereas the lines are detected at 2θ ) 12.9 and 19.8° in the aged ones (Figure 9c). To obtain confirmation of the permanent stabilization effect, the thermomechanical spectra of an EB-irradiated (700 kGy) SAU50 film conditioned at 58% RH after 1 week of aging in a wet and then in a dry ambience was recorded (Figure 10). This reference behavior was compared to that of the same sample submitted to dry ambience (2% RH) for 1 week and

This report, devoted to allylurea-starch mixtures, provides quantitative information on the initial and equilibrium composition at 58% RH of the resulting thermoplastic blends. The proposed method for preventing phase separation by radiation grafting, was shown to efficiently delay AU migration and recrystallization in the tetragonal structure, as demonstrated by X-ray diffraction monitoring. Irradiation doses of ca. 400 kGy and above were shown to immobilize AU in blends containing twice the limiting solubility of AU in amorphized potato starch. Structuring of starch in the V-type form was observed immediately after thermal processing for blends containing AU in amounts close to the compatibility limit, as well as for unstable blends with AU in excess. Some signs of retrogradation were detected for the blends treated by EB radiation at doses preventing migration and lower than 700-800 kGy. The physical stability of the blends was also confirmed by thermomechanical analysis of irradiated films including large amounts of AU and submitted to strong variations of hygrometric ambiences. Further investigations on the kinetic and mechanistic aspects of the grafting reaction are in progress and will provide a better understanding of the chemical transformations at the origin of the demonstrated physical stabilization. Acknowledgment. The assistance of Dr. U. Maschke in conducting EB-irradiation experiments is gratefully acknowledged. This work was financially supported by the Centre National de la Recherche Scientifique, the Conseil Re´gional Nord Pas de Calais, and the EU program FEDER. References and Notes (1) Doane, W. M.; Swanson, C. L.; Fanta, G. F. In Emerging Polymeric Materials Based on Starch: materials and chemicals from biomass; Rowell, E. M., Schultz, P., Narayan, R., Eds.; ACS Symposium Series 476; American Chemical Society: Washington, DC, 1992; pp 197230. (2) Forssell, P.; Mikkila¨, J.; Suortti, T.; Seppa¨la¨, J.; Poutanen, K. J. Macromol. Sci.sPure Appl. Chem. 1996, A33, 703-715. (3) Kirby, A. R.; Clark, S. A.; Parker, R.; Smith, A. C. J. Mater. Sci. 1993, 28, 5937-5942. (4) Van Soest, J. J. G.; Knooren, N. J. Appl. Polym. Sci. 1997, 64, 14111422. (5) Van der Burgt, M. C.; Van der Woude, M. E.; Janssen, L. P. B. M. J. Vinyl Addit. Technol. 1996, 2, 170-174. (6) Funke, U.; Bergthaller, W.; Lindhauer, M. G. Polym. Degrad. Stab. 1998, 59, 293-296. (7) Lourdin, D.; Coignard, L.; Bizot, H.; Colonna, P. Polymer 1997, 38, 5401-5406. (8) Willet, J. L.; Jasberg, B. K.; Swanson, C. L. Polym. Eng. Sci. 1995, 35, 202-210. (9) Shogren, R. L.; Swanson, C. L.; Thompson, A. R. Starch/Staerke 1992, 44, 335-338. (10) Lourdin, D.; Bizot, H.; Colonna, P. Macromol. Symp. 1997, 114, 179-185. (11) Ro¨per, H.; Koch, H. Starch/Staerke 1990, 42, 123-130.

Starch-Allylurea Blends (12) Poutanen, K.; Forssell, P. TRIP 1990, 4, 128-132. (13) Ruckert, D.; Cazaux, F.; Coqueret, X. J. Appl. Polym. Sci. 1999, 73, 409-417. (14) Kalichevsky, M. T.; Jaroszkiewicz, E. M.; Blanshard, M. V. Polymer 1993, 34, 346-358. (15) Sperling, L. H. Polymeric Multicomponent Materials: An Introduction; Wiley: New York, 1997. (16) Trommsdorff, U.; Tomka, I. Macromolecules 1995, 28, 6138-6150. (17) Bule´on, A.; Bizot, H.; Delage, M. M.; Multon, J. L. Starch/Staerke 1982, 34, 361-366. (18) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1991. (19) Van Soest, J. J. G.; Hollemon, S. H. D.; de Wit, D.; Vliegenthart, J.

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