Development of Biodegradable Foamlike Materials Based on Casein

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Biomacromolecules 2010, 11, 2640–2646

Development of Biodegradable Foamlike Materials Based on Casein and Sodium Montmorillonite Clay Tassawuth Pojanavaraphan,† Rathanawan Magaraphan,† Bor-Sen Chiou,‡ and David A. Schiraldi*,§ Polymer Processing and Polymer Nanomaterials Research Unit, The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand, Bioproduct Chemistry and Engineering, USDA/WRRC/ARS, 800 Buchanan Street, Albany, California 94710, and Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202 Received June 3, 2010; Revised Manuscript Received August 18, 2010

Biodegradable foamlike materials based on a naturally occurring polymer (casein protein) and sodium montmorillonite clay (Na+-MMT) were produced through a simple freeze-drying process. By utilizing DL-glyceraldehyde (GC) as a chemical cross-linking agent, the structural integrity of these new aerogels were remarkably improved when compared to those of the control system (without GC), with a minimal increase in the density from 0.11 to 0.12 g cm-3. The degree of perfection of the foamlike structures was another parameter that had a significant influence on the physical and thermal performances of the low density composites. The biodegradability of the aerogels was investigated in terms of the carbon dioxide (CO2) evolution for up to 8 weeks in compost media under controlled conditions.

Introduction Biodegradable and biocompatible polymers have received considerable attention from both academic and industrial researchers over the past decade due to the decline in available fossil resources and the increasing preference toward environmentally friendly plastics.1-4 The current generation of biobased polymers are produced primarily from renewable resources, such as sugar cane, proteins, and starches; such materials for packaging, adhesives, coatings, and biomedical applications can be produced with less overall energy consumption than their petrochemical counterparts and tend to be less toxic to the environment.3,4 Casein is a naturally occurring macromolecule that accounts for approximately 80% of the protein content of cow’s milk; it is a phosphoprotein that can be separated into various electrophoretic fractions, such as Rs-casein, κ-casein, β-casein, and γ-casein in which each constituent differs in primary, secondary, and tertiary structure, amino acid composition, and molecular weight (19-24 kDa).5-7 Due to its random coil conformation with a high degree of molecular flexibility and large amount of polar groups, casein shows good film-forming and coating properties as well as excellent barrier properties to nonpolar substances (oxygen, carbon dioxide, and aromas). This makes it an excellent candidate for numerous applications, such as paper coatings, adhesives, and food packagings.5-7 However, like other protein-based materials, casein possesses two major drawbacks: limited mechanical strength and water sensitivity, which might restrict its practical applications.5 Modification of the casein structure or blending with other materials, including plasticizers,5-7 cross-linkers,7 waxes,8 and other polymers9 have been attempted to correct its deficiencies. Formulation of nanobiocomposites has been an approach used to improve the * To whom correspondence should be addressed. Tel.: (216) 368 4243. Fax: (216) 368 4202. E-mail: [email protected]. † Chulalongkorn University. ‡ USDA/WRRC/ARS. § Case Western Reserve University.

overall thermal and mechanical properties of naturally occurring polymers, because such an approach could potentially bring about greatly improved properties and still retain material biodegradability.3 For this reason, layered silicates (such as Na+MMT) have been extensively used as reinforcements in nanocomposites systems over the past 20 years because of their high aspect ratios, which offer high interfacial areas with the matrix, as well as the ability to be organically modified by cation exchange.10 By utilizing a single freeze-drying step, it has been found that the layered silicates can be rearranged into a house of cards structure with bulk densities typically ranging from 0.01 to 0.1 gcm-3 (∼95% void volume fraction).11-20 Because the clay aerogels are relatively fragile, the incorporation of either a polymeric component or natural or synthetic fibers into the clay aerogel sample is required to improve their mechanical rigidity and to produce the foamlike structures that reflect the thermal/ mechanical properties of the matrix polymers themselves.11-20 These low density polymer/clay aerogel composites represent an interesting alternative to the typical insulating polymeric foams made from expanded polystyrene (PS) and rigid polyurethane (PU) used for packaging and cushioning applications.16 Environmentally benign foamlike materials, referred to as casein aerogels and casein/clay aerogel composites, were produced from their aqueous aerogel precursor suspensions using a simple freeze-drying process in the present study. To improve their physical, mechanical, and thermal properties, the biocompatible cross-linking agent, DL-glyceraldehyde (GC), was incorporated into the aqueous mixture, followed by heat treatment of the freeze-dried aerogels to form cross-links through reaction of the amino groups in casein with the aldehyde groups in GC. The biodegradability of such low density biobased polymer/ clay aerogel composites was investigated for the first time using a compost media under controlled conditions.

10.1021/bm100615a  2010 American Chemical Society Published on Web 08/31/2010

Biodegradable Foamlike Materials

Biomacromolecules, Vol. 11, No. 10, 2010

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Table 1. Composition of the Aqueous Suspensions and Density Values of the Freeze-Dried Aerogels density (g cm-3)

composition

a

samples

casein (wt%)

CA10 CA10G1 CA10G5 CA10G10 CA10M5 CA10G5M5 CA2.5M5 CA2.5G5M5

10 10 10 10 10 10 2.5 2.5

a

GC % (w/w)

+

Na -MMT (wt%)

w/o post curing

5 5 5 5

0.110 ( 0.001 0.108 ( 0.002 0.117 ( 0.001 0.120 ( 0.002 0.113 ( 0.002 0.109 ( 0.005 0.071 ( 0.003 0.076 ( 0.005

1 5 10 5 5

w post curing 0.111 ( 0.003 0.120 ( 0.003 0.123 ( 0.002 0.096 ( 0.012 0.078 ( 0.003

x% (w/w) indicates the weight percentage of GC relative to the total weight of casein in solution.

Experimental Section Materials. Casein (lab-grade) and sodium hydroxide (NaOH) were supplied by Fisher Scientific and used as received. DL-Glyceraldehyde, GC (FW 90.08), was purchased from Sigma Aldrich Corp. (St. Louis, MO) and used without further modification. Sodium Montmorillonite (Na+-MMT; PGW grade, cation exchange capacity (CEC) 145 meq/ 100 g) was purchased from Nanocor Inc. Deionized (DI) water was obtained using a Barnstead RoPure reverse osmosis system. Aerogel Preparation. General. Percentages of casein and clay structural components are given as percentages of total solution to be freeze-dried. Percentages of GC cross-linker are relative to the amount of casein present in the material. Casein Aerogels. Casein powder was initially dispersed in 30 mL of a 0.17 M NaOH solution with stirring at room temperature. DI water (27.5 mL) was then added and heated at 80 °C for 2 h to create a 10 wt % casein solution. After being cooled down to ambient temperature, the appropriate amount of GC was added (see Table 1) with constant stirring to minimize the generation of trapped air bubbles. Once thoroughly mixed, the resulting solution was transferred to the cylindrical polystyrene (PS) vials and immediately frozen in a solid carbon dioxide/ethanol bath (∼-80 °C) before being placed in a VirTis Advantage EL-85 lyophilizer, where high vacuum was applied to sublime the ice. After 4 days in the freeze-dryer, the samples were removed and postcured in an oven at 80 °C for 24 h to ensure maximum curing of casein aerogels. Control samples were made in a similar fashion using the neat casein aqueous solution. The quantity of casein was reduced by a factor of 4 in order to produce the analogous 2.5% solutions. Casein/Clay Aerogel Composites. Casein solutions were prepared using the 0.17 M NaOH solution, as described above, and were mixed with the GC at a 1:20 ratio of GC to casein (dry weight basis). In a separate vessel, 2.75 g of Na+-MMT was blended with 27.5 mL of DI water on the high speed setting of a Waring model MC2 mini laboratory blender for ∼1 min to create a 10 wt % clay aqueous suspension. The clay gel was then slowly added into the casein solution under constant stirring to create the casein/clay gels comprising 5 wt % Na+-MMT, 5% (w/w) GC, and either 2.5 or 10 wt % casein (expressed as wt% of the solution to be freeze-dried, see Table 1). The resulting mixtures were then subjected to the protocol mentioned above in order to create the casein/clay aerogel composites. A similar process was used to produce 2.5% casein aerogels. Note that the neat casein/clay aerogel composites were also prepared following the same procedure and served as controls. Characterization. The densities of the dried aerogels were calculated from the mass and dimension measurements using a Mettler Toledo AB204-S analytical balance and digital caliper, respectively. Fourier transform infrared (FTIR) analyses were conducted using a Perkin-Elmer FT-IR Spectrometer Spectrum GX equipped with a Universal attenuated total reflectance (ATR; single reflection diamond/ ZnSe) crystal accessory. The spectra were recorded over a wavenumber range of 4000-600 cm-1 with a resolution of 4 cm-1. Compression testing was conducted on the cylindrical specimens (∼20 mm in diameter and height) using an Instron model 5565 universal testing machine, fitted with a 1 kN load cell, at a constant crosshead

speed of 1 mm min-1. Five samples of each composition (allowed to equilibrate at room temperature/humidity, ∼25 °C/30% RH, for at least 24 h) were tested for reproducibility. The compressive modulus and toughness were calculated from the slope of the linear portion and the integrated area of the stress-strain curve, respectively. Morphological microstructure of the aerogels was examined using a Quanta 3D 200i scanning electron microscope (FE-SEM) at an acceleration voltage of 5 kV. The samples were fractured in a liquid nitrogen bath, mounted on a stub with carbon tape, and then coated with a thin layer of palladium (∼50 Å) using a Denton vacuum sputter. Thermogravimetric analyses (TGA) were conducted on a TGA Q500 (TA Instruments) under a nitrogen flow (40 mL min-1). The samples (∼5 mg) were placed in a platinum pan and heated from ambient temperature to 600 °C at a rate of 10 °C min-1. The biodegradability of aerogels was monitored on the basis of CO2 evolution using a Micro-Oxymax Respirometer System (Columbus Instruments Inc.). Cylindrical specimens were ground and mixed with 20 g of compost soil and kept in a 250 mL sample chamber at room temperature. The extent of biodegradation (%) was calculated by the following equation:

D(%) ) [(V(CO2)t - V(CO2)b)/V(CO2)th] × 100

(1)

where V(CO2)t and V(CO2)b are the amounts of CO2 (µL) released within the assay and blank reactor, respectively, and the V(CO2)th is the theoretical amount of CO2 available from the initial samples.

Results and Discussion It was previously shown that alkaline solutions have no significant effect on the physicochemical properties, such as molecular weight (MW) and thermal stability, of the casein protein.7,16 As reported earlier, freeze-drying of the aqueous casein or casein/clay solution produced durable aerogel monoliths, matching the dimensions of polystyrene vials used as sample molds.14-19 The incorporation of glyceraldehyde (GC) did not considerably alter the bulk density of the neat casein aerogels or casein/clay aerogel composites (see Table 1), independent of post freeze-drying heat treatment. In contrast, a reduction in casein concentration to 2.5 wt % caused a pronounced decrease in the bulk density from 0.11 to 0.07 g cm-3, which might affect the modulus and the extent of perfection of aerogels. After post curing at 80 °C for 24 h, there was a change in the physical appearance of the casein aerogels and casein/clay aerogel composites from white to brownish yellow, concomitant with the chemical cross-linking of GC with the amino groups in the casein macromolecular chains, as illustrated in Scheme 1. Glutaraldehyde (GT) is the most frequently used aldehyde for cross-linking protein molecules, because it is a dicarbonyl compound with two reactive moieties that react with amino groups of lysine residues, leading to a tightly cross-linked network.7,20 However, due to the toxicity

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Pojanavaraphan et al.

Scheme 1. Proposed Cross-Linking Mechanism of Casein Protein with GC through the Maillard Reaction

of GT, GC was chosen as a chemical cross-linking agent in the present study, despite its lower reactivity (lower percentage of free carbonyl compound) at ambient temperature.7,20 FTIR analyses were carried out to confirm the cross-linking reaction, as depicted in Figure 1. The characteristic peaks of casein are located at 1639, 1515, 1442, and 1229 cm-1, which corresponds to CdO stretching, both C-N stretching and N-H bending, C-H deformation, and N-H deformation, respectively.6,9 Upon the addition of 5% (w/w) GC, these bands, particularly at 1639 and 1229 cm-1, were shifted toward higher wavenumbers, most likely due to changes in protein structure resulting from addition of the cross-linking agent. A similar finding was observed in the case of casein/clay aerogel composites. These composites also have Na+-MMT characteristic peaks at 1040 and 917 cm-1, which corresponds to Si-O stretching and Al-OH bending, respectively.21 The shift of absorption peaks at 3642 and 3254 cm-1 (not shown here) toward lower wavenumbers indicated possible hydrogen bonding between the carboxylic and/or amide groups on casein protein and the hydroxyl groups at the edge of clay platelets. It was noteworthy that the presence of air bubbles within the hydrogel would cause significant structural defects to the freezedried composites as well as a deterioration in their mechanical properties.14,16 Because increasing the GC concentration qualitatively resulted in a monotonic increase in the solution viscosity, including a tendency to create trapped air bubbles within the final composites, 5% (w/w) GC was found to be the optimum concentration based on a compromise between the solution viscosity and the mechanical integrity of the final materials. The compressive stress-strain curves of the casein aerogels and casein/clay aerogel composites are shown in Figure 2a and b, respectively, whereas the modulus and toughness at 10 and 30% strain of the studied materials are depicted in Figure 3. The stress-strain curves of the casein aerogels followed the basic form of classical rigid porous foam behavior: a linear elastic deformation at low strain, followed by a densification

Figure 1. FTIR spectra of casein aerogels and the corresponding casein/clay aerogel composites.

Figure 2. Stress-strain behaviors of (A) casein aerogels and (B) casein/clay aerogel composites. The inset visualizes the compression curves of the 2.5 wt % casein/clay aerogel composites.

region beyond the yield point, as the void space collapses.16,17,19 Due to the brittle nature of casein, the corresponding aerogels exhibited nonelastomeric behavior in which original shape after compression testing was not recovered; aliphatic epoxy/clay aerogels, in contrast, behave as elastomeric foams and do recover up to 70% compression.14 On average, the incorporation of GC resulted in monotonic increases in both the compressive modulus and toughness values of the casein aerogels as shown in Figure 3. This trend implied that, at higher GC concentrations, the samples had a greater cross-link density and dissipated energy more effectively under applied stress. The casein/clay aerogel composites were found to adopt a layered architecture that had the ability to absorb and transfer more load through the alternating soft organic casein phase and hard inorganic clay aerogel phase. This architecture resulted in approximately 150 and 110% improvements in compressive modulus and toughness at 30% strain, respectively, over the neat casein aerogel (in the absence of GC), even though the clay aerogel had a compressive modulus of only ∼10 kPa.15-19 After adding 5% (w/w) GC, the compressive modulus and toughness at 30% strain of the casein/clay aerogel composites increased from 3900 to 5600 and 58 to 72 kPa, respectively. Note that these improvements were achieved with a negligible increase in the material density. Table 2 summarizes the mechanical properties of the casein/ clay aerogel composites having different casein concentrations. The polymer concentration had the most effect on the creation of the perfect foamlike structures; increasing the polymer content beyond the concentration required for formation of interpenetrating cocontinuous networks substantially improved the material properties.14,16 In the present case, it was found that the compressive modulus and toughness values of 2.5 wt % casein/clay aerogel were less than those of 10 wt % casein/clay aerogel composite, which is consistent with the above-mentioned



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Figure 4. SEM micrographs of the casein aerogels: (A) CA10, (B) CA10G1, (C) CA10G5, and (D) CA10G10.

Figure 3. Dependence of the mechanical properties of the casein aerogels and casein/clay aerogel composites: (A) compressive modulus, (B) toughness at 10% strain, and (C) toughness at 30% strain. Table 2. Comparison between Compressive Properties of the Different Casein/Clay Aerogel Composites samples

initial modulus (kPa)

toughness at 10% strain (kPa)

toughness at 30% strain (kPa)

CA2.5M5 CA2.5G5M5 CA10M5 CA10G5M5

90 ( 21 98 ( 22 3900 ( 1000 5600 ( 1000

0.3 ( 0.1 0.3 ( 0.1 13 ( 2 17 ( 2

1.4 ( 0.1 1.9 ( 0.2 58 ( 8 72 ( 12

structural perfection. The addition of GC had minimal effect on the compressive properties of the 2.5 wt % casein/clay aerogel composite. Morphological studies of the casein aerogels and casein/clay aerogel composites were conducted using SEM, and the corresponding micrographs are displayed in Figures 4 and 5, respectively. The neat casein aerogel adopted a layered architecture that followed the direction of the ice crystal growth,

Figure 5. SEM micrographs of the casein/clay aerogel composites: (A) CA2.5M5, (B) CA2.5G5M5, (C) CA10M5, and (D) CA10G5M5.

similar to the findings of Ghosh et al.,7 who observed nearly parallel and separated sheets in the microstructure of glycerol plasticized casein scaffold produced by freeze-drying. The incorporation of GC did not alter the overall isotropic structure, but rather increased the surface roughness of the casein aerogels, particularly at 10% (w/w) GC (see Figure 4d). The micrographs of the casein/clay aerogel composites (Figure 5a,c) showed that these aerogels exhibited well-defined lamellar structures, having an individual layer thickness of ∼1 µm and layer spacings of 20-50 µm. By reducing the amount of casein from 10 to 2.5 wt %, there was a noticeable structural change from the interpenetrating cocontinuous network, in which the layers were connected by the polymeric phase, to a highly lamellae architecture, in which the layers were separate. These structural features were consistent with the observed differences in mechanical properties (see Table 2). The introduction of GC did bring about an increase in the structural integrity of the composites (Figure 5b,d).

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Figure 6. (A) TGA and (B) DTG thermograms of the casein aerogels.

Previous studies indicated that the structure of the aerogels was strongly dependent on either the polymer molecular weight range or the filler concentration/type.15,17,18 Using high molecular weight polymer solutions or highly concentrated dispersions (e.g., aqueous dispersion of cellulose whiskers) affected the filler’s ability to rearrange or assemble at the grain boundaries between the growing ice crystals under a nonunidirectional freezing process. This limitation in mass mobility produced a random or disordered structure instead of a highly lamellar architecture. As the molecular weight of casein is lower than that of the poly(vinyl alcohol) previously studied, this material possessed sufficient translational mobility in the hydrogel state during solution freezing, thus, allowing the casein and clay particles to be reoriented by the growing ice front upon freezing the water, ultimately resulting in the layered architecture, as observed in SEM micrographs. The thermal stability of the casein aerogel and casein/clay aerogel composites, as shown in Figures 6 and 7, respectively, was investigated by TGA and DTG. Assuming the mass losses of the aerogel precursor suspension during the transfer between containers were negligible, this analysis provided the decomposition temperature (Td), percentage of residue (WR), and the discrepancy between the theoretical and measured values of the total organic content (see Tables 3 and 4). By assuming the reaction followed first-order kinetics, the decomposition activation energy (Ea) was calculated according to the Horowitz-Metzger method21,22 as follows:

[

ln ln(1 - R)-1] )

Eaθ RT2d

]

(2)

Figure 7. (A) TGA and (B) DTG thermograms of the casein/clay aerogel composites.

θ ) T - Td

(3)

where R is the fraction of weight loss, Ea is the activation energy for decomposition, Td is the temperature at maximum rate of weight loss, and R is the universal gas constant. From the plots of ln[ln(1 - R)-1] as a function of θ, shown in Figure 8, Ea can be calculated from the slope of the corresponding straight line. These aerogels displayed similar decomposition patterns with two main steps of weight loss. The first step, observed up to 100 °C (∼5% weight loss), was related to the removal of adsorbed or bound water, whereas the second step, occurring between 200 and 500 °C, was associated with the decomposition of the casein protein. The thermal stability of the casein aerogels increased modestly with an increase in the GC concentration. This was reflected in changes in Td values from 300 to 307 °C, weight loss of organic phase from 73 to 70%, and continuous increase in activation energy from 68 (casein) to 74 kJ mol-1 (casein/GC). This phenomenon was consistent with the structural changes observed from compression testing and SEM images. The formation of a carbonaceous surface layer during TGA analyses could hinder the flux of decomposition products and heat flowing into the underlying materials,21,23 leading to incomplete decomposition of casein aerogels with residual yields of 22.4-24.7%. The introduction of 5 wt % clay in casein/clay aerogels increased Td values and decreased peak decomposition rates (dW/dT) compared to those of the neat casein aerogel. This result was typical of composite materials where the silicate layers with high aspect ratio served as heat and mass transport barriers. A more tortuous pathway for the decomposition products to diffuse



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Table 3. Thermal Characteristics of the Freeze-Dried Aerogels samples

Tda (°C)

dW/dTb (%/°C)

weight lossc (%)

WRd (%)

Eae (kJ mol-1)


300 303 306 307 306 301 282 282

0.81 0.82 0.78 0.74 0.55 0.53 0.20 0.19

72.8 72.2 71.5 70.4 55.9 53.2 30.0 31.5

22.4 22.9 23.7 24.7 39.4 42.9 65.9 63.9

68.1 ( 3.9 68.7 ( 3.8 68.6 ( 3.5 73.5 ( 2.9 67.9 ( 2.7 69.9 ( 4.0 40.6 ( 3.3 36.9 ( 2.4

a The decomposition temperature at the maximum rate of weight loss. b The maximum rate of weight loss. c Weight loss at temperatures between 200 and 600 °C. d The weight remaining of the samples at 600 °C. e The apparent activation energy for the thermal decomposition.

Table 4. Theoretical and Measured Values of Organic Content in the Freeze-Dried Aerogels samples

theoretical wt% of organic phasea

measured wt% of organic phase


100.0 100.0 100.0 100.0 67.6 69.7 34.3 35.4

72.8 72.2 71.5 70.4 55.9 53.2 30.0 31.5

a

Comprising both the casein protein and GC components.

Figure 9. Degree of biodegradation (%) of the casein aerogels and casein/clay aerogel composites under compost at 23 ( 2 °C.

Figure 8. Plots of ln[ln(1 - R)-1] against θ for determining the Ea of the casein aerogels.

out of the polymer matrix resulted and, thus, hindered the continuous decomposition of the organic phase.21,23 If the clay content of an aerogel composite was increased beyond a critical point, an inverse effect upon thermal stability could be seen.24-26 The clay aerogel was not comprised of individual exfoliated sheets, but as rearranged clay bundles. We hypothesize that the bundles could retain the accumulated heat and accelerate the decomposition process, in conjunction with the external heat flow. Consequently, the magnitude of organic loss in the casein/ clay aerogel composite (∼82%) was somewhat higher than that of the casein system (∼72%) with no significant change in their Ea values. With the addition of GC, the slight thermal stabilization of cross-linking agent and destabilization by clay appear to effective cancel each other out, and the overall changes in decomposition behavior were negligible. Because of their highly porous internal structures, both the clay aerogel and low density polymer/clay aerogel composites exhibited superior thermal insulation characteristics, similar to those of conventional insulating foams.16 This low thermal conductivity was reflected in the Ea values, which were relatively higher than that of unprocessed casein powder (43 ( 4 kJ mol-1).

For the 2.5 wt % casein/clay aerogel composites, there appeared to be insufficient casein to produce complete interpenetrating cocontinuous networks. Consequently, this system was expected to exhibit inferior thermal stability compared to that of the 10 wt % casein composite. A reduction in their Td and Ea values as well as an increase in percentage of organic loss to ∼88% were in perfect agreement with the above conjecture. These results indicated that a lower polymer concentration led to aerogels with much lower density, inferior mechanical integrity, and reduced thermal stability. Interestingly, the addition of GC did not show any considerable improvement in the composite thermal properties, as shown by the trivial changes in Td, weight loss of casein, and Ea values between cross-linked and non-cross-linked casein/clay aerogels. This behavior was explained by the fact that the majority of surfaces within the 2.5 wt % casein composite was not completely covered with a casein layer. This increased the potential for the available GC to be adsorbed onto the clay aerogel surfaces instead of cross-linking or reacting with the casein component. Consequently, the cross-linking efficiency or reactivity of GC was reduced, minimizing the impact of the cross-linking agent in the 2.5 wt % casein/clay aerogel composite. Respirometric tests were conducted to study the biodegradation of the casein aerogels and casein/clay aerogel composites in compost media at room temperature. These results are shown in Figure 9. Wheat starch pregel was selected as a reference because its biodegradability was more than 60% after 45 days, consistent with rate of CO2 release of good biodegradable materials.27 Compared with the degradation behavior of wheat starch, it was observed that the casein aerogels and casein/clay aerogel composites exhibited relatively low degradation rates. Their biodegradability reached only 24-30%, even after 45 days of immersion in compost media. The existence of the layered “house of cards” architecture probably increased the time for degradation to occur because the interior region was more

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difficult to access for the microorganisms. Unlike a simple granular solid, the aerogels of this work present a tortuous path through which degradation microorganisms must diffuse. Most of the aerogels exhibited similar biodegradation behaviors in which there was a steep increase in the degradation rate within the first 3 weeks and a subsequent plateau after approximately 30 days. However, the biodegradation rate was higher for the CA10G5, CA10M5, and CA10G5M5 samples compared to the neat casein aerogel (CA10) sample. The CA10G5 and CA10G5M5 samples required 17 and 12 days to achieve 20% biodegradability, respectively, whereas the CA10 sample required 18 days to reach the same level of degradation. This implies that the addition of GC resulted in a different mode of attack and disruption on the casein component of the tested samples. The microorganisms in the soil directly attacked the GC component to achieve metabolism first, leading to faster biodegradation compared to that in neat casein aerogel. From a previous study, it was found that the fish gelatin films crosslinked with 0.5% (w/w) glutyraldehyde (GT) degraded at faster rate than those without GT. Also, chemically modified mammalian gelatin films degraded faster than unmodified films in soil.28,29 Furthermore, the existence of clay aerogel may change the integrity of the casein matrix, thus, promoting the biodegradation process. Changes in polymer structure, such as were observed by FTIR shifts for casein upon cross-linking in this study, can certainly play a major role in controlling rates of biodegradability.30

Conclusions The present study, inspired by the impact of persistent plastic wastes on the environment as well as the limited availability of landfill sites and petroleum resources, explored the fabrication of lightweight aerogels, comprising biologically based casein, GC, and Na+-MMT clay using an environmentally friendly freeze-drying process. It was found that increasing the GC concentration increased the mechanical strength, surface roughness, and thermal stability (modestly) of the corresponding aerogels while retaining densities in the 0.11-0.12 g cm-3 range. A GC level of 5% (w/w) based on casein appears to be the optimum concentration because the sample displayed an adequate compromise between solution viscosity and composite structural integrity. Aerogels produced from high casein concentration (10 wt % solutions that are then freeze-dried) adopted a more continuous architecture in which the layers are connected by a web of polymer. This architecture resulted in relatively superior mechanical and thermal properties compared to those of the 2.5 wt % casein samples in which the layers were separate. TGA analyses revealed two different effects associated with the clay aerogel: (1) cross-linking of the casein brings about a modest increase in thermal resistance of the foamlike material; (2) the presence of clay decreased the overall thermal stability of the structure, though it significantly increases mechanical properties. Biodegradability studies in compost media revealed distinct differences in biodegradation rates between the wheat starch and aerogels. The aerogels did in fact exhibit useful rates of biodegradation, rates that were enhanced by chemical crosslinking of the casein polymer, with the attendant changes in its protein structure. These foamlike materials hold promise for a wide range of applications where the low density and environmental friendli-


ness are of great importance; the ultra-low-density layered architectures result in favorable mechanical and thermal insulation properties. Acknowledgment. Financial support from the Royal Golden Jubilee Ph.D. Program is greatly appreciated (PHD/0088/2549). One of the coauthors, D.S., has a financial interest in a company that is commercializing the technology investigated in this research. He is an owner and officer of the company. Case Western Reserve University also has an ownership and IP interest in this technology which, if commercialized, could result in royalties for D.S. and CWRU.

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BM100615A

Development of Biodegradable Foamlike Materials Based on Casein

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