Synergetic Effect of Dual Compatibilizers on in Situ Formed Poly

To whom correspondence should be addressed. (J.Z.) Tel.: 509-335-8723. Fax: 509-335-5077. E-mail: [email protected], (L.J.) Tel.: 509-335-6362. Fax: 509...
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Ind. Eng. Chem. Res. 2010, 49, 6399–6406

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Synergetic Effect of Dual Compatibilizers on in Situ Formed Poly(Lactic Acid)/ Soy Protein Composites Bo Liu,†,‡ Long Jiang,*,‡ Hongzhi Liu,‡ and Jinwen Zhang*,†,‡ Materials Science Program, Washington State UniVersity, Pullman, Washington 99164 and Composite Materials and Engineering Center, Washington State UniVersity, Pullman, Washington 99164

In this work, biobased poly(lactic acid) (PLA)/soy protein concentrate (SPC) composites were prepared by twin screw extrusion and injection molding. Poly(2-ethyl-2-oxazoline) (PEOX) and polymeric methylene diphenyl diisocyanate (pMDI) were used sequentially as compatibilization agents to improve phase morphology and interfacial bonding. Properties of the PLA/SPC composites were significantly improved by the application of PEOX and pMDI. The SPC phase was refined and stretched into fine threads during processing under the compatibilization effect of PEOX. With only 0.5 part pMDI, the tensile strength of PLA/SPC composite increased significantly to approach that of pure PLA. With 1 part pMDI, the tensile strength was 6% higher than that of the neat PLA. Scanning electron micrographs evidenced enhanced interfacial adhesion between the two phases. Dynamic mechanical analysis tests revealed that the presence of pMDI enhanced the storage modulus of the composite, especially at temperatures above the glass-transition temperature of PLA, due to the strong interactions between the PLA and SPC phases after pMDI compatibilization. The compatibilized PLA/SPC blends also exhibited significantly reduced water uptakes. Fourier Transform Infrared Spectroscopy confirmed the occurrence of PLA grafting onto SPC molecules through pMDI compatibilization. Introduction PLA exhibits mechanical properties comparable to those of standard polystyrene, i.e., high tensile strength and modulus but low impact strength. In recent years, PLA composites comprising other polymers or inorganic materials have been extensively investigated in the hope of either toughening PLA or reducing material costs. For the latter, natural polymers such as fibrous cellulose, starch, soy protein (SP), and other agricultural residues, are often used as the inexpensive renewable polymers to form composites with PLA. In the last two decades, a great deal of research has been devoted to new industrial uses of SP, such as SP plastics.1-7 When used as a polymer material, SP can be used either as a simple filler in a polymer matrix,5 a plastic component in polymer blends,3,7 or a stand-alone melt-processable polymer material.6 To make SP melt processable, plasticization of SP by water and/or other hydrophilic plasticizers are necessary through a heat and mechanical shear process. In this process, SP gelates under heat in the presence of sufficient water. The gelated SP is plasticized by mechanical shearing in combination with the actions of plasticizers (e.g., water, glycerol) and reducing agents (e.g., Na2SO3). During the process, the globular SP molecules are unfolded and intermolecular interactions are established. Similarly, when SP contains additional water and is melt blended with other polymers, it exhibits certain plastic characteristics, i.e., flowable and deformable. Therefore, by processing SP as a plastic component during blending, the morphology of resulting blends could be greatly manipulated, hence the properties. Our recent study demonstrated that blending PLA with SPC containing extra water resulted in the formation of a percolated SPC structure in the blends, and such * To whom correspondence should be addressed. (J.Z.) Tel.: 509335-8723. Fax: 509-335-5077. E-mail: [email protected], (L.J.) Tel.: 509-335-6362. Fax: 509-335-5077. E-mail: [email protected]. † Materials Science Program, Washington State University. ‡ Composite Materials and Engineering Center, Washington State University.

blends exhibited much higher properties than those of the blends made from dry SPC in which SPC is merely used as a filler.1,2 We further noted that the addition of glycerol as a plasticizer in such a process actually coarsened the SP phase structure.2 The effect of water content in the precompounding SP on the phase structure of resulting blends was further studied in the investigation of poly(butylene adipate-co-terephthalate) (PBAT)/ SPC blends and the formation of percolated SPC thread structure was verified.3,7 It was clear that the formation of the percolated SPC network structure contributed to the higher mechanical properties of the composites containing plasticized SPC. It should be pointed out that, because extensive cross-linking occurred during the compounding process3 and water was evaporated after the compounding, the SPC domains largely lost their flowability/deformability and behaved like solids in the subsequent melt processing (i.e., injection molding). In other words, the blending resulted in the formation of in situ PLA/ SPC composites. Because of the large differences in polarity and hydrophobicity/hydrophilicity between PLA and SPC, interfacial modifiers are needed to improve interfacial bonding. PEOX, which is a water-soluble polymer and has good miscibility with SP, was found to be an effective compatibilizer in this system, resulting in the fine phase morphology of the composites and substantially improved mechanical properties of the blends.1,2 Fang et al. prepared PLA/SP blends using both sodium bisulfite (NaHSO3) and MDI as compatibilizers.5 As is the case in many other PLA/ nature fiber or natural polymer systems,8-10 isocyanate-type coupling agents also greatly increase the interfacial adhesion between PLA and SP. MDI had a much stronger effect than that of sodium bisulfite in compatibilizing PLA and SP. The blend comprising 4% MDI exhibited a tensile strength approaching that of the neat PLA. Nevertheless, the high reactivity of MDI with water demands a thorough drying of the SP before compounding, and this makes SP to be only used as filler. Although remarkable progress has been made to improve the properties of polymer composites comprising SP, the tensile strength of the PLA/SP composites is still low compared to pure

10.1021/ie100218t  2010 American Chemical Society Published on Web 06/24/2010

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Table 1. Formulationsa of PLA/SPC Blends and Control Samples sample code

PLA

SPCb

PEOX

pMDI

H2Oc

Na2SO3

control-1 pMDI-0.5 pMDI-1 PEOX-pMDI-0 PEOX-pMDI-0.5 PEOX-pMDI-1 PEOX-pMDI-2 PLA control-2

70 70 70 70 70 70 70 100 70

30 30 30 30 30 30 30 0 30

0 0 0 3 3 3 3 0 0

0 0.5 1 0 0.5 1 2 0 1

3 3 3 3 3 3 3 0 3

0.15 0.15 0.15 0.15 0.15 0.15 0.15 0 0.15

a All units are in parts. b Dry weight. c The total moisture content in SPC was adjusted to 10% on the basis of SPC dry weight.

PLA.2,5,11 In this work, synergetic effects of PEOX and pMDI on the mechanical and other physical properties of PLA/SP composites were examined. PEOX was first used to improve SPC dispersion in the PLA matrix during twin-screw compounding. Then pMDI was added in injection molding to increase interfacial bonding between the two phases. When both interface modifiers were present, PEOX content was maintained at three parts for all formulations and the PMDI content was varied to investigate its effects on tensile properties, dynamic mechanical properties, crystallization, and water resistance of the composites. This work was aimed at further improving the properties of the PLA/SP composites. Experimental Section Materials. PLA (6202D) was obtained from Nature Works. Commercial grade SPC (Arcon F) was provided by Archer Daniels Midland Company (Decatur, IL). It was composed of ∼69% protein, 20% carbohydrate, 3% fat, and 6% moisture. PMDI (Mondur 541) was obtained from Bayer Material Science LLC (Pittsburgh, PA) and it comprised 31.5 wt % NCO group. PEOX (Mw 500 kDa) was obtained from Aldrich. Sodium sulfite was purchased from J.T. Baker Chemical Company (Phillipsburg, NJ). Preparation of PLA/SPC Composites. SPC (100 parts, dry weight) was mixed with sodium sulfite (0.5 parts) and water (adjusted the moisture content to 10 parts) in a kitchen mixer and equilibrated in sealed plastic bags at room temperature for at least 8 h. This formulated SPC was then compounded with PLA or PLA and PEOX using a co-rotating twin-screw extruder (Leistritz ZSE-18HP) equipped with a volumetric feeder and a strand die. The diameter of the screw was 18 mm with a lengthto-diameter ratio (L/D) of 40. Since the formulated SPC contained water, the extrusion temperature was set as low as possible to reduce the degree of PLA hydrolysis.1 The temperature profile of the extruder was 90, 100, 130, 145, 160, 160, 155, and 155 °C (feed throat to die). The screw speed was 60 rpm. Water vapor was removed through the vent on the seventh heating zone of the barrel. The extruded strands were cooled in a water trough and subsequently pelletized. The ratio of PLA to SPC was maintained at 70:30 (w/w) for all blends. The pelletized blends were dried in a convection oven at 95 °C for 12 h before injection molding. Injection Molding. Standard tensile bars (ASTM D638, Type I) were prepared by injection molding (Sumitomo SE 50D). The barrel temperatures were set at 150, 160, 170, and 165 °C (feed throat to nozzle). A mold temperature of 40 °C and a cooling time of 30 s were employed for the process. The dried PLA/ SPC pellets were mixed with pMDI prior to being fed into the molding machine. Formulation. Table 1 gives the formulations of all the samples studied in this work. Samples compatibilized with

pMDI only were designated as pMDI followed by the pMDI concentration (parts). Samples compatibilized by dual compatibilizers were denoted as PEOX-pMDI followed by the pMDI concentration. In this case, the PEOX content was maintained at three parts. Two controls, one contained no compatibilizers and another contained only pMDI, which was added during the blending stage, were also prepared. Mechanical Testing. Tensile testing was performed on an 8.9-kN, screw-driven universal testing machine (Instron 4466) equipped with a 10 kN electronic load cell and mechanical grips. The tests were conducted at a crosshead speed of 5 mm/min with strains measured using a 25-mm extensiometer (MTS 634.12 × 10-24). All tests were carried out according to the ASTM D638. Five replicates were tested for each sample to obtain an average value. All samples were tested after one week of conditioning at 23 °C and 50% RH. Fourier Transform Infrared Spectroscopy (FTIR). Composite samples were extracted in a Soxhlet extractor (chloroform as the solvent) for 72 h to remove the free PLA. The extracted SPC particles were ground with KBr and pressed into discs for FTIR analysis. The analysis was performed using a Thermo Nicolet Nexus 670 spectrometer. The spectra were recorded from 400 to 4000 cm-1 with 32 repeated scans at a resolution of 2 cm-1. Morphology. The microstructure of PLA/SPC composites was investigated with scanning electron microscopy (SEM). Tensile fracture surfaces and microtomed smooth surfaces of the composites were examined after sputter coating. The extracted SPC particles were also examined for the morphology of the SPC phase in the composites. Differential Scanning Calorimetry (DSC). Crystallization behavior of the composites was studied by DSC (TA 2920) using the specimens sliced from injection molded samples. The specimens were crimp sealed in 40 mL aluminum crucibles. All specimens were scanned from 30 to 180 at 5 °C · min-1 without erasing their thermal history. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties of PLA and its SPC composites were studied by a Rheometrics Solids Analyzer (RSAII). DMA specimens (2 × 4 × 45 mm3) were cut from injection molded samples and tested using a dual-cantilever fixture at 1 Hz vibration frequency. All tests were conducted at a strain of 0.03% using a 2 °C · min-1 temperature ramp from -10 to 160 °C. Water Absorption. Water absorption of the composites was examined following ASTM D570-98. All samples were first dried at 50 °C for 24 h and then cooled to room temperature in a desiccator. The dried samples were immersed in distilled water at room temperature for specific intervals, removed from the water, blotted with tissue paper to remove excess surface water, and then weighed. Five replicates were tested for each sample. The water absorption was calculated on a dry sample weight basis. Results and Discussion Tensile Properties. Table 2 gives mechanical properties of the entire composite and control samples. For the samples comprising PEOX, the addition of 0.5 part pMDI caused a significant tensile strength increase (53.8 to 70.4 MPa). Higher contents of pMDI (1 and 2 parts) only led to mild strength increases over the 0.5 part content. The strengths of the three formulations were all higher than that of the pure PLA, which was significant in polymer/SP composites. Strain at break of the composites was also increased with the increasing content of pMDI. Tensile strain is largely controlled by sample defects.

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010 Table 2. Mechanical Properties of PLA/SPC Blends Prepared under Different Compatibilization Conditions sample code control-1 pMDI-0.5 pMDI-1 PEOX-pMDI-0 PEOX-pMDI-0.5 PEOX-pMDI-1 PEOX-pMDI-2 PLA control-2

strength (MPa) strain at break (%) modulus (GPa) 48.6 ( 2.4 56.7 ( 3.2 60.3 ( 2.3 53.8 ( 1.2 70.4 ( 0.7 72.5 ( 0.4 73.0 ( 1.0 68.6 ( 0.4 49.8 ( 0.6

1.13 ( 0.08 1.30 ( 0.09 1.46 ( 0.12 1.26 ( 0.03 1.85 ( 0.06 2.09 ( 0.14 2.29 ( 0.12 2.90 ( 0.12 1.20 ( 0.06

4.34 ( 0.07 4.59 ( 0.20 4.52 ( 0.13 4.42 ( 0.12 4.54 ( 0.05 4.51 ( 0.09 4.41 ( 0.07 3.48 ( 0.08 4.29 ( 0.16

The defects (cracks, voids, stress concentrators, etc.) can cause premature sample failure during sample deformation due to localized high stress induced by these defects. With increasing pMDI content, SPC inclusions became smaller and their interfacial bonding with the PLA matrix increased, which led to lower stress concentration and less interfacial cracks in the samples. As a result, strain at break of the samples increased with the increasing pMDI content. Comparing above samples with the samples comprising no PEOX (i.e., Control-1, pMDI-0.5, and pMDI-1), it was evident that the samples comprising PEOX had significantly higher tensile strength, e.g., 53.8 vs 48.6 MPa at 0 part pMDI, 70.4 vs 56.7 MPa at 0.5 part pMDI, and 72.5 vs 60.3 MPa at 1 part pMDI. Strain-at-break values of the samples comprising PEOX were also higher than those of without. The results demonstrated that PEOX and pMDI did exhibit a synergetic effect on the tensile properties of the PLA/SPC composites. The reason for this synergetic effect was made clear in the morphology study. It was worth noting that if pMDI were added before extrusion (sample Control-2), then the compatibilizer showed no compatibilization effect on the composite. The tensile strength of Control-2 was much lower (49.8 vs 60.3 MPa) than that of pMDI-1, which had essentially the same formulation but with pMDI added before injection molding. Control-2 exhibited the properties only comparable to Control-1, which contained no compatibilization agents at all. The reason for this large difference in compatibilization effect was believed to be due to pMDI’s high reactivity to moisture.12,13 When pMDI was added before extrusion, its reaction with the moisture in SPC prevented its reaction with PLA and SP. Therefore, its compatibilization effect was virtually lost. Figure 1 gives representative tensile stress-strain curves of the composite and control samples. From the tensile curves, we can see that the yield point appeared when pMDI was present. However, because of the 30 parts of SPC, the elongation of composite was not as long as that of neat PLA.

Figure 1. Tensile stress-strain curves of PLA/SPC composites with 3 parts of PEOX and various contents of pMDI. a: PEOX-pMDI-0; b: PEOX-pMDI0.5; c: PEOX-pMDI-1; d: PEOX-pMDI-2; and e: pure PLA.

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All blends show similar modulus in Table 2, despite the use of compatibilizers. The modulus of a material is calculated within the linear elastic region (small strain) of the material deformation, where the stress-strain relationship obeys Hooke’s law, and the deformation involves vibrational and reversible movement of polymer chains. However, the tensile strength is the maximum value of the material stress, which often occurs during nonlinear plastic deformation (relatively large strain) of the material and involves nonreversible chain movements, such as rotation, slip, and even chain fracture. The addition of compatibilizers into PLA/SPC blends reduced SPC inclusion size and increased interfacial bonding between the PLA and SPC phases, which reduced stress concentration and promoted stress transfer at the interface during sample deformation and consequently increased the tensile strength. However, in the initial part of the deformation (i.e., linear region), the vibrational/ reversible movement of the chains remained largely unaffected by these compatibilization effects, and therefore, the modulus of the blends remained unchanged. Table 2 also shows that there exists an optimal pMDI concentration for the blend properties. Tensile strength of the blend increased significantly when 0.5 parts of pMDI was added (e.g., PEOX-pMDI-0.5). The strength increase was negligible from 1 to 2 parts of pMDI. While increasing the pMDI concentration likely increases interfacial bonding between the PLA and SPC phases through chemical bonds, excessive pMDI could react with the moisture in the blend and produce low molecular weight urea compounds and CO2 gas. Morphology. Figure 2a-c shows the microtomed surfaces of the PLA/SPC composites comprising PEOX and increasing contents of pMDI. To better illustrate SPC morphology, the composites were also extracted using CHCl3 to remove PLA. SPC particles produced after the extraction are shown in Figure 2a′-c′. The micrographs of the microtomed surfaces could only provide the SPC shape and size information from a single layer, whereas the images of extracted SPC particles provided 3-D structural information of the SPC phase. From these micrographs, it appears that the addition of pMDI did not obviously change SPC dispersion and domain sizes but resulted in less visible voids and interfacial cracks on the composite surfaces after the compatibilization, indicating that pMDI improved wetting of SPC by PLA and interfacial bonding. The higher mechanical properties of the composites after pMDI compatibilization were likely attributed to increased interfacial bonding between the two phases. Figure 2 (especially parts a′-c′) also shows that in the presence of PEOX, the SPC phase was changed from round particles of natural SP into threads after compounding, and with the addition of pMDI in the following processing, these threads became finer. The increased fineness of the threads would usually result in an increase in the average aspect ratio. Because stress transfer from the matrix to the elongated threads could be enhanced with the increasing aspect ratio, the improved strength of the composite might also contain the contribution from the SPC phase. When neither compatibilization agents were used (Figure 3a and a′), the SPC phase appeared to be large agglomerates without sign of flow induced stretching, implying poor deformability of SPC under this condition. When only pMDI was used (Figure 3, parts b, b′, c, and c′), the agglomerates became smaller, but still with no obvious phase stretching. The reduced particle size was due to increased interfacial bonding between SPC and PLA after pMDI compatibilization. Overall, the size of the SPC phase in Figure 3 was significantly larger than that in Figure 2, where PEOX was present during extrusion blending.

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Figure 2. SEM micrographs of PLA/SPC composites with 3 parts of PEOX and various contents of pMDI. a and a′: PEOX-pMDI-0; b and b′: PEOXpMDI-1; c and c′: PEOX-pMDI-2. a-c: microtomed composite surfaces; and a′-c′: SPC phase left after chloroform extraction of PLA.

From these results, it could be concluded that PEOX was an effective compatibilizer for PLA/SPC blends and could significantly improve the dispersion of the SPC. Similar results have also been reported by Zhang et al.1 and the reason was attributed to PEOX’s compatibility to both SPC and PLA. PEOX is regarded as a derivative polymeric homologue of the aprotic polar solvent, N,N-dimethylacetamide (DMAc). DMAc can dissolve many organic polymers which are insoluble in other common organic solvents. PEOX is considered as a broadly compatible polymeric solvent or compatibilizing agent for many polymers.14 PEOX is believed to be miscible with SPC through

various molecular interactions such as H-bonding and dipoledipole interaction. In addition, PEOX is slightly basic due to its tertiary amide structure. SPC is usually prepared by precipitation at its isoelectric point (∼ pH 4.5), which makes them slightly acidic. Therefore, the acid base interaction could also contribute to their compatibilization.1 It was also showed that PEOX and PLA was immiscible but compatible through the hydrogen bonding between the carbonyl groups (PEOX) and the hydroxyl and carboxylic end groups (PLA). Therefore, the added PEOX predominantly resided in the SPC phase as a polymeric solvent. The PEOX located at the SPC/PLA interfaces

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Figure 3. SEM micrographs of PLA/SPC composites without PEOX. a and a′: control-1; b and b′: pMDI-1; c and c′: control-2. a-c: surfaces prepared by microtomy; and a′-c′: SPC phase after chloroform extraction of PLA.

interact with PLA through the hydrogen bonding and reduced interfacial tension between the two phases, which facilitated the dispersion of SPC. In addition, as a polymeric solvent, PEOX helped to maintain the extended soy protein molecular structure (as opposed to the coiled structure of natural soy protein molecules) and decreased the viscosity of the SPC phase (especially when the water content was low at the late stage of the extrusion). The lower viscosity of the SPC phase also led to its improved dispersion because lower shear stress was required to deform and break it into smaller inclusions.

With the tremendously improved SPC phase dispersion by PEOX, pMDI was then added to the system to react with SPC and PLA at their interfaces. This reaction at the interface bridged the two phases and substantially strengthened their interfacial bonding. This explained why the composites comprising sequentially added PEOX and pMDI showed exceptionally high tensile properties. Comparing Figure 3, parts b to c, it was also noticeable that the surface of Control-2 had more voids and interfacial cracks than did the surface of pMDI-1. This could contribute to the

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Figure 4. SEM micrographs of the tensile fracture surfaces of PLA/SPC composites comprising 3 parts of PEOX and various contents of pMDI compatibilizers. a: PEOX-pMDI-0; b: PEOX-pMDI-0.5; c: PEOX-pMDI-1; and d: PEOX-pMDI-2.

lower tensile properties of Control-2. The interfacial cracks on the Control-2 sample surface indicated poor interfacial bonding between SPC and PLA due to their lack of pMDI compatibilization. Figure 4 shows the tensile fracture surface of the PLA/SPC composites comprising 3 parts of PEOX and varying contents of pMDI. When there was no pMDI compatibilization (Figure 4a), individual SPC particles can be clearly seen from the surface. A large number of particles were pulled out from the surface during fracture due to poor interfacial bonding. This extensive particle pull-out left cavities on one fracture surface and protruding SPC particles on the other surface. After the addition of pMDI (Figure 4, parts b, c, and d), no particle pullout can be seen on the surface, indicating substantially increased interfacial bonding. Moreover, the surfaces show signs of plastic deformation at high pMDI concentrations (circled area in Figure 4c,d), corresponding to the increased strain at break of the composites with pMDI compatibilization. Fourier Transform Infrared Spectroscopy (FTIR). FTIR has been proven to be an effective tool to study interfacial bonding. Figure 5a-d shows FTIR spectra of pure PLA, pure SPC, the SPC extracted from the composite containing no pMDI (PEOX-pMDI-0), and the SPC extracted from the composite containing 1 part pMDI (PEOX-pMDI-1). Pure PLA had a strong absorption band at 1760 cm-1 (CdO stretching) and a weak absorption at 3500 cm-1 (O-H stretching of PLA end hydroxyl groups) (Figure 5a). Pure SPC showed strong absorption bands at 1514, 1658, and 3338 cm-1 (Figure 5b), which

Figure 5. FTIR spectra of (a) pure PLA, (b) pure SPC, (c) SPC without pMDI, and (d) SPC with 1 part pMDI.

could be attributed to the N-H bending (amide II bond), C-O stretching (amide I bond), and N-H stretching, respectively.15 The SPC extracted from PEOX-pMDI-0 exhibited a small shoulder at 1759 cm-1 (CdO stretching of PLA) and three strong peaks (1521, 1653, and 3306 cm-1) associated with SPC (Figure 5c). In comparison, the SPC extracted from PEOX-

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Figure 7. DSC thermograms of neat PLA and PLA/SPC composites. Table 3. Cold Crystallization and Melting Parameters of Neat PLA and PLA/SPC Composites cold crystallization Tg (°C)

Tcc (°C)

Hcc (J/g)a

Tm (°C)

Hm (J/g)a

PEOX-pMDI-0 PEOX-pMDI-0.5 PEOX-pMDI-1 PEOX-pMDI-2 neat PLA

55.9 57.2 57.9 57.9 57.0

94.3 96.5 97.4 98.2 99.2

23.7 22.2 24.4 24.3 21.6

161.7 162.3 162.3 161.6 164.5

40.4 35.5 36.4 35.2 31.7

a

Figure 6. Storage modulus (a) and tanδ (b) of the pure PLA and the PLA/ SPC composites comprising PEOX and different pMDI contents.

pMDI-1 showed a strong absorption peak at 1760 cm-1 and three strong absorption peaks located at 1536, 1658, and 3338 cm-1, respectively. This result indicated that more PLA was grafted to SPC through the reactions of pMDI with the hydroxyl groups of PLA and SPC and the amide and amine groups of SPC. In addition, the N-H stretching of SPC shifted from 3306 cm-1 (no pMDI) to 3338 cm-1 (1 part pMDI) (Figure 5d), again suggesting the reaction of pMDI with SPC. Dynamic Mechanical Properties. The storage modulus (E′) and tanδ of the composites as a function of temperature were shown in Figure 6. The E′ of all of the composites dropped rapidly starting from ∼60 °C due to the glass transition of PLA. For the pure PLA and PLA/SPC without pMDI, their moduli were too low to be registered by the instrument in the temperature range of ∼70-85 °C. However, for the samples containing pMDI, a modulus of ∼20 MPa was registered in the same temperature range. This reinforcement was mainly derived from the large interfacial area (homogeneous dispersion of SPC) and strong interfacial bonding (restraint on PLA molecule mobility) as a result of pMDI compatibilization. Additionally, pMDI can cross-link PLA chains through its reaction with the hydroxyl end groups of PLA. The cross-linking increased polymer molecular weight and chain entanglements, which could also contribute to the increase in storage modulus. The storage moduli increased between 85 and 100 °C for all composites due to the cold crystallization of PLA. In Figure 6b, the peak temperature of tanδ (damping peak) for the composites containing pMDI was ∼1.5 °C higher than the damping peak of the pure PLA and the composites without pMDI. Moreover, the height of the damping peaks of the pMDIcontaining composites was also lower than those of the pure PLA and the pMDI-free composites. Both results indicated the

melting

sample

Data were corrected for the percentage of PLA in the blend.

increased restraint on the PLA molecular mobility after the pMDI compatibilization. The blend with only PEOX (e.g., PEOX-pMDI-0) also showed lower damping peak value and broader transition compared to the pure PLA. The decrease in damping was because the PEOX at PLA/SPC interfaces formed hydrogen bonds with PLA molecules and therefore increased the restraint of the SPC phase on the mobility of PLA chains. In addition, the addition of PEOX increased interfacial heterogeneity, which was attributed to the broadening of the damping peak. Crystallization. DSC thermograms and crystallization and melting data of the PLA/SPC composites are given in Figure 7 and Table 3. In Figure 7, all of the samples showed three transitions, namely PLA glass transition, cold crystallization, and melting.1,9,16 From Table 3, the addition of SPC decreased the cold crystallization temperature (Tcc) and melting point (Tm) of PLA and increased the enthalpy of fusion (Hm) of the polymer. SPC, as a foreign substance in the PLA matrix, acted as a nucleation agent for PLA and promoted PLA crystallization. As a result, the cold crystallization temperature decreased and the enthalpy of fusion increased for the PLA/SPC composite (sample PEOX-pMDI-0). Interestingly, the cold crystallization temperature increased in the presence of pMDI and the increase was larger at higher pMDI concentrations. Nucleation agents promote polymer crystallization (heterogeneous nucleation) because their surface energy reduces the energy barrier to form polymer nuclei. The pMDI increased wetting of SPC by PLA, which increased the free energy needed for nucleation and therefore decreased the nucleation effect of SPC.17 As a result, the cold crystallization temperature of PLA increased with increasing pMDI concentration. Another possible contributing factor was the pMDI cross-linking of PLA chains, which impeded PLA chain diffusion and folding and, as a result, increased the PLA cold crystallization temperature. The reduced

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Acknowledgment The authors are grateful for financial support from the United Soybean Board (USB) and from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, Grant No. 2007-35504-17818. Literature Cited

Figure 8. Effect of pMDI compatiblization on water absorption of the PLA/ SPC composites.

nucleation effect of SPC also caused decreased Hm of the PLA/ SPC composites containing pMDI (Table 3). The decreased melting point of the composites might be due to the smaller thickness of PLA spherulite lamella under the influence of SPC nucleation.18,19 Water Absorption. In Figure 8, the hydrophobic PLA showed a low water uptake of ∼0.8%. The control blend (without the addition of pMDI) exhibited an ∼16% water uptake at equilibrium. However, the water adsorption was progressively reduced with increasing pMDI addition. SPC is highly hydrophilic due to the polar functional groups such as amide, amino, and hydroxyl groups. The reactions of pMDI with these functional groups significantly reduced the polarity of SPC and increased its wetting by PLA, and therefore greatly decreased its rate of water absorption. In Figure 8, the control blend appeared to reach its maximum water absorption after ∼20 days of immersion in water, but these pMDI compatibilized blends had not achieved its equilibrium of water absorption after 40 days of immersion. Conclusions We demonstrated in this work that PEOX was an effective compatibilizer for PLA and SPC blend. The compatibilizer could substantially enhance the flowability/deformability of the SPC phase, which led to the in situ formation of SPC fibers. The enhanced flowability of SPC and the improved compatibility also significantly improved the dispersion of the SPC phase. These two results led to remarkably improved PLA/SPC composite properties. On the basis of PEOX compatibilization, the subsequent addition of pMDI to the system enabled the formation of strong interfacial bonding between SPC and PLA. This further increased the tensile strength of the PLA/SPC composites to even higher than that of the pure PLA. The dynamic mechanical properties and water resistance of the composites were also substantially improved. Thus, a significant synergistic effect of PEOX and pMDI on the PLA/SPC composite properties was revealed.

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ReceiVed for reView January 29, 2010 ReVised manuscript receiVed May 26, 2010 Accepted June 10, 2010 IE100218T