Poly(methyl vinyl ether-co-maleic acid)−Polyethylene Glycol

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Biomacromolecules 2010, 11, 2660–2666

Poly(methyl vinyl ether-co-maleic acid)-Polyethylene Glycol Nanocomposites Cross-Linked In Situ with Cellulose Nanowhiskers Lee Goetz,†,‡ Marcus Foston,† Aji P. Mathew,‡ Kristiina Oksman,‡ and Arthur J. Ragauskas*,†,§ Institute of Paper Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, Division of Manufacturing and Design of Wood and Bionanocomposites, Luleå University of Technology, Luleå, Sweden, and Forest Products and Chemical Engineering Department, Chalmers University of Technology, Gothenburg, Sweden Received June 15, 2010; Revised Manuscript Received August 28, 2010

Nanocomposites were developed by cross-linking cellulose nanowhiskers with poly(methyl vinyl ether-co-maleic acid) and polyethylene glycol. Nuclear magnetic resonance (NMR) studies showed cross-linking occurs between the matrix and cellulose nanowhiskers via an esterification reaction. Proton NMR T2 relaxation experiments provided information on the mobility of the polymer chains within the matrix, which can be related to the structure of the cross-linked nanocomposite. The nanocomposite was found to consist of mobile chain portions between crosslinked junction points and immobilized chain segments near or at those junction points, whose relative fraction increased upon further incorporation of cellulose nanowhiskers. Atomic force microscopy images showed a homogeneous dispersion of nanowhiskers in the matrix even at high nanowhisker content, which can be attributed to cross-linking of the nanowhiskers in the matrix. Relative humidity conditions were found to affect the mechanical properties of the composites negatively while the nanowhiskers content had a positive effect. It is expected that the cross-links between the matrix and the cellulose nanowhiskers trap the nanowhiskers in the cross-linked network, preventing nanowhisker aggregation subsequently producing cellulose nanocomposites with unique mechanical behaviors. The results show that in situ cross-linking of cellulose nanowhiskers with a matrix polymer is a promising route to obtain nanocomposites with well dispersed nanowhiskers, tailored nanostructure, and mechanical performance.

Introduction Though nanocomposites have been reported in the literature as early as the 1950s, polymer-based nanocomposites made a marked impact in industry and academics with the report of layered nanosilicate-based nanocomposites by Toyota in 1993.1,2 Frequently, the main function of incorporating nanocompositebased materials is to increase the mechanical performance, thermal stability, and barrier properties.3,4 At the nanoscale, the nanoparticles have a large surface area and have the possibility to interfere with polymer chain mobility, thereby manipulating the matrix properties. However, it is important to understand that these expected improvements are directly dependent on the dispersion and distribution of the nanoparticles in the polymer matrix as well as the interaction between the matrix and the reinforcing phases. The optimization of the dispersion and interactions of nanoparticles in polymer matrices through mechanical and chemical approaches is a key factor that enables the development of high quality nanocomposites. In recent years there has been renewed societal interest in developing and utilizing biobased green materials. Cellulose is the most abundant biomaterial, and concern is growing in finding new ways to utilize this biopolymer to create new biobased materials.5-9 Consequently, cellulose nanowhiskers (CNW) have * To whom correspondence should [email protected]. † Georgia Institute of Technology. ‡ Luleå University of Technology. § Chalmers University of Technology.

be

addressed.

E-mail:

generated much attention recently.6,7 Cellulose nanowhiskers are rod-shaped in structure and formed when native cellulose is subjected to strong acid hydrolysis.8,9 The dimensions of the resulting nanowhiskers vary according to the source but can range from approximately 10-20 nm in diameter and 100-1000 nm in length.9-11 The modulus of cellulose nanowhiskers has been experimentally and theoretically calculated to range from 138 GPa (experimentally determined) to 167 GPa (theoretically determined).10-12 Cellulose nanowhiskers (CNW) have been incorporated in a wide variety of matrices to improve strength properties including latex,13,14 starch,15-17 cellulose acetate butyrate (CAB),18 polyhydroxyl alkanoates (PHA),19 and polylactic acid (PLA).20,21 The addition of surfactants20,21 and plasticizers15,22,23 was found to improve the dispersion of nanowhiskers in the matrix and improve physical properties. While encapsulating a matrix around CNW was reported by Samir et al.,24 cross-linking CNW with a matrix in situ to achieve good nanowhisker dispersion has not been evaluated. We have recently reported the co-cross-linking of the poly(methyl vinyl ether comaleic acid) (PMVEMA)polyethylene glycol (PEG) matrix with cellulose nanowhiskers.25 The resulting cross-linked nanocomposite was shown to have unique properties that were distinctly different from the starting components and of special interest were the hydrogel properties that facilitated up to ∼900% mass absorption of water. PMVEMA is a polycarboxylic acid containing polymer that is currently used in health care applications. However, PMVEMA has recently been investigated for its potential as a bioadhesive polymer for drug delivery options.26-32 There has

10.1021/bm1006695  2010 American Chemical Society Published on Web 09/21/2010

Nanocomposites Cross-Linked with Nanowhiskers Table 1. Composition of PMVEMA-PEG/CNW Nanocomposite Hydrogels sample code

CNW, wt %

PMVEMA-PEG, wt %

0CNW 25CNW 50CNW 75CNW 100CNW

0 25 50 75 100

100 75 50 25 0

also been interest in expanding its use with cellulosic materials. Barcus and Bjorkquist developed a procedure of reacting PMVEMA and PEG with wood fibers using a thermal dehydration reaction as an alternative method for grafting cellulose without metal catalysts to create a cross-linked material with increased water sorption properties,33 while Khutoryanskaya et al. have investigated blends of hydroxyethylcellulose and PMVEMA.34 It is expected that the in situ co-cross-linking of nanowhiskers in a water-soluble biopolymer matrix leads to the development of a well-dispersed nanocomposite with enhanced mechanical properties and stability in aqueous medium. Additionally, the crosslinking may enable the dispersed nanowhiskers to be locked within the polymer matrix and develop nanocomposite materials with tailor-made properties depending on the degree of crosslinking, nanowhiskers concentration, and relative humidity conditions. This paper investigates the mechanical properties and transverse relaxation of cross-linked PMVEMA-PEG-CNW nanocomposites and the effect of CNW concentration on these physical properties. A promising future use of these materials is in climate-controlled applications and biomedical applications, hence, the behavior of the prepared hydrogels in different humidity environments was explored.

Experimental Section Materials. PMVEMA was provided by ISP Corp (U.S.A.) as trade name Gantrez S-97. It is a water-soluble polymer with Mw of 1.98 × 106 g/mol. Poly(ethylene glycol) with a Mw of 3000 g/mol was purchased from VWR (Sweden). Microcrystalline cellulose (VIVAPUR 105) was acquired from JRS Pharma (Germany). All other chemicals were purchased from Aldrich and used as received. Preparation of Cellulose Nanowhiskers. Microcrystalline cellulose was hydrolyzed to cellulose nanowhiskers using the procedure published by Bondeson et al.35 Typical yield of cellulose nanowhiskers was ∼40%. Synthesis of PMVEMA-PEG/CNW Nanocomposite Hydrogels. The procedure for preparing the PMVEMA-PEG/CNW nanocomposite hydrogels has already been described.25 The nanocomposite hydrogels are described according to the percent nanowhiskers in each starting composition (see Table 1). The mass of PMVEMA to PEG is maintained throughout at 6.70 g PMVEMA/1.00 g PEG. An unreacted control film of 50 wt % nanowhiskers and 50 wt % matrix was also prepared using the same method as the cross-linked nanocomposite hydrogels with the exception of not being cured. The nanocomposite hydrogels were stored in desiccators at varying relative humidity (i.e., 2, 54, and 92%) for 1 week prior to physical testing. Nuclear Magnetic Resonance (NMR) Analysis. In preparation for NMR analysis, the nanocomposite hydrogels were Soxhlet extracted with water for 24 h to ensure removal of unreacted material. These samples were then oven-dried and ball milled for approximately 45 s. The NMR samples were prepared by conditioning at ambient air relative humidity/temperature or swelling the ball milled nanocomposites in 100% D2O for 4 h. This was then followed by loading the samples into 4 mm cylindrical ceramic MAS rotors. Repetitive steps of packing

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the sample into the rotor were done to fully compress and load the maximum amount of sample. 13 C CP/MAS solid-state NMR measurements were carried out on samples conditioned at ambient air relative humidity/temperature on a Bruker DSX-400 spectrometer operating at a frequency of 100.55 MHz for 13C NMR in a Bruker double-resonance MAS probe head at a spinning speed of 10 kHz. CP/MAS experiments utilized a 5 µs (90°) proton pulse, 1.5 ms contact pulse, 4 s recycle delay, and 2K scans. 1 H spin-spin (T2) NMR experiments on samples swollen in 100% D2O were performed on a Bruker DSX-400 spectrometer, operating at a frequency of 399.875 MHz for 1H NMR in a Bruker double-resonance MAS probe head at spinning speed of 2 kHz. A standard Carr-PurcellMeiboom-Gill (CPMG) sequence with a τ ) 50 us, utilized a 5 µs (90°) proton pulse, 2 s recycle delay, and 128 scans. The resulting T2 decay profiles were analyzed using a single-component exponential or two-component Gaussian-exponential model. PMVEMA, PEG, and PMVEMA-PEG were dissolved in DMSOd6 and analyzed as liquids by 13C and 1H NMR on a Bruker Avance400 spectrometer operating at frequencies of 100.59 MHz for 13C NMR and 399.875 MHz for 1H NMR. 13C spectra were acquired using an inverse-gated sequence with a 44.8 µs dwell time, 1.468 s acquisition time, 10 s recycle delay, and 10K scans per sample. 1H spectra were acquired with a 133 µs dwell time, 0.817 s acquisition time, 4 s recycle delay, and 128 scans per sample. Atomic Force Microscopy (AFM). Cellulose nanowhiskers, as well as the nanocomposites, were characterized using a Veeco MultiMode scanning probe microscope with a Nanoscope V controller. For the analysis of CNWs, a droplet of the aqueous nanowhisker suspension (0.5% by weight) was dried on a mica surface prior to AFM examination. The nanocomposite hydrogels were embedded in epoxy and ultramicrotomed before using the AFM to map the morphology. The images from the cross-section of the nanocomposite samples were collected using a tapping mode etched silicon tip with a nominal spring constant of 5 N/m and a nominal frequency of 270 kHz. Stress-Strain Measurements. The tensile measurements were performed on an Instron 4411 (U.S.A.) with a 500 N load cell. Rectangular shaped strips (0.30-0.40 mm thickness) were cut from the nanocomposites. The gauge length was 25 mm and a strain rate of 5 mm/min was applied. The maximum strength and modulus were calculated. The values given are based on 5 tests per composition. The error in the measurements was reported as the standard deviation.

Results and Discussion The preparation of the nanowhiskers and chemical characterization are detailed in a communication by Goetz et al.25 PMVEMA and cellulose nanowhiskers were shown to undergo an esterification reaction primarily between the primary hydroxyl group of the cellulose and the acid group of the PMVEMA (Figure 1). In addition, the PEG cross-links with the PMVEMA in a second esterification reaction. The prior report demonstrated that all three components, PEG, PMVEMA, and the cellulose nanowhiskers, were needed to form the reported nanocomposites. NMR analysis of the PMVEMA, PEG, PMVEMA-PEG, and cross-linked nanocomposite hydrogels was performed to further analyze the cross-linking. 13C NMR chemical shifts of the starting PEG and PMVEMA agreed with literature values and with the corresponding13C CP/MAS NMR of the cross-linked nanocomposite hydrogels.36,37 The 13C CP/MAS NMR spectra of the cellulose nanowhiskers showed the typical spectra of crystalline cellulose, and based on the integration of the cellulose C4 peak region (13C δ ∼ 80-85 and 85-92 ppm amorphous and crystalline cellulose, respectively) the nanowhiskers have a crystallinity of ∼61%, which does not change upon incorporation of PMVEMA-PEG components. Moreover, the chemical shifts of the cellulose carbons do not seem to be affected by

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Figure 1. Cross-linked PMVEMA, PEG, and cellulose nanowhiskers.

Figure 2.

13

C spectra of H2O extracted CNW nanocomposites.

incorporation of PMVEMA-PEG components, suggesting a chemical alteration of cellulose occurs on a small fraction of surface accessible glycosidic units. As can be seen in Figure 2, the samples which were Soxhlet extracted exhibited all three components due to transesterified linkages clearly displaying carbon resonances overlapping with cellulose at ∼72 and 50-65 ppm generated by PEG and PMVEMA, respectively. One of the most dramatic observations is for the NMR spectrum of a cross-linked 50CNW nanocomposite when compared to the NMR spectrum of a control unreacted 50 wt % CNW mixture with PMVEMA-PEG. The carboxylic acid shift from the PMVEMA at ∼175 ppm is almost completely removed from the uncured 50 wt % CNW mixture NMR spectrum upon Soxhlet extraction. This not only confirms the Soxhlet extraction procedure removes any noncovalently linked oligomers, but most importantly that the residual nanocomposite hydrogels are truly cross-linked.

Figure 3. 1H spectra of PEG and PMVEMA dissolved in D2O and 1H MAS 2 kHz spectrum of H2O extracted 50CNW nanocomposite swollen in D2O at 50 °C.

The 1H MAS NMR spectrum obtained for the 50CNW nanocomposite swollen in D2O is shown in Figure 3 along with the solid spectrum of pure cellulose nanowhiskers. The 1H MAS NMR spectrum shows peaks between ∼3.8 and 1.9 ppm that belong to protons on CNW, PEG, and PMVEMA. In the solid state spectrum of the nanocomposite there is one PEG-related chemical shift of note, the chemical shift of the protons on the methylene groups along the backbone of the polymer which appear at ∼3.8 ppm. There are three fairly resolved PMVEMA peaks that appear in the solid state spectrum of the nanocomposite: the chemical shift of the protons on the 1- and 2-methine groups attached to pendent carbonyls along the backbone denoted C1,2 at ∼3.06 ppm, the 3-methine group attached to a methoxy pendent group along the backbone denoted C3 at ∼3.30 ppm and the methoxy pendent group appearing at ∼3.50 ppm labeled 3-methoxy. As is evident by the spectrum of pure

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Figure 4. Spin-spin relaxation of the (open) PMVEMA protons on C3 and (closed) PMVEMA protons on C1,2 for (O) 50CNW and (0) 25CNW nanocomposites at 50 °C.

cellulose nanowhiskers, the peak at ∼3.50 ppm in the spectrum of the nanocomposite is actually an overlapping resonance of both protons from CNW and the 3-methoxy on PMVEMA. The nature of molecular mobility in a polymer system can be probed easily via a T2 NMR experiment. Spin-spin or T2 relaxation probes how fast a nucleus loses transverse magnetization. In T2 experiments, signal intensity decays as a function of local inhomogeneities in the magnetic field mainly due to perturbation by nuclei through space or dipolar interactions, and this signal attenuation or the characteristic relaxation rate for this process is called the spin-spin relaxation or T2 time. The T2 time can be used to describe molecular motion and is particularly sensitive to chain dynamics in polymer systems above Tg. Basically, the faster the rate of the decay the more rigid or fewer degrees of freedom the chemical group associated with that decay has. 1H T2 relaxation can be used on this CNW nanocomposite system to infer information on the length of chains between junctions of cross-linking points, cross-linking density, and sterical constrains from surrounding chains. A Carr-Purcell-Meiboom-Gill (CPMG) sequence was used to collect the T2 data, and Figure 4 displays the typical spin-spin decay profiles for PMVEMA protons on carbon positions C3 and C1,2 for both D2O swollen 25CNW and 50CNW nanocomposites at 50 °C. The lines represent a fit based on a twocomponent, Gaussian-exponential model commonly used in polymer systems containing distinct rigid and mobile components. The CPMG experiments were conducted at an elevated temperature in an effort to impart greater chain dynamics and differences in those dynamics. Figure 4 shows that the relaxation rate that belongs to the 50CNW nanocomposite are faster than that of the 25CNW nanocomposite indicating a much more rigid morphology and is mainly interpreted as an increase in crosslinking density. The T2 results for the various other resolvable peaks for the CNW nanocomposite are compiled in Table 2, listing both the relative intensity and the T2 time for the rigid repeat units in cross-linking junction points (described by the Gaussian component) and more mobile repeat units belonging to chains outside or between junctions of cross-linking points (described by the exponential component).

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The methylene protons on PEG were analyzed using the aforementioned two-component model. The results for the faster relaxing PEG component of the 25CNW nanocomposite show a T2 of ∼44 ms. This component describes a portion of the composite in which PEG chains have low mobility and are attributed to PEG repeat units adjacent to junction points. Whereas the slower relaxing PEG component has a T2 of ∼762 ms and because the extractable PEG had been removed in the exhaustive extractions, the order of magnitude difference suggests this component describes highly mobile PEG chain repeat units most likely part of the linkage between or outside junction points and in areas of appreciably low cross-link density. The results in Table 2 for protons on PMVEMA show that the component in the 50CNW nanocomposite has a T2 of approximately half of that of the 25CNW nanocomposite, suggesting that on average the PMVEMA components have half as many degrees of freedom. Essentially upon incorporation of additional nanowhiskers, the PMVEMA itself exhibits slower dynamics, most likely due to the formation of additional attachments to the polymer composite system. The T2 values for both junction and chain portions of the PEG in the 50CNW nanocomposite are slightly reduced when compared to the 25CNW nanocomposite, again showing upon addition of more cellulose the PEG becomes less mobile, which is due to an increased number of attachment points and an overall increase in sterical constraints. More interesting is the increase of the relative intensity of the junction component and the decrease seen in the relative intensity in the chain component. The change in the relative intensity of the junction component seems to indicate twice as many PEG-related junction points exist in the 50CNW nanocomposite than in the 25CNW nanocomposite. The chemistry involved in this PEG, PMVEMA, and CNW composite preparation suggests PEG has attachments to the composite system through functionality on PMVEMA only at oligomeric PEG end-groups. This is supported by a T2 relaxation profile that exhibits an explicitly clear two-component mobile/rigid behavior, an order of magnitude difference in relaxation times between those components, and whose changes in component relative intensity scales directly with CNW concentration. Moreover, along with this direct correlation between PEG-related junctions and CNW concentration and the aforementioned nanocomposite system chemistry, these results not only infer PEG is bonded solely directly to the PMVEMA chains but also suggest the location of PMVEMA chains within the nanocomposite are in close proximity to the CNW surface for the majority of the nanowhisker length. The methine and methyl protons on PMVEMA were also analyzed using the two-component Gaussian-exponential model. The characteristic T2 time results for the faster relaxing PMVEMA component of the 25CNW nanocomposite show a T2 of ∼28 ms for proton at carbon locations C3 and C1,2. The slower relaxing PMVEMA component had T2 times of ∼157 and 143 ms for proton at carbon locations C3 and C1,2,

Table 2. Characteristic 1H Spin-Spin Relaxation (T2) Values and Relative Component Intensitiesa 25CNW chemical group PEG PMVEMA-C3 PMVEMA-C1,2

1

H (ppm) 3.81 3.30 3.06

jun

%T2

17 26 26

chain

%T2

83 74 74

T2

jun

50CNW (ms)

44 28 28

T2

chain

(ms)

762 157 143

jun

%T2

34 16 16

chain

%T2

66 84 84

T2jun (ms)

T2chain (ms)

29 12 8

525 75 65

a Based on a Gaussian-exponential model for H2O extracted 25 and 50CNW nanocomposites swollen in D2O at 50 °C. jun: junction point describe repeat units close enough to crosslinking point to have redued dynamics, chain: chains between junctions describe repeat units between crosslinking points displaying “unrestricted” dynamics.

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Figure 5. Height image of the nanowhiskers showing the diameter measurements of the cellulose nanowhiskers.

respectively. It is evident that there must be less difference in chain dynamics between the two components for PMVEMA than PEG because of the lower difference between the relaxation rates between the junction and chain components. This most likely is because PMVEMA can bond to the composite system

Goetz et al.

at various points along its backbone. This would lead to much shorter chain portions having less mobility and more junction points relative to PEG-related junctions, which is also reflected in Table 2. The T2 values for all PMVEMA-related resonances and components decrease for the 50CNW nanocomposite with respect to the 25CNW nanocomposite, in this case, mainly because of an appreciable increase in steric constraints upon the incorporation of additional cellulose. Microscopy. The size and size distribution of the nanowhiskers used for this study is shown in Figure 5. The nanowhiskers are found as needle-shaped crystals with diameters between 15-17 nm. The diameter measurements were made using the Nanoscope V software and were determined from the height of the nanowhiskers. This methodology has been shown to be more accurate for the characterization of CNW because the broadening effect due to the AFM tip scanning is considered to be minimized in comparison to width measurements.36 It is worth mentioning that the efficiency of the measurements are restricted by the aggregation of nanowhiskers during sample preparation, leading to difficulties in obtaining a monolayer of well-dispersed whiskers for measurements. In most cases, a majority of the nanowhiskers will be only partially separated or even bundled together. An overview and detailed view of cross-linked nanocomposites with 25 and 50CNW are shown in Figure 6. These results demonstrate an equally distributed second phase in a continuous matrix phase.

Figure 6. Cross-sections of cross-linked nanocomposites with 25CNW, (a) overview, (b) detailed view; and 50CNW, (c) overview, (d) detailed view.

Nanocomposites Cross-Linked with Nanowhiskers Table 3. Mechanical Properties of Matrix and Crosslinked Nanocomposite Hydrogels at Different Relative Humidities sample code

relative humidity [%]

tensile strength [MPa]

strain [%]

E-modulus [GPa]

25CNW 50CNW 75CNW 100CNW 75CNW 75CNW 75CNW

92 92 92 92 2 54 92

2.3 ( 0.8 4.2 ( 0.2 4.0 ( 0.1 4.0 ( 1.6 36.8 ( 5.3 8.5 ( 2.7 4.0 ( 0.05

309 ( 16 227 ( 24 120 ( 2 105 ( 0 106 ( 2 116 ( 4 120 ( 2

0.09 ( 0.06 0.19 ( 0.06 2.3 ( 0.1 3.9 ( 0.8 14.3 ( 3.3 8.7 ( 0.9 2.3 ( 0.1

In the AFM micrographs, the oval-shaped structures are attributed to cellulose nanowhiskers or clusters of nanowhiskers embedded in the PMVEMA-PEG phase. A broad range of size distributions is observed for the nanowhiskers in the nanocomposite images that may be due to the fact that the nanowhiskers are cut at different angles during sample preparation. This is expected as the nanowhiskers are randomly oriented in solution casted films and the AFM image obtained is from the ultramicrotomed cross-section. In addition it was noted that the sizes of the embedded phase showes a tendency to increase as the CNW content increased from 25 to 50 wt %. This may suggest bigger agglomerates of CNWs at higher CNW content. Mechanical Properties. The stress-strain properties of the prepared hydrogels were studied as a function of CNW content and relative humidity (RH) conditions. The nanocomposite hydrogels with 25, 50, 75, and 100 wt % CNW content conditioned at 92% RH were evaluated for physical properties, and these results are shown in Table 3. It can be seen that the tensile strength was increased when the nanowhisker content increased from 25 to 50 wt %. The modulus (stiffness) of the hydrogels was very low for 25 and 50CNW but increased for the 75 and 100CNW hydrogels. All hydrogels exhibited a very high ability to strain, between 120-306%, with the highest values for lower nanowhisker content. Compared to cellulose nanofiber networks (nanopaper) toughness values reported by Henriksson et al.37 the in situ cross-linked materials have superior toughness but lower stiffness and strength. The effect of the relative humidity on the mechanical properties is demonstrated in Table 3 based on the mechanical performance of the 75CNW composites conditioned at three different relative

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humidity conditions (2, 54 and 92%). The percent elongation at break increased as the relative humidity increased, while the maximum stress at break was achieved at the lowest relative humidity. From Table 3, it is possible to see the correlation between the composition and relative humidity. For example, as the relative humidity increased, elongation at break (%) also increased, while the tensile strength (MPa) decreased as relative humidity increased. This shows that these materials are capable of a wide range of properties depending on the moisture conditions. The representative stress-strain curves of the cross-linked composites varied with different CNW content, with the 92% RH conditioned samples shown in Figure 7. As the nanowhisker content increased, the stress and modulus increased, while the strain decreased. As can be seen in Table 3 and Figure 7, the strain and the stress varied based on the composition of the materials. However the relationship between the CNW content and the mechanical properties is nonlinear. It appears as though the 50CNW at 92% RH (7c) is able to balance stress versus strain demands better than the 25CNW or the 75CNW composite nanocomposite hydrogels. In summary, each of the nanocomposite hydrogels behaved differently from the others. This furthers our belief that these materials have developed differing degrees of cross-linking between the matrix and the cellulose nanowhiskers. This provides the possibility to potentially tailor these materials to a specific set of conditions depending upon the desired application.

Conclusion In-situ co-cross-linked nanocomposite hydrogels were developed by dispersing and cross-linking cellulose nanowhiskers with poly(methyl vinyl ether-co-maleic acid)-polyethylene glycol matrix using solution casting and a subsequent crosslinking technique. Nuclear magnetic resonance studies supported the cross-linking between the matrix and the cellulose nanowhiskers via an esterification reaction. The atomic force microscopy images show that the nanowhiskers were dispersed homogenously in the matrix even at high nanowhisker concentration, indicating that in situ co-cross-linking of cellulose nanowhiskers with a matrix polymer is a promising route to prevent nanowhisker aggregation and obtain good dispersion at the nanolevel.

Figure 7. Stress-strain curves of prepared hydrogels: (a) 100CNW, (b) 75CNW, (c) 50CNW, and (d) 25CNW nanocomposite hydrogels at 92% relative humidity.

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The cross-linked nanocomposite hydrogels exhibited significantly different behavior relaxationally, mechanically, and morphologically as the material percentages were varied. The mechanical property increased with an increase in nanowhisker content and was found to decrease as relative humidity increased. The nanocomposite hydrogels with 50 wt % CNW content appear to be a balance between the mechanical properties of the nanocomposites with 25 and 75 wt % CNW content. This new methodology of nanocomposite processing is expected to provide an efficient route to develop a tailor-made nanocomposite controlled by degree of co-cross-linking, nanowhisker concentration, relative humidity conditions, and so on. Acknowledgment. The support of the National Science Foundation through the International Research and Education in Engineering Grants EEC-0525746 and EEC-0332554 is gratefully appreciated and acknowledged. The authors also thank the Member companies of the Institute of Paper Science and Technology and the IPST@GT fellowship program for financial support. Portions of this work will be used by L.G. as partial fulfillment of the requirements for the degree of Ph.D. at the Georgia Institute of Technology.

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