optimizing noncovalent interactions between lignin and synthetic polymers to develop effective...

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Full PaperMacromolecularChemistry and Physics

Optimizing Noncovalent Interactions Between Lignin and Synthetic Polymers to Develop Effective Compatibilizers

Nathan Henry, David Harper, Mark Dadmun*

Experiments are designed and completed to identify an effective polymeric compatibilizer for lignin–polystyrene blends. Copolymers of styrene and vinylphenol are chosen as the structure of the compatibilizer as the VPh unit can readily form intermolecular hydrogen bonds with the lignin molecule. Electron microscopy, thermal analysis, and neutron refl ectivity results demonstrate that among these compatibilizers, a copolymer of styrene and VPh with ≈ 20%–30% VPh most readily forms intermolecular interactions with the lignin molecule and results in the most well-dispersed blends with lignin. This behavior is explained by invoking the competition of intra- and intermolecular hydrogen bonding and functional group accessibility in forming intermolecular interactions.

1. Introduction

Lignin is an amorphous, irregularly branched biopolymer that serves to bind together the cellulose fi bers in all plant matter [ 1 ] and is generated as a byproduct in enor-mous quantities from cellulose extraction in the paper industry. [ 2 ] Lignin is also produced as a byproduct of the emerging cellulosic ethanol industry. [ 3 ] It is reported that 70 million tons are generated annually from the paper industry, [ 4 ] 98% of which is simply burned as a fuel for process heat. [ 5 ] Lignin has seen some use as a feedstock for isocyanate and epoxy resins, surfactants and in urethane foams, but lignin is still a very underutilized resource. [ 6–9 ] The economic utility of lignin could be expanded if it could be incorporated into composites with synthetic polymers

wileyonli

N. Henry , M. Dadmun Chemistry Department, University of Tennessee, Knoxville, TN 37996, USA E-mail: [email protected] D. Harper Center for Renewable Carbon, University of Tennessee, Knoxville, TN 37996, USA M. DadmunChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

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as an additive or fi ller. In addition to lignin being an abun-dant renewably sourced material, it is also biodegradable unlike many synthetic polymers. [ 10 ] Its potential has been demonstrated as an antioxidant additive for rubber [ 11 ] and it can add fl ame retardant properties to composites. [ 12 ] Suc-cessful composites, however, must incorporate signifi cant amounts of lignin without excessive negative impact to physical properties, which is a challenge, limiting the use of such composites.

Today about 30% of polymer products are made from copolymers or mixtures of homopolymers [ 13 ] and such combinations are a topic of considerable research. The most direct way to incorporate lignin into polymeric materials is by blending with another premade poly mer, producing a “polymer alloy.” This is a powerful approach; the blending with different polymers could lead to a wide variety of applications where lignin is incorporated by simple physical mixing. One of the main barriers to useful polymer blends is that the components are often incom-patible with each other: the entropic benefi t of mixing between two polymers is often small; thus absent signifi -cant intermolecular interactions, the enthalpy of mixing is typically endothermic and results in a phase-separated system. [ 14 ] This is illustrated and quantifi ed by the Flory–Huggins equation for the free energy of mixing two polymers, where the high molecular weight of polymers

nelibrary.com DOI: 10.1002/macp.201100633

Optimizing Noncovalent Interactions Between Lignin and . . .

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MacromolecularChemistry and Physics

(a) (b)

Figure 1 . Illustrations depicting (a) an interface with hydrogen bonding between com-ponents that can strengthen the interface and (b) interfacial strengthening due to the presence of entanglement across the biphasic interface.

Figure 2 . Illustrative lignin segment composed of four coupled monolignols.

( M A or M B ) reduces the contribution of the entropic terms ([ φ / M ] ln φ ) to Δ G mix , [ 15 , 16 ]

�Gmix

RT=

φA

MA

ln φA +φB

MB

ln φB + χAB φAφB

The reason that phase separation is generally undesir-able is that load transfer through the bulk is often inter-rupted across the interface between components unless the interfacial adhesion is quite strong, leading to mate-rials with poor physical properties. [ 17 ] Pucciariello et al. [ 18 ] demonstrated this for immiscible blends of polyethylene or polystyrene with lignin, correlating electron micro-graphs of a very inhomogeneous fracture surface, indica-tive of weak interfaces, with a signifi cant decrease in mechanical properties with increasing lignin loading.

To make useful composites, the intermolecular inter-actions between lignin and the matrix polymer must be strengthened: this would result in either the bulk of the composite becoming miscible with a negative Δ G mix , enhancement of the interfacial adhesion that preserves mechanical strength (Figure 1 a) or an intermediate case where limited interdiffusion occurs but an interface is retained. [ 19 ] In either of the two latter cases, if a polymer chain of suffi cient length extends and entangles across the interface (Figure 1 b), it will also increase interfacial adhe-sion from the entangled covalent bonds of the backbone. [ 20 ]

Unlike most common synthetic polymers, lignin has a complex and irregular polymer structure because of the coupling at multiple possible sites of the lignin monomer units. The “monolignols” that make up lignin are based on simple aromatic alcohols, in some types with adjacent pendant methoxy groups, bearing an unsaturated side chain terminated with a hydroxyl group. [ 21 ] The overall structure of the lignin is comprised of an assortment of monolignols bonded to different sites, the ratio of their type and bonding varying according to source plant species, and can be further changed during the lignin

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extraction process. [ 22–25 ] Therefore, the exact structure of lignin varies, and escapes simple characterization (Figure 2 ).

Lignin exists as a lightly crosslinked polymer within the plant tissue struc-ture, bearing covalent bonds between itself and to the celluloses that the lignin reinforces. All methods, which extract lignin, therefore cause chem-ical change by breaking some of these bonds to liberate macromolecules with molar masses generally between 1000 and 20 000 g mol − 1 . [ 26 ] Several lignin extraction methods are available, such as the alkali hydrolysis “soda” process, the sulfi te and Kraft processes, and

others. The different processes have different effects on lignin, such as extensive oxidation of soda lignin, [ 27 ] increased crosslinking and incorporation of sulfi tes in the sulfi te process, [ 28 ] and aliphatic thiols in Kraft lignin. [ 29 ] One relatively new process of note uses organic solvents, such as ethanol or methanol, to replace lignin ether and ester bridges with ethoxy or methoxy groups as well as dissolving the cleaved lignin macromolecules. These orga-nosolv lignins contains no sulfur, low amounts of water soluble impurities, and good processability. [ 30 , 31 ]

Lignin has abundant hydroxyl and ether functionality that are eligible for hydrogen bonding, which causes lignin molecules to associate into a loose network struc-ture of supermolecular complexes up to 200–300 nm in diameter and consisting of 10 3 –10 4 molecules. [ 32–34 ] As mentioned above, signifi cant differences exist in the spe-cifi c structure of the lignin monomer based on the source and extraction process. Obviously, the specifi c structure will impact the behavior of the lignin as a polymer.

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Figure 3 . Illustration of the ability of polymeric compatibilizers to limit droplet coalescence in the coarsening of phase-separated polymer blends.

Previous work has taken advantage of the abundant functional groups present on the lignin structure to intro-duce intermolecular hydrogen bonding with a matrix polymer bearing compatible groups, such as ether or hydroxyl groups. [ 35 ] This approach has shown some suc-cess, such as Kubo and Kadla reporting evidence for lignin intermolecular association with polyvinyl alcohol [ 36 ] and solubility with low-molecular-weight poly(ethylene terephthalate). [ 37 ] However, with few exceptions, lignin loading above approximately 40% (w/w) has not been demonstrated without signifi cant embrittlement of the composite. [ 38 , 39 ]

Although relatively successful, this strategy imposes limitations on, and may add expense to, the matrix polymer that the lignin is blended with: the requirement that its structure must be tailored for lignin compat-ibility reduces the attractiveness of such composites. An alternative approach is to add a ternary “compatibilizer” polymer, one that would lie at the interface between the two components, and would serve to mitigate the prob-lems of incompatible lignin composites in an effi cient and customizable fashion.

With such a compatibilizer as an additive, lignin could be incorporated at higher loading in composites with a wider range of polymers, as well as reduce the need for specifi cally tailored base materials. For a blend of an incompatible, immiscible polymer with lignin, a com-patibilizer with enthalpic affi nity for both materials can be induced to lie at the interface. [ 20 ] There, a polymeric compatibilizer enhances the ultimate composite in three ways: by increasing interfacial adhesion, mitigating inter-facial tension and stabilizing dispersed polymer droplets. The fi rst two effects arise from the entanglement of the interfacial modifi er with both phases, a result of the affi nity of the compatibilizer with both the synthetic polymer and the lignin. The third effect is the result of unique property of polymeric compatibilizers, that they present an entropic “cushion” that retards droplet coa-lescence in the melt. In a phase-separated system, coars-ening of phase domain size will occur by droplet coales-cence. In this coalescence process, for two droplets of like polymer to coalesce, the other phase must be pushed out of the way. With polymeric compatibilizers on the droplet, as shown in Figure 3 , there is required an additional force and entropic penalty to displace the compatibilizer polymer. This displacement requires that the compati-bilizer chains be compressed, hindering their conforma-tional freedom and decreasing their entropy (Figure 3 ), which in turn limits the coalescence process. [ 40 ]

Polymeric compatibilizers are not a new approach, and have been investigated by our group [ 41 , 42 ] and others as a method to improve the dispersion, interfacial adhe-sion, and properties of phase-separated polymer blends. For instance, Alexy et al. [ 43 ] demonstrated the ability of

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poly[ethylene- ran -(vinyl acetate)] copolymers to com-patibilize blends containing lignin and polyethylene, suc-ceeding in signifi cantly increasing both tensile strength and elongation at break.

A critical parameter in optimizing the properties of a phase-separated blend is to create a polymer that will interact equally with both components, interactions that can be modulated by the structure of a copolymeric com-patibilizer. For instance, a previous study by Kulasekere et al. [ 44 ] examined the ability of random poly[styrene- ran -(methyl methacrylate)] to compatibilize the interface between polystyrene and poly(methyl methacrylate). These results clearly showed that the interfacial strength of the compatibilized interface is strongly affected by the composition of monomers in the copolymer. Li and Sar-kanen [ 45 ] note that the mechanical properties of lignin–poly(ethylene terephthalate) composites are also strongly infl uenced by the density of ester groups.

In this work, the ability of a copolymer containing hydrogen bonding groups to compatibilize the interface between polystyrene, a model synthetic polymer, and lignin is examined. In particular, the role of the compo-sition of the copolymer, or equivalently, the amount of hydrogen bonding group present in the copolymer, on its ability to compatibilize the polystyrene–lignin inter-face is monitored, with the goal of defi ning the optimal density of hydrogen bonding groups on the polymeric compatibilizer. In this work, a series of poly(styrene- ran -4 VPh) (PS- r -4VPh) copolymers were synthesized with VPh content ranging between 10 and 50%. The interaction of these copolymers with lignin is examined via electron microscopy, differential scanning calor-imetry, and interfacial structure, and the results inter-preted to explain the optimal interacting copolymer’s behavior.

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2. Experimental Section

Organosolv lignin (Sigma-Aldrich) was used in this study. Gram quantities of protonated random copolymers of styrene and vinyl phenol (PS- ran -4VPh) were synthesized in our laboratory by loading a round bottom fl ask with varying amounts of styrene monomer and 4-acetoxystyrene monomer to 1,4-dioxane solvent in a 1:2 volume ratio with 2,2 ′ -azoisobutyronitrile as the radical initiator in a 1:400 mass ratio to the monomer. The mixture was degassed by freezing with liquid nitrogen and evacuated for 30 min to remove oxygen from the solution. These solutions were then reacted for 20 h at 60 ° C under an inert argon atmos-phere, the copolymer precipitated in cold methanol and dried for 48 h in a vacuum oven at 80 ° C. The poly[styrene- ran -(4-acetoxy-styrene)]s were then redissolved in 1,4-dioxane with 15 vol% concentrated hydrazine hydrate and stirred for 48 h at room tem-perature to convert the pendant acetoxy to hydroxyl groups. [ 46 ] These poly[styrene- ran -4-(vinyl phenol)] copolymers were then precipitated in cold methanol (10–20% 4VPh) or hexanes (30–50% 4VPh) and dried for 24 h under vacuum at 60 ° C. Small quantities of deuterium labeled copolymers were also produced by a sim-ilar route with several exceptions: deuterated styrene monomer was used, the solvent was reduced to a 1:1 volume ratio and the polymerization solution was charged into 15 mL glass ampoules and evacuated.

The molecular weights of the polymers examined in this set of experiments based on polystyrene standards are reported in Table 1 . All polymer molecular weights were determined with size exclusion chromatography using a Polymer Labs GPC-20 with Mixed-C 5 μ m columns, a refractive-index detector, and tetrahydrofuran as the eluent at 20 ° C. The ratio of styrene to 4VPh in the polymer was monitored by performing 1 H NMR on the protonated poly(styrene- ran -acetoxystyrene) intermediates. Integration of the resonances of the methyl (CH 3 �) of the acetoxy

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Table 1. Molecular weight characteristics of the polymers used in this study.

Polymer 4VPh [mol%]

M n [kDa]

M w [kDa]

PDI

protonated

0% 4VPh 0 77.3 196 2.54

10% 4VPh 10 94.8 186 1.96

23% 4VPh 23 88.0 151 1.72

30% 4VPh 30 94.3 178 1.89

43% 4VPh 43 67.6 114 1.69

50% 4VPh 50 75.8 125 1.65

deuterated

0% 4VPh 0 111 125 1.13

10% 4VPh ≈ 10 52.3 88.9 1.70

20% 4VPh ≈ 20 62.8 99.2 1.58

30% 4VPh ≈ 30 64.5 101 1.56

40% 4VPh ≈ 40 70.7 111 1.57

50% 4VPh ≈ 50 80.1 124 1.55

Macromol. Chem. Phys.© 2012 WILEY-VCH Verlag Gm

group at δ = 2.2 (3 H, singlet) relative to the phenyl ring protons at δ = 6.2–7.2 (4–5 H, multiplet) quantifi es the composition of the copolymers. Conversion of the acetoxy to hydroxyl groups was also verifi ed by the disappearance of the acetoxy group resonance in 1 H NMR.

Small quantities of copolymer–lignin blends were prepared by melt mixing to observe the morphology and phase behavior of the blends to monitor the effectiveness of each copolymer as a lignin compatibilizer. These blends, composed of 80 wt% copoly mer and 20 wt% lignin, were combined by melt mixing at 165 ° C for 15 min under an argon blanket in a single-screw Atlas LMM cup melt mixer at 60 rpm, extruding and reloading once half-way through mixing to maximize homogeneity. The short mixing time, low temperature, and inert atmosphere were used to limit lignin cross-linking and thermal decomposition. [ 4 ] The bulk morphology of lignin–copolymer composites was studied by imaging the fracture cross-sections with scanning electron microscopy (SEM). Pieces of composite blends were sectioned by cryofracturing with liquid nitrogen, coating with a ≈ 10 nm layer of gold metal to dissipate electrostatic charging and imaged with a LEO-1525 scanning electron microscope at 10 kV acceleration.

To monitor the extent of miscibility in the blends, the glass transition temperature ( T g ) of the lignin, the styrenic polymers, and the composites produced by melt mixing were measured. The T g of samples with mass ≈ 10–20 mg were determined using a Mettler-Toledo 821e differential scanning calorimeter (DSC), by scanning the temperature between 25 and 150 ° C at a ramp rate of 10 ° C min − 1 under a blanket of nitrogen gas. The T g of pure lignin was measured after annealing under vacuum for 15 min at 160 ° C to expose it to the same potential thermal degradation as lignin in the melt-mixed copolymer–lignin blends.

Neutron refl ectometry (NR) is a powerful technique used to study the interfacial structure of polymers: NR provides a depth profi le of thin fi lms, and can accurately characterize the interfacial width of many phase-separated polymer blends. [ 47 ] Contrast between layers was obtained by selective deuteration of the synthetic styrene-based polymers, whereas the natural lignin remains protonated.

Our experiment examined bilayer samples of the lignin and styrenic polymers, where the lignin–copolymer bilayers were assembled using the fi ve deuterium-labeled PSd8- ran -4VPh copolymer compatibilizers as well as deuterated polystyrene. They were made by spin-casting a lignin fi lm at 2500 rpm for 30 s from tetrahydrofuran onto thoroughly cleaned silicon wafers. Silicon wafers were cleaned by immersion for 30 min in a freshly prepared solution of 3:1 H 2 SO 4 and 30 wt% H 2 O 2 , rinsed with copious volumes of high-purity water, and dried under a stream of dry nitrogen. Copolymer fi lms were likewise spun-cast from ≈ 1.5 wt% solutions of toluene (4VPh 0–10%), methylethylketone (20% 4VPH), or tetrahydrofuran (30–50% 4VPh) onto polished NaCl monoliths. These fi lms were stacked onto lignin by fl oating the copolymer fi lms on high-purity water and lifting the copolymeric fi lms with the initial lignin fi lm. After fl oat coating, samples were dried in air and under vacuum for 24 h each to remove excess water. The thicknesses of the copolymers were monitored using a DRE GmbH model ELX-02C ellipsometer with a 632 nm laser and at a 70 ° angle of incidence on duplicate copolymer fi lms spun-cast onto silicon wafers.

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Figure 4 . SEM micrographs of the cryofractured composite cross-sections at 5000 × magnifi cation.

To give the mechanically stacked lignin–copolymer bilayer fi lms mobility to intermix, the fi lms were vapor annealed [ 48 ] for 24 h using a saturated vapor of tetrahydrofuran, which is a common solvent for the copolymers and lignin. This method was chosen to avoid further thermal degradation or crosslinking of the lignin. All samples were measured in air using the Liquids Refl ectometer at the Oak Ridge National Laboratory Spallation Neutron Source.

3. Results

3.1. SEM

Figure 4 shows the fracture surfaces of the six blend samples studied, showing the bulk morphologies of the resultant mixtures. Inspection shows that the PS-lignin sample exhibits large ( ≈ 1–2 μ m) lignin domains, whereas the lignin blend with the copolymer that contains 10% VPh exhibits very little observable lignin domains. As the amount of VPh increases to 23% in the styrenic polymer, the mixture appears to be nearly homogeneous at this magnifi cation, with no clear holes or domains in the matrix. Conversely, the blends that contain the 30 and 43% VPh copolymers clearly contain large (0.5–2 μ m) lignin domains. Interestingly, the structure of the lignin blend that contains the 50% VPh copolymer also exhibits sepa-rate lignin domains at this magnifi cation; however, the domains appear to be smaller than those found in the 30 and 43% VPH copolymer blends.

Moreover, for the samples with clear lignin domains, the domain droplets extend out of or into the surface rather

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than being cleaved along the fracture. This observation suggests that the con-duction of force was not transmitted through the lignin droplets due to frac-ture at the lignin–copolymer interface. This behavior is characteristic of poor interfacial adhesion, a key reason that composites of incompatible materials often exhibit poor mechanical properties.

The 10 and 23% 4VPh samples do not exhibit this pull-out, which indicates improved interfacial adhesion. This is consistent with the observations above, the smaller lignin droplets result from a lower interfacial energy between the copolymer and lignin, which is attrib-uted to stronger lignin–copoly mer inter-actions. Having a stronger enthalpic inter-action between lignin and copoly mer will lead to higher interfacial adhesion
and improved material properties.
3.2. DSC

DSC was also used to determine the impact of varying the amount of hydroxyl groups in the styrenic compatibilizer on the miscibility of the copolymer and lignin. This study takes advantage of the dependence of the T g on the mis-cibility in miscible polymer blends. T g varies systemati-cally with blend composition for miscible polymer blends, where the relationship between the blend composition and measured T g is given by the Fox equation,

1Tg

=wa

Tg,a+ wb

Tg,b

where w a and w b are the mass fractions of components a and b in the blend, respectively. Thus, the measured T g of a blend provides a measure of the miscibility of the two polymers. [ 49 ] As described in this equation, when polymers are completely miscible, a blend exhibits a single T g near a weighted average between the respective homopolymer transition temperatures. For blends that are only partially miscible, the two transition temperatures will shift toward this intermediate but not completely coa-lesce into a single transition. The magnitude of this shift also provides insight into the relative solubility of a com-posite blend.

Lignin has a higher T g ( ≈ 126 ° C) than the copolymers (average ≈ 102 ° C), therefore, the T g of the blend will shift to a higher temperature with increasing miscibility. Moreover, inspection shows that the compound forms a viscous liquid at the mixing temperature. The results of the DSC experiment are shown in Figure 5 and are listed in Table 2 . Figure 5 shows the DSC traces of the lignin and

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Figure 5 . The DSC traces of lignin and the 20/80 lignin/styrenic copolymer blends. The percent VPh in the styrenic copolymer is denoted next to each curve.

Figure 6 . Measured refl ectivity and model fi t for the ligninpoly-styrene bilayer sample after solvent annealing.

the 20/80 lignin–styrenic copolymer blends. Table 2 lists the T g of the blends and that of the pure styrenic polymer, as well as the difference between these two values ( Δ T g ). The largest shift is observed for the copolymer that con-tains 10 mol% 4VPh, with a shift of + 15 ° C, indicating substantial miscibility between the lignin and 10% 4VPh copolymer. The blends of 23% and 30% 4VPh copolymer and lignin also show miscibility, each with a shift of + 5–6 ° C. The 43 and 50% 4VPh copolymers show little or no shift in the T g of the blend, indicative of very little to no miscibility.

3.3. Neutron Refl ectivity

Neutron refl ectometry is used to monitor the interpenetra-tion of lignin and each styrenic copolymer when assembled as a bilayer and annealed. The degree of interpenetration, or width of the interface, offers a measure of the solu-bility between the two polymers across that interface. NR provides a 1D depth profi le of scattering length density (SLD), a measure of scattering strength, as a function of


Table 2. Glass transition temperature of the styrenic copolymer and blends.

Copolymer T g styrenic polymer [ ° C]

T g blend [ ° C]

Δ T g [ ° C]

polystyrene 107 101 − 6

PS-r-4VPh 10% 76 91 + 15

PS-r-4VPh 23% 99 104 + 5

PS-r-4VPh 30% 110 116 + 6

PS-r-4VPh 43% 120 118 − 2

PS-r-4VPh 50% 103 114 + 2

Macromol. Chem. Phys. 2© 2012 WILEY-VCH Verlag Gm

depth. The SLD depth profi le is, however, convoluted into an inverse space intensity curve ( R (q ) vs. q (z) ) which is not generally practical to directly invert into a real space depth profi le. To overcome this, depth profi les of the samples are modeled and fi t to the experimental data. To guide model construction, NR and ellipsometry measurements were performed on monolayers of all materials to obtain values for individual component SLD and layer thickness to use as starting points for modeling. The SLDs of all polymers are determined from the fi ts of the monolayers and verifi ed by the calculated SLD of the polymeric structures based on their composition. Ellipsometric measurements were used to determine the thickness of all monolayers, except lignin, as the refractive index of lignin is not accurately known. The thickness of the lignin monolayer was determined by neutron refl ectivity.

These models were then iteratively refi ned until the experimental and calculated refl ectivity curves converged using REFLfi t software, [ 50 ] as is shown in Figure 6 for the polystyrene–lignin bilayer. The data and models were also fi t with the constraint that the as-cast and annealed bilayers were mass balanced.

The depth profi les of the six bilayers are determined from the SLD depth profi le using

φ styrenic =

SLD(z) − SLDlignin

SLDstyrenic−SLDlignin

where SLD( z ) is the experimentally determined scattering length density at depth z , SLD(lignin) is the scattering length density of lignin, and SLD(styrenic) is the scattering length density of the styrene-based copolymer.

Figure 7 shows a representative composition depth profi le of the lignin-50% VPh bilayer, which has been

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Figure 7 . Depth profi le of the deuterated styrenic copolymer in the lignin-50% VPh copolymer bilayer after solvent annealing.

Table 3. Interfacial width and extent of phase separation of lignin bilayers as extracted from the neutron refl ectivity measurements.

Copolymer w I [Å]

Δ ϕ tan h

Δ ϕ Absolute

polystyrene- d 8 91 0.63 0.57

PSd8-r-4VPhh8 10% 64 0.47 0.38

PSd8-r-4VPhh8 20% 45 0.21 0.19

PSd8-r-4VPhh8 30% 250 0.86 0.58

PSd8-r-4VPhh8 40% 85 0.49 0.45

PSd8-r-4VPhh8 50% 90 0.86 0.81

Figure 8 . Interfacial width and extent of phase separation ( Δ ϕ ) of the interface between the styrenic copolymers and lignin as a function of 4VPh content in the copolymer.

depth-normalized: the conversion and normalization was completed to aid in comparing the different sample depth profi les, as each styrenic polymer has a different SLD and each bilayer varies in total thickness. The interface between lignin and the styrenic polymer is evident by the drop in φ at approximately 50% of the depth, from the copolymer-rich layer near the air surface to a copolymer-poor one near the substrate, where the lignin was depos-ited. All six bilayers were assembled with the copolymer on top of the lignin. A sharp transition with a large drop in composition is characteristic of a well-defi ned interface and poor solubility, whereas a small composition drop or a broad interface is indicative of more miscibility between the two layers. [ 51 ]

To quantitatively compare the different interfaces that exist in the various samples, the interfacial density profi le is fi t to a modifi ed hyperbolic tangent function, where the choice of this function is based on its suitability to model polymer–polymer interfaces as demonstrated by Creton, Schnell, and Stamm. [ 51 ] The hyperbolic tangent function provides a quantifi cation of the width of the interface and degree of phase separation (or mixing) across the interface. Equation 1 is the functional form used in this fi tting process. In this equation, Δ φ corresponds to the rel-ative degree of miscibility between the two layers, where smaller values indicate less phase-separated pairs. As well, w I provides a measure of the width of the interface between the lignin and the styrenic polymer. To further characterize the miscibility between the two layers, the absolute difference of copolymer concentration at the top and the bottom of the fi lm was also calculated, denoted as Δ ϕ absolute below. This last calculation became impor-tant as the hyperbolic tangent poorly approximated the PSd8- r -4VPh 30%-lignin interface.

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φ (z) =

�φ2

× tanh

(d(z)

wI

)+ φ I

(1)

The results of the characterization of the lignin inter-faces by neutron refl ectivity are shown in Table 3 and Figure 8 , which document the change in interfacial width, w i , and the extent of phase separation of the interface. Figure 8 shows that the interfacial width is a maximum ( ≈ 250 Å) for the bilayer of lignin with the 30% VPh copoly mer, demonstrating the highest degree of interpenetration between layers, which should cor-respond to a strong interface. Δ φ shows a minimum for the bilayer of lignin with PSd8- ran -4VPh 20%, exhib-iting a minimal composition difference ( ≈ 21%) across the interface. This value indicates signifi cant misci-bility between this copoly mer and the lignin, in general agreement with the electron microscopy results. These results, therefore demonstrate that the copolymer with an intermediate density of hydrogen bonding groups on the chain (20–30%) is optimum for creating a strong interface to lignin.

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4. Discussion

These results illustrate the importance of modulating the density of hydrogen bonding hydroxyl groups in a polymer chain to achieve effi cient compatibilization with lignin. For the PS- ran -4VPh system studied here, the optimum appears to be near ≈ 20% 4VPh. Below this value, a sub-optimal density of hydrogen bonding vinyl groups are present, leading to too few hydrogen bonds with lignin. In this case, the enthalpic interaction is not suffi ciently strong to signifi cantly create a strong lignin–styrenic interface.

However, it does not appear that increasing the amount of hydrogen bonding groups in the styrenic poly mer chain beyond ≈ 20–30% further improves the extent of hydrogen bonding and miscibility with lignin. This occurs for a number of reasons; the fi rst is a result of the conforma-tional and steric hindrance of the 4VPh groups because of their proximity on the polymer chain as the content of VPh increases. If several hydroxyl groups are in close proximity to each other and one is hydrogen bonded to a lignin functional group, this bond pins the chain at that point. Nearby groups on the same polymer chain then lack the rotational and/or translational freedom to fi nd an accessible lignin site and orient correctly to form another for hydrogen bond. Thus, the accessibility of the functional groups [ 52–54 ] to form hydrogen bonds decreases as more hydrogen bonding functional groups pack onto the polymer chain. This effect has been exquisitely inves-tigated by Coleman et al. [ 55 ] for blends of poly(2,3-dime-thyl butadiene- ran -4 vinyl phenol) and poly(ethylene- ran -vinyl acetate) copolymers with differing density of 4VPh groups, where the extent of intermolecular hydrogen bonds that are formed was monitored by infrared spec-troscopy. The concept of limited accessibility is illustrated in Figure 9 by the “trapped” 4VPh monomer in the center of the PS- ran -4VPh chain segment.

The competition between inter- and intramolecular hydrogen bonding by the VPh groups also plays a role in limiting the accessibility of a VPh hydroxyl group to


Figure 9 . Illustration of limited accessibility of hydrogen bonding functional groups (�OH) due to the formation of intermolecular interactions by neighboring moieties.

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form an intermolecular hydrogen bond. As the number of hydroxyl groups on a given polymer chain increases, the probability that one styrenic �OH group will form a hydrogen bond with another styrenic �OH group (i.e., an intramolecular hydrogen bond) increases. More impor-tantly, if an �OH group is participating in an intramo-lecular hydrogen bond, it is not available to form the desired intermolecular hydrogen bond. [ 56 , 57 ] Therefore, the competition between inter- and intramolecular hydrogen bonding also contributes to the decrease in miscibility of the PS- ran -VPh copolymers with lignin for the higher per-cent VPh copolymers, resulting in an optimum composi-tion of the styrenic copolymers.

Finally, the fact that lignin molecules tend to form desirable supramolecular complexes via lignin–lignin hydrogen bonding must be considered in defi ning an optimum compatibilizer. Should the formation of inter-molecular hydrogen bonds between lignin and compatibi-lizers disrupt these complexes, the mechanical strength of lignin will be signifi cantly degraded. [ 58 ] Previous work has clearly demonstrated that the strength of the interaction of the compatibilizers with the lignin must be modulated such that there is signifi cant interaction with the lignin, but not so much that it disrupts the lignin complexes. [ 45 ]

5. Conclusion

In this work, the effectiveness of copolymer compatibi-lizers to increase the interfacial adhesion with lignin has been investigated. In this study, the amount of hydrogen bonding 4VPh monomers in a styrenic copolymer is varied, and the impact of this modulation on the interaction of the copolymer with lignin is monitored. Copolymers con-taining 0%–50% 4VPh were studied by electron micro-scopy, thermal analysis, and neutron refl ectivity. The study of the blend morphology from microscopy and thermal behavior from thermal analysis shows that a copolymer with ≈ 10–23% 4VPh yields a much fi ner dispersion and more blend-like T g s than copolymers with high amounts of 4VPh. Analysis of the interfacial structure and inter-mixing between fi lms of lignin and the VPh copolymers with NR shows a maximum of mixing with copolymers of 20% 4VPh and a maximum interfacial width at 30% 4VPh. Therefore, the optimal interaction and miscibility of the styrenic copolymer with lignin is observed for copolymers that contain a minority of the interacting VPh functional group. This unexpected behavior can be explained by con-sidering the accessibility of the polymer-bound �OH func-tional group to form an intermolecular hydrogen bond with lignin. Copolymers with low% 4VPh ( < ≈ 10%) do not contain suffi cient hydrogen bonding moieties to form intermolecular interactions that can improve polymer–lignin compatibility. At the same time, copolymers with

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≈ > 30% 4VPh are limited in their ability to form intermo-lecular hydrogen bonds due to connectivity effects of the functional groups and intra-copolymer hydrogen bonding. This leads to an optimal composition of the copolymer to form intermolecular hydrogen bonds between the sty-renic polymer and lignin, which in turn predicts that these copolymers will behave as the most effective compatibi-lizers in styrene–lignin blends.

This becomes important in the quest to identify value-added lignin-based materials. Mixing of lignin with com-mercial polymers often results in inferior properties due to phase separation. However, the negative effects on material properties of meager miscibility, poor dispersion and weak interpolymer adhesion can be mitigated by the addition of a single polymeric compatibilizer. Polymeric compatibilizers can make it practical to blend different polymers to fabricate composites of low value or recycled polymers yielding new materials that add value while reducing waste. These results exemplify that the design of such compatibilizers requires the tuning of the struc-ture of the interfacial modifi er, and does not merely rely on the incorporation of interacting units. Moreover, these results provide guidelines that can be used to design effective polymeric interfacial modifi ers that effi ciently interact with the abundant biopolymer lignin, and these results should be applicable to other naturally derived macromolecules as well.

Acknowledgements : This work was supported by the Department of Energy, Offi ce of Basic Sciences, through the EPSCoR grant, DE-FG02-08ER46528. The authors also wish to acknowledge the Joint Institute for Neutron Sciences at the University of Tennessee for support of this project. The support of the Scientifi c User Facilities Division, Offi ce of Basic Energy Sciences, U.S. Department of Energy, who sponsors the Oak Ridge National Laboratory's Spallation Neutron Source is gratefully acknowledged.

Received: November 15, 2011 ; Revised: February 17, 2012; Published online: May 11, 2012; DOI: 10.1002/macp.201100633

Keywords: biopolymers; compatibilization; neutron refl ectivity; renewable resources

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