Journey to Polymeric Materials Composed Exclusively of Simple

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Journey to Polymeric Materials Composed Exclusively of Simple Lignin Derivatives Simo Sarkanen, Yi-ru Chen, and Yun-Yan Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01700 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Sustainable Chemistry & Engineering

Journey to Polymeric Materials Composed Exclusively of Simple Lignin Derivatives Simo Sarkanen*, Yi-ru Chen, Yun-Yan Wang Email: [email protected] Department of Bioproducts and Biosystems Engineering, University of Minnesota 2004 Folwell Avenue, Saint Paul, Minnesota 55108-6130, USA KEYWORDS: Lignin-based polymeric materials, Mechanical properties, X-ray powder diffraction, Atomic force microscopy ABSTRACT Between 1955 and 1960, theories about lignin configuration were vacillating between random-coil and crosslinked “microgel” representations for macromolecular lignin chains. Light scattering was important in these early studies, but it was difficult to deal adequately with lignin fluorescence at the 546 nm incident wavelength being used. Crosslinking then prevailed, largely because of the hydrodynamic compactness of high molecular weight lignin species. The conceptual ramifications of this paradigm led to 40 wt% incorporation limits (or less) for lignins in cohesive polymeric materials. In due course, however, further evidence for a random-coil description of individual lignin components materialized; it became less obvious why simple lignin derivatives could not, on their own, form promising polymeric materials. The first plastics

ACS Paragon Plus Environment

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composed solely of a (native) ball-milled softwood lignin are similar to polyethylene in tensile behavior. Blending with just 5 wt% tetrabromobisphenol A (a flame retardant) results in a material that surpasses polystyrene decisively. Prior methylation of the ball-milled lignin produces markedly better results, with and without small quantities of blend components. Even quite challenging lignin derivatives like the (polyanionic) sulfonates perform auspiciously in formulations with particular aliphatic polyesters. The macromolecular species in polymeric materials with very high lignin-derivative contents are associated lignin complexes rather than individual lignin macromolecules. The interactions of blend components with these complexes are instrumental in determining the mechanical properties of contemporary lignin-based plastics. INTRODUCTION For over 40 years, macromolecular lignin configuration has been recognized as the key point of departure in any preface to instructions for creating functional lignin-based polymeric materials. For example, when contemplating the place of “Lignin in Materials”, Ingemar Falkehag (Westvaco Research Center, 1975) declared that a “sound scientifically-based understanding of lignin as a macromolecule and its potential roles in materials systems is most desirable as a platform for applied studies”.1 This perspective was further illuminated with explicit advice. “It is quite possible that, in the attempted uses of lignin to meet polymer or materials needs, one should not just try to ‘replace’ a synthetic component, but to take new innovative approaches where the uniqueness of lignin as a macromolecule should be exploited.” Initial studies of macromolecular lignin configuration Noteworthy efforts in the mid-to-late 1950s had been focused on this central issue, but conflicting working hypotheses emerged at the very outset. In 1955, molecular-weight


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determinations for a series of (Western hemlock) ligninsulfonate fractions by 546-nm light scattering were reported.2 The diffusion coefficients of the fractions were inversely proportional to Mw0.57 (Mw denoting weight-average molecular weight). Such a relationship was consistent with a Flory–Fox random-coil model for unbranched ligninsulfonate polymer chains. This early finding may have suffered from inadequate corrections for the fluorescence and absorbance of the ligninsulfonate solutions being examined. Nevertheless, the inaugural conclusion was consistent with a 1982 size-exclusion chromatographic (SEC) calibration curve that had been determined from paucidisperse (Douglas fir) kraft lignin fractions.3 The absolute molecular weights were calculated from ultracentrifuge sedimentation equilibrium measurements; the semilogarithmic plot with respect to retention volume was parallel to that for poly(styrenesulfonate) fractions beyond the excluded limit of the column (Figure 1). More detailed studies of narrow (Jack pine) kraft lignin fractions had been carried out, by 1996, to establish how their polydispersity varies with molecular weight.4 Here, ultracentrifuge sedimentation equilibrium curves were analyzed to determine both z-average and weight-average molecular weights that, when plotted semilogarithmically with respect to retention volume, yielded parallel SEC calibration curves (Figure 2). The results showed that the polydispersity (Mz/Mw) of the fractions does not vary with the hydrodynamic volume of the kraft lignin components, a clear indication that there should be no long-chain branching nor crosslinking in the individual polymer chains. Be that as it may, a fundamental challenge had already appeared by 1960 in an alternative diagnosis of ligninsulfonate configuration that attributed distinct crosslinked microgel character to the constituent macromolecular entities.5 The investigation that led to this verdict had


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documented the polyelectrolyte expansion of ligninsulfonate fractions through viscosimetric measurements and light-scattering studies. The work revealed a linear relationship between the cube of radius of gyration (Rg3) and intrinsic viscosity ([η]) at different ionic strengths, which would be consistent with the swelling of a crosslinked microgel. A 4- to 5-fold greater (“zaverage”) Rg than ([η]-derived) hydrodynamic radius was plausibly ascribed to the polydispersity of the fractions examined—these were pre-SEC days.5 Other quantitative discrepancies with respect to contemporaneous polyelectrolyte-expansion theories prompted the idea that the negatively charged groups in individual ligninsulfonate molecules reside at or near the surface of the crosslinked assembly of polymer chain segments (Figure 3). Variation of molecular weight with hydrodynamic volume is similar for kraft lignin components and poly(styrenesulfonates).

Polydispersity of kraft lignin fractions does not vary with hydrodynamic volume.

Sephadex G100/aq. 0.10 M NaOH calibration curves for paucidisperse kraft lignin fractions and poly(styrenesulfonates)

softwood kraft lignin components exhibit neither crosslinking nor long-chain branching

5.0

4.5

kraft lignin fractions

log mol. wt.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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poly(styrenesulfonate)

Mz

4.0 Mw

3.5

pSSA: p-styrenesulfonic acid

3.0

diE: diisoeugenol GAE: guaiacylglycerol β-(2-methoxyphenyl) ether

2.5 0.0

Figure 1. Semilogarithmic plots of weight-average molecular weight (M ) versus SEC elution volume for kraft lignin and poly(styrenesulfonate) fractions. Reprinted with permission from reference 3 © 1982 American Chemical Society.

0.4

0.8

1.2

1.6

VR

Figure 2. Semilogarithmic plots of weightaverage (M ) and z-average (M ) molecular weights versus SEC elution volume for paucidisperse kraft lignin fractions. Reprinted with permission from reference 4 © 1996 American Chemical Society.


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Whatever the merits of these working hypotheses, the light-scattering data were obtained with incident wavelengths of 546 nm, at which the fluorescence of lignins is pronounced and difficult to eliminate, even with narrow bandpass interference filters (unavailable at the time). Apart from inevitable over-estimates in the molecular weights, it is difficult to assess how farreaching the other effects of errors in light-scattering intensity may have been. Nevertheless, the idea that lignins in muro are crosslinked became very persistent. Ultracentrifugal sedimentation-velocity studies as well as viscosimetric observations and diffusion behavior indicated that the higher molecular weight species in lignin derivatives possess small hydrodynamic volumes.6 The belief that these higher molecular weight entities were covalently discrete received prompt support in 1966 from indistinct effects of methylating or acetylating lignin preparations. As far as pine and birch ball-milled lignins or pine dioxane−HCl lignin were concerned, neither methylation nor acetylation were reported to liberate greater proportions of smaller components in the molecular weight distributions of the resulting derivatives,7 at least when the excluded limit of the SEC (Sephadex G50) column was relatively low. The concept of crosslinking in native lignins was extended in 1984 to a Flory−Stockmayer degelation model for sprucewood delignification with dioxane−HCl solution.8 Such an analysis relied primarily on how the weight- and number-average degrees of

Figure 3. Softwood ligninsulfonate macromolecule as a crosslinked microgel (with free charges primarily on surface) reproduced by permission from reference 5, © 1960 Elsevier Ltd.


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polymerization characterizing the dissolved lignin components vary during the course of delignification. Thus, the lignin in muro was viewed as a distribution of primary chains crosslinked through tetrafunctional branchpoints. The crosslink density of the native lignin was estimated to be 0.052, and the weight-average degree of polymerization of the primary chains in the dissolved lignin components grew to 27 as delignification proceeded.8 However, experimental alkyl aryl ether contents (determined four years later) seemed to indicate that an implied decrease in crosslinked density for the lignin components being dissolved was not consistent with a steady increase in their average molecular weight during delignification.9 First incarnation of lignin-containing polymeric materials The application of Flory−Stockmayer theory to the delignification of wood spanned a 16year period between 1970 and 1986.10 Corresponding guidelines for incorporating lignins into polymeric materials received elaboration for over a decade, essentially between 1975 and 1986. In relation to early polyurethanes produced from carboxylated kraft lignin, it was declared11 that “the extent to which lignin’s structural rigidity needs to be softened will also determine the percentage of lignin that can be utilized in the manufacture of a commercial polymeric material.” The strategy of achieving high tensile strength and modulus12 for a lignin-containing material became one of “crosslinking or reinforcing a soft segment matrix with hard segment [lignin] domains.” Indeed, at one point in the discussion, it was thought13 that the “polyfunctional character of lignin limits its use to thermosetting polymers.” Consequently, it is probably inevitable that attempts to introduce lignin derivatives into polymeric materials would initially be confounded by incorporation limits of 40 wt% or less for the lignin itself.10 Limiting trends were encountered not only when the lignin derivatives were


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covalently crosslinked14 with a difunctional reactant (Figure 4) but also when they were blended noncovalently15 with a compatible polymeric component (Figure 5). A list of tensile properties for some common engineering plastics is relevant in this context (Table 1). A range of

Table 1. Tensile Properties of Some Engineering Plastics16 Tensile strength (MPa)

Thermoplastic

Elongation at break (%)

Polystyrene

46

2.2

Styrene-acrylonitrile

72

3

Acrylonitrile-butadiene-styrene (ABS)

48

8

Flame-retardant ABS

40

5.1

Polypropylene

32

15

Polyethylene

30

9

80

polyurethanes from hydroxypropyl kraft lignin and hexamethylene diisocyanate

75

50

 max

10

5

25

E

 MPa

60

MPa

b % or

15

E GPA x 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

20

acetate butyrate hexanoate laurate

max

b

0

0

10

20

30

40

0

0 0

kraft lignin content wt%

Figure 4. Effect of kraft lignin content on Young’s modulus (E), tensile strength (σmax) and strain at break (εb) of hydroxypropylkraft-lignin containing polyurethanes.14

10

20

30

40

50

lignin ester content wt%

Figure 5. Effect of Organosolv lignin ester content on tensile strengths of multiphase blends with cellulose acetate butyrate.15


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polymeric-material behavior between polyethylene (30 MPa strength, 9% elongation-at-break) and polystyrene (46 MPa strength, 2% elongation-at-break) reveals that polyurethanes produced from hydroxypropyl kraft lignin and hexamethylene diisocyanate (Figure 4) exhibited acceptable tensile behavior only when the kraft lignin content remained below 40 wt%. The situation was more restricted for multiphase Organosolv lignin-ester blends with cellulose acetate butyrate (Figure 5), where adequate tensile strengths could be achieved only when the lignin-ester content was confined to 30 wt% or less. Central role of powerful noncovalent interactions between lignin components Any methodical approach to developing functional polymeric materials with ligninderivative contents that may reach 100% must begin with the physicochemical scrutiny of candidate lignin preparations. Apart from an analytical description of substructure frequency, the most fundamental consideration involves the molecular weight distribution of the lignin derivative being examined. Taking softwood kraft lignins as an example, the results are striking. After having been pre-incubated in aqueous alkaline solution and suitably fractionated, underivatized kraft lignin preparations exhibit roughly unimodal SEC profiles in dimethylformamide (DMF) that are distributed around an apparent molecular weight of ~1 x 108 Da (based on an extrapolated polystyrene calibration curve, as shown in Figure 6).17 The corresponding aqueous 0.10 M NaOH/Sephadex G100 elution profiles reveal Mws below 10,000, however, so that the macromolecular kraft lignin species in DMF could embody as many as 104 individual components.17 An even more remarkable result is manifested when similarly handled softwood kraft lignin samples are consecutively acetylated and methylated (Figure 6). The resulting kraft lignin


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derivatives exhibit multimodal distributions of species ranging between (roughly) 5 and 100 million Da in apparent molecular weight.18 The multimodality of the molecular weight distributions suggests that the supramacromolecular entities visible in the SEC profiles (Figure 6) are assembled in stages, individual components forming complexes that then associate with one another in a well-defined way. Indeed, the intermolecular interactions between lignin substructures are strong. The stabilization energies (7−11 kcal/mol) for complexes involving cofacially offset arrangements of guaiacyl rings19 have been estimated (at the M05-2X/6-31+G(d,p) level of density functional theory) to be higher than those for GC/CG base pairs in DNA double helices (Figure 7). The

~1 x 108 1

underivatized and acetylated methylated 50 x 106 kraft lignins with different degrees of 36 x 106 6 association in DMF 18 x 10 1 6 4.5 x 10 3

A320

3

3

1 1

3

10

15

20

25

elution volume, mL 40

10

1.0

0.1

0.01

0.001

polystyrene standard mol. wt. x 10

0.0001

-7

Figure 6. Apparent molecular weight distributions in DMF of kraft lignin samples after Sephadex LH20/aqueous 35% dioxane fractionation following association for (1) 300 h, (2) 144 h and (3) 0 h at 170 gL-1 in 1.0 M ionic strength aqueous 0.40 M NaOH;17 separately acetylated and methylated after Sephadex LH20/aqueous 35% dioxane fractionation following association for (1) 6744 h, (2) 3912 h and (3) 1632 h at 195 gL-1 in 1.0 M ionic strength aqueous 0.40 M NaOH18 (107 Å pore-size poly(styrene-divinylbenzene column).


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corresponding stabilization energies for edge-on complexes are less than half of those for the cofacial complexes (Figure 7), but they are significant nonetheless. Solvent molecules were omitted from the calculations because they do not penetrate into lignin-based polymeric-material domains. The conformations of the components participating in complexes were chosen so as to preclude hydrogen bonding, which consequently does not contribute to the estimated stabilization energies. Here, therefore, nonbonded attraction arises primarily from electron correlation since there were no obvious indications of frontier orbital effects.19

cofacial complex of veratryl methyl ether with veratryl alcohol edge-on veratryl alcohol complex

spirodienone complex

~4.1 Å

~3.7 Å

~5.0 Å

3.6 kcal/mol

8.2 kcal/mol

5–5–O–4 dibenzodioxocin complex

11.0 kcal/mol (+)-/(–)-pinoresinol complex

3.4 Å

14.4 kcal/mol

14.7 kcal/mol

Figure 7. Stabilization energies of complexes resulting from interactions between edge-on and cofacial lignin substructures calculated at the M05-2X/6-31+G(d,p) level of density functional theory.19 (Images of cofacial veratryl-methyl-ether−veratryl-alcohol complex, spirodienone−spirodienone complex and dibenzodioxocin−dibenzodioxocin complex are reproduced by permission from reference 19, © 2009 Elsevier Ltd.)


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Eventually, improving upon the tensile behavior of polystyrene with polymeric materials containing 80 wt% methylated or ethylated methylated kraft lignin was not difficult to achieve.20,21 The kraft lignin sample chosen for the purpose was a higher molecular weight fraction retained during ultrafiltration of the parent preparation through a 10 kDa nominal molecular weight cutoff membrane in aqueous 0.10 M NaOH. After alkylation, blends with 20 wt% poly(butylene adipate) or poly(ethylene glycol) surpassed polystyrene convincingly in tensile strength and elongation-at-break (Figure 8).20,21

RESULTS AND DISCUSSION Alkylation can have a considerable impact on the mechanical properties of polymeric materials with very high lignin contents. The notable tensile behavior of the ethylated and/or

20% PBA, 80% MeKL

50

20% PEG, 80% MeKL 20% PBA, 80% Et MeKL 30% PTMG, 70% MeKL

polystyrene

40

MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Et MeKL

25% PEG, 75% MeKL

30% PBA, 70% MeKL

30

20

Blends of PEG or aliphatic polyesters with alkylated higher molecular weight industrial kraft lignin fraction.

MeKL

Et MeKL: ethylated methylated kraft lignin MeKL: methylated kraft lignin

10

0

0

5

10 

15

20

Figure 8. Tensile behavior of ethylated and/or methylated kraft lignin-based polymeric materials in blends with poly(ethylene glycol) (PEG), poly(butylene adipate) (PBA), or poly(trimethylene glutarate) (PTMG).20,21


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methylated kraft lignin-based blends (Figure 8) with 20 wt% poly(butylene adipate) or poly(ethylene glycol) would not have been preserved without kraft-lignin alkylation. The polymeric materials formed under these circumstances would be far more brittle in the absence of ethylation and/or methylation. Thus, intermolecular hydrogen bonding does not usually enhance the impact of nonbonded attraction (arising from electron correlation) between cofacially offset aromatic rings (Figure 7). Formulations for functional plastics that are fully lignin-based After solution casting at 150°C, a polymeric material composed solely of ball-milled softwood lignin (BML22, Mw 2300 Da, Mw/Mn = 3.0) may exhibit a tensile strength of 34 MPa with 6% elongation-at-break, while the strength of the methylated derivative (MBML22) increases to 43 MPa at a 5.5% elongation-at-break (Figure 9A). In contrast, phenolic-hydroxylgroup methylation of softwood ligninsulfonate (LS22) to form the singly methylated derivative (sMLS22) has little impact on the ~21 MPa tensile strength of the material with its modest ~3% elongation-at-break (Figure 9A). On the other hand, small quantities of miscible blend components can have pronounced effects on the tensile behavior of materials containing very high levels of simple lignin derivatives. Thus, 5 wt% poly(ethylene glycol) (PEG) with Mn 400 Da increases the tensile strength of MBML to 65 MPa as the elongation-at-break exceeds 9% (Figure 9B), while 5 wt% tetrabromobisphenol A (a flame retardant) results in a 53.5 MPa tensile strength for the BMLbased blend with elongation-at-break approaching 8%. Concomitantly, 15 wt% poly(trimethylene glutarate) improves the tensile strengths of LS (Mw 7100 Da, Mw/Mn = 3.8) and sMLS to ~46 MPa with elongations-at-break near 7% and 11%, respectively (Figure 9B).


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The modern lignin-based plastics exemplified here are completely amorphous, but their X-ray powder diffraction patterns reveal fundamental details about the macromolecular lignin species of which they are composed. The diffuse scattered intensities can be represented as sums of two broad Lorentzian peaks that overlap with one another (Figure 10). Their maxima are centered at equivalent Bragg spacings of ~4.2 Å and ~5.6 Å after the plastics have been cast without other blend components (Figure 10). As with X-ray powder diffraction analyses of paucidisperse kraft lignin fractions,21 the two families of separation distances can be ascribed respectively to different series of cofacial and edge-on configurations of interacting aromatic rings (Figure 7). These are intrinsic to the macromolecular structures that maintain the integrity of contemporary lignin-based plastics.22

A

60 50

 MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

20

15% PTMG 85% LS cast at 115°C, then 150°C

20

100% LS cast at 115°C, then 150°C

10

15% PTMG 85% sMLS cast at 115°C, 125°C, then 150°C

30

100% sMLS cast at 115°C,125°C, then 150°C

10

0

5% TBBP-A 95% BML cast at 150°C

Polystyrene

40

100% BML cast at 150°C

30

95% MBML cast at 150°C

50

100% MBML cast at 150°C

Polystyrene

5% PEG M n400

B

60

0 0

2

4

 %

6

8

10

0

2

4

6

 %

8

10

Figure 9. Tensile behavior of polymeric materials composed of simple softwood lignin derivatives alone and nearby blends. (A) Cast materials composed solely of unmethylated (BML, Mw 2300 Da, Mw/Mn = 3.0) and methylated (MBML) ball-milled lignin, and unmethylated (LS) and methylated (sMLS) ligninsulfonate. (B) Blends of BML with 5 wt% tetrabromobisphenol A (TBBP-A), MBML with 5 wt% poly(ethylene glycol) (PEG, Mn 400), and LS and sMLS both with 15 wt% poly(trimethylene glutarate) (PTMG). BML, LS and their methylated derivatives were prepared as described previously,22 but here LS is characterized by Mw 7100 Da, Mw/Mn = 3.8.


13


A

B

uncast BML

Intensity

26 ± 9.5 nm complexes 14.7° 6.0 Å ± 1.9

44% edge-on

22.0° 4.0 Å ± 0.8

BML cast at 150°C 21.3° 4.2 Å ± 0.9

86% cofacial

56% cofacial

15.8° 5.6 Å ± 1.2

14%

edge-on

5

10

15

20

25

C

30

35

5

10

D

uncast MBML

Intensity

21.7° 4.1 Å ± 0.9

10

15

25

30

35

MBML cast at 150°C 21.6° 4.1 Å ± 0.8

14.4° 6.1 Å ± 1.6

5

20

13 ± 3 nm complexes

15.9° 5.6 Å ± 1.7

29% edge-on

66% cofacial

34% edge-on

20

25

E

5

15

71% cofacial

30

35

5

10

15

20

12 ± 3 nm complexes

22.6° 3.9 Å ± 1.0 75%

25

30

35

LS cast at 115°, then 150°C

F

uncast LS

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21.1° 4.2 Å ± 1.1 100%

cofacial

cofacial 14.6° 6.1 Å ± 1.8 25% edge-on

10

15

20

25

30

35

5

10

2deg

15

20

25

30

35

2deg

Figure 10. X-ray powder diffraction patterns of uncast and cast polymeric materials based on unmethylated and methylated softwood ball-milled lignins and an unmethylated ligninsulfonate. (A) uncast and (B) cast unmethylated ball-milled lignins (BML); (C) uncast and (D) cast ball-milled lignin successively methylated with dimethyl sulfate and diazomethane (MBML); (E) uncast and (F) cast unmethylated ligninsulfonate (LS).22 The x-ray diffraction patterns of the amorphous polymeric materials were analyzed by fitting two Lorentzian functions I(x) = I(0)/(1 + x2/hwhm2), x = 2θ – 2θk, where I(x) is the scattered intensity at x from the Bragg angle 2θk for the peak, 2θ is the scattering angle, and hwhm is the half-width at the half-maximum of the peak. Dimensions of complexes were determined by atomic force microscopy as previously described.22


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It has been proposed that the more stable arrangements of cofacial aromatic rings occupy the inner domains of associated complexes that make up the predominant macromolecular entities in polymeric materials with high lignin contents.22 On the other hand, the peripheral domains of the complexes embody a greater frequency of edge-on aromatic-ring arrangements that enable material continuity to be established between adjoining macromolecular lignin species.22 In the process of casting plastics composed of simple lignin derivatives, the proportions of cofacial and edge-on aromatic rings invariably change (Figure 10). This occurs because of the need for continuity in the cast materials, but the ultimate proportions created for the inner and peripheral domains depend on the balance between the various intermolecular forces at work. Indeed, for the same reason, the dimensions of the associated complexes themselves can vary among the plastics produced from different lignin derivatives (Figure 10). Casting of unmethylated ball-milled lignin (BML) engenders a marked increase in the contribution from inner (cofacial) domains in the plastic formed (Figure 10A and B). The dimensions of the associated complexes in the BML material are distributed around 26 nm; both hydrogen bonding and electron correlation take part in the interactions between lignin components. In contrast, casting of methylated ball-milled lignin (MBML) appreciably reduces the proportion of inner (cofacial) domains in the plastic created (Figure 10C and D). The associated (~13 nm) complexes in the MBML material are much smaller, and hydrogen bonding no longer contributes to the noncovalent forces between lignin components. In this context, casting of unmethylated softwood ligninsulfonate (LS) is accompanied by the most remarkable transformations of all. Only the inner (cofacial) domains remain in the cast


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LS material (Figure 10E and F) even though the associated (~12 nm) complexes are characterized by dimensions quite similar to those in the MBML material (Figure 10C and D). Evidently, the intermolecular forces arising from hydrogen bonding, electron correlation and dipolar interactions act in harmony (if not in synergy): differential scanning calorimetric thermograms of the LS material reveal no indication of a glass transition temperature.22 In response to a reviewer’s question about the effect of humidity on plastics with very high lignin-derivatives contents, test pieces cast from BML, MBML and LS were separately immersed in distilled water for 120 h under ambient conditions. In terms of a complete absence of color in the three aqueous phases, there was no visible sign of lignin-component dissolution from the BML, MBML or LS materials. After retrieving the test pieces and drying their surfaces, their tensile behavior revealed no reduction in strength for any of the three plastics. CONCLUSIONS In contrast to most unblended polymeric materials, the integrity of plastics based entirely on simple lignin derivatives depends pre-eminently on associated macromolecular complexes rather than individual macromolecular chains. Typically, the stability of these lignin complexes is ensured by inner domains that embody cofacially offset configurations of interacting aromatic rings. Material continuity between adjoining complexes is usually achieved through peripheral domains that possess much higher frequencies of (less stable) edge-on aromatic-ring arrangements. Miscible blend components interact preferentially (or even exclusively) with these peripheral domains; in so doing, they influence the viscoelastic behavior of the ligninbased polymeric material. A fundamental understanding of how this occurs will enable formulations for functional lignin-based plastics to be improved in a predictable way.


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AUTHOR INFORMATION Corresponding Author *Simo Sarkanen. Tel.: (+) 1 612 624 6227, E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by Agriculture and Food Research Initiative Grant no. 2011-67009-20062 from the USDA National Institute of Food and Agriculture, and a Subaward (115808 G002979) from the “Northwest Advanced Renewables Alliance” led by Washington State University and supported by the Agriculture and Food Research Initiative Competitive Grant no. 2011-6800530416 from the USDA National Institute of Food and Agriculture. X-ray powder diffraction studies and atomic force microscopy were carried out in the Characterization Facility at the University of Minnesota which receives partial support from NSF through the MRSEC program. REFERENCES (1)

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(21) Li, Y.; Sarkanen, S. Miscible blends of kraft lignin derivatives with low-Tg polymers. Macromolecules 2005, 38, 2296–2306. (22) Wang, Y.-Y.; Chen, Y.-r.; Sarkanen, S. Path to plastics composed of ligninsulphonates (lignosulfonates). Green Chemistry 2015, 17, 5069-5078.


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For Table of Contents Use Only Journey to Polymeric Materials Composed Exclusively of Simple Lignin Derivatives Simo Sarkanen, Yi-ru Chen, Yun-Yan Wang

Synopsis Profitable conversion of lignocellulose to commodity organic chemicals and liquid biofuels will benefit from functional plastics based entirely on simple lignin derivatives and nearby blends.


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Journey to Polymeric Materials Composed Exclusively of Simple

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