Exploiting Feedstock Diversity To Tune the Chemical and Tribological

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Exploiting Feedstock Diversity To Tune the Chemical and Tribological Properties of Lignin-Inspired Polymer Coatings Jillian A. Emerson,†,‡ Nikolay T. Garabedian,§ David L. Burris,§ Eric M. Furst,† and Thomas H. Epps, III*,†,∥ †

Department of Chemical & Biomolecular Engineering and §Department of Mechanical Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States ∥ Department of Materials Science & Engineering, University of Delaware, 127 The Green, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Biobased polymers present an immense opportunity to design and manufacture new coating materials largely as a result of their feedstock diversity and inherent functionality, yet unraveling the key structure/property relationships inherent in these environmentally friendly systems remains a considerable challenge. A major focus of this work was to develop functional group−property design rules for a representative library of lignin-inspired polymers. Of particular interest were the polymers’ solubilities, surface energies, and friction coefficients because of their relevance to coatings applications. The structural diversity of our bioinspired library, consisting of various polymers generated from methacrylate-functionalized lignin pyrolysis products, arose from the differing moieties at the para and ortho positions on the polymer repeat units relative to the methacrylate backbone. Polymer compatibilities with organic solvents studied herein increased with greater aliphatic content in the para functionality and decreased with the incorporation of methoxy groups ortho to the polymer backbone. The surface energies of the films followed similar trends between the interaction parameters and the functional group. By linking solvent compatibility to surface energy, it was demonstrated that changes in polar moieties, such as aldehydes and methoxies, have greater effects on solubility, surface energy, and friction than changes in the aliphatic (dispersive) groups. Thus, the target material properties can be understood and tuned through careful consideration of the pendant group functionalities inherent in the bioinspired materials, unlocking enhanced property design for next-generation coatings. KEYWORDS: Lignin, Biobased polymers, Tribology, Surface energy, Chi parameter, Friction coefficient

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polymers.9−11 Thus, lignin can become a useful alternative as it is a renewable feedstock that has substantial aromatic content and therefore has thermal properties, e.g., glass transition temperatures (Tgs), comparable to those of PS and PMMA.12 Furthermore, lignin is abundant and inexpensive; currently, it can be collected as a waste product from the paper and pulping industry at a rate of ∼70 million tons/year13 and is also obtained from other waste sources, such as sugar cane bagasse and agricultural residues.14 All together, these features make lignin-based materials particularly appealing for next-generation coatings.15,16 The synthesis of a library of lignin-inspired polymers (Table 1) and characterization of their bulk properties, such as Tg, onset and peak degradation temperatures (T0, Tp), elastic shear modulus (G′), and complex and zero shear viscosity (η*, η0), has been reported previously.17−19 Using Kraft pyrolysis, lignin

INTRODUCTION

Polymer films are prevalent as coatings for medical devices, pharmaceutical tablets, antifouling surfaces, automotive components, and food and beverage coatings, among many other applications.1−3 Innovative technologies that incorporate properties, such as scratch resistance, heat resistance, etc., are increasingly based upon a new generation of bioderived polymers and “smart” capabilities, such as ability to heal or respond to external stimuli.3 Polymers sourced from renewable feedstocks are one possible route to accelerating development of these environmentally friendly systems, with promising feedstocks for green polymer synthesis, including lignin, cellulose, vegetable oils, chitin/chitosan, and starches.4−8 Biobased or bioinspired polymers with material properties comparable to polystyrene [PS] or poly(methyl methacrylate) [PMMA] are especially attractive for various applications because of the balance of thermal and mechanical properties achieved through PS and PMMA. Unfortunately, the specific benefits of PS and PMMA are somewhat mitigated by the need to access petroleum-based sources in the generation of these © XXXX American Chemical Society

Received: February 8, 2018 Revised: April 12, 2018

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DOI: 10.1021/acssuschemeng.8b00667 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Chemical Structures of Sustainable Polymers

using a series of different liquids.37 Surface energy, as determined through contact angle measurements, can be used to describe the hydrophobic/hydrophilic nature of the polymer surface and connect the polymer chemical structure to tribological behavior.30,38−41 Finally, the friction coefficient affords the characterization of interactions between two surfaces.42 Low-friction surfaces are of interest when slip is important (e.g., touch screens and skis),43 whereas high friction is of interest when grip is important (e.g., tires and shoe treads).44 Friction (or friction coefficient) and wear (or wear rate) often are dominated by adhesive forces of attraction that depend on the chemical composition, surface energy, and interfacial energy at contact junctions.42,45 Because the solvent compatibilities, surface energies, and friction coefficients of polymers depend strongly on repeat unit structure, creating structure−property relationships for macromolecules, such as the bioinspired materials discussed herein, can provide valuable insight toward incorporating ligninderived films in next-generation coatings using a more environmentally friendly feedstock.29,45,46 For the purpose of developing such a structure−property library, a series of lignin-inspired phenyl methacrylate polymers was selected with various functionalities at the ortho or para position relative to the polymer backbone. The solubility parameters, surface energies, and friction coefficients of various homopolymer and heteropolymer films (see Table 1 for an overview of repeat unit structures) were measured and related to the pendant group functionalities. Such understanding ultimately will facilitate the incorporation of these materials into next-generation functional coatings.

can be broken down into a variety of subunits including guaiacols and syringols20−22 and easily functionalized to subsequently generate methacrylate-based polymers.17−19 The most common functionalities on the aromatic pendant groups are methoxy, aldehyde, and short chain aliphatic moieties. As one example of functional group impact, the Tg of poly(syringyl methacrylate) [PSM] was ∼100 °C higher than the Tg of poly(guaiacyl methacrylate) [PGM].18 These polymers differ only by a methoxy group ortho to the methacrylate group on the phenyl ring (see Table 1). The significantly higher Tg of PSM in comparison to PGM was attributed to the reduction in rotational freedom resulting from interactions between the carbonyl ester and the ortho groups.18 Despite the recent advances in generating and characterizing lignin-based polymers, the possible impacts of these materials for surface applications remains virtually unexplored,9−11 with some of the key properties of interest, including polymer solubility (or solvent compatibility), surface energy, friction coefficient, and wear resistance, in need of additional study. The importance of each of the properties is discussed in the following paragraphs. Solubility influences the ability to process polymers in solution and determines the solvent resistance of the final material. Solubility behavior can be predicted through the use of solubility parameters.23−27 Experimentally, one approach to determining the solubility parameters is through regular solution theory in conjunction with Flory−Huggins interaction parameters.23,26,27 The solvent−polymer Flory−Huggins interaction parameter (χs‑p) can be measured using scattering methods, osmosis, critical miscibility, vapor pressure methods, inverse gas chromatography, melting/freezing point depression, intrinsic viscosity, and swelling equilibria.23 χs‑p, which can vary with solvent concentration, can be determined over a wide composition range using vapor pressure methods and swelling equilibria.23,26 The breadth of the accessible solvent concentration range is particularly applicable to processes such as solvent casting of polymer film coatings, during which the initially high solvent volume fraction (ϕs > 0.90) is reduced to a dry film state (ϕs < 0.10) primarily through evaporation.26,28 Another property of importance in coatings applications is surface energy.3,29−34 The surface energy plays a role in the stability, adhesion, and wettability of polymer film on various substrates, as well as the wetting of other fluids on the polymer surface.29−34 Furthermore, by changing the surface energy, the phase separation in multicomponent polymer films can be manipulated.35,36 One common method for measuring the surface energy of polymer films is sessile drop contact angle

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EXPERIMENTAL METHODS

The functional groups presented in Table 1 were selected because they represent a significant fraction of useful components in lignin pyrolysis products.20−22 Materials. Tetrahydrofuran [THF] (optima), chloroform [CHCl3] (certified ACS), and dichloromethane [DCM] (certified ACS) were purchased from Fisher Scientific. Anisole (anhydrous, 99.7%) was obtained from Sigma-Aldrich. All solvents were used as received. The bioinspired polymers were synthesized through reversible addition− fragmentation chain-transfer [RAFT] polymerization and characterized as described elsewhere.12,17,18,47,48 The molecular weight, dispersity, and Tg for all polymers are given in Table 2. Additional characterization data for PPM and PSAM are provided in the Supporting Information, Figure S1. Film Casting. A flow coating apparatus was employed to cast solutions onto cleaned silicon wafers to generate films with thicknesses greater than 100 nm.49 Further details are provided in the Supporting Information. B


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ACS Sustainable Chemistry & Engineering Table 2. Molecular Weight, Dispersity, and Tg Data for Lignin-Inspired Polymers Used in the Solvent Vapor Swelling/Contact Angle Experiments and Friction Coefficient/Adhesion Studies solvent vapor swelling samplesa polymer PPM PGM PCM PEM PVM PSM PSAM

Mnb (kDa) d

41 38 34 35 36 24c 29d

D̵ b d

1.2 1.3 1.4 1.3 1.4 1.7c 1.5d

Tge (K) f

392 388 394 384 402 478 474f

The sessile contact angle (θs) was used to calculate the surface energy of our polymer films from the Owens−Wendt two-fluid method, as given by eq 3.39 The contact angle fluids used in this study were water (purified with a Milli-Q reagent water purification system; γwater = 72.8 mJ/m2, γDwater = 21.8 mJ/m2, γPwater = 50 mJ/m2) and diiodomethane (99%, stabilized, Acros Organics; γdiio = 50.8 mJ/m2, γDdiio = 50.8 mJ/m2, γPdiio = 0 mJ/m2).40,41 Further experimental details are located in the Supporting Information. Dynamic Friction Coefficient and Adhesion Measurements. Dynamic friction coefficients (μi)s were measured using a custom linear reciprocating tribometer, which was operated in a sphere-on-flat geometry, as described in the literature.52 The probe diameter was 6.35 mm, and the probe was made of either borosilicate glass or highdensity polyethylene (HDPE) depending on the experiment. Select time-dependent friction coefficient traces are shown in the Supporting Information, Figure S2. Adhesion between the probe and the film was determined by measuring the pull-off forces using the same tribometer. The adhesion was characterized by the magnitude of the pull-off force upon immediate probe retraction from the film. Additional details are located in the Supporting Information.

contact angle and friction coefficient samplesa Mnb (kDa) d

41 26 29 26 24 21

D̵ b 1.2 1.2 1.2 1.2 1.5 1.5

d

Tge (K) 392f 385 398 381 398 476

a

As will be discussed in the Results and Discussion, higher molecular weight polymers were used in solvent swelling studies to enhance film stability during solvent cycling, whereas lower molecular weight polymers were used in contact angle and friction coefficient studies (when necessary) to reduce overall film roughness. bMolecular weight and dispersities determined from size exclusion chromatography (SEC) in THF with light scattering detection, relative to PS standards.17,18,47 cbMolecular weight and dispersities determined via SEC in chloroform with refractive index detection, relative to PS standards.17,18,47 dMolecular weight and dispersities determined via SEC with refractive index detection relative to PS standards (see Supporting Information, Figure S1a). eTg from differential scanning calorimetry (DSC); second heating at 2 °C/min in N2.17,18,47 fTg from DSC; second heating at 5 °C/min in N2 (see Supporting Information, Figure S1b).

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RESULTS AND DISCUSSION Macromolecular Characterization. Homopolymer Specimens. Two sets of chemically identical polymers were Table 3. Characterization Data for the Lignin-Inspired Heteropolymers chemical constituentsa

Solvent Vapor Swelling. Solvent vapor swelling was used to determine the solvent−polymer interaction parameters for the lignininspired polymers in THF and CHCl3, employing a procedure described elsewhere.26 The solvent vapor concentration is defined as the ratio of the pressure of the solvent, pi, to the saturated partial pressure at the experimental temperature, pi,sat, The amount of solvent incorporated into the film (ϕs = 1 − ϕp, for which ϕp is the polymer volume fraction) depended on the solvent concentration in the chamber and the polymer−solvent Flory−Huggins interaction parameter, χs‑p ⎛ p ⎞ ⎛ V⎞ ln⎜⎜ i ⎟⎟ = χs − p ϕp2 + ln(1 − ϕp) + ⎜⎜1 − s ⎟⎟ϕp Vp ⎠ ⎝ ⎝ pi ,sat ⎠

CM

EM

VM

PBOM PVES PCES PES

0.25

0.23

0.27 0.23 0.34 0.95

0.25 0.22

0.18

SM

D̵ b

Tgc (K)

0.55 0.48 0.05

40.5 34.9 35.3 35.8

1.33 1.50 1.32 1.30

392 432 427 387

Weight fraction of the base components in the heteropolymers as determined by 1H NMR spectroscopy. bMolecular weight and dispersities determined from SEC with light scattering detection.18,47,48 cTg from DSC; second heating at 2 °C/min in N2.18,47,48

used in this work to study the effect of functional groups on solvent compatibilities, surface energies, and dynamic friction coefficients. Higher molecular weight polymers were used for the solvent vapor swelling and contact angle tests. The higher molecular weights provided enhanced solvent stability, enabling the measurement of polymer/solvent interactions over a wider solvent concentration range.25 Lower molecular weight polymers were used for the tribology tests. The lower molecular weights facilitated solvent casting of smooth films. The molecular weight, dispersity, and Tg data for the polymers are provided in Table 2. Heteropolymer Specimens. In addition to bioinspired homopolymers, the solvent vapor swelling behavior of several lignin-inspired heteropolymers was examined. Four heteropolymers were studied: poly(bio-oil methacrylate) [PBOM], poly(creosyl methacrylate-ran-4-ethylguaiacyl methacrylate-ransyringyl methacrylate) [PCES], poly(4-ethylguaiacyl methacrylate-ran-syringyl methacrylate) [PES], and poly(vanillin methacrylate-ran-4-ethylguaiacyl methacrylate-ran-syringyl methacrylate) [PVES]. The chemical constituents and molecular characterization data for the heteropolymers are reproduced in Table 3.18,47,48 Solvent Vapor Swelling. Homopolymers. Solvent vapor swelling experiments were performed on the bioinspired

(1)

for which γij is the interfacial tension between phases i and j, with the subscripts S, L, and V representing the solid, liquid, and vapor phases, respectively. The dispersive (D) and polar (P) contributions to the contact angle, as described by Owens and Wendt, were determined using two contact angle fluids. The Owens−Wendt two-fluid model uses a geometric mean of the polar and dispersive components39 P P γSV γLV )

GM

a

for which Vs is the molar volume of solvent, and Vp is the molar volume of the polymer. Because Vp ≫ Vs, the last term in eq 1 could be approximated as (1 − 1/N)ϕp or ϕp.50,51 (Note: polymer volume f raction was directly correlated to changes in f ilm thickness during swelling, as the f ilms were pinned to the substrate in the in-plane directions.) Contact Angle Measurements. The contact angle, θ, of each film was determined by the three-phase contact between film (material of interest), liquid (contact angle fluid), and vapor (air). Young’s equation was used to describe the balance between the various interactions γ − γSL cos θ = SV γLV (2)

D D (1 + cos(θS))γLV = 2( γSV γLV +

polymer

characterization Mnb (kDa)

(3) C


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ACS Sustainable Chemistry & Engineering

Figure 1. Swelling curves for polymer films in THF or CHCl3. Increased compatibility is indicated by reduced polymer volume fraction in the film. Solvent concentration is given by the ratio of the partial pressure to the saturated partial pressure, pi/pi,sat; values are displayed on the plots. Dashed line represents the nonswollen polymer (initial polymer volume fraction). Note: all films in the same panel were analyzed in the same experiment, while each panel represents a different experiment. (a) Swelling of PSM (red times signs), PGM (blue plus signs), PCM (orange squares), PVM (purple diamonds), and PEM (green triangles) in THF. (b) Swelling of PSM (red times signs) and PSAM (blue circles) homopolymers in CHCl3. (c) Swelling of PBOM heteropolymer (green circles) in THF in comparison to its homopolymer constituents [PGM (blue plus signs), PCM (orange squares), PVM (purple diamonds), PEM (green triangles)]. (d) Swelling of PES (magenta triangles), PCES (orangle squares), and PVES (purple diamonds) heteropolymers in THF in comparison to their homopolymer constituents [PSM (red times signs), PCM (orangle squares), PVM (purple diamonds), PEM (green triangles)].

group (PVM). Furthermore, the polymer films containing more prominent aliphatic functional groups at the para position had the highest swelling (lowest polymer volume fraction) in THF (from lowest swelling to highest swelling PGM → PCM → PEM). The solubility of the homopolymers in CHCl3 followed similar trends to those found with THF, see Figure 1b. Of the polymers tested in CHCl3, PVM had the more favorable interactions (we note that PPM, PGM, PCM, and PEM were not tested in CHCl3 due to dewetting issues as described below). PSAM, which contained both an aldehyde and an additional methoxy group, had a lower compatibility with CHCl3 than PVM but a higher compatibility that PSM. Because PSM lacks the aldehyde, these results suggest that aldehyde groups improve the compatibility of the polymer with the solvent. Similarly, comparing PSAM to PVM, the second methoxy group reduced the solubility of the polymer in CHCl3 (see Figure S3). Overall, for both THF and CHCl3, polymers containing multiple methoxy moieties had the lowest compatibilities with the solvents, and furthermore, the presence of an aldehyde group versus an aliphatic group reduced the lignin-inspired polymers’ compatibilities with the solvents.

polymers to estimate solvent−polymer interaction parameters. THF and CHCl3 were selected to capture the swelling behavior, as both were known to be fairly good solvents for our polymers, i.e., THF and CHCl3 readily dissolved the polymers. Swelling curves for the sustainable polymers in THF and CHCl3 are shown in Figure 1. To ensure reproducible data collection, these experiments began at the highest solvent concentration and progressively decreased in solvent content to avoid start-up effects, most notably those effects that result from glassiness of the films at the lower solvent contents.26 With THF, PSM had the lowest swelling (highest polymer volume fraction), suggesting that THF was the least compatible with PSM in comparison to the other polymers (Figure 1a). PGM, which differed from PSM by one methoxy group in the ortho position, had a more favorable interaction with THF. At the same solvent concentration, the swollen polymer volume fraction of PVM was between that of PSM and PGM, which indicated higher solvent compatibility (greater solubility) than PSM and lower solvent compatibility (lesser solubility) than PGM. Thus, polymers with a second methoxy group (PSM) were less compatible with THF than those with an aldehyde D


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polymers that contained mixtures of the above monomer segments with varied functionalities at the ortho or para positions. Figure 1c and 1d contains a comparison of the swelling behavior for various heteropolymers as benchmarked by the constituent homopolymer data in THF. Notably, the swelling behavior of the heteropolymers was consistent with the homopolymer constituent composition, indicating that heteropolymer solvent compatibility was tunable on the basis of the monomeric constituents of the purified lignin-based mixtures. Interaction Parameter Determination. Homopolymers. As mentioned above, it was necessary to understand the Tg behavior of the solvent-swollen polymer films to assess the appropriate thickness measurements for use in the Flory− Huggins solvent−polymer interaction parameter calculations. The polymer volume fraction (as determined by the change in film thickness relative to the neat film) was used to estimate the solvent concentration (ratio of solvent partial pressure to the saturated partial pressure) at which the polymers became nonglassy upon solvent addition. Several reports in the literature describe how solvent-induced changes in polymer Tg can be determined from DSC or other techniques;53−56 however, solvent swelling was used in this work such that Tg data can be extracted from the identical films to those used for the solvent compatibility experiments. For this work, the conditions under which these sustainable polymers had sufficient mobility to equilibrate with the solvent were determined by plotting the polymer volume fraction vs the solvent concentration, pi/pi,sat, as shown for PEM in Figure 2 (see Supporting Information, Figure S4 for the other polymers in THF). Two regimes (slopes) were noted in the data, with a crossover point that could be identified as the intersection between the two regimes. It was expected that the crossover point would correspond to the solvent concentration at which the polymer transitioned from a more “rubbery” (mobile) state to a more “glassy” (rigid) state (i.e., the effective glass transition under the experimental solvent concentration and temperature). The volume fraction corresponding to the minimum level of swelling (or maximum polymer volume fraction) for which the film still had appreciable chain mobility was defined as the crossover volume fraction, ϕc. The solvent concentration (pi/pi,sat) at which ϕc occurs was pi,c/pi,sat. To validate these experiments, the swelling of polymers in pure

Figure 2. Effect of THF concentration in the PEM film on chain mobility. Rubbery regime (green solid circles, green open circles) has a different slope than the glassy regime (red solid circles, red open circles). Closed symbols represent data from flow solvent vapor swelling experiments, whereas open symbols represent data from THF/water bell jar experiments. Data at ϕc = 1 are the results from polymer swelling in water (pi/pi,sat = 0).

Methoxy groups largely contribute polar interactions, whereas aldehyde groups add a combination of polar and dispersive interactions. In comparison, aliphatic functional groups only impact the dispersive nature of the polymer. These insights, along with the swelling results, indicate that the polymers likely have higher polar and lower dispersive character than the solvents; as such, increasing the dispersive nature of the polymer improves the solvent−polymer compatibility, whereas increasing the polar character reduces the solvent−polymer compatibility. For the homopolymers in THF, PSM swelled the least and PEM swelled the most. PSM did not significantly deswell upon solvent removal when pi/pi,sat was lower than 0.94, which suggested the polymer film was in a non-equilibrium state. This result and its implications will be discussed in further detail below, in the context of interaction parameter determination and polymer Tg. Heteropolymers. The solvent compatibility of lignininspired materials also was investigated by probing hetero-

Figure 3. (a) Crossover polymer volume fractions determined via solvent swelling. Shaded regions are the estimated crossover location calculated using eq 4 on the basis of the range of solvent Tgs found in the literature.59 (b) Minimum solvent concentration above the film at which the polymer has mobility, the crossover solvent concentration above the film (pi,c/pi,sat), is shown for PCM, PEM, PGM, and PVM. Crossover solvent concentrations and polymer volume fractions are given in Table S1. E


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also could be estimated from mixing rules between the Tgs of the neat polymer and the solvent using eq 457 mp m 1 = + s Tg,mix Tg,p Tg,s (4) in which mi is the weight fraction of component i (mix = mixture, p = polymer, s = solvent) and Tg,i is the Tg of i (mix = mixture, p = polymer, s = solvent). Literature values were used for the solvent Tg values.58,59 The resulting ranges of polymer volume fraction using eq 4 with THF at room temperature (20 °C) are represented by the shaded region in Figure 3a. The corresponding solvent concentration required to reach ϕc in these experiments, pi,c/pi,sat, is shown in Figure 3b. The ratio pi,c/pi,sat was between 0.70 and 0.75. Now, by excluding swelling data that were within the glassy regime (ϕc < 0.75), the Flory−Huggins solvent−polymer interaction parameter was calculated using eq 5 (a rearranged version of eq 1)26 ⎛ p ⎞ ⎛ V⎞ ln⎜⎜ i ⎟⎟ − ln(1 − ϕp) − ⎜⎜1 − s ⎟⎟ϕp = χs − p ϕp2 Vp ⎠ ⎝ ⎝ pi,sat ⎠

(5)

ϕp2,

By plotting the left-hand side of eq 5 as a function of the slope of the linear fit to the data was the Flory−Huggins solvent−polymer interaction parameter for the homopolymers, as shown in Figure 4a and 4b. Thus, the relative compatibilities of the solvent−polymer combinations were found; in this treatment, a higher slope indicated less favorable mixing between a particular solvent−polymer combination.26 In THF, polymers containing additional methoxy groups ortho to the polymer backbone had lower to higher compatibility: PSM → PGM → PPM. In comparison to those ortho constituents, para functional groups increased the compatibility with THF from lower to higher: PGM → PCM → PEM; PGM → PVM. Overall, the data indicated that PPM, which contained no ortho or para groups, had the highest compatibility in THF followed by PEM, which had one methoxy ortho and an ethyl group para, while PSM, which contained two methoxy moieties ortho and no functional groups para to the backbone, had the lowest compatibility with THF. PVM, PSM, and PSAM were stable up to the highest solvent swelling concentrations due to their higher Tgs and lower solubilities in CHCl3. From these data, shown in Figure 4b, PVM had the highest compatibility with CHCl3. PSAM, which contained both a second methoxy and an aldehyde group, was less compatible than PVM (aldehyde) and had slightly more favorable interactions with CHCl3 than PSM (two methoxy groups). Thus, two ortho methoxy groups appeared to detract from the polymer/solvent compatibility (PSAM vs PVM). Additionally, a polymer containing an aldehyde para to the backbone had greater solvent compatibility (comparing PSM and PSAM; PGM and PVM). The difference in swelling for PVM and PGM at the same composition is shown in the Supporting Information, Figure S3. The polymers containing only aliphatic moieties (PEM, PGM, and PCM) dewet at the highest solvent concentrations. The dewetting possibly was due to destabilization by short-range polar forces.60,61 Reducing the concentration of CHCl3 to prevent dewetting did not provide adequate polymer mobility in these films to allow equilibration with the solvent vapor environment. Hence, it was not possible to capture the CHCl3−polymer interaction parameters for PEM, PGM, PCM, and PPM.

Figure 4. Equilibrium swelling data for lignin-derived (a) homopolymers in THF and (b) homopolymers in CHCl 3 and (c) heteropolymers in THF, plotted based on eq 5. Solid lines represent the linear fits to the data, and dashed lines represent the extrapolation of the fits, which assumes that there is no composition dependence in the interaction parameter. Homopolymers shown in a and b in order of increasing compatibility (higher slope to lower slope) are PSM (red times signs), PSAM (blue circles), PGM (blue plus signs), PCM (orange squares), PVM (purple diamonds), PEM (green triangles), and PPM (grey circles). Solubility of the heteropolymers in c decreases on the basis of monomeric constituents of the polymers: PCES (orange squares) ≈ PVES (purple diamonds) < PBOM (blue circles) < PES (magenta triangles).

water also was measured (pi/pi,sat = 0, for i = THF); these tests confirmed that the polymers did not swell in water alone. The crossover volume fraction, ϕc, for each polymer with THF is plotted in Figure 3a. ϕc for solvent/polymer mixtures F


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Table 4. Flory−Huggins Solvent−Polymer Interaction Parameters for the Homopolymers with THF and CHCl3 and Heteropolymers with THF tetrahydrofuran

chloroform

homopolymer polymer PGM PVM PCM PEM PPM

heteropolymer χTHF‑p

polymer

± ± ± ± ±

PCES PVES PBOM PES

0.70 0.50 0.50 0.43 0.26

0.03 0.02 0.02 0.02 0.02

homopolymer χTHF‑p

polymer

χCHCl3‑p

± ± ± ±

PSM PSAM PVM

0.71 ± 0.01 0.64 ± 0.01 0.45 ± 0.01

0.56 0.52 0.49 0.43

0.04 0.05 0.03 0.03

The resulting Flory−Huggins solvent−polymer interaction parameters are shown in Table 4. THF was determined to be a good solvent (χ < 0.5) for PPM and PEM, a theta solvent (χ = 0.5) for PCM and PVM, and a poor solvent (χ > 0.5) for PGM. From the literature, PS evaluated by the same method gives a χs‑p = 0.41 with THF,26 which is most similar to PEM. Additionally, the CHCl3 was determined to be a good solvent (χ < 0.5) for PVM and a poor solvent (χ > 0.5) for PSAM and PSM. The solvent−polymer interaction parameter between CHCl3 and PS is 0.39, indicating PVM is slightly less compatible with CHCl3 than PS.26 Heteropolymers. The THF−polymer interaction parameters for the heteropolymers, shown in Figure 4c, fell between the values for the homopolymers comprised of the constituent monomers. PES had the highest compatibility (lowest slope), as it contained majority EM, but it was slightly less compatible than PEM likely because of the small fraction of SM segments. PCES and PVES had the same interaction parameter within error, largely as a result of their similar compositions of EM and SM, as PCM and PVM had nearly identical interactions with THF (see Table 4). PBOM, which contained almost equivalent mass fractions of EM, GM, VM, and CM, had a solubility parameter close to that of PVM, PCM, and PEM and lower than that of PGM. Polymer/Solvent Compatibility Discussion. As described previously, PSM had a higher solvent resistance to THF due to its higher Tg. However, the PSM did not swell sufficiently in THF to equilibrate with the solvent vapor environment, which was required to capture the interaction parameter accurately. Thus, rules for solvent selection must be generated to enable χs‑p determination for these new, bioderived polymers. The maximum swelling of PSM in various solvents was studied using a bell jar annealing experiment, for which pi/pi,sat = 1. These solvents had a wide range of solubility parameters (see Figure 5), enabling the screening of a variety of solvent−polymer interactions to maximize swelling. For the solvents tested, the glass transition crossover point for PSM, estimated using eq 4, was normally from ∼0.60 to ∼0.70. PSM had the highest swelling (lowest polymer volume fraction) in chlorinated solvents (CHCl3, DCM [data not shown]). Due to the greater solvent uptake of CHCl3 in PSM in comparison to the other solvents tested, CHCl3 was selected for further experiments. At similar solvent concentrations (pi/pi,sat), CHCl3 swelled PSM more than THF. Additionally, the rubbery regime was accessible in the flow setup with CHCl3 (see Supporting Information, Figure S5). Surface Energy. The contact angles are shown in Figure 6a, and the calculated surface energies are plotted in Figure 6b. Details with respect to the surface energy calculations are provided in the Supporting Information. The PEM surface energy was the lowest among the sustainable polymers, at 45

Figure 5. Solvent quality and polymer swelling data for PSM. (a) Solubility parameters for solvents tested. (b) Polymer volume fractions of PSM swollen during bell jar experiments (pi/pi,sat = 1). Gray shaded region represents the polymer volume fraction regime in which PSM has mobility, from eq 4. (c) Flow SVA of PSM in THF (top, ×) and CHCl3 (bottom, ×).

G


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Figure 6. (a) Contact angle with water (□) and diiodomethane (■). Error bars are the standard deviation in the contact angle from multiple measurements and, in some cases, are smaller than the data points. (b) Calculated total (○), dispersive (■), and polar (◆) surface energy using the two-fluid Owens−Wendt contact angle method described elsewhere.40,41 Error bars from the surface energy calculations represent the error propagated from the standard deviation of the contact angle measurements. For data points in which the error bars are not visible, the error is smaller than the data points. Total surface energy decreased with increasing aliphatic character, whereas total surface energy increased with increasing oxygen content.

dependent friction data, see Figure S2. The friction coefficients were nearly independent of polymer structure when measured with the glass probe. However, when measured with the HDPE probe, the friction coefficients increased with oxygen content, likely as a result of the change in adhesion between the probe and the film (see PVM and PSM data in Figure 7). In general, the friction coefficients in Figure 7 are similar to those for PS measured under similar conditions with glass (μPS = 0.46) and HDPE (μPS = 0.51).52 Tribology Discussion. Although increasing oxygen content per repeat unit impacted the polymer−solvent interaction parameters with THF and CHCl3, leading to both increases in solvent compatibility (PGM → PSM) and decreases in solvent compatibility (PGM → PVM) [see Figure 4], depending on the position and type of the oxygen group, the polymers containing more oxygen groups generally were less soluble in the casting solvent (THF). These differences in solvent compatibility (solubility) can influence the film roughness during casting, which could impact the measured friction coefficients. To investigate this potential effect, the root-mean-squared (RMS) roughness (shown in Figure S6) was measured using AFM. With the exception of PSM, all films had similar roughnesses and the trends in the roughness do not explain the differences in friction coefficient. For example, PSM and PVM films had similar friction coefficients with glass and HDPE; however, the roughness values for these films were the most dissimilar of all samples. Furthermore, minimal variations in friction coefficient were reported for most polymers when probed via the glass and HDPE beads, despite the fact that the probe roughnesses differed by about an order of magnitude. Thus, the reported differences in friction coefficient behavior noted in Figure 7 appear to result from the physics/chemistry associated with the nature of the polymer repeat units, as opposed to film processing conditions. Other factors could explain the differences in friction coefficients of these lignin-inspired polymers, such as differences in adhesion between the films and the probes. The adhesion between two surfaces was estimated from the

Figure 7. Time-averaged friction coefficients of films probed with HDPE (◇) and glass (◆). Glass−polymer friction coefficients were nearly identical within error for all polymer films. Error bars represent the standard deviation of the friction coefficients in time obtained from the data in Figure S2.

mJ/m2, which was only slightly higher than both PS [40 mJ/ m2] and PMMA [41 mJ/m2].23 Increasing the number of oxygen-containing groups (PPM → PGM, PCM, PEM; PGM → PSM, PVM; PSM, PVM → PSAM) increased the total surface energy, largely through increases in the polar surface energy component. Increasing the length of the short aliphatic chain substituents in the para position (PGM → PCM → PEM) decreased the dispersive component of the surface energy, which led to a decrease in the total surface energy, as the polar contribution did not change significantly between PGM, PCM, and PEM. Tribology Results. Glass and HDPE probes were used to measure the friction coefficient of the polymer films at 4.6 mm/s. The average friction coefficients for each polymer film are reported in Figure 7 and were extracted from timeH



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differences in the material surface energies upon contact (see Table S3, eqs S2 and S3, and the associated description in the Supporting Information).29 Despite the greater calculated works of adhesion between the glass probe and the films, the glass−polymer friction coefficients did not depend strongly on the chemical structure, suggesting that adhesion also did not factor significantly into the friction coefficient of the films. Thus, probing the HDPE− polymer adhesion provided additional information with respect to polymer/probe adhesion behavior. The adhesion data (pulloff forces) for all polymers with HDPE were small and of similar magnitude (see Supporting Information, Figure S7), suggesting that the probe−film interactions were not influenced strongly by the differences in surface energy between the oxygen-containing and the aliphatic para groups on the sustainable polymers. Generally, the friction coefficient was not impacted strongly by the different functional groups of the sustainable polymers. With the exception of the friction coefficient measured for PSM and PVM with the HDPE probe, the friction coefficients fell within the same range (within error) for all polymers. The similarities in the calculated work of adhesion from the surface energies (Table S3), the lack of significant differences in film roughness (Figure S6), and the low adhesion (Figure S7) all support the relative independence of the friction coefficient on processing and probe parameters presented in Figure 7. These results suggest that any of the sustainable polymers would have similar tribological performance (also similar to PS), regardless of fine details in the chemical structure of the polymer repeat unit, while simultaneously allowing tunability in solvent resistance and solution processability to optimize manufacture ease and ultimate performance.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00667. Experimental details, including characterization of PPM and PSAM, film coating methodology, and work of adhesion details; crossover concentration for sustainable polymers in THF; crossover concentration for PSM in CHCl3; comparison of PGM and PVM swelling; contact angle values for sustainable polymers using water as the test fluid; comparison of surface energy from contact angle hysteresis and static contact angles; time-dependent friction coefficient traces; calculated probe−film adhesion; RMS roughness of sustainable polymers; HDPE-film pull-off forces (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Jillian A. Emerson: 0000-0001-7536-1941 David L. Burris: 0000-0003-2687-7540 Thomas H. Epps III: 0000-0002-2513-0966 Present Address ‡

J.A.E.: Core R&D, The Dow Chemical Company, Midland, Michigan 48674, United States. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes

CONCLUSIONS The effects of ortho and para constituents on the solvent compatibilities, surface energies, and friction coefficients of lignin-inspired polymer films were examined. Predictive trends were developed to explain these key properties (solvent compatibility, surface energy, and friction coefficient) that influence the processability and wetting of these polymers for functional coatings. For example, polymers that contained more aliphatic character in the para position of the pendant phenyl ring had better solvent compatibilities and lower total surface energies in comparison to their aldehyde- and multimethoxycontaining counterparts. Within the series of monomethoxybased polymers, all materials had similar glass transition temperatures but different solubility parameters with THF, which enabled tuning of film/solvent compatibility without impacting possible use temperatures. Additionally, the solvent resistance was increased significantly by the presence of the second methoxy group, due to greatly increased glass transition temperatures. Furthermore, the inclusion of the oxygen moieties resulted in higher surface energies and friction coefficients when evaluated using an HDPE probe. Finally, heteropolymers comprised of the same functional monomers had intermediate properties in comparison to their homopolymer counterparts. Not only do these heteropolymers afford additional material tunability but they also allow for reduced separations costs. Thus, this systematic study of the material properties illustrates the effect of para and ortho function groups on the material properties of the resulting lignininspired polymer films for coatings applications.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.A.E. and T.H.E. wish to acknowledge the NSF CHE-1507010 for funding support, and E.M.F. acknowledges NSF CBET1235955 and NASA Grant No. NNX10AE44G. N.G. and D.L.B acknowledge NSF CMMI-1434435 for financial support. The authors thank Prof. T. Beebe, Jr., Department of Chemistry & Biochemistry, and Prof. M. E. Mackay, Department of Materials Science & Engineering, both at the University of Delaware (UD), for use of their contact angle measuring systems. Additionally, we acknowledge the UD W. M. Keck Microscopy Facility for use of the AFM, which was supported, in part, by the Delaware COBRE program with a grant from the National Institute of General Medical Sciences− NIGMS (5 P30 GM110758-02) from the National Institutes of Health.

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ABBREVIATIONS PCES, poly(creosyl methacrylate-ran-4-ethylguaiacyl methacrylate-ran-syringyl methacrylate); PCM, poly(creosyl methacrylate); PES, poly(4-ethylguaiacyl methacrylate-ran-syringyl methacrylate); PEM, poly(4-ethylguaiacyl methacrylate); PGM, poly(guaiacyl methacrylate); PSAM, poly(syringealdehyde methacrylate); PSM, poly(syringyl methacrylate); PVES, poly(vanillin methacrylate-ran-4-ethylguaiacyl methacrylate-ran-syringyl methacrylate); PVM, poly(vanillin methacrylate); THF, tetrahydrofuran; CHCl3, chloroform; DCM, dichloromethane; AFM, atomic force microscopy; Tg, I


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glass transition temperature; CM, creosyl methacrylate; EM, 4ethylguaiacyl methacrylate; GM, guaiacyl methacrylate; SAM, syringealdehyde methacrylate; SM, syringyl methacrylate; VM, vanillin methacrylate; ϕc, crossover polymer volume fraction; pi/pi,sat, solvent vapor concentration above the film; ϕs, solvent volume fraction; ϕp, polymer volume fraction; χs‑p, solvent− polymer Flory−Huggins interaction parameter; δ, solubility parameter; θ, contact angle; θs, sessile contact angle; γP, polar contribution to surface energy; γD, dispersive contribution to surface energy; γH, hydrogen-bonding contribution to surface energy; Wad, work of adhesion; μi, friction coefficient of component i; mi, mass fraction of component i; Vs, molar volume of solvent; Vp, molar volume of polymer; PS, polystyrene; PMMA, poly(methyl methacrylate); SVA, solvent vapor annealing; HDPE, high-density polyethylene.

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