Softwood Lignin-Based Methacrylate Polymers with Tunable Thermal and Viscoelastic Properties

Article abstract of DOI:10.1021/acs.macromol.5b02316

Softwood (guaiacylic) lignin-based methacrylate polymers (LBMPs) that exhibit excellent glass transition temperatures (T_g's), desirable thermal stabilities (greater than 100°C above T_g), and intermediate shear-flow resistances, in co

Full text of DOI:10.1021/acs.macromol.5b02316

Article
pubs.acs.org/Macromolecules
Softwood Lignin-Based Methacrylate Polymers with Tunable
Thermal and Viscoelastic Properties
Angela L. Holmberg,† Ngoc A. Nguyen,‡ Michael G. Karavolias,† Kaleigh H. Reno,†,§ Richard P. Wool,†,§
and Thomas H. Epps, III*,†,‡
‡
§
†Department of Chemical and Biomolecular Engineering, Department of Materials Science and Engineering, and Center for
Composite Materials, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: Softwood (guaiacylic) lignin-based methacrylate
polymers (LBMPs) that exhibit excellent glass transition
temperatures (Tg’s), desirable thermal stabilities (greater than
100 °C above Tg), and intermediate shear-flow resistances, in
comparison to polystyrene and poly(methyl methacrylate), are
reported herein. Different R-groups (p-position hydrogen,
methyl, ethyl, and formyl groups) in otherwise homologous
LBMPs impart distinct characteristics to the flow behavior and
thermal properties of these bio-based polymers, which permit the investigation of unique structure−property relationships. More
specifically, the zero-shear viscosities (η0’s) for the LBMPs span nearly 2 orders of magnitude as the R-group is varied, while the
characteristic degradation temperatures differ more modestly (by ≈50 °C over the same series of polymers), and the Tg’s exhibit
minimal, yet application relevant, variations between ≈110 and ≈130 °C. These property differences were probed independent
of tacticity, molecular weight, and dispersity effects due to the nature of the well-controlled macromolecules generated via
reversible addition−fragmentation chain-transfer polymerization. Furthermore, heteropolymers prepared from mixtures of the
lignin-based monomers have composition-dependent Tg’s and component-dependent thermal degradation temperatures,
thermolysis rates, and η0’s. The multicomponent materials demonstrate the enhanced tunability inherent in LBMPs. Altogether,
this versatile library of softwood lignin-based monomers, and the unique structure−property relationships intrinsic to the
resulting polymers, provides a unique platform for building potentially low-cost, high-performance, and bio-based viscoelastic
materials attractive for thermoplastic elastomer and binder applications.
copolymers may be expensive to produce, as separating
complex bio-oils into single- or two-component streams
would increase costs significantly.1,2,21,22 A more effective
means for delivering cost-effective bio-based polymers should
involve capitalizing on the exceptional molecular diversity of
bio-oils by building multicomponent polymers (heteropoly-
mers) from mixtures of bio-oil components.4,23−25 Polymeriz-
ing bio-oil components together could ameliorate separations
costs while also providing a means to tailor polymer properties,
such as Tg, thermal stability, and viscoelasticity, by adjusting
bio-oil composition.
Softwood lignin-based bio-oils, such as those from pine or
cedar trees, are ideal for these endeavors in multicomponent
bio-based polymers as they largely consist of homologous
aromatic compounds,26−28 and the aromaticity of these
molecules is expected to provide corresponding polymers
with desirable Tg’s, viscoelasticities, and mechanical
strengths.18,24,29,30 Components of softwood lignin bio-oils
bearing a single hydroxyl functionality predominantly include 4-
methoxyphenols, such as guaiacol, 4-ethylguaiacol, creosol, and
INTRODUCTION
■
Practical bio-based materials that can compete with petroleum-
based plastics in both cost and performance are of growing
interest yet are challenging to design due to trade-offs between
cost, feedstock sustainability, and macromolecular proper-
ties.1−3 An especially significant void exists for bio-based
polymers with glass transition temperatures (Tg’s) ≥ 100 °C.1
Lignin has the potential to provide monomers that can fill this
void, as it is an inexpensive, abundant, and renewable waste
product with significant aromaticity.1,4−9
Several groups have taken chemicals found in (or prepared
from) natural products or lignin-based bio-oils, such as β-
methylstyrenes10 or derivatives of butyrolactones,11−13 glu-
cose,14 terpenes,15−17 vanillin,18 and isosorbide,19 and em-
ployed controlled polymerization techniques to generate
materials with Tg’s ≥ 106 °C and other potentially desirable
properties.1,20 Controlled synthesis methods, such as reversible
addition−fragmentation chain-transfer (RAFT) polymerization,
unlock possibilities for these bio-based monomers to be
incorporated into block polymers for applications such as
thermoplastic elastomers, pressure-sensitive adhesives, and
composite binders.1 However, in these controlled syntheses
of bio-based polymers, generally only one or two chemically
distinct monomers are employed. Such homopolymers and
Received: October 22, 2015
Revised: January 16, 2016
© XXXX American Chemical Society
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vanillin.26−28 For example, 78−83 wt % of the gas
chromatography detectable components of crude Kraft lignin-
based bio-oils (23−34 wt % of the total bio-oil) as reported by
Choi et al. were 4-methoxyphenols (41−50 wt % guaiacol, 5−8
wt % 4-ethylguaiacol, 11−13 wt % creosol, 4−6 wt % vanillin,
etc.).28 These guaiacolic components can be acylated readily to
methacrylate monomers24,29 (2-methoxyphenyl methacrylate
derivatives) that have similar reactivities, allowing for the
straightforward synthesis of linear polymers with precise
molecular weights, narrow dispersities, and approximately
random distributions of monomer segments in each chain.23
However, to make the most practical materials from such bio-
oils, one needs to predict a priori the properties of
multicomponent polymers based on the polymerization
conditions and bio-oil composition.23 This goal requires an
understanding of the components’ underlying structure−
property relationships, some of which also may be applicable
to explaining the varying properties of lignins themselves.31
The prediction of polymer properties from a structural basis
is an immense challenge, especially for methacrylates, which
have characteristics that depend significantly on tacticity and
side-chain functionality.32−34 For example, the Tg’s for isotactic
versus syndiotactic poly(methyl methacrylate) (PMMA) can
differ by ≈70 °C,33,35 and the Tg’s for PMMA versus poly(ethyl
methacrylate) can differ by ≈40 °C.34 Quantitative structure−
property relationships (QSPRs) for predicting polymer
characteristics can be calibrated to sets of polymerized
methacrylates to describe these significant differences between
Tg that result from various polymer architectures, but model
accuracy is limited by the amount and quality of available
literature data.32 Bertinetto et al.32 compiled Tg and tacticity
data from the literature for 110 phenyl-group-containing
poly(meth)acrylic esters, but only two examples were derived
from methoxy phenols (and those phenols had methoxy groups
in the 3- and 4-positions instead of the 2-position as it is for
guaiacol). Some seminal works compare the Tg’s of poly(phenyl
methacrylate)s and derivatives with single methyl, isobutyl, tert-
butyl, methoxy, and other functionalities in various positions
around the phenyl ring;36,37 however, differences in molecular
weights and variable tacticities make structure−property
comparisons difficult. Thus, the relationships between side-
chain aromatic groups carrying different functional groups and
polymer properties, such as Tg, thermal stability, and
viscoelasticity, need further investigation.
Scheme 1. Color-Coded Nomenclature Utilized Herein and
the Scheme for Synthesizing Softwood Lignin-Based
Methacrylate Monomers and the Corresponding Polymers
a
a
Reagents and conditions: (i) methacrylic anhydride, catalytic 4-
(dimethylamino)pyridine, 55 °C, ≥24 h; (ii) 2-cyano-2-propyl
benzodithioate, 2,2′-azobis(isobutyronitrile), solvent (see Table S1
for details), 72 °C, ≤10 h.
methyl, ethyl, or formyl group. These minor structural
differences measurably influence the polymers’ Tg’s, thermolysis
behavior, and zero-shear viscosities (η0’s), especially in
comparisons between the hydrogen substituent and formyl
group (Tg and η0) or ethyl group (thermolysis behavior).
Altogether, this work describes the structural heat-flow stability
and polymer processability of guaiacylic methacrylate polymers
and provides insight into the component-based versus
composition-based structure−property relationships that
could facilitate future efforts in macromolecular design.
EXPERIMENTAL SECTION
■
Materials. All lignin-based monomers and polymers were
synthesized via acylation and RAFT chemistries, respectively, as
depicted in Scheme 1, described in detail elsewhere,18,23,24,29 and
overviewed in Table S1 of the Supporting Information with the
macromolecular characteristics (dispersities, molecular weights, etc.) of
each LBMP. The monomers are guaiacyl methacrylate (GM) from
guaiacol, creosyl methacrylate (CM) from creosol, 4-ethylguaiacyl
methacrylate (EM) from 4-ethylguaiacol, and vanillin methacrylate
(VM) from vanillin, all of which were prepared by reaction of the
phenol with methacrylic anhydride using a catalytic amount of base [4-
(dimethylamino)pyridine]. The corresponding polymers are referred
to as PGM, PCM, PEM, and PVM, respectively. Hyphenated numbers
are used to distinguish between the different materials, in which the
value corresponds to the polymer’s weight-average molecular weight.
Heteropolymers composed of mixtures of these monomers are
referred to as poly(bio-oil methacrylate)s or PBOMs.
Herein, the thermal and viscoelastic properties of RAFT-
synthesized softwood lignin-based methacrylate polymers
(LBMPs, Scheme 1) are reported and compared to other
common glassy polymers, viz., PMMA and polystyrene (PS).
Two of the LBMPs are heteropolymers, in which one has a
composition that approximately mimics a bio-oil based on data
from Brodin et al. (pyrolyzed Liner Permeate pine/spruce
lignin),38 and the other has approximately equimolar amounts
of each monomer to facilitate determination of component-
dependent vs composition-dependent property comparisons.
The data demonstrate the LBMPs’ beneficial Tg’s of at least 100
°C (above boiling water but not too high such that processing
and energy costs could become problematic), thermal stabilities
above 200 °C, and flow resistances that bridge the gap between
PS and PMMA (or exceed PMMA in some cases). The relative
properties of various LBMPs are examined in the context of
their different R-groups. The softwood lignin-based building
blocks that constitute the LBMPs are homologous except for
the p-position moiety, which is either a hydrogen atom or a
Nuclear Magnetic Resonance (NMR) Spectroscopy. Compo-
sitions of the heteropolymers are listed in Table 1 and were
Table 1. Composition of the PBOMs (in wt %)
monomer
PBOM-22
PBOM-54
EM
GM
CM
VM
28
40
10
22
27
25
23
25
1
determined using H NMR spectroscopy (Bruker AVIII 600 MHz
spectrometer, 26 °C, CDCl3 with 0.03 vol % TMS) and characteristic
peaks indicated in the literature.23 The compositions of the
heteropolymers do not deviate significantly from the compositions
of the monomer mixtures from which the polymers were synthesized
at all conversions, as reported previously23 while confirming the
approximately random distribution of monomer segments in each
polymer chain.
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Tacticity of the homopolymers was measured using 1H NMR
spectroscopy but with 3 vol % trifluoroacetic anhydride added to the
solvent to consume any water in the CDCl3 that can obscure α-methyl
protons. Three peaks were fit to each polymers’ α-methyl protons
using MestReNova NMR software39 to determine the relative amounts
of meso−meso (mm) isotactic, meso−racemo (mr or rm) heterotactic,
and racemo−racemo (rr) syndiotactic triads, in which the mm peak
was assumed to be downfield and the rr peak was assumed to be
upfield of the middle mr peak. This assumption follows the peak order
and approximate spreading of α-methyl protons reported for PMMA40
and poly(phenyl methacrylate).37 The fraction of racemo diads was
estimated by normalizing each mm, mr, and rr peak to the total area of
all three peaks and then adding rr to 0.5 mr. The fraction of
syndiotactic triads was determined as rr/(rr + mr + rm + mm), in
which mr + rm was obtained from a single peak. Select samples also
were studied at an elevated temperature (58 °C) with a Bruker AVIII
400 MHz spectrometer to confirm the peak locations of the triads.
Size-Exclusion Chromatography (SEC) with Light Scattering.
Molecular weight (number-average, Mn, and weight-average, Mw) and
dispersity (Đ) data for the polymers were obtained by performing size-
exclusion chromatography (SEC) experiments in tetrahydrofuran
(THF, 1.0 mL/min) using a Viscotek VE 2001 instrument equipped
with a Waters Styragel HR1 and HR4 column (7.8 × 300 mm) in
series. A Viscotek multidetector system connected to the SEC
instrument consisted of a refractive index (RI) detector (VE3580), an
ultraviolet photodiode array (UV-PDA detector), and a viscometer-
plus-light-scattering dual detector (270 dual detector), all of which
were calibrated with a solution of PS in THF (c = 1.03 mg/mL, Mw =
98 400 g/mol, dn/dc = 0.185 mL/g) immediately prior to data
acquisition. Polymer solutions used to evaluate the differential
refractive index (dn/dc) and molecular weight for each polymer
(Table S1) were prepared at four concentrations (1.0, 0.9, 0.6, and 0.4
mg/mL polymer/THF). RI detector data were used to calculate dn/dc,
and light scattering data from scattering angles of 90° and 7° were
used to determine absolute molecular weights (Mn,LS and Mw,LS) via
Zimm analysis. Hyphenated numbers following a polymer designation
correspond to that polymer’s Mw,LS (kg/mol). Example SEC data for
these polymers are shown in Figure S1.
purge). Samples first were heated at 20 °C/min to 100 °C and
annealed for 10−20 min to remove possible water, then were cooled at
10 °C/min to 50 °C and annealed for 1 min, and finally were heated at
10 °C/min to 600 °C for analysis. Percent mass-loss thermolysis
profiles were normalized to the mass measured at 100 °C during the
final heat. The final mass always matched the empty pan mass within
error. Ramp data under continuous N2 flow (50 mL/min purge for
both the balance and sample) and isothermal data under airflow (200
°C for 1 h) also were collected for reference (see Figures S2 and S3).
The characteristic degradation temperatures reported herein are the
onset degradation temperature (To, the temperature at the intersection
of tangents to the initial and steepest segments of the mass-loss data)
and the peak degradation temperature (Tp, the temperature at the
maximum of the derivative). These characteristic degradation
temperatures were chosen because they are not affected by end-
group thermolysis and therefore depend minimally on molecular
weight.42 Derivatives of the mass-loss data in the figures are smoothed
over 200 data points (100 s of run time) for visual clarity.
Dynamic Mechanical Analysis (DMA). DMA data were collected
using an ARES-G2 shear rheometer fitted with 8 mm diameter parallel
plates. The polymers for these experiments (PVM-35, PCM-48, PGM-
44, PEM-48, PBOM-54, PMMA-55, PS-31, and PS-47) were selected
for their similar molecular weights, noting also that PMMA-55 has a
similar tacticity to the LBMPs. Polymer discs (0.3 mm in thickness and
8 mm in diameter) were molded at high temperature (170 °C for the
LBMPs and PMMA and 150 °C for the PSs) and pressure (1 metric
ton per 50 mm2 sample) for 15 min. The samples were optically clear,
and the RAFT-generated LBMPs retained their pink color from the
labile benzodithioate end groups (see Figure S4), indicating that end-
group thermolysis was not significant during sample preparation.
Frequency sweeps for the LBMPs were acquired at 150, 160, 170,
and 180 °C with strain amplitudes between 0.1% and 3.0% in the
linear region. The resulting data were shifted by a factor, aT, to a
reference temperature of 150 °C using time−temperature super-
position. Frequency sweeps for PS samples, which were analyzed for
comparison, were collected at 120, 130, 140, and 150 °C with strain
amplitudes ranging from 0.5% to 5.0%, and frequency sweeps for
PMMA were collected at 150, 160, 170, 180, 190, and 200 °C with
strain amplitudes ranging from 0.4% to 5.0%. These PS and PMMA
data also were shifted to a reference temperature of 150 °C.
Temperature-dependent data obtained during dynamic shear were
obtained at a constant frequency of 6.28 rad/s and strain amplitudes
between 0.03% and 5.0%. The temperature range was 100−180 °C for
all of the polymers, except for PEM-48 (90−180 °C) and the PSs
(90−150 °C) due to the lower PEM and PS Tg’s and for PMMA-55
(120−200 °C) due to slippage between the parallel plates at lower
temperatures. The maximum temperature for the PS data acquisition
was 150 °C because the polymer flowed from between the plates at
higher temperatures. The Cox−Merz rule43 was used to estimate η0 at
the reference temperature (Tref) from the rheology data (see Figure
S5), and the dependences of these η0’s on temperature T were
estimated from the shift factors [η0(T) ≈ aT(T)η0(Tref)].
Differential Scattering Calorimetry (DSC). Tg’s for each of the
LBMPs (PVMs, PCMs, PGMs, PEMs, and PBOMs) were determined
using a differential scanning calorimeter (Discovery Series, TA
Instruments) with a cell constant and temperature profile calibrated
to an indium standard. Polymer powder (2−13 mg), dried at 40 °C
under reduced pressure for 48 h, was loaded quantitatively into at least
two 20 μL aluminum pans and hermetically sealed in air. Data were
collected at 5 °C/min under continuous N2 flow (50 mL/min) using
temperature programs that looped between 0 and 170 °C or 40 and
180 °C, for three heating and cooling cycles. The midpoint of the
inflection in the second and third heating traces is reported as the Tg,
noting that there were no significant changes between the second and
third heating. Error in the Tg measurement is reported at 95%
confidence based on DSC data collected using the above temperature
programs. The Flory−Fox equation41
RESULTS
■
K
Mn
T = T
−
g
g∞
Glass Transition Characteristics. The collective Tg’s of
the homopolymers rank from highest to lowest as PVM, PCM,
PGM, and PEM, as illustrated by the Tg,∞’s listed in Table 2
and the locations of the endothermic transitions in the example
DSC data (normalized to a slope of zero at T = 140−160 °C)
shown in Figure 1a. The Tg’s for the PBOMs (Tg,PBOM-22 = 111
1 °C, Tg,PBOM-54 = 119 1 °C) are intermediate to the Tg’s of
PCM and PGM, as evident by the corresponding molecular-
weight-dependent data points in Figure 1b (molecular weight
information for all of the polymers is available in the
Supporting Information). The Flory−Fox constants from the
linear fits to the data in Figure 1b are listed in Table 2 and show
that the molecular weight dependence of Tg (K, Table 2) is
identical at 95% confidence for all homopolymers, except
(1)
in which Tg∞ is the glass transition temperature expected at infinite
molecular weight and K is the empirical Flory−Fox coefficient, was fit
to linearized Tg data to approximate the dependence of Tg on Mn. Two
outliers (PVM-50, Tg = 118 2 °C and PEM-19, Tg = 92 1 °C)
were omitted from the Flory−Fox analysis due to degradation of
PVM-50 (see the Supporting Information for more details) and the
statistically different tacticity of PEM-19 at 95% confidence (see Table
S1).
Thermogravimetric Analysis (TGA). Thermal degradation data
for each polymer were collected using a thermogravimetric analyzer
(Discovery Series, TA Instruments). Polymer powder (≈5 mg) was
loaded quantitatively into 100 μL platinum pans and heated under
continuous airflow (10 mL/min sample purge, 5 mL/min balance
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stable, PGM is most stable, and PCM and PVM have
intermediate stabilities in air. These relative thermal stabilities
are illustrated by example data in Figure 2a, in which the onsets
and peaks in the PGM data occur at higher temperatures than
in the PEM data, and corroborated by data in the Supporting
Information (TGA data collected under N2 flow in Figure S2
and isothermal data taken at 200 °C under airflow in Figure
S3). Additionally, the multiple peaks and high-temperature tails
in the derivative data for PEM and PVM (and the asymmetry in
the data for PCM) indicate that these homopolymers have
more complicated degradation pathways in comparison to
PGM, which has a single nearly symmetric peak and no
measurable tail in the derivative data. Thus, all of these
differences in the homopolymer TGA data are attributed to the
various R-groups.
Table 2. Properties of Lignin-Based Methacrylate
Homopolymers as Determined via Flory−Fox Analysis,
a
DSC, and TGA
parameter
PEM
116
PGM
120
PCM
132
PVM
139 2
Tg∞ (°C)
1
3
4
K (g K mol−1) × 10−5 2.0 0.3
2.1 0.9
0.239
2.7 0.9
0.216
3.6 0.6
0.291
b
dΔH/dT
0.217
(mW g−1 K−1
)
To (°C)
Tp (°C)
255
281
6
5
302
4
6
273
291
3
5
264 10
327
b
294
3
a
Error reported at 95% confidence. Error is 0.005 mW g−1 K−1.
For the heteropolymers, PBOM-22 and PBOM-54 have
approximately equal thermal stabilities, with To’s of 257 and
254 °C and Tp’s of 277 °C (first peak), 327 °C (second peak)
and 278 °C (first peak), 334 °C (second peak), respectively.
Two values are reported for Tp in each PBOM example because
two distinct peaks exist in the derivatives of the thermolysis
profiles, as shown in Figure 2b. The degradation onsets for the
PBOMs match the degradation onset of PEM (the correspond-
ing curves in Figure 2a nearly overlap at lower temperatures).
Additionally, the peaks for the PBOM data are shifted to
slightly lower temperatures relative to the PEM data and
slightly higher temperatures relative to the PVM data (see the
derivative data in Figure 2a).
PBOM-22 and PBOM-54 have different volatilization rates of
the degradation products, which is indicated most clearly by the
different peak heights in the derivative data (Figure 2b). These
various characteristics of the TGA data likely depend on the
PBOM constituents’ R-groups as well as the relative
compositions of each of the constituents. Finally, all of the
LBMPs, homopolymers and heteropolymers alike, have
characteristic degradation temperatures that are significantly
higher than their Tg’s and provide adequate melt processability
to ∼200 °C.
Viscoelastic Characteristics. DMA data for five LBMPs
(PEM-48, PGM-44, PCM-48, PVM-35, and PBOM-54), two
PSs (PS-31 and PS-47), and a PMMA (PMMA-55) are
presented in Figure 3. Time−temperature superposition master
curves of elastic shear moduli (G′) vs shifted frequency (aT·ω)
are shown in Figure 3a, and corresponding loss shear moduli
(G″) master curves are in the Supporting Information. The
overlaid dashed lines in these figures have slopes that approach
two (elastic) and one (loss) at low frequencies. These slopes
are typical for linear polymers with no significant intermo-
lecular interactions or phase separation in the melt state.45 G′ vs
T data are shown in Figure 3b, and complex viscosity (η*) vs
aT·ω data are shown in Figure 3c. The slopes of the data in
Figures 3b and 3c at melt temperatures (T > Tg) and melt
frequencies (low aT·ω), respectively, are reasonably consistent
between the LBMPs at each shifted temperature (shifted
relative to Tg), indicating that the LBMPs’ rheological
properties are influenced by temperature and shear in a similar
manner. Dissipation factor (tan δ = G″/G′) vs T data are
shown in Figure 3d, and the maxima in these data occur at the
rheological Tg’s. The vertical and horizontal shifts between the
data are mainly due to these different Tα’s. Specifically, values
for G′ and η* in the melt (at high temperatures or low ω’s) in
Figures 3a−3c are lowest for the homopolymers with the
lowest Tg’s and increase in order from PS-31 and PS-47 to
Figure 1. (a) Example DSC data (5 °C/min) and (b) polymer Tg’s
used in Flory−Fox analysis plotted vs inverse Mn. Data from (a) are
mapped onto (b) at the circled positions. The data in (a) (from the
polymers also studied by DMA) are shifted vertically, normalized by
the slope between 140 and 160 °C, and overlaid with arrows indicating
measured Tg’s. The data in (b) include error bars that represent 95%
confidence in the measurement and color-matched dashed lines that
are the linear Flory−Fox fits to the data. The black-filled and black-
outlined triangles for the PBOMs represent experimental and
predicted (see eq 2) Tg’s, respectively. The experimental and predicted
Tg’s for PBOM-54 (119 1 and 120 3 °C, respectively) are nearly
identical aside from the greater error for the predicted value.
between PVM and PEM. Additionally, changes in heat flow
with respect to temperature (dΔH/dT) for the homopolymers
at T = 60−80 °C are reported in Table 2 (dΔH/dT is 0.218
0.005 mW g−1 K−1 for PBOM-54). These slopes indicate the
relative flexibility of the methacrylate esters,44 in which PVM
and PGM have more rigid ester groups than PCM and PEM.
Altogether, these DSC data suggest that a relationship exists
between homopolymer R-group, Tg, and dΔH/dT and also
between heteropolymer composition and Tg, as discussed in
further detail below.
Thermal Degradation Characteristics. On the basis of
To and Tp listed in Table 2 for the homopolymers, PEM is least
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Figure 2. Representative TGA data (10 °C/min, air) for selected LBMPs, in which mass loss (top) and the corresponding derivative (bottom) was
obtained on heating. (a) Homopolymers and one PBOM for comparison. (b) Both PBOMs for comparison.
Figure 3. Rheological data for selected LBMPs, two PSs, and a PMMA for comparison. The color-coded legend for all plots is shown in panel (a).
(a) The elastic shear modulus (G′) for each polymer plotted as a function of frequency (ω) and shifted by factor aT to a reference temperature of
150 °C using time−temperature superposition. The linear dotted lines have slopes of two to illustrate the expected behavior at low frequencies for
polymers with minimal intermolecular interactions. (b) Temperature-dependent G′ for each polymer measured at a constant ω of 6.28 rad/s, ramp
rate of 3 °C/min, and strain amplitude between 0.03% and 5.0%. (c) Complex viscosity (η*) for each polymer plotted as a function of aT·ω as
described in (a). The corresponding dynamic viscosity (η) vs shear rate plot for determining zero-shear viscosities (η0’s, Table 3) is in the Supporting
Information. (d) Temperature-dependent dissipation factor (tan δ) for each polymer determined using the conditions and data reported for (b) and
the corresponding loss moduli reported in the Supporting Information. The location of the maximum in each tan δ trace was taken as Tα, the
rheological Tg.
PEM-48, PGM-44, PCM-48, and PVM-35 to PMMA-55.
Values for G′ and η* for PBOM-54 are mostly located between
the corresponding values for PGM-44 and PCM-48 (Figures
3a−3c), as expected based on PBOM-54’s intermediate Tα to
PGM-44 and PCM-48; however, at the lowest aT·ω’s in Figures
3a and 3c, the values for PBOM-54 begin to exceed those for
PCM-48 and approach those for PVM-35. This crossover may
indicate that VM segments in PBOM-54 disproportionality
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influence the viscoelasticity of the heteropolymer in the melt, as
discussed in further detail below. Overall, the temperature- and
time-dependent changes in viscoelasticity shown in Figure 3 are
mostly independent of R-group, aside from the horizontal and
vertical shifts.
In contrast, the R-group plays a vital role in the η0’s listed in
Table 3 and shown in Figure 4. The LBMP Tα’s span 19 °C,
DISCUSSION
■
The lignin-based monomers and polymers described herein are
significant for at least four main reasons. First, these LBMPs are
considered “green” for the constituents’ low volatility and low
toxicity according to estimates made via the Environmental
Protection Agency’s Estimation Program Interface Suite (EPI
Suite)47 and for their potentially high bio-based contents (64−
69%, or 100% assuming the methacrylate group comes from
renewable resources in the future).18,24 Second, as indicated in
the above results, the bio-based monomers yield materials with
beneficial properties in comparison to PS, with respect to their
reduced tendency for deformation, and PMMA, with respect to
processability. Third, the structural differences between
polymers provide insight into structure−property relationships
that are increasingly relevant as more homologous monomers
are extracted from biomass or prepared using biological means.
Finally, the heteropolymers are expected to provide a means for
reducing costs and tuning properties, which herein involves
choosing η0’s without compromising the promising Tg’s,
thermal stabilities, and other viscoelastic properties.
Table 3. Properties of Selected Polymers as Determined
a
Using SEC (Light Scattering) and DMA
b
polymer
Mw,LS (g/mol)
Đ
Tα (°C)
η0 (kPa s)
PEM-48
PGM-44
PCM-48
PVM-35
PBOM-54
PMMA-55
PS-31
48 000
44 000
48 000
35 000
54 000
55 000
31 000
47 000
1.39
1.20
1.26
1.47
1.33
1.54
1.23
1.18
132
138
148
151
144
138
114
114
730
1300
18 000
34 000
25 000
270 000
5.0
PS-47
14
a
Thermal and macromolecular property data for these polymers are
Homopolymer Structure−Property Relationships.
Glass Transition Temperatures. The most important detail
regarding the Tg-practicality of the softwood LBMPs is that
they have Tg’s near and above that of boiling water, but the Tg’s
are not so high that processing and energy costs could become
problematic. Two polymers have Tg’s that are a close match to
shown in Figures 1−4. A full list of polymers, synthesis conditions, and
b
characteristics is available in the Supporting Information. Tref = 150
°C.
the 100 °C Tg of PS (PEM-19 and PEM-30 have Tg’s of 92
1
and 108 2 °C, respectively), while another polymer has a Tg
as high as 130 2 °C (PVM-53). The proximity of some of
these LBMP Tg’s to 100 °C is among the closest reported to
date for bio-based thermoplastics from linear and narrow-
dispersity macromolecules.1,19,20,22
Differences between the Tg’s for the LBMPs are direct
consequences of their different R-groups and molecular
weights, not of tacticity variations. This assertion was supported
by measuring the fractions of racemo diads [r/(r + m)] and
syndiotactic triads [rr/(rr + rm + mr + mm)] via the α-methyl
protons in H NMR spectroscopy data. Example spectra are
shown in Figure 5. The average racemo diad percentages were
79 3% for the PVMs, 78 2% for the PCMs, 71 1% for
1
Figure 4. Zero-shear viscosity vs. temperature data determined by
normalizing the shift factors that were used to construct master curves
(Figures 3a and 3c, Figure S6a) with the η0’s at Tref = 150 °C.
and the LBMP η0’s (at 150 °C) span nearly 2 orders of
magnitude. In both cases, PVM-35 and PCM-48 have the
highest values and PEM-48 has the lowest. This ordering is
consistent with the ordering based on Tg∞’s (Table 2).
Additionally, at 150 °C, the η0 for PEM-48 is nearly 2 orders
of magnitude larger than the η0 for PS-47 yet less than the η0
for atactic PMMA-55 and other similar PMMAs from the
literature.46 These viscosity data indicate that the LBMPs may
be less prone to deformation under normal use conditions, with
respect to time, temperature, or shear, in comparison to PS,
while maintaining the processability of PMMA.
Finally, PBOM-54’s Tα is consistent with its DSC-based Tg,
which is between the Tg’s for PGM and PCM, as listed in Table
3 and shown by the relative shifts of the data in Figure 3d;
however, PBOM-54’s η0 is intermediate to the η0’s for PCM-48
and PVM-35. This difference indicates that there may be a
disproportionate dependence between VM content and
heteropolymer properties, as mentioned above in reference to
Figures 3a and 3c.
1
Figure 5. Example H NMR spectra obtained at (a) 26 °C, 600 MHz
and (b) 58 °C, 400 MHz for the softwood LBMPs that were studied
using DMA. α-Methyl peaks utilized to quantify tacticity are shown in
the expanded region. Peaks marked with x’s in the full spectra
(trifluoroacetic acid, chloroform, etc.) are solvent peaks.
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the PGMs, and 73
8% for the PEMs. These values are
Additionally, PVM’s higher Tg in comparison to PCM results
from the reduced mobility of the methacrylate ester. Evidence
for differences in ester group mobility are available in the DSC
and 1H NMR spectroscopy data (Figures 1a and 5,
respectively) for the LBM homopolymers. First, the slope of
the DSC data for the LBMPs at T ≪ Tg (dΔH/dT, Table 2 and
Figure 1a) is steepest for PVM, suggesting that it has less ester-
group mobility than the other LBMPs.44 Second, the peaks
shown in Figure 5 that represent backbone α-methyl protons
are broader and less well-defined for PVM than for the other
LBMPs. This qualitative difference is a possible indication that
the PVM backbone is not as flexible as that of the other
polymers, which may result from polarity differences between
the various homopolymers. Specifically, polymer polarity
should decrease from PVM to PGM to PCM/PEM, the same
order of decreasing slopes in the DSC data. This ranking of
polarities is consistent based on the electron-donating/electron-
withdrawing character of the R-groups;49 1H NMR data,23 in
which the chemical shifts of the vinyl protons are furthest
downfield for the most polar monomers due to the electron-
withdrawing character (polarity) of the ester group; and
hydrophobicity predictions made using Mathers et al.’s
method50 with ACD/Laboratories,51 EPI Suite,47 and Chem-
Bio3D52 software. Together, these data support that PVM, the
likely most polar macromolecule, has the most rigid
methacrylate ester group and thus a higher Tg relative to the
other LBMPs.
consistent with atactic methacrylate polymers, which have
racemo diad percentages of approximately 70%.32 The
syndiotactic triad percentages (see the Supporting Information)
were somewhat more variable, yet still consistent between
polymers, and also suggestive of atactic polymers.
To draw relevant comparisons between the R-groups and
Tg’s that could inform future QSPR models and molecular
designs, molecular weight effects also were eliminated by fitting
the Tg data with the Flory−Fox equation (eq 1, Table 2). In
order of increasing Tg∞, the polymers rank as PEM, PGM,
PCM, and PVM and exhibit Tg’s that span ≈20 °C at similar
molecular weights. This ordering is consistent with Saito et al.’s
finding that greater aliphatic and guaiacylic carbon content in
oligomeric softwood lignins corresponds to lower Tg’s.31 This
small Tg-gap indicates that although R-group influences Tg, its
significance is minor in comparison to expected tacticity
effects.32,33,35
The relative Tg’s between the LBMPs can be explained in
part by applying qualitative free volume and rigidity principles.
Molecular movements thought to contribute most to the
relative Tg’s of the LBM homopolymers are depicted in Figure
6. Flexible R-groups should increase free volume and
Overall, these thermal transition data clearly indicate that
these modest adjustments to the p-position R-groups
measurably influence Tg’s. Other R-groups, from bio-based or
synthetic materials, may affect aromatic polymer Tg’s in similar
ways, so the reported structure−Tg relationships herein can
provide guidance for future macromolecular designs.
Thermal Degradation Temperatures. The characteristic
thermal degradation temperatures for the LBMPs (Table 2) are
at least 100 °C greater than the Tg’s (Figure 1), indicating that
these polymers all should be amenable to melt processing and
thermoforming in air unless temperatures greater than 250 °C
are required. As with Tg, the relative thermal degradation
characteristics of the LBMPs (Figure 2a) result from differences
in R-groups. PEM is least stable, PGM is most stable, and PCM
and PVM have intermediate stabilities. PEM degrades at the
lowest temperature likely because the CH2−CH3 bond is the
weakest bond in any of the R-groups; the lowest R-group-
associated bond dissociation energy is 301 kJ/mol for a
C6H5CH2−CH3 (EM) bond, 356 kJ/mol for a C6H5CH2−H
(CM) bond, 364 kJ/mol for a C6H5C(O)−H (VM) bond,
and 473 kJ/mol for a C6H5−H (GM) bond.49,53 Conversely,
PGM is the most thermally stable of the softwood LBMPs and
likely degrades by random scission of the polymer backbone
due to the R-group’s high bond dissociation energy. This
hypothesis is supported in that PGM’s peak degradation
temperature of ∼330 °C (10 °C/min under flowed air; between
370 and 380 °C at 10 °C/min under flowed N2, as shown in
Figure S2) is independent of Mw and similar to that of RAFT-
generated PMMA’s higher peak degradation temperature of
∼360−380 °C (5 °C/min under N2) that is thought to
correspond to a chain-scission mechanism.54 Additionally, the
unimodal peak in the derivative of the TGA data for PGM
suggests that only a single polymer thermolysis process is
significant in air. The N2-based thermolysis data in Figure S2
are more complicated due to the elimination of oxygen that can
consume radicals as they are generated and hence the greater
Figure 6. Space-filling models to illustrate the differences between
monomer R-groups, which are highlighted in color and by black
ellipses. Red lines signify important axes of rotation or bending, black
lines and ellipses are visual cues to R-group size, single-headed curved
arrows represent rotational movement, the double-headed bent arrow
indicates flexural movement, and the single-headed straight arrow
denotes polarity. The aromatic rings and methacrylate esters are
oriented approximately parallel and perpendicular to the page,
respectively.
consequently lower Tg;48 this relationship is consistent for
PEM’s Tg in comparison to PGM’s Tg. The ethyl moiety can
bend along at the R-group’s CH3−CH2 bond and rotate around
the CH2−phenol bond to open up free space, whereas the
hydrogen atom (and the methyl and formyl groups) lacks these
degrees of freedom for movement. In contrast, rigid R-groups
should reduce free volume by sterically hindering side-chain
motion and consequently raise Tg; this relationship also is
consistent for PCM’s and PVM’s Tg’s in comparison to PGM’s
Tg. In these examples, the bulky, yet stiff, methyl (PCM) and
aldehyde (PVM) groups restrict rotation of the phenolic group
around the phenol−ester linkage, decreasing backbone
flexibility.
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possibility for unzipping from the end groups.54 In both the air
and N2 TGA data, the comparably complicated and multimodal
thermolysis profile for PVM likely is a direct consequence of
the formyl group. Aldehydes are inherently reactive, so the
complexity of the thermolysis data could arise from multiple
reaction types being involved in the breakdown of the polymer.
Some of the reactions likely result in the formation of
temporary char, the source of the significant tail in the TGA
data for PVM at T > 400 °C (Figure 2a).55 Overall, these TGA
data indicate that the most stable of the studied softwood
LBMPs has a hydrogen atom as an R-group, the least stable has
an n-alkyl R-group, and the most char-forming has a formyl R-
group.
Viscoelasticity. The LBMPs have increased η0’s and
otherwise consistent temperature- and frequency-dependent
viscoelastic performance relative to PS, but the η0’s and
rheological properties deviate from those of PMMA-55 by
significantly differing amounts. These characteristics indicate
that the LBMPs may be less prone to deformation from natural
temperature- and time-dependent flow processes in comparison
to PS, but LBMP deformation in comparison to PMMA
depends largely on R-group, discussed more in later paragraphs.
In general, the LBMPs are less or similarly viscous as PMMA-
55 (even after normalization by Mw,LS and considering that the
higher Đ of PMMA-55 lowers its η0 in comparison to the other
polymers) and other PMMAs from literature,46 so they could
be preferable for certain applications. Mainly, these viscosities
that are intermediate to those of PS and PMMA may provide a
means for balancing trade-offs between materials longevity and
processability for various applications in adhesives, thermo-
plastic elastomers, or building and vehicle components.
Although the LBMPs have generally attractive η0’s, the
entanglement molecular weight (Me) for the LBMPs may be
somewhat less desirable. Evidence for the different Me’s
between PS, PMMA, and the LBMPs is found mainly in the
G′ data (Figure 3b). Specifically, the PS traces in Figure 3b
have sharper inflections and shallower slopes (features of a
plateau modulus that start to become apparent at Mw > 2Me)
than the LBMP traces of similar Mw polymers, and these
features are even more pronounced for PMMA-55. A significant
difference in Me is expected because Me generally increases with
increasing side-chain bulkiness; atactic PS and atactic poly(4-t-
butylstyrene) have Me’s of 17 000 and 38 000 g/mol,
respectively,56 while atactic poly(methyl methacrylate) has an
Me near 7000 g/mol.46
prevent polymer chains from sliding past each other, either
through possible physical or (for PVM) dipole−dipole
interactions. Regardless, a number of quantitative yet semi-
empirical models57,58 support that Tg and η0 are interrelated,
consistent with the assertion herein that both Tg and η0 are
measurably dependent on R-group for reasons of rigidity, free
volume, and possibly polarity.
Overall, the LBMPs have mostly intermediate flow character-
istics in comparisons to PSs and PMMAs of similar molecular
weights, making the LBMPs excellent candidates as glassy
blocks, but to differing degrees, as the η0’s span almost 2 orders
of magnitude. The elastic and loss shear moduli are significantly
less R-group-dependent than η0, allowing one to choose a
desired η0 without significantly changing other mechanical
shear properties. Hence, functionalized components of soft-
wood lignin bio-oils provide a convenient platform for
accessing a range of thermal and viscoelastic polymer properties
without compromising other beneficial material characteristics.
Heteropolymer Composition−Property Relationships.
Glass Transition Temperatures. The Tg for a heteropolymer
can be predicted by utilizing compositional information.
Various forms of the Fox equation empirically relate the weight
fraction of each monomer type within a heteropolymer to the
Tg of the heteropolymer. In this work, the following form was
used:
w
T
g,i
1
i
=
∑
T
g,PBOM
(2)
in which Tg,PBOM (K) is the Tg of the heteropolymer, Tg,i (K) is
the Tg for a homopolymer composed of monomer i estimated
via eq 1 and the Flory−Fox constants from Table 2, and wi is
the weight fraction of monomer i in the heteropolymer (Table
1). This method yields predicted Tg’s for PBOM-22 and
PBOM-54 of 109
match the measured Tg’s of 111
3 and 120
3 °C, respectively, which
1 and 119 1 °C,
respectively. The agreement between the measured and
predicted Tg’s supports the assertion that the tacticities of the
homopolymers and heteropolymers are similar. These features
of predictability and consistent tacticity are invaluable if precise
Tg’s are important for a particular application.
Thermal Degradation Temperatures. Heteropolymers
likely are as stable as the least stable component, as suggested
by comparing the consistent To’s of 254−257 °C for PBOM-22,
PBOM-54, and the PEMs, which all contain segments with
ethyl R-groups. Consequently, some level of bio-oil fractiona-
tion may be necessary if a heteropolymer for a given application
requires a higher thermal stability than Tg + 100 °C.
The degradation rates (the derivative TGA data [Figure 2],
which herein are the release rates of volatiles) of the
heteropolymers likely depend on the composition of the
overall chains, not just EM content. PBOM-22 degrades faster
than PBOM-54; it has the higher peak degradation rate in the
derivative data shown in Figure 2b. Thermolysis can begin at
chain ends,54 so the higher end group concentration in PBOM-
22 may be one reason that this heteropolymer degrades faster
than PBOM-54; however, composition likely is a significant
factor as well. In this example, PBOM-54 (25 wt % VM)
contains more VM segments than PBOM-22 (22 wt % VM),
and the mass remaining is consistently higher for PBOM-54
than PBOM-22 (by 3−8 wt % units from 260 to 400 °C). As
char-formers (e.g., VM as discussed in the corresponding
homopolymer section) in polymers can reduce material
No definitive relationships exist between the studied LBMP
R-groups and the shear moduli (or differential changes to the
shear moduli) with respect to temperature or frequency, as
shown in the rheology data (Figure 3a,b,d and Figure S6). Both
the magnitudes and slopes of the shear moduli data for the melt
are similar relative to Tα given the slight variations in molecular
weight and dispersity, as was described in the Results section.
These similarities were expected given that the LBMPs likely
have similar Me’s.
Unlike the modulus data, the η0’s significantly depend on R-
group, spanning nearly 2 orders of magnitude between PEM-48
and PVM-35. The ranking of polymers based on η0’s is
consistent with their ranking based on Tg, so the qualitative free
volume and polarity arguments mentioned above (e.g., Figure
6) likely apply here as well. Interestingly, the large differences
between η0’s compared to the ≈20 °C differences between Tg’s
suggest that viscosity may be more sensitive to R-group than
Tg. For example, the rigid methyl and aldehyde groups may
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flammability by disproportionately reducing volatiles formation
and release,55 these different VM contents may be responsible
for (at least partially) the different TGA-based degradation
rates. Furthermore, PBOM-22 and PBOM-54 contain similar
mass fractions of EM segments (Table 1), and PCM and PGM
have nearly the same peak degradation rates (Figure 2a), so
EM, CM, and GM components likely are not contributing
significantly to the relative degradation rates of the PBOMs.
Overall, the thermal stabilities and degradation rates of PBOMs
can be tuned by eliminating the least stable components (EM)
and adjusting the content of char-formers (VM), as the factors
that influence heteropolymer thermolysis are likely both
component-based and composition-based.
inexpensive bio-based platform for the synthesis of practical
new materials.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b02316.
Polymer synthesis conditions and macromolecular
characteristics; example SEC data; photograph of DMA
samples; and additional TGA and DMA data (PDF)
Viscoelasticity. The viscosity and shear modulus data for
PBOM-54 exhibit the same differential changes with respect to
temperature and frequency as the LBM homopolymers
(Figures 3 and 4, Figures S5 and S6), yet the η0 is
disproportionately dependent on VM content based mainly
on η0 for PBOM-54 being closest to η0 for PVM. Additionally,
averages weighted by Mw (η0 is proportional to Mw for Mw <
Me)45 and composition consistently underestimated η0,
suggesting that η0 may be more component-dependent than
composition-dependent. The cause for VM’s disproportionate
effect on η0 may be its polarity (or possibly dipole−dipole
interactions). Overall, these DMA data suggest that viscoelastic
properties of heteropolymers likely are more component-
dependent than composition-dependent.
AUTHOR INFORMATION
Corresponding Author
*E-mail: thepps@udel.edu (T.H.E.).
Notes
The authors declare no competing financial interest.
R.P.W.: Deceased 24 March 2015.
■
ACKNOWLEDGMENTS
■
The authors from the University of Delaware (UD) Depart-
ment of Chemical and Biomolecular Engineering acknowledge
NSF Grant CHE-1507010 and AFOSR-PECASE Grant
FA9550-09-1-0706 for partial financial support. The authors
from the UD Center for Composite Materials acknowledge a
SERDP WP-1758 through the Cooperative Agreement
W911NF-06-2-001 for partial financial support. N.A.N.
acknowledges the UD Department of Materials Science and
Engineering and NIST Award 70NANB10H256 through the
UD Center for Neutron Science for financial support. The UD
NMR facility used for this project was supported by the
Delaware COBRE program with a grant from the National
Institute of General Medical Sciences−NIGMS (1 P30
GM110758-01) from the National Institutes of Health. The
authors thank Scott R. Horton and Prof. Michael T. Klein for
helpful discussions; Prof. Joseph F. Stanzione, III, for providing
some monomer; Prof. Michael E. Mackay for use of the
rheometer; and the UD Advanced Materials Characterization
Lab for use of the DSC and TGA instruments.
CONCLUSION
■
As demonstrated by this library of softwood lignin-based
methacrylate monomers, structural differences between the p-
position R-groups in guaiacylic methacrylates measurably affect
the glass transition, thermal degradation, and viscoelastic
properties of corresponding polymers. All of the polymers are
stable thermally to at least 100 °C above the Tg, suggesting that
LBMPs should be amenable to melt processing. PVM and
PCM, with formyl and methyl functional groups, respectively,
have the most rigid R-groups and consequently the highest Tg’s
and η0’s in comparison to PGM and PEM. Interestingly, the R-
group affects η0 much more than it affects Tg, To, and Tp.
Finally, all of the LBMPs, homopolymers and heteropolymer
alike, have superb viscoelastic character for improving the shear
flow stability of plastics.
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DOI: 10.1021/acs.macromol.5b02316
Macromolecules XXXX, XXX, XXX−XXX