Sustainable poly(ether amide)s from lignin-derived precursors

Article abstract of DOI:10.1002/pola.29175

In recent years, there have been concerted efforts to replace petrochemical products with those from renewable sources due to the unsustainability of petroleum feedstock, and the continued volatility in the price. This work describes the synthesis and thermal properties of two new lignin-derived poly(ether-amide)s as alternative thermoplastics to petroleum-based commodities. Poly-4-(2-aminoethoxy)benzoate (PEAB) and poly-4-(2-aminoethoxy)-3-methoxybenzoate (PEAV) are synthesized by a melt polycondensation and characterized by ¹H NMR spectroscopy and thermal analysis. The number average molecular weight (M_n) of the polymers are estimated from the ¹H NMR spectroscopy analysis, and were shown to be 4100 and 12,000 g/mol for PEAB and PEAV respectively. The PEAB had a higher decomposition temperature (T_d) as well as glass transition temperature (T_g) compared to PEAV; albeit, with a lower molecular weight. The polymers’ T_d were in the range of 330 °C–380 °C and the T_g were between 100 °C and 120 °C. The thermal properties of the polymers are in the desirable range for thermoplastic materials used in the packaging, storage, and coating industries. Furthermore, the polymers are susceptible to degradation under acidic conditions in a short period; a property that is highly desirable for degradable polymers.

Full text of DOI:10.1002/pola.29175

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Sustainable Poly(Ether Amide)s from Lignin-Derived Precursors
Guery Saenz, Colleen Scott
Department of Chemistry, Mississippi State University, Starkville, Mississippi
Correspondence to: C. Scott (E-mail: cscott@chemistry.msstate.edu)
Received 7 June 2018; Accepted 23 June 2018; published online 0 Month 2017
DOI: 10.1002/pola.29175
ABSTRACT: In recent years, there have been concerted efforts to
replace petrochemical products with those from renewable
sources due to the unsustainability of petroleum feedstock,
and the continued volatility in the price. This work describes
the synthesis and thermal properties of two new lignin-derived
poly(ether-amide)s as alternative thermoplastics to petroleum-
based commodities. Poly-4-(2-aminoethoxy)benzoate (PEAB)
and poly-4-(2-aminoethoxy)-3-methoxybenzoate (PEAV) are
synthesized by a melt polycondensation and characterized by
1H NMR spectroscopy and thermal analysis. The number aver-
age molecular weight (Mn) of the polymers are estimated from
the 1H NMR spectroscopy analysis, and were shown to be 4100
and 12,000 g/mol for PEAB and PEAV respectively. The PEAB
had a higher decomposition temperature (Td) as well as glass
transition temperature (Tg) compared to PEAV; albeit, with a
lower molecular weight. The polymers’ Td were in the range of
330 ꢀC–380 ꢀC and the Tg were between 100 ꢀC and 120 ꢀC. The
thermal properties of the polymers are in the desirable range
for thermoplastic materials used in the packaging, storage, and
coating industries. Furthermore, the polymers are susceptible
to degradation under acidic conditions in a short period; a
property that is highly desirable for degradable polymers.
© 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2018
KEYWORDS: bio-based; degradable; lignin; poly(ether-amide)
(implants, sutures, and drug encapsulation), short-term food
packaging and agriculture.7–10 Another highly abundant bio-
derived polymer that is receiving increased attention is lignin.11
It is the second most abundant polymer in nature, only being
exceeded by cellulose. The complex chemical structure of lignin,
which comprises of a rich source of aromatic building blocks,
makes it an attractive resource to explore for many applications.
For example, lignin has been derivatized and explored for many
decades as fillers,12 stabilizer against the photooxidative degra-
dation of polymers,13 and for the production of polyesters and
polyurethanes.14 Recently, lignin was also shown to be an excel-
lent nucleating agent in poly(ϵ-caprolactone)-grafted-lignin
copolymers.15
INTRODUCTION The worldwide consumption of plastics
between the years 2014 and 2020 is predicted to increase by
29%.1 This expected increase in the consumption of plastics
will cause a demand on the mining of fossil fuel for petrochem-
icals, thus continuing the ongoing concern for the limited
source of fossil fuel and volatile cost of petroleum.
Additionally, the products produced from these raw materials
are non-biodegradable, and when discarded improperly, result
in pollution.2 Subsequently, waste disposal creates an addi-
tional cost to the production of commodities from petrochemi-
cals. As a result, polymers produce from bio-based raw
materials are being pursued as a sustainable and environmen-
tally friendly alternative to fossil fuel products.3,4 In fact, the
Biomass R&D Technical Advisory Committee commissioned by
congress, proposed to replace the current U.S. petroleum con-
sumption up to 30% with biofuels by 2030.5
Bio-based polymers have been produced by different techniques
such as, the chemical or physical modification of natural poly-
mers, the synthesis of materials from biomass feedstock, or the
synthesis of materials from a combination of bio-based mono-
mers with non-bio-based molecules.16,17 Over the last decade,
several bio-based materials have been developed as alternative
to those made from petroleum. For example, suberin is a natural
polymer that is used to produce novel linear and crosslinked
bio-polyesters through a transesterification process.2 Bio-based
polymers have also been synthesized from vegetable oils,18
including castor oil,19,20 and soybean oil,21 for the production of
Bio-based polymers are described as polymers where some or
all parts of the monomer units are derived from renewable feed-
stock.6 They have attracted much attention in the polymer field
due to their tendency to degrade under ambient to moderate
conditions, thus eliminating the potential of accumulating toxic
waste and adding to the plastic pollution problem. Biodegradable
polymers such as polylactic acid and polyhydroxybutyrate have
been widely used in applications such as biomedical devices
Additional supporting information may be found in the online version of this article.
© 2018 Wiley Periodicals, Inc.
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polyesters. Recently, lignocellulosic biomass (lignin polyol) was
functionalized with propylene carbonate (PC) for use as replace-
ment to conventional monomers for the synthesis of polyure-
thanes and polyesters.22
dissolved in deuterated chloroform, while the polymers were
dissolved in CF3CO2D/CD2Cl2 (1:1) solvent.
Number Averaged Molecular Weight (Mn)
The Mn of the polymers were determined from the 1H NMR
spectroscopy data using end group analysis. The number of
the protons associated with the polymer and those associated
with the terminal groups of the monomer were determined
by the areas of the peaks. The following equations reported
by Izunobi et al.30 were used to determine n and Mn:
In addition to bio-based polymers, bio-based monomers have
also been used in the synthesis of new polymers.6,23–27 For
example, Miller and coworkers explored furandicarboxylic
acid (FDCA), a monomer that is obtained from lignin biomass,
as a potential replacement for terephthalic acid in the produc-
tion of poly(ethylene terephthalate) (PET),28 one of the most
abundant commodity plastic. Moreover, Mialon et al. also
reported some polyalkylenehydroxybenzoates (PAHBs) that
have thermal properties comparable to PET.28 The thermal
properties of the polymers were shown to vary with the num-
ber of alkoxy side chains and the length of the alkyl chain in
the polymer backbone. For example, the thermal processes
[glass transition (Tg), melting (Tm), and decomposition (Td)
temperatures] decreased with increasing methoxy side chains
(pHB > VEB > SEB), and increasing alkyl chain length. It was
previously shown that ꢀpolyethylene vanillate (PEV) with a Tg
ap ꢁme ꢁne
np =
ð1Þ
ð2Þ
ae ꢁmp
Mn = npMp + Me
where, np is the number of repeating units (DP), ap and mp
are the area of the 1H NMR and number of the protons for the
repeating units, respectively. Likewise, me, ne, and ae are the
number of protons, number of units, and area of the
unreacted end group protons, respectively. In the formula for
the calculation of Mn, Mp is the molecular weight of one
repeating unit, and Me is the combine molecular weights of
the end-groups.
ꢀ
≈ 71 C and Tm ≈ 239 C displayed thermal properties closest
29
ꢀ
ꢀ
to PET (Tg ≈ 67 C and Tm ≈ 265 C).
Thermogravimetric Analysis (TGA)
Interest in our group is focused on developing degradable poly-
mers from renewable resources as alternatives to the petroleum-
based ones. Inspired by the work of Miller and coworkers, we
investigated the ether-amide functionality of PAHBs derived from
lignin as a way to further tune their thermal properties toward
applications for packaging, consumer goods, and coatings mate-
rials. Critical to these applications are the thermal properties (Td,
Tg, and Tm). In addition to developing materials for our particular
applications interest, we desired to develop materials that are
environmentally friendly, and can be subjected to degradation
under mild environmental conditions. This manuscript presents
the synthesis, thermal properties and degradability of two
poly(ether amide)s that were synthesized from lignin-derived
monomers. The polymers were characterized by 1H NMR, molec-
ular weight (using 1H NMR end-group analysis), and FTIR.
TGA analyses were carried out using a Q50 (TA Instrument)
to test the thermal stability of the monomers and polymers.
The samples were loaded in the form of a powder (3–10 mg).
Samples were heated from room temperature to 600 C under
dry nitrogen at a constant heating rate of 10 ꢀC/min.
ꢀ
Differential Scanning Calorimetric (DSC)
DSC runs were carried out on a Q200 (TA Instrument) at a
ꢀ
heating and cooling rate of 10 C/min. The thermal history
ꢀ
was erased by heating the samples up to 200 C and holding
it for 3 min. Then the samples were cooled to 0 C to create a
standard thermal history. Subsequently, a second heating run
was performed up to the desired temperature, which was
used in the analysis.
ꢀ
Fourier Transform Infrared Spectroscopy (FTIR)
EXPERIMENTAL
ATR-FTIR was recorded on a Cary 630 (Agilent technologies)
fitted with a Diamond Attenuated Total Reflectance (ATR).
The powdered sample was used without modification.
Materials
Methyl 3-methoxy-4-hydroxybenzoate (99% purity; Sigma-
Aldrich), methyl 4-hydroxybenzoate (97% purity; Sigma-
Aldrich), 2-chloroethyl amine hydrochloride (98%; Acros
Organics), antimony (III) oxide (99.6% purity; Alfa Aesar), and
di-tert-butyl dicarbonate (Oakwood Chemicals) were pur-
chased and used without further purification. All solvents were
reagent grade and used as such unless otherwise mentioned.
Dry and degassed CH2Cl2 was obtained from a VAC solvent
purification system.
Degradation Study
The polymer sample (PEAV) was diluted with various acidic
solutions to a final concentration of 1 mg/mL and sonicated
for 30 min. The polymer solutions were allowed to stir at
room temperature for 72 h. Following the degradation time
period, the polymer samples were filtered and washed with
copious amount of water then dried under high vacuum
before being analyzed.
Characterization
Nuclear Magnetic Resonance Spectroscopy (1H NMR and
13C NMR)
Synthesis of Monomer
Synthesis of Tert-Butyl (2-Chloroethyl) Carbamate (3)
To a solution of BOC2O (2 equiv.) and 2-chloroethyl amine
hydrochloride (1.0 equiv.) in dry CH2Cl2 at room temperature
1
The H NMR spectra were recorded using Bruker AVANCE III
300 MHz spectrometer. Compounds 3, 5, and
6 were
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was added Et3N (174 mmol) and the reaction was stirred
under inert atmosphere for 72 h. At the end of the reaction,
the solution was washed with 2.0 M NaOH, water, dried over
anhydrous MgSO4, filtered, and concentrated under reduced
pressure to give compound 3 as a viscous liquid that solidifies
at temperature slightly below room temperature in 98% yield.
1H NMR (300 MHz, CDCl3) δ 4.94 (s, 1H), 3.59 (t, J = 5.5 Hz,
2H), 3.58–3.45 (m, 2H), 1.45 (s, 9H).
TABLE 1 Polymerization Results and Characterization of PEAB
and PEAV
Final Polym
Time,
Entry Polymer h/Tp 5 (ꢀC)
T
50% (ꢀC)
T
g (ꢀC) DP
Mn
1
2
3
4
5
6
PEAB
1/220
1/245
2/245
2/220
5/200
2/225
366
350
375
312
331
354
115
120
124
89
9
1660
4160
21
ND* ND*
Synthesis of Methyl 4-{2-[(Tert-Butoxycarbonyl)Amino]
Ethoxy} Benzoate (5a) and Methyl 4-{2-[(Tert-
Butoxycarbonyl)Amino] Ethoxy}-3-Methoxybenzoate (5b)
PEAV
18
25
63
3690
105
118
5045
To a solution of compound 3 (1.0 equiv.) and methyl 4-hydro-
xybenzoate (4a) or methyl 3-methoxy-4-hydroxybenzoate
(4b) (0.8 equiv.) in DMF was added anhydrous potassium car-
12,410
*Mn was not determined due to the partial solubility of the polymer in
the NMR solvent.
ꢀ
bonate (2.0 equiv.) and the reaction was stirred at 80 C over-
night. The reaction mixture was diluted with water and
extracted with EtOAc. The combine organic layer was washed
with 2.0 M NaOH, dried over anhydrous Na2SO4, filtered and
concentrated under reduced pressure to give the product. (5a,
83%) 1H NMR (300 MHz, CDCl3, δ): 7.99 (d, J = 8.9 Hz, 2H,
Ar), 6.91 (d, J = 8.9 Hz, 2H, Ar), 4.07 (t, J = 5.1 Hz, 2H, CH2),
3.89 (s, 3H, CH3), 3.55 (dd, J = 10.6, 5.3 Hz, 2H, CH2), 1.45 (s,
9H, 3CH3). 13C NMR (500 MHz, Acetone-d6, δ): 167.0, 162.9,
157.1, 131.2, 122.3, 113.9, 78.9, 66.8, 51.0, 39.5, 27.3 (5b,
ꢀ
250 C. The reaction mixture was held at its final temperature
according to the temperature time reported in Table 1. The
polymer samples were dissolved in 1:1 dichloromethane/tri-
fluoroacetic acid (CD2Cl2/TFA) solution and precipitated from
methanol. The polymers were re-precipitated two times from
ethanol and then dried under vacuum for 24 h before
analyzed.
1
96%) H NMR (300 MHz, CDCl3, δ): 7.65 (dd, J = 8.4, 1.8 Hz,
RESULTS AND DISCUSSION
1H, Ar), 7.56 (d, J = 1.6 Hz, 1H, Ar), 6.90 (d, J = 8.3 Hz, 1H,
Ar), 4.13 (t, J = 5.1 Hz, 2H, CH2), 3.92 (s, 3H, CH3), 3.90 (s, 3H,
CH3), 3.58 (dd, J = 10.8, 5.5 Hz, 2H, CH2), 1.45 (s, 9H, 3CH3).
13C NMR (500 MHz, Acetone-d6) δ: 167.0 157.0, 152.5, 148.9,
123.4, 122.6, 112.3, 111.9, 78.9, 67.7, 55.1, 51.1, 39.4, 27.3.
Polymers Synthesis
The synthesis of the polymers follow a very simple three-step
procedure (Scheme 1). Methyl-4-hydroxybenzoate (4a) and
methyl 3-methoxy-4-hydroxybenzoate (4b), purchased from
Sigma-Aldrich and used as a substitute for the lignin-derived
precursors, were alkylated at the phenolic oxygen with tert-
butyl (2-chloroethyl) carbamate (3) forming methyl 4-{2-[(tert-
butoxycarbonyl)amino] ethoxy}benzoate (5a) and methyl
4-{2-[(tert-butoxycarbonyl)amino]- ethoxy)-3-methoxy-benzoate
(5b), respectively. The Boc-protected amino groups were depro-
tected with an acidic mixture of HCl in ethyl acetate to give the
monomers, methyl 4-(2-aminoethoxy)benzoate (EAB, 6a) and
methyl 4-(2-aminoethoxy)-3-methoxybenzoate (EAV, 6b) follow-
ing deprotonation of the amine salt.31 Common deprotection
conditions such as TFA/CH2Cl2 and HCl/MeOH resulted in low
yield of the amine. The products, which are AB monomers were
then subjected to the ꢀmelt polycondensation under dynamic
Synthesis of Methyl 4-(2-Aminoethoxy)Benzoate (6a) and
Methyl 4-(2-Aminoethoxy)-3-Methoxybenzoate (6b)
Compound 5a or 5b (1 equiv.) was dissolved in a solution of
HCl/EtOAc (4 equiv. 12.1 M) and stirred at room temperature
for 1–3 h. The mixture was then taken up in water until all
the solids were completely dissolved and then washed with
EtOAc. The aqueous layer was basified using 1 equiv. 2.0 M
NaOH, solution and then extracted with EtOAC, dried with
MgSO4, filtered and concentrated under reduced pressure to
give compound 6 as a white crystalline solid. (6a, 81%) 1H
NMR (300 MHz, CDCl3, δ): 7.99 (d, J = 8.8 Hz, 2H, Ar), 6.92 (d,
J = 8.8 Hz, 2H, Ar), 4.05 (t, J = 5.1 Hz, 2H, CH2), 3.88 (s, 3H,
CH3), 3.12 (t, J = 5.0 Hz, 2H, CH2). 13C NMR (500 MHz, CDCl3,
δ): 166.9, 162.7, 131.6, 122.7, 114.1, 70.3, 51.9, 41.4. (6b,
ꢀ
vacuum between 210 C and 245 C catalyzed by antimony
1
85%) H NMR (300 MHz, CDCl3, δ): 7.65 (dd, J = 8.4, 1.6 Hz,
1H, Ar), 7.55 (d, J = 1.6 Hz, 1H, Ar), 6.89 (d, J = 8.4 Hz, 1H,
Ar), 4.11 (t, J = 5.0 Hz, 2H, CH2), 3.91 (s, 3H, CH3), 3.89 (s, 3H,
CH3), 3.18 (t, J = 5.0 Hz, 2H, CH2,). 13C NMR (500 MHz, CDCl3,
δ): 166.9, 152.4, 149.0, 113.5, 112.9, 112.4, 111.9, 71.2, 56.0,
52.0, 41.3.
Synthesis of Poly(Ether Amide)s
Compounds 6a and 6b with Sb2O3 (0.07 equiv.) were sepa-
rately dried on high vacuum overnight. The mixtures were
transferred to a Schlenk tube, and the contents were heated at
ꢀ
140 C under inert atmosphere for 2 h followed by applica-
SCHEME 1 Synthetic scheme for preparing the PEAB and PEAV
monomers and poly(ether amide)s PEAV and PEAB.
ꢀ
tion of dynamic vacuum while slowly heating to 220 C–
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oxide. Following polymerization, the polymers were dissolved in
a minimal amount of TFA:CH2Cl2 (1:1) solution and precipitated
from methanol. This process was repeated twice with precipita-
tion from ethanol. The precipitates were dried in a vacuum oven
1
for 24 h before H NMR and FTIR spectroscopy, TGA, and DSC
analyses were performed.
1H NMR
1
The H NMR spectra for PEAB monomer (6a) and its polymer
(7a) are shown in Figure 1(A). The chemical shifts for the cor-
responding protons are identified in the spectra. Of particular
interest to our analysis are the protons whose chemical shifts
changed significantly following the polymerization process
due to the reactivity of their neighboring groups. In that
regards, the methylene protons ( CH2 NH ) labeled “a” in
1
the H NMR spectrum of the monomer resonating as a broad
singlet at 3.75 ppm have shifted downfield. These protons
1
labeled a0 resonate around 4.05 ppm in the H NMR spectrum
of the polymer. This chemical shifting in the peak for proton
“a” is a result of the free amine group reacting with the
methyl ester to form an amide bond in the polymer. The peak
for the methoxy group of the ester in the monomer (labeled
“g”) has decreased dramatically in the polymer; however, it is
still detectable suggesting a moderate conversion in the poly-
merization. Another set of protons that shifted significantly
after the polymerization are the protons labeled “d” resonat-
ing at 8.05 ppm that are ortho to the carbonyl group on the
benzene ring in the monomer. These protons are labeled d0 in
the polymer and resonate at 7.85 ppm, which is an up field
shift compared to the protons from the monomer. The other
FIGURE 1 1H NMR assignment of: PEAB (A) and PEAV (B).
protons representing the CH2
O
(labeled “b”) and the two
C H protons ortho to the alkoxy group on the benzene ring
(labeled “c”) in the monomer did not have significant change
in their chemical shifts going form monomer to polymer. A
similar result is seen for the PEAV series [Fig. 1(B)]. The
chemical shifts for the methylene protons ( CH2 NH )
labeled “a” in the monomer resonate as a broad singlet in
TFAd:CD2Cl2 (1:1) around 4.07 ppm have also shifted down-
field underneath the methoxy peak labeled “g” and “f” in the
monomer and g0 and f0 in the polymer. The protons “d” and
“e” ortho to the carbonyl group on the benzene ring, reso-
nated around 7.75 and 7.85 ppm, respectively, in the mono-
mer, shifted up field, and resonate as a broad peak around
7.50 ppm corresponding to both protons labeled d0 and e0,
respectively, in the polymer. Once again, all the other protons
present in the structure did not have significant chemical
shifts going from monomer to polymer. However, the methoxy
groups from the methyl ester in the monomer decreased in
intensity; albeit, it is difficult to see due to its overlap with
proton a0 and f0 in the polymer.
quickly evident that our polymers with the highest percent
conversion were only partially soluble in the solvent systems
that were investigated, which led to inconsistent results. The
solvent systems that were investigated for intrinsic viscosity
include TFA, 1,2-dichloroethane, and phenol in various combi-
nations. We also investigated concentrated sulfuric acid as a
solvent for viscosity measurements, as reported in several lit-
eratures;32 however, in our case, decomposition of the poly-
mers occurred leading to irreproducible results. Consequently,
the molecular weights of the polymers were estimated using
end-group analysis based on the area of the integrated peaks
of the unreacted end groups and the polymer repeat unit in
the 1H NMR spectrum. To estimate Mn, we first determined
the number of repeating units (n), which was used to calcu-
late the Mn by the summation of the atomic masses of the con-
stituents of the polymer.30 The 1H NMR spectra that were
used in the analysis of Mn are shown in the Supporting Infor-
mation, and the protons that were used in the calculation are
based on the aromatic protons adjacent to the carbonyl group,
that is, “d” compared with d0 for PEAB, and “d” and “e” com-
pared with d0 and e0 for PEAV. These protons are well-
resolved, with a significant change in their chemical shifts
upon formation of the product, thus making them good candi-
dates for the analysisꢀ. When the polymerization was carried
out for PEAB at 220 C while applying a steady vacuum for
1 h, a Mn of approximately 1600 g/mol was observed
Molecular Weight Analysis
Polymerization optimization was performed using several
melt and polymerization temperatures and times (Supporting
1
Information). The reactions were monitored by H NMR spec-
troscopy to determine the percent conversion. It is worth not-
ing that viscosity measurements were explored to determine
the intrinsic viscosity of the polymers; however, it became
4
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(Table 1, entry 1). However, when the polymerization temper-
ature was increased to 245 C while applying a steady vac-
observed for our polymers. All polymers displayed one major
transition signifying a uniform polymeric structure. In the
PEAB series, the highest molecular weight polymer has a Td of
ꢀ
uum for 1 h, the calculated Mn for PEAB under these
conditions was increased to approximately 4100 g/mol
(Table 1, entry 2). Actually, when the polymerization time
ꢀ
375 C at 50% weight loss and an onset of decomposition at
ꢀ
ꢀ
300 C. The Td for the PEAV series at 50% weight loss ꢀis
ꢀ
was increased to 2 h at the 245 C polymerization tempera-
352 C with an onset of decomposition also around 300 C
ture, the polymer became partially insoluble in the NMR sol-
vent and only the lower molecular weight polymers were
observed in the H NMR spectrum; therefore an accurate esti-
[Fig. 2(A,B)].
1
The DSC results are consistent with what is seen by the TGA
data. The DSC data were collected after a heat-cool-heat cycꢀle,
where the first heating temperature was ramped up to 200 C
and held for 3 min before cooling and ramping a second time
mation of the Mn was not possible with this technique
(Table 1, entry 3). A similar result was obtained for the PEAV
polymer, where the polymerization was carried out at the
desired temperature, while applying a steady vacuum for
2 h. The highest Mn obtained for this polymer was calculated
to be 12,400 g/mol with DP of 63 units (Table 1, entry 6).
ꢀ
up to 300 C [Fig. 3(A,B)]. The second heating cycle was used
to observe the and Tg the Tm. A clear Tg was observed for all
the polymers that follows the trend in the Mn. We were
ꢀ
pleased to see that the Tg for our polymers exceeded 100 C,
which is comparable to most materials used in the packaging,
storage, and coating industry. The polymers seem to be very
amorphous with very small endothermic transitions above
Thermal Properties: TGA and DSC
The thermal properties of the polymers were investigated.
Since several factors such as sample type, size, morphology,
and heating rate can affect the shape and transition tempera-
tures of the decomposition curve, we endeavored to control
as many factors as possible. Decomposition temperatures typi-
cally increased with increased Mn and this trend was also
ꢀ
200 C that could represent melting. However, due to the
small size of the transitions and the close proximity to the
onset of decomposition temperatures, we cannot conclude
that those transitions are a result of melting. Consequently,
FIGURE 2 TGA thermograms of: PEAB (A) and PEAV (B). [Color
figure can be viewed at wileyonlinelibrary.com]
FIGURE 3 The second heating DSC curves of: PEAB (A) and
PEAV (B). [Color figure can be viewed at wileyonlinelibrary.com]
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we will further explore these transitions using a more definite
technique such as wide-angle X-ray scattering (WAXS). The
sharp peaks in the curves for entries 4 and 6 go away upon
heating the polymers above their Tg for 1 h. In general, the
thermal processes (Td and Tg) of the PEAB series occurred at
a higher temperature than the PEAV series; albeit, the Mn of
the PEAB polymers were estimated to be smaller than the
PEAV polymers. These results correlate well with the work of
Miller and coworkers,28 where the methoxy groups on the
benzene ring in the vanillate and syringate series resulted in a
decrease of the thermal properties, probably due to a less
ordered structure of those polymers.
The Fourier Transform Infrared (FTIR) Spectroscopy
The FTIR spectroscopy data is shown in Figure 4. The promi-
nent bands in the FTIR spectra are very similar for both poly-
mers. The carbonyl stretching frequency (C O stretch) of the
ester group around 1710 cm−1 in the monomer (Supporting
Information) has disappeared, while the formation of a new
peak around 1640 cm−1 representing the amide I band for
secondary amides in the solid state is observed for both PEAB
and PEAV. The amide II band around 1570–1515 cm−1 can
also be observed around 1540–1530 cm−1 in both spectra.
Other common features in the FTIR spectra that represent the
expected functional groups are also present. For example, the
N H stretching frequency around 3200–3300 cm−1, as well as
the sp3 C H stretch around 2930 cm−1 and the sp2 C H
stretch from the aromatic ring around 3078 cm−1 are also evi-
dent in the spectra.
FIGURE 5 1H NMR spectra of PEAV after acidic treatment.
1
chemical stability under these conditions. The H NMR analy-
sis (Fig. 5) and the TGA (Fig. 6) were used to observe the
degradation.
The results revealed that PEAV degrades substantially in 4 M
H2SO4, HNO3, and TFA as shown by the decreased in the aro-
1
matic protons’ intensity in the H NMR spectra of these poly-
mers, as well as broadening or decrease in peak intensity in
the aliphatic region. The structures of the resulting materials
are undeterminable from the 1H NMR spectra; however, it is
well known that amide bonds are susceptible to hydrolysis
under acidic conditions and thus we propose this mechanism
for the degradation process.
Acid Degradation Study
During the characterization of the polymers, it was found that
they were susceptible to degradation in concentrated TFA
solution over time (Supporting Information). This prompted
us to investigate this phenomenon further. PEAV was sub-
jected to four different acidic solutions at 4 M concentration
and one buffer solution at pH 4 for 72 h to determine its
The Td for the H2SO4, HNO3, and TFA treated PEAVs have also
decreased significantly in their onset of decomposition tem-
perature compared to the untreated polymer (Fig. 6). We con-
cluded that the drastic change in the thermal integrity of the
polymer under these conditions is a result of a substantial
change in the polymer’s structure due to degradation. On the
other hand, no degradation was observed for the HCl and the
buffer treated PEAV based on both the 1H NMR and Td data. It
is our belief that the lack of degradation in the 4 M HCl and
pH 4 buffer solution is due to a lack of solubility of the
FIGURE 4 FTIR spectra of PEAB and PEAV. [Color figure can be
viewed at wileyonlinelibrary.com]
FIGURE 6 TGA thermograms of PEAV after acidic treatment.
[Color figure can be viewed at wileyonlinelibrary.com]
6
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JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ORIGINAL ARTICLE
9 S. Mitsuhiro, O. Koichi, T. Naozumi, Y. Ryutoku, T. Hiroyuku,
Macromol. Mater. Eng. 2003, 288, 35.
polymer in these solvents. The slightly higher degradation
temperatures of the polymer treated with 4 M HCl and pH
4 buffer solution compared with the untreated polymer is
unclear. Importantly, these studies revealed that PEAV is
degradable in strong acidic conditions in a very short time
frame. An extensive study will be conducted to determine the
process of degradation and the extent of degradation at rele-
vant environmental conditions.
10 K. Van de Velde, P. Kiekens, Polym. Test. 2002, 21, 433.
11 B. M. Upton, A. M. Kasko, Chem. Rev. 2016, 116, 2275.
12 J. Lora, In Monomers, Polymers and Composites from
Renewable Resources; M. N. Belgacem, A. Gandini, Eds.; Else-
vier Ltd: Amsterdam, The Netherlands, 2008; pp 225–242.
13 A. B. Gandini, M. N. Belgacem, In Monomers, Polymers and
Composites from Renewable Resources, M. N. Belgacem,
A. Gandini, Eds. Elsevier Ltd.: Amsterdam, The Netherlands,
2008; pp 243–271.
CONCLUSIONS
14 Polyethylene Terephthalate Global Production and Market.
We have synthesized two poly(ether amide)s from lignin-
derived precursors and showed that they are thermally stable
http://www.chemmarket.info/en/home/article/2422/
4/17/2018).
(accessed
ꢀ
up to 250 C with less than 5% of the polymer decomposed.
15 R. A. Pérez–Camargo, G. Saenz, S. Laurichesse, M. T. Casas,
J. Puiggalí, L. Avérous, A. J. Müller, J. Polym. Sci. Part B: Polym.
Phys. 2015, 53, 1736.
ꢀ
The polymers also exhibit Tg above 100 C; therefore, their
thermal properties are ideal for potential applications in the
packaging, storage, and coating industry. In addition to their
great thermal properties, we have shown that PEAV can be
significantly degraded in acidic condition in a short time, mak-
ing that polymer an ideal candidate to be explored as an alter-
native sustainable thermoplastic with degradable features for
use in the packaging and storage industry.
16 A. Gandini, T. M. Lacerda, Prog. Polym. Sci. 2015, 48, 1.
17 Y. Jiang, K. Loos, Polymers 2016, 8, 243.
18 N. F. Abd. Rahim, K. Watanabe, H. Ariffin, Y. Andou,
M. A. Hassan, Y. Shirai, Chem. Lett. 2014, 43, 1517–1519.
19 X. Kong, H. Qi, J. M. Curtis, J. Appl. Polym. Sci. 2014, 131,
40579.
20 X. Hu, X. Shen, M. Huang, C. Liu, Y. Geng, R. Wang, R. Xu,
H. Qiao, L. Zhang, Polymer (United Kingdom) 2016, 84, 343.
ACKNOWLEDGMENTS
21 T. A. Hassan, V. K. Rangari, S. Jeelani, J. Appl. Polym. Sci.
2013, 130, 1442.
The authors thank Mississippi State University for support of
this project.
22 S. L. Bell, Bio-Based Polymers; New York: IHS Chemical Pro-
cess Economics Program, 2013.
23 P. Gopalakrishnan, S. Narayan-Sarathy, T. Ghosh,
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