Pressure-sensitive adhesives from renewable triblock copolymers

Article abstract of DOI:10.1021/ma102216d

ABA triblock copolymers were prepared using the renewable monomers menthide and lactide, by sequential ring-opening polymerizations. Initially, hydroxy telechelic polymenthide was synthesized by the diethylene glycol-initiated and tin(II) ethylhexanoate-catalyzed polymerization of menthide. The resulting 100 kg mol^-1 polymer was used as a macroinitiator for the tin(II) ethylhexanoate-catalyzed ring-opening polymerization of d,l-lactide. Two polylactide-polymenthide-polylactide triblock copolymers were prepared with 5 and 10 kg mol^-1 polylactide end blocks. Transesterification between the two blocks, and polylactide homopolymer formation were minimized, and triblock copolymers with narrow molecular weight distributions were produced. Microphase separation in these systems was corroborated by differential scanning calorimetry and small-angle X-ray scattering measurements. The triblocks were combined with up to 60 wt % of a renewable tackifier, and the resulting mixtures were evaluated using probe tack, 180° peel adhesion, and shear strength tests. Maximum values of peel adhesion (3.2 N cm^-1) and tack (1.1 N) were obtained at 40 wt % of tackifier. These new materials hold promise as renewable and hydrolytically degradable pressure-sensitive adhesives.

Full text of DOI:10.1021/ma102216d

Macromolecules 2011, 44, 87–94 87
DOI: 10.1021/ma102216d
Pressure-Sensitive Adhesives from Renewable Triblock Copolymers
Jihoon Shin, Mark T. Martello, Mona Shrestha, Jane E. Wissinger,
William B. Tolman,* and Marc A. Hillmyer*
Department of Chemistry and Center for Sustainable Polymers University of Minnesota, 207 Pleasant Street
SE, Minneapolis, Minnesota 55455-0431, United States
Received September 24, 2010; Revised Manuscript Received November 18, 2010
ABSTRACT: ABA triblock copolymers were prepared using the renewable monomers menthide and lactide,
by sequential ring-opening polymerizations. Initially, hydroxy telechelic polymenthide was synthesized by the
diethylene glycol-initiated and tin(II) ethylhexanoate-catalyzed polymerization of menthide. The resulting
100 kg mol-1 polymer was used as a macroinitiator for the tin(II) ethylhexanoate-catalyzed ring-opening
polymerization of D,L-lactide. Two polylactide-polymenthide-polylactide triblock copolymers were prepared
with 5 and 10 kg mol-1 polylactide end blocks. Transesterification between the two blocks, and polylactide
homopolymer formation were minimized, and triblock copolymers with narrow molecular weight distributions
were produced. Microphase separation in these systems was corroborated by differential scanning calorimetry
and small-angle X-ray scattering measurements. The triblocks were combined with up to 60 wt % of a renewable
tackifier, and the resulting mixtures were evaluated using probe tack, 180° peel adhesion, and shear strength tests.
Maximum values of peel adhesion (3.2 N cm-1) and tack (1.1 N) were obtained at 40 wt % of tackifier. These new
materials hold promise as renewable and hydrolytically degradable pressure-sensitive adhesives.
Polymeric materials derived from renewable resources have
received significant attention due to environmental and other
concerns associated with the use of petroleum-based products.1-3
Currently, renewable resource polymers are used for biomedical
applications, food packing, and disposable items, but their some-
what limited property profiles limit their widespread application.
Pressure-sensitive adhesives (PSAs) represent a potential applica-
tion (e.g., tapes, labels, stamps, and sticky notes) that would
benefit from the use of renewable materials.4-6 A distinctive
characteristic of PSAs is that they exhibit permanent tackiness
and the ability to wet surfaces on contact. PSAs are typically
formulated with tackifiers, plasticizers, and fillers mixed with an
elastomeric polymer such as a polyacrylate, natural rubber, a
silicone, or a styrenic block copolymer (SBC).7,8 Commonly used
SBCs9,10 have an ABA architecture comprised of a soft, rubbery,
low-Tg segment (B) flanked by two hard (high-Tg) end blocks
(A).11 At ambient temperature when microphase separated, the
end blocks can act as physical cross-links to form a network of
elastically effective B-strands responsible for superior resistance
to flow.12 For removable PSAs this property is essential to
minimize residue after the adhesive is removed. Tackifying resins
miscible with the midblock improve PSA performance,13 while
plasticizers reduce cost, decrease hardness, and improve low-
temperature flexibility.14 PSAs include solvent-based, hot-melt,
and emulsion-based formulations.15
large particles and dispersible/soluble PSAs which could be
dissolved in the recycling process. Recently, polymeric materials
from cationic butyl acrylates17 and sugar-based acrylates18,19
have been explored as water-soluble PSAs.
Herein, we report the preparation and characterization of
a new PSA system composed of all renewable polylactide-
polymenthide-polylactide (PLA-PM-PLA) triblock copolymers
combined with a renewable rosin ester tackifier. In previous
work, the ring-opening polymerization of menthide (M) was demon-
strated,20 and PLA-PM-PLA triblocks containing 20-50 wt %
PLA were synthesized using ZnEt2 and AlEt3 catalysts in a two-
step polymerization process.21 These microphase-separated tri-
blocks behaved as thermoplastic elastomers and were shown to be
susceptible to hydrolytic degradation.22,23 On the basis of previous
literature, we hypothesized that materials suitable as elastomers
for PSA formulations could be attained if the PM segment were
lengthened and the PLA content minimized.12,24 We also sought to
improve the protocol for synthesis of such triblocks to render the
process greener and amenable to large-scale preparations needed
for property testing of PSA formulations. Herein we report (a) new
methods for preparing PLA-PM-PLA triblocks with 100 kg mol-1
PM segments with narrow molecular weight distributions, (b)
1
characterization of these triblocks using H and 13C NMR
spectroscopy, size exclusion chromatography (SEC) coupled with
multiangle light scattering (MALS), differential scanning calorim-
etry (DSC), tensile testing, and small-angle X-ray scattering
(SAXS), and (c) combination of these triblocks with variable
amounts of a renewable rosin ester tackifier to yield all-renewable
PSA formulations with excellent adhesive performance as shown
by peel adhesion, probe tack, and shear strength tests.
In paper recycling PSAs attached to the paper can break down
into small pieces during repulping, resulting in a so-called
“stickies” problem for the recycling industry.16 Macrostickies
are effectively removed during screening and cleaning processes,
but microstickies are still left in the aqueous pulp solution,
leading to deposition on equipment and reduced paper quality.
To address these problems, two types of PSAs have been studied:
screenable/removable PSAs which are broken into relatively
Results and Discussion
Synthesis of Triblock Copolymers. a. Synthesis of Menthide.
In previous studies we showed that (-)-menthone could
be effectively oxidized to M using m-chloroperbenzoic acid
(m-CPBA) in a chlorinated solvent.20,21,25 We now report
*Corresponding authors. E-mail: wtolman@umn.edu (W.B.T.);
hillmyer@umn.edu (M.A.H.).
r 2010 American Chemical Society
Published on Web 12/07/2010
pubs.acs.org/Macromolecules

88 Macromolecules, Vol. 44, No. 1, 2011
Shin et al.
Scheme 1
the Baeyer-Villiger oxidation of (-)-menthone using
Oxone (potassium peroxymonosulfate) in an ethanol/water
mixture as a potentially greener alternative to the m-CPBA/
CHCl3 (Scheme 1) process with ∼80% conversion of starting
material; following distillation and sublimation, M was
obtained in greater than 70% isolated yield.26 Characteriza-
tion data for M were consistent with previous literature
reports.20,27
b. Preparation of Telechelic Polymenthide Midblock. R,ω-
Hydroxyl-functionalized polymenthide (PM) was prepared
from the ring-opening polymerization of M using tin(II)
ethylhexanoate (Sn(Oct)2) as the catalyst and diethylene
glycol (DEG) as a bifunctional initiator (Scheme 1). The
polymerization was carried out in the presence of a minimal
amount of toluene at 135 °C under an inert atmosphere for
1
72 h on a 50 g scale (see Experimental Section). H NMR
spectroscopy was used to monitor monomer conversion
(typically ≈95%), determine DEG chain connectivity, and
calculate Mn for PM.20,21 Conversion of the DEG into the
corresponding ester upon addition of M resulted in the
quantitative downfield shift of both sets of methylene reso-
nances (Hb and Hc, Figure 1). The methine protons at the
termini of the PM chains were also observed (Ha). Compar-
ison of the integration values for the methylene resonances
from the DEG initiator to the methine protons at the PM
chain ends gave a ratio of about 4 to 1, consistent with two
hydroxyl groups per chain and indicative of suppression of
adventitious initiation. On the basis of conversion (95%) and
the initial monomer-to-initiator content (635:1), the theore-
tical Mn for PM(100)A was 103 kg mol-1 (Table 1). A Mn
value of 100 kg mol-1, in good agreement with the theoretical
Figure 1. Expanded 1H NMR spectra (500 MHz, CDCl3) of (a)
diethylene glycol (DEG), (b) PM(100)A, and (c) PLA-PM-PLA-
(10-100-10)A.
c. Synthesis and Characterization of PLA-PM-PLA Tri-
block Copolymers. Typical SBCs used in PSA formulations
contain 10-30 wt % polystyrene (PS) as the minority com-
ponent.12,24 This requires the overall molecular weight, and
the PS block length, be large enough such that the PS
domains form hard physical cross-links. Accordingly, PLA
end blocks with Mn values of 5 and 10 kg mol-1 were
targeted. PM(100)A was used as a macroinitiator in the
ring-opening polymerization of DL-lactide (LA) catalyzed
by Sn(Oct)2 at 135 °C (20 h) in a minimal amount of toluene
under an inert atmosphere on a 20 g scale (Scheme 1). Upon
completion of the reaction, the signal for the terminal
methine protons of PM at δ 3.33 ppm was no longer present
in the 1H NMR spectrum, and a signal for the new terminal
methine protons of the PLA end blocks appeared at δ 4.36
ppm (Hd, Figure 1), consistent with efficient initiation by the
PM chain ends. As illustrated in Figure 1C, the relative
integration between the terminal PLA methine protons and
the DEG methylene protons corroborates the ABA archi-
tecture and suggests that contamination by PLA homo-
polymer is insignificant.
1
value, was determined by H NMR spectroscopy using the
relative integration between the methylene protons of the
DEG initiator fragment (8 protons in total) and the methine
protons from the polymenthide repeat units, assuming one
DEG moiety per chain. A similar result was obtained if the
integration values for the methine protons from the hydroxyl
end groups were used in the calculation. By SEC using PS
standards, the Mn = 95 kg mol-1 (PDI = 1.07) (Table 1 and
Figure 2a). The absolute Mn was measured by SEC equipped
with a MALS detector which gave a value of 91 kg mol-1
.
The molecular weight distribution was monomodal and
narrow with a PDI of 1.03 (Table 1). In sum, this simple
method for the preparation of PM with Sn(Oct)2 represents a
significant advance over our previously reported synthetic
scheme that relied on the use of diethylzinc.20,21

Article
Macromolecules, Vol. 44, No. 1, 2011 89
Table 1. Characterization Data for PM and PLA-PM-PLAa
Mn(theo)
(kg/mol)d
Mn(NMR)
(kg/mol)e
Mn(SEC)
(kg/mol) (PDI)f
Mn(SEC-LS)
(kg/mol) (PDI)g
h
polymer
[M]0/[I]0
fPLA
PM(100)A
PM(100)Bi
PLA-PM-PLA(5-100-5)A
PLA-PM-PLA(10-100-10)A
PLA-PM-PLA(10-100-10)Bi
635b
635b
104c
190c
190c
103
102
112
124
126
100
105
111
122
123
95 (1.07)
101 (1.09)
102 (1.07)
109 (1.09)
112 (1.13)
91 (1.03)
95 (1.04)
102 (1.04)
96 (1.05)
0.08
0.15
0.13
a See Experimental Section for details. b M is menthide and I is diethylene glycol (DEG) as a initiator. c M is D,L-lactide and I is polymenthide (PM) as a
macroinitiator. d Calculated on conversion from monomer to polymer on 1H NMR spectroscopy. e Calculated from relative integrations of PM
repeating units and DEG for PM and relative integrations of PLA and PM repeating units for PLA-PM-PLA by 1H NMR spectroscopy. f Determined
with size exclusion chromatography (SEC) in CHCl3 at 35 °C relative to polystyrene standards. g Determined by SEC with multiangle laser light
scattering (MALS). h Calculated volume fraction of PLA using the densityof PLA and PM as 1.248 and 0.975 g/cm3 at 25 °C, respectively. i Performed by
one-pot, two-step polymerization (sequential addition).
Figure 3. 13C NMR spectra for carbonyl carbons of (a) PM(100)A, (b)
PLA-PM-PLA(5-100-5)A, (c) PLA-PM-PLA(10-100-10)A, (d) PLA-
PM-PLA(10-100-10)B by one-pot, two synthetic process, and (e) PLA
(as a reference; poly( -lactide)).
Figure 2. Size exclusion chromatography data (1 mg/mL CDCl3) for
PM samples and PLA-PM-PLA triblock copolymers.
D,L
Prior to purification, 1H NMR spectroscopy of the crude
reaction mixture showed 79% and 88% conversion of LA
corresponding to a triblock composition of 11.5 wt % PLA
and 19.7 wt % PLA for PLA-PM-PLA(5-100-5)A and PLA-
PM-PLA(10-100-10)A, respectively. The purified triblock
compositions were 10.1 wt % PLA and 18.2 wt % PLA,
respectively. The theoretical total PLA Mn based on 79%
To simplify the triblock synthesis further, we developed a
one-pot two-step method. As shown in Scheme 1, M was
first polymerized using the DEG/Sn(Oct)2/toluene system to
give the corresponding macroinitiator, which was not iso-
lated. Instead, LA and additional toluene were then added to
the reaction mixture, and heating was continued. The con-
1
version of M by H NMR spectroscopy just prior to the
conversion of LA for PLA-PM-PLA(5-100-5)A was 12 kg mol-1
,
addition of LA was 94%; this observation is consistent with
the M polymerization reaching its equilibrium conversion
(95% at 135 °C).20 The conversion of LA reached 87% after
20 h at 135 °C. Similarly, the preparation of PLA-PM-
PLA(10-100-10)A reached 88% conversion under nearly
identical conditions. The residual M content was a constant
∼5% before and after the polymerization of LA, suggesting
that M present during the polymerization of LA is not
incorporated in the PLA end blocks. These results are similar
to those reported for ε-caprolactone/LA copolymerizations
where significant ε-caprolactone conversion does not take
place until almost all of the LA has been depleted.30 PLA-
PM-PLA(10-100-10)B also possessed only two distinct re-
sonances for PM and PLA at 172.9 and 169.3 ppm in the 13C
NMR spectrum, indicating minimal transesterification
(Figure 3d) and little incorporation of unreacted M into
the PLA end blocks.30 The Mn and PDI values (SEC, PS
standards) of PMand PLA-PM-PLA by the one-pot method
were determined to be 101 kg mol-1 (PDI 1.09) and 112 kg mol-1
(PDI 1.13) (Figure 2d,e), respectively. These values are
nearly identical to those obtained by the “two pot, two step
procedure” described above. Furthermore, the SEC data are
consistent with negligible PLA homopolymer byproducts
from fortuitous initiators.
which compares well to a measured PLA Mn of 11 kg mol-1
calculated using the integration values for the PLA and PM
backbone methine protons. For PLA-PM-PLA(10-100-
10)A, the theoretical total PLA Mn values based on 88%
conversion of LA and the measured PLA Mn were 24 and
22 kg mol-1, respectively. Analysis of the triblocks by SEC/
SEC-LS showed a shift toward lower elution volumes com-
pared to the PM homopolymer, and the absence of low
molecular weight material that would be indicative of PLA
homopolymer (Figure 2).
Transesterification between the PLA end blocks and the
PM midblock would compromise the ABA triblock archi-
tecture.28 To evaluate this possibility, 13C NMR spectra of
PM(100)A, PLA-PM-PLA(5-100-5)A, PLA-PM-PLA(10-
100-10)A, PLA-PM-PLA(10-100-10)B, and a PLA homo-
polymer (Mn = 19.4 kg mol-1 and PDI = 1.08) were compared
(Figure 3). The triblock copolymers possess only two distinct
resonances (172.9 and 169.3 ppm) associated with the carbonyl
groups of the PM and PLA blocks, consistent with minimal
transesterification (additional PM carbonyl peaks would be
observed if transesterification were significant).29 This
conclusion is also supported by with the observed narrow
molecular weight distributions (Figure 2).

90 Macromolecules, Vol. 44, No. 1, 2011
Shin et al.
Figure 5. Expanded storage moduli (G0) for PLA-PM-PLA tri-
Figure 4. DSC analysis for PM and PLA-PM-PLA triblock copolymers.
block copolymers at 1 rad s-1, a strain around 1%, and a ramp
rate of 3 °C min-1
.
In a separate experiment, the reaction mixture was held at
135 °C for 1 week after the addition of LA. Although some
broadening of the molecular weight distribution was observed
relative to the above experiment (PDI=1.14), no transesterifica-
tion was evident by 13C NMR spectroscopy (Figure S1). After
the 1 week at 135 °C, a second aliquot of LA was added to the
reaction mixture and allowed to react for an additional 20 h. The
PLA end blocks increased in molecular weight, confirming that
the tin catalyst was still active. Again there was some broadening
of the molecular weight distribution (PDI=1.18), but no evi-
dence for transesterification between the PM midblock and the
PLA end blocks. The reaction was then held at 135 °C for
another week, and no evidence for transesterification or incor-
poration of unreacted M into the PLA chains was observed
(PDI = 1.18). Collectively, these results are consistent with no
reaction of the propagating PLA chains with either M or PM.
Thermal Properties and Morphology. The microstructures
adopted by the triblock copolymers were studied by differ-
ential scanning calorimetry (DSC) and small-angle X-ray
scattering (SAXS). DSC analyses (Figure 4) of the triblock
copolymers show two glass transition temperatures at -22
and 53 °C, corresponding to the PM midblock and the PLA
end blocks, respectively. This suggests that the PLA end
blocks are not mixed in the PM phase. SAXS analysis
(Figure S2) at room temperature exhibited a weak principal
reflection that was also consistent with microphase separa-
tion of the triblock copolymer. The principal domain spa-
cings for PLA-PM-PLA(5-100-5)A and PLA-PM-PLA(10-
100-10)A were determined to be 59 and 62 nm, respectively.21
Evidence from both DSC and SAXS support a microphase-
separated structure lacking long-range order, where the PLA
segments form discrete glassy domains embedded in a rub-
bery PM matrix at room temperature.
Dynamic Mechanical Properties. An important property
for PSAs is the dynamic elastic modulus. To promote wetting
and contact between adhesive and substrate at the use
temperature PSAs should exhibit a dynamic elastic modulus
below the Dahlquist criterion (shear modulus G0 e 3 ꢀ 105 Pa
at 1 s-1 or alternatively tensile (Young’s) modulus, E e
1 ꢀ 105 Pa).31 The storage moduli (G0) of the two triblock
copolymers as a function of temperature at low strain (1%)
are shown in Figure 5. The precipitous drop in G0 at ∼ -20 °C
occurs around the Tg of the PM phase. As the temperature
is raised, a plateau in the storage modulus is observed until
the onset of a more gradual drop in G0 due to the glass to
rubber transition in the PLA block. The plateau moduli
(although not established with great precision)32 at 25 °C for
the two PLA-PM-PLA(5-100-5 and 10-100-10)A samples
are 2.0 ꢀ 105 and 3.5 ꢀ 105 Pa, respectively. From the plateau
Figure 6. DSC analysis for PLA-PM-PLA(10-100-10)A/rosin ester
(RE) tackifier blends.
modulus values at 25 °C, the entanglement molecular weight
Me ≈ 11-14 kg mol-1 for PM was approximated using the
Guth-Smallwood equation.33,34 These values are larger
than the Me reported for PI (∼7 kg mol-1), the midblock
in SIS block copolymers, and thus less additive would be
necessary to achieve even further reduction in the plateau
modulus and comparable PSA properties.35,36 To access
more effective PSA’s, the use of tackifiers was explored in
order to dilute the entanglements and thus lower the modulus
further.12,13,37
Adhesive Behavior. Through simple compatibility tests, we
determined that the rosin ester (RE) tackifier (Sylvalite 2E
80HP) was miscible with the triblocks (Figure S3). Eight PSA
formulations were prepared by the combination of PLA-
PM-PLA(5-100-5)A and PLA-PM-PLA(10-100-10)A with
0, 20, 40, and 60 wt % RE tackifier. DSC measurements
showed two glass transitions for these mixtures (Figure 6 and
Figure S4). Adding more tackifier led to broadening of the
glass transition temperatures and an increase in the Tg of
the PM midblock in the blends, relative to that of the pure
triblock copolymer. However, there was little change in the
Tg of the PLA end blocks in the blends (Figure S4). This is
consistent with our conclusion that the tackifier is miscible
with only the PM midblock, as expected given the more
hydrophobic nature of the PM midblock.37,38

Article
Macromolecules, Vol. 44, No. 1, 2011 91
Figure 9. Effect of RE tackifier content on the shear strength of the
PSA systems.
Figure 7. Effect ofRE tackifier content on the peel adhesion of the PSA
systems. In the case marked by an asterisk, the adhesive separated from
the backing (i.e., interfacial or adhesive failure occurred).
PSA, actually reducing tack.42 Finally, similar values from
the shear tests were observed regardless of the amount of
tackifier (Figure 9). Taken together, the probe tack, peel
adhesion, and shear strength test data demonstrate that
mixtures of PLA-PM-PLA triblock copolymers and com-
mercially available (and renewable) tackifiers are not only
effective PSAs but also exhibit adhesive properties that are
competitive with commercial samples (see Figures S5-S7).
Conclusions
A pressure-sensitive adhesives system was designed using
renewable triblock copolymers derived from natural products.
Using a green chemical approach, M was prepared using Oxone
and aqueous ethanol. The synthesis of the end-functionalizedPM
with controlled molar mass involved the ring-opening polymer-
ization of M by Sn(Oct)2 as the catalyst, giving the corresponding
macroinitiator. Subsequently, PLA-PM-PLA triblock copoly-
mers were synthesized as elastomer bases for PSA formulations,
using the same catalyst. The polymers were characterized by
NMR spectroscopy and size exclusion chromatography. All
polymers showed narrow molecular weight distributions. Phase
separation was elucidated with small-angle X-ray scattering and
differential scanning calorimetry. We determined that transester-
ification was minimized during polymerization of lactide from a
polymenthide macroinitiator, thus providing well-defined block
polymers. We also reported the one-pot, two-step polymerization
for the triblock copolymers. The elastomers were formulated with
a renewable rosin ester tackifier, and probe tack, peel adhesion,
and shear strength tests were carried out to evaluate the adhesive
properties. The adhesion results were consistent with excellent
adhesive performance, thus demonstrating a new application for
the all-renewable PLA-PM-PLA triblock copolymers.
Figure 8. Effect of RE tackifier content on the tack of the PSA systems.
Tack is the property that controls the instant formation of a
bonding interaction between adhesive and substrate when they
are brought into contact, peel strength is the force needed to
remove an adhesive from a substrate, and shear strength is the
internal resistance of an adhesive to flow under an applied
load.4,7 Results of these three types of adhesive tests for the eight
PSA systems are shown in Figures 7-9 (see Supporting In-
formation for details and comparisons to commercial PSA
tapes).39-41 The triblock copolymers exhibited some degree of
peel adhesion and tack without adding tackifier (consistent with
the low modulus values show in Figure 5), and the adhesive
properties increased with increasing RE tackifier content, ulti-
mately reaching values of peel adhesion = 3.2 N cm-1, tack =
1.1 N, and shear strength ≈ 2500 min at 40 wt % tackifier. By
comparison, tests on commercial duct, paper, and electrical
tapes under identical conditions gave values of peel adhesion =
1.9-4.2 N cm-1, tack = 0.4-0.6 N, and shear strength ≈ 1500
min (for most samples) as shown in Figures S5-S7. These
results for the adhesive properties of our renewable PSA show
that they are comparable to commercial samples.
Experimental Section
Materials. All air- or moisture-sensitive compounds were
handled under a nitrogen atmosphere in a glovebox, as indi-
cated. Toluene was dried using sodium and benzophenone,
distilled under reduced pressure, and stored in a nitrogen-filled
glovebox. Diethylene glycol (Aldrich) used for polymerizations
was distilled under reduced pressure over sodium and stored in a
glovebox. Tin(II) 2-ethylhexanoate (Aldrich) used for polymer-
ization was also distilled using a Kugelrohr apparatus followed
by storage in a glovebox. D,L-Lactide (Purac) was recrystallized
twice from toluene prior to being stored in a glovebox. (-)-
Menthone (90%), Oxone, and m-chloroperoxybenzoic acid
(m-CPBA, e77%) were used as received from Aldrich without
further purification. The rosin ester tackifier, Sylvalite 2E 80HP,
and the polyterpene tackifier, Sylvares TR B115, were obtained
After the peel test of samples with 60% RE, interfacial
failure (adhesive failure) was evident (residue left on the
stainless steel test plate). Also, the probe tack test results for
this blend showed a large standard deviation. Evidently,
while tackifiers can improve the cohesive strength in PSAs
by the formation of a tackifier-rich soft phase,12,37,38 too
much tackifier (60 wt %) can lead to increases in the Tg of the

92 Macromolecules, Vol. 44, No. 1, 2011
Shin et al.
from Arizona Chemical. All other solvents and reagents were
used as received from the commercial source without further
purification. The backing for the adhesive testing was poly-
(ethylene terephthalate) sheet (PET, thickness: 50 μm).
Oxone (49.2 g, 160 mmol) in water (200 mL) was added via
syringe pump over 48 h. After the addition was complete, the
reaction was allowed to stir for an additional 8 h. The reaction
mixture was vacuum filtered through Celite to remove the solids.
The filtrate was concentrated to ∼10 mL via rotary evaporation
under reduced pressure. The product was diluted with ethyl
acetate (100 mL), dried over anhydrous magnesium sulfate, and
vacuum filtered through Celite. The product was purified by
vacuum distillation (65-70 °C, 0.6 mmHg) followed by sub-
limation (40-42 °C, 0.3 mmHg) to yield M, which is a white
crystalline solid (4.0 g, 23.5 mmol and overall yield of 58.8%).
GC/MS (m/z): 56, 69, 81, 98, 128. 1H NMR (CDCl3): δ 4.04 (dd,
J = 9.2 and 4.4 Hz, 1H), 2.57-2.45 (m, 2H), 1.97-1.81 (m, 4H),
1.64-1.56 (m, 1H), 1.33-1.25 (m, 1H), 1.04 (d, J = 6.7 Hz, 3H),
0.97 (t, 6.8 Hz, 6H). 13C{1H} NMR (CDCl3) δ 174.0, 83.8, 42.1,
36.8, 32.9, 30.7, 30.1, 23.6, 18.1, 16.6.
Synthesis of PM(100)A. In a nitrogen-filled glovebox, M (48.0 g,
282 mmol) was dissolved in toluene (22.4 mL). The menthide
solution was passed through activated basic alumina (Fisher
Scientific). Without passing M through the alumina column,
attempts to prepare high molecular weight PM polymers resulted
in an apparent bimodal molecular distribution and a reduction in
the molecular weight determined by SEC. Employing the alumina
column resulted in a unimodal molecular weight distribution. We
attribute the bimodal distribution to an impurity in the monomer
feed, possibly the hydrolysis product, that acts as an adventitious
monofunctional initiator during the polymerization. The filtered
solution was quantitatively transferred to a pressure vessel
(350 mL) equipped with a stir bar. Diethylene glycol (42.1 μL, 0.444
mmol) was injected into the reaction vessel followed by the addition
of tin(II) 2-ethylhexanoate (180 mg, 0.444 mmol). The reaction
vessel was sealed, taken out of the glovebox, and placed in a
thermostatic oil bath at 135 °C for 72 h (95% conversion of
monomer). The reaction was quenched by exposure to the air.
The crude product was diluted with chloroform (ca. 400 mL). The
polymer solution was precipitated into methanol at -78 °Cinorder
to recover PM and remove the remaining monomers. It was dried
at 90 °C in a vacuum oven (36.5 g recovered, 80% isolated yield
based on product weight calculated by conversion). Density: 0.975
g/cm3. dn/dc: 0.064 mL/g in THF at 25 °C. 1H NMR (CDCl3): δ
4.72 (m, 589H, Ha from the repeating unit of PM), 4.22 (q, J = 4.5
Hz, 4H, Hb from the incorporated initiator), 3.68 (t, J = 4.5 Hz,
4H, Hc from incorporated initiator), 3.33 (br, 2H, Haterminal from
the end unit of PM), 2.30 (dd, J = 15.3 and 5.5 Hz, 589H from the
repeating unit of PM), 2.07 (dd, J = 14.4 and 8.5 Hz, 589H from
the repeating unit of PM), 1.94 (m, 589H from the repeating unit
of PM), 1.82 (m, 589H from the repeating unit of PM), 1.53 (m,
1178H from the repeating unit of PM), 1.33 (m, 589H from the
repeating unit of PM), 1.18 (m, 589H from the repeating unit
of PM), 0.94(d, J= 6.6 Hz, 1767H from the repeating unit of PM),
0.88 (dd, J = 6.8 and 2.0 Hz, 3534H from the repeating unit
of PM). 13C{1H} NMR (CDCl3): δ 172.9, 78.2, 41.9, 32.6, 31.1,
30.3, 28.4, 19.7, 18.6, 17.5.
Measurements. 1H and 13C NMR spectra were recorded using
Varian Inova-500 spectrometer at room temperature. The sam-
ples of the polymers were prepared at a concentration of approxi-
mately 10 and 100 mg/mL, respectively, in CDCl3 (Cambridge).
Molecular weights (Mn and Mw) were determined by size exclusion
chromatography (SEC) using polystyrene standards. Chloroform
was the mobile phase, and the flow rate was set at 1.0 mL/min at
35 °C using a Hewlett-Packard high-pressure liquid chromatography
equipped with three Jordi poly(divinylbenzene) columns of 104,
103, and 500 A pore size and a HP1047A differential refractometer.
To measure the absolute molecular weights of the polymers, SEC
coupled with MALS was also performed in THF at ambient
temperature with an eluent flow rate of 1 mL/min using an Alltech
426 HPLCpumpwitha 100μL injection loop, a Wyatt Technology
Corp. Dawn DSP-F laser photometer, and a Wyatt Technology
Corp. Optilab DSP interferometric refractometer with three Phe-
nomonex Phenogel columns (pore sizes 5 ꢀ 103, 5 ꢀ 104, and
5 ꢀ 105 A). The differential refractive index increment (dn/dc) was
determined by pumping THF solutions of the polymers of known
concentration through the Wyatt Technology Corp. Optilab DSP
interferometric refractometer, with a wavelength of 633 nm and a
temperature of 25 °C throughout the measurement. All GC-MS
experiments were conducted on an Agilent Technologies 7890A
GC system and 5975C VL MSD. Differential scanning calorimetry
(DSC) measurements were performed using a TA Instruments
Q1000 under a nitrogen atmosphere. The polymer samples were
heated to 120 °C, held there for 1 min to avoid the influence of
thermal history, cooled to -100 °C, held there for 1 min, and
then reheated to 120 °C. The rates of heating and cooling were
10 °C/min. The values reported were obtained from the second
heating cycle. An indium standard was used for calibration. Samples
weighing 7.0-9.0 mg were loaded into aluminum pans. SAXS mea-
surements for samples of PLA-PM-PLA(5-100-5 and 10-100-10)A
were performed at the Advanced Photon Source (APS) on beam-
line 5 ID-D, which is maintained by the DuPont-Northwestern-
Dow Collaborative Access Team (DND-CAT). The X-ray source
operated at a wavelength of 0.8856 A with a sample-to-detector
distance of 8552 mm calibrated with silver behenate. The flight tube
was evacuated. Two-dimensional diffraction images were recorded
using a Mar 165 mm CCD X-ray detector at a resolution of 2048 ꢀ
2048. The two-dimensional images were azimuthally integrated
and reduced to the one-dimensional form of scattered intensity
versus the spatial frequency q. Rheological testing was performed
on the as-synthesized samples with an ARES (Rheometric Scien-
tific, Piscataway, NJ) with 8 mm parallel plates. The dynamic
moduli were measured as a function of temperature at a frequency
of 1 rad s-1, a strain around 1%, and a ramp rate of 3 °C min-1
.
Low-temperature measurements were performed by heating the
sample above the glass transition of the PLA, increasing the
distance between the parallel plates to ca. 7 mm, thereby stretching
the sample, and subsequently cooling to the test temperature. This
allowed for the measurement of the moduli in the glassy region, in
which the stiffness of the sample would have otherwise caused
compliance errors and transducer resonance.
Synthesis of PLA-PM-PLA(5-100-5)A. In a nitrogen-filled
glovebox, a pressure vessel (350 mL) was charged with PM-
(100)A (17.7 g, 0.177 mmol), tin(II) 2-ethylhexanoate (71.5 mg,
0.177 mmol), and toluene (35.0 mL). The reaction vessel was
sealed and taken out of the glovebox to dissolve the mixture at
135 °C for 30 min. The vessel was cooled to room temperature and
again brought into the glovebox. D,L-Lactide (2.65 g, 18.4 mmol)
was added into the reaction vessel. It was sealed and taken out of
the glovebox to stir at 135 °C for 20 h (79% conversion of monomer).
The reaction was quenched by exposure to the air, diluted
with chloroform (ca. 100 mL), and precipitated into methanol
at -78 °C. The recovered PLA-PM-PLA was dried at 90 °C in a
vacuum oven (19.4 g recovered, 98% isolated yield based on prod-
uct weight calculated by conversion). dn/dc: 0.058 mL/g in THF at
25 °C. 1H NMR (CDCl3): δ 5.20 (m, 78H, Hd from the repeating
unitofPLA) 4.72(m,589H,Ha from the repeating unit of PM),4.36
(br, 2H, Hdterminal from the end unit of PLA), 4.22 (q, J=4.5 Hz,
Synthesis of (-)-Menthide (M). Two Baeyer-Villiger reac-
tion protocols were employed to prepare M in one step starting
from (-)-menthone. As previously reported by Zhang et al.,20
m-chloroperbenzoic acid in chloroform converted the starting
1
material to M with 85% conversion determined by H NMR
spectroscopy. Following isolation and purification, M was
obtained in >75% gravimetric yield on a 50 g scale. The follow-
ing procedure has been developed for the preparation of M
using Oxone in an aqueous ethanol solution. A round-bottom
flask (1 L) was loaded with (-)-menthone (6.170 g, 40 mmol),
sodium bicarbonate (26.9 g, 320 mmol), and ethanol (200 mL).
The reaction was stirred at room temperature as a solution of

Article
Macromolecules, Vol. 44, No. 1, 2011 93
4H, Hb from the incorporated initiator), 3.68 (t, J = 4.5 Hz, 4H, Hc
from incorporated initiator), 2.30 (dd, J = 15.3 and 5.5 Hz, 589H
from the repeating unit of PM), 2.07 (dd, J= 14.4 and 8.5 Hz, 589H
from the repeating unit of PM), 1.94 (m, 589H from the repeating
unit of PM), 1.82 (m, 589H from the repeating unit of PM), 1.53 (m,
1178H from the repeating unit of PM and 234H from the repeat-
ing unit of PLA), 1.33 (m, 589H from the repeating unit of PM),
1.18 (m, 589H from the repeating unit of PM), 0.94 (d, J = 6.6 Hz,
1767H from the repeating unit of PM), 0.88 (dd, J= 6.8 and 2.1 Hz,
3534H from the repeating unit of PM). 13C{1H} NMR (CDCl3):
δ 172.9, 169.3, 78.2, 69.2, 41.9, 32.6, 31.1, 30.3, 28.4, 19.7, 18.6,
17.5, 16.7.
same amounts of two solutions were mixed in a vial using a stir
bar for 3 h. The solution was transferred to a test tube,
allowed to stand for 10 days at room temperature, and then
observed visually. Second, the 40 wt % toluene solutions of
the adhesive systems were cast on a PET sheet. After evapor-
ating toluene at room temperature, the transparency of the
films was observed visually. The resulting thickness of the
adhesive systems layer was ca. 20 μm. The 50 wt % of tackifier
content was used.
b. Samples Preparation for Adhesive Tests. Eight samples
from our PSAs system were prepared for performing adhesive
tests and measuring the thermal properties. PLA-PM-PLA(5-
100-5 and 10-100-10)A were used as elastomers and the rosin
ester was used as a tackifier. The concentration of PLA-PM-
PLA/tackifier toluene solution was 30 wt %. The tackifier
concentration was 0, 20, 40, and 60 wt % based on the total
solid content. Selected amounts of eight solution samples were
dried at 90 °C in a vacuum oven to completely evaporate toluene
and anneal overnight. They were used for preparing DSC
samples. The solutions of the PSAs in toluene were coated on
the PET sheets (thickness: 50 μm), using a drawdown caster and
a wire wound rod, and were dried in a 100 °C oven for 10 min.
The cast films were transferred to a room with controlled
temperature and humidity of 22 ( 1.5 °C and 50 ( 2% room
humidity and remained in the room for at least 12 h before
testing. The target thickness of the cast adhesive after drying was
ca. 20 μm. PSA sample performance was evaluated using the
following adhesive tests.
Synthesis of PLA-PM-PLA(10-100-10)A. In a nitrogen-filled
glovebox, a pressure vessel (350 mL) was charged with PM-
(100)A (17.1 g, 0.171 mmol), tin(II) 2-ethylhexanoate (69.2 mg,
0.177 mmol), and toluene (34.0 mL). The reaction vessel was
sealed and taken out of the glovebox to dissolve the mixture at
135 °C for 30 min. The vessel was cooled to room temperature
and again brought into the glovebox. D,L-Lactide (4.68 g, 32.5
mmol) was added into the reaction vessel. It was sealed and taken
out of the glovebox to stir at 135 °C for 20 h (88% conversion of
monomer). The reaction was quenched by exposure to the air.
The crude product was diluted withchloroform(ca. 100 mL). The
polymer solution was precipitated into methanol at -78 °C in
order to recover the PLA-PM-PLA and remove the remained
monomers. It was dried at 90 °C in a vacuum oven (20.3 g
recovered, 96% isolated yield based on product weight calculated
1
by conversion). dn/dc: 0.065 mL/g in THF at 25 °C. H NMR
(CDCl3): δ 5.20 (m, 153H, Hd from the repeating unit of PLA)
4.72 (m, 589H, Ha from the repeating unit of PM), 4.36 (br, 2H,
Hd terminal from the end unit of PLA), 4.22 (q, J = 4.5 Hz, 4H, Hb
from the incorporated initiator), 3.68 (t, J = 4.5 Hz, 4H, Hc from
incorporated initiator), 2.30 (dd, J = 15.3 and 5.5 Hz, 589H from
the repeating unit of PM), 2.07 (dd, J = 14.4 and 8.5 Hz, 589H
from the repeating unit of PM), 1.94 (m, 589H from the repeating
unit of PM), 1.82 (m, 589H from the repeating unit of PM), 1.53
(m, 1178H from the repeating unit of PM and 459H from the
repeating unit of PLA), 1.33 (m, 589H from the repeating unit of
PM), 1.18 (m, 589H from the repeating unit of PM), 0.94 (d, J =
6.6 Hz, 1767H from the repeating unit of PM), 0,88 (dd, J = 6.8
and 2.1 Hz, 3534H from the repeating unit of PM). 13C{1H}
NMR (CDCl3): δ 172.9, 169.3, 78.2, 69.2, 41.9, 32.6, 31.1, 30.3,
28.4, 19.7, 18.6, 17.5, 16.7.
One-Pot, Two-Step Polymerization for Synthesis of PLA-PM-
PLA(10-100-10)B. In a nitrogen-filled glovebox, M (2.50 g, 14.7
mmol) was dissolved in toluene (1.2 mL) and filtered through
activated basic alumina. The filtered solution was transferred to
a pressure vessel (35 mL) with a stir bar. Diethylene glycol (2.20
μL, 0.023 mmol) was injected into the reaction vessel followed
by the addition of tin(II) 2-ethylhexanoate (9.30 mg, 0.023 mol).
The reaction vessel was sealed and taken out of the glovebox and
heated to 135 °C for 72 h (94% conversion of monomer). After
cooling the vessel to room temperature, it was brought into the
glovebox. The vessel was charged with toluene (3.8 mL) and
reheated to 135 °C for 30 min to dissolve the polymer. The vessel
was cooled to room temperature, and D,L-lactide (634 mg, 4.40
mmol) was added into the reaction vessel in the glovebox. The
vessel was sealed and returned to 135 °C for 20 h (87%
conversion of monomer). The reaction was quenched by ex-
posure to the air, diluted with chloroform (ca. 5 mL), and
precipitated into methanol at -78 °C. The recovered PLA-
PM-PLA was dried at 90 °C in a vacuum oven (2.3 g recovered,
78% recoverybasedonproductweight calculatedbyconversion).
Adhesive Behaviors. a. Compatibility Tests. The compatibility
of the tackifier and the elastomer was estimated by the following
simple tests. First, the liquid phase separation of the adhesive
solution in toluene was examined. Individual solutions of PLA-
PM-PLA(10-100-10)A in toluene (40 wt %) and rosin ester or
polyterpene in toluene (40 wt %) were prepared, to make the
tackifier content 50 wt % based on the total solid content. The
c. Peel 180° Test. The peel strength of the PSAs was measured
in the controlled room, using an IMASS SP101B slip/peel tester
at a peel rate of 12 in. min-1. One inch wide PET backed films
were peeled from Pressure-Sensitive Tape Council, PSTC-grade
polished stainless steel panels as an adherend. The strip on the
stainless steel was pressed with a 5.0 lb ASTM quality manual
roller to develop good contact between the adhesive and the steel
plate. The average peel force and standard deviation from at
least three tests were reported for each sample.
d. Probe Tack Test. Tack was also measured in the controlled
room, using the IMASS SP101B slip/peel tester with a 10 N load
cell. A stainless steel probe with a diameter of 5 mm contacted
the film, was held at a constant force for 15 s, and then raised
from the surface at a fixed rate. The resulting maximum force
was recorded. The probe was cleaned before each test. The
average tack force and standard deviation from at least 10 tests
were reported for each sample.
e. Shear Test. Shear test strips were prepared with a contact
area of 0.5 ꢀ 0.5 in. between PSTC-grade polished stainless steel
panels and the adhesive. Samples were rolled with a 5 lb ASTM
quality manual roller. 500 g weights were attached to the bottom
of the test strips. The average time in minutes for the weight to
pull the PET-backed films from the plate for at least three tests
was reported for each sample.
Acknowledgment. This work was supported by the Center
for Sustainable Polymers at the University of Minnesota through
a grant from the Institute for Renewable Energy and the Envi-
ronment (IREE RL-0009-09). The authors greatly appreciate
Arizona Chemical for samples of the tackifiers, Dr. Carolyn L.
Wanamaker for helpful discussions and for some experimental
support, and Dr. Steve Severtson and Dr. Jiguang Zhang for help
with adhesive testing.
Supporting Information Available: 13C NMR spectra for
sequential additions, SAXS profiles for PLA-PM-PLA triblock
copolymers, compatibility tests details with rosin ester and
polyterpene tackifiers, and adhesive test details and compari-
sons to commercial PSA tapes. This material is available free of
charge via the Internet at http://pubs.acs.org.

94 Macromolecules, Vol. 44, No. 1, 2011
Shin et al.
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