Melt-phase synthesis and properties of triptycene-containing copolyesters

Article abstract of DOI:10.1021/ma2004025

A new triptycene diol (TD), triptycene-1,4-hydroquinone-bis(2-hydroxyethyl) ether, was synthesized and was used to prepare a series of copolyesters with dimethyl 1,4-cyclohexanedicarboxylate (1,4-DMCD) by melt polycondensation. Straight chain aliphatic spacers, including ethylene glycol (EG), 1,4-butanediol (BD), and 1,6-hexanediol (HD), were used as codiols with TD to explore the effects of straight chain flexible spacers on copolyester properties. A concomitant series of non-triptycene copolyesters based on hydroquinone bis(2-hydroxyethyl) ether (HBE), bis[4-(2-hydroxyethoxy)phenyl] sulfone (BHPS), 1,1-bis[4-(2-hydroxyethoxy)phenyl]cyclohexane (BHPC), or 1,1-bis(2- hydroxyethoxy)phenyl-3,3,5-trimethylcyclohexane (BHPT) were prepared for comparison. The results demonstrated that the triptycene-containing polyesters in this study have higher thermal stability and higher glass transition temperatures (T_g's) than the corresponding non-triptycene analogues. For triptycene-containing copolyesters, the mechanical properties were found to be dependent on the types and compositions of comonomer diols. A 1,4-butanediol-based triptycene copolyester was observed to have a significant increase in T_g and modulus while maintaining high elongation at ambient temperature (23 °C). However, all the studied 1,4-butanediol-based copolyesters were brittle and had comparable modulus values at low temperatures (-25 or -40 °C).

Full text of DOI:10.1021/ma2004025

ARTICLE
pubs.acs.org/Macromolecules
Melt-Phase Synthesis and Properties of Triptycene-Containing
Copolyesters
Yanchun Liu,† S. Richard Turner,*,†,‡ and Garth Wilkes‡,§
†Department of Chemistry, ‡Macromolecules and Interfaces Institute (MII), and §Department of Chemical Engineering, Virginia Tech,
Blacksburg, Virginia 24061, United States
S Supporting Information
b
ABSTRACT: A new triptycene diol (TD), triptycene-1,4-hydroquinone-
bis(2-hydroxyethyl) ether, was synthesized and was used to prepare a series
of copolyesters with dimethyl 1,4-cyclohexanedicarboxylate (1,4-DMCD) by
melt polycondensation. Straight chain aliphatic spacers, including ethylene
glycol (EG), 1,4-butanediol (BD), and 1,6-hexanediol (HD), were used as
codiols with TD to explore the effects of straight chain flexible spacers on
copolyester properties. A concomitant series of non-triptycene copolyesters
based on hydroquinone bis(2-hydroxyethyl) ether (HBE), bis[4-(2-hydro-
xyethoxy)phenyl] sulfone (BHPS), 1,1-bis[4-(2-hydroxyethoxy)phenyl]cyclo-
hexane (BHPC), or 1,1-bis(2-hydroxyethoxy)phenyl-3,3,5-trimethylcyclohex-
ane (BHPT) were prepared for comparison. The results demonstrated that the
triptycene-containing polyesters in this study have higher thermal stability and
higher glass transition temperatures (Tg’s) than the corresponding non-
triptycene analogues. For triptycene-containing copolyesters, the mechanical properties were found to be dependent on the types and
compositions of comonomer diols. A 1,4-butanediol-based triptycene copolyester was observed to have a significant increase in Tg and
modulus while maintaining high elongation at ambient temperature (23 °C). However, all the studied 1,4-butanediol-based copolyesters
were brittle and had comparable modulus values at low temperatures (ꢀ25 or ꢀ40 °C).
’ INTRODUCTION
significantly enhanced glass transition temperatures and increased
brittleness as evidenced by the reported brittle nature of cast
films.12,13 Both of these early reports were based on incorporation
of a triptycene monomer with a 9,10 functionality for polymeri-
zation into the various polymer backbones studied. In contrast, the
Swager and Thomas work is based on the use of 1,4-hydroquinone
triptycene structure which significantly changes the monomer
structure and polymerbackbonestructure. Also, in therecentwork
a long aliphatic spacer was found to be necessary to bring this
proposed mechanical interlocking mechanism into operation. The
combination of decanediol and the 1,4-hydroquinone triptycene
units in the polyester chain led to these unusual properties.
Our goal is to explore the properties of a series of copolyesters
that incorporate the 1,4-hydroquinone triptycene group via a
new primary diol triptycene derivative which permits the facile
melt-phase preparation of copolyesters. We are interested in
raising the Tg values of aliphatic polyesters based on 1,4-
cyclohexane dicarboxylic acid (via 1,4-DMCD) without nega-
tively impacting the mechanical properties of these materials. In
this research the number of methylene groups in the aliphatic
codiol was varied from two to six (ethylene diol to hexane diol),
and selected thermal and mechanical properties of the resulting
Material scientists have been pursuing the enhancement of
mechanical properties to produce high performance polymers for
a wide range of applications. Incorporation of rigid structures
into a polymer backbone to enhance properties has been studied
extensively by many researchers.1ꢀ3 Polymers containing such
rigid building blocks usually show not only increased Tg but also
decreased ductility. For example, adamantyl building blocks
usually raise the Tg but also lower the ductility of a polymer by
reducing the flexibility of the polymer backbone and intermole-
cular chain entanglements.4ꢀ7 A variety of bisphenol derivatives
are also well-known to produce high-Tg polymers.8ꢀ10 However,
a recent report from Swager and Thomas et al.11 shows that
incorporation of triptycene, a rigid aromatic cyclic structure,
gives an increase in both modulus and ductility even at a low
temperature of ꢀ30 °C when incorporated into certain polyester
backbones. The authors hypothesized that neighboring chains
can lie in a V-shaped cleft of the triptycene units and that this
provides a mechanism for molecular interlocking and is the origin
of these normally divergent mechanical properties.11 Unlike
common intermolecular interactions, such as hydrogen bonding
and ionic interactions, this “mechanical interlocking” is a novel
concept, and it is not known if this interaction is operative in
other polymer structures.
Received: February 21, 2011
Previous work on triptycene containing polyesters in the late
1960s from DuPont and Eastman Kodak resulted in polymerswith
Revised:
Published: May 03, 2011
April 19, 2011
r
2011 American Chemical Society
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|

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ARTICLE
Scheme 1. Synthesis of Triptycene-1,4-hydroquinone-bis(2-hydroxyethyl) Ether (3)
copolyesters were studied. Furthermore, the incorporation of other
bulky hydroxyethoxylated bisphenol derivatives into identical
polyester backbones was investigated, and the mechanical proper-
ties, without the triptycene architecture, of these polyesters were
compared to those of corresponding triptycene polyesters.
samples were unchanged from the original samples before compression
molding. The film samples were dried in vacuum 24 h and then were cut
to a dog bone shape at 40 ꢁ 4 ꢁ 0.3 mm (length ꢁ width ꢁ thickness)
for tensile tests. The samples were tested at a rate of 15 mm/min using
an initial grip-to-grip separation of 15 mm. Young’s modulus was
calculated from the linear part of the initial slope. All reported tensile
data were averaged from at least three independent measurements, and a
standard deviation was also reported. An X-ray diffractometer was used
to determine if any crystallinity existed within the cast films. For this
experiment, the original films or stretched films were fixed on the
platform and the X-ray diffraction was observed from the surface of films
by use of the reflection mode.
Synthesis of Triptycene-1,4-quinone (1) and Triptycene-
1,4-hydroquinone (2). Triptycene 1,4-quinone (1) and triptycene-
1,4-hydroquinone (2) were prepared according to the published
literature.14,15 The detailed procedures are described in the Supporting
Information.
Synthesis of Triptycene-1,4-hydroquinone-bis(2-hydroxye-
thyl) Ether (3). Synthesis of (3) is shown in Scheme 1. A 500 mL two-
necked flask charged with 30 g (0.105 mol) triptycene-1,4-hydroquinone
(2) and 1.45 g of K2CO3 (0.0105 mol) in 250 mL of N,N-dimethylforma-
mide (DMF) was heated to reflux temperature (165 °C) under argon;
18.46 g (0.210 mol) of ethylene carbonate in 100 mL of DMF was added
into the flask drop by drop (about 1 h). After that, the reaction mixture was
stirred at reflux temperature for another 2 h and then cooled to room
temperature. A fine precipitate was obtained by pouring the reaction
mixture into 800 mL of deionized water. The solid was filtered and washed
completely with deionized water. A fine white solid was obtained after
recrystallization from methanol and drying under vacuum oven overnight.
Yield: 89%, mp 238ꢀ239.3 °C. 1H NMR (400 MHz; DMSO-d6) δ ppm:
3.74ꢀ3.77 (m, 4H, CH2OH), 3.92ꢀ3.96 (m, 4H, ArOꢀCH2), 4.94ꢀ4.97
(t, 2H, OꢀH), 5.95 (s, 2H, ArꢀCH), 6.64 (s, 2H, ArꢀH), 6.97ꢀ6.70 (m,
4H, ArꢀH), 7.15ꢀ7.17 (d, 4H, ArꢀH). 13C NMR (DMSO-d6, 100 MHz)
δ ppm: 46.81, 60.21, 72.11, 112.09, 124.13, 125.25, 135.71, 145.93, 148.64.
Elemental analysis calculated: C, 76.99; H, 5.92. Found: C, 77.03; H, 5.89.
Abbreviation of Polyesters. The polymer nomenclature used in
this article is based on a polyester containing 100 mol % of diester and 100
mol % of diol. For example, the polymer designated by poly[100-
(DMCD)75(EG)25(TD)] means that this targeted polymer contains
100 mol %DMCDasthediester units and75 mol % EGand 25 mol % TD
as the diol units. The letters stand for various monomers’ abbreviation and
the numbers indicate targeted mol % of monomers, respectively.
Melt-Phase Polymerization. The triptycene diol (TD) (3) and
the comonomer EG were copolymerized with DMCD by melt poly-
condensation reaction. The detailed procedure in Scheme 2, which is
similar to a published method,9 is as follows (for poly[100(DMCD)75-
(EG)25(TD)]): 10 g (0.05 mol) of DMCD consisting of a trans/cis
’ EXPERIMENTAL SECTION
Materials. Anthracene (97%) was purchased from Aldrich and recrys-
tallized from xylene. Ethylene glycol (g99%), 1,4-butanediol (99%), 1,6-
hexanediol (99%), p-benzoquinone, hydroquinone bis(2-hydroxyethyl)
ether (98%), and 4,40-cyclohexylidenebisphenol (98%) were purchased
from Aldrich and used as received. Dimethyl 1,4-cyclohexanedicarboxylate
(1,4-DMCD) (cis/trans = 3/1) and 1,1-bis(hydroxyphenyl)-3,3,5-tri-
methylcyclohexane were donated by Eastman Chemical Co. and Hi-Bis
GmbH, respectively. Titanium(IV) n-butoxide (>98%) was purchased from
Alfa Aesar, and the titanium catalyst solution was prepared by mixing
titanium n-butoxide with dry n-butanol in a dry bottle under nitrogen at a
concentration of 0.06 g/mL based on Ti.
Instrumentation. All measurements were performed in Virginia
Tech (Blacksburg, VA) except for the elemental analysis, which was
done by Atlantic Microlab, Inc. (Norcross, GA). NMR spectra were
determined at 25 °C at 400 MHz with an INOVA spectrometer.
Molecular weights of the synthesized polymers were determined using
size exclusion chromatography (SEC) with a refractive index (RI)
detector and viscometer DP detector and using a polystyrene standard.
SEC measurements were performed at 30 °C in chloroform with a
sample concentration 5.00 mg/mL at a flow rate of 1.00 mL/min.
Thermogravimetric analysis (TGA) was conducted under nitrogen from
25 to 600 °C at a heating rate of 10 °C/min using a TGA Q500 of TA
Instruments. Differential scanning calorimetry (DSC) was conducted
using a DSC Q2000 of TA Instruments. DSC data were obtained from
ꢀ20 to 300 °C at heating/cooling rates of 20 °C/min under nitrogen
circulation. The glass transition temperature was determined from
analysis of the second heating cycle. Dynamic mechanical analysis
(DMA) of samples was conducted using a DMA Q800 of TA Instru-
ments at a heating rate of 5 °C/min from ꢀ150 to 100 °C while they
were deformed (10 μm amplitude) in the tension mode at a frequency of
1 Hz under nitrogen. Tensile measurements at room temperature were
performed on an Instron Model 4400 Universal Testing System
equipped with a 1KN load cell. Tensile measurements at low tempera-
tures (ꢀ25 or ꢀ40 °C) were performed on an Instron 5800R and
Thermotron Testing System equipped with a load capacity of 1KN. The
film samples were prepared using a PHI Model GS 21-J-C-7 compres-
sion molding press at 70 °C above Tg for 15 min. After the film sampl-
es were cooled down in ambient air, they were stored in a desiccator
at ambient temperature. The molecular weights of the prepared film
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ARTICLE
Scheme 2. Synthesis of
Poly[100(DMCD)(100ꢀx)(EG)x(TD)]
Synthesis of Poly[100(DMCD)75(BD)25(BHPS or BHPC or
BHPT)]. The other non-triptycene analogues (depicted in Scheme 4)
with the same compositions were synthesized for comparative purposes.
The same experimental procedures were applied.
’ RESULTS AND DISCUSSION
Selection of Monomers. The synthetic route to triptycene-1,4-
hydroquione-bis(2-hydroxyethyl)ether(3)isshowninScheme1.
Anthracene was reacted with quinone across the 9,10-position to
yield triptycene-1,4-quinone (1).14,15 When treated with HBr in
glacial acetic acid, triptycene-1,4-quinone (1) gives triptycene-
1,4-hydroquinone (2) with high yield.14 This bisphenol can be
readily and inexpensively converted to the primary alcohol
triptycene-1,4-hydroquione-bis(2-hydroxyethyl) ether (3) in
high yield by reaction of the phenol OH group with ethylene
carbonate.9 To the best of our knowledge, (3) has not been
reported in the literature and is a new monomer. Triptycene (3)
with primary alcohol groups isrequiredfor polyesters in diolꢀdie-
ster polycondensations because it is well know that the direct melt
polycondensation of bisphenols is a low yield reaction.9 1,1-
Bis[4-(2-hydroxyethoxy)phenyl]cyclohexane (BHPC) was ob-
tained in much higher yield than the literature.16 1,4-DMCD
(cis/trans ratio of 3 to 1) as the diacid unit was used in this study
to provide amorphous polyesters with improved solubility, while
maintaining the linear 1,4-enchainment mode. Thus, the intro-
duction of cyclohexane units to the main chain of the polyesters
does not significantly decrease the mechanical properties due to
the rigidity of the alicyclic structure.
Scheme 3. Structures of Non-Triptycene
Poly[100(DMCD)(100ꢀx)(EG)x(HBE)]
Copolyester Composition by 1H NMR Spectroscopic Anal-
1
ysis. Figure 1 shows the H NMR spectra of a representative
copolyester based on DMCD, EG, and TD with a targeted molar
ratio of 100:75:25. The diols within the copolyester chains are
assumed to react in a random fashion. In brief, peak “a”, “a0”, “b”,
and “c” are assigned to the protons of the triptycene group. Peaks
“d” and “e” are the methylene group adjacent to the oxygen at the
TD unit. Peak “f” is assigned to the methylene group adjacent to
the oxygen at the EG unit (single peak), and the cis/trans ratio of
DMCD was determined by comparing the R-hydrogens on the
cis and trans isomers. Some isomerization of DMCD from cis to
trans occurred during polymerization (final mole ratio of cis/
trans = 1/1). These broad peaks “h” come from the protons on
carbons in the rings. The peak area ratio of “f” to “e” give 74% EG
and 26% TD. The 1H NMR spectrum of each of these polymers
showed good agreement of its actual composition with the
targeted composition.
Thermal Property Analysis of Copolyesters Poly[100-
(DMCD)(100ꢀx)(EG)x(TD)]. All polymers became highly viscous
when the polymerization proceeded to high conversion, which
typically took about 6 h. Some of the copolyesters started to take
on a yellow color due to the titanium catalyst. The semiaromatic
copolyester samples were soluble in common chlorinated sol-
vents, such as dichloromethane and chloroform, as expected
from the amorphous polyester structure. In order to minimize
the effect of physical aging of the respective polyesters on thermal
and mechanical properties, all polyester film samples were run as
soon as possible (in 24 h) after they were made by compression
molding. The thermal properties, the molecular weights and
tensile properties at ambient temperature (23 °C) are summar-
ized in Table 1.
(1/3 molar ratio) mixture, 4.66 g (0.075 mol) of EG (100% excess), and
4.68 g (0.0125 mol) of TD were charged to a two-necked 50 mL reaction
vessel equipped with a mechanical stirrer, nitrogen inlet, and condenser.
The reactor was placed in a Belmont metal bath with a temperature
controller. Titanium n-butoxide catalyst (100 ppm with respect to the
targeted polyester) was added via a syringe under nitrogen. A multistep
temperature procedure was used for the reaction; i.e., the reaction
mixture was heated and stirred at 190 °C for 2 h, 220 °C for 2 h, and
275 °C for 0.5 h. Methanol was collected in a receiving flask. At the end,
high vacuum (0.1ꢀ0.2 mmHg) was applied to drive the reaction to high
conversion for an additional 2 h. Then the vacuum was discontinued,
and nitrogen was passed through the system. The polymer was allow-
ed to cool to room temperature and was removed from the reaction
flask. The polymer was dissolved in chloroform and precipitated into
methanol. The solid precipitate was obtained by vacuum filtration
and was dried under vacuum at 30ꢀ60 °C overnight to yield 12.7 g
(92%) of dry copolyester. The same procedure was employed to prepare
poly[100(DMCD)(100ꢀx)(EG)x(TD)].
Synthesis of Poly[100(DMCD)(100ꢀx)(EG)x(HBE)] for Com-
parative Purposes. Non-triptycene analogues also were synthesized
for comparison. Hydroquinone bis(2-hydroxyethyl) ether (HBE) was
used as a corresponding diol shown in Scheme 3. The experimental
procedures are the same as described above.
Synthesis of Poly[100(DMCD)75(BD or HD)25(TD or HBE)].
Copolyesters were prepared by the above-mentioned procedure except
that the contents of straight-chain alkanediol and TD (or HBE) were
fixed at 75 and 25 mol %, respectively. 1,2-Ethanediol was replaced by
1,4-butanediol or 1,6-hexanediol, which was used in 30 mol % excess.
An examination of Table 1 shows that most of the copolyesters
displayed high molecular weights as well as PDIs of 2.5ꢀ3.1,
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ARTICLE
Scheme 4. Structures of Poly[100(DMCD)75(BD)25(BHPS or BHPC or BHPT)]
Figure 1. 1H MNR spectra of poly[100(DMCD)(74)(EG)26(TD)].
which are typical for melt polymerization polyesters. The SEC
trace of poly[100(DMCD)74(EG)26(TD)] by the refractive
index detector is shown in Figure 2 as representative of these
samples. The presence of small peaks, following the main sharp
peak, suggests the presence of some cyclic oligomers in the
polyester. Molecular weights of polyesters, containing DMCD,
EG, and TD, decrease with decreasing EG content. However, Tg
increases with increasing TD content. As shown in Table 1, the
5% weight loss (Td) for all TD-containing polymers was higher
than those of the corresponding non-TD analogues, as expected
from the more highly aromatic structure of TD. For example,
poly[100(DMCD)74(EG)26(TD)] showed a Td at 384 °C,
whereas poly[100(DMCD)74(EG)26(HBE)] displayed a Td
of 372 °C. These data indicate that copolyesters containing
TD have marginally higher thermal stability than non-triptycene
analogues. When the triptycene unit was incorporated into the
polyester, the Tgs were remarkably increased when compared to
those of the non-triptycene polyesters. From Table 1, the
incorporation of 26 mol % HBE into the DMCD/EG backbone
only raised the Tg 8 °C, whereas TD at the same incorporation
level raised the Tg 54 °C. This is consistent with the bulky
structure of TD. The data in Table 1 also show that, as expected,
the Tg of triptycene copolyesters decreases when the carbon
number of the linear aliphatic codiol increases to 4 (BD) and 6
(HD). Average molecular weights also decreased with the longer
chain aliphatic diols because of their lower volatility than EG,
making it more difficult to drive the conversion. The TD-containing
polymers poly[100(DMCD)75(BD or HD)25(TD)] exhibited
higher thermal stabilities than the corresponding non-triptycene
analogues poly[100(DMCD)75(BD or HD)25(HBE)]. The ab-
sence of a melting peak in all DSC traces indicates that the
copolyesters are amorphous, in contrast to the results reported by
Swager and Thomas et al.11 (their triptycene-containing polyesters
were semicrystalline). It is difficult to obtain high molecular weight
copolyesters with an incorporation level of 70 mol % or more TD
due to the nonvolatility of TD.
Mechanical Property Analysis of Copolyesters Poly[100-
(DMCD)75(EG or BD or HD)25(TD or HBE)]. Tensile data
at ambient temperature revealed that poly[100(DMCD)74-
(EG)26(TD)] has a higher modulus and yield stress than poly-
[100(DMCD)74(EG)26(HBE)], which is a flexible material
(Figure 3 and Table 1). But the low elongation to break of
poly[100(DMCD)74(EG)26(TD)] indicated that this tripty-
cene polyester is brittle under these same conditions. Unlike
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Table 1. Characterization Results of Triptycene and Non-Triptycene Copolyesters
polyester composition
(1H NMR)
TGA Td. 5%
SEC Mn
DSC Tg
(°C)
tensilea stress at break
(MPa)
tensilea strain
at break (%)
modulusa
(MPa)
weight loss (°C)
(g/mol)
Mw/Mn
b
b
b
100(DMCD)100(EG)
307
384
384
385
372
372
372
375
368
370
368
360
346
331
359
357
51 000
54 000
17 000
9 500
2.7
3.1
2.7
2.9
3.1
2.5
2.6
2.2
1.9
2.0
2.0
2.5
2.1
1.9
2.0
2.0
15
69
99
118
23
27
31
44
4
100(DMCD)74(EG)26(TD)
100(DMCD)49(EG)51(TD)
100(DMCD)26(EG)74(TD)
100(DMCD)74(EG)26(HBE)
100(DMCD)49(EG)51(HBE)
100(DMCD)26(EG)74(HBE)
100(DMCD)74(BD)26(TD)
100(DMCD)74(BD)26(HBE)
100(DMCD)75(HD)25(TD)
100(DMCD)65(HD)35(TD)
100(DMCD)50(HD)50(TD)
100(DMCD)75(HD)25(HBE)
100(DMCD)74(BD)26(BHPS)
100(DMCD)74(BD)26(BHPC)
100(DMCD)75(BD)25(BHPT)
48 ( 4
4.5 ( 0.7
1475 ( 137
c
c
c
c
c
c
43 000
33 000
106 000
25 500
20 000
24 000
27 800
24 300
19 000
20 100
20 400
19 500
6 ( 0.7
1920 ( 76
0.9 ( 0.1
b
b
b
b
b
b
32 ( 2
0.39 ( 0.03
11 ( 1
319 ( 14
737 ( 33
494 ( 47
1169 ( 21
1.0 ( 0.2
50 ( 3
25
47
77
ꢀ6
32
26
36
43 ( 3
10 ( 2
734 ( 27
c
c
c
0.24 ( 0.01
20 ( 2.0
15 ( 0.7
19 ( 0.7
339 ( 32
433 ( 41
688 ( 27
249 ( 18
1.4 ( 0.4
316 ( 30
4 ( 0.2
673 ( 46
a Tensile tests were done at 23 °C. b Tensile tests were not run. c Samples failed during preparation of dogbone specimens.
in a brittle material with low elongation to break (10%) at ambient
temperature. When the HD content was lowered to 50 mol %, poor
films were obtained and tensile tests were not possible for this
triptycene copolyester composition. The TD-containing polyesters
poly[100(DMCD)75(BD or HD)25(TD)] also exhibited much
higher modulus at 23 °C when compared to the HBE analogues
poly[100(DMCD)75(BD or HD)25(HBE)], which are highly
flexible with a modulus of about 1 MPa as shown in Table 1.
Mechanical Property Analysis of Copolyesters Containing
BHPS, BHPC, or BHPT. Numerous bisphenol derivatives have been
synthesized and incorporated into polymer backbones to increase
the Tg for high-performance materials.9,17 BHPS is commercially
available; BHPC and BHPT were synthesized as described in the
synthesis section. In this study, they were incorporated into
identical polyester backbones by replacing the triptycene units
with the respective bisphenol derivatives, BHPS, BHPC, or BHPT.
The properties of their copolyesters are shown in Table 1. We can
see that poly[100(DMCD)74(BD)26(TD)] still has the highest
thermal stability and highest modulus among these copolyesters.
These data thus confirmed that the incorporation of TD into the
polyester backbone can increase Tg due to its rigid structure. The
glassy solid BHPT possesses an amorphous and bulky structure due
to the three pendent methyl groups. When the BHPT concentra-
tion was also fixed at 25 mol %, the modulus of the corresponding
copolyester was significantly less than the 25 mol % TD copolye-
ster. From the tensile test data of poly[100(DMCD)74(BD)26-
(BHPC)], we conclude that it has some elastic-like properties with
a low modulus (only 4 MPa) since Tg is very close to the ambient
temperature, at the temperature where the tensile measurements
were run. The copolyester based on BHPS, poly[100(DMCD)74-
(BD)26(BHPS)], has a lower Tg and modulus and a higher
elongation than the corresponding TD copolyester.
Figure 2. SEC trace of poly[100(DMCD)(74)(EG)26(TD)].
the polyesters in the paper of Swager and Thomas et al., which
use long chain diols, this polyester poly[100(DMCD)74(EG)26-
(TD)] based on a short chain diol, EG, does not demonstrate
ductile behavior.
Therefore, we replaced EG with a longer diol, BD, and kept the
TD composition the same. A similar polymer was also prepared
with a longer diol, HD, so that the effects of the length of aliphatic
spacers could be elucidated. We observed significantly different
properties with the longer straight chain diols BD and HD.11 These
characterization results are also summarized in Table 1. Tensile
data revealed that poly[100(DMCD)74(BD)26(TD)] has a high-
er modulus and stress to yield than poly[100(DMCD)75(HD)-
25(TD)], which has more elastic-like properties. High elonga-
tion to break (319%) and high modulus (1.17 GPa) of poly[100-
(DMCD)74(BD)26(TD)] indicated that it is ductile and a rela-
tively tough material. Therefore, BD is considered to possess a
suitable chain length to demonstrate enhanced ductility and
enhanced modulus of triptycene polyesters. A decrease of HD
content in the triptycene copolyester from 75 to 65 mol % resulted
From the above tensile curves for poly[100(DMCD)75(BD)25-
(BHPT)] (Figure 4), the copolyester based on BHPT is found to be
a high modulus and ductile material with a yield point, although no
necking was observed during the deformation. However, when
compared to poly[100(DMCD)75(BD)25(BHPT)], the triptycene
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Figure 3. Tensile properties of poly[100(DMCD)74(EG)26(TD)] (left) and poly[100(DMCD)74(EG)26(HBE)] (right) at 23 °C.
Figure 4. Ambient temperature tensile properties of poly[100(DMCD)74(BD)26(TD)] (left) and poly[100(DMCD)75(BD)25(BHPT)] (right).
copolyester poly[100(DMCD)74(BD)26(TD)] exhibits the
synergistic effect observed by Swager and Thomas et al. because
both the elongation to break and modulus of the triptycene
copolyester are improved. The distinct upward turn in both tensile
curves in Figure 4 is due to strain hardening, which begins at a fairly
high level of strain. Necking formation is also observed for poly-
(100(DMCD)74(BD)26(TD) and is similar to the observation of
Swager and Thomas et al.11 However, the polyesters of this study
did not show crystallinity in contrast to Swager and Thomas et al.11
In order to verify if crystallization is induced during the deformation,
we obtained the X-ray diffraction of the elongated polymer film,
which was maintained in the stretched condition during the
measurement. Figure 5 shows the X-ray diffraction traces for both
the original and stretched samples. No significant difference be-
Figure 5. X-ray diffraction traces of poly(100(DMCD)74(BD)26(TD)
films before and after stretching.
tween these films is observed. The broad diffuse peaks (almost
across 20°) indicate that both samples are amorphous.
In order to better compare the mechanical properties of
triptycene polyesters to non-triptycene analogues in their glassy
states, tensile measurements at low temperatures were carried
out, and the results are shown in Table 2. The tensile properties
of these non-triptycene polyesters were measured at ꢀ25 °C,
except for the HBE-containing polyester, which was measured at
ꢀ40 °C due to its low Tg.
in regards to ductility and modulus when compared to the other
copolyesters of this study containing rigid and bulky units
(BHPS, BHPC, and BHPT) measured at well below Tg. These
data differ from the earlier results of Swager and Thomas et al.,11
who observed ductile behavior for their triptycene-containing
copolyesters at ꢀ30 °C.
All copolyesters in Table 2 exhibit comparable glassy modulus
values and are brittle at temperatures well below their Tg’s. Since
the tensile elongation to break of the triptycene copolyester
poly[100(DMCD)74(BD)26(TD)] is greatly decreased at
ꢀ25 °C, this triptycene polyester does not behave differently
DMA spectra of triptycene and various non-triptycene polye-
sters are presented in Figure 6. The plots of storage modulus
versus temperature indicate that all polyesters possess an ex-
pected glassy storage modulus (above 1 GPa) except for the
HBE-containing polyester. The R peaks in the tan δ curves,
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Table 2. Tensile Properties of Various Copolyesters at ꢀ25 or ꢀ40 °C
various copolyesters with similar compositions (Tg)a
tensile stress at break/yield (MPa)
tensile strain at break (%)
modulus (MPa)
DMCD/BD/BHPS (32) 100/74/26
DMCD/BD/BHPC(26) 100/74/26
DMCD/BD/BHPT(36) 100/75/25
DMCD/BD/TD(44) 100/74/26
DMCD/BD/HBE(4) 100/74/26
46 ( 6 (break)
50 ( 3 (yield)
51 ( 4 (yield)
69 ( 3 (break)
37 ( 4 (break)
6 ( 1
19 ( 2
10 ( 1
8 ( 0.4
24 ( 5
1406 ( 96
1448 ( 65
1526 ( 82
1688 ( 61
952 ( 66
a Tg values are shown in parentheses. The tensile tests of poly(DMCD/BD/HBE) was run at ꢀ40 °C.
Figure 6. DMA of poly[100(DMCD)75(BD)25(TD/BHPT/BHPC/BHPS/HBE)].
accompanied by a sharp decrease in modulus, correspond to the
Tg’s of the respective polyesters. The triptycene polyester dis-
plays the highest glassy storage modulus (above 1.8 GPa) and Tg
(66 °C from tan δ), together with the widest glassy plateau well
past room temperature among these polyesters, while the HBE-
containing polyester shows the lowest glassy DMA modulus of
0.7 GPa and Tg of 21 °C. The tan δ Tg was about 20 °C higher
than the DSC Tg as expected. All copolyesters exhibit the
existence of a secondary relaxation tan δ peak from ꢀ57
to ꢀ51 °C with about the same intensity. The conformational
changes of the cyclohexyl units in the polyester backbones
are the origin of sub-Tg loss peak. It is of interest to note that
other cyclohexyl group-containing polyesters, such as the
poly(1,4-cyclohexylenedimethylene terephthalate) (PCT),
were confirmed by Yee et al.18 to have a weak transition in
this region as well.
ambient temperature modulus and enhanced ductility. However,
our results do not confirm the synergistic effect observed by Swager
and Thomas et al. since the triptycene copolyester does not show
enhanced ductility when compared to other copolyesters contain-
ing rigid and bulky units (BHPS, BHPC, and BHPT) at ꢀ25 °C.
These polyesters, including triptycene polyesters, are brittle and
display similar modulus values at temperatures well below their
DMA (tan δ) Tg’s. Poly[100(DMCD)74(BD)26(TD)] in this
study differs from the triptycene-containing copolyesters
of Swager and Thomas et al.,11 which still exhibited ductile
behavior at ꢀ30 °C.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures for tripty-
b
cene 1,4-quinone (1), triptycene-1,4-hydroquinone (2), BHPC
and BHPT, the synthetic scheme of poly[100(DMCD)75(BD or
HD)25(TD or HBE)]. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ CONCLUSION
In summary, we have described the melt-phase synthesis of
triptycene-containing polyesters using a new primary triptycene
diol, and we have also characterized some of the properties of these
new materials. All TD-containing polymers in this study have
higher thermal stability by TGA and higher Tg’s than the corre-
sponding non-triptycene analogues. The Tg of TD-containing
polymers increased with increasing TD content. The results from
tensile tests revealed that poly[100(DMCD)74(EG)26(TD)] is
rigid but brittle at ambient temperature. However, the copolyester
poly[100(DMCD)74(BD)26(TD)] was found to simultaneously
possess high modulus and excellent ductility at ambient tempera-
ture. It indicates that its short flexible spacer (butane unit) in
combination with triptycene units can also promote an improved
’ AUTHOR INFORMATION
Corresponding Author
*Tel (540)231-4552; Fax (540)231-3971; e-mail srturner@
vt.edu.
’ ACKNOWLEDGMENT
The authors are grateful to Eastman Chemical Company for
donating DMCD and Dr. Liang Chen for synthesis of the
monomer BHPT. We gratefully acknowledge Prof. McGrath’s
group for allowing us to use their TGA and DSC equipment. We
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also thank Prof. Riffle’s group for SEC measurement and Gilles
Divoux in Prof. Robert Moore’s group for the X-ray diffraction
measurements.
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