Dynamic Cross-Linking of Polyethylene via Sextuple Hydrogen Bonding Array

Article abstract of DOI:10.1021/acs.macromol.8b01715

Multiple hydrogen bonding motifs are promising tools for polymer functionalization to obtain adaptable networks combining advantages of permanently cross-linked systems with processability of thermoplastics. Here we describe the use of a new multiple hydrogen bonding motif to impart increased tensile strength, stiffness, barrier properties, and a plateau modulus after melting to functional polyolefins, while retaining adaptability of the polymer network. The cross-linked nature of these polymers was elucidated by thermal and mechanical analysis, revealing a raised glass transition and rheology similar to permanently cross-linked polymer matrices. The apolar polymer matrix was found to stabilize the new hydrogen bonding motif at elevated temperatures. The resulting polymer showed thermal resistance superior to ureidopyrimidone (UPy) motif functionalized materials, the most commonly employed synthetic multiple hydrogen bonding motif to date.

Full text of DOI:10.1021/acs.macromol.8b01715

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Dynamic Cross-Linking of Polyethylene via Sextuple Hydrogen
Bonding Array
‡
,‡
Jonathan Tellers,† Stefano Canossa,† Roberta Pinalli,† Maria Soliman, Jerome Vachon,*
́
̂
and Enrico Dalcanale*,†
†Department of Chemistry, Life Sciences and Environmental Sustainability and INSTM UdR Parma, University of Parma, Parco
Area delle Scienze 17/A, 43124 Parma, Italy
‡SABIC Europe B.V., Urmonderbaan 22, 6160 AH Geleen, The Netherlands
S
* Supporting Information
ABSTRACT: Multiple hydrogen bonding motifs are promis-
ing tools for polymer functionalization to obtain adaptable
networks combining advantages of permanently cross-linked
systems with processability of thermoplastics. Here we
describe the use of a new multiple hydrogen bonding motif
to impart increased tensile strength, stiffness, barrier proper-
ties, and a plateau modulus after melting to functional
polyolefins, while retaining adaptability of the polymer
network. The cross-linked nature of these polymers was
elucidated by thermal and mechanical analysis, revealing a
raised glass transition and rheology similar to permanently cross-linked polymer matrices. The apolar polymer matrix was found
to stabilize the new hydrogen bonding motif at elevated temperatures. The resulting polymer showed thermal resistance
superior to ureidopyrimidone (UPy) motif functionalized materials, the most commonly employed synthetic multiple hydrogen
bonding motif to date.
molecules with a plethora of different structures were
developed with the ability to homo- or heterodimerize.20−22
Using multiple hydrogen bonding moieties, properties such as
controlled degradation,23 optical responsiveness,24 high
strength and stiffness,25 increased peel strength,26 self-
healing,27 miscibility between immiscible polymers,28 and
reversible chain extension29,30 can be introduced into
polymers. The increased strength of these hydrogen bonding
arrays can lead to their assembly even in water,31,32 thus
making them an excellent tool for the preparation of humidity
and moisture resistant materials.
Among the different types of multiple hydrogen bonding
array molecules, the most prominent is the ureidopyrimidone
(UPy)29 motif, a self-dimerizing quadruple hydrogen bonding
motif characterized by a high association constant,33 ease of
synthesis,34 and readily available derivatives for functionaliza-
tion of polymers.35 To date, UPy has been the most important
studied synthetic multiple hydrogen bonding motif.36 Research
on polymers functionalized with UPy has largely been focused
on telomers at lab scale, while functionalization of long chain
polyolefins remains largely unexplored. Coates et al. poly-
merized 1-hexene with alkene functionalized UPy, obtaining a
completely amorphous, wax-like polyolefin with improved
tensile properties over nonfunctionalized homopolymer.37
INTRODUCTION
■
Polyethylene (PE) is the most widely employed synthetic
polymer in the world,1,2 owing to its low cost and excellent
properties such as chemical resistance, processability, moisture
resistance, and low conductivity.3 However, there is an
ongoing interest to overcome some of the flaws of PE such
as wear,4,5 poor oxygen barrier properties,6,7 low strength,8,9
and creep,8,10 relying mostly on fillers as property enhancing
tool.
The introduction of hydrogen bonding in polymers can lead
to extraordinary properties. For example, nylon, possibly
described as a PE backbone chain with equally spaced
difunctional hydrogen bonding moieties (amides), has superior
tensile, barrier, fatigue, and heat resistance properties.11−13
Ethylene vinyl alcohol (EVOH) shows superior barrier
properties ascribed to a high crystallinity resulting from the
considerable cohesive energy between chains provided by the
large amount of hydrogen bonds present.14 Other notable
examples of polymers that own some of their remarkable
properties to hydrogen bonding are polyurea and polyur-
ethanes.15 The properties of these polymers are however
profoundly dependent on moisture, as it can interfere with the
hydrogen bonds.16−19
More recently, arrays of multiply hydrogen bonds have been
explored to improve polymer properties. These are molecules
inspired by DNA base pairs with a favorable arrangement of
hydrogen bonding donor (D) and acceptor (A) groups,
allowing for organized self-assembly.20 Following this concept,
Received: August 8, 2018
Revised: September 7, 2018
© XXXX American Chemical Society
A
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Single Crystal X-ray Diffraction (SCXRD). SCXRD analysis was
performed at the ELETTRA Synchrotron facility (XRD1 beamline).41
Diffraction data were collected using a monochromatic 0.7 Å
wavelength at 100 K, using an Oxford Cryostream 70042 cooling
device. Diffraction data sets were processed using CrysAlisPro
software ver. 1.171.38.43.43 Structure solution and refinement were
conducted using Olex2 software ver. 1.2.9.44 CCDC 1825490
contains crystallographic data relative to the described phases. This
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via https://www.ccdc.cam.ac.uk/structures.
Gel Permeation Chromatography (GPC). GPC was performed on
a Polymer Char GPC-IR system using a Polymer Char IR 5 infrared
detector as concentration detector and a Wyatt MALLS detector
Helios II. As column set two Polymer laboratories 20 μm PLgel
MIXED ALS (300 × 7.5 mm) were used. System performance is
safeguarded by measuring the statistical process control sample (SPC)
NBS 1475A (or PE52K) and STAM 7625 before every sample series.
Samples were dissolved in o-dichlorobenzene, and the runs were
performed at 160 °C.
Differential Scanning Calorimetry (DSC). DSC was performed on
a TA Instruments Q20 equipped with a RCS 90 cooling system.
About 3−5 mg of polymeric sample was weighed inside an aluminum
pan and subjected to DSC measurements under a nitrogen
atmosphere. Unless otherwise noted, polymers were screened twice
from −90 to 200 °C at a constant heating/cooling rate of 10 K/min.
Melting and glass transition (Tg) temperatures and the enthalpy of
melting were determined from the second heating, crystallization
temperature from the second cooling cycle.
Nojiri et al. reported a UPy functionalized polypropylene
(PP),38 where melt viscosity was increased after functionaliza-
tion with UPy units. The reported dissociation temperature of
UPy moieties is at 190 °C, which is about 40 °C higher than
the degradation temperature of UPy moieties reported in the
literature.35,39 Additionally, they described the compatibiliza-
tion of isotactic polypropylene functionalized with UPy and
ethylene-co-propylene functionalized with Napy, using the
ability of these moieties to form heterodimers.40 Here, melt
mixing of polymers was performed at 170 and 210 °C,
exceeding the reported temperature of degradation of UPy in
polymers by 20 and 60 °C, respectively. Together, this raises
concerns about the chemical integrity of the analyzed polymer
samples.
The development of alternative, more temperature stable H-
bonding motifs is desirable. Herein, we report the development
of the novel self-dimerizing multiple hydrogen bonding array
1-(7-oxo-7,8-dihydro-1,8-naphthyridin-2-yl)urea (ODIN, Fig-
ure 1, compounds 3 and 4) with a DDADA binding motif able
Tensile Tests. Tensile tests were performed on a Zwick type Z100
tensile tester equipped with a 100 N load cell. Specimens were
obtained by cutting strips of 8 cm length and 1 cm width from square
films (10 × 10 cm2) with an average thickness of 150 μm. Sample
strips were clamped, and a preload of 0.3 MPa was applied before
stretching at a speed of 200 mm/min until sample failure occurred.
Compression Molding. Polymer films and samples for DMTA and
rheology were obtained by following the subsequent protocol:
Appropriate molds were filled with the polymer powder, followed
by treating the material in a preheating step for 5 min at 150 °C, then
for 10 min at 150 °C with an applied force of 50 kN, and finally
cooling at a rate of 15 K/min until a temperature of 40 °C was
reached while maintaining the applied force.
DMTA Measurements. Rectangular samples suitable for DMTA
were cut to dimensions of 3 × 5 × 0.5 mm2 (length × width ×
thickness). Samples were measured on a TA Instruments Q800 in
tensile mode. The storage modulus (E′) and loss modulus (E″) were
monitored while screening the samples during a temperature sweep
from −100 to 200 °C at 3 K/min. An oscillation frequency of 1 Hz
with an oscillation amplitude of 10 μm was applied.
Rheology. Samples for rheology were prepared via compression
molding to obtain disk shaped specimens (diameter 25 mm, thickness
1 mm). Samples were measured using a TA Instruments DHR-2,
equipped with a parallel plate geometry. Samples were measured with
a frequency sweep from 100 to 0.01 rad/s using a strain amplitude of
0.4% at a temperature of 150 °C.
Oxygen and Water Transmission. At least two films were
measured for each sample, and obtained transmissions are reported
as the average of both measurements. Oxygen transmission rates were
determined according to ISO 15105-2 at 23 °C, 0% relative humidity
(rh) and at 38 °C, 50% rh. Water vapor transmission rates were
determined at 23 °C and 85% rh, adhering to the ISO 15106-3
standard. The thickness of films, ranging from 100 to 120 μm, was
measured at varying locations, and the average was used to report
thickness corrected oxygen transmission rates.
Figure 1. Overview of employed naphthyridine derivatives.
to form dimers in the solid state. Introduction of ODIN into
poly(ethylene-co-(2-hydroxyethyl methacrylate)) (PE-
HEMA), a functional polyethylene with pendant hydroxy
groups, gives rise to improved mechanical and barrier
properties, an increased melt strength, and improved thermal
stability compared to UPy functionalized polymers.
METHODS
■
General. Nuclear Magnetic Resonance (NMR). NMR spectra
were recorded on a Bruker AVANCE 400 (400 MHz) spectrometer
and a Bruker AVANCE 300 (300 MHz) at room temperature.
1
Chemical shifts are reported in parts per million (ppm). H NMR
chemical shifts are given in reference to the residual solvent peak at
7.26 ppm in CDCl3 and at 2.50 ppm in DMSO-d6. High-temperature
NMR spectra were recorded on a Bruker AVANCE III (500 MHz)
equipped with a cryogenically cooled probe head at 120 °C in
1
deuterated tetrachloroethane (TCE-d2). H NMR chemical shifts are
given in reference to the residual solvent peak of TCE at 6.00 ppm.
Infrared Spectroscopy (IR). IR spectra were recorded on a
PerkinElmer Spectrum One equipped with a Golden Gate accessory
(diamond ATR). Variable temperature IR (VTIR) spectra were
recorded on a Bruker Vertex 70 spectrometer equipped with a
diamond ATR accessory (GladiATR 300 by PIKE Technologies).
First, spectra were recorded of the empty crystal at all employed
temperatures. These spectra were subsequently used as background
spectra for the actual measurements to compensate for the change in
transmission of diamond at elevated temperatures.
Elemental Analysis (EA). A small amount of sample is burned in
the presence of oxygen. The nitrogen content was determined by
means of thermal conductivity (TC) and volumetric analysis. The
hydrogen and carbon contents were determined by NIR.
Synthesis. Unless otherwise specified, chemicals and solvents were
purchased from Sigma-Aldrich and used as received. The poly-
(ethylene-co-(2-hydroxethyl methacrylate)) copolymers were pro-
vided by SABIC. All solvents employed were laboratory grade and
used as received. An overview of the prepared naphthyridine
derivatives is displayed in Figure 1.
B
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7-Amino-1,8-naphthyridin-2(1H)-one (1). Naphthyridine 1 was
synthesized according to the literature.45 Ground diaminopyridine
(109.13 g, 0.98 mol, 1 equiv) and malic acid (145 g, 1.08 mol, 1.1
equiv) were placed in a three-neck flask and cooled in an ice bath.
Sulfuric acid (500 mL) was slowly added while maintaining an
internal temperature below 45 °C. The obtained mixture was slowly
heated to 110 °C and reacted for 2.5 h. Subsequently, after cooling
the mixture, the lukewarm solution was poured over ice, and
concentrated ammonia (∼1.5 L) was slowly added while stirring until
a pH of ∼7−8 was reached. The precipitate was filtered and first
washed with ∼9 L of water and then with a mixture of H2O and
MeOH (9:1) until the wash was colorless. The solid residue was dried
under vacuum at 100 °C to obtain the product as a deep brown solid
(133.2 g, 84% yield). The characterization gave values consistent with
the literature.45 1H NMR (300 MHz, DMSO-d6) δ: 11.85 (s, 1H),
7.65 (d, J = 9.3 Hz, 2H), 7.64 (d, J = 8.6 Hz, 1H), 6.34 (d, J = 8.5 Hz,
1H), 6.11 (d, J = 9.2 Hz, 1H).
hydroxy groups) was added as a solid, followed by addition of 4 drops
of dibutyltin dilaurate (DBTDL) as catalyst. The temperature of the
mixture was raised to 100 °C and stirred for 4 h. The hot suspension
was filtered through a preheated (140 °C) Buchner funnel to filter off
residual unreacted 4 and immediately precipitated into isopropanol.
The precipitate was dried under vacuum to obtain polymers P1−P4
as off-white to yellow solids.
RESULTS AND DISCUSSION
■
Design and Preparation of 1-(7-Oxo-7,8-dihydro-1,8-
naphthyridin-2-yl)urea (ODIN), a Novel Multiple Hydro-
gen Bonding Motif. As the most ubiquitous self-dimerizing
multiple hydrogen bonding motif to date,47,48 we first
considered the quadruple hydrogen bonding motif ureidopyr-
imidinone (UPy)49 (Figure 2A) for the preparation of
7-Amino-4-butyl-1,8-naphthyridin-2(1H)-one (2). The synthesis
for 2 was adapted from the literature.46 A three-neck flask equipped
with a thermometer, mechanical stirrer, and dropping funnel was
charged with diaminopyridine (12.67 g, 1 equiv) and methyl 3-
oxoheptanoate (20.20 g, 1.1 equiv) and stirred for 1 h. The mixture
was then cooled to 0 °C, and sulfuric acid (65 mL) was slowly added
while maintaining a temperature below 45 °C. After full addition the
dropping funnel was replaced with a reflux condenser, and the mixture
was heated for 1.5 h at 120 °C. The solution was then cooled to room
temperature, poured over ∼100 g of ice, and brought to pH ∼ 7−8 by
slow addition of a 30% ammonia solution (∼125 mL). The formed
precipitate was filtered, washed with water, then triturated with a 1:9
methanol/water mixture, filtered again, and washed with methanol/
acetone until the wash was colorless. The product was dried under
vacuum and obtained as a yellow solid (10 g, 40% yield), which was
used immediately without further purification. MS (ESI) m/z: [M +
H]+ calcd for C12H15N3O, 218.13; found, 218.09.
1-(5-Butyl-7-oxo-7,8-dihydro-1,8-naphthyridin-2-yl)-3-(4-
ethylphenyl)urea (3). 2 (0.1 g, 0.46 mmol, 1 equiv) and 4-
ethylphenyl isocyanate (0.3 mL, 2.1 mmol, 4.6 equiv) were combined
in a 10 mL vial and stirred overnight at 110 °C. A 1:1 hexane/acetone
mixture (5 mL) was added, and the precipitate was filtered off and
washed with acetone until the wash was colorless. The compound was
dried under vacuum and obtained as a tan powder (0.106 g, 63%
yield). Crystals shaped like needles suitable for X-ray crystallography
were obtained from 3 dissolved in chloroform and layered with
acetone. 1H NMR (300 MHz, DMSO-d6) δ: 12.26 (s, 1H), 10.56 (s,
1H), 9.87 (s, 1H), 8.11 (d, J = 8.7 Hz, 1H), 7.62 (d, J = 8.5 Hz, 2H),
7.16 (d, J = 8.4 Hz, 2H), 7.07 (d, J = 8.7 Hz, 1H), 6.24 (s, 1H), 2.74
(t, J = 7.6 Hz, 2H), 2.58 (q, J = 7.6 Hz, 2H), 1.64−1.51 (m, 2H), 1.38
(m, 2H), 1.18 (t, J = 7.6 Hz, 3H), 0.92 (t, J = 7.3 Hz, 3H). MS (ESI)
m/z: [M + H]+ calcd for C21H24N4O2, 365.20; found, 365.23.
1-(6-Isocyanatohexyl)-3-(7-oxo-7,8-dihydro-1,8-naphthyridin-2-
yl)urea (4). Naphthyridine 1 (50 g, 0.31 mol, 1 equiv) and
hexamethylene diisocyanate (500 mL, 3.1 mol, 10 equiv) were
combined and stirred. The resulting slurry was heated for 15 h at a
temperature of 118 °C (temperature of oil bath). After cooling to
room temperature, the solid was filtered over a Buchner funnel and
washed with hexane and acetone until the wash was colorless. The
solid residue was dried under vacuum at 60 °C, yielding the product
Figure 2. Structures of (A) ureidopyrimidinone (UPy) motif,49 (B) 7-
acylamino-[1H]-2-oxo-1,8-naphthyridine (NAPY),54 and (C) the
naphtyridine based motif ODIN. On the right, the respective
arrangements of donor (D) and acceptor (A) moieties as well as
attractive (green dotted) and repulsive (red dotted) secondary
electrostatic interactions (according to Jorgensen55,56) are illustrated.
functional polyolefins. However, because of concerns about
thermal stability due to the UPy urea bond being cleaved at
elevated temperatures50,51 and a degradation temperature of
UPy moieties in polymers given at ∼150 °C,35 we desired an
alternative. Some of the UPy degradation compounds have
been identified as isocyanate species,50,52 which form as a
result of the thermoreversible nature of the urea moiety.53
It has been shown that urea thermal stability and extent of
electron delocalization in aryl-substituted urea can be
correlated.57 Thereby, extending the conjugation of the aryl
substituent (in the case of UPy the substituent can be viewed
as isocytosine) might result in a more temperature stable urea.
One way to extend the conjugated system is by employing
1
as a tan powder with minor impurities (72.7 g, 71% yield). H NMR
(400 MHz, DMSO-d6) δ: 12.18 (s, 1H, Ar−NH), 9.65 (s, 1H, Ar−
NH−CO−NH), 8.99 (s, 1H, NH−CO−NH−Ar), 7.91 (d, J = 8.4
Hz, 1H), 7.78 (d, J = 9.4 Hz, 1H), 6.84 (d, J = 8.5 Hz, 1H), 6.34 (d, J
= 9.4, 1.8 Hz, 1H), 3.39−3.31 (m, 2H), 3.20 (dd, J = 6.7 Hz, 2H),
1.62−1.48 (m, 4H), 1.43−1.19 (m, 4H). MS (ESI) m/z: [M + H]+
calcd for C17H23N5O4 (methanol adduct), 362.18; found, 362.26.
Synthesis of Polymers P1−P4. PE−HEMA copolymers were
functionalized according to the following general procedure: PE−
HEMA (4 g) was dissolved under a nitrogen atmosphere in toluene
(200 mL) at 80 °C for 1 h inside a custom-made glass reactor
equipped with mechanical stirring, cooling, and gas inlet. The
apparatus was flushed with nitrogen, and 4 (1.3 equiv with respect to
C
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fused aromatic species.58 Naphthyridines, a class of hetero-
cycles made up of two fused aromatic rings,59 could provide a
suitable alternative to UPy. For example, a known quadruple
hydrogen bonding motif can be obtained by acylation of
naphthyridine 1 (Figure 2B).54,60 NAPY dimerizes via an
alternating DADA arrangement of donors and acceptors with
an association constant (Ka) of ∼750−1900 M−1 (in CHCl3),
dependent on the side chain.54 The comparatively weak
association constant was desired because already 1−2 mol % of
weakly hydrogen bonded dimers is sufficient to affect the
mechanical behavior in thermoplastics substantially.61,62 An
array of at least four hydrogen bonds should be maintained to
ensure association in the presence of water.31,32 To facilitate
the introduction of these hydrogen bonding arrays into
functional polymers with little synthetic effort, an excess of
diisocyanate was employed to react with the free amine group
of 1, forming 4 with an urea link and free aliphatic isocyanate
available for functionalization. The introduction of the urea
group provides an additional hydrogen bonding donor that can
interact with one of the oxygen lone pairs of the lactam,
resulting in a self-assembly mode involving six hydrogen bonds
(Figure 2C, the ODIN motif). The assembly mode of ODIN
was elucidated by synthesizing derivative 3 with an increased
solubility in organic solvents, needed to grow single crystals.
Compound 3 presents the same DDADA arrangement of
donor and acceptor groups as 4, specifically an urea group with
two N−H as donor, an aromatic N as acceptor, and a lactam
N−H donor, CO acceptor moiety. The Ka of ODIN was
investigated by fluorescence measurements of 3 dissolved in
CHCl3 (see Supporting Information for method and Figure
S1), from which a Ka of 4 × 104 M−1 (in CHCl3) was
determined for ODIN dimerization. The crystal structure of 3
reveals the formation of dimers that stack head to tail driven by
the self-complementary DDADA arrangement (Figure 3A). As
consequence the molecules are coplanar, and their mutual
disposition allows for the formation of two bifurcated
hydrogen bonds between both urea N−H donor moieties
and the oxygen of the lactam, while the aromatic nitrogens are
involved with the aromatic N−H groups of the respective
molecular counterparts of the dimer. The only heteroatoms
not involved in the intradimer interactions are the oxygen
atoms of the urea group, which accept weak hydrogen bonds
from the aliphatic C−H groups belonging to adjacent dimers
(Figure 3B).
The intermolecular interaction energy of the dimer of 3
observed in the solid state was calculated using the program
Crystal Explorer.63 According to the adopted methods (see
Supporting Information), the total energy of the dimeric unit
amounts to −166.8 kJ/mol. Analogous calculations were
performed considering the two different dimeric forms
discussed previously (A and B in Figure 2), showing −116.3
kJ/mol for the DADA and −203.5 kJ/mol for the DDAA type.
As a matter of fact, the most stabilizing interaction can only be
achieved avoiding alternation of donor and acceptor groups.
Indeed, although for the dimer of 3 the greater number of
interactions could potentially lead to a better stabilization, both
studies in the solid state and in solution reveal that the
DDADA sequence results into a less effective association than
the DDAA (UPy Ka = 2 × 107 M−1 in CHCl3).33 This aspect
has already been addressed by Jorgensen et al. on the basis of
ab initio computational studies,55 experimentally confirmed by
Zimmerman et al.,56 and the present evidence further confirms
the applicability of this rule to the cited systems.
Synthesis of Multiple Hydrogen-Bonding Function-
alized PE Polymers. For the preparation of a multiple
hydrogen bonding cross-linked polyolefin, polymers bearing
moieties suitable for grafting of the hydrogen bonding motifs
are required. Because ODIN motif 4 (Figure 1) bears an
aliphatic isocyanate, polymers with pendant hydroxy or amino
groups were considered. PE−HEMA, a copolymer of ethylene
and 2-hydroxyethyl methacrylate (structure in Figure 4A), is
available from SABIC with HEMA content ranging from 1.6 to
12.4 mol %. The pendant hydroxy groups of HEMA allow for
easy grafting of the isocyanate 4.64−66 A homopolymeric PE
sample, prepared employing the same reaction conditions that
were used to synthesize PE−HEMA, was used as a reference
material. The provided copolymers used in this study, their
HEMA content in mol % determined via NMR, their
molecular weight determined by GPC (traces reported in
Figure S2), Tm determined via DSC (graphs in Figure S3), and
Tβ obtained from DMTA are reported in Table 1.
The synthesis pathway for the formation of multiple
hydrogen bonded semicrystalline LDPE is depicted in Figure
4A. The reaction was carried out for 4 h at 100 °C in toluene
with DBTDL acting as a catalyst. An overview of the prepared
functional polymers P1−P4, the content of introduced ODIN
determined via either NMR or elemental analysis (EA), and
some characteristic thermal data are given in Table 2.
Molecular weight was not determined after functionalization
due to solubility issues in o-dichlorobenzene. Because of the
nature of cross-linking via multiple hydrogen bonding moieties,
the virtual molecular weight should increase, reflected by the
decreased solubility.68 1H NMR of P1−P4 in TCE-d2 at
120 °C revealed the characteristic, strongly downfield shifted
signals corresponding to amine protons participating in
multiple hydrogen bonding (Figure 4B).69 The degree of
functionalization could be investigated via 1H NMR by
following the intensity of the HEMA methylene signals (δ ∼
Figure 3. Dimeric structure of 3 (A) and its interactions with adjacent
dimers (B), as observed in the crystalline phase. The intradimer and
interdimer hydrogen bonds are highlighted in light green and orange,
respectively.
1
3.85 and 4.25 ppm). A selected portion of the H NMR
spectrum for P1, detailing the hydroxyethylene methylene
signals, is shown in Figure 4C. The two distinct methylene
D
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Figure 4. (A) Synthesis of hydrogen bonding functionalized PE−HEMA; cartoon depicts semicrystalline nature of resulting polymer cross-linked
1
via multiple hydrogen bonding moieties. (B) Selected portion of P1 H NMR (120 °C, in TCE-d2) highlighting the hydrogen bonding region;
1
signal of CH−NH is superimposed by the residual solvent TCE signal. (C) Selected portion of P1 H NMR (120 °C, TCE-d2), highlighting the
HEMA hydroxyethyl signal region; the methylene groups of functionalized HEMA give rise to only a single signal.
1
For functional polymers P3 and P4, H NMR was not
successful in determining the ODIN content due to only
partial solubility in tetrachloroethane (TCE) at 120 °C.
Therefore, elemental analysis (EA) was performed to
determine the amount of incorporated ODIN. The EA
determined ODIN content was in good agreement with the
1H NMR determined content for soluble polymers P1 and P2
and will be used for subsequent comparison of these materials.
Thereby, polymers with ODIN content ranging from 1 to 6
mol % could be obtained (Table 2). If we assume that the
number of repeating units per chain does not change with
functionalization, the average number of hydrogen bonding
motifs per chains nHB can be calculated according to the
following relation:70
Table 1. HEMA Content, GPC Data, and Thermal Data for
PE−HEMA Copolymers
a
bc
,
d
e
HEMA
Mw
Tm
Tβ
b
entry
LDPE
PE−HEMA 1
PE−HEMA 2
PE−HEMA 3
PE−HEMA 4
(mol %)
(kg/mol)
PDI
(°C)
(°C)
0
200
270
85
37
35
7.7
9.2
6.3
3.8
119.5
108.3
97.9
91.5
80.5
n.d.
8.8
−0.3
−3.5
−10.8
1.6
5.15
6.8
12.4
3.6
c
a
b
Determined via NMR. Determined via GPC. Decreasing molecular
weight due to HEMA acting as chain transfer agent.67 Determined
d
e
via DSC. Determined via DMTA. For details, see the Methods
section.
Table 2. Overview of Prepared ODIN−PE−HEMA
MnxHB
nHB
=
Polymers
MHEMAxHEMA + MEt(1 − xHEMA
)
(1)
a
b
d
e
ODIN
ODIN
Tm
Tβ
with Mn as the number-average molecular weight of parent
PE−HEMA, xHB and xHEMA as the molar fraction of ODIN
(HB) and HEMA, and MHEMA and MEt as the molecular weight
of HEMA and ethylene. According to eq 1, nHB was
determined to be 9.9, 9.5, 9.5, and 14.3 for polymers P1, P2,
P3, and P4, respectively. Given that there are many more than
two moieties per chain, a supramolecular network should be
formed as long as the association of dimers is thermodynami-
cally favored.
entry parent polymer
(mol %)
(mol %)
(°C)
(°C)
P1
P2
P3
P4
PE−HEMA 1
PE−HEMA 2
PE−HEMA 3
PE−HEMA 4
0.96
2.16
0.98
2.25
3.45
107.3
95.5
86.5
79.4
17.2
20.0
33.0
48.2
c
3.06
c
2.98
6.05
a
b
c
Determined via NMR. Determined via EA. Not fully dissolved in
TCE-d2 at 120 °C. Determined via DSC. Determined via DMTA.
d
e
Hydrogen Bonding in ODIN−PE−HEMA Polymers.
The influence of the introduction of the multiple hydrogen
bonding moieties was investigated via DSC measurements on
functional polymers (graphs in Figure S4). Compared to the
pristine polymers, Tm and ΔHm of functionalized materials
decrease. The bulky motif grafted onto the polymer backbone
hinders crystallization of the polymer main chain, reducing
crystallinity and Tm.38,71−73 In addition, Tβ increases for each
signals of nonfunctional HEMA combine to a single peak after
formation of the carbamate (δ ∼ 4.3 ppm). From the ratio of
these two signals (nonfunctionalized and ODIN-functionalized
HEMA) the degree of functionalization can be determined.
About 40−60% of hydroxy groups underwent functionaliza-
tion, with increasing reaction time having no considerable
influence on grafting percentage.
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of the samples after functionalization (see Tables 1 and 2).
This is due to the multiple hydrogen bonding facilitated
formation of physical cross-links between chains, known to
restrict chain movement and therefore increasing Tg.70,74,75
To showcase the adaptable nature of the hydrogen bonding
functionalized system, stress relaxation was performed on P2 at
140 °C in tensile mode. A strain of 0.5% was applied to P2,
and the resulting stress, normalized for the initial stress, was
monitored over time (Figure S5). A slow decline of the
normalized stress to 20% of the initial value is observed within
1000 s, with full relaxation expected to be reached within
10000 s. Even though relaxation is slow, the stress of the
physically cross-linked material can relax over time. In
addition, it was possible to prepare defect-free samples suitable
for mechanical analysis via compression molding at 150 °C
(Figure S6), revealing indeed adaptability of our hydrogen
bonding network. Compression molded samples of P1−P4
and their respective PE−HEMA counterparts were analyzed
via DMTA.
copolymers such as the ones employed here, usually three main
relaxations, α, β, and γ, are observed during DMTA.
Specifically, α is close to the melting temperature, β close to
room temperature, and γ below −100 °C, respectively.76,77
The γ relaxation was not observed within the available
measurement regime between −100 and 200 °C. A non-
polyethylene related relaxation is observed for P2−P4 and
their corresponding pristine counterparts at −70 °C. Because
no such transition is visible for P1 and PE−HEMA 1, we
believe that it is likely motion induced by breaking and re-
forming hydrogen bonds of the HEMA side chain.78
Associated with main chain movements in the amorphous
region,79 recent evidence denotes the β relaxation as the glass
transition of polyethylene and its copolymers.80 This relaxation
has been found to change in intensity and location with the
degree of crystallinity because of hindered amorphous chain
segments as a result of the physical cross-links provided by the
polymer crystallites.81,82 Consequently, any types of additional
branching points or cross-links should raise Tβ. In fact, despite
large molecular weight differences between pristine PE−
HEMA polymers, DMTA determined ΔTβ increases linearly
with ODIN content between pristine and functional polymers
(Figure S8). This reflects a behavior previously observed for
UPy functionalized methacrylate, where the glass transition
increased linearly with UPy content for polymers of equal
molecular weight.83,84 A similar relationship is only partially
verified for DSC determined ΔTg because of the inaccuracy in
evaluating Tg via DSC. The cross-links were still in effect for
the region between Tβ and Tα (T > 50 °C), where a greater E′
was measured for P2−P4, with a less pronounced difference
observed for P1.85
The data obtained for P2 and the corresponding PE−
HEMA 2 are presented in Figure 5A (DMTA graphs for P1−
P4 in addition to their respective pristine PE−HEMA
polymers are given in Figure S7). In polyethylene and related
The α transition, usually related to main chain movements
that become possible at the onset of melting of the crystalline
phase,76,82,86 is observed as a stark increase in tan δ for all of
the pristine polymers until melting of the sample specimen
leads to measurement stop. For functionalized samples, a more
gradual increase of tan δ is observed leading into a peak for
P2−P4, with tan δ of P1 forming a shoulder around the
melting temperature.
Remarkably, heating above Tm did not lead to measurement
stop for functionalized samples, and the elastic character was
retained, revealing a rubbery plateau after melting, character-
istic for cross-linked polymers.87−89 We attribute the formation
of this plateau modulus to the cross-linking via multiple
hydrogen bonding arrays, which is still effective above Tm. This
was unexpected because these arrays of hydrogen bonds are
expected to dissociate at elevated temperatures,90 and
adaptability of our network was apparent from stress relaxation
measurements and from the possibility to obtain defect-free
samples via compression molding. In contrast to cross-linked
rubber, where E′ usually remains constant above Tg,91−93 E′
decreases gradually after Tm up until 190 °C for functional
polymers. At 190 °C, the polymer structure changes involving
the formation of permanent cross-links, causing an increase in
E′, which will be discussed at a later stage. Because the
hydrogen bonding moieties are in equilibrium between
dissociated and associated states, we propose that the
increasing temperature causes a shift from the dimeric to
monomeric state, thus decreasing cross-link density.52,75,84 The
conformational stability of functional samples at elevated
temperatures is easily visualized by looking at the DMTA
sample strips after measurement displayed in the inset picture
of Figure 5A. Melting compromised the shape of pristine
Figure 5. (A) DMTA sweep of P2 (black) and PE-HEMA 2 (red); E′
is represented by closed symbols and δ by open symbols. The light
red square highlights the temperature range above melting of the
crystalline region; inset shows DMTA strips after measurements: P2
(top) and PE-HEMA 2 (bottom). (B) Rheology frequency sweep
from 0.01 to 100 rad/s of P2 (black) and PE-HEMA 2 (red) at 150
°C. G′ is represented by closed symbols, G″ by half symbols, and
phase angle by open symbols.
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(bottom), but not of functional sample (top). To investigate
the properties of ODIN functional polymers in the melt state,
P2 was subjected to a frequency sweep at 150 °C, which is the
temperature of compression molding, using parallel plate
rheology, and compared to the pristine PE-HEMA 2. The data
for both polymers are shown in Figure 5B. At 150 °C, the
pristine sample (red line) was completely molten and
displayed typical behavior of entangled polymer melts.94 G″
is above G′ over the entire frequency range, typical for non-
cross-linked thermoplastics in the melt. No crossover point is
observed for the pristine polymer within the measured
frequency range, likely because it would occur outside of the
measurement range. G″ and G′ are strongly dependent on the
observed time scale, displaying Newtonian-like flow behavior
at long time scales (low frequencies) and more elastic behavior
at short time scales (higher frequencies). However, the
frequency sweep of the functional polymer reveals a completely
different behavior. G′ is above G″ over the entire frequency
range, displaying solid, not melt-like, behavior. Additionally,
the moduli are largely unaffected by frequency. Both are
characteristic properties of cross-linked polymer systems,
revealing the effective cross-linking via multiple hydrogen
bonding in the melt.94
To confirm our theory that the multiple hydrogen bonding
motifs are largely associated at elevated temperatures, variable
temperature IR (VTIR) was performed. VTIR is a useful tool
to investigate these types of interactions because the bands of
amine and carbonyl groups involved in hydrogen bonding shift,
as they dissociate.35,95−97 VTIR was performed on P3,
measuring first blank spectra as background and then taking
spectra in intervals, starting from 25 °C up to 175 °C. These
spectra are compared to ODIN functionalized reversible cross-
linked Jeffamine T3000, a trifunctional polyether amine
(JeffODIN; Figure S9). Here, functionalization of the liquid
Jeffamine T3000 results in a supramolecular polymer able to
form flexible films due to linkage of the low molecular weight
species via sextuple hydrogen bonding. The VTIR spectra for
JeffODIN and P3 are shown in Figure 6. When looking at the
temperature influence on the absorption peaks of JeffODIN,
clear shifts are apparent in the CO stretching and N−H
bending region. Specifically, with increasing temperature the
band at 1610 cm−1 loses intensity, and the absorption band
observed at 1570 cm−1 shifts significantly toward lower
wavenumbers. Together, these shifts indicate the progressive
dissociation of hydrogen bonds at elevated temperature.97 This
is in line with visual observations, where viscous flow can be
observed for JeffODIN starting at around 140 °C. On the
contrary, for spectra of P3, only one peak can be observed to
clearly change around 720 cm−1, corresponding to the melting
of PE crystalites.98 No clear influence of temperature on peaks
involved in hydrogen bonding is apparent for ODIN
functionalized polymers. This can be attributed either to the
influence of the apolar polymer matrix known to increase the
strength of hydrogen bonds,30,36 or by the cooperative
associative effect due to the increased amount of moieties
per chain. This effect was previously elucidated by Long et al.,
showing that more thermal energy was required to disrupt
association of species with increasing numbers of UPy
moieties.99 Based on these results, multiple hydrogen bonding
is clearly still in effect for ODIN functional polyolefin after
melting of the crystalline regions and responsible for the
observed plateau moduli after melting.
Figure 6. VTIR spectra of (A) ODIN functional polymer P3 and (B)
trifunctional supramolecular polymer JeffODIN. Samples were heated
stepwise starting from 25 °C up to 150 °C in the case of JeffODIN
and 175 °C for P3. The shifting absorption bands are indicated with
arrows. Spectra are normalized for the C−H stretch region around
2830 cm−1.
Thermal Stability of ODIN−PE−HEMA Polymers. One
of the main motivations for developing the ODIN motif was
the desired improved thermal stability over UPy functionalized
polymers. To investigate the temperature of degradation, heat
cycling DSC measurements were performed, repeatedly
heating in steps of 10 °C/cycle from 160 to 250 °C using a
heating rate of 10 K/min (Figure S10). The evolution of Tm
was followed, and its shift was interpreted as degradation of the
polymer structure. Indeed, for ODIN functionalized PE−
HEMA, a Tm shift occurs at about 190 °C, placing the
degradation temperature around 40 °C higher than the
degradation temperature of UPy.35,39 For some UPy function-
alized polymer systems, thermal stability above 200 °C is
claimed in the literature, based on TGA results alone.51,100
Given the contradicting nature of these claims with other
reported thermal stabilities of UPy functionalized polymers,35
we suspected that TGA would give a false sense about the
thermal stability of ODIN functional polymers, as it relates to
the formation of volatiles.
To investigate whether TGA would report thermal stability
higher than 190 °C for ODIN functionalized polymers,
polymers P3 and corresponding PE-HEMA 3 were subjected
to TGA and TGA-DSC measurements. Two major weight loss
steps were observed for the functionalized polymer at 300 and
440 °C (Figure S11). During the first step, a mass loss of
24.76% was observed, in line with the theoretical weight loss of
24.58% for full abstraction of ODIN-motif in P3, cleaved at the
carbamate linkage. Consequently, we surmise that during the
first weight loss step ODIN and the linker chain degrade into
volatiles, whereas the second weight loss step is caused by the
polymer main chain degradation. This is coherent with the
measurement of parent PE-HEMA 3, where only one weight
loss step, in line with the second weight loss step of the
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functional polymer, was observed (Figure S12). No weight loss
or peak in DSC-TGA was observed at 190 °C, the temperature
of degradation determined via DSC.
As previously discussed, the urea group in UPy is
thermoreversible, so that elevated temperature leads back to
formation of amine and isocyanate species. Likewise, we
assumed that the urea moiety of ODIN motif is thermorever-
sible. Therefore, thermal treatment of functional polymer
species between 190 and 300 °C would not lead to degradation
as such, but to formation of permanent cross-links facilitated
by isocyanate−amine reactions. To investigate this theory,
DMA was performed on P4 at 190 °C, the degradation
temperature determined via DSC, while monitoring E′, E″, and
tan δ as a function of time. P4 was chosen because it has the
largest amount of ODIN, and changes should be seen easily.
The obtained data are shown in Figure 7. After 2 min at 190
Figure 8. Measured average tensile strength and Young’s modulus for
reference LDPE, P1−P4, and related parent PE−HEMA 1−4
polymers determined from at least three test specimens.
HEMA 4 was improved 3-fold, displaying the potential of
ODIN motif to improve mechanical properties of polymeric
species.37 Interestingly, no improvement of tensile strength and
only slightly increased stiffness were observed between P3 and
P4, despite the latter being functionalized with almost twice
the amount of ODIN, revealing diminishing returns of ODIN
functionalization. Compared to reference PE, tensile properties
seem to be on par or slightly increased. This is due to the
significantly reduced crystallinity of functionalized samples
compared to the semicrystalline reference PE.105
Multiple Hydrogen Bonding Cross-Linked Polyethy-
lene as Oxygen Barrier Material. To reduce gas
permeability through semicrystalline polymers, the free volume
available for permeation may be reduced.106 Reduction of the
polymer free volume can be achieved by affecting either size or
shape of polymer crystals or by reducing the chain mobility in
the amorphous regions.14 By introducing cross-links in the
amorphous phase, the cohesive energy density between chains
can be increased, which in turn reduces the oxygen
permeability.107 The formation of an efficient hydrogen
bonding network should provide such cross-links resulting in
increased barrier properties.
To investigate the effect of HEMA content and ODIN
functionalization on the water and oxygen barrier properties,
standard measurements108 were performed to determine
oxygen and water vapor transmission rates of selected
polymers. The thickness corrected values for oxygen trans-
mission obtained at 38 °C, 50% rh and water transmission
measured at 23 °C, 85% rh are illustrated in Figure 9
(tabulated data and graph for oxygen transmission measured at
23 °C, 0% rh are reported in the Supporting Information). For
PE−HEMA copolymers, a HEMA content dependent increase
in both water and oxygen transmission rate is observed. This is
due to the water penetrating the polymer matrix, revoking the
interchain hydrogen bonds, therefore progressively affecting
polymers with greater HEMA content.14 By contrast, the
measurements at near zero humidity reveal an almost constant
oxygen transmission rate between different PE−HEMA
polymers because the hydrogen bonding remains intact
(Figure S13). The increased HEMA content should decrease
the oxygen transmission due to network formation, but the
advantageous effects are likely off set by the loss in crystallinity
caused by the introduction of HEMA (see Figure S3). Upon
Figure 7. Graph of E′, E″, and tan δ of P3 measured in DMA tensile
mode over 16 min at a constant temperature of 190 °C.
°C, an increasing E′ accompanied by a decrease of tan δ is
observed. It is known that the formation of permanent cross-
links is related to an increase in E′.101 We can deduct that
permanent cross-linking takes place in the polymer sample. We
theorize that this is the result of re-formation of isocyanates
and amines followed by immediate cross-linking of these
species. On the basis of these results, together with results
obtained from DSC and TGA, we believe that TGA alone
cannot be the sole technique on which to claim thermal
stability and structural integrity for these aryl urea type
multiple hydrogen bonding species attached to polymers.
Tensile Properties of Functionalized Polymers. To
assess the influence of cross-linking via ODIN on tensile
properties, functionalized polymers P1−P4 were subjected to
tensile tests and compared to their respective pristine
counterparts as well as the reference PE sample. The tensile
strength and Young’s modulus data for the measured polymers
are given in Figure 8. Not shown are the data for strain at
break, which generally decreases for functionalized polymers
due to the formed cross-links.102,103 The tensile strength of our
reference LDPE material was within commonly obtained
values.104 For P1, no improved strength or increased stiffness
was observed after functionalization, likely because of the
reduced crystallinity of the functionalized sample without yet
forming a strong enough hydrogen bonding network. For PE-
HEMA 2, 3, and 4, however, the functionalization with ODIN
tends to increase tensile strength and significantly increases
stiffness of the soft polymers. The tensile strength of PE-
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after melting of the crystalline lattice, as revealed by parallel
plate rheology and DMTA, where a rubbery plateau modulus
was observed after melting, revealing a remarkably increased
melt strength. This increased strength of hydrogen bonds is
attributed to the apolar polymer matrix, elucidated via VTIR
measurements. In comparison to UPy functionalized polymers,
degradation involving permanent cross-linking of ODIN
functional polyolefins occurs about 40 °C later. Moreover,
compared to single hydrogen bonds, an increased resilience of
the sextuple hydrogen bonding motif toward humidity was
revealed during oxygen and water transmission measurements.
For subsequent research, the influence of the polymer matrix
must be considered in the design of multiple hydrogen
bonding functionalized polymers. An appropriate matrix
polymer could be used to tune the strength of hydrogen
bonding moieties, making dimers with weaker association
constants attractive choices. The increased thermal stability of
ODIN should provide a useful tool for functionalization of
polyolefins, but a key aspect will be to introduce moieties
without adversely affecting the crystallinity of these polymers.
Figure 9. Oxygen and water vapor transmission rates of pristine and
ODIN functional polymers. Oxygen transmission was measured at 38
°C, 50% rh and water vapor transmission at 23 °C, 85% rh.
Corresponding pristine (open circles) and ODIN functional (closed
squares) polymers share the same color. The arrows highlight the
trend in transmission rates with respect to increasing HEMA and
ODIN content for pristine and ODIN functional polymers.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01715.
functionalization of PE−HEMA polymers with ODIN, a
gradual decrease in oxygen transmission is observed between
pristine and functional polymers measured at 38 °C, 50% rh.
Also, with increasing ODIN content, a lower oxygen
transmission rate is detected, not being adversely affected by
humidity in the same manner as pristine polymers with higher
HEMA content were.
In addition, an almost equal water vapor transmission rate
can be observed for all functional samples, showcasing the
increased stability of these hydrogen bonding arrays toward
humidity compared to single hydroxy hydrogen bonds.
Experimental data, synthesis of supramolecular polymer,
NMR spectra, oxygen and water transmission data
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*(J.V.) E-mail: jerome.vachon@sabic.com.
*(E.D.) E-mail: enrico.dalcanale@unipr.it.
ORCID
Stefano Canossa: 0000-0002-6817-0810
Roberta Pinalli: 0000-0002-0000-8980
Enrico Dalcanale: 0000-0001-6964-788X
■
CONCLUSIONS
■
Using the UPy motif, a plethora of multiple hydrogen bonding
functionalized materials have been developed in the past 20
years. However, functionalization of polyolefins has been
largely unexplored, with few examples including a fully
amorphous LDPE material37 and functional PP probed way
above the degradation temperature of the UPy motif.35,38 To
overcome the thermal stability issues, the novel self-
complementary multiple hydrogen bonding motif ODIN was
developed. The structure of this new motif was elucidated via
X-ray crystallography, revealing the formation of dimers in the
solid state involving two regular and two bifurcated hydrogen
bonds, resulting in a total of six hydrogen bonds. By reacting
ODIN bearing pendant aliphatic isocyanate functionality with
PE−HEMA, multiple hydrogen bonding array functionalized
polyolefins with an ODIN content ranging from 1 to 6 mol %
were obtained. The adaptable nature of the hydrogen bonding
system was revealed via stress relaxation measurements and
compression molding of thin films and sample strips. The
formation of a cross-linked network was apparent from (i) the
increased glass transition temperature determined via DMTA,
(ii) the increased storage modulus of functionalized polymers
above Tg, and (iii) the near frequency independent loss and
storage moduli determined via parallel plate rheology.
Remarkably, cross-linking via ODIN motif was still relevant
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the SUPRAmolecular polyolefins as
oxygen BARRIER materials (SUPRABARRIER) Marie Skło-
dowska Curie project, which is funded through the European
Union Eighth Framework Program (H2020-MSCA-ITN-
2014) under the Grant Agreement No. 642929. We acknowl-
edge Francesca Guagnini and Chiara Massera for their help in
preparing suitable crystals. The financial support from SABIC
is gratefully acknowledged. We thank Peter Neuteboom for
providing the PE−HEMA polymers used in this study.
ABBREVIATIONS
■
PE−HEMA, poly(ethylene-co-(2-hydroxyethyl methacrylate));
PE, polyethylene; PP, polypropylene; DSC, differential
scanning calorimetry; rh, relative humidity; UPy, ureidopyr-
imidinone; A, acceptor; D, donor; TGA, thermogravimetric
analysis; DMTA, dynamic mechanical thermal analysis; ppm,
parts per million; IR, infrared spectroscopy; VTIR, variable
temperature infrared spectroscopy; EA, elemental analysis;
TCE, tetrachloroethane; tan δ, tangent delta.
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