doi.org/10.26434/chemrxiv.10000232.v4
Mechanical and structural consequences of associative dynamic
cross-linking in acrylic diblock copolymers
Jacob Ishibashi, Ian Pierce, Alice Chang, Aristotelis Zografos, Yan Fang, Steven Weigand, Frank S. Bates,
Julia Kalow
Submitted date: 21/12/2020 • Posted date: 22/12/2020
Licence: CC BY-NC-ND 4.0
Citation information: Ishibashi, Jacob; Pierce, Ian; Chang, Alice; Zografos, Aristotelis; Fang, Yan; Weigand,
Steven; et al. (2019): Mechanical and structural consequences of associative dynamic cross-linking in acrylic
diblock copolymers. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.10000232.v4
The composition of low-T
g
n-butylacrylate-block-(acetoxyaceto)ethyl acrylate block polymers is investigated
as a strategy to tune the properties of dynamically cross-linked vinylogous urethane vitrimers. As the
proportion of the cross-linkable block is increased, the thermorheological properties, structure, and stress
relaxation evolve in ways that cannot be explained by increasing cross-link density alone. Evidence is
presented that network connectivity defects such as loops and dangling ends are increased by microphase
separation. The thermomechanical and viscoelastic properties of block copolymer-derived vitrimers arise from
the subtle interplay of microphase separation and network defects.
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Mechanical and structural consequences of
associative dynamic cross-linking in acrylic
diblock copolymers
Jacob S. A. Ishibashi,
1
Ian C. Pierce,
1
Alice B. Chang,
2
Aristotelis Zografos,
2
Yan Fang,
1
Steven J. Weigand,
3
Frank S. Bates,
2
and Julia A. Kalow
1
*
1
Department of Chemistry, Northwestern University, Evanston, IL 60208
2
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis,
Minnesota 55455
3
Argonne National Laboratory, 9700 Cass Ave, Lemont, IL 60439
For Table of Contents use only:
Abstract
The composition of low-T
g
n-butylacrylate-block-(acetoxyaceto)ethyl acrylate block polymers is
investigated as a strategy to tune the properties of dynamically cross-linked vinylogous urethane
vitrimers. As the proportion of the cross-linkable block is increased, the thermorheological properties,
structure, and stress relaxation evolve in ways that cannot be explained by increasing cross-link density
alone. Evidence is presented that network connectivity defects such as loops and dangling ends are
increased by microphase separation. The thermomechanical and viscoelastic properties of block
block
prepolymers
tunable vitrimers
+
soluble
particles
- THF
network
defects
microphase
separation
copolymer-derived vitrimers arise from the subtle interplay of microphase separation and network
defects.
Introduction
Block copolymers that combine rubbery and glassy segments self-assemble into nanostructured
materials with emergent properties not seen in the individual homopolymers. These properties are
highly tunable by adjusting the volume fraction of each block and their architecture.
1
When the low-
T
g
segment is the majority block, microphase separation produces a rubbery matrix physically cross-
linked by spherical glassy domains. Thermoplastic elastomers (TPEs), the most commercially
successful example of this phenomenon, combine the processability of thermoplastics and the
elasticity of chemically cross-linked rubbers.
2
As the volume fraction of the glassy block is increased,
high-impact thermoplastics with bicontinuous morphologies are formed.
3
Above the T
g
of the glassy
block, these block copolymers behave as viscous liquids, enabling extrusion and injection molding of
virgin and recycled material; however, this transition also defines the upper operating temperature of
the material. Under stress, chain pull-out from the hard domains can also occur, ultimately leading to
failure. To replace or reinforce these physical cross-links without sacrificing processability, researchers
have explored various supramolecular interactions, including H-bonds,
4
,
5
metal-ligand interactions,
6
,
7
,
8
π-π stacking,
9
and ionic interactions.
10
,
11
Here, we explore the incorporation of dynamic covalent bonds in rubbery diblock copolymers,
varying their composition to create vitrimers with highly tunable properties. Vitrimers are an emerging
class of covalent adaptable networks (CANs)
12
that also bridge thermoplastics and thermosetting
polymers through associative exchange reactions.
13
Unlike TPEs, supramolecular networks, and many
dissociative CANs, vitrimers do not experience a viscosity drop at elevated temperatures because
reconfiguration of the network topology occurs without loss of covalent connectivity. Furthermore,
the associative mechanism is thought to provide higher solvent resistance, since many vitrimers can
swell in good solvents without dissolving. Following pioneering work by Bowman and Leibler,
14
many
researchers have adapted various associative exchange reactions for use in vitrimers. These efforts
have focused on controlling vitrimer flow through applying new reactions,
15
catalyzing bond
exchange,
15b
or modifying reactivity of existing mechanisms.
16
Macromolecular structure and assembly
might also be harnessed to tune vitrimer properties. For example, cross-link density,
17
chain mobility,
18
and phase separation
19
have all been investigated in conjunction with vitrimers.
While the first generation of vitrimers focused on high-T
g
thermoset-like materials, our group and
others have become interested in the challenge of designing catalyst-free elastomeric vitrimers.
20
With
high-T
g
materials, even if the small-molecule exchange reaction can occur at low temperatures, it will
not proceed appreciably within the network until T
g
is exceeded.
15c
For low-T
g
materials, however, the
associative exchange reaction must be carefully considered to enable reprocessing at reasonable
temperatures but avoid creep under service conditions. Additionally, the identity and structure of the
rubbery polymer matrix can influence the mechanism of the exchange reaction
20b
and the rate of flow.
21
The majority of elastomeric vitrimers have used telechelic or randomly grafted prepolymers, resulting
in vitrimers with a relatively uniform distribution of the exchangeable groups. Segregating the
exchangeable groups into one segment of a block copolymer provides new opportunities to tune the
properties of the resulting vitrimer while keeping the chemical identity of the components constant.
In particular, self-assembly of the vitrimer by microphase separation into cross-link-rich and cross-
link-poor domains can be programmed by the structure of the block prepolymer to dramatically
influence the vitrimer’s thermomechancical properties. We envisioned that minority blocks bearing
exchangeable groups could serve an analogous role to the glassy domains of TPEs, while conferring
the superior solvent and temperature resistance common to vitrimers.
During the course of our study,
22
Sumerlin, Epps, and co-workers reported a glassy vitrimer derived
from the symmetric block copolymer poly(butyl methacrylate)-block-poly(2-(acetoxyaceto)ethyl
methacrylate), or BMA-b-AAEMA, wherein dynamic cross-linking occurs solely in the AAEMA
block
23
using the vinylogous urethane dynamic cross-link developed first by Du Prez and co-workers.
24
They attributed differences in the viscoelastic properties, relative to a statistical copolymer-derived
vitrimer, to the presence of phase separation. This initial contribution focused on thin-film structure
and suggested the potential of block copolymer-derived vitrimers by comparing their high-
temperature stress relaxation and creep with those of a statistical copolymer-derived vitrimer.
To build a more complete picture of the design space surrounding block copolymers as vitrimer
precursors, we vary the composition of a low-T
g
diblock copolymer to study the effect of weight
fraction of the cross-linking domain and study a poly(n-butylacrylate)-block-poly(2-(acetoxyaceto)ethyl
acrylate) (PnBA-b-PAAEA) system. We provide the full details of vitrimer synthesis,
thermomechanical characterization, stress relaxation, and microphase separated structure. Using
dynamic light scattering (DLS) and diffusion-ordered NMR spectroscopy (DOSY NMR), we show
that under the influence of dynamic cross-linking, block copolymers evolve from hyperbranched
polymers to infinite networks. Dynamic mechanical thermal analysis (DMTA) and stress relaxation
measurements reveal that rubbery plateau modulus and flow rates do not increase as expected when
cross-linker is increased. We employ small-angle x-ray scattering (SAXS) to assess the microstructure
of the prepolymers and vitrimers. By studying this series of vitrimers using this broad suite of
techniques, we provide experimental comparisons to recent theories describing vitrimer stress
relaxation and elucidate general strategies for tuning vitrimer properties with block copolymer
composition.
Results
