Synthesis of high molecular flexibility polycarbonates for solid polymer electrolytes

Article abstract of DOI:10.1016/j.electacta.2015.01.074

A new self-plasticizing aliphatic polycarbonate comprising flexible alkyl and alkyl ether side groups was designed and synthesized from six-membered cyclic carbonate monomers with the aim of producing a material with high molecular flexibility (low T_g) and concomitant high ionic conductivity when used as a polymer electrolyte. The T_g of the novel polycarbonate was determined to be -49.4 °C at a molecular weight of 34 400 g mol^-1, which is the lowest reported T_g to date for a substituted poly(trimethylene carbonate). UV-crosslinked polymer electrolytes were produced based on this novel material together with LiTFSI salt and showed ionic conductivities in the range of 2 × 10^-8 to 2 × 10^-7 S cm^-1 at room temperature and 1 × 10^-6 to 1 × 10^-5 S cm^-1 at 100 °C. The limited ionic conductivities of these electrolytes indicate that high molecular flexibility alone does not guarantee fast ion transport in solid polymer electrolytes and that other factors, such as the polarity of the polymer host material, will also influence the transport properties of the electrolyte.

Full text of DOI:10.1016/j.electacta.2015.01.074

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Accepted Manuscript
Title: Synthesis of high molecular flexibility polycarbonates
for solid polymer electrolytes
Author: Jonas Mindemark Laura Imholt Daniel Brandell
PII:
S0013-4686(15)00094-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2015.01.074
EA 24126
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
19-11-2014
17-1-2015
17-1-2015
Please cite this article as: Jonas Mindemark, Laura Imholt, Daniel Brandell, Synthesis of
high molecular flexibility polycarbonates for solid polymer electrolytes, Electrochimica
Acta http://dx.doi.org/10.1016/j.electacta.2015.01.074
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Synthesis of high molecular flexibility polycarbonates for solid polymer electrolytes
Jonas Mindemark*, Laura Imholt, Daniel Brandell
Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE-751 21
Uppsala, Sweden
* Corresponding author. E-mail: jonas.mindemark@kemi.uu.se; tel.: +46 18 471 37 36.
Graphical abstract
Highlights
 A new polycarbonate was synthesized and showed a record-low Tg of −49.4 °C.
 Crosslinking provided mechanically stable polymer electrolyte films.
 Ionic conductivity up to 2×10−7 S cm−1 at room temperature was demonstrated.
Abstract
A new self-plasticizing aliphatic polycarbonate comprising flexible alkyl and alkyl ether side
groups was designed and synthesized from six-membered cyclic carbonate monomers with the
aim of producing a material with high molecular flexibility (low Tg) and concomitant high ionic
conductivity when used as a polymer electrolyte. The Tg of the novel polycarbonate was
determined to be −49.4 °C at a molecular weight of 34 400 g mol−1, which is the lowest reported
Tg to date for a substituted poly(trimethylene carbonate). UV-crosslinked polymer electrolytes
were produced based on this novel material together with LiTFSI salt and showed ionic
conductivities in the range of 2×10−8 to 2×10−7 S cm−1 at room temperature and 1×10−6 to
1×10−5 S cm−1 at 100 °C. The limited ionic conductivities of these electrolytes indicate that high
molecular flexibility alone does not guarantee fast ion transport in solid polymer electrolytes and
that other factors, such as the polarity of the polymer host material, will also influence the
transport properties of the electrolyte.
Keywords: polycarbonate; ring-opening polymerization; glass transition temperature; polymer
electrolyte; ionic conductivity
1. Introduction
The development of new, functional materials to help us deal with current or future challenges
and enable new technological advancements is a continuously ongoing effort. In this respect,

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aliphatic polycarbonates constitute an interesting class of polymers that has recently seen a surge
in scientific attention [1]. This can be associated with the idea that these materials can be
synthesized through sequestering carbon dioxide, thereby fixating this greenhouse gas and
preventing its release into the atmosphere [2, 3]. Also contributing to the current interest in
aliphatic polycarbonates are the numerous recent examples of their use in biomedical
applications [4-7].
Apart from synthesis through copolymerization of epoxides with carbon dioxide, ring opening
polymerization is a broadly utilized technique for synthesizing aliphatic polycarbonates [1, 8].
Particularly six-membered cyclic carbonates are well-suited for controlled ring-opening
polymerization under mild conditions using a variety of different initiator/catalyst systems [1, 9].
Together with ample possibilities for post-polymerization functionalization and facile methods
for monomer synthesis, this has led to a great diversity of materials derived from an equally
impressive array of different monomers [7, 10].
While a lot of attention has been directed toward the use of aliphatic polycarbonates for
biomedical applications, it has recently been shown that polycarbonate materials are also
promising candidates for solid polymer electrolytes for Li-ion batteries by virtue of their ability
to dissolve and conduct lithium ions [11-15]. However, the application of solid polymer
electrolytes, including aliphatic polycarbonates, in Li-ion batteries is currently limited by the
insufficient ionic conductivity of the materials [16]. In a typical SPE, the mobility of dissolved
ions is inherently linked to the segmental motion of the host polymer chains [17, 18]. Thus, in
order to achieve high ionic conductivity, the molecular flexibility of the host polymer should be
as high as possible, i.e., the glass transition temperature, Tg, of the material should be as low as
possible.
The simplest aliphatic polycarbonate derived from a six-membered cyclic carbonate monomer is
−16
poly(trimethylene carbonate) (PTMC). While PTMC has a relatively low Tg of
°C at high
molecular weights [13], it is nowhere near the commonly used SPE host material poly(ethylene
oxide) (PEO; Tg close to −60 °C [19]) in terms of molecular flexibility. While several examples
can be found of PTMC derivatives with an increased crystallinity, rigidity and Tg [20-24], few
deliberate efforts seem to have been made to create more molecularly flexible materials. Some
examples of low-Tg PTMC derivatives include the allyl-functional poly(5-allyloxy-1,3-dioxan-2-
one) with a Tg of −40 °C at moderate molecular weights [25] and poly(2-allyloxymethyl-2-

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ethyltrimethylene carbonate) with a Tg of −30 °C, at an unspecified molecular weight [26]. These
polymers all comprise pendent allyl ether functionalities that function as plasticizers to lower the
Tg of the materials [27]. There are, however, other possible functional moieties that may
plasticize the material and lower the glass transition temperature even more than the allyl ether
group. Through inspiration from traditional low-molecular-weight plasticizers and careful
monomer design, essentially self-plasticizing polymers can be synthesized by incorporating
flexible side chains that increase the free volume of the material [28]. In an effort to improve the
ionic conductivity of polycarbonate-based SPEs, we have used this approach to create a novel
aliphatic polycarbonate with alkyl and alkyl ether side groups that give the material an
exceptionally low Tg, and demonstrate its applicability for use in solid electrolytes for Li-ion
batteries.
2. Experimental
2.1 Materials
All chemicals were obtained from commercial sources and used as received unless otherwise
noted. Dry dichloromethane (DCM; Fisher Scientific) was obtained by storing over activated 4 Å
molecular sieves. Trimethylolpropane monoallyl ether was a gift from Perstorp AB, Sweden.
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU; Acros Organics) was distilled under reduced
pressure. Lithium bis(trifluoromethanesulfonimide) (LiTFSI; Purolyte, Ferro Corporation) was
dried in vacuo at 120 °C for 24 h before use.
2.2 Instrumentation
1H NMR spectra were recorded at 25 °C on a JEOL Eclipse+ 400 MHz NMR spectrometer using
the residual solvent signal as an internal standard.
Molecular weight determination was performed through gel permeation chromatography (GPC)
on a Verotech PL-GPC 50 equipped with a refractive index detector and two PolarGel-M organic
GPC columns. Samples were injected using a PL-AS RT autosampler and chloroform was used
as the eluent at a flow rate of 1 ml min−1. Flow rate fluctuations were corrected by an internal
standard and the system was calibrated against narrow polystyrene standards.
Thermal properties were measured using differential scanning calorimetry on a TA Instruments
DSC Q2000. Samples were hermetically sealed in aluminium pans in a glovebox under an argon
atmosphere. These were then rapidly cooled to –70 °C followed by heating at 10 °C min−1 to
100 °C for measurement.