Meltable copolymeric elastomers based on polydimethylsiloxane with multiplets of pendant liquid-crystalline groups as physical crosslinker: A self-healing structural material with a potential for smart applications
Graphical abstract
Introduction
Polysiloxane copolymers with liquid crystalline (mesogenic) building blocks are compounds which can offer fascinating material properties, due to the combination of the highly flexible and hydrophobic polysiloxane with the phase behavior of the mesogenic (LC) units [1], [2]. These materials attract deserved research interest since Finkelmann’s pioneering works in the 1980 s [3], [4], [5], [6], [7]. Since then, most of the studies were dedicated to liquid-crystalline siloxane polymers (LCPs) with mesogens as side chains, whereas main-chain copolymers were studied much less frequently (see e.g. [8], [9], [10], [11]). In 1991, Finkelmann prepared mono-domain-oriented nematic LCPs (with side-chain mesogens) via a two-step-crosslinking process [12]. A structural variation of the ‘side-chain polysiloxane LCPs’ are copolymers with side-on bonded pendant mesogens, i.e. via a linker in the central part of the rod-like LC, which leads to a different ordering behaviour [13]. Long pendant LC chains consisting of multiple mesogen units have also been attached to functionalized polydimethylsiloxane (PDMS), namely via ATRP grafting polymerization [14]. All the above-discussed polysiloxane LCPs are very rich in the LC component (typically one LC per siloxane repeat unit), which makes up a dominant volume fraction, and the behaviour of the usually more or less fixed (via chemical crosslinking) mesogens is responsible for practically all the material properties. In contrast to that, in the presented work, the mesogen makes up a relatively small volume fraction of the copolymer. The elastic product properties originate from the PDMS component, while melting, solidification, sensitivity to strain damage, as well as self-healing are controlled by the phase behaviour of the mesogen.
Polysiloxane LCPs are most frequently synthesized via hydrosilylation (as is the case in this work), namely by grafting vinyl-functional mesogens onto hydrido-functional (Si–H) PDMS. Alternative routes include the use of alkynyl-functional mesogens in the hydrosilylation reaction [15], leading to a more rigid link between PDMS and the mesogen, the thiol-ene addition using vinyl-functionalized PDMS and mesogenic thiol [13], [29], [32], or the azide-yne cycloaddition (Huisgen ‘click reaction’) [28], [16].
From the application point of view, the polysiloxane LCPs have been investigated as electro-optic- [17], as light-emitting- [18], gas-separation- (membrane-) [19], [20] and chromatography materials [21], as well as actuators (see e.g. [11], [22], [23], [24]). The latter, often referred-to as ‘liquid-crystalline elastomers’ (LCEs), gained by far the most research attention, especially in recent years. The mechanisms of stimulus response include UV-triggered cis/trans isomerization of azobenzene mesogens (e.g. [13], [25], [26], [27], [28]), or the nematic → isotropic transition of polyaromatic mesogens (T-triggering), e.g. [11], [29], [30], [32]). Other remarkable polysiloxane LCEs include electrostrictive [31], multiple-stimuli-responsive [32], or stimulus-converting systems [30], as well as actuators with programmable shape-memory [25]. In contrast to that, in this work, the polysiloxane LCPs were studied as potential structural smart materials.
The aggregation and micro-phase-separation of the pendant mesogenic (LC) units in the copolymers, which are studied in this work, results in an organic-organic nanocomposite morphology. This morphology plays a key role in the copolymers’ material properties, especially in physical crosslinking and in thermo-mechanical transitions. In their previous work, the authors studied in detail the effects of aggregation behaviour in epoxy-POSS nanocomposites. A general advantage of such nano-composites in comparison to conventional ones consists in the small size of the reinforcing phase, which allows for the use of the same processing techniques like for filler-free matrices (or thermoset precursors), while optical transparency often can be preserved [33], [34]. Additionally, the finely dispersed nanofillers can provide specific chemical [35], [36], optical [37], [38], electrical [39], [40], magnetic [41], [42], or gas barrier [43], [44], [45] properties to the hybrid material. Last but not least, a marked mechanical reinforcement can be achieved with small amounts of nanofillers, due to their high specific surface [46], [47], [48], [49], [50]. In the mentioned studies of the epoxy-POSS hybrids which contained covalently bonded POSS nano-building blocks of molecular type [51], [52], [53], [54], the authors demonstrated the key importance of POSS − POSS aggregation. This effect could lead to true nanocomposite morphology (with relatively large inorganic POSS nano-domains) and to very strong mechanical reinforcement, as well as to thermal stabilization. The mutual interactions of organic substituents which covered the POSS surface were found to control the aggregation: The substituents with the strongest crystallization tendency were the most efficient for achieving nano-aggregation and reinforcement. Similar results concerning aggregation were observed also for the heavier homologue of POSS, the stannoxane dodecamer cages (Sn-POSS), which additionally introduced the chemical reactivity of the filler phase (matrix-repair reactions) under oxidizing conditions [55], [56], [57], [58], [59], [60], [61], [62].
The experience with the physical crosslinking via aggregation of crystallizing inorganic molecular nano-building blocks, as well as the previous study of liquid-crystalline copolymer networks [42], inspired the authors to the present work, in which the crystallization of mesogenic building blocks is used to reversibly crosslink an elastomer.
The aim of this work was the synthesis of side-chain-type LCP elastomers with reversible physical crosslinks, which could be disconnected by heat or by a suitable solvent. The polymers were based on linear polydimethylsiloxane chains (PDMS) grafted with quartets of mesogens, in which the mesogen-free PDMS segments act as elastic spacers between the grafts. Nano-crystallization of the attached mesogen, further enhanced by PDMS/mesogen incompatibility, was expected to serve as the physical cross-linker, similarly like POSS aggregation in previous works. It was planned to obtain products which combine the mechanical properties of covalently crosslinked elastomers with the processability (fusibility, solubility) of linear polymers. This would open the potential application of such non-covalent networks in the field of 3D-printing, or as viscoelastic coupling materials. A more fundamental aim of this work was to elucidate the structure–property relationship in the PDMS-LC elastomers, namely the efficiency as physical crosslinker of the relatively small mesogen units, as well as the influence of the thermotropic properties of the pendant LC groups on the whole copolymer.
Section snippets
Commercial chemicals
The dimethylsiloxane-methylhydrosiloxane(25–35%) copolymer “HMS 301” (Mn = 1 900–2 100 g/mol) and the dimethylsiloxane-methylhydrosiloxane(4–8%) copolymer “HMS 064” (Mn = ca. 60 000–65 000 g/mol; for both structures see Scheme 3 were purchased from Gelest. Chlorofom (solvent) was purchased from Sigma-Aldrich, while a 2% solution of the Karstedt‘s catalyst (see Scheme 4) was purchased from Merck. All the commercial products were used as received. Prior to use, however, the equivalent molecular masses
Synthesis
Strongly physically crosslinked meltable rubbers were synthesized in this work, the general structure of which is depicted in the (simplified) Scheme 2. Such elastomers offer an application promise as passive smart materials.
The prepared copolymers are based on hydrido-functional polydimethylsiloxane (PDMS) chains (see Scheme 3 top) tethered with pendant liquid-crystalline (LC) building blocks named “BAFKU” (Scheme 3 bottom). The pendant BAFKU units occur in separated quartets (see Scheme 4,
Conclusions
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Strongly physically cross-linked rubbers were successfully prepared, based on alternating polydimethylsiloxane (PDMS) spacer segments and pendant quartets of mesogenic building blocks (LC) of azobenzene type called “BAFKU”.
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These materials are structurally related to the well-studied polymers with pendant-chain LC units (light sensitive actuators), but are generally highly different: The LC units make up only a small volume fraction and do not generate elastic energy upon irradiation, but they
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors thank Dr. Sabina Krejčíková for the PLM analyses and Ms. Eva Miškovská for the XRD experiments, as well as the Czech Science Foundation, project Nr. 19-04925S, for the financial support of this work.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
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