Polydiacetylene–Polyurethane Crisscross Elastomer as an Intrinsic

Letter Cite This: ACS Macro Lett. 2019, 8, 409−413

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Polydiacetylene−Polyurethane Crisscross Elastomer as an Intrinsic Shape Memory Conductive Polymer Xiao-Qi Xu, Yonglin He, Haocheng Liu, and Yapei Wang* Department of Chemistry, Renmin University of China, Beijing 100872, PR China

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S Supporting Information *

ABSTRACT: Owing to the high phase transition temperature and incompatibility with thermoplastic elastomers, conjugated polymers are hardly formulated as shape memory materials. This work presents a crisscross polymer composed of polyurethane and polydiacetylene. Phase separation is completely avoided based on the photoinduced polymerization of polydiacetylene from polyurethane chains. The two backbones are intercrossed and covalently linked to each other. Particularly, polyurethane acts as a soft segment to provide elastic performance, and the rigid polydiacetylene provides conductive pathways. Such a crisscross topology, combined with soft and rigid compositions, renders the possibility to serve the polymer as an intrinsically elastic conductive polymer. Intriguingly, the polymer possesses shape memory performance, meanwhile retaining the reliable conductivity. Electrical tests demonstrate that the shape memory conductive polymer is one attractive candidate for exploiting shape-customized strain sensors.

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exploited as a colorimetric sensor based on its configuration change against external stimuli.37−41 The past few years also witnessed an explosive development of PDA as optoelectronic materials.42 This polymer is typically made from diacetylene under high energy treatments, including UV irradiation, heating, or X-ray irradiation. Such an easy polymerization process affords great convenience to generate conducting PDA pathways into another matrix. In this work, we proposed a concept of crisscross elastomer which serves as an intrinsic shape memory conductive polymer. Different from conventional cross-linked polymers, the crisscross topology refers to a structure consisting of two polymer backbones that are intercrossed and covalently linked to each other (Scheme 1).

hape memory polymers (SMPs), which have been conceived as a particular class of constituents for intelligent materials and devices, are offering great promises in the areas of clinical treatments, electronics, aeronautics, and textiles.1−18 Besides the efforts to encode the shape change in a predesigned way, intense investigations were also delivered to shape memory materials with special functions.19−29 Particularly, conductive SMPs are rising as an innovative branch of SMPs in terms of their potentials in fabricating smart electronics.30−35 The conductivity was routinely improved through the choice of loading conductive particles within SMPs. This strategy, as similar to other heterogeneously blended systems, requires the loading content of conductive particles with a value above the percolation threshold. Yet, troubles of particle agglomeration, chain reorganization, and conductivity loss may be encountered upon manipulating shape changes under thermal treatment. So far, there are few examples of intrinsic shape memory conductive materials which are fully composed of conjugated polymers or polymers entangled with conductors at the molecular level.36 Solely conjugated polymers are difficult to be formulated as SMPs because they generally possess high phase transition temperature due to strong intermolecular interactions. In principle, it is possible to interpenetrate a conjugated polymer within a SMP, hopefully leading to a polymer product with reasonable phase transition temperature and ideal conductivity. However, conjugated polymers are rarely compatible with thermoplastic polymers as serious phase segregation happens once they are mixed with each other. Polydiacetylene (PDA) is a special conjugated polymer with double and triple alternating bonds, which has been widely © XXXX American Chemical Society

Scheme 1. Schematic Illustration of Conventional CrossLinked Polymer and Crisscross Polymer

Received: March 9, 2019 Accepted: March 25, 2019

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DOI: 10.1021/acsmacrolett.9b00179 ACS Macro Lett. 2019, 8, 409−413

Letter

ACS Macro Letters

absorbance that appeared at 553 nm illustrates the formation of a π-conjugated structure, which should be attributed to the topochemical polymerization of DA units into PDA. Such a light-induced polymerization is supposed to be rapid as the absorption reaches maximum under UV irradiation for 10 min. According to the optical bandgap (1.94 eV) calculated from the visible absorption onset at 640 nm, the efficient πconjugated length of PDA is estimated as 19 double bound units, which is supported by the density functional theory (DFT) calculation (Figure S10). Additional DSC and PXRD studies give further understandings of the crisscross PODD− PEG−HDI polymer (Figures S11 and S12). The viscoelastic property of the crisscross polymer was characterized by dynamic mechanical thermal analysis (DMA) experiments. Storage modulus and loss factor (tan θ) as a function of temperature are related to the glass transition temperature (Figure 2a, Figures S13−S14). As the temperature

Herein, PDA is formulated into an elastic polyurethane (PU) network. The crisscross structure is beneficial for cooperating PDA with PU, leading to a conducting polymer with the ability of shape memory. In order to build a crisscross network of PDA and PU without the appearance of phase separation, the DA monomer is chemically inserted into the PU backbone, allowing the growth of PDA from PU main chains under UV light initiation (Figure 1a). To do that, three building blocks, including the

Figure 1. (a) Schematic illustration of ODD−PEG−HDI (left) and PODD−PEG−HDI (right). (b) 1H NMR spectrum of ODD−PEG− HDI in d-DMSO. (c) UV−vis absorption spectra of ODD−PEG− HDI thin film irradiated by 253 nm UV light for different times. (d) Optical images of the ODD−PEG−HDI thin film (left), the patterned thin film (middle), and the PODD−PEG−HDI thin film (right).

Figure 2. Dynamic mechanical analysis of the PODD−PEG−HDI film. (a) Loss factor change and storage modulus of PODD−PEG400− HDI film within a temperature range from −40 to 40 °C. (b) Stress− strain curves of PODD−PEG−HDI films. (c) Elastic shape memory performance of PODD−PEG400−HDI with activating temperature at 85 °C and shape fixing temperature at 0 °C. (d) Normalized temperature-dependent stress relaxation curves of PODD−PEG400− HDI. Inset: Activation energy (Ea) of the stress relaxation process calculated by an Arrhenius formula.

DA monomer of 3,5-octadiyne-1,8-diol (ODD), hexamethylene diisocyanate (HDI), and polyethylene glycol (PEG), were copolymerized into PU resins and slightly cross-linked by the trimer of hexamethylene diisocyanate (THDI). As the soft segments in the final PU, PEGs with different molecular weight (200 g/mol, 400 g/mol, and 600 g/mol) were compared to optimize mechanical performance at a given feed ratio. Typically, the PU resins consisting of ODD, HDI, and PEG200/400/600 were characterized by 1H NMR and 13C NMR (Figures S3−S8). For PU made from PEG400, the chemical shift at 4.56 ppm assigned to the terminal hydroxyl groups of PEG is barely observed, indicating a high degree of PEG consumption in the polymerization process (Figure 1b). On the other hand, the Fourier infrared spectrum (FTIR) confirms an almost complete conversion of carbimide groups of THDI (Figure S9). However, there are still a remarkable amount of hydroxyl groups remaining when PEG800 is used, which may be due to the worse chain diffusion of PEG with higher molecular weight. Thin PU films were prepared via drop-casting method and thermally annealed at 120 °C for 30 min before further use. The colorless PU films turned red when they were exposed to UV irradiation. The patterned color change in the presence of a photomask is indicative of a photochemical reaction under UV irradiation (Figure 1d). As further monitored by UV−vis spectroscopy in Figure 1c, the

is increased above −10 °C, the storage modulus of PODD− PEG200−HDI drastically decreases to a plateau at 20 °C. Correspondingly, tan θ presents a wide transition range with a main peak at 14.8 °C. In the same measurement conditions, the glass transition temperatures of PODD−PEG400−HDI and PODD−PEG600−HDI are estimated as −10.4 °C and −25.6 °C, respectively. The tendency of Tg change satisfied with the principle that increases the length of soft segments is generally beneficial to looser polymer chains. Therefore, the PODD− PEG200−HDI film exhibits a relatively higher Young’s modulus of 0.86 MPa and lower breaking elongation of 49.2% at ambient temperature, in comparison with the other two polymers (Figure 2b). Notably, PODD−PEG400−HDI has a breaking elongation over 200%. However, for PODD− PEG400−HDI with longer soft segments, it has similar Young’s modulus with PODD−PEG400−HDI and poorer elongation ability. It is assumed that crystallization may be enhanced in the crisscross polymer having longer PEG chains. 410

DOI: 10.1021/acsmacrolett.9b00179 ACS Macro Lett. 2019, 8, 409−413

Letter

ACS Macro Letters

accelerated if temperature is increased. The phenomenon of stress relaxation exhibits the viscoplasticity of PODD− PEG400−HDI, which can only be explained by the disruption and reorganization of cross-linked structure. Regarding that the exchange of urethane bonds is the main driving force for stress relaxation, the activation energy of chain reorganization is estimated as 121.7 kJ/mol. In terms of considerable elasticity and acceptable plasticity, temporary shape and permanent shape for shape memory applications could be renewed at low and high temperature, respectively. As shown in Figure 3b, a straight PODD−PEG− HDI stripe was mechanically stretched to a strain of 50% at 85 °C. The mechanical stretching was kept until the stripe was rapidly cooled to 0 °C. The temporary shape as the form of a prolonged stripe was preserved after the stress was released, while it was retracted into the original length once it was reheated at 85 °C. As a proof-of-concept demonstration, the crisscross elastomer stripe of PODD−HDI−PEG was exploited as a versatile electrical sensor. In practice, π-conjugated PDA is more conductive after it is well doped by iodine. Such an iodine-doping treatment is ascribed to chemical oxidation of PDA backbones which is supposed to increase the number of charge carriers. Accordingly, PODD−HDI−PEG is like a ptype semiconductor, and its conductivity is positively relevant to temperature change. As recorded in Figure 3c, the conductivity change (ΔG/G0), which is deduced by the current change of ΔI/I0, is rapidly increased when the polymer is subjected to thermal heating. To be specific, ΔG/G0 increases from 0% (at 25 °C) to 72% and 1819% at temperature of 30 and 70 °C, respectively. The increased number of carrier concentrations is plausibly responsible for the increased conductivity at higher temperature. Notably, the conductivity change keeps almost the same upon multiple cycles of on−off heating and cooling treatments, exhibiting good sensing stability and reproducibility. In addition to thermal sensing, the elastic PODD−PEG−HDI could also be applied as a tensile stain sensor. Before the elongation breaking at a strain of 150%, the resistance change (ΔR/R0) is increased against tensile elongation and fully recovers once the tensile stress is released (Figures 3d and S17). Concretely, the resistance increases to 87.8% under a strain of 60%. The deformation of conductive pathways and the enhanced carrier scattering are assumed to account for the increase of resistance under tensile stretching. To describe its full potential as shape-customized electrical sensors, a PODD−HDI−PEG stripe was folded into different geometries with shape of alphabetic letters, “V” and “W”, which are expected to serve as diverse sensors (Figure 4a). Typically, the character of “V” was prepared at 120 °C, which is considered as a new permanent shape, instead of its mother permanent shape of the straight stripe. Following this new permanent shape, a “W”-like temporary shape was formulated at 85 °C, which could be fully recovered to “V” shape in terms of shape memory transition. Intriguingly, the shape change causes little influence on the conductive performance as both the permanent “V” shape and the temporary “W” shape possess similar conductivity with the original stripe shape (Figure S18). However, these special shapes endow the PODD−HDI− PEG sensors with the ability of testing compressive strains that can hardly be achieved by the shape of a straight stripe. As illustrated in Figure 4b, the conductivity change is positively relevant to the compressive strain as the “V”-shaped sensor is

Considering the best stress−strain performance, PODD− PEG400−HDI was chosen as the representative material for further studies on shape memory and stress relaxation. As revealed in Figure 2c, when the PODD−PEG400−HDI film was cooled from 85 to 0 °C under uniaxial stretching with a pressure of 0.7 MPa, the strain was still maintained at 43% with 90% shape fixity after the stress was suddenly removed under the cooling conditions. Upon heating to 85 °C again, spontaneously, the temporary length of PODD−PEG400− HDI film was fully recovered to its original length. It is assumed that both vitrification-induced chain immobilization and hydrogen bonding interaction among urethane bonds account for the physical fixing at temporary states. To confirm this assumption, insights were provided into the shape fixing process at temperature notably below Tg or above Tg. The temporary shape was completely retained upon releasing the tensile stress if the shape fixing was achieved at −25 °C, suggesting a better vitrification than the case with shape fixing temperature at 0 °C (Figure S15a). However, at the shape fixing temperature of 25 °C which is far beyond Tg, the preservation of temporary shape becomes worse because of the lower vitrification, yet it still has a 60% shape fixity (Figure S15b). Such a shape fixity without enough chain immobilization based on vitrification justifies the great contribution of hydrogen bonding in the shape memory system. Additional dynamic mechanical analysis illustrates that reducing the hydrogen bond rearrangement, as a result of thermally activating the PODD−PEG400−HDI film at lower temperature, decreases the elongation capability and also causes the loss of shape fixity (Figure S16). In addition to serving as a thermoset material, it is expected that PODD−PEG400−HDI can serve as a thermadapt material at temperature high enough to allow the exchange reaction of urethane bonds in the presence of DBTDL (Figure 3a).43,44 According to the DMA studies (Figure 2d), the stress was gradually relaxed when the PODD−PEG400−HDI film was thermally treated at high temperature. Specifically, the stress relaxation at 115 °C is apparently faster than that at 100 °C. It could be further

Figure 3. (a) Thermally induced transcarbamoylation reaction of urethane bonds. (b) Demonstration of the shape memory performance of PODD−PEG−HDI. (c) Thermal responses against the increase of temperature ranging from 30 to 80 °C with an interval of 5 °C. Room temperature is 25 °C. Inset: On−off cycles of the thermal sensing between room temperature and 50 °C. (d) Resistance change of PODD−PEG400−HDI elastomer under various strains from 10% to 60%. Inset: the photographs of the stripe before and after elongation on a stretcher which are captured at room temperature. 411

DOI: 10.1021/acsmacrolett.9b00179 ACS Macro Lett. 2019, 8, 409−413

Letter

ACS Macro Letters

permanent shapes. It is believed that this breakthrough has laid groundwork for exploiting a new generation of flexible electronics with customized demands. It is also envisioned that the concept of crisscross polymers may be readily extended to other polymer systems, opening opportunities in optoelectronics, reinforcement, and encapsulation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00179.

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General information; synthesis and characterization of polymers; and computational studies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yapei Wang: 0000-0001-5420-0364 Notes

Figure 4. (a) Visual demonstration of shape manipulation of the PODD−PEG−HDI stripe arising from the plasticity and the elasticity. (b) Electrical response of “V”-shaped PODD−PEG400−HDI against the compressive strains between −20% and −100%. (c) Two on−off cycles of the electrical response of the compressive stress response of “V”-shaped PODD−PEG400−HDI under the strain of −80%. (d) Electrical response of “W”-shaped PODD−PEG400−HDI against the compressive strains from 0% to −80%. (e) One on−off cycle of the electrical response of “W”-shapeed PODD−PEG400−HDI against the compressive strain of −60%.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21825503 and 21674127). Prof. Daming Wang at Jilin University and Prof. Lei Tao at Tsinghua University were acknowledged for their support on DMA measurements.

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deformed into “Y” shape. To be specific, ΔG/G0 is increased from 7% to 132% when the compressive strain is strengthened from −20% to −100%. The on−off cycles reveal that the conductivity change undergoes a progressive increase or decrease, which is ascribed to the gradient contact between the two sides of the “V” shape, thus presenting a considerable continuous sensing signal (Figure 4c). In contrast to the analog signal performed by the “V” shape, however, the “W” shape can simulate a digital signal owing to the point contact, as shown in Figure 4d and 4e. The conductivity change of ΔG/G0 remains unchanged before the compressive strain of “W” shape reaches −60%. Predictably, after the “W” shape is further compressed into “Ω” shape, the ΔG/G0 value drastically jumps to 53% and maintains the same value until the strain of −80%. It clearly provides two distinct bands corresponding to two states of “true” and “false”, which is regarded as a type of digital signal simulation. It is worth noting that both “V” and “W” shape sensors exhibit excellent repeatability against 100 cycles of on− off mechanical compressing at a strain of −80% (Figures S19− S20), rendering great possibilities for long-term service. In conclusion, we presented a versatile strategy for developing polydiacetylene elastomers as an intrinsic shape memory conductive polymer. The unique crisscross topology as the combination of two different polymers not only ensures less phase segregation but also tailors the energy dissipation against mechanical manipulations. Benefiting from these advantages, the conjugated polymer with poor flexibility was successfully formulated as a conductive part within an elastomer. Brilliant conversion between thermoset and thermoplastic affords great promise to manipulate the temporary and

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