The Effect of the Degree of Polymerization and Polymer Composition on the Temperature Responsiveness of Cholesteric Semi-Interpenetrating Networks

Abstract

Cholesteric liquid crystal oligomers and polymers are promising materials for creating materials and devices with stimuli-responsive structural color, and the cholesteric to smectic pre-transition effect is of particular interest as it leads to a strong redshift in the reflected color upon cooling. Cholesteric polymers can be stabilized by the formation of semi-interpenetrating networks to obtain more robust photonic materials, but this tends to strongly suppress the pre-transition effect. Here, we show that the pre-transition effect in semi-interpenetrating networks based on main-chain cholesteric oligomers can be amplified by incorporating a smectic monomer and by increasing the degree of polymerization of the oligomers. This amplification counteracts the suppressing effect of the semi-interpenetrating network, and the resulting materials still show a significant band shift upon cooling. Presumably, both methods lead to the formation of more smectic domains in the cholesteric helix, which causes an amplified pre-transitional effect. The results bring us closer to the use of cholesteric semi-interpenetrating cholesteric networks for applications in smart sensing, healthcare, and safety devices.

1. Introduction

Photonic materials with structural color are promising for the development of new devices and consumer products. Structural color arises in materials with a periodic structure that reflects light, such as the wings of butterflies [1,2] and the shells of beetles [3,4], but also in artificial systems such as embossed surfaces [5,6], inverse opals [7,8,9], and distributed Bragg’s reflectors [9,10,11]. Cholesteric liquid crystal polymers are a particularly promising material for creating materials and devices with structural color [10]. These materials have a periodic helical structure, where the pitch length is the distance over which the molecules make a full rotation. In cholesteric polymer materials, the pitch length can be manipulated by changing the temperature [12,13,14], applying physical forces [15,16,17], solvent swelling [18,19], etc. This leads to a shift in the reflected wavelength and can be used to achieve stimuli-responsive color, which could find widespread applications in consumer products such as smart packaging and biosensors [20,21,22].

Cholesteric materials, which change their reflective properties in response to a change in temperature, have been developed and employ various mechanisms [21]. The most straightforward way such a color change takes place in crosslinked polymers is due to a volume expansion upon temperature increase, which leads to a redshift in the reflection band. However, in many applications, a redshift upon cooling is desired instead. An interesting and promising mechanism, which can cause such a shift to happen, is the cholesteric to smectic pre-transitional effect [23,24,25,26]. In this case, cooling of the cholesteric material leads to the formation of smectic domains in the cholesteric helix, which causes a rapid unwinding of the helix as the cholesteric–smectic transition temperature is approached [27,28,29]. This effect is often present in cholesteric polymers, as polymerization stabilizes the smectic phase, and is often observed even if no clear transition to smectic is present [30,31,32]. However, crosslinking of the polymer strongly inhibits the pre-transitional effect [26,33,34], while uncrosslinked cholesteric polymers are often unstable with heating to the isotropic phase or at longer timescales. A compromise can be reached by the preparation of a semi-interpenetrating network, which is made by adding a small amount of crosslinkable monomer to the polymers, followed by a polymerization step [26,30,35]. This can lead to the formation of materials that are stable and still show some redshift upon cooling, though the band shift per °C is typically small. Being able to amplify the sensitivity to temperature would make these materials ideal for the development of applications in smart photonics.

In previous work, we demonstrated that the sensitivity to temperature in main-chain cholesteric oligomers due to the pre-transitional effect, which can be enhanced by incorporating a smectic monomer [30]. Here, we used a similar material to prepare temperature-responsive semi-interpenetrating networks. Oligomers with no smectic monomer and 25% smectic monomer of various degrees of polymerization were prepared, and the temperature response of the reflection band of the films of the oligomers and the corresponding semi-interpenetrating networks was investigated. We found that a higher degree of polymerization and the presence of smectic monomer both enhanced the sensitivity and led to a stronger temperature response.

2. Materials and Methods

2.1. Materials

Monomer 1 and monomer 3 were purchased from Jiangsu Hecheng Advanced Materials Co. Ltd., Jiangsu, China. Monomer 2 was obtained from Jiangsu Creative Electronic Chemicals, Jiangsu, China. 2,2′-(ethylenedioxy)diethanethiol (EDDET) 4, butanethiol 5, dipropylamine (DPA) 6, tetrahydrofuran, dichloromethane, and deuterated chloroform were all purchased from Sigma-Aldrich, China. Photoinitiator 7 was purchased from Heowns Biochem Technologies LLC, Tianjin, China. The surfactant 2-(N-ethylperfluorooctanesulfonamido) ethyl methacrylate was purchased from Shenzhen Reagent Biotechnology Co., Ltd., Shenzhen, China. The chemical structures are shown in Figure 1a.

2.2. Substrates

The polycarbonate substrates (175 µm thickness, BASF) were used for coating. No pretreatment was required.

2.3. Preparation of the Acrylate End-Capped Oligomer

Monomer 1, monomer 3, dipropylamine, and in some cases monomer 2, were added in a vial, dissolved in DCM, and stirred at room temperature for 5 min. EDDET was added while stirring, and the mixture was stirred at 55 °C for another 5–7 h. The DCM had mostly evaporated after oligomerization and was further removed in a vacuum oven at 55 °C overnight.

2.4. Preparation of the Thiol End-Capped Oligomer

The acrylate end-capped oligomer, dipropylamine, and DCM were mixed at room temperature and allowed to fully dissolve. Excess butanethiol was added, and the mixture was stirred at 60 °C for 12 h. After evaporating the DCM, the final oligomer was dried at 60 °C under vacuum for 12 h.

2.5. Preparation of the Polymer Films

The thiol end-capped oligomer and surfactant were mixed with THF at 50 °C and allowed to fully dissolve. Then, THF was removed until the mixture was at a workable viscosity. The mixture was then applied to a PC substrate using an automatic coating machine (BEVS) at 50 °C. The thickness of the coating was controlled using an 8 µm gap applicator at a speed of 15 mm/s. The coating was kept at 50 °C for 30 min to further remove the THF.

When an interpenetrating network was prepared, monomer 1 and a photoinitiator were added to the oligomer mixture before coating. The coating was put in a nitrogen box, and photopolymerization was carried out using an EXFO Omnicure S2000 mercury lamp at 50 °C for 45 min with an intensity of 20 Mw/cm².

2.6. Characterization

The NMR spectra were measured using a 400 MHz Varian AS400. The DSC curves were recorded using a Mettler Toledo DSC 1 from Mettler Toledo. A rate of 5 °C/min was used for both the heating and cooling cycles. The phase behaviors were examined using a Leica CTR6000 polarized optical microscope, equipped with a Leica DFC 420C camera and Linkam PE95/T75 temperature controller. Both DSC curves and phase behavior information were collected from the second heating and cooling cycle. Transmission spectra were measured using a UV–Vis–NIR spectrophotometer (Perkin Elmer Lambda 950, Perkin Elmer) with a temperature controller.

3. Results

Oligomers with acrylate terminal groups were synthesized via a thiol-acrylate Michael addition reaction by mixing diacrylate liquid crystal monomers, the chain extender EDDET, and the catalyst DPA. Monomer 1 (I-119-N-30-Cr) is a commonly used nematic diacrylate monomer, while monomer 2 is a diacrylate monomer with complex smectic behavior (I-137-N-117-SmC-87-SmB-62-SmX) [30]. Addition of a small amount of chiral diacrylate monomer 3 is required to form a cholesteric phase. To investigate the effect of the presence of monomer 2 on the behavior of the oligomers and the corresponding semi-interpenetrating networks, two kinds of oligomer were prepared: one containing no monomer 2, and one containing 25 mol% monomer 2 with the remaining 75 mol% represented by monomers 1 and 3 (not counting EDDET and DPA). By varying the ratio between diacrylate and dithiol, the degree of polymerization (DP) of the oligomers could be controlled. The component ratios of the oligomers with and without monomer 2 are listed in

Table 1. Compositions of the oligomers with no monomer 2.

No.	Compositions (mol%)				Average DP
	Monomer 1	Monomer 3	EDDET 4	DPA 6
1	62.4	2.9	32.7	2.0	2.0
2	55.7	2.9	39.0	2.4	3.0
3	51.2	2.9	43.3	2.6	5.0

and

Table 2. Compositions of the oligomers with 25% monomer 2.

No.	Compositions (mol%)					Average DP
	Monomer 1	Monomer 2	Monomer 3	EDDET 4	DPA 6
1	46.0	15.3	3.4	32.0	3.3	2.0
2	41.3	13.7	3.4	38.9	2.7	3.0
3	38.1	12.7	2.9	42.8	3.5	6.3

, respectively. The structures of the acrylate end-capped oligomers with no smectic monomer and 25% monomer were determined using nuclear magnetic resonance (NMR) (Figures S1 and S2 in the Supplementary Materials). The average DP of the oligomers was calculated based on the peak integrals corresponding to the protons in the diacrylate end groups. As it is difficult to exactly control the DP of the oligomers at higher DP values, the oligomers without monomer 2 had DP values of 2, 3 and 5, while the ones with 25% smectic monomer had DP values of 2, 3 and 6.3. The chemical structures of the materials used are shown in Figure 1a, and a schematic representation of the polymerization process is shown in Figure 1b.

The acrylate end groups of the oligomers are reactive, which reduces their stability in the presence of UV light. To avoid this instability, butanethiol was used to react with these acrylate groups in the presence of DPA to deactivate them through the same Michael addition reaction used for the polymerization (Figure 1b). The degree of conversion of all samples was also characterized by NMR (Figures S3 and S4 in the Supplementary Materials). No acrylate peaks were detected, which means all the acrylate groups were deactivated. Instead of this deactivation step, we could also have opted to create oligomers with thiol end groups, which would not be unstable in the presence of UV light, but these groups would react with the monomers used to generate the interpenetrating networks. Differential scanning calorimetry (DSC) was used to reveal the phase transition temperatures of the oligomers (Figure 2). Two obvious exothermic peaks were found in every DSC curve, corresponding to the cholesteric–isotropic phase transition and cholesteric–smectic phase transition. For the oligomers without monomer 2, a small peak for the cholesteric–smectic transition was found between 5 and 20 °C, and a larger peak for the cholesteric–isotropic transition was found between 55 and 60 °C, with both peaks shifting to higher temperatures at a higher DP. For the oligomers with monomer 2, the peak for the cholesteric–isotropic transition was similar to the one observed in oligomers without monomer 2, but the peak corresponding to the cholesteric–smectic transition was much more obvious, appeared at higher temperatures, and was strongly DP dependent. This illustrates the more smectic character of the oligomers in which the smectic monomer 2 was present.

Thin films of the oligomers were investigated using UV-Vis spectroscopy to observe the reflection band at various temperatures. A coating formulation was prepared by dissolving the oligomers and a surfactant in THF. The coating formulation was stirred at 50 °C to dissolve the oligomers, and the solvent was evaporated until the viscosity was suitable for the coating process. The coating formulation was then applied to a PC substrate at 50 °C using blade coating with an 8 µm gap applicator at a speed of 15 mm/s. Afterwards, the coating was dried, and investigated using UV-Vis spectroscopy (Figure 3). The films prepared from oligomers containing no monomer 2 showed a gradual redshift of the reflection band as the temperature was reduced, starting at around 600 nm at 60 °C and ending between 800 nm and 1100 nm at room temperature. This shift became larger as the DP of the oligomer was increased. The reflection bands of the films of the smaller DP stayed sharp at any temperature, while the oligomer of DP 5 broadened at lower temperatures. In comparison, the oligomers of similar chain length containing 25% monomer 2 showed a larger band shift over a narrower temperature range and showed more significant broadening at lower temperatures. In fact, at lower temperatures the reflection bands could not be measured as they had shifted out of the measurement range or had become too broad and shallow. The redshift of the reflection band upon cooling towards the cholesteric–smectic transition was caused by the formation of smectic domains that pushed apart the cholesteric helix and increased the pitch. Both the incorporation of monomer 2 and the increase in the DP stabilized the smectic phase, so the increase in band shift and the broadening of the reflection band were assumed to be caused by an amplified formation of smectic domains.

Semi-interpenetrating networks were formed by adding a small amount of reactive mesogen and a photoinitiator to the oligomers, followed by UV-photoinitiation to induce polymerization and generate a crosslinked network. To the oligomers containing no smectic monomer, 3% reactive mesogen was added, and to the oligomers containing 25% smectic monomer, 5% reactive mesogen was added. After the photo-polymerization was complete, the films were analyzed by measuring the UV-Vis spectra at various temperatures. To better compare the band-shifts, the wavelength values of the reflection band centers were normalized by dividing the values by the value at the highest temperature to obtain the relative band-shift at various temperatures. The results were compared to the oligomer films without network, the result of which is summarized in Figure 4.

The oligomers with no smectic monomer showed a slow but significant shift over a long temperature range when no semi-IPN was present (Figure 4a). For the oligomer with a DP of 2, this shift was entirely inhibited upon formation of the semi-IPN (the apparent slight blueshift upon cooling was so small it could be attributed to experimental error), while for the oligomer with a DP of 3, the shift was still present but was very small. For the oligomer with a DP of 5, the shift was less strongly inhibited. The oligomers with 25% smectic monomer showed a similar shift in the reflection wavelength compared to the oligomers with no smectic monomer but over a shorter temperature range, which meant that the steepness of the curve in nm per °C was larger (Figure 4b). For all the examined oligomers, the shift in reflection band was less strongly inhibited by the formation of the semi-IPN when compared to the oligomers which contained no smectic monomer, even though the amount of network was larger. However, the effect was less obvious at larger DP values, which also reduced the inhibiting effect of the semi-IPN when no smectic monomer was present. Similar to the oligomers which were not part of a semi-IPN, a stronger band shift in the semi-IPNs was caused by both the incorporation of monomer 2 and the increase in DP. It seems reasonable to conclude that the amplification of the cholesteric–smectic pre-transition effect can counteract the inhibiting effect of stabilizing the oligomers by the formation of a semi-IPN.

4. Conclusions

To summarize, we prepared unreactive cholesteric main-chain oligomers of various DPs, without a smectic monomer and with 25% smectic monomer. Comparing the band-shifting behavior of the oligomers and semi-interpenetrating networks based on the oligomers, we found that both the incorporation of the smectic monomer and the higher DP values increased the band-shifting in nm/°C, which counteracted the inhibiting effect of the formation of the semi-IPN. However, increasing the DP of the oligomers also increased the viscosity, which can cause problems for the processing of the materials. The incorporation of smectic monomers is a more convenient way to obtain semi-interpenetrating networks with a stronger temperature response. Further finetuning of the composition of the mixtures used to make polymer films based on cholesteric semi-IPNs could be conducted using different monomers [36,37] or other additives. If successful, temperature-responsive photonic devices, which combine good stability and strong responsiveness, as well as their various applications in smart sensing, healthcare, safety devices, etc., come within reach.