STIMULI-RESPONSIVE POLYMER FILM OR COATING PREPARED BY MIXING IN A SUITABLE FASHION A SIDE CHAIN LIQUID CRYSTALLINE POLYMER WITH REACTIVE MESOGENS AND RESPONSIVE DEVICES. PROCESS FOR PREPARING THE SAME

Abstract

The limitation of the different classes of responsive liquid crystals such as volatility in case of low molecular weight liquid crystals (LMWLCs) can be overcome by the development of a responsive film based on polymerliquid crystals (PLCs) and reactive mesogens (RMs or reactive liquid crystal monomers) to create a responsive film or coating material which appears to be easily alignable and processable. That coating material shows a large response of which the properties can be tuned in a modular approach. In this way, the advantages of both materials, PLCs and RMs, were combined, yielding stable films, which can be aligned when desired and which stimuli-responsive properties can be tuned by the choice of RMs. Thus mixtures of PLCs with RMs open the doors to a wide variety of stimuli-responsive coating systems, without the need of time consuming trial-and-error synthesis of PLCs and closed liquid crystal cells. By choosing chiral RMs, cholesteric LC coatings can for instance be fabricated, while a light responsive RM could provide a light responsive coating. In addition, one could use similar methods as were used for LMWLCs with RMs in closed cells to prepare for example broadband reflectors or patterned coatings that change topography by a stimulus.

Claims

1. Stimuli-responsive polymer film or coating material consisting of a side chain liquid crystalline polymer embedded in a liquid crystal polymer network formed with the use of a reactive mesogen (RM).

2. Stimuli-responsive polymer film or coating material according to claim 1 wherein the achiral liquid crystalline polymer is a side chain liquid crystalline polysiloxane (SCLCP).

3. Stimuli-responsive polymer film or coating material according to claim 1, wherein the RM is selected from the group consisting of nematic di-acrylate or a blend of nematic di-acrylate and nematic mono-acrylate monomers.

4. Stimuli-responsive polymer film or coating material according to claim 3 wherein at least one of the acrylate monomers is a chiral molecule.

5. Stimuli-responsive polymer film or coating material according to claim 1 wherein a suitable photo initiator and a suitable surfactant are mixed together with a chiral side chain liquid crystalline polymer and an RM.

6. Stimuli-responsive polymer film or coating material according to claim 1 for use in at least one from the group consisting of broadband reflectors and coatings.

7. Stimuli-responsive polymer film or coating material according to claim 1 for use in coatings for self-cleaning and antifouling.

8. Stimuli-responsive polymer film or coating material according to claim 1 for coatings that change topography.

9. Stimuli-responsive polymer film or coating material according to claim 1 containing a percentage of liquid crystal polymer which is in the range of 5 to 98 w % and preferably in the range of 20 to 90 w %.

10. Stimuli-responsive polymer film or coating material according to claim 1 which is processed to a film and cured by photopolymerization.

11. Stimuli-responsive polymer film or coating material according to claim 1 in which the switchable polymer has a phase transition from nematic to isotropic at a temperature between 0 and 35 C.

12. A responsive device or product consisting of a transparent substrate with a coating of a stimuli-responsive polymer film or coating material according to claim 1.

13. The responsive device or product according to claim 12 switching by light exposure or another stimulus.

14. The responsive device or product according to claim 12 switching by an electric field.

15. Process for preparing a stimuli-responsive polymer film or coating material according to claim 1.

16. (canceled)

17. The responsive device or product according to claim 12, switching one from the group consisting of reflective to transparent, narrowband to broadband, and one from the group consisting of shifting its wavelength by temperature and by a change in temperature.

Description

EXAMPLE AND DESCRIPTION OF FIGURES

[0024] As an example of the possible systems mentioned hereinabove, a chiral RM was introduced into an achiral SCLCP by mixing (together with some photo initiator and surfactant), to prepare a reversible temperature responsive CLC, reflective coating (FIG. 1A). These mixtures showed a cholesteric to isotropic phase transition around 50 C., independent of the ratio between SCLCP and RM. The reflective wavelength of these mixtures can be tuned by the amount of chiral RM. The mixtures were coated in the CLC phase using an automated gap applicator and cured using UV-light to polymerize the RMs present in the mixture. This results in a coating in which the SCLCP is not crosslinked and therefor it has still the freedom to go to the isotropic phase upon increasing the temperature, resulting in a decrease in reflection (FIGS. 1B and 1D). This process appears to be reversible over multiple temperature cycles and stable up to at least 120 C. Further it has been found that the degree of reflection decrease upon heating, depends on the concentration of SCLCP in the system; the more SCLCP, the more material will go to the isotropic phase, the more the reflection decreases (FIG. 1C).

[0025] FIG. 1(A)Components used in the mixtures including their individual phase behaviour. G refers to glassy, SmC to smectic C, SmA to smectic A, Cr to crystalline, N* to cholesteric and I to isotropic. 1(B) Vis-IR spectra at 30 C. and 120 C. for coatings reflecting in green, red and IR The values below the spectra represent the wt % of chiral RM-1 used in the various mixtures. At 475 nm the sequence of the graphs from bottom to top is; M1 30 C., M1 120 C., M3 30 C., M2 30 C., M2 120 C., M3 120 C.

[0026] 1(C) The reflection decrease relative to the initial value as a function of temperature averaged over two temperature cycles. 1(D) Photographs of the red reflecting coating at 30 C. and 120 C. on a black background.

[0027] By reducing the crosslink density of the network by replacing some diacrylates to monoacrylates the network was able to contract when the SCLCP side chains loses their order and blue shift occurred. By storing the coatings several hours at a temperature just below the clearing temperature the SCLCP side chains were able to organize themselves thus the reflection band red shifted to some extent By changing the concentration of chiral RM, the initial reflective wavelength could be tuned as well (FIG. 3). By varying the concentration of diacrylate and monoacrylate the wavelength range between which the reflection band shift takes place could be influenced. A higher concentration of diacrylates led to a smaller blue shift, but increased the red shifting capabilities of the coatings till a certain plateau. The influence of the monoacrylate concentration showed a similar trend, although the influence is weaker. This way coatings could be prepared with a desired colour change, which is interesting for optical sensor applications.

[0028] Coatings were prepared on 33 cm glass plates, which show a decrease in reflection upon increasing the temperature. (FIG. 2 A). This process is reversible.

[0029] In addition, coatings with temperature responsive surface topographies on 33 cm glass plates were prepared. FIGS. 2 B-3D images of the temperature responsive surface topographies of a coating obtained by (A) single mask and (B) dual mask photopolymerization induced diffusion.

[0030] FIG. 3(A)Components used in the mixtures including their individual phase behavior. G refers to glassy, SmC to smectic C, SmA to smectiv A, Cr to crystalline, N* to cholesteric and I to isotropic. FIG. 3(B) Coating prepared from mixture A (SCLCP/RM-2/RM-3/RM-4 77/5/15.8/) shifting from 735 to 537 nm reversibly. At 525 nm the sequence of the graphs from bottom to top is; 45 C. end, 80 C. t=63 min, 21 C. initial, 80 C. t=0 min FIG. 3(C). Coating prepared from mixture B (SCLCP/RM-2/RM-3/RM-4 77/5/11/5) shifting from 1119 to 731 nm reversibly. At 1100 nm the sequence of the graphs from bottom to top is; 21 C. initial, 23 C. end, 80 C. t=0 min, 80 C. t=63 min.

[0031] So far fast and large responsive, patternable, modular and stable polymer films do not exist. By combining PLCs and RMs it appears to be possible to prepare such polymers.

[0032] Using the present process an SCLCP can be embedded in an anisotropic polymer matrix to fabricate a thermally stable coating. This provides a new and easy way to tune the stimuli-responsive properties of SCLCPs over the conventional method of synthesizing SCLCPs with the desired (responsive) properties by trial-and-error. As an example a cholesteric LC RM mixture has been introduced in an achiral SCLCP, resulting in a reversible temperature-responsive coating.

[0033] In addition, by mixing RMs with SCLCPs it appears that a facile route can be provided to align SCLCPs using conventional coating methods (e.g. knife coating), which is amongst others necessary for cholesteric reflective coatings. The RM also provides a memory effect for the SCLCP to return to its planar alignment after the stimulus is removed.

[0034] Mixing RMs with SCLCPs also opens the possibility to create patterns and gradients in the films. As an example surface topographies with modulated crosslink density using a photo mask during polymerization have been prepared.

[0035] A couple of alternatives of stimuli-responsive liquid crystal systems are known, such as micro-encapsulated droplets of cholesteric LMWLC, thermochromic cholesteric LMWLCs in a closed cell environment, or SCLCPs in an external isotropic polymer matrix. The first two are limited to closed systems, while the latter lacks the possibility of cholesteric coatings, since these require alignment. The combination of the advantages of both alternatives has not been found in prior publications.