Solid Opalescent Films Originated from Cellulose Derivatives

Chapter 3

Cellulose Color Effects Copied from Nature with Natural Materials: Solid Opalescent Films Originated from Cellulose Derivatives

Downloaded by FUDAN UNIV on December 2, 2016 | http://pubs.acs.org Publication Date: February 15, 2001 | doi: 10.1021/bk-2001-0786.ch003

Georg Maxein, Manfred Müller, and Rudolf Zentel* Department of Chemistry and Institute of Materials Science, Gauss Strasse 20, D-42097 Wuppertal, Germany

Solid opalescent films, which owe their color to Bragg reflection of visible light, can be prepared from cholesteric cellulose derivatives. Both thermotropic and lyotropic systems can be used. They are accessible from commercial products by simple reactions and a subsequent photo polymerization (crosslinking). We found cellulose carbanilates and hydroxypropylcellulose esters most promising. By careful selection of the substitutents, the degree of substitution and the molecular weight, systems with brilliant reflection colors are available.

The cholesteric phase (chiral nematic phase) of liquid crystals shows selective reflection of light, i f the pitch of the cholesteric helix coincides with the wavelength of light within the material (λ = η * ρ) (see figure 1). Since the reflection conditions vary with the angle between the cholesteric helix and the incident light, different reflection colors are seen depending on the observation angle. Recently highly crosslinked cholesteric pigments have found a lot of interest as dye pigments for cars or as "copy safe" colors for documents or money (1). These cholesteric pigments have so far been prepared from cholesteric monomers or oligomers by a photocrosslinking process (2,3). © 2001 American Chemical Society Gross and Scholz; Biopolymers from Polysaccharides and Agroproteins ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

61


62

Figure

1: Schematic representation of the cholesteric phase and it's optical

properties. . ( λ: wavelength of the reflected light; n : average refractive indicex of a n

nematic layer; P: pitch; a angle of reflection)

Helical biopolymers offer the same potential and a great deal is known about thermotropic or lyotropic phases of polypeptides or cellulose derivatives. In this case crosslinking is possible either by crosslinking a suitable thermotropic system (4) or by the crosslinking of vinyl monomers used as solvents for a lyotropic phase, thus producing a semi-interpenetrating network. The last example has been successfully demonstrated for polyglutamates (5). Among cellulose derivatives many chiral nematic phases are known, both lyotropic (6-8) and thermotropic (9-11). The mesogens of these liquid crystals are usually commercial products or accessible by simple reactions. If the conditions are right, a great number of these liquid crystals exhibit selective reflection between 400 and 800 nm. In addition, only moderately polar solvents such as ketones, glycol ethers, and glycol acetates are necessary in order to obtain colored mesophases from

Gross and Scholz; Biopolymers from Polysaccharides and Agroproteins ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

63


the methanes of cellulose (12,13) (many technologically used derivatives of acrylic and methacrylic acid are of similar moderate polarity and of similar structure). In this work we describe both lyotropic and thermotropic systems based on cellulose derivatives. These systems can be prepared from commercial products by simple reactions. They show excellent optical properties and can be photo patterned. Lyotropic mesophases were preparedfromaryl urethanes of cellulose in commercially available mono- or afunctional derivatives of acrylic and methacrylic acids (see Scheme 1). To obtain solid films and to conserve the selective reflection the solvents were polymerized photochemically, thus yielding a semi-interpenetrating network of cellulose urethanes in polyacrylates. Thermotropic systems were preparedfromesters of hydroxypropylcellulose (HPC) with propionic acid and acrylic acid (see Scheme 2).

Lyotropic Cholesteric System In the search for lyotropic cholesteric systems (described in detail in ref. 14) cellulose carbanilates in mono- and bisacrylates of oligoethylen glycols gave the best results (see Scheme 1). Lyotropic mesophases formed by these compounds orient well and show a sharp absorbance of light due to selective reflection (see figure 3). Photo polymerization transforms this lyotropic cholesteric phase into a solid film. However, the sample remains clear and no signs of phase separation are seen. The selective reflection (absorption) after crosslinking is as sharp as before, but it is shifted to a shorter wavelength, presumably because of a volume shrinkage during polymerization. A crucial point in the preparation of cholesteric films is the orientation of the cholesteric phase in the planar texture (helical axis perpendicular to the film surface). Such an orientation takes a very long time, if the DP (degree of polymerization) of the cellulose derivatives is high. A misalignment leads to broad reflection bands and a great deal of scattering in the sample. In order to overcome these problems cellulose with a DP of 100 or 50 was used, which is accessible by saponification of cellulose acetate or propoinate. It was later transformed into the cellulose carbanilates under investigation (see Scheme 1). Especially cellulose carbanilates prepared from 3-trifluoromethyl-phenylisocyanate proved to be advantages, as their viscosity in the about 45 weight % solutions, necessary to prepare a cholesteric phase with selective reflection in the visible range, is rather low. Nevertheless, orientation times of half an hour are necessary to obtain a perfect orientation of the cholesteric phase and narrow reflection bands. To reduce the viscosity and the polydispersity of the cholesteric solutions further, a sole gel fractionation of the cellulose carbanilates (system acetone/ water) was performed. It reduced the polydispersity to 1.5 and produced samples ranging in molecular weight


64


Synthesis of Cellulose Urethanes

OCONHR

CH OCONHR 2

2-ethoxyethyl acrylate

OCONHR

n = 1,2,3 and 4

Scheme 1


65 ( M : peak maxima, G P C in T H F against polystyrene) from M = 21,000 to 134,000 (see figure 2). A l l samples gave lyotropic cholesteric samples, which showed the reflection wavelengths displayed in figure 2 for crosslinked samples. Two results are noteworthy: 1.) A decrease of the molecular weight leads to a blue shift (shrinkage of the cholesteric helix) of the reflected light: 2.) Even the sample with the lowest molecular weight ( M = 21,000) produces a 100% cholesteric solution. This behavior is different from that of cellulose carbanilates samples, for which the low molecular weights had been prepared by acidic hydrolysis of high molar mass polymers. For these systems with a broad polydispersity (about 2.7) the samples with a molecular weight below M = 50,000 do - no longer - give rise to lyotropic cholesteric phases. P S

P S

P S


P S

400 I ι ι ι ι ι—ι—ι—ι ι ι ι ι ι—ι ι ι ι ι—ι—I—ι—ι—ι ι ι—m—|—• 140 120 100 80 60 40 20 0

Mps/1000 Figure 2: Influence of the molecular weight on the maxima of the selective reflections. The cholesteric mixtures contained 45 wt. % of trifluormethyl phenyl urethanes in diethylene glycol dimethacrylate. The measurements were carried out at room temperature after cross linking.

Lyotropic solutions prepared from the low molar mass system obtained by fractionation ( M = 21,000) orient perfectly within a few minutes. B y changing the temperature it is - in addition - possible to observe selective reflection throughout the whole visible range. Figure 3 shows some films - sometimes as big as 100 cm obtained by crosslinking these mixtures at temperatures ranging from 24 to 53°C. B y performing the photochemical crosslinking at various temperatures it is possible to pattern films as described in ref. 14. P S

2



66

Q

Γΐ

400

I I I

I

I I I I

I

I

500

I I I

I

I I I

ill

600

I I I

I

I I I

I

I

I I I I

I

I I I

700

II

800 λ/nm

Figure 3: Influence of the temperature on the maxima of the selective reflections. The cholesteric mixtures contained 45 wt. % of trifluormethyl phenyl urethanes in diethylene glycol dimethacrylate. The cholesteric mixture was irradiated at: 24°C (I), 2fC (II), 31°C (III), 38°C (IV), 41°C (V), 46°C (VI) and 53°C (VII). The Spectra were recorded at room temperature.

Thermotropic Cholesteric Systems Based on the well-known liquid crystalline properties of 2-Hydroxypropyl cellulose (HPC) derivatives, a thermotropic system could be realized as described in detail in ref. 15. There are two major advantages with this concept: a) cellulose represents a renewable raw material with almost unlimited accessibility b) the resulting polymeric liquid crystal is a single component cholesteric system, containing no evaporatable or toxic low molecular weight compounds. The "2-step-single-pot" polymer analogous reaction of HPC with acrylic and propionic acid chloride in acetone yields statistically substituted HPC mixed esters, which represent highly crosslinkable thermotropic liquid crystalline macromers. The naturally chiral cellulose backbone induces the desired selective reflecting cholesteric structure that can be permanently frozen in by curing films of the material with a photo initiator and UV-light (see Scheme 2).


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ο ci

HPC +

acetone, Q

reflux

Cl


Scheme 2

UV/VIS spectra on thin uncrosslinked films of these derivatives show that the selective reflection wavelength and thus the cholesteric pitch (see Figure 2), increases linearly with temperature. This is analogous to the lyotropic systems. The reflection wavelength is -however- more strongly determined by polymer specific parameters. So it was found that a smaller degree of substitution (DS) as well as a smaller degree of polymerization (DP) of the HPC result in a significant "red-shift" of the reflection wavelength (see Figures 4 and 5).

0,3 H PC17 (DP=150)

50°C;DS=2.0

PC19 (DP=190) PC18 (DP=220)

0,2 Δλ,/2 = 69 nm

0,1

&k = 73 nm iJ2

Δλ|/2 = 121 nm ^ 0,0

1*

300

400

500 600 wavelength [ran]

700

800

Figure 4: Dependence of the selective reflection λ οη the degree ofpolymerization DP: A smaller DP leads to a more intensive reflection and a smaller half width of the reflection peak. 0


68 Figure 4 shows the high sensibility of the cholesteric phase towards small changes in the DS, the increase by 0.5 in DS shifts the samples reflection color from red to blue (see Figure 4 at 70°C). Furthermore, a smaller D P exhibits more intense and more brilliant reflection colors (high and narrow peaks) (see Figure 5). This is most probably due to a more perfect alignment of the cholesteric phase during annealing. For bulky substituents one finds red-shifted, but flat and broadened, reflection signals.

750-

-•--PC23(DSf=1,51) - A -


[nm] 700

-·-•-

650-

PC24(DS=1,79) PC25 (DS=1,90) PC26(DS=1,99)

600550-

500 450 400

DP=cal60

350

1

30

35

1

1

40

1

1

45

.

j

50

1

1

55

1

1

60

1

1

65

1

1

70

1

1

75

1

1

80

r

85

temperature [°C\ Figure 5: Influence of the degree of substitution DS on λ : Significant "blue-shift"for the higher substituted HPC-derivatives. 0

Whereas the DP is more or less controlled by the initial H P C , the DS can be controlled by the reaction conditions and time respectively. Finally only the use of H P C with a low degree of polymerization in combination with a mere partial substitution of the H P C - O H - groups in a esterification reaction free of a proton trap (in situ: acid catalyzed depolymerization possible) yields cholesteric phases, which orient well to give brilliant reflection colors. Due to the temperature dependence of the cholesteric phase an exact adjustment of the color effect prior to the crosslinking process is possible. This is illustrated in figure 6. Here the circles represent the linear relation between temperature and reflection color for an uncrosslinked film (photo initiator already added. Photo crosslinking at 60°C or 90°C respectively yields in permanently locked in cholesteric phases (filled squares) that reflect blue or red light respectively. As can be seen from figure 6, the crosslinking process causes almost no change in the cholesteric phase. The previously adjusted reflection color only suffers a small blue shift (5 nm) due to volume shrinkage. This effect is much smaller than in the lyotropic systems. The


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90°C red

5001


4501 60°C blue

4001

3501 30

40

50

60

70

80

90

100

temperature [°C] Figure 6: Fixation of the cholesteric phase by crosslinking: * represents the thermotropic phase that allows precise color adjustment. resulting solid phases exhibit a excellent thermal stability of their reflection colors over the whole temperature range (filled squares in figure 6). The original aim for the development of cholesteric systems is their angle dependent selective reflection. This effect makes a coated surface change their color appearance to a spectator, who varies his viewing angle towards the surface. Angle dependent UV/VIS-spectroscopy proves this desired optical effect (see figure 7). The samples color changes from almost red for the nearly orthogonal view (β = 10°) on the surface to blue for a flat angle (β = 70°). In conclusion we have shown that also the cellulose based systems are suitable for achieving the desired optical properties of crosslinked cholesteric films.

Acknowledgment Financial support from the D F G (Schwerpunkt : Cellulose) is highly appreciated.

References [1] [2] [3]

Spiegel 1995, 49, 256. Daimler-Benz A G , Wacker-Chemie GmbH, D E 44 18 076 A1, 1995 B A S F - A G , D E 4342280 A1, 1995. Gross and Scholz; Biopolymers from Polysaccharides and Agroproteins ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Solid Opalescent Films Originated from Cellulose Derivatives

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