Synthesis, Characterization, and Mesophase Formation of

A series of phenylacetoxy cellulosics with degrees of substitution (DS) between 1.4 ... of the DS, in contrast to CH2Cl2 solutions which display liqui...
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Biomacromolecules 2004, 5, 74-80

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Synthesis, Characterization, and Mesophase Formation of Phenylacetoxy Cellulose and Its Halogenated Derivatives Qizhou Dai, Richard D. Gilbert, and John F. Kadla* College of Natural Resources, North Carolina State University, Raleigh, North Carolina 27695-8005 Received July 17, 2003; Revised Manuscript Received September 17, 2003

A series of phenylacetoxy cellulosics with degrees of substitution (DS) between 1.4 and 3.0 and different halogenation (2-chloro, 3-chloro, 4-chloro, 2,4-dichloro, 3,4-dichloro, and 4-bromo) were synthesized. All the prepared phenylacetoxy cellulosics were soluble in dimethylformamide (DMF) and DMAc. The solubility increased with increasing DS. Mesophases were observed for all of the phenylacetoxy cellulosics with low to medium DS (DS < 2.5) in DMF and DMAc. Non- or mono-halogeneated phenylacetoxy cellulosics with high DS (DS > 1.9) were soluble in methylene chloride (CH2Cl2), whereas those with very low DS or di-halogenation on the phenyl ring were only slightly swollen or partially soluble in CH2Cl2. Non- and mono-halogenated phenylacetoxy cellulosics were soluble in DMSO and formed liquid crystals regardless of the DS, in contrast to CH2Cl2 solutions which display liquid crystalline behavior at medium to high DS (DS > 1.9) only. The solubility of the di-halogenated phenylacetoxy cellulosics in DMSO was limited to ∼40 wt %. Introduction Cellulose and many of its derivatives form liquid crystalline phases in solutions and melts.1,2 Because of the chirality of the cellulose backbone, cellulosic liquid crystalline phases form chiral nematic structures.3 Chiral nematic mesophases possess a unique structure in which the alignment of molecular sheets is at a slight angle to one another resulting in a helicoidal supramolecular structure.4 It can be described by a pitch p (or its inverse, the twist p-1); p ) λ0/n˜ where λ0 is the reflection wavelength and n˜ is the mean refractive index of a sheet, and the corresponding handedness of the twist; right-handed helicoidal structures being assigned to a positive pitch (p > 0) and left-handed helicoidal structures to a negative pitch (p < 0).1,5,6 The type of supramolecular arrangement results in unique optical 7 and physical properties.8 As a result, many lyotropic cellulosic systems have been developed with potential applications ranging from liquid crystalline displays9 to high modulus high strength regenerated cellulose fibers.10 In recent years, systems which display a reversal of their helical twisting sense by solvent11,12 or chemical modification13-15 have been of particular interest. One such system displaying a structural dependent reversal of helical twisting sense is cellulose tricarbanilate (CTC). Zugenmaier and coworkers15,16 found that by introducing specific halogenated substitutents on the side chain of cellulose to form CTC derivatives, both right-handed mesophase (3-Cl-CTC) and left-handed liquid crystals (CTC or 4-Cl-CTC) were formed in the same solvent. Moreover, by introducing different substituents into the same polymer, a pseudonematic liquid crystalline phase was formed. * To whom correspondence should be addressed. Phone: (919) 5132455. Fax: (919) 515-6302. E-mail: [email protected].

It is highly desirable to produce high modulus and high strength cellulosic fibers and films from a pseudonematic liquid crystalline phase, as it may minimize or eliminate the orientational disruption of the polymer chains during the phase transformation (nematic f chiral nematic) in fiber spinning or film casting. Unfortunately, the utilization of CTC and the various halogenated derivatives in the production of regenerated cellulose fibers or films is precluded by the labile side chain. A structurally similar cellulose ester, phenylacetoxy cellulose, has however been shown to form both thermotropic and lyotropic mesophase and to readily regenerate into cellulose.17,18 However, the effect of the degree of substitution on the properties of the phenylacetoxy cellulose mesophases were not studied. Thus, the potential exists to manipulate the helical twisting sense of the cellulosic polymer and thus that of the resulting regenerated cellulose fibers and films by structurally changing the phenylacetoxyl group via the introduction of specific halogenated substituents. Halogenated phenylacetoxy cellulose derivatives have not been previously synthesized, and the formation and properties of mesophases of these derivatives have not been reported. Here we present the synthesis and mesophase properties of a number of halogenated phenylacetoxyl cellulose derivatives. The effect of the degree of substitution and position of halogenation on liquid crystallinity is discussed. Experimental Section Materials. Cellulose (microgranular; degree of polymerization (DP), 200) was purchased form Aldrich Co. To facilitate dissolution in N,N-dimethylacetimide (DMAc)/LiCl (9% LiCl), cellulose was first activated through a series of solvent exchange processes.19 Accordingly, cellulose (10 g)

10.1021/bm034245q CCC: $27.50 © 2004 American Chemical Society Published on Web 10/25/2003

Biomacromolecules, Vol. 5, No. 1, 2004 75

Phenylacetoxy Cellulosic Liquid Crystals

Table 1. Structure of Phenylacetoxy Cellulose Derivatives

2a 2b 2c

Figure 1. Reaction scheme for the preparation of phenylacetoxy cellulose (2a). Reagents and conditions: (i) SOCl2, 80 °C, 2 h, 100%; (ii) Et3N or C6H5N, DMAC/LiCl, 80 °C.

was suspended in deionized water (100 mL), stirred for 24 h, and centrifuged, and the water was removed. The cellulose was washed twice with methanol (20 mL) and then resuspended in fresh methanol (100 mL). The methanolcellulose suspension was stirred for 24 h and then centrifuged, and the methanol was removed. The procedure was repeated using dried DMAc. The resulting cellulose was then mixed with DMAc/LiCl (9% LiCl) at a 5% (w/w) solution concentration and stirred for 7 days. The solution was filtered and stored over 4 Å molecular sieves. Thionyl chloride (99%, Aldirch) and pyridine (99%+, Aldrich) were distilled before use. Lithium chloride (ACS grade, Aldrich) was used after drying at 105 °C overnight. Other chemicals, N,N-dimethylacetamide (DMAc, 99+%), N,N-dimethylformamide (DMF, 99+%), phenylacetyl chloride (98%), phenylacetic acid (99%), 2-chlorophenylacetic acid (99%), 3-chlorophenylacetic acid (98%), 4-chlorophenylacetic acid (99%), 4-bromophenylacetic acid (98%), 2,4-dichlorophenylacetic acid (99%), 3,4-dichlorophenylacetic acid (98%), ACS grade methanol, methylene chloride (DCM), and triethylamine (TEA), were purchase from Aldrich and used as received. Preparation of Phenylacetoxy Cellulose (2a). Phenylacetoxy cellulose was prepared by reaction of cellulose with phenylacetyl chloride using TEA or pyridine as the catalyst (Figure 1). In a typical reaction, the cellulose/DMAc/LiCl solution (0.012 mole of cellulose) was diluted to 2.5% by adding dry DMAc. Triethylamine (22.5 mL, 0.162 mole) or pyridine (13 mL, 0.162 mole) was added and the solution was heated to 80 °C under N2. Phenylacetyl chloride (13.5 mL, 0.108 mole) was added dropwise to the cellulose solution in three equal portions at 0, 1, and 2 h. The reaction was stirred at 80 °C for 9 h. Upon completion of the reaction, the reaction was allowed to cool to room temperature, and the product was precipitated in methanol, filtered, and washed with methanol/water (50/50, vol/vol). The raw product was dissolved in N,N-dimethylformamide (DMF), filtered and precipitated in methanol, and dried under vacuum at room temperature. The color of the polymer ranged from white (low DS) to lightly yellow (high DS). The phenylacetoxy content of the product depended on the amount of reagent and catalyst added. The degree of phenylacetoxyl group substitution was determined by titration according to method

X

Y

Z

H H H

H H Cl

H Cl H

2d 2e

X

Y

Z

Cl Br

H H

H H

2f 2g

X

Y

Z

Cl Cl

H Cl

Cl H

ASTM D871-72 (DSTitr) and verified by the elemental analysis (DSElem). 13C NMR (DMSO) δ: 39.7 (C-8), 63.3 (C-6), 72.1 (C-2, C-3, C-5), 80.0 (C-4), 99.9 (C-1), 126.1 (C-12), 127.7 (C-11, C-13) 129.4 (C-10, C-14), 134.3 (C-9), 171.0 (C-7). FT-IR: 3479 cm-1 (O-H), 1744 cm-1 (CdO), 1217 cm-1 (C-O-C,ester). Elemental Analysis: C 68.44, H 5.32. DSTitr ) 2.48, DSElem ) 2.56. Preparation of Halogenated Phenylacetoxy Cellulosics. The halogenated PACs were prepared according to the general procedure outlined above using TEA as the catalyst and the corresponding halogenated phenylacetic acid (2chlorophenylacetic acid, 3-chlorophenylacetic acid, 4-chlorophenylacetic acid, 2,4-dichlorophenylacetic acid, 3,4dichlorophenylacetic acid, and 4-bromophenylacetic acid) with an excess (2-equivelents) of thionyl chloride at 80 °C under N2 for 2 h. The excess thionyl chloride was removed under vacuum at 80 °C. The procedure of preparing halogenated phenylacetoxy cellulosics was similar to that of preparing phenylacetoxy cellulose (Figure 1). All phenylacetyl chlorides were used within 48 h of preparation. The color of the polymer ranged from brown yellow to dark brown. The substituted phenylacetoxy content of the product was dependent on the amount of reagent and catalyst added. The degree of substitution of halogenated phenylacetoxyl group was determined by titration according to method ASTM D871-72 and verified by elemental analysis. Table 1 shows the structure of the various Phenylacetoxy cellulose derivatives. 2-Chlorophenylacetoxy Cellulose (2b). 13C NMR (DMSO) δ: 38.2 (C-8), 61.7 (C-6), 72.3 (C-2, C-3, C-5), 73.4 (C-4), 99.7 (C-1), 127.9 (C-13), 129.5 (C-11, C-12) 132.2 (C-14), 134.5 (C-9, C-10), 170.1 (C-7). FT-IR: 3480 cm-1 (O-H), 1750 cm-1 (CdO), 1215 cm-1 (C-O-C,ester). Mn ) 10 300 amu. Elemental Analysis: C 57.20, H 4.05, Cl 15.89. DSTitr ) 2.27, DSElem ) 2.29. 3-Chlorophenylacetoxy Cellulose (2c). 13C NMR (DMSO) δ: 39.7 (C-8), 63.8 (C-6), 73.6 (C-2, C-3, C-5), 77.8 (C-4), 100.2 (C-1), 128.9 (C-12), 130.0 (C-14), 131.2 (C-13), 133.4 (C-10), 138.2 (C-9, C-11), 171.9 (C-7). FT-IR: 3480 cm-1 (O-H), 1745 cm-1 (CdO), 1217 cm-1 (C-O-C,ester). Mn ) 7700 amu. Elemental Analysis: C 56.33, H 4.96, Cl 14.63. DSTitr ) 1.86, DSElem ) 1.80. 4-Chlorophenylacetoxy Cellulose (2d). 13C NMR (DMSO) δ: 40.2 (C-8), 61.7 (C-6), 71.8 (C-2, C-3, C-5), 75.9 (C-4), 99.3 (C-1), 127.7 (C-11, C-13), 130.2 (C-10, C-14), 130.8 (C-12), 131.1 (C-9), 170.2 (C-7). FT-IR: 3480 cm-1 (OH), 1746 cm-1 (CdO), 1217 cm-1 (C-O-C,ester). Mn )

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9100 amu. Elemental Analysis: C 56.21, H 4.39, Cl 14.69. DSTitr ) 1.91, DSElem ) 1.82. 4-Bromophenylacetoxy Cellulose (2e). 13C NMR (DMSO) δ: 40.1 (C-8), 63.2 (C-6), 72.0 (C-2, C-3, C-5), 76.9 (C-4), 101.2 (C-1), 120.9 (C-12), 131.9 (C-10, C-11, C-13, C-14), 133.0 (C-9), 171.9 (C-7). FT-IR: 3480 cm-1 (O-H), 1745 cm-1 (CdO), 1217 cm-1 (C-O-C,ester). Mn ) 7800 amu. Elemental Analysis: C 47.58, H 3.34, Br 29.54. DSTitr ) 2.14, DSElem ) 2.20. 2,4-Dichlorophenylacetoxy Cellulose (2f). 13C NMR (DMSO) δ: 37.8 (C-8), 63.9 (C-6), 72.8 (C-2, C-3, C-5), 81.0 (C-4), 99.7 (C-1), 127.6 (C-11, C-13), 129.2 (C-14) 132.1 (C-9), 133.9 (C-12), 136.2 (C-10), 170.0 (C-7). FTIR: 3485 cm-1 (O-H), 1749 cm-1 (CdO), 1214 cm-1 (CO-C,ester). Elemental Analysis: C 49.23, H 3.43, Cl 26.16. DSTitr ) 1.88, DSElem ) 1.92. 3,4-Dichlorophenylacetoxy Cellulose (2g). 13C NMR (DMSO) δ: 39.2 (C-8), 63.1 (C-6), 71.4 (C-2, C-3, C-5), 80.4 (C-4), 102.4 (C-1), 131.1 (C-10, C-13, C-14), 131.6 (C-12), 132.8 (C-9), 133.5 (C-11), 170.5 (C-7). FT-IR: 3479 cm-1 (O-H), 1744 cm-1 (CdO), 1218 cm-1 (C-O-C, ester). Mn ) 9300 amu. Elemental Analysis: C 49.28, H 3.20, Cl 27.13. DSTitr ) 2.19, DSElem ) 2.17. Preparation of Lyotropic Liquid Crystalline Solutions. The liquid crystalline solutions were prepared by adding a certain amount of polymer and solvent to a 7 mL glass vial to prepare ∼40% (w/w) solution. The vial was capped and sealed with Teflon tape. The mixture was heated at 50 °C until the polymer was fully penetrated by the solvent. The vial was then centrifuged back and forth until the polymer was fully dissolved. The vial was unsealed, and the solvent was slowly evaporated at room temperature in a desiccator until a desired concentration was reached. The vial was then resealed, and the solution was aged in the dark at room temperature for at least one week. General Analysis. Infrared spectra of thin cellulosic films were measured with a Perkin-Elmer 16PC FTIR spectrophotometer; and 32 scans were measured for each sample with a resolution of 2 cm-1. The phenylacetoxy cellulose and halogenated cellulosic films were cast from dilute (∼5%) CH2Cl2 solutions (2-3 drops of the phenylacetoxy cellulosic solution was spread on a ZnSn window, and the solvent was evaporated by blowing dry air on the surface for about 1 min). The FT-IR spectra of CH2Cl2 insoluble derivatives were measured as KBr pellets. 13C NMR analyses were recorded with a Bruker AVANCE 500 Hz spectrometer (1996) using an Oxford narrow bore magnet (1989). The polymers (∼30 mg) were dissolved in 0.75 mL of DMSO-d6; the DMSO peak at 39.7 ppm was used as the internal reference. Conditions for analysis were as the follows: temperature of 300°K, a 90° pulse with 10µs and a 1.0 s pulse delay (d1) and with 2000-3000 transients. Gel permeation chromatography (GPC) analyses were performed on a Waters HPLC system at ambient conditions using a µ-Styragel column (HR4), calibrated using polystyrene standards. DMF was the eluent (0.5 mL/min), and fractions were monitored using refractive index (Waters refractometer model no. 410) and UV absorbance at 280 nm (Waters UV spectrometer model no. 484).

Dai et al.

Figure 2. Effect of phenylacetyl chloride to cellulose hydroxyl group ratio on the degree of substitution (DS) of phenylacetoxy cellulose (2a) (*phenylacetic acid was used as the starting chemical).

The liquid crystalline phases were observed using a polarizing optical microscope (Olympus BH2-UMA). The liquid crystalline solutions were sandwiched between a microscope slide and a cover slide with a thickness of approximately 0.1 mm. The edges were sealed with a fast cured epoxy resin adhesive (Extra Setting Epoxy Adhesive from Super Glue, Inc.) to prevent solvent loss. The sandwiched samples were relaxed for at least 3 days before microscopic observation. The birefringence of the liquid crystalline solutions was measured at room temperature (23 °C) using the procedure of Laivins and Grey20 and an Abbe Refractometer (Milton Roy Inc.). Results and Discussion Phenylacetoxy Cellulose Synthesis. The preparation of cellulose esters is typically accomplished by the reaction of an acid chloride or anhydride with the hydroxyl groups of the cellulose molecule. These reactions can be run either under heterogeneous or homogeneous reaction conditions, with the homogeneous reaction system producing higher degrees of substitution (DS) in shorter reaction times with improved yields. In many systems, acid acceptors such as tertiary amines are utilized to improve yields. Previously, Pawlowski et al.21 used pyridine as the acid scavenger in the synthesis of phenylacetoxyl cellulose. Unfortunately, the product discolors, turns brownish at the reaction temperature (80 °C) due to side reactions involving the pyridine. The color of the polymer will significantly affect the optical properties of liquid crystalline solutions. To address this, Gilbert et al.18 used triethylamine (TEA) as the acid scavenger. TEA is not as easily discolored as pyridine, and the product was only slightly colored. However, in both studies, no details regarding the effect of acid scavenger on DS were reported. In the present work, cellulose esters were prepared by the reaction of an acid chloride with cellulose dissolved in N,N-dimethylacetamide/lithium chloride (DMAc/ LiCl) solution. Pyridine and triethylamine (TEA) were utilized as acid scavengers (Figure 1). By varying the ratio of acid chloride to hydroxyl group of the cellulose, phenylacetoxyl cellulose (2a) with different degrees of substitution (DS) were obtained. Figure 2 shows the effect of acid

Phenylacetoxy Cellulosic Liquid Crystals

Biomacromolecules, Vol. 5, No. 1, 2004 77

Figure 3. FT-IR Spectra of cellulose 1, phenylacetoxy cellulose 2a (DS ) 1.40 and DS ) 2.6), and 2,4-dichlorophenylacetoxy cellulose 2f (DS ) 1.9).

scavenger and acid chloride:hydroxyl group on the DS of 2a. For both the TEA and pyridine reaction systems, increasing the acid chloride/hydroxyl group lead to an increase in DS. Interestingly, the pyridine system produced a higher DS at the same acid chloride/hydroxyl group ratio. Under the reaction conditions utilized in this study, we found that the TEA was readily lost, stripped away by the argon purge. The loss of TEA would lower the efficiency as an acid acceptor and may increase the acidity of the reaction system, which may explain the observed difference in DS. Unfortunately, an increase in the TEA charge was not possible to compensate the loss; the TEA charge could not exceed 5-6 equiv of the cellulose in the reaction solution. Increasing the TEA concentration precipitated the cellulose from solution, destroying the homogeneous reaction system. In addition, it was observed that the TEA formed a DMAc insoluble salt with the phenylacetyl chloride. The reaction is in fact heterogeneous. The salt formed by pyridine and phenylacetyl chloride is soluble in DMAc during the entire reaction. The reaction is homogeneous (in fact, pyridine can be increased to at least 10 equivalents without causing cellulose to precipitate). As a result, under the same conditions, the DS of 2a using pyridine is higher than that using TEA. However, as the pyridine system produced discolored phenylacetoxy cellulose and the TEA system did not, therefore TEA was utilized for the synthesis of the other phenylacetoxy cellulosics (2b-2g). The FT-IR spectrum of 2a shows a decrease in the absorbance around 3400 cm-1 indicating the substitution of hydroxyl groups (Figure 3). Compared with cellulose (sample 1, Figure 3), as the DS increases the appearance of a strong absorption at 1745 cm-1 is evidence of the carbonyl functionality of the ester. The signal due to the C-O

Figure 4. Effect of phenylacetyl chloride to cellulose hydroxyl group ratio on the degree of substitution (DS) of phenylacetoxy cellulose and the various halogenated derivatives.

stretching (1000-1300 cm-1) of the ester is somewhat overlapped by the C-O stretching of cellulose itself. In addition, the doublet at ∼1500 cm-1 is indicative of aromatic CdC stretching associated with the phenylacetoxy substituent. As with 2a, the various halogenated PACs produced distinct FT-IR spectra. A decrease in the hydroxyl group stretching region along with an increase in the carbonyl stretching region was observed. The halogenated phenyl ring produced distinct absorption bands relative to 2a. 2,4Dichlorophenylacetoxy cellulose (2f) showed signals at 830900 cm-1, an out-of-plane bending characteristic of 1,2,4 trisubstituted aromatic rings, and ∼1100 cm-1 for C-Cl stretching (Figure 3). In addition, the aromatic CdC stretching (1530 cm-1) was a strong single absorption band.

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Figure 5.

13C

Dai et al.

NMR Spectrum of 2,4-dichlorophenylacetoxy cellulose 2f (DS ) 1.9) in DMSO-d6 at 25 °C.

All of the phenylacetoxy derivatives were soluble in DMSO at moderate concentration, and well resolved 13C NMR spectra were obtained. Figure 5 shows the 13C NMR spectrum of 2,4-dichlorophenylacetoxy cellulose. All of the carbon signals, with the exception of C11 and C13, are clearly resolved. The signals appearing around 125-130 ppm and 170-172 ppm are assigned to the aromatic carbons and carbonyl carbon, respectively, in the phenylacetoxy substituent. The benzyl methylene carbon (C8) appears at around 40 ppm. The signals between 65 and 100 ppm are the carbons on the cellulose backbone (C1 at 101 ppm, C4 at 75-80 ppm, C2, C3, and C5 at 70-75 ppm, and C6 at 60-65 ppm). Solubility of Phenylacetoxy Cellulosics. Cellulosic polymers must have sufficiently high solubility in a solvent to obtain the critical concentration for lyotropic liquid crystal formation. Phenylacetoxy cellulose 2a forms a lyotropic mesophase in CH2Cl2.22,23 However, the solubility is greatly affected by the DS, with only those of a DS greater than 1.9 having good solubility in CH2Cl2 and dissolving completely to form homogeneous solutions. When the DS is between 1.5 and 1.9, 2a only partially dissolves in CH2Cl2. When the DS is below 1.5, 2a is only swollen in CH2Cl2. Phenylacetoxy cellulose with different DS has good solubility in dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and DMAc. In all solvents tested, increasing DS facilitated dissolution, likely due to decreased interchain hydrogen bonding by replacing the hydroxyls with phenylacetoxy groups. The solubility of the halogenated phenylacetoxy cellulosics in various solvents was dependent on the DS as well as halogen substitutions. Mono-halogenated phenylacetoxy cellulosics (2b-2e) with a medium to high degree of substitution dissolve completely in CH2Cl2. Di-halogenated cellulosics (2f and 2g) do not dissolve in CH2Cl2, regardless of

Table 2. Solubility of Phenylacetoxy Cellulose Derivatives in Various Solvents cellulosics

CH2Cl2

DMF

DMAc

DMSO

2a 2b 2c 2d 2e 2f 2g

DS > 1.9 DS > 1.6 DS > 1.4 DS > 1.7 DS > 1.7 insoluble insoluble

soluble soluble soluble soluble soluble soluble soluble

soluble soluble soluble soluble soluble soluble soluble

soluble soluble soluble soluble soluble < ∼40% < ∼40%

the DS. All of the halogenated phenylacetoxy cellulosics with DS greater than 1.4 are completely soluble in DMF and DMAc, and the solubility increases as the DS increases. All of the halogenated phenylacetoxy cellulosics readily dissolve in DMSO to form dilute solutions. Only the monohalogenated phenylacetoxy cellulosics reach high concentrations in DMSO. The di-halogenated phenylacetoxy cellulosics have a solubility limit of ∼40 wt % in DMSO. The solubility of the phenylacetoxy cellulosics is summarized in Table 2. Mesophase Formation in Concentrated Solutions. High solubility of rigid or semirigid polymers in a certain solvent does not guarantee the formation of a lyotropic liquid crystalline state. Mesophase formation is a delicate balance of polymer-solvent interactions.24 If a polymer is too readily soluble, mesophase formation will not occur, rather forming a gel at a higher concentration. Therefore, the formation and physical properties of lyotropic cellulosic mesophases are strongly influenced by the main chain properties (i.e., degree of polymerization), side chain properties (e.g., the nature of substituents, degree of substitution and the distribution of substitution along the cellulose chains), and solvent(s) used. A series of high concentration phenylacetoxy cellulosic solutions in different solvents were prepared, and lyotropic

Biomacromolecules, Vol. 5, No. 1, 2004 79

Phenylacetoxy Cellulosic Liquid Crystals Table 3. Effect of Solvent and Degree of Substitution on Mesophase Formation for the Various Phenylacetoxy Cellulose Derivatives

Table 4. Effect of Degrees of Substitution on the Critical Concentrations (wt %) of Phenylacetoxy Cellulose 2a and 2,4-Dichlorophenylacetoxy Cellulose 2f in DMF and DMAc

cellulosics

CH2Cl2

DMF

DMAc

DMSO

DS

1.4

1.9

2.5

2.8

2a 2b 2c 2d 2e 2f 2g

DS > 1.9 DS > 1.6 DS > 1.4 DS > 1.7 DS > 1.7 no no

DS < 2.7 DS < 2.8 DS < 2.5 DS < 2.8 DS < 2.5 DS < 2.6 DS < 2.6

DS < 2.7 DS < 2.8 DS < 2.5 DS < 2.8 DS < 2.5 DS < 2.6 DS < 2.6

all DS range all DS range all DS range all DS range all DS range no no

2a in DMF 2a in DMAc 2f in DMF

42

48 53 58

56 59

>75 >75 >75b

a

51a

DS ) 1.5. b DS ) 2.6.

Figure 6. Chiral nematic birefringence (∆n) vs concentration of phenylacetoxy cellulose (2a) with DS 1.88 in DMF (open circles), phenylacetoxy cellulose (2a) with DS 2.48 in DMF (filled squares), and phenylacetoxy cellulose (2a) with DS 1.88 in DMAc (filled triangles).

mesophase formation was determined by the appearance of strong birefringence. Solutions that did not exhibit birefringence were considered isotropic. The texture of the lyotropic solutions was observed at room temperature (∼23 °C) with a polarized microscope. Lyotropic mesophases were observed for phenylacetoxy cellulose 2a and the various mono-halogenated phenylacetoxy cellulosics (2b-2e) in DMF, DMAc, CH2Cl2, and DMSO. The degree of substitution had a dramatic effect on mesophase formation and was dependent on solvent used. In CH2Cl2, the birefringence was only observed for medium to high degree of substitution, whereas in DMSO, the birefringence was observed over the entire DS range. The di-halogenated compounds (2f and 2g) on the other hand did not form mesophases in CH2Cl2 or DMSO due to solubility issues. The effect of solvent and DS on mesophase formation is summarized in Table 3. Figure 6 shows the chiral nematic birefringence (∆n ) n⊥ - n|) for various phenylacetoxy cellulose 2a concentrations. All lyotropic solutions exhibit a positive birefringence with ∆n > 0. The positive ∆n indicates that the refractive index of axial direction n| is smaller than that of tangential direction n⊥.20 The larger refractive index in the tangential direction may be due to the larger polarizability of the substitution groups as compared to the smaller refractive index in the axial direction caused by the cellulose backbone.24 The critical concentrations for phase separation of lyotropic phenylacetoxy cellulose solutions were determined by the chiral nematic birefringence. The onset concentrations for the appearance of birefringence vs the DS of 2a and 2f

Figure 7. Polarized photomicrographs of liquid crystalline solutions. (i) Phenylacetoxy cellulose (2a) DS ) 2.48 in DMF (70wt %/wt); (ii) phenylacetoxy cellulose (2a) DS ) 2.48 in DMAc (70wt %/wt); (iii) 2,4-dichlorophenylacetoxy cellulose (2f) DS ) 1.88 in DMF (wt %/wt); (iv) 2,4-dichlorophenylacetoxy cellulose (2f) DS ) 1.88 in DMAc (wt %/wt); and (v) phenylacetoxy cellulose (2a) DS ) 1.88 in DMF (55wt %/wt).

are summarized in Table 4. In DMF and DMAc, when the concentration of the solution is above 75% (w/w), the viscosity of the solution is very high, and the solution forms a gel-like material. In general, the critical concentration, C*, for the onset of the anisotropic phase was found to depend on the DS of the cellulose derivative.25 According to Flory’s theory on phase separation of lyotropic liquid crystals, a longer polymer chain persistence length results in a lower critical concentration.26 As shown in Table 4, as the DS is increased, the critical concentration of the various phenylacetoxy cellulosics increases. This indicates a decrease in the persistence length, which may be due to the substitution of hydroxyl groups on the cellulose chain; intramolecular hydrogenbonding in the cellulose is reduced, and the rigidity of the polymer chain is decreased. Also shown in Table 4, C* increased by changing the solvent system from DMF to DMAc. This suggests that there is a change in the flexibility of 2a as a result of a change in polymer-solvent interactions. The critical concentration determination was not possible for the CH2Cl2 system.

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Because of the high volatility of CH2Cl2, it is difficult to determine the precise concentration when the solution is observed under the microscope or with the Abbe refractometer. The concentration is estimated to be between 60% and 80% (w/w). The halogenated phenylacetoxy cellulosics with low to medium degree of substitution (DS < 2.5) form mesophases in DMAc and DMF. At high DS, the anisotropic phase can only be observed at high concentrations. For 2f with a DS ) 2.6, C* is greater than 75% (w/w) (Table 4). The various phenylacetoxy cellulosics display textures typical of chiral nematic liquid crystals. Figure 7 shows the polarized microscopic images obtained for various systems. Polygonal (Figure 7, parts i, iii, and iv) and streak (Figure 7ii) textures are observed for the lyotropic solutions of 2a and 2f. Figure 7v displays a characteristic fingerprint pattern for lyotropic cellulosics. The pitch of 2a (DS of 1.9) at 55% (w/w) in DMF is approximately 7 µm. This value is comparable to the pitch of 2a in CH2Cl2.21

Dai et al.

References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

Conclusions Various phenylacetoxy cellulose derivatives with different degrees of substitution and halogenation were synthesized under homogeneous reaction conditions by varying the reactants and chemical charge. Polymer solubility and mesophase formation were strongly dependent on the DS, on the nature of the substituent, and on the solvent used. The critical concentration for mesophase formation increased with DS and chlorination of the phenyl ring. All of the phenylacetoxy cellulose derivatives formed chiral nematic mesophases in DMF and DMAc and displayed typical chiral nematic liquid crystalline textures under crossed polarizers.

(16) (17) (18) (19) (20) (21) (22) (23) (24) (25)

Acknowledgment. Funding for this study was provided by the U.S. Department of Agriculture (USDA-NRI 200101561).

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