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Aliphatic acid estersof (2-hydroxypropyl) cellulosed Effect of sidechain length on properties of cholesteric liquid crystalsBin Huanga, JasonJ. Geb,1, Yonghong Lia, Haoqing Houa,*aCollege of Chemistry and Chemical Engineering, Jiangxi Normal University, 99 Ziyang Street, Nanchang, Jiangxi 330022, ChinabMaurice Morton Institute and Department of Polymer Science, University of Akron, Akron, OH 44325-3909, USAReceived 13 August 2006; received in revised form 12 November 2006; accepted 15 November 2006Availableonline 4 December 2006AbstractA series of aliphatic acid estersof (2-hydroxypropyl)cellulose (CnPC) were synthesized via the esteri?cation of aliphatic acid chloride and(2-hydroxypropyl) cellulose (HPC). The liquid crystalline (LC) phasesand transitions were investigated using differential scanning calorimetry,wide-angle X-ray diffraction (WAXD), andpolarized light microscope (PLM) techniques. This seriesof CnPC polymers exhibited characteristicfeaturesof cholesteric LC phasesbetween their glasstransition and isotropization temperatures.The cholesteric LC characteristics were studiedutilizingan ultraviolet/visible/nearinfrared spectrometer in a re?ection mode. It was con?rmed that, with an increase in the number ofmethylene units in the side chains of this seriesof CnPC polymers, there wasan increase,on the scale of nanometers,in the layer spacingvaluesfor the cholesteric LC phasesmeasured by WAXD. This periodic layer spacing representsthe thickness of the neighboring twisted layers ina helical structure. Basedon a unique signi?cant red shift of the maximum re?ection peakfor the LC phasesin this seriesof CnPCs, it is evidentthat the pitch distance in the helical structure also increaseswith an increasein the length of methylene units in the side chains. 2006 Elsevier Ltd. All rights reserved.Keywords: Cholesteric liquidcrystal; Cellulose derivative; Side chain1. IntroductionCholesteric liquidcrystals (ChLCs) are of great scienti?cand technological interest due to their unique selective re?ec-tion of light at characteristic wavelengths with a speci?c pitchdistance. Importantapplications of ChLCs have been identi-?ed, including rigid or ?exible re?ective liquid crystal displaysand re?ective polarization ?lms for usein ?at panel displays toimprovebrightness1e 4.Theselectivere?ection(l ?nPocosq) is caused by the molecularhelical structure witha pitch distance (Po) in cholesteric liquid crystalline(ChLC)phaseswhere the average refractive index (n) is equal to (nk2t2nt2)/30.5and the incident angle is q 1 . Interestingly, mostof the cellulosederivatives formcholesteric LC phases insolutions withincertain concentration regions (lyotropic)orin the bulk within certain temperature regions (thermotropic)5e 18. Generallyspeaking, it is understood that physicalpacking (chiral arrangement) schemes of stiff cellulose chainscould be considered playing important roles in inducing theformationof cholesteric LC phases. In particular, when the?exibleside chains are connected onto the cellulose back-bones, the resulting hairy-rod cellulose polymers start formingthese cholesteric LC phases.It is believed that the attachmentof ?exible side chains onto the rigid cellulose molecules canfacilitatethe orientationalorder of the semi-rigidhairy-rodbackbones with side chains with increasing the chain mobilityand enhancing the solubility. Among the cellulose derivatives,(2-hydroxypropyl)cellulose (HPC) derivatives are of speci?cinterest because they can form either cholesteric lyotropic orthermotropicLC phases 18e 24. A particularlyinterestingproperty of the cellulose derivatives is that, based on differentchemical structures, the materials can re?ect light at a speci?clightwavelength and in a speci?c temperature region. For* Correspondingauthor. Tel.: t 86 791 8120389; fax: t 86 791 8120536.E-mail address: (H. Hou).1Present address: AltuglasInternational,Arkema, 900 1st Avenue, King ofPrussia, PA 19403, USA.0032-3861/$ - see front matter 2006 Elsevier Ltd. Allrights reserved.doi:10.1016/j.polymer.2006.11.033Polymer 48 (2007) 264e - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 6 页 - - - - - - - - - example, (2-ethoxypropyl)cellulose (EPC) 15 re?ects visi-ble lightat 130e 160 C, while (2-acetoxy-propyl)cellulose(C2PC) 18, (2-propionyloxy)-propylcellulose (C3PC) 19and (2-butyrionyloxy)propylcellulose (C4PC) 24 displayiridescent color at ambient temperature due to the selectivere?ections of light from the cholesteric LC phases.One of the most importantapproaches towards obtainingcellulose derivatives exhibitingcholesteric LC phase transi-tions is via the esteri?cation of HPC 19,24 . These cellulosederivatives can possess different side chain lengths based ondifferent numbers of methylene units. The motivationof thiswork is to design a series of HPC derivatives (CnPC, n ? 5,6, 7 and 10) to further improve the systematic understandingof the side chain effect on the formation of thermotropic cho-lesteric LC phasesand phase transitions aswell astheir uniqueselectivityfor color re?ections and the pitch distance in thecholesteric LC phases.2. Experimentalpart2.1. Materials2-Hydroxylpropylcellulose (HPC) (Aldrich,weight aver-age molecularweight,Mw? 100,000 g/mol)was driedat50C(30 mbar)priortouse.Aliphaticacidchloride(CH3(CH2)n2COCl, n? 2, 3, 4, 5, 6, 7, 10, purchased fromAldrich)was used as received. Acetone (Aldrich)was driedover CaCl2for 2 days and was distilledin an N2atmospherebefore use.The synthetic procedure of CnPC is described here usingC5PCas anexample.HPCof5.0 g(correspondingto41.71 mmol of hydroxyl groups) was added to 30 mL acetoneunder a dry nitrogen atmosphere and was dissolved by heatingthesolution.Then15.1 mL(125.13 mmol)valericacidchloride was swiftlyadded to the solution of HPC by usinga syringe. After 2 h of re?ux, the reaction mixture was pouredinto 200 mL distilledwater. After removing the liquid phase,a cream-colored, sticky material was obtained and then dis-solved in 80 mL acetone and precipitated by adding 5 mL ofwater to the solution. The pasty product was liberated fromthe acetone/water by decantation. The second solution precip-itation was repeated for 5 times. Finally, the product was driedat 60C (30 mbar, 48 h). The amount of 6.57 g of the ?nalpolymer material was obtained with a yield of 76.8%.Gel permeation chromatographic (GPC) results showed thatMw? 6.83? 104. The degreeof substitution (DS) 23 was de-termined as 2.67 by1H nuclear magnetic resonance(NMR) andas 2.79 by saponi?cation. Infrared (IR) spectroscopic resultsshowed absorption at 3300e 3600 cm1for e OH stretching,2850e 3000 cm1for the CeH stretching, and 1731.2 cm1for the CO stretching. The results obtained in1H NMR (inCDCl3) analysisare2.8e 4.5ppm for the protons of the anhydro-glucose ring; 4.95 ppm for -OCH2CH(CH3)OCH2CH(CH3)-(OOC(CH2)3CH3); 3.48 ppm for -OCH2CH(CH3)OC H2CH-(CH3)(OOC(CH2)3CH3);1.08ppmfor-OCH2CH(CH3)-OCH2CH(CH3)(OOC(CH2)3CH3); 2.23 ppm for -OCH2CH-(CH3)OCH2CH(CH3)(OOCCH2CH2CH2CH3);1.63 ppmfor-OCH2CH(CH3)OCH2CH(CH3)(OOCCH2CH2CH2CH3);1.29 ppm for -OCH2CH(CH3)OCH2CH(CH3)(OOCCH2CH2-CH2CH3); 0.92 ppm for -OCH2CH(CH3)OCH2CH(CH3)-(OOCCH2CH2CH2CH3);1.17 ppm for-OCH2CH(CH3)-OCH2CH(C H3)(OOCCH2CH2CH2CH3) ( stands forthestructure of cellulosic chain).The rest of the CnPC polymers with different lengths in theside chains were synthesized using the same procedure asdescribed for C5PC,similar to Refs. 18,24 . Molecular weightvalues and yields are given in Table 1.2.2. Equipments and experimentsInfrared (IR) spectroscopy was performed using a PerkinElmerFT-IR1600 in a transmissionmode. The sampleswere dissolvedin CHCl3at the concentrationof 5% andthen solutioncast ontoKBrplates. The IR spectra wererecorded after the solvent was evaporated completely. Solution1H NMR spectra (400 MHz) were recorded on a Bruker AC400 while the samples were dissolved in CDCl3at room tem-perature. Differentialscanning calorimetric(DSC) measure-ments were conducted on pasty samples using A Mettler-30DSCinstandard aluminumpans witha coolingrate of15 C/minfrom the isotropicmelt. Polarizinglightmicros-copy (PLM)was performed using a Leitz Orthoplan micro-scope equippedwitha MettlerFP hotstage. ThePLMsamples were prepared in the form of ?lm with about 1 mmthickness by, respectively, putting 0.5 g of each CnPC pastyproduct on a clean glass slide and keeping the slides withCnPC samples in a baking oven at 60 C overnight for makingthe sample thickness more uniformand for developing LCphases. Gel permeation chromatography(GPC) was carriedout using a Viscotek detector and a set of 5 mm mixed beadcolumns. The calibration was done with a series of standardpolystyreneswithknownmolecularweight.The sampleswere dissolved in tetrahydrofuran (THF) at ambient tempera-ture and ?ltrated with a 0.4 mm PTFE membrane ?lter. Wide-angle X-ray diffraction(WAXD)experiments were conductedusing a Siemens D5000 (Cu Ka). The pasty samples were usedin WAXD experiments in a re?ection mode. The samples wereprepared in the way of preparing PLM samples. The UV/vis/Table 1Reaction conditions, yield, degree of substitution,and molecular weight forCnPCsTime(h)Cn 1COCl(mL)Yielda(%)DSbDScMw(? 104)Mw/MnC2PC29.170.72.702.829.622.63C3PC211.572.02.692.837.801.27C4PC213.377.52.672.808.651.43C5PC215.276.82.642.796.831.24C6PC217.779.02.592.786.381.47C7PC219.678.52.642.766.381.37C10PC226.580.62.652.817.061.56aYield was calculated on the basis of 100% esteri?cation of HPC with DEof 3.4.bDegree of substitution (DS) was determined by NMR 23 .cDetermined by saponi?cation23 .265B. Huang et al. / Polymer 48 (2007) 264e 269名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 6 页 - - - - - - - - - NIR optical spectra were recorded on a Perkin Elmer Lambda19 UV/vis/NIRphotospectrometer with an integrating spheredetector in a re?ection mode where the ?lm samples were pre-pared like the PLM samples by using about 0.1 g CnPC prod-uct for each sample but for C6PC using 0.5 g. The refractiveindex of the polymer?lms was measured with a Metricon2010 opticalprismcoupler at a wavelengthof 632.8 nm.The polymer ?lms (at the thickness of 2e 5 m m) were formedby a squeeze of quartz prism on the CnPC pasty masson cleanSiwafersubstrateswhilethemeasurementwasbeingconducted.3. Results and discussion3.1. Molecularstructure and chain packingTheoretically,the esteri?cationofHPC is of molecularweightaccretion.However, the data of molecularweightsshown inTable 1 indicatethatthe Mwvalues ofCnPCmeasured in THFusing GPC are lowerthan that of HPC(100,000 g/mol)beforetheesteri?cationreactions.Thisdeviation could come fromthe differencein solubilityandhydrodynamicvolumes ofmore rigidHPC and less rigidCnPC molecules. This could also implythat the backbonesof the cellulose derivatives may suffer from chain scissionsduring the esteri?cationin acetone. For a CnPC molecularstructure, the e OH groups of HPC have been substituted bythe aliphatyloxy groups (RCOOe ) after the esteri?cation reac-tion. The molecular structure of HPC has been described inprevious publications18,19,24 . Gray et al. 18 illustratedthe molecular structure of CnPC on the basis of the idealizedmolecularstructure, as shown in Fig. 1. In thisidealizedstructure, every anhydroglucose unit contains three ?exiblealiphaticester side chains. The long backbone consists ofcyclic rings with three ?exible, pendent side chains. The over-all chain structure becomes semi-rigid; therefore, thermotropicor lyotropiccholesteric LC phases may be developed in bulkor in high CnPC concentration solutions.The chain packing scheme of the thermotropic LC phase inCnPCs can be deduced based on structural characterizationmethods such as the WAXDtechnique. Fig. 2 shows a set ofone-dimensionalWAXDpatternsdetectedat roomtem-perature for this series of CnPCs with differentnumbers ofmethylene units in the side chains. An intense diffractionpeak was observed in the low 2q angle region between 3and 8in each WAXDsample. Furthermore, with increasingthe number of methylene units, the d-spacing of the diffractionpeak increases from 1.20 nm in C2PC to 1.37 nm in C5PC to1.86 nm in C10PC, as listed in Table 2. These diffraction peaksrepresent the existence of short range or quasi-long-range po-sitional order. The physical origin of thesediffractionpeaks isassociated with the smectic layered structure in LC phasesalthough we do not know whether it belongs to smectic A(a layer normal along the ?ber direction in the oriented ?berpattern) or smectic C (a layer normal tilt to the ?ber direction).It is very interesting to determine the packing structure in thereal spacein three dimensions in our future work. Besides, thescattering halos at around 2q ? 20(d-spacing of 0.444 nm)are related to the average lateral distance between the neigh-boring chains. With increasing the length of methylene units,the birefringence (Dn) of CnPC starts to decrease gradually,in which the birefringence(Dn ? n(TM)n(TE) is measuredat 633 nm from the difference between the TM (out-of-plane)and TE (in-plane)modes. The average refractiveindex ofC2PC is 1.4682 slightlyhigher than the others while the restCH3(CH2)n-2COCH2CHCH3OOC(CH2)n-2CH3C(CH2)n-2CH3OCH2CHCH3OOCH2CHCH3OOOOxFig. 1. An idealized chemical structure of CnPC with three substituents 18 .C3PC510152025302C2PCC4PCC5PCC6PCC7PCC10PCFig. 2. 1D WAXDpowder diagram: CnPC bulk at room temperature on a glassslide in a re?ection mode.Table 2Physical properties of CnPCsCnPCnBirefringenceDn? 103lmax(nm)Po(nm)d-Spacing(nm)C2PC1.46824.51.197C3PC1.46474.2284193.91.281C4PC1.46393.3405276.71.323C5PC1.46533.1673459.31.374C6PC1.46472.21440983.11.480C7PC1.46372.018611271.41.55018271248aC10PC1.46431.025741758a1.860aCalculated on the basis of the layer line interval distance of ?ngerprinttexture and magni?cation.266B. Huang et al. / Polymer 48 (2007) 264e 269名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 6 页 - - - - - - - - - of them are more or less constant. Both of the re?ection peakwavelengths (lmax) at the maximum height and the pitch dis-tance (Po? l /n) increase with the increase in the length ofmethylene units, which willbe further discussed later.3.2. Thermal transition propertiesFig. 3 shows a set of DSC cooling diagrams for this seriesof CnPCs. It is evident that by increasing the length of the sidechains, both the DHiand Tidecrease.The transition enthalpychanges (LC phase to the isotropicmelt, DHi) and the LCphase to the isotropicmelt transitiontemperatures (Ti) ofthis series of CnPC are listed in Table 3. These importantexperimental results indicate that not only the backbones butalso the side chains are involvedin these transitions;thetransitionsare dependent on the length of the side chainssigni?cantly.Further coolingleads to a vitri?cationof thematerials (as shown in Fig. 3). The glass transition temperature(Tg) of CnPC also decreasesas the number of methylene unitsin the side chains increases. Allthese observation