8178
Macromolecules 1995,28, 8178-8181
Thermotropic Liquid Crystalline Behavior of an Amphiphilic Polymer Lacking Mesogens Hongbin Li, Xi Zhang, Ruifeng Zhang, and Jiacong Shen” Department of Chemistry, Jilin University, Changchun, 130023, People’s Republic of China
Bing Zhao and Weiqing Xu Key lab of the molecular spectrum and molecular structure, Jilin University, Changchun, 130023, People’s Republic of China Received April 20, 1995; Revised Manuscript Received July 11, 1995@
ABSTRACT: The thermotropic liquid crystalline behavior of a new kind of amphiphilic polymer without mesogens is reported. The existence and structure for the polymeric mesophase are confirmed and characterized by DSC, X-ray diffraction, optical polarized microscopy, and temperature-dependent FTIR. Due to strong interactions between pendent long alkyl chains, we conclude that some hydrophobic semirigid rods form and are responsible for mesophase formation. Moreover, the flexible polymer network might act as a unique “spacer”group and also assist in mesophase formation.
Introduction It is well known that molecules with a form-anisotropic architecture, such as a rod- or disk-shaped
Experimental Section
Amphiphilic polymers were synthesized according to Figure 1: stearic acid was reacted with a large excess of ethylenediamine under melting conditions t o produce the monoamide, structure, often exhibit thermotropic liquid crystalline which was purified by recrystallization from ethanol. Then behavior. Likewise, amphiphilic molecules often form the monoamide and ethylenediamine with different molar lyotropic liquid crystals in aqueous systems due to the ratios were mixed under melting conditions, and epichlorohyhydrophobic effect. Combining the molecular design of drin was added to the well-stirred reaction mixture over a thermotropic a n d lyotropic mesogens, i.e., incorporating period of 20 min t o proceed with melt polymerization. Final t h e typical thermotropic liquid crystalline building product was purified by precipitating ethanol solutions of the blocks (rod- or disk-shaped) into amphiphiles, amphopolymer into ether twice to ensure removal of all unpolymertropic liquid crystals a r e f0rmed.l Many amphotropic ized substances. The product was dried under vacuum for 24 liquid crystalline materials have been i n v e ~ t i g a t e d , ~ - ~ h at 40 “C. a n d it seems that all thermotropic liquid crystal behavIR (KBr): 3301 (OH), 2955 (CH3), 2848,2919 (CHz), 1648, ior of amphiphilic molecules originates from these 1637 cm-I (C=O, in amide). amphotropic liquid crystal materials. I n this paper we The structural data are characterized and shown in Table report thermotropic liquid crystal behavior for a n am1. The number-average molecular weight was determined by membrane osmometry (Knauer model). The alkyl chain phiphilic polymer not bearing mesogens. To t h e best content of the amphiphilic polymer was determined by weighof our knowledge, this is t h e first report on this subject. ing the amount of stearic acid obtained by thorough hydrolysis we discovered a new kind of In previous of 0.5 g of polymer in HCl solution. The stearic acid obtained amphiphilic polymer composed of hydrophobic microgel was washed with water and dried under vacuum for 24 h at cores a n d hydrophilic chains, or vice versa. They self50 “C. rearrange at t h e airlwater interface a n d are readily Liquid crystalline phases were characterized by X-ray transferable as so-called “duckweed” or “reversed duckdiffraction (D/max yA, using copper K a radiation of waveweed” polymeric Langmuir-Blodgett (LB) films to solid length 1.542A), by optical polarized microscopy (Opton R Pol, substrates. The t e r m “duckweed” means that t h e Germany),by differential scanning calorimetrywith a heating hydrophobic microgels a r e floating onto t h e surface of rate of 5 Wmin (Perkin-Elmer DSC 2 4 , and by temperaturewater, a n d the hydrophilic grafting chains are projecting dependent FTIR (Bruker IFS66V). The FTIR temperature into the water; “reversed duckweed” means t h a t t h e controller is homemade and gives a temperature stability of better than 0.1 K when the sample is heated to the desired hydrophilic networks extend downward into water a n d temperature (20 min wait for equilibration). The method for t h e hydrophobic grafting chains a r e upward packing determining the FTIR band half-width and peak position is away from t h e surface of water. Their unique feature the second-derivative method (program supplied with the is t h e combination of order a n d stability, a n d they can Bruker IFS66V FTIR instrument). be used as a matrix for assembling functional or composite ultrathin 2D films. The amphiphilic polymer we use here was composed of a flexible hydrophilic epichlorohydrin-ethylenediamine cross-linked network a n d several hydrophobic stearic chains (SA). We found that this polymer self-organizes at the airlwater interface to form a monolayer with a ”reversed-duckweed” structure a n d t h a t it could be easily transferred onto solid substrates to form LB multilayers which were highly ordered.” I n this paper, we report t h e thermotropic liquid crystalline behavior of this kind of amphiphilic polymer. @
Abstract published in Advance ACS Abstracts, October 15,
1995.
0024-9297/95/2228-8178$09.00/0
Results and Discussion Liquid crystal behavior is generally induced either by temperature or by t h e influence of a given solvent. Here, t h e thermotropic liquid crystal behavior for this amphiphilic polymer was analysed by DSC, X-ray diffraction, optical polarized microscopy, a n d FTIR. Figure 2 shows t h e heating DSC scans for three samples, exhibiting phase behavior ranging from 280 to 430 K. From t h e DSC data, we found that sample ES-1 h a d a transition peak corresponding to t h e melting process, T = 360.4 K with AH = 94.4 Jlg; ES-2 h a d two transition peaks, T1 = 368.7 K, AH1 = 121.5 Jlg, TZ= 386.5 K, A H 2 = 3.0 Jlg; a n d ES-3 had three transition 0 1995 American Chemical Society
LC Behavior of an Amphiphilic Polymer 8179
Macromolecules, Vol. 28,No. 24, 1995 CI13(CAZ) ,6COOH
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