Carbohydrate Polymers 125 (2015) 224–231

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Effect of the degree of substitution in the transition temperatures and hydrophobicity of hydroxypropyl cellulose esters Delia López-Velázquez a , Armando R. Hernández-Sosa a , Ernesto Pérez b,∗ a Fac. de Ciencias Químicas, BUAP. Blvd. de la 14 Sur y Av. San Claudio, Cd. Universitaria (Antiguo edificio de la Fac. de C. Quím.), Col San Manuel, Puebla Pue., C.P. 72570. Mexico b Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 10 July 2014 Received in revised form 25 November 2014 Accepted 8 December 2014 Available online 26 February 2015 Keywords: Hydroxypropyl cellulose esters Homogeneous acylation Hydrophobicity Liquid crystals. Chemical compounds studied in this article: Benzyl 6-bromohexanoate (PubChem CID: 562258) Tetramethylammonium fluoride trihydrate (PubChem CID: 2735143) N,N -Carbonyldiimidazole (PubChem CID: 68263)

a b s t r a c t We have synthesized and characterized five members of a homologous series of side chain polymers of hydroxypropyl cellulose esters obtained by homogeneous esterification with 6[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid. Two acylation procedures were studied. One procedure involved the acid chloride derivative and the other one was the activation of that acid with ¯ The N,N -carbonyldiimidazol. The second method yielded esters with higher degree of substitution, DS. ¯ ranging from 26 to about 66%, were characterized by FTIR, NMR, solution viscometry, esters, with DS TGA, DSC, polarized optical microscopy, and X-ray diffraction, in order to study the effect of the degree of substitution on the hydrophobicity, on the transition temperatures and on their potential liquid crystal properties. It has been found that the hydrophobicity and the transition temperatures of the HPC deriva¯ values showed liquid tives are very much dependent on the degree of acylation. The esters with high DS crystal properties. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Hydroxypropyl cellulose (HPC) is a water soluble polymer and a renewable thermoplastic. HPC is widely used in food, pharmaceutical, paint, and adhesives industries. Since the hydroxypropyl cellulose backbone has reactive hydroxyl groups, therefore, the architecture of HPC can be appropriately modified (LópezVelázquez, Bello, & Pérez, 2004) to tailor the final properties and to open up new opportunities for HPC usages. Thus, the preparation of hydrophobically modified water-soluble polymers is an interesting subject for different applications (Just & Majewicz, 1985; Landoll, 1982; Marconi, Cordelli, Napoli, & Piozzi, 1999; Marstokk & Roots, 1999) taking advantage of the specific interactions between the hydrophobic moieties. For instance, the modification of water-soluble polar carbohydrates with non-polar water-insoluble hydrocarbons will produce

∗ Corresponding author. Tel.: +34 912587577; fax: +34 915644853. E-mail address: [email protected] (E. Pérez). http://dx.doi.org/10.1016/j.carbpol.2014.12.086 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

amphiphilic molecules with rather interesting properties, and, specifically, the possibility to form lyotropic liquid crystals (Jeffrey, 1986). Moreover, different HPC esters were reported do display interesting lyotropic or thermotropic phases (Bianchi, Marsano, Picasso, Matassini, & Costa, 2003; Hou, Reuning, Wendorff, & Greiner, 2000; Rusig et al., 1994; Steinmeier & Zugenmaier, 1988; Wang, Dong, & Tan, 2003; Wojciechowski, 2000; Yamagishi et al., 1994) and even they have found applications as electro-optical sensors after appropriate crosslinking (Costa, Filip, Figueirinhas, & Godunho, 2007). As part of a research program, we are investigating the modification of HPC with biphenyl derivatives. The biphenyl group is one of the simplest mesogenic units, leading to thermotropic liq˜ uid crystalline structures, mainly of the smectic type (Bello, Perena, ˜ Pérez, & Bello, 1993; Pérez, & Benavente, 1994; Benavente, Perena, López-Velázquez, Hernández-Sosa, Bernes, Pérez, & FernándezBlázquez, 2008; Meurisse, Nöel, Monnerie, & Fayolle, 1981; ˜ Benavente, Pérez, Marugán, & VanderHart, 1993; Pérez, Perena, & Bello, 1997; Pérez, del Campo, Bello, & Benavente, 2000). The present work is concerned with the esterification of HPC with

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225

2.2.3. Fourier transformed infrared spectroscopy (FTIR) Attenuated total reflectance (ATR) infrared spectroscopy was obtained using a Perkin Elmer model Spectrum One FT-IR spectrometer with a diamond cell. The degree of substitution was ascertained by this technique. Fig. 1. Chemical structure of 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid.

6-[4 -(Ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid by the classic acyl chloride method as well as the activation of the acid with N,N -carbonyldimidazol (Heinze, Liebert, & Koschella, 2006). We report here the synthesis and characterization of five 6[4 -(Ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid esters of HPC with different degree of substitution, in order to assess the influence of the hydrophobic biphenyl derivative on the properties of HPC. The maximum degree of substitution, referred to as ¯ attained in this study was 66.6%. The 6-[4 -(Ethoxycarbonyl) DS, diphenyl-4-yloxy]hexanoic acid will be simply abbreviated as acid monomer. The 6-[4 -(Ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid esters of hydroxypropyl cellulose will be referred to as HPC¯ ester or HPC-E6-DS.

2. Experimental 2.1. Materials The hydroxypropyl cellulose used was a gift of Hercules– Mexicana (Klucel-EF, Mw = 80,000), with an average molar substitution (hydroxypropyl groups) MS = 3. It was dried under vacuum at 95 ◦ C overnight before use. Benzyl 6-bromo hexanoate was prepared in the laboratory (López-Velázquez, Hernández-Sosa, Pérez, & Castillo-Rojas, 2012). 6-[4 -(Ethoxycarbonyl)biphenyl-4yloxy]hexanoic acid was also prepared in the laboratory (LópezVelázquez & Mendoza, 2013). N,N -carbonyldiimidazol (CDI), tetrabutylammonium fluoride trihydrate, TBAF, and dimethyl sulfoxide (DMSO) were obtained from Aldrich and used without treatment. In order to optimize possible liquid crystalline properties, the 6[4 -(Ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid (C21 H24 O5 ) was designed (Fig. 1) with an aliphatic segment (flexible spacer) and a biphenyl mesogenic rigid core with a single polar terminal-group (CH3 -CH2 CO-O-). This monomer is a white crystalline solid (López-Velázquez and Mendoza, 2013), and soluble in DMSO, DMF, THF, acetone, and ethyl acetate, but insoluble in H2 O. It melted at 140 ◦ C without showing mesomorphic phases (see below).

2.2.4. Differential scanning calorimetry (DSC) The phase transition temperatures were determined by means of a differential scanning calorimeter (Perkin Elmer DSC7), equipped with an auto-cool accessory and thermal analysis data station. Transition temperatures were collected during heating and cooling scans, under N2 atmosphere, at a rate of 10 ◦ C/min. The sample size ranged from 2 to 10 mg. The temperature calibration was performed with high-purity standards of n-dodecane, indium and zinc, while the enthalpy calibration was also performed with indium (H = 28.45 J/g). Estimated errors are the following: ±0.2 ◦ C in temperature and 1 J/g in enthalpy. 2.2.5. Thermogravimetric analysis The thermal stability of the different samples was studied by thermogravimetric analysis (TGA) by using a TA model TGA Q50 instrument. Samples were examined at a temperature ramp rate of 10 ◦ C/min, from room temperature to 500 ◦ C, under nitrogen gas purge. 2.2.6. Polarizing optical microscopy Optical textures were observed in a polarizing microscope (Olympus BH-2), equipped with a Linkam TMH 600 hot stage. 2.2.7. Viscosity measurements Viscosity measurements at constant temperature (20 ◦ C) were carried out with an Ubbelohde viscometer inserted in an automatic viscometer PVS-1 LAUDA. Solutions of HPC-E6 in THF at a concentration of 0.5 g/dl were prepared. As usual, the relative viscosity is determined by the ratio between the viscosity of the solution and that of the solvent, approximated by the ratio between the corresponding elution times. 2.2.8. X-ray diffraction X-ray diffraction patterns were recorded in the reflection mode by using a Bruker D8 advance diffractometer provided with a PSD Vantec detector (from Bruker, Madison, Wisconsin). Cu K␣ radiation ( = 0.1542 nm) was used, operating at 40 kV and 40 mA. The parallel beam optics was adjusted by a parabolic Göbel mirror with horizontal grazing incidence Soller slit of 0.12◦ and LiF monochromator. The equipment was calibrated with different standards. A step scanning mode was employed for the detector. The diffraction scans were collected within the range of 2 = 3–36◦ , with a 2 step of 0.024◦ and 0.2 s per step. 2.3. Synthesis of HPC esters

2.2. Characterization techniques 2.2.1. NMR spectroscopy 1 H- and 13 C-NMR spectra were acquired with a Varian spectrometer (400 MHz). The NMR samples were prepared as 10–20% (w/v) solutions in CDCl3 . Spectra were usually recorded at room temperature, and TMS served as an internal standard. As seen below, the degree of substitution was determined by 1 H-NMR spectroscopy.

2.2.2. Elemental analysis Elemental analysis was performed by the Microanalytical Laboratory of the Universidad Autónoma de Hidalgo (UAEH), México, on a Perkin Elmer 2400 analyzer.

It should be mentioned that HPC is soluble in solvents as H2 O, ethanol, methanol, THF, DMSO, acetone or CHCl3 , and insoluble in CH2 Cl2 . On the other hand, 6-[4 -(Ethoxycarbonyl)biphenyl-4yloxy]hexanoic acid is soluble in DMSO by heating. We attempted the esterification of HPC by the two routes shown in Scheme 1: Route I: Acylation of HPC with 6-[4 -(ethoxycarbonyl)biphenyl4-yloxy]hexanoyl chloride, anhydrous pyridine, and THF as solvent in a similar procedure as it was described in the literature for alkyl acid (López-Velázquez et al., 2004). Route II: Acylation of HPC with the 6-[4 -(ethoxycarbonyl) biphenyl-4-yloxy]hexanoic acid, CDI, TBAF, and DMSO as solvent was made following a procedure described in detail elsewhere (Hussain, Liebert, & Heinze, 2004).

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Scheme 1. Different routes for the synthesis E6 = C2 H5 OCO   OC5 H10 COO HPC.

of

6-[4 -(Ethoxycarbonyl)biphenyl-4-yloxy]hexanoic

The HPC esters prepared by the acyl chloride method (Route I) rendered relatively low degrees of substitution (27, 31%); they are called HPC-E6-27, HPC-E6-31. On the contrary, by using the ¯ (39, 57, 66) were system CDI/TBAF/DMSO (Route II) higher DS obtained. The following is an example of the reaction of HPC with 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid using CDI. ¯ (66.6) was prepared by using dried HPC-E6-66 with the highest DS HPC (0.35 gr), TBAF (1.15 g, 3.64 mmol), monomeric acid (1.11 g, 3.11 mmol), CDI (0.60 g, 3.74 mmol), and DMSO (60 ml). Procedure: The acid was dissolved in DMSO (30 ml) at 50 ◦ C, then CDI was added. This mixture was maintained at 50 ◦ C and stirred 24 h to form the 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid imidazolide. Then, it was added to a solution of HPC dissolved in DMSO (30 ml)/TBAF at 50 ◦ C. The reaction mixture was stirred for 72 h at 100 ◦ C under N2 . The homogeneous reaction mixture was cooled down to room temperature, subsequently precipitated in ethanol (800 ml), filtered off, washed with ethanol, redissolved in methylene chloride (15 ml), and then precipitated into ethanol (200 ml) and dried in vacuum at 65 ◦ C for 18 h. Finally, the product was again redissolved in methylene chloride (15 ml), and then, poured in ethanol and dried in vacuum at 65 ◦ C for 18 h. Yield = 0.30 g and degree of substitution = 66% (determined by means of 1 H-NMR spectroscopy). The FTIR spectrum (solid state, cm−1 ) of the acid (Fig. 2) presents the following characteristic absorption bands: ␯ (C O, Aliph.) 1703; ␯ (C O, COOH) 1694; ␯ (C C, Ar) 1600. As the HPC is a carbohydrate, it does not contain neither carbonyl, nor C C, so that the carbonyl and C C absorptions are absent in its FTIR spectrum, while the intense broad absorption of the hydroxyl groups on the glycosidic ring is really characteristic of the HPC (Bhadani & Gray, 1983), around 3450 cm−1 . In the FTIR spectrum of the HPC-E6-66,

acid

esters

of

hydroxypropyl

cellulose,

HPC-

Transmittance (u. a.)

226

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000

800

cm-1 Fig. 2. FTIR spectra corresponding to the original HPC, ester HPC-E6-26, ester HPCE6-66 and the acid, from top to bottom, respectively.

the presence of the ester side chains was confirmed by the intense broad absorption band of the carbonyl groups around 1711 cm−1 , which are overlapped with each other. It also shows an intense peak at 1600 cm−1 and 3450 cm−1 corresponding to the double bond absorption (C C, stretching) of the aromatic ring, and a broader absorption band of the unreacted hydroxyl group of the glycosidic ring of HPC (evidently, the intensity of this band due to the remaining unreacted OH, decreases in parallel with the increase of the degree of substitution in HPC, as shown below. The C C is also confirmed from the following peaks 1527, 1497 and 828 (characteristic of disubstituted aromatic rings).

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Fig. 3. Idealized chemical structure of 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid esters of hydroxypropyl cellulose.

Other HPC-esters were synthesized by the procedure above described, but the acylation was carried out at 48 and 24 h of reac¯ values were 57.1 and 39, respectively. tion times. The resulting DS These products are called HPC-E6-57, HPC-E6-39. As mentioned above, high degrees of esterification are easily obtained by route 2. Other important advantage of this route is that all the esterifications produced well soluble HPC esters. On the contrary, by route I the esterifications may afford mainly insoluble HPC-Esters (but also soluble in some cases). The idealized chemical structure of these HPC esters is shown in Fig. 3. The HPC structure is not so simple, since on an average there are 3-hydroxypropyl groups per cellulose ring, but it does not mean that necessarily those 3-hydroxypropyl groups are attached to each one of the 3-OH groups of cellulose (as sketched in the idealized structure): some of these OH groups may be unsubstituted but then approximately the same amount of hydroxypropyl groups will be attached to the OH at the end of another hydroxypropyl group. Anyway, this will not affect the calculations, since the real chemical formula of HPC will be unchanged. All the FTIR spectra of HPC-esters prepared by the two methods (Fig. 2) show the same absorption bands: ␯ (C O, Aliph.) 1711; ␯ (C C, Ar) 1600 and 3450 cm−1 . Moreover, a significant decrease of the intensity of the absorption hydroxyl band is clearly observed in these spectra, owing to the partial modification of HPC. Fig. 4 shows the proton NMR spectra of HPC-E6-31 and HPCE6-66, compared with that of the acid and the original HPC. In the esters, the characteristic peaks arising from both HPC (the peaks due to the cellulose skeleton protons are broad and indistinguishable from others) and acid (López-Velázquez and Mendoza, 2013) are clearly observed. The success of the acylation is confirmed by the following resonances assigned to the protons of the ester moieties: (a) the isolated signals of the biphenyl group (much broader than in the initial acid) in the spectral region from 6.9 to 8.2 ppm, which provide an analytical approach for the determination of the degree of substitution of the HPC-esters; (b) also, the CH2 isolated signal of the ethoxy group, appearing at 4.37 ppm (H2 , CH2 O CO Ph); (c) besides, at 5.0 ppm can be seen a new small broad peak that is assigned at the methine proton( CH(CH3) OCO ) of the propoxy group connected to the ester side chain. It shifts upward due to the acylation of the propoxy unit of the HPC.

3. Results and discussion We report the esterification of HPC with 6-[4 -(ethoxycarbonyl) biphenyl-4-yloxy]hexanoic acid. New esters of HPC with different degree of substitution, ranging from 26 to 66% were prepared. The chemical structures of all the HPC esters synthesized were analyzed by a combination of nuclear magnetic resonance spectroscopy (NMR) and infrared spectroscopy (FTIR). The results obtained from these techniques were found to be consistent with the predicted structures of the products. Spectrometric data of the HPC esters are similar between each other. The degree of modification was determined in a reliable way by 1 H-NMR spectroscopy (Fig. 4) since the HPC esters so formed had adequate solubility and their acyl substituents include aromatic signals with no interference with the other signals, as shown above. The average degree of substitution in the HPC esters is, therefore, determined straightforwardly by considering the idealized chemical structure shown in Fig. 3, with an average molar substitution (hydroxypropyl groups) of 3 in the original HPC. It was found that the substitution (acylation) of HPC with 6-[4 (ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid is more effective with CDI, TBAF/DMSO, and it afforded easily handled soluble HPC esters with degree of substitution > 35%. On the contrary, when ¯ < 35% were using the acyl chloride method, esters of HPC with DS obtained, which require more aggressive solvents. 3.1. Solubility It has been found that the hydrophobicity of the HPC derivatives is very much dependent on the degree of acylation. Thus, very small amounts of incorporated hydrophobic groups (C2 H5 OCO   OC5 H10 COO ) have a profound effect on the solubility of HPC esters in water. Hence, even the HPC-E with a ¯ = 2.2 was already insoluble in water. HPC esters with DS ¯ ≤ 31% DS were insoluble in water but soluble in ethanol. Furthermore, the ¯ ranging from 39 to 66 were not only insoluHPC esters with DS ble in water, but also in warm ethanol. The decrease of solubility in water and ethanol resulted from the fact that acylation facilitated the isolation of the HPC moieties. It should be pointed out that the 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid is soluble in ethanol, CH2 Cl2 , CHCl3 , THF, and DMF, but insoluble in

228

D. López-Velázquez et al. / Carbohydrate Polymers 125 (2015) 224–231 Table 1 ¯ at 20 ◦ C in THF 0.5 g/dl. Relative viscosities of HPC-E6-DS ¯ (%) HPC-E6-DS

r

HPC (HF) HPC-E6-26 HPC-E6-31 HPC-E6-39 HPC-E6-57 HPC-E6-66

1.70 1.59 1.44 1.47 1.35 1.30

¯ = degree of substitution, HPC (HF) = macromonomer; r = relative viscosity DS

Table 2 Thermogravimetric data of HPC, acid monomer, and HPC-E6 esters. Sample

Decomposition temperature Td10% (◦ C)

HPC HF Et OCO   OC5 H10 COOH HPC-E6-26 HPC-E6-31 HPC-E6-39 HPC-E6-57 HPC-E6-66

346 304 351 353 357 361 364

Td10% = temperature at which 10% weight loss was recorded by TGA at a heating rate of 10 ◦ C/min in nitrogen.

3.2. Relative viscosity The relative viscosity values of HPC esters here synthesized were smaller than those corresponding to unmodified HPC, as observed in Table 1, which indicates, most probably, less hydrogen bond interactions since some hydroxyl groups have been esterified. It is well known, however, that HPC esters may undergo important degradation problems, causing significant changes in their properties (Rusig et al., 1992, 1994). In the present case, no appreciable changes with time have been observed, probably due to the careful purification of the products (see above).

3.3. Thermal behavior of the acid and of the new HPC esters

Fig. 4. 1 H-NMR spectra (CDCl3 ; 400 MHz) corresponding to the acid, the original HPC, ester HPC-E6-31 and ester HPC-E6-66, from top to bottom, respectively. Proton assignments: see Fig. 3.

¯ water. The increase of the hydrophobicity with the increase of DS is the result of the substitution of OH (glycosidic units) by the ester side chains which destroy the intermolecular interaction of the HPC chains. One of the more important consequences is that these 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid esters of hydroxypropyl cellulose are hydrophobic and form transparent films from organic solutions.

The thermal stability of unacylated HPC and HPC-E6 esters was evaluated in the temperature range from room temperature to 600 ◦ C. Table 2 shows the 10% weight loss for these macromolecules. When comparing unacylated HPC with the HPC-E6, it is observed a light increase of the 10% weight loss with the increase of ¯ (side chains containing biphenyl moieties). Their decomposition DS was single stage type. These results indicate that the thermal transitions of the HPC-E6 esters can be analyzed without the interference of their thermal degradation. The DSC curves of the acid are shown in Fig. 5 for the first melting, subsequent cooling, and second melting. Only one transition is observed, which is reversible and does not change on repeated heating and cooling cycles. It appears centered at 114.0 ◦ C on cooling, comprising a total enthalpy of 32.8 kJ/mol (92 J/g). In the second melting, the peak temperature is 140.6 ◦ C, with an enthalpy of melting of 33.8 kJ/mol (95 J/g). Since only one transition is observed, it will correspond, most probably, to the melting of a crystalline structure, and no liquid crystalline phases are developed for this compound, contrary to the complicated polymesomorphism exhibited by other materials including the biphenyl unit (López-Velázquez et al., 2008). It can be explained by the dimeric structure of the acid (López-Velázquez and Mendoza, 2013) in the solid state, which is formed by hydrogen bonds between acid groups.

D. López-Velázquez et al. / Carbohydrate Polymers 125 (2015) 224–231 Table 3 DSC resultsa (second melting) of the original HPC and different esters.

12

8

heat flow (W/g)

1st melting 4

Compound

¯ (%) DS

Tg (◦ C)

Tm (◦ C)

Hm (J/g)

HPC HPC-E6-26 HPC-E6-31 HPC-E6-39 HPC-E6-57 HPC-E6-66

− 26 31 39 57 66

− 21 21 27 40 43

195 56 58 83 95, 113, 142 96, 118, 146

4 2 3 5 21 22

a

2nd melting

¯ ±1; temperatures: ±1 ◦ C; enthalpies: ±1 J/g. Estimated errors: DS:

cooling 0

-4

-8 100

120

140

T (°C) Fig. 5. DSC curves of 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid corresponding to the first melting, subsequent cooling and second melting.

1.0

HPC-E-66

0.8

HPC-E-57 Heat Flow (W/g)

229

0.6

HPC-E-39 0.4

HPC-E-31 HPC-E-26

0.2

HPC 0.0 0

30

60

90

120

150

180

210

T (°C) Fig. 6. DSC curves corresponding to the second melting of the original HPC and the indicated HPC esters.

3.4. Mesomorphic behavior and transitions temperatures of the HPC esters As mentioned in the previous section, the acid monomer does not exhibit liquid crystalline character. In this section, the effect of substitution on the mesomorphic behavior and on the transition temperatures will be discussed. The DSC curves corresponding to the second melting of the HPC esters (and of the original HPC sample) are presented in Fig. 6. As reported before (López-Velázquez et al., 2004), the second melting of HPC shows only a small endothermic effect at 195 ◦ C, attributed to the melting (or isotropization) of the sample. The small enthalpy

involved indicates that either the crystallinity is rather small, or a low-ordered phase is present. That small endothermic peak of the original HPC sample rapidly ¯ similarly disappears in the esters for relatively small values of DS, to the findings (López-Velázquez et al., 2004) for the substitution of HPC with different amounts of all-aliphatic palmitoyl chloride side chains. For higher degrees of substitution, however, a different transition is observed in the DSC curves, with a melting temperature varying from 56 to 83 ◦ C and with increasingly higher enthalpies of melting (Fig. 6 and Table 3). This transition is related to the fact that when the degree of substitution is sufficiently high, the side branches are able to produce lateral crystallization. Finally, for very ¯ (samples HPC-E6-57 and HPC-E6-66) two or three high values of DS endothermic peaks, highly overlapped, are observed on melting. Table 3 shows the DSC thermal transitions (deduced on the second melting) of the original HPC and several esters with dif¯ It is observed that these transition temperatures depend ferent DS. rather markedly on the degree of substitution of the acid monomer. Firstly, higher values of the glass transition temperature, Tg , are obtained with increasing grafting of the lateral substituents (C2 H5 OCO   OC5 H10 CO ). Interestingly, the values of Tg are close to room temperature and to that of the human body, and can be tailored somehow with the degree of substitution. Unlike the HPC backbone, the aliphatic segment of the side chain cannot undergo intrapolymeric hydrogen bonding, and besides they are relatively short and semiflexible. When HPC bears a small ¯ values smaller than 10%), number of side chains (probably for DS the majority of them are randomly isolated from each other, so they can not disturb significantly the thermal behavior of the HPC backbone, but they can respond more quickly to the changes of ¯ the temperature than the backbone chain of the HPC. At higher DS, however, the side chains come in close proximity and can organize from each other through hydrophobic interactions. Besides, and having into account the mesogenic character of the biphenyl units, the side chains may arrange in layered liquid-crystalline structures with different degree of interdigitation, as sketched in Fig. 7. ¯ (samples HPCAs mentioned above, for very high values of DS E6-57 and HPC-E6-66) two or three endothermic peaks, highly overlapped, are observed. Moreover, now the enthalpies involved are considerably high (around 21–22 J/g), but yet they are significantly lower than those found for the monomeric acid (95 J/g). All these features indicate, most probably, the presence of liquid crystal phases. In order to asses this aspect, optical textures of polymer HPCE6-66 were observed in a polarizing microscope, POM, at variable temperatures. Fig. 8 shows different birefringent textures from 26 ◦ C up to the clearing point at about 150 ◦ C, then cooling down. Additional information has been obtained from X-ray diffraction experiments, which are shown in Fig. 9. Two rather wide peaks are observed for HPC, which were interpreted (López-Velázquez et al., 2004) as indicative of a mostly disordered structure. These two peaks arise from the axial asymmetry of the HPC chains, similar to the case of amorphous poly(1-olefins) (Turner-Jones, 1964).

230

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¯ values, showing different degrees of interpenetration. Fig. 7. Examples of possible arrangements of the side chains in the HPC esters with high DS

Very important changes in the diffractograms are observed for ¯ valthe HPC esters, as observed in Fig. 9, especially for the higher DS ues. Thus, several relatively narrow diffractions are present, both in the high and low angle regions of the diffractograms. These features are indicative of lateral crystallization and/or a layered structure, with a relatively high degree of order. Especially interesting is the low angle peak appearing at about ¯ values higher than 26%. 5.5 degrees, which is observed for DS

That peak corresponds to a distance or around 1.6 nm, which is close to the length of the branch in Figs. 1 and 3, so that, most probably a smectic layer structure is formed, with interdigitated branches. One important feature is related to the nature of the different transitions observed by DSC for HPC-E-57 and HPC-E-66. In order to analyze this aspect, real-time variable temperature synchrotron experiments are planned.

Fig. 8. Birrefringent textures, under crossed polarizers, of HPC-E-66. Heating: a. 26.2 ◦ C, b. 132 ◦ C, c. 150 ◦ C, and d. cooling, 128 ◦ C. Scale bar: 100 ␮m.

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231

Acknowledgement

0.6

This work was partly supported by VIEP-BUAP (Internal project 2012–2013), PIFI 2010–2013. We also thank the CONACYT-Mexico (I0006-5560) and MICINN-Spain (project MAT2010-19883) for financial support. Thermogravimetric analyses were obtained at de ICN-UNAM.

HPC 0.4 norm int

HPC-E-26

HPC-E-39

References

0.2 HPC-E-57

HPC-E-66 0.0 5

10

15

20

25

30

35

2θ Fig. 9. X-ray diffractograms, at room temperature, of the original HPC and the indicated HPC esters.

4. Conclusions We explored the homogeneous esterification of HPC with 6-[4 (ethoxycarbonyl)biphenyl-4-yloxy]hexanoic acid. It was found that the acylation of HPC with the activated biphenyl acid (imidazolide) ¯ values considerably higher than in the yielded HPC esters with DS case of the acylation via acid chloride. These two synthesis paths afforded new esters of HPC with different degree of substitution, ranging from 26 to 66%. The resulting materials showed hydrophobic properties and form transparent films. The relative viscosity values of HPC esters here synthesized were smaller than those of corresponding to unmodified HPC, which indicates, most probably, less hydrogen bond interactions since some hydroxyl groups have been esterified. It has been found that the hydrophobicity and the transition temperatures of the HPC derivatives are very much dependent on the degree of acylation. Therefore, depending on the degree of substitution (and on the nature of the side groups), the hydrophilichydrophobic character of these HPC derivatives can be tailored. Moreover, and for the present esters of HPC, high degrees of substitution are required for these polymers to behave as a liquid crystal. Other 6-[4 -(ethoxycarbonyl)biphenyl-4-yloxy]alcanoic acids can be attached to the hydroxypropyl cellulose backbone, so it is possible to prepare a wide range of such HPC derivatives with different flexible spacers. Some of these new acid esters of hydroxypropyl cellulose are under preparation and further study of structure–property relationships.

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