Introduction

As a very important class of chemical molecules liquid crystals exhibit the features of both the solid and the liquid states. They have the ordering properties of solids, but they flow like liquids. They are still being developed in order to provide effective solutions for the problems encountered in some applications, such as liquid crystal thermometers, liquid crystal displayer, electrical, optical, and switching materials [1,2,3,4,5,6,7,8]. Molecular shape is an important factor in the preparation of mesogenic molecules. Generally, mesogenic units having one terminal chain on the aromatic ring give rise to calamitic mesomorphism, whereas the presence of a large number of terminal chains causes the molecule to adopt a discotic or star shaped structure. Star shaped liquid crystals has received much attraction in recent years. The mesogenic groups of these type, which are generally located within the branches or the mesogenic units with terminal flexible groups are linked to original molecules. Cyclotriphosphazene core, (N3P3Cl6), is a good candidate as a core to obtain star shaped molecules [9,10,11].

Liquid crystalline polymers (LCPs) exhibit the advantageous of combination of the anisotropy of LCs with the desirable bulk properties and processing probabilities of polymers. In recent years, depending on the phase structures and transitions of LCPs, synthesis and design of LCPs became important for potential new applications including engineering plastics, optical data storage, electro-optics displays, sensing, templates, nonlinear optic devices, stimuli responsiveness [12,13,14,15,16,17,18,19,20]. LCPs can be splited mainly into two kinds depending on the locations of mesogens: side-chain liquid crystalline polymers (SCLCPs) and main-chain liquid-crystalline polymers (MCLCPs) [20, 21]. MCLCPs consist of linear chains where flexible polymers with rigid mesogenic units are incorporated into the main chain [22, 23]. On the basis of the way in which mesogens are either laterally or terminally linked to the polymer backbones, SCLCPs can be classified into either side-on SCLCPs or end-on SCLCPs [20, 23,24,25]. On the other hand, there have been few studies of the synthesis of star-shaped LCPs due to their crucial applications and unique properties [26, 27]. Star-shaped side-chain liquid crystalline polymers (SSCLCPs) was first studied by Pugh and co-workers [28,29,30]. They prepared three and six-armed SSCLCPs via atom transfer radical polymerization (ATRP). Tang et al. synthesized four-armed SCLCPs containing azobenzene with various terminal substituents via ATRP to contribute much to this area [31]. In these studies, it was reported that SSCLCPs have a similar LC behavior in comparison with their linear analogue, but phase transition temperature lowers slightly. Star-shaped main-chain liquid crystalline polymers (SMCLCPs) were also reported. Unlike SSCLCPs, SMCLCPs showed higher glass transition temperature and lower crystallinity owing to its branched structure [32,33,34].

Cholesterol (Chol) based small or macromolecules show liquid crystal properties because of its well-known mesogenic nature, that is, its ability to self-organize into liquid crystalline substances [35]. LCPs containing Chol moiety have attracted interest as they have many applications in various fields such as biomedical, nonlinear optical or electro-optical matters [36,37,38,39,40,41,42,43]. Chol groups could be introduced both inside and main chain in polymers [37,38,39, 44,45,46]. SCLCPs bearing Chol include backbone (methacrylate, acrylate, siloxane, urethane, norbornene, etc.) and flexible spacers (methylene, siloxane etc.). Majority of SCLCPs bearing Chol were prepared by the radical polymerization of (meth)acrylic type monomers bearing cholesterol with flexible spacers. On the other hand, MCLCPs having Chol are synthesized by Chol moieties that are able to initiate polymerization of monomers or cholesterol was introduced by post-polymerization reaction between the cholesteryl chloroformate or cholesterol and end-groups of the precursor polymers [46].

Biodegradable polymers have wide applications in biomedical and pharmaceutical fields. Among them, aliphatic polyester family has a leading position as it combines both biocompatibility and biodegradability with satisfactory mechanical performances [47, 48]. Especially, biodegradable PCL is one of the most widely used aliphatic polyesters. It has been widely applied in the field of biomedicine as surgical sutures and drug delivery systems due to its high permeability to drugs and less acidic degradation products as compared to other aliphatic polyesters [49,50,51,52,53].

With all these backgrounds in mind, we synthesized a series of Chol end-capped PCL polymers to investigating of mesogenic properties of polymer structures containing Chol end groups. To the best of our knowledge, this is the first report describing cholesterol end capped liquid crystal lineer and star-shaped polymers. Well-defined LPCL having Chol end group(s) at one and both ends and Chol end-capped three, four, and six-armed SPCLs were prepared via a combination of CUAAC and ROP techniques. 1H NMR, 31P NMR, Fourier transform infrared (FTIR), and gel permeation chromatography (GPC) were employed for the characterization of the modified polymers and the intermediates at various stages of synthesis. Mesogenic properties of the polymers were examined via WAXS, SAXS, DSC and POM.

Experimental

Materials

Six-armed phosphazene initiator was prepared according to the literature method [22]. was used as supplied. ε-caprolactone (ε-CL, 98%), purchased from Aldrich, was dried over anhydrous calcium hydride (CaH2, Alfa Aesar, 90–95%) for 24 h at room temperature and then distilled at 97 °C under argon atmosphere and vacuo (10 mmHg), and stored over actived 4 Å molecular sieves. Hexachlorocyclotriphosphazene (N3P3Cl6; Otsuka Chemical Co.) was purified by fractional crystallization from dry hexane. Tetrahydrofuran (THF, Merck, 99.8%) was freshly distilled after being refluxed over sodium/benzophenone for 1–2 h under argon. Sodium hydride (NaH, 60% in mineral oil, Merck) was washed with dry hexane just prior to use. Sodium borohydride (NaBH4, 98%, Alfa Aesar), ethanol (≥99.9%, Fluka), 1,1,1-tris(hydroxymethyl)propane (98%, Aldrich), tetrabutylammonium bromide (99%, Aldrich), tin(II) 2-ethylhexanoate (Sn(Oct)2, 95%, Aldrich), ethylene glycol (99.8%, Sigma–Aldrich), 2-bromo-2-methylpropanoyl bromide (98%, Aldrich),, cholesteryl chloroformate (C28H45ClO2, Alfa Aesar, 98%), 4-hydroxybenzaldehyde (98%, Alfa Aesar), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), sodium azide (NaN3, 99.5%, Sigma–Aldrich), dichloromethane (DCM, 99.8%, Sigma–Aldrich), copper(I) bromide (99%, Alfa Aesar), triethylamine (TEA, ≥99.5%, Fluka), dimethylformamide (DMF, 99.8%, Sigma–Aldrich) and pentaerythritol (≥99%, Aldrich) were used as received.

Measurements

1H and 31P NMR spectra were registered in d6-DMSO and CDCl3 solutions on a VarianUNITY INOVA 500 and 202 MHz spectrometer, respectively. Infrared spectra were taken on Perkin–Elmer Paragon 1000 FTIR spectrometer utilizing the attenuated total reflectance (ATR) method in the range of 4000–600 cm−1. Gel permeation chromatography (GPC) measurements for molecular weight distributions and average molecular weights of the polymers were predicted on an Agilent GPC Instrument (Model 1100) comprising of a refractive index detector, a pump, and two Waters Styragel column (HR 5E) using THF as eluent at a flow rate of 0.5 mL/min at 23 °C and toluene as an internal standard. Average molecular weights were calculated on the basis of a calibration curve drawn utilizing monodisperse polystyrene (PS) standard of known molecular weights. Mass spectra measurements were recorded on Bruker MicroTOF LC–MS (electron sprayionization) and Bruker microflex LT MALDI-TOF MS (nitrogen UV-laser ionization at 337 nm) spectrometers. DSC was carried out on Mettler Toledo DSC 822 calorimeter under a dry nitrogen flow of 10 mL/min to detect crystallization temperatures (Tc) and the melting points (Tm) of the polymers. All the samples were first heated from −25 °C to 100 °C with a heating rate of 10 °C/min and held there for 5 min to erase the thermal history, then cooled to −25 °C at 10 °C/min, and finally heated to 100 °C at 10 °C/min. TGA was recorded on a Mettler Toledo TGA/SDTA 851 thermo gravimetric analyzer from room temperature to 700 °C with a heating rate of 10 °C/min under dry argon atmosphere. The phase transition behaviour of the species was detected by means of polarized optical microscopy at 45 °C (POM, Leitz Wetzler Orthoplan-pol.) equipped with a hot stage (Linkam TMS 93) and a temperature controller (Linkam LNP). X-ray diffraction (XRD) patterns of thermal threated films were attained at 45 °C on a Rigaku SmartLab X-ray diffractometer utilizing Cu Kα radiation with a wavelength of 1.54 A° over a 2θ range from 0° to 30° at speed of 0.1o min−1. The voltage and current were set to 40 Kv and 30 mA, respectively.

Synthesis of cholesterol end-capped PCLs

Cholesterol end-capped linear PCLs at one end (LPCL-Chol) and both ends (LPCL-(Chol)2) and cholesterol end-capped star-shaped PCLs (SPCL-(Chol)x where x = 3,4, and 6) were synthesized according to the literature methods [54, 55].

Synthesis of alkyne functional cholesterol (cholesteryl 2-propyn-1-yl carbonate)

Alkyne-functional cholesterol was prepared according to the literature method with minor modifications [56].

Result and discussion

Synthesis of cholesterol end-capped PCLs

The cholesterol end-capped were synthesized by the core-first approach, which is based on growing polymer arms out of the initiators having different numbers of OH functional groups in a four-step procedure. ROP and CUAAC click reaction techniques were used in the first and final steps, respectively. As depicted in Scheme 1 and Table 1, firstly, linear and star-shaped PCLs were synthesized using pentaerythritol, ethanol, ethylene glycol, hexakis (p- (hydroxymethyl) phenoxy) cyclotriphosphazene, and 1,1,1- tris (hydroxymethyl) propane as the initiator compounds via ROP of ε-CL catalyzed by stannous octoate (Sn(Oct)2) at 120 °C in bulk. The further end-group modification of these polymers was accomplished by the derivatization of OH end-functionalities. In the last step, CUAAC reaction between azide end-groups of the PCLs and alkyne-cholesterol provided the cholesterol end-capped linear and star-shaped PCLs.

Scheme 1
scheme 1

General synthetic procedure for the preparation of cholesterol end-capped PCLs

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Table 1 Synthesized polymers
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PCLs were characterized by FT-IR and 1H NMR spectroscopic techniques. FT-IR and 1H NMR spectra of three-armed PCLs were given in Figs. 1, 2. Also, 1H NMR shift values of synthesized polymers can be seen in Table 2. The number- and weight-average molecular weights of linear and star shaped PCL shaving -OH end-functional groups were estimated via GPC and the results are summarized in Table 3.

Fig. 1
figure 1

FT-IR spectra of three-armed PCLs. a 1,1,1-tris(hydroxymethyl)propane, b 3SPCL-OH, c 3SPCL-Br, d 3SPCL-N3, and e 3SPCL-Chol

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Fig. 2
figure 2

1H NMR spectra of three-armed PCLs. a 3SPCL-OH, b 3SPCL-Br, c 3SPCL-N3, and d 3SPCL-Chol

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Table 2 31P NMR and 1H NMR parameters for cholesterol end-capped PCLs
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Table 3 Conditions and results of PCLs with OH end functional groups
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Mesomorphic properties

Theoretically, it is expected that Chol end-functionalized poylmers/oligomers should exhibit liquid crystal behaviour in particular temperature ranges. The mesomorphic properties of the Chol end-capped PCLs were examined via DSC, POM, and XRD techniques and the results clearly showed that all the polymers have liquid crystal property.

The thermal behaviours of the polymers were studied using DSC. DSC thermograms and temperature range of the liquid crystallinity of the polymers are displayed in Fig. 3 and Table 4, respectively. They gave the reproducible DSC curves for heating and cooling over a few cycles [39, 57]. From the DSC characterization, it was found that LPCL-(Chol) showed a broad peak between around 43–58 °C. Similar behaviour was also observed for other polymers. However, only one peak was detected during heating in DSC curves. The results obtained from DSC measurements clearly demonstrated that all the polymers have a mesomorphic character and the temperature range of the liquid crystallinity becomes widen with the increasing amount of Chol in the polymers [40].

Fig. 3
figure 3

DSC thermograms of the linear and star-shaped PCLs in the second heating run (a) and cooling run (b)

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Table 4 Phase transition temperatures of cholesterol end-capped PCLs from DSC
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The phase identification of the polymers was examined using POM together with XRD studies. The samples for POM investigation were prepared as thin film between two glass plates via melting before visual inspection. The images for all the polymers exhibited the characteristic liquid crystal texture under POM both on heating and on cooling process. From the POM characterization, it was found that LPCL-(Chol)1 shows a transition of solid state to liquid crystalline state at 43 °C and a transition of solid state to isotropic state at 58 °C. The other polymers also exhibit the similar behaviour and liquid crystal textures. Thermal transition temperatures of the polymers were given in Table 4. The polarising optical micrographs of the polymers which taken at 45 °C for all polymers were shown in Fig. 4. The images of the polymeric liquid crystals are focal conic textures, which are reminiscent of smectic liquid crystals [14, 43, 58].

Fig. 4
figure 4

POM images for the synthesized PCLs. a 1 (45 °C), b 2 (45 °C), c 3 (45 °C), d 4 (45 °C), and e) 5 (45 °C) (magnification 25x)

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X-ray diffraction analysis provided more detailed information regarding the mesogenic structures and liquid crystal phases. Generally, a diffractogram examines two regions as SAXS and WAXS. For smectic phase, a sharp and strong peak is seen at SAXS region (1° < 2ϴ < 10°), whereas a broad peak is observed at WAXS region (10° < 2ϴ < 20°). In addition, this broad peak implies the amorphous part of the compounds. The strong sharp peak observed in WAXS indicates that the compounds are arranged in regularly spaced layers and d1 value gives the distances between the first and second layers. Nematic and cholesteric liquid crystals have only broad peak in WAXS region [11, 14, 59,60,61,62]. SAXS region of the synthesized polymers was examined at 45 °C in order to obtain more information on the organisation of layers. For this purpose, thermally treated film samples were prepared for analysis. At SAXS region a strong broad peak was observed for all of the polymers and their 2ϴ and corresponding d-spacing values are given in Table 5. In addition, XRD curves of the polymers can be seen in Fig. 5. The patterns of SAXS regions have encouraged the liquid crystallinity of the synthesized polymers [63,64,65,66,67,68]. WAXS region of LPCL-Chol is given as an example in Fig. 6. The results obtained from both XRD diffractograms and POM images clearly showed that liquid crystallinity type of all the polymers are smectic phase.

Table 5 2 Theta and d spacing value of the polymers
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Fig. 5
figure 5

SAXS region of XRD diffractograms of polymers

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Fig. 6
figure 6

WAXS region of XRD diffractograms of LPCL-Chol

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Conclusion

The cholesterol end-capped linear and star-shaped PCLs were synthesized in a four-step reaction sequence using core-first approach. ROP and CUAAC click reaction techniques were used in the first and final steps, respectively. Hydroxyl end-groups of PCLs prepared via ROP of ε-CL were then successfully converted into Br, N3, and Chol groups in turn. Chol end groups were introduced into the polymers via “CUAAC click reaction” between the linear and star-shaped PCL polymers having azide chain ends and Chol derivative with alkyne functionality. The structures of polymers were characterized using 1H NMR, FT-IR and GPC methods. A comparative study of mesomorphic properties of the PCLs were carried out by using DSC, POM, and SAXS regions of the XRD diffractograms. Because of the mesogenic property of cholesterol, all the polymers exhibited liquid crystallinity. According to POM micrographs, PCLs have focal conic textures which are characteristic image of the smectic liquid crystals. The diffractograms of the PCLs bearing the specific peak of smectic phase have encouraged the mesomorphism. In addition, the temperature range in which liquid crystallinity is observed becomes widen with the increasing number of polymer arms (from lineer to star-shaped topology). Also, because of biocompatible of Chol-end functional PCLs can be employed as promising materials in cell proliferation studies.