This is the published version: Peng, Shuhua, Hartley, Patrick G., Hughes, Timothy C. and Guo, Qipeng 2015, Enhancing thermal stability and mechanical properties of lyotropic liquid crystals through incorporation of a polymerizable surfactant., Soft matter, vol. 11, no. 31, pp. 6318‐6326.
Available from Deakin Research Online: http://hdl.handle.net/10536/DRO/DU:30075547 Reproduced with the kind permission of the copyright owner Copyright : 2015, Royal Society of Chemistry
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Enhancing thermal stability and mechanical properties of lyotropic liquid crystals through incorporation of a polymerizable surfactant Shuhua Peng,ab Patrick G. Hartley,* b Timothy C. Hughes* b and Qipeng Guo* a We present a facile method to prepare thermally stable and mechanically robust crosslinked lyotropic liquid crystals (LLCs) through incorporation of a polymerizable amphiphile into a binary LLC system comprising commercially available surfactant Brij 97 and water. Thermal stability and mechanical properties of the polymerized LLCs were significantly enhanced after polymerization of the incorporated polymerizable surfactant. The eff ect of incorporating a polymerizable amphiphile on the phase behavior of the LLC system
Received 6th July 2015, Accepted 6th July 2015
was studied in detail. In situ photo- rheology was used to monitor the change in the mechanical properties of the LLCs, namely the storage modulus, loss modulus, and viscosity, upon polymerization. The retention of
DOI: 10.1039/ c5sm01646k
the LLC nanostructures was evaluated by small angle X- ray scattering (SAXS). The ability to control the thermal stability and mechanical strength of LLCs simply by adding a polymerizable amphiphile, without
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tedious organic synthesis or harsh polymerization conditions, could prove highly advantageous in the preparation of robust nanomaterials with well- defined periodic structures.
Introduction As on e of th e m ost com m on self-assem bled system s, lyotropic liquid crystals (LLCs) are ubiquitous in n ature an d th ey h ave immediate relevance in biology due to the self-organized biological bilayer membranes in living systems.1 LLCs often exhibit the low viscosity characteristic of fluids while maintaining a highly ordered m orph ology m ore typical of solid m aterials.2 LLCs prepared by self-assem bly of am ph iph illic surfactan ts an d lipids h ave been widely employed to synthesize interesting nanostructured materials with con trollable structures an d properties.1–7 A variety of LLC ph ases, i.e., cubic, lam ellar, an d h exagon al, can be form ed depen din g on th e con cen tration an d n ature of th e surfactan ts an d solven t. Th e geom etric diversity of th ese LLC m esoph ases h as th e poten tial to prepare a variety of n an om aterials with differen t arch itectures.8–11 Aside from th eir use as tem plates for th e syn th esis of m esoporous in organ ic m aterials,12,13 LLCs h ave received m uch atten tion because of th eir poten tial application s in biological studies, such as th e capture an d replication of viruses,14 drug delivery,15–17 m edical im agin g,18,19 an d protein crystallisation .20–22 However, the lack of thermal stability and mechanical strength h as greatly lim ited th e use of LLCs in m aterials application s a
Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia. E-mail:
[email protected] b CSIRO Materials Science and Engineering, Bayview Avenue, Clayton South, Victoria 3169, Australia. E-mail:
[email protected],
[email protected]
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wh ere m ech an ically durable an d th erm ally stable structures are required. Polymerizable or cross-linkable LLCs provide a feasible solution to this problem.1 The resulting covalently crosslinked LLCs with robust networks have been used for controlled particle synthesis,23 drug delivery agents,24 heterogeneous catalysis and separations,6 and templates for nanocomposites and anisotropic m etallization .25,26 Alth ough th e use of polym erizable LLCs h as been dem on strated, an eff ective m eth od to im prove th e m ech an ical stren gth an d th erm al stability of th eir precursors an d reten tion of th e paren t LLC n an ostructure th rough out th e polym erization reaction can be ch allen gin g.5,27,28 Th erm odyn am ically driven ph ase separation even ts often occur durin g polym erization , leadin g to polym er n etworks with larger an d m ore disordered structures th an th e origin al LLC tem plates. Eff orts to elucidate th e factors in cludin g in itiation rate, ph otoinitiator mobility, reaction speed, temperature, monomer segregation behavior, and variations in polymerization rate have been extensively investigated to provide considerable control over the final state of polymerized LLCs structures by manipulating environment during the polymerization reaction.2,4–7,27–36 It was reported by Gin et al. th at th e order of th e origin al LLC template can be largely retained when polymerizable amphiphiles (i.e., surfactants) were employed as the LLC m esogen s.37–39 Catalytic fun ctional groups were incorporated into these polym erizable am phiph iles and the crosslinked LLC assem blies with well-defined nano-chan nels and/or pores h ave been successfully applied for h eterogen eous catalysis and selective separation.6 However, the design and synthesis of these polym erizable
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amphiph ilic m on om ers requires careful con sideration of their mesogenic behavior, amphiphilic self-assembly, and polymerization activity, which makes these polymerizable LLCs scarcely accessible. Thus, there is a need for new polymerizable LLCs with the capability of full retention of their parent nanostructures that could be easily prepared from existing chemicals and/or simple organic synthesis. In th is work, a sim ple polym erizable LLC system based on a combination of polymerizable amphiphile and nonpolymerizable surfactants was developed to prepare polym erized LLCs with enhanced thermal stability and m echanical strength. The newly designed and synthesized siloxane macromonomer, from a onestep reaction, is an amphiphile that forms part of the polym erizable LLC system. Real-time photo-rheology measurements were performed to follow the dynamic changes of moduli and viscosity during the course of LLC polymerization. The elastic and viscous moduli of polymerized LLCs were quantitatively evaluated at a constant strain rate as a function of UV exposure time. The structures of LLCs were investigated before and after polym erization to determine the retention of the parent LLC nanostructures. In addition, the thermal stability of the polymerized LLCs located in diff erent phase regions was also demonstrated.
Experimental section Materials Triblock copolymer poly(ethylene oxide)-block-polydimethylsiloxaneblock-poly(eth ylen e oxide) (PEO–PDMS–PEO, DBE-C25, Mw = 3500–4500, 60 wt% n on -siloxan e) was purch ased from Gelest an d was dried usin g 4 Å m olecular sieves for at least 24 h before use. 2-Isocyan atoeth yl m eth acrylate (IEM), purch ased from Am trade In tern ation al Pty. Ltd, was purified by distillation under reduced pressure, and stored in a closed vessel under dark condition at 20 1C prior to use. Dibutyltin dilaurate (DBTDL) was obtain ed from Merck Co; surfactan t Brij 97 an d ph otoin itiator (2-h ydroxy-2-m eth ylpropioph en on e, 97%) supplied by Sigm aAldrich were used as received. Syn th esis of polym erizable PEO–PDMS–PEO dim eth acrylate Polymerizable PEO–PDMS–PEO dimethacrylate (MA–PEO–PDMS– PEO–MA) was prepared by a sim ilar m ethod reported in our previous work32 and the synthesis route is shown in Scheme 1. Briefly, hydroxyl-term inated PEO–PDMS–PEO (12.0 g, 0.012 mol) an d IEM (4.0 g, 0.026 m ol) were added in to a roun d-bottom flask fitted with a Teflon -coated m agn etic stirrer bar. Th e flask was sealed an d stirred vigorously to produce a h om ogen eous solution . After 10 m in , th e catalyst DBTDL (20 mL) was added in to th e flask an d th e solution was stirred for 24 h to en sure a com plete reaction in th e absen ce of ligh t at room tem perature. Proton n uclear m agn etic reson an ce (1H NMR) spectra of MA– PEO–PDMS–PEO–MA were recorded on a Bruker NMR spectrom eter at 400 MHz usin g deuterated ch loroform as th e solven t. 1 H NMR ch em ical sh ifts (d) in parts per m illion s (ppm ) were referen ced relative to ch loroform (d = 7.26 ppm ) as an in tern al stan dard.
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Scheme 1 Synthesis route of polymerizable amphiphile MA– PEO– PDMS– PEO– MA.
Polarised optical m icroscopy (POM) Birefrin gen t textures were observed with a Nikon Eclipse 80i cross-polarised optical m icroscope equipped with a Lin kam h ot stage an d con troller (LTS 120 with PE94 con troller, Lin kam UK). Im ages were captured with a Nikon Ds-Fi1 CCD cam era equipped with DS-U2 con troller (Nikon Australia Pty. Ltd., Melbourn e, Australia). Sm all an gle X-ray scatterin g (SAXS) SAXS experim en ts were perform ed at the Australian Syn ch rotron on the small/wide an gle X-ray scattering beamline as previously described.31,40 Briefly, sam ples were inserted into 1.0 m m glass capillaries wh ich were then sealed. The backgroun d correction was perform ed by m easurin g the scattering of an em pty capillary and correctin g for sam ple absorption . Asliver beh en ate was used to calibrate the sam ple to detector distan ce. Data reduction (calibration and integration) of data collected using a 2D detector was achieved using AXcess, a custom-written SAXS analysis program written by Dr Andrew Heron from Imperial College, London.41 Determ in ation of ph ase diagram Th e partial tern ary ph ase diagram of Brij 97/water/MA–PEO– PDMS–PEO–MA m ixture system at room tem perature was determ in ed by com bin ation of SAXS an d POM. In order to prepare th e system s of varyin g com position s, differen t ratios of Brij 97 an d MA–PEO–PDMS–PEO–MA were prepared firstly an d th en th e required am oun t of water was added to th e m ixture of Brij 97 an d MA–PEO–PDMS–PEO–MA. Th e tern ary m ixtures were th orough ly m ixed usin g a Vortex m ixer after th ey were gen tly h eated above th e ph ase-separation tem perature wh ere th e m ixtures exh ibited a rath er low viscosity.42 After th at, th ey were son icated an d cen trifuged repeatedly un til h om ogen eous sam ples were obtain ed. Lyotropic ph ase region of th e tern ary system was iden tified by both POM an d SAXS with a con cen tration accuracy of 5 wt% water con ten t. Polym erization of LLCs by UV LLCs form ulation s were polym erized usin g 1 wt% ph otoin itiator (2-h ydroxy-2-m eth ylpropioph en on e, 97%), wh ich was based on th e weigh t of polym erizable MA–PEO–PDMS–PEO–MA.
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Th e fresh LLCs were firstly loaded in to 1.0 m m borosilicate glass capillaries and then the sealed capillaries were put into the UVbox for polym erization. Photo-polym erization was carried out in a UV box working at a wavelength of 365 n m (with an output of 10 m W cm 2) for about 30 m in at room tem perature.
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In situ m on itorin g of polym erization of LLCs by ph oto-rh eology In situ m on itorin g of th e UV curin g process for polym erization of LLC ph ases was con ducted usin g an ARES ph oto-rh eom eter (TA In strum en ts, USA) conn ected to an EXFO Acticure 4000 ligh t source workin g at 100 m W cm 2 (365 n m ). A Peltier tem perature con troller was also con n ected to th e rh eom eter to m aintain the cure tem perature at 25 1C, thereby ensuring that both rheological an d SAXS experim en ts were perform ed un der the same con dition s for phase beh aviour study. A description of these fixtures is given in our previous work.9,32 The sam ples were loaded in the cen tre of two parallel plates of 20 m m in diam eter. The gap between the two plates was set at 0.3 m m. The in situ cure kin etics was studied at a con stan t tem perature of 25 1C for 10 m in . For the first 2 m in , the ligh t source was con trolled in off m ode to get a baselin e an d autom atically con verted to on-m ode for 18 m in . Storage shear m odulus (G0), the loss shear m odulus (G00), an d complex viscosity (Z*) were m easured as a fun ction of tim e at a con stan t frequen cy of 100 rad s 1 and a strain of 1.0%.
Fig. 1 1H NMR spectra of HO– PEO– PDMS– PEO– OH and MA– PEO– PDMS– PEO– MA in CDCl3.
Results and discussion Ch aracterization of MA–PEO–PDMS–PEO–MA Th e 1H NMR spectra of HO–PEO–PDMS–PEO–OH an d MA– PEO–PDMS–PEO–MA are respectively sh own in Fig. 1. Con version of HO–PEO–PDMS–PEO–OH to MA–PEO–PDMS–PEO–MA was eviden ced by th e upfield sh ift of proton sign als from m eth ylen e n ext to OH group. All th e peaks were assign ed an d th e in tegral ratios of th e peaks m atch ed th e assign ed structure, dem on stratin g th at polym erizable MA–PEO–PDMS– PEO–MA was successfully syn th esized. Ph ase diagram of th e Brij 97/Water/MA–PEO–PDMS–PEO–MA On th e basis of visual in spection , POM, an d SAXS an alysis of aroun d 120 sam ples, th e isotropic ph ase an d lyotropic ph ase for th e Brij 97/water/MA–PEO–PDMS–PEO–MA tern ary system h ave been iden tified. An approxim ate partial ph ase diagram , determ in ed at 25 1C, is illustrated in Fig. 2. It m ain ly exh ibits two lyotropic m esoph ases: lam ellar ph ase La an d n orm al h exagon al ph ase H 1. Th e ph ase boun dary of th ese lyotropic ph ases was establish ed with an accuracy of 5 wt% water con ten t. As sh own in Fig. 2, it can be clearly seen th at th ese lyotropic ph ases are close to th e h igh Brij 97 con ten t region . Th is is presum ably due to the tem platin g effect of Brij 97. Th ere h ave been exten sive studies usin g Brij 97 as a tem plate for th e syn th esis of m esoporous in organ ic m aterials.43,44 Th e polym erizable MA–PEO–PDMS–PEO–MA with 60 wt% h ydroph ilic blocks can n ot self-assem ble in to well-defin ed LLCs because of its good water solubility. However, with an appropriate ratio of Brij 97, th e m ixture of Brij 97/MA–PEO–PDMS–PEO–MA
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Fig. 2 Partial lyotropic phase behaviour of the Brij 97/water/ MA– PEO– PDMS– PEO– MA at room temperature. Three water dilution lines E, F, and G were indicated. Three formulations with constant water contents (25 wt%, E25, F25, and G25) and three formulations with increasing water content (F20, F25, and F40) along dilution line F were indicated for the following mechanical and thermal stability study.
exh ibits th e ability to form lyotropic m esoph ases. Two large lyotropic ph ase region s were observed for th e Brij 97/water/MA– PEO–PDMS–PEO–MA tern ary system in Fig. 2. A water dilution lin e wh ich wen t th rough both lam ellar an d h exagon al ph ases was selected for th e followin g polym erization study due to th e relatively h igh polym erizable MA–PEO–PDSM– PEO–MA con ten t (dilution lin e F, Fig. 2). As sh own in Fig. 3, th e form ulation s alon g dilution line F in th e lyotropic region s exhibited brightness between crossed polarized film s because of their anisotropic characteristics. In contrast, for the formulations with 10 wt% and 60 wt% water corresponded to the isotropic and
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Fig. 3 Samples along water dilution line F with diff erent water contents were observed between cross polarized films.
n on -birefrin gen t ph ases did n ot sh ow brigh tn ess un der cross polarized ligh t. Mean wh ile, two represen tative POM optical textures for th e Brij 97/water/MA–PEO–PDMS–PEO–MA tern ary system with 20 wt% an d 40 wt% water were presen ted in Fig. 4, respectively. Ch aracteristic streaked texture for lam ellar ph ase was observed for th e tern ary system with 20 wt% water con ten t. Th e tern ary m ixture with 40 wt% water sh owed a typical parallel striation texture con sisten t with th e form ation of a h exagon al ph ase.42 SAXS m easurem en ts were furth er used to iden tify diff eren t LLC m esoph ases based on th e ch aracteristic diff raction pattern of th e Bragg peaks.43 Specifically, th e relative peak position s for a lam ellar ph ase (La ) are 1 : 2 : 3 : 4 an d for a h exagon al ph ase are 1 : O 3 : O 4 : O 7. As sh own in Fig. 5, a broad SAXS peak was observed for the formulation along water dilution line F with 15 wt% water. Mean wh ile, lack of birefringen ce under crossed polarizers and low viscosity together suggested the presence of reverse m icelles for the Brij 97/water/MA–PEO–PDMS–PEO–MA ternary m ixture. With increasin g water con tent to 20 wt%, the m ixture evolved to lamellar phase, evidencin g by the existen ce of two sharp SAXS peaks with relative position s 1 : 2. As the water con tent was increased to 30 wt%, these SAXS peaks becam e m uch broader an d expressed the onset of phase transition. Three well developed SAXS peaks with the relative positions of 1 : O 3 : O 4 were observed when the water con tent reach ed to 40 wt% an d 50 wt%, which is characteristic of well-defined h exagon al structures. Moreover, an alysis of these SAXS pattern s
revealed that the lattice param eter increased from 6.23 n m to 6.79 n m as the water con tent was increased from 40 wt% to 50 wt%. The increase of lattice param eter seem s to be due to the swellin g effect of the n ormal h exagon al (H 1) phase by increasing water con tent.9,43 With furth er increase of water con tent to 60 wt%, the ternary m ixture tran sformed to the isotropic phase with broad SAXS peaks. Real-tim e ph oto-rh eology m easurem en t durin g polym erization Real-tim e ph oto-rh eology m easurem en ts were perform ed to follow th e LLC polym erization process. Elastic an d viscous m oduli of polym erizin g LLCs were quan titatively evaluated when they were plotted as a function of UV exposure time at a constant frequency. In this study, the macromonomer (MA–PEO– PDMS–PEO–MA) formulations were cross-linked by exposing LLC formulations to UVlight. As shown in Fig. 6, the dynamic changes of storage modulus (G0), loss modulus (G00), and viscosity (Z*) with cure time were clearly observed for the lamellar phase with 25 wt% water content along the water dilution line F (Fig. 2) during the course of polymerization. The initial exposure of the mixture to UVlight was intentionally delayed for 2 m in to obtain a baseline for the measurement of the rheological properties. No change in moduli and viscosity was observed before the m ixture was exposed to UVlight. Upon exposure to UV light a fast increase of moduli and viscosity of the polymerizing LLC was observed and the plateau values were reached within 15 min. Two orders of magnitude increase of storage modulus (G0) and viscosity (Z*)
Fig. 4 Representative POM optical textures for the Brij 97/ water/ MA– PEO– PDMS– PEO– MA ternary system along water dilution line F with different water contents: (a) F20; (b) F40.
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Fig. 7 Storage modulus (G0) for the lamellar phases with constant water content (25 wt%) and variable polymerizable MA– PEO– PDMS– PEO– MA contents (E25, 37.5 wt%; F25, 30 wt%; G25, 22.5 wt%) and Brij 97 contents (E25, 37.5 wt%; F25, 45 wt%; G25, 52.5 wt%) during polymerization. The compositions of these lamellar phases are indicated in Fig. 2.
Fig. 5 SAXS patterns for the formulations along dilution line F and water contents are indicated in square brackets.
Fig. 6 Photo- rheology profile of cure process for a lamellar phase having 25 wt% water, 30 wt% MA– PEO– PDMS– PEO– MA, and 45 wt% Brij 97 (F25 in Fig. 2) following dilution line F.
were observed after polymerization. The increase of moduli and viscosity was presumably due to the cross-linking of polymerizable MA–PEO–PDMS–PEO–MA. Meanwhile, Fig. 7 illustrates how the mechanical properties of the polymerized LLCs were affected by their corresponding precursor LLC phase behaviour. A series of polymerizable lam ellar phases (E25, F25, and G25 in Fig. 2) with constant water content (25 wt%) and variable MA–PEO–PDMS– PEO–MAcontents were selected for this study. As shown in Fig. 7, decreasing polymerizable MA–PEO–PDMS–PEO–MA content from 30 wt% (F25) to 22.5 wt% (G25) resulted in the reduced density of cross-links of the polymerized LLCs and a corresponding decrease of G0 for G2.5 was observed.45 It should be noted that, however, a lower plateau value of G0 for the lamellar phase with higher polymerizable MA–PEO–PDMS–PEO–MA content (E25, 37.5 wt%) was recorded in Fig. 7. This may be due to the intrinsic structural
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characteristic of this lamellar phase (E25). The bilayer lamellar nanostructure of E25 was not likely to be well-developed because it was located near the phase boundary between lyotropic lamellar and isotropic micellar phase (Fig. 2). In addition, as shown in Fig. 7, a rather low initial G0 value for E25 (compared with that of F25 and G25) was observed before polymerization, underscoring the intermediate phase behaviour of E25. Therefore, lack of welldeveloped bilayer lamellar structure for E25 resulted in a lower G0 after polymerization even there was higher polymerizable content in its corresponding precursor LLC phase. Importantly, it also should be noted that initial lamellar and hexagonal phases were retained after polymerization (details will be given in the following section) and the increasing G0 was merely due to the cross-linking of the polymerizable MA–PEO–PDMS–PEO–MA. Th e eff ect of polym erization on th e m ech an ical property of polym erizable LLCs in term s of sh ear storage m odulus G0 with in diff eren t m esoph ase region s was sum m arized in Fig. 8. Gen erally, it can be clearly seen th at G0 in creased at least on e order of m agn itude for all th e LLC sam ples upon polym erization . More th an th ree orders of m agn itude in crease of G0 was observed for th e lam ellar ph ase with h igh polym erizable MA–PEO–PDMS–PEO–MA con ten t (E25, 37.5 wt%). As for th e polym erizable h exagon al ph ases, it sh ould be n oted th at th e in itial G0 was m uch h igh er th an th at of lam ellar ph ases due to th eir h igh ly ordered structures with array of lon g parallel cylin ders.9 Polym erization with in h exagon al ph ases results in aroun d on e order of m agn itude in crease of G0. Th e effect of polym erization on th e m ech an ical property of polym erized LLCs with differen t m esoph ases was closely correlated with polym erizable surfactan t con ten t an d in trin sic arch itecture of LLC m esoph ase. Com pared with lam ellar ph ase, h exagon al ph ase h as h igh er surface area an d h en ce th e n um ber of cross-lin kers per surfactan t surface is also h igh er. Furth er results an d discussion about th e effect of polym erization on the therm al stability of polym erized LLCs are given the following section .
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Fig. 8 Storage modulus (G0) for the lamellar phases with 25 wt% water content and hexagonal phases with 40 wt%water content before and after polymerization.
Reten tion of LLC ph ases after polym erization As discussed earlier, th e lack of th erm al stability of LLCs h as greatly lim ited th eir application s an d it is ch allen gin g to polymerize LLCs while fully retaining the precursor nanostructure. Polym erizable LLC system s with con ven tion al polym eizable m on om ers like poly(eth ylen e glycol) diacrylate (PEGDA) an d 1,6-h exan ediol diacrylate (HDDA) exh ibited disrupted n an ostructures due to m on om er segregation an d ph ase separation during polymerization.36 The preservation of LLC nanostructure tem plates after polymerization of polymerizable MA–PEO–PDMS– PEO–MA was dem onstrated by comparison of SAXS patterns and POM images for the samples before and after polym erization, respectively. Two representative sam ples from lam ellar and h exagon al phase regions were inten tion ally selected for this study. As sh own in Fig. 9(a), the SAXS profiles exhibit a ratio between the primary and secondary reflections of 1 : 2 for the sam ple with 20 wt% water conten t alon g dilution lin e F before polym erization. After polym erization, these SAXS peaks with the relative positions of 1 : 2 were retained, indicatin g the preservation of lam ellar phase during polym erization. Sim ilar results for the h exagon al phase with 40 wt% water con tent alon g dilution lin e F were also observed. As sh own in Fig. 9(b), the characteristic peaks for h exagon al phase with relative positions of 1 : O 3 : O 4 were retained upon polym erization. Analysis of the SAXS pattern s yields essen tially equivalent lattice param eters for the LLC phases before and after polym erization. Specifically, lattice param eters were 5.47 n m and 5.65 n m for the lam ellar phase an d 6.23 n m and 6.38 n m for the h exagon al phase, indicatin g the reten tion of nan ostructure in these system s after polym erization. However, as indicated by POM (data n ot shown ), dilution of the polym erized LLCs with water resulted in the disappearan ce of LLC structure, suggestin g that internal crosslin king of LLCs did n ot take place. POM results furth er confirm th e reten tion of LLC ph ases after polym erization . As sh own in Fig. 10(a) an d (b), after polym erization , th e streaked texture for th e lam ellar ph ase
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Fig. 9 Representative SAXS patterns for the LLC phases along dilution line F before and after cross- linking: (a) lamellar with 20 wt% water (F20 in Fig. 2); (b) hexagonal with 40 wt% water (F40 in Fig. 2).
was preserved without significant loss of birefringence, indicating the lamellar phase was retained after polymerization. The retention of streaked texture for the lamellar phase is comparable to the ternary LLC system with polymerizable poly(ethylene glycol) (PEG), dodecyltrimethylammonium bromide (DTAB), and water.34 Similar POM results for the retained hexagonal phase with parallel striation texture were shown in Fig. 10(c) and (d). Taken together with the SAXS results in Fig. 9, it can be con cluded th at polym erization of polym erizable MA–PEO–PDMS–PEO–MA did n ot alter th e n an ostructure of th e paren t LLCs. En h an ced th erm al stability of polym erized LLC Th e eff ect of polym erization on th e th erm al stability of th ese LLCs was in vestigated by SAXS m easurem en t at diff eren t tem peratures. Fig. 11(a) sh ows th at th e SAXS profiles of th e sam ple with 20 wt% water con ten t before polym erization at various tem peratures ran gin g from 25 to 80 1C. At 25 1C, th ere were two clear scatterin g peaks with relative position s of 1 : 2, in dicatin g a well-developed lam ellar structure. From 25 to 55 1C, th e sh arpn ess an d in ten sity of th ese ch aracteristic SAXS peaks from th e lam ellar ph ase decreased. At 60 1C th e first prim ary peak distin ctly broaden ed an d th e secon d h igh er-order peak becam e diffi cult to observe. Above 60 1C, a sin gle broad SAXS peak was observed due to th e m eltin g of th e lam ellar ph ase. Cross-lin kin g of th e polym erizable MA–PEO–PDMS–PEO–MA in th e lam ellar ph ase clearly en h an ced its th erm al stability, wh ich was eviden ced by th e SAXS pattern s in Fig. 11(b). Th e SAXS profiles of polym erized lam ellar ph ase with relative peak position s of 1 : 2 could be observed even up to 80 1C, th e h igh est tem peratures tested. Th e ph otopolym erization of hexagon al ph ase was carried out for Brij 97/water/MA–PEO–PDMS–PEO–MA tern ary m ixture with 40 wt% water alon g dilution lin e F. Fig. 12(a) sh ows th at th e SAXS pattern s for th e m ixture before polym erization from 25 to 65 1C. At 25 1C, it exh ibits th ree clear scatterin g peaks with relative position s of 1 : O 3 : O 4 for th e h exagon al ph ase. Wh en th e tem perature was in creased from 30 to 40 1C, th e sh arpn ess an d in ten sity of th ese peaks decreased sim ilarly to th ose of for
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Fig. 10 POM optical textures for the Brij 97/ water/ MA– PEO– PDMS– PEO– MA ternary system before and after polymerization: (a) F20 before polymerization; (b) F20 after polymerization; (c) F40 before polymerization; (d) F40 after polymerization.
Fig. 11 SAXS patterns plotted as a function of temperature for the formulation along dilution line F with 20 wt% water content (F20 in Fig. 2): (a) before polymerization; (b) after polymerization.
Fig. 12 SAXS patterns plotted as a function of temperature for the formulation along dilution line F with 40 wt% water content (F40 in Fig. 2): (a) before polymerization; (b) after polymerization.
th e lam ellar ph ase (Fig. 11(a)). At 45 1C, th e SAXS pattern evolved in to two distin ctly broaden ed peaks, in dicatin g th e m eltin g of h exagon al ph ase an d th e on set of m icelles. Th e
6324 | Soft Matter, 2015, 11, 6318--6326
broader peak was presum ably due to th e fact th at, with in creased th erm al fluctuation s, th e closely packed m icelles in LLC ph ase sim ply break m ore often an d becom e in creasin gly
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Table 1 Thermal stability of diff erent LLC mesophases before and after polymerization in terms of their melting points with an accuracy of 5 1C was determined by SAXS measurements
Com position (wt%)
Meltin g poin t (1C)
Sam ple/LLCs
Lattice param eters (n m ) (before/after)
Water
Brij 97
MA–PEO–PDMS–PEO–MA
Before
After
E25/La F25/La G25/La F20/La F35/H 1 F40/H 1 F45/H 1
5.54/5.68 5.46/5.61 5.51/5.64 5.47/5.65 6.01/6.14 6.23/6.38 6.59/6.72
25 25 25 20 35 40 45
37.5 45 52.5 48 39 36 33
37.5 30 22.5 32 26 24 22
50 55 60 55 45 45 55
4 4 4 4
80 80 80 80 60 60 60
Scheme 2 Schematic illustration of polymerizable surfactant was incorporated into hexagonal phase and the initial structure was retained after polymerization.
sh orter un til th ey reach an isotropic m icellar state.46,47 As shown in Fig. 12(b), the clearly improved therm al stability of the h exagon al phase was again indicated by the increased m elting point, from 45 to 60 1C after polym erization of the ternary m ixture. It is n oted that the m elting point of the polym erized h exagon al phase was lower than that of lam ellar phase. This m ay be due to the different polym erizable MA–PEO– PDMS–PEO–MAcon tents in the ternary m ixture. For the lam ellar phase with 20 wt% water con tent, the con tent of polym erizable MA–PEO–PDMS–PEO–MA in the m ixture is 32 wt%. The polym erizable con tent decreases to 24 wt% for the h exagon al phase with 40 wt% water. The h igher con centration of polym erizable com poun d in the lam ellar phase resulted in m ore cross-linked points in the polym erized m ixture, thus enabling h igher therm al stability of the system after polym erization . Another possible explanation for the diff erent thermal stability of lamellar and hexagonal phases after polymerization could be due to their diff erent intrinsic interfacial architectures. Compared with highly curved hexagonal phase, non-curved planar lamellar phase makes the polymerizable double bonds more closely aligned, resulting in the polymerized lamellar phase being more stable than the hexagonal phase. It has been reported that polymerization kinetics is closely related to LLC phase geometry and that faster polymerization rates and higher retention of parent LLC phases was observed for a lamellar phase.27,35 Thermal stability of seven LLC samples with four La phases and three H 1 phases before and after polymerization was summarized in Table 1. It can be clearly seen that the melting points of polymerized La phases are consistently higher than the polymerised H 1 phases even though the content of polymerizable MA–PEO–PDMS–PEO–MA was lower in the La phase (G25, 22.5 wt%), indicating that the intrinsic interfacial architectures of LLCs may play more pronounced role in affecting polymerization efficiency and thermal stability. Finally, a sketch illustrates the role of incorporated polymerizable surfactant
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and the effect of polymerization on the phase of LLCs was given in Scheme 2.
Conclusions This work presents a facile method to prepare thermally stable and mechanically robust LLCs by simply incorporating a polymerizable amphiphile. The polymerizable MA–PEO–PDMS–PEO–MA was synthesized and incorporated into the binary Brij 97/water LLC system. The phase behaviour of the ternary mixture of Brij 97/ water/MA–PEO–PDMS–PEO–MA was intensively studied by POM and SAXS and two LLC phase regions including lamellar and hexagonal were identified. SAXS measurements indicated that all of the parent LLC nanostructures reported here can be retained after polymerization of MA–PEO–PDMS–PEO–MAwithin the lamellar and hexagonal phases respectively. Significant improvement of mechanical properties was observed and two orders of magnitude increase of storage modulus and viscosity were obtained after polymerization of the lamellar phase structure. Improved thermal stabilities of polymerized LLCs were observed. However, the degree of induced stability varied according to different phase behaviour of the precursor LLC templates. The higher concentration of polymerizable MA–PEO–PDMS–PEO–MA in the lamellar phase resulted in more cross-linked points in the polymerized mixture, thus enabling higher thermal stability of the system after polymerization. The ability to enhance the thermal stability and mechanical strength of LLCs simply by adding a polymerizable amphiphile has been shown to be a simple and effective way to prepare robust nanomaterials with well-defined periodic structures.
Acknowledgements SAXS/WAXS research was un dertaken at th e Australian Syn ch rotron , Victoria, Australia an d th e auth ors th an k Dr Nigel Kirby
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for h is assistan ce. S.P. gratefully ackn owledges Deakin Un iversity an d CSIRO for provision of a collaborative Ph D sch olarsh ip.
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