Thermal Behavior and High- and Low-Temperature Phase Structures

Article pubs.acs.org/Langmuir

Thermal Behavior and High- and Low-Temperature Phase Structures of Gemini Fluorocarbon/Hydrocarbon Diblocks Caroline de Gracia Lux,† Bertrand Donnio,‡ Benoit Heinrich,‡ and Marie Pierre Krafft*,† †

Systèmes Organisés Fluorés à Finalités Thérapeutiques (SOFFT), Institut Charles Sadron (ICS, UPR CNRS 22), Université de Strasbourg, 23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 2, France ‡ Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS, UMR 7504), Université de Strasbourg, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: The thermal behavior and phase structure of two series of gemini fluorocarbon/hydrocarbon diblock amphiphiles with the general formula (CnF2n+1CH2)(Cm − 2H2m − 3)CH−CH(CnF2n+1CH2)(Cm − 2H2m − 3), with n = 8, 10 and m = 6, 12, 14, 16, 18, 20 (abbreviated as di(FnHm)), have been investigated by differential scanning calorimetry, polarized optical and freeze−fracture transmission electron microscopies, dilatometry, and small-angle X-ray scattering (SAXS). The various terms of the series exhibit the same thermal behavior, essentially composed of two exothermal transitions, a low-temperature event that corresponds to the melting of the hydrocarbon chains at TH and a high-temperature transition associated with the melting of the fluorocarbon chains at TF. Below TH, a disordered plastic rotator phase, MLT, and above TH, a lamellar phase, MHT, were determined by SAXS experiments. Above TF, the compounds eventually clear into the isotropic liquid. In the MHT phase, both the Fn and Hm blocks are segregated from each other, forming sublayers with sharp interfaces, as revealed by the five lamellar orders and remarkable sharpness of the SAXS peaks. In the MLT phase, the partial crystallization of the aliphatic blocks when the temperature is lowered leads to the disruption of the aliphatic sublayers into rows of ribbons arranged according to pseudohexagonal and/or rectangular arrangements with different lattice sizes (p2gg symmetry). The Fn segments form the fluorinated continuum. In support of SAXS, molecular packing models of the tetrablocks are proposed on the basis of the temperature/volume variations of di(F10H20) and di(F10H16) in both high- and low-temperature phases, as determined by dilatometry. It is notable that the arrangements found for di(FnHm) are completely different from those previously reported for FnHm diblocks, revealing the influence of the linker unit on the solid-state behavior of the tetrablocks. when transferred onto solid substrates.3,4 They also provided the first example of stacking of self-assembled discrete objects in ultrathin films. Di(FnHm) tetrablocks can be considered to consist of two semifluorinated alkanes CnF2n+1CmH2m+1 (e.g., FnHm diblocks) covalently coupled by a carbon−carbon single bond. Because they comprise both rigid (Fn blocks) and flexible segments (Hm blocks), interconnected through a hydrocarbon linker, these modulable nonpolar, highly hydrophobic amphiphiles are expected to exhibit rich and controllable self-assembly behavior with possible applications in surface nanopatterning.2,3,5 Fluorinated chains (F chains) are known to promote selfassembly2,3,6−8 and stabilize surface films and membranes.9−16 FnHm diblock amphiphiles allow the stabilization of vesicles, tubules, and other supramolecular constructs in water and other liquids.17−19 One should note that the four-carbon linker that

I. INTRODUCTION A series of gemini (F-alkl)alkyl diblock amphiphiles di(FnHm) (C n F 2 n + 1 CH 2 )(C m − 2 H 2 m − 3 )CH−CH(C n F 2 n + 1 CH 2 )(Cm − 2H2m − 3), with n = 8, 10 and m = 6, 12, 14, 16, 18, 20, have recently been synthesized (Scheme 1).1 These compounds, Scheme 1. Molecular Structure of Gemini Diblocks Di(FnHm)a

The aliphatic cross-linker onto which the antagonist fluorinated and hydrogenated chains are grafted is enclosed in the dashed square.

a

although devoid of polar headgroups, display a definite dipole moment as well as amphiphilic, amphisteric, and amphidynamic character2,3 and are therefore prone to organized self-assembly behaviors. In particular, they were recently shown to form stratified films of surface micelles both at the water interface and © 2013 American Chemical Society

Received: February 11, 2013 Revised: April 2, 2013 Published: April 5, 2013 5325

dx.doi.org/10.1021/la400565h | Langmuir 2013, 29, 5325−5336

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holds the two diblocks together provides substantial flexibility for conformational adjustments. The thermal behavior and mesophase structure of related linear diblocks CnF2n+1CmH2m+1, FnHm, have been extensively investigated.2,12,16,20−36 They were found to exhibit a variable number of mesophase transitions, reflecting the various degrees of ordering of the Fn and Hm segments before complete clearing in the isotropic liquid, with the latter event being dominated by the complete breakdown of the regular Fn block packing (at temperature TF). Numerous different dense layered arrangements that minimize the contact between the two types of chains have been suggested and discussed in the literature both below and above the main phase transition (parallel and antiparallel chains, tilt, interdigitation, cylinders, ribbons, double-layered undulating lamellae, and herringbone arrangements).2,7 In spite of extensive investigations, however, no general clear and unambiguous picture of the molecular organization of FnHm diblocks in their various phases has yet emerged. Successful small-angle X-ray scattering (SAXS) reflection assignments were achieved for two series of diblocks, namely, for two (perfluorodecyl)alkanes (e.g., perfluorodecyloctane (F10H8) and perfluorodecyldecane (F10H10)22,32) and for four (perfluorododecyl)alkanes (F12Hm, m = 6, 8, 10, 12).29,31 It is noteworthy that only the structure of the mesophase (within the temperature range delimited by TH, the melting temperature of the Hm block, and TF) has been elucidated. The mesomorphous state of diblock F10H10 was investigated by SAXS at 50 °C in the smectic B liquid-crystalline phase.22 A well-developed lamellar structure, with a sharp interface

between the segregated Fn and Hm blocks and a periodicity of 28.0 Å, was determined. In this organization, the Fn blocks are rigid and oriented normal to the layer plane, and the Hm blocks are in a disordered liquidlike state. Further SAXS studies conducted on F12Hm (m = 6, 8, 10) above TH concluded that they also form a simple lamellar structure consisting of singlelayer stacks.29,31 An investigation of the F12Hm (m = 8, 10, 12) compounds below TH led to complex diffractograms, which prohibited indexation in the high-angle region. However, for F12H6, the monolayered lamellar arrangement with tilted molecular axes with respect to the layer normal was still proposed. Concerning F12Hm (m = 8, 10, 12), double-layered undulating lamellae with interdigitating hydrocarbon chains were suggested as a possible arrangement.31 More recently, a different structure was published for F10H10 and F10H8 above TH.32 By contrast with the earlier study, the lamellar phase was characterized by a segregation of stretched Fn chains in the middle of a smectic layer, and the orientation of the Hm blocks alternates up and down in order to fill the remaining space. For both compounds, the 00l peaks reflected the superimposition of high-electronic-density lamellar regions (Fn chains) and electron-poor sublayers (Hm chains) along the normal of the layers. The lamellar spacing periodicities were 24.8 and 27.6 Å for F10H8 and F10H10, respectively. The hk0 reflections revealed the 2D packing as being a rectangular herringbonetype arrangement of Fn chains adopting single helicoidal axis 21 perpendicular to the chain axis (P21 as the structural model). The present study concerns the thermal behavior of the di(FnHm) tetrablocks in the bulk solid and the structure of the various phases encountered with temperature. We intended to

Figure 1. DSC thermograms measured upon cooling for (a) di(F10Hm) and (b) di(F8Hm).

Table 1. Thermal Characteristics of the Low- and High-Temperature Phases (MLT and MHT)a low-temperature phase MLT di(FnHm) di(F10H20) di(F10H18) di(F10H16) di(F10H14) di(F8H20) di(F8H18) di(F8H16) di(F8H14) di(F8H12)b

THh 58 46 32 26 40 29 10 −13

THc 48 39 26 14 36 22 5 −21

ΔHHh 57 33 13 2 25 26 23 23

ΔHHc 55 32 12 2 27 28 21 22

high-temperature phase MHT ΔSHh

ΔSHc

173 104 43 8 79 86 80 88

172 102 41 7 87 94 75 85

TFh

ΔHFh

TFc 68 69 70 70 60 54 47 36 20

65 61 62 63 54 44 39 26 7

8 36 61 73 55 38 41 25 14

ΔHFc 17 42 63 69 61 41 42 24 13

ΔSFh 23 106 178 212 165 115 127 80 48

ΔSFc 51 126 187 206 186 129 136 82 48

Peak transition temperatures, TH and TF (°C), and corresponding enthalpies, ΔHH and ΔHF (kJ mol−1), and entropies, ΔSH and ΔSF (J K−1 mol−1), for the di(FnHm) tetrablocks upon heating and cooling (h and c, respectively). bFor di(F8H12), only the high-transition mesophase (MHT) could be detected in the explored temperature ranges. (See the text.) a

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molecular fragments. The origin of this structural feature was further investigated by dilatometry and SAXS and will be discussed below. The variations of the TF temperature as a function of the total number of carbon atoms, for different molecules containing fluorinated and hydrogenated linear segments such as diblocks (F8Hm, F10Hm, and F12Hm), have been compared to those of the two tetrablocks series di(F8Hm) and di(F10Hm) (Figure 3). The isotropization temperatures of

retrieve all differences in behavior between both the di(FnHm) tetrablocks and the FnHm diblocks to determine the influence of the linker unit. The structures of the low- and hightemperature mesophases, MLT and MHT below and above TH, respectively, for the two series of di(FnHm) tetrablocks, with n = 8 or 10 and m = 16, 18, 20, have been elucidated by SAXS. Quite different supramolecular arrangements were deduced with respect to those previously reported for FnHm diblocks.

II. RESULTS AND DISCUSSION A. Thermal Behavior of the Di(FnHm) Tetrablocks. The thermal behavior of the gemini diblocks was investigated by DSC from −40 to 100 °C (Figure 1). The values of TH and TF, recorded on both heating and cooling, along with the corresponding enthalpies and entropies are collected in Table 1. For essentially all of the di(FnHm) compounds, the thermograms are similar and present two important transition enthalpies, except di(F8H12), which features only one thermal event, and di(F8H6), for which no peak was recorded, suggesting that it remains in the isotropic liquid phase throughout the explored temperatures range. All transitions are also rather broad and span a few degrees, with this broadening being more obvious for the high transition temperature. By analogy to segmented diblock polymers (vide infra), the low-temperature event likely corresponds to the partial melting of the flexible Hm block, referred to as TH, whereas the high-temperature transition, which is gradual, is evidently associated with the melting of the rigid Fn blocks, referred to as TF. Above TF, the compounds are in the isotropic liquid state (Iso). Significant hysteresis in the transition temperatures (between heating and cooling cycles) is observed for all nine tetrablocks investigated, as illustrated in Figure 2 for di(F8H20). The shift

Figure 3. Isotropization temperatures upon heating F alkanes (black circles), H alkanes (black triangles), F8Hm diblocks (magenta squares), F10Hm diblocks (green squares), F12Hm diblocks (blue squares), di(F8Hm) tetrablocks (purple triangles), and di(F10Hm) tetrablocks (red triangles) as a function of the total number of carbons.

F alkanes and H alkanes have also been plotted for comparison. For all diblocks, TF shows little (or no) dependence on the length of the Hm segment, with this dependence being further partially modulated by the length of the Fm chain. This is in stark contrast to the variations in the clearing temperatures of both F alkanes and H alkanes, which show a substantial linear increase with the increasing lengths of the segments. Furthermore, the TF values of the diblocks are inside the temperature ranges delimited by the clearing temperatures of F alkanes and H alkanes, and their onset is solely governed by the length of the Fn segment (e.g., the longer the F segments, the higher the onset temperatures). The behavior of the tetrablocks is more difficult to comprehend solely on the basis of these straight structural criteria. Indeed, whereas TF increases steadily in the di(F8Hm) series, indicating that both blocks play a role in the clearing process, the clearing behavior of the di(F10Hm) tetrablocks is almost invariant with the size of the Hm block. Moreover, the behavior of di(F10Hm) is almost superimposable with that of the F10Hm diblocks whereas that of di(F8Hm) is intermediate between that of the H alkanes and the F8Hm diblocks. It appears that for both FnHm diblocks and di(FnHm) tetrablocks TF is little influenced by the Hm chain length when long Fn segments are present and thus that the clearing behavior of the di(FnHm) tetrablocks seems to be primarily affected by the state of order of the Fn chains. An investigation of the thermal behavior of the di(FnHm) tetrablocks uncovered other significant differences compared to that of their FnHm diblock analogues. Although the latter present a strong, sharp melting-peak transition at TF and a much weaker, broader peak at TH, most di(FnHm) (n = 8, 10, m = 20, 18, 16) present endotherms of similar shapes and intensities for both phase transitions, as shown in Figure 4.

Figure 2. DSC thermograms of di(F8H20) recorded (1) during the cooling run and (2) for the subsequent heating run. It is worth mentioning that prior to the DSC measurement the sample was heated from −40 to 100 °C at a rate of 20 °C min−1.

in the peak temperatures is of the same order of magnitude for both transitions, (e.g., ∼4−13 °C for TF and ∼4−11 °C for TH) when recorded at a scanning rate of 20 °C min−1. However, the determination of the onset temperatures is not very accurate because the heat flow varies very progressively on the lowtemperature side of both transition peaks. This asymmetric broadening occurs on the same peak side on both heating and cooling and reproduces similarly for slower ramps (down to 0.5 °C min−1), thus excluding kinetic origins (vide infra). Such a continuous specific heat variation for a single-component system means that the internal energy continuously changes in the mesophase domain, limited by TH and TF, likely over a progressive and reversible evolution of the interactions between 5327


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series (Figure 1, Table 1). These differences can be explained by the properties of each block, which are supposed to reproduce roughly those of the corresponding F and H alkanes. Therefore, C8F18 melts to the isotropic liquid at −25 °C (i.e., far below H alkanes (from 6 to 36 °C from C14H30 to C20H42)), but C10F20 clears at 36 °C (i.e., above alkanes (Figure 3)). The isotropization temperatures of the tetrablocks lie in both cases beyond those of the individual blocks, and the influence of the aliphatic block then appears to be logically more important with respect to the fluorinated chain stiffening at low temperature. This effect is compatible with the predominant role of the fluorinated moieties in the persistence of the high-temperature phase (MHT), as suggested by the variations of TF, ΔHF, and ΔSF in the di(F10Hm) series. In contrast, the TH values, limiting the low-temperature phase (MLT), decrease in a comparable manner with both chain lengths in the di(F8Hm) series, following the trend of the individual block properties. Quite noteworthy, and for both series of tetrablocks, is the regular decrease in the MHT mesophase temperature range as the portion of the aliphatic block increases, with respect to the expansion of the domain of the MLT phase (Figure 5). This behavior is likely reflected by the tendency of the aliphatic chains to be more crystalline as their length increases, thus further stabilizing the MLT phase. Very similar thermal profiles were observed for the various di(FnHm) samples of solution-crystallized material and for samples cooled from the melt. No additional transitions were observed in the former material. Thus, unlike for the F12H20 diblock,26 the thermal history of the sample does not appear to have a significant impact on the thermal behavior of the di(FnHm) tetrablocks. B. Dilatometry Studies. The molecular volumes of di(F10H20) and di(F10H16) were measured as a function of temperature (Figure 6a,b) in order to provide information relative to the molecular packing of the compounds in the two identified phases. The experiments were performed between 25 and 100 °C, a range that spans the two phase transitions. The complete reversibility of the changes in volume was assessed by running two successive heating−cooling cycles. The molecular volume versus temperature profiles confirm the existence of two phase transitions for both di(F10H20) and di(F10H16). These temperatures are slightly higher than the ones determined by DSC because of the spreading of the transition over an extended range (i.e., dilatometry detects the end of the transition on heating and the start on cooling, which are different from the DSC onset temperatures; Figure 1).

Figure 4. DSC thermograms recorded upon cooling for F8H20 and di(F8H20).

In the di(F10Hm) series, the molar enthalpies (ΔHH, ΔHF) and entropies (ΔSH, ΔSF) recorded at temperatures TH and TF show clearly opposite dependences upon chain length, with ΔHH and ΔSH increasing steeply with increasing aliphatic content (Table 1). This confirms that the high-temperature transition of the tetrablocks is primarily associated with the disordering of the Fn chains, acting as mesogenic parts, whereas the low-temperature event is mainly driven by the influence of the alkyl chain length, tuning the microphase segregation and the gaps between fluorinated domains. Moreover, at these lower temperatures, a partial crystallization of Hm segments might occur and distort the segregated structure. A complete crystallization of chains is obviously excluded because the process releases around 4.1 kJ per mole of methylene groups in paraffins and liquid crystal series,37 which largely exceeds the measured enthalpy changes (0.1 to 1.4 kJ per mole of CH2). Nevertheless, these values would be compatible with the onset of an intermediate state of ordering between Hm segments, as in mesophase organizations with hexagonally crystallized aliphatic chains. (For the related rotator phases of n alkanes, enthalpy changes associated with the transition to the liquid are a little larger: ∼2.2 kJ per mole of CH2.38) Different trends appear for the di(F8Hm) series. The low-temperature transitions provide similar ΔHH and ΔSH, whatever the length of the Hm moiety, and ΔHF and ΔSF are reduced for the lowest aliphatic content, contrary to the di(F10Hm) series. This phenomenon occurrs in conjunction with the collapse of transition temperatures, and TF is quite constant in the other

Figure 5. Diagrams of the mesomorphic behaviors of both series of tetrablocks [(a) di(F10Hm) and (b) di(F8Hm)] as a function of the aliphatic chain length and temperature during both heating (filled symbols) and cooling (open symbols). TH (circles) and TF (triangles); MHT and MLT refer to the structures identified above and below TH, respectively, and Iso refers to the isotropic state (above TF). 5328


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Figure 6. Variation of the molecular volume of (a) di(F10H20) and (b) di(F10H16) as a function of temperature, as determined by dilatometry. Red and blue symbols refer to the heating and cooling scans, respectively. Partial volume (c) VCF2 and transverse cross section (d) σF in MHT and isotropic phases for di(F10H20) (black squares) and di(F10H16) (red discs); partial volume (c) VCH2 and transverse cross section (d) σH in MLT. (Black open squares) MHT and isotropic phases for di(F10H20). (Blue dotted line) Comparison with the variation in the isotropic phase for the FnHm diblock series. (Black dotted line) Extrapolation of VCF2 used for calculation.

The progressive melting of the fluorine chains is best evidenced by the variation of the partial volume of CF2 groups, VCF2, which is directly deduced from dilatometry curves by subtracting from the molecular volume the contribution of molten aliphatic chains and dividing by the total number of CF2 and CF3 groups (with the volume ratio of CF3 to CF2 having been considered in the calculation).39 The minimum section of fluorinated chains σF then results from the ratio of VCF2 and the average distance between CF2 units along the stretched chain axis (1.31 Å measured on crystal structures from the Cambridge database) (Figure 6c,d). The first molecular volume jump (at TH) is rather sharp, particularly in the case of di(F10H20), and occurs within a few degrees and with a relatively small molecular volume increase. Only limited superfusion is seen (Figure 6a,b, open symbols). This first volume jump represents the melting of the Hm blocks. Starting at ∼53 °C for di(F10H20) and at ∼33 °C for di(F10H16), that is, at their TH values, a gradual increase in molecular volume is observed. This behavior is in agreement with the view that the Hm blocks of the molecules are totally molten whereas the Fn chains lose their organization progressively over a much larger temperature range (over ∼20 °C) than the Hm chains. This gradual melting of the Fn chains is corroborated by 19F NMR studies of fluorinated phospholipid bilayers and has been explained by the successive onset of motions of the CF3 and CF2 groups along the Fn chains.40 Clearing is encountered in the dilatometry profiles when the temperature reaches ∼75 °C for di(F10H20) and ∼71 °C for di(F10H16). Thermal agitation then becomes too high, and the Fn chains melt completely.

A knowledge of the molecular volumes, Vmol, measured between TH and TF for di(F10H16) and di(F10H20) helps, in conjunction with small-angle X-ray diffraction studies, to determine the organization of the tetrablocks in the various phases. Partial volumes and minimum sections in the isotropic phase and their dependences upon temperature determined for both tetrablocks are close to those deduced from empirical relationships on the diblock FnHm series,39 confirming the ratio of 1.5 between cross sections of fluorinated and aliphatic chains in the molten state (roughly 32 vs 21 Å2, close to that at room temperature). Below the isotropization, this ratio is lowered by the continuous decrease in σF as the crystallization process of fluorinated tails goes on. In the depth of the MHT domain, the process progressively stops with the approach of an asymptotic σF value (∼27 Å2) close to the cross sections in room-temperature crystal phases of perfluoroeicosane (σF = 28.1 Å2)41 and PTFE (σF = 27.7 Å2).42 When the transition to the MLT phase is reached, the alkyl chains start to crystallize in turn. In di(F10H16), this happens very close to room temperature and the process freezes readily, as shown by the small degree of contraction and in agreement with the smaller enthalpy changes noticed for the short-chain terms of the series (see above). In contrast, the transition occurs at substantially higher temperature in di(F10H20) and gives rise to a progressive contraction resembling that in the MHT domain. Close to room temperature, σH then approaches the cross sections of hexagonally crystallized chains in mesophases (σH = 20.0 Å2)43 and of long-chain n-alkanes in rotator phases (σH = 19.2 to 20 Å2).44,45 To resume, the DSC peak asymmetry and the nonlinear specific volume variations are explained by the progressive 5329


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Table 2. Indexation of the Reflections Collected by SAXS for the Di(FnHm) Tetrablocks above TH dmeas (Å)a

00lb

I (a.u.)c

di(F8H16): MLT 10 °C; MHT 47 °C; 47.92 001 VS (sh) (23.9) D+(002) M (br) 16.08 003 M (sh) 12.04 4.0−5.5

004

S (sh) VS (br)

di(F8H18): MLT 29 °C; MHT 54 °C; 52.56 001 VS (sh) (26.08) D+(002) M (br) 17.32 003 W (sh) 12.98 4.92 4.0−5.5

004

M (sh) W (sh) VS (br)

dcalc (Å)a Iso 48.11 (24.05) 16.03 12.03 hFliq + hHliq

Iso 51.94 (25.97) 17.31 12.99 hF hFliq + hHliq

di(F8H20): MLT 40 °C; MHT 60 °C; Iso 55.05 001 VS (sh) 54.99 27.55 002 S (sh) 27.49 27 D M (br) 18.16 13.81 11.04 4.9 4.0−5.5 a

003 004 005

W (sh) S (sh) W (sh) W (sh) VS (br)

18.33 13.75 10.99 hF hFliq + hHliq

parametersd

dmeas (Å)a

T = 30 °C (MHT) d = 48.11 Å Vmol = 1576 Å3 (ρ = 1.356 g cm−3) f F = 0.412 Amol = aF = aH = 32.8 Å2 σF = 27.7 Å2 σH = 21.4 Å2

00lb

I (a.u.)c

di(F10H16): MLT 32 °C; MHT 70 °C; 54.03 001 VS (sh) (27.0) D+(002) M (br) 18.04 003 M (sh)

Iso 53.98 (27.0) 17.99

13.45 5 4.0−5.5

13.49 hF hFliq + hHliq

004


di(F10H18): MLT 46 °C;MHT 69 °C; 58.78 001 VS (sh) (29.34) D+(002) M (br) 19.61 003 M (sh)


14.68 5.11 4.5−5.5


dcalc (Å)a

004

Iso 58.77 (29.39) 19.59



di(F10H20): MLT 58 °C; MHT 68 °C; 60.6 001 VS (sh) (30.4) D+(002) M (br) 20.11 003 M (sh)

Iso 60.57 (30.28) 20.19

15.2 4.95 4.5−5.5


004


parametersd T = 50 °C (MHT) d = 53.98 Å Vmol = 1779 Å3 (ρ = 1.408 g cm−3)* f F = 0.463 Amol = aF = aH = 33.0 Å2 σF = 28.0 Å2 σH = 21.7 Å2 T = 50 °C (MHT) d = 58.77 Å Vmol = 1863 Å3 (ρ = 1.385 g cm−3) f F = 0.432 Amol = aF = aH = 31.7 Å2 σF = 27.5 Å2 σH = 21.7 Å2 T = 60 °C (MHT) d = 60.57 Å Vmol = 1977 Å3 (ρ = 1.342 g·cm−3) f F = 0.408 Amol = aF = aH = 32.6 Å2 σF = 27.6 Å2 σH = 21.9 Å2

dmeas and dcalc are the measured and calculated diffraction spacings [d = dcalc = (Σ00l × d00l)]/Nl. hH + hF and hF are the maxima of the diffuse scattering due to lateral distances between molten and/or partially aliphatic and fluorinated tails. bMiller indices. cI corresponds to the intensity of the reflections (VS, very strong; S, strong; M, medium; W, weak; VW, very weak; br and sh stand for broad and sharp). dPhase and molecular parameters: d is the lamellar spacing; Vmol is the molecular volume determined by dilatometry (*) or calculated; Amol = Vmol/d is the molecular area; aH = aF = 2Amol/2 is the molecular area per chain; σF is the cross section of the half-crystallized fluorinated chains deduced from the dilatometry study; σH represents the cross sections of the molten aliphatic chains from reference dilatometry according to σH = 20.91 + 0.01593 × T (with temperature T in °C); f F is the volume fraction of the fluorinated moiety; D is additional diffuse scattering discussed in the section following the structural analysis of the MLT phase.

All of the di(FnHm) tetrablocks exhibit fibrous textures. It is known that F12H20 can crystallize in well-aligned bundles with fibrillar morphology.26 The fibers are typically several hundred micrometers in length and have a rather constant diameter of ∼1 μm. Although the fibers formed by di(F8H14) and di(F8H16) tend to aggregate in densely interwoven arrays, those of di(F8H12) appear to be more isolated, possibly because the material is primarily involved in smaller fibers that are below the resolution of optical microscopy. Fiber melting was determined by microscopy to occur at temperatures in good agreement (TF ± 2 °C) with the heating-scan DSC data. For all of the tetrablocks, melting led to clear, isotropic liquids. An investigation of the morphology of the fibers below TH for di(F10H20), di(F10H18), and di(F8H20) did not reveal any obvious morphological change. Observations below TH could not be achieved for di(F10Hm) with m = 16 and 14 and for di(F8Hm) with m = 18, 16, 14, and 12 because of water condensation at the low temperatures required for these compounds. D. Mesophase Structures and Small-Angle X-ray Scattering Studies (SAXS). X-ray scattering patterns were collected at various temperatures for di(F10Hm) and di(F8Hm) with m = 16, 18, or 20 in order to determine the various mesophase structures. For five compounds, diffractograms were acquired both below and above the transition temperature TH, corroborating the existence of two mesophases, hereafter referred to as the low-temperature phase

melting/crystallization of fluorinated chains and aliphatic chains below the upper and lower transition temperatures, respectively. The Hm chains are thus still molten in the MHT phase that can therefore be considered to be a mesophase, whereas the process stops at an incomplete crystallized stage in the MLT phase. The molecular volumes of di(F10H20) and di(F10H16) were provided by dilatometry studies and were also used in the calculations of the molecular volumes of the other tetrablocks, hypothesizing the same temperature variations of the volume for all of the terms of the series, differing only in the number of fluoromethylene and methylene groups according to the procedure described in the experimental part (Supporting Information). In summary, the volume in the isotropic phase was classically determined by using dilatometric reference measurements and assuming partial molecular volume additivity. Then, both dilatometries were fitted by an empirical expression of the partial volume contraction as a function of temperature and both transition temperatures, and this mathematical relationship was then used to correct all molecular volumes from the estimated contractions in MHT and MLT domains. All of the Vmol values are collected in Tables 2 and 3. C. Optical Microscopy. Optical microscopy was used to observe the morphology of the di(FnHm) gemini diblocks in the bulk. The micrographs presented in Figure 7 were performed between TH and TF. 5330


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Table 3. Indexation of the Reflections Collected by SAXS for the Di(FnHm) Tetrablocks below TH di(F8H18)

di(F8H20)

di(F10H16)

di(F10H18)

dmeas (Å)a

hkb

I (au)c

49.05 37.87 28.7 26.89 16.85 14.05 12.4 4.97 4.0−5.5 4.13 52.33 44.75 27.41 22.01 17.7 14.94 13.58 11.03 10.7 8.95 4.97 4.0−5.5 4.13 52.13 39.56 29.74 28.8 17.66 15.55 13.38 4.96 4.0−5.5 4.18 52.48 50.23 30.54 18.06 14.5

11 = 20 21 02/31 12 33/60 04/62 44/80

VS (sh) M (sh) M (sh) M (sh) M (sh) W (sh) M (sh) M (sh) M (br) M (sh) VS (sh) S (sh) M (sh) W (sh) W (sh) W (sh) M (sh) VW (sh) VW (sh) VW (sh) M (sbr) W (br) W (sbr) VS (sh) M (sh) M (sh) W (sh) W (sh) W (sh) W (sh) M (sh) W (br) W (sh) VS (sh) S (sh) M (sh) M (sh) W (sh)

11 20 31 40 33 60 53/62 73/64 06 75/66

11 = 20 21 02/31 12 33/60 04/62 44/80

11 20 02 33 71/53 24/62

4.95 4.0−5.5 4.14 di(F10H20)

52.75 48.33 32.11 19.27 17.83 15.03 4.94 4.0−5.5 4.16

11 20 02 42 33 24

dcalc (Å)a 37.1 28.3 27.2 16.35 14.16 12.26 hF hFliq + hHliq hH

27.08 22.37 17.44 14.91 13.75/13.54 11.0/10.95 10.75 9.08/8.72 hF hFliq + hHliq hH 39.41 30.1 28.92 17.38 15.05 13.03 hF hFliq + hHliq hH

M (sh) M (br) W (sh)

30.77 17.49 14.35/ 14.7 hF hFliq + hHliq hH

VS (sh) M (sh) W (sh) M (sh) M (sh) M (sh) M (sh) W (br) W (sh)

31.47 19.17 17.58 14.96 hF hFliq + hHliq hH

parametersd T = 20 °C (MLT) a = 98.10 Å b = 56.60 Å Vmol = 1593 Å3 (ρ = 1.4 g cm−3) f F = 0.389 S (Z = 2) = 5552 Å2 σFWA = 28.6 Å2 σHWA = 19.7 Å2 ⟨hF⟩ = 5.22 Å NF = NH = 18.2 T = 20 °C (MLT) a = 89.50 Å b = 64.50 Å Vmol = 1693 Å3 (ρ = 1.37 g.cm−3) f F = 0.361 S (Z = 2) = 5773 Å2 σFWA = 28.5 Å2 σHWA = 19.7 Å2 ⟨hF⟩ = 5.21 Å NF = NH = 17.8

T = 20 °C (MLT) a = 104.26 Å b = 60.20 Å Vmol = 1642 Å3 (ρ = 1.50 g cm−3) f F = 0.469 S (Z = 2) = 6276 Å2 σFWA = 28.4 Å2 σHWA = 20.2 Å2 ⟨hF⟩ = 5.20 Å NF = NH = 19.9 T = 20 °C (MLT) a = 104.60 Å b = 61.54 Å Vmol = 1744 Å3 (ρ = 1.47 g cm−3) f F = 0.435 S (Z = 2) = 6182 Å2 σFWA = 28.3 Å2 σHWA = 19.8 Å2 ⟨hF⟩ = 5.20 Å NF = NH = 18.4 T = 30 °C (MLT) a = 96.66 Å b = 62.95 Å Vmol = 1855 Å3 (ρ = 1.43 g cm−3) f F = 0.416 S (Z = 2) = 6085 Å2 σFWA = 28.2 Å2 σHWA = 20.0 Å2 ⟨hF⟩ = 5.19 Å NF = NH = 17.0

a dmeas and dcalc are the measured and calculated diffraction spacings [d = dcalc = (Σ00l × d00l)]/Nl. hH + hF and hF are the maxima of the diffuse scattering due to lateral distances between molten and/or partially aliphatic and fluorinated tails. bMiller indices. cI corresponds to the intensity of the reflections (VS, very strong; S, strong; M, medium; W, weak; VW, very weak; br and sh stand for broad and sharp). dsbr, slightly broad; S, area of the bidimensional lattice containing Z ribbons; σFWA and σHWA are cross sections of hexagonally packed fluorinated and aliphatic chains from WAXS reflections hF and hH according to σ = 2/31/2 × h2; ⟨hF⟩ is the average distance toward the first neighbor’s shell according to ⟨hF⟩ = 0.9763(σ1/2);49 NF = NH is the average number of fluorinated and aliphatic chains associated in a ribbon section, according to N = S/Z × ⟨hF⟩/(Vmol/2).

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Figure 7. Optical micrographs (scale bar, 10 μm; enlargment, ×40) of (a) di(F10H20) at 60 °C, (b) di(F10H18) at 50 °C, (c) di(F10H16) at 40 °C, (d) di(F10H14) at 25 °C, (e) di(F8H20) at 50 °C, (f) di(F8H18) at 30 °C, (g) di(F8H16) at 20 °C, and (h) di(F8H14) and (i) di(F8H12) at 15 °C.

alternating sublayers formed by Fn and Hm blocks. Moreover, the intensities of the (002) and (003) reflections are inverted in the series, likely in relation to the fluorinated volume fractions lying between 1/3 and 1/2 (Table 2). As a matter of fact, in a simple two-state model with flat interfaces and high- and lowelectronic-density sublayers associated with fluorinated and hydrogenated moieties, respectively, the structure factor cancels for the second order, for a volume fraction of 1/2, and for the third order, for a volume fraction of 1/3, reproducing roughly the variation in the series. However, the model fails to reproduce the enhanced intensity of the (004) reflection, indicating that the real density profile is more complicated. Structure factor modulations resembling the experimental intensity modulation could be generated by introducing mixing zones between both sublayers, but this approach was not developed further because of the many ways to modify density profiles with respect to the zone bridging both moieties, the molten and crystallized zones, and also the sublayer undulations. Indeed, the small-angle region contains an additional weak diffuse scattering (periodicity D between 25 and 30 Å for a correlation length on the order of 100 Å), which is more or less visible because of the overlap with the (002) reflection. Presumably, this modulation of the electronic density is due to the different sections of fluorinated and aliphatic moieties, with the adaptation near the interface generating sublayer undulations by the same mechanism as in smectic phases of polycatenar materials.46,47 Except for di(F8H16), the five other tetrablocks give rise to a wide-angle region composed of the overlapping of the diffuse scatterings due to molten aliphatic chains and fluorinated chain segments (hH and hF) with a sharp summit becoming narrower and more intense at lower temperature. The location of this sharp summit (between 4.9 and 5.1 Å) coincides with the location of the first-order reflection of hexagonally crystallized chains (4.94 Å in crystalline perfluoroeicosane, vide supra). Conversely, both types of chains are completely molten in

(MLT) and the high-temperature phase (MHT) before clearing into the isotropic molten liquid (Supporting Information). The sixth compound, di(F8H16), shows a different high-temperature phase (MHT′) with the same average segregated structure but with a lower symmetry as a result of a less-well-defined organization of fluorinated tails that could not be elucidated. Moreover, DSC reveals for this compound the existence of an MLT phase below room temperature (i.e., beyond the measuring range of our equipment). 1. Structure of the Mesophase above TH (MHT). The SAXS patterns of the tetrablocks recorded on cooling from the isotropic liquid in the high-temperature interval delimited by TF and TH, thus corresponding to the MHT phase domain of stability (Figures 8 and

Figure 8. Small-angle X-ray scattering plot of di(F10H20) measured at 60 °C, chosen as a representative example of the MHT mesophase.

S1−S5), are all very similar for the six compounds investigated, suggesting little influence of the chain length on the structure. These diffraction patterns all show consistently at least three sharp small-angle reflections corresponding to the (00l, l = 0−4) Bragg reflections of a lamellar organization. Up to five lamellar orders could be detected in one case, di(F8H20), indicating that sharp interfaces separate the 5332


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identical terminal chains, the area per chain is then simply given by achain = aH = aF = 2 × Amol/2 = Amol (Figure 9).

the isotropic phase, and both contributions overlap in a unique scattering signal with a rounded shape. In the MHT phase, the scattering signal of the fluorinated chains is thus intermediate between the hump shape typical of liquidlike ordering and the isolated sharp reflection of a hexagonal cylinder packing, confirming the progressive “crystallization” of the fluorinated chains deduced from dilatometry and DSC. This crystallization occurs without setting up the correlation in a position between fluorinated tails of different sublayers, as shown by the absence of any further reflection in the WAXS region of the five tetrablocks. This feature makes the MHT structure analogous to a smectic B phase in which the partially crystallized fluorinated chains would play the role of the mesogens. The case of di(F8H16) is particular, where numerous badly defined reflections overlap in the WAXS region. This reveals the setting up of longrange correlations between positions of fluorinated parts across the molten aliphatic sublayers and suggests the classification of the phase among the mesophases with 3D ordering.43 Above TH, the Fn segments stay rigid whereas the Hm chains have melted. Therefore, in terms of a reduction of the energy mismatch between the two amphisteric moieties, the lamellar packing is the most favorable because of the relatively freeflowing melted Hm chains. Although the SAXS data definitely indicate layered arrangements, the repeat distance does not correspond to the calculated contour length of the molecules (lcal) or to twice lcal (Table S1). This discrepancy trivially results from the difference in bulkiness of both molecular moieties: the tetrablocks adopt the bilayer arrangement optimizing the microsegregation and in doing so impose their minimum cross section on the whole layer stack. Within their sublayers, the liquidlike aliphatic tails spread to compensate for their smaller cross section and fill the entire space between fluorinated sublayers (Scheme 2). This spreading of the

Figure 9. Molecular area per chain achain vs temperature in the MHT and MHT′ phases of the di(FnHm) series (n, m = 8, 16, black squares; 8, 18, red discs; 8, 20, dark-green up triangles; 10, 16, blue down triangles; 10, 18, magenta diamonds; 10, 20, purple crosses). Comparison with the cross sections σF of fluorinated chains in the molten state (dark-green dashed line) or hexagonally crystallized state (blue dotted line) and with the cross section of molten aliphatic chains σH39 (black mixed dotted and dashed line).

Experimental areas turned out to be nearly constant throughout the series (32.4 to 34 Å2) with a small thermal expansion (∼13 × 10−4°C−1) of about twice the volume expansion in the isotropic phase (∼7 × 10−4 °C−1). Surprisingly, these areas and expansion coefficients are the same as if the fluorinated chains were molten in the MHT phase, in contradiction to the experimental and theoretical evidence that the mesophase properties appear only when the crystallization starts and confers to the system a minimum of rigidity. This naturally implies that the crystallization process does not occur homogenously but that a portion of the fluorinated moiety stays in the liquid state whereas another portion stiffens in a conformation and packs with a compactness similar to that of a crystal. The larger bulkiness of the liquid portion would then logically determine the minimum molecular area of the system, and the crystallized amount would compensate for the discrepancy in sections by random tilting or by the formation of islands surrounded by a molten periphery. Experimental data do not provide insight into the nature of molten and crystallized domains, but it is tentatively suggested that the fluorinated chain portion close to the aliphatic sublayer crystallizes/melts at a different temperature than the internal portion of the sublayer. This proposition follows the certitude that interfaces between sublayers cannot be completely flat and may even undulate, as suggested by the additional diffuse scattering in the small-angle region (D, Figure 8, Table 2). In the zone close to the interface, both types of chains would then partially mix and therefore exhibit another “stiffening” temperature as in the bulk of the sublayers. In the average structure, there should not be defined zones but a progressive trail away from the influence of the interface with its distance in the sublayer. This would then lead to a dissymmetric blurring of the transition, as observed by DSC, dilatometry, and SAXS, and to the persistence of a molten stratum in the molecular area during the whole “crystallization” process. This interpretation would accurately fit all results but remains speculative in the absence of further experimental verification. 2. Structure of the Solid Phase below TH (MLT). When the MLT phase is entered, the patterns change drastically. In the smallangle region, the first order reflection shifts toward wide angles by

Scheme 2. Model of Molecular Organization in the MLT Phase (Left) and MHT Phase (Right) of Di(FnHm)a

The fluorinated moiety is drawn in blue, and the aliphatic moiety is drawn in grey. MLT phase: fluorinated bilayers refold and disrupt aliphatic bilayers into rows of ribbons arranged in a rectangular p2gg lattice involving two sublayers and two ribbons. Both types of chains are crystallized, with a large tilt angle inverting from row to row, between the aliphatic chains’ axes and lattice vector a. MHT phase: molten aliphatic chains spread to fill the space between the still partially crystallized fluorinated bilayers, giving rise to an alternation of continuous bilayers of periodicity d. An additional diffuse reflection D is attributed to peristaltic undulations reminiscent of the sublayer disruptions in the MLT phase. a

aliphatic medium reduces the sublayer thickness in proportion and causes the discrepancy between 2 × lcal and d. The spreading extent of the aliphatic sublayers and the variation of the molecular length both determine the layer spacing, but the former contribution can be evaluated separately in the molecular area, consisting of the ratio of the molecular volume to the layer spacing: Amol = Vmol/d. For a bilayer arrangement with two 5333


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remaining a single peak (di(F8H18), di(F10H16)) or splitting into a doublet (di(F8H20), di(F10H18), and di(F10H20)), whereas the appearance of supplementary sharp higher-order reflections evidence the interruption of the lamellae into objects arranged in a lattice of higher dimensionality (Figures 10 and S1−S5, Table 3).

Figure 11. Areas of the fluorinated sublattice σF (red discs) and of the aliphatic sublattice σH (black squares) from WAXS signals in the MLT phase. Variation for molten fluorinated chains (blue dotted line) and molten aliphatic chains (black mixed dotted and dashed line). Figure 10. Small-angle X-ray scattering plot of di(F10H20) measured at 30 °C, chosen as a representative example of the MLT mesophase.

and rodlike. The geometric constraints here become analogous to polycatenar calamitic systems when the overall aliphatic section exceeds the section of the rodlike mesogens. In these reference systems, the tilting of the mesogens realizes the compensation up to a certain discrepancy, but beyond this limit, mesogen sublayers are periodically disrupted, most often into unidimensional ribbons arranged in a bidimensional lattice.46−48 As the reference mesogens, the crystallized aliphatic tails have to tilt apart from the layer normal imposed by the fluorinated sublayers in order to realize the adaptation of projection areas in the plane of the layers. The tilting preserves lamellae but competes with a regular lateral packing between crystallized aliphatic chains, which may finally compromise by setting up periodic sublayer disruptions. Indeed, these disruptions are evidenced here by the small-angle region of patterns, indexed in a bidimensional lattice containing two ribbons with an uncentered rectangular symmetry, likely p2gg, as deduced from the presence of numerous reflections with noninteger (h2 + 3k2)/4 and the absence of reflections (h0) and (0k) with odd h or k. The first terms of both series, di(F8H18) and di(F10H16), show pseudohexagonal geometry, but all other tetrablocks show lattices shrunken along lattice vector a, with an opposite influence of both chain lengths on the lattice parameter values. (Shortening aliphatic chains and lengthening fluorinated chains systematically expand the lattice along a.) This evolution of the lattice geometry suggests that fluorinated chains point in a direction close to the a vector whereas aliphatic chains tilt apart with a large angle, as known from the different cross section of both crystallized chains. Along the ribbons, the distance between molecules should at least be the average between the most bulky fluorinated fragments ⟨hF⟩, deduced from the wide-angle reflection (Table 3). The number of chains of each type associated in a ribbon section, NF = NH, can then be evaluated by comparing the volume of a slice of ⟨hF⟩ thickness to Vmol. The calculation gives chain numbers between 17 and 20 (i.e., between 8.5 and 10 couples of chains involved in bilayers). Scheme 2 summarizes these results in a model of packing within the bidimensional lattice plane. In this view, the disrupted aliphatic bilayers form rows of ribbons alternating with continuous fluorinated bilayers, which refold in between ribbon rows. Fluorinated chain axes distribute around the a vector, and aliphatic chains point in strongly tilted directions, reducing the symmetry to p2gg. This structure of disrupted lamellae is clearly related to the rigid rod conformation of crystallized chains, selecting this particular type of area

The wide-angle region is now constituted of two quite sharp reflections dominating a residual scattering of molten parts. These WAXS signals are the only ones visible in the di(F10Hm) series and characterize the 2D organization within sublayers, excluding positional correlations between fragments from different sublayers. On the contrary, the WAXS region contains additional weak and badly defined reflections for both di(F8Hm) compounds, revealing the presence of such positional correlations. These latter structures, as the MHT′ structure of di(F8H16), are therefore reminiscent of 3D crystals, and the structures of the di(F10Hm) tetrablocks can be considered to be real mesophase organizations of segregated domains. Concerning the organizations inside domains, the lowestangle peak is quite strong and sharp in all patterns (Scherrer correlation length ξ between 130 and 210 Å in the di(F10Hm) series). Its location (hF ≈ 4.95 Å) coincides with the sharp WAXS summit in the MHT phase and with the first-order reflection in crystalline perfluoroeicosane, in agreement with its assignment to the sublattice formed by the packing of fluorinated chains. The second quite sharp peak (hH ≈ 4.15 Å; intensities 5 to 10 times smaller, ξ between 70 and 100 Å in the di(F10Hm) series) coincides with the first-order reflection of hexagonally crystallized chains and is attributed to the lateral packing within aliphatic domains. In comparison to molten chains, the lattice areas are considerably shrunken and explain the unusual dilatometry traces regarding the 10% volume contraction with respect to the isotropic liquid (Figure 11). Indeed, in common mesophase organizations with molten chains, volume jumps are on the order of 1% or less.43,48 Nevertheless, experimental data also show that a fraction of molten chains subsist in the MLT phase range. As expected from the transitions occurring at lower temperature, this amount is larger for the tetrablocks with short chains, which traduces in SAXS patterns a slightly stronger residual diffuse scattering and a slightly weaker hH peak. Although a direct comparison with SAXS is not pertinent because of the different time scale of the experiments, the residual molten amount is also traced in DSC by the reduced enthalpy change determined for tetrablocks with short chains. In the MHT phase, the liquidlike aliphatic chains spread to compensate for the discrepancy of the section with the fluorinated moiety, but this mechanism of adaptation is excluded in the MLT phase because chains are mainly crystalline and thus stiff 5334


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adaptation. However, the disruption is only the ultimate state of compensation involving peristaltic undulations,46 which may pre-exist in the MHT phase in association with the spreading of the molten chain. This type of undulations is likely at the origin of the D scattering in MHT patterns, which is all the more supported by the closeness of its location (between 25 and 30 Å) to that of the (002) reflection in MLT structures. E. Freeze−Fracture Transmission Electron Microscopy. The bulk of the waxy di(F8H16) solid was investigated by freeze−fracture TEM, which revealed a lamellar morphology. All of the images were obtained at room temperature, that is to say, in the high-temperature phase, and showed large domains of a well-developed lamellar phase with long-range order (Figure 12).

Article

ASSOCIATED CONTENT

S Supporting Information *

Materials and Methods, including X-ray patterns of the di(FnHm) tetrablocks in their different phases. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +33 3 88414060. Fax: +33 3 88414099. E-mail: krafft@ unistra.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Marc Schmutz (ICS, Strasbourg) for the electron microscopy and the Centre National de la Recherche Scientifique (CNRS) for support. C.d.G.-L. acknowledges the Région Alsace for research fellowships and the GIS-Fluor for a travel grant.

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REFERENCES

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Figure 12. (a) Transmission electron microscopy of a replica taken from a freeze−fracture surface of the di(F8H16) tetrablock above TH (MHT). (b) Fourier transform of image a.

The main periodicities were found to be ca. 80−85 and 40−45 Å. The second distance, which can be measured in two distinct orientations, is in agreement with the SAXS periodicity of the layered structure, at the precision of the technique. Intuitively, the double periodicity viewed by freeze−fracture should be related to the additional diffuse scattering in the small region. This signal is attributed to peristaltic undulations in the plane of layers that should effectively double the periodicity (Scheme 2). However, this doubling is invisible by SAXS because odd lamellar order reflections of double periodicity would be extinct by symmetry.

III. CONCLUSIONS We have determined the structure and thermal properties of two sets of gemini fluorocarbon/hydrocarbon diblock amphiphiles [(C n F 2 n + 1 CH 2 )(C m − 2 H 2 m − 3 )CH−CH(C n F 2 n + 1 CH 2 )(Cm − 2H2m − 3) (n = 8 or 10; m = 12, 14, 16, 18 or 20)]. The thermal behavior of the various terms of the series essentially reveals two exothermal transitions, the first one corresponding to the melting of the hydrocarbon chains and the second one associated with the melting of the fluorocarbon chains and the clearing of the materials into the isotropic liquid. In the lowtemperature range, a disordered plastic rotator phase, MLT, and above a lamellar phase, MHT, were confirmed by SAXS experiments. The structure of the MHT phase consists of segregated sublayers of Fn and Hm blocks and resembles a smectic B type of organization. The structure of the MLT phase consists of rows of ribbons resulting from the partial crystallization of the aliphatic blocks on lowering the temperature and then their reduced ability to spread or tilt to compensate for the larger bulkiness of the fluorinated moiety. For both phases, the Fn segments form a continuum. The large domains of a wellordered lamellar phase with a periodicity of ∼40−45 Å observed by freeze−fracture TEM for di(F8H16) also support these models for the supramolecular organization of the tetrablocks. 5335


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