1598
J. Phys. Chem. B 2002, 106, 1598-1605
Investigation of the Solid State Behavior of a Semifluorinated n-Alkane by Means of NMR, Calorimetric, and Dielectric Techniques Marco Geppi, Silvia Pizzanelli, and Carlo Alberto Veracini* Dipartimento di Chimica e Chimica Industriale, UniVersita` degli Studi di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
Camillo Cardelli and Elpidio Tombari Istituto di Fisica Atomica e Molecolare, CNR, Via G. Moruzzi 1, 56100 Pisa, Italy
Pierandrea Lo Nostro Dipartimento di Chimica and CSGI, UniVersita` degli Studi di Firenze, Via Gino Capponi 9, 50121 Firenze, Italy ReceiVed: July 9, 2001; In Final Form: December 3, 2001
The solid-state behavior of the semifluorinated n-alkane C8F17-C16H33 (F8H16) has been characterized by temperature-modulated calorimetry, dielectric spectroscopy, and 1H and 19F nuclear magnetic resonance. Three solid-solid phase transitions were observed below the melting point (325 K) at about 258, 275, and 297 K. Wide-line 1H and 19F NMR suggests that the first transition is related to the onset of a dynamic process occurring in the hydrocarbon chain, which is, however, mainly involved in the central transition. Contributions from the fluorinated segment to the transitions at 275 and 297 K can be revealed. NMR relaxation times, temperature-modulated calorimetry, and dielectric spectroscopy agree in suggesting that the motional processes activated at the phase transitions remain quite slow in the whole solid range and are characterized by maximum frequencies of the order of some mHz.
Introduction Semifluorinated n-alkanes CnF2n+1-CmH2m+1 (usually indicated as FnHm) are made up of a hydrogenated and a fluorinated chain connected by a covalent bond. Hydrocarbons (HCs) and fluorocarbons (FCs) do not mix below certain temperatures but phase separate because of their different molecular conformations: in fact, FCs are arranged in either 15/7 or 13/6 helices depending on temperature,1,2 whereas HCs possess the wellknown typical zigzag all-trans conformation. In FnHm, the two moieties are covalently bound, and this peculiar molecular architecture results in particular properties, both in the solid and in the liquid states. These short diblock copolymers can show several solid-solid phase transitions as well as liquid crystalline smectic phases.2,3 The two chains establish different interactions with selective solvents; in fact, the fluorinated moiety will dissolve in the fluorocarbons, whereas the hydrogenated segment will mix with other hydrocarbons. As a result of such incompatibility between fluorocarbons and hydrocarbons, FnHm molecules are able to produce supramolecular micellar-like structures and gels both in HC and FC fluids, with a well-defined microsegregation of the two moieties.4,5 Being formed by two incompatible chains, FnHm have been named as “primitive” surfactants, because they possess the molecular quintessence of amphiphiles.6 Moreover, phase diagrams and static light scattering experiments show that the addition of a certain amount of FnHm to a FC/HC mixture significantly lowers the phase separation * To whom correspondence should be addressed. Fax: +39-050-918260. E-mail:
[email protected].
temperature of the liquid, indicating that semifluorinated nalkanes reduce the interfacial energy between the two fluids.4,7 The structure and phase behavior of FnHm in the solid state have been extensively investigated by different authors through Raman spectroscopy, SAXS, WAXD, DSC, birefringence, and electron microscopy; however, further studies are still needed in order to get a clear picture of the molecular organization in the condensed phase. For example, pure F10Hp (with 9 e p e 11) and F(CF2)9CF2-CH2-CHI-(CH2)q-2H (with 6 e q e 12) exhibit liquid crystal phase transitions.3,8,9 Ho¨pken and Mo¨ller’s work is a milestone for the study of the morphology and solidstate properties of the F12Hm series: according to their work,10,11 for m e 2 the compound behaves as a pure fluorocarbon, whereas for a larger m, the structure is less crystalline and more amorphous and new endothermic transitions appear. In particular, F12H20 shows a peculiar phase behavior that has been studied by NMR and DSC, both for the samples crystallized from the solution and from the melt.10 The fluidity of the hydrogenated segment affects the solid-solid transitions, whereas the melting processes are ruled by the disordering of the fluorinated chain. NMR experiments confirm that the hydrocarbon tails possess liquidlike conformational freedom and mobility, already below the melting point.10 SAXS profiles, diffraction densities, packing constraints, and fluorocarbon/hydrocarbon interactions indicate the presence of several different structures with some overlapping of the two incompatible segments.3 The structure of crystallized F12H20 obtained either from the melt or from the solution was discussed,10,12 but later SAXS experiments performed on F8H16 showed that a ribbonlike network of molecules
10.1021/jp0125956 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/22/2002
Behavior of a Semifluorinated n-Alkane should be preferred to a cylindrical arrangement.13 Far from being entirely solved, the issue of semifluorinated n-alkanes' molecular organization still needs further experimental data from independent techniques. F12Hm molecules have been extensively studied by smallangle X-ray scattering and show different crystal structures, depending on the length of the hydrogenated chain, and present two solid-solid phase transitions below the melting temperature.11,14 For short hydrogenated segments (2 < m < 6), the SAXS profiles indicate only one single sharp peak, with a spacing that depends on m, and that is lower than the length of the semifluorinated alkane. In these cases, the molecules are tilted with respect to the surface normal for T lower than the solid-solid transition temperature (Tc), and perpendicular for T > Tc. From packing considerations, diffraction densities, and unfavorable fluorocarbon/hydrocarbon interactions, several different structures with some overlapping of the two chains can produce scattering profiles such as those detected experimentally.14 When m ) 8, 10, or 12, three or four peaks appear, because of the presence of two crystal packings that interchange by a sliding of the chains along the major molecular axis. In this case, crystals form lamellar bilayers, with a spacing that is proportional to the sum of the hydrogenated block length and twice the length of the fluorinated moiety. When Hm chains become longer, they are often tilted.15 For 14 < m < 20, only one single sharp peak and a diffuse maximum appear, indicating a bilayer structure. In particular, F12H20 shows the presence of a bilayered structure with indented hydrogenated tails.10 The characterization of phase transitions that occur below the melting point is important, as these molecules may provide a tool for predicting the behavior of their longer polymeric analogues, such as block copolymers of the type -[-(CF2)n(CH2)m-]p-. The presence of several phase transitions has been revealed by differential scanning calorimetry (DSC) studies on FnHm molecules having different n and m values. The application of temperature modulation to conventional scanning calorimetry (TMSC) is quite recent16 and has proved to be very powerful for the study of the reversibility and the kinetics of phase transitions.17 The recent combination of sinusoidal temperature modulation and DSC by Reading et al.18 made TMSC a method of investigation of great interest for the study of the reversibility and the kinetics of phase transitions.19 In practice, TMSC simultaneously measures the mean heat flow which is due to the average temperature scanning rate and the “in-phase” and “out-of-phase” oscillating heat flow, because of the sinusoidal temperature modulation. The combination of the different components of the heat flow allows the irreversible processes to be distinguished from the reversible ones and the characteristic equilibration time associated to the transition to be determined. At first, TMSC was successfully applied in the study of the glass transition,20 a kinetic transition of the relaxation processes in the material, that is better characterized in the frequency domain as done by dielectric and dynamic-mechanical techniques. Only recently, TMSC has been extensively applied to other phase transitions.21,22 It is very interesting to perform TMSC studies on semifluorinated n-alkanes because of their multiphase behavior. Dielectric spectroscopy (DS) is a valid technique for the study of electric dipole variations that are due to structural variations, such as conformational changes occurring at the phase transitions. FnHm have a dipole located along the CF2-CH2 covalent bond, because of the large difference in electronegativity between fluorine and hydrogen, that results in a remarkable
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1599 displacement of electronic density on the two carbon atoms and therefore in a detectable dipole moment. Because phase transitions are expected to modify, either directly or indirectly, the mobility of the part of the molecule where the dipole is located, we can reasonably expect a change in the macroscopic polarizability of FnHm. Nuclear magnetic resonance (NMR) spectroscopy represents a particularly powerful technique for the investigation of phase transitions in the solid state and associated structural and dynamic behavior, because information arising both from different nuclei and measurable properties can be exploited. The observation of different nuclei allows one to measure properties corresponding to defined positions within the molecule: in the case of FnHm, the different behavior of the hydrogenated and fluorinated chains can be investigated straightforwardly. Moreover, the several NMR observables contain information on both structural and dynamic chemical physical properties. The dynamics, in particular, can be investigated over a broad range of characteristic frequencies, mostly thanks to the measurement of different relaxation times: spin-spin (T2) and spin-lattice in the rotating frame (T1F) and in the laboratory frame (T1) relaxation times are affected by motions with characteristic frequencies in near to zero and tens of kHz and tens of MHz ranges, respectively. The spin-spin relaxation times are particularly suitable for the individuation and characterization of phase transitions. With all of this considered, an accurate choice of experiments can shed light on the structural and dynamic behavior of individual molecular moieties. In this paper, the phase transitions and the dynamics in the solid state of F8H16 have been investigated by means of TMSC, DS, and NMR measurements. Because this class of compounds exhibits a behavior dependent on thermal history,4 samples crystallized directly from the liquid solution as well as from the melt have been studied. Materials and Methods The synthesis and purification of F8H16 were carried out according to the literature.14 For our purposes, F8H16 was either crystallized from the solution at the final stage of the synthesis process (indicated as S), or from the melt after several thermal cycles (indicated as M). Temperature Modulated Scanning Calorimetry (TMSC). A single-cell modulated adiabatic scanning calorimeter instrument (MASC), which has already been described elsewhere,23,24 was used. MASC is a homemade calorimeter equipped with a calorimetric cell having a right cylindrical symmetry and surrounded by an active thermal shield. The shield is used to control the heat flux to the cell and to minimize the temperature gradients. Both the cell and the shield have an electrical heater and a temperature sensor that are uniformly distributed all along their cylindrical surfaces. The temperature is measured with an accuracy of 0.01 K and sensitivity is better than 0.0001 K s-1. The total heat flow to the cell is controlled within (30 µW. In TMSC experiments, the cell temperature T is programmed as
T(t) ) T0 + βt + Tm cos(ωt)
(1)
where T0 is a constant value, β is the average scanning rate, Tm is the amplitude of the temperature modulation, and ω is the angular frequency of modulation defined as ω ) 2π/period. The resulting heat flow to the sample, P(t), necessary to impose the temperature profile in eq 1, can be expressed as a
1600 J. Phys. Chem. B, Vol. 106, No. 7, 2002
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Figure 1. Temperature and its time-derivative versus time used in the TMSC experiments. The average temperature scanning rate is 1/600 K s-1, with a superimposed sinusoidal modulation of period 300 s (f ) 3.33 mHz) and amplitude 0.5 K. The dashed line shows the average temperature scanning rate.
superposition of three components:
P(t) ) Pav + P′ cos(ωt) + P′′ sin(ωt)
(2)
where Pav(t) is the moving average of P(t) over the time period of modulation and P′ and P′′ are the amplitude of the “in-phase” and “out-of-phase” oscillating components of P(t), respectively. These three components of P(t) allow us to determine25 the time dependent heat capacity Cscan measured at scanning rate β:
Cscan ) Pav/β and the complex heat capacity, C*(ω) ) C′(ω) - iC′′(ω), in response to temperature modulation:
C′ ) P′′/ωTm C′′ ) -P′/ωTm In the studies here reported, the temperature was programmed according to eq 1 with β ) (1/600 K s-1, Tm ) 0.5 K, and period ) 300 s (frequency of 3.33 mHz). T and its time derivative dT/dt are reported in Figure 1 as a function of time. The temperature profile imposed to the sample was made with alternating heating and cooling cycles in order to verify the reversibility of the transitions. All measurements were performed on sample M. The sample container was a Pyrex cylindrical tube (o.d. ) 3 mm; i.d. ) 2.2 mm; L ) 90 mm), partially filled with 0.316 g of F8H16 in the liquid state and then flame sealed. Before the TMSC study, the sample was kept for 5 min at 350 K and then cooled to 250 K at 5 K min-1. Dielectric Spectroscopy (DS). Dielectric measurements were performed with a simultaneous impedance and thermal analyzer (SITA), specifically developed for carrying out studies of thermal analysis and dielectric spectroscopy in real time, during chemical and/or physical transformations.26 The sample holder consists of a pair of concentric electrodes acting as a dielectric cell of 8 pF nominal capacitance. The length of the cell is larger than the internal electrode in order to create a reservoir of material in the top of the volume. The frequency range covered by the instrument is between 30 Hz and 0.5 MHz, and the time taken to collect the data for 25 frequencies in this range is 70 s. The sample holder, filled with 1.287 g of F8H16, is contained in a calorimetric cell for which the heat flow and the temperature profile are measured and controlled. The measurements were carried out with temperature scanned at the rate of 1/600 K s-1 (the same as in TMSC). A computer-interfaced Precision RLC
Figure 2. Average heat flow to the sample (crystallized from the melt) as a function of the temperature. In the insert, an enlargement of the central part of the thermogram is reported.
Meter (QuadTech model 7400) was used for the impedance measurement part of SITA. All measurements were performed on sample M, which was introduced in the SITA cell in the liquid state in order to fill completely the volume between the electrodes. When the sample became solid during the cooling, part of the volume contraction was compensated by the reservoir; however, the complete filling was not guaranteed. As in the TMSC study, the sample was kept in the liquid state at 350 K for a few minutes and then cooled to 250 K at a rate of 5 K min-1 before starting the measurements. NMR. A single-channel Varian XL-100 spectrometer interfaced with a DS-NMR Stelar acquisition system was used for low-resolution 1H and 19F NMR investigations. All of the experiments were performed on-resonance, working at a reduced magnetic field, corresponding to a Larmor frequency of 20 MHz for 1H and 18.816 MHz for 19F. The 90° pulse was 2.1 µs for 1H and 2.2 µs for 19F. 1H and 19F free induction decays (FIDs) were recorded through the application of a solid-echo pulse sequence27 with a dwell time of 1 µs; 1H and 19F spin-spin relaxation times (T2) were determined by FID fitting through a suitable combination of decay functions. 1H spin-lattice relaxation times in the laboratory frame (T1) were measured by detection in the time domain using an inversion-recovery technique followed by solid-echo. 1H spin-lattice relaxation times in the rotating frame (T1F) were measured by detection in the time domain through a variable spin-lock time technique followed by solid-echo, using a spin-lock field of 119 kHz. In all cases, the echo delay was 12 µs. The relaxation delay was 2∼8 s for 1H and 2∼13 s for 19F, to achieve a complete spinlattice relaxation. In the case of multiexponential decays, the population weighted rate average (PWRA) quantity was calculated in order to rule out the spin diffusion effects.28 Relaxation times were obtained from relaxation decays by means of nonlinear least-squares fit routines run on a PC. All of the measurements were performed on heating in the temperature range 232∼330 K. For sample M, a procedure similar to that used for TMSC and DS was followed; that is, the sample was kept in the liquid state at 350 K for a few minutes and then cooled to 232 K at a rate of about 5 K min-1 before starting the measurements. The temperature was controlled within 0.1 K. Results and Discussion Temperature Modulated Scanning Calorimetry (TMSC). The calorimeter revealed the presence of two transitions in F8H16 crystallized from the melt below the melting temperature. The heat flow supplied to the cell averaged over one oscillation period is reported in Figure 2 as a function of T. In addition to
Behavior of a Semifluorinated n-Alkane
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1601 TABLE 1: Characteristic Times, τ(T), for Two Transitions of F8H16 at the Indicated Temperaturesa transition at 274.5K
Figure 3. Cscan, C′, and C′′ for the solid-solid phase transition at 274.5 K. C0 is calculated from C′ and C′′ according to eq 7. The dashed lines are the baselines for the real and the imaginary parts of Cbaseline.
transition at 297K
T/K
τ/s
T/K
τ/s
271.75 272.25 272.75 273.25 273.75 274.25 274.75 275.25 275.75 276.25 276.75 277.25
38 38 48 60 76 103 78 57 46 47 37 34
293.75 294.25 294.75 295.25 295.75 296.25 296.75 297.25 297.75 298.25 298.75 299.25
17 21 8 25 34 12 24 13 35 24 18 29
a The values are calculated by eq 6 from experimental data reported in Figures 3 and 4.
that of the phase transition. Following this hypothesis, we can separate the real and the imaginary part in eq 3, obtaining
C′ - Cbaseline ) ∆C0/(1 + ω2τ2)
(4)
C′′ ) ∆C0ωτ/(1 + ω2τ2)
(5)
and the combination of eqs 4 and 5 yields
τ ) C′′/ω(C′ - Cbaseline) ∆C0 ) Figure 4. Cscan, C′, and C′′ for the solid-solid phase transition at 297 K. C0 is calculated from C′ and C′′ according to eq 7. The dashed lines are the baselines for the real and the imaginary parts of Cbaseline.
the melting, appearing with a large peak at 325 K and heat of fusion 72.0 J g-1, a transition at 274.5 K of weak intensity (6.2 J g-1) is visible along with a very weak one (0.6 J g-1) at 297 K (see the insert in Figure 2). The application of TMSC to the study of first order phase transitions is currently a matter of debate because of the difficulties rising when long delays of the heat diffusion time into the sample are present.22 This problem takes place in the melting of simple compounds and/ or when large heat exchange occurs in a narrow T interval. Because of the small heat involved in the two F8H16 transitions below the melting point, the heat diffusion time results negligible as compared to the modulation period that we used in the present study. This favorable condition leads us to analyze and explain the complex heat capacity data reported in Figures 3 and 4 for the two transitions revealed by TMSC. In the hypothesis that the transition provoked by a temperature variation is fully reversible, it has a linear response and occurs with a characteristic time τ (in analogy to the electrical polarization observable under the application of an electric field), and the following equation can be written for the complex heat capacity:
C*(ω) - Cbaseline ) ∆C0/(1 + iωτ)
(3)
where Cbaseline ) Cbaseline(T) is the heat capacity baseline (see Figures 3 and 4), ∆C0 ) ∆C0(T) is the low-frequency limit of the amplitude of the process (i.e., at the equilibrium), and ω is the angular frequency of the oscillating component of the heat flow and of T. It has to be outlined that Cbaseline is a real quantity, representing all of the contributions to the heat capacity except
C′ - Cbaseline C′′ 1+ C′ - Cbaseline
(
(6)
)
2
(7)
When the experimental data and the extrapolated baseline values are substituted in eq 6, the characteristic times are obtained in the range τ ) 35∼100 s for the transition at 274.5 K and τ ) 10∼30 s for the transition at 297 K (see Table 1). The data are spreaded in quite large time intervals owing to the instrumental noise and to the weak transitions signal. In Figures 3 and 4, the measured time-dependent heat capacity Cscan, and the calculated values of ∆C0 via eq 7 are reported for the two transitions below the melting point. In both cases, Cscan is larger than ∆C0, indicating that the two transitions do not reversibly follow the temperature modulation; that is, the transformations are partially irreversible. From the analysis of the shape of the transitions as they appear in the heat flow (see Figure 2) or in the Cscan (see Figures 3 and 4), we can estimate widths of about 5 and 10 K for the transitions at 274.5 and 297 K, respectively. Considering the amount of heat involved in the two processes, we conclude that these widths are not due to the heat diffusion delay into the sample, but they are intrinsic. This result suggests that the solidsolid transitions from one kind of structural order to another occur gradually in a finite temperature interval, that is typical of the λ transitions. Dielectric Spectroscopy (DS). The Cscan calorimetric results obtained with SITA experiments were in agreement with those obtained from MASC data, although the former are intrinsically more noisy because of the particular sample holder used in SITA. These results are not presented in this paper. In Figure 5, the real part, ′, and the imaginary part, ′′, of the complex dielectric constant, * ) ′ - i′′, for F8H16 are reported versus temperature for different frequencies. The sample analyzed was prepared by crystallization from the melt inside the cell. The possible presence of air gaps between the electrodes, created
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Figure 6. d′/dT (in double scale) as a function of temperature of the data in Figure 5A at f ) 14.67 kHz. The arrows mark the peaks of the transitions.
Figure 5. ′ (A) and ′′ (C) of F8H16 measured versus temperature at the different frequencies indicated. B is an expansion of A.
during the solidification of the sample, is probably responsible for the low values of ′ observed at low temperatures. The highest increase in ′, to which both dipolar and electronic polarization contribute, is observed around the melting point, near 325 K. The permanent dipole associated to the semifluorinated molecule gains full mobility in the transition from the solid to the liquid state. In simple liquids above the melting point the reorientational characteristic time is of the order of 10-9 or 10-12 s. For liquid F8H16, ′ is frequency-independent in the kHz range because it equals the static dielectric constant, s. ′′ is non zero and carries the contribution of ionic conductivity, a factor that is usually present in polar liquids because of some impurities. Above the melting, ′ is expected to decrease with increasing temperature, as confirmed by the experimental results. In the solid state, ′ shows a continuous increase (at all frequencies) with increasing temperature: this indicates a continuous increment in the mobility of dipoles. In particular, near the two transitions below the melting point revealed by TMSC, ′ shows a larger slope versus temperature, indicating a partial recover of mobility when the material undergoes a change of the internal order. This larger increase of mobility occurs at temperatures that are independent of the frequency, at least in the range covered by SITA experiments. On the contrary, our results show that different frequencies produce different changes in polarizability. The entity of the ′ change at 274.5 K is larger than that at 297 K for frequencies above 0.316 kHz: this result is in agreement with the calorimetric data and confirms that the transition at lower temperature is more intense (see Figure 2). At low frequency (′ at 68 Hz) the second transition shows a larger intensity in ′. This result is in accordance with TMSC results which probed a relaxation time τ ) 35-100 s (corresponding to a characteristic frequency f of 1.5-4.5 mHz) for the transition at 274.5 K and τ ) 10-30 s (f ) 5-15 mHz) for the transition at 297 K. The faster
transition, being closer to the frequencies used in the dielectrical constant measurements, gives a larger contribution at lower frequency. As in each transition region, the loss peak associated to dipole relaxation is at frequencies much lower than those used here (more than three decades), and the higher frequencies results are too far to show the effect because of difference in the characteristic times of the transitions. The explanation of the trend of ′′ as a function of frequency and of its relationship to the transitions below the melting point are harder tasks. As obtained from calorimetric data, the characteristic relaxation times are out of the DS frequency window. In the solid phases, we can observe only the highfrequency tails of relaxations. When ′′ versus frequency at different temperatures is analyzed, it results that ′′ decreases monotonically with power law exponents of -0.25, -0.40, -0.50, and -0.65 at 260, 280, 290, and 310 K, respectively. This may indicate that, as the temperature increases, relaxation mechanisms with a narrower distribution of relaxation times take place. To confirm this hypothesis, it is necessary to perform the DS down to the mHz or µHz frequency region. Provided the higher sensitivity of dielectric spectroscopy in the kHz range, the data show a higher signal-to-noise ratio as shown in Figure 5. To better investigate the variation of ′ as a function of T, in Figure 6, we plotted d′/dT for the 14.7 kHz frequency in two different scales. The profiles and the positions of the transitions at 274.5, 297, and 325 K are clearly shown in the magnified plot, along with another peak centered at 258 K. The latter signal was not revealed by TMSC, but it was observed by NMR, as discussed in the forecoming paragraphs. Comparing the d′/dT curves to the heat flow (see Figure 2), we observed a strong analogy between the two trends, i.e., between d′/dT and Cscan ) dH/dT. The specific change in polarizability is very similar to the specific change of enthalpy when T is increased. This finding confirms that any enthalpy increment (either vibrational or configurational contribution) is accompanied by a molecular mobility increment, as detected by ′, which indicates the ability of the dipoles to follow the electric field. It is worth focusing the attention on the transition shape at 274.5 K in Figure 6, where d′/dT has the profile of a λ transition between two solid states of different order and molecular mobility, in a more pronounced way. For the other two solid-solid transitions, it is harder to rise any hypothesis because of the weakness of the measured d′/dT or dH/dT signals. NMR: FID Analysis. 1H and 19F FIDs have been recorded every 2 K in the temperature range of 232∼330 K for both samples M and S. Both 1H and 19F FIDs were well fitted by a linear combination of a Gaussian and an exponential function at all temperatures. The weight percentage of the exponential
Behavior of a Semifluorinated n-Alkane
Figure 7. Weight percentage of the exponential component in the 1H FID versus temperature. Empty circles and full down triangles refer to sample M and S, respectively.
Figure 8. Weight percentage of the exponential component in the 19F FID versus temperature. Empty circles and full down triangles refer to sample M and S, respectively.
component is reported, as a function of temperature, in Figures 7 and 8 for 1H and 19F FIDs, respectively. The 1H FID is purely Gaussian at the lowest temperature and purely exponential above the melting point: this is an expected result, as Gaussian and exponential functions are typical of crystalline and liquid phases, respectively. The main step in going from the Gaussian to the exponential relaxation behavior takes place near the melting process in a 15 K range, although a small but detectable exponential component is revealed above 275 and 260 K for samples M and S, respectively. This component has a weight of about 2∼5% which remains substantially constant up to the melting process. The 19F FID shows the same behavior, but the presence of the exponential components cannot be revealed below 295 and 283 K for samples M and S, respectively. If, on one hand, it is easy to explain the dramatic change in the weight of the exponential component at about 315 K in terms of the melting process, it is more difficult to understand the presence of a small percentage of an exponential component at lower temperatures. A possible, reasonable explanation takes into account that a small amount of the sample (either a fragment of the molecule or a portion of the phase) experiences a higher mobility. Below the melting point, the T2 of both 1H and 19F exponential components are of the order of several hundreds microseconds, which are typical of solid mobile phases, such as, for instance, rubbery polymers.29 However, an alternative explanation of the presence of a long-T2 exponential component could be given in terms of the “memory effect”.30 The 1H T2 trend of the Gaussian component versus temperature, reported in Figure 9, highlights the presence of a quite different behavior for samples M and S. In both cases, the T2
J. Phys. Chem. B, Vol. 106, No. 7, 2002 1603
Figure 9. Trend of the proton spin-spin relaxation time (T2) of the Gaussian component versus temperature. Empty circles and full down triangles refer to sample M and S, respectively. Lines are drawn to highlight changes in the slope and the phase transitions at 257 and 272-274 K, as described in details in the text.
Figure 10. Trend of the fluorine-19 spin-spin relaxation time (T2) of the Gaussian component versus temperature. Empty circles and full down triangles refer to sample M and S, respectively. Lines are drawn to highlight changes in the slope and the phase transitions at 275 and 297 K, as described in details in the text.
value ranges from 12 µs at 232 K to 22∼23 µs at 325 K. In the case of the sample crystallized from the solution (S), only one phase transition, centered at about 274 K, can be clearly observed below the melting, with a steep increase of T2 from 14 µs (at 271 K) to 19 µs (at 278 K), preceded and followed by slow and regular increasing trends with temperature. For the sample recrystallized from the melt (M), the same transition, centered at about 272 K, is clearly observed in the range of 268∼276 K. In addition, a less evident transitional process, centered at about 257 K, can be revealed in the range of 246∼263 K. Finally, at about 291 K, the slope of the curve seems to increase, possibly indicating the occurrence of a third transitional process that, however, is not fully achieved before the melting. Also in the case of 19F, a different trend of the T2 of the Gaussian component versus temperature can be observed for the two samples (Figure 10). As in the case of 1H, the T2 of the Gaussian component assumes substantially equal values at the limits of the investigated temperature range, about 24∼25 and 38∼39 µs at 234 and 325 K, respectively. Sample S shows a remarkable increase in the range of 267∼307 K. It is rather difficult to distinguish the two phase transitions revealed by the calorimetric and dielectric techniques in this temperature range, although the data are compatible with two distinct transitions, one being centered at about 275 K and the other around 297 K, as shown in the figure. These two transitions can be even more hardly identified in the T2 trend of sample M. Never-
1604 J. Phys. Chem. B, Vol. 106, No. 7, 2002
Figure 11. Trend of the proton spin-lattice relaxation time in the laboratory frame (T1) versus temperature for sample M.
theless, the smooth changes in the slope above 258 K, combined to the fact that almost equal T2 values are observed for samples M and S at the limits of the temperature range investigated, suggest for sample M the occurrence of similar, but more broadened, transitional processes with respect to sample S. On the basis of the 1H and 19F FID analyses performed on both M and S samples, it can be stated that some solid-solid phase transitions are clearly present in the temperature range investigated. Although a detailed characterization of these solidsolid transitions appears to be rather difficult, their attribution to either the hydrogenated or the fluorinated chain can be attempted. In particular, the most intense transition at 275 K can be reliably ascribed to both segments, even if it is more clearly revealed by 1H data. A second phase transition, previously revealed by TMSC and dielectric spectroscopy at about 297 K, can also be observed by NMR data, albeit its characterization suffers from the proximity of the melting point. This transition seems to be especially ascribable to the fluorinated moiety, although a contribution from the hydrogenated tail cannot be excluded. A third, less intense phase transition, already identified by dielectric measurements, can be observed at about 257 K only from the 1H T2 trend of sample M, therefore ruling out any involvement of the fluorinated chain. It is clear that the behavior of the samples strongly depends on their thermal history: the transition at about 275 K is much sharper for the sample crystallized from solution, while a more continuous change of the spin-spin relaxation data is observed for the sample recrystallized from the melt, revealing more complex and less defined transitional processes. NMR: Spin-Lattice Relaxation Times. To investigate the mid-kHz and MHz dynamic ranges, 1H spin-lattice relaxation times in the rotating (T1F) and laboratory (T1) frames have been measured in sample M. Proton T1’s exhibit a monoexponential decay at all temperatures, as a result of the total averaging effect performed by the spin diffusion process. Within a simplified theoretical framework (BPP model31), a single motional process gives rise to a typical T1 curve versus temperature, with a minimum at ω0τc ≈ 0.62, being ω0 the Larmor frequency and τc the correlation time characteristic of the motion.32 The proton T1’s measured as a function of temperature, reported in Figure 11, qualitatively follow the typical theoretical trend, showing a minimum at about 250 K. This highlights the presence of a motional process in the tens of MHz region that affects the proton spin-lattice relaxation in the whole temperature range investigated, also below the first solid-solid transition observed. Such motion could be identified with the methyl group rotation around its ternary symmetry axis, that is usually active even at
Geppi et al. very low temperatures.33 A rough analysis based on the BPP model gives an estimate of about 30 ns for the correlation time of the motion under investigation at 250 K. An extreme narrowing regime is present at higher temperatures, and no effect is observed at the phase transitions identified through the FID analysis, thus indicating that the motion already active at lower temperatures remains the main contribution to the relaxation throughout the solid range. Proton T1F’s always exhibit a biexponential decay, as a result of an incomplete averaging process because of spin diffusion. To rule out the spin diffusion effects, we calculated the PWRA quantity, which is equal for intrinsic and measured relaxation times.28 However, no interesting trend of T1F PWRA versus temperature was observed. It is important to highlight that the T1F values are unusually long (the two components being about 10 and several hundreds of ms, with a very short PWRA of the order of 10-100 s-1), thus suggesting that no molecular motions with characteristic frequencies in the mid-kHz region are present. The absence of any effects on both proton spin-lattice relaxation times at the solid-solid phase transitions suggests that these transitions are associated to the onset of motions which strongly affect the spectral densities at near to zero frequency (and therefore the spin-spin relaxation times) but do not affect proton spin-lattice relaxation times in both the rotating (sensitive to spectral densities at the spin-lock and Larmor frequencies) and the laboratory (sensitive to spectral densities at the Larmor frequency only) frames, thus indicating that such motions remain very slow in the whole solid range. Conclusions In this paper, the solid-solid phase transitions of the semifluorinated n-alkane C8F17-C16H33 have been characterized by TMSC, dielectric measurements, and NMR spectroscopy. Three solid-solid phase transitions were identified from the experimental results. Dielectric measurements, performed on the sample recrystallized from the melt, showed a higher signal-to-noise ratio with respect to the TMSC measurements, indicating the presence of three phase transitions at 258, 274.5, and 297 K, in addition to the melting at 325 K. The latter is accompanied by an increasing contribution of fast dipolar relaxation processes centered at frequencies higher than 1 MHz, typical of the liquid phase, as shown by the increase of ′′ above 100 kHz. 1H and 19F NMR FID analysis was consistent with the presence of these phase transitions and, in addition, allowed an assignment in terms of dynamic processes occurring either in the hydrocarbon or fluorocarbon chains to be attempted. The transition at 258 K was ascribed to the hydrocarbon tail only, which is also involved in the most intense transition at 275 K. The fluorinated chain contributes to the transitions occurring at 275 and 297 K. Moreover, the different behavior between the sample directly crystallized from the solution and the one recrystallized from the melt could be highlighted by NMR. TMSC and DS gave information on the order of magnitude of the characteristic frequency of the motions taking place at each phase transition. Information on the characteristic frequencies could also be obtained from the trend of proton spin-lattice relaxation times versus temperature. Usually fast motional processes, such as the rotation of methyl groups, not detectable by dielectric spectroscopy, were found by NMR to be already active below the temperature range investigated. The motional processes responsible for the studied solid-solid transitions are instead characterized by low frequencies of the order of some mHz and can be tentatively identified with rotation of molecular fragments. The shape and the width of the transitions observed
Behavior of a Semifluorinated n-Alkane by TMSC and DS, in agreement with NMR observations, suggest that these solid-solid transitions (and particularly those at 258 and 297 K) do not occur sharply at a specific temperature but they would rather occur gradually, along with the change of internal order and dynamics. Acknowledgment. The authors acknowledge MURST and CSGI (Florence, Italy) for partial financial support. References and Notes (1) Viney, C.; Twieg, R. J.; Russell, T. P.; Depero, L. E. Liq. Cryst. 1989, 5, 1783. (2) Cho, H. G.; Strauss, H. L.; Snyder, R. G. J. Phys. Chem. 1992, 96, 5290. (3) Viney, C.; Russell, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63. (4) Lo Nostro, P. AdV. Colloid Interface Sci. 1995, 56, 245. (5) Lo Nostro, P.; Chen, S. H. J. Phys. Chem. 1993, 97, 6535. (6) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1993, 110, 7797. (7) Lo Nostro, P.; Ku, C. Y.; Chen, S. H.; Lin, J. S. J. Phys. Chem. 1995, 99, 10858. (8) Viney, C.; Twieg, R. J.; Russell, T. P. Mol. Cryst. Liq. Cryst. 1990, 182B, 291. (9) Viney, C.; Twieg, R. J.; Russell, T. P.; Depero, L. E. Liq. Cryst. 1989, 5, 1783. (10) Ho¨pken, J.; Mo¨ller, M. Macromolecules 1992, 25, 2482. (11) Ho¨pken, J.; Pugh, C.; Richtering, W.; Mo¨ller, M. Makromol. Chem. 1988, 189, 911. (12) Ho¨pken, J.; Fluorocarbon-Hydrocarbon Molecules. Structural Components of Self-Organizing Materials, Ph.D. Thesis, University of Twente, The Netherlands, 1991. (13) Ku, C. Y.; Lo Nostro, P.; Chen, S. H. J. Phys. Chem. B 1997, 101, 908.
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