Monotropic Polymorphism in Ester-Based Phase Change Materials

Article pubs.acs.org/crystal

Monotropic Polymorphism in Ester-Based Phase Change Materials from Fatty Acids and 1,4-Butanediol S. Cabus,† K. Bogaerts,† J. Van Mechelen,‡ M. Smet,† and B. Goderis*,† †

Chemistry Department, Polymer Chemistry and Materials, Catholic University of Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium ‡ Faculty of Sciences (FNWI), Laboratory for Crystallography, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands S Supporting Information *

ABSTRACT: This article reports on the synthesis, thermal behavior, and morphology of phase change materials based on diesters from 1,4butanediol with palmitic (BD16), stearic (BD18), or behenic (BD22) acid. The crystalline contents and melting enthalpies of all substances exceed 90% and 180 J/g, respectively. The molecules display monotropic polymorphism. The metastable β′ form is built from outstretched molecules, whereas the denser, stable β form is composed of molecules with a kinked conformation. BD18 and BD22 crystallize into the β′ form during cooling. Only BD18 transforms into the β form during subsequent heating via a solid−solid transition. The resistance of BD22 to convert into the β form is believed to originate from the lack of mobility associated with longer aliphatic chains. In contrast, the polymorphic conversion is thought to be very efficient and to take place during cooling for the shorter BD16 molecules. The hypothesis is put forward that the original BD16 β′ nuclei are rapidly overgrown by the β phase by which crystal growth is postponed to larger supercooling where the β phase can grow from the melt. Consequently, BD16 only occurs in the β form. The melt crystallization of BD18 and BD22 into the β′ form hardly requires any supercooling.

1. INTRODUCTION Phase change materials (PCMs) are characterized by a large latent heat during solid−liquid, liquid−gas, or solid−solid phase transitions and can therefore be used for the storage and release of thermal energy. Nowadays, PCMs are used to increase the comfort and to optimize the energy management of buildings or in textiles for clothing, for the smoothening of exothermic temperature peaks in chemical reactions or the recovery of waste heat, in medical hot−cold therapies, in solar heating systems and green houses, as protective shielding in heat releasing, high-performance electronic equipment, etc.1,2 Exploiting solid−liquid transitions in PCMs seems economically most attractive and since different applications require storage capacity at different temperatures, a wide variety of PCMs exists.2 Materials to be used for phase change thermal energy storage must have a large latent heat and high thermal conductivity. They should have a melting temperature lying in the practical range of operation, melt congruently, and crystallize with minimum supercooling, be chemically stable, low in cost, nontoxic, and noncorrosive.3 Given the current climate changes and the growing awareness that the exploitation of fossil resources should be limited, it is worth investigating PCM materials based on renewable resources such as, for example, fatty acids.4 In this context, besides pure fatty acids and their blends,5,6 also fatty acid derivatives have received considerable attention. © 2013 American Chemical Society

Esters of fatty acids with different types of (poly)alcohols have been studied to get rid of the fatty acid typical corrosivity, bad odor, and sublimation during heating, while maintaining reasonably high latent heats, narrow melting ranges, and little supercooling.7−12 In addition, such esters are thermally very reliable, withstanding numerous thermal cycles without loss of performance.9−12 The downside of current lipid-based PCMs is that they are limited to low temperature applications and that they display low thermal conductivities, typically in the range between 0.15 and 0.2 W/mK.13 For comparison, conductivity values readily exceed 0.15 W/mK for inorganic substances.13 Currently, attempts are being made to increase the thermal conductivity of lipid-based PCMs by adding highly conductive fillers.14 Altering the length of the fatty acids or the alcohol residues, as well as blending different species, allows for a finetuning of transition temperatures and melting enthalpies.7−10,12 To further widen the application range for lipid PCMs also amide-15,16 and carbonate-based17 derivatives have been reported. The review by Sarier and Onder on organic PCMs in textile applications contains a comprehensive overview of contemporary lipid-based PCMs.14 Received: March 5, 2013 Revised: June 13, 2013 Published: June 14, 2013 3438

dx.doi.org/10.1021/cg400339z | Cryst. Growth Des. 2013, 13, 3438−3446

Crystal Growth & Design

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dry dichloromethane (DCM) (30 mL) at room temperature. The mixture was stirred for another 4 h prior to washing with diluted HCl and NaHCO3. After drying over MgSO4, DCM was removed using a rotavapor. All compounds were purified using a silica column with 100% DCM and appeared white. They are denoted as BDz, with z being the number of carbon atoms in the saturated fatty acid residue (i.e., BD16, BD18, and BD22 for the compounds based on palmitic, stearic, and behenic acids, respectively.) 1H-NMR (BD18, 300 MHz, CDCl3, 298K): ∂ 0.88 (t, J = 6 Hz, 6H), ∂ 1.25 (s, 55H), ∂ 1.60 (t, J = 2 Hz, 5H), ∂ 1.70 (q, J = 3 Hz, 4H), ∂ 2.29 (t, J = 7 Hz, 4H), ∂ 4.09 (t, J = 6 Hz, 4H). 2.2. Differential Scanning Calorimetry (DSC). DSC measurements were performed on a TA Instruments Q2000 in combination with a RCS90 cooling device. The device was purged with nitrogen and calibrated with sapphire and indium for the temperature and the enthalpy. Samples of approximately 5 mg, enclosed in Tzero Aluminum hermetic pans, were characterized during heating (first heating, referred to as ramp A), cooling (ramp B), and heating (second heating, ramp C) at 10 °C/min. Data were also collected during cooling at 10 °C/ min (ramp B) followed by heating at 1 °C/min (ramp D). The experimental heat flow data were converted into specific heat, cp(T), values by subtracting an empty pan measurement and subsequent normalization to the sample mass and ramp rate. The melting enthalpy, ΔHm, was calculated by integrating the area between the cp(T) heating curve and a linear extrapolation from the melt cp(T) data.21 Crystallization temperatures, Tc, were determined at the onset of crystallization. The determination of melting peak onsets was not unambiguous. Therefore, melting peak maxima are reported as a measure for the melting temperature, Tm. 2.3. Polarized Optical Microscopy (POM). Microscopic analyses using crossed polarizers were conducted with an Olympus BHS optical microscope (Olympus Belgium N.V., Aartselaar Belgium). The temperature was controlled using a Linkam TMS 600 hot stage and a Linkam TMS 91 controller (Linkam, Surrey, United Kingdom). A droplet of liquid material was applied to the carrier glass at high temperature and covered with a glass coverslip. The samples were imaged during cooling at 10 °C/min (ramp B) every 6 s with a JVC TK-C138 color video camera, using a script within the Leica Qwin software (Leika, Germany). This protocol results in one image per 1 °C. Data were also collected every °C for BD18 in a temperature profile composed of ramp B followed by ramp D. 2.4. X-ray Diffraction. 2.4.1. In-House X-ray Powder Diffraction and Crystal Structure Determination. X-ray powder patterns were collected on an X’Pert MPD diffractometer (PANalytical, Almelo, The Netherlands) equipped with a sealed Cu tube, a focusing mirror, 0.01 rad soller slits, and an X’Celerator strip detector. The samples were loaded in a 0.7 mm glass capillary and were spinning continuously during data collection. Data were collected at room temperature right after synthesis and also during a ramp D protocol [see Differential Scanning Calorimetry (DSC)] (i.e., during heating at 1 °C/min after cooling at approximately 10 °C/min from the melt). The temperature was controlled by a Compact Cryostream (Oxford Instruments, Apdingdon, U.K.). Determining crystal structures from powders of long-chain organic compounds is quite laborious and not straightforward22,23 and was therefore limited to one representative sample at room temperature (i.e., BD18). BD18 exists in two 2polymorphic forms, which were solved by using direct space

When browsing the literature, it is striking that the optimization of novel lipid-based PCMs does not make use of crystallographic information, although it is well-known that fatty acids and their derivatives display rich polymorphism.18 The present paper wants to illustrate that gathering knowledge on polymorphism is indeed relevant. The design of the studied PCMs was inspired by the work of Li and Ding19 who considered 1,4-butanediol distearate (BD18) as PCM because of its rather high melting enthalpy (182 J g1−). In later work, this group also studied distearates with other linear diols,8 whereas Alkan and co-workers explored the impact of replacing the ester moyeties by amide groups.15,16 This chemical versatility allows for systematic studies on how molecular structure, crystal packing, and polymorphism relate to the PCM thermal behavior. In a first effort, we explored the impact of altering the saturated fatty acid length (z in Figure 1) on the crystallization and melting behavior of esters with 1,4-butanediol. Note that fatty acids and 1,4 butanediol can be obtained from renewable resources.20

Figure 1. Chemical structure of the PCM materials (abbreviated as BDz), with z being the number of carbon atoms in the saturated fatty acid residue.

Interestingly, the melting enthalpy values of products with shorter (palmitic acid) and longer (behenic acid) fatty acids were found to be higher than for BD18. Moreover, the PCM based on palmitic acid (BD16) displayed considerable supercooling in crystallization, whereas supercooling effects were absent in PCMs based on behenic (BD22) and stearic (BD18) acid. Explanations were found in polymorphism, crystal perfection, and the pathway followed by the different species to reach a given crystal form. The thermal behavior was explored by means of differential scanning calorimetry (DSC), the PCM crystal structures, and polymorphism by means of temperature resolved in house and synchrotron X-ray powder diffraction. Crystal morphology and nucleation efficiency were addressed through optical microscopy.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Stearic acid was bought from Fluka; palmitic and behenic acid were bought from Acros Organics. They were used without further purification. Solvents were of analytic grade and dried over calcium hydride. Thionylchloride, 1,4-butanediol, and triethylamine were purchased from Acros Organics and also used without further purification. In a first step, 1 equivalent of a fatty acid [palmitic acid (z = 16), stearic acid (z = 18) or behenic acid (z = 22), respectively, with z being the number of carbon atoms in the fatty acid] was added to 2 equivalents of thionylchloride together with a drop of dimethylformamide (DMF). The solution was refluxed and stirred for three hours. Excess thionylchloride was removed using a rotary evaporator (rotavapor). The resulting liquid was dripped to a solution of 0.5 equivalents 1,4-butanediol and 1 equivalent triethylamine in 3439



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search techniques as described by David and Shankland.24 For indexing the program McMaille was used, which generates a list of potential unit cells, taking into account restrictions to the cell dimensions.25 With the program Chekcell, a further selection and cell refinement was carried out.26 For the ensuing structure solution with FOX, a starting model with a flat molecular conformation was built in a Z-matrix description.27 At the startup of the structure solution process (parallel tempering), only translational and rotational freedom was given. At a later stage, more flexibility was allowed while keeping some restraints to all bond distances and bond angles. The two long saturated acyl chains were restrained to be flat by dihedral restraints of 180°. No dihedral angle restraints were applied to the central C4 moiety. Structure refinement was carried out with the program GSAS.28 The background was modeled by a Chebyshev polynomial. By using profile function 4, (hkl)-dependent peak broadening as well as low-angle peak asymmetry was taken into account. Soft restraints were applied to all distances and angles. Soft planar restraints were applied to the saturated acyl chains at either side of the central C4 moiety. Atomic displacement parameters were not refined. Figures showing the agreement between calculated and experimental powder diffraction data are available in the Supporting Information. Figures, representing crystal structure features, were made, using the program Mercury 3.0. Scattering patterns are represented as a function of the scattering vector modulus, q = (4π/λ) sin(θ), with θ being half the scattering angle and λ, the X-ray wavelength. 2.4.2. Time-Resolved Synchrotron X-ray Powder Diffraction. Temperature-resolved synchrotron X-ray powder diffraction was performed at DUBBLE, the Dutch-Belgian beamline (BM26) at the European Synchrotron Radiation Facility (ESRF; Grenoble, France), using a wavelength, λ, of 1.24 Å. The PCM samples were presented in a rubber ring, sandwiched between thin mica sheets. The sandwich constructions were kept together in crimped, perforated aluminum Perkin-Elmer DSC pans. The small-angle X-ray scattering (SAXS) parts of the powder patterns were measured on a two-dimensional (2D) multiwire gas-filled detector placed at 1.3 m from the sample after an evacuated tube. The wideangle X-ray diffraction (WAXD) parts were collected on a Pilatus 300K−W detector from Dectris put closely to the sample. The scattering angles were calibrated using the inhouse X-ray powder diffraction data of the materials. The SAXS and WAXD patterns were normalized to the intensity of the incoming beam, and measured by a photodiode at the beam stop close to the SAXS detector. The 2D data were azimuthally averaged using ConeX,29 corrected for the empty holder scattering, and represented as a function of the scattering vector modulus q. X-ray patterns were collected in consecutive frames of 6 s during heating, cooling, and (second) heating at 10 °C/ min (i.e., a B ramp followed by a C ramp), which results in one frame per 1 °C. Temperature was imposed through a Linkam HFS 191 heating/freezing stage.

Figure 2. DSC cp(T) data: (A) first heating at 10 °C/min, (B) subsequent cooling at 10 °C/min, (C) subsequent heating at 10 °C/ min, and (D) heating at 1 °C/min after cooling at 10 °C/min for BD16 (top), BD18 (middle), and BD22 (bottom). Heating runs are plotted upward, and cooling runs are plotted downward. The Greek letters refer to the melting or crystallization of a given polymorphic form. The arrow in the middle panel points to some exothermic heat developing during heating at 1 °C/min. The curves C and D are shifted upward with 20 and 40 cp(T) units, respectively, for clarity.

Table 1. DSC-Based Thermal Properties of All Lipid-Based PCM, Obtained during Heating after (A) Synthesis, (B) Subsequent Cooling, and (C) Heating at 10 °C/min.a BD16

BD18

BD22

△Hm0 (J g−1)

62(A) 62(C) 51(B) 188(A) 188(C) 0.92(A) 0.92(C) 204(A)

△Sm0 (J g−1 K−1)

0.61(A)

58(A) 54(C) 52(B) 186(A) 181(C) 0.93(A) 0.91(C) 199(A) 198(C) 0.60(A) 0.60(C)

74(A) 70(C) 67(B) 209(A) 199(C) 0.97(A) 0.94(C) 215(A) 212(C) 0.62(A) 0.62(C)

Tm (°C) Tc (°C) △Hm (J g−1) crystalline fraction

3. RESULTS AND DISCUSSION Figure 2 represents the DSC cp(T) A, B, C, and D ramps for all PCM materials collected between 20 and 80 °C. The crystallization onset, Tc, and melting peak, Tm, temperatures are listed in Table 1 together with the melting enthalpy, ΔHm, values for the A and C ramps. All heating and cooling runs of BD16 are unimodal. The first (ramp A) and second (ramp C) heating runs are virtually identical (see also data in Table 1). The endothermic melting

a

Crystalline fractions are extracted from the synchrotron WAXD data at 20 °C.

signal is narrower and the melting peak, therefore, occurs at slightly lower temperatures in ramp D compared to in ramps A 3440



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and C, which is expected when lower heating rates are used. The melting onset temperatures are not affected by the heating rate. The cooling ramp B may seem odd as the exothermic peak describes a loop as a function of the temperature. The heat developed during the crystallization of BD16 makes the sample temperature rise temporarily. This effect, which can be captured thanks to the Tzero technology, is only possible for samples that crystallize at high supercooling and the natural tendency to return to the thermodynamic melting point.30 For BD16, the difference between Tc and Tm is 11 °C. For the other samples, there is hardly any supercooling and therefore also no looping of the crystallization peak. The ramp A synchrotron WAXD data in Figure 3 illustrate that only one crystal type, referred to

Figure 3. Selection of synchrotron X-ray WAXD data during ramp A for BD16 (data are presented using a linear intensity scale). The righthand side numbers refer to the temperature in °C. At the left, the crystal type or the melt state is indicated.

Figure 4. Synchrotron X-ray SAXS and WAXD data, taken at 25 °C during the A ramps (left column panels) and C ramps (right column panels) for the different BD samples, as indicated. The strongest reflections are highlighted with vertical lines and are labeled with their Miller indices. Note that a logarithmic intensity scale is used. The patterns in the left and right columns are typical for β and β′ crystals, respectively.

as the β form, exists for BD16 and that it converts into a melt where DSC produces an endothermic peak. Similar WAXD patterns are observed in the synchrotron B and C ramps and in the in-house X-ray D ramp. Figure 4 contains both the synchrotron SAXS and WAXD data for BD16 taken in ramp A at 25 °C. The dominant reflections are labeled with the β crystal Miller indices, of which the corresponding crystal structure will be discussed further below. The melting point and enthalpy of BD18 during the first heating run (ramp A) are both higher compared to in the second heating run (ramp C) (see Table 1). The synchrotron X-ray pattern of freshly synthesized BD18 strongly resembles that of BD16, as illustrated in Figure 4, by means of the 25 °C SAXS and WAXD patterns of ramp A. During heating in ramp A, this BD18 β crystal form melts just like BD16 does, albeit at a slightly lower temperature. However, upon cooling in ramp B, another crystal form, the β′ form, is generated with a scattering

pattern, as illustrated in Figure 4, via the 25 °C patterns of ramp C. The synchrotron X-ray data further reveal that this β′ form simply melts during subsequent heating at 10 °C/min. The ramp C endothermic peak in Figure 2 is labeled accordingly. Clearly, the melting point difference between ramps A and C for BD18 is related to polymorphism. When heated at 1 °C/min after a ramp B cooling, the BD18 melting peak splits into two contributions (ramp D). Moreover, a small exothermic signal appears shortly before the melting onset, as indicated by the arrow in Figure 2. The corresponding in-house X-ray data in Figure 5 reveal that the cooling-induced β′ form converts into the more stable β structure exactly in the temperature range of the exothermic DSC signal. This 3441



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Figure 5. Selection of in-house X-ray data during ramp D for BD18 (data are presented using a linear intensity scale). The right-hand side numbers refer to the temperature in degrees Celcius. At the left, the crystal type is indicated.

conversion seems to be fairly complete as judged from the Xray data. In DSC, however, the small endothermic peak at 54 °C points to some remaining β′ phase that melts prior to the converted β fraction. Just like for BD16, the melting peak of this β fraction occurs at a slightly lower temperature when heated at 1 °C/min compared to when heated at 10 °C/min (compare with the BD18 β peak in ramp A). The absence of a melting event prior to the conversion exotherm as well as the absence of meltlike features in the X-ray patterns in this temperature range demonstrate that the β′ to β transition is a solid−solid transition. This notion is further supported by the optical microscopy images in Figure 6. Except for small changes in birefringence, no modifications to the crystals are observed after the transition between 40 and 50 °C. In a melt-mediated process, crystals with a totally different orientation would have been created. Clearly, in BD18, the β form is less easily created compared to in BD16. This resistance to forming the β form is further enhanced in BD22. The BD22 synchrotron X-ray patterns in Figure 4 at 25 °C in ramp A, which represents the situation right after the synthesis, clearly display β characteristics. The DSC melting trace in ramp A, however, is bimodal (see Figure 2), revealing the melting of some β′ material in the lowtemperature peak in front of that of the β fraction. The high-temperature synchrotron WAXD data in Figure 7 support this explanation. From above approximately 50 °C, the β′ form 040 reflection (see Figure 4 for the assignment of the reflections) starts to drift to smaller q values, away from below the β 110 reflection. This small 040 feature is shaded black in Figure 7 and disappears completely by melting before reaching 69 °C, in agreement with the DSC data. The β′ phase also contributes some less conspicuous intensity at q = 1.5 Å−1,

Figure 6. Optical microscopy images, using crossed polarizers, for BD18 at selected temperatures during heating at 1 °C/min (ramp D). Prior to heating, the material was cooled at 10 °C/min (ramp B). Note that the black patches in the images at 20.5 and 40 °C are not due to the presence of liquidlike material but to crystals that are rotated so that their optical axis runs parallel the polarizer or analyzer direction. Black in the images at 57 and 58 °C truly corresponds to molten material.

which is also highlighted in Figure 7. Although certainly present, no efforts were made to highlight the β′ features at temperatures below 50 °C due to overlap with the dominating β peaks. Since the β′ fraction is vanishingly small, only the β melting peak temperature is included in Table 1 for ramp A. When cooled down at 10 °C/min, BD22 converts into the β′ form. During subsequent heating and irrespective of whether it happens at 10 (ramp C) or 1 °C/min (ramp D), no conversion into the β form takes place prior to melting. Indeed, the conversion from β′ to β seems more difficult for BD22 compared to that for BD18, where crystallization from a solvent (which is what happens during the synthesis) or the application of long-term high-temperature protocols suffice for a complete conversion into the β form. In BD16, the β form dominates completely. Noteworthy is that the optical microscopy morphology of BD16 is more fine compared with that of BD18 or BD12, as illustrated in Figure 8 for the room temperature situation after cooling at 10 °C/min. The fact that the β′ phase converts into the β phase via an exothermic solid−solid transition makes the β′ and β phases monotropically related.31 In accordance with the enthalpy of 3442



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value for BD18 in Table 1, corresponding to the A ramp (β form) is higher than that of the C ramp (β′ form). However, in comparing melting enthalpies, one needs to ensure that all samples are fully crystalline, or alternatively, that deviating crystallinities are accounted for. Therefore, crystallinity values were estimated from the synchrotron WAXD data at 25 °C. At first, a linear background connecting the X-ray values at q = 1 and q = 2 Å−1 was subtracted. Next, a Gaussian was used to approximate the liquid scattering. The ratio of the crystalline contribution to the total background corrected WAXD intensity in a q2 weighted intensity representation yielded a crystallinity estimate, which was further multiplied with 1.032 and 1.025 for the β and β′ phases, respectively, to account for the crystalline contributions at q values below q = 1 Å−1. These correction factors originate from an analysis of the theoretical BD18 β′ and β scattering patterns that in turn result from the crystal structures of the BD18 polymorphs, discussed further below. Liquid and crystalline contributions above q = 2 Å−1 were neglected, assuming that their shares are proportional to those in the range 0 < q < 2 Å−1. Normalizing the ΔHm values to the corresponding crystallinities, yield the ΔHm0 values of fully crystalline material, represented in Table 1. The ramp A values, representative for the β phase (predominantly the β phase for BD22) are still higher compared to the ramp C values that correspond to the β′ phase melting. The differences are, however, strongly reduced compared to when no crystallinity corrections were applied. The ratio of the ΔHm0 values to the corresponding melting temperatures (in K) results in melting entropy values, ΔSm0, that are very similar for the different BD samples and polymorphic forms (Table 1). These ΔSm0 values agree with those calculated by means of the group contribution method described by Dannenfelser et al.,32,33 yielding 0.61 J g−1 K−1, irrespective of the molecule at hand. The similarity between the calculated and experimental ΔSm0 values implies that the PCM chains gain full flexibility upon melting. That a disordered melt is created can be deduced from the absence of a broad band in the SAXS region (data not shown).18 The crystallographic study served at finding molecular explanations for some of the thermal characteristics listed above. The crystallographic data, collected in Table 2, for BD18 were obtained via a full crystallographic analysis as explained in Experimental Section. Details are available in the Supporting Information (.cif file format). Data on the other samples are limited to the crystalline unit cell, assuming space groups identical to their BD18 β′ or β analogues. To obtain the unit cell dimensions in these cases, the labeled reflections in Figure 4 were used. The cell optimization procedure started from a first guess, based on the altered molecular length. Figure 9 illustrates the crystallographically determined molecular conformations in the BD18 β′ and β phases at 20 °C. In the BD18 β′ phase, the two aliphatic chains of the molecules are configured coaxially and the central butanediol moiety is bent. In the β form, the aliphatic axes are displaced by a kink at the central moiety. Figure 10 contains projections of the BD18 crystal structures. There are 4 and 2 symmetry-related molecules within the β′ and β unit cells, respectively. Both crystal forms are organized as stacks of layers with a thickness given by the 001 spacing, which for the β′ and β forms equals 51.88 Å and 46.78 Å, respectively. Clearly, the chain tilt in the β form is larger than in the β′ form. In both crystal forms, the aliphatic chains do not adopt a perfect all-trans configuration. Although their packing is

Figure 7. Selection of synchrotron X-ray WAXD data during ramp A for BD22 (data are presented using a linear intensity scale). The righthand side numbers refer to the temperature in degrees Celcius. The black-shaded areas highlight the β′ crystal form contribution. Except for the melt pattern at 78 °C, all patterns are dominated by reflections typical for the β crystal form.

Figure 8. Optical microscopy images taken at 20 °C after cooling at 10 °C/min for (a) BD16, (b) BD18, and (c) BD22.

fusion rule, the higher melting polymorph of a monotropic pair should also have the higher melting enthalpy. Indeed, the ΔHm 3443



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Table 2. Summary of the Crystallographic Unit Cell Parameters BD16 (20 °C) system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) volume (Å3) molecules in cell density (g/dm3)

BD18 (20 °C)

BD22 (20 °C)

β

β′

β

β′

β

triclinic P1̅ 4.49 8.62 45.16 104.63 84.90 99.42 1665.51 2 1.130

monoclinic P21/a 5.05 14.79 51.93 90 92.46 90 3876.853 4 1.067

triclinic P1̅ 4.52 9.05 50.24 110.97 82.27 100.75 1878.489 2 1.101

monoclinic P21/a 5.02 14.71 62.17 90 91.62 90 4592.348 4 1.062

triclinic P1̅ 4.60 8.60 60.45 105.02 75.12 101.08 2210.87 2 1.103

detect them. This happens in a temperature range where the more stable β phase does not nucleate as a result of the kinetic barriers associated with the formation and stacking of kinked molecules. In accordance with classical nucleation theory, for a crystal nucleus to be stable, it needs to reach a minimum size before it can overcome the free-energy penalty associated with the formation of crystal surfaces. On this account, a rather high number of molecules would have to adopt a kinked conformation in a concerted way, which seems highly improbable. This number of molecules is particularly large at low supercooling, where the driving force for crystallization is low. Assuming that growth occurs via a process of secondary nucleation, the growth of β phases at low supercooling is also prohibited. In contrast, the β′ nucleation is easier as it happens from the collection of chains in their straight configuration. However, at low supercooling, the rate of β′−β conversion in BD16 is so fast that it happens before the β′ nuclei were able to substantially grow. Moreover, as soon as the β′−β conversion reaches the β′ crystal growth front, its growth is promptly arrested because only β phases are facing the melt. This scenario may happen very frequently during the cooling process, by which BD16 becomes loaded with a high number of β nuclei. These β nuclei only grow at a sufficiently high degree of supercooling where, as a result of the higher crystallization driving force, smaller secondary nuclei can be formed. Indeed, the nucleation density for BD16 is very high (Figure 8a) and crystallization at a supercooling of 11 °C occurs very massively, giving rise to the looping phenomenon observed in DSC (Figure 2). When the β form is grown from the β′ form via a solid−solid transition (such as in the BD18 and BD22 cases), one may imagine a number of stacking defects where the conformational change was only partially successful. The presence of such defects will lower the density. If, on the other hand, the β form is grown directly from the melt, such as in the BD16 case, the amount of defects is expected to be much lower and the density higher. The higher crystal density for BD16 (see Table 2) is mirrored in an unexpectedly high ΔHm0 and Tm (see Table 1). Unexpected indeed, since in even normal paraffins ΔHm0 increases with increasing chain length.35 Such a trend is visible for BD18 and BD22, for the β′ as well as for the β form. The ΔHm0 value and melting point of the BD16 β phase do not follow this trend. Given the crystallographic density values, however, it seems that the BD16 β phase ΔHm0 and Tm values are not exceptionally high but that the values for the other two BD samples are rather low, as a result of crystal defects. Note that the presence of defects is also reflected in larger peak

Figure 9. Crystallographically determined molecular conformations in the BD18 β′ and β phases at 20 °C (red represents oxygen, gray is carbon, and white is hydrogen).

not perfectly parallel, it is fair to state that the chain packing subcell of the β′ phase resembles an orthorhombic O⊥ packing and that of the β phase the more dense triclinic T ∥ packing.18 In accordance with the lipid crystal nomenclature, this justifies labeling the two structures as β′ and β, respectively.18 It is conceivable that the stacking of coaxial units is easier than that of kinked objects. That a conversion into the metastable, less dense β′ form is kinetically favored above the stable β phase is in agreement with Ostwald’s rule of stages,34 at least for BD18 and BD22. For monotropically related pairs, the equilibrium interconversion temperature lies above the melting point of both polymorphs and is thus virtual.31 Therefore, in most cases, interconversions are melt-mediated. Solid−solid conversions are only possible when the molecules within the metastable solid phase display a minimum mobility and when the interconversion is not too involving. The change of the central butanediol configuration, together with a shearingrotation movement of the aliphatic chains seems to belong to this category. Because of frictional forces, such a movement will be more difficult the longer the molecules are and explains why BD22 is more resistant to the β′−β conversion compared to BD18. Furthermore, this solid−solid process involves nucleation, which takes time. When cooled and heated at 10 °C/min, insufficient time is spent in the temperature range where the mobility within the β′ phase is high enough for β nucleation in BD18. This does not hold for heating at 1 °C/min, where ample time is provided for nucleation. This time seems to be insufficient for the less mobile BD22. Along this line of thought, the β′−β conversion for shorter molecules, such as BD16, should be quite efficient and could even be happening during cooling. Ultimately, this conversion could be so fast that it escapes the observation. The following hypothesis therefore seems plausible. When cooling BD16 from the melt, small β′ nuclei are formed at such a low concentration that DSC or X-rays cannot 3444



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Figure 10. Projections of the BD18 crystal structures at 20 °C. The vertical direction in each image runs parallel to the 001 direction. The images A and D are projections along the unit cell a axis (red axis), whereas the projections in B and C are along the unit cell b axis (green axis). The crystallographic c axis is represented blue. The images A and B correspond to the BD18 β′ form, the images C and D to the β form.

widths for BD18 and BD22 compared to in BD16 (see Figure 4). The data in Table 2 also reveal that the higher ΔHm0 values for the β form (compared to the β′ form) correlate with the higher densities for the β phase. This observation complies with the density rule, stating that the polymorph with the highest density is the stable one. Note that the densities of the different polymorphic forms remain fairly constant over the temperature range studied (data not shown).

cooling in crystallization, whereas supercooling effects are absent in PCMs based on behenic (BD22) and stearic acid. BD18 and BD22 display monotropic polymorphism with the β phase being stable and the β′ phase metastable. Both BD18 and BD22 occur completely or to a large extent in the β form when crystallized from solution during purification after synthesis but crystallize into the β′ form during cooling from the melt. A detailed crystallographic study revealed that the β′ form is built from the layer-wise stacking of outstretched molecules. During subsequent heating, only BD18 transforms into the more stable β form via a solid−solid transition, provided a slow heating rate is used. In this conversion, the molecules adopt a kinked configuration at the central butanediol moiety and stack at an increased tilt angle with respect to the layer normal. The β form is the stable form due to its higher crystal density and melting enthalpy. The resistance of BD22 to convert into the more stable β form during heating is believed to originate from a lack of mobility as a result of the longer aliphatic chain length. BD16 only occurs in the β form, even when cooled from the melt.

4. CONCLUSIONS This article reported on the synthesis, thermal behavior, and temperature-dependent morphology of three phase change materials (PCM) based on diesters from 1,4-butanediol with saturated fatty acids of different lengths. All substances crystallize rather well with crystalline contents exceeding 90%. The melting enthalpy values of products with shorter (palmitic acid) and longer (behenic acid) fatty acids are higher than for products based on stearic acid (BD18). Moreover, the PCM based on palmitic acid (BD16) display considerable super3445



Article

(21) Mathot, V. B. F. In Calorimetry and Thermal Analysis of Polymers, Mathot, V. B. F., Ed.; Hanser Publishers: New York, 1994; Chapter 5, pp 105−167. (22) Van Mechelen, J. B.; Peschar, R.; Schenk, H. Acta Crystallogr. 2006, B62, 1121−1130. (23) Van Mechelen, J. B.; Peschar, R.; Schenk, H. Acta Crystallogr. 2006, B62, 1131−1138. (24) David, W. I. F.; Shankland, K. Acta Crystallogr. 2008, A64, 52− 64. (25) Le Bail, A. Powder Diffr. 2004, 19, 249−254. (26) Laugier, J.; Bochu, B. Chekcell (http://www.ccp14.ac.uk/about. htm), 2004. (27) Favre-Nicolin, V.; Cerný, R. J. Appl. Crystallogr. 2002, 35, 734− 743. (28) Larson, A. C.; Von Dreele, R. GSAS; Report No. LA-UR-86748; Los Alamos National Laboratory: New Mexico, USA, 1987. (29) Gommes, C. J.; Goderis, B. J. Appl. Crystallogr. 2010, 43, 352− 355. (30) Sanders, B.; Rekstad, J. Sol. Energy 2006, 80, 616−625. (31) Henck, J.-O.; Kuhnert-Brandstatter, M. J. Pharm. Sci. 1999, 88, 103−108. (32) Dannenfelser, R. M.; Yalkowsky, S. H. Ind. Eng. Chem. Res. 1996, 35, 1483−1486. (33) Dannenfelser, R. M.; Yalkowsky, S. H. J. Pharm. Sci. 1999, 88, 722−724. (34) Ostwald, W. F. Z. Zeitschrift für Physikalische Chemie 1897, 22, 289−330. (35) Domanska, U.; Wyrzkykowska-Stankiewicz, D. Thermochim. Acta 1991, 179, 265−271.

The hypothesis is put forward that when BD16 is cooled from the melt, small β′ nuclei are nevertheless formed, but that the rate of β′−β conversion is very rapid. As a result, the original β′ crystal growth front is rapidly overgrown by the β phase by which the growth of the nucleus is postponed to larger supercooling, where the driving force for growth in the β form is sufficiently large.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic information files (CIF) are available for the β and β′ polymorph of compound BD18, as well as images showing the agreement between the calculated and experimental powder diffraction data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +32 16327806. Fax: +32 16327990. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank FWO-Vlaanderen for supporting DUBBLE at the ESRF through the Big Science Project G.0707.08. S.C. thanks the Flemish Agency for Innovation by Science and technology (IWT) for a Ph.D. scholarship.

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REFERENCES

(1) Sharma, A.; Tyahi, V. V.; Chen, C. R.; Buddhi, D. Renewable Sustainable Energy Rev. 2009, 13, 318−345. (2) Zalba, B.; Marin, J. M.; Cabeza, L. F.; Mehling, H. Appl. Therm. Eng. 2003, 23, 251−283. (3) Farid, M. M.; Khudhair, A. A.; Razack, S. A. K.; Al-Hallaj, S. Energy Convers. Manage. 2004, 45, 1597−1615. (4) Menoufi, K.; Castell, A.; Farid, M. M.; Boer, D.; Cabeza, L. F. Renewable Energy 2013, 51, 398−403. (5) Kaygusuz, K.; Sari, A. Energy Sources, Part A 2006, 28, 105−116. (6) Kenisarin, M. M.; Kenisarina, K. M. Renewable Sustainable Energy Rev. 2012, 16, 1999−2040. (7) Suppes, G. J.; Goff, M. J.; Lopes, S. Chem. Eng. Sci. 2003, 58, 1751−176. (8) Li, W.-D.; Ding, E.-Y. Mater. Lett. 2007, 61, 4325−4328. (9) Sari, A.; Biçer, A.; Karaipekli, A.; Alkan, C.; Karadag, A. Sol. Energy Mater. Sol. Cells 2010, 94, 1711−1715. (10) Sari, A. Energy Convers. Manage. 2012, 64, 68−78. (11) Aydin, A. A. Sol. Energy Mater. Sol. Cells 2003, 108, 98−104. (12) Aydin, A. A. Sol. Energy Mater. Sol. Cells 2013, 113, 44−51. (13) Tasidjodoung, P.; Le Pierrès, N.; Luo, L. Renewable Sustainable Energy Rev. 2013, 18, 327−349. (14) Sarier, N.; Onder, E. Thermochim. Acta 2012, 540, 7−60. (15) Canik, G.; Alkan, C. Sol. Energy 2010, 84, 666−672. (16) Alkan, C.; Canik, G.; Dünya, H.; Sari, A. Sol. Energy Mater. Sol. Cells 2011, 95, 1203−1207. (17) Kenar, J. A. Sol. Energy Mater. Sol. Cells 2010, 94, 1697−1703. (18) Larsson, K. In The Lipid Handbook; Gunstone, F. D., Hardwood, J. L.; Padley, F. B., Eds.; Chapman and Hall: London, 1986, Chapter 8, pp 321−384. (19) Li, W.-D.; Ding, E.-Y. Mater. Lett. 2007, 61, 1526−1528. (20) Yim, H.; Haselbeck, R.; Niu, W.; Pujol-Baxley, C.; Burgard, A.; Boldt, J.; Khandurina, J.; Trawick, J. D.; Osterhout, R. E.; Stephen, R.; Estadilla, J.; Teisan, S.; Schreyer, H. B.; Andrae, S.; Yang, T. H.; Lee, S. Y.; Burk, M. J.; Van Dien, S. Nat. Chem. Biol. 2011, 7, 445−452. 3446


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