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polyethylene blends as form-stable phase change materials
Accepted Manuscript Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials Fang Chen, Michael P. Wolcott PII: DOI: Reference:
S0014-3057(13)00487-4 http://dx.doi.org/10.1016/j.eurpolymj.2013.09.027 EPJ 6251
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
1 August 2013 25 September 2013 28 September 2013
Please cite this article as: Chen, F., Wolcott, M.P., Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials, European Polymer Journal (2013), doi: http://dx.doi.org/10.1016/j.eurpolymj.2013.09.027
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Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials Fang Chen, Michael P. Wolcott Composite Materials and Engineering Center, Washington State University, Pullman, WA 99163. United States Corresponding author: Michael P. Wolcott (Tel: +1 509-339-3531 e-mail: [email protected]) Mailing Address: Composite Materials and Engineering Center, WSU Research Park. PO Box 641806. Pullman, Washington 99164-1806
Abstract Thermal energy storage is useful to promote energy conservation in buildings or machinery. One means of achieving a form stable phase change materials (PCMs) with polymers is to utilize immiscible blend pairs, governed by blend miscibility. The degree of miscibility between the polymer pairs may also influence energy efficiency in applications. Binary polyethylene-paraffin blends were melt compounded at different ratios of high-density polyethylene (HDPE), lowdensity polyethylene (LDPE), and linear low-density polyethylene (LLDPE) using a parallel corotating twin screw extruder. The miscibility of the paraffin in the three types of polyethylene was evaluated using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The DSC data demonstrated two melting temperatures with a depression of the equilibrium melting temperature for the polyethylene in the mixture. Two distinct and an intermediate phase were evident in the atomic force microscopy images. This structure verified the partial miscibility of paraffin in polyethylene. Interaction parameters between paraffin and polyethylene were obtained through melting point depression analysis via the Flory-Huggins approximation for thermodynamic mixing of two components. The crystallinity of each component depends upon the blend concentration. Two paraffin crystallization peaks for the PE/paraffin blends were observed, with the enthalpy of one peak increasing at the expense of the other. The lowest paraffin miscibility in polyethylene is found in paraffin/HDPE blend. 1
Key words miscibility paraffin, polyethylene, form-stable, phase change materials 1. Introduction Thermal energy can be stored as sensible or latent heat in materials that undergo phase changes near the operating temperatures of interest. In such uses, phase change materials (PCMs) can possess high-energy storage density and isothermal operating characteristics that make them efficient materials for utilizing latent heat [1-3]. Organic, inorganic, and eutectics forms of PCMs [1] have been widely investigated for storage of passive solar energy for deployment in the walls or floors of buildings. In this application, they act as a temperature buffer for energy conservation in the building. When the building’s interior temperature approaches the melt temperature of the PCM, the PCM changes from solid to liquid and, in doing so, absorbs energy. Later, when the ambient temperature drops, the PCM begins to crystallize, releasing stored thermal energy to the building and stabilizing the interior temperature. The PCM temperature will be maintained closer to the desired temperature during each phase transition period until the phase change is complete. In this manner, the PCM decreases interior temperature fluctuations, maintaining human comfort [2] while conserving energy through this reversible phase change. The major obstacle to the wide application of PCMs is the leakage of the molten PCM.
Leakage might be economically addressed by dispersing the PCM throughout a polymer support matrix with a much higher melting temperature than that of the PCM. In such a matrix, the melting points of both the matrix and the PCM may be altered resulting from a change in crystal perfection after mixing. Loss of the PCM might be reduced in this method because the matrix polymer remains in a solid state as long as the temperature remains below its melting point. Support materials that have been investigated include: inorganic materials, e.g. silicon dioxide
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[3], and polymers, e.g. high density polyethylene (HDPE) [7], low density polyethylene (LDPE) [8], styrene-butadiene-styrene copolymer [9], and poly (methyl methacrylate) [10].
Among the various kinds of PCM, paraffin wax, typically with a chain of 18-50 carbons, has been widely used as a phase change material due to its high heat of fusion, range of phase change temperatures, chemical resistance, commercial availability, and low cost [4-6]. Polyethylene is the most commonly used commodity polymer, possessing many useful properties including light weight, low cost, low dielectric constant and losses, high chemical resistance, and good processability [4]. Blended with other materials, polyethylene improves the properties of the blend. Using morphology characterization [5], Lee and Choi demonstrated the durability of HDPE/paraffin blends as shape-stabilized energy storage phase change materials. Cai et al. improved the thermal stability and flame retardant properties of HDPE/paraffin blends using expanded graphite and ammonium polyphosphate as additives [6]. Luyt et al. blended polyethylene with soft and hard Fischer-Tropsch paraffin wax as well as with wood flours to investigate the influence of paraffin type and content on blend properties. Using thermal fractionation (stepwise cooling), Luyt observed the formation of co-crystals after mixing [7]. In a comparison of three different blends of PE and soft Fischer-Tropsch paraffin, Luyt found that the polyethylene/wax blends were immiscible for samples with 30-50% wax [8]. The miscibility of soft and hard Fischer-Tropsch paraffin in LDPE was also compared and they found that compared with soft paraffin wax, the hard paraffin wax was more miscible with LDPE because of co-crystallization [9]. Finally, Luyt created composites of paraffin, PE, and natural fiber， and studied the changes in structure and thermal properties [10-13]. Estep et al. claimed that HDPE forms an interconnected phase throughout the HDPE/paraffin blend, and maintains the
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products shape during the PCM phase changing. In a chloroform extraction test, Estep reported increased paraffin leakage with higher paraffin content [14]. There is a limit on paraffin content (~77%) in the blend, above which rapid paraffin seepage occurs in the liquid state during mixing [15]. In addition, they found that immiscibility of HDPE and paraffin led to the formation of PCM pockets. Hence, the mechanism of PCM leakage from a polymer matrix is closely related to the miscibility of the matrix with the PCM. Furthermore, it is known that the PE chain structure and morphology substantially influence its miscibility with paraffin wax [9, 10], which in turn, dramatically affects the thermal storage capacity of the blend. The AFM phase images were able to show component dispersion [17-20], and the interaction parameter can be calculated to identify the degree of miscibility. While evaluating the nature of paraffin interactions with PE for future form-stable PCMs, our specific objectives are as follows: (1) Characterize the dispersion morphology of paraffin in 3 types of PE, (2) Distinguish changes on paraffin crystallization and potential co-crystallization with the PE in blends, and (3) Compare the equilibrium melt temperature depression of paraffin in PE blends. Leakage and form-stable studies will be presented in another manuscript.
2. Materials and method 2.1. Materials A technical grade of octadecane paraffin was obtained from commercial sources (Roper Thermals). The paraffin contains 91.61% octadecane (C18) and 7.30% branched octadecane; 0.44% has a carbon number of less than eighteen, while the carbon number is greater than eighteen in 0.65% of the paraffin. As specified by the producer, this paraffin possessed a density of 0.6485 g/cm3 and melting and boiling temperatures of 28.2 oC and 316.7 oC, respectively.
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Three forms of polyethylene were evaluated: high-density (HDPE), low-density (LDPE), and linear low-density (LLDPE). Their properties are listed in Table 1. Table 1. Properties of HDPE, LDPE, and LLDPE. Name
Mode
Form
Source
Density (g/cm3)
MFR (g/10min)
Mw
HDPE
HP54-60 Flake
Powder
Ineos
0.954
0.55
~180,000 to 220,000
1.0
~110,000
1.0
~130,000
LDPE
LLDPE
(ASTM D4883)
Petrothene® Powder NA960000
Lyondell Chemical
Pellets
Lyondell
0.918
Basell
(ASTM D1505)
Petrothene GA501021
0.920 (ASTM D1505)
2.2. Blending The PE/paraffin blends with different weight ratios were compounded using an 18-mm parallel co-rotating twin-screw extruder with an L/D ratio of 40 (Leistritz ZSE-18HP). To prepare the mixture for extrusion, the PE solids were placed in a beaker and molten paraffin was gradually added by stirring with a glass rod for at least one minute. This mixture was immediately transferred into the extruder, which was controlled at 180°C and a screw speed of 80 rpm. Following extrusion, the compound was air cooled at room temperature.
2.3. Characterization 2.3.1. Atomic force microscopy (AFM)
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The surface morphology of the PE/paraffin blends was investigated in the tapping mode with a Veeco Multimode AFM equipped with a Nano ScopeIIIa controller (Digital Instruments Inc.). In preparation, the extruded blends were hot pressed at 180oC for 5 min to produce a flat film, then cooled at room temperature. The film surfaces were scanned in air using Si tips (Digital Instruments Inc.).
2.3.2. DSC The thermal transitions of PE, paraffin, and their blends were investigated using a liquid nitrogen cooled, Mettler Toledo 822e differential scanning calorimeter (DSC). Samples of approximately 6 to 8 mg were sliced from the extruded blends and the raw materials were sealed in an aluminum crucible. To eliminate thermal history, the samples were quenched to -10ºC, ramped up to 150ºC at 10ºC/min, held for two minutes, and then quenched again at the same speed. The heating process was then repeated and data from the second scan was recorded. To determine the equilibrium melting temperature with DSC, the following procedure was followed. The samples were heated to an annealing temperature of 150oC for two minutes to allow the samples melt under a nitrogen atmosphere, then quenched with liquid nitrogen to the preset Tc, and then isothermally crystallized until the crystallization was complete. Finally, the samples were heated to 150oC at 10oC/min; the peak melting temperature was selected as the melting point T'm [1, 2].
2.3.3. Interaction parameter The crystallization and melting behaviors of various polymer blends have been widely studied. The blends fall into three categories as summarized by Mandelkern et al. [16]: (1) two polymers with different chemical structures where at least one component crystallizes; (2) polymer-low
6
molecular weight diluents; (3) a mixture of two chemically identical species that differ only in molecular weight [17]. Long-chain polyethylene and short chain paraffin are chemically identical species differing only in molecular weight as described in (3), however the relatively low molecular weight of the paraffin provides some similarities to the polymer-diluent system of (2). Considering case (3), mixing should be athermal with strong combinatorial entropy contributions due to the short paraffin chains [12, 14]. The miscibility of the components in polymer blends can directly affect the mechanical, thermal, and optical properties of the final materials as well [18]. The Flory-Huggins’s equation for melting temperature depression is the simplest method for studying miscibility. The entropy of mixing for two polymers of high molecular weight are generally ignored based on the Nish-Wang theory [19]. However, the entropy of mixing is not negligible in a polymer-diluent system as the low carbon chain paraffin and polyethylene blend approaches. This relatively large contribution of entropy on the interaction parameter should be considered.
3. Results and discussion 3.1. Morphology The dispersion of paraffin in three types of PE was characterized by AFM in tapping mode. Figure 1 shows the phase images of the thirty percent paraffin blends. Two distinct phases were found for each sample, with the dark phase corresponding to the PE and the light phase corresponding to the paraffin. The paraffin is well dispersed in the PE, though the blends are not uniform as can be seen by the shape and size of the paraffin domains. The clear phase separation supports the notion that PE and paraffin are not totally miscible. The intermediate phase is formed from the limited miscibility between HDPE and paraffin and is indicated by green
7
regions. Although this region is barely visible for HDPE blends, it is clearly evident in the paraffin blends with LDPE and LLDPE blends. Based on the strength of this intermediate phase, we conclude that paraffin is less miscible in HDPE than in LDPE or LLDPE. Figure 1 AFM phase images of PE/paraffin blends.
(a) HDPE70/paraffin30
8
(b) LDPE70/paraffin30
(c) LLDPE70/paraffin30
9
3.2. Crystallization and crystallinity 3.2.1. Paraffin crystallization temperature in the blends The crystallization thermographs as determined with DSC are presented in Figure 2 for the various blends. Two distinct low temperature exotherms are evident in many of the thermograms. As the melting temperature of the polyethylenes used in the study are all in excess of 100oC, the dual low-temperature exotherms can be attributed to some form of paraffin or paraffin-blend. The endotherm I centered from 20-25oC and to some extent were independent of the PE content with a high paraffin content which are at least 80% for HDPE, 70% for LDPE and LLDPE, and therefore likely attributed to the formation of a eutectic [17, 20]. With the enthalpy of exotherm I increased at the expense of the exotherm II counterpart. While studying the influence of heating rate (from 1 to 20oC/min) on melting behavior, we found that the crystallization temperatures decreased with increased heating rates, likely from the thermal lag. Yet the shapes of the exotherms were independent of scan rate in all cases. This finding demonstrates that the multiple crystallizations did not result from recrystallization during cooling [17].
In contrast, the position of the endotherm II, varied with concentration, indicating the paraffin was influenced by the presence of the PE, e.g. miscibility or confinement of the paraffin in the PE networks [21]. With decreasing paraffin content, the magnitude of the peak shifts to a lower temperature in the following order: HDPE/paraffin < LDPE/paraffin < LLDPE/paraffin. The effect of LDPE on paraffin crystallization is, therefore, greater than that of HDPE, but less than that of LLDPE, based on the extent of the crystallization peak shifts. The miscibility of the
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paraffin in LDPE is stronger than in HDPE, but weaker than in LLDPE. This supports the trend observed in the AFM experiments above.
Figure 2 Crystallization of paraffin in various blends at a cooling rate of 10oC/min (e.g. P90 indicates 90% paraffin and 10% PE)
(a) HDPE/paraffin
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(b) LDPE/paraffin
(c) LLDPE/paraffin
3.2.2. Crystallinity of PE and paraffin 12
The percent crystallinity was calculated using the heat of fusion measurement normalized by the component mass fraction compared to the heat of fusion for neat component material; i.e. 293J/g for PE [22] and in this study, we used 240.7J/g for paraffin based on experimental DSC test. This is consistent with literature values 61.5 and 60.484 kJ/mol [23, 24]. The PE and paraffin crystallization temperatures were well separated and therefore the heat of fusion for PE and paraffin was easily resolved from the DSC thermograms. The crystallinity of PE and paraffin in the mixtures is shown in Figure 3. There was an obvious decrease in the degree of crystallinity for the paraffin with a increasing PE content. Based on the overall intensity of the crystallinity decrease, we concluded that the order of paraffin miscibility with polyethylene is HDPE < LDPE < LLDPE. Crystallinity is calculated from enthalpy, and the enthalpy of paraffin in the blends are shown in Table 2 for your reference. There was an obvious increase in paraffin enthalpy with a decreasing PE content. Figure 3 Crystallinity of paraffin (a) and PE (b) according to weight fraction of paraffin (%)
(a) Crystallinity of paraffin 13
(b) Crystallinity of PE Table 2 Paraffin enthalpy (kJ/kg) Formulation 90PE10Paraffin 80PE20Paraffin 70PE30Paraffin 60PE40Paraffin 50PE50Paraffin 40PE60Paraffin 30PE70Paraffin 20PE80Paraffin 10PE90Paraffin
LDPE/Paraffin HDPE/Paraffin 6.2 117.2 29.0 120.8 80.5 160.1 86.6 189.6 178.2 181.7 238.3 198.0 237.2 224.6 233.6 236.1 240.0 235.4
LLDPE/Paraffin 29.4 39.7 61.3 100.7 189.6 186.9 228.3 240.3 231.9
3.3. Equilibrium melting temperature and melting point depression Analyzing the melting behavior of crystalline polymer blends is an important method for assessing blend miscibility. It is predicted that the chemical potential of a polymer will be decreased by the addition of a miscible diluent based on thermodynamic considerations. According to this decline of chemical potential, the equilibrium melting temperature will also 14
decrease if the polymer is able to crystallize. The Hoffmann-Weeks equation correlates the equilibrium melting temperature, Tom, the observed experimental melting point, T'm , and the isothermal crystallization temperature, Tc , as follows in Eq. (1) where γ is the ratio of the initial thickness to the final thickness of a chain-folded lamella thickening ratio. T'm= Tc/γ + (1-1/ γ) Tom (1) In this equation, the relationship between T'm and Tc is linear.
Figure 4 Method for determining the equilibrium melting temperature of LDPE in LDPE/paraffin blends.
Classic Hoffman-Weeks plot is represented in Figure 4 for LDPE/paraffin blends. The best fit of T'm versus Tc lines for each formulation and the equilibrium melting point Tom is obtained from 15
the intersection of these lines with the line of the equation T'm =Tc, which implies an extrapolation to infinite thickness of the chain-folded lamella [25]. The values of Tom obtained from the extrapolations (Figure 4) for different blends are reported in Table 3.
Table 3 Equilibrium melting temperature (Tom) and thickening ratio (γ) of PE in paraffin
HDPE/paraffin H100 H90P10 H80P20 H70P30 H60P40 H50P50 H40P60 H30P70 H20P80 H10P90
Tom (oC) 136 134 131 129 126 122 120 118 114 112
γ
LDPE/paraffin
2.60 2.00 2.15 2.03 1.94 1.66 1.82 1.47 1.77 1.62
L100 L90P10 L80P20 L70P30 L60P40 L50P50 L40P60 L30P70 L20P80 L10P90
Tom (oC) 113 111 109 108 106 100 98 97 95 92
γ
LLDPE/paraffin
4.59 4.29 3.76 3.44 3.32 3.13 3.01 2.50 2.60 2.74
LL100 LL90P10 LL80P20 LL70P30 LL60P40 LL50P50 LL40P60 LL30P70 LL20P80 LL10P90
Tom (oC) 123 122 121 120 120 116 114 109 109 108
γ 8.70 4.69 4.35 3.82 3.66 2.68 3.00 2.46 2.22 2.31
From Table 3 we can indicate that the Tom of each of the three PE in paraffin blends decreases with increasing paraffin content. It has been reported that lamellar thickness, crystal type, degree of crystal perfection, and other morphological properties all influence the melting temperatures of polymer blends [26]. This explains why both of the thickening ratio and equilibrium melting temperature decrease with increasing paraffin fraction, as shown in Table 3. The rate of decrease is in the order HDPE < LDPE < LLDPE. Weeks found that increased thickness increased the melting point of a crystal [27], and that molecular motion is involved in thickening. High-density polyethylene has the highest molecular weight (180,000 to 220,000) of the three types of polyethylene studied, followed by LLDPE (130,000) and LDPE (110,000), and thus has the most restricted mobility in a paraffin blend. Furthermore, the long and short branching of the LDPE and LLDPE result in larger surface area for contacting the paraffin molecule compared to the 16
more linear HDPE. This might explain why HDPE is less miscible with paraffin and has a smaller thickening ratio in the blend than LDPE or LLDPE.
Similar melting point depression phenomena have been reported for other polymer blends exhibiting some degree of miscibility [28]. The melting point of the crystalline component is commonly lower than the pure crystalline polymer due to thermodynamically favorable interactions [29]. The melting point depression was likely induced by morphological effects such as changes in the lamellar thickness, the degree of crystalline perfection, the physical nature of the amorphous phase surrounding the crystalline phase, and the thermodynamics of polymer mixing [30]. For blends with similar paraffin content (Table 4), the equilibrium melting temperature of paraffin in the LLDPE decreased more dramatically than in LDPE or HDPE compared with pure paraffin. This influences the interaction parameter comparison, discussed in the next section. The thickening ratio followed a similar trend, which corroborates the finding that the miscibility of paraffin in PE is in the order of HDPE
Type Paraffin HDPE LDPE LLDPE
Slope 0.02 0.06 0.05 0.03
Intercept 30.05 25.33 25.30 24.00
Thickening ratio 31.1 8.7 9.8 15.1
Tom 30.6 26.9 26.7 24.8
4. Interaction parameter To describe many polymer-diluent systems, equation (2) has been used satisfactorily used, including n-paraffin-polyethylene mixtures [21, 22]. 1/ Tm -1/ Tom = -[(RV2u)/(∆H2u V1u) ][-(1-V2 )+χ12 (1-V2 )2 ] (2) For paraffin (C-18), we consider m1=18 and m1→∞ , and Eq. (2) can be written as Eq. (3). 17
-(1/ Tm -1/ Tom)(∆H2uV1u)/(RV2u)+ V1/18= χ12 (1-V2 )2 (3) To apply Eq. (3), the molar volume of paraffin and PE repeat units are assumed to be the same, that is, V1u=V2u. Substituting experimental data into Eq. (3), the plot representing LDPE/paraffin blends is shown in Figure 5 (a). Theoretically, they should be a straight line passing through the origin to assume concentration independence. This is not the case, rather a quadratic relationship was observed in Figure 5 (a). This nonlinearity likely occurs because the interaction parameter is concentration dependent and because the residual entropic effect was neglected in the derivation of Eq. (3). If a straight line is drawn through the origin and each point in the graph, a slope is obtained that changes with concentration and corresponds to the interaction parameter for each blend. The X-axis was changed to V1, and a straight line in Figure 5 (b) was obtained. It is very close to pass through the origin point. Thus the slope in the graph is considered as the interaction parameter we sought due to the linear relationship and negligible intercepts of -0.0001, 0.0002, and -0.0001 for the respective blends of HDPE, LDPE and LLDPE with paraffin.
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Figure 5 Fitting the Flory-Huggin’s approximation Eq.(3) for LDPE/paraffin blend.
(a)
19
(b)
It has been shown that the interaction parameter between polyethylene and n-paraffin decreases with an increasing number of carbon atoms in the n-alkane; for chains with n>18, this parameter approaches zero [12-14, 32-34]. Nakajima and Hamada reported an interaction parameter of 0.37 for n-C6H14 and 0 for n-C32H66 [31]. According to Coran and Anaonostopoulos [32], the interaction parameter decreased from 0.09 for n-C7H16 to -0.45 for n-C20H42 by using ∆H2u =922cal/mol. Paul and Gardner similarly found that the dissolution temperature (melting temperature) for a polymer in the alkanes, increased with increasing carbon number [33] due to a reduced entropy of mixing [34]. Ke reported an interaction parameter of -1.3 at 135oC for polyethylene-n-C33H66 as determined by differential thermal analysis of the melting temperature [35]. Flory et al. reported a value of 0.02 for the polyethylene-n-C28H58 system at 140oC. The value was determined by comparing the degree of swelling of the polyethylene network in nC28H58 with the elastic retroactive force exhibited by the same network when stretched. This is in good agreement with Nakajima and Hamada’s results [33, 37]. The difference in numerical value may originate with sample conditions (purity, crystallization sufficiency) and preparation methods [31]. Most studies used fractionated samples and considered paraffin a diluent. However, in the broad application area of phase change materials, components are not purified before mixing. Based on our knowledge, no thermodynamic interaction literature could be found for technical grade polyethylene-paraffin phase change materials with 91.61% octadecane (C18) and 7.30% branched octadecane . This is a thorough analysis of the thermodynamic interactions of this polyethylene/paraffin system from the point of view of phase change materials for energy saving in buildings.
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The interaction parameter value for linear polyethylene and n-C18H38 is 0.10 at 410oK, as determined by Nakajima and Hamada [31]. Our value is similar to this result. The small difference might be due to the samples themselves. Nakajima and Hamada used fractionated linear polyethylene Sholex-6009 (polyethylene of Marlex-50 type manufactured by Japan Olefin Chem. Co.), with fractionation carried out by fractional precipitation in xylene-triethylene glycol at 130oC, and a viscosity-average degree of polymerization of the fraction of 2000. The product that they used was an 18-carbon paraffin purified by distillation at reduced pressure. The PE and paraffin that we used were unpurified and they were used immediately after purchase.
The small positive value of χ12 indicates that PE and paraffin are partially miscible due to their similar chemical structures. The values of χ12 play a notable role in miscibility, with a smaller χ12 indicating better miscibility. Though the difference is small, comparing the values of χ12, 0.0614, 0.0612 and 0.0596 for HDPE, LDPE and LLDPE blend with paraffin, we once again show that the paraffin miscibility with polyethylene is in the order of HDPE
Conclusion High-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) are partially miscible with paraffin. The HDPE/paraffin blend has the weakest miscibility of the three systems. Because of the influence of miscibility on the thermal behavior of paraffin, we suggest using HDPE in PE/paraffin form-stable phase change materials to maintain the energy saving behavior of paraffin in building applications for reducing interior temperature fluctuations.
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Acknowledgment
This work was supported by the Louisiana Pacific Quasi-Endowment Fund. The authors appreciate Dr. Jinwu Wang's help on editing.
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Graphical Abstract
Title: Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials Authors: Fang Chen, Michael P. Wolcott
26
Highlights AFM phase images verified partial miscibility of paraffin in polyethylene. The crystallinity of each component depends upon the blend concentration. Two paraffin crystallization peaks for polyethylene/paraffin blends were observed. The lowest paraffin miscibility in polyethylene is found in paraffin/HDPE blend.
27
S0014-3057(13)00487-4 http://dx.doi.org/10.1016/j.eurpolymj.2013.09.027 EPJ 6251
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
1 August 2013 25 September 2013 28 September 2013
Please cite this article as: Chen, F., Wolcott, M.P., Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials, European Polymer Journal (2013), doi: http://dx.doi.org/10.1016/j.eurpolymj.2013.09.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials Fang Chen, Michael P. Wolcott Composite Materials and Engineering Center, Washington State University, Pullman, WA 99163. United States Corresponding author: Michael P. Wolcott (Tel: +1 509-339-3531 e-mail: [email protected]) Mailing Address: Composite Materials and Engineering Center, WSU Research Park. PO Box 641806. Pullman, Washington 99164-1806
Abstract Thermal energy storage is useful to promote energy conservation in buildings or machinery. One means of achieving a form stable phase change materials (PCMs) with polymers is to utilize immiscible blend pairs, governed by blend miscibility. The degree of miscibility between the polymer pairs may also influence energy efficiency in applications. Binary polyethylene-paraffin blends were melt compounded at different ratios of high-density polyethylene (HDPE), lowdensity polyethylene (LDPE), and linear low-density polyethylene (LLDPE) using a parallel corotating twin screw extruder. The miscibility of the paraffin in the three types of polyethylene was evaluated using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The DSC data demonstrated two melting temperatures with a depression of the equilibrium melting temperature for the polyethylene in the mixture. Two distinct and an intermediate phase were evident in the atomic force microscopy images. This structure verified the partial miscibility of paraffin in polyethylene. Interaction parameters between paraffin and polyethylene were obtained through melting point depression analysis via the Flory-Huggins approximation for thermodynamic mixing of two components. The crystallinity of each component depends upon the blend concentration. Two paraffin crystallization peaks for the PE/paraffin blends were observed, with the enthalpy of one peak increasing at the expense of the other. The lowest paraffin miscibility in polyethylene is found in paraffin/HDPE blend. 1
Key words miscibility paraffin, polyethylene, form-stable, phase change materials 1. Introduction Thermal energy can be stored as sensible or latent heat in materials that undergo phase changes near the operating temperatures of interest. In such uses, phase change materials (PCMs) can possess high-energy storage density and isothermal operating characteristics that make them efficient materials for utilizing latent heat [1-3]. Organic, inorganic, and eutectics forms of PCMs [1] have been widely investigated for storage of passive solar energy for deployment in the walls or floors of buildings. In this application, they act as a temperature buffer for energy conservation in the building. When the building’s interior temperature approaches the melt temperature of the PCM, the PCM changes from solid to liquid and, in doing so, absorbs energy. Later, when the ambient temperature drops, the PCM begins to crystallize, releasing stored thermal energy to the building and stabilizing the interior temperature. The PCM temperature will be maintained closer to the desired temperature during each phase transition period until the phase change is complete. In this manner, the PCM decreases interior temperature fluctuations, maintaining human comfort [2] while conserving energy through this reversible phase change. The major obstacle to the wide application of PCMs is the leakage of the molten PCM.
Leakage might be economically addressed by dispersing the PCM throughout a polymer support matrix with a much higher melting temperature than that of the PCM. In such a matrix, the melting points of both the matrix and the PCM may be altered resulting from a change in crystal perfection after mixing. Loss of the PCM might be reduced in this method because the matrix polymer remains in a solid state as long as the temperature remains below its melting point. Support materials that have been investigated include: inorganic materials, e.g. silicon dioxide
2
[3], and polymers, e.g. high density polyethylene (HDPE) [7], low density polyethylene (LDPE) [8], styrene-butadiene-styrene copolymer [9], and poly (methyl methacrylate) [10].
Among the various kinds of PCM, paraffin wax, typically with a chain of 18-50 carbons, has been widely used as a phase change material due to its high heat of fusion, range of phase change temperatures, chemical resistance, commercial availability, and low cost [4-6]. Polyethylene is the most commonly used commodity polymer, possessing many useful properties including light weight, low cost, low dielectric constant and losses, high chemical resistance, and good processability [4]. Blended with other materials, polyethylene improves the properties of the blend. Using morphology characterization [5], Lee and Choi demonstrated the durability of HDPE/paraffin blends as shape-stabilized energy storage phase change materials. Cai et al. improved the thermal stability and flame retardant properties of HDPE/paraffin blends using expanded graphite and ammonium polyphosphate as additives [6]. Luyt et al. blended polyethylene with soft and hard Fischer-Tropsch paraffin wax as well as with wood flours to investigate the influence of paraffin type and content on blend properties. Using thermal fractionation (stepwise cooling), Luyt observed the formation of co-crystals after mixing [7]. In a comparison of three different blends of PE and soft Fischer-Tropsch paraffin, Luyt found that the polyethylene/wax blends were immiscible for samples with 30-50% wax [8]. The miscibility of soft and hard Fischer-Tropsch paraffin in LDPE was also compared and they found that compared with soft paraffin wax, the hard paraffin wax was more miscible with LDPE because of co-crystallization [9]. Finally, Luyt created composites of paraffin, PE, and natural fiber， and studied the changes in structure and thermal properties [10-13]. Estep et al. claimed that HDPE forms an interconnected phase throughout the HDPE/paraffin blend, and maintains the
3
products shape during the PCM phase changing. In a chloroform extraction test, Estep reported increased paraffin leakage with higher paraffin content [14]. There is a limit on paraffin content (~77%) in the blend, above which rapid paraffin seepage occurs in the liquid state during mixing [15]. In addition, they found that immiscibility of HDPE and paraffin led to the formation of PCM pockets. Hence, the mechanism of PCM leakage from a polymer matrix is closely related to the miscibility of the matrix with the PCM. Furthermore, it is known that the PE chain structure and morphology substantially influence its miscibility with paraffin wax [9, 10], which in turn, dramatically affects the thermal storage capacity of the blend. The AFM phase images were able to show component dispersion [17-20], and the interaction parameter can be calculated to identify the degree of miscibility. While evaluating the nature of paraffin interactions with PE for future form-stable PCMs, our specific objectives are as follows: (1) Characterize the dispersion morphology of paraffin in 3 types of PE, (2) Distinguish changes on paraffin crystallization and potential co-crystallization with the PE in blends, and (3) Compare the equilibrium melt temperature depression of paraffin in PE blends. Leakage and form-stable studies will be presented in another manuscript.
2. Materials and method 2.1. Materials A technical grade of octadecane paraffin was obtained from commercial sources (Roper Thermals). The paraffin contains 91.61% octadecane (C18) and 7.30% branched octadecane; 0.44% has a carbon number of less than eighteen, while the carbon number is greater than eighteen in 0.65% of the paraffin. As specified by the producer, this paraffin possessed a density of 0.6485 g/cm3 and melting and boiling temperatures of 28.2 oC and 316.7 oC, respectively.
4
Three forms of polyethylene were evaluated: high-density (HDPE), low-density (LDPE), and linear low-density (LLDPE). Their properties are listed in Table 1. Table 1. Properties of HDPE, LDPE, and LLDPE. Name
Mode
Form
Source
Density (g/cm3)
MFR (g/10min)
Mw
HDPE
HP54-60 Flake
Powder
Ineos
0.954
0.55
~180,000 to 220,000
1.0
~110,000
1.0
~130,000
LDPE
LLDPE
(ASTM D4883)
Petrothene® Powder NA960000
Lyondell Chemical
Pellets
Lyondell
0.918
Basell
(ASTM D1505)
Petrothene GA501021
0.920 (ASTM D1505)
2.2. Blending The PE/paraffin blends with different weight ratios were compounded using an 18-mm parallel co-rotating twin-screw extruder with an L/D ratio of 40 (Leistritz ZSE-18HP). To prepare the mixture for extrusion, the PE solids were placed in a beaker and molten paraffin was gradually added by stirring with a glass rod for at least one minute. This mixture was immediately transferred into the extruder, which was controlled at 180°C and a screw speed of 80 rpm. Following extrusion, the compound was air cooled at room temperature.
2.3. Characterization 2.3.1. Atomic force microscopy (AFM)
5
The surface morphology of the PE/paraffin blends was investigated in the tapping mode with a Veeco Multimode AFM equipped with a Nano ScopeIIIa controller (Digital Instruments Inc.). In preparation, the extruded blends were hot pressed at 180oC for 5 min to produce a flat film, then cooled at room temperature. The film surfaces were scanned in air using Si tips (Digital Instruments Inc.).
2.3.2. DSC The thermal transitions of PE, paraffin, and their blends were investigated using a liquid nitrogen cooled, Mettler Toledo 822e differential scanning calorimeter (DSC). Samples of approximately 6 to 8 mg were sliced from the extruded blends and the raw materials were sealed in an aluminum crucible. To eliminate thermal history, the samples were quenched to -10ºC, ramped up to 150ºC at 10ºC/min, held for two minutes, and then quenched again at the same speed. The heating process was then repeated and data from the second scan was recorded. To determine the equilibrium melting temperature with DSC, the following procedure was followed. The samples were heated to an annealing temperature of 150oC for two minutes to allow the samples melt under a nitrogen atmosphere, then quenched with liquid nitrogen to the preset Tc, and then isothermally crystallized until the crystallization was complete. Finally, the samples were heated to 150oC at 10oC/min; the peak melting temperature was selected as the melting point T'm [1, 2].
2.3.3. Interaction parameter The crystallization and melting behaviors of various polymer blends have been widely studied. The blends fall into three categories as summarized by Mandelkern et al. [16]: (1) two polymers with different chemical structures where at least one component crystallizes; (2) polymer-low
6
molecular weight diluents; (3) a mixture of two chemically identical species that differ only in molecular weight [17]. Long-chain polyethylene and short chain paraffin are chemically identical species differing only in molecular weight as described in (3), however the relatively low molecular weight of the paraffin provides some similarities to the polymer-diluent system of (2). Considering case (3), mixing should be athermal with strong combinatorial entropy contributions due to the short paraffin chains [12, 14]. The miscibility of the components in polymer blends can directly affect the mechanical, thermal, and optical properties of the final materials as well [18]. The Flory-Huggins’s equation for melting temperature depression is the simplest method for studying miscibility. The entropy of mixing for two polymers of high molecular weight are generally ignored based on the Nish-Wang theory [19]. However, the entropy of mixing is not negligible in a polymer-diluent system as the low carbon chain paraffin and polyethylene blend approaches. This relatively large contribution of entropy on the interaction parameter should be considered.
3. Results and discussion 3.1. Morphology The dispersion of paraffin in three types of PE was characterized by AFM in tapping mode. Figure 1 shows the phase images of the thirty percent paraffin blends. Two distinct phases were found for each sample, with the dark phase corresponding to the PE and the light phase corresponding to the paraffin. The paraffin is well dispersed in the PE, though the blends are not uniform as can be seen by the shape and size of the paraffin domains. The clear phase separation supports the notion that PE and paraffin are not totally miscible. The intermediate phase is formed from the limited miscibility between HDPE and paraffin and is indicated by green
7
regions. Although this region is barely visible for HDPE blends, it is clearly evident in the paraffin blends with LDPE and LLDPE blends. Based on the strength of this intermediate phase, we conclude that paraffin is less miscible in HDPE than in LDPE or LLDPE. Figure 1 AFM phase images of PE/paraffin blends.
(a) HDPE70/paraffin30
8
(b) LDPE70/paraffin30
(c) LLDPE70/paraffin30
9
3.2. Crystallization and crystallinity 3.2.1. Paraffin crystallization temperature in the blends The crystallization thermographs as determined with DSC are presented in Figure 2 for the various blends. Two distinct low temperature exotherms are evident in many of the thermograms. As the melting temperature of the polyethylenes used in the study are all in excess of 100oC, the dual low-temperature exotherms can be attributed to some form of paraffin or paraffin-blend. The endotherm I centered from 20-25oC and to some extent were independent of the PE content with a high paraffin content which are at least 80% for HDPE, 70% for LDPE and LLDPE, and therefore likely attributed to the formation of a eutectic [17, 20]. With the enthalpy of exotherm I increased at the expense of the exotherm II counterpart. While studying the influence of heating rate (from 1 to 20oC/min) on melting behavior, we found that the crystallization temperatures decreased with increased heating rates, likely from the thermal lag. Yet the shapes of the exotherms were independent of scan rate in all cases. This finding demonstrates that the multiple crystallizations did not result from recrystallization during cooling [17].
In contrast, the position of the endotherm II, varied with concentration, indicating the paraffin was influenced by the presence of the PE, e.g. miscibility or confinement of the paraffin in the PE networks [21]. With decreasing paraffin content, the magnitude of the peak shifts to a lower temperature in the following order: HDPE/paraffin < LDPE/paraffin < LLDPE/paraffin. The effect of LDPE on paraffin crystallization is, therefore, greater than that of HDPE, but less than that of LLDPE, based on the extent of the crystallization peak shifts. The miscibility of the
10
paraffin in LDPE is stronger than in HDPE, but weaker than in LLDPE. This supports the trend observed in the AFM experiments above.
Figure 2 Crystallization of paraffin in various blends at a cooling rate of 10oC/min (e.g. P90 indicates 90% paraffin and 10% PE)
(a) HDPE/paraffin
11
(b) LDPE/paraffin
(c) LLDPE/paraffin
3.2.2. Crystallinity of PE and paraffin 12
The percent crystallinity was calculated using the heat of fusion measurement normalized by the component mass fraction compared to the heat of fusion for neat component material; i.e. 293J/g for PE [22] and in this study, we used 240.7J/g for paraffin based on experimental DSC test. This is consistent with literature values 61.5 and 60.484 kJ/mol [23, 24]. The PE and paraffin crystallization temperatures were well separated and therefore the heat of fusion for PE and paraffin was easily resolved from the DSC thermograms. The crystallinity of PE and paraffin in the mixtures is shown in Figure 3. There was an obvious decrease in the degree of crystallinity for the paraffin with a increasing PE content. Based on the overall intensity of the crystallinity decrease, we concluded that the order of paraffin miscibility with polyethylene is HDPE < LDPE < LLDPE. Crystallinity is calculated from enthalpy, and the enthalpy of paraffin in the blends are shown in Table 2 for your reference. There was an obvious increase in paraffin enthalpy with a decreasing PE content. Figure 3 Crystallinity of paraffin (a) and PE (b) according to weight fraction of paraffin (%)
(a) Crystallinity of paraffin 13
(b) Crystallinity of PE Table 2 Paraffin enthalpy (kJ/kg) Formulation 90PE10Paraffin 80PE20Paraffin 70PE30Paraffin 60PE40Paraffin 50PE50Paraffin 40PE60Paraffin 30PE70Paraffin 20PE80Paraffin 10PE90Paraffin
LDPE/Paraffin HDPE/Paraffin 6.2 117.2 29.0 120.8 80.5 160.1 86.6 189.6 178.2 181.7 238.3 198.0 237.2 224.6 233.6 236.1 240.0 235.4
LLDPE/Paraffin 29.4 39.7 61.3 100.7 189.6 186.9 228.3 240.3 231.9
3.3. Equilibrium melting temperature and melting point depression Analyzing the melting behavior of crystalline polymer blends is an important method for assessing blend miscibility. It is predicted that the chemical potential of a polymer will be decreased by the addition of a miscible diluent based on thermodynamic considerations. According to this decline of chemical potential, the equilibrium melting temperature will also 14
decrease if the polymer is able to crystallize. The Hoffmann-Weeks equation correlates the equilibrium melting temperature, Tom, the observed experimental melting point, T'm , and the isothermal crystallization temperature, Tc , as follows in Eq. (1) where γ is the ratio of the initial thickness to the final thickness of a chain-folded lamella thickening ratio. T'm= Tc/γ + (1-1/ γ) Tom (1) In this equation, the relationship between T'm and Tc is linear.
Figure 4 Method for determining the equilibrium melting temperature of LDPE in LDPE/paraffin blends.
Classic Hoffman-Weeks plot is represented in Figure 4 for LDPE/paraffin blends. The best fit of T'm versus Tc lines for each formulation and the equilibrium melting point Tom is obtained from 15
the intersection of these lines with the line of the equation T'm =Tc, which implies an extrapolation to infinite thickness of the chain-folded lamella [25]. The values of Tom obtained from the extrapolations (Figure 4) for different blends are reported in Table 3.
Table 3 Equilibrium melting temperature (Tom) and thickening ratio (γ) of PE in paraffin
HDPE/paraffin H100 H90P10 H80P20 H70P30 H60P40 H50P50 H40P60 H30P70 H20P80 H10P90
Tom (oC) 136 134 131 129 126 122 120 118 114 112
γ
LDPE/paraffin
2.60 2.00 2.15 2.03 1.94 1.66 1.82 1.47 1.77 1.62
L100 L90P10 L80P20 L70P30 L60P40 L50P50 L40P60 L30P70 L20P80 L10P90
Tom (oC) 113 111 109 108 106 100 98 97 95 92
γ
LLDPE/paraffin
4.59 4.29 3.76 3.44 3.32 3.13 3.01 2.50 2.60 2.74
LL100 LL90P10 LL80P20 LL70P30 LL60P40 LL50P50 LL40P60 LL30P70 LL20P80 LL10P90
Tom (oC) 123 122 121 120 120 116 114 109 109 108
γ 8.70 4.69 4.35 3.82 3.66 2.68 3.00 2.46 2.22 2.31
From Table 3 we can indicate that the Tom of each of the three PE in paraffin blends decreases with increasing paraffin content. It has been reported that lamellar thickness, crystal type, degree of crystal perfection, and other morphological properties all influence the melting temperatures of polymer blends [26]. This explains why both of the thickening ratio and equilibrium melting temperature decrease with increasing paraffin fraction, as shown in Table 3. The rate of decrease is in the order HDPE < LDPE < LLDPE. Weeks found that increased thickness increased the melting point of a crystal [27], and that molecular motion is involved in thickening. High-density polyethylene has the highest molecular weight (180,000 to 220,000) of the three types of polyethylene studied, followed by LLDPE (130,000) and LDPE (110,000), and thus has the most restricted mobility in a paraffin blend. Furthermore, the long and short branching of the LDPE and LLDPE result in larger surface area for contacting the paraffin molecule compared to the 16
more linear HDPE. This might explain why HDPE is less miscible with paraffin and has a smaller thickening ratio in the blend than LDPE or LLDPE.
Similar melting point depression phenomena have been reported for other polymer blends exhibiting some degree of miscibility [28]. The melting point of the crystalline component is commonly lower than the pure crystalline polymer due to thermodynamically favorable interactions [29]. The melting point depression was likely induced by morphological effects such as changes in the lamellar thickness, the degree of crystalline perfection, the physical nature of the amorphous phase surrounding the crystalline phase, and the thermodynamics of polymer mixing [30]. For blends with similar paraffin content (Table 4), the equilibrium melting temperature of paraffin in the LLDPE decreased more dramatically than in LDPE or HDPE compared with pure paraffin. This influences the interaction parameter comparison, discussed in the next section. The thickening ratio followed a similar trend, which corroborates the finding that the miscibility of paraffin in PE is in the order of HDPE
Type Paraffin HDPE LDPE LLDPE
Slope 0.02 0.06 0.05 0.03
Intercept 30.05 25.33 25.30 24.00
Thickening ratio 31.1 8.7 9.8 15.1
Tom 30.6 26.9 26.7 24.8
4. Interaction parameter To describe many polymer-diluent systems, equation (2) has been used satisfactorily used, including n-paraffin-polyethylene mixtures [21, 22]. 1/ Tm -1/ Tom = -[(RV2u)/(∆H2u V1u) ][-(1-V2 )+χ12 (1-V2 )2 ] (2) For paraffin (C-18), we consider m1=18 and m1→∞ , and Eq. (2) can be written as Eq. (3). 17
-(1/ Tm -1/ Tom)(∆H2uV1u)/(RV2u)+ V1/18= χ12 (1-V2 )2 (3) To apply Eq. (3), the molar volume of paraffin and PE repeat units are assumed to be the same, that is, V1u=V2u. Substituting experimental data into Eq. (3), the plot representing LDPE/paraffin blends is shown in Figure 5 (a). Theoretically, they should be a straight line passing through the origin to assume concentration independence. This is not the case, rather a quadratic relationship was observed in Figure 5 (a). This nonlinearity likely occurs because the interaction parameter is concentration dependent and because the residual entropic effect was neglected in the derivation of Eq. (3). If a straight line is drawn through the origin and each point in the graph, a slope is obtained that changes with concentration and corresponds to the interaction parameter for each blend. The X-axis was changed to V1, and a straight line in Figure 5 (b) was obtained. It is very close to pass through the origin point. Thus the slope in the graph is considered as the interaction parameter we sought due to the linear relationship and negligible intercepts of -0.0001, 0.0002, and -0.0001 for the respective blends of HDPE, LDPE and LLDPE with paraffin.
18
Figure 5 Fitting the Flory-Huggin’s approximation Eq.(3) for LDPE/paraffin blend.
(a)
19
(b)
It has been shown that the interaction parameter between polyethylene and n-paraffin decreases with an increasing number of carbon atoms in the n-alkane; for chains with n>18, this parameter approaches zero [12-14, 32-34]. Nakajima and Hamada reported an interaction parameter of 0.37 for n-C6H14 and 0 for n-C32H66 [31]. According to Coran and Anaonostopoulos [32], the interaction parameter decreased from 0.09 for n-C7H16 to -0.45 for n-C20H42 by using ∆H2u =922cal/mol. Paul and Gardner similarly found that the dissolution temperature (melting temperature) for a polymer in the alkanes, increased with increasing carbon number [33] due to a reduced entropy of mixing [34]. Ke reported an interaction parameter of -1.3 at 135oC for polyethylene-n-C33H66 as determined by differential thermal analysis of the melting temperature [35]. Flory et al. reported a value of 0.02 for the polyethylene-n-C28H58 system at 140oC. The value was determined by comparing the degree of swelling of the polyethylene network in nC28H58 with the elastic retroactive force exhibited by the same network when stretched. This is in good agreement with Nakajima and Hamada’s results [33, 37]. The difference in numerical value may originate with sample conditions (purity, crystallization sufficiency) and preparation methods [31]. Most studies used fractionated samples and considered paraffin a diluent. However, in the broad application area of phase change materials, components are not purified before mixing. Based on our knowledge, no thermodynamic interaction literature could be found for technical grade polyethylene-paraffin phase change materials with 91.61% octadecane (C18) and 7.30% branched octadecane . This is a thorough analysis of the thermodynamic interactions of this polyethylene/paraffin system from the point of view of phase change materials for energy saving in buildings.
20
The interaction parameter value for linear polyethylene and n-C18H38 is 0.10 at 410oK, as determined by Nakajima and Hamada [31]. Our value is similar to this result. The small difference might be due to the samples themselves. Nakajima and Hamada used fractionated linear polyethylene Sholex-6009 (polyethylene of Marlex-50 type manufactured by Japan Olefin Chem. Co.), with fractionation carried out by fractional precipitation in xylene-triethylene glycol at 130oC, and a viscosity-average degree of polymerization of the fraction of 2000. The product that they used was an 18-carbon paraffin purified by distillation at reduced pressure. The PE and paraffin that we used were unpurified and they were used immediately after purchase.
The small positive value of χ12 indicates that PE and paraffin are partially miscible due to their similar chemical structures. The values of χ12 play a notable role in miscibility, with a smaller χ12 indicating better miscibility. Though the difference is small, comparing the values of χ12, 0.0614, 0.0612 and 0.0596 for HDPE, LDPE and LLDPE blend with paraffin, we once again show that the paraffin miscibility with polyethylene is in the order of HDPE
Conclusion High-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) are partially miscible with paraffin. The HDPE/paraffin blend has the weakest miscibility of the three systems. Because of the influence of miscibility on the thermal behavior of paraffin, we suggest using HDPE in PE/paraffin form-stable phase change materials to maintain the energy saving behavior of paraffin in building applications for reducing interior temperature fluctuations.
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Acknowledgment
This work was supported by the Louisiana Pacific Quasi-Endowment Fund. The authors appreciate Dr. Jinwu Wang's help on editing.
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Graphical Abstract
Title: Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials Authors: Fang Chen, Michael P. Wolcott
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Highlights AFM phase images verified partial miscibility of paraffin in polyethylene. The crystallinity of each component depends upon the blend concentration. Two paraffin crystallization peaks for polyethylene/paraffin blends were observed. The lowest paraffin miscibility in polyethylene is found in paraffin/HDPE blend.
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