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Polyurethanes as solid–solid phase change materials for thermal energy storage
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Solar Energy 86 (2012) 1761–1769 www.elsevier.com/locate/solener
Polyurethanes as solid–solid phase change materials for thermal energy storage ¨ mer F. Ensari b, Derya Kahraman b Cemil Alkan a,b,⇑, Eva Gu¨nther a, Stefan Hiebler a, O a
Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meissner-Str. 6, 85748 Garching, Germany b Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey Received 8 September 2011; received in revised form 29 February 2012; accepted 19 March 2012 Available online 11 April 2012 Communicated by: Associate Editor Halime Paksoy
Abstract Polyurethane polymers (PUs) have been synthesized as solid–solid phase change materials for thermal energy storage using three different kinds of diisocyanate molecules and polyethylene glycols (PEGs) at three different molecular weights. PEGs and their derivatives are usually used as phase change units in polymeric solid–solid phase change materials due to the hydroxyl functional groups. 1000, 6000, and 10,000 g/mol number average molecular weight PEGs are used as working element as hexamethylene, isophorone, and toluene diisocyanates are used as hard segment at the backbone. The effects of molecular weight of PEG and type of diisocyanate on the thermal energy storage properties have been discussed. Only two of the produced polymers show solid–liquid phase change as the rest show solid–solid phase transitions. The produced PUs with a solid–solid phase transitions have potential to be used in thermal energy storage systems. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Energy storage; Phase change material; Solid–solid phase transition; Polyurethane
1. Introduction Energy demand is increasing on a worldwide basis, as the sources are decreasing (International Energy Outlook, 2006). The consumption of energy sources are planned to be decelerated by exploiting renewable energy sources and using the present energy sources more economically. Thermal energy storage using phase change materials (PCMs) is one of the most attracting means of energy saving. Therefore there are many scientists working to generate more efficient materials. Solid–solid PCMs are advantageous among all of the phase change materials due to following reasons: (1) no leakage of melted PCM during phase change process; (2) no additional storage container for encapsulation; (3) ⇑ Corresponding author at: Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey. Tel.: +90 3562521616; fax: +90 3562521285. E-mail address: [email protected] (C. Alkan).
0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.03.012
terminating the reactivity of PCM with the outside of environment; (4) easily preparing in desired dimensions and therefore, being feasible for some heating applications in buildings such as under floor space heating and reducing electric peak load in heating using polymeric SSPCM boards or coverings. Many solids undergo reversible phase changes in the solid state, but only very few have sufficient latent heat to be a potential latent heat energy storage material. These solid state PCMs can be classified in three groups (Xi et al., 2009): inorganic solid–solid PCMs (Li et al., 1999; Busico et al., 1980; Landi and Vacatello, 1975; Ruiyun et al., 1990; Ruan et al., 1995), certain hydrocarbon molecular crystals (Benson et al., 1985), and polymer based solid– solid PCMs (Xi et al., 2009; Qi and Liu, 2006; Jiang et al., 2002; Li and Ding, 2007; Gu et al., 2010; Pielichowska and Pielichowski, 2010). The object of this study is to develop a novel group of polymeric solid–solid PCMs
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based on poly(ethylene glycol) (PEG), i.e., substances of the third group. PEG is a well-defined macromolecule with the characteristics of non toxicity, good biocompatibility, biodegradability, hydrophilicity, and ease of chemical modification, which have led to widespread applications of PEG in chemical, biomedical, and biotechnological fields (Han et al., 1997). Moreover, PEG is one of the promising working materials for PCMs because of its relatively large fusion heat, congruent melting behavior, resistance to corrosion (Harris and Zalipsky, 1997). PEG and their derivatives are usually used as phase change units of polymer based solid–solid phase change materials due to their superior properties (Xi et al., 2009): PEG and monomethylether PEG possesses high heat of fusion, melting temperature variability, and wide molecular weight selectivity (Wang et al., 2009). Novel types of PEG based polymeric solid– solid PCMs have been prepared using the method of direct graft copolymerization (Jiang et al., 2002) or step growth polymerization (Chen et al., 2008). However PEG based polymeric solid–solid PCMs do not serve satisfying thermal energy storage properties until the molecular weight of PEG or their derivatives is higher than 4000 according to literature (Li and Ding, 2007; Su and Liu, 2006; Cao and Liu, 2006). For the fabrication of shape stabilized PCMs, physical blending (Liang et al., 1995; Alkan and Sari, 2008; Sari et al., 2006; S ß entu¨rk et al., 2011) and chemical modification (Cao and Liu, 2006; Li and Ding, 2007; Sarıer ¨ nder, 2008) are the main methods. and O In the present work, polyurethane polymers are produced from PEGs at three different average molecular weights in N,N-dimethylformamide (DMF) as solid–solid PCMs simply using hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), and 2,4-toluene diisocyanate (TDI) as coupling reagents. Equimolar amount of diisocyanate and diol molecules were used to get maximum molecular weight leading to entanglements or physical crosslinking points so that physical interactions would prevent melting of PUs. These new PUs were characterized using fourier transform-infrared (FT-IR) spectroscopy, polarized optical microscopy (POM), and differential scanning calorimetry (DSC) methods. Also a special technique to determine the total amount of enthalpy stored between determined temperature values was used. 2. Experimental 2.1. Materials Three types of PEGs; PEG 1000, PEG 6000, and PEG 10,000 used as soft segments were technical grade chemicals from Merck and used without further purification. HMDI and TDI from Sigma and IPDI from Aldrich were analytical grade and used without further purification. DMF obtained from Sigma was also analytical grade and used as received.
2.2. Synthesis For the synthesis of PUs, a general route as shown in Fig. 1 is followed. For each of PU preparations, 3 g of PEG and stoichiometrically equimolar amount of diisocyanate compound is dissolved in DMF separately and diisocyanate solution is added to PEG solution dropwise by vigorous mixing. After complete addition, the reaction media is mixed for 3 h and solvent is evaporated under vacuum. Then temperature of the resultant polymers is kept constant at 100 °C for an hour. The resultant polymers are called as condensates due to that the molecular weight of the resultant polymers increases as a result of coupling with diisocyanates without any bi-product. 2.3. Analysis Spectroscopic analysis of pristine PEGs and PU polymers is performed on KBr disks using a Jasco 430 model FT-IR spectrophotometer. The KBr pressed disc technique (around 2 mg of sample and 200 mg of KBr) is used for sample preparation. A visual observation is performed with POM on a Leica DM EP polarizing optical microscope (Germany, 2010) equipped with a camera and a computer. The sample is placed between a microscope glass and a cover slip and heated with a hot stage. To characterize the thermal behavior of PEGs and PUs, analyses are conducted on a TA Instruments DSC Q2000 V24.4 type DSC (Eschborn, Germany, 2010). The instrument is calibrated for measurements performed at different heating rates. During analyses of PEGs, test specimens are heated from 0 to 80 °C at a rate of 2 °C min 1 in closed crucibles under a nitrogen atmosphere (50 mL min 1). The DSC analyses of pristine PEGs are conducted at least three times to determine its capacity of heat storage for repetitive heating cycles. PEG-diisocyanate condensates are also tested by DSC at the same manner but between 20 and 100 °C. 3. Results and discussion 3.1. FTIR-analysis PUs are commonly produced by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl groups easily. They are widely used in various applications like high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts due to their considerably wide spectrum of properties changing upon the raw materials. So synthesis of polyurethane and characterization is very
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Fig. 1. Reaction mechanism to synthesis of the polyurethanes.
common. Fig. 2 shows FTIR spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates as a sample of characterization of polyurethane polymers produced. Hydroxyl groups of PEG are commonly strong and observed at around 3440 cm 1 due to hydrogen bonding interactions. Another relatively strong absorption peak of 3280 cm 1 is due to the N-H stretching vibration and single sharp peaks at 1618 cm 1, 1629 cm 1, and 1616 cm 1 in the spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates is another evidence of NH groups. N-H bonds form as a result of urethane producing reaction and it is not present in PEG or diisocyanates. The carbonyl group is another new formed group and its stretching vibration peak is appearing at around 1705 cm 1 , 1712 cm 1, 1725 cm 1 for PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates respectively. Similar bond formations have been observed in also PUs produced using the other PEG polymers at average molecular weights of 6000 and 10,000 proving urethane linkages in those polymers. On the other hand the intensity of CH2
Fig. 2. FT–IR spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates.
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vibration peaks are generally higher in pristine PEG polymers at different molecular weights as compared to the strong absorption peak of OH groups around 3440 cm 1. However its intensity decreases to values below the intensity of OH groups in its urethane compounds. According to Fig. 2, the CH2 peak intensity is lower than the OH stretching peak intensity only in PEG 1000-HMDI condensate while it is almost equal in PEG 1000-IPDI condensate and higher in PEG 1000-TDI condensate. This result shows that the characteristic peaks in PEG 1000-TDI condensate is the most similar ones to PEG 1000 compound among all the urethane polymers synthesized in this work. 3.2. Form stability/leakage tests The structural consistency of PUs above solid–solid transition temperature was determined applying thermal treatment. For this reason, produced PUs were grouped into three: PEG–HMDI condensates (Fig. 3a), PEG–IPDI condensates (Fig. 3b), and PEG–TDI condensates (Fig. 3c). The derivatives of each urethane with average molecular weights of 1000, 6000, and 10,000 g/mol of PEG were placed from left to right by increasing average molecular weights. The samples were heated to 100 °C, i.e., well above the solid–solid phase transition temperature. The tubes were photographed after waiting for 20 min to remove thermal history. It is clear from Fig. 3 that PEG 1000-IPDI and PEG 1000-TDI condensates melt at 100 °C completely. So they were classified as solid–liquid PCMs, while the rest were classified as solid–solid PCMs. Mobility of the polymer chains depends on the density of chain ends since chain ends prevent the molecules to get close, decreasing secondary interactions between the chains. As the number of chain ends per unit volume increases for decreasing molecular weight of the polymer, the phase change temperature and enthalpy decrease because the range of interactions decreases. In PUs, PEG polymers as phase changing elements are dispersed between urethane linkages and bonding PEG chain ends via the urethane linkage decreases the chain end effect. On the other hand, the six membered rings of IPDI and TDI introduce a constant separation of the PEG chains. Therefore the melting temperature and enthalpies of PEG 1000-IPDI and PEG 1000-TDI condensates are lower than PEG 1000 and these two polymers are solid–liquid phase change materials. At higher molecular weights of PEG, the range of interaction between the PEG chains is long enough for crystallization. Here, the urethane bonding of the PEG chains results in entanglements of the chains preventing melting of the matrix. Contrary to IPDI and PEG–TDI condensates, PEG–HMDI condensates are linear and symmetric along the backbone of the molecule inducing crystallinity to polyurethane polymers. Also HMDI is a molecule not creating steric effect for PEG segments of polyurethane polymer to get close. As a result PEG–HMDI condensates are all solid–solid PCMs.
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Fig. 3. Photographs of the polyurethanes after thermal treatment (a) HMDI–PEG 1000, HMDI–PEG 6000, and HMDI–PEG 10,000 condensates respectively (b) IPDI–PEG 1000, IPDI–PEG 6000, and IPDI–PEG 10,000 condensates respectively (c) TDI–PEG 1000, TDI–PEG 6000, and TDI–PEG 10,000 condensates respectively.
3.3. Thermal analysis The DSC data for all investigated materials are given in Table 1. The phase change temperature corresponds to the peak onset temperature as obtained by standard DSC evaluation. The latent heat of fusion was evaluated by peak integration using the software of the DSC manufacturer. In addition to the onset temperatures and enthalpy determined by standard DSC evaluation, in this study, the total capacity of thermal storage of pristine PEGs and PU polymers were also determined according to Mehlings’ method which is based on the summation of sensible heat and latent heat (Mehling et al., 2009). This evaluation directly shows the heat storage capacity for usage as thermal storage material. Figs. 4 and 5 show the DSC curves of PEG polymers used to produce polyurethane polymers in this study and their total amount of stored heat. It is seen that PEGs store latent heat at different temperatures depending upon their
average molecular weights to some extent. However, when the molecular weight reaches a threshold value, phase change temperature and latent heat do not change significantly. Judging from the measurement data as shown in Fig. 4 and Table 1, the threshold value of the molecular weight is situated above 1000 g/mol but below 6000 g/ mol: the phase change temperature of the used polymers is increased strongly when the molecular weight is increased from 1000 g/mol to 6000 g/mol, but there is only a small increase when it goes from 6000 g/mol to 10,000 g/ mol. The same tendency is observed for the latent heat. Looking at Fig. 5, also the total enthalpy storage value depends on the molecular weight of PEG. However, the inclination of the curves outside the phase transition regions are almost same for each of the PEGs, which means that sensible heat storage capacities are almost the same. The total energy stored is between 300 J/g and 315 J/g between 0 °C and 80 °C. In this representation of the measurement data, overcooling is clearly visible as the distance
Table 1 DSC data for pristine PEGs and PU polymers. Sample
PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG
1000 6000 10,000 1000-HMDI 6000-HMDI 10,000-HMDI 1000-IPDI 6000-IPDI 10,000-IPDI 1000-TDI 6000-TDI 10,000-TDI
Phase transition
S-L S-L S-L S-S S-S S-S S-L S-S S-S S-L S-S S-S
DH (J/g)
Transition temperature
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
153 176 179 109 176 171 92.5 166 169 69.5 161 162
155 176 184 113 177 173 91.7 165 168 65.6 164 162
32.4 57.4 59.7 19.0 59.9 57.7 23.3 57.4 58.8 26.9 57.0 57.1
30.7 47.3 50.1 26.3 47.2 48.9 20.4 41.2 46.7 3.8 41.4 46.0
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Fig. 4. DSC curves (exo down) of PEGs at average molecular weights of 1000, 6000, and 10,000 g/mol.
Fig. 5. Total enthalpy curves of PEGs between 0 °C and 80 °C. Solid lines/filled symbols denote heating curves, dashed lines/open symbols cooling curves.
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Fig. 6. Total enthalpy plots for PEG1000 and PEG1000-diisocyanate condensates.
Fig. 7. Total enthalpy plots for PEG6000 and PEG6000-diisocyanate condensates.
in temperature between heating and cooling curves. It can be seen that overcooling is larger for PEG 6000 and PEG 10,000 compared to PEG 1000. In Figs. 6–8, total enthalpy plots are shown for the PEGs and the corresponding three PUs for each molecular weight of PEGs in a separate graph. The results are discussed with respect to the key characteristics, namely (1) phase change temperature, (2) enthalpy change and (3) overcooling, in the following paragraphs. (1) The phase change temperature as function of diisocyanate is almost invariant for PEG 6000 and PEG 10,000, but not for PEG 1000. Here, the melting temperature is reduced for IPDI and HMDI materials by about 2 K and by about 4 K for the TDI material. (2) Similarly to the melting temperature, also the enthalpy change is most affected for the PEG 1000 materials in the order HMDI–IPDI–TDI. For the later, the phase change enthalpy is reduced to less than 50% (see Table 1). As to the PEG 6000 materials, the enthalpy change is reduced by roughly 15% at most for the HMDI–PU.
Fig. 8. Total enthalpy plots for PEG10,000 and PEG10,000-diisocyanate condensates.
Here, the TDI- and IPDI-materials show less to no change in enthalpy. As to the PEG 10,000 materials, the total enthalpy change is almost invariant with respect to the di-isocyanate component for all investigated materials.
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(3) As to the overcooling, no effect at all is observed for PEG 6000-HMDI condensate. For all other materials, overcooling of the PU’s is stronger than for the PEGs. The amount of change in overcooling is about 2 K for PEG 10,000-TDI and PEG 1000-HMDI, about 4 K for PEG 10,000-IPDI and -HMDI, about 5 K for PEG 6000-IPDI and –TDI, about 10 K for PEG 1000-IPDI and about 28 K for PEG 1000-TDI. In summary, overcooling is increased less for the higher molecular weight PEG and for HMDI. As for IPDI and TDI materials, no clear trend is visible from our data. While the change in overcooling is clearly more pronounced in the low molecular weight PEG, the absolute overcooling is 15–20 K for most materials with the exception of PEG 1000-HMDI (about 7 K) and PEG 1000-TDI (about 32 K). For PEG 10,000, the slope of the total enthalpy curves is different for the different PU’s: The melting occurs at a very narrow temperature range for the pristine PEG, but this range grows for IPDI polyurethane and even more for HMDI and TDI polyurethanes. Pristine PEGs consist of ethyl–ether linked segments with active hydroxyl end groups that easily form intermolecular hydrogen bonds, crystallize easily and have high transformation enthalpies for melting [22]. Expectedly, the DSC curves of PU polymers were similar to that of pristine PEG because their structure contained PEG units. The phase change enthalpies of PU polymers are also considerably high, which means that PU polymers have potential to be used as latent heat thermal energy storage materials. However the number of segments available for the crystalline regions reduced because of the presence of hard segments at the backbone of the PU polymers, which consequentially led to the decline of phase change temperature and enthalpy in general.
In general, the phase change temperatures of PU polymers are lower than that of pristine PEGs. The segments near hard segment sites are confined after active hydroxyl end groups of PEG reacted with diisocyanate. Consequently the arrangement and orientation of PEG molecules are partially suppressed by the steric effect and the crystalline regions become smaller, which causes the transition point and enthalpy to fall to a certain degree. This observation that an introduction of hard segments results in a decrease of the phase change temperature and enthalpy agrees with observations previously found by many researchers (Su and Liu, 2006; Cao and Liu, 2006). Our study however quantifies this effect for the selected diisocyanates and PEG molecular weights. As a result of our leakage test PEG 1000-IPDI condensate was identified as the polymer with solid–liquid phase changing property. Moreover it was observed that overcooling is at maximum in this condensate. This is probably due to the steric effect preventing the chains to get close. This effect is less observable in PEG–HMDI condensates due to linear and symmetrical structure of HMDI and at high molecular weight PEGs because of decreasing impact of steric hindrance on long molecules. IPDI forms the biggest hard segment among the other diisocyanates used in this study. So it produces the maximum steric hindrance for PEG segments. Due to that PEG 1000 is the smallest molecule among the used ones in this study, its transition temperatures and enthalpies are affected mostly. PEG 6000-IPDI and PEG 10,000-IPDI condensates are also affected by the blocking reaction even the integral heat values for these two are higher than corresponding PEG polymers. Toluene diisocyanate which is generally in 2,4-TDI form is one of the most commonly used counterpart in industrial polyurethane production so it is chosen to be used in this study. 2,4-TDI is not a linear molecule like IPDI therefore there is steric hindrance effective on crystallization of its
Fig. 9. POM images of polyurethanes produced using PEG 1000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
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Fig. 10. POM images of polyurethanes produced using PEG 6000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
Fig. 11. POM images of polyurethanes produced using PEG 10,000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
condensate with PEG 1000. However as in the case of PEG–IPDI condensates, PEG–TDI condensates are not affected by steric affects at higher molecular weights. Furthermore the total enthalpy stored by PEG 1000-TDI condensate is much lower among PEG–TDI condensates between 20 and 80 °C. In addition overcooling in PEG 1000-TDI condensate is observed at maximum. 3.4. POM investigation Figs. 9–11 show the optical microscopy images of pristine PEGs and polyurethane polymers produced using PEGs and diisocyanates below and above phase transition
temperatures. The observed morphological changes are discussed in the following. Fig. 9 shows the spherulitic structure of PEG 1000 together with polarized microscopy images of the three investigated PEG 1000 condensates below and above their phase transition temperature. In Figs. 10 and 11, the same arrangement of pictures is given for PEG 6000 and PEG 10,000 and their respective polyurethanes. It can be observed that both PEG and PU polymers have spherulitic structures but PUs’ spherulites are smaller in diameter than corresponding PEGs are. For the higher temperatures, the spherulitic structure disappears in all samples. That means, the blocking of PEG polymers using
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diisocyanate molecules did not alter the crystal form of PEG, but it limited the degree of crystallization. The missing spherulites in the POM micrographs of PU polymers at 80 °C demonstrate that the crystal PEG completely transforms to an amorphous state. It can be concluded that in any of the PU polymers a phase transition of PEG from the crystalline solid to the amorphous solid state takes place. According to the DSC data as presented in Section 3.3, during this phase transition of the PEG segments a significant amount of latent heat is absorbed and released. The synthesis of PUs is designed to produce PUs at maximum molecular weights theoretically so synthesized PUs are expected to show solid–solid phase changing property. However PEGs are synthetic polymers and synthetic polymers are always polydispersed. Under these conditions taking a definite molar amount of a synthetic polymer is almost impossible. This leads to a decrease in molecular weights. As a result the resultant PU derivatives had probably grown according to the experimental conditions. At this point it can at least be said that it is possible to produce PU polymers with solid–solid phase changing property. Most of the synthesized PCMs remained in the solid state above transition temperatures because of the hard segments connecting PEG chain ends to each other and so restricting free movements of them. Su et al. produced PU polymers from a hard segment produced reacting butanediol with 2 methylene diisocyanate molecules and PEG 4000 and reported its results (Su and Liu, 2006) which are very similar to our work. Polymers are viscoelastic materials and PEG blocks at the backbone makes them more viscous. To improve mechanical properties, diisocyanate ended PU blocks (considerably long hard segments) could be developed and used. Our further studies to produce PUs with better physical properties for applications are going on. 4. Application outlook The transition temperatures of PUs were suitable for many applications such as solar energy storage, waste heat recovery, and temperature adaptable greenhouses. The novel solid–solid PCMs with high phase change enthalpy will be promising heat storage materials and may take place of other similar energy storage materials. The phase change temperatures of PEG–HMDI condensates could be better suitable for solar space heating applications and air conditioning. 5. Conclusion Preparations of various type of PUs were conducted via the condensation reaction of PEGs with diisocyanates. The results showed that produced PUs had typical solid–solid phase transition properties with high enthalpies reaching 179 kJ/kg, and transition points between 19 and 60 °C. The resultant blocking copolymers could be potentially
used as solid–state PCMs for thermal energy storage and temperature-control applications if the PEG molecular weight is high enough to crystallize under different conditions. PUPCMs with the special solid–solid phase change behaviors of high thermal energy storage capability, suitable phase transition temperature, reversible latent heat transition, and good thermal stability can be used as a new kind of solid–solid PCMs for thermal energy storage and temperature control. Acknowledgements We would like to thank the German Federal Ministry of Education and Research (project ENFoVerM/FKZ 0327851D) and Scientific and Technical Research Council of Turkey TUBITAK (project code: 108T865-TBAG) for their financial support for this study. References Alkan, C., Sari, A., 2008. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Solar Energy 82, 118–124. Benson, D.K., Webb, J.D., Burrows, R.W., McFadden, J.D.O., Christensen, C., 1985. Materials research for passive solar systems: solid– state phase-change materials. report prepared for the US. Department of energy, Contract No. EG-77-C-01-4042. Busico, V., Carfagna, C., Salerno, V., 1980. The layer perovskites as thermal energy storage systems. Solar Energy 24, 575–579. Cao, Q., Liu, P., 2006. Hyperbranched polyurethane as novel solid–solid phase change material for thermal energy storage. European Polymer Journal 42, 2931–2939. Chen, C., Wang, L., Huang, Y., 2008. Morphology and thermal properties of electrospun fatty acids/polyethylene terephthalate composite fibers as novel form stable phase change materials. Solar Energy Materials and Solar Cells 92, 1382–1387. Gu, X., Xi, P., Cheng, B., Niu, S., 2010. Synthesis and characterization of a novel solid–solid phase change luminescence material. Polymer International 59, 772–777. Han, S., Kim, C., Kwon, D., 1997. Thermal/oxidative degradation and stabilization of polyethylene glycol. Polymer 38, 317–323. Harris, J.M., Zalipsky, S., 1997. Introduction to Chemistry and Biological Applications of Poly(ethylene glycol). American Chemistry Society, Washington, DC. International Energy Outlook, 2006. Energy information administration office of integrated analysis and forecasting US. Department of Energy. Available from:. Jiang, Y., Ding, E., Li, G., 2002. Study on the transition characteristics of PEG/CDA solid–solid phase change materials. Polymer 43, 117–122. Landi, E., Vacatello, M., 1975. Metal-dependent thermal behaviour Ln (nCnH2n+1NH3)2MCl4. Thermochimica Acta 13, 441–447. Li, W., Zhang, D., Zhang, T., Wang, T., Ruan, D., Xing, D., Li, H., 1999. Study of solid–solid phase change of (n-CnH2n+1NH3)2MCl4 for thermal energy storage. Thermochimica Acta 326, 183–186. Li, W.D., Ding, E.Y., 2007. Preparation and characterization of crosslinking PEG/MDI/PE copolymer as solid–solid phase change heat storage material. Solar Energy Materials and Solar Cells 91, 764–768. Liang, X.H., Guo, Y.Q., Gu, L.Z., Ding, E.Y., 1995. Crystalline– amorphous phase- transition of a poly(ethylene glycol) cellulose blend. Macromolecules 28, 6551–6555. Mehling, H., Gu¨nther, E., Hiebler, S., 2009. A new measurement and evaluation method for DSC of PCM samples. In: Proceedings of
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Polyurethanes as solid–solid phase change materials for thermal energy storage ¨ mer F. Ensari b, Derya Kahraman b Cemil Alkan a,b,⇑, Eva Gu¨nther a, Stefan Hiebler a, O a
Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meissner-Str. 6, 85748 Garching, Germany b Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey Received 8 September 2011; received in revised form 29 February 2012; accepted 19 March 2012 Available online 11 April 2012 Communicated by: Associate Editor Halime Paksoy
Abstract Polyurethane polymers (PUs) have been synthesized as solid–solid phase change materials for thermal energy storage using three different kinds of diisocyanate molecules and polyethylene glycols (PEGs) at three different molecular weights. PEGs and their derivatives are usually used as phase change units in polymeric solid–solid phase change materials due to the hydroxyl functional groups. 1000, 6000, and 10,000 g/mol number average molecular weight PEGs are used as working element as hexamethylene, isophorone, and toluene diisocyanates are used as hard segment at the backbone. The effects of molecular weight of PEG and type of diisocyanate on the thermal energy storage properties have been discussed. Only two of the produced polymers show solid–liquid phase change as the rest show solid–solid phase transitions. The produced PUs with a solid–solid phase transitions have potential to be used in thermal energy storage systems. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Energy storage; Phase change material; Solid–solid phase transition; Polyurethane
1. Introduction Energy demand is increasing on a worldwide basis, as the sources are decreasing (International Energy Outlook, 2006). The consumption of energy sources are planned to be decelerated by exploiting renewable energy sources and using the present energy sources more economically. Thermal energy storage using phase change materials (PCMs) is one of the most attracting means of energy saving. Therefore there are many scientists working to generate more efficient materials. Solid–solid PCMs are advantageous among all of the phase change materials due to following reasons: (1) no leakage of melted PCM during phase change process; (2) no additional storage container for encapsulation; (3) ⇑ Corresponding author at: Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey. Tel.: +90 3562521616; fax: +90 3562521285. E-mail address: [email protected] (C. Alkan).
0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.03.012
terminating the reactivity of PCM with the outside of environment; (4) easily preparing in desired dimensions and therefore, being feasible for some heating applications in buildings such as under floor space heating and reducing electric peak load in heating using polymeric SSPCM boards or coverings. Many solids undergo reversible phase changes in the solid state, but only very few have sufficient latent heat to be a potential latent heat energy storage material. These solid state PCMs can be classified in three groups (Xi et al., 2009): inorganic solid–solid PCMs (Li et al., 1999; Busico et al., 1980; Landi and Vacatello, 1975; Ruiyun et al., 1990; Ruan et al., 1995), certain hydrocarbon molecular crystals (Benson et al., 1985), and polymer based solid– solid PCMs (Xi et al., 2009; Qi and Liu, 2006; Jiang et al., 2002; Li and Ding, 2007; Gu et al., 2010; Pielichowska and Pielichowski, 2010). The object of this study is to develop a novel group of polymeric solid–solid PCMs
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based on poly(ethylene glycol) (PEG), i.e., substances of the third group. PEG is a well-defined macromolecule with the characteristics of non toxicity, good biocompatibility, biodegradability, hydrophilicity, and ease of chemical modification, which have led to widespread applications of PEG in chemical, biomedical, and biotechnological fields (Han et al., 1997). Moreover, PEG is one of the promising working materials for PCMs because of its relatively large fusion heat, congruent melting behavior, resistance to corrosion (Harris and Zalipsky, 1997). PEG and their derivatives are usually used as phase change units of polymer based solid–solid phase change materials due to their superior properties (Xi et al., 2009): PEG and monomethylether PEG possesses high heat of fusion, melting temperature variability, and wide molecular weight selectivity (Wang et al., 2009). Novel types of PEG based polymeric solid– solid PCMs have been prepared using the method of direct graft copolymerization (Jiang et al., 2002) or step growth polymerization (Chen et al., 2008). However PEG based polymeric solid–solid PCMs do not serve satisfying thermal energy storage properties until the molecular weight of PEG or their derivatives is higher than 4000 according to literature (Li and Ding, 2007; Su and Liu, 2006; Cao and Liu, 2006). For the fabrication of shape stabilized PCMs, physical blending (Liang et al., 1995; Alkan and Sari, 2008; Sari et al., 2006; S ß entu¨rk et al., 2011) and chemical modification (Cao and Liu, 2006; Li and Ding, 2007; Sarıer ¨ nder, 2008) are the main methods. and O In the present work, polyurethane polymers are produced from PEGs at three different average molecular weights in N,N-dimethylformamide (DMF) as solid–solid PCMs simply using hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), and 2,4-toluene diisocyanate (TDI) as coupling reagents. Equimolar amount of diisocyanate and diol molecules were used to get maximum molecular weight leading to entanglements or physical crosslinking points so that physical interactions would prevent melting of PUs. These new PUs were characterized using fourier transform-infrared (FT-IR) spectroscopy, polarized optical microscopy (POM), and differential scanning calorimetry (DSC) methods. Also a special technique to determine the total amount of enthalpy stored between determined temperature values was used. 2. Experimental 2.1. Materials Three types of PEGs; PEG 1000, PEG 6000, and PEG 10,000 used as soft segments were technical grade chemicals from Merck and used without further purification. HMDI and TDI from Sigma and IPDI from Aldrich were analytical grade and used without further purification. DMF obtained from Sigma was also analytical grade and used as received.
2.2. Synthesis For the synthesis of PUs, a general route as shown in Fig. 1 is followed. For each of PU preparations, 3 g of PEG and stoichiometrically equimolar amount of diisocyanate compound is dissolved in DMF separately and diisocyanate solution is added to PEG solution dropwise by vigorous mixing. After complete addition, the reaction media is mixed for 3 h and solvent is evaporated under vacuum. Then temperature of the resultant polymers is kept constant at 100 °C for an hour. The resultant polymers are called as condensates due to that the molecular weight of the resultant polymers increases as a result of coupling with diisocyanates without any bi-product. 2.3. Analysis Spectroscopic analysis of pristine PEGs and PU polymers is performed on KBr disks using a Jasco 430 model FT-IR spectrophotometer. The KBr pressed disc technique (around 2 mg of sample and 200 mg of KBr) is used for sample preparation. A visual observation is performed with POM on a Leica DM EP polarizing optical microscope (Germany, 2010) equipped with a camera and a computer. The sample is placed between a microscope glass and a cover slip and heated with a hot stage. To characterize the thermal behavior of PEGs and PUs, analyses are conducted on a TA Instruments DSC Q2000 V24.4 type DSC (Eschborn, Germany, 2010). The instrument is calibrated for measurements performed at different heating rates. During analyses of PEGs, test specimens are heated from 0 to 80 °C at a rate of 2 °C min 1 in closed crucibles under a nitrogen atmosphere (50 mL min 1). The DSC analyses of pristine PEGs are conducted at least three times to determine its capacity of heat storage for repetitive heating cycles. PEG-diisocyanate condensates are also tested by DSC at the same manner but between 20 and 100 °C. 3. Results and discussion 3.1. FTIR-analysis PUs are commonly produced by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl groups easily. They are widely used in various applications like high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts due to their considerably wide spectrum of properties changing upon the raw materials. So synthesis of polyurethane and characterization is very
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Fig. 1. Reaction mechanism to synthesis of the polyurethanes.
common. Fig. 2 shows FTIR spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates as a sample of characterization of polyurethane polymers produced. Hydroxyl groups of PEG are commonly strong and observed at around 3440 cm 1 due to hydrogen bonding interactions. Another relatively strong absorption peak of 3280 cm 1 is due to the N-H stretching vibration and single sharp peaks at 1618 cm 1, 1629 cm 1, and 1616 cm 1 in the spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates is another evidence of NH groups. N-H bonds form as a result of urethane producing reaction and it is not present in PEG or diisocyanates. The carbonyl group is another new formed group and its stretching vibration peak is appearing at around 1705 cm 1 , 1712 cm 1, 1725 cm 1 for PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates respectively. Similar bond formations have been observed in also PUs produced using the other PEG polymers at average molecular weights of 6000 and 10,000 proving urethane linkages in those polymers. On the other hand the intensity of CH2
Fig. 2. FT–IR spectra of PEG 1000-HMDI, PEG 1000-IPDI, and PEG 1000-TDI condensates.
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vibration peaks are generally higher in pristine PEG polymers at different molecular weights as compared to the strong absorption peak of OH groups around 3440 cm 1. However its intensity decreases to values below the intensity of OH groups in its urethane compounds. According to Fig. 2, the CH2 peak intensity is lower than the OH stretching peak intensity only in PEG 1000-HMDI condensate while it is almost equal in PEG 1000-IPDI condensate and higher in PEG 1000-TDI condensate. This result shows that the characteristic peaks in PEG 1000-TDI condensate is the most similar ones to PEG 1000 compound among all the urethane polymers synthesized in this work. 3.2. Form stability/leakage tests The structural consistency of PUs above solid–solid transition temperature was determined applying thermal treatment. For this reason, produced PUs were grouped into three: PEG–HMDI condensates (Fig. 3a), PEG–IPDI condensates (Fig. 3b), and PEG–TDI condensates (Fig. 3c). The derivatives of each urethane with average molecular weights of 1000, 6000, and 10,000 g/mol of PEG were placed from left to right by increasing average molecular weights. The samples were heated to 100 °C, i.e., well above the solid–solid phase transition temperature. The tubes were photographed after waiting for 20 min to remove thermal history. It is clear from Fig. 3 that PEG 1000-IPDI and PEG 1000-TDI condensates melt at 100 °C completely. So they were classified as solid–liquid PCMs, while the rest were classified as solid–solid PCMs. Mobility of the polymer chains depends on the density of chain ends since chain ends prevent the molecules to get close, decreasing secondary interactions between the chains. As the number of chain ends per unit volume increases for decreasing molecular weight of the polymer, the phase change temperature and enthalpy decrease because the range of interactions decreases. In PUs, PEG polymers as phase changing elements are dispersed between urethane linkages and bonding PEG chain ends via the urethane linkage decreases the chain end effect. On the other hand, the six membered rings of IPDI and TDI introduce a constant separation of the PEG chains. Therefore the melting temperature and enthalpies of PEG 1000-IPDI and PEG 1000-TDI condensates are lower than PEG 1000 and these two polymers are solid–liquid phase change materials. At higher molecular weights of PEG, the range of interaction between the PEG chains is long enough for crystallization. Here, the urethane bonding of the PEG chains results in entanglements of the chains preventing melting of the matrix. Contrary to IPDI and PEG–TDI condensates, PEG–HMDI condensates are linear and symmetric along the backbone of the molecule inducing crystallinity to polyurethane polymers. Also HMDI is a molecule not creating steric effect for PEG segments of polyurethane polymer to get close. As a result PEG–HMDI condensates are all solid–solid PCMs.
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Fig. 3. Photographs of the polyurethanes after thermal treatment (a) HMDI–PEG 1000, HMDI–PEG 6000, and HMDI–PEG 10,000 condensates respectively (b) IPDI–PEG 1000, IPDI–PEG 6000, and IPDI–PEG 10,000 condensates respectively (c) TDI–PEG 1000, TDI–PEG 6000, and TDI–PEG 10,000 condensates respectively.
3.3. Thermal analysis The DSC data for all investigated materials are given in Table 1. The phase change temperature corresponds to the peak onset temperature as obtained by standard DSC evaluation. The latent heat of fusion was evaluated by peak integration using the software of the DSC manufacturer. In addition to the onset temperatures and enthalpy determined by standard DSC evaluation, in this study, the total capacity of thermal storage of pristine PEGs and PU polymers were also determined according to Mehlings’ method which is based on the summation of sensible heat and latent heat (Mehling et al., 2009). This evaluation directly shows the heat storage capacity for usage as thermal storage material. Figs. 4 and 5 show the DSC curves of PEG polymers used to produce polyurethane polymers in this study and their total amount of stored heat. It is seen that PEGs store latent heat at different temperatures depending upon their
average molecular weights to some extent. However, when the molecular weight reaches a threshold value, phase change temperature and latent heat do not change significantly. Judging from the measurement data as shown in Fig. 4 and Table 1, the threshold value of the molecular weight is situated above 1000 g/mol but below 6000 g/ mol: the phase change temperature of the used polymers is increased strongly when the molecular weight is increased from 1000 g/mol to 6000 g/mol, but there is only a small increase when it goes from 6000 g/mol to 10,000 g/ mol. The same tendency is observed for the latent heat. Looking at Fig. 5, also the total enthalpy storage value depends on the molecular weight of PEG. However, the inclination of the curves outside the phase transition regions are almost same for each of the PEGs, which means that sensible heat storage capacities are almost the same. The total energy stored is between 300 J/g and 315 J/g between 0 °C and 80 °C. In this representation of the measurement data, overcooling is clearly visible as the distance
Table 1 DSC data for pristine PEGs and PU polymers. Sample
PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG
1000 6000 10,000 1000-HMDI 6000-HMDI 10,000-HMDI 1000-IPDI 6000-IPDI 10,000-IPDI 1000-TDI 6000-TDI 10,000-TDI
Phase transition
S-L S-L S-L S-S S-S S-S S-L S-S S-S S-L S-S S-S
DH (J/g)
Transition temperature
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
153 176 179 109 176 171 92.5 166 169 69.5 161 162
155 176 184 113 177 173 91.7 165 168 65.6 164 162
32.4 57.4 59.7 19.0 59.9 57.7 23.3 57.4 58.8 26.9 57.0 57.1
30.7 47.3 50.1 26.3 47.2 48.9 20.4 41.2 46.7 3.8 41.4 46.0
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Fig. 4. DSC curves (exo down) of PEGs at average molecular weights of 1000, 6000, and 10,000 g/mol.
Fig. 5. Total enthalpy curves of PEGs between 0 °C and 80 °C. Solid lines/filled symbols denote heating curves, dashed lines/open symbols cooling curves.
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Fig. 6. Total enthalpy plots for PEG1000 and PEG1000-diisocyanate condensates.
Fig. 7. Total enthalpy plots for PEG6000 and PEG6000-diisocyanate condensates.
in temperature between heating and cooling curves. It can be seen that overcooling is larger for PEG 6000 and PEG 10,000 compared to PEG 1000. In Figs. 6–8, total enthalpy plots are shown for the PEGs and the corresponding three PUs for each molecular weight of PEGs in a separate graph. The results are discussed with respect to the key characteristics, namely (1) phase change temperature, (2) enthalpy change and (3) overcooling, in the following paragraphs. (1) The phase change temperature as function of diisocyanate is almost invariant for PEG 6000 and PEG 10,000, but not for PEG 1000. Here, the melting temperature is reduced for IPDI and HMDI materials by about 2 K and by about 4 K for the TDI material. (2) Similarly to the melting temperature, also the enthalpy change is most affected for the PEG 1000 materials in the order HMDI–IPDI–TDI. For the later, the phase change enthalpy is reduced to less than 50% (see Table 1). As to the PEG 6000 materials, the enthalpy change is reduced by roughly 15% at most for the HMDI–PU.
Fig. 8. Total enthalpy plots for PEG10,000 and PEG10,000-diisocyanate condensates.
Here, the TDI- and IPDI-materials show less to no change in enthalpy. As to the PEG 10,000 materials, the total enthalpy change is almost invariant with respect to the di-isocyanate component for all investigated materials.
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(3) As to the overcooling, no effect at all is observed for PEG 6000-HMDI condensate. For all other materials, overcooling of the PU’s is stronger than for the PEGs. The amount of change in overcooling is about 2 K for PEG 10,000-TDI and PEG 1000-HMDI, about 4 K for PEG 10,000-IPDI and -HMDI, about 5 K for PEG 6000-IPDI and –TDI, about 10 K for PEG 1000-IPDI and about 28 K for PEG 1000-TDI. In summary, overcooling is increased less for the higher molecular weight PEG and for HMDI. As for IPDI and TDI materials, no clear trend is visible from our data. While the change in overcooling is clearly more pronounced in the low molecular weight PEG, the absolute overcooling is 15–20 K for most materials with the exception of PEG 1000-HMDI (about 7 K) and PEG 1000-TDI (about 32 K). For PEG 10,000, the slope of the total enthalpy curves is different for the different PU’s: The melting occurs at a very narrow temperature range for the pristine PEG, but this range grows for IPDI polyurethane and even more for HMDI and TDI polyurethanes. Pristine PEGs consist of ethyl–ether linked segments with active hydroxyl end groups that easily form intermolecular hydrogen bonds, crystallize easily and have high transformation enthalpies for melting [22]. Expectedly, the DSC curves of PU polymers were similar to that of pristine PEG because their structure contained PEG units. The phase change enthalpies of PU polymers are also considerably high, which means that PU polymers have potential to be used as latent heat thermal energy storage materials. However the number of segments available for the crystalline regions reduced because of the presence of hard segments at the backbone of the PU polymers, which consequentially led to the decline of phase change temperature and enthalpy in general.
In general, the phase change temperatures of PU polymers are lower than that of pristine PEGs. The segments near hard segment sites are confined after active hydroxyl end groups of PEG reacted with diisocyanate. Consequently the arrangement and orientation of PEG molecules are partially suppressed by the steric effect and the crystalline regions become smaller, which causes the transition point and enthalpy to fall to a certain degree. This observation that an introduction of hard segments results in a decrease of the phase change temperature and enthalpy agrees with observations previously found by many researchers (Su and Liu, 2006; Cao and Liu, 2006). Our study however quantifies this effect for the selected diisocyanates and PEG molecular weights. As a result of our leakage test PEG 1000-IPDI condensate was identified as the polymer with solid–liquid phase changing property. Moreover it was observed that overcooling is at maximum in this condensate. This is probably due to the steric effect preventing the chains to get close. This effect is less observable in PEG–HMDI condensates due to linear and symmetrical structure of HMDI and at high molecular weight PEGs because of decreasing impact of steric hindrance on long molecules. IPDI forms the biggest hard segment among the other diisocyanates used in this study. So it produces the maximum steric hindrance for PEG segments. Due to that PEG 1000 is the smallest molecule among the used ones in this study, its transition temperatures and enthalpies are affected mostly. PEG 6000-IPDI and PEG 10,000-IPDI condensates are also affected by the blocking reaction even the integral heat values for these two are higher than corresponding PEG polymers. Toluene diisocyanate which is generally in 2,4-TDI form is one of the most commonly used counterpart in industrial polyurethane production so it is chosen to be used in this study. 2,4-TDI is not a linear molecule like IPDI therefore there is steric hindrance effective on crystallization of its
Fig. 9. POM images of polyurethanes produced using PEG 1000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
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Fig. 10. POM images of polyurethanes produced using PEG 6000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
Fig. 11. POM images of polyurethanes produced using PEG 10,000 at 25 °C (top line) and at 80 °C (bottom line) (about 100x magnification).
condensate with PEG 1000. However as in the case of PEG–IPDI condensates, PEG–TDI condensates are not affected by steric affects at higher molecular weights. Furthermore the total enthalpy stored by PEG 1000-TDI condensate is much lower among PEG–TDI condensates between 20 and 80 °C. In addition overcooling in PEG 1000-TDI condensate is observed at maximum. 3.4. POM investigation Figs. 9–11 show the optical microscopy images of pristine PEGs and polyurethane polymers produced using PEGs and diisocyanates below and above phase transition
temperatures. The observed morphological changes are discussed in the following. Fig. 9 shows the spherulitic structure of PEG 1000 together with polarized microscopy images of the three investigated PEG 1000 condensates below and above their phase transition temperature. In Figs. 10 and 11, the same arrangement of pictures is given for PEG 6000 and PEG 10,000 and their respective polyurethanes. It can be observed that both PEG and PU polymers have spherulitic structures but PUs’ spherulites are smaller in diameter than corresponding PEGs are. For the higher temperatures, the spherulitic structure disappears in all samples. That means, the blocking of PEG polymers using
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diisocyanate molecules did not alter the crystal form of PEG, but it limited the degree of crystallization. The missing spherulites in the POM micrographs of PU polymers at 80 °C demonstrate that the crystal PEG completely transforms to an amorphous state. It can be concluded that in any of the PU polymers a phase transition of PEG from the crystalline solid to the amorphous solid state takes place. According to the DSC data as presented in Section 3.3, during this phase transition of the PEG segments a significant amount of latent heat is absorbed and released. The synthesis of PUs is designed to produce PUs at maximum molecular weights theoretically so synthesized PUs are expected to show solid–solid phase changing property. However PEGs are synthetic polymers and synthetic polymers are always polydispersed. Under these conditions taking a definite molar amount of a synthetic polymer is almost impossible. This leads to a decrease in molecular weights. As a result the resultant PU derivatives had probably grown according to the experimental conditions. At this point it can at least be said that it is possible to produce PU polymers with solid–solid phase changing property. Most of the synthesized PCMs remained in the solid state above transition temperatures because of the hard segments connecting PEG chain ends to each other and so restricting free movements of them. Su et al. produced PU polymers from a hard segment produced reacting butanediol with 2 methylene diisocyanate molecules and PEG 4000 and reported its results (Su and Liu, 2006) which are very similar to our work. Polymers are viscoelastic materials and PEG blocks at the backbone makes them more viscous. To improve mechanical properties, diisocyanate ended PU blocks (considerably long hard segments) could be developed and used. Our further studies to produce PUs with better physical properties for applications are going on. 4. Application outlook The transition temperatures of PUs were suitable for many applications such as solar energy storage, waste heat recovery, and temperature adaptable greenhouses. The novel solid–solid PCMs with high phase change enthalpy will be promising heat storage materials and may take place of other similar energy storage materials. The phase change temperatures of PEG–HMDI condensates could be better suitable for solar space heating applications and air conditioning. 5. Conclusion Preparations of various type of PUs were conducted via the condensation reaction of PEGs with diisocyanates. The results showed that produced PUs had typical solid–solid phase transition properties with high enthalpies reaching 179 kJ/kg, and transition points between 19 and 60 °C. The resultant blocking copolymers could be potentially
used as solid–state PCMs for thermal energy storage and temperature-control applications if the PEG molecular weight is high enough to crystallize under different conditions. PUPCMs with the special solid–solid phase change behaviors of high thermal energy storage capability, suitable phase transition temperature, reversible latent heat transition, and good thermal stability can be used as a new kind of solid–solid PCMs for thermal energy storage and temperature control. Acknowledgements We would like to thank the German Federal Ministry of Education and Research (project ENFoVerM/FKZ 0327851D) and Scientific and Technical Research Council of Turkey TUBITAK (project code: 108T865-TBAG) for their financial support for this study. References Alkan, C., Sari, A., 2008. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Solar Energy 82, 118–124. Benson, D.K., Webb, J.D., Burrows, R.W., McFadden, J.D.O., Christensen, C., 1985. Materials research for passive solar systems: solid– state phase-change materials. report prepared for the US. Department of energy, Contract No. EG-77-C-01-4042. Busico, V., Carfagna, C., Salerno, V., 1980. The layer perovskites as thermal energy storage systems. Solar Energy 24, 575–579. Cao, Q., Liu, P., 2006. Hyperbranched polyurethane as novel solid–solid phase change material for thermal energy storage. European Polymer Journal 42, 2931–2939. Chen, C., Wang, L., Huang, Y., 2008. Morphology and thermal properties of electrospun fatty acids/polyethylene terephthalate composite fibers as novel form stable phase change materials. Solar Energy Materials and Solar Cells 92, 1382–1387. Gu, X., Xi, P., Cheng, B., Niu, S., 2010. Synthesis and characterization of a novel solid–solid phase change luminescence material. Polymer International 59, 772–777. Han, S., Kim, C., Kwon, D., 1997. Thermal/oxidative degradation and stabilization of polyethylene glycol. Polymer 38, 317–323. Harris, J.M., Zalipsky, S., 1997. Introduction to Chemistry and Biological Applications of Poly(ethylene glycol). American Chemistry Society, Washington, DC. International Energy Outlook, 2006. Energy information administration office of integrated analysis and forecasting US. Department of Energy. Available from:
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