Use of electrohydrodynamic processing to develop nanostructured materials for the preservation of the cold chain

Innovative Food Science and Emerging Technologies 26 (2014) 415–423

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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset

Use of electrohydrodynamic processing to develop nanostructured materials for the preservation of the cold chain Rocío Pérez-Masiá, Amparo López-Rubio, Maria José Fabra, Jose María Lagaron ⁎ Novel Materials and Nanotechnology Group, IATA-CSIC, Avda. Agustin Escardino 7, Paterna, Valencia 46980, Spain

a r t i c l e

i n f o

Article history: Received 21 January 2014 Accepted 27 October 2014 Available online 11 November 2014 Editor Proof Received Date 17 November 2014 Keywords: Electrospinning Encapsulation Phase change material (PCM) Polycaprolactone Polylactic acid Polystyrene

a b s t r a c t In this study Rubitherm-RT5®, a PCM with a phase transition temperature around 5 °C, was encapsulated inside PCL, PLA and PS by means of electrohydrodynamic processing in order to develop thermal energy storage systems for refrigeration applications. The effect of the morphology of the encapsulation structures (fibrillar or spherical) on thermal properties and encapsulation efficiency was evaluated. The morphology of the structures played an important role on the energy storage capacity, since PCM was better encapsulated in fibrillar structures, providing higher energy storage capacity. The greater encapsulation efficiency was achieved for the fibers, which showed that ~80–90 wt.% of the incorporated PCM, effectively remained within the polymeric matrices. These hybrid structures are of great interest for the development of active packaging systems aimed at improving food quality preservation under refrigeration conditions. Industrial relevance: As a response to the consumers for more safety foodstuffs and low environmental impact technologies, this work presents a novel methodology to develop energy storage materials. These materials buffer thermal variations and, thus, could be used in packaging systems to better preserve the cold chain. Furthermore, the incorporation of these materials in refrigeration equipment could reduce energy consumption of existing refrigeration technologies. © 2014 Published by Elsevier Ltd.

1. Introduction Conventional food packaging has been used to enable marketing of products and to provide passive protection against environmental contaminations or influences that affect the shelf life of foodstuffs (de Abreu, Cruz, & Losada, 2012). However, consumers are claiming higher quality, safer and fresher products and, thus, new functions are demanded to the packaging structures. Therefore, over the last decade, there has been an increased research interest in the development of different active packaging systems, such as oxygen scavengers, moisture absorbers, flavor and odor absorbers/releasers or antimicrobials systems, which have an active role in food preservation, extending the shelf life or improving the conditions of packaged food through the retention or release of compounds from/to the food or package headspace. A more recent concept of packaging structure having an active role in food preservation is packaging with heat management properties. It is well-known that temperature is a critical parameter which activates chemical reactions and microorganisms' growth. In this sense, refrigeration temperatures have preservative effects on perishable products and thus, by controlling the temperature along the different stages of the food production, a great number of deteriorative processes could be avoided and the shelf life of products could be increased. However, ⁎ Corresponding author. Tel.: +34 963900022; fax: +34 963636301. E-mail address: [email protected] (J.M. Lagaron).

http://dx.doi.org/10.1016/j.ifset.2014.10.010 1466-8564/© 2014 Published by Elsevier Ltd.

temperature fluctuations during storage and commercialization of foods can result in a number of quality drawbacks, such as crystal ice growth in products like meat or ice-cream. A possible approach to control thermal variations during storage and distribution of food, maintaining the preservation temperature constant and thus, preventing temperature fluctuations which can derive in food quality loss is through packaging structures with thermal energy storage capacity. This can be attained through the incorporation of phase change materials (PCMs) into the packaging structures. PCMs are able to absorb or release a great amount of energy during their melting/crystallization process and, as a consequence, they are able to buffer the thermal variations of the environment, and thus, they could provide thermal protection to the packaged food. The use of PCMs in energy storage systems has been recently applied in different fields such as building materials, air conditioning applications, solar energy storage systems, greenhouses, temperature regulating textiles, electronic devices and biomedical systems (Sarier, Onder, Ozay, & Ozkilic, 2011). Specifically in the food packaging area, PCMs are replacing dry ice containers used during the transport and storage of perishable foodstuffs. Some of the advantages of using PCMs include the weight reduction of the containers in comparison with dry ice and, their reusability during many thermal cycles. Among all the compounds available as phase change materials, paraffins are one of the most studied, since they are chemically stable and do not degrade after repeated cycling. Paraffins are highly compatible with a wide variety of materials and have a latent heat of fusion of

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around 200 kJ/kg depending on the particular paraffin selected (Ehid & Fleischer, 2012). However, the use of paraffins for thermal energy storage applications also presents some drawbacks. On one hand, paraffins have a low thermal conductivity, which limits the energy that can be extracted from them. Another problem is their handling, since they are liquid at ambient temperature and, what is more important, they need to undergo a phase change (i.e. from liquid to solid and vice versa) at the target temperature to exert the desired functionality. Some strategies to overcome these difficulties are the reduction of the PCM particles' diameter in order to achieve a very high surface area to volume ratio, thus increasing their heat flow rate due to a higher exchange area per unit of volume (Alkan, Sari, & Karaipekli, 2011). Furthermore, the encapsulation of these particles in a solid matrix can solve the handling problems of the PCMs. The most common paraffin encapsulation techniques are interfacial polymerization (Chu et al., 2003), emulsion polymerization (McDonald & Devon, 2002), in situ polymerization (Yang, Xu, & Zhang, 2003), layer by layer deposition of polyelectrolytes (Sukhorukov et al., 2004), spray drying and coacervation (Hawlader, Uddin, & Khin, 2003). However, only small amounts of PCM can be effectively encapsulated by some of these techniques, and they render too large particle sizes for some applications (SánchezSilva et al., 2010). Therefore, a rather novel route for PCM encapsulation, based on the electrospinning process (Lagaron, Perez-Masia, & LopezRubio, 2011), is proposed in this work. The electrospinning process uses high voltage electric fields to produce electrically charged jets from viscoelastic polymer solutions which on drying, by the evaporation of the solvent, produce ultrathin polymeric structures (Li & Xia, 2004). This technique has recently proven to encapsulate different materials with significant yielding and flexibility in design, obtaining submicron sized structures (Torres-Giner, Gimenez, & Lagaron, 2008; Fernandez, Torres-Giner, & Lagaron, 2009; López-Rubio & Lagaron, 2012). Although there are a few works using electrospinning to develop PCM-containing materials, most of them deal with room and human comfort range temperatures (Alay, Göde, & Alkan, 2010; Arecchi, Mannino, & Weiss, 2010; Cai et al., 2012; Chen, Wang, & Huang, 2007; McCann, Marquez, & Xia, 2006; Seifpoor, Nouri, & Mokhtari, 2011). The aim of this work was to use the electrospinning technology to obtain novel food packaging energy storage systems of interest in refrigeration applications. Two biopolyesters, polylactic acid (PLA) and polycaprolactone (PCL), were used as shell materials since they have attracted great interest among the existing biodegradable matrices because of their good physical properties (when compared to other biodegradable and renewable polymers), excellent biocompatibility and commercial availability (Cohn & Hotovely Salomon, 2005; Garkhal, Verma, Jonnalagadda, & Kumar, 2007; Huang, Günther, Doetsch, & Mehling, 2010; Pamula & Menaszek, 2008). Furthermore, the encapsulation systems based on these biodegradable materials were compared with others obtained with polystyrene (PS), since this petro-based polymer is currently used in refrigerating equipments and food packaging. Regarding the PCM, Rubitherm-RT5® (RT5) was encapsulated in the different polymers. RT5 is a commercial mixture of paraffins, specifically C14H30 (33 wt.%), C15H32 (47 wt.%), C16H34 (16 wt.%), C17H36 (3 wt.%) and C18H38 (1 wt.%), which presents a phase transition at around 5 °C. This temperature is commonly used to keep the refrigerating conditions of foodstuffs in retail and display cabinets at supermarkets or in household refrigerators. For each material, the effect of encapsulation morphology (fibrillar or spherical) on the molecular organization and on the thermal properties of the capsules was evaluated. 2. Materials and methods 2.1. Materials Rubitherm-RT5® (RT5) was purchased from Rubitherm (Germany). A semicrystalline extrusion grade of polylactide (PLA) (Natureworks) with a D-isomer content of approximately 2 wt.% was used. It had

a weight-average molecular weight (Mw) of 150,000 g/mol and a number average molecular weight (Mn) of ca. 130,000 g/mol. The polycaprolactone (PCL) grade FB100 was kindly supplied in pellet form by Solvay Chemicals (Belgium). This grade had a density of 1.1 g/cm3 and a mean molecular weight of 100,000 g/mol. Polystyrene (PS) commercial grade foam was supplied by NBM (Spain). N,Ndimethylformamide (DMF) with 99% purity and trichloromethane were purchased from Sigma-Aldrich (Spain) and Panreac Quimica S.A. (Spain), respectively.

2.2. Preparation of polymer/RT5 solutions Based on previous studies, the different polymers and RT5 were dissolved under magnetic stirring, in a solvent prepared with a mixture of trichloromethane:N,N-dimethylformamide (85:15 w/w) in order to produce fibrillar (fibers) or spherical (beads) capsules according to the conditions presented in Table 1.

2.3. Characterization of the solution properties The viscosity and surface tension of the pure RT5, the solvent mixture used and all the biopolymeric solutions were characterized before the electrospinning process. The viscosity of the solutions was measured using a rotational viscosity meter Visco Basic Plus L from Fungilab S.A. (Spain) using a Low Viscosity Adapter (spindle LCP). The surface tension was measured using the Wilhemy plate method in an EasyDyne K20 tensiometer (Krüss GmbH, Germany). Both measurements were done, in triplicate, at 25 °C.

2.4. Preparation of the polymer-PCM fibers or beads through electrospinning Fibers and beads were obtained using the electrospinning technique. The electrospinning apparatus, equipped with a variable high-voltage 0–30 kV power supply, was a FluidNatek® basic setup assembled and supplied by BioInicia S.L. (Spain). The polymer/PCM solutions were electrospun under a steady flow-rate using a stainless-steel needle situated toward a metallic grid used as collector. The needle was connected through a PTFE wire to the polymer solution kept in a 5 mL plastic syringe which was disposed horizontally lying on a digitally controlled syringe pump. The electrospinning conditions were the following: 1 mL/h flow-rate, 12.5 kV voltage and 10 cm tip-to-collector distance for all the solutions.

2.5. Scanning electron microscopy (SEM) The morphology of the electrospun structures was examined using scanning electron microscopy (SEM; Hitachi S-4100) after having been sputtered with a gold–palladium mixture under vacuum. All SEM experiments were carried out at 10 kV. Fibers and bead diameters were measured by means of the Adobe Photoshop CS4 software from the SEM micrographs in their original magnification. Table 1 Polymers and PCM loadings. Type of Polymer

Polymer concentration (wt.%)

Polymer: PCM weight ratio

Morphology

PCL PLA PS PCL PLA PS

13 5 10 1.5 2.5 2.5

55:45 80:20 55:45 55:45 55:45 55:45

Fibers Fibers Fibers Beads Beads Beads

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the type and concentration of polymer did not considerably affect the surface tension values, except in PCL/RT5 where a slight decrease in this parameter was observed as the polymer concentration increased. These results indicate that, for this study, the viscosity of the solutions was the main parameter that influenced the morphology of the encapsulation systems. The morphology of the electrospun structures was analyzed through scanning electron microscopy (SEM). Fig. 1 shows the SEM images of the different encapsulated systems and the corresponding size distribution of the fibers or beads. For the fibers, it was observed that PCL and PS led to fibers with an average diameter greater than 1 μm, although PS structures presented some beaded areas. In the case of PLA, this polymer produced very thin structures (with an average diameter of 214 nm) also with some beaded areas. The differences in average fiber diameters could be attributed to both the viscosity of the solutions and the solvent–polymer interactions. The different fiber diameters could also explain the total amount of PCM that could be theoretically incorporated within the electrospun structures. Thus, only 20 wt.% of RT5 could be added to the thin PLA structures, while the thicker fibers obtained from the PCL and PS solutions favored the encapsulation of up to 45 wt.–wt.% of PCM. It is worth noting that the maximum weight ratios of the polymer/PCM tested were typically near 50/50. Greater PCM loadings tended to destabilize the encapsulation process for the selected polymer/PCM systems. In order to have a better insight about RT5 distribution within the fibers, cross-sections of cryofractured PCL and PS fibers are shown in Fig. 2. From these images, it can be observed that incorporation of the PCM led to formation of multiple channels where the PCM was supposed to be allocated. For the PLA fibers containing RT5 it was not feasible to obtain detailed SEM images of the cross-sections due to the smaller fiber diameter. Nevertheless, it could be assumed that the distribution of the PCM along the structures occurred in a similar way as in the other electrospun structures. For the beaded structures, the average diameter of the capsules was around 2–3 μm regardless of the polymer used, although the size distribution was wider in the case of PCL/RT5 and PS/RT5 materials, where beads from 500 to 5000 nm were obtained. It is worth noting that the decrease of the polymer concentration in the case of the beads, together with the addition of the PCM, complicated the intermolecular entanglements of the polymers during the capsule's formation. This fact resulted in the collapse of the spherical structure and the production of wrinkled particles as observed in Fig. 1 for PLA and PS bead structures, which seemed to be empty (Hong, Li, Yin, Li, & Zou, 2008). Moreover, it has also been reported that oil evaporates from the structures during the vacuum conditions used during the SEM experiments, which also explains the collapsed appearance of the capsules shown in Fig. 1 (Angeles, Cheng, & Velankar, 2008). In the case of PCL/RT5 solutions it was difficult to obtain only beads and, even when the polymer concentration was reduced to 1.5 wt.%, a large amount of thin fibers were also observed. However, PCL concentrations below 1.5 wt.% were not suitable for electrospinning because of a continuous dripping of the solution. The PS/RT5 diluted solution provided beads with a wider diameter size range together with some thin fibers. In contrast, beads with a

2.6. Attenuated total reflectance infrared spectroscopy (ATR-FTIR) ATR-FTIR spectra were collected at 25 °C in a FTIR Tensor 37 equipment (Bruker, Germany). The spectra were collected in the different materials by averaging 20 scans at 4 cm−1 resolution. The experiments were repeated twice to verify that the spectra were consistent between individual samples. 2.7. Differential scanning calorimetry (DSC) Thermal analyses of electrospun structures were carried out on a DSC analyzer (Perkin Elmer DSC 7, US) from −25 to 25 °C in a nitrogen atmosphere using a refrigerating cooling accessory (Intracooler 2, Perkin Elmer, US). The scanning rate was 2 °C/min in order to minimize the influence of this parameter in the thermal properties. The amount of material used for the DSC experiments was adjusted so as to have a theoretical PCM content of 1–2 mg approximately. Measurements were done in triplicate. 2.8. Evaluation of the energy storage ability of the PCL/RT5 fibers PCL fibers and PCL/RT5 fibers were stored at −18 °C overnight and then put in a controlled-temperature chamber at 15 °C. The temperature evolution of the material was registered with an infrared thermometer MS Plus (PCE Instruments, Spain). 2.9. Statistical analysis Statistical analysis of data was performed through analysis of variance (ANOVA) using Statgraphics Plus for Windows 5.1 (Manugistics Corp., Rockville, MD). Homogeneous sample groups were obtained by using LSD test (95% significant level). 3. Results and discussion 3.1. Solution properties and morphology of the encapsulated materials Based on previous knowledge about the parameters that affect the electrospinning process (Fong, Chun, & Reneker, 1999), and on screening studies on the electrospinning of the polyesters/PCM solutions, different polymer/RT5 solutions were prepared in order to produce fibrillar or spherical structures for the encapsulation of the commercial PCM. Table 1 compiles the different polymer concentrations and polymer/PCM ratios used in this work, where it can be observed that the most concentrated solutions were used to obtain fibers, while lower polymer concentrations were selected for the production of beads. Table 2 shows the viscosity and surface tension values of the different polymer/PCM solutions, the pure PCM and the solvent mixture. As expected, the viscosity of all polymer/PCM solutions was greater than that of pure PCM due to the presence of the high molecular weight polymers. Furthermore, the viscosity of the polymer/PCM solutions was greater in the solutions used to obtain fibers than in those used to form beads, as the amount of polymer in the former was greater. However,

Table 2 Viscosity (μ) and surface tension (σ) of the different solutions used for electrospinning. Fibers

PCL/RT5 PLA/RT5 PS/RT5 RT5 Solvent

Beads

Polymer:RT5

η (cP)

55:45 80:20 55:45 0:100 –

1148.11 45.27 83.44 2.29 1.00

σ (mN/m) ± ± ± ± ±

54.41a 4.01b 5.01c 0.11d 0.11e

28.00 28.50 27.95 26.73 28.34

± ± ± ± ±

0.11a 0.11b 0.21a 0.21c 0.11b

Polymer:RT5

η (cP)

55:45 55:45 55:45 0:100 –

11.07 7.07 4.23 2.29 1.00

σ (mN/m) ± ± ± ± ±

0.42a 0.52b 0.12c 0.11d 0.11e

28.87 28.53 27.87 26.73 28.34

± ± ± ± ±

0.22a 0.21a 0.21b 0.21c 0.11a

a–e: Different superscripts within the same column indicate significant differences among biopolyester/PCM structures (p b 0.05). 1–2: Different superscripts within the same parameter indicate significant differences for a determined polymer/PCM due to the morphology reached (fibers or beads) (p b 0.05).

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Fig. 1. Selected SEM images and size distribution of fibers (up) and beads (down) of: a) PCL/RT5; b) PLA/RT5; c) PS/RT5. Scale markers correspond to 10 μm.

Fig. 2. Cross-section SEM images of pure PCL/RT5 (left) and PS/RT5 (right) fibers. Scale markers correspond to 3 μm.

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homogeneous size distribution were achieved for the solutions of PLA/RT5. The fact that PLA formed micrometric beads, with an average diameter of ~3.2 μm could explain the greater capacity of these structures for encapsulating the PCM when compared to PLA fibers, since the latter presented an average diameter of only ~0.2 μm. The morphological differences among the studied polymer/PCM systems were again attributed to the viscosity of the solutions and the solvent–polymer interactions. When the polymer concentration was reduced, less polymer entanglements were obtained and thus beads or beaded fibers were obtained. Nevertheless, the spinnability of a polymer is also related to the intermolecular interactions between the polymer chain and the solvent molecules which could contribute to the formation of fibers instead of beads. In fact, when the polymer–solvent interaction is favored, higher molecular chain entanglements can be achieved and the formation of beads is hindered (Luo, Stride, & Edirisinghe, 2012). This fact could explain the greater viscosity of the PCL solutions, even with a lower polymer concentration, and the difficulty in producing bead structures with this polymer.

3.2. ATR-FTIR analysis ATR-FTIR analyses were carried out in order to corroborate the presence of RT5 in the capsules, as well as to study the molecular organization of the materials. The experiments were done at room temperature (25 °C), at which RT5 was in liquid state, and at −25 °C, at which RT5 was crystallized. Fig. 3a shows the ATR-FTIR spectra of the pure RT5 at 25 °C (black line) and at − 25 °C (gray line) from 2800 to 3000 cm−1, where the most characteristic bands of the PCM can be found. This region is associated to the stretching vibrations of methyl groups, characteristics of alkane compounds. From this figure it can be observed that while the liquid RT5 presented spectral bands at 2957 cm− 1, 2922 cm−1 and 2853 cm−1, when the PCM was crystallized, these bands were shifted toward lower wavenumbers due to an increase in intermolecular attractive forces in the solid state, which decreased the intramolecular vibrational transition energies (Porter,

Fig. 3. ATR-FTIR spectra of the pure RT5 at 25 °C (black line) and at −25 °C (gray line) from 2800 to 3000 cm−1 (a) and from 750 to 670 cm−1 (b).

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Jeffries, & Hanson, 2009). Moreover, in the crystalline state the bands arose sharper and more intense mainly because of the higher degree of homogeneity of the intermolecular interactions. This leads to less dispersion of vibrational levels related to a higher molecular order (Wolkers, Oliver, Tablin, & Crowe, 2004). From Fig. 3b it is observed that the spectral band at 722 cm− 1 attributed to the methyl rocking vibration present in the liquid RT5, split in two upon paraffin crystallization. This doublet is typical of odd alkanes with orthorhombic structure (MacPhail, Strauss, Snyder, & Eiliger, 1984). The encapsulated systems were also analyzed using ATR-FTIR at 25 and − 25 °C. Fig. 4 shows the ATR-FTIR spectra from 2800 to 3000 cm−1 at 25 °C of the pure RT5, the pure polymers, the hybrid fibers and the hybrid beads. This figure shows that in this specific spectral region, the polymers also presented some characteristic bands. Nevertheless, they were not completely overlapped with those of the PCM and, thus, it was possible to identify the RT5 bands in the hybrid systems. From this figure it was also observed that the PCM was effectively present in all the polymeric matrices (both fibers and beads), since the RT5 bands were evident in all the structures. At 25 °C the rocking vibration band (722 cm−1) of the PCM was difficult to see due to the contribution of other spectral bands from the polymers in this region (data not shown). The hybrid materials were also analyzed at −25 °C in order to study the molecular organization of the encapsulated PCM upon crystallization. Fig. 5 shows the ATR-FTIR spectra of RT5 and the capsules at − 25 °C in the regions of 2800–3000 cm−1 and 670–750 cm−1. For PCL/RT5 fibers it was observed that the bands attributed to the PCM arose at the same wavenumbers as those of pure RT5, which suggested that there were no interactions between the PCL fibers and the PCM

Fig. 4. ATR-FTIR of polymer (gray), RT5 (black), fibers (dashed) and beads (dotted): a) PCL-RT5; b) PLA-RT5; c) PS-RT5.

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and, thus, RT5 could crystallize within the fibers. However, it was seen that the methyl stretching vibration bands arose broader than those of the pure PCM, probably because of a lower molecular order of the PCM molecules in the electrospun structures, as well as to the contribution of PCL, which also presented a spectral band nearby this region. On the contrary, when studying this region in the PCL/RT5 beads, it was seen that the bands were slightly displaced toward higher wavenumbers than in the spectrum of pure RT5. Moreover, the intensity of the doublet band at 722 cm − 1 was significantly lower than that of the pure PCM. This result suggested that a lower amount of PCM was incorporated in PCL beaded capsules, probably because of the heterogeneous structures (thin fibers with beaded areas) obtained from this biopolymeric solutions. For PLA/RT5 and PS/RT5 materials, it was seen that in the 2800–3000 cm− 1 area, the PCM bands arose at higher wavenumbers and slightly broader than the pure RT5. This fact indicates lower molecular order of the PCM molecules probably due to the presence of the polymers which could be hindering or modifying the crystallization of the PCM. It was also observed that the doublet at 722 cm− 1 was present in PLA/RT5 beads and PS/RT5 fibers, while it did not arose in PLA/RT5 fibers and PS/RT5 beads. This result could be related to the PCM amount finally incorporated in these hybrid structures, i.e. the low amount of PCM present in the latter structures cannot be detected in this spectral range, as it will be confirmed below.

3.3. Thermal properties Thermal properties (enthalpy values, melting and crystallization temperatures and supercooling degree) of the hybrid electrospun materials analyzed by DSC at 2 °C/min are given in Table 3 (fibers) and Table 4 (beads). The supercooling degree is the lag between the melting and crystallization temperatures, calculated as the difference between the last melting and the first crystallization peaks detected. From these data it was observed that pure RT5 melted at 6.52 °C and crystallized at 5.26 °C, having a melting enthalpy of 142.2 J/g. It was observed that the onset temperature of the melting process for all the fibers did not significantly differ from that of pure RT5, but the maximum of melting increased when RT5 was encapsulated within these structures. This result indicates that the melting process occurred over a broader temperature range in the hybrid structures than in the pure PCM, as it can be observed from the peak width. This fact probably occurred because in the encapsulated systems there was a greater heterogeneity of crystals due to the different fiber diameters and also because the polymers themselves could be hindering heat transfer across the structures (Mathot, 1994). Moreover, while in the PCL/RT5 fibers, a single melting peak of the PCM was observed at 7.4 °C, for the PLA/RT5 and PS/RT5 hybrid structures, two different melting peaks of the PCM were observed. This fact could be attributed to the formation of two different crystal populations as a consequence of the confined state of the

Fig. 5. ATR-FTIR of RT5 (black), fibers (dashed) and beads (dotted) from 2800 to 3000 cm−1 and from 750 to 670 cm−1 of PCL-RT5 (a and b); PLA-RT5 (c and d); PS-RT5 (e and f).

Table 3 Thermal properties of the various polymer/RT5 fibers developed. Melting

Crystallization

System

Onset (°C)

RT5 PCL/RT5 PLA/RT5 PS/RT5

2.83 2.92 3.22 3.46

± ± ± ±

0.3a 0.2a 0.4a 0.3a

Tm1 (°C) 6.52 7.37 7.46 7.97

± ± ± ±

0.4a 0.1b 0.5 b 0.3b

Tm2 (°C)

Width (°C)

– – 10.73 ± 0.2a 10.78 ± 0.3a

3.92 5.78 5.59 7.90

± ± ± ±

0.6a 1.0b 0.2b 0.1c

ΔHmeas (J/g) 142.23 61.35 24.94 50.75

± ± ± ±

9.7a 8.5 b 3.5 c 7.3 b

Onset (°C) 5.83 6.27 10.15 9.24

± ± ± ±

0.5a 0.3a 0.1b 0.5 c

Tc1 (°C) 5.26 4.11 9.37 8.72

± ± ± ±

0.3a 0.5b 0.1c 0.1d

Tc2 (°C)

Tc3 (°C)

Width (°C)

– – 4.07 ± 0.4a 3.61 ± 0.4a

– – −5.28 ± 0.3a −9.32 ± 0.4b

1.08 6.98 17.39 22.35

± ± ± ±

0.2a 1.4b 1.0c 1.9d

ΔHmeas (J/g) −144.62 −61.948 −25.21 −56.68

± ± ± ±

Supercooling (°C) 6.8a 7.1b 3.8c 5.3d

1.3 3.26 1.36 2.06

± ± ± ±

0.4a 0.5b 0.3a 0.2c

Tm1 and Tm2: first and second peak temperatures respectively; Tc1, Tc2, Tc3: first, second and third crystallization peak temperatures respectively. a–d: Different superscripts within a column indicate significant differences between biopolyester/PCM fibers (p b 0.05).

Melting

Crystallization

System

Onset (°C)

RT5 PCL/RT5 PLA/RT5 PS/RT5

2.83 8.85 5.36 6.54

± ± ± ±

0.3a 1.3b 1.0c 1.6c

Tm1 (°C) 6.52 13.20 10.81 11.78

± ± ± ±

0.4a 0.8b 1.0c 1.9c

Tm2 (°C)

Width (°C)

– – – 15.02 ± 1.7

3.92 5.50 6.52 9.63

± ± ± ±

0.6a 1.0b 0.4b 0.8c

ΔHmeas (J/g) 142.23 21.01 42.89 24.96

± ± ± ±

9.7a 1.5b 6.8c 3.5b

Onset (°C) 5.83 12.09 9.67 12.91

± ± ± ±

0.5a 0.4b 0.3c 1.5b

Tc1 (°C) 5.26 11.20 8.66 12.03

± ± ± ±

0.3a 0.9b 0.3c 1.6b

Tc2 (°C)

Tc3 (°C)

Width (°C)

– – 5.52 ± 0.5a 6.95 ± 1.3a

– – – −4.87 ± 1.6

1.08 4.86 8.58 21.05

Tm1 and Tm2: first and second peak temperatures respectively; Tc1, Tc2, Tc3: first, second and third crystallization peak temperatures respectively. a–d: Different superscripts within a column indicate significant differences between biopolyester/PCM beads (p b 0.05).

± ± ± ±

0.2a 1.0b 1.9c 0.8d

ΔHmeas (J/g) −144.62 −19.81 −42.95 −26.44

± ± ± ±

Supercooling (°C) 6.8a 0.9b 6.9c 2.8d

1.3 2.00 2.15 3.00

± ± ± ±

0.4a 0.1a 0.9a 0.5b

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Table 4 Thermal properties of the various polymer/RT5 beads developed.

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paraffins. This might be related to a greater stability of the rotator phase transitions occurring in paraffins as it will be further described below. Regarding the crystallization, the thermal behavior of the PCL/RT5 fibers was similar to that of the non-encapsulated PCM, i.e. in both cases the beginning of the process was around 6 °C and the PCM crystallized in only one peak. Nevertheless, in the PCL/RT5 fibers the crystallization peak was broader, probably due to the greater heterogeneity of crystals and also because PCL could be impeding the heat transfer, as it was commented before. In contrast, for PLA and PS hybrid fibers the crystallization occurred in multiple stages. This behavior could be attributed to the different crystallization mechanisms, as well as to the different populations of crystals found along the fibers. It has been reported that when some paraffins are microencapsulated, they can undergo what is called a rotator phase transition (Zhang, Fan, Tao, & Yick, 2005). A rotator phase is defined as lamellar crystals which exhibit long-range order in the molecular axis orientation and center-of-mass position, but lack rotational degrees of freedom of the molecules about their long axis (Kraack, Sirota, & Deutsch, 2000). When this transition takes place, various crystallization temperatures can be detected through DSC, corresponding to the heterogeneously liquid-rotator transition, the rotator-crystal transition and the homogeneously liquid-crystal transition, although sometimes the first and the second mechanisms could happen together (Zhang et al., 2005). Therefore, the thinner fibers obtained for PLA/RT5 and PS/RT5 fibers could be favoring these crystallization mechanisms. Moreover, as it was commented before, it has been observed that, in the confined state, rotator phases which are unstable in bulk, became metastable (Fu, Liu, Liu, Su, & Wang, 2011), a fact that would explain the presence of two melting peaks in these developed materials. Concerning the supercooling effect, it was greater in the PCL/RT5 fibers than in the pure RT5, probably due to a delayed heat transfer through the biopolyester matrix. However, for PLA and PS hybrid fibers, it was seen that the last melting peak temperature and the first crystallization peak temperatures almost matched, so the supercooling effect was reduced in these structures. With regard to the thermal properties of the beaded or spherical morphologies, it was seen that the melting process was delayed and the peak temperature increased for all the polymeric matrices. This again seems to be related to the thermal lag associated to the presence of polymer shells, as well as to potential interactions between the PCM and the polymers as suggested by spectral band shifts (cf. Fig. 5 and Section 3.2). However, this complex thermal behavior, which could also be a consequence of different crystal morphologies, needs a more thorough study which is out of the scope of the present work. The crystallization of the PCM in the hybrid PLA/RT5 and PS/RT5 beads was also characterized by several transition events which again could be attributed to the different crystallization mechanisms (rotator phase, heterogenous and homogeneous). Regarding the supercooling, it increased when compared to the non-encapsulated RT5. It is wellknown that the undercooling required to obtain crystallization significantly increases when reducing the droplet size in comparison to the bulk material as each droplet must be nucleated separately and the nucleation mechanism shifts from heterogeneous to homogeneous nucleation (Montenegro, Antonietti, Mastai, & Landfester, 2003). Therefore, from the data obtained, it can be concluded that the use of these hybrid beads as encapsulation morphologies would not be suitable for refrigeration applications, since the phase transition temperatures obtained for these structures did not match the temperature required in the refrigeration systems.

hybrid structures (both fibers and beads), the initial amount of PCM added to the solutions and the calculated total amount of RT5 effectively encapsulated in the systems. Results showed that fiber structures presented greater encapsulation efficiency than beads, which may be ascribed to the fact that continuous fiber structures contributed to retain greater amounts of PCM than discontinuous beads. When comparing among the various polymers used as encapsulating matrices, PCL and PS fibers demonstrated a greater ability to retain PCM than PLA fibers, probably due to the diameter of the structures obtained thereof. From Fig. 1 it was seen that PCL and PS formed fibers with an average size higher than 1 μm, while PLA fiber mean diameter was around 200 nm and, thus, it is expected that the PCM was more easily encapsulated in the thicker PCL and PS structures. The greatest encapsulation efficiency was achieved for PCL/RT5 fibers (~96%), meaning that around 43 wt.% of the material obtained was able to provide energy storage capacity. On the contrary, only 36 wt.% and 18 wt.% of the hybrid PS/RT5 and PLA/RT5 fiber structures, respectively, had energy storage capacity. In contrast, for the compositions that gave rise to beaded structures, PLA was the most efficient matrix, being ~30 wt.% of the material able to provide energy storage capacity. However, changing from fiber to bead morphology significantly reduced the encapsulation efficiency of PCL and PS structures, showing that only an 18–19 wt.% of the capsules provided energy storage capacity. This could be explained by the different bead morphology reached depending on the matrix used, since PLA led to a continuous mat of beads, while PCL and PS formed bead structures together with very thin fibers, which probably were not able to fully encapsulate the PCM. 3.5. Evaluation of the energy storage ability of the PCL/RT5 fibers From Table 5 it was observed that PCL/RT5 fibers were the most efficient energy storage material providing ~60 J/g. Therefore, this system was selected for the evaluation of the energy storage ability of the material developed. For this aim, a piece of 0.5 g of the hybrid PCL/RT5 fiber mat and the same amount of pure PCL fibers for comparison purposes were stored at − 18 °C and then they were exposed to 15 °C. Fig. 6 shows the temperature evolution of pure PCL fibers and PCL/RT5 fibers. From this figure it can be seen that the PCL fibers without PCM reached room temperature in 80 s. However, the hybrid fibers were able to keep the temperature at around 4–6 °C during 1.5 min and room temperature was reached after 3.5 min, i.e. incorporation of the PCM within the polymer fibers almost tripled the time needed to reach room temperature, thus demonstrating the heat management capacity of this type of structures. Although promising, when considering these types of structures for real applications, the energy storage capacity needs to be increased so that the amount of material used can be kept to a minimum. Several strategies are under consideration to increase the heat management properties of these structures and will be presented elsewhere. 4. Conclusions In this work different polymeric capsules containing RubithermRT5®, a phase change material (PCM) with a phase transition at around

Table 5 Encapsulation efficiency and PCM loading of the various polymer/RT5 capsules developed.

3.4. Evaluation of the PCM loading and thermal energy storage capability The encapsulation efficiency was calculated by dividing the experimental melting enthalpy obtained for the hybrid materials by the experimental melting enthalpy obtained for pure RT5, considering the quantity of the PCM added to the electrospinning solutions. Table 5 shows the encapsulation yield derived from the DSC results of the

Fibers

Beads

System

Efficiency (%)

Initial PCM (%)

PCM loaded (%)

RT5

100

100

100

PCL/RT5 PLA/RT5 PS/RT5 PCL/RT5 PLA/RT5 PS/RT5

95.85 87.67 79.29 42.62 61.45 38.0

45 20 45 45 45 45

43.13 17.53 35.68 19.18 27.7 17.1

± ± ± ± ± ±

13.3 12.2 11.3 12.9 6.5 6.2

± ± ± ± ± ±

6.0 2.4 5.1 5.8 2.9 2.8

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Fig. 6. Temperature evolution of PCL and PCL/RT5 fibers.

5 °C, were developed by means of the electrohydrodynamic processing. Different encapsulation morphologies, fibrillar (fibers) and spherical (beads) were attained depending on the properties of the polymer/ PCM solutions. ATR-FTIR results showed that the PCM could be encapsulated in both types of morphologies, although fibers seemed to be more suitable for energy storage applications since the thermal properties of the PCM retained in the fiber structures were not as modified as within the beads. Furthermore, the encapsulating efficiency and the energy storage capacity of fibers were also greater than in the beads. Thus, it can be concluded that materials comprising electrospun fibers are a promising alternative to develop active packaging materials for foods stored under refrigeration conditions. Besides, among all the polymers tested, PCL was the most efficient matrix with ~ 43 wt.% of RT5 encapsulated. Moreover, these fibers showed the ability of temperature buffering at the phase transition temperature of the PCM for some time. Thus, the incorporation of these hybrid fibers in the food packaging materials or in the walls of refrigeration equipment can be a suitable strategy to buffer the thermal fluctuations with the aim of improving the quality and safety of foodstuffs.

Acknowledgments The authors would like to acknowledge the Central Services for Experimental Investigation Supporting (SCSIE) of the University of Valencia. M.J. Fabra is a recipient of a Juan de la Cierva contract from the Spanish Ministry of Economy and Competitiveness (JCI-2011-08977). The authors would also like to acknowledge the EU project of the FP7 FRISBEE (FP7-KBBE-2009-3) for the financial support.

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