Heat Storage and Dimensional Stability of Poly(vinyl alcohol) Based

Article pubs.acs.org/IECR

Heat Storage and Dimensional Stability of Poly(vinyl alcohol) Based Foams Containing Microencapsulated Phase Change Materials Irene Bonadies,† Adolfo Izzo Renzi,‡ Mariacristina Cocca,*,† Maurizio Avella,† Cosimo Carfagna,† and Paola Persico§ †

Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli, Italy Department of Chemical, Materials and Industrial Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy § Institute for Macromolecular Studies, National Research Council of Italy, Via Bassini 15, 20133 Milano, Italy ‡

ABSTRACT: The footwear market is addressing eco-sustainable solutions to the continuous demand for materials that at the same time show lighter weight with higher comfort. Toward this aim, a biodegradable matrix, poly(vinyl alcohol) (PVA), was used to obtain foams through an eco-sustainable methodology based on high speed mixing, without the use of chemical blowing agents. Moreover, considering that good thermoregulating performance in terms of heat transfer and perspiration is required for good quality shoes, microencapsulated phase change materials (PCMs) were incorporated into PVA foams. Foam morphologies investigated by SEM reveal that PCMs are finely dispersed through the PVA matrix. Mechanical properties of foams were analyzed in terms of compression behavior and shape memory recovery, triggering the dimensional changes by both temperature and relative humidity. The thermoregulating properties confirm that the presence of microcapsules affects the heat transfer phenomenon, since part of the heat provided by the source is involved in the wax phase transition.

1. INTRODUCTION Foamed plastics, due to their unique structure and properties, are normally used in all those applications for which lightweight, thermal/acoustic barrier effects and shock-impact absorption are requested, ranging from the automotive sector to packaging systems, and from construction to technical footwear. As a matter of fact, the footwear market is addressing sustainable solutions to the continuous demand for materials that at the same time show lighter weight with higher comfort, durability, and dimensional stability. In this framework, ethylene vinyl acetate (EVA) flexible foams are becoming standard materials in many athletic, ladies high heel, and casual shoes, for fabrication of insoles, midsoles, and unisoles, since light weight, comfort, aesthetic, low cost, and good performance are required. Polyurethane (PU) foams are also commonly used in shoe manufacturing owing to their excellent long-term mechanical properties.1,2 Nevertheless, despite the already high performance of today’s foams, these polymer foams possess serious environmental problems at end use due to their non-biodegradability and difficult disposal. For this reason, developing greener products that replace many petroleum-based plastics with ones based on materials derived from renewable resources and more sustainable business processes have become a priority for many shoe companies.3 For example, the shoe designer Simple Factory Group in collaboration with the plastics supplier Bayer MaterialScience have realized a microcellular PU elastomer system for outer soles and midsoles in which the proportion of renewable raw materials is as high as 70%.4 However, “green” foams developed so far still require more work for practical application. © XXXX American Chemical Society

In this study, a biodegradable matrix, poly(vinyl alcohol), was used to obtain foamed structures through an eco-sustainable methodology without the use of chemical blowing agents. PVA was chosen and employed owing to its ability to provide a porous matrix through an eco-sustainable methodology based on high speed mixing, as well as its biodegradability.5−7 As a consequence, problems concerning the waste disposal, the recyclability, and the effect of blowing agents on the environment could be overcome. Moreover, considering that good thermoregulating performance in terms of heat transfer and perspiration is required for good quality shoes, microencapsulated phase change materials were incorporated into PVA foams in order to find out the buffering function of the system against temperature changes. In fact, as reported in the literature, PCMs are materials that can absorb or release the energy equivalent with their latent heat when the temperature undergoes or overpasses the phase change temperature associated with them.8,9 Therefore, PCMs can provide to the foams the ability to storage the heat energy produced during walking, thus prolonging the comfort and well-being. The effects of PCMs on foam microstructure, water vapor absorption, thermal and mechanical properties were analyzed. In addition, another challenging aspect of the obtained foam, concerning its ability to retain the original shape and size after mechanical deformation by varying environmental conditions, that is its shape memory effect (SME), was investigated. The SME is the feature of shape memory materials (SMMs), which Received: June 17, 2015 Revised: August 4, 2015 Accepted: August 31, 2015

A

DOI: 10.1021/acs.iecr.5b02187 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

The mixture was heated at 90 °C and stirred for 2 h, until a transparent solution was obtained, and then the solution was cooled at room temperature. In the cooled solution 20 wt % MPCM 28, with respect to PVA, was dispersed, and the mixture, about 200 mL, was sonicated for 5 min using a microtip probe sonicator with 500 W, 20 kHz of energy output (VC 505, Vibracell Sonics, Newtown, CT, USA). The amount of MPCMs was selected on the basis of the data reported in the literature related to the evaluation of thermoregulating properties of textiles and to the defoaming process due to the high amount of loaded MPCMs.32,33 Dispersions were mixed into a high speed mixer for 4 min at about 3000 rpm. Finally, these foamed dispersions were poured into cylindrical molds (40 mm diameter and 30 mm height) and in Petri dishes (80 mm diameter and 15 mm height), frozen in liquid nitrogen, kept for 48 h at −30 °C, and successively lyophilized at a temperature of −80 °C and pressure of 5 × 10−2 Torr in an Edwards Modulyo freeze-dryer for 48 h. The same preparation method was also used to obtain neat PVA foams. Neat PVA foams and foams containing MPCMs were coded as Pk6 and Pk6M respectively. In the codes used, “P” refers to PVA, “k” means Kuraray, “6” is due to the PVA concentration in the water solution, and “M” is related to microencapsulated phase change materials. Samples were conditioned at 25 °C and 50% relative humidity in a climatic chamber (Climacell 222, München, Germany) for 24 h before characterization. 2.3. Characterization. Morphological analysis was performed by means of a FEI Quanta 200 FEG environmental scanning electron microscope (ESEM; Eindhoven, The Netherlands) in high vacuum mode, using an accelerating voltage ranging between 15 and 20 kV and a secondary electron detector (Everhart-Thornley detector). The inner microstructure of the foams was analyzed without any surface coating. Inner surfaces were obtained through cryogenic fracturing in the direction normal to the cylinder axis. Water swelling behaviors of the foams were investigated through a gravimetric determination of the swelling ratio (Sw), calculated using eq 1:

are able to virtually hold a temporary shape unless the right stimulus is applied to trigger the shape recovery.10−12 A variety of shape memory polymers (SMPs) have been realized and well documented in the literature that can be triggered for shape recovery by various external stimuli and even multiple stimuli simultaneously.13,14 Depending on the material used, stimuli can be temperature (so-called switching temperature), light (UV and infrared light),15 and chemical (moisture, solvent, and pH change).16,17 Generally, SMPs are polymer networks equipped with suitable switching structures.18 The net points can be physical entanglements of polymer chains or chemical cross-links, while the switching structure can be either crystalline or amorphous.19 Shape memory polymers, such as foams and hydrogels, have received great attention in both industry and academia, throughout the last years.20 Often in the literature, the SME is discussed together with another shape change phenomenon, i.e., shape change effect (SCE). Different from the SME, in which a material is able to virtually keep the quasi-plastic deformation forever, the SCE refers to a kind of elastic or superelastic behavior in response to the right stimulus.21 For instance, the combination of creep and thermoresponsive shape recovery was observed in ether−vinyl acetate copolymer; in this case creep is an SCE for which the energy barrier is lower than that for the SME.22 As for hydrogels, instead, the shape memory effect or shape change effect is moisture-dependent: at lower water/moisture contents, the hydrogel shows a thermoresponsive SME and moistureinduced SME, while at higher water/moisture contents, the hydrogel has a rubber-like SCE and water-induced SCE.23,24 Poly(vinyl alcohol) is a type of easy-processable and biocompatible SMP. For PVA-based SMPs, the cross-linked network through hydrogen bonds and microcrystallites contributes to the permanent phase, while the amorphous phase acts as the stimulus sensitive phase.25,26 As reported in the literature, PVA-based SMPs exist mostly in the form of dense bulk polymers.27,28 In addition, porous PVA-based SMPs and shape-memory hydrogels have been developed to realize additional functions not offered by solid dense SMPs.29−31 The underlying molecular principle of the various forms of SMPs is identical to that of dense bulk SMPs. Notwithstanding the numerous investigations performed on shape-memory hydrogels, actually, very few articles deal with shape memory behavior of PVA-based foams. The aim of this study was to realize PVA-based foams containing PCMs and to analyze their morphological, thermal, and mechanical properties investigating in particular whether PVA foam presents a shape memory property and if this property is altered by the presence of microPCMs.

Sw = 100

Ws − Wd Wd

(1)

where Ws is the weight of the sample in the swollen state and Wd is the weight of the dried sample. In order to measure Sw values, preweighed dried samples were placed in a climatic chamber at 25 °C and 50% relative humidity (RH) and the weight of the swollen samples was measured after 15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 10 h, 24 h, and 48 h. For each sample, six specimens were tested and the average value was reported. Thermal properties of PVA-based foams were investigated using a Mettler DSC 822e calorimeter (Mettler-Toledo, Inc.) equipped with a liquid-nitrogen accessory for fast cooling. The calorimeter was calibrated in temperature and energy using indium. Dry nitrogen was used as purge gas at a rate of 30 mL/ min during all measurements and thermal treatments. A small piece of foam, weighting about 5 mg, was encapsulated in standard aluminum 40 μL pans. Measurements on PVA foams were carried out according to the following thermal program: 30 to 250 °C at 10 °C/min; 250 to −70 °C at 10 °C/min; −70 to 250 °C at 10 °C/min.

2. METHODS 2.1. Materials. PVA, 99−99.8% hydrolyzed, Mw 145.000, MOWIOL 23-99, was kindly supplied by Kuraray America Inc.; glycerol, ≥99.5%, was purchased from Sigma-Aldrich; microencapsulated phase change material, MPCM 28, water suspension, phase change temperature 28 °C, and size 17−20 μm, was purchased from Microtek Laboratories. MPCM 28 contains 85−90 wt % n-octadecane, as PCM, and 10−15 wt % polymer shell which was identified by Fourier transform infrared spectroscopy as melamine formaldehyde resin. 2.2. Foam Preparation. PVA was dispersed in deionized water at a concentration of 0.06 g/mL; glycerol (gly), 30 wt % with respect to PVA, was added to the mixture as plasticizer. B


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deflection (CD) was calculated at the end of the first thermomechanical cycle, as the ratio of the load at 25% deflection and the area of the tested sample. The thermoregulating properties of PVA-based foams were evaluated by a self-designed equipment (Scheme 1, VIRC)34

The shape memory behavior was investigated through a sequence of thermomechanical experiments consisting of a compression step performed on an Instron testing machine (Model 4505) equipped with a thermochamber, and a stressfree recovery step. The shape recovery experiment was carried out on cylindrical samples (40 mm diameter and 30 mm height) conditioned at 25 °C and 50% RH for 24 h. Each sample was first heated in the compression device to a temperature 20 °C over its glass transition temperature (i.e., 50 °C), then it was compressed at a loading speed of 12.5 mm/ min to a maximum strain εm = 30% of its original length, the strain was maintained for 5 min, then under constant strain it was cooled with forced air to 20 °C to fix the compressed foam shape (temporary shape), maintained for 5 min at this temperature, and finally unloaded. After the removal of the strain a rapid recovery of the deformation was recorded and expressed by the shape fixity coefficient, Sf, following eq 2:

Sf =

Hu ·100 He

Scheme 1. Self-Designed Equipment (VIRC) To Evaluate the Thermoregulating Properties of Samples

(2)

where He is the height of the sample at 30% deformation and Hu is the height of the sample immediately after strain removal. To allow the strain recovery, the samples were heated to the loading temperature (T = 50 °C) for 1 h in an oven and their heights were measured after 10, 20, 30, and 60 min. Since PVAbased foams are very moisture sensitive, after 1 h in the oven, the specimens were located in the climatic chamber at 25 °C and 50% RH overnight prior to perform a new cycle. These steps complete one thermomechanical cycle. For each sample, five specimens were tested and three thermomechanical cycles were performed. The shape recovery coefficient, Sr, describes the ability of the material to recover its original shape depending on the molecular and shape parameters of the foam as well as the environmental condition. In our case, both the thermal (T) and the relative humidity (RH) influence the shape recovery behavior of foams. The stress-free shape recovery for each cycle due to the thermal effect, Sr(T), was calculated by eq 3: Sr(T ) =

Hu − HrT ·100 H u − H0

composed of a Peltier effect device coupled with an infrared thermocamera (FLIR Systems ThermoVision A40 M Researcher) having a thermal sensitivity of 0.08 °C, a wavelength of 7.5−13 μm, a measurement range of −40 to 2000 °C, a frame rate of 1/50 s, and an imager with a resolution of 320 × 240 pixels. Two samples (rectangular shaped, 4.5 mm thick) were located on the Peltier plate; thus they were subjected, in the same room condition, to the same cyclic temperature program: Tamb to 6 °C at 3 °C/min; 6 to 32 °C at 5 °C/min; 32 to 6 °C at 3 °C/min. The thermal cycle is controlled by an electronic shield interfaced to a thermocouple located on the Peltier plate. At the same time, the average temperature distribution on the opposite surface of the tested material was measured using the infrared thermocamera. The system acquisition data (SAD) is based on an electronic shield (Arduino MEGA2560), and it is composed of an actuator−conditioning−elaboration (ACE) signal system, a transmission/reception (Tx/Rx) data system, and self-designed PC software developed in Visual Basic that allows the user to set the test parameters and to record the temperature values.

(3)

where Hu is the height of the sample immediately after strain removal, HTr is the height recovered after 60 min at 50 °C, and H0 is the height measured at the beginning of each compression test. The stress-free shape recovery for each cycle related to the moisture absorption, Sr(RH), was calculated by eq 4: Sr(RH) =

Hu − HrRH ·100 H u − H0

3. RESULTS AND DISCUSSION PVA-based foams were prepared by using an eco-friendly process through which air was entrapped in the polymeric dispersion during the high-speed mixing, and the viscosity of the medium avoided its flowing out. The foamed dispersions were frozen and foams were obtained by a lyophilization process. This procedure was used to prepare either neat PVA foams, coded as Pk6, or PVA foams containing microencapsulated PCMs, coded as Pk6M. Foam morphology and microstructure as well as PCM dispersion in PVA matrix were evaluated by means of scanning electron microscopic (SEM) analysis. In Figure 1, SEM

(4)

where HRH is the height after 24 h in the climatic chamber. r The absolute recovery, Rabs, was calculated as reported in eq 5: R abs =

Hrf − He ·100 He

(5)

is the final height of the specimen after three thermomechanical cycles. Compressive modulus values (E) were measured as the slope of the stress−deformation curve at 50 °C. Compression

H rf

C


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Figure 2. Swelling ratios of PVA-based foams as a function of time. Figure 1. SEM micrographs of PVA-based foams: (A, B) Pk6; (C, D) Pk6M.

weight after 24 h (Sw = 8.09 ± 0.43% at 24 h), easily ascribed to the hydrophilic nature of PVA matrix along with the morphology of this foam. On the other hand, for Pk6M, Sw after 24 h is lower with respect to that of neat PVA foam (Sw = 6.61 ± 0.12% at 24 h). Probably the presence of microcapsules, finely dispersed and well-fixed within the foam structure, reduced the number of free −OH groups available for water hydrogen bond formation, since most of them are already involved in strong interactions with microcapsule surfaces. Thermal properties of foams were evaluated by differential scanning calorimetric (DSC) analysis. DSC cooling and heating curves of PVA-based foams, conditioned at 25 °C and 50% relative humidity, are reported in Figure 3. It is to be underlined that the first heating cycle was discarded to minimize the effect of the water. The glass transition temperature of Pk6 is at about 30 °C, that of Pk6M is undeterminable due to the superposition of the endothermic peak related to the microencapsulated PCM phase transition23 and the glass transition temperature of PVA; the inset in Figure 3 reports a magnification of the glass transition step. The crystallization temperature (Tc), the onset crystallization temperature (Tc_onset), the melting temperature (Tm), the corresponding heat of fusion (ΔHm), and the degree of crystallinity (Xc) calculated from DSC thermograms are listed in Table 2. The degree of crystallinity of the material was determined using eq 6:

micrographs of cryogenically fractured surfaces of Pk6 and Pk6M samples are shown, respectively. As expected, Pk6 foam is characterized by a dual-pore structure: large pores are due to the air entrapped in the polymer matrix while small pores are due to the water removal during the freeze-drying.35 Large pores and small pores are partially interconnected. The addition of microencapsulated PCMs does not alter the foam microstructure. In fact, SEM micrographs of Pk6M, Figure 1C,D, reveals that the microstructure is similar to that of Pk6; microcapsules appear not damaged, well embedded in the polymer matrix and homogeneously distributed along the cell walls. This result, together with the complete absence of agglomeration zones, can be ascribed to the good interaction between the microcapsule shell, having hydrophilic groups on the surface,36 and PVA chains. Pore size and cell density values, determined by means of the public domain software ImageJ (release 1.43u),37,38 show that the cell density and cell diameter are similar for both samples; see data reported in Table 1. Small pores, derived from the water sublimation during the lyophilization, present an average diameter lower than 5 μm, in agreement with data reported in the literature.7 Table 1. Average Diameter and Cell Density of PVA-Based Foams sample

cell density (no./mm2)

cell diam (μm)

Pk6 Pk6M

18 ± 3 19 ± 5

58 ± 18 54 ± 16

Xc =

ΔHm 100 ΔHm0 w

ΔH0m

(6) −1

where = 161.6 J g is the melting enthalpy of a 100% crystalline PVA sample, taken from the ATHAS Data Bank, w is the weight fraction of the matrix in the foam. Crystallization of neat PVA foam occurs at 174 °C, starting at about 178 °C and being complete at about 115 °C. In Pk6M sample, the presence of microencapsulated PCMs induced a decrease of the crystallization temperature, Tc = 153 °C, and the related onset temperature of PVA was shifted toward 162 °C. The exothermic peak at 20 °C is related to the crystallization of microencapsulated n-octadecane wax.39 It is worth noticing that the crystallinity value calculated for Pk6 sample is 32.7% and that for Pk6M is 25%. This

In order to evaluate the effect of foam composition on the water swelling behavior of the prepared samples, the swelling kinetics was studied following the procedure described above. The water swelling behaviors of PVA-based foams were investigated by determining the swelling ratio (Sw) reported in Figure 2. The water swelling kinetics are comparable for both samples, while the total amount of absorbed water is different between Pk6 and Pk6M. In fact, Pk6 shows a higher increase of the D


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Figure 3. DSC heating (A) and cooling (B) curves of PVA-based foams, conditioned at 25 °C and 50% relative humidity. In the inset a magnification of the glass transition step is reported.

Table 2. Thermal Data for PVA-Based Foams sample

Tg (°C)

Tc_onset (°C)

Tc (°C)

Tm_onset (°C)

Tm (°C)

ΔHm (J g−1)

Xc (%)

Pk6 Pk6M

30 −

178.0 161.9

173.9 153.0

184.4 173.1

203.8 201.8

52.9 32.2

32.7 25.0

crystallinity decrease could be explained as the result of limited intermolecular interactions among PVA chains through hydrogen bonding caused by the presence of microcapsules that affect the capability of PVA chains to form large crystalline domains.40 Thermal energy storage of Pk6M was determined by integrating the endothermic peak due to the solid−liquid phase transition of n-octadecane.41 The melting enthalpy, 35.7 J/g, was normalized according to the real amount of microcapsules in the foam. The obtained value, 178.5 J/g, is in agreement with the data reported in the MPCM technical data sheet, ΔHm = 180−195 J/g, as well as the measured value of 185 J/g (data not reported). This result suggests that PVA matrix nearly does not influence the thermal proprieties of pure MPCM. Thanks to the thermal properties of the Pk6M foams, they should be used as materials for energy storage applications with an improved function against temperature changes such as in the functional clothing and packaging sectors.42−44 In Figure 4 the stress−strain curves of Pk6 and Pk6M, obtained during compression tests, are reported. In the curves three regions are noticeable: (I) linear elasticity at low stress, (II) a wide plateau due to cell collapse, and (III) a steep stress increase after cells collapse. In Table 3, the compressive moduli (E) of foamed systems determined at 50 °C after the first thermomechanical cycle (N) are reported. As it is possible to observe, the E value related to Pk6 is higher than that of Pk6M, although the systems show the same morphology. This result can be explained taking into account the higher crystallinity of Pk6, compared to Pk6M as revealed by DSC analysis. Moreover, by heating Pk6M at 50 °C, the microencapsulated wax melts and the liquid state of PCM inside the shell affects the mechanical properties of materials lowering the compressive modulus of this sample.45 As reported by Su et al., the temperature affects the mechanical properties of melamine formaldehyde (MF) microcapsule shells. In fact, the shell is a thin membrane so that the physical

Figure 4. Compressive stress−strain curves of PVA-based foams.

state of paraffin greatly influences the mechanical behavior of MPCMs. In a similar way compression deflection (CD) decreases due to the presence of PCMs. As reported in the literature,46 even non-cross-linked polymers present a shape memory effect; in these materials the physical net points together with the crystalline phase contribute to the permanent phase, while the amorphous phase acts as the stimulus sensitive phase. In this respect the ability of the foams to retain the original shape and size after mechanical deformation was evaluated. PVA shape memory properties were investigated according to the procedure reported in the section Characterization and summarized in Scheme 2 on cylindrical samples of 40 mm diameter and 30 mm height. The mechanism reported by Sun et al., set up for polymers without cross-linking and applicable to most thermoplastic polymers, was used to describe the shape memory occurring in PVA foams: upon compression, at temperatures above the glass transition temperature (Tg), the amorphous phase is highly flexible, and the requested deformation was reached; during cooling below Tg, the mobility of polymer chains is reduced, and the inter- and intramolecular interactions through hydrogen bonding are frozen, thus maintaining the distorted shape. The shape recovery is triggered first by temperature and then E


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Industrial & Engineering Chemistry Research Table 3. Density Value and Mechanical Parameters of PVA-Based Foams at 50% RH Pk6

Pk6M

N

E [kPa] (d = 0.122 ± 0.02 g·cm−3)

CD [kN/m2]

E [kPa] (d = 0.115 ± 0.01 g·cm−3)

CD [kN/m2]

1

144.3 ± 61.9

33 ± 10

56.7 ± 14.6

20 ± 4

Scheme 2. Thermomechanical Cycle Imposed to Foams

Table 4. Shape Memory Properties of Pk6 and Pk6M Samples as a Function of Number of Cycles (N) Pk6

Pk6M

N

Sf

Sr(T)

Sr(RH)

Sf

Sr(T)

Sr(RH)

1 2 3

109 ± 5 112 ± 4 113 ± 0

48.9 ± 3.4 61.2 ± 10.6 75.5 ± 6

66.9 ± 3.1 98.7 ± 3.3 100.2 ± 1.0

113 ± 1 116 ± 2 120 ± 1

45.2 ± 4.8 63.6 ± 1.7 81.7 ± 7.1

45.7 ± 2.4 79.7 ± 5.4 98.2 ± 3.7

Figure 5. Height recovery at 50 °C as a function of time: (left) Pk6; (right) Pk6M.

A slight rise of Sf, by increasing the number of cycles, is recorded for Pk6M. This result could be due to the presence of MPCM that hinder the hydrogen bonding formation; as a consequence, the foam has a slightly reduced capability to keep its temporary size. Values of the shape recovery ratio induced by temperature, Sr(T), for both Pk6 and Pk6M are less than 50% in the first cycle and increase to about 80% after three cycles. Values of the shape recovery ratio induced by relative humidity, Sr(RH), for both Pk6 and Pk6M are higher than Sr(T) values for each cycle and are close to 100% after three cycles, thus indicating that water can favor samples to regain their original shape. These results suggest that humidity favors the shape recovery of foams more than temperature. It is well-known that PVA is a typically hydrophilic polymer: the hydroxyl groups exhibit high activity and have strong affinity toward water molecules. Moreover, taking into account the swelling behavior of PVA-based foams, it is possible to correlate the differences in Sr(RH) values between Pk6 and Pk6M with Sw. In order to better understand the mechanism of shape recovery, the height recovery due only to the temperature (50 °C) as a function of time, recorded for each thermomechanical cycle, is reported in Figure 5. It is noticeable that the recovery triggered by temperature occurs almost immediately during the first few minutes in the oven and then it reaches a plateau. On the other hand, during

by humidity; the molecular mobility is increased upon heating and favors the initial shape recovery, after that the plasticizing effect of the absorbed water enhances almost the total recovery. The shape memory properties of Pk6 and Pk6M samples are listed in Table 4. The shape memory effect is described by two important parameters: the shape fixity (Sf) and the shape recovery (Sr). Sf describes the ability of the material to fix the temporary shape achieved by the sample during the compression at 30% of its original height. The shape recovery ratio, Sr, is used to quantify the extent of the applied strain recovered during the shape memory transition from one cycle to the next. In PVA-based foams these parameters could be affected by both temperature (T) and relative humidity (RH)47 so that the shape recovery behavior of foams was correlated to both the environmental conditions. As can be seen from Table 4, the shape fixity for both foams, in the first cycle, is close to 100% and its value remains almost constant also for the other compression cycles. The fixity of the temporary shape is governed by chain movements around the glass transition temperature of the material. When the sample is deformed at a temperature well above Tg and then cooled below Tg, the mobility of polymer chains is reduced, the interand intramolecular interactions through hydrogen bonding are frozen, and thus a high strain fixity value is kept. F


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Industrial & Engineering Chemistry Research A = A 0σ

the following free-recovery step in the climatic chamber, the samples achieved their original height. These findings can be explained considering that the systems in the oven are almost dried; thus the inter- and intramolecular hydrogen bonds fix the macromolecular chain movements and do not allow the complete recovery of the original height. On the contrary, when samples are in the climatic chamber, the absorbed water molecules penetrate into the polymer matrix and act as plasticizers. This effect increases the chain mobility allowing the recovery of the foams’ original shape. As a conclusion, the Rabs values calculated at the end of the three cycles, Rabs(Pk6) = 45.0 ± 0.3 and Rabs(Pk6M) = 43.9 ± 1.9, indicate a significant extent of recovery suggesting an appreciable shape memory behavior for the prepared foams. Thermoregulating Properties. Figure 6 shows temperature profiles corresponding to the signal of the source, the

(7)

where A is the peak-to-peak amplitude of the attenuated thermal wave, A0 is the peak-to-peak amplitude of the thermal wave imposed (signal of the source), and σ is the attenuation factor. It is clear from the temperature profiles, reported in Figure 6, that both foams contributed to a reduction in amplitude of the thermal wave of the source. Pk6 induces a decrease of the temperature at the maximum of the thermal wave of about 8 °C; moreover, Pk6M further reduces the peak temperature of about 3 °C. The last data is due to the latent heat storage capacity of MPCMs used (melting point 28 °C and heat of fusion 180−195 J/g). The attenuation factors calculated for Pk6 and for Pk6M samples are 0.50 and 0.44, respectively. These results indicate that the foamed system can affect the heat transfer phenomenon and this ability is higher when microPCMs are used. The thermal response to a temperature periodic program of the foamed systems has been also evaluated by recording color images using the infrared thermocamera, as displayed in Figure 7. Temperature magnitude in a thermograph can be visualized by a color scale in which each color is related to a temperature range. From the hottest to the coldest temperature, the colors are so scaled: white, red, yellow, green, and blue, with the exact value depending on the experimental set fixed. In this experiment, the image was equivalent to the temperature range from 6 to 32 °C corresponding to the thermal wave signal of the source. When the cooling/heating cycle starts, the samples are at the same temperature (step 1), both showing green color (T = 18 °C). During the cooling ramp, the sample containing PCMs (left side, SX) turns cyan, while the sample without PCMs (right side, DX) is blue (step 2). Such a color difference indicates that the temperature of the sample containing PCMs is higher than that of neat foam. In this process, PCMs act as a thermal buffer material by releasing the stored heat delaying the foam cooling. During the heating ramp, the sample with PCMs is yellow, while the sample without PCMs is orange (step 3). This color difference indicates that the temperature of the sample with

Figure 6. Sinusoidal thermal waves of the source, Pk6, and Pk6M.

thermal wave, and the output signals recorded after the passage through the samples. The temperature profiles recorded depend on the heat capacity of the sample, so it is possible to observe an attenuation of the thermal wave imposed. This phenomenon can be described with eq 7:

Figure 7. Infrared camera images of samples with (SX) and without (DX) PCMs during a thermal cycle program from room temperature down to 6 °C and then up to 32 °C. G


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PCMs is lower than that not containing them, since part of the available heat is used for wax melting. It is to be underlined that the darker color on the left border of the DX sample is related to the distance between samples; in this zone the thermal wave is not going through the sample but through the holder.

4. CONCLUSIONS In this work, new biodegradable and eco-sustainable foams based on PVA were prepared by means of a high speed mixing methodology, without chemical blowing agents. In order to improve the thermal insulation capability of the systems, microencapsulated phase change materials (PCMs) were added. Foam morphologies, investigated by SEM, reveal that microcapsules are finely dispersed and well-fixed through the foam structure, without altering it. Mechanical properties of foams were analyzed in terms of compression behavior and shape memory recovery after three thermomechanical cycles, triggering the dimensional changes by both temperature and relative humidity. This last parameter was found more effective than temperature in favoring the recovery of initial shape, since water molecules interact with PVA chains. The absolute recovery value, Rabs, is about 50%, thus suggesting interesting shape memory performance. The experimental results were interpreted taking into account also the swelling ratio values and the degree of crystallinity. It was evidenced that microcapsules influence the amount of water uptake and the formation of crystalline domain, affecting the number of intraand intermolecular hydrogen bonds since several −OH groups of PVA interact with microcapsule shells. The thermoregulating properties of samples were studied using a self-designed equipment composed of a Peltier effect device coupled with an infrared thermocamera. The attenuation factor measured confirmed that the presence of microcapsules affects the heat transfer phenomenon, since part of the heat provided by the source is involved in the wax phase transition.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +39 081 8675363. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the PON 01_00485 2007/13 MATECON project, founded by the Italian MIUR. REFERENCES

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Heat Storage and Dimensional Stability of Poly(vinyl alcohol) Based

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