Thermo-Responsive Shape Memory Effect and Conversion Of

Article Thermo-Responsive Shape Memory Eﬀect and Conversion of Porous Structure in a Polyvinyl Chloride Foam

Tao Xi Wang , Lu Lu Chang, Yun Hui Geng and Xing Shen * State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China; [email protected] (T.X.W.); [email protected] (L.L.C.); [email protected] (Y.H.G.) * Correspondence: [email protected]; Tel.: +86-025-84895702

Received: 20 August 2020; Accepted: 2 September 2020; Published: 4 September 2020

Abstract: In this paper, a thermo-responsive shape memory effect in a polyvinyl chloride thermoset foam is characterized. Excellent shape recovery performance is observed in foam samples programmed both at room temperature and above their transition temperature. The conversion of porous structures in the foam from closed-cell to open-cell after a shape memory effect cycle is revealed via a series of specially designed oil-dripping experiments and optical images of the micro pores. Programming the strain higher than 20% results in an apparent increase in open-cell level, whereas programming temperatures have almost no influence.

Keywords: shape memory eﬀect; polyvinyl chloride; thermoset foam; porous structure

1. Introduction Purely synthetic thermoset polymeric foam has a wide range of applications in daily lives due to its light weight and tailorable mechanical properties [1,2]. In terms of cellular types, foam materials can be divided into two groups: closed-cell foam and open-cell foam. The difference between these two kinds of porous structures is how the air particles are entrapped within the solid. In closed-cell foams, cells are completely enclosed between each other by thin walls or membranes so that the air/liquid particles are restricted to that particular space inside the cell and are not able to move around from one cell to another. Cells are interconnected in open-cell foams, allowing liquid/air particles to pass through and move around between cells within the foam [3]. Compared to closed-cell foam, open-cell foam is light-weight and has low thermal conductivity, high soundproofing, and good water/air absorption and penetration. For these reasons, open-cell foam has a number of applications, such as acoustical engineering [4], footwear [5], packaging [6], etc. Distinct differences can be found in the fabrication of these two types of foam. The manufacturing process of closed-cell foam is simpler. Casting/molding is used to create 3D shapes specific to the design and decomposition of foaming agents to produce clear pores [3,7]. For open-cell foams, the traditional vapor deposition [8] provides better control over the pore size, distribution, and interconnectivity. Compared with thermoplastic polymers that can rebuild their physical shapes and structures upon melting and solidification, most foams are thermoset so that their porous structures are completely fixed once fabricated. Switching between closed-cell and open-cell structures is traditionally not feasible. However, in this paper, we report a new finding that could realize structural conversion from closed-cell to open-cell in a commercial rigid polyvinyl chloride (PVC) foam using its shape memory effect (SME). SME refers to the capability of a severely or quasi-plastically deformed material to recover its original shape in the presence of a certain stimulus [9–11]. Materials that undergo the SME are called

Polymers 2020, 12, 2025; doi:10.3390/polym12092025 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 16 Polymers 2020, 12, 2025 2 of 16 shape memory materials (SMMs). A complete SME cycle includes two processes: programming, duringshape memorywhich the materials SMM is (SMMs).deformed Ato completea required/d SMEesired cycle shape, includes and twoshape processes: recovery, programming, which can be eitherduring free which (no the external SMM is constraint) deformed toor a constrained required/desired [12,13]. shape, The and external shape stimulus recovery, required which can to be trigger either thefree shape (no external recovery constraint) can be heat or constrained(thermo-responsive [12,13]. TheSME) external [14–16], stimulus chemicals required (chemo-responsive to trigger the shapeSME) [10,17–21],recovery can light be heat(photo-responsive (thermo-responsive SME) SME) [22,23], [14 or–16 even], chemicals magnetic (chemo-responsive fields (magneto-responsive SME) [10,17 SME)–21], [24,25].light (photo-responsive SME) [22,23], or even magnetic ﬁelds (magneto-responsive SME) [24,25]. SMMs can be further categorized into two majormajor groups: shape memory alloy (SMA) [25,26] [25,26] and shape memory polymer (SMP) [[27–2927–29]. In comparison with SMAs, SM SMPsPs usually have much higher recoverable strain,strain, easilyeasily tailorable tailorable thermo-mechanical thermo-mechanical properties, properties, and and better bette biocompatibilityr biocompatibility [30 –[30–32]. 32].For theseFor these reasons, reasons, SMPs SMPs have have attracted attracted considerable considerab attentionle attention in the in past the decade.past decade. So far, So SMEs far, SMEs in both in boththermoset thermoset and thermoplastic and thermoplastic polymers polymers have been have reported been inreported the literature in the and literature many engineeringand many polymersengineering have polymers been shown have been to have shown intrinsically to have intrinsically superior shape superior memory shape performance memory performance [10,33–38]. [10,33–38].In the case In of foamthe case materials, of foam many materials, foamed many polymers foamed (such polymers as polyurethane (such as polyurethane (PU) [39], epoxy (PU) [39], [40], epoxyethylene-vinyl [40], ethylene-vinyl acetate copolymer acetate copolymer (EVA) [41 ],(EVA) etc.) [4 have1], etc.) been have reported been reported with excellent with excellent SME when SME whencompression compression is used is in used programming. in programming. In this work, the SME of the selected PVC foam was systematically characterized. characterized. Additionally, Additionally, the conversion of its porous structure after an enti entirere SME cycle was revealed via a series of specially designed oil-dripping experiments.

2. Materials, Sample Preparat Preparation,ion, and Experimental Methods As a combination of high mechanical strength and low density, the Divinycell H series PVC closed-cell thermosetthermoset foam foam from from Diab Diab Group, Group, Sweden Swed wasen usedwas widelyused widely as the coreas the material core inmaterial sandwich in sandwichcomposite composite structures. structures. Divinycell Divinycell H35 with H35 38 kg with/m3 density38 kg/m was3 density used was in this used study. in this Figure study.1 depicts Figure 1this depicts foam. this foam.

Figure 1. Thethe polyvinyl polyvinyl chloride chloride (PVC) (PVC) foam foam used used in in this this study. study.

Figure 22 showsshows thethe didifferentialfferential scanning calorimetrycalorimetry (DSC) results of this foam, which were obtained from aa Q200Q200 DSCDSC machinemachine (TA(TA Instruments,Instruments, New New Castle, Castle, DE, DE, USA) USA) under under nitrogen nitrogen flow flow at at a aheating heating/cooling/cooling speed speed of of 10 10◦C °C/min/min for for two two thermal thermal cycles cycles to to determine determine its its thermal thermal property. property. The purpose of the first cycle was to eliminate the thermal history of the testing sample; thus, only the second cycle was used for analysis. As the figure shows, no clear melting/crystallization transition (peak and trough) was observed. A zoom-in view of the curve (inset of Figure2) shows a step between 70 and 90 ◦C upon heating, which indicates the temperature range of glass transition (Tg). A simple demonstration of the thermo-responsive SME of the foam is shown in Figure3. The cubic-shaped sample was initially finger-indented at room temperature (23 ◦C) on the central area of its upper surface. In Figure3a 1,a2, clear indentation can be seen in both top and side views. A heat gun was then used to heat the indented surface (Figure3b 1). In the infrared photo (Figure3b 2), the highest surface temperature is about 121 ◦C, which is well above its Tg. During heating, gradual flattening was

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Polymers 2020, 12, 2025 3 of 16 observed on the indented area. After heating for around 30 s, the indentation completely disappeared andPolymers the 2020 sample, 12, x recoveredFOR PEER REVIEW its original ﬂat surface. 3 of 16

Figure 2. DSC results of the PVC foam. Inset: zoom-in view of the glass transition range during heating.

The purpose of the first cycle was to eliminate the thermal history of the testing sample; thus, only the second cycle was used for analysis. As the figure shows, no clear melting/crystallization transition (peak and trough) was observed. A zoom-in view of the curve (inset of Figure 2) shows a step between 70 and 90 °C upon heating, which indicates the temperature range of glass transition (Tg). A simple demonstration of the thermo-responsive SME of the foam is shown in Figure 3. The cubic-shaped sample was initially finger-indented at room temperature (23 °C) on the central area of its upper surface. In Figure 3a1,a2, clear indentation can be seen in both top and side views. A heat gun was then used to heat the indented surface (Figure 3b1). In the infrared photo (Figure 3b2), the highest surface temperature is about 121 °C, which is well above its Tg. During heating, gradual flattening was observed on the indented area. After heating for around 30 s, the indentation completely disappeared and the sample recovered its original flat surface. Figure 2. DSC results of the PVC foam. Inset: zoom-in view of the glass transition range during heating. Figure 2. DSC results of the PVC foam. Inset: zoom-in view of the glass transition range during heating.

Figure 3. Demonstration of SME in the PVC foam. (a1) Top view of the indented foam sample; (a2) side Figure 3. Demonstration of SME in the PVC foam. (a1) Top view of the indented foam sample; (a2) view; (b1) foam sample during heating; (b2) infrared image of sample during heating; (c) top view of side view; (b1) foam sample during heating; (b2) infrared image of sample during heating; (c) top view the recovered foam sample. of the recovered foam sample. In this study, the investigation of the PVC foam had two parts. The ﬁrst part focused on the characterization of its SME, whereas the second part introduced the SME-based structural conversion from closed-cell to open-cell. In the ﬁrst part, cubic shaped samples were prepared with dimensions of 25 mm (length) 25 mm × (width) 30 mm (thickness). As illustrated in Figure4, a complete SME cycle contained two processes × and three steps in this study: (a) At room temperature or 80 ◦C, the sample was uniaxially compressed to a required maximum 2 strain (εm) of 20%, 50%, or 80% at a constant strain rate of 10− /s. (b) After cooling back to room temperature (if necessary), the load was gradually released at the same strain rate. The residual strain after unloading is denoted as ε1. This represents the end of programming process.

Figure 3. Demonstration of SME in the PVC foam. (a1) Top view of the indented foam sample; (a2)

side view; (b1) foam sample during heating; (b2) infrared image of sample during heating; (c) top view of the recovered foam sample.

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In this study, the investigation of the PVC foam had two parts. The first part focused on the characterization of its SME, whereas the second part introduced the SME-based structural conversion from closed-cell to open-cell. In the first part, cubic shaped samples were prepared with dimensions of 25 mm (length) × 25 mm (width) × 30 mm (thickness). As illustrated in Figure 4, a complete SME cycle contained two processes and three steps in this study: (a) At room temperature or 80 °C, the sample was uniaxially compressed to a required maximum strain (ɛm) of 20%, 50%, or 80% at a constant strain rate of 10−2/s. (b) After cooling back to room temperature (if necessary), the load was gradually released at the Polymers 2020, 12, 2025 4 of 16 same strain rate. The residual strain after unloading is denoted as ɛ1. This represents the end of programming process. (c) Samples were reheated to 100 °C (above Tg) for 10 min for recovery. The residual strain after (c) Samples were reheated to 100 ◦C (above Tg) for 10 min for recovery. The residual strain after recovery is denoted as ɛ2. This is the shape recovery process. Note that a 100% shape recovery could recovery is denoted as ε2. This is the shape recovery process. Note that a 100% shape recovery could be achieved if ɛ2 = 0. be achieved if ε2 = 0.

. Figure 4. Illustration of a typical shape memory cycle. Figure 4. Illustration of a typical shape memory cycle. Thus, we can define the shape fixity ratio (Rf) to evaluate the capability of the material to maintain the deformedThus, we shape,can define the shape fixity ratio (Rf) to evaluate the capability of the material to maintain the deformed shape, R f = ε1/εm (1) 𝑅 = ε ⁄ε and the shape recovery ratio (Rr) for the capability of the material to restore its original shape, (1) and the shape recovery ratio (Rr) for the capability of the material to restore its original shape, Rr = (ε ε )/ε . (2) 1 − 2 1 𝑅 =(ε − ε)⁄ε. (2) Here, unless otherwise stated, the compression/programming was conducted using an Instron Here, unless otherwise stated, the compression/programming was conducted using an Instron 5565 machine with an integrated temperature-controlled chamber. For simplicity, all strain and stress 5565 machine with an integrated temperature-controlled chamber. For simplicity, all strain and stress mentioned are meant for engineering strain and engineering stress. mentioned are meant for engineering strain and engineering stress. In addition, a conical indenter with a 120 cone angle and a spherical indenter with a diameter of In addition, a conical indenter with a 120°◦ cone angle and a spherical indenter with a diameter 1.588 mm were used to produce local indentation on foam surfaces. The sample was then heated at of 1.588 mm were used to produce local indentation on foam surfaces. The sample was then heated 100 C for 5 min to induce local shape recovery. The surface topography of these two samples was at 100◦ °C for 5 min to induce local shape recovery. The surface topography of these two samples was recorded after both indentation and recovery using a Sensofar S neox 3D optical profilometer. recorded after both indentation and recovery using a Sensofar S neox 3D optical profilometer. In the second part, a series of oil-dripping experiments were carried out in the foam. Cubic foam In the second part, a series of oil-dripping experiments were carried out in the foam. Cubic foam samples that were 4 mm thick with a width and length of 20 mm were prepared and initially samples that were 4 mm thick with a width and length of 20 mm were prepared and initially programmed to εm values of 20%, 50%, and 80% at either room temperature or 80 ◦C. After heating for programmed to ɛm values of 20%, 50%, and 80% at either room temperature or 80 °C. After heating shape recovery, two drops of silicone oil were then dripped onto the upper surfaces of each sample. for shape recovery, two drops of silicone oil were then dripped onto the upper surfaces of each An original foam sample (without treatment) with the same size was also used as reference. A piece of sample. An original foam sample (without treatment) with the same size was also used as reference. color paper was placed underneath each sample so that clear staining could be observed once the oil A piece of color paper was placed underneath each sample so that clear staining could be observed flowed through the sample and reached the bottom. The staining results were captured by camera after 4, 8, 14, and 24 h upon dripping. With the help of the software Adobe Photoshop, the open-cell levels of these samples were quantified using the nominal stained area S, which is defined by:

S = Nd/Np (3)

where Nd and Np represent the pixel number of dripped area and the entire paper, respectively. Optical images of the foam samples both before and after structural conversion were taken using a microscope. The foam samples that recovered from εm of 80% were compressed to 80% once again at room temperature to reveal the mechanical strength of the foam after structural conversion. Polymers 2020, 12, x FOR PEER REVIEW 5 of 16 once the oil flowed through the sample and reached the bottom. The staining results were captured by camera after 4, 8, 14, and 24 h upon dripping. With the help of the software Adobe Photoshop, the open-cell levels of these samples were quantified using the nominal stained area S, which is defined by:

𝑆=𝑁⁄𝑁 (3) where Nd and Np represent the pixel number of dripped area and the entire paper, respectively. Optical images of the foam samples both before and after structural conversion were taken using a microscope. The foam samples that recovered from ɛm of 80% were compressed to 80% once again at room temperature to reveal the mechanical strength of the foam after structural conversion. Polymers 2020, 12, 2025 5 of 16

3. Experimental Results and Analysis 3. Experimental Results and Analysis 3.1. Characterization of Shape Memory Effect (SME) 3.1. Characterization of Shape Memory Effect (SME) The typical stress versus strain relationship of the foam under uniaxial compression to a maximumThe typical strain stress of 20% versus at both strain room relationship temperatur of thee and foam 80 under °C are uniaxial plotted compression in Figure 5a. to aThe maximum figure showsstrain ofthat 20% the at residual both room strain temperature for the sample and 80 programmed◦C are plotted at inroom Figure temperature5a. The figure was shows10.88%, that which the wasresidual much strain higher for (18.68%) the sample for programmed the sample programmed at room temperature at 80 °C. was In terms 10.88%, of whichstress, wasthe two much samples higher behaved(18.68%) almost for the samplethe same programmed before a strain at 80of ◦1%.C. In Howe termsver, of stress,from 2% the onwards, two samples the stress behaved of the almost sample the programmedsame before a at strain 80 °C of became 1%. However, much lower from and 2% onwards,the stress theafter stress strain of of the 6% sample was about programmed half of the at stress 80 ◦C ofbecame the one much programmed lower and the at stressroom after temperature. strain of 6% The was difference about half ofin thestress stress resulted of the one from programmed the glass transitionat room temperature. induced softening. The diff erence in stress resulted from the glass transition induced softening.

(a)

(c)

(b)

FigureFigure 5 5.. TypicalTypical stress vs.vs. strainstrain relationshipsrelationships in in uniaxial uniaxial compression compression to to a maximuma maximum strain strain of 20%of 20% (a), (50%a), 50% (b), and(b), 80%and (c80%) at room(c) at temperature room temperature and 80 ◦ Cand (followed 80 °C by(followed cooling by back cooling to room back temperature) to room temperature)and then unloading. and then unloading.

A similar trend was observed in samples with εm of 50% in Figure5b. After strain of around 30%, slight hardening occurred in the sample at 80 ◦C, whereas it maintained a rather stable stress level at room temperature. After unloading, the sample programmed at 80 ◦C maintained a much higher residual strain compared with the one programmed at room temperature (42.00% vs. 29.25%). Figure5c presents the results of samples with an εm of 80%, conﬁrming that high testing temperature causes lower compressive stress but better shape ﬁxing. In addition, severe stress hardening was observed in both samples when compression was more than 50%. The evolution in thickness of two typical samples after heating for shape recovery is presented in Figure6; Figure6a,b represent the samples programmed to 20% at room temperature and 80% at 80 ◦C, respectively. Polymers 2020, 12, x FOR PEER REVIEW 6 of 16

A similar trend was observed in samples with ɛm of 50% in Figure 5b. After strain of around 30%, slight hardening occurred in the sample at 80 °C, whereas it maintained a rather stable stress level at room temperature. After unloading, the sample programmed at 80 °C maintained a much higher residual strain compared with the one programmed at room temperature (42.00% vs. 29.25%). Figure 5c presents the results of samples with an ɛm of 80%, confirming that high testing temperature causes lower compressive stress but better shape fixing. In addition, severe stress hardening was observed in both samples when compression was more than 50%. The evolution in thickness of two typical samples after heating for shape recovery is presented in Figure 6; Figure 6a,b represent the samples programmed to 20% at room temperature and 80% at Polymers 2020, 12, 2025 6 of 16 80 °C, respectively.

FigureFigure 6. 6. EvolutionEvolution of of thickness thickness of of the the foam. ( (aa11)) Original Original sample; sample; ( (aa22)) sample sample programmed programmed to to ɛεmm ofof 20%20% at at room room temperature; temperature; ( (aa33)) recovered recovered sample; sample; ( (b11)) original original sample; sample; ( (b22)) sample sample programmed programmed to ɛεmm ofof 80% 80% at at 80 80 °◦C;C; ( (bb3)3 )recovered recovered sample. sample.

AsAs we we can can see from (a(a33), thethe samplesample seeminglyseemingly returned returned to to its its original original thickness thickness upon upon heating heating for forrecovery. recovery. However, However, a tiny a tiny residual residual strain strainε2 (1.83%)ɛ2 (1.83%) was was determined determined after after precise precise measurement measurement by bya digitala digital Vernier Vernier caliper. caliper. For For the the sample sample in (b),in (b), residual residual strain strainε2 wasɛ2 was 6.64% 6.64% after after recovery. recovery. With With the theexception exception of theseof these two two samples, samples, the theε 2ɛ2of of tested tested samplessamples waswas alsoalso recorded and the the results results are are summarizedsummarized together together with with ɛε1 1inin Table Table 11 forfor further further investigation. investigation.

TableTable 1. ɛε11 andand ɛε2 2ofof all all tested tested foam foam samples. samples.

20%20% 50% 50% 80%80%

ɛ1 ε1 ɛ2 ε2 ɛ1ε 1 ɛε2 ε1ɛ1 ε2 ɛ2 RoomRoom temperature temperature 10.88%10.88% 1.83%1.83% 29.95% 29.95% 3.27% 3.27% 44.05% 4.24%4.24% 80 °80C ◦C 18.68%18.68% 2.39%2.39% 42.00% 42.00% 3.98% 3.98% 59.84% 6.64%6.64%

With ɛ1 and ɛ2, the shape fixity ratio (Rf) and shape recovery ratio (Rr) were then obtained and With ε1 and ε2, the shape ﬁxity ratio (Rf) and shape recovery ratio (Rr) were then obtained and the results are presented in Figure 7. the results are presented in Figure7. Clearly, programming at 80 °C causes much higher Rf regardless of the deformation level. Clearly, programming at 80 ◦C causes much higher Rf regardless of the deformation level. However, However, Rf at 80 °C keeps decreasing in nearly linearly with the increase in maximum strain. The Rf at 80 ◦C keeps decreasing in nearly linearly with the increase in maximum strain. The highest Rf

was 93.4% occurring at an εm of 20%, which dropped to only 74.8% at an εm of 80%. The situation was quite diﬀerent in room temperature programming as the corresponding Rf (black dash-line) was almost deformation-independent (around 55%). Rr was higher than 80% in all samples. Unlike Rf, not much diﬀerence was observed in Rr between room temperature programming and 80 ◦C programming. Additionally, Rr for 80 ◦C programming stayed high and stable (between 87% and 91%) regardless of εm. Therefore, we conclude that the foam has quite outstanding shape recovery performance in both room temperature and 80 ◦C programming. Polymers 2020, 12, x FOR PEER REVIEW 7 of 16 Polymers 2020, 12, x FOR PEER REVIEW 7 of 16

highest Rf was 93.4% occurring at an ɛm of 20%, which dropped to only 74.8% at an ɛm of 80%. The highest Rf was 93.4% occurring at an ɛm of 20%, which dropped to only 74.8% at an ɛm of 80%. The situation was quite different in room temperature programming as the corresponding Rf (black dash- situation was quite different in room temperature programming as the corresponding Rf (black dash- line) was almost deformation-independent (around 55%). line) was almost deformation-independent (around 55%). Rr was higher than 80% in all samples. Unlike Rf, not much difference was observed in Rr Rr was higher than 80% in all samples. Unlike Rf, not much difference was observed in Rr between room temperature programming and 80 °C programming. Additionally, Rr for 80 °C between room temperature programming and 80 °C programming. Additionally, Rr for 80 °C programming stayed high and stable (between 87% and 91%) regardless of ɛm. Therefore, we conclude programming stayed high and stable (between 87% and 91%) regardless of ɛm. Therefore, we conclude that the foam has quite outstanding shape recovery performance in both room temperature and 80 that the foam has quite outstanding shape recovery performance in both room temperature and 80 Polymers°C programming.2020, 12, 2025 7 of 16 °C programming.

FigureFigure 7. 7.Shape Shape ﬁxity fixity ratios ratios (black (black color) color) and and shape shape recovery recovery ratios ratios (grey (grey color) color) of all of tested all tested samples samples as Figure 7. Shape fixity ratios (black color) and shape recovery ratios (grey color) of all tested samples a functionas a function of εm of. ɛm. as a function of ɛm. TheThe evolution evolution of of the the surface surface topography topography of of the the indented indented foam foam samples samples is is presented presented in in Figure Figure8, 8, The evolution of the surface topography of the indented foam samples is presented in Figure 8, wherewhere Figure Figure8a,b 8a,b represent represent indentation indentation by by conical conica andl and spherical spherical indenters, indenters, respectively. respectively. Subscripts Subscripts where Figure 8a,b represent indentation by conical and spherical indenters, respectively. Subscripts 1 and1 and 2 refer2 refer to to the the sample sample after after indentation indentation and an recovery,d recovery, respectively. respectively. As As shown shown in in Figure Figure8a 18a,b11,b, 1, 1 and 2 refer to the sample after indentation and recovery, respectively. As shown in Figure 8a1,b1, thethe maximum maximum depths depths ofof conical and and sphe sphericalrical indentation indentation were were 288 288 and and 107 107μm,µ respectively.m, respectively. After the maximum depths of conical and spherical indentation were 288 and 107 μm, respectively. After Afterheating heating for recovery, for recovery, the values the values reduced reduced to 69.5 and to 69.5 34.4 andμm, 34.4whichµm, indicated which a indicated 76% and 68% a 76% recovery and heating for recovery, the values reduced to 69.5 and 34.4 μm, which indicated a 76% and 68% recovery 68%in depth, recovery respectively. in depth, respectively. In terms of Inoverall terms surface of overall topography, surface topography, although slight although surface slight unevenness surface in depth, respectively. In terms of overall surface topography, although slight surface unevenness unevennesswas still observable was still observable in both samples in both after samples recove afterry, recovery,the indentation the indentation disappeared. disappeared. Figure 9 shows Figure 9the was still observable in both samples after recovery, the indentation disappeared. Figure 9 shows the shows2D cross-sectional the 2D cross-sectional view of the view indented of the indentedsurfaces, and surfaces, clear andsurface clear flattening surface ﬂatteningwas also observed was also in 2D cross-sectional view of the indented surfaces, and clear surface flattening was also observed in observedboth samples in both after samples recovery. after recovery. both samples after recovery.

(a1) (a2) (a1) (a2) Figure 8. Cont.

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Polymers 2020, 12, 2025 8 of 16 Polymers 2020, 12, x FOR PEER REVIEW 8 of 16

(b1) (b2)

Figure 8. Evolution(b of1) surface topography of the indented foam samples.(b (2a) 1) After indentation with

2 1 Figurea conical 8. Evolution indenter; of surface(a ) upon topography heating ofat the100 indented °C for 5 foam min; samples. (b ) after (a 1indentation) After indentation with a withspherical a Figure 8. Evolution of surface topography of the indented foam samples. (a1) After indentation with conicalindenter; indenter; (b2) (upona2) upon heating heating at 100 at 100°C for◦C 5 for min. 5 min; (b1) after indentation with a spherical indenter; a conical indenter; (a2) upon heating at 100 °C for 5 min; (b1) after indentation with a spherical (b2) upon heating at 100 ◦C for 5 min. indenter; (b2) upon heating at 100 °C for 5 min.

(a)

(b)

Figure 9. Comparison between cross-sections of foam samples after indentation (black solid lines) Figure 9. Comparison between cross-sections( bof) foam samples after indentation (black solid lines) and and recovery (grey solid lines). (a) Sample indented with conical indenter; (b) sample indented with recovery (grey solid lines). (a) Sample indented with conical indenter; (b) sample indented with spherical indenter. Figurespherical 9. Comparison indenter. between cross-sections of foam samples after indentation (black solid lines) and 3.2. Structuralrecovery Conversion (grey solid from lines). Closed-Cell (a) Sample to Open-Cellindented with conical indenter; (b) sample indented with 3.2. sphericalStructural indenter. Conversion from Closed-Cell to Open-Cell The results of the oil-dripping experiment on the untreated foam sample are presented in Figure 10, in3.2. which StructuralThe Figure results 10Conversiona–c of the were oil-dripping fr recordedom Closed-Cell right experiment after to Open-Cell dripping, on the untreated after 2 h upon foam dripping, sample are and presented after 24 h in upon Figure dripping,10, in which respectively. Figure 10a–c As revealed were recorded in Figure right 10a 2afte, oilr dropdripping, was onafter the 2 foamh upon in dripping, a nearly ﬂat and circular after 24 h The results of the oil-dripping experiment on the untreated foam sample are presented in Figure shapeupon due dripping, to its hydrophobic respectively. surface. As revealed Four greyin Figure reference 10a2, dots oil drop on the was paper on the underneath foam in a the nearly foam flat 10, in which Figure 10a–c were recorded right after dripping, after 2 h upon dripping, and after 24 h

upon dripping, respectively. As revealed in Figure 10a2, oil drop was on the foam in a nearly flat

Polymers 2020,, 12, x 2025 FOR PEER REVIEW 99 of of 16 16 circular shape due to its hydrophobic surface. Four grey reference dots on the paper underneath the foamwere drawnwere drawn to indicate to indicate the position the position of the of foam. the foam. As shown As shown in Figure in Figure 10b 110b,b2,1,b oil2, oil was was found found to beto befully fully absorbed absorbed into into the the foam foam after after 2 h upon2 h upon dripping. dripping. However, However, no staining no staining was found was found on the on paper the paper(Figure (Figure 10b3). 10b The3). same The same result result was obtainedwas obtained after after 24 h 24 (Figure h (Figure 10c1 10c–c31). to Therefore, c3). Therefore, we conclude we conclude that thatthe oilthe was oil was trapped trapped inside inside the pores,the pores, indicating indicati theng closed-cell the closed-cell porous porous structure structure of the of foam. the foam.

Figure 10. Oil-drippingOil-dripping experiment experiment in in an an untreated foam sample. ( (aa11) )Top Top view view of of sample sample upon dripping; ((aa22)) sideside view view of of sample sample upon upon dripping; dripping; (a3) top(a3) view top ofview the paperof the underneath paper underneath upon dripping; upon dripping;(b1) top view (b1) oftop sample view of after sample 2 h uponafter dripping;2 h upon dripping; (b2) side view(b2) side of sample view of after sample 2 h uponafter 2 dripping; h upon dripping;(b3) top view (b3) oftop the view paper of the underneath paper underneath after 2 h upon after dripping; 2 h upon (dripping;c1) top view (c1) oftop sample view of after sample 24 h uponafter 24dripping; h upon ( cdripping;2) side view (c2 of) side sample view after of 24sample h upon after dripping; 24 h upon (c3) topdripping; view of ( thec3) top paper view underneath of the paper after underneath24 h upon dripping. after 24 h upon dripping.

The experimental experimental results results of of the the foam foam cubes cubes afte afterr SME SME cycles cycles are arepresented presented in Figure in Figure 11. The 11. correspondingThe corresponding εm andεm andprogramming programming temperature temperature were were marked marked above above each each sample. sample. Top Top and and side views of all samples right after dripping are shown in FigureFigure 1111aa11,a2,, respectively. respectively. Red Red circles circles were were drawn on the pictures to indicate the positions of oil drops. Results after after 4, 4, 8, 8, 14 14 and and 24 24 h h upon upon dripping dripping are are presented presented in in (b), (b), (c), (c), (d) (d) and and (e), (e), respectively. respectively. As shown, oil staining staining was was found found on on each each of of them, them, which which indicated indicated the the interconnection interconnection between between pores pores so that oil could flow ﬂow from one pore to another and eventually outflow outﬂow the foam body. Therefore, Therefore, open cells were created inside the foam after a SME cycle. The relationship between nominal staining area (S) and time of all samples is plotted in Figure 12. Samples with an ɛm of 20% had the lowest open-cell level and their S grew almost linearly (dashed lines), regardless of programming temperature. The open-cell level in samples with an ɛm of 50% and 80% were quite similar (solid and dash-dot lines) but apparently higher in the one with ɛm of 20%. This phenomenon is also evidenced in Figure 11e, where almost the entire contact area on papers for samples with ɛm of 50% and 80% were stained after 24 h, whereas the unstained region was still

Polymers 2020, 12, x FOR PEER REVIEW 10 of 16

Polymersobvious2020 for, 12those, 2025 with ɛm of 20%. However, programming temperature seemingly has no apparent10 of 16 influence on open-cell level as the S values in samples with same ɛm are all quite close.

(a1)

(a2)

(b)

Figure 11. Cont.

Polymers 2020, 12, 2025 11 of 16 Polymers 2020, 12, x FOR PEER REVIEW 11 of 16

(c)

(d) Figure 11. Cont.

Polymers 2020, 12, 2025 12 of 16 Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

(e)

Figure 11.11. Oil-drippingOil-dripping experimentsexperiments in in foam foam samples samples after after shape shape recovery recovery from from programming programming at room at roomtemperature temperature (80 ◦C) (80 to °C)εm toof εm 20%, of 20%, 50% 50% and and 80%. 80%. (a1 ()a Samples1) Samples upon upon dripping; dripping; ( a(a2)2) sideside viewview upon dripping; (b–e) top views of of the the papers papers underneath underneath after after 4, 4, 8, 8, 14, 14, and and 24 24 h h upon upon dripping, dripping, respectively. respectively.

Red circles in (a) and (a1)) indicate indicate the the oil drops.

The relationship between nominal staining area (S) and time of all samples is plotted in Figure 12. Samples with an εm of 20% had the lowest open-cell level and their S grew almost linearly (dashed lines), regardless of programming temperature. The open-cell level in samples with an εm of 50% and 80% were quite similar (solid and dash-dot lines)(e but) apparently higher in the one with εm of 20%. This phenomenonFigure 11. Oil-dripping is also evidenced experiments in in Figure foam 11samplese, where after almost shape recovery the entire from contact programming area on papersat for samplesroom temperature with εm of (80 50% °C) andto εm80% of 20%, were 50% stained and 80%. after (a1) 24Samples h, whereas upon dripping; the unstained (a2) side region view upon was still obviousdripping; for those (b–e) with top viewsεm of of 20%. the papers However, underneath programming after 4, 8, 14, temperature and 24 h upon seemingly dripping, hasrespectively. no apparent inﬂuenceRed circles on open-cell in (a) and level (a1) asindicate the S thevalues oil drops. in samples with same εm are all quite close.

Figure 12. Nominal stained area vs. time (in hours) upon dripping.

Optical images of the cross-section of the foam are presented in Figure 13, in which Figure 13a,b refer to the original foam and the foam recovered from programming with εm of 80% at room temperature, respectively. As shown in Figure 13a, thin membranes that are highly reflective to lights were seen within each cell. The membranes separate pores so that air/liquid could only be trapped inside instead of flowing around. The recovered foam sample in Figure 13b is different. The reflective membranes seemingly disappeared, and the pores looked much clearer in the image, which visually confirmed the interconnection of the pores after a SME cycle. With such structural conversion, Figure 12. Nominal stained area vs. time (in hours) upon dripping. flowing of air/liquidFigure inside 12. the Nominal foam body stained becomes area vs. possible,time (in hours) resulting upon indripping. the staining phenomenon observed in Figure 11. Optical images of the cross-section of the foam are presented in Figure 13, in which Figure 13a,b refer to the original foam and the foam recovered from programming with εm of 80% at room temperature, respectively. As shown in Figure 13a, thin membranes that are highly reflective to lights were seen within each cell. The membranes separate pores so that air/liquid could only be trapped inside instead of flowing around. The recovered foam sample in Figure 13b is different. The reflective membranes seemingly disappeared, and the pores looked much clearer in the image, which visually confirmed the interconnection of the pores after a SME cycle. With such structural conversion, flowing of air/liquid inside the foam body becomes possible, resulting in the staining phenomenon observed in Figure 11.

Polymers 2020, 12, 2025 13 of 16

(a) (b)

Figure 13. Optical images of the cross-section of the original foam (a) and the foam recovered from Figure 13. Optical images of the cross-section of the original foam (a) and the foam recovered from programming to 80% at room temperature (b). The scale bar is 1 mm. programming to 80% at room temperature (b). The scale bar is 1 mm. Figure 14 compares the mechanical behaviors of the original foam and shape-recovered foam Figure 14 compares the mechanical behaviors of the original foam and shape-recovered foam under uniaxial compression to 80%. Note that for the shape-recovered samples, the εm was 80% under uniaxial compression to 80%. Note that for the shape-recovered samples, the ɛm was 80% and and programming at both room temperature and 80 ◦C are included (grey dashed and solid lines). Resultsprogramming show only at a both slight room diﬀerence temperature between and the 80 recovered °C are included samples, (grey which dashed indicates and that solid programming lines). Results temperaturesshow only a have slight almost difference no inﬂuence between on the the reco mechanicalvered samples, strength which of the indicates foam. On that the programming other hand, thetemperatures stress of these have two almost samples no atinfluence strain of on 20% the were mech 0.3522anical and strength 0.3215 of MPa, the foam. respectively, On the whichother hand, are 67.0%the stress and 61.2%of these of two that samples of the original at strain sample of 20% at were room 0.3522 temperature and 0.3215 (0.5251 MPa, MPa). respectively, In terms which of the are maximum67.0% and stresses 61.2% of (at that 80% of strain), the original the ratios sample were at room approximately temperature 90.5% (0.5251 and MPa). 84.3%, In respectively. terms of the maximum stresses (at 80% strain), the ratios were approximately 90.5% and 84.3%, respectively. Additionally, compared to the original sample at 80 ◦C, shape-recovered samples had slightly higher stressAdditionally, during loading. compared The to the results original indicate sample that at despite80 °C, shape-recovered the porous structure samples being had convertedslightly higher to open-cell,stress during the mechanical loading. The strength results of indicate the foam that after des apite SME the cycle porous is still structure considerable. being converted to open- cell, the mechanical strength of the foam after a SME cycle is still considerable.

Figure 14. Comparison of stress and strain relationships under uniaxial compression to 80% strain between original samples at room temperature and 80 °C, and shape-recovered samples with ɛm of 80% at room temperature.

Polymers 2020, 12, x FOR PEER REVIEW 13 of 16

(a) (b)

Figure 13. Optical images of the cross-section of the original foam (a) and the foam recovered from programming to 80% at room temperature (b). The scale bar is 1 mm.

Figure 14 compares the mechanical behaviors of the original foam and shape-recovered foam under uniaxial compression to 80%. Note that for the shape-recovered samples, the ɛm was 80% and programming at both room temperature and 80 °C are included (grey dashed and solid lines). Results show only a slight difference between the recovered samples, which indicates that programming temperatures have almost no influence on the mechanical strength of the foam. On the other hand, the stress of these two samples at strain of 20% were 0.3522 and 0.3215 MPa, respectively, which are 67.0% and 61.2% of that of the original sample at room temperature (0.5251 MPa). In terms of the maximum stresses (at 80% strain), the ratios were approximately 90.5% and 84.3%, respectively. Additionally, compared to the original sample at 80 °C, shape-recovered samples had slightly higher stress during loading. The results indicate that despite the porous structure being converted to open- Polymers 2020, 12, 2025 14 of 16 cell, the mechanical strength of the foam after a SME cycle is still considerable.

Figure 14. Comparison of stress and strain relationships under uniaxial compression to 80% strain Figure 14. Comparison of stress and strain relationships under uniaxial compression to 80% strain between original samples at room temperature and 80 ◦C, and shape-recovered samples with εm of between original samples at room temperature and 80 °C, and shape-recovered samples with ɛm of 80% at room temperature. 80% at room temperature. 4. Discussion

The oil-dripping experiment and optical images of the cross-section prove the closed-cell to open-cell conversion of the porous structure of such foam. A potential interpretation may be that the compression of the foam during programming squeezes the pores and destroys the thin membranes. Upon heating, the compressed pores recover their original sizes without membranes, which consequently creates many open cells. Although the foam used in this study was PVC-based, such structural conversion may not be limited to this material. This technique can be considered as a potential general technique to produce open-cell structure even for other types of polymeric foam, since SME is an intrinsic property of many engineering polymers [11]. From a manufacturing point of view, the fabrication of rigid open-cell polymeric foam may also be simpliﬁed by such structural conversion because producing foam with closed-cell structures is currently much easier in foam industry. H35 PVC foam is widely used as the core material of industrial composite sandwich structures; thus, deformation caused by external load is likely to happen in real applications. Deformed structures lose some of their mechanical rigidity and have a higher risk of causing accidents, and thus are considered unsuitable for carrying on work. However, replacing local deformed parts is impractical. With the experimental results of this study, a potential solution based on SME may have emerged. From Figures7 and 14, the foam is able to not only recover the sizes via heating, but also restore most of its mechanical strength, so that reusing the deformed foam becomes possible. Moreover, re-process thermosets remains challenging, so reusing the foam could consequently reduce the concern of recycling as well.

5. Conclusions In this paper, a systematic characterization of the SME in a commercial PVC foam was presented. After programming at either 80 ◦C or room temperature, excellent shape recovery was always observed upon heating. The programming destroyed the membranes between pores so that after shape recovery, the original closed-cell porous structure of the foam converted to open-cell. An apparent increase of the open-cell level is conﬁrmed in foam with εm higher than 20%. In addition, the mechanical strength Polymers 2020, 12, 2025 15 of 16 was also revealed to be largely maintained after structural conversion. However, programming temperatures seemingly have almost no inﬂuence on both the open-cell level and mechanical strength of the foam. Such structural conversion may also be extended to other types of polymeric foam as a new general technique to fabricate open-cell structures. The capabilities of both shape and strength recovery may allow the deformed foam to be reused in real applications.

Author Contributions: T.X.W. and L.L.C. conducted some of the experiments; T.X.W. and Y.H.G. helped in data analysis and discussions; T.X.W., L.L.C., and X.S. prepared the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This project was supported by the National Natural Science Foundation of China (Grant No. 11872207), Aeronautical Science Foundation of China (Grant No. 20180952007) and State Key Laboratory of Mechanics and Control of Mechanical Structures (Grant No. MCMS-I-0520G01). Conﬂicts of Interest: The authors declare no conﬂict of interest, and manuscript is approved by all authors for publication.

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