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Polymer 87 (2016) 98e107

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Polymer

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On the successful fabrication of auxetic polyurethane foams: Materialsrequirement, processing strategy and conversion mechanism

Yan Li a, b, Changchun Zeng a, b, *

a High Performance Materials Institute, Florida State University, Tallahassee, FL 32310, USAb Department of Industrial and Manufacturing Engineering, FAMU-FSU College of Engineering, Tallahassee, FL 32310, USA

a r t i c l e i n f o

Article history:Received 9 October 2015Received in revised form15 January 2016Accepted 27 January 2016Available online 1 February 2016

Keywords:Auxetic foamNegative poisson's ratioPolyurethaneGlass transitionStress relaxation

* Corresponding author. High Performance MateUniversity, Tallahassee, FL 32310, USA.

E-mail address: zeng@eng.fsu.edu (C. Zeng).

http://dx.doi.org/10.1016/j.polymer.2016.01.0760032-3861/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

This study aims to provide fundamental understandings in several issues critical to the fabrication ofauxetic polyurethane (PU) foams by thermal compression process: conversion and auxetic structurefixation mechanisms, materials characteristics essential for the successful auxetic conversion, andoptimal conditions for auxetic conversion. Three flexible PU foams with similar morphology wereselected for the study. First the commonalities as well as the differences between these foams in terms oftheir chemical composition, microstructure and thermomechanical properties were thoroughlyanalyzed. This is followed by the auxetic convertibility study of these three foams. Mechanisms for fixingthe structure were elucidated and the windows for processing were interpreted in the context ofpolymer relaxation. Guided by these understandings, a series of auxetic foams were manufactured at awide range of conditions. The processing conditions agreed very well with those suggested from themechanistic investigation, validating the proposed auxetic conversion and structure fixation mecha-nisms. The mechanism was further confirmed by direct observation of the morphology of the fabricatedauxetic foams.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Auxetics refer to a family of materials possessing negativePoisson's ratio (n) [1e4]. They expand in the transverse directionwhen being stretched, while shrink under compression [1e4].Before the pioneering work by Lakes et al. on the manufacturing ofartificial auxetic materials, [1] therewas little interest in this type ofmaterials in spite of many intriguing properties they possess,because their rarity in nature [5]. Since then there has been sig-nificant interest in the development of auxetic materials, because ofthe novel properties and promising application potential theyexhibited, such as enhanced indentation resistance for applicationsin protective equipment and biomedical devices [6e8], improvedbending stiffness and shear resistance for structural integrity con-struction elements [9e13], optimal dynamics, acoustic and dielec-tric properties for damping application and wave absorbers[14e17].

Auxetic polyurethane (PU) foams is a class of auxetic materials

rials Institute, Florida State

that can bemanufactured from conventional flexible PU foams via athermal mechanical process [1,18e22]. This process, first proposedby Lakes et al., involves applying tri-axial compression on a neat PUfoam to partially buckle the cell struts and induce the re-entrantmorphology. The compressed foam was then heated above thesoftening temperature of the polymer, followed by cooling in thecompression state to fix the intended re-entrant structure [1,23,24].A schematic of this process is shown in Fig. S1 in supplementalinformation.

While there are a great deal of study on the effects ofmanufacturing process parameters (temperature, time andcompression ratio) on the structure and properties of auxetic PUfoams [1,10,20,24e31], several key questions remains. First, resultsfrom different researchers show large discrepancies. For example,published studies have shown that for successful auxetic conver-sion using the seemingly simple process, the required processingtemperature may differ substantially from 130 to 220 �C, and theprocessing time may vary from 6 to 60 min. While qualitatively thecombination of higher temperature/shorter time was considered ageneral requirement for the successful conversion, the questionwhy would certain combination would work while others wouldnot, remains unanswered. Most researchers attributed these to the

Y. Li, C. Zeng / Polymer 87 (2016) 98e107 99

difference in the equipment used and properties of the starting PUfoams (e.g., pore size), which only lead to more unansweredquestions. For example, it was reported that PU foams with similarpore size could behave dramatically different in terms of theirauxetic convertibility and associated processing conditions [32]. Arecent review by Critchley et al. [32] provided a comprehensiveaccount of the discrepancies in process parameters and auxeticproperties. Moreover, the microscopic mechanism for the auxeticstructure fixation is unknown, and the vaguely referenced “soft-ening temperature’, critical for auxetic conversion, lack clearmeaning in the context of the materials.

To date a surprising omission in the study of auxetic PU foams isthe lack of understanding of the effects of the chemistry andmicrostructure of the starting PU foams, as they dictate both thematrix PU properties and the cellular structure and properties. It isconceivable they are both critically important in determining 1) thesuitability of auxetic conversion; 2) appropriate windows of pro-cess parameters for the conversion; and 3) the auxetic properties ofthe PU foams. Comprehensive understanding of the chemistry andmicrostructure will 1) facilitate the establishment of the selectioncriteria for starting PU foams for auxetic foam manufacturing, 2)elucidate definitively the physical meaning of the currently vaguelyreferred “softening temperature” and the molecular mechanism forthe auxetic structure fixation, and 3) suggest optimal processingtemperature/time combination that is based on the firm under-standing of the behavior of the materials systems under consider-ation. This would provide a comprehensive framework thatfacilitates transforming the PU auxetic foam manufacturing fromthe current trial-and-error and user-dependent process to engi-neering practices that are based on fundamental polymer scienceand processing principle.

In this study three flexible PU foams with similar morphologywere investigated. First their chemical composition, microstructureand thermomechanical properties were thoroughly analyzed toidentify the common and different characteristics in these foams.This is followed by the auxetic convertibility study of these threefoams. Mechanisms for fixing the structurewere elucidated and thewindows of processing were interpreted in the context of polymerrelaxation. Guided by these understandings, a series of auxeticfoamsweremanufactured at awide range of conditions that agreedvery well with those suggested from the mechanistic investigation.Results from the study have the potential to serve as the generalprinciple for fabrication of auxetic foams of consistent morphologyand properties under optimal conditions.

2. Experimental section

2.1. Materials

In this study, three commercially available flexible PU foamswith similar nominal cell diameters of 480 mmwere used and theyare identified as I, II and III. The densities of the three foams werealso similar (44.8 kg/m3, 44.8 kg/m3 and 48.1 kg/m3, respectively).They were dried in an air-flow oven at 80 �C for at least 12 h beforeuse. Dimethylformamide (DMF) was purchased from Fisher Scien-tific and was used as received.

2.2. Morphology and microstructure characterizations

2.2.1. Sol-Gel analysisSamples (ca. 2 g) were immersed in 500 ml of Dimethylforma-

mide (DMF). After 48 h, they were removed dried in a vacuum ovenat 40 �C for 24 h and then at 80 �C for an additional 24 h. The so-lution, which contained the solvent DMF and the sol fraction fromthe PU foams, were dried to evaporate the solvent. Samples were

thenweighed to determine the soluble fraction of each sample. Thesoluble fractions for I, II and III were found to be 5 wt%, 9 wt% and18 wt%, respectively.

2.2.2. Scanning electron microscopy (SEM)Morphologies of the samples were investigated using field

emission scanning electron microscope (SEM) (JEOL 7401F). Sam-ples were cut using a razor, and the surfacewas sputter-coatedwitha thin layer of gold before observation.

2.2.3. Fourier transform infrared (FTIR) spectroscopic analysisFourier transform infrared (FTIR) spectra were performed using

a Nicolet NEXUS 470 FTIR-spectrometer (ThermoFisher Ltd.) withthe KBr pellet technique in a range from 4000 to 400 cm�1 at aresolution of 4 cm�1. Data were collected as average of 32 scans.FTIR with attenuated total reflectance (ATR) spectra were carriedout in a spectral range from 4000 to 650 cm�1 utilizing a SmartGolden Gate reflectance attachment and recorded 64 scans at aresolution of 2 cm�1. All spectra had been normalized using the CH2peak at 1969 cm�1 as an internal reference peak.

2.2.4. X-ray scatteringSimultaneous small- and wide-angle X-ray scattering (SAXS/

WAXS) measurements were obtained using a Bruker NanoSTARsystem, operating at 45 kV and 650 mA with Ims microfocus X-raysource (Cu Ka, l¼ 0.15412 nm). The SAXS patternwas recorded by aHiStar 2D multi-wire area detector. The WAXS pattern was recor-ded by a Fuji Photo Film image plate, and the plate was read with aFuji FLA-7000 scanner.

2.3. Thermal, thermomechanical and mechanical characterizations

2.3.1. Differential scanning calorimetry (DSC)Differential scanning calorimetry was performed using a TA

Q100 (TA Instruments) under nitrogen atmosphere. The foamsamples (ca. 10 mg) were firstly maintained at 225 �C for 5 min inorder to eliminate possible thermal history effect. Subsequentlythey were rapidly cooled down to �80 �C, and then reheated to200 �C at a heating rate of 10 �C/min.

2.3.2. Dynamic mechanical analysis (DMA)The 7 mm � 7 mm � 25 mm rectangular DMA samples were

machined using a CO2 laser (VersaLASER, Universal Laser Systems).DMA analysis was conducted using a TA Instruments Q800 Dy-namic Mechanical Analyzer in tension model using a deformationof 0.2% strain, a frequency of 1 Hz, a force track of 150%, and apreload force of 0.05 N. The test was run in the temperature rangeof �100 to 200 �C using a heating rate of 1 �C/min.

2.3.3. Uniaxial compression testCompression experiments were conducted using a TA In-

struments Q800 Dynamic Mechanical Analyzer in compressionmodel with a 15 mm compression clamp at a strain rate of0.01 min�1 and 30 �C. Disk sample, 15 mm in diameter and 5 mmthick, were machined using a CO2 laser (VersaLASER, UniversalLaser Systems).

2.4. Structure convertibility study

Structural convertibility characteristics were quantified viastrain-controlled compression tests performed on an ARES-LS3rheometer with 25 mm parallel plate fixture (TA instruments).Disk samples, with a diameter of 25 mm and 10 mm thick, weremachined using a CO2 laser (VersaLASER, Universal Laser Systems).They were heated to the experiment temperatures and allowed to

Fig. 1. Cross-sectional SEM images of the three flexible PU foams in the study. Top row:the three foams showed similar cellular structure; Middle row: high magnificationSEM micrographs showed that the rib/strut region of I had a smooth surface, whilethere were particles present in II and III; Bottom row: high magnification SEM mi-crographs of the rib/strut regions of the three foams after subjected to DMF extraction.The morphology of I remained unchanged, while holes were observed in II and III.These hole appeared to result from the particles being extracted from the samples. Theholes in both II and III were considerably fewer than the original particles, suggestingsome particles are not extractable by DMF.

Y. Li, C. Zeng / Polymer 87 (2016) 98e107100

equilibrate for 10 min, and then compressed to a strain of 40% or70% at a rate of approximately 0.5 min�1. The compressed sampleswere then allowed to equilibrate at the testing temperature fordifferent time. After being cooled at room temperature with thestrain still imposed for an additional 10 min, the samples wereremoved from the fixture and stored for 24 h to allow for thecompletion of relaxation. Finally, the sample thickness wasmeasured, and the structure convertibility (Rf) was calculated usingthe following equation:

Rf ¼ ε=εload

(1)

where ε is the strain after unloading and εload is the initial loadingstrain. Values averaged from three separate measurements wereused for calculation.

2.5. Auxetic foam fabrication and characterization

2.5.1. Fabrication of auxetic foamAuxetic foams were fabricated by the thermo-mechanical pro-

cess described in the literature [19,22]. Samples with initial di-mensions of 32mm in diameter and 80mm in length were insertedinto a metallic tube mold and compressed to 20 mm diameter and50mm long. Themoldwas then placed in a forced-convention ovenat desired temperatures for predetermined periods of time. Themold was then removed from the oven and cooled down for 1 h inambient environment. Subsequently the converted foams wereretrieved from the mold. A schematic of the process is shown inFig. S2.

2.5.2. Poisson's ratio determinationVideos were acquired from a video extensometer system (Shi-

madzu DV-201) machine during tensile tests using a tensile tester(Shimadzu ASG-J, with a strain rate of 6 mm/min and maximumstrain of 10%) (See Fig. S3 in supplemental information). For thecalculation of Poisson's ratio, the videos were first transformed intoa series of static images (for different strains) via the softwareMATLAB R2013b. A MATLAB routine we developed [33] was used tocalculate the length (l) and diameter (d) of the sample for everyimage. Then, the transverse strain (εx) and longitudinal strain (εy)were calculated using the following equations, respectively:

εx ¼ Dllo

(2)

εy ¼ Dddo

(3)

where, l0 is the original length and d0 is the original diameter. Thesection for l0 determination was selected such that it is away fromboth clamps to eliminate the end effect. Finally, the average Pois-son's ratio was calculated from the strainestrain curve by theclassical definition of Poisson's ratio [34].

v ¼ �εx

εy(4)

3. Results and discussion

3.1. Characterization of chemistry and structure

Morphology of the three PU foams were studied by SEM. Fig. 1shows the results. All foams showed similar cellular structure

(Fig. 1, top row). However, a clear difference was identified in thehighermagnification images taken from the rib/strut regions (Fig. 1,middle row). While foam I showed a smooth surface, both II and IIIcontained well dispersed sub-micron sized spherical particles. Theamount of particles was higher in III than in II. After extraction byDMF, the morphology of I remained unchanged, while holes wereobserved in both II and III. The number of holes appeared to beconsiderably less than that of the particles in the respective sam-ples before extraction. Nevertheless the locations of the holesappeared to coincide with those of the particles with similar sizes.There were more holes in the extracted III than in II.

The flexible PU foams are chemically and structurally complexmulti-scale, multiphase materials formed from two competing re-action between a diisocyanate and both polyol and water [35,36].Aside from the obvious macroscale cellular structure, microscopi-cally the flexible PU matrix shows a dominant phase-separatedstructure consisting of hard-segment domains dispersed in andlinked to a continuous soft-segment phase by covalent bonds andphysical interactions [37e41]. If a high water content is used in theformulation, aggregates of the hard segments or “urea balls”, whichare rich in urea and much larger in size and higher in crystallinity,may form [35,36]. Particles such as styrene and acrylonitrilecopolymer (SAN) copolymer particles may also be added in order toimprove the foams' load bearing property and cell openness [41].

Fig. 2a shows the ATR-FTIR spectra of the original foams. In theNH stretching vibration region (from 3500 to 3150 cm�1), all foamsshowed a peak around 3295 cm�1, which was attributed to the NHgroup that is hydrogen bonded with the ether oxygen (NH—O)(hard e soft segment hydrogen bonding) [37]. In the carbonyl (C]O) stretch region (from 1800 to 1600 cm�1), all samples showed

Fig. 2. (a) ATR-FTIR spectra of flexible PU foams. (b) FTIR spectra of the extractants of flexible PU foams by DMF.

Fig. 3. DSC thermograms for the three PU foams under investigation.

Y. Li, C. Zeng / Polymer 87 (2016) 98e107 101

two peaks. The first peak at 1720 cm�1 is assigned to the freeurethane C]O. The second peak around 1642 cm�1 is associatedwith the urea C]O group hydrogen bonded to NH group in theordered hardehard segments (C]O—HN, urea aggregates) [37e39].The FITR results not only confirmed all three PU foams possess thetypical phase-separated structure of flexible PU foams [37e41], butidentified in all materials the presence of two types of hydrogenbonds, one between hard - soft segment, and the other betweenhard - hard segment. I has slightly less hard-soft segment hydrogenbonding than II and III. On the other hand, III has significantly lowerfree urethane content and higher content of hard-segment do-mains, evidenced by the much lower relative intensity of the freeurethane C]O and much higher relative intensity of the orderedC]O—HN peak. Despite the slight differences the overall phase-separated structure are similar in all three foams.

Aside from the common features, FTIR also revealed one keydifference between foam I, and foams II and III. A nitrile (CN) peakcentered at 2240 cm�1 was present in II and III, but absent in I. Thispeak is more distinct in the extracted sol of both II and III (Fig. 2b). Itis known that SAN copolymer particles were often used in PU for-mulations to improve the load bearing property and cell opennessof flexible PU foam [41], and the FTIR provided a direct evidencethat SAN particles were present in II and III but not in I. Comparingto II, III has a higher CN signal, indicating a higher SAN content. Inlight of this, it can be concluded that the particles observed on thecell walls of II and III (Fig. 1, middle row) are SAN particles, whichare of significantly higher number in III than in II.

Furthermore, the connection of the SAN particles to the PUmatrix can be inferred from the morphology of the foams afterextraction (Fig. 1, bottom row) and the FTIR spectra of the extractedsol (Fig. 2b). SAN particles were present in the extracted sol and theamount of extractable SAN was substantially higher in III than in II.At the same time, the sol from III also contained considerablyhigher amount of soluble hydrogen-bonded C]O (hard-segment)than both II and I. These results suggest that some SAN copolymerparticles are linked to the PU matrix by physical means such ashydrogen bonding [40,41]. They can be extracted from the PUmatrix, leaving behind holes observed on the cell walls of II and III(Fig. 1, bottom row). Moreover the fact that in both II and III, theholes were considerably fewer than the particles in the originalfoam suggests majority of the SAN particles are linked to the PU

matrix by chemical grafting, and are not extractable by DMF. Theseobservations are all in good agreement with previous studies onflexible PU foams involving SAN copolymer particles [40,41].

Structural information was further probed by DSC. Fig. 3 showsthe DSC temperature scans. Melting peakwas not detected in any ofthe foams in temperature range up to 200 �C. This suggests thatunlike some flexible PU foams prepared with high water concen-tration that contain large urea-rich aggregates or “urea balls”structure [35,36], all three foams studied herein lack sizable ureaaggregates and the resultant crystallinity. All foams show a tran-sition temperature of about �50 �C, which is the glass transition ofthe soft segment [41] and is denoted as Tg, soft. An additionaltransition was observed in II and III at a temperature of about103e104 �C, but not detected in I. This temperature, which agreeswell with previous reports [40e42], was the glass transition tem-perature of SAN (denoted as Tg, SAN). The DSC results thus furtherconfirmed the presence of SAN copolymers in II and III.

Y. Li, C. Zeng / Polymer 87 (2016) 98e107102

The transition behaviors were also investigated by dynamicmechanical analysis (See Supplemental Information Fig. S4).Consistent with the DSC results, all foam showed a soft segmentglass transition, while the SAN glass transitions were observed inboth II and III but was absent in I. Furthermore by considering thereinforcement effect of the SAN particles, the volume fractions ofSAN particles in the foams were calculated using the Guth equation[43] to be 0 vol % in foam I, 12 vol % in foam II and 32 vol % in foamIII. Foam III has almost three times the amount of SAN particles asfoam II. Details on the calculation can be found in supplementalinformation.

To further understand the fine features of the foams' micro-structure, we employed simultaneous small and wide-angle x-rayscatteringmeasurements (SAXS andWAXS). Fig. 4a shows the SAXSprofiles for the three foams I, II and III. All samples display ashoulder at 0.5e1 nm�1, suggesting a phase separated structurewith weak interconnectivities between hard-segments [44]. Fig. 4bshows the 2D WAXS patterns for the three foams. In all cases onlyan amorphous halo at 2q ~20� were observed. This demonstratesthat in these foams very little orders exist in the hard-segmentdomains [45]. Using Scherrer equation (D z kl/bcosq, where k isa material parameter, commonly 1 for polymer, l is the wavelengthof the X-ray, and b is the full width at half-maximum of the WAXSpeak, the hard segment domain size, estimated by the correlationlength D, was found to be about 1 nm for all three foams. This is farless than the smallest structural heterogeneity resolvable throughTg measurements (~10 nm) [46]. Note also that the presence of theSAN particles in II and III has at most, very limited effect on the hardsegment e soft segment phase separated structure, as both SAXSand WAXS results for the three foams are similar.

From the systematic characterizaton studies, the structuralfeatures of the three PU foams were reavled and confirmed, andsummerized herein. All three foams possess a phase separatedstructure consisting of large fraction of soft-segments with hard-segments dispersed within, typical of felxible PU foams. The hardsegment domain has a size of ~1 nm with limited structural order.Large-size urea-rich aggregates are not present in any of thesefoams. There is one distinct difference between Foam I, and Foams

Fig. 4. (a) SAXS scattered intensity profiles, (b) WAXS patterns and (

II and III. Foams II and III contains sub-micron sized SAN copolymerparticles, while the particles are absent in Foam I. The SAN copol-ymer particles are embedded in the PU matrix by both chemicaland physical crosslinkings. Their presentce does not impact thebasic phase-separated structure of the PU matrix. Foam III has asubstantially higher amount of SAN than II. The muti-scale, muli-phase structures of the three foams are schematically shown inFig. 5.

3.2. Structure convertibility

Two compressive strains, 40% and 70%, were used for the studyof structural convertibility (Rf). Both are beyond the yield point onthe compressive stressestrain curve (the strains are in collapseplaeau regime and densification regime, respectively. See Fig. S5 insupplimental information). This is important because it is in theseregimes that bulkling of the cell cells and ribs/struts and theircontact would take places, which allow for deformation that can beretained (fixed) after removal of the stress. Fig. 6 shows the resultsfrom the convertiblity studies. For all three materials, the structureconvertibility benefited from the use of higher preload strain,which helped achieving a higher Rf in a shorter amount of time.This effect was the most prominent for I, only moderate for II, andbarely noticeable for III. At either preload strain, III showed excel-lent convertibility. Within minutes the structural transformationwould be completed with near perfection (Rf approaches to 1). Bycomparision, I had poor structural convertibility (long conversiontime and low Rf), particularly at lower preload strain. The structuralconvertiblity and fixing rates of II are both inferior to III, but su-perior to I.

Albeit somemajor, distinct difference, auxetic foams share somesimilarity with thermally induced shape memory polymer (SMP)and SMP foams. Both concerns shape fixing using external stress. InSMP the fixed shape is temporary and the major interest is in theelastic shape change between the two states (temporary andpermant shape) and associated charactersitics such as fixity, re-covery dynamics and percent of recovery, hyterisis etc. [47,48] Bycontract, the auxetic foams are used exclusively in the fixed shape

c) WAXS scattered intensity profiles for three flexible PU foams.

Fig. 5. Schematics illustrating the structure features in foams I, II, and III. The basic microstructure of the PUmatrix is a phase separated structure with small hard segment domainsdispersed in a continuous soft segment phase. This is typical of flexible PU foams. The hard segment domain has a size of ~1 nm with limited structural order. Large-size urea-richaggregates are not present in any of these foams. Aside from the common features, II and III contains SAN particles that are linked to the PU matrix by both physical and chemicalgrafting.

Fig. 6. Structural convertibility of the three foams at 135 �C.

Y. Li, C. Zeng / Polymer 87 (2016) 98e107 103

and concerned mostly fixity and the properties of the fixed auxeticstate.

The auxetic foam conversion process resemble the “dual-shapecreation process” (DSCP) for thermally induced SMP. During DSCPthe thermally induced SMP are deformed from the unperturbedinitial state to fixed temporary state by application of externalstress. The fixation relies on the presence of so called “switchingdomains”, and the utilization of cooling the materials below acharacteristic thermal transition temperature of the switchingdomains so that these domains solidify and form physical

crosslinks [47,48]. These additional crosslinks dominate the net-points dermining the permanent shape, thereby enabling thetemporary fixation of an elastic deformation that can be recoveredby reheating. The most commonly encounted switching mecha-nisms are glass transition of amorphous domains and melting/crystallization of crystalline domains.

It is plausbile that the auxetic conversion follows the similaroverall change scheme with its unique fixing mechanisms. Thesemechanisms are more appropriately called “fixing” rather than“switching” because 1) some mechanisms are not entirely revers-ible, and 2) in auxetic foams, one tries to use the materials in thefixed state exclusively, and avoid triggering the return to the initialpermanent shape.

Under the experimental conditions, in the case of I with low pre-strain (40%), the primary fixing mechanism is the hard-segment esoft-segment hydrogen bonding interactions. These hydrogen bondsweaken as temperature increases, allowing the hard-segmentphase to move relative to the soft-segment phase. When the tem-perature is reduced, the hard- and soft-segment phases re-established their hydrogen bonding interactions in the newdeformed geometry, thereby fixing the structure. These forces arehowever, normally weak relative to the overall strength of theelastic matrix's restoring force, resulting in low fixing rate and lowRf. The hydrogen bonds also are easily affected by humidity [39],which decreases the interaction and reduce the convertibility andstability.

At 70% preload strain, buckling of cell walls and ribs took place(Fig. S5). This may lead to possible weak van der Waals interactionsor even contacts between neighboring structures at the experi-mental temperature. Such weak adhesion may have facilitated (tosome degree) the improvement in structure fixation. However itshould be noted that even the adhesion may play a role, it is

Y. Li, C. Zeng / Polymer 87 (2016) 98e107104

ineffective in promoting the fixity and may cause problems in theprocess. First, the van der Waals forces are too weak to offset theelastic restoring force. In addition, fixation by establishment ofcontacts during conversion is intrinsically a random process. It isalso highly individual foam materials dependent and highly un-controllable. It would fundamentally alter the cellular structure andexcessive contacts may completely destroy the foam structure.Therefore for consistent auxetic fabrication, processing conditionsshall be selected to prevent adhesion mechanism taking place.

While the two aforementioned mechanisms were also presentin the conversion of II and III, additional mechanisms must exist,which dominate the conversion process and are responsible for therapid conversion and high Rf, particularly in III. As elucidated in thestructure analysis section, substantial amount of SAN copolymerare present in II and III. These copolymer exist as particles and arelinked to the PUmatrix by both physical and chemical bonds. At theexperimental temperature, ca. 30 �C above the glass transitiontemperature of the SAN copolymer, the particles are in the rubberystate. The compressive stresses not only deformed the foam cellularstructure and PU cell walls and ribs, but also cause the deformationof the viscoelastic SAN particles via time-dependent stress relaxa-tion. The deformed SAN particles, whose shape changed fromspherical from ellipsoidal, were observed in samples afterconvertibility study. Fig. 7a shows an example image and Fig. 7bshows a schematic illustrating this process. Such stressed inducedparticle deformation has been previously reported for polystyrene

Fig. 7. (a) An example SEM micrograph showing the deformed SAN particles in the PUfoam after compression at 135 �C; (b) a schematic showing the concurrent occurrenceof the deformation of both the foam cellular structure and the SAN particles.

(PS) filled poly(dimethylsiloxane) (PDMS) system [49,50]. Whenthe system was cooled down to below the glass transition tem-perature of SAN, the particles were vitrified fixing the overall ge-ometry of the foam samples. Thus in foam III the SAN particlesserves the same role as the switching domain in SMP. Their relax-ation behavior dominate the overall materials response. The stiffSAN particles (at T < Tg, SAN) would withstand and completely offsetthe elastic restoring force, maintaining the foam in the convertedgeometry and thereby an excellent Rf (close to 1). The effects of theother two mechanisms became negligible. In the auxetic conver-sion of II, all three mechanisms may play a role. The SAN particleswhile greatly expedite the structure fixing and substantially higherRf than I, the effect is not as dominant as that in III.

To study in detail the essential role of the SAN particles on theauxetic conversion in II and particularly in III, we conducted sys-tematic convertibility study at different temperature using thesestwo foams. A preload strain of 40% was used so that the effect ofadhesion on the structural convertibility is negligible. As shown inFig. 8a and b, either a longer heating time or a higher temperatureresulted in a higher structural convertibility, providing furthersupport for a SAN particle relaxation controlled process.

To better comprehend the particle relaxation behavior, wemakeuse of a well-known empirical relationship, the Kohlrausch, Wil-liams andWatts (KWW) stretched exponential function [51,52]. It iswidely used to describe the structural relaxation in amorphoussystems and takes the following form.

1� Rf ¼ exp

"��

ttðTÞ

�b#

(5)

where b (0 < b � 1) is the stretch exponent, t is the relaxation timeand T is the temperature.

Data were replotted in double-logarithmic as log[-ln(1-Rf)] vs.log(t) and were shown in Fig. 8c and d. The data for both foams (atall temperatures) exhibited good linear relationships and fit wellusing the KWW stretched exponential, from which the tempera-ture dependent relaxation times for both foams were calculatedand are shown in Fig. 8e and f. For both foams the relaxation timesexhibited a critical “slowing down” when the temperature ap-proaches to the glass transition temperature of SAN, agreeing wellwith the argument that the relaxation behavior observed in theseexperiments originated from the SAN copolymer.

While displaying similar temperature dependence, the particlerelaxation times in III are several magnitude lower than those in II.The relaxation process was extremely rapid unless the temperaturereached to close the SAN glass transition temperature. Weemphasize that the relaxation times discussed here (Fig. 8e and f)are not the SAN copolymer molecular chain relaxation times.Instead, they are particle relaxation times in the two foams, thecharacteristic times (t) for the particles to reach particular levels ofdeformations (εt,p). As mentioned earlier, the deformations of theSAN particles collectively fix the foams in the deformed state. Theyprovide a mechanism for “restraining stress” that can counter/offset the spring-back force originated from the elasticity of the PUfoams upon removal of the external stresses after auxetic conver-sion. As a first order estimation, such restraining stress F can beconsidered equal to the stress necessary to return the deformedSAN particles to the original undeformed state at the auxetic foamservice temperature, under which the SAN copolymer is typically inglassy state. Fff(ESAN,glassy,fp, εt,p), F is a function of the molus ofSAN particle in the glassy state (ESAN,glassy), the number of the SANparticles (or volume fraction of SAN particles, fp), and the charac-teristic deformation of each SAN particle (εt,p). The characteristictimes deduced from the convertibility study are the relaxation

Fig. 8. Structural convertibility of PU foam (a) II and (b) III at different shape holding temperature (shape strain 40%); Plots of log[-ln(1-Rf)] vs. log(t) for PU foam (c) II and (d) III. Thedash lines are the KWW stretched exponential fits; Relaxation time as a function of temperature for PU foam (e) II and (f) III. The values of t were calculated by fitting the data in adoubleelogarithmic plot presented in (c) and (d).

Y. Li, C. Zeng / Polymer 87 (2016) 98e107 105

times to achieve a particular εt,p. As the stress is contributedcollectively by all SAN particles, to empower the same level ofrestraining stress the required deformation εt,p from each particlewould decrease with increasing particle volume fraction fp. Forfoams containing large amount of SAN particle such as foam III inthe present study, the required deformation εt,p may be very small,and could be rapidly achieved at the processing conditions.

To summarize, in this sectionwe aim to elucidate the underlyingmechanisms responsible for the structural convertibility of flexiblePU foams. Our study suggests that the structure fixation may berealized by: 1) hydrogen bonding between hard- and soft-segments; 2) adhesion between the cell walls/ribs from van derWaals interactions and contacts; and 3) stressed induced defor-mation and relaxation of SAN particles. The first two forces arerather weak and inefficient in structural fixation. By contrast, thestress induced SAN particles deformation and relaxation is apowerful means to fix the structure whose effectiveness is pro-foundly affected by the amount of SAN particle in the materials.Extremely rapid structure conversion with Rf/1 can be achieved.

These findings would provide valuable insights to guide auxeticfoams fabrication.

3.3. Auxetic foams manufacture

Aiming to utilize the mechanisms illustrated in the previoussection to guide the auxetic foam manufacturing, we used the tri-axial compression process to fabricate a series of auxetic samplesusing foams I, II and III. It was found that conversion of foam I toauxetic foam was not possible under any of the experiment con-ditions. This coincides with the previous finding of the poorstructural convertibility of this material. The hard-soft segmentshydrogen bonding is too weak and inept for auxetic conversion.Fabrication of auxetic foams using II and III were successful under avariety of processing time/temperature combination. Fig. 9 showsthe results. The auxetic conversion of III was substantially fasterthan that of II. Notably, the processing times (Fig. 9) are comparableto the particle relaxation times (Fig. 8). The slight difference may beattributed to the difference in the equipment used and their

Fig. 9. Auxetic foams were fabricated under a variety of processing conditions (time/temperature) from II and III. Numbers in the graph are measured Poisson's ratio.

Y. Li, C. Zeng / Polymer 87 (2016) 98e107106

heating capability. The relaxation time experiments utilized a DMAthat can provide almost instantaneous heating whereas thermal lagwas much more prominent for the oven used for the auxetic foamfabrication. Nevertheless to a large extent results from the auxeticfoam fabrication study mirror those from the structural convert-ibility study, further confirming the critical role of the SAN particlesand their stress induced deformation/relaxation in determining thesuccess of auxetic conversion and the selection of conversionconditions.

Fig. 10 shows the SEM images of a fabricated auxetic foam. Thetypical re-entrant structure was clearly prevailing in the foam. Highmagnification images (Fig. 10c and d) showed that the SAN parti-cles, which were ellipsoidal in shape, aligned in a preferential di-rection (plausibly along the local stress direction). Indeed thedeformed SAN particles effectively serve as curing/locking agents

Fig. 10. SEM images of an auxetic foam fabricated from foam III. (a) A typical imageshowing the re-entrant foam structure; (b) a higher magnification view of the cell wallregions; (c) and (d) high magnification views of circled regions in (b) showing that theellipsoidal SAN particles preferentially align in one direction (presumably the localstress direction).

and “freeze” the re-entrant structure formed when the foam iscooled to below the SAN glass transition temperature.

4. Conclusions

By a plethora of detailed characterization and analysis that re-veals the materials microstructures, chemistries, morphologies,and thermomechanical properties of several flexible PU foams, thisstudy affords answers to questions critical in manufacturing ofauxetic PU foams from flexible PU foams. The study, for the firsttime, elucidated several microscopic, molecular level mechanismsfor auxetic conversion. Both hydrogen bonding interaction andadhesion by van der Waals interaction and contacts are ineffectiveand highly uncontrollable in achieving auxetic conversion duringprocessing and maintaining the auxetic structure thereafter. Thesignificant role of SAN copolymer particles in the structural con-version of the flexible PU foams was identified and thoroughlyanalyzed for the first time. It was found that stress induced defor-mation and relaxation of the SAN particles is an extremely powerfulmechanism for successful conversion of the PU foam to auxeticfoams with high convertibility and high time efficiency. SAN con-taining flexible PU foams are therefore excellent choices for use inthe fabrication of auxetic PU foams. The processing time-etemperature relationship of SAN containing PU foams largelycoincide with that of the SAN particle relaxation time e tempera-ture relationship, which can be attained by utilizing a simple yethighly useful stretched exponential function and used to design theauxetic foammanufacturing process. Themethodology can serve asa general guideline for manufacturing of auxetic PU foams fromSAN containing flexible PU foams.

The study also unequivocally demonstrated that how PU foamswith similar macroscopic pore morphology would differ tremen-dously in the chemical and physical characteristics, which in turnwould enable different conversion mechanisms that dictate thesuitability of, and associated processing conditions for auxeticconversion using a particular flexible PU foam. The “softening”temperatures discussed in the literature is ill-defined and vaguebecause they may be related to different thermal transition phe-nomena in different materials systems. For rational design ofauxetic foam manufacturing process and product consistency,critical for the adoption of this type of materials, a thorough un-derstanding of the starting PU foam from the materials perspectiveis a indispensable.

Acknowledgment

This work was supported by the U.S. Department of VeteransAffairs (VA118-12-C-0066). The authors are grateful for Md DeloyerJahan and Hui Wang in assisting Poisson's ratio determination.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2016.01.076.

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