In-situ polymorphic alteration of filler structures for biomimetic

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Applications of Polymer, Composite, and Coating Materials

In-situ polymorphic alteration of filler structures for biomimetic mechanically adaptive elastomer nanocomposites Tamil Selvan Natarajan, Shigeru Okamoto, Klaus Werner Stöckelhuber, Sven Wiessner, Uta Reuter, Dieter Fischer, Anik Ghosh, Gert Heinrich, and Amit Das ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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ACS Applied Materials & Interfaces

In-situ polymorphic alteration of filler structures for biomimetic mechanically adaptive elastomer nanocomposites Tamil Selvan Natarajan1,2, Shigeru Okamoto3, Klaus Werner Stöckelhuber1, Sven Wießner1,2, Uta Reuter1, Dieter Fischer1, Anik Kumar Ghosh1,2, Gert Heinrich1,5, Amit Das1,4* 1

Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, D-01069 Dresden, Germany Technische Universität Dresden, Institut für Werkstoffwissenschaft, D-01069 Dresden, Germany 3 Nagoya Institute of Technology, Department of Materials Science and Engineering, Nagoya, Japan 4 Technical University of Tampere, Korkeakoulunkatu 16, Fi-33101 Tampere, Finland 5 Technische Universität Dresden, Institut für Textilmaschinen und Textile Hochleistungswerkstofftechnik, D-01069 Dresden, Germany 2

Abstract: A mechanically adaptable elastomer composite is prepared with reversible soft-stiff properties that can be easily controlled. By the exploitation of different morphological structures of calcium sulphate, which is acting as active filler in soft elastomer matrix, the magnitude of filler reinforcement can be reversibly altered that will be reflected in changes of the final stiffness of the material. The higher stiffness, in other words, the higher modulus of the composites, is realized by the in-situ development of fine nano-structured calcium sulphate di-hydrate crystals, which are formed during exposure to water and, further, this highly reinforcing crystals can be transformed to a non-reinforcing hemihydrate mesocrystalline structure by simply heating the system in a controlled way. The Young’s modulus of the developed material can be reversibly altered from ~6 MPa to ~17 MPa, and the dynamic stiffness (storage modulus, at room temperature and 10 Hz frequency) alters its value in the order of 1000 %. As the transformation is related to presence of water molecules in the crystallites, a hydrophilic elastomer matrix was selected, which is a blend of two hydrophilic polymers, namely epichlorohydrin-ethylene oxide-allyl glycidyl ether ter-polymer (GECO) and a ter-polymer of ethylene oxide-propylene oxide-allyl glycidyl ether (GEPO). For the first time, this method also allows a route to regulate the morphology and structure of calcium sulphate nano-crystals in a confined ambient of crosslinked polymer chains. Keywords: Mechano-adaptive Elastomers, Reversible Mechanical Properties, Biomimetic Composites, Nanocomposites, Calcium Sulphate, Water Responsive

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Corresponding author email: [email protected] Introduction: Stimuli responsive polymers are now receiving an enormous attention because of their reversible physical, mechanical, optical, electrical properties that can be tuned with temperature, electric field, light, water, solvents, ions, glucose, strain or enzymes1-6. The past decade has seen the commercialization of some of the stimuli responsive polymer composites like shape memory polymers, sensors and actuators in the field of aerospace, biomedical and drug delivery applications710

. Similarly, mechanically adaptable polymer composites are a class of stimuli responsive polymer

composites which can reversibly change its mechanical properties when it comes into contact with external stimuli like temperature, electric field, light, water, solvents, ions, enzymes11-15. Mechanical adaptable elastomer composites were inspired from biological creatures such as sea cucumber dermis, which has the ability to reversibly change the stiffness of their dermis when it is immersed in water16. To mimic this behavior, epichlorohydrin and polyvinyl acetate were blended with cellulose whiskers with the help of a special technique like the three dimensional template approach17. The rate of mechanical adaptability was varied by modifying the surface of cellulose nanowhiskers with different charge densities14. In another report, the chemo-mechanical adaptability combined with entropic elasticity of the elastomer has been also extended to obtain rapidly switchable water sensitive shape memory effect, in which the shape recovery in a material was realized by a simple wetting process and fixing through a simple drying process with cellulose nanowhiskers filled elastomeric thermoplastic polyurethanes18. Similarly, a nanocomposite consisting of an electrospun mat of PVA (poly vinyl alcohol) and a polymer matrix consisting of either polyvinyl acetate (PVAc) or ethylene oxide-epichlorohydrin copolymer (EO-EPI) was fabricated by solution casting method19. PVAc/PVA composite exhibited a reversible modulus reduction by a factor of 280 upon exposure to water, whereas EO-EPI/PVA showed a similar reduction of tensile modulus upon water uptake, but with incomplete restoration when dried19-21.

In contrast to cellulose based composites, reactive

composites comprised of hydrogenated nitrile butadiene rubber (HNBR)/cement composite were reported

to

have

water

responsive

mechanically

adaptable


properties.

Ionic

crosslink

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formation/solvation between carboxylate anions and the cations of the cement (Ca2+) during dry/wet process was found to be reversible even after several wet/dry cycles leading to mechanical adaptability between moduli values from 150 MPa to 400 MPa22-23. Similarly, magnesium oxide (MgO) was also used as novel water responsive reinforcing filler for HNBR composites24. Being one of the most abundant minerals found in natural environment, CaSO4 is widely used in the construction, ceramics and medical industries. Although several crystal phases of calcium sulphate were claimed (Table 1), mainly the crystal phases can be classified into anhydride (CaSO4), hemihydrate (CaSO4 · 0.5 H2O) and di-hydrate (CaSO4 · 2 H2O). Industrially, hemi-hydrate and anhydride are prepared by heating gypsum (di-hydrate, DH) to temperatures between 100 to 200 °C to remove the water of hydration25-26. Three anhydrite forms can be obtained depending on the heating temperature. Anhydrite III (hexagonal, γ-CaSO4) form is metastable and commercially used. Other types of anhydride (II orthorhombic, β-CaSO4; I cubic, α-CaSO4) are formed at higher temperatures (300 – 1180 °C). They are very stable and cannot be hydrated again. There are two hemi-hydrate (HH) forms; one is α-HH and the other one is hydrated CaSO4 powder. Among these two types, α-HH is the most commonly used material for different applications, because of fast setting time, good workability, and high strength of its hydrated state. Actually, hemi-hydrate and anhydride (III) can be rapidly converted to gypsum on addition of water (hydration). The hardened mass is a highly porous material with a relatively large internal surface consisting of interlocking crystals in the form of needles, plates, and rods 27-28.



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*Table 1: Crystalline phase systems of calcium sulphate and their properties29, 30

Chemical formula of Phase

CaSO4·2H20

CaSO4·0.5 H20

CaSO4 III

CaSO4 II

CaSO4 I

Designation

Calcium sulphate dihydrate

Calcium sulphate hemi-hydrate

Calcium sulphate anhydride III α -AIII β - AIII

Calcium sulphate anhydride II

Calcium sulphate anhydride II AI - insoluble

α form β form Forms Crystal water Density

20.92 2.31

6.21 2.619 (β) 2.757 (α) 145.15

172.17 Molecular mass Monoclinic Monoclinic/ Crystal Rhombohedral system < 40°C >45-110°C Stability 2.05 8.8 β Solubility in 6.7 α Water at 20°C (g/l) *Gips-Datenbuch, Bundesverband der Gipsindustrie August 18. 2017

0 2.580 136.14 Orthorhombic/ hexagonal 110-300°C 8.8 β 6.7 α

AII – sparingly soluble AII – insoluble AII – plaster cast 0 2.93 2.97 136.14

0 2.93 136.14

Orthorhombic

Cubic

300-1180°C 2.7

1180°C 0

e.V., Berlin, Germany, www.gips.de accessed

More recently, time resolved sample quenching along with high-resolution microscopy was used to demonstrate that gypsum forms via a three-stage process: (i) homogeneous precipitation of nanocrystalline/meso-crystalline

hemi-hydrate31, (ii) self-assembly of meso-crystalline into elongated

aggregates co-oriented along their c axis, and (iii) transformation into di-hydrate gypsum32-33. On the other hand, a gypsum or di-hydrate (DH) form can be converted into α-HH by a solvo-thermal recrystallization process that is a first dissolution of the ions with a subsequent recrystallization into a different form. On heating gypsum under wet condition the α-HH form can be obtained, whereas heating at ambient condition at relatively higher temperature (~140 °C) the β-HH is formed. The dehydration process can be taken place at relatively lower temperature (< 100 °C) if some external salts or acids are used in the process34. High temperature, for example more than 250 °C autoclave heating may give rise the insoluble anhydrite35. The temperature and kinetics of dehydration process ACS Paragon Plus Environment

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can be adjusted by varying the partial pressure, heating method (vacuum oven, autoclave, hot air oven), salt type and its concentration in the solution36-38. As far as the shape and size of gypsum is concerned, a considerable effort has been given to control the size and shape of calcium sulphate crystals in order to fulfil different demands, for example, to control the mechanical properties of gypsum, overcome scale deposition in oil and industrial water treatment plant, in construction, as medical supplements, tissue engineering and implants.

28, 39-42

. Using organic media, Song et al were

successful to generate calcium sulphate nanocrystals with different morphology like small nanoparticles, nano-sheets and nano-whiskers43. While remaining in the soft rubber matrix as active filler, the morphological transformation of calcium sulphate from crystalline to mesocrystalline, and vice versa, leads to alternating mechanical properties of the composites. Obviously, for this phase transformation process the presence of water inside the polymer plays a crucial role. Available literature states that hydrophilic elastomers were mostly prepared by mixing hydrophobic elastomers (chlorinated polyethylene, silicone rubber, natural rubber) with hydrogels (polyacrylic acid, poly-acrylic acid-acrylic amide) which are highly incompatible. The use of compatibilisers like polyethylene glycol or amphiphilic polymer leads to an improved mechanical performance of the composites, but still results in an unstable water swelling characteristics leading to loss of hydrogel particles from the elastomer composites44-46. Owing to the presence of oxygen containing backbone the elastomeric terpolymer GECO (chlorohydrin (CO), ethylene oxide (EO) and allyl glycidyl ether (AGE)) offers a hydrophilic characteristics, but the water absorption capacity of this polymer is limited. On the other hand, another terpolymer (GEPO) with oxygen containing backbone (propylene oxide (PO), ethylene oxide (EO) and allyl glycidyl ether (AGE)) shows a good water absorption capacity. But the elastic nature of this polymer is poor due to presence of some polyoxyethylene crystalline domains44, 47. In this present work, we have developed a new strategy in designing a stable hydrophilic and crosslinked elastomer composite by mixing GECO and GEPO terpolymers and subsequently crosslinking the blend by sulphur curatives. Calcium sulphate was selected as active filler inside the soft elastomer matrix. The reversible alteration of the stiffness of the elastomers which is associated



with the reinforcing and non-reinforcing nature of different crystalline states of calcium sulphate has been characterized with the help of Wide-angle X-ray Scattering (WAXS), Raman spectroscopy, dynamic mechanical properties and transmission electron microscopic studies.

Figure 1: Chemical structure of the base polymer

Experimental Materials: The terpolymer of chlorohydrin (CO), ethylene oxide (EO) and allyl glycidyl ether(AGE) - (GECO) (Hydrin T3108, Tg -51 °C, density -1.26 g/cc), and the terpolymer of propylene oxide (PO), ethylene oxide (EO) and allyl glycidyl ether (AGE)-(GEPO) (Zeospan 8030, Tg= -57 °C, density – 1.16 g/cc, Tm =40 °C) were obtained from Zeon Chemicals. Fig. 1 describes the chemical structure of the raw polymers. Calcium sulphate (99 % anhydrous) was obtained from Acros Organics. This calcium sulphate anhydrous used in our study is manufactured by the dehydration process of the calcium sulphate di-hydrate. All other rubber chemicals like zinc oxide (3 phr), stearic acid (2 phr), sulphur (1 phr), MBT (2-mercaptobenzothiazole) (1 phr), TMTD (tetramethylthiuram disulfide) (2.5 phr) used were of rubber grade. ‘phr’ stands here parts per hundred by weight. The composition of the rubber compounds are shown in Table 2.


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Table 2: The composition of the rubber compounds Materials*

GECO

GEPO

CaSO4

100 GECO

100

-

-

75GECO/25GEPO

75

25

-

50GECO/50 GEPO

50

50

-

25GECO/75GEPO

25

75

-

75GECO/25GEPO 10 CS

75

25

10

75GECO/25GEPO 30CS

75

25

30

75GECO/25GEPO 50CS

75

25

50

100 GECO 50CS

100

-

50

50GECO/50 GEPO 50CS

50

50

50

25GECO/75GEPO 50CS

25

75

50

100 GEPO 50CS

100

-

50

*All compounds contain ZnO- 3 phr, stearic acid- 2 phr, TMTD- 2.5 phr, MBTS- 1 phr, sulphur-1 phr. ‘phr’ stands for (weight) parts per hundred of rubber.

Preparation of the composite: The compounding of rubbers with additives and calcium sulphate was performed with the help of an internal mixer (Haake, Rheomix, Thermo Electron GmbH, Karlsruhe) by two stages mixing. In the first stage, rubber (GECO), hydrophillic polymer (GEPO), calcium sulphate, zinc oxide, stearic acid are mixed at a rotor speed of 60 rpm and temperature of 60 °C for 10 min. In the second stage, the compound was mixed with accelerators (MBT, TMTD) and sulphur at rotor speed of 60 rpm at 60 °C for 3-4 mins. Subsequently, the mixture is sheeted out with the help of laboratory size tworoll mixing mill (Polymix 110 L, size: 203 × 102 mm, Servitech GmbH, Wustermark, Germany). After compounding, all the samples were allowed to mature for 24 h at room temperature. After that, the rheometric study was conducted using a moving die rheometer at 160 °C and at a frequency of 1.67 Hz (SIS V-50, Scarabeus GmbH, Germany). The curing parameters like maximum torque (MH), minimum torque (ML) and the ultimate rheometric torque (difference between MH and ML) were recorded. Optimum curing time (t90) was calculated as the time required to attain 90 % of the ultimate



Page 8 of 30

torque (MH − ML). The matured samples were then cured in a heated press at 160 °C up to their respective optimum cure times obtained from the rheometer.

Sample preparation: To get a di-hydrate crystalline phase of calcium sulphate the moulded samples are immersed in water for different times (1-24 h). Then the sample is removed from water and dried at room temperature for 72 h. It is expected that during room temperature drying process the calcium sulphate di-hydrate stable nano-crystals are formed inside the rubber matrix. To alter again the di-hydrate to hemi-hydrate crystalline phase the water treated sample is exposed at different ambient temperatures (100-200 °C) using hot air oven. The evaluation of the mechanical properties and other investigations of the samples were carried out at different stages of the process.

Characterisation techniques: The water swelling/absorption characteristics were established by an immersion-gain method. Firstly, the initial weight of a sample was recorded (W0), and then the sample was immersed in water (deionized water, Millipore MPK01). After some time, the sample was taken out and its weight was measured (Wt) after removing the surface water by blotting with tissue paper, until a stable value. Swelling/absorption ratio =

× 100

(1)

The tensile tests were performed with DIN S2 dumbbell specimens as per the DIN 53504 using Zwick/Roell-Z010 material testing machine fitted with an optical elongation sensor. The cross-head speed rate was 200 mm/min which was operated at room temperature. Dynamic mechanical analysis was performed using a dynamic mechanical thermal spectrometer (Gabo Qualimeter, Ahlden, Germany, model Eplexor 150 N) in tension mode at rectangular specimen (10 mm × 2 mm × 35 mm). The isochronal frequency employed was 10 Hz and the heating rate was 2 °C/min with a dynamic load at 0.5 % strain and static load at 1 % strain. Amplitude sweep measurements were performed on Eplexor 2000 N in tension mode at room temperature, at a constant frequency of 10 Hz, static load at 60 % pre-strain and varying dynamic load at 0.01–30 % dynamic strain. ACS Paragon Plus Environment

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Differential scanning Calorimetry (DSC) investigations were carried out for rubber blends using Q100 (TA Instruments, USA) coupled with an auto-sampler in the temperature range of -80 °C to +80 °C at the scanning rate of 10K/min under nitrogen atmosphere The morphology of filler particles in the nanocomposites on hydration were also analyzed by Transmission Electron Microscopy (TEM). Ultra-thin sections of the rubber composite were cut by ultra-microtome at -150 °C and the images were captured by Libra 120 transmission electron microscope (ZEISS, Oberkochen, Germany) with an acceleration voltage of 120 kV. WAXS measurements were performed at an ambient temperature on Beamline 03XU at Spring 8, Japan. The X-ray wavelength and the camera length between a detector and a sample were 0.15 nm and 4000 mm, respectively. Two kinds of detectors, Pilatus 3S 1M (Dectris Ltd.) and SOPHIAS detector48 were used. The beam size at the detector position was 120 × 100 µm. Sample thicknesses were ca. 1 mm. The scattering intensity was corrected for absorption due to samples and air scattering. Raman spectroscopy was further utilized to confirm the phase transformation achieved during the hydration and dehydration process. Raman-spectra were recorded using a Confocal Raman Microscope alpha 300 R (WITec GmbH, Ulm, Germany) equipped with a laser with an excitation wavelength of 785 nm and a laser power of 200 mW. Samples were measured with a 20× objective and an integration time of 0.5 s for a single scan in the wavelength region from 200 to 3500 cm−1. For each spectrum 200 accumulations were done. Results and Discussion: Preparation and properties of the hydrophilic and crosslinked elastomers: To achieve a soft hydrophilic elastomeric material with good elastic properties, like higher elongation at break values, as well as good water absorption capabilities, two types of elastomers (GECO and GEPO) are blended and compounded with sulphur curatives which is required for the crosslinking process. Fig. 2a shows the development of torque of the elastomers during sulphur curing at 160 °C. As both of the polymers comprised with double bonds where allyl glycidyl unit remains as a pendant, they can undergo sulphur crosslinking process. The steep rising of the torque values indicates that all the compounds are experiencing with sulphur crosslinking process. ACS Paragon Plus Environment


b)

16

0.2

14

0.1

Heat Flow (W/g)

Torque (dNm)

a)

12 10 8 6

100 GECO 75 GECO 25 GEPO 50 GECO 50 GEPO 25 GECO 75 GEPO 100 GEPO

4 2 0

5

10

15

20

25

30

35


0.0 -0.1 -0.2

Glass Transition -0.3

exo -0.5 -100 -80 -60 -40 -20

40

Time (min)

c)

2nd heating

-0.4

0

0

20

40

d)

3

102

80 100


- 34 °C

2.0

Tan delta

E' (MPa)

10


60

Temperature (°C)

2.5

1.5 1.0

101

0.5 - 46 °C 100

-60

-40

-20

0

20

40

60

0.0

80

-60

Temperature (°C)

e)

Water intake (%)

2 100 GECO 75 GECO 25 GEPO 50 GECO 50 GEPO 25 GECO 75 GEPO 100 GEPO

-20

0

20

40

60

80

Temperature (°C) 100 GECO 75 GECO 25 GEPO 50 GECO 50 GEPO 25 GECO 75 GEPO 100 GEPO

600

1

-40

f)

3

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500 400 300 200 100 0

0 0

50

100

150

200

250

0

2

Strain (%)

4

6

8

10

Time (days)

Figure 2: a) Development of moving die rheometric torque during crosslinking of the elastomers, b) differential scanning calorimetric plot of the elastomers, c) plots of storage modulus (E′) against temperature of the pure polymer and its blends, d) plots of loss tangent (tan δ) against temperature of the pure polymer and its blends, e) stress-strain properties of the crosslinked elastomers, and f) water intake capabilities with time of the crosslinked elastomers (without filler).

To maintain a good dimensional stability during swelling experiments with water this crosslinked network structure is desirable, as well as this network structure can offer good elastic properties of the


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materials. It can be observed from DSC study (Fig. 2b) that the glass transition temperature of the pure rubber as well their blends remains in the range of -60 to -50 °C. After the glass transition, at higher temperature range the pure GECO and the 75 GECO/ 25 GEPO blend with higher content of GECO did not show any melting behavior, but pure GEPO and other blends 50 GECO/ 50 GEPO and 25 GECO/ 75 GEPO show endothermic melting behavior very clearly. This endothermic behavior is associated with the melting of semi-crystalline phase of GEPO polymer. Besides glass transition absence of any characteristic thermal process confirms the amorphous nature of GECO. The dynamic mechanical spectra of these compounds are depicted in Fig. 2c and Fig. 2d. The storage modulus of pure GECO and 75 GECO/ 25 GEPO blend drops sharply around ~ -40 °C because of the steady glass transition process of GECO. But the other samples did not follow this trend and a gradual decrease of the storage modulus can be seen up to 50 °C. This processes associated with pure GEPO and the blends with higher content of GEPO is due to gradual melting of the crystalline phase in GEPO component during heating. Nevertheless, after 50 °C the entire rubber samples, irrespective of their composition, show a rubbery plateau indicating a stable crosslinked network structure. As far as the tan δ-temperature plots are concerned, all the rubber samples show one single glass transition and their glass transition temperatures are remaining in the range of -46 °C to -34 °C. A single glass transition temperature of the rubber blend indicates the compatibility of the polymers in the blend systems49-51. The presence of allyl glycidyl ether in both of the polymers (GECO and GEPO) facilitates the co-vulcanisation process and structural similarities between the polymers might be the major reason for this good compatibility. Stress-strain properties of the GECO/GEPO composites are shown in Fig. 2e. The blending of GEPO with GECO has led to increased Young’s modulus, tensile strength and decreased elongation at break values. The blending of 25, 50, 75 % of GEPO in GECO has led to 25, 85, 170 % improvement in Young’s modulus respectively.

The water swelling

absorption characteristics of the elastomer were shown in Fig. 2f. The pure GECO and GEPO can absorb water about 20 wt.% and 420 wt.%, respectively. Although the main polymer backbones of the two polymers are identical, the water absorption capacity is different. Dramatic improvement in water



absorption capability of GECO is enhanced by blending GEPO into the GECO elastomer matrix. Water absorption of about 50-400 % can be achieved by varying the GEPO content in the blends. The mechanical properties of the crosslinked gum compounds were also evaluated. It can be observed from Fig. 3a that the freshly prepared sample, the water treated sample as well as the heat treated sample provide very similar types of storage modulus patterns as a function of temperature. A careful observation can find a small peak in the E´ vs. temperature profile of the heat treated sample. Most probably, the exposure to high temperatures during the heat treatment process could have led to a reduced interaction between the GECO and GEPO phases and thus leading to the formation of crystalline (cold crystallization) GEPO domains with melting temperature above 0°C. This issue can be further illuminated in our forthcoming study. The stress-strain properties of those gum samples (freshly prepared, water treated and thermally treated samples) were also performed and as expected all of the samples show a similar trend (Fig. 3b). This finding is indicating that the gum rubber without any filler, but after water treatment, did not show any alteration of its mechanical properties, confirming the result that the crosslinking density of the gum rubber is not influenced either by the water treatment or heat treatment processes.

a)

b) 3,0 freshly prepared water treated thermal treated

2,0

1000

2,5

Tan δ

1,5 1,0 0,5

100

0,0 -90

-60

-30

0

30

60

90

Temperature (°C)

10 freshly prepared water treated thermal treated

Stress (MPa)

E′ (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2,0 1,5 1,0 0,5

75 GECO 25 GEPO

1 -90

-60

-30

75 GECO/25 GEPO

0

30

60

90

0,0 0

50

Temperature (°C)

100

150

200

250

Strain (%)

Figure 3: Dynamic and mechanical characterization of the gum rubber- a) storage modulus as a function of temperature, (the inset displays tanδ δ vs. temperature, without any filler), b) stressstrain properties of the gum samples.


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Development of mechanically adaptive elastomeric composites For the preparation of mechano-adaptive elastomer composites, calcium sulphate (hemihydrate/anhydride III, as purchased form) was melt mixed with GECO/GEPO systems. The alteration of the mechanical properties was realized by water treatment and subsequent controlled heating of the water treated samples in a cyclic way. For easy understanding the sample designations are: untreated freshly prepared, water treated sample and thermal treated sample will be followed. As can be observed from the stress-strain plots depicted in Fig. 4a, the curves become steeper and stronger when the sample was treated with water. Again, after dehydration (heat treatment) the curves become weaker, which indicates an alteration of mechanical properties. The same sample can be treated with water to attain again higher mechanical performance. The Young’s modulus was only 6.2 MPa, but after water treatment this value was increased to 16.8 MPa. After heat treatment at 200 °C the modulus drops down again to 7.5 MPa. The reason of these altering mechanical properties is associated with the morphological transformation process of the calcium sulphate. The stronger stress strain properties at same filler loading are possible, if the filler inside the rubber becomes reinforcing after water treatment and/or the rubber-filler interactions are remarkably enhanced. It is fact that a further heat treatment the filler is again converted into a non-reinforcing form. Most probably, the higher reinforcing character can be ascribed by the transformation of calcium sulphate hemi-hydrate to more defined and fine nanocrystals (di-hydrate form) along with good rubber to filler interaction52. By heating the sample, the reinforcing nanocrystals are transforming to non-reinforcing calcium sulphate in hemi-hydrate state. Obviously, these hemi-hydrate particles are neither fully crystalline nor amorphous but exists in the meso-crystal form31. A direct microscopic analysis shown in Fig. 4b-4d reveals the fact more clearly. The TEM image of freshly prepared calcium sulphate (hemi-hydrate) containing sample is shown in Fig. 4b, where calcium sulphate is seen as a dark area because electron density is higher than polymers, i.e., GECO and GEPO. The calcium sulphate particles have microsized morphology of an irregular shape with wide size distribution. No defined crystal structure can be noticed from this picture. But, once the rubber composites are treated with water, a clear morphological transformation can be observed (Fig. 4c and 4d). In these images some amorphous-type



structure of the filler is also seen. Furthermore, after hydration the calcium sulphate is converted into nano-rods or nano-whiskers, which can be clearly observed from Fig. 4d.

a)

b)

freshly prepared water treated thermal treated

4

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

2

1 75 GECO/25 GEPO 50 CaSO4

0 0

100

200

300

Strain%

Reinforcing c)

Nonreinforcing d)

Figure 4: a) Stress-strain plots of the GECO-GEPO 75 : 25 composites filled with 50 phr calcium sulphate, transmission electron micrographs b) of 50 phr calcium sulphate filled GECO-GEPO 75 : 25 blend (fresh sample, hemi-hydrate), c) and d) water treated sample (di-hydrate) with same composition.

It is expected that the formation of nano-sized needles and nano-rods could lead to significant improvement in the mechanical properties as the aspect ratio of the fillers are increased27 to a considerable extent. As far as our knowledge is concerned, this is the first report about the formation of calcium sulphate nanocrystal inside a polymer matrix. The limited supply of water in the polymer matrix and confined space within the free volume of polymer network allow toform very tiny calcium ACS Paragon Plus Environment

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sulphate di-hydrate crystals. It is well known that the formation of interlocking crystals on hydration/water treatment is the unique and inherent property of calcium sulphate. The mechanism/theory behind this process is still controversial. Two different types of theory are mostly believed/reported in literatures: i) crystallization theory, ii) colloidal theory. In crystallization theory, the calcium sulphate hemihydrate/anhydride dissolves in water to form a highly super-saturated solution and then the gypsum (calcium sulphate di-hydrate) crystallizes from the solution as interlocking 1D/2D crystals. In case of colloidal theory, the calcium sulphate particles form a colloidal state when mixed with water (sol-gel mechanism) and then slowly gypsum is formed.

b)

a) 16

45 freshly prepared water treated


f = 7.2

E' (MPa)

12

E'/E'0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 4

30

15

f = 1.2 0 0,00

0,04

0,08

0,12

0,16

0,1

Vol. fraction (φ)

1

10

Dynamic Strain (%)

Figure 5: a) Modified Bachelor-Green plot for the untreated and hydrated 75/25 GECO/GEPO composites filled with 50 phr calcium sulphate. b) storage modulus as a function of dynamic strain amplitude of calcium sulphate filled 75/25 GECO/GEPO blend (untreated, water treated, thermal treated) measured at 60°C.

The aspect ratio of di-hydrate and hemi-hydrate particles present in the water and thermal treated composites are evaluated in a very simple manner following modified Bachelor and Green equation and introducing aspect ratio, which can be written as53-54

where

is the tensile storage modulus of the filled composites,

the gum sample (without any filler),

is the tensile storage modulus of

is the volume fraction of the solid particles, and

is the shape

factor. From Fig. 5a it can be seen that after water treatment the aspect ratio/shape factor of the ACS Paragon Plus Environment


calcium sulphate particles has been ~6 times increased whereas, the shape factor of hemi-hydrate particles remains in the range of ~1.2 indicating more or less spherical/square type of structure. This finding attributed that the higher reinforcing nature of the calcium sulphate particles are mostly associated with the rod-like nature and higher aspect ratio of the crystals. Here, it will be interesting to discuss the non-linear strain dependent dynamic mechanical behavior of the composites. Generally, a gum rubber (without any filler) does not show dependencies of modulus (storage modulus, E’) on dynamic strain amplitude as a filled rubber does. In a filled system, after certain critical strain the filler-filler networks are broken which is reflected in the dynamic mechanical properties of crosslinked elastomers. In the present case it is observed that the freshly prepared sample comprised with hemihydrate shows a little dependency, but after water treatment a strong downward change of the storage modus can be seen with increasing dynamic strain amplitude. This effect can be explained by the insitu development of a strong three dimensional filler-filler network which is associated with the in situ transformation of hemi-hydrate to highly anisotropic calcium sulphate particles in di-hydrate form. This phenomenon can be correlated with the formation of solid plaster (plaster of Paris) from calcium sulphate. A three dimensional network structure with mechanical interlocked di-hydrate crystal particles are involved to maintain such structural rigidity. But in the rubber system, the rod-like nanosized crystals are formed due to inhibition of crystal growth and controlled release of the water by the hydrophilic rubber matrix. Ultimately, these crystals are forming a filler-filler network inside the rubber matrix. However, during further heat treatment the crystal structures collapsed and, consequently, the filler-filler network is also destroyed. Due to the formation of hemi-hydrate particles with poor crystalline nature (meso-crystalline) the calcium sulphate is no longer capable to maintain a percolating structure of the fillers, what can be explained by the dynamic strain sweep analysis. Motivated by the considerable difference between the Young’s moduli values of the soft hemi-hydrate and stiff di-hydrate samples, a detailed dynamic mechanical characterization was carried out with the goal to further enhance the differences of the dynamic properties between stiff and soft state. Fig. 6a describes the dynamic mechanical spectrum of the composites as a function of temperature. A sharp decrease of the dynamic storage modulus of the all the composites has been noted due to rubber to glassy transition around ~ - 40 °C region. From 20 °C to 80 °C clear rubbery plateaus have been ACS Paragon Plus Environment

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appeared for all the samples. In this plateau region the storage modulus was increased from ~8 MPa to ~64 MPa after water treatment indicating water induced stiffening of the materials. After heat treatment, the modulus again revers back to the previous range. These results strongly demonstrate the reversible soft to stiff transition of the materials which is associated with reversible alteration of the filler structure from hemi-hydrate to di-hydrate state. However, an unusual behavior of the dehydrated sample was noticed at the temperature region close to 0 °C. This effect may be associated with some extra relaxation of the entrapped polymer chains by complex polymorphic crystals of the calcium sulphate. A similar observation was realized in gum compounds which might be associated with cold crystallization process of phase separated GEPO.

a)

b) 10000

2,0


1000

100


-37 °C

1,5

Tan Delta

E' (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-37 °C

1,0 -39 °C

0,5

10 -80 -60 -40 -20

0

20

40

60

80

100

0,0 -80

T (°C)

-60

-40

-20

0

20

40

60

80

100

T (°C)

Figure 6: a) Effect of water on the storage modulus (E′- temperature sweep) of 75 GECO/25 GEPO/50 CaSO4 composites. For the di-hydrate sample, the sample was immersed in water for 3 h and then dried at room temperature for 72 h. Thermal treated sample (regenerated hemihydrate) was prepared by heating the sample for 1 h at 200 °C. b) plots of loss tangent (tan δ) as a function of temperature.

Fig. 6b shows the loss tangent behavior of the rubbers against temperature. The glass transition temperatures, as designated by peak position, of the composites do not alter that much, but a slight reduction of the Tg of the water treated composites (di-hy) has been noticed. As the rubber used here is actually blend system, some of the calcium sulphate di-hydrate crystals may be preferentially accumulated in the GEPO phase which has a lower Tg as compared with GECO. Furthermore, in the



di-hydrated sample some water molecules may act as plasticizers and can decrease the glass temperature. Since the reversible change of the stiffness is the main concern of this material, the condition to turn stiff elastomer to a soft state is also optimized. The water treatment time here is an important issue as it finally determines the hemi-hydrate to di-hydrate transformation process. The modulus was found to attain a maximum value when the sample was treated with water for 3 h at room temperature (Fig. 6a). Further prolonged water treatment most probably the dissolution of the calcium sulphate into soluble form took place and the total volume fraction of the filler can be decreased which results into less reinforced (softer) elastomer matrix with lower modulus values. Fig. 7b shows the development of tensile storage modulus (T= 60°C at 10 Hz) of the samples as a function of different conditions. This curve can serve as an indication for an in-situ transformation from di-hydrated to hemi-hydrate state. It is observed that a relatively higher temperature (~200°C) is suitable to bring back the modulus of the materials to its initial range of values. After 1 hour thermal treatment at 200 °C temperature the modulus dropped from 50 MPa to 6 MPa. Though the transformation from di-hydrate to hemi-hydrate takes place starting from ~ 40 to 45 °C, but the process is too slow to be realized for the alteration of the stiffness values. Taking advantage of GECO and GEPO for their temperature resistance behavior, the rubber composite can be thermally treated at 200 °C in the range of less than one hour without noticing any degradation behavior.


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b)

10 hemi-hydrate

10 0

0 0

5

10

15

20

c)

220°C for 20 min

Conditions

d)

Time (h)

200°C for 1 h

20

150°C for 7d

20

30

120°C for 48 h

30

regeneration of the hemi-form

40

100 °C for 72 h

40

50 di-hydrate

di-hydrate

hemi

50

Storage Modulus (MPa)


a)

di-hydrate

50 40 30

di-hydrate to hemi-hydrate 20 dehydration at 200°C

10 0

20

40

60

80

di-hydrate

70 60 50 40

hemi-hydrate (regenerated) hemi-hydrate (fresh)

30 20 10 0

modulus of the initial dry state (hemi-hydrate)

0


80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

120

0

10

Time (mins)

20

30

40

50

60

70

80

GEPO (%)

Figure 7: a) storage modulus of the sample those were immersed in water (23°C) with different time, b) effect of different dehydration conditions (di-hydrated to hemi-hydrate transformation) on the storage modulus values. The di-hydrate sample was prepared by immersion of the freshly prepared sample into water for 3 h. c) plot of storage modulus as a function of thermal treatment time at 200 °C. All the samples used here (a, b, c) are comprised with 75 GECO/25 GEPO/50 CaSO4. In curve d) effect of GEPO content in the blend on the maximum hydrated modulus achieved all the samples are filled with 50 phr calcium sulphate, the water treatment time was 2 h, heat treatment was carried out at 200°C for 2 h. The tensile storage modulus was considered at 60 °C and 10 Hz frequency.

Fig. 7c shows tensile storage modulus as a function of heat treatment time at 200 °C. A steady decrease of the storage modulus can be observed, which is finally approaching to the modulus values of the untreated fresh sample. After 2 h the degree of the conversion from hydrated to hemi-hydrate is quite satisfactory as the sample shows again lower modulus values which can be compared with the values before water treatment. The effect of blend ratio is also considered in the experimental plans as the water absorption capacity of the rubber is directly related with GEPO content. That means the final water content can also influence the in-situ polymorphic crystalline process. Fig. 7d shows the storage ACS Paragon Plus Environment


modulus of the freshly prepared sample, water treated and heat treated samples. 100 % GECO composites offers a little effect on the modulus values by water and heat treatment process. However, with the addition of GEPO in the composite, the hydrated modulus increases and reaches a maximum at 20 % GEPO content. It is clear that the addition of GEPO increases water absorption of the composite as already shown in Fig. 2f. A higher amount of imbibed water may facilitate the calcium sulphate transformation to the di-hydrate state and could lead to a higher modulus. In contrast, at more than 20% GEPO content an adverse effect can be seen, as the modulus of the water treated sample decreases rapidly. Higher content of GEPO means a higher amount of imbibed water and these extra water molecules may dissolute the calcium sulphate resulting in a decrease of the total amount of calcium sulphate in the rubber matrix. This finding also corroborates with the results of water treatment time for di-hydrate formation (Fig. 7a). A further detailed study is required to understand this effect. It could be mentioned here that a 100 % conversion of hemi-hydrate to di-hydrate state and vice versa is not expected to be taken place, but water and thermal treatment have a direct influence on the mechanical behavior that can only explained by the reversible polymorphic transformation of hemi-hydrate to di-hydrate of the calcium sulphate crystals. WAXS analysis, Raman Spectroscopy: The different crystal forms of calcium sulphate in pure form as well as in composites can be analyzed through Wide-angle X-ray Scattering (WAXS) and Raman Spectroscopy. WAXS from i) as-supplied calcium sulphate powder-hemi-hydrate, ii) di-hydrate, iii) gum rubber without any filler, iv) rubber/calcium sulphate-untreated, v) rubber/calcium sulphate-water treated, and v) rubber/ calcium sulphate-thermally treated are shown in Fig. 8. Here, q is the magnitude of scattering vector defined as q = (4π sinθ) / λ, where 2θ and λ are the scattering angle and the wavelength, respectively. Calcium sulphate (as supplied) used for the composite preparation were found to have two strong scattering maxima at q = 10.3, 17.9 nm-1 that are attributed to (1 0 0), (1 1 0) planes of hemi-hydrate hexagonal crystal, whereas three scattering maxima at q = 8.15, 14.56, 16.43 nm-1 can be referred to (0 2 0), (0 2 1), (0 4 0) planes of monoclinic crystal structure of hydrated (di-hydrate) calcium sulphate as shown by the WAXS profiles i) and ii), respectively. The calcium sulphate powder (di-hydrate), the small maxima at around 14.56, 16.43 nm-1 that are characteristic to the above-mentioned monoclinic calcium ACS Paragon Plus Environment

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sulphate dihydrate were still seen, indicating some amount of unreacted calcium sulphate anhydride remained along with the one hemi-hydrate. Gum rubber also showed a broad amorphous peak at ca. 15 nm-1 with some subtle weak peaks that might be due to the presence of other ingredients like zinc oxide, curatives etc. as shown by the WAXS profile (iii). Untreated GECO/GEPO/CaSO4 (iv) showed clearly the scattering maxima at the same q values around 10.5, 18.0 nm-1 as well as the broad amorphous peak. This indicates that the calcium sulphate anhydride and calcium sulphate hemihydrate phases successfully remained even after the mixing and molding process (at 160°C) of the elastomer composites. Note that three scattering peaks at 16.0, 16.3 and 17.6 nm-1 are artifacts and should be ignored (indicated in red circle). After hydration (GECO/GEPO/CaSO4 water treated), a series of the scattering maxima at ca. 8.23, 14.62, 16.45 nm-1 from (0 2 0), (0 2 1) and (0 4 0) clearly emerged, which indicates the formation of monoclinic calcium sulphate di-hydrate (CaSO4 . 2H2O), and a considerable amount of unreacted calcium sulphate anhydride and hemi-hydrate were also present as evidenced by another series of the maxima at 10.2 and 17.55 nm-1 as shown by the WAXS profile (v). This result is consistent with the TEM observation in which unreacted calcium sulphate coexisted with nano-rods of calcium sulphate crystals. Note that more scattering maxima are seen at 18.27 nm-1 in the WAXS profile (iv) and 7.93, 8.54 and 14.12 nm-1, and that the peak at ca. 15 nm-1 on the WAXS profiles (v) and (vi) are much broader than the other peaks: Actually, the two peaks consist of a few peaks. All of these peaks are considered to come from somehow distorted crystals or crystals with different lattice constants possibly because of the network structure of the nano-rods discussed above in the TEM images. However, this is beyond the scope of this work. Further when the composite is dehydrated/thermal treated, the series of the scattering maxima at ca. 8.23, 14.62, 16.45 nm-1 totally extinct (the WAXS profile (vi)), which confirms the reversible transition between hemihydrate/anhydride III and di-hydrate structure. The reversibility of the mechanical properties is based on this reversible emergence/extinction of monoclinic calcium sulphate di-hydrate that is interwoven as a network structure of nano-rods as revealed by the TEM images, regardless of whether anhydride and/or hemi-hydrate CaSO4 exist. Actually, the latter exists in all the composite samples, but contribute much less to the mechanical properties as described above. So, WAXS studies confirmed



the reversible phase transformation of the calcium sulphate crystals during hydration and dehydration of the elastomer composites. Similarly Raman spectroscopy also can be used to clearly elucidate the phase transformation process particularly, in the region of 800 cm-1 - 1100 cm-1. Due to the presence of several crystalline structures of calcium sulphate, an in depth analysis of Raman spectra is very interesting to understand the in-situ reversible phase changing characteristics of calcium sulphate. Prieto-Taboada30 et al. critically analyzed calcium sulphate using Raman Spectroscopy and in this work, the peak around at 1025, 1015, 1008 cm-1 can be attributed for identification of anhydride (III), hemi-hydrate and di-hydrate phases in the composites30,

55

. These peaks are associated with υ1 symmetric stretching of SO42-

(sulphate) tetra-hedra. The chemical environment (water molecules) and the covalent bonds of the sulphate ions are responsible for such vibration. As the sulphur-oxygen bonds of sulphate tetra-hedra are easily affected by hydrogen bonding with water molecules. It can be seen from Fig. 8b that freshly prepared rubber samples as well as the pure calcium sulphate powder (as supplied) both show a major, but broad peak around 1015-1017 cm-1 indicating the presence of hemi-hydrate and a little amount of anhydride III crystal phases in the elastomer composite. On water treatment, major peak of 1007 cm-1 and a minor peak of 1015 cm-1 is obtained. The major and minor peak explained the formation of di-hydrate and hemi-hydrate phase respectively. Further on heat treatment, 1015 cm-1 peak was identified, which indicates the reversible formation of hemihydrate (regenerated) phase.


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b)

a)

Intensity (a.u.)

1015-1017 cm-1 thermal treated rubber

1008 cm-1 water treated rubber untreated rubber

CS powder (di-hydrate) CS powder (hemi)

960

980

1000

1020

1040

-1

wave number (cm )

c)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water treated

60

Dihydrate CaSO4 . 2H2O

50 40 30

Hemihydrate CaSO4 . 1/2 H2O

water

20

heat

10 0

freshly prepared 10

thermal treated

30

50

Calcium Sulphate (phr)

Figure 8: a) X-ray crystallographic analysis of gypsum powder (hydrate), calcium sulphate hemi-hydrate powder, pure rubber without any filler (75 GECO/25 GEPO), composite with untreated (75 GECO/25 GEPO- 50 phr calcium sulphate, hemihydrate), hydrated (75 GECO/25 GEPO- 50 phr calcium sulphate, gypsum), dehydrated (75 GECO/25 GEPO- 50 phr calcium sulphate, regenerated hemi-hydrate, b) Raman Spectroscopy of the rubber composites with the same composition used in X-ray scanning, c) tensile storage modulus (as a measure of the stiffness of the materials) as a function of calcium sulphate loading. This picture also demonstrates the mechanism of stiff-soft transition of the elastomer. The tensile storage modulus was considered at 60 °C and 10 Hz frequency.

Now it is confirmed that reversible transformation of various types of crystalline structures are responsible for the mechanical adaptation of the crosslinked elastomers. However, a significant magnitude of the stiff-soft transition can be realized if the volume fractions of the fillers are sufficiently high to establish a secondary network inside the rubber matrix. Elastomer composites with various loading of calcium sulphate were prepared (50 GECO/50 GEPO and 10, 30, 50 phr calcium



sulphate). The stiff and softness character was quantified by dynamic mechanical analysis and shown in Fig. 8c. Addition of calcium sulphate at low volume fraction provide little effect in soft-stiff transition, however, at 50 phr calcium sulphate loading the effect is very pronounced. The tensile storage modulus of the gum rubber (75 GECO/ 25 GEPO) was found to be ~ 4 MPa (Fig. 2c) and after incorporation of 50 phr calcium sulphate (as supplied) the value becomes ~ 6 MPa. At this loading, the marginal changes only suggests the non-reinforcing feature of the as suppled calcium sulphate. However, after water treatment the value is increased to 60 MPa. This finding strongly suggests the formation of highly reinforcing hydrated calcium sulphate particles which exist with higher anisotropic character. Concurrently, these particles form a three dimensional filler-filler secondary network within the polymeric network resulting to significant stiffer elastomers. After further heat treatment the di-hydrate nanoparticles are broken into non-reinforcing fillers (hemi-hydrate) and three dimensional networks is also disturbed resulting the elastomer matrix in the soft state. Conclusion: Mechanically adaptive elastomer composites have received huge interest in soft robotics and soft matter technologies which composes mostly of elastomers, gels and colloidal suspensions. Due to the similar elastic and rheological properties of the soft materials with the biological counter parts, these types of soft materials/machines become an automatic choice for the robotic materials56. Thus these types of mechanically adaptive elastomer materials play a key role in sensors, actuators57, artificial muscle and skin materials used in humanoid robots. One of the areas where these types of materials find immediate applications is vibration control58. For example, cyclic stiffness modulation occurring at the helicopter blade root has been used to significantly reduce the vibrations in the helicopter59. In this context, adaptive elastomer composites find more and more applications, e.g. in mounts, dampers, buffers, etc. Additionally, mechanically adaptive composites could also be used in several areas like dynamic implants, active orthoses60-61, protective clothing, switchable membranes, smart seating, smart tyres, deployable structures and smart sealing applications62. The adopted strategy for the preparation of hydrophilic elastomers by blending a structurally similar hydrophilic polymer (GEPO) with the GECO elastomer is successfully utilized and leads to new kinds of water ACS Paragon Plus Environment

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induced mechanically adaptable elastomer composites. The mechanical adaptability was achieved by utilizing the reversible polymorphic phase transformation process that occurred in-situ during the hydration and dehydration process of the calcium sulphate filled composites. This method also demonstrates the preparation of calcium sulphate nanoparticles by a polymer network template route. Other thermoplastic polymers with a hydrophilic character, like polyvinyl alcohol, or polyacrylic acid, could be utilized for the development of mechanical adaptable composites. Future challenges are also identified when the samples were subjected to multiple cyclic soft-stiff transitions. Acknowledgement The author would like to acknowledge Zeon Chemicals (Joseph Fields) for providing the raw polymers. The authors would like to give special thanks to Holger Scheibner for performing mechanical studies and Judith Nelke for graphical arts. The synchrotron radiation experiments were performed at the BL03XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016B7251, and 2017A7201) for the beamtime at SPring8). This work was (partially) supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan for the detector, Pilatus3S 1M (Dectris Ltd.) at SPring8). The authors appreciate T. Kudo and T. Hatsui for the provision of the SOPHIAS detector Reference: 1. Stuart, M. A. C.; Huck, W. T.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M., Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9 (2), 101-113. 2. Liu, F.; Urban, M. W., Recent Advances and Challenges in Designing Stimuli-Responsive Polymers. Prog. Polym. Sci. 2010, 35 (1), 3-23. 3. Rapoport, N., Physical Stimuli-Responsive Polymeric Micelles for Anti-Cancer Drug Delivery. Prog. Polym. Sci. 2007, 32 (8), 962-990. 4. Kim, P.; Zarzar, L. D.; He, X.; Grinthal, A.; Aizenberg, J., Hydrogel-Actuated Integrated Responsive Systems (HAIRS): Moving Towards Adaptive Materials. Curr. Opin. Solid State Mater. Sci. 2011, 15 (6), 236-245. 5. Natarajan, T. S.; Eshwaran, S. B.; Stöckelhuber, K. W.; Wießner, S.; Pötschke, P.; Heinrich, G.; Das, A., Strong Strain Sensing Performance of Natural Rubber Nanocomposites. ACS Appl. Mater. Interfaces 2017, 9 (5), 4860-4872. 6. Bhagavatheswaran, E. S.; Parsekar, M.; Das, A.; Le, H. H.; Wiessner, S.; Stöckelhuber, K. W.; Schmaucks, G.; Heinrich, G., Construction of an Interconnected Nanostructured Carbon Black Network: Development of Highly Stretchable and Robust Elastomeric Conductors. J. Phys. Chem. C 2015, 119 (37), 21723-21731. ACS Paragon Plus Environment


7. Hu, J.; Liu, S., Responsive Polymers for Detection and Sensing Applications: Current Status and Future Developments. Macromolecules 2010, 43 (20), 8315-8330. 8. Hoffman, A. S., Rtimuli-Responsive Polymers: Biomedical Applications and Challenges for Clinical Translation. Adv. Drug Delivery Rev. 2013, 65 (1), 10-16. 9. Mishra, Y. K.; Adelung, R., ZnO Tetrapod Materials for Functional Applications. Mater. Today 2017. 10. Shree, S.; Schulz-Senft, M.; Alsleben, N. H.; Mishra, Y. K.; Staubitz, A.; Adelung, R., Light, Force, and Heat: A Multi-Stimuli Composite that Reveals its Violent Past. ACS Appl. Mater. Interfaces 2017, 9 (43), 38000-38007. 11. Mendez, J.; Annamalai, P. K.; Eichhorn, S. J.; Rusli, R.; Rowan, S. J.; Foster, E. J.; Weder, C., Bioinspired Mechanically Adaptive Polymer Nanocomposites with Water-Activated Shape-Memory Effect. Macromolecules 2011, 44 (17), 6827-6835. 12. Annamalai, P. K.; Dagnon, K. L.; Monemian, S.; Foster, E. J.; Rowan, S. J.; Weder, C., WaterResponsive Mechanically Adaptive Nanocomposites Based on Styrene-Butadiene Rubber and Cellulose Nanocrystals-Processing Matters. ACS Appl. Mater. Interfaces 2014, 6 (2), 967-976. 13. Shanmuganathan, K.; Capadona, J. R.; Rowan, S. J.; Weder, C., Biomimetic Mechanically Adaptive Nanocomposites. Prog. Polym. Sci 2010, 35 (1), 212-222. 14. Jorfi, M.; Roberts, M. N.; Foster, E. J.; Weder, C., Physiologically Responsive, Mechanically Adaptive Bio-Nanocomposites for Biomedical Applications. ACS Appl. Mater. Interfaces 2013, 5 (4), 1517-1526. 15. Natarajan, T. S.; Stöckelhuber, K. W.; Malanin, M.; Eichhorn, K.-J.; Formanek, P.; Reuter, U.; Wießner, S.; Heinrich, G.; Das, A., Temperature-Dependent Reinforcement of Hydrophilic Rubber Using Ice Crystals. ACS Omega 2017, 2 (2), 363-371. 16. Capadona, J. R.; Shanmuganathan, K.; Tyler, D. J.; Rowan, S. J.; Weder, C., Stimuli-Responsive Polymer Nanocomposites Inspired by the Sea Cucumber Dermis. Science 2008, 319 (5868), 13701374. 17. Shanmuganathan, K.; Capadona, J. R.; Rowan, S. J.; Weder, C., Bio-Inspired MechanicallyAdaptive Nanocomposites Derived from Cotton Cellulose Whiskers. J. Mater. Chem. 2010, 20 (1), 180-186. 18. Zhu, Y.; Hu, J.; Luo, H.; Young, R. J.; Deng, L.; Zhang, S.; Fan, Y.; Ye, G., Rapidly Switchable Water-Sensitive Shape-Memory Cellulose/Elastomer Nano-Composites. Soft Matter 2012, 8 (8), 2509-2517. 19. Stone, D. A.; Wanasekara, N. D.; Jones, D. H.; Wheeler, N. R.; Wilusz, E.; Zukas, W.; Wnek, G. E.; Korley, L. T., All-Organic, Stimuli-Responsive Polymer Composites with Electrospun Fiber Fillers. ACS Macro Lett. 2011, 1 (1), 80-83. 20. Wanasekara, N. D.; Stone, D. A.; Wnek, G. E.; Korley, L. T., Stimuli-Responsive and Mechanically-Switchable Electrospun Composites. Macromolecules 2012, 45 (22), 9092-9099. 21. Wanasekara, N. D.; Korley, L. T., Toward Tunable and Adaptable Polymer Nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2013, 51 (7), 463-467. 22. Musso, S.; Robisson, A.; Maheshwar, S.; Ulm, F.-J., Stimuli-Responsive Cement-Reinforced Rubber. ACS Appl. Mater. Interfaces 2014, 6 (9), 6962-6968. 23. Robisson, A.; Maheshwari, S.; Musso, S.; Thomas, J. J.; Auzerais, F. M.; Han, D.; Qu, M.; Ulm, F.-J., Reactive Elastomeric Composites: When Rubber Meets Cement. Compos. Sci. Technol. 2013, 75, 77-83. 24. Han, D.; Qu, M.; Yue, C. Y.; Lou, Y.; Musso, S.; Robisson, A., Swellable Elastomeric HNBR–MgO Composite: Magnesium Oxide as a Novel Swelling and Reinforcement Filler. Compos. Sci. Technol. 2014, 99, 52-58. 25. Badens, E.; Llewellyn, P.; Fulconis, J.; Jourdan, C.; Veesler, S.; Boistelle, R.; Rouquerol, F., Study of Gypsum Dehydration by Controlled Transformation Rate Thermal Analysis (CRTA). J. Solid State Chem. 1998, 139 (1), 37-44. 26. Azimi, G.; Papangelakis, V. G., Mechanism and Kinetics of Gypsum–Anhydrite Transformation in Aqueous Electrolyte Solutions. Hydrometallurgy 2011, 108 (1), 122-129.


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27. Singh, N.; Middendorf, B., Calcium Sulphate Hemihydrate Hydration Leading to Gypsum Crystallization. Prog. Cryst. Growth Charact. Mater. 2007, 53 (1), 57-77. 28. Osterwalder, N.; Loher, S.; Grass, R. N.; Brunner, T. J.; Limbach, L. K.; Halim, S. C.; Stark, W. J., Preparation of Nano-Gypsum from Anhydrite Nanoparticles: Strongly Increased Vickers Hardness and Formation of Calcium Sulfate Nano-Needles. J. Nanopart. Res. 2007, 9 (2), 275-281. 29. Bundesverbandder Gipsindustrie, e. V., Gips Databook. 2013. 30. Prieto-Taboada, N.; Gómez-Laserna, O.; Martínez-Arkarazo, I.; Olazabal, M. Á.; Madariaga, J. M., Raman Spectra of the Different Phases in the CaSO4–H2O System. Anal. Chem. 2014, 86 (20), 10131-10137. 31. Chen, Q.; Jia, C.; Li, Y.; Xu, J.; Guan, B.; Yates, M. Z., α-Calcium Sulfate Hemihydrate Nanorods Synthesis: A Method for Nanoparticle Preparation by Mesocrystallization. Langmuir 2017, 33 (9), 2362-2369. 32. Van Driessche, A.; Benning, L.; Rodriguez-Blanco, J.; Ossorio, M.; Bots, P.; García-Ruiz, J., The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation. Science 2012, 336 (6077), 69-72. 33. Wang, Y.-W.; Meldrum, F. C., Additives Stabilize Calcium Sulfate Hemihydrate (Bassanite) in Solution. J. Mater. Chem. 2012, 22 (41), 22055-22062. 34. da Silva, N. M. P.; Rong, Y.; Espitalier, F.; Baillon, F.; Gaunand, A., Solvothermal recrystallization of α-calcium sulfate hemihydrate: Batch reactor experiments and kinetic modelling. J. Cryst. Growth 2017, 472, 46-53. 35. Powell, D., Transformation of the α-and β-Forms of Calcium Sulphate Hemihydrate to Insoluble Anhydrite. Nature 1958, 182 (4638), 792-792. 36. Combe, E.; Smith, D., Studies on the Preparation of Calcium Sulphate Hemihydrate by an Autoclave Process. J. Appl. Chem. (Hoboken, U. S. A.) 1968, 18 (10), 307-312. 37. Ball, M.; Urie, R., Studies in the System Calcium Sulphate–Water. Part II. The Kinetics of Dehydration of β-CaSO 4, ½H 2 O. J. Chem. Soc. A 1970, 528-530. 38. Ostroff, A., Conversion of Gypsum to Anhydrite in Aqueous Salt Solutions. Geochim. Cosmochim. Acta 1964, 28 (9), 1363-1372. 39. Boisvert, J.-P.; Domenech, M.; Foissy, A.; Persello, J.; Mutin, J.-C., Hydration of Calcium Sulfate Hemihydrate (CaSO4·1/2H2O) into Gypsum (CaSO4·2H2O): The Influence of the Sodium Poly (Acrylate)/Surface Interaction and Molecular Weight. J. Cryst. Growth 2000, 220 (4), 579-591. 40. Smith, B.; Alexander, A., The Effect of Additives on the Process of Crystallization II. Further Studies on Calcium Sulphate (1). J. Colloid Interface Sci. 1970, 34 (1), 81-90. 41. Klepetsanis, P. G.; Koutsoukos, P. G., Kinetics of Calcium Sulfate Formation in Aqueous Media: Effect of Organophosphorus Compounds. J. Cryst. Growth 1998, 193 (1), 156-163. 42. Amjad, Z., Calcium Sulfate Dihydrate (Gypsum) Scale Formation on Heat Exchanger Surfaces: The Influence of Scale Inhibitors. J. Colloid Interface Sci. 1988, 123 (2), 523-536. 43. Song, X.; Sun, S.; Fan, W.; Yu, H., Preparation of Different Morphologies of Calcium Sulfate in Organic Media. J. Mater. Chem. 2003, 13 (7), 1817-1821. 44. Wang, C.; Zhang, G.; Dong, Y.; Chen, X.; Tan, H., Study on a Water-Swellable Rubber Compatibilized by Amphiphilic Block Polymer Based on Poly (Ethylene Oxide) and Poly (Butyl Acrylate). J. Appl. Polym. Sci. 2002, 86 (12), 3120-3125. 45. Wang, G.; Li, M.; Chen, X., Preparation and Water-Absorbent Properties of a Water-Swellable Rubber. J. Appl. Polym. Sci. 1998, 68 (8), 1219-1225. 46. Zhang, G.; Zhang, Z.; Xie, F.; Hu, X.; Luo, X.; Chen, X., Chlorohydrin Water Swellable Rubber Compatibilized by an Amphiphilic Graft Copolymer. I. Synthesis and Characterization of Compatibilizer PVA-g-PBA. J. Appl. Polym. Sci. 2000, 75 (8), 977-986. 47. Jiang, X.; Hu, K.; Yang, P.; Ren, J., Study on Preparation and Properties of Water Swellable Rubber Modified by Interpenetrating Polymer Networks. Plast., Rubber Compos. 2013, 42 (8), 327333. 48. Hatsui, T.; Omodani, M.; Kudo, T.; Kobayashi, K.; Imamura, T.; Ohmoto, T.; Iwata, A.; Ono, S.; Kirihara, Y.; Kameshima, T. In A Direct-Detection X-Ray CMOS Image Sensor with 500 μm Thick High Resistivity Silicon, Proc. Int. Image Sensor Workshop, 2013. ACS Paragon Plus Environment


49. Utracki, L.; Favis, B., Polymer Alloys and Blends. Marcel Dekker: New York: 1989; Vol. 4. 50. Varughese, K.; De, P.; Sanyal, S.; De, S., Miscible Blends from Plasticized Poly (Vinyl Chloride) and Epoxidized Natural Rubber. J. Appl. Polym. Sci. 1989, 37 (9), 2537-2548. 51. Dehbari, N.; Zhao, J.; Peng, R.; Tang, Y., Neutralisation and Compatibilisation Effects on Novel Water-Swellable Rubber Composites. J. Mater. Sci. 2015, 50 (15), 5157-5164. 52. Strydom, C.; Hudson-Lamb, D.; Potgieter, J.; Dagg, E., The Thermal Dehydration of Synthetic Gypsum. Thermochim. Acta 1995, 269, 631-638. 53. Domurath, J.; Saphiannikova, M.; Heinrich, G., The Concept of Hydrodynamic Amplification in Filled Elastomers. Kautsch. Gummi Kunstst. 2017, 70 (1-2), 40-43. 54. Domurath, J.; Saphiannikova, M.; Férec, J.; Ausias, G.; Heinrich, G., Stress and Strain Amplification in a Dilute Suspension of Spherical Particles Based on a Bird-Carreau Model. J. NonNewtonian Fluid Mech. 2015, 221, 95-102. 55. Pérez-Alonso, M.; Castro, K.; Madariaga, J. M., Investigation of Degradation Mechanisms by Portable Raman Spectroscopy and Thermodynamic Speciation: The Wall Painting of Santa Maria de Lemoniz (Basque Country, North of Spain). Anal. Chim. Acta 2006, 571 (1), 121-128. 56. Wolf, S.; Hirzinger, G., A New Variable Stiffness Design: Matching Requirements of the Next Robot Generation, Robotics and Automation, 2008. ICRA 2008. IEEE International Conference, IEEE: 2008; pp 1741-1746. 57. Tonietti, G.; Schiavi, R.; Bicchi, A. In Design and Control of a Variable Stiffness Actuator for Safe and Fast Physical Human/Robot Interaction, Robotics and Automation, 2005. ICRA 2005. Proceedings of the 2005 IEEE International Conference, IEEE: 2005; pp 526-531. 58. Walsh, P.; Lamancusa, J., A Variable Stiffness Vibration Absorber for Minimization of Transient Vibrations. J. Sound Vib. 1992, 158 (2), 195-211. 59. Anusonti-Inthra, P.; Gandhi, F., Helicopter Vibration Reduction through Cyclic Variations in Rotor Blade Root Stiffness. J. Intell. Mater. Syst. Struct. 2000, 11 (2), 153-166. 60. Brister, M.; Brauker, J., Transcutaneous Medical Device with Variable Stiffness, U.S. Patent 7,783,333 B2, Aug. 24, 2010. 61. Stalker, K. C., Variable Stiffness Stent, U.S Patent 6,652,576 B1, Nov. 25, 2003. 62. Shahinpoor, M.; Schneider, H.-J., Intelligent Materials, Royal Society of Chemistry, Cambridge, UK, 2008.


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For Table of Contents Only

In-situ polymorphic alteration of filler structures for biomimetic mechanically adaptive elastomer nanocomposites

Tamil Selvan Natarajan1,2, Shigeru Okamoto3, Klaus Werner Stöckelhuber1, Sven Wießner1,2, Uta Reuter1, Dieter Fischer1, Anik Kumar Ghosh1,2, Gert Heinrich1,5, Amit Das1,4*

1

Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, D-01069 Dresden, Germany Technische Universität Dresden, Institut für Werkstoffwissenschaft, D-01069 Dresden, Germany 3 Nagoya Institute of Technology, Department of Materials Science and Engineering, Nagoya, Japan 4 Technical University of Tampere, Korkeakoulunkatu 16, Fi-33101 Tampere, Finland 5 Technische Universität Dresden, Institut für Textilmaschinen und Textile Hochleistungswerkstofftechnik, D-01069 Dresden, Germany 2

Graphical abstract



76x44mm (300 x 300 DPI)


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In-situ polymorphic alteration of filler structures for biomimetic

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