Three-Dimensional Printing of Shape Memory Composites with Epoxy

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3D Printing of Shape Memory Composites with Epoxy-Acrylate Hybrid Photopolymer Ran Yu, Xin Yang, Ying Zhang, Xiaojuan Zhao, Xiao Wu, Tingting Zhao, Yulei Zhao, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13531 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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ACS Applied Materials & Interfaces

3D Printing of Shape Memory Composites with Epoxy-Acrylate Hybrid Photopolymer Ran Yu,1,* Xin Yang,1 Ying Zhang,1 Xiaojuan Zhao,1 Xiao Wu,1,2 Tingting Zhao,1,2 Yulei Zhao,1 Wei Huang1,* 1

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s

Republic of China. 2

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of

China. KEYWORDS: 3D printing, stereolithography, shape memory polymer, epoxy, acrylate

ABSTRACT: 4D printing, a new process to fabricate active materials through 3D printing developed by MIT’s Self-Assembly Lab in 2014, has attracted more and more research and development interests recently. In this paper, a type of epoxy-acrylate hybrid photopolymer has been synthesized and applied to fabricate shape memory polymers through stereolithography 3D printing technique. The glass- to- rubbery modulus ratio of the printed sample determined by Dynamic Mechanical Analysis is as high as 600 indicating that it may possess good shape memory properties. Fold-deploy 1

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and shape-memory cycles tests are applied to evaluate its shape memory performance. The shape fixity ratio and the shape recovery ratio in ten cycles of fold-deploy tests are about 99 % and 100%, respectively. The shape recovery process takes less than 20 s indicating its rapid shape recovery rate. The shape fixity ratio and shape recovery ratio during 18 consecutive shape memory cycles are 97.44 ± 0.08 % and 100.02 ± 0.05 %, respectively, showing that the printed sample has high shape fixity ratio, shape recover ratio and excellent cycling stability. Tensile test at 62 °C demonstrates that the printed samples combine a relatively large break strain of 38 % with a large recovery stress of 4.73 MPa. Besides, mechanical and thermal stability tests prove that the printed sample has good thermal stability and mechanical properties including high strength and good toughness.

1. INTRODUCTION 3D printing, a popular term for additive manufacturing, refers to various processes to synthesize three-dimensional objects. Recently, increasing research interests has been attracted by 3D printing due to its advantages.1-6 Compared with traditional manufacturing processes, it can be faster, more flexible and less expensive especially when producing relatively small quantities of parts. More importantly, it can fabricate models with very complicated structures which are not available through traditional techniques. Until now, many 3D printing techniques have been developed, including stereolithography, fused deposition modeling, selective laser melting, direct metal laser sintering, selective laser sintering, etc. 2


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Recently, 3D printing is applied to fabricate active materials which can subsequently change the configuration in response to some environmental stimuli. This adds the 4th dimension of the time-dependent configuration to 3D printing, which has firstly been termed as 4D printing by Tibbits.7 Tibbits and his colleagues reported the successful synthesis of self-deforming structures through multiple material inkjet printer (Stratasys Connex 500). They utilized hydrophilic materials which can expand up to 200 % of its original volume as submerged in water to introduce shape changes. Shape memory polymers (SMPs), one of the most extensively studied active materials, have attracted more and more interests of researchers all over the world due to their potential applications in various areas. Qi and his colleagues have done a lot of work on fabricating shape memory composites through 3D printer (Objet 260 Connex, StrataSys). They designed and manufactured SMPs by 3D-printing of multiple digital photopolymers supplied by 3D printer manufacturer.8-12 There are also reports of fabricating SMPs with thermal-responsive hydrogel10, 13-14 or thermoplastic polymers15-18 through 3D printing. SMPs are smart polymers which are able to fix a deformed shape and recover their original shape in response to an external stimulus.13-14, 19 Heat-activated SMPs in which polymers can deform due to temperature change gain the most attentions since it is easy to operate. Heat- activated SMPs have already been applied in various areas. 20-24 To date, a lot of SMPs including thermoplastic and thermoset polymeric materials have been reported.25-29 Thermoset SMPs, compared with thermoplastic SMPs, usually have better shape memory performance due to their thermal and structural stability. 3



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Epoxy resin is one type of the most commonly used thermoset resins. Due to its outstanding properties including high mechanical strength, thermal stability and low shrinkage, it has been widely applied in many fields, like coating, adhesives and composite matrix, etc.30-31 Recently, shape-memory epoxy resins (SMEPs), are attracting more and more attentions due to their excellent shape-memory properties.32-33 SMEPs based on diglycidyl ether of bisphenol A epoxy resin were successfully fabricated by controlling the crosslink density of the material, which is necessary for thermoset polymers to possess shape memory properties.33 However, the inherent brittleness of epoxy resin limited their application as SMPs, especially when large strain is necessary in some cases. Researchers have developed several approaches to improve the toughness of epoxy systems and meanwhile obtain suitable crosslink density to achieve good shape memory properties. One is to introduce toughening agents like rubbery elastomers to the system. Kavitha reported that introduction of rubbery CTBN-phase to epoxy system can largely improve the thermomechanical cycling property of SMEPs.34-35 Another approach is to change the chain structure of epoxy resin by chemical modification. This can be done through modification of epoxy molecules or curing agents. In the first method, many researches have been done to intrinsically toughen epoxy resin by introducing flexible groups to the backbone of epoxy molecules, such as oxyethyl, oxypropyl groups or polyether chains.3639

In the second method, curing agents with flexible chain like polyether amine can be

applied.32, 40-48

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An approach to fabricate SMPs through SLA 3D-printing technique is described in the present work. SLA is one of the most accurate and widely used 3D printing techniques. It is based on the UV-curing reaction of liquid photopolymers with irradiation. Typically, a photopolymer consists of a mixture of multifunctional monomers, oligomers and photo-initiators. Generally, there are two routes for photoinitiation: free radical and ionic. Monomers and oligomers which can be initiated by free radical photoinitiators are typically functionalized by an acrylate while epoxide and oxetanes are the most widely used cationic photoinitated compounds. They both have advantages and disadvantages, respectively. Firstly, free radical polymerization has the drawback of oxygen inhibition effect while ionic polymerization is insensitive to oxygen. Secondly, the curing rate of free radical polymerization is much faster than that of ionic polymerization. Thirdly, the volume shrinkage of ionic polymerization upon ring-opening polymerization of epoxide compounds is significantly below those of acrylates. Thus, hybrid photopolymers composed of both epoxide and acrylate monomers gaining the high curing rate from acrylates and meanwhile low volume shrinkage together with insensitivity to oxygen of epoxides have attracted more and more attentions from material scientists. Moreover, studies showed that free radical photo-initiators promoted cationic polymerization and an interpenetrating polymer networks (IPN) structure was formed upon dual-curing mechanism.49 In the present work, a type of acrylate-epoxy hybrid photopolymer is applied to fabricate SMPs. In previous reports where photopolymers were applied to fabricate SMPs through 3D-printing, multiple materials with different properties were applied to print 3D 5



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objects.8-12 The choice of these digital materials, provided by 3D printer manufacturers was very limited. Moreover, the fabrication processes needed complicated and precise geometric design to place these multiple materials at each exact tiny position within the design domain. In our work, only one kind of material, acrylate-epoxy hybrid photopolymer is applied to fabricate SMPs through 3D printing. The printed 3D structures show good mechanical properties including high strength and good toughness due to the composition and structure of the hybrid photopolymer. The excellent shape memory performance of the printed samples is proven by fold-deploy experiments and shape memory cycles tests. 2. EXPERIMENTAL SECTION 2.1 Synthesis Materials Diglycidyl ether of bisphenol A (DGEBA) with an epoxide equivalent value of 196 g/mol (Wuxi Diaish Epoxy Co.), Diglycidyl ether of hydrogenated bisphenol A (DGEHBA) with an epoxide equivalent of 233 g/mol (Yantai Aolifu Chemical Industry Co.) and 3-ethyl-3-hydroxymethyl oxetane (OXT) (Shanghai Ginray Co.) are applied as cationic monomers. Bisphenol A diglycidyl ether diacrylate (DGEDA) and ethylene oxide modified trimethylol propane triacrylate (EO3-TMPTA) (DSM) are used as free radical monomers. 1-hydroxycyclohexyl phenyl ketone (184) and Bis(4-methylphenyl) iodonium hexafluorophosphate (IPF) (Sigma-Aldrich) are applied as photoinitiators for the hybrid photopolymer. Maleic anhydride (MA), 2-ethylhexyl acrylate (EHA), 2,2'6


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azobisisobutyronitrile (AIBN) from Sinopharm Chemical Reagent Co. are used to synthesize preformed particles (PP) dispersion. Synthesis of preformed particles Preformed particles were fabricated through in-situ polymerization of acrylate in epoxy resin. DGEBA (450 g, 1.15 mol), MA (30 g, 0.306 mol) were added to reaction tank equipped with a thermometer and a mechanical stirrer. The mixtures were heated to 120 °C and stirred at this temperature for 6 h. Then, it was cooled down to 80 °C. Afterwards, 1050 g of DGEBA, 30 g of EHA (0.163 mol) and 3 g of AIBN were added to the system and the mixtures was kept stirred at 80 °C for 8 h. Finally, preformed particles, composed of polyacrylate-co-epoxy particles dispersed in epoxy resin were obtained. Photopolymer The content of photoinitiators should be precisely controlled to make the hybrid photopolymer to have high curing rate and suitable curing depth in order to be applied in SLA technique. The detailed composition of the hybrid photopolymer applied in our work is shown in Table 1.

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Table 1. The composition of the hybrid photopolymer applied in our work. Components

Quantity (g)

Percentage of each part (%)

PP-DGEBA

1300

24.98

DGEBHA

1384

26.59

OXT

816

15.68

DGEDA

410

7.88

EO3-TMPTA

1090

20.94

184

102

1.96

IPF

102

1.96

Fabrication of test samples with the photopolymer SL200 Stereolithography (ZRapid Tech) was used to print test samples. The horizontal resolution of printing 3D structures is 40 µm and the vertical resolution is 1 µm. In the present work, layer thickness was set to 100 µm. The wavelength of laser irradiation is 355 nm and the laser power was set to 420 mW. The scanning speed was set to 6000 mm/s. Test samples with the different dimensions for DMA and mechanical tests were printed with the hybrid photopolymer through SL200 3D printer. Fabrication of test samples with the neat acrylate or epoxy resin The neat acrylate and epoxy resin polymers were fabricated through a home-made mold (size: 100× 100 × 1 mm) with Phillip UV lamp (QVF135 HAL-TDS, 500W). Test

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samples were made through cutting the cured thin sheet to a definite dimension required for specific tests. The detailed composition of the neat acrylate and epoxy resin are shown in Table 2 and Table 3, respectively. Table 2. The composition of neat acrylate applied in our work.

Components

Quantity (g)


DGEDA

7.88

26.8

EO3-TMPTA

20.94

71.2

184

0.59

2

Table 3. The composition of neat epoxy applied in our work.

Components

Quantity (g)


PP-DGEBA

24.98

25

DGEBHA

26.59

26.59

OXT

15.68

15.68

IPF

1.37

1.96

2.2 Characterization FTIR spectroscopy was carried out with a Bruker Tensor-27 FTIR spectrometer. 1HNMR measurements were performed with a Bruker Avance 400 MHz NMR spectrophotometer.

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Dynamic mechanical analysis (DMA) was carried out with a TA Q800 instrument. Rectangular test samples (size: 60 mm × 15 mm × 2 mm) were used and measurements were done in double cantilever mode at a frequency of 1 Hz and a heating rate of 5 °C min-1. The neat acrylate and epoxy polymer samples (size: 30 mm × 4 mm × 0.9 mm) were measured in film tension mode. The storage modulus and glass transition temperature (Tg, DMA), defined by the tanδ (visco-elastic character) peaks of those samples were obtained from the DMA measurement. Mechanical properties were determined with an Instron 3365 Universal Tester. Rectangular samples (size: 80 mm × 10 mm × 4 mm) were directly printed for flexural tests and the deformation speed was set to 1 mm min-1. Dumbbell-shaped samples (size: 180 mm × 10 mm × 4 mm) were printed for tensile tests with a deformation speed of 2 mm min-1. Determination of impact toughness was performed with a JC-25 impact tester (Chengde Precise Tester, China) and unnotched test samples with a size of 80 mm × 10 mm × 4 mm were used. The mechanical properties results were the average of at least five measurements. Tensile properties of the printed samples at high temperatures were measured on the same TA Q800 DMA Instrument in film tension mode. The size of specimens applied in the test is with a thickness of 1.0 mm and a width of about 3.5-4.0 mm. Through stressstrain curves at different temperatures, the temperature when the printed samples have the largest elongation at break was detected.

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Photo-DSC was performed on DSC 204F1 Phoenix (Netzsch) with OminiCure S2000SC as UV curing lamp to study the UV-curing process and the curing extent of the photopolymer upon laser irradiation. Measurements were carried out in N2 atmosphere at 30 °C with intermittent laser irradiation. The strength of irradiation was set to 0.5 W/cm2. The irradiation lasted 10 s each time and repeated until no reaction heat released. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out on a Netzsch STA 449F3A-1137-M in N2 atmosphere at 10 °C min-1. DSC and TGA were used to investigate the glass transition temperatures and thermal stability of the printed samples, respectively. Scanning electron microscopy (SEM) was performed with a Hitachi S4800 microscope to investigate the fracture surface morphology of printed samples. An alkaline solution of 10% KOH in methanol was used to selectively remove the acrylate part in the printed samples. The dimension of preformed particles was characterized with a Olympus BX53 optical microscope. Evaluation of the shape memory performance Several parameters including the shape fixity ratio (Rf) and shape recovery ratio (Rr), shape recovery rate, shape recovery stress are used to evaluate shape memory performance of SMPs. The shape fixity ratio and the shape recovery ratio demonstrate the ability of SMPs to fix their deformed shape and the ability to recover their original shape, 11



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respectively. The shape-memory performance of the printed samples is evaluated through fold-deploy tests and shape memory cycles (SMCs) measurements in the present work. Fold-deploy test. Fold-deploy test was carried out as follows, a straight specimen with a size of 150 ×12× 2 mm was immersed in a hot water bath with a certain temperature(>Tg) for 1 min. Afterwards, it was deformed manually into a ‘‘U’’ shape and the bending angle of 180° was recorded as θmax. The deformed sample was fixed by quickly immersing it in cold water while maintaining the deformation force and the bending angle after this process was recorded as θfixed. For recovery process, the deformed sample was put back into the hot water bath and the final bending angle was measured as θi. The experiment was repeated for 10 times. The shape recovering time for SMPs at different temperatures is recorded. The shape fixity ratio (Rf) and shape recovery ratio (Rr) were determined by Eqs. (1) and (2): R = θ /θ × 100%

(1)

R = (θ − θ )/θ × 100%

(2)

Shape memory cycles (SMCs). The shape-memory properties of the printed samples were quantitatively evaluated through SMCs tests. It was performed with a Q800 DMA in a force controlled mode as reported previously.32, 38, 50 Firstly, the deformation temperature (Td) used in the consecutive SMCs test has to be found. In the present work, Td is set at temperature when the printed material has the largest break elongation. The stress-strain performance of material is greatly influenced by temperature and generally, at the glass transition temperature, the material has the largest elongation at break.50 12


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Stress

(ii) Ts

(iii)

(i) !

Strain

(iv) Td

"

Temperature

Scheme 1. Sketch of the performance of the printed samples in SMCs test. Afterwards, shape memory cycles test was carried out in four steps as shown in Scheme 1. (i) Deformation: The sample was equilibrated at Td for 10 min and then stretched with increasing force (force increasing rate 0.5 N min-1) until the stress of the sample reached to 3 MPa. Afterwards, it was equilibrated at Td for 10 min and the strain at the end of the step was recorded as ; (ii) Cooling: Maintaining the external force, the sample was cooled to Td – 30 °C and equilibrated for 10 min; (iii) Fixing: The external force was removed, and the sample was maintained at Td – 30 °C for 5 min, the strain after this step was recorded as ; (iv) Recovery: The temperature was increased back to Td + 20 °C with heating rate of 3 °C min-1and then equilibrated at Td for 10 min and the strain of the sample was recorded as . The shape memory cycles were programmed to repeat for 18 times. The shape fixity (Rf) and shape recovery (Rr) can be determined from Eqs. (3) and (4): ()

() = () × 100%

(3)

13



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# () =

()$% () ()$% ($&)

× 100%

(4)

3. RESULTS AND DISCUSSION 3.1 Characterization of the printed SMPs The successful synthesis of preformed particles dispersion was proved through FTIR and NMR analysis (see Figure S1 and S2 in supporting information). All the test samples were fabricated via SL200 stereolithography apparatus (ZRapid Tech, China) with a resin vat charged with preformed particles modified hybrid photopolymer. The composition of the printed sample is proven by FTIR spectroscopy (Figure 1). The characteristic absorption features of acrylate (stretching vibration of carbonyl group at 1720 cm-1 and C=C at 1635 cm-1) and epoxy resin (asymmetrical stretching of epoxy group at 913 cm-1) are clearly detectable on the FTIR spectrum of the hybrid photopolymer. The absorption peaks of C=C (peak a) and epoxy group (peak b) disappear in the cured samples. Moreover, comparison of the FTIR spectra of cured hybrid photopolymer and the neat acrylate and epoxy polymers, no new absorption peaks emerge in cured hybrid photopolymer, which means that no reaction but only physical blending occurred between acrylate and epoxy resin.

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d i r b y h d e r u c

a

b

y x o p e d e r u c

Transmittance (%)

d i r b y h e t a l y r c a d e r u c

3500 3000

1800 1600 1400 1200 1000 800 -1

600

Wavenumber (cm ) Figure 1. FTIR spectra of the printed sample, hybrid photopolymer before curing, the neat acrylate and epoxy polymers. - 256.39 J/g - 14.07 J/g

(a)

- 7.24 J/g

- 5.83 J/g - 5.71 J/g

0

100

- 6.02 J/g

-5

-10

(b)

99

- 5.76 J/g - 5.71 J/g

Curing Extent (%)

- 9.26 J/g

DSC (mW/mg)

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


98 97 96 95

-15

94

5

10

15

20

25

30

Time (min)

1

2

3

4

5

6

Numbers of Irradiation

Figure 2. (a) Photo-DSC curve of photopolymer under UV irradiation, (b) the curing extent of photopolymer as a function of numbers of irradiation (the irradiation with the 15


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energy density of 0.5 W/cm2 lasted 10 s each time and repeated until no reaction heat released). The UV-curing process and the curing extent of the photopolymer upon laser irradiation have been studied with photo-DSC and the result is shown in Figure 2. It shows that the curing extent of the hybrid photopolymer gets to 100 % after 8 times of irradiation. Moreover, the curing extent has already reached to 94.7 % in about 3 min after the first irradiation. So, compared with traditional thermal curing of epoxy or acrylate, the rate of UV-curing of the hybrid photopolymer is much higher. In the present system, acrylate monomers polymerized through free radicals while epoxides polymerized with cationic mechanism. During the process of UV-curing of the hybrid formulation, crosslinking will lock the acrylate and epoxide polymers together non-covalently but irreversibly. Most likely, IPN structure will be formed in the printed sample. IPN is a special type of polymer blends in which both polymers generally are in network form.51-54 Polymerization of a mixture with monomers reacting through different mechanism is one of the most common methods to generate IPNs.52 Jansen et al. reported IPN structure formed from radically initiated divinyl resins and amine-cured epoxy resin53,

55

, Griffini et al. studied a sequential IPN obtained by co-formulation of a

photocurable acrylic resin with a thermocurable epoxy resin49 and Decker et al. studied IPNs by UV-radiation curing of acrylate/epoxide system52 which is similar as the present system.

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Figure 3. Photographs and SEM micrographs of cured hybrid photopolymer without preformed particles through home-made mold (a, b) and printed sample from preformed particles modified hybrid photopolymer (c, d): (a) and (b) are cured samples, (c) and (d) are samples after etching with 10% KOH in methanol at 60 °C for 1h. SEM is applied to characterize the morphology of the printed sample. The morphology cured sample without preformed particles is also characterized for comparison. Photographs and SEM micrographs of these two samples are present in Figure 3. Photograph of the sample without preformed particles shows that the sample remains perfectly transparent after UV-curing. It implies that the compatibility of epoxides and acrylate polymers is good and no phase separation happened during UVcuring process. SEM micrographs of a and c show that both samples form uniform 17



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structure after curing process. Micrographs Figure 3b and d show that no distinct changes happen on the sample without preformed particles while widely distributed microvoids appear on the printed sample after etching. The average dimension of these microvoids is 0.86 ± 0.25 µm. The dimension of preformed particles is around 0.8 µm observed with optical microscopy (shown in Figure S3), so these microvoids are due to preformed particles after etching in alkaline solution. Micrograph b implies that acrylate and epoxides polymers interconnected closely and no large acrylate domain existed in the cured sample so alkaline solution is difficult to penetrate into the sample to etch acrylate polymers. Thus, a co-continuous structure, most possibly an IPN structure obtains through UV-radiation curing of the hybrid photopolymer. Micrographs of samples after etching with alkaline for longer time are shown in Figure S4. 3.2 DMA characterization The shape memory performance of thermoset SMPs is due to its special two phase network structure, fixed phase and reversible phase. The fixed phase, usually the crosslinking network of thermoset SMPs, is responsible for the original permanent shape. The reversible phase, usually the switching chain segments, which will get soft as the temperature is higher than the glass transition temperature (Tg), is responsible for the temporarily deformed shape. During the glass transition process of thermoset SMPs, the movements of the switching segments become increasingly unimpeded. Under the applied external force, the soften switching segments can experience large movement while cross-linking can prevent the slipping and flow of polymer chains. As a result, the 18


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sample can fix the transient conformation below Tg while recover its original conformation when the external stress released above Tg. 2500

(a)

0.8

acrylate epoxy hybrid photopolymer post-cured hybrid

2000

(b)

acrylate epoxy hybrid photopolymer post-cured hybrid

0.6

1500 0.4

Tanδ

Storage modulus (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


1000

0.2

500 0.0

0 20

40

60

80

100 120 140 160 180 200 220

20

40

60

80

100 120 140 160 180 200 220

Temperature (°C)

Temperature (°C)

Figure 4. DMA curves of the printed sample and corresponding epoxy and acrylate, (a) Storage modulus as a function of temperature and (b) tanδ as a function of temperature (post-cured samples are printed samples after thermal treated at 80°C for 30 min). The glass- to- rubbery modulus ratio (E′g/ E′r) is usually applied to roughly evaluate the shape memory performance of thermoset polymers. Generally, the E′g/ E′r should be above 100 for thermoset polymers to have good shape memory performance.33, 44 DMA was applied to measure the glass transition temperature and the glass- to- rubbery modulus ratio of the printed sample. The neat acrylate and epoxy polymers were also characterized for comparison. Figure 4 shows storage modulus (E′) and loss factor (tanδ) of three samples as a function of temperature. Due to the uncontrolled radical chain growth mechanism, the photo-curing rate of the acrylate is very high and a highly 19



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irregular network is formed after curing. So the glass transition of the acrylate is very broad. The peak of the tanδ-temperature curve is at 94 °C. In contrast, epoxides polymerize by a cationic mechanism and the curing rate is slower than that of the acrylate, so a more regular network is obtained. In consequence, a sharp and tall peak of Tg at 78 °C can be seen. Furthermore, hybrid systems consisting of acrylate and epoxy monomers are very common way to regulate the final polymer architecture of the acrylate polymers.56 Both the curing rate and distribution of crosslinks change in the hybrid photopolymer and finally it evolves in a fairly regular structure which is similar as that of epoxides. So, tanδ- temperature curve of the printed hybrid photopolymer is similar as that of epoxides. The printed sample has two Tgs (79 °C and 94 °C) after printing and only one Tg (82 °C) after post-curing. This behavior can possibly be attributed to postcuring process of the printed samples. One characteristic of cationic polymerization of epoxides is that the cationic polymerization of epoxy will continue once initiated even when UV irradiation is stopped. Due to the high printing rate, the printed samples may not be cured completely. The peak of 94 °C of the printed sample is due to post-curing process during DMA test. The only one Tg after post-curing can also prove the good compatibility of acrylate and epoxy polymers and the highly interconnected IPN structure of the printed sample. The storage modulus of the printed sample decreased sharply with the increase of temperature. The value of E′g/ E′r is calculated to be about 600 indicating that it has great potential to be applied as SMP. Clearly, storage modulus of the printed sample falls in between that of epoxy and acrylate. E′g is mainly determined by the compacting density 20


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of polymer chains. Due to the structure of interpenetrating polymer network, E′g value of the printed sample is between its corresponding epoxy and acrylate parts and it is 2016.1 MPa at 33 °C. E′r is largely corresponds to crosslinking density of materials. The addition of preformed particles decreases the crosslinking density of the cured hybrid photopolymer, which leads to a relatively low E′r of 4.03 MPa at 104 °C. In contrast, E′r of the cured sample without preformed particles is 15.1 MPa at 95°C (DMA curves are shown in Figure S4). 3.3 Shape-memory properties Fold-deploy test results of the printed samples are presented in Figure 5. Figure 5a presented the change of recovery angles with time at different temperatures for the printed samples. It shows clearly that the sample recovers faster at higher temperature due to the increased mobility of switching segments. Furthermore, it recovers faster in the recovery angles from 0° to 150° than that in 150°- 180°. This is because only a little of the stored strain energy is left in the recovery angles of 150°- 180°. The value of Rf and Rr obtained from this method are not accurate, but it can roughly reflect the shape memory performance of the printed samples. Figure 5b shows that Rf and Rr keeps at around 99% and 100%, respectively, and shape recovery time also keeps constant with experimental times. Figure 5c and d show that any object printed with the hybrid photopolymer prepared in our lab through SLA technique can possess shape memory properties. The deformed thin sheet test sample recovers to its original shape in a 85 °C water bath in 10 s and the shape recovery process of an deformed Eiffel Tower model (8 21



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cm tall) takes only 8 s in air with a dryer. The experiment results proved structural and thermal stability and excellent shape memory properties of the printed samples.

Figure 5. Fold-deploy test results for printed samples: (a) recovery angle with time in water bath at different temperatures; (b) shape fixity and recovery ratio, shape recovery time with number of times in a 85 °C water bath; (c) visual demonstration of printed sample’s shape recovery process after nine shape recovery cycles in a 85 °C water bath; (d) the shape recovery process of the printed Eiffel Tower with a dryer. The stress-strain performance of material is greatly influenced by temperature and generally, at the glass transition temperature, the material has the largest elongation at 22


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break.50 In our experiments, DSC measurement and tensile tests at different temperatures are applied to detect the temperature at which the printed sample has the largest elongation at break. The glass transition temperature obtained from DSC was about 61 °C (see Figure S6 in supporting information). Due to the frequency effect of DMA test, Tg, MA (82 °C) is about 11 °C higher than Tg, DSC. Tensile tests of printed samples were conducted at different temperatures (Figure 6). The breaking strain firstly increased from 13 % at 25 °C to 38 % at 62 °C and then decreased to 16 % at 70 °C. The printed sample has the largest break strain at 62 °C, which fits quite well with Tg,DSC. More importantly, stress-strain curve at 62 °C shows that the tensile stress of the printed sample at the strain of 38 % corresponding to the shape recovery stress at that point reached up to 4.73 MPa which is relatively high for SMPs.

28 °C 62 °C

60 °C

5

30 4 20

3 65 °C

10

2

70 °C 1

0 0

5

10

15

20

25

30

35

0 40

Tensile strength at HT (MPa)

6

40

Tensile strength at RT (MPa)

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Strain (%)

Figure 6. Stress-strain curves of the printed samples at different temperatures.

23



25

101

(a)

(b)

100

20

Strain (%)

15

Rf and Rr

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10 5

Rf

98

97

0 30

Tem40 50 per 60 atu re 70 80 (°C )

Rr

99

3 2

a) MP ( ss tre

1 0

S

96

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Number of times

Figure 7. (a) Stress-strain-temperature diagram from SMCs test for the printed samples; (b) Rf and Rr during 18 consecutive shape-memory cycles. The values of Rf and Rr of the printed samples were precisely determined through SMCs measurements with Q800 DMA under the controlled force mode. Figure 7 shows the stress-strain-temperature diagram of the printed samples from SMCs test and the value of Rf and Rr calculated in each cycle. During the consecutive SMCs test, the evolutional process of the strain in each cycle remained quite constant, especially after the first several cycles for the printed samples. Moreover, a small residue strain, about 5%, can be detected after the first cycle and it remains nearly unchanged for the subsequent cycles. This irreversible deformation might be attributed to the plastic deformation from directional rearrangement of polymer chains upon the external force.42 The evolutional process of the strain and stress of the printed sample with time during SMCs test has also been drawn into two-dimensional diagram (see Figure S7 in supporting information).

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The strain at the end of deformation step (εm), after releasing the force (εu), the final strain of the sample (εf) in each cycle were recorded and subsequently, Rf and Rr were calculated (see Table S1 in supporting information). The average Rf and Rr during 18 consecutive shape memory cycles are 97.44 ± 0.08 % and 100.02 ± 0.05 %. Plenty of thermoset SMPs have been studied in the previous literatures.19,

32, 34-35, 38, 40, 42, 57-58

Rousseau et al. reported the successful synthesis of a series of newly developed SMEPs with excellent shape fixity and recovery. Rf and Rr of these SMEPs evaluated through SMCs test ranged from 85.6 to 102.3% and 97.4 to 99.8%, respectively.40, 42 Fan et al. synthesized a series of SMEPs with diglycidylether of propoxylated bisphenol-A and Rf and Rr evaluated also through SMCs test are 98.88 ± 0.04 and 96.67 ± 6.91 %, respectively.38 In the previous work of our lab, a new type of SMEP with diglycidyl ether of 9, 9′-bis [4-(2-hydroxyethoxy) phenyl] fluorine was successfully synthesized and the shape memory properties were measured through tensile-shrinkage test. Both Rf and Rr are about 99%.37 A lot of SMPs have been synthesized by other researchers, but only traditional test like fold-deploy test was applied to roughly evaluate the shape memory performance.34,

36, 45, 57

A critical comparison with the recovery performance of other

SMPs reported in these literatures indicates the excellent shape memory properties of the printed sample in the present work. In summary of the results from fold-deploy experiments and SMCs , the printed sample through SLA technique has excellent shape memory performance including good shape fixity, shape recovery, high recovery rate, high recovery stress and excellent endurance due to its thermal and chemical stability. 25



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3.4 Mechanical properties and thermal stability The mechanical properties of the printed samples are listed in Table 4. Tensile and flexural strength of the printed samples are 36.28 and 63.96 MPa, respectively, showing that the printed samples have high strength. The high elongation at break of 15.09 % combined with the high impact strength of 38.83 KJ m-2 indicate good toughness of the printed test samples. This can be attributed to two reasons. The first one is that the chain structure of the cured hybrid photopolymer is flexible. Acrylate applied in our experiment has flexible ethyoxyl groups and cationic photopolymer including oxetane and hydrogenated epoxy has flexible chain structure. Another reason is attributed to the addition of preformed particles in the photopolymer. Compared with traditional heatactivated curing of epoxy resin, the reaction rate of photopolymerization is much higher. Traditional reactive liquid rubber may not be able to toughen the resin, since there may not be enough time for phase separation to occur to get rubber particles. So in our experiment, preformed particles are applied. They do not require phase separation and remain in the shape when they are added to the resin. The toughening effects of these preformed particles might possibly be attributed to the strong interactions between rubbery particles and matrix due to the chemical reactions between the matrix and preformed particles dispersion. The good interfacial adhesion can improve stress transfer while the rubbery phase uniformly distributed throughout the matrix can damp the stresses imposed to the sample leading to reduction of the stress at the point of impact tip. Also, the rubbery phase can cause some localized plastic shear yielding.59-61 Moreover, the dimension of the particles with a diameter of about 0.8 µm is known to improve 26


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impact properties.62 These effects above might be reasons for the observed increased toughness. So the flexible chain structure of the hybrid photopolymer and the addition of preformed particles are responsible for the good toughness of the printed sample. Table 4. Mechanical properties of the printed test samples. Items

Printed sample

Tensile strength (MPa)

36.3 ± 0.6

Elongation at break

15.2 ± 3.1 %

Flexible modulus (MPa)

2182.3 ± 52.5

Flexible strength (MPa)

64.0 ± 1.7

Impact strength (KJ m-2)

38.8 ± 6.7

TGA was applied to determine the thermal stability of the printed sample (see Figure S8 in supporting information). The temperatures at 5 and 10 % weight loss (T5% and T10%) are 286 °C and 350 °C, respectively, which are enough for many applications. In conclusion, the printed SMP sample has good mechanical and thermal stability properties. 4. CONCLUSIONS Here the outlined new strategy for the fabrication of SMP through 3D printing took the advantages of cationic and free radical polymerization. The glass- to- rubbery modulus ratio of the printed sample is as high as 600 indicating its great potential application as shape memory materials. Fold-deploy experiments and shape-memory 27



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cycles tests are applied to evaluate the shape memory performance. The shape fixity and recovery ratios in ten cycles of fold-deploy tests are about 99 % and 100%, respectively. The shape recovery process takes less than 20 s indicating rapid shape recovery rate. In the SMCs test, the shape fixity ratio and shape recovery ratio during 18 consecutive cycles are 97.44 ± 0.08 % and 100.02 ± 0.05 %, respectively, showing that the printed sample has excellent shape fixity, shape recovery performance and good cycling stability. Tensile test at 62 °C demonstrates that the printed sample has a relatively large break strain of 38 % and meanwhile a quite high recovery stress of 4.73 MPa. Mechanical and thermal stability tests prove that the printed sample has good thermal stability and mechanical properties including high strength and good toughness. With the advantage of an easy fabrication, excellent shape memory performance and good mechanical properties, the printed SMPs have great potential applications in many areas. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization including FTIR spectrum, NMR spectrum and optical micrograph of preformed particles dispersion, SEM micrographs of cured hybrid photopolymer after etching with alkaline solution, DSC and TGA curves of printed sample with hybrid photopolymer, more detailed results from SMCs test and DMA curves of cured hybrid photopolymer without preformed particles for comparison in

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Figure S1-S8 and Table S1 (PDF). A video showing the shape recovery process of the printed Eiffel Tower with a dryer (video). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study is financially supported by the National Natural Science Foundation of China (No. 51573189 and No. 51603208). REFERENCES (1) Muth, J. T.; Vogt, D. M.; Truby, R. L.; Menguec, Y.; Kolesky, D. B.; Wood, R. J.; Lewis, J. A. Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers. Adv. Mater. 2014, 26 (36), 6307-6312. (2) Abbadessa, A.; Blokzijl, M. M.; Mouser, V. H. M.; Marica, P.; Malda, J.; Hennink, W. E.; Vermonden, T., A Thermo-Responsive and Photo-Polymerizable Chondroitin Sulfate-Based Hydrogel for 3D Printing Applications. Carbohyd. Polym. 2016, 149, 163174. (3) Bakarich, S. E.; Gorkin, R.; in het Panhuis, M.; Spinks, G. M. Three-Dimensional Printing Fiber Reinforced Hydrogel Composites. ACS Appl. Mater. Interfaces 2014, 6 (18), 15998-16006.

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(60) Kinloch, A. J.; Shaw, S. J.; Tod, D. A.; Hunston, D. L. Deformation and FractureBehavior of a Rubber-Toughened Epoxy 1. Microstructure and Fracture Studies. Polymer 1983, 24 (10), 1341-1354. (61) Kinloch, A. J.; Shaw, S. J.; Hunston, D. L. Deformation and Fracture-Behavior of a Rubber-Toughened Epoxy 2. Failure Criteria. Polymer 1983, 24 (10), 1355-1363. (62) Chikhi, N.; Fellahi, S.; Bakar, M. Modification of Epoxy Resin Using Reactive Liquid (ATBN) Rubber. Eur. Polym. J. 2002, 38 (2), 251-264.

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Three-Dimensional Printing of Shape Memory Composites with Epoxy

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