Shape memory properties of interpenetrating polymer networks (IPNs) based on hyperbranched polyurethane (HBPU)

Journal Pre-proofs Shape memory properties of interpenetrating polymer networks (IPNs) based on hyperbranched polyurethane (HBPU) Cong Wang, Yaoming Zhang, Jianming Li, Zenghui Yang, Qihua Wang, Tingmei Wang, Song Li, Shoubing Chen, Xinrui Zhang PII: DOI: Reference:

S0014-3057(19)31719-7 https://doi.org/10.1016/j.eurpolymj.2019.109393 EPJ 109393

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

22 August 2019 26 November 2019 27 November 2019

Please cite this article as: Wang, C., Zhang, Y., Li, J., Yang, Z., Wang, Q., Wang, T., Li, S., Chen, S., Zhang, X., Shape memory properties of interpenetrating polymer networks (IPNs) based on hyperbranched polyurethane (HBPU), European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj.2019.109393

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Shape memory properties of interpenetrating polymer networks (IPNs) based on hyperbranched polyurethane (HBPU) Cong Wang1,2, Yaoming Zhang1,2, Jianming Li3, Zenghui Yang1, Qihua Wang1,2, Tingmei Wang1,2, Song Li1,2, Shoubing Chen1,2, Xinrui Zhang1,2* 1 State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. 2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China. 3 Petro China Lubricating Oil R&D Institute, Lanzhou 730060, China. Correspondence to: Xinrui Zhang (E-mail: [email protected])

Abstract In this article, a type of hyperbranched polyurethane (HBPU) with triazine structure was successfully synthesized. Afterwards, HBPU was applied through grafting epoxy resin (EP) to fabricate the interpenetrating polymer networks (IPNs) with different ratio of HBPU/EP. The structure of HBPU prepolymer and the formation of the HBPU/EP IPNs were characterized and confirmed by Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopy, and the nanoscale phase separation in IPNs composite was analyzed by Atomic Force Microscope (AFM). Besides, the thermal and mechanical results indicated that the IPNs structure enables the material with high strength and thermal stability. Shape memory cycles test demonstrated that the HBPU/EP INPs possess an excellent shape memory performance, and the best shape fixity and recovery were presented by IPNs composite with 20 wt% HBPU.

Key words: Hyperbranched polyurethane; interpenetrating polymer networks; phase separation; thermal and mechanical properties; shape memory performance

1. Introduction

Shape memory polymers (SMPs) which allow their shapes regulation in response to the external stimuli, such as heat, light, moisture, electromagnetic fields, pH and so on, have attracted extensive attentions in academia and industry as smart material recently[1-5]. SMP with especial transition temperature, excellent thermal properties and mechanical strength has widely applied in field of medical devices, textiles, high temperature actuators, aerospace and etc[6-9]. As a kind of most common SMP, shape memory polyurethane (SMPU) has become to a hot research topic all over the world in recent years owing to excellent comprehensive performance[10, 11]. However, low stiffness and modulus limits their application in fields which requires high mechanical property, such as the aerospace materials. Lots of researchers found the thermal, mechanical properties can be improved by adding either inorganic or organic fillers. Fu et. al successfully modified polyurethane (PU) by 1,3,5,7-tetrahydroxy adamantane and found that the thermal stability of the PU was greatly improved[12]. When fillers are incorporated into SMPU matrix, the compatibility and dispersibility of the filler determines the composites properties in certain level. A.V et. al introduced high aspect ratio nanoparticles into porous polyurethane film by using the phase inversion method, and the results showed that nanoparticles could improve the shielding effectiveness of PU at the expense of reducing shape memory performance[13]. Because fillers blocking the movement of the polymer chains and result in a delay in the shape recovery process, the shape memory effect (SME) is usually depleted[14]. Therefore, the modification of polymer without filler is desired for SMP. Interpenetrating polymer networks (IPNs) is a new polymer blending consisting of two or more polymers in a network form. On the premise of good compatibility between the two polymers, they held together by physical entanglements due to their strong polarity of the rigid molecular segments. And graft–IPNs exist a spot of occasional covalent bonds (chemical crosslinking) between the chains of the two different types[15, 16]. The performance of polymeric materials can be tuned by selecting raw materials, changing the ratio of the components and improving the processing technology. R. Ballestero et. al generated chemical crosslink points between

two networks of polyurethane and acrylic/ester, and found that the method of graftIPNs could minimize phase separation of polymeric system[17]. Chen et. al fabricated the IPNs composite by PU polyblends with Epoxy resin (EP), and the mutual solubility of PU and EP enables the IPNs composite with excellent damping performance[18]. Yu et. al blended PU into EP to form the IPNs composite and found the significant improvement of mechanical properties in terms of tensile strength and impact strength[19]. Besides, the IPNs technology can yield the optimization of recovery of the shape memory performance due to cross-linking network. The IPNs material is a kind of very attractive composite material, numerous studies mainly focus on the mechanical properties and damping properties in the previous research, on the contrary, the shape memory performance is rarely considered. In this work, hyperbranched polyurethane (HBPU) with s-triazine unit in the structure is synthesized. The most attractive property of hyperbranched polymer is the extremely high density of functional groups on the surface[20, 21], and it could be beneficial for the grafting reaction with HBPU and EP. Meanwhile EP owns excellent properties in adhesion and mechanical properties, easy processing and molding, as well as pretty good compatibility with PU[22, 23]. Therefore, EP and HBPU prepolymer with different composition ratios were used to prepare the HBPU/EP IPNs composite. The effects of the content of HBPU on the thermal and mechanical properties of IPNs composites were studied. And the shape memory performance was evaluated by Dynamic Mechanical Thermal Analysis (DMTA). The facile and feasible method provides a potential way to design a high performance composite material and it is expected that the present work is also significant for shape memory performance of the HBPU/EP IPNs.

2. Experimental 2.1 Materials Polytetramethylene ether glycol (PTMEG) with molecular weight of 2000 g·mol1

was purchased from Jining Letian Crafts Co., Ltd, China. 2,4-tolylene diisocyanate

(TDI) and 1,3,5-Tris(2-hydroxyethyl)isocyanurate (THEIC) were purchased from Saen Chemical Technology (Shanghai) Co., Ltd, China. Epoxy resin (E-51, epoxide number = 0.48–0.54 eq·100 g-1) was received from Nantong Xingchen Synthetic Material Co., Ltd, China. The curing agent of 4,4'-Methylene bis(2-chloroaniline) (MOCA) was purchased from Chemtura Shanghai, Co., Ltd, China. All the chemical reagents were used as-obtained unless a notation was made.

2.2 Preparation of HBPU/EP IPNs The detailed synthesis route of the HBPU/EP IPNs composite is outlined in Scheme 1. First of all, certain PTMEG was added into a three neck flask containing TDI, the molar ratio of -OH:-NCO was set to 3 : 8. The reaction was vigorously stirred for 1.5 h at 60 oC under nitrogen protection atmosphere. Then THEIC (molar ratio of the functional group to TDI = 1:8) was added to the flask, followed with heating up to 80 oC and was stirred for 5 h to obtain the HBPU prepolymer. After the chain extentsion, a stoichiometric quantity of EP and MOCA were sequentially added into the HBPU prepolymer and stirred. Finally, the HBPU/EP was poured into the mold and cured in the oven at 130 oC for 8 h to allow the solidification for the HBPU/EP IPNs materials. The HBPU/EP IPNs with 20, 30, 40, 50 wt% HBPU were defined as IPN-1, IPN-2, IPN-3, IPN-4, respectively.

2.3 Characterization The chemical structure of HBPU prepolymer was characterized by a Bruker 400MHz superconducting Nuclear Magnetic Resonance (NMR) spectrometer (Bruker, Switzerland). Deuterated chloroform (DCCl3) and tetramethylsilane (TMS) was used as solvent and internal reference, respectively, to analyze the proton 1H MNR spectra. Fourier Transform Infrared (FTIR) spectra were recorded by a Nexus 870 infrared spectrometer (Nicolet, America) in attenuated total reflection (ATR) infrared mode. Multimode8 Atomic Force (AFM) microscope (Brooke, USA) at tapping mode was used to characterize the microstructure by phase separation analysis. Uniaxial tension test was performed on an MTS Universal Materials Testing Machine (Criterion Model

41). The stretch rate was set to 5 mm·min-1 at room temperature. The film specimens were prepared in dogbone shape according to standard of ISO-527-2/1BB with the size of 30 mm × 2 mm (middle) × 0.3 mm. At least five specimens were tested to obtain average values. Thermal stability data of the samples was obtained by a STA449F3 Synchronous Thermal Analyzer (Netzsch Inc, Germany). The temperature range from 30 oC to 500 oC at heating rate of 10 oC·min-1 under nitrogen atmosphere was applied. Dynamic Mechanical Thermal Analyses (DMTA, DMA242E, German Necker scientific instrument, Germany) were used to test the samples at tension mode with a frequency of 1 Hz. The temperature range is 30 oC to 150 oC at a heating rate of 5 oC·min-1.

The films were cut into rectangle shape with the size of 20 mm × 3 mm × 0.3

mm.

2.4 Shape memory properties test Shape memory properties test of the prepared samples were performed on DMA242E under controlled force mode. The process of one cycle of shape memory performance testing consists the following steps: (1) The initial strain of the sample was defined as ε0 under the room temperature. When sample was heated to 20 oC above Tg, a force of 1N was loaded to elongate the film for 3 minutes; (2) In the following process of cooling down the sample to room temperature, the force of 1N was maintained loading and the max strain was defined as εm; (3) While the sample was retained at the room temperature for 5 minutes, removed external force a decreased strain could be obtained and the strain at this moment was defined as εf; (4) After sample was heated to 20 oC above Tg at a heating rate of 5 oC min-1, the sample would recovery under the shape memory force, the recovery strain εi was defined after the sample kept 30 min at Tg + 20 oC. Shape fixing rate (Rf) and shape recovery rate (Rr) were calculated according to following equations: Rf =

εf ― ε0 εm ― ε0

× 100%

(1)

Rr =

(a)

εm ― εi

× 100%

εm ― ε0

(2)

NCO

NCO H

+

O

O

NCO

pH

N2

O

60°C

C

PTMEG (Mn=2000)

CH3

CH3

N H

O

O

TDI

R

OCN

pC O

H N

CH3

NCO

NCO

NCO

O

O

O

C

C

O

NH R

O

NH C

O

O

N N

+

O N H

R

N H

C

O HO NCO

O

N2

N

N

80°C

O

OH

O

O

N O

N

O HN C OCN

HN R

OH

O

THEIC

O C

O HBPU prepolymer

(b)

HBPU prepolymer

+

MOCA

H H2 H2C C C O O

CH3 C CH3

H H2 OCH2C C O n OH

CH3 C

0
CH3

H OCH2C CH2 O

HBPU/EP graft-IPNs

130°C

EP

Scheme 1. Detailed synthesis route of the HBPU/EP IPNs composite

3. Results and discussion 3.1 Structures of HBPU/EP IPNs The HBPU/EP IPNs was synthesized through the grafting reaction of –NCO terminal group of HBPU prepolymer with epoxy resin to form the network crosslink point. Firstly, the HBPU prepolymer was synthesized by incorporation of TDI, PTMEG and THEIC. The 1H NMR spectrum of the HBPU prepolymer is shown in the Fig. 1 (a). The peak at δ = 2.25 ppm represents the -Bz-CH3, and it indicates the existence of TDI in HBPU prepolymer. The peak at δ = 1.61 and 3.61 are associated to -CH2- and O-CH2- in PTMEG, respectively. The peak at δ = 4.16 is attribute to the -CH2 connected to triazine ring. The characteristic peaks at δ = 10.91, 10.83, and 10.8 ppm represent the secondary –NH protons of the urethane linkages, which refer to terminal (T), linear

(L) and dendritic (D) structural units respectively. In conclusion, the HBPU prepolymer was synthesized. The degree of bifurcation (DB) determinates the structural characteristic of hyperbranched polymers, it can be calculated by the computational method of Frechet equation[24]: DB = (D + T) / (D + T + L)

(3)

When DB closes to zero, linear is the dominate structure of the prepolymer, and when DB closes to 0.5, the prepolymer is hyperbranched structure. Integrating peak areas of dendritic, terminal and linear structural units on high resolution NMR and DB of the HBPU prepolymer is calculated to be 0.64 according to equation (3). Thereby a highly branched structure of HBPU prepolymer was obtained.

Fig. 1 (a) 1H NMR of HBPU prepolymer and (b) FT-IR spectrum of unmodified EP, HBPU prepolymer and IPN-4.

To prove the generation of the HBPU/EP IPNs, FTIR was adopted to track the reaction. Fig. 1 (b) shows the spectrum of unmodified EP, HBPU prepolymer, and IPN4. The peak at around 3500 cm-1 in EP spectrum proves the existence of secondary hydroxyl group. In the HBPU prepolymer spectrum, the characteristic peak at around 1700 cm-1 and 3300-3400 cm-1 are attributed to strong -C=O stretching and -NH stretching vibration, respectively, it suggests the formation of urethane linkage. Besides, the peak at 2270 cm-1 indicates the presence of unreacted -NCO in the HBPU prepolymer. After the polymerization, the disappearance of characteristic peak of secondary hydroxyl (3500 cm-1) from EP and -NCO (2270cm-1) in IPN-4 indicates the successful reaction between HBPU prepolymer and EP.

Fig. 2 AFM micrographs of the IPNs samples: (a) IPN-1 and (b) IPN-3

In crosslinked IPNs, the formation of physical crosslink usually accompanied with phase separation, which determines the morphology and properties of IPNs. In previous reports[25], nanoscale phase separation of hard and soft domains in crosslinked polymer network regulates shape memory behavior and mechanical properties. AFM is a common tool to observe the microphase separation, the incompatible hard phase and soft phase can be discerned with differential contrast in phase image[26]. As shown in Fig. 2, the bright phase represents hard phase and the dark phase represents soft phase. The morphology of nanoscale two-phase can be distinguished clearly in two series of IPNs, it indicates two phases of the IPNs materials strongly interpenetrated due to the excellent compatibility. In IPN-1 (Fig.2a), partial bright regions which represent EP are evenly distributed in the interconnected clusters of HBPU region, most epoxy regions presents in continuous phase. Besides, the domain size of hard phase scattered in dark region in IPN-1 shows∼8 nm dimensions, which is much smaller than IPN-3 of ∼40 nm. The domain size below 50 nm was agreement with the phase domain sizes generally reported for the IPNs material[26] and IPNs-1 shows better compatibility.

3.2 Thermal and mechanical properties The mechanical and thermal properties of thermal-responsive materials are very important for their applications. In selecting different series of IPNs polymers, we give

priority to their shape memory performance, so we choose HBPU/EP IPNs with 20-50 wt% HBPU. Table 1. Characteristic temperature at thermal degradation

Samples IPN-1 IPN-2 IPN-3 IPN-4 HBPU

HBPU/EP

T5 (℃)

T20 (℃)

T70 (℃)

20/80 30/70 40/60 50/50 100/0

369.4 360.1 355.4 350.8 283.1

397.7 397.1 394.9 392.6 346.2

448.0 443.4 434.2 434.1 417.9

Residual weights at 500 ℃(%) 27.7 26.0 22.6 20.9 12.4

T5, temperature of 5% weight loss. T20, temperature of 10% weight loss. T70, temperature of 70% weight loss

Fig. 3 TGA thermograms of HBPU and HBPU/EP IPNs.

Based on TGA analysis, the thermal stability of HBPU and the HBPU/EP IPNs composites were ascertained. The thermograms are presented in Fig. 3 and characteristic temperature of HBPU and IPNs at thermal degradation are summarized in Table 1. Pure HBPU shows two typical degradation processes[27, 28] with the turning point of 390 oC. Two steps degradation should ascribe to thermal degradation of hard segment of TDI and THEIC and soft segments of PTMEG, respectively. The pyrolysis of soft segment leads to the release of carbon dioxide which happened in the range from 283 oC to 390 oC, the pyrolysis of hard segment releases hydrogen cyanide, aromatic carbonitrile and ether which happened from 390 oC to 450 oC. When the temperature is 500 °C, 87.6 % of HBPU has been decomposed. Comparing with pure

HBPU, the thermal stability (5% weight loss) of HBPU/EP IPNs is obviously improved from 283 oC to 369.4 oC. At 500 °C, residual weights of IPNs are 20.9-27.7%. This may be the fact that EP owns good thermal properties and the grafting reaction of HBPU and EP that increases the degree of cross-linking and yield improving of thermal stability[29]. Between 380 oC and 450 oC, the weight loss of IPNs materials decreases sharply, the increase of temperature may accelerate the decomposition of hard segment in HBPU. With the increase of HBPU content, the thermal stability of IPNs decreases slightly, which may be related to the decrease of degree of cross-linking of IPNs.

Fig. 4 (a) The reprensentive stress–strain curves of HBPU/EP IPNs with different composition ratios, (b) the tendency of tensile strength, elongation at break and tensile modulus of HBPU/EP IPNs with the increasing of HBPU

The stress–strain curves and the data of HBPU/EP IPNs with different composition are shown in Fig. 4. Comparing with the HBPU, the introducing of EP to HBPU/EP IPNs, the tensile strength from 12.5 MPa to 57.7 MPa and tensile modulus from 4.8 MPa to 1522.1 MPa, respectively, are remarkable enhanced. The high modulus of EP could be one reason, that the adding of EP improved the mechanical strength and elastic modulus of HBPU-based IPNs. Besides, the formation of interpenetration network is another reason, crosslinking points are formed between HBPU and EP so that the polymer chains are less likely to slip under the action of external force at the room temperature. With the increase of HBPU content in HBPU/EP IPNs, elongation at break raises while tensile strength and tensile modulus decreases gradually. The results are consistent with the phase diagram results in AFM, continuous hard phase of

EP in IPN-1 result in the the brittleness of IPN-1, and the elongation at break decreases with the decrease of HBPU content. However, magnifying of PU domains in the composites with the increase of PU content, and as well-known, increasing of polyurethane with soft molecular chain could lead to decrease of tensile strength. These results suggest that the formation of crosslinking points between HBPU and EP can enhance their mechanical strength and elastic modulus, which depends on the mass ratio of HBPU/EP. It also lays the foundation of excellent mechanical properties for the HBPU-based IPNs shape memory polymer. 3.3 DMTA characterization The detail DMTA results of HBPU/EP IPNs with different component ratios at 1 Hz is summarized in Fig. 5 and Table 2. As you can see in figure 5a, the tanδ peaks of all the IPNs associated with the glass transition temperature (Tg) which exhibits a monodistribute peak. It is attributed to the uniformity of the composition as described in the AFM results. With the increasing of HBPU component, the value of tanδ decreased gradually, the Tg of the composites move towards low temperature. It may be a fact that the movement of the chain segments is restricted by the intermolecular interaction in the process of the thermal transition. The lower the crosslink density and entanglement, the lower the temperature is required to reduce the rigid segment and unwrap the entangled node, so IPNs with lower crosslink density and entanglement have lower Tg. In the shape memory cycle, the high glass state modulus (Eg) makes the sample with high fixity at the stage of fixing shape and the rubber state modulus (Er) is related to elastic recovery at the stage of recovering[30]. Figure 5b shows the relationship between storage modulus and temperature of the IPNs. Generally the Eg of the IPNs composites at temperature of 30 oC is quite high, Eg of the IPN-1 composite reach to 2600 MPa. The presence of aromatic structures and triazine rings structure in IPNs, as well as physical chain entanglement and chemical bonds between HBPU and EP result in relatively high storage modulus in glass state of the HBPU/EP IPNs. As shown in figure 5b, the Er value of rubber platform decreases with the increase of HBPU content in IPNs. The Er of the IPNs is determined by the network cross-linking and

entanglement. The higher Er of the sample, the more elastic strain energy can be stored in the programming process of shape memory, which endows a big recovery stress and good shape memory behaviors. The cross-linking density is calculated according to the reported formula[31], and it is shown in Table 2. The crosslinking density decreases with the increase of HBPU content, which may cause the gradually decrease of shape recovery rate of IPNs materials. And as shown in Table 2, near the glass transition temperature (Tg + 30 oC and Tg - 30 oC), the large fall of storage modulus (E') between glass state and rubber state has a significant effect on the shape memory properties of the polymer, and it foretell that the sample is very sensitive to varying of temperature. Table 2. DMTA data of HBPU/EP IPNs with different component ratios at 1 Hz

Samples HBPU/E P

tanδ (max)

Tg (℃)

E'Tg-30 (MPa)

E'Tg+30 (MPa)

IPN-1 IPN-2 IPN-3 IPN-4

0.70 0.57 0.47 0.40

96.1 89.1 77.1 60.6

1370 827 897 461

18.9 16.4 22.5 17.3

20/80 30/70 40/60 50/50

cross-linking density (kmol −3 m ) 1.96 1.85 1.80 1.68

E′

cross-linking density (ν) = 3RT, where E' is the storage modulus at rubbery platform, R is the universal gas constant (8.314 J K−1 mol−1) and T = 125 ℃ (=398K)[31].

Fig. 5 (a) Loss factor (tanδ) and (b) storage modulus of HBPU/EP IPNs with different component ratios at 1 Hz

3.4 Shape memory properties In the process of a shape memory cycle, the material was deformed at temperature

above Tg, and its temporary shape was fixed when it was cooled to room temperature, as shown in Fig. 6a, two rectangle film were twistwd and the scrolls could be fixed at different stages. When reheated the material above the Tg, the scrolls recovered to the initial rectangle shape. The visual SME of IPNs demonstrates the excellent recovery ability.

Fig.6 (a) Visual shape memory effect of (IPN-1) IPNsmaterial, (b) the trend of Rf and Rr of IPNs materials, (c) the trend of seven cycles of shape memory of IPN-1.

The shape memory performance of polymers is usually evaluated by Rf and Rr, the Rf and Rr values of IPNs is was obtaind by thermomechanical three cycles tensile test under the condition of Tg + 20 oC and 1N force loaded. The first cycle usually shows poor shape recovering performance, which is due to the residual strain in the sample processing[32, 33]. So the averaged Rr and Rf from the second and third cycles were used to evaluate the shape memory properties. As shown in Fig. 6b, the Rf and Rr of all IPNs materials are above 96 %, and IPN-1 presidents the highest one with Rr of 99.4 % and Rf of 99.7 %, respectively. These indicate that HBPU/EP IPNs could be tailored with excellent shape memory performance. The shape memory properties of polymers are related to physical entanglement and covalent network points. The hard domains as

physical crosslink (entanglement) of the polymer minimize the permanent deformation which was caused by the creeping of the polymer chain, and it is crucial for polymers to provide high shape memory retention. However, the physical and chemical crosslinking points are strong enough to restore the polymer to original state and without significant creep. These are attributed to high Eg, Er and cross-linking density which are obtained from DMTA characterization. As shown in Fig. 6b, the Rf and Rr of the samples decrease with the increase of HBPU content, it is ascribed to the relative reduction of aromatic ring in hard phase and the reduced cross-linking density caused by the increasing of soft domain. The sample with high HBPU content allows a higher elongation during heating under stretching, and the storage strain energy becomes less during cooling. When the sample is reheated, the release of the strain energy is not sufficient to recover more of the non-relaxed polymer chains. Figure 6c is the Rf and Rr trend diagram of IPN-1 within seven cycles. The Rf is invariant to remain 99.3 % and the Rr is above about 96 % from the second to the seventh cycles, which indicates the excellent SME stability of IPN-1.

4. Conclusions HBPU prepolymer which containes triazinyl ring structure was synthesized, and IPNs was formed by grafting reaction of HBPU and EP. The microphase separation of IPNs was observed through AFM analysis and the result showed the good compatibility of two phases. The formation of interpenetrating network increased the degree of crosslinking and entanglement of IPNs materials, which was beneficial to the improvement of thermal, mechanical and shape memory properties of IPNs materials. All of our HBPU/EP IPNs materials exhibited excellent shape memory performance with Rf > 96 % and Rr > 96 %. Due to higher Eg and Er of IPN-1, it presented best shape memory property, Rf is 99.3% and the Rr is 99.7%. This study could pave the way for the SMPs molecular design and IPNs material further-covering SMP.

Acknowledgments

This work was financially supported by the Key Research Program of Frontier Science, Chinese Academy of Sciences (Grant QYZDJ-SSW-SLH056), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2018457) and the National Natural Science Foundation of China (Grant 51673205, 51875549).

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Graphical Abstract Shape memory properties of interpenetrating polymer network (IPNs) based on hyperbranched polyurethane (PU)

Highlights · Hyperbranched polyurethane was applied through grafting epoxy resin to obtain interpenetrating polymer networks. ·Crosslinking network points help enhancing thermal and mechanical properties. ·All interpenetrating polymer networks express excellent shape memory effect.

We would like to submit the enclosed manuscript entitled “Shape memory properties of interpenetrating polymer network (IPNs) based on hyperbranched polyurethane (PU)” for consideration by “European Polymer Journal”. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.

Author Statement Cong Wang1,2, Yaoming Zhang1,2, Jianming Li3, Zenghui Yang1, Qihua Wang1,2, Tingmei Wang1,2, Song Li1,2, Shoubing Chen1,2, Xinrui Zhang1,2*

The above authors have made outstanding contributions in this paper and All the authors have no objection. The details are as follows: Cong Wang is the first author of this article and mainly contributes to preparation of SMP, characterizes properties and writes the article. Yaoming Zhang assists me in analyzing data of AFM Jianming Li assists me in analyzing data of shape memory properties. Zenghui Yang assists me in the synthesis of polymers. Song Li assists me in the tensile test and helps to analyze data. Shoubing Chen assists me in analyzing data of FTIR. Qihua Wang, Tingmei Wang and Xinrui Zhang assist me in designing experimental, revising paper and guiding experiment, respectively.