Creating Poly(tetramethylene oxide) Glycol-Based Networks with

Jun 19, 2017 - Creating Poly(tetramethylene oxide) Glycol-Based Networks with Tunable Two-Way Shape Memory Effects via Temperature-Switched Netpoints ...
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Creating Poly(tetramethylene oxide) Glycol-Based Networks with Tunable Two-Way Shape Memory Effects via Temperature-Switched Netpoints Hui Xie, Chuan-Ying Cheng, Xiao-Ying Deng, Cheng-Jie Fan, Lan Du, Ke-Ke Yang,* and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), College of Chemistry, Sichuan University, Chengdu 610064, China ABSTRACT: Polymers with two-way shape memory effect (2W-SME) are of great potential in real applications due to the reversible nature. Though 2W-SME has been already realized in some semicrystalline networks by the crystallization-induced elongation (CIE) and melting-induced contraction (MIC), more adaptable 2W-SME systems are highly expected. In this work, to achieve tunable 2W-SME with different reversible deformation ranges, we designed a semicrystalline poly(tetramethylene oxide) glycol (PTMEG) network with temperature-switched netpoints. This adaptable network was constructed by photo-cross-linking of the PTMEG-based copolymer containing amorphous poly(D,L-lactide) (PDLLA) segments in main chain and photosensitive anthracene groups in side chain. The photo-cross-linking degrees of networks were adjusted by controlling irradiation time. Remarkably, 2W-SME in the current system was realized by the CIE and MIC of PTMEG segments for the first time. Here, along with the photo-cross-links, the low content of PDLLA served as a switchable netpoint which was controlled by high temperature (Thigh). Differential scanning calorimetry (DSC) analysis revealed that all the photo-cross-linked networks exhibited desirable crystallinity. Dynamic mechanical analysis (DMA) indicated that there were two distinct declines of storage modulus (E′) when temperature crossed Tm,PTMEG and Tg,PDLLA, which guided us to choose 45 and 70 °C as two specific Thighs. 2W-SME was investigated by DMA under constant stress; in detail, the effects of applied stress, Thigh, and photo-cross-linking degree synergizing with the switchable netpoints structure on the characteristics of 2W-SME (εi, εNon‑CIE, εCIE, Ract(σ), and Rrec(σ)) have been disclosed.

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INTRODUCTION Shape memory polymers (SMPs), as an emerging class of smart materials, have the capable of responding to external stimuli (such as heat, light, electricity, and so on).1−4 This kind of excellent property endows SMPs with a wide variety of applications, ranging from simple actuators to biomedical devices.5−8 Up to now, thermally induced SMPs (TSMPs) have attracted great interests due to their advantages of readily being designed and programmed. Generally, a TSMP consists of netpoint and molecular switch; the former determines the permanent shape, and the latter is responsible for the shape recovery.9 Commonly, a shape memory cycle under stress conditions proceeded as follows: (1) deforming TSMP under external force at a high temperature (Thigh) which is above the phase transition temperature (Ttrans, including melting temperature Tm, glass transition temperature Tg, and liquid crystalline clearing temperature Tcl);10−17 (2) cooling TSMP to a low temperature (Tlow) below Ttrans (especially if Ttrans = Tm, Tlow may be lower than crystalline temperature Tc) and then removing the external force, generating a stable temporary shape; (3) reheating © XXXX American Chemical Society

TSMP to Thigh, leading to the shape recovery process. In fact, the shape memory behavior described above belongs to oneway shape memory effect (1W-SME), since reprogram is essentially required in the successive cycles.18 This irreversible nature of 1W-SME greatly limits the real applications of conventional SMPs in remote-control systems or shielding devices. Recently, the reversible shape memory effect or the so-called two-way shape memory effect (2W-SME) has been found in liquid-crystalline elastomers and semicrystalline polymer networks successively. For a semicrystalline polymer network, 2WSME is realized by the crystallization-induced elongation (CIE) and melting-induced contraction (MIC) under constant stress or stress-free conditions.19,20 Compared with one-way shape memory systems, the SMPs with 2W-SME need not to be reprogrammed in successive cycles, so they are a good candidate for sensors, actuators, and artificial muscles.21,22 Up Received: December 23, 2016 Revised: June 2, 2017

A

DOI: 10.1021/acs.macromol.6b02773 Macromolecules XXXX, XXX, XXX−XXX

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prepared from PDLLA, PTMEG, and BHEAA, and the chemical structures of these materials are presented in Scheme 1a. Note that the

to now, polymeric materials based on poly(cyclooctene) (PCO),22−24 poly(ethylene-co-vinyl acetate) (EVA),18 poly(εcaprolactone) (PCL),25−31 poly(ω-pentadecalactone) (PPD),19 poly(1,4-butylene adipate) (PBA),32 poly(octylene adipate) (POA),33−35 and some others36 have been employed as matrix in two-way shape memory systems. In our previous work, we developed a triple-shape memory photo-cross-linked network based on poly( D , L -lactide) (PDLLA) and poly(tetramethylene oxide) glycol (PTMEG). We found that PTMEG exhibited distinct CIE behavior when cooling below to its crystallization temperature (Tc) in a typical 1W-SME cycle,37 and this phenomenon was only observed for the network with low PDLLA content (about 20%) because of the adequate crystallization of PTMEG segment. This unexpected discovery inspired us to employ crystalline PTMEG segment to build a network possessing 2W-SME, which would be a good supplement to current two-way shape memory systems. Moreover, the photo-cross-linking of anthracene groups has been considered as a fast and facile strategy to fabricate shape memory networks. In the present work, linear PDLLA−PTMEG copolymer dangling anthracene groups was prepared at first, and the content of PDLLA was set as 20% to keep the balance between adequate netpoints and efficient CIE according to our previous work. Furthermore, a series of PDLLA−PTMEG networks with different photocross-linking degrees were well constructed by simply tuning the photoirradiation time. Just like a general network with 2WSME, PTMEG segments perform as a “molecular switch” and photo-cross-links of anthracene groups act as netpoints, but one thing different is that a small amount of introduced PDLLA segments act as an assistant on−off netpoint which is switched by high temperature (Thigh). In detail, when Tg,PDLLA > T > Tm,PTMEG, both PDLLA segment and photo-cross-links perform as netpoints, and when T > Tg,PDLLA, only photo-cross-links act as netpoint. This feature endows the two-way SMP developed in our current work with tunable temperature-switched netpoints so that it could be conducted at two different temperature ranges. The influence of the switchable netpoints structure on the performance of 2W-SME was investigated. For better understanding, the strain actuation that was not induced by CIE (εNon‑CIE) was addressed thoroughly, and it was found to highly depend on Thigh and applied stress. More importantly, applied stress, high temperature (Thigh), and photo-crosslinking degree were also taken into account to systematically investigate the 2W-SME.

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Scheme 1. Preparation of NW-PDLLA-PTMEG Networks by Photo-Cross-Linking of the Anthracene Groups: (a) Chemical Structures of PDLLA, PTMEG, and BHEAA; (b) Simplified Structure of AN-PDLLA20-PTMEG; (c) Network Formed by Photo-Cross-Linking of the Anthracene Groups upon 365 nm UV Light Irradiation; (d) Photo-CrossLinking Reaction of Anthracene Groups

Mn of PDLLA was 6300 g mol−1, as obtained from its 1H NMR (the spectrum is not shown; however, two specific positions, h and g, are marked in the PDLLA’s chemical structure) and calculated by eq 1 according to our previous works.4,37

Mn =

Ih × 2 × 2 × 72 + 2 × 72 + 90 Ig

(1)

The targeted product was coded as AN-PDLLA20-PTMEG, and its simplified structure is displayed in Scheme 1b. The synthesis procedures were carried out as follows. (1) A mixture of PDLLA (0.80 g, 0.13 mmol) and PTMEG (3.20 g, 1.10 mmol) was put into a reaction flask and dehydrated at 50 °C under vacuum for 3 h, during which an exhausting−refilling process was repeated three times. (2) 1,2-Dichloroethane (C2H4Cl2) (16.0 mL) was injected into the former reaction as solvent, and then the temperature was raised to 75 °C. (3) HDI (0.40 mL, 2.50 mmol) and five drops of DBTL were added to terminate the prepolymers as soon as PDLLA and PTMEG dissolved completely, and then the reaction was kept for 3 h. (4) BHEAA (0.20 g, 0.68 mmol) and 1,4-butanediol (BD) (0.05 mL, 0.58 mmol) were added to start the chain-extension reaction, and the reaction was kept for another 4 h. (5) After being cooled to room temperature, the resulting solution was precipitated by a large amount of cold ether/ hexane (1/4, v/v) to obtain AN-PDLLA-PTMEG, which was a yellow flocculent solid. The final product was dried in a vacuum oven at room temperature for 48 h. Preparation of a Series of Photo-Cross-Linked PDLLAPTMEG Networks with Different Photo-Cross-Linking Degrees. To investigate the effect of photo-cross-linking degree on 2W-SME, a series of networks were fabricated by photo-cross-linking of ANPDLLA20-PTMEG, and the irradiation time of 365 nm UV light was varied (Scheme 1c). First of all, a thin film of AN-PDLLA20-PTMEG (thickness: 0.13 mm) was prepared by solution casting from chloroform (CHCl3), and subsequently, three square films were prepared after being dried completely. Then, 365 nm UV light (the intensity to the films was 7.10 mW cm−2) was employed to irradiate both the two sides of the films. The irradiation time for each side was set as 0.5, 1.0, and 2.0 h, and the corresponding networks were coded as NW-PDLLA20-PTMEG/1, NW-PDLLA20-PTMEG/2, and NWPDLLA20-PTMEG/4, respectively. Swelling Tests of NW-PDLLA20-PTMEG Networks. The gel fractions of photo-cross-linked networks were calculated by the

EXPERIMENTS AND METHODS

Materials. Hydroxyl-telechelic poly(tetramethylene oxide) glycol (PTMEG, Mn = 2900 g mol−1), dibutyltin dilaurate (DBTL), and stannous octoate (Sn(Oct)2, 95%) were purchased from SigmaAldrich. D,L-Lactide and 9-hydroxymethylanthracene were provided by Daigang Biomaterial Company and J&K Company, China, respectively. 1,6-Hexamethylene diisocyanate (HDI) was from Alfa Aesar. Other common reagents and solvents were purchased from Chengdu Kelong Reagent Company, China. All the raw materials were used as received or purified before use as reported by our previous works. Poly(D,L-lactide) (PDLLA) was synthesized from the ring-opening polymerization (ROP) of D,L-lactide according to our previous work.37 The photoresponsive monomer N,N-bis(2-hydroxyethyl)-9anthracenemethanamine (BHEAA) was synthesized from 9-hydroxymethylanthracene, phosphorus tribromide (PBr3), and diethanolamine (DEA) according to the previous work.4 Synthesis of PDLLA-PTMEG Copolymer Containing Anthracene Groups in the Side Chains (AN-PDLLA-PTMEG). In this part, a linear PDLLA-PTMEG copolymer with 20% of PDLLA was B

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Macromolecules following steps: four pieces of each NW-PDLLA20-PTMEG sample were prepared, then swelled and extracted by chloroform at room temperature for 48 h. Three key parameters were recorded: the mass of original samples (mo), the mass of swelled extracted samples (ms), and the mass of dried extracted samples (md). The gel fraction (G) and swelling ratio (Sd) were calculated according to the following equations: m G (%) = d × 100% mo (2)

Sd (%) =

ms × 100% md

previous works. The test program was performed in tension under DMA controlled force mode using a DMA Q800. For 1W-SME, the procedures were carried out as follows. (1) The sample with an original strain (εp(N−1)) was heated to Thigh (45 or 70 °C) and deformed under external stress (0.45 MPa). (2) Then cooling the sample to Tlow (−35 °C) at a rate of 5 °C/min and keeping for 20 min. As a consequence, a maximum deformed strain (εd(N)) can be achieved. (3) Removing the external stress, and the so-called fixed strain (εf(N)) was obtained. (4) Reheating to Thigh at a rate of 5 °C/ min, the sample recovered to its original strain with a residual strain (εp(N)). The shape memory cycle were conducted for four cycles. The shape fixity ratio (Rf) and shape recovery ratio (Rr) were calculated by the following equations: εf(N ) − εP(N − 1) Rf = × 100% εd(N ) − εp(N − 1) (9)

(3)

Determination of the Cross-Linking Density of NW-PDLLA20PTMEG Networks. The cross-linking density (v) of photo-crosslinked networks was calculated by the Flory−Rhener equation:38,39

v=

− [ln(1 − v2) + v2 + χv2 2]

(

Vs v21/3 −

2v2 5

Rr =

)

(4)

md /(msρpolymer ) md /(msρpolymer ) + (ms − md)/(msρsolvent )

(5)

where md and ms represent the dried extracted mass and the swollen mass obtained from the swelling test, respectively. ρsolvent and ρpolymer refer to the densities of solvent and polymer (networks), respectively. Meanwhile, in eq 4, χ represents the polymer−solvent interaction parameter and can be calculated by eq 6: χ = 0.34 +

Vs (δpolymer − δsolvent)2 RT

δpolymer

MPTMEG

⎠

(7)

where n is the ratio of the number of repeat units of PTMEG and PDLLA, which can be obtained from the 1H NMR spectrum of ANPDLLA20-PTMEG. δPTMEG and δPDLLA represent the densities of PTMEG and PDLLA, respectively. The solubility parameters of PTMEG and PDLLA, δPTMEG and δPDLLA, can be calculated by eq 8:41,42

δ PTMEG(or PDLLA) =

∑F V̅

(10)

RESULTS AND DISCUSSION Preparation of AN-PDLLA20-PTMEG Copolymer and NW-PDLLA20-PTMEG Networks. To create the targeted photo-cross-linked networks, a well-defined linear copolymer AN-PDLLA20-PTMEG with 20 wt % PDLLA segments was prepared at first. The synthesis route was briefly described in the followings: (1) end-capping of the PDLLA and PTMEG prepolymers with NCO group using HDI; (2) chain extension of the former NCO-terminated prepolymers using BHEAA and BD. The number-average molecular weight (Mn) of the product was 8.0 × 104 g mol−1, as determined by GPC. The chemical structure of AN-PDLLA20-PTMEG was demonstrated by the 1 H NMR spectrum (Figure 1). Also, the content of PDLLA segments was calculated as 20.5% according to the character peak area of its repeat unit in the 1H NMR spectrum, which was close to its theoretical value. Then, the NW-PDLLA20-PTMEG networks were constructed by simply photo-cross-linking of the anthracene groups in the linear AN-PDLLA20-PTMEG copolymer.

(6)

1/2 ρPTMEG 2⎞ δ MPTMEG PDLLA ⎟ ρPTMEG ⎟

× 100%

■

where R is the gas constant, T is the absolute temperature, and δsolvent is the solubility parameters of chloroform, while δpolymer represents the solubility parameters of polymer (networks), and it can be determined by eq 7:40 2 ⎛ n ρPDLLA δ PTMEG + M = ⎜⎜ PDLLA ρPDLLA nM + ⎝ PDLLA

εf(N ) − εp(N − 1)

For 2W-SME, the test program was performed as follows. (1) The sample was heated to Thigh (45 or 70 °C), and a specific stress (0.15, 0.30, and 0.45 MPa) was applied using a rate of 0.10 MPa/min. This stress was kept constant during the whole experiment. (2) Cooling the sample to Tlow (−35 °C) at a rate of 5 °C/min and keeping for 20 min, during which the crystallization-induced elongation (CIE) phenomenon could be observed. (3) Reheating the sample to Thigh at a rate of 5 °C/min, and the shape recovery process could be activated by melting-induced contraction (MIC). Characteristics of 2W-SME are calculated and discussed in the Results and Discussion section.

In eq 4, Vs is the molar volume of solvent and v2 represents the volume fraction of the swollen polymer in the state of swollen equilibrium, which can be computed by eq 5:

v2 =

εf(N ) − εP(N )

(8)

where V̅ is the molar volume of polymer and F is the molar attraction constant of each group in a repeat unit. The essential data and parameters for eqs 4−8 can be consulted from a professional book.42 Thermal Behavior of NW-PDLLA20-PTMEG Networks. Differential scanning calorimetry (DSC) measurements were conducted on a TA Instruments DSC Q200 to investigate the melting and crystallization behavior of networks. The experiments were run from −60 to 120 °C using a heating and cooling rate of 5 °C/min. Dynamic Mechanical Properties of NW-PDLLA20-PTMEG Networks. A dynamic mechanical analyzer (DMA) (DMA Q800, TA Instruments, USA) was employed to evaluate the storage modulus (E′) of networks. The experiments were run from −90 to 90 °C using a heating rate of 3 °C/min, and the test frequency was 1 Hz. Shape Memory Behavior of NW-PDLLA20-PTMEG Networks. In this section, 1W-SME and 2W-SME were investigated according to

Figure 1. 1H NMR spectrum of AN-PDLLA20-PTMEG. C

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Macromolecules Table 1. Swelling Test Results of Photo-Cross-Linked NW-PDLLA20-PTMEG Networks sample name

Sd (%)

G (%)

v (× 10−6 mol cm−3)

NW-PDLLA20-PTMEG/1 NW-PDLLA20-PTMEG/2 NW-PDLLA20-PTMEG/4

10470 ± 371 7061 ± 315 6215 ± 616

21.2 ± 0.84 40.9 ± 1.79 59.0 ± 1.59

1.14 ± 0.11 2.52 ± 0.18 3.56 ± 0.02

Generally, the photo-cross-linking degree highly relies on the geometric dimension of sample and the irradiation time (with the same irradiate intensity) until it reaches its reaction equilibrium. For the sake of better photo-cross-linking efficiency, thin films were employed in this work. To generate networks with different photo-cross-linking degrees, the irradiation time of each side was controlled as 0.5, 1.0, and 2.0 h, and the corresponding samples were coded as NWPDLLA20-PTMEG/1, NW-PDLLA20-PTMEG/2, and NWPDLLA20-PTMEG/4, respectively. The swelling ratio (Sd) and gel fraction (G) were investigated through the typical swelling tests. Additionally, swelling test is also one of the effective methods to determine the cross-linking density (v), as reported previously.43,44 The relevant data are listed in Table 1. The G values increased from 21.2% to 59.0% with the increase of irradiation time. As expected, Sd decreased along with the increase of gel fractions. In order to characterize the architecture of targeted photo-cross-linked networks more clearly, the cross-linking density v was also determined. As seen from Table 1, the v values are much lower than those in the other studies38 even though the gel fractions are comparable. This difference is due to the longer chain length between photo-cross-links achieved in this study as a consequence of fewer anthracene moieties in each copolymer chain. Qualitatively, the average functionality of AN-PDLLA20-PTMEG is about 5 ( f ≈ 5) in the current system. However, the v values increased from 1.14 × 10−6 to 3.56 × 10−6 mol cm−3 with the increase of irradiation time. As all networks were generated from the same copolymer AN-PDLLA20-PTMEG, the difference between their v values was merely due to the difference in the amount of photo-cross-links in respective networks that reflects the different photo-cross-linking degree of anthracene groups. Consequently, the longer the irradiation time was, the higher the photo-cross-linking degree was and eventually donated the higher cross-linking density. These results not only confirmed the effective formation of networks by photocross-linking of the anthracene groups but also demonstrated the fact that longer irradiation time gave higher photo-crosslinking degree as expected. However, the G values of networks in current system are lower than those formed by chemical cross-linking using some coupling reagents, such as HDI and DCP. Even though, it is still worth to investigate the 1W-SME and 2W-SME for the current PTMEG-based networks. Thermal Behavior of NW-PDLLA20-PTMEG Networks. As 2W-SME in this system strongly relies on the crystallization and melting behavior of PTMEG segments, the thermal properties of NW-PDLLA20-PTMEG networks with different photo-cross-linking degrees were evaluated through DSC analysis (Figure 2). The data of crystallization temperature (Tc) and corresponding enthalpy (ΔHc), melting temperature (Tm), and its corresponding enthalpy (ΔHm) are listed in Table 2. Note that the glass transition temperature of PDLLA (Tg,PDLLA) was not obviously detected due to its low content in polymers. As shown in Figure 2a, which represents the cooling scan, the crystallization behavior of PTMEG segments was clearly observed for both linear copolymer and photo-cross-

Figure 2. DSC curves of linear copolymer and its corresponding NWPDLLA20-PTMEG networks with different photo-cross-linking degrees: (a) cooling scan and (b) heating scan. Heating and cooling rate: 5 °C/min.

Table 2. DSC Data of Photo-Cross-Linked NW-PDLLA20PTMEG Networks PTMEG segments sample name

Tc (°C)

ΔHc (J g−1)

Tm (°C)

ΔHm (J g−1)

PTMEG2900 AN-PDLLA20-PTMEG NW-PDLLA20-PTMEG/1 NW-PDLLA20-PTMEG/2 NW-PDLLA20-PTMEG/4

13.2 −32.2 −29.7 −29.0 −29.0

97.8 21.4 21.4 20.0 19.0

23.3 24.5 24.2 23.8 23.4

102 36.8 34.0 32.2 29.9

linked networks. In detail, Tc of the original copolymer without any irradiation was detected at −32.2 °C. As for networks, it ranged from −29.7 to −29.0 °C when the photo-cross-linking degree increased. Meanwhile, the values of corresponding ΔHc slightly decreased from 21.4 to 19.0 J/g. In heating scan (Figure 2b), likewise, the Tm of networks slightly decreased from 24.2 to 23.4 °C, while the corresponding ΔHm decreased from 34.0 D

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photo-cross-links in polymer networks. For the rest of two networks which have higher photo-cross-linking degrees, slow plateaus of E′ appeared. Overall, the higher the photo-crosslinking degree of the network was, the higher the E′ value in the high temperature region (T > Tg,PDLLA) was. This phenomenon was due to the fact that the photo-cross-links were the only netpoint that accounted for the strength of materials when T > Tg,PDLLA. In summary, E′ values had a great change when temperature crossed Tm,PTMEG and Tg,PDLLA, and this feature gave us guidance to choose 45 and 70 °C as two specific Thighs in terms of the concept of temperature-switched netpoints. The effects of Thighs on 1W-SME and 2W-SME are addressed in detail thereafter. One-Way Shape Memory Property. 1W-SME, as the most general type of SME, was briefly studied at first. In this part, NW-PDLLA20-PTMEG/4 was chose to investigate 1WSME using DMA analysis, and the test programs were addressed in the Experiments section. Here, only two key points should be highlighted: one is that the experiments were conducted at two different Thighs (45 and 70 °C) under the same condition, and another one is that the low temperature (Tlow) utilized to fix the temporary shape was −35 °C, which was below the Tc of PTMEG segment to allow its adequate crystallization. Four consecutive 1W-SME cycles for each Thigh were recorded (Figure 4), and the relevant data are listed in Table 3. For 1W-SME under 45 and 70 °C, as we expected, an obvious crystallization-induced elongation (CIE) was clearly observed when cooling to Tlow. As seen from the results of DSC analysis, all networks have obvious crystallization and melting phenomenon, even though their crystallinity was slightly affected by the photo-cross-linking degree. The appearance of

to 29.9 J/g with the increase of photo-cross-linking degree. From the structure of linear copolymer, most of the anthracene groups were dangled into PTMEG segments as determined by synthetic strategy and specific feed ratio, so the photo-crosslinks mainly formed between the PTMEG chains. As for NWPDLLA20-PTMEG networks, the formed photo-cross-links affected the chain mobility of PTMEG segments to some extent. The higher the photo-cross-linking degree was, the more the photo-cross-links were and eventually led to the greater influence on crystallization. Though the photo-crosslinks have affected the thermal behavior of materials, all NWPDLLA20-PTMEG networks exhibited adequate crystallinity which would enable the networks to show 1W-SME and 2WSME. Dynamic Mechanical Properties of NW-PDLLA20PTMEG Networks. DMA analysis was employed to investigate the change of storage modulus (E′) of the networks when temperature ramped up. The curves of linear copolymer ANPDLLA20-PTMEG and three networks NW-PDLLA20-PTMEG are displayed in Figure 3. Generally, E′ decreased when

Figure 3. Storage modulus plots of linear copolymer AN-PDLLA20PTMEG and its corresponding NW-PDLLA20-PTMEG networks with different photo-cross-linking degrees.

temperature increased, but more distinct change could be observed when it crossed the thermal transition temperatures of the materials. In current system, the DMA curves can be divided into three main stages (I, II, III) which covered the glass transition temperature of PTMEG (Tg,PTMEG), Tm,PTMEG, and Tg,PDLLA, respectively. In stage I, when the temperature crossed Tg,PTMEG, polymer chains in amorphous regions started to move freely, and as a consequence, the E′ value decreased. In stage II, E′ decreased more sharply when temperature crossed Tm,PTMEG. For the sample AN-PDLLA20-PTMEG without cross-linking, a slow decline zone could be observed about −40 °C between stage I and stage II. In fact, the Tg,PTMEG of networks would be extended after photo-cross-linking; meanwhile, all samples showed a very wide melting temperature range which confirmed by DSC analysis. So there was no distinct boundary between stage I and stage II for all networks. A valuable change of E′ at high temperature above the glass transition region of PDLLA was found (stage III). Here, for the networks with different photo-cross-linking degrees, the situations were different. For AN-PDLLA20-PTMEG, its E′ value dropped quickly and almost corrupted. A similar situation was observed for NW-PDLLA20-PTMEG/1, which has the lowest photo-cross-linking degree, in other words, the least

Figure 4. 1W-SME curves of NW-PDLLA20-PTMEG/4 network under different Thigh recorded from DMA: (a) Thigh = 45 °C and (b) Thigh = 70 °C. E

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Macromolecules Table 3. Data for 1W-SME of NW-PDLLA20-PTMEG/4 at Different Thighs (Cycles 2−4) Thigh (°C)

strain (%)

Rf (%)

Rr (%)

45 70

60.8 ± 0.9 147 ± 6.4

98.2 ± 1.4 99.2 ± 0.03

98.1 ± 2.8 98.9 ± 1.9

CIE in NW-PDLLA20-PTMEG/4 indicated it may have good 2W-SME, for which CIE is the fundamental requirement. Because of the residual stress from processing, which generated obvious irreversible strain,45 the shape recovery ratios (Rr) in the first cycle were 80.5% and 89.6% for Thigh = 45 and 70 °C, respectively, even though the shape fixity ratios (Rf) were all up to about 99.0%. In Table 3, the results of cycles 2−4 (Rf and Rr for Thigh = 45 and 70 °C) were evaluated to probe the effect of the different netpoints structure. When Thigh = 45 °C, both photo-cross-links of network and vitrified PDLLA segments served as netpoints. When Thigh = 70 °C, the PDLLA segments converted to elastic state, so only photo-cross-links served as netpoints. In all, the network exhibited repeatable 1W-SME cycling performance with quite high Rf and Rr (more than 98%) no matter which Thigh was employed, which indicated that photo-cross-links were sufficient for reasonably good 1W-SME. However, the maximum deformation strain under 70 °C was much higher than that under 45 °C when the same stress was applied. This phenomenon can be explained as the different rigidity of materials at 45 and 70 °C as shown in the DMA curves. Two-Way Shape Memory Property. The 2W-SME of NW-PDLLA20-PTMEG networks is the most important property that we concern with. Quantitative experiments were performed on DMA to evaluate 2W-SME under different experimental conditions. Four consecutive cycles were recorded, and the final results were concluded based on the data of cycles 2−4. As we know, 2W-SME is realized through the CIE and MIC of PTMEG segments. However, as shown in Figure 5a, the real actuation strain during cooling was not totally caused by CIE (εCIE). There was a small part of strain before the εCIE, which could be differentiated by the turning point, and we defined it as εNon‑CIE for comparison. When increasing the temperature, a recovered strain (εREC) would be induced by MIC. It should be noted that the three parameters mentioned above are all absolute magnitudes of strain change during a whole 2W-SME cycle. For strain actuation, the parameter Ract(σ)22 (here, expressed as Ract‑A(σ) in eq 11) was used to evaluate the absolute magnitude. To evaluate the strain actuations in different cases, for example, different applied stresses, the relative magnitude (Ract‑R(σ)) was also calculated by comparing to the baseline strain at Thigh.18 Hence, we utilize Ract‑R(σ) to evaluate strain actuation for different initial strains generated in different cases and use Rrec(σ) to determine the strain recovery magnitude. For each 2W-SME cycle, the two important characteristics, Ract‑R(σ) and Rrec(σ), were calculated by eqs 12 and 13: R act − A(σ ) = εlow − εhigh = εNon − CIE + εCIE

R act − R (σ ) =

R rec(σ ) =

εNon − CIE + εCIE × 100% εi

εREC × 100% εNon − CIE + εCIE

Figure 5. (a) Schematic diagram of a typical 2W-SME process to show εNon‑CIE, εCIE, εREC, and εi (taken from fourth cycle of NW-PDLLA20PTMEG/4 (stress: 0.45 MPa)). (b) 2W-SME curves of NWPDLLA20-PTMEG/4 under different external stresses (cycle 4, Thigh = 45 °C).

Here, εi represents the initial strain at high temperature, that is, at the beginning of each cycle before cooling. In this work, our attention was not only focused on Ract‑R(σ) and Rrec(σ) as most of the previous works did but also aimed at εNon‑CIE and εCIE which would make practical sense for potential applications. The effects of applied stress, Thigh, and photo-cross-linking degree on 2W-SME were investigated systematically. Applied Stress. Typically, choosing NW-PDLLA20-PTMEG/ 4 as an example, the stresses were set as 0.15, 0.30, and 0.45 MPa, and Thigh was fixed at 45 °C in four consecutive cycles. The strain curves (cycle 4) under different stresses are illustrated in Figure 5b, and the relevant data based on cycles 2−4 are summarized in Table 4. When a constant stress was applied, an original strain εi was created, and the εi values increased along with the increase of applied stress. This feature offers us some guidance to choose proper external stress to achieve the specific deformation. Then a cyclic temperature programming was conducted, and the reversible shape-memory effect was observed. Apparently, both εCIE and εNon‑CIE increased with the increase of applied stress, as presented in Table 4. As we know, CIE is essentially caused by the stretched strain under constant stress; higher stress generates higher stretched strain and finally leads to a higher εCIE.46 During cooling, εNon‑CIE gradually increased with the decrease of temperature until to the Tc of PTMEG, that is, before the polymer chains were fully crystalline. This situation can be explained as the temperature dependence of modulus of

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(12)

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Macromolecules Table 4. Data for 2W-SME of NW-PDLLA20-PTMEG/4 under Different Applied Stresses (Cycles 2−4, Thigh = 45 °C) applied stress (MPa)

Ract‑A(σ) (%)

Ract‑R(σ) (%)

εNon‑CIE (%)

εCIE (%)

Rrec(σ) (%)

0.15 0.30 0.45

2.02 ± 0.03 8.35 ± 0.14 13.7 ± 0.18

24.1 ± 0.4 45.4 ± 1.1 49.0 ± 0.9

−0.23 ± 0.01 1.32 ± 0.06 3.07 ± 0.10

2.25 ± 0.04 7.03 ± 0.13 10.6 ± 0.19

90.1 ± 4.4 95.1 ± 0.4 93.7 ± 1.7

εi, εNon‑CIE, and εCIE under 70 °C were much higher than those under 45 °C. Apparently, the modulus of material at 70 °C was much lower than that at 45 °C (see Figure 3), since the PDLLA chains were in an elastic state at 70 °C but in a glassy state at 45 °C. Therefore, it makes sense that a larger strain would be obtained under the same applied stress. However, it is noteworthy that the Rrec(σ) values for Thigh = 70 °C, which ranged from 62.6 to 78.6%, were much lower than those for Thigh = 45 °C. Just as mentioned above, when Thigh = 70 °C, the network only exhibited a normal netpoint structure contributed by photo-cross-links, which were not enough to maintain the initial strain due to the relatively low gel fractions constrained by the feature of photo-cross-linking reaction. When shifting Thigh to 45 °C, the 20% PDLLA became vitrified, and it served as netpoint synergizing with photo-cross-links (schematically illustrated in Figure 7). Therefore, the networks possessed very desirable Rrec(σ) which were all above 92.9% when Thigh = 45 °C. In all, the 2W-SME was successfully realized under Thigh = 45 and 70 °C, which indicated that the structure of temperature-switched netpoints could make sense to choose proper programming condition for a 2W-SME system. Photo-Cross-Linking Degree. In this part, 2W-SME of networks with different photo-cross-linking degrees under different Thighs was investigated (applied stress was fixed at 0.30 MPa). Four consecutive 2W-SME cycles were recorded and eventually illustrated as strain−time curves in Figure 8. The relevant data are available in Table 5. From Figure 8a which represents the 2W-SME when Thigh = 45 °C, no significant difference of 2W-SME between the three networks was observed. In detail, εNon‑CIE and εCIE for each network were close to each other even though the Rrec(σ) values slightly increased with the increase of photo-cross-linking degree. In this case, although both PDLLA segments and photo-crosslinks performed as netpoints, the former one played a dominant role leading to the depression of the impact of photo-crosslinking degree. It also can be proven by the fact that there is no obvious difference in E′ value at 45 °C. So only a slight increase of Rrec(σ) was observed. However, when Thigh was adjusted to 70 °C, only photo-cross-links acted as netpoints determining the 2W-SME of networks. In Figure 8b, apparently, the higher the photo-cross-linking degree was, the lower the initial strain (εi). Meanwhile, εNon‑CIE and εCIE showed a decreasing trend with the increase of photo-cross-linking degree, while Rrec(σ) showed an increasing trend overall. On one hand, a higher photo-cross-linking degree contributed higher modulus (rigidity) for networks when Thigh = 70 °C as demonstrated by DMA analysis, resulting in lower εi (with the same temperature and external stress) and eventually led to the decrease of εNon‑CIE and εCIE according to the discussions before; on the other hand, a higher photo-cross-linking degree donated more photo-cross-links which were expected to memory initial strain, so the Rrec(σ) values increased.

entropy elasticity; in other words, the temperature-related modulus of entropy elasticity decreased in accordance with the decrease of temperature, and as a result, a significant but gradual elongation was generated so as to maintain the stress.22,47 For reheating process, a slight increment in strain was observed at first due to the thermal expansion of crystalline domains before polymer chains fully melting, and subsequently, a recovery process of actuation strain was obviously found. Moreover, compared εREC with εCIE (Figure 5b), which obviously resulted in εREC > εCIE, we can conclude that εNon-CIE was partially recovered beside the residual irreversible strain. In Table 4, excellent Rrec(σ) values ranging from 90.1% to 95.1% were obtained even though different stresses were applied. As an absolute parameter of strain actuation, the values of Ract‑A(σ) increased dramatically along with the increase of applied stress, which was only related to εNon‑CIE and εCIE. However, as different external stresses generate different initial strains (εi), we prefer to utilize Ract‑R(σ), a relative magnitude involving the effect of εi, to evaluate the strain actuation. Ract‑R(σ) values also showed an increase trend along with the increase of applied stress, which indicated that a greater external stress would be beneficial to a significant strain actuation for 2W-SME system. From the view of SME in real applications, absolute elongation and contraction are more meaningful because they account for the consecutive shape deformation and recovery in a 2W-SME model merely under temperature change conditions (cooling and heating) after preprogramming. Meanwhile, both Ract‑A(σ) and Ract‑R(σ) essentially involve εNon‑CIE and εCIE, so the discussions afterward are mainly focused on absolute strain change magnitudes (εNon‑CIE, εCIE) and Rrec(σ). High Temperature (Thigh). In this part, 45 and 70 °C were defined as two Thighs and individually used in the 2W-SME cycle, which correspond to two different netpoints. DMA was employed to investigate 2W-SME conducted at each Thigh, and the typical strain curves of NW-PDLLA20-PTMEG/4 are displayed in Figure 6. The relevant data for all the three networks are listed in Table 5. From Figure 6, one can find that

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CONCLUSIONS In this work, a series of adaptable NW-PDLLA-PTMEG networks were fabricated by photo-cross-linking of the

Figure 6. Strain curves for 2W-SME of a typical sample NWPDLLA20-PTMEG/4 when varying Thigh (45 and 70 °C). G

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Macromolecules

Table 5. Data for 2W-SME of NW-PDLLA20-PTMEG Networks with Different Photo-Cross-Linking Degrees at Different Thigh (Cycles 2−4, Stress 0.30 MPa) Thigh = 45 °C

Thigh = 70 °C

sample

εNon‑CIE (%)

εCIE (%)

Rrec(σ) (%)

εNon‑CIE (%)

εCIE (%)

Rrec(σ) (%)

NW-PDLLA20-PTMEG/1 NW-PDLLA20-PTMEG/2 NW-PDLLA20-PTMEG/4

1.18 ± 0.10 1.66 ± 0.10 1.32 ± 0.06

6.80 ± 0.15 6.34 ± 0.28 7.03 ± 0.13

92.9 ± 2.9 94.1 ± 1.5 95.1 ± 0.4

21.5 ± 4.3 14.6 ± 0.3 4.80 ± 0.4

11.7 ± 1.3 10.0 ± 0.5 6.70 ± 0.3

66.3 ± 10 62.6 ± 4.5 78.6 ± 5.8

PTMEG-based copolymer which contained low content of poly(D,L-lactide) (PDLLA) segment (20%) in main chain and photosensitive anthracene groups in side chain. The photocross-linking degrees of networks were adjusted by simply varying the photoirradiation time. The CIE and MIC of PTMEG were utilized to realize 2W-SME under constant stress, and the photo-cross-links along with the PDLLA segments constituted switchable netpoints which were controlled by high temperature (Thigh). Differential scanning calorimetry (DSC) analysis revealed that all photo-cross-linked networks exhibited desirable crystallinity, and dynamic mechanical analysis (DMA) showed two distinct declines of storage modulus (E′) when temperature crossed Tm,PTMEG and Tg,PDLLA, which provided us evidence to choose 45 and 70 °C as two specific Thighs. The phenomenon of CIE was observed in a conventional 1W-SME cycle, which satisfied the requirements for 2W-SME. Then, undoubtedly, tunable 2W-SME with different reversible deform amplitudes was achieved under two different Thighs. Systematic evaluations turned out that applied stress, Thigh, and photo-cross-linking degree have great influence on the characteristics of 2W-SME. Under the same Thigh and photo-cross-linking degree, the greater the applied stress was, the higher the two strain increments (εNon‑CIE and εCIE) were. When Thigh = 45 °C, both photo-cross-links and vitrified PDLLA segment acted as netpoints, networks with different photo-cross-linking degrees exhibited reversible strain in relative small-amplitude but with very high Rrec(σ) ranging from 92.9% to 95.1% (stress: 0.30 MPa). When switching Thigh from 45 to 70 °C, only photo-cross-links acted as netpoint and the networks presented reversible strain in relative large amplitude, but Rrec(σ) decreased dramatically since the irreversible strain caused by partial εNon‑CIE was enlarged. The great influences of photo-cross-linking degree on εNon‑CIE, εCIE, and Rrec(σ) were observed especially when Thigh = 70 °C; a higher photo-cross-linking degree donated better Rrec(σ) but lower εNon‑CIE and εCIE. Because of the fact that CIE occurs at approximately −30 °C and the structure of temperatureswitched netpoints, this adaptable PTMEG-based network with tunable 2W-SME is highly expected to apply in low temperature range and the areas where different deformation amplitudes are necessary.

Figure 7. Mechanism diagram of temperature-switched netpoints at different Thigh: 45 and 70 °C.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax 86-28-85410755; Tel 86-2885410755 (K.-K.Y.). Figure 8. Strain−time curves of 2W-SME of NW-PDLLA20-PTMEG networks with different photo-cross-linking degrees: (a) Thigh = 45 °C and (b) Thigh = 70 °C.

ORCID

Ke-Ke Yang: 0000-0002-7019-6059 Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (51473096, 51421061), the International S&T Cooperation Project of Sichuan Province (2017HH0034), and the State Key Laboratory of Polymer Materials Engineering (sklpme2016-2-06).

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