A Facile Strategy To Construct PDLLA-PTMEG Network with Triple

May 9, 2016 - Shape Effect via Photo-Cross-Linking of Anthracene Groups. Hui Xie .... the solid state was utilized to construct triple-shape network i...
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A Facile Strategy To Construct PDLLA-PTMEG Network with TripleShape Effect via Photo-Cross-Linking of Anthracene Groups Hui Xie, Chuan-Ying Cheng, Lan Du, Cheng-Jie Fan, Xiao-Ying Deng, Ke-Ke Yang,* and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), College of Chemistry, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Covalently cross-linked network has been widely applied in triple-shape memory polymers (TSPs), and fabricating triple-shape memory networks with the optional shapes through a facile and fast way is highly expected in the real applications. In this study, a “preshaped and post-cross-linking” strategy has been put forward to fabricate the triple-shape networks via fast photo-cross-linking in solid state. The photoresponsive anthracene group was first employed to develop a poly(D,L-lactide)−poly(tetramethylene oxide) glycol (PDLLA-PTMEG) network via UV light irradiation. Two steps were involved in network fabrication: first, linear copolymers (AN-PDLLA-PTMEG) containing anthracene groups on the side chains with different mass ratio of PDLLA segments were synthesized, and then PDLLA-PTMEG networks (NW-PDLLA-PTMEG) were formed by 365 nm UV light irradiation under an argon atmosphere. The structures of all the precursors were determined by 1 H NMR, and all networks were evaluated by swelling tests. The results of tensile tests show that the content of PDLLA segments has a crucial effect on the mechanical performance of the materials. Differential scanning calorimetry (DSC) analysis combined with dynamic mechanical analysis (DMA) reveals that all the NW-PDLLA-PTMEG’s display two thermal transitions (Tm,PTMEG and Tg,PDLLA), which can be utilized as Ttrans to trigger triple-shape memory behavior. The cyclic thermal mechanical testing for triple-shape effects of NW-PDLLA-PTMEG, which was performed by DMA, demonstrates that the mass ratio of two segments has a great effect on the shape fixity and shape recovery. Moreover, a practical application as heat-shrinkable tube (or film) has been put forward.

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and recovery.13 For a TSMP, the reversible thermal transitions of the polymer, including glass transition temperature (Tg),14,15 melting temperature (Tm),16−19 or liquid crystalline clearing temperature (Tcl),20−22 may serve as transition temperature (Ttrans) of the molecular switch. In fact, aliphatic polyesters and polyether, including poly(D,L-lactide) (PDLLA),23−26 poly(pdioxanone) (PPDO), poly(ε-caprolactone) (PCL), poly(ethylene glycol) (PEG), and poly(tetramethylene oxide) glycol (PTMEG),17,27,28 have been widely applied in design of TSMPs in view of their desirable mechanical performance,

INTRODUCTION

Shape memory polymers (SMPs), as one of promising intelligent materials, exhibit the capacity for responding to appropriate external stimuli (heat, light, magnetic, or electricity and so on).1−5 Up to now, SMPs have been proposed for a variety of applications ranging from simple actuators to biomedical devices for holding distinctive advantages such as light weight, ease to processing, and high recovery strain compared to shape memory alloys (SMAs).6−12 Among all kinds of SMPs, thermally induced SMPs (TSMPs) have attracted significant attention as they can be easily obtained and programmed. In general, a SMP consists of netpoints and molecular switch; the former determine the permanent shape, and the latter is responsible for shape fixing © XXXX American Chemical Society

Received: February 21, 2016 Revised: April 28, 2016

A

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the influence of the composition on its thermal properties, mechanical properties, and triple-shape memory effect is addressed.

outstanding biocompatibility, suitable transition temperature, and easy tailoring. Recently, triple-shape polymers (TSPs) arouse increasing concern since they can memorize two temporary shapes in one SME cycle.13,29,30 And it is a mature strategy to create tripleshape memory polymers (TSPs) by incorporate two discrete thermal transitions into one system.31−34 Although both physically and chemically cross-linking can act as the netpoints of a SMP, the latter is the most favorite pattern to develop a TSP for its easy tailoring and better performance. In our previous works, we designed a series of co-networks and interpenetrating networks, which of course are TSPs, based on crystalline aliphatic polyesters (PPDO and PCL) and aliphatic polyethers (PEG and PTMEG).27,28,35,36 Commonly, the shape memory polymer networks can be formed through chemical cross-linking by using some couple reagents; this approach often takes quite a long time. Moreover, it is not convenient to create an optional sample especially in complicated architecture from a cross-linked network. Though some works focused on facile ways to design SMP have been reported,37,38 few of them concerned about shape memory network. Then, we attempt to apply a post-cross-linking approach on the preshaped sample in a fast way to combine the virtues of the thermoplastic and networks. Actually, photocross-linking of some photosensitive moieties under UV light irradiation can be identified as a facile and fast method to create a network.39−41 However, there are only a few reports about this kind of photo-cross-linked shape memory network.42−45 Zhou and co-workers44 prepared a triple-shape memory TSMPU, which contains a Tm-type segment (PCL) as well as a Tg-type segment (polyurethane), based on PCLU with cinnamon groups on the side chains via photochemical crosslinking reaction under λ > 280 nm UV light irradiation. However, all the experiments involved were carried out in solvent. Here, what we are concerned with is anthracene, another photoresponsive group that can photodimerize when exposure to λ > 300 nm UV light as shown in Scheme 1. It also can be utilized to develop the photo-cross-linking networks, which has been proved by many previous researches.46−50

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EXPERIMENTS AND METHODS

Materials. Poly(tetramethylene oxide) glycol (PTMEG, Mn = 2900 g mol−1) was purchased from Aldrich Co., and the D,L-lactide was purchased from Daigang Biomaterial Co. Ltd. (China). 9-Hydroxymethylanthracene was supplied by J&K (China). 1,6-Hexamethylene diisocyanate (HDI) from Alfa Aesar as well as dibutyltin dilaurate (DBTL) and stannous octoate (Sn(Oct)2, 95%) from Sigma-Aldrich were used without further purification. After being diluted with dry toluene, the Sn(Oct)2 solution was stored in glass ampules under nitrogen. 1,4-Butanediol (BD), diethanolamine (DEA), 1,2-dichloroethane (C2H4Cl2), and other solvents were provided by Kelong Reagent Corp (China). BD was dried over CaH2 and distilled under reduced pressure before use, C2H4Cl2 was dehydrated by CaH2, refluxed, and distilled before use, whereas other solvents were used as received or purified as reported before use. Synthesis of N,N-Bis(2-hydroxyethyl)-9-anthracenemethanamine (BHEAA). The photoresponsive monomer BHEAA was synthesized by two steps, as shown in Scheme 2a. The synthesis procedures are presented in the Supporting Information. Synthesis of Poly(D,L-lactide) Diol (PDLLA). PDLLA was prepared by the ring-opening polymerization (ROP) of D,L-lactide following the literature method, as shown in Scheme 2b. The synthesis procedures are presented in the Supporting Information. Synthesis of PDLLA-PTMEG Copolymer with Pendent Anthracene Groups (AN-PDLLA-PTMEG). A series of AN-modified photosensitive AN-PDLLA-PTMEG’s with different ratio of PDLLA segment were designed according to Table 1 and prepared via a two-step route (Scheme 2c): terminating the PDLLA and PTMEG diols with NCO group at first and then chain-extending with BHEAA and BD without interruption. In a typical experiment, a mixture of PDLLA (2.00 g, 0.34 mmol) and PTMEG (2.00 g, 0.69 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. The C2H4Cl2 was injected into the reaction vessel as the solvent, and the temperature was then raised to 75 °C. After PDLLA and PTMEG dissolved completely, HDI (0.33 mL, 2.06 mmol) and five drops of dibutyltin dilaurate (DBTL) were added to terminate the prepolymers. After 3 h, BHEAA (0.20 g, 0.68 mmol) and a predetermined amount of BD (0.03 mL, 0.35 mmol) were added to start the chain-extending reaction for another 4 h. Finally, the resulting solution was poured into a large amount of cold ether and hexane to give AN-PDLLA-PTMEG which was a yellow flocculent solid and dried in a vacuum oven. Fabrication of Photo-Cross-Linked PDLLA-PTMEG Networks. The mechanism for constructing the network is schematically illustrated in Scheme 2d. First, thin films of 0.12−0.14 mm of thickness were prepared by solution cast of AN-PDLLA-PTMEG. Then the crosslinked PDLLA-PTMEG networks were obtained after being irradiated with 365 nm UV light (8 W, Spectroline E-series, USA) under an argon atmosphere to eliminate the photodegradation of the polymer. Incident light intensity to the films was measured with a digital radiometer/photometer. The irradiation time was 1.5 h for each side, and the light intensity to the film was 6.5 mW cm−2. Characterization and Methods. Ultraviolet−Visible Spectrophotometer (UV−Vis). The UV spectra of thin AN-PDLLA35-PTMEG film (30 um) were recorded on a Hitachi U-1900 spectrophotometer (Varian, USA) in the wavelength range 200−600 nm. The tests were conducted at room temperature. Gel Content and Swelling Ratio. The swelling test was carried out by the following steps: all of the PDLLA-PTMEG network samples were cut into pieces, then swelled, and extracted by chloroform at room temperature for 24 h. The masses of the original samples (m0), the swelled extracted samples (ms), and the dried extracted samples (md) were recorded. The swelling ratio (S (%)) and gel content (G (%)) were calculated by the following formulas:

Scheme 1. Photodimerization Reaction of Anthracene Groups

In this work, a photo-cross-linking of the anthracene group in the solid state was utilized to construct triple-shape network in a facile strategy; PDLLA and PTMEG segments with suitable transition temperature difference are embedded in matrix to act as two molecular switches. To achieve the triple-shape memory network with well-defined architecture, a stepwise synthetic strategy was carried out: First, the linear copolymers ANPDLLA-PTMEG with pendent anthracene groups with different composition were prepared by coupling PDLLA and PTMEG together with anthracene-functionalized monomer; then, the films of linear copolymers were irradiated by 365 nm UV light to excite the photo-cross-linking reaction between anthracene groups and finally produce the target PDLLAPTMEG network. Additionally, a systematical investigation of B

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Macromolecules Scheme 2. Synthesis Routes of (a) BHEAA, (b) PDLLA, (c) AN-PDLLA-PTMEG, and (d) NW-PDLLA-PTMEG

Table 1. Composition of Linear Copolymers AN-PDLLA-PTMEG’s Obtained from 1H NMR

a

sample name

feed ratio PDLLA:PTMEG (wt:wt)

real composition PDLLA:PTMEG (wt:wt)

Mna (×104 g mol−1)

PDIa

AN-PDLLA20-PTMEG AN-PDLLA35-PTMEG AN-PDLLA50-PTMEG AN-PDLLA65-PTMEG PDLLA5800 PTMEG2900

20:80 35:65 50:50 65:35

21.9:79.1 36.6:63.4 51.9:48.1 66.8:33.2

5.36 3.60 3.55 6.48 0.95 0.60

2.48 2.06 2.00 2.66 1.45 2.05

Determined by GPC. S (%) =

ms × 100% m0

(1)

G (%) =

md × 100% m0

(2)

in Scheme 3. Typically, taking NW-PDLLA50-PTMEG as an example, the processes were carried out as follows. In step 1, a sample (shape A, ε2p(N−1), specially, original strain (εp(0)) for cycle 1) was heated to a high temperature Thigh (75 °C) and kept for 10 min before extending the sample to a specified strain (ε1d(N)). In step 2, the sample was cooled to a middle temperature Tmid (40 °C) to fix the temporary shape maintained in step 1. After the release of the external stress, the sample obtained a shape B with a strain (ε1f(N)). In step 3, similar to step 1, after being kept at Tmid, the sample was then extended to a specified strain (ε2d(N)). In step 4, after cooling the sample to a low

Shape Memory Behavior. The triple-shape memory effect (TSE) of the PDLLA-PTMEG network was measured in tension using a DMA Q800 under a controlled force mode according to the previous works. The test program in one TSE cycle is schematically illustrated C

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through a step-feed strategy (Scheme 2c): first, the PDLLA and PTMEG diols were terminated with excessive HDI ([NCO]: [OH] = 2:1) and then chain extended with photoresponsive BHEAA and BD ([NCO]:[OH] = 1:1). The AN-PDLLAPTMEG’s with different composition were referred to as ANPDLLAx-PTMEG, where the subscript x is the mass fraction of PDLLA in feed. The detailed molecular characteristics of the series of AN-PDLLA-PTMEG are summarized in Table 1. Figure 1c shows the 1H NMR spectrum of the AN-PDLLA65PTMEG. The characteristic peak of −CH2 in the PTMEG repeat units appears at 3.41 ppm (m) and 1.62 ppm (n), and the weak typical aromatic resonances in the range of 7.6−8.4 ppm should be assigned to the aromatic protons from anthracene rings. Compared with PDLLA diol, PDLLA segments in the AN-PDLLA-PTMEG exhibited the same chemical shifts, which appear at 5.19 ppm (h) and 1.57 ppm (i). Also, the real composition of all the AN-PDLLA-PTMEG’s can be approximately calculated by determining the peak intensities of repeat units in the PDLLA and PTMEG segments according to the individual 1H NMR spectrum. It was found that the real composition of the samples coincided with the feed ratio quite well. All these results indicate that AN-PDLLAPTMEG’s have been prepared successfully. Preparation of PDLLA-PTMEG Network (NW-PDLLAPTMEG). The curing of NW-PDLLA-PTMEG was realized via photo-cross-linking of anthracene groups in thin AN-PDLLAPTMEG films by using 365 nm UV light irradiation under an argon atmosphere. To confirm the photo-cross-linking reaction of anthracene moieties in film, thin films of AN-PDLLA35PTMEG were selected as examples, and a UV−vis spectrophotometer was employed. As seen from Figure 2, the anthracene group shows a maximum absorption around 370 nm, and the intensity of the absorption peak decreased with the increasing of irradiation time under 365 nm UV light. The result demonstrates that the anthracene groups in the film underwent the photo-cross-linking reaction, and the photoconversion of anthracene moieties to dimers in film was calculated up to 62.1 ± 8.20% by determining the changes of the maximum absorption of anthracene groups at 0 and 1.5 h. In order to further confirm the formation of NW-PDLLA-PTMEG, the swelling tests were carried out. The swelling ratio (S) and gel content (G) of the networks are listed in Table 2. In detail, the samples show the S value ranged from 1170% to 1800% and the G value ranged from 22.6% to 40.0%. Limited by efficiency of photoreaction, especially in bulk, the G value of the networks was much lower than that of the networks produced through chemically cross-linking by using a couple reagent such as HDI in our previous works.35,36 But the photo-cross-links are still adequate to be acted as the netpoints for the material in a triple-shape programming and recovery procedure, and this view has been confirmed by the following investigation of shape memory effect. Thermal Behavior. From the view of molecular design, in this work, the glass transition temperature of PDLLA (Tg,PDLLA) and the melting temperature of PTMEG (Tm,PTMEG) serve as transition temperature (Ttrans) for a triple-shape material. Therefore, a full understanding of its thermal behavior is very important in both the program designing for the shape memory effect and the optimization of the shape memory performance. Here, DSC and DMA were employed to investigate the thermal behavior of AN(NW)-PDLLA-PTMEG. First, the effect of photo-cross-link on the crystallization behavior of PTMEG segment was concerned. Figure 3 shows

Scheme 3. Schematic Diagrams of the Test Processes of TSE

temperature Tlow (−20 °C), also a shape C with a strain (ε2f(N)) can be obtained upon releasing the stress. In step 5, when reheating the sample to Tmid, it returned to its shape B with a residual strain (ε1p(N)), and then when further heated to Thigh, its original shape recovered with a strain (ε2p(N)). Five TSE cycles (from step 1 to step 5) were examined. The shape fixity ratio Rf and shape recovery ratio Rr were calculated according to eqs 3−6.51

R f,A − B =

εf(1 N ) − εp(2 N − 1) 1 2 εd( N ) − εp(N − 1)

R f,B − C =

εf(2N ) εd(2 N )

R r,C − B =

εf(2N ) εf(2N )

R r,C − A =

2 εf(2N ) − εP( N) εf(2N ) − εp(2 N − 1)

− − − −

× 100% (3)

εf(1 N ) εf(1 N )

× 100%

1 εP( N) εf(1 N )

× 100%

(4)

(5) × 100% (6)

Here, for cycle 1, ε2p(N−1) is equal to its original strain (εp(0)). More characterization and methods are available in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis of PDLLA. PDLLA diol was synthesized by the ROP of D,L-LA monomer, using BD as an initiator (Scheme 2b), and the structure of resultant PDLLA was confirmed by the 1H NMR spectrum (Figure 1b). The characteristic chemical shifts 5.17 ppm (h) and 1.59 ppm (i) are related to the −CH and the −CH3 of the PLA repeat units, respectively, and the chemical shifts 4.39 ppm (k) and 1.41 ppm (j) can be attributed to the corresponding protons in the terminal group of PDLLA. Meanwhile, the signals that appear at 1.70 ppm (f) and 4.18 ppm (g) are the characteristic peaks of BD. Based on these assignments, the molecular weights (Mn) of the PDLLA can be calculated as follows: I M n = h × 2 × 72 + 2 × 72 + 90 Ig (7) In the formula, Ih and Ig are the areas of peaks h and g, respectively, 72 is the molar mass of a LA repeat unit, and 90 is the molar mass of the BD molecule. According to the above formula, the Mn of PDLLA was determined as 5800 g mol−1. GPC was also employed to determine the Mn of PDLLA, which is listed in Table 1. Preparation of AN-PDLLA-PTMEG. The AN-PDLLAPTMEG’s with pendent anthracene groups were synthesized D

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Figure 1. 1H NMR spectra of (a) photoresponsive monomer BHEAA, (b) PDLLA diol, and (c) linear copolymer AN-PDLLA65-PTMEG.

crystalline enthalpies (ΔHc) and melting enthalpies (ΔHm) of PTMEG segments were decreased after photo-cross-linking. As for the sample containing 65% of PDLLA, ΔHc and ΔHm were

the DSC curves of two typical samples, AN-PDLLA35-PTMEG and AN-PDLLA65-PTMEG, before and after its photo- crosslinking. For the sample containing 35% of PDLLA, the E

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Figure 2. UV−vis spectra of AN-PDLLA35-PTMEG film which are obtained at different irradiation times (top down: 0, 0.5, 1.0, and 1.5 h) with 365 nm UV light.

Table 2. Swelling Test Results of NW-PDLLA-PTMEG’S sample name NW-PDLLA20-PTMEG NW-PDLLA35-PTMEG NW-PDLLA50-PTMEG NW-PDLLA65-PTMEG

swell (%) 1775 1170 1173 1801

± ± ± ±

139 62.0 27.0 92.0

gel (%) 40.0 22.9 22.6 33.3

± ± ± ±

1.5 1.8 1.1 1.9

Figure 4. DSC curves of the NW-PDLLA-PTMEG with different composition: (a) cooling scan and (b) heating scan. Heating and cooling rate: 10 °C min−1.

Table 3. Relevant Data Recorded from DSC and Partially from DMA PTMEG sample name PDLLA5800 NWPDLLA20PTMEG NWPDLLA35PTMEG NWPDLLA50PTMEG NWPDLLA65PTMEG

Figure 3. DSC curves of samples before and after irradiation (samples containing 35% and 65% PDLLA segments act as representative examples).

sharply decreased after irradiation, especially in the cooling scan; it seems no crystalline exothermic peak of PTMEG. These results showed that the cross-links do limit the ability of chain motion, which should result in the decrease of crystallinity of segments compared with the corresponding linear polymers. Additionally, the effect of the composition is also distinct. So, a comprehensive research focused on this factor was conducted. Figure 4 illustrates the DSC traces of the networks with different composition in the cooling scan (a) and heating scan (b). For a clearer view, the relevant thermal parameters and their corresponding values are summarized in Table 3. Varying the weight content of PDLLA from 20 to 65 wt %, one can find that the melting temperature (Tm) of PTMEG decreased from 26.0 to 19.3 °C and the relevant melting enthalpies (ΔHm) ranged from 32.5 to 8.46 J g−1. At the same time, the crystalline exothermic peaks of PTMEG declined, especially for the

PDLLA

Tg (°C)

Tcb

(°C)

ΔHc (J g−1)

Tm (°C)

ΔHm (J g−1)

−c −56.1

− −29.0

− 21.5

− 26.0

−57.4

−24.0

18.3

−59.7

−28.0

−67.4

−

a

b

Tga (°C)

Tgb (°C)

− 32.5

− −

47.8 −

25.8

26.5

67.3

−

10.5

24.4

15.7

69.1

48.5

−

19.3

8.46

69.7

48.3

a

Measured by DMA. bRecorded from DSC during second cooling run and second heating run. c(−) Does not exist or is not clear.

sample NW-PDLLA65-PTMEG which contained lowest PTMEG segments, the exothermic peak of PTMEG was almost invisible. Accordingly, the relevant crystalline enthalpies (ΔHc) decreased apparently. From Table 3, we also found that the glass temperature of PDLLA is higher than the crystalline temperature of PTMEG, so it is easy to understand that the incorporation of PDLLA segment into the networks markedly restrained the crystalline of PTMEG segment, because it not only destroyed the regularity of the PTMEG polymer chains F

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NW-PDLLA20-PTMEG was not obvious due to the low content of PDLLA. In Figure 5b, two obvious drops in E′ occurred when the temperature crossed the Tm,PTMEG and Tg,PDLLA. In detail, when T was below Tg,PTMEG, there was no significant difference in E′ between all the samples because all the polymer chains had been frozen under this condition. When T rose above the Tg,PTMEG but still below the Tg,PDLLA, the sample contained higher PDLLA segment keep at higher E′ value because the PDLLA segment was in glassy state. When T rose above the Tg,PDLLA further, E′ decreased due to the crosslinking structure even all the polymer chains can move freely. Generally, a great distinction in storage modulus below and above the transition temperature (Ttrans) is the most essential property to afford the shape memory effect of the materials, so we are highly concerned the difference in E′ of the target networks below and above the Tm,PTMEG and Tg,PDLLA. The Tm,PTMEG and Tg,PDLLA are around 25 and 70 °C, respectively, so four typical modulus E′10 °C, E′40 °C, E′65 °C, and E′75 °C are noted, and the storage modulus ratios (E′10 °C/E′40 °C and E′65 °C/E′75 °C) are calculated accordingly (Table 4). Here, it is clearly seen that all samples display two plateaus corresponding to Tm,PTMEG and Tg,PDLLA, and the decrease in amplitude in E′ below and above both the Ttranss is highly dependent on its composition. According to the result of DSC analysis, increasing the PTMEG segment will enhance the crystallization of PTMEG, and consequently the polymers show higher E′10 °C/E′40 °C value, and on the other hand, increasing the content of PDLLA segment will result in higher E′65 °C /E′75 °C value. On the basis of the distinction in E′ below and above these two Ttrans, we can predicate that the NW-PDLLAPTMEG possess of an ideal thermal mechanical feature to program the TSE. Static Tensile Properties. As a kind of applied shape memory material, the mechanical properties of the PDLLAPTMEG networks also should be taken into account. In this part, static tensile testing was carried out. The engineering stress−strain curves of corresponding networks (NW-PDLLAPTMEG) are displayed in Figure 6, and the relevant Young’s modulus (E), tensile strength (σm), and elongation at break (εb) are summarized in Table 5. In Table 5, the fact that the weight content of PDLLA segments (PDLLA wt %) has a crucial effect on mechanical properties can be clearly observed. In detail, E increased from 4.58 to 433 MPa when PDLLA wt % increased from 20% to 65%. This phenomenon also found in σm value, which increased from 5.37 to 13.2 MPa in an overall trend with PDLLA wt % increasing. In Figure 6, the networks with lower PDLLA wt % (20% and 35%) exhibited elastomerlike tensile behavior, while the other samples with higher PDLLA wt % (50% and 65%) behaved as plastics. As we known, pure PDLLA possesses much higher strength and modulus than those of PTMEG, so it makes sense that the NW-PDLLA-PTMEG networks with higher PDLLA wt % exhibit higher E and σm.

seriously but also the PDLLA domains in glass state sharply restrict the chain movement of PTMEG segments. Generally, we can obtain the Tg of PTMEG and PDLLA segment from DSC analysis; however, it is not the most sensitive method. When one segment in network has low content, the corresponding Tg can hardly be detected. Nevertheless, the analysis of network performed by DMA hereafter will bring more information on this parameter. Dynamic Mechanical Analysis. Figure 5 illustrates the typical curves of loss factor (tan δ) (a) and storage modulus

Figure 5. DMA curves of NW-PDLLA-PTMEG: (a) loss factor (tan δ) and (b) storage modulus E′.

(E′) (b) versus temperature (T) recorded by DMA, and the relevant data are listed in Table 3. Two peaks in tan δ curve (Figure 5a) were observed: the peak in low temperature area, which is attributed to the Tg of PTMEG (Tg,PTMEG) segment, decreased from −56.1 to −67.4 °C along with the decreasing of the content of PTMEG segment; however, the other peak around 67.3−69.7 °C, which assign to the Tg of PDLLA segments (Tg,PDLLA), has no obvious change with the variation of PDLLA content. Here, it should be noted that the Tg,PDLLA of

Table 4. Data for the NW-PDLLA-PTMEG Networks Resulting from the DMA Analysis storage modulus E′ (MPa) sample

10 °C

40 °C

65 °C

75 °C

E′10 °C/E′40 °C

E′65 °C/E′75 °C

NW-PDLLA20-PTMEG NW-PDLLA35-PTMEG NW-PDLLA50-PTMEG NW-PDLLA65-PTMEG

99.0 286 439 770

4.36 79.0 196 504

2.16 11.8 37.6 92.6

1.37 1.73 3.65 5.32

22.7 3.62 2.24 1.53

1.58 6.82 10.3 17.4

G

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Figure 7. TSE of NW-PDLLA50-PTMEG recorded from cyclic thermal mechanical test. Figure 6. Typical engineering stress−strain curves for NW-PDLLAPTMEG with different PDLLA wt %.

higher content of PDLLA segments dominates better shape fixing. As for Rf,B−C, the good results benefit from the outstanding crystallization of PTMEG segments without significant differences. During the shape recovery program, increasing the PDLLA content, that is decreasing the content of PTMEG segments, the Rr of shape C−B (Rr,C−B) will deceased from 98.1% to 54.3%, and Rr,C−A were all above 95.0% without significant change. The shape recovery from shape C−B relies on the chain motion of PTMEG segments, which was strongly affected by the PDLLA segments. As a consequence of that Rr,C−B decreased with the content of PDLLA segments increasing. The excellent performance in Rr,C−A, which is determined by the cross-linked netpoints, reveals that the photo-cross-linking of anthracene groups in solid state are strong enough to achieve excellent overall recovery performance. Overall, the sample NW-PDLLA50 -PTMEG with equivalent segment content exhibits optimum performance in TSE. The TSE of NW-PDLLA50-PTMEG is also testified visually as Figure 8. The strip sample in its original shape (S0) was heated at 75 °C for 10 min and then deformed; a temporary shape (S1) was obtained after being moved to a 40 °C oven and released the external force. Then, deformed shape S1 at 40 °C, subsequently cooled to −20 °C, and then removed stress, after which another temporary shape (S2) was constructed. Reheating the sample to 40 and 75 °C recovered the shape S1 and shape S0, respectively. Qualitatively, under these thermal conditions, the sample displayed an excellent triple-shape memory property. Thanks to the excellent triple-shape memory properties and desirable mechanical properties of NW-PDLLA-PTMEG, it shows potential applications such as heat-shrinkable tube (or film) for packaging or connection use. The model, including the procedure and description, is displayed in Figure 9. Some movies are available in the Supporting Information.

Table 5. Relevant Data (Young’s Modulus (E), Tensile Strength (σm), and Elongation at Break (εb)) of PDLLAPTMEG Networks Obtained from Static Tensile Testing sample name NW-PDLLA20-PTMEG NW-PDLLA35-PTMEG NW-PDLLA50-PTMEG NW-PDLLA65-PTMEG

E (MPa) 4.58 51.0 189 433

± ± ± ±

0.14 12.1 10.8 12.8

σm (MPa) 7.42 5.37 13.0 13.2

± ± ± ±

0.32 0.45 1.55 2.35

εb (%) 627 436 551 354

± ± ± ±

28.1 12.1 36.7 82.5

Shape Memory Behavior. From the view of the mechanism of TSE, the PTMEG and PDLLA segment have separate Ttrans (Tm,PTMEG and Tg,PDLLA, respectively) with the temperature difference more than 20 °C, which allow them dominant two temporary shapes without obvious mutual interference; meanwhile, the cross-linked points formed by [4 + 4] cycloaddition between anthracene groups bring a strong netpoint for the system. We investigated the triple-shape memory properties of all the samples thoroughly using the DMA Q800 under a controlled-force mode. In this section, NW-PDLLA50-PTMEG was chosen as the typical sample. And the programming temperatures were chosen as follows: Thigh = 75 °C (Tg,PDLLA + 5 °C), which ensures that both PDLLA segments and PTMEG segments have a good chain mobility; Tmid = 40 °C (Tm,PTMEG + 15 °C), which ensures the PDLLA segments in a glassy state while the PTMEG segment in a molten state; Tlow = −20 °C, at which temperature both PDLLA segments and PTMEG segments are frozen. The testing procedure was performed as described in the Experiments section. Figure 7 shows the results of the triple-shape programmed cyclic thermal mechanical tests for the typical sample, and the corresponding data for all the samples are summarized in Table 6. We determined the Rf and Rr based on the results of cycles 3−5, which can reflect the SME of the material to the greatest extent because the results of former one or two cycles may influenced by the residual stress from the processing history of the samples even though we had processed to eliminate thermal history before the tests. From Table 6, one can find that the Rf and Rr were strongly influenced by the composition of NWPDLLA-PTMEG. In the shape fixing program, the Rf of shape A−B (Rf,A−B) increased from 44.7% to 98.5% as the PDLLA segments content increased from 20% to 65%, while Rf,B−C were all above 97.4% with no obvious change. Rf,A−B is determined by the glassy transition of PDLLA segments, so

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CONCLUSIONS In present work, a facile approach to construct triple-shape memory networks based on PDLLA and PTMEG via photocross-linking of anthracene groups in solid state was well established, which can be further expanded to “preshaped and post-cross-linking” fabricating of shape memory networks with complicated shapes. First, the functionalized monomer BHEAA and PDLLA prepolymer were well prepared, and then PDLLA and PTMEG prepolymers were terminated with a NCO group by HDI in solvent and then chain extended with BHEAA and H

DOI: 10.1021/acs.macromol.6b00382 Macromolecules XXXX, XXX, XXX−XXX

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Table 6. Triple-Shape Memory Performance of the NW-PDLLA-PTMEG Networks Recorded from Cyclic Thermal Mechanical Tests Carried on DMA sample name NW-PDLLA20-PTMEG NW-PDLLA35-PTMEG NW-PDLLA50-PTMEG NW-PDLLA65-PTMEG

Rf,A→Ba (%) 44.7 79.2 91.7 98.5

± ± ± ±

1.2 3.6 0.5 0.2

Rf,B→Ca (%) 98.9 97.4 98.4 97.5

± ± ± ±

Rr,C→Ba (%)

0.1 0.4 0.1 1.2

98.1 94.0 83.2 54.3

± ± ± ±

1.3 0.2 0.3 5.9

Rr,C→Aa (%)

Rr,tot,C→Ab (%)

± ± ± ±

89.7 84.8 90.8 92.5

95.9 95.0 97.4 96.4

2.0 1.6 1.5 2.9

Rf and Rr are based on the results of cycles 3−5. bRr,tot,C→A = (ε2f(5) − ε2P(5))/(ε2f(5) − ε2p(2)) × 100% represents the total shape recovery ratio during cycles 3−5.

a

employed to investigate the triple-shape memory effect. The results show that the fixity ratio of shape A−B which is determined by the glassy transition of PDLLA segments was strongly influenced by the PDLLA segments content. Moreover, this factor also affected the recovery ratio of shape C−B which is relies on the chain motion of PTMEG segments because the PDLLA segments sharply restricted the movements of PTMEG chains. From the view of the composition of all the samples, the networks containing moderate PDLLA segments content, in detail, 35% and 50%, have a better shape memory capability than the others. A typical sample of NW-PDLLA50PTMEG exhibits excellent triple-shape memory properties with the average R of cycles 3−5: Rf,A−B = 91.7 ± 0.5%, Rf,B−C = 98.4 ± 0.1%, Rr,C−B = 83.2 ± 0.3%, and Rr,C−A = 97.4 ± 1.5%. In terms of the excellent triple-shape memory properties, a potential application as heat-shrinkable tube (or film) for packaging or connection use has been prospected. More applications required complex three-dimensional shapes can be achieved by combining this photo-cross-linking facile fabrication strategy with some advanced polymer processing technologies.

Figure 8. Images of a TSE cycle of the typical sample NW-PDLLA50PTMEG.

BD; finally, the target network was obtained after 365 nm UV irradiation. The structures of all the precursors were determined by 1H NMR, and the networks were evaluated by the swelling test. The results of DSC analysis reveal that the cross-links and the composition of NW-PDLLA-PTMEG have an effect on the crystallization behavior of PTMEG segments. Two discrete thermal transition Tm,PTMEG and Tg,PDLLA were confirmed by DSC and DMA, which can endow the networks with triple-shape memory properties. The tensile tests reveal that PDLLA wt % has a crucial effect on mechanical properties. Cyclic thermal mechanical tests carried on DMA were

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00382.

Figure 9. Demonstration of a practical application of the triple-shape memory PDLLA-PTMEG networks as heat-shrinkable tube (or film). (a) Deforming process: stretched at 75 °C (I to II) and 40 °C (II to III). (b) Shrink process at 40 °C. Bottom: a SMP cycle making from the “strip” sample (III). Middle: compared to a glass tube (no. 1, diameter: ∼20 mm). Top: the SMP cycle sticks to the no. 1 tube; see Movie S1 for a panoramic view. (c) Shrink process at 75 °C. Top: removed from the bigger tube (no. 1) carefully. Middle: compared to a smaller tube (no. 2, diameter: ∼16 mm). Bottom: the SMP cycle fits snugly to the no. 2 tube; see Movie S2 for a panoramic view. I

DOI: 10.1021/acs.macromol.6b00382 Macromolecules XXXX, XXX, XXX−XXX

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Synthesis routes of BHEAA and PDLLA; Not listed characterization and methods (PDF) Movie S1 of the heat-shrinkable behavior after being treated at 40 °C (panoramic view) (AVI) Movie S2 of the heat-shrinkable behavior after being treated at 75 °C (panoramic view) (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Fax 86-28-85410755; Tel 86-2885410755 (K.-K.Y.). *E-mail [email protected]; Fax 86-28-85410259; Tel 86-2885410259 (Y.-Z. W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Science Foundation of China (51473096, 51421061), and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026).

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