Ionically Cross-Linked Shape Memory Polypropylene - ACS Publications

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Ionically Cross-Linked Shape Memory Polypropylene Thomas Raidt, Robin Hoeher, Monika Meuris, Frank Katzenberg, and Joerg C. Tiller* Biomaterials and Polymer Science, Department of Biochemical and Chemical Engineering, TU Dortmund, 44221 Dortmund, Germany S Supporting Information *

ABSTRACT: An ionically cross-linked syndiotactic polypropylene (ix-sPP) was synthesized by subsequent grafting of maleic anhydride (MA) to the polymer followed by compounding with ZnO. The polymer network was investigated by X-ray scattering, transmission electron microscopy, and various thermal and mechanical analyses. The optimized polymer network with 1 wt % MA grafting and 20 wt % ZnO exhibits a crystal melting temperature of 125 °C and rubber elastic behavior up to 203 °C and becomes a viscous polymer melt at higher temperatures. This process is fully reversible. Further, ix-sPP is an exceptionally stable ionic polymer network that matches the stability of the respective covalently cross-linked polymer in terms of shape memory properties. Additionally, the ionic cross-linking affords thermoplastic processability and shape memory assisted self-healing.

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INTRODUCTION

Materials. Syndiotactic polypropylene (sPP, EOD 96-30, Mw = 180 000 g mol−1, PDI = 2.5, concentration of rrrr pentad = 78%)33 was kindly provided by FINA Oil & Chemical Company. Dicumyl peroxide (DCP, 98% purity; Sigma-Aldrich), maleic anhydride (MA, 99% purity; Fluka), zinc oxide (ZnO, 99.5% purity; VWR International), and stearic acid (StAc, 97% purity; Merck) were used for grafting and cross-linking without further purification. Sample Preparation. Ionically cross-linked sPP (ix-sPP) was prepared by reactive extrusion, followed by heat curing.31 For this purpose sPP was mixed with 2 wt % DCP and 4 wt % MA in a twin-screw extruder (DSM Research) operated at 120 rpm and 200 °C for 120 s.31,34 In accordance with literature the grafting efficiency is about 1 wt % MA.34 Subsequently, 5, 10, 15, 20, and 25 wt % ZnO and 1 wt % StAc were added and mixed for further 240 s.35−37 After extrusion the mixture was compression molded to a sheet using a heating press and a mold with the dimensions of l = 60 mm, w = 30 mm, and d = 1 mm by applying a force of 80 kN at 220 °C for 20 min under exclusion of air. The formed sheet was cut into suitable sample geometries with regard to the different analysis methods. Dynamic Mechanical Analysis. Classical modulus versus temperature plots were recorded in order to determine if the sPP samples were successfully ionically cross-linked as well as the temperature at which the ionic bonds dissolve. To this end, samples were cut into pieces of 30 mm × 2 mm × 1 mm (length × width × depth), mounted to a film tension clamp of a DMA 2980 (TA Instruments, Inc.), and heated from 50 to 250 °C using a temperature ramp of 5 K min−1, a frequency of 1 Hz, a preload force of 0.01 N, and an amplitude of 10 μm. A thermoplastic ionic network was assumed to be formed when the Young’s modulus first remains constant above the melting temperature but decreases rapidly when the ionic bonds dissolve at higher temperatures.

Polymer networks that contain thermally degradable netpoints distinguish themselves by thermal reprocessability (e.g., by extrusion, injection molding, etc.) and exhibit exceptional effects like self-healing.1−5 Different approaches have been reported that use hydrogen bonds,1,2,6,7 ionical bonds,8−11 and reversible covalent bonds12−18 as netpoints, or copolymers with hard and soft segments,19−22 homopolymers with extremely high molecular weights,23−25 where the hard segments, or entanglements, respectively, act as netpoints. Only a few examples are known that combine this approach with shape memory polymers.26 Networks with hydrogen or ionic cross-links, hard segment stabilized copolymers, and entanglement networks suffer from poor recovery ratios of less than 90%. A more efficient approach was demonstrated by Defize et al.,27 who created a network that combines excellent shape memory properties (Rr = 100%, εstored = 180%) with thermal reprocessability by covalent bonding of star-shaped, furan and maleimide end-functionalized poly(ε-caprolactones) (PCL) using thermoreversible Diels− Alder (DA) adducts.28 However, this polymer network has to be tempered for 72 h at 65 °C after each plasticizing to recover the network structure. A faster approach for obtaining a thermally reprocessable shape memory polymer would be the use of thermoreversible ionic netpoints.29,30 The goal of this study is to design a thermoreversible ionically cross-linked SMP with excellent shape recovery and fast network re-formation after thermal reprocessing. To this end, we ionically cross-linked syndiotactic polypropylene (ix-sPP) and compared its properties to that of previously reported covalently crosslinked (x-sPP).31,32 © XXXX American Chemical Society

EXPERIMENTAL SECTION

Received: June 29, 2016 Revised: September 2, 2016

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DOI: 10.1021/acs.macromol.6b01387 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Unit Cell Parameters of Syndiotactic Polypropylene56 form

conformation

lattice system

a [nm]

b [nm]

c [nm]

form I49−53 form II54,55 mesomorph46−48

(ttgg)n (ttgg)n tttt

orthorhombic orthorhombic pseudohexagonal

1.450 1.450

1.120 0.560

0.740 0.740 0.505

Determination of Net Chain Molecular Weight. The Mooney− Rivlin equation (eq 1) for uniaxial stress was used to obtain the net chain molecular weight Mc of the cross-linked polypropylenes.38−40 ⎛ 2C 2 ⎞⎛ 1⎞ ⎟⎜ λ − ⎟ σN = ⎜2C1 + ⎝ ⎠ ⎝ λ λ2 ⎠

constant ambient temperature was needed. Accumulation times between 60 and 1800 s per frame were used. The literature-known unit cell parameters of the crystal modifications of sPP are listed in Table 1. Figure 1 exemplarily shows the WAXS patterns of stretched x-sPP

(1)

Here, 2C1 is connected with the shear modulus G (eq 2) and thus responsible for the description of the influence of covalent network, while the coefficient 2C2 takes entanglements, distribution of chain length, and free chain tails into account. According to the rubber elasticity theory for a covalent network with a functionality of network nodes f = 4, the correlation between shear modulus and net chain molecular weight is as follows, while Flory’s loose-end-correction is taken into account:41,42

⎛ 1 2 ⎞ 2C1 = G = ρRT ⎜ − ⎟ Mn ⎠ ⎝ Mc

Figure 1. WAXS patterns of stretched ix-sPP samples, crystallized in (a) disordered mesomorphic trans-planar and (b) helical (form I/II) crystal modification.

(2)

Here, T is the absolute temperature, ρ the polymer density, R the gas constant (8.314 J mol−1 K−1), and Mn the number-average of the molecular weight of the initial polymer. The melt density of the PP network at 160 °C was calculated using the Taid equation to 0.77 g cm−3.43,44 The samples (dimensions: l = 20 mm, w = 3 mm, d = 1 mm) were clamped into a DMA 2980 using the film tension clamp, and an increment force of 0.01 N was applied for 20 min at a constant temperature of 160 °C to achieve a constant value of λ for the corresponding force. This procedure was repeated until a strain of 400% was reached, and λ was recorded in dependence on σN after each 20 min. In order to determine the coefficients 2C1 and 2C2, the Mooney−Rivlin equation (eq 1) was converted to a linear relationship between the reduced stress σ* = σN(λ − λ−2)−1 and the reciprocal stretch ratio λ−1 (eq 3).

σ * = 2C1 +

2C 2 λ

samples, crystallized in (a) disordered mesomorphic trans-planar46−48 and (b) helical (form I49−53/II54,55) crystal modification.56 In order to detect meridional reflections, the samples were tilted by an angle of 17.7° for WAXS analysis. Mechanical Analysis. Classical stress−strain plots were recorded using a tensile tester 3343 (Instron, Inc.). To this end, samples were cut into pieces of 60 mm × 4 mm × 1 mm (length × width × depth), mounted to the clamp, and stretched with 250% s−1 to εprog,cold = 300%. Subsequently, the strain was reduced until stress becomes zero. Since the strain measured by crosshead displacement was inaccurate, marks on the samples were used for strain correction. Programming Procedure and Determination of Shape Memory Parameters. Thermomechanical cycles were carried out for determination of the shape memory parameters of ionic cross-linked sPP samples (dimensions: l = 30 mm, w = 3 mm, d = 1 mm) using a custom-made stretching apparatus operated in strain-controlled mode.57 Samples were hot- as well as cold-programmed. For hotprogramming ix-sPP samples were heated to a temperature of 160 °C, subsequently stretched to programming strain εprog,hot (which is 95% of the predetermined fracture strain) using a strain rate of 250% s−1, and cooled to 20 °C. Thereafter, the strain of the samples was decreased until the stress becomes zero to determine the value of the fixed strain εfix,hot (temporary shape). Subsequently, the samples were heated again to 160 °C in order to melt the temporary shape stabilizing crystals before the clamps of the stretching apparatus were set back to the original distance of 0% strain. The samples were kept at this temperature for 5 min to ensure that the retraction process took place completely. For coldprogramming ix-sPP samples were quenched amorphously first. To this end, the samples were heated to a temperature of 160 °C and quenched in iced water. Subsequently, the specimens were stretched at room temperature to a programming strain εprog,cold using a strain rate of 250% s−1. Thereafter, the strain of the samples was decreased until the stress becomes zero to determine the value of the fixed strain εfix,cold. Subsequently, the samples were heated again to 160 °C before the clamps of the stretching apparatus were set back to the original distance of 0% strain. The samples were kept at this temperature for 5 min to ensure that the retraction process took place completely. The strains εprog, εfix, and εperm related to the initial length of the sample and were controlled by observing marks on each sample. The accuracy of the length measurement was ±0.5 mm. The fixity and recovery ratios were obtained by using the following equations:42,57

(3)

By means of linear regression of the corresponding legs of the Mooney plots for elongations of 1.5 < λ < 2.5, 2C1 was obtained at the ordinate intercept in order to calculate Mc. The value of 2C2, which can be related to the number of the elastically effective trapped entanglements, was determined from the slope.42,45 Transmission Electron Microscopy. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) investigations were carried out on a Philips CM200 transmission electron microscope operated at 200 kV. Ultrathin sections of the ionic cross-linked sPP were prepared by usage of an ultramicrotome Ultracut UCT (Leica Microsystems GmbH) at 77 K. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) experiments were conducted on a DSC 2910 (TA Instruments, Inc.). All measurements were carried out using a constant heating and cooling rate of 10 K min−1 within a temperature range of −20 to 200 °C. Sample sizes were 10 ± 1 mg. The maximum rate of melting and crystallization were used to define the melting temperature Tm and the recrystallization temperature Tc, respectively. The glass transition temperature Tg was determined at the inflection point of the baseline shift. Wide-Angle X-ray Scattering. Wide-angle X-ray scattering (WAXS) patterns were recorded using a VANTEC-2000 detector and a micro focus X-ray source (IμS, Incoatec GmbH) with Cu-anode and integrated Montel Optic operated at 50 kV and 0.600 mA (Bruker Nanostar). The X-ray wavelength was 1.5406 Å. Al2O3 standard was used for calibration. Sample-to-detector distances of 4.60 and 13.25 cm were used. A distance of 13.25 cm was used whenever a defined or B


Article

Macromolecules R f (N ) =

R r(N ) =

εfix (N ) εprog(N )

(4)

εprog(N ) − εperm(N ) εprog(N ) − εperm(N − 1)

(5)

The stored strain for each cycle εstored(N) was calculated as follows. The permanent length of each sample after a cycle was used as initial length for the next cycle. For this reason, the stored strain εstored(N) results in42,57 εstored(N ) =

lfix(N ) − lperm(N ) lperm(N − 1)

(6)

Figure 2. ATR-FTIR analysis of the initial sPP, the initial sPP with 20 wt % ZnO and 1 wt % StAc (sPP/ZnO/StAc), the grafted sPP (sPP with anhydride side groups; sPP-g-MA), and the grafted and ZnO blended sPP (sPP with zinc carboxylate side groups; sPP-g-MA/ZnO/StAc).

In this equation, lfix(N) stands for the fixed length of the specimen in the Nth cycle and lperm(N − 1) and lperm(N) stand for the permanent length of the sample in cycle N − 1 and N, respectively. Determination of Trigger Temperature and Retraction Process. Trigger temperature Ttrig and the retraction process were obtained by using a thermal mechanical analyzer TMA 2940 (TA Instruments, Inc.). For this purpose, the penetration probe was used with a preload force of 0.01 N to measure the thickness increase during retraction of cold-programmed ix-sPP samples while the temperature was increased with 1, 10, and 100 K min−1. The average trigger temperatures were determined at the inflection point of the corresponding plots of thickness increase against temperature.42,57 ATR-FTIR. ATR-FTIR analyses were carried out with Bruker Alpha spectrometer.

carboxylate originating from partially hydrolyzed anhydrides are fully converted into the respective zinc salt. In order to check if this successful zinc salt formation leads to ionic cross-linking, classical Young’s modulus versus temperature plots were recorded using a dynamic mechanical analyzer (DMA). The samples were assumed to be ionically cross-linked when the Young’s modulus first does not change significantly above the melting temperature but decreases rapidly when the ionic clusters dissolve at higher temperatures (see Figure 3).31,42

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RESULTS AND DISCUSSION In this study we followed an ionic cross-linking strategy for syndiotactic polypropylene (sPP) to obtain a thermally reprocessable shape memory material. In order to compare the properties of ionically cross-linked syndiotactic polypropylene (ix-sPP) with the previously reported covalently cross-linked analogue, x-sPP,31,32 we chose the same sPP as used in previous works with a medium stereoregularity of probably 78% (rrrrpentads) and low crystal melting temperature of 125 °C.33 Zinc oxide maleic anhydride based thermoreversible ionic cross-linking was used for network formation35−37 since this reaction results in thermoreversible netpoints, referred to as ionic clusters, that allow a thermal reprocessing above a temperature of at least 190 °C, as reported by Antony et al.35 and Airinei et al.36 for ionically cross-linked maleic acid anhydride grafted ethylene propylene diene−elastomer (EPDM-g-MA). Ionically cross-linked syndiotactic polypropylene samples were prepared by reactive extrusion and compression molding as described in the Experimental Section. ATR-FTIR analysis using a Bruker Alpha spectrometer was carried out to investigate if the sPP was successfully grafted and if the grafted anhydrides were successfully converted into the respective zinc carboxylate. The resulting plots of absorbance against wavenumber are shown in Figure 2. The ATR-FTIR analysis of grafted sPP (c) shows signals at wavenumbers of 1712, 1743, and 1775 cm−1 and a weak signal at 1840 cm−1. The last two signals are typical for acid anhydrides. The first signal originates from a carboxylate, indicating that the anhydride is partially hydrolyzed during the grafting process. The band at 1743 cm−1 is typically found in oxidative degraded polypropylene.58 The spectrum of grafted and of subsequently ZnO blended sPP (d) shows a new signal at 1540 cm−1, which can be attributed to a carboxylate, originating from the zinc salt formation of the carboxylic groups. The anhydride specific signals and the signal of the acid at 1712 cm−1 disappear in this spectrum. This indicates that the anhydride functions and the

Figure 3. Young’s modulus versus temperature plots of grafted (sPP-gMA) containing different amounts of ZnO.

As seen in Figure 3, 15 wt % of ZnO are sufficient to ionically cross-link the MA-grafted sPP. However, the ionic bonds are cleaved at 190 °C, which is close to the crystal melting temperature of sPP (125 °C). A higher thermal stability of the ionic bonds (203 °C) results when increasing the ZnO content to 20 wt %. Further increase of the ZnO content to 25 wt % does not improve the thermal stability of the ionic bonds but downgrades the mechanical properties of the network; i.e., the materials becomes susceptible to cracking. Thus, the optimal material was obtained by ionically cross-linking MA-grafted sPP with 20 wt % ZnO. This material was used for all following investigations. In order to characterize the formed ionically cross-linked syndiotactic polypropylene (ix-sPP) network in further detail, the net chain molecular weight Mc was determined from stress− strain experiments at 160 °C to Mc = 33 400 g mol−1 according to the Mooney−Rivlin equation.38,39 Given an effective MA grafting of 1 wt % and complete conversion to the respective zinc salt, this value suggests a crosslinking efficiency of some 30%. To further characterize the network structure, ultramicrotomy cuts were prepared of liquid nitrogen cooled ix-sPP and investigated with transmission electron microscopy (TEM). C


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Macromolecules Figure 4 shows the TEM bright-field micrograph of an ix-sPP sample.

Figure 5. DSC plot of ix-sPP after quenching from 160 °C in iced water.

sPP.31,32 Thus, the high amount of ZnO does not significantly affect the ability to quench ix-sPP amorphously. Since sPP is known to crystallize at room temperature69,70 within a short time, we subsequently investigated the stability of the amorphous state of ix-sPP at room temperature. To this end, wide-angle X-ray scattering (WAXS) measurements were carried out. Figure 6a shows the WAXS pattern of an ix-sPP sample

Figure 4. TEM bright-field micrograph of an ultramicrotomy cut of ixsPP.

As seen in Figure 4, particles of about 200 nm in diameter are visible. EDX analysis revealed that the dark particles are composed of 50.0 at. % Zn, 44.5 at. % O, and 5.5 at. % C. It is assumed that these particles are composed of ZnO with some adhered stearate. The bright matrix is composed of 98.8 at. % C, 1.0 at. % O, and 0.2 at. % Zn. We suggest that the larger particles do not play any role for network formation, since they are too coarsely distributed and the ratio of zinc to oxygen in the matrix suggests that the maleic anhydride to Zn2+ is between one and two, indicating molecular cross-linking. This is in accordance with comparable work of Wouters et al.59 on maleated ethylene− propylene copolymer networks that shows that ionic clusters of sizes between 2 and 3 nm are not formed in ionomer networks with less than 3 wt % effective grafts. After successful ionic cross-linking of sPP, the resulting shape memory properties of the formed network were investigated. As reported in previous work,31 covalently cross-linked x-sPP is amorphously quenchable due to its low stereoregularity and is capable of cold-programming via strain-induced crystallization. Strain-induced crystallization (SIC) is a rare phenomenon in polymer materials majorly described for natural rubber60−62 and very few other polymers such as un-cross-linked33,63−67 and covalently cross-linked sPP.31,32 The great advantage of SIC is that polymer networks are transformed from an amorphous to a semicrystalline state during deformation, which results in strain hardening and thus a high deformation energy density.68 In terms of shape memory polymers, SIC offers the possibility of coldprogramming and enhanced storable strain and deformation energy. In order to check if the same is possible for ionically crosslinked ix-sPP, we first tried to quench ix-sPP to a fully amorphous state. To this end, the sample was heated to a temperature of 160 °C, quenched in iced water, and immediately examined by DSC (see Figure 5). The DSC plot of the quenched ix-sPP shows a glass transition at −2.5 °C, a recrystallization peak at 40.7 °C, and a melting peak at 124.3 °C. Since the observed melting enthalpy (ΔHm) is practically equal to the crystallization enthalpy (ΔHc), we assume that the sample was fully amorphous directly after quenching. This is similar to the behavior of covalently cross-linked x-

Figure 6. WAXS patterns of ix-sPP (a) directly after quenching (recorded at −10 °C to avoid crystallization) and (b) after quenching and storage at 20 °C for 1 h.

recorded immediately after quenching. In order to avoid any crystallization during data accumulation, the pattern was recorded at −10 °C. The obtained WAXS pattern exhibits only an amorphous halo and no discrete Debye−Scherrer rings and confirms the DSC result, that the ix-sPP sample is fully amorphous immediately after quenching. Figure 6b shows the WAXS pattern of a quenched ix-sPP sample after 1 h storage at room temperature. The pattern shows an unoriented helical (form I) crystal modification, as indicated by homogeneous diffraction rings of the (200)h and (020)h reflections at 2θ positions of 12.2° and 15.8°, respectively.56 This thermal crystallization at room temperature was investigated in situ by WAXS in further detail. After 4 min the first crystal reflections appear. In order to explore if an amorphous ix-sPP sample can crystallize strain-induced similar to un-cross-linked33,63−67 and covalently cross-linked31,32 sPP, we stretched a freshly quenched sample to a strain of 300% using a tensile tester. After releasing the stretching force, the sample retained at a fixed strain of about 200%. As expected from the literature,46−48,56,71,72 the WAXS analysis of this sample revealed strain-induced, oriented crystals in mesomorphic trans-planar modification as indicated by the occurrence of the trans-planar reflections at 2θ = 17.0° and 2θ = 23.7°, respectively (see Figure 7). Thus, amorphously quenched ix-sPP is cold-programmable similar to its covalently cross-linked analogue, x-sPP.31,32 D


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Macromolecules

until the sample stabilizes at the fixed strain (2). The amount of crystals decreases simultaneously as indicated by the decreasing peak area of the trans-planar reflection at 2θ = 17°. Typically, the crystallinity versus strain curves of strain-induced crystallized elastomers, such as natural rubber62,73−75 and x-sPP,31 show a hysteresis (see Supporting Information Figures S1 and S2). However, this is not the case for ix-sPP (see Figure 9b). In order to evaluate the shape memory properties of coldprogrammed ix-sPP, the typical shape memory parameters, being maximal stored strain εstored, the trigger temperature Ttrig, the recovery ratio Rr, and fixity ratio Rf, were determined using a custom-made stretching apparatus.57 To this end, shape memory cycles were carried out composed of (1) heating the ix-sPP to 160 °C and quenching it in iced water, (2) stretching to various strains at room temperature, (3) releasing the stretching force to determine the fixed strain, and (4) triggering upon heating to 160 °C to obtain the recovered permanent shape. As shown in Figure 10, the programming

Figure 7. WAXS pattern of a cold-programmed ix-sPP sample.

The process of cold-programming of ix-sPP is visualized in Figure 8 by means of a stress−strain plot. In order to get further

Figure 10. Recovery ratio in dependence on strain applied for coldprogramming.

Figure 8. Stress−strain plot of a freshly quenched ix-sPP sample and respective WAXS patterns at the indicated strains: (1) stretched from εperm = 0% to εprog,cold = 300% and (2) subsequently relaxed, resulting in a strain εfix of about 200%.

strain εprog,cold strongly affects the recovery ratio of the coldprogrammed ix-sPP. When stretching the network above 300%, the recovery is less than 100% while even samples programmed by stretching to 500% still show a good recovery ratio of 97.8%. The same experiments were carried out with the previously published x-sPP. Comparing the results shows that the covalently cross-linked x-sPP fully recovers up to its maximum programmable strain of 515% while ix-sPP shows minor plastic deformation when programmed to strains above 300%. Shape memory parameters over five cycles were determined for ix-sPP using a programming strain of 300%. As shown in Table 2, cold-programmed ix-sPP fully recovers its permanent

insights into the strain-induced crystallization while coldprogramming, WAXS patterns were recorded of ix-sPP samples. To this end, ix-sPP samples were stretched at room temperature to different strains (see Figure 8) and instantly quenched to and kept at −10 °C to avoid further crystallization. Figure 9a illustrates how the WAXS patterns were evaluated to gain qualitative information on increasing crystallinity during stretching. Figure 9b shows the obtained peak area plotted against the respective strain. It is seen that the amount of crystals increases with increasing strain (1). By releasing the stretching force, the strain decreases

Figure 9. (a) Intensity of the trans-planar reflections at 2θ = 17° against azimuthal angle, exemplarily for a 300% stretched ix-sPP sample. Gray marked regions indicate the integration areas. (b) Peak area of the trans-planar reflection at 2θ = 17° in dependence on strain, while (1) stretching from εperm = 0% to εprog = 300%, (2) relaxing from εprog = 300% to the fixed strain εfix ≈ 200%, and (3) stretching again from εfix ≈ 200% to εprog = 300%. E


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Macromolecules shape and does not alter its fixity ratio for at least five shape memory cycles, which is similar to x-sPP.32

DSC analysis revealed that this is due to a crystal transformation from the mesomorphic trans-planar to the helical (form I) modification, which was also found for x-sPP (see Supporting Information Figure S3).31,32 Similar to x-sPP, this crystal transformation can be affected by the heating rate used for triggering, as shown in Figure 12. A

Table 2. Shape Memory Parameters of Cold-Programmed, Ionically and Covalently Cross-Linked sPP Samples Measured over Five Shape Memory Cycles sample

εprog,cold [%]

εstored,cold [%]

Rf,cold [%]

Rr,cold [%]

ix-sPP x-sPP31

300 515

195−200 335−340

65−66 65−66

100 100

Hot-programming of ix-sPP was explored by heating the network to a temperature of 160 °C, stretching to 400%, which refers to 95% of the predetermined fracture strain, and constraint cooling to room temperature. Similarly to the covalent crosslinked analogue x-sPP, ix-sPP ruptures within the second shape memory cycle. The shape memory properties are listed in Table 3. Since hot-programmed ix-sPP fails after one shape memory cycle, we focused on cold-programmed ix-sPP in the following. Figure 12. TMA plots for one and the same cold-programmed ix-sPP sample recorded with a heating rate of 1, 10, and 100 K min−1.

Table 3. Shape Memory Parameters of Hot-Programmed ixsPP and x-sPP sample

εprog,hot [%]

εstored,hot [%]

Rf,hot [%]

Rr,hot [%]

ix-sPP x-sPP31

400 700

320 550

80 80

100 98.5

heating rate of 100 K min−1 completely prevents the formation of the helical phase. Thus, ix-sPP exhibits equally to its covalently cross-linked analogue x-sPP32 a dual- or triple-shape memory effect that strongly depends on the heating rate used for triggering. Since it is known from the literature that trans-planar crystals completely transform to helical crystals upon heating leading to a higher crystallinity,76 we explored if this effect is usable to increase the fixity ratio of a cold-programmed ix-sPP as well as xsPP by recrystallization under constraint condition. To this end, freshly quenched ix-sPP and x-sPP samples were stretched to

In order to explore the trigger process in further detail, we heated a cold-programmed ix-sPP sample in a thermalmechanical analyzer (TMA) to 160 °C using a heating rate of 1 K min−1. As seen in Figure 11, cold-programmed ix-sPP exhibits similar to x-sPP a triple-shape memory with average trigger temperatures of 58 and 124 °C, respectively.32 WAXS and

Figure 11. TMA and DSC plots of a quenched and cold-programmed ix-sPP sample with the corresponding WAXS patterns at 20, 60, 80, 100, 115, 124, 130, and 150 °C. F


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Macromolecules

As seen in Figure 14, thermal reprocessing of ix-sPP does not alter its Young’s modulus versus temperature profile.

300% and 515%, respectively, and kept constrained upon heating to a temperature of 110 °C for 2 h. After cooling to room temperature and declamping, the ix-sPP and x-sPP samples retain a strain of 300% and 515%, respectively. Thus, the fixity ratios of both cold programmed sPP networks improves from 66% to 100% using the as-described thermal treatment. DSC and WAXS analysis revealed that this is indeed due to the transformation of the originally shape fixing mesomorphic trans-planar crystals to helical (form I/II) crystals (see Figure 13b).56,76 Furthermore,

Figure 14. Young’s modulus versus temperature plots of (a) freshly prepared ix-sPP and (b) ix-sPP after thermal reprocessing by extrusion and compression molding.

Cyclic thermomechanical measurements showed that the shape memory properties of thermally reprocessed ix-sPP are exactly the same as those of freshly prepared samples. The shape memory parameters are presented in Tables S1−S3 (see Supporting Information). Thus, thermal reprocessing of ix-sPP does affect neither its mechanical nor its shape memory properties. Another interesting feature of shape memory polymers is shape memory assisted self-healing (SMASH).4 In order to explore if this is also possible for ix-sPP, a sample was notched (Figure 15a) and cold-programmed to a strain of 200% at room temperature. This leads to an extensive propagation of the notches (Figure 15b). The obviously damaged sample was unclamped, heated to 180 °C to allow shape recovery (Figure 15c), held isothermally for 30 min to proceed self-healing, cooled to room temperature, and kept there for further 30 min. After this procedure the notches disappeared (Figure 15d) and did not reappear even after renewed cold-programming by stretching to 300% (Figure 15e). The Young’s modulus versus temperature plots of ix-sPP of an un-notched, notched, and shape memory assisted self-healed sample were recorded. No significant differences were found between the original and the healed sample (see Figure 16). Thus, damaged ix-sPP can be fully repaired by shape memory assisted self-healing.

Figure 13. (a) TMA and DSC plots of a quenched ix-sPP sample stretched to εprog = 300% and constrained tempered for 2 h at 110 °C. WAXS patterns of a quenched x-sPP sample stretched to εprog = 300% (b) before and (c) after constrained tempering for 2 h at 110 °C.

the as-described heat treatment results in a significantly narrower trigger process, as shown in Figure 13a. This effect is also independent of the kind of cross-linking, as seen for x-sPP in Figure S4 (Supporting Information), which shows the same improvement of fixity ratio and trigger process. When comparing these results to literature known experiments on non-crosslinked sPP, it becomes obvious that cross-linking is mandatory to achieve a change in crystal modification from trans-planar to helical without massive plastic deformation.72 The shape memory parameters of cold-programmed and thermally retreated ix-sPP and x-sPP were determined over five cycles and are listed in Table 4. Table 4. Shape Memory Parameters over Five Cycles of Ionically and Covalently Cross-Linked SPP Samples, Amorphously Quenched and Stretched to 300% (ix-sPP) and 515% (x-sPP), Respectively, and Constrained Tempered for 2 h at 110 °C sample

εprog,cold [%]

εstored,cold [%]

Rf,cold [%]

Rr,cold [%]

ix-sPP x-sPP31

300 515

300 515

100 100

100 100

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As shown above, ionically cross-linked sPP exhibits similar mechanical and physical properties as covalently cross-linked sPP. Because of its ionic character, it should have some properties such as thermal reprocessability and self-healing that are superior to x-sPP. These are investigated in the following. Similar to comparable ionic networks containing maleic acid and zinc oxide, ix-sPP flows above a temperature of 203 °C according to the Young’s modulus versus temperature plot shown in Figure 3. In order to explore if ix-sPP can be reprocessed without losing its mechanical performance, it was intensively mixed using a double-screw extruder at 60 rpm for 5 min. The extrudate was reshaped to sheets by compression molding at 230 °C examined concerning its mechanical as well as shape memory properties.

CONCLUSIONS

We have shown that ionically cross-linked syndiotactic polypropylene (ix-sPP) is cold-programmable and exhibits a heating rate sensitive dual and triple-shape memory effect similarly to its covalently cross-linked analogue, x-sPP. The excellent shape memory parameters of ix-sPP (Rf = 80%, Rr = 100%) can be further improved by thermal retreating coldprogrammed similar ix-sPP as well as x-sPP. Thermal reprocessing of ix-sPP at a temperature above 217 °C does not significantly alter its shape memory parameters. Furthermore, we found that the effect of shape memory assisted self-healing, known from other SMPs with reversible netpoints, is also available for repairing a damaged ix-sPP without significant loss of its mechanical as well as shape memory properties. G


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Macromolecules

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Figure 16. Young’s modulus versus temperature plots of un-notched, notched, and shape memory assisted healed ix-sPP.

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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.6b01387. Figures S1−S4 and Tables S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.C.T.). Author Contributions

T.R. and R.H. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from funding provided by the Deutsche Forschungsgemeinschaft DFG under Grant TI 326/41.

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