poly(ester urethane) with varying polyester chain length: polymorphism and shape-memory behavior

Full PaperMacromolecularChemistry and Physics

Poly(ester urethane) with Varying Polyester Chain Length: Polymorphism and Shape-Memory Behavior

Martin Bothe , Franziska Emmerling , Thorsten Pretsch*

The swelling, viscoelastic, and mechanical behavior of phase-segregated poly(ester urethane) (PEU) block copolymers, composed of 4,4′-methylenediphenyl diisocyanate, 1,4-butanediol as a chain extender, and crystallizable poly(1,4-butylene adipate) (PBA) with molecular weights between 1330 and 4120 g mol −1 , are investigated. Wide-angle X-ray scattering (WAXS) is employed to study the overall PEU crystallinity, which increases from 8.6 to 13.6% at higher PBA contents. The existence of two crystalline, polymorphic PBA phases, a thermodynamically stable α phase and a metastable β phase, is confi rmed by further WAXS measurements. Calo-rimetric and thermomechanical investigations give evidence for controllable PBA polymor-phic behavior. The crystallization conditions, like the cooling rate, affect the emerging polymorphic mixture, whereas the storage conditions either promote or inhibit the polymorphic ( β to α ) transition. The introduced concepts represent a new approach for gaining control over programmable thermo-responsiveness, which may be transferable to other shape-memory polymers with polymorphic switching segments.

1 . Introduction

Shape-memory polymers (SMPs) are able to respond to an external stimulus like heat with a shape change. [ 1–9 ] As a prerequisite for such behavior, the SMP must be ther-momechanically pretreated (programmed). In case of phase-segregated polyurethane block copolymers with crystallizable switching segments like poly( ε -caprolactone) (PCL), [ 10 ] poly(1,4-butylene adipate) (PBA) [ 11 ] and trans -polyisoprene, [ 12 ] programming usually consists in deforma-tion of the SMP in the viscoelastic state (above a segmental

wileyonlin

M. Bothe, Dr. T. PretschBAM Federal Institute for Materials Research and Testing , Division 6.5 Polymers in Life Science and Nanotechnology , Unter den Eichen 87 , 12205 , Berlin , GermanyE-mail: [email protected] Dr. F. EmmerlingBAM Federal Institute for Materials Research and Testing , Division 1.3 Structure Analysis , Richard-Willstätter-Straße 11 , 12489 , Berlin , Germany

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melting transition), followed by cooling the polymer under constrained conditions to the so-called shape fi xity tem-perature (below a segmental crystallization transition) and thus into the semi-crystalline polymer regime. Once stabi-lized through crystallization, the polymer keeps the tem-porary shape until heating above the segmental melting transition triggers the shape-memory effect. As a result, an almost complete recovery of the original (permanent) shape takes place.

SMPs offer a broad range of thermal and mechanical properties for a large number of applications. The shape-memory effect is technologically applicable where one-time shape changes are needed. Applications spread from medical implants like biodegradable stents, [ 13–17 ] self-folding polymer fi lms, [ 18 ] pressure and micro-actu-ators [ 19–21 ] to wrinkle-free fabrics [ 22,23 ] and switchable information carriers. [ 24,25 ]

In search of thermoplastic materials, which exhibit pronounced shape-memory properties, we studied the shape-memory behavior of three physically cross-linked, phase-segregated PEUs with differing hard to soft segment

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ratios. The hard segments were composed of 4,4′-methyl-enediphenyl diisocyanate (MDI) and a 1,4-butanediol (BD) chain extender. The soft segments were built up by PBA as obtained from condensation reaction of BD with adipic acid. Following a special feeding method (soft seg-ment pre-extension process), [ 26 ] the poly(adipate) diols reacted with reduced excess of MDI, thus forming reactive pre-extended polymer species, before further quantities of MDI and BD were added. Pre-extension of PBA chains was identifi ed to improve the processing behavior of PEU same as the heat stability. [ 26 ] From a structural point of view, the hard segment domains cross-linked the soft segment, preventing viscous fl ow above the PBA melting transition and assuring dimensional stability inside the block copolymer. In this regard, one focus of our studies was the infl uence of soft segment content (average PBA chain length) on the effective cross-link density at tem-peratures below and above the PBA melting transition and on the crystallization and melting behavior within the PEU series. The shape-memory properties of the PEUs were closely examined in thermomechanical measure-ments, using PBA as a crystallizable/meltable switching segment. Thermomechanical programming was used to transform the polymers into deformed, semicrystalline states, whose thermoresponsiveness was later studied both under free strain and constant strain conditions. In particular in these thermomechanical approaches, we aimed at making an assignment of PBA crystallization and melting events for the respective crystalline forms and took this as basis for controlling the polymorphic and thus the thermomechanical behavior of the polymers in several ways.

2 . Experimental Section

2.1 . Materials

Three segmented thermoplastic poly(ester urethane) elastomers were fabricated by Bayer MaterialScience AG as injection molded plaque samples. The hard segment was built up by 4,4′-methylen-ediphenyl diisocyanate (MDI) and 1,4-butanediol (BD) as a chain extender. The soft segment was composed of poly(1,4-butylene adipate) (PBA) with varying chain lengths, which resulted for the as-fabricated states in different hard to soft segment ratios. The starting molecular weight of PBA used to synthesize the PEUs was 4100 g mol −1 . PEUs were termed “PEU-h(ssc)”, “PEU-m(ssc)” and “PEU-l(ssc)” in line with the highest (h), medium (m), and lowest (l) soft segment content.

2.2 . Swelling Experiments

The weight of rectangular samples with a dimensioning of 33 mm × 5 mm × 2 mm and an initial mass m 0 in between 1.03 and 1.26 g was determined in air and water at 23 °C with a Sartorius AC 211 balance. Then, the samples were stored for 6 h

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at 60 °C in ethyl acetate ( V EA = 98.2 cm 3 mol –1 , M̄w ,EA = 88.1 g mol –1 , ρ EA = 0.894 g cm –3 ), [ 27 ] which was used as a swelling agent, before the weighing procedure was repeated for one more time. The sample weights were directly determined after immersion at a sample temperature of 60 °C and after drying on air for 5 d at 23 °C and a relative humidity of 50%. The volumes of the untreated and swollen state and the density of the PEUs ρ PEU were calculated using the buoyancy-fl oating method. The frac-tion of the mass of the swollen sample m sw and dry sample m d was then used to determine the gel content G according to:

G = md

msw (1)

The degree of swelling Q was calculated by means of

Q = 1 +ρ

ρ1msw

/md − 1

)

(2)

in which ρ = ρ PEU was the specifi c PEU density and ρ 1 = ρ EA the solvent density. From the Flory–Rhener theory, which applies to polymers with 3D network structures, the molecular weight M̄w of swollen, cross-linked polymer chains and the cross-link density ν c can be determined: [ 28,29 ]

vc =ln 1 − φ2

)+ φ2 + φ2

2χ12

V1

(φ22 −φ

132

) =ρ

Mw

(3)

Here, ϕ 2 is the volume fraction of the polymer in the swollen system. It is defi ned by the ratio of dry volume ( V d ) to swollen volume ( V sw ). Beyond that, V 1 = V EA is the molar solvent volume and ρ = ρ PEU is the density of the untreated polymer.

For simplicity, PEU swelling was expected to be mostly domi-nated by the swelling of amorphous PBA segments, whereas the hard segment contribution to swelling and the deviation of the spatially extended hard segment domains from theoretically point-shaped network nodes were neglected. We selected the Flory polymer–solvent interaction parameter χ 12 for PBA in ethyl acetate, which is χ PBA-EA = 0.43 at 120 °C. [ 30 ]

2.3 . Differential Scanning Calorimetry

Calorimetric measurements were performed with an EXSTAR DSC7020 from Seiko Instruments Inc. The sample weight was approximately 5 mg. At the beginning of the DSC measurement, the sample was cooled to –80 °C and kept there for 2 min. The fi rst heating to 100 °C was carried out with a rate of 10 °C min –1 , followed by cooling to –80 °C with a rate of –10 °C min –1 . After 2 min at –80 °C, a second heating run to 100 °C was conducted with 10 °C min –1 .

Further differential scanning calorimetry (DSC) scans were run on thermally pretreated and subsequently annealed samples of PEU-h(ssc), having a weight of 5 mg each. Thermal pretreat-ment consisted in heating from 23 to 60 °C, keeping that tem-perature constant for 5 min, cooling from 60 to –20 °C with a rate of –2.4 °C min −1 , temperature holding for further 5 min and fi nal heating to 23 °C. For sample annealing, PEU-h(ssc) was adjacently kept at 25 °C for time intervals between 0 and 168 h, before DSC

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measurements were run. Latter consisted in sample cooling to 20 °C, 2 min temperature holding, heating to 100 °C, cooling to –20 °C, 2 min temperature holding and fi nal heating to 100 °C with heating/cooling rates of 10 °C min −1 . For the untreated samples, melting peak deconvolution of the heating scans was carried out with a least-squares fi t of the superposition:

F (T ) = E1(T ) + E2(T ) + c (4)

with a constant offset c and the two exponentially modifi ed Gaussian distributions E y ( T ) as defi ned according to:

Ey(T ) =A

2βexp

α2

2β 2+

Tm − T

βerf

Tm − T√2α

− α√2β

+β

|β | (5)

where T is the temperature, A is the area of the distribution, T m the temperature-position of the peak maximum, α is the peak width at the inversion point, β is the peak distortion and erf the error function.

2.4 . Wide-Angle X-Ray Scattering

Diffraction studies were conducted on the injection-molded plaques of the three different PEUs, using an INEL CPS 120 dif-fractometer equipped with an 120° curved position sensitive detector and setup in Debye–Scherrer geometry. The detector allowed a simultaneous data collection in 4096 bins over a range of 120° in 2 θ . The X-ray source was a 1500Q sealed tube with a Cu target. The sample holder was rotated around the axis defi ned by the planar surface of the sample to average the crystallite refl ections, contributing to the azimuthally averaged diffraction pattern. The crystalline structure of the PEUs was investigated in two different scenarios: 1) after long-term storage (more than 7 d) at 23 °C and 2) within 1 h after thermal pretreatment, including sample heating to 60 °C, temperature holding for 5 min, cooling to –20 °C at a rate of –10.8 °C min −1 , further sample holding for 5 min and subsequent heating to 23 °C.

High-intensity wide-angle X-ray scattering (WAXS) measure-ments were carried out on the injection-molded PEU plaques at the synchrotron micro focus beamline μ Spot (BESSY II of the Helmholtz Centre Berlin for Materials and Energy). The experi-mental setup was employed as described by Paris et al. [ 31 ] The experiments were run at a wavelength of 0.99987 Å and an exposure time of 120 s. The scattered intensities were collected 200 mm behind the sample position with a 2D X-ray detector (MarMosaic, CCD 3072 × 3072). The obtained scattering images were processed and converted into diagrams of scattered intensi-ties versus scattering angle 2 θ (with respect to the wavelength of the Cu K α -line) using the computer program FIT2D. [ 32 ]

Peak deconvolution of the scattered intensities allowed their assignment to crystalline and amorphous regions. As a criterion, a refl ex was attributed to crystallinity when its half width at half maximum did not exceed 1°. The degree of PEU crystallinity χ c was estimated by comparison of the integrated intensity from crystals I cryst with the totally scattered intensity I total :

χc =IcrystItotal

(6)

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2.5 . Tensile Tests

The fracture mechanical properties of PEU tensile bars (EN ISO 527–2: 1996) were determined with a Zwick 1464 device at 23 °C and a relative humidity of 50%. In order to obtain reproducible testing results, the tensile bars were stored for 24 h at 23 °C before testing. After clamping with a gauge length of 10 mm, a preload of 5 N was applied. Subsequently, the specimen was stretched until fracture with a rate of 0.5% s –1 . From the obtained data sets, the Young’s modulus, specimen toughness, elongation-at-break, and maximum tensile strength were determined.

2.6 . Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was conducted with a Netzsch DMA 242, operating in single cantilever bending mode. Here, PEU samples with dimensions of 5 mm × 2 mm × 2 mm were used. The temperature was ramped from –100 to 60 °C with a heating rate of 1 °C min –1 . In parallel, the storage modulus E ′ and the loss modulus E ′′ were determined. Since DMA results were found to be highly sensitive to the thermal history of PEU (e.g., storage conditions), all the PEU samples were annealed for 10 min at 60 °C and stored at 23 °C (50% air humidity) for at least 1 week, before their viscoelastic properties were studied.

2.7 . Shape-Memory Measurements

Cyclic thermomechanical measurements were conducted with an MTS Insight 10 electromechanical testing system, equipped with a Thermcraft thermochamber and a Eurotherm 2404 tem-perature controller unit. Dumbbell-shaped type 5 B tensile bars (EN ISO 527–2: 1996) were punched out of the 2 mm thick PEU plaques and clamped at an initial gauge length of 10 mm into the pneumatic grips of the electromechanical testing system, using a clamping pressure of 3 bar. In subsequent measurements, the specimen strain ε was directly determined from cross-head dis-placement and changes in tensile force were recorded with a 100 N load cell. The nominal tensile stress σ was calculated by normalizing the measured force to the initial cross section of the specimen, which by standard was 4 mm 2 . The heating rates were set to the maximum rates achievable by the given experimental setup, resulting in (19.5 ± 0.3) °C min −1 from –20 to 23 °C and (17.4 ± 1.1) °C min −1 from 23 to 60 °C. Beyond that, cooling rates of (–3.8 ± 0.5) °C min −1 from 60 to –20 °C and (–4.3 ± 1.0) °C min −1 from 60 to 23 °C were employed.

In shape-memory measurements, shape programming con-sisted in specimen heating from 23 to 60 °C, 5 min temperature holding and subsequent stretching to ε m = 100% with a rate of d ε /d t = 0.5% s –1 , culminating in a maximum loading stress σ l . By cooling to –20 °C, the stress decreased to a minimum stress σ min , whereupon the applied deformation could be fi xed. At –20 °C, a 5 min temperature holding interval was added before the spec-imen was unloaded within exactly 1 min, using a dynamically adapted linear stress release rate. Then, the specimen was heated to 23 °C. One fi nal 5 min temperature holding step constituted the end of shape programming. The residual specimen strain at 23 °C was defi ned as ε u .

Heating of programmed PEU specimens from below to above the PBA melting temperature allowed examining the thermally

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induced recovery behavior with respect to the liberation of strain and stress, respectively. Every programming/recovering couple gave one complete thermomechanical cycle of which in total N = 5 cycles were measured.

In case of shape-memory measurements under free strain conditions, the specimen behavior was investigated under zero force by heating to 60 °C. To ensure specimen contraction to the greatest possible extent, the “high” temperature was kept con-stant for 5 min. Then, the thermomechanical cycle was fi nalized by cooling the polymer to 23 °C and keeping it there for 5 min to determine the strain ε p of the recovered shape. The strain fi xity ratio R f ( N ), the strain recovery ratio R r ( N ), and the total strain recovery ratio R r,tot ( N ) were calculated according to:

Rf (N) =εu(N)ε m

(7)

Rr(N) =εm − εp(N)

εm − εp(N − 1) (8)

and:

Rr,tot(N) =εm − εp(N)

εm (9)

Furthermore, the strain recovery temperature T trans was deter-mined from the strain-temperature-derivative d ε /d T . Here, d ε /d T showed an absolute maximum value during shape recovering.

The second recovery scenario was under constrained condi-tions, in which the stress response of the programmed polymer on heating from 23 to 60 °C was studied. Therefore, the clamping distance after temperature holding at 23 °C was maintained and the stress evolution, culminating in a maximum stress σ r,max , was investigated. After a temperature holding time of 5 min at 60 °C, the stress was linearly reduced within exactly 1 min to 0 MPa and the thermomechanical cycle was completed with specimen cooling to 23 °C and one fi nal 5 min temperature holding step.

3 . Results and Discussion

Hereafter, we elucidate the network structures of three-phase-segregated PEU block copolymers, differing in

Table 1. Swelling behavior of PEU with high (h), medium (m), and lowtion of the polymer density ρ , degree of swelling Q , gel content G , aveand cross-link density ν c according to Equation 1–3.

Sample ρ [g cm –3 ]

Q G [%]

PEU-h(ssc) 1.190 4.02 97.9

(±0.001) (±0.02) (±0.1

PEU-m(ssc) 1.193 3.08 98.8

(±0.005) (±0.03) (±0.1

PEU-l(ssc) 1.197 2.61 99.0

(±0.002) (±0.02) (±0.1

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soft segment (PBA) content. We report on the PBA crys-tallization and melting characteristics, followed by the assignment of the individual polymorphic forms, the determination of overall PEU crystallinity and the analysis of the mechanical and, in particular, the shape-memory behavior. Table 1 provides an overview on the main results of our swelling experiments.

As experimentally verifi ed, the increase in hard seg-ment content resulted in an increase in polymer density ρ . In return, at growing soft segment content, the degree of swelling Q increased, implying that the amorphous soft segments served as kind of solvent sponge. Gel contents G close to 100% indicated successful polymerization of the reactants. Notably, slightly rising G values were detected with increasing hard segment content, substantiating that hard segments served as physical cross-links for PBA chains within the network structures. The decrease in soft segment content corresponded to a decrease in PBA chain length from 4120 to 1330 g mol –1 , equivalent to a statistical number of PBA repeating units between 21.1 and 6.6. Conversely, PEU-l(ssc) had a three times higher cross-link density ν c compared with PEU-h(ssc).

In DSC measurements, the PEU block copolymers exhibited PBA melting peaks at temperatures in between 42 and 47 °C and a glass transition close to −48 °C (Figure 1 ). The assigned phase transition temperatures were in good agreement with those of a recently studied PEU. [ 33,34 ] As expected from studies of similar poly(ester urethanes), [ 35 ] the PBA melting and crystallization enthalpies systemati-cally increased with growing soft segment chain length (Figure 1 a, Table S1, Supporting Information). The higher degree of PBA crystallinity may be related to a higher con-formational degree of freedom, which should also result in a larger number of crystallizable PBA units. In line with the general temperature-dependence of the glass tran-sition in polymers, [ 36,37 ] T g was found to increase from –49 to –47 °C with growing soft segment chain length (Figure 1 b).

As homopolymer, PBA (M̄w = 40 000 g mol –1 ) is known to crystallize in at least two crystal systems, which are either

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(l) soft segment (PBA) content in ethyl acetate at 60 °C. Determina-rage PBA molecular weight M̄w , number of PBA repeating units (R.U.)

M̄w [g mol –1 ]

R.U. ν c [×10 –4 mol cm –3 ]

4120 21.1 2.9

) (±50) (±0.2) (±0.1)

2090 10.4 5.7

) (±90) (±0.4) (±0.2)

1330 6.6 9.0

) (±10) (±0.1) (±0.1)

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Figure 2. DSC thermograms of PEU-h(ssc), showing the fi rst heating run after thermal pretreatment and different times of room temperature annealing, including the assignment of the PBA melting peaks to the β and α crystal form.

Figure 1. DSC thermograms of PEU-h(ssc), PEU-m(ssc), and PEU-l(ssc). a) First heating and cooling scan. b) PBA glass transition during second heating (solid line) and the corresponding time derivatives (dashed lines). c) PBA melting during second heating (solid lines) and deconvolved melting curves (dotted and dashed lines) to assign the PBA β and α form.

monoclinic ( α crystal form) or orthorhombic ( β crystal form). [ 38–40 ] In the kinetic process of fast cooling [ 40,41 ] and during annealing at relatively low temperatures, [ 39 ] PBA homopolymer preferentially develops a meta-stable β morphology. In contrast, at higher annealing tempera-tures [ 39,42 ] or strong deformations, [ 43 ] PBA more likely crystallizes in the thermodynamically stable α form. In an attempt to investigate the presence of those different PBA



crystal morphologies in the considered block copolymers, the heat fl ow in the second DSC heating run was closely examined (Figure 1 c). The DSC scans unveiled a broad PBA melting signal with a shoulder at lower tempera-ture. Peak deconvolution (Equation 4 and 5) gave the pres-ence of two distinct transitions (double melting behavior) (Figure 1 c and Table S2, Supporting Information). In the deconvolved thermograms, a strong signal between 40 and 44 °C exhibited a broad shoulder at about 35 °C. In agreement with the assignment by Gan et al. [ 39 ] for PBA homopolymer (M̄w = 40 000 g mol −1 ), we assigned the lower temperature signal to β and the higher temperature signal to α crystallite melting.

In order to learn about the temperature stability of crystalline PBA, we carried out further DSC studies on PEU-h(ssc). Samples were thermally pretreated to obtain comparable initial states, at which the PBA phase was completely molten, before crystallizing it and keeping the polymer for different times at 25 °C. Then, we studied the PBA melting behavior (Figure 2 ).

Again, a broad endothermal signal was found in the fi rst DSC heating run of the non-annealed sample. With extended room temperature annealing, a steady decrease in the low temperature shoulder and in parallel an increase in the high temperature signal were found. This progressive shift from PBA β to α form proved the sensi-tivity of the polymorphic mixture toward room tempera-ture annealing. Since the lower temperature shoulder almost completely disappeared within 168 h at 25 °C, the PBA β crystallites must have been largely transformed into PBA α crystallites. As pointed out by Gan et al., [ 39 ] PBA β crystals have a fairly low thermal stability, which in our case could have resulted in a PBA β to α crystal transform-ation (Figure 2 ). With these considerations in mind, the endothermic peaks in the DSC thermograms during fi rst heating (Figure 1 a) suggest that all three (untreated) PEUs

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Figure 3. DSC thermograms of PEU-h(ssc). a) Non-isothermal crystallization of PBA at cooling rates between –0.1 and –10 °C min –1 (fi rst cooling) and b) the ensuing, second heating at a uni-form heating rate of 10 °C min –1 (dashed and dotted lines resulted from peak deconvolution, illustrating the melting-related contri-butions of the PBA β and α crystal form).

Figure 4. WAXS diffraction patterns of untreated and thermally pretreated PEU-h(ssc) with refl exes assigned to the PBA α and β crystal form. Thermal pretreatment consisted in PBA melting and crystallization.

were initially in almost pure PBA α crystalline states. The exothermic signal on fi rst cooling (Figure 1 a) indicated crystallization in a single crystalline form, most likely the formation of PBA β crystallites. As discussed above, the adjacent heating scan (Figure 1 c) implied the presence of both PBA β and α crystallites.

We continued our non-isothermal crystallization studies by investigating the infl uence of the cooling rate upon the PBA crystallization and melting behavior (Figure 3 ).

For PEU-h(ssc), a systematic lowering of the cooling rate from 10 to 0.1 °C min −1 gave a shift in PBA crystallization signal toward higher temperatures up to 27 °C (Figure 3 a).

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In the ensuing DSC heating run, two melting signals were found (Figure 3 b). At lower cooling rates, PBA melting peak deconvolution showed an increase in the fraction of α crystals and at the same time a decrease in the fraction of β crystals. The dependency of PBA melting behavior on the cooling rate is in good agreement with the kinetic behavior of PBA homopolymer (M̄w = 12 000 g mol −1 ). [ 41 ]

Wide-angle X-ray scattering was used to verify the gen-eral assignment of the polymorphic PBA phases. The dif-fractograms of untreated and thermally pretreated PEU-h(ssc) (Section 2.4) were recorded at 23 °C (Figure 4 ).

Independent of polymer composition and thermal his-tory, the diffraction peaks were superimposed on a broad halo, indicative for the presence of an amorphous phase. As expected, the signals from untreated PEU-h(ssc) (Figure 4 , black line) could be attributed to the PBA α crystalline structure with a monoclinic unit cell ( a = 6.73 Å, b = 7.94 Å, c = 14.20 Å, β = 45.5°). [ 39,43 ] Similar diffraction behavior was detected for the other two polymers (not shown herein). This proves that room temperature storing fi nally gave PEUs in a purely PBA α crystalline state with charac-teristic melting behavior (Figure 1 a).

By contrast, directly after thermal pretreatment, the WAXS pattern exhibited no refl ections, which derived from the PBA α crystal form, but some other refl exes (Figure 4 , gray line). These refl ections originated from the kinetically preferential β crystal form ( a = 5.06 Å, b = 7.35 Å, c = 14.67 Å). [ 39 ] This fi nding suggests that PBA crystallized exclusively in the β crystal form during fi rst DSC cooling (Figure 1 a). Because α crystal melting could be detected in all the DSC thermograms, to some extent a β to α crystal transformation must have taken place during second DSC heating (Figure 1 c) and during room temperature annealing (Figure 2 , 0 h annealing time). Thus, the polymorphic behavior of the block copolymers was comparable with the one in PBA homopolymer

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(M̄w = 40 000 g mol −1 ), where β crystal melting is also accompanied by a spontaneous crystal–crystal trans-formation from the thermodynamically metastable β to the room temperature stable α morphology. [ 39 ] To briefl y summarize, long-term storage of the PEUs at 23 °C gave PBA in exclusively α crystalline form, whereas thermal pretreatment was useful to crystallize PBA in a meta-stable β phase. Higher-intensity synchrotron data (BESSY II) was collected for the untreated PEUs (Figure S1, Sup-porting Information). The degree of hard and soft seg-ment crystallinity (Equation 6) declined from highest to lowest soft segment content as evidenced by overall crystallinities of χ C = 13.6% (PEU-h(ssc)), 11.1% (PEU-m(ssc)) and 8.6% (PEU-l(ssc)). This trend is in agreement with fi ndings from Hojabri et al. [ 35 ] Again, almost all the refl exes could be assigned to the crystal lattice of the monoclinic PBA α morphology. A signal at 2 θ = 19.8° in the WAXS pattern of PEU-l(ssc) suggested the presence of crystalline hard segment domains. Diffraction studies by Hwang et al. [ 44 ] on MDI-BD hard segment model compounds showed a similar diffraction peak, which they assigned to refl ection (–106) of the triclinic unit cell ( a = 5.05 Å, b = 4.67 Å, c = 37.9 Å, α = 116°, β = 116°, γ = 83.5°). However, the contribution of that refl ection to the overall diffraction pattern was negligible so that the overall PEU crystallinity was dominated by the soft segment.

The mechanical properties of the untreated, semi-crystalline PEUs were determined in tensile tests at 23 °C (Table 2 and Figure S2, Supporting Information).

As evidenced by the Young’s modulus E ’, higher hard segment contents gave an increase in polymer stiffness, whereas increasing soft segment contents favored larger tensile strains at break. Both can be explained with the coexistence of highest fractions of crystallizable PBA seg-ments and amorphous, highly fl exible PBA segments. Slightly higher values for tensile stress at break and frac-ture toughness were attributed to the increasing degree of soft segment crystallinity as verifi ed by our synchro-tron data.

Next, we used DMA to investigate the three block copolymers in their untreated, almost pure PBA α crystal-line states.

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Table 2. Young’s modulus E ′, tensile strain at break ε B , tensile stresmined from tensile tests at 23 °C (type 5 B tensile bars according EN

Sample E ′[MPa]

ε B [%]

PEU-h(ssc) 39 (±3) 1927 (±38)

PEU-m(ssc) 40 (±3) 1811 (±61)

PEU-l(ssc) 47 (± 3) 1707 (±82)

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The DMA plots showed two consecutive phase changes (Figure S3, Supporting Information). The lower-temper-ature transition was assigned to the PBA glass transition and the higher-temperature transition was assigned to PBA melting. Changes in the slope of the storage modulus E ′ started at about –50 and 30 °C, respectively. At lower temperatures, slightly raised elastic moduli were found for PEU-h(ssc) due to the higher fractions of crystalliz-able and vitrifi able PBA segments. In agreement with our DSC data, the PBA glass transition temperature T g —here associated with E ′′—was about –47 °C for all three PEUs (Figure S3, Supporting Information). At approximately 30 °C, the PEUs were consistently semicrystalline. Here, higher E ′ values indicated increasing resistance toward viscous fl ow at higher PBA content, since PBA α crystals exerted a reinforcing effect. Final heating to 60 °C and thus above the soft segment melting transition resulted in a strong decrease of the elastic modulus below the detectable threshold. In the PEU series, the inversion point of the storage modulus E ′ during its second drop, corresponding to the PBA α crystal melting temperature, was in between 37 and 40 °C. The storage modulus ratio E ’(23 °C)/ E ’(60 °C) decreased at growing hard segment content with values of 20 (PEU-h(ssc)), 14 (PEU-m(ssc)), and 10 (PEU-l(ssc)).

Thermomechanical measurements were conducted to investigate the infl uence of average PBA chain length and polymorphic behavior upon the shape-memory proper-ties of the PEUs. Figure 5 and Table 3 summarize the pro-gramming-related characteristics of the specimens.

Elongation of 100% gave the highest loading stress at highest hard segment content since shorter soft seg-ment chains were earlier approaching a state of full chain extension compared with longer ones, thus anticipating a stress transfer onto the hard segment domains. Sub-sequent shape fi xing was achieved by cooling the PEUs below the PBA crystallization transition. This resulted in effi cient stress reduction as evidenced by the ratios of loading to minimum stress σ l / σ min ( Table 3 ). The stress-temperature derivatives of cooling indicated the presence of two signals at around 18 and 9 °C (not shown herein), which we assigned to PBA crystallization in α and β form, respectively. At the end of programming, PEU-h(ssc) had

2689

s at break σ B , and fracture toughness of the untreated PEUs as deter- ISO 527–2: 1996, strain rate = 0.5% s –1 and preload = 5 N).

σ B [MPa]

Fracture Toughness [MJ m −3 ]

73 (±3) 63.5 (±3.4)

74 (±5) 62.0 (±5.5)

71 (±4) 57.5 (±4.9)

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Figure 5. Shape programming of type 5 B tensile bars (EN ISO 527–2: 1996) of PEU-h(ssc), PEU-m(ssc), and PEU-l(ssc). The evolu-tion in strain ε , stress σ , and temperature T is plotted above time t .

Figure 6. Free strain and constrained stress recovery behavior of programmed type 5 B tensile bars (EN ISO 527–2: 1996) of PEU-h(ssc), PEU-m(ssc), and PEU-l(ssc). Evolution in a) strain and b) stress above temperature ( N = 1) and the respective strain- and stress-temperature derivatives (dotted lines).

Table 3. Infl uence of PEU composition on shape programming-related parameters, including loading stress σ l , minimum stress σ min during cooling, the ratio of loading to minimum stress σ l / σ min and the strain fi xity ratio R f (Equation 7). All the param-eters are given for the fi rst cycle ( N = 1).

Sample σ l [MPa]

σ min [MPa]

σ l / σ min R f [%]

PEU-h(ssc) 1.5 0.3 5.0 99 (±1)

(±0.1) (±0.1) (±0.1)

PEU-m(ssc) 2.1 0.8 2.6 96 (±1)

(±0.1) (±0.1) (±0.1)

PEU-l(ssc) 2.6 1.3 2.0 92 (±1)

(±0.1) (±0.1) (±0.1)

the highest σ l / σ min ratio, indicating most distinct PBA crystallization. As a result, the polymer exhibited very good strain fi xity with R f being close to 100%, whereas systematically lower R f values were detected for the other two polymers. A decrease in shape fi xity at growing hard segment content is well known from PEUs having crystal-lizable soft segments. [ 45 ]

The programmed specimens were heated from 23 to 60 °C to investigate the strain- and stress-related shape recovery characteristics (Figure 6 and Table 4 ).

0Macromol. Chem. Phys. 20

© 2013 WILEY-VCH Verlag Gmb

At growing soft segment content, higher recovery strains ε u – ε p were detected. This fi nding is in line with our DMA results, in which stronger drops in E ′ were found within the PBA melting transition region (Figure S3, Sup-porting Information). In any case, recovering started with an immediate, slight decrease in strain at 23 °C (Figure 6 a), indicating a melting of PBA β crystallites due to their lower temperature stability compared with PBA α crys-tallites. As visible in the strain-temperature derivatives of melting, a distinct signal appeared which we assigned to PBA α crystal melting; the signal shifted toward higher temperatures at growing soft segment content. As a result, a coincidental shift in T trans toward higher values was detected.

Under constant strain recovery conditions, again instantaneous material responses to heating were found at around 23 °C, but this time in form of stress increases in the stress-temperature diagram, giving a fi rst maximum in the associated stress-temperature derivative at about 27 °C (Figure 6 b). At higher temperature, a second melting signal with a maximum at around 43 °C appeared. Thus, the overall stress recovery behavior was indicative for the


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Table 4. Infl uence of PEU composition on the shape-memory properties including free strain recovery behavior ( ε u – ε p , R f , R r , R r,tot, according Equations 7-9 and T trans ) and the constant strain recovery characteristics ( σ r,max ). All the parameters are listed for the fi rst cycle ( N = 1) and cycle-averaged ( N = 2–5).

Sample ε u – ε p [%]

R f [%]

R r [%]

R r,tot [%]

T trans [°C]

σ r,max [MPa]

N = 1 N = 2–5 N = 2–5 N = 1 N = 2–5 N = 1 N = 2–5 N = 1 N = 2–5

PEU-h(ssc) 79 100 97 80 73 41 41 1.3 1.2

(±1) (±1) (±3) (±1) (±1) (±1) (±1) (±0.1) (±0.1)

PEU-m(ssc) 76 97 97 80 74 38 39 1.7 1.6

(±1) (±1) (±2) (±1) (±1) (±1) (±1) (±0.1) (±0.1)

PEU-l(ssc) 70 93 98 78 76 36 36 1.9 1.8

(±1) (±1) (±1) (±1) (±1) (±1) (±1) (±0.1) (±0.1)

Figure 7. Infl uence of cooling rate variation during programming on the stress recovery characteristics of type 5 B tensile bars (EN ISO 527–2: 1996) from PEU-h(ssc) ( N = 1). The evolution of stress (solid lines) and stress-temperature derivatives (dotted lines) is shown with rising temperature.

crystalline coexistence of both the PBA β and α form in the temporary shapes of the three PEUs. Having our pre-vious fi ndings in mind, PBA β crystal melting was again expected to occur prior to α crystal melting. Since stress increases resulted from polymorphic melting, both PBA β and α crystals must have served as physical cross-links in the elongated network structures. As expected from our DMA results, both loading and maximum recovery stresses increased at higher hard segment content (Table 3 and 4 ).

In the next step, we focused on PEU-h(ssc) due to the distinctive crystallization of its PBA phase. Lowering the cooling rate from –5.5 to –2.0 and –0.6 °C min –1 gave a slight increase in the onset stress recovery temperature from 23 to 24 °C (Figure 7 ). In parallel, the stress-tempera-ture derivatives showed a decrease of the lower tempera-ture signal and at the same time an increase of the higher temperature signal, implying preferential formation of PBA α crystallites at lower cooling rates. This fi nding is in good agreement with our cooling-rate depending DSC measurements (Figure 3 ).

In one further strain recovery approach, we extended the holding times after shape programming for PEU-h(ssc). By selecting holding times of 3 and 6 h at 23 °C, we were able to raise the stress recovery onset tempera-ture from 23 to 29 °C (Figure S4, Supporting Informa-tion). Here, virtually complete crystal–crystal ( β to α ) transformation must have taken place at 23 °C. Remark-ably, the recovery path did only slightly differ from that one we detected in the standard measurement, when storing a programmed PEU-h(ssc) specimen for 1 week at –2 °C and thus below the onset of the PBA β melting transition. We conclude that low temperature storage prevented the crystal–crystal transformation. Therefore, the surrounding temperature turned out to be a decisive parameter for the crystalline PBA α to β ratio by either promoting or preventing the polymorphic transition. However, the specimen strains remained unaffected by

www.MaterialsViews.comMacromol. Chem. Phys. 2


that polymorphic transformation, implying that the new PBA α crystals served as reliable crystalline netpoints. The crystal–crystal transformation was less time intensive compared to the one we detected in the DSC experiments (Figure 2 ). In other words, β crystals seemed to have lower temperature stability in highly ordered systems (strongly elongated specimens) compared with more arbitrary morphological arrangements. Since the totally recovered stress remained nearly unchanged, complete crystalline netpoint melting must have occurred in the physically cross-linked structure.

In a multiple cycling experiment on PEU-h(ssc), two stage stress reduction took place as indicated by the pres-ence of two signals in the respective stress-temperature derivatives of cooling – one at 18 °C, the other one at 8 °C (Figure S5, Supporting Information)

Especially in cycles 2–5, these signals are clearly vis-ible, suggesting that PBA crystallized in two polymor-phic forms, which contrasts our observations from the DSC cooling runs. Apparently, the constantly applied

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mechanical load during cooling on the one hand and repeated specimen cycling on the other hand were pro-moting polymorphic PBA crystallization, whereupon cyclically stable crystallization paths could be observed (Figure S5a, Supporting Information). As deducible from the respective heating curves under constant strain con-ditions, melting of PBA β crystallites initiated stress increase, before further stress growth was driven by PBA α crystal melting. In course of cycling, the posi-tion of the PBA β and α crystal melting peaks remained almost unchanged. In parallel, the maximum stress con-verged to lower, constant values (Figure S5b, Supporting Information).

Returning to our PEU series, we must note that slight cyclic changes were evident both in the free strain and stress recovery behavior (Table 4 ).

Under cyclic conditions, R f ( N = 2–5) values increased a little compared with the fi rst cycle (Table 3 ), which indi-cates progressive PBA crystallization. R r ( N = 2–5) values ≥97% and almost unaffected T trans values gave evidence for the good strain recoverability and reliable shape-memory properties. As commonly detected in segregated polyurethane block copolymers, higher R r,tot ratios are favored at increased hard segment contents. [ 46,47 ] This is in agreement with the present R r,tot , whose deteriora-tion from the fi rst to the preceding cycles was weakest at growing hard segment content. Apparently, the higher cross-link density provided by hard segment domains prevented viscous fl ow during repeated deformation and supplied higher maximum recovery stresses.

4 . Conclusion

The physical properties of three PEU block copolymers have been investigated. Swelling and uniaxial tensile tests gave evidence for the different compositions and the resulting network structures. As shown by DSC and DMA, the increase in soft segment content gave larger fractions of meltable and crystallizable PBA units, whose melting and crystallization temperatures shifted toward higher values. Every single PEU had its particular thermome-chanical specifi cations; higher soft segment contents sup-plied better strain fi xities and strain recoveries. In return, the increase in cross-link density favored higher loading stresses and facilitated more pronounced stress recovery behavior.

The thermal properties of PEU depended on the poly-morphic behavior of the PBA phase. DSC measurements proved that a lowering of the cooling rate, or alternatively, the extension of room temperature holding time after PBA crystallization were useful techniques to increase the amount of thermodynamically stable PBA α crys-tals by in parallel reducing the fraction of meta-stable β

2Macromol. Chem. Phys. 2


crystals, structurally verifi ed by WAXS. By transferring these fi ndings to our programming routes, the onset tem-perature of stress recovering could be raised by 6 °C. In return, we were able to show for PEU-h(ssc) that storing programmed specimens at –2 °C is an adequate way to prevent the crystal–crystal ( β to α ) transformation, such that a programmed thermoresponsiveness can be main-tained. Finally, we would like to note that thermome-chanical measurements and the resulting stress-temper-ature derivatives seem to be an appropriate analytical technique for studying the thermal stability of polymor-phic crystals in semicrystalline elastomers, which at least in the melting paths allows for an accurate assignment of polymorphic phases.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : The authors acknowledge fi nancial support from the German Federal Ministry of Education and Research (BMBF, project funding reference number 03V0043). The authors thank Dietmar Neubert for conducting the DSC measurements, Petra Fengler for the DMA measurements, Carsten Vogt and Dietmar Schulze for their support regarding the swelling experiments, Franz-Georg Simon for providing the equipment to determine the fracture mechanical properties of the PEUs, Klaus-Jürgen Wenzel for the WAXS measurements and Andreas Thünemann and Wolfgang Stark for the fruitful discussions and acknowledge the kind collaboration with Bayer MaterialScience AG.

Received: July 10, 2013; Revised: August 8, 2013; Published online: September 11, 2013; DOI: 10.1002/macp.201300464

Keywords: shape-memory polymers ; stimuli-sensitive polymers ; poly(ester urethane) ; polymorphism

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poly(ester urethane) with varying polyester chain length: polymorphism and shape-memory behavior

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