Structure and Shape Memory Properties of Polyurethane Copolymers

This article was downloaded by: [Hong Kong Polytechnic University] On: 19 November 2012, At: 23:05 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Macromolecular Science, Part B: Physics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lmsb20

Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments a

a

a

Feng Long Ji , Jin Lian Hu , Wing-Man Winnie Yu & Stephen SinYin Chiu

b

a

Institute of Textile and Clothing, Hong Kong Polytechnic University, Hong Kong, P.R. China b

Department of Chemistry, Hong Kong University, Hong Kong, P.R. China Accepted author version posted online: 27 Jun 2011.Version of record first published: 14 Nov 2011.

To cite this article: Feng Long Ji, Jin Lian Hu, Wing-Man Winnie Yu & Stephen Sin-Yin Chiu (2011): Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments, Journal of Macromolecular Science, Part B: Physics, 50:12, 2290-2306 To link to this article: http://dx.doi.org/10.1080/00222348.2011.562091

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

R Journal of Macromolecular Science
, Part B: Physics, 50:2290–2306, 2011 Copyright © Taylor & Francis Group, LLC ISSN: 0022-2348 print / 1525-609X online DOI: 10.1080/00222348.2011.562091

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments FENG LONG JI,1 JIN LIAN HU,1 WING-MAN WINNIE YU,1 AND STEPHEN SIN-YIN CHIU2 1

Institute of Textile and Clothing, Hong Kong Polytechnic University, Hong Kong, P.R. China 2 Department of Chemistry, Hong Kong University, Hong Kong, P.R. China Shape memory polyurethanes are usually fabricated with low-molecular weight polyols through a two-step copolymerization, which often results in difficulty attaining both desired shape memory switch temperature and optimal thermomechanical properties. Here we present a series of shape memory polyurethane copolymers having urethane chains as soft segments. The structure and shape memory properties of copolymers were investigated with differential scanning calorimetry, dynamic mechanical analysis, small angle x-ray scattering, and thermomechanical tests. Increasing the length of the urethane soft segments enhanced phase separation, while it brought little change to the glass transition temperature (Tg ). Based on the urethane soft segments, some rigid chain extenders could be readily introduced into the backbone of copolymers, resulting in better phase separation. All polyurethane copolymers exhibited more than 90% of shape recovery. The shape recovery of the materials was proved to be inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The copolymers containing longer soft and hard segments and rigid chain extenders exhibited higher deformation stress and thus larger recovery stress. The copolymerization employing urethane chains as soft segments can greatly expand flexibility for molecular design and favor the optimization of shape memory properties. Keywords copolymerization, phase separation, polyurethane, recovery stress, SAXS, shape memory

Introduction As a class of smart materials, shape memory polyurethanes (SMPUs) are considered promising for many applications such as actuators,[1] tools for disabled persons,[2] biomedical materials,[3] biodegradable sutures,[4,5] and smart fibers.[6,7] Typically, SMPUs are segmented polyurethanes comprising hard and soft segments, which tend to separate into hard and soft phases, forming physically cross-linked polymer networks.[8–13] The hard domains play the role of a fixed phase for maintaining polymer networks and memorizing the permanent shape of materials, while the soft phase serves as reversible phase whose thermal transition temperature (T trans ) is the switch temperature for trigging shape memory effect.[8,10,13] The Received 10 June 2010; revised 27 November 2010; accepted 30 November 2010. Address correspondence to Jin Lian Hu, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P.R. China. E-mail: [email protected]

2290

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

SMPU with Urethane Chains as Soft Segments

2291

reversible phase can be either amorphous[8,9,12–16] or crystalline[10,11,17–20] and thus T trans can be either a glass transition temperature (T g ) or a melting transition temperature (T m ). These two types of SMPUs are designated as T g -SMPUs and T m -SMPUs, respectively. Several investigations have been made on T g -SMPUs.[8,9,12,14] The properties, including T g , shape recovery, shape fixity, and recovery stress, of SMPUs are defined by the characteristics of the phases and the morphological structure such as phase compositions, domain sizes, and phase interconnectivity. T g of the T g -SMPUs is usually close to or above room temperature. Higher T g of the soft phase can be attained either by employing short polyols, which result in the pronounced phase mixing, or by increasing the hard-segment content (HSC), which enhances the physical cross-linking density. T g -SMPUs are usually synthesized from polyols having a number average molecular weight of 300–1000 through a typical two-step copolymerization route, wherein the polyols are first end-capped with isocynate (NCO) groups followed by chain extension with short diols.[8–10] As illustrated in Fig. 1, the T g -SMPUs synthesized through the typical two-step copolymerization route have shorter hard segments, which is unfavorable for phase separation and the formation of hard domains. Hence, increasing the length of hard segments and thus raising the stability of hard domains are desired for the development of high-performance T g -SMPUs. By changing the species of monomers and chemical compositions, the T g of SMPUs can be readily located at a specific temperature. However, in order to synthesize T g -SMPUs with desired T g as well as optimal overall shape memory properties, researchers need a series of polyols having different lengths and chemical structures. Therefore, it is necessary to develop an expansion of the molecular design flexibility by varying the copolymerization route with the limited types of commercially available polyols. This report presents a modified two-step copolymerization route employing urethane chains, made of short polyols and 4,4 -diphenylmethane diisocyanate (MDI) as soft segments, which is anticipated to expand the flexibility of molecular design of SMPUs. At first, polypropylene glycols with M w ≈ 400 (abbreviated hereafter as PPG400) were used to alternatively copolymerize with MDI to produce NCO group-terminated urethane prepolymers. Afterwards, the prepolymers were reacted with stoichiometric MDI and chain extenders to produce the T g -SMPUs. The “lone” MDI between polypropylene glycol diols is considered miscible with polyols.[21] Hence, the urethane chains composed of PPG400 polyols and MDIs can be viewed as soft segments in T g -SMPUs, as shown in Fig. 1. The block lengths of both soft and hard segments are increased through modified

Figure 1. Structural views of T g -SMPUs synthesized through different copolymerization routes.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2292

F. Long Ji et al.

copolymerization. In order to raise the stability of hard domains, some rigid chain extenders have been utilized in the synthesis of T m -SMPUs.[22,23] In contrast, no such investigation has been made on T g -SMPUs. We have used the rigid chain extender 4,4 -Methylene-bis(2chloroaniline) (MOCA), MDI, and PPG400 polyol to synthesize T g -SMPUs through the typical two-step copolymerization. But the resultant polymers exhibited much higher T g (>90◦ C). This could be attributed to the prominent phase mixing of short hard segments and PPG400 polyols. Here we used bis(2-hydroxyethyl)ether (HQEE) and MOCA to replace 1,4-butanediol (BDO) in addition to synthesizing T g -SMPUs through the modified two-step copolymerization route. We characterized the morphological structure of T g -SMPUs by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and small angle x-ray scattering (SAXS). The shape memory properties, including shape recovery, shape fixity, and recovery stress, of T g -SMPUs were investigated through a series of thermomechancial tests. Use of urethane chains as soft segments was proved to result in the formation of stable hard domains and thus raise the recovery stress. Through modified copolymerization, the rigid chain extenders can be readily incorporated into T g -SMPUs without raising T g dramatically. This study is expected to cast some light on the fundamentals of developing high-performance T g -SMPUs.

Experimental Materials and Synthesis The PPG400 polyols were dried at 100◦ C under vacuum of 5–10 mm Hg for 2 h prior to usage. MDI (Aldrich, St. Louis, MO, USA), HQEE (Aldrich, St. Louis, MO, USA), and MOCA (Suzhou Xiangyuan Special Fine Chemical Co., Suzhou, China) were used as received. N,N -dimethylformamide (DMF, Aldrich, St. Louis, MO, USA) and BDO (International Laboratory, San Bruno, CA, USA) were dried with 4 Å molecular sieves in advance for usage. T g -SMPUs were synthesized through a two-step copolymerization route; the detailed formulations are shown in Table 1. The isocyanate-terminated prepolymers were first prepared by reacting dried PPG polyols with mole excessive MDI in DMF solution at 65◦ C for 3 h. Then the prepolymers were reacted with stoichiometric MDI and the chain extenders BDO, HQEE, or MOAC at 80–100◦ C for another 4 h. A series of T g -SMPUs having urethane chains as soft segments are designated as iPPG400-b-xx or iiPPG400-b-xx, where “b” refers to the T g -SMPUs chain extended with BDO, “xx” is the weight percentage of HSC (hereafter the notation HSC of the copolymers produced through modified twostep copolymerization refers to HSC2 as shown in Table 1). Likewise, iPPG400-h-xx and iPPG400-m-xx denote the T g -SMPUs chains extended by HQEE and MOCA, respectively. Characters i and ii, respectively, refer to the urethane prepolymers (MDI-PPG400)4 -MDI and (MDI-PPG400)9 -MDI. As a control, a polyurethane copolymer was prepared in a typical two-step copolymerization and is designated as PPG400–60 (see Table 1). Film Preparation Thin films of the T g -SMPUs were fabricated by casting DMF solutions into rectangular Teflon molds and drying in a vacuum oven at 80◦ C for 24 h. To eliminate thermal history, the films were heated up to 120◦ C and cooled gradually to room temperature for another 24 h. The thickness of the films was about 0.1–0.2 mm.

SMPU with Urethane Chains as Soft Segments

2293

Table 1 Feed ratios of T g -SMPUs

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

Samples iPPG400-b-35 iPPG400-b-40 iPPG400-b-45 iiPPG400-b-35 iiPPG400-b-45 iPPG400-h-35 4 iPPG400-h-40 4 iPPG400-m-35 iPPG400-m-40 PPG400–60 ∗

1st 2nd (mole) (mole) PPG400 MDI MDI 4 4 4 9 9 5 5 4 4 1

5 5 5 10 10 1.27 1.94 5 5 —

1.98 2.87 3.92 7.13 11.00 — — 0.96 1.55 1.78

BDO 2.98 3.87 4.92 8.13 12.00 2.27 2.9 — — 0.78

HSC1 ∗ HSC2 ∗∗ HQEE MOCA (wt%) (wt%) — — — — — — — — 2.55 —

— — — — — 55.76 59.15 1.96 59.16 —

55.72 59.14 62.55 58.20 63.74 35.00 40.00 55.71 40.00 60.00

35.00 40.00 45.00 35.00 45.00

35.00 —

HSC1 refers to the sum of weight percentages of MDI and chain extenders. HSC2 refers to the weight percentage of hard segments.

∗∗

Characterization The thermal properties of T g -SMPU DSC tests were investigated using the DSC (Perkin–Elmer, Waltham, MA, USA) purged with nitrogen gas and cooled with an intracooler. The specimens made from polyurethane films were scanned from −50◦ C to 250◦ C at a heating rate of 10◦ C/min. DMA measurements were carried out on the Perkin–Elmer Diamond DMA in tensile mode at a fixed frequency of 1 Hz. The specimens were heated from −50◦ C to 150◦ C at a heating rate of 3◦ C/min whereby the quantities of storage modulus and loss modulus were recorded. SAXS experiments were performed on an SAXS equipment Nanostar (Bruker Co. Ltd., Karlsruhe, Germany), which employs Cu (Kα wavelength, λ = 0.154 nm) as radiation source. The collimation system consisted of two cross-coupled Gobel Mirrors and four pinholes. The detector was a Bruker AXS HI-STAR position sensitive area detector. The incident beam had a diameter of about 1 mm. The sample to detector distance was about 1.061 m. The scattering range was q = 0.1–2.8 nm−1, where q = (4π /λ) sin (θ /2) is the scattering vector (θ is the scattering angle). The shape memory effect tests were done through a series of thermomechanical cyclic tensile tests, which were conducted on an Instron 5566 accompanied by a constant temperature chamber. The samples of the polyurethane thin film were firstly elongated to 100% of strain at a speed of 50%/min at T g + 15◦ C. Afterwards the samples were quenched down to room temperature (ca. 22◦ C) and kept for 30 min. After removing the external force, the sample underwent shape recovery by being heated for 20 min at a recovery temperature, T rec = T g + 15◦ C, where the next cycle started. In the Nth cycle, the shape fixity Rf (N) and the shape recovery Rr (N) were calculated with the following equations: Rf (N) =

εu (N ) × 100%, εm

(1)

Rr (N) =

εm − εp (N ) × 100%, εm

(2)

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2294

F. Long Ji et al.

where εm denotes the maximum strain in the cyclic tensile tests; εu represents the residual strain after unloading at T room , and εp refers to the residual strain after shape recovery. The long-term shape fixing behaviors of T g -SMPUs after unloading were examined with the Instron. At first, a sample was elongated to εm = 100% followed by being cooled down to T room and kept for 30 min prior to unloading. Subsequently, the upper clamp of the Instron returned to its original position. By manually adjusting the upper clamp to make film sample just straight, the shape fixity changing with time could be recorded. Likewise, the shape recovery of the samples was recorded by heating the temporarily fixed samples at a rate of 2◦ C/min from 22◦ C to a temperature high enough where no more shape recovery could be observed. In order to evaluate recovery stress, the sample was fixed in advance into a temporary shape. The shape programming conditions were identical to those for shape recovery tests. Then the sample was adjusted to be just straight. Being held at this fixed strain, the sample was heated at a rate of 2◦ C/min from 22◦ C up to a sufficiently high temperature where the stress apparently decreased. Hence, the recovery stress released in heating was recorded by the Instron.

Results and Discussion DSC Figure 2 presents the DSC thermograms of the T g -SMPUs; the thermal properties, including T g , heat capacity change at T g (Cp ), melting temperature (T m ), and heat of fusion (H m ), are summarized in Table 2. The copolymer PPG400–60 was synthesized through a typical two-step copolymerization, while iPPG400-b-40 was prepared through a modified one. These have similar chemical compositions, i.e. the same as HSC1 (see Table 1). A deflection appears at ca. 34◦ C on the thermogram of iPPG400-b-40, which is attributed to the glass transition of the soft phase (see Fig. 1(a)).[8,9,14,24] A broad and weak endothermic peak observed at ca. 150◦ C is ascribed to the melting of the hard phase.[8,9,14] In contrast, PPG400–60 exhibits no endothermic behavior above 100◦ C and shows only a glass transition at ca. 38◦ C. The different thermal properties of PPG400–60 and iPPG400-b40 suggest that the modified copolymerization enhances phase separation and promotes the formation of hard domains because of longer soft and hard segments. According to the investigations of Cooper and coworkers,[25,26] the endotherm in the temperature range 120–190◦ C should be attributed to disruption of the long-range order of the hard phase. Here the melting transitions of the T g -SMPUs containing MDI/BDO hard segments were all in this range. Hence, the hard phase of the T g -SMPUs should be considered as long-range ordered instead of crystalline. As would be expected, T m and H m rose gradually with the growth of HSC (see Figs. 2(b) and (c) and Table 2), resulting from the increasing fraction of hard phase. Meanwhile, T g was also raised gradually, which primarily arises from the increasing limitation imposed on the soft segments by hard domains. The Cp values decreased with the increase of HSC owing to the decreasing fraction of soft segments. The T g values of the T g -SMPUs having rigid chain extenders are close to those of the chains extended by BDO (see Figs. 2(d) and (e) and Table 2). This suggests that incorporating rigid chain extenders into the T g -SMPUs through the modified two-step copolymerization did not raise T g significantly. The iPPG400-h-xx polyurethanes exhibited endothermic behaviors in the range 220–230◦ C, indicating the melting of the hard phase. In contrast, the DSC traces of the iPPG400-m-xx T g -SMPUs displayed no endothermic peak below 250◦ C.

2295

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

SMPU with Urethane Chains as Soft Segments

Figure 2. DSC traces of T g -SMPUs.

DMA Figure 3(a) illustrates the plots of dissipation factor (tanδ) against temperature obtained from the DMA tests of PPG400–60 and iPPG400-b-40. The T g value, corresponding to the peak maximum of the tanδ plot, of PPG400–60 is centered at ca. 59◦ C, while that of iPPG400-b-40 is situated at ca. 53◦ C (see Fig. 3(a) and Table 2). This is consistent with the DSC results, suggesting that employing urethane chains as soft segments gives rise to a lower T g owing to less phase-mixing. Moreover, the tanδ plot of PPG400–60

2296

F. Long Ji et al.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

Table 2 Thermal properties and inter-domain spacings of T g -SMPUs

Samples

Tg (◦ C)

DSC CP [J/(g × ◦ C)]

Tm (◦ C)

DMA H m (J/g)

Tg (◦ C)

SAXS E +15 (Mpa)

d (nm)

iPPG400-b-35 iPPG400-b-40 iPPG400-b-45 iiPPG400-b-35 iiPPG400-b-45 iPPG400-h-35 iPPG400-h-40 iPPG400-m-35 iPPG400-m-40 PPG400–60

34.11 35.24 37.43 32.22 36.10 32.89 34.56 30.57 32.36 —

0.62 0.48 0.43 0.58 0.44 0.53 0.42 0.43 0.40 —

156.91 166.66 173.84 159.97 174.89 221.89 228.56 — — —

2.65 3.48 6.72 3.29 8.68 9.20 8.12 — — —

49.61 52.86 59.85 48.56 58.11 48.15 52.26 53.32 55.51 58.71

34.1 58.2 187.2 56.1 231.8 87.9 231.8 109.8 245.8 2.2

16.75 15.86 13.97 16.02 13.47 15.54 13.95 — 14.08 —

rises abruptly at ca. 80◦ C, whereas that of iPPG400-b-40 increases dramatically above 120◦ C. Some researchers have attributed the rise of a tanδ plot appearing above T g of the soft phase to the glass transition of hard phase.[10,27] Since iPPG400-b-40 has long hard segments, we conclude that the tanδ rise of iPPG400-b-40 appearing above 120◦ C is related to the glass transition or melting transition of the hard phase. On the other hand, the tanδ rise of PPG400–60 should be ascribed to the disassociation of weak physical interactions, such as hydrogen bonds, because it is homogeneous and has no hard phase domains. With the increasing HSC, the T g peak maxima shift to higher temperature (see Figs. 3(b) and (c)), which is in agreement with the DSC results. The iPPG400-h-xx and iPPG400-m-xx copolymers exhibit the same tendency (see Figs. 3(d) and (e)). The T g s of iiPPG400-b-xx were slightly lower than those of iPPG400-b-xx with identical HSC (see Table 2). This suggests that increasing block lengths of soft and hard segments reduces phase mixing, resulting in the decrease of T g of the soft phase. Moreover, the incorporation of rigid chain extender HQEE brought little change to T g (see Table 2). In contrast, the incorporation of rigid chain extender MOCA raised T g s slightly (see Table 2). This is because the presence of MOCA can increase the physical cross-linking density, which intensifies the limitation imposed on soft segments resulting in the rise of T g s. But as a whole, incorporating rigid chain extenders through the modified two-step copolymerization route did not raise T g dramatically. Figure 4 presents the plots of storage modulus against temperature of the T g -SMPUs from which the storage modulus at T g + 15◦ C (E +15 ) were extracted and are listed in Table 2. The rubbery modulus of segmented polyurethanes is considered to reflect their physical cross-linking density.[10] The rubbery modulus plateau of iPPG400-b-40 was both higher and longer than that of PPG400–60 (see Fig. 4(a)), resulting from the presence of the hard phase as physical cross-links in the former. With the increase of HSC, the rubbery plateau of the T g -SMPUs was elevated steadily (see Figs. 4(b), (c), (d), and (e)), arising from the increasing physical cross-linking density. Moreover, the E +15 values were raised by increasing the length of soft and hard segments or by employing rigid hard segments (see Table 2), also due to the increasing physical cross-linking density.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

SMPU with Urethane Chains as Soft Segments

2297

Figure 3. Plots of tanδ against temperature of T g -SMPUs.

SAXS Figure 5 illustrates the SAXS profiles of T g -SMPUs. A distinct scattering peak appears in the curve of iPPG400-b-40, indicating the presence of two-phase morphology (see Fig. 5(a)). On the other hand, PPG400–60 gave no such scattering peak, suggesting that hard segments were dissolved in the amorphous soft matrix. This is consistent with the DSC and DMA results and verifies that employing the urethane chains as soft segments promotes phase separation. The other copolymers exhibited more or less phase separation (see Figs. 5(b), (c), (d), and (e)). With the increase of HSC, the scattering peak maxima shifted toward higher angles, suggesting that the long period (d) of copolymers decreased

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2298

F. Long Ji et al.

Figure 4. Plots of storage modulus against temperature of T g -SMPUs.

resulting from the increasing number of hard domains, i.e. physical cross-linking density. The iiPPG400-b-xx T g -SMPUs had smaller d in comparison with the iPPG400-b-xx ones (Table 2), indicating that longer soft and hard segments enhanced phase separation and thus promoted the formation of hard domains. The d value iPPG400-m-40 was smaller than that of iPPG400-b-40, indicating that the concentration of hard domains in the former was higher than that in the latter. This is because the MOCA chain extenders can enhance phase separation and favor the formation of the hard domains in comparison with the BDO chain extenders. Likewise, the iPPG400-h-xx materials exhibited smaller d values as compared

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

SMPU with Urethane Chains as Soft Segments

2299

Figure 5. SAXS profiles of T g -SMPUs.

with the iPPG400-b-xx ones that correspondingly have equal HSC, since the presence of HQEE chain extenders promotes the formation of hard domains.

Thermomechanical Cyclic Tensile Tests According to the polymer viscoelasticity theory, the deformation strain ε of the T g -SMPUs is given by[18] ε = εe + εvis + εir ,

(3)

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2300

F. Long Ji et al.

where εe refers to the elastic deformation strain, which can be recovered instantly after the removal of external force; εvis is the viscoelastic deformation strain, which can be restored gradually by increasing temperature or time; and εir is the irreversible deformation strain, which cannot be restored. We found that in the large deformation of SMPUs, εe , i.e. the initial linear region of the deformation curves, usually occupied a small part. As will be shown in the following section, εir was no more than 10%. Hence, the shape memory behaviors of the T g -SMPUs were largely dependent on the generation, fixing, and recovery of εvis . As physical cross-links, the hard domains are not point cross-links and may occupy a very large volume and are deformable.[28] Thus, εvis arises from both the viscoelastic deformation strain of the soft phase (εvis (SS)) and the hard phase (εvis (HS)). Generally, εvis (SS) can be recovered above T g , whereas εvis (HS) needs to be recovered at higher temperature because of the lower molecular mobility of the hard phase. As a result, the shape recovery region can be broadened if εvis (HS) accounts for a high percentage. The phase composition and the modulus of soft and hard phases underline the fractions of εvis (SS) and εvis (HS) and eventually define the shape memory behaviors. Figure 6 demonstrates the thermomechanical cyclic tensile tests of copolymers wherefrom the shape memory parameters, including shape recovery Rr (1), Rr (4), and shape fixity Rf (1) calculated from Equations (1) and (2), are summarized in Table 3. The Rr (1) and Rf (1) values of all copolymers were more than 90% (see Table 3). The shape recovery values of the materials were reduced by 2–4% after four thermomechanical cycles. The iiPPG400-b-xx copolymers exhibited lower Rr (1) in comparison with the iPPG400-b-xx ones having identical HSC (see Figs. 6(a) and (c) and Table 3). This is because the higher fraction of hard phase in iiPPG400-b-xx samples gave rise to larger εvis (HS), which cannot be recovered at T g + 15◦ C. Likewise, because of their higher fraction of hard phase domains, the iPPG400-h-xx and the iPPG400-m-xx copolymers showed lower Rr (1) in comparison with the iPPG400-b-xx ones (see Figs. 6(a) and (b) and Table 3). Moreover, the values of Rr (1) basically decreased with the increasing HSC, also due to the increasing

Figure 6. Thermomechanical cyclic tensile tests of T g -SMPUs (Color figure available online).

SMPU with Urethane Chains as Soft Segments

2301

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

Table 3 Thermomechanical and shape memory properties of T g -SMPUs

Samples

Rr (1) (%)

Rr (4) (%)

Rr (1)– Rr (4) (%)

Rf (1) (%)

Rf 120 (%)

εir (%)

σ r max (Mpa)

σ r max / σ 100 (%)

iPPG400-b-35 iPPG400-b-40 iPPG400-b-45 iiPPG400-b-35 iiPPG400-b-45 iPPG400-h-35 iPPG400-h-40 iPPG400-m-35 iPPG400-m-40

97.3 94.5 93.6 90.5 92.7 93.1 93.1 96.1 93.0

95.0 90.8 91.3 88.8 89.2 89.1 89.1 93.8 90.0

2.3 3.7 2.3 1.7 3.5 4.0 4.0 2.3 3.0

99.0 99.1 98.8 99.1 98.5 99.2 98.8 98.7 98.8

85.4 89.2 95.5 92.2 96.6 88.4 91.4 93.5 95.0

3.2 5.5 7.0 5.4 7.0 5.5 5.5 7.1 7.6

2.16 2.73 3.70 2.52 4.86 2.91 4.37 2.42 5.87

61.19 63.46 70.53 65.14 72.10 66.90 68.88 61.15 66.32

fraction of hard phase domains (Table 3). The Rr (1) values of iPPG400-m-xx were higher than those of iPPG400-h-xx because of the stronger MDI/MOCA hard domains, which resulted in lower εvis (HS). Therefore, it can be concluded that Rr (1) is inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The deformation stresses at 100% of elongation strain (σ 100 ) of T g -SMPUs are listed on the thermomechanical graphs. σ 100 increased monotonically with the increase of fraction of hard phase and the strength of hard domains. For example, iiPPG400-b-xx, iPPG400h-xx, and iPPG400-m-xx copolymers exhibited higher σ 100 values in comparison with the iPPG400-b-xx ones having identical HSC (see Figs. 6(a), (b), and (c) and Table 3). Moreover, σ 100 increased with the increase of HSC (see Table 3). It can be seen that the rigid chain extenders can indeed raise σ 100 and the effect was more prominent as HSC increased from 35 to 40% (see Figs. 5(a) and (b)), indicating that the rigid hard segments should be long enough to have a pronounced effect for reinforcing T g -SMPUs. Shape Fixing The aforementioned shape fixity refers to the instant shape fixity values derived from the unloading process in thermomechanical cyclic tensile tests. It is reported that some shape memory polyurethane foams could not hold their shape fixity and gradually recovered their original shape at room temperature.[29] Figure 7 shows the shape fixity evolution with time after unloading at room temperature of the iPPG400-b-xx material. With the elapse of

Figure 7. Evolution of the shape fixity of iPPG400-b-xx with time after unloading.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2302

F. Long Ji et al.

time, the shape fixity curves of T g -SMPUs decrease progressively, which can be explained as follows. By being cooled below T g , the micro-Brownian motions of soft segments are mostly limited. Nevertheless, the molecular mobility of the soft segments is not entirely suppressed regardless of their long characteristic times. In addition, some soft segments may be in the rubbery state if the shape fixing temperature is in the glass transition region, which may result in the rapid decrease of shape fixity. It was also found that the shape fixity of the iPPG400-b-xx copolymers decreased more and more slowly with the increasing time. We estimated the shape fixity at 120 min after unloading (Rf 120 ) from curve fitting based on an exponential decay model. Because of higher T g , the polyurethane copolymers with higher HSC exhibited higher Rf 120 , as shown in Table 3. We also found that T g -SMPUs with longer soft and hard segments and those having rigid hard segments exhibit higher Rf 120 owing to higher fraction of hard phase or higher T g . In short, at 120 min after unloading, all T g -SMPUs preserve more than 85% of shape fixity. Shape Recovery Figure 8 illustrates the plots of shape recovery versus heating temperature where the starting shape recovery values of the curves are roughly equal to 100% Rf 120 . With the increase of temperature, the shape recovery of a polyurethane copolymer remains constant or increases slowly at the beginning followed by an abrupt rise in a narrow temperature range. The shape recovery grows up to above 90% and stops increasing at that temperature where the irreversible deformation strain (εir ) is attained. With the increase of HSC, the shape recovery

Figure 8. Shape recovery curves of T g -SMPUs.

SMPU with Urethane Chains as Soft Segments

2303

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

plots of T g -SMPUs shift toward higher temperatures, which is caused by the increasing T g of T g -SMPUs (see Fig. 8(a)). In addition, as compared with those of iPPG400-b-xx copolymers, the shape recovery curves of iiPPG400-b-xx, iPPG400-h-xx, and iPPG400-mxx T g -SMPUs move toward higher temperatures because of either larger fraction of hard phase or higher T g (see Figs. 8(b), (c), and (d)). This suggests that changing the block lengths of soft and hard segments and employing rigid chain extenders gave rise to the variation of morphological structure of T g -SMPUs and also influenced their shape memory behaviors. In other words, modifications of a typical two-step copolymerization can expand the flexibility of molecular design. Recovery Stress Recovery stress is essential for the applications of shape memory polymers (SMPs) because many of them are based on shape fixing ability, shape recoverability, and recovery stress or their combinations.[1,2] As compared with shape memory alloys, SMPs generate much lower recovery stress, which is their major disadvantage. They have always been considered useful in the applications involving lower recovery force or free recovery.[30] A number of attempts have been made to study the recovery stress of T m -SMPUs,[17,31,32] but we know of no investigations that have been done on the recovery stress of T g -SMPUs. Figure 9 demonstrates the evolution of recovery stress of T g -SMPUs with the increase of temperature. The recovery stress of T g -SMPUs should arise from the entropic part

Figure 9. Recovery stress curves of T g -SMPUs.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2304

F. Long Ji et al.

of their deformation stress. The preservation of entropic energy is dependent on their shape fixity. Moreover, the stress may inevitably decrease more or less during storage and release because of stress relaxation. Furthermore, the modulus of polymers decreases with the increase of temperature, which can lead to the decay of recovery stress. Therefore, the recovery stress evolution involves complex processes, including stress release, stress relaxation, and modulus depression. The recovery stress of a copolymer appears at certain temperature and rises steadily with the increasing temperature up to a peak point where a maximum stress (σ rmax ) is attained. After the peak maximum it decreases gradually as temperature increases further, which is caused by stress relaxation and the decreasing modulus. As compared with iPPG400-b-xx T g -SMPUs, iiPPG400-b-xx ones exhibited higher σ rmax values, primarily arising from their higher deformation stress (see Figs. 9(a) and (c)). Likewise, iPPG400-h-xx and iPPG400-m-xx copolymers showed higher σ rmax values in comparison with iPPG400-b-xx ones (see Figs. 9(a) and (b)). It was also noted that σ rmax increased with the increase of HSC (see Table 3). The σ rmax values of copolymers varied in the range 2–6 MPa with the changing lengths of soft and hard segments and types of chain extenders. The parameter stress conversion ratio (σ rmax /σ 100 ) is proposed to reflect the amount of deformation stress converting into recovery stress. It can be seen that the σ rmax /σ 100 values covered the range of ca. 60–70%. In short, by changing the block length and employing rigid chain extenders, the physical cross-linking density can be adjusted. The T g -SMPUs having higher HSC and longer and more rigid hard segments gave rise to higher stress conversion.

Conclusions A series of T g -SMPUs having urethane chains as soft segments were prepared. In order to strengthen the physical cross-links, the rigid chain extenders HQEE and MOCA were incorporated into the hard segments of T g -SMPUs. DSC, DMA, SAXS, and thermomechanical tests were conducted to investigate the structure and shape memory properties of the T g -SMPUs. As compared with typical two-step copolymerization, employing urethane chains as soft segments can greatly enhance phase separation. Increasing the urethane chain length or using rigid chain extenders can improve phase separation and thus can increase deformation stress. The T g -SMPUs exhibited more than 90% of shape recovery at T g + 15◦ C. The shape recovery decreased with the rising fraction of hard phase and rose with the increasing strength of hard domains. After removing external force, the shape fixity of T g -SMPUs decreased. But it was still above 85% after 120 minutes. All T g -SMPUs recovered most of the deformation strain (more than 80%) in narrow temperature regions. The T g -SMPUs have 61–72% of deformation stress converted into recovery stress, which changed from 2 MPa to 6 MPa with varying lengths of soft and hard segments and types of chain extenders. The maximum recovery stress σ rmax can be elevated by increasing the length of soft and hard segments or by employing rigid chain extenders. In short, employing urethane chains as soft segments can expand the molecular design potential for developing high-performance SMPUs.

Acknowledgment This study was financially supported by the project “High Performance Advanced Materials for Textile and Apparel (Ref: GHS/088/04)” from Hong Kong Innovation Technology Funding (ITF).

SMPU with Urethane Chains as Soft Segments

2305

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

References 1. Koerner, H.; Price, G.; Pearce, N.A.; Alexander, M.; Vaia, R.A. Remotely actuated polymer nanocomposites-stress-recovery of carbon-nanotube-filled thermoplastic elastomers. Nat. Mater. 2004, 3, 115. 2. Tobushi, H.; Hara, H.; Yamada, E.; Hayashi, S. Thermomechanical properties in a thin film of shape memory polymer of polyurethane series. Smart Mater. Struct. 1996, 5, 483. 3. Meng, Q.H.; Hu, J.L.; Liu, B.H.; Zhu, Y. A low-temperature thermoplastic anti-bacterial medical orthotic material made of shape memory polyurethane ionomer: Influence of ionic group. J. Biomat. Sci. Polym. E. 2009, 20, 199. 4. Lendlein, A.; Schmidt, A.M.; Langer, R. AB-polymer networks based on oligo(ε-caprolactone) segments showing shape-memory properties. PNAS 2001, 98, 842. 5. Lendlein, A.; Langer, R. Biodegradable, elastic shape memory polymers for potential biomedical applications. Science 2002, 296, 1673. 6. Zhu, Y.H.J.; Yeung, L.; Liu, Y.; Ji, F.; Yeung, K. Development of shape memory polyurethane fiber with complete shape recoverability. Smart Mater. Struct. 2006, 15, 1385. 7. Meng, Q.H.; Hu, J.L.; Zhu, Y.; Lu, J.; Liu, Y. Morphology, phase separation, thermal and mechanical property differences of shape memory fibres prepared by different spinning methods. Smart Mater. Struct. 2007, 16, 1192. 8. Lin, J.R.; Chen, L.W. Study on shape-memory behavior of polyether-based polyurethane II. Influence of the hard-segment content. J. Appl. Polym. Sci. 1998, 69, 1563. 9. Lin, J.R.; Chen, L.W. Study on shape-memory behavior of polyether-based polyurethane II. Influence of the hard-segment content. J. Appl. Polym. Sci. 1998, 69, 1575. 10. Kim, B.K.; Lee, S.Y.; Xu, M. Polyurethanes having shape memory effects. Polymer 1996, 37, 5781. 11. Kim, B.K.; Lee, S.Y.; Lee, J.S.; Baek, S.H.; Choi, Y.J.; Lee, J.O.; Xu, M. Polyurethanes ionomers having shape memory effects. Polymer 1998, 39, 2803. 12. Lee, B.S.; Chun, B.C.; Chung, Y.C.; Sul, K.I.; Cho, J.W. Structure and thermomechanical properties of polyurethane block copolymers with shape memory effect. Macromolecules 2001, 34, 6431. 13. Lendlein, A.; Kelch, S. Shape-memory polymers. Angew. Chem. Int. Ed. 2002, 41, 2034. 14. Takahashi, T.; Hayashi, N.; Hayashi, S. Structures and properties of shape-memory polyurethane block copolymers. J. Appl. Polym. Sci. 1996, 60, 1061. 15. Kim, B.K.; Lee, J.S.; Lee, Y.M.; Shin, J.H.; Park, S.H. Shape memory behavior of amorphous polyurethanes. J. Macromol. Sci., Phys. 2001, 40, 1179. 16. Park, S.H.; Kim, J.W.; Lee, S.H.; Kim, B.K. Temperature-sensitive amorphous polyurethanes. J. Macromol. Sci., Phys. 2004, 43, 447. 17. Ping, P.; Wang, W.; Chen, X.; Jing, X. Poly(ε-caprolactone) polyurethane and its shape-memory property. Biomacromolecules 2005, 6, 587. 18. Ji, F.L.; Hu, J.L.; Li, T.C.; Wong, Y.W. Morphology and shape memory effect of segmented polyurethanes. Part I: With crystalline reversible phase. Polymer 2007, 48, 5133. 19. Yang, J.H.; Chun, B.C.; Chung, Y.C.; Cho, J.H. Comparison of thermal/mechanical properties and shape memory effect of polyurethane block-copolymers with planar or bent shape of hard segment. Polymer 2003, 44, 3251. 20. Li, F.; Zhang, X.; Hou, J.; Xu, M.; Luo, X.; Ma, D.; Kim, B.K. Studies on thermally stimulated shape memory effect of segmented polyurethanes. J. Appl. Polym. Sci. 1997, 64, 1511. 21. Garrett, J.T.; Runt, J.; Lin, J.S. Microphase separation of segmented poly(urethane-urea) block copolymers. Macromolecules 2000, 33, 6353. 22. Jeong, H.M.; Lee, J.B.; Lee, S.Y.; Kim, B.K. Shape memory polyurethane containing mesogenic moiety. J. Mater. Sci. 2000, 35, 279. 23. Rabani, G.; Luftmann, H.; Kraft, A. Synthesis and characterization of two shape-memory polymers containing short aramid hard segments and poly(3-caprolactone) soft segments. Polymer 2006, 47, 4251.

Downloaded by [Hong Kong Polytechnic University] at 23:05 19 November 2012

2306

F. Long Ji et al.

24. Bogart, V.; John, W.C.; Gibson, P.E.; Cooper, S.L. Structure-property relationships in polycaprolactone-polyurethanes. J. Polym. Sci. Polym. Phys. Ed. 1983, 21, 65. 25. Hesketh, T.R.; Van Bogart, J.W.C.; Cooper, S.L. Differential scanning calorimetry analysis of morphological changes in segmented elastomers. Polym. Eng. Sci. 1980, 20, 190. 26. Van Bogart, J.W.C.; Bluemke, D.A.; Cooper, S.L. Annealing-induced morphological changes in segmented elastomer. Polymer 1981, 22, 1428 27. Zhu, Y.; Hu, J.L.; Yeung, K.W.; Choi, K.; Liu, Y.Q.; Liem, H.M. Effect of cationic group content on shape memory effect in segmented polyurethane cationomer. J. Appl. Polym. Sci. 2006, 103, 545. 28. Petrovic, Z.S.; Ferguson, J. Polyurethanes elastomers. Prog. Polym. Sci. 1991, 16, 695. 29. Tey, S.J.; Huang, W.M.; Sokolowski, W.M. Influence of long-term storage in cold hibernation on strain recovery and recovery stress of polyurethane shape memory polymer foam. Smart Mater. Struct. 2001, 10, 321. 30. Wei, Z.G.; Sandstrom, R.; Miyazaki, S. Review shape-memory materials and hybrid composites for smart systems. J. Mater. Sci. 1998, 33, 3743. 31. Ping, P.; Wang, W.; Chen, X.; Jing, X. The influence of hard segments on two-phase structure and shape memory properties of PCL-based segmented polyurethanes. J. Polym. Sci. Polym. Phys. 2007, 45, 557. 32. Wang, W.; Ping, P.; Chen, X.; Jing, X. Polylactide-based polyurethane and its shape-memory behavior. Eur. Polym. J. 2006, 42, 1240.