Research Article Received: 1 May 2018
Revised: 7 June 2018
Accepted article published: 11 June 2018
Published online in Wiley Online Library: 16 July 2018
(wileyonlinelibrary.com) DOI 10.1002/pi.5656
Mechanically strong shape memory polyurethane for water vapour permeable membranes Suman Thakur,† Md Anwar Jahid† and Jinlian Hu* Abstract Water vapour permeable polymeric thin films possess significant importance in miscellaneous applications such as packaging, medical devices, controlled-release systems, electronics and biosensors. In this work, a series of shape memory polyurethanes (SMPUs) were synthesized by a two-step pre-polymerization technique with variations in hard to soft segments and molecular weight of macroglycol. DSC, Fourier transform infrared spectra, dynamic TGA and tensile testing were carried out to characterize and evaluate the properties of these synthesized SMPUs. The effect of the soft segment and the molecular weight of macroglycol on the thermal properties, mechanical properties and water vapour permeability of the synthesized SMPUs were investigated to achieve a good water vapour permeable membrane. We found that the synthesized SMPUs demonstrated a good water vapour transmission rate of over 1460 g m−2 day –1 as well as robust mechanical properties with tensile strength 19.8 MPa indicating a promising permeable polymeric thin film for many potential applications, especially as protective clothing. © 2018 Society of Chemical Industry Supporting information may be found in the online version of this article. Keywords: water vapor permeability; shape memory polyurethane; segmented polymer; thin film; breathable textile
INTRODUCTION
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Water vapour permeable polymeric thin films have attracted great attention from material scientists and have turned into a vital material in various applications, including protective garments/shoes, chemical/medical equipment, sophisticated electronics and construction materials.1–4 However, the main application of such permeable thin films is in garments, where they aid in addressing problems of heat build-up and condensation of moisture.3 Thus, breathable polymer thin films play a crucial role in outdoor garments. The target to incorporate such film in garments is to deliver the best possible compromise between protection and comfort, under extreme climatic conditions and the widest range of activities.5 In particular, it is required for a garment to promptly transport water vapour from the skin to the outside for maintaining physical and psychological health. This is known as water vapour permeability (WVP). Garments generally have a porous structure; therefore, they have WVP.6 However, it is difficult to attain adequate WVP to provide protection against rain and wind as well as under extreme climatic conditions. Various approaches have been adopted to achieve this, such as coating, finishing and laminating polymer films to a fabric for an improved WVP.3,7,8 In this context, it is important to mention that shape memory polyurethanes (SMPUs) demonstrate good WVP.9–11 These SMPUs are generally phase-segregated multi-segmented polyurethane wherein the soft or reversible phase is flexible and can be deformed elastically while the hard phase memorizes the original shape.12–15 Usually, SMPUs are defined by their transition temperature which corresponds to their glass transition temperature (T g ) or melting temperature (T m ) depending on the Polym Int 2018; 67: 1386–1392
amorphous or crystalline structure, respectively. It is reported that the mobility of the soft segments is increased above the transition temperature and helps to diffuse the moisture.16 The mobility of the soft segment is less below the transition temperature; therefore SMPUs behave as an insulator layer to maintain warmth and moisture. This study suggests that WVP may be significantly influenced by the soft segment and crystalline region. WVP mainly depends on the dissolution of water molecules in the thin films and subsequent diffusion of vapour due to the presence of a concentration gradient. Our earlier study showed that the permeability of an SMPU membrane decreases with the presence of crystallinity of the soft segments, as vapour is mainly diffused through amorphous regions.9 However, several studies have been carried out on SMPU based WVP membranes to investigate the effect of crystallinity and free volume. A detailed study of an SMPU based water vapour permeable membrane to adjust the WVP by tuning soft segments and the molecular weight of macroglycol is highly desirable. In this study, a series of SMPUs with different hard to soft segment and macroglycol contents were synthesized by a two-step pre-polymer technique. The thermal and mechanical properties, shape memory effect and WVP of these SMPUs were investigated.
∗
Correspondence to: J Hu, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China. E-mail:
[email protected]
† These authors contributed equally to this work. Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China
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Table 1. Composition of the reactants for the SMPUs Raw materials (g mol−1 ) Soft:hard segment
Sample
PEG1000
PEG2000
BDO
MDI
SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6
0.1 0.1 0.1 – – –
– – – 0.1 0.1 0.1
0.0245 0.052 0.084 0.122 0.178 0.243
0.1245 0.152 0.184 0.222 0.278 0.343
75:25 70:30 65:35 75:25 70:30 65:35
EXPERIMENTAL Materials Polyethylene glycol (PEG) (Alfa Aesar, Tewksbury, USA; molecular weight 1000 and 2000 g mol−1 ), 1,4-butanediol (BDO) (Acros Organics, New Jersey, USA) and methylene diphenyl diisocyanate (MDI) (Alfa Aesar, Fisher Scientific, New Jersey, USA) were purchased and used as received. BDO and N,N-dimethylacetamide (Alfa Aesar, Fisher Scientific, Tewksbury, USA) were vacuum distilled and kept over 4A-type molecular sieves before use. Other chemicals and solvents were of reagent grade and were used without further purification.
Synthesis of SMPU All experimental glass apparatus was dried in a furnace at 400 ∘ C for 8 h prior to synthesis. SMPUs were synthesized via a pre-polymerization technique. PEG was placed in a three-necked flask and degassed at 80 ∘ C for 1.5 h to remove entrapped moisture. After that, MDI was slowly added in the reaction flask and the reaction was continued for 3 h at 80 ∘ C under a nitrogen environment. After formation of the pre-polymer, the temperature was cooled to room temperature and BDO was slowly added to the pre-polymer with high speed stirring in a mechanical stirrer for homogeneous mixing. Finally, the mixture was heated for 1 h at 60 ∘ C for the chain extension process. The prepared SMPU was poured into a pre-heated (100 ∘ C) polytetrafluoroethylene mould for the drying process up to 24 h at 80 ∘ C. Following the same procedure, other sets of SMPUs were also synthesized using different chemical compositions as tabulated in Table 1.
Figure 2. FTIR spectra of SMPUs: curve (a), SMPU1; curve (b), SMPU2; curve (c), SMPU3; curve (d), SMPU4; curve (e), SMPU5; curve (f ), SMPU6.
Characterization The Fourier transform infrared (FTIR) spectra of the SMPUs were measured over the wavenumber range 4000–500 cm−1 with a Perkin-Elmer model Spectrum 100 FTIR spectrometer. Spectra were recorded with a resolution of 4 cm−1 and a scan number of 4. TGA was carried out using a simultaneous thermal analyser (Mettler Toledo TGA) with the temperature increasing from 25 ∘ C to 700 ∘ C at a heating rate of 10 ∘ C min−1 under a 50 mL min−1 nitrogen atmosphere. The melting behaviours of the samples were tested by DSC. DSC was carried out using a Perkin-Elmer diamond differential scanning calorimeter in a controlled nitrogen environment with an intracooler. The specimens were scanned from −60 ∘ C to 90 ∘ C at a scanning rate of 10 ∘ C min−1 and kept at −60 ∘ C for 1 min. Mechanical properties were investigated using an Instron-5566 universal tensile tester. The width of the specimens employed was 10 mm and the distance between the two clamps was 20 mm; the thickness of the SMPU films (around 30 μm) was measured with the aid of a micrometer (Mitutoyo, Japan). The tensile stress at maximum, Young’s modulus, the tensile stress and tensile strain at break were recorded for the mechanical properties. Thermomechanical cyclic test Thermomechanical cyclic testing was carried out using an Instron-5566 tensile tester equipped with a controlled heating chamber. The tensile load and displacement of gauge length were measured from the load cell (500 N) and moving crosshead respectively. The specimen dimensions of the tested film were 80 mm × 10 mm × 0.3 mm. A gauge length of 50 mm was maintained during testing. The total strain was 100% with an extension ramp of 10 mm min−1 . Cooling at ambient temperature and heating at 60 ∘ C were done at a thermal ramp of 5 ∘ C min−1 . The shape fixity (Rf ) and shape recovery ratio (Rr ) were calculated using shape fixity (%) =
shape recovery (%) =
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𝜀m − 𝜀p 𝜀m
× 100
where 𝜀u is the fixed strain, 𝜀m is the maximum strain and 𝜀p is the plastic or residual strain.
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Figure 1. The apparatus for measuring the WVP of SMPU film.
𝜀u × 100 𝜀m
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S Thakur, MA Jahid, J Hu
Figure 3. DSC curves: curve (a), SMPU1; curve (b), SMPU2; curve (c), SMPU3; curve (d), SMPU4; curve (e), SMPU5; curve (f ), SMPU6.
filled with deionized water as shown in Fig. 1. The membrane was placed over the top of the cup and secured to give a perfect seal between the cup and the membrane. The gap between the membrane and the water surface was about 4 mm. The cup was placed in a constant temperature chamber at the desired temperature. The difference in weight after 24 h was measured. The result for the WVP was calculated using the formula WVP =
G tA
where G is the weight change in grams, t is the duration of the test in hours and A is the test area in square metres. During all the WVP measurements, the air surrounding the membranes had a constant temperature and relative humidity. Sample thicknesses for all measurements were in the range 80–100 μm. On average three different readings were used for each WVP measurement, expressed in units of g m−2 day−1 (24 h). Figure 4. Stress–strain profiles of the SMPUs.
RESULT AND DISCUSSIONS
Table 2. Thermomechanical cyclic properties of the SMPUs
Sample
Plasticity, 𝜀p (%)
Shape fixity, Rf (%)
SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6
15.9 14.6 13.9 16.5 15.4 14.2
78.3 79.7 84.2 76.3 78.1 82.8
Shape recovery, Rr (%) 84.1 85.4 86.1 83.5 84.6 85.8
The thermomechanical cycle includes (i) stretching of the SMPU composite above T trans ; (ii) holding the SMPU composite under constraint; (iii) cooling the SMPU composite under constraint; (iv) relaxing the employed constraint; (v) reheating the SMPU composite for shape recovery. No plastic strain was observed after repeating five cycles. This finally ensures the structural integrity of the polymeric network for its prolonged and repetitive use with no plastic deformation.
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Water vapour permeability testing WVP was measured as per ASTM method E 96-80B. Briefly, a round mouthed plastic cup with diameter 60 mm and height 90 mm was
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Characterization of SMPUs FTIR spectra of SMPU1, SMPU2, SMPU3, SMPU4, SMPU5 and SMPU6 are shown in Fig. 2. The absorption band due to NCO groups disappeared as no intense and sharp band at about 2250–2270 cm−1 for the polymers was found, and this confirmed the completion of the reaction. The presence of characteristic bands at 1060–1090 cm−1 (N — H deformation vibrations), 1140–1175 cm−1 (C–O stretching vibrations), 1557–1580 cm−1 (C — N stretching and N — H bending character), 1720–1730 cm−1 (C=O stretching vibrations of urethane groups) and 3430 cm−1 (O–H free and N — H stretching of urethane group stretching vibrations) clearly confirmed the formation of the urethane linkage –NH–C(=O)–O– in the SMPUs.17 It is well established that the position and intensity of these vibrations are extremely sensitive to the strength and specificity of the hydrogen bonds that form.18 The phase separation in SMPU can be characterized by the measurement of the intensity and position of the hydrogen-bonded NH stretching vibrations. Usually, a significant amount of N–H···O=C (urethane) hydrogen bonding indicates extensive phase separation. Thus, the tendency of phase separation increases with increase in the hard segment content. The C=O stretching frequencies are very complex for such polyurethanes as they are influenced by C–N stretching and N–H bending. In the case of –C=O stretching frequency a gradual shift from 1710 to 1695 cm−1 was observed with increase in the hard segment content in the SMPUs.19 This suggested that
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Mechanically strong shape memory polyurethane
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Figure 5. (a) Tensile modulus and (b) toughness of the SMPUs.
Figure 6. Thermomechanical cyclic test curves of the SMPUs.
with the increase in hard segment content the extent of H bonding increases and it demonstrated the different phase separation behaviours of the SMPUs. The effect is more pronounced in PEG1000 based SMPUs.
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Mechanical properties The mechanical properties of the SMPUs were evaluated to examine their suitability as membranes. Generally, mechanical properties depend on several factors, namely the presence of hydrogen bonds, polar groups within the polymeric chains, the existence of intermolecular and intramolecular interactions,
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Thermal properties In the DSC curves (Fig. 3), we noticed that PEG2000 based SMPU showed a peak for T m whereas no such T m was observed for PEG1000 based SMPU. This suggests a higher molecular weight of the macroglycol form crystalline structure in SMPU. The thermal stability of SMPU was evaluated by TGA. The thermograms of the SMPUs are shown in Fig. S1 (Supporting Information). All the SMPUs showed two-step degradation patterns which indicate
that the hard segment content and the molecular weight of macroglycol have no influence on the degradation mechanism. However, the thermal stability was improved with increasing hard segment content and higher molecular weight of the macroglycol. This is due to the restricted motion of SMPU chains in the higher hard segment content.
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Table 3. WVPs of the SMPUs at different temperatures and relative humidities WVP (g m−2 day –1 )
Sample
Temp 5 ∘C RH 96%
Temp 5 ∘ C RH 50%
Temp 9 ∘ C RH 50%
SMPU1 SMPU2 SMPU3 SMPU4 SMPU6 SMPU6
43 51 39 11 12 8
52 65 48 32 35 31
75 90 71 40 50 33
Temp 25 ∘ C RH 70% 293 337 285 152 242 133
RH, relative humidity.
S Thakur, MA Jahid, J Hu significantly, it shows the highest toughness of the synthesized SMPUs. Shape memory properties The shape memory properties of the synthesized SMPUs were investigated through a series of thermomechanical cyclic tests. The thermomechanical cyclic testing results are summarized in Table 2 and Fig. 6. The shape recovery of the synthesized SMPUs was increased with an increase in the content of hard segments in the SMPUs. The shape memory behaviour is mainly considered as an entropic process.24 The SMPU chains are oriented in a random coil formation in the permanent shapes of the SMPU films, i.e. at their highest entropic state.25,26 During heating above the transition temperature, the chain of the SMPU is activated and therefore it is easily deformed into a temporary shape to lower the entropic energy. Then, the temporary shaped SMPU was cooled to low temperature to fix the programmed shape by kinetically restricting the motion of the polymeric chains. At the moment when the mechanical stress was released, the SMPU chains did not have adequate entropic energy to convert the deformation shape. During reheating of the SMPU above the transition temperature, the mobility of the SMPU chains was again activated which permitted them to obtain the desired entropy and return to the permanent shape. SMPU3 showed higher shape recovery compared to the other synthesized SMPUs due to presence of high stored entropic energy in the system. The presence of higher interaction and uniform distribution of the hard segment is a crucial factor for the higher entropy. All the synthesized SMPUs demonstrated excellent shape fixity. When SMPU is heated and stressed, the SMPU chains are elongated which guides dislocation of the net points and a change in orientation of the SMPU chains. The excellent shape fixity may be a result of the presence of a high amount of physical interactions between the new oriented SMPU segments upon cooling.
Figure 7. WVPs of the SMPUs at different temperatures.
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entanglement of chains, compositions and nature of reactants, molecular weight, rigidity of the polymer etc.20 The stress–strain profiles of the SMPUs are shown in Fig. 4. The presence of yield and necking points in all the SMPUs indicates the typical elastomeric nature. It was observed that the tensile strength increases with increase in hard segment content in the SMPUs. This might be due to the increase in intermolecular and intramolecular interactions, the degree of hydrogen bonding and the number of aromatic moieties which increase with hard segment content.21 All SMPUs show adequate flexibility as they can be bent in a mandrel with a diameter of 1 mm. This flexibility is attributed to the flexible PEG moiety. It is also noticed that PEG1000 based SMPUs showed higher tensile strength than PEG2000 based SMPUs. Long chain macroglycol (PEG2000) generally provides more flexibility compared to short chain macroglycol (PEG1000),22 and this may affect the mechanical properties of the SMPUs. Thus, SMPU3 showed the highest tensile strength among the synthesized SMPUs. Similar to tensile strength, SMPU3 also showed the highest tensile modulus (568 MPa) and toughness (88.9 MJ m−3 ) of the studied SMPUs (Fig. 5). This is attributed to the presence of more physical interaction and the short chain macroglycol. The toughness of the polymer is a combination of flexibility and strength.23 As SMPU3 has the highest strength and flexibility and does not deteriorate
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Water vapour permeability The dependence of the WVP of the different SMPUs on relative humidity and temperature was measured according to ASTM E96-80B and is summarized in Table 3. The WVP in compact polymer thin films is a measure of the actual mobility of penetrant moisture molecules in the polymer matrix. The diffusion of water vapour is restricted when the size of the free volume is almost the same as the moisture molecules. Further, vapour can easily diffuse through the thin film with a large diffusion constant if the size of the free volume is significantly large compared with the moisture molecules. This is the main reason for the increase in WVP as a function of free volume hole size. The influence of the soft segment content on WVP is attributed to the free volume as well as the hydrophilicity of the membranes. From Table 3, it is clear that PEG1000 based SMPU has a much higher WVP than PEG2000 based SMPU, which is attributed to its much higher free volume. Another reason for the lower WVP is the presence of crystal structure in PEG2000 based SMPUs as confirmed from the DSC study. The presence of such crystal structure hinders the diffusion of vapour molecules in the thin film.9,27 It is quite general that the WVP becomes high if the presence of soft segments is increased as there is more free volume. However, this improvement has a certain limit. We studied and tuned the exact soft to hard segment ratio for higher WVP. Also, we noticed
© 2018 Society of Chemical Industry
Polym Int 2018; 67: 1386–1392
Mechanically strong shape memory polyurethane
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Figure 8. Relationship between WVP, temperature and relative humidity of (a) SMPU1, (b) SMPU2, (c) SMPU3, (d) SMPU4, (e) SMPU5 and (f ) SMPU6.
that SMPU with 70:30 (soft:hard) segment ratio provides a higher WVP than SMPUs with 75:25 and 65:35 ratios. This is the interesting finding of this study. SMPU2 showed the highest WVP compared to other synthesized SMPUs. The WVP of the SMPUs is highly dependent on temperature and relative humidity. The WVP of SMPU thin films is also observed (Fig. 7) to increase as the temperature is raised to above 40 ∘ C. This indicates that SMPU thin films can regulate the WVP. The WVP was low up to 30 ∘ C and then started to rise dramatically. At temperatures below the transition temperature, the SMPU is in a glassy state which is very stiff. As the temperature increases above the transition temperature, it becomes rubbery and assist to diffuse the vapour molecules. Thus, the WVP increases drastically above 30 ∘ C. The WVP of SMPU membranes can be tuned by the simultaneous effect of temperature and relative humidity. Therefore, with the aim of identifying the relationship between WVP, temperature and relative humidity, we measured the WVP of the SMPUs in different conditions (Fig. 8). It is clear from Fig. 8 that increasing the temperature and decreasing relative humidity will bring about enhancement of the WVP. The image of the SMPU thin film was shown as Fig. S2 (Supporting Information).
CONCLUSION
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The authors would like to acknowledge the financial support from the projects fund, Development of Environmental Friendly and Self-Adaptive Breathable Polyurethane Film (GHX/001/16GD), and the Science and Technology Planning Project of Guangdong Province, China (Project number 2016A050503013).
SUPPORTING INFORMATION Supporting information may be found in the online version of this article.
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We have synthesized a series of SMPUs by a two-step pre-polymer technique with variations in hard to soft segments and the molecular weight of macroglycol. PEG1000 based SMPU (hard:soft = 30:70) showed higher tensile strength than PEG2000 based SMPU. SMPU3 showed higher shape recovery compared to other synthesized SMPUs due to the presence of high stored entropic energy in the system. The synthesized SMPUs demonstrated a good water vapour transmission rate of over 1460 g m−2 day –1 as well as robust mechanical properties with a tensile strength of 19.8 MPa indicating a promising permeable polymeric thin film for many potential applications, especially as protective clothing.
ACKNOWLEDGEMENT
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