Studies of the Thermal and Mechanical Properties of Poly(urethane

Article pubs.acs.org/IECR

Studies of the Thermal and Mechanical Properties of Poly(urethane− siloxane)s Cross-Linked by Hyperbranched Polyesters Jasna V. Džunuzović,*,† Marija V. Pergal,† Rafał Poręba,‡ Sanja Ostojić,§ Nada Lazić,§ Milena Špírková,‡ and Slobodan Jovanović∥ †

Institute of Chemistry, Technology and Metallurgy (ICTM)−Center of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ‡ Institute of Macromolecular Chemistry AS CR, v.v.i. (IMC), Nanostructured Polymers and Composites Department, Heyrovskeho nam. 2, 16206 Praha 6, Czech Republic § Institute of General and Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ∥ Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia ABSTRACT: Polyurethane networks based on Boltorn hyperbranched polyesters (HBPs) of different pseudogenerations as cross-linkers, α,ω-dihydroxy(ethylene oxide−poly(dimethylsiloxane)−ethylene oxide) (EO−PDMS−EO) and 4,4′-methylenediphenyl diisocyanate were synthesized using a two-step polymerization reaction in solution. The influence of the pseudogeneration number of Boltorn HBPs on the thermomechanical and thermal properties and morphologies of polyurethane networks was investigated using FTIR, DMTA, TGA, hardness measurements, SEM, and SAXS. Synthesized polyurethanes showed higher cross-linking density and hardness and lower thermal stability with increasing pseudogeneration number of HBP. Samples with lower EO−PDMS−EO content exhibited slightly better thermal stability and higher hardness. All utilized characterization methods yielded consistent results and confirmed the existence of microphase separated morphology with the appearance of certain microphase mixing between segments as the pseudogeneration number of HBP increased. The obtained results revealed that the synthesized polyurethanes have good thermal and thermomechanical properties, which can be tailored by changing the pseudogeneration number of the HBP or the EO−PDMS−EO content.

1. INTRODUCTION Hyperbranched polymers attract much attention due to their specific three-dimensional structure, large numbers of end functional groups, and interesting properties such as high solubilities in different solvents, low degrees of chain entanglements in melt, low viscosities in solution and in melt, and good compatibilities with other materials.1−3 The synthesis of hyperbranched polymers can be performed at a reasonable cost, while adequate modification of their numerous end functional groups can be utilized to tailor and adjust the final material properties to meet different application requirements.4−6 Commercially available Boltorn hyperbranched polyesters (HBPs) based on 2,2-bis(hydroxymethyl)propionic acid as monomer and a tetrafunctional ethoxylated pentaerythritol core are probably the most investigated hyperbranched polymers.1,7−9 These aliphatic hydroxy-functional HBPs have already found practical uses as components of coating resins, for toughening of epoxy blends, for the synthesis of polymer nanocomposites, for the preparation of tumor-targeted drug delivery carriers, etc.10−12 Recently, different Boltorn hyperbranched polyesters have been used together with polyester or polyether macrodiols for the synthesis of polyurethanes (PUs).13−16 In our previous papers we have shown that poly(dimethylsiloxane) (PDMS) macrodiols can also be used for the synthesis of PU networks based on Boltorn HBPs.17−19 Poly(dimethylsiloxane)s have unique properties, such as good thermal, oxidative, and hydrolytic stabilities, high flexibility, good biocompatibility, low surface energy, good © 2012 American Chemical Society

insulating properties, low moisture and good gas permeability and UV stability, very low glass transition temperatures, and consequently broad service temperature ranges.20 However, due to the low glass transition temperature, the mechanical properties of PDMS are relatively poor at room temperature, unless PDMS is for example cross-linked and reinforced with adequate fillers.21 Furthermore, PDMS is incompatible with almost all organic polymers due to the very low solubility parameter, i.e., its nonpolar nature. Nevertheless, it has been shown that the adequate incorporation of PDMS into PUs can impart certain desirable properties of PDMS and avoid unwanted macroscopic phase separation during the polymerization, without significantly altering the good mechanical properties of PUs.20,22−24 According to the literature, the incorporation of PDMS into PUs improves their thermal and surface properties, water resistance, and fire resistance.20,24,25 Polyurethane networks based on HBPs and PDMS macrodiols can assemble and combine properties of all components, especially those significant for the coating applications. Namely, the presence of HBPs which have numerous end −OH groups provides fast curing and formation of the highly cross-linked material with good mechanical properties and chemical resistance. On the other hand, the presence of PDMS improves Received: Revised: Accepted: Published: 10824

April 9, 2012 July 21, 2012 July 26, 2012 July 26, 2012 dx.doi.org/10.1021/ie300927z | Ind. Eng. Chem. Res. 2012, 51, 10824−10832

Industrial & Engineering Chemistry Research

Article

numbers, HN, were determined by a titration method (Table 1).9,28 From the obtained Mn and HN values, average functionalities, f, of Boltorn HBPs were calculated (Table 1). 4,4′-Methylenediphenyl diisocyanate (MDI), supplied by Aldrich (purity > 98%), was used as received. Using the titration method, the content of the −NCO groups in MDI was checked (33.6 wt %).29 Stannous octanoate (Sn(Oct)2, from Aldrich) was used as a diluted solution in anhydrous N-methyl2-pyrrolidone (NMP). NMP was purchased from Acros, distilled prior to use, and kept over molecular sieves (0.4 nm). Tetrahydrofuran (THF) was supplied by J. T. Baker, dried over lithium aluminum hydride, and distilled before use. 2.2. Preparation of PU Networks. Three series of PU networks based on EO−PDMS−EO, MDI, and Boltorn HBPs were synthesized by a two-step polymerization in solution, using an NMP/THF mixture as solvent and stannous octanoate as a catalyst. Cross-linkers for the samples of series PU2 (PU240 and PU2-30), PU3 (PU3-40 and PU3-30), and PU4 (PU440 and PU4-30) were BH-20, BH-30, and BH-40, respectively. Each series of the synthesized PUs consists of two samples of differing EO−PDMS−EO content (40 and 30 wt %), which is marked by the last two numbers in the names of the prepared samples. The total molar ratio of −NCO and −OH groups was for all samples kept constant (NCO/OH = 1.05).24,25,29 All PUs were synthesized in a four-neck round-bottom flask, placed in a silicone oil bath and connected to an inlet for dry argon, a mechanical stirrer, a dropping funnel, and a reflux condenser. As an example, the synthesis of PU3-30 is described. EO− PDMS−EO (3.3 mmol, 4.00 g), dissolved in the mixture of solvents NMP/THF (25.5 mL/3.0 mL), was placed in the flask. Then, MDI (21.3 mmol, 5.32 g) was added and the reaction mixture was stirred and heated to 40 °C under an argon atmosphere. After reaching 40 °C, catalyst solution in NMP (0.33 mL; 0.15 mol % based on EO−PDMS−EO) was added into the flask.24,25,29 The reaction mixture was continuously stirred at 40 °C, and after approximately 30 min NCOterminated prepolymer was obtained.24,25,29 A standard dibutylamine back-titration method was used to control the content of −NCO groups.30 In the second step of the polymerization, BH30 (1.3 mmol, 4.01 g) dissolved in NMP (34 mL) was added dropwise through the dropping funnel into the reaction mixture. After stirring for another 10 min at 40 °C, the obtained reaction mixture was cast in a Petri dish, previously lubricated with silicone oil. The cross-linking of the PUs was continued in a force-draft oven at 80 °C for 45 h and at 110 °C for 1 h, and finally at 50 °C for 10 h in a vacuum oven. Brown PU samples with a thickness of 1.5 ± 0.2 mm were obtained. All synthesized PUs were kept in desiccators at room temperature for 2 weeks before testing. The chemical structure of the synthesized PU samples was confirmed by Fourier transform infrared (FTIR) spectroscopy. 2.3. Characterization. FTIR spectra of the investigated PU networks were recorded on a NICOLET 380 FTIR spectrometer, using attenuated total reflection (ATR) mode. Dynamic mechanical thermal analysis (DMTA) was carried out on an ARES G2 rheometer (TA Instruments). The viscoelastic behavior of three synthesized PUs (PU2-30, PU330, and PU4-30) was investigated at a frequency of 1 Hz, strain 0.1%, with a heating rate of 3 °C/min and in the temperature range from −135 to 180 °C. The measurements were performed on rectangular specimens (15.0 mm × 7.5 mm × 1.5 mm ± 0.2 mm), under torsion mode, using torsion fixture (rectangle) geometry.

thermal and surface properties and brings elasticity in such highly cross-linked systems, due to its low glass transition temperature. Investigation of the properties of such PU networks, prepared from HBPs and PDMS macrodiols, can therefore reveal possibilities for their application in the coating industry. In our previous work we have shown that, during the synthesis of PUs based on PDMS macrodiol and Boltorn HBP of the second pseudogeneration (BH-20) in the melt, macroscopic phase separation occurs, leading to the formation of a heterogeneous network.17 PU networks were then synthesized by a relatively simple two-step polymerization in solution, using a mixture of N-methyl-2-pyrrolidone and tetrahydrofuran as a reaction medium, in order to improve the compatibility between reactants.18,19 Despite that, obtained PU networks based on BH-20 still showed relatively low crosslinking densities, high percent of sol fractions, and relatively good thermal stabilities. The present work represents a continuation of our efforts to prepare novel poly(urethane− siloxane) networks with good thermal and thermomechanical properties. Therefore, the aim of this paper is the synthesis of PU networks based on Boltorn HBP of the fourth pseudogeneration, α,ω-dihydroxy(ethylene oxide−poly(dimethylsiloxane)−ethylene oxide) and 4,4′-methylenediphenyl diisocyanate, in order to investigate their structure− property relationship and compare their properties with PU networks based on Boltorn HBPs of the second and third pseudogenerations. The influence of the Boltorn HBP pseudogeneration number on the thermal and thermomechanical properties and surface morphologies of the polyurethane networks was investigated, since, to our knowledge, there is no such study available so far on these particular polyurethanes.

2. EXPERIMENTAL SECTION 2.1. Materials. α,ω-Dihydroxy(ethylene oxide−poly(dimethylsiloxane)−ethylene oxide) (EO−PDMS−EO) was purchased from ABCR and dried over molecular sieves (0.4 nm) before the synthesis. The number-average molecular weight of EO−PDMS−EO was calculated from the 1H NMR results, and the obtained value, Mn = 1200 g/mol, was used for the reaction mixture calculations.26 The molecular weight of the central PDMS block is Mn = 1090 g/mol, while the end ethylene oxide sequences are composed of one unit each. Boltorn hydroxy-functional aliphatic hyperbranched polyesters of the second (BH-20), third (BH-30), and fourth (BH-40) pseudogenerations were kindly supplied by Perstorp Specialty Chemicals AB (Sweden) and dried at 50 °C under vacuum for 48 h before use. Boltorn HBPs were synthesized from 2,2bis(hydroxymethyl)propionic acid and ethoxylated pentaerythritol using a pseudo-one-step procedure.27 The polydispersity indexes, Mw/Mn, of the Boltorn HBPs were determined by GPC and are listed in Table 1.9,28 The number-average molecular weights of Boltorn HBPs were determined using vapor pressure osmometry, while the values of the hydroxyl Table 1. Properties of Boltorn Hydroxy-Functional Aliphatic Hyperbranched Polyesters

a

sample

Mna (g/mol)

Mw/Mna

HNa (mg of KOH/g)

f

f theor

BH-20 BH-30 BH-40

1340 3080 2720

1.4 1.9 2.8

501.1 474.1 470.5

12 26 23

16 32 64

Results presented in refs 9 and 28. 10825

dx.doi.org/10.1021/ie300927z | Ind. Eng. Chem. Res. 2012, 51, 10824−10832


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Figure 1. Chemical structures of (a) EO−PDMS−EO, (b) MDI, (c) BH-20, (d) BH-40, and (e) simplified chemical structure of the synthesized polyurethane network based on BH-20 as cross-linking agent.

3. RESULTS AND DISCUSSION Six PU samples based on EO−PDMS−EO and BH-20 (PU2 series), BH-30 (PU3 series), and BH-40 (PU4 series) were synthesized by a two-step polymerization in solution. Each series of the synthesized PUs is composed of two samples of different EO−PDMS−EO contents. The chemical structures of the selected reactants and simplified structure of the synthesized polyurethane network based on BH-20 as crosslinking agent are presented in Figure 1, while values of the number-average molecular weight, polydispersity index, hydroxyl number, calculated average, and theoretical functionality of Boltorn HBPs are listed in Table 1. The calculated Mn and average functionality of BH-40 are slightly lower than the Mn and f of BH-30 and quite lower than the theoretical ones, indicating that a great extent of side reactions occurred during the synthesis of BH-40, which led to the formation of HBP with higher polydispersity, lower molecular weight, and consequently lower functionality, compared to the theoretical values.9,28 Similar results were obtained by other authors. From the SEC-MALS measurements, using a solution of BH-40 in the mixture THF/MeOH (9:1), Ž agar and Ž igon determined Mn = 2625 g/mol and consequently f = 22.7 Thomasson et al. reported a functionality of 24.6 for a Boltorn hyperbranched polyester of the fourth pseudogeneration.8 3.1. FTIR Spectroscopy of Synthesized PU Networks. FTIR spectroscopy was applied to investigate the chemical structure of the synthesized PU networks. Infrared spectra of the selected PU samples are shown in Figure 2. For all prepared networks the locations of the absorption bands assigned to different groups were similar. The absorption peaks which can be observed around 1258 cm−1 and 1538 cm−1 are associated with amide II and amide III vibrations, respectively. Other characteristic bands can be observed around 790 cm−1 (Si− CH3 linkage), 1596 and 1412 cm−1 (aromatic CC), 2961, 2903, and 2876 cm−1 (symmetric and asymmetric −CH2− and −CH3), and 3309 cm−1 (hydrogen-bonded −NH stretching

The density of the synthesized PUs was determined at room temperature (20.1 °C) using a pycnometer and with distilled water as the medium. The average of four measurements, which corresponded to two different test samples of each PU network, was used. The hardness measurements of the synthesized PUs were performed on a Shore A apparatus (Hildebrand, Germany). The results of the measurements were recorded 1 s after penetration of the needle into the sample. The average of at least five measurements was used. Differential scanning calorimetry (DSC) was performed on a DSC Q1000 V9.0 Build 275 thermal analyzer. The DSC measurements were run under a dynamic nitrogen atmosphere, in the temperature range from −90 to 200 °C, at a heating rate and cooling rate of 10 and 5 °C/min, respectively (two scans were performed for each sample). Values of the glass transition temperature of the synthesized PUs were determined from the second heating run. Thermogravimetric analysis (TGA) of the synthesized samples was carried out on a TGA Q500 V6.3 Build 189 instrument under nitrogen, in the temperature range from 25 to 700 °C and at a heating rate of 10 °C/min. The scanning electron micrographs of the PUs were recorded on a JEOL JSM-6610 scanning electron microscope (SEM). Small-angle X-ray scattering (SAXS) experiments were carried out using a three-pinhole camera (Molmet/Rigaku), attached to a multilayer aspherical optics (Osmic Confocal Max-Flux), which monochromatizes and concentrates the beam of a microfocus X-ray tube (Bede) operating at 45 kV and 0.66 mA (30 W). The camera was equipped with a multiwire, gasfilled two-dimensional detector with a 0.2 m diameter of an active area (Gabriel design). Peak positions were employed to obtain the characteristic length D (interdomain spacing) according to Bragg’s law, D = 2π/q, where q is the scattering vector. 10826



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temperature dependences of the loss modulus, G″, the storage shear modulus, G′, and the loss factor, tan δ, are presented respectively, while the obtained DMTA results are listed in Tables 2 and 3. Three peaks can be observed in the temperature dependence of G″ (Figure 3), indicating the existence of microphase separated morphology, which is mainly a consequence of the immiscibility of PDMS with other reactants used for the synthesis of the investigated PUs. The temperature associated with the peak of the loss modulus between −128 and −123 °C is defined as the glass transition temperature of the soft EO−PDMS−EO segment, TgS, (Table 2). The peak which can be observed in the region between −75 and −72 °C (T2 in Table 2) is associated with the subglass relaxation process and is probably a consequence of the movement of the part of the chain which contains urethane groups connected to the Boltorn HBP.15 Due to the steric hindrance, these urethane groups are not involved in the hydrogen bond formation and are therefore more mobile.15 The third peak detected in the region between 34 and 63 °C is ascribed to the glass transition temperature of the hard MDI− HBP segments, TgH (Table 2). It can be observed that the value of the TgH decreases with increasing generation number of the Boltorn HBPs from the second to the third. The length of the arms, their flexibility, and the mobility of the chain ends increase with increasing pseudogeneration number of hyperbranched polymers. The Boltorn HBP of the second pseudogeneration has relatively short (Figure 1) and stiff arms. On the other hand, due to the increased length and mobility of the BH-30 arms, the TgH of PU3-30 is lower than the TgH of PU2-30. However, the TgH of the PU cross-linked with BH-40 is higher than the TgH of PU3-30 (Table 2). The reason for this behavior could be a slightly lower average functionality of BH-40 in comparison with the BH-30. However, this difference in f values is not very pronounced and there is probably an additional reason which contributes to this specific behavior. Namely, due to the higher polydispersity index (Table 1), BH-40 contains a certain amount of relatively high molecular weight fractions, which was confirmed by different authors.7,28 Such fractions are most likely composed of longer arms than those present in BH-30 and their increased flexibility and mobility can promote back folding of the branches, which are then trapped, leading to the slight increase of the TgH of PU4-30.16 The temperature associated with the relaxation process observed between −75 and −72 °C and the glass transition temperature of the soft segment follow the same trend as TgH, since the mobility of the soft segment chains and urethane groups connected to the cross-linking agent is greatly influenced by the pseudogeneration number of HBP. However, the difference between TgS values of different samples is much smaller than between TgH values, due to the fact that the length of the soft segment is the same for all synthesized PUs. This further allowed better insight into the influence of the

Figure 2. FTIR spectra of selected synthesized PU networks.

vibration). The absorption bands around 1014 and 1080 cm−1 are ascribed to the overlapped bands of Si−O−Si and C−O−C groups. The peaks which are ascribed to the CO stretching vibrations are detected in the region 1645−1735 cm−1. The absorption bands belonging to the isocyanate (2270 cm−1) and hydroxyl groups (3300 cm−1) were not detected in the FTIR spectra of the prepared PU networks, indicating their complete conversion during the reaction. 3.2. Dynamic Mechanical Thermal Analysis of Synthesized PU Networks. The influence of the number of pseudogenerations of Boltorn hyperbranched polyesters on the viscoelastic properties of polyurethane networks based on poly(dimethylsiloxane) was investigated by dynamic mechanical thermal analysis of three synthesized PU samples with the same EO−PDMS−EO content (30 wt %). In Figures 3−5

Figure 3. Loss modulus of selected synthesized PUs versus temperature.

Table 2. Temperatures Corresponding to the Three Peaks Observed in G″ Temperature Dependence, Values of G′ at 0 °C, (G′)0, at 70 °C, (G′)70, and in the Rubbery Plateau Region at TgH + 90 °C, (G′)RP, Cross-Linking Density, ν, and Molecular Weight of Polymer Chain between Cross-Links, Mc, of Selected Synthesized PUs sample

TgS (G″) (°C)

T2(G″) (°C)

TgH (G″) (°C)

(G′)0 (MPa)

(G′)70 (MPa)

(G′)RP (MPa)

ν × 104 (mol/cm3)

Mc (g/mol)

PU2-30 PU3-30 PU4-30

−123 −128 −124

−72 −75 −72

63 34 47

499 680 430

82 20 45

2.9 4.7 6.3

8.19 14.32 18.48

1280 800 650

10827



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Table 3. Temperatures Corresponding to tan δ Peaks, tan δ Values, and the Ratio Ts/TgH(tan δ)a of Selected Synthesized PUs

a

sample

TgS(tan δ) (°C)

T2(tan δ) (°C)

TgH(tan δ) (°C)

(tan δ)1

(tan δ)2

(tan δ)3

Ts/TgH(tan δ)

PU2-30 PU3-30 PU4-30

−120 −126 −121

−69 −67 −66

90 50/73 83

0.05 0.03 0.05

0.04 0.04 0.05

0.48 0.39/0.33 0.33

0.572 0.444 0.438

Ts is the softening point calculated as the extrapolated onset of the drop of G′.

than the values determined for some other cross-linked poly(urethane−siloxane) structures.14,31 On the other hand, shape memory PUs based on BH-30 and poly(butylene adipate) glycol exhibited much higher plateau moduli.13 According to the results given in Figure 4 and Table 2, as the pseudogeneration number of the cross-linking agent increases, a higher plateau modulus appears, indicating higher cross-linking density and increase of the microphase mixing in the synthesized PUs, probably due to the higher content of the urethane bonds and higher degree of hydrogen bonding.23 The difference in the G′ value before the glass transition of hard segments and in the rubbery plateau is also related to the crosslinking density.32 Namely, the sample with the smallest G′ difference, which is observed for PU4-30, has the highest crosslinking density. The obtained DMTA results indicate that a certain amount of species with higher functionality than 23 exists in BH-40, which consequently led to a somewhat higher plateau modulus. Values of the cross-linking density can be easily calculated from the rubbery plateau modulus. Based on the theory of rubber elasticity, the equilibrium shear modulus, G, is correlated with the cross-linking density, ν, i.e., the molar number of elastically effective network chains per cubic centimeter, in the following manner:33,34

pseudogeneration number of HBP cross-linking agent on the properties of PUs. From DMTA results presented in Figures 3 and 4, one can see that for all three PU samples the values of G′ are in the

Figure 4. Storage modulus of selected synthesized PUs versus temperature.

G = (ν − hμ)RT

whole investigated temperature region higher than values of the loss modulus. This observation indicates that cohesion and stability of the PU network are not destroyed under the investigated experimental conditions.31 In addition, Figure 4 shows that during the glass transition of the soft segment, the G′ of the synthesized PUs slightly decreases, while in the glass transition region of the hard segments G′ rapidly decreases as a consequence of the increased mobility of the chains. On the other hand, the presence of the chemical cross-links restricts the flow of the polymer in the rubbery plateau (around 130 °C). In Table 2 are given G′ values at three different temperatures. The first two temperatures (0 and 70 °C) are chosen in order to compare G′ values of different PUs before and in the glass transition regions of the hard segments. The third G′ value is actually the rubbery plateau modulus at TgH + 90 °C. From these results it can be observed that sample PU330 has the highest G′ value before and the lowest G′ value in the glass transition region of the hard segments. According to Figure 4, the softening point of PU3-30, Ts (calculated as the extrapolated onset of the G′ decrease in the glass transition region of the hard segments), is the lowest. However, the situation is quite different in the rubbery plateau region. All the rubbery plateau regions of the investigated samples appear above 120 °C. The slight increase of the G′ values of PU3-30 and PU4-30, which occurred at around 150 °C, is probably a consequence of the additional curing (hardening) of the synthesized PUs.31 Relatively high values of the rubbery plateau modulus are obtained, and they are higher than the values determined for some similar PUs synthesized with the same cross-linking agents but different macrodiols and higher

(1)

where h is an empirical parameter, μ represents the concentration of elastically active cross-links, R is the universal gas constant, and T is the temperature in kelvin. In the case of a phantom network, the cross-links can fluctuate about their mean positions due to the Brownian motion and h is equal to 1.33 Due to the high functionality of the network and the existence of steric hindrance, synthesized PUs show affine behavior where these fluctuations do not exist and the movement of network chains and cross-links is proportional to the macroscopic deformation. Since h is equal to 0 for an affine network and if we assume that G is equal to the rubbery plateau modulus, G′, eq 1 can be written as ν=

G′ RT

(2)

where G′ is the storage modulus of the synthesized PUs at 90 °C above TgH and T = TgH + 90 °C. The number-average molecular weight of the polymer chain between cross-links, Mc, can be evaluated as ρ Mc = (3) ν where ρ is the density of the synthesized PUs, which is between 1.05 and 1.19 g/cm3. Values of ν and Mc are collected in Table 2, and it can be observed that the cross-linking density of the synthesized PUs increases, while Mc decreases with increasing pseudogeneration number of the utilized cross-linking agent. Calculated values of Mc are relatively low due to the existence of both physical and chemical cross-linking. 10828



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In the tan δ temperature dependence, given in Figure 5, three peaks of different magnitudes can be observed. Analogous to

Table 4. Glass Transition Temperatures of Hard Segments Determined by DSC, (TgH)DSC, and Hardness of Synthesized PUs sample

(TgH)DSC (°C)

hardness (deg Shore A)

PU2-40 PU2-30 PU3-40 PU3-30 PU4-40 PU4-30

50 65 52 57 50 57

88 88 − 88 94 96

and on the EO−PDMS−EO content, PUs exhibited hardness values between 88 and 96° Shore A. The increase of the crosslinking density and decrease of the EO−PDMS−EO content increased the hardness values of investigated PUs, which is in agreement with results obtained by DMTA measurements. 3.4. Thermal Stability of Synthesized PU Networks. The thermal stability of the synthesized PU samples was examined by thermogravimetric (TG) analysis in a nitrogen atmosphere, at a heating rate of 10 °C/min. In Figures 6 and 7 are presented obtained TG and derivative thermogram (DTG) curves for samples with 40 and 30 wt % EO−PDMS−EO, respectively. Furthermore, in Table 5 are listed characteristic temperatures of thermal degradation T10, T40, T60, and T90 (at 10, 40, 60, and 90% weight loss, respectively). The results listed in Table 5 and presented in Figures 6 and 7 show that initial thermal degradation is slower for PUs with lower EO−PDMS− EO content. In the whole temperature region the thermal stabilities of PUs from the series PU2 and PU3 with 30 wt % EO−PDMS−EO are higher than the thermal stabilities of PU240 and PU3-40. The same thermal behavior was observed for the samples PU4-40 and PU4-30 up to the temperature which corresponds to 60% weight loss. According to the results presented in Figures 6 and 7 and in Table 5, it can be concluded that the increase of the pseudogeneration number of the Boltorn HBPs has a quite complex influence on the thermal stabilities of these networks. For the PU samples with 40 wt % EO−PDMS−EO it is not possible to find some specific trend (Figure 6a and Table 5), except in a very short region between T40 and T60 where distinct differences in thermal stability between PU2-40, PU340, and PU4-40 can be observed. Thermal stabilities of these three PUs in this region increase with a decrease of the Boltorn HBP pseudogeneration number. The same trend, but much more pronounced, can be observed for the PUs with 30 wt % EO−PDMS−EO at temperatures higher than T40 (Figure 7a). The possible explanation for this specific behavior can be found by examining DTG curves of these PUs. For all synthesized PU samples thermal decomposition in nitrogen occurs between approximately 270 and 650 °C, via a four-step process (Figures 6b and 7b). The first DTG peak, around 300 °C, corresponds to the temperature at the maximum rate of weight loss during the first step of the thermal degradation, which occurred due to the decomposition of the urethane groups (the thermally weakest bond).36−38 During the second step of thermal degradation (around 330 °C) ester components decomposed, while in the third step (around 410 °C) EO−PDMS−EO degraded. In the temperature region between 500 and 650 °C aromatic compounds decomposed. As the number of pseudogeneration of HBPs increases, the third peaks in DTG curves of PUs becomes less pronounced and diffused (Figures 6b and 7b), indicating

Figure 5. Values for tan δ of selected synthesized PUs versus temperature.

the G″ temperature dependence, temperatures of different tan δ peaks are ascribed to the TgS (peak 1), the movement of the urethane groups attached to the Boltorn HBP (peak 2), and TgH (peak 3). Temperatures of the tan δ peaks are somewhat higher than the temperatures corresponding to the G″ peaks (Figure 3 and Table 2), and they are listed in Table 3 together with the tan δ peak values. The loss tangent peak which corresponds to the TgH of sample PU3-30 has one shoulder around 73 °C, and therefore two temperatures and values of the (tan δ)3 peak are written in Table 3. The reason for the splitting of the (tan δ)3 peak is probably the existence of more than one relaxation process in this case. According to Figure 5 and Table 3, the tan δ peak associated with the glass transition of the hard segments is the most pronounced and it decreases with increasing pseudogeneration number of HBP, which is in agreement with results obtained by other authors.14 There is no significant difference between the (tan δ)2 values of different PUs, while (tan δ)1 has the lowest value for sample PU3-30. In Table 3 values of the ratio Ts/TgH(tan δ) are also listed, since they are correlated with the width of the (tan δ)3 peak.16,35 With increasing pseudogeneration number of Boltorn HBP, the ratio Ts/TgH(tan δ) decreases and the tan δ peak becomes broader, indicating an increase of cross-link nonuniformity and the existence of more than one relaxation.16 3.3. DSC Analysis and Hardness Measurements of Synthesized PU Networks. Thermal properties of the synthesized PUs were also examined by DSC. In the investigated temperature region (−90 to 200 °C) only glass transition temperatures of hard segments were observed and the obtained values are collected in Table 4. Values of (TgH)DSC decrease with increasing pseudogeneration number of the HBP and with increasing EO−PDMS−EO content. The transition around −72 °C was not observed in DSC curves, indicating that DMTA is the more sensitive method for thermal analysis of the synthesized PUs. The obtained difference in the TgH values determined by different methods is due to the frequency dependence. Crystallization and melt peaks were not detected in DSC curves of prepared PU networks. The Shore A hardness of the synthesized PUs is listed in Table 4. Depending on the Boltorn HBP used for the synthesis 10829



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Figure 6. (a) TG and (b) DTG curves of synthesized samples with 40 wt % EO−PDMS−EO, determined at a heating rate of 10 °C/min, in nitrogen atmosphere.

Figure 7. (a) TG and (b) DTG curves of synthesized samples with 30 wt % EO−PDMS−EO, determined at a heating rate of 10 °C/min, in nitrogen atmosphere.

urethane components, the immiscibility of PDMS with other components, and its low surface energy, PDMS predominates on the surface even when it is present in low concentrations, leading to the presence of microphase separated structures.40 From Figure 8 it can be observed that EO−PDMS−EO forms spheres of different diameters (0.5−6 μm) on the surface of sample PU2-30. As the pseudogeneration number of the Boltorn HBPs increases, phase mixing morphology appears and PU samples show increased homogeneity. The results obtained by SEM analysis reveal the existence of microphase separated morphology of the investigated PU networks and are consistent with DMTA and TGA results. The microphase separated state of the synthesized PU networks was further investigated using results obtained by SAXS experiments. The scattering profiles for the series containing 30 wt % EO−PDMS−EO (i.e., for samples PU230, PU3-30, and PU4-30) are shown in Figure 9. All PUs exhibited pronounced scattering peaks in the SAXS curves, suggesting a microphase separated morphology. The value of interdomain spacing of hard segments, calculated from the peak position, is 10.5 nm for PU2-30 and 13.1 nm for both PU3-30 and PU4-30. The obtained results indicate that interdomain spacing is a function of the pseudogeneration number of the

Table 5. Characteristic Temperatures of Thermal Degradation, T10, T40, T60, and T90 (at 10, 40, 60 and 90% Weight Loss, Respectively), Obtained from TG Curves of PU Samples sample

T10 (°C)

T40 (°C)

T60 (°C)

T90 (°C)

PU2-40 PU2-30 PU3-40 PU3-30 PU4-40 PU4-30

277 287 274 291 278 278

317 324 311 323 309 320

385 405 389 394 365 376

594 594 596 600 590 573

improved phase mixing between segments and consequently lower thermal stability.16 Another reason for this specific thermal behavior of the synthesized PUs could be the fact that samples cross-linked with higher pseudogeneration HBPs have higher contents of weak urethane bonds, leading to lower thermal stability.39 3.5. SEM and SAXS Measurements of Synthesized PU Networks. Scanning electron micrographs of the surfaces of selected synthesized PUs are presented in Figure 8. Due to the large difference between the solubility parameters of PDMS and 10830



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Figure 8. SEM micrographs of surfaces of samples PU2-30, PU3-30, and PU4-30.

Dynamic mechanical thermal analysis revealed the existence of two glass transition temperatures: one between −128 and −123 °C, ascribed to the soft segment glass transition, and another between 34 and 63 °C associated with the glass transition of hard segments. The third peak, observed in the region between −75 and −72 °C, is ascribed to the movement of urethane groups close to the HBP. As the pseudogeneration number of Boltorn HBPs increases, values of the cross-linking density, hardness, and phase mixing between segments increase, while the thermal stability, determined in the nitrogen atmosphere, decreases. The increase of the EO−PDMS−EO content induced lower hardness and thermal stability. SEM micrographs and SAXS experiments confirmed the presence of microphase separated morphology, which is consistent with results obtained by DMTA and TGA. The overall results indicate that combination of EO− PDMS−EO and Boltorn HBPs, utilized to prepare PUs, yielded better thermomechanical and thermal properties in comparison to some similar PU networks presented in the literature and prepared from another type of macrodiols. In addition, thermomechanical and thermal properties of the synthesized PUs can be tailored and optimized for the specific application requirements (such as in coatings) by simply changing the ratio between soft and hard segments or the pseudogeneration number of the hyperbranched cross-linking agent.

Figure 9. SAXS profiles of PU2-30, PU3-30, and PU4-30.

hyperbranched cross-linking agent. Namely, the longer length of the arms of Boltorn HBP of the higher pseudogeneration number is responsible for higher interdomain spacing. For the network PU2-30 the scattering intensity follows Porod’s low at q > qmax, i.e., it scales as q−n, where n is equal to 4, which is typical for well microphase separated structures with sharp phase boundaries.41,42 The scattering vector qmax represents the value of q at the peak position. However, the values of n for PU3-30 and PU4-30 are 2.5 and 2.4, respectively, indicating that the scattering intensities for these two networks decrease more slowly than q−4 at high q.42 According to the literature, PUs which have n values lower than 4 have also lower degrees of microphase separation.42 The obtained SAXS results are in good agreement with DMTA, TGA, and SEM results, giving in this manner a more complete picture about the microphase separated state of the synthesized PU networks. The increase of the pseudogeneration number of the Boltorn HBP increases the interdomain spacing of hard segments, causing disordering of the hard domains and an increase in the compatibility between the soft EO−PDMS−EO and hard MDI−HBP segments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education and Science of the Republic of Serbia (Project No. 172062) and the Czech Science Foundation (GACR, Project No. P108/10/0195). The authors also thank Ms. Hana Šandová (IMC) for the experimental work (SAXS analysis).

4. CONCLUSIONS Three series, containing six PU networks based on Boltorn hyperbranched polyesters and α,ω-dihydroxy(ethylene oxide− poly(dimethylsiloxane)−ethylene oxide), were prepared to investigate the influence of the cross-linking agent on the thermomechanical and thermal properties and morphologies of the synthesized PU networks. Boltorn HBPs of the second, third, and fourth pseudogenerations were utilized as crosslinkers for the PUs of different series.

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REFERENCES

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Studies of the Thermal and Mechanical Properties of Poly(urethane

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