Influence of Dangling Chains on Molecular Dynamics of Polyurethanes

Article pubs.acs.org/Macromolecules

Influence of Dangling Chains on Molecular Dynamics of Polyurethanes Wenwen Yu,‡ Miao Du,*,†,‡ Dezhi Zhang,§ Yu Lin,‡ and Qiang Zheng*,†,‡ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou 310027, China Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China § Hangzhou Applied Acoustic Institute, Hangzhou 310012, China ‡

ABSTRACT: The effect of dangling chains on phaseseparated microstructure and molecular dynamics for polyurethanes (PUs) was investigated. PUs with different dangling chain lengths and polar groups were prepared through changing the types of diol extender. The molecular dynamics was studied by a combination of dynamic mechanical analysis (DMA) and broadband dielectric relaxation spectroscopy (BDRS). Four relaxations (processes), namely, a secondary relaxation (β), the soft phase segmental relaxation (α), the I process associated with hydrogen bond, and Maxwell− Wagner−Sillars (MWS) interfacial polarization process caused by charge accumulation at hard/soft phase interfaces, were detected. The I process occurred in temperatures lower than that of MWS process but higher than α relaxation in general. The β relaxation remains unaffected with changing dangling chain lengths or polar groups. However, the glass transition temperature (Tg) of the soft phase shifts to lower temperature, and the segmental motion becomes faster with increasing dangling chain length, while the introduction of a polar ester group into the dangling chains makes it slow down, corresponding to a higher Tg, and results in a higher fragility. On the other hand, there is an absence of I process, and the MWS process shifts to higher frequencies when longer dangling chain is introduced. In the case of increasing the hard segment content, the I process reappears and the MWS process slows down. It is suggested that these results are related to the H-bond interactions within hard segments and the micromorphologies of PUs. influences the properties of PUs.10 These dangling chains result in imperfections in the network structure and could function as plasticizers that reduce polymer rigidity and improve polymer flexibility.11 On the other hand, rotational tunneling of dangling chains that are independent of the backbone or the unattached free chains employed as relaxation components can cause heterogeneous dynamics in the polymers.12−15 The influence of dangling chains on the dynamics of PUs has been probed over the past years to evaluate the structure and properties of PUs with dangling chains. Most dangling chains are introduced in the soft segments of PUs and thus have a strong effect on the morphology and properties of PUs. Zlatanić et al.14 synthesized PUs from triolein, with and without dangling chain (C9), and found that the introduction of dangling chains in soft segments decreases the glass transition temperature (Tg) of PUs. Xu et al.15 found that the presence of short carbon dangling chain of six carbons in each repeating unit of the soft segments does not limit the overall bulk properties. Petrovic et al.10 fabricated segmented PUs based on ricinoleic acid soft segments and found that the

1. INTRODUCTION Polyurethanes (PUs) are a family of elastomeric materials whose chains are composed of relatively flexible (rubbery) soft segment and relatively rigid (solid-like) hard segment. The soft segment normally consists of flexible macrodiol, whereas the hard segment is formed by the reaction of a rigid diisocyanate with either diol or diamine molecules acting as chain extenders. PUs usually exhibit a phase-separated structure with rigid isocyanate domains (hard phase) embedded within a matrix of flexible chains (soft phase),1−4 and hard segment chains form strongly aggregated domains because of strong polarity of urethane bonds, and these domains function as reinforcing fillers. By contrast, if the cohesion force between hard segments is not strong enough, PUs exhibit a partially miscible state. The development of PUs based on vegetable oils has recently attracted a growing interest,5−9 which is economically driven because vegetable oils are cheap renewable resources. The chain lengths in these oils are 6−18 carbon atoms usually, which can be either saturated or unsaturated. It is known that PUs produced by using vegetable oils present several excellent properties such as enhanced hydrolytic and thermal stability.9 However, when functionalized vegetable oils react with isocyanate, introduction of dangling chains with lengths of C6 to C18 into PUs can be achieved, which significantly © XXXX American Chemical Society

Received: June 18, 2013 Revised: August 16, 2013

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dx.doi.org/10.1021/ma401260d | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Formation of PUs with Different Dangling Chains and the Diol Structures

carbons (C18) are expected to influence remarkably the molecular dynamics of PUs, thereby leading to the changes in properties. In view of the fact that H-bond is one of the important factors that strongly influence the morphology and properties of polymers,17,18 we try to design PUs with and without an ester group in the dangling chains to examine the effect of H-bond interaction on polymer dynamics via broadband dielectric relaxation spectroscopy (BDRS), which is believed to be an effective method to characterize the relaxation behavior of polymers with polar groups, such as polyureas.19−24 Also, the dynamics of PUs containing dangling chains with different graded lengths, functional groups, and contents will be probed by a combination of dynamic mechanical analysis (DMA) and BDRS.

influence of dangling chains on polymer properties could hardly be delineated from the effect of molecular weight. These dangling chains apparently have a plasticizing effect by precluding the crystallization of the soft segment and by lowering the elasticity of the samples. Oprea et al.16 introduced dangling chains into the hard segments by using castor oil as chain extenders as well as a cross-linker. The formation of hydrogen bonds between urethane groups of physical networks has been found to control molecular mobility; however, the chemical nature and content of castor oil within hard segments strongly influence molecular relaxation, and corresponding relaxation peaks shift to high temperatures with increasing castor oil contents. It is noted that even though some studies on dangling chains have been conducted, it is necessary for us to study their effects on PUs, such as the effect of dangling chains based on vegetable oils in hard segments on the molecular relaxations of PUs as well as the determination of dominant factors such as length or content of the dangling chains. To our knowledge, a systematic study and detailed understanding at a molecular dynamics level of the relationship between dangling chains and macroscopic performance has not been reported up to date. In the present article, we attempt to prepare PUs with different lengths of dangling chains through changing the diol chain extender. The dangling chain structures in the hard segment of PUs are given in Scheme 1, among which monolaurin (ML) and stearic acid monoglyceride (SAM) are obtained from vegetable oils. These dangling chains with graded length of 0 carbon (C0) to 18

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(propylene glycol) (PPG-1000) with weight average molecular weight (Mw) being 1000, trimethylolpropane (TMP), 1,2-octanediol (OG), 1,2-tetradecanediol (TG), ML, and stearic acid SAM obtained from vegetable oils were purchased from Aladdin Reagent Co. (Shanghai, China). Toluene diisocyanate (TDI) (80:20 mixture of 2,4-TDI and 2,6-TDI isomers) was supplied by TCI Co. (Japan). Ethanediol (EG), 1,2-propanediol (PG), methyl ethyl ketone (MEK), and N-methyl-2-pyrrolidone (NMP) were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). 2.2. Syntheses of PUs with Dangling Chain. A weighed amount of PPG-1000 was placed in a round-bottomed flask, heated to 75 °C, and then thoroughly mixed with a predetermined amount of TDI. The reaction system was vigorously stirred with a Teflon-coated magnetic stir bar in a dry nitrogen atmosphere for about 2 h to form a urethane B


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temperature to −80 °C, held at this temperature for 10 min, and heated to 120 °C at a heating rate of 3 °C/min. 2.5. Differential Scanning Calorimetry (DSC). Tg was determined using a differential scanning calorimeter (Q100, TA Instruments). The response was measured over the temperature range from −80 to 200 °C at a heating rate of 10 °C/min at a nitrogen flow rate of 50 mL/min. The soft phase Tg from the first heating scan was considered as the inflection point in the DSC thermogram for ensuring consistence with the thermal history of BDRS measurements. 2.6. Atomic Force Microscopy (AFM). Phase-separated morphologies were characterized using a Nanoscope IIIa Multimode microscope (Digital Instruments) in tapping mode. Phase images were obtained at ambient temperature by using Nanosensors PPP-NCH AFM probes under the condition of spring constant being 10−130 N m−1 and resonance frequency being 280−361 kHz. All images were acquired in ambient conditions with the same tapping force (rsp) value of 0.8, where rsp = A/A0 (set point amplitude/free amplitude of oscillation). 2.7. BDRS. The samples with thicknesses from 0.4 to 0.5 mm were sandwiched between brass electrodes with a top electrode diameter of 20 mm in a parallel-plate capacitor configuration. Isothermal relaxation spectra were measured in nitrogen by using a Novocontrol Alpha highresolution dielectric analyzer and spectrometer (Novocontrol GmbH Concept 40, Novocontrol Technology, Germany) from 0.1 Hz to 10 MHz in temperatures from −120 to 100 °C. Dielectric relaxation strength (Δε) and characteristic relaxation time (τHN) were determined for each relaxation process by fitting the dielectric loss (ε*HN) to the appropriate form of the Havriliak− Negami (HN) function25 as given by

prepolymer. Chain extender diols with dangling chains and the crosslinking agent TMP that was dissolved in MEK were added to the system. The mixture was degassed in vacuum for several minutes and then poured and pressed into the preheated Teflon molds. Filled molds were heated at 60 °C for 24 h, at 80 °C for 6 h, and 100 °C for 2 h. Here, PUs with EG, PG, OG, TG, ML, and SAM as chain extenders were prepared in the same way. The synthesis reactions of PUs with dangling chain and chemical structures of the chain extenders are given in Scheme 1, and the diol chain extender, dangling chain length, the presence of ester group, and the calculated hard segment content (mass percentage) for PUs with various dangling chains are listed in Table 1.

Table 1. Diol Chain Extender, Dangling Chain Length, the Presence or Not of Ester Group, and the Calculated Content of Hard Segment for PUs with Various Dangling Chains samples

diol chain extender

dangling chain length

the presence of ester group?

hard segment content (%)

PU-EG PU-PG PU-OG PU-TG PU-ML PU-SAM

EG PG OG TG ML SAM

0 1 6 12 12 18

no no no no yes yes

28.7 29.0 30.6 30.6 33.3 31.3

2.3. Fourier-Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were measured using a Nicolet spectrometer model 560 (Nicolet Instruments). The samples were dissolved in NMP (2 wt % PUs), and the solutions were cast onto KBr windows. The specimens were dried overnight in ambient conditions and further dried for 4 h at 80 °C in a vacuum. The samples were scanned 32 times at a resolution of 4 cm−1. 2.4. Dynamic Mechanical Analysis (DMA). The samples with dimensions of 20 mm × 6.3 mm × 0.5 mm were subjected to dynamic mechanical analysis (Q800, TA Instruments) by using a mode of tensile film clamp. A frequency ( f) of 10 Hz and a strain of 0.2% were used for testing samples. The samples were cooled from ambient

ε*HN (ω) = ε∞ +

Δε σ −i s a b ε (1 + (iτHNω) ) 0ω

(1)

where ω is the angular frequency (ω = 2πf), ε∞ is the unrelaxed value (ω = ∞) of the dielectric constant, a and b are shape parameters (0 < a, ab < 1), σ is the direct current conductivity constant, ε0 is the dielectric permittivity of vacuum, and the coefficient s (0 < s < 1) reflects the conduction mechanism. τHN is related to the maximum frequency of maximum loss ( f max) as given by

Figure 1. Tapping mode AFM phase images of (a) PU-EG(C0), (b) PU-PG(C1), (c) PU-OG(C6), (d) PU-TG(C12), (e) PU-ML(C12), and (f) PU-SAM(C18) with scan sizes of 2 μm × 2 μm in which the hard domains appear as bright regions. C


Macromolecules fmax =

1/ a 1 ⎡ sin(πa(2 + 2b)) ⎤ ⎢ ⎥ 2πτHN ⎣ sin(πab(2 + 2b)) ⎦

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(2)

Above Tg, the contribution of ohmic conduction from ionic impurities dominates the dielectric loss (ε″), which potentially masks dipolar processes. A straightforward derivative method was applied, wherein the ohmic-conduction-free loss (εD″ ) was determined from the logarithmic derivative of dielectric constant to analyze the isothermal data further26

εD″ = −

π ∂ε′(ω) 2 ∂ ln ω

(3)

It is noted that εD″ in eq 3 is approximately equal to the real ohmicconduction-free dielectric loss. The generalization of this approach to broadened relaxation processes (non-Debye response) that are observed here has been discussed in detail by Wubbenhorst and van Turnhout.27,28 The appropriate derivative form of the HN function was used to fit the processes that were resolved by this method.

3. RESULTS AND DISCUSSION 3.1. Microstructure. Figure 1 shows the microphase separation morphologies of PUs with different dangling chains characterized by tapping-mode AFM. For a moderate force image, the hard phase usually corresponds to a high modulus. Hence, hard phases appear as bright regions and soft phases appear as dark regions for PUs.29 Moreover, all PUs exhibit typical microphase separation morphology with dispersed hard domains in a continuous soft phase as is reported.15 It is noted that the sizes of hard domains decrease from 150 to 20 nm when the dangling chain length increases from C0 (PU-EG) to C12 (PU-TG), despite the almost identical content of hard segment as listed in Table 1. Dangling chains within hard segments seem to improve the compatibility between hard and soft segment domains. As shown in Scheme 1, two types of dangling chain are introduced as chain extenders: one is −(CH2)n−CH3, and the other is −CH2−OCO−(CH2)n−2− CH3 with an ester group near the backbone. Compared with PU-TG, the microphase separation morphology (∼40 nm) of hard domain for PU-ML appears larger with the same dangling chain length (C12) and an ester group in the dangling chain. When the dangling chain length increases to C18 (PU-SAM with an ester group in dangling chain), the longer soft dangling chain results in an ambiguous interface between hard and soft phases (Figure 1f). The degree of phase separation for PUs is influenced by both dangling chain length and the accompanying group. It is believed that the introduction of a polar group into the dangling chains has an influence on the formation of H-bond within PUs, thereby affecting the microphase separation. 3.2. State of H-Bonding Associations from FT-IR. Structural characteristics obtained using FT-IR have been widely used to characterize the H-bonds in PUs to evaluate Hbond interactions within PUs with various dangling chains.30,31 The stretching modes of H-bonded carbonyls and N−H groups have distinct wavenumber assignments in the free, ordered, and disordered states. Figure 2 presents the results of FT-IR spectra. The N−H band assignments are ∼3450, ∼3300, and ∼3260 cm−1 for free, disordered, and ordered H-bonded N−H groups, respectively. The carbonyl band assignments are ∼1725, ∼1700, and ∼1660 cm−1 for free, disordered, and ordered Hbonds, respectively. N−H and carbonyl absorption peaks are slightly weakened when short dangling chains are introduced. However, the free N−H absorption peak intensity enhances from a small shoulder peak to a clear splitting peak, and the

Figure 2. FT-IR spectra in the (a) N−H and (b) carbonyl stretching regions illustrating the state of hydrogen bonding. Data are shifted vertically for clarity.

total carbonyl absorption peak intensity decreases when the dangling chain length is further increased. The H-bond between N−H and carbonyl groups in the hard domain weakens with increasing dangling chain length. For PU-ML with an ester group near the urethane group, both carbonyls and N−H group-ordered H-bond absorption peaks have stronger intensities than those of PU-TG with the same length of dangling chain. The H-bond in hard segments is strengthened when polar groups are introduced to the dangling chain, leading to enhancement of the microphase separation, which is consistent with the results in Figure 1. Besides the morphologies of PUs, various H-bond interactions caused by the introduction of dangling chains would have major impact on their relaxation behaviors. 3.3. Dynamic Mechanical Behavior. Figure 3 gives the dynamic mechanical spectra of PU systems. Only the relaxations near 10 °C reflecting Tg of the soft microdomain can be observed in the testing temperature range for several PUs, while the Tg of virgin PPG-1000 is −70 °C.32 Moreover, hard segments are more or less homogeneously distributed throughout the continuous soft phase, and a boundary layer of the mixed phases between the soft and the hard phase is expected (Figure 1), which leads to increasing Tg of the soft phase for PUs. Figure 4 depicts dependence of the corresponding Tg and the value of loss factor (tan δ) peak value obtained from Figure 3 on the length of dangling chain. Tg decreases and the tan δ peak value increases with increasing dangling chain length. In general, the shape and position of tan δ peak are known to be affected by many factors such as crossD


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the polymer network may be effectively transferred to the dangling chain by the chain coupling mechanism,34 wherein the polymer is subjected to external vibration near Tg and the dangling chain might wriggle into the backbone chains that affects their relaxation, which leads to an increase in damping capacity and temperature range. The values of tan δ at 80 °C from around 0.28 to 0.64 practically widen the damping temperature range; hence, PUs with long dangling chains have great potential as damping materials. The PU-TG sample becomes too soft to test above 30 °C because of the weak interaction of molecular chains. The dynamic mechanical spectra of PUs actually reflect relaxation units with various sizes, which are related to the molecular dynamics. In order to confirm the results above, the broadband dielectric relaxation spectroscopy of PUs with different dangling chains will be discussed later. 3.4. Broadband Dielectric Relaxation Behavior. One may wonder if the presence of dangling chains only leads to an increased intermolecular internal friction between relaxed units without affecting the segmental relaxation mechanism. BDRS was used to investigate the molecular dynamics over a broad frequency and temperature range. Figure 5 displays the frequency and temperature dependence of dielectric loss processes for PU-EG and PU-SAM as representative examples.

Figure 3. Dynamic mechanical spectra of polyurethanes with different dangling chain lengths at 10 Hz.

Figure 4. Relationship between tan δ peak value, Tα, and the length of dangling chain without (open symbols) or with (solid symbols) ester group.

linking density, plasticizer or filler content, and orientation of molecular structure, and the dangling chain may function as an interplasticizer, resulting in the decrease of Tg. The degree of microphase separation decreases (Figure 1) and Tg shifts to a lower temperature with increasing dangling chain length. Different from our result, Xu et al.15 found that a visible lowering of Tg occurs with decreasing soft segment content for PUs with dangling chains in the soft segment when the degree of microphase separation increase, suggesting that the microstructure influences the segmental motion of the soft phase, but it is not the only factor. The onset of large-scale motion of a polymer network with long dangling chains near Tg requires a higher energy and free volume than those of ideal network, thereby increasing the value of tan δ peak. Yamazaki et al.33 studied the influence of many dangling chains with uniform lengths on poly(butyl acrylate) elastomers and found a high mechanical damping (tan δ > 1) at a characteristic frequency. Compared with PU-TG, PU-ML exhibits a lower tan δ peak value and higher Tg. These phenomena are concerned in the strong H-bond formed between the ester carbonyl group in the dangling chains and N−H of urethanes in PU-ML (Figure 2). A plateau with a high tan δ value is observed at hightemperature regions for vegetable-based PU-ML (C12) and PU-SAM (C18) (Figure 3). In this case, the energy loading on

Figure 5. Frequency and temperature dependence of the conduction free dielectric loss ε″D for (a) PU-EG and (b) PU-SAM. E


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increases with increasing temperature, which is the same as that in previous reports,35 and Δεβ slightly increases with increasing dangling chain length in PUs at the same temperature. In such a low temperature range, both PU-SAM and PU-ML, which contain polar ester groups, exhibit a slightly higher Δε than the others, indicating a stronger interaction between hard and soft segments. The dielectric responses of PUs are very sensitive to water content.37,38 Capaccioli et al.39 have found that the process with activation energy about 50 kJ/mol in aqueous mixtures is associated with motion of water. Previous work on PU samples at different hydration levels has indicated significant effects of water on β relaxations, and β relaxations can be clearly enhanced by the presence of water.22 Although PUs are dried prior to measurement, residual water still remains because of the strong interaction between water molecules and the polar hard segment as well as the ether linkages of the PPG chain segments. As far as vegetable oil-based PUs with ester groups in the dangling chain are concerned, such as PU-SAM and PU-ML, polar hard segments attach more water molecules because of the greater free volume and the polar group in PUs, which is ascribed to the high Δε to a particular extent. 3.4.2. α Relaxation. In intermediate temperatures, obvious α relaxations are observed in all PUs. Unlike DMA experiments, the α relaxation is believed to be associated with the cooperative segmental motion of the soft segment.38 Figure 7

Several relaxations are observed in all dielectric spectra, namely, local glassy state motions (β) in low temperatures, segmental motion of the soft phase (α), and MWS interfacial polarization in relatively high temperatures. An additional low-frequency/ high-temperature relaxation (process I) is formed in PU-EG but is absent in PU-SAM sample. Similar to DMA results, no peaks corresponding to the glass transition of hard domains can be detected over the measured regions owing to the low content of hard segments. The influences of dangling chains with different lengths and groups on the relaxation need to be discussed in detail. 3.4.1. β Relaxation. The β relaxation is the lowest temperature process and is due to a well-known effect of the local motion of oxygen-containing ether groups in PPG soft segments in glassy state.22,35 However, in DMA experiments of several PUs, no clear relaxation peak is observed in the temperature range of −80 to −40 °C. Figure 6a shows the

Figure 7. Dielectric permittivity εD″ as a function of frequency for PUs with different dangling chain lengths. The data have been normalized to the maximum and rescaled for clarity.

presents the frequency dependence of normalized dielectric loss processes of PUs at 30 °C. The α relaxation moves toward a high frequency with increasing alkyl dangling chain length. However, it turns to low frequency when an ester group is introduced in the dangling chain, indicating the reduced mobility in the PU-ML (C12) compared with PU-TG (C12). These results are in agreement with the change trend of Tg from DMA curves. Figure 8 shows the f max and Δε of the α relaxation (Δεα) as a function of temperature. The f max of the α processes exhibits a non-Arrhenius temperature-dependence behavior as expected for segmental relaxation (see Figure 8a) but follows a Vogel− Fulcher−Tammann (VFT) form as given by

Figure 6. (a) Frequency maxima and (b) dielectric relaxation strengths of β local relaxations as a function of temperature.

profile of f max versus 1/T for β relaxations of PUs with various dangling chains. It can be seen that f max never changes significantly with increasing dangling chain length and shows Arrhenius temperature dependence, and its activation energy is about 56 kJ/mol, which is similar to previously reported values,18,39 meaning that the local dynamics of PPG segments could hardly be affected by changing length of dangling chains and dangling groups in hard segments. Dielectric strength (Δε), which is related to the fraction of mobile dipoles involved in the relaxation process, reflects the molecular interaction to a particular degree.36 Figure 6b gives dependence of Δε for β process (Δεβ) on temperature. It is seen that Δεβ slightly

⎛ B ⎞ fmax = f0 exp⎜ − ⎟ ⎝ T − T0 ⎠ F

(4)


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presence of polar interactions may lead to slower segmental dynamics because of higher friction between relaxed units. The dynamic fragility caused by deviations from the Arrhenius temperature-dependence behavior of mass transport or relaxation properties (e.g., relaxation time, viscosity, fluidity) has been widely used to investigate the dynamics of macromolecules. The index of dynamic fragility or steepness index (m) characterizes the rapidity in which a liquid’s dynamic properties change as it approaches Tg and is quantified as41 m=

∂(log χ ) ∂(Tg /T )

= T = Tg

BTg ln 10(Tg − T0)2

(5)

where χ is a dynamic variable, such as viscosity (η) or relaxation time (τ = 1/2πf) in this study. The material is classified as a fragile glass when m has a high value.42,43 Figure 9 gives the

Figure 8. (a) Frequency maxima and (b) dielectric relaxation strengths of α relaxation (open symbols) and I process (solid symbols); α processes with VFT fits (solid line) and I processes with Arrhenius fits (dotted line).

Figure 9. Relationship between dielectric relaxation strengths of α relaxation at 10 °C, fragility index, and the length of dangling chain without ester group (open symbols) and with ester group (solid symbols).

where T0 is the Vogel temperature and f 0 is relevant to vibration lifetimes40 and is fixed to 1.59 × 10−11 Hz (τ0 = 10−12 s) for the α process to reduce fitting uncertainties. The temperature coefficient (B) is related to the apparent activation energy and fragility. The fitting parameters B and T0 are listed in Table 2. Tg can be estimated from segmental relaxation by

value of m calculated from eqs 4 and 5. It is seen that an increase in the dangling chain length leads to a decrease in m for PUs. The presence of dangling chains increases the PU backbone flexibility, and a long dangling chain results in a high backbone flexibility. The effect of polar group on segmental dynamics strongly depends on the location of this group relative to the backbone, whether it is directly attached to the backbone or separated from the backbone by a large spacer group. In the present case, PU-ML with a polar group in the dangling chain exhibits a slightly higher m than that of PU-TG. The intermolecular interactions that are related to the polymer structure should be understood as involved in the influence of a polar group on segmental dynamics in polymers. The molecular cohesive energy (ξ) depends on the intermolecular interactions induced by dipole−dipole interactions and London dispersion forces.44 The approximate empirical relationship between the dipole moment (μ0) of a molecule and ξ of dense phase materials is expressed as ξ = (5 ± 1) + 0.6 μ02.45 The dipole moment can be estimated from the group contribution method.46 Hence, owing to introduction of an ester group into dangling chain, the dipole moment increases from 1.50 to 1.64 D for the hard phase of PU-TG and PU-ML, ξ of the hard segment increases from 6.35 to 6.61 kcal/mol and the introduction of polar interactions may slow down segmental

Table 2. Relevant Fitting Parameters for the VFT Equation, Tg from DSC, and Tref at τ = 100 s for PUs samples

B ± 20

T0 (K) ±2K

PU-EG PU-PG PU-OG PU-TG PU-ML PU-SAM

1488 1472 1546 1545 1560 1552

215.5 213.1 204.5 199.3 203.0 202.4

Tg (K) (DSC) ± 3K

Tg (K) (DMA at 10 Hz)

Tref (K) (at τ = 100s) ± 5 K

257.2 256.0 254.9 248.5 249.4 247.7

284.4 280.2 277.2 275.7 277.5 275.1

262.7 260.6 254.1 245.8 253.6 251.8

extrapolating the VFT fit to τ = 100 s, which is denoted as Tref (Table 2). Tg determined from DSC experiments and Tref are in good agreement with each other within experimental error. Even a big gap between Tref and Tg determined by DMA at 10 Hz is observed; the downward trend with increasing length of dangling chain appears to be similar. On the other hand, Tg of PU-ML is higher than that of PU-TG, suggesting that the G


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dynamics, resulting in a significant increase in Tg and fragility (m: from 68.5 to 69.2). Moreover, PU-TG with a longer dangling chain has greater flexibility than that of PU-EG. Although PU-ML with a polar dangling chain exhibits higher m compared with PU-TG, it still has a lower m value than that of PU-EG without a dangling chain. It is believed that chain flexibility seems to have more dominant effect than polarity. Unlike the β process, the Δεα of PUs increases with elevating temperature and reaches a maximum at around 10 °C and then gradually decreases. On the basis of the DMA results, the main transition temperature is about 10 °C. Figure 9 shows the relationship between Δεα and dangling chain length at 10 °C. Δεα increases with increasing dangling chain length. Taking the α relaxation into account, we believe that it is caused by the relaxation of the soft segment including both unrestricted soft segments in the soft segment-rich phase and segments that are constrained by their attachment to hard domains.37 An increase in Δεα is considered to be primarily due to the decreased restriction of segmental rotation by the attachment of dangling chain to the hard segment while no dangling chain exists on the soft segments. Compared with PU-TG, the Δεα of PU-ML is lower because the ester group of ML favors the formation of Hbond between ester group and −NCO group within the hard segment and facilitates the microphase separation between soft and hard segments. As a result, the interaction between soft and hard segments decreases to a particular extent. The effect of a polar group in dangling chains in the hard segment on the segmental dynamics of the soft domains in PUs is secondary effect as compared with dangling chain length (Figure 9). 3.4.3. Origin of I Processes. As is aforementioned, the I process, as an additional low-frequency/high-temperature relaxation, appears in PU-EG but is absent in PU-SAM. Comparison of relaxation peaks of PUs obtained using different techniques suggests that both mechanical and calorimetric glass transitions correspond to α processes whereas the I process has no clear corresponding signature. Moreover, the absence of the I process in mechanical and calorimetric measurements could hardly be immediately explained. Dielectric spectroscopy seems to be sensitive to molecular motions in these materials because of relatively large dipole moment of urethane linkages that are trapped or mixed in the soft phase. On the other hand, the I process can be observed in the ε″D spectra where losses caused by impurity ion conduction (which significantly increases loss above the soft phase process) are removed from the spectra. The I process is expected to be a α process of hard domain; however, it is observed only in frequencies lower than those responsible for the soft segmental relaxation, and exhibits Arrhenius temperature-dependence behavior over the measured frequency range with an activation energy of about 43.2 kJ/mol (Figure 8a). Considering its Arrhenius temperature dependence, the origin of I process is associated with local reorientations of strong H-bonds within the hard domains. The same relaxation behavior in time scale was observed by Castagna et al.35,47 and has a higher activation energy of about 60 kJ/mol when ordered 4,4′-diphenylmethane diisocyanate and 1,4-butanediol were used to form the hard segment. It is found that the I process appears apparent in PU-EG and PU-PG with much shorter dangling chains, whereas it is absent in the ε″D spectra of PU-OG with a dangling chain length of C6 and others with longer dangling chains. The weakening of the H-bond between soft and hard phases due to long dangling chains is believed to result in the absence of the I process. The I process of PU-PG with short dangling chain is faster than that

of PU-EG, and the loss peak shifts to a low frequency because the introduction of methyl causes more network defects. ΔεI increases monotonically with increasing temperature (Figure 8b), which is probably due to the gradual release of constraints from the H-bond with increasing temperature. For PU-ML and PU-SAM that both contain polar groups in the dangling chains, the enhanced H-bond interaction between the ester and −NCO groups within the hard segment seems not enough to balance the long dangling chain, and as a result, no I process can be observed. 3.4.4. MWS Interfacial Polarization. The accumulating charges behave similarly to a macroscopic dipole, which results in dielectric loss peak (MWS polarization). The MWS process appears in the frequency range of the loss dominated by dc conductivity of ionic impurities in the soft phase. Derivative formalism is applied to resolve this process, and a maximum MWS process in the εD″ spectra19 can be obtained. In heterogeneous materials with different characteristics of dielectric permittivity or conductivity, interfacial polarization occurs because of the accumulation of charges at the interfaces.48−51 The strength of the MWS process is demonstrated to provide a rather sensitive indicator of the microphase mixing process. Good phase mixing causes a decrease in the amount of boundary charges, i.e., a decrease in the strength of MWS process. Figure 10 gives the f max and Δε of the MWS process (ΔεMWS) as functions of temperature. Both f max and ΔεMWS depend on the “contrast” between soft

Figure 10. (a) Frequency maxima and (b) dielectric relaxation strengths of MWS processes as a function of temperature. H


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and hard domains, i.e., the difference between their respective dielectric constants and conductivities. With increasing dangling chain length, the MWS process shifts to high frequencies (Figure 10a). However, the strength significantly decreases (Figure 10b), which reflects the declining “contrast” between soft and hard phases. When the polar ester group in the dangling chain is introduced, the dielectric constants of the hard segments increase and the MWS process relaxation times of PU-ML become longer compared with PUTG. The ΔεMWS of PU-EG and PU-PG exhibits greater values than those of PUs with other longer dangling chains in low temperatures (Figure 10b), which means that long dangling chains improve the compatibility between soft and hard segment and reduce the degree of microphase separation (Figure 1). Unlike other processes, the ΔεMWS of PU-EG and PU-PG rapidly decreases with increasing temperature. However, the ΔεMWS of PUs with relatively long dangling chains initially decreases slightly and then increases with increasing temperature. PU-EG and PU-PG obviously undergo phase mixing process,52 which results in the rapid decrease of ΔεMWS at elevated temperatures. The ΔεMWS of PU-PG decreases faster than that of PU-EG, implying that the methyl group in the hard segment can speed up the microphase mixing process. For PUs with long dangling chains, their phase mixing is slowed down by the barrier effect of long dangling chains. When the temperature is increased further, the dangling chain units start to participate in the accumulation of charges at the interfaces, which may lead to the increase of ΔεMWS in high temperatures. The increased interphases for PU with dangling chains contribute to the high value of tan δ in a high temperature region observed by DMA. 3.5. Effect of Hard Segment Content. The effect of content for hard segment containing dangling chains on molecular dynamics of PUs was investigated to explore further the relationship between the dangling chain and relaxation behavior. Andrady et al.53 studied the dependence of ultimate properties on dangling chain irregularities of model poly(dimethylsiloxane) and found that Tg is generally insensitive to the content of dangling chains over a range from 0 to 41.1 wt %. In the present study, PUs with different contents of hard segment (containing long dangling chain (C18) obtained from vegetable oil) were synthesized, and the hard segment content was controlled by changing the content of total TDI-SAM hard segment. In other words, increase of the hard segment content implies the synchronous increase of dangling chain content. Figure 11 demonstrates the dynamic mechanical spectra of PUs with different TDI-SAM hard segment contents. The Tg of soft segment shifts to a high temperature, and its transition temperature range becomes wider with increasing content of TDI-SAM hard segment, whereas the value of tan δ peak decreases. Figure 12 presents the profiles of ε″D of PUs with a TDI-SAM hard segment content of 43.1% as a function of frequency and temperature. It can be found that the I process re-emerges, and its strength also increases with increasing temperature. As mentioned above, the I process is related to local reorientations of strong H-bond within hard domains. For a hard segment, content of 43.1% is probably enough to form more H-bond within hard phases to balance the influence of the long dangling chain, thereby leading to the reappearance of the I process. Figure 13 gives the frequency-dependent ε″D for PUs with different hard segment contents. It is clear that the strength of the α relaxation decreases with increasing hard segment content (Figure 13), and its breadth significantly

Figure 11. Dynamic mechanical spectra of polyurethanes with different hard segment contents.

Figure 12. Dielectric permittivity εD″ as a function of frequency and temperature for PU-SAM containing hard segments of 43.1%.

Figure 13. Dielectric permittivity ε″D as a function of frequency for PUs with different hard segment contents.

increases. These results are similar to those of DMA spectra (Figure 11) where the HN parameter a decreases from 0.66 to 0.35 with increasing hard segment content, indicating that segments in the soft phase become relaxed in an increasing heterogeneous manner.47,54 It should be noted that even the hard segment content is as high as 49.1 wt %; its Tg as well as the melting point could hardly be detected by either DMA or I


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faster with increasing dangling chain length. Long dangling chains also cause the index of dynamic fragility of PUs to decrease, which indicates a high flexibility. The dangling chain length seems to dominate the I process and makes the I process absent for PUs with a long dangling chain length (C6 to C18). MWS processes become faster after longer dangling chains are introduced. The polarity of dangling chains also remarkably influences the dynamics of PUs. The introduction of ester groups into the dangling chain slows down the segmental dynamics, while it increases the value of m and the degree of microphase separation. Polar groups in long dangling chains enhance H-bond interaction between the ester and −NCO groups within hard segment, but it is not strong enough to balance the effect of long dangling chains. Therefore, I process could hardly be observed. The MWS process relaxation weakens when ester groups are introduced. Tg shifts to a high temperature, and its range becomes wider with increasing hard segment contents. The I process reemerges provided a hard segment content reaches 43.1%, and its activation energy increases with increasing hard segment content, indicating that a strong H-bond within the hard phase can counteract the influence of the long dangling chain. Moreover, the MWS process shifts to low frequencies, and its strength increases because of the high degree of microphase separation. Furthermore, the dependence of molecular dynamics on dangling chain length, polar group, and content suggests that the properties of PUs can be tailored not only by the typical modification of network backbone but also by controlling architecturing of dangling chains and introducing polar groups.

BDRS owing to the narrowly employed temperature range here. Figure 14 gives the f max of all relaxation processes as a

Figure 14. Frequency maxima of the β (horizontal lines symbols), α (open symbols), I (solid symbols), and MWS (cross symbols) processes as a function of reciprocal temperature for PUs with various hard segment contents; α processes fit to VFT equation and β, I processes fit to Arrhenius equation.

function of reciprocal temperature. The β relaxation appears almost independent of TDI-SAM hard segment content. However, both α and I processes shift to somewhat low frequencies with increasing hard segment content. As expected, the α process is endowed with a VFT temperature-dependent behavior and the I process an Arrhenius temperaturedependent behavior. The activation energies of the I process increases from 40.5 to 74.4 kJ/mol with increasing hard segment content, which is believed to be due to the strengthened H-bond interaction within the hard phase. In previous studies on PUs with dangling chains in the soft segment,55,56 similar trends of H-bond interaction were observed by using FT-IR, in which the H-bond interaction was enhanced by increasing the hard segment content. PUSAM with a hard segment content of 43.1% has activation energy of 40.5 kJ/mol similar to 43.2 kJ/mol for PU-EG (hard segment = 28.7%), suggesting the close intensity of H-bond interaction. Combining with Figure 2, the intensities of H-bond interactions can be tuned by varying the hard segment contents and dangling chain lengths. In addition, the peaks of the MWS process are strengthened, whereas those of the α relaxations are reduced because the TDI-SAM hard segment contents increase (Figure 13). An increase of TDI-SAM is believed to favor the microphase separation between soft and hard segments, which makes the microphase mixing near the MWS process difficult and the dynamic restrictions more drastic. Therefore, a high degree of microphase separation actually slows MWS process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.D.); [email protected] (Q.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 20674072) and the Program for Zhejiang Provincial Innovative Research Team (Grant No. 2009R50004).

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REFERENCES

(1) Hepburn, C. Polyurethane Elastomers, 2nd ed.; Elsevier: London, 1991. (2) Oertel, G. Polyurethane Handbook, 2nd ed.; Hanser: Munich, 1993. (3) Harrell, L. L. Macromolecules 1969, 2 (6), 607−6012. (4) Petrović, Z. S.; Ferguson, J. Prog. Polym. Sci. 1991, 16, 695−836. (5) Carme Coll Ferrer, M.; Babb, D.; Ryan, A. J. Polymer 2008, 49 (15), 3279−3287. (6) Kong, X. H.; Narine, S. S. Biomacromolecules 2008, 9 (8), 2221− 2229. (7) Narine, S. S.; Kong, X. H.; Bouzidi, L.; Sporns, P. J. Am. Oil Chem. Soc. 2007, 84 (1), 55−63. (8) Oprea, S. J. Mater. Sci. 2011, 46 (7), 2251−2258. (9) Dušek, K.; Dušková-Smrčková, M.; Fedderly, J. J.; Lee, G. F.; Lee, J. D.; Hartmann, B. Macromol. Chem. Phys. 2002, 203 (13), 1936− 1948. (10) Petrović, Z. S. Polym. Rev. 2008, 48 (1), 109−155. (11) Erman, B.; Mark, J. E. Structure and Properties of Rubberlike Networks; Oxford University Press: New York, 1997.

4. CONCLUSION The effect of dangling chains on phase-separated microstructure and molecular dynamics for polyurethanes (PUs) has been studied. The dangling chain exerts significant effects on the morphology, dynamic mechanical properties, and molecular dynamics of PUs. β relaxations in a low-temperature range remain unaffected by increasing dangling chain length or polar groups. However, both long dangling chain and polar group in vegetable oil have a pronounced effect on segmental motion, reflecting the α relaxation of soft segments in PUs. Tg shifts to a lower temperature, and segmental dynamics becomes J


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(12) Lee, Y. L.; Sung, P.-H.; Liu, H. T.; Chou, L. C.; Ku, W. H. J. Appl. Polym. Sci. 1993, 49 (6), 1013−1018. (13) Yamazaki, H.; Takeda, M.; Kohno, Y.; Ando, H.; Urayama, K.; Takigawa, T. Macromolecules 2011, 44 (22), 8829−8834. (14) Zlatanić, A.; Lava, C.; Zhang, W.; Petrović, Z. S. J. Polym. Sci., Part B: Polym. Phys. 2004, 42 (5), 809−819. (15) Xu, Y. J.; Petrovic, Z.; Das, S.; Wilkes, G. L. Polymer 2008, 49 (19), 4248−4258. (16) Oprea, S.; Potolinca, O.; Oprea, V. High Perform. Polym. 2011, 23 (1), 49−58. (17) Jin, X.; Zhang, S.; Runt, J. Macromolecules 2003, 36 (21), 8033− 8039. (18) Masser, K. A.; Zhao, H. Q.; Painter, P. C.; Runt, J. Macromolecules 2010, 43 (21), 9004−9013. (19) Czech, P.; Okrasa, L.; Boiteux, G.; Mechin, F.; Ulanski, J. J. NonCryst. Solids 2005, 351 (33−36), 2735−2741. (20) Lee, H. K.; Fragiadakis, D.; Martin, D.; Milne, A.; Milne, J.; Runt, J. Macromolecules 2010, 43 (7), 3125−3127. (21) Fragiadakis, D.; Runt, J. Macromolecules 2010, 43 (2), 1028− 1034. (22) Georgoussis, G.; Kyritsis, A.; Pissis, P.; Savelyev, Y. V.; Akhranovich, E. R.; Privalko, E. G.; Privalko, V. P. Eur. Polym. J. 1999, 35 (11), 2007−2017. (23) Raftopoulos, K. N.; Pandis, C.; Apekis, L.; Pissis, P.; Janowski, B.; Pielichowski, K.; Jaczewska, J. Polymer 2010, 51 (3), 709−718. (24) Vallance, M. A.; Yeung, A. S.; Cooper, S. L. Colloid Polym. Sci. 1983, 261 (7), 541−554. (25) Havriliak, S.; Negami, S. Polymer 1967, 8, 161−210. (26) Jiménez, M. L.; Arroyo, F. J.; van Turnhout, J.; Delgado, A. V. J. Colloid Interface Sci. 2002, 249 (2), 327−335. (27) Wübbenhorst, M.; van Turnhout, J. J. Non-Cryst. Solids 2002, 305 (1−3), 40−49. (28) van Turnhout, J.; Wübbenhorst, M. J. Non-Cryst. Solids 2002, 305 (1−3), 50−58. (29) Cascaval, C. N.; Rosu, D.; Rosu, L.; Ciobanu, C. Polym. Test. 2003, 22 (19), 45−49. (30) Lee, H. S.; Wang, Y. K.; Hsu, S. L. Macromolecules 1987, 20 (11), 2089−2095. (31) Yu, W.; Zhang, D.; Du, M.; Zheng, Q. Eur. Polym. J. 2013, 49, 1731−1741. (32) Petrović, Z. S.; Javni, I. J. Polym. Sci., Part B: Polym. Phys. 1989, 27 (3), 545−560. (33) Yamazaki, H.; Takeda, M.; Kohno, Y.; Ando, H.; Urayama, K.; Takigawa, T. Macromolecules 2011, 44 (22), 8829−8834. (34) Sears, J. K.; Darby, J. R. The Technology of Plasticizers; John Wiley & Sons, Inc.: New York, 1982. (35) Castagna, A. M.; Fragiadakis, D.; Lee, H.; Choi, T.; Runt, J. Macromolecules 2011, 44 (19), 7831−7836. (36) Marion, J. B. Classical Dynamics of Particles and Systems; Academic: New York, 1965. (37) Fragiadakis, D.; Gamache, R.; Bogoslovov, R. B.; Roland, C. M. Polymer 2010, 51 (1), 178−184. (38) Cerveny, S.; Alegría, Á .; Colmenero, J. Phys. Rev. E 2008, 77 (3), 031803−1−031803−5. (39) Capaccioli, S.; Ngai, K. L.; Shinyashiki, N. J. Phys. Chem. B 2007, 111 (28), 8197−8209. (40) Schonhals, A.; Kremer, F. Broad Band Dielectric Spectroscopy; Springer: Berlin, 2002. (41) Santangelo, P. G.; Roland, C. M. Macromolecules 1998, 31 (14), 4581−4585. (42) Wu, J. R.; Huang, G. S.; Qu, L. L.; Zheng, J. J. Non-Cryst. Solids 2009, 355, 1755−1759. (43) Qin, Q.; McKenna, G. B. J. Non-Cryst. Solids 2006, 352 (28− 29), 2977−2985. (44) Longster, G. F.; Walker, E. E. Trans. Faraday Soc. 1953, 49, 228−233. (45) Agapov, A. L.; Wang, Y. Y.; Kunal, K.; Robertson, C. G.; Sokolov, A. P. Macromolecules 2012, 45 (20), 8430−8437.

(46) Van Krevelen, D. W. Properties of Polymers - Their Estimation and Correlation with Chemical Structure; Elsevier: New York, 1976. (47) Castagna, A. M.; Pangon, A.; Choi, T.; Dillon, G. P.; Runt, J. Macromolecules 2012, 45 (20), 8438−8444. (48) Okrasa, L.; Zigon, M.; Zagar, E.; Czech, P.; Boiteux, G. J. NonCryst. Solids 2005, 351 (33−36), 2753−2758. (49) van Beek, L. K. H. Dielectric Behavior of Heterogeneous Systems; Heywood Books: London, 1967; Vol. 7. (50) North, A. M.; Pethrick, R. A.; Wilson, A. D. Polymer 1978, 19, 913−922. (51) Hayward, D.; Pethrick, R. A.; Siriwittayakorn, T. Macromolecules 1992, 25 (5), 1480−1486. (52) Pongkitwitoon, S.; Hernández, R.; Weksler, J.; Padsalgikar, A.; Choi, T.; Runt, J. Polymer 2009, 50 (26), 6305−6311. (53) Andrady, A. L.; Llorente, M. A.; Sharaf, M. A.; Rahalkar, R. R.; Mark, J. E.; Sullivan, J. L.; Yu, C. U.; Falender, J. R. J. Appl. Polym. Sci. 1981, 26 (6), 1829−1836. (54) Oprea, S. J. Appl. Polym. Sci. 2011, 119 (4), 2196−2204. (55) Ning, L.; De-Ning, W.; Sheng-Kang, Y. Polymer 1996, 37 (16), 3577−3583. (56) Lu, Y.; Larock, R. C. Biomacromolecules 2008, 9 (11), 3332− 3340.

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Influence of Dangling Chains on Molecular Dynamics of Polyurethanes

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