NMR Study of the Microstructures and Water–Polymer Interactions in

Article pubs.acs.org/Macromolecules

NMR Study of the Microstructures and Water−Polymer Interactions in Cross-Linked Polyurethane Coatings Haijin Zhu,*,†,‡ Hendrik P. Huinink,*,† Olaf C. G. Adan,† and Klaas Kopinga† †

Department of Applied Physics, Transport in Permeable Media, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Institute for Frontier Materials, Deakin Univerity, Geelong, Victoria 3220, Australia S Supporting Information *

ABSTRACT: The microstructure of a polymer coating plays an important role in the water uptake behavior. This paper aims to correlate the molecular mobility and the water− polymer interactions with the microstructures of a highly cross-linked PU system. GARfield NMR imaging was used to monitor in situ the water uptake of the PU coating at different temperatures. The results of continuum T2 fitting show that at temperatures below the enthalpy relaxation temperature (65 °C) the PU coating uptakes water, whereas the polymer matrix is not plasticized by the presence of water. At higher temperatures, however, the polymer matrix is significantly mobilized by the presence of water molecules as indicated by the appearance of the longer T2 component. The water content in the PU coating is monitored by GARfield NMR at different temperatures. The results show that the water content decreases in two steps as the temperature decreases from 85 °C to the room temperature. This result is explained in combination with the molecular relaxation phenomenon probed by the DSC. A microstructure model was formulated based on the experimental results.

1. INTRODUCTION Polyurethanes (PU) are an important class of industrial materials and widely applied as coatings, fabrics, binder resins, and high-performance elastomeric products.1,2 However, a big issue for these materials is that they suffer from weathering problems when applied in a wild nature environment. Water is one of the most important reasons for the material failure. The water−polymer interactions in polymer coatings have been studied by many authors. Carbonini et al. studied the water uptake of a polymeric multilayer system and found that the water absorption and chemical degradation are highly dependent on the chemical properties of each layer and the position among different layers.3 Baukh et al. investigated the water− polymer interaction in a two-layered base coat/top coat polyurethane coating system.4 On the basis of the highresolution NMR imaging results, they pointed out that water is loosely bonded to the polymer matrix at high water contents. The polymeric dispersant in the coating plays an important role in the water sorption by the coating. These studies have made a big step toward an understanding of the water uptake process in polymer coatings. However, most of the studies focused on formulating a kinetic or thermodynamic model to describe the water uptake process. What is generally ignored is the fact that the water uptake and transport is closely related to the microstructure of the polymer coating. In this work, water uptake behavior of a polyurethane coating will be discussed in relation to its microstructure. The phase structures of the linear segmented PU elastomers (which are generally known as thermal plastic polyurethanes, TPU) have been intensively studied since the past century.5−7 © XXXX American Chemical Society

It is well-known that it is the thermodynamic incompatibility between the “hard” polyurethane segments and the “soft” polyether or polyester segments drives their microphase separation into hard and soft domains.5,6,8 The hard domain plays the role of physical cross-link and acts as a higher modulus filler in the low modulus soft matrix, whereas the soft domain gives excellent impact resistance and extensibility to the materials.9 The hard domain in the thermal plastic PU (TPU) elastomers is usually associated with hydrogen bonding between the hard segments, which is a weak interaction and it breaks down easily at a higher temperature. Therefore, these materials behave like cross-linked polymers at the temperature of use, and significant deterioration of the mechanical properties occurs as the temperature approaches the softening or melting temperature of the hard domain. For the material considerations, controlled cross-linking via chemical bonding not only improves dimensional stability at higher temperatures but also improves the mechanical and thermal properties of the material.10−13 The covalent crosslinks can prevent extensive disruption and reorganization of the domains. Therefore, it has a restrictive influence on the size of microdomains, especially when the cross-linking is prepared in a one-stage process. Most of the published literature concerns the microstructure of linear TPU systems. Only a few studies are dedicated to the cross-linked polyurethanes. Krakovský et al. prepared cross-linked polyurethanes using poly(butadiene)Received: June 18, 2013 Revised: July 22, 2013

A

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Figure 1. Schematic illustration of the synthesis process of (a) the polyester (PNI) and (b) polyurethane (PU).

2. EXPERIMENTAL SECTION

diol (PBD), 4,4′-diphenylmethane diisocyanate (MDI), and poly(oxypropylene)triol (POPT).13,14 An inhomogeneous structure on the scale of 70−120 Å was observed by smallangle X-ray scattering (SAXS). Based on their observations, a two-phase structure in the polyurethane network was assumed. One phase consists of PBD chains while the other of densely cross-linked POPT/MDI network. Many experimental techniques have been used to probe the water uptake of the coatings and polymeric films, such as gravimetry (TGA), 1 5 , 1 6 electrochemical impedance spectroscopy(EIS),17−19 Fourier transform infrared spectroscopy,20,21 fluorescence spectroscopy,22 etc. These techniques only provide information about the total amount of water uptake in the coatings. Recently, a high spatial resolution magnetic resonance imaging (MRI) profiling technique based on the socalled GARfield approach was developed by Glover et al.23 The experimental MRI setup has been described in details somewhere else.24 The beauty of this technique is that it not only probes the water distribution in depth direction with high spatial resolution but also provides information about the molecular dynamics by T2 relaxation analysis. It is a very powerful technique which can be used to detect the molecular mobility of a multilayer coating system. This study aims at understanding the polymer mobility and water uptake behavior in relation to the microstructures of a highly cross-linked PU coating. The enthalpy relaxation behavior of the PU coatings as detected by DSC will be explained in combination with the molecular mobility of polymer network probed by using T2 analysis of the NMR data. Based on these results, a tentative inhomogeneous microstructure model is proposed.

2.1. Sample Preparation. The poly(neopentyl isophthalate) (PNI) used in this study was provided by DSM. This polyester was prepared from isophathalic acid and neopentyl glycol in a bulk polycondensation process as shown in Figure 1a. The details of the synthesis steps have been described somewhere else.11 The densely cross-linked polyurethane coating samples were prepared by mixing the hexamethylene diisocyanate trimer (Tolonate HD-LV2, Perstorp) and poly(neopentyl isophthalate) (PNI) with functional group molar ratio of 1:1.25 In terms of polymer network structure, this molar ratio means that the polymer system is of the highest cross-link density and lowest free-dangling ends density. The synthesis processes of the polyester (PNI) component and the cross-linking procedure are schematically illustrated in Figure 1. Both components are dissolved in N-methyl-2-pyrrolidone (NMP) at about 100 °C with concentration of 40% (wt %), respectively. The dissolved components are mixed together under stirring, and then a polymer coating layer with thickness of about 150 μm was applied on an aluminum substrate. The coating was transferred immediately into the oven after preparation, and kept at 125 °C for 30 min under vacuum. Finally, the cured coating was scratched from the substrate for the DSC. In the following discussion, the PU sample exposed in the room temperature and air humidity conditions will be referred to as “dry” sample in contrast to the H2O- and D2O-saturated samples. 2.2. Differential Scanning Calorimetry (DSC). The DSC measurements were performed on a Mettler 822e DSC apparatus. About 6 mg of the PU sample was scratched from the coating panel and sealed in an aluminum pan. The temperature program for the DSC measurements is shown in Figure 2. The thermal history of the sample was eliminated by holding the temperature at 120 °C for 10 min. Subsequently, the sample was cooled to room temperature and annealed at room temperature for 2 days. The annealed sample was then heated up to 120 °C, and the first heating endotherm was recorded. During the second temperature cycle, the sample was first held at 120 °C for 10 min to remove the thermal history, then it was cooled down to room temperature and heated up immediately after 5 min equilibrium time, and the second heating endotherm was recorded B

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related to its microstructures. Therefore, knowledge of microstructure in the polymer coating is a must to make a next step toward a full understanding of the process. The molecular relaxation phenomena in polymers have long been investigated to explore structure changes occurring in a polymer system at temperatures lower than glass transition. The term “enthalpy recovery” is usually used to quantify this molecular relaxation process. The DSC temperature program used for measuring this enthalpy recovery is shown in Figure 2. The first heating run records the sample enthalpy that is related to the prior thermal history where the sample was annealed at room temperature for 2 days. Then after erasing the thermal history at 120 °C, the second heating run was performed immediately after the cooling scan (5 min before heating scan was waited to stabilize the temperature), and it measures the thermal response of the unaged material. The DSC melting endotherms of the annealed and unannealed sample are presented in Figure 4. For the unannealed sample, a glass

Figure 2. Temperature program for the DSC measurements. during the heating scan. The heating and cooling rates are 10 °C/min for all the scans. 2.3. NMR. The NMR imaging setup used in the present study consists of a GARfield magnet and a homemade acquisition system which has been described elsewhere by Kopinga and Pel.25 The static field B0 of the magnet at the position of the sample is 1.5 T, and the gradient field strength G is 40 T/m. All the signal profiles were measured with a pulse sequence θx−[te/2−θy−te/2−echo]n. A pulse duration of 1 μs and an inter-echo time te of 50 μs were specified in all measurements. The theoretical spatial resolution that can be achieved by this inter-echo time setting is about 15 μm. Each signal profile was averaged 4096 times, and the long delay between two subsequent pulse sequences was set to 1 s. The experimental setup is shown in Figure 3. The PU coating was first applied on glass coverslips with

Figure 4. DSC melting endotherms of the unannealed PU sample and the PU sample that has been annealed at room temperature for 2 days. The full temperature program for the measurements is shown in Figure 2. Figure 3. Schematic illustration of the GARfield NMR experimental setup.

transition at about 60 °C can be identified from the step change of the heat flow (or heat capacity). This is attributed to the glass transition of the amorphous region in the PU coating. The glass transition temperature (Tg) of a real material always occurs over a temperature range depending on the kinetic nature of the heterogeneity of the molecular (segments) mobility. The reported Tg value is the inflection point of change in the DSC heat flow curve. The annealed sample, however, shows a first-order transition (shown as an endothermic peak) superimposed on the glass transition (endothermic step) of the unannealed sample. The peak temperature of the first-order transition is about 65 °C, which is slightly higher than the glass transition temperature. This first-order transition is generally termed as “enthalpy recovery”, and it is ascribed to the molecular relaxation at the annealing temperature.26 From the molecular structure point of view, this molecular relaxation is a consequence of intra- and/or intermolecular interaction, e.g., van der Waals force, hydrogen bonding, etc., and it leads to a more organized or ordered structure. It worth mentioning that this locally ordered structure is not identical to the long-range ordered crystalline structure which can be clearly identified by the sharp diffraction peaks in the X-ray diffraction (XRD) pattern. This is confirmed by the XRD pattern of the annealed

sizes of 18 mm × 18 mm × 100 μm. A glass tube was glued on the top of the coating and filled with water or heavy water (D2O) to saturate the sample. A single-sided RF coil with a diameter of about 3 mm was used for excitation and receiving, and as a result, the RF field is not homogeneous throughout the sample volume. There will be distribution of flip angle θ in the sample. In order to correct the position dependence of the NMR signal intensity caused by the RF field inhomogeneity, all the signal profiles are corrected by dividing the signal intensity of the 0.01 mol/L copper sulfate (CuSO4) solution measured at the same temperatures. Note that the amplitudes of the corrected signal are relative densities of proton nuclei with respect to the proton density in water. In the temperature-variable experiments, the time between two subsequent temperature points was 30 min in order to equilibrate the temperature. The measuring time for each single experiment is about 1.5 h. 2.4. Continuum T2 Fitting. The T2 relaxation spectra are fitted with multiexponential decay function using an in-house program written in Matlab.

3. RESULTS AND DISCUSSION 3.1. Thermal Properties. As mentioned in the Introduction, water uptake behavior of polymer coatings is closely C

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Figure 5. NMR signal profiles of (a) dry coating, (b) heavy water saturated coating, and (c) water saturated coating. The same PU coating sample was repeatedly used for all the measurements. The intensities of the profiles are averaged throughout the coating thickness and are plotted against temperature in (d). The dashed lines in (a) indicate the position of interfaces. The arrows in the figures denote the intensity change with increasing temperature.

corresponding to the bottom of the coating (as shown in Figure 5a). The NMR signal on the top of the polymer coating in Figure 5c is attributed to the H2O on the top of the polymer coating. No signal was detected in the same position of Figures 5a and 5b because the polymer coatings are covered by air and deuterated water, respectively. The higher intensity between 200 and 150 mm in Figure 5c is attributed to the water on the top of the coating. Figures 5a and 5b are profiles of the dry and the D2O saturated coating, respectively. No water is presented on the top. Therefore, there is no intensity between 200 and 150 mm. The NMR intensities of these samples are plotted against temperature in Figure 5d. For the dry sample, the intensity increases slightly with temperature in the whole temperature region studied. Although significant relaxation processes were observed in the temperature range of 50−70 °C from the DSC results in section 3.1, no abrupt intensity increase is observed in this temperature region. A possible explanation is that due to the restriction from the dense cross-links, the polymer chain keeps low mobility even above the glass transition and enthalpy relaxation temperatures. Therefore, the T2 of the dry PU sample is so short in the whole temperature range studied that most of the signal decayed already at the first echo. In presence of water, however, the NMR signal of the profiles is higher than that of the dry sample at all temperatures. This signal increase compared to the dry sample may be attributed to two reasons: the water ingression which could give extra signal from the water and/or an increase of the polymer mobility which could also increase the NMR signal of the polymer matrix due to longer T2. If the increased signal originates exclusively from water, it means that the polymer

sample which shows a broad hump instead of sharp diffraction peaks. The enthalpy recovery can be quantified using the difference in enthalpy between the first and second heating scans, which is measured to be about 4 J/g. Assuming the ideal equilibrium state for the PNI molecules is a perfect crystalline (100%) structure, one can estimate how far is the current state away from the equilibrium by dividing the “melting” enthalpy of the current state by that of the perfect crystal. To the best of our knowledge, the melting enthalpy of a perfect PNI crystal is not available in the literature. However, comparing with that of a similar molecular structure polyester, poly(ethylene terephthalate) (PET), for example, may also give a semiquantitative estimation of the amount of the ordered structure. The melting enthalpy of the perfect PET crystal is about 140 ± 20 J/g.27 Therefore, it can be estimated that the enthalpy recovery is less than 5% of the melting enthalpy of the perfect PET crystal. 3.2. NMR Signal Profiles and the Mobility of the Polymer Network. The DSC results in section 3.1 have shown that, while increasing the temperature, the PU sample undergoes a glass transition at about 60 °C and a first-order enthalpy relaxation at about 65 °C. In order to get the information on the molecular mobility of the cross-linked polymer network and the interaction between polymer and water molecules, the NMR profiles of the dry, heavy water saturated, and water saturated PU samples were measured at different temperatures and shown in Figures 5a, 5b, and 5c. The x-axis (labeled as position) in Figure 5 is corresponding to the vertical direction of the polymer coating illustrated in Figure 3. The left-hand side of the profile is corresponding to the top of the coating, and the right-hand side of the profile is D

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Figure 6. T2 spectra of dry, D2O saturated, and H2O saturated PU coatings measured at different temperatures. In order to achieve equilibrium, the sample temperatures were first heated up to 85 °C and then decreased stepwise to room temperature (25 °C). A 30 min equilibrium time was introduced between two measurements, and each measurement takes approximately 50 min.

3.3. Water−Polymer Interactions. In order to study the interaction between water and polymer network, it is a prerequisite to distinguish these signals of water molecules and different polymer components. However, in the NMR profiles, both the polymer and water contribute to the NMR signals, and to distinguish them in space (frequency domain) is not possible. Continuum T2 fitting provides a possibility to distinguish the NMR signals of different components in the time domain.4,30−32 Consider a sample slice consists of N distinct 1H pools, the time domain NMR signal can be expressed by

matrix is not plasticized by the presence of water molecules, and the interaction between water/polymer matrix could be very weak. In order to understand which effect dominates the signal increase in this case, heavy water was used to saturate the PU coating instead of water. Because the deuterium cannot be probed, only signal of the polymer matrix is measured. It is well-known that in most polymer systems heavy water behaves in a similar way to the water molecules.28,29 Therefore, it is reasonable to assume that the PU coating can be plasticized by D2O in a similar way to the H2O. It is observed in Figure 5d that the H2O saturated sample shows higher intensity compare to the other two samples. This is a clear indication of water uptake. The intensity difference between the H2O and D2O saturated sample gives the information about the amount of water in the coating. At temperatures lower than 65 °C, the D2O saturated sample shows almost the same signal intensity as the dry sample. While the temperature increases beyond 65 °C, however, the signal intensity of D2O saturated sample increases abruptly compare with that of the dry sample. Because the D2O signal is not detected by NMR, the signal increase at higher temperature is attributed to the increased molecular mobility as a result of the plasticization effect of D2O. This result suggests that the polymer matrix is not plasticized by D2O when temperature is lower than 65 °C, whereas is plasticized significantly at higher temperatures. Note that 65 °C is the first-order enthalpy relaxation temperature as discussed in the section 3.1 (Figure 4). Comparing the DSC and NMR results, it can be concluded that (1) the PU coating takes up water below the enthalpy relaxation temperature of 65 °C and (2) the PU coating is plasticized by water molecules only at temperatures above the enthalpy relaxation temperature, whereas at lower temperatures, plasticization is not observed.

N

S(t ) =

∑ Ckρk exp(−t /T2,k) k=1

(1)

where k refers to the number of 1H pools and Ck is a prefactor for the NMR signal of the kth 1H pool. It includes the effects of longitudinal relaxation during the repetition time, RF pulse excitation and coil sensitivity, and evolution of coherent pathways in the used pulse sequences. ρk [mol/m3] is the proton density of the kth 1H pool. T2,k is the spin−spin relaxation time of the kth 1H pool which characterize the mobility of the proton species. The T2 relaxation spectra can be obtained from the continuum fitting of the NMR signal S(t) at each position in the coating.30,33 Figure 6 shows the T2 spectra of dry, D2O saturated, and H2O saturated PU coatings measured at different temperatures. The dry PU coating shows almost the same T2 spectra for all the different temperatures in the measured temperature range (Figure 6a). It worth mentioning that the inter-echo time for the T2 measurements was 50 μs, and the long tails at T2 < 50 μs are the values extrapolated by the fitting program and therefore are not reliable. For the water saturated sample, two peaks can be E

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identified from the T2 spectra of 45 °C and above. The longer T2 peak (right side) is assigned to the absorbed water, and shorter T2 peak (left side) is assigned to the polymer network. Compared to the dry sample, the water saturated PU coating shows significant longer T2 component at temperature of 75 °C and above. While temperature is lower than 70 °C, the water saturated PU coating shows similar T2 spectra compare to the dry sample. It is interesting to notice that the water signal can be clearly observed until the temperature cooled down to 45 °C. This means that the polymer network is plasticized by water only at temperature higher than 75 °C. At temperature range of 70−45 °C, water molecules remain in the polymer network but does not mobilized (or plasticize) the polymer network. At a temperature below 35 °C, water signal cannot be observed in the T2 spectra. This does not necessarily mean that the water is completely expelled out of the coating, but it could also be possible that the water signal is so low that it is below the detection limit of our NMR equipment and embedded in the noise level. To exclude the signal from water and detect the influence of water on the polymer matrix, the PU coating sample was saturated with D2O instead of H2O, and all the other experimental parameters and conditions are the same. The T2 spectra are shown in Figure 6c. Because the signal from D2O is invisible, the T2 spectra only show the signal of the polymer component. Similar to the H2O saturated sample (left peak in Figure 6b), the longer T2 component of the D2O saturated polymer disappears as temperature decreased to 70 °C and lower. 3.4. Temperature Dependency of Water Uptake. In section 3.2, it has been mentioned that the amount of water absorbed by the coating can be monitored by the difference between the intensities of the profiles of H2O and D2O saturated PU coatings. However, this method is based on the assumption that the heavy water plasticizes the polymer matrix in a similar way to the water molecules. T2 fitting provides another possibility to quantify the amount of water in the PU coating. Integral of the peaks in the T2 spectra (Figure 6b) gives the information about the mole percentage of the protons in the corresponding species, and this mole percentage can be easily translated into weight percentage by considering the molar mass (of a repeat unit) and number of protons per molecule (or repeat unit). In order to exclude the hysteresis effect during the water uptake process, temperature was decreased from 85 to 25 °C. Figure 7 shows the plot of

water content in the PU coating against the temperature. The temperature dependence of the water uptake can be roughly divided into three temperature regions. In region I, the water content almost keeps constant in the initial stage (temperature lower than 35 °C) and then increases abruptly up to 2 wt % at temperature of ca. 50 °C. In region II, the water content almost keeps constant. While temperature increases to 65 °C and above (region III), the water content increase again with increasing temperature. Generally, the water uptake behavior of a polymer material is tightly related to its phase structures and the transitions. The phase structure of the linear segmented PU has been studied extensively. Such materials are composed of the microphaseseparated “hard” polyurethane domains and the “soft” polyether or polyester domains. The cross-linked PU, however, is generally treated as a homogeneous polymer network, and the phase inhomogeneity was seldom studied in literatures. The difficulty of probing the inhomogeneity in a cross-linked PU lies in the low contrast between different phases and the small sizes of microdomains as a result of restriction from the covalent cross-links which can largely prevent reorganization of the domains. A possible solution to these problems may be the use of water as a “contrast agent” to distinguish different phases. The change of water content with temperature can be correlated with the phase transitions of the PU. In order to explain the temperature dependence of the water content shown in Figure 7, an inhomogeneous phase model has to be introduced. For this consideration, a tentative two-phase model was proposed and is shown in Figure 8. The PU coating is intrinsically a microphase-separated phase structure due to the incompatibility between the polyester (PNI) and the crosslinkers. PNI molecules (or segments) and the cross-linkers prefer to stay together with their own kind and form the “PNI rich” domains and the “cross-linker rich” domains. It is difficult to quantify the size of the domains by the NMR techniques; however, it can be estimated that the size is smaller than lower limit of the wavelength of the visible light which is about 400 nm because the coating is transparent and no scattering effect was observed. Further XRD study did not show any periodic structure, which means that no crystalline structure was detected. It is observed in Figure 7 that the water content increases in two steps as the temperature increases from room temperature to 85 °C. According to this plot, the water uptake of PU coating is roughly divided into three temperature regions. This temperature dependence of the water uptake can be explained by the phase transition of the two domains (“PNI rich” domain and “cross-linker rich” domain) in the coating as shown in Figure 8. At room temperature, the molecular mobility of both domains is low due to the strong inter/intramolecular hydrogen bonding, and therefore the water uptake of the PU coating is negligible. This explains that in the initial stage of temperature region I, i.e. below 35 °C, the water content is around 0%, which means that it is below the detection limit (0.2 wt %) of our setup, as discussed in the Experimental Section. When the temperature increases to ca. 50 °C, the water uptake increases abruptly to about 2 wt %. The inflection point of this water uptake process is about 40 °C (Figure 7). This is attributed to the microstructural transition (both glass transition and enthalpy relaxation) in the “PNI rich” domain. This transition is clearly observed in DSC thermograms (Figure 4) which shows an endotherm with inflection point of ca. 55 °C. This transition temperature obtained from DSC is apparently higher

Figure 7. Absorbed water content in the PU coating as a function of temperatures as obtained from the integrals of the water peaks in Figure 6b. F

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Figure 8. Schematic illustration of the microphase structure of the PU coating and the water uptake behavior at different temperatures. TIII,II and TII,I refer to the transition temperature between regions III and II and regions II and I in Figure 7.

increased molecular dynamics and the increased water− polymer interactions in the “cross-linker rich” domain, the water uptake increases with temperature in region III. The water uptake process of the PU coating is shown in Figure 7, and the related phase transition behavior is schematically illustrated in Figure 8. The PU coating is intrinsically a microphase-separated system with “PNI rich” phase and “cross-linker rich” domain. At T < 35 °C, “PNI rich” domain is of locally ordered structure. Because of the low molecular mobility and limited free volume, neither domain is accessible to water molecules. As temperature increases further to TII,I, the “PNI rich” domain gets more disordered and consequently becomes accessible to water molecules. This is the reason that the water uptake increases with increasing temperature in temperature region I (Figure 7). As the temperature increases further and goes across TIII,II, molecular mobility of the “cross-linker rich” domain substantially increases, and moreover, polymer network starts to interact with water molecules, which leads to the further water uptake in the temperature region III.

than that obtained from NMR results. This is reasonable considering the fact that the water content in Figure 7 was obtained during the cooling process in order to avoid hysteresis effect during the water uptake process, whereas the DSC thermograms were obtained during heating scan. In order to directly compare the DSC data with the NMR results, the pure (un-cross-linked) PNI sample was immersed in water and sealed in an aluminum pan for measurement. The DSC curve was recorded during the cooling scan with the cooling rate of 10 °C/min, and the result is shown in the Supporting Information (Figure S1). The DSC curve shows an apparent glass transition with an inflection point temperature of about 30 °C. This temperature is slightly lower than the temperature of the first water uptake process shown in Figure 7, which is about 40 °C. This discrepancy is rational considering the fact that the PNI in the PU sample is cross-linked and therefore is of relatively higher glass transition temperature than the pure PNI. Comparison of these NMR and DSC results strongly suggests that the first water uptake process at temperature of 40 °C is related to the enthalpy relaxation process of the “PNI” domain. The enthalpy relaxation is related to the breakdown of the ordered local structures in the “PNI rich” domain, and the glass transition is explained by the increase of the free volume in the disordered “PNI rich” domain. As a consequence of both processes, the “PNI rich” domain becomes more accessible to water molecules, and therefore the water uptake increases with temperature in this region. The water content is essentially constant with temperature in temperature region II. As temperature increases further to 85 °C (region III), the mobility of the “cross-linker rich” phase increases, and therefore it becomes accessible to water molecules. Because of the hydrophilic nature of the urethane group, the water molecules interact strongly with the cross-linker though hydrogen bonding. This interaction between water molecules and polymer in the “cross-linker rich” domain leads to a significantly increased mobility of the cross-linker segments. This is also evidenced by the longer T2 component in the T2 spectra of the H2O and D2O saturated samples (Figure 6b,c), which is not observed for the dry sample (Figure 6a). As a result of the

4. CONCLUSION This paper aims at understanding the water/polymer interactions and the microstructures of a highly cross-linked PU coating. GARfield NMR is used to probe the molecular mobility of the water molecules and the polymer network. Continuum T2 fitting analysis of the NMR data provides a possibility to distinguish different components in the time domain and to probe the molecular mobility of each component. Temperature had a considerable effect on the plasticization effect of water molecules in the PU coating. The coating uptakes water at temperatures below the enthalpy relaxation temperature of 65 °C, whereas the polymer matrix is plasticized by the water molecules only above the enthalpy relaxation temperature. It is also observed that the absorbed water content in the PU coating increases in two steps as the temperature increases from the room temperature to 85 °C, implying an inhomogeneous microstructure in the PU coating. A microG

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structure model has been formulated to explain the experimental results. At temperature lower than 35 °C, neither “PNI rich” domain nor the “cross-linker rich” domain is accessible to water molecules because of the low molecular mobility and limited free volume. As temperature increases, the “PNI rich” domain gets more disordered and consequently becomes accessible to water molecules, and therefore the water uptake increases with increasing temperature in region I. In temperature region II, the water content remains constant with temperature because the molecular dynamics of both domains are essentially unchanged. In temperature region III, molecular mobility of the “cross-linker rich” domain substantially increases, and the polymer network becomes more hydrophilic, which leads to the further water uptake in this temperature region. This inhomogeneous microstructure model may be helpful for the further studies of such materials, e.g., the modeling of the weathering process of the PU coatings and to predict the service lifetime of the coatings.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Z.); [email protected] (H.P.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is funded by DPI. Haijin Zhu acknowledges the financial support from the Australian Research Council (ARC) through the Australian Laureate Fellowship Schemes. Leendert G. J. van der Ven, Hans Dalderop, and Jef Noijen from TU/e are thanked for sample preparation and technical support and assistance.

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REFERENCES

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dx.doi.org/10.1021/ma401256n | Macromolecules XXXX, XXX, XXX−XXX