Water sorption on a thin film of stereocontrolled poly ( N-ethylacrylamide ) and poly ( N , N-diethylacrylamide )

The tacticity effects on the water-vapor sorption of the thin films of poly(N-ethylacrylamide) (PNEAm) and poly(N,N-diethylacrylamide) (PNdEAm) are investigated by the quartz crystal microbalance method and infrared spectroscopy. The quantity of sorbed water on the spin-coated films of isotactic-rich PNEAm is larger than that of atactic one. For PNdEAm, the syndiotactic-rich polymer film absorbs more water. The amide I or C=O stretching band of the polymers are monitored during water sorption on the spin-coated films. The result indicates that the water-sorption site is also influenced by the tacticity of the polymers. The annealing of these films wipes out the tacticity dependence of water sorption, indicating that the water-sorption sites in the spin-coated films are voids around C=O groups that are made by evaporation of solvent during spin-coating (so-called, the “solvent-imprint” effect). The isotactic-rich PNEAm is more influenced by the solvent-imprint effect than the syndiotactic-rich one, while an opposite tendency is found for PNdEAm.


Introduction
Controlling the water sorption of polymer thin films is important for designing soft biomaterials.In fact, when a foreign material such as a synthetic polymer comes into contact with blood, the water sorption is the first event before protein sorption [1].Thermoresponsive polymers have attracted keen interest of scientists in this field [2], because of their potentials for developing intelligent soft biomaterials.Poly(N-substituted acrylamide) is a representative of thermosensitive polymers in aqueous media [3].In general, the thermo-responsiveness of acrylamide polymers is concerned with lower critical solution temperature (LCST) type phase behaviour in the aqueous medea.Poly(N-isopropylacrylamide) (PNiPAm) is the most popular thermoresponsive polymer and has extensively been studied [2][3][4][5][6].The interaction between the polymer chain and water is a fundamental factor to induce the thermo-responsiveness of the polymers.In this context, the effects of salt and co-solvent on the phase behaviour have been investigated.On the other hand, several experimental results have implied that the intramolecular interaction among the side chains plays an important role in its phase behaviour.This is clearly shown when the phase separation behaviour of PNiPAm is compared with poly(N,N-diethylacrylamide) (PNdEAm).The co-solvent effect on the phase behaviour of PNiPAm is different from that of PNdEAm; the addition of methanol to the aqueous solutions causes the rise of the cloud point (T c ) for PNdEAm [7], but the decrease of T c for PNiPAm [8].The tacticity dependence of T c is also opposite for these two polymers: The increase of the meso diad (m) fraction in the polymer chain causes the rise of T c for PNdEAm [9,10], but the decrease of T c for PNiPAm [11,12].These phenomena cannot be explained only by taking account of the interaction between the side chain and water, and thus we have to also consider the contribution of the intramolecular interaction among the side chains [13].A remarkable difference in the chemical structures of PNdEAm is the lack of the N−H group.The side chains of PNiPAm can form an intramolecular C=O⋅⋅⋅H−N hydrogen bond (H-bond), whereas those of PNdEAm cannot.In the previous paper [14], we revealed that the tacticity dependence on the water solubility of the dimer model of PNiPAm is owing to the changes in the intramolecular interaction between the diastereomers.This clearly indicates that the intramolecular C=O⋅⋅⋅H−N H-bond of the amide groups affects the solubility of the polymer in water.In this paper, we have examined how the tacticity of polymers affects the water-vapor sorption of the thin films of poly(N-ethylacrylamide) (PNEAm) and PNdEAm.
The preparation of N-ethylacrylamide can be found in elsewhere [15].N,N-dietylacrylamide was kindly provided by Kojin and purified by distillation under reduced pressure.The polymerizations were carried out in the absence or presence of Yttrium(III) trifluoromethanesulfonate (Y(OTf) 3 Lewis acid) [12,16].Chemical structures of PNEAm and PNdEAm were shown in figure 1.The weight-average molecular weight (M w ) and polydispersity (M w /M n ) were determined by a size exclusion chromatography.The m content was determined by 1 H NMR [12,16,17].The cloud point (T c ) of a 1 wt % aqueous solution was estimated by the transmittance curve [12].
IR spectra were recorded at a resolution of 2 cm − 1 with a co-addition of 512 scans on a Jasco FT/IR-615 spectrometer equipped with a liquid-nitrogen cooled HgCdTe (MCT) detector.CaF 2 plates were used as a window material.Quartz crystals (9 MHz) with Cr/Au electrodes and the electronic circuit oscillators (QSET -9E-H) were purchased from Tama-devise co.ltd.The Sauerbrey equation was used for correlating changes in the oscillation frequency of the QCM sensor with the mass deposited on it [19].Polymers dissolved in acetone (0.2 wt%) were spincoated on CaF 2 and QCM sensors with 2000 rpm, and then the samples were dried in a desiccator overnight before being placed in a vacuum chamber.The chamber was vacuumed by a rotary pump (LS63P, Edwards).The vapor pressure of water was monitored by a strain-gauge manometer (D357-26 strain-gauge with D395-57 monitor, Edwards).The experiment was carried out at room temperature.After the system was vacuumed, the needle valve was opened to introduce the D 2 O vapor.

Results and Discussion
Table 1 compiles the characterization of prepared samples and their notations.For both PNEAm and PNdEAm, the polymerization in the presence of Y(OTf) 3 yields an isotactic-rich polymer, which contains a high meso diad (m) configuration in the main chain.The T c of the aqueous solution of PNdEAm increases with increasing m content as reported previously [10].The tacticity dependence of T c for PNEAm, on the other hand, is similar to that observed for PNiPAm [12], which is opposite to that for PNdEAm.These results indicate that an isotactic-rich PNdEAm is hydrophilic, whereas an isotactic-rich PNEAm is hydrophobic.

Tacticity dependence on the quantity of water sorption for the polymer films
Figure 2 shows the water-vapor sorption of the thin films of PNEAm with different tacticity (E-m47 and E-m78) at each water-vapor pressure.For the spin-coated film as prepared, the water sorption of E-m78 is a little greater than that of E-m47.When the 0.2 µg of D 2 O is sorbed on 1.0 µg of PNEAm, one monomer unit of the polymer interacts with one water molecule on an average.The difference between the quantity of water sorption between E-m78 and E-m47 is about 0.03 µg/µg (0.015 D 2 O molecule per monomer unit) at any vapor pressure.Even though the difference is not quite big, the result seems to be counter intuitive, because T c of the aqueous solutions insists that E-m78 is more hydrophobic.After annealing the films at 140 °C for 15 min, the water sorption on the film was measured.For E-m47, the quantity of sorbed water is almost identical between the films before and after annealing, while that for E-m78 markedly decreases.This may be caused by a solventimprint effect on the spin-coated film of E-m78.In the previous paper [19], we concluded from the IR measurement of stereocontrolled PNiPAm that solvents around the amide groups of the polymer evaporate quickly before the reorientation of the amide groups occurs, in which a portion of the carbonyl groups would be left isolated.If the water-sorption site is concerned with these free carbonyl groups, the result obtained here suggests that the solvent-imprint effect of E-m78 is more prominent.The water sorption of PNdEAm films as a function of water-vapor pressure is represented in figure 3. The fraction of water molecule per one monomer unit is about 0.6 on an average at where 0.1 µg of D 2 O is sorbed on 1.0 µg of PNdEAm.The quantity of the sorbed water on the PNdEAm film is smaller by twice than that on the PNEAm film.This implies that PNdEAm is more hydrophobic than PNEAm.In fact, T c of PNdEAm in water is lower by 30-40 °C than that of PNEAm.It is worth noting that the functional type for the water sorption isotherm of PNdEAm seems to be different from that of PNEAm.For the spin-coated films, the water sorption of D-m55 is slightly greater than that of D-m90.Interestingly, the result is in good harmony with that for the PNEAm films, because D-m55 has a lower T c and absorbs more water than D-m90 in a solid state.The annealing effects on the PNdEAm film also show a similar tendency with those of PNEAm.The quantity of sorbed water of D-m90 is almost identical between the films before and after annealing, while that for D-m55 discernibly decreases.It is worth reminding that for PNEAm the annealing effect of E-m47 on the water sorption is small in contrast to E-  m78.The result supports again the existence of solventimprint effects in a spin-coated film of acrylamide polymers.Moreover, the solvent-imprint effect is always large for a polymer that has a lower T c in aqueous media, i.e. for a polymer that has poor solubility to water.

Water-sorption sites in PNEAm films
IR spectra of the polymer thin films at each water-vapor pressure were measured in order to investigate the watersorption site.Figure 4 shows changes in the amide I envelope of PNEAm depending upon the water-vapor pressure.There are at least three bands in this region at 0 hPa (Dry);1675, 1650, and 1625 cm -1 .These bands are associated with the proton accepting number of the C=O groups (n H ). According to the previous papers [13,20], the bands at 1675, 1650, and 1625 cm -1 are assignable to n H =0, 1, and 2, respectively.It is worth noting that the bands due to n H =1 and 2 contain contributions not only from an intermolecular H-bond between the C=O group and water (C=O⋅⋅⋅H-O) but also from an intramolecular C=O⋅⋅⋅H-N H-bond among the side chains of the polymer.
For both E-m47 and E-m78, the relative intensity of the 1675 cm -1 band decreases with increasing vapor pressure, while that of the 1625 cm -1 band increases (figure 4).This indicates that D 2 O molecules sorbed in the film form a Hbond with C=O groups of the polymer.The relative intensity of the 1625 cm -1 band for E-m78 at 20 hPa is a little stronger than that for E-m47, indicating that the amount of H-bonded C=O groups with n H =2 for E-m78 is larger.This is in good agreement with the QCM results; the quantity of water sorption of E-m78 is larger than that of E-m47.Further, the result supports the hypothesis that the solvent-imprint effect for E-m78 is more emphasized than that for E-m47.It is interesting that the difference in the amide I envelopes between the two polymers at 0 hPa is smaller than that at 20 hPa.This may be attributed to the fact that the relative intensity of a free amide I band is much smaller than that of a Hbonded amide I band, i.e.H-bonding at a C=O group enlarges the relative strength of the amide I mode.
The annealing of the films marginally modifies the amide I envelope at a dry state (0 hPa of water vapor), but causes the reduction of the relative intensity near 1625 cm -1 when the vapor pressure increases (see figure 5).
After annealing the films, the amide I envelopes of E-m47 and E-m78 are almost identical even at 20 hPa.By carefully inspecting the spectra shown in the left panel of figure 5, the band at 1675 cm -1 for E-m78 at 0 hPa before annealing is stronger than that for E-m47.It is therefore considered that the capacity of water-sorption depends upon the amount of the free C=O groups (n H =0) in the film.After annealing, the shoulder near 1675 cm -1 of the amide I band of E-m78 decreases its relative intensity, and the band profile becomes almost identical with that for E-m47.This can explain well the QCM results that the quantity of water sorption of E-m78 decreases after annealing.The amount of free C=O groups of E-m78 may be reduced by forming an intramolecular H-bond during the annealing process, and then the quantity of watersorption sites decreases.It is suggested by the QCM results that the quantities of water sorption for E-m47 and E-m78 films after annealing are almost same at a vapor pressure higher than ~20 hPa (figure 2).Again, the QCM and IR results are well correlated.
As mentioned above, QCM results indicate that at 25 hPa of vapor pressure one monomer unit of PNEA interacts with one water molecule on an average.C=O groups are generally capable to accept two protons at least.Thus, we inferred that the water-sorption of the PNEAm films is caused by forming a direct H-bonding between C=O groups and water.In contrast to PNEAm, the IR spectrum of D-m55 is markedly different from that of D-m90 even at 0 hPa (dry state).The peak of dry D-m55 and D-m90 films is located at 1637 and 1634 cm -1 , respectively.This indicates that the interaction among the C=O groups in the dry D-m90 film is slightly stronger than that in the dry D-m55 [21].
The interaction in a dry PNdEAm film should be attributed to a dipole interaction among the neighbouring amide groups.When the water-vapor pressure becomes high, a shoulder on the low wavenumber side of the main peak increases its relative intensity.According to the previous paper [21], the hydration of the C=O groups of PNdEAm gives rise to the bands at 1616 and 1595 cm -1 , which are associated with the weakly and strongly hydrated C=O groups, respectively.Therefore, the result obtained here indicates that the water molecules directly interacts with the C=O groups in the polymer after being absorbed by the films.Figure 7 shows the curve fitting results of D-m55 and D-m90 at 20 hPa of water vapor.The peaks of the bands due to hydration are fixed at 1616 and 1595 cm -1 , respectively.The curve-fitting results suggest that the relative intensity of the 1595 cm -1 band for D-m55 is stronger than that for D-m90.On the other hand, the 1616 cm -1 band of D-m90 is more emphasized.The hydration of the C=O groups of D-m90 may be more interfered by the interaction among the C=O groups.In the current state, it is difficult to find a clear correlation between the quantity of sorbed water and the watersorption sites of PNdEAm, unlike those we found for PNEAm.

Conclusion
For PNEAm films, the QCM and IR results suggest that the majority of water molecules in the films exist around the C=O groups and form a H-bond with those.The amount of water-sorption for the spin-coated films of PNEAm and PNdEAm are markedly influenced by the tacticity of polymers, which is possibly induced by the solvent-imprint effects.The solvent-imprint effect is always large for the polymer that has poor solubility to water.The annealing treatment makes the tacticity effects small.The results obtained here suggest that the solvation structure and polymer chain geometry of PNEAm and PNdEAm in solution are varied depending on the tacticity.

Fig. 2 .
Fig. 2. Water sorption of PNEAm films.The line was drown as guides for the eyes.

Fig. 3 .
Fig. 3. Water sorption of PNdEAm films.The line was drown as guides for the eyes.

ICOMF1403001-p. 3 3. 3 .
Figure 6 represents the changes in the C=O stretching envelope of PNdEAm depending upon the water-sorption.In contrast to PNEAm, the IR spectrum of D-m55 is markedly different from that of D-m90 even at 0 hPa (dry state).The peak of dry D-m55 and D-m90 films is located at 1637 and 1634 cm -1 , respectively.This indicates that the interaction among the C=O groups in the dry D-m90 film is slightly stronger than that in the dry D-m55[21].The interaction in a dry PNdEAm film should be attributed to a dipole interaction among the neighbouring amide groups.When the water-vapor pressure becomes high, a shoulder on the low wavenumber side of the main peak increases its relative intensity.According to the previous paper[21], the hydration of the C=O groups of PNdEAm gives rise to the bands at 1616 and 1595 cm -1 , which are associated with the weakly and strongly hydrated C=O groups, respectively.Therefore, the result obtained here indicates that the water molecules directly interacts with the C=O groups in the polymer after being absorbed by the films.Figure7shows the curve fitting results of D-m55 and D-m90 at 20 hPa of water vapor.The peaks of the bands due to hydration are fixed at 1616 and 1595 cm -1 , respectively.The curve-fitting results suggest that the relative intensity of the 1595 cm -1 band for D-m55 is stronger than that for D-m90.On the other hand, the 1616 cm -1 band of D-m90 is more emphasized.The hydration of the C=O groups of D-m90 may be more interfered by the interaction among the C=O groups.In the current state, it is difficult to find a clear correlation between the quantity of sorbed water and the watersorption sites of PNdEAm, unlike those we found for PNEAm.

Fig. 6 .
Fig. 6.Dependence of the C=O stretching envelope of D-m55 and D-m90 spin-coated films on the water-vapor pressure.

Table 1 .
Characterization of polymer samples