3.1. Synthesis and Physico-Chemical Characterization of MBC Based on PPEG and D4
The progress of the copolymerization of PPEG with D4 initiated by the terminal –O–K− groups was investigated primarily by the homogenization of the reaction system. In the case of homopolymerization of D4, the resulting polydimethylsiloxane (PDMS) and PPEG are not mixed due to their thermodynamic incompatibility. As a result, the phase separation is clearly traced. During the course of copolymerization, the viscosity of the reaction system increases and a homogeneous opaque mass is formed. Before the studies, the copolymers were preheated under vacuum pressure until they reached a constant mass. D4 conversion was calculated gravimetrically, and in all cases, it exceeded 98 wt.%.
The possibility of copolymerization PPEG with D4 initiated by terminal potassium-alcoholate groups was also investigated by FTIR spectroscopy.
For comparison, the FTIR spectra of D4
and polydimethylsiloxane are shown in Figure 2
. An analytical band in the 787 cm−1
region, corresponding to the stretching vibrations of the Si–C bond and bands in the regions of 1011 and 1073 cm−1
due to the stretching vibrations of the Si–O–Si bond, is present in the FTIR spectrum of PDMS.
The D4 FTIR spectrum contains an analytical band at 802 cm–1, corresponding to the stretching vibrations of the Si–C bond and the band at 1057 cm−1, due to the stretching vibrations of the Si–O–Si bond.
The FTIR spectra of the products of the interaction of PPEG and D4
) at a low D4
]:[PPEG] = 1 and 2) show the band at 810 cm−1
, corresponding to the stretching vibrations of the Si–C bond as part of D4
. With an increase in the mole fraction of D4
]:[PPEG] = 2, the band at 791 cm−1
is observed in the spectra, which corresponds to the stretching vibrations of the Si–C bond in PDMS.
The formation of polydimethylsiloxane chains as a result of the PPEG-initiated polyaddition of D4 is confirmed by FTIR spectroscopy—the appearance of the band at 791 cm−1 and its intensity increasing with increasing mole fraction of D4 in the reaction system. In addition, the bands at 1011 and 1076 cm−1 appear due to the stretching vibrations of the Si–O–Si bond in PDMS for the D4 polyaddition products to PPEG.
In the range of 1003–1096 cm−1, unusual changes are observed, which are manifested in the FTIR spectra of PPEG and D4 copolymers obtained with a relatively high molar excess of D4 disappearing at 1096 cm−1, which characterizes the C–O–C bonds in the composition of the used PPEG. With a relatively low molar excess of D4, on the contrary, only bands characterizing the C–O–C bonds in the composition of the used PPEG are found in the FTIR spectra.
To explain the observed phenomenon, it should be noted that the measurements were carried out in the mode of incomplete reflection. That is, infrared rays affected only the surface of the sample. Considering the amphiphilicity of the PPEG- and D4-based multiblock copolymers, it can be assumed that, due to the incompatibility of the hydrophilic polyether and hydrophobic polydimethylsiloxane components in the melt of the formed MBC, microphase separation processes occur. As a result, with an excess of D4, polydimethylsiloxane chains are predominantly located on the surface of the supramolecular formation (possibly a micellar structure). When D4 is deficient, on the contrary, the micelle surface consists of polyether chains. Thus, the conducted FTIR spectroscopic studies confirm the formation of MBC during the interaction of PPEG with D4.
The use of amphiphilic ASiP particles affects the manifestation of the FTIR spectra of the products of the interaction of PPEG with D4
). Since the band, due to stretching vibrations of the Si–C bond in the region of 795 cm−1
, does not change its intensity, in this case, there is no reason to assert that ASiP contributes to an increase in the D4
conversion. For all ASiP amounts used, the analytical band at 1096 cm−1
, due to stretching vibrations of the C–O–C bond of the polyether component, does not appear in the obtained BC, but the intensities and intensity ratios of the bands at 1018 and 1077 cm−1
, corresponding to stretching vibrations of the Si–O–Si bond, change. With an increase in the ASiP content, a shift in the band at 783 cm−1
, corresponding to stretching vibrations of the Si–C bond in PDMS, to 798 cm−1
, is also observed.
Such changes are difficult to relate to the degree of conversion of D4. The most probable reason for such an uneven change in the position and intensities of absorption bands in the FTIR region of the spectrum can be attributed to the appearance of intermolecular interactions, of which ASiP are a participant, and their significant effect on the processes of supramolecular organization of the synthesized multi-block copolymers.
An analysis of the FTIR spectra allows us to conclude that, during the interaction of the PPEG and D4, the D4 polyaddition initiated by potassium–alcoholate groups occurs. The MBC formed in this case is the basis for further processes of microphase separation and the formation of supramolecular formations. The use of amphiphilic ASiP particles in the synthesis of MBC does not contribute to an increase in D4 conversion, but it affects the processes of the supramolecular organization of the obtained block copolymers.
3.2. Surface Active Properties of MBC
As is known, amphiphilic block copolymers consist of regions of different chemical nature and, due to this, exhibit micelle formation ability. To confirm that the formation of MBC occurs during the interaction of PPEG with D4 and the influence of the molar excess of D4 on the length of the polydimethylsiloxane component in MBC is established, their surface-active properties in the aqueous medium were studied.
In the case of the formation of MBC, hydrophobic (PDMS components) and hydrophilic (PPEG components) blocks undergo microphase separation. During micelle formation in an aqueous medium, hydrophobic blocks are associated with the formation of a core region, while the position of the hydrophilic segments will be between the core and the external aqueous medium. As a result, the hydrophobic core is stabilized by a hydrophilic shell, which serves as the interface between the bulk aqueous phase and the hydrophobic domain.
According to Figure 5
, noticeable changes in the concentration dependences of surface tension (σ) for MBC are observed with a twofold molar excess of D4
relative to PPEG. The results of measurements of the values of σ allow us to state that during the interaction of PPEG with D4
, the D4
polyaddition initiated by potassium–alcoholate groups occurs with the subsequent formation of MBC. An increase in the comparative content of D4
leads to an increase in the size of the polydimethylsiloxane block in the composition of MBC.
The results of the surface tension measurements in the aqueous medium are consistent with the changes in particle size, measured in toluene (Figure 6
). Thus, a decrease in particle size with an increase in the [D4
]:[PPEG] ratio is a consequence of MBC micelle formation, the amphiphilicity of which increases with an increase in the comparative content of polydimethylsiloxane block in in their composition.
Particle size distributions for MBC based on [D4
]:[PPEG] = 15 with different ASiP content were also measured (Figure 7
). Changing the ASiP content has a significant effect on the particle size distribution. The observed changes in the course of the particle size distribution with a change in the amount of ASiP used to modify the polymer-forming system confirm the findings of FTIR spectroscopic studies on the effect of ASiP on the macromolecular structure of MBC.
Changing the ASiP content has a significant effect on the concentration dependencies of surface tension for MBC obtained based on [PPEG]:[D4
] = 1:15 (Figure 8
). According to Figure 7
and Figure 8
, with an increase in the ASiP content, the values of surface tension and critical micelle concentration noticeably increase, while the particle size decreases. This is a consequence of the fact that ASiP does not lead to an increase in the molecular weight of the PDMS block in MBC, but is the reason for its structuring. Structuring can occur due to the transetherification reaction of terminal silanol groups and ASiP (Scheme 2
3.3. Polymers Characterization
As a next step, the polymers based on [PPEG]:[D4]:[TDI] with the different content of D4
were prepared. According to Figure 9
, during the interaction of MBC with TDI, isocyanate groups are completely involved in the process. This is evidenced by the absence of bands in the area of 2275 cm−1
. The formation of polyisocyanurates is evidenced by the bands in the region of 1700 and 1410 cm−1
, due to stretching vibrations of the C=O bond present in their structure. The formation of a small number of urethane groups can be judged by the presence of a shoulder of low intensity in the region of 1730 cm−1
, due to stretching vibrations of the corresponding C=O bonds.
The results obtained allow us to describe the synthesis of amphiphilic poly(dimethylsiloxane-ethylene-propylene oxide)-polyisocyanurate cross-linked block copolymers. The formation of multi-block copolymer proceeds by the opening of octamethylcyclotetrasiloxane initiated by terminal potassium-alcoholate groups according to Scheme 3
The migration of potassium ions in the zone of exposure to isocyanate groups of TDI creates active centers that cause the formation of isocyanurates and the formation of terminal silanol groups (Scheme 4
As a result of a sequence of chemical reactions, isocyanurate cycles, the initiated formation of which occurs at the active centers of MBC, are combined into a single polyisocyanurate network, creating a core along the periphery of which a shell consisting of MBC is “laid”.
3.4. Dielectric Loss Tangents
Investigations of the temperature dependences of dielectric losses make it possible to establish the temperature of the onset of segmental mobility (α
- and β
-transitions) for segments merging into their own microphase, which are components of MBC (Figure 10
In this research the reaction conditions are created where TDI enters into the PPEG-initiated TDI reaction, accompanied by the formation of rigid polyisocyanurate structures that formed the “core”. A flexible multiblock copolymer component is assembled around the “core”, creating a “shell” in the macromolecular architecture.
The polymers obtained with a small molar excess of D4
were investigated. According to Figure 10
, for the control sample synthesized based on [PPEG]:[TDI] = 1:12 without using D4
, one region of the α-transition is observed.
The use of even a small amount of D4 caused a significant change in the supramolecular organization of polymers, which was reflected in the manifestation of the temperature dependences of the tgδ of the polymers obtained with [PPEG]:[D4]:[TDI] = 1:2:12. An important consequence of the measurements is also the fact that the use of ASiP in the synthesis of the corresponding MBCs affects the microphase separation of the considered polymers.
When the excess D4
is increased to the molar ratio [PPEG]:[D4
]:[TDI] = 1:15:8, two temperature regions of α
-transitions arise, indicating the existence of microphase separation involving polyoxyethylene (POE) and polyoxypropylene (POP) segments (Figure 11
and Figure 12
). For the same ratio, a β-transition region is observed in the temperature range from −150 to −80 °C, due to the release of the polydimethylsiloxane block into its own microphase.
With an increase in the excess of TDI to the molar ratio [PPEG]:[D4
]:[TDI] = 1:15:15, the temperature regions of the α
-transitions are averaged, and the region of the β
-transition is also absent in this case (Figure 11
). The change in the curve of the temperature dependence of the tgδ
for the polymer obtained on the basis of [PPEG]:[D4
]:[TDI] = 1:15:15 can be explained by the fact that an increase in the content of TDI leads not only to an increase in the size of the “core” in the “core-shell” structure, but also an increase in the number of nodes of the spatial polymer network due to the high content of polyisocyanurates in the polymer. As a result, the implementation of a supramolecular structure built on the basis of the “core-shell” type becomes impossible.
The use of the ASiP modifier affects the processes of supramolecular organization of polymers obtained by [PPEG]:[D4
]:[TDI] = 1:15:8. According to Figure 12
, as the severity of the β
-transition increases, the nature of the manifestation of α
-transitions changes. The results obtained allow us to describe the principle of building the supramolecular architecture of the studied polymers as follows.
The molar excess of TDI used with respect to PPEG at a molar ratio of [PPEG]:[D4]:[TDI] = 1:15:8 is sufficient for crosslinking sites to be formed in the polymer matrix. Moreover, the ratio of the sizes of the polyisocyanurate “core” and the multi-block copolymer “shell” in the “core-shell” structure allows microphase separation of POE and POP segments and polydimethylsiloxane segments.
Due to the fact that the resulting polydimethylsiloxane component does not directly bind to the polyisocyanurate rigid “core”, but is located on the periphery of the supramolecular structure constructed as a “core-shell”, it extends the PPEG macrochain to an ever-greater distance from the “core”. As a result, the first layer of the “shell” consists of POE segments directly connected to the rigid polyisocyanurate core. The next is the layer of associated POP segments. The terminal POE segments, at the ends of which the initiated opening of the D4
cycles occurred, are pulled out from the common flexible chain PPEG due to its thermodynamic incompatibility with the polydimethylsiloxane component of the MBC chain (Scheme 5
3.6. Gas Transport Properties of Obtained Polymer Films
The resulting polymer films were investigated as gas transport membrane materials. The obtained mass transfer parameters, namely permeability, selectivity, diffusion and sorption coefficients, for the studied polymers, are given in Table 2
, Table 3
, Table 4
and Table 5
, respectively. As the kinetic diameters of considered gases increase as following CO2
, the diffusion coefficient values are arranged vice versa. With regard to the sorption coefficients of studied polymers, in the case of CO2
, these values are at least twice higher than in case of CH4
and almost three times higher than for N2
. The polymers have an affinity to CO2
, which allows for a higher sorption coefficient.
According to the work [45
], the reaction conditions have a significant effect on the mechanism of TDI polyaddition initiated by the terminal potassium alcoholate groups of PPEG. Therefore, when carrying out the reaction at relatively low temperatures and in the presence of acidic cocatalysts, isocyanate groups of the para-position in the TDI are opened via the carbonyl component, followed by the formation of coplanar acetal-type polyisocyanate blocks. As a result, the supramolecular structure of such polymers acquires a cellular character. In connection with the results obtained earlier in the work [45
], Table 2
shows the values of the permeability coefficient for a number of gases using a polymer with a cellular supramolecular structure obtained with [PPEG]:[TDI] = 1:15 in the presence of acidic cocatalysts as a membrane. Table 3
shows the corresponding ideal selectivity values.
Under the synthesis conditions used in this work, the opening of isocyanate groups proceeds by the usual mechanism, that is, via the N=C bond, and is accompanied by the formation of polyisocyanurates. The result of such a reaction, as shown above, is the formation of the “core-shell” type supramolecular structure. On the surface of the shell is a polydimethylsiloxane component. The use of the ASiP modifier in the synthesis leads to an additional lengthening of the polydimethylsiloxane component.
According to the analysis of the results shown in Table 2
and Table 3
, polymers with a cellular supramolecular structure exhibit lower values of carbon dioxide permeability in comparison with polymeric film materials whose supramolecular structure is constructed on the basis of the “core-shell” principle. An additional attachment of a polydimethylsiloxane component to the surface of the shell and an increase in the size of the shell lead to a sequential increase in the coefficient of permeability of carbon dioxide. In order to put results into the context of the state-of-art membrane technology, the gas transfer properties of the obtained membranes were compared to Robeson’s upper bound [53
] (Figure 16
and Figure 17