3.1. Synthesis of Block-Copolymers
The commercially available (α,ω) hydroxyl terminated silicone oligomers free of ethoxylated end groups were chosen for this study. PDMS and more pronounced polytrifluoropropylmetylsiloxane (PTFPS) have poor mechanical properties, making their reinforcement essential. LAD III having remaining reactive silanol end and inner groups with their ladder-like structure seem the best additive for the improvement of mechanical properties of siloxanes [
12]. The molecular structure of POSS largely depends on the synthetic methods and processing, which can be classified into randomly branched, ladder-like, cage and partial cage structures. These structural variations govern the properties of the resulting block-copolymers. The pathway of synthesis of block-copolymers IV and V is presented on
Scheme 1. Polyfunctionality of LAD III leads to the formation of a complex branched structure of block-copolymers IV and V.
Scheme 1 presents only some examples from a numbers of possible variants. However, the selected molar ratio between LAD III and PDMS/PTFPS oligomers, which is much lower than equivalent prevents the formation of crosslinked structures and leads to formation of soluble products. The use at synthesis of the same weight of PDMS and PTFPS, despite the large difference in the mass of monomer units, has been determined the difference in molar mass of the prepared block-polymers because bigger difference in the number of hydroxyl groups in LAD III and PTFPS oligomer. The molar mass of the prepared block-polymers was determined with liquid chromatography (for IV M
w = 9.0 × 10
4, M
w/M
n = 1.97; for V M
w = 4.2 × 10
4, M
w/M
n = 2.32). The obtained values support the formation of multiblock-copolymers composed from each of three or four initially used oligomeric units. The formation of multiblock-copolymers, but not a mixture of starting components is supported by a relatively narrow product peak in chromatograms. The formation of a mixture should lead to the observation of a broad signal with the presence of initially low molecular components. The narrow molecular mass distributions of block-copolymers IV and V also speak to the higher reactivity of silanol groups under the chosen conditions.
The structure of multiblock-copolymers was well confirmed by IR spectroscopy. As one can see in
Figure 1, the spectra of block-copolymers IV and V appear as superpositions of IR spectra of starting blocks.
Only in the spectrum of oligomeric III can one observe the absorption of silanol OH-groups at 3500–3200 cm
−1 (free and H-bonded groups) [
13]. The IR spectra contain the vibrations of C–H, C–C, Si–O, Si–C, and C–F bonds in accordance with common spectroscopic analysis [
13].
The solution
1H NMR spectra of block copolymers IV and V (
Figure 2) confirm the structure of copolymers.
Therefore, the presence of OH groups signals relates to the defected ladder-like structure of the ladder block (the presence of silanol units). In block-copolymers, this signal is located around 1.6 ppm. Therefore, this signal is common with water presented in deuterochloroform (1.56 ppm). To support the presence of silanol OH groups in block-copolymers, titration was performed using of Grignard reagents. The presence of silanol units was confirmed in both block-copolymers. The fraction of OH groups in block-copolymer L-PMFS V is about four-fold higher than in L-PDMS IV, in accordance with the mole ration of LAD and PDMS/PMFS at synthesis.
A similar situation is observed in the 1H NMR spectrum of LAD III. The downfield shift of the OH-groups signal is connected with the shielding effect.
The presence of defects in this block was confirmed by solution
13C NMR spectra (
Figure 3).
The redundant and broad signals of ipso carbon of phenyl groups indicates the presence of defects in the ladder structure. This can be confirmed by
29Si NMR spectroscopy. The solid state NMR spectra are given in
Figure 4.
As one can see, these spectra contain not only signals from T
3 units (centered at −82 ppm) that correspond to a perfect ladder structure, but also minor signal from −69 to −72 ppm related to T
2 units of cage-like structure or silanol fragments [
14]. Hence, we certify the structure of block-copolymers IV and V (the chemical shift of fluorine in trifluoromethyl group of V is −65.5 ppm, see
Supplementary Materials). The most important result of this study is the fact that the ladder-like block of copolymers contains defects with silanol groups that can result in a decrease of the contact angle, deterioration of mechanical properties, and to an increase of adhesion.
Another important result obtained in the NMR study entails data concerning molecular dynamics. As one can see from solution
1H NMR spectra of block copolymers IV and V (
Figure 2), the signals of aromatic protons are very broad. This is connected to the restricted molecular mobility in ladder-like fragments. This is consistent with the solution
13C NMR spectra, where broad signals of aromatic units coexist with sharp signals of quartet of CF
3 group, which can freely rotate without restriction (
Supplementary Materials). The distribution of rigid and flexible fragments of block-copolymers can be obtained from solid state
1H NMR spectra (
Figure 5).
As one can see, the spectra of block-copolymers IV and V contain both rigid (broad) and mobile (sharp) parts. The mobile part contains the protons with different resonances which are related not only to the different fragments, but to different mobile phases as well. The exact assignment of these resonances is very complex. The
13C CP/MAS spectra of polymers III–V (
Figure 6) indicating fragments with restricted mobility contain the signals from both blocks used at synthesis. Hence, the formal rigid regions contain both ladder and linear units.
The solid state
1H NMR spectrum of oligomer III contains mostly rigid components (
Figure 7). However, the presence of mobile components allows us to calculate the fraction of defects (silanols) in these oligomers, which is less than 10%.
Hence, we synthesized multiblock-copolymers with a complex rigid–soft structure with defect units containing silanol groups.
3.3. The Surface Properties of the Films of Copolymers IV-V
For the investigation of water contact angle of copolymers IV and V, their solutions in THF were casted on metal plate (steel, copper, aluminum) and annealed at 70 °C. Because the annealing temperature was much higher than the glass transition temperatures of copolymers, the casting solvent did not play any role in the surface formation of casting films. For copolymer IV, the measured water contact angle of film did not depend on water and is 107.5°. For copolymer V the following dependence is observed: water contact angle is 110.5° on aluminum, 111.3° on steel, and 115.7° on copper. However, SEM images of surfaces did not allow to find any differences (
Figure 10a,b).
We measured an adhesion to these casting films. No obvious connection with the values of water contact angle of films was observed. The adhesion of copolymer IV to copper is 0.71 MPa, 0.96 MPa to aluminum, and 1.70 MPa to steel (with deviation between five samples less than 0.05 MPa). A similar range was obtained for copolymer V: 0.41, 0.46, and 0.60 MPa, correspondingly. Hence, obviously, low surface energy leads to poor adhesion. Adhesion is also connected with the mechanical properties of block-copolymers. Copolymers IV have good mechanical performance as compared to polysiloxanes [
16]. Their tensile strength is 1.3 ± 0.1 MPa at elongation 370 ± 20%. Clearly, this result is connected with the presence of ladder units, which play the role of nanofibers. The mechanical performance of copolymer V is far worse. It is impossible to perform a test using dumb-bell shaped specimens on a breaking machine. Accordingly, we opted for mechanical puncturing using a texture analyzer. The tensile strength of copolymer V is 78 ± 5 kPa at elongation 150 ± 10%.
3.4. Solubility and Morphology of Block-Copolymers
After synthesis, copolymer IV is a white-yellowish powder and copolymer V is white rubber. As one can see from
Table 1, the copolymer IV is well soluble in most polar solvents. The behavior of copolymer V is more complex. It can form emulsions in many solvents (
Figure 11).
There is a possibility to manipulate the morphology of casted films using different solvents and solution states.
The annealed films of both copolymers form the smooth films on the surface of support for recording SEM images distinctly replicating the dashes (
Figure 10a,b). The difference in appearance of these two films casted from toluene is caused by the quality of this solvent for each copolymer. The toluene is a very good solvent for copolymer IV and, as a result, the film casted from it does not show any signs of phase separation or structuring. The opposite situation is observed for copolymer V. The poor solvent allows the process of phase separation of different blocks and their aggregation to occur (
Figure 10b).
The formation of block copolymer nanostructures is an intriguing phenomenon. However, these processes have been impaired by annealing at the drying stage of casting films. One of the most frequently employed strategies to overcome it is crosslinking at casting from solution [
16]. We used such a popular silicone crosslinker as VOS. It was added to polymer solution in the amount of 1 wt.% to polymer just before casting.
We start this pathway from copolymer IV. The good solubility of this polymer in many solvents limits the possibility for the setup of self-organization. However, we prepared the films from solution in chloroform and recorded their images from the surface and the chip (
Figure 10c,d). As one can see, the surface is smooth, but the bulk structure is complex and consists of grains and layers. If the casted copolymer V film was crosslinked from solution in good solvent with respect to one block (butyl acetate), a similar picture (
Figure 10e,f) was observed. Thereby, it was shown that the use of selectivity of solvent to any one block of copolymers opens a possibility to manipulate the self-organization of block copolymers. In the case when a solvent is good for both blocks, the situation is much simpler. When the film was crosslinked from such a solvent as hexafluorobenzene, the self-organization was not fixed (
Figure 10g,h), and both the surface and chip of films are smooth.
The formation of emulsion (or micellar solution, any kind of latex) presents a new possibility for the manipulation of self-organization. First of all, the crosslinking allows to fix a globular structure which is formed in solution (
Figure 10i). Interestingly, at slow drying, such a structure can be transformed in a regular long-strip-like surface (
Figure 10j,k). Similar structures have been formed with the preparation of films from latexes of polysiloxane/poly (fluorinated acrylate) block-copolymers [
17]. The authors connected the formation of such structures to the core/shell segregation of block copolymers in solution. Hence, in our case, the observed film structures also confirm the segregation of blocks in solution. Thereby, the possibility for the fabrication of well-ordered block copolymer thin films by the variation of solvents used at film casting was demonstrated.