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Article

Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties

by
Yulia S. Dyuzhikova
1,
Anton A. Anisimov
1,*,
Alexander S. Peregudov
1,
Mikhail I. Buzin
1,
Galina G. Nikiforova
1,
Viktor G. Vasil’ev
1,
Sergey A. Kostrov
1,2,
Alexander I. Buzin
3,
Alexei A. Stupnikov
4,
Yulia N. Malakhova
4,5,
Olga I. Shchegolikhina
1 and
Aziz M. Muzafarov
1,3
1
Institute of Organoelement Compounds of the Russian, Academy of Sciences, Vavilov St. 28, 119991 Moscow, Russia
2
Department of Physics, Moscow State University, Leninskie Gory 1-2, 119991 Moscow, Russia
3
Institute of Synthetic Polymeric Materials of Russian, Academy of Sciences, Profsoyuznaya St. 70, 117393 Moscow, Russia
4
National Research Center “Kurchatov Institute”, Akademika Kurchatova Square. 1, 123182 Moscow, Russia
5
M. V. Lomonosov Institute of Fine Chemical Technologies, MIREA–Russian Technological University, Prospekt Vernadskogo 86, 119571 Moscow, Russia
*
Author to whom correspondence should be addressed.
Polymers 2022, 14(2), 285; https://doi.org/10.3390/polym14020285
Submission received: 7 December 2021 / Revised: 22 December 2021 / Accepted: 31 December 2021 / Published: 12 January 2022
Browse Figures
Versions Notes

Abstract

:
New non-crystallizable low-dispersity star-shaped polydimethylsiloxanes (PDMS) containing stereoregular cis-tetra(organo)(dimethylsiloxy)cyclotetrasiloxanes containing methyl-, tolyl- and phenyl-substituents at silicon atoms and the mixture of four stereoisomers of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane as the cores were synthesized. Their thermal and viscous properties were studied. All synthesized compounds were characterized by a complex of physicochemical analysis methods: nuclear magnetic resonance (NMR), FT-IR spectroscopy, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), viscometry in solution, rheometry, and Langmuir trough study.

Graphical Abstract

1. Introduction

Creation of polymers with new macromolecular architecture is one of the principal driving forces in the development of polymer science. The structure–properties relationship, which is the cornerstone of polymeric chemistry guarantees that polymer of unusual architecture would possess an unusual complex of properties [1,2]. Among a vast variety of macromolecular structures, a big class of branched high-molecular compounds is distinguished. Bright representatives of this class are the star-shaped polymers (SSP). These are branched macromolecules in which the arms (linear polymers) ‘grow’ from one branching center (core). So, an atom, a molecule or a macromolecule can act as the branching center. At the same time, it is supposed that the length of arms is identical [3]. An important parameter for such polymers is the number of arms, their functionality and molecular weight. The main feature of SSPs distinguishing them from linear analogs of identical molecular masses is their compact structure (smaller hydrodynamic volume, and, therefore, less viscosity) and possible wider functionality [4,5,6].
At present, there are various approaches to SSP synthesis. They can be divided into three main types: “core-first”, “arm-first”, and “grafting-onto” [6]. Each of these approaches has a certain number of advantages and disadvantages.
Our interest in synthesis and research of siloxane SSP properties is determined by their valuable properties, such as stability at high and low temperatures, water repellency, biocompatibility, high gas permeability, UF radio-resistance, and a number of other unique characteristics [7,8,9,10]. So far, the majority of the available publications on SSP synthesis describe polymers where only one component has siloxane nature—it is either arms [11,12,13,14,15,16,17,18,19,20], or a core [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. The synthesis and study of properties of totally siloxane star-shaped systems [38,39,40,41,42] are described in only some of publications.
We chose the grafting-onto method for the formation of star-shaped siloxane polymers. This approach allows high-molecular compounds with the highest level of structural control to be obtained, since SSP core and arms can be synthesized and characterized separately, even before obtaining a target product. By using this method, we synthesized a series of low dispersity polydimethylsiloxane SSPs. Their molecules contain flexible stereoregular phenylcyclosilsesquioxanes [PhSi(O)OSiMe2H]n of various sizes, structures, and functionality (n = 4, 5, 6, 8, 12), such as the branching center (core), as well as the cage octahedral silsesquioxane playing the role of rigid point center [43,44]. It should be noted that only two of six obtained polymers had three-dimensional spatial structures, traditional for SSP, in which the arms spread in all directions from the branching center. These polymers contain tris-cis-tris-trans-dodeca[phenyl(dimethylsiloxy)]cyclododecasiloxane and octakis[(dimethylsiloxy)]octasilsesquioxane as the branching center. Other four polymers with the molecules containing cis-configuration cycles as the core had absolutely different spatial geometry, where all polydimethylsiloxane arms were arranged on one side of the branching center plane. Similar star-shaped structures were not known before. In our previous publication, we assessed the influence of size, branching cyclosiloxane center and arms number on the properties of SSPs on their basis. In this paper, we were interested in the influence of the frame in cyclosiloxane core, and also the stereoregularity of arms arrangement relative to the cycle plane. For objective comparison, we obtained the mixture of four stereoisomers of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane, and stereoregular cyclotetrasilsesquioxanes, containing methyl-, tolyl-, and phenyl-substituents at silicon atom. The synthesized cyclotetrasilsesquioxanes were used as cores for SSP synthesis.

2. Experimental Part

2.1. Materials

Solvents were prepared according to earlier described technique [45].
n-BuLi (1.6 M solution in hexane), vinyldimethylchlorosilane—commercial products (Acros). Hexamethylcyclotrisiloxane (D3); Karsted’s catalyst (solution of platinum complex (0) with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane in a xylene, Pt~2%mass.)—commercial products (Sigma-Aldrich, St. Louis, MI, USA); sulfocationite (Amberlyst 15)—a commercial product (abcr, Karlsruhe, Germany).
Functional stereoregular organocyclosilsesquioxanes were synthesized according to earlier described technique [46,47,48,49,50].

2.2. Methods

NMR spectra were registered on Bruker Avance™ 600 spectrometer (Bruker, Berlin, Germany) operating at 600.22, 150.93 and 119.26 MHz for 1H, 13C, and 29Si cores, respectively, and Bruker Avance II 300 (Bruker). 1H and 13C chemical shifts were measured relative to residual signals of corresponding solvents and calculated to tetramethylsilane. 29Si chemical shifts were measured relative to external standard - tetramethylsilane.
IR-spectra were registered with the use of FT-IR spectrometer Bruker Tensor 37 (Bruker, Berlin, Germany). Samples were prepared by pressing KBr pellets.
High-resolution mass spectra (HRMS) were measured using Bruker micrOTOF II instrument with electrospray ionization (ESI) (Bruker, Berlin, Germany).
The analysis by gel permeation chromatography (GPC) method was carried out on chromatographs “Shimadzu” (Kyoto, Japan), detector—RID refractometer-20 A, column—PSS SDV analytical 100 000 A (size (300 × 8 mm)); eluent—toluene; “Shimadzu” (Kyoto, Japan), detectors—RID refractometer-20 A and photodiode detector SPD-M20A, column—Phenogel 500 A (the size (300 × 7,8 mm)); eluent—tetrahydrofuran.
The study by DSC method was conducted on DSC-822e device (Mettler-Toledo, Greifensee, Switzerland) at 10 °C/min heating and cooling rates.
The study by TGA method was conducted on Derivatograph-C device, (MOM, Mateszalka, Hungary) in air and in argon at 10 °C/min heating rate.
Rheological studies were conducted on Anton Paar MCR 302 rheometer (Graz, Austria), in the mode of constant shear rate, plane–plane measuring mode, plane diameter 25 mm.
The reduced viscosity of diluted solutions of obtained polymers was measured with an Ubbelohde suspended level capillary viscometer in the concentrations range of 0.25–1 dl/g at 25 ± 0.05 °C.
Formation and study of Langmuir layer properties was carried out on Minitrough Extended (KSV, Espoo, Finland) with maximum area of the interphase surface equal to 558 cm2. Compression and expansion speed was 15 cm2 min−1. As a subphase, purified and demineralized water with a specific resistance of 18.2 MOhm cm (at 25 °C) thermostatically maintained at 20 °C with the use of Milli-Q (Millipore, Burlington, MA, USA) integrated water purification system was utilized. The studied star-shaped copolymers were dissolved in chloroform. Surface pressure was measured by Wilhelmy’s method with the use of a rough platinum plate with 0.1 mN m−1 accuracy. Surface potential was measured by method of vibrating electrode (KSV, Espoo, Finland) with 1 mV accuracy. The Langmuir layers morphology directly on water surface was visualized by Brewster angle microscope BAM-300 (KSV, Espoo, Finland). The images obtained, corresponding to 200 × 200 μm2 interface surface, were geometrically corrected taking into account Brewster angle of water (53.1°). Confidence intervals for the values obtained from surface pressure and surface potential isotherms are 0.1 mN m−1 for surface pressure, 30 Å2 for the area per a molecule, 0.3 Å2 for the area per unit of a dimethylsiloxane and 5 mV for surface potential.

2.2.1. Reaction of Cis-Tetra[phenyl(dimethylsiloxy)cyclotetrasiloxane Isomerization

Briefly, 0.3 g (0.4 mmol) of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane and 0.015 g (5 masses. %) of sulfocationic resin were loaded into a two-neck flask. The reaction in the ultrasonic bath continued for 4 h at 70 °C. To remove the sulfocationite, the reaction mass was dissolved in hexane and filtered through the paper filter. The yield: 0.27 g (90%) after solvent removing
1H NMR (600 MHz, CDCl3, ppm): δ 7.87–7.19 (m, Ph), 4.98–4.61 (m, SiH), 0.37–0.02 (m, Si(CH3)2), 7.21–7.23.
13C NMR (150 MHz, CDCl3, ppm): δ 0.15, 0.23, 0.35, 0.44, 0.54, 0.63, 127.56, 127.60, 127.63, 127.67, 127.70, 127.74, 130.0, 130.05, 130.10, 130.15, 130.22, 130.26, 132.47, 132.51, 132.68, 132.71, 132.90, 132.98, 134.0, 134.06, 134.09, 134.15, 134.18, 134.22.
29Si NMR (119 MHz, CDCl3, ppm): δ-78.46,-78.35,-78.33,-3.91,-3.83,-3.79,-3.75,-3.68,-3.66.
IR (ν/cm−1): 3095, 3073, 3053, 3016, 3007, 2961, 2902, 2134, 1430, 1253, 1135, 1066, 900, 836, 771, 697, 594, 485.
Mass spectrometry (ESI) of m/z is calculated for: C32H52NO8Si8, [(M + NH4)+]: 802.18, found 802.1842.
Elemental analysis found, %: C, 49.18; H, 6.15; Si, 28.35. Calculated for C32H48O8Si8, %: C, 48.94; H, 6.16; Si, 28.61.

2.2.2. Synthesis of PDMS-15

Briefly, 132 mL of hexane, 45.06 g (202.5 mmol) of D 3 and 18.1 mL of n-BuLi (28.9 mmol, 1.6 M solution in hexane) were loaded into a one-neck flask supplied with a magnetic stirrer. In 12 h, 75 mL of tetramethylene oxide (THF) was mixed into the system. In 6 h after adding THF, 7 g (57.9 mmol) of vinyldimethylchlorosilane was added dropwise. The reaction mass was filtered off from LiCl through the paper filter and solvents were removed to constant weight. The yield: 42.85 g (95%) of a viscous transparent liquid.
1H NMR (600 MHz, CDCl3, ppm): 6.25–5.75 (m, SiVin); 1.41–1.32 (m, SiBu); 0.96–0.92 (m, SiBu); 0.62–0.57 (m, SiBu); 0.21–0.10 (m, Si (CH3)2).
IR (ν/cm−1): 3052, 2963, 2905, 2875, 2860, 2799, 1944, 1596, 1445, 1410, 1260, 1191, 1092, 1022, 958, 862, 798, 702, 687, 669, 517.
GPC: Mn = 2.4 kDa, PDI = 1.13.

2.2.3. Synthesis of Star-Shaped Siloxane Polymers

General Synthesis Technique

Toluene, organocyclotetrasilsesquioxane, PDMS-15 and Karsted’s catalyst were loaded into a one-neck flask supplied with a magnetic stirrer. Stirring continued for 2 days. Then toluene solution was filtered through silica gel to remove Pt and concentrated. All polymers were purified by method of preparative chromatography.

2.2.4. Synthesis of Me4-15 Star-Shaped Polymer

Toluene (22 mL), cis-tetra[methyl(dimethylsiloxy)]cyclotetrasiloxane (0.2 g, 0.38 mmol), PDMS-15 (2 g, 1.6 mmol) and Karsted’s catalyst (4 µL). The yield: 1.99 g (91%) of a viscous transparent liquid.
1H NMR (600 MHz, CDCl3, ppm): δ 1.36–1.31 (m, SiBu); 0.92–0.89 (m, SiBu); 0.57–0.54 (m, SiBu); 0.49–0.44 (m, SiCH2 CH2); 0.16–0.06 (m, Si(CH3)2, SiCH3).
IR (ν/cm−1): 2963, 2911, 2801, 2054, 1946, 1732, 1602, 1447, 1408, 1261, 1090, 1028, 799, 695.
GPC: Mn = 6.6 kDa, PDI = 1.08.

2.2.5. Synthesis of Ph4-15 Star-Shaped Polymer

Toluene (23 mL), cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane (0.3 g, 0.38 mmol), PDMS-15 (2 g, 1.6 mmol) and Karsted’s catalyst (4 µL). The yield: 2.11 g (96%) of a viscous transparent liquid.
1H NMR (600 MHz, CDCl3, ppm): δ 7.30–7.24 (m, SiPh); 7.09–7.06 (m, SiPh); 1.37–1.30 (m, SiBu); 0.92–0.89 (m, SiBu); 0.57–0.54 (m, SiBu); 0.50–0.39 (m, SiCH2CH2); 0.21–0.02 (m, Si(CH3)2).
IR (ν/cm−1): 2962, 2910, 1409, 1260, 1091, 1027, 801, 740, 697.
GPC: Mn = 6.7 kDa, PDI = 1.14.

2.2.6. Synthesis of Phr4-15 Star-Shaped Polymer

Toluene (23 mL), mixture of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane (0.32 g, 0.41 mmol) isomers, PDMS-15 (2 g, 1.6 mmol) and Karsted’s catalyst (4 µL). The yield: 2 g (91%) of a viscous transparent liquid.
1H NMR (600 MHz, CDCl3, ppm): δ 7.77–7.04 (m, SiPh); 1.34–1.24 (m, SiBu); 0.88–0.85 (m, SiBu); 0.54–0.50 (m, SiBu); 0.42–0.25 (m, SiCH2CH2); 0.21–0.21 (m, Si(CH3)2).
IR (ν/cm−1): 3062, 2962, 2910, 2800, 1409, 1260, 1088, 1027, 800, 739, 697.
GPC: Mn = 6 kDa, PDI = 1.11.

2.2.7. Synthesis of Tol4-15 Star-Shaped Polymer

Toluene (42 mL), cis-tetra[tolyl(dimethylsiloxy)]cyclotetrasiloxane (0.58 g, 0.7 mmol), PDMS-15 (3.84 g, 3.1 mmol) and Karsted’s catalyst (6 mcl). The: 3.95 g (98%) of a viscous transparent liquid.
1H NMR (600 MHz, CDCl3, ppm): δ 7.21–7.18 (m, SiTol); 6.69–6.89 (m, SiTol); 6.69–6.89 (m, SiTol); 1.36–1.30 (m, SiBu); 0.91–0.89 (m, SiBu); 0.57–0.54 (m, SiBu); 0.47–0.37 (m, SiCH2CH2); 0.19–0.02 (m, Si(CH3)2).
IR (ν/cm−1): 2962, 2912, 2800, 1408, 1260, 1091, 1027, 800, 690.
GPC: Mn = 7.9 kDa, PDI = 1.13.

3. Results and their Discussion

3.1. Cores Synthesis

Synthesis of cis-tetraorganocyclotetrasilsesquioxanes containing methyl-, tolyl-, and phenyl- groups was carried out according to Scheme 1.
The respective organotrialkoxysilane was treated by equimolar quantity of sodium hydroxide or potassium hydroxide in presence of equimolar amount of water. n-Butanol was used as solvent in case of tolyl- and phenyl- substituent; in case of methyl- substituent, methanol and hexane mixture with 1/7 ratio was used. For further use of these cycles as SSP cores, they were treated by dimethylchlorosilane according to Scheme 1.
The isomerization of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane was carried out in mass in presence of sulfocationite (Amberlyst 15) at 70 °C for 4 h (Scheme 2). As a result, the mixture of all four isomers in equal quantities with 90% yield was formed.
The kinetics of isomerization process was monitored by 1H NMR method (Figure 1).
Reaction mass samples were taken after 1, 2, and 4 h from the beginning of reaction.
According to the 1H NMR spectroscopy data of initial compound, cis-tetra[phenyl (dimethylsiloxy)]cyclotetrasiloxane (a violet curve), in the field of 4.9 ppm we observe a signal that corresponds to SiH-group. As isomerization goes on, the emergence of new signals in the field of 4.5–5 ppm (turquoise and green curves), which correspond to SiH isomers groups is observed. The appearance of signals of equal intensity for all SiHgroups in the region of 4.5–5 ppm (burgundy curve) shows the moment when the reaction ends. The isomerization reaction is completely over (stopped) 4 h after the start.
Composition of mixture obtained and structure of isomers were identified by 1H NMR-spectroscopy (Figure 2).
According to GPC data, the hydrodynamic radius of isomers in the mixture and of initial cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane coincide (Figure 3).

3.2. Synthesis of Arm

The monofunctional PDMS-arm with n = 15 polymerization degree was synthesized by method of living anionic polymerization of hexamethylcyclotrisiloxane in presence of n-BuLi, with subsequent blocking by vinyldimethylchlorosilane (Scheme 3).

3.3. Assembly of Star-Shaped Polydimethylsiloxanes

For SSP synthesis on the basis of various tetracyclic cores, the reaction of hydrosilylation was carried out in the presence of Karsted’s catalyst in toluene (Scheme 4).
The course of the reaction was monitored by 1H NMR based on the disappearance of SiH signals in the initial organocyclosiloxane. Four SSPs with identical quantity and length of arms but with different cores were synthesized as a result. Molecular-mass characteristics of the polymers obtained are presented in Table 1. All polymers have narrow molecular-mass distribution.

3.4. Thermal Properties

The synthesized SSP and the initial arm (PDMS-15) were studied by TGA and DSC methods.
In Figure 4, DSC curves for Phr4-15, Me4-15, Ph4-15 и, and Tol4-15 polymers differing in organic substituent at Si atom and stereoregularity of the cyclic core, (Phr4-15 and Ph4-15 polymers) and initial PDMS-15 are presented. According to DSC data obtained, full suppression of PDMS-arm crystallization process is observed in all SSPs. Similarly, crystallization of side chains was not observed for SSPs with 21 PDMS units per arm [44]. Thus, introduction of a cyclic fragment as the branching core suppresses the ability of polymeric chains of the target products to crystallize.
Glass-transition temperatures for all SSPs are close and are within −124 to −122 °C that is characteristic of classical linear PDMS [51].
TGA data obtained for Phr4-15, Me4-15, Ph4-15, and Tol4-15 polymers and PDMS-15 arm are presented in Table 2 and in Figure 5. They show SSP improved thermal and thermo-oxidative stability compared with their linear arm. The destruction onset temperatures in argon and in air are within temperature limits typical for linear PDMS of similar molecular weight [51].

3.5. Rheological Properties

In Solution

Intrinsic viscosity [η] depends on solvent quality, i.e., on its thermodynamic affinity to polymer. The macromolecular coil in various solvents swells differently. The “better” is the solvent the bigger is its size that, in turn, results in bigger hydrodynamic resistance to flow and [η] increase. Heptane is a “good” solvent for PDMS that is confirmed by value χ = 0.409 defined in [52]. In [53], the constants of Mark–Kuhn–Houwink equation for linear PDMS in heptane at 25 °C [η] = 1.207 × 10−4 M0.741 were found.
The measured values of intrinsic viscosity for SSP and calculated values of intrinsic viscosity of their linear analogs are presented in Table 3 and Figure 6.
In Figure 6, concentration dependencies of reduced viscosity of diluted solutions in heptane are presented. The values of intrinsic viscosity [η] defined at C → 0 are given in Table 3.
As presented data show (Table 3 and the Figure 6), all SSPs have small intrinsic viscosity values irrespective of macromolecule branching-out center structure that might evidence rather dense SSP macromolecule packing in solution.
The results of SSP rheological study in bulk are presented in Table 3. Low viscosity values in both solution and block are characteristic of practically all studied samples, and still viscous flow activation energy values “feel” the cyclic branching-out center. For all studied samples, viscous flow has high power consumption.
Flow curves of SSP and of their linear analog PMS-200 are presented in Figure 7. As Figure 7 shows, SSP viscosity practically does not depend on shift rate that indicates the Newtonian character of the flow.
For linear polymer, the Newtonian flow begins at shear rates over 100 s−1. At smaller shift rates, viscosity decreases as the shift rate grows that is related to gradual orientation of polymer macromolecules in the shear field. SSP viscosity in the whole interval of shear rates is less than for linear polymer, and Me4-15 has the least viscosity among SSPs.
Flow curves of Ph4-15 at various temperatures are presented in Figure 8. Similar dependencies were also obtained for other SSPs.
The viscosity temperature dependencies in Arrhenius equation coordinates (Figure 9) were plotted.
They have linear shape that allows to calculate viscous flow activation energy (Ea) for all SSPs.
Based upon the data obtained, it can be assumed that the macromolecule coil size in bulk is smaller than for the polydimethylsiloxane analog. On the other hand, as PDMS-arms have much smaller length, they orient quicker in the shift field that is confirmed by the Newtonian character of their flow at all shear rates.

3.6. Langmuir Layers

Amphiphilic siloxane SSPs form Langmuir monolayers at the air–water interface after spreading of the solutions and evaporation of the solvent. The macromolecules contain identical hydrophilic fragments and differ by hydrophobic substituents. Hydrophilic covalent-ionic Si-O bonds are directed into water subphase and form hydrogen bonds with water molecules. Hydrophobic butyl-, methyl-, phenyl-, or tolyl- groups are directed into air phase. SSPs studied in this paper differ from their analogs with bigger number of arms [44]. They have strictly four arms of 15 Si-O units instead of 21. Their cores contain both similar [44] phenyl-groups, and methyl- or tolyl- groups. Such SSP selection allows the discovery of the following: (1) whether SSP behavior in Langmuir monolayers is influenced by the type of hydrophobic groups in the core (among Me4-15, Ph4-15, and Tol4-15); (2) how regular cis-SSP features manifest themselves in comparison with their irregular analog, while all their other molecular parameters are identical (Phr4-15, and Ph4-15).
Dependencies of surface pressure (π) and surface potential (ΔU) on the area of interphase surface per molecule (A) for Me4-15 are given in Figure 10, for Phr4-15, Ph4-15, and Tol4-15 in Figure 11. A–D points on curve 1 in Figure 10 correspond to traditional [54,55,56] designation of π-A isotherm characteristic points of PDMS. Values of surface pressure, surface areas both per molecule and per repeating dimethylsiloxane unit in points A–D, and surface potential ranges (defined similarly as shown for curve 1 in Figure 10) are summarized in Table 4. Calculation of changes in surface areas per repeating dimethylsiloxane unit allows SSPs to be compared with each other, with PDMS, and with SSPs [44]. Brewster angle microscopy images for SSP Langmuir layers are shown in Figure 12.
Me4-15 contains butyl groups at the ends of arms as well as methyl groups as substituents at Si atoms similarly to PDMS. The shape of π-A isotherm and the range of surface pressure change between the points C and D (0.5 mN m−1) for Me4-15 are similar to the ones observed for linear PDMS. In Brewster angle microscopy images in the area of zero surface pressure (Figure 12a), the border of darker water surface, and lighter Me4-15 monolayer, is visible. At the same time, prior to surface pressure rise, the surface potential isotherm exhibits a jump from zero to positive ΔU values in a narrow interval of surface area change (curve 3 in Figure 10). On compression of a monolayer in the A–B region of π-A isotherm (curve 1 in Figure 10), the water surface is completely covered with a monolayer (Figure 12b); the conformational transition in siloxane arms in the B–C region of the π-A isotherm does not change Langmuir monolayer surface morphology (Figure 12c). Two hypotheses [57] of conformational transformations of siloxane chain in the B–C region of π-A isotherm of PDMS are known: the formation of horizontal folds from odd quantity of chain segments, or a helix with six dimethylsiloxane units per one turn (analog of Damaschun helicoid at crystallization). However, the surface pressure values in Me4-15 collapse point are lower than those typical for PDMS [57,58]. It can result from both Me4-15 rather low molecular mass as well as formation of less stable layer due to formation of helices consisting of only two coil turns assumed by each SSP arm, or steric difficulties caused by attachment of one of PDMS-arms end to the central cycle.
After the final collapse of Langmuir layer in point D (Figure 12d), Me4-15 excess accumulates into lenses, similar to [44]. Brewster angle microscopy image shows bright domains on a dark surface. In the range A–D, surface potential fluctuates irrespective of conformational transformations of the siloxane chain and Langmuir film collapse (curve 3 Figure 10).
An increase in size of organic substituents attached to the core, in Ph4-15, Phr4-15, and Tol4-15 results in surface pressure decrease in points B and C of π-A isotherm together with its increase in point D in comparison with Me4-15 (Figure 10 and Figure 11). At the same time, the surface-pressure jump between points C and D increases to 1.4 mN m−1 for Ph4-15, Phr4-15, and Tol4-15. It exceeds the range of 0.9 mN m−1, which was typical for Ph4-15 analog with arm length of 21 dimethylsiloxane units [44]. Morphological changes in Langmuir monolayers of Ph4-15, Phr4-15, and Tol4-15 polymers according to Brewster angle microscopy are similar to presented for Me4-15 in Figure 12. The surface potential obtained on compression of Ph4-15, Phr4-15, and Tol4-15 in the A–D region fluctuates in the range of 150–180 mV, which is reduced in comparison to 180–200 mV typical for linear PDMS and Me4-15 polymer.
The shape of surface pressure isotherms obtained on expansion of Me4-15 and Phr4-15 Langmuir layers (curves 2 in Figure 10 and Figure 11) are similar to those obtained under compression. The hysteresis in compression–expansion cycle is observed in the whole range of surface areas. After Langmuir layers expansion, the excess of polymers collapsed in lenses spreads in a monolayer with conformational transformations of helix or folded chains into straightened ones. The π-A and ΔU-A isotherm shapes, as well as the change in Langmuir layers surface morphology in compression–expansion cycle, indicate their liquid aggregate state.

4. Conclusions

The isomerization of cis-tetra[(phenyl)(dimethylsiloxane)]cyclotetrasiloxane was carried out for the first time and the mix of all four isomers in equal quantities was obtained with 90% yield. Four new narrow-dispersity non-crystallizable star-shaped polydimethylsiloxanes were synthesized. Their molecules contain identical number of arms of identical length, but have differences in the branching-out center. Cis-tetratolyl-, cis-tetraphenyl-, and cis-tetramethylsilsesquioxane cycles serve as the branching-out center in three SSPs, respectively. One SSP has the mix of four stereoisomers as a core. SSP viscosimetric research showed that their macromolecules are small-size dense coil in both solution and in bulk.
SSP study in Langmuir layers at the air–water interface showed that the increase in the size of organic substituents in cyclic core is a major factor for increased stability of Langmuir layer before a collapse. Thus, Me4-15 polymer forms less stable Langmuir layers by 1 mN m−1 than PDMS due to macromolecule structure change from linear to star-shaped and smaller molecular weight. Replacement of only four methyl groups in a SSP cyclic core to phenyl or tolyl without considerable change in molecular-mass characteristics, strikingly changes the form of the surface pressure isotherm and increases the stability of the layer by 0.5 mN m−1.
At the same time, as the general tendency in behavior of studied SSP, we note the determinative influence of PDMS-arms in comparison with the features of the branching-out center structure. It seems advisable to continue the assessment of influence of various elements of the SSP structure with an even bigger reduction in arms length that would allow the influence of branching-out center features on SSP properties to be more accurately revealed.

Author Contributions

Y.S.D. carried out experiments and wrote the paper; A.A.A. and O.I.S. conceived, designed and wrote the paper; A.S.P. recorded the 1H and 29Si NMR spectra; M.I.B. conducted a study using the TGA and DSC methods; G.G.N., S.A.K. and V.G.V. conducted physical and mechanical measurements; A.I.B., A.A.S. and Y.N.M. carried out studies of surface properties at the water/air interface and interpreted the results; O.I.S. wrote the paper and discussion of the results; A.M.M. led the research. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the grant of the Russian Science Foundation (project No. 21-73-20225). The elemental analysis of cyclotetrasiloxanes was carried out in the laboratory of microanalysis of INEOS, RAS; rheological study was conducted in the laboratory of polymer physics of INEOS, RAS, with financial support of the Russian Federation Ministry of Science and Higher Education. Langmuir layers formation and research was carried out using equipment of Kurchatov Resource Center of Organic and Hybrid Materials «Polymer». The study was supported by the Russian Foundation for Basic Research (grant no. MK 19-29-13031) in part of the synthesis of alkaline metal siloxanolates.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Polymers 14 00285 sch001 550
Scheme 1. General scheme of cis-tetraorganocyclotetrasilsesquioxanes synthesis.
Scheme 1. General scheme of cis-tetraorganocyclotetrasilsesquioxanes synthesis.
Polymers 14 00285 sch001
Polymers 14 00285 sch002 550
Scheme 2. Scheme of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane isomerization.
Scheme 2. Scheme of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane isomerization.
Polymers 14 00285 sch002
Polymers 14 00285 g001 550
Figure 1. 1H NMR-spectra of initial compound (violet), after 1 h from the beginning of reaction (turquoise), after 2 h (green) and after 4 h (burgundy).
Figure 1. 1H NMR-spectra of initial compound (violet), after 1 h from the beginning of reaction (turquoise), after 2 h (green) and after 4 h (burgundy).
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Polymers 14 00285 g002 550
Figure 2. 1H NMR-spectrum of isomers mixture.
Figure 2. 1H NMR-spectrum of isomers mixture.
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Polymers 14 00285 g003 550
Figure 3. GPC curves of the initial cis-tetra[phenyl(dimethylsiloxi)]cyclotetrasiloxane and its mix of isomers.
Figure 3. GPC curves of the initial cis-tetra[phenyl(dimethylsiloxi)]cyclotetrasiloxane and its mix of isomers.
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Polymers 14 00285 sch003 550
Scheme 3. Scheme of PDMS-arm synthesis.
Scheme 3. Scheme of PDMS-arm synthesis.
Polymers 14 00285 sch003
Polymers 14 00285 sch004 550
Scheme 4. The scheme of SSP synthesis on the basis of tetra[organo(dimethylsiloxy)]cyclotetrasiloxane.
Scheme 4. The scheme of SSP synthesis on the basis of tetra[organo(dimethylsiloxy)]cyclotetrasiloxane.
Polymers 14 00285 sch004
Polymers 14 00285 g004 550
Figure 4. DSC curves for PDMS-15 (1) and Me4-15 (2), Tol4-15 (3), Ph4-15 (4), Phr4-15 (5) star-shaped polymers at 10 °C/min heating rate.
Figure 4. DSC curves for PDMS-15 (1) and Me4-15 (2), Tol4-15 (3), Ph4-15 (4), Phr4-15 (5) star-shaped polymers at 10 °C/min heating rate.
Polymers 14 00285 g004
Polymers 14 00285 g005 550
Figure 5. TGA curves for PDMS-15 (1) and Ph4-15 (2), Phr4-15 (3), Tol4-15 (4), Me4-15 (5) star-shaped polymers both in air (a) and in argon (b) at 10 °C/min heating rate.
Figure 5. TGA curves for PDMS-15 (1) and Ph4-15 (2), Phr4-15 (3), Tol4-15 (4), Me4-15 (5) star-shaped polymers both in air (a) and in argon (b) at 10 °C/min heating rate.
Polymers 14 00285 g005
Polymers 14 00285 g006 550
Figure 6. Concentration dependences of reduced viscosity of dilute solutions of star-shaped Phr4-15, Me4-15, Phr4-15 and Tol4-15 in heptane.
Figure 6. Concentration dependences of reduced viscosity of dilute solutions of star-shaped Phr4-15, Me4-15, Phr4-15 and Tol4-15 in heptane.
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Polymers 14 00285 g007 550
Figure 7. Phr4-15, Me4-15, Tol4-15, Ph4-15, and PMS-200 flow curves at 20 °C.
Figure 7. Phr4-15, Me4-15, Tol4-15, Ph4-15, and PMS-200 flow curves at 20 °C.
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Polymers 14 00285 g008 550
Figure 8. Flow curves of star-shaped Ph4-15 polymer at 20, 30, 40, 50, 60, and 70 °C.
Figure 8. Flow curves of star-shaped Ph4-15 polymer at 20, 30, 40, 50, 60, and 70 °C.
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Polymers 14 00285 g009 550
Figure 9. Defining Ea for Ph4-15 polymer.
Figure 9. Defining Ea for Ph4-15 polymer.
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Polymers 14 00285 g010 550
Figure 10. π-A (1,2) and ΔU-A (3) compression (1,3) and expansion (2) isotherms of Me4-15 Langmuir layer. T = 20 °C. Letters A–D denote characteristic points on the surface pressure isotherm.
Figure 10. π-A (1,2) and ΔU-A (3) compression (1,3) and expansion (2) isotherms of Me4-15 Langmuir layer. T = 20 °C. Letters A–D denote characteristic points on the surface pressure isotherm.
Polymers 14 00285 g010
Polymers 14 00285 g011 550
Figure 11. π-A (1–4) and ΔU-A (5–7) compression (1,3–7) and expansion isotherms (2) of Phr4-15 (1,2,5), Ph4-15 (3,6), and Tol4-15 (4,7) Langmuir layers. T = 20 °C.
Figure 11. π-A (1–4) and ΔU-A (5–7) compression (1,3–7) and expansion isotherms (2) of Phr4-15 (1,2,5), Ph4-15 (3,6), and Tol4-15 (4,7) Langmuir layers. T = 20 °C.
Polymers 14 00285 g011
Polymers 14 00285 g012 550
Figure 12. Brewster angle microscopy images of Me4-15 Langmuir layer surface obtained at compression up to the following values of surface pressure and area per a molecule: 0 mN m−1 and 1890 Å2 (a), 3.8 mN m−1 and 1610 Å2 (b), 7.5 mN m−1 and 1030 Å2 (c), 8.4 mN m−1 and 490 Å2 (d).
Figure 12. Brewster angle microscopy images of Me4-15 Langmuir layer surface obtained at compression up to the following values of surface pressure and area per a molecule: 0 mN m−1 and 1890 Å2 (a), 3.8 mN m−1 and 1610 Å2 (b), 7.5 mN m−1 and 1030 Å2 (c), 8.4 mN m−1 and 490 Å2 (d).
Polymers 14 00285 g012
Table
Table 1. Molecular-mass characteristics of a linear arm and star-shaped polymers.
Table 1. Molecular-mass characteristics of a linear arm and star-shaped polymers.
SampleMnNMR, kDaMnGPC, kDaMwGPC, kDaPDIOutput, %
PDMS-151.32.42.71.1395
Ph4-155.96.77.61.1496
Phr4-155.966.71.1191
Tol4-155.97.98.91.1398
Me4-155.56.67.11.0891
Table
Table 2. Thermal characteristics of SSP obtained.
Table 2. Thermal characteristics of SSP obtained.
SampleTg, CTcc, CTm, CTd5%, CM, Mas. %
AirArgonAirArgon
PDMS-15−133−83−60234281434
Ph4-15−123--3314104311
Phr4-15−122--3374205013
Tol4-15−124--3574392016
Me4-15−123--322413469
Table
Table 3. Rheological properties of SSP in solution and in bulk.
Table 3. Rheological properties of SSP in solution and in bulk.
SampleStar/
Linear Polymer		Pa*s
Star		Ea, kJ/mol
Ph4-150.049/0.0910.08116.3
Phr4-150.044/0.0830.08016.6
Tol4-150.047/0.1000.08016.3
Me4-150.054/0.0860.07315.8
Table
Table 4. Characteristic points A–D of π-A and ΔU-A isotherms for Me4-15, Ph4-15, Phr4-15, Tol4-15.
Table 4. Characteristic points A–D of π-A and ΔU-A isotherms for Me4-15, Ph4-15, Phr4-15, Tol4-15.
ABCD
PDMS [57,58]Area per (OSi(CH3)2) unit, Å219.015.08.07.0
Surface pressure, mN m−107.08.59.0
Surface potential range, mV190–200
Me4-15Area per a molecule, Å217801410790690
Area per (OSi(CH3)2) unit, Å218.514.68.27.2
Surface pressure, mNm−106.87.68.1
Surface potential range, mV180–195
Ph4-15Area per a molecule, Å219001530850700
Area per (OSi(CH3)2) unit, Å218.514.98.36.8
Surface pressure, mN m−106.27.28.6
Surface potential range, mV150–175
Phr4-15Area per a molecule, Å216801340750610
Area per (OSi(CH3)2) unit, Å218.614.88.36.7
Surface pressure, mN m−106.17.08.4
Surface potential interval, mV160–175
Tol4-15Area per a molecule, Å2223018001010800
Area per (OSi(CH3)2) unit, Å218.614.98.46.6
Surface pressure, mN m−1067.08.4
Surface potential interval, mV155–180
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Dyuzhikova, Y.S.; Anisimov, A.A.; Peregudov, A.S.; Buzin, M.I.; Nikiforova, G.G.; Vasil’ev, V.G.; Kostrov, S.A.; Buzin, A.I.; Stupnikov, A.A.; Malakhova, Y.N.; et al. Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties. Polymers 2022, 14, 285. https://doi.org/10.3390/polym14020285

AMA Style

Dyuzhikova YS, Anisimov AA, Peregudov AS, Buzin MI, Nikiforova GG, Vasil’ev VG, Kostrov SA, Buzin AI, Stupnikov AA, Malakhova YN, et al. Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties. Polymers. 2022; 14(2):285. https://doi.org/10.3390/polym14020285

Chicago/Turabian Style

Dyuzhikova, Yulia S., Anton A. Anisimov, Alexander S. Peregudov, Mikhail I. Buzin, Galina G. Nikiforova, Viktor G. Vasil’ev, Sergey A. Kostrov, Alexander I. Buzin, Alexei A. Stupnikov, Yulia N. Malakhova, and et al. 2022. "Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties" Polymers 14, no. 2: 285. https://doi.org/10.3390/polym14020285

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