Article pubs.acs.org/JPCB
Thermodynamic Properties of Carbosilane Dendrimers of the Sixth Generation with Ethylene Oxide Terminal Groups Semen S. Sologubov,† Alexey V. Markin,*,† Natalia N. Smirnova,† Natalia A. Novozhilova,‡ Elena A. Tatarinova,‡ and Aziz M. Muzafarov‡ †
Lobachevsky State University of Nizhni Novgorod, 23/5 Gagarin Av., 603950 Nizhni Novgorod, Russia Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 70 Profsoyuznaya St., 117393 Moscow, Russia
‡
S Supporting Information *
ABSTRACT: The temperature dependences of heat capacities of carbosilane dendrimers of the sixth generation with ethyleneoxide terminal groups, denoted as G6[(OCH2CH2)1OCH3]256 and G6[(OCH2CH2)3OCH3]256, were measured in the temperature range from T = (6 to 520) K by precision adiabatic calorimetry and differential scanning calorimetry (DSC). In the above temperature range the physical transformations, such as glass transition and hightemperature relaxation transition, were detected. The standard thermodynamic characteristics of the revealed transformations were determined and analyzed. The standard thermodynamic functions, namely, heat capacity Cp°(T), enthalpy H°(T) − H°(0), entropy S°(T) − S°(0), and Gibbs energy G°(T) − H°(0) for the range from T → 0 to 520 K, and the standard entropies of formation ΔfS° of the investigated dendrimers in the devitrified state at T = 298.15 K, were calculated per corresponding moles of the notional structural units. The standard thermodynamic properties of dendrimers under study were discussed and compared with literature data for carbosilane dendrimers with different functional terminal groups.
1. INTRODUCTION Dendrimers are highly branched monodisperse macromolecules characterized by a unique tree-like 3D architecture and surface functionality. They can be prepared by divergent, convergent, or a combination of these two synthetic methodologies; it provides a controllable increase in their molecular weight, size, and number of functional terminal groups.1−3 The nature of these groups plays a key role in different properties of dendrimers. Dendrimers are one of the most extensively studied materials because of their perfect topological structure and versatile physicochemical properties. Therefore, dendrimers have been widely and successfully applied to fields of catalysis, medicinal chemistry, and nanotechnologies.4−8 In recent years, there has been a rapid growth in the discovery and application of dendrimers. In particular, carbosilane dendrimers have been widely used because of their catalytic inertness and accessibility. 9 Additionally, carbosilane dendrimers typically possess low glass-transition temperatures, high solubility in majority of solvents, and quite low viscosity in solutions as most of dendrimers do. They are kinetically and thermodynamically stable owing to the low polarity and high strength of the Si−C bond.10 The investigation of standard thermodynamic properties of carbosilane dendrimers with different terminal groups in a wide temperature range by precision adiabatic calorimetry and DSC allows us to determine and analyze their dependences on composition and structure.11−18 The discovery of structural © 2015 American Chemical Society
anomalies for carbosilane dendrimers of lower generations11−13,17,18 and high-temperature relaxation transitions for carbosilane dendrimers of higher generations13−17 is a significant and important result of the calorimetric studies. This work continues previous studies of thermodynamic properties of carbosilane dendrimers with different terminal groups on the outer layer. The heat capacity and thermodynamic properties of dendrimer G6[(OCH2CH2)3OCH3]256 were previously studied in the range from T = (6 to 350) K.11 The aim of this work was to study the temperature dependences of heat capacities of carbosilane dendrimers of the sixth generation with ethyleneoxide terminal groups G6[(OCH2CH2)1OCH3]256 in the temperature range from T = (6 to 520) K and G6[(OCH2CH2)3OCH3]256 in the interval from T = (350 to 520) K by precision adiabatic calorimetry and DSC; to reveal possible physical transformations on heating and cooling and to determine their thermodynamic characteristics; to calculate the standard thermodynamic functions for the range from T → 0 to 520 K and the standard entropies of formation of dendrimers in the devitrified state at T = 298.15 K per corresponding moles of the notional structural units; and to compare the standard thermodynamic properties of the Received: July 14, 2015 Revised: October 22, 2015 Published: October 23, 2015 14527
DOI: 10.1021/acs.jpcb.5b06786 J. Phys. Chem. B 2015, 119, 14527−14535
Article
The Journal of Physical Chemistry B
Figure 1. (a) Scheme of carbosilane dendrimers of the sixth generation with ethyleneoxide terminal groups (n = 1, 3; i = the modifying agent containing ethyleneoxide units, Karstedt catalyst, toluene). (b) Structure of carbosilane dendrimer of the third generation with butyl terminal groups.
Figure 2. Scheme of synthesis of carbosilane dendrimers with ethyleneoxide terminal groups.
macromolecules, and z is the number of terminal groups. For example, the structure of carbosilane dendrimer of the third generation with butyl terminal groups G3[Bu]32 is illustrated in Figure 1b. The core is one of basic architectural components of dendrimers.19,20 Because dendrimers are formed by series of reiterative or generational reactions, they are typically identified by the number of generations to which they have been reacted (e.g., G3, G6). Under current nomenclature,19 the dendrimer core with the first set of branches attached thereto is referred to as a zeroth generation (G0 dendrimer). Once the second set of
investigated dendrimers with literature data for carbosilane dendrimers with different functional terminal groups.
2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Dendrimers. The notional schemes of the investigated carbosilane dendrimers G6[(OCH2CH2)1OCH3]256 and G6[(OCH2CH2)3OCH3]256 are presented in Figure 1a. The general designation of dendrimers is GX[Y]z, where G is the generation of dendrimer, X is the number of generation, Y is the terminal groups on the outer layer of dendrimer 14528
DOI: 10.1021/acs.jpcb.5b06786 J. Phys. Chem. B 2015, 119, 14527−14535
Article
The Journal of Physical Chemistry B
Table 1. Designations and Molar Masses of the Notional Structural Units Selected for Carbosilane Dendrimers of the Sixth Generation with Ethyleneoxide Terminal Groups compound
notional structural unit
M/g·mol−1
G6[(OCH2CH2)1OCH3]256 G6[(OCH2CH2)3OCH3]256
[(CH2)3Si(CH3){(CH2)3Si(CH3)2OSi(CH3)2(CH2)3(OCH2CH2)1OCH3}] [(CH2)3Si(CH3){(CH2)3Si(CH3)2OSi(CH3)2(CH2)3(OCH2CH2)3OCH3}]
376.726 464.825
branches is attached to the first set of branches, it is a first generation (G1 dendrimer). The core of the tested dendrimers is a regular carbosilane structure where silicon atoms are branching centers connected by propylene bridges. The samples of carbosilane dendrimers G6[(OCH 2 CH 2 ) 1 OCH 3 ] 256 and G6[(OCH 2 CH 2 ) 3 OCH 3 ] 256 were synthesized at the Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences (Moscow). The scheme of synthesis of dendrimers under study is shown in Figure 2. Under standard conditions the investigated dendrimers were transparent colorless wax-like solid substances. The composition and structure of the synthesized samples were proved by data of elemental analysis and 1H NMR spectroscopy. For G6[(OCH2CH2)1OCH3]256: Found (%): C, 54.39; H, 10.62; Si, 22.20. Calculated (%): C, 54.17; H, 10.70; Si, 22.35. 1 H NMR (CDCl3) δ: − 0.09 (s, 756 H, CH2Si(CH3)CH 2 CH 2 CH 2 Si); 0.02 (d, 3072 H, CH 2 Si(CH 3 ) 2 OSi(CH3)2CH2, J = 3.7 Hz); 0.44−0.59 (m, 2544 H, CH2SiCH2, SiOSiCH2CH2CH2O); 1.24−1.38 (m, 1016 H, Si C H 2 C H 2 C H 2 S i ) ; 1 . 5 5− 1 . 6 6 ( m , 5 1 2 H , S i O SiCH2CH2CH2O); 3.40 (t, 512 H, SiOSiCH2CH2CH2O, J = 6.7 Hz); 3.36 (s, 768 H, OCH3); 3.50−3.59 (m, 1024 H, OCH2CH2O). For G6[(OCH2CH2)3OCH3]256: Found (%): C, 53.79; H, 10.33; Si, 17.53. Calculated (%): C, 54.10; H, 10.38; Si, 17.58. 1 H NMR (CDCl3) δ: −0.09 (s, 756 H, CH2Si(CH3)CH 2 CH 2 CH 2 Si); 0.02 (d, 3072 H, CH 2 Si(CH 3 ) 2 OSi(CH3)2CH2, J = 4.3 Hz); 0.44−0.59 (m, 2544 H, CH2SiCH2, SiOSiCH2CH2CH2O); 1.24−1.38 (m, 1016 H, Si C H 2 C H 2 C H 2 S i ) ; 1 . 4 7− 1 . 6 3 ( m , 5 1 2 H , S i O SiCH 2 CH 2 CH 2 O); 3.36−3.43 (m, 1280 H, SiOSiCH 2 CH 2 CH 2 O, OCH 3 ); 3.51−3.65 (m, 3072 H, OCH2CH2O). The gel permeation chromatography (GPC) curves show that dendrimers under study are characterized by a narrow monomodal distribution. The purity of the tested dendrimers was confirmed by gas−liquid chromatography (GLC) results. The mole fraction purities for G6[(OCH2CH2)1OCH3]256 and G6[(OCH2CH2)3OCH3]256 were, respectively, 0.9933 and 0.9972. The molecular masses of the synthesized dendritic macromolecules were determined by the static light scattering method. The previously listed data are presented in detail elsewhere.21 The designations and molar masses of the notional structural units selected for dendrimers under study are given in Table 1. The molar masses of these units were calculated from the International Union of Pure and Applied Chemistry (IUPAC) table of atomic weights.22 The standard thermodynamic functions of the investigated dendrimers were calculated per corresponding moles of the notional structural units. 2.2. Adiabatic Calorimetry. A precision automatic adiabatic calorimeter (Block Calorimetric Thermophysical, BCT-3) with discrete heating was applied to measure the heat capacities in the temperature range from T = (6 to 350) K. The liquid helium and nitrogen were used as cooling reagents.
The calorimeter design and the operation procedure are described in detail elsewhere.23 All measurements were performed with a computer-controlled measuring system comprising an analog-to-digital converter, a digital-to-analog converter, and a switch. The calorimetric cell is a thin-walled cylindrical vessel (with a volume of 0.0015 dm3) made from titanium. The sensitivity of the thermometric circuit was 1 × 10−3 K; the sensitivity of the analog-to-digital converter was 0.1 μV. The energy introduced into the sample cell and the equilibrium temperature of the cell after the energy input were automatically recorded and processed online by a computer. The reliability of the calorimeter was verified by heat capacities measurements of the reference standard samples, K-2 benzoic acid, and α-Al2O3.24,25 The measurement uncertainty of heat capacities of the investigated samples was within 2 × 10−2 Cp° at temperatures from T = (6 to 15) K; then, it decreased to 0.5 × 10−2 Cp° in the temperature range from T = (15 to 40) K, and it was equal to 2 × 10−3 Cp° in the interval from T = (40 to 350) K. The standard uncertainty for the temperatures of transformations was u(T) = 0.01 K. 2.3. Differential Scanning Calorimetry. A differential scanning calorimeter DSC 204 F1 Phoenix with μ-sensor (Netzsch−Gerätebau, Germany) was applied to measure the heat capacities of dendrimers under study in the temperature range from T = (300 to 520) K and to investigate their thermal behavior in the above interval. The calorimeter was calibrated and tested against melting of standard calibration set (indium, bismuth, tin, zinc, mercury, biphenyl, potassium nitrate, and cesium chloride). The heating and cooling rates were 5 K· min−1; the measurement was carried out in an argon atmosphere. It was established that the measurement procedure allows us to determine the temperatures of transformations with the standard uncertainty u(T) = 0.5 K and the enthalpies of transitions with the relative standard uncertainty ur(ΔtrH) = 0.01. The temperatures and enthalpies of transformations were evaluated in accordance with the standard Netzsch Proteus Software procedure. The technique for determining of these characteristics is described in detail elsewhere26,27 and in the Netzsch Proteus Software. The heat capacities of the investigated dendrimers were determined by the ratio approach; sapphire was used as the reference standard sample. The measurement uncertainty of
