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From ultra-high molecular weight polydimethylsiloxane to super-soft elastomer
European Polymer Journal 120 (2019) 109243
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From ultra-high molecular weight polydimethylsiloxane to super-soft elastomer Codrin Tuguia, Vasile Tironb, Mihaela Dascalua, Liviu Sacarescua, Maria Cazacua, a b
T
⁎
Department of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487 Iasi, Romania Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I no. 11, Iași 700506, Romania
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high molecular weight Anionic polymerization Soft elastomer Polydimethylsiloxane Sustainable chemistry
A polydimethylsiloxane-α,ω-diol (UHMW-PDMS) with unusually high molecular weight (Mw = 1,395,000 g/ mol) and narrow polydispersity (PDI = 1.49), free of monomer or catalyst traces, was obtained in high yield by a scalable process, easy to achieve (bulk anionic ring-opening polymerization of octamethylcyclotetrasiloxane catalyzed by tetramethylammonium hydroxide). Spectral and thermogravimetric analysis confirmed the theoretical structure, purity and thermal stability of the polymer. Individual or aggregated macromolecules were assessed by dynamic light scattering (DLS) and visualized by microscopic methods (TEM, AFM). The mechanical, dielectric, and thermal performances of application interest for this polymer were evaluated on samples of chemically defined elastomer cUHMW-PDMS prepared by condensation crosslinking through the polymer chain ends with tetraethylorthosilicate. The results were compared with those obtained on a lower molecular weight (Mw = 720,100 g/mol, PDI = 1.87) homologue (PDMS) prepared and processed in identical conditions, but also with a commercial silicone elastomer. UHMW-PDMS-based elastomer shows superior results on elastic recovery and stress decay at high elongation, as compared with reference samples.
1. Introduction Silicones, with the main representative member polydimethylsiloxane, although well-known since mid 20th century, still enjoy a huge interest, both in terms of fundamental research and science applied in most branches of modern industry [1]. Silicones have a unique combination of features: good surface wetting and spreading capabilities, long-term elasticity, flexibility, weather resistance, stability under challenging conditions (extreme temperatures, thermal shock, chemicals and oxidation), resistance to aging, chemical and biological inertness, water repellence, strong adhesion properties, excellent dielectric properties. As a result, they can perform a wide range of functions, from adhesives and sealants, antifoams, surfactants, to rubbers, waxes, gels, resins, etc. Because of their special properties and the vast range of applications, polysiloxanes and especially polydimethylsiloxanes, both the cyclic and linear molecular chains, have stimulated over the years the interest of researchers for their study in all aspects: structure and chemical, physical, thermal, mechanical, rheological, tribological, etc. behaviors. Silicones are invaluable tools for developing sustainable and durable materials. Their versatile chemistry allows them to be structurally customized and engineered into a wide variety of shapes and properties
⁎
to meet a wide range of requirements [2,3]. This can be accomplished by manipulating the length of the macromolecular chain, the crosslinking pattern, the addition of fillers, by their chemical modification in the sense of associating with co-chained, grafted, interpenetrated organic partners, or by addition of organic groups to give them specific functions (surfactant, sensor, ligand, etc.). Polydimethylsiloxanes are produced in a wide range of molecular masses, with reported values from a few hundred to even more than a million Da, polymers close to this limit are considered ultra-high molecular weights (UHMWs) [4–10]. Polydimethylsiloxanes are relatively low in viscosity, flowing even at very high molecular masses, in comparison with their organic polymers counterparts, due to the flexibility of the chain and the extremely weak intermolecular forces. Virtually they do not have the energy barrier for rotation, resulting in polymers with the lowest transition temperature [10,11]. They are colorless, transparent fluids whose viscosity can vary, depending on the length of the chain, from 0.65 cSt for the shortest member of the series (hexamethyldisiloxane), which is thinner than water (0.89 cSt), to several tens of millions cSt (e.g. 50,000,000 cSt [12]) when viewed as semisolid but without dimensional stability over time. At molecular masses greater than 30,000, the entanglement of the chains begins to occur resulting in the leveling of changes in physical properties in dependence
Corresponding author. E-mail address: [email protected] (M. Cazacu).
https://doi.org/10.1016/j.eurpolymj.2019.109243 Received 3 April 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 07 September 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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The NMR spectra were recorded with a 5 mm direct detection, four nuclei (1H, 13C, 19F and 29Si) probe. In the 29Si NMR spectrum, the broad signal centered at −112.20 ppm is the background signal from the glass of the NMR tube and probe. Steady-State Flow curves of bulk PDMS samples were recorded with the RHEOPLUS MCR501 equipment, in the range 0.001–100 s−1, 25 °C, in the system cone-plate, d = 0.05 mm. Gel permeation chromatography (GPC) analysis was performed in eluent chloroform at 25 °C and at a flow rate of 1 mL/min using a WGE SEC/GPC chromatograph equipped with four in-line detectors (RI, VI, UV, MALS) and two Agilent PLgel 5 μm columns, MIXED D and MIXED C capable to separate molecular weights within 200 to 200,0000 g/mol. The molecular weights were calculated by the standard calibration method. For this purpose, ten polystyrene standards (Agilent) having the polymer nominal Mp within 1000 to 200,0000 g/ mol were used. The calibration curve log (Mp) versus elution volume was fitted to a third order polynomial equation. The size and distribution of aggregates in chloroform solution was determined by dynamic light scattering (DLS) using a Malvern Instruments Autosizer LoC 7032 Multi-8 Correlator (Malvern Instruments, UK). The solution previously filtered through 0.5 µm filter, without further dilution was irradiated with red light (HeNe laser, wavelength λ = 632.8 nm) and the intensity fluctuations of the scattered light analysed to obtain an autocorrelation function. The transmission electron microscopy (TEM) analysis was performed with Hitachi High-Tech HT7700 equipment operated at 100 kV accelerating voltage in high-contrast mode. The polymer solution in chloroform was placed on carbon-coated copper grids with 200-mesh size and then the solvent was removed under vacuum. The morphology of UHMW-PDMS polymer was studied by means of Atomic Force Microscopy (AFM), with a NT-MDT Solver Pro-M equipment. All the AFM measurements were carried out in air (solvent was removed under vacuum condition), at room temperature, using the same cantilever (NSG01 from NT-MDT) in a non-contact mode to avoid the damage of the polymer’s surface. The AFM images were processed using Nova software from NT-MDT. Thermogravimetric (TG) measurements were conducted on a STA 449 F1 Jupiter device (Netzsch, Germany). About 10 mg of each sample was weighed and heated in alumina crucibles. Nitrogen was purged as inert atmosphere at a flow rate of 50 mL min−1. Samples were heated from 25 °C to 700 °C at a heating rate of 10 °C min−1. DSC measurements were conducted with a DSC 200 F3 Maia (Netzsch, Germany). About 10 mg of sample was heated in pressed and punched aluminium crucibles at a heating rate of 10 °C min−1. Nitrogen was used as inert atmosphere at a flow rate of 100 mL min−1. Dielectric measurements were carried out in normal conditions on a Novocontrol Concept 40 apparatus, in the frequency range of 100–106 Hz. The tensile test experiments were performed using an Instron 3365 machine with a crosshead speed of 20 mm min−1. Dumbbell-shaped specimens with a gauge width of 4 mm and a gauge length of 50 mm were prepared using a cutting press. Three independent tests were performed for each film. The elastic recovery and stress decay were determined from 100 cycles stress-strain within 0–100% strain and 10 cycles within 0–500%. The stationary time at maximum and minimum strain was 2 s.
on viscosity [11]. In general, the polydimethylsiloxane-α,ω-diols currently provided by the silicones suppliers are of moderate molecular weights, up to the order of several hundred thousand. They are available both as such and processed in various forms: oils, greases, rubbers, resins. Of these basic silicone grades, thousands of unique silicone products have been designed to meet the needs of designers, manufacturers, people and municipalities around the world. Besides these, polydimethylsiloxanes are used extensively in the plastics industry as additives for improving the processing and surface properties of plastics. In this aim, mainly low molecular weight PDMS (viscosity < 1000 cSt) are used as external additives. However, by incorporating higher molecular weight PDMS (viscosities ranging from 10,000 to 60,000 cSt) as internal additive, their external application is avoided, and processing advantages are provided in addition to surface property improvement. A recent embodiment in the field of additives is the use of PDMS of ultra-high molecular mass, with viscosities between 10 × 106 and 50 × 106 cSt, which allows for increased loading in the additive from 20 to 50% and eliminates the “bleed-out” phenomenon on the surface that happens in the case of small molecular PDMS [13]. Commercial Super-High Viscosity Pure Silicone Fluids are linear with viscosities ranging from 300,000 to 20,000,000 cSt, characterized by their high damping action, excellent lubricity, inertness to plastics, rubbers and metals, high dielectric strength, high resistance to shear, thermal stability, wide service temperature range and low viscosity change at temperature. They are used in a wide range of military and industrial applications [14]. Although they are of great practical importance, in the field of silicone research, studies on polymers of extra-high molecular mass are still rare. In this paper, it is reported obtaining a polydimethylsiloxane-α,ωdiol of weight average molecular mass slightly above 1,000,000 (1,395,000) g/mol with rather narrow dispersity (PDI = 1.49) by bulk polymerization in the presence of a conventional catalyst, tetramethylammonium hydroxide (TMAH). This is one of the so-called transient catalysts, which readily decompose into total volatile components when raising the temperature of the reaction mixture above 130 °C leaving a clean and thermally stable polymer without traces of catalyst [15]. Thus, the chemistry underlying this polymer, itself known to be environmentally friendly, is a sustainable one without the production of waste. Raw polymer was analyzed structurally and thermally and its crosslinked form was evaluated for thermal, dielectric and mechanical behaviour compared to a smaller molecular weight homologue obtained in identical conditions and with a commercial silicone elastomer. 2. Experimental 2.1. Materials Octamethylcyclotetrasiloxane, [(CH3)2SiO]4 (D4), purity > 99% (GC) supplied by Fluka was dried over Na wire and freshly distilled before use. Dibutyltin dilaurate (DBTL) 95% and tetraethyl orthosilicate (TEOS) 98% were supplied by Alfa Aesar and used as such. Tetramethylammonium hydroxide solution ~10 wt% in methanol was purchased from Sigma Aldrich. Dimethylformamide (Fluka AG) was dried over calcium hydride and distilled under vacuum before use. The Elastosil films with a thickness of 100 μm were purchased from Wacker Chemie, Germany.
2.3. Procedure 2.3.1. Synthesis of high molecular weight polydimethylsiloxane-α,ω-diols (UHMW-PDMS, PDMS) Into a three-necked reaction vessel were introduced, under nitrogen atmosphere, 0.1 mL of 10 wt% TMAH solution in methanol, after which the vessel was placed in an oil bath heated to 80 °C and connected to the distillation equipment, first under normal pressure (15 min) and then under vacuum (10 mm Hg for 15 min). Then, the reaction vessel was disconnected from the distillation line and a mechanical stirrer and a reflux condenser were attached to it. Over the TMAH remained as a white solid dot on the bottom of the reactor, were added, also under nitrogen atmosphere, first 0.1 mL of DMF and after 5 min 25 mL of D4,
2.2. Measurements Fourier transform infrared (FT-IR) spectra were recorded using a Bruker Vertex 70 FT-IR spectrometer. The experiments were performed in the transmission mode in the range 400–4000 cm−1 at room temperature with a resolution of 2 cm−1 and accumulation of 32 scans. The NMR analysis was performed on a Bruker Avance NEO 400 spectrometer, operating at 400.1 and 79.4 MHz for 1H and 29Si, respectively. 2
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3. Results and discussions
both freshly dried. The reaction mixture was stirred at 80 °C under an inert gas atmosphere. After the first 15 min, the reaction mixture gained consistency of viscous oil, and after 30 min the mechanical stirrer began to encounter spin resistance until 50 min when the viscosity of the reaction mixture stopped increasing, but the mixture was maintained under the same conditions for up to 1.5 h. Thereafter, the reaction mixture was heated for 1 h at 150 °C to decompose the catalyst, then brought to room temperature and vacuumed at 10 mm Hg for 1 h to remove the decomposition products of the catalyst, after that again the temperature is raised to 150 °C and vacuum is applied to remove low molecular species. 3.5 mL liquid was collected by distillation in these conditions, representing 7 wt% volatile fractions. IR spectrum (KBr pellet), νmax (cm−1): 2966 m, 2907vw, 1445vw, 1412w, 1263 s, 1099vs, 1020vs, 868 m, 800vs, 698 m, 669w, 500vw, 410 m; 1H NMR, (400.13 MHz, δ (ppm), CDCl3): 0.07 (s, 3H, CH3). 29Si NMR, (79.4 MHz, δ (ppm), CDCl3): 21.95 (s, Si, (CH3)2SiO); Molecular weight (GPC, elution with chloroform): Mw = 1,395,000 g/mol; PDI = 1.49 (Fig. S1a). Mv = 976,045 g/mol determined with Haug and Meyerhoff relationship: [η] = 8.28 × 10−3 × M0.72 [4,16], where intrinsic viscosity, [η] = 170 dL/g, was determined from the plot of the reduced viscosity, extrapolated to zero concentration in toluene. Zero-shear (apparent) viscosity determined by a standard extrapolation procedure of the linear low shear parts of flow curve of the bulk UHMW-PDMS at 25 °C is 37,600,000 cSt (Fig. S2). A polydimethylsiloxane-α,ω-diol (PDMS) prepared in similar conditions but with higher amount of catalyst, having Mw = 720,100 g/ mol, PDI = 1.874 (Fig. S1b), zero-shear viscosity = 4,580,000 cSt (Fig. S2), was chosen as reference.
3.1. Ultra-high molecular weight polydimethylsiloxane-α,ω-diol (UHMWPDMS) 3.1.1. Polymer synthesis In the context of the known specificity (conferred by the nature of siloxane bonding) of the cyclosiloxanes polymerization, which occurs either under thermodynamic control, at equilibrium, or under kinetic control by living species, in dependence on the nature of the monomer but mainly of the catalyst [17–23] (see more in SI), ammonium and phosphonium hydroxides, also known as transient catalysts, are an excellent choice. They are thermally labile, decompose into volatile products easily removed by simply raising the temperature without requiring neutralization and repeated washing processes with the consumption of large amounts of water and solvents and their release as wastes, as in the case of polymerization catalyzed with alkali metal compounds or acids (sulfuric acid, for example) [22]. Furthermore, when tetramethylammonium hydroxide is used as catalyst, the amount of cycles formed is very small because, according to some opinions [17,19], at low concentrations of the catalyst, the process would be a pseudo-living one, the polymerization degree being approximately inversely related to the catalyst concentration [19]. Comparative studies have shown that tetramethylammonium and tetrabutylphosphonium hydroxides are more effective than potassium hydroxide, due to the greater degree of dissociation of the former and possibly their better solubility in the monomer mass even if the polymerization is conducted at a lower temperature (i.e. 80 °C imposed by its thermal stability). However, this has the disadvantage that, the system is not always homogeneous and undefined carbonate quantities can also be present [24]. In this work, we have chosen as catalyst the tetramethylammonium hydroxide commercially available as a 10% solution in methanol, which allows easier dosing of the very small amount of catalyst required and minimizes the risk of carbonation during handling. The use of the methanolic solution has also the advantage that it is no longer necessary to entrain water as azeotrope with benzene or toluene as is the case for the aqueous solution. Initially, at room temperature, tetramethylammonium hydroxide is insoluble in nonpolar cyclic dimethylsiloxane. As temperature of the reaction medium rises to 80 °C, tetramethylammonium hydroxide melts and acts on SieO bond from D4 forming soluble tetramethylammonium siloxanolate, which becomes the catalyst for further polymerization of cyclosiloxane in homogeneous medium. To end the reaction, the temperature is increased to > 130 °C, and tetramethylammonium siloxanolate decomposes into trimethylamine and methanol, according to some authors [22,25], or dimethyl ether after others [26], easily volatile products which can be removed in vacuum and/or temperature leaving a clean product free of catalyst residues that could affect subsequent behavior of the polymer [25]. The dipolar aprotic solvent N,N-dimethylformamide (DMF), a weak base and hydrogen bond acceptor, has been added in the reaction mixture as a promoter for polymerization. This has the role of complexing the cation of the catalyst and reducing the amount of alkoxide or silanolate aggregates, thus leading to an increased propagation rate of the reaction by involving anionic reagents and has the added benefit of avoiding side reactions [20]. Cations, such as R3NH+, are hydrogenbond donors, but the common interactions to be considered are cationdipole [27]. Different from other reports [24], we have carried out the polymerization in one step, in bulk, with the in-situ formation of the siloxanolate, without its isolation. The possible reactions are shown in Scheme 1, adapted according to Ref. [22]. In the case of a water-free system, it has been reported that propagation by binding of D4, resulting in molecular weight and viscosity increase, occurs especially up to a conversion of about 60–70% cycles. At higher conversion rate, the molecular mass begins to decrease due to redistribution reactions and equilibrium is established. However, these
2.3.2. Crosslinking polydimethylsiloxane-α,ω-diols into soft films (cUHMW-PDMS, cPDMS) 1 g of UHMW-PDMS/PDMS was dissolved in 15 mL toluene with magnetic stirring for 24 h at room temperature. 0.07 mL TEOS and 0.01 mL DBTDL were added and the mixture was further stirred until a homogeneous solution was obtained. The reaction mixture solution was poured on a Teflon substrate and left for 24 h to allow the curing process. However, to complete the crosslinking reactions and to evaporate the volatile by-products, the obtained films of about 150 µm in thickness were deposited at room temperature for two weeks before use. The soluble fractions, Ws, for cUHMW-PDMS, cPDMS and commercial Elastosil were determined by extraction with toluene for 48 h under continuously rotation shaking, the leakage of excess solvent from samples, washing with fresh toluene, draining again excess solvent, after that the samples were dried under vacuum at 80 °C for 48 h. It has been used the following equation: Ws = (mi − mf)/mi * 100 (where mi is the initial weight, and mf is the final weight, after extraction) to calculate soluble fraction percentage, the results being presented in Fig. S3. The swelling behavior of the elastomers with respect to toluene and chloroform was evaluated by immersing the calibrated samples in respective solvents for 24 h and weighing them in swollen state. The swelling ratio, Q, of crosslinked polymers in equilibrium swollen state was calculated using the following equations: mdry
Q: =
ρp
+
(msw − mdry ) ρs mdry ρp
where: mdry is the weight of dry elastomer, msw is the weight of swollen elastomer, ρp is the density of elastomer and ρs is the density of solvent. The obtained values are showed in Fig. S4.
3
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Scheme 1. A picture of the possible reactions in the TMAH-catalysed ring-opening polymerization of D4 [22].
whose size is so large that it does not imply major spectral differences from a linear structure. The IR spectrum (Fig. 1) recorded in the transmittance mode reveals the existence of only the characteristic bands for any linear polydimethylsiloxane structure: 2966, 2907 cm−1 (CeH asymmetric, νas, and symmetric, νs, stretching in SieCH3), 1263 cm−1 (CeH symmetric bending, δs, in Si-CH3), 1099–1020 (SieOeSi asymmetric stretching, νas), 868 cm−1 (asymmetric CeH rocking, δas, in Si(CH3)2), 800 cm−1 (asymmetric SieC stretching in Si(CH3)2), 698 (SieC symmetric stretching in Si(CH3)2) [32–34]. It is known that PDMSs with low viscosities (up to 1000 cSt) have Newtonian behavior, their viscosity remaining constant, regardless of shear rates (up to 10,000 s−1). As viscosity increases, Newtonian's behavior domain is restricted to a certain limit of gradient velocity. Beyond this value, PDMS becomes shear thinning, i.e. the apparent viscosity decreases below the real one with strain-rate and their ratio is no longer constant. This behavior is called “pseudoplastics” and is perfectly reversible. Critical shear rate (s−1) decreases as the real viscosity of the silicone rises [11]. Thus, high molecular weight
studies were conducted on D4 systems and hexamethyldisiloxanes [22]. In our case, in the presented conditions, after 1.5 h, the volatile fraction constituted 7 wt% and the GPC chromatogram of the remaining polymer indicated an ultra-high molecular weight with narrow dispersity without small molecular species (Fig. S1a). 3.1.2. Structural and rheological characterization In the 1H NMR spectrum (Fig. 1), a single peak is present at 0.07 assigned to the protons in the dimethylsiloxane groups. Due to the very long chains, there are no visible peaks corresponding to the final dimethylsiloxane groups to which the OH groups (about 1.2 ppm) and H in the OH (3.7–3.8 ppm [28]) are attached. A single peak also, that at 21.95 ppm attributed to the silicon atoms in the dimethylsiloxane units on the chain, is present in 29Si NMR spectrum (Fig. 1). The NMR spectra do not indicate the presence of detectable cyclic species, which in 1H NMR spectrum should show a peak at 0.095 ppm (D4) [29] or 0.168 ppm (D3) [30], while in 29Si NMR it should show peaks in the region from −19.4 to −19.7 [6,31]. However, this does not permanently exclude the existence of cyclic species. There may be cycles but 4
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Fig. 1. NMR and FTIR spectra for UHMW-PDMS.
the hypothesis regarding the high purity of the synthesized compound and corresponds to the expectation that degradation in the inert atmosphere consists of depolymerization with the formation of low molecular weight cyclic and linear volatile oligomers [39]. The thermostability of UHMW-PDMS is slightly lower as compared with lower molecular weight PDMS, which show Ton 426 °C and Tpeak 483 °C. The DSC curves (Fig. 2 right) indicate clear crystallization processes (−78.9 °C) during cooling, as well as glass transition (−122.5 °C) and melting (−38.9 °C) during heating, processes which, under the applied measurement conditions, are reproduced in successive cooling/heating cycles (Table 1). Logically, the Tg and Tc values are slightly lower and Tt slightly higher than in the lower molecular weight homologue PDMS.
polymers show dramatic deviations from Newtonian behavior at high shear rates due to the orientation and/or the flow mechanism involving smaller jump units [35]. The flow curve recorded for our polymer UHMW-PDMS fit well Bird-Carreau model [36] and indicates Newtonian behavior on a fairly narrow shear rate range of 0.01–0.0316 s−1, almost half the corresponding range for PDMS with lower molecular weight, while zero shear rate viscosity is 37,600,000 cSt for UHMWPDMS and only 4,580,000 cSt for PDMS (Fig. S2). Molecular weight of the polymer was first estimated by viscometric measurements in solution using successive dilution method. The inherent viscosity thus measured was 170 dL/g and the molecular weight calculated according to Ref. [16] was 976,045 g/mol, which, as is normal, is within the estimated Mn and Mw values by GPC. However, the best method for determining the average molecular weight of polymers and its distribution is GPC, which provides key information to predict the processability and material properties because these parameters control many physical properties. Thus, increasing Mw leads to increase of strength, toughness, brittleness, melt viscosity, chemical resistance and decreased solubility. However, the drastic reduction in polydispersity makes polymers more difficult to process. The chloroform eluting curve displayed by the refractive index (RI) detector (Fig. S1a) indicate considerable value of the average molecular weight relative to polystyrene standards, Mw = 1,395,000, and a polydispersity index, PDI = 1.490.
3.1.4. Raw polymer morphology Dynamic light scattering (DLS) was used to determine the particle size distribution of the polymer in solution and the average particle diameter. Measurements were made on solutions prepared by successive dilutions in both chloroform and toluene, the average values estimated by extrapolation at the near zero concentration being 3.3 and 5.0 µm, respectively (Fig. 3). The lower values of the aggregates in the case of the chloroform are due to its higher ability to dissolve the PDMS, despite the fact that it has a solubility parameter (9.2 cal1/2 cm−3/2) more different from that of the PDMS (7.3 cal1/2 cm−3/2) than toluene (8.9 cal1/2 cm−3/2) [41]. This mismatch is explained by the polarity of the solvent (µCHCl3 = 1.0 D as compared to only 0.4 D for toluene), which is found as polar component δp, together with the dispersion component δd, and hydrogen-bonding forces δh, in the Hansen's total solubility parameter (HSP) δt (δt2 = δp2 + δd2 + δh2) [41,42] The closer the distributions of the solubility parameter components in the total solubility parameter of the polymer and solvent are, the more the polymer (in this case PDMS) will swell more [41]. The degree of similarity between the HSP of the polymer and the solvent is expressed as the factor Ra (distance in Hansen space) calculated with the relation: [42,43]
3.1.3. Thermal analysis According to the most of the literature sources [37,38], polydimethylsiloxanes are stable under high vacuum or in inert atmosphere up to least 350–400 °C, the thermostability being negatively affected by factors including acidic or basic impurities, oxygen, water, solvent, fillers, and residual catalyst but also end-group functionality [38,39]. Depolymerization at high temperature can occur by three mechanisms: unzipping mechanism, random scission and impurity catalyzed mechanism [37]. It has been found that, while at lower molecular mass predominates the unzipping mechanism, with the increase of the molecular mass, intramolecular and intermolecular redistributions, which have the effect of lowering the thermal stability, become predominant [40]. The silanol-terminated polydimethylsiloxanes are less stable as compared with trimethylsilyl-terminated homologues because the terminal hydroxyl groups can participate in a ‘‘back-biting’’ reaction [39]. As the thermogravimetric analysis data show (Fig. 2 left, Table 1), UHMW-PDMS begins to decompose at 405 °C losing almost the entire mass (97.08%) in a single step around 456 °C. This behavior supports
Ra =
4(δdp − δds )2 + (δpp − δps )2 + (δhp − δhs )2
with the values δp, δd, δh for PDMS: 15.9, 0.1, 4.7; for toluene: 18.0, 1.4, 2.0; and for chloroform: 17.8, 3.1, 5.7, respectively [41,42]. For the case of PDMS-toluene and PDMS-chloroform pairs, Ra's values are 5.2 and 4.9 respectively, which indicates a higher degree of swelling in chloroform because the lower this value, the higher the degree of swelling [42]. Higher value of the PDMS-CHCl3 interaction parameter (0.470) compared to PDMS-toluene (0.465) [44] also supports the 5
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Fig. 2. Thermograms (left) and DSC traces (right) comparatively presented for: (a)-UHMW-PDMS; (b)-PDMS; (c)-cUHMW-PDMS; (d)-cPDMS; (e)-Elastosil.
analysis XRR of thin films of PDMS melt deposited on hydroxylated silicon substrates, as “a heap of spaghetti” [46]. A more detailed analysis of the sample (Fig. 5(d–f), image size of 10 × 10 µm2) has revealed, besides the horizontal belt-shaped structures, a prevailing conelike morphology. The surface's topography mainly consists of cones with quite different sizes, with the basis laying on the glass surface. As a result of the analysis of AFM micrographs, the in-plane projection diameter ranges from 20 to 200 nm, while the height ranges from 8 to 16 nm. We assume that these cone-like structures are the vertical variants of either macromolecules or smaller belts compressed in the form of a cone under the action of its own weight. Both belt-shaped and conelike structures are also highlighted in the AFM phase image (Fig. 5f). By using the grains analysis function from image processing software (Nova), the average size (diameter) of cone-like structures was estimated to 55 nm close to the value (46.6 nm) revealed by the DLS analysis at high concentrations in toluene (Fig. S5a). Therefore, we can state that the AFM measurement results fit well to DLS results (Fig. S5). A more detailed AFM analysis is presented in Fig. 6, which shows a 2D AFM topography image (size of 1 × 1 µm2) and the topographic cross sections along and across a belt-shaped structure. In topographical AFM image there are visible four adjacent thin belts, which look like backbones. From topographic profile across the belt, a width of about 50 nm (taking into account the curvature radius of the AFM tip – 30 nm) and height of about 8 nm can be estimated. The topographic profile along the belt shows that the macromolecular chain is an ordered structure consisting of multiple collinear “chain-links”, as those illustrated in Fig. 4 – right. The thicker and broader belts shown in Fig. 5 can be a bundle of multiple thin belts, as those described above, or can be an individual chain-belt structure containing more coplanar chain-links connected together, as those illustrated in Fig. 4 – left. One of the most important fundamental properties of polysiloxane family is the highly pronounced inherent conformational flexibility of their completely inorganic main-chain backbones, O[Si-O]n, which enables the unusually high mobility of their segments and entire molecules [4,46]. But the arrangement of the molecules on the substrate is determined by the ratio between the entropic factors associated with the molecular conformation and the energy interactions with the surface [46], depending on it the packaging or arrangement of the molecules could be from random coil to complete nematic packing. This may explain the differences between the conformations highlighted by the two techniques, TEM and AFM. While for the TEM, the sample was deposited on a non-polar surface, the AFM sample was deposited on the glass which, having hydroxyl groups on the surface, allowed interaction with the molten polymer molecules [46]. Monolayer studies of PDMS have indicated that, depending on the forces developed at the interface, the molecule may adopt a conformation from “flat” to more collapsed, with successive coiling of polymer chains into a helical conformation.
Table 1 Details of the thermal decomposition stages and the thermal transitions identified by thermogravimetric analysis and differential scanning calorimetry, respectively. Sample
Thermogravimetric analysis data
DSC data
Ton
Tpeak
Tend
Mass change, wt%
% rez at 700 °C
Tg, oC
Tc, oC
Tt, oC
UHMWPDMS Step I Step II
30
–
520
97.50
1.25
−122.52
−78.87
−38.90
30 405
37 456
67 520
0.42 97.08
PDMS Step I Step II Step III
39 39 427 516
– 44 483 556
591 75 516 591
96.48 0.68 75.98 19.82
2.95
−120.24
−74.96
−42.54
cUHMWPDMS Step I Step II Step III Step IV
35
–
580
99.75
0.00
−122.64
−68.06
−41.17
35 323 380 466
40 333 417 552
72 380 434 580
3.41 4.63 12.73 78.98
cPDMS Step II Step III
38 38 402
– 46 501
590 83 590
92.20 1.28 90.92
7.64
−121.96
−69.07
−39.02
Elastosil Step I Step II Step III
27 27 388 503
– 47 442 508
700 91 470 528
81.41 3.15 76.24 2.02
18.41
–
–
–
higher swelling capacity in chloroform than in toluene. Obtaining a PDMS of the ultra-high molecular mass, which means large molecules, our idea was to try to visualize such a molecule, mainly using the imaging techniques TEM and AFM. For this, the solution of the polymer was deposited on the carbon-coated copper grid for TEM and on the glass substrate for AFM analysis, the acquired images being shown in Figs. 4–6. TEM images (Fig. 4) indicate macromolecular chains with multiple knots or few entangled chains (thread-like structure) whose diameter is estimated to about 10 nm. Taking into account that the width of the PDMS chain is about 0.7 nm, it is clear that these macromolecular structures are crowded from several PDMS wrapped chains [45]. The AFM image of the polymer deposed on glass substrate, this time from toluene solution, (Fig. 5(a–c), image size of 20 × 20 µm2) clearly indicates the presence of multiple belt-shaped structures with length ranging from 3 to 5 µm, width ranging from 300 to 800 nm, while the height is between 8 and 24 nm. The belt-shaped structure corresponds to that described in literature, based on synchrotron X-ray reflectivity 6
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Fig. 3. Dependence of mean particle diameter estimated by DLS depending on the concentration of UHMW-PDMS polymer solution in: (a) - chloroform; (b) - toluene.
the helical conformation is due to the tendency of the SieO dipole to perform intermolecular compensation, and this is sustained by changes in the dipole moment with increasing molecular weight. Many physical properties can be explained due to helical conformation (hydrophobicity, compressibility, surface properties, etc.) [47]. 3.2. Super-soft silicone elastomer 3.2.1. Crosslinking, extraction and swelling with/in organic solvents The UHMW-PDMS thus synthesized was processed as film and crosslinked by treating in solution (in toluene) with an excess of TEOS as a crosslinking agent in presence of DBTDL as a catalyst. The crosslinking pattern through the very long chain ends (Scheme 2) logically leads to a low cross-link density and increased probability of remaining free chains reflected by higher soluble fraction found, 9.05% versus 6.25% for PDMS with the same crosslinking pattern but with smaller molecular weight or 1.15% for the commercial elastomer based on another crosslinking system (Fig. S3).
Fig. 4. TEM image of the fingerprint left by the solution of polymer in chloroform on copper grid solution.
The 6/1 helix structure was confirmed to be the lowest energy conformation in the crystalline state of PDMS, while a more extended helical structure was suggested by solid state NMR study [2]. Adoption of
Fig. 5. AFM images for the polymer remained on glass substrate after solvent evaporation from the polymer solution in toluene. 7
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Fig. 6. AFM images for the polymer and topographic cross sections along and across a belt-shaped structure.
The swelling measurements made on crosslinked polymers reveal swelling ratio (Q) values higher in chloroform (23.9 for cUHMW-PDMS and 14.2 for cPDMS) than in toluene (19.3 for cUHMW-PDMS and 11.8 for cPDMS) (Fig. S4). In both chloroform and toluene, c-UHMW-PDMS that is based on the higher molecular weight polymer swells more strongly than c-PDMS. Elastomeric films were analyzed for thermal, dielectric and mechanical behaviors compared to those based on lower molecular weight polydimethylsiloxane and a commercial elastomer. By crosslinking, thermal stability is significantly reduced as compared with raw polymers, UHMW-PDMS and PDMS (Fig. 2 left). The crosslinking pattern could also be the cause of this, but also the prevalent depolymerization mechanism. The presence in the material of the acid catalyst, DBTDL, used for the cross-linking process could facilitate breakage of the siloxane bond. Due to the rare crosslinks, the DSC curves are very similar to those of raw polymers, while the commercial elastomer does not show any transition in the analyzed field.
Fig. 7. Dielectric permittivity in dependence on frequency comparatively showed for the two silicone elastomers prepared in lab, cUHMW-PDMS and cPDMS, and commercial one.
3.2.2. Dielectric behaviour The dielectric spectrum (Fig. 7, Table 2) of the cUHMW-PDMS reveals slightly higher values for the dielectric permittivity across the entire range of frequencies studied, when compared with Elastosil. This increase of dielectric permittivity could have two causes: (a) the high degree of physical crosslinking by inter- and intramolecular entanglement of long chains between the chemical crosslinking nodes, which could reduce the free volume; (b) the presence of non-condensed Si-OH groups formed by the hydrolysis of excess cross-linking agent. In the case of reference cPDMS with shorten chains, these values are even slightly higher.
3.2.3. Mechanical behaviour The stress-strain curves of the crosslinked UHMW-PDMS processed as films were recorded and presented in Fig. 8 side by side with those obtained in similar conditions for cPDMS and Elastosil. The films show common elastomer stress-strain behaviour with a steeper slope at low deformations, a rubbery domain and a strain hardening region at high strains. The sample cUHMW-PDMS shows the highest strain at break up to 1337% (Fig. 8a), while cPDMS and Elastosil show elongations up to 836% and 568%, respectively (Fig. 8b and c). The average Young’s
Scheme 2. The crosslinking pathway used for UHMW-PDMS and PDMS. 8
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Table 2 Main dielectric and mechanical parameters of the analyzed silicone elastomers. Sample cUHMW-PDMS cPDMS Elastosil
ε' at 104 Hz, 25 °C 2.61 2.62 2.53
ε'' at 104 Hz, 25 °C −4
5.6 ∙ 10 8.4 ∙ 10−4 2.6 ∙ 10−3
Maximum elongation at break, %
Young modulus, MPa
1336 840 568
0.3 0.3 1.1
Fig. 8. Stress strain curves for: (a) cUHMW-PDMS; (b) cPDMS; (c) commercial elastomer and the two synthetized elastomers.
the case of cPDMS sample, as the number of cycles increase, a stress relaxation process occurs. For example, the maximum stress for the first cycle was 0.20 MPa, while for the last cycle the maximum stress was 0.17 MPa. This behavior is less evident for the cUHMW-PDMS and Elastosil samples. The stress relaxation process is time-dependent and is closely related to the elastomer microstructure. The relaxation processes occur mainly due to the disentanglement of physical nodes, viscous flow, bond interchange or chain scission. Since the cPDMS elastomer has shorter molecular chains, consequently a higher chemical cross-linking density, a better mechanical behavior would have been expected as compared with the cUHMW-PDMS elastomer. Surprisingly, on the contrary, the latter showed superior mechanical properties. It is assumed that the very long macromolecular chains of the UHMW-PDMS polymer are highly tangled, generating numerous physical entanglements that act as physical cross-linking nodes. Moreover, the sol
modulus values determined from the linear part of the stress-strain curves indicate that the prepared in lab silicones have similar Young’s modulus, around 0.3 MPa, while the commercial film is more rigid having a modulus of about 1.1 MPa. The high strains and low Young’s modulus found for cUHMW-PDMS and cPDMS can be attributed to the very long molecular chains correlated with the cross-linking system through the chains ends. In order to evaluate the viscoelastic behaviour, the elastomeric films were subjected to 100 repeated stress-strain cycles (Fig. 9). After the firs cycle, a stress-softening behavior known as Mullins effect is observed. This phenomenon is typical for elastomers and normally appears after the first elongation cycle due to the rearrangement of the macromolecular chains. In Fig. 9d–f is presented the variation of the applied force at 100% strain for each stress-strain cycle. It can be noticed that, especially in
Fig. 9. Cyclic stress-strain curves (a–c) and the corresponding stress relaxation (d–f) for: cUHMW-PDMS (a and d); cPDMS (b and e) and Elastosil (c and f), respectively. 9
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Fig. 10. Cyclic stress-strain curves of: (a) cUHMW-PDMS; (b) cPDMS; (c) Elastosil, and the corresponding stress relaxation (d–f) at 500% cyclic strain.
Elastosil reference sample seems to show a better recovery as compared with cUHMW-PDMS. The cPDMS sample presents the highest mechanical losses, after 100 cycles recovering only 92% of its original length (Fig. 11a). Moreover, it is worth noting that, after 10 cycles within 0 – 500% strain, the cUHMW-PDMS film has an elastic recovery of about 99%, while the Elastosil and cPDMS samples failed (Fig. 11b). The elastic recovery and stress decay values evaluated in various testing conditions (1, 10, 100 cycles at 100 and 500% strain) centralized in Table 3 for cUHMW-PDMS and the two references clearly reveal the performance and limits of the former.
fraction of cPDMS elastomer is higher as compared with cUHMWPDMS. The free molecular chains will move past one another and will increase the viscous dissipation. Therefore, the elastomer based on ultra-long molecular chains will provide superior mechanical properties. Furthermore, when the elastomers are subjected to 10 stretching cycles at 500% (Fig. 10), only cUHMW-PDMS film did survived to this experiment and behaves almost purely elastic (Fig. 10d), while cPDMS and Elastosil films failed after the first and third cycles, respectively (Fig. 10e and f). As expected, the Mullins effect is more pronounced when the elastomers are subjected to repeated stress-strain cycles up to 500% strain (Fig. 10a and b). Excluding the Mullins effect, the cyclic tests reveal that at both 100% and 500% strain, the cUHMW-PDMS film shows very small hysteresis loops indicating low viscoelastic losses (Figs. 9c and 10c). Fig. 11a shows the elastic strain recovery of tested materials when subjected to 100 stress-strain cycles within 0 – 100% strain. Up until 50 cycles, the cUHMW-PDMS film has the best elastic recovery, of about 96.3%, this being greater than for the two reference samples, cPDMS and Elastosil, which have an elastic recovery of 93.2% and 96.1%, respectively (Fig. 11a). However, after 50 cycles, the
4. Conclusions Tetramethylammonium hydroxide is a valuable catalyst for a clean procedure to polymerize especially cyclosiloxanes to very high molecular masses. The separated polymer is one of narrow molecular weight dispersity, free of catalyst and cyclic compounds, as GPC and spectral analyses showed. The zero-shear viscosity was estimated at 37,600,000 cSt and, as a result, the Newtonian field is very narrow. Both isolated and aggregated macromolecules were emphasized by DLS and imaging TEM and AFM techniques. In thin films the conformation
Fig. 11. Elastic recovery of cUHMW-PDMS as compared with Elastosil and cPDMS during stress-strain cycles within 0 – 100% strain (a) and elastic recovery of cUHMW-PDMS during stress-strain cycles within 0 – 500% strain (b). The solid lines are guides to the eye; the first stress-strain cycle in which the Mullins effect is manifested is not plotted. 10
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Table 3 Elastic recovery and stress decay of cUHMW-PDMS, cPDMS, and Elastosil. Sample
Test elongation %
Elastic recovery (1 cycle) %
Elastic recovery (10 cycles) %
Elastic recovery (100 cycles) %
Stress decay (1 cycle) %
Stress decay (10 cycles) %
Stress decay (100 cycles) %
cUHMW-PDMSr*
100 500
98.9 99.6
97.3 99.5
95.4 nd
0.9 1.5
2.7 1.8
6.4 nd
cPDMS*
100 500
98.7 98.9
96.1 Failed
91.6 nd
1.4 1.6
6.4 Failed
11.9 nd
Elastosil*
100 500
100 Failed
97.6 Failed
96.6 nd
0.7 Failed
4.2 Failed
6.1 nd
* The first stress-strain cycle, where the Mullins effect is manifested, was ignored; nd - not determined.
of the macromolecules depends on the nature of the substrate. Thus, while on hydrophobic substrate there is a tendency to disperse at the molecular level as tangled chains, on a polar support (i.e., ordinary glass) they arrange like both vertical cones and belt-shaped horizontal structures. High length of the chains but also crosslinking adversely affect thermal stability. The latter, along with dielectric permittivity, are slightly lower than those of cPDMS but much better than commercial elastomer. The crosslinked films swell more strongly in chloroform than in toluene. The cUHMW-PDMS showed a strain at break up to 1337%, much higher than that of the cPDMS and the commercial elastomer of 836% and 568%, respectively, taken as references, while the average Young’s modulus value (determined from the linear part of the stress-strain curves) is around 0.3 MPa, similar as for cPDMS but much lower than 1.1 MPa found for commercial elastomer. The modulus of elasticity is close to the values (generally less than 0.1 MPa) that characterize ultra-soft silicones of interest, for example in substrate cell culture for mimicking biological stress–strain behaviour [48–50]. At 100% elongation, elastic recovery and stress decay values are comparable for each of the three analyzed samples. However, at higher elongation (500%), the two samples taken as a reference fail during either the first elongation cycle (Elastosil) or 3rd cycles (cPDMS), while the cUHMW-PDMS sample shows an elastic recovery of 99%. Values are better even than those reported for ExSil100 Resin, a commercial elastomer presented as having a 5000% elongation [51,52].
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Acknowledgements This work was supported by a grant of Romanian Ministry of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4ID-PCCF-2016-0050 (5DnanoP), within PNCDI III. Grateful thanks to dr. Alina Nicolescu for NMR spectra and dr. Cristian-Dragos Varganici for thermal analysis. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109243. References [1] Z. Kőkuti, K. van Gruijthuijsen, M. Jenei, G. Tóth-Molnár, A. Czirják, J. Kokavecz, P. Ailer, L. Palkovics, A.C. Völker, G. Szabó, Appl. Rheol. 24 (2014) 63984. [2] C. Kim, M.C. Gurau, P.S. Cremer, H. Yu, Langmuir 24 (2008) 10155. [3] E. Maaskant, K. Tempelman, N.E. Benes, Eur. Polym. J. 109 (2018) 214. [4] E.A. Büyüktanir, Z. Küçükyavuz, J. Polym. Sci. Part B Polym. Phys. 38 (2000) 2678. [5] P.R. Dvornic, J.D. Jovanovic, M.N. Govedarica, J. Appl. Polym. Sci. 49 (1993) 1497. [6] A.C.M. Kuo, In Polymer Data Handbook, Oxford University Press, 1999, pp. 411–435. [7] F. Normand, X.W. He, J.M. Widmaier, G.C. Meyer, J.E. Herz, Eur. Polym. J. 25 (1989) 371. [8] J.G. Zilliox, J.E.L. Roovers, S. Bywater, Macromolecules 8 (1975) 573. [9] P.J. Flory, L. Mandelkern, J.B. Kinsinger, W.B. Shultz, J. Am. Chem. Soc. 74 (1952) 3364. [10] Technology of Pressure-Sensitive Adhesives and Products; Benedek, I.; Feldstein, M., Eds.; CRC Press: London, 2009.
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From ultra-high molecular weight polydimethylsiloxane to super-soft elastomer Codrin Tuguia, Vasile Tironb, Mihaela Dascalua, Liviu Sacarescua, Maria Cazacua, a b
T
⁎
Department of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487 Iasi, Romania Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I no. 11, Iași 700506, Romania
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high molecular weight Anionic polymerization Soft elastomer Polydimethylsiloxane Sustainable chemistry
A polydimethylsiloxane-α,ω-diol (UHMW-PDMS) with unusually high molecular weight (Mw = 1,395,000 g/ mol) and narrow polydispersity (PDI = 1.49), free of monomer or catalyst traces, was obtained in high yield by a scalable process, easy to achieve (bulk anionic ring-opening polymerization of octamethylcyclotetrasiloxane catalyzed by tetramethylammonium hydroxide). Spectral and thermogravimetric analysis confirmed the theoretical structure, purity and thermal stability of the polymer. Individual or aggregated macromolecules were assessed by dynamic light scattering (DLS) and visualized by microscopic methods (TEM, AFM). The mechanical, dielectric, and thermal performances of application interest for this polymer were evaluated on samples of chemically defined elastomer cUHMW-PDMS prepared by condensation crosslinking through the polymer chain ends with tetraethylorthosilicate. The results were compared with those obtained on a lower molecular weight (Mw = 720,100 g/mol, PDI = 1.87) homologue (PDMS) prepared and processed in identical conditions, but also with a commercial silicone elastomer. UHMW-PDMS-based elastomer shows superior results on elastic recovery and stress decay at high elongation, as compared with reference samples.
1. Introduction Silicones, with the main representative member polydimethylsiloxane, although well-known since mid 20th century, still enjoy a huge interest, both in terms of fundamental research and science applied in most branches of modern industry [1]. Silicones have a unique combination of features: good surface wetting and spreading capabilities, long-term elasticity, flexibility, weather resistance, stability under challenging conditions (extreme temperatures, thermal shock, chemicals and oxidation), resistance to aging, chemical and biological inertness, water repellence, strong adhesion properties, excellent dielectric properties. As a result, they can perform a wide range of functions, from adhesives and sealants, antifoams, surfactants, to rubbers, waxes, gels, resins, etc. Because of their special properties and the vast range of applications, polysiloxanes and especially polydimethylsiloxanes, both the cyclic and linear molecular chains, have stimulated over the years the interest of researchers for their study in all aspects: structure and chemical, physical, thermal, mechanical, rheological, tribological, etc. behaviors. Silicones are invaluable tools for developing sustainable and durable materials. Their versatile chemistry allows them to be structurally customized and engineered into a wide variety of shapes and properties
⁎
to meet a wide range of requirements [2,3]. This can be accomplished by manipulating the length of the macromolecular chain, the crosslinking pattern, the addition of fillers, by their chemical modification in the sense of associating with co-chained, grafted, interpenetrated organic partners, or by addition of organic groups to give them specific functions (surfactant, sensor, ligand, etc.). Polydimethylsiloxanes are produced in a wide range of molecular masses, with reported values from a few hundred to even more than a million Da, polymers close to this limit are considered ultra-high molecular weights (UHMWs) [4–10]. Polydimethylsiloxanes are relatively low in viscosity, flowing even at very high molecular masses, in comparison with their organic polymers counterparts, due to the flexibility of the chain and the extremely weak intermolecular forces. Virtually they do not have the energy barrier for rotation, resulting in polymers with the lowest transition temperature [10,11]. They are colorless, transparent fluids whose viscosity can vary, depending on the length of the chain, from 0.65 cSt for the shortest member of the series (hexamethyldisiloxane), which is thinner than water (0.89 cSt), to several tens of millions cSt (e.g. 50,000,000 cSt [12]) when viewed as semisolid but without dimensional stability over time. At molecular masses greater than 30,000, the entanglement of the chains begins to occur resulting in the leveling of changes in physical properties in dependence
Corresponding author. E-mail address: [email protected] (M. Cazacu).
https://doi.org/10.1016/j.eurpolymj.2019.109243 Received 3 April 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 07 September 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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The NMR spectra were recorded with a 5 mm direct detection, four nuclei (1H, 13C, 19F and 29Si) probe. In the 29Si NMR spectrum, the broad signal centered at −112.20 ppm is the background signal from the glass of the NMR tube and probe. Steady-State Flow curves of bulk PDMS samples were recorded with the RHEOPLUS MCR501 equipment, in the range 0.001–100 s−1, 25 °C, in the system cone-plate, d = 0.05 mm. Gel permeation chromatography (GPC) analysis was performed in eluent chloroform at 25 °C and at a flow rate of 1 mL/min using a WGE SEC/GPC chromatograph equipped with four in-line detectors (RI, VI, UV, MALS) and two Agilent PLgel 5 μm columns, MIXED D and MIXED C capable to separate molecular weights within 200 to 200,0000 g/mol. The molecular weights were calculated by the standard calibration method. For this purpose, ten polystyrene standards (Agilent) having the polymer nominal Mp within 1000 to 200,0000 g/ mol were used. The calibration curve log (Mp) versus elution volume was fitted to a third order polynomial equation. The size and distribution of aggregates in chloroform solution was determined by dynamic light scattering (DLS) using a Malvern Instruments Autosizer LoC 7032 Multi-8 Correlator (Malvern Instruments, UK). The solution previously filtered through 0.5 µm filter, without further dilution was irradiated with red light (HeNe laser, wavelength λ = 632.8 nm) and the intensity fluctuations of the scattered light analysed to obtain an autocorrelation function. The transmission electron microscopy (TEM) analysis was performed with Hitachi High-Tech HT7700 equipment operated at 100 kV accelerating voltage in high-contrast mode. The polymer solution in chloroform was placed on carbon-coated copper grids with 200-mesh size and then the solvent was removed under vacuum. The morphology of UHMW-PDMS polymer was studied by means of Atomic Force Microscopy (AFM), with a NT-MDT Solver Pro-M equipment. All the AFM measurements were carried out in air (solvent was removed under vacuum condition), at room temperature, using the same cantilever (NSG01 from NT-MDT) in a non-contact mode to avoid the damage of the polymer’s surface. The AFM images were processed using Nova software from NT-MDT. Thermogravimetric (TG) measurements were conducted on a STA 449 F1 Jupiter device (Netzsch, Germany). About 10 mg of each sample was weighed and heated in alumina crucibles. Nitrogen was purged as inert atmosphere at a flow rate of 50 mL min−1. Samples were heated from 25 °C to 700 °C at a heating rate of 10 °C min−1. DSC measurements were conducted with a DSC 200 F3 Maia (Netzsch, Germany). About 10 mg of sample was heated in pressed and punched aluminium crucibles at a heating rate of 10 °C min−1. Nitrogen was used as inert atmosphere at a flow rate of 100 mL min−1. Dielectric measurements were carried out in normal conditions on a Novocontrol Concept 40 apparatus, in the frequency range of 100–106 Hz. The tensile test experiments were performed using an Instron 3365 machine with a crosshead speed of 20 mm min−1. Dumbbell-shaped specimens with a gauge width of 4 mm and a gauge length of 50 mm were prepared using a cutting press. Three independent tests were performed for each film. The elastic recovery and stress decay were determined from 100 cycles stress-strain within 0–100% strain and 10 cycles within 0–500%. The stationary time at maximum and minimum strain was 2 s.
on viscosity [11]. In general, the polydimethylsiloxane-α,ω-diols currently provided by the silicones suppliers are of moderate molecular weights, up to the order of several hundred thousand. They are available both as such and processed in various forms: oils, greases, rubbers, resins. Of these basic silicone grades, thousands of unique silicone products have been designed to meet the needs of designers, manufacturers, people and municipalities around the world. Besides these, polydimethylsiloxanes are used extensively in the plastics industry as additives for improving the processing and surface properties of plastics. In this aim, mainly low molecular weight PDMS (viscosity < 1000 cSt) are used as external additives. However, by incorporating higher molecular weight PDMS (viscosities ranging from 10,000 to 60,000 cSt) as internal additive, their external application is avoided, and processing advantages are provided in addition to surface property improvement. A recent embodiment in the field of additives is the use of PDMS of ultra-high molecular mass, with viscosities between 10 × 106 and 50 × 106 cSt, which allows for increased loading in the additive from 20 to 50% and eliminates the “bleed-out” phenomenon on the surface that happens in the case of small molecular PDMS [13]. Commercial Super-High Viscosity Pure Silicone Fluids are linear with viscosities ranging from 300,000 to 20,000,000 cSt, characterized by their high damping action, excellent lubricity, inertness to plastics, rubbers and metals, high dielectric strength, high resistance to shear, thermal stability, wide service temperature range and low viscosity change at temperature. They are used in a wide range of military and industrial applications [14]. Although they are of great practical importance, in the field of silicone research, studies on polymers of extra-high molecular mass are still rare. In this paper, it is reported obtaining a polydimethylsiloxane-α,ωdiol of weight average molecular mass slightly above 1,000,000 (1,395,000) g/mol with rather narrow dispersity (PDI = 1.49) by bulk polymerization in the presence of a conventional catalyst, tetramethylammonium hydroxide (TMAH). This is one of the so-called transient catalysts, which readily decompose into total volatile components when raising the temperature of the reaction mixture above 130 °C leaving a clean and thermally stable polymer without traces of catalyst [15]. Thus, the chemistry underlying this polymer, itself known to be environmentally friendly, is a sustainable one without the production of waste. Raw polymer was analyzed structurally and thermally and its crosslinked form was evaluated for thermal, dielectric and mechanical behaviour compared to a smaller molecular weight homologue obtained in identical conditions and with a commercial silicone elastomer. 2. Experimental 2.1. Materials Octamethylcyclotetrasiloxane, [(CH3)2SiO]4 (D4), purity > 99% (GC) supplied by Fluka was dried over Na wire and freshly distilled before use. Dibutyltin dilaurate (DBTL) 95% and tetraethyl orthosilicate (TEOS) 98% were supplied by Alfa Aesar and used as such. Tetramethylammonium hydroxide solution ~10 wt% in methanol was purchased from Sigma Aldrich. Dimethylformamide (Fluka AG) was dried over calcium hydride and distilled under vacuum before use. The Elastosil films with a thickness of 100 μm were purchased from Wacker Chemie, Germany.
2.3. Procedure 2.3.1. Synthesis of high molecular weight polydimethylsiloxane-α,ω-diols (UHMW-PDMS, PDMS) Into a three-necked reaction vessel were introduced, under nitrogen atmosphere, 0.1 mL of 10 wt% TMAH solution in methanol, after which the vessel was placed in an oil bath heated to 80 °C and connected to the distillation equipment, first under normal pressure (15 min) and then under vacuum (10 mm Hg for 15 min). Then, the reaction vessel was disconnected from the distillation line and a mechanical stirrer and a reflux condenser were attached to it. Over the TMAH remained as a white solid dot on the bottom of the reactor, were added, also under nitrogen atmosphere, first 0.1 mL of DMF and after 5 min 25 mL of D4,
2.2. Measurements Fourier transform infrared (FT-IR) spectra were recorded using a Bruker Vertex 70 FT-IR spectrometer. The experiments were performed in the transmission mode in the range 400–4000 cm−1 at room temperature with a resolution of 2 cm−1 and accumulation of 32 scans. The NMR analysis was performed on a Bruker Avance NEO 400 spectrometer, operating at 400.1 and 79.4 MHz for 1H and 29Si, respectively. 2
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3. Results and discussions
both freshly dried. The reaction mixture was stirred at 80 °C under an inert gas atmosphere. After the first 15 min, the reaction mixture gained consistency of viscous oil, and after 30 min the mechanical stirrer began to encounter spin resistance until 50 min when the viscosity of the reaction mixture stopped increasing, but the mixture was maintained under the same conditions for up to 1.5 h. Thereafter, the reaction mixture was heated for 1 h at 150 °C to decompose the catalyst, then brought to room temperature and vacuumed at 10 mm Hg for 1 h to remove the decomposition products of the catalyst, after that again the temperature is raised to 150 °C and vacuum is applied to remove low molecular species. 3.5 mL liquid was collected by distillation in these conditions, representing 7 wt% volatile fractions. IR spectrum (KBr pellet), νmax (cm−1): 2966 m, 2907vw, 1445vw, 1412w, 1263 s, 1099vs, 1020vs, 868 m, 800vs, 698 m, 669w, 500vw, 410 m; 1H NMR, (400.13 MHz, δ (ppm), CDCl3): 0.07 (s, 3H, CH3). 29Si NMR, (79.4 MHz, δ (ppm), CDCl3): 21.95 (s, Si, (CH3)2SiO); Molecular weight (GPC, elution with chloroform): Mw = 1,395,000 g/mol; PDI = 1.49 (Fig. S1a). Mv = 976,045 g/mol determined with Haug and Meyerhoff relationship: [η] = 8.28 × 10−3 × M0.72 [4,16], where intrinsic viscosity, [η] = 170 dL/g, was determined from the plot of the reduced viscosity, extrapolated to zero concentration in toluene. Zero-shear (apparent) viscosity determined by a standard extrapolation procedure of the linear low shear parts of flow curve of the bulk UHMW-PDMS at 25 °C is 37,600,000 cSt (Fig. S2). A polydimethylsiloxane-α,ω-diol (PDMS) prepared in similar conditions but with higher amount of catalyst, having Mw = 720,100 g/ mol, PDI = 1.874 (Fig. S1b), zero-shear viscosity = 4,580,000 cSt (Fig. S2), was chosen as reference.
3.1. Ultra-high molecular weight polydimethylsiloxane-α,ω-diol (UHMWPDMS) 3.1.1. Polymer synthesis In the context of the known specificity (conferred by the nature of siloxane bonding) of the cyclosiloxanes polymerization, which occurs either under thermodynamic control, at equilibrium, or under kinetic control by living species, in dependence on the nature of the monomer but mainly of the catalyst [17–23] (see more in SI), ammonium and phosphonium hydroxides, also known as transient catalysts, are an excellent choice. They are thermally labile, decompose into volatile products easily removed by simply raising the temperature without requiring neutralization and repeated washing processes with the consumption of large amounts of water and solvents and their release as wastes, as in the case of polymerization catalyzed with alkali metal compounds or acids (sulfuric acid, for example) [22]. Furthermore, when tetramethylammonium hydroxide is used as catalyst, the amount of cycles formed is very small because, according to some opinions [17,19], at low concentrations of the catalyst, the process would be a pseudo-living one, the polymerization degree being approximately inversely related to the catalyst concentration [19]. Comparative studies have shown that tetramethylammonium and tetrabutylphosphonium hydroxides are more effective than potassium hydroxide, due to the greater degree of dissociation of the former and possibly their better solubility in the monomer mass even if the polymerization is conducted at a lower temperature (i.e. 80 °C imposed by its thermal stability). However, this has the disadvantage that, the system is not always homogeneous and undefined carbonate quantities can also be present [24]. In this work, we have chosen as catalyst the tetramethylammonium hydroxide commercially available as a 10% solution in methanol, which allows easier dosing of the very small amount of catalyst required and minimizes the risk of carbonation during handling. The use of the methanolic solution has also the advantage that it is no longer necessary to entrain water as azeotrope with benzene or toluene as is the case for the aqueous solution. Initially, at room temperature, tetramethylammonium hydroxide is insoluble in nonpolar cyclic dimethylsiloxane. As temperature of the reaction medium rises to 80 °C, tetramethylammonium hydroxide melts and acts on SieO bond from D4 forming soluble tetramethylammonium siloxanolate, which becomes the catalyst for further polymerization of cyclosiloxane in homogeneous medium. To end the reaction, the temperature is increased to > 130 °C, and tetramethylammonium siloxanolate decomposes into trimethylamine and methanol, according to some authors [22,25], or dimethyl ether after others [26], easily volatile products which can be removed in vacuum and/or temperature leaving a clean product free of catalyst residues that could affect subsequent behavior of the polymer [25]. The dipolar aprotic solvent N,N-dimethylformamide (DMF), a weak base and hydrogen bond acceptor, has been added in the reaction mixture as a promoter for polymerization. This has the role of complexing the cation of the catalyst and reducing the amount of alkoxide or silanolate aggregates, thus leading to an increased propagation rate of the reaction by involving anionic reagents and has the added benefit of avoiding side reactions [20]. Cations, such as R3NH+, are hydrogenbond donors, but the common interactions to be considered are cationdipole [27]. Different from other reports [24], we have carried out the polymerization in one step, in bulk, with the in-situ formation of the siloxanolate, without its isolation. The possible reactions are shown in Scheme 1, adapted according to Ref. [22]. In the case of a water-free system, it has been reported that propagation by binding of D4, resulting in molecular weight and viscosity increase, occurs especially up to a conversion of about 60–70% cycles. At higher conversion rate, the molecular mass begins to decrease due to redistribution reactions and equilibrium is established. However, these
2.3.2. Crosslinking polydimethylsiloxane-α,ω-diols into soft films (cUHMW-PDMS, cPDMS) 1 g of UHMW-PDMS/PDMS was dissolved in 15 mL toluene with magnetic stirring for 24 h at room temperature. 0.07 mL TEOS and 0.01 mL DBTDL were added and the mixture was further stirred until a homogeneous solution was obtained. The reaction mixture solution was poured on a Teflon substrate and left for 24 h to allow the curing process. However, to complete the crosslinking reactions and to evaporate the volatile by-products, the obtained films of about 150 µm in thickness were deposited at room temperature for two weeks before use. The soluble fractions, Ws, for cUHMW-PDMS, cPDMS and commercial Elastosil were determined by extraction with toluene for 48 h under continuously rotation shaking, the leakage of excess solvent from samples, washing with fresh toluene, draining again excess solvent, after that the samples were dried under vacuum at 80 °C for 48 h. It has been used the following equation: Ws = (mi − mf)/mi * 100 (where mi is the initial weight, and mf is the final weight, after extraction) to calculate soluble fraction percentage, the results being presented in Fig. S3. The swelling behavior of the elastomers with respect to toluene and chloroform was evaluated by immersing the calibrated samples in respective solvents for 24 h and weighing them in swollen state. The swelling ratio, Q, of crosslinked polymers in equilibrium swollen state was calculated using the following equations: mdry
Q: =
ρp
+
(msw − mdry ) ρs mdry ρp
where: mdry is the weight of dry elastomer, msw is the weight of swollen elastomer, ρp is the density of elastomer and ρs is the density of solvent. The obtained values are showed in Fig. S4.
3
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Scheme 1. A picture of the possible reactions in the TMAH-catalysed ring-opening polymerization of D4 [22].
whose size is so large that it does not imply major spectral differences from a linear structure. The IR spectrum (Fig. 1) recorded in the transmittance mode reveals the existence of only the characteristic bands for any linear polydimethylsiloxane structure: 2966, 2907 cm−1 (CeH asymmetric, νas, and symmetric, νs, stretching in SieCH3), 1263 cm−1 (CeH symmetric bending, δs, in Si-CH3), 1099–1020 (SieOeSi asymmetric stretching, νas), 868 cm−1 (asymmetric CeH rocking, δas, in Si(CH3)2), 800 cm−1 (asymmetric SieC stretching in Si(CH3)2), 698 (SieC symmetric stretching in Si(CH3)2) [32–34]. It is known that PDMSs with low viscosities (up to 1000 cSt) have Newtonian behavior, their viscosity remaining constant, regardless of shear rates (up to 10,000 s−1). As viscosity increases, Newtonian's behavior domain is restricted to a certain limit of gradient velocity. Beyond this value, PDMS becomes shear thinning, i.e. the apparent viscosity decreases below the real one with strain-rate and their ratio is no longer constant. This behavior is called “pseudoplastics” and is perfectly reversible. Critical shear rate (s−1) decreases as the real viscosity of the silicone rises [11]. Thus, high molecular weight
studies were conducted on D4 systems and hexamethyldisiloxanes [22]. In our case, in the presented conditions, after 1.5 h, the volatile fraction constituted 7 wt% and the GPC chromatogram of the remaining polymer indicated an ultra-high molecular weight with narrow dispersity without small molecular species (Fig. S1a). 3.1.2. Structural and rheological characterization In the 1H NMR spectrum (Fig. 1), a single peak is present at 0.07 assigned to the protons in the dimethylsiloxane groups. Due to the very long chains, there are no visible peaks corresponding to the final dimethylsiloxane groups to which the OH groups (about 1.2 ppm) and H in the OH (3.7–3.8 ppm [28]) are attached. A single peak also, that at 21.95 ppm attributed to the silicon atoms in the dimethylsiloxane units on the chain, is present in 29Si NMR spectrum (Fig. 1). The NMR spectra do not indicate the presence of detectable cyclic species, which in 1H NMR spectrum should show a peak at 0.095 ppm (D4) [29] or 0.168 ppm (D3) [30], while in 29Si NMR it should show peaks in the region from −19.4 to −19.7 [6,31]. However, this does not permanently exclude the existence of cyclic species. There may be cycles but 4
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Fig. 1. NMR and FTIR spectra for UHMW-PDMS.
the hypothesis regarding the high purity of the synthesized compound and corresponds to the expectation that degradation in the inert atmosphere consists of depolymerization with the formation of low molecular weight cyclic and linear volatile oligomers [39]. The thermostability of UHMW-PDMS is slightly lower as compared with lower molecular weight PDMS, which show Ton 426 °C and Tpeak 483 °C. The DSC curves (Fig. 2 right) indicate clear crystallization processes (−78.9 °C) during cooling, as well as glass transition (−122.5 °C) and melting (−38.9 °C) during heating, processes which, under the applied measurement conditions, are reproduced in successive cooling/heating cycles (Table 1). Logically, the Tg and Tc values are slightly lower and Tt slightly higher than in the lower molecular weight homologue PDMS.
polymers show dramatic deviations from Newtonian behavior at high shear rates due to the orientation and/or the flow mechanism involving smaller jump units [35]. The flow curve recorded for our polymer UHMW-PDMS fit well Bird-Carreau model [36] and indicates Newtonian behavior on a fairly narrow shear rate range of 0.01–0.0316 s−1, almost half the corresponding range for PDMS with lower molecular weight, while zero shear rate viscosity is 37,600,000 cSt for UHMWPDMS and only 4,580,000 cSt for PDMS (Fig. S2). Molecular weight of the polymer was first estimated by viscometric measurements in solution using successive dilution method. The inherent viscosity thus measured was 170 dL/g and the molecular weight calculated according to Ref. [16] was 976,045 g/mol, which, as is normal, is within the estimated Mn and Mw values by GPC. However, the best method for determining the average molecular weight of polymers and its distribution is GPC, which provides key information to predict the processability and material properties because these parameters control many physical properties. Thus, increasing Mw leads to increase of strength, toughness, brittleness, melt viscosity, chemical resistance and decreased solubility. However, the drastic reduction in polydispersity makes polymers more difficult to process. The chloroform eluting curve displayed by the refractive index (RI) detector (Fig. S1a) indicate considerable value of the average molecular weight relative to polystyrene standards, Mw = 1,395,000, and a polydispersity index, PDI = 1.490.
3.1.4. Raw polymer morphology Dynamic light scattering (DLS) was used to determine the particle size distribution of the polymer in solution and the average particle diameter. Measurements were made on solutions prepared by successive dilutions in both chloroform and toluene, the average values estimated by extrapolation at the near zero concentration being 3.3 and 5.0 µm, respectively (Fig. 3). The lower values of the aggregates in the case of the chloroform are due to its higher ability to dissolve the PDMS, despite the fact that it has a solubility parameter (9.2 cal1/2 cm−3/2) more different from that of the PDMS (7.3 cal1/2 cm−3/2) than toluene (8.9 cal1/2 cm−3/2) [41]. This mismatch is explained by the polarity of the solvent (µCHCl3 = 1.0 D as compared to only 0.4 D for toluene), which is found as polar component δp, together with the dispersion component δd, and hydrogen-bonding forces δh, in the Hansen's total solubility parameter (HSP) δt (δt2 = δp2 + δd2 + δh2) [41,42] The closer the distributions of the solubility parameter components in the total solubility parameter of the polymer and solvent are, the more the polymer (in this case PDMS) will swell more [41]. The degree of similarity between the HSP of the polymer and the solvent is expressed as the factor Ra (distance in Hansen space) calculated with the relation: [42,43]
3.1.3. Thermal analysis According to the most of the literature sources [37,38], polydimethylsiloxanes are stable under high vacuum or in inert atmosphere up to least 350–400 °C, the thermostability being negatively affected by factors including acidic or basic impurities, oxygen, water, solvent, fillers, and residual catalyst but also end-group functionality [38,39]. Depolymerization at high temperature can occur by three mechanisms: unzipping mechanism, random scission and impurity catalyzed mechanism [37]. It has been found that, while at lower molecular mass predominates the unzipping mechanism, with the increase of the molecular mass, intramolecular and intermolecular redistributions, which have the effect of lowering the thermal stability, become predominant [40]. The silanol-terminated polydimethylsiloxanes are less stable as compared with trimethylsilyl-terminated homologues because the terminal hydroxyl groups can participate in a ‘‘back-biting’’ reaction [39]. As the thermogravimetric analysis data show (Fig. 2 left, Table 1), UHMW-PDMS begins to decompose at 405 °C losing almost the entire mass (97.08%) in a single step around 456 °C. This behavior supports
Ra =
4(δdp − δds )2 + (δpp − δps )2 + (δhp − δhs )2
with the values δp, δd, δh for PDMS: 15.9, 0.1, 4.7; for toluene: 18.0, 1.4, 2.0; and for chloroform: 17.8, 3.1, 5.7, respectively [41,42]. For the case of PDMS-toluene and PDMS-chloroform pairs, Ra's values are 5.2 and 4.9 respectively, which indicates a higher degree of swelling in chloroform because the lower this value, the higher the degree of swelling [42]. Higher value of the PDMS-CHCl3 interaction parameter (0.470) compared to PDMS-toluene (0.465) [44] also supports the 5
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Fig. 2. Thermograms (left) and DSC traces (right) comparatively presented for: (a)-UHMW-PDMS; (b)-PDMS; (c)-cUHMW-PDMS; (d)-cPDMS; (e)-Elastosil.
analysis XRR of thin films of PDMS melt deposited on hydroxylated silicon substrates, as “a heap of spaghetti” [46]. A more detailed analysis of the sample (Fig. 5(d–f), image size of 10 × 10 µm2) has revealed, besides the horizontal belt-shaped structures, a prevailing conelike morphology. The surface's topography mainly consists of cones with quite different sizes, with the basis laying on the glass surface. As a result of the analysis of AFM micrographs, the in-plane projection diameter ranges from 20 to 200 nm, while the height ranges from 8 to 16 nm. We assume that these cone-like structures are the vertical variants of either macromolecules or smaller belts compressed in the form of a cone under the action of its own weight. Both belt-shaped and conelike structures are also highlighted in the AFM phase image (Fig. 5f). By using the grains analysis function from image processing software (Nova), the average size (diameter) of cone-like structures was estimated to 55 nm close to the value (46.6 nm) revealed by the DLS analysis at high concentrations in toluene (Fig. S5a). Therefore, we can state that the AFM measurement results fit well to DLS results (Fig. S5). A more detailed AFM analysis is presented in Fig. 6, which shows a 2D AFM topography image (size of 1 × 1 µm2) and the topographic cross sections along and across a belt-shaped structure. In topographical AFM image there are visible four adjacent thin belts, which look like backbones. From topographic profile across the belt, a width of about 50 nm (taking into account the curvature radius of the AFM tip – 30 nm) and height of about 8 nm can be estimated. The topographic profile along the belt shows that the macromolecular chain is an ordered structure consisting of multiple collinear “chain-links”, as those illustrated in Fig. 4 – right. The thicker and broader belts shown in Fig. 5 can be a bundle of multiple thin belts, as those described above, or can be an individual chain-belt structure containing more coplanar chain-links connected together, as those illustrated in Fig. 4 – left. One of the most important fundamental properties of polysiloxane family is the highly pronounced inherent conformational flexibility of their completely inorganic main-chain backbones, O[Si-O]n, which enables the unusually high mobility of their segments and entire molecules [4,46]. But the arrangement of the molecules on the substrate is determined by the ratio between the entropic factors associated with the molecular conformation and the energy interactions with the surface [46], depending on it the packaging or arrangement of the molecules could be from random coil to complete nematic packing. This may explain the differences between the conformations highlighted by the two techniques, TEM and AFM. While for the TEM, the sample was deposited on a non-polar surface, the AFM sample was deposited on the glass which, having hydroxyl groups on the surface, allowed interaction with the molten polymer molecules [46]. Monolayer studies of PDMS have indicated that, depending on the forces developed at the interface, the molecule may adopt a conformation from “flat” to more collapsed, with successive coiling of polymer chains into a helical conformation.
Table 1 Details of the thermal decomposition stages and the thermal transitions identified by thermogravimetric analysis and differential scanning calorimetry, respectively. Sample
Thermogravimetric analysis data
DSC data
Ton
Tpeak
Tend
Mass change, wt%
% rez at 700 °C
Tg, oC
Tc, oC
Tt, oC
UHMWPDMS Step I Step II
30
–
520
97.50
1.25
−122.52
−78.87
−38.90
30 405
37 456
67 520
0.42 97.08
PDMS Step I Step II Step III
39 39 427 516
– 44 483 556
591 75 516 591
96.48 0.68 75.98 19.82
2.95
−120.24
−74.96
−42.54
cUHMWPDMS Step I Step II Step III Step IV
35
–
580
99.75
0.00
−122.64
−68.06
−41.17
35 323 380 466
40 333 417 552
72 380 434 580
3.41 4.63 12.73 78.98
cPDMS Step II Step III
38 38 402
– 46 501
590 83 590
92.20 1.28 90.92
7.64
−121.96
−69.07
−39.02
Elastosil Step I Step II Step III
27 27 388 503
– 47 442 508
700 91 470 528
81.41 3.15 76.24 2.02
18.41
–
–
–
higher swelling capacity in chloroform than in toluene. Obtaining a PDMS of the ultra-high molecular mass, which means large molecules, our idea was to try to visualize such a molecule, mainly using the imaging techniques TEM and AFM. For this, the solution of the polymer was deposited on the carbon-coated copper grid for TEM and on the glass substrate for AFM analysis, the acquired images being shown in Figs. 4–6. TEM images (Fig. 4) indicate macromolecular chains with multiple knots or few entangled chains (thread-like structure) whose diameter is estimated to about 10 nm. Taking into account that the width of the PDMS chain is about 0.7 nm, it is clear that these macromolecular structures are crowded from several PDMS wrapped chains [45]. The AFM image of the polymer deposed on glass substrate, this time from toluene solution, (Fig. 5(a–c), image size of 20 × 20 µm2) clearly indicates the presence of multiple belt-shaped structures with length ranging from 3 to 5 µm, width ranging from 300 to 800 nm, while the height is between 8 and 24 nm. The belt-shaped structure corresponds to that described in literature, based on synchrotron X-ray reflectivity 6
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Fig. 3. Dependence of mean particle diameter estimated by DLS depending on the concentration of UHMW-PDMS polymer solution in: (a) - chloroform; (b) - toluene.
the helical conformation is due to the tendency of the SieO dipole to perform intermolecular compensation, and this is sustained by changes in the dipole moment with increasing molecular weight. Many physical properties can be explained due to helical conformation (hydrophobicity, compressibility, surface properties, etc.) [47]. 3.2. Super-soft silicone elastomer 3.2.1. Crosslinking, extraction and swelling with/in organic solvents The UHMW-PDMS thus synthesized was processed as film and crosslinked by treating in solution (in toluene) with an excess of TEOS as a crosslinking agent in presence of DBTDL as a catalyst. The crosslinking pattern through the very long chain ends (Scheme 2) logically leads to a low cross-link density and increased probability of remaining free chains reflected by higher soluble fraction found, 9.05% versus 6.25% for PDMS with the same crosslinking pattern but with smaller molecular weight or 1.15% for the commercial elastomer based on another crosslinking system (Fig. S3).
Fig. 4. TEM image of the fingerprint left by the solution of polymer in chloroform on copper grid solution.
The 6/1 helix structure was confirmed to be the lowest energy conformation in the crystalline state of PDMS, while a more extended helical structure was suggested by solid state NMR study [2]. Adoption of
Fig. 5. AFM images for the polymer remained on glass substrate after solvent evaporation from the polymer solution in toluene. 7
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Fig. 6. AFM images for the polymer and topographic cross sections along and across a belt-shaped structure.
The swelling measurements made on crosslinked polymers reveal swelling ratio (Q) values higher in chloroform (23.9 for cUHMW-PDMS and 14.2 for cPDMS) than in toluene (19.3 for cUHMW-PDMS and 11.8 for cPDMS) (Fig. S4). In both chloroform and toluene, c-UHMW-PDMS that is based on the higher molecular weight polymer swells more strongly than c-PDMS. Elastomeric films were analyzed for thermal, dielectric and mechanical behaviors compared to those based on lower molecular weight polydimethylsiloxane and a commercial elastomer. By crosslinking, thermal stability is significantly reduced as compared with raw polymers, UHMW-PDMS and PDMS (Fig. 2 left). The crosslinking pattern could also be the cause of this, but also the prevalent depolymerization mechanism. The presence in the material of the acid catalyst, DBTDL, used for the cross-linking process could facilitate breakage of the siloxane bond. Due to the rare crosslinks, the DSC curves are very similar to those of raw polymers, while the commercial elastomer does not show any transition in the analyzed field.
Fig. 7. Dielectric permittivity in dependence on frequency comparatively showed for the two silicone elastomers prepared in lab, cUHMW-PDMS and cPDMS, and commercial one.
3.2.2. Dielectric behaviour The dielectric spectrum (Fig. 7, Table 2) of the cUHMW-PDMS reveals slightly higher values for the dielectric permittivity across the entire range of frequencies studied, when compared with Elastosil. This increase of dielectric permittivity could have two causes: (a) the high degree of physical crosslinking by inter- and intramolecular entanglement of long chains between the chemical crosslinking nodes, which could reduce the free volume; (b) the presence of non-condensed Si-OH groups formed by the hydrolysis of excess cross-linking agent. In the case of reference cPDMS with shorten chains, these values are even slightly higher.
3.2.3. Mechanical behaviour The stress-strain curves of the crosslinked UHMW-PDMS processed as films were recorded and presented in Fig. 8 side by side with those obtained in similar conditions for cPDMS and Elastosil. The films show common elastomer stress-strain behaviour with a steeper slope at low deformations, a rubbery domain and a strain hardening region at high strains. The sample cUHMW-PDMS shows the highest strain at break up to 1337% (Fig. 8a), while cPDMS and Elastosil show elongations up to 836% and 568%, respectively (Fig. 8b and c). The average Young’s
Scheme 2. The crosslinking pathway used for UHMW-PDMS and PDMS. 8
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Table 2 Main dielectric and mechanical parameters of the analyzed silicone elastomers. Sample cUHMW-PDMS cPDMS Elastosil
ε' at 104 Hz, 25 °C 2.61 2.62 2.53
ε'' at 104 Hz, 25 °C −4
5.6 ∙ 10 8.4 ∙ 10−4 2.6 ∙ 10−3
Maximum elongation at break, %
Young modulus, MPa
1336 840 568
0.3 0.3 1.1
Fig. 8. Stress strain curves for: (a) cUHMW-PDMS; (b) cPDMS; (c) commercial elastomer and the two synthetized elastomers.
the case of cPDMS sample, as the number of cycles increase, a stress relaxation process occurs. For example, the maximum stress for the first cycle was 0.20 MPa, while for the last cycle the maximum stress was 0.17 MPa. This behavior is less evident for the cUHMW-PDMS and Elastosil samples. The stress relaxation process is time-dependent and is closely related to the elastomer microstructure. The relaxation processes occur mainly due to the disentanglement of physical nodes, viscous flow, bond interchange or chain scission. Since the cPDMS elastomer has shorter molecular chains, consequently a higher chemical cross-linking density, a better mechanical behavior would have been expected as compared with the cUHMW-PDMS elastomer. Surprisingly, on the contrary, the latter showed superior mechanical properties. It is assumed that the very long macromolecular chains of the UHMW-PDMS polymer are highly tangled, generating numerous physical entanglements that act as physical cross-linking nodes. Moreover, the sol
modulus values determined from the linear part of the stress-strain curves indicate that the prepared in lab silicones have similar Young’s modulus, around 0.3 MPa, while the commercial film is more rigid having a modulus of about 1.1 MPa. The high strains and low Young’s modulus found for cUHMW-PDMS and cPDMS can be attributed to the very long molecular chains correlated with the cross-linking system through the chains ends. In order to evaluate the viscoelastic behaviour, the elastomeric films were subjected to 100 repeated stress-strain cycles (Fig. 9). After the firs cycle, a stress-softening behavior known as Mullins effect is observed. This phenomenon is typical for elastomers and normally appears after the first elongation cycle due to the rearrangement of the macromolecular chains. In Fig. 9d–f is presented the variation of the applied force at 100% strain for each stress-strain cycle. It can be noticed that, especially in
Fig. 9. Cyclic stress-strain curves (a–c) and the corresponding stress relaxation (d–f) for: cUHMW-PDMS (a and d); cPDMS (b and e) and Elastosil (c and f), respectively. 9
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Fig. 10. Cyclic stress-strain curves of: (a) cUHMW-PDMS; (b) cPDMS; (c) Elastosil, and the corresponding stress relaxation (d–f) at 500% cyclic strain.
Elastosil reference sample seems to show a better recovery as compared with cUHMW-PDMS. The cPDMS sample presents the highest mechanical losses, after 100 cycles recovering only 92% of its original length (Fig. 11a). Moreover, it is worth noting that, after 10 cycles within 0 – 500% strain, the cUHMW-PDMS film has an elastic recovery of about 99%, while the Elastosil and cPDMS samples failed (Fig. 11b). The elastic recovery and stress decay values evaluated in various testing conditions (1, 10, 100 cycles at 100 and 500% strain) centralized in Table 3 for cUHMW-PDMS and the two references clearly reveal the performance and limits of the former.
fraction of cPDMS elastomer is higher as compared with cUHMWPDMS. The free molecular chains will move past one another and will increase the viscous dissipation. Therefore, the elastomer based on ultra-long molecular chains will provide superior mechanical properties. Furthermore, when the elastomers are subjected to 10 stretching cycles at 500% (Fig. 10), only cUHMW-PDMS film did survived to this experiment and behaves almost purely elastic (Fig. 10d), while cPDMS and Elastosil films failed after the first and third cycles, respectively (Fig. 10e and f). As expected, the Mullins effect is more pronounced when the elastomers are subjected to repeated stress-strain cycles up to 500% strain (Fig. 10a and b). Excluding the Mullins effect, the cyclic tests reveal that at both 100% and 500% strain, the cUHMW-PDMS film shows very small hysteresis loops indicating low viscoelastic losses (Figs. 9c and 10c). Fig. 11a shows the elastic strain recovery of tested materials when subjected to 100 stress-strain cycles within 0 – 100% strain. Up until 50 cycles, the cUHMW-PDMS film has the best elastic recovery, of about 96.3%, this being greater than for the two reference samples, cPDMS and Elastosil, which have an elastic recovery of 93.2% and 96.1%, respectively (Fig. 11a). However, after 50 cycles, the
4. Conclusions Tetramethylammonium hydroxide is a valuable catalyst for a clean procedure to polymerize especially cyclosiloxanes to very high molecular masses. The separated polymer is one of narrow molecular weight dispersity, free of catalyst and cyclic compounds, as GPC and spectral analyses showed. The zero-shear viscosity was estimated at 37,600,000 cSt and, as a result, the Newtonian field is very narrow. Both isolated and aggregated macromolecules were emphasized by DLS and imaging TEM and AFM techniques. In thin films the conformation
Fig. 11. Elastic recovery of cUHMW-PDMS as compared with Elastosil and cPDMS during stress-strain cycles within 0 – 100% strain (a) and elastic recovery of cUHMW-PDMS during stress-strain cycles within 0 – 500% strain (b). The solid lines are guides to the eye; the first stress-strain cycle in which the Mullins effect is manifested is not plotted. 10
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Table 3 Elastic recovery and stress decay of cUHMW-PDMS, cPDMS, and Elastosil. Sample
Test elongation %
Elastic recovery (1 cycle) %
Elastic recovery (10 cycles) %
Elastic recovery (100 cycles) %
Stress decay (1 cycle) %
Stress decay (10 cycles) %
Stress decay (100 cycles) %
cUHMW-PDMSr*
100 500
98.9 99.6
97.3 99.5
95.4 nd
0.9 1.5
2.7 1.8
6.4 nd
cPDMS*
100 500
98.7 98.9
96.1 Failed
91.6 nd
1.4 1.6
6.4 Failed
11.9 nd
Elastosil*
100 500
100 Failed
97.6 Failed
96.6 nd
0.7 Failed
4.2 Failed
6.1 nd
* The first stress-strain cycle, where the Mullins effect is manifested, was ignored; nd - not determined.
of the macromolecules depends on the nature of the substrate. Thus, while on hydrophobic substrate there is a tendency to disperse at the molecular level as tangled chains, on a polar support (i.e., ordinary glass) they arrange like both vertical cones and belt-shaped horizontal structures. High length of the chains but also crosslinking adversely affect thermal stability. The latter, along with dielectric permittivity, are slightly lower than those of cPDMS but much better than commercial elastomer. The crosslinked films swell more strongly in chloroform than in toluene. The cUHMW-PDMS showed a strain at break up to 1337%, much higher than that of the cPDMS and the commercial elastomer of 836% and 568%, respectively, taken as references, while the average Young’s modulus value (determined from the linear part of the stress-strain curves) is around 0.3 MPa, similar as for cPDMS but much lower than 1.1 MPa found for commercial elastomer. The modulus of elasticity is close to the values (generally less than 0.1 MPa) that characterize ultra-soft silicones of interest, for example in substrate cell culture for mimicking biological stress–strain behaviour [48–50]. At 100% elongation, elastic recovery and stress decay values are comparable for each of the three analyzed samples. However, at higher elongation (500%), the two samples taken as a reference fail during either the first elongation cycle (Elastosil) or 3rd cycles (cPDMS), while the cUHMW-PDMS sample shows an elastic recovery of 99%. Values are better even than those reported for ExSil100 Resin, a commercial elastomer presented as having a 5000% elongation [51,52].
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Acknowledgements This work was supported by a grant of Romanian Ministry of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4ID-PCCF-2016-0050 (5DnanoP), within PNCDI III. Grateful thanks to dr. Alina Nicolescu for NMR spectra and dr. Cristian-Dragos Varganici for thermal analysis. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109243. References [1] Z. Kőkuti, K. van Gruijthuijsen, M. Jenei, G. Tóth-Molnár, A. Czirják, J. Kokavecz, P. Ailer, L. Palkovics, A.C. Völker, G. Szabó, Appl. Rheol. 24 (2014) 63984. [2] C. Kim, M.C. Gurau, P.S. Cremer, H. Yu, Langmuir 24 (2008) 10155. [3] E. Maaskant, K. Tempelman, N.E. Benes, Eur. Polym. J. 109 (2018) 214. [4] E.A. Büyüktanir, Z. Küçükyavuz, J. Polym. Sci. Part B Polym. Phys. 38 (2000) 2678. [5] P.R. Dvornic, J.D. Jovanovic, M.N. Govedarica, J. Appl. Polym. Sci. 49 (1993) 1497. [6] A.C.M. Kuo, In Polymer Data Handbook, Oxford University Press, 1999, pp. 411–435. [7] F. Normand, X.W. He, J.M. Widmaier, G.C. Meyer, J.E. Herz, Eur. Polym. J. 25 (1989) 371. [8] J.G. Zilliox, J.E.L. Roovers, S. Bywater, Macromolecules 8 (1975) 573. [9] P.J. Flory, L. Mandelkern, J.B. Kinsinger, W.B. Shultz, J. Am. Chem. Soc. 74 (1952) 3364. [10] Technology of Pressure-Sensitive Adhesives and Products; Benedek, I.; Feldstein, M., Eds.; CRC Press: London, 2009.
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