Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Comb Polymer Network of Polydimethylsiloxane with a Novolac Stem: Synthesis via Click Coupling and Surface Morphology Architecturing by Solvents K. Sunitha,† S. Bhuvaneswari,‡ Dona Mathew,† G. Unnikrishnan,§ and C. P. Reghunadhan Nair*,∥ †
Polymers and Special Chemicals Division and ‡Analytical and Spectroscopy Division, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India § Department of Chemistry, National Institute of Technology, Calicut 673601, India ∥ Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682022, India S Supporting Information *
ABSTRACT: Cuprous iodide-catalyzed azide−alkyne cycloaddition (CuAAC) offered an easy route for the synthesis of organic−inorganic comb polymer networks with a short stem composed of novolac and arms made of lightly cross-linked polydimethylsiloxane (PDMS). This was achieved by the click reaction between novolac propargyl ether (NPE) and azide telehelic PDMS (PDMS-AZ) in a single step at near-ambient conditions. The precursors were absolutely characterized. The reaction, monitored by differential scanning calorimetry and rheometry, implied the cross-linking occurring at relatively low temperatures. The gelation occurred at a lower conversion than predicted by the Flory−Rehner equation. The reaction was facilitated by polar solvents. Dynamic mechanical analysis (DMA) implied biphasic behavior for the cross-linked polymer, the transitions of the soft and hard segments appearing at −110 and 26 °C, respectively. The calculated cross-link densities tallied well with the expected network structure. Solvents influenced the reorganization of the soft and hard segments as reflected in the water contact angle of these resins cast from different solvents. The morphological analyses by scanning electron microscopy and atomic force microscopy substantiated these findings.
1. INTRODUCTION Advances in different polymerization techniques enable the development of a variety of polymeric architectures such as linear, graft, dendritic, comb or star/comb-like, hyperbranched, brush, and cyclic polymers.1 Among them, comb and star/comb polymers represent a group of polymers having enhanced physical properties and are distinctly different compared to their linear counterparts due to unique chemical design that permit their use in areas like life sciences and nanotechnologies.2,3 Several approaches are employed for the synthesis of comb and star/comb copolymers.4−7 Among these, though the ones based on anionic or controlled radical polymerizations offer methods for deriving well-defined comb and star/comb polymers, they demand stringent reaction conditions.8,9 “Coupling onto” and “grafting to” techniques by means of highly efficient organic coupling reactions such as click reactions10,11 are more efficient methods for the preparation of well-defined star and comb polymeric architectures.12−14 Reactions such as thiol−yne,15 oxime ligation,16 and coppercatalyzed azide−alkyne reaction17 are often exploited for the coupling chemistry. “Click chemistry” captured the attention of researchers due to its versatility, simplicity, efficiency, high yield, and tolerance to a variety of functional groups.18−20 This © XXXX American Chemical Society
reaction opens wide possibilities for macromolecular engineering enabled by a nearly perfect reaction.21−23 Phenolic novolac resin represents a rigid polar oligomeric species with tremendous possibilities for polymer chain buildup by either graft-on or graft-from methods.24−27 Polydimethylsiloxane (PDMS), on the other hand, is a soft and flexible polymer with little possibility for strong intermolecular interactions. Novolac and PDMS have contradictory characteristics in many respects, arising from their opposing polarities. Hence, it was of interest to design a copolymer of the two, resorting to the click reaction. Comb polymers have found widespread application in different fields owing to their novel properties arising from the structural features of their stem and arms.28−33 We envisaged comb copolymer with a short stem composed of polar novolac and arms constituted by hydrophobic PDMS. The arms are cross-linked to result in a network comb copolymer. Polymers with similar constitution have been synthesized previously by the hydrosilylation reaction.34−36 However, the silylation reaction is adversely affected by moisture, and it is difficult to achieve 100% reaction in this Received: October 1, 2017 Revised: November 18, 2017
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DOI: 10.1021/acs.macromol.7b02046 Macromolecules XXXX, XXX, XXX−XXX
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Dimethylformamide (80 mL) was used as the solvent. The reaction flask was covered with a dark cloth and kept at room temperature for 2 days, followed by heating at 50 °C for 1 day with magnetic stirring. The product was extracted with diethyl ether and then washed several times with water to remove the unreacted reactants such as sodium azide and ammonium chloride. The yellow viscous liquid thus obtained was dried under vacuum at 60 °C for 24 h. The product was characterized by FTIR, NMR, and for epoxy and nitrogen contents. As the potential impurities are highly water-soluble unlike the product of interest, the contamination of the product with these nitrogenous impurities is absolutely improbable. FTIR: 2100 cm−1 (stretching of NN), 3420 cm−1 (−OH group). 1 H NMR (ppm, CDCl3): 3.6 ppm (−O−CH2), 3.99 ppm (−CH− OH), 2.3 ppm (−CH2N3). 2.4. Synthesis of Novolac Propargyl Ether (NPE). Novolac propargyl ether was synthesized from novolac according to a previously reported procedure.24 The required novolac was synthesized from phenol (25 g, 0.27 mol) and formaldehyde (5.425 g, 0.18 mol, 15.5 mL of 35% (w/v) solution) in the presence of oxalic acid (1.24 g) by heating in an oil bath at 90 °C for 8 h. It was washed repeatedly with hot water (90 °C) to remove unreacted phenol and low molecular weight fractions, if any. Drying was done by azeotropic distillation using toluene followed by vacuum drying at 60 °C. The precursor novolac was characterized by intrinsic viscosity and 13C NMR. The final product NPE was characterized by FTIR, NMR, and hydroxyl value as per method, ASTM E222-10. FTIR (cm−1): 3288 (H−C), 3424 (−OH), and 2120 (−C C−). 1 H NMR (ppm, CDCl3): δ = 6.94 (m, Ar), 4.6−4.9 (−OCH2), 3.45 (CHC−). 2.5. Casting of Slabs in Different Solvents. “Click coupling” was performed using stoichiometric amounts of PDMS-AZ (equivalence based on azide groups) and NPE (equivalence based on propargyl groups) in the presence of Cu2I2 as catalyst (0.1 wt %). To study the effect of solvent on the phase organization of the formed polymers, slabs were prepared by conducting the coupling reaction in solution of the desired solvents. Acetonitrile (ACN), acetone, tetrahydrofuran (THF), dichloromethane (DCM), and toluene− acetonitrile mixtures (TACN) were used for the study. The curing was continued for 24 h at room temperature. Subsequently, the slabs were vacuum-dried at 60 °C for 8 h. These dried polymer films were used for the examination of morphology and for contact angle measurements.
chemistry. Rutnakornpituk et al. attempted the curing reaction between novolac and a mixture of epoxy- and cyanidefunctional PDMS and evaluated the influence of polarity of the modified siloxane on their microphase separation characteristics. These reactions are not as facile as the click reaction and cannot be expected to go to completion, risking the formation of the final polymer of expected structure. Chen et al. synthesized comb-shaped copolymers composed of hydrophobic and hydrophilic segments, which self-assembled in aqueous solution.37 Yoshihiro et al. prepared A−B−A triblock bottle brush polymer of polystyrene and poly[2(dimethylamino)ethyl methacrylate] capable of self-assembly into discrete aggregates with well-defined structures. Chakrabarty et al. investigated the morphological characteristics of trifluoroethoxy(methyl)oxetane) segment incorporated PDMS hybrid network.38 Luo et al. studied the influence of Flory− Huggins interaction parameters on the self-assembly features of PDMS-b-PEO copolymer.39 In the present study, we employed the “click method” for synthesizing the network comb polymers. The comblike polymers were envisaged to possess a novolac stem and PDMS arms linked through a triazole moiety. Chemical, spectral, thermoanalytical, and rheological characterizations of the precursor system and dynamic mechanical and surface characteristics of the cross-linked systems have been evaluated. The influence of solvent polarity on reaction kinetics as well as on the morphology of the formed network has also been examined. To the best of our knowledge, this is the first ever report of a novolac−silicone comblike elastomer realized through azide− alkyne click coupling and studies on the special features of this hybrid organic−inorganic network.
2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals used for the synthesis were of AR grade and were used as such except in the case of potassium carbonate, which was dried at 120 °C/4 h prior to use. Other chemicals include diglycidyloxypropyl-terminated poly(dimethylsiloxane) (Alfa Aesar), sodium azide (99%, Spectrochem, India), ammonium chloride (99.8%, High Purity Laboratories, India), phenol (99.5%, Merck, India), formaldehyde solution (34−37% w/v in water, Merck, India), propargyl bromide (97%, 80% w/w in toluene, Alfa Aeser), benzyltriethylammonium chloride (98%, Spectrochem, India), and cuprous iodide (99.9%, Sigma-Aldrich, USA). Analytical grade solvents such as acetonitrile, dimethylformamide, tetrahydrofuran, dichloromethane, toluene, and acetone were purchased from SRL, India. 2.2. Instruments. Nitrogen content was determined using a PerkinElmer 2400 CHN analyzer. FTIR spectra were recorded using a PerkinElmer spectrum GXA spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker Avance spectrometer (400 MHz). DSC was carried out on a TA Instruments model 2920 modulated DSC at a heating rate of 10 °C/min under N2 atmosphere. Rheological experiments were performed using a DHR-3-5332-0102 using a 20 mm parallel plate assembly at a frequency of 1 Hz and a strain of 4%. The water contact angle was measured by the sessile drop method using a Data Physics contact angle instrument OCA-15EC, using deionized water. Morphological studies were carried out in a scanning electron microscope (SEM) model Carl Zeiss SMT EVO 50. AFM analyses were performed in tapping mode using an Agilent 5500 scanning probe microscope. The morphology and phase changes of cocured films of PDMS-AZ and NPE processed in various solvents were examined. 2.3. Synthesis of Azide-Terminated PDMS (PDMS-AZ). Diglycidyloxypropyl-terminated poly(dimethylsiloxane) (11 g, 0.0125 mol), sodium azide (6.24 g 0.096 mol), and ammonium chloride (5.136 g, 0.096 mol) were taken in a 250 mL round-bottom flask.
3. RESULTS AND DISCUSSION Azide telechelic polydimethylsiloxane (PDMS-AZ) was synthesized from diglycidyloxypropyl-terminated polydimethylsiloxane by reacting it with sodium azide/ammonium chloride mixture in dimethylformamide medium as illustrated in Scheme 1. The extent of “azidation” (azide addition) was found to be near complete from the nitrogen content estimation. After azidation in its FTIR spectrum characteristic of azide and hydroxyl groups, respectively (SI1). Azidation was further confirmed from 1H NMR signals corresponding to the methylene group containing the azide group (−CH2−N3, 2.3 ppm). Novolac resin was characterized by molecular weight, degree of branching, ortho−para substitution ratio, and intrinsic viscosity (for molecular weight). It possessed a number-average molecular weight (Mn) of 834 with a degree of branching of around 23%. From the analyses results, it was concluded that novolac possessed an almost linear configuration (branching probability = 0.23) with statistical ortho−para (2:1) substitution. The degree of polymerization was found to be nearly 8 (see SI2). B
DOI: 10.1021/acs.macromol.7b02046 Macromolecules XXXX, XXX, XXX−XXX
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Cu2I2 as catalyst. Typical reactions between PDMS-AZ and NPE are shown in Scheme 3. The click reaction was confirmed by thermal and spectroscopic characterizations. The triazole-linked elastomeric product was characterized by FTIR, DSC, and 13C NMR. FTIR spectra showed the absence of peaks characteristic of azide and propargyl groups after the reaction, while the peak corresponding to triazole ring appeared at 1411 cm−1 as seen in Figure 1. The cure reaction between NPE and PDMS-AZ was monitored by DSC (Figure 2), too. It was observed that the cure reaction, initiated at around 3 °C, peaked at 40 °C and ended by 60 °C. Thus, effectively, the reaction gets completed at ambient temperature. The reaction between the terminal azides of PDMS-AZ and NPE through the click reaction was further confirmed from solid-state 13C NMR spectra (Figure 3). The characteristic signals of triazole rings appeared in the range of 128 ppm (merged with the aromatic peaks) and at 142 ppm (not present in the precursors). The most remarkable feature of this chemistry is that one easily gets the hybrid elastomer through click ligation at room temperature in a single step. The resultant elastomer slabs are translucent and flexible. 3.2. Rheological Characterization. With a view to understand the effect of advancement of cure reaction on the flow properties of the resin mix, the precursor resins mixed in the appropriate ratio were subjected to rheometry. In isothermal rheology (Figure 4), the cure stagnates in about 15 min at 30 °C, indicating completion. Modulus increased continuously, peaked at ∼15 min (completion of reaction), and then it stagnated. The gel point in both cases is the time at which the change in viscosity is high. The extent of conversion at the gel point was calculated theoretically and also experimentally and was found to be 41% and 25%, respectively (SI4). The experimentally observed gel point is lower than the one predicted by Flory− Stockmayer theory. This can occur due to two reasons: (i) fundamental error in assuming that storage modulus is a linear function of conversion and (ii) the complex configuration of the NPE “monomer” possessing seven reactive groups on a novolac template. All these groups are presumed to have identical reactivity, which may not be correct. The proximity of the unreacted group on the once reacted template increases the chance of reaction of another pair of reactants, thus advancing the gelation to lower conversion. Similar observations have been reported for a thiol−yne click reaction involving multifunctional thiol templates and triple-bonded ynes.40 3.3. Glass Transition Temperature and Viscoelastic Properties of NCP. Dynamic mechanical analysis (DMA) of the cured NCP is demonstrated in Figure 5. It manifests a biphasic behavior. Two Tgs, one corresponding to the flexible and the other to the rigid part, could be identified. The low temperature transition at −110 °C is ascribed to PDMS segments and that at 25−30 °C to the hard segments constituted by novolac and triazole. Molecular weight between cross-links (Mc) calculated from the G′ value was found to be 1170 g/mol (SI5). This is almost matching with the segmental length of PDMS-AZ (1183, determined by NMR). The structure of the network is thus proposed as per Scheme 3. 3.4. Effect of Solvent on the Morphology and Association of NCP Hybrid Network. Amphiphilic block copolymers exhibit tendencies for microphase separation in solution, and the behavior is strongly influenced by the polarity
Scheme 1. Synthesis of PDMS-AZ
Novolac propargyl ether (NPE) was synthesized form novolac as per a reported procedure24 and was characterized. FTIR showed characteristic peaks of propargyl groups (H− C bond) at 3288 cm−1, 2120 cm−1 (−CC−), and the −OH absorption appeared at 3424 cm−1. Proton NMR signals at 3.45 ppm correspond to propargyl groups. The extent of “propargylation” was estimated from the hydroxyl value and proton NMR was found to be similar (85%). The remaining 15% is non-etherified OH groups (SI2). As the degree of polymerization of novolac is around 8 and that the extent of propargyl etherification is 85%, there are about seven phenol units in novolac that are etherified. In all probability, the leftover phenol group has to be the one located on the o/o′ substituted phenolic moieties as this OH group needs higher activation for reaction with an electrophile due to steric hindrance. Also, it was seen that the ortho/para substitution ratio is 2 as statistically expected. Hence, the structure of NPE can very well be represented as in Scheme 2. 3.1. Synthesis of Network Comb Polymer (NCP). NCP was prepared by reacting stoichiometric amounts of azide and propargyl groups; weights were taken based on azide or propargyl ether equivalent weights (see SI 3) of PDMS-AZ and NPE in the respective solvents at room temperature using Scheme 2. Synthesis of Novolac Propargyl Ether (NPE)
C
DOI: 10.1021/acs.macromol.7b02046 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 3. Schematics of Ideal Reaction between PDMS-AZ and NPE through Click Reaction
and hydrophobic blocks. These features of such diblock copolymers depend mainly on three factors: (i) the volume fractions of the blocks, (ii) total degree of polymerization, and (iii) Flory−Huggins polymer−solvent interaction parameter (χ). The Flory−Huggins parameter determines the degree of incompatibility between the blocks that drives the phase separation. This is directly linked to the solubility parameter difference between the polymer and the solvent,42 which is further related to the polarity of the solvent. The solvent polarity can influence the rate of advancement of the reaction, which in turn also affects the rate of phase separation and the domain size. The kinetics of polymerization was done in three solvents of high, medium, and low polarity indices. The reaction was followed rheometrically using dispersion of the reactant mixtures in these solvents. The apparent rates of polymerization were estimated from the increase in storage modulus and applying the formula conversion =
Figure 1. FTIR spectra of triazole-linked NCP and its precursor mix.
G G′st
where G′st is the G′ at the stagnating region in the rheogram. The slope of the plot of this ratio against time provided the conversion rate. The rate so calculated was found to be directly proportional to the polarity index as seen in Figure 6. Thus, solvent affects bulk morphology through the rapidity of the molecular mass buildup process as well as by the polymer−solvent interaction. 3.5. Microscopic Characterization. In order to understand the influence of solvent medium on the surface morphology of network comb polymer, SEM images of the NCP cast from different solvents of varying polarity indices43 [such as acetonitrile (5.8), acetone (5.1), tetrahydrofuran (4.0), toluene−acetonitrile mixture (3.8), and dichloromethane (3.1)] were taken. Details of the cast polymers and the polarity indices of the respective solvents are listed in Table1 . Microstructures of the NCP cast in different solvents are given in Figure 7a−e. SEM images of the amphiphilic block copolymers give an indication of the miscibility of the blocks.44 It is seen that the NCP prepared in DCM (Figure 7e) showed an even distribution of the two phases, NPE and PDMS, along the surfaces. But in the case of THF, though it serves as the common solvent for both the blocks, microphase separation is facilitated (Figure 7d), and it exhibits a segregated morphology. Similar to NCPTHF, NCPTACN and NCPACN also showed phaseseparated morphologies with varying domain sizes of the blocks (Figure 7a,c,d). In addition to this, NCPTACN showed an
Figure 2. Typical DSC trace of “click” reaction of NCP (heating rate: 10 °C/min).
of the blocks, chemical structure of the copolymer, interactions between the blocks and solvents, etc.41 In the case of block copolymers containing two immiscible blocks, addition of solvents having varying polarity during the network formation can lead to special molecular architectures for the hydrophilic D
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Figure 3. Comparison of 13C NMR spectra (a) PDMS-AZ, (b) NPE (both in liquid state), and (c) cross-linked elastomer, NCP (solid state).
Figure 4. Isothermal rheogram (at 30 °C; heating rate: 5 °C/min). Figure 5. Dynamic mechanical behavior (tan δ of the NCP; heating rate: 5 °C/min).
ordered, fibrillar morphology with an array of domains embedded over the fibers. While in the case of NCPacetone, layered arrangement of the phases is observed without any visual agglomeration (Figure 7b). These observations were further supported by AFM images of the samples, recorded in tapping mode. Figure 8a−e shows the topographies and phase images of the slabs cast in TACN, acetone, ACN, THF, and DCM (SI6). Surface roughness (Sa), which is the mean value of surface relative to the center plane, was calculated from the topographic images in all the cases. Figure 8a shows the morphology of the films (AFM) cast from the toluene−acetonitrile mixture. Interconnected cylindrical domains of size ranging from 40 to 400 nm with Sa = 36 nm are observed. In the topographical image, there are well-
defined projections of brighter segments. The brighter projections are the hydrophilic blocks due to higher interaction with the cantilever. On the other hand, isolated cylindrical nanodomains (Figure 8b,c) of average sizes below 100 and 80 nm, respectively were obtained with Sa 8.6 and 5.4 nm when the media were acetone and acetonitrile, respectively. The corresponding phase image in the case of NCPacetone (Figure 8b) depicts spherical features with a bright shell corresponding to the hydrophilic segment and a dark core due to the PDMS arms. It is also reported that star/comb polymers can possess core−shell structures and appear as soft spheres.45 The core− shell features, though visible in the phase image in Figure 8c, E
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size ranges from 30 to 75 nm with the least Sa of 2.86 nm. THF and DCM, though both act as common solvent for NPE and PDMS, there is variation in the domain sizes of the resultant NCPs. This can be due to the difference in the interfacial energy between the solvent and the micellar core.46 Different morphologies and sizes of the self-assembled micelle arise from the difference in the micelle free energy. This free energy depends on three factors such as stretching of the core-forming blocks, the intercoronal interactions, and surface tension between the core-forming block and the solvent.47 Surface roughness of the differently prepared networks of NCP obtained from AFM followed the order NCPTHF > NCPTACN > NCPacetone > NCPACN > NCPDCM. 3.6. Contact Angle Measurements. Water contact angle measurements of the differently prepared NCP were carried out by the sessile water drop method (Figure 9). It was observed that NCPTACN and NCPacetone showed contact angle values in the hydrophilic regime. But in the case of samples prepared in DCM and THF, the water contact angle (WCA) appeared in the hydrophobic range, while that of acetonitrile appeared in the margin. In general, the surface roughness and the morphology of the material influence the contact angle.48 The slab prepared using THF possess a WCA of 109° with higher surface roughness (topography image Figure 8d). On the contrary, NCP prepared in DCM (Figure 8e) shows hydrophobic character with the least surface roughness. Hence from our studies, it is concluded that not only Sa but the self-assembly nature of material will also influence the contact angle. It is proven experimentally that variation in the solvent polarity bears a direct impact on the segregation of the NCP blocks, leading to changes in surface characteristics. Polymerization in the presence of solvents triggering self-assembly in the cured networks leads to orientation of the polar and nonpolar parts in relation to the solvent. This is confirmed from the contact angle measurements. SEM and AFM pictures confirmed that the self-assembly in the solvent−nonsolvent mixture imparted a regular ordered surface morphology in bringing the NPE segments to the surface. It is hence implied
Figure 6. Conversion rate (%/min) vs polarity index plot.
Table 1. Details of the Cast Polymers and the Polarity Indices of the Solvents solvent
polarity index
notation of polymer cast
acetonitrile (ACN) acetone tetrahydrofuran (THF) toluene−acetonitrile (TACN) dichloromethane (DCM)
5.8 5.1 4.0 3.8 3.1
NCPACN NCPacetone NCPTHF NCPTACN NCPDCM
the hydrophilic and hydrophobic segments could not be distinguished. The topographic image in Figure 8d corresponds to NCPTHF, which appeared as a large agglomerated microsphere with Sa = 54 nm. The phase image showed a brighter rodlike network with a semicircle structure, attributed to the mixing up of the segments by the common solvent. Topographic and phase images of NCPDCM (Figure 8e) are similar to those of NCPTHF (Figure 8d), except that the domains are isolated and are in the nano regime. The domain
Figure 7. SEM images of NCP samples cast from different solvents: (a) NCPTACN, (b) NCPacetone, (c) NCPACN, (d) NCPTHF, and (e) NCPDCM. F
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Figure 8. Topographic and phase images of (a) NCPTACN, (b) NCPacetone, (c) NCPACN, (d) NCPTHF, and (e) NCPDCM.
that in the case of NCP cast in toluene−ACN mixture segregation of blocks occur, and in effect, polar groups project
toward the air surface and nonpolar groups are pulled back as demonstrated in Figure 10. In general, the polarity of the G
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Figure 9. Water contact angle (WCA) of the NCP prepared in different solvents: (a) NCPTACN (86 ± 1°); (b) NCPacetone (81 ± 1°); (c) NCPACN (95 ± 1°); (d) NCPTHF (100 ± 1°); (e) NCPDCM (109 ± 1°).
These types of polymer systems can be utilized as adhesives and coatings with solvent-architectured surface morphologies and consequent surface properties.
medium leads to even distributed small domains while solvents of low polarity does the reverse.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02046. Figures S1−S14 (PDF)
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Figure 10. Schematic representation of segregation behavior of NCPTACN.
AUTHOR INFORMATION
Corresponding Author
*Tel +91 484 2575723; fax +91 484 2577747; e-mail cprnair@ gmail.com (C.P.R.N.). ORCID
Though a regular surface morphology was absent in samples cast from polar solvents such as acetone and ACN, there exists the presence of NPE blocks on the surface as implied by the contact angle values (Figure 9). While in the case of NCPDCM and NCPTHF, mixing up of blocks resulted in the formation of the hydrophobic surface.
S. Bhuvaneswari: 0000-0001-8531-8904 G. Unnikrishnan: 0000-0001-6257-7217 C. P. Reghunadhan Nair: 0000-0003-2618-949X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge Director, VSSC, for granting permission to publish this work. They also thank the Analytical and Spectroscopy Division and Material Characterisation Division of VSSC for the analytical support.
4. CONCLUSIONS Lightly cross-linked, well-defined comb copolymers of silicone with a short novolac stem could be synthesized at room temperature by the copper-catalyzed azide−alkyne reaction of propargyl-functionalized novolac and azide telechelic PDMS. The arm linking polymerization reaction proceeded at ambient temperature and was accelerated by polar solvents. The gelation behavior and cross-link densities conformed to a structure predicted from the molecular characteristics of the precursors. The network system showed a biphasic behavior corresponding to soft and hard segments. Solvents used for casting the NCP films had a profound influence on their surface morphology which was reflected in the water contact angle values. Surface characteristics of the hybrid network built in the presence of solvent−nonsolvent mixture (TACN) showed an interconnected cylindrical surface morphology with segregation of hydrophilic and hydrophobic blocks to result in contact angle values in the hydrophilic range. Variations in the polarity indices of the solvents had an impact on the surface characteristics of NCP.
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REFERENCES
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DOI: 10.1021/acs.macromol.7b02046 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b02046 Macromolecules XXXX, XXX, XXX−XXX
