Triggered Precision Benzoxazine Film Formation by Thermally

Article pubs.acs.org/Macromolecules

Triggered Precision Benzoxazine Film Formation by Thermally Induced Destabilization of Benzoxazine Nanodroplets Using a LCSTBearing Surfactant Kevin Chiou,† Pablo Froimowicz,*,‡ Katharina Landfester,‡ Andreas Taden,‡,§ and Hatsuo Ishida*,† †

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany § Adhesive Technologies R & D Henkel AG & Co. KGaA, 40191 Duesseldorf, Germany ABSTRACT: A novel method for film formation has been developed utilizing miniemulsion properties. The process destabilizes emulsion droplets in a controlled fashion and deposits the droplets onto a substrate to form a resin film. By utilizing a polymeric stabilizer with lower critical solution temperature (LCST) behavior as functional protective colloid, the emulsion is designed to destabilize when the system reaches a set temperature. Using LCST-bearing protective colloids, benzoxazine resin films are formed and polymerized. By design, the utilized polymeric stabilizer contains benzoxazine groups, which are covalently incorporated into the matrix as a reactant. The resulting film is similar to those formed following a solution casting method. The novel process offers the possibility of applying precision micro-optic coating eliminating the edge effect as well as forming homogeneous film thickness aided by the near monodisperse emulsion particle sizes. most notably the improved stability,3,4 possibilities for novel processing techniques,5,6 and benign properties to the environment.7 Because of its unique properties, the field of miniemulsions grew rapidly in the recent years in fabrication techniques, utilizations, and overall physical theories of miniemulsions. Utilizing the pseudostable state of emulsions, miniemulsion droplets can be destabilized to induce droplet deposition onto a substrate to form films. In this paper, benzoxazine miniemulsions were prepared and deposited onto submerged substrates. However, pure polybenozoxazine derived only from monomeric precursors often suffers from brittleness as is the case for all other thermosetting resins. As a way to counteract polybenzoxazine’s brittleness, the miniemulsion process was smartly exploited by using reactive protective colloids not only to stabilize the droplets but also to use it as a reactive additive or plasticizer.8 This hypothesis is based on reactive surfmers, which are known for exploiting this covalent immobilization of the surfactant’s reactive part to avoid its migration either in the droplet or during subsequent film formation while allowing functionalization onto the surface of droplets and particles.9−11 Thus, the polymeric stabilizer tends to locate primarily on the surface of droplets and guarantee a

1. INTRODUCTION Typical polymer film formation techniques focus on two concepts: physical melt deformation and solution cast film formation. Physical melt deformation involves well-known methods such as extrusion and film blowing. Solution cast film formation evaporates the solvent from the polymer solution depositing the solute as a film. Thermosets could be cast into a film by the solution cast methods. Evaporation of solvent induces instability in film thickness, in particular at the edge, and it is not suitable for micro-optical coating, for example. Thus, a new method is presented to thermally induce deposition of thermoset resins onto a substrate with minimal evaporation of the solvent. After deposition, the deposited film can be removed from the remaining phase and polymerized, thus generating the desired thermoset. This method, however, is not limited to depositing emulsions made of liquids and is equally applicable to solid particle suspensions.1 An emulsion is a mixture of two or more immiscible liquids referred to as the continuous phase and the dispersed phase. In stable emulsions, droplets of the dispersed phase are often stabilized by surface active molecules (surfactants and/or protective colloids) to avoid the droplets from merging together. A miniemulsion is a type of emulsion where the droplets of the dispersed phase are in the nanometer scale (30− 500 nm); the high stability of narrowly distributed droplets is obtained by using an osmotic pressure agent.2 The reduced domain sizes offer many advantages over traditional emulsions, © 2014 American Chemical Society

Received: February 20, 2014 Revised: April 30, 2014 Published: May 9, 2014 3297

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otherwise stated. Protective colloid (BA-jeff) and cross-linkable monomeric benzoxazine (BA-hex) were synthesized following reported methods.10 2.2. Synthesis. 2.2.1. Assembly of Benzoxazine Miniemulsion. BA-jeff (0.5 g) was dissolved in 25 mL of distilled water to prepare the continuous phase. BA-hex (6.25 g) was separately dissolved in an equivalent weight of chloroform (6.25 g) to prepare the dispersed phase. The continuous phase and dispersed phase were then added to a 50 mL flask and pre-emulsified by stirring at 1250 rpm using a stirring hot plate at ambient temperature for 80 min. The mixture was then sonicated at 20.0 kHz and 280 W using a Branson Sonifier S-450 at 2 s alternating interval pulses for a total of 4 min in an ice bath. The emulsion was then left under open atmosphere for 18 h while stirring to remove chloroform from benzoxazine miniemulsion droplets, to leave benzoxazine droplets suspended in the water continuous phase. Miniemulsions are then abbreviated as BZX-p. The sizes of the droplets in the different miniemulsions were in average 354 nm, and free surfactant was not detected. 2.2.2. Deposition of the Benzoxazine Droplets via Destabilization of LCST Surfactants. The concentration of BZX-p was adjusted to the designated solid content of 0.1%, 1.0%, 10%, 25%, and 50%. A glass substrate of 2.5 cm × 4 cm × 0.1 cm was placed within a cylindrical container of 2.5 cm radius. The container was then filled with BZX-p suspension (30 mL) to thoroughly cover the glass substrate. Each suspension was then heated to the temperature as specified by each trial as 45, 60, 75, and 90 °C. All these temperatures were higher than the measured LCST of the respective protective colloid used, which was 38 °C. The solid content of the suspensions within the cylindrical container was measured at time periods of 5, 10, and 20 h. After 20 h of heating, the glass substrate was removed and rinsed once with distilled water. The deposited material was then polymerized at 200 °C for 1 h. 2.2.3. Control Experiment for the Thermally Induced Destabilization of Miniemulsions Using Polystyrene NP and LCST as Protective Colloids. Synthesis. Styrene (6.01 g) and benzophenone (200 mg) were dissolved in hexadecane (250 mg), forming the dispersed phase. BA-jeff (1 g) was dissolved in distilled water (25 mL), forming the continuous phase. The two phases were mixed using a magnetic stirring bar at 1250 rpm for 80 min. The mixture was then sonicated at 20.0 kHz and 280 W using a Branson Sonifier S-450 at alternating interval pulses of 2 s for a total of 4 min in an ice bath. The miniemulsion was then photopolymerized (at 350 nm) for 24 h with constant stirring and finally characterized. Thermal Destabilization. Polystyrene nanoparticles were thermally destabilized following the same procedure as described for the benzoxazine systems. 2.3. Instruments. Fourier transform infrared spectroscopic analysis (FT-IR) was performed on a PerkinElmer Spectrum BX, at a resolution of 4 cm−1, with 32 scans and using KBr pellets. Nuclear magnetic resonance (NMR) spectroscopic analysis was performed on a Bruker Avance spectrometer 250 at a proton frequency of 250 MHz. Deuterated chloroform was used as a solvent for NMR analyses. Turbidity experiments were carried out on a Metrohm 662 turbidity photometer. Transmission electron microscopy (TEM) was performed using a Philips CM12 instrument. Sample staining was done with RuO4. Atomic force microscopic (AFM) images were taken using a Veeco Instruments Multimode Nanoscope IIIa. The cantilever used was a OMCLAC 160 TS, with a resonance frequency of 300 kHz and a spring constant of 42 N m−1. The polymerization process was followed by differential scanning calorimetry (DSC) (TA Instruments DSC Model 2920). The DSC thermograms were obtained at a temperature ramp rate of 10 °C/min under a nitrogen flow rate of 40 mL min−1. A few mg of sample was crimped in a hermetic aluminum pan with lid. Thermal stability of samples was studied by thermogravimetric analysis (TGA) using a TA Instruments Model High Res TGA 2950 at a heating rate of 5 °C min−1 under nitrogen purge rate of 90 mL min−1. Approximately 5 mg of samples was used for each analysis.

proper mixing. Moreover, those additional functionalities specifically located onto the surface of the droplets may be exploited afterward for further chemical/physical functionalization or to induce responsiveness to different stimuli.12 Polybenzoxazines are a class of relatively new commercial polymers with properties comparable to that of current high performance resin.13−17 Polybenzoxazines have various unusual and excellent properties, such as near-zero volume shrinkage upon polymerization,18,19 great thermal stability and high char yield,20,21 and ultralow surface energy.22−27 Recently, many polybenzoxazines based on natural materials were synthesized and demonstrated to exhibit comparable properties to their synthetic phenolic counterparts,28 showing the versatilities of polybenzoxazines. Moreover, most of the favorable polybenzoxazine properties can be added into a product by incorporating (poly)benzoxazines at different locations, such as side groups,29−31 forming telechelic polymers,32−34 or even as constituents of the backbones.35−38 The latter, for instance, presents the chemical composition of the functional polymeric stabilizer utilized throughout this contribution as polymerizable protective colloid, which is also thermoresponsive thanks to the other segments forming the polymer main chain, poly(propylene oxide)-co-poly(ethylene oxide). Thermal responsiveness of the protective colloid was achieved exploiting the lower critical solution temperature (LCST), which is the temperature above which a polymer is no longer soluble in the solvent.39 The physical phenomenon of the LCST is complex and different for each system. Uses of LCST have been reported, for example, for hydrogel and nanoparticle formation.40−43 In aqueous systems below the LCST, poly(propylene oxide)-co-poly(ethylene oxide) is solvated by water via hydrogen bonding.44 Above the LCST, the hydrogen bonding between the polymer and water weakens, diminishing the solvating action of the water, forcing the extended polymer to collapse and thus losing its solubility and decreasing. Using this LCST feature, one can control the ability to precipitate a solute from a solution and even destabilize the dispersed phase from an emulsion to form a controlled temperature-responsive colloidal system. In this paper, a smart, precision thermoresponsive film casting method is presented, which is based on the destabilization of miniemulsions containing both the thermoset precursor and the plasticizer. To this end, a protective colloid was designed and synthesized to have reactive benzoxazines and water-soluble LCST-bearing segments in the main chain. While the hydrophobic benzoxazine moieties of the protective colloid are exploited to covalently react with the monomeric benzoxazines within the droplets, the soft water-soluble poly(ethylene oxide)-co-poly(propylene oxide) parts exposed to the water phase are responsible for both the stabilization and the thermo-induced destabilization of the droplets. This soft component of the protective colloid also acts toughener in the final product after polymerization. Comparisons of the properties exhibited by the films here generated with those previously obtained by melt casting are also presented.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (99%), benzophenone (99.5%), and chloroform were supplied by Fisher Scientific, and sodium dodecyl sulfate (SDS) (ACS reagent, ≥99.0%) was from Sigma-Aldrich. Styrene was uninhibited by passing through a column of silica gel, and all other chemicals were used without further purification unless 3298

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Figure 1. (a) Chemical structures of the cross-linkable monomeric benzoxazine resin BA-hex (top) and the protective colloid BA-jeff (bottom). (b) Thermoresponse of the protective colloid BA-jeff in water. While the extended structure is soluble, the collapsed one is insoluble. (c) Preparation of benzoxazine miniemulsions stabilized by the protective colloid and subsequent thermally induced destabilization to form films on demand.

3. RESULTS AND DISCUSSION 3.1. Experimental Design. The protective colloid was synthesized as a multifunctional main-chain benzoxazine copolymer consisting of alternating moieties of the thermoresponsive polyether block and benzoxazine units. A key point in this design is that, while the benzoxazine moiety is hydrophobic, the water solubility of the carefully chosen thermoresponsive polyether can be altered by changing the temperature. The synthesized protective colloid (BA-jeff) presents a LCST of 38 °C, being soluble at lower temperatures and insoluble at higher ones. Thus, when forming the miniemulsions, the hydrophobic benzoxazine moieties of the BA-jeff will be mixed with the cross-linkable monomeric benzoxazine (BA-hex) on the outer part of the droplets,

whereas the soft polyether segments will remain exposed to the water phase being responsible for both the stabilization and the thermo-induced destabilization of the droplets. The latter will cause precipitation of the droplets. However, as one can intentionally induce this precipitation by increasing the temperature, this process can be seen as a thermo-induced deposition of the thermoset precursors forming films to be then polymerized. The entire process is summarized in Figure 1. 3.2. Amount Deposited at Each Condition. Thermal destabilization of benzoxazine miniemulsions was used to deposit the thermoset precursors onto a glass substrate. Quantification was done by measuring the remaining solid content in the emulsions. The protective colloid LCST in water is 38 °C; therefore, above this temperature the hydrophilic 3299

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Figure 2. Plot of the residual solid content (SC) of the benzoxazine miniemulsions as a function of the heating time at (a) 45, (b) 60, (c) 75, and (d) 90 °C. In every plot the different original SC for each miniemulsion system studied is indicated as follows: (◆) 0.10% SC, (■) 1.00% SC, (▲) 10.00% SC, (∗) 25.00% SC, and (×) 50.00% SC. (e) Deposition of polystyrene nanoparticles at 75 °C used as the control experiment.

segments begin to turn increasingly hydrophobic until the point of being no longer soluble. Figures 2a−d show the residual solid content (SC) of different miniemulsions (0.1, 1.0, 10, 25, and 50% in SC) as a function of heating time at a given temperature (45, 60, 75, and 90 °C, respectively). These figures clearly show the successful thermal-induced miniemulsion destabilization. It is worth mentioning that the mechanism for destabilization is beyond the scope of this paper and that the focus was put on finding an efficient method to induce a smart and precision deposition of the polybenzoxazine’s precursors toward the final thermoset. Figure 2a shows that thermal destabilization of the nanodroplets is already achieved when performed at 45 °C, although the process is not that efficient. Except for the 1.00% original SC system, which presented a decreased SC of 40% after 20 h, all systems were slightly destabilized diminishing their SC up to 70−85%. When the same set of experiments is carried out at 60 °C (Figure 2b), destabilization begins to

enhance notably its efficiency since it is upon this temperature that full deposition was achieved. For instance, the original 0.1% SC system fully deposited its nanodroplets after 5 h, while the original 1.00% SC system did it after 10 h and the original 10% SC system did it after 20 h. Results are slightly more efficient when performing the experiments at 75 °C instead of at 60 °C, as it can be seen in Figure 2c. It should be noted that nanodroplets of 0.1% solid content were deposited faster than 1% solid content at 60 °C. The difference in speed may be smaller at higher temperatures to further suggest greater efficiency at higher temperatures. Finally, the last studied temperature of 90 °C shows the best results where all systems, regardless their original solid content, fully deposit upon thermal induced destabilization (see Figure 2d). The results show that miniemulsions are entirely destabilized at temperature near 60 °C for the time range studied. This is expected since the LCST of BA-jeff is 38 °C, and the driving force to destabilize very close to this temperature is not expected to be high. Despite that the hydrophilic segments of 3300

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present in this system at a lower temperature. Nevertheless, due to severe peak overlapping, detailed polymerization activity of each component and possible interaction could not be studied at this point. It must be emphasized, that the miniemulsiondestabilized films presented similar polymerization profile as those formed by a bulk-casting process.8 Complementary TGA studies were carried out to gain deeper insights. To this end, not only the thermally induced deposited film was investigated but also its individual components, the monomer BA-hex and the protective colloid BA-jeff. The results are presented in Figure 4, where direct TGA and the corresponding first derivative (DTG) curves are shown for each system. Figure 4a shows that BA-jeff experienced a relatively simple thermal degradation process, exhibiting a T5% value of 304 °C (where T5% is the temperature at which a 5% weight loss is measured), a degradation onset temperature (Tonset) of 339 °C, and a final char yield at 800 °C of 5%. These results indicate an excellent thermal stability. Although the monomer’s thermal stability and reactivity is well established, we studied its profile to enhance our understanding on the entire system developed here. Unlike the protective colloid, it can be seen in Figure 4b that monomer BA-hex underwent a progressive multistep thermal decomposition occurring in several closely related but undistinguished stages. It is possible, however, to assume the presence of three main visible processes observed at 275, 320, and 416 °C. Moreover, the BA-hex T5% and Tonset were measured and determined to be 269 and 231 °C, respectively, with a final char yield at 800 °C of 6%. Figure 4c shows the TGA and DTG for the thermally induced deposited film. As expected, the thermal decomposition of this more complex system presented a degradation profile which is comparable to the net sum of its precursor’s degradation profiles. Thus, a simplified interpretation of the profile displays three main processes observed at 277, 329, and 415 °C similar to the case of pure BA-hex, which is the main component in the thermally induced deposited film. However, the important difference in the magnitudes for each process is caused by the presence of BA-jeff (minor component) in the same film. Finally, the thermally induced deposited film T5% and Tonset were also measured and determined to be 250 and 216 °C, respectively, with a final char yield of 19% at 800 °C. 3.4. Compatibility Effects of Benzoxazine Core and Protective Colloid. Effects of the LCST deposition on the compatibility between benzoxazine core (BA-hex) and protective colloid (BA-jeff) were studied by examining transparency of the film before and after polymerizations. In the monomeric state, the film’s slight opaqueness is possibly due to the presence of water remaining from the deposition process or the morphology of the deposited benzoxazine droplets. Figure 5b shows how the legend “ABC 123” is slightly blurred for the nonpolymerized cast film. After polymerization, the polybenzoxazine film gains transparency with the characteristic yellow color of polymerized BA-hex or BA-jeff. The improved transparency is likely due to both the elimination of water during polymerization and the copolymerization of the two types of benzoxazine, homogenizing the remaining heterogeneity that scatters light. If there were any heterogeneity present, the dimension must be sufficiently smaller than the wavelength of the visible light. Polymerization of benzoxazine groups consisted of heating the samples at 200 °C for 1 h. These conditions are certainly in favor of further mixing and

the protective colloid start turning hydrophobic and, therefore, collapsing onto the surface of the droplets, some poly(ethylene oxide) portions may still maintain a few hydrogen bonds. This may be the result of different factors, such as small temperature gradient between 38 and 60 °C, morphology of the droplets, and/or protective colloid concentration since BA-jeff was used in a higher concentration than needed to produce films where the protective colloid may also act as an effective toughener. A control experiment was performed to corroborate that LCST of BA-jeff is indeed the driving force causing the thermal destabilization of the miniemulsions. To this end, polystyrene nanoparticles (PS-NPs) were synthesized using the same protective colloid and then studied in regards to their thermal behavior using the same condition as before. As a way to validate the control experiment, it must be mentioned that heating for 1 day PS-NPs synthesized using SDS surfactant, a non-LCST surfactant, normally causes insignificant destabilization and is frequently performed to remove cosolventssuch as chloroformfrom the PS-NPs loaded with hydrophobic substances. Figure 2e shows that the BA-jeff-containing PS-NPs deposits steadily when heated at 75 °C. Thus, destabilization on the model PS-NPs is assumed to be solely caused by thermally surpassing the LCST of the protective colloid. This result also suggests that applying this same protective colloid (BA-jeff) to other systems or manipulating other surfactants is a viable way to design systemssuch as miniemulsionswith controllable destabilization. An additional interesting result is the fact that the previously mentioned PS-NPs could indeed be synthesized. As shown in Figure 1a, the protective colloid (BA-jeff) bears a phenolic OH, which is a functional group known to act as a radical trap. This result suggests that the benzoxazine moieties of the protective colloid may remain on the outer part of both the PS-droplets and the PS-NPs, thus affecting only slightly the photoinitiated radical polymerization of styrene. 3.3. Thermal Characterization of the Deposited Film. Thermal properties of the polymerized films cast by the LCST deposition method are similar to those of films formed by melt processing of the two benzoxazine monomers. Figure 3 shows

Figure 3. DSC thermograms for BA-jeff (a), BA-hex (b), and a miniemulsion-destabilized system (c).

the presence of exothermic peaks at 240 °C associated with polymerization of benzoxazine groups for all three films generated by BA-jeff, BA-hex, and the miniemulsion-destabilized system (thermograms a, b, and c, respectively). However, important differences regarding the onset temperatures are seen for BA-jeff (182 °C), BA-hex (200 °C), and the thermally destabilized systems (about 180 °C in average). The latter shows an onset similar to the one exhibited by BA-jeff and likely causes catalysis during polymerization of BA-hex also 3301

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Figure 4. TGA (black) and corresponding first derivative (DTG, red) curves for (a) protective colloid, BA-jeff; (b) monomer, BA-hex; and (c) thermally induced deposited film.

Figure 5. Pictures of the glass substrate (a) and the cast films formed by the thermo-induced miniemulsion destabilization method before (b) and after (c) polymerizations.

Figure 6. TEM images of a polymerized film formed by the thermo-induced miniemulsion destabilization method at different magnifications.

the droplet morphology from miniemulsions has apparently collapsed during film casting and polymerization. This nanophase separation with collapsed nanodroplets is consistent with the visible light observation of transparency shown in Figure 5. AFM images show the topography and phase contrast picture of the surfaces, which are rough with many grooves of approximately 200 nm long and 20 nm wide (Figure 7). The grooves appear to be regions that have sunk down within the material, thus evidencing the softness of these areas. The presence of these grooves may be understood regarding the copolymeric composition of the films. Whereas polybenzoxazine generates the undeformable rigid regions, poly(propylene

copolymerizing between BA-jeff and BA-hex, evaporation of trapped water, and dissociation of any possible morphology within the film, and therefore in favor of the generation of more transparent films. While inspection by the naked eye served as a quick analysis, TEM (Figure 6) and AFM (Figure 7) images helped to study the microscopic textures of the film. TEM images show significant nanophase separation with domain sizes of around 20 nm. Although phase separation exists in the system, the domain sizes are small compared to other polymer blends of similar composition.45 The small separation size is due to covalent bonding during copolymerization and intermolecular interactions between polymer matrix and additives. Moreover, 3302

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Figure 7. AFM images of a cured film formed by the thermo-induced miniemulsion destabilization method. Left: topography. Right: phase contrast.

Scheme 1. Simplified Chemical Design for Synthesizing Thermoplastics Consisting of Alternating Main-Chain Benzoxazine and Flexible Polyether Blocks

to trigger responsiveness at different temperature was also evaluated. To this end, new copolymers consisting of alternating main-chain benzoxazine and flexible polyether blocks were synthesized exploiting the simple chemical design and using mixtures of two different Jeffamines, ED-2003 and ED-900. The main reasons for selecting Jeffamine ED-900 were that (a) its chemical structure and reactivity are essentially the same as for ED-2003, except for lower molecular weight; (b) its high hydrophilicity is similar to ED-2003, thus minimizing the disruption on the hydrophilic segments; and (c) its lack of LCST that will affect the thermal responsiveness of the resulting protective colloids. The new copolymers were synthesized following exactly the same synthetic procedure, using the mentioned Jeffamines in the following ED-2003:ED-900 stoichiometric ratios: 100:0%, 87.5:12.5%, and 75:25%. The thermal response of these newly synthesized potential protective colloids was evaluated by measuring their LCST and comparing to the one exhibited by BA-Jeff. The results are shown in Figure 8, where it can be seen that increasing the amount of ED-900 induces the lowering in their respective LCST. This result evidence the successful tunability of the benzoxazine-containing protective colloids since their LCST can be regulated by adjusting the Jeffamine ratios during their synthesis. Tunable LCST bearing polymers have been a subject of active investigation in recent years.46−49

oxide)-co-poly(ethylene oxide) forms the soft areas, subsequently generating the grooves due to collapsing and sinking. Such morphology is potentially very useful to improve toughness of the material without significantly altering the bulk properties of the component polybenzoxazines as the phase separation size is much smaller than the typical Griffith cracks, thus allowing avoidance of crack initiation while minimizing the dissolution of the soft component into polybenzoxazine phase to soften the rigid phase. Once again, comparing AFM images of miniemulsion destabilized benzoxazine films to previously reported melt processed benzoxazine mixture films of the same composition,45 the overall bulk structures of the final polymers are similar to the same kind and degree of phase separation. These results suggest that the composition of the film dominates its surface properties. Based on the good compatibility between the components and the efficiency of the thermally induced destabilization method, the use of a reactive protective colloid completely eliminates the need for additional additives that may be cause heterogeneities in the films. 3.5. Tunability of the Protective Colloid. Protective colloids having various properties can be synthesized by copolymerizing main-chain-type polybenzoxazine and flexible polyether block, such as the Jeffamine used in the current paper, using the same synthetic procedure.8,45 Scheme 1 shows a simplified chemical design, presenting this flexibility in the synthesis. For example, unlike in the protective colloid BA-Jeff presented here, hydrophobic PTMO-based diamine XTJ-542 was also used to obtain a fully hydrophobic backbone containing benzoxazine moieties. This prepolymer was then functionalized to incorporate terminal blocks of a hydrophilic polyether, thus forming a protective colloid.10 As the focus of this work was to establish a novel smart and precision method for film deposition using a LCST-bearing protective colloid, the possibility of tuning the thermal response

4. CONCLUSION A novel film casting method exploiting the LCST of a polymer to destabilize miniemulsions containing the thermoset precursors was studied and compared to well-known melt processing samples. This new method used a reactive protective colloid which, after polymerization, is covalently attached to the final material, thus avoiding the need of additional plasticizers. Compatibility studies of the films show similar phase separations for polymerized samples obtained following the 3303

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Figure 8. Turbidity measurements of the three synthesized copolymer as potential protective colloids, using Jeffamines in the following ED2003:ED-900 stoichiometric ratios: 100:0% (green), 87.5:12.5% (blue), and 75:25% (red). The abrupt decrease in transmission is associated with the lower critical solubility temperature (LCST) which occurred at ∼28, ∼33, and ∼38 °C for the studied systems.

two mentioned procedures, and thermal analysis also evidenced similar properties. The combination of those results suggests that the materials behave in the same fashion once cast via either method. Thus, both methods produce similar products using different methodologies for mixing and film formation. It needs to be highlighted that while the final products may be similar, the methodologies to prepare the precursors and the films are extremely different. This new thermally induced procedure succeeds in preserving all the good features of the final thermosets. The novel film casting method mixes the material using the physical arrangement of emulsified systems, exposing the thermally sensitive segments of the protective colloid to the water phase. Once emulsion system is controllably destabilized and droplets fell, the nanoparticles form a macroscopically homogeneous film during the drying and polymerization process. Finally, although complementary studies need to be conducted, the successful tunability of the protective colloids makes of this method a potential technological tool to be considered in the near future for industrial applications. Potential film thickness control method relies on heating duration of the emulsion system to control the amount of material deposited.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.I.). *E-mail [email protected] (P.F.). Present Address

P.F.: Pagora - Grenoble Institute of Technology, 461, rue de la Papeterie, Grenoble, St-Martin-d’Hères, Cedex 38402, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.C. gratefully acknowledges the financial support of Case Alumni Association and Department of Macromolecular Science and Engineering of Case Western Reserve University and the Max Planck Institute for partial financial support. We also thank Katrin Kirchhoff (MPI-P) for her assistance in obtaining TEM images and Uwe Rietzler (MPI-P) for his assistance in obtaining AFM images. 3304

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