Article pubs.acs.org/IECR
Preparation of Highly Uniform Polyurea Microspheres through Precipitation Polymerization and Their Characterization Xubao Jiang, Xiumei Li, Xiaoli Zhu, and Xiang Zheng Kong* College of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China S Supporting Information *
ABSTRACT: Uniform polyurea (PU) microspheres were prepared through precipitation polymerization of isophorone diisocyanate (IPDI) in water− acetone mixed solvent, under mechanical oscillation and quiescent conditions. Higher yield with better uniformity for the microspheres was achieved under the quiescent process. The preparation was therefore optimized for the quiescent process. The maximal IPDI loading reached 11.0 wt % with the yield of the microspheres of 88.5%. With acetone replaced by acetonitrile, this yield was increased further to 93.5% combined with also a higher IPDI loading of 15.0 wt % at the same time. The chemical structure of PU was studied using nuclear magnetic resonance. PU microspheres, insoluble in most of organic solvents tested, were dissolved in m-cresol at 30 °C and in acetic acid at 66 °C. These results showed that the PU microspheres consisted of only linear polymers. This work provides therefore a simple and promising protocol for large-scale production of highly uniform polymer microspheres through precipitation polymerization without any additives.
1. INTRODUCTION Uniform polymer microspheres have been widely applied in drug delivery and release,1 chromatography analysis,2 enzymes immobilization,3 and liquid crystal display.4 Conventional methods for the preparation of these polymer microspheres include emulsion and dispersion polymerizations,5,6 as well as Shirasu porous glass membrane emulsification with a postpolymerization.7 A common problem of these methods is the need of surfactants or stabilizers, which are very difficult to be fully removed after microsphere preparation. This has imposed a variety of restrictions for the application of the polymer microspheres in many fields, particularly in biological and medical fields where microspheres with an absolutely clean surface are preferred. In contrast, no surfactants or stabilizers were required in precipitation polymerization. This process has become a powerful tool for preparation of uniform microspheres, mainly based on free radical mechanism of polymerization in its earlier stage of development.8−14 A great variety of uniform polymer microspheres with different functions have been achieved through this approach.15−18 However, a severe drawback of the process has been the extreme low monomer concentration, which is generally limited to 2 wt %,9−17 owing to limited stability of the polymer microspheres because of the absence of surfactant and stabilizers. At the same time, monomer conversion is low because of the low monomer concentration that the process allowed to attain. Monomer conversion is indeed increased by increasing either the amount of the crosslinker monomer or that of the free radical initiator.13,14 However, with these measures taken, the size of the microspheres becomes smaller, with aggregation of the © XXXX American Chemical Society
microspheres often observed, particularly in the early stage of the process. The uniformity of the microspheres is therefore deteriorated. It is recently reported that monomer loading of up to 20 wt %, with the yield of microspheres of 93.7%, was attained by solvothermal process combined with precipitation polymerization under quiescent mode;19 the uniformity of the microspheres was not as good as that by simple precipitation polymerization. Under this circumstance, we have reported recently a novel protocol of precipitation polymerization in water−acetone cosolvent under oscillation.20 Different from free radical mechanism, the polymerization was based on step-growth polymerization of isophorone diisocyanate (IPDI) with isophorone (di)amine, which was in situ formed from the reaction of IPDI with water as depicted in Figure 1. Under optimized condition, IPDI loading was raised up to 8.0 wt % with a yield of microspheres of up to 86%, a much higher productivity in comparison with that based on free radical polymerization.9−17 This process was advanced later by a quiescent polymerization, i.e., a polymerization process with the reactor standing still without any oscillation or stirring.21 The monomer concentration was further increased up to 11 wt % with the polymerization accomplished within 2 h, combined also with a higher yield of microspheres of 88.5% relative to the monomer loading. In this paper, the two processes of precipitation polymerization, i.e., under oscillation and Received: Revised: Accepted: Published: A
September 11, 2016 October 6, 2016 October 18, 2016 October 18, 2016 DOI: 10.1021/acs.iecr.6b03526 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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under vacuum. To optimize the process, the polymerization was also carried out at different temperatures, with varied IPDI loadings and solvent compositions. 2.3. Characterization and Instrumentation. The size of microspheres was examined using scanning electron microscopy (SEM, Hitachi S-2500) as previously described.13,22 The number-average size (Dn) and the weight-average size (Dw) were obtained by counting at least 200 microspheres on SEM pictures. The size distribution (Dw/Dn) of the microspheres was calculated. Infrared analysis was done on a PerkinElmer FTIR spectrometer (Spectrum GX) with the sample compressed in KBr pellets. Thermal gravimetric analysis (TGA) was carried out using a PerkinElmer Diamond TG/DTA instrument at heating rate of 10 °C/min in nitrogen atmosphere. Nuclear magnetic resonance (NMR) analysis of high resolution for the samples was carried out using a 400 M Bruker instrument (Advance III), with DMSO-d6 as the solvent and tetramethylsilane as the internal reference. Solubilization test for PU microspheres was conducted by dispersing 2 g of the powder microspheres into 98 g of solvent, followed by oscillating the samples at 120 osc/min for 6 h. Light transmittance of the dispersion was tracked by a Metrohm 662 photometer at a wavelength of 525 nm. At the end of the test, the dispersion was centrifuged and the solid dried up. The solids obtained were weighed and observed under SEM.
Figure 1. Chemical reactions involved in formation of PU microspheres through IPDI reaction with water.
quiescent conditions, are compared to elucidate the mechanism of the polymerization. On the basis of the results, the protocol of quiescent polymerization was optimized.
2. EXPERIMENTAL SECTION 2.1. Materials. Acetone, hexane, ethyl acetate, acetic acid, and acetonitrile were purchased from Tianjin Fuyu Chemicals. Isopropyl alcohol, ethylene glycol (EG), toluene, pyridine, N,Ndimethylformamide (DMF), and tetrahydrofuran (THF) were purchased from Tianjin Kermel Chemicals. m-Cresol was purchased from Tianjin Guangcheng Chemicals. All these chemicals were analytically pure grade and used as received. Isophorone diisocyanate (industrial grade) was provided by Keju New Materials Co. Ltd., Beijing. Full deuterated dimethyl sulfoxide (DMSO-d6, 99.9%) was from Sigma-Aldrich. Water used was double-distilled in the laboratory. 2.2. Preparation of PU Microspheres by Precipitation Polymerization. A typical protocol for the precipitation polymerization is described as follows: to a glass bottle of 120 mL capacity was charged 92.0 g of premixed H2O/acetone at mass ratio of 30/70 and 8.0 g of IPDI. The bottle was sealed off, hand-shaken to make the mixture homogeneous, and located into a water bath oscillator operating at 120 osc/min for the polymerization under oscillation, whereas for the process of quiescent polymerization, the reaction bottle was left standing still in a water bath at a set temperature, without any oscillation or stirring. The polymerization was continued for 2 h in general. At the end of the polymerization, samples were taken and centrifuged for 5 min at 12 000 rpm. The separated microspheres were washed twice with a mixture of H2O/ acetone (mass ratio of 1/4) and dried up at 80 °C for 12 h
3. RESULTS AND DISCUSSION 3.1. Comparison of Polymerization under Oscillation and Quiescent Conditions. To prepare polyurea using IPDI as the only monomer in the mixed solvent of H2O−acetone, the reactions involved are quite simple as depicted in Figure 1. IPDI reacts with water to yield its monoamino-substituted derivative or isophorone diamine in a first step with release of carbon dioxide; the in situ formed isophorone amines react with IPDI to form PU. PU microspheres were prepared with two different polymerization processes, i.e., under oscillation and quiescent conditions, with varied monomer loadings in the binary solvent of different compositions and at different temperatures. The results are given in Table 1. It is to note that, for each set of polymerization under a given condition (oscillation, 30 °C, for example), the first line of the data gives the highest IPDI loading at and below which the highly uniform microspheres (Dw/Dn ≤ 1.01) are effectively obtained, while in the second line is given the (lowest) IPDI loading; at this point the
Table 1. Parallel Comparison of Microsphere Preparation by Precipitation Polymerization of IPDI under Oscillation and Quiescent Conditions at Different Temperatures in H2O/Acetone at Mass Ratio of 30/70 process and temp
IPDI solubility (wt %)
IPDI loading (wt %)
sphere yield (wt %)
Dn (μm)
Dw/Dn
oscillation 30 °C
7.2 7.2 7.2 7.2 8.6 8.6 8.6 8.6 11.0 11.0 11.0 11.0
6.6 6.8 7.2 7.4 8.0 8.2 8.6 8.8 6.0 8.0 11.0 12.0
85.12 85.60 86.55 86.23 88.75 86.23 88.75 88.99 81.06 86.39 88.46 89.05
10.19 9.60 9.86 9.79 11.25 12.51 6.91 8.02 6.89 6.07 10.45 8.58
1.008 1.027 1.006 1.065 1.008 1.035 1.006 1.028 1.003 1.010 1.006 1.206
quiescent 30 °C oscillation 50 °C quiescent 50 °C oscillation 70 °C quiescent 70 °C
B
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on the mechanism of the particle formation in this process:21,23,24 Two steps were basically involved in the process, the nucleation of the primary particles and their growth. Oligomers were first produced by polymerization, and reached their critical length, where they became insoluble in the solvent, leading to the nucleation of primary particles. Growth of these particles was followed either by continuous polymerization of the monomers with the oligomers adsorbed on the surface of the primary particles, or by continuous capture of the oligomers formed after the nucleation of the particles. Note that these oligomers were most likely captured by the existing particles before they could reach their critical length. Otherwise, new particles ought to have nucleated, leading to polydisperse microspheres. Note that IPDI droplets must be present when IPDI loading was above its solubility; IPDI on the interface of the droplets would polymerize in a way similar to the interface polymerization of diisocyanate,25 in competition with the monomer molecules dissolved in the solvent. This would lead to large microspheres with their size similar to those of the droplets, significantly larger than the primary particles formed by nucleation from the oligomers. In addition, the size of the droplets, formed by IPDI in excess of its solubility, was different one from another and was also in constant change at the start of the polymerization owing to lack of stabilization by surfactant or stabilizer; multisized microspheres were therefore produced under this circumstance. The polymerization was started at low temperature (30 °C) with H2O/acetone mass ratio of 40/60 in the mixed solvent. The results demonstrated that the solubility of IPDI was only 1.8 wt % at this low temperature combined with the rich water content in the solvent, too low for the process to be interesting, although highly uniform microspheres (Dw/Dn ≤ 1.01) were obtained (Supporting Information, Table S1). To achieve higher productivity of the microspheres, the polymerization, under oscillation and quiescent conditions, was carried out in the binary solvent with H2O/acetone mass ratio of 30/70. IPDI solubility was increased by nearly 3-fold, from 1.8 to 7.2 wt %, by this increase of 10 wt % in acetone content in the solvent. However, highly uniform microspheres were not obtained with 7.2 wt % of IPDI under oscillation, but at a lower IPDI concentration of 6.6 wt % (Table 1, data line 1, Figure 2A1), the microspheres showed broader dispersity even at 6.8 wt % of IPDI loading (Table 1, data line 2). In contrast, highly uniform microspheres were indeed obtained with 7.2 wt % of IPDI under quiescent condition (Table 1, data line 3, and Figure 2B1). Uniformity of the microspheres was severely deteriorated by increasing acetone content further to 80 wt % (see Figure S1 in the Supporting Information). It was easy to conceive that the solubility would also increase with increased temperature. The polymerization was then carried out at higher temperature, while keeping the same binary solvent composition at 30/70 for H2O/acetone mass ratio. Indeed, IPDI solubility was promoted from 7.2 wt % at 30 °C to 8.6 wt % at 50 °C, and further to about 11.0 wt % at 70 °C. When polymerized at 50 °C, highly uniform microspheres were obtained with 8.0 wt % of IPDI loading under oscillation (Table 1, data line 5, and Figure 2A2) and with 8.6 wt % of IPDI loading under quiescent condition (Table 1, data line 7, and Figure 2B2). By increasing the polymerization temperature from 50 to 70 °C, the productivity of the uniform microspheres was further enhanced with IPDI loading reached 11 wt % under quiescent condition (Table 1, data line 11, and Figure 2B3), whereas no perceptible difference was detected for the process
uniformity of the microspheres became broader. With further increase in IPDI loading above this critical point, the microspheres, at least part of them, were aggregated with lumps of polymer observed at the end of polymerization. Selective SEM photos of the microspheres are given in Figure 2 for visual illustration for the uniformity of the microspheres.
Figure 2. SEM photos of PU microspheres prepared under oscillation (A1, A2, A3) and quiescent conditions (B1, B2, B3 and C1, C2, C3) in solvent of H2O/acetone with mass ratio at 30/70 at different temperatures and IPDI loadings (A1, B1, and C1 prepared at 30 °C with IPDI loading of 6.6%, 7.2%, and 7.4%; A2, B2, and C2 prepared at 50 °C with IPDI loading of 8.0%, 8.6%, and 8.8%; A3, B3, and C3 prepared at 70 °C with IPDI loading of 8.0%, 11.0%, and 12.0%).
With regard to the microsphere yield, it is to point out that, although the reactions involved were simple (Figure 1) and the monomer IPDI was confirmed to have fully polymerized to yield PU, it was not that straight to determine the conversion of IPDI and the yield of the microspheres, because the amount of IPDI reacted with H2O relative to that reacted with the in situ formed isophorone amines was unknown. In addition, the mass of the PU produced was different from the monomer charged because of the CO2 release in the reaction process. However, taking into account the PU structure and the mechanism of PU formation, one can conclude that, for 100 g of IPDI loaded, about 88.3 g of PU should be obtained with IPDI fully polymerized. This conclusion was achieved by ignoring the terminal effect of PU chains, i.e., by assuming that PU chains were of an ultimately high degree of polymerization. The microsphere yield was obtained this way, and the difference between this yield and 100% was the yield of the soluble oligomers not precipitated at the end of the polymerization. In the preliminary polymerization, it was found that the formation of PU microspheres and their size distribution, particularly the latter, were closely dependent on IPDI concentration. Clearly homogeneous solution was immediately obtained by hand-shaking the reaction bottle when IPDI loading was below its solubility in H2O−acetone solvent, while a slightly milky mixture (IPDI−H2O−acetone) was obtained once IPDI loading was above its solubility in the binary solvent. It was also observed that, with polymerization carried out above IPDI solubility, the microspheres obtained were always polydisperse, with some aggregated microspheres or even polymer coagulum appearing. This is easily understood based C
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Table 2. Preparation of Polyurea Microspheres at Different Temperatures with Varied H2O/Acetone Ratios and IPDI Loadings by Quiescent Polymerization H2O/acetone mass ratio
temp (°C)
IPDI solubility (wt %)
IPDI loading (wt %)
sphere yield (wt %)
Dn (μm)
Dw/Dn
30/70 26/74 26/74 24/76 24/76 30/70 26/74 26/74 30/70 26/74 26/74 30/70b 20/80b 15/85b
30 30 30 30 30 50 50 50 70 70 70 30 30 30
7.2 7.8 7.8 9.0 9.0 8.6 9.0 9.0 11.0 13.0 13.0 9.0 17.0 26.0
7.2 7.8 8.0 9.0 10.0 8.6 9.0 10.0 11.0 11.0 12.0 9.0 15.0 15.0
86.55 87.53 N/Aa 88.39 N/Aa 88.75 87.32 N/Aa 88.46 87.65 N/Aa N/Aa 93.50 91.23
9.86 9.04 9.46 9.11 12.17 6.91 8.33 8.39 10.45 11.63 12.24 9.57 7.11 8.95
1.006 1.007 1.093 1.006 1.104 1.006 1.006 1.095 1.006 1.007 1.088 1.024 1.009 1.087
a
N/A indicates that microspheres partially or fully aggregated; lumps of polymer observed. bPolymerization conducted in H2O/acetonitrile instead of H2O/acetone for the rest.
the particles was also benefited from there because increasing the polymerization temperature or acetone content in the solvent was also in favor of the formation of the protective gel layer of the microspheres, since the polymer terminals were more compatible with acetone than with water and they were also more flexible at higher temperature. On the basis of the analysis, it became also easy to explain the observation that more uniform microspheres were achieved with higher yield under quiescent condition than under oscillation. Without any protective agents added, the stability of the particles in this process, provided only by the solventswollen surface layer, ought to be fragile. Any external shearing introduced, as oscillation for instance, may well constitute a supplementary driving force to destabilize the microspheres, leading to a precocious aggregation of the microspheres. Obviously, by increasing polymerization temperature or acetone amount in the solvent, the benefit for the microsphere formation is limited. The primary particles would be hard to nucleate or they may become prone to aggregate when the temperature or the content of acetone in the binary solvent was too high, where the oligomers became largely soluble and the microspheres were overswollen. The fact that no uniform microspheres were obtained when the process was done at temperatures above 70 °C or in the solvent with 80 wt % of acetone or more may be understood this way. 3.2. Supplementary Assays for Microsphere Preparation. In order to elucidate the experimental conditions to get a higher productivity for the microspheres, more polymerization was carried out with refined polymerization temperature and solvent composition under quiescent condition. The results are given in Table 2. For easy comparison, three sets of data in Table 1 (H2O/acetone mass ratio of 30/70 at temperature 30, 50, and 70 °C) were duplicated in Table 2. The polymerization was started at 30 °C by increasing acetone content in the solvent, from 70 to 74 wt % and further to 76 wt %, corresponding to H2O/acetone mass ratio of 30/70, 26/74, and 24/76, respectively (Table 2). By increasing acetone content in the solvent to 76 wt % (H2O/acetone mass ratio at 24/76), the highest IPDI loading to get uniform microspheres was 9.0 wt % with the yield of microspheres of 88.39%. Keeping this H2O/ acetone ratio, the attempt to prepare uniform microspheres at IPDI loading above 11 wt % failed with polymerization
under oscillation (Table 1, data lines 9 and 10, and Figure 2A3). With the polymerization conducted at 80 °C, broad size distribution for the outcome microspheres was always detected regardless of the process and the IPDI concentration (SEM pictures are given in Figure S2 in the Supporting Information). From the above results, several points are to be remarked for the preparation of the PU microspheres: One is that the uniformity of the microspheres was closely related to the amount of IPDI allowed to dissolve in the solvent. Fabrication of uniform microspheres was only possible with IPDI loading below its solubility. The second point is that, by increasing either the acetone content in the solvent or the temperature, IPDI solubility was increased, so was the upper limit of IPDI concentration to get uniform microspheres. The third point is that higher yields of uniform microspheres were achieved under quiescent condition in comparison with oscillation, a key merit of this study. On the basis of the mechanism of the microsphere formation aforementioned, it is believed that two key points may be taken as the critical prerequisite for the production of the uniform microspheres. One is that the nucleation of the primary particles should occur at a later stage of the polymerization when the oligomers were accumulated to a higher concentration, and it should also occur within a quite narrow period of time, i.e., at the same time without continuous or second nucleation. The other point is that the primary particles or the microspheres should be granted with a reasonable stability during the polymerization. In accordance with the entropy mechanism in this precipitation polymerization,23 the particle surface is solventswollen at any instant during the polymerization to form a gel layer or a fluffy surface layer, which protects the particles from coagulation to keep the polymerization going on, leading to the formation of uniform microspheres. In other words, the particles are sterically stabilized via this entropy mechanism, quite similar to the stabilization of the particles by nonionic stabilizers in suspension polymerization and to that by nonionic surfactant in emulsion polymerization. Under such circumstance, it is readily understandable that, by increasing the polymerization temperature and acetone content in the solvent, not only the solubility of the oligomers was enhanced, so that to delay the nucleation of the primary particles, the stability of D
DOI: 10.1021/acs.iecr.6b03526 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research conducted at 50 and 70 °C; partial aggregation of the microspheres was always observed. However, keeping the polymerization temperature at 70 °C and lowering acetone content in the solvent by 2 wt % to 74 wt %, i.e., with H2O/ acetone mass ratio at 26/74, 13 wt % of IPDI was dissolved, a solubility slightly higher (by 2 wt %) than that in the binary solvent with H2O/acetone mass ratio at 30/70 at the same temperature. Nevertheless, highly uniform microspheres were achieved only at 11 wt % of IPDI loading, with the yield of microspheres of 87.65%. In comparison to the best result in Table 1, this IPDI loading of 11 wt % was equal to that achieved at 70 °C in the solvent of H2O/acetone ratio of 30/70 (Tables 1 and 2), and the corresponding yield of 87.65% was even slightly lower than that (88.46%) observed at 70 °C in the solvent of H2O/acetone ratio of 30/70. All these results indicate that, either by changing the solvent composition or the polymerization temperature, the highest productivity for uniform microspheres by this precipitation polymerization was achieved with IPDI loading of 11 wt %, combined with a yield of about 88.5% for the microspheres. It was noticed that acetonitrile was reportedly used as solvent in precipitation polymerization for preparation of uniform polymer microspheres.18,23,26,27 Acetonitrile was known to be a good solvent for IPDI owing to their similarity in molecular polarity. A test done in our lab indicated that a higher amount of IPDI was dissolved without phase separation in a binary mixture of H2O/acetonitrile than in H2O/acetone at the same mass ratio (Table 2, last three lines). An attempt to prepare uniform microspheres was therefore carried out in H2O− acetonitrile. Results revealed that no microspheres were observed in the mixed solvent when H2O content was below 5 wt % (H2O/acetonitrile ≤5/95). The polymerization system (H2O−acetonitrile−IPDI) remained as a clear solution through the entire process of polymerization, probably owing to good solubility of the PU in the solvent. Microspheres started to appear, along with polymeric lumps, when H2O content was increased to 10% (H2O/acetonitrile = 10/90). With H2O content in the solvent further increased to 15 wt %, up to 26 wt % of IPDI was dissolved in without observed phase separation. However, well-separated microspheres, with broad size distribution and free of polymer lumps, were obtained only with IPDI loading of 15 wt % or lower (bottom line in Table 2, Figure 3C). With further increase in water content in the binary
to the formation of a large number of primary particles, which might aggregate later in the polymerization process, producing therefore the microspheres with different sizes as seen in Figure 3A. In summary, from these results on precipitation polymerization of IPDI in binary solvent of H2O−acetonitrile or H2O− acetone with the objective to prepare highly uniform microspheres, the yield was significantly increased with the quiescent process in comparison with the polymerization by oscillation, and the yield of the uniform microspheres was further increased by replacing acetone in the binary solvent by acetonitrile. For a batch of 1000 g conducted in H2O−acetone, for example, the maximal IPDI loading was 8.0 wt % in the process under oscillation, combined with the yield of 88.75%, 71.0 g of microspheres were obtained by one batch.20 For the same batch by quiescent polymerization, with the highest allowed IPDI loading of 11.0 wt % and the yield of 88.5%, 97.3 g of microspheres were prepared. With the same batch done in mixed solvent of H2O−acetonitrile, where the maximal IPDI loading was 15.0 wt % under the quiescent process with the yield of 93.5% (Table 2), 140.3 g of uniform microspheres were effectively achieved. This represented an increase of about 44%, i.e., (140.3 − 97.3)/97.3 = 44%, in the productivity in comparison to the quiescent polymerization with H2O− acetone as the solvent, and this productivity (140.3 g) almost doubled that in the oscillation process in H2O−acetone. 3.3. Characterization of PU Chemical Structure. FTIR analysis was done for the PU polymer of the microspheres (the spectra are given in Figure S3, Supporting Information). Virtually identical spectra were obtained for all selected samples prepared in solvents of different compositions and at different temperatures. On the basis of the reported studies and the chemical structures of PU,28−30 the strong absorption peak at 2247 cm−1 in the IPDI spectrum was assigned to the stretching vibration of isocyanate group (NCO), which disappeared in PU, indicating that IPDI was fully reacted. The peak at 1637 cm−1 on the PU spectrum was assigned to the stretching vibration of urea carbonyl. The peaks at 3371 and 1558 cm−1 were attributed to the stretching and plane bending vibration of NH. Their high intensities indicate the abundant presence of amino groups in the polymer. PU of the microspheres was subjected to 13C NMR analysis with DMSO-d6 as the solvent. The spectra are given in Figure 4. In order to better understand the results, the spectrum for the monomer IPDI was also supplied along with the chemical structures of IPDI and PU polymer. All the peaks of chemical shifts were easily assigned based on the reported studies,30−34 as listed in Table 3.
Figure 3. SEM photos of microspheres prepared by polymerization at 30 °C in H2O/acetonitrile mixed solvent (H2O/acetonitrile mass ratio and IPDI loading: (A) 30/70, 9.0 wt %; (B) 20/80, 15.0 wt %; (C) 15/85, 15.0 wt %).
solvent, highly monodisperse microspheres were obtained only at H2O/acetonitrile ratio of 20/80 (last second line in Table 2, Figure 3B). With H2O/acetonitrile ratio of 30/70 (last third line in Table 2, Figure 3A), the uniformity of the microspheres was deteriorated, because of the presence of high water content in the solvent, which might entrain too early precipitation of the oligomers. This early oligomer precipitation ought to lead
Figure 4. 13C NMR spectra of IPDI and PU of the microspheres. E
DOI: 10.1021/acs.iecr.6b03526 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 3. Assignment of Chemical Shift Peaks for 13C NMR Spectra of IPDI and PU chemical shifts, IPDI
chemical shifts, PU
C atoms
cis isomer
trans isomer
cis isomer
trans isomer
3, 3′ 7, 7′ 9, 9′ 1, 1′ 2, 2″ 4, 4′ 5, 5′ 6, 6′ 8, 8′ 10, 10′ 12, 12′ 11, 11′
31.47 27.11 22.68 48.36 47.22 45.63 35.98 42.19 34.36 56.06 122.40 121.53
31.35 26.56 29.21 48.05 46.97 45.44 36.11 42.91 34.26 50.07 122.40 121.47
31.24 27.34 23.00
31.11 26.88 29.64 46.94 46.47 42.30 35.83 42.15 34.75 53.17, 48.79 158.39, 157.50, 156.52
water. The dispersion was located into a water bath shaker and shaken for 6 h at 120 osc/min and 30 °C. Light transmittance of the dispersion was measured at the beginning and the end of the test. The dispersion (or mixture) of the microspheres in the solvent tested was turbid and the light transmittance was about 4% for all the samples, particularly at the beginning of the test, because the microspheres were neither solubilized nor swollen at this time. The transmittance of the microsphere dispersion remained the same (about 4%) after shaking for 6 h at 30 °C for all the tested solvents with exception of acetic acid and mcresol. In acetic acid, the transmittance of the dispersion rose from 4% at the start to 84% at the end of the test, and a 100% transmittance was detected within 3 h of shaking for the test with m-cresol, indicating a full dissolution of the microspheres. To get a better understanding, the samples were centrifuged, the solid collected, dried up under vacuum, weighed, and compared to the weight before the test. For the sample tested with m-cresol, no centrifugate was obtained, in good agreement with the light transmittance test. For the sample tested with acetic acid, less than 2 wt % of the microspheres was lost, whereas for the rest of the samples, the mass of the microspheres was kept exactly the same as that prior to the test. The solid collected after the test was also examined under SEM (Figure 5). It was confirmed that, for the sample tested
The chemical shift of C in NCO was expected to appear around 121−123 as observed on the IPDI spectrum. These peaks disappeared in the PU spectrum, confirming that NCO groups were full reacted as expected, in good agreement with FTIR analysis. By reacting IPDI with cyclohexanemethylamine and cyclohexylamine, respectively, Désilets et al. synthesized two model compounds of urea with the acylamide in the urea unit linked to different carbon atoms (i.e., methylene, −CH2− or methine, −CH−) and subjected them to 13C NMR analysis using deuterated chloroform as the solvent. They found three chemical shifts at 160.3, 159.2 and 158.2, and assigned the shift at 160.3 to the carbonyl atom with the urea amides connected to methylene group at both sides (CH2−NHCONH−CH2), the shift at 158.2 to methine group at both sides (CH− NHCONH−CH), and the one at 159.2 to methine at one side and to methylene at the other side (CH2−NHCONH−CH).34 With DMSO-d6 as the solvent and the urea inserted in polymer chains in this study, the three chemical shifts appeared at 158.4, 157.5, and 156.5 (Figure 4 and Table 3), slightly lower than the reported values, while the order of their chemical shifts was kept.35 The same authors revealed also a peak at chemical shift of 155.2 and attributed it to the carbonyl carbon in a biuret unit, which was assumingly present only in polyureas with cross-linked structure formed by the reaction of the urea amide with an isocyanate group. No any peak was observed at 155.2 and nearby in Figure 4, an indication that biuret unit was absent, i.e., the microspheres were consisting of only linear PU. 3.4. Thermal Property of the Microspheres and Interaction with Organic Solvents. The PU microspheres were also subjected to TGA test (the TGA curve is given in Figure S4 in the Supporting Information). The results revealed that the sample remained stable up to about 350 °C, showing a high thermal stability of microspheres. A mass loss of about 5% was seen at 365 °C. Afterward, the sample started to degrade dramatically, with only about 20% of its initial mass remaining at 400 °C. This PU demonstrated a better thermal stability in comparison with the PU based on toluene diisocyanate and the cross-linked one based on IPDI and diethylenetriamine.29,36 It is to note that this high degradation temperature was not a characteristic for this particular PU, but rather a common behavior for all PU materials.37,38 Dissolubility or swelling of the microspheres with different organic solvents was also tested by dispersing 2 g of the powder sample in 98 g of solvent for each. Twelve solvents were tested in total, which included hexane, toluene, THF, acetone, acetic acid, pyridine, m-cresol, acetonitrile, DMF, DMSO, EG, and
Figure 5. SEM photos of the residua of the PU microspheres after being dispersed in different solvents followed by shaking for 6 h at 30 °C, centrifugation, and drying (the sample with m-cresol was done on the polymer obtained by m-cresol removal by evaporation from the solution).
with acetic acid, the shape of the microspheres was dramatically changed with a great portion aggregated. For the sample done with m-cresol, the solid was the residua obtained by removal of the solvent in the solution through simple evaporation. The SEM photo showed a smooth film without any granular structure, whereas for the rest, the microspheres kept exactly the same shape and size as those prior to the test (Figure 5, in THF as an example). All these tests demonstrated that PU F
DOI: 10.1021/acs.iecr.6b03526 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research microspheres were full dissolved in m-cresol, largely swelled in acetic acid, and remained intact in the rest of the tested solvents, indicating that these PU microspheres were neither dissolved nor swollen in these solvents. It is to point out that this dissolution test was conducted at 30 °C. As a matter of fact, PU microspheres were dissolved in DMSO at 60 °C by the same test. The sample for NMR analysis was done this way in this study. PU was also effectively dissolved in acetic acid when the same test was done at 66 °C or higher as shown in Figure 6, which demonstrated that the
production of highly uniform polymer microspheres through precipitation polymerization without need for any additives.
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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.iecr.6b03526. Comparison of the precipitation polymerization done under oscillation and quiescent conditions with H2O/ acetone of 40/60 at 30 °C, SEM photos of PU microspheres prepared under oscillation and quiescent conditions with H2O/acetone mass ratio of 20/80 at 30 °C, SEM photos of PU microspheres prepared under oscillation and quiescent conditions with H2O/acetone mass ratio of 30/70 at 80 °C, Fourier transform infrared spectra of PU microspheres, and thermal gravimetry analysis of PU microspheres (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 531 82767725. Fax: +86 531 87161600. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 6. Light transmittance of PU microsphere dispersion in acetic acid after shaking for 6 h at different temperatures.
ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (nos. 21274054, 21304038, and 51473066) and by the Natural Science Foundation of Shandong Province, China (ZR2012BQ019).
PU microspheres were practically dissolved, yielding a light transmittance of about 98% with the microsphere dispersion in acetic acid shaken for 6 h at 66 °C, in agreement with the reported test done at 70 °C for similar microspheres.29 In that study, the microspheres that consisted of cross-linked PU prepared using IPDI and diethylenetriamine remained intact. These results provided therefore a solid support that PU in this work consisted of linear polymers, in agreement with the conclusion from 13C NMR analysis.
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REFERENCES
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