Preparation and Properties of Hydrophilic Rosin-Based Aromatic Polyurethane Microspheres

Abstract

Hydrophilic aromatic polyurethane (HAPU) microspheres were prepared through dispersion polymerization of a rosin-based polyurethane dispersion with C=C and styrene (St). The effects of the monomer ratio (i.e., waterborne rosin-based aromatic polyurethane (WRPU) to St), dispersant level, and reaction temperature on the properties of the microspheres were investigated; the effects of pH and adsorption temperature on the adsorption capacity of Orange II were also studied. The microspheres were characterized using Fourier transform infrared spectroscopy, energy-dispersive spectrometry, thermogravimetric analysis, laser particle size analysis, and scanning electron microscopy. The results showed that HAPU microspheres have been successfully synthesized and the produced microspheres exhibited good thermal stability and monodispersion. The optimum reaction conditions for the preparation of the microspheres were determined as a monomer ratio (m_WRPU/m_St) of 6:4 with 8 wt % poly(vinyl pyrrolidine) (on the basis of the mixed monomer) at 80 °C for 8 h. Under these conditions, the average particle size of the synthetic microspheres was 120 nm and the particle size distribution index was 0.442. The microspheres’ adsorption capacity for Orange II reached 17.53 mg·g^–1 when the solid–liquid ratio was 1 g·L^–1, with an initial concentration of 100 mg·L^–1 at pH 5, and the adsorption was conducted at 313 K for 3 h.

Introduction

Polymer microspheres are a kind of spherical or nearly spherical particles. They are widely used in separation and adsorption and can also be used as specific carriers of proteins, enzymes, antibodies, or drugs for clinical examination, injection, oral drug administration, and other applications. Polyurethane (PU) microspheres were first reported in the 1970s, obtained directly from a separation column by Hileman et al.¹ PU microspheres have a greater elastic behavior compared to those of conventional biodegradable polymers, such as polylactic acid (PLA) and poly(lactic-co-glycolic acid). In addition, the performance of the PU microspheres can be controlled by changing the proportion and type of hard and soft segments. PU microspheres can be prepared through chemical synthesis as well as physical processing methods. The former method includes (inverse) suspension polymerization, dispersion polymerization, and emulsion polymerization.² Of these, dispersion polymerization has been found to be an efficient method for producing monodisperse polymer particles with particle sizes in the range of several hundred nanometers to micrometers.³

So far, the fabrication of PU resin typically requires some amount of organic solvents. Waterborne PU has become a hot topic of many studies in recent years with respect to sustainable environmental development and ensuring full use of the characteristics of PU. However, the study of waterborne PU has mainly focused on its use in coatings,⁴ composites,⁵ microcapsules,⁶ leather, and adhesive industries; there are still few reports on the preparation of PU microspheres. As a new type of organic polymer material, PU microspheres contain many polar chemical groups, such as −NHCOO, which exhibit high adsorption, chelation, and ion exchange, as well as good stability in an aqueous solution. At the same time, the larger specific surface area provides storage places for target molecules, which could be used in biomedical applications.⁷ For example, Budriene et al.⁸ synthesized PU microspheres with a good thermal stability and narrow particle size distribution, using 4,4′-diphenyl-methane diisocyanate and poly(tetrahydrofuran) as raw materials, and the fixation rate of maltogenase reached 97.5%. Liu et al.⁹ successfully prepared two kinds of PU microspheres with a shape memory function using polycarbonate and isophorone diisocyanate or hexamethylene diisocyanate as raw materials for vascular embolization, respectively.

Degradability is a prerequisite for materials used in environmental protection and medicine. At present, biodegradable materials are mainly classified into biopolymers, bioceramics, biological derivatives, and hybrid materials. PU is an important component of the polymer-degradable material, with increasing research focus on biodegradable PU. Desai et al.¹⁰ prepared a corresponding PU material using starch and a plant component (i.e., lignin, molasses, and wood powder filler) as primary raw materials and verified its degradation using the soil burial method. Zhang et al.¹¹ synthesized a biodegradable castor oil-based PU, which was eventually used as a carrier for drug release. Sullad et al.¹² prepared blended microspheres of chitosan and PU through a water-in-oil emulsion cross-linking method and used them to encapsulate two water-soluble cardiovascular drugs. As one of the most common natural renewable resource, rosin amounts to more than 1 billion tons of annual production worldwide. The bulky hydrophenanthrene ring of rosin can improve the mechanical strength of the resulting material, and the double bonds and carboxyl group can be used as reactive centers for modification; thus, it has been widely applied in chemistry, medicine, and other fields.¹³ However, the synthesis of biodegradable PU microspheres with rosin as a raw material has not been reported. Due to rosin’s biodegradability, special structure, and excellent compatibility with the human body, rosin-based PU microspheres have great potential for responsive behaviors, such as adsorption and drug release.

To combine the characteristics of PU with those of rosin, stable, uniform, and well-shaped PU microspheres were prepared through the dispersion polymerization method using rosin as a raw material; the effects of monomer ratio, dispersant level, and reaction temperature on the properties of the microspheres were investigated. Furthermore, the adsorption properties of the resulting microspheres, applied on Orange II, were studied under different conditions.

Conclusions

The rosin-based aromatic hydrophilic PU microspheres with good thermal stability and monodispersion were successfully prepared through dispersion polymerization using biodegradable polyester as a soft segment. The optimal conditions for the preparation of the PU microspheres were determined. Under these conditions, the resulting microspheres exhibited good sphericity and surface morphology, with an average particle size of 120 nm and a polydispersity index (PDI) of 0.442. The optimal adsorption conditions of the prepared microspheres for Orange II were also determined, and the maximum adsorption capacity of golden Orange II reached 17.53 mg·g^–1. In addition, on account of the electrostatic adsorption mechanism, the pH value and temperature exerted a large impact on the adsorption. The study provides a new way for the preparation of nanoscale biomass polyurethane microspheres that can be potentially applied in the adsorption field.

Experimental Section

Raw Materials and Chemicals

Industrial-grade rosin was provided by Guilin Xingsong Forest Chemical Co. Limited (Guangxi, China). 2,4-Tolylene diisocyanate (TDI) was purchased from Alfa Aesar. Analytical-grade reagents acetone, di-n-butylamine, triethylamine (TEA), and azodiisobutyronitrile (AIBN) were all purchased from Xilong Scientific Co. Chemically pure grade acrylic acid, styrene (St), and di-n-butyltin dilaurate (DBTDL) were purchased from Xilong Scientific Co. 2,2-Dimethylol propionic acid (DMPA) and glycidyl methacrylate (GMA, 98%) were purchased from Aladdin Industrial Co. The indicator bromocresol green was purchased from Xilong Scientific Co. Chemically pure grade poly(vinyl pyrrolidone) (PVP) and Orange II were purchased from Sinopharm Chemical Reagent Co., Ltd. DMPA was dehydrated at 120 °C for 4 h prior to use. St was purified through a chromatographic column and kept in a cold refrigerator.

Synthesis of an Adduct (RA) of Rosin and Acrylic Acid

RA was synthesized through the Diels–Alder reaction (Scheme 1). Rosin was placed in a three-necked flask equipped with a thermometer, a condenser, a stirrer, and a nitrogen protection device. The flask was slowly heated until the rosin melted completely. Acrylic acid (rosin/AA = 1:1.2, molar ratio) was then added dropwise at 230 °C for 1 h; by ending the reaction when the acid value remained the same, the adduct of acrylic acid and rosin was obtained.

Scheme 1. Synthesis of RA with Rosin and Acrylic Acid.

Synthesis of an Ester (RAG) of GMA and RA

An acrylic rosin-based polyester diol (RAG) was synthesized through open-loop esterification of GMA with RA (Scheme 2). RA was added to a three-necked flask equipped with a stirrer, a thermometer, a condenser, and a nitrogen protection device. The flask was immersed in an oil bath and maintained at 120 °C until the RA was dissolved in toluene. Thereafter, GMA (RA/GMA = 1.0:2.0, molar ratio), catalyst triethylamine (0.5 wt % of RA), and inhibitor hydroquinone (0.5 wt % of GMA) were added to the system. After the acid value remained unchanged, the reactor was cooled down and the reaction mixture was filtered (i.e., removed the hydroquinone); the ester of GMA and RA was obtained.

Scheme 2. Synthesis of RAG with RA and GMA.

Preparation of WRPU Dispersion

RAG and DBTDL (0.5 wt % of reactive monomer) were dissolved in acetone. A three-necked flask containing the reaction mixture, fitted with a stirrer, a reflux condenser, and a nitrogen inlet, was carefully heated to approximately 30 °C. Next, TDI was introduced into the system for the reaction. The −NCO content was determined using the acetone-di-n-butylamine method. When the −NCO content was reduced to the theoretical value, DMPA (11 wt % of the reaction monomer) was added and the reaction mixture was heated to 55 °C. When the −NCO content dropped to zero, the system was cooled down to 18 °C with ice water. Subsequently, triethylamine (n_TEA/n_DMPA = 1.0:1.0) was added as a neutralizer under stirring for 10 min. Finally, distilled water was added at 1300 rpm to the reactor for emulsification for 30 min. The aromatic rosin-based waterborne PU (WRPU) dispersion was obtained after removing acetone using a rotary evaporator.

Preparation of WRPU Microspheres (Hydrophilic Aromatic Polyurethane, HAPU)

A designed amount of WRPU dispersion, St, dispersant PVP, and initiator AIBN was dispersed with distilled water, using an ultrasonicator for 10 min, and mixed uniformly (Scheme 3). Then, the mixture was added to a 100 mL three-necked flask and heated to the specified temperature under N₂ protection for 8 h. After centrifugation of the mixture at 10 000 rpm for 15 min, washing with distilled water, secondary centrifugation, and air drying, the rosin-based aromatic hydrophilic PU (HAPU) microspheres were obtained.

Scheme 3. Synthesis of WRPU Microspheres with RAG, TDI, TEA, and St.

Absorption of Orange II by HAPU Microspheres

The HAPU microspheres (m = 0.03 g, solid-to-liquid ratio of 1 g·L^–1) were placed into several conical flasks, and Orange II with different pH values (V = 30 mL), with an initial concentration of 100 mg·L^–1, was added. After oscillation adsorption for 3 h at different temperatures, the absorbance value of the solution at the maximum absorption wavelength (λ_max = 484 nm) was determined. Finally, the adsorption amount of Orange II by the microspheres was calculated according to formula 1.

1

where Q represents the adsorption of Orange II by the microspheres (mg·g^–1); C₀ and C are the concentrations of Orange II before and after adsorption (mg·L^–1), respectively; V is the volume of Orange II (mL); and m is the quantity of Orange II (g).

Characterization

Fourier transform infrared (FT-IR) spectra were recorded on an IS10 FT-IR spectrometer. Thermal gravimetric analysis (TGA) was carried out with an SDT-Q600 system from 30 to 600 °C with a heating rate of 10 °C·min^–1 under a nitrogen atmosphere. Scanning electric microscopy (SEM) was carried out using JSM-6380LV to investigate the size and morphology of the HAPU. A digital pH meter (PHS-25) was used to regulate different pH values of Orange II solution. The absorbance of Orange II solution was measured by a UV–vis spectrophotometer (PerkinElmer instruments Co. Ltd., Shanghai). An energy-dispersive spectrometer (equipped on JSM-6380LV) was used to analyze the types and contents of elements in the microspheres and to study the pellet-forming mechanism by comparing the particle size distribution. The particle size and distribution were tested by a laser diffraction particle size analyzer (ZS-90, Malvern, U.K.).

The surfaces of HAPU microspheres were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi).

Results and Discussion

Effects of Monomer Ratios

We investigated the effect of the monomer ratio on the performance of HAPU microspheres under the condition of a PVP content of 6 wt % (on monomer weight, the same hereafter), a monomer concentration of 8 wt %, and a reaction temperature of 85 °C. The particle size and particle size distribution of the HAPU microspheres prepared using different monomer ratios were determined, and the results are shown in Table 1 and Figure 1. With the increase of WRPU content, the average particle size increased, whereas the particle size distribution narrowed. The WRPU molecules are linear structures and are mainly aggregated through hydrogen bonds. As the content of St increased, the cross-linking density of the segments increased due to their chemical bonds, resulting in the reduction of the particle size.¹⁴ The increase in the St content also reduced the content of hydrophilic WRPU in each colloidal particle and increased the amount of small particle size, resulting in a wider particle size distribution. In addition, Figure 1 shows two peaks of different sizes below and above 100 nm and the intensity of the peak for the larger particle sizes increased with the WRPU content; this suggested that the number of the large microspheres increased, whereas the number of the small microspheres decreased.

Table 1. Effects of Monomer Ratios on HAPU Microspheres.

mWRPU/mSt	average particle size (nm)	PDI
8:2	194	0.255
7:3	187	0.256
6:4	161	0.445
5:5	141	0.512
4:6	133	0.528

Figure 1.

Particle size distribution diagram of the HAPU microspheres synthesized with different monomer ratios (m_WRPU/m_St: (a) 8:2; (b) 7:3; (c) 6:4; (d) 5:5; and (e) 4:6).

As shown in Figure S3a, when m_WRPU/m_St = 8:2, the microspheres exhibited poor sphericity and were mostly irregular. The reason is that a high content of WRPU yielded insufficiently rigid microspheres and eventually led to irregular collapse. From Figure S3b–e, we can observe that the surface of the microspheres becomes severely rougher as the content of WRPU increases, which could be attributed to the low content of St or excessive WRPU attached to the surface of the micelles, forming an irregular state. From the comprehensive consideration of the utilization rate of rosin and WRPU in the microspheres, as well as the final exhibition of the sphericity, it was concluded that the optimal m_WRPU/m_St reaction condition was 6:4.

Effects of Dispersant Amount

When the polymerization conditions were fixed (m_WRPU/m_St = 6:4, a monomer concentration of 8 wt %, and a reaction temperature of 85 °C), the performance of the HAPU microspheres was determined by varying the level of the dispersant PVP. As shown in Table S1 and Figure 2, the particle size of the microspheres decreased with the increase of PVP content. According to the classic theoretical relationship of Smith–Ewart (N_p = K[I]^0.4[S]^0.6[M]), the number of emulsion colloidal particles, N_p, is proportional to the concentration of the dispersant [S]^0.6; when the concentration of the dispersant increases, the number of emulsion colloidal particles will also increase and the nucleation rate of the WRPU and St mixture increases, resulting in an increase in the number of the microspheres and a decrease of the particle size. The particle size distribution curve, with 2 wt % PVP, is unimodal. However, with an increase in the amount of PVP, the particle size distribution curves are divided into two peaks; a small peak area shows a trend of increase, and the particle size distribution becomes wider. An increase of the PVP level decreases the particle size of the microsphere and increases the chances of collision between colloidal particles, which means that the probability of “secondary nucleation” increases and the particle size distribution becomes wider.

Figure 2.

Particle size distribution curves of HAPU microspheres prepared with different dispersant levels ((a) 2%, (b) 4%, (c) 6%, and (d) 8%).

As seen from Figure S4a, many microspheres connected and grooves appeared on some microspheres. This was attributed to the lower concentration of PVP and the poorer dispersion ability of WRPU and St in the early stage of nucleation; therefore, the microspheres easily aggregated together and the small ball nuclei were adsorbed on the surface of the large ball nuclei. At the end of the reaction, the small microspheres detached from the surface of large microspheres and formed a relatively smooth groove. With the increase in the PVP level, there was no agglomeration of the microspheres and the microspheres became smooth overall. As a surfactant, PVP could reduce the interfacial tension of the WRPU/St micelle, making the emulsion stable. Thus, improving the PVP content affords the microspheres a good dispersion and smoothness.

Effects of Reaction Temperature

The performances of the HAPU microspheres were determined with respect to reaction temperature under fixed polymerization conditions (m_WRPU/m_St = 6:4, a monomer concentration of 8 wt %, and PVP contents of 6 wt %). As observed from Table S2 and Figure 3, the particle size of the microspheres decreased as the reaction temperature increased whereas the particle size distribution widened. When the reaction temperature was 70 °C, the particle size distribution curve was unimodal. With increasing temperature, the curve splits from a single peak to bimodal; the shape of the small peak grew gentler and the peak area also increased, whereas the large peak decayed. Meanwhile, the microspheres became less uniform and the particle size distribution became wider. This was attributed to an increase in the reaction temperature, which accelerated the decomposition rate of initiator AIBN and increased the generation rate of free radicals, thus leading to the increase of spherical nuclei and decrease of particle size. At the same time, the adherence of PVP to the surface of the emulsion colloidal particles weakened and the surface of the colloidal particles became thinner; the aggregation ability between colloidal particles also increased, with some microspheres exhibiting larger particle sizes, and the particle size distribution became wider. As seen from Figure S5, when the temperature was 70 °C, the surface of the microspheres was rough and the microspheres aggregated together. As the temperature increased, the surface of the microspheres became smooth and the aggregation phenomenon decayed. This was attributed to the incomplete polymerization of the monomer at low temperatures, resulting in a lack of rigidity of the microspheres and easy aggregation. On the basis of various factors, the optimal reaction temperature was 80 °C.

Figure 3.

SEM images (left) and particle size distribution curves (right) of the HAPU microspheres synthesized at different reaction temperatures ((a) 70 °C, (b) 75 °C, (c) 80 °C, and (d) 85 °C).

Thermal Resistance Analysis

The thermal gravimetric analysis (TGA) and differential thermal gravity (DTG) curves of WRPU and HAPU are shown in Figure 4. Three thermal degradation stages of WRPU and HAPU are observed in the TGA curves. The first stage of degradation occurring in the temperature range of 100–150 °C is related to the volatilization of the residual solvent and water. The second stage of degradation occurring at approximately 320 °C is attributed to the decomposition of the hard segment (i.e., urethane). The third stage of degradation at approximately 443 °C is due to the decomposition of the soft segment (i.e., RAG) and St, and the amount of decomposition clearly increases. The temperature of HAPU at T_50% increases from 393 to 424 °C, whereas the temperature at the maximum weight-loss rate in the third stage also increases from 438 to 447 °C. The results indicate that the enhancement of the cross-linking density of HAPU achieved through increasing the content of St can improve its thermal stability, which is attributed to the interpenetrating network structure obtained from copolymerization between WRPU and St, increasing the energy required for thermal decomposition of the segment.¹⁵

Figure 4.

TGA and DTG curves of WRPU and HAPU microspheres.

Adsorption of Orange II with the HAPU Microspheres

Figure 5a displays the effect of the pH value of the HAPU microsphere in water on the adsorption capacity of Orange II. Note that the adsorption capacity first increases and then decreases with the increase of the pH value; the maximum adsorption amount reaches 17.53 mg·g^–1. The reason is that when the pH value was relatively low, due to its high H⁺ concentration in the solution, the dissociation degree of the sulfonic acid group in golden Orange II was low and the active site of interaction with the microsphere was reduced, resulting in a lower adsorption amount. When the pH value of the adsorbent was 5, the −NH on the surface of the microspheres protonates to −NH₂⁺; in this case, −SO₃^– dissociated from the Orange II was adsorbed through electrostatic interaction and the maximum adsorption amount was obtained. As the pH value went up further, the degree of −NH protonation decreased, thus reducing the amount of adsorption. Hence, a pH value of 5 was the preferred adsorption condition.

Figure 5.

Effect of pH values (a) and temperature (b) on the adsorption of Orange II.

Effect of Temperature on Adsorption Capacity

As observed in Figure 5b, when the temperature increased, the adsorption amount of the microspheres to Orange II first increased and then decreased. The effect of changing temperature on the amount of adsorption is the result of the combined effects of chemical kinetics and thermodynamics. On one hand, as the temperature rises, the motion of −SO₃^– dissociated from the golden Orange II molecule accelerated and the collision probability increased with −NH₂⁺, resulting in an increase in the adsorption capacity of the microspheres. However, when the temperature increased further, the adsorption capacity of the microspheres decreased because the adsorption of −NH₂⁺ into the molecular structure of the Orange–Orange II with the HAPU microspheres was an exothermic reaction and the rise in the temperature did not favor the adsorption of the golden Orange molecule to the polymer microspheres, resulting in a decrease in the amount of adsorption.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03334.

• Fourier transform infrared spectra; X-ray photoelectron spectroscopy; (Figure S1) IR spectra of TDI (a), RAG (b), WRPU (c), and HAPU (d); (Figure S2) XPS spectra of HAPU before and after adsorption of Orange II; (Figure S3) SEM diagram of waterborne HAPU microspheres synthesized with different monomer ratios; (Figure S4) SEM diagram of HAPU microspheres synthesized with different dispersant levels; (Figure S5) SEM diagram of HAPU microspheres synthesized at different reaction temperatures; (Table S1) effects of dispersant level on HAPU microspheres; (Table S2) effects of different reaction temperatures on HAPU microspheres (PDF)

Author Contributions

Prof. Caili Yu and Faai Zhang conceived the idea and supervised the project; J.S. and F.B. performed the experiment; J.S., Y.Z., and F.Z. wrote the manuscript and discussed with all authors; all authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.