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Synthesis and characterization of core–shell polyacrylate particles containing hindered amine light stabilizers
Colloids and Surfaces A: Physicochem. Eng. Aspects 392 (2011) 365–370
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and characterization of core–shell polyacrylate particles containing hindered amine light stabilizers Bin You, Daojun Zhou, Fan Yang, Xiancheng Ren ∗ College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Accepted 19 October 2011 Available online 28 October 2011 Keywords: HALS Seeded emulsion polymerization Core–shell latex particle
a b s t r a c t A polymerizable hindered amine light stabilizer (HALS) 1,2,2,6,6-pentamethylpiperidin-4-yl acrylate (PMPA) was synthesized through transesterification of 1,2,2,6,6-pentamethylpiperidin-4-ol (PMP) with methyl acrylate (MA). Core–shell latex particles containing HALS moieties in the shell phase were prepared by two-stage seeded emulsion polymerization from n-butyl acrylate (BA), methyl methacrylate (MMA) and PMPA. The Fourier transformed infrared (FTIR) and nuclear magnetic resonance (1 H NMR) analysis showed that PMPA monomer was successfully prepared and was effectively involved in the polyacrylate particles. The surface composition was studied by X-ray photoelectron spectroscopy (XPS), and the results indicated that HALS-containing groups could be distributed on the surfaces of the particles. Transmission electron microscopy (TEM) analysis revealed that the particles obtained presented a core–shell structure with a particle size around 100 nm. Two glass transition temperatures (Tg ), assigned to the core phase and the shell phase of the particles, respectively, were observed for both HALScontaining and HALS-free particles, as determined by differential scanning calorimetry (DSC). In addition, the Tg value for the shell phase of HALS-containing particles was 13 ◦ C lower than that of HALS-free particles, indicating the presence of random copolymer between MMA monomer and PMPA comonomer in the shell phase. The thermogravimetry analysis (TGA) and differential thermal gravimetric (DTG) results showed that HALS-containing particles provided an improvement in thermal stability in comparison to HALS-free particles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric materials exposed to sunlight undergo degradation, resulting in the discoloration, cracking of surface, stiffening, and decrease in the mechanical properties, which shortens their service life as a consequence of photo-oxidation. That is why the photostability of polymers is one of their most important properties, especially for those in outdoor applications. As a possible way to solve the problem of polymer stabilization, a number of different light stabilizers have successfully been used to increase stability against photo-oxidation, such as benzophenones, benzotriazoles, triazines and hindered amine light stabilizers (HALS). Among them, HALS are the most effective existing photostabilizers against deterioration induced by ultraviolet light for many polymers. HALS consist of tetramethylpiperidine or derivatives, which can prevent the deterioration of plastics by a multifunctional mechanism including mainly free radicals scavenging, deactivation of peroxidic species and quenching of singlet oxygen [1–9].
∗ Corresponding author. Tel.: +86 288546178686; fax: +86 2885405402. E-mail address: [email protected] (X. Ren). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.10.019
Most important of all, however, stabilizers must be effective over long periods of time. It is important that they do not volatilize, be leached out or can otherwise be removed from plastic materials. In addition, diffusion of the stabilizers to the surface layers is important for protection of polymers, especially for UV ageing. However, low molecular weight photostabilizers vaporize easily and have poor extraction resistance, decreasing their photostabilization effect during long-term use. These disadvantages caused by the low molecular weight stabilizers can overcome through the use of the polymeric stabilizers or polymer bound stabilizers [2,3]. A series of investigations on the synthesis of polymerizable photostabilizer derivatives and their copolymers with styrene, methyl methacrylate, acrylonitrile and olefins were reported [10–13]. However, the stabilizing fragments deep inside the matrix are not able to migrate to the surface where they are most needed and cannot protect the polymer surfaces from damage by sunlight. Very few studies are reported for the preparation of copolymers enriched with stabilizers in the surface phase via emulsion polymerization. In the present study, polyacrylate latex particles bound hindered amine light stabilizing fragments with a core–shell structure, in which the core phase and the shell phase consisted of poly(n-butyl acrylate) and poly(MMA–PMPA) copolymer, respectively, were prepared through two-stage semicontinuous
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seeded emulsion polymerization. The resulting HALS-containing core–shell particles were investigated by Fourier transformed infrared (FTIR) spectrometry, nuclear magnetic resonance (1 H NMR) spectrometry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA).
Monomers MMA (13.0 g), PMPA (4.5 g) were emulsified using SDS (0.24 g), OP-10 (0.12 g) and deionized water (50 ml). KPS aqueous solution (0.07 g of KPS in 10 ml water) together with the shell pre emulsion was added dropwise to the core latex within 2 h. The reaction was carried out for additional 2 h at 80 ◦ C. Poly(BA–MMA) core–shell latex particles was prepared in the same way except that PMPA was not involved.
2. Experimental 2.4. Measurements and characterization 2.1. Materials 1,2,2,6,6-Pentamethyl-4-piperidinol (PMP, purity above 98.0%) provided by Jinchun Meibang Chemical (Wuhan, China) was an industrial product with recrystallization refinement. 2,3,5Trimethylhydroquinone (THQ, purity above 99.0%) purchased from Tianjin Zhongxin Chemtech Co., Ltd. (Tianjin, China), was used as received without further purification. Analytical reagent grade sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS), alkylphenol polyoxyethylene (OP-10), potassium persulfate (KPS), tetrahydrofuran (THF), calcium chloride (CaCl2 ), sodium bicarbonate (NaHCO3 ), divinyl benzene (DVB), tetrabutyl titanate (TNBT), methyl acrylate (MA) were provided by Kelong Chemical Reagent Factory of China (Chengdu, China). Methyl methacrylate (MMA, analytical reagent) and n-Butyl acrylate (BA, analytical reagent), which were provided by Tianjin Bodi Chemical Co., Ltd. of China (Tianjin, China), were distilled before application. 2.2. Monomer synthesis PMPA monomer was prepared by transesterification of methyl acrylate with PMP according to [6]. The detailed synthesis of PMPA is described below: a solution of 17.1 g (0.1 mol) of 1,2,2,6,6pentamethyl-4-piperidinol, 0.06 ml tetrabutyl titanate and 0.5 g 2,3,5-trimethylhydroquinone in 36 ml (0.4 mol) of methyl acrylate was refluxed for 8 h. Methanol produced in the course of reaction was distilled of through a Vigreux column. At the end of reaction the excess of methyl acrylate was distilled off under reduced pressure and the product was purified by vacuum distillation.
FTIR measurements were carried out on a Nicolet iS10 FTIR spectrometer (Nicolet Instruments Company, USA) in the range from 4000 to 400 cm−1 in transmission. The samples were purified by precipitation of chloroform solution into methanol. The precipitate was dried and dissolved in chloroform to prepare FTIR sample. 1 H NMR spectrum was recorded on a Varian INOVA-400 spectrometer (Varian Company, USA), the emission frequency was 400 MHz and the scanning range was 0–15 of 400 MHz. The samples were purified as mentioned above. Deuterated chloroform and tetramethylsilane were used as solvent and internal standard, respectively. Transmission electron microscopy (TEM) micrographs of the core–shell polyacrylate latex particles were taken with HITACHI H-600 transmission electron microscope (Hitachi Company, Japan) at an accelerating voltage of 100 kV. The sample was diluted with deionized water and stained with 3% phosphotungstic acid (PTA) solution. The X-ray photoelectron spectra (XPS) analysis was performed on an XSAM800 (Kratos Analytical Company, UK) with Al K␣ (Xray) lamp-house. The latex particles were dried to remove all the water under vacuum condition at room temperature. For the DSC analysis, a TA Q200 (TA Instruments Company, USA) with nitrogen as the purge gas was used. The samples were heated from −90 ◦ C to 200 ◦ C at a rate of 20 ◦ C/min. Thermogravimetric analysis (TGA) was performed with a TG 209F1 Iris (NETZSCH Company, Germany) under the nitrogen atmosphere at a heating rate of 10 ◦ C/min from 50 to 800 ◦ C. 3. Results and discussion
2.3. Preparation of core–shell latex particles 3.1. FTIR analysis Poly(BA–MMA–PMPA) and poly(BA–MMA) were obtained through seeded emulsion polymerization using deionized water as reaction medium. The syntheses were carried out in a 250 ml four-neck round-bottom flask equipped with reflux condenser, mechanical stirrer and constant pressure dropping funnels. The flask was heated with a water bath. The core–shell latex was synthesized by two-stage, namely, core and shell formation.
FTIR spectra obtained from poly(BA–MMA) and poly(BA–MMA–PMPA) are shown in Figs. 1 and 2. The C–H stretching vibrations of saturated aliphatic hydrocarbons at 2958 and 2870 cm−1 , a very strong absorption peak at 1732 cm−1 associated with C O ester carbonyl stretching vibration, the C–H asymmetric and symmetric deformation vibrations of saturated
2.3.1. Synthesis of crosslinked poly(n-butyl acrylate) (PBA) core latex particles To prepare the crosslinked PBA core latex particles, the following procedure was used: The mixture of BA (16.0 g), DVB (0.8 g), SDS (0.22 g), OP-10 (0.11 g), NaHCO3 (0.20 g) and deionized water (50 ml) was vigorously stirred in a 250 ml three-necked flask for 0.5 h at 50 ◦ C. 0.02 g KPS was added to 1/5 of the resulting mixture (pre emulsion) in a 250 ml four-necked flask under moderate stirring. The reaction temperature was increased to 80 ◦ C. After additional 0.5 h, KPS aqueous solution (0.04 g of KPS in 10 ml water) and remaining 4/5 of the pre emulsion were dropped into the flask within 2 h, simultaneously. After completion of feeding, the reaction was carried out for 2 h at 80 ◦ C. 2.3.2. Synthesis of core–shell latex particles In the shell formation step, core particle synthesized above was used as core and shell monomers were mixture of MMA and PMPA.
Fig. 1. FTIR spectra of (a) poly(BA–MMA) and (b) poly(BA–MMA–PMPA).
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Fig. 2. 2000–1000 cm−1 region of the FTIR spectra. (a) Poly(BA–MMA) and (b) poly(BA–MMA–PMPA). Fig. 4.
aliphatic hydrocarbons at 1450 and 1380 cm−1 , two strong absorption bands at 1240 and 1160 cm−1 assigned to C–O–C stretching vibrations from ester groups and the –CH2 – rocking vibrations centered at 755 cm−1 also are observed for poly(BA–MMA) (Fig. 1). As expected, the C–H stretching vibrations (2958, 2870 cm−1 ), the carbonyl absorption (1723 cm−1 ), C–H asymmetric and symmetric deformation vibrations (1450, 1380 cm−1 ), C–O–C stretching bands (1240, 1160 cm−1 ), –CH2 – rocking vibrations (755 cm−1 ) are similar to those observed for poly(BA–MMA). However, a significant difference between the spectra of poly(BA–MMA) and poly(BA–MMA–PMPA) is observed. It is that a doublet at 1378 and 1360 cm−1 attributed to the –C(CH3 )2 symmetric bending vibrations, which is the characteristic absorption peak for PMPA, appears in the spectrum for poly(BA–MMA–PMPA). Therefore, it could be concluded that PMPA is successfully incorporated into the poly(BA–MMA–PMPA) latex particles. 3.2.
1H
NMR analysis
The chemical structure of PMPA and chemical composition of poly(BA–MMA–PMPA) are evaluated by 1 H NMR, and the spectra for the monomer and the copolymer are demonstrated in Figs. 3 and 4. It could be seen from Fig. 3, that the practical calculate relative peak area ratio is corresponding with the theoretic value, and coincides with the literature [6], proving that PMPA monomer is successfully prepared.
Fig. 3.
1
H NMR spectrum of PMPA monomer.
1
H NMR spectrum of poly (BA–MMA–PMPA).
In Fig. 4, the chemical shifts of –O–CH3 protons in MMA and –O–CH2 protons in BA are observed at 3.60 and 4.03 ppm [14–16], respectively. A weak peak around 5.04 ppm assigned to the chemical shift of –O–CH proton in PMPA is observed (as shown in Fig. 3), indicating that PMPA has been successfully incorporated into the polyacrylate latex particles. Their relative peak areas are 1.00 (–O–CH), 12.11 (–O–CH2 ) and 20.61 (–O–CH3 ), respectively. Based on the number of protons and the integration of these peaks, the mole ratio of n(PMPA):n(BA):n(MMA) can be calculated, i.e. n(PMPA):n(BA):n(MMA) = 1:6.06:6.87. And the mass ratio of m(PMPA):m(MMA):m(BA) could be calculated by the molecular weight of monomers, i.e. m(PMPA):m(BA):m(MMA) = 1:3.45:3.05. The result is consistent with the mass ratio of feeding monomers (m(PMPA):m(BA):m(MMA) = 1:3.56:2.89). Therefore, 1 H NMR analysis provides evidence that PMPA, BA and MMA monomers have effectively participated in the seeded emulsion polymerization. 3.3. Morphology of core–shell poly(MMA–BA–PMPA) latex particles The microstructure of poly(BA–MMA–PMPA) latex particles is provided in Fig. 5. A clear core–shell structure of the particles has been observed due to the difference of electron penetrability to the core and shell phases, as shown in Fig. 5. The white and gray regions in the particles correspond to PBA core and poly(MMA–PMPA) shell, respectively, which confirms the core–shell morphology of the
Fig. 5. TEM micrograph of core–shell latex particles of poly(BA–MMA–PMPA).
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Table 1 Element content on the surfaces of the HALS-containing polyacrylate latex particles. Element
Peak area/CPS
Sensitivity factor
Determined/%(mass)
Theoretical/(mass)
C 1s O 1s N 1s
9071.3 6671.3 142.2
0.25 0.66 0.42
72.35 26.87 0.78
74.75 23.50 1.75
Fig. 7. DSC curves of (a) poly(BA–MMA–PMPA) and (b) poly(BA–MMA). Fig. 6. XPS spectrum of C 1s region for poly(BA–MMA–PMPA).
particles. Furthermore, the presence of the particles is sphericallike in shape, with an average diameter around 100 nm (magnifying power is 60,000). Hence, it proves that the core–shell latex particles have been obtained as desired through two-stage seeded emulsion polymerization in water media. 3.4. Surface analysis of core–shell poly(BA–MMA–PMPA) latex particles XPS analysis gives some insight into the chemical composition on surfaces of the core–shell poly(BA–MMA–PMPA) latex particles and the results are presented in Fig. 6 and Table 1. The survey spectra reveal the characteristic signals of carbon (C 1s at 284.760, 285.930, 287.080 and 288.715 eV), oxygen (O 1s at 532.152 and 533.450 eV) and nitrogen (N 1s at 399.350 eV) as expected [17]. In Fig. 6, the main peaks are attributed to the aliphatic carbon atoms, the ester carbon atoms and the C–N contribution [18], respectively. It could be observed from Table 1 that the nitrogen content (0.78%) on the surfaces of poly(BA–MMA–PMPA) latex particles is less than one half of the theoretical weight average value (1.75%) in the shell phase. The possible reason is that MMA is more hydrophilic than PMPA, which results in the enrichment of MMA chain segments on the particle–water interface during the formation of the shell. All in all, the XPS results indicate that the PMPA could locate at the most up surface of the latex particles prepared by seeded emulsion polymerization using water as reaction media. 3.5. DSC analysis Fig. 7 illustrates the DSC curves for the latex particles of poly(BA–MMA) and poly(BA–MMA–PMPA). The curves present two steps in the baseline of the recorded DSC signal at −41.1, 112.6 ◦ C for poly(BA–MMA) and −41.3, 99.3 ◦ C for poly(BA–MMA–PMPA),
indicative of the presence of two glass transition temperatures (Tg ) corresponding to the core and the shell microphases in the latex particles as shown in the TEM (Fig. 5). The Tg value observed for the PBA core phase is about 8 ◦ C higher than the earlier study [19–21]. It is associated with the mild crosslinking by DVB, which hinders the sub-chain motion of PBA. The Tg value for PMMA shell phase of poly(BA–MMA) particles is in accordance with the previous study [19,20]. It is noteworthy, however, that the Tg value for the shell phase of poly(BA–MMA–PMPA) particles is 13 ◦ C lower than that of poly(BA–MMA), owing to the presence of poly(MMA–PMPA) copolymer in the shell phase. Generally, polymers with asymmetrical substituents have higher Tg values than that with only one substituent, if they have similar backbone structures [22]. Random copolymers have only one Tg value and lie in between the higher and lower Tg values of the two homopolymers [22]. Therefore, the Tg value for the shell phase of poly(BA–MMA–PMPA) is lower than that of poly(BA–MMA). It proves that PMPA comonomer have copolymerized with MMA via random copolymerization. The DSC analysis also verifies the microstructure of the latex particles as directly confirmed by TEM. 3.6. Thermal stability The effect of HALS fragments on the thermal stability of the latex particles is investigated by thermogravimetric analysis (TGA) and differential thermal gravimetric (DTG). Figs. 8 and 9 provide the TGA/DTG curves for the polyacrylate latex particles. As shown in Table 2, the onset decomposition temperature (Tonset ), the temperature at 10% (T10 ), 50% (T50 ) and 90% weight loss (T90 ), and the temperature at maximum weight loss rate (Tmax ) are all increased except the end decomposition temperature (Tend ). It can be seen from Figs. 8 and 9 that both poly(BA–MMA) and poly(BA–MMA–PMPA) exhibit a two-step degradation pattern [23].
Table 2 Thermal decomposition temperatures for HALS-containing and HALS-free polyacrylate latex particles. Sample
Tonset /◦ C
T10 /◦ C
T50 /◦ C
T90 /◦ C
Tmax /◦ C
Tend /◦ C
Poly(BA–MMA) Poly(BA–MMA–PMPA)
160 230
327 345
382 391
415 421
383 392
445 446
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onset decomposition temperature and enhanced the thermal stability of the core–shell polyacrylate latex particles. Acknowledgement We would like to thank the generous financial support by the following grant: National Natural Sciences Foundation of China, grant no. 50873069. References
Fig. 8. TGA/DTG curves of poly(BA–MMA).
Fig. 9. TGA/DTG curves of poly(BA–MMA–PMPA).
For poly(BA–MMA), the lower temperature step involves unzipping of the chains starting at both the vinylidene end groups and the weaker head-to-head linkages of PMMA in the shell [24,25]. Nevertheless, poly(BA–MMA–PMPA) provides a 70 ◦ C improvement in Tonset , owing to the random copolymerization between MMA monomer and PMPA comonomer in the shell phase, which substantially reduces the relative abundance of vinylidene end groups and the weaker head-to-head linkages in pure PMMA. The second stage decomposition involves random scission of the polymer chains [24–27]. It is worth noting that the temperature at maximum weight loss rate (Tmax ) for poly(BA–MMA–PMPA) increases 9 ◦ C, compared with poly(BA–MMA), which is associated with the scavenging of radical species by PMPA [28,29]. It is obvious that the random copolymerization between MMA and PMPA contributes to a remarkable improvement in Tonset and enhances the thermal stability for the polyacrylate latex particles. 4. Conclusions Polymerizable HALS monomer was synthesized by transesterification and was used as comonomer to prepare HALS-containing core–shell polyacrylate latex particles via two-stage semicontinuous seeded emulsion polymerization process in water media, in which the moderately crosslinked PBA was served as the core, and poly(MMA–PMPA) copolymer as the shell. The latex particles presented spherical core–shell structure with the particle size about 100 nm in diameter, as was directly confirmed by TEM. The shell phase composed of MMA and PMPA copolymer was enriched with HALS-containing groups, as revealed by 1 H NMR. And the HALS fragments could locate at the outmost surface of the latex particles (about half of the average in the shell). Random copolymerization between MMA monomer and PMPA comonomer substantially reduced the weaker end groups and linkages of the molecular chains of pure PMMA and provided a great improvement in the
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and characterization of core–shell polyacrylate particles containing hindered amine light stabilizers Bin You, Daojun Zhou, Fan Yang, Xiancheng Ren ∗ College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Accepted 19 October 2011 Available online 28 October 2011 Keywords: HALS Seeded emulsion polymerization Core–shell latex particle
a b s t r a c t A polymerizable hindered amine light stabilizer (HALS) 1,2,2,6,6-pentamethylpiperidin-4-yl acrylate (PMPA) was synthesized through transesterification of 1,2,2,6,6-pentamethylpiperidin-4-ol (PMP) with methyl acrylate (MA). Core–shell latex particles containing HALS moieties in the shell phase were prepared by two-stage seeded emulsion polymerization from n-butyl acrylate (BA), methyl methacrylate (MMA) and PMPA. The Fourier transformed infrared (FTIR) and nuclear magnetic resonance (1 H NMR) analysis showed that PMPA monomer was successfully prepared and was effectively involved in the polyacrylate particles. The surface composition was studied by X-ray photoelectron spectroscopy (XPS), and the results indicated that HALS-containing groups could be distributed on the surfaces of the particles. Transmission electron microscopy (TEM) analysis revealed that the particles obtained presented a core–shell structure with a particle size around 100 nm. Two glass transition temperatures (Tg ), assigned to the core phase and the shell phase of the particles, respectively, were observed for both HALScontaining and HALS-free particles, as determined by differential scanning calorimetry (DSC). In addition, the Tg value for the shell phase of HALS-containing particles was 13 ◦ C lower than that of HALS-free particles, indicating the presence of random copolymer between MMA monomer and PMPA comonomer in the shell phase. The thermogravimetry analysis (TGA) and differential thermal gravimetric (DTG) results showed that HALS-containing particles provided an improvement in thermal stability in comparison to HALS-free particles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric materials exposed to sunlight undergo degradation, resulting in the discoloration, cracking of surface, stiffening, and decrease in the mechanical properties, which shortens their service life as a consequence of photo-oxidation. That is why the photostability of polymers is one of their most important properties, especially for those in outdoor applications. As a possible way to solve the problem of polymer stabilization, a number of different light stabilizers have successfully been used to increase stability against photo-oxidation, such as benzophenones, benzotriazoles, triazines and hindered amine light stabilizers (HALS). Among them, HALS are the most effective existing photostabilizers against deterioration induced by ultraviolet light for many polymers. HALS consist of tetramethylpiperidine or derivatives, which can prevent the deterioration of plastics by a multifunctional mechanism including mainly free radicals scavenging, deactivation of peroxidic species and quenching of singlet oxygen [1–9].
∗ Corresponding author. Tel.: +86 288546178686; fax: +86 2885405402. E-mail address: [email protected] (X. Ren). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.10.019
Most important of all, however, stabilizers must be effective over long periods of time. It is important that they do not volatilize, be leached out or can otherwise be removed from plastic materials. In addition, diffusion of the stabilizers to the surface layers is important for protection of polymers, especially for UV ageing. However, low molecular weight photostabilizers vaporize easily and have poor extraction resistance, decreasing their photostabilization effect during long-term use. These disadvantages caused by the low molecular weight stabilizers can overcome through the use of the polymeric stabilizers or polymer bound stabilizers [2,3]. A series of investigations on the synthesis of polymerizable photostabilizer derivatives and their copolymers with styrene, methyl methacrylate, acrylonitrile and olefins were reported [10–13]. However, the stabilizing fragments deep inside the matrix are not able to migrate to the surface where they are most needed and cannot protect the polymer surfaces from damage by sunlight. Very few studies are reported for the preparation of copolymers enriched with stabilizers in the surface phase via emulsion polymerization. In the present study, polyacrylate latex particles bound hindered amine light stabilizing fragments with a core–shell structure, in which the core phase and the shell phase consisted of poly(n-butyl acrylate) and poly(MMA–PMPA) copolymer, respectively, were prepared through two-stage semicontinuous
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seeded emulsion polymerization. The resulting HALS-containing core–shell particles were investigated by Fourier transformed infrared (FTIR) spectrometry, nuclear magnetic resonance (1 H NMR) spectrometry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA).
Monomers MMA (13.0 g), PMPA (4.5 g) were emulsified using SDS (0.24 g), OP-10 (0.12 g) and deionized water (50 ml). KPS aqueous solution (0.07 g of KPS in 10 ml water) together with the shell pre emulsion was added dropwise to the core latex within 2 h. The reaction was carried out for additional 2 h at 80 ◦ C. Poly(BA–MMA) core–shell latex particles was prepared in the same way except that PMPA was not involved.
2. Experimental 2.4. Measurements and characterization 2.1. Materials 1,2,2,6,6-Pentamethyl-4-piperidinol (PMP, purity above 98.0%) provided by Jinchun Meibang Chemical (Wuhan, China) was an industrial product with recrystallization refinement. 2,3,5Trimethylhydroquinone (THQ, purity above 99.0%) purchased from Tianjin Zhongxin Chemtech Co., Ltd. (Tianjin, China), was used as received without further purification. Analytical reagent grade sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS), alkylphenol polyoxyethylene (OP-10), potassium persulfate (KPS), tetrahydrofuran (THF), calcium chloride (CaCl2 ), sodium bicarbonate (NaHCO3 ), divinyl benzene (DVB), tetrabutyl titanate (TNBT), methyl acrylate (MA) were provided by Kelong Chemical Reagent Factory of China (Chengdu, China). Methyl methacrylate (MMA, analytical reagent) and n-Butyl acrylate (BA, analytical reagent), which were provided by Tianjin Bodi Chemical Co., Ltd. of China (Tianjin, China), were distilled before application. 2.2. Monomer synthesis PMPA monomer was prepared by transesterification of methyl acrylate with PMP according to [6]. The detailed synthesis of PMPA is described below: a solution of 17.1 g (0.1 mol) of 1,2,2,6,6pentamethyl-4-piperidinol, 0.06 ml tetrabutyl titanate and 0.5 g 2,3,5-trimethylhydroquinone in 36 ml (0.4 mol) of methyl acrylate was refluxed for 8 h. Methanol produced in the course of reaction was distilled of through a Vigreux column. At the end of reaction the excess of methyl acrylate was distilled off under reduced pressure and the product was purified by vacuum distillation.
FTIR measurements were carried out on a Nicolet iS10 FTIR spectrometer (Nicolet Instruments Company, USA) in the range from 4000 to 400 cm−1 in transmission. The samples were purified by precipitation of chloroform solution into methanol. The precipitate was dried and dissolved in chloroform to prepare FTIR sample. 1 H NMR spectrum was recorded on a Varian INOVA-400 spectrometer (Varian Company, USA), the emission frequency was 400 MHz and the scanning range was 0–15 of 400 MHz. The samples were purified as mentioned above. Deuterated chloroform and tetramethylsilane were used as solvent and internal standard, respectively. Transmission electron microscopy (TEM) micrographs of the core–shell polyacrylate latex particles were taken with HITACHI H-600 transmission electron microscope (Hitachi Company, Japan) at an accelerating voltage of 100 kV. The sample was diluted with deionized water and stained with 3% phosphotungstic acid (PTA) solution. The X-ray photoelectron spectra (XPS) analysis was performed on an XSAM800 (Kratos Analytical Company, UK) with Al K␣ (Xray) lamp-house. The latex particles were dried to remove all the water under vacuum condition at room temperature. For the DSC analysis, a TA Q200 (TA Instruments Company, USA) with nitrogen as the purge gas was used. The samples were heated from −90 ◦ C to 200 ◦ C at a rate of 20 ◦ C/min. Thermogravimetric analysis (TGA) was performed with a TG 209F1 Iris (NETZSCH Company, Germany) under the nitrogen atmosphere at a heating rate of 10 ◦ C/min from 50 to 800 ◦ C. 3. Results and discussion
2.3. Preparation of core–shell latex particles 3.1. FTIR analysis Poly(BA–MMA–PMPA) and poly(BA–MMA) were obtained through seeded emulsion polymerization using deionized water as reaction medium. The syntheses were carried out in a 250 ml four-neck round-bottom flask equipped with reflux condenser, mechanical stirrer and constant pressure dropping funnels. The flask was heated with a water bath. The core–shell latex was synthesized by two-stage, namely, core and shell formation.
FTIR spectra obtained from poly(BA–MMA) and poly(BA–MMA–PMPA) are shown in Figs. 1 and 2. The C–H stretching vibrations of saturated aliphatic hydrocarbons at 2958 and 2870 cm−1 , a very strong absorption peak at 1732 cm−1 associated with C O ester carbonyl stretching vibration, the C–H asymmetric and symmetric deformation vibrations of saturated
2.3.1. Synthesis of crosslinked poly(n-butyl acrylate) (PBA) core latex particles To prepare the crosslinked PBA core latex particles, the following procedure was used: The mixture of BA (16.0 g), DVB (0.8 g), SDS (0.22 g), OP-10 (0.11 g), NaHCO3 (0.20 g) and deionized water (50 ml) was vigorously stirred in a 250 ml three-necked flask for 0.5 h at 50 ◦ C. 0.02 g KPS was added to 1/5 of the resulting mixture (pre emulsion) in a 250 ml four-necked flask under moderate stirring. The reaction temperature was increased to 80 ◦ C. After additional 0.5 h, KPS aqueous solution (0.04 g of KPS in 10 ml water) and remaining 4/5 of the pre emulsion were dropped into the flask within 2 h, simultaneously. After completion of feeding, the reaction was carried out for 2 h at 80 ◦ C. 2.3.2. Synthesis of core–shell latex particles In the shell formation step, core particle synthesized above was used as core and shell monomers were mixture of MMA and PMPA.
Fig. 1. FTIR spectra of (a) poly(BA–MMA) and (b) poly(BA–MMA–PMPA).
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Fig. 2. 2000–1000 cm−1 region of the FTIR spectra. (a) Poly(BA–MMA) and (b) poly(BA–MMA–PMPA). Fig. 4.
aliphatic hydrocarbons at 1450 and 1380 cm−1 , two strong absorption bands at 1240 and 1160 cm−1 assigned to C–O–C stretching vibrations from ester groups and the –CH2 – rocking vibrations centered at 755 cm−1 also are observed for poly(BA–MMA) (Fig. 1). As expected, the C–H stretching vibrations (2958, 2870 cm−1 ), the carbonyl absorption (1723 cm−1 ), C–H asymmetric and symmetric deformation vibrations (1450, 1380 cm−1 ), C–O–C stretching bands (1240, 1160 cm−1 ), –CH2 – rocking vibrations (755 cm−1 ) are similar to those observed for poly(BA–MMA). However, a significant difference between the spectra of poly(BA–MMA) and poly(BA–MMA–PMPA) is observed. It is that a doublet at 1378 and 1360 cm−1 attributed to the –C(CH3 )2 symmetric bending vibrations, which is the characteristic absorption peak for PMPA, appears in the spectrum for poly(BA–MMA–PMPA). Therefore, it could be concluded that PMPA is successfully incorporated into the poly(BA–MMA–PMPA) latex particles. 3.2.
1H
NMR analysis
The chemical structure of PMPA and chemical composition of poly(BA–MMA–PMPA) are evaluated by 1 H NMR, and the spectra for the monomer and the copolymer are demonstrated in Figs. 3 and 4. It could be seen from Fig. 3, that the practical calculate relative peak area ratio is corresponding with the theoretic value, and coincides with the literature [6], proving that PMPA monomer is successfully prepared.
Fig. 3.
1
H NMR spectrum of PMPA monomer.
1
H NMR spectrum of poly (BA–MMA–PMPA).
In Fig. 4, the chemical shifts of –O–CH3 protons in MMA and –O–CH2 protons in BA are observed at 3.60 and 4.03 ppm [14–16], respectively. A weak peak around 5.04 ppm assigned to the chemical shift of –O–CH proton in PMPA is observed (as shown in Fig. 3), indicating that PMPA has been successfully incorporated into the polyacrylate latex particles. Their relative peak areas are 1.00 (–O–CH), 12.11 (–O–CH2 ) and 20.61 (–O–CH3 ), respectively. Based on the number of protons and the integration of these peaks, the mole ratio of n(PMPA):n(BA):n(MMA) can be calculated, i.e. n(PMPA):n(BA):n(MMA) = 1:6.06:6.87. And the mass ratio of m(PMPA):m(MMA):m(BA) could be calculated by the molecular weight of monomers, i.e. m(PMPA):m(BA):m(MMA) = 1:3.45:3.05. The result is consistent with the mass ratio of feeding monomers (m(PMPA):m(BA):m(MMA) = 1:3.56:2.89). Therefore, 1 H NMR analysis provides evidence that PMPA, BA and MMA monomers have effectively participated in the seeded emulsion polymerization. 3.3. Morphology of core–shell poly(MMA–BA–PMPA) latex particles The microstructure of poly(BA–MMA–PMPA) latex particles is provided in Fig. 5. A clear core–shell structure of the particles has been observed due to the difference of electron penetrability to the core and shell phases, as shown in Fig. 5. The white and gray regions in the particles correspond to PBA core and poly(MMA–PMPA) shell, respectively, which confirms the core–shell morphology of the
Fig. 5. TEM micrograph of core–shell latex particles of poly(BA–MMA–PMPA).
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Table 1 Element content on the surfaces of the HALS-containing polyacrylate latex particles. Element
Peak area/CPS
Sensitivity factor
Determined/%(mass)
Theoretical/(mass)
C 1s O 1s N 1s
9071.3 6671.3 142.2
0.25 0.66 0.42
72.35 26.87 0.78
74.75 23.50 1.75
Fig. 7. DSC curves of (a) poly(BA–MMA–PMPA) and (b) poly(BA–MMA). Fig. 6. XPS spectrum of C 1s region for poly(BA–MMA–PMPA).
particles. Furthermore, the presence of the particles is sphericallike in shape, with an average diameter around 100 nm (magnifying power is 60,000). Hence, it proves that the core–shell latex particles have been obtained as desired through two-stage seeded emulsion polymerization in water media. 3.4. Surface analysis of core–shell poly(BA–MMA–PMPA) latex particles XPS analysis gives some insight into the chemical composition on surfaces of the core–shell poly(BA–MMA–PMPA) latex particles and the results are presented in Fig. 6 and Table 1. The survey spectra reveal the characteristic signals of carbon (C 1s at 284.760, 285.930, 287.080 and 288.715 eV), oxygen (O 1s at 532.152 and 533.450 eV) and nitrogen (N 1s at 399.350 eV) as expected [17]. In Fig. 6, the main peaks are attributed to the aliphatic carbon atoms, the ester carbon atoms and the C–N contribution [18], respectively. It could be observed from Table 1 that the nitrogen content (0.78%) on the surfaces of poly(BA–MMA–PMPA) latex particles is less than one half of the theoretical weight average value (1.75%) in the shell phase. The possible reason is that MMA is more hydrophilic than PMPA, which results in the enrichment of MMA chain segments on the particle–water interface during the formation of the shell. All in all, the XPS results indicate that the PMPA could locate at the most up surface of the latex particles prepared by seeded emulsion polymerization using water as reaction media. 3.5. DSC analysis Fig. 7 illustrates the DSC curves for the latex particles of poly(BA–MMA) and poly(BA–MMA–PMPA). The curves present two steps in the baseline of the recorded DSC signal at −41.1, 112.6 ◦ C for poly(BA–MMA) and −41.3, 99.3 ◦ C for poly(BA–MMA–PMPA),
indicative of the presence of two glass transition temperatures (Tg ) corresponding to the core and the shell microphases in the latex particles as shown in the TEM (Fig. 5). The Tg value observed for the PBA core phase is about 8 ◦ C higher than the earlier study [19–21]. It is associated with the mild crosslinking by DVB, which hinders the sub-chain motion of PBA. The Tg value for PMMA shell phase of poly(BA–MMA) particles is in accordance with the previous study [19,20]. It is noteworthy, however, that the Tg value for the shell phase of poly(BA–MMA–PMPA) particles is 13 ◦ C lower than that of poly(BA–MMA), owing to the presence of poly(MMA–PMPA) copolymer in the shell phase. Generally, polymers with asymmetrical substituents have higher Tg values than that with only one substituent, if they have similar backbone structures [22]. Random copolymers have only one Tg value and lie in between the higher and lower Tg values of the two homopolymers [22]. Therefore, the Tg value for the shell phase of poly(BA–MMA–PMPA) is lower than that of poly(BA–MMA). It proves that PMPA comonomer have copolymerized with MMA via random copolymerization. The DSC analysis also verifies the microstructure of the latex particles as directly confirmed by TEM. 3.6. Thermal stability The effect of HALS fragments on the thermal stability of the latex particles is investigated by thermogravimetric analysis (TGA) and differential thermal gravimetric (DTG). Figs. 8 and 9 provide the TGA/DTG curves for the polyacrylate latex particles. As shown in Table 2, the onset decomposition temperature (Tonset ), the temperature at 10% (T10 ), 50% (T50 ) and 90% weight loss (T90 ), and the temperature at maximum weight loss rate (Tmax ) are all increased except the end decomposition temperature (Tend ). It can be seen from Figs. 8 and 9 that both poly(BA–MMA) and poly(BA–MMA–PMPA) exhibit a two-step degradation pattern [23].
Table 2 Thermal decomposition temperatures for HALS-containing and HALS-free polyacrylate latex particles. Sample
Tonset /◦ C
T10 /◦ C
T50 /◦ C
T90 /◦ C
Tmax /◦ C
Tend /◦ C
Poly(BA–MMA) Poly(BA–MMA–PMPA)
160 230
327 345
382 391
415 421
383 392
445 446
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onset decomposition temperature and enhanced the thermal stability of the core–shell polyacrylate latex particles. Acknowledgement We would like to thank the generous financial support by the following grant: National Natural Sciences Foundation of China, grant no. 50873069. References
Fig. 8. TGA/DTG curves of poly(BA–MMA).
Fig. 9. TGA/DTG curves of poly(BA–MMA–PMPA).
For poly(BA–MMA), the lower temperature step involves unzipping of the chains starting at both the vinylidene end groups and the weaker head-to-head linkages of PMMA in the shell [24,25]. Nevertheless, poly(BA–MMA–PMPA) provides a 70 ◦ C improvement in Tonset , owing to the random copolymerization between MMA monomer and PMPA comonomer in the shell phase, which substantially reduces the relative abundance of vinylidene end groups and the weaker head-to-head linkages in pure PMMA. The second stage decomposition involves random scission of the polymer chains [24–27]. It is worth noting that the temperature at maximum weight loss rate (Tmax ) for poly(BA–MMA–PMPA) increases 9 ◦ C, compared with poly(BA–MMA), which is associated with the scavenging of radical species by PMPA [28,29]. It is obvious that the random copolymerization between MMA and PMPA contributes to a remarkable improvement in Tonset and enhances the thermal stability for the polyacrylate latex particles. 4. Conclusions Polymerizable HALS monomer was synthesized by transesterification and was used as comonomer to prepare HALS-containing core–shell polyacrylate latex particles via two-stage semicontinuous seeded emulsion polymerization process in water media, in which the moderately crosslinked PBA was served as the core, and poly(MMA–PMPA) copolymer as the shell. The latex particles presented spherical core–shell structure with the particle size about 100 nm in diameter, as was directly confirmed by TEM. The shell phase composed of MMA and PMPA copolymer was enriched with HALS-containing groups, as revealed by 1 H NMR. And the HALS fragments could locate at the outmost surface of the latex particles (about half of the average in the shell). Random copolymerization between MMA monomer and PMPA comonomer substantially reduced the weaker end groups and linkages of the molecular chains of pure PMMA and provided a great improvement in the
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