Abstract Filming process of polystyrene nano-latex (NPS) particles was studied by a combination of various methods. For a constant annealing time of 1 h, the ...
NOTES nm to 0.19687 nm (table 5) with average value of 0.19643 nm, which is about equal to the corresponding value in Cu2(CH2= CH--COO)d(urea)2 (0.1967 nm) and slightly shorter than that in Cu2(CH2= CH-C00)4(H20)2 (0.01972 nm). The average distance of Cu(1)-0 (carboxyl) is much shorter than the Cu(1) - O(axia1) distance, 0.21506(12) nm, due to the Jahn-Teller effect of the 8 electron configuration of Cu( 11). The angles of O(3)--€u(l)-0(1), O(3)--Cu(l)0(2A), O(1)-Cu(1)-O(4A) and O(2A)-Cu(1)--0(4A) are in the range of 90 2 " (table 5), indicating the square pyramidal geometry about the copper(I1) The and O(3)-Cu(1)bond angles of O(1)-Cu(1)-O(2A) O(4A) are 168.69" and 168.48" . Atoms 0(5), Cu(1) and Cu(1A) are not linear, where the angle of O(5)-Cu(1)Cu(1A) is 175.05" . The Cu(l), 0(1), 0(2), C(l), and Cu(1A) atoms are on one plane (average deviation: 0.0003, plane 2), and Cu(l), 0(3), 0(4), C(5), and Cu(1A) are on another plane (average deviation: 0.0010, plane 3). Planes 1 and 2 are nearly vertical with the dihedral angle of 89.5" , and it is 89.0" between planes 1 and 3, 89.1" between planes 2 and 3.
+
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 29831010) and the Foundation of the State Key Laboratory of Coordination Chemistry of Nanjing University.
References King, R. B., Encyclopedia of Inorganic Chemisty, New York: John Wiley & Sons Ltd., 1994, 829. Mathrubootham, V., Rathinam, V., Mallayan, P. et al., Copper(I1) complexes with unusual axial phenolate coordination as structure models for the active site in galactose oxidase: X-ray crystal structures and spectral and redox properties of [Cu(bpnp)X] complexes, Inorg. Chem., 1998, 37: 6418. Wang Yaoyu, Shi Qian, Shi Qizhen et al., Synthesis, characterization, crystal structure and forming mechanism of copper( I1 )a,P unsaturated carboxylate complexes with imidazole, Polyhedron, 1999, 18: 2009. Wang Yaoyu, Shi Qian, Shi Qizhen et al., Synthesis, crystal structure and forming mechanism of two novel copper( I1 ) amethacrylate complexes with benzimidazole, Science in China, Ser. B, 1999,42(4): 363. Wang Yaoyu, Shi Qian, Shi Qizhen et al., Synthesis, thermal decomposition and crystal structure of copper( 11 ) a$ -unsaturated carboxylate with urea, Chinese Science Bulletin, 1999,44(7): 602. Wang Yaoyu, Shi Qian, Shi Qizhen et al., Syntheses, magnetic properties and crystal structure of supramolecular copper( I1 ) complexes with a,P-unsaturated carboxylates, Acta Chimica Sinica (in Chinese), 1999,57(6): 541. Gao Yici, Wang Yaoyu, Shi Qizhen et al., Crystal structure of bis(a-methacrylato)-2,2 '-bipyridine-monohydrate copper( I1 ), Polyhedron, 1991, lO(16): 1893. Gao Yici, Wang Yaoyu, Shi Qizhen et a]., Synthesis, characterization and crystal structure on ternary complexes of copper( I1 ) a,P unsaturated carboxylic acids with 1,lO-phenanthroline, Chemical Journal of Chinese Universities (in Chinese), 1997, 18(3): 348. Wang Yaoyu, Shi Qian, Shi Qizhen et al., Synthesis and crystal
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10.
1I .
12.
13. 14. 15.
structure of a new mixed-valence complex of copper( I , 11) a-methacrylate with hiphenylphosphine and methanol, Chinese Chemical Letters, 1998, 19(11): 1069. Chen, X. M., Feng, X. L., Yu, X. L. et al., A chain-like polymeric copper(I1) complex bridged simultaneously by carboxylato, hydroxo and aqua ligands, Inorg. Chim. Acta, 1997,266: 121. Melnik, M., Koman, M., MacaSkovi, L. et al., Copper(I1) propionates: crystal and molecular structure of bis(propionato) copper( 11) di(methy1-3-pyridylcarbamate), J. Coord. Chem., 1998.43: 159. Holz, R. C., Bradshaw, J. M., Bennett, B., Synthesis, molecular structure, and reactivity of dinuclear copper( I1 ) complexes with carboxylate-rich coordination environments, Inorg. Chem., 1998, 37: 1219. Melnik, M., Study of the relation between the structural data and magnetic interaction in 0x0-bridged binuclear copper( I1 ) compounds, Coord. Chem. Rev., 1982,42: 259. Kato, M., Factors affecting the magnetic properties of dimeric copper( 11) complexes, Coord. Chem. Rew., 1988,92: 45. Earnshaw, A., Introduction to Magnetochemistry, London: Academic Press, 1968. (Received August 30,2000)
Novel sintering behavior of polystyrene nano-latex particles in filming process QU Xiaozhong, TANG Yalin, CHEN Liusheng & JIN Xigao State Key Laboratory of Polymer Physics & Chemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Correspondence should be addressed to Jin Xigao (e-mail: jinxg@ infoc3.icas.ac.cn)
Abstract Filming process of polystyrene nano-latex (NPS) particles was studied by a combination of various methods. For a constant annealing time of 1 h, the AFM images showed that the deformation and sintering temperatures for NPS particles were ca. 90C and 100% respectively. In spin-lattice relaxation measurements of solid state NMR, it is found that TiL, TIS and Z l d o increased significantly after annealing at 90C and above. DSC results showed that there was an exothermic peak near T, after annealing for 1 h at the selected temperatures below 95T; otherwise, the exothermic peak disappeared after annealing at 100C or above. The apparent density of NPS increased suddenly in the temperature range. The results implied that the macromolecules in NPS particles are in a confined state with higher conformational energy and less cohensional interactions which are the drive force for the sintering at a lower temperature compared with the multichain PS particles and the bulk polymer. Keywords: polystyrene, nano-latex, filming process.
The film formation process of polymer latex can
generally be divided into three steps. The first step is water evaporation and subsequent packing of polymer particles. The second step is the deformation of the latex particles at or above the minimum filming temperature (MFT). The third step starts above its T, and relates to the interdiffusion and interpenetration of polymer chains among adjacent particles to form a homogeneous film[''. It is known that temperature plays an important role in the film formation process of polymer latex, especially in the second and third steps. In recent years, more attention has been paid to the particle size effect on sintering temperature of latex filming. Goudy et aLr2' observed that a smaller particle latex resulted in a faster rate of the interparticle fusion for the polystyrene (PS) latex particles sized in the range of 0.24-1.05 ym. Mazur et al?' confirmed the existence of a maximum particle size and a minimum packing fraction in the sintering of acrylic copolymer lattices without any contribution from viscous flow. The sintering behavior for nano-latex particles has not yet received enough attention. The filming process for the PS latex with a meaning size of 1.05 ym was investigated by scanning electromicroscopy (SEM) and laser confocal fluorescence microscopy (LCFM) in our previous work. It was noted that the particle deformation and sintering temperatures were at ca. 120°C (20°C higher than its T,) and 140°C (corresponding to the flow temperature) after annealing for 1 h, re~ ~ e c t i v e l In ~ ' ~this ' . work, we focused on the sintering behavior of the film formation of nano-latex particles. By microemulsion polymerization, we synthesized polystyrene nano-latex particles (NPS) in which only several polymer chains are contained in each particle. The condensation characteristics of the macromolecules in NPS are significantly different from that of random-coils in a multichain particle or in bulk polymer'5'. The deformation temperature, sintering temperature of NPS particles and the molecular motion behavior during the filming process were studied by atomic force microscopy (AFM), I3c high-resolution solid state NMR, differential scanning calorimetry (DSC) and dilatometer, etc. Furthermore, the novel sintering behavior of the nano-latex particles and their size effect in nanoscale were discussed.
1 Experimental ( i ) Materials.
Styrene (Shanghai Chemical Reagent Co.) was washed with 10% NaOH aqueous solution for 3 times and dried over anhydrous sodium sulfate after repeated washing with distilled-deionized water (DDI water, with the electroconductivity of about 1 yS/cm). Further purification was carried out by distillation at 35°C under reduced pressure in a nitrogen atmosphere prior to use. Sodium dodecyl sulfate (SDS, Chinese Medical Co.) was recrystallized from DDI water and methanol, respectively. Potassium persulfate (KPS) and sodium bicarbon-
ate (NaHC03, Beijing Chemical Reagents Co.), were recrystallized twice from DDI water. ( i i ) Polymerization. 2 mL styrene, 3 g SDS and 0.01 g KPS were mixed into 100 mL 10 mmol/L NaHC03 aqueous solution. The microemulsion system was polymerized in a flask with an electromagentic stirrer at 70°C for 20 h under a nitrogen atmosphere. ( i i i ) Characterization. The polystyrene particles produced by microemulsion polymerization were coagulated by adding methanol with a volume ratio of 10 : 1. The NPS particles were harvested and purified by centrifugation and washing for several times with methanol and DDI water until the electroconductivity of the centrifugate less than 10 yS/cm, and allowed to dry under vacuum at room temperature. On an average, the diameter of the NPS particles is ca. 29 nm, measured by transmission electronic microscopy (TEM) and its M, is 1.64X lo6, obtained by gel permeation chromatography (GPC). Based on the laser-light scatter (LLS) measurement, it was estimated that there are ca. 4 polystyrene chains in each NPS particles. The details of synthesis and purification of the NPS were described in a previous workL5'.Part of the purified NPS particles were re-dispersed into DDI water. The films were prepared by spreading a drop of the diluted NPS dispersion directly onto freshly cleaned mica, followed by drying in air for 72 h, and then in vacuum for 24 h at room temperature. The dried films and NPS solid powder were annealed in a convection oven for exactly 1 h at the temperatures of 80, 90, 100, 110 and 120°C, respectively. After annealing, the morphology of the NPS films was imaged by the tapping-mode AFM at room temperature. With NPS powder, I3cspin-lattice relaxation times (TI) were obtained by high-resolution solid state NMR and fitted as a double exponential model for resolving into longer (TIL)and shorter (Tls) relaxation time terms. Thermal behavior was studied by DSC with a heating rate of 1O0C/min. In the temperature range of 30-200°C with a heat rate of l0C/min and the temperature maintained constant for 0.5 h in each of the selected temperature steps, the density of the NPS particles was measured in a dilatometer evacuated to less than 0.1 3 Pa before filling mercury.
2 Results and discussion ( i ) AFM images. Fig. 1 shows the height images of NPS latex films prepared at room temperature (fig. l(a)) and annealed for 1 h at the temperatures of 80 and 90°C ((b) and (c)). The particle boundary is clearly seen whenever the annealing temperature was below or at 80°C.The deformation of the particle contours could be observed obviously after annealing at 90°C, which is much lower than the deformation temperature of multichain PS (MCPS) particles (it is noted that the deformation tem-
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Fig. 1.
AFM images of NPS latex film at room temperature (a) and after annealing at 80'C (b), 90'C ( c ) for 1 h.
Fig. 2. AFM images of NPS latex film after annealing at 100°C for 1 h. (a) Height image. (b) Phase image.
perature is ca. 120°C for the particles with diameter of 1.05 pm)[51,even lower than the T, for bulk PS. Otherwise, the particle boundaries were still visible, this implies the interparticle diffusion for NPS is limited after annealing at 90°C. While the film was annealed at 100°C for 1 h, though the distinct contract was obviously among the homogeneous domains, the particle boundaries were blurred, as shown in the height image (fig. 2(a)). It is indicated that the interdiffusion and sintering took place to a certain extent. In the phase image (fig. 2(b)), which is related to surface stiffness and viscoelasticity, some homogeneous domains with the dimension much more than one particle size could be observed; in other words, the interfaces between those adjacent particles almost disappeared in the areas. Fig. 3 shows that the surface topology Chinese Science Bulletin Vol. 46
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of the film after annealing at 110°C for 1 h was fairly flat and homogeneous, indicating that the interdiffusion of the polymer chains and sintering were almost complete. All of the results implied that the sintering temperature of the nano-latex particles is obviously lower than that of multichain particles with a larger diameter (+m granularity). ( ii ) I3c solid state NMR measurements. The spin-lattice relaxations of the samples after annealing for 1 h at various temperatures are listed in table 1, in which the longer (TIL)and shorter (Tls) time terms correspond to the motion of entangled (networked) and free chain segments, respectively. The large values of TILand Ill/loresult from the interlocking structure of the side aromatic rings16]. It is found that TIL and IILllo increased significantly after annealing from 90°C to 100°C for both aromatic and aliphatic carbons in NPS chains, but changed 993
Fig. 3. AFM images of NPS latex film after annealing at 110°C for 1 h. (a) Height image. (b) Phase image. Table 1
Longer and shorter spin-lattice relaxation time of the carbons in PS chains after annealing at selected temperatures for I h
p-aromatic carbons
Idlo (%) TIsls
69.78 4.94
Aliphatic carbons CH
I L / (%) ~
83.66 9.69 16.34
TIsls Id10 (70) a) RT: Room temperature.
12.76
little at the annealing temperatures higher than 100°C. This is because the heat treatment at 90°C causes the particle deformation, leading to a loss of the free volume of NPS. The increase of TILand IIL/IO resulted from the increase of the interaction and the decrease of the mobility of the aromatic rings and the main polymer chain. For the annealing temperatures higher than 10O0C, the TILand IIL/IOvalues were larger than those of annealing at 90°C, meaning that the PS chains among adjacent particles penetrated extensively and formed a multichain polymer system. (iii) DSC measurements. The DSC traces of the NPS particles are shown in fig. 4. It is noted that there was a characteristic exothermic peak near T,'~," after annealing for 1 h at the selected temperatures below 95°C. The exothermic maximums peaked from 106.0 to 110°C and shifted forward higher as the annealing temperature increased. The exothermic enthalpy of the peaks was ca. 2 J/g for the samples annealed below 90°C while it decreased to ca. 1.7 J/g for the samples annealed at 9 5 C . However, the exothermic peak disappeared after annealing at 100°C or above. Ming et a ~ . ' found ~' that PS micro994
spheres displayed an exotherm above its T, on the DSC trace, which was interpreted by the sintering of PS rnicrospheres, i.e. the energy releases due to the loss of surface area. Our results implied that the condensed state of NPS transfers to that of multichain one because of interdiffusion and interpenetration of PS chains among adjacent particles in 90-10O0C, confirmed by NMR data and the AFM images.
Fig. 4. DSC traces of NPS particles after annealing at different temperatures for 1 h (heat rate: 10eC/min).
(iv) Dilatometer measurements. ChineseScience Bulletin Vol. 46
Fig. 5 shows the denNo. 12 June 2001
NOTES sity data of the solid NPS aggregates and the re-heated specimen. Every measurement in fig. 5 was carried out after annealing for 0.5 h at the corresponding temperatures. It is shown that the apparent density of NPS aggregates at room temperature was ca. 0.57 g/cm3 involving the void volume among particles in which mercury could not be filled completely. By dividing the packing fraction @, 0.58 and 0.74 corresponding to disordered and close packed particles respectively, the density of NPS particles was calculated to be 0.80 - 0.97 g/cm3. The value is close to that measured by porosimetryr71.As shown in fig. 5, the apparent density of NPS aggregates is larger than 1.0 g/cm3 at 102°C and approach the bulk one at ca. llO0C, which means the interdiffusion camed out at the annealing temperature higher than 100°C.
of the NPS particle is only 29 nm and the density of the particle is ca. 10 % lower than that of bulk PS. As M, = 1.64 X lo6, the root-mean end to end distance should be ca. 80 nm, implying that the polymer chains in the NPS particle are in a confined state. So the macromolecules contain less entanglements and more free volume, leading to higher conformational energy than that of bulk PS and MCPS chains. Furthermore, the higher conformational energy is the major driving force for the NPS particles sintering at a lower temperature. 3 Conclusion
In summary, the filming process of NPS particles was studied by a combination of various methods. It is found that the deformation and interdiffusion temperatures of NPS particles are at ca. 90°C and 100°C respectively, which are much lower than those of MCPS particles. The results implied that the macromolecules in NPS particles are in a confined state with higher conformational energy and less cohensional interactions which are the driving force for the sintering at a lower temperature compared with the bulk PS and the multichain PS particles. Acknowledgements This work was supported by the National Key Project for Fundamental Research, the State Science and Technology Commission of China (Grant No. 95-1 I), and the National Natural Science Foundation of China (Grant Nos. 2000401 1 and 20023003).
References
Fig. 5. Apparent density of NPS in the annealing process measured by dilatometer. I , NPS: 2, bulk PS.
( v ) Discussion. In general, the molecular motion and interpenetration of polymers occur around their flow temperature (Tf). ~ o ~ e rprovided '~' the concept of liquid-liquid relaxation temperature (Tll) above its T, for a polymer, i.e. the beginning temperature for the mobility of mass center and diffusion of a polymer chain. It relates to the excitation of segments motion between entanglement points (M,), if M,,>M, (M, of PS is ca. 200--300 units and its TI, is ca. 150-16O0C). This is in agreement with our previous experiments on the filming process of MCPS particles141. But the sintering temperature of NPS is much lower than that mentioned above. Li et al.'lO1simulated the conformation of a single polystyrene chain (SPSC, M, = 4.16 X lo6) in 8 solution by the RIS-Monte Carlo model. With a particle diameter of 24 nm, it is shown that the conformational energy of the single chain particle is 0.93 J/g higher than that of a free state (Gaussian coil) PS chain. On the other hand, if the particle diameter is larger than 120 nm or there are 20 PS chains contained in a particle within a diameter of 24 nm, the conformation energy of the SPSC is equal to that of a Gaussian PS chain. As mentioned above, the diameter
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