ISSN e-polymers 2010, no. 099 Yanyan Wei, * Chengzhong Zong, Fufang Wang

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1 e-polymers 2010, no ISSN Preparation of vinyl polymer/polyurethane hybrid latex particles and their application in nano-sized vinyl polymer particle/thermoplastic polyurethane blends Yanyan Wei, * Chengzhong Zong, Fufang Wang * Key Laboratory of Rubber & Plastics (Qingdao University of Science & Technology), Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, China; tel: ; fax: ; yywei@ qust.edu.cn. (Received: 25 May, 2009; published: 09 September, 2010) Introduction Abstract: Vinyl polymer/polyurethane hybrid latex particles with various compositions were successfully prepared via emulsion polymerization of vinyl monomer in the presence of self-emulsified polyurethane dispersion without using any surfactant. Studies were carried out on polymerization kinetics, characterization of the hybrid particles and the physical properties of nano-sized vinyl polymer particle/thermoplastic polyurethane blends. It was found that the maximum content of vinyl polymer in polyurethane hybrid particles was up to 80 percent and all of the vinyl polymer/polyurethane hybrid particles were less than 120 nm. Infrared spectroscopy, thermal gravimetric analysis and differential scanning calorimetry analysis indicated the influence of vinyl polymer on the polyurethane hybrid particles. With the polyurethane shells outside and, therefore, good compatibility with polyurethane matrix, the hybrid particles can be easily blended into polyurethane matrix. Some unusual changes of dynamic mechanical properties in the low temperature region were found in the blends of hybrid particles and thermoplastic polyurethane. With the addition of only 3%, the mechanical properties of these blends did not show a significant change. This study provided a new method to prepare hybrid particles in the absence of surfactant and made an attempt on application of vinyl polymer/polyurethane hybrid particles in the blending modification. Key words: Vinyl polymer/polyurethane hybrid latex particle; nano-sized; thermoplastic polyurethane blends; dynamic mechanical properties. The synthesis of polyurethane (PU) hybrid nano/microspheres is an active area of research with the applications in the biomaterial, luminescence technology, inkjet printing, etc., [1-3] and several methods for the preparation of PU hybrid nano/microspheres have been studied and developed in recent years [4-6]. Miniemulsion polymerization process can be used for preparing nano-sized PU hybrid particles, but this process is rather complex and surfactant cannot be avoided. In our experiments, PU hybrid latex particles were produced by emulsion polymerization of vinyl monomer in the presence of PU dispersion. The core-shell morphology was formed with the hydrophobic vinyl polymers as the core and the water-borne PU as the shell. The shell can be cross-linked during the emulsion polymerization due to the unsaturated bond in PU molecules. This method is relatively easy for industry manufacture and nano-sized or micron-sized hybrid 1

2 Solid content /% Solid content /% particles can be obtained. Besides, there is no extra emulsifier used in the synthesis. There were only PU and vinyl polymer remained in the particle after being dried. PU hybrid nano/microspheres have received considerable attention as versatile materials in various applications, and most of them are applied in the fields of coatings, adhesives and inks. [7-8] However, the application of PU hybrid nano/microspheres in the blending modification is seldom reported. It is known that the optimal combination of properties of two different polymers can often be better achieved with structured particles than by blending the polymers. PU has a poor compatibility with vinyl polymers, whereas vinyl polymer/pu hybrid particles can gain the optimal combination of properties of PU and vinyl polymer. It is believed that vinyl polymer/pu hybrid particles with PU as the outside shell have a good compatibility with PU matrix. Due to the good compatibility, the vinyl polymer/pu hybrid particles can be dispersed into PU matrix homogeneously by blending. The slightly cross-linked PU shells can be considered as being dissolved in the surrounding PU matrix. So the vinyl polymer particles/pu blends can be formed by this means. If the cores of the hybrid particles are nanoscale, nano-sized vinyl polymer particle/thermoplastic polyurethane (TPU) blends can be accomplished. We made an attempt to apply the vinyl polymer/pu hybrid particles synthesized in our experiments in the blending modification of TPU, and several interesting phenomena were found. Results and discussion Emulsion polymerization of monomer in PU dispersions Since polyurethane acrylate (PUA) has good emulsification ability with a certain amount of hydrophilic groups in its chain, the droplets with PUA as the shell and monomer as the core were formed after emulsification. Small part of MMA monomer was dissolved into the water phase due to its relatively high water-solubility Time/min (a) St content 50% St content 65% St content 80% Time/min (b) MMA content 50% MMA content 65% MMA content 80% Fig. 1. The change of solid content during synthesis of vinyl polymer/polyurethane hybrid latex particles. (a) Synthesis of PS-PU hybrid particle. (b) Synthesis of PMMA- PU hybrid particle. 2

3 It can be seen from Fig.1 that the solid content of emulsion increased continuously up to 25 wt.% solid content, which is the total mass content of PUA and monomers. It was clearly that the conversion of monomer became nearly 100% when the solid content reached to 25 wt.%. From the result of the final solid content, it can be concluded that most monomer was converted into hydrophobic polymer and became the core of PU hybrid particles during the process of emulsion polymerization. As described, different amounts of monomer were polymerized into PU hybrid latex particles. The maximum content of monomer which could be emulsified by PUA was up to 80 percent. The high emulsification ability of PUA was attributed to the high carboxylic content and its flexible backbone. The reaction time in Fig.1 increased with the increasing of monomer content. Furthermore, it was found that the polymerization of St in PUA dispersion consumed longer reaction time than that of MMA. Characterization of vinyl polymer/pu hybrid latex particles The particle sizes of PU hybrid latex particles in Tab. 1 shows an increasing trend with increasing of monomer content, whereas particle size distributions does not show a clear trend with the change of monomer content. As previously described, PUA had a relatively high carboxylic content. This is the reason why the particle sizes of the hybrid latexes are so small. Especially, all of the PMMA-PU hybrid particles are under 100 nm. It was found that the type of monomer also had an important effect on particle size. The PS-PU hybrid particles had larger particle sizes than PMMA-PU hybrid particles at the same monomer content. PS-PU hybrid latexes were opaque and that of PMMA-PU were semitransparent with bluish cast. Tab. 1. Particle size and particle size distribution of vinyl polymer/pu hybrid latex particles synthesized with different monomer content. Monomer Content Poly Index Z-Average(nm) MMA 50% MMA 65% MMA 80% St 50% St 65% St 80% FT-IR spectra of vinyl polymer/pu hybrid particles are shown in Fig.2. The broad peak at the range of 3425 cm -1 ~3443cm -1 is assigned to the hydroxyl group of carboxyl group. It is believed that the water sorption of carboxylate broadened this peak. The peak located at 1726 cm -1 is assigned to the carbonyl group derived from MMA and the peaks at about cm -1, cm -1 are characteristic peaks of carbonyl group derived from PMMA. The characteristic absorption peaks of aromatic ring of styrene are located at about 757 cm -1, 698 cm -1. There is also an absorption peak at 1726 cm -1 in PS-PU spectra, which is attributed to the carbonyl group on PU chain. In addition, PUA exhibited the characteristic peaks at about 2852~2950 cm -1 (-CH 2 -CH 2 - CH 2 -CH 2 -) and cm -1 (-NH-). 3

4 weight (%) ATR Units MMA content 80% St content 80% Wavenumber cm -1 Fig. 2. The FTIR spectra of vinyl polymer/pu hybrid particles MMA content 65% MMA content 80% St content 80% Temperature ( 0 C) Fig. 3. Thermogravimetric analysis of vinyl polymer/pu hybrid particles. The thermal stability of PU hybrid particles had been investigated by TGA. Below C, the thermal weight loss of all samples was about 3 wt.%~4 wt.%, which was mainly attributed to the release of absorption water in polymer. PMMA/PU hybrid nanospheres had the obvious weight losses at about C and C, which was due to the degradation of PMMA present in the PU particles. In the same way, PS/PU hybrid nanosphere had a weight loss at about C, which was due to the degradation of PS. The decomposition of PUA occured in the temperature range of 200~400 0 C. Due to the influence of PUA, PMMA/PU hybrid nanospheres with the monomer content of 65 wt.% had a higher weight loss than that with monomer content of 80 wt.% in this temperature range. Vinyl polymer/pu hybrid particles showed a prominent thermal transition in their DSC curves (Fig. 4). PMMA/PU hybrid nanospheres with the monomer content of 80% showed a glass-transition temperature at C, which was near to the T g of pure PMMA. When the monomer content decreased to 65%, the T g of PMMA was reduced to C. The decreasing of T g with the monomer content was ascribed to the chemical cross-linking between PUA and PMMA. The DSC thermogram of PS/PU hybrid nanosphere showed the same pattern as PMMA/PU hybrid nanospheres. 4

5 Heat Flow (mw/mg) MMA 80%:Glass transition C MMA 65%: Glass transition C St 80%: Glass transition C exo Temperature ( 0 C) St 80% MMA 80% MMA 65% Fig. 4. Differential scanning calorimetry of vinyl polymer/pu hybrid particles. Morphology of vinyl polymer/pu hybrid particles Fig. 5 shows TEM photographs of vinyl polymer/pu hybrid particles with different kinds and contents of monomers. The dark region in the photos indicates the area where PU is predominantly localized. They are actually the collapsed shells of the particles. Some PU particles without vinyl polymer cores were thought to be contributors of the dark region. The light particles mostly surrounded by the dark region are deemed to be the PMMA or PS core of the hybrid particles. The particle size of PS/PU hybrid particles are bigger than that of PMMA/PU in TEM photos, which was consistent with the results got by quasi-elastic light scattering. (a) (b) Fig. 5. TEM photos of vinyl polymer/pu hybrid particles. (a) MMA content is 80 wt.% (scale bar=100 nm,magnification=50 k); (b) St content is 80 wt.% (scale bar=100 nm, magnification=50 k). The diisocyanate used to synthesize PUA was HDI, which had a soft (CH 2 ) 6 segment in its molecule. There were no aromatic rings in PUA hard segment, so the PUA shells of these particles had a too low T g to maintain a certain shape at room temperature. It was so soft that it could form a continuous film even it was slightly cross-linked. That is, we cannot see their core-shell morphology at room temperature by TEM, because their shells tend to stick to each other when the latexes were dried at room temperature [9]. The schematic presentation of film-forming process of vinyl polymer/pu hybrid particles is shown in Fig. 6. 5

6 E' (MPa) E'' (MPa) Fig. 6. Schematic presentation of film-forming process of vinyl polymer/pu hybrid latexes. Effect of vinyl polymer/pu hybrid particles on the dynamic mechanical properties of thermoplastic polyurethane blends With the PU shells outside, the vinyl polymer/pu hybrid particles were believed to have good compatibility with PU polymers. So the vinyl polymer/pu hybrid particles were dispersed into TPU matrix by blending. And some unusual changes of the dynamic mechanical properties and mechanical properties were found in our experiments. Dynamic mechanical thermal analysis (DMTA) was performed with the three typical samples: TPU blended with PMMA/PU hybrid nanosphere with monomer content of 65 wt.%, TPU blended with PMMA/PU hybrid microsphere with monomer content of 80 wt.%, TPU blended with PS/PU hybrid microsphere with monomer content of 80 wt.%. The concentration of hybrid particle in TPU is 3 wt.% (Figure 7) TPU PMMA-0.8 blend PS-0.8 blend PMMA-0.65 blend TPU PMMA-0.8 blend PS-0.8 blend PMMA-0.65 blend Temperature ( 0 C) Temperature ( 0 C) (a) (b) Fig. 7. Temperature dependence of dynamic storage modulus E and dynamic loss modulus E at a frequency of 10 Hz, where the concentration of hybrid particles is 3 %wt. In this figure, PMMA-0.65 blend refers to the TPU blended with PMMA/PU hybrid particle (MMA content is 65 wt.%). PMMA-0.8 blend refers to the TPU blended with PMMA/PU hybrid particle (MMA content is 80 wt.%). PS-0.8 blend refers to the TPU blended with PS/PU hybrid particle (St content is 80 wt.%). In the low temperature region especially below 0 0 C, both elastic modulus and loss modulus of TPU blends with PMMA/PU hybrid nanospheres were improved. This tendency became more obvious with the decreasing of temperature. With the 6

7 increasing of MMA content, the improvement of elastic modulus and loss modulus became larger. However, the elastic modulus and loss modulus of TPU in low temperature was reduced by blending with PS/PU microspheres. This may be ascribed to the different nature of PMMA and PS. Tab. 2. Mechanical properties of hybrid particle / TPU blends. Specimen TPU PMMA-0.65 blend* PMMA-0.8 blend* PS-0.8 blend* Tensile strength (MPa) Elongation (%) Tear strength (N/mm) Hardness (Shore-A) * PMMA-0.65 blend, PMMA-0.8 blend and PS-0.8 blend refer to the same samples in Fig.7. The mechanical properties of PU hybrid particle / TPU blends are shown in Table 2. PMMA/PU hybrid particle and PS/PU hybrid particle have different effects on the mechanical properties of TPU blends. TPU blend with PMMA/PU hybrid particles exhibited a higher tensile strength and tear strength, but a lower elongation than TPU. However, TPU blend with PS/PU hybrid particles showed a lower tensile strength, but a higher tear strength and elongation than TPU. Due to the addition amount of 3 wt.%, the changes of tensile strength, elongation and tear strength were not very significant. There was little change in hardness with addition of hybrid particles. Conclusions This work demonstrates that vinyl polymer/polyurethane hybrid latex particles can be prepared via emulsion polymerization of vinyl monomer in the presence of selfemulsified PU dispersion. Nano-sized PMMA/PU hybrid latex particles were obtained and the particle sizes of PS/PU hybrid latex particles were under 120 nm. The polymerization kinetics was influenced by the type of monomer. The polymerization rate of PMMA/PU hybrid latex was faster than that of PS/PU hybrid latex. IR analysis indicates the presence of vinyl polymer in the hybrid particles. DSC and TGA analysis show that the hybrid particles with different kind of vinyl polymers have different thermal behaviors, mainly affected by the nature of vinyl polymer. It was found that vinyl polymer/polyurethane hybrid particles have changed the dynamic mechanical properties of TPU blend with the addition amount of only 3 wt.%, especially in the low temperature region. The mechanical properties were changed slightly at this addition amount. Experimental part Materials 1,6-Hexamethylene diisocyanate(hdi, >99.5%) was used without further purification. Dimethylol propionic acid (DMPA, >99%), polytetramethylene glycol (PTMG, M w =1000 g/mol) were maintained in vacuum at 110 o C for 2 hours to remove the moisture. 2-Hydroxyethyl methacrylate(hema, >98%), N-methyl-2-pyrrolidone(NMP, 7

8 analytical pure) and triethylamine (TEA, analytical pure) were dried by molecular sieves. Methyl methacrylate (MMA,>98%) and styrene (St, >99%) was vacuum distilled before use. Dibutyltin dilaurate (DBTL, analytical pure) was used as catalyst. P-Hydroxyanisole (analytical pure) was used as an inhibitor. Ammonium persulfate (APS, analytical pure) was used as received. Thermoplastic polyurethane from Bayer (The trade name is TPU8685) was dried in 110 o C for 4 hours before use. Synthesis of polyurethane acrylate (PUA) The reaction was carried out in a dry flask with a stirrer, a reflux condenser and a thermometer under N 2 purge. After the nitrogen gas inlet for 10 minutes to eliminate the residual moisture, HDI and PTMG were poured into the reactor. The reaction temperature was raised to 80 o C. When the amount of the NCO group reached to the end point of the reaction, which the residual NCO group was 3.28 mmol/g in this experiment, the reaction temperature was reduced to 60 o C. At this reaction temperature, DMPA was added to extend the polymer chain and incorporate carboxyl as the hydrophilic group. NMP was used as the solvent to dissolve DMPA. In the end, HEMA was poured into the reactor to react with the residual NCO group and incorporate unsaturated C=C group into the polymer. The carboxyl content of PUA was 0.73 mmol/g. PUA was neutralized with TEA for 15 minutes. CH 3 H 2 C=CCOOCH 2 CH 2 OCONH COO - COO - COO - CH 3 NHCOOCH 2 CH 2 OCOC=CH 2 Fig. 8. The molecular structure of polyurethane acrylate. Preparation of vinyl polymer/polyurethane hybrid particles After neutralized with TEA for 15 minutes, PUA was mixed with a certain amount of MMA/St monomer. The PUA/monomer mixture was transferred into a plastic tank under FLUKO F25 homogenizer. The dionized water was poured into the mixture under the agitation speed of rpm. The emulsificaiton was mantained at 25 0 C for 5 minutes. Finally, a semi-transparent dispersion with the solid content of 25 wt.% was obtained. The dispersions were poured into a glass reactor with a stirrer, a reflux condenser, a thermometer. The dispersions were stirred and heated to 65 0 C under N 2 purge. 0.4 wt.% of APS was added into the reactor and initiated the polymerization. The reaction lasted about 5 hours. The resulting latex concentration was approximately 25 wt.% by weight. M M M M M Emulsion Polymerization Monomer swollen particle PU hybrid nano/microsphere Fig. 9. Schematic representation of PU hybrid particle synthesis. 8

9 Vacuum freeze-drying of vinyl polymer/pu hybrid latex The emulsions were kept at C for 2 hours. Then they were transferred into a vacuum freeze-drier for 48 hours to get rid of the water and solvent. The freezedrying was performed at 0.28 kpa with a condenser temperature of C. After freeze-drying at vacuum, the latex was turned into dried powders. The powders were used for testing and blending. In the following characterization, the dried powders of vinyl polymer/pu hybrid particles were used for IR, TGA and DSC testing. Blending and molding 1.5 g powder obtained by freeze-drying and 50 g TPU was mixed in an internal mixer (HAAKE Rheometer) at C and 50 rpm for 8 min. Then the vinyl polymer particle/tpu blends were molded into a sheet with 2 mm thickness in a molding machine. The sheet was cut into certain shapes for dynamic mechanical thermal analysis and mechanical testing. Measurement and characterization Monomer conversion was measured gravimetrically. The emulsion was inhibited by P-hydroxyanisole, dried in vacuum, and weighed. The conversion of the vinyl monomer was estimated by abstracting the PUA weight from the total solid content. Quasi-elastic light scattering (Zetasizer 3000HSA from Malvern Instrument) was used to investigate the particle size and particle size distribution. The PU hybrid latex was diluted to about 0.3 wt.% by the pure water for test. Transmission electron microscopy (TEM) measurements were carried out by JEM- 2000EX. The emulsion was diluted to 0.5 wt.% by distilled water. TEM specimens were made by staining one drop of diluted emulsion on copper grids. After 20 minutes, the sample was stained using 3 wt.% phosphotungstic acid (PTA) aqueous solution for 1 minute, leaving the cores bright while staining the polyurethane shell dark. Extra solution was absorbed by filter paper. The stained specimens were dried at room temperature before observation. Fourier transform infrared attenuated total reflectance (FTIR-ATR) was used to characterize the structure of PU hybrid microsphere. This experiment was performed on a Fourier Transform Infrared Analyzer (VERTEX 70, from Bruker company). The latex sample was vacuum freeze-dried before testing. Thermal Gravimetric Analysis (TGA) was performed using TG 209 F1 from NETZSCH. The sample was heated at 10 K/min from 30 0 C to C in a stream of nitrogen. The latex sample was vacuum freeze-dried before testing. Differential Scanning Calorimetry (DSC) was performed using DSC 204 F1 from NETZSCH. The sample weighing 10 mg was heated at 10 K/min from C to C. The sample was heated from 25 0 C to C before testing to eliminate thermal history. The latex sample was vacuum freeze-dried before testing. Dynamic Mechanical Thermal Analysis (DMTA) was carried out by NETZSCH DMA 242. The samples were prepared by cutting the vinyl polymer particle/tpu blend sheets into certain shapes. The sample was tested in dual cantilever mode at a frequency of 10 Hz. The temperature sweep was from C to C at 3.0 K/min. 9

10 The tensile strength,elongation and tear strength were measured using a testing instrument(instron Tensile Strength Teste), at a crosshead spead of 500 mm/min.all experiments were carried out at 25 0 C. Acknowledgements This research was supported by three foundations: a Project of Shandong Province Higher Educational Science and Technology Program under Award J09LB05; Promotive Research Fund for Excellent Young and Middle-aged Scientisits of Shandong Province under Award BS2009CL020; the foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education. References [1] Soutar, I.; Swanson, L. Langmuir 2006, 22, [2] Wu, Q.X.; Wu, Z.S.; Tian, H.F.; Zhang, Y.; Cai, S.L. Ind. Eng. Chem. Res. 2008, 47, [3] Phadtare, S.; Kumar, A.; Vinod, V.P.; Dash, C.; Palaskar, D.V.; Rao, M.; Shukla, P.G.; Sivaram, S.; Sastry, M. Chem. Mater. 2003, 15, [4] Li, M.; Daniels, E.S.; Dimonie, V.; Sudol, E.D.; El-Aasser, M.S. Macromolecules 2005, 38(10), [5] Haschick, R.; Mueller, K.; Klapper, M.; Muellen, K. Macromolecules 2008, 41(14), [6] Crespy, D.; Stark, M.; Hoffmann-Richter, C.; Ziener, U.; Landfester, K. Macromolecules 2007, 40(9), [7] Faust, H.U.; McLennan, A.J. Schofield, K. US [8] Irle, C.; Blum, H.; Kremer, W; Roschu, R. US [9] Wei, Y.Y.; Luo, Y.W.; Li, B.F.; Li, B.G. Colloid Polym. Sci. 2006, 284(10),