TY - JOUR
AU - Sadhukhan,, Pat
AB - Abstract There has been considerable interest, both academic and industrial, in developing synthesis processes for making polymeric nanoparticles. Our effort relied on the nanoassembly concepts of block macromolecules in solutions to prepare particles with a hard core made of crosslinked plastics and a soft shell made of low Tg elastomer. By a suitable variation of the composition, polymer molecular weight and solute concentration, we were able to produce spherical, ellipsoidal, cylindrical, and chain-like nanoparticles. Under microscopes, the chain-like nanoparticles displayed very rich conformational features in diluted and dense states. Our observation on the conformation characters of the nanochains in 3D diluted state agreed well with the proposition of the self-avoid coil model. However, in 2D dense state, our observation on the nanochains appeared to be in contradiction with the segregated globule model proposed by de Gennes. microscopy, styrene, butadiene, polymer, nanoparticles, morphology, conformation Introduction Methods that allow the preparation of polymeric nanoparticles with varied and elaborate structures and functions, similar to those of biological systems, have drawn much attention during the past decade [1–9]. This interest is largely based on the potentials that polymeric nanoparticles ranging in size from 10 to more than 100 nm and having new and enhanced physical, chemical, biological and mechanical properties could be designed and commercially fabricated in large scales [11–15]. They would, of course, have to be suitably tailored to the task they are to perform in specific applications [16–20]. Moreover, the market will require that they be made reproducibly in relatively high volumes and at a cost commensurate with the value of the benefit they are expected to impart in different applications. Liu et al. [1,2] first reported a process to make polymeric nanoparticles by taking advantage of the self-assembly of diblock copolymer in the solid state. Their approach was based on the synthesis of diblock copolymer with a photo-crosslinkable block which allowed them to form an ordered structure of either nanosized spherical or cylindrical domains. After crosslinking the core of these self-assembled structures and dispersing the resultant material in a solvent, they obtained core-shell structured nanospheres [1] and nanofibers [2]. Applying the same concept, Wooley et al. [3–5] used diblock copolymers to form amphiphilic nanoparticles of core-shell structure. In their experiments, the micelles were formed in a dilute solution and subsequently stabilized by the crosslinking of the outer shell layer. Similar results were obtained by Akashi et al. [6–9] with comb-type polymers prepared by dispersion polymerization of a crosslinkable monomer and a macromonomer. The macromonomer, prepared separately and carefully purified [7] prior to its use, acted as a steric stabilizer to provide colloidal stability to the micelles formed in a solvent and eventually it became a part of the surface layer of particles. The spherical nanoparticles generated had a core-shell structure with short hydrophilic brushes. More recently, Zheng et al. [10] prepared core-shell type nanoparticles by a multistep process involving the initial synthesis of crosslinked nanosized globules by conventional microemulsion copolymerization of polystyrene with divinylbenzene (DVB) and ethylvinylbenzene. Then, polybutadiene of about 17 kg/mol in molecular weight was surface grafted onto the preformed core particle by anionic polymerization after they were washed, dried and subsequently suspended in hexane. It is well known that polymer chains in solution, depending on their miscibility, can self-assemble into domains of various structures. Our first attempt, dated back to the 1990s [11–20], extended the nanoassembly concepts of macromolecules in solutions to prepare nanosized particles with a hard core made of crosslinked plastics and a soft shell made of low Tg elastomer. Our approach relied on the thermodynamic-driven force of certain heterogeneous macromolecules placed in a solvent to self-assemble into micelles of predominated size and various shapes. Our primary effort focused on the anionic living polymerization of styrene and butadiene in hydrocarbon solutions. Additional steps included stabilizing (crosslinking the core) the micelles to form the nanoparticles that could have various designs in the shapes and internal structures. The synthesized nanoparticles showed unique merits as performance-enhancing additives and novel reinforcing elements in rubber compounds [14]. In this study, we report an investigation of the nanoparticles using microscopes and particularly focus on the conformation behavior of those particles in various states. Experimental methods Synthesis of spherical nanoparticles The synthesis of spherical nanoparticles involves using a diblock copolymer with a molecular weight of 50 kg/mol and a butadiene (BD)/styrene weight ratio of 50/50, which was made by using lithium-initiated living anionic polymerization by a sequential addition of first butadiene and then styrene after all the butadiene has been polymerized as shown in Fig. 1a. In a hexane solution of about 12% solid, this living diblock aggregate forms spherical micelles, with the styrene block directed toward the center of the micelle and the butadiene block as tail extending therefrom. This occurs because the polystyrene blocks of the copolymer are largely insoluble in hexane whereas the polybutadiene blocks are very soluble in this solvent. After forming the micelles, 8 wt% (of the diblock polymer) DVB was added. The DVB having a strong affinity for styrene diffuses to the center of the micelles, where it reacts with the lithium anion on the polystyrene chain ends to form a highly crosslinked region in the polystyrene core of the nanoparticle. In this way, the nanoparticle is permanently fixed in the shape of the self-assembled micelle structure. The following is the detail synthesis procedure. Fig. 1. Open in new tabDownload slide Formation of nanoparticles in anionic polymerization process: (a) Spherical nanoparticles. (b) Chain-like nanoparticles. Fig. 1. Open in new tabDownload slide Formation of nanoparticles in anionic polymerization process: (a) Spherical nanoparticles. (b) Chain-like nanoparticles. A 2-gallon reactor was first charged with 0.508 kg of hexane, and then with 1.043 kg of the butadiene/hexane blend (22 wt% of butadiene). The batch was then heated to 57°C. After the temperature stabilized, polymerization was initiated by adding 5.0 ml of 1.6 M n-butyllithium in hexane. After 2 h, the reactor was charged with 0.66 kg of styrene/hexane blend that contained 33 wt% styrene. After an additional 2 h reaction, the reactor was charged with 1.81 kg of hexane and then with 50 ml of DVB. A batch temperature of 57°C was maintained during the entire polymerization process. The product was then dropped into a solution of isopropanol and butylated hydroxytoluene (BHT), and subsequently dried in vacuum. The Gel Permeation Chromatography (GPC) analysis of the product showed two distinct peaks. One was from the micelle nanoparticles (∼95%), and the other was from some unreacted diblock copolymer (∼5%). The molecular weight of the diblock was about 49.4 kg/mol. The polydispersity of the nanoparticles was 1.15. Synthesis of chain-like nanoparticles The synthesis of chain-like nanoparticles involves the use of relatively high St/BD ratio block living copolymer at relatively high solute concentration. In this case, the diblock copolymer was designed to have a molecular weight of 47 kg/mol, comprising 30% by weight butadiene and 70% by weight styrene. In hexane solution of about 30% solid, this living diblock aggregate forms cylindrical micelles with the styrene block directed toward the longitudinal axis of the micelle and the butadiene block as tail extending therefrom, as shown in Fig. 1b. These chain-like particles can also be subsequently stabilized by crosslinking the styrene portion of the cylindrical micelles using reactions involving DVB and the Li located at the ends of the polystyrene blocks. Linear chains having a diameter of about 20 nm and a length of over 10 μm can be produced in such a manner. In this way, the nanoparticle is fixed in the shape of the self-assembled cylindrical micelle structure, rather than a globular aggregate. Depending on the amount of the DVB usage, the synthesized material can be either flexible chains or rigid cylinders. In this study, we prefer to make nanosized flexible chains by using only 6% DVB. The following is the detail synthesis procedure. A 2-gallon reactor was charged with 1.19 kg of a butadiene/hexane blend (22 wt% butadiene) and heated to 57°C. After the temperature had stabilized, polymerization was initiated with 10.2 ml of a 1.5 M butyllithium/hexane solution. The batch temperature was maintained at 57°C for the duration of the polymerization. After 2 h, the reactor was charged with 1.81 kg of a styrene/hexane blend (33 wt% styrene). After an additional 2 h reaction time, the reactor was charged with 55 ml of DVB. Ten minutes later the agitation was stopped and the temperature was maintained at 57°C for an additional 2 h. The batch was then discharged into a 95/4/1 blend of acetone, isopropanol and BHT, and subsequently dried in vacuum. Microscopy visualization Electron microscopic observations were carried out with a Philips CM–12 transmission electron microscope (TEM). About 10 ml of the polymer solution was taken from a final polymerization batch and further diluted with hexane solvent to about 10−4 wt% of the polymer solid. A drop of the diluted solution was then deposited on a carbon coated copper grid. After the solvent was evaporated, the grid was stained with OsO4, and examined by electron microscopy. Atomic force microscopic (AFM) observation was carried out with a Dimension-3000 microscope made by Digital Instruments. The AFM was used in tapping mode with an etched silicon tip. A small drop of the diluted solution (∼10−4 wt%) was placed on a newly cleaved graphite surface of about 10 mm × 10 mm size. The examination commenced when the solvent had completely evaporated. Results and discussion In the following section, we will describe our microscope investigations on the nanoparticles that are produced from the styrene/butadiene block copolymers using an anionic polymerization in hexane solutions. The composition and structure information on the nanoparticles studied in this report are summarized in Tables 1 and 2. Table 1. Characterization of poly(styrene/butadiene) nanoparticles studied . PBD shell Mn . Shell . PBD/PS ratio . DVB content . Particle . Particle . Particle . Particle . Sample . (kg/mol) . Mw/Mn . in particle . in particle (%) . purity (%) . diameter (nm) . length (nm) . polydispersity . Spherical 24.7 1.05 50/50 8 95.0 20 n/a 1.15  nanoparticles Chain-like 14.2 1.03 30/70 6 >95 20 100∼10 000 n/a  nanoparticles . PBD shell Mn . Shell . PBD/PS ratio . DVB content . Particle . Particle . Particle . Particle . Sample . (kg/mol) . Mw/Mn . in particle . in particle (%) . purity (%) . diameter (nm) . length (nm) . polydispersity . Spherical 24.7 1.05 50/50 8 95.0 20 n/a 1.15  nanoparticles Chain-like 14.2 1.03 30/70 6 >95 20 100∼10 000 n/a  nanoparticles Open in new tab Table 1. Characterization of poly(styrene/butadiene) nanoparticles studied . PBD shell Mn . Shell . PBD/PS ratio . DVB content . Particle . Particle . Particle . Particle . Sample . (kg/mol) . Mw/Mn . in particle . in particle (%) . purity (%) . diameter (nm) . length (nm) . polydispersity . Spherical 24.7 1.05 50/50 8 95.0 20 n/a 1.15  nanoparticles Chain-like 14.2 1.03 30/70 6 >95 20 100∼10 000 n/a  nanoparticles . PBD shell Mn . Shell . PBD/PS ratio . DVB content . Particle . Particle . Particle . Particle . Sample . (kg/mol) . Mw/Mn . in particle . in particle (%) . purity (%) . diameter (nm) . length (nm) . polydispersity . Spherical 24.7 1.05 50/50 8 95.0 20 n/a 1.15  nanoparticles Chain-like 14.2 1.03 30/70 6 >95 20 100∼10 000 n/a  nanoparticles Open in new tab Table 2. Characterization of PBD shell microstructures PBD shell microstructure . 1,2-Addition (%) . 1,4-Addition (%) . 1,4-trans (%) . 1,4-cis (%) . Spherical nanoparticles 8.8 91.2 54.1 37.1 Chain-like nanoparticles 9.2 90.8 53.8 37.0 PBD shell microstructure . 1,2-Addition (%) . 1,4-Addition (%) . 1,4-trans (%) . 1,4-cis (%) . Spherical nanoparticles 8.8 91.2 54.1 37.1 Chain-like nanoparticles 9.2 90.8 53.8 37.0 Open in new tab Table 2. Characterization of PBD shell microstructures PBD shell microstructure . 1,2-Addition (%) . 1,4-Addition (%) . 1,4-trans (%) . 1,4-cis (%) . Spherical nanoparticles 8.8 91.2 54.1 37.1 Chain-like nanoparticles 9.2 90.8 53.8 37.0 PBD shell microstructure . 1,2-Addition (%) . 1,4-Addition (%) . 1,4-trans (%) . 1,4-cis (%) . Spherical nanoparticles 8.8 91.2 54.1 37.1 Chain-like nanoparticles 9.2 90.8 53.8 37.0 Open in new tab Figure 2a presents a TEM micrograph of spherical nanoparticles. Our synthesis procedure usually produces monodispersed nanoparticles of perfect spherical shape and of about 20 nm in diameter [11]. The product can be further hydrogenated using a Ni catalyst [13], where the outside polybutadiene layer was converted into polyethylene layer, an even sharper image is obtained (see Fig. 2b). By a judicious variation of St/BD ratio, polymer molecular weight, solid volume fractions and temperatures, we were able to produce ellipsoidal and chain-like nanoparticles containing polybutadiene shell and polystyrene core (see in Fig. 2c and d). The formation of chain-like nanoparticles has been of great interest to us as one can anticipate many useful applications of such structures as reinforcing particles for rubber compounds, etc. They can be synthesized by the use of relatively high molecular weight diblock copolymers of high St/BD ratio at relatively high solute concentration. The detail procedure has been described in the experimental methods section. The synthesized nanosized chain has a chain diameter (or the characteristic size a) of about 20 nm and a contour length (L) over 10 μm (see Fig. 3a and b). It is possible, however, to obtain nanochains of relatively narrow distributions by physical selections. A typical fractionation procedure of the nanochains consists of solubilizing the crude polymer in toluene, filtering the solution through glass wool to remove insoluble materials and then centrifuging at different speeds to fractionate chains of different contour lengths. Fig. 2. Open in new tabDownload slide TEM micrographs of polymeric nanoparticles. (a) Spherical PS/PBD core-shell nanoparticles. (b) Hydrogenated spherical PS/PBD nanoparticles (negatively stained). (c) Ellipsoidal PS/PBD core-shell nanoparticles. (d) Cylindrical PS/PBD core shell nanoparticles. Fig. 2. Open in new tabDownload slide TEM micrographs of polymeric nanoparticles. (a) Spherical PS/PBD core-shell nanoparticles. (b) Hydrogenated spherical PS/PBD nanoparticles (negatively stained). (c) Ellipsoidal PS/PBD core-shell nanoparticles. (d) Cylindrical PS/PBD core shell nanoparticles. Fig. 3. Open in new tabDownload slide TEM micrographs of linear nanochains. (a and b) Mixture of spherical particles and nanochains. (c and d) Single nanochains in solution (the sample prepared by freeze-drying of a dilute benzene solution). Fig. 3. Open in new tabDownload slide TEM micrographs of linear nanochains. (a and b) Mixture of spherical particles and nanochains. (c and d) Single nanochains in solution (the sample prepared by freeze-drying of a dilute benzene solution). It is now generally believed that a polymeric chain in three dimensional diluted state will adopt a self-avoid coil-like conformation. There are tremendous experimental evidences collected from light and neutron scattering in support of this coil-like proposition; however, the real situation has never been visualized. Kumaki and Hashimoto once using AFM visualized the conformation change of an isolated synthetic chain on a mica surface [21]. The conformation they observed is typical 2D conformation of diluted state. Chains in a real 3D situation have never been visualized before. There are some reports on performing observations of chain conformation using double-stranded DNA chains. However, the DNA technique suffers from the complicated ion interactions and the conformation is not strictly random. Therefore, it is the natural choice that the nanochain is used for this study. In order to see the real chain conformation, we used the freeze-drying technique on a diluted benzene solution at about −70°C that contained the nanochains. Figures 3c and d displays the typical TEM picture of single nanorope coils obtained via the freeze-drying process. As can be seen, these chains adopt the self-avoid random flying conformation, a clear evidence in supporting the proposition predicted by theory, long time ago. The ratio between the chain length, which can be obtained by scrutinizing the whole trial of the long chain molecule shown in Fig. 3c, and the globule size of the chain usually gives us an approximate estimation of the persistence length using a self-avoid chain model. The estimated persistence length is about 60–70 nm. By applying the AFM, it is possible to visualize not only the shape of the nanoparticles but also the softness of the polymeric layer structures. Figure 4 is an AFM image of the nanochains on a graphite surface. The chains were purposely stretched using spin-coating technique. On the left side of the figure, bump lines appear in the topography image, each of them presumably corresponding to a single nanochain molecule. The elasticity image, on the right side of the figure, reveals that each of the bump lines is soft relative to the graphite substrate. This behavior comes from the fact that the surface layer of these nanochains are made from polybutadiene. The polybutadiene in this case has a Tg as low as −90°C, and thus behaves as soft elastomer at room temperature. As shown in Fig. 4, the nanosized chain can have a diameter about 25 nm and a contour length over 10 μm. Fig. 4. Open in new tabDownload slide AFM micrographs of linear nanochains. The sample was prepared by spin-coating a diluted solution on graphite surface. Fig. 4. Open in new tabDownload slide AFM micrographs of linear nanochains. The sample was prepared by spin-coating a diluted solution on graphite surface. It has been a long history of study of the conformation of many chains in 2D dense state [22–28]. de Gennes [26] theoretically postulated that a chain in 2D dense state would have a radius comparable to the ideal dimension because the repulsion parameter derived from scaling argument was of the order unity in 2D. Based on the scaling result, he predicted that a single-chain glubule in 2D would be enough to build up a concentration comparable with the total concentration, and chains in 2D dense state would be segregated. However, the existence of such a segregated state containing 2D Gaussian globules continues to be questionable [22–28]. Very little data are available on chain conformation in strict 2D dense state. A major obstacle in resolving the problem is the lack of suitable experimental methods needed to obtain the information required for many chains in confinement of thickness comparable with a monomer diameter (or the characteristic monomer size a). Previous experiments were mostly performed [22–25] by using scattering techniques on thin films with thickness (h) of tens of nanometers, in spite of a typical monomer size is about ∼5 Å (i.e. for styrene). Therefore, the nanochains synthesized are very suitable for this study because they are large in molecular sizes (i.e. with diameter a ≈ 20 nm, contour length L of tens of micrometers), relatively flexible between segments (i.e. with persistence length l ≈ 60 nm) and observable under microscopes. Thus, the actual conformations of many chains on 2D dense states can be obtained by scrutinizing the trails of the chains. In order to prepare a well-equilibrated 2D monolayer film, the nanosized chains were first fractionized and well dispersed in toluene, then a small drop of the diluted solution (10−5 wt%) was placed on a newly cleaved graphite surface of about 10 mm × 10 mm size. To make the deposit uniform, we restricted the evaporation of the toluene from the graphite surface by covering the wet solution with a lid that had only a small hole over the center through which the solvent vapor could escape. Completing the evaporation of toluene at room temperature depended on the size of the hole and could be controlled to take several days. Figure 5 is an AFM image of the nanorope chains in 2D dense state. The chains shown in the image were fractionated. The picture displays only parts of the deposit, the actual size of the deposit is several millimeters. A closer look of the chains in the 2D dense state show that the conformation of an individual chain is perturbed by the presence of its neighbors, and the long-chain molecules in 2D dense state are strongly interpenetrated, though they are not entangling each other. In small scales, these chains seem to arrange themselves in colinear succession, in which neighboring segments lie parallel to one another. In large scales, they are still completely disordered. This phenomenon is due to the avoidance of competition in 2D for occupying the same site by sequences of segments in different chains. Fundamental statistical mechanics tells us that the time-average of a dynamic event of a real system is equal to the ensemble average of its instant static microstates. Therefore, it is probable to analyze the chain conformation in a 2D deposit, as long as we are able to take many pictures on various microsections of the deposit. Figure 5 records only a small part of the chain conformation of the whole deposit, represents the typical behavior of many chains in the 2D dense state. This result seems in contradiction with the segregated globule model predicted by de Gennes [26–28]. Fig. 5. Open in new tabDownload slide AFM micrographs of equilibrium conformation of linear nanochains in 2D dense state. The deposit was prepared by restricted evaporation of the toluene from a graphite surface for over 2 days by covering the wet solution with a lid that had a small hole over the center through which the solvent vapor could escape. Fig. 5. Open in new tabDownload slide AFM micrographs of equilibrium conformation of linear nanochains in 2D dense state. The deposit was prepared by restricted evaporation of the toluene from a graphite surface for over 2 days by covering the wet solution with a lid that had a small hole over the center through which the solvent vapor could escape. One reason could be that the polydispersity of the nanorope chains, which is about 1.25 after fractionation. The polydispersity could have some effects on the way that chains pack with each other, since small chains could accommodate voids which might be formed by the packing of larger globules. This could lead to some interpenetration of chains that might be absent in monodisperse systems. However, it is unlikely that this limited polydispersity would explain the observed large-scale interpenetration between long chains, as shown in these pictures. The so-called swelling effects by small chains are visually small. Another possible reason could be that the nanochain structure included phase separation and multiple chain coassembly, while the 2D dense state conformation of de Gennes considered only the homopolymer chains. The nanochains are at a different level of complexity as compared with the chain packing of homopolymers. However, to the best of our knowledge, the phase separation should not be the cause as long as the chains are present in the toluene solvent and rearrange themselves on 2D surface before the solvent evaporates completely. It is noteworthy that precaution in sample preparation is very important because the conformation of nanochains can be artificially altered under high flow rates of solvent. An example is the preparation of the deposit using fast evaporation. In this case, the 2D deposit develops a lot of holes and stretched chains, as shown in Fig. 6. Fig. 6. Open in new tabDownload slide AFM micrographs of nonequilibrium conformation of linear nanochains in 2D dense state. The deposit was prepared by fast evaporation of the toluene from a graphite surface within 5 min. Fig. 6. Open in new tabDownload slide AFM micrographs of nonequilibrium conformation of linear nanochains in 2D dense state. The deposit was prepared by fast evaporation of the toluene from a graphite surface within 5 min. Conclusion We reported the using of industrial viable synthesis processes to produce spherical, ellipsoidal, cylindrical and chain-like nanoparticles. Under microscopes, these nanoparticles displayed very rich conformational features in diluted and dense states. Our observation on the conformation characters of the nanochains in 3D diluted state agreed well with the proposition of the self-avoid coil model. However, in 2D dense state, our observation on the nanochain conformation appeared to be in contradiction with the segregated globule model proposed by de Gennes. References 1 Guo A , Lui G , Tao J . Star polymers and nanospheres from cross-linkable diblock copolymers , Macromolecules , 1996 , vol. 29 pg. 2487 Google Scholar Crossref Search ADS WorldCat 2 Liu G , Qiao L , Guo A . Diblock copolymer nanofibers , Macromolecules , 1996 , vol. 29 pg. 5508 Google Scholar Crossref Search ADS WorldCat 3 Wooley KL . From dendrimers to knedel-like structures , Chem. Eur. J , 1997 , vol. 3 pg. 1397 Google Scholar Crossref Search ADS WorldCat 4 Thurmond KB , Kowalewski T , Wooley KL . Water-soluble knedel-like structures: The preparation of shell-cross-linked small particles , J. Am. Chem. Soc , 1996 , vol. 118 pg. 7239 Google Scholar Crossref Search ADS WorldCat 5 Wooley KL . Shell crosslinked polymer assemblies: Nanoscale constructs inspired from biological systems , J. Polym. Sci. A: Polym. Chem , 2000 , vol. 38 pg. 1397 Google Scholar Crossref Search ADS WorldCat 6 Akashi M , Kirikihira I , Miyauchi N . Synthesis and polymerization of a styryl terminated oligovinylpyrrolidone macromonomer , Angew. Makromol. Chem , 1985 , vol. 132 pg. 81 Google Scholar Crossref Search ADS WorldCat 7 Chen M-Q , Serizawa T , Kishida A , Akashi M . Graft copolymers having hydrophobic backbone and hydrophilic branches. XXIII. Particle size control of poly(ethylene glycol)-coated polystyrene nanoparticles prepared by macromonomer method , J. Polym. Sci. A: Polym. Chem , 1999 , vol. 37 pg. 2155 Google Scholar Crossref Search ADS WorldCat 8 Serizawa T , Takehara S , Akashi M . Transmission electron microscopic study of cross-sectional morphologies of core-corona polymeric nanospheres , Macromolecules , 2000 , vol. 33 pg. 1759 Google Scholar Crossref Search ADS WorldCat 9 Chen M-Q , Kishida A , Serizawa T , Akashi M . Graft copolymers having hydrophobic backbone and hydrophilic branches – 27. Nanosphere formation in copolymerization of methyl methacrylate with poly(ethylene glycol) macromonomers , J. Polym. Sci. A: Polym. Chem , 2000 , vol. 38 pg. 1811 (2000) OpenURL Placeholder Text WorldCat 10 Zheng L , Xie A , Lean J . Polystyrene nanoparticles with anionically polymerized polybutadiene brushes , Macromolecules , 2004 , vol. 37 pg. 9954 Google Scholar Crossref Search ADS WorldCat 11 Wang Xr , Hall J , Warren S , Krom J , Magistrelli J , Rackaitis M , Bohm GGA . Manufacture and commercial uses of polymeric nanoparticles , ACS Proceeding: PMSE , 2006 , vol. 231 392; and (2007) Synthesis, characterization, and application of novel polymeric nanoparticles. Macromolecules40, 499 OpenURL Placeholder Text WorldCat 12 Krom J , Wang Xr . Nano-particle preparation and applications , USP6437050 , 2002 OpenURL Placeholder Text WorldCat 13 Wang Xr , Foltz VJ , Sadhukan P . Crystalline polymer nano-particles , USP6689469 , 2004 OpenURL Placeholder Text WorldCat 14 Wang Xr , Foltz VJ . Multi-layer nano-particle preparation and applications , USP6872785 , 2005 OpenURL Placeholder Text WorldCat 15 Wang Xr , Lin CC , Hall J , Warren S , Krom J , Kondo H , Morita K . Nano-particle preparation and applications , USP6956084 , 2005 OpenURL Placeholder Text WorldCat 16 Wang Xr , Foltz VJ , Brumbaugh D . Polymer nano-particle with polar core and method for manufacturing same , US Pub US2006/0173130 , 2005 OpenURL Placeholder Text WorldCat 17 Wang Xr . Polymer nano-strings , USP6875818 , 2005 OpenURL Placeholder Text WorldCat 18 Wang Xr , Hall J , Bohm GGA , Lin CC . Nano-sized polymer-metal composites , USP7112369 , 2006 OpenURL Placeholder Text WorldCat 19 Wang Xr , Ozawa Y , Hall JE . Polymeric nano-particles of flower-like structure and applications , USP7205370 , 2007 OpenURL Placeholder Text WorldCat 20 Wang Xr , Foltz VJ . Chain conformation in two-dimensional dense state , J. Chem. Phys. , 2004 , vol. 16 pg. 121 OpenURL Placeholder Text WorldCat 21 Kumaki J , Hashimoto T . Conformational change in an isolated single synthetic polymer chain on a mica surface observed by atomic force microscopy , J. Am. Chem. Soc. , 2003 , vol. 125 16 pg. 4907 Google Scholar Crossref Search ADS PubMed WorldCat 22 Shuto K , Oishi Y , Kajiyama T , Han CC . Aggregation Structure of a 2-dimensional ultrathin polystyrene film prepared by the water casting method , Macromolecules , 1993 , vol. 26 pg. 6589 Google Scholar Crossref Search ADS WorldCat 23 Krause J , Muellar-Buschbaum P , Kuhlman T , Schubert D , Stamm M . Confinement effects on the chain conformation in thin polymer films , Europhys. Lett. , 2000 , vol. 49 pg. 210 Google Scholar Crossref Search ADS WorldCat 24 Brulet A , Boue F , Menelle A , Cotton J . Conformation of polystyrene chain in ultrathin films obtained by spin coating , Macromolecules , 2000 , vol. 33 pg. 997 Google Scholar Crossref Search ADS WorldCat 25 Jones RL , Kumar SK , Ho DL , Briber RM , Russell TP . Chain conformation in ultrathin polymer films , Nature , 1999 , vol. 400 pg. 146 Google Scholar Crossref Search ADS WorldCat 26 de Gennes PG . , Scaling Concepts in Polymer Physics , 1977 , vol. 61 Chapter II Ithaca and London : Cornell University Press Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC 27 Calvert P . Polymers – When a thick film is thin , Nature , 1996 , vol. 384 pg. 311 Google Scholar Crossref Search ADS WorldCat 28 Frank CW , Rao V , Despotopoulou MM , Pease RFW , Hinsberg WD , Miller RD , Rabolt JF . Structure in thin and ultrathin spin-cast polymer films , Science , 1996 , vol. 273 pg. 912 Google Scholar Crossref Search ADS PubMed WorldCat © The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org Oxford University Press
TI - Under Microscopes the Poly(styrene/butadiene) Nanoparticles
JO - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfm025
DA - 2007-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/under-microscopes-the-poly-styrene-butadiene-nanoparticles-604pX4TJ1v
SP - 209
EP - 216
VL - 56
IS - 6
DP - DeepDyve
ER -