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Nano-Particle Structural Characteristics

We modeled polymer nano-particles which should closely correspond to the ones that can be experimentally created fiom a droplet generator that efficiently generates nearly perfect spherical submicron particles of arbitary composition Using the molecular dynamics technique, PE particles with chain lengths of 100 beads generated with up to 60,000 atoms have almost a spherical shape (a symmetry parameter -0.1 at a temperature of 10 K) as shown in Fig. 3b, in good agreement with our experimental results. To interpret properties of the polymer fine particles differing fiom their bulk solid phase, we first counted the surface atoms using a three-dimensional grid method in Cartesian coordinates and the ratio of the surface atoms to the total number of atoms are obtained (the diameter is the average value of distance from a center of mass to the surface atoms). Since the smaller sized particles have more surface atoms than the larger ones, a decrease of the diameter increases the ratio as shown in Table 2. The large ratio of surface atoms to the total number of atoms provides reduction of the nonbonded interactions for the surface layer; hence the cohesive energy is dramatically dependent on the size. In addition, the ratio of surface chain ends to total number of chain ends for the particles is much larger than that of the bulk system, leading to enrichment of chain ends at surface. This observation is consistent with analysis ofthin films.18-20 With regard to an effect ofthe side-atoms, the increase in the side-atoms corresponds to a decrease m the ratio of surface atoms and therefore represents an increase of cohesive energy of the system.

The surface area and volume are calculated using the contact-reentrant surface method21 with a probe radii R p= 1.4 A The large proportional surface area defined by S^o = (surface area)/(volume) leads to large surface free energy, which is described by per unit of surface area (J/nm2. The Sratio ofthe particles is large compared with that of the bulk so that the surface area and surface free energy are large. The surface of the particles is also characterized by the fractal dimension which describes a degree of irregularity of a surface.22 The values ofD are smaller for the particles than the value ofthe bulk ( D = 2.72). This indicates that the surface is irregular and has many cavities which may introduce unique (catalytic or interpenetrating) properties of polymer fine particles. This predicts that nano-scale polymer particles are loosely packed and can show dynamical flexibility (e.g. Compressive modulus of the particles is much smaller than that of the bulk system). The free volume (cavities) and molecular packing can be important in

Figure 3 Creation of polyethylene droplets for initial conditions: a) randomly coiled 6 chains with chain length of 100 beads, b) a particle for 600 atoms with those 6 chains, and c) the particle and a new set of6 chains. Before the set is slung, the particle was randomly rotated. d) 60000 atom of PE particle with a chain length of 100 beads

Figure 3 Creation of polyethylene droplets for initial conditions: a) randomly coiled 6 chains with chain length of 100 beads, b) a particle for 600 atoms with those 6 chains, and c) the particle and a new set of6 chains. Before the set is slung, the particle was randomly rotated. d) 60000 atom of PE particle with a chain length of 100 beads a diffusion rate of a small molecule trapped in the particle.23 The structural characteristics of the PE particles up to 60,000 atoms with a chain length of 100 beads are shown in Table 2.

TABLE 2. Characteristics of polymer nano-partii

No. of

Diameter

Ratio of

Ratio of

Areaa)

Volume "

^ration

Fractal

atoms

(nm)

- chain

surface

(nm2)

(nm3)

(nm"1)

Dim. (nm-')b)

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