Polystyrene hole size

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

Figure 11.7 Hole-size distribution and mean size near the surface of a polystyrene film on a Cu substrate [10].

G. Dlubek, A. P. Clarke, H. M. FretweU, S. B. Dugdale, M. A. Alam, Positron lifetime studies of free volume hole size distribution in glassy polycarbonate and polystyrene, Phys. Stat. Sol. A, 157, 351 (1996). 

Using PALS, Dammert et al. (92) and Yu et al. (93) examined the hole volume of a series of polystyrenes of different tacticity. They arrived at a free-volume hole size distribution maximum of around 110 at room temperature see Rgure 8.24 (92). This corresponds to an effective spherical hole radius of approximately 3 A. While this radius is somewhat larger than the theoretical value of 1.5 A found above, if the holes are actually irregular in shape the values are seen to agree quite well. 

Figure 8.24 PALS study of free volume hole size distribution in polystyrenes and poly(p-methylstyrenes) calculated from lifetime distributions of o-positronium.

In a GPC experiment, the polymer is separated in a column which is filled with a swollen, uniformly packed resin ( gel , called stationary phase, while the solvent which passes through the column is called mobile phase). The gel beads are usually made of crosslinked polymers (in particular polystyrene but also various inorganic porous materials) with little holes and pores of different size where the pore diameter is of the dimension of the size of the solvated polymer coils, i.e., the pore-size distribution is approx. 10-10 nm. 

The larger free volume and distribution also indicates a larger fraction of free volume near the surface than in the bulk. According to the WFL theory [43], a larger free volume leads to a lower Tg. Indeed a significant Tg depression (as much as 70 °C) has been reported in the surface of polystyrene by using PAL method [10]. Other studies of polymer surfaces have shown that the size of the free volume holes near the surface of polyethylene [47] and polypropylene [48] are larger than the bulk. 

Only recently first reports appeared describing the potential of the nanostructured thin block copolymer films for lithographic etching. A thin film of polystyrene-block-polybutadiene with a hexagonal cylindrical morphology where the poly-(butadiene) cylinders were oriented perpendicular to the substrate was deposited on a silicon wafer and selectively decomposed by treatment with ozone or converted with osmium tetroxide. By a subsequent reactive ion etching process the pattern could be inscribed into the surface of the silicon wafer yielding small holes or islands with a lattice constant of 27 nm and hole/island sizes of 13 nm [305,312]. 

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