Polystyrene molecular orientation

A type of angle-dependent x-ray photoemission spectroscopy was used to investigate the molecular orientation at the surface of sulfonated polystyrene as a function of reaction depth. A model based on these measurements indicates that at a critical sulfonation depth the aliphatic hydrocarbon backbone becomes exposed preferentially at the surface. These results are consistent with surface energy and tribo-electric charging measurements, which also reveal the effects of associative interactions in the form of conversion dependencies. 

Table 1. Effect of Molecular Orientation on CO, Transport Behavior in Polystyrene at 25 C t...

In order to deal with the four non-crystalline forms in a unified way, we define a network chain in a crosslinked system, as the section of network between neighbouring crosslinks (Fig. 3.6). The shape of both a network chain in a rubber, and a molecule in a polymer melt, can be changed dramatically by stress, and both can respond elastically. However, when the polymer is cooled below Tg, the elastic strains are limited to a few per cent (unless a glassy polymer yields), so the molecular shape is effectively fixed. If the melt or rubber was under stress when cooled, the molecular shape in the glass is non-equilibrium. This molecular orientation may be deliberate, as in biaxially stretched polymethylmethacrylate used in aircraft windows, or a by-product of processing, as the oriented skin on a polystyrene injection moulding. Details are discussed in Chapter 5. 

Problem 7 Molecular orientation during flow increases with the polymer melt elasticity and the flow rate. If the polymer molecular weight is kept to a minimum, the melt elasticity is minimised. Polycarbonate has a low melt elasticity compared with polystyrene. For CD manufacture the mould is filled in between 0.2 and 0.4 s, which is a low flow rate. The skin thickness can be reduced by having a very hot melt at 340 °C and a mould temperature of 95 °C, to reduce the solidification during mould filling. 

Some polymer surface studies that have been reported recently are the detection of the molecular orientation at polymer film surfaces. For instance, the spectrum of iostactic polystyrene is different from that of the atactic material [82]. The spectra of thin films of poly(methylmethacrylate) (PMMA) cast on An, Al, or Cu were also different especially the intensity of the C = 0 stretching band at 1710 cm" varied considerably [83]. Thus HREELS seems to be capable of identifying molecular long-range order in polymeric surfaces. 

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

In passing the dependence of the molecular orientation on film thickness was also reported for polystyrene film prepared from solution [82]. 

The molecular orientation of the polymer in a fabricated specimen can significantly alter the stress-strain data as compared with the data obtained for an isotropic specimen, eg, one obtained by compression molding. For example, tensile strengths as high as 120 MPa (18,000 psi) have been reported for PS films and fibers (8). Polystyrene tensile strengths below 14 MPa (2000 psi) have been obtained in the direction perpendicular to the flow. 

When an amorphous pol3oner is stretched the molecules may be preferentially aligned along the stretch direction. In pol3nnethyl methacrylate and polystyrene such molecular orientation may be detected by optical methods, which measure the small difference between the refractive index in the stretch direction and that in the perpendicular direction. X-ray diffraction methods still reveal no evidence of three-dimensional order, so the structure may be regarded as a somewhat oriented tangled skein (Figure 1.9(b)) that is oriented amorphous but not crystalline. 

Fig. 6 (a) Fluorescence image of single DiD molecules embedded in a polystyrene film (5x5 xm ). The pseudo color scale shows the polarization direction of the fluorescence, indicating the molecular orientation (b) Fluorescence decay curves for one DiD molecule at different times. The red lines are the single-exponential functions fitted to the data (c) Time traces of fluorescence intensity and lifetime with 100 ms time interval. Reprinted with permission of [30], copyright (2004) American Chemical Society... 

Fig. 5.17. Fracture energy of polystyrene at 23 °C after drawing above the glass transition at 10 s. showing effect of molecular orientation. The crack plane is parallel to the draw direction (after L. J. Broutman and F. J. McGarry).

The natural draw ratio for amorphous polymers is very sensitive to the degree of preorientation, i.e. the molecular orientation in the polymer before cold-drawing. This was reported for polyethylene terephthalate by Marshall and Thompson [20] and for PMMA and polystyrene by Whitney and Andrews [26]. 

Mine, P.J., Duckett, R.A. and Read, D.J. (2007) Influence of molecular orientation and melt relaxation processes on glassy stress-strain behavior in polystyrene. Macromolecules, 40, 2782. 

In another type of application a low molecular fluorescent probe is added to a system containing macromolecules. As would be expected, the rotation of a small species is insensitive to the molecular weight of high polymers, but depends on the "microscopic viscosity" which is a function of free volume. For instance, Nishijima has shown that the microscopic viscosity of liquid paraffin hydrocarbons levels off for molecular weights above 1000 and that the microscopic viscosity of polystyrene containing 10 volume"/ benzene is only 200 times as high as that of benzene (15). Nishijima also showed that the emission anisotropy is a useful index of molecular orientation. Since both the excitation and the emission are anisotropic, the method yields the fourth moment of the distribution function of orientations, while other optical properties (dichroism, birefringence) depend on the second moment (15). 

Typical Experimental Results Consider first an amorphous polymer polystyrene [134]. This first example has been chosen to show the effect of flow on the final orientation. The molecular orientation in the part thickness has been characterized by shrinkage measurements (Fig. 15.33). It is maximum at the surface, decreases, passes through a secondary maximum, and finally tends to zero at the core. 

T. T. Jones Effect of Molecular Orientation on the Mechanical Properties of Polystyrene, Rep. lUP AC Working Party, lUPAC Internat. Symposium on Macro molecules, 10-14 Sept. 1973, 41-57. 

The effect of molecular orientation on the crazing behavior of polystyrene is primarily associated with the nucleation stage of the crazing process. 

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