Polymers chain surface orientation

Figure 11.6. Schematic illustrations of brittle fracture, (a) Idealized limiting case of perfectly uniaxially oriented polymer chains (horizontal lines), with a fracture surface (thick vertical line) resulting from the scission of the chain backbone bonds crossing these chains and perpendicular to them. This limit is approached, but not reached, in fracture transverse to the direction of orientation of highly oriented fibers, (b) Isotropic amorphous polymer with a typical random coil type of chain structure. Much fewer bonds cross the fracture surface (thick vertical line), and therefore much fewer bonds have to break, than for the brittle fracture of a polymer whose chains are perfectly aligned and perpendicular to the fracture surface, (c) Illustration of a defect, such as a tiny dust particle (shown as a filled circle), incorporated into the specimen during fabrication, which can act as a stress concentrator facilitating brittle fracture.

The disadvantages of these materials in addition to their anisotropy are their high price and poor abrasion resistance. Because of the oriented nature of the polymer chains, surface fibrillation can occur quite easily. 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

Having said this, it was felt therefore that there is a need for a book addressing analysis and characterisation of polymers from the point of view of what we wish to call the primary analytical question. Many excellent textbooks and reference works exist which address one or more individual analytical techniques, see, for example, references [1-10]. These books form the basis of the knowledge of the technique expert. They also contain many excellent and varied examples on successful applications of analytical techniques to polymer analysis and characterisation. There are also books which address the multitude of analytical techniques applied in polymer analysis, see, for example, references [11-24], However, a synthetic chemist may wish to know the constitution of his/her polymer chain, a material scientist may want to find out the reasons why a fabricated sample had failed. What technique is best or optimal to study chain constitution will depend on the situation. Polymer failure may result from morphological features, which needs to be avoided, a contaminant, a surface property degradation, etc. When a sample has been processed, e.g., a film blown, molecular orientation may be the key parameter to be studied. A formulation scientist may wish to know why an additive from a different supplier performs differently. It is from such points of view that polymer analysis and characterisation is addressed in this book. 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

When a 7i-conjugated polymer has long alkyl or alkoxy side chains and forms the stacked structure, it tends to be aligned on the surface of substrates (e.g., Pt plate) with the alkyl or alkoxy chains oriented toward the surface of the substrate [95,128,134,135], presumably because of a strong tendency for the alkyl chain to stand upright on the surface of substrates [95,136]. Actually, the PAEs shown in No. 19 in Table 2 are aligned on a Pt plate with the OR chains oriented toward the surface of the Pt plate [95]. A model for the alignment is depicted in Fig. 1. 

Atomistic simulation of an atactic polypropylene/graphite interface has shown that the local structure of the polymer in the vicinity of the surface is different in many ways from that of the corresponding bulk. Near the solid surface the density profile of the polymer displays a local maximum, the backbone bonds of the polymer chains develop considerable parallel orientation to the surface [52]. This parallel orientation due to adsorption can be one of the reasons for the transcrystallinity observed in the case of many anisotropic filler particles. 

Electron diffraction measurements indicate that polymer chains are generally oriented normal or very nearly normal to the plane of the lamellae [13,14]. As the molecules in the polymer are at least 1000 A long and the lamellae are only about 100 A thick, the most plausible explanation is that the chains are folded [13,15], Figure 22.3 illustrates the proposed models of the fold surface in polymer lamellae [13,16],... 

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