Machine direction, molecular orientation

Wet processes involve mixing a hydrocarbon liquid or some other low-molecular -weight substance with a polyolefin resin, heating and melting the mixture, extruding the melt into a sheet, orientating the sheet either in the machine direction or biaxi-ally, and then extracting the liquid with a volatile solvent [6-8]. 

Film can be produced either by extrusion tubular blowing or flat process. Each has its advantages and disadvantages. These processes result in film with a molecular orientation predominantly in the machine direction (MD). As reviewed later, orienting the film can be in two orthogonal directions that develop superior optical, mechanical, and physical properties. The process is known as biaxial orientation and it can be applied to both tubular and flat film. 

Matsuwaki [5] prepared polyimide films, (V), having molecular orientation in the machine direction that were used in high-density mounting of flexible printed circuit boards. 

One approach to characterising the molecular orientation in both uniaxially and biaxially oriented samples of PET makes use of the ratio of the absorption bands near 1250 and 1725 cm, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus, it is possible to determine individually the orientation functions with respect to the machine and transverse directions (131). 

Compare the molecular orientation and the film tensile and tear strengths in the machine and transverse directions. 

This equation is used to calculate the ratio of film speed to melt speed (i.e., take-up ratio). It provides quantification of the degree of machine direction (MD) stretching imparted on the melt by the process conditions. MD stretching is related to MD molecular orientation. Melt speed is difficult to measure, but its value is not required in order to calculate take-up ratio as described in the next section (Conservation of Mass). 

Compared to most isotropic polymers, thermotropic LCPs possess outstanding mechanical properties due to their rigid rod-like backbones. It has been reported that LCPs have tensile strength in a range of 150 to 400 MPa and tensile modulus from 13 to 25 GPa, while LCP fibers have tensile strength ranging from 500 to 650 MPa and tensile modulus from 50 to 100 GPa. When LCPs are blended with thermoplastics in a process where an elongational flow field exists, the LCP phase will deform into molecularly oriented fibers that reinforce the thermoplastic matrix. So, until now, many isotropic polymers have been blended with LCPs. Table 7.5 summarizes various in situ composites reported, with their machine direction mechanical properties. 

Figure 11.2 Molecular orientation of uniaxial LCP film results in weak transverse strength. Biaxially-oriented film has equal transverse and machine direction strengths.

The material properties being dependent on the direction. Most multiphase polymeric systems show some degree of anisometry. The mechanical performance in the machine direction can be as much as a hundred times higher than those in the transverse direction. In homopolymers, the anisotropy is a reflection of the molecular orientation in either a glassy or a semicrystalline state (see Birefringence ). 

Molecular orientation is another important factor influencing mechanical properties of coextruded films (11). Biaxial orientation can greatly improve film strength. However, uniaxial or highly unbalanced orientation causes poor transverse properties, which result in easy splitting of coextruded films in the machine direction. This tendency may occur even when a relatively thin layer responds to unidirectional orientation and propagates failure to thicker adjacent layers. 

Subsequently, a second or even more (15) machine direction stretching stations may be used. The web may have to be further heated before it is stretched a second time. This process can produce higher molecular orientation in films than... 

SGqUGntial Biaxial OriGntation. The standard process for sequential biaxial orientation is machine direction orientation followed by transverse orientation. This is a continuous process. The machine direction stretch occurs similar to the process described in the monoaxial section (Fig. 5). However, a second draw station is typically not present in this process. In the pure monoaxial orientation case, the process often tries to maximize the molecular orientation in the... 

Broadline nmr, relating melt order with polymer composition and temperature, also provided evidence supporting the thermotropic nature of these materials. The physical properties of molded parts of certain compositions showed a definite mechanical anisotropy. Strength and modulus values in the machine direction were considerably higher than those values transverse to the machine direction, that is, perpendicular to the direction of flow. Again, facile and high molecular orientation was noted for these polymers in the flow direction and this became more pronounced as part thickness was reduced. Further, the melt viscosities of the X7G copolyesters, at about equivalent molecular weights, were... 

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