Cooling molecular orientation

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

EOY speeds are the most recent development in PET spinning (78). Properties are similar to HOY and appear to be limited by the differential cooling rate from filament surface to filament core. This leads to radial distribution of viscosity, stress, and, consequentiy, molecular orientation (75). Eiber tensde strength is limited. Nevertheless, speeds up to 7000 m /min are commercial and forecasts are for speeds up to 9000 m /min by the year 2000 (79). Speeds to 9000 m/min have been studied (68,80,81). 

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

Ordered dialkoxy PPV derivative has been prepared by Yoshino et al. [491. oly(2 -nonoyloxy-1,4-phenylene vinylene) 27a forms a nematic liquid-crystalline phase upon melting. The material retains its order upon cooling to room temperature, and its band gap (2.08 eV) is measurably smaller than in an unoricnted sample. Oriented electroluminescence may be achieved by rubbing a thin fdin of the material to induce molecular orientation [50],... 

The next stop is to cool the nylon below its Tg without removing the stress, retaining its molecular orientation. The nylon becomes rigid with a much higher elastic modulus in the tension direction [15,000 to 20,000 MPa (2 to 3 x 106 psi)]. This is nearly ten times the elastic modulus of the unoriented nylon-66 plastic. The stress for any elastic extension must work against the rigid backbone of the nylon molecule and not simply unkink molecules. This procedure has been commonly used in the commercial production of man-made fibers since the 1930s via DuPont. 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

Nylons are semicrystalline polymers whose properties are controlled primarily by their amide concentration, molecular orientation, crystallization conditions, and the level of absorbed water. As discussed earlier, the level of crystallinity and hence product stiffness, is maximized by high concentrations of amide groups, high orientation, slow cooling, and the absence of absorbed water. 

When a crystal of ice is cooled to very low temperatures it is caught in some one of the many possible configurations but it does not assume (in a reasonable period of time) a uniquely determined configuration with no randomness of molecular orientation. It accordingly retains the residual entropy k In IF, in which k is the Boltzmann constant and W is the number of configurations accessible to the crystal. 

However, molecular orientation takes place in any case in some degree in any process involving melt flow and is frozen into the objects when they cool and the material solidifies. Such orientation can cause difficulties (like local brittleness and a tendency for the objects to distort), and will be especially troublesome if in the course of production the pattern of orientation changes. Orientation and strains in transparent objects can be studied conveniently using a source of polarized light (when they can be seen in different colours), but is more difficult with opaque items. 

Not only may the rate of cooling be slow it also may vary from one part of the moulding to another, and this can have marked effects on the crystalline morphology, molecular orientation, shrinkage—and consequently on the qualities and appearance of products. If, for example, the rate of crystallization differs in different parts of a moulding the sizes of crystals also will differ, and its clarity will be impaired. 

As known [7,8], the thermal expansion coefficient is reduced in the direction of the molecular orientation obtained by stretching of a thermoplastic polymer during or directly after its processing. In special cases thermotropic polyesters are applied to facilitate the process of molecular orientation [9]. However, in all these cases solidification must proceed either by cooling down from the melt or by evaporation of the solvent. These relatively slow processes are not suited for on-line optical fiber coating. 

The data in this work were obtained by quenching either uniaxially stretched samples for which the low thickness of the specimens allows rapid cooling, or samples deformed in simple shear where higher thickness of the specimens was required in order to perform the scattering experiments along the three principal shear directions. For both types of flow, special devices were developed to control the flow kinematics in the molten state as well as the quenching process which freezes the molecular orientation. These devices are briefly described in the next paragraphs. 

The clusters we study are known from constant energy simulations [6,16,18,40] and experimental data [41] to undergo at least two temperature-driven orientational order-disorder transformations below the freezing point. The current study confirms the finding [16] that there are two successive transformations First, when the cluster is cooled below its range of solidification, it exhibits a discontinuous solid-solid transformation, involving partial molecular orientation and lattice reconstruction. A further cooling induces... 

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