Nonoriented polymers

Amorphous, nonoriented polymers are not optically birefringent, because their optically anisotropic monomeric units are randomly ordered with respect to one another. A birefringence first occurs when the chains are oriented or placed under strain. Generally, the following relation holds ... 

It was also found [6], that in this case experimentally determined values of athermic fracture stress turn out to be essentially (2 3 times) smaller than theoretically calculated ones. A small values k 0.2 1.0) is one more important feature of nonoriented polymers fracture in impact tests. This means, that the stress on breaking bonds is essentially lower than nominal fracture stress of bulk sample. And at last, it was found out [7], that the value k reduces at testing temperature growth and the transition from brittle fiactuie to ductile (plastic) one. These effects explanation was proposed in Refs. [4-7], but development of fractal analysis ideas in respect to polymers lately and particularly, Alexander and Orbach woik [8] appearance, which introduced the fraction notion, allows to offer the major treatment of polymer fracture process [9, 10], including the dilaton concept [1-3] as a constituent part. 

Let us consider the reasons of the adduced in Table 7.1 values k < 1 for nonoriented polymers. As it has been shown in chapter two, the value characterizes molecular mobility (deformability) level of the indicated chain part [17]. At = 1.0 this mobihty is suppressed completely and at =2.0 it reaches the greatest possible level, typical for rubber-like state. Molecular mobility intensification results to corresponding stress relaxation intensification, applied to chain part between entanglements and, as consequence, to its reduction lower than macroscopic sample fracture stress [18], Such treatment assumes availability of the correlation between parameters K and This assumption is confirmed by the plot of Fig. 7.1, where the dependence for 10 polymers, pointed out in table 7.1, is adduced... 

E reduction at large for compositions UHMPE/Al is due to not molecular mechanisms, but macroscopic ones (interfacial boundaries polymer matrix-filler fi acture) [16]. The linear dependence K2(Xj jj) is obtained, which at = 1 is extrapolated to = 0. This means, that polymer Ifacture is impossible without chain preliminary drawing. In other words, this assumes definite nonzero failure strain for nonoriented polymers samples. As it has been shown in Ref [22], the greatest value X j ( ) can be determined... 

Let us consider further reasons of pol5rmer chains breaking at so small stresses, which can be on order lower than ftacture macroscopic stress (i.e., at h5rpothetical k = 0.1). The reasons were pointed for the first time in Refs. [1, 26]. Firstly, anharmonicity intensification in fracture center gives the effect, identical to mechanical overloading effect [26]. Quantitatively this effect is expressed by the ratio of thermal expansion coefficient in fracture center and modal thermal expansion coefficient [5]. The second reason is close inter communication of local yielding and fracture processes [ 1]. This allows to identify fracture center for nonoriented polymers as local plasticity zone [27, 28]. The ratio uJ(X in this case can be reached -100 [5]. This effect compensates completely k reduction lower than one. So, for PC ala 70, K- = 0.44, a. = O.IE. 700 MPa and fiien o = o a /K,a 23 MPa, that by order of magnitude corresponds to experimental value Oj. for PC, which is equal approximately to 50 MPa at T= 293 K [7]. 

Thus, the stated above results demonstrated, that fractal analysis application for polymers fracture process description allowed to give more general fracture concept, than a dilation one. Let us note, that the dilaton model equations are still applicable in this more general case, at any rate formally. The fractal concept of polymers fracture includes dilaton theory as an individual case for nonfractal (Euclidean) parts of chains between topological fixation points, characterized by the excited states delocalization. The offered concept allows to revise the main factors role in nonoriented polymers fracture process. Local anharmonicity ofintraand intermolecular bonds, local mechanical overloads on bonds and chains molecular mobility are such factors in the first place [9, 10]. 

An amorphous polymer in a state of molecular alignment is not a stable structure - it is metastable. It can uansition either to a more perfectly ordered, crystalline structure, or to a more disordered, nonoriented structure In either case, the free energy of the system is reduced. Given enough time and/or thermal energy, an oriented amorphous polymer will transition in either or both of these directions. 

When polymer melts are accelerated during the manufacturing processes such as injection molding and afterwards cooled down quickly, the stretched molecular chains cannot relax, which leads to orientations in the direction of melt flow. These oriented polymer chains act differently toward ESC influences as compared to nonoriented molecules. Orientation perpendicular to the crack propagation direction may result in a higher ESCR, as will be shown in the following discussion. 

In order to gain information on the elecronic structure of polymer sulfur nitride, (SN)X, we investigated the photoemission properties of this interesting material. The measurements were obtained on nonoriented (SN)X films using x-ray and variable ultraviolet photoemission spectroscopy (XPS and UPS) (1), (2). Table 1 shows the nitrogen and sulfur core levels in (SN)X and in neutral elements. 

A partially crystalline polymer that is not fully crystallized during processing will shrink when heated to a certain temperature. When the chain mobility is increased enough for the chain to begin to form into crystallites, the polymer will shrink, since the crystallites occupy less volume than the amorphous regions. This behavior can be seen if one heats a nonoriented PET or nylon film. Most often it is associated with nonpackaging applications like the shrinkage of polyester fabrics when washed in water that is too hot. 

As in the case of crystallinity, the sample is assumed to be composed of the oriented and nonoriented fractions, defined by / and 1-/, respectively. Using the observed dichroic ratio of a partially oriented polymer film or fiber, the fraction of oriented material is given by [19]... 

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