Polymers hard elastic

STRESS-RELATED SURFACE TENSION EFFECTS IN HARD ELASTIC POLYMERS... 

The first hard elastic polymers were made from crystalline lamellar materials, and were processed via melt spinning and stress crystallization, followed by annealing under tension. The common morphology of these polymers is shown for hard elastic polypropylene [3] in Figure 2a. The structure consists of rows of lamellae oriented perpendicularly to the direction of draw, o Between these lamellae are microfibrils (ca. 100-500A... 

The precise function of microfibrils in hard elastic polymers is not well understood. Their importance in various models of crystalline polymers has ranged from mere tie points [4,5] to the fundamental element [9]. The recent disclosure of hard elastic behavior in crazed HIPS [11] clearly revealed the connection between microfibrillar superstructure and hard elastic behavior, since no lamellae exists in this material. Polymers with a fibrillar structure are not necessarily hard elastic,however. Gore-Tex has an extensive fibrillar domain as shown in Figure 2c. Its loading cycle (Figure 5), however, clearly reveals an inelastic material. Hence, the necessary criterion of microfibrillar structure for a hard elastic polymer must be established. 

Microscopic investigations of hard elastic polymers indicated that extensive void and fibril formation occurs... 

The stress sensitivity of hard elastic polymers to changes in environmental surface tension has been well documented [6,10,11]. Because of their exposure to the environment, the surface contribution to the stress is primarily due to the microfibrils. Brown and Kramer [12], using a cylinder as a model for craze fibrils related the change in the surface component of the stress in crazed polystyrene to the change in surface tension, as shown below ... 

One may conclude, therefore that the microfibrils in the structure must have a sufficiently low diameter in order to induce hard elastic behavior. There are two justifications for this conclusions. Firstly, we have observed that hard elastic polymers contain a large surface tension component in their retractive stress, an observation supported by Miles et. al. [6] and postulated by others [7-9]. The surface component of the stress in Gore-Tex as shown by our experimental evidence and predicted by Equation 1, is simply too small to induce a suitable retractive force. 

Hard elastic polymers consist of numerous interconnecting pores whose void volume is highly dependent on the processing history and strain imposed on the materials. Mass transport of fluids into these pores is greatly affected by the viscosity of the environmental liquid, decreasing with increasing viscosity. 

Fig. 12. Diagram of fibrillar and bulk phase in hard elastic polymers.

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

ABA-type triblock copolymerization of MMA/BuA/MMA should give rubberlike elastic polymers. The resulting copolymers should have two vitreous outer blocks, where the poly(MMA) moiety (hard segment) associates with the nodules, and the central soft poly(BuA) elastomeric block provides rubber elasticity. Ihara et al. [35] were the first to synthesize an AB-type block copolymer, with MMA (190 equivalents of initiator) first polymerized by... 

Softening as a result of micro-Brownian motion occurs in amorphous and crystalline polymers, even if they are crosslinked. However, there are characteristic differences in the temperature-dependence of mechanical properties like hardness, elastic modulus, or mechanic strength when different classes of polymers change into the molten state. In amorphous, non-crosslinked polymers, raise of temperature to values above results in a decrease of viscosity until the material starts to flow. Parallel to this softening the elastic modulus and the strength decrease (see Fig. 1.9). 

Eiser et al. (2009) have recently reported on the pH-induced transformation of a hard-boiled egg from a white, brittle particulate gel to a transparent, elastic polymer gel (see Figure 6.20). Eggs were incubated in their hard protective shells for up to 26 days in a strong alkaline solution (0.9 M NaOH + 0.5 M NaCl, pH 12) at room temperature. These harsh experimental conditions are apparently rather similar to those used in a traditional Chinese method developed over two thousand years ago as a way of preserving eggs so that they would remain safely edible for many months. 

Hard-Elastic Fibers. Hard-clastic fibers arc prepared by annealing a moderately oriented spun yam at tiigli temperature under tension. They are prepared from a variety of olefin polymers, acetal copolymers, and polypivalolactone. 

In view of these complexities, it is remarkable that Eq. 4.1-4 represents numerous metal-metal, dry frictional data rather well, for both the static and sliding cases. Polymers, on the other hand, exhibit an even more complex frictional behavior on metal. This is, perhaps, not surprising, since the physical situation involves a relatively soft, viscoelastic, and temperature-dependent material in contact with a hard, elastic, and much less temperature- and rate-dependent material. Empirical evidence of these complexities is the nonlinear relationship between the frictional force and the normal load... 

So far the micro-mechanical origin of the Mullins effect is not totally understood [26, 36, 61]. Beside the action of the entropy elastic polymer network that is quite well understood on a molecular-statistical basis [24, 62], the impact of filler particles on stress-strain properties is of high importance. On the one hand the addition of hard filler particles leads to a stiffening of the rubber matrix that can be described by a hydrodynamic strain amplification factor [22, 63-65]. On the other, the constraints introduced into the system by filler-polymer bonds result in a decreased network entropy. Accordingly, the free energy that equals the negative entropy times the temperature increases linear with the effective number of network junctions [64-67]. A further effect is obtained from the formation of filler clusters or a... 

A third observation obtained from Figure 27 is the disappearance of these fissures as soon as the film is locally separated (Fig. 27 a, arrow B). It must be assumed, therefore, that the deformation lines close upon stress relieve just as in hard-elastic materials. A characteristic feature of some of those highly crystalline, highly oriented polymers is the fact that they can be extended by 50-100%, this extension being practically completely and inmiediately reversible... 

Still further differences are observed for stress/strain diagrams of what are known as hard elastic or springy polymers. These polymeric states should exhibit a large energy-elastic component which is attributed to a special network structure (see Figure 38-10). However, electron microscopic studies do not provide any evidence for the proposed network structure. 

A hydrophilic polymer (especially the aoss-linked form) may transition from hard and rigid to soft and elastic when immersed in aqueous media. A good example of this is cross-linked poly(2-hydroxyethyl methacrylate) (pHEMA), the original soft contact lens polymer. When dehydrated, pHEMA is a hard, hrittle polymer. When hydrated, it is a soft elastomer. The hydrated (swollen) form of cross-linked pHEMA contains about 40% by weight of water. Polymers that swell to an equiUhrium level in aqueous solutions are referred to as hydrogels. 

The changes made by these modifications, however, are quite modest in comparison to those possible by the planned formation of suitable fiberforming chain molecules. The first representative of this class of elastic polymers for textile use was Spandex. Spandex is a segmented polyurethane in which the hard segments serve as cross-linking points for the rubberlike... 

But other polymers such as it-poly(propylene) or poly(oxymethylene) can also be converted to what are known as hard-elastic fibers by suitable physical post-treatments (see also Section 38.3.1). At the present time, these energy elastic fibers are in the evaluation stage. 

C H Du, B K Zhu, Y Y Xu, Effects of stretching on crystalline phase structure and morphology of hard Elastic PVDF fibers , J of Appl Polymer Sd, 2007104 2254-2259. 

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