Brittleness of polymer

Structural Parameters Affecting the Brittleness of Polymer Glasses and Composites... 

In this paper we discuss (1) small main-chain motions and their effect on the flow processes, (2) the embrittlement of polycarbonate, (3) the formation of microvoids from sample preparation and their effect on the brittleness of polymer glasses, and (4) the modification of the degree of brittleness of polymer glasses at the filler interface in polymer composites. 

Plasticizers reduce the hardness and brittleness of polymers. They increase the distance between the molecular chains, thus reducing the secondary valence forces and shifting the transition interval to lower temperamres, see Sect. 1.1. There are two ways to reach these goals, internal and external plasticization. 

Brostow, W., Hagg Lobland, H.E., Narkis, M., 2006. Sliding wear, viscoelasticity, and brittleness of polymers. Journal of Materials Research 21, 2422—2428. 

These additives are used to overcome the inherent brittleness of polymers such as polystyrene, polypropylene, and Terylene. The additives are incorporated into the polymer during manufacture. Pigments, extenders, fillers, nucleating agents, hydrocarbon oils, waxes and rubbers are all used as a means of improving the impact strength of these polymers. 

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

Engineering design with polymers starts with stiffness. But strength is also important, sometimes overridingly so. A plastic chair need not be very stiff - it may be more comfortable if it is a bit flexible - but it must not collapse plastically, or fail in a brittle manner, when sat upon. There are numerous examples of the use of polymers (luggage, casings of appliances, interior components for automobiles) where strength, not stiffness, is the major consideration. 

For many applications polystyrene might be considered to be too brittle a polymer. Because of this, polystyrene manufacturers have made a number of attempts to modify their products. 

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

A typical example of this class of polymer may be obtained by reaeting ethylenediamine and dimer fatty aeid , a material of inexact structure obtained by fractionating heat-polymerised unsaturated fatty oils and esters. An idealised strueture for this acid is shown in Figure 18.21. These materials are dark coloured, ranging from viscous liquids to brittle resins and with varying solubility. 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

Rubber-modified amorphous polymers The brittleness of amorphous polymers has been a hindrance in their commercial development. Fortunately, for reasons still not fully understood, the addition of rubbery polymers as dispersed droplets, or sometimes in a network form, into the glassy polymer can often lead to substantial increases in impact strength, albeit usually at... 

Thermoplastic polymers, such as poly(styrene) may be filled with soft elastomeric particles in order to improve their impact resistance. The elastomer of choice is usually butadiene-styrene, and the presence of common chemical groups in the matrix and the filler leads to improved adhesion between them. In a typical filled system, the presence of elastomeric particles at a level of 50% by volume improves the impact strength of a brittle glassy polymer by a factor of between 5 and 10. 

The second phase of polymer degradation is characterized by a decrease in the rate of chain scission (Fig. 19) and the onset of weight loss. Weight loss has been attributed to (1) the increased probability that chain scission of a low molecular weight polymer will produce a fragment small enough to diffuse out of the polymer bulk and (2) the breakup of the polymer mass to produce smaller particles with an increased probability of phagocytosis. The decrease in the rate of chain scission, as well as the increased brittleness of the polymer, is the result of an increase in the crystallinity of PCL,... 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

---