Polymers, linear amorphous

Most amorphous polymers linearly shrink about 0.4-0.6%. Crystalline polymers such as high density polyethylene and polypropylene linearly shrink about 2.0% (42). Thermoformed parts shrink away from female molds and onto male molds. Male molds must have typical draft angles of 2-5°, but sufficiently great enough to allow release of the formed parts. Female molds need minimal, if any, draft angles. 

Figure 4.7 Various representations of the properties of a mixture of crystalline and amorphous polymer, (a) The monitored property is characteristic of the crystal and varies linearly with 0. (b) The monitored property is characteristic of the mixture and varies linearly with 0 between and P, . (c) X-ray intensity is measured with the sharp and broad peaks being P. and P., respectively.

Linear-amorphous polymers (like PMMA or PS) show five regimes of deformation in each of which the modulus has certain characteristics, illustrated by Fig. 23.1. They are ... 

Fig. 23.2. A schematic of o linear-amorphous polymer, showing the strong covalent bonds (full lines) and the weak secondary bonds (dotted lines). When the polymer is loaded below Tg, it is the secondary bonds which stretch.

Fig. 23.7. A modulus diagram for PMMA. It shows the glassy regime, the gloss-rubber transition, the rubbery regime and the regime of viscous flow. The diagram is typical of linear-amorphous polymers.

The above information is conveniently summarised in the modulus diagram for a polymer. Figure 23.7 shows an example it is a modulus diagram for PMMA, and is typical of linear-amorphous polymers (PS, for example, has a very similar diagram). The modulus E is plotted, on a log scale, on the vertical axis it runs from 0.01 MPa to... 

Orientation in linear amorphous polymers Crystalline Polymers... 

Below T, most amorphous polymers show a more or less linear stress-strain curve with no yield point (Fig. 18.8)... 

The glass transition is a phenomenon observed in linear amorphous polymers, such as poly(styrene) or poly(methyl methacrylate). It occurs at a fairly well-defined temperature when the bulk material ceases to be brittle and glassy in character and becomes less rigid and more rubbery. 

Monnerie, L., Laupretre, F. and Halary,. L. Investigation of Solid-State Transitions in Linear and Crosslinked Amorphous Polymers. Vol. 187, pp. 35-213. 

Contents Chain Configuration in Amorphous Polymer Systems. Material Properties of Viscoelastic Liquids. Molecular Models in Polymer Rheology. Experimental Results on Linear Viscoelastic Behavior. Molecular Entan-lement Theories of Linear iscoelastic Behavior. Entanglement in Cross-linked Systems. Non-linear Viscoelastic-Properties. 

Hyperbranched polymers are often referred to as amorphous polymers since the branching of the backbone reduces the ability to crystallize in the same manner as linear polymers. Some exceptions have, however, been presented where the polymers have been modified to induce liquid crystallinity [34,35] or crystallinity [36] (Sect. 3.6.1). 

Based on the results obtained to date, which have been summarized above for several different semicrystalline polymers— linear and low density (branched) polyethylene, polytrimethylene oxide, polyethylene oxide and cis polyisoprene—it is concluded that the relatively fast segmental motions, as manifested in Tq, are independent of all aspects of the crystallinity and are the same as the completely amorphous polymer at the same temperature. Furthermore, it has previously been shown that for polyethylene, the motions in the non-crystalline regions are essentially the same as those in the melts of low molecular weight ii-alkanes. (17)... 

When the crystallinity of polyethylenes is increased, the gas permeability through the film decreases. The factors involved are the tortuosity of the gas path through the amorphous phase, and the effect of the crystals in restricting the mobility of the amorphous polymer chains (chain immobilisation factor). The logarithm of the permeability of nitrogen, argon and carbon dioxide decreased almost linearly with increased crystallinity of PE, with the ratio of the gas values remaining almost constant for a particular PE. 

Examples of crystalline polymers are nylons, cellulose, linear polyesters, and high-density polyethylene. Amorphous polymers are exemplified by poly(methyl methacrylate), polycarbonates, and low-density polyethylene. The student should think about why these structures promote more or less crystallinity in these examples. 

Amorphous polymers with irregular bulky groups are seldom crystallizable, and unless special techniques are used even ordered polymers are seldom 100% crystalline. The combination of amorphous and crystalline structures varies with the structure of the polymer and the precise conditions that have been imposed on the material. For instance, rapid cooling often decreases the amount of crystallinity because there is not sufficient time to allow the long chains to organize themselves into more ordered structures. The reason linear ordered polymers fail to be almost totally crystalline is largely kinetic, resulting from an inability of... 

According to the more widely used Williams, Landel, and Ferry (WLF) equations, all linear, amorphous polymers have similar viscoelastic properties at Tg and at specific temperatures above Tg, such as Tg + 25 K, and the constants Ci and C2 related to holes or free volume, the following relationship holds ... 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

If crystallization is carried out from concentrated solutions, multilamellar aggregates are formed. In particular, melt crystallization of polyethylene gives bunched-up lamellae with an overall spherical symmetry. The space between the lamellae contains uncrystallized amorphous polymer. These objects are called spherulites, and their radii grow linearly with time, in spite of their intricate morphological features [9]. Another remarkable feature of spheruhtes formed by linear polyethylene is that they are gigantically chiral, although the molecules are achiral. 

The use of C-labeled triethylaluminum allowed us to demonstrate that the quantity of —C2HB groups [deriving from A1(C2Hb)3] which is bound to the non-atactic polymer at the end of the polymerization, when operating with a constant amount of titanium trichloride, is a linear function of the square root of the alkylaluminum concentration (Fig. 22), in the considered range of experimental conditions (4 ). Similar results have been obtained by analyzing the fraction of amorphous polymer (Fig. 23) (38). 

As the temperature is increased above the rubbery plateau, the linear amorphous polymer assumes a viscous state and may undergo irreversible flow, i.e., flows such that the original shape is lost. The flow of the viscous liquid may approach a Newtonian flow, i.e., its flow properties may be estimated from Newton s law for ideal liquids. 

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