Amorphous polymer .

Polymers with fixed liquid crystalline structure — if the polymerization temperature was below the glass ti ansition temperature ( ) of the polymer. These polymers exhibit a liquid crystalline structure in the solid glassy phase because the structure of the ordered monomer phase was frozen in by polymerization. By heating the polymer these frozen structures were irreversibly lost above the Tq An additional effect was observed starting from 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

In conventional LC phases the motion of the molecule is restricted only by the anisotropic interactions with its neighbors. This leads to the formation of the orientational long range order and in the case of smectic phases, to an additional lamellar sti ucture. However, completely different conditions normally exist in a liquid crystalline polymer. 

These two tendencies conflict, and steric hindrance in the system determines which tendency will dominate. The steric hindrance results from the direct linkage of the main chain to the side chain. A direct coupling of motions of the main chain to the rigid mesogenic side chains can be assumed. Therefore, an amorphous liquid polymer is obtained if the anisotropic orientation of the side chain is hindered or disturbed by the main chain. On the other hand, a solid polymer with a LC structure is obtained if the anisotropic-ordered rigid side chains hinder the normal motions of the main chain and thus tend to restrict the main chain motions. 

Homopolymers. Phenylesters of benzoic acid were chosen as mesogenic groups for the synthesis of suitable monomers. The acryloyl- and the methacryloyl moieties were used as polymerizable groups. The flexible spacer was an alkyl chain or an alkyloxy chain of varying length. Thus a homologous series of p- (2-methylpropenoyloxyalkyoxy) benzoic acid -p substituted phenyl esters 1 was prepared. The monomers were synthesized by the standard methods as follows  

A real amorphous polymer usually exhibits more than one transition. As indicated above, there is a high-temperature transition, usually labelled a or Ota, which is the glass transition and corresponds to the onset of main-chain segmental motion, as discussed in sections 5.7.5 and 7.5.3, and secondary transitions at lower temperatures. These are assigned to various types of motion, such as motions of side groups, restricted motion of the main chain or motions of end groups, some of which are discussed in detail in sections 5.7.4 and 5.7.5. The secondary relaxations often show up more clearly in the loss modulus or tan S. 

While amorphous polymers constitute a significant fraction of the polymer sector, they are much less studied by vibrational spectroscopy. It is often easier to obtain the spectra than for crystalline systems but the interpretation is much more difficult. This is because the lack of local and long range order means that the group theoretical tools that are the backbone of vibrational analysis are largely inapplicable. Computational study is also difficult because the flexible nature of the polymer chains means that there are often many conformations with similar energy. In 

FIGURE 3.9 (a) Specific volume versus temperature for A, an amorphous polymer, and B, a partly crystalline polymer, (b) Volume-temperature curve for pure poly(V,At -sebacoyl piperazine), for which is 82°C and is 180°C-181°C. (Data from Flory, P. J., and H. K. Hall, J.Am. Chem. Soc., 73, 2532, 1951.) 

Experimentally, itself depends on the timescale of the experiment in which it is measured. As pointed out by Bueche [17], a 100-fold increase in heating rate when volume change is being measured increases the apparent of PS only about 5°C. Molecular weight affects also. Theoretical equations of some complexity have been proposed [18]. An empirical correlation can be obtained in the form 

Tg for a finite molecular weight M is related to that for an infinitely long polymer. For example, the data of Beevers and White [18] for PMMA can be fitted by = 2.1 x 10 °C mol/g and = 114°C when M exceeds 10 . Likewise, PS data are fitted by = 1.7 x 10 °C mol/g and = 100°C down to molecular weights of less than 3 x 10 [17]. In both cases the maximum difference in due to differences in M are small above M = 5 X lOL 

Transition Temperatures for Selected Polymers T, Total Solubility Parameter 

Polymer Handbook, 3rd edn., Wiley, New York, 1989 Lewis, O. Linear Homopolymers, Springer-Verlag, New York, 1968. G., Physical Constants of 

Tg of the butadiene (—77°C) and the Tg of PS are readily evident in the tan 8 plots for the two materials. The fact that we see two distinct 7 s immediately tells us that the blend is immiscible. This is generally the case with most polymers, where only limited solubility of one polymer within the other occurs. In the few polymers that are miscible (poly(styrene) and poly(phenylene oxide) for example) only a single is observed, whose temperature hes in between the two component Tg s, in proportion to the composition. 

An introduction to the extensive experimental studies of linear viscoelastic behaviour in polymers falls conveniently into three parts, in which amorphous polymers, crystalline polymers and temperature dependence are discussed in turn. 

Mechanical Properties of Solid Polymers, Third Edition. I. M. Ward and J. Sweeney. 2013 John Wiley Sons, Ltd. Published 2013 by John Wiley Sons, Ltd. 

The observed plateau in the rubbery region is a consequence of high molecular mass, as the long molecules tend to entangle, with the formation of physical cross-links that restrict molecular flow through the formation of temporary networks. At long times such physical entanglements are usually labile and lead to some irreversible flow, in contrast with the situation for permanent chemical cross-links, such as those introduced when mbber is vulcanised. It follows directly from the theory of rubber elasticity (Chapter 4) that the value 

Kargin and his school in a large number of papers, have discussed the structural features of polymers as revealed by examination under the electron microscope. It is suggested that, even in amorphous glasses, aggregation into long, thin bundles of chains is possible. 

On the basis of the foregoing and other recent evidence, it seems reasonable to assume that there is a basic structural unit in the so-called amorphous polymers. This is the molecular bundle, which may or may not include some chain folding. If one postulates the existence of such 

The observed plateau in the rubbery region is a consequence of high molecular 

When their chemical and stereochemical structure is not sufficiently regular, polymer chains cannot organise themselves into crystals. They remain disordered, or amorphous. A great number of industrial polymers fall into this category  

The first two of these, PI and SBR, are elastomers. SBR is one of the components used in car tyre treads. The last two are solids at ordinary temperatures. In expanded form, PS is used for thermal insulation panels and food containers, whereas in solid form, it is used for transparent cups and yoghurt pots. PMMA, better known by its commercial name Plexiglas, is used for windows, but also for wash basins, baths and street lights. 

Whether these products are flexible or rigid depends largely on the temperature. A hollow ball made of PI or SBR and cooled in liquid nitrogen becomes hard and brittle. A solid ball in the same conditions has a greatly reduced bounce relative to ordinary temperatures (by a factor of 2 or 3). Likewise, when immersed in boiling water, PS becomes soft and easily deformed. Deformations inflicted at high temperatures remain upon cooling. 

We have so far been concerned principally with the structure of crystalline polymers which can readily be studied by using standard X-ray and electron diffraction methods. However, there is an important category of polymers which have not yet been considered which can be completely non-crystalline. They are generally termed amorphous and include the well-known polymer glasses and rubbers. Although the properties of these materials have been studied at length, very little is known about their structure. This is because there is no well-defined order in the structure of amorphous polymers and so they cannot be analysed very easily using standard diffraction techniques. 

Amorphous polymers can be thought of simply as frozen polymer liquids. Over the years there have been many attempts to analyse the structure of liquids of small molecules and they have met with a varied success. As may 

It was pointed out in Section 3.13 that small-angle neutron scattering has been used to show that in a pure amorphous polymer, the polymer molecules adopt their unperturbed dimensions (Section 3.3.3) as predicted originally by Flory. 

It is more convenient to talk in terms of fractional free volume,/, which is defined as / = Vf/V, At and below the Tg,fis given by fg= Vf/V and can be considered as being effectively constant. Above the Tg there will be an important contribution to Vf from the expansion of the melt. The free volume above Tg is then given by 

The effect of the chemical nature of the polymer chain upon Tg is similar to the effect it has upon Tr (Section 4.3.5). The most important factor is chain flexibility which is governed by the nature of the chemical groups which constitute the main chain. Polymers such as polyethylene (—CH2— 

Information on the conformations of polymer chains in amorphous polymers has been obtained by using small-angle neutron diffraction. The technique involves measurement of the coherent scattering of neutrons by mixtures of deuterated and protonated polymer molecules. Analysis of the scattering data leads to an estimation of the radius of gyration (Section 

2) of the polymer molecules. It has been clearly demonstrated that in both polymer glasses and polymer liquids the polymer chains, to a first approximation, have their unperturbed dimensions. This means that the molecules have the same conformations in the bulk as they have in a Theta solvent. This behaviour was predicted many years ago by Flory and the simple reason for this is that a molecule in the bulk state will interfere with itself, but if it expands to decrease this interaction there is an increase in the interaction with its neighbours and so it adopts the unperturbed conformation. 

There are some polymers, such as poly(ethylene terephthalate) and 

If the melt of a non-crystallizable polymer is cooled it becomes more viscous and flows less readily. If the temperature is reduced low enough it becomes rubbery and then as the temperature is reduced further it becomes a relatively hard and elastic polymer glass. The temperature at 

Polymer molecules that can be packed closely together can more easily form crystalline structures in which the molecules align themselves in some orderly pattern. Commercially crystalline polymers have up to 80% crystalline structure and the rest is amorphous. They are identified technically as semicrystalline TPs. Polymers with 100% crystalline structures are not commercially produced. 

Amorphous TPs have no crystalline structure. Their molecules form no patterns. These TPs have no sharp melting points. They are usually glassy and transparent, such as acrylontrile-butadiene-styrene (ABS), acrylic (PMMA), polycarbonate (PC), polystyrene (PS), and polyvinyl chloride (PVC). Amorphous plastics soften gradually as they are heated during processing. If they are rigid, they may become brittle unless modified with certain additives. 

During the melting process as the symmetrical molecules approach each other within a critical distance, crystals begin to form. They form first in the areas where they are the most densely packed. This crystallized area becomes stiff and strong. The noncrystallized, amorphous, area is tougher and more flexible. With increased crystallinity, other effects occur such as with polyethylene (crystalline plastic) there is increased resistance to creep. 

Side groups also influence chain flexibility. As the size and the rigidity of the side groups increase, the chain flexibility is lowered and the glass transition temperature is increased [16]. In contrast, introduction of lengthy flexible nonpolar side groups decreases Tg [15,17]. 

Plasticizers are liquids that are added to plastics to soften them [1,8,15]. The softening is brought about by the plasticizer dissolving in the polymer and lowering its glass transition temperature [15]. 

Fillers are frequently added to polymers to lower their cost [1], As a result of the presence of filler, the mechanical properties of the material are changed [1]. Impact and elongation are usually decreased whereas tensile strength, hardness 

If polymer molecules are adsorbed on the filler surface their mobility is restricted [1]. As a result, the glass transition of the adsorbed polymer will increase [1]. Often two transitions can be seen, corresponding to the glass transition of the unadsorbed and adsorbed polymer [20]. 

As an illustration of the tendency for polymer end groups to segregate to the surface we will consider the case of a polystyrene that has fluorine end caps. The polymer was produced by using a dilithium initiator, and the end cap was 

Polymer molecules in the melt are very coiled up in fact, they are often approximately Gaussian coils. In other words, the radius of gyration for a linear chain is given by Eq. (2.2), which also applies to an isolated coil in solution. In the melt, the density of surrounding molecules prevents an individual molecule from stretching, and it adopts a compact conformation. 

An alternative way of observing the time-dependent visco-elasticity of a polymer melt is to perform creep compliance experiments. Here a constant elongational stress is applied to a polymer melt (or concentrated solution) and the extension (strain) is measured as a function of time. The compliance J t) (Eq. 1.40) initially increases rapidly before reaching a plateau region, where the material has a rubbery response. After a time To, the deformation becomes irreversible and the polymer exhibits plastic flow. At times T tq the deformation is reversible (elastic). The crossover time To is called the characteristic relaxation time of the polymer. It can 

In the rubbery state the chains may be entangled and the plateau modulus can be used to obtain the entanglement molar mass. If Ge is the plateau modulus, the entanglement molar mass is defined by Me = Ge, where p is the density, K is the gas constant and T is the temperature. 

The viscosity of a polymer melt depends strongly on molar mass. A typical curve of zero-shear viscosity as a function of molar mass on a double logarithmic scale is shown in Fig. 2.21. There are two regimes. The viscosity 

Two distinct models have been introduced to describe the motion of unentangled and entangled polymers, the Rouse model and reptation model respectively. 

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Tittel J, Kettner R, Basche T, Brauchle C, Quante FI and Mullen K 1995 Spectral diffusion in an amorphous polymer probed by single molecule spectroscopy J. Lumin. 64 1-11... 

Gentile F T and Suter U W 1993 Amorphous polymer miorostruoture Materials Science and Technology, Structure and Properties of Polymers vol 12, ed E L Thomas (Weinheim VCFI) pp 33- 77... 

Kotelyanskii M 1997 Simulation methods for modelling amorphous polymers Trends Polym. Sol. 5 192-8... 

Kotelyanskii M, Wagner N J and Paulaitis M E 1996 Building large amorphous polymer struotures atomistio simulation of glassy polymers Macromolecules 29 8497- 506... 

This is a fairly reasonable way to describe man-made amorphous polymers, which had not been given time to anneal. For polymers that form very quickly, a quick Monte Carlo search on addition can insert an amount of nonoptimal randomness, as is expected in the physical system. 

B. Bom, H. W. Spiess, Ah Initio Calculations of Conformational Effects on NMR Spectra of Amorphous Polymers Springer-Verlag, New York (1997). 

Irregularities such as branch points, comonomer units, and cross-links lead to amorphous polymers. They do not have true melting points but instead have glass transition temperatures at which the rigid and glasslike material becomes a viscous liquid as the temperature is raised. 

Figure 3.16a shows the storage and loss components of the compliance of crystalline polytetrafluoroethylene at 22.6°C. While not identical to the theoretical curve based on a single Voigt element, the general features are readily recognizable. Note that the range of frequencies over which the feature in Fig. 3.16a develops is much narrower than suggested by the scale in Fig. 3.13. This is because the sample under investigation is crystalline. For amorphous polymers, the observed loss peaks are actually broader than predicted by a...

The loop formed by the chain as it emerges from the crystal, turns around, and reenters the crystal may be regarded as amorphous polymer, but is insufficient to account for the total amorphous content of most crystalline polymers. 

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.

All amorphous polymers show remarkably similar results when values of log ay are plotted versus T - Tq. Manipulation of these data shows that the empirically determined shift factors can be fitted by the expression... 

Amorphous adsorbents Amorphous nylon Amorphous nylons Amorphous polymers... 

To a large extent, the properties of acryUc ester polymers depend on the nature of the alcohol radical and the molecular weight of the polymer. As is typical of polymeric systems, the mechanical properties of acryUc polymers improve as molecular weight is increased however, beyond a critical molecular weight, which often is about 100,000 to 200,000 for amorphous polymers, the improvement is slight and levels off asymptotically. 

Some representative backbone stmctures of PQs and PPQs and their T data are given in Table 1. As in other amorphous polymers, the Ts of PQs and PPQs are controlled essentially by the chemical stmcture, molecular weight, and thermal history. Several synthetic routes have been investigated to increase the T and also to improve the processibiUty of PPQ (71). Some properties of PPQ based on 2,3-di(3,4-diaminophenyl)quinoxaline and those of l,l-dichloro-2,2-bis(3,4-diaminophenyl)ethylene are summarized in Table 2. 

L. C. E. Stmik, PhjsicalAging in Amorphous Polymers and Other Materials, Elsevier, Amsterdam, the Netherlands, 1978. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial appHcation in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of sphemUtes by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find appHcations in packaging and barrier resins markets (191). 

Polycarbonates are an unusual and extremely useful class of polymers. The vast majority of polycarbonates are based on bisphenol A [80-05-7] (BPA) and sold under the trade names Lexan (GE), Makrolon (Bayer), CaUbre (Dow), and Panlite (Idemitsu). BPA polycarbonates [25037-45-0] having glass-transition temperatures in the range of 145—155°C, are widely regarded for optical clarity and exceptional impact resistance and ductiUty at room temperature and below. Other properties, such as modulus, dielectric strength, or tensile strength are comparable to other amorphous thermoplastics at similar temperatures below their respective glass-transition temperatures, T. Whereas below their Ts most amorphous polymers are stiff and britde, polycarbonates retain their ductiUty. 

Solubility and Solvent Resistance. The majority of polycarbonates are prepared in methylene chloride solution. Chloroform, i7j -l,2-dichloroethylene, yy -tetrachloroethane, and methylene chloride are the preferred solvents for polycarbonates. The polymer is soluble in chlorobenzene or o-dichlorobenzene when warm, but crystallization may occur at lower temperatures. Methylene chloride is most commonly used because of the high solubiUty of the polymer (350 g/L at 25°C), and because this solvent has low flammabiUty and toxicity. Nonhalogenated solvents include tetrahydrofuran, dioxane, pyridine, and cresols. Hydrocarbons (qv) and aUphatic alcohols, esters (see Esters, organic), or ketones (qv) do not dissolve polycarbonates. Acetone (qv) promotes rapid crystallization of the normally amorphous polymer, and causes catastrophic failure of stressed polycarbonate parts. 

Structure and Crystallinity. The mechanical—optical properties of polycarbonates are those common to amorphous polymers. The polymer may be crystallized to some degree by prolonged heating at elevated temperature (8 d at 180°C) (16), or by immersion ia acetone (qv). Powdered amorphous powder appears to dissolve partially ia acetone, initially becoming sticky, then hardening and becoming much less soluble as it crystallizes. Enhanced crystallization of polycarbonate can also be caused by the presence of sodium phenoxide end groups (17). 

Pure amorphous polymers, being homogeneous materials, are transparent. Atactic polystyrene is a good example. The crystalline syndiotactic form is not transparent. Alack of transparency does not necessarily indicate crystallinity, however. It can also be caused by inorganic fillers, pigments, gas bubbles (as in a foam), a second polymer phase, etc. 

Analytical and Test Methods. In addition to the modem spectroscopic methods of detection and identification of pyrroles, there are several chemical tests. The classical Runge test with HCl yields pyrrole red, an amorphous polymer mixture. In addition, all pyrroles with a free a- or P-position or with groups, eg, ester, that can be converted to such pyrroles under acid conditions undergo the Ehrlich reaction with p-(dimethylamino)henzaldehyde to give purple products. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

The shift factor required to superimpose a set of data for an amorphous polymer is described mathematically by the WLF equation (eq. 8) (24), which becomes... 

As-polymerized PVDC does not have a well-defined glass-transition temperature because of its high crystallinity. However, a sample can be melted at 210°C and quenched rapidly to an amorphous state at <—20°C. The amorphous polymer has a glass-transition temperature of — 17°C as shown by dilatometry (70). Glass-transition temperature values of —19 to — 11°C, depending on both method of measurement and sample preparation, have been determined. 

Barrier Properties. VinyUdene chloride polymers are more impermeable to a wider variety of gases and Hquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeabiUty. A more subtle factor may be the symmetry of the polymer stmcture. It has been shown that both polyisobutylene and PVDC have unusually low permeabiUties to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values Hsted in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

Stereoregular Polymerization. Chemists at GAF Corporation were first to suggest that stereoregularity or the lack thereof is responsible for both nontacky and crystalline or tacky and amorphous polymers generated from IBVE with BF2 0(C2H )2, depending on the reaction conditions (22,23). In addition, it was shown that the crystalline polymer is actually isotactic (24). Subsequentiy, the reaction conditions necessary to form such polymers have not only been demonstrated, but the stereoregular polymerization has been extended to other monomers, such as methyl vinyl ether (25,26). 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

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