Amorphous Polymer

9 Key excerpts on "Amorphous Polymer"

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  eBook - ePub

  Carraher's Polymer Chemistry

  • Charles E. Carraher Jr.(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  m and do not give a clear x-ray diffraction pattern. Amorphous Polymer chains have been likened to spaghetti strands in a pot of spaghetti, but in actuality, the true extent of disorder that results in an Amorphous Polymer is still not fully understood.

  Section 13.4 contains a discussion of a number of techniques employed in the search for the real structure of the amorphous bulk state. Briefly, evidence suggests that little order exists in the amorphous state with the order being similar to that observed with low-molecular-weight hydrocarbons. There is evidence that there is some short-range order and for long-range interactions the chains approximate a random coil with some portions paralleling one another. In 1953, Flory and Mark suggested a random coil model whereby the chains had conformations similar to those present if the polymer were in a theta solvent. In 1957, Kargin suggested that Amorphous Polymer chains exist as aggregates in parallel alignment. Models continue to be developed, but all contain the elements of disorder/order suggested by Flory and Mark and the elements of order suggested by Kargin.

  2.5 Polymer Structure–Property Relationships

  Throughout the text, we will relate polymer structure to the properties of the polymer. Polymer properties are related not only to the chemical nature of the polymer, but also to factors such as extent and distribution of crystallinity, distribution of polymer chain lengths, and nature and amount of additives, such as fillers, reinforcing agents, and plasticizers, to mention a few. These factors essentially influence all the polymeric properties to some extent, including hardness, flammability, weatherability, chemical stability, biological response, comfort, flex life, moisture retention, appearance, dyeability, softening point, and electrical properties.

  Materials must be varied to perform the many tasks required of them in today’s society. Often, they must perform them repeatedly and in a “special” manner. We get an idea of what materials can do by looking at some of the behavior of giant molecules in our body. While a plastic hinge must be able to work thousands of times, the human heart, a complex muscle largely composed of protein polymers (Section 10.6), provides about 2.5 billion beats within a lifetime moving oxygen (Section 16.8) throughout the approximately 144,000 km of the circulatory system with (some) blood vessels the thickness of hair and delivering about 8000 L of blood every day with little deterioration of the cell walls. The master design allows nerve impulses to travel within the body at the rate of about 300 m/min; again polymers are the “enabling” material that allows this rapid and precise transfer of nerve impulses. Human bones, again largely polymeric, have a strength of about five times that of steel (on a weight basis). Genes, again polymeric (Sections 10.10 and 10.11), appear to be about 95% the same between humans, with 5% functioning to give individuals the variety of size, abilities, etc. that confer uniqueness. In the synthetic realm, we are beginning to understand and mimic the complexities, strength, preciseness, and flexibility that are already present in natural polymers.

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  eBook - ePub

  Introduction to Physical Polymer Science

  • Leslie H. Sperling(Author)
  • 2015(Publication Date)
  • Wiley-Interscience

    (Publisher)

  CHAPTER 5

  THE AMORPHOUS STATE

  The bulk state, sometimes called the condensed or solid state, includes both amorphous and crystalline polymers. As opposed to polymer solutions, generally there is no solvent present. This state comprises polymers as ordinarily observed, such as plastics, elastomers, fibers, adhesives, and coatings.

  While Amorphous Polymers do not contain any crystalline regions, “crystalline” polymers generally are only semicrystalline, containing appreciable amounts of amorphous material. When a crystalline polymer is melted, the melt is amorphous. In treating the kinetics and thermodynamics of crystallization, the transformation from the amorphous state to the crystalline state and back again is constantly being considered. The subjects of amorphous and crystalline polymers are treated in the next two chapters. This will be followed by a discussion of liquid crystalline polymers, Chapter 7. Although polymers in the bulk state may contain plasticizers, fillers, and other components, this chapter emphasizes the polymer molecular organization itself.

  A few definitions are in order. Depending on temperature and structure, Amorphous Polymers exhibit widely different physical and mechanical behavior patterns. At low temperatures, Amorphous Polymers are glassy, hard, and brittle. As the temperature is raised, they go through the glass–rubber transition. The glass transition temperature (Tg ) is defined as the temperature at which the polymer softens because of the onset of long-range coordinated molecular motion. This is the subject of Chapter 8.

  Above Tg , cross-linked Amorphous Polymers exhibit rubber elasticity. An example is styrene–butadiene rubber (SBR), widely used in materials ranging from rubber bands to automotive tires. Rubber elasticity is treated in Chapter 9. Linear Amorphous Polymers flow above Tg .

  Polymers that cannot crystallize usually have some irregularity in their structure. Examples include the atactic vinyl polymers and statistical copolymers.

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  eBook - ePub

  Food Packaging Materials

  Testing & Quality Assurance

  • Preeti Singh, Ali Abas Wani, Horst-Christian Langowski(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  1994 ).

  3.5.1.1 Crystallinity

  Crystalline materials have their molecules arranged in repeating patterns. The crystallization takes place between glass transition and the melting state; it is always exothermic and a stabilization process of polymeric molecules. Amorphous materials, by contrast, have their molecules arranged randomly and in long chains that twist and curve around one another, making large regions of highly structured morphology unlikely. An amorphous solid is formed when the chains have little orientation throughout the bulk polymer. Solid plastic polymers are composed of crystalline and/or amorphous regions (disordered arrangements of randomly coiled and entangled chains). Thermoplastics, for example, usually are semicrystalline, a combination of crystalline and amorphous regions. The properties of thermoplastics are strongly influenced by their morphology. The fraction of the ordered molecules in polymer is characterized by the degree of crystallinity (i.e., the volume fraction of crystalline regions in a polymer). Typically, the value of the degree of crystallinity is in the 10% to 80% range, depending on the crystallization conditions (Gowariker et al. 1986 ; Carraher 2012 ). The degree of crystallinity affects the optical, thermal, and mechanical properties of a polymer. Although crystallinity is a powerful tool for improving strength and stiffness, such strengthening is always directional—that is, the properties are not the same in all directions (Allcock and Lampe 1981 ) (Figure 3.7 ). A crystalline polymer will be much stronger in the direction of molecular alignment and much weaker at right angles to that alignment. Amorphous Polymers are, in general, clear and transparent. On the other hand, visible light can pass through them, and semicrystalline polymers can be recognized easily because they are normally opaque and hazy. Light is not transmitted through the polymers; rather, it is reflected or scattered by the crystalline planes (Carraher 2012 ). The different properties of Amorphous Polymers and semicrystalline polymers are shown on Table 3.2

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  eBook - PDF

  Materials Science of Polymers

  Plastics, Rubber, Blends and Composites

  • A. K. Haghi, Eduardo A. Castro, Sabu Thomas, P. M. Sivakumar, Andrew G. Mercader, A. K. Haghi, Eduardo A. Castro, Sabu Thomas, P. M. Sivakumar, Andrew G. Mercader(Authors)
  • 2015(Publication Date)
  • Apple Academic Press

    (Publisher)

  They considered semicrystalline polymer as composite, in which matrix is the amorphous and the crystallites are the filler. The authors [1] also supposed that other polymers, for example, hybride polymer systems, in which two components with different mechanical properties were present obviously, can be simulated by a similar method. In paper [2] it has been pointed out, that the most important consequence from works by supramolecular formation study is the conclusion that physical-mechanical properties depend in the first place on molecular structure, but are realized through supramolecular formations. At scales interval and studies methods resolving ability of polymers structure, the nanoparticle size can be changed within the limits of 1 ¸ 100 and more nanometers. The polymer crystallite size makes up 10 ¸ 20 nm. The macromolecule can be included in several crystallites, since at molecular weight of order of 6 ¸ 10 4 its length makes up more than 400 nm. These reasonings point out that macromolecular formations and polymer systems in virtue of their structure features are always nanostructural systems. However, in the cited above works the amorphous glassy polymers consider-ation as natural composites (nanocomposites) is absent, although they are one of the most important classes of polymeric materials. This gap reason is quite enough (i.e., polymers amorphous state quantitative model absence). However, such model appearance lately [3–5] allows to consider the amorphous glassy polymers (both lin-ear and cross-linked ones) as natural nanocomposites, in which local order regions (clusters) are nanofiller and surrounded by loosely packed matrix of Amorphous Polymers structure which is matrix of nanocomposite.

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  eBook - ePub

  Ullmann's Polymers and Plastics

  Products and Processes

  • (Author)
  • 2016(Publication Date)
  • Wiley-VCH

    (Publisher)

  Polymer properties are influenced not only by the chemical structure (constitution, molar mass, configuration, microconformation) but also by the physical structure of polymers. These structures may range from totally irregular arrangements of chain segments over shorter or longer parallelizations of chains, to voids and other defects in otherwise highly organized assemblies of polymer molecules. Two possible ideal structures exist in the solid state: Perfect crystals and totally Amorphous Polymers. Polymer molecules are perfectly ordered in ideal crystals. They convert at the thermodynamic melting temperature into melts, which ideally are totally disordered. Amorphous Polymers can be viewed as frozen-in polymer melts. They are polymer glasses that convert to melts at the glass transition temperature.

  1.1. Noncrystalline States

  1.1.1. Structure

  Isolated polymer coils possess approximately a Gaussian distribution of chain segments; their segment density decreases with increasing chain length. However, the macroscopic densities of polymer melts do not change with chain lengths if end group effects on small molar mass molecules are neglected. Coils must therefore overlap considerably in polymer melts. At the glass transition temperature, cooperative segmental movements freeze in, and the physical structure of the melt is conserved. Small-angle neutron scattering studies have shown that the radii of gyration of Amorphous Polymers are indeed essentially identical for their melts, glasses, and solutions in theta solvents.

  The absence of long-range order in melts and Amorphous Polymers does not exclude the presence of short-range order in these states. Because of the persistence of polymer chains, a parallelization of short segments seems probable, as is found, for example, for alkanes according to X-ray investigation. This local order does not exceed 1 nm in each direction.

  The packing of chain segments cannot be perfect. Amorphous Polymers thus possess “free volumes”, which are regions of approximately atomic diameters. The volume fraction of this free volume is ca. 2.5% at the glass transition temperature and is independent of polymer constitution. Polymer segments in melts can move more freely than in the glassy state; the densities of melts are thus higher than the densities of glasses at the same temperature.

  1.1.2. Orientation

  Polymer segments, polymer molecules, and crystalline domains may be oriented along the machine direction by drawing or other mechanical processes. The orientation of chain segments need not necessarily lead to crystallization, however. An example is injection-molded polystyrene, which shows optical birefringence due to the orientation of segments but no X-ray crystallinity.

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  eBook - ePub

  Injection Moulding

  A Practical Guide

  • Vannessa Goodship(Author)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  6  Amorphous and semi-crystalline thermoplastics

  6.1  Introduction

  Thermoplastics can be subdivided into two distinct classes based upon differences in molecular structure. These differences can have a significant effect on the behaviour of the material during processing as well as impacting on the performance of moulding parts in service.

  Materials such as polystyrene (PS), polycarbonate (PC), acrylics [polymethyl methacrylate (PMMA)], acrylonitrile-butadiene-styrene (ABS) and polyvinyl chloride (PVC) are said to be amorphous thermoplastics. This signifies that in the solid state their molecular structure is random and disordered, the long chain molecules that make up the structure are all entangled; rather like solidified spaghetti.

  Materials such as most of the nylons [polyamides (PA)], polyacetal [polyoxymethylene (POM)], polypropylene (PP), polyethylene (PE) and thermoplastic polyesters [polyethylene terephthalate (PET)] have a much more ordered structure in the solid state, with a considerable proportion of the long chain molecules closely packed in regular alignment; these materials are known as semi-crystalline thermoplastics. It should be noted however, that at sufficiently high temperature (this is when the material is in its melt state) the molecular structure of both semi-crystalline and amorphous materials is amorphous. Table 6.1 classifies some common materials into these two groups.

  Table 6.1: Classifying plastics.

  Amorphous	Semi-crystalline
  Acrylic	Acetal
  PVC	Nylon
  SAN	Polyester
  PS	PE
  PC	PP
  ABS	PTFE

  PTFE: Polytetrafluoroethylene SAN: Styrene-acrylonitrile

  Most amorphous thermoplastics are transparent in their natural, unpigmented form, although ABS, for example, is an exception, whereas most semi-crystalline thermoplastics in their solid unpigmented form are translucent or an opaque white colour. It is interesting to observe (for example, during the purging stage in injection moulding machines) that fully molten natural PP or acetal are initially transparent, but as the melt cools they cloud over becoming translucent in the case of PP, and opaque white in the case of acetal. This clouding is due to the molecular structure of the material gradually rearranging itself from the tangled amorphous state in the melt to the more ordered semi-crystalline state in the solid state.

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  eBook - ePub

  Handbook of Industrial Hydrocarbon Processes

  • James G. Speight(Author)
  • 2010(Publication Date)
  • Gulf Professional Publishing

    (Publisher)

  Branching of polymer chains affects the ability of chains to slide past one another by altering intermolecular forces, in turn affecting bulk physical polymer properties. Long-chain branches may increase polymer strength, toughness, and the glass transition temperature due to an increase in the number of entanglements per chain. The effect of such long-chain branches on the size of the polymer in solution is characterized by the branching index. On the other hand, random length and atactic short chains may reduce polymer strength due to disruption of organization and may likewise reduce the crystallinity of the polymer.

  Increased crystallinity is associated with an increase in rigidity, tensile strength, and opacity (due to light scattering). Amorphous Polymers are usually less rigid, weaker and more easily deformed. They are often transparent.

  Three factors that influence the degree of crystallinity are: (1) chain length, (2) chain branching, and (3) inter-chain bonding. The importance of the first two factors is illustrated by the differences between low-density polyethylene and high-density polyethylene.

  Low-density polyethylene is composed of smaller and more highly branched chains which do not easily adopt crystalline structures. This material is therefore softer, weaker, less dense and more easily deformed than high-density polyethylene. Generally, mechanical properties such as ductility, tensile strength, and hardness rise and eventually level off with increasing chain length. On the other hand, high-density polyethylene is composed of very long unbranched hydrocarbon chains. These pack together easily in crystalline domains that alternate with amorphous segments, and the resulting material, while relatively strong and stiff, retains a degree of flexibility. Low-density polyethylene (LDPE) has significant numbers of both long and short branches, is quite flexible, and is used in applications such as plastic films, whereas high-density polyethylene (HDPE) has a very low degree of branching, is quite stiff, and is used in applications such as milk jugs.

  In contrast, natural rubber is a completely Amorphous Polymer and the potentially useful properties of raw latex rubber are limited by temperature dependence; however, these properties can be modified by chemical change. The cis-double bonds in the hydrocarbon chain provide planar segments that stiffen, but do not straighten, the chain. If, these rigid segments are completely removed by hydrogenation the chains lose all constraints, and the product is a low melting paraffin-like semisolid of little value. If, instead, the chains of rubber molecules are slightly cross-linked by sulfur atoms (vulcanization

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  eBook - PDF

  Polymer Viscoelasticity

  Stress and Strain in Practice

  • Evaristo Riande, Ricardo Diaz-Calleja, Margarita Prolongo, Rosa Masegosa, Catalina Salom(Authors)
  • 1999(Publication Date)
  • CRC Press

    (Publisher)

  Unlike main-chain LC polymers, side-chain LCs do not possess such good mechanical properties, but their optical behavior— particularly their response to electric and magnetic fields—makes them useful as materials for nonlinear optics (mixers, amplifiers, and frequency modulators). Crystalline and Amorphous States 57 CHOLESTERIC Figure 2.21 Side-chain LCPs organized in different mesophases. 2 .8 GLASSY STATE The introduction to this chapter made reference to the absence of long-range molecular order in Amorphous Polymers as being the characteristic differ­ entiating them from crystalline polymers. Depending on the temperature and structure, Amorphous Polymers exhibit different types of physical and mechanical behavior. In a molten polymer the molecular segments inter­ change places because of the high possibility of conformational changes caused by thermal excitation. In a solid Amorphous Polymer, the movements of the chain segments are vibrations around fixed positions. When the tem­ perature increases, the amplitude of the vibrations increases, transmitting a rise in tension to the intermolecular interactions. If the temperature con­ tinues to increase, a growing fraction of chain segments acquire enough energy to overcome these intermolecular interactions. Stronger modes of movement appear that involve the rotation and translation of chain term­ inals and chain segments or loops incorporating about 10 bonds (Fig. 2.22). These movements are an important mechanism for energy absorption, thereby imparting toughness to the material. A temperature can be assigned to each polymer at which, during the observation time of the experiment, these movements start to be detected. This temperature is the glass transi­ tion temperature, Tg (7,24,25). At temperatures below Tg, the polymer maintains the disordered nature of the melt but lacks molecular mobility; 58 Chapter 2 Figure 2.22 Movements of chain terminals, loops, and segments in the glass transition temperature range.

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  eBook - ePub

  Fundamentals of Polymer Science

  An Introductory Text, Second Edition

  • Michael M. Coleman, Paul C. Painter(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  There are three things about the picture obtained from X-ray diffraction experiments that concern us here. First, for simple, low molecular weight materials, it is often possible to obtain good single crystals that provide a wealth of data (diffraction spots) from which an accurate picture of the arrangements of atoms and molecules can be determined. Synthetic polymers cannot be obtained in the form of large single crystals and it is necessary to obtain a diffraction pattern from a drawn or stretched fiber, where the chains are aligned to some degree (depending on the amount of stretching) in the fiber direction. Only a few spots are observed (usually broadened into arcs) and the process of determining structure is much more difficult. Second, even low molecular weight materials form crystals that contain defects and these can profoundly affect properties. We shouldn’t think that crystalline materials have a completely perfect structure. (Anybody who buys a diamond usually learns this very quickly.) Finally, in low molecular weight materials a molecule is smaller than the size of a unit cell. In polymers this is not so and individual chains pass through many unit cells.

  The details of the determination of polymer crystal structure are beyond the scope of what we want to cover here. We will simply observe that X-ray crystallography played a key role in early studies of polymers and the establishment of the macromolecular hypothesis and proceed to describe two or three typical examples of polymer unit cell structures. This will make it immediately clear why some polymers crystallize and others do not.

  Figure 7.22 shows a representation of the unit cell of polyethylene. There are three things you should notice. First, as we have just mentioned, only a small part of each chain lies in a unit cell. Accordingly, a knowledge of the arrangement of chains in the unit cell is a sort of local knowledge, in the sense that we do not know what sections of the rest of the chain are doing. Are all the segments also in the crystal or are some in those amorphous regions that we know are also present in polymers? Second, the chains are in the preferred, minimum energy, all trans or zig-zag conformation. This is generally, but not always, the rule for polymer crystals, particularly if there are several conformations of almost equal energy. Finally, the crystal structure is close packed, as one might expect if intermolecular attractions are to be maximized. This means that defects, such as short chain branches, generally cannot be accommodated in a crystal lattice (some small defects are occasionally incorporated into certain polymer crystals, but these naturally distort the lattice). Accordingly, just small amounts of branching in polyethylene serve to reduce the degree of crystallinity

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