Glass Transition Temperature

12 Key excerpts on "Glass Transition Temperature"

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  eBook - PDF

  Prediction of Polymer Properties

  • Jozef Bicerano(Author)
  • 2002(Publication Date)
  • CRC Press

    (Publisher)

  CHAPTER 6 TRANSITION AND RELAXATION TEMPERATURES 6.A. Background Information 6.A.1. Operational Definition of the Glass Transition At a relatively simple-minded practical and operational (and thus theoretically nonrigorous) level of treatment, we can define the Glass Transition Temperature (T g ) as the temperature at which the forces holding the distinct components of an amorphous solid together are overcome by thermally-induced motions within the time scale of the experiment, so that these components become able to undergo large-scale molecular motions on this time scale, limited mainly by the inherent resistance of each component to such flow. The practical effects of the glass transition on the processing and performance characteristics of polymers are implicit in this definition. In most polymeric as well as non-polymeric amorphous materials, the ability to undergo large-scale molecular motions implies the freedom to flow, so that the material becomes a fluid above T g . However, in the special class of polymers commonly described as “thermosets”, covalent crosslinks limit the ability to undergo large-scale deformation. Consequently, above T g , thermosets become “elastomers” (also known as “crosslinked rubbers”). On the experimental time scale, above T g , non-thermoset amorphous materials are viscous fluids. Their glass transitions can then be viewed as transitions, over the experimental time scale, from predominantly elastic “solid-like” to predominantly viscous “liquid-like” behavior. In fact, traditionally, the glass transition has often been identified in practical terms (not only for polymers, but also for amorphous inorganic materials) as taking place when the viscosity reaches a threshold value (most commonly taken to be 10 13 Poise). The glass transition occurs in the reverse direction if the temperature is instead lowered from above to below T g , with the material then undergoing “vitrification”.

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  Polymer Chemistry

  • Timothy P. Lodge, Paul C. Hiemenz(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  gauche conformers and thus chain extension should be more facile in such cases; this would lower the critical yield stress. On the other hand, crazing requires cavitation. In addition to the surface energy penalty noted above, the formation of a cavity requires either chain scission or pullout across the interface where the cavity forms. The higher the entanglement density, the shorter will be the dangling entanglement strands at the ends of chains and the smaller the extension ratio of one entanglement strand. Both factors increase the critical stress for crazing, and thus more highly entangled chains favor yielding. Simply put, a higher entanglement density corresponds to a material that is more tightly stitched together, and therefore more resistant to localized yielding (i.e., crazing).

  12.8Chapter Summary

  In this chapter, we have examined the transition between the liquid state and the glassy state, which takes place over a range of temperatures near a characteristic Glass Transition Temperature, Tg . The principal points are the following:

  1. The glass transition is a kinetic transition, but it approximates a second-order thermodynamic transition. A completely satisfactory theory of the glass transition is not yet available.
  2. The Glass Transition Temperature may be located in a variety of ways, but the most common tools are DSC and rheology.
  3. The Glass Transition Temperature is the single most important parameter in determining whether a given polymer may be suitable for a certain application. There is no simple way to correlate Tg with a particular chemical structure, although some general rules exist.
  4. The value of Tg may be modified by changing molecular weight or by blending; the molecular weight and composition dependences of Tg are generally straightforward.
  5. The concept of free volume is a particularly useful and physically intuitive way to understand the glass transition, and the profound effect that proximity to Tg has on the temperature dependence of any viscoelastic or transport property. The free volume approach provides a natural explanation for the widely used Vogel–Fulcher–Tammann-Hesse and Williams–Landel–Ferry equations, which describe the temperature dependence of viscoelastic properties above Tg .
  6. The principle of time–temperature superposition is an essential ingredient in the study of polymer viscoelasticity because small changes in temperature produce large changes in the polymer relaxation times. Consequently measurements over a finite range of time or frequency at one temperature can be superposed with measurements at other temperatures to generate master curves of dynamic response, which can extend over as many as 20 orders of magnitude in reduced time or frequency.

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  Thermosetting Polymers

  • Jean-Pierre Pascault, Henry Sautereau, Jacques Verdu, Roberto J. J. Williams(Authors)
  • 2002(Publication Date)
  • CRC Press

    (Publisher)

  Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. In this chapter the main characteristics of the glass transition are analyzed and equations relating the Glass Transition Temperature with the conversion of the thermosetting polymer are discussed. Then, CTT and TTT transformation diagrams are derived and examples of their practical use provided. Glass Transition and Transformation Diagrams 121 4.2 GLASS TRANSITION 4.2.1 Phenomenological Aspects From the practical point of view, the glass transition is a key property since it corresponds to the short-term ceiling temperature above which there is a catastrophic softening of the material. For amorphous polymers in gen-eral, and thus for thermosets, one can consider that the Glass Transition Temperature, T g , is related to the conventional heat deflection temperature (HDT) (usually, HDT is 10-15°C below T g , depending on the applied stress and the criterion selected to define T g ). In the field of processing, the glass transition is rather a floor tem-perature because the polymerization (cure) exhibits a very slow or even negligible rate in the glassy state (see Chapter 5). Many important proper-ties, such as the yield stress or the fracture toughness at a temperature T are sharply linked to (T g —T). Some qualitative and important quantitative differences between the glassy and rubbery states are listed in Table 4.1. The glass transition is usually characterized as a second-order thermo-dynamic transition.

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  Engineering Properties of Foods

  • M.A. Rao, Syed S.H. Rizvi, Ashim K. Datta, Jasim Ahmed, M.A. Rao, Syed S.H. Rizvi, Ashim K. Datta, Jasim Ahmed(Authors)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  The glassy state of matter and the glass transition itself have still remained unsolved problems in various disciplines of science and engineering. The exis-tence of glassy or rubbery state is related to the molecular motion inside the material that is mostly governed by temperature, timescale of observation, plasticization, and other fac-tors. A glassy material lies below the temperature at which molecular motions exist on the timescale of the experiment, and a rubbery material is above the temperature at similar conditions (Andrews and Grulke, 1999). A glassy material is formed when a melt or liquid is cooled below its crystalline melting temperature, T m , at a faster rate to avoid crystalliza-tion. The change between rubbery liquid and glassy behavior is known as the glass transi-tion, and the critical temperature, which separates glassy behavior from rubbery behavior is known as the Glass Transition Temperature, T g (Figure 4.1). This transition occurs with no change in order or structural reorganization of the liquid and is not a thermodynamic first-order process since there is no change in entropy, enthalpy, or volume (Haward, 1973). The transition is considered as a thermodynamic second-order phase transition, which implies a jump in the heat capacity or expansivity of the sample that occurs over a tem-perature range. The glass transition is the most important property of amorphous materi-als, both practically and theoretically, since it involves a dramatic slowing down in the motion of chain segments, which rarely one can observe in the static state. Glass transition leads to affect many physical properties including density, specific heat, heat flow, specific volume, mechanical modulus, viscosity, dielectric properties, and so on (Andrews and Grulke, 1999). The technological importance of the glass transition in amorphous food products is enormous.

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  Introduction to Physical Polymer Science

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

    (Publisher)

  9 Pa. Often the Glass Transition Temperature is defined as the temperature where the thermal expansion coefficient (Section 8.3) undergoes a discontinuity. (Enthalpie and dynamic definitions are given in Section 8.2.9. Other, more precise definitions are given in Section 8.5.)

  Qualitatively, the glass transition region can be interpreted as the onset of long-range, coordinated molecular motion. While only 1 to 4 chain atoms are involved in motions below the Glass Transition Temperature, some 10 to 50 chain atoms attain sufficient thermal energy to move in a coordinated manner in the glass transition region (9,11–14) (see Table 8.4 ) (9,15). The number of chain atoms, 10–50, involved in the coordinated motions was deduced by observing the dependence of Tg on the molecular weight between cross-links, Mc . When Tg became relatively independent of Mc in a plot to Tg versus Mc , the number of chain atoms was counted. It should be emphasized that these results are tenuous at best.

  The Glass Transition Temperature itself varies widely with structure and other parameters, as will be discussed later. A few Glass Transition Temperatures are shown in Table 8.4 . Interestingly, the idealized map of polymer behavior shown in Figure 8.2 can be made to fit any of these polymers merely by moving the curve to the right or left, so that the Glass Transition Temperature appears in the right place.

  Table 8.4

  Glass transition parameters (9,15)

  Polymer	Tg , °C	Number of Chain Atoms Involved
  Poly(dimethyl siloxane)	–127	40
  Poly(ethylene glycol)	–41	30
  Polystyrene	+100	40–100
  Polyisoprene	–73	30–40

  8.2.3 The Rubbery Plateau Region

  Region 3 in Figure 8.2 is the rubbery plateau region. After the sharp drop that the modulus takes in the glass transition region, it becomes almost constant again in the rubbery plateau region, with typical values of 2 × 107 dynes/cm2 (2 × 106

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  Phases of Matter and their Transitions

  Concepts and Principles for Chemists, Physicists, Engineers, and Materials Scientists

  • Gijsbertus de With(Author)
  • 2023(Publication Date)
  • Wiley-VCH

    (Publisher)

  The glass transition tem- perature T g , located approximately in the middle of the transition range, characterizes the transition, while the overall behavior is denoted as viscoelastic. Thus, unlike during melt- ing/freezing, where several properties change abruptly at the melting temperature (in this chapter labeled) T m , properties for a glass change gradually from one regime to another in the glass transition range, as illustrated for the specific volume in Figure 18.1b. Whether crystallization occurs or not, depends on the balance between nucleation and relaxation. The T g is usually determined by extrapolation of the behavior of the undercooled liquid and vitreous regions. Characteristic is that T g depends on the cooling rate q * used: the lower q * , the lower the T g obtained. Qualitatively, this behavior can be understood through relaxation. At high q * only limited relaxation of the liquid structure can occur before the temperature has decreased so far that further relaxation is very slow. At lower q * more relaxation can take place, thus continuing Phases of Matter and their Transitions: Concepts and Principles for Chemists, Physicists, Engineers, and Materials Scientists, First Edition. Gijsbertus de With. © 2024 WILEY-VCH GmbH. Published 2024 by WILEY-VCH GmbH. 526 18 The Glass Transition V Physical aging Fast cooling Slow cooling T T g1 T g2 T m Crystalline solid Vitreous state Undercooled liquid state Free volume Occupied volume T ∞ Liquid (a) (b) Figure 18.1 (a) Two-dimensional (2D) schematic of an amorphous structure with no long-range order and a crystalline structure with long-range order. (b) The (idealized) change in specific volume V for the glass transition at cooling rate 1 > cooling rate 2 leading to Glass Transition Temperatures T g1 and T g2 . Also shown is the normal melting behavior at melting point T m . The enthalpy shows similar behavior. the liquid-like regime to lower temperature and hence leading to a lower T g .

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  Physics and Mechanics of Amorphous Polymers

  • Jo Perez(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  VII Glass Transition

  Glass technologists have long known that during cooling a gradual increase in viscosity of liquids consisting of mixtures of oxides takes place to form glass; this process is schematically depicted in Figure VII.1. It was natural, therefore, to adopt a convention that distinguishes between the liquid state and the solid glassy state. We thus define the temperature of liquid-glass transition as that temperature at which the liquid has a viscosity of 1012 Pa·s (1013 poises), for which Tammann gave the symbol Tg (‘glass temperature’). Simon concomitantly suggested the term ‘range of freezing’, this domain coinciding with the optimal range of temperature for the curing of glasses. Such a coincidence emphasises the technological interest in glass transition. It also reveals more fundamentally that at this temperature the characteristic time of structural reorganisation is comparable to the time of experimental observation. Since that time, scientific interest about glass transition has grown. It is now well established that the condensed phases so chemically differing such as the chalcogenic elements and various alloys based on them, molecular organic liquids (glycerol, pyridine, O-terphenyl), some mixtures of oxides, of which at least one is of the type ‘formator’ (SiO2 , B2 O3 …), and some metallic alloys, show during cooling not only an increase in viscosity, but also around Tg , the same type of sufficiently rapid modification of properties. So it is clear that glass transition is a specific manifestation of the glassy state. This justifies the present effort to understand the fundamental physical aspects of this manifestation; as evidenced by the numerous papers published during the 1980s. Conforming to the theme of this book, we shall confine ourselves to the case of amorphous polymers, an important class of glassy solids; however, this will not impart the characters of generality of the concepts used. These concepts, developed in Chapter II and widely used in subsequent chapters, explicitly account for glass transition since section II.5.2

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  Principles of Polymer Science and Technology in Cosmetics and Personal Care

  • E. Desmond Goddard, James V. Gruber, E. Desmond Goddard, James V. Gruber(Authors)
  • 1999(Publication Date)
  • CRC Press

    (Publisher)

  The relative importance of each of these factors is illustrated by the examples listed in Table 2. a. Measurement of the Glass Transition Temperature . The glass temperature of a polymer can be observed experimentally by measuring basic thermodynamic, physical, mechanical, or electrical properties as a function of temperature. Thermal methods are Table 2 Relationships Between the Structure of Polymers and Their Glass Transition Temperature ( T g) Polymer structure Main chain unit Tg(°C) Chain flexibility Polyethylene - n Poly(a-vinylnaphthalene) m + 135 Size of the substituent group Poly(methyl methacrylate) — [CH2CH(COOCH3)]n— + 105 Poly(ethyl methacrylate) — [CH2CH(COOCH2CH3)]n— +65 Poly(propyl methacrylate) — [CH2CH(COOCH2CH2CH3)]n— +35 Interchain forces (H-bonds, dipole-dipole interactions) Polyethylene — (CH2CH2)n— -125 Polyoxymethylene — (OCH2)n— -85 Polypropylene — [CH2CH(CH3)]n— -15 Polyvinylchloride — (C H2CHCl)n— +80 Polyacrylonitrile — [CH2CH(CN)]n— + 105 Polycaprolactone — [(CH2)^COOL— -60 Polycaprolactam (nylon 6) — [(CH2)5—CONH]n— +60 18 Winnik used routinely (48,49). Two closely related thermal methods dominate, an older method, differential thermal analysis (DTA), and a newer method, differential scanning calorimetry (DSC) (50). In making DTA measurements, the temperature of a sample is compared with a reference material, such as powdered alumina. Both the sample and the reference are heated at a uniform rate, typically 10-20°C/min. Since the two materials have different heat capacities, each maintains slightly different temperatures throughout the scan. The difference in temperature, AT, is monitored as a function of the temperature T.

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  Phases of Matter and their Transitions

  Concepts and Principles for Chemists, Physicists, Engineers, and Materials Scientists

  • Gijsbertus de With(Author)
  • 2023(Publication Date)
  • Wiley-VCH

    (Publisher)

  Although glasses do not show a clear melting point where rigidity disappears, they do show with increasing temperature, a gradual change from elastic to viscous behavior in the transition range. The Glass Transition Temperature T g, located approximately in the middle of the transition range, characterizes the transition, while the overall behavior is denoted as viscoelastic. Thus, unlike during melting/freezing, where several properties change abruptly at the melting temperature (in this chapter labeled) T m, properties for a glass change gradually from one regime to another in the glass transition range, as illustrated for the specific volume in Figure 18.1 b. Whether crystallization occurs or not, depends on the balance between nucleation and relaxation. The T g is usually determined by extrapolation of the behavior of the undercooled liquid and vitreous regions. Characteristic is that T g depends on the cooling rate q * used: the lower q *, the lower the T g obtained. Qualitatively, this behavior can be understood through relaxation. At high q * only limited relaxation of the liquid structure can occur before the temperature has decreased so far that further relaxation is very slow. At lower q * more relaxation can take place, thus continuing the liquid‐like regime to lower temperature and hence leading to a lower T g. A glass cooled at a high rate can, when kept for a sufficiently long time at a sufficiently high temperature T < T g in the transition regime, relax to its undercooled liquid state at T, a process often denoted as physical aging when it occurs undesirably, or as annealing when it occurs by design. Such a glass is called a stabilized glass with a fictive temperature T fic = T, and during annealing at T < T fic it contracts, while at T > T fic it expands. For a heating rate close to the initial q *, T fic ≅ T g. Physical aging stands in contrast to chemical aging where a slow chemical reaction modifies the chemical constitution

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  Plastics Materials

  • J A Brydson(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  One object of research into the physical properties of plastics material is to determine the locations of tough-brittle transitions for commerical polymers. As with other physical properties the position of the Glass Transition Temperature and the facility with which crystallisation can take place are fundamental to the impact strength of a material. Well below the Glass Transition Temperature amorphous polymers break with a brittle fracture but they become tougher as the Glass Transition Temperature is approached. A rubbery state will develop above the glass transition and the term impact strength will cease to have significance. In the case of crystalline materials the toughness will depend on the degree of crystallinity; large degrees of crystallinity will lead to inflexible masses with only moderate impact Bibliography 71 strengths. The size of the crystalline structure formed will also be a significant factor, large spherulitic structures leading to masses with low impact strength. As indicated in the previous chapter spherulitic size may be controlled by varying the ratio of nucleation to growth rates. The valuable characteristics of polyblends, two-phase mixtures of polymers in different states of aggregation, were also discussed in the previous chapter. This technique has been widely used to improve the toughness of rigid amorphous polymers such as PVC, polystyrene, and styrene-acrylonitrile copolymers. References 1. S W A L L O W , j. c , /. Roy. Soc. Arts, 99, 355 (1951) 2. G O R D O N , M . , High Polymers, Iliffe, London (1963) 3. J E N C K E L , E . , and U E B E R R E I T E R , Κ . , Z. Phys. Chems., A182, 361 (1939) 4. B R Y D S O N , J. A . , Chapter entitled 'Glass Transition, Melting Point and Structure' in Polymer Science (Ed. J E N K I N S , A . D . ) , North-Holland, Amsterdam (1972) 5. E D G A R , o. B . , and H I L L , R . , / . Polymer Sci., 8, 1 (1952) Bibliography B I L L M E Y E R , F.

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  Low-Temperature Behaviour of Elastomers

  • Bukhina, Kurlyand(Authors)
  • 2007(Publication Date)
  • CRC Press

    (Publisher)

  Mechanical Properties of Elastomers near the Glass Transition Temperature The results of the comparison of T g values obtained by different techniques show a signi-ficant difference between the values determined by physical and mechanical methods. As we have pointed out, an important role can be played both by the frequency in the mechan-ical measurements and by the loading. Besides, depending on the service conditions of elas-tomeric materials, they can be characterized by a low temperature boundary T low ; this boundary is determined by the properties of the material itself and the level of the properties required to provide the operation of elastomeric goods. This issue will be considered in detail in Chapter 6; here we would only note that T low and T g can differ significantly. Here-with, (2.1) is usually positive, i.e., T low is higher than T g ; for particular applications, however, the con-dition can be achieved when T low < T g (see below and also Chapter 6). The temperature T low is determined not only by T g but also by the level of the mechan-ical properties near this temperature. Therefore, when considering the low-temperature be-haviour of rubbers, it is important not only to know T g and understand how it changes under the action of various factors, but also to consider all the mechanical properties of elastomers near (both higher and lower than) T g and the character of their change with the temperature and under the action of various factors. 2.1 Mechanical properties in the region of transition from the rubberlike to the glassy state When considering the effect of one factor or another, one should bear in mind that all glass transition-related changes in the mechanical properties of elastomers at a temperature de-crease are determined by a deceleration of the relaxation processes and, as a consequence, an increase of the ratio of elastic deformation and rubberlike deformation.

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  Thermoforming

  A Plastics Processing Guide, Second Edition

  • Geza Gruenwald(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  EIGHT Thermoforming-Related Material Properties B ASICALLY, ALL THERMOPLASTIC materials should be suitable for thermoforming processes. Such materials, when heated, will exhibit a reduction in their modulus of elasticity, their stiffness, and their load-bearing capacity. To understand these relationships it becomes necessary to know how temperature changes affect the physical properties of plastics. We are too much accustomed to assume that our everyday materials, such as wood, concrete, glass, metals, and textiles, remain unchanged between 0 and 200°F. Glass Transition Temperature Low molecular weight or atomic crystalline solids will, upon raising the temperature, melt at a certain point by forming a low-viscosity liquid (water, salts, metals, and the like). The common metals become as fluid beyond their melting points as mercury is at room temperature. High molecular weight amorphous materials (most thermoplastics) or materials that exhibit strong ionic bonds, such as glass, can appear to be rigid solids as long as their chain links remain immobilized. Depending on their chemistry, these links may, upon heating, become at one point able to rotate and translate. This characteristic point has been named Glass Transition Temperature. Above this temperature the material remains a coherent solid but exhibits some flexibility resembling a “leathery” or “rubbery” texture. Starting with an extremely high viscosity (in the range of 10 12 poise 1), the polymeric material will change with increasing temperature to a viscous substance but will never become a fluid liquid because the weak cohesive forces acting between each link of the long polymer chain will prevent the molecules from easily sliding past each other

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