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Previous Lecture Lennard-Jones potential energy for pairs of atoms and for pairs within molecular crystals Evaluation of the Young’s (elastic) modulus for molecular crystals starting from the L-J potentials Response of soft matter to shear stress: Hookean (elastic) solids versus Newtonian (viscous) liquids Description of viscoelasticity with a transition from elastic to viscous response at a characteristic relaxation time, An important relationship between elastic and viscous components: = G o
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3SM Lecture 4 Time Scales, the Glass Transition and Glasses 4 March, 2010 See Jones’ Soft Condensed Matter, Chapt. 2 and 7
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Response of Soft Matter to a Constant Shear Stress: Viscoelasticity t Slope: We see that 1/G o ( 1 / ) is the relaxation time An alternative expression for viscosity is thus G o
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Relaxation and a Simple Model of Viscosity »When a “simple” liquid is subjected to a shear stress, immediately the molecules’ positions are shifted but the same “neighbours” are kept. Thereafter, the constituent molecules re-arrange to relax the stress, and the liquid begins to flow. A simple model of liquids imagines that relaxation takes place by a hopping mechanism, in which molecules escape the “cage” formed by its neighbours. Molecules in a liquid vibrate with a frequency,, comparable to the phonon frequency in a solid of the same substance. Thus can be considered a frequency of attempts to escape a cage. But what is the probability that the molecule will escape the cage? F
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0 Molecular configuration Potential Energy (per molecule) Intermediate state: some molecular spacings are greater: thus higher w(r)! The frequency, f, at which the molecules overcome the barrier and relax is an exponential function of temperature, T. Molecular Relaxation in Simple Viscous Flow
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Molecular Relaxation Times is the energy of the higher state and can be considered an energy barrier per molecule. Typically, 0.4 L v /N A, where L v is the heat of vapourisation per mole and N A is the Avogadro number. A statistical argument tells us that the probability P of being in the high energy state is given by the Boltzmann distribution: P exp(- / kT) T is the temperature of the reservoir. As T 0, then P 0, whereas when T , then P 1 (100% success) Eyring proposed that the frequency of successful escapes, f, is then the product of the frequency of attempts ( ) and the probability of success (P): The time required for a molecule to escape its cage defines a molecular configurational relaxation time, config, which is comparable in magnitude to the macroscopic relaxation time. And so, config = 1/ f.
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Arrhenius Behaviour of Viscosity In liquids, the relaxation time, , is very short, varying between 10 -12 and 10 -10 s. Hence, as commonly observed, stresses in liquids are relaxed nearly instantaneously. In melted polymers, can be on the order of several ms or s. We can approximate that G o , where G o is the shear modulus of the corresponding solid. Hence an expression for can be found from the Eyring relationship: Alternatively, an expression based on the molar activation energy E can be written: This is referred to as an Arrhenius relationship.
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Non-Arrhenius Temperature Dependence Liquids with a viscosity that shows an Arrhenius dependence on temperature are called “strong liquids”. An example is melted silica. “Fragile liquids” show a non-Arrhenius behaviour that requires a different description. An example is a melted polymer. The viscosity of a melted polymer is described by the Vogel-Fulcher (VF) relationship: We see that diverges to , as the liquid is cooled towards T o. It solidifies as temperature is decreased. In the high-temperature limit, approaches o - a lower limit. where B and T o are empirical constants. (By convention, the units of temperature here are usually °C!)
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Temperature-Dependence of Viscosity P = Poise Arrhenius Vogel- Fulcher
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Configurational Re-Arrangements As a liquid is cooled, stress relaxation takes longer, and it takes longer for the molecules to change their configuration, as described by the configurational relaxation time, config. From the Vogel-Fulcher equation, we see that: We see that the relaxations become exceedingly slow ( becomes v. large) as T decreases towards T o.
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An Example of the Glass Transition Video: Shattering polymer
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Experimental Time Scales To distinguish a liquid from a solid, flow (or other liquid-like behaviour) must be observed on an experimental time scale, exp. A substance will appear to be a solid on short time scales but a liquid on long time scales! For example, if a sample is being cooled at a rate of 1 K per min., then exp is ~1 min. at each temperature increment. At higher temperatures, exp > config, and flow is observed on the time scale of the measurement. Debonding of an Adhesive Flow can be observed on long time scales, exp
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Oscillatory Stress Apply a shear strain (or stress) at an angular frequency of = 1 / exp t ss 2/2/
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The Glass Transition At higher temperatures, exp > config, and so flow is observed on the time scale of the measurement. As T is lowered, config increases. When T is decreased to a certain value, known as the glass transition temperature, T g, then config ~ exp. Below T g, molecules do not change their configuration sufficiently fast to be observed during exp. That is, exp < config. The substance appears to be solid-like, with no observable flow. At T = T g, is typically 10 13 Pa-s. Compare this to = 10 -3 Pa-s for water at room temperature.
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Competing Time Scales Reciprocal Temperature (K -1 ) Log(1/ ) =1/ vib f = 1/ config 1/ exp 1/T g config < exp config > exp Melt (liquid) glass Molecular conformation does not change when passing through the glass transition.
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Are Stained-Glass Windows Liquid? Window in the Duomo of Siena Some medieval church windows are thicker at their bottom. Is there flow over a time scale of exp 100 years?
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Thermodynamics of Phase Transitions At equilibrium, the phase with the lowest Gibbs’ free energy will be the stable phase. How can we describe this transition? b a The “b” phase is stable below the critical temperature, T c. TcTc
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Temperature, T Free energy, G Free Energy of the Melting/Freezing Transition Crystalline state Liquid (melt) state TmTm Below the melting temperature, T m, the crystalline state is stable. The thermodynamic driving force for crystallisation, G, increases when cooling below T m. During a transition from solid to liquid, we see that (dG/dT) P will be discontinuous. GG Undercooling, T, is defined as T m – T.
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Classification of Phase Transitions A phase transition is classified as “first-order” if the first derivative of the Gibbs’ Free Energy, G, with respect to any state variable (P,V, or T) is discontinuous. An example - from the previous page - is the melting transition. In the same way, in a “second-order” phase transition, the second derivative of the Gibbs’ Free Energy G is discontinuous. Examples include order-disorder phase transitions in metals and superconducting/non-SC transitions.
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Thermodynamics of First-Order Transitions Gibbs’ Free Energy, G: G = H-ST so that dG = dH - TdS - SdT Enthalpy, H = U + PV so that dH = dU + PdV + VdP Substituting in for dH, we see: dG = dU + PdV + VdP - TdS - SdT The central equation of thermodynamics tells us: dU = TdS - PdV Substituting for dU, we find: dG = TdS - PdV + PdV + VdP - TdS - SdT Finally, dG = VdP-SdT H = enthalpy S = entropy U = internal energy
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Thermodynamics of First-Order Transitions dG = VdP - SdT In a first order transition, we see that V and S must be discontinuous: V T liquid crystalline solid TmTm Viscosity is also discontinuous at T m. There is a heat of melting, and thus H is also discontinuous at T m. (Or S)
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Thermodynamics of Glass Transitions V T Crystalline solid TmTm Liquid Glass TgTg
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Determining the Glass Transition Temperature in Polymer Thin Films Poly(styrene) h o ~ 100 nm TgTg Melt Glass Keddie et al., Europhys. Lett. 27 (1994) 59-64 ~ Thickness
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Is the Glass Transition Second-Order? Thus in a second-order transition, C P will be discontinuous. Recall that volume expansivity, , is defined as: And V = ( G/ P) T. So, Expansivity is related to a second differential of G, and hence it is likewise discontinuous in a second-order phase transition. Note that S is found from -( G / T) P. Then we see that the heat capacity, C p, can be given as:
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Experimental Results for Poly(Vinyl Acetate) Data from Kovacs Expansivity is not strictly discontinuous – there is a broad step.
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Effect of Cooling Rate on T g T g is not a constant for a substance. When the cooling rate is slower, exp is longer. For instance, reducing the rate from 1 K min -1 to 0.1 K min -1, increases exp from 1 min. to 10 min. at each increment in K. With a slower cooling rate, a lower T can be reached before config exp. The result is a lower observed T g. Various experimental techniques have different associated exp values. Hence, a value of T g depends on the technique used to measure it.
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Data from H. Utschick, TA Instruments Glass Transition of Poly(vinyl chloride) Heat flow ~ heat capacity Sample is heated at a constant rate. Calorimeter measures how much heat is required. T Heat capacity is not strictly discontinuous – the step is broad.
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Structure of Glasses There is no discontinuity in volume at the glass transition and nor is there a discontinuity in the structure. In a crystal, there is long-range order of atoms. They are found at predictable distances. But at T>0, the atoms vibrate about an average position, and so the position is described by a distribution of probable interatomic distances, n(r).
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Atomic Distribution in Crystals 12 nearest neighbours And 4th nearest! FCC unit cell (which is repeated in all three directions)
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Comparison of Glassy and Crystalline Structures 2-D Structures Going from glassy to crystalline, there is a discontinuous decrease in volume. Local order is identical in both structures Glassy (amorphous) Crystalline
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Simple Liquid Structure r
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Structure of Glasses and Liquids The structure of glasses and liquids can be described by a radial distribution function: g(r), where r is the distance from the centre of a reference atom/molecule. The density in a shell of radius r will have atoms per volume. For the entire substance, let there be o atoms per unit volume. Then g(r) = (r)/ o. At short r, there is some predictability of position because short-range forces are operative. At long r, (r) approaches o and g(r) 1.
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R.D.F. for Liquid Argon Experimentally, vary a wave vector: Scattering occurs when: (where d is the spacing). Can vary either or in experiments
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R.D.F. for Liquid Sodium Compared to the BCC Crystal 4r2(r)4r2(r) r (Å) 3 BCC cells Each Na has 8 nearest neighbours.
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Entropy of Glasses Entropy,S, can be determined experimentally from integrating plots of C P /T versus T (since C p = T ( S/ T ) P ) The disorder (and S) in a glass is similar to that in the melt. Contrast this case to crystallisation in which S “jumps” down at T m. Since the glass transition is not first-order, S is not discontinuous through the transition. S for a glass depends on the cooling rate. As the cooling rate becomes slower, S becomes lower. At a temperature called the Kauzmann temperature, T K, we expect that S glass = S crystal. The structure of a glass is similar to the liquid’s, but there is greater disorder in the glass compared to the crystal of the same substance.
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Kauzmann Paradox Crystal Glass Melt (Liquid)
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Kauzmann Paradox S glass cannot be less than S crystal. Yet by extrapolation, we can predict that at sufficiently slow cooling rate, S glass will be less than S crystal. This prediction is a paradox! Paradox is resolved by saying that T K defines a lower limit to T g as assumed in the V-F equation. Experimentally, it is usually found that T K T o (V-F constant). Typically, T g - T o = 50 K. This is consistent with the prediction that at T=T o, config will go to . T g equals T K (and T o ) when exp is approaching , which would be obtained via an exceedingly slow cooling rate.
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Problem Set 2 1. Calculate the energy required to separate two atoms from their equilibrium spacing r o to a very large distance apart. Then calculate the maximum force required to separate the atoms. The pair potential is given as w(r) = - A/r 6 + B/r 12, where A = 10 -77 Jm 6 and B = 10 -134 Jm 12. Finally, provide a rough estimate of the modulus of a solid composed of these atoms. 2. The latent heat of vaporisation of water is given as 40.7 kJ mole -1. The temperature dependence of the viscosity of water is given in the table below. (i) Does the viscosity follow the expectations of an Arrhenius relationship with a reasonable activation energy? (ii) The shear modulus G of ice at 0 C is 2.5 x 10 9 Pa. Assume that this modulus is comparable to the instantaneous shear modulus of water G o and estimate the characteristic frequency of vibration for water,. Temp ( C)01020304050 (10 -4 Pa s)17.9313.0710.027.986.535.47 Temp ( C)60708090100 (10 -4 Pa s) 4.674.043.543.152.82 3. In poly(styrene) the relaxation time for configurational rearrangements follows a Vogel-Fulcher law given as = o exp(B/T-T o ), where B = 710 C and T o = 50 C. In an experiment with an effective timescale of exp = 1000 s, the glass transition temperature T g of poly(styrene) is found to be 101.4 C. If you carry out a second experiment with exp = 10 5 s, what value of T g would be obtained?
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Liquid Crystals Rod-like (= calamitic) molecules Molecules can also be plate-like (= discotic)
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LC Phases Isotropic Nematic Smectic Temp. Density The phases of thermotropic LCs depend on the temperature. N = director Attractive van der Waals’ forces are balanced by forces from thermal motion.
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Order in LC Phases Isotropic Nematic Smectic Density N = director Orientational Positional None High None weak 1-D
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LC Characteristics LCs exhibit more molecular ordering than liquids, although not as much as in conventional crystals. LCs flow like liquids in directions that do not upset the long- ranged order. When there is a shear stress along the director, a nematic LC will flow. In a “splay” deformation, order is disrupted, and there is an elastic response with an elastic constant, K From RAL Jones, Soft Condensed Matter
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LC Orientation Distribution function, f( ) Director NN n Higher order Lower order 0
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Order Parameter for a Nematic- Isotropic LC Transition Discontinuity at T c : Therefore, a first-order transition Isotropic Nematic S The molecular ordering in a LC can be described by a so-called order parameter, S: 1 0 With the greatest ordering, = 0° and S = 1. The order parameter is determined by the minimum in the free energy, F. Disordering increases S and decreases F, BUT intermolecular energies and F are decreased with ordering.
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From RAL Jones, Soft Condensed Matter, p. 111 Experimental Example of First-Order Nematic-Isotropic Transition Data obtained from birefringence measurements (circles) and diamagnetic anisotropy (squares) of the LC p-azoxyanisole. TcTc
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Scattering Experiments d = molecular spacing
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Diffraction from LC Phases L a nematic isotropic smectic
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Polarised Light Microscopy of LC Phases Nematic LC Why do LCs show birefringence? (That is, their refractive index varies with direction in the substance.)
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Birefringence of LCs The bonding and atomic distribution along the longitudinal axis of a calamitic LC molecule is different than along the transverse axis. Hence, the electronic polarisability ( o ) differs in the two directions (longitudinal ( l ) versus transverse ( t )). Polarisability in the bulk nematic and crystalline phases will mirror the molecular. The Clausius-Mossotti equation relates the molecular characteristic to the bulk property ( or n 2 ): In the isotropic phase: With greater LC ordering, there is more birefringence.
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IsotropicNematicPerfect nematic N N S = 1 S = 0 0< S <1
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Crossed polarisers Crossed Polarisers Block Light Transmission http://www.kth.se/fakulteter/TFY/kmf/lcd/lcd~1.htm
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Twisted nematic LC: No applied field and light is transmitted E > E crit : Light is blocked Liquid Crystal Displays The director rotates by 90° going from the top to the bottom of the LC. A strong field aligns the LC director in the same direction - except along the surfaces. d
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Why does the Glass Transition Occur? Adam and Gibbs (1965) proposed that as the temperature of a liquid is lowered, more and more atoms must co-operatively re-arrange. If the number of atoms/molecules required for co- operativity is z*, and the barrier for each molecule to move is , then will vary with T as: Z* = 9
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Viscous Response of Newtonian Liquids A A y F xx There is a velocity gradient (v/y) normal to the area. The viscosity relates the shear stress, s, to the velocity gradient. The viscosity can thus be seen to relate the shear stress to the shear rate: The top plane moves at a constant velocity, v, in response to a shear stress: v has S.I. units of Pa s. The shear strain increases by a constant amount over a time interval, allowing us to define a strain rate: Units of s -1
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Relaxation and a Simple Model of Viscosity »When a liquid is subjected to a shear stress, immediately the molecules’ positions are shifted but the same “neighbours” are kept. Thereafter, the constituent molecules re-arrange to relax the stress, and the liquid begins to flow. A simple model of liquids imagines that relaxation takes place by a hopping mechanism, in which molecules escape the “cage” formed by its neighbours. Molecules in a liquid vibrate with a frequency,,comparable to the phonon frequency in a solid of the same substance. Thus can be considered a frequency of attempts to escape a cage. But what is the probability that the molecule will escape the cage? F
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0 Molecular configuration Potential Energy Need to consider the probability of being in a higher state with an energy of . Intermediate state: some molecular spacings are greater
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Arrhenius Behaviour of Viscosity In liquids, is very short, varying between 10 -12 and 10 -10 s. Hence, as commonly observed, stresses in liquids are relaxed nearly instantaneously. In melted polymers, is on the order of several ms or s. From our discussion of viscoelasticity, we know that G o . Hence an expression for can be found from the Eyring relationship: Alternatively, an expression based on the molar activation energy E can be written: This is referred to as an Arrhenius relationship.
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Non-Arrhenius Temperature Dependence Liquids with a viscosity that shows an Arrhenius dependence on temperature are called “strong liquids”. An example is melted silica. “Fragile liquids” show a non-Arrhenius behaviour that requires a different description. An example of a fragile liquid is a melted polymer, which is described by the Vogel-Fulcher relationship: We see that diverges to , as the liquid is cooled towards T o. It solidifies as temperature is decreased. In the high- temperature limit, approaches o - a lower limit. where B and T o are empirical constants. (By convention, the units of temp. here are usually °C!)
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Temperature-Dependence of Viscosity P = Poise Arrhenius V-F
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Configurational Re-Arrangements As a liquid is cooled, stress relaxation takes longer, and it takes longer for the molecules to change their configuration, as described by the configurational relaxation time, config. From the Vogel-Fulcher equation, we see that: We see that the relaxations become exceedingly slow ( becomes v. large) as T decreases towards T o.
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An Example of the Glass Transition: NOT freezing or crystallisation! Video: Shattering polymer
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Experimental Time Scales To distinguish a liquid from a solid, flow (or other liquid-like behaviour) must be observed on an experimental time scale, exp. A substance will appear to be a solid on short time scales but a liquid on long time scales! For example, if a sample is being cooled at a rate of 1 K per min., then exp is ~1 min. at each temperature increment. At higher temperatures, exp > config, and flow is observed on the time scale of the measurement. Debonding of an Adhesive Flow can be observed on long time scales, exp
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Oscillatory Stress Apply a shear strain (or stress) at an angular frequency of = 1 / exp t ss 2/2/
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The Glass Transition At higher temperatures, exp > config, and so flow is observed on the time scale of the measurement. As T is lowered, config increases. When T is decreased to a certain value, known as the glass transition temperature, T g, then config ~ exp. Below T g, molecules do not change their configuration sufficiently fast to be observed during exp. That is, exp < config. The substance appears to be solid-like, with no observable flow. At T = T g, is typically 10 13 Pas. Compare this to = 10 -3 Pa-s for water at room temperature.
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Competing Time Scales Reciprocal Temperature (K -1 ) Log(1/ ) =1/ vib f = 1/ config 1/ exp 1/T g config < exp config > exp Melt (liquid) glass
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Thermodynamics of Glass Transitions V T TmTm Glass Crystalline solid Liquid TgTg Faster-cooled glass T fcg T g is higher when there is a faster cooling rate. We see that the density of a glass is a function of its “thermal history”.
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Effect of Cooling Rate on T g T g is not a constant for a substance. When the cooling rate is slower, exp is longer. For instance, reducing the rate from 1 K min -1 to 0.1 K min -1, increases exp from 1 min. to 10 min. at each increment in K. With a slower cooling rate, a lower T can be reached before config exp. The result is a lower observed T g. Various experimental techniques have different associated exp values. Hence, a value of T g depends on the technique used to measure it.
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Experimental Results for Poly(Vinyl Acetate) Data from Kovacs
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Thermodynamics of Glass Transitions V T TmTm Glass Crystalline solid Liquid TgTg Faster-cooled glass T fcg T g is higher when there is a faster cooling rate. We see that the density of a glass is a function of its “thermal history”.
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Data from H. Utschick, TA Instruments Glass Transition of Poly(vinyl chloride) Heat flow ~ heat capacity Sample is heated at a constant rate. Calorimeter measures how much heat is required. T
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