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Lecture11Notes - Lecture 11 Glass transitions and Liquid...

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Unformatted text preview: Lecture 11 Glass transitions and Liquid Crystalline Behavior 1 Wednesday, November 3, 2010 Glass Transition Temperature (Tg) 2 Wednesday, November 3, 2010 Order of Phase Transitions We can plot free energy as a function of temperature for a crystalline (or glassy) material, and free energy as a function of temperature for a melted material In the crystalline state, the configurational entropy is small because they are arranged in an ordered conformation and locked into that position At low temperatures, the attractive forces between the chains dominate the free energy; at higher temperatures, the entropic portion increases, and the liquid form becomes more favorable At the melting point, the free energy of the crystal and the melt are equal Free energy is continuous as a function of temperature, but there is a shift from one curve to the other at the melting point This graph is usually confusing to students 3 Wednesday, November 3, 2010 Order of Phase Transitions A first order phase transition occurs when the first derivative of free energy wrt T is discontinuous (volume, enthalpy, entropy) Example: the slope of the curves of crystallization and melting are different when they meet at the melting point, leading to an abrupt change Tm is first order; Tg is not 4 Wednesday, November 3, 2010 Order of Phase Transitions A second order phase transition occurs when the second derivative of free energy wrt T is discontinuous (heat capacity, CP, compressibility, κ, and α, thermal expansion coefficient) Polymer crystallization and melting are true first order transitions, and the glass transition may be related to a second-order transition (pseudo-second order) 5 Wednesday, November 3, 2010 Motion In the gaseous state, translational motion is easy to picture In the crystalline state, translational motion is restricted, but there are still oscillations about an average or mean position In the liquid state, motions are coupled 6 Wednesday, November 3, 2010 Motion and Polymers Polymers are not well approximated by spheres Chains can interpenetrate (remember theta conditions) and entangle Large scale motion in the melt is prevented by random close packing of the chains 7 Wednesday, November 3, 2010 Nature of Tg Is Tg a thermodynamic or kinetic phenomenon? The observed Tg depends on thermal history, so there is a kinetic component But is there an underlying thermodynamic transition? 8 Wednesday, November 3, 2010 Free Volume In a crystalline lattice, there is empty volume (volume between atoms); this is NOT what we mean when we discuss free volume Molecules have thermal motion For a material in the glassy state, we can think of this motion in terms of a sphere which oscillates in a cage of its neighbors These oscillations create some free volume over and above the empty space characteristic of random close packing For polymers, the sphere represents a segment, not a whole chain 9 Wednesday, November 3, 2010 Free Volume Free volume is not unoccupied or empty volume Free volume increases with increasing temperature, as the oscillations (and their amplitude) increase Free volume is not shared equally by all molecules, but fluctuates; at some instant of time one molecule may be trapped in a local close-packed cage of its neighbors while another has sufficient free volume available to it that it bounces into a new position The idea that the Tg corresponds to the point where the free volume falls below a critical value was suggested by Fox and Flory 10 Wednesday, November 3, 2010 Free volume, viscosity and the glass transition • • • • • • Viscosity is the resistance of fluid to flow, and as temperature increases, viscosity decreases At low temperatures, when a material is hard, rigid and brittle, there is essentially no flow and the material is infinitely viscous Viscosity should change dramatically in the vicinity of the glass transition temperature At high temperatures, molecules have a lot (relative) of free volume and can move around easily Below the Tg, the molecules don’t have enough room to move around Empirical relationships have been used to describe the relationship of viscosity to Tg and/or free volume 1 −B /V f ] = Ae[ η Doolittle equation η = A' e[ B ' /(T −T0 )] Vogel-Fulcher equation 11 Wednesday, November 3, 2010 MW and Tg The chain ends have more freedom of motion than the segments in the center of the chain, and, crudely, can be thought of as having more free volume Low molecular weight chains have more ends per unit volume than long chains, hence a higher free volume, hence a lower Tg Fox and Flory used these simple free volume arguments to derive the following equation: K Tg = Tg∞ − Poly(styrene) Mn 12 Wednesday, November 3, 2010 Chain Stiffness and Tg Chain stiffness affects chain mobility If there are bulky side groups, there is a high energy barrier to rotations, which then only occur at higher T Increasing chain stiffness 13 Wednesday, November 3, 2010 Effect of Bulky Side Groups The presence of bulky side groups raises the Tg through steric hindrance to bond rotations As the pendant group gets larger, the Tg increases; however, at some point the attached side groups no longer get in the way of rotations further and further away from the chain The effect of attaching a methyl group to the main chain of polystyrene is greater than effect of increasing the size of the aromatic unit because the close proximity of this group to the polymer backbone introduces a higher degree of steric hindrance 14 Wednesday, November 3, 2010 Effect of Flexible Side Groups Tg decreases with increasing side chain length Substituents closest to the chain provide the bulk of steric hindrance The rest of the attached side chain can get out of the way of motions of the main chain through rotations around side-chain bonds Because side chains increase free volume through their effect on the packing of the chains, the Tg is lowered 15 Wednesday, November 3, 2010 Molecular Interactions and Crosslinking Strong intermolecular attractions raise the Tg Here, the -Cl and -CH3 have approximately the same effect on bond rotations The polar character of the Cl leads to stronger forces of attraction between chains, so that on average, these groups are closer The free volume is less and the Tg is higher (although it is more complicated than just this effect) Crosslinking, similarly, decreases free volume (crystalline domains work in the same way – act as crosslinks) The chains are more closely tied together and the Tg increases 16 Wednesday, November 3, 2010 Diluents and Copolymerization In a miscible (single phase) mixture, a Tg intermediate to that of the two components is seen If the two components are immiscible (two phases), two Tgs are seen What would happen with a block copolymer? Random copolymer? The Tg of a mixture is rationalized on the basis of free volume arguments to give the Fox eqn: 1 w1 w 2 = + Tg Tg1 Tg 2 17 Wednesday, November 3, 2010 Liquid Crystalline Polymers MAIN-CHAIN LCPs SIDE-CHAIN LCPs terminally attached laterally attached CC N O N N O C O Polar Substituents: -OCH3 , -CN, -F, -Cl, -NO2 n least ordered sA sC sB sE sF sG sH sI sJ sK most ordered 18 Wednesday, November 3, 2010 Liquid Crystalline Structure can mimic Biological Structures Percec research group, U. Penn Wednesday, November 3, 2010 19 Kevlar 20 Wednesday, November 3, 2010 Characterization of Crystallinity and LC Behavior 21 Wednesday, November 3, 2010 Amorphous vs. Semi-Crystalline • • • • • • • • • X-Ray Diffraction (XRD) is used to look at crystallinity If atoms are arranged in a periodic array, they will scatter x-rays to give a regular pattern of spots that are observed on photographic film The structure of the crystal can be re-constructed from the pattern of spots In this technique, you usually need a macroscopic size single crystal to get the maximum amount of information In polymers, we typically do not obtain a macroscopic crystal, but rather a collection of randomly arranged smaller crystals (i.e. a powder) Instead of spots, the XRD of the powder is a series of concentric rings This pattern is translated into a graph by moving outwards from the center of the film and measuring the intensity of the scattered radiation Large crystals give narrow peaks, and small crystals give broader peaks (the width of the bands depends inversely on size) For a melted (amorphous) sample, the diffraction pattern does not completely disappear, as there is still some local order (there is a reasonably well-defined average number of neighbors, and scattering from these gives the first peak) 22 Wednesday, November 3, 2010 Amorphous vs. Semi-crystalline 23 Wednesday, November 3, 2010 Amorphous vs. Semi-crystalline 24 Wednesday, November 3, 2010 Amorphous vs. Semi-crystalline Crystalline n-alkanes n-alkanes in the melt Polyethylene 25 Wednesday, November 3, 2010 XRD of Polyethylene 26 Wednesday, November 3, 2010 Rosalind Franklin • • An x-ray diffraction expert, she obtained key images of DNA that led to the double helix structure of DNA by Watson and Crick Watson and Crick won Nobel prizes, but gave no credit to her pivotal images, which they had seen without her permission or knowledge 27 Wednesday, November 3, 2010 Differential Scanning Calorimetry Power Supplies Thermometers Sample Reference Measure the dierence in energy required to maintain both samples at the same temperature 28 Wednesday, November 3, 2010 Density • • Calibrated glass beads float in a salt solution whose density varies linearly with height Samples sink to the point of neutral buoyancy, from which the density can be determined 29 Wednesday, November 3, 2010 Infrared Spectroscopy Infrared radiation stimulates molecular vibrations. Infrared spectra are traditionally displayed as %T (percent transmittance) versus wavenumber (4000-400 cm-1). Useful in identifying presence or absence of functional groups Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra The most useful regions are as follows: 1680-1750 cm-1: C=O stretches feature very strongly in IR spectra and the type of carbonyl group can be determined from the exact position of the peak. 2700-3100 cm-1: different types of C-H stretching vibrations. 3200-3700 cm-1: various types of O-H and N-H stretching vibrations 30 Wednesday, November 3, 2010 Infrared Spec and Crystallinity 31 Wednesday, November 3, 2010 Circular Dichroism • • • If a molecule is chiral or asymmetric, it will absorb left-and right-circularly polarized light differently CD (absorptive) is observed for λ where the compound absorbs light; at other λ, optical rotation (dispersive) occurs Optical rotation is a consequence of a different RI for left- and rightcircularly polarized light 32 Wednesday, November 3, 2010 NMR •Detects nuclei with magnetic moments (1H and 13C most common) •Nuclei flip in a magnetic field of the correct strength •Position in ppm (0 for TMS internal standard) 33 Wednesday, November 3, 2010 Polarized Light Microscopy Can be used to detect anisotropy Typically used for liquid crystals, or to find boundaries between substances of dierent RI Beam of light gets split by material, and reunited later z x y δ = phase shift =(2π/ λ)Δn⋅d 34 Wednesday, November 3, 2010 Scanning Probe Microscopy/Atomic Force Microscopy A small cantilever scans the surface of a material; the deflection of this tip is measured as it goes over the surface and this signal is translated into topography AFM can measure mechanical contact forces, vDW forces, chemical bonding, electrostatics… Position sensitive detector Laser 35 Wednesday, November 3, 2010 Small Angle Neutron Scattering • The technique of small-angle neutron scattering (SANS) is used for studying the structure of a material on length scale of 10 to 1000 Å using scattering of a monochromatic beam of neutrons from the sample and measuring the scattered neutron intensity as a function of the scattering angle(~ 0.5–10°) • Deuterium can be used as contrast • Neutrons can penetrate a sample far better than other scattering techniques • There are some 37 neutron sources in 21 countries and of these 23 are in Europe, 10 in North America (including Canada), 2 in Japan and 1 in each of Australia & India. 36 Wednesday, November 3, 2010 Disadvantages of SANS • SANS is a routine technique available at neutron-scattering facilities associated with research nuclear reactors (have to book beam time, no source at your research facility, typically) • Neutron sources are very expensive to build and to maintain - millions of dollars annually to operate a nuclear research reactor and a ton in electrical bills alone to run a pulsed neutron source. • Neutron flux is very low. Presently, the neutron flux available at these reactors is equivalent to the X-ray flux available in the 1940s. This makes for long measurement times and increased demand from researchers to use the facilities. • Interaction of neutrons with matter is weak. Therefore, large samples are required. Although small angle scattering techniques provide beneficial data, these limitations exclude their use 37 Wednesday, November 3, 2010 Why are Tm, Tg and LC behavior important for biomedical applications? 38 Wednesday, November 3, 2010 Prion Diseases • • • • Prion diseases (transmissible spongioform encephalopathies) have long incubation periods and result in spongioform changes associated with neuronal loss (no inflammatory response) Prion: Proteinaceous and infectious (analagous to virion) The protein that prions consist of is found throughout the body; in contrast to the normal protein, the prion has a different folding pattern and is resistant to proteases (PrPC vs PrPSc) PrPSc converts PrPC to a different conformation (increased β-sheet content) 39 Wednesday, November 3, 2010 Prion Diseases • • • • • • • • • Resistant to denaturation by protease, heat, radiation and formalin Causes fatal familial insomnia, scrapie, kuru (“tremble with fear”), Creutzfeldt-Jakob disease, bovine spongiform encephalopathy (mad cow disease); sometimes genetic link, sometimes not Non-disease function of a prion? Virus responsible for prion production? (PNAS 2007 Manuelidis) Prions are a stable form of protein, seems to cause refolding of proteins in the brain Going from α-helix to β-sheet leads to large holes - swiss cheese β-sheets lead to amyloid aggregation Transmission - kuru, CJD, BSE transmitted to humans who have eaten the meats or brains of infected animals (Kuru-cannabalism) In the GI tract, proteins are normally digested to amino acids Prions remain intact, accumulate in the distal ileum 40 Wednesday, November 3, 2010 Tg and Proteins and Cells • • • An important approach to stabilizing proteins and preventing denaturation is to freeze them in a glassy state for storage, typically using an aqueous solution containing some sort of multifunctional alcohol (sugars, glycerol) When we freeze down cells, its imperative to keep them glassy and not have crystallization occur, so usually DMSO is added Proteins exhibit a Tg that depends partly on its interaction with water and partly on its own chain 41 Wednesday, November 3, 2010 Liquid Crystallinity and Biology • • • Cholesterol is a natural liquid crystal; can cause pathological changes in arteries when deposited as a lyotropic mesophase from LDL circulating in blood Cell membranes and other structures in biology exhibit LC behavior Viruses can influence/control liquid crystal behavior, so a sensor for a specific virus could be based on LC 42 Wednesday, November 3, 2010 ...
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This note was uploaded on 11/29/2010 for the course BME 104 taught by Professor Kasko during the Fall '10 term at UCLA.

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