Physical Chemistry Of Polymers [PPT]

Jul 10, 2019
  • G. S. Mandal's Maharashtra Institute of Technology Aurangabad Department of Plastic & Polymer Engineering
  • Course Code: PPE253 Course: Physical Chemistry of Polymers Pre-requisite: Organic Chemistry, Chemistry Credits: 4 Teaching Scheme: Theory: 4 hrs/week Class Test: 20 Marks Theory Examination: 80 Marks Theory Examination (Duration): 3 hrs Course Coordinator: Ms. S.Mandal Contact: Room no : 210 Contact hours: 12.15 pm to Ipm 3.00 pm to 3.15 pm Email: [email protected]
  • Thermodynamics of Polymer Solutions: Laws of Thermodynamics, Enthalpy, entropy, Gibbs free energy, Helmholtz free energy, Clausius inequality, thermodynamic condition for solubility, Flory-Huggins theory, Phase diagrams of binary solution; Upper and lower critical solution temperature with examples of each kind. Thermodynamic and kinetic flexibility of polymer chains, Factors determining chain flexibility, Practical importance of chain flexibility. (12 h) Unit-IV
  • Thermodynamics. Predicts whether the reaction is thermally favorable. The energy the products and reactants are taken as the guiding principle, The equilibrium will be in favor of products when the product energy is lower. ' Molecule with lowered energy posses enhanced stability.
  • Laws of Thermodynamics The First Law of Thermodynamics The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. For example, turning on a light would seem to produce energy; however, it is electrical energy that is converted. A way of expressing the first law of thermodynamics is that any change in the internal energy (AE) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by the surroundings: AE=q+w This law says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system. Since both heat and work can be measured and quantified, this is the same as saying that any change in the energy of a system must result in a corresponding change in the energy of the surroundings outside the system. In other words, energy cannot be created or destroyed. If heat flows into a system or the surroundings do work on it, the internal energy increases and the sign of q and w are positive. Conversely, heat flow out of the system or work done by the system (on the surroundings) will be at the expense of the internal energy, and q and w will therefore be negative. Ref: https://www.khanacademy.orq/science/bioloqy/enerqy-and-enzymes/the-laws-of- thermodynamics/v/first-law-of-thermodynamics-introduction
  • Laws of Thermodynamics The Second Law of Thermodynamics The second law of thermodynamics says that the entropy of any isolated system always increases. ' Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system. More simply put: the entropy of the universe (the ultimate isolated system) only increases and never decreases. A simple way to think of the second law of thermodynamics is that a room, if not cleaned and tidied, will invariably become more messy and disorderly with time — regardless of how careful one is to keep it clean. When the room is cleaned, its entropy decreases, but the effort to clean it has resulted in an increase in entropy outside the room that exceeds the entropy lost. Ref: http://npte/
  • Laws of Thermodynamics The Third Law of Thermodynamics The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero. The entropy of a system at absolute zero is typically zero, and in all cases is determined only by the number of different ground states it has. Specifically, the entropy of a pure crystalline substance (perfect order) at absolute zero temperature is zero. This statement holds true if the perfect crystal has only one state with minimum energy.
  • Enthalpy, Entropy and Free Energy Essential terms Free energy change (AG) — Overall free energy difference between the reactant and the product ' Enthalpy (AH) — Heat contentof a system under a given pressure. ' Entropy (AS) — The energy of disorderness, not available for work in a thermodynamic process of a system.
  • The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system which can be attained only in a completely reversible process. The Gibbs free energy change at temperature T is expressed as, In terms of standard states, when reactants and products at 1 M concentrations (or 1 atmosphere pressure), the free energy change is expressed as, A A HO-T A so
  • FREE ENERGY (AG) ' For a reaction to be spontaneous The overall free energy at any concentrations of reactant and product is: AG = AGO+ RT ln[product]/[reactant] Where R(gas constant)= 8.314 JmoI-1K-1and T(temperature) = in 0K At equilibrium, AGO+ RT ln[product]/[reactant] AGO= -RT Inl
  • For a reaction, equilibrium shifts in the direction of lower species. Hence , GO(reactant ) > GO(product) then, reaction is spontaneous since A O. i.e. negative and Keq> 1 GO(reactant ) < GO(product) then, reaction is nonspontaneous since A O i.e. positive and Keq < 1
  • ENTHALPY (AH) ' Enthalpy can be regarded as thermodynamic potential. ' It is a state function and an extensive quantity. ' It also refers to the difference in bond energies between the reactant and product. ' In short enthalpy is 'the heat absorbed (or released) by a chemical reaction'.
  • CALCULATION OF ENTHALPY Enthalpy is a measure of difference in bond energy between reactants and products. E.g. Combustion of methane. +2 02 -CO +2 (l) = Enp H(reactants) - Enr H(products) reaction +2 AHO -2 AHO comb f,methane f, oxygen f, water {carbon dioxide = O as oxygen is a pure element.) foxygen = -891 KJ mol-I . AHO is +ve i•vhen reaction is AHO is -ve when reaction is endothermic (endergonic). exothermic (exergonic).
  • SOME FACTS ABOUT ENTROPY (AS) The second law of thermodynamics states that the entropy of any closed system, not in thermal equilibrium, will almost always increases. Entropy is a thermodynamic property, it is the measure of energy not used to perform work but is dependent on temperature as well as volume. Entropy is directly proportional to Spontaneity. Comparison of entropies: Gases >liquids>solids (bromine gas has greater entropy than when in liquid state) Entropy is greater for larger atoms (as we move down in groups in periodic table) and molecules with larger number of atoms. Entropy is a measure of the number of ways particles as well as energy can be arranged. More configurations (different geometries), more will be the entropy. Entropy of a irreversible system always increases.
  • Zeroth Law: FilSt Law: Second Law: Third Law: Laws of Thennodynamics Heat flows from hot to cold Energy and matter are consented Matter tends towards chaos Entropy of a pure crystal at 0K is zero Thennodynamic Tenns 1%at does each term tell us? Enthalpy (AR) Entropy (AS) Free energy (AG) Equilibrium (K ) Energy content Disorder Themnodynamically favored or not favored Extent of reaction + endothennic increase m the dispersal of matter + not thennodymmically favored >1 reaction favors products — exothennic decrease in the dispersal of matter themuodynamically favored
  • Internal Energy (AE) and Heat Flow Refers to all of the energy contained within a chemical system. Heat flow between the system and its surroundings involves changes in the internal energy of the system. It will either increase or decrease Increases in internal energy may result in a temperature increase chemical reaction starting phase change Decreases in internal energy may result in a a decrease in temperature phase change Note: even though the change in internal energy can assume several different forms, the amount of energy exchanged between the system and the surroundings can be accounted for ONLY by heat (q) and work (w) AE = q + w Work (w) refers to a force acting on an object; in chemical processes this acting force is done by a gas through expansion or to a gas by compression. This is referred to as "pressure/volume" work Thus, w = - PAV Where P is constant external pressure (atm) and AV (L) is the change in volume of the system
  • Calculating Heat (q) ' Heat (q) gained or lost by a specific amount of a known substance can be calculated using the heat capacity of the substance and the change in temperature the system undergoes. ' Calorimetry The process of experimentally measuring heat by determining the temperature change when a body absorbs or releases energy as heat.
  • Coffee-cup calorimetrg -use a Styrofoam cup, mm reactant that begin at the same temperature and look far change m temperature; the heat transfer is calculated from the change m temp. q=mCåT q = quantity of heat (Joules) is the change 111 temperature Ar=rf-Ti (final-initial) • watch the sign; ifthe system loses heat to the sumundmgsthentheAT- Cp = specific heat capacity (J/g'C) m = mass l.n grams the specific heat of water (liquid) = 4.184 J gec Aln note q = -AH at constant pressure
  • Enthalpy Heat content; AH Endothennic (+) ar Exothenmc Calculatin Enthal S Wa Ca101ime (see above) Enthal v offonnation, AH/ (usin table of standard values Hess's Law Stoicluom Bond Energ:es Enthalpy of Formation Production of ONE MOLE afcompoundFROM ITS ELEMENTS ill states C) zero (0) for ELEMENTS m 25-C (298 K), 1 IU 3 Al(s) + — (Alz03(s) + AICII(s) +3 NO') + 6 Substance AIC13(s) -295 -1676 704 90.0 -242
  • Entropy (AS) Dis ersal of matter Less di ersal or More dif ersal (+) Calculatin Entro • Table of standard values • Hess's Law Entropy Increases when: Gases are formed from solids or liquids (most imponant!!!!) E:ocs) C(s) + coz(s) 2 cot) A solution is formed Volume is increased in a gaseous system (energy is more effciently dispersed) LMore complex molecules are formed BigMamma Equation n: 2 so:t) + 02') — 12 sol') Substance SO(JK-lm01-l) 248.1 205.3 256.6
  • Free Energy (A G) Free Energy Thermod amic favorabili of the reaction Thermo amicall favorable (—åG')or thermo mamicall unfavorable C+AG') Calculate: • Table of standard values • Hess's Law BigMamma Equation 111: AH', AS', and AGO may all be calculated fram tables of standard values, from Hess' Law or fram the Gibb's equation: Cannectians to AH • and Granddaddy of Them All: AH'— TAS' Caution on units: and åG'are typically given m W mol whereas -AS' typically given as J K mol-I Conditions of AG Spontaneous at all tem Spontaneous athi h tem Spontaneous ( ) at low tem Non-spontaneous (+) at all tem
  • AH (when ressvvre is canstant/coffee cu (same value; o — AH is exothermic; +åHis endothermic Cheat Relationshi s TAS Wim01KforR)and watch our units for AG:th will he in W (96,500 far at equlllbnum (including phase change) libium and direction change Be cautious of which system component is losing heat Use AG = AH— TSSequation to justify and which is gainmg heat. Assi +/— signs accordinglv. Kinetics — reaction diagrams thermodynamic favorability. Discuss AH -overtaking the TSS term and vice versa. Connections Electrachem: SG = —n E' Stoichiomeåy — Energy values are usually kJ/mol so if you have other than 1 mole adjust accordingly Potential Pitfalls iB usually in W mar-I (that's per mol ofrxn) åHi is usually in W marl Cp — Jig-C (specific heat units) LNITS CAUTION: this calculation gives w in units of CL•ann) not Joules (or k"! !!! and 1 L = 0.001 ml 1 101,325 — 2 1 L-atm= 101.3 101.3 J ALL PAW calculations for work need to be x 101B to convert to Joules, J AS is in J'K not m kJ like AH and åG AG must be negative for thermodynamr favorability Watch and know what th mean
  • Helmholtz free energy In thermodynamics, the Helmholtz free energy is a thermodynamic potential that measures the "useful" work obtainable from a closed thermodynamic system at a constant temperature and volume. For such a system, the negative of the difference in the Helmholtz energy is equal to the maximum amount of work extractable from a thermodynamic process in which temperature and volume are held constant. Under these conditions, it is minimized at equilibrium. The Helmholtz free energy was developed by Hermann von Helmholtz and is usually denoted by the letter A (from the German "Arbeit" or work), or the letter F . The IUPAC recommends the letter A as well as the use of name Helmholtz energy. In physics, the letter F is usually used to denote the Helmholtz energy, which is often referred to as the Helmholtz function or simply "free energy.'
  • Helmholtz free energy While Gibbs free energy is most commonly used as a measure of thermodynamic potential, especially in the field of chemistry, the isobaric restriction on that quantity is inconvenient for some applications. ' For example, in explosives research, Helmholtz free energy is often used since explosive reactions by their nature induce pressure changes. It is also frequently used to define fundamental equations of state in accurate correlations of thermodynamic properties of pure substances.
  • Helmholtz free energy The Helmholtz energy is defined as: where • A is the Helmholtz free energy (Sl:joules, CGS: ergs), • U is the internal energy of the system (Sl: joules, CGS: ergs), • Tis the absolute temperature (kelvins), • S is the entropy (Sl: joules per kelvin, CGS: ergs per kelvin). The Helmholtz energy is the negative Legendre transform with respect to the entropy, S, of the fundamental relation in the energy representation, IJ(S, V, N). The natural variables of A are T, V, N.
  • Clausius inequality Assume reversible and irreversible paths between two states. Reversible path produces more work. dlJ is the same for both the paths. dlJ = dq + dw = -dqrev + dwrev dqrev dq = dw - dwrev 2 0 dq/T dS dq/T System is isolated. dS20 Clausius inequality Clausius inequality
  • How do we derive conditions for equilibrium and spontaneity? For an isolated system AS > 0, > sign for a spontaneous process and = for equilibrium. In the case of open or closed system, there are two ways 1. Evaluate AS for systems and surroundings. AS -AS + AS total system surroundings AS > O
  • 2. Other way is to define entropy change of the system alone. dStotal = dSsystem + dSsurroundings Clausius inequality Consider constant volume: (IS - dUT20 Tds 2 dlJ (constant 'V and so no work due to expansion) At constant U or at constant S, the expression is: Criterion of spontaneity 1. is the common statement of second law. 2. Entropy is unchanged, for sponteneity, entropy of the surroundings must increase for which U of the system as to decrease.
  • At constant pressure, TdS dl-l Interpretations are the same. The inequalities mean dlJ - TdS s o dkl - TdS s o We define A= U — TS Helmoltz energy G = H —TS Gibbs energy -TdS dG = -TdS (dG)T O
  • Thermodynamic Condition for Solubility Thermodynamics of Polymer Solution (Ref: Solution + Properties P Ghosh) Thermodynamics of Polymer Solution 1 (Ref: Chapter 12) (Reference files are uploaded )
  • Flory-Huggins theory Flory-Huggins theory (Ref: Chapter 4, Lec 24t) (Reference files are uploaded )
  • Refe re n s enzymes/the-laws-of-thermodynamics/a/the-laws-of- thermodynamics Retrieved 2007- 11-04. FUNDAMENTALS OF POLYMER SCIENCE, Solution Properties, Prof. Premamoy Ghosh, Polymer Study Centre, Kolkata- 700078 , (21.09.2006) chemistry/chapter/the-laws-of-thermodynamics/
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