Hanyang Univ. Spring 2007 Unlike small molecules, polymers are typically a mixture of differently sized molecules. Only an average molecular weight can be defined. Chap 5. Characterization of MW In case of Low MW

Hanyang Univ. Spring 2007 Measurements of average molecular weight (M.W.) Weight Average Molecular Weight,Mw Number average molecular weight M n considers the number of molecules of each size, Mi, in the sample The weight average, Mw, considers the mass of molecules of each size within the sample Viscosity average M.W.,Mv Average determined by viscosity measurements. Closer to Mw than Mn

Hanyang Univ. Spring 2007 For further details, CLICK the following link. RELATED LINK: High MW sensitive Low MW sensitive seMonodisper1 Mn M sePolydisper1 Mn M = > w w : :Mw Polydispersity : Polydispersity = Mw/ Mn

Hanyang Univ. Spring 2007 Influence of Increasing Molar Mass on Properties

Hanyang Univ. Spring 2007 Influence of Molecular Weight on Mechanical Properties.

Hanyang Univ. Spring 2007 Methods of Molecular Weight Determination Number Average Molecular Weight End-group analysis determine the number of end-groups in a sample of known mass Colligative Properties most commonly osmotic pressure, but includes boiling point elevation and freezing point depression Weight Average Molecular Weight Light scattering translate the distribution of scattered light intensity created by a dissolved polymer sample into an absolute measure of weight-average MW Viscosity Average Molecular Weight Viscometry the viscosity of an infinitely dilute polymer solution relative to the solvent relates to molecular dimension and weight. Molecular Weight Distribution Gel permeation chromatography fractionation on the basis of chain aggregate dimension in solution

Hanyang Univ. Spring 2007 Weight Average Molecular Weight Light scattering Determination of Molecular Weight Large particles in solution/suspension scatter light. Responsible for phenomena such as refraction, Tyndall effect. Extent of scattering is a function of the size and shape of the particles. Consequently yields information about Mw. Measuring Light scattering Ar or He/Ne laser source Photometer/goniometer measures light intensity at various angles (θ)

Hanyang Univ. Spring 2007 Determination of Molecular Weight The concentration dependence is observed to follow this functional dependence: Neglect higher order term in c. Plot of h sp /c vs c will yield [h] as y-intercept. k = Huggins constant k=2 for rigid uncharged spheres k=0.35 for flexible polymers y-intercept Viscosity Average Molecular Weight Viscometry

Hanyang Univ. Spring 2007 Virtual Experiment Case Western Reserve Univ. Polymer and Liquid Crystals Viscosity Measurements Click the next homepage, experiment part If you have the trouble viewing this site, See this page rpages_mmcom/default.html K and a are empirical parameters characteristic of a polymer and a solvent - a=0.5 for a well-coiled polymer in a poor solvent - a=1.7 for rigid rod-like polymer

Hanyang Univ. Spring 2007 Solvent flow carries molecules from left to right; big ones come out first while small ones get caught in the pores. It is thought that particle volume controls the order of elution. But what about shape? For further details CLICK the following link, Surfing to the internet Size Exclusion Principle Molecular Weight Distribution Gel permeation chromatography Determination of Molecular Weight

Hanyang Univ. Spring 2007 Simple GPC degas pump injector DRI VeVe log 10 M c c c

Hanyang Univ. Spring 2007 Three factors are of general interest 1. What Solvents will dissolve what polymer? 2. How does the polymer-solvent interaction influence the solution properties/ 3. To what applications do the interesting properties of polymer solutions lead? General rules foe polymer solubility 1. Like dissolve like; Polar polymers- polar solvents Nonpolar polymer-nonpolar solvent. ex) PVA in water, PS in toluene 2. Solubility will decrease with increasing molecular weight at const. temp. 3. Crystallinity decreases solubility. 4. Crosslinking eliminates solubility. 5. The rate of solubility increases with short branches, allowing the solvent molecules to penetrate more easily. 6. The rate of solubility decreases with longer branches, because the entanglement makes it harder for individual molecules to separate. Chap 6. Polymer Solubility and Solutions

Hanyang Univ. Spring 2007 ΔGm=ΔHm-TΔSm < 0 Where, ΔGm = the change in Gibbs free energy in the process ΔHm = the change in enthalpy in the process ΔSm = the change in entropy in the process Only if ΔGm is negative will the solution process be feasible. A positive ΔH  solvent and polymer “prefer their own company”, the pure materials are in a lower energy state. A negative ΔH  the solution is in the lower energy state, specific interactions are formed between the solvents and polymer molecules. Thermodynamics Basics  G mix < 0  G mix > 0 AB Immicible AB solution + [Ref : H.Tompa; polymer solutions, Butterworths,London, 1956,chapter 7]

Hanyang Univ. Spring 2007 The solubility parameter is defined as:  H   E =  1  2 (  1   2 ) 2 (cal/cm 3  soln)  1,  2 : vol. frac.  1 : polymer solubility parameter.  2 : solvent solubility parameter.  = (CED) 1/2 = (  E v / v ) 1/2 = (cal/cm 3 ) 1/2  E v / v : molar vol. of liq. rule of thumb  1   2   0.5 for solubility. Solubility Parameter For further details, Click next homepage. yauth/burke/solpar/ Surfing to the internet Likely crosslinked polymer Uncrosslinked polymer : Polymer solvent solubility parameter Swelling Fig. Determination of(polymer ) by swelling

Hanyang Univ. Spring 2007 Lattice model of solubility Filled circles : Solute Open circles : Solvent (a) Low-molecular-weight solute (b) polymeric solute.

Hanyang Univ. Spring 2007 internal energy on vaporization  E v =  E h +  E d +  E p Hansen’s 3-D Solubility Parameter The cohesive energy, δ 2, is divided into three parts: 1)Dispersion forces (δ d ) 2)Polar forces ( δ p ) 3)Hydrogen bonding ( δ H )

Hanyang Univ. Spring 2007 SOLVENT Chloroform THF MEK m-cresol Methanol Toluene H2OH2O PS Styrene Hansen’s 3-D Solubility Parameter

Hanyang Univ. Spring 2007 Properties of dilute solutions -Good solvent : Solubility parameter closely matches that of the polymer. The secondary forces between polymer segments and solvent molecules are strong, and the polymer molecules are strong, and the polymer molecules will assume a spread-out conformation in solution. -Poor solvent : the attractive forces between the segments of the polymer chain are greater than those between the chain segments and the solvent. Ex) polystyrene (δ = 9.0 ~ 9.3 ) Chloroform (δ = 9.2 ) Then non-solvent is added, methanol( δ = 14.5 ),the mixed solvent(Chloroform + methanol) becomes too”poor” to sustain solution, and the polymer precipitates out. When ΔG = 0 and ΔH = TΔS (θ condition)

Hanyang Univ. Spring 2007 Glass Transition Polybutadiene Polyethylacrylate rubbery at RT, when cooled in Liq. N 2 Polyisoprene become rigid and glassy, shatters when break At low temperatures, all amorphous polymers are stiff and glassy, sometimes called as the Vitreous State, especially for inorganic polymers. On Warming, polymers soften in a characteristic temperature range known as the glass-rubber transition region. The glass transition temperature (Tg), is the temperature at which the amorphous phase of the polymer is converted between rubbery and glassy states. Tg constitutes the most important mechanical property for all polymers. In fact, upon synthesis of a new polymer, the glass transition temperature is among the first properties measured. Chap 7. Transitions in Polymers PMMA, PS  hard, rigid glassy plastics at RT when heated to 125  C, become rubbery

Hanyang Univ. Spring 2007 Amorphous regions of the material begin to exhibit long-range, cooperative segmental motion Molecular motion available to polymer chains below their Tg are restricted primarily to vibrational modes. Above Tg there is sufficient thermal energy for the chains, through cooperative rotational motion about the backbone bonds, to flow under an applied stress. The presence of this large-scale segmental motion (20–50 atoms moving in concert) above Tg produces an increase in the free volume of the polymer. Molecular Motions in an Amorphous Polymer

Hanyang Univ. Spring 2007 Differential Thermal Analysis (DTA) Principal components of the experiment include: an oven for the controlled heating of the samples separate temperature sensing transducers for both the analysis and reference samples DTA Design

Hanyang Univ. Spring 2007 Differential Scanning Calorimeter (DSC) For further details, Click next homepage. Surfing to the internet Two samples, each heated independently. Temperature difference is monitored. Control heat flow into analysis sample (adjusting heater power) to keep the temperature difference ∆T = 0. This is a null experiment with feedback.

Hanyang Univ. Spring 2007 DTA & DSC Images and Sample data DTA DTA sample data DSC sample data

Hanyang Univ. Spring Free volume of the polymer v f. Free volume v f = v  v s v : specific volume of polymer mass v s : volume of solidly packed molecules 2. Attractive forces between molecules.  Tg  Main factor of Tg

Hanyang Univ. Spring Internal mobility of chains (= freedom to rotate about bond)  TgEoEo Silicone ~ 0 PE PTFE6.2>204.7 Example (1)

Hanyang Univ. Spring Stiffness of the materials Young’s Modulus E may be written as: σ = Eε σ = Tensile stress; ε = Tensile strain Young’s modulus is a fundamental measure of the stiffness of the material. The higher its value, the more resistant the material is to being stretched. Unit of E: dynes/cm2 (10 dynes/cm2 = 1 Pascal) 5. Chain Length.

Hanyang Univ. Spring 2007 T : Kelvin temperature 1, 2 : polymer 1 and 2 Tg of Copolymer For further details about Tg, Click next homepage. & Surfing to the internet

Hanyang Univ. Spring 2007 Nylon n  Tm   T  S  water cont  Example (2) T g Comparision Example (3) H-bonding  H m  PU :  O  (Swivel)  flexibility Extra NH in polyurea  more extensive H-bond.  H m

Hanyang Univ. Spring  C, PET rapidly cooled to RT rigid and transparent heated to 100  C maintained at that temp and becomes translucent then cooled down to RT again now, it is translucent rather than transparent. Example (4)

Hanyang Univ. Spring 2007 Homogeneous, amorphous Amorphous and glassy, rigid Influence of Copolymerization on Properties