Biomass Fundamentals Module 6: Fundamental Principles of Polymer Chemistry A capstone course for BioSUCCEED: Bioproducts Sustainability: a University Cooperative Center of Excellence in EDucation The USDA Higher Education Challenge Grants program gratefully acknowledged for support

This course would not be possible without support from: USDA Higher Education Challenge (HEC) Grants Program

Polymer Chemistry Macromolecules Polymer Structure/Classification Molecular Weight Definitions Molecular Weight Distribution Viscocity Polymer Morphology

The Macromolecular Hypothesis In the late 1800’s it was hypotheses that large molecules - “Macromolecules” existed as a result of covalently linked smaller units, and possessed unique physical and chemical properties. The Macromolecular Hypothesis However the scientific community at that time was unwilling to accept such a notion, explaining high MW-molecules as being the result of inferior methodology and/or molecular association of smaller molecules.

Polymer Structure skeletal structures Polymers can exist with various skeletal structures - such as linear, branched or cross- linked or network polymers. LinearBranched Network

Polymer Structure Variations in skeletal structure give rise to major differences in polymer properties. –linear polyethylene has a melting point 20 o C higher than that of a branched polyethylene. –unlike most linear polymers and branched polymers, network polymers do not melt upon heating, and will not dissolve

Polymer Classification Polymers are commonly classified based on their underlying molecular structure. Polymers ElastomersThermosetsThermoplastics Crystalline Amorphous

Thermoplastics Plastics Often referred to as just “Plastics” are linear or branched polymers which soften upon heating. They can be moulded (and remoulded) into virtually any shape –injection moulding, extrusion and constitute the largest portions of the polymers used in industry semicrystalline crystallineamorphous Thermoplastics never achieve 100% crystallinity, but instead are semicrystalline with both crystalline and amorphous domains.

Thermoplastics crystalline The crystalline phases of such polymers are characterized by their melting temperature (T m ). amorphous Many thermoplastics are completely amorphous and incapable of crystallization, these amorphous polymers (and amorphous phases of semicrystalline polymers) are characterized by their glass transition temperature (T g ). –the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft).

Thermoplastics Glass transition temperature (T g ) This transition corresponds to the onset of chain motion below the T g the polymer chains are unable to move and are “frozen” in position. Both T g and T m increase with increasing chain stiffness and increasing forces of intermolecular attraction

Elastomers Elastomers Elastomers - crosslinked rubbery polymers - rubber networks - that can be easily stretched to high extensions ( 3x to 10x original dimensions) –the rubbery polymer chains become extended upon deformation but are prevented from permanent flow by crosslinking, and driven by entropy, spring back to their original positions on removal of the stress.

Thermosets Thermosets Thermosets - normally rigid materials - network polymers in which chain motion is greatly restricted by a high degree of crosslinking As with elastomers, they are intractable once formed and degrade rather than melt upon the application of heat.

Polysaccharides ,000 monosaccharide units The size of polysaccharide molecules can vary, occurring as polydispersed molecules that have a range of 100 to 100,000 monosaccharide units –MW 16, ,000,000 daltons There are a number of methods used to determine the molecular weight of polysaccharides –viscosity*, light scattering, ultracentrifugation, osmometry and titration are most common (*viscosity is routinely used, but is not an absolute method and can be used only in conjunction with one of the other methods)

Molecular Weight Distribution number-average molecular weight (  n ) The simplest, most common molecular weight is the number-average molecular weight (  n ) –end-group analysis or colligative properties (b.p. elevation, osmotic pressure, etc)  w  z   others commonly used are weight-average molecular weight (  w ), z-average molecular weight (  z ) and viscosity-average molecular weight (   ) –light scattering (  w ), sedimentation equilibrium (  z ) and solution viscosity (   )

Number-average molecular weight (  n ) –based on methods of counting the number of molecules in a given weight of polymer the total weight of a polymer sample, w, is the sum of the weights of each molecular species present N = number of molecules M = molecular weight

Weight-average molecular weight (  w ) determination of molecular weight based on size rather than the number of molecules –the greater the mass, the greater the contribution to the measurement w = weight fraction M = molecular weight N = number of molecules

Z-average molecular weight (  z )  z some molecular weight determination methods (e.g. sedimentation equilibrium) yield higher molecular weight averages -  z w = weight fraction M = molecular weight N = number of molecules

Example Example - a polymer sample consists of 9 molecules of mw 30,000 and 5 molecules of mw 50,000 Number-average molecular weight (  n )

Consider the previous example Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000 Weight-average molecular weight (  w )

Consider the previous example Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000 Z-average molecular weight (  z )

A Typical Molecular Weight Distribution Curve M i ( g mol -1 ) 10 4 w i  n = g mol -1  w = g mol -1  z = g mol -1

Molecular Weight Determination colligative properties light scattering In measurements of colligative properties, each molecule contributes regardless of weight, whereas in light scattering, the larger molecules contribute more because they scatter light more effectively. For this reason,  w are greater than  n, except when all molecules are of the same weight and  w =  n

Molecular Weight Distribution The narrower the molecular weight range, the closer are the values of  w and  n, and the ratio  w /  n may thus be used as an indication of the breadth of the molecular weight range in a polymer sample. polydispersity index polydispersed The ratio is called the polydispersity index, and any system having a range of molecular weights is said to be polydispersed

A Typical Molecular Weight Distribution Curve M i ( g mol -1 ) 10 4 w i  n = g mol -1  w = g mol -1  z = g mol -1

Polymer Solution Viscosity When a polymer is dissolved in a solvent and then subjected to flow through a narrow capillary it exerts a resistance to that flow. This resistance is very informative. It provides information on the size of the polymer Its Flexibility and shape in solution Its interactions with the solvent it is disolved in.

Polymer Solution Viscosity For dilute solutions the ratio between flow time of a polymer solution (t) to that of the pure solvent (t o ) is effectively equal to the ratio of their viscosity (  /    As this has a limiting value of unity, a more useful quantity is specific viscosity (  sp )

Intrinsic Viscosity [η] specific viscosity intrinsic viscosity [  To eliminate concentration effects, the specific viscosity (  sp ) is divided by concentration and extrapolated to zero concentration to give intrinsic viscosity [  Thus plotting  sp /c vs c, the intercept is the intrinsic viscosity [  and from the slope, K H (Huggins constant, typically between ) can be determined

Intrinsic Viscosity Determination C ( g dl -1 )     [[ KH[η2]KH[η2]

Mark-Houwink-Sakurada Equation Intrinsic viscosity [  ] can be related to molecular weight by the Mark-Houwink-Sakurada Equation Applicable for a given polymer-solvent system at a given temperature Viscosity-Molecular Weight Relations log Ka Log [  ] vs log M (  w or  n ) for a series of fractionated polymers produces log K (intercept) and a (slope) A wide range of values have been published –a –a ~ 0.5 (randomly coiled polymers) ~ 0.8 (rod-like, extended chain polymers) –K –K between and 0.5

Typical Mark-Houwink-Sakurada Equation Constants for Several Polysaccharides

Typical Intrinsic Viscosities, a and K values for Several Naturally Occurring Polymeric Materials The degree of expansion or shape of the molecular coils of a polymer can be ascertained from its a values (Table 2) lignin (Newtonian sphere), cellulose (non- freedraining coil) and xylan (freedraining coil)

Viscosity-average molecular weight (   ) –viscosity, like light scattering, is greater for the larger- sized polymer molecules than the smaller ones, and is much closer to M w than M n w = weight fraction N = number of moles M = molecular weight a = A constant –When a = 1,   =  w, usually a ~ – a is a measure of the the hydrodynamic volume of the polymer – varies with polymer, solvent and temperature

Einsteins equation Intrinsic viscosity [  ] is a measure of the effective hydrodynamic volume of the molecule. For a hard non- swelling sphere, Einsteins equation is valid Polymer Properties Where V is the specific volume of the material in the sphere. In a linear solvent-swollen polymer like cellulose, V and thus also [  ] is much larger. A low [  ] value means that the molecule is compact and thus occupies a relatively small volume

Polymer Morphology The ultimate properties of any polymer (plastic, fiber, or rubber) result from a combination of molecular weight and chemical structure. Polymers require a particular MW, which depends largely on the chemical structure, to have desirable mechanical properties. Molecular Weight Mechanical Property

Polymer Morphology The mechanical properties result from attractive forces between molecules –dipole-dipole interactions, H-bonding, induction forces, London forces or ionic bonding, ion-dipole interactions A lower MW polyamide will produce good fiber properties as compared to the polyester  H-bonding H-bonding dipole-dipole

Hydrogen Bonding –A dipole-dipole interaction for hydrogens bonded to electronegative elements Electrostatic InteractionElectrostatic Interaction Polymer Morphology Weak bond ~ 5 kcal mol -1 (c-c ~ 81 kcal mol -1 ) Require short bond distance ~ 2.5Å (c-c ~ 1.46Å) very important in cellulose

Polymer Morphology Intermolecular forces drop off very rapidly with distance  important polymer molecules be able to pack together closely to achieve maximum cohesive strength. ex. Natural Rubber unstretched state - molecules are randomly distributed  low modulus stretched state - molecules become aligned, at 600% elongation  high modulus (2000 times higher than unstretched) amorphouscrystalline unstretched - amorphous / stretched - crystalline