Week 03 Molecular Weight and Molecular Weight Distribution

Week # 03 Molecular Weight and Molecular Weight Distribution Polymer Science and Engineering (TM-2052) By: Muhammad Haseeb Iqbal National Textile University

Concepts to be covered in this week What is molecular weight and molecular weight distribution § Different forms of molecular weights and their calculation § What is polydispersity and its effect on polymer properties § How do we name different polymers (Polymer nomenclature) § 2

Polymerization Reactions There are four types of polymerisation reactions; § • • Addition or chain growth polymerisation Coordination polymerisation Condensation or step growth polymerisation Copolymerization 3

MOLECULAR WEIGHT • The molecular weight of a polymer is of prime importance in the polymer’s synthesis and applications • Chemists usually use the term molecular weight to describe the size of a molecule. • The more accurate term is molar mass, usually in units of g /mol. • The term molecular weight is the ratio of the average mass per formula unit of a substance to 1/12 th of the mass of an atom of 12 C and is dimensionless (IUPAC, 1991, in press).

• The interesting and useful mechanical properties that are uniquely associated with polymeric materials are a consequence of their high molecular weight. • Most important mechanical properties depend on and vary considerably with molecular weight as seen in following figure

�There is a minimum polymer molecular weight (A), usually a thousand or so, to produce any significant mechanical strength at all. Above A, strength increases rapidly with molecular weight until a critical point (B) is reached. Mechanical strength increases more slowly above B and eventually reaches a limiting value (C). The critical point B generally corresponds to the minimum molecular weight for a polymer to begin to exhibit sufficient strength to be useful. �Most practical applications of polymers require higher molecular weights to obtain higher strengths. The minimum useful molecular weight (B), usually in the range 5000– 10, 000, differs for different polymers.

MOLECULAR WEIGHT DISTRIBUTION • Polymers have wide distribution of the chain lengths i. e. there is no symmetry in chains, some of very large size and other are of very small • The plot b/w no of moles of chains (Ni) and their mol wt (Mi) is called as “Mol wt Distribution”

AVERAGE MOLECULAR WEIGHT �Exact breadth of the mol wt distribution depends upon type of polymerization �Some olefins result in very broad mol wt distribution while some like PS give narrow mol wt distribution �The reason for the polydispersity of polymers lies in the statistical variations present in the polymerization processes. �Due to this diversity we have to define an average mol wt to characterize individual polymer �The control of molecular weight and molecular weight distribution (MWD) is often used to obtain and improve certain desired physical properties in a polymer product.

Mol wt determination • Various methods based on solution properties are used to determine the average molecular weight of a polymer sample. These include methods based on colligative properties, light scattering, and viscosity etc. • The various methods do not yield the same average molecular weight. • Different average molecular weights are obtained because the properties being measured are biased differently toward the different -sized polymer molecules in a polymer sample.

• Some methods are biased toward the larger-sized polymer molecules, while other methods are biased toward the smaller-sized molecules. • The result is that the average molecular weights obtained are correspondingly biased toward the larger- or smaller-sized molecules. • Generally average mol wt is defined as • Ni= no of moles of molecules with mol wt Mi • α = weighting factor determining particular average of mol wt distribution • For Mn, Mw and Mz α is 1, 2 and 3 respectively

Number average molecular weight: Mn § The number average molecular weight is the statistical average molecular weight of all the polymer chains in the sample.

Weight average molecular weight: Mw

Higher average molecular weights: Mz, Mz+1 •

Dispersity •

Polymer nomenclature § IUPAC Commission on Macromolecular Nomenclature • • § Common Names Source based names Structure based names Linkage based names Chemical abstracts based nomenclature

Polymer nomenclature Poly(methyl 2 -methylpropenoate) Polyester

Polymer nomenclature

Polymer nomenclature

Polymer nomenclature (Seniority)

Thank You Best of Luck!