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Lecture# 3Molecular Mass and Chain Microstructure

• Mass vs. Weight• Molecular “Weight” and Distribution• Averages• Polydispersity• Property Implications

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Polymer Chain Length

• Polymer Chain Length– Polymer notation represents the repeating group

• Example, -[A]-n where A is the repeating monomer and n represents the number of repeating units.

• Molecular Weight– Way to measure the average chain length of the polymer – Defined as sum of the atomic weights of each of the atoms in the

molecule. Example:

– Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole

– Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole

– Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole

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MOLECULAR WEIGHT• Molecular weight, M: Mass of a mole of chains.

Low M

high M

• During the polymerization NOT ALL chains in a polymer grow to the same length, so there is a distribution of molecular weights.

• The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species.

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xi = number fraction of chains in size range i

MOLECULAR WEIGHT DISTRIBUTION

iiw

iin

MwM

MxM

wi = weight fraction of chains in size range i

Mi = mean (middle) molecular weight of size range iMn = the number average molecular weight (mass)

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Polydispersity

• By virtue of its definition, Mw cannot be less than Mn. It is influenced by the high molecular weight fraction of the material to a greater degree than Mn.

• The ratio of Mw to Mn,

defines the polydispersity of a molecular weight distribution.

• Low polydispersity (PD=Mw/Mn 2) generates

higher melt viscosity, higher tensile strength and better toughness inpolyethylene.

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Degree of Polymerization

Ex. Calculate the degree of polymerization if polyethylene (PE) has a molecular weight of 56,000 g/mol.

The degree of polymerization refers to the total number of repeat units in the chain.

C CH H

H H

n

Polyethylene (PE)

Mrepeat unit = 2(atomic wt. of C) + 4(atomic wt. of H) = 2(12) + 4(1) = 28

Degree of Polymerization = 56,000/28 = 2,000

DP = Mn/Mo

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Property Implications of MW

• Higher MW increases• Tensile Strength, impact toughness, creep resistance,

and melting temperature.

– Due to entanglement, which is wrapping of polymer chains around each other.

– Higher MW implies higher entanglement which yields higher mechanical properties.

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and Molecular Weight Distribution

• Broader MWD decreases strength • Broad MW distribution represents polymer with many

shorter molecules which are not as entangled and slide easily.

• Broader MWD decreases crystallinity– Shorter chains are too short to fold into crystalline

domains• Broader MWD increases melt flow rate

• Shorter chains flow more easily and act as plasticizer.

Example 1.1:What is the molecular weight of polypropylene (PP), with a degree of polymerization of 3×104 ?

Solution:Structure of the repeating unit for PP

Molecular weight of repeat unit = (3×12 + 6×1) = 42Molecular weight of polypropylene = 3×104×42 = 1.26×106

Example 1.2:Nylon 11 has the following structure

If the number-average degree of polymerization, X n, for nylon is 100 and M w= 120,000, what is its polydispersity?

Example (3.1): a. To Find: (a) The number-average molecular weight (b) The weight-average molecular weight (c) The degree of polymerization and P.Dfor the given polypropylene material

CLASSIFICATION OF POLYMERSPolymers can be classified in many different ways. The most obvious classification is based on the origin of the polymer, i.e., natural vs. synthetic. Other classifications are based on the polymer structure, polymerization mechanism, preparative techniques, or thermal behavior.

A. NATURAL VS. SYNTHETICPolymers may either be naturally occurring or purely synthetic. All the conversion processes occurring in our body (e.g., generation of energy from our food intake) aredue to the presence of enzymes. Life itself may cease if there is a deficiency of these enzymes. Enzymes, nucleic acids, and proteins are polymers of biological origin. Their structures, which are normally very complex, were not under stood until very recently., etc. Each family itself has subgroups.

Starch — a staple food in most cultures — cellulose, and natural rubber, on the other hand, are examples of polymers of plant origin and have relatively simpler structures than those of enzymes or proteins. There are a large number of synthetic (man-made) polymers consisting of variousfamilies: fibers, elastomers, plastics, adhesives

B. POLYMER STRUCTURE1. Linear, Branched or Cross-linked.

2.Amorphous or Crystalline

Examples of crystalline polymers include polyethylene , polyacrylonitrile

poly(ethylene terephthalate) , and polytetrafluoroethylene

Poly(methyl methacrylate)

polycarbonate

3. Homopolymer or Copolymer

Polymers composed of only one repeating unit in the polymer molecules are known as homopolymers

Polymers composed of two different repeating units in the polymer molecule are defined as copolymers. An example is the copolymer formed when styrene and acrylonitrile are polymerized in the same reactor..

There are several types of copolymer systems:

See lecture notes