Polymers: what they are and how they work? Michael R. Kessler School of Mechanical and Materials Engineering Washington State University

What are Polymers Examples of polymers and where they are used. Often Synthetic Polymers are called Plastic. The volume of polymers produced is three times larger than that of all metals! Poly (many) mer (parts)  made up of molecules of very long chains.

What is a Polymer? Poly (many) mer (parts)  made up of molecules of very long chains. repeat unit repeat unit repeat unit C H Polyethylene (PE) Cl C H Poly(vinyl chloride) (PVC) H Polypropylene (PP) C CH3

Ancient Polymers Originally natural polymers were used Wood – Rubber Cotton – Wool Leather – Silk Oldest known uses Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark

Polymer Composition Most polymers are hydrocarbons – i.e., made up of H and C Saturated hydrocarbons Each carbon singly bonded to four other atoms Example: Ethane, C2H6

Chemistry and Structure of Polyethylene Polymer- can have various lengths depending on number of repeat units Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene

Bulk or Commodity Polymers Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

Bulk or Commodity Polymers (cont)

Bulk or Commodity Polymers (cont)

Analogy for the Structure of Polymers

Properties of Polymers Glass transition temperature (snake pit analogy) Strength—measured in maximum stress (force per unit area) Stiffness—measured in stress per unit strain Damping—ability to absorb energy Properties are often temperature and rate dependent (Viscoelastic)

Properties (Demonstration) Happy Ball Sad Ball Compare the damping properties for two apparently identical polymer balls by bouncing them on the floor. How do their damping behaviors vary with temperature? Happy Ball = Neoprene (polychloroprene) Sad Ball = Norsorex (polynorbornene)

MOLECULAR WEIGHT • Molecular weight, M: Mass of a mole of chains. Low M high M Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes Not all chains in a polymer are of the same length — i.e., there is a distribution of molecular weights

MOLECULAR WEIGHT DISTRIBUTION Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes Mi = mean (middle) molecular weight of size range i xi = number fraction of chains in size range i wi = weight fraction of chains in size range i

Degree of Polymerization, DP DP = average number of repeat units per chain DP = 6 Chain fraction mol. wt of repeat unit i

Types of Polymers Thermoplastics Amorphous Semi-crystalline Thermosets Crystalline Region Amorphous – Examples include polystyrene, polyvinyl chloride, low density polyethylene (LDPE) … Semi-crystalline – Examples include high density polyethylene (HDPE), Nylon, Kevlar … Thermosets – Examples include epoxy, cyanate ester, natural rubber … Crosslinks

Molecular Structures for Polymers B ranched Cross-Linked Network Linear secondary bonding

Polymers – Molecular Shape Molecular Shape (or Conformation) – chain bending and twisting are possible by rotation of carbon atoms around their chain bonds note: not necessary to break chain bonds to alter molecular shape

Chain End-to-End Distance, r

Tacticity Tacticity – stereoregularity or spatial arrangement of R units along chain isotactic – all R groups on same side of chain syndiotactic – R groups alternate sides

atactic – R groups randomly positioned Tacticity (cont.) atactic – R groups randomly positioned

Copolymers two or more monomers polymerized together random – A and B randomly positioned along chain alternating – A and B alternate in polymer chain block – large blocks of A units alternate with large blocks of B units graft – chains of B units grafted onto A backbone A – B – random alternating block graft

Crystallinity in Polymers Ordered atomic arrangements involving molecular chains Crystal structures in terms of unit cells Example shown polyethylene unit cell

Polymer Crystallinity Crystalline regions thin platelets with chain folds at faces Chain folded structure 10 nm

Polymer Crystallinity (cont.) Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned crystalline region • Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase. amorphous region

Semicrystalline Polymers Some semicrystalline polymers form spherulite structures Alternating chain-folded crystallites and amorphous regions Spherulite structure for relatively rapid growth rates Spherulite surface

Photomicrograph – Spherulites in Polyethylene Cross-polarized light used -- a maltese cross appears in each spherulite Adapted from Fig. 14.14, Callister & Rethwisch 8e.