BE 220 Study Guide - Midterm Guide: Coble Creep, Viscous Liquid, Ductility
BE 220: Midterm 2 Study Guide
Two main types of polymers
1. Thermoplastics- long chain molecules, no 3D cross-likig, saturated, Va’t Der Waals fores
hold chains together
a. Can be heated and cooled repeatedly (amorphous, crystalline)
2. Thermosets: Unsaturated, 3D primary bond structures (cross-linking), permanently hard
a. Upon heating- o’t softe, retais struture → if high enough- cross link bonds break,
polymer degradation
b. Cross link density has direct effect on mechanics- more cross links = harder
Polymer Structure (thermoplastics)
- Amorphous: short range order only, less efficient packing, random arrangement, branching, ring
structures
- Crystalline/Semi-crystalline: both short range order and long range order
- Factors that affect the ability of polymers to form crystals
o Side groups size- large groups prevent neighboring chains from coming together
o Chain branching- branched polymers also harder to come together, have lower percent
crystallinity than linear polymers
o Tacticity- isotactic (same side) and syndiotactic (alternating sides) more conducive than
atactic (random)
o Intermolecular interactions- polar side groups can promote interactions→ promote
crystallinity
- Chain folded model of polymeric crystallinity
o Poler hais geerall are’t straight, fold ito laella
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o Poler hais do’t fold perfetl ito laella- tie molecules stick out, form
amorphous layer that connect adjacent lamella
o Lamella connect in polymer to form spherulites-lamellae form radial lines, folds oriented
perpendicular to the radial direction, lamellae separated by amorphous regions that
include chain folds and tie molecules
Polymer point defects and Impurities
- Point defects/vacancies = space between where one chain ends and next begins
- Less of an effect on overall properties compared to metals/ceramics
Thermal Transitions
Metals/ceramics- Tm = melting point, behaves like liquid above, solid below
- High temp → low temp = formation of materials
- Speed of quenching (cool down) determines long range order- if you quench a ceramic really
fast, you get a glass (amorphous)
Amorphous ceramics (glasses)- no distinct Tm
- Aorphous solid- more and more viscous with decreasing temp until treated like a solid
- Tg = glass transition temperatures, temp below which a material is considered to be glass (solid)
Thermal transitions in polymers- Tg < Tc < Tm
- Factors that influence Tm- concentrate on polymers ability to form secondary bonds
o Increase % crystallinity- amount of secondary bonds between the polymer chains, bulky
side chains, branching, tacticity
o Decreasing molecular weight lowers Tm (more chain ends = less energy required to
break secondary bonds between chains) → as MW increased, Tm increases
- Tg affected by polymer chain vibration and flexibility- more flexible chains = lower Tg
o Consider ability of backbone bonds to rotate (ex. C-O rotates more easily than C-C)
o Side and reactivity of side groups- larger side groups= more difficult rotation, more
polar= harder to vibrate because of polar interactions
o Increased molecular weight = harder to rotate = higher Tg
- Tc = crystallization temp, polymer chains have sufficient energy to move into highly ordered
crystalline state (exothermic process)
o Want lots of crystals- cool liquid to Tc very slowly (quench rate) to give it time to form
long range order
Differential Scanning Calorimetry
- Phase transitions
o First order phase transitions- change in enthalpy, solid/liquid/vapor/crystal
o Second order phase transitions- no change in enthalpy associated, glass transition
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- Sample and reference cells heated by individual heaters- DSC sensors compared power needed
to keep equal temp
- Tg: middle of the upward slope
o Heat capacity of sample increases, energy absorbed by chain rearrangement
- T: edotheri, eltig of ost polers ours oer rage of teps (polers all
different sizes)
o Area under curve is energy need- more area = more crystalline
- Tc: polymer chains rearrange into crystalline state (lower state of energy- exothermic) as they
cool from melted liquid state
o Cold crystallization- amorphous regions forming crystals
Ductility- ability of material to plastically deform (as opposed to rupture) under tensile load
Brittle- material that ruptures with little/no plastic deformation
Elastic deformation
Molecular causes of elastic deformation
- Small changes in atomic spacing and stretch of bonds (bond force curves)
o Anisotropic- if directionally dependent (isotropic- same in all directions)
- Polymers elastic at low temps (limits chain sliding), high temps if cross-linked or sufficiently
entangled, often anisotropic
o Along axis of chain- stretching of primary (covalent) bonds
o Transverse to chain= secondary forces, much lower properties
Plastic Deformation and Defects- permanent displacement of atoms in crystals from applied load, force
large enough to displace atoms (translates macroscopically to yield stress)
Metals and crystalline ceramics
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Document Summary
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