PKSC 4160 Exam 1

What are macromolecules?

-very large molecules
-can be defined as compounds made of a very large number of atoms chemically connected by covalent bonds

What are polymers?

-large molecules made up from many, chemically-linked monomers

Polymers form a large group of materials that include:

-plastics
-adhesives
-rubbers
-fibers
-surface coatings
-cellulose
-DNA
-RNA

What is a monomer?

-the basic unit of structure from which a polymer is constructed

What are monomers mostly from?

-crude oil and natural gas (carbon backbones and four primary bonds)
-silicones (special class of polymers with silicon backbones and have four primary bonds)

What is a constituent unit?

-the smallest chemical unit whose repetition completely describes the main chain structure
-may comprise of more than one monomer

What is a constituent unit also known as?

-constitutional unit
-repeating unit

What are plastics?

-a special class of polymers that can be made to flow and can be shaped and molded at relatively low temperatures compared to glass and metals

What is a copolymer?

-a polymer made from two different constitutional units
-Example: Ethylene Vinyl Acetate

What is degree of polymerization?

-described by n, the number of monomeric units linked together in the polymer chain
-n\=DP

What is a vinyl polymer?

-addition polymers formed from monomers with the structure CH2\=CHX or CH2\=CXY

What are inter-molecular forces?

-forces that bond atoms together forming molecules
-connect atoms like C, H, O, and N
-"primary bonds"
-covalent bonds
-ionic bonds

What are covalent bonds?

-most common
-strong bonds
-shared electrons
-predominant bonding linking the backbone of polymer chains

What are ionic bonds?

-rare
-electrons transferred between molecules
-sometimes present in polymer side chains

What are intra-molecular forces?

-when atoms and molecules approach one another closely, they begin to exert forces on one another that do not result from the sharing or transfer of electrons
-"secondary bonds or forces" \= Van der Waals forces

What are dispersion forces?

-the least strong
-time varying partial positive and negative charges
-most prevalent type

What are induction forces?

-polar atoms induce charge fluctuations in non-polar atoms
-found only when polar atoms are present creating temporary polarity
-EVA

What are dipole forces?

-strongest
-permanent partial positive and negative charges from polar bonds
-PVC
-hydrogen bonds

What are primary bonds important for?

-they are important for determining the molecular structure of the material

What are secondary forces responsible for?

-they are responsible for the physical nature
-gases have weak secondary forces
-liquids have stronger secondary forces
-solids have the strongest secondary forces

What happens when a solid is heated in regards to secondary forces?

-when a solid is heated and melts, the heat is providing the energy necessary to disrupt the secondary forces sufficiently to allow flow

What are the types of plastics?

-thermoplastics
-thermosets
-elastomers

What are thermoplastics?

-repeatedly undergo shape changing treatment (used more often)
-molecules are "linked" by secondary chemical bonds (hydrogen bonds and Van der Waals bonds)
-fuse and can be shaped at some temperature
-can be dissolved or swollen in solvents
-can be soft, ductile, or hard at room temperature

What are thermosets?

-molecules are linked by a high density of primary chemical bonds, called cross-links (strong)
-infusible and temperature resistant (once formed, it cannot be reformed)
-cannot be dissolved and difficult to swell
-cannot be plastically deformed

Why can thermosets only be shaped once?

-they can be shaped only once because they form irreversible covalent bonds between chains during the "setting" process and consequently will no longer melt and flow

What are elastomers?

-molecules that are lightly cross-linked by primary chemical bonds
-cannot be melted or dissolved
-can be swollen by solvents
-generally, in a soft, elastic condition at room temperature (like rubber band)

What are amorphous thermoplastics?

-not crystalline
-generally, are clear like glass
-soften in a certain temperature range, but do not have a measurable melting point

What are semi-crystalline thermoplastics?

-under the right conditions, they form highly ordered, repeating crystal structures
-look milky and opaque when the crystals are large enough to scatter light
-demonstrate a measurable melting temperature
-they will have amorphous regions within it

Draw a sketch of a linear polymer

-look at review
-monomers form a single long backbone chain
-example: HDPE

Draw a sketch of a branched polymer

-look at review
-some monomers become part of side chains
-example: LDPE

Draw a sketch of a cross-linked polymer

-look at review
-chains are linked by chemically-bonded other chains (primary bonds)

Draw a diagram of atactic tacticity

-look at review
-back and forth, random

Draw a diagram of isotactic tacticity

-look at review
-occur on the same side

Draw a diagram of syndiotactic tacticity

-look at review
-alternating
-symmetrical and same interval

A molecule of polyethylene has a molecular weight of 578,000. What is its approximate degree of polymerization?

578,000 / 28 \= 20,643

What is polyethylene?

-it is a polyolefin
-long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom

Is linear or branched polyethylene stronger?

-linear polyethylene (HDPE) is much stronger than branched polyethylene (LDPE)

\___________ \___________ (LDPE) is cheaper, easier to make, and more flexible

branched polyethylene

What is a homopolymer?

-when a polymer is made by linking only one type of small molecule or monomer together

What is the number average molecular weight?

-it is just the total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample (Mn)

What is the weight average molecular weight?

-it is based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do (Mw)

What is tacticity?

-the way pendant groups are arranged along the backbone chain of a polymer

When polymers have a \____________ arrangement of their atoms, it is very \__________ for them to pack together into crystals and fibers.

1. regular
2. easy

What is a crystal?

-polymer molecules tend to move towards an arrangement which is in the lowest possible energy state
-any material in which the molecules are arranged in a regular order or pattern

What is the lowest energy level that a compound can achieve?

-crystal form

What is a spherulite?

-when lamellae grow in a bulk specimen they form spherical structures called spherulites
-the branches extend from the crystal nucleus and the regions in between are amorphous

What is a unit cell?

-the smallest unit of a crystal
-the regular order and pattern (the shape) the crystal forms

What is crystalline?

-polymers that form crystals
-not commonly used in packaging

What is semi-crystalline?

-polymers that can form crystals, but are called "semi" because they still have some areas of amorphousness

What is amorphous?

-polymers that do not form crystals

How do we know polymer chains in a crystalline lamella are folded back and forth? (hint: how was it discovered?)

-folding was discovered by crystallizing polymers from solution
-we get lamellar structures
-the lamella are about 100A thick
-since the molecules are about 1000A long, the chains must fold back on themselves as they crystallize

Are all polymer chains incorporated perfectly into crystalline lamella? What else happens?

-most often the chains cannot really make up their minds
-the chains end up outside the lamella or turn around and re-enter the crystal structure

Sketch the polymer chains in a spherulite

-look at review

What factors enhance polymer crystals?

-orderly structures
-chain branching
-hydrogen bonding
-polarity

Sketch how the lamellae rearrange when they are stretched

-look at review

What is a lamella?

-folded chains in the polymer

What is a switchboard model?

-some lamellae have chains that go out for a while and then come back in

What are tie molecules?

-when chains start in one lamella, cross the amorphous region, and then join another lamella

What does crystallinity do to a material?

-makes it stronger
-makes it brittle

What do amorphous regions give polymers?

-toughness
-ability to bend without breaking

What is a plasticizer?

-small molecules that keep chains apart and allow more movement at lower temperatures

What is heat capacity?

-the amount of heat required to raise the temperature of one gram of matter one degree Celsius
-important in extrusion, forming/molding and sealing
-heat capacity changes at Tg

What is thermal conductivity?

-measures how well polymers can conduct heat

What is coefficient of thermal expansion?

-describes how the size of an object changes with a change in temperature

What is Tg?

-glass transition temperature
-exhibited only by amorphous polymers or amorphous areas of semi-crystalline polymers (not by crystals)
-Below Tg\=glassy state (hard and brittle)
-above Tg\=rubbery state (soft and flexible)

What is Tc?

-crystallization temperature

What is Tm?

-exhibited only by crystals, not amorphous regions or amorphous polymers
-at melting point, polymer chains fall out of their crystal structure and become a disordered liquid

How do "warm" chains respond to an external disturbance or stress?

-when you try to move "warm" chains they are already moving around
-they can change their positions easily to remove the stress you place on them

How do "cold" chains respond to an external disturbance or stress?

-when you try to move "cold" chains they cannot easily change position, they're either strong enough to resist the stress or they break due to the stress

What kind of molecular motion begins in the region above the Tg? (remember only in amorphous)

-long range segmental motion
-segments of the chains can wriggle around, swing to and fro, or turn like a corkscrew
-it is overcoming inertia

What kind of molecular motion begins in the region below the Tg? (remember only in amorphous)

-long range segmental motion ceases

How does backbone flexibility affect Tg?

-it lowers it

How do hook like pendant groups affect Tg?

-it increases it

How do bulky pendant groups affect Tg?

-it lowers it

How do plasticizers affect Tg?

-it lowers it

Sketch the change in heat capacity that takes place at Tg and at Tm

-look at review

Why do plasticizers have more effect on Tg than on Tm?

-plasticizers are small molecules that keep chains apart and allow more movement at lower temperatures
-they increase the free volume so chains can slide around past each other more easily
-it takes less energy to get into motion

Any change brought about by heat and a latent heat involved is called a \______________

-first order transition

When the glass transition involves change in heat capacity, but it doesn't involve a latent heat, it is called a \_________________

-second order transition

What is thermal analysis?

-the generic name given to several analytical techniques for measuring properties of a small quantity of material while it is being heated, cooled, or held at a constant temperature

What is DSC?

-differential scanning calorimetry
-measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature
-measurements provide quantitative and qualitative information about physical and chemical changes
-changes involve endothermic and exothermic processes, or changes in heat capacity

What is TGA?

-thermogravity analysis
-measures the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere

What is TMA?

-thermomechanical analysis
-measures the linear or volumetric change in dimensions of a sample as it is heated, cooled, or held at a constant temperature
-measures the stress or strain response of a sample held under constant strain or stress as it is heated, cooled, or held at a constant temperature

What is thermal expansion coefficient?

-describes how the size of an object changes with a change in temperature

What is the vicat softening point?

-test to study where polymer starts softening

What is the difference between exothermic and endothermic reactions?

-exothermic reactions release heat (Tc)
-endothermic reactions absorb heat (Tg and Tm)

How does DSC work?

-the DSC adds (or removes) heat to maintain a constant temperature change rate
-if the test sample undergoes an endothermic event, the DSC has to add more heat to keep up the rate
-if the test sample undergoes an exothermic event, the DSC has to reduce heat to maintain the rate

How does TMA work?

-the TMA adds heat to maintain a constant temperature change rate
-it measures force and displacement (distance)
-usually holds one constant and measures the other

How does TGA work?

-the TGA adds heat to maintain a constant temperature change rate
-it weighs the sample as oxidation, etc cause changes in the weight

What is stress?

-it is a load (force) applied over an area
-stress\=load/area
-psi, grams/cm^2, newtons/mm^2, pascals

What is strain?

-the deformation compared to the original size of the sample
-always dimensionless because dividing the same units

What is a modulus?

-a material property measuring stiffness
-how well a material resists deformation
-same units as stress
-E

What is elongation?

-a type of deformation
-simply a change in shape that anything undergoes under stress
-stretches and becomes longer

What is stress-strain?

-measures the change in stress
-deform a plastic at a fixed rate
-as we deform it, stress results
-as strain increases, stress curve changes slope depending on nature of polymer

What is creep?

-place a fixed load on a plastic
-with time, the plastic will deform
-the amount of deformation depends on the initial load and the temperature

What is stress relaxation?

-deform a plastic a fixed amount
-as we deform it, a certain stress results
-with time, that stress will decrease (relax)
-the rate of stress relaxation will depend on the initial deformation and on the temperature

Describe the stress-strain test

-constant rate of deformation
-measures the change in stress
-strain is the input
-stress is the output
-check slides for picture

Describe the stress relaxation test

-deform sample a fixed amount
-measures the change in stress
-strain is the input
-stress is the output

Describe the creep test

-impose a fixed load
-measures the change in strain
-stress is the input
-strain is the output

What is a resin?

-pellet formed plastic