Engineering Science 1021A/B Lecture Notes - Lecture 29: Glass Transition, Tangled, Tacticity
17 May 2018
School
Department
Professor
Bonding along the "backbone" of the chain is covalent
•
In the simple thermoplastic polymers, the chains are bound to each other by
weaker Van der Waal's forces and mechanical entanglement
•
Therefore, the chains are relatively strong, but it is relatively easy to slide and
rotate the chains over each other
○
•
Amorphous Thermoplastics
Glass Polymers (
𝑇" < " 𝑇
$
)
Below the Glass Transition Temperature, amorphous polymers are hard
and brittle
○
•
Rubbery or Leathery Polymers (
𝑇
$" < "𝑇" < 𝑇
%
)
Between
𝑇
$
and
𝑇
%
, when a stress is applied, the polymer deforms
elastically and plasticlly at the same time
○
When stress is removed, the elastic deformation is recovered, but the
polymer is permanently deformed because of the movement of the chains
○
•
Liquid Polymers (
𝑇 > 𝑇
%
)
Bonds between chains are very weak
○
Chains slide past each other with almost no force
○
•
Crystalline Thermoplastics
Linear polymers never crystallize
However, some polymers partially crystallize
○
•
Neighbouring chains become aligned and fold back on themselves to form thin
plates
These plates are connected to each other by amorphous chains and often
form spherulites
○
•
○
○
•
The ability of a polymer to crystallize is affected by:
Complexity of the chain
Crystallization is easier for simple polymers and harder for complex
polymers
§
○
Cooling rate
Slow cooling allows more time for the chains to align
§
○
Annealing
Heating to just below the 𝑇
%can allow chains to allign and form
crystals
§
○
Degree of Polymerization
It is harder to crystallize longer chains
§
○
Deformation
Slow deformation between 𝑇
$and 𝑇
%can straighten the chains
allowing them to get closer together
§
○
•
Controlling the Strength of Thermoplastics
Plastic deformation in thermoplastics is due to the rotation and sliding of chains
over each other
•
To increase the strength of a thermoplastic, we have to make it harder
3 ways that we can control this:
Alter the length of the chains
§
Change the strength of the bonds within the chains
§
Change the strength of the bonds between the chains
§
○
•
Chain Length
If the polymer chains are made longer (increased degree of polymerization),
they become more tangled and harder to pull apart
The strength is increased
○
•
•
Forces Between Chains
The degree of crystallinity influences the strength of polymers•
Higher crystallinity means more chains are aligned in crystals
Stronger Van der Waal's forces
○
•
•
Changing some or all of the side groups in the polymer chain can alter the bonds
strength between chains and make it harder for the chains to rotate,
disentangle and slide over each other
•
•
Polymer chains with many side branches can not pack as tightly together and
the bonds between chains are not as strong
•
•
Tacticity: for polymers with non-symmetrical repeat units, the location of the
non-symmetrical side group affects the properties
Actactic Syndiotactic Isotactic
Least regular•
Poor packing•
Low strength and
stiffness
•
Regular alternation of
the side groups
promotes close packing
and crystallization
• Favours east
crystallization
•
Chains form helices that
nest together
•
•
Forces Within Chains
Chains formed from more complex monomers often have much stronger bonds
and resist rotation and sliding
•
Higher bond strength also means:
○
•
Thermoplastics
Bonding along the "backbone" of the chain is covalent•
In the simple thermoplastic polymers, the chains are bound to each other by
weaker Van der Waal's forces and mechanical entanglement
•
Therefore, the chains are relatively strong, but it is relatively easy to slide and
rotate the chains over each other
○
•
Amorphous Thermoplastics
Glass Polymers (𝑇" < " 𝑇
$)
Below the Glass Transition Temperature, amorphous polymers are hard
and brittle
○
•
Rubbery or Leathery Polymers (𝑇
$" < "𝑇" < 𝑇
%)
Between 𝑇
$and 𝑇
%, when a stress is applied, the polymer deforms
elastically and plasticlly at the same time
○
When stress is removed, the elastic deformation is recovered, but the
polymer is permanently deformed because of the movement of the chains
○
•
Liquid Polymers (𝑇 > 𝑇
%)
Bonds between chains are very weak
○
Chains slide past each other with almost no force
○
•
Crystalline Thermoplastics
Linear polymers never crystallize
However, some polymers partially crystallize
○
•
Neighbouring chains become aligned and fold back on themselves to form thin
plates
These plates are connected to each other by amorphous chains and often
form spherulites
○
•
○
○
•
The ability of a polymer to crystallize is affected by:
Complexity of the chain
Crystallization is easier for simple polymers and harder for complex
polymers
§
○
Cooling rate
Slow cooling allows more time for the chains to align
§
○
Annealing
Heating to just below the 𝑇
%can allow chains to allign and form
crystals
§
○
Degree of Polymerization
It is harder to crystallize longer chains
§
○
Deformation
Slow deformation between 𝑇
$and 𝑇
%can straighten the chains
allowing them to get closer together
§
○
•
Controlling the Strength of Thermoplastics
Plastic deformation in thermoplastics is due to the rotation and sliding of chains
over each other
•
To increase the strength of a thermoplastic, we have to make it harder
3 ways that we can control this:
Alter the length of the chains
§
Change the strength of the bonds within the chains
§
Change the strength of the bonds between the chains
§
○
•
Chain Length
If the polymer chains are made longer (increased degree of polymerization),
they become more tangled and harder to pull apart
The strength is increased
○
•
•
Forces Between Chains
The degree of crystallinity influences the strength of polymers•
Higher crystallinity means more chains are aligned in crystals
Stronger Van der Waal's forces
○
•
•
Changing some or all of the side groups in the polymer chain can alter the bonds
strength between chains and make it harder for the chains to rotate,
disentangle and slide over each other
•
•
Polymer chains with many side branches can not pack as tightly together and
the bonds between chains are not as strong
•
•
Tacticity: for polymers with non-symmetrical repeat units, the location of the
non-symmetrical side group affects the properties
Actactic Syndiotactic Isotactic
Least regular•
Poor packing•
Low strength and
stiffness
•
Regular alternation of
the side groups
promotes close packing
and crystallization
• Favours east
crystallization
•
Chains form helices that
nest together
•
•
Forces Within Chains
Chains formed from more complex monomers often have much stronger bonds
and resist rotation and sliding
•
Higher bond strength also means:
○
•
Thermoplastics
Bonding along the "backbone" of the chain is covalent•
In the simple thermoplastic polymers, the chains are bound to each other by
weaker Van der Waal's forces and mechanical entanglement
•
Therefore, the chains are relatively strong, but it is relatively easy to slide and
rotate the chains over each other
○
•
Amorphous Thermoplastics
Glass Polymers (𝑇" < " 𝑇
$)
Below the Glass Transition Temperature, amorphous polymers are hard
and brittle
○
•
Rubbery or Leathery Polymers (𝑇
$" < "𝑇" < 𝑇
%)
Between 𝑇
$and 𝑇
%, when a stress is applied, the polymer deforms
elastically and plasticlly at the same time
○
When stress is removed, the elastic deformation is recovered, but the
polymer is permanently deformed because of the movement of the chains
○
•
Liquid Polymers (
𝑇 > 𝑇
%
)
Bonds between chains are very weak
○
Chains slide past each other with almost no force
○
•
Crystalline Thermoplastics
Linear polymers never crystallize
However, some polymers partially crystallize
○
•
Neighbouring chains become aligned and fold back on themselves to form thin
plates
These plates are connected to each other by amorphous chains and often
form spherulites
○
•
○
○
•
The ability of a polymer to crystallize is affected by:
Complexity of the chain
Crystallization is easier for simple polymers and harder for complex
polymers
§
○
Cooling rate
Slow cooling allows more time for the chains to align
§
○
Annealing
Heating to just below the 𝑇
%can allow chains to allign and form
crystals
§
○
Degree of Polymerization
It is harder to crystallize longer chains
§
○
Deformation
Slow deformation between 𝑇
$and 𝑇
%can straighten the chains
allowing them to get closer together
§
○
•
Controlling the Strength of Thermoplastics
Plastic deformation in thermoplastics is due to the rotation and sliding of chains
over each other
•
To increase the strength of a thermoplastic, we have to make it harder
3 ways that we can control this:
Alter the length of the chains
§
Change the strength of the bonds within the chains
§
Change the strength of the bonds between the chains
§
○
•
Chain Length
If the polymer chains are made longer (increased degree of polymerization),
they become more tangled and harder to pull apart
The strength is increased
○
•
•
Forces Between Chains
The degree of crystallinity influences the strength of polymers•
Higher crystallinity means more chains are aligned in crystals
Stronger Van der Waal's forces
○
•
•
Changing some or all of the side groups in the polymer chain can alter the bonds
strength between chains and make it harder for the chains to rotate,
disentangle and slide over each other
•
•
Polymer chains with many side branches can not pack as tightly together and
the bonds between chains are not as strong
•
•
Tacticity: for polymers with non-symmetrical repeat units, the location of the
non-symmetrical side group affects the properties
Actactic Syndiotactic Isotactic
Least regular•
Poor packing•
Low strength and
stiffness
•
Regular alternation of
the side groups
promotes close packing
and crystallization
• Favours east
crystallization
•
Chains form helices that
nest together
•
•
Forces Within Chains
Chains formed from more complex monomers often have much stronger bonds
and resist rotation and sliding
•
Higher bond strength also means:
○
•
Thermoplastics
Document Summary
Bonding along the backbone of the chain is covalent. In the simple thermoplastic polymers, the chains are bound to each other by weaker van der waal"s forces and mechanical entanglement. Therefore, the chains are relatively strong, but it is relatively easy to slide and rotate the chains over each other. Below the glass transition temperature, amorphous polymers are hard and brittle. Rubbery or leathery polymers ($ < < %) Between $ and %, when a stress is applied, the polymer deforms elastically and plasticlly at the same time. When stress is removed, the elastic deformation is recovered, but the polymer is permanently deformed because of the movement of the chains. Chains slide past each other with almost no force. Neighbouring chains become aligned and fold back on themselves to form thin plates. These plates are connected to each other by amorphous chains and often form spherulites. The ability of a polymer to crystallize is affected by:
