Polymer Processing II

Question 1

What is layer multiplying coextrusion?

Answer 1

repeatedly slicing and restacking layered polymer flow to increase the number of layers

often referred to as forced assembly

Question 2

How does layered multiplying coextrusion work?

Answer 2

a series of 2x multipliers applied to 3-layer flow. It takes 2 side-by-side flows and rearranges them so that they are stacked on top of one another

Question 3

How does multipliers influence number of layers and size of layers?

Answer 3

2 - 8 layers (16 micrometers)
4 - 32 layers (4 micrometers)
6 - 128 layers (1 micrometer)

*layers thin as more layers are created

Question 4

How can layered multiplying coextrusion be used to create different shapes

Answer 4

different orientations of the extruder layer multiplier can create nanofibers

Question 5

Issues with layer multiplying coextrusion

Answer 5

1. Elastic layer rearrangement due to imbalances of normal forces from square/rectangle nozzles
   - try to fix wither high T and slow flow
   - or use sacrificial layer on outside that will take all the expressive forces
2. viscosity mismatch - end up with polymer encapsulation
   - try to control temperature and shear rate to get similar viscosities

Question 6

What happens to the glass transition temperature as layer multiplying coextrusion creates many layers

Answer 6

• as the layer thickness decreases, the glass transition temperatures of the 2 immiscible polymers become more and more similar until they overlap
• happens because we create a larger interface between immiscible polymers and eventually with decreasing layer thickness, the thickness approaches the scale of an interphase

Question 7

What happens to the mechanical properties as layer multiplying coextrusion creates many layers

Answer 7

increases the ductility of blend systems
- increases interphase and interfacial area which causes more cohesive loading and deformation of system (and changes in failure mode)
- local elongation increases where metal might otherwise be brittle

Question 8

What happens to crystalization as layer multiplying coextrusion creates many layers

Answer 8

We get confined crystallization
- the layer thickness impacts the kinetics and dimensionality of crystal growth
- the more we confine a semicrystalline layer, the less freedom the chains have to move in the z-axis

• with sufficient confinement, crystals can be forced into planar growth (stacked in-plane lamellae to even single crystals)
  *these in-plan crystals are highly desirable for barrier film applications since the crystal lamellae are highly impermeable

Question 9

What happens to optical properties as layer multiplying coextrusion creates many layers

Answer 9

By offsetting volume ratios in multiplier channels, it is possible to create nanolayer films with a gradient of layer thickness
- this allows the polymer to have a gradient refractive index (uses to create polymer lenses) which improve field of view and focusing power

• can also control reflection and transmission of light - used to create periodic variation in refractive index (create layered structure to tailor narrow-band filters) - used for privacy screens

Question 10

What is in situ nanofibrillation?

Answer 10

a flow-induced structuring technique, well suited for producing polymer-polymer composites

uses extreme elongational flow of a multi-phase polymer blend to convert droplets into nanofibrils

Question 11

How is in situ nanofibrillation done?

Answer 11

elongational flow is induced using extrusion melt spinning techniques (such as melt blowing and spunbound fibrillation), a hot air pushes out polymer and it stretched into the collection, creating polymer fibres. Polymer is cooled as its stretched to trap the system in the thermodynamically unfavoured state

Question 12

What effect are we working against when doing in situ nanofibrillation?

Answer 12

Plateau-Rayleigh instability

• after certain distances, streams break apart into droplets due to perturbations

The thinner the polymer fibre, the more difficult it is to prevent these perturbations from breaking apart the fibre

Question 13

What conditions do we need to design around for in situ nanofibrillation?

Answer 13

1. melt strength
2. viscosity ratio and bulk viscosity
3. interfacial tension and coupling
   - decreased interfacial tension and increase coupling allows for smaller droplet formation
4. processing window (once nanofibrils are created, want to produce useful items while preserving the morphology)
5. polymers with lots of branching structure, will hold together better

Question 14

What is the fibril morphology?

Answer 14

with appropriate process design and material selection, possible to create polymer nanofibrils with diameters as low as 80-100 nm

• fibrils can be exposed and characterized using selective solvent etching to remove the surrounding matrix

Question 15

How does in situ nanofibrillation impact the physical properties?

Answer 15

They become functional composites
- tough without compromising stiffness

• tensile toughness increases drastically while the elasticity remains same as spherical additives

Question 16

How does in situ nanofibrillation impact the viscosity?

Answer 16

At low frequencies (or shear rates), the viscosity actually increases rather than have a Newtonian plateau
- this is because the nanofibrils resist deformation and cause a higher viscosity, but they break/are disentangled at higher shear rates and act like the polymer with droplets - liquid-like behaviour)

Question 17

How does in situ nanofibrillation impact the modulus?

Answer 17

• loss modulus remains same behaviour
• but storage modulus is higher at low shear rates

*this can also be seen in the Van Gurp-Palmen plots where the phase angle becomes closer to 0 (more solid-like) and increases as the nanofibrils break before returning to polymer behaviour as it returns to solid-like behaviour

**gel point also changes with concentration (seen with Winter-Chambon criterion)

Question 18

How does crystal nucleation change for in situ nanofibrillation?

Answer 18

• polymer nanofibrils are highly effective at nucleating crystals in a surrounding crystallizable matrix (increases rate of crystallization and crystal nuclei density) to droplets as they promote epitaxial growth around nanofibrils
• only the nanofibrils are melted into droplets, the nucleation density much decreases, as can be seen by the significantly lower heat flow during crystallization temperatures

Question 19

How does in situ nanofibrillation impact tensile properties?

Answer 19

As fibrils diameter reduces, the individual fibrils tend to becomes significantly stronger and grow more rigid.

chain alignment and orientation in the spinning direction significantly increases the fibrils’ resistance to deformation

Also have less structural defects per unit cross-sectional area as the fibril diameter decreases

Question 20

What are self-reinforced composites? (in situ nanofibrillation)

Answer 20

process of improving compatibility between matrix and dispersed phase with self-reinforced composites wehre the matrix and dispersed phase are molecularly similar

• approach can be combined with in situ nanofibrillation to create extremely fine nanofibrils that significantly enhance thermomechanical properties

Question 21

What do we see when we combine self-reinforced composites with in situ nanofibrillation?

Answer 21

• at first perceive 2 discrete domains, but repeat heating and cooling leads to complete intermolecular diffusion and loss of separate melting peaks (at high temperature self-similar polymers tend to form a solution due to extremely low interfacial tension)
• change in crystalline structures, since nanofibrils are extremely efficient nucleating agents
• the change in crystalline structure improves optical transparency since the size of individual crystalline domains is reduced below the wavelength of visible light, limiting the ability of crystals to diffract light