Polymer Blends Flashcards
What is polymer blending?
Blending 2 or more resins is widely used in commercial applications to meet performance goals
- similar to ‘alloying’, mixing 2 dissimilar polymers with complimentary properties to meet our end goals
-the properties of the blend change with the composition of each polymer
What is a polymer blend made of?
- A matrix phase - continuous, surrounds the dispersed phase
- Dispersed phase - discrete particles or fibres surrounded by the matrix
What makes a good blend? And what makes a bad blend?
- high interactions between blend. The blend will be:
1. relatively homogeneous
2. well-dispersed with small droplet sizes
3. form a strong interface between the blend components (phases) - poor blending creates stage locations where polymer will fail. The blend will be:
1. very heterogeneous
2. have large droplet sizes
3. have poor adhesion/interaction between the components
What are the 3 factors in influence blend ability?
- miscibility
- measure of interfacial tension between matrix and dispersed phase
- want low interficial tension which allows droplets to break into smaller sizes
- high miscibility gets good blend mixing = smaller droplet size
2.viscoelastic properties
- if mismatched becomes to mix efficiently
- less viscous gets most shear when mixed (thus thermodynamically separating them)
- relative viscosity and elasticity of each component dictates ability to effectively transfer stress and break up droplets
- Process Design (processing conditions)
- use proper equipment to properly mix and disperse a polymer blend
- cool quickly to keep the blend from coalescing after mixing
What are homogeneous blends?
fully miscible blends
- polymers with similar molecular structures and low interfacial tension can form true solutions under the right conditions
- generally performed with polymer with the same monomer structure but different Mw or branching density
- can also occur if have same function groups that like to interact (ex. aromatic groups like to stack regardless of monomer group)
The properties:
- merge glass transition processes
- some degree of crystal formation suppression
- no observable phase separation
- viscoelastic relaxation processes are coupled
What are heterogeneous blends?
immiscible blends
- form phase-separated structures with domain size governed by volume fraction, viscoelastic properties, mixing efficiency, and interfacial tension
- since phases are separated, generally see that phase transitions and relaxation processes occur independently (ie. 2 Tgs and 2 Tms, ect.)
What are the different forms of heterogeneous blends?
- sea-island: one is matrix and other is dispersed (high concentration of matrix and low concentration of other), get islands of the dispersed within the matrix
- co-continuous (similar concentrations of both), creates sample spanning networks of both polymers)
- can be terrible for properties (has 2 matrix polymers)
- but can be good for other cases such as barriers
- region depends on volume of the 2, the miscibility, and the viscoelasticity
What are the 2 types of mixing to create heterogeneous blends?
- distributive mixing - achieves good mixing
- needed in most situations to have uniform thermomechanical properties across the sample - dispersive mixing - reduces domain size of the discontinuous phase (makes small droplets)
*ideally want both
What is the viscosity ratio?
the viscosity of the droplets divided by the viscosity of the matrix
- value helps determine how well the system will mix
How to break up droplets in the matrix?
requires specific conditions to be favourable. There is an energy cost associated with deforming a droplet and increasing the interfacial area.
- shear is applied to break droplets apart but interfacial tension holds the droplets together
What is capillary number?
a measure of the balance of the shear stresses and interfacial tensions.
- the critical capillary number dictates the where the balance of the two forces are equal (above that value, the droplet breaks apart, below it does not)
- value changes with the viscosity ratio (which changes with temperature, speed, ect.) and flow type (shear flow is significantly higher than elongational flow) - less stress needed to break apart polymers in elongational flow
What are the best viscosity ratios for breaking up droplets?
1-4 (thus want to match viscoelastic properties of 2 polymers as much as possible)
*and elongational flow not shear
What is interfacial tension?
is the tension and associated stresses between 2 phases and depends on the surface free energy of each component
- surface free energy can be determined by putting droplet of liquid on the polymer surface and measuring the contact angle. (increasing angle = increased surface free energy = increased interfacial tension)
the interfacial tension is the sum of surface free energy of a polar component and a dispersive component. These are found with tests with a polar liquid (water) and a non-polar liquid (diiodomethane)
polymer-polymer interfacial tension can be calculated with Owens-Wendt method
What sort of structures can occur if more than 2 polymers in blend?
- complete wetting (1 encapsulates another which is encapsulated by another), occurs when the interfacial tension between 2 is less than 1 and the other two is greater than 1
- partially wetting (both dispersive compounds mixed with each other and matrix), occurs when all have interfacial tensions less than 0 with each other
- separation (dispersive components all separated from each other) - occurs when all interfacial tensions between dispersive components are greater than interfacial tensions between dispersive and matrx
What is the interphase vs interface?
interphase is a region where 2 polymers meet and merge into each other through a constant gradient
- considered a 3rd component that bridges the other 2
- interphase increases as the interfacial tension lowers
- results in improved mechanical properties (so want to maximize this region)
