Test 2

Melt flow indexer

1) Die diameter is known 2) Weight is known (P). Apply weight at different T. 3) Measure the mass flow rate ( ): grams of extrudate in 10 minutes.

Shear Modulus

A term describing a solid's resistance to shear stress, denoted by the letter G and measured by the ratio of shear stress (F/A) to strain (x/h).

C1,C2

C1=17.4, C2=51.6

Newtonian fluid

Fluids for which the rate of deformation is linearly proportional to the shear stress.

relaxation modulus

For viscoelastic polymers, the time-dependent modulus of elasticity. It is determined from stress relaxation measurements as the ratio of stress (taken at some time after the load application—normally 10 s) to strain.

Retractive force

In the case of the elastomer the major effect of the deformation is the stretching out of the network chains, which substantially reduces their entropy. Thus, the retractive force arises primarily from the tendency of the system to increase its entropy toward the (maximum) value it had in the undeformed state. Increase in temperature increases the chaotic molecular motions of the chains and thus increases the tendency toward the more random state. As a result there is a decrease in length at constant force, or an increase in force at constant length.

Viscous flow

Is caused by (linear) polymer chains moving past one another. This relaxation mechanism becomes important when the temperature increases and the chains gain mobility, especially near or above the glass transition temperature. The same is true for relaxation through molecular motion. Relaxation is mainly caused by rotation of bonds, for example by a "crankshaft" mechanism. Near the Tg the chains relax at about the same rate as the time frame of the experiment. The motions can be grouped in main-chain and side-chain motions. In general, low-temperature main-chain motions absorb energy better than equivalent side-chain motions.

Flow Curve

It must be emphazised that these types are an idealization of the real flow behavior of fluids. Most polymer solutions and melts exhibit shear thinning, that is, they belong to the class of pseudoplastic materials, whereas shear-thickening or dilatant behavior is rarely observed. Some common examples of shear-thickening fluids are cornstarch in water and nanoparticles dispersed in a (polymer) solution.

Viscosity curve

It must be emphazised that these types are an idealization of the real flow behavior of fluids. Most polymer solutions and melts exhibit shear thinning, that is, they belong to the class of pseudoplastic materials, whereas shear-thickening or dilatant behavior is rarely observed. Some common examples of shear-thickening fluids are cornstarch in water and nanoparticles dispersed in a (polymer) solution.

Maxwell Model

Mechanical analog model consisting of a spring and a dashpot in series for describing viscoelastic material properties.

Bond interchange

Often occurs in polyesters and polysiloxanes. In this case, stress relaxation is achieved by interchange of bonds perpendicular to the stress vector, that is, the stress is relaxed in a similar manner as chain scission, only that two pairs of neighboring "broken" chain ends form two new (not load bearing) chains.

Thixotropic

Property of a liquid to flow more readily under mechanical force.

Rheopectic

Rheopectic is very similar to dilatant in that when shear is applied, viscosity increases. The difference here is that viscosity increase is time-dependent.

Rubber elasticity

Rubber elasticity may be operationally defined as very large deformability with essentially complete recoverability. In order for a material to exhibit this type of elasticity, three molecular conditions must be met: (1) the material must consist of polymeric chains, (2) the chains must be joined into a network structure (3) the chains must have a high degree of flexibility (1-3), The first requirement arises from the fact that the molecules in a rubber or elastomeric material must be able to alter their arrangements and extensions in space dramatically in response to an imposed stress, and only a long-chain molecule has the required a very large number of spatial arrangements of very different extensions.

Bingham liquid

Some other fluids require a threshold shear stress before they start to flow. This kind of fluid is called a plastic fluid and if the flowing liquid has a constant viscosity it is called a Bingham liquid. However, such behavior is not observed in ordinary polymer melts and solutions. Typical examples for plastic flow behavior are polymer/silica micro- and nanocomposites. The solid-like behavior at low shear stress can be explained by the formation of a silica network structure arising from attractive particle-particle interactions due to hydrogen bonding between silanol groups. Once the particle network breaks down upon application of a critical yield stress (τy), the polymer shows normal flow behavior.

Kelvin Model

Spring and dashpot in parallel

Stress Relaxation

Stress-Relaxation Testing. This test method is used to determine a sample's creep properties when subjected to a prolonged tensile or compressive load at a constant temperature. The rate of deformation of a sample to stress at a constant temperature is known as the creep rate.

Boltzman Superposition Principle

The Boltzmann superposition principle states that the response of a material to a given load is independent of the response of the material to any load, which is already on the material. The deformation of a specimen is directly proportional to the applied stress, when all deformations are compared to equivalent times. It is only valid in linear viscoelastic region. For the case of creep, the total strain may be expressed by D(t)=1/E(t), the compliance function, which is a characteristic of the polymer at a given temperature and initial stress.

DYNAMICAL MECHANICAL ANALYSIS

The DMA technique is based on a rather simple principle; when a sample is subjected to a sinusoidal oscillating stress, its response is a sinusoidal oscillation with similar frequency provided the material stays within its elastic limits. When the material responds to the applied oscillating stress perfectly elastically, the responding strain wave is in-phase (storage or elastic response), while a viscous material responds with an out-of-phase strain wave (loss or a viscous response). For a Newtonian liquid the phase angle will be 90 degrees and for Hookean solid it will be 0 degrees, whereas the phase angle of viscoelastic material falls in between these two extremes.

Rubbery Plateau

The appearance of a rubbery plateau is the result of entanglements or crosslinks. The rubbery plateau is also related to the degree of crystallinity in a material, that is, the temperature behavior of the modulus of a semicrystalline polymer is qualitativley similar to that of a high molecular weight amorphous polymer, only the modulus is typically higher in the secondary plateau due to the reinforcement effect of the crystalline domains dispersed in the amorphous rubbery phase above the Tg but below the melting point (Tm).

Shear thinning fluid

The observed shear thinning of polymer melts and solutions is caused by disentanglement of polymer chains during flow. Polymers with a sufficiently high molecular weight are always entangled (like spagetti) and randomly oriented when at rest. When sheared, however, they begin to disentangle and to allign which causes the viscosity to drop. The degree of disentanglement will depend on the shear rate. At sufficiently high shear rates the polymers will be completely disentangled and fully alligned. In this regime, the viscosity of the polymer melt or solution will be independent of the shear rate, i.e. the polymer will behave like a Newtonian liquid again.2 The same is true for very low shear rates; the polymer chains move so slowly that entanglement does not impede the shear flow. The viscosity at infinite slow shear is called zero shear rate viscosity (η0).

Crossover Point

The point that tells you which material is viscous and which material is elastic.

shear viscosity

The shear viscosity of a system measures its resistance to flow.

creep compliance

The strain e(t) in a viscoelastic material will increase with time. The ratio: J(t) = ( e(t) / s(0)) is called the creep compliance. In linearly viscoelastic materials, the creep compliance is independent of stress level.

Creep Relaxation

When viscoelastic materials are subjected to stress they undergo deformations by molecular rearrangements and by viscoelastic flow. To study these deformations, creep experiments are often conducted. In these experiments, a sample is subjected to an instantaneous load at time t0 and the strain (creep) is recorded as a function of time at constant temperature.

tensile creep compliance

apply a constant tensile stress to the sample and measure the tensile deformation

shear thickening

increase in viscosity when a shear stress is applied (quicksand)

Chain scission

is often caused by oxidation and hydrolysis and/or an applied stress at elevated temperature, that is, the reduction in stress is caused by breaks in load bearing chains. This relaxation is of little importance at low temperature, but gains importance with increasing temperature and exposure time and is the dominating relaxation mechanism when the temperature approaches the decomposition temperature.

plastic deformation

permanent deformation caused by strain when stress exceeds a certain value

Disentanglement

physical knots and trapped entanglements in elastic networks is caused by chain diffusion and can be explained with the Rouse-Bueche or the de Gennes Reptation model, that describes polymer chain diffusion as a snake-like motion within a tube formed by "fixed" obstacles, i.e. by other chains and other particles (plasticizer molecules, etc.).

elastic deformation

region where the material will return to its original shape when the stress is removed

shear stress

stress that occurs when forces act in parallel but opposite directions, pushing parts of a solid in opposite directions

Rheology

study of flow

Non-Newtonian Fluid

the shearing stress is not proportional to the shearing rate

volumetric flow rate

the volume of liquid traveling through the die per unit time

relaxation time

time from peak tension until relaxation is complete

shear thinning

when stress is applied viscosity decrease ex: ketchup, quicksand