Mechanical properties of polymers: an overview. Suryasarathi Bose Dept. of Materials Engineering, IISc, Bangalore

Transcription

1 Mechanical properties of polymers: an overview Suryasarathi Bose Dept. of Materials Engineering, IISc, Bangalore UGC-NRCM Summer School on Mechanical Property Characterization- June 2012

2 Overview of polymer science Thermal transitions in polymers Structure-property relationship in polymers Dynamic mechanical properties of polymers Rheology

3 Cellulose nitrate The first man made polymer Poly(isoprene)

4 What is a polymer?

5 How big are polymers?

6 CLASSIFICATION Structural shape of polymer molecules, which may be: Linear Branched Cross-linked

7 THERMOPLASTICS and THERMOSETS Thermoplastics Soften when heated and harden when cooled irrespective of the number or times the process is repeated Flow with pressure and heat Temperature of use limited to softening temperature No chemical changes upon heating Thermosets Once heated they react irreversibly to give strong intermolecular bonding Do not soften or flow upon application of heat and pressure Use temperature is higher and is governed by degradation temperature

8 Polymer can be synthesized to yield desired mechanical behavior by appropriate combination of: Glass Transition Temperature, Tg Crystallinity Melting Temperature, Tm Branching or Cross Linking These determine the use of a specific polymer as Elastomer (Rubber) Flexible Plastic Rigid Plastic Fiber

9 Thermal transitions in polymers Translation, Vibration, Rotation Local segment mobility besides vibration & rotation B Only Vibration E G Specific Volume F I H C D Amorphous Semi-crystalline Crystalline Temperature T g T m

10 FACTORS AFFECTING GLASS TRANSITION TEMPERATURE 1. Bulky side groups 2. Long side chains 3. Intermolecular bonding forces of attraction 4. Chain stiffness 5. Copolymers 6. Blending 7. Plasticization 8. Crystallinity

11 Molecular interpretation Free volume theory Motions in the rubbery state require larger free volume than the short range order in the glassy state. The rise in the relative free volume with increasing temperature above glass transition leads to higher observed expansion coefficient in the region. Above Tg the segments can move, rearrange and relieve stress and the polymer will flow

12 Upon application of stress Above Tg - the segments can move, rearrange and relieve stress and the polymer will flow. Physical state soft, rubbery, sticky & can flow easily. Below Tg the segments are frozen, the polymer will not flow. It will show ductile behavior, resulting in cohesive brittle failure. Physical state hard, brittle, rigid & not easily deformable.

13 DYNAMIC mechanical analysis (DMA) DYNAMIC mechanical analysis (DMA)-is a thermal analysis technique that measures the properties of materials as they are deformed under periodic stress. Specifically, in DMA a variable sinusoidal stress is applied, and the resultant sinusoidal strain is measured. If the material being evaluated is purely elastic, the phase difference between the stress and strain sine waves is 0 (i.e., they are in phase). If the material is purely viscous, the phase difference is 90. However, most real-world materials including polymers are viscoelastic and exhibit a phase difference between those extremes. DMA continuously monitors material modulus with temperature and, hence, provides a better indication of long-term, elevated temperature performance.

14 DMA - Measure response of material to periodic stress - Can apply stress (strain) in tension, compression, shear, bend - Also measure the phase difference or lag (δ) between two sine waves Force applied stress F dynamic material response phase angle (δ) amplitude Temperature δ E* E E F static (δ) phase difference, phase lag or dissipation factor

15 There are several components that are critical to the design and resultant performance of a dynamic mechanical analyzer. Those components are the drive motor (which supplies the sinusoidal deformation force to the sample material), the drive shaft support and guidance system (which transfers the force from the drive motor to the clamps that hold the sample), the displacement sensor (which measures the sample deformation that occurs under the applied force), the temperature control system (furnace), and the sample clamps.

16 DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA) Can detect weaker thermal transitions:

17 At sufficiently low temperatures when chain-and and chain segment mobility are frozen in, that is below the glass-transition temperature, polymers behave like common elastic materials. The (elastic) deformations in that state are characterised by changes of bond length and bond angles. The only in macromolecular substances observed rubber elasticity is not caused by an energetic distortion of bond length or bond angles but by entropic effects: perturbation of a random coil leads to a state of lower entropy since the number of accessible quantum states (conformations) is restricted by e. g. an extension. Rubber elasticity can be observed at temperatures higher than the glass transition temperature if the polymer chains are long enough and if cross-links of any kind are present. The cross-links can be permanent or temporary, chemical or physical of nature. They cause phenomena like relaxation and creep (retardation).

18 Rheology of polymer melts -Deformation and flow - Gives information on the structure Mechanical Spectroscopy: to study polymer morphology and structure and relate these to end-use performance. Rheological properties can be measured continuously as the material undergoes temperature-induced changes from amorphous to crystalline

19 Rheology The study of deformation and flow characteristics of substances is called rheology Ideal solids deform elastically- energy required to deform is fully recovered Ideal fluids (liquids/gases) deform irreversibly- they flow; energy required to deform is dissipated within the fluid as heat (viscous). In reality- bodies are somewhere between ideal solids and ideal fluids Polymers- viscous and elastic- viscoelastic Time scale of any deformation process Characteristic time factor: λ (infinite for ideal elastic solids and almost 0 for liquids) Deformation process is related to a characteristic value: t Deborah No.: λ/t (high Deborah No. indicates solid-like like and low D No. indicates liquid like)

20 Dynamic Viscosity: As a fluid moves, a shear stress is developed in it, the magnitude of which depends on the viscosity of the fluid. Shear stress, denoted by the Greek letter (tau), τ,, can be defined as the force required to slide one unit area layer of a substance over another. Thus, τ is a force divided by an area and can be measured in the units of N/m 2 (Pa) The fact that the shear stress in the fluid is directly proportional to the velocity gradient can be stated mathematically as: τ= µ γ

21 Newtonian Fluids and Non-Newtonian Newtonian Fluids Any fluid that behaves in accordance with called a Newtonian fluid. Conversely, a fluid that does not behave in accordance with the above equation is called a non-newtonian fluid. Two major classifications of non-newtonian fluids are time- independent and time-dependent fluids. As their name implies, time-independent independent fluids have a viscosity at any given shear stress that does not vary with time. The viscosity of time dependent fluids, however, changes with time.

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23 Drop break-up Coalescence η & mrγ Ca= α capillary number Ca that represents the ratio of deforming viscous to restoring interfacial stresses. η m the matrix viscosity, γ the shear rate, R the droplet radius, α the interfacial tension and η d the droplet viscosity viscosity ratio, λ Drops can break when 1< Ca <2 ηd λ = η m If λ >4, no break-up

24 Thank You!

25 For deformed droplets and fibrils, the interfacial tension drives retraction towards a spherical shape. In the case of ellipsoidal Newtonian droplets in a Newtonian matrix, the characteristic time τ d for this process can be obtained from the linear viscoelastic model of Palierne: R ηm (19λ+ 16)(2λ+ 3 2ϕ( λ 1)) τd = 4 α 10( λ+ 1) 2ϕ(5λ + 2) If the droplet elongation exceeds a certain critical value, the droplet or fibril will break up after cessation of the flow The increase of the dimensions of the dispersed phase are caused by coalescence and Ostwald ripening. Ostwald ripening is purely caused by thermodynamics, which drives the content of small droplets to diffuse through the matrix into the bigger droplets. The dynamics of droplet growth by Ostwald ripening obeys the following kinetics as a function of time t: R 3 = R b t R 0 is the initial droplet radius and b is a factor that depends, among others, on the diffusion coefficient of the molecules of the dispersed phase in the matrix material. In co-continuous continuous blends, the rate of coarsening dr/dt dt is given by: dr a α = dt η blend Due to the large viscosity and matched densities of polymers, the driving forces for droplet approach are rather limited, which makes that quiescent coalescence is mainly observed in blends with a large volume fraction of dispersed phase.