Mechanism Properties of Polymers, Chemistry tutorial

Introduction

The mechanical properties of a polymer involve its behavior under stress. Such properties said a polymer scientist or engineer many of the things he or she needs to know when considering how a polymer can be used.

  • How strong is the polymer? How much can you stretch it before it breaks?
  • How stiff is it? How much does it curve when we push on it?
  • Is it brittle? Does it break easily if you hit it hard?
  • Is it hard or soft?
  • Does it hold up well under repeated stress?

The mechanical properties of polymers are one of the characteristics that distinguish them from small molecules.

Mechanical Properties of Polymers  

Mechanical properties refer to those properties or characteristics of a material which determine its response to an applied stress or strain. The stress may be applied in increments, in flexure, tension or shear. Or it might be applied extremely abruptly or in cyclic (alternating) manner.  Mechanical properties are manifested in the ability of the material to resist failure and/or its deformational behaviour. Polymers being viscoelastic substances, their mechanical properties in contrast to other materials, depend on time factor, temperature, structure, nature and previous history. Time factor means elapsed time under load, rate of loading, rate of deformation etc. Structure means molar mass, branching, crosslinking and crystallinity. Nature means kinds of additives, defects, blending and orientation. Previous history covers method of fabrication and example preparation.

Other factors depend on the nature and structures of the polymer that as well influence the mechanical properties are:

Effects of Additives - Additives are usually incorporated into polymers for one reason or another. Several, these as water and monomer, might be unavoidably present. Such additives might have significant effect on the mechanical properties and this must be taken account especially in design consideration. In general, solid additives these as fillers and reinforcing agents, advance the impact strength, flexural strength, tensile strength and heat distortion temperature of polymers. Plasticizers have a considerable effect on the mechanical properties of polymers.

Addition of plasticizer lowers the glass transition temperature. Modulus and yield stress drop through rising plasticizer content. Consequence of Blending and Copolymerization - The require for polymers having exact or improved properties is sometimes met not through synthesizing new polymers but through copolymerizing two or more monomers or by mixing homopolymers, copolymer and terpolymers of different structures. Therefore, when vinyl acetate or acrylonitrile is copolymerized with increasing amounts of vinyl chloride, the glass transition temperature increases from 303K for poly (vinyl acetate) or drops from about 380K for polyacrylonitrile to about 353K for poly (vinyl chloride). The copolymers become increasingly softer and tougher at room temperature but the softening point is lowered.

Consequence of Molar Mass - The huge molar mass of polymers is responsible for most of their helpful properties. However, as well huge a molar mass can make a polymer hard to procedure and fabricate. It follows then that a balance has to be originate between simple procedure ability and maximum physical properties. Molar mass has little consequence on yield stress and modulus. Increasing molar mass improves the rupture properties and impact resistance of a polymer. As the molar mass increases, more tie molecule hold the crystallite together, there is more flexibility and therefore greater toughness.      

The mechanical properties of polymers are generally described using the subsequent terms; elasticity, plasticity, strength, toughness, ductility, brittleness and hardness. Elasticity - This terms to the ability of substance to recover to its initial shape and dimensions when the applied stress is eliminated.

Hardness refers to the ability of a substance to surface deformation, indentation or abrasion. Dimensional stability - Is the ability of a polymer to withstand progressive elimination of material from its surface as a consequence of mechanical action of rubbing, scrapping or corrosive nature; for example eraser and chalk contain low scratch and abrasion resistance.   

Plasticity - A substance is said to exhibit plasticity if on releasing the applied stress it keeps a permanent deformation.

Brittleness - A fragile substance shatters suddenly with no noticeable plastic deformation preceding failure.

Ductility - A ductile substance undergoes huge irrecoverable deformation before rupture. Strength terms to the ability of a material to withstand a slowly applied stress without rupture. There is more than one type of strength. The type of strength a polymer has based on how well the polymer holds up to whatever we are trying to do to it. For example;

1)  Tensile strength is the stress required to break a example. It is a significant property for polymers that are going to be stretched. 

2)  Compressive strength of a polymeric substance refers to the ability of a material to resist crushing when a squeezing force is applied to it. An instance of material through elevated compressive strength is foam substance.

3)  Impact strength calculates the ability of a substance to resist sharp blows or shock. For example Glass has zero collision strength. Here are several types of strength, starting through tensile strength. Each one depends on what we are trying to do by the example.

Table: Different kinds of Strength exhibited by polymers

2058_Different kinds of Strength exhibited by polymers.jpg

Stress - strain properties 

The mechanical behaviour of a polymer can be assessed via its stress - strain properties. Several of the terms utilized in describing the response of substance to stress - strain comprise:  

1) Ultimate elongation: is the extent of elongation at the point of failure. The elongation-to-break is the strain on an example when it breaks. This generally is stated as a percent. Fibers have a low elongation-to-break and elastomers have an elevated elongation-to-break.

1259_elongation.jpg

2) Tensile strength: the stress required to rupture the object. This is the most significant indicator of strength of a polymer and it is the force utilized to stretch an instance, separated through the cross-sectional area. Tensile strength is stated in Pascals or psi (pounds per square inch). 1 MPa = 145 psi  

955_tensile strength.jpg

3) Young modulus: the resistance to deformation. This is specified via the ratio of initial stress to initial strain. It as well is described the modulus of elasticity or the tensile modulus. Young's modulus is the slope of stress-strain curve. Stress-strain curves are frequently not straight-line plots, indicating that the modulus is changing through the amount of strain. In this case the initial slope generally is utilized as the modulus, as is demonstrated in the diagram below. Rigid substances, these as metals, have an elevated Young's modulus. In common, fibers have elevated Young's modulus values, elastomers have low values, and plastics lie somewhere in between.

2349_strain.jpg

4)  Toughness: The toughness of a substance is the area under a stress-strain curve. The stress is proportional to the tensile force on the material and the strain is proportional to its length. The area under the curve then is proportional to the integral of the force over the distance the polymer stretches before breaking.

Area α ∫ F (L) dL

This integral is the work (energy) required to break the sample. The toughness is computed of the energy an instance can absorb before it breaks. 

There is a dissimilarity between toughness and strength, as is demonstrated in the three plots at the below. A substance that is strong but not tough is said to be brittle. Brittle substances are strong, but can't deform very much. Polystyrene (PS) is brittle, for instance. Elevated impact polystyrene (HIPS), a blend of polystyrene and polybutadiene (a rubbery polymer above its glass transition temperature) is said to be rubber-toughened.

Table: Typical Materials - Mechanical Properties

2155_Typical Materials - Mechanical Properties.jpg

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