Chemical Structure vs Properties

When selecting the correct material for your application the overall molecular structure, physical/thermal properties, and chemical reactivity must be considered. Questions such as what are the mechanical requirements of this application, what type of temperature will the polymer be exposed to, and what kind of chemicals will come into contact with my product are the foundation for beginning the material selection process. The molecular composition of commercial polymers largely dictates the overall functionality of the end use application. Within this technical overview, we will consider the basic fundamentals of material selection with an emphasis on chemical structure versus desired properties.

Melting Point

In general, elevated polymer melting points are associated with highly regular structures, rigid molecules, close packing capability, and strong inter-chain attraction. In accordance to the packing postulate for considering polymer physical properties, the general rule is that structures that are the most tightly bonded (high bond energy), fitting the closest together, and held in place with the highest rigidity (intermolecular forces) will have the highest melting point.

Structural Regularity

In the case of EVOH or EVA, the addition of a copolymer content reduces the melting point of the parent structure. The addition of copolymer reduces the structural regularity; moreover, the addition of PE in the EVOH causes steric interactions increasing bulk lattice constants and reducing close pack ability. This ultimately accounts for reduced oxygen barrier at higher ethylene content as shown in the following section.

Bond Flexibility / Inter-Chain Attraction

As the number of carbon atoms (non-polar moieties) increase per amide bond, the bond length is increased and overall chain flexibility is increased. With reduced molecular rigidity the backbone flexibility is increased while the melting point is reduced. Next, the addition of hydrophobic methylene characteristics leads to a reduced water absorption at equilibrium within the nylon structure. The increased distance between the amide groups within the backbone causes the interchain attraction to become reduced; increased aliphatic character increases steric hindrance and the hydrogen bonding (intermolecular force) effect is reduced.

Glass Transition Temperature

The rigidity of the internal molecular bonding within a structure directly influences the glass transition temperature. Bonds with lower steric hindrance (atoms pushing on each other in space) upon rotation will ultimately have lower glass transition temperatures; moreover, the temperature upon which thermal energy can be translated into conformational (spatial) changes is by definition the glass transition temperature. Below the glass transition temperature the polymer is considered glassy or brittle with no inter-chain/segmental motion. The chart below shows an example of several classes of polymers. We note the structure differences in the image after and note how sterically hindered side chain groups affect glass transition temperature.

FTIR Spectroscopic Functionality

In many cases, it is convenient to identify a material by its chemical signature on FTIR. FTIR is the acronym for Fourier Transform Infrared Spectroscopy. This analytical testing method allows scientists to measure the absorption/transmittance of inferred light as it passes through a sample. There are very specific absorbance ranges for different chemical structures within a polymer. An example table can be seen below. The FTIR spectrum allows us to determine a general polymer family that a sample belongs to.

Chemical Reactivity – Resistance Chart / Solvent Resistance

The general trends of reactivity are common within each polymer family. There are several signs of chemical attack on plastic products: stress cracking, embrittlement, swelling, plasticization, and discoloration. Typical organic reactions between polyesters and polyamides with strong acids are common in consumer applications. Selecting polymer types that have resistance to chemical attack and provide barrier to diffusion across a cross section for a given application is vital. As a general rule of thumb, utilizing a chemical resistance chart is recommended as a starting point.

ESCR Resistance

One of the most common causes of premature failure of polymer products in the field is environmental stress cracking/embrittlement. While general chemical resistance is part of the tendency of a material to stress crack, the molecular weight distribution, crystalline grain size, and molecular architecture also play a role in stress cracking. Stress cracking begins to form as microscopic craze domains that act as dislocations in the polymer matrix. Below, we show tensile strength retention data for various polymer families when exposed to medical solvents (known stress cracking agents). It is important to note that PC and ABS are especially prone to chemical stress cracking and thereby have significantly reduced tensile strength post exposure.

UV Stability

Within each polymer family, the UV stability (resistance to molecular damage after exposure to UV radiation) is highly dependent on the chemical structure of the material. Generally, chemicals with high bond energy and without UV absorbing groups are the most stable; furthermore, if a chemical has weak bond energies and UV absorbing groups, they are more likely to exhibit UV instability.

The reactivity and steric hindrance to attack of the hydrogen bonded on secondary and tertiary carbon atoms follows linearly with UV stability in PIB, PP, and PE. In general the C-H bonds in these groups do not absorb UV radiation; however, any impurities that exist within the polymer matrix can catalyze free radical reactions that lead to chain scission, cross linking, and secondary oxidation.

Barrier / Permeability

As shown previously, the functionality of a polymer system can be influenced by the existence of varying copolymer content. Image one below shows how an increased lattice constant in EVOH leads to reduced barrier performance. The lower the PE content, the better the barrier (but increased difficulty to process) the EVOH becomes.

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