Chapter Guide
Physical Properties and Physical Chemistry of Polymers
Polymer physical properties are governed by chain structure, chain mobility, intermolecular forces, molecular weight, morphology, and test conditions. This guide explains why lookup values are ranges and how to compare them responsibly.
Structure-Property Relationships
Every repeat unit contributes flexibility, polarity, volume, and intermolecular attraction. Because that unit repeats thousands of times, modest chemical differences can produce large changes in Tg, density, refractive index, solubility, and melt behavior.
| Structural Factor | Property Impact | Check On Site |
|---|---|---|
| Backbone flexibility | Flexible chains lower Tg and support rubbery behavior. | Silicones |
| Bulky side groups | Can raise Tg, reduce packing, and change optical behavior. | Polystyrene |
| Polarity and hydrogen bonding | Increase cohesive energy, water sensitivity, and solvent selectivity. | Acrylics, cellulosics |
| Aromaticity | Raises stiffness and often increases refractive index. | RI guide |
| Halogens or high-mass atoms | Can increase density and polarizability. | PVC, fluorinated polymers |
| Branching | Changes crystallinity, melt flow, density, and mechanical response. | Polyolefins |
Amorphous State and Glass Transition
An amorphous polymer lacks long-range crystalline order. Below Tg, chain segments are sluggish and the material behaves glassy. Above Tg, segmental motion increases and the material becomes softer, rubberier, or more leathery depending on molecular weight and crosslinking.
- Tg is not a single universal constant; it depends on molecular weight, plasticizer, moisture, heating rate, thermal history, and measurement method.
- Plasticizers lower Tg by increasing free volume and chain mobility.
- Bulky, rigid, polar, or aromatic groups often raise Tg by restricting segmental motion.
- Crosslinking can raise apparent service temperature and prevent viscous flow, but it does not make the polymer immune to thermal degradation.
Crystalline and Semicrystalline States
Many polymers are semicrystalline: they contain ordered crystalline regions and disordered amorphous regions. Crystallinity affects density, modulus, melting behavior, permeability, opacity, shrinkage, and chemical resistance.
| Variable | Effect | Practical Note |
|---|---|---|
| Tacticity | Regular stereochemistry can permit ordered packing. | Critical for polypropylene and some styrenics. |
| Branching | Branches disrupt packing and often reduce crystallinity. | LDPE versus HDPE is a useful comparison. |
| Cooling rate | Fast cooling can reduce crystal size or crystallinity. | Molded parts can differ from annealed plaques. |
| Orientation | Drawn fibers and films can be anisotropic. | Measure properties in relevant directions. |
| Comonomers | Irregular units can disrupt packing. | Copolymer composition changes density and Tm. |
Viscoelasticity, Elasticity, and Rheology
Polymer response depends on time and temperature. A material may behave elastic in a fast test, viscous in a slow test, and brittle below Tg. Melt and solution rheology are especially sensitive to molecular weight, branching, concentration, and entanglement.
- Elastic response: Deformation recovers when chains are stretched but not permanently rearranged.
- Viscous response: Chains slide or flow, producing permanent deformation.
- Viscoelastic response: Most polymer solids and melts combine elastic and viscous behavior.
- Time-temperature superposition: Faster tests can resemble colder tests, and slower tests can resemble warmer conditions.
- Entanglements: Long chains physically constrain one another and increase melt strength, viscosity, and toughness.
Density and Refractive Index Context
Density and refractive index are not isolated table values. Density is controlled by mass per unit volume, packing, crystallinity, voids, fillers, plasticizers, and temperature. Refractive index is controlled by polarizability, wavelength, temperature, crystallinity, orientation, and sample form.
Polymer Density Chart
Use the chart as a screening table, then verify grade, crystallinity, filler, and test method.
Refractive Index Guide
Interpret RI with wavelength, temperature, sample form, and formulation state.
Density, Thermal, and Mechanical Research
Open-access papers for thermal history, crystallinity, rheology, and property measurement.
Physical Property Checklist
- Record polymer identity, grade, additives, filler, plasticizer, and residual solvent.
- Record sample form: powder, pellet, molded plaque, film, fiber, foam, solution, or coating.
- Record temperature, humidity, conditioning, thermal history, and measurement method.
- Distinguish amorphous, crystalline, semicrystalline, oriented, and crosslinked states.
- Treat values as ranges unless the exact grade and test conditions are known.