The fraction of a crystalline polymer occupied by crystalline regions. It is determined from density, the heat of fusion by DSC, XRD and so on; higher crystallinity raises density and modulus but lowers transparency.
Japanese name: 結晶化度
Crystallinity appears in 5 practice questions across 3 topics, including Polymers and composites, Thin films, surfaces and nanomaterials, Biomaterials.
Field: Polymers and composites
The long chains of a polymer are entangled, so the whole material cannot become perfectly ordered. On cooling from the melt, plate-like crystals about 10 nm thick in which the chains are folded (lamellae) alternate with amorphous regions between them, and these further gather radially into spherulites. Crystallinity is the fraction of this crystalline part. The more regular the chain structure, the more readily it crystallizes: lightly branched high-density polyethylene has high crystallinity, while atactic polystyrene and PMMA, with irregularly oriented side groups, hardly crystallize at all.
In DSC, crystallinity is found by dividing the specimen's heat of fusion ΔHm by the heat of fusion of a perfect crystal ΔHm°. For polyethylene, ΔHm° of about 293 J/g is commonly used, so a heat of fusion of 176 J/g gives a crystallinity of about 60%. For specimens such as PET that crystallize during heating (cold crystallization), that exotherm is subtracted. Other methods use density, or separate the areas of the crystalline peaks and the amorphous halo in XRD. Each method counts a different range as crystalline, so values are compared using the same method.
Even for the same resin, fast cooling leaves no time for crystals to grow and gives low crystallinity, while slow cooling or holding below the melting point (annealing) raises it. PET bottles are made by quenching to form a transparent amorphous preform and then stretch-blow molding to align and crystallize the chains, raising strength and gas-barrier performance while staying transparent. In polypropylene, nucleating agents are added to make the spherulites finer, improving transparency and stiffness. When considering the properties of a molded part, the processing conditions matter as much as the resin grade.
Crystalline regions do not melt until the melting point Tm, so even above Tg they tie the chains together and keep the shape. This is why polyethylene and polypropylene, at room temperature above their Tg, are soft yet hold their shape. Higher crystallinity raises modulus, heat resistance, chemical resistance and gas-barrier performance, but tends to lower impact resistance and transparency. Crystalline resins also tend to show larger molding shrinkage than amorphous resins because of the volume decrease on crystallization, which is taken into account in mold design.
Answer: C. About 54%
Assuming the volumes of the crystalline and amorphous regions are additive, the crystallinity by mass is Xc = ρc(ρs − ρa)/[ρs(ρc − ρa)] = 1.000 × (0.925 − 0.850)/[0.925 × (1.000 − 0.850)] = 0.075/0.13875 ≈ 0.54, about 54%. About 50% is the volume fraction obtained from (ρs − ρa)/(ρc − ρa), which differs from the mass fraction by the ratio of densities. About 46% is the mass fraction of the amorphous regions (1 − 0.54). The density method is a simple way to measure crystallinity; it can also be obtained from the heat of fusion by DSC or the ratio of crystalline peak areas in X-ray diffraction. The density of the perfect crystal can be calculated from the lattice parameters of the crystal structure.
Answer: A. Density and modulus increase, and transparency decreases
In crystalline regions the chains are packed regularly and densely, so higher crystallinity raises the density and strengthens interchain interactions, improving modulus, yield strength, heat resistance and chemical resistance. This is why high-density polyethylene (HDPE) is stiffer than low-density polyethylene (LDPE). On the other hand, crystalline and amorphous regions differ in refractive index, and spherulites and other features comparable to or larger than the wavelength of light scatter it, so transparency falls and the material turns milky. Gases also pass through crystalline regions with difficulty, so gas permeability decreases. A decrease in density is the opposite of what happens, and high transparency is characteristic of amorphous polymers such as PMMA and polycarbonate.
Tg is the temperature at which molecular motion begins in the amorphous regions, and Tm the temperature at which the crystalline regions melt. Higher crystallinity makes the change at Tg smaller and the heat of fusion at Tm larger. A completely amorphous polymer has no Tm.
Not in ordinary molded products. Single crystals grown from dilute solution come close, and highly drawn fibers of ultra-high-molecular-weight polyethylene can reach around 90%, but entanglements and chain ends prevent a perfect crystal.
Use a consistent literature value for the heat of fusion of a perfect crystal, subtract the exotherm of cold crystallization during heating, and note that values change with heating rate and thermal history. The first heating reflects the as-molded state, and the second the state after cooling under fixed conditions.
Assuming the volumes of the crystalline and amorphous regions are additive, the crystallinity by mass is Xc = ρc(ρs − ρa)/[ρs(ρc − ρa)] = 1.000 × (0.925 − 0.850)/[0.925 × (1.000 − 0.850)] = 0.075/0.13875 ≈ 0.54, about 54%.
Take the density of perfectly crystalline polyethylene as 1…In crystalline regions the chains are packed regularly and densely, so higher crystallinity raises the density and strengthens interchain interactions, improving modulus, yield strength, heat resistance and chemical resistance.
For the same type of polymer, what generally happens as the…PVC has low crystallinity, and the plasticizer acts mainly by lowering the Tg of the amorphous regions, so it is not that only the melting point falls.
When a plasticizer is added to poly(vinyl chloride) (PVC), …In vacuum there is no convective heat exchange, but improving film crystallinity is not the main purpose of high vacuum.
What is the main reason vacuum evaporation is carried out i…Raising the crystallinity makes it harder for water to enter, slowing degradation.
A bone fixation pin of crystalline poly-L-lactic acid (PLLA…XRD (X-ray diffraction) Glass transition temperature Rule of mixtures Thermosetting resin Viscoelasticity