Semi-Crystalline Polymers PDF

16. Semi-crystalline polymers Features of semi-crystalline polymers -containing both crystalline and amorphous components. 15.1. Spherulites. If a melt-crystallized polymer is prepared in the form of a thin film, either by sectioning a bulk sample or casting the film directly, and then view...

16. Semi-crystalline polymers Features of semi-crystalline polymers -containing both crystalline and amorphous components. 15.1. Spherulites. If a melt-crystallized polymer is prepared in the form of a thin film, either by sectioning a bulk sample or casting the film directly, and then viewed in an optical microscope using polarized light, a characteristic structure is normally obtained. It consists of entities known as spherulites that show a characteristic Maltese cross pattern in polarized light. Analysis of the Maltese cross patterns has indicated that the molecules normally are aligned tangentially in polymer spherulites. It has been shown that the b crystals axis is radial in polyethylene spherulites and the a and c crystallographic directions are tangential. 1 It exposes the crystals radiating from the central nucleus and terminating at the spherulite boundaries. Close examination of the micrograph shows that the crystals are lamellar. When viewed edge-on, they appear as fine lines of the order of 10 nm thick and as flat areas when they are parallel to the sample surface. Lamellar structures: chain folding + inter-crystalline links There are thought to be small segments of fairly sharp chain folding and a large number of inter- crystalline links with a given molecule shared between at least two crystals. 15.2. Degree of crystallinity Melt-crystallized polymers are never completely crystalline. This is because there are an enormous number of chain entanglements in the melt and it is impossible for the amount of organization required to form a 100% crystalline polymer to take place during crystallization. The degree of crystallinity is of great technological and practical importance and several 2 methods have been devised to measure it, although they do not always produce precisely the same results. 15.2.1. Density method The crystallization of a polymer from the melt is accompanied by a reduction in specimen volume due to an increase in density. This is because the crystals have a higher density than the molten or non-crystalline polymer and this effect provides the basis of the density method for the determination of the degree of crystallinity. The technique relies upon the observation that there is a relatively large and measurable difference (up to 20%) between the densities of the crystalline and amorphous regions of the polymer. This method can yield both the volume fraction of crystals ϕc and the mass fraction xc from measurement of sample density ρ. If Vc is the volume of crystals and Va the volume of amorphous material, then the total specimen volume V is given by Similarly the mass of the specimen W is given by 𝜌𝑉 = 𝜌𝑐 𝑉𝑐 + 𝜌𝑎 𝑉𝑎 𝑉𝑐 𝜌 − 𝜌𝑎 𝜙𝑐 = = 𝑉 𝜌𝑐 −𝜌𝑎 The mass fraction xc of crystals is similarly defined as 𝑊𝑐 𝜌𝑐 𝑉𝑐 𝜌𝑐 (𝜌 − 𝜌𝑎 ) 𝑥𝑐 = = = 𝑊 𝜌𝑉 𝜌(𝜌𝑐 −𝜌𝑎 ) The density of the polymer sample can readily be determined by flotation in a density-gradient column. The density of the crystalline regions ρc can be calculated from knowledge of the crystal structure. The term ρa can sometimes be measured directly if the polymer can be obtained in a completely amorphous form, for example by rapid cooling of a polymer melt. Otherwise, it can be determined by extrapolating either the density of the melt to the temperature of interest or that of a series of semi-crystalline samples to zero crystallinity. 3 15.2.2. Wide-angle X-ray scattering (WAXS) Flat-plate X-ray diffraction patterns obtained from isotropic melt-crystallized polypropylene. The amorphous halo is then traced in either from knowledge of the scattering from a purely amorphous sample or by estimation using experience with other polymers. The mass fraction of crystals xc is then given to a first approximation by where Aa is the area under the amorphous halo. Ac is the area remaining under the crystalline peaks. Other methods: differential scanning calorimetry (DSC) This involves determining the change in enthalpy ΔHm during the melting of a semi-crystalline polymer. Spectroscopic methods such as NMR and infrared spectroscopy. 15.3. Crystal thickness and chain extension Crystal thickness can be determined by using small-angle X-ray scattering (SAXS) 4  𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃 where n is an integer λ the wavelength of the radiation d is now the periodicity of the array θ the diffraction angle. Factors influencing crystal thickness. a. Crystallization temperature For a given solvent, the lamellar thickness is found to increase with increasing crystallization temperature. Crystallization from solution: the crystallization process is controlled by the difference between the crystallization temperature Tc and solution temperature Ts, rather than the actual temperature of crystallization. Crystallization from melt: at high supercoolings, the thickness of solution and melt-crystallized lamellae are comparable. At low supercoolings, however, there is a rapid rise in lamellar thickness with crystallization temperature that is not encountered with solution crystallization. It 5 is thought that this is due to an isothermal thickening process whereby the crystals become thicker with time when held at a constant temperature close to the melting temperature. b. Molar mass The molar mass of the polymer has a strong effect upon lamellar thickneXss only when the length of the molecules is comparable to the crystal thickness. Normally, the molecular length is many times greater than the fold length and so even large increases in molar mass produce only correspondingly small increases in lamellar thickness. c. Pressure The melting temperature of polymers increases rapidly with pressure and it is found that crystallization of polyethylene at low supercoolings at pressures above about 3 kbar (300 MPa) can produce crystal lamellae in which some of the molecules are in fully extended conformations. Crystals of up to 10 μm thick have been reported for high-molar-mass- polymers. In the chain-extended lamellae, it is thought that the molecules fold backwards and forwards several times as it is known that the molecular length is somewhat greater than the average crystal thickness. Chain-extended morphologies can have extremely high degrees of crystallinity, sometimes in excess of 95%. However, their mechanical properties are extremely disappointing. Chain-extended polyethylene is relatively stiff but absence of inter- crystalline link molecules causes it to be very brittle and it crumbles when deformed mechanically. 6 7