Polymer Thermal Transitions: Crystallization, Tm and Tg 高分子專題演講 黃耀正 張豐志教授實驗室 交通大學應用化學所 2009/10/19 Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Glass Transition Temperature (Tg) & Crystalline Melting Point (Tm) Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Thermal transitions of polymers Completely amorphous polymer: glass transition temperature (Tg) only Completely crystalline: crystalline melting point (Tm) only Semi-crystalline: Both Tg & Tm Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Semicrystalline polymers Two major types of thermal transitions: Crystalline melting point (Tm) Above Tm, polymer is liquid Below Tm, polymer forms flexible crystalline solid Glass transition temperature (Tg) Above Tg, polymers are rubbery Below Tg, polymers are glassy Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Glass transition temperature (Tg) From glassy state to rubbery state The temperature where polymer loses its glasslike properties and assumes those more commonly identified with a rubber. Glassy glassy transition rubbery plateau Rubbery flow Viscous flow From glassy state to rubbery state Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Factors affecting Tg Chain length Chain flexibility Side groups Branching Cross-linking ‧ etc. Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Chain length Each chain end has some free volume associated with it. A polymer with shorter chains will have more chain ends per unit volume, so there will be more free volume. Hence Tg' for shorter chains will be lower than Tg for long chains. Note that the shorter-chained polymer also has more free volume frozen in below Tg than the long-chained polymer. Free volume A small amount of unfilled volume is associated with the end of a polymer chain. This volume is called the free volume. Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

. Chain Flexibility Side Groups Branching Cross-linking A polymer with a backbone that exhibits higher flexibility will have a lower Tg. This is because the Ea for conformational changes is lower. Therefore, conformational changes can take place at lower temperatures. Side Groups Larger side groups can hinder bond rotation more than smaller ones, and therefore cause an increase in Tg. Polar groups such as Cl, CN or OH have the strongest effect. Branching Polymers with more branching have more chain ends, so have more free volume, which reduces Tg, but the branches also hinder rotation, like large side groups, which increases Tg. Which of these effects is greater depends on the polymer in question, but Tg may rise or fall. Cross-linking Cross-linking reduces chain mobility, so Tg will be increased. It also affects the macroscopic viscosity of the polymer, since if there are cross-links between the chains, then they are fixed relative to each other, so will not be able to slide past each other. . Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Crystalline Melting Point (Tm) Further increase in temperature, provides more energy to the molecules and the rubber will now melt (For simplicity we are assuming that the hose is made out of thermoplastic, non cross linked material). This is the melt phase. The temperature at which the molecules transition from a rubbery state to a melt state is called the melt temperature or the Tm. Tm Tm Tmo Tmo Tc 1/lc Lc∞ Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Factors affecting Tm Same factors affect both Tg & Tm. Usually high Tg & high Tm are found together and vice versa. High polarity and hydrogen bonding increases Tm. Molecular symmetry leads to high Tm. Eg: Polyethylene has high Tm due to tightly packed crystals although Tg is low due to flexibility of the chain. Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Crystallization Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Crystalline and amorphous polymers Amorphous polymers are generally found in a random coil conformation and have a disordered chain structure. Crystalline polymers are predominantly in the all-trans conformation, and the chains are arranged in lamellae, as below: Folded chains into a two-dimensional lamella One-dimensional chain-folded sequences Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Structure of Crystalline Polymers The Fringed Micelle Model According to the fringed micelle model, the crystallites are about 10 nm long. The chains are long enough to pass several crystallites, binding them together. amorphous crystalline Interconnected by amorphous regions Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Polymer Single Crystals 1957 Keller prepared single crystals of PE. The Folded-Chain Model --- Composed of polymer chains folded back on themselves Adjacent reentry 10~20 nm thick Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

The Unit Cell of Polyethylene Orthorombic TEM image ED pattern Top view Side view Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Single crystal of Nylon 6 Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

PEO-b-PS : the crystals reject the amorphous portion (PS), which appears on the surfaces of the crystals. Optical micrograph Electron micrograph Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Switchboard Model In the switchboard model the chains do not have a reentry into the lamellae by regular folding; they rather reenter more or less randomly. tie molecule lc Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Spherulitic Morphology Polarizing Optical Microscope  Maltese cross Formation 1° nucleus 2° column 3° additional 4° fill When the spherulites are nucleated simultaneously, the boundaries between them are straight. However, when the spherulites have been nucleated at different times, their boundaries form hyperbolas. Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

SEM Image Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Mechanism of Spherulite Formation Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Kinetics of Crystallization Experimental Observations of Crystallization Kinetics Slope = grown rate(μm/min) Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Thermodynamic control Poor chain mobility High supercooling degree (ΔT) Good chain mobility Low supercooling degree (ΔT) ΔT = Tf - Tc Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Example Calculation of Percent Crystallinity Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

PEO-b-PS Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Ac: Intergrated area of crystalline region Aa: Intergrated area of amorphous region Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Methods of Determining Crystal Structure (1) X-Ray Methods (2) Electron Diffraction of Single Crystals (3) Infrared Absorption (4) Raman Spectra Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU

Thanks for your attention Chang’s group, Polymer Research Center Institute of Applied Chemistry, NCTU