Physical and Mechanical Properties of Polymers

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1 MATE 453/MSE 553 Physical and Mechanical Properties of Polymers Guided Lecture Notes for Fall 2012 Prof. Michael Kessler Department of Materials Science and Engineering Iowa State University

2 PHYSICAL AND MECHANICAL PROPERTIES OF POLYMERS Section 1 Step-Growth and Chain Growth Polymerization W elcome to Physical and Mechanical Properties of Polymers. These notes are intended to summarize some of the material that we ll be covering this semester. Several sections are intentionally left blank for you to fill in during the lecture. The best approach will be to follow along during the lectures, filling in the empty sections, as well as writing additional notes on blank paper to insert with these pages. You may want to obtain a three ring binder to hold the notes in one place as we progress through the semester. In addition to these formal notes, many classes will not have notes prepared, and you ll need to prepare your own notes from scratch based on the lecture and discussion. Additional material from the text book and references will also be assigned that may not be covered in these notes. Polymer Synthesis Polymers can be categorized by their and by their. ICON KEY Valuable information Interesting fact Definition Chemistry of Synthesis Mechanism of Chain Growth

3 2. 2. Step-growth polymerization involves Chain-growth polymerization involves. 2

4 Step-growth Polymerization Step-growth & Condensation Polymerization Condensation polymerizations (i.e., polycondensation reactions) are those which yield polymers with repeat units having atoms than those present in the monomers from which they are formed. The coupling of reactive end groups from each monomer (i.e., formation of covalent linkage) results in the formation of molecular weight product (H 2 O, HCl, NH 3, etc). Examples of Condensation Reactions (step growth) I) diol + diacid (Polyester) II) diamine + diacid (Polyamide) III) self-condensation of an A-B monomer 3

5 Step-growth & Non-condensation Polymerization No liberation of small molecules I) diisocyanate + diol (Polyurethane) II) Nylon 6 (from ring) Examples of polymers formed by step-growth polymerization 4

and M M w X w = xw =, where M w is weight-average molecular

6 Degree of Polymerization in Condensation Polymerization M n X n = xn =, where Mo is molecular weight of monomer molecule M o and M M w X w = xw =, where M w is weight-average molecular weight of polymer M o n is number-average molecular weight of polymer 5

7 Degree of Polymerization as a Function of Conversion p x n 1 = 1 p Chain-growth Polymerization polymerization). Growth of the polymer involves the successive addition of monomers through an (radical in free radical polymerization, anion in anionic polymerization, and cation in cationic 6

8 The polymerization of monomers (e.g., monomers with carbon-carbon double bond) typically involves a chain reaction. There are three major stages that control addition polymerization: a) ; b) ; c). *: denotes the active center Four Chain Growth Polymerizations 1. Free Radical, R (unpaired electron) 2. Carbocation Ion, R 3. Carbanion, R 4. Coordination Complex 7

concentration of monomer throughout the polymerization. Free Radical Polymerization Kinetics Step 1a.

9 Rapid, preferential growth of initiated polymer chains; high DP attained at start of the reaction. Monomer is capable of reacting only with an active end group (or initiator), resulting in a steadily decreasing (but significant) concentration of monomer throughout the polymerization. Free Radical Polymerization Kinetics Step 1a. Initiation: Initiator can decompose and generate upon thermolysis (as shown in the above), or photolysis (UV radiation ~360 nm), or high-energy radiation (including electrons, gamma rays, x-rays, and slow neutrons) Step 1b. Initiation: 8

10 Step 2. Step 3. Combination: Involves the together of two growing chains to form a single polymer molecule: Disproportionation: A hydrogen atom can be from one chain to the other. 9

11 Kinetic Rate Expression 10

12 Gel Effect Also known as the At higher conversion, the termination rate decreases due increase in viscosity which leads to a lower diffusion rate of growing chains. As a result, their lifetime increases. The diffusion of monomer is also decreasing, but to a much smaller extent. The rate of polymerization and molecular weight significantly increase in a nonlinear fashion. 11

Inhibition and Retardation Certain substances react with the freeradical sites to produce species which are incapable of re-initiating polymerization.

13 Inhibition and Retardation Certain substances react with the freeradical sites to produce species which are incapable of re-initiating polymerization. If such reaction is highly efficient, the polymerization is and the substance is called (see curve c). When the reaction yields species that polymerization, the rate of polymerization is reduced and the substance is referred to as a (see curve b) Comparison and Example 12

14 Example. Lactic Acid and Acrylic Acid a) Lactic Acid b) Acrylic Acid Both can be polymerized, write the repeat unit for each. 13

Macromolecular Chemistry

Macromolecular Chemistry Lecture 5 Step Growth Chain Growth Paul Flory Clears Things Up Polymer Structure is distinct from polymerization process Addition Polymerization H H Condensation Polymerization