Stress-strain behavior of some polymers
The influence of temperature on the stress-strain characteristics of polymethyl methacrylate
Part Four Structure and Properties
The Physical Properties of a polymer material are largely determined by:
A list of physical properties might include: melting point; boiling point; solubility; melt viscosity; tensile strength; cohesion.
Melting point (softening point): the melting point of a polymer does not occur over a sharp temperature (1-2 oC) as is observed for small organic molecules. If a polymer becomes a melt, there is usually a range of as much as 50 oC over which the viscosity of the polymer slowly changes from that of a solid to that of a liquid. Note that for a polymer melt, a polymer must be a thermoplastic.
The glass transition temperature refers to a point where
there is a change in polymer molecular chain motion which has drastic effects on
strength. Tg is sometimes called the "glass-rubber" transition. The crystalline
melt temperature (Tm) is higher than Tg, and at Tm the crystalline domains of a
polymer melt to become amorphous.
Solubility Most polymers are insoluble in water.
Some polymers can be soluble in strong organic solvents. Polymer nonsolubility
is an advantage for a finished product. However, it may present a tiresome
problem for the engineer who is trying to manufacture a product.
Melt viscosity
Tensile strength-
Tensile strength numbers (psi)- 145 psi = 1 MPa
polyethylene (low to medium density) 1,000- 2,400
poly(tetrafluoroethylene) (a.k.a. teflon) 3,500
polyethylene (high density) 4,400
poly(dimethylsiloxane) 5,000
polypropylene 5,000
poly(vinylidene chloride) 8,000
polystyrene 8,000
polyamides 9,000 to 12,500
polycarbonate 9,500
polyesters (cast- as opposed to molded) ~10,000
polysulfone 10,200- 12,000
poly(phenylene oxide) 10,500
The higher the chain
interaction, the higher the molecular cohesion.
The following is the molecular cohesion per 5 angstroms of polymer chain. Values
around 5 or higher on the scale below should be considered "high."
polyethylene 1.0 kcal/mole
polyisobutylene 1.1 kcal/mole
rubber 1.3 kcal/mole
poly(vinyl chloride) 2.6 kcal/mole
poly(vinyl acetate) 3.2 kcal/mole
poly(styrene) 4.0 kcal/mole
poly(vinyl alcohol) 4.2 kcal/mole
Chapter 10 Morphology and order in crystalline polymers
A. Configuration of Polymer Chains
I. Configuration Involving an Asymmetric Carbon Atom
Simple linear polymers tends to in the fully extended (all trans) planar zigzag conformation.
For examples:
poly(a-olefins), polystyrene, poly(vinyl isobutyl ether) and poly(methacrylate) can be made of isotactic configuration
poly(methyl methacrylate) and 1,2-addition of polybutadiene can be made syndiotactic.
Whether a polymer is isotatic or syndiotactic usually determines its crystal structure.
II. Configuration Involving a Carbon-Carbon Double bond
Various configurations exist, for example, isoprene from monosubstituted butadiene
III. Optically Active Polymers -- A polymer that can rotate the plane of polarization of light.
They can be prepared in a several ways:
1) Polymerization of optically active monomers in such a way that their asymmetric centers are preserved
2) Introduction of activity into optically inactive polymers by reactions that place asymmetric centers on side chains.
3) Stereospecific polymerization of optically inactive monomers with optically active catalyst in such a manner that the repeat units have one or more asymmetric carbon atoms of a given configuration
4) Polymerization of racemic monomer mixtures to yield polymer from only one of the monomer (as by using a selective initiator) or from both polymerizing simultaneously and independently to yield a mixture of dextro-rotatory and levorotatory polymer which can be separated.
B. Crystal Structures of Polymers
I. Structural Requirements for Crystallinity
Many polymers are partially crystalline.
There is a close relation between regularity of molecular structure and crystallizability. Typical crystalline polymers are those whose molecules are chemically and geometrically regular in structure. Occasional irregularities, such as branching of polyethylene, limit the extent of crystallization but do not prevent its occurrence.
Typical noncrystalline polymers include those in which irregularity of structure occurs; copolymer with significant amounts of two or more quite different monomer constituents or atactic polymers.
It is concluded that CH2, CHOH, CF2, and C=O groups are all fit into similar crystal lattices, and the structures of polymers from these monomers are similar. Others those absent of crystallinity are due to their large size substituent and their atactic structure.
II. Structures Based on Extended Chains
Polyethylene. Near perfect linear zigzag chain (figure shown as 10-3)
It exhibits polymorphism -- more than one crystal structure, depending on conditions
Poly(vinyl alcohol). Similar to that of polyethylene
Poly(vinyl chloride) No crystallinity. Teh polymer is primarily syndiotactic but with considerable irrgularity.
Polyamides, including poly(hexamethylene adipamide) (66-nylon), poly(hexamethylene sebacamide) (610-nylon), and polycaprolactam (6-nylon), etc. Fully extend chains linked by H-bonds to form sheets that giving two crystal structures. oxygen atoms of one molecule are always found opposite NH group of a neighboring molecule, with the N-H...O distance of 2.8A. Other nylons (99, 106, 1010, and 11) contains chains slightly distorted from the planar zigzap form.
Cellulose. Many forms of derivatives of cellulose have crystal structures.
III. Distortion from fully extended Chains
Polyesters. In most aliphatic polyestes and poly(ethylene terephthalate), the polymer chains are shortened by rotation about the C-O bonds to allow close packing. As a result, the main chain are no longer planar.
Polyisoprene and Polychloroprene. They have similar crystal structures.
Polypeptides: two crystal structures
β-keratin structure--nearly extended polypeptide chains arranged into sheet through hydrogen bonding.
α-keratin structure--polymers with bulky substituents closely spaced along the chain often take on a helical conformation in the crystalline phase to minimize the energy.
Many isotatic polymers crystallize with helical conformation. A couple of helix is show as figure 10-4.
C. Morphology of crystalline Polymers
I. Polymer Single Crystals and Lamellae-- Some single crystal polymers may be made.
All the single crystals reported have the same general appearance -- a thin, flat platelet (Lamellae) about 100A thick and often many micrometers in lateral dimensions
The most prominent organization in polymers on a scale larger than lamellae is the spherulite, a spherical aggregate ranging from submicroscopic in size to millimeters in diameters in extreme cases.
Schematic representation of the detailed structure of a spherulite.
D. Crystallization and Melting
I. Crystallization Kinetics
Experimental: 1) A property varying with total amount of crystallinity, such as specific volume (and enthalpy etc.), is observed as a function of time at constant temperature. 2) Observed directly with the microscope.
II. Degree of Crystallinity
Higher degree of crystalliniy could improve the polymer properties such as stiffness and strength, solvent resistance and dimensional stability.
Ways including: specific volume, x-ray diffraction, IR and DSC.
Specific volume:
wc--weight fraction of material crystallized; V--specific volumes of the specimen; Vc--specific volumes of pure crystals; Va--completely amorphous materials;
The crystalline specific volume is determined from x-ray unit cell dimensions, while Va is usually obtained by extrapolation of the specific volumes of polymer melts.
or
where r is the density of the sample (as determined by a gradient column or dilatometry, ra is the density of density of the amorphous phase, and rc is the density of the crystalline phase.
x-ray diffraction
An x-ray diffraction
spectrograph consists of a plot of x-ray counts received by a detector vs. the
scattering angle of the detector.
The computer performs a mathematical deconvolution from which the true area of
the crystalline peaks, and the amorphous peak can be determined. Notice the read
and orange crystalline peaks.
If it is determined that the area of one crystalline peak is 5.0, the area under
the other is 10, and the area under the amorphous peak is 50, then the percent
crystallinity is:
5 + 10 15
----------- = ---- = 0.23 23% relative crystallinity
5 + 10 + 50 65
The amount of crystallinity in a polymer depends on the following:
the physical treatment of the polymer
the thermal history of the polymer
shows an illustration of semicrystalline polymer (type
'crystalline-amorphous' into the find function when you arrive at the site)
Examples of crystallinity values:
polyethylene, high density (HDPE) 50- 90%
teflon 95%
poly(vinyl chloride) (PVC) 5%
trans-poly(1,4-butadiene) 80%
cis-poly(1,4-butadiene) 0%
OTHER CRYSTALLINITY INFORMATION: Crystallinity in these polymers can be as
high as
Linear polyethylene is 90% crystalline
Isotactic polypropylene is 90% crystalline
Linear random poly(ethylene-co-propylene) has 0% relative crystallinity. Thus,
the random copolymerization of two monomers which produce highly crystalline
homopolymers produces an amorphous copolymer.
Tacticity (see isotactic, syndiotactic, and atactic) is important:
Isotactic polypropylene is 90% crystalline, but atactic polypropylene is 0%
crystalline.
Examples of polymer melt temperatures:
poly(ethylene glycol adipate) 45 deg C
poly(ethylene oxide) 66 deg C
poly(propylene oxide) 77 deg C
linear polyethylene 130 deg C
polypropylene 160 deg C
poly(vinylidine chloride (PVC) 210 deg C
poly(chlorotrifluoroethylene 210 deg C
polystyrene 230- 240 deg C
nylon 66 235 deg C
teflon 327 deg C
Infrared Absorption
Suitable IR bands used for determining amorphous and crystalline structure.
DSC method:
Hm—Energy absorbed during the melting, Hc—energy released during the crystallization, Hc* (or Hm*) – specific heat of melting.
E. Strain-induced morphology (omited)
Homework:
1. Draw structure formulas indicating the stereoregular chain configuration in a) atactic polystyrene b) isotatic polypropylene c) syndiotactic PVC.
2. Why poly(vinyl acetate) can't be crystallized?
3. One example of degree of crystallinity calculation.
4. Compare the density of crystalline polymer and its amorphous counterpart.