Polymers

Question 1

Differential Scanning Calorimetry

Answer 1

A means of determining thermodynamic data about polymers eg phase changes. The area under the peak is equivalent to ∆H. A sharp, narrow peak indicates a first order thermodynamic transition.
Peaks that represent the same transition upon heating vs cooling do not occur at the same time as there is a thermal memory of the sample.
Multiple runs must be made to determine mp accurately due to the possibility that the polymer is in a kinetically trapped state.

Question 2

X-ray Crystallography

Answer 2

The measurement of coherently scattered radiation that can be used to determine any repeating long-range order of a solid, or can be used to infer the degree of crystallinity based on the sharpness of the peak.

Question 3

WAXS

Answer 3

Wide-angle x-ray scattering occurs when the spacing between atoms is small, resulting in a wide scattering. The scattered rays form a halo-esque shape around the path of the incident beams for non-crystalline materials. Degree of crystallinity can be inferred by how smeared-out the scattered rays are, rather than being discrete, ordered points.
1-2 Å spacing.

Question 4

SAXS

Answer 4

Small-angle x-ray scattering looks at longer-range scattering, on the order of 10-1000 Å spacing. Can look at longer-range ordering.

Question 5

Scherrer Effect

Answer 5

Line-broadening effect in polymers in XRD. Small (nano) size crystalites form in the polymer, and when they are of the same order of magnitude size as the x rays, the sharp line becomes a broad peak. The equation is:
tau=K (lamda)/(ħcos(theta))
K=shape constant
tau=crystal size
ß=full-height half maximum
theta=Bragg scattering angle

Question 6

Paracrystallinity

Answer 6

Structural nature of polymer leads to poor long-range translational order that creates a highly defective crystal matrix. Results in ill-defined grain boundaries

Question 7

Polymer density

Answer 7

Use gradient column with gradient of density solute to determine density of polymer sample. The real density of the polymer is the average of the column density and the crystalline density determined through XRD

Question 8

Solid State NMR

Answer 8

Dissolving a sample removes dipolar interaction and chemical shift aniosotropy but gives no information about degree of crystallinity. Solid samples spun at an angle 54.74 to the applied magnetic field do not experience these line broadening effects.
The resulting line widths indicate degree of crystallinity. Relaxation time can also be used to measure crystallinity.

Question 9

Structural impact on polymer crystallinity

Answer 9

A crystalline polymer must be:
Symmetrical with regular structure
Have functionalized chains with strong interactions to promote packing
or
Must be stereoregular if possible

Small side groups do not hinder packing but larger ones can provide attractive interactions between chains