Page 336 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 313

to higher intermolecular forces. This means that mechanical properties such as modulus and strength
increase with crystallinity. Ductility decreases with crystallinity since polymer chains have less room
to slide past each other.
The primary requirement for crystallinity is an ordered repeating chain structure. This is why stere-
oregular polymers are often crystalline and their irregular counterparts are amorphous. Stereoregular
polymers have an ordered stereostructure: either isotactic or syndiotactic. Isotactic polymers have the
same configuration at each stereo center, while configuration alternates for syndiotactic polymers (see
Fig. 13.4). Atactic polymers have no pattern to their stereostructure. Polypropylene (PP) is a classic
example of a polymer whose crystallinity and properties change drastically depending on stere-
ostructure. Syndiotactic and isotactic PP have a high degree of crystallinity, while atactic PP is com-
pletely amorphous. Isotactic PP has excellent strength and flexibility due to this regular structure and
makes excellent sutures. The atactic PP is a weak, gumlike material. Recent advances in polymer syn-
thesis have made available new polymers with well-controlled tacticity based on olefins. It is likely
that they will find use as biomaterials in the future.

Me H Me H Me H Me H Me H

Isotactic polypropylene Me H Me H H Me H Me Me H

Me H H Me Me H H Me Me H Atactic polypropylene

Syndiotactic polypropylene
FIGURE 13.4 Stereoisomerism in polypropylene: Me = CH .
3
Crystallinity plays a large role in the physical behavior of polymers. The amorphous regions play
perhaps an even greater role. Some amorphous polymers such as PMMA are stiff, hard plastics at
room temperature, while polymers such as polybutadiene are soft and flexible at room temperature.
If PMMA is heated to 105°C, it will soften and its modulus will be reduced by orders of magnitude.
If polybutadiene is cooled to –73°C, it will become stiff and hard. The temperature at which this hard
to soft transformation takes place is called the glass transition temperature T .
g
Differential thermal analysis (DTA) or a similar technique called differential scanning calorimetry
(DSC) can be used to determine the temperature at which phase transitions such as glass transi-
tion temperature and melting temperature T occur. DTA involves heating a polymer sample along
m
with a standard that has no phase transitions in the temperature range of interest. The ambient tem-
perature is increased at a regular rate and the difference in temperature between the polymer and the
standard measured. The glass transition is endothermic; therefore, the polymer sample will be cooler
compared to the standard at T . Similarly, melting is endothermic and will be detected as a negative
g
temperature compared to the standard. If the polymer was quenched from melt prior to DTA analysis,
it may be amorphous even though it has the potential to crystallize. In this case, the sample will crys-
tallize during the DTA run at a temperature between the T and T . Figure 13.5 shows a schematic DTA
g
m
curve for a crystalline polymer quenched for melt. A polymer that does not crystallize would show
a glass transition only and the crystallization and melting peaks would be absent. These measure-
ments can be made to identify an unknown plastic, or to aid in the synthesis of new polymers with
desired changes in mechanical properties at certain temperatures.
Random copolymers have no pattern to the sequence of monomers. A random copolymer
using repeat units A and B would be called poly(A-co-B). The term alternating copolymer is