Page 199 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

|78 Chapter 7 Polymers: Structure, General Properties, and Applications

material is a composite of high-molecular-weight An illustration of fully set bone cement is shown in
PMMA particles interconnected by PMMA chains. Fig. 7.7.

|` PMMAparticle

9

°
,
°
9 Q Polymerized MMA
I
matrix
Q Q e 0 l @--l°MMAdissoIved
o 6 . in monomer
° e
- 9 5

FIGURE 1.7 Schematic illustration of the microstructure of
polymethylmethacrylate cement used in dental and medical
applications.

7.2.2 Crystallinity
Polymers such as PMMA, polycarbonate, and polystyrene are generally amorphous;
that is, the polymer chains exist without long-range order (see also amorphous
alloys, Section 6.14). The amorphous arrangement of polymer
ai chains often is described as being like a bowl of spaghetti or
1 * g
Q 2, Qc Q Amorphous
s s* s as like worms in a bucket, all intertwined with each other. In some
* L 'Q ‘ , an region
n * fp e polymers, however, it is possible to impart some crystallinity
i
'io geggtfjs 1 *‘,ti, 'Q'
Q* ** '
f“.’%'l“
s-;
*
- QW*
_°5§°** Q 4 ¢*)'G=* Q e and thereby modify their characteristics. This arrangement may
n
,.
ug
*a .
¢ XY* x. at 4, ¢ ' e be fostered either during the synthesis of the polymer or by de-
is
Q 1 - s is ‘ * g an ¢ 9 in if , in
7* ,_ g as as 4 formation during its subsequent processing.
,
Q*
1
¢
G 'QQ ,,¢‘,s~i= Q ¢ as
* E The crystalline regions in polymers are called crystallites
1 isgia ¢s’¢si¢¢¢a ‘_ Q
S`°»n|;al=¢ns¢* (Fig. 7.8). These crystals are formed when the long molecules
,ss
. 1 ‘xl ,ing-;¢»l** Crystalline arrange themselves in an orderly manner, similar to the folding of
gg* nx=¢x1»1=** * region a fire hose in a cabinet or of facial tissues in a box. A partially
Q
C G ¢»|*,,¢.;a¢¢’ *"
' 0 crystalline (semicrystalline) polymer can be regarded as a two-
1. a T t * Q * Q
z phase material, one phase being crystalline and the other
Q 5
Q I
amorphous.
By controlling the fatC of solidification during cooling and
FIGURE 1.8 Amorphous and crystalline regions the chain structure, it is possible to impart different degrees of
in a polymer. The crystalline region (crystallite) has
an orderly arrangement of molecules. The higher crystallinity to polymers, although never 100%. Crystallinity
the crystallinity, the harder, stiffer, and less ductile ranges from an almost complete crystal (up to about 95% by
the polymer. volume in the case of polyethylene) to slightly crystallized