Crystallinity creates tightly packed, highly ordered regions in a polymer. These crystalline domains restrict how much the chains can move and reduce the free volume between them, so the material becomes harder to deform. In a loaded polymer, those stiff, lattice-like regions carry more of the applied stress, boosting the overall elastic modulus and hence the stiffness. The amorphous parts are more mobile and compliant, so increasing the fraction of crystalline material makes the whole structure stiffer. The other ideas don’t fit: crystallinity doesn’t make diffusion easier (it actually hinders it), it doesn’t increase free volume (it decreases it), and it doesn’t change the chain length, which is a fixed molecular property.

Grain boundaries are planar defects: they form the interface between two differently oriented crystal grains in a polycrystal. Because they lie as a plane that extends through the material, they are a two-dimensional feature. Point defects are localized atoms missing or extra within the lattice, and line defects are dislocations that run through the crystal. Volume defects are three-dimensional features like voids or inclusions. Since grain boundaries are the boundary between grains and act as an interface rather than a localized point or a line, they are classified as interfacial (planar) defects.

Plastic deformation in metals mainly happens when dislocations glide along specific slip systems, which are particular planes and directions where atoms can slide past one another. For a crystal to deform easily in any direction, it needs several independent slip systems that can be activated under a range of stresses. Hexagonal close-packed metals have far fewer easily activated slip systems than face-centered cubic or body-centered cubic metals. The easiest slip mode in HCP crystals is glide on the basal plane, but slip on non-basal planes requires much higher stresses and is not readily activated at room temperature, so the number of usable slip systems is limited. With fewer available slip systems, the crystal cannot accommodate arbitrary plastic strains as readily, making HCP metals less deformable (less ductile) under typical conditions. Temperature or alloying can activate additional slip systems and improve deformability, but even then HCPs tend to be less ductile than FCC or BCC metals. The idea that diffusion governs slippage would apply to diffusion creep at very high temperatures, not to the ordinary dislocation glide that controls room-temperature plasticity in metals.

In materials terminology, the term for long, thread-like filaments with a high length-to-diameter ratio that can be made from polymers or ceramics is fibers. This name emphasizes the shape and usage of the material rather than its exact composition. You can have polymeric fibers (like nylon) or ceramic fibers (like silicon carbide); their internal structure can be amorphous or polycrystalline, yet they’re still called fibers because of their slender, filamentary form. Wires describe metallic conductors and aren’t defined by this fiber geometry; whiskers are typically single-crystal filaments used for fracture studies; polymers are a broad material class, not the specific elongated-filament category. So the correct class name is fibers.