Bleach's combat system is largely built around soul-cutting swords known as zanpakuto. Soul Reapers are the primary warriors who wield such weapons in battle, though there are exceptions. A zanpakuto is essential for a Soul Reaper's duty to slay monstrous Hollows and escort the souls of the dead to the Soul Society, but what about an Arrancar's zanpakuto?
In very broad strokes, an Arrancar's zanpakuto is similar to a Soul Reaper's, but there are some functional and thematic differences that set the two apart -- and not always in a good way. In fact, it could be said that Arrancar zanpakuto is the ultimate expression of futility and denial of one's own fate.
Soul Reaper & Arrancar Zanpakuto: The Functional Differences
The basic concept of a zanpakuto is to have a bladed weapon that can release its true power, which will be unique from one character to another. That is the only true similarity between Soul Reaper and Arrancar zanpakuto, however. Most Soul Reaper zanpakuto are shaped like katanas, with minor differences in blade length, the hilt's color, and the handguard's pattern. An Arrancar's sealed zanpakuto may take a wide variety of shapes, from a regular katana all the way to a battle-ax or even a squid-like appendage. Some zanpakuto are actually two weapons in one, such as Gantenbainne Mosqueda's, which appears as a pair of brass knuckles with fang-like attachments on them.
A Soul Reaper's zanpakuto is a separate being from its owner, a unique spirit with its own mind and personality. In this sense the two are really partners, rather than a warrior and their weapon, which may remind Bleach viewers of the living weapons in Soul Eater. By contrast, an Arrancar's zanpakuto is more of a vessel. Once released, the sword vanishes, and the wielder's body will be altered to closely mimic their Hollow form. There is no partnership here.
Soul Reaper zanpakuto will grow stronger as the wielder forms a deeper and more meaningful relationship with its inner spirit. This allows them to first unlock the zanpakuto's shikai mode, and if there is potential, its bankai mode will be mastered as well.
More than once, a Soul Reaper only unlocked their weapon's true power once they set aside their pride and attuned themselves closely to their zanpakuto spirit. Renji Abarai once did this to achieve his bankai, Hihio Zabimaru, and Kenpachi Zaraki later unlocked his shikai, Nozarashi. The latter's achievement came at a heavy price though -- Kenpachi had to slay Captain Retsu/Yachiru Unohana in combat to truly become one with his zanpakuto and learn its name at long last. Arrancars don't go through this process; their zanpakuto's power is ready from the start.
Soul Reaper & Arrancar Zanpakuto: The Thematic Differences
The nature of these weapons, and that of their wielders, make Arrancar and Soul Reaper zanpakuto quite different on a philosophical level. Soul Reapers enrich themselves by speaking to their zanpakuto and befriending them, matching traditional shonen values of forming friendships and bonds of trust. It's a good lesson to teach viewers, while Arrancars have no such opportunity. Captain Sosuke Aizen merely used the Hogyoku's power to turn Hollows into Arrancar and seal their powers within. There's no room for growth beyond that point.
More importantly, though, Arrancar zanpakuto represent the futile attempt to escape the fate of a Hollow. The empty nature of Hollows and Arrancars is one of Bleach's most persistent themes since Hollows seek a worthwhile future when there is none to be had. Many Arrancar, such as the former king Baraggan Luisenbairn and the scientist Szayelaporro Grantz, try to find salvation and purpose through sheer power, but this is futile. Aizen strengthened his Hollow minions by unmasking them and giving them Soul Reaper-style swords, but he merely saw his Arrancar followers as tools.
No matter how hard they try, mimicking a Soul Reaper won't allow an unmasked Hollow to find satisfaction in life. By contrast, a Soul Reaper's zanpakuto has plenty of room for growth and development, thanks to their Soul Reaper partners guiding them.