100% cotton vs cotton/poly blend-which is cooler?

All-cotton is cooler than a synthetic or cotton-synthetic blend (assuming the fabrics are about the same weight).

This is getting 'way out there, I know , but if you're really concerned about keeping cool and could sew your own, linen is great for keeping cool. No one makes linen scrubs or uniforms, but you can get linen fabric in fabric shops.

Pure cotton wrinkles badly and must be ironed. Linen is one big perpetual wrinkle.

I've been wearing only natural fibers for decades, and I rarely iron anything. If you get cotton stuff out of the dryer quickly, and hang or fold it neatly, and don't leave it sitting in a heap, it doesn't need to be ironed (unless you are particular about wanting it to look ironed). Linen? The wrinkles are part of its charm.

Sassy Scrubs makes linen scrubs. Linen is highly “hydroscopic” meaning it rapidly absorbs and gives up moisture. Adsorbing water as quickly as a pond surface, before giving a feeling of being wet, linen cloth can absorb as much as 20% of its dry weight. This explains why linen cloth always feels fresh and cool.

http://www.sassyscrubs.com/fabric_view.php#view_top

for me it is 100% Cotton, some a little hard to keep looking decent, even with starch and ironing, others almost need no pressing at all...... but the all cotton breaths for me.........

I wear Sassy Scrubs cotton scrubs or linen scrubs. They're much cooler than poly/cotton and the color holds up well. If you take your cotton or linen scrubs out of the dryer and don't let them dry in a heap, they don't really need ironing. Linen scrubs are VERY hard, almost impossible, to find, but Sassy Scrubs has them here http://www.sassyscrubs.com/fabric_view.php?cat=0&col=45 They are well worth it for the comfort you gain while you work!