Cellulose thinners won't harm your brushes, warm to hot water will destroy them instantly, as will not cleaning them thoroughly after every painting session. The main culprit is build up at the base of the ferrule, and this starts as soon as you use a brand new brush - and its this that forces the hairs to divide into two or more bundles. Easy to clean, just takes time. When the main body of paint has been washed out, switch to a clean source of thinners, soak, the brush, then work your thumbnail firmly, from the ferrule edge, towards the point of the brush, working your way round the whole circumference of the ferrule, then dip back into the thinners, to wash away what you've just disturbed, then press carefully into a paper towel. Repeat this a few times, and if you've been doing this from new, it'll keep them good for years. Note, this takes a lot longer to explain than to do!