In short the answer to this question is yes. Under direct sun light you will feel the rattan become more flexible. It is not noticeable on your chairs as they will just be a little more pliable when touched. There is no visible change to the rattan on the chairs. In our experience we have only every have customers asking about the rattan on top of their tables. The rattan on the top of the dining tables but underneath the toughened glass can often appear a little slack when exposed to direct sun light.
The movement in the rattan under direct sunlight is very much the norm throughout the rattan garden furniture industry and is not a sign of any lasting problem. The rattan can appear to be a little wavy under the glass of the table top when exposed to direct sunlight. As you can imagine the glass acts as a magnifying glass and heats the underneath of the table. When the sun goes down the rattan returns to its original position and the there is very little evidence of the any movement.
I though t I would write a little blog on this subject as only today I had a customer asking my about it. I am going to start writing more blogs in our Q and A section as we do get very similar questions being asked quite a lot. We do also have a Q and A section on the main website along with a rich selection of rattan information on our about rattan garden furniture page.