Chain conformation and packing of isopentylcellulose in thin films

Multilayers of isopentylcellulose (systematic name: tri‐O‐isopentyl cellulose, IPC) were investigated by means of electron diffraction. Only the persubstituted cellulose ether showed some order in a multilayer as deposited. IPCs of lower degree of substitution (DS) form ordered layers when annealed at temperatures ≤150°C. IPC molecules adopt helix conformation; the lattices show in this temperature range 3‐fold screw axes. From analogy with known structures of cellulose derivatives we suggest left‐handed 32‐helices. At a temperature of about 210°C IPC multilayers undergo a transition into the liquid‐crystalline state and the feature of a stack of individual layers disappears. This was investigated for an IPC of DS = 3. By subsequent cooling a phase transition into a highly crystalline state takes place. From the huge increase of the crystalline density from initially 1.02 g/cm3 in the multilayer system to 1.40 g/cm3 in the bulk crystal at ambient temperature after having passed the liquid‐crystalline state we infer that the transition of the layered system into the liquid‐crystalline state is accompanied by selection of adjacent chains according to their polarity. Simultaneously the chain conformation is changed into a 4‐fold screw with 2 symmetry related chemical repeats forming the asymmetrical unit. Such a chain conformation is not known for other cellulose derivatives, only trimethylamylose adopts a similar conformation with a distorted 43‐helix.