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 less than or equal to 150 degrees 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 3(2)-helices. At a temperature of about 210 degrees 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/cm(3) in the multilayer system to 1.40 g/cm(3) 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 4(3)-helix.