No abstract

Combining the concept of "flexible spacer" which can bring liquid crystalline (LC) properties to the side-chains and the side-group "jacketing" effect which can result in main-chain with rod-like conformation, we have synthesized a new combined main-chain/side-chain LC polymer based on radical polymerization, poly(2,5-bis{[6-(4-methoxy-4 0oxy-azobenzene)hexyl]oxycarbonyl}styrene) (denoted as P 1 ) with two azobenzene groups per repeating unit. The chemical structures of P 1 and the corresponding monomer were characterized using various techniques with satisfactory analysis data. The phase structures and transitions of P 1 were investigated using differential scanning calorimetry, polarized optical microscope, and one-and two-dimensional (1D and 2D) wide-angle X-ray diffraction. We identify that P 1 can form a hierarchically ordered structure with double orderings on both the nanometer and subnanometer length scales. Most likely, the thick main-chains of P 1 obtained by "jacketing" the central rigid portion of terephthalate side-chain to the polyethylene backbone construct a 2D centered rectangular scaffold, which is stable until the sample becomes completely isotropic. The packing of side-chains inside the main-chain scaffold undergoes the transitions of smectic B-(SmB-) like T smecitc A (SmA)-like T isotropic. The confinement arising from the scaffold induces the SmB-like packing and enhances the stability of SmA-like structure. The hierarchically ordered structure of P 1 renders a biaxial orientation with the side-chains perpendicular to the main-chains. We compared P 1 with an end-on side-chain LC polymer of poly(4-{[6-(4-methoxy-4 0 -oxy-azobenzene)hexyl]oxycarbonyl}styrene) (denoted as P 2 ). P 2 bearing one mesogenic group per repeating unit forms a monolayer SmA phase, with the transition temperature much lower than that of P 1 . Upon UV irradiation, in contrast to that P 2 will become isotropic, P 1 can still exhibit LC behavior after the azobenzene groups adopt cis conformation.

Changes in the crystal growth mechanism were observed in pseudo-dewetted, thin layers of low molecular weight (MW) poly(ethylene oxide) (PEO) on a hydrophilic mica surface. The studies were conducted using two PEO fractions (HPEO and MHPEO) with similar MW's (number-average MW of around 4500 g/mol) but different end-group chemistries. For the HPEO, both ends of the chain are capped by −OH groups. For the MHPEO, one end is capped by an −OH group, while the other end is capped by an −OCH3 group. Utilizing in-situ atomic force microscopy, the growth of single crystals as a function of time (t) was monitored at different crystallization temperatures (T x ). Depending on the end-group chemistry and T x applied, two growth laws were observed, which state that the crystal lateral size r or the crystal volume V can be linearly proportional to t (r ∝ t or V ∝ t). Combined with morphological observations of the single crystals, it could be deduced that when r ∝ t, the crystal growth was nucleation-limited (NL), while in the case of V ∝ t, and thus r ∝ t 0.5 at constant crystal thickness, the crystal growth was diffusion-limited (DL). A change of the crystal growth mechanism from the NL to the DL process was also observed with decreasing T x in the MHPEO sample.